In this paper, deep level defects in high quality continuous GaN films grown over a cracked Si-doped GaN template has been studied using digital deep level transient spectroscopy (DLTS) and transmission electron microscopy (TEM). From TEM observation, it is found that the density of pure screw dislocations have been effectively suppressed while pure edge dislocations remained in substantial quantity. From DLTS measurement, trap levels at Ec -ET ∼ 0.11-0.12 eV, 0.24-0.27 eV, 0.60-0.63 eV were detected in the high quality GaN layer. DLTS measurement was also carried out on the underlying cracked Si-doped GaN template after the top high quality continuous GaN film was removed by plasma etching. An additional defect level at Ec-Et ∼ 0.37 eV was detected which we attributed to defect decoration at screw dislocation. Both the trap levels Ec-ET ∼ 0.24–0.27 eV, 0.60-0.63 eV are believed to originate from mixed screw/edge dislocation based on observation of the logarithmic capture behavior. Trap level at Ec -ET ∼ 0.24-0.27eV, however, experiences a more drastic increase in transient capacitance (i.e. in trap concentration) compared to that of Ec -ET ∼ 0.60-0.63 eV after plasma etching, illustrating that the latter is related to a higher proportion of edge dislocation. The 0.11-0.12 eV trap level, which exhibits an exponential capture kinetic, is believed to be related to nitrogen vacancies. This high quality continuous GaN layer can be used as a template to grow any device structure and the underneath cracked Si-doped GaN layer may help to release stress for the top continuous GaN layer. This can bring about a cracked free epilayer for subsequent device fabrication.

Reactive Ion Etching (RIE) induces defects in semiconductor materials. These defects can serve as local non-radiative recombination centres for electron-hole pairs, affecting the radiative lifetimes and luminescence efficiencies of the semiconductors. Argon (Ar) and sulphur hexafluoride (SF6) gases were used as etching gases to investigate the influence of ion energy on the RIE induced optical damage of Gallium Nitride (GaN). The significant result of etching by Ar and SF6 gases was that these etching induce defects, but as the total PL does not greatly change, it appears that this process is not increasing the density of nonradiative centres.

Electron injection into p-type GaN and related compounds leads to a pronounced increase in the minority carrier lifetime. This increase is manifested in a multiple-fold elongation of the minority carrier diffusion length as is evident from the Electron Beam Induced Current (EBIC) measurements in-situ in a Scanning Electron Microscope. Minority carrier transport enhancement as a result of electron injection is consistent with the changes observed in the material's luminescent properties. Based on the activation energy for the electron injection-induced effects, we ascribe this phenomenon to charging of Mg-acceptor related levels. In addition, we demonstrate an impact of electron injection on responsivity of GaN p-i-n photodetectors.

We have studied the effects of sapphire surface treatments and nitridation on GaN nucleation grown using the hydride vapor phase epitaxy technique. The surface treatments used were airannealing at 1400°C, etching in pure H2SO4 at 250°C, and etching in a 3:1 mixture of H2SO4:H3PO4 solution at 250°C. A nitridation step was carried out using 20% NH3 in N2 gas mixture at 1100°C. GaN nucleation and the early stage s of growth was investigated by short time growth and quench experiments. Atomic force microscopy and double crystal x-ray diffraction were used to examine the sapphire surface morphology, GaN island density, and GaN island structure. A lower density of GaN islands was grown on the air-annealed sapphire compared to the H2SO4-etched and 3:1 H2SO4:H3PO4-etched sapphire. GaN islands grown on the 3:1 H2SO4:H3PO4-etched sapphire had a broad mosaic spread due to preferential growth on surface pits. Sapphire nitridation resulted in a higher GaN island density with a smaller mosaic spread. A high density and uniform nucleation of GaN islands is critical in producing high quality thick GaN films. The H2SO4-etched sapphire and nitridation resulted in a high density of uniform GaN islands.

Mn doped ZnO thin films were grown using pulsed laser deposition technique on (001) Al2O3 substrates. The x-ray diffraction data confirmed highly c-axis oriented and hexagonal structure of ZnO and Mn doped ZnO thin films. However, an unidentified secondary peak was observed at very close to ZnO (002) peak. Micro Raman spectra of ceramics as well as thin films showed disorder induced Raman bands besides standard wurtzite ZnO modes. The intensity of several LO modes of ZnO was increased with the increase of Mn concentration. Optical absorption data showed an additional absorption band towards lower than bandgap energy (3 eV) with the increase in Mn content. The incorporation of Mn in ZnO thin films reduced the value of resistivities and mobilities with an increase in carrier concentrations.

