Bound Exciton Lines Research Articles

A distinctive bound exciton line induced by electron-beam irradiation and subsequent annealing of an unintentionally doped GaN layer

Abstract An undefined bound exciton line (UDX) with a photon energy 0.6 meV lower than the Si-donor bound exciton line was observed by photoluminescence in GaN layers annealed at 1100 °C after electron beam (EB) irradiation at an energy of 137–2000 keV. The UDX was not observed in samples not subjected to EB irradiation or annealing but was found in a sample annealed after EB irradiation at an energy of 137 keV, where only nitrogen atoms are displaced in GaN. The origin of the UDX was presumably formed by a thermal reaction of defects containing nitrogen-displacement-related defects.

Luminescence spatial characteristics of ZnSe:Fe

Two types of thermal-diffusion-based technological routes have been realized to obtain ZnSe:Fe plates demanded for laser application in the mid-IR spectral range. For the technological routes used, the iron doping process is shown to lead to the formation of auxiliary luminescence bands including (1) luminescence of residual tetrahedral donors and acceptors; (2) unknown bound exciton line located at 459 nm; (3) broad bands with maxima at 490, 520, 670, and 820 nm; and (4) IR band at 960 nm previously assigned to centers related to transition metals. Using the measurements of photoluminescence under conditions of two-photon excitation and measurements of low-temperature microphotoluminescence, the intensity profiles of the bands have been shown to be uncorrelated with the concentration profile of the optically active (Fe2+) iron. This trend does not depend on the technological routes used. Thus, heavily Fe doped areas, as well as adjacent regions in ZnSe:Fe plates produced by thermal-diffusion-based techniques, contain auxiliary centers governing luminescence in visible and near IR spectral ranges. These centers are not directly related to the optically active (Fe2+) iron.

Exciton luminescence in AlN triggered by hydrogen and thermal annealing

Exciton recombination bands in homoepitaxial AlN layers are strongly dependent on the presence of hydrogen. By thermal treatment under hydrogen-free and hydrogen-rich ambient, respectively, several sharp bound exciton lines are modulated in intensity reversibly. In contrast, the exciton bound at the neutral donor silicon remains unaffected. The mechanism causing these effects is most probably hydrogen in- and out-diffusion into the AlN sample. The main factor determining hydrogenation of AlN layers is found to be molecular H2 in contrast to NH3. We find hints that carbon incorporation into AlN may be closely related with that of hydrogen. Besides photoluminescence spectra of exciton bands, our model is supported by theoretical reports and comparison to the case of hydrogen in GaN.

Importance of the dielectric contrast for the polarization of excitonic transitions in single GaN nanowires

We investigate the polarization of excitonic transitions of single dispersed GaN nanowires with a diameter of about 50 nm. We observe donor-bound exciton transitions with a linewidth narrower than 250 μeV at 10 K, whereas the luminescence from free excitons exhibits a width of up to 5 meV. This broadening is larger than that observed for free excitons in the as-grown nanowire ensemble and is the result of inhomogeneous strain introduced by the nanowire dispersion. This strain lowers the symmetry of the lattice structure and allows A excitons to emit light polarized parallel to the nanowire axis. The polarization anisotropy of A excitons, however, is found to largely vary from one nanowire to another. In addition, the various bound-exciton lines in a given nanowire do not show the same polarization anisotropies. These findings can be explained by the dielectric contrast between the nanowire and its environment, but only when taking into account the strong variations of the dielectric function of GaN at the near band-edge.

