Thermal annealing of proton-irradiated silicon solar cells

Solar cells made from 1.5- and 10-Ωċcm p-type silicon, with silver-titanium evaporated electrodes, were irradiated by 4.6-MeV protons at room temperature to fluences ranging from 1 × 10¹⁰to 1 × 10¹²protons/cm². The photovoltaic current-voltage characteristics, the photovoltaic spectral response, and the minority carrier diffusion length were studied as the solar cells were annealed isochronally to temperatures up to 600°C. The proton radiation damage annealed in two stages, the first occurring between 50° and 150°C, and the second between 350° and 450°C. The removal of proton damage in this manner differs markedly from the annealing reported for 1-MeV electron damage, where practically no recovery of the photovoltaic properties is observed below 350°C. At any selected annealing temperature, the 10-Ω ċ cm cells were observed to recover to a slightly greater degree than the 1.5-Ω ċ cm type.     

High temperature characteristics of bipolar mode power JFET operation

The high-temperature operating characteristics of high-voltage JFET devices operated in the bipolar mode are evaluated. Good forward blocking capability with no degradation in blocking gain is observed at up to 200°C. The on-resistance and gate turnoff time of the devices was found to double from 25°C to 200°C, and the current gain was found to decrease by 30 percent. Despite the increase in gate drive requirements with increasing temperature, these devices should still be attractive for high-speed power switching applications because their on-resistance per unit area is at least 10 time lower than that of the power MOSFET.     

Effects of high temperature annealing on the device characteristics of Ga0.52In0.48P/GaAs and Al0.52In0.48P/GaAs heterojunction bipolar transistors

GaInP/GaAs and AlInP/GaAs heterojunction bipolar transistor (HBT) structures were grown by low pressure metalorganic vapor phase epitaxy and annealed at various temperatures up to 675°C for 15 min. Subsequent comparisons with HBTs fabricated on both annealed and unannealed control samples showed no effects for annealing up to and including 575°C, but significant changes in the electrical characteristics were observed at an annealing temperature of 675°C. For the GaInP/GaAs devices, the base current increased by a significant amount, reducing the gain and increasing the base current ideality factor from 1.07 to 1.9. Photoluminescence and electrical measurements on the structures indicated that both the emitter and base were affected by an increase in the recombination times in those regions. These effects were attributed to an out-diffusion of hydrogen from the base during annealing. The emitter of the AlInP/GaAs HBT was affected less by the hydrogen diffusion because of the larger bandgap. These observations have important implications for device performance dependence on the details of the temperature/time profile subsequent to the base growth.     

Resistance of diamond optics to high-power fiber laser radiation

The resistance of optical windows and mirrors that are made of polycrystalline CVD diamonds (with a 25-mm diameter and 1.3-mm thickness) with a heat conductivity of 1970 W/mK is studied upon the radiation of the CW ytterbium laser (?? = 1.07 ??m and the power is 10 kW). It is determined that the window withstands a power density of 11.7 MW/cm², and a 25-layer interference mirror on a diamond plate was destroyed (without cooling) at 8.2 MW/cm². The simulation of the water-cooled window showed that its maximum heating will not exceed 100°C at an incident power of 35 kW. It is shown that unique properties of the CVD diamond allow one to consider this material promising for applications in high-power lasers.     

Physical,thermal and optical characterization of rhodium(III1) acetylacetonate

Rhodium(III) acetylacetonate was investigated for its physical, thermal and optical properties. Ultraviolet-visible absorption spectra show absorptions at 320 nm, 260 nm, and 210 nm. Density measurements yielded a value of 1.75 g/cm³. Thermal characteristics were evaluated using differential thermal analysis (DTA). It was found that the compound is not volatile, decomposing upon heating. If heating rates are rapid enough,e.g. > 2° C/min, melting at 267° C can be observed. If heating rates are slower, decomposition is complete below the melting point. If the compound is annealed at 267° C for four hours decomposition is complete, yielding 99% pure rhodium metal. In an oxidizing atmosphere, on the other hand, the compound decomposes into a product containing 75% rhodium metal which appears to be RhO₂.     

