JP3549728B2 - Method of forming group III nitride semiconductor crystal and semiconductor device including the same - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a selective formation technique for forming a crystal composed of a group III-V compound semiconductor containing nitrogen as a constituent element (group III nitride semiconductor) in a selective region of a surface of a group III-V compound semiconductor crystal. About. In addition, the present invention relates to a technique for forming a semiconductor device having a III-V compound semiconductor crystal layer containing the crystal.
[0002]
[Prior art]
(Semiconductor device using microstructure)
In general, a Quantum Well structure is known as an example of a super lattice structure formed by laminating ultra-thin films having a thickness of several nanometers (nm) to several tens of nm (Tokomi Deep Sea). Supervisor, "Semiconductor Engineering" (March 20, 1993, 1st edition, 7th edition, published by Tokyo Denki University Press), pp. 258-260). For example, a quantum effect device having this quantum well structure as an active layer includes, for example, an SQW or MQW having a light emitting layer having a single quantum well (Single QW: SQW) or a multiple quantum well structure (Multi QW: MQW). Optical devices such as a laser diode (abbreviation: LD) are known (Yasuharu Suematsu, “Optical Device” (May 15, 1997, published by Corona Co., Ltd.), pp. 62-63. reference).
[0003]
In addition to the above-described semiconductor elements using quantized carriers that behave in a two-dimensional plane space, recently, devices using one-dimensionally quantized (low-dimensional) carriers have been proposed. The invention described in Japanese Patent Application Laid-Open No. 9-64476 discloses a gallium arsenide (GaAs) / GaInAs heterostructure utilizing a gallium indium arsenide (GaInAs) semiconductor microcrystal having a small volume and a small volume and having a small spatial extent. A junction light emitting device is disclosed. In general, from a light emitting layer having a quantum well structure that provides quantized carriers, there is an advantage that monochromatic light emission is emitted as compared with a case where the half width is reduced. Recently, in the indium arsenide (InAs) / GaAs quantum LD, emission of a narrow half width is actually consequently achieved, and further reduction in threshold voltage and improvement in emission efficiency are expected by utilizing the quantum dot effect. (See “Applied Physics,” Vol. 67, No. 2, (1998), pp. 172-175.)
[0004]
A conventional quantum well structure, for example, an SQW or MQW structure is formed by a method in which crystal layers formed by an epitaxial film forming technique are stacked one by one. Gallium arsenide (chemical formula: GaAs) or aluminum gallium arsenide (Al_xGa_yAn SQW or MQW structure can be formed relatively easily from a general III-V compound semiconductor such as As: 0 <x ≦ 1, x + y = 1). This is because indium phosphide (chemical formula: InP) is used as a barrier layer and a gallium-indium arsenide mixed crystal (for example, Ga_0.47In_0.53This is because, as in the case of forming an SQW or MQW structure using As) as a well layer, a large change in film formation temperature is not required.
[0005]
On the other hand, when forming an SQW or MQW structure from a group III nitride semiconductor growth layer, a complicated film forming operation is required. Conventionally, a MQW structure type light emitting layer for a quantum effect light emitting device employing a group III nitride semiconductor growth layer has been generally used as a gallium nitride-indium mixed crystal (Ga)._bIn_cN: 0 <c ≦ 1, b + c = 1) as a well layer, and a mixed crystal of aluminum nitride and gallium (Al_aGa_bN: 0 ≦ a ≦ 1, a + b = 1) as a barrier layer. However, Ga_bIn_cN mixed crystal and Al_aGa_bAn appropriate film forming temperature is separated from the N mixed crystal by several hundred degrees (see J. Crystal Growth, 145 (1994), pp. 209 to 213). Therefore, in order to form the SQW or MQW structure, a complicated film forming operation is required such that the film forming temperature needs to be frequently changed.
[0006]
In addition, semi-one-dimensional quantum structures such as a quantum wire structure and a quantum dot structure are conventionally formed mainly using an epitaxial growth technique, particularly a selective epitaxial technique. For example, sapphire (α-Al₂O₃) Is covered with a mask material having a selectively opened region, and a group III nitride semiconductor crystal having a shape suitable for acting as a quantum dot or a quantum line is formed selectively in this opening region. (Jpn. J. Appl. Phys., 1997), Vol. 36 (1997), pages L552-L535). However, in order to form a low-dimensional group III nitride semiconductor quantum structure, it is necessary to further surround such a dot-shaped or linear crystal with a group III nitride semiconductor crystal having a different band gap. There is.
[0007]
For example, Ga as a short-wavelength visible light emitting medium_bIn_cN (0 <c ≦ 1, b + c = 1) is a quantum dot, and Al_aGa_bFIG. 10 is a schematic cross-sectional view of a laminated structure including a quantum structure layer in which N (0 ≦ a ≦ 1, a + b = 1) surrounds the dots. The steps of forming the laminated structure will be described in order. For example, the buffer layer 102 and the Al_aGa_bThe N lower cladding layer 103 is formed once. After the film formation, the surface of the lower cladding layer 103 is coated with silicon dioxide (SiO 2).₂) Or silicon nitride (Si₃N₄) And the like. The mask material is opened only in a region where formation of quantum dots or quantum lines is desired. With the film for selective epitaxial growth still covered, the operation is shifted to the film formation operation again, and Ga is formed on the surface of the lower cladding layer 103 in a dot shape or a line (band) shape._bIn_cGrow N crystals. In order to provide a structure in which quantized carriers are provided, a group III nitride semiconductor layer serving as the light emitting layer 104 is formed so as to surround the quantum dot-like crystal 107 so as to surround it. On the light emitting layer 104, for example, Al_aGa_bA double hetero (DH) junction type light emitting unit 10 including a crystal layer (quantum structure layer) having a quantum structure made of a group III nitride semiconductor crystal is formed by covering the upper cladding layer 105 made of N.
[0008]
Regardless of either a two-dimensional quantum structure such as a quantum well or a one-dimensional quantum structure such as a quantum dot, a conventional quantum structure made of a group III nitride semiconductor has a structure similar to that described above. Are formed by laminating them.
[0009]
In the conventional method of forming a quantum structure by a stacking method, the temperature at the time of layer stacking may be as high as over 1000 ° C. (“optical”, 22 (1993), No. 11, pages 670-675). For this reason, sapphire (α-Al₂O₃Crystal materials having excellent heat resistance, such as single crystals, have been used as substrates. However, sapphire, like hexagonal silicon carbide (SiC) and many oxide crystals, does not exhibit clear cleavage. For this reason, it is difficult to form a smooth and flat end face of the element. This means that, for example, in a quantum well structure LD, an optical resonance surface that requires smoothness and flatness cannot be formed by using cleavage of a substrate crystal. At present, in a short-wavelength group III nitride semiconductor LD using sapphire as a substrate, precise etching is used (see Japanese Patent Application Laid-Open No. 9-283861), or the crystal plane orientation of the sapphire substrate is changed or the chip is chipped. The optical resonance surface is formed by a technique that requires a precise control of the cutting direction for the formation (see Japanese Patent Application Laid-Open No. 9-275243).
[0010]
[Problems to be solved by the invention]
For example, a conventional problem when forming a quantum structure composed of quantum boxes or quantum dots is caused by the complexity of the epitaxial growth method in which the growth layers need to be stacked one by one to form a quantum structure, or the selective epitaxial growth method. are doing. If a method for easily forming a group III nitride semiconductor quantum structure is provided, it is possible to overcome at least the problems associated with the conventional method for forming a group III nitride semiconductor microstructure described above.
[0011]
A first object of the present invention is to provide a method for easily producing a group III nitride semiconductor crystal which can act as a quantum dot or a quantum line, without using a conventional group III nitride semiconductor epitaxial film formation method. Is to provide.
[0012]
A second object of the present invention is to provide a semiconductor device provided with a group III-V compound semiconductor crystal layer having a crystal of this type of group III nitride semiconductor.
