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.

A simple and novel spectroscopic photo I–V method of diagnosing the homogeneity of electrically-active defect distribution in the large area AlGaN/GaN HEMT (high electron mobility transistor) epi-structure grown on 6-inch silicon wafers is reported. It is of utmost importance to produce the HEMT epi-structure electrically homogeneous across the wafer if devices with uniform electrical characteristics are to be constructed. AlGaN/GaN HEMT epi structures were grown on a silicon substrate via metal–organic chemical vapour deposition. An array of circular semi-transparent Ni Schottky contacts was prepared on top of the diced AlGaN/GaN HEMT structure substrates, which were selected from different locations of the 6-inch wafer. The information of the electrical homogeneity across the wafer was elucidated from the spectral dependences of the I–V characteristics collected from different locations of the AlGaN/GaN HEMT wafer. It is successfully demonstrated that the proposed spectroscopic photo I–V measurement technique can be employed to diagnose electrical homogeneity of the electrically-active defect distribution in the AlGaN/GaN HEMT epi structure constructed on Si with minimum sample preparation steps.

Deleterious effects of high concentration of defects on the performance of AlGaN/GaN high electron mobility transistors (HEMTs) cannot be disregarded. Therefore, Raman spectroscopy, photoluminescence (PL), and spectroscopic photo current-voltage measurements were performed to characterize the surface/interface and bulk defects. An array of Schottky contacts were prepared on the piece of the HEMT wafer, and 2 devices with a similar dark current characteristics (sample A and B) were chosen for this investigation. Raman spectroscopy displayed that two samples have similar crystalline quality and stress. In addition, PL measurement revealed that samples possessed strong near band edge peak at around 3.42eV and broad blue luminescence spreading from 3.17eV to 2.55eV with a peak maximum at around 2.79eV. The spectroscopic photo current-voltage (I-V) measurements with sub-bandgap illumination exhibited the existence of sub-bandgap defects with different activation energies for each sample. The depth-resolved UV spectroscopic photo current-voltage (I-V) (DR-UV-SPIV) measurements revealed that the sample A contains higher concentration of defects at depth ~70nm and ~90nm from the surface of AlGaN/GaN HEMTs heterostructure than the sample B. It was demonstrated that even though electrically active defects for two devices on the same pieces of AlGaN/GaN HEMTs heterostructures cannot be distinguished in Raman spectroscopy and PL measurement, they can be differentiated with the use of spectroscopic photo I-V with sub-bandgap illumination and DR-UV-SPIV measurements. It was concluded that DR-UV-SPIV and spectroscopic photo I-V measurements with sub-bandgap illumination can be used to assess the uniformity of in-depth and spectral distribution of sub-band gap defects across the AlGaN/GaN wafers, respectively. Consequently, these techniques can play an important role as a diagnostic tool for optimizing materials properties of AlGaN/GaN HEMTs heterostructure, and will provide wafer vendors a valuable information for the assessment of wafer quality in terms of the distribution of electrically active defects.

Time-resolved photocurrent (TRPC) spectroscopy with a variable-wavelength sub-bandgap light excitation was used to study the dynamics of the decaying photocurrent generated in the heterostructures of the AlGaN/GaN high electron mobility transistors (HEMTs) layers. In AlGaN/GaN HEMTs, reliability of the device is degraded due to the prevalence of current collapse. It is recognized that electrically active deep level defects at the surface/interfaces and the bulk in the HEMTs layers can contribute to the unwanted current collapse effect. Therefore, it is of great importance to analyze the deep level defects if the reliability of the HEMTs device is to be improved. In this research, TRPC spectroscopy was used to elucidate the origin and nature of the deep level defects by analyzing the time evolution of the photocurrent decay excited at different wavelengths of light. The two devices that show similar characteristics for wavelength-dependency on photocurrent generation were chosen, and TRPC spectroscopy was conducted on these devices. Although the two samples show similar characteristics for the wavelength-dependency on photocurrent generation, they exhibited dissimilar time-dependent photocurrent decay dynamics. This implies that TRPC spectroscopy can be used to distinguish the traps which have different origins but have the same de-trapping energy.

We have theoretically investigated the change of refractive index of nanocomposites composed of Ga-doped ZnO nanoparticles (Ga-ZnO NPs) dispersed in conductive polymer Poly(3,4-ethylenedioxythiophene) and Poly(styrene sulfonic acid) (PEDOT:PSS) matrix by tuning the fraction of the Ga-ZnO NP inclusions and concentration of Ga dopant in Ga-ZnO, by using Maxwell-Garnett, Bruggeman and Lorentz-Lorenz effective medium theories. The dependence of the effective refractive index of Ga-ZnO NPs embedded in PEDOT:PSS composite on the Ga doping concentration and its volume fraction of the inclusions are presented. Ga-ZnO NPs can increase the effective refractive index. This can be used to engineer a desired effective index of refraction for the enhancement of light extraction and absorption efficiency in optical devices such as organic light emitting diodes (OLEDs) and organic solar cells (OSCs). It is crucial to match the refractive indices of conductive anode electrode and the emissive layer.

The effect of gamma-ray (γ-ray) irradiation on the material characteristics of nanometre scale films of molybdenum disulphide (MoS2) has been investigated. 3.2, 4.5, and 5.2 nm thick MoS2 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 MoS2 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 E2g1 and A1g modes after irradiation. XPS revealed that the MoS2 crystal structure was converted to molybdenum oxide (MoOx). It is hypothesised that S vacancies are formed due to the γ-ray irradiation, which subsequently transforms MoS2 to MoOx.

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 acceptor like 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.

A comparative study on the direct-current (dc) electrical performance and optical characteristics of unirradiated and 120 MRad 60Co-gamma-ray (c-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 thepresence 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 subbandgap 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 letter, we present a study of the condensation of exciton-polaritons in large etched pillar structures that exhibit shallow edge trapping. The ~100 um x100 um pillars were fabricated using photolithography and a BCl3/Cl2 reactive ion etch. A low energy region emerged along the etched edge, with the minima ~ 7 um from the outer edge. The depth of the trap was 0.5 -1.5 meV relative to the level central region, with the deepest trapping at the corners. We were able to produce a Bose-Einstein condensate in the trap near the edges and corners by pumping non-resonantly in the middle of the pillar. This condensate began as a set of disconnected condensates at various points along the edges, but then became a single mono-energetic condensate as the polariton density was increased. Similar edge traps could be used to produce shallow 1D traps along edges or other more complex traps using various etch geometries and scales.