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Flip-flops are fundamental building block in designing the digital circuits. They contribute significant amount of power consumption in the digital systems. In the modern era of VLSI design, power consumption has become crucial factor. As the technology is scaled down below 65nm, leakage power contributes significantly to overall power consumption of digital circuits. Since, Flip-flop is a fundamental building block in designing digital systems, reducing the power consumption of Flip-flop is required to achieve low power consumption of digital systems. Recently single-phase clocking technique has been employed in designing FFs to achieve low power consumption. This technique reduces excess loading on the clock signal. In this article performance of recent single-phase clocked FFs is compared with the conventional transmission-gate Flip-flop. Also, drawbacks of these FFs are discussed. For the comparison of performance of single-phase-clocked FFs with conventional transmission-gate FF, all the simulations are performed at 45-nm predictive technology on Tanner EDA tool.

Large scale manufacturing of CdTe PV modules at the GW/yr level may be constrained due to the limited availability of the relatively rare (Te) element and the volume of potentially hazardous (Cd) material being used in the typically 3–8 μm thick CdTe absorber layer. However, we find that it is possible to reduce the CdTe layer thickness without much compromise in efficiency. The CdS/CdTe solar cells were fabricated using magnetron sputtering with ultra-thin CdTe layers in the range of 0.5–1.28 μm. The ultra-thin films and cells were characterized using X-ray diffraction (XRD), optical transmission, scanning electron microscopy (SEM), current–voltage and quantum efficiency measurements. These results were compared with those of standard 2.3 μm thick CdTe sputtered cells. Different post-deposition processing parameters were required for cells with ultra-thin and standard CdTe thicknesses to achieve high efficiency. Ultra-thin CdTe cells showed crystallographic texture and CdTe1−xSx alloy formation after CdCl2 treatment very similar to standard CdTe cells. Optimization of the post-deposition CdCl2 treatment and back-contact processing yielded cells of 11.2% efficiency with 0.7 μm CdTe compared to 13.0% obtained with standard 2.3 μm CdTe cells.

Cd1−xZnxTe alloy films with 1.6 and 1.7 eV band gaps were deposited by RF magnetron sputtering from targets made either of mixed powders or alloys of CdTe and ZnTe (25% and 40%). High-quality polycrystalline films with the (1 1 1) preferred orientation were obtained. The films were characterized using X-ray diffraction (XRD), scanning electron microscopy, resistivity, optical absorption, Raman, and photoluminescence. The EDS, XRD, and optical absorption analysis indicated that the x-value of the as-grown films were typically ∼0.20 and 0.30 for films sputtered from 25% and 40% ZnTe containing targets, respectively. The as-deposited alloy films exhibit quite low photovoltaic performance when used to make cells with CdS as the hetero-junction partner. Therefore, we have studied various post-deposition treatments with vapors of chlorine-containing materials, CdCl2 and ZnCl2, in dry air or H2/Ar ambient at ∼390 °C. The best performance of a Cd1−xZnxTe cell ( V OC = 737 mV , J SC = 19 mA / cm 2 ) was found for treatment with vapors of the mixed CdCl2+0.5%ZnCl2 in an H2/Ar ambient after pre-annealing at ∼520 °C in pure H2/Ar.

Polycrystalline II–VI semiconductor materials show great promise for efficient, low-cost photovoltaics. Large-area deposition of the II–VI semiconductors such as CdTe is possible by a variety of methods but the use of a plasma-based method such as magnetron sputtering can have significant advantages. Here we present recent results in the fabrication of CdS/CdTe cells using rf magnetron sputtering and discuss some of the advantages that accrue from the use of sputtering methods in this class of materials. Some of these advantages are particularly relevant as the polycrystalline thin-film PV community addresses issues related to the fabrication of tandem cells with efficiencies over 25%. Our best results have been obtained with sputtered ZnO:Al to achieve a CdTe solar cell having 14.0% efficiency at one sun with an air mass 1.5 global spectrum. We have also studied reactive sputtering of ZnTe:N which shows promise for use as a transparent back contact or as a recombination junction with ZnO:Al for II–VI based alloy top cells in a tandem solar-cell configuration. Finally, some advances have been made in substrate-configuration CdTe cells on Mo using sputter deposition that hold promise for flexible CdTe-based PV.

