JP3763701B2 - Gallium nitride semiconductor light emitting device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a gallium nitride based semiconductor light emitting device, and more particularly to an n-type semiconductor layer in a gallium nitride based semiconductor light emitting device structure.
[0002]
[Prior art]
Gallium nitride semiconductors are capable of high-efficiency recombination due to direct transition optical transition, and have been developed as high-efficiency light-emitting element materials such as semiconductor lasers and high-intensity LEDs. LEDs from blue to green using gallium nitride based semiconductor light emitting devices have been put into practical use. FIG. 2 is a sectional view showing the structure of a conventional gallium nitride based semiconductor light emitting device. In the conventional gallium nitride based semiconductor light emitting device, a buffer layer 22, an n ⁻ GaN contact layer 23, an InGaN active layer 24, a p ⁻ AlGaN cladding layer 25 and a p ⁻ GaN contact layer 26 are sequentially formed on a sapphire substrate 21. . A p ⁻ side electrode 27 is provided on the p ⁻ GaN contact layer 26, and an n ⁻ side electrode 28 is provided on a part of the n ⁻ GaN contact layer 23.
[0003]
[Problems to be solved by the invention]
In the conventional gallium nitride based semiconductor light emitting device shown in FIG. 2, the operating voltage is low. For example, when a blue light emitting LED (450 nm) is taken as an example, the energy gap (about 2.7 V) of the active layer and the resistance component inside the device structure Is estimated to be about 3V. However, devices that are put into practical use exhibit a high operating voltage of about 3.6 to 4.0V. This is due to the high contact resistance of the p-type semiconductor layer with the metal electrode for current injection and the high resistivity of the n-type semiconductor layer. The voltage in the n ⁻ GaN contact layer 23 contributes to an increase in operating voltage. The surface contact type element (p and n electrodes are taken from the element surface) as shown in FIG. 2 is in the n type contact layer 23 as compared with the upper and lower contact type elements (one electrode is taken on the back side of the substrate). The moving distance of carriers is several hundred times, and the voltage drop in the n ⁻ GaN contact layer 23 becomes relatively large. The resistance component estimated from the shape and crystal characteristics is about 30Ω, and the voltage drop at 20 mA is as large as 0.6V. For the n-type contact layer 23, GaN or AlGaN is mainly used. These materials can be doped with impurities to reduce the resistivity. Usually, Si is used for n-type impurities. However, high concentration doping causes the generation of defective crystals. If the concentration exceeds about 5E18 cm ⁻³ , hexagonal pyramid pits appear in the crystal, which causes the flatness to be impaired. Therefore, since the impurity cannot be doped at a high concentration into the n-type contact layer, the voltage drop in the n-type contact layer causes the operating voltage to increase.
[0004]
Further, the conventional structure has a problem that the uniformity of light emission is low. In general, gallium nitride-based semiconductors have a property that current is less likely to flow than group 3 and 4 compound semiconductors such as GaAs and GaP. Therefore, electrons supplied from the n ⁻ side electrode are difficult to spread in the lateral direction in the n ⁻ GaN contact layer 23 and concentrate on a part of the active layer reaching through the shortest distance from the n ⁻ side electrode. Therefore, a compound semiconductor light emitting device has been devised that emits light uniformly by providing a high carrier concentration semiconductor layer containing In in an n-type semiconductor and making carrier injection into the active layer uniform (JP-A-8-23124). ). FIG. 3 is a cross-sectional view showing the structure of the gallium nitride based semiconductor light emitting device. A buffer layer 32 and an n ⁻ GaN contact layer 33 are sequentially provided on the sapphire substrate 31, and an n ⁻ InAlGaN layer 34 is provided on the n ⁻ GaN contact layer 33. On the n ⁻ InAlGaN layer 34, an n ⁻ AlGaN cladding layer 35, an InGaN active layer 36, a p ⁻ AlGaN cladding layer 37, and a p ⁻ GaN contact layer 38 are sequentially formed. p ^- p on the GaN contact layer 38 ^- have a side electrode ^39, p - GaN contact layer ^38, p - AlGaN cladding layer 37, InGaN active layer ^36, n - AlGaN cladding layer ^35, n - InAlGaN layer 34 and the n ^The n ⁻ side electrode 40 is provided on the n ⁻ GaN contact layer 33 exposed by etching a part of the GaN contact layer 33. Incidentally, the layer thickness of the gallium nitride-based semiconductor light-emitting element is ⁿ - GaN contact layer 33 is 1 m to 5 ^m, n - InAlGaN layer 34 is ^10Å~1μm, n - AlGaN cladding layer 35 and the ^p - AlGaN cladding layer 37 and p ^- The GaN contact layer 38 is 50 to 1 μm, and the InGaN active layer 36 is 50 to 0.5 μm. In this structure, electrons supplied from the n-type contact layer spread uniformly through the n-type InAlGaN having a high carrier concentration, and the uniformity of light emission in the active layer can be improved. However, since the n-type InAlGaN layer 34 is a thin film of 1 μm or less, the resistance component in this layer is also relatively large, and the operating voltage of this element at 20 mA is 3.3V. Therefore, even in this structure, the improvement effect on the operating voltage is still insufficient.
