JP2002363000A - Method for growing nitride semiconductor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for growing a nitride semiconductor, by which the nitride semiconductor having a good crystal state can be grown on a crystalline nitride semiconductor substrate. SOLUTION: The method of growing a crystal of the nitride semiconductor on the crystalline nitride semiconductor substrate comprises supply of gaseous raw materials onto the substrate while keeping the temperature of the substrate within the temperature range of 400 to 600 deg.C so as to form a low temperature buffer layer composed of the nitride semiconductor material on the surface of the substrate. Thereafter, supplying the gaseous raw materials onto the substrate while keeping the temperature of the substrate at a predetermined temperature higher than 600 deg.C so as to grow the nitride semiconductor on the low temperature buffer layer formed on the substrate.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for growing a nitride semiconductor, and more particularly to a method for growing a nitride semiconductor on a substrate made of a crystalline nitride semiconductor.

[0002]

2. Description of the Related Art A nitride semiconductor used for a light-emitting element such as a light-emitting diode or a laser diode, or a light-receiving element such as an optical sensor is manufactured by growing it on a heterogeneous substrate such as sapphire or silicon carbide. .

In this case, a low-temperature buffer layer made of amorphous gallium nitride is formed on a heterogeneous substrate made of sapphire or silicon carbide, and a crystalline nitride semiconductor is formed on the low-temperature buffer layer. Further, in order to prevent dislocation of the formed crystalline nitride semiconductor, after forming the nitride semiconductor on the low-temperature buffer layer, the nitride semiconductor is etched in a stripe shape, and the nitride semiconductor is grown again. By doing so, the crystal growth of the nitride semiconductor is performed on the part grown in the lateral direction by utilizing the fact that the dislocation from the lower layer has little effect.

[0004]

However, even with the above-described growth method, dislocation of the nitride semiconductor could not be sufficiently prevented.

[0005] Under such circumstances, with the development of the hydride vapor phase epitaxy method and the organometallic vapor phase epitaxy method, it has become possible to produce a crystalline substrate made of a nitride semiconductor having a relatively large thickness. By growing a nitride semiconductor crystal on the crystalline nitride semiconductor substrate obtained in this way, the above-described dislocation problem, reduction of cleavage and defects, and improvement of thermal conductivity are achieved. It is considered that a nitride semiconductor having better crystallinity can be obtained.

However, growth conditions for growing a nitride semiconductor in a good crystalline state on a crystalline nitride semiconductor substrate have not been established yet.

Accordingly, the present invention provides a method for growing a nitride semiconductor capable of growing a nitride semiconductor having a good crystalline state, which is suitably used for a light emitting element and a light receiving element, on a crystalline nitride semiconductor substrate. The purpose is to provide.

[0008]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention provides a method for growing a nitride semiconductor on a substrate made of a crystalline nitride semiconductor. Gas is supplied and the substrate is
By maintaining the temperature in the range of from about 600 ° C. to about 600 ° C., after forming a low-temperature buffer layer made of a nitride semiconductor material on the surface of the substrate, a source gas is supplied to the substrate and the substrate is heated to a temperature lower than By maintaining the temperature at a high predetermined temperature, a nitride semiconductor crystal is grown on the surface of the substrate on which the low-temperature buffer layer is formed.

In such a method, 400 ° C. to 600 ° C.
The temperature of the substrate having the low-temperature buffer layer formed in
In the step of growing the crystal of the nitride semiconductor, the temperature is further raised to a predetermined temperature higher than 600 ° C., and in this temperature raising process, micro-growth nuclei are formed in the low-temperature buffer layer.
For this reason, when crystal-growing a nitride semiconductor, the two-dimensional growth of the nitride semiconductor is accelerated centering on the fine growth nucleus, and a nitride semiconductor having a flat surface is formed on the upper portion of the substrate.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the method for growing a nitride semiconductor according to the present invention will be described below in detail with reference to the drawings.
Here, gallium nitride (Ga) is used as the nitride semiconductor.
An embodiment for growing N) will be described.

