JPS6356199B2 - - Google Patents

Description

[Detailed description of the invention]

This invention relates to a method for producing high quality aluminum-gallium-arsenic (AlGaAs) crystals using molecular beam crystal growth. AlGaAs can have a wider forbidden band width than GaAs and has a smaller refractive index, so it is used in a heterojunction with GaAs to form a semiconductor laser, but in order to improve the laser characteristics, it is of high quality with fewer non-emissive centers. AlGaAs crystals are required. An example of a molecular beam crystal growth apparatus for manufacturing the above-mentioned AlGaAs crystal will be explained with reference to FIG. 1. In an ultra-high vacuum chamber 12, a GaAs substrate crystal 1 is attached to a molybdenum block 2 using indium. Heater 3 installed on the back of block 2
The substrate is heated through the block. A thermocouple 4 is further provided on the back surface of the block 2, and the heating temperature of the heater 3 is controlled by the temperature of the back surface of the block. On the inner peripheral surface of the vacuum chamber 12 facing the substrate 1, there are base material evaporation sources 5, 6, such as As, Ga, Al, etc.
7 and impurity material evaporation sources 8 and 9 such as Be and Si are provided to heat, evaporate, or sublimate the base material and impurities. A shutter is provided at the evaporation port of each evaporation source, and the evaporated and sublimated molecules are ejected from the evaporation port that is opened and incident on the heated substrate, where crystals grow on the substrate. The intensity of the emitted molecular beam is controlled by the heating temperature of each evaporation source. In the above molecular beam crystal growth, Al, Ga and
If the molecular beam intensity ratio of As is appropriate and the heating temperature of the substrate is appropriate, AlGaAs single crystals with good crystallinity will grow, but until now it has not been possible to detect the specific growth conditions for forming high-quality crystals. The method was unknown. In other words, although the temperature of the substrate is measured using a thermocouple or a radiation thermometer, the temperature of the substrate surface where the actual crystal grows cannot be accurately known in either case, and depends on the crystal growth conditions. Then,
Since the substrate surface temperature varies slightly, it is difficult to estimate it, and values measured by thermocouples and radiation thermometers cannot be used as parameters for crystal growth conditions. Regarding the relationship between molecular beam intensity ratio and crystal quality,
In Appl.Phys.Lett.39(6), (1981) pp486-487, we measured the photoluminescence spectra of crystals grown with the molecular beam intensity ratio As/Ga low and 8 times higher. It is said that when the intensity ratio is low, band edge emission is observed, so high quality crystals can be obtained. However, the molecular beam intensity ratio in this case is said to be a condition for As richness, and the specific conditions have not been clarified. Furthermore, in J.Appl.Phys.52(9), (1981) p5792, when the molecular beam intensity ratio As/(Al+Ga) is increased to ~10, the quality of the crystal evaluated from the photoluminescence spectrum is poor; It is said that the best spectrum can be obtained when the line intensity ratio is 2, but the specific growth conditions and accurate substrate surface temperature are not disclosed, resulting in poor reproducibility. In this way, until now it has not been possible to identify specific growth conditions for obtaining high-quality AlGaAs crystals, and crystal growth has been carried out based on empirically obtained growth conditions. If the growth conditions are insufficient and the growth conditions change even slightly, the quality of the crystals formed will be affected, making it impossible to produce high-quality AlGaAs crystals at high yields. An object of the present invention is to provide a method for producing high-quality AlGaAs crystals with excellent optical and electrical properties using molecular beam crystal growth with good reproducibility. When the substrate surface was irradiated with an electron beam during AlGaAs crystal growth and the diffraction image was observed, it was found that there was a clear correlation between the diffraction image, the crystal growth conditions, and the quality of the formed crystal. The present invention was completed based on the discovery that the conditions under which high-quality AlGaAs crystals can be obtained can be determined using diffraction images. In molecular beam crystal growth, the main growth conditions that influence the quality of the AlGaAs crystal to be formed are, as mentioned above, the molecular beam intensity ratio of Al and Ga to As and the substrate temperature. Figure 2 shows the photoluminescence spectrum from an AlGaAs crystal grown at a GaAs substrate temperature of 750°C, and Figure 2a shows the molecular beam intensity of As and the Al and Ga
Figure 2b is the spectrum of a crystal grown with the molecular beam intensity ratio As/(Al+Ga) added to 3.4, and Figure 2b is the spectrum of a crystal grown with the above molecular beam intensity ratio of 1.0, as indicated by the respective arrows. This peak is the emission peak due to bound excitons. The narrower the width and the higher the intensity of this emission peak, the fewer defects there are and the higher the optical quality of the crystal. Therefore, it can be seen from FIG. 2 that when the molecular beam intensity ratio is decreased from 3.4 to 1.0, the quality of the crystal tends to improve. In addition, the substrate temperature used in this specification means the temperature measured by pressing a thermocouple onto the back side of a molybdenum block to which a substrate is attached, and the molecular beam intensity is measured by placing an ion gauge at the position of the substrate at the time of crystal growth. This is the value obtained by measuring the partial pressure and using equation (1). In the formula, x is the type of molecule, P _x is the partial pressure, Tx is the temperature of the evaporation source, Mx is the weight of the molecule per mol, and ηx is
It is known from experience that the ionization rate of molecule x is given by equation (2) when the ionization rate of N ₂ is set to 1. ηx=0.6ΣZ/14+0.4...(2) In the formula, Z is the number of atoms, and ΣZ means the total number of atoms constituting the molecule x. Moreover, the molecular beam intensity ratio means J _As4 /(J _Ga + J _Al ). FIG. 3 is a graph showing the relative values of the emission intensity of bound excitons from AlGaAs crystals grown with various GaAs substrate temperatures and molecular beam intensity ratios. From the graph in Figure 3, the emission intensity of bound excitons increases as the substrate temperature increases, and even at the same substrate temperature, As/
It can be seen that the smaller the molecular beam intensity ratio of (Al+Ga), the more the emission intensity of bound excitons increases. When the substrate temperature is 720℃ and 750℃, the molecular beam intensity ratio is 1.0.
When the substrate temperature is 780°C, the highest emission intensity is obtained when the molecular beam intensity ratio is around 2.5.
However, when the molecular beam intensity ratio is further reduced, Al and Ga tend to precipitate alone on the crystal surface, resulting in deterioration of crystallinity. From the above, the AlGaAs crystal growth conditions are such that when the substrate temperature is approximately 750°C, the molecular beam intensity ratio is around 1.0, and as the substrate temperature increases, the molecular beam intensity ratio is increased, and when the substrate temperature is around 780°C, the molecular beam intensity ratio is approximately 1.0. By setting the line intensity ratio in a range of around 2.5, an AlGaAs crystal of excellent quality can be grown. The hole mobility and free electron concentration of the sample determined to be a high quality crystal by the above-mentioned bound exciton emission intensity measurement were measured, and the results were as shown in the following table.

