JP2010040971A - Method of forming thin film semiconductor layer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for forming a thin film semiconductor layer having a high carrier concentration, wherein a semiconductor layer having a carrier concentration much higher than that of the semiconductor layer is formed on the surface of the semiconductor layer.
A carrier layer having a higher carrier concentration than that of a semiconductor layer 1 is obtained by bonding a natural oxide film on the surface of a semiconductor layer 1 or an oxide film 4 formed at a low temperature of 250 ° C. or less to an activated metal element formed by reduction. In addition, the thin film compound semiconductor layer 2 having a band gap larger than that of the semiconductor layer is formed. On this, it is preferable to provide a protective layer 3 made of SiN: H in order to protect the thin film.
[Selection] Figure 1

Description

  The present invention relates to a method for forming a thin film semiconductor layer with a high carrier concentration, which can be formed with a high carrier concentration even in a semiconductor layer having a large band gap and a thin layer capable of tunneling electrons. More specifically, even a semiconductor layer having a very large band gap such as gallium nitride (GaN) compound or silicon carbide (SiC) is formed by reduction substitution on a semiconductor layer having a greatly different lattice constant. The present invention relates to a method for forming a thin film semiconductor layer having a high carrier concentration, which can be formed as a strained lattice layer without using a very thin and expensive facility, and which can greatly reduce the contact resistance with an electrode.

  Since GaN and SiC have a large band gap, high electron mobility, high heat resistance, and high radiation resistance, they are materials for short-wavelength light-emitting elements and microwaves and higher-frequency devices. However, it is also expected for power devices and in-vehicle devices. However, such a semiconductor layer has a problem that it is difficult to increase the carrier concentration, the contact resistance with the electrode material increases, a voltage drop occurs, and the device performance deteriorates. In particular, the P-type semiconductor layers of these materials cannot increase the carrier concentration, and even when LED (Light Emitting Diode) is used, the current cannot be spread over the entire chip, and only the vicinity of the electrodes emits light. In addition, there is a problem that it is difficult to obtain an ohmic contact between the P-type layer and the electrode.

  For example, a GaN-based compound semiconductor (in addition to GaN, a compound in which a part or all of gallium (Ga) is substituted with another group III element such as aluminum (Al), also referred to as a nitride semiconductor, hereinafter the same) A structure of a HEMT (High Electron Mobility Transistor) using the above is known as shown in FIG. That is, on a channel layer 51 made of GaN that is laminated on a sapphire substrate or the like (not shown) and is made of GaN having a very low carrier concentration, the carrier voltage is large, the band gap is large, and a desired threshold voltage is obtained. An AlGaN-based compound semiconductor layer 52 having a thickness of about 20 to 30 nm is provided, and a source electrode 53, a drain electrode 54, and a Schottky junction gate electrode 55 are formed on the surface thereof. With such a configuration, a two-dimensional electron gas is generated on the interface side of the channel layer 51 with the AlGaN-based compound semiconductor layer 52, and electrons move at high speed through the channel layer 51 with few impurities, resulting in high speed and low noise. It operates as a transistor.

However, in such a structure, since the contact resistivity between the source / drain electrodes 53 and 54 and the AlGaN-based compound semiconductor layer 52 cannot be sufficiently lowered, there is a problem that output power, power load efficiency, and operating frequency are lowered. In order to obtain a low-resistance ohmic contact, the thickness of at least the contact portion of the AlGaN-based compound semiconductor layer 52 with the electrodes 53 and 54 is reduced, and the two-dimensional electron gas accumulated at the heterointerface is used as the electrode. It has been proposed to lower the contact resistance with the electrodes 53 and 54 by tunneling (see cited document 1).
Japanese Patent Laid-Open No. 2003-100778

In a structure in which a two-dimensional electron gas is generated at the interface by bonding a semiconductor layer having a large band gap and a large carrier concentration on a semi-insulating or very low carrier concentration semiconductor layer as in the HEMT described above. The higher the carrier concentration of a semiconductor layer with a large band gap, the higher the Fermi level, the more likely it is to generate a two-dimensional electron gas, and the lower the contact resistance with the electrode formed on the semiconductor layer with the large band gap. Therefore, it is preferable to form a high carrier concentration. However, materials having a large band gap, particularly SiC and GaN-based compound semiconductors, cannot sufficiently increase their carrier concentration, and even the N type has a problem that it is difficult to exceed the order of 10 ¹⁸ cm ^−3. is there. This tendency becomes more prominent as the Al mixed crystal ratio increases.

Further, in the P-type semiconductor layer having a large band gap, the limit of the carrier concentration is further reduced by about one digit or more, and as described above, in a semiconductor light emitting device such as an LED or LD (laser diode) using a GaN compound semiconductor. However, there is a problem that the current spread from the electrode is reduced, or the series resistance is increased and the drive voltage is increased. In addition, the SiC impatt diode also has a large loss in output power due to its large contact resistance with the electrode of the P-type layer, and this specific contact resistance is lowered to about 10 × 10 ⁻⁵ Ω · cm ² or less. If possible, an output improvement of about 1000 times in the pulse output is expected.

  The present invention has been made in view of such circumstances, and provides a method for forming a thin film semiconductor layer having a high carrier concentration, which forms a semiconductor layer having a carrier concentration much higher than that of the semiconductor layer on the surface of the semiconductor layer. The purpose is to do.

