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Since the order–disorder phase transition easily occurs during the growth of optoelectronic II–IV–V2 ternary compounds, cation–cation antisites were always considered as the major point defects, while anion-related defects did not attract sufficient attention. In this paper, based on first-principles simulations, the anion–cation antisites are revealed to be comparable to or even dominate over the cation–cation antisites in II–IV–V2 phosphides and arsenides. These antisite defects are predicted to have significant impacts on the optoelectronic properties because they can either act as nonradiative recombination centers or enhance the p-type carrier concentration. Furthermore, based on the calculated defect properties and band alignments, we propose that the alloy ZnGe(P,As)2 can be an efficient p-type solar cell absorber. Its maximal open circuit voltage is effectively enlarged by the low valence band edge; meanwhile, the dominating anion–cation antisites are electrically benign. These results highlight the necessity of considering the anion–cation antisites in the defect engineering of II–IV–V2 phosphides and arsenides. [ABSTRACT FROM AUTHOR]

In ternary semiconductors, both stacking disorder and cation disorder can cause the formation of polytypes and extended defects. We perform density functional theory calculations to investigate the stability of polytypes in six ternary sulfides, such as CuIn S 2 , CuGa S 2 , CuAl S 2 , AgIn S 2 , AgGa S 2 , and AgAl S 2 . The formation energy of polytypes generated by stacking disorders is used to generate the anisotropic next-nearest neighbor Ising models. The estimated stacking fault energy by the model is in good agreement with the calculated stacking fault energy. The incorporation of Ga and Ag tends to suppress and promote the formation of stacking faults, respectively. On the other hand, the electronic bandgap of the polytypes generated by cation disorder is negatively correlated with the formation energy, resulting in the trapping of charge carriers at antisite domain boundaries. The formation of antisite domain boundaries can be suppressed by the incorporation of Ag and Ga. [ABSTRACT FROM AUTHOR]

Composed of inexpensive and naturally abundant elements, the chalcopyrite mineral CuFeS 2 has received attention as a potentially useful thermoelectric. We use first-principles electronic structure and Boltzmann transport theory calculations to investigate thermoelectric properties of n -doped CuFeS 2. We find that energy-dependent carrier lifetimes that are inversely proportional to the electronic density-of-states are crucial for reproducing experimental data on the transport properties, including the measured values of the Seebeck coefficient, α. The heavy-effective-mass conduction band promotes high values of α , but it also leads to low mobility due to strong electron-acoustic-phonon scattering. Low mobility forces one to rely on high carrier concentration to achieve high conductivity, which decreases α and limits the achievable power factor. Our calculations predict that ideally doped CuFeS 2 that has been nanostructured to an average grain size d ≈ 20 nm can attain thermoelectric figures of merit z T = 0.25 to 0.8 for T = 300 to 700 K, respectively, due to a reduction in the lattice thermal conductivity. [ABSTRACT FROM AUTHOR]

Raman scattering measurements have been performed in cadmium digallium sulphide (CdGa2S4) with defect chalcopyrite structure up to 25 GPa in order to study its pressure-induced phase transitions. These measurements have been complemented and compared with lattice-dynamics ab initio calculations including the TO-LO splitting at high pressures in order to provide a better assignment of experimental Raman modes. In addition, experimental and theoretical Grüneisen parameters have been reported in order to calculate the molar heat capacity and thermal expansion coefficient of CdGa2S4. Our measurements provide evidence that CdGa2S4 undergoes an irreversible phase transition above 15 GPa to a Raman-inactive phase, likely with a disordered rock salt structure. Moreover, the Raman spectrum observed on downstroke from 25 GPa to 2 GPa has been attributed to a new phase, tentatively identified as a disordered zinc blende structure, that undergoes a reversible phase transition to the Raman-inactive phase above 10 GPa. [ABSTRACT FROM AUTHOR]

Buried wurtzite structures composed by stacking faults of the {111} planes in zinc-blende and {112} planes in chalcopyrite structures can result in barriers for charge carrier transport. A precise understanding of stacking fault annihilation mechanisms is therefore crucial for the development of effective deposition processes. During co-evaporation of Cu(In,Ga)Se2—a photovoltaic absorber material showing record efficiencies of up to 22.9% for thin film solar cells—a reduction of stacking faults occurs at the transition from a Cu-poor to a Cu-rich film composition, parallel to grain growth, which is suggesting that the two phenomena are coupled. Here, we show by in situ synchrotron X-ray diffraction during annealing of Cu-poor CuInSe2 thin films that stacking faults can be strongly reduced through annealing, without passing through a Cu-rich film composition. We simulate the evolution of the X-ray diffraction stacking fault signal with a simple numerical model of grain growth driven by stacking fault energy and grain boundary curvature. The results support the hypothesis that the stacking fault reduction can be explained by grain growth. The model is used to make predictions on annealing times and temperatures required for stacking fault reduction and could be adapted for polycrystalline thin films with similar morphology. [ABSTRACT FROM AUTHOR]

Understanding the impact of impurities in solar absorbers is critical to engineering high-performance in devices, particularly over extended periods of time. Here, we use hybrid functional calculations to explore the role of hydrogen interstitial (Hi) defects in the electronic properties of a number of attractive solar absorbers within the chalcopyrite and kesterite families to identify how this common impurity may influence device performance. Our results identify that Hi can inhibit the highly p-type conditions desirable for several higher-band gap absorbers and that H incorporation could detrimentally affect the open-circuit voltage (Voc) and limit device efficiencies. Additionally, we find that Hi can drive the Fermi level away from the valence band edge enough to lead to n-type conductivity in a number of chalcopyrite and kesterite absorbers, particularly those containing Ag rather than Cu. We find that these effects can lead to interfacial Fermi-level pinning that can qualitatively explain the observed performance in high-Ga content CIGSe solar cells that exhibit saturation in the Voc with increasing band gap. Our results suggest that compositional grading rather than bulk alloying, such as by creating In-rich surfaces, may be a better strategy to favorably engineering improved thin-film photovoltaics with larger-band gap absorbers. [ABSTRACT FROM AUTHOR]

