JP3809749B2 - Nitride semiconductor light emitting device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an LED element made of a nitride semiconductor (In _X Al _Y Ga _1-XY N, 0 ≦ X, 0 ≦ Y, X + Y ≦ 1).
[0002]
[Prior art]
Conventionally, red LEDs and yellow-green LEDs have been put to practical use as visible LEDs. Recently, blue, blue-green LEDs have been developed with nitride semiconductors, and for the first time, B, G, and R3 colors are used. The full color LED display that appeared.
[0003]
However, the current green LED has a light emission wavelength in the yellow-green region near 560 nm and is not a pure green LED near 520 nm, so the color reproduction region is narrow. Moreover, since the brightness of the blue and red LEDs is only 1/10 or less, there is a drawback that the number of yellow-green LEDs must be increased in order to achieve white balance. In order to solve this problem, a pure green LED having a light intensity of 2 cd or more is required.
[0004]
We have developed a high-brightness LED that emits pure green light that can solve this problem, and have already announced it (Jpn.J.Appl.Phys. Vol.34 (1995) pp.L797-L799). FIG. 2 shows the structure of the LED. 21 is a substrate made of sapphire, 22 is a buffer layer made of GaN having a thickness of 30 nm, 23 is a Si-doped n-type GaN layer having a thickness of 4 μm, 24 is a Si-doped n-type Al0.1Ga0.9N layer having a thickness of 0.1 μm, 25 Is a 50 nm thick Si-doped n-type In0.05Ga0.95N layer, 26 is a 2 nm thick non-doped In0.43Ga0.57N active layer, 27 is a 0.1 μm thick Mg-doped p-type Al0.1Ga0.9N layer, and 28 is 0 .5 μm thick Mg-doped p-type GaN layer. This LED has an active layer having a single quantum well (SQW) structure, and is molded with a resin having a lens shape with a main emission wavelength of 525 nm, a light emission output of 1 mW, and a directivity angle of 10 ° at a forward current of 20 mA. The luminous intensity is 4 cd. As a result of the development of this LED, one pixel can be constructed for each of G, B, and R, and a display with a wide color reproduction region can be realized.
[0005]
[Problems to be solved by the invention]
However, since the LED having the above structure has a complicated stacked structure, the growth process of the nitride semiconductor is complicated. Accordingly, an object of the present invention is to realize a light-emitting element such as a high-brightness LED with a minimum laminated structure of nitride semiconductors.
[0006]
[Means for Solving the Problems]
The nitride semiconductor LED of the present invention comprises a current injection layer / carrier confinement layer made of at least an n-type Al _b Ga _1-b N having a b value of 0.05 or less and an In thickness of 7 nm or less on a substrate. an active layer having a quantum well structure including _{x Ga 1-x N (0} <X <1) well layer, a p-type cladding layer y value is 0.05 or more p-type _Al y _{Ga 1-y} N It has the laminated structure which has these in this order . In the present invention, InGaN, AlGaN, GaN, etc. do not necessarily refer to nitride semiconductors that are only ternary mixed crystals or only binary mixed crystals, but for example, in the case of InGaN, a small amount of Al, within a range that does not change the action of InGaN. It goes without saying that other impurities are included within the scope of the present invention.
[0007]
FIG. 1 is a schematic cross-sectional view showing the structure of the LED element of the present invention. In this figure, 11 is a substrate, 12 is a buffer layer, 13 is a clad layer made of n-type In _a Al _b Ga _1-ab N (0 ≦ a, 0 ≦ b, a + b ≦ 1), an n-type contact layer, 16 _{in X Ga 1-X N (} 0 ≦ X <1) than become active layer having a single quantum well or multiple quantum well structure, 17 denotes a p-type _{Al Y Ga 1-Y N (} 0 ≦ Y <1) A p-type cladding layer 18 is a p-type contact layer made of p-type GaN.
[0008]
The substrate 11 includes sapphire (including Al ₂ O ₃ , A plane, R plane, C plane), spinel (MgAl ₂ O ₄ ), SiC (including 6H, 4H, 3C), ZnS, ZnO, GaAs, Conventional materials proposed for growing nitride semiconductors such as GaN can be used.
