Photodiode properties of molecular beam epitaxial InSb on a heavily doped substrate

Photodiodes of InSb were fabricated on an epitaxial layer grown using molecular beam epitaxy (MBE). Thermal cleaning of the InSb (0 0 1) substrate surface, 2° towards the (1 1 1) B plane, was performed to remove the oxide. Photodiode properties of МВЕ-formed epitaxial InSb were demonstrated. Zero-bias resistance area product (R₀A) measurements were taken at 80 K under room temperature background for a pixel size of 100 μm × 100 μm. Values were as high as 4.36 × 10⁴ Ω/cm², and the average value of R₀A was 1.66 × 10⁴ Ω/cm². The peak response was 2.44 (A/W). The epitaxial InSb photodiodes were fabricated using the same process as bulk crystal InSb diodes with the exception of the junction formation method. These values are comparable to the properties of bulk crystal InSb photodiodes.     

Effects of trench oxide and field plates on the breakdown voltage of SOI LDMOSFET

To improve the breakdown voltage, we propose a SOI-based LDMOSFET with a trench structure in the drift region. Due to the trench oxide and underneath boron implanted layer, the surface electric field in the drift region effectively reduced. These effects resulted in the increment of breakdown voltage for the trenched LDMOS more than 100 V compared with the conventional device. However, the specific on-resistance, which has a trade-off relationship, is slightly increased. In addition to the trench oxide on the device performance, we also investigated the influence of n− drift to n+ drain junction spacing on the off-state breakdown voltage. The measured breakdown voltages were varied more than 50 V with different n− to n+ design spaces and achieved a maximum value at L_DA = 2.0 μm. Moreover, the influence of field plate on the breakdown voltage of trench LDMOSFET was investigated. It is found that the optimum drain field plate over the field oxide is 8 μm.     

Characterization of photodiodes,made from a p-type epitaxial layer grown on n-type InSb <1 1 1> by LPE method

In this article the performance of photodiodes made from epitaxially grown layers of p-InSb on n-type InSb substrates is reported. The effect of increasing Cd atomic weight percent on p-type carrier concentration and mobility at 77 K is also discussed. In our epitaxial growth method, a ramp cooling technique was used. Finally by improving growth parameters such as growth temperature, prior cleaning of B face (Sb) n-InSb substrates and cooling rate, adequate epitaxial layers of p-InSb on n-InSb <1 1 1> and consequently highly sensitive photodiodes have been obtained.A high detectivity photodiodes fabricated for p-InSb on n-InSb substrate by liquid phase epitaxy (LPE) was measured using optoelectronic tests and the detectivity of the diodes was compared with n-InSb on p-InSb. Several other tests such as Hall effect, thickness measurements, I–V and X-ray diffraction (XRD) were also performed and morphology images will be presented in this paper.     

Growth of Sr2CrReO6 epitaxial thin films by pulsed laser deposition

We report the growth, structural, magnetic, and electrical transport properties of epitaxial Sr₂CrReO₆ thin films. We have succeeded in depositing films with a high crystallinity and a relatively large cationic order in a narrow window of growth parameters. The epitaxy relationship is Sr₂CrReO₆ (SCRO) (0 0 1) [1 0 0]∥SrTiO₃ (STO) (0 0 1) [1 1 0] as determined by high-resolution X-ray diffraction and scanning transmission electron microscopy (STEM). Typical values of saturation magnetization of M_S (300 K)=1 μ_B/f.u. and ρ (300 K)=2.8 mΩ cm have been obtained in good agreement with previous published results in sputtered epitaxial thin films. We estimate that the antisite defects concentration in our thin films is of the order of 14%, and the measured Curie temperature is T_C=481(2) K. We believe these materials be of interest as electrodes in spintronic devices.     