Variable range hopping (VRH) conduction of Mott type for a constant and non-vanishing density of states at the Fermi level is observed over a wide range of temperature between 45 and 210K in the ordered defect compound CuIn3Te5 (CIT135). For this type of electrical conduction at a very high temperature, not reported before in elemental, II-VI, and I-III-VI2 compound semiconductors, we employed three different methods to analyze the data and to confirm this behavior. The occurrence of VRH at very high temperatures is explained as due to the presence of the electrically inactive (InCu+2 + VCu-1)) donor-acceptor defect pairs in CIT135. This defect-pair partially annihilates the shallow acceptor defect state Cu vacancy, which is responsible for the activated electrical conductivity observed in p-type ternary I-III-VI2 chalcopyrite compounds in the temperature range of liquid nitrogen. In such conditions, the only acceptor level available for electrical conduction in CIT135 is a deep acceptor state whose activation energy is about 200meV and cannot be activated below about 200K. Hence, the VRH conduction mechanism dominates the electrical properties of this material up to about 200K. The study of the temperature and magnetic field dependence of the magnetoresistance (MR) up to 27 T is made by taking into consideration different theoretical models. To explain the negative MR at lower fields, the theory based on quantum interference is used. At higher magnetic fields, the MR becomes positive and is explained with the model based on the shrinkage of the wave function. [ABSTRACT FROM AUTHOR]

Cerium doped CuInS2 thin films were successfully fabricated by a powder metallurgy method. X-ray diffraction and scanning electron microscope measurements showed that the as-prepared CuIn1-xCexS2 samples are of good crystallinity and crystallize with chalcopyrite structure when sintering at 550 °C. The presence of Ce3+ in host material was conformed by X-ray photoelectron spectroscopy. Two subband photon absorption peaks were observed at 1710 nm (0.73 eV) and 1955 nm (0.63 eV) in the UV-Vis-NIR absorption spectrum. This behavior could suggest that an intermediate band forms in the forbidden band of CuInS2 semiconductor due to cerium incorporation. The optical bandgap of CuIn1-xCexS2 films was tuned in the range of 1.38 eV to 1.23 eV with increasing cerium content. And the electrical conductivity could be improved if doped moderate cerium content, especially x=0.1. [ABSTRACT FROM AUTHOR]

Currently, Cu-containing chalcopyrite-based solar cells provide the highest conversion efficiencies among all thin-film photovoltaic (PV) technologies. They have reached efficiency values above 20%, the same performance level as multi-crystalline silicon-wafer technology that dominates the commercial PV market. Chalcopyrite thin-film heterostructures consist of a layer stack with a variety of interfaces between different materials. It is the chalcopyrite/buffer region (forming the p-n junction), which is of crucial importance and therefore frequently investigated using surface and interface science tools, such as photoelectron spectroscopy and scanning probe microscopy. To ensure comparability and validity of the results, a general preparation guide for "realistic" surfaces of polycrystalline chalcopyrite thin films is highly desirable. We present results on wet-chemical cleaning procedures of polycrystalline Cu(In1-xGax)Se2 thin films with an average x=[Ga]/([In][Ga])=0.29, which were exposed to ambient conditions for different times. The hence natively oxidized sample surfaces were etched in KCN- or NH3-based aqueous solutions. By x-ray photoelectron spectroscopy, we find that the KCN treatment results in a chemical surface structure which is - apart from a slight change in surface composition - identical to a pristine as-received sample surface. Additionally, we discover a different oxidation behavior of In and Ga, in agreement with thermodynamic reference data, and we find indications for the segregation and removal of copper selenide surface phases from the polycrystalline material. [ABSTRACT FROM AUTHOR]

In this study, we investigate the effect of different elemental selenium to elemental sulfur ratios on the chalcopyrite phase formation in Cu(In,Ga)(S,Se)2 thin films. The films are formed by the stacked elemental layer process. The structural and electronic properties of the thin films and solar cells are analyzed by means of scanning electron microscopy, glow discharge optical emission spectrometry, X-ray diffraction, X-ray fluorescence, Raman spectroscopy, spectral photoluminescence as well as current-voltage, and quantum efficiency measurements. The influence of different S/(S+Se) ratios on the anion incorporation and on the Ga/In distribution is investigated. We find a homogenous sulfur concentration profile inside the film from the top surface to the bottom. External quantum efficiency measurements show that the band edge of the solar cell device is shifted to shorter wavelength, which enhances the open-circuit voltages. The relative increase of the open-circuit voltage with S/(S+Se) ratio is lower than expected from the band gap energy trend, which is attributed to the presence of S-induced defects. We also observe a linear decrease of the short-circuit current density with increasing S/(S+Se) ratio which can be explained by a reduced absorption. Above a critical S/(S+Se) ratio of around 0.61, the fill factor drops drastically, which is accompanied by a strong series resistance increase which may be attributed to changes in the back contact or p-n junction properties. [ABSTRACT FROM AUTHOR]

Element-specific unoccupied electronic states of Cu(In, Ga)S2 were studied as a function of the In/Ga ratio by combining X-ray absorption spectroscopy with density functional theory calculations. The S absorption edge shifts with changing In/Ga ratio as expected from the variation of the band gap. In contrast, the cation edge positions are largely independent of composition despite the changing band gap. This unexpected behavior is well reproduced by our calculations and originates from the dependence of the electronic states on the local atomic environment. The changing band gap arises from a changing spatial average of these localized states with changing alloy composition. [ABSTRACT FROM AUTHOR]

