Determination of the surface band bending in InxGa1−xN films by hard x-ray photoemission spectroscopy

Core-level and valence band spectra of In_xGa_1−xN films were measured using hard x-ray photoemission spectroscopy (HX-PES). Fine structure, caused by the coupling of the localized Ga 3d and In 4d with N 2s states, was experimentally observed in the films. Because of the large detection depth of HX-PES (∼20 nm), the spectra contain both surface and bulk information due to the surface band bending. The In_xGa_1−xN films (x = 0–0.21) exhibited upward surface band bending, and the valence band maximum was shifted to lower binding energy when the mole fraction of InN was increased. On the other hand, downward surface band bending was confirmed for an InN film with low carrier density despite its n-type conduction. Although the Fermi level (E_F) near the surface of the InN film was detected inside the conduction band as reported previously, it can be concluded that E_F in the bulk of the film must be located in the band gap below the conduction band minimum.     

A new method for determining the sharpness of InGaAs/GaAs heterojunctions by auger depth profiling

It is established for the first time that the phenomenon of ion-stimulated surface segregation can be used to increase the depth resolution of Auger profiling during analysis of the In_x Ga_{1 − x} As/GaAs heterostructures. It is demonstrated that, by varying the energy of the sputtering Ar⁺ ion beam from 1 to 0.5 keV in the region of the GaAs/InGaAs heterojunction, the junction sharpness can be estimated at a resolution on the order of 0.5 nm determined by a difference in the projected range of Ar⁺ ions and independent of the escape depth of the Auger electrons.     

Optical properties of nanocrystalline Ga1−xInxSb/SiO2 films

Ga_1−xIn_xSb (x=0.19, 0.38, 0.63) nanoparticles embedded in a SiO₂ matrix were grown on the glass substrates by radio-frequency magnetron co-sputtering. X-ray diffraction patterns strongly support the existence of nanocrystalline Ga_1−xIn_xSb in the SiO₂ matrix. The changes in binding energies with Ga_1−xIn_xSb nanocrystals deposition have been directly observed by X-ray photoemission spectroscopy, and these show the existence of Ga_1−xIn_xSb nanocrystals in the SiO₂ matrix. Room-temperature Raman spectra show that the Raman peaks of the Ga_1−xIn_xSb-SiO₂ composite film have a larger red-shift of about 95.3 cm⁻¹ (longitudinal-optical mode) and 120.1 cm⁻¹ (transverse-optical mode) than that of bulk GaSb, suggesting the existence of phonon confinement and tensile stress effects. Additionally, the room-temperature optical transmission data exhibit a large blue-shift with respect to that of the bulk semiconductor due to the strong quantum confinement effect.     

Experimental determination of the valence band structure of the InxGa1-x-As(001) surface

In_xGaj_1-xAs layers were grown on GaAs(00l) substrates by molecular beam epitaxy. The structure and quality of the surface and the layer thickness were monitored in situ by dynamical reflection high-energy electron diffraction (RHEED oscillations). Angle-resolved photoemission spectroscopy studies using He-I radiation were performed on the epitaxial layers. The energy-distribution curves of the photoelectrons were determined along the high symmetric directions in the surface Brillouin zone. The experimental band structure of In_xGa_1-xAs was determined with the help of the experimental and theoretical band structures of GaAs and In As. This article was submitted by the author in English.     

Ion sputtering rates of C, CrxCy, and Cr at different Ar ion incidence angles

To study the ion sputtering of a layered structure with different layer densities and ion sputtering yields a trilayer structure of C-graphite(46 nm)/Cr_xC_y(60 nm)/Cr(69 nm) was sputter deposited onto smooth silicon substrates. The ion sputtering rates of amorphous carbon, amorphous Cr_xC_y and polycrystalline Cr were determined by means of Auger electron spectroscopy depth profiling as a function of the angle of incidence of two symmetrically inclined 1 keV Ar⁺ ion beams in the range between 22° and 82°. The sputtering rates were calculated from the known thicknesses of the layers and the sputtering times necessary to remove the individual layers. It was found that the sputtering rates of C-graphite, Cr_xC_y carbide and Cr were strongly angle dependent. The experimental sputtering yields were in agreement with the theoretical results obtained by calculation of the transport of ions in solids, but the sputtering yields of C-graphite measured at ion incidence angles larger than 29° were smaller than the simulated ones.     