Optical properties of C-doped bulk GaN wafers grown by halide vapor phase epitaxy

Freestanding bulk C-doped GaN wafers grown by halide vapor phase epitaxy are studied by optical spectroscopy and electron microscopy. Significant changes of the near band gap (NBG) emission as well as an enhancement of yellow luminescence have been found with increasing C doping from 5 × 1016 cm−3 to 6 × 1017 cm−3. Cathodoluminescence mapping reveals hexagonal domain structures (pits) with high oxygen concentrations formed during the growth. NBG emission within the pits even at high C concentration is dominated by a rather broad line at ∼3.47 eV typical for n-type GaN. In the area without pits, quenching of the donor bound exciton (DBE) spectrum at moderate C doping levels of 1–2 × 1017 cm−3 is observed along with the appearance of two acceptor bound exciton lines typical for Mg-doped GaN. The DBE ionization due to local electric fields in compensated GaN may explain the transformation of the NBG emission.

Effects of photoinduced charge redistribution on excitonic states in Zn(Cd)Se/ZnMgSSe quantum wells

Photoinduced charge redistribution processes in Zn(Cd)Se/ZnMgSSe/GaAs quantum-well structures are studied using steady-state photoluminescence, photoreflectance, and scanning spreading resistance microscopy with an additional illumination. It is shown that an above-barrier optical pumping leads to the accumulation of electrons in the quantum wells. The resulting concentration of excess electrons in the quantum wells is several orders of magnitude higher than the concentration of photoexcited electron-hole pairs. These excess electrons induce broadening of excitonic resonances and, furthermore, cause an enhancement in the photoluminescence quantum yield and an increase in the relative intensity of the bound-exciton emission line. The additional below-barrier illumination at temperatures about 100 K leads to a decrease in the excess electron concentration in the quantum wells. The observed phenomena are explained in terms of a simple model considering the formation of a barrier in the conduction band near the ZnMgSSe/GaAs heterointerface.

Morphological, electrical and optical properties of highly oriented undoped and doped zinc oxide and cadmium oxide films grown by atmospheric-pressure chemical vapor deposition

Morphological, electrical and optical properties of undoped and Ga-doped ZnO and undoped CdO films grown on sapphire (Al2O3) substrates by the atmospheric-pressure CVD method using Zn, Cd, H2O and GaCl3 as source materials were investigated. The influences of the deviation from the stoichiometric composition were clearly observed on photoluminescence (PL), photoluminescence excitation (PLE) and photoacoustic (PA) spectra of the undoped ZnO films. The carrier concentration n in the undoped ZnO film was found to be dominated by the hydrogen related donors rather than native defects. The Ga doping in ZnO film led to the degradation of surface flatness, the increase in carrier concentration n and the decrease in Hall mobility μ. With increasing carrier concentration n, the Ga-related neutral donor bound exciton line of the Ga-doped ZnO film shifted towards shorter wavelengths, accompanied by asymmetric broadening. This is probably due to the screening of the Coulomb potential by heavily introduced donors and free-electrons. Highly oriented CdO films with smooth surfaces were successfully grown on r-Al2O3 substrates. Transmittance values for the CdO/r-Al2O3 films were higher than 70% in the range of 700–2500nm. Under the assumption that the reflectance is neglected, the direct and indirect optical gap energies of the CdO films determined from the transmittance spectra were 2.34–2.38eV and 1.98–2.06eV, respectively. PA measurements for the CdO films revealed that the tail states associated with the residual impurities and/or the structural defects reduce the direct optical gap energy. A maximum Hall mobility μ of 178cm2/Vs was achieved on the CdO/r-Al2O3 film with the carrier concentration n of 4.2×1019cm−3.

Microstructure and point defects in CdTe nanowires for photovoltaic applications

Defects in Au-catalysed CdTe nanowires vapour–liquid–solid-grown on polycrystalline underlayers have been critically evaluated. Their low-temperature photoluminescence spectra were dominated by excitonic emission with rarely observed above-gap emission also being recorded. While acceptor bound exciton lines due to monovalent metallic impurities (Ag, Cu or Na) were seen, only deeper, donor–acceptor-pair emission could be attributed to the Au contamination that is expected from the catalyst. Annealing under nitrogen acted to enhance the single crystal-like PL emission, whilst oxidizing and reducing anneals of the type that is used in solar cell device processing caused it to degrade. The incidence of stacking faults, polytypes and twins was related only to the growth axes of the wires (〈111〉 50%, 〈112〉 30% and 〈110〉 20%), and was not influenced by annealing. The potential electrical activity of the point and extended defects, and the suitability of these nanowire materials (including processing steps) for solar cell applications, is discussed. Overall they have a quality that is superior to that of thin polycrystalline films, although questions remain about recombination due to Au.

