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Objectives This study aimed to evaluate whether myocardial blood flow (MBF) quantified with dynamic computed tomography perfusion imaging (CTP) has an incremental prognostic value over coronary CT angiography (CTA) for major adverse cardiac events (MACEs) in patients with suspected coronary artery disease (CAD). Background The incremental prognostic value of CTP over CTA is unclear. The quantification of MBF with dynamic CTP may potentially enhance risk stratification. Methods A total of 332 patients (67% men; age: 67 ± 10 years) with suspected CAD who underwent CTA and dynamic CTP was analyzed. A MACE was defined as cardiac death, nonfatal myocardial infarction (MI), unstable angina, or hospitalization for congestive heart failure. A summed stress score (SSS) was calculated by adding scores of all myocardial segments according to normalized MBF values. Abnormal perfusion was defined as SSS ≥4. Obstructive CAD was defined as ≥50% stenosis in ≥1 vessel on CTA. Results During a median follow-up of 2.5 years, 19 patients had a MACE. Multivariate analysis showed that, when adjusted for obstructive CAD on CTA, abnormal perfusion was significantly associated with hazards for MACEs (hazard ratio [HR]: 5.7; 95% confidence interval [CI]: 1.9 to 16.9; p = 0.002), with a significant improvement in the prognostic value. Abnormal perfusion was an independent predictor even when adjusted for ≥70% stenosis in ≥1 vessel (HR: 5.4; 95% CI: 1.7 to 16.7; p = 0.003) or adjusted for ≥50% stenosis in ≥2 vessels (HR: 6.5; 95% CI: 2.2 to 18.9; p = 0.001). In the setting of obstructive CAD, annualized event rates showed a significant difference between the patients with and without abnormal perfusion for all events (12.2% vs. 1.5%; p = 0.002) and for cardiac death and nonfatal MI (4.2% vs. 0%; p = 0.015). Conclusions MBF quantified with dynamic CTP has an incremental prognostic value over CTA. The addition of dynamic CTP to CTA allows improved risk stratification of patients with CTA-detected stenosis.

Atomic layer deposition of Al2O3 was used to fabricate rear surface passivation with lithographed point contact Cu(In,Ga)Se2 (CIGS) solar cells. We successfully demonstrated the use of photolithography to fabricate point contact holes of small size and fine pitch. The efficiency of the Al2O3 rear-surface-passivated ultra-thin CIGS solar cells with absorber layers of 1.89 μm and with a 5-nm-thick Al2O3 rear surface passivation layer and local point contact holes with a pitch of 1 μm and a diameter of 500 nm was 19.3% (total area = 0.519 cm2). The efficiency of the Al2O3 rear-surface-passivated ultra-thin CIGS solar cells with absorber layers of 380 nm was 11.3% (active area = 0.514 cm2) when an antireflection layer was used. Compare with unpassivated CIGS solar cells, higher efficiencies were found for the Al2O3 rear-surface-passivated CIGS solar cells, mainly due to the reduced carrier recombination and enhanced rear internal light reflection.

B-doped ZnO (ZnO:B) films with improved visible and near-infrared transparency have been applied as transparent conductive oxide (TCO) electrodes in Cu(In,Ga)Se2 submodules. The use of ZnO:B instead of conventional Al-doped ZnO (ZnO:Al) improved the short-circuit current density (Jsc) and the resulting conversion efficiency without decreasing the fill factor. The detailed optical and electrical analysis of the modules revealed that (i) the improvement in Jsc was due to lower parasitic absorption owing to free carriers in the TCO layer and (ii) the series resistance of the ZnO:B module originating from the TCO and TCO/Mo contacts is comparable to that of the ZnO:Al module.

The relationship between the composition and conversion efficiency of Cu 2 ZnSnSe 4 (CZTSe)-based solar cells was investigated. CZTSe films were grown by thermal deposition using a coevaporation method. It was found that the composition of CZTSe and Na concentration [Na] have strong correlations to the conversion efficiency, and higher-efficiency samples were obtained with the composition ratios closer to Zn/Sn ~ 1.6, Cu/(Zn + Sn) ~ 0.8, and [Na] ~ 2%. The highest conversion efficiency ( η = 2.1%) was obtained in the composition region, and the composition was significantly nonstoichiometric. The CZTSe composition was automatically fixed on a tie line between Cu 2 ZnSnSe 4 and ZnSe, and it was demonstrated that this composition can be expressed by one parameter. The structural and electrical properties of the high- and low-efficiency samples were also investigated. No significant differences in such properties were observed between the high- and low-efficiency samples by surface, x-ray diffraction, and Raman measurements. However, the low-efficiency sample showed a high carrier concentration.

