JPH09246666A - Manufacture of nitride semiconductor laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the threshold current of a laser to make the continuous oscillation of the laser possible at room temperatures by a method wherein a mask is formed into a shape having a striped open part and an active layer consisting of a second nitride semiconductor layer is grown in the open part. SOLUTION: An N-type optical confinement layer 3 is grown and thereafter, a mask 100 is formed into a stripe form. After the formation of a mask, an N-type light guide layer 4 consisting of an N-type Si-doped GaN layer is grown. Then, an active layer 5 is grown. This active layer 5 is constituted of an In- containing nitride semiconductor layer and it is desirable that the layer 5 is formed into a layer containing a ternary mixed crystal Inx Ga1-x N(O<x<1). After the layer 5 is grown, a P-type cap layer consisting of a P-type Mg-doped AlGaN layer is grown. Then, a P-type light guide layer 6 consisting of a P-type Mg-doped GaN layer is grown. Susequently, a P-type optical confinement layer 7 consisting of an Mg-doped AlGaN layer is grown. Subsequently, a layer consisting of a P-type Mg-doped GaN layer is grown and a P-type contact layer 8 consisting of a P-type GaN layer is grown.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a nitride semiconductor (In).
_X Al _Y Ga _1-XY N, 0 ≦ X, 0 ≦ Y, X + Y ≦ 1).

[0002]

2. Description of the Related Art A nitride semiconductor is known as a material for a laser device that emits light in the ultraviolet to blue region, and the present applicant has recently used this material to generate a laser oscillation of 410 nm at room temperature under pulse current. Announced (for example, Jpn.J.
Appl.Phys. Vol.35 (1996) pp.L74-76). The disclosed laser device is a so-called electrode stripe type laser device, in which the stripe width of the nitride semiconductor layer including the active layer is set to several tens of μm, and laser oscillation is performed.

The active layer, which is the stripe waveguide of the laser element, is formed by etching, and the side surface of the stripe is exposed to the atmosphere. Therefore, the threshold current of the laser element is 1 to 2 A. Further, when the stripe waveguide is formed by etching, it is difficult to align the photomask and it is difficult to reduce the stripe width. The width of the waveguide is preferably as narrow as possible in order to reduce the threshold current.

[0004]

At present, when pulse oscillation of a short wavelength laser of 410 nm has been confirmed, continuous oscillation at room temperature is urgently desired. Therefore, the present invention has been made in view of such circumstances, and an object of the present invention is to provide a method for manufacturing a nitride semiconductor laser device capable of easily manufacturing an active region having a narrow stripe width. Therefore, the threshold current of the laser element is reduced to achieve continuous oscillation at room temperature.

[0005]

The method for manufacturing a laser device according to the present invention is roughly divided into two kinds of modes.
In the embodiment, a mask made of a material having a property that a nitride semiconductor thin film does not easily grow and having a refractive index smaller than that of the active layer is formed on the first nitride semiconductor layer having the first conductivity type in a stripe shape. It is characterized in that it is formed in a shape having an opening, and an active layer made of a second nitride semiconductor is grown in the opening. For the nitride semiconductor having the first conductivity type, an n-type nitride semiconductor containing Al is selected, and the active layer includes at least a nitride semiconductor containing In, and more preferably includes a nitride semiconductor layer containing In. When an active layer having a quantum well structure is grown, the difference in refractive index between the nitride semiconductor layers becomes large and the output also becomes large.

In the first aspect, after the active layer is grown, a third nitride semiconductor having the second conductivity type is grown in the opening. The nitride semiconductor having the third conductivity type is most preferably a p-type nitride semiconductor containing Al because the difference in refractive index from the active layer becomes large.