[0005]
In the conventional structure shown in FIGS. 2 and 3, since the active layers InGaN 5 and 36 are thin, they grow in lattice matching with the relatively thick n ⁻ GaN contact layers 3 and 33, It is formed including distortion. Since InGaN has a larger lattice constant than GaN, compressive strain occurs in the crystal of the InGaN active layer lattice-matched to the GaN contact layer. As a result, a piezoelectric field is generated in InGaN, and carriers (electrons and holes) existing in the crystal are spatially separated, which is accompanied by a decrease in recombination probability. This reduces the internal quantum efficiency in the LED and is undesirable. As described above, the conventional gallium nitride-based light emitting device has some problems. The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a gallium nitride-based light emitting device having a low operating voltage, uniform light emission and high light emission efficiency, and a method for manufacturing the same.
[0006]
[Means for Solving the Problems]
A gallium nitride based semiconductor light emitting device according to the present invention is formed on a buffer layer formed on a substrate, a first n-type semiconductor layer formed on the buffer layer, and the first n-type semiconductor layer. A second n-type semiconductor layer, an active layer formed on the second n-type semiconductor layer, a p-type semiconductor layer formed on the active layer, and the first n-type semiconductor layer And the first n-type semiconductor layer contains indium and has an electron carrier concentration and a layer thickness larger than those of the second n-type semiconductor layer.
The first n-type semiconductor layer has a layer thickness of 1 μm or more.
In the method for producing a gallium nitride based semiconductor light emitting device according to the present invention, a layer made of indium gallium nitride is formed at a growth rate of 0.5 μm / h or higher and a temperature of 800 ° C. or higher and 1000 ° C. or lower by metal organic vapor phase epitaxy. It is characterized by.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described using examples. FIG. 1 is a cross-sectional view showing the structure of a gallium nitride based semiconductor light emitting device according to the present invention. This example n contained In ^- has a contact layer, the n ^- different from the conventional example in that it has a side electrode ^- n on the contact layer. The structure will be described in detail. A buffer layer 2 is provided on the sapphire substrate 1, and an n ⁻ InGaN contact layer 3 containing In is formed on the buffer layer 2. On the n ⁻ InGaN contact layer 3, there are four n ⁻ AlGaN cladding layers. On the n ⁻ AlGaN cladding layer 4, an InGaN active layer 5, a p ⁻ AlGaN cladding layer 6, and a p ⁻ GaN contact layer 7 are sequentially formed. An n- electrode having an anode electrode 8 on the p ⁻ GaN contact layer 7 and etching and exposing a part of the p ⁻ AlGaN cladding layer 7, the InGaN active layer 5, the n ⁻ AlGaN cladding layer 4 and the n ⁻ InGaN contact layer 3. An n ^- side electrode 9 is provided on the InGaN contact layer 3. Regarding the layer thickness, the n ^- InGaN contact layer 3 is 4 μm, the n ^- AlGaN cladding layer 4 and the p ^- AlGaN cladding layer 6 and the p ^- GaN contact layer 7 are 0.1 μm, and the InGaN active layer 5 is 20 μm. The composition ratio of the n-InGaN contact layer 3 is 0.9 for Ga and 0.1 for In.
