Inx Ga1-x Research Articles

High-Resolution Mapping of Strain Partitioning and Relaxation in InGaN/GaN Nanowire Heterostructures.

Growing an In x Ga1− x N/GaN (InGaN/GaN) multi‐quantum well (MQW) heterostructure in nanowire (NW) form is expected to overcome limitations inherent in light‐emitting diodes (LEDs) based on the conventional planar heterostructure. The epitaxial strain induced in InGaN/GaN MQW heterostructure can be relaxed through the sidewalls of NW, which is beneficial to LEDs because a much larger misfit strain with higher indium concentration can be accommodated with reduced piezoelectric polarization fields. The strain relaxation, however, renders highly complex strain distribution within the NW heterostructure. Here the authors show that complementary strain mapping using scanning transmission electron microscopy and dark‐field inline holography can comprehend the strain distribution within the axial In0.3Ga0.7N/GaN MQW heterostructure embedded in GaN NW by providing the strain maps which can cover the entire NW and fine details near the sidewalls. With the quantitative evaluation by 3D finite element modelling, it is confirmed that the observed complex strain distribution is induced by the strain relaxation leading to the strain partitioning between InGaN quantum disk, GaN quantum well, and the surrounding epitaxial GaN shell. The authors further show that the strain maps provide the strain tensor components which are crucial for accurate assessment of the strain‐induced piezoelectric fields in NW LEDs.

Structural Characteristics and Photoluminescence of Thin Films of Cu(In1–xgax)(Syse1–y)2 Solid Solutions

The phase composition and structural and optical characteristics of thin films of Cu(In1–xGax)(SySe1–y)2 solid solutions with the chalcopyrite structure were investigated. The unit-cell constants according to x-ray diffraction analysis were a ~ 5.720 A and c ~ 11.52 A. The elemental compositions were x = Ga/(Ga + In) ~ 0.14 and y = S/(S + Se) ~ 0.11 for Cu(In1–xGax)(S ySe1–y)2 thin films. The optical band gap of Cu(In1–xGax)(S ySe1–y)2 solid solutions determined from measurements of photoluminescence spectra and luminescence excitation spectra at ~4.2 K was Eg ~ 1.122 eV. The mechanism of radiative recombination of nonequilibrium charge carriers in thin films of Cu(In1–xGax) (S ySe1–y)2 solid solutions is discussed.

Control of electronic structure in Cu(In, Ga)(S, Se)2 for high-efficiency solar cells

We have made a three-dimensional (3D) map of the band-gap energy of the Cu(In1–xGax)(SySe1–y)2 (CIGSSe) quinary system. The band-gap energy of the CIGSSe system monotonically increased from CuInSe2 (x = 0.0 and y = 0.0) with simultaneously increasing Ga/(Ga+In) (GGI, x) and S/(S + Se) (SSSe, y) ratios. We also made a 3D map of the energy levels of the valence band maxima (VBMs) and conduction band minima (CBMs) of the CIGSSe system from the ionization energies measured by photoemission yield spectroscopy and the band-gap energies. The substitution effect of S for Se in CuInSe2 was different from that of Ga for In. The VBM level of the CIGSSe system did not change significantly and the CBM level increased with an increasing GGI ratio, x. On the other hand, the VBM level decreased and the CBM level increased with an increasing SSSe ratio, y.

High-Brightness Blue Light-Emitting Diodes Enabled by a Directly Grown Graphene Buffer Layer.

Single-crystalline GaN-based light-emitting diodes (LEDs) with high efficiency and long lifetime are the most promising solid-state lighting source compared with conventional incandescent and fluorescent lamps. However, the lattice and thermal mismatch between GaN and sapphire substrate always induces high stress and high density of dislocations and thus degrades the performance of LEDs. Here, the growth of high-quality GaN with low stress and a low density of dislocations on graphene (Gr) buffered sapphire substrate is reported for high-brightness blue LEDs. Gr films are directly grown on sapphire substrate to avoid the tedious transfer process and GaN is grown by metal-organic chemical vapor deposition (MOCVD). The introduced Gr buffer layer greatly releases biaxial stress and reduces the density of dislocations in GaN film and Inx Ga1-x N/GaN multiple quantum well structures. The as-fabricated LED devices therefore deliver much higher light output power compared to that on a bare sapphire substrate, which even outperforms the mature process derived counterpart. The GaN growth on Gr buffered sapphire only requires one-step growth, which largely shortens the MOCVD growth time. This facile strategy may pave a new way for applications of Gr films and bring several disruptive technologies for epitaxial growth of GaN film and its applications in high-brightness LEDs.

