Second-harmonic and sum-frequency generation in KTiOAsO4

Tungstate flux-grown KTiOAsO₄ (KTA) has been found to be phase matchable for type-2 SHG and SFG down to 0.5174 and 0.4193 μm at 20.0°C. Sellmeier's equations and nonlinear optical constants of this crystal are reported     

CuInSe2 cells and modules

Recent CuInSe₂ photovoltaic technology advances are discussed. 14.1% active area efficient test cells and the fabrication of monolithic integrated modules with power outputs of 112 W/m² on 940 cm² and 91.4 W/m² on 3900 cm² have been achieved. Packaged modules are stable outdoors. Studies indicate a recombination controlled junction mechanism and imply a wide CIS compositional range over which high-efficiency junctions are possible. Processing improvements already demonstrated on test cells and 940 cm² modules will yield 52-W, 3900-cm² CIS modules     

Pre-breakdown in thin SiO2 films

The on-off fluctuations of the tunnel current in 5.6 nm SiO₂ films before dielectric breakdown are analyzed in detail. For this purpose, a low noise measurement system has been realized which allows detection of pre-breakdown phenomena and interruption of the stress before catastrophic failure occurs. A spectral analysis of these fluctuations is presented along with preliminary results of the experiments made possible, for the first time, by the new measurement system     

Deposition of CuInSe2 films by a two-stage processutilizing E-beam evaporation

The deposition technique involves first depositing Cu and In metallic layers onto a substrate using the electron-beam evaporation method and then selenizing these layers in an H₂Se atmosphere. High-quality CuInSe₂ films of chalcopyrite phase have been obtained by this technique, and the films have been characterized by scanning electron microscopy, X-ray diffraction, electron microprobe, and resistivity measurements. Solar cells have been fabricated using these CuInSe₂ films, and active-area conversion efficiency values approaching 11% have been demonstrated for these devices     

Post deposition UV-induced O2 annealing of HfO2 thin films

UV-assisted annealing processes for thin oxide films is an alternative to conventional thermal annealing and has shown many advantages such as low annealing temperature, reducing annealing time and easy to control. We report in this work the deposition of ultra-thin HfO₂ films on silicon substrate by two CVD techniques, namely thermal CVD and photo-induced CVD using 222 nm excimer lamps at 400 °C. As-deposited films of around 10 nm in thickness with refractive indices from 1.72 to 1.80 were grown. The deposition rate measured by ellipsometry was found to be about 2 nm/min by UV-CVD, while the deposition rate by thermal CVD is 20% less than that by UV-CVD. XRD showed that the as-deposited HfO₂ films were amorphous. This work focuses on the effect of post deposition UV annealing in oxygen on the structural, optical and electrical properties of the HfO₂ films at low temperature (400 °C). Investigation of the interfacial layer by FTIR revealed that thickness of the interfacial SiO₂ layer slightly increases with the UV-annealing time and UV annealing can convert sub-oxides at the interface into stoichiometric SiO₂, leading to improved interfacial qualities. The permittivity ranges in 8–16, are lower than theoretical values. However, the post deposition UV O₂ annealing results in an improvement in effective breakdown field and calculated permittivity, and a reduction in leakage current density for the HfO₂ films.     

Second-harmonic generation efficiency in periodically poled LiNbO3 waveguides

Quasi-phase-matched second-harmonic generation efficiency in periodically poled LiNbO₃ using titanium indiffusion, has been theoretically investigated. This permits clarifying how the involved guided modes convert the triangle-shaped form of the ferroelectric polarization in the crystal, via the overlap integral between this form and these modes, into an effective periodic nonlinear coefficient. As a consequence, the periodic function that really permits the quasi-phase-matching operation is the modulation of this effective nonlinear coefficient, the shape of which can be far from a triangular shape, as shown using different cases of mode profiles     

Electrical properties of CuInSe2 single crystals

Various bulk electrical properties and device characteristics have been measured. It has been shown that the majority carrier type is dependent on crystal stoichiometry. Mobilities of 660 cm²/V sec and 30 cm²/V sec have been measured for n-and p-type samples, respectively. Rectifying contacts and p-n junctions have been investigated by small signal analysis and the associated doping levels and equilibrium band diagrams have been determined. Photovoltage measurements on rectifying contacts have shown that the band-gap has a value of 0.95 ± 0.01eV.     

Structural properties of fluorinated SiO2 thin films

Fluorinated SiO₂ (SiOF) films, prepared by plasma enhanced chemical vapour deposition from SiH₄, N₂O and CF₄ precursors, have been analysed by infrared (IR) spectroscopy and X-ray photoelectron spectroscopy (XPS) to extract chemical and structural information. Notwithstanding XPS reveals that fluorine concentrations are quite low (less than 4 at.%), the analysis of the Si–O–Si vibration modes in the IR spectra indicates that CF₄ addition involves a deeper modification of the film structure, than the simple formation of Si–F bonds. In particular, by increasing the F concentration in the oxides, the stretching frequency of the Si–O–Si bonds increases, while the bending frequency decreases. On the basis of the central force model, both observations are consistent with the occurrence of a Si–O–Si bond angle relaxation phenomenon, the importance of which increases with the fluorine concentration in the films.     