A forced gas-cooled single-disk window using silicon nitride composite for high power CW millimeter waves

Silicon nitride composite as a candidate of a window material for high power CW (Continuous Wave) millimeter-waves was high power tested especially with a surface cooling by impinging gas nitrogen jets on the single-disk surface. Gas-cooling dramatically suppressed the temperature of the window disk even with gas flow rate of around 100 l/min. With gas cooling of 465l/min., 130kW CW power of HE₁₁ mode could be transmitted through the silicon nitride window with a diameter of 88.9mm. The peak window temperature was completely saturated on 123.6 °C. Without gas-cooling it did not saturate and reached 323 °C during 30 seconds pulse. A possibility of 1MW CW single disk Brewster windows with a forced gas-cooling is discussed, resulting in convinced prospects of the windows with realistic size and thickness.     

小型激光器新颖放电结构的研究

根据微空心阴极自持放电(MCSD)基本结构设计的一种新型放电结构是把多个MCSD并联在一起构成多级放电链,产生高气压、大体积、高电流密度的均匀辉光放电等离子体,用来作为小型激光器的增益介质。利用该放电结构进行了空气放电实验,记录了放电等离子体图片,测量了放电的伏安特性曲线,发现该结构在整个放电区域都具有正的微分电阻系数。在气压P=26.7 kPa,放电电流I=40 mA时,估算放电等离子体中电流密度、电子密度和功率密度分别为6.4 A/cm2, 3.7×1015 cm-3, 4.67 kW/cm3。实验结果表明利用多个MCSD并联构成多级放电链制作小型激光器是可行的。     

Structure and Optical Properties of CeO2 Nanoparticles Synthesized by Precipitation

Cerium dioxide (CeO₂) has special electrical and optical properties, and chemical and thermal stability. It has been used in semiconductor devices and as a luminescent material. In this work, CeO₂ nanoparticles were synthesized by the precipitation method and the product annealed at various temperatures. Thermogravimetric analysis (TGA)/differential scanning calorimetry (DSC) results show that the optimum annealing temperature for fabrication of CeO₂ nanoparticles is greater than 500°C. When the calcination temperature is increased from 550°C to 1050°C, Fourier-transform infrared (FTIR) results show that the water and impurities are almost completely removed, after calcination at 750°C. The x-ray diffraction (XRD) results suggest that the synthesized CeO₂ exhibits a cubic fluorite structure. The crystallite size of the CeO₂ increases from 8 nm to 75 nm when the calcination temperature is increased from 550°C to 1050°C. The absorption spectrum in the ultraviolet (UV) region from 372 nm to 395 nm demonstrates their applicability as UV-filter materials, and the shift of the estimated E _g,eff from 3.21 eV to 3.65 eV demonstrates their applicability in photoelectric devices. CeO₂ would be potentially important for applications such as insulator structures, stable capacitor devices, and light-emitting diodes (LEDs).     

The GaAlAsSb quaternary and GaAlSb ternary alloys and their application to infrared detectors

GaAlAsSb quaternary alloys and GaAlSb ternary alloys have been grown by liquid phase epitaxy (LPE) at 550°C and 450°C, respectively. The material compositions and epitaxial structures are suitable for fabricating photodiodes sensitive in the1.0-1.8 mum wavelength range. Various avalanche photodiode structures fabricated in these materials are discussed. The ion implanted GaAlAsSb APD's exhibit a high gain of 100, 82 percent quantum efficiency, and a FWHM of 400 ps. The heterojunction GaAlAsSb APD's have a gain of 40, 92 percent quantum efficiency, and a FWTM of 150 ps. The only remaining material problem which limits the performance of these devices is the high-surface leakage current.     

Parametric study of modal gain and threshold power density in electrically pumped single-layer organic optical amplifier and laser diode structures

In this paper, a model to calculate the modal gain in organic optical amplifiers and the laser threshold power density in organic laser diode structures is presented. We consider a single-layer design to investigate the dependence of the modal gain and threshold power density on electron and hole mobility, injection barriers, the thickness of the active layer, as well as exciton dissociation at the injecting contacts. A figure of merit is introduced to quantify the influence of absorption by polarons in optical amplifiers. We show that equal charge carrier mobilities are of crucial importance to achieve appreciable gain on the order of 1/cm at a power density of P= 50 kW/cm/sup 2/ for the considered poly[2-methoxy, 5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV)-like model material. Increasing the injection barriers to /spl phi//sub b//spl ap/ 0.3 eV decreases the gain marginally but is beneficial in terms of polaron absorption. Regarding modal gain, there is an optimum thickness for the active layer of d/spl ap/ 200 nm, if different devices are compared on the basis of equal power density. We derive power laws for the dependence of modal gain on mobility and power density, which can serve as guidelines for future device design considerations. We determine the maximum allowed polaron absorption cross section /spl sigma//sub abs/ relative to the cross section /spl sigma//sub stim/ for stimulated emission that may not be exceeded to achieve positive net gain necessary for optical amplification. For the most favorable parameters, /spl sigma//sub abs/ has to be at least 20 times smaller than /spl sigma//sub stim/. The dependence of the laser threshold power density on all of the above-mentioned parameters is investigated. We show that, in the optimum case considered, the power density necessary for lasing is 40 times higher than the highest value reported in the literature.     

