JPH0864791A - Epitaxial growth method - Google Patents

Abstract

PURPOSE: To provide a lattice non-matching epitaxial growth method of obtaining an epitaxial growth layer which has a low dislocation density, high quality, and is suitable for manufacturing a semiconductor light emitting device such as a light emitting diode, a laser diode or the like. CONSTITUTION: An amorphous GaN film 35 is grown on a sapphire substrate 31 in an initial crystal growth stage. The amorphous GaN film 35 is formed into stripes by etching. A GaN film 34 is epitaxially grown on the amorphous GaN film 35 in a second crystal growth stage. By this setup, lattice defects or dislocations are concentrated in a specific region 36, so that the active region of a required semiconductor light emitting device is capable of lessening relatively in defect density.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lattice-mismatched epitaxial growth method represented by epitaxial growth of gallium nitride-based compound semiconductor on a sapphire substrate.

[0002]

2. Description of the Related Art The problem of a semiconductor light emitting device using a III-V group nitride mixed crystal epitaxial layer such as GaN is G.
An aN bulk substrate crystal cannot be easily prepared, and a bulk crystal having a lattice constant close to that of GaN, which can be used as an alternative substrate, cannot be obtained. Conventionally, α-Al ₂ O ₃ (sapphire), SiC, Si, GaAs, etc. have been used as an alternative crystal substrate.
It is mainly manufactured by MOCVD (metal organic chemical vapor deposition) or halide VPE (chemical vapor deposition). α
-Al ₂ O ₃ (sapphire) is most widely used because the (0001) C-plane is close to the lattice constant of GaN.
It has an extremely large lattice mismatch of 13.8%, misfit dislocations (hereinafter simply referred to as dislocations) tend to occur due to strain stress applied to the crystal lattice during growth, and there is a problem that a high-quality epitaxial growth layer cannot be obtained. .

[0003]

According to the above-mentioned prior art, when GaN is grown on a sapphire substrate, first, 6
It is generally considered that a three-dimensional growth of prismatic GaN occurs, and initially a small hexagonal prismatic crystal gradually grows, bonds and disappears to grow into a larger crystal. However, in this process, although some of the strain stress generated at the interface with the substrate is relaxed, high-density dislocations are still generated and grown in the epitaxial layer along the growth direction (0001), and high-quality dislocations are obtained. The epitaxial layer could not be grown.

It is an object of the present invention to provide a semiconductor crystal growth method suitable for producing a semiconductor light emitting device such as a light emitting diode or a laser diode which has few lattice defects such as dislocations and is provided with a high quality epitaxial growth layer.

[0005]

SUMMARY OF THE INVENTION The essence of the present invention is to concentrate lattice defects and dislocations generated in a specific region in a lattice-mismatched epitaxial growth typified when GaN is grown on a sapphire substrate. By relatively reducing the defect density in the active region of a desired semiconductor light emitting device, it is possible to provide a semiconductor light emitting device of higher quality than the conventional growth method.

[0006]

In the conventional method, defects randomly generated locally due to lattice strain at the interface with the substrate become dislocations,
As the growth of the epitaxial layer progresses, it propagates in the crystal while meandering in the same direction as the growth direction, and is generated with a uniform density in the plane of the epitaxial layer after the growth.

It is known that the generated dislocations move due to stress even during crystal growth. The external stress required to move dislocations is very small, probably 10 ⁵ dyn / cm ²
It is said that the dislocation motion can be easily promoted by applying external stress to the substrate crystal.

Dislocations generally extend in the growth direction, but when stress is applied in the in-plane direction, the motion component of the dislocation in the in-plane direction increases. Once the external stress reaches the epitaxial layer grown on the amorphous layer patterned on the substrate, the dislocation motion ceases. Since the epitaxial layer grown on the amorphous layer is also in a state close to an amorphous state, external stress is less likely to be applied as compared with the crystalline portion, so that the dislocation reaching there does not move further from that region.

Usually, dislocations do not break inside the crystal and always form loops or reach the edge portion of the substrate during crystal growth. When a closed loop is formed, it does not propagate to the upper layer of the epitaxial layer. When the dislocation reaches the edge portion, the dislocation disappears and the dislocation density decreases.

Even in the case of a metal material, the movement of dislocations is promoted by a thermal cycle and the dislocations are often reduced by allowing them to escape to the free surface. The principle of the present invention is based on the above operation. That is, by stacking an amorphous layer or a crystal with a very high defect density on a specific part of the substrate and concentrating defects / dislocations to form a closed loop of dislocations, or by providing a role equivalent to the substrate edge. It is intended to reduce the dislocation density of the epitaxial layer in a desired region.

