As-grown Samples Research Articles

In-plane anisotropy in van der Waals epitaxial MoS2 on MoO2(010)

Transition metal dichalcogenide based 2D/2D or 2D/3D van der Waals heterostructures exhibit superior properties for high-performance electronics, tunneling transistors, and catalysts. Here, we report on the fabrication of high quality 2D/3D MoS2/MoO2 heterostructures with an atomic clean interface by one-step chemical vapor deposition. Optical microscopy, atomic force microscopy, Raman spectroscopy, scanning electron microscopy, and energy dispersive spectroscopy measurements reveal the high-quality of as-grown samples and the specific epitaxial relationship between MoS2 and MoO2: MoS2[1¯1¯20]//MoO2[001] and MoS2[1¯100]//MoO2[201]. Photoluminescence and Kelvin probe force microscopy measurements combined with density functional theory calculations confirm the interfacial charge transfer from MoS2 to the underlying MoO2. Furthermore, MoO2 induced in-plane anisotropy in MoS2 was revealed using angle-resolved polarized Raman and photoluminescence spectroscopy with anisotropic ratios of 1.27 (Raman) and 1.29 (photoluminescence), respectively, which is most possibly attributed to anisotropic interfacial charge interactions. Our findings provide an excellent platform for the investigation on interfacial effects. Moreover, the in-plane anisotropy in MoS2 induced by MoO2 has expanded the application of isotropic MoS2 in the polarization-dependent fields.

Annealing effect on structural and optical properties of glad synthesized Gd2O3 nanorod

Gadolinium oxide nanorod (Gd2O3 NR) was fabricated on an n-type silicon substrate by catalyst-free glancing angle deposition (GLAD) technique. The as-grown sample was annealed at 500 °C to investigate the impact of annealing on structure and optical properties. XRD analysis reveals an improvement in the crystal structure after annealing. Field emission scanning electron microscope (FE-SEM) and energy dispersive X-ray (EDX) confirm the successful growth of vertically aligned Gd2O3 NR with the presence of Gd, O, and Si. Transmission electron microscope (TEM) showed the crystalline nature of the grown Gd2O3 NR. Finally, UV–vis spectroscopy was performed to study the optical properties. A significant enhancement in the photon absorption (200–300 nm) was observed, which can be attributed to increased grain size, improved crystallinity, and large generation of electron–hole pairs. In addition, the Tauc plot showed a decrease in the bandgap from 5.05 eV to 4.98 eV after annealing at 500 °C.

Electrical characterization of Au/Ni Schottky contacts on GaN synthesized using electrodeposition

Gallium nitride thin films have attracted attention due to their prospects in semiconductor devices and technology. In this study, we investigate the electrical properties and perform deep level transient spectroscopy (DLTS) on Au/Ni Schottky diodes fabricated on gallium nitride thin films that were synthesized by electrodeposition on a Si(111) substrate from a solution containing gallium nitrate (Ga(NO3)3) and ammonium nitrate (NH4(NO3)) using current densities of 1 and 3 mA cm−2. The thin films were found to crystallize in the wurtzite hexagonal structure with crystallite sizes of approximately 20 nm. Scanning electron microscopy and atomic force microscopy were used to characterize the microstructure of the GaN thin films. The Schottky diodes had good rectifying properties, corresponding to n-type material. The diodes had a IV barrier heights of 0.76 eV and 0.60 eV; a CV barrier heights of 0.92 eV and 0.71 eV; and carrier densities of 1.2 × 1016 cm−3, and 1.7 × 1016 cm−3, for material deposited under 1 mA cm−2 and 3 mA cm−2, respectively. By fitting a model taking both thermionic emission as well as conduction through the polycrystalline bulk into account, it was found that the grain boundary potential of the GaN crystallites was 0.29 V. The DLTS study revealed different dominant DLTS peaks in each as-grown sample with activation energies of 0.49 and 0.48 eV with capture cross-sections of 9 × 10−15 and 4 × 10−16 cm2, in the material grown under 1 mA cm−2 and 3 mA cm−2 current density, respectively. The defects seem to be similar to defects found in heavily damaged GaN and GaN grown on highly mismatched or cracked substrates, and are probably not simple defects.