[0009]
For example, GaN, AlN, GaAlN, SiC, or the like is known as the buffer layer 12 and is usually grown to a thickness of about 5 nm to 1 μm in order to alleviate the lattice mismatch between the substrate and the nitride semiconductor. For example, JP-B-59-48794 and JP-B-4-15200 describe a method using AlN as a buffer layer, and JP-A-60-173829 and JP-A-4-297023 disclose a GaN buffer layer. Is described. In particular, when a substrate having a lattice constant close to that of a nitride semiconductor or a substrate having the same lattice constant is used, the buffer layer may not be formed.
[0010]
Next, the laminated structure which is a feature of the present invention will be described. The n-type cladding layer 13 may have any composition as long as it is a nitride semiconductor represented by In _a Al _b Ga _1-ab N (0 ≦ a, 0 ≦ b, a + b ≦ 1), but is particularly preferable. Is preferably GaN, In _a Ga _1-a N having an a value of 0.5 or less, or Al _b Ga _1-b N having a b value of 0.5 or less. This is because, as shown in FIG. 1, when the n-type cladding layer 13 is also used as a contact layer for forming an n-electrode, a certain thickness is required. Even if the nitride semiconductor is grown to a thickness of, for example, 1 μm or more, a crystal with good crystallinity can be obtained, so that an n-electrode and a good ohmic can be obtained as a contact layer. Moreover, it is difficult to obtain an LED element with high output unless the next active layer, p-type cladding layer, etc. are laminated on the n-type cladding layer having good crystallinity. The film thickness of the n-type cladding layer is not particularly limited, but it is desirable to grow the film to a thickness of about 0.5 μm to 5 μm in order to serve as a contact layer as described above. Nitride semiconductors have the property of becoming n-type due to nitrogen vacancies formed in the crystal even when they are not doped, but the carrier concentration is usually high by doping donor impurities such as Si, Ge, Se, etc. during crystal growth A preferred n-type can be obtained.
[0011]
Next, the active layer 16 has a single quantum well (SQW) structure or a multi-quantum well (MQW) structure, In _x Ga _1-X N (0 ≦ X <). 1). When the SQW structure or the MQW structure is used, a light emitting element having a very high output can be obtained. SQW and MQW indicate the structure of an active layer that can emit light between quantum levels due to non-doped InGaN. For example, in SQW, the active layer is composed of In _X Ga _1-X N (0 ≦ X <1) having a single composition. a configuration with a layer, in _X Ga _1-X N film thickness 10nm or less, more preferably a strong light emission between quantum levels is obtained by the 7nm or less. MQW is a multilayer film in which a plurality of thin films of In _x Ga _1-X N (including X = 0 and X = 1 in this case) having different composition ratios are stacked. In this way, by setting the active layer to SQW and MQW, light emission of about 365 nm to 660 nm can be obtained by light emission between quantum levels. As described above, the thickness of the quantum well layer is preferably 7 nm or less. In the multiple quantum well structure, the well layer is preferably composed of In _x Ga _1-X N, and the barrier layer is also composed of In _Y Ga _1-Y N (Y <X, including Y = 0 in this case). Particularly preferably, when the well layer and the barrier layer are formed of InGaN, the active layer having good crystallinity can be obtained because it can be grown at the same temperature. When the thickness of the barrier layer is 15 nm or less, more preferably 12 nm or less, a high-power light-emitting element can be obtained.
[0012]
As described above, when the thickness of the quantum structure well layer is 7 nm or less, more preferably 5 nm or less, an element having a high light emission output can be realized. This indicates that this film thickness is less than the critical film thickness of the InGaN active layer. In InGaN, the Bohr radius of electrons is about 3 nm, and the quantum effect of InGaN appears below 7 nm. Similarly, in the case of the multiple quantum well structure, it is desirable to adjust the thickness of the well layer to 7 nm or less, while adjusting the thickness of the barrier layer to 15 nm or less.
