K. P. Chik

Properties of a-boron films prepared by low pressure chemical vapour deposition

Properties of LPCVD a-B films are found to depend strongly on substrate temperature Ts. Hardness increases but H content and ESR linewidth (Δ) decrease with increasing Ts. Δ decreases with increasing measuring temperature, T. Narrowing of Δ may be due to the higher charge carrier mobility (and therefore motional narrowing) at higher T.

Calorimetric studies of the heat capacity and relaxation of amorphous Si prepared by electron beam evaporation

The heat capacity of a‐Si thin film prepared by electron beam evaporation method was measured from 360 to 820 K by a differential scanning calorimeter. For the as‐prepared a‐Si specimen, two novel irreversible endothermic processes and one irreversible broad exothermic reaction were found. The origins of the endothermic reactions were not known. It is suggested that they may be caused by a change in the number and distribution of voids that occurs at approximately 465 K and the creation of dangling bonds at the higher temperature regime (≳620 K). The exothermic reaction is attributed to heat release during structural relaxation. When measuring the heat capacity of a‐Si, Ca‐Sip, these irreversible reactions were first eliminated by annealing the specimens at high temperatures. The heat capacity of crystalline Si, Cc‐Sip, was also measured and the difference, ΔCSip=Ca‐Sip −Cc‐Sip, was used to evaluate the thermodynamic melting temperature of the a‐Si, Tal, which is determined to be 1400 K.

New amorphous silicon-boron alloy

Abstract A new amorphous Si1−xBx alloy with composition x ranging from .05 to .5 was produced in an LPCVD system. This material has good semiconducting properties. Dark conductivity of the alloy is typically 10 −1 Ω −1 − cm −1 . The Hall mobility at room temperature ranges from 1 to 10 cm2V−1s−1.

Epitaxial regrowth of Ge films on (001) GaAs by in situ thermal pulse annealing of evaporated amorphous germanium

Germanium thin films have been epitaxially regrown on (001) GaAs by in situ thermal pulse annealing of evaporated amorphous germanium under <102 Watt/cm2 broad-band irradiation in high vacuum. Epitaxial regrowth was found to occur only when the duration of the thermal pulse (te) was greater than a critical value tc (≃3.20 s). The crystal quality of the resultant film was examined by high resolution x-ray diffraction technique (HRXRD) and grazing-incidence x-ray diffraction technique (GIXRD). All rocking curves were found to have a full width at half-maximum of about 0.02°. Both HRXRD and GIXRD confirmed the Ge overlayer was grown epitaxially as well as pseudomorphically on the substrate. Scanning electron microscopy and atomic force microscopy revealed the very different surface morphologies resulting from different te. For te<tc, columnar germanium grains with a four-fold symmetry and a high uniformity in size were found, while for te⩾tc, epitaxial regrowth was observed. It is suggested that epitaxial re...

Thermoelectric power and electronic transport in thermal-LPCVD amorphous silicon-boron films

Abstract The temperature dependence of the thermoelectric power S and the electrical conductivity σ of thermal low-pressure chemical vapour deposited (LPCVD) a-Si:B films have been measured. The films contain a solid-phase boron concentration between 2 and 40at.%. To explain the temperature dependence of S and σ quantitatively, we propose a three-path-conduction model, involving conduction via the valence band, the band tail and a boron-related impurity band. By introducing suitable functions for the density of states and the microscopic mobility, both σ and S can be calculated numerically using the Kubo formalism. An excellent fit between calculated and experimental data has been obtained. It is found that the impurity band plays a dominant role in the transport properties of these heavily doped films. The impurity band, assumed to have a Gaussian shape, peaks at about 0·2 eV above Ev.

Gap states study of evaporated silicon films before and after hydrogenation

Abstract The distribution of gap states in evaporated amorphous silicon films before and after hydrogenation is investigated by the space-charge-limited current (SCLC) method. Broad peaks of gap states with density around 3 × 10 17 cm −3 eV −1 are observed at energy between 0.73 and 0.53eV below E c . Hydrogenated samples show an exponential band tail with gap state density decreasing smoothly to about 5 × 10 15 cm −3 eV −1 around 0.4eV below E c .