  Another object of the present invention is to provide a method of forming a thin film semiconductor layer on the surface of a semiconductor layer, which has both a band gap and a carrier concentration larger than that of the semiconductor layer and can tunnel electrons.

  Still another object of the present invention is to improve the carrier concentration of a semiconductor layer in contact with an electrode not only in an N-type layer but also in a P-type layer in a device using a GaN-based compound semiconductor or SiC, thereby making contact with the electrode. An object of the present invention is to provide a method for forming a thin film semiconductor layer having a high carrier concentration capable of greatly reducing the resistance.

  The method for forming a thin film semiconductor layer according to the present invention combines an activated metal element formed by reducing a natural oxide film on the surface of a semiconductor layer or an oxide film generated at a low temperature of 250 ° C. or less with another element. Thus, a thin film compound semiconductor layer having a carrier concentration higher than that of the semiconductor layer and a band gap larger than that of the semiconductor layer is formed.

  Here, the semiconductor layer means a semiconductor layer on the surface when one or more kinds of semiconductor layers are epitaxially grown on the substrate, and means a surface portion of the substrate when only the semiconductor substrate is used. It may be semi-insulating, N-type or P-type.

  Specifically, for example, the semiconductor layer is made of a semiconductor layer containing indium (In) as a constituent element of the compound, and after the oxide film on the surface of the semiconductor layer containing In is reduced and cleaned, the oxide is immediately An indium oxide layer can be formed on the surface of the semiconductor layer by immersing the semiconductor layer in a generating liquid or exposing it to an oxidizing atmosphere at a low temperature of 250 ° C. or lower.

  “Contained as a constituent element of a compound” herein means an element that is contained as a constituent element of a compound, such as In and P of indium phosphide (InP) or In and As of indium arsenide (InAs). Not intended to be included. The term is the same for other compounds. The oxide generation liquid can oxidize activated metal elements on the surface of the semiconductor layer such as high purity water (including the case of boiling in high purity water), hydrogen peroxide water, ozone-containing water, oxygen-containing water, and the like. A liquid means an oxidizing atmosphere means a UV-excited ozone-containing gas, an oxygen plasma treatment at a low temperature of 250 ° C. or lower, and the like.

  After forming the indium oxide layer, if a protective film made of silicon nitrogen hydrogen alloy (SiN: H) is formed on the indium oxide layer by immediately introducing a monosilane gas and a nitrogen atom-containing gas, A stable oxide layer such as indium oxide can be stably maintained without disappearing.

  Another specific method is to reduce the oxide film formed on the surface of the semiconductor layer with hydrogen ions or halogen ions generated by gasification of the gas, and to combine the activated metal element with the raw material of the element The thin film compound semiconductor layer having a high carrier concentration can be formed by introducing a source gas of a gas and a dopant element into plasma and combining it with the activated metal element.

  The semiconductor layer comprises a semiconductor layer containing Ga as a constituent element of the compound, and the oxide film on the surface of the semiconductor layer containing Ga is reduced and cleaned by hydrogen (H) ions generated by plasma treatment with ammonia gas. And a thin film compound semiconductor comprising a compound containing Ga and N as constituent elements by combining activated Ga exposed by the reduction with nitrogen (N) ions generated by the plasma treatment of the ammonia gas. By forming the layer, a thin film of a GaN-based compound semiconductor layer having a high carrier concentration can be formed.

  Here, the semiconductor layer containing Ga as a constituent element of the compound includes, for example, GaAs, GaP, and mixed crystals thereof, GaN, etc., and part of these Ga is replaced with other group III elements such as Al and In. It means a semiconductor layer containing mixed crystals.

  By introducing a VIB group element gas as the dopant element, an N-type thin film compound semiconductor layer made of a compound containing Ga and N can be formed.

  After forming the thin film compound semiconductor layer, a monosilane gas is further introduced in the same apparatus to form a protective film made of silicon nitrogen hydrogen alloy (SiN: H) on the thin film compound semiconductor layer, thereby forming Ga and When the surface of the compound semiconductor layer containing N is protected without forming an oxide film, for example, when MISHEMT or the like is formed, a gate electrode can be formed on the surface using this protective film as an insulating layer. Here, the silicon-nitrogen hydrogen alloy (SiN: H) is silicon nitride containing about several at% to about 10 at% of H.

  The semiconductor layer is made of Si or SiC, and a hydrocarbon compound gas is introduced as a source gas of an element to be combined with the activated metal element, and carbon (C) ions generated by plasma treatment and Si activated by the reduction Can be combined to form a thin film compound semiconductor layer made of SiC.

  By introducing a hydride of a group III element as a source gas for the dopant element, a thin film compound semiconductor layer made of P-type SiC having a high carrier concentration can be formed. Contact resistance can be greatly reduced, and output characteristics and the like can be greatly improved.

  A thin film compound semiconductor layer made of high carrier concentration N-type SiC can be formed by introducing ammonia gas as a source gas of the dopant element.

  After forming the thin film compound semiconductor layer, a protective film made of silicon nitrogen hydrogen alloy (SiN: H) may be formed on the thin film compound semiconductor layer by further introducing ammonia gas and monosilane gas in the same apparatus. In addition, a thin film semiconductor layer that is easily lost by oxidation or the like can be stably held.

