Hydrostatic and uniaxial stress dependence and photo induced effects on the channel conductance of n-AlGaN/GaN heterostr
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aDepartment of Electrical and Computer Engineering, University of Minnesota, Minneapolis, MN 55455, [email protected], [email protected] bRockwell International Science Center, Thousand Oaks, CA 91358 CSVT Associates, Eden Prairie, MN 55344 dEpitronics/ATMI, Phoenix, Arizona 85027
eAPA Optics, Blaine, Minnesota 55449
fSchool of Electrical Engineering, Cornell University, Ithaca, NY 14853 Abstract We measure the hydrostatic stress, uniaxial stress, and photo induced dependence of the channel conductance of two-dimensional electron gas AIGaN/GaN heterostructures grown on c-axis sapphire. The structures examined are grown by nitrogen-plasma molecular beam epitaxy and metal organic chemical vapor deposition. Electrical conductance measurements are made with four point probes on Hall bar samples. Both, hydrostatic stress and uniaxial stress result in changes in the conductance. Moreover, these changes in conductance have long settling times after the stress is applied and may be due to deep level defects, the energy levels of which change with stress. Stress coefficients extracted from the samples are partially attributed to deep level defects and to the piezoelectric effect resulting from different piezoelectric coefficients of GaN and AIN. Photo induced changes of the two-dimensional electron gas are also observed. We find that pulsed illumination produces long transient times in the conductance. These transients are reduced by thermal heating in some samples. However, they can still be present at 153TC.
Introduction Stress experiments are useful in the characterization of semiconductors. They can be used to determine piezoelectric constants and they can reveal the presence of deep defects having energy levels that change with pressure. 2 Recent efforts have utilized applied static stress/strain to 0iezoelectrically modify the two-dimensional (2D) electron concentrations at AIGaAs/GaAs and AlGaN/GaN heterostructure interfaces. The changes in channel conductance can be related to differences in the piezoelectric constants and to differences in the stress/strain profiles of the materials that constitute the heterostructures. Pressure has also been2 used to quantify the amount of energy shift of deep levels through the changes in conductance and photoluminesce. 5 By the application of piezoelectric effects, and through the understanding of the behavior of defects on the properties of semiconductors, novel and improved device structures can be implemented.6' 7 In this study we report on hydrostatic and uniaxial stress effects in the 2D electron gas channel conductance of AlGaN/GaN heterostructures. We find that the channel conductance exhibits a slow (on the order of 102 to 103 seconds) change with the application of pressure. This slow transient follows a very fast change that occurs on the time scale of the application or removal of pressure (a few seconds). We also examine the photo response of the samples in attempts to gain more information about the cause of these long pressure induced transients. 495 Mat. Res.
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