Effective-mass States Research Articles

Shallow and Deep States of Beryllium Acceptor in GaN: Why Photoluminescence Experiments Do Not Reveal Small Polarons for Defects in Semiconductors.

Currently, only one shallow acceptor (Mg) has been discovered in GaN. Here, using photoluminescence (PL) measurements combined with hybrid density functional theory, we demonstrate that a shallow effective-mass state also exists for the Be_{Ga} acceptor. A PL band with a maximum at 3.38eV reveals a shallow Be_{Ga} acceptor level at 113±5 meV above the valence band, which is the lowest value among any dopants in GaN reported to date. Calculations suggest that the Be_{Ga} is a dual-nature acceptor with the "bright" shallow state responsible for the 3.38eV PL band, and the "dark," strongly localized small polaronic state with a significantly lower hole capture efficiency.

Unusual properties of the RY3 center in GaN

The investigation and identification of point defects in GaN is crucial for improving the reliability of light-emitting and high-power electronic devices. The RY3 defect with a characteristic emission band at about 1.8 eV is often observed in photoluminescence (PL) spectra of $n$-type GaN grown by hydride vapor phase epitaxy, and it exhibits unusual properties. Its emission band consists of two components: a fast (10-ns lifetime) RL3 with a maximum at 1.8 eV and a slow (100--300 \ensuremath{\mu}s lifetime) YL3 with a maximum at 2.1 eV and zero-phonon line at 2.36 eV. In steady-state PL measurements, the YL3 component emerges with increasing temperature from 90 to 180 K, concurrently with a decrease in the RL3 intensity. The activation energy of both processes is about 0.06 eV. In time-resolved PL, the YL3 intensity abruptly rises when the RL3 intensity begins to saturate. These and other phenomena can be explained using a model of an acceptor with two excited states. A delocalized, effective-mass state at about 0.2 eV above the valence band captures photogenerated holes. These holes transition to the ground state, which produces the RL3 component with a lifetime of \ensuremath{\sim}10 ns. Alternatively, they may nonradiatively transition over a 0.06 eV-high barrier to a localized excited state with a level at 1.13 eV above the valence band. Recombination of free electrons or electrons at shallow donors with the holes at this localized excited state is responsible for the YL3 component. The relative intensities of the RL3 and YL3 components are dictated by the probabilities of holes at the shallow excited state to transition to the ground or to the localized excited states. Transition metals and complex defects are considered as the main candidates for the RY3 center.

Discriminating a deep gallium antisite defect from shallow acceptors in GaAs using supercell calculations

For the purposes of making reliable first-principles predictions of defect energies in semiconductors, it is crucial to distinguish between effective-mass-like defects, which cannot be treated accurately with existing supercell methods, and deep defects, for which density functional theory calculations can yield reliable predictions of defect energy levels. The gallium antisite defect ${\mathrm{Ga}}_{As}$ is often associated with the 78/203 meV shallow double acceptor in Ga-rich gallium arsenide. Within a conceptual framework of level patterns, analyses of structure and spin stabilization can be used within a supercell approach to distinguish localized deep defect states from shallow acceptors such as ${\mathrm{B}}_{As}$. This systematic approach determines that the gallium antisite supercell results has signatures inconsistent with an effective mass state and cannot be the 78/203 shallow double acceptor. The properties of the Ga antisite in GaAs are described, total energy calculations that explicitly map onto asymptotic discrete localized bulk states predict that the Ga antisite is a deep double acceptor and has at least one deep donor state.

Steady state and transient electron transport properties of bulk dilute GaNxAs1−x

Two valley ensemble Monte Carlo simulations have been performed to investigate the electronic transport properties of bulk GaNxAs1−x alloys where the nitrogen concentration x ≤ 0.02 (2%). We have investigated these properties using two separate approaches, 1) through simulation of GaAs using the standard Kane model with the addition of isolated and pair state nitrogen scattering mechanisms and 2) approximating the lower “mixed state” band that arises from the use of band-anticrossing model with an analytic function that implements the inflection point (for concentrations >1%). From the steady-state properties, we find that the nitrogen scattering model produces a better fit with other results, both theoretical and experimental. We also comment on the transient properties of GaNxAs1−x, noting that the velocity overshoot peaks are of a much lower velocity than is found in GaAs at comparable field strengths, and through the use of model 2, that negative effective mass states have a significant role in the transient behavior. We find that the system takes much longer to reach equilibrium when compared to bulk GaAs in both models, and through the use of model 2, there is a significant population of negative effective mass states when the system is subjected to higher fields.