  According to the method for forming a thin film semiconductor layer of the present invention, an element constituting a desired compound and an optional dopant element are directly bonded to an element having a dangling bond exposed by reducing an oxide film on the surface of the semiconductor layer. Therefore, even if it is a compound semiconductor layer that does not lattice match with the semiconductor layer, it has a superlattice structure. Thus, it can be formed as a strained lattice layer. In addition, the strained lattice layer can be formed as a thin film simply by turning the gas into plasma, and it is very inexpensive without requiring expensive and difficult equipment such as conventional MBE equipment or MOCVD equipment. A thin film semiconductor layer can be formed. Note that by forming the oxide film on the surface of the semiconductor layer by oxidizing with ozone or the like, a thicker oxide film can be formed, and a semiconductor layer formed by reducing the oxide film can also be formed thicker. However, since it is oxidized at a low temperature, it does not exceed 10 nm, all oxide films can be reduced and replaced, and no oxide film remains. Further, if reduction replacement is performed with only the natural oxide film, a very thin semiconductor layer of about 1 to 3 nm can be formed.

As a result, as filed separately by the inventors of the present application, an N ⁺ type thin film semiconductor layer having a larger band gap and a higher carrier concentration than the N type semiconductor layer is formed on the N type semiconductor layer. Then, by generating a two-dimensional electron gas at the interface and tunneling the electrons of the two-dimensional electron gas, a diode having a very low forward rise voltage and a very high reverse breakdown voltage is formed. be able to. In addition, a HEMT in which a semiconductor layer having a larger band gap and a higher carrier concentration is formed on a Si semiconductor layer or a GaAs semiconductor layer can be easily formed. Furthermore, since the element having dangling bonds is combined with other elements, the dopant element is similarly combined with the elements having dangling bonds by mixing the dopant element, and the carrier concentration is significantly increased. The contact resistance with the electrode can be greatly reduced.

Further, when the semiconductor layer contains In as a constituent element of the compound, only the surface side of the semiconductor layer containing In is oxidized, and when the oxide film is reduced, the V group element originally combined with In Oxides such as oxides evaporate due to high vapor pressure even at low temperatures, and because they are water-soluble, they dissolve and disappear during the washing process. As a result, since only In existing on the surface side has a dangling bond and is newly oxidized, a very thin indium oxide film can be formed. In addition, oxygen deficiency is likely to occur because the oxide film is not forcedly formed at a high temperature but is oxidized in a liquid such as high-purity water (may be boiled) or in an oxygen atmosphere at a low temperature of about 250 ° C. or lower. It becomes an oxide film and can be an N-type semiconductor layer having a very high carrier concentration. In addition, a new oxide film is formed with high-purity water in a state in which a natural oxide film with impurities on the surface is reduced and cleaned, so that a very pure indium oxide (In ₂ O ₃ ) layer is formed. can do.

  Since this indium oxide layer is very thin and easy to reduce, it cannot be stably maintained unless care is taken not to eliminate it. For example, when a metal layer such as an electrode is formed thereon, if a metal having a reducing property such as aluminum (Al) or titanium (Ti) is deposited, it is immediately reduced. It is necessary to form an electrode. From these viewpoints, it is preferable to form a protective film made of, for example, silicon nitrogen hydrogen alloy (SiN: H) on the surface immediately after the indium oxide layer or the like is formed. In the case where the exposed indium oxide layer disappears when the opening is provided to form the electrode, the indium oxide layer can be regenerated only at the electrode formation place by performing oxidation treatment again.

In addition, when forming a thin-film semiconductor layer other than indium oxide, ions of a dopant element are introduced together with a gas of an element constituting the semiconductor layer, and ions formed into plasma are combined with a metal element activated by reduction of the oxide film. Therefore, the activated metal element combines with the dopant element in the same manner, and it becomes very easy to incorporate the dopant element, and the carrier concentration can be increased. In addition, since the dopant element incorporated in this manner is present in a reliable bond with the elements constituting the semiconductor layer, it is difficult to diffuse even after the subsequent heat treatment, and the semiconductor layer has a very stable high carrier concentration. become. The ratio of the dopant incorporation can be adjusted by changing the flow ratio of the semiconductor constituent gas and the dopant element gas within a range of the order of 10 ¹⁹ to 10 ²¹ cm ⁻³ . It can be obtained with a carrier concentration. Also in this case, since the surface of the generated semiconductor layer such as GaN is oxidized and may be lost if the oxide film is removed, a protective film is immediately formed on the surface. It is preferable to form or proceed to the next step as soon as possible.

  By using such a method, as in the case of the indium oxide layer described above, even when the wide-gap semiconductor layer thin film is formed, for example, even if the lattice constant is significantly different from that of the semiconductor layer, the surface oxide film can be formed. Since it has a superlattice structure with a very thin layer of 10 nm or less of only the reduced and replaced portion, it is formed as a strained lattice layer. As a result, a GaN-based compound thin film layer can be formed on the GaAs semiconductor layer, or a SiC thin film layer can be formed on the Si semiconductor layer. A semiconductor layer having a much wider gap than that can be formed on the semiconductor layer. It can be formed easily, and can be useful for a diode or HEMT using the above-described two-dimensional electron gas.

