Acceptor-bound Exciton Research Articles

Direct evidence of high efficiency of the Auger nonradiative recombination of acceptor bound excitons in Cd1 −xMnxTe

The magnetization of Cd1 −xMnxTe can be reduced by the Mn2+ magnetic resonance. The decrease of the magnetization affects all the “edge‘ emissions of the material studied, which shift towards higher energies. It also affects the relative efficiencies of the free exciton (FE) and the acceptor bound exciton (ABE) emissions. The latter decays mostly nonradiatively, which is due to a competing Auger transition. The large Auger recombination rate of the ABE is directly confirmed by the present optically detected magnetic resonance (ODMR) investigations.

Nitrogen doping of molecular beam epitaxially grown CdTe with a radio-frequency plasma source

We report on the use of an Oxford Applied Research MPD21 Atom/Radical RF source to dope CdTe layers with nitrogen (N). The layers were grown in a VG V80H MBE system, using CdTe as the source material. Photoluminescence (PL) measurements give evidence of N incorporation in our CdTe:N layers, as signified by an increase in intensity of the N acceptor bound exciton (ABE) line and the associated donor-acceptor pair (DAP) band, with increasing plasma RF power. The presence of N was confirmed using secondary ion mass spectrometry (SIMS). In addition, PL and SIMS results show the presence of significant levels of antimony (Sb) in the N-doped layers. The Sb acceptor depth was found to be 52 meV from observation of the associated free to bound transition. The unintentional presence of Sb is tentatively attributed to enhanced autodoping from the substrate, caused by the presence of N. Like the undoped layers, the N-doped samples are found to be n-type, but the carrier concentrations are <10 15 cm -3.

Shallow impurity- and defect-related complexes in undoped ZnSe crystals

The electronic structure of bound excitons involving native-as well as point-defects is investigated by optical spectroscopy using highly crystalline ZnSe samples grown by the Markov method. We demonstrate that the interactions of attendant single particles is resposible for the splitting of the (D 0, X) and (A 0, X) states. From these splitting the hole-hole interaction parameter γ and the cubic crystal-field parameter β for the Li bound exciton complex has been determined unambigously for the first time. The I d 1 emission line shows a fine structure which is similar to the I Li 1 exciton line. I d 1 is therefore assigned to an acceptor-bound exciton recombination with an associate formed by a zinc vacancy and an impurity donor combined acting as an acceptor.

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.

Dynamic Jahn-Teller effect for a double acceptor or acceptor-bound exciton in semiconductors: Mechanism for an inverted level ordering.

The dynamic Jahn-Teller effect provides a simple mechanism for inverting the ordering of the J=0,2 levels of the two equivalent holes in a neutral double acceptor or an exciton bound at a neutral acceptor. Whereas Hund's rule places the J=2 level below J=0, Jahn-Teller coupling to E and/or ${\mathit{T}}_{2}$ vibrational modes tends to shift J=0 below J=2, while splitting J=2 into its ${\mathrm{\ensuremath{\Gamma}}}_{3}$ and ${\mathrm{\ensuremath{\Gamma}}}_{5}$ components. Alternative proposed explanations involving central-cell, Stark, and strain effects fail to introduce the attractive interaction needed to offset the holes's Coulomb repulsion.

Nitrogen radical doping during metalorganic vapor phase epitaxy of ZnSe

The availability of nitrogen radical doping in low-pressure metalorganic vapor phase epitaxy of N-doped ZnSe layers is reported. Active nitrogen generated by low-frequency dielectric discharge was supplied alternately with source materials. The narrow and distinct peak for the acceptor-bound exciton, together with donor-acceptor pair peaks, is predominant in the low-temperature cathodoluminescence spectra for the layers prepared with alternate growth and doping.

