JP2006032739A - Light emitting element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light emitting element whose light emission efficiency is improved by obtaining a GaN compound thin film with high crystal quality. <P>SOLUTION: In the light emitting element 10, a GaN buffer layer 12 consisting of GaN is formed on a Ga<SB>2</SB>O<SB>3</SB>substrate 11 consisting of β-Ga<SB>2</SB>O<SB>3</SB>single crystal and shows n-type conductivity. The GaN compound thin film is grown up on the GaN buffer layer 12. An n electrode 18 is formed below the Ga<SB>2</SB>O<SB>3</SB>substrate 11 by bringing it contact with the Ga<SB>2</SB>O<SB>3</SB>substrate 11. A reflective layer 19 which reflects luminescence light passing through the Ga<SB>2</SB>O<SB>3</SB>substrate 11 and the n electrode 18 to a light emitting layer 14-side is formed on the lowest layer. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a light-emitting element such as a light-emitting diode and a laser diode, and more particularly to a light-emitting element that has improved luminous efficiency by obtaining a GaN-based compound thin film with high crystal quality.

Conventional semiconductor layer, and the Al ₂ O ₃ substrate of Al ₂ O _3, epitaxial growth AlN layer formed on the Al ₂ O ₃ surface of the substrate, on the AlN layer by MOCVD (Metal Organic Chemical Vapor Deposition) method (See, for example, Patent Document 1).

According to this semiconductor layer, by forming the AlN layer between the Al ₂ O ₃ substrate and the GaN growth layer, it is possible to reduce the mismatch of lattice constants and suppress the deterioration of crystal quality.
Japanese Patent Publication No. 52-36117

  However, according to the conventional semiconductor layer, the lattice constants of the AlN layer and the GaN growth layer cannot be completely matched, and it is difficult to further improve the crystal quality of the GaN growth layer. Further, according to the conventional semiconductor layer, when applied to a light emitting element, the crystallinity of the light emitting layer is deteriorated, and the light emission efficiency may be reduced.

  Accordingly, an object of the present invention is to provide a light emitting device having improved luminous efficiency by obtaining a GaN-based compound thin film with high crystal quality.

In order to achieve the above object, the present invention provides a substrate made of a Ga ₂ O ₃ -based compound semiconductor exhibiting n-type conductivity, a buffer layer exhibiting n-type conductivity formed on the substrate, and the buffer layer And a GaN-based compound thin film made of a GaN-based compound formed thereon.

  It is preferable to form a reflective layer that reflects emitted light on the outside of the electrode formed on the substrate side or the electrode formed on the layer opposite to the substrate. Moreover, it is preferable that a reflection layer has electroconductivity. Moreover, it is preferable that the electrode formed in the board | substrate side or the electrode formed in the layer on the opposite side to a board | substrate is transparent.

The light emitting device according to the present invention is formed on the buffer layer by laminating the buffer layer showing n-type conductivity on the substrate made of a Ga ₂ O ₃ -based compound semiconductor showing n-type conductivity. When the crystal quality of the GaN-based compound thin film is not deteriorated and this is used for a light emitting device, the light emission efficiency can be increased.

[First Embodiment]
FIG. 1 shows a cross section of a light emitting device according to a first embodiment of the present invention. This light-emitting element 10 has a low-temperature buffer layer GaN buffer layer 12 made of GaN on a Ga ₂ O ₃ substrate 11 made of β-Ga ₂ O ₃ single crystal and showing n-type conductivity, and exhibits n-type conductivity. _{In (1-x) Ga x} n light-emitting layer 14 having n-GaN cladding layer ₁₃ made of _{GaN, In (1-x)} a layer containing Ga _x n multiple quantum well structure (MQW) (however, 0 ≦ x ≦ 1), p-Al _y Ga _x N cladding layer 15 made of Al _y Ga _x N exhibiting p-type conductivity (where 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ x + y ≦ 1), A p-GaN contact layer 16 made of GaN exhibiting p-type conductivity and a p-electrode 17 are sequentially stacked. The lower surface of the Ga ₂ O ₃ substrate 11, n electrode 18 is provided in contact with the _Ga 2 _{O 3} substrate 11, the bottom layer, the reflective layer 19 for reflecting the emitted light is provided.