FIG. 1 is a schematic view showing an example of an MOCVD (metal organic-CVD) apparatus used for growing a nitride semiconductor according to the embodiment. This MOCVD apparatus includes a reaction tube 1 for accommodating a substrate W on which a nitride semiconductor growth process is performed in a sealed state. In the reaction tube 1, a substrate W
The susceptor 2 for supporting the substrate W is provided, and the internal atmosphere and the temperature of the substrate W supported by the susceptor 2 can be controlled freely. The reaction tube 1 is connected to a supply line 3 for supplying gas into the reaction tube 1 and a vent line 4 for exhausting gas from the reaction tube 1. It is connected.

The supply pipe line 3 and the vent line 4
Ammonia gas supply means 5 for supplying ammonia gas as a nitrogen (N) raw material, hydrogen gas supply means 6, gallium (G
a) A first supply pipe 7 for supplying trimethylgallium (TMG) as a raw material and a second supply pipe 8 for supplying tetraethylsilane as a silicon (Si) raw material are connected to each other. Note that silicon is an n-type impurity introduced into a nitride semiconductor made of gallium nitride.

The ammonia gas supply means 5 is connected to the supply pipe line 3 and the vent line 4 via valves V5a and V5b. The ammonia gas supply means 5 is provided with a flow control unit (mass flow controller) F5 upstream of the valves V5a and V5b.

Further, the hydrogen gas supply means 6 is provided with a hydrogen purifier 6a. Flow controllers F6a and F6b are provided between the hydrogen purifier 6a and the supply pipe line 3 and between the hydrogen purifier 6 and the vent line 4, respectively. Is provided.

The first supply pipe 7 is provided with a valve V7a,
V7b is connected to the supply pipe line 3 and the vent line 4. The first supply pipe 7 is connected to a bubbler 11 in which a raw material solution (here, TMG) is stored. A branch pipe from the hydrogen gas purifier 6a is connected to the bubbler 11 via a flow control unit F7, and the tip of the branch pipe is disposed in the TMG stored in the bubbler 11.

Similarly, the second supply pipe 8 is connected to a valve V8.
a, V8b are connected to the supply pipe line 3 and the vent line 4. The second supply pipe 8 is connected to a bubbler 1 in which a raw material solution (here, tetraethylsilane) is stored.
2 are connected. A branch pipe from the hydrogen gas purifier 6a is connected to the bubbler 12 via a flow control unit F8, and the tip of the branch pipe is disposed in tetraethylsilane stored in the bubbler 12.

In the MOCVD apparatus configured as described above,
Ammonia supplied from the ammonia gas supply means 5 is introduced into the reaction tube 1 via the supply pipe line 3 in a state where the flow rate is controlled by the flow rate control unit F5. At this time, switching of the introduction of ammonia between the supply pipe line 3 and the vent line 4 is performed by the valves 5a and 5b.

Further, hydrogen gas highly purified by the hydrogen purifier 6a is introduced into the reaction tube 1 through the supply pipe line 3 while the flow rate is controlled by the flow control unit F6a. Further, the hydrogen gas is also supplied to the vent line 4 while the flow rate is controlled by the flow rate control unit F6b.

The hydrogen gas highly purified by the hydrogen purifier 6a is supplied to TMG in the bubbler 11 and tetraethylsilane in the bubbler 12 as a carrier gas. Thereby, from each of the bubblers 11 and 12,
The raw material gas corresponding to the vapor pressure is supplied to the supply pipe line 3 via the first supply pipe 7 or the second supply pipe 8 together with the carrier gas (hydrogen gas), and further supplied into the reaction pipe 1. At this time, the amount of the source gas generated is controlled by adjusting the flow rate of the hydrogen gas using the flow control units F7 and F8 provided in the branch pipe of the hydrogen purifier 6a. The source gas generated in the bubbler 11 and collected in the first supply pipe 7 is supplied to the valve 7.
Switching between the supply pipe line 3 and the vent line 4 is performed by a and 7b. Further, the raw material gas generated in the bubbler 12 and collected in the second supply pipe 8 is supplied to the valves 8a and 8b.
Is switched between the supply line 3 and the vent line 4.

On the other hand, the substrate W to be processed is held by the susceptor 2 in the reaction tube 1 and is kept at a predetermined temperature in a state of being housed in the reaction tube 1 in a closed state. Then, as described above, each source gas is supplied to the substrate W maintained at a predetermined temperature at a flow rate adjusted respectively, and the processing of the substrate W is performed.