[0013]
[Means for Solving the Problems]
In order to achieve the above object, the present invention provides a simple heat treatment which results in thermal alteration due to stoichiometric imbalance of the surface of a group III-V compound semiconductor crystal without using an epitaxial film forming technique. The present invention provides a method for forming a group III nitride semiconductor microcrystal in a selected region by utilizing the method. That is, in the present invention,No. III Contains at least one of Al, Ga, and In as a group-constituting element, and includes As or P as a group V-constituting element III -The surface of the group V compound semiconductor crystalheatingMeltingThen, the group V constituent element in the III-V compound semiconductor crystal is volatilized, and a droplet mainly containing the group III constituent element as a main component is partially formed on the surface of the III-V compound semiconductor crystal. And a III-V compound semiconductor crystal having the droplet.Heating in an atmosphere containing nitrogen,Part or all of the group V constituent element in the droplet is replaced with nitrogenA second step of converting into a group III nitride semiconductor crystal;RukoAnd a method for forming a group III nitride semiconductor crystal.
[0014]
In the present invention, a group III nitride crystal is formed after at least two of the first and second steps. In the first step, a group III-V compound semiconductor bulk crystal and a vapor-phase- or liquid-phase-grown group III-V compound semiconductor crystal layer deposited on the surface thereof are subjected to a heat treatment, and the surface is treated. A droplet containing a Group III constituent element as a main component is formed. The droplets are heated by heating to volatilize Group V constituent elements preferentially from the surface of the group III-V compound semiconductor crystal and intentionally shift the stoichiometric composition of the surface in the direction of enriching the Group III element. Form. Therefore, it is possible to desirably use a group III-V compound semiconductor crystal having a property that the group V element can be volatilized by heating at an appropriate temperature and that efficiently generates droplets.
[0015]
In the present invention, it is intended that a group III nitride semiconductor crystal is constituted by using a liquid droplet containing a group III element as a main component. Group III nitride semiconductor fine particles are not formed from solid particles mainly containing a group III element. The structure of the group III-V compound semiconductor crystal is generally represented by composition formula B_kAl_lGa_mIn_nQ_1-rN_r(0 ≦ k, l, m, n ≦ 1, k + 1 + m + n = 1, 0 <r ≦ 1, and the symbol Q represents a Group V element other than nitrogen). Among the group III elements constituting these group III-V compound semiconductor crystals, the melting point of boron (element symbol: B) is 2300 ° C. (edited by National Astronomical Observatory of Japan, “Physical Chronological Table 1994” (Heisei 9). (November 30, 3rd, published by Maruzen Co., Ltd.), p. 484). For this reason, even if the group V element bonded to boron can be efficiently volatilized, solid boron is consequently obtained at a temperature lower than the melting point of boron. Conversely, at a high temperature equal to or higher than the melting point of boron, which is expected to form droplets containing boron as a main component, most III-V compound semiconductors melt. Therefore, a group III-V compound semiconductor crystal containing boron as a group III constituent element is not a desirable crystal in the present invention.
[0016]
On the other hand, according to a published publication (see Iwanami “Physical and Chemical Dictionary” (published on April 5, 1976, 3rd edition, 7th edition, published by Iwanami Shoten Co., Ltd.)), the melting point of aluminum (element symbol: Al) is 660. 2 ° C. The melting point of indium (element symbol: In) is 156.4 ° C. The melting point of gallium (element symbol: Ga) is 29.78 ° C., which is even lower than that of Al or In. The boiling points of Al, In, and Ga are about 2060 ° C., 2100 ° C., and 2300 ± 150 ° C., respectively. Thus, Al can exist as a liquid in the range of about 660C to about 2060C. The temperatures at which In and Ga can exist as liquids are in the range of about 156 ° C to 2100 ° C and about 30 ° C to about 2300 ° C, respectively. Here, for example, the melting point of gallium arsenide (GaAs) is 1238 ° C., and that of indium phosphide (InP) is 1060 ° C. The melting point of aluminum arsenide (AlAs) is 1760 ° C. (See Iwao Teramoto, “Introduction to Semiconductor Devices” (March 30, 1995, first edition, published by Baifukan Co., Ltd.), p. 28). That is, Al, Ga, and In can exist as liquids at a temperature lower than the melting point (represented by the symbol M (° C.)) of the group III-V compound semiconductor crystal containing the group III constituent element.
[0017]
Therefore, in the present invention, it is preferable to use a III-V compound semiconductor crystal containing Al, Ga, and In as Group III elements and As and P as Group V elements. In particular, a group III-V compound semiconductor crystal containing Ga having a large temperature difference between the melting point and the boiling point as a group III constituent atom can be conveniently used for forming a droplet composed of a group III constituent element. Specifically, available III-V compound semiconductor crystal materials include aluminum arsenide / gallium mixed crystal (Al_xGa_1-xAs: 0 ≦ x ≦ 1) gallium-arsenide / indium mixed crystal (Ga_yIn_1-yAs: 0 ≦ y ≦ 1) gallium-indium phosphide mixed crystal (Ga_yIn_1-yP: 0 ≦ y ≦ 1) or aluminum phosphide / gallium mixed crystal (Al_xGa_1-xP: 0 ≦ x ≦ 1). In the temperature range where Al, Ga and In can exist as a liquid, desorption of arsenic (element symbol: As) or phosphorus (element symbol: P), which is a group V constituent element, from the crystal surface occurs conveniently. It is also a temperature. Arsenic (As) and phosphorus (P) have a higher vapor pressure than antimony (Sb) (see RCA Review (Jun. 1969), pages 285-305). Therefore, a group III-V compound semiconductor crystal containing arsenic (As) or phosphorus (P) as a group V constituent element is a material that can be conveniently used for generating droplets. In particular, arsenic has the advantage of lower fluidity at high temperatures and the ability to form uniform and clear droplets as compared to phosphorus. That is, Al_xGa_1-xAs (0 ≦ x ≦ 1) or Ga_yIn_1-yAs (0 ≦ y ≦ 1) is a particularly suitable crystal material in the present invention.
[0018]
In the present invention, in the first step, the above-described III-V compound semiconductor crystal is heated to generate droplets on the surface of the crystal. Droplets are generated by volatilizing the group V constituent elements. Therefore, heating is performed at a temperature equal to or higher than the temperature at which desorption of group V constituent elements from the crystal surface starts. Desorption of the group V element starts at a temperature of 450 ° C. or higher as a whole. Further, the desorption becomes significant at about 500 ° C. or more. Taking an InP single crystal having a crystal plane orientation of {001} as an example, in a heat treatment at a temperature lower than 500 ° C., mainly due to the evaporation of phosphorus (P), as shown in FIG. Although cracks 106 composed of substantially parallel linear holes are generated, they obviously do not sufficiently generate droplets containing indium as a main component. Clear drop formation is observed at temperatures of 550 ° C. and above.
[0019]
Conversely, in the heat treatment at an extremely high temperature, the evaporation of the group V constituent elements becomes more conspicuous, so that the droplet develops greatly, and as a result, almost the entire crystal surface is covered with the droplet. . In the present invention, which intends to form a group III nitride semiconductor microcrystal in a selective region with a droplet formed partially on the crystal surface as a nucleus, a droplet is generally generated on the crystal surface. The situation is not favorable. Therefore, heat treatment at an extremely high temperature is disadvantageous because droplets are generated at a high density on the crystal surface. In order to prevent such a situation, it is effective to set the heating temperature to M-250 (° C.) or less based on the melting point (M (° C.)) of the group III-V compound semiconductor crystal. Further, when the upper limit temperature is M-300 (° C.), the droplets can be dispersed on the crystal surface at an interval of several μm to several hundred μm appropriately and conveniently. Therefore, in the first step, a preferable range of the temperature at which droplets can be conveniently generated from the viewpoint of the existing density and the size thereof is a range from 500 ° C. to M-300 ° C.
