The effect of gamma-ray (γ-ray) irradiation on the material characteristics of nanometre scale films of molybdenum disulphide (MoS 2) has been investigated. 3.2, 4.5, and 5.2 nm thick MoS 2 films (measured by atomic force microscopy) were grown on Si by using a two-step synthesis method (sputtering of Mo, followed by sulphurisation). The samples were subsequently exposed to γ-ray irradiation (dose of 120 MRad). Dramatic chemical changes in the MoS 2 films after irradiation were characterised by micro-Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and optical microscopy. Micro-Raman spectroscopy showed the disappearance of the E 2g 1 and A 1g modes after irradiation. XPS revealed that the MoS 2 crystal structure was converted to molybdenum oxide (MoO x). It is hypothesised that S vacancies are formed due to the γ-ray irradiation, which subsequently transforms MoS 2 to MoO x .
Proton irradiation-induced effects on AlGaN/GaN high electron mobility transistors (HEMTs) were studied by emulating a certain space radiation environment (upstream of the earth's bow shock) using a relatively low energy (100 keV) proton beam with fluences of 1 × 1010, 1 × 1012, and 1 × 1014 protons/cm2. In order to isolate radiation-induced effects produced by the modification of the epi-layer from the effects produced by the change in the device structure (such as contacts), the epi-layers were irradiated prior to device fabrication, followed by material/device characterization. Proton irradiation-induced sub-gap traps were detected by spectroscopic photo current-voltage measurement. Raman study revealed that the proton irradiation had induced strain relaxation on the AlGaN/GaN HEMTs epi-layer. No substantial change in the crystal quality of the epi-layer was indicated by Raman and PL studies. With increasing proton fluences, increasing charge carrier density was observed, which was estimated via Raman spectroscopy and the charge-control model analysis. The magnitude and direction of the transistor threshold voltage shift were also dependent on proton fluence. Overall, degradation of transistor output characteristics of the fabricated HEMTs was observed with increasing proton fluence. However, based on the observed performance and the level of influence on material/device characteristics by 100 keV protons, it can be suggested that the AlGaN/GaN HEMTs have high endurance for exposure to relatively high fluences of the low-energy proton beam.
A comparative study on the direct-current (dc) electrical performance and optical characteristics of unirradiated and 120 MRad 60Co-gamma-ray (γ-ray) irradiated AlGaN/GaN high electron mobility transistors (HEMTs) was performed. The devices fabricated on an irradiated HEMT epilayer structure show slight degradation/alteration in the dc characteristics such as source–drain current–voltage (IDS-VDS), transfer (IDS-VGS), transconductance, and gate current–voltage, indicating the presence of radiation-induced defects. Also, a shift in flat band voltage was observed from the capacitance-voltage measurements. Micro-Raman spectroscopy and photoluminescence (PL) spectroscopy were used to compare the crystal quality of the heterojunction. No shift in the Raman peak frequency position was observed in both the unirradiated and irradiated samples, which implies that the irradiation did not produce an additional strain to the HEMT layers. However, the full width at half maximum of the Raman and near-band-edge PL peaks has increased after irradiation, which suggests the degradation of crystal quality. The spectroscopic photocurrent–voltage study with sub-bandgap and above bandgap illumination confirmed the pre-existence of sub-bandgap defects in the heterostructure and revealed the possibility of their rearrangement or the introduction of new defects after the irradiation. It was concluded that AlGaN/GaN HEMTs are relatively resistant to high dose (120 MRad) gamma-ray irradiation, but they can introduce additional traps or reconfigure the pre-existing traps, influencing the electrical and optical characteristics of AlGaN/GaN HEMTs.
In this paper, the authors report the device instability of solution based ZnO thin film transistors by studying the time-evolution of electrical characteristics during electrical stressing and subsequent relaxation. A systematic comparison between ambient and vacuum conditions was carried out to investigate the effect of adsorption of oxygen and water molecules, which leads to the creation of defects in the channel layer. The observed subthreshold swing and change in field effect mobility under gate bias stressing have supported the fact that oxygen and moisture directly affect the threshold voltage shift. The authors have presented the comprehensive analysis of device relaxation under both ambient and vacuum conditions to further confirm the defect creation and charge trapping/detrapping process since it has not been reported before. It was hypothesized that chemisorbed molecules form acceptorlike traps and can diffuse into the ZnO thin film through the void on the grain boundary, being relocated even near the semiconductor/dielectric interface. The stretched exponential and power law model fitting reinforce the conclusion of defect creation by oxygen and moisture adsorption on the active layer.
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