Growth Character Study Of InAsSb Mid-infrared Materials By MOCVD

The ternary antimonide alloys InAs1-xSbx could have the smallest bandgap among allⅢ-Ⅴmaterials with the value between 0.1~ 0.36e V by adjusting its solide composition x. The narrow band-gap InAsSb ternary alloys have received considerable interest because of its potential applications to optoelectronic devices such as lasers, light emitting diodes, detectors etc, in the wavelength ranges of 3~5μm and 8~14μm, where the atmospheric absorption is minimum. Photodetectors operation between 8~14μm wavelength range are of great importance for applications in infrared (IR) thermal imaging. Recently, the experimental results have demonstrated that the cutoff wavelength of InAsSb material can be extended to longer than 12.5μm at 300K. In addition, a very high speed response of the devices can be achieved due to small electron effective mass and consequent high electron mobility of the InAsSb alloys.Metal-Organic Chemical Vapor Deposition (MOCVD), a non-equilibrium technology, has established itself as a unique and important epitaxial crystal growth technology. Many studies about growth characters of InAsSb thin films deposited by MOCVD technique have been done now, but mostly in the conditions of atmospheric pressure and high temperature(more than 520℃). For the low pressure and low temperature MOCVD system, there are some differences. Compared with atmospheric pressure MOCVD, Low- pressure MOCVD system has many more superiority performances. In our work, we have studied the growth characters of mid-infrared InAsSb materials by LP-MOCVD system. Firstly, the InAsSb materials achieved, the technology to growth InAsSb materials by MOCVD was mastered; and then, the characters of InAsSb epitaxial layer samples were measured with several material assessment techniques. The surface morphology of the sample was observed by optical microscopy (×1000). The crystal quality and structure characters of the samples were examined by X-ray diffraction. X-ray is well established and commonly used as a first-line non-destructive analytical tool to acquire much valuable information such as its quality, lattice parameter, and composition of a crystal. In this study, we also used the lattice parameter got from X-ray diffraction to calculate the lattice mismatch between epitaxy layers and substrates. The electrical characters of the samples were obtained by Van der Pauw Hall measurements. Combining the conclusions of every testing and the growth parameters, we have made a detail discussion of the effect of growth parameters to the characters of InAsSb materials, and got the conclusions as follows:(1). There are several significant differences between the growth techniques used for the MOCVD of standard nitrides, arsenides or phosphides and the techniques needed for the high quality antimonides. These differences are primarily determined by differences in the fundamental properties of the antimonides compared to the more extensively examined nitrides, arsenides or phosphides. There are four major fundamental differences as follows: (1). the low vapor pressure of antimony over the growing surface; (2). the lack of stable groupⅤhydride; (3). the kinetically controlled nature of growth; (4). lack of an insulating antimonide substrate. These fundamental properties introduce many difficulties to MOCVD of antimonides.(2). We have made a detail discussion about the influence of growth parameters such as growth temperature, vaporⅤ/Ⅲratio in reactor and lattice mismatch on the crystal quality of InAsSb epitaxial layer prepared by MOCVD. Firstly, the GaAs substrates and the InAsSb alloys are big mismatched (14.6%~7.15%), which has serious influence on the quality of epitaxial layers; Secondly, under the typical growth conditions the MOCVD growth process of InAsSb epitaxial layer is a"kinetically controlled regime"for the special properties of the antimonides, which makes the growth process very difficult to contral. Both growth temperature and vaporⅤ/Ⅲratio are key growth parameters because they influence greatly the crystal quality of the epitaxial layers. The optimized growth temperature and vaporⅤ/Ⅲratio range both are limited by two factors and consequently narrow. By our discussion, optimized growth temperature range is 450-520℃and vaporⅤ/Ⅲratio range is 1.0-2.0, respectively.(3). The composition of InAs1-xSbx epitaxial layer is discussed in detail. Firstly, because the bandgap of InAs1-xSbx alloy is a particularly sensitive function of composition it is necessary to be able to predict and accurately control the composition during the growth, however, in the case of"kinetically controlled regime"the compositions are sensitive to the kinetic factors, so it is necessary to maintain the stabilization of growth parameters during growth. Secondly, for its special properties, the antimony atoms are difficult to be incorporated into the solid, so it is difficult to achieve epitaxial layer with high antimony content. It is impotant to get high vapor-solid distribution coefficient ratio of antimony (KSb). Both growth temperature and vaporⅤ/Ⅲratio influence the KSb. In low pressure MOCVD system, growth temperature range of 450~520℃and vaporⅤ/Ⅲratio range of 1.0-2.0 could get high KSb ratio. High quality InAs1-xSbx epitaxial sample #0327 with the X-ray diffraction peak FWHM(Full-Width of Half Maximum) 0.294°was achieved by choosing the growth tempreture of 450℃and vaporⅤ/Ⅲratio of 1.43 in low pressure MOCVD system, its solide composition x is 0.215, which is the highest Sb content InAs1-xSbx epitaxial layer sample achieved in our work.(4). The mobility and carrier concentration of InAs1-xSbx epitaxial layer were measured by Van der Pauw Hall measurements. Firstly, the big lattice mismatch between the epitaxial layers and the substrates introduced a high density of misfit dislocations, the scattering due to misfit dislocations reduced mobility significantly, and growth a buffer layer may reduce the effect of misfit dislocations. The highest mobility we achieved is 4755 cm2/V.s, lower than the ratio reported from international, because we have not done anything to reduce the effect of lattice mismatch; Secondly, the undoped InAs1-xSbx epitaxial layers with higher Sb content with higher background carrier concentration indicated more impurities, this can be interpreted as more alkyl antimony compound source lead to more carbon incorporation. The carbon incorporation could be reduced by properly increase the arsine flux.In this work, we have mastered the technology of MOCVD InAs1-xSbx epitaxial layers and some material testing methods, and achieved the optimized growth parameters. Our major work in the future is the study of MOCVD of GaInAsSb quaternary alloys and the fabrication of the thermophotovoltaic devices based on the alloys. The growth techniques used for GaInAsSb quaternary alloys are very similar to InAsSb ternary alloys, so the study of InAsSb ternary alloys today established the foundation for our future work.