JP2000299496A - Manufacture of gallium nitride compound semiconductor layer and semiconductor device manufactured thereby - Google Patents

Abstract

PROBLEM TO BE SOLVED: To promote increase in the density of dots to improve luminance of light emission by forming position charges working as the nuclei for concentration of In by replacing a part of nitrogen with oxygen during the growth of mixed crystal of three elements. SOLUTION: A substrate suscepter 2 is arranged within a reactor 1 and a crystal substrate 3 is held thereon. A high frequency coil 5 is arranged around the substrate suscepter 2, and it is then connected to a high frequency power supply 4 to heat the substrate suscepter 2 and substrate 3 with the high frequency power source voltage. A gas flowing path is provided above the substrate 3, and nitrogen gas or hydrogen gas is independently supplied from the upper flowing path 10. From an intermediate flowing path 11, trymethylaluminum is supplied from a first vessel 14, trimethylgallium from a second vessel 16, and trymethylindium from a third vessel 15. Moreover, from the lower flowing path 12, ammonium and ammonium bubbling the water of the oxygen supply source are supplied from a fourth vessel 13. The reaction gas is supplied through independent control with each gas exchange valve.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor device composed of a gallium nitride-based compound, and more particularly to manufacturing a dot structure for forming a dot structure which is expected to be confined by quantum dots. The present invention relates to a method and a semiconductor device utilizing the dot structure.

[0002]

2. Description of the Related Art In recent years, semiconductor devices using electrons or excitons having quantum mechanical effects confined in quantum structures such as quantum wells, quantum wires, and quantum boxes have been developed.
In particular, in the field of light-emitting elements, development is underway for the purpose of increasing the emission luminance. Semiconductor lasers whose wavelengths are going to reach the blue region are required to have not only improved oscillation efficiency but also performance that can operate stably even at large temperature changes. To achieve these performances,
It is necessary that the quantum structure that exhibits the quantum mechanical effect by reducing the size of the active layer be a quantum box that can confine 0-dimensional electrons.

This is because when the size of the active layer, that is, the size of the quantum box becomes as small as the effective bore radius of the exciton,
Coulomb interaction based on the overlap of wave functions of electrons and holes is strengthened, and the effect of increasing the binding energy of excitons and the oscillator density of excitons is used. For this reason, in the semiconductor laser, the oscillation threshold current density is small, and the carrier temperature distribution is suppressed,
Since the differential gain increases, stable temperature characteristics can be obtained.

Further, an InGaN layer, which is an active layer of a gallium nitride based semiconductor light emitting device, is composed of a dot having a large In composition and a peripheral region having a smaller In composition around the dot. Large dots contribute to light emission. This dot has a size that has a quantum effect (dots with a diameter of 1 nm or more and less than 10 nm).
On the other hand, when the size is such that the quantum effect is not exhibited (the diameter is 10 nm or more), the dot has high crystallinity and has few defects. The luminous efficiency is remarkably improved as compared with the case where the active layer has a layer structure. Therefore, in order to improve the efficiency of the gallium nitride-based semiconductor light emitting device and realize the low threshold current oscillation of the gallium nitride-based semiconductor laser, it is required to form the dots with high density.

Many proposals have been made and developed as a method of realizing an active layer having a dot structure, such as a formation method based on lithography using an electron beam and a formation method utilizing lateral growth on an inclined substrate. However, due to processing accuracy and complicated processes, there are some strict steps to achieve desired performance. At present, as a practical method, a method of forming a quantum box with a high In composition mixed crystal part by agglomeration of In, which can be said to be self-forming based on a strain relaxation phenomenon, has been attempted (Japanese Patent Application Laid-Open Nos. 8-88345 and 8-88345, 9-331116). According to this method, high-purity processing technology and complicated steps are not required compared to other methods,
In addition, there is no contamination of harmful impurities due to its derivative effect,
This is the most realistic and promising quantum box realization method. However, this method has excellent simplicity in the sense that minute dots can be spontaneously formed, but as a means for achieving desired device performance, the controlling factors of In aggregation are still unclear. However, the control factors such as the size and density of the quantum box are poorly grasped, and it is not yet a means for realizing the aim.

