Growth and Fabrication of High Reverse Breakdown Heterojunction n-Gan: p-6H-Sic Diodes
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GROWTH AND FABRICATION OF HIGH REVERSE BREAKDOWN HETEROJUNCTION n-GAN: p- 6H-SIC DIODES A.V. Sampath, A. Bhattacharyya, R. Singh, C.R. Eddy, P. Lamarre*, W.F. Stacey*, R.S. Morris*, T.D. Moustakas ECE Department, Boston University, 8. St. Mary’s Street, Boston MA, 02215 *Viatronix Inc., 40 Amherst Avenue, Waltham, MA 02451 ABSTRACT Wide band gap semiconductors are attractive for developing high power switching devices because of their ability to operate at both higher temperatures as well as higher frequencies than conventional Si. In this paper we report on the growth and fabrication of GaN/SiC np heterojunction diodes by depositing Si doped n-GaN films by plasma-assisted molecular beam epitaxy directly on SiC without the use of GaN or AlN buffers. Careful ex-situ and in-situ preparation of the Si terminated 6H- SiC surface was necessary to produce high quality diodes. Vertical circular diodes were fabricated with sizes varying from 200 microns to 1mm in diameter. Mesas were formed by ICP etching of the MBE deposited n- GaN layer using Cl2. A Ti/Al/Ni/Au metal stack was employed as an n-ohmic contact to the GaN layer and an Al/Ti/Au metal stack was employed as a backside p-ohmic contact to the 6H- SiC layer. The diodes were characterized by I-V and C-V measurements. The 1 mm diameter diodes exhibited almost ideal behavior under forward bias with an ideality factor of 1.6, and a reverse saturation current of 10-19 A/cm2. Under reverse bias, these devices were driven up to 1000 V with a measured leakage current of 5x 10-7 A. and a dynamic resistance varying from 1010 to 109 ohms with increasing reverse bias. The built-in potential in these n-p heterojunctions was determined from C-V measurements to be 2.25 V. From these values we determined that the heterojunction is of Type II with conduction and valence bands offsets calculated to be 0.65 and 1.1 eV respectively. INTRODUCTION The III-Nitrides are an attractive family of semiconductors for the development of high power electronic devices due to the high breakdown fields and large saturation velocities found in these materials. However, these films are generally deposited on sapphire or 6H-SiC substrates due to the lack of a readily available and large area GaN substrates. 6H SiC substrates are advantageous for the growth of III-Nitrides power electronic devices due to their small lattice mismatch with GaN, ~ 4%, and high thermal and electrical conductivity. As a result, these devices can be fabricated as vertical diodes in contrast to the case of a GaN homojunction deposited on c-plane sapphire. Torkiv et al have reported on GaN/SiC n-p heterojunctions fabricated by depositing n-GaN by molecular beam epitaxy (MBE) directly on 6H-SiC substrates [1]. The authors demonstrated 100x100 mm area diodes with nearly ideal forward IV characteristics, h = 1.2, and low reverse saturation current, Isat = 10-32 A/cm2 but high reverse leakage currents ~5x10-8 A/cm2 at –10V. Danielsson et al reported on similar diodes fabricated by depositing n-GaN by hydride vapor phase e
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