[0013]
Next, the p-type cladding layer 17 in contact with the active layer 16 needs to be p-type Al _Y Ga _1-Y N (0 ≦ Y <1), and particularly preferably a high output when the Y value is 0.05 or more. An element is obtained. Furthermore, AlGaN is easy to obtain a p-type with a high carrier concentration, is not easily decomposed during growth, and has an effect of suppressing decomposition of the InGaN active layer 16. In addition, the band offset and the refractive index difference can be made larger than those of other nitride semiconductors with respect to the InGaN active layer 16, which is the most excellent. If the first p-type cladding layer is p-type GaN, the light emission output is reduced to about 1/3 compared to p-type AlGaN. This is presumed that AlGaN tends to be p-type compared to GaN, or that the InGaN active layer is decomposed during GaN growth. Therefore, as the p-type cladding layer, Mg-doped p-type Al _Y Ga _1-Y N having a Y value of 0.05 or more is most preferable.
[0014]
The thickness of the p-type cladding layer 17 is desirably 1 nm or more and 2 μm or less, more preferably 5 nm or more and 0.5 μm or less. If the thickness is less than 1 nm, the p-type cladding layer 17 is almost absent, and the light emission output tends to decrease. If the thickness is more than 2 μm, the p-type cladding layer itself tends to crack during crystal growth. This is because even if the next layer is stacked on the layer containing, a semiconductor layer with good crystallinity cannot be obtained and the output tends to decrease. In order to obtain a p-type nitride semiconductor, it can be obtained by doping an acceptor impurity such as Mg, Zn, C, Be, Ca, Ba during crystal growth. More preferably, after doping with an acceptor impurity, annealing is performed at 400 ° C. or higher in an atmosphere of an inert gas such as nitrogen or argon (Japanese Patent Laid-Open No. 5-183189). By performing annealing, a carrier concentration of 1 × 10 ¹⁷ to 1 × 10 ¹⁹ / cm ³ is usually obtained with p-type AlGaN. In addition, you may perform the electron beam irradiation process shown by Unexamined-Japanese-Patent No. 3-218625.
[0015]
Next, the p-type contact layer 18 is p-type GaN, particularly preferably Mg-doped p-type GaN. Since p-type GaN is a layer in contact with an electrode, it is important to obtain ohmic contact in the case of a light emitting device such as an LED or LD. P-type GaN is most preferable as a contact layer because it can easily form ohmic with many metals. As the electrode material, for example, ohmic can be obtained from Ni—Au, Ni—Ti or the like. The thickness of the p-type contact layer is not particularly limited, but it is usually desirable to grow with a thickness of about 50 nm to 2 μm.
[0016]
The nitride semiconductor can be grown by vapor phase growth methods such as metal organic vapor phase epitaxy (MOVPE), halide vapor phase epitaxy (HDVPE), and molecular beam vapor phase epitaxy (MBE). Among them, the MOVPE method can quickly obtain a crystal with good crystallinity. In the MOVPE method, TMG (trimethylgallium) and TEG (triethylgallium) are used as the Ga source, TMA (trimethylaluminum) and TEA (triethylaluminum) are used as the Al source, TMI (trimethylindium) and TEI (triethyl) are used as the In source. A trialkyl metal compound such as indium) is often used, and a gas such as ammonia or hydrazine is used as a nitrogen source. As the impurity source, silane gas is used for Si, germane gas is used for Ge, Cp2Mg (cyclopentadienylmagnesium) is used for Mg, and DEZ (diethyl zinc) is used for Zn. In the MOVPE method, these gases are supplied to the surface of a substrate heated to, for example, 600 ° C. or more, and decomposed, whereby In _X Al _Y Ga _1-XY N (0 ≦ X, 0 ≦ Y, X + Y ≦ 1) can be epitaxially grown.
[0017]
[Action]
The light-emitting element of the present invention has a minimum necessary structure, and an element excellent in light emission output can be obtained. This is because each layer works effectively. First, the n-type cladding layer serves as both a current injection layer and a carrier confinement layer. Since the active layers of SQW and MQW have good crystallinity, they become very efficient layers as light emitting layers. The p-type cladding layer is a high-concentration layer as a carrier confinement layer, and further as a carrier confinement layer, a high light emission output can be obtained. Further, since the p-type contact layer can provide a preferable ohmic with the electrode material, the forward voltage of the LED element is lowered to improve the light emission efficiency.