[0006]

In order to realize dots in the InGaN active layer, it is important to precisely control the size and density of the dots, but it has been difficult to realize them by the conventional technology. As an alternative forming method, as described in the above-mentioned publication, InGaAs / GaAs,
InAs / GaAs, InGaN / GaN and SiC
It is a quantum dot that has been observed for example. This is a self-generated method that is generally obtained in a Stranski-Krastanov mode that grows while relaxing the strain, and is epitaxially grown on a lattice-mismatched substrate. However, even with this method, there is a problem in the controlling factor of the aggregation of In and the size and density of the dots.

SUMMARY OF THE INVENTION It is an object of the present invention to promote the aggregation of In, which is an eye for forming quantum dots, by means of doping oxygen into the active layer, and to control the amount of In by controlling the doping amount of oxygen.
Provided are a method of forming dots having any size and density by increasing controllability of the size and density of a dot having a composition ratio larger than its surroundings, and a gallium nitride-based compound semiconductor device having the dot structure. is there.

[0008]

A semiconductor device according to the present invention has a first region made of a gallium nitride-based compound semiconductor epitaxially grown on a surface of a semiconductor substrate capable of growing a single crystal, and a disordered region in the first region. Is formed so as to include a dot-shaped second region in which the composition ratio of the constituent elements included in the first region is different, but at the start of or during the epitaxial growth of the first region, the strain relaxation of the first region can be more uniformly reduced. By introducing oxygen as a promoting element that induces and locally concentrates In at a high density, a dot-shaped second region having a higher In density is provided. In the case where the first region is a GaN-based mixed crystal which is a group III-V compound semiconductor, oxygen is easily replaced by a nitrogen lattice point, and
Since the ion radius of oxygen is about 20% smaller than the ion radius of nitrogen, the introduction of oxygen causes greater strain in the in-plane direction in the InGaN epitaxial growth layer having a lattice mismatch with the substrate polycrystalline surface. The strain caused by the lattice mismatch is partially reduced by reducing the In composition of the InGaN layer, which is apt to cause mixed unsaturation, so as to substantially match the lattice constant of the substrate single crystal. At the same time, a phenomenon of local aggregation of In is induced. The region where the In composition is increased is spherical and a minute region is formed in order to disperse and reduce the increased intrinsic strain, and is stabilized in a state where the strain energy inherent in the growth layer is minimized.

According to the mechanism described above, the relaxation of the intrinsic strain is caused by the aggregation due to the movement of In, but the movement distance of In is reduced by the oxygen substituted at the nitrogen lattice position. This is because oxygen substituted at the nitrogen lattice point has the property of stabilizing by combining with the gallium vacancy,
When n encounters a gallium vacancy, it easily fits into the lattice point, and thus promotes the aggregation of In. When the number of oxygen contained in the nitrogen vacancies is about 2 × 10 ²¹ cm ⁻³ , regularity based on the crystal structure becomes apparent in the aggregation of In, and regularity appears in the arrangement of dots. Based on these oxygen effects, the dot-shaped region having a high In composition ratio is
It can be formed finer and more regular than in the case where oxygen is not introduced. Therefore, the size of the dot area is
Although it tends to increase in proportion to the layer thickness of the first region, it becomes possible to form a fine dot-shaped region corresponding to a layer thickness smaller than the conventional one, and the preferable layer thickness is 1.5 to 40 nm can be controlled and the density can be increased. The In composition ratio of the dot region depends on the amount of oxygen introduced and the growth conditions,
It becomes easy to control the composition ratio to be larger than the n composition ratio, for example, 0.15.

Since the dot-shaped region has a larger composition ratio and a smaller energy gap than In of the first region, the dot-shaped region is a dot capable of zero-dimensional quantum confinement.

[0011]

Embodiments of the present invention will be described below with reference to the drawings. Here, a method of self-forming dots of an In _x Ga ₁ -xN mixed crystal on a gallium nitride-based compound semiconductor surface by metal organic chemical vapor deposition (MOCVD) will be described. However, the examples described below exemplify the technical concept of the present invention, and the method of the present invention does not specify the composition of the material, the crystal growth conditions, the type of the vapor growth gas, etc. Absent.