Assessment of surface recombination in mid-wave infrared InAsSb nBn detectors using transient microwave reflectance

Investigation of surface recombination is an important area for infrared detectors as the demand for smaller pixels increases. In this study, we use transient microwave reflectance to characterize the minority carrier lifetime of InAsSb nBn structures under three conditions: As-Grown, blanket Barrier-Etched, and SU-8 Passivated blanket barrier-etched. A qualitative comparison of these three samples shows that the minority carrier lifetime decreases for the Barrier-Etched sample compared to the As-Grown sample, indicating that the minority carrier lifetime is sensitive to changes in the sample surface, specifically the introduction of surface states. We compare these samples quantitatively using a polynomial fit (A−1 + Bn + Cn2). We find for the As-Grown sample A = 1.22 ± 0.45 µs, B = 2.5 ± 0.2 × 10−12 cm3/s, and C = 5.004 ± 4.996 × 10−31 cm6/s, for the Barrier-Etched sample A = 1.17 ± 0.40 µs, B = 9.9 ± 0.2 × 10−12 cm3/s, and C = 9.502 ± 9.498 × 10−30 cm6/s, and for the Passivated sample A = 1.34 ± 0.45 µs, B = 5.3 ± 0.5 × 10−12 cm3/s, and C = 5.500 ± 4.500 × 10−33 cm6/s.

Synthesis and Characterization of Highly Crystalline Bi-Functional Mn-Doped Zn2SiO4 Nanostructures by Low-Cost Sol-Gel Process.

Herein, we demonstrate a process for the synthesis of a highly crystalline bi-functional manganese (Mn)-doped zinc silicate (Zn2SiO4) nanostructures using a low-cost sol-gel route followed by solid state reaction method. Structural and morphological characterizations of Mn-doped Zn2SiO4 with variable doping concentration of 0.03, 0.05, 0.1, 0.2, 0.5, 1.0, and 2.0 wt% were investigated by using X-ray diffraction and high-resolution transmission electron microscopy (HR-TEM) techniques. HR-TEM-assisted elemental mapping of the as-grown sample was conducted to confirm the presence of Mn in Zn2SiO4. Photoluminescence (PL) spectra indicated that the Mn-doped Zn2SiO4 nanostructures exhibited strong green emission at 521 nm under 259 nm excitation wavelengths. It was observed that PL intensity increased with the increase of Mn-doping concentration in Zn2SiO4 nanostructures, with no change in emission peak position. Furthermore, magnetism in doped Zn2SiO4 nanostructures was probed by static DC magnetization measurement. The observed photoluminescence and magnetic properties in Mn-doped Zn2SiO4 nanostructures are discussed in terms of structural defect/lattice strain caused by Mn doping and the Jahn-Teller effect. These bi-functional properties of as-synthesized Zn2SiO4 nanostructures provide a new platform for their potential applications towards magneto-optical and spintronic and devices areas.

Second harmonic generation and simplified bond hyperpolarizability model analyses on the intermixing of Si/SiGe stacked multilayers for gate-all-around structure

Si/SiGe stacked multilayers are key elements in fabrication of gate-all-around (GAA) structures and improvement of electrical properties, with the evolution of the Si/SiGe interfaces playing a crucial role. In this work, a model is developed based on the simplified bond hyperpolarizability model (SBHM) to analysis the anisotropic reflective second harmonic generation (Ani-RSHG) on a three-period stacked Si/Si1−x Ge x multilayer, which builds on Si(100) diamond structures. The C 4v symmetry of the Si(100) structure enables the second harmonic generation (SHG) contribution from the bonds to be simplified and the effective hyperpolarizabilities of the interfacial and bulk sources to be obtained. The effective interface dipolar and bulk quadrupolar SHG hyperpolarizabilities in the Si1−x Ge x sample with various Ge concentration profiles are modeled by interpreting the concentration of a component element as the probability of the element occupying an atomic site. On the basis of the developed model, the Ani-RSHG spectra of the as-grown samples with various Ge ratios for each layer and the samples annealed at 850 °C and 950 °C are analyzed to inspect the change in Ge distribution and its gradient in depth. The ani-RSHG analysis on as-grown samples showed difference in Ge distribution in samples with the multi Si/SiGe structure, which is not well observed in synchrotron x-ray diffraction (XRD) spectra. For the annealed samples, the response to changes in Ge concentration and its gradient in depth reveal the Si/Si1−x Ge x interface intermixing. Results of high-angle annular dark-field scanning transmission electron microscopy and energy dispersive x-ray spectroscopy agree well with the Ani-RSHG with SBHM findings. Compared with the Raman and synchrotron XRD spectra, the Ani-RSHG with SBHM simulation result demonstrates much better response to changes in compositions of the Si/Si1−x Ge x stacked multilayered structures, verifying the potential for characterizing the concentration distribution in stacked multilayered thin films for GAA structures.

