Acceptors In ZnSe Research Articles

Behavior of antimony in ZnSe grown by a closed Bridgman method

High-quality and large-size p-ZnSe single crystals doped with antimony (Sb) have been grown by a Bridgman method using a closed double crucible. Photoluminescence spectra at low temperatures indicate that Sb works as both a shallow acceptor and a deep complex donor. By studying the temperature and excitation intensity dependences of the free-to-acceptor and donor-acceptor pair emissions, we estimated that the ionization energy of Sb as a shallow acceptor in ZnSe is (112–114)meV above the valence band, and that of the deep complex donor related to Sb is 37 ±1meV below the conduction band.

Local vibrational modes of Zn–H–As defects in GaAs, ZnSe and ZnTe

The hydrogenation of the zinc acceptor in GaAs (Zn Ga), and As acceptor in ZnSe (As Se) and in ZnTe (As Te), is studied by computer modeling using an ab initio pseudopotential density-functional method. We found that the lowest energy location for hydrogen is nearly bond-centered (closer to As) between Zn and As atoms. Also for GaAs:ZnH, ZnSe:AsH and ZnTe:AsH, antibonding As–H units were found to be metastable by 0.4, 0.5 and 0.7 eV, respectively. The calculated local vibrational modes and deuterium isotopic shifts agree within less than 4% of the experimental data. The correct ordering of the As–H stretch modes in GaAs and ZnSe is only reproduced when anharmonic effects are taken into account.

Configuration of Li acceptor in ZnSe determined by infrared magnetoabsorption measurements

Based on the experimental results of the infrared magnetoabsorption measurements, we propose that a Li acceptor is displaced from its proper substitutional site in ZnSe. Infrared absorption spectra in magnetic fields reveal additional splitting of acceptor levels that cannot be understood in the framework of cubic symmetry. The splitting is caused by the symmetry lowering induced by the displacement of the Li atom. The splitting of the ${1S}_{3/2}$ ground state is estimated to be 2.9 meV. The ${2P}_{3/2},$ ${2S}_{3/2},$ and ${2P}_{5/2}({\ensuremath{\Gamma}}_{8})$ states split into two states, and the magnitude of the splitting is estimated to be 5.5, 5.3, and 5.6 meV, respectively.

Self-Compensation of the Phosphorus Acceptor in ZnSe

ZnSe epilayers doped with plasma-activated phosphorus have been investigated by means of optical and electrical measurements. It is found that PSe forms a shallow acceptor with binding energy of 85 meV which is identified in the optical emission spectra. In these samples we also observe an energy level near 90 meV from the valence band using deep level transient spectroscopy. We assign it to the PSe acceptor identified in photoluminescence. Also, deeper levels were observed with binding energies up to 450 meV. Some of them are metastable and give rise to persistent photoconductivity. We discuss their relevance taking into account that all samples investigated in the study were highly resistive.

Displaced Substitutional Phosphorus Acceptors in Zinc Selenide

Spin-flip Raman scattering experiments reveal that shallow phosphorus acceptors in ZnSe grown epitaxially on GaAs are subject to a local crystal field of trigonal symmetry. In relaxed layers, the trigonal field dominates the macroscopic biaxial tensile strain field. The trigonal field is also observed in pseudomorphic layers, where the sign of the overall macroscopic biaxial strain is reversed. In both cases, the wavefunction of the hole bound at the acceptor appears to be sufficiently localised for the effects of the macroscopic strain field to be severely reduced. This increased localisation relative to nitrogen acceptors and the appearance of the trigonal field are in excellent agreement with the results of previous total energy pseudopotential calculations, which predict that substitutional phosphorus acceptors should be displaced from selenium sites in 〈111〉 directions.

Direct evidence for the trigonal symmetry of shallow phosphorus acceptors in ZnSe

Spin-flip Raman scattering has been used to provide direct experimental evidence that shallow phosphorus acceptors in relaxed ZnSe epitaxial layers grown on GaAs lie at sites of predominantly trigonal local symmetry. As a consequence, the energy of the heavy-hole bound exciton is about 0.45 meV less than that of the light exciton. This splitting is significantly smaller than (and of opposite sign and different symmetry to) the valence-band splitting of 3 meV observed due to the macroscopic biaxial tensile strain in the same specimens. The behavior is in complete contrast to that of nitrogen acceptors, for which no trigonal field is observed. The experiments provide confirmation of the behavior predicted by previous pseudopotential total-energy calculations for the shallow acceptor states formed by these two different group-V dopants when substituting at selenium sites.

