Coaxial Group III−Nitride Nanowire Photovoltaics
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Basic Research Needs for Solar Energy Utilization, Report of the Basic Energy Sciences Workshop on Solar Energy Utilization. April 18−21, 2005;US Department of Energy,Washington, DC, 2005, (http://www.er.doe.gov/bes/reports/abstracts.html#SEU).
Davydov V. Y., 2002, Phys. Status Solidi B, 229, R1, 10.1002/1521-3951(200202)229:3<R1::AID-PSSB99991>3.0.CO;2-O
Coaxial n-GaN/i-InxGa1−xN/p-GaN core/shell/shell nanowires were synthesized as follows: 0.01 M nickel nitrate solution was deposited on a sapphire substrate and placed in a MOCVD reactor (Thomas Swan Scientific Equipment Ltd.). n-type GaN cores were grown in hydrogen (H2) at 950 °C and 700 Torr for 4800 s using trimethylgallium (TMG, 22 μmol min−1) and ammonia (NH3, 67 mmol min−1), while silane (100 ppm in H2, 2 sccm) was used as the n-type dopant. The intrinsic InGaN layer was sequentially deposited in nitrogen at 715−775 °C and 300 Torr for 500 s, using TMG (5.3 μmol min−1) and trimethylindium (TMI, 6.5 μmol min−1) as Ga and In sources, respectively. Lastly, the p-GaN outer shell was grown in H2at 960 °C and 100 Torr for 400 s using bis(cyclopentadienyl)magnesium (MgCp2, 0.65 μmol min−1) as p-type dopant. Our III-nitride nanowires have triangular cross sections, a fact which has been confirmed through HRTEM studies reported previously.(22-27)The triangular side length of the p-i-n nanowires was ca. 1−1.25 μm and the thicknesses of the i-InGaN and p-GaN layers were ca. 80−100 and 100−125 nm, respectively. The lengths of the nanowires after transfer to the substrate for device fabrication were 15 to 40 μm. The coaxial n-GaN/p-GaN core/shell nanowire synthesis was the same as above except that the InGaN layer deposition step was eliminated.
The nanowires were dispersed on silicon substrates (100 nm oxide/200 nm nitride, 1−10 Ω·cm resistivity) for electrical and optoelectronic measurements. Electron beam lithography (EBL) was used to define windows at nanowire ends, and then the shells were etched using a Unaxis Shuttleline ICP RIE at constant process parameters of 10 sccm BCl3, 10 sccm Ar, and 3 sccm N2flow, 550 W ICP power, 200 W RIE power, 2 mTorr pressure, and 23 °C temperature for 2 min. The etching rate was determined to be ∼2 nm·s−1. Electrical contacts were defined in two separate EBL steps, where Ni/Au (150/150 nm) and Ti/Al/Ti/Au (20/100/30/250 nm) were deposited for p-type shell and n-type core contacts, respectively. The contacts were annealed in nitrogen at 550 °C for 2 min.I−Vdata were recorded using an Agilent semiconductor parameter analyzer (model 4156C) and EL spectra were recorded using a 300 mm spectrometer (150 lines·mm−1grating) and a liquid nitrogen cooled charge-coupled device detector with a diode forward bias of 6 V. Standard solar illumination was provided by a Newport Solar Simulator (model 96000) with air mass global, AM 1.5G filter. UV light illumination was carried out using a Spectra Physics Q-switched 266 nm Nd:YVO4laser (model J40-BL6-266Q) with 35 kHz repetition rate, and 7 ns pulse duration. Illumination intensities were calibrated with a power meter (Coherent, Field Master).