E. Ahmed

Impact of annealing processes on the properties of CuIn0.75Ga0.25Se2 thin films

Abstract The efficiency of a solar cells utilizing polycrystalline Cu(In,Ga)Se 2 is to a large extent limited by crystalline defects of the semiconductor. Depending on the fabrication process the density of grain boundaries and dislocations, can vary considerably. However, the material properties can be improved significantly by the subsequent processing steps. In this paper results obtained using various post deposition methods to improve the structural and electro-optical properties of CuIn 0.75 Ga 0.25 Se 2 (CIGS) thin films have been presented and discussed. Films deposited by the evaporation of pre-reacted polycrystalline CIGS onto glass substrate were subsequently processed under several sets of conditions including vacuum, selenium, inert and forming gas ambients at different temperature and times. The structural and electro-optical properties of both as-deposited and annealed films were studied using a variety of analytical techniques. X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies of the films showed a columnar structure with strong orientation, which after heat treatments was relaxed to form a chalcopyrite structure. Raman analysis showed that full wave half maximum (FWHM) value reduced from 20 to 10 cm −1 with the annealing process indicating a change in both film composition and microstructure. In addition, investigations using energy dispersive X-ray analysis (EDAX), X-ray fluorescence (XRF) and Rutherford backscattering spectroscopy (RBS) revealed that the composition was approaching that of the starting polycrystalline material. Both n- and p-type conductivities were observed and gave resistivity values in the range 10 −1 to 10 6 Ω cm. Annealing in selenium changed the observed n-type conductivity of the as-deposited films to p-type. Photoacoustic spectroscopy (PAS) have also been applied to verify the improvement in the optical properties of annealed films.

Optical properties of flash-evaporated CuIn0.75Ga0.25Se2 thin films by photoacoustic spectroscopy

Abstract A high-resolution near-infrared photoacoustic spectrometer of the gas-microphone type is used for room-temperature analysis (in the subgap region of the spectrum) of non-radiative defect levels in as-grown CuIn0.75Ga0.25Se2 thin films. Films were grown by flash evaporation onto glass substrates at 200 °C. The absorption coefficient has been derived from the photoacoustic spectra to determine the gap energy and establish the activation energies for several defect-related energy levels. We also present preliminary results relating to the influence of post-deposition heat treatments in a selenium atmosphere on the photoacoustic spectral response. The improvements in the photoacoustic spectrum following annealing are directly correlated with the sample compositional, structural and electrical properties. Finally, the effect of interference on the photoacoustic spectra is discussed.

Deposition and characterization of copper indium gallium diselenide films by laser ablation and flash evaporation for use in solar cells

Copper indium gallium diselenide (CIGS) thin films have shown considerable promise for use as an absorber layer in high-efficiency solar cells. The initial results obtained from the preparation of CIGS films via laser ablation and flash evaporation are presented along with a comparison of the two deposition processes. The as-deposited CIGS films have been characterized by a variety of techniques, namely Rutherford back scattering and energy dispersive analysis using X-rays for composition measurements X-ray diffraction and Raman spectroscopy for structure elucidation, SEM for surface examination, and the four-point probe for resistivity measurements. In essence, good-quality coatings of CIGS were produced from both deposition processes in terms of their stoichiometry, electrical and structural properties.

The influence of annealing processes on the structural, compositional and electro-optical properties of CuIn0.75Ga0.25Se2 thin films

Crystalline defects, such as the density of voids, grain boundaries and dislocations, in Cu(In,Ga) Se2 absorber layers depend on the fabrication conditions and determine to a large extent the efficiency of photovoltaic devices. The material properties, however, can be improved significantly by using post-deposition processes. In this paper, the effects of post-deposition heat treatments on properties of CuIn0.75Ga0.25Se2 (CIGS) thin films are investigated. Selected flash evaporated samples were subsequently processed under several sets of conditions, including vacuum, selenium, inert (argon) and forming gas (a 9:1 mixture of N2:H2) ambients, at different temperature and times. Structural, compositional and electro-optical properties of both as-deposited and annealed films were studied using a variety of analytical techniques. X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies of the films showed a columnar grain structure with strong < 112 > preferred orientation, which after heat treatments relaxed to give a chalcopyrite structure. Raman analysis showed that the annealing process reduced the full-wave half-maximum (FWHM) value from 20 to 10 cm−1 indicating a change in both film composition and microstructure. In addition, X-ray fluorescence (XRF) and Rutherford backscattering spectroscopy (RBS) revealed that the composition was approaching that of the polycrystalline starting material. Both n- and p-type conductivities were observed with resistivity values in the range 10−1 to 106 Ω cm. Annealing in selenium atmosphere altered the n-type conductivity to p-type. To confirm improvements in optical properties of annealed films, photoacoustic spectroscopy (PAS) was employed.