Measuring magnetic profiles at manganite surfaces with monolayer resolution

The performance of manganite-based magnetic tunnel junctions (MTJs) has suffered from reduced magnetization present at the junction interfaces that is ultimately responsible for the spin polarization of injected currents; this behavior has been attributed to a magnetic “dead layer” that typically extends a few unit cells into the manganite. X-ray magnetic scattering in resonant conditions (XRMS) is one of the most innovative and effective techniques to extract surface or interfacial magnetization profiles with subnanometer resolution, and has only recently been applied to oxide heterostructures. Here we present our approach to characterizing the surface and interfacial magnetization of such heterostructures using the XRMS technique, conducted at the BEAR beamline (Elettra synchrotron, Trieste). Measurements were carried out in specular reflectivity geometry, switching the left/right elliptical polarization of light as well as the magnetization direction in the scattering plane. Spectra were collected across the Mn L_2,3 edge for at least four different grazing angles to better analyse the interference phenomena. The resulting reflectivity spectra have been carefully fit to obtain the magnetization profiles, minimizing the number of free parameters as much as possible. Optical constants of the samples (real and imaginary part of the refractive index) in the interested frequency range are obtained through absorption measurements in two magnetization states and subsequent Kramers–Krönig transformation, allowing quantitative fits of the magnetization profile at different temperatures. We apply this method to the study of air-exposed surfaces of epitaxial La_2/3Sr_1/3MnO₃ (0 0 1) films grown on SrTiO₃ (0 0 1) substrates.     

Ultrafast relaxation of hot optical phonons in monolayer and multilayer graphene on different substrates

Hot carrier cooling in few-layer and multilayer epitaxial graphene on SiC, and chemical vapor deposition (CVD) grown graphene transferred onto a glass substrate was investigated by transient absorption spectroscopy and imaging. Coupling to the substrate was found to play a critical role in charge carrier cooling. For both multilayer epitaxial graphene and monolayer CVD graphene, charge carriers transfer heat predominantly to intrinsic in-plane optical phonons of graphene. At high pump intensity, a significant number of optical phonons are accumulated, and the optical phonon lifetime presents a bottleneck for charge carrier cooling. This hot phonon effect did not occur in few-layer epitaxial graphene because of strong coupling to the substrate, which provided additional cooling channels. The limiting charge carrier lifetimes at high excitation densities were 1.8 ± 0.1 ps and 1.4 ± 0.1 ps for multilayer epitaxial graphene and monolayer CVD graphene, respectively. These values represent lower limits on the optical phonon lifetime for the graphene samples.     

Homoepitaxial growth of MOD-YBCO thick films on evaporated and MOD templates

We have prepared metal organic deposition (MOD)-YBCO thick films by repeating the coating-pyrolysis-crystallization procedure onto ～100-nm-thick evaporated and MOD templates. Surface morphology of the template was found to strongly affect the homoepitaxial growth of MOD-YBCO layers on the template; namely, the epitaxial growth of MOD-YBCO on the evaporated template was much easier than that on the MOD template. A 220-nm-thick epitaxial MOD-YBCO film was successfully prepared on the 100-nm-thick evaporated-YBCO template to obtain a 320-nm-thick YBCO film, which exhibited J_c = 2.44 MA/cm² and I_c = 78 A/cm. The I_c value has significantly increased from 37 A/cm for the evaporated-template.     

Off-state breakdown characteristics of InGaP-based high-barrier gate heterostructure field-effect transistors

In this work, the off-state breakdown characteristics of two different types InGaP-based high-barrier gate heterostructure field-effect transistors are studied and demonstrated. These devices have different high-barrier gate structures, e.g. the i-InGaP layer for device A and n  ⁺ - GaAs/p ⁺  -InGaP/n-GaAs camel-like structure for device B. The wide-gap InGaP layer is used to improve the breakdown characteristics. Experimentally, the studied devices show high off-state breakdown characteristics even at high temperature operation regime. This indicates that the studied devices are suitable for high-power and high-temperature applications. In addition, the off-state breakdown mechanisms are different for device A and B. For device A, off-state breakdown characteristics is only gate dominated at the temperature regime from 30 to 180  ^∘ C. For device B, off-state breakdown characteristics are gate and channel dominated at 30  ^∘ C and only gate dominated within 150 to 210  ^∘ C.     

