JP2001102678A - Gallium nitride compound semiconductor element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize a low-threshold current, a low operating voltage and a high reliability by reducing the contact resistance of an n-type GaN contact layer to an n-side electrode. SOLUTION: On a sapphire substrate 11 an n-GaN contact layer 13, an n- AlGaN clad layer 14, an MQW active layer 16, a p-AlGaN clad layer 18 and a p-GaN contact layer 19 are laminated, a p-side electrode 20 is formed on the p-GaN contact layer 19, the layers 14, 16, 18, 20 are partly removed to expose the n-GaN contact layer 13 and an n-side electrode 30 is formed on the n-GaN contact layer 13, thus forming a blue semiconductor laser. The n-side electrode 30 is formed in a TiN-Ti-Al laminated structure, and semiconductor layer crystals 35 having a wurtzite structure and a cubic structure are mixed in the contact portion between the n-GaN contact layer 13 and the n-side electrode 30, and the wurtzite structure abundance ratio is less than 1%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a gallium nitride-based compound semiconductor device such as a gallium nitride-based blue-violet semiconductor laser or a gallium nitride-based high-brightness blue / green light-emitting diode, and more particularly to a gallium nitride-based compound semiconductor layer and a metal contact. The present invention relates to a gallium nitride-based compound semiconductor device in which a contact portion with a layer is improved.

[0002]

2. Description of the Related Art Heretofore, a semiconductor laser using an InGaAlP material has been already put to practical use as a light source in a 600 nm band with its characteristics improved to a level at which both reading and writing of an optical disk are possible. In recent years, blue semiconductor lasers having shorter wavelengths have been actively developed to further improve the recording density. As a constituent material of the blue semiconductor laser, Ga
Gallium nitride-based compound semiconductors such as N, InGaN, GaAlN, and InGaAlN have attracted attention.

For example, in a semiconductor laser using a GaN-based material, continuous oscillation at a wavelength of 380 to 417 nm has been confirmed. However, at present, satisfactory characteristics cannot be obtained, and the threshold voltage at room temperature oscillation is a high value of about 10 V and has large variations, and the characteristics as a semiconductor laser are not sufficient. This is because crystal growth of the gallium nitride-based compound semiconductor layer is difficult and element resistance is large.

In particular, in a gallium nitride-based compound semiconductor device, a large voltage drop is caused due to the inability to form a p-type gallium nitride-based compound semiconductor layer having a high carrier concentration and a high p-side electrode contact resistance. Even there is a problem that heat generation and deterioration due to metal reaction occur.
Various proposals have been made on this issue, and there are potential solutions.

On the other hand, with respect to the resistance on the n-side, the n-type semiconductor can realize a sufficient carrier concentration as compared with the p-side resistance, and the electrode contact resistance has not been a serious problem. However, as the improvement of the resistance on the p-side has progressed, it has been found that there is a point to be improved on the n-side as a whole resistance. For example, the n-side electrode metal is n
Already reacts when deposited on the p-type GaN layer,
Even a slight temperature rise during the process of 0 ° C. or during operation of the laser causes deterioration. Since the resistance is high and unstable, continuous oscillation of the laser at room temperature may be difficult.

[0006]

As described above, in a gallium nitride compound semiconductor laser expected to be applied to an optical disk or the like, the contact resistance between the n-side electrode and the gallium nitride compound semiconductor layer becomes high and unstable. . For this reason,
A large voltage drop occurs at the contact portion, and the operation becomes unstable, and it is difficult to realize a device having a low threshold current, a low operating voltage, and high reliability. Further, since the n-side electrode and the gallium nitride-based semiconductor layer deteriorate when energized, continuous oscillation of the laser at room temperature is difficult.

The present invention has been made in consideration of the above circumstances, and has as its object to reduce the contact resistance generated between an n-side electrode and a gallium nitride-based compound semiconductor layer, and further reduce the operating resistance. An object of the present invention is to provide a gallium nitride-based compound semiconductor device which can stabilize heat resistance against heat generation of a semiconductor device, does not deteriorate at a low threshold current and a low operating voltage, and has excellent reliability. .

[0008]

(Structure) In order to solve the above-mentioned problem, the present invention employs the following structure.