Improving the short-wave infrared response of strained GeSn/Ge multiple quantum wells by rapid thermal annealing

In this work, the evolution of structural, optical and optoelectronic properties of coherently strained Ge0.883Sn0.117/Ge multiple quantum wells (MQWs) grown by molecular beam epitaxy under rapid thermal annealing (RTA) is systematically investigated. The MQW structure remains fully-strained state with RTA at 400 °C or below and disrupts at higher annealing temperatures due to Sn segregation and interdiffusion of Ge and Sn atoms. The GeSn well layers exhibit the strongest absorption in 2.0–2.4 μm after annealing at 400 °C and become transparent above 1.8 μm after RTA at 600 °C or beyond due to serve Sn segregation. Owing to improved crystal quality after RTA at 400 °C, the dark current of the fabricated metal-semiconductor-metal photodetector is effectively lowered by more than two times. Additionally, the responsivities at 1.55 and 2.0 μm are improved by 4.15 and 3.78 folds, respectively, compared to those of the as-grown sample. The results can be an insightful guidance for the development of high-performance short-wave infrared photonic devices based on Sn-containing group-IV low-dimensional structures.

Effect of annealing on forming-free bipolar resistive switching of Gd2O3 thin films

Oxide-based resistive switching devices are a promising candidate for non-volatile memory applications. Here, we present a Gd2O3-based non-volatile memory device deposited using e-beam evaporation. In order to know the effect of annealing on the device performance, the as-grown sample was annealed at 500 °C. The capacitance (C) – voltage (V) and conductance (G) – voltage (V) measurements were investigated in the range of 200 kHz to 1 MHz. It was observed that the C-V and G-V changes with frequency due to the presence of interface states. The effect of series resistance (Rs) and interface state density (Dit) was also investigated for both devices. Furthermore, the resistive switching was recorded at± 5 V. Both devices exhibit bipolar resistive switching with charge conduction mechanism ascribed to ohmic, space charge limited conduction, and trap-filled space charge limited conduction. Consequently, the annealed device displayed an improved resistance ratio of 102 and a stable endurance of up to 1000 cycles with better data retention of over 103 s.

Enhanced light output of Eu, O-codoped GaN caused by reconfiguration of luminescent sites during post-growth thermal annealing

Luminescence efficiency of Eu-related emission from Eu, O-codoped GaN (GaN:Eu, O) strongly depends on the local structure of Eu ions. Growth at relatively low temperature (∼960 °C) not only enables high Eu doping concentration but also elevates Eu-clustering due to its low diffusion coefficient, which results in formation of a large number of inefficient luminescent sites. We have studied the impact of post-growth thermal annealing at high temperatures on elimination of Eu clusters by photoluminescence measurements. These clarify that thermal annealing at high temperatures induces changes in the structural conformation and converts inefficient luminescent sites to efficient ones. As a result, the sample annealed at 1100 °C shows increased luminescence efficiency with a maximum of 5.1 times that of the as-grown sample. Post-growth thermal annealing offers a way to improve the efficiency of GaN:Eu, O further for practical application in III-nitride-based monolithic three-primary colors' light-emitting diodes.