When growing gallium nitride into which boron (B) is introduced as a nitride semiconductor, triethylboron is used as a source solution for a boron source gas.
Gallium nitride (AlGaN, AlGaInN) into which aluminum (Al) is introduced as a nitride semiconductor;
Alternatively, when growing aluminum nitride (AlN), trimethyl aluminum is used as a source solution for an aluminum source gas. Further, gallium nitride (InG) into which indium (In) is introduced as a nitride semiconductor is used.
aN, AlGaInN) or indium nitride (In)
When growing N), trimethylindium is used as a raw material for the indium raw material gas. These raw materials are stored in a bubbler like TMG which is a raw material solution for gallium raw material gas, and supplied into the reaction tube 1 together with a carrier gas (here, hydrogen gas). Since trimethylindium is a solid raw material, it is used after being dissolved in a bubbler. In addition to the above, a raw material (raw material solution) for supplying a group III raw material gas according to the material constituting the nitride semiconductor is used in the same manner. Furthermore, here, the case where tetraethylsilane is used as the silicon (Si) raw material has been described, but the silicon raw material is not limited to tetraethylsilane, and monosilane or disilane may be used.

FIG. 2 is a graph showing the temperature sequence of the substrate and the timing of introducing each source gas according to the first embodiment of the present invention. The first embodiment of the nitride semiconductor growth method will be described below with reference to this graph and FIG.

First, the substrate W is mounted and fixed on the susceptor 2 in the reaction tube 1 of the MOCVD apparatus, and the inside of the reaction tube 1 is sealed. This substrate W is a nitride semiconductor substrate and is made of crystalline gallium nitride or a crystalline gallium nitride layer formed on a heterogeneous substrate such as sapphire.

Next, the heating of the substrate W is started, and the supply of ammonia gas (NH ₃ ) into the reaction tube 1 is started while the gas in the reaction tube 1 is exhausted. The supply of the ammonia gas into the reaction tube 1 may be started from a room temperature state in the reaction tube 1, and after the temperature of the substrate W has increased to some extent before the supply of TMG thereafter. Is also good.

Then, while supplying ammonia gas into the reaction tube 1, the temperature of the substrate W is raised at a predetermined speed to the first growth temperature Tg1. At this time, the first growth temperature Tg
1 is set in the range of 400 ° C. to 600 ° C.

Then, the temperature of the substrate W is set at 400 ° C. to 600 ° C.
C., and supply of the group III raw material (here, TMG) into the reaction tube 1 is started.
A low-temperature buffer layer made of the same material as the substrate W (that is, gallium nitride) is formed. This low temperature buffer layer becomes an amorphous layer or a polycrystalline layer, or a mixed layer thereof. The supply of tetraethylsilane is started simultaneously with the supply of TMG, and a low-temperature buffer layer having the same composition as the crystalline nitride semiconductor to be grown in the subsequent steps is grown.
Also, the start of supply of TMG and tetraethylsilane
After the temperature of the substrate W reaches 300 ° C.,
It will be set after reaching 0 ° C. This makes it possible to supply TMG after the ammonia supplied as the nitrogen source is decomposed, and to form a low-temperature buffer layer in which gallium and nitrogen are preferably combined.

Here, the flow rate of each source gas is, for example, 10 slm (standard liter / minute) of ammonia gas.
s: liter / minutes under standard conditions), TMG is 50 μm
ol / min, 1.5 × 10 ^{−3 of} tetraethylsilane
It is set to μmol / min.

After forming the low-temperature buffer layer with a thickness of 30 to 50 nm, preferably 38 to 40 nm, the temperature of the substrate W is raised at a predetermined rate to the second growth temperature Tg2. At this time, the second growth temperature T
g2 is higher than 600 ° C. and 900 ° C. or higher and 1300 ° C.
Below (900 ° C. to 1300 ° C.), preferably 10
The temperature is set in the range of 00 ° C to 1200 ° C.

During the temperature rise period from the first growth temperature Tg1 to the second growth temperature Tg2, the supply of ammonia gas into the reaction tube 1 is continued. This allows
Prevents escape of nitrogen from the low temperature buffer layer. However, the supply of TMG and tetramethylsilane may be stopped or continued during this heating period. However, when the supply of TMG and tetramethylsilane is stopped between the heated substrates, the supply of TMG and tetramethylsilane is performed before the temperature of the substrate W reaches the second growth temperature Tg2 and exceeds 20 minutes. Will be restarted.
By setting the re-supply time of TMG in this manner, detachment of the nitride semiconductor constituent material from the surface of the low-temperature buffer layer is prevented.