Compensation of nitrogen acceptor in ZnSe : N/ZnSe grown by MOCVD

It has been proved that the net acceptor concentration as high as 2×10 17 cm −3 can be obtained when ZnSe : N homoepitaxial layers grown by metalorganic chemical vapor deposition (MOCVD) are annealed in molten zinc. However, it was found that the emission intensity of two sets of donor–acceptor pair (DAP), shallow DAP (D sAP) and deep DAP (D dAP), increased with the increase in annealing temperature. In order to clarify the compensation effect, the study on the origin and the concentration of deep donor related to the D dAP emission should be carried out. The ionization energies of acceptor and deep donor were estimated to be 101±1 and 44±5 meV, respectively, by examining the dependence of their emission peaks with the excitation intensity. Furthermore, the upper limit of the deep donor concentration was estimated to be 1×10 16 cm −3. This is one magnitude smaller than the net acceptor concentration obtained from the annealed ZnSe : N homoepitaxial layer in our previous paper. By studying on the behaviors of deep donor, the main reason to lead to a low net acceptor concentration in the ZnSe : N epitaxial layers is not attributed to deep donor complex containing nitrogen atoms, but hydrogen passivation, although a part of doped-nitrogen atoms combine with other defects to form complicated defects.

Fano resonance of LO-phonon-coupled excited states of Li acceptors in ZnSe and Luttinger parameters

Fano resonance between LO-phonon-coupled excited states of a Li acceptor and the valence-band state has been observed in bulk ZnSe through the infrared-absorption measurements. For the ${2P}_{5/2}({\ensuremath{\Gamma}}_{7})$ state of a Li acceptor, we found that the coupling strength of the Fano resonance $f(h{\ensuremath{\nu}}_{0})$ is 2.8 ${\mathrm{cm}}^{\ensuremath{-}1},$ the distortion parameter q is 1.07, and the spectral width of the coupled state $\ensuremath{\Gamma}$ is $16.8 {\mathrm{cm}}^{\ensuremath{-}1}.$ It is concluded that a significant feature of the resonance peak is due to strong hole LO-phonon coupling in ZnSe. The Luttinger parameters ${\ensuremath{\gamma}}_{1},$ ${\ensuremath{\gamma}}_{2},$ and ${\ensuremath{\gamma}}_{3}$ are estimated to be 6.44, 2.58, and 2.74, respectively.

Doping of ZnSe during molecular beam epitaxial growth using an atomic phosphorus source

Zinc selenide films were doped with phosphorus during molecular beam epitaxial (MBE) growth by employing a novel, valved, three-zone solid-source radio frequency (rf)-cracker unit manufactured by Oxford Applied Research (OAR). Optical emission spectroscopy analysis of the plasmas produced in the rf chamber of the unit showed that the apparatus was capable of generating a flux of atomic phosphorus. By suitably adjusting the operating conditions of the unit, phosphorus concentrations in ZnSe epilayers were varied over the range of 1×1016 cm−3 to high-1018 cm−3, in a highly controlled and reproducible fashion. Phosphorus atoms, in contrast to P4 molecules, were found to be highly chemically reactive at the growing ZnSe surface at a normal growth temperature (around 300 °C), and the OAR unit was found to be eminently suitable for the provision of atomic P for MBE growth. Doping using atomic P was found, at least in lightly to moderately doped ZnSe, to provide an acceptor state that gave rise to a neutral-acceptor bound exciton emission located at 2.7919 eV, in keeping with that of other substitutional acceptors in ZnSe. Also, compensation in atomic P doped ZnSe appears to occur by virtue of the formation of shallow donor states rather than via the production of deep level states, which dominate in the case of most of the previously reported phosphorus-doped ZnSe studies.

