Charge deep-level transient spectroscopy study of high-energy-electron-beam-irradiated hydrogenated amorphous silicon

Abstract

We present a study of changes in the defect density of states in hydrogenated amorphous silicon ͑a-Si: H͒ due to high-energy electron irradiation using charged deep-level transient spectroscopy. It was found that defect states near the conduction band were removed, while in other band gap regions the defect-state density increased. A similar trend is observed for a-Si: H which has been subjected to light soaking, but in that case the majority of defect states are created around midgap, whereas with electron-beam degradation more defect states are created near the valence-band tail. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2221876͔ Hydrogenated amorphous silicon ͑a-Si: H͒ solar cells show great promise for space application, particularly in high-radiation missions, due to their relatively high radiation tolerance and low-temperature annealing properties. 1,2 Various studies have addressed the degradation behavior of a -Si: H due to high-energy charged particle irradiation, and it is generally accepted that changes in the defect density of states govern the degradation, although trapping of the incident charge on preexisting defects is also considered. [3][4][5] The underlying degradation mechanism, however, is still under debate. Danesh et al.,6 amongst others, showed that there is a strong similarity between high-energy charged particle irradiation and the Staebler-Wronski effect, 7 i.e., reversible changes in electronic properties of a-Si: H under light exposure. In that case, it is expected that the defect-state distribution after light soaking is similar to the distribution after high-energy electron-beam irradiation. In this letter we report on changes in defect-state distribution of a-Si: H due to electron-beam irradiation measured by charge deep-level transient spectroscopy ͑Q-DLTS͒. These results are compared to findings from light-soaking experiments in order to investigate the reported similarities in the defect-creation mechanism further.Q-DLTS has been utilized to study the defect-state distribution in as-deposited as well as in light-soaked a-Si: H. This technique gives direct information about the energy distribution of the gap states and is very sensitive to changes in the gap-state distribution. According to the defect-pool model by Powell and Deane,8 Q-DLTS measurements were performed on a-Si: H based metal-oxide-semiconductor ͑MOS͒ structures, consisting of a 1-m-thick a-Si: H layer deposited on n ++ crystalline Si. The fabrication method of the MOS structure is described by Durný et al. 11 The measurements were carried out using a bias voltage of −3 V, excitation pulses of 6 V, and a rate window of 100 s −1 . Prior to electron-beam irradiation or light soaking, the samples were annealed at 200°C for 30 min. A Van de Graaff accelerator, with a beam current density of 1.5ϫ 10 12 electrons cm −2 s −1 , was employed for the 1-MeV electron irradiation using a fluence of 2 ϫ 10 15 and 2 ϫ 10 16 electrons cm −2 . Light soaking was performed for 6 and 60 min using a red light laser havin...

Effects of irradiation with electrons of different energies on the dark conductivity and the network of hydrogenated amorphous silicon films

“…Many researchers have investigated the degradation of the photo-electrical properties and the generation of dangling bonds in undoped and lightly-doped a-Si:H films by prolonged irradiation time [14][15][16][17]. However, there are few reports on the effects on the properties of heavily-doped a-Si:H films of irradiation with electrons having different energies.…”