[0008]
Next, a method for manufacturing a gallium nitride based semiconductor light emitting device in this example will be described. The gallium nitride based semiconductor light emitting device in this example was formed by metal organic chemical vapor deposition (MOCVD). Specifically, crystal growth is performed by the following process. First, the sapphire substrate 1 is cleaned in a hydrogen atmosphere at 1100 ° C. for 10 minutes. The sapphire substrate 1 is cooled to 500 ° C., and the growth material is supplied with a hydrogen carrier gas to form the buffer layer 2. The substrate temperature is raised to 1000 ° C., and TMG, TMI, NH ₃ and SiH ₄ are supplied with a nitrogen carrier gas to form the n ⁻ InGaN contact layer 3. In order to form the n ⁻ InGaN contact layer 3 with a thickness of 4 μm, it is necessary to increase the growth rate than usual. Since the supply amount of TMG almost determines the growth rate in the growth of the InGaN layer, in this embodiment, the growth rate is set to 8.3 × 10 ⁻⁵ mol / min with respect to the growth rate of 1 μm / h. The electron carrier concentration of the formed InGaN contact layer is 5 × 10 ¹⁹ cm ⁻³ . Next, the substrate temperature is raised to 1050 ° C., and TMG, TMA, NH ₃ , and SiH ₄ are supplied by a hydrogen carrier gas to form the n ⁻ AlGaN cladding layer 4. Thereafter, the substrate temperature is lowered to 750 ° C., and TMG, TMI, and NH ₃ are supplied with a nitrogen carrier gas to form the InGaN active layer 5. The substrate temperature is raised to 1050 ° C., and TMG, TMA, NH ₃ , and Cp ₂ Mg are supplied with a hydrogen carrier gas to form the p ^- AlGaN cladding layer 6. TMG without changing the substrate _temperature, the NH 3, _Cp 2 Mg is supplied with hydrogen carrier gas to form ^p - GaN contact layer 7. Next, the p ^- GaN contact layer 7, the p ^- AlGaN clad layer 6, the InGaN active layer 5, the n ^- AlGaN clad layer 4, and a part of the n ^- InGaN contact layer 3 are removed by etching the obtained structure. An n ⁻ side electrode is formed on the exposed n ⁻ InGaN contact layer 3. p ^- is on the GaN contact layer 7 p ^- and an element to form a side electrode structure.
[0009]
The structure of the gallium nitride based semiconductor light emitting device in this example is different from the conventional example in that the n ⁻ contact layer 3 uses InGaN mixed crystal containing In and has a layer thickness of 4 μm.
In the conventional example shown in FIG. 3, since the film thickness is limited to 1 μm or less from the viewpoint of crystal quality of the In-containing n-type layer 34, the operating voltage cannot be sufficiently reduced as described above. In the n ⁻ InGaN contact layer in this example, the electrical characteristics are a carrier concentration of 5 × 10 ¹⁹ cm ⁻³ and a resistivity of 0.001 Ωcm, which is 0.008 Ωcm (carrier concentration: Better than 5 × 10 ¹⁸ cm ⁻³ ). Therefore, the voltage drop in the n ⁻ contact layer 3 can be reduced, and the operating voltage of the element can be reduced.
Furthermore, the laminated structure of the n ⁻ InGaN contact layer 3 and the n ⁻ AlGaN cladding layer 4 also reduces the resistance. This is because the movement of carriers toward the interface is promoted by the large energy gap difference between InGaN and AlGaN. Since current spreading in the interface direction, that is, in the lateral direction is facilitated, uniform carrier injection into the active layer is performed. As a result, uniform light emission of the element can be realized.
