I. Chambouleyron

Laser crystallization and structuring of amorphous germanium

The short-pulse laser crystallization and interference structuring of amorphous germanium films were investigated by time resolved reflection measurements and Raman spectroscopy. We demonstrate that submicrometer crystalline structures with very sharp lateral interfaces can be produced by laser interference crystallization of nonhydrogenated samples. In hydrogenated films, on the other hand, the film surface disrupts upon laser exposure leading to the formation of a free-standing crystalline membrane. The Raman spectra of laser crystallized germanium display effects of finite crystallite size and stress.

Short-pulse laser-induced crystallization of intrinsic and hydrogenated amorphous germanium thin films

We report on the laser crystallization of intrinsic (a-Ge) and hydrogenated (a-Ge:H) amorphous germanium thin films using short, i.e., ns range, laser pulses. The influence of hydrogen on the phase transitions was investigated by monitoring the reflectance of the sample during laser irradiation. We determined the thresholds for melting (36 mJ/cm2) and for surface damage (66 mJ/cm2) of the a-Ge film. In a-Ge:H, hydrogen effuses on a short time scale (10 ns) upon laser irradiation. The effusion leads to the formation of a lifted-off (100 nm thick) crystalline Ge membrane, leaving behind a rough and incompletely crystallized surface. In a-Ge, on the other hand, no surface disruption is observed. The Raman spectra of hydrogenated samples are dominated by stress effects, while those corresponding to non-hydrogenated samples are dominated by crystallite size distribution effects. We also conclude that laser-induced annealing, carried out by applying several pulses with increasing intensity, can be used as a too...

Light-induced metastability in a-Ge" H

Abstract Light-induced metastable changes in the conductivity of hydrogenated amorphous germanium (a-Ge : H) thin films deposited by rf-sputtering are reported. The films are photoconductive with room temperature photo-to-dark conductivity ratio σph/σd = 1.0–1.5 under AM1 illumination (1.0 mW/cm2). The dark conductivity and the photoconductivity of the films decrease after exposure to AM1 irradiation, which is attributed to light-induced defect formation. The changes are metastable and the conductivity returns to its original value after some hours in the dark. In order to establish the kinetics of defect formation and annealing, a differential set-up was used to measure conductivity transients under different temperatures and illumination conditions. The time evolution of the defect generation and annealing processes follows a stretched exponential behavior similar to the one observed for a-Si : H. Both processes are temperature activated, with activation energies of 0.40 and 0.45 eV, respectively. This behavior indicates that the kinetics of defect formation involve excitation over a wide distribution of barrier heights separating the annealed and the metastable states. The connection between defects kinetics and dispersive hydrogen diffusion is analyzed.

Compact hydrogenated amorphous germanium films by ion-beam sputtering deposition

Abstract We explore reactive ion-beam sputtering deposition (IBSD) for the growth of a-Ge:H films. It is shown that compact a-Ge:H films can be obtained by IBSD at substrate temperatures between 180°C and 220°C by minimizing the ion bombardment of the growth surface. The infrared (IR) spectra of the best materials, as far as device applications are concerned, so-far obtained show no poly-hydride nor surface-like contributions to the Ge–H dipole vibration bands. Positron annihilation (PA) spectroscopy studies of these samples reveal smaller valence ( S ) parameters and larger core ( W ) parameters as compared with the films grown under less-favorable conditions, which indicate a relatively smaller concentration of the largest voids, the annihilation process being controlled mainly by trapping at small vacancy clusters or monovacancies. Similar IR and PA measurements on in situ ion-bombarded IBSD and RF-sputtered samples indicate that ion irradiation is a main factor in large void formation.

Column III and V elements as substitutional dopants in hydrogenated amorphous germanium

Abstract The doping properties of group III (B, Al, Ga, In) and V (N, P, As) impurities in a-Ge:H films deposited by rf sputtering were systematically studied. The Fermi level (EF), band-gap (E04), Urbach (E0) tail energies, density of midgap defects, ND, and hydrogen content were determined as a function of the impurity concentration by standard methods. It was found that, for a constant amount of different dopant impurities, different EF shifts are obtained, indicating different doping efficiencies. For Al, Ga, and In, the defect density displays a common behavior as a function of Nimp, ND increasing linearly with Nimp for Nimp>2×1019 cm−3. In contrast, for B and all group V elements a ND∝(Nimp)1/2 relationship is found. These findings indicate that, for the case of p-type heavy metal doping of a-Ge:H, deep defects are induced by inactive impurities, whereas n-type doping appears to be consistent with a charge-induced bond breaking mechanism. A series of different doping-induced effects highlights the importance of the chemical aspects of substitutional doping in a-semiconductors.