[Table] From the table above, the crystal grown under conditions with a smaller molecular beam intensity ratio exhibits higher hole mobility and has better optical properties, i.e., a crystal with a molecular beam intensity ratio of 2.3 at a substrate temperature of 780°C. All crystals grown under the conditions of a substrate temperature of 750°C and a molecular beam intensity ratio of 1.0 exhibit excellent electrical properties, confirming that the growth conditions for AlGaAs crystals are optimal. However, although the control of the molecular beam intensity ratio is relatively reliable, the substrate temperature is the temperature measured by a thermocouple from the back side of the block as described above, and the correspondence relationship with the actual temperature of the substrate surface is Depending on the structure of the block being held, there is no guarantee that the temperature on the surface of the substrate will always be the same even if the same temperature is measured from the back side of the block between different devices. Therefore, in this invention, the GaAs substrate is
Observe the high-speed electron diffraction image while the AlGaAs layer is growing. That is, in FIG. 1, an electron gun 10 for electron beam diffraction is attached to a vacuum chamber 12, and the electron gun 1 is
An electron beam from zero irradiates the substrate surface, and an electron beam diffraction screen 11 is provided on the inner wall of the vacuum chamber on the opposite side where the reflected light reaches, and a diffraction image is observed. As described above, AlGaAs was grown on the substrate by changing the GaAs substrate temperature and the molecular beam intensity ratio As/(Al+Ga), and the high-speed electron diffraction image was observed. It was found that it depends on the intensity ratio. For example, electron beam (001)
The diffraction image when the beam is incident along the (110) direction of the GaAs substrate, the substrate temperature is 700° C., and the molecular beam intensity ratio is 1.7 or more shows a twice periodic structure as shown in FIG. 4a. The diffraction image when the substrate temperature is 750° C. and the molecular beam intensity ratio is 1.7 shows a 1 times periodic structure as shown in FIG. 4b. Furthermore, when the substrate temperature is 780° C. and the molecular beam intensity ratio is 1.7, the diffraction image shows a three-fold periodic structure as shown in FIG. 4c. FIG. 5 shows the relationship between the observed diffraction image, substrate temperature, and molecular beam intensity ratio. In the figure, the vertical axis is the molecular beam intensity ratio, the horizontal axis is the substrate temperature, and 3 times the periodic structure.
◎ indicates a region where a once periodic structure was observed, and a region where a double periodic structure was observed. From Figure 5, the crystal grown under the conditions of low substrate temperature and high molecular beam intensity ratio has a twice periodic structure, and as the substrate temperature increases and the molecular beam intensity ratio decreases, the diffraction image gradually becomes a once periodic structure. It can be seen that the structure shifts to a three-fold periodic structure. From the graph in Figure 3, the quality of crystals grown under conditions ranging from a molecular beam intensity ratio of approximately 1.0 at a substrate temperature of around 750°C to a molecular beam intensity ratio of approximately 2.5 at a substrate temperature of 780°C is found to be poor. Since it is the best, this range is shown in FIG. 5 as indicated by the dotted line. That is,
This is a condition when the observed diffraction image moves from a 1 times periodic structure to a 3 times periodic structure, and if crystal growth is performed under these conditions, a crystal with the highest quality will be obtained. Specifically, the method for producing AlGaAs crystals according to the present invention is described by setting a GaAs substrate in a vacuum chamber, creating a predetermined vacuum in the chamber, heating the substrate, and releasing Al and Ga as base materials from an evaporation source. , fires an As molecular beam. If necessary, molecular beams of impurities such as Be and Si may also be emitted. Although it is desirable that the heating temperature of the substrate and the molecular beam intensity ratio be set to growth conditions such that a diffraction image of the grown crystal having a one-time periodic structure is observed, this is not strictly required. Once a certain amount of AlGaAs layer is formed on the substrate,
Irradiate with an electron beam and observe the diffraction image. If the observed diffraction image has a 1 times periodic structure, the substrate temperature is slightly increased. Also, if necessary, the molecular beam intensity ratio is made slightly smaller. When the crystal is grown while changing the growth conditions in this manner, and the observed diffraction image changes from a 1 times periodic structure to a 3 times periodic structure, crystal growth is continued under the same conditions. If the initially observed crystal diffraction image has a 2x periodic structure, increase the heating temperature of the substrate and reduce the molecular beam intensity ratio as necessary to change the diffraction image from a 1x periodic structure to a 3x periodic structure. Crystal growth is performed under the conditions at the time of transition. If the initially observed diffraction image of the crystal has a 3 times periodic structure, lower the substrate temperature and if necessary increase the molecular beam intensity ratio to grow the crystal, until the diffraction image becomes a 1 times periodic structure. At that point, the substrate temperature is raised again. An AlGaAs crystal of excellent quality is formed by making the temperature increase smaller than the temperature decrease and performing crystal growth under conditions such that the diffraction pattern returns from the 1 times periodic structure to the 3 times periodic structure. At this time, the molecular beam intensity ratio may also be controlled to be small. Although control of the substrate temperature has been mainly described as a means for shifting the above-mentioned diffraction image to a triple periodic structure, the molecular beam intensity ratio may also be controlled mainly. Once the substrate temperature and molecular beam intensity ratio can be controlled to optimal crystal growth conditions based on the high-speed electron beam of the crystal growing as described above, crystal growth is performed based on the crystal growth conditions. Until now, in the molecular beam crystal growth method, the optimal conditions for crystal growth have not been known, and even if the optimal substrate temperature and molecular beam intensity ratio were known,
It is almost impossible to actually grow a crystal under such conditions, but according to the present invention, it is possible to grow a crystal even if the temperature of the substrate surface on which the crystal grows cannot be accurately measured. Even if the exact value of the molecular beam intensity ratio is not known, crystal growth can be performed by guiding the substrate temperature and molecular beam intensity ratio to the optimum conditions, and the method of controlling the above optimum conditions is also easy and reliable. As a result, high-quality AlGaAs crystals can always be produced with good reproducibility.

