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.     

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     

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.     

Influence of mechanical stress on light-induced creation of defects in hydrogenated amorphous silicon

Abstract

The effect of mechanical stress on light-induced changes in photoconductivity has been studied in films of plasma-deposited hydrogenated amorphous silicon on different substrates. The experimental results reveal no correlation between the degradation of the photoconductivity and the mechanical stress in these films. These findings contradict the observations of Stutzmann but agree with those of Guha, den Boer, Agarwal and Hack.     

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.     

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.     

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.     

Electron spin resonance of hydrogenated amorphous silicon-carbon alloy films prepared by magnetron sputtering

Abstract

New electron spin resonance (ESR) lines with g¹ = 2·0017 and g₂ and g₃ = 2·0006 have been found in the ESR spectra of as-deposited a-Si_1–x C_x:H films prepared by magnetron sputtering of silicon in the gas mixtures of methane and argon. Similarities between the observed spectra and those for the E′ centre in glassy SiO₂ are discussed.     

Recrystallization of amorphous ion-implanted silicon carbide after thermal annealing

Single crystals of 6H–SiC were implanted at room temperature with 4-MeV Au ions to a fluence of 10^15?cm^?2. Raman spectra showed that full amorphization was achieved. The recrystallization process was studied by micro-Raman spectrometry after isochronal thermal annealing between 700 and 1500°C. The spectra permitted the evolution upon annealing of Si–C bonds, and also of Si–Si and C–C bonds, to be followed. Amorphous phase relaxation takes place below 700°C; then recrystallization of the 6H polytype sets in at 700°C. At 900°C crystallites with different crystalline states are formed. Moreover, Raman spectra provide evidence of graphitic nanocluster formation at 1500°C.     

Electrical properties and local structure of n-type conducting amorphous indium sulphide

An n-type amorphous chalcogenide, In₄₉S₅₁, having a band gap of 1.9eV, has been found. The conductivity in as-prepared films was ～10^?4?S?cm^?1, which increased to 1?×?10^?1?S?cm^?1 on post-annealing at 125°C in vacuum, accompanied by a reduction in the sulphur content of the films. TEM observations showed the amorphous nature of the films before and after annealing. Both Seebeck and Hall coefficients are negative, indicating that the major carriers are electrons. The Hall mobility can be as large as 26?cm²?V^?1?s^?1 at 300?K. No significant changes to the optical absorption were observed upon annealing. Analysis of the X-ray radial distribution function reveals that the sulphur atoms have four-fold coordination, making the structure more rigid than conventional amorphous chalcogenides in which the chalcogen is alloyed to elements of group IV or V of the periodic table. We tentatively associate the electron carrier generation with the formation of sulphur vacancies.     

Transition from diffusive to ballistic capture related to hydrogen incorporation in amorphous silicon

Three intersection mechanisms with the gliding planes (111)_TB, (001)_TB and (115)_TB respectively have been observed by high-resolution electron microscopy in the type-I twin intersection of gamma-TiAl. It was found that the intersection mechanism that occurred was related to the thickness of the incident twin. The accommodation mechanism on the (111)_TB atomic plane is preferred when the incident twin becomes very thin. The dislocation dissociations of the (111)_TB plane accommodation are the most energetically unfavourable of the dissociations of the three intersection mechanisms; however, the resultant dislocations on the (111)_TB planes are the easiest to propagate away from the intersection area. Accordingly, (111)_TB atomic plane accommodation is considered to be the only mechanism allowing shear transmission under the small local stress of the pile-up of the incident twinning partials.     

Thermomagnetic gravimetry study of crystallization behaviour in amorphous Fe-Si-B-Nb alloys

The effect of the Nb content on the crystallization process of amorphous Fe84- xSi6B10Nbx alloys has been investigated using thermomagnetic gravimetry (TMG). In the TMG measurement, the sample weight was electronically zerobalanced, and then an apparent weight (AW) applied by placing a magnet beneath the sample pan. The heating treatment was carried out over temperature range between 373 and 1093 K, in order to detect the formation of a ferromagnetic crystallization-induced phase. The form of the TMG curve changed markedly in the vicinity of 2-3 at.% Nb. In the alloys containing less than 2 at.% Nb, a two-stage increase in AW was observed with increasing temperature, but only a single-stage increase occurred for the alloys containing more than 3 at.% Nb. On the basis of these results, it is inferred that the Nb content plays a key role in the structure of the amorphous phase, supporting our recent claim for a mechanism of nanocrystallization.     

Thermomagnetic gravimetric study of the crystallization process in amorphous Fe-Si-B-Cu alloys

The thermal properties of amorphous Fe_84-xSi₆B₁₀Cu_x and Fe_77.5-xSi_13.5B₉Cu_x alloys have been investigated using thermomagnetic gravimetry (TMG). After electronically cancelling the sample weight, an apparent weight (AW) was applied by placing a magnet beneath the sample pan. Heating was then carried out to detect the formation of ferromagnetic crystallization-induced phases. In both Cu-free Fe₈₄Si₆B₁₀ and Fe_77:7Si_13:5B₉ alloys, a two-stage increase in AW was observed on the TMG curve. The addition of Cu to these alloys caused a decrease in the crystallization temperatures as well as a marked increase in AW in the above two-stage process. It is presumed that such behaviour originates from the lack of metalloid elements in the amorphous matrix, which is caused by their distribution to Cu-enriched clusters.     

X-ray study of phason strain in Al-based quasicrystals

Abstract

X-ray powder diffraction spectra have been measured for melt-spun Al-Si–Mn, Al-Cu–Fe and Al-Li–Cu and Bridgman-grown Al-Li-Cu quasicrystals to investigate the phason strain frozen during the solidification process. For the melt-spun samples, most of the peaks have a shoulder or a tail which is attributed to the anisotropic linear phason strain. The peak shapes for the three melt-spun samples are essentially the same, which indicate that the same type of linear phason is quenched in these samples. In contrast, symmetric peak shapes without shoulders nor tails are seen for the Bridgman-grown Al-Li–Cu, indicating that it contains little linear phason strain.

Our findings are discussed with respect to the three linear phason models which can be derived by degradation of the symmetry from ideal icosahedral group m35 to its maximal subgroups 3m, m3 and 5m. Of these three, the model for 3m reproduces best the observed shapes and widths of the peaks.