Electrical properties at p-ZnSe/metal interfaces

In order to prepare low resistance ohmic contacts to p-ZnSn by the “deposition and annealing (DA)” technique which has been extensively used for GaAs and Si-based devices, formation of a heavily doped layer by the p-ZnSe/metal reaction is required. For p-ZnSe/Ni contacts, Ni and Se reacted preferentially at the ZnSe/Ni interface upon annealing at temperatures higher than 250°C. However, capacitance-voltage measurements showed that the net acceptor concentration (NA-ND) close to the p-ZnSe/Ni interface was reduced upon the Ni/ZnSe reaction, resulting in high contact resistance. For p-ZnSe/Au contacts, neither Au/ZnSe reaction nor reduction of the acceptor concentration were observed after annealing at temperatures lower than 300°C. This indicates that although the metal/p-ZnSe reaction is mandatory to prepare a heavily doped layer, the reaction induced an increase in the compensation donors in the p-ZnSe substrate. In order to increase the acceptor concentration in the vicinity of the p-ZnSe/metal interface through diffusion from the contact materials, Li or O which was reported to play the role of an acceptor in ZnSe was deposited with a contact metal and annealed at elevated temperatures. Ni or Ag was selected as the contact metal, because these metals were expected to enhance Li or O doping by reacting with ZnSe. However, the current density-voltage characteristics of the Li(N)/Ni and Ag(O) contacts exhibited rectifying behavior, and the contact resistances increased with increasing annealing temperature. The present results indicated that, even though the acceptor concentration in the p-ZnSe substrate increased by diffusion of the dopants from the contact elements, an increment of the compensation donors was larger than that of the acceptors. The present experiments indicated that preparation of low resistance ohmic contacts by forming a heavily doped intermediate layer between p-ZnSe and metal is extremely difficult by the DA technique.

Spin-flip Raman spectroscopy of nitrogen acceptors in ZnSe layers with different biaxial strains

Resonant spin-flip Raman scattering (SFR) was used to investigate systematically the effects of strain on the J = 3 2 spin states of nitrogen acceptors in p-type ZnSe grown on GaAs. Specimens from several MBE and MOVPE sources were studied and, by choosing specimens with different thicknesses of ZnSe, the in-plane biaxial strain could be chosen to be tensile or compressive, according to the extent of relaxation. For large tensile or large compressive strain, the acceptor states that lead to scattering are, respectively, the light- and heavy-hole states, which can be distinguished by the very different anisotropy of the corresponding SFR signals as the direction of the magnetic field is altered. The investigation also included specimens with a range of intermediate layer thicknesses for which the strain splitting of the hole states is comparable with the Zeeman splitting. In this case the field-induced lh/hh mixing is significant and the spectra are very sensitive to the exact value of the strain. The spin-Hamiltonian parameters κ ( = − 0.50 ± 0.05) and q ( = − 0.05 ± 0.01) were determined accurately by using specimens with small strains.

The phosphorus acceptor in ZnSe

Investigation of acceptor impurity levels in phosphorus implanted- and plasma-doped ZnSe/GaAs epitaxial layers grown by molecular-beam epitaxy has been performed using low temperature photoluminescence (PL) spectroscopy. It is shown that at least for weak P doping, the properties of phosphorus are similar to those of other effective mass substitutional acceptors in ZnSe.

Fluorine as a shallow acceptor in ZnSe

Fluorine-doped ZnSe epilayers were grown by metalorganic atomic layer epitaxy using a selective doping method and were characterized by photoluminescence spectrosopy. The photoluminescence spectrum of the fluorine-doped ZnSe epilayer exhibits two emission peaks when fluorine is supplied onto the Zn surface. The origin of these emission peaks is suggested to be due to the excitons bound to neutral fluorine atoms and the donor-to-fluorine acceptor emission. This is confirmed through the change in the energy positions of the emission peaks with the excitation intensity. These results suggest that fluorine is incorporated as an acceptor with an energy level of about 90 meV.

Direct observation of nitrogen acceptor passivation in ZnSe by hydrogen plasma

Passivation effects of nitrogen acceptor in hydrogenated ZnSe are investigated with low-temperature photoluminescence. Nitrogen-doped samples were exposed to a hydrogen plasma produced by an rf plasma cell. The significant PL spectral change indicates that hydrogen neutralizes the activity of nitrogen acceptors in ZnSe. This is the first observation of intentional hydrogen passivation of nitrogen acceptors in ZnSe, which plays an important role in determining the p-type conductivity of ZnSe grown by metalogranic chemical vapor deposition.

First-principles study of the compensation mechanism for nitrogen acceptors in ZnSe.

We present a mechanism for the compensation of N acceptors in ZnSe through first-principles pseudopotential calculations. In Se-rich conditions, the formation of ${\mathrm{N}}_{2}$ molecules that are electrically inert neutralizes the acceptor activity, with the maximum acceptor density achievable with N dopants being about ${10}^{18}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$, in good agreement with experiments. Going to Zn-rich conditions, the hole density is increased by an order of magnitude, suggesting a promising low-resistance p-type doping; however, a self-compensation still occurs due to a [100]-split interstitial N-N complex occupying a Se site, which behaves as a double donor.