In a second aspect, an active layer made of a second nitride semiconductor is grown on a first nitride semiconductor layer having the first conductivity type, and the active layer is nitrided. A thin semiconductor thin film having a property of being hard to grow and having a smaller refractive index than that of the active layer is formed in a shape having a stripe-shaped opening, and a third conductivity type having a second conductivity type in the opening is formed. And growing the nitride semiconductor of Also in this aspect, the first conductivity type nitride semiconductor grows an n-type nitride semiconductor containing Al, and the active layer contains at least a nitride semiconductor containing In, more preferably a nitride semiconductor layer containing In. When an active layer having a multiple quantum well structure is grown and the nitride semiconductor having the third conductivity type is a p-type nitride semiconductor containing Al, the refractive index difference from the active layer becomes large, and the output becomes high.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The manufacturing method of the present invention and its operation will be described below in detail with reference to the drawings. 1 to 4 are schematic cross-sectional views showing the structure of the main part of the nitride semiconductor wafer obtained in each step in the manufacturing method according to the first aspect of the present invention. Although the following method shows a growth method by a metal organic chemical vapor deposition (MOVPE) method, the method of the present invention is not limited to the MOVPE method, and may be, for example, molecular beam vapor phase epitaxy (MBE) or halide vapor deposition. Phase growth method (HDV
It goes without saying that it is applicable to all known methods for growing a nitride semiconductor, such as PE).

Example 1 (First Mode) A substrate 1 having a spinel (MgAl ₂ O ₄ ) 111 surface as a main surface
Is placed in a reaction vessel of a MOVPE apparatus, TMG (trimethylgallium) and ammonia are used as source gases, and a buffer layer made of GaN is grown to a thickness of 200 Å on the surface of the substrate 1 at a temperature of 500 ° C. . In addition to spinel, sapphire having A-plane, R-plane, and C-plane as main surfaces can be used for the substrate 1, and SiC, M
A conventionally known substrate made of a single crystal such as gO, GaN, Si or ZnO is used. Since the buffer layer can be deleted depending on the type of substrate, the growth method, etc., it is not shown in the figure.

Then, the temperature is raised to 1050 ° C.
TMG, ammonia, SiH as a donor impurity
_FourConsisting of Si-doped GaN using (silane) gas
The n-type contact layer 2 is grown to a film thickness of 4 μm. n-type
Contact layer 2 is In_XAl_YGa _1-XYN (0 ≦ X, 0 ≦
Y, X + Y ≦ 1), especially GaN, I
nGaN, especially Si-doped GaN
Gives an n-type layer with a high carrier concentration, and
A laser element can be obtained because a favorable ohmic contact with the pole is obtained.
The threshold current of the child can be reduced. Negative electrode
Materials are Al, Ti, W, Cu, Zn, Sn, In
A metal or alloy such as
You.

Next, the temperature is set to 750 ° C., TMG, TMI (trimethylindium) and ammonia are used as source gases, and silane gas is used as an impurity gas.
50 crack prevention layer (not shown) consisting of 1 Ga 0.9 N
It is grown to a film thickness of 0 angstrom. The crack prevention layer is an n-type nitride semiconductor containing In, preferably InG.
Growing with aN makes it possible to grow a thick film of the n-type optical confinement layer 312 made of a nitride semiconductor containing Al to be grown next, which is very preferable. LD
In this case, it is necessary to grow the layers serving as the light confinement layer and the light guide layer to have a film thickness of, for example, 0.1 μm or more. Conventionally, a thick film of AlGaN is directly formed on the GaN and AlGaN layers.
However, it is difficult to fabricate the device because the AlGaN grown later tends to be cracked. However, this crack prevention layer can prevent the optical confinement layer to be grown next from cracking. it can. The crack preventing layer is preferably grown to a thickness of 100 Å or more and 0.5 μm or less. When the thickness is less than 100 Å, it is difficult to act as a crack preventive as described above, and when the thickness is more than 0.5 μm, the crystal itself tends to turn black. Although this crack prevention layer can be omitted depending on the growth method, growth apparatus, etc., it is not particularly shown, but it is preferable to grow it when manufacturing a laser element.

