(Invited) Analysis of Defect Levels at GaAs/GaAs Surface-Activated Bonding Interface for Multi-Junction Solar Cells
Authors: Sugiyama Masakazu , Yamashita Daiji , Watanabe Kentaroh , Fujino Masahisa , Suga Tadatomo , Nakano Yoshiaki 
Published: February 2020

Room-temperature surface-activated wafer bonding between bare III-V semiconductor surfaces has become a key technology for high-efficiency multi-junction solar cells, where the reduction of interfacial electrical resistance is of crucial importance for achieving highest efficiency. In the bonding process, surface cleaning using fast atom beam (FAB) of noble gas elements is vital for successful bonding but it damages the surface, resulting in numerous crystal defects at the bonded interface and increases electrical resistance. We here developed quantitative evaluation of such defects introduced by FAB treatment. The surface of n-GaAs was treated with the FAB using Ne, Ar and Kr, and Au Schottky electrodes were formed on the surfaces. Capacitance of a Schottky diode as a function of both probe frequency and DC bias allowed us to characterize both energy depth of the defects and their density profile along the physical depth from the GaAs surface. The results indicated that atoms with the smaller diameter generate high-density defects to the deeper region from the surface. When the defect density exceeding the doping level of GaAs spreads to wider than 5 nm, significant Schottky characteristics appears in the interfacial current-voltage characteristics, as suggested by simulations. Such a tendency was semi-quantitatively in good agreement with the measured current-voltage characteristics of the n-GaAs/n-GaAs bonded interfaces treated with the FAB of Ne, Ar and Kr, suggesting that the capacitance analysis of the FAB-treated surface provides us a direction for optimizing the surface-activated bonding process using FAB.
As a simple structure for clarifying the impact of SAB process conditions on the electrical conduction property of the bonded interface, two n-GaAs wafers were bonded by SAB process. Prior to the bonding process, AuGe/Ni ohmic electrode was formed on the backside of n-GaAs wafers. The wafers were then diced into chips and their surfaces were treated with the FAB of Ne, Ar and Kr in a high-vacuum chamber, with an acceleration voltage of 1.4kV and a duration of 3 min. For the admittance spectroscopy to analyze the defects introduced by the FAB treatment, the n-GaAs diced substrates were taken out of the vacuum chamber and Au Schottky electrode was evaporated on the surface. For the conductivity measurement of the boded interface, two GaAs dices after the FAB treatment were put together and pressurized to complete bonding. Subsequently, the bonded dices were further cut into 2×2 mm2 chips and current-voltage characteristics were measured by 4 terminal method.
From the dependence of the junction capacitance on both AC frequency and DC bias, with an assumption of a value of carrier-capture cross section by a defect, it is possible to estimate both the energy position and the density profile along the depth direction of the defects existing in the surface vicinity of the substrate, or in the depletion region of the Schottky junction. Remarkably high defect density (around 1019 cm-3) was obtained exceeding the doping level of the substrate and the depth profile indicates that the FAB of the smaller atom introduces defects into the deeper region from the surface. The energy level of the defects were estimated to be 0.8 eV below the conduction-band edge.
Taking such distribution of interfacial defects into account, the current-voltage characteristics at the junction was simulated. It was predicted that the defect region thicker than 4 nm results in nonlinear, Schottky-like conduction behavior, which we should avoid in high-efficiency cells. The measured current-voltage characteristics at the n-GaAs/n-GaAs junctions by the SAB using the FAB of Ne, Ar and Kr were compered with the prediction. The tendency of Schottky-like conduction becomes more apparent in the sequence of Ne, Ar and Kr. This is the sequence of smaller atomic radius. In the defect density profile, if we focus on a certain defect density level such as 0.5×1019 cm-3, the penetration depth of the defects is larger also in the sequence of Ne, Ar and Kr.

It is strongly suggested that the high-density defects at the interface by wafer bonding distorts band edges and makes potential barrier for electrical conduction. In SAB process, FAB treatment introduces the defects in 1019 cm-3 level and it has significant impact on the interfacial electrical conduction. A suggestion for reducing such introduction of defects is the use of larger atoms in FAB treatment, which reduces penetration depth of defects. It is clear that the condition of FAB such as acceleration voltage of primary ions and exposure time impacts the density and distribution of defects. The optimization of such process parameters is quite possible using the admittance spectroscopy proposed in this work.

Figure 1