[0018]
FIG. 3 is a diagram showing the relationship between the film thickness of the active layer having a single quantum well structure according to the light emitting device of the present invention and the light emission output as a relative value. Specifically, FIG. 3 shows the LED device shown in Example 1. It shows the structure. As described above, the light emitting device of the present invention can obtain a high output light emitting device by setting the well layer to 7 nm or less.
[0019]
【Example】
Hereinafter, the LED element according to the present invention will be described in detail with reference to FIG. The process described below is based on the MOVPE method.
[0020]
[Example 1]
The well-washed sapphire substrate 11 is set in a reaction vessel, and after the inside of the reaction vessel is sufficiently substituted with hydrogen, the temperature of the substrate is raised to 1050 ° C. while flowing hydrogen to clean the sapphire substrate.
[0021]
Subsequently, the temperature is lowered to 510 ° C., hydrogen is used as the carrier gas, ammonia and TMG (trimethylgallium) are used as the source gas, and a buffer layer 12 made of GaN is grown on the sapphire substrate 11 to a thickness of 20 nm.
[0022]
After growing the buffer layer, only TMG is stopped and the temperature is raised to 1030 ° C. When the temperature reaches 1030 ° C., similarly, TMG and ammonia gas are used as the source gas, and silane gas is used as the dopant gas, and an n-type GaN layer doped with Si of 1 × 10 ²⁰ / cm ³ is grown to 4 μm as the n-type cladding layer 13.
[0023]
After the growth of the n-type GaN layer, the source gas and the dopant gas are stopped, the temperature is set to 800 ° C., TMG, TMI (trimethylindium) and ammonia are used as the source gas, and the In 0.43 Ga 0 is formed as the active layer 16 having a single quantum well structure. A .57N layer is grown by 3 nm.
[0024]
Next, stop the source gas and dopant gas, raise the temperature to 1020 ° C again, use TMG, TMA (trimethylaluminum) as the source gas, ammonia and Cp2Mg (cyclopentadienylmagnesium) as the dopant gas, and p-type cladding As the layer 17, a p-type Al0.1Ga0.9N layer doped with 2 × 10 ¹⁹ / cm ³ of Mg is grown by 50 nm.
[0025]
After stopping the TMA gas, a p-type GaN layer doped with 1 × 10 ¹⁹ / cm ^{3 of} Mg is grown as a p-type contact layer 18 by 1 μm.
[0026]
After the growth of the p-type GaN layer, the substrate is taken out of the reaction vessel, and annealed at 700 ° C. for 20 minutes in a nitrogen atmosphere with an annealing apparatus to further reduce the resistance of the p-type cladding layer and the p-type contact layer.
[0027]
Part of the p-type contact layer 18, the p-type cladding layer 17 and the active layer 16 of the wafer obtained as described above is removed by etching to expose the n-type cladding layer 13, and Ni-- is formed on the p-type contact layer. An ohmic electrode made of Au, a p-type GaN layer, and Ti—Al—Au is provided, cut into a 350 μm square chip, placed on a lead frame having a cup shape, molded with epoxy resin, and a lens directivity angle of 10 A LED element of ° was produced.
[0028]
When this LED was made to emit light at a forward current of 20 mA and its spectrum was measured, it showed pure green light emission with an emission peak of 525 nm and a half width of 45 nm, an emission output of 1.5 mW, a quantum efficiency of 2.5%, and more than conventional GaP. The light output of 40 times or more was shown with respect to yellow-green LED which becomes.
[0029]
[Example 2]
In the step of growing the active layer 16, an In0.40Ga0.60N layer having a thickness of 3 nm is grown as a well layer at 800 ° C. using TMG, TMI (trimethylindium) and ammonia as source gases, and a barrier layer is formed thereon. Then, an In0.2Ga0.4N layer having a thickness of 6 nm is grown, and an active layer 16 having a multi-quantum well structure having a five-layer structure (well + barrier + well + barrier + well) is grown.