Specific features of Hall effect in capped multilayer pHEMT structure of AlxGa1 − xAs/InxGa1−xAs/GaAs

Electrical properties of capped multilayer pHEMT (pseudomorphic High Electron Mobility Transistor) structure of Al_xGa_1−xAs/In_xGa_1−xAs/GaAs have been studied using a Hall effect method. In the device structure In_xGa_1−xAs layer was prepared as a channel layer and Al_xGa_1−xAs as the supply and spacer layers. Mole fraction, x, for the alloys varied from 19% to 20% (for In_xGa_1−xAs) and 20% to 24% (for Al_xGa_1−xAs). From the measurement carried out at room temperature, it was found that increasing In content in the channel layer will be followed by decreasing electron mobility of multilayer properties of pHEMT structure. This phenomenon is found to be in contradiction with the calculation for the alloy concerned. This phenomenon supports the consideration that electrical properties of supply and spacer layer affect the channel layer properties. From the Hall measurement, relationships between Al content in the supply and spacer with respect to the electron mobility and sheet resistance were obtained.     

Quantum-confinement effect on recombination dynamics and carrier localization in cubic InN and InxGa1 − xN quantum boxes

In this study, the quantum confinement effect on recombination dynamics and carrier localization in cubic InN (c-InN) and cubic In_xGa_1 − xN (c-In_xGa_1 − xN) low dimensional structures are theoretically examined. The small InN and In-rich In_xGa_1 − xN low dimensional structures show a strong quantum confinement effect, which results in ground states away from the band edge and discrete eigen-states. Depending on composition, temperature, and size of the InN and In_xGa_1 − xN low dimensional structures, quantum confinement effect can affect the exciton dimensions (D). In InN quantum cubes, the strong quantum confinement effect leads to temperature-dependent radiative lifetimes showing a large size variation. The nearly-temperature-independent and shorter radiative lifetimes in small InN and In-rich In_xGa_1 − xN low dimensional structures suggest that the strong quantum confinement leads to 0 D carrier confinement, stronger carrier localization, and high recombination efficiency. Reported radiative lifetimes were found to match very well with our simulation results of In-rich quantum cubes, which indicates that spontaneous emissions come from the radiative recombination of localized excitons in In-rich In_xGa_1 − xN clusters. The simulation results could provide important information for the designs and interpretations of c-InN and c-In_xGa_1 − xN devices.     

Effect of GaAs substrate misorientation on InxGa1 − xAs crystalline quality and photovoltaic performance

This paper presents the quality of In_xGa_1 − xAs (0 < x < 0.2) layers grown on GaAs substrate with different miscut angle (2° and 15°) by metal organic chemical vapor deposition. The crystalline quality of In_xGa_1 − xAs layers was found to strongly depend on indium content and substrate misorientation. The In_0.16Ga_0.84As solar cells with PN structure were grown on a 2°- and 15°-off GaAs substrates. It was found that the photovoltaic performance of In_0.16Ga_0.84As solar cell grown on 2°-off GaAs substrate was better than that of In_0.16Ga_0.84As grown on a 15°-off GaAs substrate, because the In_xGa_1 − xAs films grown on 15°-off GaAs substrate shows a highly strain relaxation in active layer of solar cell, which causes the high dislocation density at the initial active layer/In_xGa_1 − xAs graded layer interface.     

