We present a theoretical simulation of near-band-edge emission spectra of highly conducting n-InN assuming the model of 'free-to-bound' radiative recombination (FBRR) of degenerate electrons from the conduction band with nonequilibrium holes located in the valence band tails. We also study experimental photoluminescence (PL) spectra of highly conducting InN epitaxial layers grown by MBE and MOVPE with electron concentrations in the range (7.7 × 1017-6 × 1018) cm-3 and find that the energy positions and shape of the spectra depend on the impurity concentration. By modeling the experimental PL spectra of the InN layers we show that spectra can be nicely interpreted in the framework of the FBRR model with specific peculiarities for different doping levels. Analyzing simultaneously the shape and energy position of the InN emission spectra we determine the fundamental bandgap energy of InN to vary between Eg = 692 meV for effective mass mn0 = 0.042m0 and Eg =710 meV for mn0 = 0.1m0. © 2004 Elsevier Ltd. All rights reserved.
Effects of Ga incorporation on electrical, structural and optical properties of ZnO epilayers are systematically studied by employing structural and optical characterization techniques combined with electrical and secondary ion mass spectrometry measurements. A non-monotonous dependence of free electron concentrations on Ga content is observed and is attributed to defect formation and phase separation. The former process is found to dominate for Ga concentrations of around 2-3x1020 cm-3. corresponding defects are suggested to be responsible for a broad red emission, which peaks at around 1.8 eV at K. Characteristic properties of this emission are well accounted for by assuming intracenter transitions at a deep center, of which the associated Huang-Rhys factor and mean phonon energy are determined. For higher Ga doping levels, the phase separation is found to be significant. It is that under these conditions only a minor fraction of incorporated Ga atoms form shallow donors, which leads to the observed dramatic decrease of carrier concentration.
The vibrational properties of InN films with different strain have been studied using Infrared ellipsometry and Raman scattering spectroscopy. We have established a correlation between the phonon mode parameters and the strain, which allows the determination of the deformation potentials and the strain-free frequencies of the InN E1(TO) and E2 modes. The LO phonons and their coupling to the free-carrier plasmon excitations are also discussed in relation to the carrier concentration in the films. © 2004 Elsevier Ltd. All rights reserved.
Dynamic properties are studied for the AlGaN/GaN quantum well (QW) structures with and without the coating of colloidal ZnO nanocrystals (NCs). The QW exciton recombination rate was reduced in such hybrids compared to the bare QW structure only in the sample with the thinnest cap layer of 3 nm. Assuming that one of the recombination mechanisms in this hybrid is non-radiative resonant energy transfer (NRET) between the QW and the energy acceptor material i.e. ZnO NCs, the maximum pumping efficiency was estimated to be similar to 42% at 60 K. The NRET effect is, however, vanished after several months despite that the hybrid structures are composed of chemically stable components. (C) 2015 Published by Elsevier Ltd.
Using the hydride vapour phase epitaxy technique, we have grown 2-inch diameter bulk GaN material with a thickness up to 2 mm. The growth was performed in a vertical hot-walled reactor at atmospheric pressure. In this geometry, the process gases are distributed from the bottom upwards through the reactor. We present recent results on growth and characterization of the bulk GaN material. The structural and optical properties of the layers have been studied using decorative etching, optical microscopy, scanning electron microscopy, X-ray diffraction, cathodoluminescence, and low temperature photoluminescence. © 2006 Elsevier Ltd. All rights reserved.
ZnO films and nanostructures were deposited on Si substrates by MOCVD using single source solid state zinc acetylacetonate (Zn(AA)) precursor. Doping by silver was realized in-situ via adding 1 and 10 wt. % of Ag acetylacetonate (Ag(AA)) to zinc precursor. Influence of Ag on the microstructure, electrical and optical properties of ZnO at temperature range 220-550 degrees C was studied by scanning, transmission electron and Kelvin probe force microscopy, photoluminescence and four-point probe electrical measurements. Ag doping affects the ZnO microstructure via changing the nucleation mode into heterogeneous and thus transforming the polycrystalline films into a matrix of highly c-axis textured hexagonally faceted nanorods. Increase of the work function value from 4.45 to 4.75 eV was observed with Ag content increase, which is attributed to Ag behaviour as a donor impurity. It was observed, that near-band edge emission of ZnO NS was enhanced with Ag doping as a result of quenching deep-level emission. Upon high doping of ZnO by Ag it tends to promote the formation of basal plane stacking faults defect, as it was observed by HR TEM and PL study in the case of 10 wt.% of Ag. Based on the results obtained, it is suggested that NS deposition at lower temperatures (220-300 degrees C) is more favorable for p-type doping of ZnO. (C) 2018 Elsevier Ltd. All rights reserved.