Copper indium diselenide single-crystal and thin-film infrared sensors

Copper indium diselenide (CIS) and copper indium gallium diselenide (CIGS) are now well established as exceptionally efficient semiconductors with potential applications in the fields of solar cells, infrared radiation monitors and fibre optic infrared detectors. CIGS can be doped both p- and n-type and has a direct band gap which can be varied between 1.02 eV and 1.68 eV. Single-crystal CIS infrared detectors were produced on p-type substrates. The devices displayed photosensitivity significantly superior to that of silicon in the infrared at wavelengths up to 1.3 μm. Thin films were deposited by pulsed laser ablation of polycrystalline CIS targets using a XeCl excimer laser. The electrical and photo response of the thin film devices, while inferior to that from single crystals, indicated that laser ablated thin-film CIS sensors have potential for use as infrared detectors.

Synthesis and Characterization of Hexagonal Shaped Nanocrystalline Zinc Oxide Powders

Nanocrystalline ZnO powders are synthesized using a quick, simple and inexpensive combustion method. Glycine and zinc nitrate, used as fuel and oxidant/cation sources respectively, were mixed together at room temperature to form a slurry or gel. A series of experiments were performed to synthesize nanoparticles for different fuel to oxidant ratios. The rate of heating and stirring greatly influenced the combustion process. The as-synthesized powder was heat treated at elevated temperatures for varying time to remove adsorbed impurities from the surface of the ZnO nanoparticles. Both the as-synthesized and heat treated powders were characterized using a variety of analytical techniques. The hexagonal wurtzite phase of the ZnO powder was revealed from x-ray diffraction measurements. A decrease in the x-ray diffraction density (Dx) and the bulk density (D) resulted in an increase in percentage porosity (P) of as-synthesized and heat treated ZnO powders. The range of fuel to oxidant ratios used in the present work indicates that they had a strong influence on the grain size of ZnO powders. The average grain size estimated by using the Scherer formula indicated an approximate value of 6-21 nm for the as-synthesized and heat treated samples. Copyright © 2012, IGI Global.

Ion implantation and in situ doping of silicon

Abstract For many applications, both amorphous and polycrystalline deposited silicon films need to be doped in order to alter their electrical characteristics to suit VLSI devices. The two main methods of impurity introduction, ion implantation and in situ doping, have been considered in this study. The effects of the main processing parameters and thermal annealing on the electrical, surface and structural properties have been investigated. The choice of doping technique employed is dictated by considerations such as the application requirements, cost/benefit and ease of use. In both methods heat treatment is necessary to achieve optimum device characteristics. The process of rapid thermal annealing is beneficial in reducing diffusion of dopant and has also been studied.

Surface Engineering of CuIn0.75Ga0.25Se2 Thin Films

In this paper the effects of post-deposition annealing followed by hydrogen ionimplantation on the properties of CuIn0.75Ga0.25Se2 thin films have been investigated. The samples were grown by flash evaporation onto glass substrates heated at temperature between room temperature and 200°C. Selected samples were subsequently processed under several sets of conditions, including vacuum, selenium, inert (argon) and forming gas (a 9:1 mixture of N2:H2) followed by hydrogen ion-implantation. A high resolution near-infrared photoacoustic spectrometer of the gas-microphone type was used for room temperature analysis of non-radiative defect levels in the as-grown, annealed and hydrogen implanted thin films. The absorption coefficient has been derived from the PA spectra to determine the gap energy and to establish the activation energies for several defect-related energy levels. The changes observed in the PA spectra following annealing and ionimplantation has been directly correlated with the compositional and structural properties of the samples.

A photoacoustic study on the effect of Se content on defect levels in CuInSe2 single crystals

Abstract The objective of this paper is to investigate the impact of changes in the selenium content of CuInSe 2 single crystals on their optical properties in the subgap region of the infrared spectrum. A high resolution near-infrared photoacoustic spectrometer of the gas-microphone type is used for room temperature analysis of non-radiative defect states in as-grown n- and p-type CIS crystals. Samples with an excess and a deficiency of Se (5% off the stoichiometric composition) were grown from the melt by the vertical Bridgman technique. The absorption coefficient has been derived from photoacoustic spectra in order to establish activation energies for several defect-related energy levels. These results are compared with similar data obtained from near-stoichiometric single crystals which were annealed under maximum and minimum selenium vapour pressures.

Chemical vapour deposition (CVD) of borophosphosilicate glass films

Chemical vapour deposition (CVD) has become the standard method for the fabrication of microelectronic devices for use in the semiconductor industry. In this investigation, it has been used to grow films of silicon dioxide (SiO2) and borophosphosilicate glass (BPSG) at both atmospheric and low pressures under various conditions. The growth behaviour of SiO2 and BPSG films has been investigated as a function of the O2/SiH4 ratio. Both processes give a similar trend, with the growth rates of BPSG being somewhat higher than SiO2. The variation in the growth rate with O2/SiH4 ratio has been explained in terms of relative transport and kinetic reaction rates. The effects of temperature on the deposition rate have also been studied and the activation energy calculated showed two distinct regions corresponding to mass transport control and kinetic control regimes. Both BPSG and SiO2 have been annealed under various furnacing conditions. It has been shown that the addition of boron and phosphorous results in much lower reflow temperatures and times. This has a significant bearing on the performance characteristics of devices. Initial results from rapid thermal annealing (RTA) work are also presented, and RTA is shown to be a viable annealing process.