Positron annihilation spectroscopy is used to study point defects in Zn1-xMnxGeAs2 crystals with low Mn content 0 ≤ χ ≤ 0:042 with disordered zincblende and chalcopyrite structure. The role of negatively charged vacancies and non-open-volume defects is discussed with respect to the high p-type conductivity with carrier concentration 1019 ≤ p ≤ 1021 cm-3 in our samples. Neutral As vacancies, together with negatively charged Zn vacancies and non-open-volume defects with concentrations around 1016 - 1018 cm-3, are observed to increase with increasing Mn content in the alloy. The observed concentrations of defects are not sufficient to be responsible for the strong p-type conductivity of our crystals. Therefore, we suggest that other types of defects, such as extended defects, have a strong influence on the conductivity of Zn1-xMnxGeAs2 crystals. [ABSTRACT FROM AUTHOR]

The structural and optical properties of pentenary alloy (Ag,Cu)(In,Ga)Se2 polycrystalline thin films were characterized over the entire compositional range at a fixed (Cu+Ag)/(In+Ga) ratio. Films deposited at 550 °C on bare and molybdenum coated soda lime glass by elemental co-evaporation in a single-stage process with constant incident fluxes exhibit single phase chalcopyrite structure, corresponding to 122 spacegroup (I-42d) over the entire compositional space. Unit cell refinement of the diffraction patterns show that increasing Ag substitution for Cu, the refined ao lattice constant, (Ag,Cu)-Se bond length, and anion displacement increase in accordance with the theoretical model proposed by Jaffe, Wei, and Zunger. However, the refined ao lattice constant and (In,Ga)-Se bond length deviated from theoretical expectations for films with mid-range Ag and Ga compositions and are attributed to influences from crystallographic bond chain ordering or cation electronegativity. The optical band gap, derived from transmission and reflection measurements, widened with increasing Ag and Ga content, due to influences from anion displacement and cation electronegativity, as expected from theoretical considerations for pseudo-binary chalcopyrite compounds. [ABSTRACT FROM AUTHOR]

CuIn1-xCdxTe2 materials (x=0, 0.02, 0.05, and 0.1) are prepared using melting-annealing method and the highly densified bulk samples are obtained through Spark Plasma Sintering. The X-ray diffraction data confirm that nearly pure chalcopyrite structures are obtained in all the samples. Due to the substitution of Cd at In sites, the carrier concentration is greatly increased, leading to much enhanced electrical conductivity and power factor. The single parabolic band model is used to describe the electrical transport properties of CuInTe2 and the low temperature Hall mobility is also modeled. By combing theoretical model and experiment data, the optimum carrier concentration in CuInTe2 is proposed to explain the greatly enhanced power factors in the Cd doped CuInTe2. In addition, the thermal conductivity is reduced by extra phonon scattering due to the atomic mass and radius fluctuations between Cd and In atoms. The maximum zTs are observed in CuIn0.98Cd0.02Te2 and CuIn0.9Cd0.1Te2 samples, which are improved by over 100% at room temperature and around 20% at 600 K. [ABSTRACT FROM AUTHOR]

Optical absorption spectra have been measured on the single-crystalline chalcopyrite semiconductor AgInS2 using polarized light at T=10-300K. The bandgap energy Eg of AgInS2 shows unusual temperature dependence at low temperatures. The resultant temperature coefficient ∂Eg/∂T is found to be positive at T<130K and negative above 130K. This result has been successfully explained by considering the effects of thermal expansion and electron-phonon interaction. The free-exciton emission of photoluminescence spectra also indicates positive temperature dependence of the peak energies at low temperatures. The exciton binding energy of AgInS2 is determined to be 26.4meV. [ABSTRACT FROM AUTHOR]

The anisotropic thermal anharmonicity of CdSiP2 and ZnGeP2 has been studied by calculating the a- and c-axial Grüneisen parameters separately to cast light on the mechanism of anisotropic thermal expansivity of ABC2 chalcopyrite compounds. Both the Debye model and lattice dynamics theory were implemented to calculate the axial Grüneisen parameters. The variation of shear modulus, calculated from the Debye model, demonstrated normal behavior for ZnGeP2 but abnormal behavior for CdSiP2, and was thus assumed to be the most important parameter that determines anisotropic thermal anharmonicity. Using phonon frequency-based lattice dynamics, the axial mode Grüneisen parameters were calculated for not only the Γ-point but also for other K-points in the first Brillouin zone. The lowest B1 and B2 modes of both compounds were found to be new soft modes that were not observed in previous studies of volume-dependent mode Grüneisen parameters. The larger magnitude of these soft mode Grüneisen parameters of CdSiP2 was responsible for its greater abnormal axial thermal anharmonicity in the low-temperature range. The Grüneisen parameters became positive at 110 K for CdSiP2 and 80 K for ZnGeP2. [ABSTRACT FROM AUTHOR]

Electronic and transport properties of CuGaTe2, a hole-doped ternary copper based chalcopyrite type semiconductor, are studied using calculations within the Density Functional Theory and solving the Boltzmann transport equation within the constant relaxation time approximation. The electronic band structures are calculated by means of the full-potential linear augmented plane wave method, using the Tran-Blaha modified Becke-Johnson potential. The calculated band gap of 1.23 eV is in agreement with the experimental value of 1.2 eV. The carrier concentration- and temperature dependent thermoelectric properties of CuGaTe2 are derived, and a figure of merit of zT = 1.69 is obtained at 950 K for a hole concentration of 3.7·1019 cm-3, in agreement with a recent experimental finding of zT = 1.4, confirming that CuGaTe2 is a promising material for high temperature thermoelectric applications. The good thermoelectric performance of p-type CuGaTe2 is associated with anisotropic transport from a combination of heavy and light bands. Also for CuSbS2 (chalcostibite), a better performance is obtained for p-type than for n-type doping. The variation of the thermopower as a function of temperature and concentration suggests that CuSbS2 will be a good thermoelectric material at low temperatures, similarly to the isostructural CuBiS2 compound. [ABSTRACT FROM AUTHOR]