Band offsets at ZnO/SiC heterojunction: Heterointerface in band alignment

Pulsed laser deposited ZnO layers on 6H-SiC substrates showed the six-fold symmetry, indicating a two-dimensional epitaxial growth mode. X-ray photoelectron spectroscopy was employed to study the valence band discontinuity and interface formation in the ZnO/6H-SiC heterojunction. The valence band offset was measured to be 1.38 ± 0.28 eV, leading to a conduction band offset value of 1.01 ± 0.28 eV. The resulting band lineup in epitaxial ZnO/6H-SiC heterojunction is determined to be of staggered-type alignment.     

Effect of oxygen gas pressure on orientations of Cu2O nuclei during the initial oxidation of Cu(100), (110) and (111)

The orientations of oxide nuclei during the oxidation of Cu(100), (110) and (111) surfaces have been examined by in situ transmission electron microscopy. Our results indicate that the epitaxial nucleation of oxide islands on these surfaces cannot be maintained for a whole range of oxygen gas pressure varying from 10^? 5 Torr to 750 Torr. The critical oxygen gas pressure, pO₂, leading to the transition from nucleating epitaxial to non-epitaxial oxide nuclei shows a dependence on the crystallographic orientations of the Cu substrates with pO₂⁽¹⁰⁰⁾ > pO₂⁽¹¹¹⁾ > pO₂⁽¹¹⁰⁾. By fitting the experimentally determined critical oxygen pressures to a kinetic model, we find that such dependence can be attributed to the effect of surface orientations of the Cu substrates on the oxygen surface adsorption and diffusion, which dominate the kinetic processes of oxide nucleation.     

InSb grown on Cd0.955Zn0.045Te by liquid phase epitaxy

InSb has been grown by liquid phase epitaxy using indium rich solutions with a supercooling of 2–5 °C onto (1 1 1) oriented Cd_0.955Zn_0.045Te substrates at 400–405 °C. The resulting epitaxial layers were extensively characterized using X-ray diffraction, optical microscopy, Raman spectroscopy and photoluminescence.     

High resolution measurement and modelling of magnetic domain structures in epitaxial FePd (0 0 1) L10 films with perpendicular magnetisation

Magnetic domain structures in two 50 nm thick chemically-ordered FePd (0 0 1) epitaxial films with different perpendicular anisotropies have been studied using Lorentz microscopy. Domain and domain wall structures vary significantly according to the magnitude of the anisotropy. For lower anisotropy films, a stripe domain structure with a period of ≈100 nm is formed in which there is a near-continuous variation in orientation of the magnetisation vector. By contrast, in the film with higher anisotropy, a maze-like domain structure is supported. The magnetisation within domains is perpendicular to the film plane and adjacent domains are separated by narrow walls, less than 20 nm wide. Micromagnetic modelling is generally in good quantitative agreement with experimental observations and provides additional information on the domain wall structure.     

The effect of junction temperature on the optoelectrical properties of InGaN/GaN multiple quantum well light-emitting diodes

Thermal effects on the optoelectrical characteristics of green InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs) have been investigated in detail for a broad temperature range, from 30 °C to 100 °C. The current-dependent electroluminescence (EL) spectra, current–voltage (I–V) curves and luminescence intensity–current (L–I) characteristics of green InGaN/GaN MQW LEDs have been measured to characterize the thermal-related effects on the optoelectrical properties of the InGaN/GaN MQW LEDs. The experimental results show that both the forward voltages decreased with a slope of ?3.7 mV/K and the emission peak wavelength increased with a slope of +0.02 nm/K with increasing temperature, indicating a change in the contact resistance between the metal and GaN layers and the existence of a band gap shrinkage effect. The junction temperature estimated from the forward voltage and the emission peak shift varied from 25.6 to 14.5 °C and from 22.4 to 35.6 °C, respectively. At the same time, the carrier temperature decreased from 371.2 to 348.1 °C as estimated from the slope of high-energy side of the emission spectra. With increasing injection current, there was found to be a strong current-dependent blueshift of ?0.15 nm/mA in the emission peak wavelength of the EL spectra. This could be attributed to not only the stronger band-filling effect but also the enhanced quantum confinement effect that resulted from the piezoelectric polarization and spontaneous polarization in InGaN/GaN heterostructures. We also demonstrate a helpful and easy way to measure and calculate the junction temperature of InGaN/GaN MQW LEDs.     

