On Compensation and Impurities in State-of-the-Art GaN Epilayers Grown on Sapphire
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D.S.
SIMONSt and P.H. CHIt * Naval Research Laboratory, Laboratory for Advanced Material Synthesis, Washington, D.C. 20375 ** Currently at Hewlett-Packard, San Jose, CA 95131 t National Institute of Standards and Technology, Gaithersburg, MD 20899
ABSTRACT A comparison between 300 K electron transport data for state-of-the-art wurtzite GaN grown on sapphire substrates and corresponding theoretical calculations shows a large difference, with experimental mobility less than the predicted mobility for a given carrier concentration. The comparison seems to imply that GaN films are greatly compensated, but the discrepancy may also be due to the poorly known values of the materials parameters used in the calculations. In this work, recent analysis of transport and SIMS measurements on silicon-doped GaN films are shown to imply that the compensation, NA/ND, is less than 0.3. In addition, the 3 16 determination of an activation energy of 34 meV in a GaN film doped to a level of 6X10 cm16 6X10 of between a level at suggests either that a second, native donor exists in the doped films cm- 3 and 1X10 17 cm- 3 , or that the activation energy of Si in GaN is dependent on the concentration, being influenced by impurity banding or some other physical effect. GaN films grown without silicon doping are highly resistive. INTRODUCTION Significant advances have recently been reported on the performance of devices based on the 111-N material system for both opto-electronic as well as high power and high temperature electronic applications. For the fabrication of high frequency field effect transistors (FET's),
thin, high mobility active layers, typically grown on highly resistive buffer layers, are required.
Optimal device performance is dependent upon optimization of the transport properties in the active layer, hence a comparison of active layer transport properties to thick film state-of-the-art GaN properties is desired. Previously, we have reported the growth of high mobility silicondoped FET structures on highly resistive GaN buffer layers 1 . In the present work the properties of lightly Si-doped and unintentionally doped thick (- 3 gim) GaN layers are investigated as a preliminary step toward achieving the stated objective. EXPERIMENT An inductively heated, water-cooled, vertical organometallic vapor phase epitaxy reactor operated at 57 torr (7,600 Pa) was used for the growth of the GaN films as previously described in the literature 2 . The films discussed in this paper were grown on a-plane (1120) sapphire substrates. An AIN nucleation layer of approximately 20 nm thickness was deposited at 450 'C, using triethylaluminum and ammonia (NH3) as the source reagents. The GaN films were grown using trimethylgallium (TMG) and NH3 at a growth temperature of 1040 'C. Disilane, in a concentration of 8 ppm in H2, was used as the dopant source. Variable temperature Hall measurements were performed using the van der Pauw technique with a cloverleaf geometry and indium contacts at a magnetic field of 0.2 T. The current-voltage characte
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