Solar cells based on kesterite absorbers consistently show lower voltages than those based on chalcopyrites with the same bandgap. We use three different experimental methods and associated data analysis to determine minority-carrier lifetime in a 9.4%-efficient Cu2ZnSnSe4 device. The methods are cross-sectional electron-beam induced current, quantum efficiency, and time-resolved photoluminescence. These methods independently indicate minority-carrier lifetimes of a few nanoseconds. A comparison of current-voltage measurements and device modeling suggests that these short minority-carrier lifetimes cause a significant limitation on the voltage produced by the device. The comparison also implies that low minority-carrier lifetime alone does not account for all voltage loss in these devices. [ABSTRACT FROM AUTHOR]

X-ray diffraction measurements of tetragonal chalcopyrite cadmium germanium arsenide crystal are carried out over the range 25-450 °C. The collected patterns show that the powder neither yields a new phase nor structurally transforms to any other architecture. Accurate lattice parameters (including two lattice constants a and c, cell volume V, axial ratio k, and tetragonal distortion δ) are determined by means of Cohen's least square method. Polynomial fit approximation is adopted to quantitatively describe the temperature dependence of lattice parameters. These results are the groundwork that allows the evaluation of thermal expansion coefficients defined as αR = R-1(dR/dT) where R represents lattice parameters. The average thermal expansion coefficients are evaluated, 13.9 × 10-6 K-1 for αa and 2.8 × 10-6 K-1 for αc, respectively. It is also found that αa increases while αc decreases with temperature, which in consequence brings forth the large anisotropy of thermal expansion between the two main axes (a-axis and c-axis). Detailed discussion reveals that the anisotropy behavior is in tight association with the nearest neighboring bonds. Namely, the thermal expansion coefficient perpendicular to c-axis (αa) depends to a large extent on the ionic Cd-As bonds, while that parallel to c-axis (αc) depends on the covalent Ge-As bonds. Finally, the Grüneisen parameters at different temperatures are evaluated and also exhibit anisotropic behavior (γa > γc). It is found that γa is temperature-dependent and increases with temperature, while γc tends to be temperature-independent. [ABSTRACT FROM AUTHOR]

High-pressure Raman scattering measurements have been carried out in ZnGa2Se4 for both tetragonal defect chalcopyrite and defect stannite structures. Experimental results have been compared with theoretical lattice dynamics ab initio calculations and confirm that both phases exhibit different Raman-active phonons with slightly different pressure dependence. A pressure-induced phase transition to a Raman-inactive phase occurs for both phases; however, the sample with defect chalcopyrite structure requires slightly higher pressures than the sample with defect stannite structure to fully transform into the Raman-inactive phase. On downstroke, the Raman-inactive phase transforms into a phase that could be attributed to a disordered zincblende structure for both original phases; however, the sample with original defect chalcopyrite structure compressed just above 20 GPa, where the transformation to the Raman-inactive phase is not completed, returns on downstroke mainly to its original structure but shows a new peak that does not correspond to the defect chalcopyrite phase. The pressure dependence of the Raman spectra with this new peak and those of the disordered zincblende phase is also reported and discussed. [ABSTRACT FROM AUTHOR]

The defect-related levels of CuInS2 thin films prepared by sulfurization using ditertiarybutylsulfide [(t-C4H9)2S:DTBS] have been investigated by photoluminescence (PL) spectroscopy. The PL spectra exhibit four peaks at 1.43, 1.27, 1.21, and 1.17 eV. On the basis of the PL spectra observed at various excitation intensities and temperatures, the peak emissions are attributed to donor-acceptor pair transitions. The ionization energies of donors in CuInS2 thin films are determined to be 66, 330, 300, and 150 meV, and the ionization energies of acceptors are estimated to be 100, 170, and 390 meV, respectively. As a result of the different sulfurization conditions and times, the donors corresponding to these ionization energies are due to indium interstitial (Ini), indium atoms-occupied sulfur vacancies (InS), sulfur atoms-occupied copper vacancies (SCu), and sulfur vacancies (VS), and the acceptors are determined to be due to copper vacancies (VCu), sulfur interstitial (Si), and sulfur atoms-occupied indium vacancies (SIn), respectively. Using these data, a band diagram for the defect levels of CuInS2 thin films has been proposed. [ABSTRACT FROM AUTHOR]

The electronic structures of chalcopyrite CuIn1-xGaxSe2 have recently been reported to have strongly anisotropic and non-parabolic valence bands (VBs) even close to the Γ-point VB maximum. Also, the lowest conduction band (CB) is non-parabolic for energies 50-100 meV above the CB minimum. The details in the band-edge dispersion govern the material's electrical properties. In this study, we, therefore, analyze the electronic structure of the three uppermost VBs and the lowest CB in CuIn1-xGaxSe2 (x = 0, 0.5, and 1). The parameterized band dispersions are explored, and the density-of-states (DOS) as well as the constant energy surfaces are calculated and analyzed. The carrier concentration and the Fermi energy EF in the intrinsic alloys as functions of the temperature is determined from the DOS. The carrier concentration in p-type materials is modeled by assuming the presence of Cu vacancies as the acceptor type defect. We demonstrate that the non-parabolicity of the energy bands strongly affects the total DOS. Therefore, it is important to take into account full band dispersion of the VBs and CB when analyzing the free carrier concentration, like for instance, in studies of electronic transport and/or measurements that involve strong excitation conditions. [ABSTRACT FROM AUTHOR]