Then, by supplying TMG to the substrate W maintained at the second growth temperature Tg2, the same material as the substrate W (ie, gallium nitride) was formed on the surface of the substrate W on which the low-temperature buffer layer was formed. A nitride semiconductor crystal is grown. When the thickness of the crystalline nitride semiconductor reaches a predetermined value, the supply of each source gas into the reaction tube 1 is stopped, the temperature of the substrate W is lowered, and a series of nitride semiconductor growth steps is performed. Terminate. However, the ammonia gas serving as the nitrogen raw material may be continuously supplied even when the temperature of the substrate W is falling.

In the above-described method for growing a nitride semiconductor, the substrate W made of a crystalline nitride semiconductor is
After forming a low-temperature buffer layer composed of the same material as the substrate W by low-temperature processing at 0 ° C., a nitride semiconductor composed of the same material as the substrate W is crystallized by processing at a higher temperature than the formation of the low-temperature buffer layer. Growing. For this reason, the substrate having the low-temperature buffer layer is further heated to the second growth temperature Tg2 higher than 600 ° C., and in this heating process, minute growth nuclei are formed in the low-temperature buffer layer. Then, when growing the crystal of the nitride semiconductor, the two-dimensional growth of the nitride semiconductor is accelerated centering on the micro-growth nucleus, whereby a nitride semiconductor having a flat surface on the upper surface of the substrate W is formed. Become.

As a result, on the substrate W made of a crystalline nitride semiconductor, it is possible to grow a crystal of the nitride semiconductor having good surface properties and crystalline state and suitable for use in a light emitting element and a light receiving element. become.

FIG. 3 shows the result of comparing the surface roughness of the nitride semiconductor formed on the substrate W with and without the step of forming the low-temperature buffer layer.

Here, when the low-temperature buffer layer is formed, the substrate W is heated to the first growth temperature Tg1 = 500 ° C.
When the temperature was raised to, the supply of TMG was started and 40 n
m, a supply of TMG is temporarily stopped, and the substrate W is cooled to the second growth temperature Tg2 = 1.
The temperature was raised to 100 ° C., and after 10 minutes, TM
The supply of G was started to form a crystalline nitride semiconductor having a thickness of 2 μm.

On the other hand, when the low-temperature buffer layer was not formed, the supply of TMG was started when the substrate W was heated to the growth temperature Tg = 1100 ° C., and the crystalline nitride having a thickness of 2 μm was formed. A semiconductor was formed.

The surface roughness was measured by measuring the surface step of the obtained nitride semiconductor with a surface step meter, and the distance between the top and bottom of the surface irregularities was indicated as a surface step Δd (μm).

As shown in the graph of FIG. 3, when the crystalline nitride semiconductor is grown after forming the low-temperature buffer layer on the substrate W, the low-temperature buffer layer is not formed on the substrate W. It was confirmed that a nitride semiconductor having a small surface step Δd and a good surface state was formed on the substrate W as compared with the case where a crystalline nitride semiconductor was grown.

FIG. 4 shows the results of comparison of the emission intensity of the nitride semiconductor formed on the substrate W with and without the low-temperature buffer layer forming step.
The growth procedure of the nitride semiconductor in each case is the same as that of the nitride semiconductor in which the surface step Δd was measured in FIG. 3, and the nitride semiconductor containing silicon as an impurity at a concentration of 2 × 10 ¹⁸ / cm ^3. Was homoepitaxially grown on the substrate W to a thickness of 2 μm.

Then, photoluminescence measurement was performed on the nitride semiconductor grown on the substrate W. At this time, a He-Cd laser (wavelength 3
25 nm) at room temperature. FIG. 4 shows the integrated emission intensity in the photoluminescence measurement, where the highest value is 1.0%.
The relative values are shown as follows.

As shown in the graph of FIG. 4, a low-temperature buffer layer made of the same material was formed on a substrate W made of a crystalline nitride semiconductor, and then the crystalline nitride semiconductor was grown. In this case, compared to the case where a nitride semiconductor is homoepitaxially grown on a substrate without forming a low-temperature buffer layer, the integrated emission intensity of photoluminescence is high, that is, a nitride having a good crystalline state with few non-emission centers It was confirmed that the semiconductor was formed on the substrate W.