[0020]
For example, in the case of GaAs having a melting point (M ° C.) of 1238 ° C., a preferable range is 500 ° C. or more and 938 ° C. or less. In GaAs, a more suitable upper limit temperature for generating gallium (Ga) droplets while maintaining the smoothness of the crystal surface is less than 900 ° C. (T. Udagawa, et al., “Semi-Insulating III-V Materials”. (SHIBA PUBLISHING LTD., 1980), pp. 108-114). FIG. 9 illustrates the shape and existence of gallium (Ga) droplets generated by exposing a GaAs single crystal at 850 ° C. in a hydrogen atmosphere for 30 minutes. Gallium (Ga) droplets 109 are generated on the surface 108 of the GaAs single crystal whose plane orientation is (001) at intervals of approximately 50 μm. Most of the Ga droplets 109 are present in the depressions 110 surrounded by ridges parallel to the [011] crystal direction. Due to the presence of the depression 110, the Ga droplet 109 is stored therein and does not ooze out onto the crystal surface. That is, the location of the liquid Ga can be specified. The shape of the Ga droplet may be a square or a rectangle reflecting the shape of the bottom surface of the pit. Further, since Ga filled in the pit becomes substantially spherical due to surface tension, a substantially spherical droplet may be formed. The shape of the pit (pit) differs depending on the orientation of the crystal plane constituting the surface, and the pit generated on the {111} -GaAs crystal surface has a triangular shape.
[0021]
In the first step, means for selectively and efficiently desorbing the group V constituent element from the crystal surface is important for efficiently generating droplets mainly composed of the group III constituent element. In the present invention, this is achieved by selecting the configuration of the atmosphere in which the heat treatment is performed. Examples of the configuration of the atmosphere during the heating include a vacuum environment, an atmosphere composed of an inert gas such as argon (Ar) and helium (He), and an atmosphere composed of a reducing gas such as hydrogen.
[0022]
However, by heating in a vacuum atmosphere, evaporation of not only Group V constituent elements but also Group III constituent elements is uniformly promoted. For this reason, it is sometimes difficult to form droplets containing a Group III element as a main component. In a vacuum environment, it is preferable to coat other regions with a coating so that droplets can be generated only in the region where formation is desired. In an atmosphere composed only of an inert gas such as argon, nitrogen or helium, a drop in the generation efficiency of droplets is observed. In contrast, in an atmosphere composed of a reducing gas such as hydrogen, the generation of droplets is promoted. This is because a surface reaction with hydrogen (atoms) changes the group V element into a substance that is more easily vaporized, and creates a situation in which the group V element is preferentially eliminated. In other words, the stoichiometric composition of the crystal surface has an advantageous effect on enriching the group III element side, which is effective in forming droplets containing a group III element as a main component. . Therefore, in the present invention, the atmosphere is composed of a gas containing a hydrogen atom. The gas containing a hydrogen atom is, for example, hydrogen (H₂), And a mixed gas containing hydrogen in a volume ratio of approximately more than 50%. Examples of the configuration of the mixed gas include hydrogen-nitrogen (N₂), Hydrogen-argon, hydrogen-helium, hydrogen-nitrogen-argon and the like. In the case of a gas mixture with hydrogen, if the gas is mixed with the hydrogen gas, for example, if the volume ratio of the inert gas is increased, the development speed of the droplet, in other words, the size (bottom area) of the pit containing the droplet Expansion speed is reduced. Further, it helps to maintain the smoothness of the crystal surface in the region other than the region where the droplet is formed. It is known that a mixed gas of helium and hydrogen can be used as an atmosphere gas when etching the surface of a GaAs crystal uniformly (Inst. Phys. Conf. Ser., No. 24 (Inst. of Phys., London, 1975), 362-368).
[0023]
Depending on the type of the group III-V compound semiconductor crystal to be subjected to the heat treatment, other components may be contained in the droplet formed on the crystal surface and containing a group III constituent element as a main component. For example, a droplet mainly composed of gallium (Ga) formed on the surface of a GaAs crystal doped with zinc (Zn) may contain Zn as a dopant. Since Zn is easily dissolved in Ga (see “THE CHEMISTRY OF GALLIUM” (ELSEVIER PUB. COMPANY, 1966), page 214), a liquid containing Zn at a weight concentration of several% by weight due to aggregation of Zn into Ga droplets. Droplets may be formed in some cases. Also, silicon (Si) doped Al_xGa_1-xOn the As mixed crystal surface, droplets containing Si and containing Al and Ga as main components can be formed. In some cases, in addition to the doped impurities, impurities remaining in the crystal (residual impurities) are taken in the droplets. The droplets are generally composed of more than 80 to 90% by weight of Group III elements. I have.
[0024]
The size of the droplet mainly composed of a group III element is controlled by adjusting the heating temperature and the heating time in the first step. In the case where the heating process is performed using an atmosphere having the same configuration and for a fixed time, the size of the droplet (the size of the pit) increases as the heating temperature is set to a higher temperature. When the processing temperature is increased, the bottom area is further expanded, and a pit having a greater depth is formed. Therefore, the volume of the droplet that can be accommodated in the pit increases. In the case where processing is performed using an atmosphere having the same configuration and a constant heating temperature, droplets (sizes of pits) develop and increase with heating time. As an example, when a Zn-doped (001) -GaAs single crystal is exposed to a hydrogen stream at 900 ° C. for 5 minutes, on average, a rectangular [011] having a width of about 20 μm and a length of about 50 μm is obtained. Ga droplets containing Zn arranged substantially parallel to the crystal direction can be formed. In general, heating for an excessively long time exceeding one hour is not preferable because it causes loss of the droplet itself due to evaporation and remarkable deterioration of the crystal surface state. In order to form a low-dimensional quantum structure such as a quantum dot, a quantum box, or a quantum line via a droplet, a droplet having a size of several nm to several tens nm (approximately equal to the bottom area of the pit) is suitable. . Therefore, in order to form droplets that meet this purpose, it is necessary to reduce the heating temperature, shorten the heating time, or implement both of these measures.
[0025]
In forming a droplet mainly containing a Group III element in the first step, if a general selective masking technique is used, the droplet is formed on the crystal surface, for example, regularly or at a specific position. Can be formed. For example, after a film having an opening at a selected position or a specific position is applied to the surface of a group III-V compound semiconductor crystal and then subjected to a heat treatment, the surface of the crystal corresponding to the opening may be formed. A limited number of droplets can be formed. The coating is made of silicon dioxide (SiO₂  ) Or silicon nitride (Si₃N₄), Etc., can be made of an inorganic material having low volatility and heat resistance. Further, it promotes the evaporation of the group V constituent elements from the surface of the group III-V compound semiconductor exposed in the opening, and suppresses the volatilization of the group V element from the group III-V compound semiconductor in the non-opening. For example, arsenic (As) -doped SiO₂  Film or SiO doped with phosphorus (P)₂  A membrane is available. As-doped SiO₂  The film is made of GaAs or Ga containing As as a group V constituent element._yIn_1-yIt is particularly effectively used for As mixed crystals. The coating is aluminum gallium nitride (Al_lGa_mN: 0 ≦ l ≦ 1, l + m = 1) and the like.
[0026]
As one technique for forming droplets in a desired region of a crystal surface, a so-called laser annealing is performed in which a specific region is irradiated with laser light focused in a small spot (spot) to heat a specific region. ) Techniques are conceivable. This is a method in which heating is locally applied to a specific region to promote evaporation of the group V element in the region, thereby generating droplets containing a group III element as a main component. However, in this technique, it is necessary to scan the laser beam with sufficient controllability of the irradiation position for each selected region. In addition, due to the intensive irradiation of the laser light to the minute area, a temperature difference from the surrounding non-irradiation area occurs, and a problem that thermal distortion occurs occurs. Further, this method has a disadvantage that pits having clear ridge lines cannot be formed due to uneven conduction of heat from the center of the beam irradiation to the peripheral region. Therefore, in the present invention, a means for forming a droplet in a specific region by simple heating using the above-described selective masking method using a coating film is recommended. Examples of a method that the present invention calls a simple heating method include a high-frequency heating method and a resistance heating method. Such a heat treatment method has an advantage in that droplets can be generated at once in a batch over the entire surface of the object to be heated without scanning such as a laser annealing technique.
