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Highly efficient growth of a centimeter-scale MoS

A newly developed oxide scale sublimation chemical vapor deposition (OSSCVD) technique for 2D MoS

Monolayer MoSe2(0001) films are epitaxially grown on the (111)B-, (110)-, and (001)-oriented GaAs substrates by molecular-beam epitaxy. We found that the lattice mismatch is not a significant facto...

Purcell effect enhancement of spontaneous emission rates is demonstrated for isoelectronic trap single-photon emitters. Two-dimensional photonic crystal slabs with L3 defects were fabricated in nit...

There is increasing demand for the ability to form ohmic contacts without lossy intermediate layers on both the top and bottom sides of metal-semiconductor-metal plasmoelectronic devices such as quantum cascade lasers and metasurface photodetectors. Although highly Si-doped n-GaAs surfaces can allow an ohmic contact without alloying, conditions for realizing nonalloyed ohmic contacts to other n-GaAs surfaces, originally buried inside but exposed by removing the substrate, have yet to be studied. We discovered that nonalloyed ohmic contacts to initially buried surfaces with a practically low contact resistivity down to 77 K can be realized by fulfilling certain requirements, specifically keeping the Si-doping concentration within a narrow range of 7.5 × 1018 to 1.25 × 1019 cm-3 and setting the growth temperature of the succeeding upper layers to a low value of 530 °C.

Two‐dimensional (2D) transition metal dichalcogenides (TMDCs) with unique electrical properties are fascinating materials used for future electronics. However, the strong Fermi level pinning effect at the interface of TMDCs and metal electrodes always leads to high contact resistance, which seriously hinders their application in 2D electronics. One effective way to overcome this is to use metallic TMDCs or transferred metal electrodes as van der Waals (vdW) contacts. Alternatively, using highly conductive doped TMDCs will have a profound impact on the contact engineering of 2D electronics. Here, a novel chemical vapor deposition (CVD) using mixed molten salts is established for vapor–liquid–solid growth of high‐quality rhenium (Re) and vanadium (V) doped TMDC monolayers with high controllability and reproducibility. A tunable semiconductor to metal transition is observed in the Re‐ and V‐doped TMDCs. Electrical conductivity increases up to a factor of 108 in the degenerate V‐doped WS2 and WSe2. Using V‐doped WSe2 as vdW contact, the on‐state current and on/off ratio of WSe2‐based field‐effect transistors have been substantially improved (from ≈10–8 to 10–5 A; ≈104 to 108), compared to metal contacts. Future studies on lateral contacts and interconnects using doped TMDCs will pave the way for 2D integrated circuits and flexible electronics., A chemical vapor deposition method using mixed molten salts is established for vapor–liquid–solid growth of Re‐ and V‐doped transition metal dichalcogenide (TMDC) monolayers. Tunable semiconductor to metal transition is observed in the as‐grown Re‐ and V‐doped TMDC monolayers. Using heavily V‐doped WSe2 as van der Waals contact, the performance of WSe2‐based field‐effect transistors is improved by 1–3 orders of magnitude compared to Au and Pd.

We investigated the optical properties of nanoscale WS2 monolayers treated with organic solvents. The photoluminescence spectra changed significantly before and after methanol treatment. The sharp spectral peak of the neutral exciton in the methanol-treated sample was shifted to the higher energy side by 18 meV compared to the peak in untreated sample. This shift made the emission peak due to charged excitons clearly visible. Detailed fitting analysis revealed that the methanol treatment relaxed the tensile strain in the nano-sized WS2 monolayer grown on SiO2/Si substrate. In addition to the measurements in air, the results in vacuum and their temperature dependence also support this interpretation. Since this methanol treatment is simple, does not cause sample loss, and does not reduce luminescence efficiency, it may be an effective means to relax strain from nanoscale transition metal dichalcogenides monolayers.

