Solid-source doping of hydrogenated amorphous silicon

Abstract

Arsenic doping of hydrogenated amorphous silicon (a-Si: H) from a solid source in a remote hydrogen plasma reactor is reported. Arsenic incorporation in the deposited films was directly demonstrated with secondary-ion mass spectrometry, and doping efficiency was determined from d.c. conductivity. The electronic properties of the doped films were indistinguishable from those of conventional r.f. glow discharge deposited a-Si:H. The technique should be applicable to other dopant and alloying elements     

An explanation for the low-temperature H evolution peak in hydrogenated amorphous silicon films deposited 'on the edge of crystallinity'

The puzzling existence of a sharp low-temperature (T = 400°C) H evolution peak in compact hydrogenated amorphous silicon (a-Si : H) films deposited 'on the edge of crystallinity' is examined. From infrared absorption and X-ray diffraction (XRD) measurements, we show that none of the standard methods used to explain the existence of this peak in a-Si : H materials is applicable to the present films. From the Si-H wag-mode peak frequency, we postulate the existence of very small Si crystallites contained within the amorphous matrix. While the crystallite volume fraction is too small to be detected by XRD in the as-grown films, crystallization is observed for this material at anneal temperatures as low as 500°C. It is proposed that these crystallites catalyse the crystallization of the remainder of the amorphous matrix upon moderate annealing, enabling H surface desorption and H₂ out-diffusion to the sample surface along newly formed grain boundaries at low anneal temperatures.     

Dependence of the persistent photoconductivity in doping-modulated a-Si: H multilayers on the exposure temperature

Abstract

We find that the photo-induced excess conductivity (persistent photoconductivity, PPC) in doping-modulated hydrogenated amorphous silicon (a-Si: H) is thermally activated for exposure temperatures above 220 K in agreement with Kakalios and with Hundhausen and Ley. However, we find a temperature regime between 80 and 220 Kin which the PPC measured at 300 K is independent of excitation temperature. Whether the PPC is excited in the thermally activated regime above 220 K or in the temperature-independent regime, the annealing occurs at 410 K. The pre-exponential factor and the activation energy of PPC are related by the Meyer-Neldel rule independent of the excitation temperature.     

Effects of irradiation with electrons of different energies on the dark conductivity and the network of hydrogenated amorphous silicon films

The effects of irradiation with electrons having energies in the range 0.7–1.7 MeV on the dark conductivity and the network of hydrogenated amorphous silicon (a-Si:H) thin films have been studied. The dark conductivity measurements show that electron irradiation leads to a degradation in the dark conductivity, with the degradation being greater at lower electron energies. The Raman results suggest that the irradiation induces structural defects in the a-Si:H films, with the lower energy electrons producing more disorder in the amorphous network.     

Distribution of the midgap density of states and their capture cross-sections in hydrogenated amorphous silicon deduced from space-charge-limited conduction in the dark and under illumination

Space-charge-limited current measurements in the dark and under illumination offer the means of obtaining the density of states N(E) and their capture cross-sections s(E) in a range of energy E above the equilibrium Fermi level in hydrogenated amorphous silicon (a-Si:H) n⁺–i–n⁺ structures. N(E) and s(E) have been measured in the energy region from 0.65 to 0.5?eV below the conduction-band edge E _c. The decrease on s(E) observed as the energy E increases agrees with recent observations and is attributed to the coexistence of two types of defect having different capture cross-sections. This is confirmed by light soaking of a-Si:H which makes the defects with high s(E) predominant.     

The effect of hydrogen plasma on the properties of a-Si:H/a-Si1-xNx:H superlattices

Abstract

A hydrogen plasma introduced during the interrupting interval when alternating two gases affects the properties of a-Si:H/a-Si_1-xN_x:H superlattices resulting in the creation of fewer interface defects than the superlattices prepared without the hydrogen plasma.     

Reflectance of multilayer amorphous semiconductor films with random thickness layers

Abstract

Reflectance and transmittance spectra of amorphous semiconductor multilayer films were computed for layers of random thickness. This choice was prompted by the experimental results recently reported on hydrogenated amorphous silicon/silicon-nitride films in which the thickness of the lower bandgap material layers is randomly chosen. The calculation clearly elucidates the origin of a peculiar behaviour of the reflectivity observed in the spectral region of strong absorption.     