The GaN buffer layer 12 is formed on the Ga ₂ O ₃ substrate 11 using an MOCVD (metal organic chemical vapor deposition) apparatus.

The In _(1-x) Ga _x N light-emitting layer 14 is formed of, for example, a semiconductor made of non-doped InGaN to which no impurity is added, and has a single quantum well or multiple quantum well structure (MQW). The emission wavelength can be changed by changing the band gap of the In _(1-x) Ga _x N light-emitting layer 14 by adjusting the composition ratio of In and Ga or by using p-type or n-type conductivity. it can.

  The p electrode 17 and the n electrode 18 will be described separately. The p electrode 17 is formed on the p-GaN contact layer 16 with a material that can provide ohmic contact by vapor deposition, sputtering, or the like. As a material of the p-electrode 17, a simple metal such as Au, Al, Be, Ni, Pt, In, Sn, Cr, Ti, Zn, or at least two kinds of alloys thereof (for example, Au—Zn alloy, Au—Be) Alloys), those forming these in a two-layer structure (for example, Ni / Au), ITO, or the like can be used. The p electrode 17 is preferably transparent. The p electrode 17 may be a pad electrode or formed on the entire surface of the p-GaN contact layer 16.

As a material for the n-electrode 18, a simple metal such as Au, Al, Co, Ge, Ti, Sn, In, Ni, Pt, W, Mo, Cr, Cu, Pb, or at least two kinds of these alloys (for example, (Au—Ge alloy), those formed in a two-layer structure (for example, Al / Ti, Au / Ni, Au / Co), or ITO can be used. The n electrode 18 may be a pad electrode or may be formed on the entire surface of the Ga ₂ O ₃ substrate 11.

<Substrate formation method>
Next, a method for forming the Ga ₂ O ₃ substrate 11 will be described. First, a β-Ga ₂ O ₃ single crystal as a material for the Ga ₂ O ₃ substrate 11 is prepared. This β-Ga ₂ O ₃ single crystal is manufactured by the FZ (floating zone) method. First, a β-Ga ₂ O ₃ seed crystal and a β-Ga ₂ O ₃ polycrystalline material are prepared.

The β-Ga ₂ O ₃ seed crystal has a prismatic shape with a square cross section cut out from a β-Ga ₂ O ₃ single crystal by use of a cleavage plane or the like, and its axial direction is the a-axis <100> orientation, the b-axis It is in the <010> orientation or the c-axis <001> orientation.

The β-Ga ₂ O ₃ polycrystalline material is obtained, for example, by filling a rubber tube with 4N purity Ga ₂ O ₃ powder, cold-compressing it at 500 MPa, and sintering at 1500 ° C. for 10 hours. .

Next, in an atmosphere of a mixed gas of nitrogen and oxygen (changed between 100% nitrogen and 100% oxygen) having a total pressure of 1 to 2 atm in a quartz tube, β-Ga ₂ O ₃ seed crystal and β The tips of the —Ga ₂ O ₃ polycrystalline material are brought into contact with each other, and the contact portions are heated and melted. When the melted β-Ga ₂ O ₃ polycrystalline material is cooled, the β-Ga ₂ O ₃ single crystal is converted into the same direction as the axial direction of the β-Ga ₂ O ₃ seed crystal (a-axis, b-axis, or c direction). Furthermore, the β-Ga ₂ O ₃ polycrystal is dissolved in a direction away from the seed crystal, and the dissolved β-Ga ₂ O ₃ polycrystal is cooled to obtain a β-Ga ₂ O ₃ single crystal. When the β-Ga ₂ O ₃ single crystal is grown in the b-axis <010> orientation, the cleavage of the (100) plane becomes strong, so that the β-Ga ₂ O ₃ single crystal is cut along a plane perpendicular to the plane parallel to the (100) plane. Thus, a β-Ga ₂ O ₃ substrate is manufactured. In addition, when crystal growth is performed in the a-axis <100> orientation or the c-axis <001> orientation, the cleaving properties of the (100) plane and the (001) plane are weakened. There is no restriction on the cut surface.

As a result of measuring the specific resistance of the Ga ₂ O ₃ substrate 11 thus produced, a value of 0.1 Ω · cm or less was obtained at room temperature.