The usage of molten salts, e.g., Na2MoO4 and Na2WO4, has shown great success in the growth of two-dimensional (2D) transition metal dichalcogenides (TMDCs) by chemical vapor deposition (CVD). In comparison with the halide salt (i.e., NaCl, NaBr, KI)-assisted growth (Salt 1.0), the molten salt-assisted vapor-liquid-solid (VLS) growth technique (Salt 2.0) has improved the reproducibility, efficiency and scalability of synthesizing 2D TMDCs. However, the growth of large-area MoSe2 and WTe2 is still quite challenging with the use Salt 2.0 technique. In this study, a renewed Salt 2.0 technique using mixed salts (e.g., Na2MoO4-Na2SeO3 and Na2WO4-Na2TeO3) is developed for the enhanced CVD growth of 2D MoSe2 and WTe2 crystals with large grain size and yield. Continuous monolayer MoSe2 film with grain size of 100-250 {\mu}m or isolated flakes up to ~ 450 {\mu}m is grown on a halved 2-inch SiO2/Si wafer. Our study further confirms the synergistic effect of Na+ and SeO32- in the enhanced CVD growth of wafer-scale monolayer MoSe2 film. And thus, the addition of Na2SeO3 and Na2TeO3 into the transition metal salts could be a general strategy for the enhanced CVD growth of many other 2D selenides and tellurides., Comment: 35 pages, 11 figures and 1 table

We have systematically studied the strain relaxation processes in GaSb heteroepitaxy on GaAs(111)A using thin InAs interlayers. The growth with 1 ML- and 2 ML-InAs leads to formation of an InAsSb-like layer, which induces tensile strain in GaSb films, whereas the GaSb films grown with thicker InAs layers (≥3 ML) are under compressive strain. As the InAs thickness is increased above 5 ML, the insertion of the InAs layer becomes less effective in the strain relaxation, leaving residual strain in GaSb films. This leads to the elastic deformation of the GaSb lattice, giving rise to the increase in the peak width of X-ray rocking curves.

We investigate the carrier transfer in stacked droplet epitaxially grown GaAs quantum dots (QDs) in experiments and calculations. While in the Stranski-Krastanov growth mode, QDs align due to stain propagation, droplet epitaxy QDs pose a difficulty for achieving coupled stacked QDs due to their random positioning. We demonstrate that carrier transfer is possible in such structures by designing their size and areal density. We achieve a significant geometrical overlap between stacked QDs by employing an areal density of 3.9×1010 dots/cm2 and an average QD diameter of 45.5 nm. A clear redshift in the position of the photoluminescence peak is observed when the separation layer’s thickness is reduced from 16 nm to 2.5 nm. Theoretical calculations of the electronic states of the stacked QDs with varying degrees of misalignment confirm that this red-shift is mainly caused by a lowering of the ground state energy due to coupling. To separately analyze the effect of vertical carrier transfer between QDs, we investigate samples with two layers of stacked QDs of different sizes. We demonstrates in photoluminescence experiments that carriers readily transfer to the larger QD when the barrier thickness is reduced to a degree where tunneling is possible.

We successfully grow vertically aligned quantum ring-dot structures by Multiple Droplet Epitaxy technique. The growth is achieved by depositing GaAs quantum rings in a first droplet epitaxy process which are subsequently covered by a thin AlGaAs barrier. In a second droplet epitaxy process, Ga droplets preferentially position in the center indentation of the ring as well as attached to the edge of the ring in [ 1 1 ¯ 0 ] direction. By designing the ring geometry, full selectivity for the center position of the ring is achieved where we crystallize the droplets into quantum dots. The geometry of the ring and dot as well as barrier layer can be controlled in separate growth steps. This technique offers great potential for creating complex quantum molecules for novel quantum information technologies.