We studied the wavefunctions and energy positions of the confined states in the valance band in an asymmetric double quantum well. We found these to be extremely sensitive to both the carrier density and the height of the confining barriers, which allows us to control the so-called Spin Dependent Hole Delocalization. The theoretical results are shown to be in excellent agreement with experiments. (C) 1997 Academic Press Limited
Due to a constant increase in demands for transparent electronic devices the search for alternative transparent conducting oxides (TCO) is a major field of research now. New materials should be low-cost and have comparable or better optical and electrical characteristics in comparison to ITO. The use of n-type ZnO was proposed many years ago, but until now the best n-type dopant and its optimal concentration is still under discussion. Ga was proposed as the best dopant for ZnO due to similar atomic radius of Ga3+ compared to Zn2+ and its lower reactivity with oxygen. The resistivity ρ of ZnO:Ga/Si (100) films grown by PEMOCVD was found to be 3×10-2 Ω cm. Rapid thermal annealing (RTA) was applied to increase the conductivity of ZnO:Ga (1 wt%) films and the optimal regime was determined to be 800 {ring operator}C in oxygen media for 35 s. The resistivity ratio ρbefore / ρafter before and after the annealing and the corresponding surface morphologies were investigated. The resistivity reduction (ρbefore / ρafter ≈ 80) was observed after annealing at optimal regime and the final film resistivity was approximately ≈4×10-4 Ω cm, due to effective Ga dopant activation. The route mean square roughness (Rq) of the films was found to decrease with increasing annealing time and the grain size has been found to increase slightly for all annealed samples. These results allow us to prove that highly conductive ZnO films can be obtained by simple post-growth RTA in oxygen using only 1% of Ga precursor in the precursor mix. © 2007 Elsevier Ltd. All rights reserved.
Zinc oxide (ZnO) is a promising semiconductor material with a great variety of applications, for example for highly conductive films for transparent electronics. Recently, Ga has been proposed as a dopant, exhibiting the advantages of a very similar atomic radius compared to Zn, a smaller reactivity, and a higher resistivity to oxidation compared to its competitor Al. In this study ZnO films, doped by Ga, were produced on Al2O3(0001) substrates by PEMOCVD. The doping was realized with 1, 3, 5 and 10 wt% gallium precursor content in the mixture. The resistivity of the prepared films, as well as the morphology and the transmittance, was investigated. All the deposited films have demonstrated a high optical transmittance above 93% in the range between 400 and 800 nm. A strong correlation between the electrical resistivity and the optical band gap depending on the Ga content was observed. An AFM analysis demonstrated highly uniform and smooth surfaces. The average grain size and route mean square roughness decreased with increasing Ga content. © 2005 Elsevier Ltd. All rights reserved.
We have studied the transport properties of a device consisting of two quantum dots, defined electrostatically in a (Al,Ga)As heterostructure. In the series conductance of the two dots, we observe irregularly spaced conductance peaks of fluctuating amplitude. This behaviour results from transport in the stochastic Coulomb blockade regime. In a second experiment, we measure the charging energy of one dot through the effect of its potential (which varies in a saw-tooth fashion with gate voltage) on the conductance of the other dot. We find that the charging energy scales quadratically with the reflection probability of the tunnel barriers, in agreement with a recent theory.
We report new data on the transient photoluminescence behaviour of free and donor bound excitons in high quality bulk GaN material grown by HVPE. With 266 nm photoexcitation the no-phonon free exciton has a short decay time, about 100 ps at 2 K, assigned to nonradiative surface recombination. The LO replicas of the free exciton have a much longer decay at 2 K, about 1.4 ns, believed to be a lower bound for the bulk radiative lifetimes of the free excitons at 2 K. The donor bound exciton no-phonon lines exhibit a rather short (about 300 ps) nonexponential decay at 2 K, which appears to be dominated by a scattering process. The corresponding LO replicas and the two-electron transitions have a much longer decay. From the latter, the lower bound of the radiative lifetime of the O- and Si-bound excitons are 1800 ps and 1100 ps, respectively. © 2007 Elsevier Ltd. All rights reserved.