An optical method for evaluation of the net acceptor concentration in p-type ZnSe

We report photoluminescence (PL) results obtained on p-type ZnSe epilayers grown by molecular beam epitaxy. As an acceptor dopant, we used an active nitrogen beam produced by a free radical nitrogen source. On the basis of a detailed analysis of PL data we propose a simple semiquantitative method for a quick and contactless evaluation of the net acceptor concentration in p-type ZnSe. In particular, we show that the intensity ratio of the donor–acceptor pair (DAP) emission to the acceptor-bound exciton (I1) emission strongly depends on both the excitation power and the quality of the sample, and because of that it cannot by itself be regarded as a good measure of the net acceptor concentration. On the other hand, the intensity of the DAP emission under saturation excitation shows a simple direct proportionality to the net acceptor concentration, thus providing a reliable tool for determining the relative doping level in p-type ZnSe films.

Optical Studies of free and Acceptor-Bound Excitons in GaAs/AlGaAs Symmetric Coupled Double Quantum Well Structures

Discrete peaks, due to the excited 2S state of the n=l symmetric heavy hole (hh) exciton, are observed in the photoluminescence excitation spectra of GaAs/AlGaAs coupled double quantum well (CDQW) samples. The observations of the 2S state allow a precise determination of 1S-2S splitting, and the experimental results show a good agreement with a simple calculation which is based on a model of fractional-dimension of the exciton state. The obtained values for the 1S-2S splitting are significantly reduced compared with the corresponding single (or multiple) quantum wells (QW). Central acceptors confined in CDQWs have also been studied. Due to built in electric fields, interwell and intrawell excitons bound to neutral acceptors are simultaneously observed in weakly coupled doped double QWs. The diamagnetic shift and binding energy of the bound excitons are similar to results of the single QW’s, while the diamagnetic shift of the free exciton differs from single QW’s.

Magnetooptical Studies of Acceptors Confined in GaAs/AIGaAs Quantum Wells

AbstractMagnetooptical studies have been performed on the shallow Be acceptor confined in the central region of narrow GaAs/AlGaAs quantum wells (QWs) with the magnetic field along the growth direction. The magnetic field dependence of the acceptor transition between the 1S(Г6) hh-like ground state and the excited hh-like 2S(Г6) state has been investigated by means of two independent techniques: Two-hole transitions of the acceptor bound exciton (BE) and resonant Raman scattering. The 1S(Г6) – 2S(Г6) transition energy as a function of the magnetic field has been measured for central acceptors in QWs of widths in the range 50 – 150 Å. The energy levels for the 1S ground states and 2S excited states of the confined acceptor with a magnetic field as a perturbation have also been calculated. These calculations predict a larger splitting between the mj=+3/2 and mj=−3/2 components of the acceptor 1S(Г6) ground state in comparison with the corresponding splitting of the excited 2S(Г6) state. The experimental results are in good agreement with the theoretical predictions derived without any fitting parameters. Furthermore, the Zeeman splitting of the acceptor BE emission has been measured and it is concluded that the J = 5/2 BE state is lowest in energy, similar to shallow acceptor BEs in bulk GaAs.

Plasma-Assisted Epitaxial Growth of P-Type ZnSe in Nitrogen-Based Plasma

P-type ZnSe films have been successfully grown by plasma-assisted epitaxy in nitrogen-based plasma, and the resistivity of the film was reduced to the order of 10 Ω·cm at room temperature. With a relatively low nitrogen gas flow rate of around 0.073 sccm, low-temperature photoluminescence intensities of donor-acceptor pair emission and acceptor-bound exciton emission from N-doped ZnSe were maximum with negligible deep-level emission at the partial pressure ratio PN2/(PH2+PN2) of around 80%. Atomic nitrogen generated by the Penning effect around this partial pressure ratio was observed. It has been suggested that, while hydrogen improves the electronic property of ZnSe layers, excessive hydrogen can reduce the doping efficiency of shallow-acceptor nitrogen.

Low residual impurities assessment by photoluminescence in multistep wafer-annealed semi-insulating Czochralski-grown GaAs

Multistep wafer-annealed semi-insulating GaAs wafers (MWA) are characterized using photoluminescence (PL). The PL spectra present well-resolved near-band-edge transitions, including the doublet of the neutral acceptor-bound exciton. A detailed investigation using selective pair luminescence of samples submitted to different annealings, i.e., wafer- or ingot-annealing, single or multistep, shows that carbon is the main shallow acceptor. However, for the wafer-annealed samples, two other residual impurities found in the as-grown or ingot-annealed crystals have their estimated concentrations noticeably reduced, for Zn (e.g., around 1013 cm−3 in MWA), or are unresolved for Si. This reduction of background impurities may have direct consequences for device applications.

