Recombination processes in unintentionally doped GaTe single crystals

Emission spectra of GaTe single crystals in the range of 1.90–1.38 eV have been analyzed at different temperatures and excitation intensities by photoluminescence, photoluminescence excitation, and selective photoluminescence. A decrease in band gap energy with an increase in temperature was obtained from the redshift of the free exciton recombination peak. The energy of longitudinal optical phonons was found to be 14±1 meV. A value of 1.796±0.001 eV for the band gap at 10 K was determined, and the bound exciton energy was found to be 18±0.3 meV. The activation energy of the thermal quenching of the main recombination peaks and of the ones relating to the ionization energy of impurities and defects was analyzed. The results obtained show the existence of two acceptor levels with ionization energies of 110±5 and 150±5 meV, respectively, and one donor level with an ionization energy of 75±5 meV. The study of chemical composition by inductively coupled plasma-optical emission spectroscopy and x-ray energy dispersion spectroscopy shows the existence of Na, Li, and Si. Sodium and lithium impurities could be associated with acceptor levels at gallium substitutional sites, and silicon ones with a donor level at Ga sites, whose vacancies can also be involved in these electronic [email protected] ; [email protected]

Observation of a charge delocalization from Se vacancies in Bi2Se3: A positron annihilation study of native defects

[EN] By means of positron annihilation lifetime spectroscopy, we have investigated the native defects present in Bi2Se3, which belongs to the family of topological insulators. We experimentally demonstrate that selenium vacancy defects (VSe1) are present in Bi2Se3 as-grown samples, and that their charge is delocalized as temperature increases. At least from 100 K up to room temperature both V0Se1and V+Se1 charge states coexist. The observed charge delocalization determines the contribution of VSe1defects to the n-type conductivity of Bi2Se3. These findings are supported by theoretical calculations, which show that vacancies of nonequivalent Se1 and Se2 selenium atoms are clearly differentiated by positron annihilation lifetime spectroscopy, enabling us to directly detect and quantify the most favorable type of selenium vacancy. In addition to open-volume defects, experimental data indicate the presence of defects that act as shallow traps, suggesting that more than one type of native defects coexist in Bi2Se3. As will be discussed, the presence of a dislocation density around 1010cm−2 could be the source of the detected shallow traps. Understanding the one-dimensional defects and the origin of the charge delocalization that leads Bi2Se3 to be an n-type semiconductor will help in the development of high-quality topological insulators based on this material.This work is supported for the Basque Government Grant IT-443-10 and partially supported by the Spanish Ministry of Economy and Competitiveness (MINECO) under the project TEC2014-60173 and by the Generalitat Valenciana under the projects Prometeo II 2015/004 and ISIC/2012/008. I. Unzueta also wants to acknowledge financial support from the Basque Government Grant PRE-2014-214. V. Marín-Borrás thanks the University of Valencia for its pre-doctoral fellowships. Finally we would also like to thank BCMaterials for its economic support

Structural characterization of a-plane Zn1−xCdxO (0 < x <0.085) thin films grown by metal-organic vapor phase epitaxy.

Zn1−xCdxO(11math0) films have been grown on (01math2) sapphire (r–plane) substrates by metal-organic vapor phase epitaxy. A 800-nm-thick ZnO buffer, deposited prior to the alloy growth, helps to prevent the formation of pure CdO. A maximum uniform Cd incorporation of 8.5 at. % has been determined by Rutherford backscattering spectrometry. Higher Cd contents lead to the coexistence of Zn1−xCdxO alloys of different compositions within the same film. The near band-edge photoluminescence emission shifts gradually to lower energies as Cd is incorporated and reaches 2.93 eV for the highest Cd concentration (8.5 at. %). The lattice deformation, due to Cd incorporation, has been described using a new reference frame in which the lattice distortions are directly related to the a-plane surface structure. Cd introduction does not affect the c lattice parameter but expands the lattice along the two perpendicular directions, [11math0] and [math100], resulting in a quadratic volume [email protected] [email protected]

Effect of Growth Temperature on the Structural and Morphological Properties of MgCdO Thin Films Grown by Metal Organic Chemical Vapor Deposition

II-VI oxides ternary alloys have attracted considerable interest of the scientific community due to the possibility of modulating their interesting optoelectronic properties. Despite this interest, MgCdO has been poorly studied. In this work, by using the metal organic chemical vapor deposition method at low pressure, we have analyzed the synthesis of thin films of this alloy. Thus, for a fixed metal-organic precursors content, a change from Mg1-xCdxO (Mg-rich phase) to Cd1-xMgxO (Cd-rich phase) has been induced when decreasing the growth temperature. The temperature range where both phases coexist has been particularly analyzed. Using X-ray diffraction analysis and scanning electron microscopy, a structural and morphological study of the samples has been carried out. In addition, the composition of the alloy has been measured by using energy dispersive X-ray analysis, and the behavior of the lattice parameter as a function of Cd content has been studied

Strong optical nonlinearities in gallium and indium selenides related to inter-valence-band transitions induced by light pulses

A nonlinear optical effect is shown to occur in gallium and indium selenides at photon energies of the order of 1.5 eV. It corresponds to transitions from a lower-energy valence band to the uppermost one when a nonequilibrium degenerate hole gas is created in the latter by a laser pulse. This inter-valence-band transition is allowed by crystal symmetry. Its oscillator strength is estimated through the f-sum rule and turns out to be about two orders of magnitude higher than that of the fundamental transition. The intensity of this effect is stronger when the pump pulse photon energy is close to that of the inter-valence-band transition; a condition that can be fulfilled only in indium selenide. The transient behavior of the sample transmittance is shown to be controlled by the balance between absorption and stimulated emission, which depends on the hole quasi-Fermi level and the gap renormalization due to Coulomb interaction in the electron-hole gas generated by the pump