[0009] The present invention, n-type gallium nitride-based compound semiconductor layer _{(Ga x In y Al z N} : x + y + z = 1,0
≦ x, y, z ≦ 1) and a gallium nitride-based compound semiconductor device having an n-side electrode formed on the compound semiconductor layer, a semiconductor layer at a contact portion between the compound semiconductor layer and the n-side electrode. The crystal is characterized by a mixture of a wurtzite structure and a cubic structure.

Here, preferred embodiments of the present invention include the following. (1) At least the semiconductor layer side of the n-side electrode is made of TiN. (2) The n-side electrode has a three-layer structure of TiN / Ti / Al.

(3) Cubic structure (c-GaInAlAl) at the contact portion of the gallium nitride-based compound semiconductor layer with the n-side electrode
N) and the wurtzite structure (w-GaInAlN) are 0
% <(C-GaInAlN) / (w-GaInAlN) <1%.

[0012] The present invention, on a substrate, each gallium nitride-based compound semiconductor layer _{(Ga x In y Al z N} : x +
y + z = 1, 0 ≦ x, y, z ≦ 1), an n-type contact layer, an n-type cladding layer, an active layer, a p-type cladding layer,
A p-type contact layer is laminated, and p-type
A side electrode is formed, and a part of the n-type cladding layer from the p-type contact layer is partially removed.
A gallium nitride-based compound semiconductor device having a side electrode formed thereon, wherein at least the semiconductor layer side of the n-side electrode is TiN
In the semiconductor layer crystal at the contact portion between the n-type contact layer and the n-side electrode, a wurtzite structure and a cubic structure are mixed, and the cubic structure (c-GaInAlN) and the wurtzite structure (w -GaInAlN) is characterized in that 0% <(c-GaInAlN) / (w-GaInAlN) <1%.

(Operation) FIG. 5 shows an element structure of a conventional blue semiconductor laser. In the figure, 11 is a sapphire substrate, 12
Is a GaN buffer layer, 13 is an n-GaN contact layer,
14 is an n-AlGaN cladding layer, 15 is a GaN waveguide layer, 16 is an active layer having a multiple quantum well (MQW) structure, 17
Is a GaN waveguide layer, 18p-AlGaN cladding layer, 19
Is a p-GaN contact layer, 20 is a p-side electrode, 30 is n
A side electrode, 32 indicates a Ti layer, and 33 indicates an Al layer.

In the conventional device, as shown in FIG.
Ti is generally used as a contact metal between the n-side electrode and the gallium nitride-based compound semiconductor layer, and has exhibited a little ohmic property in a non-alloy state. It is also reported that the Ti / GaN structure is maintained in this state. However, the resistance tends to increase during the device manufacturing process or during the operation of the device, resulting in a high resistance. This increase in resistance is considered to be due to the fact that non-alloy and slightly ohmic contact are changed to Schottky contact at a low temperature of 100 ° C. to 150 ° C.

For this reason, in many cases, as shown in FIG. 5 (b), by forming Al on Ti and sintering at 600 ° C. or more, the TiAl alloy has good ohmic properties as a contact, I use this. The present inventors have also confirmed that Schottky properties may return to ohmic properties by sintering.

However, the sintering temperature at which the ohmic property is completely and better than that in the non-alloy state is greatly different depending on the growth conditions of the gallium nitride-based compound semiconductor layer and the processing and deposition conditions before the deposition of the contact metal. It is often difficult to improve. Further, as described in other reports, a stable ohmic characteristic can be obtained with a sinter uniformly at 600 ° C. or higher, but a semiconductor crystal is deteriorated in a process at 600 ° C. or higher. Sintering at higher temperatures must be avoided.

Therefore, the present inventors set the Al / Ti lamination to n.
Using a structure deposited on a p-type GaN layer as an example, it was investigated what caused unstable phenomena such as Schottky under various conditions. As a result, from elemental analysis, the element constituting the semiconductor and the metal Ti used as the electrode were already reacting with nitrogen at the time of deposition, and the profile in the depth direction did not change. However, when the reaction product and the crystal structure were examined by X-ray diffraction (FIG. 6), the gallium nitride-based compound semiconductor showed a good ohmic property, and the wurtzite structure and the cubic structure were mixed in the stable sample. Found that the cubic structure was less than 1%.

That is, when a metal electrode is laminated on a gallium nitride-based compound semiconductor having a wurtzite structure of 100%, not only good ohmic characteristics but also instability was obtained. However, if the layer where the cubic structure of less than 1% is mixed with the wurtzite structure comes into contact with the electrode metal,
Low resistance ohmic properties were stably exhibited.