When the semiconductor layer contains Ga as a constituent element of the compound, the ammonia gas is decomposed into N ions and H ions by introducing ammonia gas and performing plasma treatment, and the oxide which is an oxide on the surface of the semiconductor layer by H ions. Gallium (Ga ₂ O ₃₎ or the like is reduced, and the activated Ga and N ions are combined to form a compound such as GaN on the surface. At this time, together with ammonia gas, for example, hydrogen sulfide (H ₂ S) which is a gas of sulfur (S) as a dopant is mixed at a certain ratio, so that S ions generated by decomposition of the gas have an unstable structure. Therefore, it enters into the Ga deficient portion and acts as an N-type semiconductor layer. Note that when the reduction of the oxides, oxides of Group V elements such as GaAs and GaP of As or P is also reduced by the presence, they are vaporized due to the high vapor pressure as As ₂ O ₃ Evaporates and disappears, or dissolves in water during the washing process, such as phosphorus oxide (P ₂ O ₅₎ . When a P-type semiconductor layer is formed, Si or Ge enters the V group site by introducing a hydride of a group IV element such as monosilane (SiH ₄ ) or monogermane (GeH ₄ ) as a dopant gas. Acts as P-type. Since Si and Ge are combined with activated Ga in the same manner as N, a P-type GaN compound semiconductor layer having a high carrier concentration can be surely obtained.

By being able to form such a thin film of GaN-based compound semiconductor with a high carrier concentration, it is possible to obtain a diode having a very low rise voltage and a high reverse breakdown voltage using a GaN-based compound semiconductor. In addition, the contact resistance between the GaN-based compound semiconductor layer and the electrode can be greatly reduced, and the characteristics of a semiconductor light emitting device using a GaN-based compound semiconductor can be greatly improved. In particular, the carrier concentration of the P-type layer can be very high, on the order of 10 ¹⁹ to 10 ²¹ cm ⁻³ , and even if it is formed on the surface of the P-type layer, the contact resistance with the P-side electrode can be lowered. In addition, in the case of an LED, the current can be easily diffused throughout the chip, which is very effective.

Further, when the semiconductor layer is Si or SiC, for example, by introducing a hydrocarbon gas such as methane gas into plasma, H ions and C ions are generated, and the silicon oxide film on the surface is decomposed by the H ions. Si having dangling bonds can be combined with C ions to form SiC. At this time, a hydride of a group III element such as diborane (B ₂ H ₆ ) is introduced as a dopant gas as a dopant. Thus, as in the above-described examples, a part of Si or carbon is bonded to B, so that a P-type SiC layer having a very high carrier concentration can be obtained. That is, even a semiconductor layer made of SiC can be made to have high carrier concentration SiC by reducing and replacing the oxide film on the surface thereof. Since SiC is difficult to oxidize, when a natural oxide film is not sufficiently formed on the surface, it can be similarly performed by forcibly forming an oxide film by exposure to an ozone gas atmosphere. Further, in order to obtain an N-type SiC layer, if a hydride of a group V element such as ammonia gas is partially introduced as a dopant gas, an N ⁺ -type SiC layer having a high carrier concentration can be obtained similarly. In these cases as well, when the surface oxide is formed, the C oxide, that is, carbon dioxide gas or carbon monoxide, is a gas, so it is evaporated and exhausted and disappears from the surface. Therefore, only the Si oxide remains on the surface of the oxidized SiC.

  By being able to form such a thin film of SiC semiconductor with a high carrier concentration, it is possible not only to obtain a diode having a very low rising voltage and a high reverse breakdown voltage using the SiC semiconductor, It is also possible to configure a HEMT by bonding Si and SiC. Furthermore, even in the P-type SiC semiconductor layer, the contact resistance with the electrode can be significantly reduced, the characteristics of an SiC diode using an SiC semiconductor can be greatly improved, and the output power is 1000 times that of the conventional one. More than about. In other words, the conventional oscillation efficiency was 0.01% or less, but it is expected to be about 10%, which is the original impatting efficiency, and a very high output of kilowatt order can be obtained. This has the effect of opening the way to fully solid-state radar. In particular, the carrier concentration of the P-type layer can be made very high, and even if it is formed on the surface of the P-type layer, the contact resistance with the electrode can be lowered, and the effect is very great.

  Next, a method for forming a thin film semiconductor layer of the present invention will be described with reference to the drawings. The method for forming a thin film semiconductor layer according to the present invention is generated at a low temperature of 250 ° C. or lower or a natural oxide film on the surface of the semiconductor layer 1 as shown in FIG. The oxide film 4 is combined with an activated metal element formed by reduction to form a thin film compound semiconductor layer 2 having a higher carrier concentration than the semiconductor layer 1 and a band gap larger than that of the semiconductor layer. It is characterized by that.