Next, using TMG, TMA (trimethylaluminum), ammonia as a source gas, and silane gas as an impurity gas, an n-type optical confinement layer 3 made of Si-doped n-type Al0.3Ga0.7N having a thickness of 0.5 μm is formed. Grow thick. The n-type optical confinement layer 3 is made of an n-type nitride semiconductor containing Al, and is preferably a binary mixed crystal or a ternary mixed crystal of A.
By setting l _Y Ga _1-Y N (0 <Y ≦ 1), good crystallinity can be obtained, and the refractive index difference from the active layer can be increased to effectively confine the laser light in the vertical direction. is there. This layer is usually preferably grown to a thickness of 0.1 μm to 1 μm. If it is thinner than 0.1 μm, it is difficult to act as an optical confinement layer, and if it is thicker than 1 μm, even in AlGaN grown on the crack prevention layer, cracks are likely to occur in the crystal, which makes device fabrication difficult. There is a tendency. FIG. 1 shows a cross-sectional view of a wafer in which an n-type optical confinement layer 3 has been grown on a substrate 1. In the present embodiment, the first
The first nitride semiconductor layer having the conductivity type corresponds to this n-type optical confinement layer 3.

After the growth of the n-type optical confinement layer 3, the wafer is taken out of the reaction vessel and transferred to a CVD apparatus, and a mask 100 made of SiO ₂ is deposited to a thickness of 1 μm on the surface of the n-type optical confinement layer 3 as shown in FIG. The film is formed into a stripe shape. The mask interval is 3 μm. That is, the interval between the stripe-shaped masks is set to 3 μm so that the n-type optical confinement layer 3 is exposed with a stripe width of 3 μm. The SiO ₂ used for the mask has a property that a nitride semiconductor thin film is hard to grow, and has a smaller refractive index than an active layer to be formed later, and therefore is very effective for lateral light confinement of the active layer. . Such other as a mask having a property, for example, silicon oxide other than SiO ₂ (Si _X
O _Y ), silicon nitride (Si _X N _Y ), and the like. In this embodiment, the mask 100 is used as the n-type optical confinement layer 3
However, it may be formed on the surface of the n-type contact layer 2 or may be formed on the surface of the n-type optical guide layer 4 described later.

After the mask 100 is formed, the wafer is transferred to the reaction vessel again, the temperature is set to 1050 ° C., TMG, ammonia, and silane gas are used as impurity gas, and Si-doped n is used.
The n-type light guide layer 4 made of n-type GaN is grown to a film thickness of 500 Å. The n-light guide layer 4 grows on the n-type light confinement layer 3 exposed in the opening of the stripe-shaped mask, but only hexagonal columnar crystals are formed on the mask 100, and a thin film is formed. Grows little. The n-type optical guide layer 4 is composed of an n-type nitride semiconductor containing In or n-type GaN, preferably In _X Ga _1-X N (0 ≦ X ≦ 1) of ternary mixed crystal or binary mixed crystal. To do. It is usually desirable to grow this layer to a film thickness of 100 angstrom to 1 μm, and particularly by using InGaN or GaN, it becomes easy to make the next active layer 104 a quantum structure.

Next, the active layer 5 is grown by using TMG, TMI, and ammonia as source gases. The active layer 5 has a temperature of 7
While maintaining the temperature at 50 ° C., first, a well layer made of non-doped In0.2Ga0.8N is grown to a thickness of 25 Å. Next, the barrier layer made of undoped In0.01Ga0.95N is formed at the same temperature by only changing the TMI molar ratio.
It is grown to a film thickness of 0 angstrom. This operation is 1
Repeat 3 times, and finally grow the well layer to a total film thickness of 0.1μ
An active layer 5 having a multiple quantum well structure with a thickness of m is grown.