Polycrystalline Hg1−xCdxTe films were investigated for their potential as bottom cells of a CdTe-based tandem solar cell. The films were deposited by RF sputtering from a cold pressed target containing 30% HgTe + 70% CdTe. The as-deposited films were highly resistive with (111) preferred orientation and a bandgap of ∼1.0 eV. Various thermal treatment schemes were investigated under different conditions of ambient and temperature to reduce the resistivity. The film properties were analyzed using infrared transmission spectra, energy dispersive X-ray spectra and X-ray diffraction. N doped p-HgCdTe films were also prepared by reactive sputtering in a N2/Ar ambient. P-n junction solar cells were fabricated with CdS films as the heterojunction partner. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The CdTe-ZnTe or CdTe–MnTe alloy system is evaluated for application in high efficiency polycrystalline thin film multijunction solar cells. CdZnTe alloy films with 1.6 and 1.7 eV band gap and CdMnTe with 1.6 and 1.8 eV were deposited by RF magnetron sputtering from targets made of mixed powders of CdTe and ZnTe (25% and 40%) or MnTe (13% and 25%), respectively. Without postdeposition treatment, however, both of these alloy films exhibit quite low photovoltaic performance when used to make cells with CdS as the heterojunction partner. Therefore, we have searched for an appropriate postdeposition treatment to improve the efficiency of these alloy films. As-grown films have been postdeposition treated with vapors of chlorine-containing materials such as HCl gas, CdCl2, ZnCl2 or MnCl2 in dry air or H2+Ar atmosphere. When the CdCl2 + ZnCl2 or MnCl2 vapor was used, the ratio of CdCl2 to ZnCl2 or MnCl2 was adjusted to match the stoichiometry of alloy films. The temperature of the treatment was chosen to represent conditions employed for those alloy films/CdS structures. As a result, the postdeposition treated CdZnTe and CdMnTe cell efficiency can be improved by a factor of over 100. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Polycrystalline II-VI semiconductor materials show great promise for thin-film photovoltaic cells and modules. Large-area deposition of these II-VI semiconductors such as CdTe is possible by a variety of methods but the use of a plasma-based method such as magnetron sputtering can have significant advantages. Here we present recent results in the fabrication of CdS/CdTe cells using rf magnetron sputtering and discuss some of the advantages that appear possible from the use of sputtering methods in this class of materials. Some of these advantages are particularly relevant as the polycrystalline thin-film PV community addresses issues related to the challenges of fabricating high efficiency tandem cells with efficiencies over 25%. Our best results have been obtained with sputtered ZnO:Al to achieve a CdTe solar cell with 14.0% efficiency at one sun for an air-mass-1.5 global spectrum. In addition, we have studied reactive sputtering of ZnTe:N which shows promise for use as a transparent back contact or recombination junction for alloyed II-VI-based top cells in a tandem solar-cell configuration.

CuInSe2 films have been prepared using the selenization technique. Preparation of the precursor as well as selenization were carried out by the vacuum evaporation technique. The sequence of copper and Indium layer deposition during precursor preparation affects the morphological and structural properties of precursor which directly have effects on the properties of selenized CIS films. A thin layer of amorphous selenium at the substrate/film interface has been used to improve the adherence of the film. The effect of the Se under-layer has been studied on the layers of copper, indium, CuIn precursors and CIS films, using structural, morphological and optical properties. The surface morphology of a single layer of copper and indium, with and without the Selenium under-layer, are quite different and drastically affect the properties of the precursor and selenized films. The Se under-layer does not take part in the chemical reaction of CIS formation during the selenization process. The modified CIS films are uniform, single phase, polycrystalline, chalcopyrite with (1 1 2) preferred orientation showing an energy band gap of 0.99 eV and an absorption coefficient of ∼105 cm−1, and have good adherence to the substrate for the scotch tape test.

