Mg-doped and In-Mg co-doped p-type GaN epilayers were grown using the metal organic chemical vapour deposition technique. The effect of In co-doping on the physical properties of p-GaN layer was examined by high resolution x-ray diffraction (HRXRD), transmission electron microscopy (TEM), Hall effect, photoluminescence (PL) and persistent photoconductivity (PPC) at room temperature. An improved crystalline quality and a reduction in threading dislocation density are evidenced upon In doping in p-GaN from HRXRD and TEM images. Hole conductivity, mobility and carrier density also significantly improved by In co-doping. PL studies of the In-Mg co-doped sample revealed that the peak position is blue shifted to 3.2 eV from 2.95 eV of conventional p-GaN and the PL intensity is increased by about 25%. In addition, In co-doping significantly reduced the PPC effect in p-type GaN layers. The improved electrical and optical properties are believed to be associated with the active participation of isolated Mg impurities.

We report the effects of high-energy Ni-ion irradiation on the properties of the InAlGaN/GaN heterostructure grown on c-plane sapphire substrates by using a low-pressure metal organic chemical vapor deposition system. The heterostructure was irradiated with 100 MeV Ni ions for a fluence of 5 x 10(12) cm(-2) at room temperature. The X-ray rocking curve (0002) full width at half maximum (FWHM) value of the as-grown InAlGaN layer was measured to be similar to 250 arcsec and was not affected by Ni-ion irradiation. However, the PL peak intensity of the heterostructure was completely quenched after ion irradiation, and its electrical properties turned to be highly insulating clue to irradiation-induced defects. These results indicate that optical and electrical isolation is feasible in InAlGaN/GaN devices without damaging the crystalline quality by using the Ni-ion irradiation. The thermal annealing process could partly recover the optical and electrical properties of the heterostructure whereas, the junction properties of Schottky contacts fabricated on InAlGaN layer could not be restored.

We report an amelioration of the electrical, thermal and optical performances of green InGaN/GaN multiple-quantum-well light-emitting diodes (LEDs) with AlGaN/GaN short-superlattice (SSL)-inserted structure grown using metal-organic chemical vapor deposition. According to an atomic force microscopic study, the GaN template with the SSL-inserted structure shows a significant reduction in pit density compared to the conventionally-grown template. The insertion of the SSL is also found to alleviate the effect of threading dislocations on the degradation of the electrical performance and promotes the stability of the K-factor and a low thermal resistance under a long-term acceleration test. A relatively higher optical output power is obtained for SSL-inserted InGaN/GaN green LEDs at high injection currents.

Surface morphology of green InGaN/GaN multi-quantum wells (MQWs) grown by using metal organic chemical vapor deposition has been analyzed by using atomic force microscopy, scanning electron microscopy, and cathodoluminescence (CL). Effects of barrier growth temperature, indium composition, and background threading dislocation (TD) density on the evolution of the MQW surface morphology have been studied. Low temperature GaN barrier growth generates a high density of V-pits and inclusions embedded within V-pits. Using trimethylindium flow prior to InGaN well growth, In segregation on GaN barrier is shown to be the prime cause for the formation of some inclusions which appear as bright spots in CL mapping while some inclusions are related to low temperature barrier growth. High temperature GaN barrier growth at 910°C completely suppresses these inclusion defects. In high indium containing InGaN/GaN MQWs, the large lattice mismatch induced strain in the MQW plays the key role in producing V-pits as well as inclusion defects rather than the background TD density of GaN templates.

Systematic development and mechanistic studies of sensing materials are critical to the design of higher performance gas sensing elements and arrays. Polycrystalline metal-oxide semiconductors such as SnO2 and TiO2 are among the most widely used materials for thin film-based conductometric gas sensors. The mechanistic steps responsible for the gas-induced conductance changes of polycrystalline metal-oxide sensors have been investigated. Results are presented for TiO2 gas sensing films. The TiO2 films experience an increase in conductance upon exposure to ammonia. Reduction of surface oxygen is proposed as the dominant mechanism for the increase in conductance in TiO2 sensing films upon exposure to ammonia. Here TiO2 films of low thickness prepared using DC magnetron sputtering were employed for sensing applications. A suitable operating temperature, sensitivity, response and recovery time of the TiO2 thin film gas sensor was studied for sensing ammonia.

