Si Delta Doped GaN Grown by Low-Pressure Metalorganic Chemical Vapor Deposition
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INTRODUCTION In the past decade, though many inherent problems originating from the heteroepitaxial growth in wide-bandgap III-V nitride materials exist, there have been considerable developments that have led to the remarkable demonstrations of high-brightness blue light emitting diodes [1,2], laser diodes [3], and high-frequency field-effect transistors (FETs) [4,5]. In particular, highfrequency FETs using GaN/AIGaN heterostructures have been realized using uniformly Si doped layers. In Ill-V compound semiconductors such as GaAs, FETs with characteristics superior to those of conventionally doped transistors have been fabricated using the delta-doping technique [6], which can achieve very narrow doping distribution along the epitaxial growth direction and up to now has been studied in a variety of materials and devices [7-9]. It was recently demonstrated that the inclusion of delta doping could improve laser stability in the face of temperature variations, in comparison with conventional quantum well lasers [10]. Even though the growth temperature of GaN in metalorganic chemical vapor deposition (MOCVD) is much higher than that of conventional Ill-V compound semiconductors, it is considered to be important to investigate delta-doping in III-V nitride materials for highperformance device applications. To our knowledge, up to now there are no reports about the application of delta-doping on Ill-V nitrides. In this letter, we demonstrate Si delta doping in GaN grown by low-pressure MOCVD and investigate the effects of delta-doping time and post-deltadoping purge step on carrier concentration.
G 3.49 Mat. Res. Soc. Symp. Proc. Vol. 537 © 1999 Materials Research Society
EXPERIMENTAL The samples used in this study were grown in a vertical rotating MOCVD reactor operating at 200 Torr. Trimethylgallium (TMG), ammonia (NH 3), and 100 ppm silane (SiH 4) in hydrogen were used as Ga, N, and Si delta-dopant precursors, respectively. Substrates used in this experiment were c-plane sapphires, which were degreased in an organic solvent and then slightly etched in a hot solution of 3H2SO 4 : H3PO 4 for 10 min. After the thermal cleaning in H2 environment at 1070 °C for 10 min, a 20-nm-thick GaN buffer layer was deposited at 520 °C. Finally, the substrate temperature was raised to 1040 °C to grow the GaN overlayer. The V/I1 ratio for the GaN overlayer was 1560. Under these conditions, background doping was 8 x 1016 cm3 n-type. The growth procedure for Si delta-doped GaN was as follows. First, an undoped GaN layer (1.7 ýtm) was grown, after which TMG supply was stopped for 10 s on top of the GaN layer, while at that time ammonia was kept flowing. At this pre-delta-doping purge step, the surface became N-rich. This was followed by Si deposition for several tens of seconds under a Ga-free condition. The SiH 4 flow rate was 2-4 nmol/min. Then, we deposited an undoped GaN cap layer by two different methods: (1) post-delta-doping purge step for 10 s with flowing ammonia followed by the growth of a cap layer; and (2) direct growth of
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