High-Resolution Transmission Electron Microscopy: An Essential Characterization Technique for Optimization of Semiconduc
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HIGH-RESOLUTION TRANSMISSION ELECTRON MICROSCOPY: AN ESSENTIAL CHARACTERIZATION TECHNIQUE FOR OPTIMIZATION OF SEMICONDUCTOR EPITAXY AND INTERFACES J.B. POSTHILL, G.G. FOUNTAIN, R.A. RUDDER, S.V. HATTANGADY, G.S. SOLOMON, M.L. TIMMONS and R.J. MARKUNAS Research Triangle Institute, Research Triangle Park, North Carolina 27709-2194 ABSTRACT High-resolution transmission electron microscopy has been utilized to critically assess semiconductor-based epitaxial films and interfaces. These results then provide a sound, scientific basis for subsequent process modification. Materials optimization is therefore achieved more efficiently. INTRODUCTION An appropriate ex situ method of examining the interfaces resulting from the formation of semiconductor heterostructures via novel processes is high-resolution transmission electron microscopy (HRTEM). Heteroepitaxial layers of Ge-on-GaAs [1,2,3], Ge-on-Si [4], and ZnGeAs 2-on-GaAs [5] have been most recently investigated in our laboratory. Representative results from these heteroepitaxial systems are presented that emphasize the utility of HRTEM in the context of electronic materials research and development. Also, results that pertain to Ge- and GaAs-based samples with a thin Si interlayer ('-• 15A) grown prior to Si0 2 deposition are presented [6,7]. EXPERIMENTAL PROCEDURES The details concerning the fabrication of the materials discussed herein can be found in the appropriate references. Cross-section transmission electron microscopy (TEM) samples were prepared in the usual manner [8]. It was found to be necessary to hold GaAs-based samples at reduced temperature during final Ar+ ion milling (4-6kV) using a liquid nitrogen cooled stage to minimize the deleterious effects of ion bombardment [9]. This procedure was also employed for Ge-based samples. Once a suitably thin region was identified, the sample was tilted to a -type zone axis in order to view the interface edge on. An appropriate through-focal photographic series was taken at 200kV, Cs1l.2mm, and a=lmRad. Ge-on-GaAs HETEROEPITAXY The motivation for this materials study is to investigate and optimize Ge-on-GaAs heteroepitaxy for application in a high mobility transistor (HMT) structure, where the active channel is in the form of a 2-D electron (HEMT) or hole (HIHMT) gas in the lowband gap Ge. Enhanced carrier mobility can only be achieved when the concentration of microstructural defects is minimized. Furthermore, carrier scattering can also occur at impurity atom sites (independent of device operating temperature); meaning that a low temperature epitaxial growth process is required to minimize the diffusion of As or Ga into the Ge epitaxial layer. Remote plasma-enhanced chemical vapor deposition (RPECVD) has been used to grow Ge on GaAs (111) wafers over the 250'C to 400'C temperature range [1,2,3]. The use of RPECVD for Ge epitaxial growth is described
Mat.Res. Soc. Symp. Proc. Vol.139. 91989 Materials Research Society
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elsewhere [10]. Figure 1 shows HRTEM results from this series of growth runs. The heteroepita
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