Free Exciton Binding Energy Research Articles

Low residual doping level in homoepitaxially grown ZnO layers

ZnO homoepitaxial growth is shown to change the luminescent properties of ZnO in a substantial way. Temperature dependent (4–300K) photoluminescence properties of the films grown by metal organic vapor phase epitaxy or by liquid phase epitaxy are compared to those of the hydrothermal ZnO substrate used for the growth. The intensity ratio between the free exciton and the donor bound exciton strongly increases, following homoepitaxial growth. The activation energy of the band edge emission intensity also increases from 13meV, which corresponds to the donor bound exciton localization energy, up to a value of 60meV, which corresponds to the free exciton binding energy. This indicates that homoepitaxial growth favors free exciton emission instead of donor bound exciton emission.

Photoluminescence from highly excited AlN epitaxial layers

Excitonic optical properties of AlN epitaxial layers have been studied by means of photoluminescence and optical reflectance spectroscopies. The binding energy of free excitons was estimated to be 57meV on the basis of the energy separation between the n=1 ground and n=2 excited states. In addition, the luminescence line due to radiative recombination of biexcitons was observed under high density excitation. The energy separation between free-exciton luminescence and biexciton luminescence was 19meV, which corresponded to the binding energy of biexcitons. Therefore, the ratio of the biexciton binding energy to the exciton binding energy was approximately 0.33.

Optical spectroscopy of free excitons in a CuInS2 chalcopyrite semiconductor compound

The spectra of reflectance and luminescence of high-quality CuInS2 single crystals grown by oriented crystallization are studied at the temperature 4.2 K. In the region of the fundamental absorption edge, the two excitonic resonance reflectance peaks, nondegenerate peak A at the energy ∼1.5356 eV and doubly degenerate peak BC at the energy ∼1.5567 eV, and the luminescence signal produced by free and bound excitons are observed. The luminescence lines, A UPB at ∼1.5361 eV and A LPB at ∼1.5347 eV, with a half-width ∼1 meV, are attributed to exciton-polariton recombination. From the experimentally observed energy position of the exciton ground state and excited states, the binding energy of free excitons is determined to be ∼18.5 meV. In studying the photoluminescence in magnetic fields up to 10 T, a diamagnetic shift of the ground state of free excitons A is observed.

Impact of strain on free-exciton resonance energies in wurtzite AlN

The strain dependence of the free-exciton resonance energies in AlN epilayers is presented and the values are analyzed using an appropriate Hamiltonian assuming equibiaxial stress for the wurtzite crystal structure in order to obtain valence band parameters. Based on the results, we study the strain dependence of the valence band ordering, optical transition probability, and free-exciton binding energy. As a result of these calculations, the following strain-free values are obtained for the energy gap, averaged dielectric constants, and ordinary and extraordinary dielectric constants: Eg=6.095 eV at T=11 K, ϵ=7.87, ϵ⊥=7.33, and ϵ∥=8.45, respectively. A brief discussion of the valence band ordering in bulk AlxGa1−xN is also presented.

Optical band gap studies on lithium aluminum silicate glasses doped with Cr3+ ions

Lithium aluminum silicate glass system (LAS) implanted with chromium ions is prepared. The reflectance and transmittance measurements are used to determine the dispersion of absorption coefficient. The optical data are explained in terms of the different oxidation states adopted by the chromium ions into the glass network. It is found that the oxidation state of the chromium depends on its concentration. Across a wide spectral range, 0.2–1.6μm, analysis of the fundamental absorption edge provides values for the average energy band gaps for allowed direct and indirect transitions. The optical absorption coefficient just below the absorption edge varies exponentially with photon energy indicating the presence of Urbach’s tail. Such tail is decreased with the increase of the chromium dopant. From the analysis of the optical absorption data, the absorption peak at ground state exciton energy, the absorption at band gap, and the free exciton binding energy are determined. The extinction coefficient data are used to determine the Fermi energy level of the studied glasses. The metallization criterion is obtained and discussed exploring the nature of the glasses. The measured IR spectra of the different glasses are used to throw some light on the optical properties of the present glasses correlating them with their structure and composition.

