C. Longeaud

Midgap density of states in hydrogenated polymorphous silicon

When silicon thin films are deposited by plasma enhanced chemical vapor deposition in a plasma regime close to that of the formation of powder, a new type of material, named polymorphous silicon (pm-Si:H) is obtained. pmSi:H exhibits enhanced transport properties as compared to state-of-the-art hydrogenated amorphous silicon (a-Si:H). The study of space-charge-limited current in n(+)-i-n(+) structures along with the use of the modulated photocurrent technique, of the constant photocurrent method and of steady-state photoconductivity and dark conductivity measurements allows us to shed some light on the origin of these improved properties. It is shown that the midgap density of states in the samples studied here is at least ten times lower than in a-Si:H, and the electron capture cross section of deep gap states is also expected to be lower by a factor of 3-4 to account for photoconductivity results. An interesting field of theoretical research is now open in order to link these low densities of states and capture cross sections to the peculiar structure of this new material

Properties of a new a-Si:H-like material : hydrogenated polymorphous silicon

Abstract A new a-Si:H-like material has been obtained in a radio frequency-powered plasma-enhanced chemical vapor deposition system (RF-PECVD). This material prepared with dilution of silane into He or H2, under high total pressure (≈132 Pa) and high RF power exhibits enhanced electronic transport properties. The room temperature electronic mobility-lifetime product is increased by a factor up to 200 compared to hydrogenated amorphous silicon (a-Si:H) prepared under standard deposition conditions (lower pressure, lower RF power). The density of states measured by modulated photocurrent and the deep defect density measured by the constant photocurrent method are both less than that of standard a-Si:H. These transport properties are linked to the structure of this new material deposited under conditions close to those for powder formation. This structure seems to result in a decrease of the deep defect density and capture cross sections.

Very low densities of localized states at the Fermi level in hydrogenated polymorphous silicon from capacitance and space-charge-limited current measurements

The density of states at the Fermi level N(E-F) has been measured on hydrogenated polymorphous (pm-Si:H) silicon samples using both capacitance measurements on Schottky barriers and space-charge-limited current measurements on n(+)/i/n(+) structures. From both techniques, N(E-F) values of 7-8 x 10(14) cm(-3) eV(-1) have been obtained, which is significantly lower than reported in the literature for hydrogenated amorphous silicon (a-Si:H). Such values demonstrate that pm-Si:H is a very low defect density material which should be able to replace a-Si:H in the field of applications like photovoltaics

Low band gap amorphous silicon deposited under He dilution in the γ regime of an rf glow discharge: properties and stability

Abstract A new type of hydrogenated amorphous silicon film having variable bandgap (1.7–1.5 eV) has been developed in an rf powered plasma enhanced chemical vapor deposition system using a mixture of silane and helium at a subtrate temperature of 210°C. The deposition conditions were chosen so that the rf glow discharge occurs in the γ regime, usually avoided because of powder formation. The influence of the chamber pressure, on the optical gap, the hydrogen content and the electronic properties is presented. Increasing the pressure up to 1.8 Torr is found to decrease the optical gap down to 1.5 eV. The densities of states of these films were measured by electron spin resonance, constant and modulated photocurrent techniques. The density of states above the Fermi level is found to be two orders of magnitude less than that of standard amorphous silicon. Moreover, unusually fast kinetics of degradation are observed. This new material could be a good alternative to amorphous silicon germanium alloys.

Some electronic and metastability properties of a new nanostructured material: hydrogenated polymorphous silicon

When silicon thin films are deposited by plasma enhanced chemical vapour deposition in a plasma regime close to that of the formation of powder, a new type of material, called polymorphous silicon (pm-Si), is obtained. We present here the optoelectronic and stability properties of pm-Si films deposited from a mixture of silane diluted with hydrogen at total gas pressures in the range 800-1600 mTorr. A comparison with the properties of standard hydrogenated amorphous silicon (a-Si:H) is made. While some properties of both materials are similar, many others differ in a striking manner. Characterizations of as-deposited pm-Si films show that the best samples exhibit enhanced transport properties, such as the fact that the quantum efficiency-mobility-lifetime product eta mu tau is increased by a factor of 200-700 compared with that measured on a-Si:H under the same conditions. This correlates with a lower density of deep states. The kinetics of creation of defects, performed under 670 mW cm(-2) white light illumination and at a high temperature (100 degrees C) in order to attain a final steady state, have been studied, pm-Si samples exhibit faster kinetics of creation as well as of annealing of metastable defects than do a-Si:H samples. In their light-soaked state the best pm-Si samples exhibit eta mu tau products of the same order as those measured on device-grade a-Si:H in the annealed state. These enhanced transport properties, new properties and better stability are linked to the peculiar structure of pm-Si, namely ordered silicon nanoparticles embedded in an amorphous matrix.