[0030]
Thereafter, an LED element was obtained in the same manner as in Example 1. As a result, an excellent LED having an emission peak wavelength of 520 nm, an emission output of 1.9 mW, and a quantum efficiency of 3% was obtained.
[0031]
[Example 3]
In Example 1, a green LED element was obtained in the same manner except that the thickness of the active layer 16 having a single quantum well structure was set to 7 nm. As a result, a green LED having a light emission output of 1.3 mW and a quantum efficiency of 2.1% was obtained. LED was obtained.
[0032]
[Example 4]
After growing the buffer layer, the temperature was raised to 1030 ° C., and then TMG and TMA were used as source gases, ammonia gas was used as the dopant gas, and silane gas was used as the n-type cladding layer 13 to be doped with Si at 1 × 10 ²⁰ / cm ³ An LED element was produced in the same manner as in Example 1 except that the type Al0.05Ga0.95N layer was grown by 4 μm. As a result, the emission wavelength and emission output showed the same characteristics as in Example 1.
[0033]
[Example 5]
An LED element was produced in the same manner as in Example 1 except that the thickness of the p-type cladding layer 17 was set to 2 μm. As a result, a green LED having a light emission output of 1.0 mW and a quantum efficiency of 1.7% was obtained.
[0034]
[Example 6]
In Example 1, the composition of the active layer 16 is grown to a thickness of 2.5 nm for a well layer made of non-doped In0.4Ga0.6N and a thickness of 5 nm for a barrier layer made of non-doped In0.01Ga0.99N. This operation was repeated 13 times. Finally, a well layer was stacked to grow an active layer having a total thickness of 1000 angstroms. Thereafter, an LED element was obtained in the same manner as in Example 1. As a result, an excellent green LED having 520 nm, a light emission output of 2.5 mW, and a quantum efficiency of 3.2% was obtained.
[0035]
【The invention's effect】
As described above, the LED of the present invention can obtain a green LED having a very low light emission output with a minimum necessary structure without a complicated laminated structure. Further, other nitride semiconductor layers disclosed in the present invention may be placed between or outside the laminated structure without departing from the spirit of the present invention.
[0036]
As described above, by using the LED of the present invention, the LED full-color display conventionally requires a plurality of GaP-based LEDs in order to increase the luminous intensity, but one pixel is formed for each of B, G, and R one by one. As a result, a high-definition screen can be obtained. Further, if the chip LED is used, a smaller pixel can be realized, and a wall-mounted TV can also be realized.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view showing the structure of an LED element according to an embodiment of the present invention.
FIG. 2 is a schematic cross-sectional view showing the structure of a conventional LED element.
FIG. 3 is a graph showing the relationship between the thickness of the well layer and the output of the element according to the light emitting element of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 11 ... Substrate 12 ... Buffer layer 13 ... n-type cladding layer 16 ... Active layer 17 ... p-type cladding layer 18 ... p-type contact layer

Claims (6)

At least, b n-type value is 0.05 or less _{Al b Ga 1-b} and the current injection layer and a carrier confinement layer made of N, the following film thickness _{7nm In x Ga 1-x N} (0 <X <1) Nitride semiconductor light emitting device having a stacked structure having an active layer having a quantum well structure including a well layer and a p-type cladding layer made of p-type Al _y Ga _1-y N having a y value of 0.05 or more in this order . The nitride semiconductor light-emitting element according to claim 1, wherein a film thickness of the n-type Al _b Ga _1-b N is 1 μm or more.   The nitride semiconductor light emitting device according to claim 1 or 2, wherein the p-type cladding layer has a thickness of 5 nm to 0.5 µm. 4. The nitride semiconductor light emitting device according to claim 1, wherein the well layer is In _x Ga _1-x N (0.40 ≦ x ≦ 0.43).   The light emission peak wavelength in the said nitride semiconductor is 520-525 nm, The nitride semiconductor light-emitting device as described in any one of Claim 1 to 4.   The nitride semiconductor light emitting device according to claim 1, wherein the active layer has a multiple quantum well structure.

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