The bandgap value of InN was analyzed, with reference to optical data on single crystalline thin films samples grown on sapphire. It was found that the line shape of the photoluminescence spectra could be quite well reproduced in a model for the optical transitions from the conduction band states to localized states above the valence band. It was also found that the conduction band transition requires the presence of deep donor at a concentration close to 1020 cm∓3. The results show that the bandgap of pure InN should be about 0.69 eV at 2 K.
We have studied optical properties of nonpolar a-plane GaN layers grown on r-plane sapphire by metalorganic chemical vapor deposition and hydride vapor phase epitaxy using different nucleation schemes. Several emission bands, which are not typical for c-plane GaN, are observed in the photoluminescence spectra and their excitation-intensity, temperature, and polarization dependencies are examined. In addition, the spatial distribution of the emissions was examined by cathodoluminescence imaging and relations of the different emissions with particular structural features in the layers are revealed. The results are discussed with emphasis on the origin of the emission line and particular recombination mechanisms. © 2006 Elsevier Ltd. All rights reserved.
Freestanding GaN layers of various thicknesses grown by HVPE have been studied by time-resolved spectroscopy combined with structural and electrical measurements. We have observed an increase of the PL lifetime with increasing layer thickness, however, a saturation of the recombination times has been detected for the GaN layers thicker than 400 µm. We explain the observed thickness-dependent behavior of the decay times by competition of two nonradiative mechanisms, namely, for layers with thickness less than 400 µm the main nonradiative channel is related to the structural defects, while in thicker layers the recombination decay time is limited by impurities and/or vacancies. © 2007 Elsevier Ltd. All rights reserved.
Needs in more-efficient visible light sources based on quantum wells (QWs) requires the diversification of traditional optoelectronics materials as well as development of the cost-effective approaches for reliable quantum confinement engineering. Interdiffusion approach has a great potential to become a simple method for controlling the optical properties of QWs and diminishing the quantum confined Stark effect (QCSE). In this work we theoretically study the effect of Zn-Cd interdiffusion in ZnCdO/ZnO QWs on their band structure, optical matrix elements and spontaneous emission properties. The QW intermixing leads to improving both the transverse electric (TE) and transverse magnetic (TM) optical matrix elements due to enhancement of the overlap integral between electron and hole wave functions and modification of the confinement potential from triangle-shaped to parabolic-like. The optimized diffusion length 4 angstrom provided by the annealing at 700 K during 60s was determined for 2 nm-thick Zn0.85Cd0.15O QW, which offers higher spontaneous emission rate in comparison to conventional QW. The reasonable interpretation of the interdiffusion effect on the optical properties of QWs is proposed in terms of low diffusion length and high diffusion length regimes. Thus, suitable combination of annealing duration and annealing temperature with the geometrical/compositional parameters of QWs can be the efficient way for improving the optical performance of ZnO-based QWs.
Vertically well aligned zinc oxide nanorods (ZnO NRs) were grown on p-GaN by electrodeposition (ED) and aqueous chemical growth (ACG) techniques and the structures were employed to fabricate white light emitting diodes (LEDs). Room temperature current voltage (I–V), photoluminescence (PL), and electroluminescence (EL) measurements were performed to investigate and compare both LEDs. In general, the I–V characteristics and the PL spectra of both LEDs were rather similar. Nevertheless, the EL of the ED samples showed an extra emission peak shoulder at 730 nm. Moreover, at the same injection current, the EL spectrum of the ED light emitting diode showed a small UV shift of 12 nm and its white peak was found to be broader when compared to the ACG grown LED. The broadening of the EL spectrum of the LED grown by ED is due to the introduction of more radiative deep level defects. The presented LEDs have shown excellent color rendering indexes reaching a value as high as 95. These results indicate that the ZnO nanorods grown by both techniques possess very interesting electrical and optical properties but the ED is found to be faster and more suitable for the fabrication of white LEDs.