In the gallium nitride based compound semiconductor device of the present invention, a gallium nitride based compound semiconductor light emitting device will be described as an example. As a substrate, general sapphire is used, but other than MgAl ₂ O ₄ , GaN, MgO, Si
C, Si, ZnO and the like are also used.

In the MOCVD method, for example, a GaN buffer layer is grown on a sapphire substrate by using, as a carrier gas, hydrogen or nitrogen, a raw material gas such as ammonia and trimethylgallium (TMG). N ⁺ -type GaN layer, n-type GaAlN layer,
A GaInN layer, a p-type GaAlN layer, a p ⁺ -type GaN layer;
Each layer is continuously grown as a layer having a carrier concentration and a thickness suitable for the light emitting element.

The present invention relates to a method for self-forming a dot structure in an InGaN active layer, wherein a part of nitrogen is replaced with oxygen during the ternary mixed crystal growth to serve as a nucleus for In aggregation.
The formation of electric charges promotes an increase in dot density, and is characterized in that emission luminance is improved.

A ternary mixed crystal I which is a mixed crystal of GaN and InN
During the growth of nGaN, the incorporation of In into the solid phase is greatly affected by the growing hydrogen. For example, when the hydrogen partial pressure is high, nGaN has the same gas supply composition. It is extremely difficult to control the composition of mixed crystals under the conditions of growing gallium nitride-based mixed crystals with a high In content, such as the presence of a compositionally unstable region that will result in multiple mixed crystals with different compositions. Includes. Furthermore, it is extremely difficult to grow an InGaN mixed crystal active layer having a controlled composition ratio, size, or density in forming dots suitable for the performance of a semiconductor device. If this is the case, it is possible to construct a controlled self-forming dot structure.

FIG. 1 schematically shows an MOCVD apparatus used to form a dot structure by the method of the present invention.

A substrate susceptor 2 made of graphite is arranged in a reaction vessel 1, and a crystal substrate 3 is held on the substrate susceptor 2. A high-frequency coil is arranged around the substrate susceptor 2, which is connected to a high-frequency power supply 4 so that the substrate susceptor 2 and the substrate 3 can be heated at a high frequency. The gas flow path is a portion located above the substrate 3 and has a three-layer structure made of quartz, so that nitrogen gas or hydrogen gas can be supplied independently from the upper flow path 10. The mixed gas can also be supplied. From the middle flow channel 11, trimethylaluminum (TMA) from the first container 14, trimethylgallium (TMG) from the second container 16, and trimethylindium (TMG) from the third container 15 with hydrogen as a carrier gas.
I) can be supplied. Further, from the lower channel 12, ammonia and ammonia obtained by bubbling water as a supply source of oxygen can be supplied from the fourth container 13, or only ammonia can be supplied independently. The reaction gas can be supplied from each of the gas passages 10, 11, and 12 by controlling the gas switching valve independently, and is mixed immediately before the substrate susceptor 2. In addition, these gases are sent to the substrate 3 in the reaction vessel 1 while controlling the flow rate through a mass flow controller (not shown).

Next, a method of forming the dot structure shown in FIG. 2 using the above-mentioned MOCVD apparatus will be described.
After using the sapphire substrate 20 as the above-mentioned substrate 3 and washing it sufficiently, it is placed on the substrate susceptor 2 in the MOCVD reaction vessel 1 shown in FIG.
By heating to 00 ° C., the sapphire substrate 20 is thermally cleaned. After this treatment, the temperature in the reaction vessel 1 was lowered to 500 ° C., and hydrogen was used as a carrier gas, and T was used as a source gas.
A GaN buffer layer 21 having a thickness of 20 nm is grown on the surface of the sapphire substrate 20 using MG and ammonia.

Thereafter, the TMG gas is stopped, the temperature of the sapphire substrate 20 is raised to 1030 ° C., and the raw material gas is
Silane gas (SiH ₄ ) for the purpose of simultaneously adding Si as a donor dopant as MG and ammonia
To supply n ⁺ -type G having a carrier concentration of 1 × 10 ¹⁹ cm ^−3.
An electrode contact layer 22 of an aN layer is grown to a thickness of 3.5 μm. Subsequently, in order to reduce the amount of Si added, SiH
The flow rate of step ₄ is reduced, and a 0.5 μm thick n-type Al _0.2 Ga _0.8 N cladding layer 23 having a carrier concentration of 1 × 10 ¹⁸ cm ⁻³ is grown.