In this paper, we report a recent progress in Cu(In,Ga)Se2 (CIGS) solar cells and mini-modules demonstrated on soda-lime glass (SLG) substrates, which are one of the most common and cost effective substrate materials for chalcopyrite photovoltaics. CIGS absorber layers grown by the three-stage co-evaporation process was used with an optimized [Se]/[metal] flux ratio and preferable substrate temperatures for SLG substrate thicknesses. SLG substrates of 0.28–1.1 mm in thickness, which are thinner than that generally used in laboratories and industries (∼2–3 mm), were employed to develop techniques for further reductions of material cost and module weight. Solar cell and mini-module efficiencies of >19% and >17%, respectively, with relatively high open circuit voltages (Voc) and fill factors (FF) (Voc 0.77 V and FF 0.79 for cells, and Voc 0.77 V and FF 0.74 for mini-modules) were demonstrated using a composition ratio [Ga]/([In]+[Ga]) of 0.45 for the CIGS absorber layers. Key techniques and future directions toward highly efficient CIGS solar cells and modules with high values of Voc and FF are discussed.

The approaches and achievements in the development of high-efficiency CuInGaSe2 (CIGS) monolithically integrated submodules will be reported. Two types of techniques, the conventional (batch-process) and the in-line three-stage evaporation techniques, have been employed to deposit high-quality CIGS absorbers on 10 × 10-cm Mo/soda lime glass substrates. CIGS films deposited by both the conventional and the in-line evaporation techniques showed an in-depth compositional grading of In and Ga (referred as the double graded bandgap) which is a fingerprint of the three-stage evaporation process. The optimization of the integration processes such as mechanical scribing, as well as the improvement in electrical and optical properties of ZnO window layers, made possible the demonstration of high-efficency CIGS submodules with efficiencies as high as η = 16.6% (aperture area: 67.2 cm2) and η = 15.8% (aperture area: 76.5 cm2) for the conventional and the in-line evaporation process, respectively. Copyright © 2012 John Wiley & Sons, Ltd.

Coevaporated Cu(In,Ga)Se 2 (CIGS) film growth using a rf-plasma cracked Se-radical beam (R–Se) source leads to a significant reduction in the amount of raw Se source material wasted during growth and exhibits unique film properties such as highly dense, smooth surfaces and large grain size. R–Se grown CIGS solar cells also show concomitant unique properties different from conventional evaporative Se (E–Se) source grown CIGS cells. In the present work, the impact of modified surfaces, interfaces, and bulk crystal properties of R–Se grown CIGS films on the solar cell performance was studied. When a R–Se source was used, Na diffusion into CIGS layers was enhanced while a remarkable diffusion of elemental Ga and Se into Mo back contact layers was observed. Improvements in the bulk crystal quality as manifested by large grain size and increased Na concentration with the use of a R–Se source is expected to be effective to improve photovoltaic performance. Using a R–Se source for the growth of CIGS absorber layers at a relatively low growth temperature, we have successfully demonstrated a monolithically integrated submodule efficiency of 15.0% (17 cells, aperture area of 76.5 cm 2 ) on 0.25-mm thick soda-lime glass substrates.

The influence of Se beam pressure on defect properties in Cu(In 1– x ,Ga x )Se 2 (CIGS)-based solar cells fabricated by the three-stage process was investigated by admittance spectroscopy. The Se beam pressure was varied from 1.3×10 −3 to 4.4×10 −3 Pa, at the position of the samples, by varying the Se cell temperature. The spectra for all samples show three distinct peaks denoted as α , β , and ζ with activation energies of 20, 150, and 300 meV, respectively. The trap density of ζ increased from 5×10 14 to 4×10 15 cm −3 with decrease in Se beam pressure; the trap densities of α and β did not change. These results suggest that the origin of ζ is related to the Se deficiency. The density of ζ seems to correlate with the cell efficiency. Therefore, it is important to control the Se beam pressure in order to obtain highly efficient solar cells.

