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.     

Laser assisted formation of binary and ternary Ge/Si/Sn alloys

Binary and ternary Si/Ge/Sn alloys were epitaxially grown on virtual Germanium buffer layers using pulsed laser induced epitaxy with a 193 nm Excimer laser source. The role of the processing parameters on the intermixing of the components (Sn, Ge and Si) has been studied. Characterization of the resulting Ge_1 − xSn_x and Si_{1 − y − x}Ge_ySn_x alloys yield up to 1% Sn concentration in substitutional sites of the Ge or SiGe matrix.     

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.     

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.     

Efficient relaxation of strained-SiGe layers induced by thermal oxidation

The new route to fabricate compositionally graded and highly-relaxed Si_1-xGe_x layers using thermal oxidation at high temperatures is investigated. Ge atoms behavior during thermal oxidation of Si_1-xGe_x layers is strongly dependent on the oxidation temperature. For low temperature oxidation processes Ge is incorporated as GeO₂ in the grown oxide layer, while for higher temperatures it accumulates below the grown oxide into a layer with a higher concentration than the initial Si_1-xGe_x. However, Si_1-xGe_x layers oxidized at 1000 °C did not show such an accumulation layer because Ge diffusion efficiently occurred, resulting in the formation of a compositionally graded and relaxed Si_1-xGe_x layer. Such layers could be used as virtual substrates for the strained-Si and relaxed-SiGe applications.     

Growth of homogeneous polycrystalline Si1-xGex and Mg2Si1-xGex for thermoelectric application

Homogeneous polycrystalline Si_1-xGe_x were grown using a Si(seed)/Ge/Si(feed) sandwich structure under the low temperature gradient less than 0.4 °C/mm. It was found that the composition of the Si_1-xGe_x was controlled by the growth temperature. The homogeneous Mg₂Si_1-xGe_x was synthesized by heat treatment of the homogeneous Si_1-xGe_x powders under Mg vapor. The Mg₂Si_1-xGe_x sample with the relative density of 95% was synthesized by spark plasma sintering technique. The resistivity and the Seebeck coefficient of the Si, Ge, Si_1-xGe_x and Mg₂Si_1-xGe_x samples were evaluated as a function of temperature. It indicated that Seebeck coefficients of the Si_1-xGe_x and Mg₂Si_1-xGe_x samples were higher than those of Si and Ge. Moreover, the Seebeck coefficient of Mg₂Si_0.7Ge_0.3 sample was higher than that of Mg₂Si_0.5Ge_0.5 and Si_0.5Ge_0.5 samples.     

Properties of La0.75Sr0.25MnO3 films grown on Si substrate with Si1−xGex and Si1−yCy buffer layers

The structural and electrical properties of La_0.75Sr_0.25MnO₃ (LSMO) film on Bi₄Ti₃O₁₂ (BTO)/CeO₂/YSZ buffered Si_1−xGe_x/Si (0.05 ≤ x ≤ 0.2 for compressive strain), blank Si, and Si_1−yC_y/Si (y = 0.01 for tensile) were studied. X-ray high resolution reciprocal lattice mapping (HRRLM) and atomic force microscopy (AFM) show that structural properties of LSMO and buffer oxide layers are strongly related to the strain induced by amount of Ge and C contents. The RMS roughness of LSMO on Si_1−xGe_x/Si has a tendency to increase with increasing of Ge content. Electrical properties of LSMO film with Ge content up to 10% are slightly improved compared to blank Si whereas higher resistivity values were obtained for the samples with higher Ge content.     

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.     

Synthesis of strained SiGe on Si(100) by pulsed laser induced epitaxy

Pulsed laser induced epitaxy (PLIE), based on melting/solidification processes induced by nanosecond laser pulses, is used to synthesize pseudomorphic Si_1 − xGe_x epilayers from 20 to 80 nm thick Ge layers evaporated on a Si(100) wafer. Ge concentration and strain are characterized by transient reflectivity, energy dispersive X-ray analysis and X-ray diffraction from symmetric (004) and asymmetric (224) reflections. For a low Ge thickness or a high laser fluence, PLIE builds up only a pseudomorphic strained Si_1 − xGe_x layer with a graded Ge composition reaching x ≈ 19% near the surface. When the Ge amount is in excess to achieve this situation, PLIE forms additionally a relaxed Si_1 − xGe_x layer with x values up to ≈40% over the previous pseudomorphic layer.     

