Low‐temperature photoluminescence studies of In‐rich InAlN nanocolumns

High‐quality In_x Al_1–xN (0.71 ≤ x_In ≤ 1.00) nanocolumns (NCs) have been grown on Si(111) substrates by rf‐plasma‐assisted molecular‐beam epitaxy (rf‐MBE). Low‐temperature photoluminescence (LT‐PL) spectra of various In‐rich InAlN NCs were measured at 4 K and single peak PL emissions were observed in the wavelength region from 0.89 µm to 1.79 µm. Temperature‐dependent PL spectra of In_0.92Al_0.08N NCs were studied and the so‐called “S‐shape” (decrease–increase–decrease) PL peak energy shift was observed with increasing temperature. This shift indicates the carrier localization induced by the In segregation effect and is different from the anomalous blue shift frequently observed in InN films and nanowires with high residual carrier concentra‐ tions. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electrocaloric effect in antiferroelectric PbZrO3 thin films

Antiferroelectric PbZrO₃ thin films have been deposited on Pt(111)/Ti/SiO₂/Si substrate by polymer modified sol–gel route. Temperature dependent P –E hysteresis loops have been measured at 51 MV/m within a temperature range of 40 °C to 330 °C. The maximum electrocaloric effect ～0.224 × 10^–6 K mV^–1 has been observed near the dielectric phase transition temperature (235 °C) of the thin films. The electrocaloric effect and its strong temperature dependence have been attributed to nearly first‐order phase transition. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

原料纯度对VPE ZnSe单晶薄膜发光和电学性能的影响

本文叙述了原料纯度对ZnSe外延膜的发光和电学性能的影响。随原料纯度提高,ZnSe外延膜的电阻率增大,同时PL光谱中与自由激子发射有关的Es带不断增强。这就表明,在我们实验条件下,ZnSe外延层的电导载流子主要来自于原料中施主杂质的污染而不是本征施主,当衬底温度较低时,原料中施主杂质的污染要比衬底中的Ga自扩散重要得多。随着原料纯度的提高,ZnSe外延膜电阻率增加,正是由于原料中施主杂质的减少。     

MBE growth of ZnSe nanowires on oxidized silicon substrate

We report the growth of single crystalline ZnSe nanowires on oxidized Si(100) substrates by molecular-beam epitaxy using Au nano-particles as the catalysts. It was found that average length decreased while the average diameter increased as we increased the temperature from 230 to 320 ^°C. It was also found that crystal quality of the ZnSe nanowires prepared at 320 ^°C was poorer than the ZnSe nanowires prepared at 230 ^°C and 280 ^°C.     

Photoluminescence blue shift of indium phosphide nanowire networks with aluminum oxide coating

This paper describes our finding that optical properties of semiconductor nanowires were modified by depositing a thin layer of metal oxide. Indium phosphide nanowires were grown by metal organic chemical vapor deposition on silicon substrates with gold catalyst resulting in three‐dimensional nanowire networks, and optical properties were obtained from the collective nanowire networks. The networks were coated with an aluminum oxide thin film deposited by plasma‐enhanced atomic layer deposition. We studied the dependence of the peak wavelength of photoluminescence spectra on the thickness of the oxide coatings. A continuous blue shift in photoluminescence spectra was observed when the thickness of the oxide coating was increased. The observed blue shift is attributed to the Burstein–Moss effect due to increased carrier concentration in the nanowire cores caused by repulsion from intrinsic negative fixed charges located at the inner oxide surface. Samples were further characterized by scanning electron microscopy, Raman spectroscopy, transmission electron microscopy, and selective area diffractometry to better understand the physical mechanisms for the blue shift. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The Ag+ induced solution–liquid–solid growth, photoluminescence and photocatalytic activity of twinned ZnSe nanowires

Cubic ZnSe nanowires with periodically alternating twins along the wire growth direction are synthesized in the ZnCl₂–Na₂SeO₃–AgNO₃–ethylenediamine (EN)-ethylene glycol (EG)-polyvinyl–pyrrolidone (PVP) solvothermal system at 180°C for 12 h. The twinned ZnSe nanowires have diameters of 75±10 nm and lengths of >10 micrometers, and grow along 〈111〉 direction. The role of AgNO₃ in the formation of ZnSe nanowires was investigated, and an Ag⁺ induced solution–liquid–solid growth mechanism is also proposed to account for the conversion of microspheres assembled from ZnSe nanocrystallites into ZnSe nanowires. Compared with ZnSe microspheres, the as-prepared twinned ZnSe nanowires exhibit stronger band edge emissions of the wurtzite- and zinc-blende-structured ZnSe and lower deep defect related emission, and their photocatalytic ability is weaker than that of ZnSe microspheres. The results suggest that this simple, mild, one-step solution approach to fabricate ZnSe nanowires may be employed for the synthesis of other selenium compounds with one dimensional nanostructures, and provides opportunities for both fundamental research and technological applications.     

