Description
Date/Time: 04-23-2019 - Tuesday - 05:00 PM - 07:00 PM
M. Hadi Tavakoli Dastjerdi1 Ray LaPierre2

1, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
2, McMaster University, Hamilton, Ontario, Canada

III-V semiconductors are the suitable alternatives to the currently dominant Si flat panel solar cells, thanks to their high conversion efficiencies. Integration of III-V solar cells with inexpensive Si substrate results in dramatic reduction of the overall fabrication costs. Although the III-V materials cannot be grown directly on Si due to the large lattice miss match, the III-V nanowire (NW) structures can be grown dislocation-free thanks to the elastic relaxation of the lattice mismatch strain at the NW free surface. In addition, NWs have large optical absorption due to their optical antenna effect . Moreover, growing III-V NWs on Si allows for tandem solar cell designs in which the underlying Si substrate performs as a sub-cell.
Employing the self-assisted vapor–liquid–solid (VLS) method we have grown core-shell NWs on Si (111) substrate. To prepare the wafer for growth, the Si substrate is primarily coated with a thin layer of SiO2. Electron beam lithography (EBL) and oxide etching are then used to create an array of holes in the oxide film. Ga droplets collect in the oxide holes which serve as the NW nucleation centers for the NW growth. The growth is done in a gas source molecular beam epitaxy (MBE) system starting with a GaP segment to initiate the vertical growth followed by grading of GaP to GaAs to grow the Be-doped p-core section. To grow the shell layers, the droplet was consumed under As2 flow. We have previously shown that using a lower group V flow during the droplet consumption step results in pure zincblende tip without stacking faults which leads to enhanced blue response as confirmed by measuring the external quantum efficiency (EQE).
The grown NWs are about 2 µm long and they are grown in arrays that are 100 µm x 100 µm. The measured dark and light current-voltage curves show short-circuit current denisty (JSC) of 23.4 mA/cm2 and open-circuit voltage (VOC) of 0.5 V with 4.83% efficiency.
Off-axis electron holography was employed to find the electrical potential distribution for doping assessment which has also revealed that p-type Be dopants introduced in situ during MBE growth of the NWs were distributed inhomogeneously in the nanowire cross-section, perpendicular to the growth direction. The active dopants showed azimuthal distribution along the (111)B flat top of the NWs, which is attributed to preferred incorporation along 3-fold symmetric truncated facets under the Ga droplet. The new doping mechanism has not been previously observed.

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