High temperature thermopower and electrical resistivity studies in the transparent conducting oxide Sn doped MgIn2O4

Sn doping in an n-type transparent conducting oxide MgIn₂O₄ is carried out and its effect on the high temperature transport properties viz. thermopower and electrical resistivity is studied. A solid solution exists in the composition window Mg_1+xIn_2−2xSn_xO₄ for 0 < x ≤ 0.4. The band gap as well as the transport properties increases with increasing Sn concentration. The high temperature resistivity properties indicate degenerate semiconducting behavior for all the compositions. The highest figure of merit obtained is 0.12 × 10⁻⁴ K⁻¹ for the parent compound at 600 K.     

Growth mode control for direct-gap core/shell Ge/GeSn nanowire light emission

Germanium-tin is a promising semiconductor alloy system for novel light emitting devices and optical sensors in the mid-IR region. For sufficiently high Sn compositions, the material has a direct band-gap near 0.5 eV, and could have applications either as a detector or an emitter. Although high Sn compositions have been achieved in Ge_1−xSn_x through a variety of growth strategies, the understanding of how chemical vapor deposition conditions affect Sn composition and optical properties in core–shell Ge/Ge_1−xSn_x nanowires is still lacking. In this study, gas precursor partial pressures and shell growth temperatures are systematically varied to provide guiding principles to overcome obstacles for higher Sn incorporation. We achieve a direct band-gap material using an elastically compliant Ge core nanowire substrate. In the course of the growth study, we demonstrate several findings regarding the Ge_1−xSn_x shell growth mechanism. First, we observe an H₂ passivation effect in which higher H₂ to SnCl₄ partial pressure ratio results in a concurrent increase in axial wire growth and decrease in radial growth. Second, we find that Ge_1−xSn_x shell growth in the studied CVD process is mass transport limited. Third, our results suggest that low shell growth temperature and high shell growth rate facilitate high Sn composition through metastable Sn solute trapping due to suppressed surface diffusion relative to the velocity of advancing shell surface steps. In this work, we demonstrate single nanowire photoluminescence at room temperature from core-shell Ge/Ge_0.88Sn_0.12 nanowires. Understanding the Ge_1−xSn_x shell growth mechanism via chemical vapor deposition (CVD) facilitates achieving minimal residual strain in the shell and the high crystalline quality and large Sn composition necessary for the observed optical properties. The results are universally applicable to Ge_1−xSn_x thin film epitaxy on compliant substrates including grown or etched nanowires, nanosheets, or free-standing 2D crystals.     

Radiation damage of Si1-xGex S/D p-type metal oxide semiconductor field effect transistor with different Ge concentrations

The 2-MeV electron radiation damage of Si_1-xGe_x source/drain (S/D) p-type metal oxide semiconductor field effect transistor (p-MOSFET) with different Ge concentrations is studied. After irradiation at fluences below 2 × 10¹⁷ e/cm², the drain current and the maximum hole mobility decrease with increasing electron fluence for all Ge concentrations. It suggests that lattice defects are introduced by electron irradiation. In the case of Si_1-xGe_x S/D p-MOSFET, there are two locations for lattice defects, namely, the Si channel and SiGe stressor regions (S/D). Below 2 × 10¹⁷ e/cm² irradiation, no clear correlation between the radiation degradation and Ge concentration has been observed. It suggests that this degradation is mainly due to lattice defects in the Si channel, and the effects of the compressive-strain induced by the SiGe stressors on the enhancement of the hole mobility still remains after irradiation at 2 × 10¹⁷ e/cm². In the case of 5 × 10¹⁷ e/cm² irradiation, the drain current drastically decreases after irradiation for all Ge concentrations. Moreover, after 5 × 10¹⁷ e/cm² irradiation, the maximum hole mobility of x = 0.2 is close to x = 0, and in the case of x = 0.3, the maximum hole mobility drastically decreases. This fact suggests the contributions of the lattice defects, which are in the SiGe stressors, are prominent after 5 × 10¹⁷ e/cm² irradiation and dependent on Ge concentration. In addition, it provides evidence that the compressive strain in the Si-channel is relaxed by high fluence electron irradiation.     

Microstructure and thermoelectric properties of Sn-doped Bi2Te2.7Se0.3 thin films deposited by flash evaporation method

(Bi_1 − xSn_x)₂Te_2.7Se_0.3 thermoelectric thin films with thickness of 800 nm have been deposited on glass substrates by flash evaporation method at 473 K. The structures, morphology of the thin films were analyzed by X-ray diffraction and field emission scanning electron microscopy respectively. Effects of Sn-doping concentration on thermoelectric properties of the annealed thin films were investigated by room-temperature measurement of Seebeck coefficient and electrical resistivity. The thermoelectric power factor was enhanced to 12.8 μW/cmK² (x = 0.003). From x = 0.004 to 0.01 Sn doping concentration, the Seebeck coefficients are positive and show p-type conduction. The Seebeck coefficient and electrical resistivity gradually decrease with increasing Sn doping concentration.     

