Why MOCVD and GaAs nanowires? MOCVD, because: Commercially available technology for semiconductor devices High reproducibility Large substrate size Nanowires, because: Integration with Si (stress management) Superior optical properties (1D structure) Lower material cost GaAs, because: Ideal material for photovoltaic (bandgap) Lower cost compare to In precursor (vs InP) Integration with Si solar cell for tandem applications

VLS – Vapor-liquid-solid growth Typical features: Low temperature reactions (400C) Low V/III ratio Substrate preparation: Oxide, Nitride, Au film Patterning or self-assembly Droplet formation: Ga or Au droplet Absorption and diffusion of the precursors as a vapor Nanowire growth: Supersaturation Nucleation at the liquid/solid interface SAG – selective area growth sputtering a layer of dielectric mask onto the substrate Openings in the mask by e-beam or lithography Nanowire growth through the openings Typical features: Typical MOCVD thin film growth temperature (750C) High V/III ratio

Axial or radial p-n junction? Advantages of NWs: reduced reflection extreme light trapping relaxed interfacial strain single-crystalline synthesis on foreign substrates Radial Axial Advantages of radial pn junction: - radial charge separation - lower surface recombination

State of the art for GaAs solar cells The only technology for device development - MOCVD Best GaAs cells Efficiency, % Area, mm2 Voc, V Jsc, mA/cm2 FF, % Technology Description GaAs Thin film 28.8 +/-0.9 100 1.122 29.68 86.5 MOCVD 2011, Alta Devices [1] GaAs NWs 15.3 1 0.906 21.3 79.2 MOCVD, axial p-n junction 2016, Sol Voltaics [2] GaAs NWs 7.58 0.565 21.08 63.6 2014, Univ. of Southern California [3] GaAs NWs 6.63 0.25 0.44 24.3 62 MOCVD, radial p-n junction 2013, UCLA Limited solar cell area due to patterning time and cost Lower Voc due to large surface area of nanowire – surface passivation Issues with transparent front contact – lower FF and current density [1] IEEE Journal of Photovoltaics 6(1), 185 (2016) [2] Nano Lett. 14(6), 3293 (2014) [3] Nature Communications 4, 1497 (2013)

Simulations of diameter, pitch and efficiency GaAs NW solar cell alone Single nanowire Tandem configuration with Si Krogstrup et al, Nat. Phot. (2013) Diameter - 150 nm Pitch – 400 nm L = 3.5 um Alarcon-Llado et al, in prep.

Why moving to MOCVD Va = -1 V Va = 0 V Va sweep Va = 0.5 V Va = 1 V

E-beam patterning: Fabrication of hole array 1 2 3 SiO2 1- 30 nm 2- 350 nm a 700 nm 3- 10 a 20 nm

Process in MOCVD (Si substrate) Horizontal reactor Substrate – n+-Si (111) Method – SAG Mask – SiO2 20 nm Growth steps: Annealing step in H2 at 925°C for 5 min Growth (core) - Growth temperature – 750°C - Flows - TMGa 1.0x10-6 atm, AsH3 2.5x10-4 atm - Time – 1 hour 3. Growth (shell – AlGaAs) - Growth temperature – 700°C - Flows - TMGa 8.2x10-7 atm, TMAl – 1.2x10-6 atm, AsH3 1.3x10-4 atm - Time – 5 min Cooling to RT in AsH3 up to 300°C The dopants (for shell): n-doping – Si (silane SiH4) 2.5x10-8 atm p-doping – Zn (di-ethyl zinc (C2H5)2Zn) 2.8x10-6 atm Other: 0.1 atm – low pressure system; H2 flow – 5.75 slm Fukui group [1] 7 steps growth: Include modification of the Si surface with AsH3 and LT-GaAs by Horizontal MOVPE Expected doping density from planar tests: n = 3.5x1017 p = 4.0x1018 [1] ACS Nano 10, 2424 (2016)

Process in MOCVD (Si substrate) Vertical reactor Substrate – n+-Si (111) Method – SAG Mask – Si3N4 Growth steps: Annealing step in H2 at 925°C for 5 min Growth - Growth temperature – 700 – 790°C (~ 760°C) - Flows - TMGa 7.6x10-7 atm, AsH3 2.1x10-4atm - Time – 1 hour - H2 flow – 5.75 slm Cooling to RT in AsH3 up to 300°C The dopants: n-doping – Si (di-silane Si2H6) p-doping – Zn (di-ethyl zinc (C2H5)2Zn) Other: 0.1 atm – low pressure; 7 slm of H2 Zhou group [1] 7.58 % 5 steps growth by Vertical Thomas Swan MOCVD Vertical vs lateral growth: Low AsH3 pressure and high Tg  vertical growth High AsH3 pressure and low Tg  lateral growth [1] Nanotechnology 20, 145302 (2009)

Process in MOCVD (radial NWs on GaAs substrate) Substrate – n+-GaAs (111) B Method – SAG using Emcore vertical flow MOCVD Mask – SiO2 Growth parameters: Sources TMGa – 900 torr, 0°C, 3 sccm of H2 AsH3 – 700 torr, 20°C, 10 sccm of H2 Growth temperature – core n-doped 735°C – shell p-doped 600°C Main dopants: n-doping – Sn (tetra-ethyl tin (C2H5)4Sn) p-doping – Zn (di-ethyl zinc (C2H5)2Zn) Pressure: 60 torr – low pressure Huffaker group [1] 6.63 % Expected doping density from planar tests: n = 1.0x1017 p = 1.0x1018 [1] Nature Communications 4, 1497 (2013)