Tilt of InGaN layers on miscut GaN substrates

We report on the crystallographic orientation of InGaN layers grown on GaN substrates with a miscut with respect to c ‐planes up to 2.5°. The samples were examined using high‐resolution X‐ray diffraction (HRXRD) and atomic force microscopy (AFM). Because of the large (up to about 2% in this study) lattice mismatch between InGaN and GaN, an additional tilt between the c lattice planes of InGaN and GaN was observed and explained by using the Nagai model [J. Appl. Phys. 45 , 3789 (1974)]. We observed that for part of the samples, this tilt is about 10% smaller compared to the one predicted by the model. The experimental data are important for understanding the microstructure of InGaN layers grown on substrates of non‐perfect morphology. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optimization of ZnSe film growth conditions for p-type doping

Wide bandgap semiconductors such as ZnSe and ZnO have attracted great interest due to their applications in solar cells, light emitting diodes, and lasers. However, these wide bandgap semiconductors are frequently difficult to be doped to heavy concentrations, greatly limiting their application. A substrate holder with a natural temperature gradient was developed for batch growth of films at different deposition temperatures, in order to investigate ZnSe film growth and doping challenges. Thin ZnSe films were grown by pulsed laser deposition and characterized using X-ray diffraction, optical transmission and reflection, Raman spectroscopy, and Energy Dispersive X-ray analysis. Deposition temperature and film stoichiometry (Zn:Se) are shown to be significant factors affecting ZnSe growth and doping. ZnSe films with improved crystallinity have been obtained by enriching with selenium and depositing at an optimized temperature. Heavily p-type ZnSe films with hole concentrations of ~2.7 × 10¹⁹ cm^?3 and resistivities of ~0.099 Ohm cm have been obtained (compared with previous reports of ~1 × 10¹⁸ cm^?3 and ~0.75 Ohm cm). The results, which are consistent with previous theoretical prediction of compensating defects in ZnSe films, can help to optimize ZnSe growth conditions and understand doping challenges in wide bandgap semiconductors.     

MOVPE growth and characterization of a ‐plane AlGaN over the entire composition range

We report the metal organic vapor phase epitaxy (MOVPE) growth and characterization of non‐polar (11 0) a ‐plane Al_x Ga_1–xN on (1 02) r ‐plane sapphire substrates over the entire composition range. Al_x Ga_1–xN samples with ～0.8 μm thick layers and with x = 0, 0.18, 0.38, 0.46, 0.66, and 1.0 have been grown on r ‐plane sapphire substrates. The layer quality can be improved by using a 3‐stage AlN nucleation layer and appropriate V/III ratio switching following nucleation. All a ‐plane AlGaN epilayers show an anisotropic in‐plane mosaicity, strongly influenced by Al incorporation and growth conditions. Careful lattice parameter measurements show anisotropic in‐plane strain that results in an orthorhombic distortion of the hexagonal unit cell, making Al composition determination from X‐ray diffraction difficult. In general lower Al incorporation is seen in a ‐plane epilayers compared to c ‐plane samples grown under the same conditions. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Polytypism in GaAs nanowires: determination of the interplanar spacing of wurtzite GaAs by X‐ray diffraction

In GaAs nanowires grown along the cubic [111]_c direction, zinc blende and wurtzite arrangements have been observed in their stacking sequence, since the energetic barriers for nucleation are typically of similar order of magnitude. It is known that the interplanar spacing of the (111)_c Ga (or As) planes in the zinc blende polytype varies slightly from the wurtzite polytype. However, different values have been reported in the literature. Here, the ratio of the interplanar spacing of these polytypes is extracted based on X‐ray diffraction measurements for thin GaAs nanowires with a mean diameter of 18–25 nm. The measurements are performed with a nano‐focused beam which facilitates the separation of the scattering of nanowires and of parasitic growth. The interplanar spacing of the (111)_c Ga (or As) planes in the wurtzite arrangement in GaAs nanowires is observed to be 0.66% ± 0.02% larger than in the zinc blende arrangement.     