Radio-frequency (rf)-sputtered Al-doped ZnO was used as the transparent front contact in the fabrication of high efficiency superstrate configuration CdS∕CdTe thin-film solar cells. These cells had CdS and CdTe layers also deposited by rf sputtering at 250°C with the highest processing temperature of 387°C reached during a post-deposition treatment. The devices were tested at National Renewable Energy Laboratory and yielded an efficiency of 14.0%, which is excellent for a CdTe cell using ZnO and also for any sputtered CdTe solar cell. The low-temperature deposition process using sputtering for all semiconductor layers facilitates the use of ZnO and conveys significant advantages for the fabrication of more complex multiple layers needed for the fabrication of tandem polycrystalline solar cells and for cells on polymer materials.

The effect of temperature on the degree of alloy formation between Cu and In layers has been studied for the preparation of CuInSe 2 films by the selenization technique. The mechanism of alloy formation is different in bilayers annealed at temperatures lower and higher than the melting point of In, or prepared by deposition of Cu and In at 200°C and 150°C, respectively. While the annealing of the precursor up to 200°C produces an alloy in the interface region of Cu and In layers, annealing at 500°C completely changes its morphology. The effect of alloy formation in the precursor on the selenized CuInSe 2 films is studied using structural, morphological, optical and compositional properties.

Thin films of copper indium diselenide are deposited on Ti substrates by a simple and inexpensive electroless technique. The compositional, structural and morphological properties of the as such and annealed films are studied. The films are of single phase and chalcopyrite structure with (112) preferred orientation. The morphology of the films is found sensitive to the Cu/In ratio.

The present work focuses on the optimization of an interconnect junction for II-VI tandem solar cells. We have used ZnTe:N/ZnO:Al and ZnTe:Cu/ZnO:AI as interconnect junctions between a CdS/CdTe top cell and a CdS/HgCdTe bottom cell. Separate annealing studies of ZnTe:Cu show electrical and optical properties that are stable to 387/spl deg/C when the ZnTe:Cu is sputter deposited at /spl sim/320/spl deg/C. J-V measurements show that the V/sub OC/ of the tandem cell with ZnTe:Cu is much lower (/spl sim/820 mV) than the V/sub OC/ of the tandem cell (/spl sim/1031 mV) with ZnTe:N. One of the possible reasons is the interdiffusion of Cu into CdS, probably making it intrinsic and changing the junction properties of the top cell. Quantum efficiency measurements of the tandem cell with ZnTe:N show that the current is limited by the bottom cell, while the current seems to be limited by both the top cell and the bottom cell in the case of the tandem cell with ZnTe:Cu/ZnO:AI as the interconnect junction.

Large scale (gigawatts/yr) manufacturing of CdTe PV modules may be constrained due to the limited availability of the relatively rare (Te) element and the volume of potentially hazardous (Cd) material being used in 3-8 /spl mu/m thick CdTe absorber layer. This work is focused on cells with much thinner CdTe layers. Magnetron sputtering was used to prepare a series of CdS/CdTe cells with CdTe thickness from 0.5 to 1.28 /spl mu/m. Optimization of the post-deposition CdCl/sub 2/ treatment and back contact processing for the thinner layers improves the efficiency to very close to thicker CdTe cells. An efficiency of 11.8% has been achieved using only 0.87 /spl mu/m of CdTe. This cell showed V/sub OC/ of 772 mV, J/sub SC/ of 21.98 mA/cm/sup 2/ and FF of 69.7%.

