Growth and Characterization of GaPNAs on Si
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Growth and Characterization of GaPNAs on Si John Geisz, J. M. Olson, W. E. McMahon, T. Hannappel,1 K. Jones, H. Moutinho and M. M. Al-Jassim National Renewable Energy Laboratory Golden, CO 80401, U.S.A. 1 Hahn-Meitner Institute Berlin, Germany ABSTRACT The dilute nitrogen alloy GaPNAs can be lattice matched to silicon with band gaps ranging from 2.3 eV to less than 1.7eV making it of special interest for photovoltaic applications. We have studied the growth and structural quality of the alloy grown on vicinal Si(001) and GaP(001) substrates by MOCVD. Using a particular nucleation scheme, we have deposited 1µm thick layers that are crack-free and exhibit narrow x-ray line widths. The FWHM of the (004) x-ray reflection from a GaP1-xNx epilayer decreases dramatically from ~300 arcsec for x=0 to 18 arcsec for x = 0.021. The band gap of this alloy is 1.96 eV. With the addition of As (and more N), the x-ray line widths tend to increase slightly to 27 arcsec. INTRODUCTION The prospect of high efficiency multijunction solar cells on Si has long been a dream of researchers in the field of photovoltaics.1-5 This is because the ideal two-junction solar cell is the combination of a top cell with a band gap of 1.7 eV and a silicon bottom cell (with a 1.1 eV band gap).1 Prior to this paper, virtually all of the work on such a structure focused on III-V top cells that were lattice mismatched to the silicon bottom cell. Under the best conditions this leads to a threading dislocation density on the order of 106 cm-2 in the III-V top cell.6 Under most conditions, this dislocation density decreases the electronic quality of the top cell to the point that a tandem solar cell with a less optimum band gap combination, albeit lattice matched, (i.e. the GaInP/GaAs tandem solar cell) is much more efficient.7, 8 Recently, however, it has been shown that the quaternary GaPNAs can be lattice-matched to silicon while independently varying the bandgap over a range of 1.6 eV to 2.1eV,9, 10 and high quality layer have been grown on Si by MBE.11 The low band gaps are the result of the large band gap bowing associated with dilute nitride III-V alloys.12 In the following we present details on the growth of this alloy on Si(100) using metalorganic chemical vapor deposition (MOCVD). We show that the structural quality as measured by x-ray and transmission electron diffraction is a function of the lattice mismatch and the nucleation conditions prior to the growth of the GaPNAs epilayer. EXPERIMENTAL DETAILS The GaPNAs layers were grown in an atmospheric pressure (620 torr) MOCVD apparatus. The system uses conventional source materials, t-butylarsine (TBA), PH3, dimethylhydrazine (DMH), triethylgallium (TEG) and trimethylindium (TMI). The substrates are Si(001) misoriented 2˚ towards (111). Prior to each run they are cleaned in an ammonia peroxide solution, rinsed in deionized water and spin-dried. The nucleation process begins with a 10-min
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soak in H2 at 1000˚C. The Si surface is then exposed to PH3 for about 15 s after which the TEG
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