Type-II Band Profile of GaAs/Si Hetero Junctions by Surface Activated Bonding for Hybrid Tandem Cells

III-V-on-Si tandem cells fabricated in the hybrid, or wafer bonding, approach are promising as high-efficiency (>30%) solar cells. The present authors reported on the characteristics of InGaP-on-Si tandem cells that had been fabricated by surface-activated bonding (SAB) [1]. The key issue in fabricating such tandem cells lies in formation of low-parasitic-resistance tunnelling junctions made of III-V-materials, typically GaAs, and Si. In the present work, we investigate the electrical properties of p-GaAs/n-Si and p-Si/n-GaAs hetero junctions by SAB, which are one of the key factors determining the resistance across the tunnelling junctions.

p-GaAs/n-Si and p-Si/n-GaAs junctions were made of (100) p-GaAs (concentration of acceptors (N_A) of ~1E19 cm^-3),  (100) n-Si (concentration of donors (N_D) of 4.8E16 cm^-3), (100) n-GaAs (N_D of 1.1E18 cm^-3), and (100) p-Si (N_A of 2.4E17 cm^-3) substrates by SAB. Ohmic contacts were formed by evaporating AuZn/Ti/Au, Ti/Au, AuGe/Ni/Ti/Au, and Al/Ni/Au films on the respective substrates and annealing (400 deg., 1min. in N2 ambient). Their current-voltage (I-V) and capacitance-voltage (C-V) characteristics were measured at room temperature. The frequency for C-V measurements was 100 kHz.

The I-V characteristics of both junctions showed rectifying properties. The onset in the characteristics for forward bias voltages was found to be 0.26 and 0.4 V for the p-GaAs/n-Si and p-Si/n-GaAs junctions, respectively. The flat-band voltage was estimated to be 0.42 and 1.60 V in the p-GaAs/n-Si and p-Si/n-GaAs junctions, respectively, by linearly extrapolating each of 1/C²-V characteristics. The lower flat-band voltage in the p-GaAs/n-Si junction was in accordance with the smaller onset in its I-V characteristics. The extracted flat-band voltage values imply that the conduction band minimum (E_C) in GaAs is higher than that in Si by 0.83 and 0.58 eV for the p-GaAs/n-Si and p-Si/n-GaAs junctions, respectively. Such large values of the conduction-band discontinuity (ΔE_C) suggest that the band lineup in GaAs/Si junctions reveals type-II features irrespective of their polarity.

This result disagrees with the estimation based on a simple model that ΔE_C in GaAs/Si hetero junctions is expressed by the difference in the electron affinities of GaAs and Si, both of which are ~4 eV. The disagreement is attributable to the formation of dipole due to the charge imbalance at the hetero interface. Electrical charges in the interface states possibly formed during Ar beam irradiation in the SAB process might explain the difference in ΔE_Cvalues between the two hetero junctions. Practically, the type-II features in the band lineup of GaAs/Si systems, which means that the height of potential barriers that carriers pass through in p-GaAs/n-Si tunnelling junctions is effectively reduced, play an important role in making high-efficiency Si-based tandem cells.

This work was supported by "Creative research for clean energy generation using solar energy" project of JST/CREST, Japan.

[1] N. Shigekawa, et al. Jpn. J. Appl. Phys. 53, 04ER05 (2014).