Impact of the Ga concentration on the microstructure of CuIn1–x Gax Se2

Cross‐sectional samples of CuIn_1–x Ga_x Se₂ layers grown by a three‐stage process were studied by means of electron backscatter diffraction (EBSD) in completed thin‐film solar cells. The microstructural analysis reveals a dependence of the average grain size on the gallium content x = [Ga]/([Ga] + [In]), with a maximum at x = 0.23. This result is correlated with structural measurements on CuIn_1–x Ga_x Se₂ powder samples showing that the ratio of the lattice constants c /a is equal to 2 for about the same x value. The pseudocubic crystal structure at about x = 0.23 may lead to reduced strain in the growing CuIn_1–x Ga_x Se₂ layer and therefore larger average grain sizes. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis of phosphorus‐carbide thin films by magnetron sputtering

Phosphorus‐carbide, CP_x (0.025 ≤ x ≤ 0.1), thin films have been synthesized by magnetron sputtering from pressed graphite–phosphorus targets. The films were characterized by X‐ray photoelectron spectroscopy, transmission electron microscopy and diffraction, and nanoindentation. CP_0.1 ex‐hibits C–P bonding in an amorphous structure with elements of curved graphene planes, yielding a material with unique short range order. These features are consistent with what has been predicted by our results of theoretically modeled synthetic growth of CP_x . The films are mechanically resilient with hardness up to 24 GPa and elastic recovery up to 72%. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Extremely narrow luminescence linewidth in GaAs single quantum wells by insertion of thin AlAs smoothing layers

We propose a new method to considerably reduce the overall growth interruption for high-quality GaAs single quantum wells during molecular beam epitaxy. The insertion of ultrathin AlAs smoothing layers at the constituent GaAs/Al _x Ga_1–x As heterointerfaces and growth interruptions of not more than 15 s yields an improvement of the luminescence linewidth (FWHM) to 0.56 meV for a 13 nm wide GaAs well and to a value as low as 0.195 meV for a 27 nm wide GaAs well. In addition, no Stokes shift between absorption and emission and no line splitting due to monolayer fluctuations in the well width is observed.     

Structural properties of CuInSe2 films prepared by selenization of metallic precursors on MoNx film substrates

The aim of this work was to study the effect of MoN_x film substrates on the structural properties of CuInSe₂ films prepared by selenization of metallic Cu-In alloy precursors. MoN_x films were prepared by reactive dc-magnetron sputtering. All the CuInSe₂ films exhibit single phase chalcopyrite structure with (1 1 2) preferred orientation, which can be explained by the reduction of lattice mismatch between CuInSe₂ and MoN_x. The bulk composition of selenized CuInSe₂ films are near stoichiometric, but the surface composition analysis suggests Cu deficiency on surface area. Furthermore, ordered defect compound, CuIn₂Se_3.5 is found on the surface of CuInSe₂ films. The results will be helpful for fabricating Cd-free ZnO buffer layer CuInSe₂ and Cu(In_1−xGa_x)Se₂ based thin film solar cells.     

Simulations of structures of amorphous SixC1−x films

Amorphous Si_xC_1−x films possess the potential to improve wear performance in humid atmospheres and at higher temperatures. But some experimental work on the films showed that silicon contents greatly influenced their microstructures and mechanical properties. Therefore, simulations of molecular dynamics were carried out to predict structures of the Si_xC_1−x films at different silicon contents. The results show that the sp³/sp² ratio of all the films increases, but the stiffness of the films is decreasing with an increase in silicon contents. Moreover, silicon atoms are almost surrounded by carbon atoms, which is in agreement with the experiments.     

Incorporation of Sn into epitaxial GaAs grown from the liquid phase

The incorporation of Sn into LPE GaAs was studied as a function of the atomic fractionx _Sn ^l of Sn in the liquid (1.6×10⁻⁴≤x _Sn ^l ≤0.54), the growth temperatureT _K and the cooling rate α. The diffusion coefficient of As in Ga for moderate Sn-doping was deduced from the growth velocities to beD _As (760° C)=(3.3±1.0)×10⁻⁵ cm²/s. The epitaxial layers were analyzed after van der Pauw with special emphasis on the sources of experimental error. With the aid of current mobility theories the concentrations of the ionized donors and acceptors were derived. From their dependence onx _Sn ^l , on α and onT _K combined with the Schottky-barrier model of Sn incorporation it can be concluded that the melt and the growing crystal surface were in thermal equilibrium. The diffusion coefficient of Sn in GaAs is about 8×10⁻¹⁴ cm²/s at 760° C. The distribution coefficient for Sn increases from 4.4×10⁻⁵ to 12.3×10⁻⁵ in the temperature range from 690 to 800° C. The total Sn incorporationx _Sn ^s was measured using the atomic absorption spectroscopy for the first time down tox _Sn ^s =10¹⁷/cm³. From these data it can be concluded that up tox _Sn ^l =0.54 the dopant Sn is incorporated as donor and as acceptor only and that within the experimental scatter there is no indication of incorporation as a neutral species.     