Photoluminescence of 8H-SiC

8H-SiC epilayers grown on small 8H-SiC Lely platelets are investigated optically using photoluminescence spectroscopy. At low temperature the near band gap emission detected in the 2.78 to 2.67 eV range contains sharp lines associated to nitrogen-bound-exciton recombination. Three different no-phonon lines are detected accompanied by their phonon replicas. Free-exciton replicas are also observed which allows the determination of the excitonic band gap. The binding energy of the bound excitons can thus be determined and the ionization energies of the three nitrogen levels in 8H-SiC are estimated and found to be rather shallow compared to the values for other hexagonal polytypes. Additional bound-exciton lines are observed when the experimental photoluminescence temperature is increased.

Effects of annealing temperature on excitonic emissions from Na-implanted ZnO nanorods

The effects of annealing temperature on excitonic emissions from Na+ ion implanted ZnO nanorods are studied and annealing between 600 and 800°C can effectively repair the implantation as evidenced by the enhanced excitonic emissions. A well-resolved bound exciton line at 3.352eV with a linewidth of ∼2meV emerges from 800°C annealed sample and could be related to the formation of NaZn acceptor. When the annealing temperature is increased, the intensity of the I6−8 line decreases while that of I3 increases, suggesting enhanced ionization of neutral donors at elevated temperature.

Investigation of the thermal annealing effect on the defects structure in γ-irradiated CdZnTe crystals by photoluminescence method

The influence of the thermal annealing at 100 and 200°C on the low-temperature photoluminescence properties of different doses γ-irradiated Cd0.95Zn0.05Te crystals is investigated. It was obtained such thermal annealing induces increasing of the amplitude of donor bound exciton lines and A-centers line which were reduced by preliminary γ-irradiation. Such effect can be explained by the reconstruction of the initial donor defects structure. In this case the reconstruction process is caused by the transmission of the additional thermal energy which leads to the substantial movement of the radiation induced vacancies and interstitials ions with their further annihilation.

Effect of annealing and hydrogen plasma treatment on the luminescence of hydrothermally grown bulk ZnO

The optical properties of hydrothermally grown bulk ZnO is investigated by low temperature photoluminescence (PL) spectroscopy. The effects of annealing in an argon atmosphere, as well as the influence of hydrogen plasma exposure, on the PL of as-grown material are studied. The 11K PL spectrum of the as-grown ZnO shows different excitonic lines in the NBE region: several bound exciton lines are clearly visible. The origin of these bound excitons is discussed, as well as the influence of annealing on these lines. A shift in the deep level emission (DLE) is also observed with an increase in annealing temperature: a red shift is detected when the annealing temperature is increased up to 650°C, while a subsequent blue shift is observed upon an increase in the annealing temperature. The involvement of Li and Cu in this phenomenon will be discussed and the effect of hydrogen on this DLE will also be studied.

Optical identification of silicon as a shallow donor in MOVPE grown homoepitaxial AlN

AbstractAluminum nitride (AlN) layers doped intentionally with different concentrations of silicon atoms acting as shallow donors were grown by MOVPE on bulk c‐plane AlN to minimize dislocations in the doped layers; typical values for the dislocation density in the bulk AlN substrates are less than 104 cm−2. The actual silicon concentration was confirmed by secondary ion mass spectroscopy (SIMS) analysis and ranging from 1017 to 1019 cm−3. In highly resolved low temperature photoluminescence (PL) investigations, we found dominating bandgap‐related sharp emission lines with linewidth lower than 500 µeV for undoped samples. We tentatively assign silicon as the shallow donor causing the bound exciton line with 28.5 meV exciton localization energy, for which we found an intensity ratio relative to the free exciton line being linearly dependent on the silicon concentration. For increasing Si content, the respective emission band also broadened asymmetrically and shifted to lower energies as expected from the analogy with other semiconductor systems.

