Babar A. Khan

Novel technique for p‐type nitrogen doped ZnSe epitaxial layers

We report a novel technique to obtain p‐type ZnSe layers doped with nitrogen. The layers were grown in a low‐pressure metalorganic vapor phase epitaxy system using ammonia as the dopant source. A rapid thermal anneal was used to enhance the activation of the nitrogen acceptors. Net acceptor concentration values as high as 3×1016/cm3 were obtained from capacitance‐voltage measurements and the profile was uniform over the thickness of the epitaxial layers. The 7 K photoluminescence spectrum was dominated by the acceptor bound exciton peak; the donor‐acceptor pair spectra were also observed.

Activation energy of source-drain current in hydrogenated and unhydrogenated polysilicon thin-film transistors

The activation energy of the drain current in polysilicon thin-film transistors (TFTs) and the effects of hydrogenation on this energy are discussed. The activation energy data are fitted using different models of the density of states in the material. It is shown that a model which assumes a distribution of brand tail states and localized deep states can account for the activation energy data of unhydrogenated polysilicon TFTs. However, the activation energy data on hydrogenated TFTs cannot be explained with the band tail model. Instead, a simple model of deep states localized at the grain boundary can fit this data quite accurately. Also, it is shown that there is a characteristic kink in the activation energy data of the hydrogenated TFTs which is a signature of the location of the deep states relative to the valence band edge. Analysis indicates that these deep states are located approximately 0.36 eV from the valence band edge. This value is consistent with that obtained from absorption measurements using photothermal deflection spectroscopy. >

Influence of the size dispersion on the emission spectra of the Si nanostructures

A systematic study of the size dependence of the photoluminescence spectra of Si quantum nanocrystals in the SiO2 matrix has been performed. The results have been fitted to a quantum‐confinement model that includes the nanocrystal size dispersion rather than a specific size of the nanocrystal. This, in conjunction with the results [Z. H. Lu, D. J. Lockwood, and J.‐M. Baribeau, Nature 378, 258 (1995)] for amorphous Si layers serves as a strong confirmation of the confinement‐induced nature of the photoluminescence.

Low-temperature polysilicon TFT with gate oxide grown by high-pressure oxidation

Polysilicon thin-film transistors (TFTs) were fabricated with the maximum processing temperature limited to 650 degrees C. Best results were obtained when the gate oxide was grown by a two-step high-pressure oxidation process, using high-pressure steam and then dry oxygen both at 15 atm and 650 degrees C. The TFTs exhibit a mobility of 34 cm/sup 2//V*s, threshold voltage of 3.5 V, leakage current below 0.01 pA/ mu m, subthreshold slope of 0.18 V/decade, and an ON/OFF ratio of over eight orders of magnitude. These values are comparable to those obtained with conventional polysilicon TFTs using high-temperature thermal oxidation.<<ETX>>

Observation of hysteresis, transients, and photoeffects in the electrical properties of ZnSe/GaAs heterojunctions

Electronic transport properties of a Au/n‐ZnSe/n+GaAs structure are examined by studying the results of current‐voltage (I‐V) and capacitance‐voltage (C‐V) measurements at temperatures between 77 and 300 K in the dark and under illumination. C‐V data at 77 K in the dark show a large hysteresis indicating long‐time constants for reaching a steady state. The data also show a large shift in the peak capacitance towards higher‐bias values as compared to room‐temperature measurements. All 77‐K capacitance values in the dark are considerably lower than both the ones at 77 K under illumination and the ones at room temperature. These results suggest charging of the lattice‐mismatch‐induced extended defect states in ZnSe near the heterojunction interface as well as macroscopic effects such as barrier‐limited electron currents flowing into the ZnSe potential well. Furthermore, as additional evidence, 77‐K capacitance transients in the data show two distinct time constants which are due to an initial charge rearrang...

Investigation Of Efficiency Improvement on Silicon Solar Cells Due to Porous Layers

Observation of light emission from porous Si has demonstrated that the optical properties of Si can be drastically altered by the quantum size effects. We have investigated the improvement of absorption properties of Si material by forming a porous Si layer. Shallow-junction commercial crystalline as well as polycrystalline Si solar cells without anti-reflective coatings have been processed into porous Si solar cells by a wet chemical etching technique. Our best results have demonstrated more than 15% improvement in short-circuit current with no change in open-circuit voltage. The performance of the porous Si solar cells has been found to be sensitive to the porous layer thickness. The efficiency can be reduced when the porous layer is relatively deep, presumably due to the penetration of pores through the shallow junction. We believe porous Si can be optimized for photovoltaic applications by properly controlling its porosity and thickness.

Leakage Currents in P-Channel Accumulation Mode Tft's

We have fabricated accumulation mode MOSFETS in poly-silicon thin films and studied the effect of passivation of the leakage currents in these devices. We have used C-V measurements ot determine the effective doping concentration and depletion layer widths in the passivated and unpassivated devices. We find that passivation reduces the effective doping concentration and therefore increases the depletion layer width. Although this change in effective doping concentration is small (less thean a factor of 2), the increase in depletion layer width is sufficient to pinch off the passivated device but not the unpassivated one. This leads to the dramatic reduction in leakage currents after passivation.

High Performance Polysilicon Thin Film Transistors

Polysilicon TFT characteristics are shown to be dependent on the nature of the polysilicon film used as well as the TFT fabrication process. Best results were obtained when silicon self-implant and regrowth techniques were used together with plasma hydrogenation. TFTs exhibiting a mobility of 115 cm 2 /V-sec, subthreshold slope of 0.27 V/decade, leakage current below 0.01 pA/μ;m, and an on-off ratio of 10 orders of magnitude have been fabricated.

Microfabricated high intensity discharge lamps

Miniaturization of high intensity discharge lamps can lead to cheaper, more efficient, and novel lighting systems for the lighting industry. We are applying micromachining and techniques used for manufacturing integrated circuits, to develop such miniature lamps. We have fabricated electrodeless and electroded high-pressure mercury discharge lamps in quartz substrates using integrated circuit and micromachining techniques, such as photolithography, etching, and wafer bonding. Patterned quartz wafers were bonded together using fusion wafer bonding. The resulting cavities are strong enough to withstand the high pressures (124 atm) and high temperatures (1000 °C) in these lamps. Lamps containing varying amounts of mercury were fabricated to obtain high-pressure discharges in the range of 10–175 atm. The electrodeless lamps were excited by a microwave source, operating at 2.45 GHz and the electroded lamps with a dc voltage. Lamp efficacies over 40 lumens (lm)/W were obtained for the electrodeless lamps and ov...

HIGH PRESSURE DISCHARGES IN CAVITIES FORMED BY MICROFABRICATION TECHNIQUES

High pressure discharges are the basis of small high intensity light sources. In this work, we demonstrate the formation of high pressure discharges, in cavities formed by applying micromachining and integrated circuit techniques to quartz substrates. Cavities containing varying amounts of mercury and argon were fabricated to obtain high pressure discharges. A high pressure mercury discharge was formed in the electrodeless cavities by exciting them with a microwave source, operating at 2.45 GHz and in the electroded cavities by applying a dc voltage. The contraction of the discharge into a high pressure arc was observed. A broad emission spectrum due to self-absorption and collisions between excited atoms and normal atoms, typical of high pressure mercury discharges, was measured. The light output and efficacy increased with increasing pressure. The measured voltage was used to estimate the pressure within the electroded cavities, which is as high as 127 atm for one of the two cavities discussed in this w...