ZnO films were deposited by Pulsed Laser Deposition (PLD) onto silicon substrates to serve as a buffer layer for GaN films grown by MOCVD. A ZnO buffer layer was found to improve the quality of GaN grown on Si. The thermal stability of ZnO as a buffer layer was also examined. It was determined that exposure of ZnO/Si to NH3 at high temperature (> 600°C) results in the decomposition of ZnO and subsequent poor nucleation of GaN. The ZnO layer thickness on GaN quality was found to be important.

We have studied luminescence and degradation of a ZnCdMgSe/ZnCdSe/ZnCdMgSe QW LED grown by MBE and lattice matched to its InP substrate. Results have been obtained from time-resolved and bias-dependent cathodoluminescence spectroscopy and from electroluminescence spectroscopy. Reversible and irreversible variations in luminescence intensity were observed during bias cycling and electron bombardment. Cathodoluminescence from the QW LED was modeled by calculating current generation and transport, bias-dependent energy levels, envelope wave functions, overlap integrals, and carrier escape rates. Results of these calculations agree qualitatively, but not quantitatively, with bias-dependent intensity and wavelength changes observed in the experiments.

Nowadays, large area, flat panel solid state detectors are being investigated for digital radiography. In this paper, development and evaluation of a selenium-based flat-panel digital xray detector are described. The prototype detector has a pixel pitch of 139μm and a total active imaging area of 7″×8.5″, giving a total of 1.9 million pixel. This detector include a x-ray imaging layer of amorphous selenium as a photoconductor which is evaporated in vacuum state on a TFT flat panel, to make signals in proportion to incident x-ray. The film thickness was about 500μm. To evaluate the imaging performance of the digital radiography (DR) system developed in our group, sensitivity, linearity of the response of exposure, the modulation transfer function(MTF) and detective quantum efficiency(DQE) of detector was measured. The measured sensitivity was 4.16×106 ehp/pixel mR at the bias field of 10 V/μm: The beam condition was 41.9 KeV. Measured MTF at 2.5 lp/mm was 52%, and the DQE at 1.5 lp/mm was 75%.

Effect of small dose gamma-irradiation on electrical characteristics of AlGaN/GaN high electron mobility transistors has been investigated. Decreasing of the leakage current and its noise has been registered after dose of 1×106 Rad. As-grown heterostructures used in further for the device fabrication have been examined after the same radiation treatment. The small dose radiation results are explained within a model that takes into account relaxation of elastic strains and structural-impurity ordering occurring in the barrier layer under irradiation.

Laser direct-write (LDW) is used for in-situ metallization in single crystal 4H- and 6H-SiC wafers without metal deposition. Nanosecond-pulsed Nd:YAG (λ= 1064 and 532 nm) and excimer (λ = 193, 248 and 351 nm) lasers are utilized to create metal-like conductive phases in both n-type and p-type SiC wafers. Frequency-doubled Nd:YAG irradiation(Ephoton < Eg) induces a carbon rich conductive phase due to thermal decomposition of SiC. However, pulsed excimer laser irradiation (Ephoton > Eg) produces a Si- rich conductive phases due to carbon photo ablation. The Schottky barrier heights (SBH) between the laser-metallized layer and the original n-type SiC (ND = 1018 cm-3) is determined to be 0.8 eV and 1.0 eV by the current-voltage and capacitance-voltage measurements at room temperature, respectively. Linear transmission line method pattern is directly fabricated in n-type doped (ND=1018cm-3) SiC substrate by pulsed laser irradiation allowing to extract the specific contact resistance (rc)of the laser fabricated metal-like tracks (rc= 0.04-0.12 Ωcm2).The specific contact resistance is unchanged after annealing up to 3 hrs at 950°C.

ZnO thin films and ZnO-Si-ZnO multi-layer thin films have been deposited by pulsed laser deposition (PLD). And then, the films have been annealed at 300°C in oxygen ambient pressure. The optical and structural properties changed by Si layer in ZnO thin film. UV and visible peak position was shifted by Si layer. Electrical properties of the films were improved slightly than ZnO thin film without Si layer. The optical and structural properties of ZnO thin films and ZnOSi-ZnO multi-layer thin films were characterized by PL (Photoluminescence) and XRD(X-ray diffraction method), respectively. Electrical properties were measured by van der Pauw Hall measurements.