[0011]

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

FIG. 1 is a schematic sectional view of a growth apparatus used in one embodiment of the present invention. In the figure, 11 is a quartz reaction tube.
A raw material gas is introduced into the reaction tube 11 through the gas introduction port 12. A susceptor 13 made of carbon is placed in the reaction tube 11, and the sample substrate 14 is placed on the upper surface of this susceptor. The susceptor is provided with a rotation mechanism in order to obtain in-plane uniformity of composition and film pressure of the epitaxial layer. The susceptor is induction-heated by the high-frequency coil 15 arranged around the reaction tube. A thermocouple 16 located in the susceptor allows the substrate heating temperature to be monitored and controlled. The gas exhaust port 17 is connected to a vacuum pump 18 so that the pressure inside the reaction tube can be adjusted and the gas can be exhausted.

Next, the mechanism of the susceptor for imparting strain during growth to the sample substrate, which is the main part of the present invention, will be described. An oscillator 20 that moves up and down by an electromagnetic coil 19 is arranged at the center of the susceptor that comes into contact with the bottom of the substrate, and distortion is given by pushing up the substrate. At that time, the periphery of the substrate is fixed by a ring-shaped substrate holder 21 made of molybdenum so that the entire sample substrate does not move.

Next, a crystal growth method using the above apparatus will be described. First of all, α of the plane orientation (0001) was obtained by cleaning the surface with an organic solvent, hydrochloric acid chemical treatment and washing with pure water.
An -Al ₂ O ₃ (sapphire) substrate 14 is placed on the susceptor 13 and fixed by a substrate holder 21. Gas inlet
High-purity hydrogen gas that has passed through the purifier from 12 is introduced to replace the atmosphere in the reaction tube 11. After introducing hydrogen gas for several minutes, the vacuum pump 18 is operated to keep the pressure in the tube at 10 Torr. When the pressure is stable, the susceptor is induction-heated by the high-frequency coil 15, and is held for about 10 minutes after the temperature of the sample substrate 14 reaches 1200 ° C. to clean the substrate surface. Next, after lowering the substrate temperature to 400 ° C, the raw material gas TM
G (trimethylgallium) and NH ₃ (ammonia)
Is introduced from the gas introduction port 12 and an amorphous GaN film is deposited until the film pressure reaches 0.1 μm. At this time, considering that the substrate temperature is lower than that under normal growth conditions and the decomposition efficiency of NH ₃ is low, the flow rate ratio of NH ₃ and TMG is 10000:
Set to 1. At this time, if the growth temperature is higher than the above temperature,
Dimensional growth, that is, hexagonal prismatic island-shaped growth occurs, and a uniform amorphous GaN film cannot be obtained.

After the substrate temperature has dropped after the deposition, the sample substrate is once taken out from the reaction tube 11 and is subjected to a photolithography process as shown in FIG. 2 to form a stripe-shaped GaN deposition film in a direction orthogonal to the R plane of the sapphire substrate. Leave. The stripe width and spacing are 5 μm and 50 μm, respectively. After sufficiently washing with pure water, the sample substrate is returned to the reaction tube 11 again, and this time, while flowing NH ₃ gas instead of hydrogen gas, the sample substrate 14 is heated until the temperature reaches 1100 ° C. as described above. , Clean the sample substrate surface.

Next, TMG and NH 3 are introduced into the gas inlet 1
The GaN film is epitaxially grown by the ordinary two-step growth method introduced from step 2. That is, lower the substrate temperature to 600 ℃,
Three-dimensional growth, that is, hexagonal prismatic island-shaped growth is promoted up to a film thickness of 0.05 μm, and then the substrate temperature is raised to 1050 ° C. to continue epitaxial growth to a film thickness of 5.0 μm. At this time, the flow rate ratio of NH3 and TMG is 300: 1. FIG. 2 explains this in an easy-to-understand manner.

As shown in FIG. 2A, the amorphous GaN film 3 is formed on the sapphire substrate 31 by the first crystal growth.
5 is grown, and the amorphous GaN film 35 is processed into a stripe shape as shown in FIG. Next, in the second crystal growth, the GaN film 34 is epitaxially grown on the amorphous GaN film 35 (c).
As a result, lattice defects and dislocations are concentrated in a specific region 36, and the defect density in the active region of a desired semiconductor light emitting element can be relatively reduced.

GaN obtained by the above method
The crystal quality of the epitaxial layer will be described.

FIG. 3 shows a transmission electron of a 5 μm thick GaN epitaxial layer 33, 34 section grown on an α-Al ₂ O ₃ (sapphire) substrate 31 having a plane orientation (0001) by a conventional two-step growth method. The distribution of dislocations obtained from the microscope image is shown. Dislocations 32 uniformly generated from the interface 37 with the substrate 31 due to strain due to lattice mismatch extend to the surface of the epitaxial layer while meandering in the epitaxial growth direction. In the figure,
The dislocations that are visible or disappear in the middle are out of the field of view of the transmission electron microscope because the dislocations extend in the direction perpendicular to the cross section, and the dislocations do not disappear. When the dislocation density is estimated from a transmission electron microscope image, dislocations of 10 ⁹ / cm ² or more are uniformly generated, which is a lattice-matched epitaxial growth of Ga on a GaAs substrate.
The density of dislocations in epitaxial growth of As and AlGaAs is increased by 6 to 7 digits.

