Effects of Defects on Metal-Insulator-Semiconductor Properties of HgCdTe Films Grown by Liquid Phase Epitaxy
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EFFECTS OF DEFECTS ON METAL-INSULATOR-SEMICONDUCTOR PROPERTIES OF HgCdTe FILMS GROWN BY LIQUID PHASE EPITAXY Dipankar Chandra* and Michael W. Goodwin* *Infrared Devices Laboratory, Texas Instruments, Incorporated; Dallas, Texas
ABSTRACT A comprehensive study of all materials parameters influencing metal-insulatorsemiconductor (MIS) properties of n-type HgCdTe films grown by liquid phase epitaxy from tellurium rich melts was conducted. When the epitaxy process was optimized to grow films free of inclusions and terracing, the first indications of the MIS properties to be expected could be obtained from the temperature dependence of the Hall electron mobility. Films displaying an anomalous dependence of the Hall mobility on temperature yielded non-classical low frequency MIS properties with little or no measurable 'dark' storage times (< 2x10-6 second). The MIS performance of these films appeared relatively independent of other materials parameters; remaining, for example, virtually unaffected by the film dislocation density changing between 5x 104 and 2 x 106/cm 2 . Films displaying a classical dependence of the Hall mobility on temperature yielded drastically improved classical high frequency MIS properties. For these films, the MIS performance appeared almost exclusively dependent on dislocation density levels as long as the donor density remained lower than 1x10 15/cm 3. The dark storage time of classical films increased continuously with decreasing dislocation density levels, rising to lO1,sec for a dislocation density level of 1 X105 /cm 2 for materials with a 77°K cutoff wavelength of 10.5 pm. A simple monotonic relationship could be established between the MIS performance parameters and the dislocation density over the entire measurement span: from 5x10 4 to 2.5x10 6/cm 2 . INTRODUCTION An extensive series of investigations has been in progress at Texas Instruments to grow epitaxial thin films of mercury cadmium telluride using liquid phase epitaxy. The prime objective is to produce films which will fulfill MIS photocapacitor performance requirements within the LWIR range (8-12pm at 7rK). It is therefore essential to establish the materials obstacles limiting device performance, with particular emphasis placed on prioritizing these factors. The thrust can then be concentrated towards solving the most important limitation. Once this is achieved, it can then be redirected towards a solution of the next most important problem and so on.
MATERIALS ISSUES The only information available from device physics of relevance to materials growth scientists has been the desired upper limits in carrier concentrations for ntype and p-type materials respectively [1]. To this could be added the data base Mat. Res. Soc. Symp. Proc. Vol. 161. 01990 Materials Research Society
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collected from bulk materials establishing the importance of dislocation density levels. Prior to the initiation of the present investigations, net donor densities in films grown by liquid phase epitaxy were comparable to the magnitudes observed in b
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