High Resolution and Analytical Electron Microscopy of Multilayer Heteroepitaxial Semiconductors
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HIGH RESOLUTION AND ANALYTICAL ELECTRON MICROSCOPY OF MULTILAYER HETEROEPITAXIAL SEMICONDUCTORS H.-J. KLEEBE*, W.J. HAMILTON**, W.L. AHLGREN**, S.M. JOHNSON**, and M. RUJHLE*,+ *UCSB, Materials Dept., Eng. III, Santa Barbara, CA; **Santa Barbara Research Center, Goleta, CA. +Now at MPI ftir Metallforschung, Stuttgart, FRG. ABSTRACT Thin-film epitaxial layers on silicon substrates are being actively developed as an alternative to bulk substrates for infrared detectors. Success in this development is expected to yield improvements in size, strength, monolithic signal processing, and cost of the detector arrays. To provide feedback to the thin-film growth process, this study has investigated the structural and chemical properties of these films. CdTe or CdZnTe was grown on GaAs on Si wafers by metalorganic chemical vapor deposition (MOCVD) followed by the growth of HgCdTe by liquid-phase epitaxy (LPE). Development of TEM specimen preparation techniques permitted investigation of materials with up to four layers. Cross sectional TEM and AEM investigations resulted in the following observations: Selected area diffraction and CTEM show relations between the layers and structural defects. High resolution imaging of the multilayers reveals that most of the heteroepitaxial misfit is accomodated by misfit dislocations. Many, but not all, stacking faults and dislocations which propagate across the films appear to be related to structural interface defects. The evidence indicates that there is little, if any, contamination at MOCVD and LPE interfaces. Interface defects mainly appear to be extremely minute pits or hillocks. AEM investigations of interface regions have demonstrated a redistribution of material close to the CdZnTe / HgCdTe interface. INTRODUCTION A key issue in the reliability of infrared detectors is their survival under repeated thermal cycling from room temperature to cryogenic operating temperatures. Thermal mismatch between the II-VI detector elements and the silicon chips used for signal readout causes destructive stresses during cycling. In addition, significant cost savings should be realized with the use of silicon wafer automated processing technology. However, growth of single crystal, high-quality HgCdTe materials directly upon silicon has been found to be extremely difficult. Therefore, thin-film buffer layers have been grown on the silicon prior to growth of the active detector layers.1 Single-crystal, thin-film layers of GaAs have been successfully grown on silicon and are now available commercially (Kopin Corporation). The large lattice mismatch between GaAs and HgCdTe (HCT) requires first a growth of a CdTe or CdZnTe (CZT) layer as a lattice-matching buffer layer. These films have been prepared by molecular beam epitaxy (MBE)1, hot-wall epitaxy 2 , evaporation",3, and metalorganic chemical vapor deposition (MOCVD). 4 .5 HgCdTe thin-film layers for IR detectors have 8 7 6 been grown on CdTe or CZT by liquid-phase epitaxy (LPE), MOCVD and MBE. Previously, TEM has been used successfully to character
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