Hydrogen bonding and void microstructure of a‐Ge:H films

This article reports on the microvoid structure of hydrogenated amorphous germanium films, as determined from small angle x‐ray scattering data and infrared transmission spectroscopy, and its dependence on three deposition parameters, namely, the substrate temperature, the particle bombardment during film growth, and the partial pressure of hydrogen in the deposition chamber. The structure of the alloys depends on the first two deposition parameters and not on the partial pressure of hydrogen. The dependence of the optical gap on hydrogenation and microstructure is established for a‐Ge:H films for a wide range of deposition conditions.

Short-pulse laser crystallization and structuring of a-Ge

Laser pulses in the nanosecond range were used to crystallize and structure (lateral dimensions≤1 μm) amorphous germanium thin films. The crystallized material consists of grains with sizes increasing from about 5 to more than 20 nm as a function of laser pulse energy. Arrays of polycrystalline Ge dots (diameter ∼1 μm, period ∼5 μm) were produced by bringing three laser beams to interference on the sample surface. These arrays can be used as seeds for solid-phase growth of polycrystalline areas by thermal annealing below 450°C.

Steady-state photoconductivity of gallium- and indium-doped hydrogenated amorphous germanium thin films

The effects of gallium and indium p-type doping on the photoconductivity of hydrogenated amorphous germanium (a-Ge:H) thin films deposited by the rf-sputtering method are reported. The quantum efficiency-mobility-lifetime (ημτ) product was determined at room temperature as a function of the dark Fermi energy EF on samples with a relative dopant concentration range between ≈3×10−5 and ≈10−2. A decrease of ημτ is observed with the increase of the Ga concentration until a minimum is reached for compensated samples (EF close to midgap level), where ημτ is about 16 times lower than the value obtained for intrinsic samples. This behavior is followed by an ημτ increase as EF crosses the midgap level. Then, for higher Ga doping levels, ημτ decreases again. For In-doped samples, on the other hand, a monotonic decrease of ημτ is measured for all the impurity concentration range. These results are consistent with a model which assumes that the dangling bond is the main recombination path, and give independent eviden...

Hydrogen and deuterium incorporation and release processes in r.f.-sputtered hydrogenated or deuterated amorphous germanium films

Abstract In this work the out-diffusion mechanisms of H and D in r.f.-sputtered amorphous Ge films are presented. The H (D) profiles, concentrations and bonding characteristics have been studied in samples annealed at different temperatures and for various annealing times. For this purpose, elastic recoil detection analysis, scanning electron microscopy, infrared transmission spectroscopy and H (D) thermal effusion techniques were used. The main result of the present work is that two kinds of H (D) motion process coexist in r.f.-sputtered hydrogenated (deuterated) amorphous Ge films: a fast process, most probably due to the presence of voids and pinholes in the film, and a slow process due to the dispersive-like diffusion of atomie H (D) in the amorphous network. It has also been found that thermal annealing of the samples above the deposition temperature (210°C) induces structural changes. The diffusion coefficient D H of H is found to be time dependent (dispersive), as expected with a dispersion paramet...

Temperature dependence of the photoconductivity of arsenic-doped hydrogenated amorphous germanium thin films

Abstract We report a study of the photoconductivity (IPC) of As-doped a-Ge:H thin films as a function of temperature (T) and photon-flux (F). The experimental data discussed in this paper refer to the behavior of IPC at a fixed photon energy (1.3 eV) and at a fixed photon-flux (F≈2.4×10 16 cm −2 s −1 ) in the 14–380 K temperature range, as well as to the dependence of the exponent γ(IPC∝Fγ) on T (55–380 K) and on As-doping level, in the 8×10 14 –3×10 16 cm −2 s −1 photon-flux range. The measurement of IPC as a function of T indicates the presence of three distinct regions: (I) For T K ,I PC is poorly activated with T (activation energy ∼ few meV), (II) for 150 K K ,I PC is thermally activated with activation energies ranging between 122 and 167 meV, and (III) for T>260 K , a thermal quenching (TQ) of IPC is measured in all the As-doped a-Ge:H samples, the intensity of the TQ being enhanced with As doping. The measurement of IPC as a function of the photon-flux, at varying T, indicates the presence of a minimum for the exponent (γmin) at a Tmin. Both γmin and Tmin have their extreme values in the undoped sample and vary as the Fermi energy shifts from mid-gap to the conduction band edge.