InGaN Thin Films Grown by ENABLE and MBE Techniques on Silicon Substrates
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1068-C06-02
InGaN Thin Films Grown by ENABLE and MBE Techniques on Silicon Substrates Lothar A. Reichertz1,2, Kin Man Yu1, Yi Cui1, Michael E Hawkridge1, Jeffrey W Beeman1, Zuzanna Liliental-Weber1, Joel W Ager III1, Wladyslaw Walukiewicz1, William J Schaff3, Todd L Williamson4, and Mark A. Hoffbauer4 1 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720 2 University of California at Berkeley, Berkeley, CA, 94720 3 School of Electrical and Computer Engineering, Cornell University, Ithaca, NY, 14853 4 Chemistry Division, Los Alamos National Laboratory, Los Alamos, NM, 87545 ABSTRACT The prospect of developing electronic and optoelectronic devices, including solar cells, that utilize the wide range of energy gaps of InGaN has led to a considerable research interest in the electronic and optical properties of InN and In-rich nitride alloys. Recently, significant progress has been achieved in the growth and doping of InGaN over the entire composition range. In this paper we present structural, optical, and electrical characterization results from InGaN films grown on Si (111) wafers. The films were grown over a large composition range by both molecular beam epitaxy (MBE) and the newly developed “energetic neutral atomic-beam lithography & epitaxy” (ENABLE) techniques. ENABLE utilizes a collimated beam of ~2 eV nitrogen atoms as the active species which are reacted with thermally evaporated Ga and In metals. The technique provides a larger N atom flux compared to MBE and reduces the need for high substrate temperatures, making isothermal growth over the entire InGaN alloy composition range possible. Electrical characteristics of the junctions between n- and p-type InGaN films and n- and p-type Si substrates were measured and compared with theoretical predictions based on the band edge alignment between those two materials. The predicted existence of a low resistance tunnel junction between p-type Si and n-type InGaN was experimentally confirmed. INTRODUCTION The band gap of InGaN can be tuned from 0.65 eV to 3.4 eV [1]. This makes it a very attractive material for optoelectronic devices and, in particular, for solar cells, as this energy range is a perfect match to the part of the solar spectrum that reaches the surface of the Earth. InGaN is therefore a material of choice for a “full-spectrum-photovoltaic” development and there is a considerable research interest in the electronic and optical properties of InN and In-rich nitride alloys [2]. Growing InGaN on Si is interesting for various reasons. First of all, InGaN structures could be imbedded into the mature Si technology, and an obvious application in PV would be to improve the efficiency of a standard single junction Si solar cell by adding a matched InGaN top cell. The thermodynamic efficiency limit [3] of 29% for a Si solar cell under 1 sun standard illumination (AM1.5G) will increase to 42.5% already for only one additional cell with an optimum band gap of 1.8 eV [4]. To obtain this band gap, an InGaN film with a composition o
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