Molecular-beam epitaxial growth of p- and n-type ZnSe homoepitaxial layers

Undoped, p- and n-type ZnSe homoepitaxial layers have been successfully grown by molecular beam epitaxy on dry-etched substrates. ZnSe substrates were etched with a BCl 3 plasma to eliminate polishing damage. The full width at half maximum of an X-ray rocking curve for a homoepitaxial layer has reached 21 arc sec. N-doped ZnSe homoepitaxial layers grown with the active-nitrogen doping technique have shown p-type conduction which is the evidence of p-type conduction in ZnSe. The n-type homoepitaxial layers have been grown with Cl doping. The Cl-doped homoepitaxial layers have shown high Hall mobilities relative to heteroepitaxial layers. These undoped, N-doped and Cl-doped layers have exhibited strain-free photoluminescence properties; free exciton emission, neutral acceptor-bound exciton emission and neutral donor-bound exciton emission showed a single peaks at 2.804, 2.7931 and 2.7980 eV, respectively.

Planar doping of p-type ZnSe layers with lithium grown by molecular beam epitaxy

High quality p-type ZnSe layers with hole concentrations of 3 × 10 17 − 1.4 × 10 18 cm -3 have been grown using a planar doping technique employing lithium during molecular beam epitaxy (MBE). The Li-doping is investigated by reflection high energy electron diffraction (RHEED). It is found that islands of Li 2Se are formed on the growing ZnSe surface in the case of heavy Li doping. Secondary ion mass spectroscopy (SIMS) and photoluminescence (PL) are used to characterize the layers grown. SIMS analysis shows an abrupt change in Li concentration at the interface between highly doped (mid 10 19 cm -3) and undoped layers. This is the first report of the successful suppression of Li diffusion. PL spectra at 4.2 K show dominant acceptor-bound exciton emission (I 1). The line shape of I 1 is noticeably broadened with a tail towards the low energy side. This may be explained in terms of a Stark effect on bound excitons arising from charged impurities.

Growth and Optical Characterization of ZnMnTe Grown by Molecular Beam Epitaxy

ABSTRACTWe have successfully grown ZnMnTe alloys by molecular beam epitaxy using GaAs as a substrate. Bulk MnTe has the wurtzite crystal structure but the structural phase of the material was confirmed to be zinc-blende by standard θ-2θ x-ray diffraction techniques. The composition was also determined using x-ray diffraction techniques. Manganese cancentration was also estimated from magnetization measurements taken as a function of temperature. Magneto-luminescence studies were performed at 1.4K on the acceptor-bound exciton in the semimagnetic semiconductor ZnMnTe alloys. As expected, the photoluminescence peak energy decreased with increasing magnetic field.

Spectroscopic Investigation of Li and P-Doped ZnSe Grown by Molecular Beam Epitaxy

ABSTRACTA conduction band-to-acceptor (e-A°) peak at 2.7060 eV has been identified for the first time in 1.7 K PL spectra of Li-doped ZnSe/GaAs. Photoluminescence (PL) measurements as a function of temperature and excitation level have been employed to confirm the identification and to study the behavior of this peak. This peak was previously misidentified as the “R”-band characteristic of bulk material, which has been associated with transitions between preferentially paired interstitial Li donors and substitutional Li acceptors on Zn sites. We find no direct evidence of Li interstitials in the PL data. The splitting of the Li acceptor-bound exciton is dramatically reduced by removal of the GaAs substrate, which proves that the splitting is due to strain and not to the hole-hole exchange interaction. A P-doped ZnSe/GaAs sample exhibits a shallow P bound exciton peak nearly superimposed on the donor-bound excitons. It is distinguished by its strong LO phonon replica and low energy undulation wing, which are both unique characteristics of shallow acceptor-bound excitons. Transitions involving the conduction band to split P acceptor light and heavy hole levels have been detected for the first time in P-doped heteroepitaxial ZnSe. Using the temperature dependence of the (e-A°) peak positions, accurate binding energies of 89.1±0.6 and 114.4±0.4 meV have been obtained for shallow P and Li acceptor levels in unstrained ZnSe, respectively