  The example shown in FIG. 1 is an example in which an N-type semiconductor layer containing indium, for example, an N-type InP layer 1 is formed in common with a substrate (N-type InP substrate 1). As shown in FIG. 1A, an N-type InP substrate 1 having a natural oxide film 4 formed on the surface is prepared. When indium is contained as a constituent element of the compound, it is very easily oxidized, so that the indium oxide layer 4 is formed on the surface only by being placed in the air, but a sufficient indium oxide layer 4 is formed. If not, the thin indium oxide layer 4 is formed at an extremely low temperature of about room temperature. The indium oxide layer 4 is formed at a very low temperature by a thickness of about several nanometers, for example, by boiling oxidation in high pure water, ozone oxidation, ozone hydroxylation, oxidation with hydrogen peroxide, or oxidation with low temperature plasma. An indium oxide layer 4 can be formed.

The semiconductor layer 1 is not limited to N-type InP, and some of the indium and P may be replaced with other elements. However, the replaced element is an insulating material such as Ga or Al. It is not preferable to contain an excessive amount of elements forming the oxide film. In addition, even when some of the elements are replaced with other elements, it is necessary to contain indium at least 30 at% or more in order to form activated indium on the surface of the semiconductor layer in order to form indium oxide. The material and carrier concentration are appropriately selected according to the semiconductor device to be manufactured, and the carrier concentration is also set according to the purpose. For example, the carrier concentration is about 1 × 10 ¹⁵ to 1 × 10 ¹⁸ cm ^−3. Formed. As long as the semiconductor layer 1 containing indium is formed on at least the surface, the substrate may be another semiconductor substrate or an insulating substrate such as sapphire. Further, the InP substrate may be an InP layer or the like obtained by epitaxially growing an InP layer or the like on the surface so as to have a desired impurity concentration.

  Next, as shown in FIG. 1B, a natural oxide film formed on the surface of the N-type InP substrate 1 is obtained by immersing the N-type InP substrate 1 in a solution for cleaning treatment such as hydrochloric acid. Remove deposits. As a result, the In layer 5 having dangling bonds activated by reduction of oxygen atoms in the natural oxide film is exposed on the surface. As a cleaning method, it is preferable to put electrolytic polishing in an acidic aqueous solution.

Thereafter, an oxide film is formed immediately after washing with water, but immediately after washing with water, it is immersed in, for example, high purity water. As a result, as shown in FIG. 1C, the activated indium on the surface of the N-type InP substrate 1 combines with oxygen in high purity water to form a very thin indium oxide (In ₂ O _{3) of} about 1 to 3 nm. ) Layer 2 is formed. This oxidation of the surface of the N-type InP substrate 1 is not forced oxidation performed by raising the temperature in an oxygen atmosphere, so that it does not penetrate into the inside of the N-type InP substrate 1 and only the activated indium on the surface. Is oxidized. Therefore, the indium oxide layer 2 is formed as a very thin oxide film of about several nm. Indium oxide originally tends to cause oxygen defects and is oxidized only by bonding with oxygen ions in a liquid such as high-purity water, so that it is formed in an N ⁺ type having many oxygen vacancies and a high carrier concentration. The immersion time in the high purity water is about 1 to 10 minutes so that an indium oxide layer 2 of about several nm is formed. However, as described above, only the oxidation of indium activated on the surface is performed. Even if the immersion time is too long, the oxide film does not become too thick. In this oxidation treatment, an oxide of a group V element is also formed, but by performing such immersion (wet) treatment, the oxide of the group V element is easily dissolved in water. Only the oxide remains.

  The oxidation treatment of the surface of the N-type InP substrate 1 may be a solution that oxidizes activated indium without using high-purity water. For example, hydrogen peroxide water, ozone-containing water, oxygen-containing water, or the like may be used. it can. Since these solutions have a relatively strong oxidizing power, the indium oxide layer 2 is formed to have a thickness of about several nanometers when immersed for about 0.5 to 2 minutes. The film never gets too thick. Further, this oxidation treatment can be performed not by oxidation by wet treatment but also by exposure to an oxidizing atmosphere such as a gas containing ultraviolet-excited ozone and an oxygen plasma treatment at a low temperature of 250 ° C. or lower. Low temperature oxidation with nitric oxide (NO) may also be used.

The indium oxide (In ₂ O ₃ ) layer 2 thus formed is an N ⁺ type layer having a high carrier concentration of about 1 × 10 ¹⁸ to 1 × 10 ²¹ cm ^{−3 due} to oxygen vacancies as described above. In addition, as shown in the band diagram of FIG. 2A at the time of thermal equilibrium (voltage is 0), it becomes a layer having a very large band gap (Eg) as compared with InP. For this reason, the bottom of the In ₂ O ₃ conduction band is at a high position, electrons accumulate on it, and the carrier concentration of the InP semiconductor layer 1 is lowered, so that the In ₂ O ₃ electrons flow into the InP side. A two-dimensional electron gas layer is formed as shown in FIG. By forming a pair of electrodes on In ₂ O ₃ and InP, a diode having a low rise voltage and a high reverse breakdown voltage of about 3 V or more is used as described above by using a two-dimensional electron gas layer. Can be formed. In FIG. 2, χ represents electron affinity. With this configuration, when a forward voltage (positive on the In ₂ O ₃ layer side) is applied, electrons of the two-dimensional electron gas begin to flow through the In ₂ O ₃ layer at about 0.2V. Since the band gap of In ₂ O ₃ is large, a voltage close to 4 V is required for the thermal electrons to flow, so this is not a problem in practice.

