Low temperature formation of Si1 − x − yGexSny-on-insulator structures by using solid-phase mixing of Ge1 − zSnz/Si-on-insulator substrates

We proposed the low temperature formation technique of strain-relaxed Si_{1 − x − y}Ge_xSn_y-on-insulator (SGTOI) structures. We found that the solid-phase reaction and the formation of single and uniform Si_{1 − x − y}Ge_xSn_y layer on an insulator after annealing SiO₂/Ge_1 − zSn_z/SOI structures even at a temperature as low as 400 °C. We characterized the crystalline structure of SGTOI, and investigated the effects of annealing, Sn incorporation, and a SiO₂ cap layer on the solid-phase reaction between Ge_1 − zSn_z and SOI layers. The solid-phase reaction is enhanced with a higher Sn content and a thicker SiO₂ cap layer, and then Si_{1 − x − y}Ge_xSn_y layers are more rapidly formed. The SGTOI layer exhibits very low mosaicity and have good crystallinity.     

Composition dependent thermoelectric properties of sintered Mg2Si1−xGex (x = 0 to 1) initiated from a melt-grown polycrystalline source

Fabrication of Mg₂Si_1−xGe_x (x = 0-1.0) was carried out using a spark plasma sintering technique initiated from melt-grown polycrystalline Mg₂Si_1−xGe_x powder. The thermoelectric properties were evaluated from RT to 873 K. The power factor of Mg₂Si_1−xGe_x with higher Ge content (x = 0.6-1.0) tends to decrease at higher temperatures, and the maximum value of about 2.2 × 10^− 5 Wcm^− 1K^− 2 was observed at 420 K for Mg₂Si and Mg₂Si_0.6Ge_0.4. The coexistence of Si and Ge gave rise to a decrease in the thermal conductivity in the Mg₂Si_1−xGe_x. The values close to 0.02 Wcm^− 1K^− 1 were obtained for Mg₂Si_1−xGe_x (x = 0.4-0.6) over the temperature range from 573 to 773 K, with the minimum value being about 0.018 Wcm^− 1K^− 1 at 773 K for Mg₂Si_0.4Ge_0.6. The maximum dimensionless figure of merit was estimated to be 0.67 at 750 K for samples of Mg₂Si_0.6Ge_0.4.     

Behavior of N atoms after thermal nitridation of Si1 − xGex surface

Behavior of N atoms after thermal nitridation of Si_1 − xGe_x (100) surface in NH₃ atmosphere at 400 °C was investigated. X-ray photoelectron spectroscopy (XPS) results show that N atomic amount after nitridation tends to increase with increasing Ge fraction, and amount of N atoms bonded with Ge atoms decreases by heat treatment in H₂ at 400 °C. For nitrided Si_0.3Ge_0.7(100), the bonding between N and Si atoms forms Si₃N₄ structure whose amount is larger than that for nitrided Si(100). Angle-resolved XPS measurements show that there are N atoms not only at the outermost surface but also beneath surface especially in a deeper region around a few atomic layers for the nitrided Si(100), Si_0.3Ge_0.7(100) and Ge(100). From these results, it is suggested that penetration of N atoms through around a few atomic layers for Si, Si_0.3Ge_0.7 and Ge occurs during nitridation, and the N atoms for the nitrided Si_0.3Ge_0.7(100) dominantly form a Si₃N₄ structure which stably remains even during heat treatment in H₂ at 400 °C.     

Forbidden energy band gap in diluted a-Ge1 − xSix:N films

By means of electron gun evaporation Ge_1 − xSi_x:N thin films, in the entire range 0 ≤ x ≤ 1, were prepared on Si (100) and glass substrates. The initial vacuum reached was 6.6 × 10^− 4 Pa, then a pressure of 2.7 × 10^− 2 Pa of high purity N₂ was introduced into the chamber. The deposition time was 4 min. Crucible-substrate distance was 18 cm. X-ray diffraction patterns indicate that all the films were amorphous (a-Ge_1 − xSi_x:N). The nitrogen concentration was of the order of 1 at% for all the films. From optical absorption spectra data and by using the Tauc method the energy band gap (E_g) was calculated. The Raman spectra only reveal the presence of SiSi, GeGe, and SiGe bonds. Nevertheless, infrared spectra demonstrate the existence of SiN and GeN bonds. The forbidden energy band gap (E_g) as a function of x in the entire range 0 ≤ x ≤ 1 shows two well defined regions: 0 ≤ x ≤ 0.67 and 0.67 ≤ x ≤ 1, due to two different behaviors of the band gap, where for x > 0.67 exists an abruptly change of E_g(x). In this case E_g(x) versus x is different to the variation of E_g in a-Ge_1 − xSi_x and a-Ge_1 − xSi_x:H. This fact can be related to the formation of Ge₃N₄ and GeSi₂N₄ when x ≤ 0.67, and to the formation of Si₃N₄ and GeSi₂N₄ for 0.67 ≤ x.     

