Bound Exciton Lines Research Articles

Incorporation of nitrogen in ZnSe grown by metalorganic vapour phase epitaxy

The incorporation processes and efficiencies of nitrogen doping for p-type conductivity in metalorganic vapour phase epitaxy (MOVPE) grown ZnSe/GaAs epilayers are investigated by means of time-integrated and time resolved photoluminescence (PL) spectroscopy. Two nitrogen-doping methods are compared, plasma-enhanced doping during growth, and ion implantation of nitrogen with annealing after growth. Both types of doped layers exhibit the I N 1 transition from a neutral acceptor bound exciton complex (A 0 N, X), indicating an effective nitrogen embedding on selenium sites. With increasing nitrogen doping rates, a deeper bound exciton line I C 1 appears, lowering the intensity of the I N 1. An observed reduction of the I N 1 and I C 1 lifetimes for higher nitrogen doping concentrations results from an enhanced overlap of the bound exciton wave functions with those of other impurity centres.

Photoluminescence Of Zincselenide Single Crystals Annealed In Zinc Or Selenium Atmosphere

ABSTRACTZincselenide single crystals grown by the sublimation method are annealed in Zn or Se atmosphere. The annealing effects are examined by means of photoluminescence (PL) and reflection spectroscopy at 4.2 K. In the PL spectrum for the as-grown crystal, bound exciton lines (I2, I1) are observed. For the Zn-annealed crystal, the free exciton line is clearly observed. For the Se-annealed crystal, peak positions of all lines shift to the higher energies and all lines become sharp, compared with the spectra for the as-grown crystal. It is concluded that Sc-vacancies affect the bandgap energy more than Zn-vacancies do.

Effects of heat treatment on the 0.8 eV photoluminescence emission in GaAs grown by molecular beam epitaxy at low temperatures

We report 0.8 eV photoluminescence (PL) emission of GaAs grown at low temperatures between 325 and 400°C by molecular beam epitaxy. Effects of heat treatments of the 0.8 eV emission are compared with those of the 1.467 eV sharp bound exciton lines. This allows us to attribute the 0.8 eV emisson to the As-VGa center. We discuss the assigning of the Asi-VGa center to the well-known EL6. The PL intensity variation of 0.68 eV EL2 and 0.8 eV Asi-VGa seen in substrate materials is explained in terms of dislocation−mediated Asi-VGa transformation to EL2 whereas the PL intensity variation of 0.8 eV Asi-VGa for molecular beam epitaxy layers can be attributed to the growth condition.

Cathodoluminescence spectroscopy of free and bound excitons in chemical-vapor-deposited diamond.

Cathodoluminescence (CL) spectroscopy was used to study exciton recombination at temperatures from 80 to 280 K in undoped and boron-doped diamond particles grown by hot-filament chemical vapor deposition. Spectral lines due to free and bound excitons were observed in the near-band-gap region, 4.6--5.5 eV. (The band gap of diamond is at 5.49 eV.) In the undoped particles, free-exciton lines were observed at 5.27 and 5.12 eV. Another set of lines, not previously reported, was observed between 4.7 and 5.0 eV, with the most intense lines at 4.757, 4.832, and 4.950 eV. The latter set of lines is attributed to excitons bound to lattice defects, possibly dislocations. In the boron-doped particles, excitons bound to boron acceptors were found to dominate the near-band-gap CL spectrum at low temperature. The most intense acceptor-bound-exciton line is at 5.20 eV. In both boron-doped and undoped particles, the exciton lines were much less intense in {111} crystal-growth sectors than in {100} sectors. Quenching of the exciton luminescence due to nonradiative recombination is believed to be the cause of the reduced intensity in the {111} sectors. The temperature dependence of the intensities, peak positions, and peak widths of the exciton lines was examined. The free and acceptor-bound excitons in the boron-doped particles were found to be in thermal equilibrium with each other. In the undoped particles, the free and defect-bound excitons were not in thermal equilibrium; the higher-energy bound-exciton lines (at 4.832 and 4.950 eV) decreased more rapidly with increasing temperature than either the lowest-energy bound-exciton line (at 4.757 eV) or the free-exciton lines.This behavior is attributed to thermally activated transitions from the higher-energy bound-exciton states to the lowest-energy state. The full width at half maximum (FWHM) of the free-exciton line, ${\mathit{W}}_{\mathrm{FE}}$, increased with temperature at about the rate predicted for excitations thermalized near the bottom of a parabolic energy band: ${\mathit{W}}_{\mathrm{FE}}$(T)=${\mathit{W}}_{\mathrm{FE}}$(0)+1.795kT. The FWHM's of the defect-bound- and acceptor-bound-exciton lines also increased with temperature, but at slower rates than for the free exciton. The peak positions of the exciton lines increased slightly with increasing temperature.

