D.W. Lane

Growth of Ti:sapphire single crystal thin films by pulsed laser deposition

This paper documents the growth of single crystal Ti:sapphire thin films, typically 10µm thick, on undoped sapphire substrates using Pulsed Laser Deposition from a Ti:sapphire single crystal target with a doping level of 0.1 % wt Ti2O3. These thin films are shown to have very high crystal quality using Ion Beam Channelling and X-Ray Diffraction Techniques. The degree of titanium incorporation into the films is investigated using Inductively Coupled Plasma Mass Spectrometry and Particle Induced X-ray Emission. These techniques show that levels of up to 0.08% wt Ti2O3 are present in the deposited layers.

Electrodeposition and characterisation of CdTe films for solar cell applications

Thin film CdS/CdTe solar cells have been prepared by electrodeposition of CdTe on CdS coated conducting glass from an acidic electrolyte containing a high concentration of Cd2+ and a low concentration of TeO2. Deposition of a 2 μm CdTe film from stirred solutions typically requires 3 h. High quality CdTe films have been grown much more rapidly using a channel flow cell: 2 μm films were deposited in around 24 min. The CdTe|CdS thin film structures obtained in this way were characterised by photocurrent spectroscopy, electrolyte electroreflectance/absorbance spectroscopy (EER/A), XRD and AFM. CdS|CdTe films prepared by both methods were annealed at 415°C to effect type conversion of the CdTe layer. As deposited CdTe is generally n-type and exhibits strong preferential 〈111〉 orientation. Type conversion is not necessarily accompanied by recrystallisation: most of the CdTe films deposited from stirred solution did not recrystallise. Recrystallisation did occur for films grown by pulsing the potential periodically from 50 mV to>350 mV versus Cd2+/Cd during deposition. Evidence for sulphur and tellurium diffusion leading to alloy formation during annealing was obtained from bandgap shifts detected by photocurrent spectroscopy and EER/A and from changes in lattice parameters measured by XRD. The composition of the annealed electrodeposited structures approached CdS0.95Te0.05|CdTe0.95S0.05 after 15 min. Test solar cells with AM 1.5 efficiencies approaching 6% were fabricated. Recrystallised samples gave higher solar cell efficiencies than non-recrystallised samples.

Structural and spectroscopic studies of CdS/CdTe heterojunction cells fabricated by electrodeposition

CdTe films have been electrodeposited on CdS|SnO2(F)|glass in a stirred cell and in a channel flow cell. The influence of deposition conditions on recrystallization during annealing treatment has been studied. Films were characterised by XRD, photocurrent spectroscopy and electroreflectance/absorbance spectroscopy using electrolyte contacts to allow illumination from the CdTe or CdS sides. As-deposited CdTe is generally n-type and tends to show strong preferential 〈111〉 orientation. Thermal treatment in air brings about type conversion. Type conversion is not necessarily accompanied by recrystallization; most of the CdTe films deposited from stirred solution did not recrystallize. Recrystallization did occur for films grown by pulsing the potential periodically from 50 mV to >350 mV vs. Cd2+/Cd during deposition. In this case, diffusion of sulfur led to formation of CdTe0.95S0.05. CdTe films were also electrodeposited rapidly in a channel flow cell. Deposition times shorter than 30 min for 2 μm were achieved, and the films recrystallized fully during thermal annealing. The flow cell films produced in this way gave reasonable solar cell performances, holding out the promise of more rapid fabrication of CdS/CdTe cells by electrodeposition.

Optical properties and structure of thermally evaporated tin oxide films

Abstract Tin oxide films have been grown by thermal evaporation of SnO 2 powder from a resistively heated alumina crucible. Analysis by X-ray diffraction and Rutherfold backscattering spectrometry has shown that the films are of an amorphous nature, with a typical stoichiometry of SnO 1.3 . Detailed optical measurements have shown that the films have a refractive index close to two and a negligible extinction coefficient over the 2–10 μm wavelength range. Such films appear to be well suited to the fabrication of simple, single layer anti-reflection coatings for use on both germanium and silicon in the infrared.

SOME OBSERVATIONS ON THE TRACE ELEMENT CONCENTRATIONS IN HUMAN DENTAL ENAMEL

The concentration of trace elements has been measured for dental enamel from 86 healthy human teeth using particle-induced X-ray emission (PIXE). The majority of the teeth (n = 70) were collected from dentists in the county of Oxfordshire in the United Kingdom, although a smaller group (n = 16) were collected from Cornwall. The elements K, Ca, Mn, Fe, Co, Ni, Cu, Zn, Sr, Pb, and Hg have been detected and statistically analyzed by grouping according to sex, age, and geographical location. The concentrations of Fe and Cu were found to be lower in the teeth from female donors (P < 5%) and are believed to result from the continued burden of blood loss during menstruation. Strong positive correlations (P < 0.1%) were found between Ca, Co, Ni, and Zn for all groups; these elements were also found to exhibit a negative correlation (P < 1%) with age for teeth from female donors. This is believed to be related to decalcification during the menopause. Pb was found to exhibit a positive correlation (P < 5%) with age for both sexes, and is believed to substitute for Ca in the Ca hydroxy apatite (HAP) within the dental enamel.