Wetting and reaction promoted by ultrasound between sapphire and liquid Al–12Si alloy

Ultrasonic-assisted wetting between sapphire bulks and liquid Al–12Si alloy in an atmospheric environment at 620 °C is carried out in this study. Complete, rather than partial, wetting and joining can be achieved with the aid of ultrasound. Growth of epitaxial alumina on sapphire bulks is promoted dramatically during ultrasonic-assisted wetting comparing to that during hot-dipping without ultrasound. XRD results show that the epitaxial alumina is non-crystalline. This indicates that the temperature on the surface of the sapphire substrate is not more than 1200 °C even though the collapse of acoustic cavitation bubbles could theoretically produce extremely high temperature. The bonding force at the interface between the Al–Si alloy and sapphire is strengthened because of the epitaxial alumina. The interfacial shear strength of sapphire/Al–Si alloy can reach as high as 60–65 MPa. The fracture morphology shows that cracks initiated at the interface between Si grains and the epitaxial alumina on sapphire. This result is especially useful for the joining of metals and ceramics.     

Mid-infrared photoconductive properties of heavily Bi-doped PbTe p–n homojunction diode grown by liquid-phase epitaxy

We fabricated a heavily Bi-doped (x_Bi ≈ 2 × 10¹⁹ cm⁻³) PbTe p–n homojunction diode that detects mid-infrared wavelengths by the temperature difference method (TDM) under controlled vapor pressure (CVP) liquid phase epitaxy (LPE). The photocurrent density produced by the heavily Bi-doped diode sample is approximately 20 times and 3 times greater than that produced by an undoped and heavily In-doped sample, respectively. By varying the ambient temperature from 15 K to 225 K, the detectable wavelength is tunable from 6.18 μm to 4.20 μm. The peak shift of the detectable wavelength is shorter in the heavily Bi-doped sample than in the undoped sample, consistent with our previously proposed model, in which Bi–Bi nearest donor–acceptor pairs are formed in the heavily Bi-doped PbTe liquid phase epitaxial layer. Current–voltage (I–V) measurements of the heavily Bi-doped diode sample under infrared exposure at 77 K indicated a likely leak in the dark current, arising from the deeper levels. From the dark I–V measurements, the activation energy of the deep level was estimated as 0.067 eV, close to the energy of the deep Tl-doped PbTe acceptor layer. We conclude that the deep level originates from the Tl-doped p-type epitaxial layer.     

Phase transition of Sr on Si (0 0 1): First principles prediction and experiment

The ability to understand and predict the phase diagrams of surface phases from first principles can be valuable for developing processes for growth of epitaxial structures. In the growth of epitaxial oxides on Si (0 0 1), a submonolayer phase of Sr plays a key role. The physical structure for this phase, which has 2 × 3 symmetry and occurs at 1/6 monolayer Sr coverage, was recently elucidated using both first principles theory and diffraction experiments [J.W. Reiner, K.F. Garrity, F.J. Walker, S. Ismail-Beigi, C.H. Ahn, Role of strontium in oxide epitaxy on silicon (0 0 1), Phys. Rev. Lett. 101 (10) (2008) 105503.]. Our approach to understanding the broader Sr/Si phase diagram combines density functional theory with a thermodynamic analysis of the phase equilibrium between a Sr lattice gas and the 2 × 3 structure. We use reflection high energy electron diffraction (RHEED) to experimentally determine the phase diagram, finding good agreement with theoretical predictions.     