Electronic structure and dynamical behaviour of different bound-exciton complexes in ZnSe bulk crystals

High-quality ZnSe crystals grown by the high-pressure Bridgman method show emissions of the free exciton (X) in the excitonic energy range as well as bound exciton lines I 1 and I 2. The I 2i-lines, which are observed in excitation spectra of the (D 0, X)-complex, are explained taking into account a three-particle model with excited | n, l〉 hole states for the (D 0, X)-complex. Otherwise the (A 0, X) ground state is split by the ( j, j)-coupling of the two Γ 8-holes into two separate levels. From time-resolved luminescence the specified rise and decay times of different bound excitons complexes are determined.

Resonant photoluminescence measurements in As- and P-doped ZnTe epilayers

The near-gap photoluminescence (PL) of MOVPE ZnTe epilayers grown on (0 0 1) GaAs substrates has been extensively studied using resonant-excitation methods. Heteroepitaxial ZnTe layers show a small biaxial strain which splits excitonic transitions. We obtained the Luttinger parameter γ 1 = 3.8 from transition of the split free exciton 1s and 2s levels. Shallow acceptor states were observed in free standing As- and P-doped ZnTe layers and the Luttinger parameters γ 2 = 0.72 and γ 3 = 1.3 were derived. The magnetic field splitting of the 1S 3 2 acceptor ground state unambiguously identifies the acceptor bound exciton line I 1 a as double light-hole state ( A 0 a1, X 1). Furthermore the magnetic parameters κ A = -0.27 and q A = -0.015 were determined.

Band edge and deep level photoluminescence of fully strained Si 1− xGe x/Si alloys

A detailed photoluminescence (PL) analysis of fully strained SiGe layers grown by molecular beam epitaxy on Si (001) reveals several independent recombination bands. We observe SiGe band edge related free and bound exciton lines, and two deep bands L1 at about 800 meV and L2 at about 580 meV. In addition, at low excitation a weak band about 33 meV below the SiGe band edge PL is found. The temperature and laser excitation dependence of all bands and their changes on annealing are analyzed.

Photoluminescence from Si1−xGex strained layers grown by atmospheric pressure chemical vapor deposition

Low temperature photoluminescence due to the self-annihilation of bound excitons has been observed in Si1−xGex strained layers grown using atmospheric pressure chemical vapor deposition. Samples were grown at temperatures near 1000 °C with growth rates up to 1000 nm per minute allowing short growth times, thus preventing extensive interdiffusion at layer interfaces. Well-resolved, bulk-like photoluminescence spectra with narrow no phonon linewidths were observed from strained SiGe material indicating it to be of suitable electronic quality. For a sample consisting of 120 nm of Si0.92Ge0.08 capped with 90 nm of Si on a Si(100) substrate, the photoluminescence spectrum exhibited Si1−xGex bound exciton lines with resolved no phonon and transverse optic phonon components.

Zeeman spectroscopy of an axial-double-acceptor bound exciton in GaAs grown by molecular-beam epitaxy.

Zeeman-, resonant-, and polarization-spectroscopic investigations of an exciton bound to an axial, double-acceptor-type defect in gallium arsenide grown by molecular-beam epitaxy are presented. With nonresonant excitation, two bound-exciton recombination lines, ${\mathit{Y}}_{1}$ and ${\mathit{Y}}_{1}^{\mathrm{*}}$ appear in the near-gap region. Selective excitation of ${\mathit{Y}}_{1}$ and ${\mathit{Y}}_{1}^{\mathrm{*}}$ reveals 1s, 2s, and 3s two-hole replicas consistent with the presence of two holes in the bare neutral complex. The ionization energy of the first hole is calculated to be 214 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ (26.5 meV)---almost equal to the effective-mass value. ${\mathit{Y}}_{1}$ and ${\mathit{Y}}_{1}^{\mathrm{*}}$ are polarized along the [1\ifmmode\bar\else\textasciimacron\fi{}10] and [110] directions, respectively. The anisotropy of the Zeeman splittings also demonstrates preferential incorporation of the defect along the [110] crystallographic axis. A spin Hamiltonian containing three free parameters is used to model the Zeeman measurements.