Structural dynamics in CdS–CdTe thin films

Abstract The performance of CdS–CdTe heterojunction solar cells depends critically upon the structures formed during thin film deposition and any subsequent processing. We have undertaken a detailed examination of solar cell materials (in particular CdTe and CdS) which has enabled some correlation between their fundamental properties and structural behaviour as thin films. In particular we have determined the Vegard coefficients and phase diagram for the CdS–CdTe system. We have also examined the diffusion characteristics of both single-crystal and polycrystalline CdTe and CdS with respect to Te and S in order to define the rate at which any intermixed region may grow. Thus we have determined several fundamental properties of CdTe and CdS which were either not available or apparently anomalous. These data have been used to underpin and interpret findings from studies of the structural and electronic changes that occur during the type conversion anneal of CdTe. In particular, we have shown how an intermixed region forms during the heat treatment and that this could be mediated by the initial, as-deposited structures. We have also been able to contrast the behaviour of CdTe films produced by PVD and electrodeposition. In order to characterise the structure of these thin films it has been essential to develop novel depth profiling methods based upon our primary analytical methods, i.e. X-ray diffraction and ion-beam analysis. These techniques, when used with the fundamental material properties, are shown to provide complementary information that has allowed us to build models of the CdTe and CdS layers that may allow the formation of the intermixed region to be controlled during the fabrication process.

Combinatorial optimization of Al-doped ZnO films for thin-film photovoltaics

A rapid screening methodology for the development of transparent conducting oxides is presented. The methodology, based on a combination of spectrophotometry, ellipsometry and 4-point probe measurements, was used to map out the opto-electronic properties over a co-sputtered ZnO : Al2O3 film deposited from separate ceramic targets of ZnO and Al2O3. Clear distributions for the carrier density, ne, and mobility, μe, are determined as a function of wt%. Al2O3 content within the film. A minimum resistivity value of 7.6 × 10−4 Ω cm was achieved for a composition of 1.5 wt%. Al2O3.

Optical Design and Fabrication of Fully Sputtered CdTe/CdS Solar Cells

A model is presented for optimising the integrated intensity of light passing into the absorber layers of thin-film solar cells. Dispersion data for each of the transparent layers in the cells was obtained from variable angle spectroscopic ellipsometry. This data was then used in a transfer matrix model of multi-layer optics to determine the intensity spectrum passing over the interface from the transparent to the absorbing parts of the cell. Application of the method to the case of CdTe/CdS/ZnO/ITO/glass solar cells is described in detail, including the fitting of oscillator models to extract the complex dielectric function from the ellipsometry data. It was found that light coupling into the absorber was particularly sensitive to the CdS thickness but that tuning the ZnO (highly resistive transparent) layer was also influential. All-sputtered cells of this type were demonstrated as having efficiencies of 12.5% for 5 × 5 mm2 contacts.

Optical and structural characterization of CdSxTe1-x thin films for solar cell applications

The optical constants have been determined for films of CdSxTe1-x over the wavelength range 250-3200 nm. The films were prepared by vacuum evaporation from solid solutions. Rutherford backscattering spectrometry has been employed to determine the thickness of the films, which is in the range 1390-2430 nm, and x-ray diffraction has been used to determine the phase and lattice parameters. The films were found to be cubic for x 0.65. Reflectance and transmittance measurements have been made over the wavelength region 250-3200 nm, and a model, which includes the effects of coherent scattering, has been used to determine the complex refractive index in the transparent region. A singly subtractive Kramers-Kronig algorithm is derived for use with reflectance data of equal wavelength spacing. This novel Kramers-Kronig transform has been used to determine the optical constants in the opaque region. Polynomial functions are supplied which describe the variation of refractive index and extinction coefficient with wavelength. The photon energies required for both direct and indirect transitions have been found by least-squares fitting to the absorption spectra. The results are compared with previous work on 40 nm thick films.

The analysis of trace elements in human teeth collected from the Oxfordshire area in the UK

Abstract The concentrations of trace elements in the dental enamel from human teeth collected from the Oxfordshire area of the UK have been measured using PIXE. Trace elements heavier than Ca were analyzed and correlated to a number of parameters including the health of the teeth and the age and gender of the donor. The concentration of Fe, Sr and Cu were found to correlate with dental health, and the mean concentration of Pb was found to be 7 μgg −1 for under 25 year olds and increased with age. The uniformity of the trace elements, and its effect on the statistical distribution of their concentrations is discussed.