Temperature dependence of field-controlled thyristor characteristics

The dependence of the characteristics of field-controlled thyristors upon the ambient temperature has been examined in the range of -30 to 200°C. Unlike conventional thyristors, these devices have been found to continue to exhibit forward blocking capability up to the highest measurement temperature (200°C). In fact, it is shown here that the forward blocking capability as well as the blocking gain improve with increasing temperature with the usual scaling of the leakage current for power devices. The reverse blocking capability is also retained. The forward voltage drop of the device in the conducting state decreases with increasing temperature. This behavior is shown to be similar to that of conventional rectifiers and thyristors operated at high injection levels. Further, the force gate turn-off time of the devices has been found to increase with increasing temperature. This has been correlated with a measured increase in the minority-carrier lifetime. The results of this study demonstrate that field-controlled thyristors are capable of being operated at higher temperatures than conventional thyristors.     

SiC field-effect devices operating at high temperature

Field-effect devices based on SiC metal-oxide-semiconductor (MOS) structures are attractive for electronic and sensing applications above 250°C. The MOS device operation in chemically corrosive, high-temperature environments places stringent demands on the stability of the insulating dielectric and the constituent interfaces within the structure. The primary mode of oxide breakdown under these conditions is attributed to electron injection from the substrate. The reliability of n-type SiC MOS devices was investigated by monitoring the gate-leakage current as a function of temperature. We find current densities below 17 nA/cm² and 3 nA/cm² at electric field strengths up to 0.6 MV/cm and temperatures of 330°C and 180°C, respectively. These are promising results for high-temperature operation, because the optimum bias point for SiC MOS gas sensors in near midgap, where the field across the oxide is small. Our results are valid for n-type SiC MOS sensors in general and have been observed in both the 4H and 6H polytypes.     

Study of Ti/W/Cu,Ti/Co/Cu,and Ti/Mo/Cu multilayer structures as schottky metals for GaAs diodes

Schottky structures with copper and refractory metals as diffusion barrier for GaAs Schottky diodes were evaluated. These structures have lower series resistances than the conventionally used Ti/Pt/Au structure. Based on the electrical and material characteristics, the Ti/W/Cu and Ti/Mo/Cu Schottky structures are thermally stable up to 400°C; the Ti/Co/Cu Schottky structure is thermally stable up to 300°C. Overall, the copper-metallized Schottky structures have excellent electrical characteristics and thermal stability, and can be used as the Schottky metals for GaAs devices.     

Efficient generation of continuously tunable coherent radiation in the 2460-2650-Å spectral range

Continuously tunable coherent radiation in the 2460- 2650-Å spectral range has been achieved by second harmonic generation (SHG) of a dye laser in a 90° phase-matched ADP crystal, temperature tuned between-116°C and 52°C. The 8.4-percent conversion efficiency obtained at a fundamental power of 9.6 kW is approximately 5 times larger than that reported earlier for SHG at these wavelengths.     

Growth-Temperature-Controlled Optical Properties of Textured MgxZn1?xO Thin Films

Pulsed laser deposition was used to grow magnesium zinc oxide thin films on amorphous fused silica substrates at several temperatures between room temperature and 750°C. In this study, the effect of growth temperature on the optical properties of textured Mg _x Zn_1−x O thin films was examined. The optical properties of the films were measured using absorption and photoluminescence spectrometry. Absorption spectra revealed that the bandgap values of textured Mg _x Zn_1−x O thin films were enhanced in films grown at higher temperatures. The absorption spectra near the absorption edge were fitted using the Urbach equation in order to investigate the effects of growth temperature on exponential band tail and bandgap. The photoluminescence spectra were measured for magnesium zinc oxide thin films deposited at 250°C, 350°C, 450°C, 550°C, and 650°C. The film grown at 350°C provided the highest excitonic peak intensity. On the other hand, the film grown at 250°C exhibited the lowest excitonic peak intensity. The excitonic peak intensity was considerably reduced in magnesium zinc oxide thin films grown at temperatures greater than 350°C. The ability to perform substrate-temperature-dependent bandgap engineering of Mg _x Zn_1−x O will enable use of this material in next-generation optical and optoelectronic devices.     

