High Performance HFET Devices on Sapphire and SiC: Passivation with AlN
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High Performance HFET Devices on Sapphire and SiC: Passivation with AlN Jennifer A. Bardwell, Ying Liu, James B. Webb, Haipeng Tang, Stephen J. Rolfe and Jean Lapointe National Research Council Canada, Institute for Microstructural Sciences, Ottawa, ON, K1A 0R6, Canada ABSTRACT AlGaN/GaN two dimensional electron gas (2DEG) heterostructures were grown by ammonia-MBE on sapphire and SiC substrates. Devices fabricated from these optimized HFET layers, with optically defined gates showed excellent characteristics, e.g. a maximum drain current density of 1.3 A/mm, maximum transconductance of 220 mS/mm, fT of 15.6 GHz and fMAX of 58.1 GHz was measured for devices with 0.9 µm gate length and 40 µm gate width. Shorter gate length devices exhibited higher frequency responses: fT of 68 GHz and fMAX of 125 GHz for 0.25 µm gate length and fT of 103 GHz and fMAX of 170 GHz for 0.15 µm gate length. However, these devices showed “current collapse” when subjected to load pull measurements. Current collapse was also observed in sequentially repeated DC measurements in the dark, both on sapphire and SiC substrates, although the degree of collapse varied greatly from one wafer to another. One method of reducing the current collapse was to apply a thin (100 - 6000 Å) magnetron sputtered AlN passivation layer (over the gates) or a 500 Å layer under the gates so that MISFET devices were obtained. The electrical characteristics of the passivated and unpassivated devices are discussed. INTRODUCTION GaN/AlGaN HFETs are of great interest for high power, high frequency devices. The mobility and carrier concentration obtained for these structures has improved markedly over the past few years, particularly on substrates such as SiC which offer higher thermal dissipation characteristics and smaller lattice mismatch compared to that of sapphire. HFET structures with electrical performance characteristics comparable to or better than those grown on sapphire substrates have been obtained [1-5]. In this paper we report the use of ammonia-MBE for the growth of AlGaN/GaN (2DEG) structures on insulating 4H-SiC (0001) and sapphire substrates using a magnetron sputter epitaxy (MSE) deposited AlN buffer layer and carbon doped channel isolation layers [6]. Excellent device performance is routinely obtained; nevertheless, current collapse [5,7,8] remains a problem. The use of room temperature magnetron sputtered AlN passivation layers to alleviate this collapse is discussed. EXPERIMENTAL DETAILS The HFET structures were grown by ammonia-MBE. The MBE system [9] and the SiC wafer preparation prior to deposition have been described previously [10]. No ex- or in-situ high temperature hydrogen treatment of the substrate was used. The molybdenum back coated SiC wafers were degreased in chloroform vapor for 10 min. followed by a 1 min dip in 1:1 HF:H2O and a 10 min. deionized water rinse. The wafers were blown dry in N2 prior to mounting and loading into the MBE load-lock. They were then heated to 1000ºC for 10 min under an NH3
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