Growth and Properties of ZnSe Crystals by a Modified Bridgman Method

Zinc selenide crystals are grown from the melt by a modified Bridgman method under argon pressure. The electrical and optical properties of the as-grown crystals are measured as a function of the temperature of a Zn reservoir which produces a vapor of a specific Zn partial pressure. Low-resistivity n-type crystals of 0.2 to 2 Ω·cm are grown at the temperature of the Zn reservoir above 960°C. Electron Hall mobility of the crystals decreases with increasing temperature of the Zn reservoir during growth. A mobility value of 550 cm2/V·s is obtained at room temperature (290 K). The extra Zn partial pressure enhances a bound-exciton line and suppresses deep-level emission bands around 500 and 600 nm in photoluminescence spectra at 10 K.

Pseudodonor electronic excited states of neutral complex defects in silicon.

The optical-absorption spectrum of a complex neutral defect in silicon with a lowest bound-exciton line at 615.0 meV has been studied. Only transitions to s-like excited electronic states are observed and their binding energies are in good agreement with a deep pseudodonor model for the electronic structure of the defect. A comparison with optical-excitation spectra of other complex defects shows that this type of electronic structure explains the optical spectra of a large class of defects in silicon.

Growth-induced shallow acceptor defect and related luminescence effects in molecular beam epitaxial GaAs

We report a study of a defect responsible for the ‘‘g’’ bound exciton line at 1.5112 eV that is frequently detected in photoluminescence spectra of GaAs grown by molecular beam epitaxy (MBE). A direct correlation has been observed between this line and a transition at 1.4946 eV, which is shown to result from a conduction band-to-acceptor recombination involving a shallow, unidentified acceptorlike defect that is labeled ‘‘A.’’ The activation energy of the defect is 24.8±0.2 meV, about 1.7 meV lower than that of CAs acceptor. Upon hydrogenation the defect is passivated more extensively than any known shallow acceptor species in GaAs. This result is analyzed in terms of a passivation model, from which it can be inferred that the A defect is not due to a simple substitutional Group II impurity on a Ga site. Incorporation of the A defect strongly affects the luminescence properties of the material. An almost complete quenching of the donor-bound exciton lines, profound changes in the line shape and relative intensity of the free exciton recombination, and appearance of a sharp transition of unknown origin at 1.5138 eV were observed with increasing defect concentration. Apparently ‘‘donorless’’ low temperature exciton recombination spectra are reported for defect-rich p-type MBE GaAs layers with donor concentrations as high as 7×1014 cm−3 and compensation ratios of ∼0.3. The dependence of the defect incorporation on MBE growth parameters is discussed. The feasibility of MBE growth of high purity, nearly shallow defect-free p-type GaAs layers at marginally As-stabilized surface conditions over an about 1–5 μm/h range of deposition rates is demonstrated.

Excitonic photoluminescence in CuGaS2 crystals

The low-temperature photoluminescence (PL) spectra of CuGaS2 are investigated in this work. At 4.2 K, high-quality single crystals grown by both the iodine-transport and the melt-growth methods showed many sharp PL lines. Free-exciton (2.504 eV) and bound-exciton (2.501, 2.495, and 2.493 eV) lines are studied in detail with respect to transition energy, temperature, and excitation intensity. Phonon-assisted transitions have been observed in the low-energy regions of zero-phonon lines at 2.493, 2.404, and 2.397 eV. Most of them have been well interpreted on the basis of the phonon energies previously obtained by Raman measurements.

Photoluminescence characterisation of indium-doped and undoped silicon layers grown by molecular beam epitaxy

Silicon molecular beam epitaxy layers are characterised by photoluminescence. Low-energy ion doping with indium during growth has been exploited to obtain an In-doped silicon layer on a phosphorus-doped silicon substrate. The samples were grown over a range of growth temperatures, 550-850 degrees C, and the photoluminescence spectrum of the well known In-related shallow bound exciton line was detected for doping concentrations from 1*1016 to 1*1018 cm-3. Undoped silicon layers on boron-doped silicon substrates were used to study the effect of growth temperature. A previously unreported line at 884 meV is also studied.

Identification of Na acceptor in MOCVD-grown ZnS films and the effect of UV light illumination

Low-temperature photoluminescence (PL) studies have been carried out to identify a Na acceptor level in ZnS films grown by low-pressure metalorganic chemical-vapour deposition. A neutral Na acceptor bound-exciton line at 3.781 eV and related edge-emission bands at about 3.67 eV were observed. In particular, the edge-emission band is attributed to a free-to-Na acceptor transition by a time-resolved spectra experiment. These PL properties have also been investigated by changing growth and Na cell temperatures. The ionization energy of the Na acceptor is estimated to be about 170 meV which is in good agreement with the theoretical value calculated by Baldereschi and Lipari. The effect of UV light illumination was preliminary done to incorporate Na impurities into ZnS films during epitaxial growth. It was found that the Na acceptor bound-exciton line can be significantly enhanced in intensity.