The microstructure, local indium composition and photoluminescence in green-emitting InGaN/GaN quantum wells.

In this work, we analyse the microstructure and local chemical composition of green-emitting Inx Ga1-x N/GaN quantum well (QW) heterostructures in correlation with their emission properties. Two samples of high structural quality grown by metalorganic vapour phase epitaxy (MOVPE) with a nominal composition of x = 0.15 and 0.18 indium are discussed. The local indium composition is quantitatively evaluated by comparing scanning transmission electron microscopy (STEM) images to simulations and the local indium concentration is extracted from intensity measurements. The calculations point out that the measured indium fluctuations may be correlated to the large width and intensity decrease of the PL emission peak.

Traceable Quantitative Raman Microscopy and X-ray Fluorescence Analysis as Nondestructive Methods for the Characterization of Cu(In,Ga)Se2 Absorber Films.

The traceability of measured quantities is an essential condition when linking process control parameters to guaranteed physical properties of a product. Using Raman spectroscopy as an analytical tool for monitoring the production of Cu(In1-xGax)Se2 thin-film solar cells, proper calibration with regard to chemical composition and lateral dimensions is a key prerequisite. This study shows how the multiple requirements of calibration in Raman microscopy might be addressed. The surface elemental composition as well as the integral elemental composition of the samples is traced back by reference-free X-ray fluorescence analysis. Reference Raman spectra are then generated for the relevant Cu(In1-xGax)Se2 related compounds. The lateral dimensions are calibrated with the help of a novel dimensional standard whose regular structures have been traced back to the International System of Units by metrological scanning force microscopy. On this basis, an approach for the quantitative determination of surface coverage values from lateral Raman mappings is developed together with a complete uncertainty budget. Raman and X-ray spectrometry have here been proven as complementary nondestructive methods combining surface sensitivity and in-depth information on elemental and species distribution for the reliable quality control of Cu(In1-xGax)Se2 absorbers and Cu(In1-xGax)3Se5 surface layer formation.

The Effect of Doping and the Thickness of the Layers on CIGS Solar Cell Efficiency

The main problemswith the use of fossil fuels is the restrictions on their access and the detrimental consequences of their use which causes a threat to human health and quality of life. Consequently, the use of other energy sources has become necessary. Renewable Energy as a permanent and clean energy source is an answer to this problem. One such energy source includes photovoltaic solar energy that is widely available as a reliable energy source. Research and Development of Photovoltaic Energy in general, will reduce costs and improve efficiency in both areas. CIGS solar cells have higher efficiency in comparison with other cells. Ion implantation and doping technique offers the unique structure of a solar cells. This paper will examine the performance of solar cells with Cu In1-x Gax Se2 structure. This willbe performed by Silvaco software. Effect of doping phosphorus (p) and Natrium (Na), as well as the value of x and thethickness of the various layers of the solar cell, on the efficiency of the cell, have been studied.

Time‐resolved electroabsorption measurement of carrier velocity in inverted polarity In1‐xGax N/GaN heterostructures due to internal electric fields

AbstractCarrier transport measurements were made in a c ‐plane inverted‐polarity n ‐GaN/i ‐In1–xGax N/p ‐GaN heterostructure with a 200 nm thick In1–xGax N layer using ultrafast spectroscopy techniques. Time‐domain THz measurements indicate that the direction of electron transport is dominated by drift towards the n ‐GaN layer. Time‐resolved electroabsorption measurements were used to determine carrier velocities by monitoring the change in transmission of a probe beam tuned to the In1‐xGax N absorption edge due to the transport of photogenerated carriers under the built‐in internal electric field. Carrier transit times associated with screening of the electroabsorption are resolved at the lowest injection level. The signal rise time shows two distinct time scales, which correspond to an electron velocity of 3.3 ± 0.5 x 107 cm/s and a hole velocity of 6.7 ± 0.3 x 106 cm/s in an internal electric field of ∼150 kV/cm. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of thickness on the properties of Cu(Inx,Ga1-x)Se2 back conduct Mo thin films prepared by DC sputtering