Potential barrier study at the InGaAs/InAs quantum dot interface in asymmetric multi quantum well structures

The photoluminescence and its temperature dependences have been investigated for the InAs quantum dots embedded in the asymmetric GaAs/In_xGa_1−xAs/In_0.15Ga_0.85As/GaAs quantum wells (dot-in-a-well, DWELL structures) as a function of the In content x (x = 0.10-0.25) in the capping In_xGa_1−xAs layer. The study of PL temperature dependences in the range of 80-120 K has revealed the potential barrier for electrons at the capping In_xGa_1−xAs/InAs QD interface. The value of mentioned barrier has varied in QD structures with the different In_xGa_1−xAs capping layer compositions and it was estimated in the studied asymmetric GaAs/In_xGa_1−xAs/In_0.15Ga_0.85As/GaAs DWELL structures. It is shown that the barrier for electrons equal to 48 and 24 meV appears in the InGaAs conduction band at the formation of InGaAs QW/InAs QD interface for the In compositions of x = 0.10 and 0.15, respectively. This barrier has not been detected in DWELLs with the In compositions x = 0.20 and 0.25. The energy gap offsets at the InAs/In_xGa_1−xAs interface in studied structures has been estimated and discussed as well.     

Structural properties of GaN and related alloys grown by radio-frequency magnetron sputter epitaxy

Single-crystalline layers of GaN and related alloys such as AlGaN and InGaN were grown on Al₂O₃ (0001) substrates by radio-frequency magnetron sputter epitaxy. The crystalline structures of these layers were studied as functions of substrate temperature, N₂ composition ratio in N₂/Ar mixture source gas and gas pressure during the growth. Surface structure of GaN layer depended on Ga/N ratio in flux density, and nitrogen-rich growth condition resulted in pyramid-type facet structure whereas Ga-rich growth produced flat surface. The crystalline quality of GaN layer improved at relatively low N₂ composition ratios, and the GaN layer grown at 30% N₂ condition was transparent and colorless. Al_xGa_1−xN layers with x = 0.06-0.08 and In_xGa_1−xN layers with x = 0.45-0.5, were obtained at 30-40% and 30-50% N₂ composition ratios, respectively. The AlN and InN molar fractions in these layers were considerably different from Al and In molar fractions in starting metal alloys (x = 0.15 in both Al_xGa_1−x and In_xGa_1−x alloys).     

Crystalline quality and photovoltaic performance of InGaAs solar cells grown on GaAs substrate with large-misoriented angle

This article reports the quality of In_xGa_1−xAs (0 < x < 0.2) layers grown on 15°-off GaAs substrate by metalorganic chemical vapor deposition. The crystalline quality of the In_xGa_1−xAs epilayers is determined by x-ray reciprocal space mapping (RSM). From the RSM results, the crystalline quality of In_xGa_1−xAs epilayers grown with small indium composition (x < 0.11) is better than that of large indium composition (x > 0.11) due to the small strain relaxation. The crystalline quality of In_xGa_1−xAs epilayer is found to strongly depend on indium content. The photovoltaic performance of p-n structure In_0.16Ga_0.84As solar cell shows the lower device performance, because the In_xGa_1−xAs films grown on 15°-off GaAs substrate show a large strain relaxation in the active layer of solar cell. It results in dislocation defects created at the initial active layer/In_xGa_1−xAs graded layer interface. The performance of In_0.16Ga_0.84As solar cell with p-n structure can be significantly improved by the p-i-n structure.     

Unusual changes observed in the photoluminescence of annealed InxGa1 − xN/GaN quantum wells explained

In_xGa_1 − xN/GaN heterostructures and quantum wells (QWs) are particularly important in the application of III-V nitride materials for light emitting diodes and laser diodes. The photoluminescence (PL) emissions from In_xGa_1 − xN/GaN QW structures have been reported, where, for successive annealing operations, the PL peak suffers a primary red shift, followed by a blue shift. The observed phenomenon remains unexplained because of its complexity. This paper is intended towards a proper explanation of the observed experimental results through suitable quantum mechanical models and computations, whether the band gap of InN is 1.95 eV or 0.7 eV.     