Photoluminescence decay times of the defect-induced bound-exciton lines in GaAs grown by molecular-beam epitaxy.

Photoluminescence decay transients of the defect-induced bound-exciton lines frequently observed in GaAs grown by molecular-beam epitaxy were studied in detail in an effort to resolve discrepancies in the published results. In particular, the same high-quality sample previously used in other measurements was obtained in order to eliminate the possibility of sample variability. The nonresonantly excited transient results presented here agree with our earlier measurements made on a different sample, but differ substantially from the results reported by others for the same sample. Finally, new resonantly excited transient results are found to be quite different from the nonresonant case and lead to a clearer picture of the dynamics of this unusual system.

Vapor phase epitaxy of CuGaS 2 using metal chlorides and H 2S sources

VPE growth of CuGaS 2 on GaAs and ZnS/GaAs substrates has been performed using metal chlorides and H 2 S+Ar or H 2 S+H 2 gas as a sulfur source. X-ray diffraction studies showed that the epitaxial growth of singly oriented crystals with their chalcopyrite c -axis normal to the (100) GaAs surface can occur in spite of the large difference in the layer and substrate lattice constants. With (100) ZnS/GaAs substrates, the dominant growth occurred with the c -axis parallel to the substrate surface. X-ray, Raman and EPMA studies indicated the necessity of further efforts to improve the layer uniformity. The low temperature photoluminescence spectrum of the grown layer was found to be surprisingly similar for position-to-position changes in the sample, and also for sample-to-sample changes. The photoluminescence of the layer on GaAs substrates exhibited a main emission peak at 2.31 eV, and three bound exciton lines in addition to a line due to free excitons. Analysis of the time-resolved spectra of the 2.31 eV emission gave the relevant donor and acceptor binding energies to be 185 ± 10 meV and 120 ± 10 meV, respectively. A brief discussion is given on the origin of this acceptor in terms of observed low-resistivity of the grown layer.

Effect of H 2 heat-treatment of excitonic emission in MOCVD-grown CdTe films on (100) GaAs substrates

CdTe epitaxial growth can be accomplished on near-oriented (100) GaAs substrates at 350 °C by low-pressure metalorganic chemical-vapour deposition (MOCVD) using dimethylcadmium and dimethyltelluride, the latter of which was thermally pre-cracked. Nominally undoped film is originally under compressive strain for a thickness of about 1.2 μm as expected from the valence-band splitting in the exciton reflectance spectra. Intense excitonic-emission lines associated with the free-exciton, principal neutral-donor and neutral-acceptor bound-exciton (BE) lines at 1.596, 1.5931, 1.5904 and 1.5985 eV appear at 4.2 K, respectively. The linewidths of the donor and acceptor BE lines are less than 0.8 meV. The effect of post-growth annealing at temperatures between 150 and 550 °C in H 2 leads to a marked reduction in intensity of the 1.5931 and 1.5904 eV BE lines, whilst no observable energy shifts and further splitting of the excitonic lines appear.

Epitaxial growth and photoluminescence characterization of ZnSe: Na films by low-pressure MOCVD

We have for the time grown ZnSe layers with intentional Na acceptor inpurities on (100) GaAs and (110) ZnSe substrates at 300°C by the low-pressure metalorganic chemical-vapour-deposition (MOCVD) method. The intense Na acceptor bound-exciton emission line at about 2.793 eV at 4.2 K appears in the hetero- and homo-epitaxial films grown using a Na 2Se source whose cell temperature was 300–560°C. Preliminary irradiation with Hg (UV) light was carried out to activate the incorporation of Na acceptors into ZnSe during growth. The Na acceptor bound-exciton and the related donor-acceptor pair band were enhanced in intensity by this photo-assisted MOCVD.

Photoluminescence of CdTe crystals annealed in Hg

Annealing effects on CdTe crystals in liquid mercury at temperatures ranging from RT to 300°C for 200 h have been investigated by means of photoluminescence characteristics at 4.2 K. The photoluminescence spectra of as-grown p-type CdTe samples by the Bridgman method exhibit a strong broad emission at about 1.4 eV and an edge emission at 1.552 eV. After prolonged annealing, a marked decrease in the intensity of both the broad band and edge emissions is observed. On the contrary, a bound exciton line at 1.590 eV becomes intense and sharp. It is tentatively proposed that the filling-in of Cd vacancies with Hg can cause a change in the luminescence properties of Bridgman grown CdTe.