N. Maley

A critical investigation of a-Si:H photoconductivity generated by subgap absorption of light

Abstract Dual beam photoconductivity (DBP) method has been used to characterize the sub-bandgap optical absorption of a-Si:H. Results are presented for different a-Si:H films which illustrate the effects of the electron quasi-Fermi level displacement due to changes in bias light intensity and temperature. These results demonstrate that DBP offers a sensitive and reliable method for studying the densities and energy distribution of intrinsic and light induced defects and their effect on recombination kinetics.

Infrared absorption and thermal evolution study of hydrogen bonding in a-SiH

Hydrogenated amorphous silicon (a‐SiH) films were prepared by reactive dc magnetron sputtering under a wide range of conditions. The amount of hydrogen in the films CH and the nature of the Si–H bond were studied using infrared (IR) absorption and thermal evolution of H2. Hydrogen content varied between 2 and 40 at. %. The trends in the IR spectra are qualitatively similar to those previously reported. For low CH the thermal evolution spectra show a single peak centered around 600 °C, and with increasing CH, two additional peaks at ∼500 and 400 °C. A comparison of the IR and evolution spectra of the sputtered films yields an oscillator strength of 2.0×1019/cm2 for the 640 cm−1 wagging mode. This is 25% higher than the value previously reported for glow discharge and rf sputtered films. The hydrogen evolving from the 400 and 500 °C peaks correlates well with the integrated absorption of the 2100 cm−1 stretching mode. However, no good correlation is found between the high‐temperature evolution peak and the ...

Effect of midgap states in intrinsic hydrogenated amorphous silicon on sub‐band‐gap photoconductivity

Photoconductivities, generated by photon energies less than the optical gap, in intrinsic hydrogenated amorphous silicon were measured over a wide range of carrier generation rates. Detailed numerical analysis of the corresponding sub‐band‐gap absorption was used to model the nature, densities, and distribution of midgap states. These derived gap state parameters are consistent with the electron lifetimes and their dependence on the generation rates. The analysis shows how the spectral response of the sub‐band‐gap photoconductivity (absorption) depends not only on the densities of midgap states but also on their location relative to the band edges. The position of these defect states can be obtained from sub‐band‐gap photoconductivity measurements providing that the corresponding occupation by electrons is taken into account.

Subsurface hydrogenated amorphous silicon to μc‐hydrogenated silicon transformation during magnetron sputter deposition determined by spectroscopic ellipsometry

We use in situ spectroscopic ellipsometry to analyze the microstructure of the μc‐Si:H/a‐Si:H interface deposited by reactive magnetron sputtering of a Si target in (Ar+H2). We increase the hydrogen pressure to promote μc‐Si:H formation and observe several effects. Initially, H penetrates ∼45 A into the a‐Si:H substrate and increases its hydrogen content. Then ∼55 A of hydrogen‐rich a‐Si:H deposits. Finally, μc‐Si:H nucleates on top of this ∼100 A thick, high H‐content a‐Si:H interface layer. As μc‐Si:H grows, the thickness of the amorphous interface layer decreases by ∼40 A; the void fraction in the μc‐Si:H layer is always ≤15 vol %, ruling out the possibility that the a‐Si:H is etched away. These results suggest that a‐Si:H can be transformed into μc‐Si:H in a subsurface region under appropriate conditions.

Silicon epitaxy at 230 °C by reactive dc magnetron sputtering and its in situ ellipsometry monitoring

We report epitaxial growth of silicon on Si(100) at 230 °C by reactive dc magnetron sputter deposition. Growth is monitored with in situ multiwavelength ellipsometry to determine the film microstructure. Film crystallinity depends on the partial pressure of hydrogen during deposition, and the best film is obtained with 2 mTorr H2. The films are found to have a bulk density deficit of a few percent and a low‐density layer at the film‐substrate interface. The microstructure is confirmed by transmission electron microscopy measurements. Infrared absorption and thermal hydrogen evolution measurements indicate that a large amount of molecular hydrogen is present at the substrate interface. We discuss the possible roles of hydrogen and particle bombardment in promoting epitaxial growth.We report epitaxial growth of silicon on Si(100) at 230 °C by reactive dc magnetron sputter deposition. Growth is monitored with in situ multiwavelength ellipsometry to determine the film microstructure. Film crystallinity depends on the partial pressure of hydrogen during deposition, and the best film is obtained with 2 mTorr H2. The films are found to have a bulk density deficit of a few percent and a low‐density layer at the film‐substrate interface. The microstructure is confirmed by transmission electron microscopy measurements. Infrared absorption and thermal hydrogen evolution measurements indicate that a large amount of molecular hydrogen is present at the substrate interface. We discuss the possible roles of hydrogen and particle bombardment in promoting epitaxial growth.

