Single Crystalline Layers Research Articles

Improved Structural and Chemical Properties of Nearly Lattice-Matched Ternary and Quaternary Barriers for GaN-Based HEMTs

A complete structural and compositional study was carried out for a series of GaN-based lattice-matched HEMT structures. As barrier materials pseudomorphic to GaN, both ternary InAlN and quaternary InAlGaN were investigated. Growths were performed using molecular beam epitaxy on GaN/sapphire or GaN/SiC templates. An abrupt triple-layer AlN/GaN/AlN nanothin spacer at the interface is crucial to improve the structural and electrical properties of the heterostructures. In all cases, this resulted in single-crystalline and single-phase barrier layers.

Study of Arsenic ion implantation of patterned strained Si NWs

A systematic study of the impact of As + ion implantation on strain relaxation and dopant activation of biaxially strained SSOI layers and uniaxially strained/unstrained NWs is presented. Three aspects are investigated: (i) the quality of the single crystalline layers and the NWs, (ii) strain relaxation of the implanted NWs and (iii) dopant activation of the layers and NWs. Optimization of the doping conditions resulted into very low contact resistivities of NiSi contacts on strained and unstrained 70 nm SOI layers and Si NWs. For NW contacts values as low as 1.2 × 10 −8 Ω cm 2 for an As + dose of 2 × 10 15 cm −2 were achieved, which is 20 times lower than for planar contacts made under the same implantation and annealing conditions.

Electrodeposition of Cu-doped ZnO nanowire arrays and heterojunction formation with p-GaN for color tunable light emitting diode applications

Copper-doped zinc oxide (ZnO:Cu) nanowires (NWs) were electrochemically deposited at low temperature on fluor-doped tin oxide (FTO) substrates. The electrochemical behavior of the Cu–Zn system for Cu-doped ZnO electrodeposition was studied and the electrochemical reaction mechanism is discussed. The synthesized ZnO arrayed layers were investigated by using SEM, XRD, EDX, photoluminescence and Raman techniques. X-ray diffraction analysis demonstrates a decrease in the lattice parameters of Cu-doped ZnO NWs. Structural analyses show that the nanomaterial is of hexagonal structure with the Cu incorporated in ZnO NWs probably by substituting zinc in the host lattice. Photoluminescence studies on pure and Cu-doped ZnO NWs shows that the near band edge emission is red-shifted by about 5 or 12 nm depending on Cu(II) concentration in the electrolytic bath solution (3 or 6 μmol l −1). Cu-doped ZnO NWs have been also epitaxially grown on Mg doped p-GaN single-crystalline layers and the (ZnO:Cu NWs)/( p-GaN:Mg) heterojunction has been used to fabricate a light-emitting diode (LED) structure. The emission was red-shifted to the visible violet spectral region compared to pure ZnO. The present work demonstrates the ability of electrodeposition to produce high quality ZnO nanowires with tailored optical properties by doping. The obtained results are of great importance for further studies on bandgap engineering of ZnO, for color-tunable LED applications and for quantum well preparation.

P-Type Ion Implantation in Tensile Si/Compressive Si0.5Ge0.5/Tensile Strained Si Heterostructures

We present a systematic study on the formation of p-type doped strained Si / strained SiGe heterostructures by B+, BF+2 and (Si++B+) ion implantation and annealing at moderate temperatures. The aim of this paper is to address the challenge of conserving the elastic strain during dopant activation. The most important result is that efficient doping combined with the conservation of strain and a good crystalline quality can only be obtained for BF2+ implants with 1×1015 ions/cm2 and anneals at 650°C. With these parameters, single crystalline layers with negligible strain relaxation and a sheet resistance of 886 Ω/sq were achieved. The implantation of B+ requires higher doses to reach low sheet resistances resulting in low layer quality and higher strain relaxation. Si+ pre-implantation yields the lowest sheet resistances, but on the expense of strain (relaxation values over 60%). Finally, the optimized ion implantation / anneal parameters were applied for strained SiGe quantum-well MOSFETs with GdScO3 high-κ gate dielectric.

