B. Y. Tong

Scattering states of the Sech-squared potential

Abstract In the scattering of -V0Sech2(αx) potential, it is shown that perfect transmission (T = 1) is for all scattering energies or none at all. A wave-packet zooms through this potential faster than traveling in the absence of the potential. Applications in semiconductor heterojunction devices and in optical systems are suggested.

Raman studies of structural order in amorphous silicon-boron alloys

Abstract A new amorphous Si 1- x B x alloy with composition of boron x ranging from 0.01 to 0.5 was produced in a low pressure chemical vapor deposition (LPCVD) system. We performed Raman scattering experiments on the a - Si 1- x B x alloys and specifically monitored the Si-Si TO-like mode at around 480cm −1 and the TA-like mode at around 150cm −1 . A pronounced broadening of the TO-like band as well as a decrease of the intensity ratio I TO / I TA are observed with an increase of boron concentration in the a - Si 1- x B x alloys. Based on the Raman spectra, the mechanisms of structural changes induced by boron incorporation in the a-Si network are discussed in the conceptual framework of the continuum random network (CRN) model. We conclude that boron incorporation enhances the structural disorder.

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.

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...

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...

Raman Studies of Microstructural Changes in Amorphous Silicon-Boron Alloys Due to Annealing

Crystallization of amorphous Si l−x .B x alloy films by annealing is studied. Amorphous Si l−x B x . alloy films with composition of boron x ranging from 0.01 to 0.5 are deposited on Si substrates at a temperature of 480° in a low pressure chemical vapor deposition (LPCVD) system. Three films with the boron contents, 1%, 7% and 45%, are used in this study. The films are annealed in a nitrogen ambient for 30 minutes at temperatures between 600°C and 900°C. Raman spectra of the silicon vibrational mode serve as a indicator for the microstructure of the Si l−x B x , alloy films. Quantitative estimates of the volume fraction of the crystalline silicon component in respect to the amorphous silicon component in the films are calculated based on the silicon TO mode. The results show that while for the film with the boron content of 1% crystallization occurs at the annealing temperature of 500°C, the annealing temperature of 7000C is required to observe crystallization in the films with the boron contents of 7% and 45%. As the annealing temperature increases, the volume fraction of the crystalline component increases. For a given annealing temperature, the rate of crystallization depends inversely on the boron content in the films.

SIMS study of the concentration of dopants in LPCVD silicon thin films

Abstract Measurements of solid phase dopant concentration ( S ) of LPCVD Si thin films as a function of substrate temperature ( T s = 500−640 ° C) and gas phase doping ratio ( R = 1 × 10 −5 −4 × 10 −2 ) by SIMS indicate different behaviors of P and B in the films. A linear relation S = b ( T ) R is observed for B-doped film with b ( T ) varying from 4 to 50 depending on T s . Boron-doped microcrystalline film has a higher doping efficiency than that of P-doped ones.

Time-resolved study of thin nickel silicide layer growth at the nickel film-Si(100) interface

Abstract A time-resolved study of the initial stages of the formation of nickel silicide by pulsed incoherent light irradiation is described. Unreacted intermixed Ni-Si layer formation is shown to be an essential first step for silicide growth. The silicide layer emerges from the Ni-Si mixed layer by a disordered-to-crystalline transformation at a threshold nickel concentration corresponding to the crystalline silicide composition.