Stability of Multilayered Semiconductor Systems
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STABILITY OF MULTILAYERED SEMICONDUCTOR SYSTEMS Y. KIM and A. OURMAZD AT&T Bell Laboratories, Holmdel, NJ 07733 ABSTRACT We have studied the atomic scale relaxation of multilyered semiconductor systems through interdiffusion, and its implications for layer and hence device stability. Our technique combines chemical lattice imaging and vector pattern recognition to measure interdiffusion coefficients as small as 10-2 cm2/s at single AiGaAs/GaAs interfaces. Our results can be sLnmarized as follows. (a) The interdiffusion coefficient and hence layer stability depend strongly on the distance from the top surface. (b) The strong influence of the surface stems from its role as the source of rative point defects (interstitials, vacancies) involved in the diffusion process. (c) The study ef the initial transients in the diffusion process can directly yield the migration and formation energies of these defects as a function of their charge state. (d) The non-linear nature of the diffusion process can have serious consequences for the low temperature stability of multilayered materials and devices. We also discuss the implications of our results for the design and fabrication of stable multilayered systems. INTRODUCTION The operation of a wide variety of modem semiconductor devices (laser, detectors, heterostructure bipolar transistors) relies on the presence of sharp compositional changes across the interfaces between many pseudomorphically grown layers. Such precipitous compositional discontinuities can only be achieved in systems that are far from equilibrium. The relaxation of such systems during annealing or processing can introduce extended defects, such as dislocation and stacking faults, or chemical intermixing. There have been a number of studies of chemical intermixing using various techniques. However, to understand the atomic scale relaxation in the multilayered systems, we need to investigate the interdiffusion at single interfaces. This is important because our earlier study has shown the interdiffusion to be strongly depth dependent. Also, since we are dealing with interdiffusion at low temperatures (around growth and processing temperatures), we need very high spatial resolution and chemical sensitivity. Here we combine chemical lattice imaging with vector pattern recognition to investigate interdiffusion at single interfaces at the atomic level. Our results shed new light on the atomic processes that govern the stability of multilayered materials and devices. EXPERIMENTAL The PGaAs/AlGaAs system consisted of 20 periods of 50A C-doped GaAs/50A undoped GaAs/50A A1 4Ga As grown at 600C on (100) GaAs. The HgCdTe/CdTe system consisted of 50 periods of 5X thick Hg0 730.30.y7 Cd Te and 100A thick Hg 0 Cdj.!5 Te grown at 180C [1,2]. Both multilayers The carbon doping 19 -'3J were grown by Molecular Beam Epitaxyd]B. level was 10 cm". Bulk samples of the GaAs/AlGaAs were annealed in the temperature range 650C to 750C in an evacuated ampule under As poor conditions, to induce enhanced intermixing in the p-type layers [
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