Deep levels in Cu(In 1 − x ,Ga x )Se 2 (CIGS) are studied by transient photocapacitance (TPC) spectroscopy by varying the Ga concentration, x , from 0.38 to 0.7. The TPC spectra of CIGS thin-film solar cells at 140 K exhibited a defect level with an optical transition energy of about 0.8 eV. The spectrum shape in the sub-bandgap region is independent of the Ga concentration. Therefore, the optical transition energy to the defect level is almost constant with about 0.8 eV from the valence band. The TPC signals for defect level are quenched by increasing temperature. The activation energy of thermal quenching is estimated to be about 0.3 eV. The thermal and optical activation processes are explained using configuration coordinate diagram.

Defect levels in Cu(In 1 − x ,Ga x )Se 2 (CIGS) were studied by using admittance spectroscopy with varying Ga concentration of x from 0.38 to 1.0. Two distinct peaks ( α and ζ ) were detected from all of CIGS samples. Peak α could be attributed to the shallow acceptors. Peak ζ was closely correlated with the surface potential of CIGS; therefore, it may have been caused by traps near the surface of CIGS such as grain boundary defects. The intensity of peak ζ increases with increasing Ga concentration. Further, this intensity was correlated with the ideality factor of the Al/CIGS Schottky junction. We suggest that the degradation of the performance of CIGS solar cell with increasing Ga concentration may be concerned with defects corresponding to peak ζ ˙

We have studied the three-stage deposition process of CuIn1−xGaxSe2 (CIGS) thin films using spectroscopic light scattering (SLS), under varied deposition conditions. The structural changes of CIGS films by (1) Ga composition, (2) Se supply and (3) low deposition temperature, were observed in situ by SLS. The largest changes in SLS profiles by the Ga composition was observed between x = 0.3 and 0.5. The SLS profiles changed significantly during stage 1 by varying the Se pressure, while the temperature profiles did not.

We have studied the effects of (i) impurity doping, (ii) rapid thermal annealing, and (iii) crystal polarity control on photoluminescence spectra in the band-edge emission region of high quality ZnO. As a result, we have discussed the possible lattice defects responsible for some photoluminescence emission peaks, namely (i) peak I8 appears at 3.3596 eV, (ii) peak I9 appears at 3.3565 eV and (iii) peak G appears at 3.3327 eV. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The crystallographic polarity of ZnO bulk crystals and epilayers were determined by X-ray diffraction (XRD) using anomalous dispersion near the Zn K-edge. The method is not destructive and is straightforward to carry out using a typical XRD measurement system. The polarity difference between the Zn (0001) and O (000) surfaces could be easily determined using a {0002} diffraction peak and the Bremstrahlung radiation from a Cu rotating anode source. By using the normalized pre and post-Zn K-edge diffraction intensity ratios of the {0002} diffraction peak, Zn polar and O polar ZnO layers could always be distinguished but, the absolute value of the ratio was found to change with layer thickness. Acid etching results confirmed the veracity of the polarity determination of the XRD measurement. To test the technique, Zn and O polar ZnO layers were grown by radical source molecular beam epitaxy on MgO buffer layers on c-sapphire substrate and O polar ZnO layers were grown on a-plane substrates and measured using the X-ray technique with excellent agreement. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Phosphorus-doped ZnO films were grown by radical source molecular beam epitaxy (RS-MBE). Zn 3 P 2 was used for the phosphorus source. ZnO:P layers were grown on an in situ annealed low temperature buffer layers, and the phosphorus concentration was changed by altering the temperature of the Zn 3 P 2 K-cell. The phosphorus concentration could be controlled from 1 x 10 18 to 2 x 10 20 cm -3 without phase separation. The importance of the ambient gas used during thermal treatments was demonstrated. All of the ZnO:P layers showed n-type conduction, however, the electron concentration of ZnO:P was found to decrease by as much as from 4.1 × 10 19 to 9.7 x 10 16 cm -3 when the samples underwent oxygen ambient rapid thermal annealing (RTA) treatment. Furthermore, a new photoluminescence peak at 3.357eV was observed after RTA treatment, which was not present in undoped ZnO layers before and after RTA treatment. These results indicate that the activation of acceptors in ZnO:P occurred in samples that underwent RTA treatment and that phosphorus is a good candidate as an acceptor dopant.

