Evaluation of the passivation effect and the first-principles calculation on surface termination of germanium detector

The point-contact high-purity germanium detector (HPGe) has the advantages of low background, low energy threshold, and high energy resolution and can be applied in the detection of rare-event physics. However, the performance of HPGe must be further improved to achieve superior energy resolution, low noise, and long-term reliability. In this study, we combine computational simulations and experimental comparisons to deeply understand the passivation mechanism of Ge. The surface passivation effect is calculated and inferred from the band structure and density of interface states, and further confirmed by the minority carrier lifetime. The first-principles method based on the density functional theory was adopted to systematically study the lattice structure, band structure, and density of state (DOS) of four different systems: Ge–H, Ge–Ge-NH$_{2}$, Ge-OH, and Ge-SiO$_{x}$. The electronic characteristics of the Ge (100) unit cell with different passivation groups and Si/O atomic ratios were compared. This shows that H, N, and O atoms can effectively reduce the surface DOS of the Ge atoms. The passivation effect of the SiO$_{x}$ group varied with increasing O atoms and Si/O atomic ratios. Experimentally, SiO and SiO$_{2}$ passivation films were fabricated by electron beam evaporation on a Ge substrate, and the valence state of Si and resistivity was measured to characterize the film. The minority carrier lifetime of Ge-SiO$_{2}$ is 21.3 μs, which is approximately quadruple that of Ge-SiO. The passivation effect and mechanism are discussed in terms of hopping conduction and surface defect density. This study builds a relationship between the passivation effect and different termination groups, and provides technical support for the potential passivation layer, which can be applied in Ge detectors with ultralow energy thresholds and especially in HPGe for rare-event physics detection experiments in future.