Structural study of Si1 − xGex nanocrystals embedded in SiO2 films

We have investigated the structural properties of Si_1 − xGe_x nanocrystals formed in an amorphous SiO₂ matrix by magnetron sputtering deposition. The influence of deposition parameters on nanocrystal size, shape, arrangement and internal structure was examined by X-ray diffraction, Raman spectroscopy, grazing incidence small angle X-ray scattering, and high resolution transmission electron microscopy. We found conditions for the formation of spherical Si_1 − xGe_x nanocrystals with average sizes between 3 and 13 nm, uniformly distributed in the matrix. In addition we have shown the influence of deposition parameters on average nanocrystal size and Ge content x.     

Enhancing epitaxial SixC1 − x deposition by adding Ge

In this work, a possible way to enhance the epitaxial growth of metastable, tensile strained Si_xC_1 − x layers by the addition of germanium is demonstrated. During ultra-high vacuum chemical vapor deposition growth, the co-mixing of germane to the Si_xC_1 − x precursors was found to enhance the growth rate by a factor of ~ 3 compared to the growth of pure Si_xC_1 − x. Furthermore, an increase of the amount of substitutional incorporated carbon has been observed. Selective Si_xGe_yC_{1 − x − y} deposition processes utilizing a cyclic deposition were developed to integrate epitaxial tensile strained layers into source and drain areas of n-channel transistors.     

Growth of Ge1 − xSnx heteroepitaxial layers with very high Sn contents on InP(001) substrates

We investigated the crystalline structures of Ge_1 − xSn_x heteroepitaxial layers with Sn contents greater than 20% grown on InP and Ge substrates. Considering the lattice mismatch between the Ge_1 − xSn_x layers and the substrates, we achieved epitaxial growth of Ge_1−xSn_x layers with very high Sn content by suppressing the Sn precipitation; in addition, we improved upon the crystalline quality of Ge_1 − xSn_x heteroepitaxial layers. As a result, we could successfully form a 130 nm-thick Ge_1 − xSn_x heteroepitaxial layer on an InP substrate with a Sn content as high as 27% without Sn precipitation. We also improved the crystalline quality of Ge_1 − xSn_x layers by annealing at a temperature as low as 290 °C.     

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.     

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.     

Low temperature (~ 250 °C) layer exchange crystallization of Si1 − xGex (x = 1-0) on insulator for advanced flexible devices

Low-temperature (~ 250 °C) layer exchange crystallization of poly-Si_1 − xGe_x (x = 1-0) films on insulators has been investigated for realization of advanced flexible devices. We propose utilization of Au as catalyst to enhance the crystallization at low temperatures. By annealing (~ 250 °C, 20 h) of the a-Si_1 − xGe_x (x = 1-0)/Au stacked structures formed on insulating substrates, the SiGe and Au layers exchange their positions, and Au/poly-SiGe stacked structures are obtained. The Ge fractions of the obtained poly-SiGe layers are identical to that of the initial a-SiGe layers, and there is no Si or Ge segregation. This low temperature crystallization technique enables poly-SiGe films on plastic substrates, which are essential to realize advanced flexible devices.     

Low-temperature growth of Ge1 − xSnx thin films with strain control by molecular beam epitaxy

High-quality Ge_1 − xSn_x thin films on InGaAs buffer layers have been demonstrated using low-temperature growth by molecular beam epitaxy. X-ray diffraction and secondary ion mass spectrometry are used to determine the strain and Sn concentration. Up to 10.5% Sn has been incorporated into the Ge_1 − xSn_x thin film without Sn precipitation, as verified by transmission electron microscopy. Roughened surfaces are found for tensile strained Ge_1 − xSn_x layers.     