In-situ analysis of positive and negative energetic ions generated during Sn-doped In2O3 deposition by reactive sputtering

Using a quadrupole mass spectrometer combined with an energy analyser, we have investigated the in-situ energy distribution of highly energetic ions generated during reactive sputtering of In-Sn alloy (IT) targets and non-reactive sputtering of Sn-doped In₂O₃ (ITO) ceramic targets. Ar⁺, In⁺, O⁺, O⁻, O₂⁻, InO⁻ and InO₂⁻ ions with kinetic energies greater than 40 eV were clearly observed. Upon increasing the O₂ flow ratio for reactive sputtering, the surface of the IT target changes from metal (metal mode) to oxide (oxide mode) via a state of mixed metal and oxide (transition region). O⁻ ions with the kinetic energy corresponding to cathode voltage are generated at the oxide layer, which expands upon the target surface with increasing O₂ flow ratio in the metal mode and the transition region. In contrast, the flux of 60-eV Ar⁺ ions decreases with increasing O₂ flow ratio. The presence of 125- and 200-eV In⁺ ions is attributed to the dissociation of InSnO₂⁻ and InO₂⁻ with the kinetic energy corresponding to cathode voltage, respectively, while the presence of 40- and 150-eV O⁺ ions is attributed to the dissociation of InO₂⁻ and O₂⁻ with the kinetic energy corresponding to cathode voltage, respectively.     

Performance study of InAs/GaAs quantum dot covered by graded InxGa1 − xAs layer

InAs/GaAs quantum dots (QDs) with graded In_xGa_1 − xAs strained-reducing layer (SRL) are grown by metal-organic chemical vapor deposition, the effects of Indium (In) composition and thickness in In_xGa_1 − xAs on QD morphological characteristics and optical properties are investigated. Compared with In_xGa_1 − xAs SRL with fixed In content, gradient In_xGa_1−xAs SRL can further improve the growth quality of InAs QDs, enhance luminescence intensity and extend emission spectrum toward longer wavelength.     

Electronic structure of amorphous InGaO3(ZnO)0.5 thin films

The electronic structure of amorphous semiconductor InGaO₃(ZnO)_0.5 thin films, which were deposited by radio-frequency magnetron sputtering process, was investigated using X-ray photoelectron spectroscopy and O K-edge X-ray absorption spectroscopy. The overall features of the valence and conduction bands were analyzed by comparing with the spectra of Ga₂O₃, In₂O₃, and ZnO films. The valence and conduction band edges are mainly composed of O 2p and In 5sp states, respectively. The bandgap of the films determined by spectroscopic ellipsometry was approximately 3.2 eV. Further, it is found that the introduction of oxygen gas during the sputter-deposition does not induce significant variations in the chemical states and band structure.     

The effect of growth temperature on the coaxial InxGa1 − xN/GaN nanowires grown by metalorganic chemical vapor deposition

We report on the growth of coaxial In_xGa_1 − xN/GaN nanowires (NWs) on Si(111) substrates by using pulsed flow metalorganic chemical vapor deposition. The coaxial In_xGa_1 − xN/GaN NWs were grown by a two step process in which the core (GaN) structure was grown at a higher temperature followed by the shell (In_xGa_1 − xN) structure at a lower temperature. Dense and well-oriented coaxial In_xGa_1 − xN/GaN NWs were grown with an average diameter and length of about 300 ± 50 nm and 1.5-2.0 μm, respectively. The coaxial In_xGa_1 − xN/GaN NW was confirmed by cathodoluminescence mapping and high-resolution transmission electron microscopy. It is proposed that the critical dissociation of precursors at an elevated growth temperature can lead to a clear formation of an outer-shell in coaxial In_xGa_1 − xN/GaN NWs.     