We study the luminescence dynamics of telecom wavelength InAs quantum dots grown on InP(111)A by droplet epitaxy. The use of the ternary alloy InAlGaAs as a barrier material leads to photon emission in the 1.55 $\mu$m telecom C-band. The luminescence decay is well described in terms of the theoretical interband transition strength without the impact of nonradiative recombination. The intensity autocorrelation function shows clear anti-bunching photon statistics. The results suggest that our quantum dots are useful for constructing a practical source of single photons and quantum entangled photon pairs., Comment: 4 pages

Strain relaxation processes in InAs heteroepitaxy have been studied. While InAs grows in a layer-by-layer mode on lattice-mismatched substrates of GaAs(111)A, Si(111), and GaSb(111)A, the strain relaxation process strongly depends on the lattice mismatch. The density of threading defects in the InAs film increases with lattice mismatch. We found that the peak width in x-ray diffraction is insensitive to the defect density, but critically depends on the residual lattice strain in InAs films.

Vapor transportation is the core process in growing transition-metal dichalcogenides (TMDCs) by chemical vapor deposition (CVD). One inevitable problem is the spatial inhomogeneity of vapors. The non-stoichiometric supply of transition-metal precursors and chalcogens leads to poor control in the products' location, morphology, crystallinity, uniformity and batch to batch reproducibility. The vapor-liquid-solid (VLS) growth method often involves molten precursors (e.g., non-volatile Na2MoO4) at growth temperatures higher than their melting points. The liquid Na2MoO4 can precipitate out solid MoS2 monolayers when saturated with sulfur vapor. Taking advantage of the VLS growth, we attained three kinds of important achievements: (i) a 4-inch-wafer-scale uniform growth of MoS2 flakes on SiO2/Si substrates, (ii) a 2-inch-wafer-scale growth of continuous MoS2 film with the grain size exceeding 100 μm on sapphire substrates, and (iii) a patterned (site-controlled) growth of MoS2 flakes and films. We clarified that the VLS growth thus paves a new way for the high-efficient and scalable synthesis of two-dimensional TMDC monolayers.

Infrared (IR) photo-responses of the Schottky-barrier gate region are investigated in an n-AlGaAs/GaAs field-effect transistor (FET). The photocurrent from the channel region to the gate is strongly enhanced by about 3.4 times when a weak second light having photon energy above the GaAs bandgap illuminates the FET locally. The enhancement effect is independent of the illumination position of the second light, and remains even when the position is far away from the gate region. The experimental results are compared with a theoretical model based on the electron drift and hole diffusion.

A new strategy using mixed salts for chemical vapor deposition of tunable Re‐ and V‐doped 2D transition metal dichalcogenides (TMDCs) is realized by Shisheng Li, Takaaki Taniguchi, and co‐workers, as described in article number 2004438. By applying the highly conductive V‐doped WSe(2) as van der Waals contacts for 2D WSe(2) based field‐effect transistor, the on‐state current and on/off ratio are substantially improved, compared to the metal contacts. [Image: see text]

We investigate the photovoltaic properties of AlGaAs solar cells with embedded GaNAs quantum wells (QWs) with N concentrations in the range of 0–3.1%, for which the QW confinement energy can be tuned by adjusting the N concentration. We systematically study the dependence of open-circuit voltage $V_{{{\rm OC}}}$ in relation to the lowest band-to-band transition energy. In samples with low N concentrations (shallow QW confinement), $V_{{\rm{OC}}}$ degrades and is limited by the lowest transition energy in the solar cell, i.e., the QW transition. With increasing N concentration, N > 0.5% (deep QW confinement), $V_{{\rm{OC}}}$ does not degrade further and is no longer limited by the QW transition energy. The highest N sample exhibits a remarkably small offset between the lowest transition energy and the achieved $V_{{\rm{OC}}}$ of 0.23 V, which is beyond the detailed balance limit of standard solar cells. $V_{{\rm{OC}}}$ dependence is explained by analyzing the current–voltage (I–V) characteristics under different illumination conditions, from which information about the balance of escape and recombination rates of carriers from the QWs is extracted. In the deeply confined QWs, tunneling and thermal carrier escape is completely suppressed, allowing the recovery of $V_{{{\rm OC}}}$ .