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.     

Advantages of using amorphous indium zinc oxide films for window layer in Cu(In,Ga)Se2 solar cells

The advantages of using indium zinc oxide (IZO) films instead of conventional Ga-doped zinc oxide (ZnO:Ga) films for Cu(In,Ga)Se₂ (CIGS) solar cells are described. The electrical properties of IZO are independent of film thickness. IZO films have higher mobility (30-40 cm²/Vs) and lower resistivity (4-5 × 10^− 4 Ω cm) compared to ZnO:Ga films deposited without intentional heating, because the number of grain boundaries in amorphous IZO films is small. The properties of a CIGS solar cell using IZO at the window layer were better than those obtained using a conventional ZnO:Ga at the window layer; moreover, the properties tended to be independent of thickness. These results indicate that use of IZO as a transparent conducting oxide layer is expected to increase the efficiency of CIGS solar cells.     

Characterization of Zn1 − xMgxO transparent conducting thin films fabricated by multi-cathode RF-magnetron sputtering

Transparent conducting thin films of Al-doped and Ga-doped Zn_1 − xMg_xO with arbitrary Mg content x were deposited on glass substrates by simultaneous RF-magnetron sputtering of doped ZnO and MgO targets, and their fundamental properties were characterized. MgO phase separation in Zn_1 − xMg_xO films was not detected by X-ray diffraction. The Zn_1 − xMg_xO films show high optical transparency in the visible region. Although the carrier density of the Zn_1 − xMg_xO films decreased with increasing x, the Zn_1 − xMg_xO films showed good electrical conductivity; electrical resistivity as low as 8 × 10^− 4 Ω ·cm was achieved for the Zn_0.9Mg_0.1O:Ga thin film.     

Electrochemical characterization of a Ge-based composite film fabricated as an anode material using magnetron sputtering for lithium ion batteries

Amorphous germanium and germanium-based films are sputter-deposited as anodes for lithium ion batteries. The structures of Ge and Ge-Mo composites are investigated using an X-ray diffractometer (XRD) and transmission electron microscopy (TEM). The surface morphologies of the electrodes are observed using a field emission scanning electron microscope (FESEM). In order to determine the influence of inactive material in the anode, cell tests are carried out on half cells (Ge/Li metal and Ge_xMo_1 − x/Li metal) and full cells (Ge/LiCoO₂ and Ge_xMo_1 − x/LiCoO₂). The Ge film electrodes prepared on rough copper foil substrates showed stable capacities of 1000 mA h g⁻¹ over 50 cycles. The Ge_0.88Mo_0.12 composite film electrode showed reversible gravimetric capacities of up to 1000 mA h g⁻¹ with 77.9% capacity retention rates of the half-cell test after 100 cycles. Therefore, it may be possible to fabricate Ge-based anode materials with high capacity and improved capacity retention. The results of this study suggest that sputtered Ge-based electrodes are promising anode materials for next generation lithium ion batteries.     

Growth and characterization of group-III impurity-doped semiconducting BaSi2 films grown by molecular beam epitaxy

Ga- or In-doped BaSi₂ films were grown on Si(111) by molecular beam epitaxy (MBE). The Ga-doped BaSi₂ showed n-type conductivity. The electron concentration and resistivity of the Ga-doped BaSi₂ depended on the Ga temperature; however, the electron concentration and resistivity could not be controlled properly. In contrast, the In-doped BaSi₂ showed p-type conductivity and its hole concentration was controlled in the range between 10¹⁶ and 10¹⁷ cm^− 3 at RT.     

Synthesis, characterization, and dielectric properties of Ba(Ti1−xSnx)O3 nanopowders and ceramics

We prepared Ba(Ti_1−xSn_x)O₃ powders and ceramics by means of the sol-gel process, with dibutyltin dilaurate as the Sn precursor. The samples were characterized by means of Fourier-transform infrared spectroscopy, X-ray diffraction, transmission electron microscopy, and scanning electron microscopy, and also determined the dielectric properties of the ceramics. The powders synthesized by means of the sol-gel process had a grain size on the nanometer scale, with the grains mainly composed of a cubic BaTiO₃ phase. Sn can disperse into BaTiO₃ more uniformly in the sol-gel technique using dibutyltin dilaurate as the Sn precursor. With increasing Sn concentration, the grain size of the Ba(Ti_1-xSn_x)O₃ ceramics increased and the maximum dielectric constant (?_max) first increased and then decreased. At a Sn concentration of 5 mol%, ?_max reached its maximum value (19,235).