ffect of rapid thermal annealing on the properties of PECVD SiNx thin films B. Karunagaran a, S.J. Chung a, S. Velumani b,∗, E.-K. Suh a,∗∗ a Semiconductor Physics Research Center and Department of Semiconductor Science and Technology, Chonbuk National University, Jeonju 561-756, Republic of Korea b Departamento de Fı́sica, Tec de Monterrey, Campus Monterrey, E. Garza-Sada #2501, Monterrey, N.L., C.P. 64849, Mexico

Silicon nitride (SiNx:H) thin films were grown on silicon by the plasma-enhanced chemical vapor deposition (PECVD) method at low temperature in order to study their optical, electrical properties and correlate these properties to the chemical composition of the layers, so that films with desired properties may be achieved for silicon solar cells. By varying the silane (SiH4) to ammonia (NH3) ratio in the plasma gas we have been able to modify the index of refraction (from 1.9 to 2.3) and also the silicon surface state passivation properties of the films. Our results indicate that the mid-gap surface state density in silicon can be reduced down to 1.1×1010cm−2eV−1 for the SiNx:H layer deposited under optimized silane to ammonia ratio. Also, an extensive study has been carried out on the effect of rapid thermal annealing (RTA) on the carrier lifetime, reflectance, chemical composition, refractive index and interface states which decides the final output of the solar cell.

We have investigated the optical properties of Al(x)Ga(1-x)N/GaN heterostructures (x=0.08, 0.15, 0.33) grown by metal organic chemical vapor deposition on sapphire using photoluminescence (PL) and persistent photoconductivity (PPC) measurements. For the Al(x)Ga(1-x)N/GaN heterostructures (HS) containing high Al composition, we observed an anomalous temperature-dependent photoluminescence and persistent photoconductivity effects. These results show a strong dependence of the physical properties of Al(x)Ga(1-x)N/GaN HS on the Al content and layer thickness. The anomalous temperature-dependent PL is usually attributed to the presence of carrier localization states. These phenomena are explained based on the alloy compositional fluctuations in the Al(x)Ga(1-x)N/GaN HS. From the PPC measurements, the photocurrent (PC) quenching was observed for Al(x)Ga(1-x)N/GaN HS and it is explained by the metastable states formed in the underlying GaN layer. Also, the mechanisms behind the PC quenching and PPC phenomena are explained in detail.

Surface morphology of green InGaN/GaN multi-quantum wells (MQWs) on a sapphire substrate with various high temperature grown GaN barriers has been evaluated. Keeping the InGaN well growth temperature constant at 740 degrees C, a series of MQWs were grown with GaN barrier temperatures varied up to 910 degrees C. GaN barriers grown below 800 degrees C lead to the generation of a high density of V-defects and inclusions embedded within V-defects as observed by atomic force microscopy. Scanning electron microscopy and cathodoluminescence (CL) studies revealed that the embedded inclusions are of two kinds: one of them appears as bright spots in CL mapping while the other appears as the surrounding region. Temperature ramping and subsequent interruption for GaN barrier growth suppresses both kinds of inclusion defects and also significantly reduces the V-defect density. An inclusion-free smooth surface is obtained for green emitting InGaN/GaN MQWs with the GaN barrier grown at 910 degrees C.

InAlGaN/GaN heterostructures with various Al compositions have been grown on sapphire substrate using the metal organic chemical vapour deposition technique. The solid-to-gas phase ratio indicates a high Al incorporation efficiency. Atomic force microscopy reveals a smooth surface with the formation of hexagonal pits. The size and the density of the hexagonal pits increase with increasing Al mole fraction. The Hall effect and the capacitance-voltage (C-V) studies show the formation of a two-dimensional electron gas (2DEG) at the InAlGaN/GaN interface. A relatively higher Hall sheet carrier density compared with the 2DEG density estimated from the C-V profile indicates parallel conduction via the underlying GaN layer. It is observed that the 2DEG density decreases as a function of the Al composition and these results are discussed based on the increasing depth of the hexagonal pit and the background donor density.

The design and measured results of the developed radar sensor for automotive applications are presented. The radar sensor is highly integrated with the transceiver module, antenna and baseband circuits. The proposed antenna is composed of a novel microstrip-line-fed horn antenna and a quasi-optic lens antenna.