The active layer 5 is composed of a nitride semiconductor containing In, and is preferably a ternary mixed crystal of In _X Ga _{1 -X} N (0 <X <
It is desirable to use a layer containing 1). InG of ternary mixed crystal
Since aN has better crystallinity than that of a quaternary mixed crystal, the emission output is improved. Among them, it is particularly preferable that the active layer has a multi-quantum well structure (MQW: Multi-quan) in which a well layer made of In _x Ga ₁ -xN and a barrier layer made of a nitride semiconductor having a band gap larger than that of the well layer are stacked.
tum-well). An In X _'G likewise ternary mixed crystal also barrier layer
a _{1−X ′} N (0 ≦ X ′ <1, X ′ <X) is preferable, and for example, wells + barrier + well + ... + barrier + well layers (or vice versa) are stacked and multiplexed. Configure a quantum well structure. As described above, when the active layer is an MQW in which InGaN is laminated,
It is possible to realize a high-power LD in the range of about 365 nm to 660 nm by light emission between quantum levels. Furthermore, when a barrier layer made of InGaN is stacked on the well layer,
The barrier layer is made of a softer crystal than GaN and AlGaN crystals. Therefore, the thickness of AlGaN in the cladding layer can be increased, so that laser oscillation can be realized. Furthermore, In
GaN and GaN have different crystal growth temperatures. For example, in the MOVPE method, InGaN is grown at 600 ° C to 800 ° C, whereas GaN is grown at a temperature higher than 800 ° C. Therefore, if the barrier layer made of GaN is to be grown after the growth of the well layer made of InGaN, the growth temperature must be raised. If the growth temperature is raised, the previously grown InGaN well layer will be decomposed, so it is difficult to obtain a well layer with good crystallinity. Further, the thickness of the well layer is only several tens of angstroms, and it becomes difficult to manufacture the MQW when the well layer of the thin film is decomposed.
On the other hand, in the present invention, since the barrier layer is also InGaN, the well layer and the barrier layer can be grown at the same temperature. Therefore,
Since the well layer formed previously is not decomposed, MQW having good crystallinity can be formed. This shows the most preferable form of MQW, but if the bandgap energy of the barrier layer is larger than that of the well layer, such as InGaN for the well layer, GaN for the barrier layer, and AlGaN, any composition may be used. good. Further, the active layer 5 may have a single quantum well structure composed of only a single well layer. Similarly, in the active layer 5, a thin film having good crystallinity can be grown on the n-type optical guide layer 4 having a stripe width of 3 μm, but it hardly grows on the mask 100.

After the growth of the active layer 5, the temperature is raised to 1050 ° C. and TMG, TMA, ammonia, and Cp 2 Mg (cyclopentadienyl magnesium) as an acceptor impurity source.
Is used to grow a p-type cap layer of Mg-doped p-type Al0.2Ga0.8N to a film thickness of 100 angstrom. By growing the p-type cap layer to a thickness of 1 μm or less, more preferably 10 angstroms or more and 0.1 μm or less, the p-type cap layer acts as a cap layer for preventing decomposition of the active layer made of InGaN. By growing a p-type nitride layer containing Al, preferably a p-type cap layer made of Al _Y Ga _{1 -Y} N (0 <Y <1) on the active layer, the light emission output is significantly improved. If the film thickness of this p-type cap layer is thicker than 1 μm,
The layer itself tends to be cracked, which makes it difficult to manufacture the device. Note that the p-type cap layer can also be omitted depending on the growth method, growth apparatus, etc., and is not particularly shown.

Next, while maintaining the temperature at 1050 ° C., T
Mg-doped p-type G using MG, ammonia, and Cp2Mg
The p-type light guide layer 6 made of aN is grown to a film thickness of 500 angstrom. The p-type optical guide layer 6 is also formed of In-containing p-type nitride semiconductor or p-type GaN, preferably binary mixed crystal or ternary mixed crystal In _X Ga _1-X N (0 ≦ X ≦).
Grow 1). It is desirable that the p-type light guide layer 6 is normally grown to a film thickness of 100 angstrom to 1 μm. In particular, by using InGaN or GaN, the next p-type light confinement layer 7 can be grown with good crystallinity.

Then, TMG, TMA, ammonia, C
The p-type optical confinement layer 7 made of Mg-doped Al0.3Ga0.7N is grown to a thickness of 0.5 μm using p2Mg.
The p-type optical confinement layer 7 is made of a p-type nitride semiconductor containing Al and is preferably a binary mixed crystal or a ternary mixed crystal of Al _a Ga _{1 -a} N (0 <a ≦ 1). By doing so, a material having good crystallinity can be obtained. Like the n-type light confinement layer 3, the p-type light confinement layer 7 is preferably grown to have a film thickness of 0.1 μm to 1 μm, and by using a p-type nitride semiconductor containing Al such as AlGaN, the active layer is formed. By increasing the difference in the refractive index between the laser light and the laser light, the laser light effectively acts as an optical confinement layer in the vertical direction of the laser light.