Wide Dynamic Range CMOS Amplifier Design for RF Signal Power Detection via Electro-Thermal Coupling

A differential temperature sensor for on-chip signal and DC power monitoring is presented for built-in testing and calibration applications. The amplifiers in the sensor are designed with class AB output stages to extend the dynamic range of the temperature/power measurements. Two high-gain amplification stages are used to achieve high sensitivity to temperature differences at points close to devices under test. Designed in 0.18 μm CMOS technology, the sensor has a simulated sensitivity that is tunable up to 210 mV/°C with a corresponding dynamic range of 13 °C. The sensor consumes 2.23 mW from a 1.8 V supply. A low-power version of the sensor was designed that consumes 1.125 mW from a 1.8 V supply, which has a peak sensitivity of 185.7 mV/°C over a 8 °C dynamic range.     

Influence of MBE growth temperature on GaAs/AlAs resonant tunneling structures

Double barrier GaAs/AlAs tunneling structures with typical 2.5:1 room temperature peak-to-valley current ratios are examined using Deep Level Transient Spectroscopy. Deep level trap concentrations are found to be much higher in samples grown at 550° C compared to those grown at 650° C. For devices grown at 550° C, an impedance switch-ing effect due to a high concentration of deep levels is observed. The peak-to-valley ratio of the tunneling devices is largely unaffected by the growth temperatures in this range, indicating that higher growth temperatures can be employed to grow resonant tunnel-ing diodes than previously suggested in the literature.     

Reliability of power GaAs field-effect transistors

The first report on a comprehensive study of the reliability of power GaAs FET's with aluminum gates and silicon-nitride passivation is presented. A total of 265 standard 6-mm-wide devices has been aged under dc-bias conditions with and without RF drive at channel temperatures of 250, 210, and 175°C. One-million device-hours have been accumulated with no catastrophic failure. A very conservative estimate predicts that the failure rate for burnout at a maximum channel temperature in normal operation of 110°C would be below 100 FIT's. Degradation in the electrical parameters has been very slow even at 250°C channel temperature. It is estimated that the failure rate for gradual degradation at 110°C would be well below 100 FIT's and most likely lower than 10 FIT's. No deterioration in the properties of gates and ohmic contacts have been observed. Diagnostic characterization has revealed that gradual degradation in the sample devices is caused by deterioration in the channel material. There has been no noticeable difference in gradual degradation between devices aged with and without RF drive at the same channel temperature for more than 3000 h. The present study has already demonstrated that the power GaAs FET's used as the samples are very reliable.     

Performance of power FET's fabricated on MBE-Grown GaAs layers

Power GaAs FET's of various sizes have been fabricated using MBE material containing a 1 µm-thick semi-insulating buffer layer. These devices, when operated between 6 and 12 GHz, exhibited state-of-the-art microwave performance. For example, the 3 mm devices gave an output power of 1.5 W with 10.9 dB associated gain at 6.4 GHz, a power-added efficiency of 42.6% with 1.4 W output power and 6.4 dB associated gain at 8 GHz. The results confirm the capability of MBE for producing high quality material with a sharp active layer/buffer interface.     

Spray deposition and characterization of zirconium-oxide thin films

Zirconium oxide films were prepared by the pyrosol process using zirconium acetylacetonate as source material onto clear fused quartz and (100) silicon at substrate temperatures ranging from 300°C to 575°C. X-ray diffraction (XRD) measurements show that samples prepared at substrate temperatures lower than 425°C are amorphous. Films deposited at higher temperatures and short deposition times show a cubic crystalline structure. However, for long deposition times, the samples show monoclinic crystalline structure. A similar phase transformation is observed on samples deposited at short time if they are annealed at high temperature. The cubic and monoclinic phases of the corresponding samples were confirmed by infrared (IR) and Raman spectroscopy, respectively. The ZrO₂ films with cubic phase show an almost stoichiometric chemical composition and refractive index values of the order of 2.1 with an energy band gap of 5.47 eV. The current-density electric-field characteristics of metal-oxide semiconductor (MOS) structures show a small ledge from 2 MV/cm to 4.5 MV/cm, indicating current injection and charge trapping. For higher electric fields, the current is associated with oxygen ion diffusion through the zirconium oxide film. The dielectric breakdown is observed at 6 MV/cm, which is a value higher than those observed in the monoclinic and tetragonal phases.