Preparation and microstructures of CoAg granular films with giant magnetoresistance

A series of Co _x Ag_1–x (0 x < 100 at.%) granular films were deposited onto glass substrates with the thickness on the order of 400 nm using the ion-beam cosputtering technique at different substrate temperatures. Systematical investigations were made on the Giant MagnetoResistance (GMR) effect and microstructures in these thin granular films. The magnetoresistance ratio strongly depends on cobalt concentration, substrate temperature, and the optimal value of GMR was observed in Co₂₂Ag₇₈ sample prepared at a temperature of 300 K. Microstructures of these films are closely related to the fabricating process, and thus influence the GMR.     

Compositional and structural characterization of strained Si/Si x Ge1−x multilayers and interfaces by high-resolution transmission electron microscopy

A method is described for the quantitative characterization of strained Si/Si _x Ge_1–x multilayers and interfaces by high-resolution transmission electron microscopy (HRTEM) in [110] and [100] crystal projections. The method relies on systematic variations of the image contrast with variations of the local composition x for certain ranges of objective lens defocus f and specimen thickness t and takes tetragonal lattice distortions fully into account. From an extensive study of the image formation process for Si _x Ge_1–x alloys and coherent Si/Si _x Ge_1–x interfaces, ranges of f and t were identified by Bloch-wave and multi-slice image simulations at 400 keV for which a quasi-linear functional relationship between the composition x and the first-order Fourier coefficients of the image intensity exists. By application of a novel image-processing algorithm, which allows a precise measurement of image Fourier coefficients in geometrically distorted lattice images, local composition values x can be determined at near-atomic resolution with an accuracy of x ±0.1 and interface sharpness can be detected at the atomic level. Recent applications of the method to the characterization of interfaces of strained Si _x Ge_1–x layers and short-period Si _m Ge _n superlattices fabricated by different deposition techniques will be presented.     

Strain analysis and optical characterization of LPE InGaAsP laser-confining layers

The compositional dependence of lattice parameters and strains induced by lattice mismatch of liquid phase epitaxial (LPE) In_1–x Ga _x AS _y P_1–y /InP with smallx andy are studied. The measured elastic strain is proportional to the lattice mismatch within certain critical limits; as much as 78% of the mismatch strain is found to be accommodated elastically under both compression and tension. The near-band edge absorption and photoluminescence measurements yield the free electron and hole recombination probability and the compositional dependence of the energy gap at low temperature. The influence of the lattice mismatch on the optical properties is discussed.On leave from Institute of Semiconductors, Chinese Academy of Sciences, Peking, ChinaOn leave from Instituto de Fisica, Universidade Estadual de Campinas, Campinas, S.P. 13100, Brazil     

Selectively dopedn-AlxGa1−xAs/GaAs heterostructures with high-mobility two-dimensional electron gas for field effect transistors

In selectively dopedn-Al_xGa_1–xAs/GaAs heterostructures with high-mobility two-dimensional electron gas (2 DEG) at the heterointerface a second conductive channel exists, if the Al_xGa_1–xAs layer is not totally depleted from free carries. The occurrence of parallel conductance has a deleterious effect on the performance of high-electron mobility transistors (HEMTs) fabricated from this material. Although in principle computable, parallel conductance depends on a large number of design parameters to be chosen for the heterostructure, which are additionally affected by the presence of deep electron traps inn-Al_xGa_1–xAs of composition 0.25n-Al_xGa_1–xAs/GaAs heterostructures is shown.     

