Anion Exchange Reactions and initial GaN Epitaxial Layer Formation Under Nitrogen Plasma Exposure of a GaAs Surface
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ABSTRACT Initial nitridation of molecular beam epitaxially (MBE) grown GaAs surfaces by means of an electron cyclotron resonance (ECR) microwave plasma source is investigated in situ through time-resolved reflection high energy electron diffraction (RHEED), and ex situ high-resolution X-ray diffraction (HRXRD) techniques. Brief (< 8-10 s) plasma exposure of GaAs (100) surfaces results in surface N-for-As anion exchange and a new, specular, commensurate (3x3) RHEED pattern which appears to correspond to up to 1 monolayer of coherently strained GaN on GaAs. Anion exchange kinetics is studied through the time-dependence of the onset and decay of the (3x3) RHEED pattern as a function of substrate temperature. For the first time, coherently strained GaNyAsl-y /GaAs heterostructures are grown and characterized. Direct evidence for thermally activated processes from both RHEED and HRXRD observations is presented, and N desorption and surface-segregation phenomena are proposed to explain the experimental results.
INTRODUCTION GaN-based wide-gap II-V refractory semiconductors have come under intensely renewed interest as a prospective short-wavelength optoelectronic device material. Several reviews of GaN-based materials have been given recently, 1-3 and a number of successful demonstrations of GaN-based blue-green 4 and blue 5 light-emitting diode (LED) devices have been reported. Owing to the considerable technological demand for blue-to-UV semiconductor LEDs and lasers, most research on GaN-based wide-gap semiconductors has emphasized light-emitting device structures. In contrast, relatively few reports have concentrated on fundamental investigation of the epitaxial growth process itself. In particular, for the case of electron-cyclotron-resonance microwave plasma assisted molecular beam epitaxial (ECR-MBE) growth of GaN/GaAs structures, the exchange of N and As atoms ("anion exchange") plays a crucial role in the initial stages of epitaxy, but, nonetheless, has received little attention. In this paper, we investigate anion exchange processes at an initial GaAs (100) surface upon nitrogen plasma (N*) exposure in an ECR-MBE system. Anion-exchange processes are monitored in situ through time-resolved reflection high energy electron diffraction (RHEED) techniques. Brief N* exposure is shown to result in the formation of a comparatively stable, coherently strained nitrided surface layer on GaAs, due to N-for-As surface exchange. This layer exhibits a previously unreported (3x3) surface reconstruction, and can be overgrown commensurately with GaAs. In this way, coherently strained GaNAs/GaAs strained-layer superlattices are fabricated for the first time, and characterized through high resolution X-ray diffraction (HRXRD). Finally, two important but previously unreported thermally activated 259
Mat. Res. Soc. Symp. Proc. Vol. 388 ©1995 Materials Research Society
processes, nitrogen desorption and surface segregation, which can be expected to have direct significance to the initial stages of GaN epitaxial growth on GaAs, ar
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