[0010]
The InGaN active layer 5 (In composition 0.25) grown in lattice matching on the n ⁻ InGaN contact layer 3 (In composition 0.1) has a compressive strain corresponding to the In composition difference. However, the amount of strain in the active layer is small compared to the structure of the conventional example based on GaN. This is because the n ⁻ InGaN contact layer 3 contains soft crystals because it contains indium, and relaxes the compressive strain generated in the InGaN active layer 5. Therefore, the piezoelectric field generated in the active layer is reduced, the recombination probability of electrons and holes is increased, and highly efficient light emission is possible.
When a current was injected by applying a bias to the light emitting element in this example, uniform light emission with a main emission wavelength of 450 nm from the active layer was observed. At a forward current of 20 mA, the operating voltage was 3.0 V and the light emission output was 8 mW.
The manufacturing method of the gallium nitride based semiconductor light emitting device according to the present invention is different from the conventional example in that the contact layer containing indium is grown at a growth rate of 1 μm / h and a high growth temperature. Thereby, a high-quality InGaN crystal having a thick film can be formed. By forming the thick n ⁻ InGaN contact layer 3, the etching freedom for forming the n-side electrode 9 is increased, and the electrode can be formed on the n ⁻ InGaN contact layer.
[0011]
Embodiments in the present invention are not limited to those shown above. The impurity of the n ⁻ InGaN contact layer 3 is Si, but the same effect can be obtained by using Ge, O, C, or the like instead. The electron carrier concentration of the n ⁻ InGaN contact layer 3 is 5 × 10 ⁻¹⁹ cm ³ , but can be changed in the range of 1 × 10 ¹⁹ cm ^{−3 to 1} × 10 ⁻²² cm ³ . Although the gallium nitride based semiconductor light emitting device in this embodiment has the n ⁻ AlGaN cladding layer 4, the InGaN active layer 5 can also be formed directly on the n ⁻ InGaN contact layer 3. However, in this case, the In composition of the InGaN active layer 5 needs to be larger than that of the n ⁻ InGaN contact layer 3. The present invention is a n-type semiconductor layer having a layer thickness of more than 1μm containing an In, n on the upper surface ^- relates gallium nitride based semiconductor light-emitting device having a side electrode, the element formation as long as it satisfies the structure The structure can be changed as long as there is no problem. The growth rate of the n ⁻ InGaN contact layer 3 is 1 μm / h. However, the growth rate is not limited to this, and can be changed as long as there is no problem in element formation. Although the growth temperature of the n ⁻ InGaN contact layer 3 is set to 1000 ° C., it can be changed within the range where there is no problem in element formation. This embodiment relates to a gallium nitride LED, but can also be applied to a gallium nitride LD. Other modifications can be made without departing from the scope of the present invention.
[0012]
【The invention's effect】
According to the structure of the gallium nitride based semiconductor light emitting device of the present invention, an element having a low operating voltage, uniform light emission of the active layer, and improved light emission output can be realized.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing the structure of a gallium nitride based semiconductor light emitting device according to the present invention;
FIG. 2 is a cross-sectional view showing the structure of a conventional gallium nitride based semiconductor light emitting device;
FIG. 3 is a cross-sectional view showing the structure of a conventional gallium nitride based semiconductor light emitting device.
[Explanation of symbols]
1 ... sapphire substrate 2 ... buffer layer 3 ^... n - InGaN contact layer 4 ^... n - AlGaN cladding layer 5 ... InGaN active layer 6 ^... p - AlGaN cladding layer 7 ^... p - GaN contact layer 8, 9 ... electrode

Claims (2)

A buffer layer formed on a substrate, a first n-type semiconductor layer formed on the buffer layer, the second n-type semiconductor layer formed in contact with said first n-type semiconductor layer on surface When the a second n-type semiconductor layer on surface active layer formed in contact, the active layer p-type semiconductor layer formed on the first n-type semiconductor electrode formed on the layer And the first n-type semiconductor layer and the active layer contain indium, and the second n-type semiconductor layer is made of AlGaN. 2. The gallium nitride based semiconductor light emitting device according to claim 1, wherein the first n-type semiconductor layer has a layer thickness of 1 μm or more.

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