P. K. John

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...

Interaction of a CO2 laser beam with a shock‐tube plasma

The results of experimental investigations of the interaction of a CO2 laser beam with plasma produced in an electromagnetic shock tube are presented. The interaction was investigated in two different configurations: with the laser beam perpendicular to the direction of propagation of the shock wave and with the laser beam parallel to the direction of the shock wave. The laser energy was 0.3 J in a 180‐nsec pulse. The plasma density was in the range 1017–1018 cm−3 and temperature was around 2 eV. Spectroscopic methods were used in the measurement of density and temperature. Direct observation of the path of the laser beam through the plasma was made by an image‐convertor camera in conjunction with a narrow‐band interference filter. The propagation of the laser through the plasma and energy absorption are discussed. The observed maximum increase in electron temperature due to the laser in the first configuration was 0.4 eV and the estimated temperature increase in the second configuration was about 2 eV.

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...

Q switching a CO2 laser by a plasma

A transversely excited pulsed CO2 laser is Q switched by means of a plasma. The plasma was produced in an electromagnetic shock tube and was used to cut off the trailing edge of the laser pulse at a predetermined time and Q switch the rest. Up to an order‐of‐magnitude relative increase in power is obtained for the Q switched portion of the output.

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.

Efficient ion beam extraction from a flowing plasma

A moving plasma with directed flow velocities v larger than the ion acoustic speed cs is used as a source of high‐current ion beams in the 10–20‐keV range of energies. Current densities up to 3 A/cm2 are obtained at the plasma boundary which is an order of magnitude larger than the limiting value of Bohm current in a stationary plasma. The observed current densities were proportional to v, unlike the Bohm current densities which are proportional to ion acoustic speed. Total ion currents up to ∼100 A were extracted from the plasma through a two electrode extraction system. Simple geometric shaping of the electrodes enabled an 8‐cm‐diam beam to be focused to ∼7 mm.