Na has deliberately been incorporated into Cu(In,Ga)Se2 ('CIGSe') chalcopyrite thin-film solar cell absorbers deposited on Mo-coated polyimide flexible substrates by adding differently thick layers of NaF in-between CIGSe absorber and Mo back contact. The impact of Na on the chemical and electronic surface structure of CIGSe absorbers with various Cu-contents deposited at comparatively low temperature (420 °C) has been studied using x-ray photoelectron and x-ray excited Auger electron spectroscopy. We observe a higher Na surface content for the Cu-richer CIGSe samples and can distinguish between two different chemical Na environments, best described as selenide-like and oxidized Na species, respectively. Furthermore, we find a Cu-poor surface composition of the CIGSe samples independent of Na content and - for very high Na contents - indications for the formation of a (Cu,Na)-(In,Ga)-Se like compound. With increasing Na surface content, also a shift of the photoemission lines to lower binding energies could be identified, which we interpret as a reduction of the downward band bending toward the CIGSe surface explained by the Na-induced elimination of InCu defects. [ABSTRACT FROM AUTHOR]

High-pressure optical absorption and Raman scattering measurements have been performed in defect chalcopyrite (DC) CdGa2Se4 up to 22 GPa during two pressure cycles to investigate the pressure-induced order-disorder phase transitions taking place in this ordered-vacancy compound. Our measurements reveal that on decreasing pressure from 22 GPa, the sample does not revert to the initial phase but likely to a disordered zinc blende (DZ) structure the direct bandgap and Raman-active modes of which have been measured during a second upstroke. Our measurements have been complemented with electronic structure and lattice dynamical ab initio calculations. Lattice dynamical calculations have helped us to discuss and assign the symmetries of the Raman modes of the DC phase. Additionally, our electronic band structure calculations have helped us in discussing the order-disorder effects taking place above 6-8 GPa during the first upstroke. [ABSTRACT FROM AUTHOR]

The defect levels in epitaxial and polycrystalline wide bandgap chalcopyrite CuGaSe2 with various stoichiometry deviations were investigated using modulated photocurrent spectroscopy. The results were analyzed as a function of light intensity and Fermi-level position. Comparison of the results from epitaxial and polycrystalline material distinguished levels belonging to intrinsic defects and their correlation with the material stoichiometry. We also compared the fingerprints of defect levels by MPC to the results derived from capacitance spectroscopy performed on Schottky diodes fabricated on both epitaxial and polycrystalline layers. This allowed us to attribute unambiguously levels observed in the capacitance response to bulk point defects. In the final conclusions we provide information on the electronic parameters of nine defect levels observed in CuGaSe2 and their correlation with the material stoichiometry. These results should help to identify intrinsic defects that are important for the photovoltaic performance of solar cells based on chalcopyrites. [ABSTRACT FROM AUTHOR]

The effect of magnetic annealing on the texture is investigated in a series of CoPt films of different thickness obtained by annealing of Co/Pt multilayers. Magnetic field is found to promote the (001) texture by selective (001) grain growth driven by the magnetic field and biaxial strain. A systematic correlation between in-plane biaxial strain and the degree of (001) texture is found only for the magnetically annealed samples. [ABSTRACT FROM AUTHOR]

Interactions between monovalent cations and anions are common in ternary AIBIIIC2VI chalcopyrite crystals, and are known to significantly influence their electronic behaviors. Density functional theory was used to investigate the effect of monovalent cations on the electronic, optical, and lattice dynamic properties of XInSe2 (X = Cu, Ag, Li) compounds. It follows that the electronic configurations of X ions, as well as the interactions between X ions and InSe4 groups, are responsible for the different electronic and optical properties of XInSe2. The special lattice dynamic properties of LiInSe2 are mainly related to the smaller cationic masses of Li cations, whereas the high ionicity of Li-Se bonds leads to the anomalistic declines of the A1 and A2 modes and the greater LO/TO splitting of the upper phonon polar modes. [ABSTRACT FROM AUTHOR]

Hybrid exchange density functional theory is used to study the wide band gap chalcopyrite CuAlS2. The formation energies of charged and neutral intrinsic defects are calculated for different environmental conditions, and it is shown that CuAlS2 is a p-type material that cannot be type inverted through the formation of intrinsic defects. The calculated band gap states associated with the different intrinsic defects are used to comment on the origin of the observed CuAlS2 photoluminescence emissions. The origin and stability of ordered defect compounds derived from CuAlS2 are investigated, and it is concluded that CuAl5S8 is a stable ordered defect compound, albeit in a small region of phase space. [ABSTRACT FROM AUTHOR]

The CdGa2Se4 layers were grown by the hot-wall epitaxy method. From the absorption measurement, the band-gap variation in CdGa2Se4 was well interpreted using Varshni's equation. After the postannealing in various ambient, the behavior of point defects in CdGa2Se4 was investigated by measuring photoluminescence (PL). Point defects originating from VCd, VSe, Cdint, and/or Seint were classified as donor or acceptor types. Thus, the Ga in CdGa2Se4 did not form native defects because the Ga existed in the form of stable bonds in CdGa2Se4. Based on these PL results, we schemed out a band diagram of the recombination process in CdGa2Se4. [ABSTRACT FROM AUTHOR]

The surface Cu-depletion of chalcopyrite thin films and its influence on the interface properties of related solar cells have been subject of a controversial debate for many years. Although the nature of this Cu-depletion and its extension in depth are crucial for the device physics, there are only a few contradictory experimental results that address this topic. To clarify this issue, we performed depth-dependent compositional analysis by angle dependent soft x-ray emission spectroscopy (AXES) on Cu(In,Ga)Se2 thin films with different integral Cu-contents. By considering depth profiles from literature and by taking the accuracy of AXES into account, our numerical AXES simulations predict a pronounced angle dependence for our samples. However, our experimental data show only a minor angle dependence, which leads to the conclusion that the Cu-depleted surface layer must be restricted to a very thin surface layer, which is not accessible by AXES. This conclusion is consistent with the result from our previous investigation by hard x-ray photoelectron spectroscopy, where we found a Cu-depleted surface layer in the subnanometer regime. Consequently the present study gives further experimental evidence for the surface reconstruction model proposed by first-principles calculations. Supported by secondary neutral mass spectroscopy, we show that the minor angle dependence in our AXES data can be attributed to a Ga-gradient in the chalcopyrite material. [ABSTRACT FROM AUTHOR]