Infrared relfectance spectroscopy of very thin films of a-SiH

Abstract We demonstrate the ability of infrared reflectance spectroscopy to monitor the nature of SiH bonding in a-SiH films only ∼10A thick. Measurements on reactive dc magnetron sputtered samples clearly show a SiH stretching mode at 2100cm −1 in the thinnest films. The results also show systematic variations with film thickness and the hydrogen partial pressure during deposition. A simple model is presented to explain the observed results.

Effect of hydrogen on the microstructural, optical, and electronic properties of a‐Si:H thin films deposited by direct current magnetron reactive sputtering

Device quality hydrogenated amorphous silicon (a‐Si:H) films have been deposited under a wide range of deposition conditions using dc magnetron reactive sputtering. The total hydrogen content (CH) has been varied from 0 to ∼40 at. % by changing the substrate temperature (Ts) or hydrogen partial pressure (PH2) independent of other deposition parameters. The films that contain CH between 10 and 28 at. % have the highest quality. The optical band gap (Eg) varies linearly with CH for all deposition conditions studied. With increasing CH the dark conductivity at 300 K decreases from ∼1×10−4 to ∼1×10−12 (Ω cm)−1; however, the photoconductivity under AM‐1 illumination, for the highest quality films, remains in the 0.8–3.5×10−5 (Ω cm)−1 range. The dark conductivity activation energy (Ea) was measured to determine the Fermi‐level (Ef) position with respect to the conduction band (Ec) and a linear correlation between Eg and Ea is found for high‐quality films. The films having low hydrogen content (10

Hydrogenated amorphous silicon films deposited by DC planar magnetron reactive sputtering

Abstract Reactive sputtering has the advantage in hydrogenated amorphous silicon (α-Si:H) deposition that it permits wide control of the hydrogen content in the films. This is in contrast to the commonly used method of plasma-assisted chemical vapor deposition, where the hydrogen enters the system on Si containing molecules, such as SiH 4 or Si 2 H 6 , and control is limited. Most reactive sputtering work has been done using RF driven planar diodes or magnetrons. RF sputtering systems have the disadvantage that the substrates can be subjected to electron and ion bombardment to a degree that is difficult to quantify. This paper describes α-Si:H films with high photo-to-dark conductivity ratios (10 5 –10 6 ) and low midgap densities of states (∼ 5 × 10 15 cm −3 −eV −1 ) that we have deposited by DC magnetron sputtering onto borosilicate glass substrates. Particular attention is given to the influence of the hydrogen partial pressure on the film properties such as the hydrogen content and bonding, the photo- and dark-conductivity, and the optical bandgap.

The improved stability of hydrogenated amorphous silicon films grown by reactive magnetron sputtering at high substrate temperature

We report the electronic properties, stability, and microstructure of a‐Si:H films grown at high substrate temperature (320–440 °C) by dc reactive magnetron sputtering. The initial defect state density, determined by the constant photocurrent method, varies from 2–5×1015 cm−3 with H content changing from 15–10 at. % as Ts increases from 320–375 °C. For 100 h of white light exposure at 1 W/cm2, the midgap state density reached an apparent saturation at 2–3×1016 cm−3 over this temperature range. By contrast, films grown at 230–300 °C saturate at 9×1016 cm−3.

Surface hydrogen release during the growth of a‐Si:H by reactive magnetron sputtering

We present a study of the hydrogen release step during growth of hydrogenated amorphous silicon (a‐Si:H) thin films. Hydrogen release must occur whenever the hydrogen fraction in the growth flux exceeds the hydrogen incorporation rate into the film, which is usually the case in a‐Si:H deposition. The films in our study are deposited by direct current magnetron sputtering of silicon in an argon–hydrogen atmosphere. In our experiment we monitor the rate of HD production with a mass spectrometer when a deuterated amorphous silicon sample is exposed to a silicon–hydrogen growth flux. Using mass balance arguments, these data, along with known bulk film hydrogen incorporation rates, allow a determination of a lower bound for the rate of reactive hydrogen impingement on the growing surface. Depending on the hydrogen partial pressure in the discharge, we find that about one reacted hydrogen per incorporated silicon arrives at the substrate for typical deposition conditions. In addition, our modeling predicts that...