The Ga ₂ O ₃ substrate 11 is basically composed of a β-Ga ₂ O ₃ single crystal, but is selected from the group consisting of Cu, Ag, Zn, Cd, Al, In, Si, Ge, and Sn. the Ga obtained by adding one or more may be constituted by Ga ₂ O ₃ system compound whose main component. By adding these elements, the lattice constant or band gap energy can be controlled. For example, by adding elements of Al and In, (Ga _x Al _y In _(1-xy) ) ₂ O ₃ (where 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ x + y ≦ 1) Can be obtained.

<Method for forming buffer layer>
Next, a method for forming the GaN buffer layer 12 made of GaN will be described with reference to FIG.

  FIG. 2 is a schematic view showing the MOCVD method, and shows a schematic cross section of a main part of the MOCVD apparatus. The MOCVD apparatus 20 includes a reaction vessel 21 connected to an exhaust unit 26 having a vacuum pump and an exhaust device (not shown), a susceptor 22 on which a substrate 27 is placed, a heater 23 that heats the susceptor 22, and a susceptor. A control shaft 24 that rotates and moves up and down 22, a quartz nozzle 25 that supplies a source gas obliquely or horizontally toward the substrate 27, a TMG (trimethylgallium) gas generator 31 that generates various source gases, and a TMA A (trimethylaluminum) gas generator 32 and a TMI (trimethylindium) gas generator 33 are provided. In addition, you may increase / decrease the number of gas generators as needed.

NH ₃ is used as a nitrogen source, He is used as a carrier gas, for example, and TMG and NH ₃ are used as source gases in order to form a GaN thin film.

Next, a method for forming the GaN buffer layer 12 using the MOCVD apparatus 20 will be described. First, in the reaction vessel 21, the substrate 27 is held by the susceptor 22 so that the surface on which the thin film is formed faces up. Then, the temperature in the reaction vessel 21 is adjusted so that the surface temperature of the substrate 27 becomes 580 ° C. ± 50 ° C. The inside of the reaction vessel 21 is depressurized to 100 torr, and TMG as a Ga supply material and NH ₃ as a nitrogen source are supplied into the reaction vessel 21 to form the GaN buffer layer 12. The plane orientation of the substrate 27 on which the GaN buffer layer 12 is formed is ± 20 ° with respect to the (100) plane. In addition, since the GaN buffer layer 12 is a low-temperature buffer layer, formation of the buffer layer 12 with flatness can be expected. Further, instead of the GaN buffer layer 12, a buffer layer 12 made of AlN may be formed. The growth plane may be a plane other than the (010) plane, (001) plane, (100) plane, for example, the (801) plane.

As the carrier gas, a rare gas such as Ar and Ne and an inert gas such as N ₂ can be used in addition to the above He. These inert gases do not react with the oxide semiconductor constituting the substrate 27. Here, when H ₂ is used as a carrier gas at a high temperature as in the prior art, the surface of the substrate 27 becomes full of holes as etched, and the flatness becomes very poor. This is considered that the oxygen of Ga ₂ O ₃ constituting the substrate 27 is reduced by hydrogen of H ₂ . Therefore, when H ₂ is used, it is difficult to grow a thin film such as GaN on the substrate 27. Therefore, it is preferable to use the aforementioned He, Ar, or the like as a carrier gas.

Further, a small amount of reducing gas that does not react with oxygen constituting the substrate 27 may be added to the carrier gas. For example, a small amount of hydrogen H ₂ may be added to the He gas.

<Method for forming GaN-based compound thin film>
An n-GaN clad layer 13, an In _(1-x) Ga _x N light-emitting layer 14, a p-Al _y Ga _x N clad layer 15, and a p-GaN contact layer 16 that are GaN-based compound thin films made of a GaN-based compound. Is formed by MOCVD as in the formation of the GaN buffer layer 12. In order to form an InGaN thin film, TMI, TMG, and NH ₃ are used as source gases, and in order to form an AlGaN thin film, TMA, TMG, and NH ₃ are used as source gases. As the carrier gas, He is used as in the buffer formation method. In order to form the GaN thin film, the same source gas and carrier gas as those used in the buffer layer forming method described above are used.

