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 .

Biocompatible solid electrolyte chitosan is introduced as a protonic/electronic electric double layer (EDL) dielectric in a multilayer MoS 2 transistor. This chitosan-bioinspired transistor can operate at a low working voltage (<3 V) and exhibits an asymmetric ambipolar behavior. A high on/off ratio ($10 4 ) was achieved for both electrons and holes, in conjunction with a very steep subthreshold swing (67 mV/dec) which is close to the theoretical limit of an ideal field-effect transistor (60 mV/dec). It was established that the on-state conductance of these devices is strongly limited by the contact resistance of the metal-MoS 2 junctions, and the conductance can be increased by a factor of $3 by using dual-gate electrostatic modulation. Further exploiting the double-gate geometry allows us to characterize the contact resistance in the on-regime. We make a numerical simulation and observe important contributions that are independently modulated by back and top gates stemming from Schottky barriers formed at the MoS 2 /metal interface and the chitosan-induced charge accumulation near the electrode, respectively. Such EDL multilayer MoS 2 transistors with bioinspired chitosan solid electrolyte can provide a new opportunity for the fabrication of low voltage and costeffective two-dimensional semiconductor devices which are also biocompatible and environment friendly.

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.

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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.