The molecular beam epitaxy (MBE) technique was used for ZnO epitaxial growth and growth properties were evaluated. Intrinsic ZnO epilayers with mobility of 120 cm 2 /(V s) and carrier concentrations of 7 × 10 16 cm −3 were obtained. ZnO on Si, bandgap engineering using Se and nitrogen doping were carried out for research of device application using this growth technique. We found a large bowing parameter of 12.7 eV in ZnOSe, new ZnOSSe semiconductor that is lattice matched to Si and able to change the bandgap from UV to IR, and co-doping phenomenon of N and Ga doped ZnO.

The properties of the defect level at 0.8 eV above the valence band in Cu(In1−x,Gax)Se2 were studied by the two-wavelength excited photoluminescence (PL) method. The two-wavelength excited PL involves two different pumping light sources, that is, a 635 nm diode laser used as an above-gap excitation and a 1550 nm diode laser whose energy corresponds to the defect level resulting in having a role of charge saturation at the defect level. The intensity of the two-wavelength excited PL was stronger than that without 1550 nm laser irradiation for all specimens at room temperature regardless of Ga concentration. The results suggest the 0.8 eV defect level acts as recombination center at room temperature.

A selenium capping and decapping process for the protection of CuInSe 2 surfaces during storage in air was investigated. The quality and cleanness of CuInSe 2 (0 0 1) surfaces restored in UHV by thermal evaporation of the covering protective Se cap was assessed. For the study, MBE-grown heteroepitaxial CuInSe 2 /GaAs (0 0 1) films were employed. The Se capping and decapping process was monitored by reflection high-energy electron diffraction. The Se cap layer is amorphous and, after its reevaporation, the initial diffraction pattern of CuInSe 2 (0 0 1) is restored. The decapping process and the quality of decapped CuInSe 2 (0 0 1) surfaces were investigated by means of photoelectron spectroscopy using synchrotron radiation with excitation energies of 1050 and 95 eV. For a decapping temperature of 390 °C, an O 1s contamination signal as small as the detection limit was found. Photoelectron spectra taken with the highest surface-sensitivity ( hν =95 eV) yield well-resolved valence band emissions confirming the low contamination level and high quality of the decapped surfaces. The investigations show that the Se capping and decapping process is an efficient means for the conservation and restoration of clean and well-defined Cu chalcopyrite surfaces in UHV.

We have studied photoluminescence spectra in the band-edge emission region of very high quality undoped ZnO, as a function of both (i) sample growth methods and conditions, and (ii) measurement temperatures. As a result, we have classified the defect types responsible for emission peaks observed in the band-edge emission region into three groups. In addition, probable lattice defects responsible for some of the emission peaks are suggested to be (i) oxygen vacancies and/or interstitial zinc, and (ii) residual aluminum impurities. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