The GaN-based compound includes an additive such as a group III element such as B, Al, In, or Tl. For example, by adding elements of Al and In, the general formula Ga _x Al _y In _(1-xy) N (where 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ x + y ≦ 1) GaN-based compounds represented can be used.

<Formation of thin films with different carrier concentrations>
In order to change the carrier concentration of GaN like the n-GaN cladding layer 13 and the p-GaN contact layer 16 by the MOCVD apparatus 20, it is performed by changing the amount of n-type dopant or p-type dopant added to GaN.

In order to form thin films having different carrier concentrations, for example, the n-GaN cladding layer 13 and the p-GaN contact layer 16, using the MOCVD apparatus 20, the following is performed. First, in the reaction vessel 21, the substrate 27 is held by the susceptor 22 so that the surface on which the thin film is formed faces up. Then, the temperature in the reaction vessel 21 is 1080 ° C., TMG is 54 × 10 ⁻⁶ mol / min, TMA is 6 × 10 ⁻⁶ mol / min, and monosilane (SiH ₄ ) is 22 × 10 ⁻¹¹ mol / min. Then, Si-doped Ga _0.9 Al _0.1 N (n-GaN cladding layer 13) is grown to a thickness of 3 μm.

Further, the temperature in the reaction vessel 21 is 1080 ° C., TMG is flowed at 54 × 10 ⁻⁶ mol / min together with bisdiclopentadienylmagnesium (Cp ₂ Mg), and Mg-doped GaN (p-GaN contact layer 16) Is grown with a film thickness of 1 μm.

In the light emitting element 10 according to this embodiment, it is represented by the general formula Ga _x Al _y In _(1-xy) N (where 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ x + y ≦ 1). One or more thin films each made of a p-type and n-type conductive material are formed.

  Instead of the n-GaN cladding layer 13, InGaN, AlGaN, or InGaAlN may be grown. In the case of InGaN and AlGaN, the lattice constant with the GaN buffer layer 12 can be substantially matched, and in the case of InAlGaN, the lattice constant with the GaN buffer layer 12 can be matched.

<Effect of the first embodiment>
The light emitting device 10 according to the first embodiment has the following effects.
A) According to the light emitting device of the present invention, the buffer layer 12 is formed by laminating the buffer layer 12 exhibiting n-type conductivity on the substrate 11 made of a Ga ₂ O ₃ -based compound semiconductor exhibiting n-type conductivity. thin GaN-based compound formed on the crystal (n-GaN cladding layer _{13, in (1-x)} Ga x n light-emitting layer _{14, p-Al y Ga x} n layer cladding 15, p-GaN contact layer 16) When the quality is not deteriorated and is used for the light emitting element 10, the light emission efficiency can be increased.
B) Since the GaN buffer layer 12 which is a low-temperature buffer layer is formed on the Ga ₂ O ₃ substrate 11, the flatness is good. Therefore, deterioration of the crystal quality of the GaN compound thin film laminated on the GaN buffer layer 12 can be suppressed, so that the light emission efficiency can be increased.
C) Since the light-emitting element 10 can grow a GaN compound thin film having a good crystal quality, the light-emitting element 10 emits light with a peak at around 410 nm.
D) Since β-Ga ₂ O ₃ has conductivity, a light emitting diode having a vertical electrode structure can be formed. As a result, the entire light emitting element 10 can be used as a current path. Therefore, the lifetime of the light emitting element 10 can be extended.
E) The reflection layer 19 reflects the emitted light reaching the n-electrode 18 to the p-electrode 17 side and emits the emitted light from the p-electrode 17 side, so that the emitted light can be emitted efficiently.
F) Since the substrate 11 is composed of a β-Ga ₂ O ₃ single crystal, the substrate 11 exhibiting high crystallinity and n-type conductivity can be formed.
G) Since the light-emitting element 10 has a multiple quantum well structure, there is a high probability that electrons and holes serving as carriers are confined in the In _(1-x) Ga _x N light-emitting layer 14 and recombined. As a result, the luminous efficiency is greatly improved.