Non-toxic selenization using thermal annealing for CuGa/In bi-layer precursors deposited by sputtering

Cu(In_xGa_1?x)Se₂ (CIGS) thin films were produced using a two-step sputtering process consisting of precursor formation and selenization. In the first stage, we prepared Cu_0.75Ga_0.25/In bi-layer precursors by direct current sputtering on Mo/soda-lime glass substrates. In the second stage, the stacked precursors were selenized using non-toxic Se pellets in a graphite box in which the temperature was controlled at 475–680 °C during rapid thermal annealing. We investigated the effect of thermal annealing temperature on Ga distribution and the crystallinity of the Cu(In_xGa_1?x)Se₂ films. Thermal annealing significantly affected the distribution of Ga atoms. At low temperatures, segregation of Ga atoms into the CIGS/Mo interface and an absence of Ga content on the surface were observed. In addition, a phase-separated CuInSe₂/CuGaSe₂ structure and incomplete selenization phases were observed. At high temperatures, CIGS films were formed with the proper distribution of Ga content.     

Epitaxy of In0.01Ga0.99As on Ge/offcut Si (001) virtual substrate

In_0.01Ga_0.99As thin films free of anti-phase domains were grown on 7° offcut Si (001) substrates using Ge as buffer layers. The Ge layers were grown by ultrahigh vacuum chemical vapor deposition using ‘low/high temperature’ two-step strategy, while the In_0.01Ga_0.99As layers were grown by metal-organic chemical vapor deposition. The etch-pit counting, cross-section and plane-view transmission electron microscopy, room temperature photoluminescence measurements are performed to study the dependence of In_0.01Ga_0.99As quality on the thickness of Ge buffer. The threading dislocation density of Ge layer was found to be inversely proportional to the square root of its thickness. The threading dislocation density of In_0.01Ga_0.99As on 300 nm thick Ge/offcut Si was about 4 × 10⁸ cm^− 2. Higher quality In_0.01Ga_0.99As can be obtained on thicker Ge/offcut Si virtual substrate. We found that the threading dislocations acted as non-radiative recombination centers and deteriorated the luminescence of In_0.01Ga_0.99As remarkably. Secondary ion mass spectrometry measurement indicated as low as 10¹⁶ cm^− 3 Ge unintended doping in In_0.01Ga_0.99As.     

Electron energy loss spectroscopy study of the effect of low-energy Ar-ion bombardment on the surface structure and composition of Pt80Co20 (1 1 1) alloy

Electron energy loss spectroscopy has been employed for investigation of the effect of 600 eV Ar⁺-ion irradiation in the dose range 7×10¹⁶-4×10¹⁷ ions/cm² on the atomic structure and surface composition of alloy Pt₈₀Co₂₀(1 1 1). A method of the layer-by-layer reconstruction of the lattice interplanar distance changes based on the analysis of the plasmon spectra excitation was proposed. The ion bombardment was shown to result in a non-monotonic variation of the lattice interplanar distance due to formation of the stable defects, with the topmost layer being in the state of compression. Using the ionization energy loss spectra, a layer-by-layer concentration profile of the alloy components was reconstructed for different doses of ion irradiation of the surface. The Ar⁺-ion bombardment of the alloy was found to result in the preferential sputtering of Co and in the enrichment of the near-surface region by Pt atoms with formation of an altered layer, which is characterized by a non-monotonic concentration profile dependent on the irradiation dose. The results obtained are discussed in the framework of the models of preferential sputtering and radiation-induced segregation.     

Study on growth and characterizations of GaxIn1−xP/GaAs solar cell structure

Ga_xIn_1?xP/GaAs solar cell (SC) structure was grown on p-type (100)-oriented GaAs substrate by using solid-source molecular beam epitaxy technique. The structural, optical and morphological properties of Ga_xIn_1?xP/GaAs SC structure have been evaluated by means of high resolution X-ray diffraction, photoluminescence, spectroscopic ellipsometry and atomic force microscopy measurements at room temperature. In addition, Ga_xIn_1?xP/GaAs SC structure was fabricated for obtaining the cell’s electrical output parameters. For this purpose, the current–voltage characteristics of Ga_xIn_1?xP/GaAs SC structure were performed and analyzed at the room temperature under both dark and illuminations by using air mass global 1.5 (AM1.5) solar simulator. The device parameters such as the open-circuit voltage, the short-circuit current (I _sc ), the fill factor and the energy convertion efficiency (η) of Ga_xIn_1?xP/GaAs SC structure were extracted from the current–voltage characteristics.