Sputtered Ge-on-Si heteroepitaxial pn junctions: Nanostructure, interface morphology and photoelectrical properties

Ge thin films are epitaxially grown onto (1 0 0) Si substrates by DC-Pulsed Magnetron Sputtering. Relaxed single crystalline layers, with slightly misoriented domains are identified by XRD, TEM and HREM. Planar defects and threading dislocations are the relevant lattice imperfections. As-deposited Ge films are p-type without the need for intentional doping, even in the absence of grain boundaries. A pronounced flatness in the near IR absorption spectra is evident, in the absence of strong interfacial strain. This could be traced to a bandgap narrowing effect due to intragap states related to defects in the interfacial region. Photoconductive response around λ = 1.5 μm is flat and an equivalent responsivity R eff| Vbias = −1V = 1.0088 A/W at λ = 1.5 μm has been estimated. DC-Pulsed Magnetron Sputtering is therefore an attractive solution, deserving further development, to build near-infrared C-MOS compatible photodetectors, particularly suitable for low-speed applications.

Solution growth of Ce–Pd–In single crystals: Characterization of the heavy-fermion superconductor Ce2PdIn8

Solution growth of single crystals of the recently reported new compound Ce2PdIn8 was investigated. When growing from a stoichiometry in a range 2:1:20–2:1:35, single crystals of CeIn3 covered by a thin (∼50 μm) single-crystalline layer of Ce2PdIn8 were mostly obtained. Using palladium richer compositions the thickness of the Ce2PdIn8 layers were increased, which allowed mechanical extraction of single-phase slabs of the desired compound suitable for a thorough study of magnetism and superconductivity. In some solution growth products also Ce4Pd10In21 and traces of phases with the stoichiometry CePd2In7, Ce1.5Pd1.5In7 (determined only by EDX) have been identified.Magnetic measurements of the Ce2PdIn8 single crystals reveal paramagnetic behaviour of the Ce3+ ions with significant magnetocrystalline anisotropy. Above 70 K the magnetic susceptibility follows the Curie–Weiss law with considerably different values of the paramagnetic Curie temperature, for the magnetic field applied along the a- (−90 K) and c-(−50 K) axis. Below the reported critical temperature for superconductivity Tc (0.69 K) the electrical resistivity drops to zero. Comparative measurements of the electrical resistivity, heat capacity and AC susceptibility of several crystals reveal that the superconducting transition is strongly sample dependent.

Interaction of localized spins in low-temperature-grown GaAs layers

Localized spins are associated with anti-site arsenic ( As Ga + )ions in a GaAs layer grown at a low temperature by molecular-beam epitaxy (LT-GaAs). In order to investigate possible interactions among these localized spins, magnetic properties of Be-doped LT-GaAs layers were studied by electron spin resonance (ESR) spectroscopy and super-conducting quantum interference device (SQUID) measurements, for the latter of which single crystalline LT-GaAs layers with thicknesses up to 20 μm were grown. In the SQUID measurements at strong fields, thick LT-GaAs layers show the Curie-type paramagnetism. At weak fields, a spin-glass like transition occurs; the temperature dependence of the magnetization under the zero-field cooling exhibits a peak, and the field dependence of the magnetization has a significant hysteresis at temperatures lower than the peak temperature. The peak temperature varies among samples, ranging from 2.7 to 5.5 K, depending on their growth temperatures and Be concentrations. Measured line-widths of ESR spectra have a close correlation with the results of SQUID measurements; a sample in which more significant effects of interactions among localized spins occur in the SQUID results exhibits a narrower ESR line and a larger increase in the line-width with a decrease in temperature.