[0027]
The shape of the opening is determined in consideration of the desired shape of the group III nitride semiconductor crystal. For example, when a group III nitride semiconductor crystal layer is formed linearly like a quantum wire, a band-shaped opening is provided in the film to form a band-shaped droplet. In order to obtain a quantum box-shaped crystal, a coating having a square or rectangular opening is used. In order to obtain quantum dot-like spherical microcrystals, for example, a film having a circular opening is used. Photolithography technology that exposes near ultraviolet rays, electron beam lithography technology, focused ion beam technology, and the like can be used for the hole forming process for providing an opening in the coating film. For example, As-doped SiO on a GaAs crystal layer₂After a general or electron beam drawing photoresist material is applied to the surface of the film, exposure is performed through a glass mask or the like on which a pattern is drawn, and the resist material is patterned. In the region where the resist material was removed by the processing, the above SiO 2 was used.₂The film is exposed. Therefore, the exposed region of SiO₂If the film is removed by a dry etching method so that the side wall of the film remains substantially vertically and clearly, a film having an opening limited to a selected region can be formed.
[0028]
In the second step of the present invention, a liquid droplet containing a Group III constituent element as a main component is impregnated with nitrogen.And replace the Group V element in the liquid with nitrogen (N) III Group III nitride semiconductorThis is a step for the purpose of The impregnated body needs to be in a liquid state in order to efficiently impregnate the substance containing nitrogen atoms into the inside thereof instead of adsorbing it on the surface of the solid. Accordingly, in the case of particles containing Al, Ga or In as the main component, in order to make them exist as droplets, it is essential to maintain the temperature at or above the melting point of the group III element. For example, since the melting point of Al is about 660 ° C., it is necessary to convert particles containing Al as a main component into droplets and impregnate with nitrogen.,aboutIt is necessary to perform the nitrogen impregnation at a temperature exceeding 660 ° C. In the case of particles containing Ga or In as a main component and not containing Al, if the temperature is raised to about 30 ° C., which is the melting point of Ga, or about 156 ° C., which is the melting point of In, the particles are present as droplets. be able to. However, at such relatively low temperatures, it is difficult to efficiently release nitrogen atoms or molecules containing nitrogen atoms. Considering the temperature at which nitrogen atoms can be efficiently released, the heat treatment temperature needs to be higher than about 350 ° C. Preferably, the temperature is 400 ° C.That is, use III Assuming that the melting point of the -V compound semiconductor crystal is M (C), the heating in the second step is preferably performed in a temperature range of 400C or more and (M-300) C or less.
[0029]
In the second step, a substance containing a nitrogen atom is penetrated and dissolved in the droplet,Substituting group V element in the liquid with nitrogen (N)Since it forms a group III nitride semiconductor crystal, it is necessary to perform heat treatment in an atmosphere composed of a gas containing a nitrogen atom. Examples of the gas containing a nitrogen atom include nitrogen (N₂), Ammonia (NH₃) And hydrazines such as dimethylhydrazine. In particular, a gas containing a molecule containing a chemical bond (N-H) of nitrogen and hydrogen can be preferably used as a gas constituting the atmosphere. The gas molecules containing N—H bonds can release nitrogen atom-containing fragments that penetrate into the droplets by pyrolysis and simultaneously release hydrogen atom-containing fragments that promote surface desorption of group V elements. Because. Examples of the gas containing an N—H bond include ammonia and hydrazine. Amines such as methylamine and dimethylamine can also be used. Heterocyclic compounds such as pyrrole can also be used. In the case of a substance containing an N—H bond, dimethylhydrazine having an asymmetric molecular structure, in which thermal decomposition starts at a temperature lower than 400 ° C., is particularly suitable as a substance constituting an atmosphere.
[0030]
Although a substance containing a nitrogen atom, for example, a nitro group (-NO₂) Or an acetamido group (-NH-CO-CH)₃Compounds containing a functional group containing an oxygen atom, such as (2), are not preferred as the nitrogen-containing substance constituting the heat treatment atmosphere of the present invention. This is because oxygen generated by thermal decomposition may function as a trap center that forms a deep level with the group III-V compound semiconductor. A compound containing a cyanide group (—CN) is also unsuitable as a nitrogen-containing substance constituting a heat treatment atmosphere from the viewpoint of toxicity.
[0031]
The processing atmosphere for penetrating nitrogen (atoms) into the droplets can be composed of a single gas composed of molecules containing nitrogen atoms. For example, heat treatment can be performed in an atmosphere consisting only of ammonia. Further, it can be composed of a mixed gas of a gas composed of a molecule containing a nitrogen atom and another gas. Examples of the configuration of the processing atmosphere composed of the mixed gas include a mixed system such as a hydrogen-ammonia system, an argon (Ar) -ammonia system, a nitrogen-ammonia system, a hydrogen-nitrogen-ammonia system, and a nitrogen-argon-hydrazine system. . When the heat treatment atmosphere is formed from the mixed gas, the volume occupation ratio of the gas containing the nitrogen-containing substance is desirably approximately 50% or more. As the processing temperature is set lower and the decomposability is lower, the partial pressure of the nitrogen-containing substance constituting the atmosphere is set higher. The atmosphere can be composed of the nitrogen-containing compound alone.
[0032]
In the second step, as a result of the penetration and impregnation of nitrogen into the interior of the droplet,Group V element in the liquid is replaced by nitrogen (N)A group III nitride semiconductor crystal is formed. Crystals mainly composed of gallium nitride (GaN) are formed from droplets mainly containing gallium (Ga). Crystals mainly composed of aluminum nitride (AlN) can be formed from droplets mainly containing aluminum (Al). A crystal mainly composed of indium nitride (InN) can be formed from a droplet mainly containing indium (In). In addition, a crystal composed of a mixed crystal of aluminum nitride and gallium can be formed from a droplet mainly composed of Al and Ga. The nitrogen content (composition ratio) of the crystal obtained by performing the nitrogen impregnation treatment can be measured by secondary ion mass spectrometry (abbreviated as SIMS) or Auger electron spectroscopy (abbreviated as AES) or the like. Is possible.
[0033]
The present invention also provides a semiconductor device including a group III-V compound semiconductor crystal layer including a group III nitride semiconductor crystal formed via a droplet. The crystal layer is, for example, a light emitting layer in a light emitting element.In addition, it is obtained according to the above-described steps, and is provided in the dispersed minute regions on the surface of the semiconductor crystal. III Formed by a method of forming a group III nitride semiconductor crystal III It is a semiconductor device having a group III nitride semiconductor crystal.
[0034]
In the method for forming a group III nitride semiconductor crystal through a droplet according to the present invention, the heating temperature or the heating time in the first step is appropriately adjusted to obtain a microscopic crystal having a size that produces a quantum effect. A droplet having an appropriate size (volume) can be formed. When a group III-V compound semiconductor crystal containing microcrystals sufficient to provide such a quantum effect is used as a light emitting layer, a quantum effect type light emitting element can be formed. For example, a quantum dot LD can be formed from an active (light-emitting) layer containing a group III nitride semiconductor microcrystal in the form of a quantum dot.
[0035]
Further, even if the crystal is not a minute crystal enough to generate a quantum effect, the group III-V compound semiconductor layer containing the crystal may have a strain due to lattice mismatch between the crystal and the surrounding semiconductor layer. Can be used as a strained lattice semiconductor crystal layer containing.
