Shallow Acceptor States Research Articles

Optical investigation of shallow acceptor states in GaN grown by hydride vapor-phase epitaxy

The evolution of low-temperature photoluminescence (PL) spectra with the thickness of the layer (3–400 μm) is investigated on high-quality GaN grown by hydride vapor-phase epitaxy. With increasing layer thickness, three acceptor bound exciton peaks are found to reduce in intensity, although the impurity concentrations, measured by secondary ion mass spectrometry, do not depend on the sample thickness. The observed acceptor transitions are attributed to intrinsic defects, originating from the substrate/layer interface and decreasing in density with the thickness of the layer. The optical properties, studied by reflectance, temperature and excitation power dependent PL, are compared to those of homoepitaxial GaN films grown by metalorganic chemical vapor deposition.

Resonant states of shallow acceptors in uniaxially deformed germanium

The states of shallow acceptors in uniaxially deformed germanium are studied theoretically. A non-variational numerical computational method is developed for determining the energy and wave functions of localized states of holes in the acceptor field as well as the states of the continuous spectrum (including resonant impurity states). The dependence of the energy of the lower resonant state on strain is studied. It is found that this state is formed from the excited 4Γ8+ state with a binding energy of 1.3 meV (in the absence of deformation) and not from the ground state. The results presented in this work may be useful in the study of the conditions for the generation of far IR radiation in deformed p-Ge, which involves optical transitions between resonant and localized acceptor states.

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.

Time- and Temperature-Resolved Photoluminescence of GaN:Mg Epitaxial Layers Grown by MOVPE

Time-integrated and time-resolved photoluminescence (PL) spectra as well as the luminescence transients of moderately doped GaN:Mg samples grown by MOVPE were studied between 80 K and 380 K at pulse excitation by a nitrogen laser beam in order to clarify the mechanism of the large blue shift of the 2.8 eV PL band above room temperature. Based on the performed study, the new band at 3.05 eV dominating in PL spectra above room temperature is attributed to the donor-to-valence band recombination. The corresponding donor ionization energy is about 290 meV. The blue shift of the spectra is therefore explained as a result of ionization of shallow acceptor states involved together with deep donors in donor–acceptor recombination forming the 2.8 eV band below room temperature.

Evidence for shallow acceptors in GaN

Two low-temperature photoluminescence lines in GaN, in the region of energies commonly interpreted as longitudinal optical-phonon replicas of free excitons, donor-bound excitons, or acceptor-bound excitons, are reinterpreted as acceptor-bound excitons (A0X’s) collapsing to n=2 and n=3 excited states, respectively, of the acceptors involved. Application of this model to two sets of A0X-related lines in hydride-vapor-phase-grown GaN gives acceptor energies of 85±1, and 115±1 meV, respectively. The existence of such shallow acceptor states, if confirmed, is of great technological importance.

Formation of shallow acceptor states in the surface region of thin film diamond

Considerable interest exists in fabrication of electronic devices from thin film polycrystalline diamond. To date, doping this material to achieve good free carrier concentrations and mobilities at room temperature has proved difficult. In this letter we report low temperature Hall effect measurements made on diamond films subjected to a hydrogenation process, such that the near surface region becomes p type without the addition of conventional dopant atoms. High carrier concentrations and mobilities can be achieved. The change in carrier concentration within the temperature range 10–300 K does not change as expected for most films, actually increasing as the temperature falls. This effect could be related to the confinement of carriers at the surface caused by the dipole provoked by adsorbed hydrogen on the diamond. However, polished films display more conventional behavior in that the carrier concentration falls with falling temperature.

Photo EPR Study of Trapping and Recombination Processes in Semi-Insulating 4H-SiC Crystals as Function of Temperature

ABSTRACTPhoto-Electron Paramagnetic Resonance (photo-EPR) measurements of semi-insulating (s.-i.) 4H SiC have been made at 37 GHz including photo excitation and photo quenching techniques in the temperature interval from 77 K to 50 K. At T = 77 K in the dark the EPR spectrum consists of a low intensity line due to boron on the cubic lattice site and a single line with isotropic g∥ = g⊥ = 2.0025 due to a carbon-related surface defect. During illumination with ultraviolet light the EPR lines of hexagonal boron and cubic nitrogen appear in the EPR spectrum and persist after the illumination is removed. Subsequent illumination of the sample with sub-band gap, visible, light resulted in the quenching of the EPR lines from nitrogen and appearance of the IP1EPR line with g∥ = 2.0048, g⊥ = 2.0030 caused by direct transfer of electrons from nitrogen donor to the P1 center. The lifetime of the photo-generated carriers trapped by the P1 centers is found to be more than 15- 20 hours after the photo-excitation was turned off. The deep donor P1 local center is suggested to be the as yet unidentified deep level located at EC – 1.1 eV which pins the Fermi level in this sample at this energy in the dark. As the temperature is lowered from 77K and the quasi Fermi level positions reach shallow donor and acceptor states, an additional EPR line, ID, with g∥ = 2.0063, g⊥ = 2.0006, appears at 50 K in the excitation EPR spectrum and is attributed to the antisite defect Si−c with an energy level shallower than nitrogen. At the same time the ratio of the photo-excited EPR line intensities responsible for boron on the cubic and hexagonal sites, IkB:IhB, returns to the value observed at 77 K and becomes equal to 0.4 at 50 K, showing that the concentration of boron in the hexagonal site is higher than on the cubic site.