Preparation of n-type semiconductor SnO2 thin films

We studied fluorine-doped tin oxide on a glass substrate at 350℃using an ultrasonic spray technique. Tin（Ⅱ） chloride dehydrate,ammonium fluoride dehydrate,ethanol and NaOH were used as the starting material, dopant source,solvent and stabilizer,respectively.The SnO₂:F thin films were deposited at 350℃and a pending time of 60 and 90 s.The as-grown films exhibit a hexagonal wurtzite structure and have（101） orientation.The G = 31.82 nm value of the grain size is attained from SnO₂:F film grown at 90 s,and the transmittance is greater than 80%in the visible region.The optical gap energy is found to measure 4.05 eV for the film prepared at 90 s, and the increase in the electrical conductivity of the film with the temperature of the sample is up to a maximum value of 265.58（Ω·cm）^-1,with the maximum activation energy value of the films being found to measure 22.85 meV,indicating that the films exhibit an n-type semiconducting nature.     

Lightweight tandem GaAs/CuInSe2 solar cells

High-efficiency, ultralightweight, mechanically stacked 4-cm² thin-film tandem solar cells are discussed. The tandem stack consists of a single-crystal, thin-film Ga(Al)As cell fabricated by the cleavage of lateral epitaxy for transfer (CLEFT) process and adhesively bonded to the top of a CdZnS/CuInSe₂ polycrystalline thin-film cell deposited on glass. Maximum tandem efficiency in a four-terminal configuration of 21.6% AM0 have been demonstrated. This represents the highest thin-film cell efficiency reported to date. Individual subcells with efficiencies of 19.5% for CLEFT GaAs and 3.0% for CuInSe₂ have also been achieved. Cell specific power as high as 600 W/kg has been achieved with a 4-cm² cell weight of 188 mg without coverglass, at an efficiency of 20.8% AM0     

Thermoelectric power of SnO2 anisotropic thin films

A calculation of thermoelectric power in fluorine-doped SnO₂ thin films is carried out using the Boltzmann transport theory and including anisotropic effects over material as well as the degeneracy grade of the samples. Curves of thermoelectric power in dependence of the temperature and concentration of F-impurities are reported. Finally, the computed values are compared with those obtained experimentally.     

Photo-CVD process for ultra thin SiO2 films

In this paper we first report the use of very low deposition rate photo-induced chemical vapor deposition process, (below 0.05 nm/min). This photo-CVD process is adequate to grow very thin and ultra thin layers of SiO₂. Details on the design of the reaction chamber, reactive gases and process parameters to obtain the desired deposition regime are presented. Dependence of deposition rate on pressure in the chamber and gas flow ratio is discussed. Deposited layers were characterized using I–V and C–V techniques.     

The role of lattice mismatch in the deposition of CdTe thin films

Cadmium telluride (CdTe) is the most well-established II–VI compound largely due to its use as a photonic material. Existing applications, as well as those under consideration, are demanding increasingly stringent control of the material properties. The deposition of high-quality thin films is of utmost importance to such applications. In this regard, we present a report detailing the role of lattice mismatch in determining the film quality. Thin films were deposited on a wide variety of substrate materials using the pulsed laser deposition (PLD) technique. Common to all substrates was the strong tendency toward the preferential alignment of CdTe’s (111) planes parallel to the substrate’s surface. X-ray diffraction analysis, however, revealed that the crystalline quality varied dramatically depending upon the substrate used with the best results yielding a single-crystal film. This tendency also manifested itself in the surface morphology with higher structural perfection yielding smoother surfaces. The film quality showed a strong correlation with lattice mismatch. Texture analysis using the [111] pole figure confirmed that improvements in the lattice mismatch led to a higher degree of in-plane alignment of the (111) grains.     

Contact potential measurements on thin SiO2 films

Contact potential measurements by the Kelvin method were performed in vacuum on silicon wafers whose thermal oxide film was etched into the shape of a wedge. A given position along the oxide corresponded directly to a given depth and by scanning a reference electrode stripe across the wafer, information about the distribution of the oxide charge through the thickness was obtained. It was found that the oxide charge was positive and concentrated within a few hundred angstroms of the SiSiO₂ interface.     