In this study, Mo thin films which used in Cu(Inx Ga1-x)Se2 (CIGS) thin film solar cells as back conduct were deposited on soda-lime glass substrates via DC magnetron sputtering under certain conditions. A series of Mo thin films prepared of various thicknesses was obtained in different sputtering deposition times. The microstructure, electrical resistivity and mechanical strain property of Mo thin films, which may be varied by controlling the thickness, were investigated by XRD, SEM, four probes technology and Scotch tape test. As the results showed, the thicknesses of the films increased linearly with the sputtering time. With increasing thickness, the films' crystal growth showed a change from (110) preferred orientation to (211) preferred orientation. The sheet resistance sharply reduced to 2 Ω/⇑ with the increase of (110) peak height and the resistivity linearly decreased to 0.96×10-4 Ω·cm due to the level of (110) preferred orientation. The films surface has porous (fish-like) grain morphology and intergranular voids. All the films are in a tensile state, and the inner strain decreased with the increase of the thickness.

ChemInform Abstract: Colloidal CIGS and CZTS Nanocrystals: A Precursor Route to Printed Photovoltaics

AbstractReview: CIGS: Cu(In1‐xGax)Se2, CZTS: Cu2ZnSnS4; 63 refs.

Density functional theory study of ordered defect Cu‐(In,Ga)‐Se compounds

AbstractCuIn1–xGaxSe2 is used as absorber material in thin‐film photovoltaics. Presence of defect arrays of 2VCu– + XCu2+ complexes (X = In or Ga) in the material results in the formation of ordered defect phases, like for instance Cu(In1–xGax)3Se5 and Cu(In1–xGax)5Se8. An understanding of these defects phases is of considerably importance since they can strongly affect the performance of the solar cell. Density functional theory calculation with a hybrid functional is carried out to study the electronic structures and optical properties of these ordered defect phases. Fundamental properties, such as the band bowing, the band alignment and the absorption coefficients, are presented, and their dependence on material composition is discussed (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Colloidal CIGS and CZTS nanocrystals: A precursor route to printed photovoltaics

This review article summarizes our research focused on Cu(In1–xGax)Se2 (CIGS) nanocrystals, including their synthesis and implementation as the active light absorbing material in photovoltaic devices (PVs). CIGS PV layers are typically made using a high temperature (>450°C) process in which Cu, In and Ga are sequentially or co-evaporated and selenized. We have sought to use CIGS nanocrystals synthesized with the desired stoichiometry to deposit PV device layers without high temperature processing. This approach, using spray deposition of the CIGS light absorber layers, without high temperature selenization, has enabled up to 3.1% power conversion efficiency under AM 1.5 solar illumination. Although the device efficiency is too low for commercialization, these devices provide a proof-of-concept that solution-deposited CIGS nanocrystal films can function in PV devices, enabling unconventional device architectures and materials combinations, including the use of flexible, inexpensive and light-weight plastic substrates.

Wafer‐Scale Strain Engineering of Ultrathin Semiconductor Crystalline Layers

The fabrication of a wafer-scale dislocation-free, fully relaxed single crystalline template for epitaxial growth is demonstrated. Transferring biaxially-strained Inx Ga1-x As ultrathin films from InP substrates to a handle support results in full strain relaxation and the Inx Ga1-x As unit cell assumes its bulk value. Our realization demonstrates the ability to control the lattice parameter and energy band structure of single layer crystalline compound semiconductors in an unprecedented way.

Renormalization of the contribution of the electron—electron interaction to the conductivity of two-dimensional electron systems

The contribution of the electron—electron interaction to the conductivity of the two-dimensional electron gas in an Inx Ga1-x As single quantum well with different disorder strengths was experimentally studied. It is shown that the data are described well within the framework of the one-loop approximation of the renormalization group theory so long as the conductivity of the system remains higher than around 15e2/μh.