We have systematically studied the atomistic growth processes of monolayer MoSe2 on GaAs(111)A and (111)B substrates. A combination of complementary techniques of reflection high-energy electron diffraction, scanning tunneling microscopy, and X-ray photoelectron spectroscopy allows us to study the evolution processes of surface and interface structures during the MoSe2 growth. Highly oriented MoSe2 films are epitaxially grown in two steps: the amorphous Mo islands are initially formed on Se-treated GaAs surfaces, which is followed by the crystallization into MoSe2 under the supply of a Se molecular beam. While the initial Mo deposition leads to the segregation of Se atoms from the Se-treated GaAs surface, the subsequent supply of the Se beam induces the reconstruction of the Se-terminated GaAs structure beneath the MoSe2 film.

We have successfully grown high quality InxGa1-xAs metamorphic layer on GaAs (111)A using molecular beam epitaxy. Inserting a thin 3.0-7.1 monolayer (ML) InAs interlayer between the In0.25Ga0.75As and GaAs allowed the formation of a nearly lattice-relaxed In0.25Ga0.75As with a very flat upper surface. However, when the thickness of the inserted InAs is thinner or thicker than these values, we observed degradation of crystal quality and/or surface morphology. We also revealed this technique to be applicable to the formation of a high quality metamorphic InxGa1-xAs layer with a range of In compositions (0.25 ≤ x ≤ 0.78) on GaAs (111)A. Cross-sectional scanning transmission electron microscope studies revealed that misfit dislocations formed only at the interface of InAs and GaAs, not at the interface of In0.25Ga0.75As and InAs. From the dislocation density analysis, it is suggested that the dislocation density was decreased by growing In0.25Ga0.75As on InAs, which effectively contribute the strain relaxation of In0.25Ga0.75As. The InGaAs/InAlAs quantum wells that were formed on the metamorphic layers exhibit clear photoluminescence emissions up to room temperature.

A source of single photons and quantum entangled photon pairs is a key element in quantum information networks. Here, we demonstrate the electrically driven generation of quantum entangled pairs using a naturally symmetric GaAs quantum dot grown by droplet epitaxy. Coincidence histograms obtained at a temperature of 10 K reveal the generation of quantum entangled pairs that have a fidelity to the Bell pairs of 0.71 ± 0.015, much beyond the classical limit. We study the temperature dependent device characteristics and estimate the maximum operation temperature to be ∼ 65 K, which is essentially limited by the weak charge carrier confinement in the present dot system. Our study offers a guideline for the fabrication of quantum entangled pair sources suitable for practical use.

Vapor transportation is the core process in growing transition-metal dichalcogenides (TMDCs) by chemical vapor deposition (CVD). One inevitable problem is the spatial inhomogeneity of the vapors. The non-stoichiometric supply of transition-metal precursors and chalcogen leads to poor control in products' location, morphology, crystallinity, uniformity and batch to batch reproducibility. While vapor-liquid-solid (VLS) growth involves molten precursors at the growth temperatures higher than their melting points. The liquid sodium molybdate can precipitate solid MoS2 monolayers when saturated with sulfur vapor. Taking advantage of the VLS growth, we achieved three kinds of important achievements: (a) 4-inch-wafer-scale uniform growth of MoS2 flakes on SiO2/Si substrates, (b) 2-inch-wafer-scale growth of continuous MoS2 film with a grain size exceeding 100 um on sapphire substrates, and (c) pattern (site-controlled) growth of MoS2 flakes and film. We clarified that the VLS growth thus pave the new way for the high-efficient, scalable synthesis of two-dimensional TMDC monolayers., Comment: 30 pages, 10 figures

Tin-dioxide nanofacets (SnO2 NFs) are crystal-engineered so that oxygen defects on the maximal {113} surface are long-range ordered to give rise to a non-occupied defect band (DB) in the bandgap. SnO2 NFs-supported platinum-nanoparticles exhibit an enhanced ethanol-electrooxidation activity due to the promoted charge-transport via the DB at the metal-semiconductor interface.