Structural Evolution from Neutron Powder Diffraction of Nanostructured SnTe Obtained by Arc Melting

Among chalcogenide thermoelectric materials, SnTe is an excellent candidate for intermediate temperature applications, in replacement of toxic PbTe. We have prepared pure polycrystalline SnTe by arc melting, and investigated the structural evolution by temperature-dependent neutron powder diffraction (NPD) from room temperature up to 973 K. In this temperature range, the sample is cubic (space group Fm-3m) and shows considerably larger displacement parameters for Te than for Sn. The structural analysis allowed the determination of the Debye model parameters and provided information on the Sn–Te chemical bonds. SEM images show a conspicuous nanostructuration in layers below 30 nm thick, which contributes to the reduction of the thermal conductivity down to 2.5 W/m·K at 800 K. The SPS treatment seems to reduce the number of Sn vacancies, thus diminishing the carrier density and increasing the Seebeck coefficient, which reaches 60 μV K−1 at 700 K, as well as the weighted mobility, almost doubled compared with that of the as-grown sample.

Towards B-doped p-BaSi2 films on Si substrates by co-sputtering of BaSi2, Ba, and B-doped Si targets

BaSi2 is one of the emerging materials for thin-film solar cell applications; hence the conductivity control by impurity doping is of great importance. The formation of B-doped p-BaSi2 films has been achieved by molecular beam epitaxy and vacuum evaporation. We fabricated B-doped BaSi2 films on Si substrates at 600 °C by co-sputtering BaSi2, Ba, and B-doped Si targets, followed by post-annealing at 900 °C or 1000 °C for 5 min in an Ar atmosphere. Contrary to expectations, as-grown sample and the sample annealed at 900 °C showed n-type conductivity, while the sample annealed at 1000 °C showed p-type conductivity. The reason for the n-type conductivity was discussed based on first-principles calculation considering the presence of oxygen atoms in the order of 1021 cm−3. The n-type conductivity for B-doped BaSi2 is possible only when both the B and O atoms being a substitution impurity are in the same Si4 tetrahedron.

The effect of an external magnetic field on the magnetic properties of Gd-substituted CaMnO3 prepared by laser floating zone

• CaMn 0.9 Gd 0.1 O 3 fibres were obtained by LFZ under an external magnetic field (Hext) • Samples with CaMnO 3 major phase with some secondary phases were obtained. • Hext in grows direction favours phase alignment and magnetic properties. • Results show the viability to apply a Hext to other oxides tuning their properties. The perovskite-type calcium manganite exhibits interesting magnetic properties that can be tuned and enhanced by the laser floating zone (LFZ) method. This work explores, for the first time, the application of an external magnetic field during the LFZ growth of CaMn 0.9 Gd 0.1 O 3 . The effects of the field direction on structural, microstructural, electrical and magnetic properties of the as-grown samples are evidenced, being observed an improvement on grain alignment and the magnetization (31.22 emu/g for 5 T at 5 K), when samples are grown under an external magnetic field in the growth direction, while the magnetic field applied in opposite direction a small improvement is observed. The results prospects to applied this process to other oxide systems for tuning their magnetic properties.

High-temperature post-annealing effect on the device performance of B-doped p-type BaSi2/n-Si heterojunction solar cells grown by molecular beam epitaxy

AstractWe fabricated B-doped p-BaSi2/n-Si heterojunction solar cells by molecular beam epitaxy, and performed post-annealing at 1000 °C in an Ar atmosphere for various annealing durations (t a) in the range 15–300 s. At t a = 15 s, the conversion efficiency (η) under AM 1.5 G illumination was degraded unexpectedly from η = 1.96% for the as-grown sample to η = 0.8%. At t a ≥ 30 s, improvement of the open-circuit voltage (V OC) was obtained. V OC was increased from 0.28 V for the as-grown sample to 0.45 V at t a = 30 s, and the η reached 5.1%. Deep level transient spectroscopy measurement indicated the presence of hole traps coming from point defects near the BaSi2/Si interface. The surface morphology remained almost unchanged at t a ≤ 60 s. However, part of the surface peeled off at t a ≥ 120 s when we separated the attached samples from each other.

Exciton spectroscopy and unidirectional transport in MoSe2-WSe2 lateral heterostructures encapsulated in hexagonal boron nitride

Chemical vapor deposition (CVD) allows lateral edge epitaxy of transition metal dichalcogenide heterostructures. Critical for carrier and exciton transport is the material quality and the nature of the lateral heterojunction. Important details of the optical properties were inaccessible in as-grown heterostructure samples due to large inhomogeneous broadening of the optical transitions. Here we perform optical spectroscopy of CVD grown MoSe2-WSe2 lateral heterostructures, encapsulated in hBN. Photoluminescence (PL), reflectance contrast and Raman spectroscopy reveal optical transition linewidths similar to high quality exfoliated monolayers, while PL imaging experiments uncover the effective excitonic diffusion length of both materials. The typical extent of the covalently bonded MoSe2-WSe2 heterojunctions is 3 nm measured by scanning transmission electron microscopy (STEM). Tip-enhanced, sub-wavelength optical spectroscopy mapping shows the high quality of the heterojunction which acts as an excitonic diode resulting in unidirectional exciton transfer from WSe2 to MoSe2.

