Interstitial Li doping of a-Si:H

Abstract

We study the process of interstitial doping of hydrogenated amorphous silicon (a-Si:H) by Li in-diffusion and thermal annealing after Li+ implantation using secondary ion mass spectrometry, dc dark conductivity [σ(T)], electron-spin resonance, and photothermal deflection spectroscopy measurements. All Li-doped a-Si:H samples were characterized by an inhomogeneous distribution of Li atoms. Doping by Li in-diffusion at 230 °C resulted in a Li-rich (up to 1021 cm−3) region at both sample interfaces, a large increase in σ(300 K), and the creation of deep paramagnetic (g=2.0061±0.0002, ΔHp.p.=5.4±0.4 G) defects with defect densities (up to 2×1018 cm−3) proportional to the interfacial Li concentration. Li+ implantation of a-Si:H at 373 K resulted in the creation of deep paramagnetic (g=2.0056±0.001, ΔHp.p.=6.0±0.5 G) defects with defect densities (up to 2×1018 cm−3) proportional to the implanted Li+ dose. Isochronal vacuum annealing of Li+-implanted a-Si:H up to 545 K resulted in an exponential increase of σ(300 K) and an activated (Ea=+0.32 eV) decrease of the spin density with increasing anneal temperatures. Both Li in-diffused and annealed Li+-implanted a-Si:H films displayed thermal equilibration behavior similar to that characteristic of P-doped a-Si:H, which suggests that the defect compensation model of substitutional doping of a-Si:H is also applicable to the case of interstitial doping. However, the defect structure of a-Si:H doped by Li in-diffusion is significantly different than that of both undoped and P-doped a-Si:H due to the precipitation of Li at the interfaces of heavily Li-doped a-Si:H. We discuss the origin of this behavior.