We studied in detail the voltage dependence of two-step photocurrent generation through a two-step process of absorbing sub-band gap photons of different photon energies in a GaAs/AlGaAs quantum dot Intermediate Band Solar Cell. Our experiments revealed that two-step photocurrent generation is largely dependent on voltage, and exhibits a maximum at −0.3 V. A notable feature is a monotonic decrease in two-step photocurrent in the forward bias region, where the operating point of the solar cell lies. Using a model of rate equations, we extracted the voltage dependence of the individual escape and recombination rates, and found that the decrease in two-step photocurrent in the forward bias region is related to a monotonic increase in recombination rate in the quantum dots with increasing bias.

The interaction between nitrogen (N) impurity states in III-V compounds plays a key role in controlling optoelectronic properties of the host materials. Here, we use scanning tunneling microscopy to characterize the spatial distribution and electronic properties of N impurity states in dilute GaNAs. We demonstrated that the N impurity states can be directly visualized by taking empty state current images using the multipass scanning method. The N impurity states broadened over several nanometers and exhibited a highly anisotropic distribution with a bowtie-like shape on the GaAs(110) surface, which can be explained by anisotropic propagation of strain along the zigzag chains of Ga and As atoms in the {110} plane. Our experimental findings provide strong insights into a possible role of N impurity states in modifying properties of the host materials.

Skeletal gold nanocages (Au NCs) are synthesized and coated with TiO2 layers (TiO2–Au NCs). The TiO2–Au NCs exhibit enhanced photodecomposition activity toward acetaldehyde under visible light (>400 nm) illumination because hot electrons are generated over the Au NCs by local surface plasmon resonance (LSPR) and efficiently transported across the metal/semiconductor interface via the defect states of TiO2.

We studied the effects of post-growth annealing on the optical properties of GaSb quantum dots (QDs). We grew GaSb QDs in GaAs by droplet epitaxy and carried out a post-growth annealing at the temperature Ta of 700 ∼ 900 oC. The photoluminescence (PL) spectra from the GaSb QDs were found at about 1.1 ∼ 1.3 eV. The PL peak shifts toward a higher energy with increasing Ta. For the samples post-annealed at low temperatures, the PL intensity significantly decreases as the measurement temperature T increases. In contrast, the intensity reduction is relatively low for the high temperature annealed samples, indicating that the annealing process improves the crystalline quality of the sample. We also compared the integrated PL intensity to a model with a loss mechanism and found that the post-growth annealing drastically decreases non-radiative centers in the sample. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report the site-selective formation of Ga droplets on a surface where self-assembled GaAs quantum dots (QDs) were formed by droplet epitaxy. On the surface with the GaAs QDs, all the Ga droplets form adjacent to the QDs and are attached to the (1 1 1)B side facets of the QDs, which was confirmed by selective etching. While some of the QDs have two small droplets on both sides, most of the QDs have one large droplet on one side. We attribute the mechanism of the site-selectivity of the droplets to the morphology of the QDs and the high surface energies of the (1 1 1)B side facet.

We investigated the growth and optical properties of GaSb quantum dots (QDs) on GaAs (311)A substrates. We mounted GaAs (311)A and (100) substrates simultaneously on a molybdenum holder and grew GaSb QDs by molecular beam epitaxy (MBE). By atomic force microscopic studies, it was found that GaSb QDs grown on (311)A are smaller in size and their density is higher than that on (100). In photoluminescence (PL) measurements, a broad and overlapping PL peak originating from QDs is observed in both (311)A and (100) samples. The peak energy of the (311)A sample is lower than that of the (100) sample, although the size of QDs on GaAs (311)A is smaller than that on (100). This indicates that less residual As atoms are incorporated into QDs in the (311)A sample than those in the (100) sample. We also grew GaSb QDs at various substrate temperatures T s and examined how T s affects the QD shape and density.