Impurity states in doubly doped systems: Investigation of donor-donor and donor-acceptor pairs.

In a recent paper, Yi and Neumark [Phys. Rev. B 48, 17 043 (1993)] determined the effective radii of donors and acceptors in ZnSe:Na from the time decay of the close pairs. The pair decay rate used is proportional to an overlap integral between donor and acceptor wave functions. We show that such an integral as well as the hopping matrix can be used for both donor-acceptor and donor-donor systems. The density of states, calculated from these integrals, is capable of predicting the shift of conductivities observed in experimental results. The calculation for the metal-nonmetal transition of Si: P,B is in good agreement with recent experimental findings.

Systematic investigation of shallow acceptor levels in ZnSe

A systematic investigation of shallow acceptor levels in ZnSe grown by molecular beam epitaxy (MBE) has been performed using low temperature photoluminescence (PL) measurements as a function of excitation level, temperature, strain, and laser energy (i.e., selectively excited donor-acceptor pair luminescence or SPL). Five of the levels are due to N, Li, As, P, and O, while the chemical origins of two levels, denoted A 1 and A 2, have not yet been determined. The A 1 level is observed in undoped material after annealing using diamond-like C (DLC) caps, while the A 2 level is observed in nominally Na-doped material. The ionization energies of these levels are accurately determined from the temperature dependence of the band-to-acceptor (e-A 0) peak positions, accounting for strain. Those energies are 114.3 +- 0.5, 114.2 +- 0.3, 111.3 +- 0.5, 106.1 +- 0.6, 95.0 +- 0.4, 88.4 +- 0.5, and 83 +- 3 meV, respectively, for As, Li, N, A 1, A 2, P, and O in unstrained material. Several excited states have been observed in SPL measurements for As, A 2, O, and P for the first time. The excited states of As, O, and A 2 fit well to effective mass theory, while those for P do not. A model for the strain splitting of shallow acceptor-bound excitions has been developed and confirmed using measurements on samples whose substrates have been removed. Hayne's Rule is shown to be inapplicable to shallow acceptors in ZnSe. A strain splitting of the (e-A 0) peak fo As or Li acceptors in annealed material is clearly resolved and modeled.

Origin of the low doping efficiency of nitrogen acceptors in ZnSe grown by metalorganic chemical vapor deposition

The origin of nitrogen acceptor compensation in ZnSe:N has been studied by secondary ion mass spectrometry (SIMS) and infrared absorption (FTIR) measurements. Nitrogen-doped ZnSe layers were grown by atmospheric pressure metalorganic chemical vapor deposition. Ammonia gas was used as a nitrogen source. SIMS analysis has revealed that hydrogen was incorporated only into the ZnSe:N layer with the same concentration as nitrogen. FTIR measurements at 11 K strongly suggest the presence of N—H bonding at 3193 cm−1. It is concluded that hydrogen passivation is responsible for the acceptor compensation.

Electrical characterization of p-type ZnSe:N and Zn1−xMgxSySe1−y:N thin films

Differential van der Pauw–Hall effect and resistivity measurements have been performed to determine the concentration and mobility of free holes in nitrogen doped ZnSe and Zn1−xMgxSySe1−y thin films. Hall data taken between 120 and 300 K, with magnetic fields up to 4 kG yielded an activation energy of the nitrogen acceptors in ZnSe:N of 104 meV. Donor compensation in the ZnSe:N samples was negligible. Compared with ZnSe:N, samples of Zn1−xMgxSySe1−y:N, exhibited a significantly lower value of room-temperature mobility of holes, and fast-carrier freeze-out at relatively high temperature, approximately T=200 K.

Acceptor doping in ZnSe versus ZnTe

It is a long-standing puzzle that ZnSe is difficult to dope p type, while ZnTe—which is very similar to ZnSe—is very easily doped p type. We report ab initio calculations which show that the solubilities of Li and Na acceptors are much greater in ZnTe than the solubilities of the same acceptors in ZnSe. We trace the origin of this difference to the bonding properties of the acceptors with the neighboring chalcogens. Our results also explain the experimentally observed dependence on dopant concentration of the dislocation density in p-type ZnSe epilayers grown on GaAs.