It is assumed that the work function of the cubic structure is smaller than the work function of the wurtzite structure, and therefore, it is considered that the ohmic property of low resistance becomes stronger at the interface with the metal. However, when the cubic structure is 1% or more, it is speculated that the unstable Schottky property may be exhibited because the boundary between the cubic structure and the wurtzite structure is amorphous and the overall crystallinity is extremely poor.
However, no clear interpretation of this phenomenon has been made.

According to the experimental results, the present inventors have found that the gallium nitride-based compound semiconductor crystal in contact with the metal contact layer has a stable and good ohmic characteristic when a certain cubic structure is mixed. Was found to be realizable. In order to actually control the cubic structure in the wurtzite structure, it is necessary to prepare a semiconductor crystal growth method, pretreatment or modification of the semiconductor surface with a beam, dry etching conditions, the degree of vacuum during metal deposition, and the state of impurities. Thus, it is possible to control the crystal structure at the electrode interface.

[0021]

Embodiments of the present invention will be described below with reference to the drawings.

(First Embodiment) FIG. 1 shows a first embodiment of the present invention.
FIG. 3 is a cross-sectional view illustrating a schematic configuration of a blue semiconductor laser according to the embodiment.

In the figure, reference numeral 11 denotes a (0001) sapphire substrate, on which a GaN buffer layer 12, n
-Type GaN contact layer 13 (Si-doped, 5 × 10 ¹⁸ c
m ⁻³ , 4 μm), n-type Al _0.2 Ga _0.8 N clad layer 1
4 (Si-doped, 5 × 10 ¹⁷ cm ⁻³ , 0.3 μm), G
aN waveguide layer (undoped, 0.1 μm) 15, active layer 16 having a multiple quantum well structure (MQW), p-type GaN waveguide layer 1
7 (Mg doped, 0.1 μm), p-type Al _0.2 Ga _0.8
N cladding layer 18 (Mg doped, 5 × 10 ¹⁷ cm ⁻³ ,
0.3 μm), p-type GaN contact layer (Mg doped 1)
× 10 ¹⁸ cm ⁻³ , 1 μm) 19 are sequentially formed to form a semiconductor multilayer structure. Each crystal of this multilayer structure is formed substantially in a wurtzite structure.

On the p-type GaN contact layer 19 having a semiconductor multilayer structure, a p-side electrode 20 is formed. Further, a part of the semiconductor multilayer structure is removed by a dry etching method to a depth reaching the n-type GaN contact layer 13, and on the exposed contact layer 13, a TiN contact layer 31, as an n-side electrode 30, is formed. Ti layer 32, A
The l-layers 33 are formed so as to be sequentially laminated. n-type G
99% or more of the aN contact layer 13 has a wurtzite structure, but the contact portion 35 with the n-side electrode 30 includes a cubic structure in some places.

Next, a method of manufacturing the semiconductor laser according to the present embodiment will be described.

First, as shown in FIG. 2A, a GaN buffer layer 12 and an n-type GaN contact layer 1 are formed on a (0001) sapphire substrate 11 by MOCVD.
3, n-type Al _0.2 Ga _0.8 N clad layer 14, GaN waveguide layer 15, active layer 16, p-type GaN waveguide layer 17, p-type A
A l _0.2 Ga _0.8 N cladding layer 18 and a p-type GaN contact layer 19 are sequentially grown and formed. Here, each layer from the GaN buffer layer 12 to the p-type GaN contact layer 19 is:
It grows continuously by one MOCVD growth.

Next, as shown in FIG.
On the surface of the aN contact layer 19, a metal contact layer and an Au electrode are sequentially deposited by sputtering on a region having a width of 10 μm,
A heat treatment is performed in a nitrogen atmosphere at a temperature of ° C. to form a p-side electrode 20.

Next, as shown in FIG. 2 (c), a mesa shape including the p-side electrode 20 is formed by etching, and the surface of the n-type GaN contact layer 13 appearing under the mesa is lightened by about 200 MeV. Irradiate with an element ion beam for 1 hour. As a result, the surface is modified and an amorphous portion is formed near the surface. Thereafter, recrystallization is performed by heat treatment at 300 ° C. At this time, a cubic structure of less than 1% is uniformly formed on the surface layer.