In addition, when forming an electrode, especially when forming an electrode directly on the indium oxide layer 2, the indium oxide layer 2 is a very thin layer, so that it is a reducing material such as titanium or aluminum. It disappears immediately. Therefore, it is necessary to select a material that can be formed at a low temperature without reducibility, such as nickel or ruthenium oxide. Not only when forming such an electrode, the indium oxide layer 2 is a very thin layer and easily disappears. Therefore, after the indium oxide layer 2 is formed, a protective layer is formed on the surface thereof. It is preferable. A process for forming such a protective layer 3 is shown in FIG. That is, the semiconductor layer 1 on which the indium oxide layer 2 is formed is immediately put into a CVD apparatus through a load lock, and the indium oxide layer is stably held by forming, for example, about 0.2 to 0.4 μm of SiO ₂ or the like. be able to.

  The above example is an example in which an oxide semiconductor layer having a high carrier concentration and a large band gap is formed over a semiconductor layer having a low carrier concentration and a small band gap. By using the idea of the present invention, an oxide semiconductor layer is formed. In addition, a semiconductor having a large band gap such as a GaN-based compound or SiC can be formed with a high carrier concentration.

FIG. 3 is a diagram showing a process of forming a thin film layer of an N ⁺ -type GaN compound semiconductor on the surface of the N-type GaAs substrate. That is, as shown in FIG. 3A, an N-type GaAs semiconductor layer 11 having a natural oxide film (Ga ₂ O ₃ ) 14 formed on its surface is prepared, or similar to the example shown in FIG. In addition, the oxide film 14 is formed at a low temperature. Then, for example, in a plasma apparatus, for example, ammonia gas and a hydride gas of a dopant element such as hydrogen sulfide (H ₂ S) (for example, a flow rate of about 1% with respect to the flow rate of ammonia) are introduced. By performing the plasma treatment, as shown in FIG. 3B, the oxide film 14 on the surface of the N-type GaAs semiconductor layer 11 is reduced by the H ions generated by the decomposition of the ammonia gas and the dopant gas, and dangling. Ga having a ring bond is formed on the surface.

At the same time, as shown in FIG. 3C, N ions generated by the decomposition of ammonia gas and S ions generated by the decomposition of the dopant gas are combined with activated Ga, so that S is highly doped. N ⁺ -type GaN layer 12 is formed. That is, since N ions and S ions are similarly bonded to activated Ga, S of the dopant element can be reliably taken in at a ratio determined in advance by the gas flow ratio, and the dopant element is also Ga and Since they are combined, a stable state can be maintained and a high carrier concentration N ⁺ -type layer can be obtained.

This series of reactions is represented by the following chemical formula.
2NH ₃ (ammonia plasma) → 6H ⁺ + 2N ^-3
Ga ₂ O ₃ (natural oxide film) + 6H ⁺ → 2Ga ⁺³ + 3H ₂ O
2Ga ⁺³ + 2N ^-3 → 2GaN
In this state, a thin film semiconductor layer having a high carrier concentration according to the present invention can be obtained. However, as in the case of the indium oxide layer described above, it is a very thin film of 10 nm or less, and thus disappears when oxidation or the like occurs. There is a concern, and it is desirable to form a protective layer on the surface. Therefore, by continuously introducing monosilane gas while the surface of the thin film semiconductor layer remains in a reducing atmosphere, as shown in FIG. 3D, the Si ions of the monosilane gas and the N ions of the ammonia plasma are combined. Then, a protective layer 13 made of a silicon nitrogen hydrogen alloy (SiN: H) containing about several to 10 at% of H is deposited on the surface with a thickness of about 0.2 to 0.4 μm.

  That is, the GaN layer 12 having a band gap much larger than that of GaAs can be formed on the GaAs semiconductor layer 11 with a high carrier concentration. As a result, even a diode or HEMT using a two-dimensional electron gas can be formed with a GaAs and GaN layer. In this case, since the GaN layer is a very thin film of about several nm, it becomes a strained lattice layer and exists stably without warping. Further, in this example, GaAs is used as the semiconductor layer 11. However, if Ga is contained, a wide gap semiconductor layer can be formed similarly with GaP and other semiconductor layers. That is, if an AlGaAs-based semiconductor containing Al, an InGaAs-based semiconductor, or an InGaAlP-based semiconductor is used as the semiconductor layer, the thin film layer can be formed with a high carrier concentration such as an AlGaN-based semiconductor, an InGaN-based semiconductor, or an AlGaInN-based semiconductor. And can be applied to the above-described HEMT. Also, by changing the dopant gas, a thin film layer having a high carrier concentration can be formed very easily in the P-type layer as well.

FIG. 4 is a diagram showing a process of forming a thin film layer of an N ⁺ type SiC compound semiconductor on the surface of the N type Si substrate. That is, as shown in FIG. 4A, an N-type Si semiconductor layer 21 having a natural oxide film (SiO ₂ ) 24 formed on its surface is prepared, or in the same manner as the example shown in FIG. The oxide film 24 is formed at a low temperature of 250 ° C. or lower. Then, for example, in a plasma apparatus, a hydrocarbon compound gas such as methane gas and a nitrogen gas or ammonia gas as a dopant gas, for example, at a rate of 0.1% with respect to the flow rate of the methane gas, Introduced with gas and turned into plasma. As a result, as shown in FIG. 4B, the oxide layer 24 on the surface of the N-type Si semiconductor layer 21 is reduced by H ions generated by decomposition of a hydrocarbon compound or the like, and the Si layer 25 having dangling bonds. Is formed on the surface.