The valence band offset of the heterojunction between CuInS2 (CIS) and chemical bath deposited CdS has been determined both by means of combined x-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) and by near-UV constant final state (CFS) yield spectroscopy. The use of the latter shows that this spectroscopic method is indeed suitable for the investigation of chalcopyrite thin films. The higher information depth due to the low excitation energies (7.5–4.0 eV) used in CFS makes it possible to obtain a signal from the valence band of the CIS substrate even after a relatively thick (∼5 nm) CdS layer has been deposited. The number of samples needed to determine the band offset is reduced from three to one and the effects of chemical changes on the CIS surface can be accounted for because the valence band edges of both materials are measured directly. The valence band offsets with the XPS/UPS and CFS methods were determined to be 1.25±0.20 and 1.45±0.20 eV, respectively. [ABSTRACT FROM AUTHOR]

We have studied vacancy defects in chalcopyrite semimagnetic semiconducting mixed Zn1-x(Mn,Co)xGeAs2 bulk crystals with alloy composition x varying between 0.052 to 0.182 using positron annihilation spectroscopy. We identified As vacancies, potentially complexed with the transition metal alloying elements, in all the studied samples, while no cation vacancy related defects were detected. The positron lifetimes for the bulk ZnGeAs2 lattice and neutral As vacancy were determined to be τB=220–230 ps and τAs=300±10 ps, respectively. Our results also show that the p-type conductivity in the samples is not due to cation vacancy related acceptor centers. The As vacancies were found to be present at such low concentrations that they cannot be responsible for the compensation of the p-type conductivity or the reduction of mobility in the Zn1-x(Mn,Co)xGeAs2 samples. [ABSTRACT FROM AUTHOR]

The size dependent optical band gap of the less-toxic ternary I-III-VI2 chalcopyrite-type semiconductor quantum dots (QDs), CuInS2, CuInSe2, CuGaS2, CuGaSe2, AgInSe2, AgGaS2, and AgGaSe2, were evaluated using the finite-depth-well effective mass approximation calculation. From the comparison of the calculation result with the experimental values for the CuInS2 case, it was shown that the calculation was highly valid to predict the size dependent optical gap of the ternary semiconductor QDs. The optical band gap of the above seven I-III-VI2 QDs covers a wide wavelength range from the near-infrared to ultraviolet. It has been shown that the I-III-VI2 semiconductor QDs have a significant potential as alternatives to the highly toxic cadmium-containing II-VI semiconductor QDs and they are applicable to the wide range of light emitting devices and solar cells. [ABSTRACT FROM AUTHOR]

X-ray diffraction measurements on the sphalerite-derivatives ZnGa2Se4 and CdGa2S4 have been performed upon compression up to 23 GPa in a diamond-anvil cell. ZnGa2Se4 exhibits a defect tetragonal stannite-type structure (I42m) up to 15.5 GPa and in the range from 15.5 to 18.5 GPa the low-pressure phase coexists with a high-pressure phase, which remains stable up to 23 GPa. In CdGa2S4, we find that the defect tetragonal chalcopyrite-type structure (I4) is stable up to 17 GPa. Beyond this pressure a pressure-induced phase transition takes place. In both materials, the high-pressure phase has been characterized as a defect-cubic NaCl-type structure (Fm3m). The occurrence of the pressure-induced phase transitions is apparently related with an increase in the cation disorder on the semiconductors investigated. In addition, the results allow the evaluation of the axial compressibility and the determination of the equation of state for each compound. The obtained results are compared to those previously reported for isomorphic digallium sellenides. Finally, a systematic study of the pressure-induced phase transition in 23 different sphalerite-related ABX2 and AB2X4 compounds indicates that the transition pressure increases as the ratio of the cationic radii and anionic radii of the compounds increases. [ABSTRACT FROM AUTHOR]

The Zn1-xMnxGa2Se4 system with 0.52Se4, retains the defective chalcopyrite structure of the parent compound. Neutron powder diffraction experiments and temperature dependent magnetic susceptibility measurements show that such dilution has a significant influence on the magnetic properties of the system. The distance between the closest magnetic atoms decreases from a to a/2 Å (where a is the lattice parameter and a≈5.6 Å), making the Curie-Weiss temperature greater than the expected value. The magnetic dilution also decreases the internal distortion, so that the structure becomes closer to that of the ideal zinc-blende arrangement. Although MnGa2Se4 behaves as an antiferromagnet below TC=6.4±0.1 K, the magnetic properties of Zn1-xMnxGa2Se4 with 0.51-xMnxGa2Se4 0.52Se4, a magnetic lattice with a body centered cubic symmetry, but a structure possessing a greater number of sites available to the magnetic ions. [ABSTRACT FROM AUTHOR]

We report on defect spectroscopy on Cu(In,Ga)Se2 based solar cells with varying gallium content. Our investigations using deep level transient and admittance spectroscopy do not reveal a pronounced qualitative difference in the defect spectra for the case of absorbers containing indium and gallium. Therefore, we conclude that there exists no detrimental defect in the bulk material that generally prohibits one to achieve an increase in efficiency even for solar cells based on absorbers with a gallium to gallium plus indium ratio (GGI) larger than 0.3. The boundary compositions with CuInSe2 and CuGaSe2 absorbers (i.e., GGI=0 and GGI=1, respectively) show additional trap signals. Additionally, a peak width analysis was performed for two defect signals that occur for all absorber compositions. The relative peak width was found to be independent of GGI whereas the activation energies show some correlation with the gallium content of the absorber layer. [ABSTRACT FROM AUTHOR]