Magnetization reversal and wall propagation velocity in single-crystalline and polycrystalline Fe wires

The propagation velocities of a magnetic domain wall (DW) in polycrystalline and single-crystalline layers of $\text{Fe}(001)/\text{Au}/{\text{Fe}}_{19}{\text{Ni}}_{81}$ wires were measured using the giant magnetoresistance effect. In a single-crystalline Fe wire, the DW propagation velocity (1 km/s in a 110 Oe field at 77 K) was about three times faster than that in a polycrystalline Fe wire. This is attributable to the increase in an effective damping derived from incoherent rotation of the magnetization, which reduced the DW's mobility in the polycrystalline wire. Magnetocrystalline anisotropy plays an important role in the nucleation of DW and the switching process. Analytic solutions for the DW's motion have been found on the basis of an approximation suggested by the Slonczewski-Walker and Becker-D\"oring models; these solutions can explain our experimental results.

Composition-Tuned ZnO−CdSSe Core−Shell Nanowire Arrays

Vertically aligned ZnO--CdSSe core-shell nanocable arrays were synthesized with a controlled composition and shell thickness (10-50 nm) by the chemical vapor deposition on the pregrown ZnO nanowire arrays. They consisted of a composition-tuned single-crystalline wurtzite structure CdS1-xSex (x=0, 0.5, and 1) shell whose [0001] direction was aligned along the [0001] wire axis of the wurtzite ZnO core. The analysis of structural and optical properties shows the formation of Zn containing alloy in the interface region between the ZnO core and shell, which can facilitate the growth of single-crystalline shell layers by reducing both the lattice mismatch and the number of defect sites. In contrast, the TiO2 (rutile) nanowire array can form the polycrystalline shell under the same condition. The photoelectrochemical cell using the ZnO--CdS photoelectrode exhibits a higher photocurrent and hydrogen generation rate than that using the TiO2-CdS one. We suggest that the formation of the CdZnSSe intermediate layers contributes to the higher photoelectrochemical cell performance of the ZnO--CdSSe nanocables.

Surface structure and surface barrier characteristics of boron-doped diamond in electrolytes after CF 4 plasma treatment in RF-barrel reactor

We report on the effects of CF 4 plasma process in technical RF barrel reactor on surface termination and the resulting electronic surface barrier of boron-doped diamond in electrolytes. The surface characteristics were evaluated for epitaxial single crystalline layers with sub-nm roughness. The capacitance–voltage characteristics of the processes electrodes implied a low electronic barrier at the fluorine-terminated areas, comparable to hydrogen termination in literature. However carbon–oxygen groups were still present on approx. 15% of the surface area after the plasma process. To analyse the electronic barrier of the fluorinated diamond, we proposed an electrical model of two parallel metal-oxide-semiconductor structures. This model included fixed parameters derived from the analysis of a diamond electrode exposed to plasma oxidation.

A thin film approach to protein crystallography

A novel approach for the investigation of proteins or macromolecules is outlined in this conceptual study. The preparation of grapho-epitaxial layers on nanotemplated substrates is proposed as an alternative to the preparation of single crystals by vapour diffusion techniques. Crystal structure investigations of such layers may then be performed via grazing-incidence diffraction (GIXRD) in the Laue mode. Quantitative expressions for the position and intensities of XRD peaks in this geometry are presented that fully consider the effects of refraction and absorption. A simulation of the Laue-GIXRD pattern of single-crystalline layers of Concanavalin A is given. The main challenges of the approach are concluded to relate to the preparation of single-crystalline protein layers. However, if those obstacles could be overcome, a 10–100-fold faster sample throughput would become possible.

Do Multilayer Crystals Nucleate in Suspensions of Colloidal Rods?

