C. Palsule

Characterization of fluorinated hydrogenated amorphous silicon nitride (a‐SiNx:H) alloys

Fluorinated a‐SiNx:H(0≤x≤0.19) alloys prepared using glow discharge decomposition of NF3 and SiH4 have been characterized using infrared absorption and the constant photocurrent method (CPM). The bonding configurations and the oscillator strengths of various bonds in these alloys have been obtained from the infrared‐absorption spectra. The density of defects and their distribution in the band gap have been obtained without making any assumptions about the form of the distribution. It is found that in this composition range the main defect is the neutral silicon dangling bond and the concentration of charged dangling bonds is either too low or they are not present in these alloys. The possible role of fluorine in these alloys is also discussed.

Electrical and optical characterization of crystalline silicon/porous silicon heterojunctions

Abstract We have investigated the photovoltage and photocurrent spectra of crystalline silicon/porous silicon heterojunctions. The porous silicon layers were prepared using anodic etching of p-type crystalline silicon at a current density of 25 mA/cm 2 . From the spectral dependence of the photovoltage and photocurrent, we suggest that the photovoltaic properties of the junction are dominated by absorption in crystalline silicon only. We have also studied the effect of increase in the thickness of porous silicon layers on these spectra. We find that the open-circuit voltage of the devices increases, but the short-circuit current decreases with an increase in the thickness of the porous silicon layers. We propose a qualitative explanation for this trend, based on the increase in the series and the shunt resistance of these devices. The effect of hydrogen passivation on the junction properties by exposing the devices to hydrogen plasma is also reported.

Dye mixtures for ultrafast wavelength shifters

Particle detectors based on scintillation processes have been used since the discovery of radium about 100 years ago. The fast signals that can be obtained with these detectors, although often considered a nice asset, were rarely essential for the success of experiments. However, the new generation of high energy particle accelerators require particle detectors with fast response time. The authors have produced fast wavelength shifters using mixtures of various Coumarin dyes with DCM in epoxy-polymers (DGEBA+HHPA) and measured the properties of these wavelength shifters. The particular mixtures were chosen because there is a substantial overlap between the emission spectrum of Coumarin and the absorption spectrum of DCM. The continuous wave and time-resolved fluorescence spectra have been studied as a function of component concentration to optimize the decay times, emission peaks and quantum yields. The mean decay times of these mixtures are in the range of 2.5--4.5 ns. The mean decay time increases with an increase in Coumarin concentration at a fixed DCM concentration or with a decrease in DCM concentration at a fixed Coumarin concentration. This indicates that the energy transfer is radiative at lower relative DCM concentrations and becomes non-radiative at higher DCM concentrations.

Electric field quenching of continuous wave photoluminescence in hydrogenated amorphous silicon

Abstract We have studied electric field quenching of continuous wave (CW) photoluminescence (PL) in device quality glow discharge deposited hydrogenated amorphous silicon (a-Si:H). Electric fields in the range of 103 V/cm to 9×104 V/cm were used for these measurements. We find large quenching of PL reaching almost 95% with the highest electric field at 100 K. The relative change in PL follows a power-law behavior with electric field with an exponent of 1.3 for fields below 6×104 V/cm and exhibits a saturation for higher electric fields. Since a one dimensional Poole-Frenkel model fails to explain these results, we propose a presence of negatively charged shallow radiative centers with a temperature and electric field dependent hole capture cross-section in a-Si:H. We also propose that the effect of these centers on PL quenching is dependent on other factors such as defect density and doping.

Characterization and study of light degradation effects in ECR a‐Si, Cl films

We have studied the electrical and optical properties along with light induced degradation of a‐Si:H, Cl films prepared by electron cyclotron resonance (ECR) plasma. We find that there is an irreversible decrease in dark conductivity of these films after vacuum anneal at 200°C. The degradation in photoconductivity of these films due to the Staebler‐Wronski effect is smaller than that observed in glow discharge prepared a‐Si:H films.