CuInSe2薄膜太阳能电池非真空印刷制备技术研究

对CuInSe2(CISe)薄膜太阳能电池的吸收层进行了非真空印刷制备技术研究。使用机械化学法合成CISe前驱粉末,采用ethyl-cellulose作为分散试剂配置印刷浆,使用丝网印刷技术沉淀CISe吸收层,对沉淀的吸收层进行N2氛围的快速热退火处理,使用XRD、UV、SEM及J-V等手段对CISe吸收层进行了分析表征。结果表明:简单高效的机械化学法可获得主(112)晶向CISe前驱粉末;经丝网印刷并干燥后的CISe吸收层中含有大量有机分散剂,退火可蒸发有机分散剂并有效改善CISe结晶度,但过长的退火会增加晶体缺陷;实验制得一典型CISe薄膜太阳能电池的短路电流密度、开路电压、填充因子和转换效率分别为4.48mA/cm2、355mV、0.41和0.65%。     

Reliability of ultra thin ZrO2 films on strained-Si

Ultra thin high-k zirconium oxide (equivalent oxide thickness 1.57 nm) films have been deposited on strained-Si/relaxed-Si_0.8Ge_0.2 heterolayers using zirconium tetra-tert-butoxide (ZTB) as an organometallic source at low temperature (<200 °C) by plasma enhanced chemical vapour deposition (PECVD) technique in a microwave (700 W, 2.45 GHz) plasma cavity discharge system at a pressure of 66.67 Pa. The trapping/detrapping behavior of charge carriers in ultra thin ZrO₂ gate dielectric during constant current (CCS) and voltage stressing (CVS) has been investigated. Stress induced leakage current (SILC) through ZrO₂ is modeled by taking into account the inelastic trap-assisted tunneling (ITAT) mechanism via traps located below the conduction band of ZrO₂ layer. Trap generation rate and trap cross-section are extracted. A capture cross-section in the range of 10⁻¹⁹ cm² as compared to 10⁻¹⁶ cm² in SiO₂ has been observed. The trapping charge density, Q_ot and charge centroid, X_t are also empirically modeled. The time dependence of defect density variation is calculated within the dispersive transport model, assuming that these defects are produced during random hopping transport of positively charge species in the insulating layer. Dielectric breakdown and reliability of the dielectric films have been studied using constant voltage stressing. A high time-dependent dielectric breakdown (TDDB, t_bd > 1500 s) is observed under high constant voltage stress.     

Spin synthesis of monolayer of SiO2 thin films

The highly ordered monolayer of submicron size silica (SiO₂) particles (235 nm) is developed on p-silicon by using three-step spin-coating in colloidal suspension, which has significant potential in various applications. The influence of three-step spin speeds, spinning time, acceleration time between different steps, concentration of SiO₂ particles in the solution, solution quantity, and the ambient humidity (relative humidity) on the properties of monolayer SiO₂ are studied in order to achieve a large area monolayer film. A relatively high surface coverage and uniform monolayer film of SiO₂ particles in the range of 85%-90% are achieved by appropriate control of the preparative parameters. We conclude that this method can be useful in industrial applications, because of the fabrication speed, surface coverage and cost of the process.     

Characterization of CuGaSe2 thin films grown by MOCVD

CuGaSe₂ thin films have been grown by metalorganic chemical vapor deposition (MOCVD), from three organometallic precursors. Samples of about 1-2 μm thick are codeposited onto Pyrex and Mo-coated soda lime glass. A large range of compositions was investigated and characterized. Stoichiometric CuGaSe₂ thin films are single-phased and their optical bandgap is about 1.68 eV. The features of the films are presented in relation with their composition. XRD spectra always exhibit a preferential orientation along the (112) plane. Secondary phases have been observed: Cu₂Se for Cu-rich films, CuGa₃Se₅ for Ca-rich films. Observation of the morphology reveals larger polyhedral grains for Cu-rich films becoming platelet-shaped and tilted for Ga-rich compounds. The optical properties are also sensitive to the compositional changes and related to the eventual presence of binary phases. The gap increases with the Ga-content. The CuGa₃Se₅, phase exhibit a gap of about 1.85 eV. All the samples have a p-type conductivity     

Electrical conduction mechanism in high-dielectric-constant ZrO2 thin films

The electrical conduction mechanism in zirconium oxide (ZrO₂) thin films as a function of temperature T and electric field E was studied. Al/ZrO₂/p-Si metal–insulator–semiconductor (MIS) capacitors were fabricated. With the Al electrode biased negative, the conduction mechanism in the electrical field of 0.81 MV/cm < E < 1.40 MV/cm and in the temperature range of 375 K < T < 450 K is found to be modified Schottky emission. The intrinsic barrier height between Al and ZrO₂ is 1.06 eV. At higher electrical fields of 1.50 MV/cm < E < 2.25 MV/cm and higher temperatures of 375 K < T < 450 K, the electrical conduction is dominated by modified Poole–Frenkel emission. The extracted trap barrier is 0.83 eV. With the Al electrode biased positive, the conduction mechanism is found to be Schottky emission at the electrical field 0.20 MV/cm < E < 0.60 MV/cm and higher temperature range of 425 K < T< 450 K. The barrier height between Si and ZrO₂ is 1.0 eV. Based on these results, an energy band diagram of the Al/ZrO₂/p-Si system is proposed.