1. 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

2. 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

3. 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

4. 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)

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

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

7. E-beam patterning: Fabrication of hole array1 2 3
SiO2
1- 30 nm
nm a 700 nm
3- 10 a 20 nm

8. 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)

9. 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, (2009)

10. 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)