Then, TMG, ammonia, Cp2Mg
Using, the p-type contact layer 8 made of Mg-doped p-type GaN is grown to a film thickness of 0.5 μm. The p-type contact layer 8 is a p-type In _X Al _Y Ga _1-XY N (0 ≦ X, 0 ≦ Y, X
+ Y ≦ 1), especially InGaN, G
When aN, and above all, p-type GaN doped with Mg, a p-type layer with the highest carrier concentration is obtained, good ohmic contact with the positive electrode is obtained, and the threshold current can be reduced. The material of the positive electrode is Ni, P
A metal or alloy having a relatively high work function such as d, Ir, Rh, Pt, Ag, Au, etc. is likely to obtain ohmic contact. The p-type contact layer 8 is formed in the stripe-shaped opening of the mask 100.
A cross-sectional view of the wafer grown up to is shown in FIG. As shown in this figure, the nitride semiconductor grows on the nitride semiconductor layer exposed in the opening of the mask 100, but hardly grows on the mask 100, and hexagonal columnar crystals are attached. Only to do.

As described above, by forming the openings of the mask 100 into stripes, it is possible to grow a nitride semiconductor including the active layer 5 having a stripe width of, for example, 5 μm or less. Moreover, since the mask 100 made of a material having a smaller refractive index than that of the active layer is formed on both sides of the stripe, it is possible to realize a refractive index waveguide type laser element, thereby reducing the threshold current. Moreover, since the stripe including the active layer 5 is formed without etching, the side surface of the active layer is not damaged by etching.

After the wafer on which the nitride semiconductors are laminated as described above is taken out from the reaction container, the nitride semiconductor crystals (hexagonal columnar crystals) adhering to the surface of the mask 100 are removed by wet etching. After removing the crystals, a new second mask is formed on the cleaned surface of the mask 100 and the surface of the p-type contact layer 8, and the mask 100 made of SiO2 is partially removed by wet etching. The surface of the n-type contact layer 2 on which the negative electrode is to be formed is exposed by a reactive ion etching (RIE) device. Next, the uppermost p-type contact layer 8 and the mask 1
The positive electrode 20 is formed on almost the entire uppermost layer of
A striped negative electrode 21 is formed on the exposed n-type contact layer 2 in parallel with the oscillation region of the active layer, that is, in parallel with the stripe.

After forming the electrodes, the wafer is transferred to a polishing apparatus, the substrate is polished to a thickness of 80 μm to be thin, and then the wafer is cleaved in a direction perpendicular to the negative electrode 21 to form a resonance surface. A resonator is manufactured by forming a reflecting mirror made of a dielectric multilayer film on a cleavage surface serving as a resonance surface using a sputtering device. Further, the wafer is diced in a direction parallel to the striped negative electrode 21 to obtain a resonator length of 500 μm.
m laser chip. FIG. 4 is a sectional view showing the structure of this laser chip. When the chip obtained as described above was placed on a heat sink to form a laser element,
The threshold current was 0.1 A DC and continuous oscillation of 410 nm was exhibited.

Example 2 (Second Mode) FIGS. 5 to 8 are also schematic cross-sectional views showing the structure of the main part of the nitride semiconductor wafer obtained by the method of the present invention. Will be described with reference to FIG.

As shown in FIG. 5, a buffer layer, an n-type contact layer 2, a crack prevention layer, and an n-type optical confinement layer 3 are laminated on a substrate 1 in the same manner as in Example 1, and then a mask is formed. Without doing so, the n-type light guide layer 4 and the active layer 5 are subsequently grown on the light confinement layer 3.