We describe the application of sputter deposition processes to the fabrication of CdS/CdTe solar cells on light weight, flexible substrates in both the substrate and superstrate configurations. We have fabricated CdS/CdTe cells on molybdenum foil and Kapton polyimide. The Mo-based devices are among the best reported for substrate configuration CdTe solar cells, with efficiencies of /spl sim/7% at AM1.5.

Cd1-xMnxTe alloy films with band gaps of 1.6 ∼ 1.8 eV have been deposited by RF magnetron sputtering for solar-cell applications. The films have been treated by chloride vapors to improve the photovoltaic performance. These as-deposited and chloride-treated CdMnTe films have been investigated by Raman spectroscopy, x-ray diffraction (XRD) and scanning electron microscopy (SEM). Raman results indicate that Te and/or TeO2 exists in the annealed samples depending on anneal conditions.

A prototype monolithic HgCdTe/CdTe superstrate tandem cell has been fabricated by RF sputtering, comprising a CdTe/CdS top cell, a ZnTe:N/ZnO:Al interconnect junction and a HgCdTe/CdS bottom cell. The Hg1−xCdxTe film as the bottom absorption layer was deposited by RF sputtering with 70% or 85% Cd content in the Hg1−xCdxTe magnetron target. Hg1−xCdxTe films with band gap from 0.98 eV to 1.45 eV were obtained by controlling the deposition temperature. CdCl2 thermal treatments were used to improve the Hg1−xCdxTe film electrical properties. A nitrogen-doped ZnTe film combined with an aluminium (Al) doped ZnO film formed a good interconnect junction. Results of Voc = 0.99 V and Jsc = 2.1 mA/cm2 were obtained in the best such tandem cell at one sun (AM1.5).

Reactively sputtered ZnTe:N is a close valence-band match to CdTe, transparent below 2.2 eV and therefore an attractive candidate for a back contact/tunnel junction in tandem cells using CdTe or CdZnTe top cells. We report on measurements of the optical emission spectra of N/sub 2/ during reactive sputtering as part of the doping optimization. For materials characterization, a series of films produced with various N/sub 2//Ar gas ratios were studied by x-ray diffraction (XRD), atomic force microscopy (AFM), Raman spectroscopy, optical absorption, variable angle spectroscopic ellipsometry (VASE), and the Hall effect. For transparent back contact fabrication, we used a ZnTe:N/ZnO:Al recombination junction bilayer.

We have used sputtered ZnTe:N and ZnO:Al as transparent electrodes for CdTe based solar cells. ZnTe:N is reactively sputtered with 3% N2 in the sputter gas. The ZnTe:N films typically have transmission near 85% above 750 nm and resistivity as low as 10 ohm-cm with fine grains of approximately 30 nm diameter. ZnO:Al is sputtered from a ZnO:Al2O3 (2%) target. The ZnO:Al films have resistivity as low as 4 × 10-4 ohm-cm with 80-95% transmission over the visible spectrum. To test the stability of the films, ZnTe:N films were annealed in argon, dry air and nitrogen at a range of temperatures from 200°C to 500°C. Annealing decreased the resistivity for temperatures up to 350°C and increased for temperatures greater than 350°C. ZnO:Al films were annealed in dry air at temperatures from 300°C to 550°C. Though the resistivity increased at higher annealing temperatures, there was no change in the transmission. Important to the function of a tandem solar cell is a transparent contact, like ZnTe:N/ZnO:Al, to replace the traditional metal contact to the CdS/CdTe solar cell. We investigate the ZnTe:N/ ZnO:Al bilayer as a possible recombination junction. The rf sputtered ZnTe:N and ZnO:Al films were characterized by AFM, STM, XRD, transmission, 4-point probe, and Hall measurements.