[0036]
That is, in the present invention, the heat treatment constituting the first step comprises (1) preferentially removing the group V constituent element of the III-V compound semiconductor crystal from the crystal surface, and (2) the chemical treatment of the crystal surface. The stoichiometric composition makes the group III side rich, and has the effect of (3) generating droplets containing a group III element as a main component.
The heat treatment in the atmosphere composed of the nitrogen-containing atomic gas in the second step is performed by (A) impregnating the droplets generated in the first step with a nitrogen-containing substance.To replace the group V element in the liquid with nitrogen (N), (B) has a function of generating a group III nitride semiconductor crystal using the droplet as a nucleus.
[0037]
BEST MODE FOR CARRYING OUT THE INVENTION
[First Embodiment]
Hereinafter, a first embodiment of the present invention will be described with reference to FIGS.
In this embodiment mode, gallium (Ga) is deposited on the surface of a zinc (Zn) -doped GaAs growth layer formed on a gallium arsenide (GaAs) single crystal substrate by a metal organic chemical vapor deposition (MOCVD) method. A method for forming a droplet as a main component will be described as an example.
FIG. 1 shows the surface state of the outermost GaAs layer after the first step in the method for forming a group III nitride semiconductor crystal of the present embodiment.Appearance perspectiveFIG.
[0038]
Trimethyl gallium ((CH 2) is formed on a substrate 101 made of p-type (001) -gallium arsenide (chemical formula: GaAs) single crystal doped with zinc (element symbol: Zn).₃)₃Ga / AsH₃/ H₂A zinc (Zn) -doped p-type GaAs vapor phase growth layer 116 was deposited by a normal pressure (atmospheric pressure) MOCVD method using a reaction system. The deposition temperature was 660 ° C. During film formation, a growth atmosphere in which the arsenic component is excessive was created in order to suppress the generation of pits due to arsenic volatilization and obtain a flat growth surface. That is, a gallium source ((CH₃)₃Ga) concentration versus arsenic source (AsH)₃), The so-called V / III ratio was set to 40. The layer thickness of the growth layer 116 is 1 μm, and the carrier concentration is 5 × 10¹⁴cm^-3Less than
[0039]
After the supply of Group III and Group V source gases to the MOCVD reactor at 660 ° C. for 15 minutes, the supply of both source gases was stopped. Thus, the gas flowing through the MOCVD reactor was only hydrogen gas as a carrier gas, and was naturally cooled to a temperature near room temperature. After confirming that no droplet was formed on the surface of the GaAs growth layer by a general differential interference optical microscope, the process was shifted to the first step of the present invention.
[0040]
First, a laminated structure 20 composed of a p-type GaAs substrate / a p-type GaAs growth layer is placed on a heatable support base 112 provided in a furnace body 111 of a vertical heating furnace 11 schematically shown in FIG. did. The inside of the furnace body 111 is 2.6 × 10 6 by a general vacuum oil rotary vacuum pump.^-2Evacuation was performed to a degree of vacuum of hectopascal (hPa). Thereafter, an argon (Ar) gas was passed through the gas inlet 113 into the furnace body 111, and the pressure inside the furnace body was returned to substantially the atmospheric pressure. The gas species flowing into the furnace body 111 from the gas inlet 113 is changed from argon to hydrogen (H₂After changing to gas, power was applied to the resistance heater 114 housed inside the support base 112, and the temperature of the laminated structure 20 was raised from room temperature to 850 ° C. The temperature of the laminated structure 20 was a temperature measured by a thermocouple 115 disposed immediately below the center of the circular support 112.
[0041]
From the point in time when the temperature of the laminated structure 20, that is, the temperature measured by the thermocouple 115 reaches 850 ° C., the flow rate of the hydrogen gas constituting the simple substance atmosphere is measured for an electronic mass flow meter for exactly 30 minutes. The mass flow controller (MFC)) controlled precisely 2 liters / minute.
[0042]
After the completion of the first step according to the present invention, that is, exactly 15 minutes after the temperature reaches 850 ° C., the supply of power to the resistance heating element 114 is stopped to lower the temperature by natural cooling. Started. At the same time when the temperature was lowered, the type of gas supplied into the furnace was instantaneously switched from hydrogen gas to argon gas. Then, the temperature of the laminated structure 20 was lowered to room temperature while the supply of the argon gas into the furnace was continued.
[0043]
As described above, after finishing the first step according to the present invention, the surface of the GaAs growth layer 116, which is the outermost layer of the multilayer structure 20, was observed with a scanning electron microscope (English abbreviation: SEM). From the SEM observation, it was confirmed that the droplets 109 as shown in FIG. 1 were dispersed on the surface of the GaAs layer 116. The ridgeline 117 of the rectangular or square pit filled with the droplet 109 was substantially parallel to the [011] crystal direction, which is the cleavage direction of the GaAs substrate 101. The size of the pit (dent) 110 was about 25 μm on the long side and about 15 μm on the short side in the substantially rectangular pit 110 a. In the approximately square pit 110b, one side was 20 μm on average. The characteristic X-ray intensity distribution obtained by an electron probe X-ray microanalyzer (abbreviation: EPMA) showed that the main component of the droplet 109 was gallium (Ga). It was analyzed that the Ga droplet contained a small amount of zinc (Zn) at a concentration of less than several percent. There was almost no difference in the main component of the droplet 109 due to the shape of the pits, and all were mainly Ga.
[0044]
After confirming the existence of a droplet mainly composed of a group III constituent element, that is, gallium (Ga), the laminated structure 20 was again stored in the MOCVD reactor. Thereafter, the temperature of the laminated structure was increased from room temperature to 700 ° C. while flowing argon gas into the reaction furnace. When the temperature reached 700 ° C., the flow of ammonia gas was started at a flow rate of 2 liters per minute. With the start of the supply of the ammonia gas, the composition of the atmosphere became argon: ammonia = 1: 6 in volume ratio. The flow of ammonia gas was continued for exactly 30 minutes after the temperature reached 750 ° C., and the above-described treatment was performed to permeate ammonia or a thermally decomposed fragment of ammonia into the Ga droplets formed on the surface of the GaAs growth layer. After 30 minutes, the temperature was immediately lowered from 750 ° C. to 600 ° C. at a rate of 30 ° C./min. When the temperature dropped to 600 ° C., the supply of ammonia gas into the MOCVD reactor was stopped, and only argon was allowed to flow. The temperature was lowered by natural cooling to a temperature close to room temperature, and the second step referred to in the present invention was completed.
[0045]
According to the SEM observation of the surface after cooling, even after performing the second step, no difference in the size and shape of the pits formed along with the droplets in the first step was visually recognized. According to the composition analysis by the EPMA method, the droplet mainly composed of Ga before the second step has a nitrogen composition ratio (q) of about 88% (0.88) to 90% (0. Gallium arsenide nitride (GaAs)_1-qN_q) Was obtained. The method of forming group III nitride semiconductor microcrystals using droplets containing Ga as a main component has been described above.
[0046]
[Second embodiment]
Hereinafter, a second embodiment of the present invention will be described with reference to FIG.
In the present embodiment, a liquid containing an indium phosphide (InP) / gallium / arsenide / indium mixed crystal layer grown by a hydride (hydride) vapor phase epitaxial (VPE) method as a material and containing a group III element as a main component is used. A method of forming a droplet and forming a group III nitride semiconductor crystal through the droplet will be described.
FIG. 3 is a schematic diagram showing the surface state of the laminated structure after the first and second steps described in the second embodiment in the method of forming a group III nitride semiconductor crystal of the present embodiment. FIG.
