CHARACTERISTICS OF MOCVD-GROWN HIGH-QUALITY CDTE LAYERS ON GAAS SUBSTRATES

CdTe epitaxial layers are grown successfully on a (100)-GaAs substrate by metalorganic chemical vapor deposition (MOCVD) using dimethylcadrnium (DMCd) and diethyltelluride (DETe) as alkyl sources. The CdTe epilayers grown between 365C and 380C possess the best surface morphology. DETe is used as the controlling species of this growth system. Typical growth rates are varied from 2.51am/hr to 5.31am/hr. Low-temperature (12K) photoluminscence (PL) measurements reveal that 380C is the best growth temperature and the full width at half maximum (FWHM) of the dominated peak is about 1.583eV by the bound-exciton emission of 9.38meV. The double crystal X-ray rocking curves (DCRC) indicate that the FWHM decreases while increasing the epilayer thickness and approaches a stable value about 80 arc sec under the growth rate of 5.21arn/hr, the growth temperature of 380C and the DETe/DMCd concentration ratio of 1.7. The value of 80 arc sec in FWHM is the smallest one ever reported to date.


INTRODUCTION
CdTe is a large bandgap, versatile II-VI compound semiconductor known for its wide use in infrared detectors1, /-ray nuclear detectors 2 and solar cells3. Because of its small lattice mismatch and the inherent high resistivity with HgxCdl_Te, CdTe is used as a substrate in preparing HgxCdl_xTe infrared detectors. However, the CdTe substrate material contains microscopic crystal defect such as subgrain boundaries and twins, which degrade the performance 4 of infrared detectors made on the wafers containing such defects. Moreover, a large and high quality HgxCdl_xTe wafer is the suitable candidate for the next generation of twodimensional infrared detector arrays. Therefore, the selection of alternative substrates, such as Si5, InSb6, GaAs 7 and sapphire8, is useful for growing CdTe thin film as a buffer layer to reach this purpose. Among these substrates mentioned above, GaAs is the most popular one due to its high resistivity and ease of *All correspondence address to: K. E YARN, EO. Box 345, Tainan, Taiwan 704, Republic of China 247 compatibility with high-speed III-V semiconductor devices. It also has been proved that high-quality CdTe epilayers could be grown on GaAs substrates, in spite of the large mismatch (14.6%) between them.
In the present work, under the consideration of easy control, lower growth temperature, and high quality of in-situ growth of HgxCdl_xTe, a promising low temperature MOCVD method is adopted for growing CdTe buffer layer. Experimental measurements including surface morphology, film growth rate, X-ray diffraction, PL, and DCRC properties are investigated to analyze the film quality.

EXPERIMENTAL
The CdTe epilayers were grown at approximately atmospheric pressure in a if-heated cold wall vertical reactor. Fig. 1 schematically shows an experimental apparatus. DETe and DMCd were used as source materials. They were separately kept at 20C and 10C in different temperature reservoirs for supplying the equilibrium vapor pressure of 7.1 torr and 16.8 torr, respectively. By using the mass flow controller, the amount of reactants put into reactor could be precisely adjusted. Palladium-diffused ultra-pure hydrogen was used as carrier gas and the total gas kept at 1.4SLM. The susceptor was rotated at 60 rpm to get the uniform epilayers. The waste gases at the outlet of the growth chamber were passed through a resistance-heated furnace held at 800C, an activated carbon cell, and a burn-off unit to eliminate the harmful residual metal alkyls. After degreasing, the GaAs substrate was etched by H2SO4 for 2-3 minutes and then loaded into the reactor at a growth temperature of 380C. In preliminary experiments, source gases with the same DETe/DMCd ratio of 1.0 were ejected from a quartz tube and then modulated by the different temperatures. After selecting the suitable growth temperature by a PL measurement, different DETe/ DMCd ratios were used to find the best growth condition.
The surface morphology and the thickness of CdTe epilayers grown on GaAs substrates were examined by using an optical microscope. To evaluate the CdTe film crystalline condition, we measured the X-ray diffraction of (400) reflection with a spot size of 2mm 2mm. Low temperature (12K) PL measurements were carried out with a 25mW argon laser source (5145/). Finally, DCRC were checked to investigate the CdTe film quality.

Surface morphology
The photomicrographs of CdTe epilayers grown on (100)-GaAs at 335C, 350C, 365C, 380C, and 395C with equal amount of DETe and DMCd are shown respectively in Fig. 2. It has been reported that the existence of an oxide layer on the GaAs surface vould lead to 3D growth for (100)-CdTe epilayers and form pyramids14. In Fig. 2, we find that the surface morphology of CdTe epilayers are strongly dependent on the growth temperature. The volume of pyramids are larger as the growth temperature increases and the best morphology is observed at 380C. At 380C, the pyramids are nearly the same and arrange in order. Below 365C, the epilayer shows a smooth surface with only a few pyramids due to the lack of surface kinetics for the deposited molecules. For growth temperature above 395C, a rugged and different volume of pyramidal surface is obtained owing to the escaping of Cd or Te atoms from the deposited surface at higher temperature.