An Al-doped ZnO front contact was successfully used for the first time for the fabrication of high efficiency CdS/CdTe thin-film solar cells. The ZnO:Al films were deposited on aluminosilicate glass by RF sputtering from a ZnO:Al2O3 target. The ZnO:Al film has ∼95% average transmission in the visible spectrum with ∼3 ohm/square sheet resistance. The CdS and CdTe thin films were also deposited by RF sputtering and devices were completed with a vapor CdCl2 treatment and evaporated Cu/Au back contacts. The highest processing temperature was 387°C, reached during the vapor CdCl2 treatment. The devices were tested at NREL with efficiency of 14.0% which is a record for an all-sputtered CdS/CdTe solar cell. The ZnO-based cell had JSC of 23.6 mA/cm2 compared to 20.7 mA/cm2 for our recent NREL-tested 12.6% cell on a commercial soda-lime-glass/SnO2:F substrate. Other parameters of the 14% ZnO based cell are: FF = 73.25% and VOC = 814 mV. The improved performance is almost entirely due to higher current because of better optical and electrical properties of ZnO:Al TCO. We report also on relative stability between devices on SnO2:F and ZnO:Al TCO, under one-sun light soak at VOC.

It is known that carriers photogenerated in the polycrystalline CdS layer of a CdS/CdTe cell are not collected. Thus, the short-circuit current (JSC) of CdS/CdTe devices should be improved if the bandgap of CdS is increased to permit better blue response. Wu, et al, showed that alloying of CdS with oxygen can increase the absorption edge of the layer. We report here on studies of this ‘oxygenated’ CdS and its use in sputtered cells. We find that at a deposition temperature of 250°C the addition of O2 to the sputter gas results in a red shift of the absorption edge (from 2.35 eV to 1.94 eV), but that room temperature deposition gives a blue shift (from 2.36 eV to 3.28 eV). The Raman spectra of the room temperature deposited films show a considerable broadening of LO phonon peaks suggesting a micro- to nano- to amorphous transition as the O2 fraction increases. XRD measurements of these films confirm the formation of an amorphous structure at high O2 fractions. The quantum efficiency measurements of CdS/CdTe device with room temperature deposited oxy-CdS show an improvement in blue response and hence increased JSC, but are accompanied by poorer junction quality so that the overall efficiency is not increased.

We report the development of nitrogen-doped ZnTe back contacts for CdS/CdTe solar cells. Reproducible p-type doping of the ZnTe was achieved by reactive RF magnetron sputtering with Ar/N/sub 2/ gas mixtures. The conductivity of the doped films was about five orders of magnitude higher than that of intrinsic ZnTe sputtered films. These films were used as contacts for glass/SnO/sub 2//CdS/CdTe solar cells. The contact structure of ZnTe/ZnTe:N/Ni showed slightly lower initial performance but improved stability compared to our evaporated Cu/Au contacts for a 3000 hr test cycle at 100/spl deg/C.

Most of the low resistance back contacts formed on high efficiency CdS/CdTe solar cells involve copper either in elemental form (such as Cu/Au back contacts) or as dopant in other material (such as Cu-doped ZnTe). But copper is also suspected to be a cause of degradation of devices in long-term stability tests due to its high diffusion coefficient in polycrystalline CdTe. In this paper, we present results on the development of nitrogen-doped ZnTe back contacts for CdS/CdTe solar cells. Reproducible N-doped p-ZnTe films were prepared using reactive RF magnetron sputtering with Ar/N2 gas mixtures. The conductivity of the doped ZnTe films was more than five orders of magnitude higher than that of intrinsic films. We find that annealing in air can further increase the conductivity. Efficiencies near 10% have been achieved with a ZnTe:N/Ni back contact.

High efficiency CdTe solar cells are typically grown with CdTe thicknesses from 3 to 15 μm, although the thickness required for 90% absorption of the incident irradiation at 800 nm is only ∼1 μm. In this paper, we present the effect of CdTe thickness reduction on the performance of CdS/CdTe solar cells in which both the CdS and CdTe films were grown by sputtering. We produced a series of cells with different CdTe thickness (from 0.5 to 3.0 μm), and held the CdS thickness and back-contact-processing constant. The effect of CdTe thickness reduction on the diffusion of CdS into CdTe was studied using optical absorption and x-ray diffraction techniques. Only slight decreases occur in open-circuit voltage, short-circuit current, and fill factor with decrease in CdTe film thickness to 1.0 μm. Almost 10% efficient cells were obtained with 1 μm CdTe. Below 1 μm, all cell parameters decrease more rapidly, including the red quantum efficiency.

