S. K. Wong

Highly stable, photosensitive evaporated amorphous silicon films

Vacuum‐evaporated pure amorphous silicon films have been successfully hydrogenated in a Theta‐pinch plasma source to give high photoconductivity. Unlike films produced by glow discharge of silane gas, these films are highly stable against heat, intense light illumination, moisture, and other atmospheric contamination. Structural change in the bulk Si matrix being absent, a comparison of properties of films before and after plasma treatment can provide valuable information on pure hydrogenation effects.

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...

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.

Mixing of nickel and silicon by incoherent-light-pulse annealing

Abstract Mixed layers of Ni and Si were formed by pulsed incoherent-light annealing of Ni thin films evaporated on Si substrates. Incoherent light pulse of 36 μs produced in an arc-discharge plasma system was used as the energy source for annealings. A thin layer (300 A) of amorphous silicon (a-Si) deposited on 800–1000 A thick Ni films was found to increase light absorption significantly. The light energy needed to form uniform mixing was measured to be about 23 J/cm2. Rutherford backscattering and Auger electron spectroscopy techniques were used for qualitative analysis of a-Si/Ni/Si samples. Optimum operating conditions of the light source was determined.

Crystal growth in amorphous silicon thin films induced by incoherent light flashes

Amorphous silicon thin films on glass substrates were crystallized by a 4‐μs light pulse from a plasma arc. Explosive crystal growth from nucleation centers was observed at 4.6 J/cm2. Large silver‐shaped streaks (0.7 mm×50 μm) were produced. Conductivity changed by six orders of magnitude and the flashed samples gave high photoconductivity. Growth patterns were different near the threshold value of 3.1 J/cm2.

Current‐voltage characteristics of n‐amorphous low‐pressure chemical vapor deposited silicon films on p‐crystalline silicon

Heterojunction devices have been fabricated by a low‐pressure chemical vapor deposition technique whereby n‐type amorphous or microcrystalline silicon films were grown on p‐type crystalline silicon substrates. Heterostructures produced under various conditions of thin film deposition were subjected to detailed I‐V curve analysis. It is seen that for amorphous‐crystalline heterojunctions the current transport is through tunneling in the low bias range and limited by electron‐hole recombination in the high bias range. For the microcrystalline‐crystalline junctions however, recombination current at the interface dominates the current transport process. Illuminated I‐V curves corresponding to films deposited at different substrate temperatures (Ts ) and dopant gas‐to‐silane ratios (R) show that the high values of the short‐circuit current (Isc), open‐circuit voltage (Voc ), and fill factor (FF) are achieved at values close to Ts ∼620 °C and R ∼4×10−3 in conformance with dark I‐V characteristic data.