Ultraviolet Raman scattering in ZnO nanowires: quasimode mixing and temperature effects

We use near‐resonance Raman scattering to investigate zinc oxide (ZnO) nanowires grown by chemical vapor deposition on Si substrates. We discuss the role of quasimode mixing on the wavenumber of the longitudinal optical (LO) bands, and we perform Raman measurements with different excitation powers to investigate possible laser heating effects. We find that in the Raman spectra of as‐deposited nanowires grown along the c‐axis of wurtzite, the LO bands are located slightly below the E₁(LO) mode of bulk ZnO. We perform a calculation of the expected LO wavenumber in an ensemble of randomly oriented nanowires. Our analysis shows that light refraction, together with the orientation‐dependent cross‐section of the nanowires for the incoming light, counterbalances quasimode mixing effects in the as‐grown product, giving rise to LO bands that are barely redshifted relative to the E₁(LO) mode. In the case of ZnO nanowires that have been mechanically removed (scratched) and subsequently deposited onto separate Si substrates, we observe clear laser‐induced heating. Temperature effects account well for the Raman wavenumber shifts displayed by the LO bands in the Raman spectra of the scratched nanowires. Copyright © 2010 John Wiley & Sons, Ltd.     

Strain control in germanium nanowires: the use of a silicon nitride shell

The core–shell geometry is a strong tool for inducing and controlling strains in nano‐objects in order to tune their optoelectronic properties. We synthesized and characterized core–shell nanostructures by depositing a non‐epitaxial silicon nitride shell around germanium nanowires. Scanning electron microscopy as well as energy dispersive X‐ray spectroscopy confirms the structural integrity of the heterostructures, and grazing incidence X‐ray diffraction measurements reveal the presence of a radial tensile strain in the Ge nanowires. A control of this strain is then demonstrated up to 0.3% by adjusting the SiN_x shell thickness versus Ge nanowire diameter. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural evolution of self‐assisted GaAs nanowires grown on Si(111)

GaAs nanowires are grown on Si(111) by self‐assisted molecular beam epitaxy, and the ratio between wurtzite and zinc‐blende phases is determined as function of nanowire length using asymmetric X‐ray diffraction. We show that under the applied growth conditions, nanowires grow in both phases during the initial stage of growth, whereas the zinc‐blende content increases with growth time and dominates in long nanowires. Compared to the zinc‐blende units, the vertical lattice parameter of the wurtzite segments is 0.7% larger, as measured by the positions of respective diffraction peaks. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The power of in situ pulsed laser deposition synchrotron characterization for the detection of domain formation during growth of Ba0.5Sr0.5TiO3 on MgO

A highly sophisticated pulsed laser deposition (PLD) chamber has recently been installed at the NANO beamline at the synchrotron facility ANKA (Karlsruhe, Germany), which allows for comprehensive studies on the PLD growth process of dielectric, ferroelectric and ferromagnetic thin films in epitaxial oxide heterostructures or even multilayer systems by combining in situ reflective high‐energy diffraction with the in situ synchrotron high‐resolution X‐ray diffraction and surface diffraction methods. The modularity of the in situ PLD chamber offers the opportunity to explore the microstructure of the grown thin films as a function of the substrate temperature, gas pressure, laser fluence and target–substrate separation distance. Ba_0.5Sr_0.5TiO₃ grown on MgO represents the first system that is grown in this in situ PLD chamber and studied by in situ X‐ray reflectivity, in situ two‐dimensional reciprocal space mapping of symmetric X‐ray diffraction and acquisition of time‐resolved diffraction profiles during the ablation process. In situ PLD synchrotron investigation has revealed the occurrence of structural distortion as well as domain formation and misfit dislocation which all depend strongly on the film thickness. The microstructure transformation has been accurately detected with a time resolution of 1 s. The acquisition of two‐dimensional reciprocal space maps during the PLD growth has the advantage of simultaneously monitoring the changes of the crystalline structure as well as the formation of defects. The stability of the morphology during the PLD growth is demonstrated to be remarkably affected by the film thickness. A critical thickness for the domain formation in Ba_0.5Sr_0.5TiO₃ grown on MgO could be determined from the acquisition of time‐resolved diffraction profiles during the PLD growth. A splitting of the diffraction peak into two distinguishable peaks has revealed a morphology change due to modification of the internal strain during growth.     