The defect levels associated with the Mg impurity in p-type GaN films were systematically investigated in terms of doping concentration by using photocurrent spectroscopy. Mg-doped GaN samples were grown on sapphire substrates by using metal-organic chemical-vapor deposition (MOCVD) and were annealed in a nitrogen atmosphere at 850 degrees C for 10 min. At room temperature, photocurrent (PC) spectra showed two peaks, one at, 3.31 and the other at 3.15 eV, associated with acceptor levels formed at 300 and 142 meV, respectively, above the valence band in the as-grown samples. However, after thermal annealing, PC spectra exhibited various additional peaks, depending on the Mg concentration. In the GaN samples with a Mg concentration around 6 similar to 7 x 10(17) cm(-3), we observed PC peaks related to Mg at 3.31 and 3.02 eV and to the carbon acceptor at 3.17 eV. For moderately Mg-doped GaN samples, i.e., a hole concentration of p = 3 similar to 4 X 10(17) cm(-3), an additional peak was observed around 0.9 eV, which could be attributed to defects related to the Ga vacancy. For relatively low Mg-doped samples whose hole concentrations are 1 similar to 2 X 10(17) cm(-3), an additional broad peak was observed around 1.3 eV. This peak might be related to the yellow band luminescence. As the Mg concentration was increased, the concentration of Ga vacancies could be reduced because the Mg occupied the substitutional sites of Ga in the GaN lattice. When the hole concentration was above 6 similar to 7 X 10(17) cm(-3), the yellow luminescence and the Ga-vacancy-related peaks disappeared completely.

We have investigated the optical properties of Al x Ga 1-x N/GaN heterostructures (x=0.08, 0.15, 0.33) grown by metal organic chemical vapor deposition on sapphire using photoluminescence (PL) and persistent photoconductivity (PPC) measurements. For the Al x Ga 1-x N/GaN heterostructures (HS) containing high Al composition, we observed an anomalous temperature-dependent photoluminescence and persistent photoconductivity effects. These results show a strong dependence of the physical properties of Al x Ga 1-x N/GaN HS on the Al content and layer thickness. The anomalous temperature-dependent PL is usually attributed to the presence of carrier localization states. These phenomena are explained based on the alloy compositional fluctuations in the Al x Ga 1-x N/GaN HS. From the PPC measurements, the photocurrent (PC) quenching was observed for Al x Ga 1-x N/GaN HS and it is explained by the metastable states formed in the underlying GaN layer. Also, the mechanisms behind the PC quenching and PPC phenomena are explained in detail.

Two different types of magnesium (Mg) diffusion onto intentionally undoped n-type GaN, resulting in p-type GaN formation, have been employed in this work. Secondary-ion mass spectroscopy (SIMS) measurements reveal a uniform Mg concentration for both methods of diffusion up to a depth of 1.5 mu m in GaN. Hall measurement data indicate that all the diffused and annealed samples consistently show p-type conductivity with hole concentrations in the range of 5 similar to 7 x 10(16)/cm(3) and with mobilities < 50 cm(2)/V.s. The measured dark current of the samples almost exponentially increases with the reciprocal temperature with an activation energy of 144 meV or 168 meV in the temperature range from 170 K to 250 K. The photoluminescence (PL) spectra of Mg diffused GaN show a broad violet emission at around 3.15 eV. This broad peak may be attributed to Mg acceptors. In the photocurrent (PC) measurements on the Mg/Au film deposited on n-type GaN, an additional broad peak was observed at around 1.2 eV, and this peak may be related to the yellow luminescence.

This paper describes a new concept of dense, dual-phase membrane consisting of two phases conducting, respectively, CO32- and electrons for selective permeation of CO2 and O-2 at high temperatures. Membranes with a molten carbonate phase in a porous stainless-steel support were synthesized by a direct infiltration method. Membrane preparation conditions were optimized to obtain stable, gas-tight dual-phase membranes at high temperatures (> 450 degrees C). The dual-phase membranes exhibit low single-gas permeance for pure CO2 and N-2 (< 5 x 10(-9) mol s(-1) m(-2) Pa-1) in 450-650 degrees C but substantially higher CO2 permeance for CO2 mixed with O-2 at high temperatures. CO2, after interacting with O-2, transports through the molten carbonate phase in a form of CO3, with electrons transporting through the porous metal support. With further improvement, the dual-phase membranes may offer application in producing O-2-enriched CO2 streams for the oxyfuel combustion process. The concept can be also extended to prepare CO2 perm-selective dense membranes for high-temperature CO2 separation.

