## 3 Results and Discussion
### Device Fabrication and Performance  
Figure 4a shows the fabrication procedure of our solar cells. We first deposited Pt/Au (10/50 nm), which served as a bonding material and ohmic contact metal, on the GaAs solar cell sample and the transfer substrate by electron beam evaporation. Then, standard photolithography was performed to define mesas. The samples were etched using a 1:1:5 mixture of phosphoric acid, hydrogen peroxide, and deionized water for a fast ELO. Subsequently, Ar plasma was irradiated for surface activation, which leaves highly reactive radical site layers that substantially enhance surface adhesion. Metal wafer bonding was conducted with a uniaxial pressure of approximately 40 kgf/cm\({}^{2}\). After the bonding process, the GaAs epitaxial layer was separated from the Si donor substrate by an ELO process in which the AlAs sacrificial layer was etched by immersion in HF-based solution [29; 30; 31]. Finally, Pt/Ti/Pt/Au (20/30/20/200 nm) was deposited on the top contact layers by electron beam evaporation. No anti-reflection coating (ARC) was applied on the cells. Figure 4b shows photographs of the GaAs solar cells transferred to a Pt/Au-coated polyimide film. The inset shows an optical microscope image of the transferred mesa epilayer. Figure 4: (**a**) Fabrication procedure for transfer process of GaAs solar cell. (**b**) Photograph of transferred GaAs solar cell to polyimide. The inset shows the optical microscope image of GaAs solar cell grown on Si substrate. Figure 3: (**a**) Photograph of a 2-inch GaAs one-junction solar cell grown on Si substrate. The red-line square shows the selected area for micro-PL mapping. (**b**) Micro-PL mapping image on the selected area in (**a**). (**c**) Raman spectra of GaAs solar cell grown on GaAs and Si substrate. Figure 5a shows the sample structures of three different GaAs solar cells: inverted p-on-n GaAs cell grown on Si with AlAs sacrificial layer, upright solar cell, and GaAs solar cell grown on GaAs. The current density (J)-voltage (V) characteristics were investigated to compare the electrical properties of the GaAs solar cells. Figure 5b shows dark J-V curves of the GaAs solar cells and all samples showed good rectifying characteristics with low dark current densities. The comparable dark current densities of 1.26 and 2.08 uA/cm\({}^{2}\) between Sample A and B indicated that our ELO process did not degrade the material quality. The inset shows the ideality factor of the GaAs solar cell, which was derived from J-V curves of samples. The ideality factor provides insight on the current flow mechanism; its value is close to 2 when the carrier transport through the p-n junction is dominated by the generation-recombination mechanism. The obtained ideality factor of Sample C grown on a GaAs substrate showed its value to be under 2 at low forward bias voltages, which suggests that the diffusion current was superior over junction recombination current at low forward bias. On the other hand, the ideality factor of Sample A and B showed values above 2 at small forward biases, indicating that the defect-induced recombination and tunneling current was the dominant origin of dark current. Figure 5c shows the J-V characteristics of our fabricated GaAs solar cell in-house under AM1.5G solar spectrum. First, it is of note that our baseline upright GaAs solar cell (Sample C) showed a conversion efficiency of 11.2%. Comparison of the upright GaAs cells grown on GaAs and Si substrate indicated that the presence of 1.8 \(\times\) 10\({}^{7}\) cm\({}^{-2}\) TDs lowered the open-circuit voltage by 0.13 V and the fill-factor by 7.6%. In addition, Sample B showed a rather lower slope, which was attributed to higher series resistance. The hetero-epitaxial lift-off process seemed to additionally decrease the open-circuit voltage slightly, from 0.79 V to 0.72 V, but it did not affect the fill-factor. Even at low open-circuit voltage, Sample A showed a comparable conversion efficiency of 10.6%, which was slightly smaller than that of the upright cell on GaAs.