Electrical properties of Ti/Al Schottky junctions formed on p-GaN layers are investigated using Hall, I-V and C-V measurements in the temperature range of 82∼500K. All data exhibit extremely strong low temperature carrier freeze out effects, assured by huge decreases of the Hall concentration, zero-bias capacitance and also diode current level as the temperature decreases below ∼150K. The dramatic decrease of current level at the low temperature range indicates that tunneling plays a more important role for the low temperature conduction and the junction shows ohmic-like I-V characteristics due to large series resistance. The barrier height estimated with the thermionic model varies with a strong temperature dependency from 1.21eV at 500K to 0.24eV at 82K. High frequency equivalent circuit analysis indicates that the corrected capacitance still shows a strong frequency dependency even after compensating the series resistance effect due to the slow response of Mg acceptor state. The transition frequency of the acceptor state, which is proportional to the emission rate of the deep level, is estimated to be 142KHz at room temperature. Low frequency C-V measurements provide a proper doping profile, revealing the ∼15nm thin layer with a high doping spike of ∼8×1018/cm3 near the sample surface and the constantly doped bulk layer with a doping level of ∼1017/cm3.

We have investigated a new method by which selective high doping concentration of GaN can be obtained using XeCl excimer laser and presented low ohmic contact resistance fabricated by the proposed method. We have irradiated silicon film deposited by sputtering on GaN using XeCl excimer laser and formed ohmic contact on the region irradiated by laser. Ohmic contact resistance by laser doping process was effectively reduced to 0.27 ohm-mm while 0.66 ohm-mm was measured in ohmic contact by conventional method. We have verified that silicon is diffused into GaN by high laser energy during laser irradiation through SIMS analysis and ohmic contact resistance is reduced due to increase of doping concentration under ohmic contact region.

Large area, flat panel detectors are being investigated for digital radiogrpahy and fluoroscopy. Theses detectors employ an x-ray conversion layer of photoconductor to detect x-rays. The amorphous selenium layer that is currently being studied for its use as an x-ray photoconductor is not pure a-Se but rather a-Se doped with 0.2-0.5% As and 10-30 ppm Cl, also known as stabilized a-Se. The suitability of the stabilized a-Se is largely determined by its charge on generating, transporting and trapping properties.

In this paper, a conventional time-of-flight measurement was carried out to analyze the transport properties of charge carriers. A laser beam with pulse duration of 5 ns and wavelength of 350 nm was illuminated on the surface of the stabilized a-Se with thickness of 400 μm. The photo response signals of the hole and electron were measured at the applied electric field of 10 V/μm as a function of time. The measured transit times of the hole and electron were about 229.17μs and about 8.73μs at 10 V/μm, respectively. The measured mobility indicated a slight dependence with respect to the applied electric field with a range of 4-10 V/μm. The experimental results showed that the measured mobility of the hole and electron was 0.04584 cm2V-1s-1 and 0.00174 cm2V-1s-1 at the electric field of 10 V/μm.

We report high-speed solar-blind AlGaN-based Schottky photodiodes. AlGaN/GaN heterostructure device layers were grown on sapphire substrate. The devices were fabricated on AlGaN/GaN heterostructuresusing a microwave compatible fabrication process. Schottky photodiodes with Au and indium-tin-oxide (ITO) Schottky contacts were fabricated. Current-voltage, spectral responsivity, and high-speed measurements were performed. Both Schottky samples exhibited very low sub-pA dark currents at high reverse bias. A bias dependent spectral responsivity was observed with a peak responsivity of 89 mA/W at 267 nm, and 44 mA/W at 263 nm for Au and ITO-Schottky devices respectively. Time-based high-frequency measurements at 267 nm resulted in pulse responses with rise times and pulse-widths as short as 13 ps and 74 ps respectively. The fastest solar-blind detector had a record 3-dB bandwidth of 1.10 GHz.

ZnO thin films were grown with different plume-substrate angles and growth times by pulsed laser deposition. The angles between plume propagation direction and substrate plane were changed from 0°, 45° to 90°. The growth time was changed in order to adjust film thickness. From the XRD pattern exhibiting dominant (002) XRD peak of ZnO, all films were found to be c-axis oriented. Optical property of ZnO thin films were investigated by PL(Photoluminescence). The ratio of UV/green luminescent intensity of ZnO thin film increased, as the angle between plume and substrate plane decrease from 90° to 0°.

AlxGa1-xN layers grown by MOCVD on sapphire substrates have been tested using the surface acoustic wave and guided optical wave techniques. Samples with molar fraction of Al up to 0.36 have been investigated. The parameters S11 of single surface acoustic wave transducers and S12 of two-port devices have been measured with the network analyzer. The values of the surface acoustic wave velocity and electromechanical coupling coefficient for the layer-substrate structure have been extracted and calculated using material parameters available from literature. The attenuation of guided optical waves along the propagation track in AlxGa1-xN layers has been measured using the CCD imaging technique. The attenuation dependence on the mode order and layer thickness has been studied. Our results show that the properties of AlxGa1-xN that determine surface acoustic wave and guided optical wave propagation are similar to the properties of GaN films within a relatively wide range of x up to x =0.36.