At the same time, as shown in FIG. 4 (c), C ions generated by the decomposition of the hydrocarbon compound gas and N ions generated by the decomposition of the dopant gas combine with the activated Si to increase the concentration of N. A doped N ⁺ -type SiC layer 22 is formed. That is, since both C ions and N ions are combined in the same manner as activated Si, N of the dopant element can be reliably taken in at a ratio determined in advance by the gas flow ratio, and the dopant element is also Si and Since they are combined, a stable state can be maintained and a high carrier concentration N ⁺ -type layer can be obtained.

This series of reactions is represented by the following chemical formula.
CH ₄ (methane gas) → 4H ⁺ + C ^-4
SiO ₂ (natural oxide film) + 4H ⁺ → Si ⁺⁴ + 2H ₂ O
Si ⁺⁴ + C ^-4 → SiC
In this state, the thin film semiconductor layer having a high carrier concentration according to the present invention can be obtained. However, unlike the case of the indium oxide layer, the SiC layer is relatively resistant to natural oxidation. However, since it is a very thin film of 10 nm or less, even if slight oxidation or the like occurs, if the oxide film is removed, the SiC film may disappear, and it is desirable to form a protective film on the surface. Therefore, by continuously introducing the monosilane gas while the surface of the thin film semiconductor layer remains in a reducing atmosphere, as shown in FIG. 4D, the Si ions of the monosilane gas and the N ions of the ammonia plasma are combined. Then, a protective layer 23 made of a silicon nitrogen hydrogen alloy (SiN: H) containing about 10 at% H is deposited on the surface with a thickness of about 0.2 to 0.4 μm. At this time, it is necessary to adjust the flow rate of the monosilane gas or increase the flow rate of the ammonia gas so that the flow rate of the monosilane gas and the flow rate of the ammonia gas become approximately the same.

As described above, a semiconductor layer having a band gap much larger than that of Si and having a high carrier concentration can be very easily formed on the surface of the Si semiconductor layer as a strained lattice layer. Similarly, a diode or HEMT using a two-dimensional gas can be formed of a Si layer and a SiC layer. That is, for example, by carbonizing Si, a SiC layer can be formed on the surface of Si. However, even if SiC is formed by such a process, it is only necessary to maintain the impurity concentration of the original Si. The carrier concentration cannot be increased more than the layer. In addition, even if impurities are diffused after carbonization, they are hardly diffused even at a very high temperature. Even if ions are implanted, the impurity concentration cannot be increased simply by breaking the crystal, and the semiconductor has a low carrier concentration. In order to form a semiconductor layer having a large band gap and a large carrier concentration on the layer, it must be epitaxially grown by an MBE apparatus, a CVD apparatus, or the like. A high carrier concentration cannot be obtained. This is the same for the above-described GaN-based compounds. However, according to the present invention, since a dopant element N and the like are bonded together with C that is reduced and simultaneously bonded to the oxide film on the surface, a SiC layer having a high carrier concentration on the order of 10 ¹⁹ cm ⁻³ is formed. Can do. In order to further increase the carrier concentration, it is preferable not to increase the flow rate ratio of the dopant gas but to perform doping using a cathode drop on the substrate by supplying a plasma current.

FIG. 5 is a diagram showing a process of forming a thin film layer of a P ⁺ type SiC compound semiconductor on the surface of the P type SiC substrate. That is, SiC is relatively difficult to oxidize, but still a natural oxide film is formed, and as shown in the above example, an oxide film can be formed by performing ozone oxidation at a low temperature, as shown in FIG. as shown in (a), a P-type SiC semiconductor layer 31 a natural oxide film on the surface (SiO ₂₎ 34 was formed. Then, for example, in a plasma apparatus, a hydrocarbon compound gas such as methane gas and a group III element hydrogen compound gas such as diborane (B ₂ H ₆ ) as a dopant gas are introduced together with a carrier gas hydrogen gas, Turn into plasma. As a result, as shown in FIG. 5B, the Si layer 35 having dangling bonds is obtained by reducing the oxide film 34 on the surface of the P-type SiC semiconductor layer 31 by H ions generated by decomposition of a hydrocarbon compound or the like. Is formed on the surface. At this time, the oxide of C evaporates and disappears.

At the same time, as shown in FIG. 5C, the C ions generated by the decomposition of the hydrocarbon compound gas and the B ions generated by the decomposition of the dopant gas are combined with the activated Si, so that B becomes a high concentration. A doped P ⁺ -type SiC layer 32 is formed. That is, since both C ions and B ions are combined in the same way as activated Si, B of the dopant element can be reliably taken in at a ratio determined in advance by the gas flow ratio, and the dopant element is also Si and Since they are combined, a stable state can be maintained and a high carrier concentration P ⁺ -type layer can be obtained. Then, as shown in FIG. 5D, it is preferable to form a protective layer 33 made of a silicon-nitrogen-hydrogen alloy (SiN: H) on this surface, as in the example shown in FIG. Thus, it can be formed by a similar method.