Comprehensively analysis of hot electron transport in as-grown and thermally annealed n-type modulation-doped Al0.15Ga0.85As/GaAs0.96Bi0.4 quantum well structure

We report experimental and analytical results on hot electron transport in as-grown and thermally annealed n-type modulation-doped Al0.15Ga0.85As/GaAs0.96Bi0.4 quantum well (QW) structures at room temperature. The drift mobility of as-grown and thermally annealed samples is 2265 and 1414 cm2/Vs at a low electric field region, respectively. The lower electric field electron mobility for the thermally annealed sample is due to the increased 2D electron density following the annealing process. The drift velocity (vdrift) of the as-grown sample tends to saturate at 6.1 × 106 cm/s, contrary to the thermally annealed sample, for which no saturation is observed. The vdrift of the annealed sample is also lower due to the higher 2D electron density caused by annealing. Analyzing the experimental results concerning the theoretical model reveals that i) hot electron transport occurs in parallel mode due to the presence of electrons in both Al0.15Ga0.85As (barrier) and QW layers, ii) hot electrons transferred from QW to the barrier layer via so-called real space transfer (RST) mechanism is suppressed at higher electric fields even though the lower energy barrier between the barrier and QW layers, and iii) transfer of electrons from QW (Γ-valley) to the nearest L-valley, inter-valley transfer (IVT), dominates on the hot electron transfer at higher electric fields even with a higher energy barrier.

Phase transformation from FeSe to Fe3Se4

We report on thermal atomic layer deposition of iron selenide (FeSe and Fe3Se4) thin films with four-inch wafer-scale uniformity using two precursors of iron bis(N, N′-diisopropylacetamidinato) [Fe(i-Pr-Me-AMD)2] and bis(triethysilyl) selenide [(Et3Si)2Se]. The temperature window is determined to be 350–400 °C. The transformation from tetragonal FeSe to monoclinic Fe3Se4 has been found. The tetragonal FeSe thin films exhibit a strong (00 l) texture deposited on amorphous silica glass, suggesting that the surface is partially parallel to the layered ab plane. By adjusting the ratio of iron and selenium, the thin films exhibit from conductors to semiconductors, and finally to insulators. All as-grown conductive samples appear metallic-like temperature dependence by low-temperature electrical transport measurements. This study opens the way to prepare various metal selenides by atomic layer deposition.

Effect of Post-Annealing on Magnetotransport and Magnetic Properties of TaCo2Te2 Single Crystals.

The extreme magnetoresistance (XMR) of some compounds, challenging our understanding of magnetoresistance, is an interesting topic in condensed-matter and materials physics and future device applications. Here, we reported magnetotransport and magnetic properties of the as-grown and post-annealed TaCo2Te2 single crystals. The resistivity evolution with temperature in the two TaCo2Te2 single crystals shows a metallic behavior. Below 50 K, the XMR effect for the two crystals is found, and MR values at 3 K under 9 T are about 3.72 × 103% for the as-grown TaCo2Te2 and 5.71 × 102% for the annealed samples, larger than that of the previous report. The studies on the Hall effect of the two TaCo2Te2 single crystals indicate the multiband feature with high carrier mobilities from a two-band model. Electron and hole concentrations and mobilities of as-grown samples are comparable, while for the annealed sample, the hole concentration and mobility are larger than the electron concentration and mobility. The carrier mobilities for the two TaCo2Te2 single crystals have the same order of magnitude, ∼103 cm2 V-1 s-1. The XMR effect may be from high carrier mobilities. Magnetization of the as-grown TaCo2Te2 decreases with increasing temperature, and a weaker magnetic transition at ∼150 K is observed. The annealed TaCo2Te2 shows no magnetic transition and just a paramagnetic behavior with rising temperature. These results indicate that defects/deficiencies may play an important role in magnetotransport and magnetic properties of the two TaCo2Te2 single crystals. These results are helpful in deeply understanding the XMR mechanism and magnetic properties in TaCo2Te2 and offer a way to study the magnetic properties of the XMR Co-Te system.