Extremely uniform bound exciton states in nitrogen δ-doped GaAs studied by photoluminescence spectroscopy in external magnetic fields

We studied the spatial localization of excitons bound to nitrogen (N) pairs in N δ-doped GaAs to make clear origin of bound exciton lines. An extremely high uniformity of the emission wavelength was achieved for the exciton bound to the N pairs because of the uniform strain field in the N δ-doped layer fabricated in the (001) plane in the atomically controlled way. The magneto-photoluminescence spectra in the Faraday configuration showed a mixing of the bright- and dark-exciton components in the exciton fine structure and diamagnetic shift. The spatial distribution of the excitons localized at different N pairs was estimated using the diamagnetic shift coefficient and confirmed by the radiative lifetime of the bright-exciton component. According to the estimated spatial distribution of bound-exciton wave function, it was found that the exciton for the 1.444-eV line is localized stronger than that for the 1.493-eV line. The strong electron confinement for the 1.444-eV line results in the reduction of exciton-phonon interaction.

Engineering of nearly strain-free ZnO films on Si(1 1 1) by tuning AlN buffer thickness

ZnO properties were investigated as a function of AlN buffer layer thickness (0–100nm) in ZnO/AlN/Si(111) structures grown by metal organic vapor phase epitaxy. A significant improvement of ZnO film crystallinity by tuning AlN buffer thickness was confirmed by x-ray diffraction, topography and photoluminescence measurements. An optimal AlN buffer layer thickness of 50nm is defined, which allows for growth of nearly strain-free ZnO films. The presence of free excitons at 10K suggests high crystal quality for all ZnO samples grown on AlN/Si(111) templates. The intensities of neutral and ionized donor bound exciton lines are found to correlate with the in-plane and out-of-plane strain in the films, respectively.

Sharp bound and free exciton lines from homoepitaxial AlN

AbstractMOCVD AlN layers grown in c‐direction on PVT bulk AlN substrates are investigated by high‐resolution photoluminescence in the energy region of the band gap. The experiments allow for observation of different donor bound exciton lines with a full width at half maximum below 0.5 meV. Assignments are suggested for the substitutional shallow donors silicon and oxygen. The visibility of an excited state of the free exciton with a hole from the highest valence band allows to determine the exciton binding energy to 52.4 meV by use of a simple hydrogen model. The resulting band gap energy is 6.094 eV at 10 K. Comparison with earlier reports on homoepitaxial AlN leads to a new identification of the bands reported there.

Combined effects of Ga, N, and Al codoping in solution grown 3C–SiC

We report on Ga-doped 3C–SiC epitaxial layers grown on on-axis (0001) 6H–SiC substrates using the vapor-liquid-solid technique and different Si1−xGax melts. The resulting samples have been investigated using secondary ion mass spectroscopy (SIMS), micro-Raman spectroscopy (μ-R) and, finally, low temperature photoluminescence (LTPL) spectroscopy. From SIMS measurements we find Ga concentrations in the range of 1018 cm−3, systematically accompanied by high nitrogen content. In good agreement with these findings, the μ-R spectra show that the Ga-doped samples are n-type, with electron concentrations close to 2×1018 cm−3. As expected, the LTPL spectra are dominated by strong N–Ga donor-acceptor pair (DAP) transitions. In one sample, a weak additional N–Al DAP recombination spectrum is also observed, showing the possibility to have accidental codoping with Ga and Al simultaneously. This was confirmed on a non-intentionally doped 3C–SiC (witness) sample on which, apart of the usual N and Al bound exciton lines, a small feature resolved at 2.35 eV comings from neutral Ga bound excitons. Quantitative analyses of the DAP transition energies in the Ga-doped and witness sample gave 346 meV for the optical binding energy of Ga acceptors in 3C–SiC against 251 meV for the Al one. The conditions for the relative observa-tion of Ga and Al related LTPL features are discussed and the demonstration of room temperature luminescence using Ga doping is done.

