Why MOCVD and Ga As nanowires MOCVD because
Why MOCVD and Ga. As nanowires? MOCVD, because: • Commercially available technology for semiconductor devices • High reproducibility • Large substrate size Nanowires, because: • Integration with Si (stress management) • Superior optical properties (1 D structure) • Lower material cost Ga. As, because: • Ideal material for photovoltaic (bandgap) • Lower cost compare to In precursor (vs In. P) • Integration with Si solar cell for tandem applications Ga. As NW by MOCVD
VLS – Vapor-liquid-solid growth Typical features: - Low temperature reactions (400 C) - Low V/III ratio Droplet formation: Substrate preparation: - Ga or Au droplet - Oxide, Nitride, Au film - Patterning or self-assembly - Absorption and diffusion of the precursors as a vapor Nanowire growth: - Supersaturation - Nucleation at the liquid/solid interface SAG – selective area growth a) sputtering a layer of dielectric mask onto the substrate b) Openings in the mask by e-beam or lithography c) Nanowire growth through the openings Typical features: - Typical MOCVD thin film growth temperature (750 C) - High V/III ratio Ga. As NW by MOCVD
Axial or radial p-n junction? Radial Axial Advantages of NWs: • reduced reflection • extreme light trapping • relaxed interfacial strain • single-crystalline synthesis on foreign substrates Advantages of radial pn junction: • - radial charge separation • - lower surface recombination Ga. As NW by MOCVD
State of the art for Ga. As solar cells The only technology for device development - MOCVD Best Ga. As cells Efficiency, % Area, mm 2 Voc, V Jsc, m. A/cm 2 FF, % Technology Description Ga. As Thin film 28. 8 +/-0. 9 100 1. 122 29. 68 86. 5 MOCVD 2011, Alta Devices [1] Ga. As NWs 15. 3 1 0. 906 21. 3 79. 2 MOCVD, axial p-n junction 2016, Sol Voltaics [2] Ga. As NWs 7. 58 1 0. 565 21. 08 63. 6 MOCVD, axial p-n junction 2014, Univ. of Southern California [3] Ga. As 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) Ga. As NW by MOCVD
Simulations of diameter, pitch and efficiency Single nanowire Ga. As NW solar cell alone Tandem configuration with Si Krogstrup et al, Nat. Phot. (2013) Diameter - 150 nm Pitch – 400 nm L = 3. 5 um Ga. As NW by MOCVD Alarcon-Llado et al, in prep.
Why moving to MOCVD Va = -1 V Va = 0. 5 V Va = 1 V Va sweep Ga. As NW by MOCVD
E-beam patterning: Fabrication of hole array 2 1 Si. O 2 3 1 - 30 nm 2 - 350 nm a 700 nm 3 - 10 a 20 nm Ga. As NW by MOCVD
Process in MOCVD (Si substrate) Horizontal reactor Substrate – n+-Si (111) Fukui group [1] Method – SAG Mask – Si. O 2 20 nm 7 steps growth: Growth steps: Include 1. Annealing step in H 2 at 925°C for 5 min modification of the 2. Growth (core) Si surface with - Growth temperature – 750°C As. H 3 and LT-Ga. As -6 -4 by Horizontal MOVPE - Flows - TMGa 1. 0 x 10 atm, As. H 3 2. 5 x 10 atm - Time – 1 hour Expected doping density from planar tests: n = 3. 5 x 1017 3. Growth (shell – Al. Ga. As) p = 4. 0 x 1018 - Growth temperature – 700°C - Flows - TMGa 8. 2 x 10 -7 atm, TMAl – 1. 2 x 10 -6 atm, As. H 3 1. 3 x 10 -4 atm - Time – 5 min 4. Cooling to RT in As. H 3 up to 300°C The dopants (for shell): n-doping – Si (silane Si. H 4) 2. 5 x 10 -8 atm p-doping – Zn (di-ethyl zinc (C 2 H 5)2 Zn) 2. 8 x 10 -6 atm Other: 0. 1 atm – low pressure system; H 2 flow – 5. 75 slm Ga. As NW by MOCVD [1] ACS Nano 10, 2424 (2016)
Process in MOCVD (Si substrate) Vertical reactor Substrate – n+-Si (111) Method – SAG Mask – Si 3 N 4 Growth steps: 1. Annealing step in H 2 at 925°C for 5 min 2. Growth - Growth temperature – 700 – 790°C (~ 760°C) - Flows - TMGa 7. 6 x 10 -7 atm, As. H 3 2. 1 x 10 -4 atm - Time – 1 hour - H 2 flow – 5. 75 slm 4. Cooling to RT in As. H 3 up to 300°C The dopants: n-doping – Si (di-silane Si 2 H 6) p-doping – Zn (di-ethyl zinc (C 2 H 5)2 Zn) Other: 0. 1 atm – low pressure; 7 slm of H 2 Ga. As NW by MOCVD Zhou group [1] 7. 58 % 5 steps growth by Vertical Thomas Swan MOCVD Vertical vs lateral growth: Low As. H 3 pressure and high Tg vertical growth High As. H 3 pressure and low Tg lateral growth [1] Nanotechnology 20, 145302 (2009)
Process in MOCVD (radial NWs on Ga. As substrate) Substrate – n+-Ga. As (111) B Method – SAG using Emcore vertical flow MOCVD Mask – Si. O 2 Growth parameters: - Sources - TMGa – 900 torr, 0°C, 3 sccm of H 2 - As. H 3 – 700 torr, 20°C, 10 sccm of H 2 - Growth temperature – core n-doped 735°C – shell p-doped 600°C Main dopants: n-doping – Sn (tetra-ethyl tin (C 2 H 5)4 Sn) p-doping – Zn (di-ethyl zinc (C 2 H 5)2 Zn) Pressure: 60 torr – low pressure Ga. As NW by MOCVD Huffaker group [1] 6. 63 % Expected doping density from planar tests: [1] Nature Communications 4, 1497 (2013) n = 1. 0 x 1017 p = 1. 0 x 1018
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