We investigate the effects of Ga deposition rate (R) and antimony flux (P) on the morphology of GaSb quantum dots (QDs). GaSb QDs are formed on GaAs at 420 °C by Stranski-Krastanov (SK) mode under various conditions of R (0.13 ∼ 0.73 monolayer (ML)/s) and P (1.0 ∼ 4.2 × 10−7 Torr). An atomic microscope study shows that the density n of GaSb QDs is markedly affected by P and R; no (or few) QDs are formed at high R (∼ 0.73 ML/s) and at low P (≤ 1.2 × 10−7 Torr). As R (P) increases, n first increases and then decreases. n reaches its maximum ( 1 × 1011 cm−2), when the V–III ratio is about 3.6. We also analyze the experimental data by using a rate equation model and discuss the dependences of n on R and P.

We report the formation of triangular GaAs quantum dots (QDs) on (111)A substrates using droplet epitaxy. Shape transition from hexagonal to triangular QDs is observed with increasing crystallizing...

We have studied the growth processes of MoSe2 thin films on Si(111) substrates by molecular-beam epitaxy. The surface termination of the Si substrate plays a crucial role in the growth of epitaxial MoSe2 films: when the Si(111) substrate is passivated with a GaSe bilayer, highly oriented MoSe2 films could be grown under optimized growth conditions. On the other hand, the growth on the Si(111)-(7 × 7) substrate results in the preferential formation of the MoSi2 phase, which prevents the formation of epitaxial MoSe2 films. We found that the As termination of the Si(111) surface is less effective in promoting the planar MoSe2 growth, despite its highly passivated nature.

The coherence of light emitted from an isolated InAs/InP quantum dot (QD) is characterized by observing the firstorder correlation functions, g(1)(τ) of the electric field. The function of g(1)(τ) at 8 K shows an exponential decay with a decay time of 130 (±5) ps, corresponding to a Lorentzian spectral shape with 10.2 (±0.4) μeV in full width at half maximum. We observed systematic variation in g(1)(τ) as a function of temperature and excitation power. Our findings suggest that the motion of carriers surrounding QDs had a crucial effect on the decoherence (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Luminescence from individual nitrogen pairs (NN) were studied in nitrogen δ-doped gallium phosphide. Density of NN pairs (NN3,4,5,6) were directly measured by counting the spot density in μ-photoluminescence images in samples with different nitrogen densities. It was found that all the NN densities depend on the square of nitrogen sheet density suggesting that NN3∼6 definitely consists of two nitrogen atoms, although quantitative discrepancy was found for NN5 and NN6. In high resolution polarized spectrum of single NN4, two different polarization patterns due to different N-N orientation were observed. Single photon generation from a single NN4 was demonstrated under pulsed optical excitation at 30 K. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Local characterizations of a quaternary group III nitride material AlInGaN were carried out using electron microscopy. The chemical composition and the lattice parameter of the sample were estimated by a combination of energy dispersive X-ray spectroscopy and scanning transmission electron microscopy analyses and high-resolution transmission electron microscopy imaging, respectively. These results correspond well with the nominal composition of bulk sample measured by SIMS with only a few percent of error, indicating the validity of these techniques for the local characterization on this kind of hetero-structural materials. Copyright © 2008 John Wiley & Sons, Ltd.

Nitrogen (N) δ-doping in GaP was studied using low-pressure metalorganic chemical vapor deposition at 650 °C. Secondary ion mass spectroscopy measurements revealed that the profile of atomic N concentrations is abrupt and symmetrical, indicating a good-quality δ-doping. For the δ-doped sample of a dilute N sheet density, spatially dependent emission spectra from N-related isoelectronic traps were observed at 30 K with micro-photoluminescence (μ-PL). Besides the well-known NNi pairs, as well as the so-called A line, a new peak was observed at 2.2847 eV. We successfully mapped the spatial distribution of NN4 pairs by using 0th order diffraction of a spectrometer with a narrow-band pass filter. By applying this technique to several samples with different N sheet densities, it was found that the densities of NNi and the new peak have square and cubic dependence on the sheet density of N atoms, respectively. After identifying a single NN4 pair using the μ-PL, strong photon antibunching under continuous optical pumping was demonstrated, which is good evidence for clear single-photon emission from an individual N–N pair.