Next, Ti and Al are sequentially deposited by sputtering, and a heat treatment is performed in a nitrogen atmosphere to form a TiN layer 31, a Ti layer 32, and an Al layer 3 which are metal contact layers.
3 are sequentially laminated to form an n-side electrode 30.

Next, the sapphire substrate 11 is mirror-polished to a thickness of 50 μm and cleaved in a direction perpendicular to the longitudinal direction of the p-side electrode 20 to form a blue semiconductor laser chip having a length of 1 mm.

The blue semiconductor laser thus formed is
Room temperature continuous oscillation was performed at a threshold current of 80 mA. The oscillation wavelength is 420 nm, the operating voltage is 7 V, and 50 ° C., 3
It operated stably even at 0 mW drive.

FIG. 3 shows the current-voltage characteristics of the blue semiconductor laser of this embodiment in comparison with the conventional laser. Curve A in FIG. 3 shows the characteristic of the blue semiconductor laser according to the present embodiment,
As a comparison, when the crystal of the contact layer is formed as a non-alloy at the interface between the n-type GaN contact layer and the n-side electrode and the wurtzite structure is 100%, the cubic structure is 1% or more of the contact area by a 300 ° C. sinter. Case C
The current-voltage characteristics of the blue semiconductor laser shown in FIG.

As is clear from FIG. 3, the present embodiment shows good diode characteristics, but the comparative example does not have perfect diode characteristics, and the voltage rise is as high as about 15 V. The emission was confirmed, but deteriorated within a few minutes.

As described above, according to the present embodiment, the surface of the n-type GaN contact layer 13 on which the n-side electrode 30 is to be formed is irradiated with a light element ion beam and then heat-treated, so that the surface layer has a cubic structure of less than 1%. To form n
The contact resistance generated between the side electrode 30 and the n-type GaN contact layer 13 can be reduced. In addition, the heat resistance against heat generation during operation can be strongly stabilized, so that a low threshold current and a low operating voltage do not cause deterioration and excellent reliability can be realized.

(Second Embodiment) FIG. 4 shows a second embodiment of the present invention.
FIG. 3 is a cross-sectional view illustrating a schematic configuration of a blue semiconductor laser according to the embodiment. The same parts as those in FIG. 1 are denoted by the same reference numerals, and detailed description thereof will be omitted.

In the blue semiconductor laser of this embodiment, a GaN thick film crystal is formed on a sapphire substrate by the H-VPE method.
Growing by 00 μm. Then, the sapphire substrate is removed by etching or the like, and the MOCV
Each layer in the figure is grown by D growth. Ga as substrate
The N thick film is an n-type GaN contact layer 43 (Si-doped, 5
× 10 ¹⁸ cm ⁻³ ), and TiN / Ti / Al is formed on a part except for the sapphire substrate to form an n-side electrode 30.

In this case, the thick film used as the n-type contact layer 43 is formed in order to bring the electrode metal into contact with a good GaN crystal.
Further, dry etching of 20 μm is performed. At this time,
Since the sapphire substrate has already been removed from the contact portion 45 of the n-type GaN contact layer 43 with the n-side electrode 30, distortion due to lattice mismatch is small, and the cubic structure has a contact area with high acceleration and long-time ion beam irradiation. Can be set to be less than 1%. This allows
A low-resistance electrode interface can be stably maintained, and the same effects as in the first embodiment can be obtained.

(Third Embodiment) Next, a blue semiconductor laser according to a third embodiment of the present invention will be described.
The element structure is the same as that of FIG.

In the present embodiment, a GaN thick-film crystal is grown by the H-VPE method, and thereafter, the other layers are grown by the MOCVD method, as in the second embodiment. As described in the second embodiment, the GaN thick film easily has a 100% wurtzite structure because the strain is released.

Therefore, in the present embodiment, an appropriate amount of pits are formed on the surface before the electrode is deposited, and then MOCVD is re-grown to actively form the cubic structure. As a result, in the formation of the n-side electrode, a good and stable ohmic contact can be obtained without any alloy. Subsequent steps are the same as in the first embodiment.

In the laser of the present embodiment thus produced, the low resistance electrode interface can be stably maintained with respect to the contact with the n-side electrode 30, and the same effects as those of the first embodiment can be obtained. .

The present invention is not limited to the above embodiments. Semiconductor layer growth method and conditions, type of contact electrode metal, treatment of semiconductor layer before deposition,
Furthermore, there are various manufacturing methods such as sintering conditions after deposition. Further, the present invention can be applied not only to a semiconductor laser but also to a light emitting diode. Further, in addition to the light emitting element, the present invention can be applied to electronic devices such as a light receiving element and a transistor. In the embodiment, the MQW is used as the active layer. However, the active layer may have a single-layer structure instead of the quantum well structure.