[Second Embodiment]
FIG. 3 shows a light emitting device according to the second embodiment of the present invention. The light-emitting element 10 includes a GaN buffer layer 12 that is a low-temperature buffer layer made of GaN, an n-type, on a part of a Ga ₂ O ₃ substrate 11 that shows n-type conductivity made of β-Ga ₂ O ₃ single crystal. n-GaN cladding layer ₁₃ made of GaN exhibiting _{conductivity, In (1-x) Ga} x n a layer comprising a multiple quantum well structure _{(MQW) In (1-x} ) Ga x n light-emitting layer 14 ( However, 0 ≦ x ≦ 1), p-Al _y Ga _x N layer clad 15 made of Al _y Ga _x N exhibiting p-type conductivity (however, 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ x + y ≦ 1) A p-GaN contact layer 16 made of GaN exhibiting p-type conductivity, a p-electrode 17 and a reflective layer 19 are sequentially stacked, and on the upper surface of the Ga ₂ O ₃ substrate 11 where the GaN buffer layer 12 is not formed. Is formed with an n-electrode 18.

This light emitting element 10 is of a flip chip type and is mounted so as to emit emitted light from the surface of the Ga ₂ O ₃ substrate 11 where the GaN buffer layer 12 is not grown.

GaN buffer layer 12, and a GaN-based compound thin film, n-GaN cladding layer _{13, In (1-x)} Ga x N light-emitting layer _{14, p-Al y Ga x} N cladding layer 15, and p-GaN contact layer 16 is formed by MOCVD as in the first embodiment.

<Effects of Second Embodiment>
B) According to the light emitting device 10 according to the _second embodiment, the GaN buffer layer 12 that is a low-temperature buffer layer is formed on the Ga ₂ O ₃ substrate 11, so that the flatness is good. Therefore, deterioration of the crystal quality of the GaN compound thin film laminated on the GaN buffer layer 12 can be suppressed, so that the light emission efficiency can be increased.
B) Since the reflection layer 19 reflects the emitted light reaching the p-electrode 17 to the Ga ₂ O ₃ substrate 11 side and emits it from the substrate 11 side, the emitted light can be emitted efficiently.
C) Since this light emitting device has a multiple quantum well structure, the probability that electrons and holes serving as carriers are confined in the In _(1-x) Ga _x N light emitting layer 14 and recombined increases. Therefore, the luminous efficiency is greatly improved.
D) Since the Ga ₂ O ₃ substrate 11 itself has conductivity, it is possible to pass a large current as compared with the case of passing a current through a conductive thin film formed on a conventional insulating substrate, and high luminance light emission. Can be obtained.

  Note that the light-emitting element 10 according to the present invention is not limited to a light-emitting diode or a laser diode, but can also be applied to a semiconductor such as a transistor, a thyristor, or a diode. Specifically, a field effect transistor, a photodiode, a solar cell, etc. are mentioned, for example.

It is sectional drawing of the light emitting element which concerns on the 1st Embodiment of this invention. It is the schematic which shows MOCVD method. It is sectional drawing of the light emitting element which concerns on the 2nd Embodiment of this invention.

Explanation of symbols

10 light-emitting element 11 _Ga 2 _{O 3} substrate 12 the buffer layer 13 n-GaN cladding layer _{14 In (1-x) Ga} x N light-emitting layer 15 _p-Al y Ga _x N cladding layer 16 p-GaN contact layer 17 n electrode 18 p electrode 19 reflective layer 20 MOCDV device 21 reaction vessel 22 susceptor 23 heater 24 control shaft 25 quartz nozzle 26 exhaust part 27 substrate 31 TMG gas generator 32 TMA gas generator 33 TMI gas generator

Claims (4)

a substrate made of a Ga ₂ O ₃ -based compound semiconductor exhibiting n-type conductivity;
A buffer layer having n-type conductivity formed on the substrate;
And a GaN-based compound thin film made of a GaN-based compound formed on the buffer layer. The GaN-based compound thin film is made of Ga _x Al _y In _(1-xy) N (where 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ x + y ≦ 1). The light emitting device according to 1. 2. The light emitting device according to claim 1, wherein the buffer layer is made of Ga _x Al _(1-x) N doped with Si or Hf (where 0 ≦ x ≦ 1).   2. The light emitting device according to claim 1, wherein the buffer layer is formed on a surface within a range of plus or minus 20 degrees with respect to a (100) surface of the substrate.