ZnO films were grown on sapphire substrates by radical source molecular beam epitaxy (RS-MBE). ZnO low temperature buffer layers were subjected to various treatments. High quality ZnO films were obtained by vacuum annealing plus nitrogen doping of the buffer layer. The carrier concentration of the ZnO film fabricated using this buffer layer was 7.5 × 1016 cm−3 with a mobility of 132 cm2/V sec at RT. Temperature dependent Hall measurements showed implied the existence of degenerate (untreated) buffer layers. Using a nitrogen-doped buffer layer to reduce the influence of the degenerate layer, a donor energy of 110 meV was estimated from temperature dependent Hall measurements. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have investigated the effect of in-situ annealing and nitrogen doping on 40 nm low-temperature buffer layers in radical-source molecular beam epitaxy (RS-MBE) as well as subsequently grown high-temperature ZnO layers grown on the buffers. All layers were grown on (11&2macr;0) sapphire substrates. High resolution x-ray reciprocal space maps in the vicinity of the ZnO (0002) reciprocal lattice point (RLP) showed a sharp reduction in diffuse scattering for the thermally annealed buffer layer implying a reduction in dislocation density. X-ray lateral coherence (LC) values obtained from asymmetric diffraction near the (10&1macr;4) RLP indicated that the LC of a undoped, high-temperature ZnO layer grown on an in-situ N-doped buffer was larger than those of high-temperature ZnO layers grown on either the in-situ annealed or untreated low-temperature buffers. The increased LC was found to be correlated with in-plane twist suggesting that dislocation bunching may be present at domain boundaries in the high-temperature ZnO grown on annealed N-doped buffer layers. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The URT(Uramoto-gun with Tanaka magnetic field)-IP(ion plating) method is a technique for depositing a thin film on a substrate. This method offers the advantage of low-ion damage, low deposition temperatures, large area deposition and high growth rates. Ga-doped ZnO thin films were grown using the URT-IP method, and the material properties were evaluated. The quality of ZnO thin films grown by the URT-IP method was found to be sensitive to oxygen supply during growth. It was observed that the saturation point of the growth rate corresponding to the optimum oxygen supply leads to the best electrical properties. The profiles of the dependence of film properties on oxygen supply revealed a part of growth mechanism of the URT-IP method.

We have grown the compound semiconductor ZnO 1− x Se x by radical source MBE. SIMS depth profile of Se concentration shows the compositional ordering of the ZnO 1− x Se x layers. The period of ZnO 1− x Se x natural compositional ordering decreases with increasing Se concentration. The lateral coherence of the compositional alternations of ZnO 1− x Se x cannot be explained without taking into account the dynamical surface processes into consideration.

Transparent and conductive Al-doped ZnO films have been deposited at room temperature by oxygen radical-assisted pulsed laser deposition.An average optical transmittance of more than 86% (0.7 mm film thickness) in the wavelength range 400–800 nm and resistivity as low as 5=10 V cm were obtained from films deposited using a radical oxygen source.The surface y4 roughness of the ZnO films measured by means of atomic force microscopy was found to be below 1 nm, suggesting that soft deposition techniques with low kinetic energy sources can make possible the deposition of high-quality transparent conducting ZnO films, even at room temperature. 2002 Elsevier Science B.V. All rights reserved.

Near-edge X-ray absorption (XANES) at the Cu K-edge was employed to study the valency of Cu (a potential p-dopant) in Cu-doped ZnO grown by molecular beam epitaxy. For a similar chemical environment, a shift in the onset of absorption can be interpreted as being due to a change in effective valency. We have studied this shift for both as-grown and argon ambient annealed (1000°C, 30 min) Cu-doped ZnO samples. The valency shift was measured against ab-initio standards calculated using feff8 as well as the standard samples Cu 2 O (+1 valence) and CuO (+2 valence). It was found for the flux region explored here that as-incorporated Cu assumes an effective valence of approximately +1 which increases towards +2 upon a 1000 °C argon ambient anneal. X-ray diffraction also shows the presence of both metallic Cu and Cu 2 O depending upon growth and annealing conditions.

We have developed a fabrication process for integrated submodules using Cu(In,Ga)Se 2 (CIGS) absorbers deposited by in-line three-stage evaporation. An in-situ monitoring system for the compositions and thicknesses of the CIGS absorbers was also developed. High-performance CIGS submodules with efficiencies as high as η =15.8% with 17 interconnected cells (aperture area: 76.5 cm 2 ) have been fabricated.

Cu(In0.52Ga0.48)Se2 solar cell performance with an MBE-grown ZnS buffer layer was studied. It was found that cell performance with the ZnS buffer layer was inferior compared to that with the CdS buffer layer; however, by insertion of a 50 nm CdS layer in addition to the ZnS buffer layer, the device performance was improved and was better than that with the CdS buffer layer. The higher device performance by using the ZnS/CdS double buffer layer may be due to the improved interface properties of the device.