After the growth of the cladding layer 23, both the source gas and the SiH ₄ gas are stopped, and preferably 740-79.
The temperature of the sapphire substrate 20, which is desirably controlled within the range of 0 ° C., is lowered to 770 ° C., and the hydrogen serving as the carrier gas is switched to nitrogen. 20 ccm of ammonia containing water vapor by bubbling is simultaneously supplied, and n-type I having a composition ratio of In of 5 nm and a composition ratio of 0.15 is supplied.
An active layer 24 of nGaN is grown. From the latter half of the growth step of the active layer 24, the supply amount of the gallium source can be reduced to promote the aggregation of indium. During the growth of the active layer 24, dot-like dots 25 are formed in the active layer.

Subsequently, the transport of the raw material gases TMI, TMG and ammonia containing water vapor is interrupted to raise the substrate temperature to 900 ° C., and the source gases of TMA, TMG, ammonia and the acceptor impurity source Cp2Mg (cyclopentadienyl magnesium) is supplied, and Mg-added p-type Al _0.15 Ga _{0.
When the} 85N evaporation prevention layer 26 is grown to a thickness of 12 nm, the supply of the source gas is stopped.

Next, after raising the substrate temperature to 1010 ° C., the raw material gas TMG, ammonia and the increased Cp 2
Supplying Mg and p ⁺ -type GaN anode contact layer 27
Was grown to a thickness of 0.2 μm.

The grown crystal obtained through the above-described growth process is formed into an anode electrode 28 on the anode contact layer 27 and a cathode electrode 29 on the cathode contact layer 22 by a generally well-known method. Was formed. With a forward current of 20 mA, a forward voltage of 3.5 V, a light emission output of 3.9 mW, and a light emission wavelength of 443 nm.
Was obtained.

When the obtained result is compared with the characteristics of a light emitting device grown under the condition that oxygen is not added, the light emission output is 2%.
mW, and the emission wavelength is shortened by about 20 nm.

FIG. 3 is a diagram showing the relationship between the density of the dots 25 in the active layer 24 and the oxygen atom density estimated by observing the cross section of the light emitting element with a TEM (transmission atomic microscope).
This figure is obtained by fixing the In ratio x of the active layer 24 to x = 0.15, keeping the growth temperature constant at 770 ° C., and changing the supply amount of water vapor-containing ammonia over 0 to 25 ccm. From FIG. 3, it can be seen that the density of dots increases with an increase in the oxygen supply amount. It can be seen that the dot density decreases rather than the oxygen supply amount of the water vapor-containing ammonia exceeding 20 ccm. The luminous output of the light-emitting element formed in a small region where the supply amount of oxygen by the water vapor-containing ammonia was 5 ccm or less was equivalent to the luminous output of the light-emitting element grown under the condition where oxygen was not added. In this region, it is presumed that factors such as the In composition ratio, the layer thickness and the growth temperature act more effectively on dot formation than the oxygen amount itself. On the other hand, the luminous output of the light-emitting element formed in the region where the oxygen supply was large dropped sharply in the region where the oxygen supply exceeded 20 ccm. This is because in this region, an increase in the amount of interstitial oxygen caused inhibition of dot formation. Amount of oxygen supplied by ammonia containing water vapor 5 ccm
The oxygen atom density is 10 ¹⁷ cm ^-3 ,
In the case of m, it is 10 ²¹ cm ^-3 .

Here, as a method of forming dots in the active layer 24 on the cladding layer 23, the composition of each mixed crystal, the In composition ratio, the thickness of each grown layer, the growth temperature, the impurity introduction amount, and the like are fixed. The description has been made using the example in which steam is used as the source. It goes without saying that the effect of the present oxygen introduction is not restricted by the present embodiment conditions. The growth temperature is not limited to the low temperature region, and even the tendency to raise the critical temperature at which the unsaturated mixed region of In occurs is observed. Can be Further, the source of oxygen is not limited to ammonia containing water vapor, and the same effect can be obtained by directly introducing oxygen. In addition, as an assisting impurity at the time of forming a dot, there is a possibility of fluorine other than oxygen.