In this paper we investigated the effect of post-growth annealing treatment on the electroluminescence (EL) of n-ZnO nanorods/p-GaN light emitting diodes. The ZnO nanorods were grown by the low temperature (less than100 degrees C) aqueous chemical growth (ACC) technique. The as-grown ZnO nanorods were annealed in nitrogen, oxygen, argon, and air ambients at 600 degrees C for 30 min. The electroluminescence (EL) measurements showed that the deep level defects related emissions in ZnO were greatly affected by the annealing of the n-ZnO nanorods in different ambients. By comparing the EL spectra of ZnO nanorods annealed in different ambients it was found that nitrogen annealing ambient is very effective in shifting the emission peak from the green region to the red region. It was also concluded that the red emission in ZnO was attributed to oxygen vacancies WO. The effect of the annealing ambient and the temperature dependence electroluminescence were discussed with relation to the intrinsic and extrinsic defects.
In this work a new plasma-based approach to hydrogen intercalation of Graphene grown on SiC is demonstrated. By optimization of the inductively coupled plasma parameters the intercalated by hydrogen Graphene has been modified gradually and transformed into the two-dimensional hydrocarbon Graphane. The intermediate stages during the transition of Graphene to Graphane were studied by means of Raman spectroscopy and AFM. The dependence of the intensities of the Raman Graphene fingerprints: D and G peaks on the hydrogen intercalation time has been studied. The changes of resistance during the hydrogen plasma treatment were parsed. The Raman (D + D ) peak corresponding to hydrogenated graphene was studied in detail. The method developed is highly reliable and flexible as well as convenient for large-scale fabrication of Graphane to be employed as a hydrogen storage material and in 2D electronics.
ZnO substrate crystals from two different sources, and epitaxial layers have been studied by SEM, AFM, photoluminescence (T=2-135K) and EPR. Although fabricated by the same growth principle, i.e. the hydrothermal technique, the substrates differ in terms of purity and structural quality. In the PL spectra of all samples the dominating emission originates from the donor bound exciton (BE) recombination positioned at about 3361 meV. The temperature dependence of the spectra confirms the assignment of the free exciton emission in the purest sample, the line at 3376 meV evolves into a broad peak at higher temperatures, probably including both A and B excitons. Another FE-related emission appears as a shoulder on the high-energy side of FEA,B above 40 K. It is expected and associated with the crystal-field split-off counterpart of the valence band. Free-exciton related emission in the less pure sample can only be seen if the temperature is above 45 K. At T=135K all bound excitons are quenched and the spectrum in both samples consists of the free exciton no-phonon lines and their replicas. However, the emission from the pure samples is several orders of magnitude stronger than that from the other sample, which indicates strong non-radiative quenching of the excitons in the latter sample. The EPR measurements reveal a possible scenario of impurity re-arrangement, e.g. annealing at 950 °C may dissociate existing complexes and release Fe as isolated ions. The AFM and SEM investigations of an epilayer grown by MOCVD on one of the studied substrates have indicated growth instabilities and structural irregularities, thus pointing to the need for substrate quality and epitaxial process optimization.
We report the post-growth heat treatment effect on the electrical and the optical properties of hydrothermally grown zinc oxide (ZnO) nanowires (NWs) Schottky white light emitting diodes (LEDs). It was found that there is a changed in the electroluminescence (EL) spectrum when post growth annealing process was performed at 600 degrees C under nitrogen, oxygen and argon ambients. The EL spectrum for LEDs based on the as grown NWs show three bands red, green and blue centered at 724, 518 and 450 nm respectively. All devices based on ZnO NWs annealed in oxygen (O-2), nitrogen (N-2) and argon (Ar) ambient show blue shift in the violet and the red emissions whereas a red shift is observed in the green emission compared to the as grown NWs based device. The color rendering index (CRI) and the correlated color temperature (CCT) of all LEDs were calculated to be in the range 78-91 and 2753-5122 K, respectively. These results indicate that light from the LEDs can be tuned from cold white light to warm white light by post growth annealing.
An equilibrium approach is used to calculate the free energy and composition distribution of InGaN/GaN quantum dot located on the InGaN/GaN pyramid. The energy balance method is adopted to predict critical conditions for quantum dot formation. We find that the formation of QD depends strongly on the size of pyramid top surface. The results can fit our experiment qualitatively.