On the other hand, FIG. 4 shows the distribution of dislocations obtained from a transmission electron microscope image of the cross section of the GaN epitaxial layer according to this example. It can be seen that considerable dislocations reach the portion 36 where the crystal defects are concentrated on the amorphous GaN film 35 formed in a stripe shape up to the thickness of 3 μm. The dislocation density in the central portion of the stripe was 10 ⁵ / cm ² or less. Compared to the conventional example shown in FIG. 3, G has extremely excellent crystallinity.
It can be seen that an aN film is obtained.

An example in which strain is applied to the substrate crystal during growth will be described. During the growth in the second stage, the electromagnetic coil 19 was operated to move the oscillator 20 up and down to apply external stress to the sample substrate. The stroke for vertical movement is 1 mm when using a 2-inch diameter sample substrate. Moreover, although the cycle of vibration varies depending on the growth rate, it is set so as to vibrate every time a few atomic layers are grown. In this case as well, the same effect was obtained, but when dislocations generated from the interface 37 with the substrate were extended in the in-plane direction from the beginning of the second-stage epitaxial layer 34, and no external stress was applied. It can be seen that the dislocations move faster and are more effectively concentrated on the crystal defects on the amorphous GaN film 35.

In this embodiment, the stripes of the amorphous GaN film are formed on the substrate, but the same effect can be obtained by using an oxide film layer such as SiO ₂ . Figure 5 shows a thickness of 0.
On a substrate with a stripe formed of 1 μm SiO ₂ film 38,
The dislocation distribution in the cross section when the GaN film is selectively grown is shown. It can be seen that the same effect as that of the above-described embodiment can be obtained.

Further, in this embodiment, α-of the plane orientation (0001) is used.
Although the GaN epitaxial growth on the Al ₂ O ₃ (sapphire) substrate has been described, the present invention is not limited to the method of this embodiment and can be carried out in any other lattice-mismatched epitaxial growth to obtain the same effect. It is what is done.

From the above, it can be demonstrated that the method according to the present invention is sufficiently effective in obtaining a high quality epitaxial layer with a low dislocation density in the epitaxial growth of a lattice mismatching system.

[0025]

As described in detail above, according to the present invention,
In epitaxial growth of lattice-mismatched systems, the generation of lattice defects and dislocations generated can be concentrated in a specific region, and the dislocation density in the desired region can be reduced, so high-quality crystallinity such as for semiconductor lasers is required. It becomes possible to manufacture the semiconductor light emitting device.

[Brief description of drawings]

FIG. 1 is a schematic sectional view of an epitaxial growth apparatus in an example of the present invention.

FIG. 2 is a schematic cross-sectional view for explaining a step of forming an amorphous GaN film in a stripe shape on the surface of a sapphire substrate crystal and an epitaxial growth step on the substrate in an example of the present invention.

FIG. 3 shows Ga grown on a sapphire substrate in a conventional example.
Distribution diagram of dislocations in N epitaxial layer cross section

FIG. 4 is a distribution diagram of dislocations in a cross section of a GaN epitaxial layer grown on a sapphire substrate according to an example of the present invention.

FIG. 5 is a distribution diagram of dislocations in a cross section of a GaN epitaxial layer grown on a sapphire substrate according to another embodiment of the present invention.

[Explanation of symbols]

11 Reaction Tube 12 Gas Inlet Port 13 Carbon Susceptor 14 Sample Substrate 15 High Frequency Coil 16 Thermocouple 17 Gas Exhaust Port 18 Vacuum Pump 19 Electromagnetic Coil 20 Transducer 21 Substrate Holder 31 Sapphire Substrate 32 Dislocation 33 GaN Epitaxial Layer (First Step) 34 GaN epitaxial layer (second stage) 35 Amorphous GaN 36 GaN epitaxial layer in which crystal defects are concentrated 37 Interface between substrate and epitaxial growth layer 38 SiO ₂ film

Claims (6)

[Claims] 1. In epitaxial growth of a system in which a substrate and an epitaxial layer grown on the substrate have a lattice mismatch, dislocations generated by the lattice mismatch between the substrate and the epitaxial growth layer are concentrated at a specific place. Epitaxial growth method. 2. The epitaxial growth method according to claim 1, wherein an amorphous layer having the same composition as the epitaxial growth layer grown on the substrate is previously grown at a predetermined position on the surface of the substrate. 3. A method of epitaxially growing a GaN layer on a sapphire substrate, wherein an amorphous layer of GaN is grown at a predetermined position on the surface of the substrate between the substrate and the epitaxial layer. Epitaxial growth method. 4. The epitaxial growth method according to claim 3, wherein a SiO ₂ or SiN _x film is grown at a predetermined position on the surface of the substrate instead of the amorphous layer. 5. The epitaxial growth method according to claim 2, wherein the predetermined portion on the surface of the substrate has a stripe shape and extends along a specific orientation of the substrate. 6. The epitaxial growth method according to claim 1, wherein strain is applied to the substrate during the growth of the epitaxial layer on the substrate.