We report here that the ball-milling process induces the phase transformation from the tetragonal structure to the disordered face-centered-cubic structure in Ni2MnGa ferromagnetic shape-memory alloys. The in situ high-energy x-ray diffraction analyses reveal that an intermediate phase, which is characterized by amorphous structure, controls the transformation kinetics during the postannealing process. Completely different from their coarse-grained counterparts, the ferromagnetic Ni2MnGa nanoparticles undergo various sequences of structural transitions that are tailored by the crystallite size, atomic order, and intrinsic magnetic structure. [ABSTRACT FROM AUTHOR]

Spectral dependence of the pseudodielectric function [variant_greek_epsilon](E)=[variant_greek_epsilon]1(E)+i[variant_greek_epsilon]2(E) as well as of the complex refractive index, extinction coefficient, absorption coefficient, and normal-incidence reflectivity of Cu2In4Se7 and CuGa3Se5 ordered vacancy compound crystals are modeled in the 0.8–4.4 eV photon energy range using a modification of Adachi’s model [T. Kawashima et al. J. Appl. Phys. 84, 5202 (1998)] for optical properties of semiconductors. Model parameters are determined using the acceptance-probability-controlled simulated annealing method. Excellent agreement with experimental data is obtained; the relative errors for the real [variant_greek_epsilon]1 and for imaginary [variant_greek_epsilon]2 part of the dielectric function are equal to 0.9%–1.5% and 3.2%–4.1% for the studied compounds. [ABSTRACT FROM AUTHOR]

Measurements of low field static magnetic susceptibility and of magnetization with pulsed magnetic fields up to 32 T have been made as a function of temperature on polycrystalline samples of the Zn1-zMnzGa2Se4 alloy system, which has a defect tetragonal chalcopyrite structure in the whole composition range. The resulting data have been used to give information on the magnetic spin-flop and magnetic saturation transitions, and details of the magnetic B-T phase diagrams were determined for the phases. The zero-field Néel temperatures TN and triple points, for the Zn1-zMnzGa2Se4 alloy system, have been found to be 8.1 K and (7.8 K, 2.2 T) for z=1, 5.8 K and (5.6 K, 1.7 T) for z=0.85, 4.5 K and (4.35 K, 1.0 T) for z=0.075, and 3.9 K and (3.85 K, 0.5 T) for z=0.7. The susceptibility χ(T) curves for B=3 and 5 T show magnetothermal effects below 4.5 K. [ABSTRACT FROM AUTHOR]

The application of Zn compounds as buffer layers was recently extended to wide-gap CuInS2 (CIS) based thin film solar cells. Using an alternative chemical deposition route for the buffer preparation aiming at the deposition of a single-layer, nominal ZnS buffer without the need for any toxic reactants such as hydrazine has helped us to achieve a similar efficiency as respective CdS-buffered reference devices. In order to shed light on the differences of other Zn-compound buffers deposited in conventional chemical baths [chemical bath deposition (CBD)] compared to the buffer layers deposited by this alternative CBD process, the composition of the deposited buffers was investigated by x-ray excited Auger electron and x-ray photoelectron spectroscopy to potentially clarify their superiority in terms of device performance. We have found that in the early stages of this alternative CBD process a thin ZnS layer is formed on the CIS, whereas in the second half of the CBD the growth rate is greatly increased and Zn(S,O) with a ZnS/(ZnS+ZnO) ratio of ∼80% is deposited. Thus, a ZnS/Zn(S,O) bilayer buffer is deposited on the CIS thin film solar cell absorbers by the alternative chemical deposition route used in this investigation. No major changes of these findings after a postannealing of the buffer/CIS sample series and recharacterization could be identified. [ABSTRACT FROM AUTHOR]

Thin layers of the pentenary material Cu(In,Ga)(S,Se)2 (CIGSSe) are studied using photoluminescence (PL) spectroscopy. The layers were produced by a two-stage deposition process, which produces thin chalcopyrite films with a high degree of compositional uniformity throughout the thickness of the layer. It is shown that potential fluctuations, caused by compensation and high concentrations of defects, dominate the PL behavior of the investigated layers. This is inferred from a number of PL studies such as variable temperature and excitation power measurements. It is deduced that the average amplitude of fluctuations increases with increasing sulfur mole fraction S/(Se+S). Thus, sulfur incorporation increases the degree of compensation (and hence the charged defect concentration). From the strong asymmetry and the observed weak shift of the low energy tails in the PL spectra with increasing sulfur content, it is also concluded that a significant fraction of defect states associated with the quaternary CIGSSe is still present in the pentenary layers. [ABSTRACT FROM AUTHOR]

Zn(O,OH) layers deposited by the ion layer gas reaction (ILGAR) technique have the potential to replace the conventionally used CdS buffer layer in Cu(In(1-X)GaX)(SYSe(1-Y))2-based thin-film solar cells. To avoid stability issues, the fraction of metastable Zn(OH)2 should be reduced in the final buffer layer. However, hydroxide-poor or -free ZnO “buffers” result in noncompetitive devices. We have therefore investigated the impact of different oxide/hydroxide ratios on the electronic band alignment at the absorber/buffer heterointerface. The surface composition as well as the position of the valence-band maximum (VBM) of respective ILGAR-Zn(O,OH) samples was determined by photoelectron spectroscopy. The position of the conduction-band minimum (CBM) was estimated using optical band gaps determined from optical reflection/transmission measurements. From the comparison of these VBM and CBM values with the respective values of the absorber surface, predictions are made in terms of valence- and conduction-band offsets at the crucial absorber/buffer interface. The results are compared with previous findings, and the drawn conclusions are correlated with the performance of respective solar cell devices. [ABSTRACT FROM AUTHOR]