Single crystals structures of MgxZn1-Te-x (0 <= x <= 0.48) were grown for various Mg mole compositions by using a vertical Bridgman method. The optical properties of the grown crystal were characterized using photocurrent (PC) and persistent photoconductivity (PPC) measurements. As a result of the PC spectra, single crystalline MgxZn1-xTe (x = 0) showed an energy band gap of 2.380 and 2.260 eV at 4.2 and 294 K, respectively. The PC peak blue-shifts with increasing Mg mole fraction and shows linear dependence of energy band gap, E-g(x) = b + (0.8)(x). The extrapolation shows the energy band gaps of MgTe as 3.18 and 3.06 CV at temperatures of 4.2 and 294 K, respectively. Furthermore, the PC peak red-shifts with increasing temperature, and the temperature coefficient is given by a value of dE(g)/dT = - (5.6 similar to 6.1) x 10(-4) eV/K for the temperature range above 100 K. In the PPC measurements, the decay times of the PPC spectra strongly depended on the Mg content. These results clearly indicate a change in crystal structure with increasing Mg mole fraction.

Two-, three-, and four-photon-pumped stimulated emission (cavityless lasing) properties of ten novel stilbazolium dyes in solution phase have been comprehensively studied. These newly synthesized multiphoton active dye compounds have the same molecular backbone but differ either in their electron donors or in their electron acceptors and can be utilized to generate highly directional stimulated emissions over a broad visible spectral range (from 490 to 618 nm) under multiphoton-pump conditions. The pump source was a powerful Ti:sapphire oscillator-amplifier system associated with an optical-parametric generator, which could specifically provide similar to 160 fs duration and similar to 775, 1320, and 1890 nm laser pulses for two-, three-, and four-photon excitation, respectively. The spectral, spatial, and temporal properties as well as the efficiency of multiphoton-pumped lasing output from different dye-solution samples have been studied. Based on the measured results, two salient features have been found: (i) the threshold pump-energy values for two-, three-, and four-photon-pumped lasing were quite close (within a factor of 3-4); and (ii) there was an obvious wavelength difference (10-30 nm) between the forward and backward lasing output under three- and four-photon-pump conditions. (c) 2005 Optical Society of America.

We have studied the characteristics of InxGa1−xN epilayers grown on sapphire substrates by using photoluminescence (PL), optical absorption (OA), photocurrent (PC) and persistent photoconductivity (PPC) measurements. For the undoped InxGa1−xN epilayers containing high In composition, we observed the Stokes shift, S-shaped temperature-dependent PL emission, and PPC effect. These results show a strong dependence on In contents in the properties of InxGa1−xN epilayers. The decay kinetics of the PPC effect has been investigated, from that, the depth of the localization caused by alloy potential fluctuations (APFs) in InxGa1−xN epilayers is determined. However, for Si-doped InxGa1−xN epilayers, the Stokes shift, S-shaped temperature-dependent PL peak shift and PPC effect have not been observed.

Titanium dioxide (TiO2) thin films were prepared by DC magnetron sputtering onto well-cleaned p-type silicon substrates. The thickness, composition and surface morphology of the films were analyzed using alpha step profilometer, Auger electron spectroscopy (AES) and atomic force microscope (AFM) respectively. The X-ray diffraction (XRD) studies reveal the amorphous nature of the deposited film. Thin film capacitors of the type Al/TiO2/Si/Al have been fabricated. Dielectric and AC conduction studies were performed at various frequencies (10kHz to 10MHz) and temperatures (300–390K). Dielectric constant value of TiO2 film of thickness 140nm was evaluated at room temperature and at a frequency of 1MHz as 5.5. The mechanisms responsible for the AC and DC conduction in these films have been identified. For the first time, the trap density, mobility values of TiO2 thin films are evaluated from the space charge limited current (SCLC) measurements as 1.637×1017cm−3, 2×10−11cm2V−1s−1, respectively. The AC and DC activation energies have been calculated as 0.097 and 0.136eV, respectively.

The dependence of the forward operating voltage of In1−xGaxN/GaN light-emitting diodes (LEDs) on Mg flow rate during the growth of the p-GaN layer has been investigated. The hole concentration in the p-GaN layer increases with Mg flow rate up to a critical value and gradually decreases for higher flow rates. We find that the forward threshold voltage of the LEDs mainly depends on the Mg concentration in the p-GaN layer and is rather independent of hole concentration. A current flow mechanism involving the impurity acceptor band, instead of the usual valence band, is proposed to elucidate this anomalous behaviour of the I–V characteristics.