We study the isotropic-to-crystal transformation in a mixture of colloidal hard rods and nonabsorbing polymer using computer simulations. We determine the height of the nucleation barrier and find that the critical cluster consists of a single crystalline layer growing laterally for all polymer fugacities considered. At lower supersaturation, the free energy of a single hexagonally packed layer increases monotonically with size, while the nucleation barrier of a second crystalline layer is extremely high. Hence, the nucleation of multilayer crystals is never observed. Multilayer crystals form only in the spinodal decomposition regime, either where, in an intermediate stage, single crystalline membranes coalesce into multilayer clusters or where, at higher polymer fugacity, smaller clusters of rods stack on top of each other to form long filaments. Eventually, these transient structures evolve into a thermodynamically stable bulk crystal phase.

Solid phase epitaxy of amorphous Ge on Si in N2 atmosphere

We demonstrate a straightforward and economical way to obtain smooth germanium layers of high quality on silicon. Thin amorphous germanium layers deposited by plasma enhanced chemical vapor deposition on Si(111) substrates are transformed into single crystalline and smooth layers by solid phase epitaxy in N2 atmosphere. The crystal orientation of the substrate has a clear influence on the crystal quality. This is most likely due to a different growth mode, namely, layer-by-layer for Si(111) and three-dimensional growth for Si(001). The amorphous germanium layer can roughen during annealing due to mobile atoms on the surface. This can be effectively suppressed by annealing in N2 ambient. Electrical measurements show high charge mobility.

Large domain growth of GaN epitaxial films on lattice-matched buffer layer ScAlMgO 4

A homologous series compound ScAlO 3(MgO) (SCAM) has a superior lattice matching as small as ∼1.4% in a-axis with GaN. This paper reports an efficient fabrication process of a single-crystalline SCAM buffer layer on a (1 1 1) yttria-stabilized zirconia (YSZ) substrate using pulsed laser deposition (PLD). A 10-nm thick ZnO epitaxial layer was used to induce solid-phase epitaxial growth of an amorphous (a-) SCAM layer formed at room temperature on (1 1 1) YSZ. It was found that the addition of excess Sc 2O 3 and ZnO to a SCAM target used for PLD was needed to obtain single-crystalline SCAM films with atomically flat terraces-and-steps surfaces. The resulting single-crystalline SCAM films were examined as buffer layers to grow GaN by molecular beam epitaxy with a plasma nitrogen source. The GaN films were grown epitaxially on the SCAM/YSZ substrates with the epitaxial relationship of [0 0 0 1] GaN||[0 0 0 1] SCAM||[1 1 1] YSZ and [1 0 0] GaN||[11-20] SCAM||[1-10] YSZ. The SCAM buffer layers enhanced lateral growth of the GaN films owing to the good lattice matching.

Atomically smooth and single crystalline Ge(111)/cubic-Pr2O3(111)/Si(111) heterostructures: Structural and chemical composition study

Engineered wafer systems are an important materials science approach to achieve the global integration of single crystalline Ge layers on the Si platform. Here, we report the formation of single crystalline, fully relaxed Ge(111) films by molecular beam epitaxial overgrowth of cubic Pr oxide buffers on Si(111) substrates. Reflection high-energy electron diffraction, scanning electron microscopy, and x-ray reflectivity show that the Ge epilayer is closed, flat, and has a sharp interface with the underlying oxide template. Synchrotron radiation grazing incidence x-ray diffraction and transmission electron microscopy reveal the type-A/B/A epitaxial relationship of the Ge(111)/cubic Pr2O3(111)/Si(111) heterostructure, a result also corroborated by theoretical ab initio structure calculations. Secondary ion mass spectroscopy confirms the absence of Pr and Si impurities in the Ge(111) epilayer, even after an annealing at 825 °C.