Optical and structural properties of m ‐plane GaN substrates grown by ammonothermal method and GaN epilayers grown on these substrates

Abstractm ‐plane oriented GaN substrates of size 6 x 8 mm have been obtained by ammonothermal method and used to growing GaN epilayers by metalorganic chemical vapor deposition. The structural and optical properties have been determined for both m ‐plane GaN substrates and GaN epilayers deposited on these substrates. Excellent structural quality of m ‐plane GaN substrates has been confirmed by very narrow XRD peaks (the full‐width‐half‐maximum for the symmetrical (10.0) and asymmetrical (20.1) peaks are 16 and 19 arcsec, respectively) and the low concentration of threading dislocation density (∼ 104 cm‐2). In addition, these substrates exhibit quite good optical quality: the energy gap‐related transition is clearly observed at room temperature in photoluminescence and contactless electroreflectance spectra. The optical quality becomes excellent for GaN epilayers deposited on these substrates. In low temperature reflectance spectra very narrow intrinsic exciton lines which are very sensitive to the optical selection rules typical for the hexagonal symmetry were observed. In low temperature photoluminescence spectra a strong bound exciton line, relatively strong FXA and FXB lines and no emission band around 3.42 eV, which could be associated with the stacking faults, were observed. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Internal structure of acceptor-bound excitons in wide-band-gap wurtzite semiconductors

We describe the internal structure of acceptor-bound excitons in wurtzite semiconductors. Our approach consists in first constructing, in the context of angular momentum algebra, the wave functions of the two-hole system that fulfill Pauli's exclusion's principle. Second, we construct the acceptor-bound exciton states by adding the electron states in a similar manner that two-hole states are constructed. We discuss the optical selection rules for the acceptor-bound exciton recombination. Finally, we compare our theory with experimental data for CdS and GaN. In the specific case of CdS for which much experimental information is available, we demonstrate that, compared with cubic semiconductors, the sign of the short-range hole-exchange interaction is reversed and more than one order of magnitude larger. The whole set of data is interpreted in the context of a large value of the short-range hole-exchange interaction ${\ensuremath{\Xi}}_{0}=3.4\ifmmode\pm\else\textpm\fi{}0.2\text{ }\text{meV}$. This value dictates the splitting between the ground-state line ${\text{I}}_{1}$ and the other transitions. The values we find for the electron-hole spin-exchange interaction and of the crystal-field splitting of the two-hole state are, respectively, $\ensuremath{-}0.4\ifmmode\pm\else\textpm\fi{}0.1$ and $0.2\ifmmode\pm\else\textpm\fi{}0.1\text{ }\text{meV}$. In the case of GaN, the experimental data for the acceptor-bound excitons in the case of Mg and Zn acceptors, show more than one bound-exciton line. We discuss a possible assignment of these states.

Stable enhancement of near-band-edge emission of ZnO nanowires by hydrogenincorporation

We report on the photoluminescence properties of ZnO nanowires treated with a mild Arplasma. The nanowires exhibited stable and strong enhancement of the near-band-edgeemission and quenching of the deep level emission. The low temperature PL revealed astrong hydrogen donor–bound-exciton line in the plasma-treated samples indicatingunintentional incorporation of hydrogen during the plasma treatment. To confirm theresults, hydrogen was implanted into the ZnO nanowires with a low ion energy of 600 eVand different fluences. The observed result can be related to the passivation of deep centersby hydrogen. The absolute photoluminescence intensity measured by an integrating sphereshowed stable and strong UV emission from the treated samples even after several weeks.