Here, the nitride semiconductor obtained through the low-temperature buffer layer forming step is compared with the nitride semiconductor obtained without the low-temperature buffer layer forming step by using an atomic force microscope. The step of the atomic step was clear, and clear surface flatness at the atomic level was confirmed. In addition, according to observation with a transmission electron microscope, a newly generated defect is hardly observed at the interface between the substrate W and the nitride semiconductor grown on the surface thereof, and a nitride semiconductor having a good crystalline state is obtained. It was confirmed that it was obtained.

In the above-described embodiment, a procedure was described in which a low-temperature buffer layer made of gallium nitride was formed on a substrate made of crystalline gallium nitride, and crystal growth of gallium nitride was performed on this low-temperature buffer layer. . However, the present invention is not limited to the case where the substrate, the buffer layer, and the nitride semiconductor layer on which the crystal is grown are made of the same composition material. For example, the substrate and the crystallinity of the nitride semiconductor layer, respectively, the general formula B _u Al _x Ga _y
In _z N (0 ≦ u ≦ 1, 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦
A different composition may be used as long as it is a nitride semiconductor of a crystal layer composed of a group III raw material and a nitride represented by z ≦ 1, u + x + y + z = 1), and similar effects can be obtained. Similarly, the low-temperature buffer layer does not need to have the same composition as the substrate and the crystalline nitride semiconductor layer as long as the low-temperature buffer layer is formed using the material constituting the nitride semiconductor represented by the above general formula.

Further, in the above-described embodiment, the procedure of forming a low-temperature buffer layer directly on a substrate made of a crystalline nitride semiconductor and then growing the crystal of the crystalline nitride semiconductor was described. However, according to the present invention, a pattern made of silicon oxide (SiO ₂ ) or the like is formed on a nitride semiconductor, and a low-temperature buffer layer is formed from a portion of the nitride semiconductor exposed from the pattern to grow a crystalline nitride semiconductor. The same effect can be obtained in the case of performing the same.

[0044]

As described above, according to the method for growing a nitride semiconductor of the present invention, a nitride semiconductor can be crystal-grown with a flat surface on a substrate made of a crystalline nitride semiconductor. It is possible to obtain a nitride semiconductor which has favorable properties and a crystalline state and can be suitably used for a light emitting element or a light receiving element.

[Brief description of the drawings]

FIG. 1 shows an MO used in a nitride semiconductor growth method of the present invention.
It is a block diagram which shows an example of a CVD apparatus.

FIG. 2 is a graph showing an example of a substrate temperature sequence for explaining a nitride semiconductor growth method in the embodiment.

FIG. 3 is a diagram showing the relationship between the presence or absence of a low-temperature buffer layer forming step and the surface roughness.

FIG. 4 is a graph showing the relationship between the presence or absence of a low-temperature buffer layer forming step and the emission intensity.

[Explanation of symbols]

 W: Substrate (nitride semiconductor substrate)

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4G077 AA03 BE15 DB01 DB13 EA02 EF03 TB05 TC06 TC14 TC19 5F041 AA40 CA34 CA40 CA65 5F045 AA04 AB14 AC08 AC12 AD09 AD15 AF04 AF09 BB12 CA12 CA13 CB02 DA53 5F088 AB07 BA18 CB04

Claims (3)

[Claims] 1. A method for growing a nitride semiconductor on a substrate made of a crystalline nitride semiconductor, the method comprising: supplying a source gas to the substrate and subjecting the substrate to a temperature range of 400 ° C. to 600 ° C. Forming a low-temperature buffer layer composed of a nitride semiconductor material on the surface of the substrate by supplying the source gas to the substrate and raising the substrate to a predetermined temperature higher than 600 ° C. Holding the low-temperature buffer layer to perform crystal growth of the nitride semiconductor on the surface of the substrate on which the low-temperature buffer layer is formed. 2. The nitride semiconductor growth method according to claim 1, wherein in the step of growing the crystal of the nitride semiconductor, the substrate is kept at a temperature in a range of 900 ° C. to 1300 ° C. Semiconductor growth method. 3. The nitride semiconductor growth method according to claim 1, wherein the step of forming the low-temperature buffer layer and the step of crystal-growing the nitride semiconductor are repeated a plurality of times. Semiconductor growth method.