[0047]
On the surface of an indium phosphide (InP) substrate 201 having a plane orientation of (001), an n-type InP vapor deposition layer 218 doped with sulfur (S) was deposited at 680 ° C. in a hydrogen atmosphere. Subsequently, a sulfur-doped n-type gallium indium arsenide / indium mixed crystal layer (Ga) having an indium composition ratio of 0.53 at the same temperature is formed on the surface of the vapor growth layer 218._0.47In_0.53As) 219 deposited and InP / Ga_0.47In_0.53The laminated structure 30 including the As hetero junction was formed. For the gallium (Ga) source and the indium (In) source, high-purity metal Ga and metal In having a purity of 7 9 (N) and 6 9 (N) were used, respectively. The arsenic source is 10% by volume arsine (AsH).₃). The source of sulfur is hydrogen sulfide (chemical formula: H₂High purity hydrogen mixed with S) was used. The thickness of the InP vapor deposition layer 218 is about 0.5 μm, and the carrier concentration is about 6 × 10¹⁶cm^-3And Ga_0.47In_0.53The thickness of the As vapor phase growth layer 219 is about 0.5 μm, and the carrier concentration is about 7 × 10¹⁷cm^-3And
[0048]
Ga as the outermost layer of the multilayer structure 30_0.47In_0.53After the formation of the As vapor phase growth layer 219 was completed, the supply of the arsenic source into the growth furnace was stopped. Thus, the atmosphere in the growth furnace was made of simple hydrogen, and the laminated structure 30 was kept at 680 ° C. for 15 minutes. The holding operation at this high temperature is performed by Ga_0.47In_0.53This is performed to form a substantially spherical droplet containing gallium (Ga) and indium (In) as main components on the surface of the As vapor phase growth layer 219. Separately, under the same heating conditions, Ga_0.47In_0.53According to the result of the test of heating the As vapor phase growth layer, Ga and In mainly contained in a substantially square pit having a rounded corner on the surface of the same layer and having a side length of about 10 μm on average were mainly formed. It is known that substantially spherical droplets are formed.
[0049]
After the heat treatment for 15 minutes in a hydrogen atmosphere for forming droplets was completed, the gas constituting the atmosphere was changed to only ammonia gas, and the laminated structure was kept at 680 ° C. for 20 minutes. . As a result, the ammonia molecules or the thermally decomposed fragments thereof were permeated and dissolved in the droplets mainly composed of Ga and In formed in the previous step. After the treatment, the temperature of the laminated structure was lowered in an ammonia stream at a flow rate of 0.5 L / min until the temperature reached about 500 ° C. When the temperature became lower than about 500 ° C., the supply of ammonia into the growth furnace was stopped. Instead, the flow of argon gas was started, and cooling was allowed to take place until the temperature dropped to a temperature near room temperature.
[0050]
FIG. 3 schematically shows the state of the surface of the laminated structure 30 that has been subjected to the first and second steps according to the present invention in the same furnace. A substantially spherical microcrystal 220 was present corresponding to the position where the pit 210 was generated. From the external shape and the existing position, it was apparent that the microcrystal was grown with the droplets provided in the pits as nuclei. The size of the pit was maintained at about 10 μm on average. The outer shape of the microcrystal 220 was approximately spherical with a diameter of about 10 μm. According to the composition analysis using EPMA, the nitrogen composition of the microcrystalline body 220 was about 62%. The composition of gallium and indium was about 55% and about 45%, respectively, according to the composition analysis by EPMA. n-type Ga_0.47In_0.53The atomic concentration in the microcrystal 220 of sulfur (S) added as a dopant during the growth of the As vapor phase growth layer 219 is 1 × 10¹⁹cm^-3Met. As described above, the method of forming the GaInAsN microcrystals from the substantially spherical droplets containing gallium (Ga) and indium (In) as main components has been exemplified.
[0051]
[Third Embodiment]
Hereinafter, a third embodiment of the present invention will be described with reference to FIGS.
In this embodiment, a method for manufacturing a light-emitting element in which a crystal layer including a group III nitride semiconductor crystal is used as a light-emitting layer will be described.
[0052]
As shown in FIG. 4, on a surface of an n-type (001) -GaAs substrate 301 doped with silicon (Si), a Si-doped GaAs layer 321 (film thickness = 0. 5 μm, carrier concentration = 1 × 10¹⁸cm^-3), Si-doped aluminum-gallium arsenide mixed crystal (Al_0.30Ga_0.70As) layer (film thickness = 1.5 μm, carrier concentration = 2 × 10¹⁸cm^-3    ) 322 and Si-doped Al_0.10Ga_0.90As layer (film thickness = 0.5 μm, carrier concentration = 1 × 10¹⁷cm^-3  323 were sequentially deposited at 700 ° C.
[0053]
After completing the formation of the three n-type crystal layers 321, 322, and 323, the irradiation of the arsenic (As) beam is stopped while the temperature of the substrate 301 is maintained at 700 ° C. in the MBE growth vessel. did. After stopping the irradiation of As molecular beam, it is about 2.3 × 10^-5Is left in the MBE growth vessel where a high vacuum environment of about_0.10Ga_0.90Desorption of arsenic (As) from the surface of the As layer 323 was promoted. Thereby, in particular, Al_0.10Ga_0.90The stoichiometric composition of the region near the surface of the As layer 323 was devised to make the Group III element side rich.
[0054]
Next, while the temperature of the substrate was maintained at 700 ° C., ammonia plasma generated by exciting ammonia gas with microwaves was introduced into the MBE growth vessel. As a result, the stoichiometrically rich Al group III element_0.10Ga_0.90Nitrogen is penetrated from the entire surface of the As layer 323 to the deeper layer 322, and Al_0.10Ga_0.90As layer is Al_0.10Ga_0.90As_0.12N_0.88  Converted into layers. Also, Al_0.30Ga_0.70The As layer 322 is made of Al_0.30Ga_0.70As_0.12N_0.88  Converted into layers.
[0055]
After cooling from 700 ° C. to room temperature, Al_0.10Ga_0.90Al formed using As layer 323 as a material_0.10Ga_0.90As_0.12N_0.88Almost the entire surface of the crystal is covered with silicon nitride (Si₃N₄) Coated with membrane. Thereafter, using the electron beam drawing method, as shown in FIG.₃N₄The coating 324 is processed, and linear openings 325 having a line width of 3000 ° are formed at equal intervals of 20 μm in the GaAs substrate 301. -1.1. > Provided parallel to the crystal orientation. As a result, only in the band-shaped opening, Al_0.10Ga_0.90As_0.12N_0.88The surface of the crystal was exposed. This coating 324 was used as a selective mask material for generating droplets in selective areas in a later step.
[0056]
Next, Si₃N₄Keeping the membrane covered again in the MBE growth vessel, about 3 × 10^-6In a high vacuum of hPa, the temperature of the substrate 301 was increased from room temperature to 800 ° C. by heating using a resistance heating method. Exactly 10 minutes at 800 ° C, Al_0.10Ga_0.90As_0.12N_0.88The surface of the crystal was exposed to a vacuum environment. Thereby, the Al at the opening of the coating 324 is_0.10Ga_0.90As_0.12N_0.88Only in the surface region of the crystal, droplets containing Al and Ga as main components were formed.
[0057]
After forming droplets containing a Group III constituent element as a main component in the belt-shaped region 325 using the selective masking technique, the temperature of the substrate was immediately lowered to 750 ° C. When the temperature reaches 750 ° C., the Al_0.10Ga_0.90As_0.12N_0.88The surface of the crystal is irradiated with a beam of ammonia molecules and magnesium (Mg), and Al doped with Mg from the above droplets._0.10Ga_0.90As_0.09N_0.91A crystal 329 having the following composition was formed. That is, a group III nitride semiconductor crystal 329 was formed by doping a droplet with a band-shaped p-type impurity (Mg) only in a region where the film 324 was selectively opened in a band shape. The group III nitride semiconductor constituting the band-shaped crystal body 329 is formed by surrounding Al_0.10Ga_0.90As_0.12N_0.88The nitrogen composition ratio was 3% larger than the crystal. The difference of 3% in the nitrogen composition ratio is_0.10Ga_0.90As_0.12N_0.88This provided a barrier of about 0.3 eV to the crystal. Thereafter, the laminated structure was taken out of the MBE growth vessel, and the silicon nitride film 324 covering the surface was removed by etching with an ammonium fluoride aqueous solution.