Electron microscopy analysis of dislocation behavior in HVPE-AlGaN layer grown on a stripe-patterned (0 0 0 1) sapphire substrate

Microstructures were investigated by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) in order to clarify the dislocation behavior in AlGaN layers HVPE-grown on a stripe-patterned sapphire (0 0 0 1) substrate. SEM observation revealed very clearly the growth process: if AlGaN starting to grow from the side-wall of patterned substrate develops, a poly-crystalline region is formed up to the top surface of thin film. When the growth from the upper side (terrace) of patterned substrate is predominant, AlGaN becomes a single-crystalline layer with a flat surface. Threading dislocations (TDs) generated from the interface to the terrace propagate upwards, inclining to the wing regions. They are scarcely merged with one another. The AlGaN layer on the patterned substrate with a wider groove has a smaller density of dislocation to be about 1×10 9 cm −2. There are four types of dislocations: (1) TDs inclining toward 〈1 1¯ 0 0〉 normal to their Burgers vector B ; (2) TDs inclining toward 〈2 1¯ 1¯ 0〉 on their slip-plane; (3) TDs inclining largely or horizontal dislocations (HDs) along 〈2 1¯ 1¯ 0〉 and (4) roundly curved HDs lying on (0 0 0 1) plane. Some TDs change the direction of inclination, suggesting that internal stress changed intricately during the growth.

Absence of amorphous phase in high power femtosecond laser-ablated silicon

As femtosecond lasers emerge as viable tools for advanced microscale materials processing, it becomes increasingly important to understand the characteristics of materials resulting from femtosecond laser microablation or micromachining. We conducted transmission electron microscopy experiments to investigate crater structures in silicon produced by repetitive high power femtosecond laser ablation. Comparable experiments of nanosecond laser ablation of silicon were also performed. We found that an amorphous silicon layer that is typically produced in nanosecond laser ablation is absent when the material is irradiated by high power femtosecond laser pulses. Instead, only a defective single crystalline layer was observed in the high power femtosecond laser-ablated silicon crater. Possible mechanisms underlying the formation of the defective single crystalline phase are discussed.

Growth of single-crystalline GaN layers in a horizontal reactor by chloride epitaxy

Chloride epitaxy of GaN layers in a horizontal reactor is studied numerically. The steady 3D fluxes of the gas mixture in the reactor are simulated with allowance for heterogeneous reactions on the substrate (growth of epitaxial GaN layers) and on the reactor walls (growth of a polycrystalline GaN deposit). Experimental data on the growth rate distribution for polycrystalline and epitaxial GaN layers are explained. It is shown that, if the diameter of the reactor is not large enough, the growth of the deposit on the walls makes the GaN growth rate distribution over the substrate more nonuniform due to the parasitic diffusion of reagents from the gas phase to the reactor walls.

Hot Wall Epitaxy (HWE)법에 의한 ZnIn2Se4단결정 후막 성장과 열처리 효과

Single crystalline layers were grown on thoroughly etched semi-insulating GaAs (100) substrate at with hot wall epitaxy (HWE) system by evaporating source at . The crystalline structure of the single crystalline thick films was investigated by the photoluminescence (PL) and Double crystalline X-ray rocking curve (DCRC). The carrier density and mobility of single crystalline thick films measured from Hall effect by van der Pauw method are and at 293 K, respectively. The temperature dependence of the energy band gap of the obtained from the absorption spectra was well described by the Varshni`s relation,

Effect of domain boundaries on the electrical properties of crystalline Gd2O3 thin films

We report on the effect of domain boundaries on the electrical properties of epitaxial Gd2O3 thin films grown on Si(001) substrates with 4° miscut along [110] azimuth. Dedicated preparation of the substrate surfaces can lead to two different microstructures of epitaxial Gd2O3 layers when grown at identical growth conditions. A substrate surface with terraces of biatomic steps height is the key point to achieve single crystalline epitaxial Gd2O3 layer. Such epi-Gd2O3 layers exhibited significantly lower leakage currents compared with the commonly obtained epitaxial layers with two orthogonal domains. However, for capacitance equivalent thickness below 1 nm, this difference disappears, indicating that for ultrathin layers direct tunneling becomes dominating. Additionally, forming gas annealing can reduce the leakage current by a few orders of magnitude. Here, thinner layers of both structural types exhibited similar electrical properties.