We report on the electrical properties of defects introduced in epitaxial 4H-SiC by 2 MeV protons using deep level transient spectroscopy (DLTS). After proton irradiation with a dose of about 1.5×1014 cm-2, the DLTS measurements were made, and the rate window shows a single broad peak between 280 and 310 K. The intensity of this peak remains unchanged after a thermal anneal at 900°C for 20 min. However, after annealing at or above 1100°C, the peak intensity gradually decreases with anneal temperature up to 1500°C, indicating a decrease in the defect concentration. Because a complete damage recovery of the SiC is not observed even after annealing at 1500°C, we believe a higher temperature annealing is necessary for a complete recovery. Using a curve fit analysis, a set of deep levels of defect centers were found with energy ranging between 567 and 732 meV. These traps do not exhibit a significant change in the trap energy or capture cross-section parameters as a function of anneal temperature, but the decrease in the trap density with increasing anneal temperature demonstrates a damage recovery.

Since the work of Favennec et al.[1] it is well known that the quenching of luminescence from rare earth ions decreases with the host band gap. This has led to a large activity with silicon implanted or doped with RE, and then GaAs was used, in hope to realize simple, cheap light emitters. With a band gap of 3.4 eV at room temperature, GaN is even better suited to such applications. As a matter of fact, Steckl et al.[2] have demonstrated a green light emitting device based on Er doped GaN. This resulted in a renewed effort in this direction, but the crystal quality still have to be mastered and the physical phenomenon involved to be understood. In this work, GaN and Er-doped GaN with various Er concentrations were grown by gas source molecular beam epitaxy on high quality GaN templates grown by metalorganic chemical vapour deposition. In order to understand the influence of the Er incorporation on the crystal quality of GaN, Er-doped GaN were grown with a concentration between 0.1% and 5%. High quality undoped GaN were also grown, as a reference material, to show how the smallest amount of Er may affect drastically the structural and optical properties. All the samples were characterized by scanning electron microscopy, atomic force microscopy and X-ray diffraction. With these measurements, we demonstrate a strong correlation between the Er concentration and the surface roughness and the crystalline quality. This study shows that the activation of the Erbium luminescence is not improved with improving crystal quality. This assumption supports the idea that Er luminescence should be related to defect center in GaN.

Nickel (Ni) contacts to n-type silicon carbide (n-SiC) are annealed to ∼950°C in order to achieve their good ohmic properties through the reaction of the Ni with Si from the SiC to form nickel silicides. Unfortunately, the physical contact, and therefore the reliability, is poor. A possible cause is that the silicidation reaction liberates C from the SiC, which then diffuses throughout the contact. The reaction also produces a poor morphology and voids form at the metal-SiC interface. To try to understand the processes that produce the good electrical properties and at the same time improve the physical properties, we studied the reactions of Si/Ni contacts with 1:1 and 1:2 stoichiometric ratios as well as Ni-only contacts on n- and p-type SiC, both visually and electrically in situ using a hot stage and microprobe-equipped scanning electron microscope (SEM). The visual observations of the Ni-only film show that it does not react with the SiC until the temperature reaches 500–550°C. For the n-type SiC, the electrical measurements show a decrease in contact resistivity as the anneal temperature is increased from 500°C to 650°C. Increasing the anneal temperature further increases the resistivity until it begins to drop precipitously as the temperature approaches 950°C and higher. The visual observations of the Si/Ni contacts show that the Si and Ni are reacting at ∼600°C, with phases nucleating and then growing laterally. The electrical measurements for the n-type samples show that the contact resistance initially drops at 100–300°C indicating that there may be reactions, unseen by the SEM, at lower temperatures. The resistance continues to rise and fall over the intervening temperatures but begins to consistently and significantly fall at temperatures above 850°C, and then reaches ohmic values at 900–950°C. Because the silicidation reactions are seen to occur at temperatures far below those required to achieve ohmic properties, it is clear that silicide formation, while it may be necessary, is not sufficient for the formation of Ni-ohmic contacts to n-SiC. In this work, it has been observed that reaction of the Ni with the SiC appears to be necessary for achieving ohmic properties. While this may form a more intimate contact, it is proposed that damaging the SiC surface with this reaction is an important part of ohmic contact formation, possibly through increased current tunneling through interface defect states.