[0058]
The laminated structure 40 having the cross-sectional structure of FIG. 4 obtained by removing the coating from the surface was transported into the MBE growth vessel three times. Thereafter, the temperature of the substrate 301 is increased to 820 ° C., and as shown in FIG. 6, a p-type gallium nitride (GaN) crystal layer 326 doped with magnesium (Mg) is formed into an Al having a band-shaped crystal body 329._0.10Ga_0.90As_0.12N_0.88Deposited on the crystal layer. The thickness of the Mg-doped GaN layer 326 is 0.1 μm, and the carrier concentration is about 8 × 10¹⁷cm^-3And The Mg-doped GaN layer 326 was provided so as to be joined (contacted) with the band-shaped crystal body 329, that is, to be electrically conductive. That is, by electrically connecting the p-type GaN layer and the strip crystal, the n-type Al_0.10Ga_0.90As_0.12N_0.88The surface area of the p-type crystal layer in contact with the crystal layer can be increased.
[0059]
Next, ohmic electrodes were formed on the front and back surfaces of the multilayer structure 40. Since the n-type ohmic electrode 328 uses a GaAs single crystal exhibiting n-type conductivity for the substrate 301, a gold (Au) -germanium (Ge) alloy (Au 95% by weight) is formed on the entire rear surface of the substrate 301. Ge (5% by weight) was formed by vacuum evaporation. An Au film was further overlaid on the surface of the Au.Ge alloy film having a thickness of about 1.2 μm by a vacuum evaporation method. The metal film was subjected to an alloying treatment at 420 ° C. for 10 minutes to give ohmic properties. Thus, an n-type ohmic electrode 328 having a thickness of about 2.2 μm as a whole was formed.
[0060]
A gold film was deposited on the surface of the p-type GaN layer 326 constituting the outermost layer of the multilayer structure 40 by a known vacuum deposition method. After patterning as a circular electrode having a diameter of 120 μm, heat treatment was performed at 420 ° C. for 3 minutes in a hydrogen stream. Thus, a p-type ohmic electrode 327 was formed. As described above, the p-type GaN layer 326 and the strip-shaped Al_{. 0.10}Ga_0.90As_0.09N_0.91The crystal 329 is used as an upper cladding layer, and n-type Al_0.10Ga_0.90Al having a band-shaped crystal body 329 made of an As crystal layer 323 as a material_0.10Ga_0.90As_0.12N_0.88The crystal layer 322 is used as a light emitting layer._0.30Ga_0.70Al formed by converting the As crystal layer 323_0.30Ga_0.70As_0.12N_0.88A light emitting device (LED) as shown in FIG. 6 was constructed using the crystal layer as a lower cladding layer.
[0061]
The LED emitted blue-green light due to forward conduction between the p-type and n-type ohmic electrodes (327 and 328). When the forward current to be applied is relatively small, that is, slightly less than about 10 mA, light emission is caused by the Al existing between the band-shaped group III nitride semiconductor crystals 329._0.10Ga_0.90As_0.12N_0.88It was visible as coming from the crystal layer. When a forward current of 20 mA was passed, the LED emitted blue-green light having a center wavelength of about 470 nanometers (nm). In addition to the main emission spectrum, a secondary spectrum having a wavelength of about 380 nm was measured. Comparing the emission intensities, the secondary spectrum was about 1/4 of the main spectrum. The forward voltage (value at 20 mA) is about 4 volts (V), and the reverse voltage (value at 10 μA) exceeds 10 V, resulting in an LED having excellent pn junction characteristics (rectifying properties). Was.
[0062]
[Fourth Embodiment]
Hereinafter, a fourth embodiment of the present invention will be described with reference to FIG.
According to the present invention, it is possible to form a group III nitride semiconductor crystal layer having a larger band gap via a droplet mainly containing a group III element constituting a group III-V compound semiconductor crystal. Can be. In this embodiment mode, an example of a semiconductor device using a group III nitride semiconductor crystal layer including a wide band gap crystal formed in such a selective region will be described.
[0063]
For example, a crystal layer that selectively includes a region having a band gap different from that of a surrounding region can improve the characteristics of a semiconductor device even if it does not have a fine structure sufficient to provide a quantum structure. Is useful as For example, a region having a width of several μm to several tens μm and a depth of less than about 0.1 μm, which corresponds to a recessed portion of a gate electrode, has a larger forbidden band width than other regions. The crystal subjected to the nitrogen substitution treatment as described above can be used as a channel layer of a field-effect transistor (FET), which improves the gate breakdown voltage.
[0064]
FIG. 7 is a schematic view of a cross-sectional structure of a modulation-doped field-effect transistor (a so-called MODFET) in which a region where a gate electrode is laid has a band gap larger than that of a surrounding crystal and a channel layer is a crystal layer. It is. In the configuration of the laminated structure for MODFET shown in FIG.⁷A high-resistance GaAs buffer layer 402 (thickness = 0.8 μm, carrier concentration <1 × 10 4) on an undoped semi-insulating (001) -GaAs substrate 401 of ohm-cm¹⁴cm^-3), N-type channel layer 430 made of high-purity GaAs (layer thickness = 0.5 μm, carrier concentration <1 × 10¹⁴cm^-3) And n-type Al with δ (delta) doping of silicon (Si)_0.30Ga_0.70As electron supply layer 431 (layer thickness = 0.2 μm, sheet carrier concentration = 1.5 × 10¹²cm^-2) Were laminated using a normal MOCVD method.
[0065]
Once the film forming operation is stopped and the temperature is lowered, the entire surface of the electron supply layer 431 is made of silicon dioxide (SiO 2) doped with arsenic (As).₂) Coated with membrane. By using a known photolithography technique, an opening was formed only in a substantially central surface area of the electron supply layer 431 having the recess structure 432. The width of the opening corresponding to the gate recess was set to 10 μm corresponding to the width between the source (source) and drain (drain) electrodes. The length was 250 μm. This rectangular opening has a longitudinal direction of <0. -1.1> It was arranged parallel to the crystal orientation. Next, SiO 2₂While keeping the film covered, it was heated at 780 ° C. for 10 minutes in a hydrogen stream in a heating furnace as shown in FIG. By this heating operation, Al exposed at the opening_0.30Ga_0.70Desorption of arsenic from the surface of the As layer was promoted, and a region 437 having a depth of approximately 0.05 μm from the surface was defined as a region where the Group III element was stoichiometrically rich.
[0066]
Thereafter, while maintaining the temperature at 780 ° C., ammonia gas was passed through the heating furnace instead of hydrogen gas to form an atmosphere consisting of ammonia alone, and then the group III element was stoichiometrically converted. A group III nitride semiconductor crystal layer was formed in the abundant region 437. The heat treatment in the ammonia atmosphere was performed for exactly 7 minutes after starting the flow of the ammonia gas into the heating furnace. Under these conditions, the composition analysis by Auger electron spectroscopy shows that Al_0.30Ga_0.70As crystal layer is Al_0.30Ga_0.70As_0.06N_0.94It is known to be converted to a crystalline layer. Al in an area other than the opening 437_0.30Ga_0.70As is arsenic-doped SiO₂Since arsenic volatilization was prevented by coating the film, Al_0.30Ga_0.70It was left as As. Al_0.30Ga_0.70The band gap of As is about 1.65 eV, and_0.30Ga_0.70As_0.06N_0.94That of the crystal is calculated to be about 3.16 eV. Therefore, in the present embodiment, Al_0.30Ga_0.70This means that a region having a band gap larger by about 1.51 eV than that is formed in the As crystal layer.