Growth rate
In an MOCVD growth processes, the epilayer growth rate plays an important role on understanding the growth mechanism and predicting film quality. Fig. 3 shows the relationship between growth rates and 9.8 10-4mole% DETe/DMCd concentrations. The growth rates almost increase linearly with temperature between 335C and 380C and approach a maximum growth rate of 41am/hr. In this temperature range, the growth rates are controlled by the reaction kinetics and the reaction activation energy is above 21.3Kcal/mole by calculating the Arrhenius equation. About 380C, the growth rates fall off gradually. The decrease of growth rate at higher temperature in this work is attributed to the increased tendency of gas phase dissociation of metal alkyls15.
To examine whether the growth of a CdTe epilayer is reached to the mass transport limit or not, the test would be done by keeping the DETe/DMCd concentration ratio at a constant value of 1.0 and the growth temperature at 380C. The result is shown in Fig. 4. Initially, the growth rate increases linearly with the total concentration of metal alkyls. Above 2.86 x 10-3mole% concentration, the growth rate decreases gradually. This indicates that the total concentration of metal alkyls reaches that of transport-limited condition. Because the films tend to degrade at a total concentration of 3.42 x 10-3mole%, we therefore selected the value of 2.86 x 10-3mole% as the total concentration of metal alkyls in this experiment. To find out the effect of DETe/DMCd ratio versus growth rate, a series of experiments were carried out at 380C. Here, we modulated the DETe concentration by fixing DMCd concentration and the result is shown in Fig. 5. Basically, the maximum growth rate occurs at the ratio of 1.5 2.0. Below the ratio of 1.5, the growth rate decreases. It is evident that DETe is the rate-controlling species during CdTe epitaxial growth. Above the ratio of 2.0, the growth rate also decreases. The reason is that the decomposition efficiency of the excess DETe alone is small at 380C when the gas phase molecules are dominated by DETe16. Besides, there are the least defects on the top of pyramids in the range of 1.5 2.0.

X-ray diffraction (XRD) analysis
To examine the orientation and crystallization of CdTe epilayers, XRD analysis is used and diffraction patterns are shown in Fig. 6. Three diffraction peaks observed at 20 27.65 , 56.95 and 66.1 under three temperature (350C, 365C, and 380C) belong to (111)-GaAs, (400)-CdTe, and (400)-GaAs, respectively. It means that the CdTe epilayers are single crystal in (100) orientation. For the three deposition temperatures examined, no correlation between film orientation and growth temperature is evident. Similarly, there is no apparent relation between growth rate and film orientation when the growth rate varies from 2.6pm/hr to 4.0pm/hr.

Optical properties
The optical properties of CdTe epilayers grown at different temperatures with equal amount of DETe and DMCd are characterized by a 5145/k argon laser with output power of 25mW under 12K. Fig. 7 shows the PL spectra of CdTe layers grown on GaAs substrates at different temperatures. A relative sharp peak at about 7820/k due to the bound exciton emission 17 and a broad peak at long wavelength of 18 8500/ due to the so-called defect luminescence are observed. They also can be seen from the ratio of exciton intensity (Iex c 1.583eV) to the defect intensity (Idel 1.463eV) shown in Fig. 8 which indicates that the best growth temperature appears at 380C. Simultaneously, Fig. 9 shows the FWHM of dominated peak at different temperatures and the minimum value of 9.38eV is obtained at 380C.

DCRC analysis
In this section, a DCRC analysis is used with a scan speed of 0.2mm/sec to identify the CdTe crystal quality. Fig. 10 shows the FWHM of the (400)-CdTe bragg reflex from (100)-CdTe/(100)-GaAs as a function of the CdTe thickness. A decrease of FWHM with increasing layer thickness is observed. Since the width of the X-ray rocking curve is sensitive to the extended crystal defects (e.g., dislocation lines) in the epilayer, thus, the data in Fig. 10 indicate a decreased density of extended defects with increasing epilayer thickness. Transmission electron microscopy (TEM) investigation of (100)-CdTe/(100)-GaAs has shown a region (--0.1pm) of very high dislocation density close to the CdTe/GaAs interface forming a regular dislocation network and a decreasing dislocation density with increasing distance from the interface. Fig. 11 shows the relation of FWHM with different DETe/ DMCd concentration ratios in 380C. It is found that the minimum value is up to 80 arc sec when the concentration ratio is 1.7.
For different positions in 10mm 10mm CdTe epilayers, a rocking curve analysis is tested and the result is shown in Fig. 12. We find that the FWHMs of the rocking curves are changed from 80 to 130 arc sec when the tested positions are moved from the center to the outside GaAs substrate. It may have resulted from the nonuniform distribution MO gas through the surface of the GaAs substrate. By adding the flow of hydrogen, it will make the MO distribution much more flatter.

CONCLUSIONS
High quality (100)-CdTe epilayers have been successfully grown on (100)-GaAs by the MOCVD method with DETe and DMCd as source materials. The activation growth energy in the reaction-limited growth region is 21.3Kcal/mole and the DETe is the key element in controlling growth rate. Basically, the growth temperature controls the volume of pyramids by supplying enough surface kinetics. The modulation of DETe/DMCd concentration ratio also can decrease the density of defects. From the low temperature PL measurement, it is found that 380C is the most suitable temperature for CdTe growth. Besides, the DCRC experimental results also show that the FWHM of CdTe films even at higher growth rate (above 5.1lam/hr) is low to 80 arc sec, which is the best value ever reported.