A pathway to p-type wide-band-gap semiconductors

Based on first-principles calculations we devise an alternative approach to p-type doping in AlN, ZnO, and ZnMgO. Instead of searching for acceptors on the left of the host atoms in the Periodic Table, we propose to search on the far right. We find that F placed at interstitial sites in AlN, ZnO, and ZnMgO acts as a shallow acceptor, leaving a hole in an effective-mass state near the valence-band maximum. We investigate the stability of F impurities and propose a procedure to selectively introduce F in the interstitial lattice sites of the above wide-band-gap semiconductors.

Formation and dynamics of muonium centers in semiconductors

Processes of muonium atom formation and disappearance via electron capture and loss by positive muons in semiconductors have been studied using μ SR techniques. Experiments in GaAs, GaP and CdS suggest that the electron is initially captured into a highly excited state. Cascading down to the ground state muonium goes through an intermediate weakly bound effective mass state (EMS) whose hydrogenic electron structure is determined by the electron effective mass and the dielectric constant of the host. We suggest that muonium dynamics in semiconductors (including the effects of electric and magnetic fields and temperature) reflect the electron dynamics in weakly bound muonium state(s) in which the electron is delocalized over distances of about 100 Å .

On the nature of Si‐doping in AlGaN alloys

The silicon doping characteristics of AlxGa1−xN were investigated over the x = 0.2–0.5 composition range. A combination of Hall effect, thermal admittance and variable temperature photoluminescence spectroscopy indicated that the Si donor level maintained a near effective-mass state behaviour over the composition range studied. Alloy fluctuations appeared to influence the Si donor level, with the activation energies increasing more steeply with increasing compositions than predicted for an effective-mass like donor. Although the Si donor state was localised to some degree within the band gap, the transformation into a DX-centre appeared to be suppressed for these compositions. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical properties of a silver-related defect in silicon

Doping crystalline silicon with silver results in a photoluminescence center with multiplet zero-phonon structure near 778.9 meV. We show that the published assignments of the vibronic sidebands are wrong, with severe implications for the relative transition probabilities of the luminescence transitions from the excited states. At low temperature, most of the luminescence intensity derives from the phonon sideband associated with a forbidden zero-phonon line through the phonon-assisted coupling of two of the excited states of the center. The effective mass of the vibration is determined from isotope effects to be close to the mass of one Ag atom. Uniaxial stress and magnetic perturbations establish that the current assignment of the electronic structure of the center is incorrect and that it is best described by a new variant on the ‘‘pseudodonor’’ model. An electron orbits in an effective Td environment, with an orbital triplet as its lowest-energy state, giving a j 53/2 electron state. A tightly bound hole has its orbital angular momentum quenched by the C3v symmetry of the center, leaving only spin angular momentum (s51/2). These particles couple to give J52,1,0 states. Using this model, the temperature dependence of both the total luminescence intensity and measured radiative decay time can be understood. These data allow an estimate to be made of the thermally induced transition rate of the electron from the effective-mass excited states into the conduction band.

Two - Color Mid-Infrared Spectroscopy of Isoelectronic Centers in Silicon

AbstractOne of the open questions in semiconductor physics is the origin of the small splittings of the excited states of bound excitons in silicon. A free electron laser as a tunable source of the mid-infrared radiation (MIR) can be used to investigate such splittings of the excited states of optical centers created by transition metal dopants in silicon. In the current study, the photoluminescence from silver and copper doped silicon is investigated by two color spectroscopy in the visible and the MIR. It is shown the PL due recombination of exciton bound to Ag and Cu is quenched upon application of the MIR beam. The time-resolved photoluminescence measurements and the quenching effects of these bands are presented. By scanning the wavelength of the free-electron laser ionization spectra of relevant traps involved in photoluminescence are obtained. The formation and dissociation of the bound excitons, and the small splittings of the effective-mass excited states are discussed. The applied experimental method allows correlation of DLTS data on trapping centers to specific channels of radiative recombination. It can be applied for spectroscopic analysis in materials science of semicondutors.