After the growth of the active layer 5, the wafer was taken out of the reaction container and the Si layer was formed on the active layer 5 in the same manner as in Example 1.
A mask 100 made of O2 having a film thickness of 0.5 μm and 3 μm
It is formed so that the stripe width of m is exposed. FIG. 6 shows a sectional view thereof. In the second aspect, the mask 100 is preferably formed in contact with the active layer 6, but if it is on the active layer, it is formed on the surface of the p-type cap layer or the p-type light guide layer 6 described later. May be.

After the mask 100 is formed, the wafer is transferred again to the reaction container, and the p-type cap layer, the p-type light guide layer 6 and the p-type light confinement layer are placed in the 3 μm stripe described above in the same manner as in the first embodiment. Layer 7 and p-type contact layer 8 are grown. FIG. 7 shows the structure after this step is completed. As described above, by selectively growing after forming the mask 100 on the active layer 5, the p-type optical confinement layer 7 and the p-type contact layer 8 have a ridge shape, so that the refractive index waveguide is substantially formed. Type laser device, the threshold current can be reduced. Further, since the mask 100 is also an insulator, it also serves as a current constriction layer.

In the same manner as in Example 1, the wafer thus grown with the nitride semiconductor was exposed to the n-type contact layer on which the negative electrode was to be formed, and the positive electrode 20 and the negative electrode 21 were formed.
When a laser element provided with was provided, the characteristics substantially equivalent to those of Example 1 were exhibited.

[0029]

As described above, according to the method of the present invention, an active layer having a narrow stripe width can be easily formed.
Further, since a mask having a smaller refractive index than that of the active layer is formed on the side surface of the stripe-shaped active layer in the resonance direction, light can be confined in the lateral direction, so that single mode laser light can be easily obtained. Therefore, the threshold current of the laser decreases, and continuous oscillation at room temperature becomes possible.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing the structure of the main part of a nitride semiconductor wafer obtained in one step of the method of the present invention.

FIG. 2 is a schematic cross-sectional view showing the structure of the main part of a nitride semiconductor wafer obtained in one step of the method of the present invention.

FIG. 3 is a schematic cross-sectional view showing the structure of the main part of a nitride semiconductor wafer obtained in one step of the method of the present invention.

FIG. 4 is a schematic cross-sectional view showing the structure of a laser device obtained by the method of the present invention.

FIG. 5 is a schematic cross-sectional view showing the structure of the main part of a nitride semiconductor wafer obtained in one step of the method of the present invention.

FIG. 6 is a schematic cross-sectional view showing the structure of the main part of a nitride semiconductor wafer obtained in one step of the method of the present invention.

FIG. 7 is a schematic cross-sectional view showing the structure of the main part of a nitride semiconductor wafer obtained in one step of the method of the present invention.

FIG. 8 is a schematic cross-sectional view showing the structure of a laser device obtained by the method of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Substrate 2 ... N-type contact layer 3 ... N-type light confinement layer 4 ... N-type light guide layer 5 ... Active layer 6 ... P-type light guide layer 7 ... p-type optical confinement layer 8 ... p-type contact layer 20 ... positive electrode 21 ... negative electrode 100 ... mask

Claims (3)

[Claims] 1. A stripe of a mask made of a material having a property that a nitride semiconductor thin film does not easily grow on the first nitride semiconductor layer having the first conductivity type and having a refractive index smaller than that of the active layer. A method for manufacturing a nitride semiconductor laser device, which is characterized in that the active layer made of a second nitride semiconductor is grown in the opening. 2. After the growth of the active layer, a second layer is formed in the opening.
The method for producing a nitride semiconductor according to claim 1, wherein a third nitride semiconductor having a conductivity type is grown. 3. An active layer made of a second nitride semiconductor is grown on a first nitride semiconductor layer having a first conductivity type, and a nitride semiconductor thin film is grown on the active layer. A mask made of a material having a property of being hard to perform and having a refractive index smaller than that of the active layer is formed in a shape having a stripe-shaped opening, and a third nitride semiconductor having a second conductivity type is formed in the opening. A method for manufacturing a nitride semiconductor laser device, which comprises growing the same.