Band gap and excitonic resonance energies of high-quality bulk single crystals, polycrystalline thin films, and epitaxial layers of CuInSe2 and CuGaSe2 were determined as a function of temperature by means of photoreflectance, optical absorption (OA), and photoluminescence measurements. OA spectra were fit including excitonic absorption from low temperature up to room temperature (RT). The band gap energy of 1.032 eV and free exciton (FE) resonance energy of 1.024 eV were obtained at RT for strain-free CuInSe2 giving an exciton binding energy of 7.5 meV. The band gap energy of both CuInSe2 and CuGaSe2 was found to be essentially independent of the molar ratio of Cu to group-III atom (Cu/III) for near-stoichiometric and Cu-rich samples. The disappearance of the FE absorption in the In-rich (Cu/In

CuInSe2 (CIS) thin films, the thickness of about 1.0 μm and composition of Cu In ratio (γ = 1.79), were grown on (0 0 1)-oriented GaAs substrate by molecular beam epitaxy (MBE) at substrate temperature of Ts, = 450°C. The samples have been characterized by means of piezoelectric photoacoustic (PPA) measurements between liquid helium (4.2 K) and room temperature (300 K). Two distinct PPA signals due to band gap of CIS and GaAs are observed in the whole temperature range from 4.2 to 300 K and the PPA signals of CIS decrease curvilinearly at the temperature range. Since the PPA signals of CIS thin films can be obtained up to room temperature, the PPA measurements are quite effective to obtain the optical characterizations, especially for the nonradiative recombination processes.

CuInSe2 (CIS) thin films with a range of CuIn ratios were grown by molecular beam epitaxy on GaAs (0 0 1) at substrate temperatures of Ts = 450–500°C and the effects of annealing under various atmospheres have been investigated. Photoluminescence spectra obtained from an ex-situ vacuum annealed CIS film at a temperature of TA = 350°C showed a red-shift and a broadening of an emission peak (peak c) which originally appeared at 0.970 eV before annealing and the red-shifted peak c was found to consist of two overlapping peaks. The excitation power dependence of these overlapping peaks indicated the radiative recombination processes associated with the emissions to be a conduction band to acceptor transition (peak at 0.970 eV) and a transition due to donor-acceptor pairs (peak at 0.959 eV), indicating the formation of a shallow donor-type defect during the vacuum annealing process. The origin of this defect has tentatively been attributed to Se vacancies. On the other hand, the molar fraction of oxygen increased with increasing annealing temperature in dry-air. An epitaxially grown In2O3 phase was found both in Cu-rich and In-rich films annealed at TA ⩾ 350°C, which was not observed in the films annealed in Ar atmosphere. Thermodynamic calculations based on the CuInSeON system showed In2O3 to be the most stable phase in good agreement with the experimental results.

The linear and nonlinear optical properties of tetragonal CuInSe2 and CuGaSe2 thin films grown epitaxially on GaAs(001) have been investigated as a function of film stoichiometry. The refractive indicies of the films have been measured between 250 and 800 nm using ellipsometry. For a fundamental wavelength of 790 nm, the second order optical nonlinearity χ(2) and optical third harmonic field anisotropy σ are typically reduced strongly in nonstoichiometric thin films and correlate well with each other. The magnitudes of the chalcopyrite film χ(3) values are relatively small, being typically only ∼1% of those of Si. Third harmonic data from an approximately stoichiometric CuInSe2 thin film is consistent with a significant second order surface optical nonlinearity cascading contribution. Spectral features in the room temperature refractive index data from approximately stoichiometric CuInSe2 and CuGaSe2 thin films correlate well with previous experimental work.

We investigate the second-order nonlinear optical (NLO) properties of tetragonal chalcopyrite-structure CuInSe/sub 2/ thin films using optical second-harmonic generation at a fundamental wavelength of 790 nm. An approximate doubling of the second-order optical nonlinearity of the CuInSe/sub 2/ thin films is achieved through changing the substrate from GaAs[001] to In/sub 0.29/Ga/sub 0.71/As[001], thereby reducing the lattice mismatch from 2.2% to 0.2%. This observation suggests that minimizing the lattice mismatch is a key requirement for the growth of high-quality NLO active chalcopyrite structure thin films and is in contrast to some other NLO active thin-film systems.