Photoluminescence properties of ZnTeO and ZnSeO alloys with dilute O concentrations

AbstractThe photoluminescence properties of ZnTeO and ZnSeO with dilute O concentrations are studied. The exciton bound to isolated O and its phonon replicas are observed in ZnTeO. The peak energies are hardly affected by O concentration, indicating the isoelectronic feature of O. On the other hand, the free exciton peak is shifted to lower energy in ZnSeO by increasing O concentration, even when the O concentration is as low as 0.015%. No isoelectronic emissions are observed. We find the increase of binding energy of free exciton due to the increase of the effective mass of the conduction band edge. This can be interpreted as the modification of the band structure by the repulsive interaction between the O localized level and the conduction band of ZnSe with the scheme of band anticrossing model. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Exciton and donor binding energies in quantum-well wires and quantum dots a fractional-dimensional space approach

A simple method for calculating the free-exciton binding energies in the fractional-dimensional-space model for single-quantum-well structure has been extended to quantum-well wires and quantum dots, in which the real anisotropic system is modelled through an effective isotropic environment with a fractional dimension. In this scheme, the fractional-dimensional parameter is chosen via an analytical procedure and involves no ansatz. We calculated the ground-state binding energies of excitons and donors in quantum-well wires with rectangular cross sections. Our results are found to be in good agreement with previous variational calculations and available experimental measurements. We also discussed the ground-state exciton binding energy changing with different shapes of quantum-well wires.

Sulfur isotope effects on the excitonic spectra of CdS

The energies of bound excitons, as well as free excitons associated with the $A,$ $B,$ and $C$ valence band edges, are compared using both photoluminescence spectroscopy and reflectivity spectroscopy for wurtzite CdS crystals containing natural Cd and S of either natural isotopic composition, or highly enriched ${}^{34}\mathrm{S}.$ In contrast to an earlier study of S isotope effects in CdS, all of the transitions, including the free exciton excited states, show nearly the same energy shifts, demonstrating that it is the isotope dependence of the band gap energy which is the dominant factor in these shifts, and not changes in the free exciton binding energies. The observed dependence of the band gap on the S mass is found to be in good agreement with earlier predictions based on the temperature dependence of the free exciton transition energies.

Excitonic transitions in β-FeSi2 epitaxial films and single crystals

Photoreflectance spectra were obtained from an epitaxial film and a bulk single crystal of β-FeSi2 at low temperatures (T⩽180 K). A model based on the results of low-temperature absorption [M. Rebien et al., Appl. Phys. Lett. 74, 970 (1999)] was used to describe the main features of the spectra. In agreement with the absorption results, transitions corresponding to the ground state and first excited state of the free exciton were observed in both the epitaxial film and single crystal. However, additional subband gap features are revealed in the photoreflectance spectra of the thin film. It is suggested that these may be related to impurity transitions or an impurity transition plus a bound exciton resonance. From the analysis of the spectra taken on the thin film, over a temperature range of 12–180 K, we extract a free exciton binding energy of (0.009±0.002) eV and a direct energy gap at T=0 K of (0.934±0.002) eV.

A Study of the Photoluminescence of ZnO Thin films Deposited by radical Beam Assisted Molecular Beam Epitaxy

II-Ⅵ ZnO compound semiconductor thin films were grown on <TEX>$\alpha$</TEX>-Al<TEX>$_2$</TEX>O<TEX>$_3$</TEX>(0001) single crystal substrate by radical beam assisted molecular beam epitaxy and the optical properties were investigated. Zn(6N) was evaporated using Knudsen cell and O radical was assisted at the partial pressure of 1<TEX>$\times$</TEX>10<TEX>$^{4}$</TEX> Torr and radical beam source of 250-450 W RF power. In <TEX>$\theta$</TEX>-2<TEX>$\theta$</TEX> x-ray diffraction analysis, ZnO thin film with 500 nm thickness showed only ZnO(0002)and ZnO(0004) peaks is believed to be well grown along c-axis orientation. Photoluminescence (PL) measurement using He-Cd (<TEX>$\lambda$</TEX>=325 nm) laser is obtained in the temperature range of 9 K-300 K. At 9 K and 300 K, only near band edge (NBE) is observed and the FWHM's of PL peak of the ZnO deposited at 450 RF power are 45 meV and 145 meV respectively. From no observation of any weak deep level peak even at room temperature PL, the ZnO grains are regarded to contain very low defect density and impurity to cause the deep-level defects. The peak position of free exciton showed slightly red-shift as temperature was increased, and from this result the binding energy of free exciton can be experimentally determined as much as <TEX>$58\pm$</TEX>0.5 meV, which is very closed to that of ZnO bulk. By van der Pauw 4-point probe measurement, the grown ZnO is proved to be n-type with the electron concentration(<TEX>$n_{e}$</TEX> ) <TEX>$1.69<TEX>$\times$</TEX>10^{18}$</TEX><TEX>$cm^3$</TEX>, mobility(<TEX>$\mu$</TEX>) <TEX>$-12.3\textrm{cm}^2$</TEX>/Vㆍs, and resistivity(<TEX>$\rho$</TEX>) 0.30 <TEX>$\Omega$</TEX><TEX>$\cdot$</TEX>cm.