Spin–orbit coupling, mass anisotropy, time-reversal symmetry, and spontaneous symmetry breaking in the spectroscopy of shallow centers in elemental semiconductors

The electronic states of shallow donors and acceptors, explored under high resolution with Fourier Transform infrared and Raman spectroscopy, underscore the intimate connection between the localized states of these impurities and the electronic band structure of the host as contemplated in the effective mass theory. Spin–orbit interaction, mass anisotropy of the conduction band minimum (donors) and of the valence band maximum (acceptors), the site symmetry of the impurity and the possible spontaneous departure from it when the electronic ground state is degenerate are the features highlighted in our discussion of the excitation spectra of donors in silicon and boron acceptors in diamond. As in atomic spectroscopy, the removal of degeneracy under external perturbation and the polarization and field dependence of the spectral components provide a wealth of insightful characterizations. We demonstrate them with the Zeeman effect of the Raman transition between the spin–orbit split counterparts of the 1s ground state of boron acceptors in diamond; g-factors and the extreme mass anisotropy of the hole emerge from these studies. They also provide beautiful examples of time-reversal symmetry, conservation laws and selection rules connecting the Stokes to the anti-Stokes Raman scattering.

Mössbauer spectroscopy at ISOLDE

Applications of radioactive ion beams produced at the ISOLDE facility for Mossbauer studies of probe atoms in solids are presented. Examples are given for a site-selective incorporation on different substitutional sites in compound semiconductors by ion implantation and thermal annealing of the radiation damage resulting from the implantation. The interactions of the probe atoms with lattice defects created in the implantation process have been studied to elucidate likely causes for the site-selective implantation mechanism. The technique has enabled to determine the electronic densities at electrically active substitutional probe atoms, having shallow donor or acceptor states as well as states deeper in the band gap. The results are in good agreement with theoretical results from local density calculations. Methodological aspects of the Mossbauer emission techniques employed at ISOLDE are compared to alternative accelerator based techniques and the consequences of the application of different precursor isotopes to the 57Fe Mossbauer isotope are treated in detail for 57Fe in silicon. Finally, results obtained for the magnetic hyperfine interactions of 5 sp impurities associated with vacancies in ferromagnetic metals are discussed.

Theoretical investigation of shallow acceptors confined in Si/Si1−xGex quantum well structures

Energy levels of the shallow acceptor states have been calculated for center-doped Si/Si1−xGex/Si quantum wells. The impurity states were calculated using an effective-mass theory that accounts for valence-band mixing as well as the mismatch of band parameters and dielectric constants between well and barrier materials. Acceptor binding energies and splitting between the acceptor 1S3/2(Γ7) and 1S3/2(Γ6) ground states were studied at various Ge concentrations and well widths. The results are discussed in comparison with the recent conclusion from the lateral transport measurements in boron-doped Si/SiGe quantum wells.

Carrier Recombination at Screw Dislocations in n-Type AlGaN Layers

We analyse the recombination and scattering at dislocations in Si doped AlGaN layers by transmission electron microscopy (TEM), electron beam induced current (EBIC) and cathodoluminescence. We show that c-type screw dislocations are nonradiative recombination centres. From temperature dependent measurements of the EBIC contrast we find a shallow acceptor state 20 meV above the valence band. The level can be understood iri terms of one-dimensional dislocation bands split off by the dislocation deformation field. In addition piezoelectric potentials play a role.

Analysis of deep levels in a phenylenevinylene polymer by transient capacitance methods

Transient capacitance methods were applied to the depletion region of an abrupt asymmetric n+−p junction of silicon and unintentionally doped poly[2-methoxy, 5 ethyl (2′ hexyloxy) paraphenylenevinylene] (MEH-PPV). Studies in the temperature range 100–300 K show the presence of a majority-carrier trap at 1.0 eV and two minority traps at 0.7 and 1.3 eV, respectively. There is an indication for more levels for which the activation energy could not be determined. Furthermore, admittance data reveal a bulk activation energy for conduction of 0.12 eV, suggesting the presence of an additional shallow acceptor state.