ZnO films were grown on low-temperature (LT) buffer layers on sapphire a-plane (11–20) substrates by radical source molecular-beam epitaxy. The LT buffer layers were found to effect the electrical properties of subsequently grown undoped ZnO films, and their presence was found to be indispensable for the growth of films with low carrier concentrations and high mobilities. Temperature-dependent Hall measurements showed the existence of a degenerate region related to the LT buffer layers. It was found that the effects of degenerate layers could be reduced by using annealing treatments and nitrogen doping of the LT buffer layers. The dominant residual donor energy of 110 meV was found to be different than previously reported. The carrier concentration of a ZnO film fabricated using a nitrogen-doped buffer layer was 7.5×1016 cm−3 with a mobility of 132 cm2/V s at room temperature.

Combined oxygen and heat exposure processes after p-CuGaSe2/n-CdS junction formation degrade CuGaSe2 solar cell efficiency, whereas such annealing processes can improve high In content Cu(In,Ga)Se2 device performance. This result is chiefly attributable to different interface structures consisting of oxygen-sensitive CuGaSe2 or relatively oxygen-insensitive Cu(In,Ga)Se2. To reduce CuGaSe2 interfacial recombination, reduction of the process temperature of the front contact layer deposition process is found to be the key. In this work, fill factor values exceeding 0.7 are reproducibly obtained from CuGaSe2 solar cells, though such high fill factor values have been very challenging to demonstrate to date using CuGaSe2 photoabsorber layers.

Photoluminescence (PL) properties of CuInSe 2 thin films with Cu/In ratio ranging from 0.81 to 1.81 grown by molecular beam epitaxy have been investigated for photovoltaic applications. The PL spectra of Cu-rich CuInSe 2 epitaxial films unlike those of polycrystalline CuInSe 2 showed well-defined emission lines, suggesting that high quality CuInSe 2 epitaxial films have been grown. Such fine PL spectra made the identification possible of phono replicas, in other words, the separation of such phonon replicas from substantive emmissions. PL properties are found to be very sensitive to the growth parameters such as Cu/In ratio and substrate temperature. A broad peak at λ ∼ 1.45 μm becomes dominant in In-rich films, and excitation power dependence of such a broad emission indicated a pair-type radiative recombination, most likely the emission due to donor-acceptor pair or their complex.

It has been recently predicted that the co-doping of an acceptor (nitrogen) and a donor (aluminum, gallium, indium) in a 2:1 ratio will dope ZnO p-type due to a reduction in the Madelung energy making the nitrogen acceptor energy level more shallow. We have been growing gallium and nitrogen co-doped ZnO films by radical-source molecular-beam epitaxy by use of oxygen and nitrogen radicals supplied via rf radical source cells. Diode-like current–voltage characteristics and donor acceptor pair-like photoluminescence emission were observed for a Ga and N doped ZnO film grown on an undoped ZnO buffer layer. However, Hall measurements revealed that the conductivity was n-type. Formation of a non-ZnO phase in the sample was confirmed by secondary ion mass spectroscopy and x-ray diffraction measurements. Zn and Zn+O secondary ion intensities fell sharply by two orders of magnitude in going from the undoped ZnO layer to the highly co-doped ZnO. X-ray diffraction measurements indicated the formation of ZnGa2O4.

Low‐temperature photoluminescence experiments were performed on high‐purity InP substrates as a function of heat treatment, flat‐profile ion implantation and subsequent annealing procedures. Whereas the well resolved near band edge luminescence proved the superior substrate quality, up to 6 sharp luminescence lines (some of them not reported so far) and one shallow broad‐band emission were observed after Si3N4 deposition and various annealing procedures. From the temperature dependence of the luminescence peak intensities the activation energies of the novel lines were determined and discussed in terms of excitons bound to processing‐induced complex defects. These assignments are supported by the observation of a strong enhancement of the luminescence after low dose Ar+ ion implantation.