Photoluminescence study of II–VI semiconductors by using radioactive 71As dopants

By means of radioactive dopants, photoluminescence signals related to the respective chemical element are identified. In the present work, the isotope 71As ( t 1/2=65.3 h) which decays to 71Ge ( t 1/2=11.2 d) and subsequently to 71Ga was implanted into CdTe and ZnSe. In CdTe, the (D, A) transition related to the As Te acceptor is identified in agreement with the literature. In addition, a new (D, A) transition caused by a shallow Ga acceptor is identified. The corresponding acceptor level having an ionisation energy of E i=42 meV is still shallower than the effective-mass like state ( E i=57 meV). It is interpreted in terms of a 71Ga Te “antisite” defect in a Jahn–Teller distorted configuration. Ab initio calculations within the framework of density functional theory confirm the existence of a Jahn–Teller relaxed stable configuration for Ga Te. In ZnSe, no clear evidence of a (D, A As) transition is found, but like in CdTe, a new shallow acceptor is observed which is supposed to originate from a 71Ga Se defect.

Photo-induced electron trapping in indirect bandgap at low temperature

A variation of photoconductivity excitation with wavelength is applied to Si-doped (indirect bandgap material) for a wide range of temperature. The lower the temperature the lower the photocurrent below 70 K. In the range 13-30 K there is a decrease in the photoconductivity spectrum slightly above the bandgap transition energy, followed by another increase in the conductivity. We interpret these results in the light of existing models and confirm the trapping by the X-valley effective mass state, which is responsible for attenuation of persistent photoconductivity below 70 K. A intermediate state which has non-negligible lifetime is proposed as responsible for the decrease in the photoconductivity with about 561 nm of wavelength of exciting light, in the investigated 13-30 K range.

The Universal Behaviour of Shallow-Deep Level Instabilities in Semiconductors

In Al0.1Ga0.9As:Ge two photoconductivity (PC) effects related to the DX− (persistent PC) and A1 (giant PC) states are studied under hydrostatic pressure. We found that the barrier for electron capture to the DX− state, responsible for the persistent PC, changes its character for p > 109 Pa and becomes pinned to the conduction band (CB) edge. Another barrier linked with giant PC, responsible for electron capture on A1 state is pinned to the CB edge for all pressures. A new universal configurational coordinate diagram of donor related levels including two types of lattice deformation, of C3v symmetry for DX− and Td symmetry for A1, is proposed. It describes both shallow–deep A1 and DX− level instabilities and explains e.g. why capture of electrons to the DX− state goes via an intermediate effective mass state, or why for extremely sharp shallow A1 state anticrossing behavior is observed.

Optical properties of InxGa1−xN alloys grown by metalorganic chemical vapor deposition

We present the results of optical studies of the properties of InxGa1−xN epitaxial layers (0<x<0.2) grown by metalorganic chemical vapor deposition. The effects of alloying on the fundamental band gap of InxGa1−xN were investigated using a variety of spectroscopic techniques. The fundamental band-gap energies of the InxGa1−xN alloys were determined using photomodulation spectroscopy measurements and the variation of the fundamental band gap was measured as a function of temperature. The effects of pressure on the band gap for InxGa1−xN samples with different alloy concentrations were examined by studying the shift of photoluminescence (PL) emission lines using the diamond-anvil pressure-cell technique. The results show that PL originates from effective-mass conduction-band states. Anomalous temperature dependence of the PL peak shift and linewidth as well as the Stokes shift between photoreflectance and PL lines is explained by composition fluctuations in as-grown InGaN alloys.

Investigation of temperature influence on photo-induced conductivity in n-type AlxGa1−xAs

We present conductance as function of temperature (G × T) under influence of monochromatic light in the range 0.5-1.5 μm for direct as well as indirect bandgap n-type AlxGa1−xAsA. Results obtained below 60 K in indirect bandgap sample show the presence of another level of trapping, besides the DX centre, probably a X-valley effective mass state. In direct bandgap samples, these G × T curves show that above bandgap light increases conductivity to higher values than at room temperature and below bandgap light is not enough to avoid trapping. Photoconductivity spectra in indirect bandgap AlxGa1−xAs show that above ≅120 K, the absence of persistent photoconductivity contributes for a very clean spectrum. The mobility of AlxGa1−xAs is modelled considering dipole scattering. Data of transient decay of persistent photoconductivity is simulated using this approach.