Influence of growth conditions on the structural,optical, and electrical properties of ZnSe films

ZnSe films were grown by chemical vapour deposition on GaAs substrates. The influence of the source temperature (between 820 and 900° C) and the substrate temperature (between 620 and 790° C) on the film properties were investigated by Hall measurements, X-ray diffraction, and photoluminescence. With respect to blue luminescent devices the ratio of excitonic to deep level transitions was found to be optimum at low growth rates when the source temperatures were kept below 840° C. P-type conduction up to a net carrier concentration of 8×10¹⁸ cm^–3 could be obtained by substrate temperatures above 700° C. Lattice contraction versus substrate temperature pointed to a reduced incorporation of donors at higher growth temperatures.     

Electron spin resonance and quantum critical phenomena in VOx multiwall nanotubes

Basing on the high frequency (60 GHz) electron spin resonance study of the VO_x multiwall nanotubes (VO_x ‐NTs) carried out in the temperature range 4.2–200 K we report: (i) the first direct experimental evidence of the presence of the antiferromagnetic dimers in VO_x ‐NTs and (ii) the observation of an anomalous low temperature growth of the magnetic susceptibility for quasi‐free spins, which obey the power law χ (T)～1/T ^α with the exponent α ≈ 0.6 in a wide temperature range 4.2–50 K. We argue that the observed departures from the Curie–Weiss behaviour manifest the onset of the quantum critical regime and formation of the Griffiths phase as a magnetic ground state of these spin species. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electrical,optical and spectral characteristics of type-II ZnSe/ZnTe/GaAs superlattice and MSM-photodetector on their base

We report on growth, fabrication and characterization of the metal–semiconductor–metal (MSM) photodiode based on type-II ZnSe/ZnTe heterostructure. Heterostructure was grown on semi-insulating GaAs substrates by MOVPE. For the first time we present the results of experimental investigations of the MSM photodetector on the base of type-II ZnSe/ZnTe superlattice. The MSM-photodetector demonstrates very low dark current, high current sensitivity and external quantum efficiency. The maximum photoresponse of the MSM-detector at the wavelength 620 nm corresponds to current sensitivity 0.22 A/W and external quantum efficiency 44%. Photoresponse of the MSM-detector shows two peaks of response located at 620 nm and 870 nm. ZnSe/ZnTe type-II superlattice structure reduces the MSM-diode dark current significantly. For the MSM-diode with finger width and gap of 3 µm and 100?×?100 µm² photosensitive area we have obtained dark current density 10^?8 A/cm² at room temperature.     

Temperature dependent magnetic properties and application potential of intermetallic Fe11–xCox TiCe

The novel quaternary compound Fe_11–xCo_x TiCe (x = 0 to 3.25) of Mn₁₂Th structure has been fabricated by arc melting. The analysis is focused on temperature dependent determination of intrinsic properties from 4 K to 750 K using domain pattern analysis and magnetometry. Above room temperature RT maximum values of anisotropy constant K₁ and saturation polarization J_s are observed for a Co content of 15 at% (x = 1.95) with K₁ and J_s of 2.15 MJ/m³ (1.22 MJ/m³) and 1.27 T (1.05 T) at RT (200 °C). At operating temperatures of 100 °C for this material magnetic properties (BH)_max= 282 kJ/m³, µ₀H_c = 0.94 T are expected. If a suitable microstructure could be processed, based on intrinsic properties of the phases the costs would be 35% per J/m³ of the costs of Dy‐free Fe–Nd–B. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Ferromagnetism in Cu-doped ZnSe semiconducting quantum dots

The projection of integrating optical, magnetic and electronic functionalities into a single material have aggravated passionate attention in mounting wide band gap diluted magnetic semiconductor (DMS) in the midst of room temperature ferromagnetism. We report the evidence of ferromagnetism in Cu-doped ZnSe quantum dots (QDs) below room temperature, grown from a single source precursor by lyothermal method with the sizes of approximately 3.2–5.14 nm. QDs mainly exhibit paramagnetic behavior between 80 and 300 K, with a weak ferromagnetic/anti-ferromagnetic exchange at lower temperature as observed by superconducting quantum interference device (SQUID) magnetometer. From the Curie–Weiss behavior of the susceptibility, Curie temperature (T _c) of Cu-doped ZnSe sample has been evaluated. From EPR, we obtain the Lande-g factor in the Zeeman interaction term as 2.060. Photoluminescence and EPR measurements support and confirm the view that Cu²⁺ substitutes for Zn²⁺ in Cu-doped ZnSe quantum dots.