Electron energy loss spectroscopy determination of Ti oxidation state at the (001) LaAlO3/SrTiO3 interface as a function of LaAlO3 growth conditions

At the (001) interface between the two band-insulators LaAlO3 and SrTiO3, a high-mobility electron gas may appear, which has been the object of numerous works over the last four years. Its origin is a subject of debate between the interface polarity and unintended doping. Here we use electron energy loss 'spectrum images', recorded in cross-section in a scanning transmission electron microscope, to analyse the Ti3+ ratio, characteristic of extra electrons. We find an interface concentration of Ti3+ that depends on growth conditions.Comment: 6 page

The susceptibility and excitation spectrum of (VO)2_2P2_2O7_7 in ladder and dimer chain models

We present numerical results for the magnetic susceptibility of a Heisenberg antiferromagnetic spin ladder, as a function of temperature and the spin-spin interaction strengths $J_\perp$ and $J_{||}$. These are contrasted with new bulk limit results for the dimer chain. A fit to the experimental susceptibility of the candidate spin-ladder compound vanadyl pyrophosphate, (VO)$_2$P$_2$O$_7$, gives the parameters $J_\perp = 7.82$ meV and $J_{||} = 7.76$ meV. With these values we predict a singlet-triplet energy gap of $E_{gap} = 3.9$ meV, and give a numerical estimate of the ladder triplet dispersion relation $\omega(k)$. In contrast, a fit to the dimer chain model leads to $J_1=11.11$ meV and $J_2=8.02$ meV, which predicts a gap of $E_{gap} = 4.9$ meV.Comment: 16 pages, 6 figures available upon request, RevTex 3.0, preprint ORNL-CCIP-94-04 / RAL-94-02

Study of radio-frequency plasma deposition of amorphous silicon for the improvement of solar cell production

Plasma enhanced chemical vapour deposition (PECVD) of thin films such as amorphous silicon has widespread applications especially in the field of photovoltaic solar cells and thin-film transistors for flat screen production. Industrial applications require high depositions rates over large areas with a good uniformity in layer thickness. In this thesis, some aspects of PECVD in large surface, industrial type, capacitive radio frequency reactor are investigated. The aim of this work is to study the plasma process conditions to increase the deposition rate of uniform, good quality, a-Si:H layer for solar cell application in a single chamber reactor. The studies realized during this thesis have necessitated the development and the comprehension of diagnostics such as deposition rate measurement by in-situ interferometry, plasma power measurement and layer density measurement by ellipsometry. During the thesis, we have also elaborated a matching-box circuit for using process frequencies of 27.12 MHz and 40.68 MHz. Deposition of a-Si:H in small electrode gap reactor has been studied. At present industrial reactors have a standard electrode gap of 2.4 cm. We modified a reactor to reach a small gap of 1.7 cm. It appears that we have obtained faster deposition rate in the small gap reactor but non-uniformity problems increase due to edge focusing and powder effects. One solution, based on a teflon jigsaw to keep the plasma away from the edge confined spaces, is proposed to suppress focusing effect but the operation parameter space is still reduced by the powder effect. Systematic measurements of a-Si:H layer density was also done by ellipsometry. It is shown that the layer density decreases when the deposition rate increases, independently of pressure, gas flow and frequency (27.12 MHz/40.68 MHz) of the plasma. At high deposition rate, 6 Å/s, only an increase of the process temperature from 200°C to 230°C can significantly improve the layer density. We have noted also a slight improvement of layer density for layers deposited in the small gap reactor. Nevertheless industrial constraints impose a process temperature of 200°C and a standard gap reactor. By optimising the process parameters, keeping the temperature process at 200°C, good quality, uniform, a-Si:H layer were deposited at 3 Å/s on 37 cm × 47 cm glass substrates at 40.68 MHz. A particular source of non-uniformity in large area reactor has been examined. In large area reactors, a perturbation in RF plasma potential, due to the electrode edge asymmetry, propagates towards the plasma center with a characteristic damping length λ. The variation of RF plasma potential at the edge implies a variation of the deposition rate across the reactor area and then a non-uniformity of the deposited layer. A theoretical study was done to understand of this phenomenon and experimental results confirmed the model. Finally, for solar cell applications, a study of the boron cross-contamination during solar cell deposition in a single chamber process has been done. During the deposition of the intrinsic layer, for a p-i-n cell, i-layer is contaminated by the residual boron radicals present in the reactor after the deposition of the boron-doped layer. This contamination decreases cell performance. Several reactor treatments have been tested to solve this contamination problem. The effectiveness of these treatments was evaluated by secondary ion mass microscopy (SIMS) measurements. It appears that an ammonia flush or a water vapour flush of a few minutes, between the deposition of the p-layer and i-layer, reduces the boron contamination at the p-i interface. The performance of cells made with these treatments, in a single chamber process, are comparable to performance of cells done in a multi-chamber process

Reduction of the boron cross-contamination for plasma deposition of p–i–n devices in a single-chamber large area radio-frequency reactor

In this article, a new treatment to reduce boron contamination of the interface between the p- and i- layer is presented. An ammonia flush, performed at 10 Pa for 1 min, after deposition of the p-layer considerably reduces the boron contamination at the p–i interface of amorphous silicon p–i–n solar cells prepared in a single-chamber reactor. This treatment avoids the need to move the substrate out of the reactor during the full deposition process of a solar cell, thereby reducing costs. The measurement of boron contamination depth profile in the i-layer was done by Secondary Ion Mass Spectroscopy and the effectiveness of the treatment was supported by quantum efficiency and I–V measurements of solar cells