Relaxed and effective-mass excited states of a quantum-dot polaron

The polaronic corrections to the first excited-state energies of an electron in a parabolic quantum dot are obtained variationally for the entire range of the electron-phonon coupling constant and for arbitrary confinement length using a canonical transformation method based on the Lee-Low-Pines-Gross formalism. Simple analytical results are obtained in some interesting limiting cases and for arbitrary values of the parameters the nature of the excited state is studied numerically. The theory is applied to two- and three-dimensional GaAs quantum dots to obtain information about the existence of both the effective mass and the relaxed excited states of a polaron in these systems.

Localized Spin Relaxation Due To Coupling To Effective Mass States

The EPR of residual Mn in CdF2 doped with Y, In and Ga is investigated. Although these donors are barely seen in EPR, they manifest themselves by a new effect: a drastic resonant reduction in the longitudinal relaxation rate of Mn which occurs only if the Zeeman splitting of the two subsystems coincide. In this situation, the saturation of those of the six Mn hyperfine lines is weakened which coincide with the shallow donor resonance.

Stability of deep donor and acceptor centers in GaN, AlN, and BN

We examine the atomic and electronic structure of substitutional Be, Mg, and C acceptor impurities and of Si, Ge, S, and O donor impurities in GaN, AlN, and BN through first-principles calculations. The small bond lengths in III-V nitrides are found to inhibit large lattice relaxations around impurities and, with a few exceptions, this leads to a significant stabilization of effective-mass states over deep centers despite the large band gaps of these materials. In particular, we find that Ge and S impurities do not have stable or even metastable DX centers in GaN, AlN, or BN, independent of crystal structure. Similarly, the effective-mass states of Be, Mg, and C acceptor impurities in GaN are found to be more stable than the corresponding deep AX centers. We propose that persistent photoconductivity in Mg-doped GaN arises from the bistability of a N-site vacancy that is accompanied by a charge-state change from +3e to +1e.

Optical and magnetic resonance characterization of undoped and doped wurtzite GaN films deposited on sapphire substrates

We have used a combination of defect-sensitive techniques to study native defects and impurities in GaN films deposited on the basal plane of sapphire. The optical and electronic properties of undoped samples are dependent on the degree of compensation between shallow donor and acceptors levels. The paramagnetic studies of undoped films reveal resonances from effective-mass and deep-donor states. Photoluminescence studies of films doped with Mg-acceptors indicate a possible dependence of the localization of the Mg-related center with the increasing concentration of activated acceptors. The optically detected magnetic resonance measurements of Mg-doped samples show that the Mg-acceptor is perturbed from its effective-mass state.

Metastable states of Si donors in AlxGa1−xAs

Donor related states in Si-doped AlGaAs with Al compositions ranging from 0.30 to 0.59 were investigated by capacitance measurements. In addition to the stable Si-DX state, two metastable states of the silicon donor were observed. Of the two metastable states, the shallower one is attributed to the X-conduction band related effective mass state arising from substitutional silicon on the group III site. The deeper one is proposed to be related to a donor configuration different from both the substitutional configuration and that of the DX state.

Manifestation of effective-mass states of secondary minima in the persistent photoconductivity related to the DX centre in

The correlation of persistent photoconductivity and photo-Hall studies of Sn- and Si-doped layers of compositions with previous photo-EPR results obtained on the same samples shows the importance of hydrogenic effective-mass states derived from secondary conduction band minima in highly () doped samples. The Fermi level pinning after low-temperature photoexcitation in direct-gap material by the resonant level in the case of Sn-doped samples, directly demonstrated by its EPR detection, explains the difference in the spin and carrier concentrations observed as well as the different kinetics for the relaxation into the initial state. The consideration of this state allows further an interpretation of the nonmonotonic changes of the free-electron mobility with the photon flux observed in highly doped () samples.