Photocurrent study of the valence band splitting of AgInS 2 epilayers on GaAs

AgInS 2 epilayers have been grown on a GaAs substrate by using the hot-wall epitaxy method. The temperature dependence of the band gap energy of AgInS 2 is determined to be E g (T)=2.1365 eV−(9.89×10 −3 eV)T 2/(2930+T) using the absorption spectra. The free exciton binding energy, Δcr, and Δso of the chalcopyrite AgInS 2 have been found to be 0.1115, 0.1541, and 0.0129 eV, respectively. This result reveals that the Δso splitting clearly exists for the Γ 5 states of the valence band in the AgInS 2 epilayer.

Deep ultraviolet picosecond time-resolved photoluminescence studies of AlN epilayers

AlN epilayers with high optical qualities have been obtained by metalorganic chemical vapor deposition on sapphire substrates. Deep UV picosecond time-resolved photoluminescence (PL) spectroscopy has been employed to study the optical transitions in AlN epilayers. Two PL emission lines associated with the donor bound exciton (D0X, or I2) and free exciton (FX) transitions have been observed, from which the binding energy of the donor bound excitons in AlN epilayers was determined to be around 16 meV. Time-resolved PL measurements revealed that the recombination lifetimes of the I2 and free exciton transitions in AlN epilayers were around 80 and 50 ps, respectively. The temperature dependencies of the free exciton radiative decay lifetime and emission intensity were investigated, from which a value of about 80 meV for the free exciton binding energy in AlN epilayer was deduced. This value is believed to be the largest free exciton binding energy ever reported in semiconductors, implying excitons in AlN are an extremely robust system that would survive well above room temperature. This together with other well-known physical properties of AlN may considerably expand future prospects for the application of III-nitride materials.

Effects of thermal annealing on ZnO films grown by plasma enhanced chemical vapour deposition from Zn(C2H5)2 and CO2 gas mixtures

To improve ZnO thin film quality, the ZnO thin films grown on silicon (100) by plasma enhanced chemical vapour deposition from Zn(C2H5)2 and CO2 gas mixtures at a low temperature of 120°C are annealed in an oxygen ambient at temperature ranging from 600°C to 1000°C. X-ray diffraction spectra indicate that ZnO films possess a polycrystalline hexagonal wurtzite structure. Atomic force microscopy results show an increase of ZnO grain size with the increase of annealing temperature. The photoluminescence is closely related to the annealing temperature. The free exciton binding energy deduced from the temperature-dependent PL spectra is about 59 meV for the ZnO film annealed at 900°C, suggesting that the film quality can be improved by annealing process.

Direct determination of free exciton binding energy from phonon-assisted luminescence spectra in GaN epilayers

Investigation of variable-temperature longitudinal optical (LO) phonon-assisted luminescence spectra of free excitons and free electrons in heteroepitaxial GaN thin films has been conducted. It is found that thermal broadening of the LO phonon-assisted photoluminescence peaks is much slower than those of the peaks of their parents so that the first-order LO peaks of the free exciton transition and the band-to-band transition can be well resolved even at room temperature, leading to a direct determination of the band A free exciton binding energy as 25.4±0.9 meV. At the same time, we demonstrate that the simple hydrogenlike model still is a good approximation to describe the energy level structure of free excitons in GaN.