[Brief explanation of drawings]

Figure 1 is a schematic cross-sectional view of the molecular beam crystal growth apparatus, Figure 2 is a spectrum diagram of the grown AlGaAs crystal, and Figure 3 is the relationship between crystals grown by varying the substrate temperature and molecular beam intensity ratio and the emission intensity. Graph showing 4th
The figure shows a high-speed electron beam diffraction image of the grown crystal, and FIG. 5 shows a periodic structure distribution diagram of the crystal diffraction image. 1...GaAs substrate, 2...Block, 3...Heater, 4...Thermocouple, 5, 6, 7, 8, 9...
Molecular beam evaporation source, 10...electron gun, 11...screen, 12...vacuum chamber.

Claims (1)

[Claims] 1. In the method of growing AlGaAs crystals by molecular beam crystal growth on a GaAs substrate with a (001) plane, first, the temperature of the GaAs substrate and the
The molecular beam intensity ratio of Al and Ga is set such that when an electron beam is irradiated in the (110) direction, a 1x periodic structure is observed in the high-speed electron diffraction image during crystal growth, and then a 3x periodic structure is observed. Change the conditions until a three-fold periodic structure is observed, and then
A molecular beam crystal growth method for AlGaAs crystal, characterized in that an AlGaAs crystal layer is grown on a GaAs substrate.