An alternative approach to the so-called “lift-off” technology is presented, in which a CuGaSe2 solar cell absorber film is detached from a Mo-coated glass substrate. The proposed lift-off takes advantage of an interfacial MoSe2 layer, acting as a sacrificial layer, which forms at the rear contact during the growth of the CuGaSe2 film. No additional processing step is thus required to proceed with the lift-off. The lift-off was carried out in ultrahigh vacuum for quality assessment, and the rear CuGaSe2 and top MoSe2 surfaces were characterized by means of surface-sensitive techniques, namely, Kelvin probe force microscopy and photoelectron spectroscopy. The cleanness of the CuGaSe2 rear surface was confirmed by the absence of Mo remnants, thus demonstrating the suitability of the proposed method for further processing of the absorber film onto alternative substrates. In addition, a quantitative analysis of surface photovoltage, doping concentration, and interface charge at grain boundaries on the absorber’s rear surface is presented, exploiting the convenience of the procedure for characterization purposes. Preliminary results regarding the device performance and identification of limiting factors are reported. [ABSTRACT FROM AUTHOR]

A simple model is introduced which determines the optical band-gap energy Eg for penternary Cu(In(1-X)GaX)(SYSe(1-Y))2 (CIGSSe) alloys from its Ga/(Ga+In) ratio as well as from its S/(S+Se) ratio. In order to verify the model the depth dependent composition of a CIGSSe sample was revealed by elastic recoil detection analysis. Applying the model, the concentration profiles were transferred in an Eg profile. Finally, these values were compared with optical band-gap energies, which were obtained directly by independent characterization methods. [ABSTRACT FROM AUTHOR]

The crystal structural and optical properties of the ordered defect compound CuGa5Se8 grown by the vertical Bridgman–Stockbarger method have been studied by x-ray diffraction and optical absorption techniques, respectively. The analysis of the x-ray diffraction data shows that the material crystallizes in a tetragonal chalcopyrite-related structure with unit cell parameters a=5.4732(8) and c=10.931(6) Å. Optical data shows that the band gap energy EG varies from 1.917 to 1.811 eV in the temperature range between 10 and 300 K. It is found that the variation of EG with T is mainly due to the contribution of optical phonons with a characteristic phonon energy of about 19 meV. The optical absorption coefficient just below the absorption edge varies exponentially with photon energy confirming the existence of the expected Urbach’s tail. The phonon energy hνp associated with Urbach’s tail, which is found to be 54 meV, is about two times higher than the highest optical phonon mode reported for ternary semiconductors of the Cu–Ga–Se system. The origin of higher energy is attributed to the contribution of localized modes produced by structural disorders of low energy formation mainly due to deviation from ideal stoichiometry. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In this work annealing and growth of CuInS2 thin films is investigated with quasireal-time in situ Raman spectroscopy. During the annealing a shift of the Raman A1 mode towards lower wave numbers with increasing temperature is observed. A linear temperature dependence of the phonon branch of -2 cm-1/100 K is evaluated. The investigation of the growth process (sulfurization of metallic precursors) with high surface sensitivity reveals the occurrence of phases which are not detected with bulk sensitive methods. This allows a detailed insight in the formation of the CuInS2 phases. Independent from stoichiometry and doping of the starting precursors the CuAu ordering of CuInS2 initially forms as the dominating ordering. The transformation of the CuAu ordering into the chalcopyrite one is, in contrast, strongly dependent on the precursor composition and requires high temperatures. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A critical review of the experimental data of elastic stiffness constants of AgGaX[sub 2] and CuInX[sub 2] (X = S,Se) chalcopyrites is given. Using the ab initio pseudopotential method, their elastic constants have been calculated from strain-energy and from strain-stress relationships. On the basis of the linear elasticity approximation, axial and total compressibilities as well as elastic anisotropy factors have been derived. The calculated results are compared with available experimental data and previous calculations based on phenomenological models. [ABSTRACT FROM AUTHOR]

A quadratic dependence of the band gap energy on the alloy composition x was quantified for CuAl(S[sub x]Se[sub 1-x])[sub 2] films grown by low-pressure metalorganic vapor phase epitaxy, by means of photoreflectance and photoluminescence excitation spectroscopies. The bowing parameter for the A-exciton energy was estimated to be 0.20 eV. Several high-quality films grown on GaAs(001) substrates exhibited excitonic photoluminescence peaks in the blue to ultraviolet spectral ranges. The flow rate of the Al precursor was found to affect the incorporation ratio of S/Se, indicating that the Al–S compound plays a key role in controlling x. All films grown on GaAs(001) showed c(001) orientation. Conversely, the epitaxitial orientation of the films on GaP(001) changed from a(100) to c(001) with an increase in x. The critical value of x was around 0.5. The preferred orientations were explained by the natural selection rule under which the lattice strain in the epilayer is minimized. The residual strain in the 0.5-μm-thick epilayers on GaAs(001) was nearly constant for all x, although the lattice mismatch between the epilayer and the GaAs substrate varied from 0.62% to 5.39% with an increase in x. Consequently the strain was attributed to thermal stress. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The AgInS[sub 2] epilayers with a chalcopyrite structure grown using a hot-wall epitaxy method have been confirmed to be a high quality crystal. From the optical absorption measurement, the temperature dependence of the energy band gap on AgInS[sub 2]/GaAs was found to be E[sub g](T)=2.1365 eV-(9.89×10[sup -3] eV)T[sup 2]/(2930+T). After the as-grown AgInS[sub 2]/GaAs was annealed in AgInS[sub 2]/GaAs has been investigated by using the photoluminescence (PL) at 10 K. The native defects of V[sub Ag], V[sub S], Ag[sub int], and S[sub int] obtained from PL measurements were classified as a donors or acceptors type AgInS[sub 2]/GaAs to an optical p type. Also, we confirmed that In in AgInS[sub 2]/GaAs did not form the native defects because In in AgInS[sub 2] did exist in the form of stable bonds. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Investigates the low-temperature charge transport in chalcopyrite n-CuGaSe2 (CGS) in zero and non-zero magnetic field. Crystal growth and experimental methods; Variable-range hopping conductivity of n-CGS for B=0; Positive magnetoresistance of n-CGS.