This embodiment is an example in which a well layer (active layer) having a single quantum well structure is formed with dots, and the same effect is obtained even if each well layer having a multiple quantum well structure is formed with dots.

Although the evaporation preventing layer 26 in FIG. 2 does not function as a barrier layer in the single dot structure, it functions not only as a cladding layer but also as a dot barrier layer.

In addition to the method in which oxygen gas is mixed with the gas as in the above embodiment, a method in which an oxygen partial pressure is left in the atmosphere for crystal growth has the same effect. For example, a small amount of air is supplied to a crystal growth furnace. Mixing may be performed, or a substance containing oxygen atoms may be generated from the inner wall of the tube that forms the crystal growth atmosphere.

The embodiments disclosed this time are to be considered in all respects as illustrative and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

[0031]

As described above, according to the present invention,
Since the size and density of the dots, which were difficult to control in the past, can be easily controlled and formed, it is an effective means for semiconductor devices with a dot structure, at least in gallium nitride-based compound semiconductors, and has a value from an industrial point of view. There is a great thing.

[Brief description of the drawings]

FIG. 1 is a schematic view of an MOCVD crystal growth apparatus used in the manufacturing method of the present invention.

FIG. 2 is a schematic sectional view of a light emitting device according to one embodiment of the present invention.

FIG. 3 is a diagram showing a relationship between an oxygen atom density and a dot density in the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Reaction container 2 Substrate susceptor 3 Substrate 4 High frequency power supply 5 High frequency coil 10 Upper flow path 11 Middle flow path 12 Lower flow path 20 Sapphire substrate 21 Buffer layer 22 Cathode contact layer 23 Cladding layer 24 Active layer 25 Dot 26 Evaporation prevention layer 27 Anode Contact layer 28 anode electrode 29 cathode electrode

Continued on the front page F-term (reference)

Claims (7)

[Claims] 1. An active layer comprising at least one active layer made of a gallium nitride-based compound semiconductor epitaxially grown on a substrate surface, the active layer including a granular region, and a composition ratio of constituent elements of the active layer and a granular region. In the method for manufacturing a gallium nitride-based compound semiconductor layer having a dot structure different from the composition ratio of the constituent elements of the region, oxygen or oxygen is contained in the reaction vessel at least during a period from the start of the epitaxial growth of the active layer to the end of the growth. A method for producing a gallium nitride-based compound semiconductor layer having a dot structure, comprising a step of flowing a compound gas containing oxygen. 2. The composition of an element in an active layer is In _x Al _y
Ga _1-xy N (0 ≦ x ≦ 1; 0 ≦ y ≦ 1; x + y ≦ 1)
The method for manufacturing a semiconductor device having a dot structure according to claim 1, wherein 3. The active layer has at least one pair of barrier layers having the same elemental composition and a smaller In composition on the surface of the active layer, and a step of adding oxygen to each barrier layer is also included. 3. The method for producing a gallium nitride-based compound semiconductor layer having a dot structure according to claim 1, wherein an active layer is also provided on the surface of the barrier layer. 4. The reaction vessel according to claim 1, further comprising means for supplying oxygen or a compound gas containing oxygen and control means therefor, wherein the amount of oxygen added can be adjusted. 3. A method for producing a gallium nitride-based compound semiconductor layer having a dot structure according to item 1. 5. An oxygen atom density ≦ 10 ¹⁷ cm ⁻³ ≦ 10 ²¹ c
The method for producing a gallium nitride-based compound semiconductor layer having a dot structure according to any one of claims 1 to ³ , wherein m- ³ . 6. The method for manufacturing a gallium nitride-based compound semiconductor layer having a dot structure according to claim 1, wherein the supply amount of the gallium source is reduced from the latter half of the active layer growth step. 7. A gallium nitride-based compound semiconductor,
A gallium nitride-based compound semiconductor device having a dot structure in which the active layer contains indium, and where a granular region in the active layer is doped with oxygen to have a larger indium composition ratio than the surrounding active layer.