Nearly background free 1.5-μm single-photon source has been demonstrated using quasiresonant pulsed excitation. We performed a micro-photoluminescence excitation together with photon-correlation experiments, and then we compared photon statistics in nonresonant and quasiresonant excitation case. When we tuned the pump laser to the quasiresonant state which is 18.6 meV higher than the ground state excitonic transmission, almost background-free luminescence was found at 1524.3 nm. The second-order correlation function at time delay of zero, g(2)(0), was reduced to 0.017 when we carefully estimated noise contributions of after-pulses and dark counts in InGaAs APD detectors. This result opens up the possibility of promoting high quality single-photon source at telecommunication wavelength. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper deposited materials using focused ion beam-induced chemical vapor deposition (FIB-CVD) in an AuSi alloyed metal source was investigated. Results using Au and Si FIB-CVD were almost the same as those using Ga FIB-CVD. It was suggested that differences in deposited materials among the ions used should be investigated in more detail.

An etching effect of N-plasma on a GaN buffer layer was found in the initial stage of InN growth by plasma-assisted molecular beam epitaxy. We proposed to predeposit 1–2 monolayers (ML) of In (referred to as “In insertion”) on the GaN buffer layer to protect it from etching, thus preserving the flat buffer surface for InN growth. Atomic force microscopy (AFM), high-resolution X-ray diffraction (HRXRD), high-resolution transmission electron microscopy (HRTEM), and photoluminescence (PL) were carried out to evaluate the GaN buffers and InN films and compare the effect of the In insertion. It has been shown that the In insertion significantly improves the structural quality and optical property of InN. A flatter InN surface, narrower XRD full-width at half-maximum, sharper InN/GaN interface, and stronger PL were observed in optimal samples with 1.8 ML In insertion. The effect of In insertion is discussed in terms of buffer surface protection and enhancement of surface migration of In adatoms.

InN films were grown on GaN buffer layer by plasma-assisted molecular beam epitaxy (PA-MBE). We found that an In layer insertion between GaN buffer and InN film significantly improves the crystal quality and optical property of InN. The comparison of the GaN buffers with different treatment indicates that GaN buffer without In deposition is easily etched by N plasma when InN growth starts, and a 1–2 monolayers (ML) In can effectively prevent GaN from etching, thus maintaining the flat buffer surface for InN growth. With the optimum coverage of In insertion of 1.8 ML, the subsequent InN epifilm shows an atomically flat surface with root-mean-square (RMS) roughness as low as 0.2 nm. The peak intensity of photoluminescence (PL) from the same sample is increased by a factor of ∼20 compared with the InN film grown without In insertion.

Defect-band-free luminescence is observed from ZnO nanoparticles fabricated by implantation of 60 keV Zn + ions in silica glass (SiO 2 ) to 1.0 × 10 17 ions/cm 2 and subsequent oxidation at 600 °C for 1 h. To clarify the mechanism, the samples were evaluated by optical absorption spectroscopy, grazing incidence X-ray diffraction (GIXRD), photoluminescence (PL) spectroscopy and sputter depth profiling using X-ray excited Auger electron spectroscopy (XAES). Based on these observations, we discuss the possible origins of defect-band-free luminescence; i.e. the stoichiometric formation of ZnO nanoparticles due to the coexistence of metallic Zn nanoparticles, and the suppression of additional transformation of ZnO to Zn 2 SiO 4 phase at the low oxidation temperature of 600 °C.