[0067]
After the formation of the group III nitride semiconductor crystal layer was completed, the temperature was lowered to room temperature. After that, SiO₂  After the film is wet-etched with a hydrofluoric acid buffer solution (buffered hydrofluoric acid), it is placed again in the MOCVD growth furnace, and the surface of the electron supply layer 431 including the region where the forbidden band width is selectively increased. Gallium arsenide nitride (GaAs) with a nitrogen composition ratio of 1%_0.99N_0.01) A crystal layer 433 was deposited. Next, while maintaining the temperature of the substrate 401 at 780 ° C., only the ammonia gas was allowed to flow at a flow rate of 0.8 liter per minute into the MOCVD furnace, thereby creating an atmosphere consisting of ammonia alone. Exposed for exactly 5 minutes after starting the flow of ammonia_0.99N_0.01A nitriding treatment was performed to increase the nitrogen composition ratio of the crystal layer. Thereby, GaAs_0.99N_0.01GaAs having a nitrogen composition ratio of 0.40 over the entire surface of the crystal layer_0.99N_0.40A crystal layer was formed.
[0068]
GaAs_0.99N_0.40After performing the patterning process for using the resulting crystal layer as a non-alloy ohmic contact layer, both ohmic electrodes of a source (source) 434 and a drain (drain) 435 are arranged on the surface of the same layer. . The gate electrode 436 is provided at the bottom of the recess 432 formed by removing substantially the center of the contact layer by etching. That is, Al_0.30Ga_0.70Convert As to Al_0.30Ga_0.70As_0.06N_0.94A gate electrode 436 having a gate length of 0.5 μm and having a multi-layer structure of titanium (Ti) and aluminum (Al) was laid on the surface of the region where the forbidden band width was large.
[0069]
As described above, since the gate electrode is laid on the region where the forbidden band width is large, the gate breakdown voltage when the drain current is 20 mA is about 5 to about 10 V in the conventional example. , About 15V. Since the leakage current was reduced along with the improvement of the gate withstand voltage, the MODFET was excellent in pinch-off characteristics. According to this embodiment, since a crystal region providing a high gate breakdown voltage can be planarly included in one crystal layer, a high resistance barrier (barrier) layer is intentionally formed by forming an electron supply to improve the gate breakdown voltage. There is no need to dispose it on a layer (immediately below a gate electrode), which is convenient for manufacturing a high gate breakdown voltage FET device.
[0070]
【The invention's effect】
As described above in detail, according to the present invention, the stoichiometric composition of the group III and group V elements constituting the group III-V compound semiconductor crystal is set so that the group III side element is rich. The group III nitride semiconductor crystal can be formed by being dispersed on the crystal surface by a method of forming a group III nitride semiconductor crystal through a droplet formed by shifting to the side to be formed. Further, a group III nitride semiconductor microcrystal can be selectively formed at a desired position on the crystal surface. Therefore, a fine heterojunction structure including a crystal can be locally arranged in a selected region of the crystal. Further, a group III nitride semiconductor crystal layer in which regions having different band gaps are present can be easily formed. For example, a crystal layer including a region whose forbidden band width is larger than the other regions is effective in providing a field-effect transistor having high gate breakdown voltage, for example.
[Brief description of the drawings]
FIG. 1 shows a surface state of a GaAs outermost layer after completion of a first step described in a first embodiment of the present invention.Appearance perspectiveFIG.
FIG. 2 is a schematic diagram showing a configuration of a heating furnace used in the embodiment.
FIG. 3 is a schematic diagram showing a state of a surface of a laminated structure after a first and second steps described in a second embodiment of the present invention.
FIG. 4 is a schematic cross-sectional view illustrating a configuration of a laminated structure according to a third embodiment of the present invention.
FIG. 5 is a schematic diagram showing an opening state of the silicon nitride selective mask material described in the embodiment.
FIG. 6 is a schematic cross-sectional view of a light-emitting element including the stacked structure described in Embodiment.
FIG. 7 is a schematic cross-sectional view illustrating a configuration of a MODFET according to a fourth embodiment of the present invention.
FIG. 8 is a schematic diagram for explaining a linear crack parallel to the [011] crystal orientation that occurs when a {001} -InP single crystal is heated at a temperature lower than 500 ° C.
FIG. 9 is a schematic diagram for explaining the shape of a Ga droplet formed by exposing a GaAs single crystal in a hydrogen stream at 850 ° C. for 15 minutes and the state of formation on the crystal surface.
FIG. 10 is a schematic cross-sectional view of a conventional laminated structure including a semiconductor layer having a general quantum dot (dot) configuration.
[Explanation of symbols]
10. DH structure light-emitting part consisting of a layer containing a quantum structure
11 heating furnace
20, 30, 40 laminated structure
101, 201, 301, 401 Crystal substrate
102,402 buffer layer
103 Lower cladding layer
104 light emitting layer
105 Upper cladding layer
106 linear slot
107 Quantum Dot Crystal
108 (001) surface of GaAs single crystal
109 Droplet containing Group III element as main component
110 [011] Depression (pit) surrounded by ridge line parallel to crystal direction
110a Substantially rectangular pit
110b A substantially square pit
111 heating furnace
112 support
113 Gas inlet
114 Resistance heating element
115 thermocouple
116 Zn-doped p-type GaAs growth layer
117 Ridge line imitating pit outline
218 n-type InP vapor phase growth layer
219 n-type Ga_0.47In_0.53As mixed crystal layer
220 Nearly spherical microcrystals
321 Si-doped n-type GaAs MBE growth layer
322 Si-doped n-type Al_0.30Ga_0.70AsMBE growth layer
323 Si-doped n-type Al_0.10Ga_0.90AsMBE growth layer
324 Silicon nitride (Si₃N₄) Coating
325 opening area
326 p-type GaN layer
327 p-type ohmic electrode
328 n-type ohmic electrode
329 Group III nitride semiconductor crystal formed in a belt shape
430 n-type GaAs channel layer
431 n-type Al_0.30Ga_0.70As electron supply layer
432 recess structure
433 GaAs_0.99N_0.01layer
434 source electrode
435 drain electrode
436 Gate electrode
437 Area for selectively increasing forbidden band width

Claims (7)

The surface of a III-V compound semiconductor crystal containing at least one of Al, Ga, and In as a group III constituent element and containing As or P as a group V constituent element is heated and melted to form the group III-V compound semiconductor. A first step of volatilizing a Group V constituent element in the compound semiconductor crystal to form a droplet mainly containing a Group III constituent element on a surface of the III-V compound semiconductor crystal; A group III-V compound semiconductor crystal having a droplet is heated in an atmosphere containing nitrogen, and part or all of the group V constituent elements in the droplet are replaced with nitrogen. A method for forming a group III nitride semiconductor crystal, comprising a second step of converting into a group III nitride semiconductor crystal. When the melting point of the group III-V compound semiconductor crystal to be used is M (° C.), the heat melting in the first step is performed in a temperature range of 500 ° C. or more and (M-300) ° C. or less. The method for forming a group III nitride semiconductor crystal according to claim 1. 3. The method for forming a group III nitride semiconductor crystal according to claim 1, wherein the heating and melting in the first step are performed in an atmosphere containing hydrogen. 4. The method according to claim 1, wherein the size of the droplet is controlled by adjusting at least one of a heating temperature and a heating time in the first step. A method for forming a group III nitride semiconductor crystal according to the above. In the first step, a protective film having an opening is applied to a portion of the surface of the III-V compound semiconductor crystal where the group III nitride semiconductor crystal is desired to be formed, and then the surface is melted by heating. The method for forming a group III nitride semiconductor crystal according to any one of claims 1 to 4, wherein a droplet is formed at a desired position. 6. The method for forming a group III nitride semiconductor crystal according to claim 5, wherein a gas atmosphere containing a molecule containing a chemical bond (N-H) of nitrogen and hydrogen is used as the nitrogen-containing atmosphere. . When the melting point of the group III-V compound semiconductor crystal used is M (° C.), the heating in the second step is performed in a temperature range of 400 ° C. or more and (M-300) ° C. or less. The method for forming a group III nitride semiconductor crystal according to any one of claims 1 to 6.

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