Photoconductivity of Lightly-Doped and Semi-Insulating 4H-SiC and the Free Exciton Binding Energy

The paper presents a study of the structure of the photoconductivity spectra of various 4H-SiC samples near the absorption edge. By means of comparison of the spectra of low doped (mid 10(14) cm(-3)), very low doped (in 10(13) cm(-3) range), and semi-insulating moderately doped samples, features in the photocurrent due to contribution from creation of free carriers (i.e., excitons in the continuum) can be recognised. This is used for determination of the free exciton binding energy, 20.5 +/- 1 meV, in agreement with a previous study. The second lowest conduction band and the spin-orbit split off valence band are also detected.

Optical properties of GaN grown on Si (111) by gas source molecular beam epitaxy with ammonia

We report a study of the optical properties of GaN grown on Si(111) by gas source molecular beam epitaxy with ammonia. Temperature dependence of edge luminescence was studied in the range of 77–495 K for samples with low background carrier concentrations, as determined by capacitance voltage profiling and Raman spectroscopy, and the results were fitted using Passler’s and Varshni’s models. We also demonstrate strong correlation between electron concentration in GaN and relative Raman intensity of A1 (longitudinal optical) and E22 modes. The binding energy of free excitons is estimated to be 29±2 meV. The contributions of different mechanisms to free exciton line broadening are discussed.

Photoluminescence characteristics of ZnTe epilayers

High quality ZnTe epilayers have been grown on semi-insulating GaAs(100) substrates by hot-wall epitaxy, and the photoluminescence characteristics were investigated. The free exciton binding energy is found to be 12.7 meV and the free exciton reduced mass to be 0.095m0 from the clearly resolved excitonic peaks. From the temperature dependence of the free exciton line, the room temperature energy gap of ZnTe epilayer is found to be 2.278 eV. The longitudinal optical phonon energy from the resonance Raman shift is determined as 26.2 meV. The binding energy of the exciton bound to the neutral acceptor is found to be 4.9 meV. The temperature dependence of the bound exciton peak intensity is explained with a two-step thermal quenching mechanism. Deep level emissions such as donor–acceptor pair emission, Y-band, and oxygen-bound emission are also studied.

Intrinsic Photoconductivity of 6H-SiC and the Free-Exciton Binding Energy

The paper presents a study of the low-temperature photoconductivity of 6H-SiC. The photocurrent at the absorption edge is assigned to Auger recombination of excitons captured to impurities. This is shown to saturate and decrease with increasing the photon energy, so the further increase in the photocurrent, observed in the purest sample, can be attributed to free excitons created in non-bound states in the exciton continuum. Thus, the exciton binding energy can be estimated, E-bx approximate to 60 +/- 5 meV in 6H-SiC.

Determination of the Exciton Binding Energy in ZnCdSe Quantum Wells by Resonant Raman Scattering

Due to the relevant role of excitonic emission in blue-green diodes and lasers based on Zn1–xCdxSe quantum wells important efforts, in theory and experiment, have been carried out to determine the excitonic binding energy as a function of composition and quantum well width. In this work we present a direct determination of the binding energy of the free exciton of a 50 Å Zn0.74Cd0.26Se quantum well confined by ZnSe barriers by means of the observation of the resonant Raman scattering (RRS) effect. We show that the RRS is due to a double resonance: the first (incoming) is produced by the matching of the exciting laser line (488 nm, 2.5407 eV) with the 1hh → 1e absorption transition of the QW; the second (outgoing) is due to the close coincidence of the exciton binding energy and the LO phonon energy. From these results we have obtained 32.6 ± 2 meV for the free exciton binding energy.

Excitonic Photoluminescence from CuGaSe2 Single Crystals and Epitaxial Layers: Temperature Dependence of the Band Gap Energy

Excitonic line spectra from CuGaSe2 single crystals and epitaxial layers are investigated as a function of temperature. Near band edge luminescence from free and bound excitons is observed at 10 K. The identification of both, free exciton ground and first excited state allows to determine the free exciton binding energy, which is found to be (13±2) meV. The bound exciton line is attributed to the recombination of an exciton bound to a neutral acceptor (A0, X). The widely discussed phenomenon of an anomalous temperature dependence of the band gap energies in different chalcopyrite-type I-III-VI2 compounds is reconsidered for CuGaSe2 on the basis of temperature dependent photoluminescence studies. No anomalous behaviour of the band gap energy as a function of temperature is found in CuGaSe2.