  In short, in this embodiment, the newly formed thin film semiconductor layer has the same material and the same band gap as the semiconductor layer underneath, but is characterized in that the carrier concentration is greatly improved. is there. That is, the above-mentioned GaN-based compounds and SiC can not be incorporated even if a large amount of dopant elements are introduced during epitaxial growth, and the carrier concentration can be increased too much even if diffusion or ion implantation is attempted. In particular, the P-type layer has a problem that the carrier concentration cannot be increased. For this reason, the contact resistance between the P-type layer and the electrode is increased, and the device characteristics are greatly deteriorated. However, as in this embodiment, by using the reduction substitution co-doping method, the surface of the P-type layer is formed. Since a layer having a very high carrier concentration can be formed, the contact resistance with the electrode can be greatly reduced. As a result, when applied to an impatt diode or the like, the oscillation efficiency can be greatly improved.

It is process explanatory drawing which forms an indium oxide layer on the InP semiconductor layer which is one Embodiment of this invention. It is a diagram showing a band diagram during thermal equilibrium between the InP and In ₂ O ₃ in FIG. It is process explanatory drawing which forms a GaN layer on the GaAs layer which is other embodiment of this invention. It is explanatory drawing which forms a SiC layer on Si layer which is other embodiment of this invention. It is process explanatory drawing which forms the SiC layer of high carrier concentration on the SiC layer which is other embodiment of this invention. It is sectional explanatory drawing which shows the conventional HEMT.

Explanation of symbols

1 N-type InP semiconductor layer (N-type InP substrate)
2 Indium oxide layer 3 Protective layer (silicon nitride layer)
4 Natural oxide film

Claims (11)

  By combining an activated metal element formed by reducing a natural oxide film on the surface of the semiconductor layer or an oxide film generated at a low temperature of 250 ° C. or lower with another element, the carrier layer has a higher carrier concentration than the semiconductor layer. And forming a thin film compound semiconductor layer having a band gap larger than that of the semiconductor layer.   The semiconductor layer is composed of a semiconductor layer containing indium as a constituent element of the compound, and after the oxide film on the surface of the semiconductor layer containing indium is reduced and cleaned, the semiconductor layer is immediately immersed in the oxide generation liquid. The method for forming a thin film semiconductor layer according to claim 1, wherein an indium oxide layer is formed on the surface of the semiconductor layer by exposure to an oxidizing atmosphere at a low temperature of 250 ° C. or lower.   3. The thin film according to claim 2, wherein after forming the indium oxide layer, a protective film made of a silicon-nitrogen hydrogen alloy (SiN: H) is formed on the indium oxide layer by introducing a monosilane gas and a nitrogen atom-containing gas immediately. A method for forming a semiconductor layer.   The oxide film generated on the surface of the semiconductor layer is reduced by hydrogen ions or halogen ions generated by gasification of the gas, and an element source gas and a dopant element source gas bonded to the activated metal element are used. The method for forming a thin film semiconductor layer according to claim 1, wherein the thin film compound semiconductor layer having a high carrier concentration is formed by introducing and forming a plasma and bonding with the activated metal element.   The semiconductor layer comprises a semiconductor layer containing gallium as a constituent element of the compound, and reduces and cleans the oxide film on the surface of the semiconductor layer containing gallium by hydrogen ions generated by plasma treatment of ammonia gas, The activated gallium exposed by the reduction is combined with nitrogen ions generated by the plasma treatment of the ammonia gas to form a thin film compound semiconductor layer made of a compound containing gallium and nitrogen as constituent elements of the compound. 5. The method for forming a thin film semiconductor layer according to 4.   6. The formation of a thin film semiconductor layer according to claim 5, wherein a thin film compound semiconductor layer made of a compound containing gallium and nitrogen is formed in a high carrier concentration N type by introducing a gas of group VIB element as the dopant element. Method.   7. After forming the thin film compound semiconductor layer, a protective film made of silicon nitrogen hydrogen alloy (SiN: H) is formed on the thin film compound semiconductor layer by further introducing monosilane gas in the same apparatus. The formation method of the thin film semiconductor layer of description.   The semiconductor layer is made of silicon or silicon carbide, a hydrocarbon compound gas is introduced as a source gas of an element to be bonded to the activated metal element, and carbon ions generated by plasma treatment and silicon activated by the reduction are The method for forming a thin film semiconductor layer according to claim 4, wherein the thin film compound semiconductor layer made of silicon carbide is formed by bonding.   9. The method of forming a thin film semiconductor layer according to claim 8, wherein a thin film compound semiconductor layer made of P-type silicon carbide having a high carrier concentration is formed by introducing a hydride of a group III element as a source gas for the dopant element.   9. The method of forming a thin film semiconductor layer according to claim 8, wherein a thin film compound semiconductor layer made of high carrier concentration N-type silicon carbide is formed by introducing ammonia gas as the dopant element source gas.   A protective film made of silicon nitrogen hydrogen alloy (SiN: H) is formed on the thin film compound semiconductor layer by further introducing ammonia gas and monosilane gas in the same apparatus after forming the thin film compound semiconductor layer. The method for forming a thin film semiconductor layer according to 8, 9 or 10.

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