Activation of N-Acceptor in MOCVD-ZnSe by Excimer Laser Annealing

ABSTRACTThe recent success in obtaining blue-green diode lasers using ZnSe-based heterostructures has been mainly due to the successful p-doping of ZnSe using a N plasma source in an MBE system. P-type ZnSe can also be grown by MOCVD, using NH3 as the dopant source. Dopant concentrations of up to 1018 cm−3 have been obtained but net acceptor concentrations are in the range of only about 1015 cm−3 Activation of the remaining N has been achieved to some extent (NA -ND∼3*1016cm−3 ) by rapid thermal annealing. However, this has had limited success. We have used fast surface annealing by an excimer laser to activate the N. We have observed an increase in the ratio of Acceptor Bound Exciton peak intensity to Free Exciton peak intensity on annealing and this effect increases as we increase the laser power density from 10 to 30 MW/cm2. Electrical measurements ( C-V ) give a net acceptor concentration of ∼2*1016 cm−3 and thus confirm the increase in carrier concentration after annealing.

Photoluminescence and Photoacoüstic Spectroscopy of Si Single-and Multi-Step Ingot- and Wafer-Annealed GaAs Crystals

ABSTRACTSI GaAs crystals submitted to single- or multi-step, ingot-or wafer-annealing are investigated using photoluminescence (PL) and photoacoustic spectroscopy (PA). The near-band-edge PL transitions are well resolved, with a neutral acceptor-bound exciton recombination displayed as a split doublet. The improvement induced by wafer-annealing is illustrated by the absence of additional defect-related transitions found after ingot-annealing. For the room temperature PA measurements, the intensity of a peak occuring at 1.39 eV is shown to lead to an estimation of the arsenic micro-defect density as evaluated by AB etching. The 1.39 eV PA band is also asserted to be the non-radiative recombination path of a 1.482 eV band found in the low-temperature PL spectra.

Optical characterization of GaAs quantum wire microcrystals

Optical properties of quantum-size GaAs wire crystals grown by organometallic vapor-phase epitaxy (OMVPE) are measured. The typical size of the wire-shaped microcrystals is 1–5 μm long and 10–200 nm wide. Photoluminescence measurements at 4 K reveal spectral features dominated by free carrier to acceptor impurity recombination. A free exciton recombination line is also observed and is more intense relative to other features than that observed from a conventional OMVPE epitaxially grown layer. Small spectral shifts (0.5 meV) of the free exciton and the acceptor-bound exciton recombination lines are considered to be due to the effects of quantum confinement on the energy levels of the system.

Quantum size microcrystals grown using organometallic vapor phase epitaxy

Needle-shaped quantum size microcrystals as thin as 10 nm have been selectively grown by employing reduced pressure organometallic vapor phase epitaxy using trimethylgallium and arsine as source materials. The microcrystals grown within a SiO2 window area have their growth axes along the [111] direction. Transmission electron diffraction analysis shows that the crystal structure of microcrystals is consistent with the zinc-blende structure of GaAs. The mechanism for growing the needle-shaped crystals is similar to a vapor-liquid-solid (VLS) equilibrium phase growth model. From photoluminescence measurements at 4.2 K, it is found that the microcrystals show a very distinct spectra for free exciton and neutral acceptor-bound exciton recombinations, meaning good crystal quality.

P-type ZnSe homoepitaxial layers grown by molecular beam epitaxy with nitrogen radical doping

We have grown high-quality homoepitaxial layers of p-type ZnSe for the first time. The layers were grown on dry-etched ZnSe substrates by molecular-beam epitaxy with nitrogen radical doping. The p-type conduction with carrier concentration of 8.9×1015 cm−3 at 300 K was confirmed by Hall measurement. Low-temperature photoluminescence from the N-doped ZnSe layers was dominated by single acceptor-bound exciton emission I1 at 2.7931 eV, which indicates a formation of a number of shallow acceptors and strain free of the homoepitaxial layers.