In addition, various modifications can be made without departing from the scope of the present invention.

[0044]

As described above, according to the present invention, n
N in contact with the side electrode (particularly, the n-side contact metal)
Low-resistance and thermally stable by making the crystal structure at the interface of the gallium nitride-based compound semiconductor layer (especially the n-type GaN contact layer) a less than 1% cubic structure in the wurtzite structure An n-side electrode can be realized. As a result, it is possible to realize a gallium nitride-based compound semiconductor device that does not deteriorate at low threshold current and low operating voltage and has excellent reliability.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an element structure of a blue semiconductor laser according to a first embodiment.

FIG. 2 is a sectional view showing a manufacturing process of the device of FIG. 1;

FIG. 3 is a diagram showing a comparison between characteristics of the embodiment laser and a conventional laser.

FIG. 4 is a sectional view showing an element structure of a blue semiconductor laser according to a second embodiment.

FIG. 5 is a sectional view showing an element structure of a conventional blue semiconductor laser.

FIG. 6 is a graph showing an X-ray diffraction result of a gallium nitride-based compound semiconductor.

[Explanation of symbols]

11 ... sapphire substrate 12 ... GaN buffer layer 13, 43 ... n-type GaN contact layer 14 ... n-type Al _0.2 Ga _0.8 N cladding layer 15 ... n-type GaN guide layer 16 ... MQW active layer 17 ... p-type GaN guide layer 18 ... p-type Al _0.2 Ga0.8 n cladding layer 19 ... p-type GaN contact layer 20 ... p-side electrode 30 ... n-side electrode 31 ... TiN contact metal 32 ... Ti layer 33 ... Al layer 35, 45 ... GaN contact layer Contact part with n-side electrode

Continuing from the front page (72) Inventor Shinya Nunogami 1 Toshiba R & D Center, Komukai, Kawasaki-shi, Kanagawa Prefecture (72) Inventor Kazuhiko Itaya 1 Toshiba-cho, Komukai Toshiba-cho, Saisaki-ku, Kawasaki-shi, Kanagawa Address F-term in Toshiba R & D Center (reference) 5F041 AA03 CA34 CA40 CA46 CA84 CA87 CA92 FF16 5F073 AA73 AA74 AB04 BA06 CA17 CB05 CB22 EA23

Claims (4)

[Claims] 1. An n-type gallium nitride-based compound semiconductor layer (Ga)
_{x In y Al z N: x} + y + z = 1,0 ≦ x, y, z ≦
1) and a gallium nitride-based compound semiconductor device including an n-side electrode formed on the compound semiconductor layer, wherein a semiconductor layer crystal at a contact portion between the compound semiconductor layer and the n-side electrode has a wurtzite structure. And a cubic structure. 2. The gallium nitride-based compound semiconductor device according to claim 1, wherein said n-side electrode has at least a semiconductor layer side made of TiN. 3. An abundance ratio of a cubic structure (c-GaInAlN) and a wurtzite structure (w-GaInAlN) at a contact portion of the compound semiconductor layer with the n-side electrode is 0% <(c-GaInAlN) / ( 3. The gallium nitride-based compound semiconductor device according to claim 1, wherein (w-GaInAlN) <1%. 4. A substrate, each gallium nitride-based compound semiconductor layer _{(Ga x In y Al z N} : x + y + z = 1,0
≤ x, y, z ≤ 1), an n-type contact layer, an n-type cladding layer, an active layer, a p-type cladding layer, and a p-type contact layer are laminated, and a p-side electrode is formed on the p-type contact layer. A gallium nitride-based compound semiconductor device in which an n-side electrode is formed on an n-type contact layer exposed by removing a part from a p-type contact layer to an n-type cladding layer, wherein the n-side electrode is at least a semiconductor. The layer side is TiN,
In the semiconductor layer crystal at the contact portion between the n-type contact layer and the n-side electrode, a wurtzite structure and a cubic structure are mixed, and the cubic structure (c-GaInA
1N) and the wurtzite structure (w-GaInAlN) are 0%
A gallium nitride-based compound semiconductor device, wherein <(c-GaInAlN) / (w-GaInAlN) <1%.