Electronic Structures of Shallow Acceptors Confined in Si/SiGe Quantum Well Structures

AbstractEnergy levels of the shallow acceptor states have been calculated for center-doped Si/Si1−xGex/Si quantum wells (QWs). The impurity states were calculated using an effective-mass theory that accounts for valence-band mixing as well as the mismatch of band parameters and dielectric constants between well and barrier materials. Acceptor binding energies and splitting between the acceptor 1S3/2(Γ7) and 1S3/2(Γ6) ground states were studied at various Ge concentrations and well widths. The results are discussed in comparison with the recent experimental data from the lateral transport measurements in boron-doped Si/SiGe quantum wells.

Defect-related luminescence of Mg-doped n-GaN grown by hydride vapour-phase epitaxy

We study photoluminescence (PL) and photoluminescence excitation (PLE) spectra of Mg-doped n-GaN grown by hydride vapour-phase epitaxy. Defect-related red and blue emission bands around 1.85 eV and 2.90 eV, respectively, are observed in addition to the yellow emission band. The red and the blue emission bands are attributed to the recombinations involving the same Mg-related defects according to the result of the thermal annealing experiment. The PLE spectra of the red, the yellow, and the blue emission bands show that all of these emission bands can be interpreted in terms of a configuration coordinate (CC) diagram. The blue emission at 2.90 eV is attributed to the transition from the conduction band to the Mg-related deep acceptors. The CC diagram shows that the yellow luminescence is due to the transition from a deep donor state to a shallow acceptor state. A possible origin of the deep donor level is also discussed.

Shallow Acceptor States in SiGe Quantum Wells

Shallow Acceptor States in SiGe Quantum Wells

Shallow Acceptor States in SiGe Quantum Wells

The temperature dependences of conductivity and hole mobility in boron doped SiGe quantum well structures were studied. The conductivity at low temperatures is shown to be due to hopping over neutral B centers while at higher temperatures, it is due to two-stage excitation including thermal activation of holes from the ground to strain-split B states following by hole tunneling into the valence band.

Raman Scattering at Shallow Acceptors in InP

Electronic Raman scattering was used to investigate excited states of shallow acceptors in Zn- and in Cd-doped p-type InP with acceptor concentrations in the range between 1016 cm−3 and 1018 cm−3. Well resolved transition lines were obtained for acceptor concentrations of about 1017 cm−3 (Zn-doped) and 5 × 1017 cm−3 (Cd-doped), at higher concentrations the levels merge into broad bands. A linear correlation between the normalized intensity of the electronic spectrum and the acceptor concentration was found. The measured acceptor energy levels were used to prove which sets of published Luttinger parameters γ1, γ2 and γ3 are compatible with our results.

Theoretical Calculations of Shallow Acceptor States in Strained Quantum Well Structures

Acceptors confined in strained InxGa1—xAs/Al0.3Ga0.7As quantum well (QW) structures are theoretically investigated. The impurity states are calculated using a four-band effective-mass theory, in which the valence-band mixing as well as the mismatch of the band parameters and the dielectric constants between well and barrier materials have been taken into account. The binding energies of acceptors and the oscillator strengths of the active infrared electronic transitions between the acceptor ground states and the different excited p-like states are calculated with different indium fractions x and different well widths. The results show clearly, that a built-in strain strongly influences the transition energies and the relative oscillator strengths. Due to the splitting of the acceptor ground states, the absorption spectra of the acceptors also show a strong temperature dependence. The calculated spectra of the acceptor electronic transitions are ready to be compared with the infrared absorption measurements.

Theoretical study of shallow acceptor states under the influence of both a confinement potential and a deformation potential

Energy levels of the ground and the excited shallow acceptor states have been calculated for center-doped strain-free ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ quantum wells (QW's) in the presence of an external pressure and in strained ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{y}{\mathrm{Ga}}_{1\ensuremath{-}y}\mathrm{As}$ QW's. The impurity states are calculated using a four-band effective-mass theory, in which the valence-band mixing as well as the mismatch of the band parameters and the dielectric constants between well and barrier materials have been taken into account. The acceptor binding energies and the internal electronic transitions are calculated. The results show that an applied external pressure in ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ QW's or built-in strain in ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{y}{\mathrm{Ga}}_{1\ensuremath{-}y}\mathrm{As}$ QW's strongly influences the acceptor states. The oscillator strengths between the acceptor ground states and the different excited $p$-like states are also calculated for the ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ system with three different indium fractions $x.$ The results show clearly that a built-in strain strongly influences the transition energies and the relative oscillator strengths.

Quantum efficiency and admittance spectroscopy on Cu(In,Ga)Se 2 solar cells

We investigate the electronic transport properties of Cu(In,Ga)Se 2 solar cells by means of quantum efficiency and temperature dependent admittance spectroscopy. A simple evaluation scheme of quantum efficiency data is introduced which accounts for recombinatoric losses in the Us buffer layer and in the Cu(In,Ga)Se 2 absorber. By admittance spectroscopy, we find that the controlled incorporation of Na into the absorber material leads to a shallow acceptor state at about 75 meV above the valence band.