A scanning electron microscopy/transmission electron microscopy study of the photocrystallization of amorphous silicon

It has been recognized that the kinetics of crystal growth under rapid solidification conditions opens up a new regime of crystal growth. Photocrystallization of amorphous silicon leads to explosive crystallization under certain conditions. This is true for both laser and incoherent light radiation. The explosive crystallization yields three regions, i.e., a pure yellow region, a reddish brown region, and an ochre‐colored region. Till now the amorphous nature or crystallinity of the recrystallized film has been established solely by color judgment. In this communication we report on a study made by using optical, transmission electron, and scanning electron microscopy of the photocrystallized films which correlates the color changes with electron diffraction patterns. The more quantitative measurements indicate that the overwhelmingly larger optical absorption coefficient of amorphous silicon, and the presence of thin overlayers can lead to erroneous judgments of the crystallinity of the regions when made...

Dendritic crystallization of amorphous germanium by in situ thermal pulse annealing

Phase transformation of amorphous-germanium thin film has been carried out on amorphous substrate by in situ thermal-pulse annealing in a high vacuum chamber directly after evaporation. The microstructure of the resultant film was shown to depend markedly both on the annealing ambient and on the time of exposure (te) for the as-deposited films under full-powered (<102 W/cm2) incoherent broadband irradiation. The heating rate was estimated to be not less than 102 K/s. The surface morphology of the sample was examined by atomic force microscopy (AFM), scanning electron microscopy, and optical microscopy. For samples annealed in air, hillock growth mode was observed, while for samples annealed in vacuum, a transition from the microgranular to dendritic grain growth, depending sensitively on te, was clearly evident. Surprisingly, the length of the crystallized dendrites could be as long as ≃104 μm, being at least ≃104 times larger than the thickness of the film. The dendritic morphology, the implied growth ra...

Effects of Si incorporation on the structural change of a‐BxSi1−x alloy films

Amorphous BxSi1−x films can be easily prepared by low‐pressure chemical vapor deposition over the whole range of x from 0 to 1. In this article, the structural change of BxSi1−x films (0≤x≤1) was studied by x‐ray diffraction and infrared (IR) absorption experiments. It was found that these two methods are complementary to each other. X‐ray results showed that when x is decreased, there is a gradual transition from the a‐B structure, through the SiB4 structure, to the amorphous silicon structure. The transition to a‐Si structure is complete at a surprisingly high boron concentration of around 40 at. %. Infrared data also revealed an unexpected result—that the presence of boron suppresses the formation of Si—H types of bonds in high silicon content films, in strong contrast to films prepared by the glow discharge method. Another interesting feature of the IR absorption spectra is the predominance of an absorption band, related to the presence of boronlike icosahedral clusters, in all films with boron concen...

Electronic transport in amorphous and microcrystalline phosphorus-doped silicon films prepared by thermal LPCVD

Abstract Phosphorus-doped silicon films have been prepared by thermal low-pressure vapour deposition method using a wide range of coating conditions. Detailed results on dark electrical conductivity, photoconductivity and thermoelectric power are presented. The films can be roughly classified, according to their properties, into four categories, namely, low-phosphorus-content amorphous films (aLP), high-P-content amorphous films (aHP), low-P-content microcrystalline films (μcLP) and high-P-content microcrystalline films (μcHP). By analysing the experimental data by suitable two-conduction-path models, the following conclusions can be drawn. μcHP films are highly degenerate semiconductors with the Fermi energy E F lying inside the conduction band. μcLP films are slightly degenerate with a phosphorus impurity band situated at about 0·05 to 0·07 eV below E C. The as-deposited aHP films are high-quality n-doped films. The best doped sample so far achieved has E F lying 0·2 eV below E C. For as-deposited aLP f...