Enhancing the thermoelectric performance of β-Zn4Sb3 via progressive incorporation of Zn interstitials

Owing to the intrinsically low lattice thermal conductivity, β-Zn4Sb3 is regarded as a promising candidate for thermoelectric applications. To further improve its thermoelectric performance, it is essential to increase the carrier mobility of β-Zn4Sb3 for enhanced electrical transport. While incorporating light ions is a useful approach for this purpose, the excess Zn ions tend to segregate in as-grown samples. Here, we present a design strategy to stabilize the excess Zn ions in the interstitial sites by applying electric current and uniaxial stress, which triggers a progressive directional migration of Zn ions towards one terminal of the sample. In this Zn-enriched end, a homogeneous distribution of excess Zn interstitials was achieved, as confirmed by atom probe tomography and transmission electron microscopy at the atomic scale. As revealed by ab initio simulations, the additional electrons brought by Zn interstitials connect the separated (Sb2)4- dimers, enhancing the carrier mobility and thus the electrical conductivity. In consequence, the detached Zn-enriched section shows a much improved thermoelectric figure of merit, a very high maximum ZT of ∼1.5 at 618 K, and an average ZT of ∼1.1 between 300 and 618 K, outperforming the majority of thermoelectric materials in this temperature range.

Layered topological semimetal GaGeTe: New polytype with non-centrosymmetric structure

GaGeTe is a layered van der Waals material composed of germanene and GaTe sublayers that has been recently predicted to be a basic Z2 topological semimetal. To date, only one polytype of GaGeTe is known with trigonal centrosymmetric structure (α phase, space group R-3m, No. 166). Here we show that as-grown samples of GaGeTe show traces of at least another polytype with hexagonal non-centrosymmetric structure (β phase, space group P63mc, No. 186). Moreover, we suggest that another bulk hexagonal polytype (γ phase, space group P-3m1, No. 164) could also be found near room conditions. Both α and β polytypes have been identified and characterized by means of X-ray diffraction and Raman scattering measurements with the support of ab initio calculations. We provide the vibrational properties of both polytypes and show that the Raman spectrum reported for GaGeTe almost forty years ago and attributed to the α phase, was, in fact, that of the secondary β phase. Additionally, we show that a Fermi resonance occurs in α-GaGeTe under non-resonant excitation conditions, but not under resonant excitation conditions. Theoretical calculations show that bulk β-GaGeTe is a non-centrosymmetric weak topological semimetal with even smaller lattice thermal conductivity than centrosymmetric bulk α-GaGeTe. In perspective, our work paves the way for the control and engineering of GaGeTe polytypes to design and implement complex van der Waals heterostructures formed by a combination of centrosymmetric and non-centrosymmetric layers of up to three different polytypes in a single material, suitable for a number of fundamental studies and technological applications.

GaN nanostructure for hydrogen gas sensor

The growth of heterostructure of n-type GaN/AlN/Si(111) is carried out using the molecular beam epitaxy (MBE) Veeco model Gen II system. The surface morphology of the as-grown GaN sample showed pits on the GaN surface in a ratio small than those found by other research groups. Porous GaN samples were synthesized by an electrochemical etching technique combined with increasing the current density to 75 mA/cm2. The formation of pore structures are of different sizes, the etched surface became hexagonal, and pore structures are confined to a smaller size. The PL results showed greater blue shift luminescence in comparison to results found by other research groups. The reduction in crystallite size is confirmed by an increase in the broadening of XRD spectra. Raman spectra also displayed a strong band at 522 cm−1 from the Si(1 1 1) substrate, and a small band at 301 cm−1. These are due to the acoustic phonons of Si. Two Raman active optical phonons are assigned to h-GaN at 139 cm−1 and 568 cm−1, due to E2 (low) and E2 (high) respectively. The sensitivity of the gas sensor is increased as a function of the hydrogen flow rate and they became much higher compared to the as-grown sample.