234 Torvik, Leksono, Pankove, Heinlein, Grepstad, and Magee 234 Special Issue Paper Journal of Electronic Materials, Vol. 28, No. 3, 1999 INTRODUCTION GaN and SiC are wide band gap semiconductors which hold excellent promise for high temperature electronic devices. 1,2 GaN/SiC n-p heterojunctions (HJ) have been used as the emitter-base junction in high temperature transistors. 3 These transistors had an extraordinary high current gain at room temperature of up to 10 7 and were able to operate at 535°C with a current gain of 100. However, in more recent work this performance could not easily be reproduced. Extensive electrical and optical characterization of these devices led to the discovery of a new parasitic deep level in p-type SiC that appeared in the current- voltage characteristics of the GaN/SiC n-p heterojunctions. 4,5 In this article, we report on several experiments aimed to study the heterojunction char- acteristics. We investigate: 1) the SiC substrate sur- face preparation conditions, 2) SiC nitridation; the effect of flowing NH 3 above 1000°C on the SiC, and 3) the SiC conductivity type and carrier concentration. The three topics will be addressed separately in the following subsections. EXPERIMENTAL PROCEDURE The GaN epitaxial layers used in this study were grown by metalorganic chemical vapor deposition (MOCVD) on commercial 6H-SiC substrates. 6 The growth temperatures ranged from 850 to 1050°C and the GaN was unintentionally doped (n~1–5 × 10 18 cm –3 ). Low-temperature amorphous GaN or AlN nucle- ation (buffer) layers 7 were not used to avoid minority carrier recombination in this amorphous layer. Near- band edge and weaker broad defect-related yellow photoluminescence was observed from the GaN. Di- ode mesa structures (50 × 50 μm 2 –300 × 300 μm 2 ) were selectively grown using an SiO 2 mask 8 or defined using reactive ion etching (RIE) with Freon 12 (CCl 2 F 2 ). RIE was performed at 50 mTorr with a 20 sccm gas flow rate at a power density of 0.42 W/cm 2 (300 W in our system). The films were then cleaned with organic solvents, dipped in buffered oxide etch (H 2 O:HF, 6:1), and rinsed in H 2 O immediately prior to being loaded into an e-beam evaporator. Metal contacts were formed (Received July 30, 1998; accepted November 6, 1998) Interfacial Effects during GaN Growth on 6H-SiC J.T. TORVIK, 1,2 M.W. LEKSONO, 1 J.I. PANKOVE, 1 C. HEINLEIN, 3 J.K. GREPSTAD, 3 and C. MAGEE 4 1.—Astralux Inc., 2500 Central Ave., Boulder, CO 80301. 2.—e-mail: jtt@indra.com. 3.—NTNU, Department of Physical Electronics, Trondheim, N-7034, Norway. 4.—Evans East, 666 Plainsboro Road, Suite 1236, Plainsboro, NJ 08536 GaN growth on 6H-SiC was investigated for heterojunction device applications. Dopant diffusion and surface reactions were discovered at the GaN/SiC heterojunction. A systematic study was therefore conducted focusing on: 1) SiC substrate preparation, 2) SiC nitridation; the effect of flowing ammonia (NH 3 ) at 1050°C on the SiC, and 3) the conductivity type and carrier concentration of the SiC substrate. Atomic force microscopy measurements revealed that the SiC substrates became smoother after the nitridation process possibly due to nitrogen chemisorption and etching. Current-voltage and capacitance-voltage measurements on Cr-Schottky diodes made on SiC revealed evidence for an increased potential barrier in the nitrided samples that can be explained by an interfacial monolayer of SiN x . Furthermore, we compared GaN/SiC heterojunction n-n and n-p diodes made from direct and selective GaN growth. Capacitance- voltage measurements on GaN/SiC n-p heterojunctions indicate that the effec- tive doping in the junction increases as the growth temperature increases. Secondary ion mass spectrometry measurements exposed a tail of Al in the GaN due to acceptor out-diffusion from the p-SiC. Key words: GaN/SiC heterojunction, nitridation