The effect of carbon impurities on lightly doped MOCVD GaN Schottky diodes
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Puneet Suvarna, Mihir Tungare, Neeraj Tripathi, and Fatemeh (Shadi) Shahedipour-Sandvik College of Nanoscale Science and Engineering, State University of New York at Albany, Albany, New York 12203 (Received 2 August 2011; accepted 3 August 2011)
Schottky diodes have been fabricated on metalorganic chemical vapor deposition GaN epitaxial layers grown on sapphire substrates. Carbon impurities limit the ability of these films to be used in high-power devices. Although its effect can be mitigated by growing the films at higher pressure, higher flow rates, and larger V/III ratios, it still effectively limits the net carrier concentration to ;1016 cm 3 and therefore the breakdown voltage to ;1200 V by acting as a compensating deep acceptor for n-type material. The net carrier concentration is smaller than the carbon concentration indicating that not all of the carbon occupies a nitrogen site acting as a deep acceptor. It is not known whether some of the carbon occupies gallium sites acting as a donor, interstitial sites creating states in the midgap region, and/or is tied up in the large number of dislocations in the films where it is not electrically active. I. INTRODUCTION
Gallium nitride is an attractive material for highpower electronics because it has a large critical field, EC 5 3.5 MV/cm,1 and a reasonably high low-field mobility, l 5 930 cm2/Vs.2 These values compare favorably with those of SiC, where EC 5 2.5 MV/cm (Ref. 3) and l 5 900 cm2/Vs.4 Seemingly small differences can be magnified by the specific on-resistance, RSPON 5 4Vb2/elEC3, where a 40% increase in EC can lead to an almost 3-fold lower RSPON because of the cubic dependence. Vb is the breakdown voltage. The factor elEC3 is an important factor in high-power electronics, and it is often referred to as the Baliga figure of merit (FOM).5 However, this assumes that breakdown will occur by avalanche breakdown in a uniformly doped active layer, usually a low-doped epitaxial film. Because most GaN films are grown heteroepitaxially on SiC or sapphire wafers, they usually contain 109–1010 dislocations/cm.2,6 Some have suggested dislocations create states in the energy gap that can cause premature breakdown,7 while others have suggested that the premature breakdown is caused by dopants decorating the dislocations and increasing the local electric field.8 Point defects can also create states in the band gap that cause premature breakdown, and they can also create traps making it difficult to achieve the low doping, nd – na, required to achieve the desired Vb. a)
Address all correspondence to this author. e-mail: [email protected] This paper has been selected as an Invited Feature Paper. DOI: 10.1557/jmr.2011.360 J. Mater. Res., Vol. 26, No. 23, Dec 14, 2011
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Background carbon from trimethyl gallium (TMGa) can be an issue because premature decomposition of the methyl groups is more likely to occur at the higher temperatures, 1000–1100 °C, used to grow GaN films. In this article, we investigate the latter
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