Systematic theoretical investigations for contribution of lattice constraint to novel atomic arrangements in alloy semiconductor thin films

The atomic arrangements in zinc blende structured GaN_xAs_1−x thin films coherently grown on V-grooved substrates are theoretically investigated using empirical interatomic potentials and Monte Carlo simulation. The resultant atomic arrangements in GaN_xAs_1−x strongly depend on concentration x and substrate lattice parameter a_sub. Surface segregation of As or N is mainly found in GaN_xAs_1−x with large lattice mismatch to the substrate. On the other hand, the novel atomic arrangements such as layered segregation or ordered structure are found in GaN_xAs_1−x at the specific region such as (x, a_sub) = (0.5, 5.3), (0.3, 5.3), and (0.3, 5.1). This specific region corresponds to that with negative excess energy and with sufficient N and As atoms remaining in thin film layers even after their surface segregation. The formation of the novel atomic arrangements is discussed in terms of bond lengths in the surface layers. These results suggest that various novel atomic arrangements in alloy semiconductor thin films appear depending on x and a_sub which control degree of lattice constraint.     

Photoluminescence of AlxGa1−xAs/GaAs quantum well heterostructures grown by molecular beam epitaxy

Low-temperature photoluminescence measurements on nominally undoped Al_xGa_1–xAs/GaAs quantum well heterostructures (QWHs) grown by molecular beam epitaxy (MBE) exemplified the exclusivelyintrinsic free-exciton nature of the luminescence under moderate excitation conditions. Neither any spectroscopic evidence for alloy clustering in the Al_xGa_1–xAs barriers nor any extrinsic luminescence due to recombination with residual acceptors has been detected in single and double QWHs when grown at 670 °C under optimized MBE growth conditions. Carrier confinement in Al_xGa_1–xAs/GaAs QWHs starts at a well width ofL _z30 nm when x0.25. The minor average well thickness fluctuation ofL _z=4×10^–2nm as determined from the excitonic halfwidth allowed the realization of well widths as low asL _z=1 nm and thus a shift of the free-exciton line as high as 2.01 eV which is close to the conduction band edge of the employed Al_0.43Ga_0.57As confinement layer. The measurements further revealed a strongly enhanced luminescence efficiency of the quantum wells as compared to bulk material which is caused by the modified exciton transition probabilities due to carrier localization.     

Buried (Fe1-xCox)Si2 layers with variable band gap formed by ion beam synthesis

Buried layers of (Fe_{1 –x} Co _x )Si₂ were prepared by sequent implantation of iron and cobalt into (100) silicon. The depth distributions of iron and cobalt and the atomic concentration ratio silicon/metal were determined by Auger Electron Spectroscopy (AES) and Rutherford Backscattering Spectrometry (RBS). The phase composition and the microstructure of the silicide layer were studied by X-ray diffraction and electron microscopy. The band gap energy was evaluated from Infrared (IR) reflection and transmission experiments. The semiconducting-FeSi₂ structure remains stable up to a cobalt fraction ofx = 0.2 if the iron silicide is stabilized by an intermediate annealing between iron and cobalt implantation. With increasing cobalt content, the electrical resistivity as well as the energy of the direct band gap of the (Fe_{1 –x} Co _x )Si₂ layer decrease. In this way, a band gap tuning between 0.84 and 0.70 eV is possible. The gap energy is found to vary quadratically with the Co content in the composition range 0 <x < 0.15.     

Analysis of the Ge1−xCx films deposited by MFMST

Ge_1−xC_x films deposited by using a medium frequency magnetron sputtering technique (MFMST) were analyzed with X-ray photoelectron and Raman spectroscopy. The deposited Ge_1−xC_x films consist of C, Ge, GeC and GeO_y. The GeC content in the Ge_1−xC_x films linearly decreases, and the C content linearly increases with increasing deposition temperature from 150 to 350 °C. The GeC content decreases from 11.6% at a substrate bias of 250 V to a lowest value of 9.6% at 350 V, then increases again to 10.4% at 450 V. While the C content increases from 49.0% at the bias of 250 V to a largest value of 58.0% at 350 V and then maintains this level at 450 V. It is found that selecting a bias parameter seems more effective than deposition temperature if we want to obtain a higher content of GeC in the deposited films. In addition, a new method is presented in this paper to estimate the changes of GeC content in the Ge_1−xC_x films by observing the shifts of Ge-Ge LO phonon peak in Raman spectra for the Ge_1−xC_x films. The related mechanism is also discussed in this paper.     

Hydrogen sulphide doping of GaAs and AlxGa1−xAs grown by molecular beam epitaxy (MBE)

H₂S gas has been used during molecular beam epitaxy (MBE) growth of GaAs and Al _x Ga_1–x As as sulphur vector forn-type doping. Doping efficiencies are less than 10^–3 at usual growth temperatures, and are limited by an incorporation competitive surface process, probably 2Ga+H₂SGa₂S+H₂. In Al_xGa_1–x As forx0.2 the doping efficiency is further reduced by carrier freeze-out at deep levels. Measured thermal activation energies depend on growth conditions and remain relatively low even up to the direct-indirect bandgap crossover for substrate temperatures in the 585–645 C range.     