Drastic changes in average molecularities (m=Cu/In) from m≫1 to m=0.92–0.93 and in hole concentrations from p≫1019 cm−3 to as low as p=7.5×1016 cm−3 have been observed in molecular beam epitaxy grown CuInSe2 after selective etching of the Cu–Se phase by a KCN aqueous solution; high hole concentrations and Cu-excess compositions of the as-grown films were attributed to the Cu–Se phase. On the other hand, well-defined photoluminescence emissions were found characteristic of intrinsic CuInSe2. The presence of the Cu–Se phase made possible the growth of high-quality CuInSe2 epitaxial films at a temperature well below the melting point of any Cu–Se compound. Surface topology measurements showed that the surface of the as-grown films was not fully covered by Cu–Se grains, leaving holes with depths of 200–300 nm after KCN etching. The enhanced two-dimensional growth and the reduced defect concentration imply that a very thin Cu-excess surface layer controls the growth of CuInSe2 when grown under Cu-excess condit...

Cu2−xSe is an important impurity phase of the ternary chalcopyrite semiconductor CuInSe2 associated with Cu/In composition ratios greater than unity. We have observed directly in a prototypical epitaxial system the formation of Cu2−xSe on Cu-rich CuInSe2 thin films epitaxially grown on GaAs (001). Atomic force microscopy measurements of the surface topology of as-grown films clearly show faceted rectangular crystallites with dimensions on the order of 100 nm. Cross-sectional transmission electron microscopy measurements of the Cu-rich CuInSe2 showed rectangular protrusions on the surface as well as wedge shaped facets in the CuInSe2 film. Two-dimensional reciprocal space x-ray mapping of the as-grown Cu-rich CuInSe2 showed the in-plane lattice constant of the Cu2−xSe phase to be partially strained to the CuInSe2 layer. The presence of the β phase of Cu2−xSe is also presented as an alternative explanation for Cu–Pt ordering reports in CuInSe2 that have appeared in the literature. Strain-related surface und...

CuInSe2 films have been grown by molecular beam epitaxy on pseudo‐lattice‐matched substrates that consist of a 1‐μm‐thick In0.29Ga0.71As layer grown on a linearly composition‐graded InxGa1−xAs buffer (0≤x≤0.29) grown in turn on GaAs (001). The properties of these films have been compared with those of the films grown directly on GaAs (001). High resolution x‐ray diffraction analysis on CuInSe2 grown on pseudo‐lattice‐matched substrates indicated substantial reduction on residual strain in the CuInSe2 films. A photoluminescence spectrum dominated by sharp free exciton emissions has been observed for the first time from CuInSe2 films indicative of significant improvement in crystalline quality and substantial reduction in the point defect density.

Photoluminescence has been used to investigate the behavior of Si‐implanted InP, submitted to a postimplantation rapid thermal annealing. Compared with the non‐implanted material, the implanted crystal displays a new broad band around 1.382 eV, in the low temperature (2 K) spectra. This band appears to be made of two unresolved recombination paths ascribed to the electron–acceptor (e,A°) and donor–acceptor pair recombinations of the silicon acceptor impurity substituted on phosphorus site. Besides the Zn‐related transitions to s and p excited states, the selective excitation of the donor–acceptor pairs reveals additional recombination paths, ascribed to transitions to 2s3/2 and 2p5/2 (Γ7 and Γ8) excited states of silicon. The acceptor behavior of Si in InP is hence given a definite support by this work.

Multistep wafer‐annealed semi‐insulating GaAs wafers (MWA) are characterized using photoluminescence (PL). The PL spectra present well‐resolved near‐band‐edge transitions, including the doublet of the neutral acceptor‐bound exciton. A detailed investigation using selective pair luminescence of samples submitted to different annealings, i.e., wafer‐ or ingot‐annealing, single or multistep, shows that carbon is the main shallow acceptor. However, for the wafer‐annealed samples, two other residual impurities found in the as‐grown or ingot‐annealed crystals have their estimated concentrations noticeably reduced, for Zn (e.g., around 1013 cm−3 in MWA), or are unresolved for Si. This reduction of background impurities may have direct consequences for device applications.

Photoluminescence experiments under resonant excitation have been performed at low temperature in Mg+‐implanted bulk InP. The energy difference between the ground 1S3/2 and excited 2S3/2 states of the Mg acceptor is accurately measured by two‐hole spectroscopy of Mg‐acceptor bound exciton. Selective excitation of donor‐acceptor pairs luminescence allows the identification of a set of 2P3/2 and 2P5/2 excited states. The measured values to be compared with similar published data obtained for Zn and C represent an additional step in the process of accurate identification of acceptors in InP.