P. Sanguino

Optical properties and transport in PLD-GaN

Abstract We present structural, optical and transport data on GaN samples grown by hybrid, two-step low temperature pulsed laser deposition. The band gap of samples with good crystallinity has been deduced from optical spectra. Large below gap band tails were observed. In samples with the lowest crystalline quality the PL spectra are quite dependent on spot laser incidence. The most intense PL lines can be attributed to excitons bounded to stacking faults. When the crystalline quality of the samples is increased the ubiquitous yellow emission band can be detected following a quenching process described by a similar activation energy to that one found in MOCVD grown samples. The samples with the highest quality present, besides the yellow band, show a large near band edge emission which peaked at 3.47 eV and could be observed up to room temperature. The large width of the NBE is attributed to effect of a wide distribution of band tail states on the excitons. Photoconductivity data supports this interpretation.

Layer-by-Layer Growth of GaN on Sapphire by Low Temperature Cyclic Pulsed Laser Deposition / Nitrogen RF Plasma

Recently we have proposed a new layer-by-layer method for deposition of group-III nitrides from elemental precursors (Ga, N2) [1,2]. This technique is based on a two-step cyclic process, which alternates Pulsed Laser Deposition (PLD), of a liquid gallium target and nitrogen plasma treatment. In this work, we proceed on the development of this flexible cyclic deposition technique and study the influence of the power and time duration of the 1 mbar nitrogen RF plasma on the GaN thin films. The layers are deposited on pre-nitridated sapphire (0001) substrates at low deposition temperature (600° C) to minimise reevaporation. The cyclic GaN thin films thus obtained are compared in terms of crystal alignment and nitrogen incorporation. X-ray diffraction and optical transmission spectra are the selected tools used to characterise and compare the deposited films.

Long-Term Stability of Microcrystalline Silicon P-I-N Solar Cells Exposed to Sun Light

The performance of an entirely microcrystalline p-i-n solar cell was monitored during a long-term outdoor test in Lisbon starting in September 1998. A small decrease of the short circuit current was observed after 5 months of operation. The open-circuit voltage remained stable around 400 mV. From the analysis of the I-V characteristic in dark and under illumination they could identify the weak points of the test structure, like large series resistance, high recombination rate, and intensity-dependent collection efficiency.

Lifetime Regime in the Electrically-Detected Transient Grating Method Applied to Amorphous and Microcrystalline Silicon Films

The minority-carrier diffusion length in thin silicon films can be extracted from the electrically-detected transient grating method, EDTG, by a simple ambipolar analysis only in the case of lifetime dominated carrier transport. If the dielectric relaxation time, τ diel , is larger than the photocarrier response time, τ R , then unexpected negative transient signals can appear in the EDTG result. Thin silicon films deposited by hot-wire chemical vapor deposition (HWCVD) near the amorphous-to-microcrystalline transition, where τ R varies over a large range, appeared to be ideal candidates to study the interplay between carrier recombination and dielectric response. By modifying the ambipolar description to allow for a time-dependent carrier grating build-up and decay we can obtain a good agreement between analytical calculation and experimental results.

Photocarrier response time scanner

We have used the dependence of the response time after short laser pulses on background light intensity in hydrogenated microcrystalline silicon to study the properties of a two-dimensional image sensor application. A coplanar readout scheme has the advantage of a simple sample structure, however, the signal-to-noise ratio is acceptable only in the perpendicular readout of a p–i–n detector. The response time map reflects the change of both the resistivity and capacitance under varying local illumination. A camera speed of 10 frames per second with a 50 by 50 pixel resolution is reached on a 2 cm 2 detector. 2002 Published by Elsevier Science B.V.