Investigation of nitrogen-related acceptor centers in indium selenide by means of photoluminescence: Determination of the hole effective mass

In this work we report on steady-state and time-resolved photoluminescence (PL) measurements in nitrogen doped p-type indium selenide in the 33-210-K temperature range. In samples with low nitrogen concentration the photoluminescence spectrum consists of exciton-related peaks and a band-to-acceptor recombination peak (2.1-μs lifetime) with LO-phonon replica. An ionization energy of 65.5 meV is proposed for the nitrogenrelated acceptor. A long-lived (18 μs) component, which consists of an asymmetric broadband centered around the acceptor peak, has been also detected by means of time-resolved PL. Samples with a higher nitrogen concentration show a PL spectrum that mainly consists of the asymmetric long-lived broadband that can be associated to a complex center. The asymmetric shape of this band is quantitatively accounted for in the framework of the configuration coordinate model for complex centers. Under the assumption that the nitrogen related acceptor is shallow, the Gerlach-Pollman theory allows an estimate of the hole's effective masses

Study of the Partial Substitution of Pb by Sn in Cs–Pb–Sn–Br Nanocrystals Owing to Obtaining Stable Nanoparticles with Excellent Optical Properties

Halide perovskites are revolutionizing the photovoltaic and optoelectronic fields with outstanding performances obtained in a remarkably short time. However, two major challenges remain: the long-term stability and the Pb content, due to its toxicity. Despite the great effort carried out to substitute the Pb by a less hazardous element, lead-free perovskite still remains more unstable than lead-containing perovskites and presents lower performance as well. In this work, we demonstrate the colloidal preparation of Cs–Pb–Sn–Br nanoparticles (NPs) where Sn is incorporated up to 18.8%. Significantly, we have demonstrated that the partial substitution of Pb by Sn does not produce a deleterious effect in their optical performance in terms of photoluminescence quantum yield (PLQY). We observed for the first time a positive effect in terms of enhancement of PLQY when Sn partially substitutes Pb in a considerable amount (i.e., higher than 5%). PLQYs as high as 73.4% have been obtained with a partial Pb replacement of 7% by Sn. We present a systematic study of the synthesis process in terms of different growth parameters (i.e., precursor concentration, time, and temperature of reaction) and how they influence the Sn incorporation and the PLQY. This high performance and long-term stability is based on a significant stabilization of Sn2+ in the NPs for several months, as determined by XPS analysis, and opens an interesting way to obtain less Pb-containing perovskite NPs with excellent optoelectronic properties

ZnMgO-based UV photodiodes: a comparison of films grown by spray pyrolysis and MBE

Detecting the UV part of the spectrum is fundamental for a wide range of applications where ZnMgO has the potential to play a central role. The shortest achievable wavelength is a function of the Mg content in the films, which in turn is dependent on the growth technique. Moreover, increasing Mg contents lead to an electrical compensation of the films, which directly affects the responsivity of the photodetectors. In addition, the metal-semiconductor interface and the presence of grain boundaries have a direct impact on the responsivity through different gain mechanisms. In this work, we review the development of ZnMgO UV Schottky photodiodes using molecular beam epitaxy and spray pyrolysis, and we analyze and compare the physical mechanisms underlying the photodetector behavior. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only

Morphology and Band Structure of Orthorhombic PbS Nanoplatelets: An Indirect Band Gap Material

PbS quantum dots and nanoplatelets (NPLs) are of enormous interest in the development of optoelectronic devices. However, some important aspects of their nature remain unclear. Recent studies have revealed that colloidal PbS NPLs may depart from the rock-salt crystal structure of bulk and form an orthorhombic (Pnma) modification instead. To gain insight into the implications of such a change over the optoelectronic properties, we have synthesized orthorhombic PbS NPLs and determined the lattice parameters by means of selected area electron diffraction measurements. We have then calculated the associated band structure using density functional theory with Perdew–Burke–Ernzerhof functional for solids and with the GW approximation, including spin–orbit interactions. An indirect band gap is found, which may explain the weak luminescence reported in experiments. We derive effective masses for conduction and valence bands and deduce that quantum confinement along the a crystallographic axis (short axis of the NPL) reinforces the indirect band gap but that along b and c axes favors a direct gap instead. Calculations for colloidal nanoplatelets of 1.8 nm thickness, carried out with k·p theory, show that excitonic effects are strong, with binding energies of about 150 meV

Controlling the Phase Segregation in Mixed Halide Perovskites through Nanocrystal Size

Mixed halide perovskites are one of the promising candidates in developing solar cells and light-emitting diodes (LEDs), among other applications, because of their tunable optical properties. Nonetheless, photoinduced phase segregation, by formation of segregated Br-rich and I-rich domains, limits the overall applicability. We tracked the phase segregation with increasing crystalline size of CsPbBr3–xIx and their photoluminescence under continuous-wave laser irradiation (405 nm, 10 mW cm–2) and observed the occurrence of the phase segregation from the threshold size of 46 ± 7 nm. These results have an outstanding agreement with the diffusion length (45.8 nm) calculated also experimentally from the emission lifetime and segregation rates. Furthermore, through Kelvin probe force microscopy, we confirmed the correlation between the phase segregation and the reversible halide ion migration among grain centers and boundaries. These results open a way to achieve segregation-free mixed halide perovskites and improve their performances in optoelectronic devices