Investigation of a New Metallization Scheme for Front Ohmic Contacts to GaAs Intermediate Band Solar Cells

Abstract

Various metallization schemes for front contact to GaAs intermediate band solar cells (IBSCs) have been investigated by specific contact resistivity measurements and surface topography examination by scanning electron microscopy. GaAs IBSCs were fabricated by molecular beam epitaxy (MBE) according to the established process for these cells. Contacts were fabricated on the cells by thermal evaporation and electrolytic deposition in a transmission line model (TLM) pattern for contact resistance measurement. In order to compare the effect of wafer doping on specific contact resistivity, three different metallization schemes (Cr/Au, Au/Zn/Au, and Cr/Zn/Au) were each deposited by thermal evaporation onto wafers of four different doping levels (NA = 4.0 x 1016, 1.0 x 1019, 4.7 x 1019, 1.7 x 1020 cm-3). The investigation of these wafers mainly concerned the formation of low resistance ohmic contacts with the Cr/Zn/Au metallization, which has not been extensively studied. In order to compare the two deposition methods, Ni contacts were deposited onto wafers of two different doping levels (NA = 5.3 x 1018 and 3.0 x 1019 cm-3) by both thermal evaporation and electrolytic deposition. Emphasis was placed on obtaining low resistance ohmic contact to the p-GaAs material by electrolytically depositing nickel. All contacts were heat treated by rapid thermal annealing (RTA) to investigate the effect of annealing temperature and time on specific contact resistivity. After annealing contacts on wafers with low doping (NA ≤ 1.0 x 1019 cm-3), ohmic contacts were realized. In more highly doped wafers, ohmic contacts were realized upon deposition. Cr/Zn/Au contacts were always characterized by specific contact resistivities comparable to those of the typical Cr/Au and Au/Zn/Au contacts. The lowest resistances measured on contacts to p-GaAs with NA = 4.0 x 1016 cm-3 wafers were 1.09 x 10-2 and 1.96 x 10-2 Ωcm2 for Au/Zn/Au and Cr/Zn/Au, respectively. For contacts to p-GaAs with NA = 1.0 x 1019 cm-3, the lowest measured resistances were 8.48 x 10-4 for Au/Zn/Au and 8.76 x 10-4 for Cr/Zn/Au. Ohmic contacts to p-GaAs with NA = 4.7 x 1019 cm-3 were characterized by minimum contact resistances of 1.03 x 10-4, 1.53 x 10-4, and 1.51 x 10-4 Ωcm2, for Cr/Au, Au/Zn/Au, and Cr/Zn/Au, respectively. For ohmic contacts to p-GaAs with NA = 1.7 x 1020 cm-3, the lowest measured specific contact resistivities were 5.82 x 10-5, 1.55 x 10-4, and 7.25 x 10-5 Ωcm2, for Cr/Au, Au/Zn/Au, and Cr/Zn/Au, respectively. Ni contacts deposited by electrolysis displayed ohmic behavior upon deposition. The specific contact resistivities of these samples were lowered after annealing, in the case of contacts to the more lowly doped wafer (NA = 5.3 x 1018 cm-3). The lowest specific contact resistivities measured on Ni contacts by electrolytic deposition were 9.91 x 10-4 and 1.74 x 10-4 Ωcm2 on wafers of NA = 5.3 x 1018 cm-3 and NA = 3.0 x 1019 cm-3, respectively. These resistance values were comparable to those measured on Ni contacts deposited by thermal…