Relaxation of electrons following trapping in the space-charge-limited conduction regime of n + -i-n + hydrogenated amorphous silicon structures

The relaxation of a space charge due to trapped electrons in an n + -i-n + hydrogenated amorphous silicon structure has been modelled by Solomon. Here we extend the model by obtaining analytical expressions for the current at the beginning of the relaxation as well as for long times when retrapping of electrons is taken into account. Our expression for the current at long times differs considerably from that given previously. Consequences for the derived values of the capture cross-sections of the gap states near the Fermi level are examined and discussed.     

Synthesis of amorphous and quasicrystal phases by mechanical alloying of Ti 45 Zr 38 Ni 17 powder mixtures,and their hydrogenation

Mechanical alloying of Ti 45 Zr 38 Ni 17 powder mixture forms an amorphous phase, but subsequent annealing causes the formation of an icosahedral ( i ) phase. The maximum hydrogen concentration that can be loaded at 573K at a hydrogen pressure of 3.8MPa is the same (\[H]/\[M] 1.5) for the amorphous and i -phase powders. With hydrogenation, the i -phase is almost stable, forming no hydrides, whereas the amorphous phase transforms to a fcc hydride. The activation energy for hydrogen desorption for the i -phase is about 127kJmol -1, which is lower than that for the amorphous phase, suggesting that the i -phase powder may have better properties for hydrogen-storage applications.     

Ion-beam synthesis of amorphous gallium nitride

Amorphous gallium nitride (a-GaN) has been synthesized for the first time by implanting gallium into amorphous silicon nitride (a-SiNx) films. The a-GaN is only formed when gallium is implanted into hydrogenated amorphous silicon nitride (a-SiNx:Hy) films with x > 1.5. The nitrogen concentration x of the substrate is varied by changing the feed-gas ratio during plasma-enhanced chemical vapour deposition of the nitride film. Using a pre-determined composition and implant condition, the implanted gallium is made to bond with the nitrogen to form a surface layer of a-GaN. Low-temperature annealing, compatible with large-area glass substrates, is then used to increase the thickness of the a-GaN and to transform more of the a-SiNx. X-ray photoelectron spectroscopy and Rutherford back-scattering spectroscopy have been used to examine the bond structure, composition and the depth profile of the synthesized material.     

Interfacial reaction between silicon nitride and aluminium

Abstract

The microstructure of a silicon nitride/aluminium interface has been identified by analytical transmission electron microscopy. HIPed silicon nitride without any additive was bonded by pure aluminium at 800°C for 15 min. A thin (～ 5000 Å) reaction layer was recognized at the interface. It consisted of two regions. One an amorphous layer facing the aluminium; it contained aluminium, silicon and oxygen. The other was on the silicon nitride side and consisted of fine particles which were less than 100 Å in diameter. These particles were considered to have β'-sialon type structure.     

Transient photoconductivity in n-type a-Si: H

Abstract

The long-time transient photocurrent decay in annealed and light-exposed P-doped a-Si: H is examined experimentally and by numerical modelling. The decay is a dispersive power law with sublinear index, extending to times longer than 1s, and the decay rate increases with temperature. Light exposure dramatically decreases the decay amplitude but does not affect the rate of decay. The phenomenon is discussed in terms of a comprehensive multiple-trapping model in which transport of thermalized electrons is essentially non-dispersive, and recombination of free carriers via defects is dispersive, owing to continued thermalization of excess holes. The slower recombination step is free-hole capture by D^? states, while the decay of the total excess ensemble is controlled by hole release from valence-band tail traps. The index of the excess photoelectron decay provides information on the valence-band tail states, which are exponentially distributed, with a characteristic energy estimated as 0·06 eV.     

Absence of photodegradation in amorphous chalcogenide films with a narrow optical bandgap

The time-dependent change in photocurrent during illumination has been studied in amorphous arsenic tritelluride (a-As Te ), which is an amorphous 2 3 chalcogenide material with a narrow optical bandgap. No photodegradation in the photocurrent is observed in this material, although photodegradation occurs in most wide-bandgap amorphous chalcogenides.     

Long-term metastability of light-induced defects in hydrogenated amorphous silicon

The decay of the peak intensity of the electron spin resonance signal associated with light-induced dangling bonds in a-Si(H) has been measured at room temperature as a function of time during very long intervals such as 3400 days after the intense pulsed illumination was turned off. The decay curve is fitted by an exponential function with a decay constant of 393 days and reaches a steady-state value smaller than the value taken before illumination. Such a long-term metastability of light-induced dangling bonds in a-Si:H is discussed in terms of reconstruction of the amorphous network occurring through hydrogen motion.     