Structural, optical, and electronic properties of hydrogenated polymorphous silicon films deposited from silane–hydrogen and silane–helium mixtures

Here we compare the hydrogen incorporation and the optoelectronic properties of hydrogenated polymorphous silicon (pm-Si:H) films deposited by the decomposition of either silane–hydrogen or silane–helium mixtures. The use of He dilution leads to higher deposition rate and films with a lower hydrogen content with respect to hydrogen dilution. On the contrary, hydrogen dilution leads to films with high hydrogen content that show a characteristic infrared stretching band at 2040 cm−1. We propose that this absorption band is due to hydrogen bonded at the surface of crystallites and clusters in a platelet-like configuration. This peculiar hydrogen bonding is related to low temperature hydrogen diffusion, which gives rise to a sharp hydrogen effusion peak at around 420u200a°C. Interestingly enough, the higher deposition rate in the films from the He-dilution series does not result in degradation of the film’s electronic and transport properties, which are strongly improved with respect to those of standard amorphou...

Investigation of defect levels in semi-insulating materials by modulated and transient photocurrent: comparison of methods

High-resolution photoinduced transient spectroscopy and the modulated photocurrent technique are compared in terms of possible application to the investigation of defect levels in semi-insulating monocrystalline materials. After a description of the theoretical and experimental aspects, the advantages, drawbacks and limitations of each method are discussed. The two techniques complement each other and their potentialities are exemplified by the measurements of trap parameters in the same samples of semi-insulating Cr-doped and undoped GaAs. From these results we deduce a possible model for the properties of the Cr defect in GaAs.

A combined EPR and modulated photocurrent study of native defects in Bridgman grown vanadium doped cadmium telluride: the case of the tellurium antisite

The electrical and photoconductive properties of Bridgman grown vanadium–zinc co-doped CdTe bulk crystals are strongly influenced by one native intrinsic defect previously attributed to the Te vacancy. In order to identify this defect and control its formation mechanism, a correlated electron paramagnetic resonance (EPR) and modulated photocurrent (MPC) study has been made. The results obtained allow us to attribute this defect to the TeCd antisite, a double donor. By EPR and MPC its +/2+ level position has been determined to Ec - 0.20 eV. Four other centres of minor concentrations were characterized by MPC in as-grown crystals. From the MPC results the density of states for CdTe:V materials has been determined.

From amorphous to polycrystalline thin films : dependence on annealing time of structural and electronic properties

Some new results about the amorphous to polycrystalline transition of silicon thin films obtained by low pressure chemical vapor deposition (LPCVD) at 550°C are presented. From in situ (monitored during the crystallization annealing) conductance, electron spin resonance, photoluminescence and modulated photocurrent experiments, the density of states is shown to increase in the so-called nucleation regime and reach a maximum just before crystal grain growth starts. These results indicate that crystallization needs the creation of a defected material with large dangling bond densities and wide band-tails. It appears that the electronic quality of the final polycrystalline material is linked to the maximum density of states reached just at the end of the nucleation phase. This level may be associated to a viscous structure.

Phenomenological characterization of photoactive centers in Bi12TiO20 crystals

We report optical and electrical measurements contributing for a better characterization of the relevant photoactive center levels in undoped photorefractive Bi12TiO20 (BTO) crystals grown in Brazil. Comparative results for Pb-doped BTO and Bi12GaO20 are also reported. A center responsible for photochromism was identified at 0.42–0.44eV, probably below the conduction band (CB). The main electron and hole donor center is detected at 2.2eV from the CB and the equilibrium Fermi level is pinned at this level. Other localized centers were identified at different positions in the band gap and their relation with the behavior of BTO under different wavelengths and operating conditions is discussed with particular attention to holographic recording.