Photoreflectance (PR) and Photoluminescence (PL) measurements have been performed at 77 K on single crystals of CuAl x Ga1-x (S0.5Se0.5)2 penternary alloys grown by the iodine chemical vapor transport method. The energies of excitons associated with uppermost valence bands have been obtained as a function of composition, and an approximately quadratic dependence has been found. Near-band-edge PL has been observed in the crystal with x≦0.77, and the PL has been discussed in terms of the free exciton.

CuInSe2 thin films were prepared on SiO2-coated sodalime glass substrate by the selenization of precursors in vacuum chamber. The deposition of In/Cu precursors was carried out with varying substrate temperature. Cu–In (In/Cu and Cu/In) precursors with or without thin Se layer (100 Å) were also prepared by varying stacking sequence of Cu and In. The structure and the morphology of In/Cu precursors have been investigated. It was found that the adhesive property of the CuInSe2 film to the substrate strongly depends on precursor preparation conditions.

Thin films of CulnS2, prepared by sulphurization of electroless deposited Culn alloy at different conversion temperatures and time durations, have been characterized by XRD and AES. The films converted at 250 and 350 °C are of mixed phase while those converted in the range 450–550 °C are of single-phase sphalerite with (112) as the preferred orientation. Detailed photoelectrochemical investigations show the best films are those obtained at a conversion temperature at 450 °C.

Real time spectroscopic ellipsometry based on rotating-compensator modulation and multichannel detection has been implemented to characterize CdTe deposition for polycrystalline thin film photovoltaic applications. The CdTe film measured and analyzed in detail in this study was prepared by magnetron sputtering onto a native oxide-covered crystalline silicon substrate. Key observations from this study include: (i) monolayer-level surface roughness thickness (/spl sim/3 /spl Aring/) at the onset of bulk layer formation and during subsequent growth to /spl sim/50 /spl Aring/; (ii) gradual development of a lower density bulk layer after /spl sim/13000 /spl Aring/ of CdTe thickness; and (iii) much broader above-gap critical points in the dielectric function for thin film polycrystalline CdTe compared to the single crystal.

We have used electroluminescence (EL), photoluminesence (PL), and synchrotron x-ray fluorescence (XRF) to study some of the properties of defects related to Cu and Na in polycrystalline thin films of CdTe. Some samples were prepared by ion implantation of CdTe single crystals followed by thermal anneal to remove the damage. Others were prepared by magnetron sputtering on quartz or borosilicate glass followed by vapor CdCl/sub 2/ heat treatment. EL was studied from sputtered CdS/CdTe solar cell structures. We find that EL is very sensitive to defects induced by light soaking; ion implantation-induced damage can be annealed at 400 C to yield good PL; and synchrotron XRF shows the presence of substantial copper in films sputtered from nominally pure CdTe.

Real time spectroscopic ellipsometry (RTSE) based on rotating-compensator modulation and multichannel detection has been implemented to characterize polycrystalline thin film CdTe deposition for photovoltaic applications. RTSE is capable of providing routine deposition information on substrate temperature T and deposition rate. It is also capable of providing detailed information on the thickness evolution of microstructure and optical properties. In this study, we highlight the differences in nucleation that occur under different CdTe deposition conditions on smooth crystalline Si wafer substrates. Differing behavior in the initial stages of deposition has been observed, ranging from layer-by-layer growth to nucleation and coalescence of 45 Å thick clusters. We also consider the thickness and substrate dependence of the microstructure, comparing depositions on smooth Si wafer and rough thin film Mo substrates.