Mechanism of Carrier Transport in Hybrid GaN/AlN/Si Solar Cells HUSEYIN EKINCI , 1,4 VLADIMIR V. KURYATKOV, 1 IULIAN GHERASOIU, 2 SERGEY Y. KARPOV, 3 and SERGEY A. NIKISHIN 1 1.—Electrical and Computer Engineering, Texas Tech University, Lubbock, TX, USA. 2.—College of Engineering, SUNY Polytechnic Institute, Utica, NY, USA. 3.—STR Group, Soft-Impact, Ltd., St. Petersburg, Russian Federation. 4.—e-mail: huseyin.ekinci@ttu.edu The particularities of the carrier transport in p–n-GaN/n-AlN/p–n-Si and n-GaN/n-AlN/p–n-Si structures were investigated through temperature-de- pendent current density and forward voltage (J–V) measurements, carrier distribution, and transport modeling. Despite the insulating properties of AlN, reasonably high current densities were achieved under forward bias. The experimental relationship between the current density and forward voltage was accurately approximated by an expression accounting for space-charge- limited current in the AlN layer and non-linear characteristics of the p–n junction formed in silicon. We suggest that extended defects throughout the AlN volume are responsible for the conduction, although the limited data available do not allow the accurate identification of the type of these defects. Key words: Carrier transport mechanism, tandem III-nitride/Si solar cells, PAMBE, III-nitride on Si substrate, space-charge-limited current, defect-mediated current INTRODUCTION It is expected that photovoltaic (PV) power gen- eration will become a vital technology for green energy production in the future by realizing low-cost and high-conversion-efficiency solar cells. At the moment, single-junction Si solar cells dominate the PV market due to lower costs, the availability of large-diameter silicon wafers, and well-developed processing procedures. However, these solar cells have almost approached their efficiency limits. 1 In order to achieve higher conversion efficiencies, two major loss mechanisms, ‘‘transparency loss’’ and ‘‘excess excitation loss’’, must be minimized. 2 At present, fabrication of multi-junction (tandem) solar cells is the most effective solution for increasing the power conversion efficiency since such solar cells are capable of efficiently harvesting photons from solar radiation by reducing both kinds of losses simulta- neously. 3,4 Unfortunately, there are a limited number of materials capable of providing both optimal bandgap combinations and lattice constant matching, which are necessary for attaining high crystal quality of the device heterostructures. Actu- ally, only solar cells based on lattice-matched arsenide/phosphide compounds grown on germa- nium substrate are commercially available, demon- strating the highest conversion efficiency, >40%. 1 Due to the direct band gap and the wide range of variation, from 0.7 eV to 6.2 eV, III-nitride com- pounds and their alloys (III-Ns) have been initially recognized as very promising materials for the fabrication of high-efficiency tandem solar cells. 5,6 Further studies have shown, however, that even single solar cells based on either bulk InGaN or InGaN/GaN multiple quantum wells with suffi- ciently high In content in the alloy suffer from a low conversion efficiency. 2,7 This originates from a high defect density inherent in III-nitride epitaxial structures, which favors shorting of the device space-charge region. 8–11 Thus, fabrication of tan- dem solar cells made entirely of III-N materials (Received August 31, 2016; accepted April 28, 2017) Journal of ELECTRONIC MATERIALS DOI: 10.1007/s11664-017-5557-y Ó 2017 The Minerals, Metals & Materials Society