Thermal-equilibrium processes and electronic transport in undoped hydrogenated amorphous silicon

Abstract

The temperature and time dependence of the d.c. conductivity of undoped hydrogenated amorphous silicon is presented. Measurements of the electronic transport are reported, with particular emphasis on the effects of annealing and cooling the samples. Two regimes of behaviour are observed. When samples are rapidly cooled from 200°C below a temperature T _E～145°C a non-equilibrium dark conductivity, higher than that corresponding to slow cooling, is observed. The electronic and atomic structure then slowly relax and the time dependence of the excess conductivity is well described by a stretched exponential function. The second regime above T _E corresponds to a relaxation time short compared to experimental times and the conductivity is independent of which order the annealing temperature is chosen. Thus the thermal equilibrium processes observed in undoped samples are qualitatively very similar to those observed in doped samples as recently reported in the literature.     

Hydrogenation of Ti50Zr25Co25 amorphous ribbons and its effect on their structural and mechanical properties

Changes in the structure and mechanical properties of Ti₅₀Zr₂₅Co₂₅ melt-spun ribbons upon electrolytic hydrogenation have been investigated. Structural analyses of the as-spun ribbons revealed the sporadic presence of icosahedral (i) quasicrystalline short-range order (SRO) in the amorphous structure. Upon cathodic charging with hydrogen, the i-SRO vanished for a hydrogen content of about 21 at.% (hydrogen-to-metal atom ratio, H/M: 0.26), resulting in a fully amorphous structure. Simultaneously, the fracture strength was found to increase while the ribbons preserved their bending ductility. However, a significant reduction in the fracture strength and ductility of the ribbons was observed for hydrogen concentrations larger than 26 at.%. Variations in the mechanical stability are discussed based on a structure-property correlation.     

Microscopic mobility in hydrogenated amorphous silicon

Abstract

A recent letter by Kakalios (1987) has criticized our suggestion that the microscopic mobility of hydrogenated amorphous silicon increases under high-level double injection. In this Letter, we show that Kakalios misinterpreted our model and misrepresented our data.     

Density-functional and shell-model calculations of the energetics of basal-plane stacking faults in sapphire

We report the generation of fullerenes and nanotubes using an arc discharge on graphite in a high-pressure nozzle. The vapour from the arc is quenched via collisions with helium gas, forming carbon clusters within a localized highpressure region. The carbon molecules are entrained in the gas jet as it expands into the vacuum and deposited onto a silicon substrate in a low-pressure (6 mT) environment. Mass spectroscopy measurements of the plasma reveal the presence of C molecules in the expanding plume. Microstructural examination of films deposited using this method revealed clustered regions of larger fullerenes and nanotubes surrounded by an amorphous matrix. Films containing fullerenes and nanotubes were found to be significantly harder and more elastic than amorphous carbon films deposited under identical parameters but without conditions for fullerene and/or nanotube formation.     

Amorphization under fracture surface in hydrogen-charged and low- temperature tensile-tested austenitic stainless steel

ABSTRACT

The microstructure just below the fracture surface in hydrogen-charged stable austenitic SUS 316L stainless steel, which was subjected to a low strain rate tensile test at ?70°C, was studied by a combination of the focused-ion-beam method and transmission electron microscopy. An amorphous region with a chemical composition almost identical to that of the polycrystalline region was found under the lath-like structure on the fracture surface, although no deterioration of tensile properties by hydrogen appeared. In the amorphous region, band-like regions with wavy contrasts were observed, which were often accompanied by cracks at the boundaries. The presence of the amorphous region with band-like regions implies that amorphization occurred due to high-density vacancies accompanied by agglomerations of excess vacancies in the hydrogen-charged SUS 316L stainless steel that was tensile-tested at low temperatures.     

EELS analysis of vacuum arc-deposited diamond-like films

Abstract

Electron energy-loss spectroscopy measurements have been made on amorphous diamond-like carbon films produced by condensing the plasma stream from a filtered vacuum arc. The results are compared with spectra from diamond, graphitized carbon and amorphous carbon. Although the energy-loss spectra reveal the presence of a small quantity of sp²-bonded material in the diamond-like film, the high plasmon frequency and shape of the K-edge show that the material is essentially an amorphous form of diamond. The fraction of sp²- bonded carbon was quantified and found to be of the order of 15%. It was not possible to determine if the sp² carbon was on the surface or throughout the bulk.