Basic properties of Sr1 − xBaxSi2

Basic properties, such as the phase relationship, crystal structure, and energy gap E_g, have been investigated in Sr-rich Sr_1 − xBa_xSi₂. Sr_1 − xBa_xSi₂ (0 ≤ x ≤ 1.0) has two phases: one with the SrSi₂-type structure and another with the BaSi₂-type structure. The SrSi₂ phase exists at x ranging from 0 to 0.13, and the BaSi₂ phase exists at x ranging from 0.24 to 1.0. The volume increases with x in both the SrSi₂ and BaSi₂ phases. A volume jump of 13.7% appears at the structural phase transition from the SrSi₂ phase to the BaSi₂ phase. E_g increases with x in SrSi₂-phase Sr_1 − xBa_xSi₂ but E_g decreases with x in the BaSi₂-phase Sr_1 − xBa_xSi₂. In Sr-rich BaSi₂-phase Sr_1 − xBa_xSi₂, Ba atoms at a specific crystallographic site, the A1 site, are preferentially substituted by Sr atoms, as well as in Ba-rich BaSi₂-phase Sr_1 − xBa_xSi₂.     

Si1 − xGex metal-oxide-semiconductor capacitors with HfTaOx gate dielectrics

Interfacial reactions and electrical properties of RF sputter deposited HfTaO_x high-k gate dielectric films on Si_1 − xGe_x (x = 19%) are investigated. X-ray photoelectron spectroscopic analyses indicate an interfacial layer containing GeO_x, Hf silicate, SiO_x (layer of Hf-Si-Ge-O) formation during deposition of HfTaO_x. No evidence of Ta-silicate or Ta incorporation was found at the interface. The crystallization temperature of HfTaO_x film is found to increase significantly after annealing beyond 500 °C (for 5 min) along with the incorporation of Ta. HfTaO_x films (with 18% Ta) remain amorphous up to about 500 °C anneal. Electrical characterization of post deposition annealed (in oxygen at 600 °C) samples showed; capacitance equivalent thickness of ~ 4.3-5.7 nm, hysteresis of 0.5-0.8 V, and interface state density = 1.2-3.8 × 10¹² cm^− 2 eV^− 1. The valence and conduction band offsets were determined from X-ray photoelectron spectroscopy spectra after careful analyses of the experimental data and removal of binding energy shift induced by differential charging phenomena occurring during X-ray photoelectron spectroscopic measurements. The valence and conduction band offsets were found to be 2.45 ± 0.05 and 2.31 ± 0.05 eV, respectively, and a band gap of 5.8 ± 05 eV was found for annealed samples.     

Study on the optical properties of Zn1−xMgxS (0 ≤ x ≤ 0.55) quantum dots prepared by precipitation method

Zn_1−xMg_xS (0 ≤ x ≤ 0.55) quantum dots (QDs) were successfully synthesized by precipitation method. The crystal structures, microstructures, and optical properties of the Zn_1−xMg_xS QDs were investigated using X-ray diffraction, scanning electron microscopy, and ultraviolet-visible and photoluminescence (PL) spectroscopy. The Zn_1−xMg_xS QDs were found to have a cubic crystal structure and an average crystallite size of 6.40-7.96 nm. It has been shown that an increase in doping Mg²⁺ concentration in Zn_1−xMg_xS QDs led to a gradual widening of the band gap and a weakening in the PL intensity of the Zn_1−xMg_xS QDs.     

Towards an electronic model for CuIn1 − xGaxSe2 solar cells

We model some aspects of highly efficient CuIn_1 − xGa_xSe₂ solar cells with x ≈ 0.3 as well as wide band gap cells with x = 1 and ask for the dominant recombination mechanism which limits the V_oc of these devices. For CuIn_1 − xGa_xSe₂ solar cells with x ≈ 0.3, interface recombination combined with Fermi-level pinning is a possible but unlikely recombination mechanism. We argue that these cells are rather limited by recombination in the quasi-neutral region (QNR) including the back contact. Using the expression for the QNR recombination rate we calculate the derivative of the collection function in the absorber at the space charge region edge which is in reasonable agreement with the experiment. It turns out that the diffusion length must approximate the absorber thickness. Based on this information, we draw a band diagram for a CuIn_1 − xGa_xSe₂ solar cells with x ≈ 0.3 and plot the simulated collection function. For cells with x = 1 (Cu-poor CuGaSe₂), the experimental activation energy of the recombination rate mostly equals the absorber band gap, i.e. E_a ≈ E_g,a = 1.67 eV. As the experimental interface band gap is smaller than E_a, interface recombination must be ruled out. Thus, the carrier lifetime in the Cu-poor CuGaSe₂ absorber should be so small that bulk recombination is more efficient than interface recombination. From this consideration, we postulate an electron lifetime value of 10⁻¹² s for CuGaSe₂.     

