Vahid Mirkhani

Depth-resolved ultra-violet spectroscopic photo current-voltage measurements for the analysis of AlGaN/GaN high electron mobility transistor epilayer deposited on Si

We have demonstrated that the depth-dependent defect distribution of the deep level traps in the AlGaN/GaN high electron mobility transistor (HEMT) epi-structures can be analyzed by using the depth-resolved ultra-violet (UV) spectroscopic photo current-voltage (IV) (DR-UV-SPIV). It is of great importance to analyze deep level defects in the AlGaN/GaN HEMT structure, since it is recognized that deep level defects are the main source for causing current collapse phenomena leading to reduced device reliability. The AlGaN/GaN HEMT epi-layers were grown on a 6 in. Si wafer by metal-organic chemical vapor deposition. The DR-UV-SPIV measurement was performed using a monochromatized UV light illumination from a Xe lamp. The key strength of the DR-UV-SPIV is its ability to provide information on the depth-dependent electrically active defect distribution along the epi-layer growth direction. The DR-UV-SPIV data showed variations in the depth-dependent defect distribution across the wafer. As a result, rapid feedback on the depth-dependent electrical homogeneity of the electrically active defect distribution in the AlGaN/GaN HEMT epi-structure grown on a Si wafer with minimal sample preparation can be elucidated from the DR-UV-SPIV in combination with our previously demonstrated spectroscopic photo-IV measurement with the sub-bandgap excitation.

Electrical characteristics and density of states of thin-film transistors based on sol-gel derived ZnO channel layers with different annealing temperatures

We report on the fabrication and electrical characterization of bottom gate thin-film transistors (TFTs) based on a sol-gel derived ZnO channel layer. The effect of annealing of ZnO active channel layers on the electrical characteristics of the ZnO TFTs was systematically investigated. Photoluminescence (PL) spectra indicate that the crystal quality of the ZnO improves with increasing annealing temperature. Both the device turn-on voltage (Von) and threshold voltage (VT) shift to a positive voltage with increasing annealing temperature. As the annealing temperature is increased, both the subthreshold slope and the interfacial defect density (Dit) decrease. The field effect mobility (μFET) increases with annealing temperature, peaking at 800 °C and decreases upon further temperature increase. An improvement in transfer and output characteristics was observed with increasing annealing temperature. However, when the annealing temperature reaches 900 °C, the TFTs demonstrate a large degradation in both transf...

Electrical and optical characteristics of gamma-ray irradiated AlGaN/GaN high electron mobility transistors

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.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...

Study of device instability of bottom-gate ZnO transistors with sol–gel derived channel layers

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 n...

Analysis of the effect of gamma-ray irradiation and low-temperature characteristics of sol-gel derived ZnO thin-film transistors

Summary form only given. In conclusion, we investigated the effects of gamma-ray irradiation and low temperature on the device performance of sol-gel derived ZnO thin-film transistors. In both cases, the degradation of the device performance was observed. In the case of radiation study, further investigation is needed to identify which part of the device is damaged upon gamma-ray irradiation. Further details and results from additional experiments will be presented.