Structural and optical properties of CuIn1−xAlxSe2 thin films prepared by four-source elemental evaporation

CuIn_1−xAl_xSe₂ (CIASe) thin films with x=0.25, 0.5 and 0.65 were prepared by four-source elemental evaporation. The structural and optical properties were investigated by X-ray diffraction, scanning electron microscopy, energy dispersive analysis, and optical transmission. The results showed that these films contain chalcopyrite structure with preferred orientation along (112) direction. The morphology, grain distribution and composition of CIASe films were studied and compared for different Al content. The optical studies revealed that the films were highly absorbing and the energy band gap calculated from transmission spectra for x=0.25, 0.5 and 0.65 were 1.2, 1.51 and 1.73 eV, respectively. The variation of Al content in the CIASe composition offered a very effective change in the optical band gap.     

Preparation and transport properties of Ba1−x Sr x CuO2+δ infinite layer films grown by molecular-beam epitaxy

Thin films of Ba_1–x Sr _x CuO₂₊ in the infinite layer structure were prepared by molecular beam epitaxy on SrTiO₃ substrates. Excellent in-plane order during growth was shown by RHEED. The lattice constant inc-direction was determined by x-ray diffraction. It changed from 0.404 nm to 0.345 nm whenx increased from 0 to 1. The film surfaces were smooth with some outgrowths as revealed by atomic force microscopy. Excellent crystal structure and epitaxy of the films was demonstrated by high resolution transmission electron microscopy. The room temperature dc resistivities of the films varied from 10^–3 cm to 10² cm, depending onx and on the oxidation conditions during growth. The resistivities of most films showed negative temperature coefficients and obeyed the conduction model of variable range hopping at low temperatures. In the composition rangex=0.5–0.8, however, an anomalous resistance dependence on temperature was observed in many samples. The resistivities started to deviate from the monotonic behaviour just below 200 K and in some cases dropped remarkably at temperatures below 140 K.     

A TEM study of crystal and domain structures of Nb-Doped 95/5 PZT ceramics

95/5 PZT ceramics with compositions of Pb(Zr_1–x Ti _x )₃+1 wt% Nb₂O₅ (x=0.025 and 0.04) are studied with TEM. The coexistence of both ferroelectric (F) and antiferroelectric (AF) phases in one ceramic grain at room temperature is confirmed for these two compositions. The AF phase is mainly tetragonal, while a pseudo-cubic arrangement of AF polarization is also possibly present. When the Ti⁴⁺ concentration is increased, the F phase becomes predominant. Direct interfaces between F and AF domains can be observed, the former expands at the expense of the latter when bombarded by 200 keV electrons.     

Low-temperature photoluminescence of MBE-grown GaAs subject to an electric field

Low-temperature photoluminescence of GaAs has been investigated in MBE-grown Al _x Ga_1–x As-GaAs single heterojunctions subject to an electric field. No peak energy shift is observed in the emission lines due to free excitons and excitons bound to isolated centers when the electric field is applied. In contrast, the excitonic lines arising from the previously described defect-induced bound exciton (DIBX) transitions exhibit a prominent low-energy shift when the electric field is increased. We attribute these lines to excitons bound to acceptor pairs. The excitons bound to distant pairs have smaller binding energies than those bound to closer pairs. They are, therefore, easily dissociated in a weak electric field. The electrons and holes thus dissociated may again be trapped by closer pairs, which results in a low-energy shift of the overall spectrum. The photocurrent measured as a function of the electric field supports Dingle's rule for the valence bandedge discontinuity.     

Dynamics of film growth of GaAs by MBE from Rheed observations

Detailed observations have been made of the intensity oscillations in the specularly reflected and various diffracted beams in the RHEED pattern during MBE growth of GaAs, Ga _x Al_1–x As and Ge. The results indicate that growth occurs predominantly in a two-dimensional layer-by-layer mode, but there is some roughening, which is enhanced by deviations from stoichiometry and the presence of impurities. In the case of the GaAs (001) –2×4 reconstructed surface a combination of dynamic and static RHEED measurements has provided firm evidence for the presence of one-dimensional disorder features as well as surface steps.