Negative thermal expansion and electrical properties of Mn3(Cu0.6NbxGe0.4 − x)N (x = 0.05-0.25) compounds

Bulk materials with the general formula of Mn₃(Cu_0.6Nb_xGe_0.4 − x)N (x = 0.05, 0.1, 0.15, 0.2, 0.25), Mn₃(Cu_0.6Ge_0.4)N and Mn₃(Cu_0.7Ge_0.3)N were fabricated by mechanical ball milling and solid state sintering. Their thermal expansion coefficients and electrical conductivities were investigated in the temperature range of 80-300 K. It is found that the temperature interval of negative temperature expansion behavior is about 95 K in the samples of Mn₃(Cu_0.6Nb_0.15Ge_0.25)N and Mn₃(Cu_0.6 Nb_0.2Ge_0.2)N, which is twice as large as that of Mn₃(Cu_0.7Ge_0.3)N. The negative thermal expansion of Mn₃(Cu_0.6Nb_0.15Ge_0.25)N can reach to − 19.5 × 10⁻⁶ K^− 1 in the temperature range of 165 to 210 K. The electrical conductivity of this series materials is in a level of about 2.5 × 10⁶ (Ω m)^− 1.     

Photoluminescence and Raman study of Cu2ZnSn(SexS1 − x)4 monograins for photovoltaic applications

The quaternary semiconductors Cu₂ZnSnSe₄ and Cu₂ZnSnS₄ have attracted a lot of attention as possible absorber materials for solar cells due to their direct bandgap and high absorption coefficient (> 10⁴ cm⁻¹). In this study we investigate the optical properties of Cu₂ZnSn(Se_xS_1 − x)₄ monograin powders that were synthesized from binary compounds in the liquid phase of potassium iodide (KI) flux materials in evacuated quartz ampoules. Radiative recombination processes in Cu₂ZnSn(Se_xS_1 − x)₄ monograins were studied by using low-temperature photoluminescence (PL) spectroscopy. A continuous shift from 1.3 eV to 0.95 eV of the PL emission peak position with increasing Se concentration was observed indicating the narrowing of the bandgap of the solid solutions. Recombination mechanisms responsible for the PL emission are discussed. Vibrational properties of Cu₂ZnSn(Se_xS_1 − x)₄ monograins were studied by using micro-Raman spectroscopy. The frequencies of the optical modes in the given materials were detected and the bimodal behaviour of the A₁ Raman modes of Cu₂ZnSnSe₄ and Cu₂ZnSnS₄ is established.     

Characterization of MgxZn1 − xO thin films grown on sapphire substrates by metalorganic chemical vapor deposition

Wurtzite-structure Mg_xZn_1 − xO materials with five different compositions of x from 0 to 0.14 were grown on sapphire substrates by metalorganic chemical vapor deposition. It was found that increasing Mg content in the Mg_xZn_1 − xO not only increased the band gap energy of the film but was also beneficial to the epitaxial growth of p-type Mg_xZn_1 − xO without using any doping sources. In addition, the combined ultraviolet photoluminescence (PL) and Raman scattering spectra were measured with PL-Raman signals obtained together, showing a blue-shift of PL band and variation of resonant Raman multi-order longitudinal optical phonon modes with an increase of Mg content.     

Study of GeYSi1 − Y:H films deposited by low frequency plasma

In this work, we report a study of the optical properties measured through spectral transmission and spectroscopic ellipsometry in Ge:H and Ge_YSi_1 − Y:H (Y ≈ 0.97) films deposited by low frequency (LF) PE CVD with hydrogen (H) dilution. The dilution was varied in the range of R = 20 to 80. It was observed that H-dilution influences in a different way on the interface and bulk optical properties, which also depend on incorporation of silicon. The films with low band tail characterized by its Urbach energy, E_U, and defect absorption, α_D, have been obtained in Ge:H films for R = 50 with E_U = 0.040 eV and α_D = 2 × 10³ cm^− 1 (hν ≈ 1.04 eV), and in Ge_YSi_1 − Y:H films for R=75 with E_U = 0.030 eV and α_D = 5 × 10² cm^− 1 (hν ≈ 1.04 eV).     

Tuning the optical properties of alloyed CdSexS1 − x nanoparticles by changing the constituent stoichiometry

The spherical particles CdSe_xS_1 − x with 30-80 nm in radius have been successfully prepared by the hydrothermal reaction at 200 °C. The structure characterization which has been carried out using X-ray diffraction (XRD) shows hexagonal crystal structure. Novel properties have been observed via UV-visual absorption spectra and photoluminescence (PL) spectra. The absorption shoulder and the luminescence emission peaks have been tuned by changing the mole ratio of Se in the CdSe_xS_1 − x samples.