Characterisation of Cu(In1–xGax)5Se8 by spectroscopic ellipsometry

AbstractOptical measurements of Cu(In1–xGax)5Se8 were performed at room temperature in the photon energy range from 0.8 to 4.7 eV. Spectral dependence of the complex dielectric functions was derived. The structures observed in the measured spectra have been analyzed by fitting the second derivative of the experimental spectrum to analytical line shapes and have been attributed to the interband transitions E0, and E1A. The energies corresponding to the electronic transitions as well as the parameters as strength and broadening were determined. Besides, the values of E0 and E1A are found to increase linearly with increasing Ga content. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Preparation and properties of In/p‐Cu(In1–xGax)(S1–ySey)2 surface‐barrier structures

AbstractCu(In1–xGax)(S1–ySey)2 (CIGSS) thin films were prepared by the simultaneous selenizarion/sulfurization of Cu‐In‐Ga metallic alloys. Full characterizations have been carried out using XRD, SEM, EDX, AES and Raman scattering measurements. Photosensitive In/p ‐CIGSS surface‐barrier rectifying structures were created by thermal deposition of pure Incontacts onto the surface of the CIGSS as grown films. The influences of the thin films chemical composition and the illumination conditions on the photo‐electric parameters of In/p ‐CIGSS structures were studied. Analysis of the results showed the availability of the prepared CIGSS films for creation of high efficiency thin films photovoltaic devices. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Comparison of the compositions and structures of electrodeposited Cu-poor and Cu-rich Cu(In1-xGax)Se2 films before and after selenization

Cu-poor and Cu-rich near stoichiometric Cu (In1-x Gax)Se2(CIGS) precursor films were obtained by one-step electrodeposition from acid aqueous baths, and the compositions and structures of the as-deposited films before and after selenization were investigated. The straightforward experimental evidence was first obtained, which showed that Cux Se compound was conglomerated as rather large crystal grains which was distributed on the surface of the film for the Cu-rich films after seleniztion. The results indicated that the film composition was changed barely after selenization. When the ratio Cu/(In+Ga)＜1.1, there were many cracks on the surface of CIGS after selenization. When Cu/(In+Ga)＞1.2, no cracks were formed on the surface of the thin films. The efficiency of solar cells bnilt directly from Cu-poor films after selenization achieved above 2%, which is pending further investigation. Furthermore, the efficiency of solar cells bult from Cu-rich CIGS films after selenization and deposition of a few In, Ga and Se by PVD has achieved 6.8%.

Interplay of impurity segregation and lattice mismatch in molecular beam epitaxy of III′–III″–V compounds plus dopant: application to the InGaAs:Sn nanoheterolayer growth

AbstractA thermodynamic model for molecular beam epitaxy of quaternary solid alloys III′–III″–V plus dopant is developed on a practical example of In1x Gax As:Sn/GaAs heterolayer growth. In the frame of the model, the interplay of surface segregation and strain during epitaxy is analyzed. It is shown Sn atoms, being segregated during the growth in the In–stabilized conditions, act as an efficient surfactant, and Sn doping may be used to improve the surface morphology of strained nanoheterolayers. It is found that high doping level and the low growth temperatures are favourable for achieving a smooth morphology. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Energy spectra and oscillatory magnetization of two-electron self-assembledInxGa1−xAsquantum rings in GaAs

The effects of the Coulomb interaction on the energy spectrum and the magnetization of two electrons in a strained Inx Ga1-x As/GaAs ringlike nanostructure are analyzed with realistic parameters inferred from the cross-sectional scanning-tunneling microscopy data. With an increasing magnetic field, the lowest spin-singlet and spin-triplet states sequentially replace each other as the ground state. This is reminiscent of the Aharonov-Bohm effect for the ringlike structures. The exchange interaction leads to a more complicated oscillatory structure of the magnetic moment of the two electrons as a function of the magnetic field as compared to the magnetization pattern for a single-electron ringlike nanostructure. We discuss the relevance of the two-electron systems for the interpretation of the Aharonov-Bohm oscillations in the persistent current observed in low temperature magnetization measurements on self-assembled Inx Ga1-x As/GaAs ringlike nanostructures.

Grain size enlargement of Cu(InGa)Se 2 thin films by Hot Filament Melting process

Hot Filament Melting (HFM) process has been newly proposed to enlarge the grain size of Cu(In1–xGax)Se2 (CIGS) films grown by a mechanochemical process (MCP). When the substrate temperature was 500 oC and 600 oC and the current through the hot filament was 15A, we confirmed encouraging the sintering of CIGS grains. Additionally, when the substrate temperature was 700 oC, the melt of the CIGS grains at the surface was observed. The crystal structure of CIGS films after the process were analyzed by X-ray diffraction and Raman spectroscopy, and it was found that the CIGS films kept the chalcopyrite structure and that the formation of hetero-phase was not observed. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)