Keda Wang

Nanovoid-related large redshift of photoluminescence peak energy in hydrogenated amorphous silicon

A large redshift of the photoluminescence (PL) peak energy is found in hydrogenated amorphous silicon films prepared by hot-wire chemical-vapor deposition with a high-growth rate ⩾50 A/s. The PL intensity is as high as that in the standard film and its temperature dependence shows thermalization behavior. The origin of the redshift is clarified by employing 1H nuclear magnetic resonance and mass density measurements. A ∼2% volume fraction of tube-like nanoscale voids is identified. The long spin-lattice relaxation time of H2 in the nanovoids implies a negligible density of silicon dangling bonds on the nanovoid surfaces. We suggest that highly strained bonds on these surfaces form broad conduction-band tail states that are responsible for the PL redshift.

Time resolved electroluminescence in hydrogenated amorphous silicon

The electroluminescence lifetime distribution of a‐Si:H is measured. We find that the distribution is double peaked. The first peak occurs at 10−6 s independent of temperature. The second peak changes from 10−3 to 10−6 s with increasing temperature. A model including the Coulomb interaction between electrons and holes and transport limited recombination is discussed.

Transient forward bias currents in a-Si:H p-i-n devices☆

Abstract We present the transient forward bias current as a function of repetition rate and reverse bias between forward bias pulses in a-Si:H p-i-n devices which have thin or thick i-layers. Little repetition rate and reverse bias effects were found in thin (0.4μm) samples compared with thick ones (> 2.0 μ m), implying that the junctions recover faster than the bulk when subjected to excess carriers due to the bias. For thick samples to reach steady state requires the pulse repetition rate to be lower than 10 −4 Hz. Further, reverse bias increases the initial space charge limited current, delays the transient rise from space-charge-limited- to recombination-limited-current but has no effect upon the final forward bias current. More pronounced delay of the rise time was found in devices which have been light-soaked. We suggest that notonly the deep traps but also defect relaxation may be responsible for these slow decay and slow recovery effects.

Light induced metastable defects in a-Si:H studied by transient space charge perturbed currents

After a transit time, the transient space charge perturbed current in an n-i-n a-Si:H device is given by j= f(t)CV 2 μ d (t)L 2 where f(t) is the fraction of electrode limitation, 0<f(t)<9/8 and μ d (t) is the time dependent drift mobility. Metastable defects located deep in the gap only affect the mobility at long times; hoever f(t) is responsive to the short time decay of the current. We present data in n-i-n and p-i-n devices showing that both f(t) and μ d (t) are affected by photodegradation

Electroluminescence: A study of non-geminate radiative and non-radiative bulk recombination in a-Si:H

Under forward bias p-i-n a-Si:H devices inject both holes and electrons resulting in electroluminescence. We have measured under constant voltage the forward bias current and the electroluminescence as a function of voltage, temperature and photodegradation. Measurements were made in the temperature range of 80°

Electroluminescence from hydrogenated amorphous silicon p-i-n diodes

Abstract The intensity of electroluminescence (EL) and its spectrum were studied as a function of generation rate, temperature, and intrinsic-layer thickness in a group of hydrogenated amorphous silicon p-i-n diodes (p = p-type, i = intrinsic, and n = n-type). It was found that: (a) the electroluminescence efficiency is as high as the photoluminescence efficiency; (b) there is a power-law dependence of the total EL intensity versus forward current density, except in the low-injection regime; (c) the defect luminescence is mainly a p-i junction effect, but the main band luminescence is a true i-layer bulk effect that is observed in thick samples at room temperature.

Electroluminescence studies of recombination in hydrogenated amorphous silicon p-i-n devices

Abstract We present experimental data on the voltage and temperature (80 K ) dependence of electroluminescence and forward bias current in hydrogenated amorphous silicon (a-Si:H) p-i-n structures. Since electrons and holes are injected from opposite sides of the sample, we are able to probe non-geminate radiative and non-radiative recombination processes in the intrinsic layer of actual device structures. We find that the effective generation rate in the electroluminescence experiment is proportional to the square of the applied voltage because the radiative recombination rate is proportional to the double-injection electron density. A simple model of electron recombination rates explains the data. The non-radiative recombination rate was found to be temperature dependent, but the radiative recombination rate is temperature independent.

Hydrogen distribution, nanostructures and optical properties of high deposition rate hot-wire CVD a-Si:H

Nuclear magnetic resonance shows that under certain growth conditions hot-wire CVD a-Si:H grown at high rates contains a large amount of nanovoids.It was found that such nanovoids are filled with H gas and the nanovoids are elongated with the 2 along axis in the growth direction.Measured on the same samples photoluminescence (PL) showed that the tail-to-tail PL peak at 80 K was red-shifted significantly from 1.36 to 1.05 eV when the growth rate increases from 10 to 55 Ays.These studies ˚ indicate that the nanostructure is important to the behavior of PL in a-Si:H. 2003 Elsevier Science B.V. All rights reserved.

A distinct recombination regime in amorphous silicon diodes under double injection

Room temperature electroluminescence efficiency as a function of forward bias voltage, VF, shows that there are two recombination regions in device quality a‐Si:H diodes. The recombination threshold voltage, Vt1, is 0.48–0.49 V. The transition voltage, Vt2, is 0.7–0.8 V. When Vt1<VF<Vt2 the electric field strongly enhances the luminescence efficiency, when VF≳Vt2 the luminescence efficiency is almost independent of the electric field. The increase of the EL efficiency in the low injection regime depends on the competition of radiative and nonradiative recombination.

Temperature and current dependence of electroluminescence in a-Si: H

Abstract Forward-bias steady-state currents in p-i-n a-Si:H structures produce significant electroluminescence (EL). We have measured the EL as a function of temperature under constant-current conditions. We find that the EL exhibits the same temperature dependence as photoluminescence (PL), namely exp(∼T/T∗0). For EL, however, T∗0 is proportional to the cube root of the current. A simple model based on the experimental results is presented.