Growth and characterization of CoSi2 films on Si (1 0 0) substrates

In the present work, a special solid phase epitaxy method has been adapted for the preparation of CoSi₂ film. This method includes an epitaxial growth of Co films on Si (1 0 0) substrate, and in situ annealing of the Co/Si films in vacuum. It has been found that at the substrate temperature of 360°C, fcc cobalt film grows epitaxially on the Si (1 0 0) surface. The crystallographic orientation relations between fcc Co film and Si substrate determined from the electron diffraction result are: (0 0 1) Co//(0 0 1) Si, [1 0 0] Co//[1 1 0]Si. Upon annealing at temperatures range from 500 to 600°C, Co film reacts with Si substrate and transforms into CoSi₂. The CoSi₂ films prepared by this way are characterized by XTEM, XPS and AFM.     

Ga induced 2D superstructural phase diagram on trenched Si(5 5 12) surface

The high index Si(5 5 12) surface offers morphological trenches, which can be interesting for epitaxial growth. In this study, the evolution of Ga adsorption at a very low flux rate of 0.03 ML/min on high index trenched Si(5 5 12) ? 2 × 1 reconstructed surface at various substrate temperatures ranging from room temperature (RT) to 600 °C has been investigated using in-situ AES, LEED and EELS. The Auger uptake curves, which plot the Ga(LMM)/Si(LVV) Auger intensity ratio with Ga adsorption time, show that Ga grows in layer plus islands mode for substrate temperatures in the RT to 350 °C range, while it grows in Volmer–Weber (3D islands) for higher substrate temperatures (> 350 °C). We also arrive at a complete 2D superstructural phase diagram for Ga/Si(5 5 12) interfacial system that shows the pathways to attain the different superstructural phases. The formation of Ga nanowires as (2 2 5), (3 3 7) phase and Ga 3D islands in the (1 1 2) ? 6 × 1, (1 1 2) ? 6 × 2 phases and other Ga induced superstructural phases like (7 7 17) + 2x(1 1 3), (2 2 5) + (3 3 7), 1 × 1 has been carefully followed. The electronic structures of each of the observed phases have been probed by EELS and each of them is shown to have characteristic features.     

Reduced crystallization time of YBCO in a fluorine-free MOD process using uv-lamp irradiation

We have studied the crystallization time dependence of the epitaxial YBCO films (t = 0.8 μm) grown on CeO₂-buffered SrTiO₃ substrates by fluorine-free metal–organic deposition using uv-lamp irradiation (uv-MOD). As increasing the time (T₀) for heat treatment at the reaction temperature (760 °C) from 0 to 90 min, J_c and the YBCO 0 0 l XRD intensity are steeply increased and reach their maximum values at T₀ = 10 min. This suggests that the heat treatment required for YBCO crystallization is significantly shortened in uv-MOD compared to conventional all-pyrolytic F-free MOD processes, which consume T₀ = 90–150 min for crystallizing 0.4–0.5-μm-thick films. Scanning electron microscope measurement revealed a drastic change in surface morphology between T₀ = 8 and 10 min, showing a good correspondence to the J_c and XRD data which suggest that the epitaxial growth reaches the film surface at the very early stage in the heat treatment.     

Control of electric field in 4H-SiC UMOSFET: Physical investigation

In this paper, we show how breakdown voltage (V_BR) and the specific on-resistance (R_on) can be improved simply by controlling of the electric field in a power 4H-SiC UMOSFET. The key idea in this work is increasing the uniformity of the electric field profile by inserting a region with a graded doping density (GD region) in the drift region. The doping density of inserted region is decreased gradually from top to bottom, called Graded Doping Region UMOSFET (GDR-UMOSFET). The GD region results in a more uniform electric field profile in comparison with a conventional UMOSFET (C-UMOSFET) and a UMOSFET with an accumulation layer (AL-UMOSFET). This in turn improves breakdown voltage. Using two-dimensional two-carrier simulation, we demonstrate that the GDR-UMOSFET shows higher breakdown voltage and lower specific on-resistance. Our results show the maximum breakdown voltage of 1340 V is obtained for the GDR-UMOSFET with 10 µm drift region length, while at the same drift region length and approximated doping density, the maximum breakdown voltages of the C-UMOSFET and the AL-UMOSFET structures are 534 V and 703 V, respectively.