Plasma Hydrogenation Studies on Low-Temperature MBE-Grown GaAs
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PLASMA HYDROGENATION STUDIES ON LOW-TEMPERATURE MBE-GROWN GaAs 0. S. Nakagawa*, S. Ashok", K. Zhang*, D. L. Miller*, and W. K.
Chung***
Pennsylvania State University, Dept. of Electrical Engineering, University Park, PA 16802 ** Pennsylvania State University, Dept. of Engineering Science and Mechanics, University Park, PA 16802 *** GaSonics/IPC, 31172 Huntwood Ave., Hayward, CA 94544 *
ABSTRACT The effect of hydrogen plasma on the electrical properties of lowtemperature-grown GaAs (LT-GaAs) is reported. LT-GaAs epitaxial layers, 2um thick were grown on a , semi-insulating GaAs (SI GaAs) substrate using molecular beam epitaxy (MBE) at 300'C. Some of the samples were annealed in- situ at 600'C for 1 hour. These LT-GaAs samples were then subjected to hydrogen plasma at 300'C in a parallel-plate, low-frequency (30KHz) system for varying times up to a maximum of 13 hours. Current through the LT-GaAs layers as a function of voltage (I-V), temperature (I-T), and time (I-t) was measured between two surface contacts formed by thermal evaporation of Au. Evidence of partical defect passivation found in the electrical characteristics of hydrogenated LT GaAs is presented. These include reduced current at low temperature (320K). I. Introduction A wide variety of semiconductors exhibit altered electrical characteristics upon exposure to hydrogen plasma created by ac-discharge. In some cases this semiconductor modification scheme has a favorable impact on the performance of a device and is used routinely in its fabrication [1]. In other cases it is an effective tool to probe defects in semiconductors due to the interaction of atomic hydrogen with such defects [2-5]. A central idea of hydrogenation in semiconductors is defect passivation, either intrinsic (dangling bonds, grain boundaries, etc.) or extrinsic (dopants, ionimplantation / reactive ion etching damage, etc.), and there have been numerous reports on hydrogenation of materials such as amorphous Si, polySi, crystalline Si, GaAs, InP. However, hydrogenation of low-temperaturemolecular-beam-epitaxial grown GaAs (LT MBE GaAs), a material of recent vintage, has never been reported. LT MBE GaAs exhibits [6], among other anomalous characteristics, high defect density, whose mechanistic origins are a focus of debate. Two distinct conduction mechanisms have been proposed, the defect-band hopping model [7] and the buried Schottky model [8], but Mat. Res. Soc. Symp. Proc. Vol. 262. (1992 Materials Research Society
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there is yet no consensus on the transport properties of the material. We have investigated this defect-driven material through the interaction of hydrogen with its defects. H. Experimental Setup The starting material for our samples was an (100)-oriented semiinsulating (SI) GaAs wafer. A 2um-thick LT-GaAs layer was grown on the substrate surface by MBE at a below-normal substrate temperature of 300°C. There was no intentional doping. The arsenic-to-gallium incorporation ratio was 1.7, and the growth rate was 0.9um/hr. This asgrown sample (Sample A) then received
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