Shallow Donor States Research Articles

Shallow donor centers in gallium nitrides

Production processes of electrically active defects in nominally undoped n-GaN irradiated with fast electrons are investigated. It has been demonstrated that the silicon impurity atoms with shallow donor states at EC – 20 meV interact with native defects neither during irradiation nor annealing at elevated temperatures. Two major kinds of defects produced by irradiation in n-GaN are revealed: donor centers at EC – 70 meV and deep acceptors centers. These defects were found to be unstable at T > 200 °C. They can be attributed to the vacancies on the nitrogen and gallium sublattices, respectively.

Shallow donor state of hydrogen in indium nitride

The nature of the electron states associated with hydrogen in InN has been inferred by studying the behavior of positive muons, which mimic protons when implanted into semiconductors. The muons capture electrons below 60 K, forming paramagnetic centers with a binding energy of about 12 meV. Together with an exceedingly small muon-electron hyperfine constant indicative of a highly delocalized electron wave function, the results confirm the recently predicted shallow-donor properties of hydrogen in InN.

Shallow donor versus deep acceptor state in II–VI semiconductor compounds

Information on the properties of the possible muonium states in II–VI semiconductor compounds is obtained in this study. In these materials, muonium may either form a shallow donor state, which is characterized by a small hyperfine interaction and a level-energy close to the conduction band, or an acceptor state, which corresponds to muonium at an interstitial site with a tightly bound electron and a hyperfine interaction close to that of free muonium. We show here that in CdS, CdSe and ZnO muonium preferentially forms a donor state whereas the acceptor state is preferred in ZnS and ZnSe. In CdTe both states are observed, indicating that the level energies are similar.

Low-temperature scanning tunneling spectroscopy ofn-type GaAs(110) surfaces

Scanning tunneling spectroscopy is used to study n-type GaAs(110) surfaces at temperatures near 10 K. Spectral lines associated with surface accumulation layer states are observed. Near dopant atoms, three additional lines are seen in the spectra. These lines are identified with the shallow donor states located near the conduction-band minimum (seen at different voltages for tunneling into and out of the states) and with a donorlike state derived from the accumulation layer.

Magnetic-field and laser effects on the electronic and donor states in semiconducting quantum dots

Light shifts induced in the electronic and shallow on-center donor states in spherical semiconductor quantum dots, including magnetic field effects, are theoretically investigated. The interaction of light with the spherical GaAs–(Ga, Al)As quantum dot is treated within a dressed-band approach in which the Kane band structure scheme is used to model the GaAs bulk semiconductor whereas the dressing by the laser field is treated through the renormalization of the GaAs energy gap and conduction/valence effective masses. This nonperturbative approach is valid far from resonances and has been successfully adopted for other confined semiconductor heterostructures. The discrete nature of the electronic and impurity states, characteristic of quantum dot systems, and the possibility of adding extra confining effects by laser and applied magnetic fields opens up a promising route of applicability and/or manipulation of quantum-dot states in recent quantum-computer proposals.

Electroceramics—the role of interfaces

The opportunities and potential problems in the development of junction-based devices composed of semiconducting oxides are examined. The relevant differences between the material properties of oxide semiconductors and conventional semiconductors are discussed. This is followed by a review of the work done on various kinds of oxide junctions, i.e., metal–semiconductor (Schottky), double Schottky barriers, p– n homo- and heterojunctions and n– n heterojunctions. The use of nanosized structures is expected to play an important role in future junction-based devices, and special attention is given to the situation where the space charge region exceeds the dimensions of the structure. Three important requirements for further developments in this emerging field are identified: (i) selective activation of shallow donor or acceptor states in normally p- or n-type conducting oxides, respectively, (ii) growth of single crystal p– n homo- and heterojunctions, and (iii) improved control of interface states.

Nuclear spin-lattice relaxation by optically bistable defects inCdF2:In

Nuclear spin-lattice relaxation rates have been measured for ${}^{113}\mathrm{Cd}$ and ${}^{19}\mathrm{F}$ in ${\mathrm{CdF}}_{2}$ containing optically bistable In impurities. In the present work, relaxation rates were measured from 25 K to 500 K for ${}^{113}\mathrm{Cd}$ and from 200 K to 400 K for ${}^{19}\mathrm{F}.$ The rates for both species are strongly enhanced in semiconducting ${\mathrm{CdF}}_{2}:\mathrm{In}$ compared with undoped materials. Except below 100 K, nuclear relaxation is attributed to relaxation by thermally excited conduction electrons for ${}^{113}\mathrm{Cd}$ and thermally excited hydrogenic shallow donor states ${\mathrm{for}}^{19}$F. From the temperature dependences of the relaxation rates, activation energies of $185\ifmmode\pm\else\textpm\fi{}10 \mathrm{meV}$ and $70\ifmmode\pm\else\textpm\fi{}10 \mathrm{meV}$ are inferred for the concentrations of conduction electrons and hydrogenic states, respectively. These results are consistent with the $\mathrm{DX}$ model of the bistable impurities and with binding energies determined previously. Following in situ white light illumination below 100 K, persistent photoenhancement of the ${}^{113}\mathrm{Cd}$ relaxation rates was observed and attributed to metastable photogenerated shallow donor states.

Shallow donor state due to nitrogen-hydrogen complex in diamond.

Based on an ab initio calculation, we propose a possible shallowing of a nitrogen (N) donor in diamond, in contrast to the traditional thinking that it is very deep. A complex defect of N and hydrogen (H), N-H-N, should be much shallower than an isolated N donor. A qualitative scenario for formation of the N-H-N defects is presented. The existence of this complex is strongly suggested by a recent discovery of a new muonium center in N-rich diamond.

Charged native point defects in GaAs and other III–V compounds

The solubility of both native point defects and dopants in semiconductors is dependent on growth temperature, on crystal stoichiometry and on Fermi level position. At high temperatures III–V compounds contain high concentrations of predominantly charged native point defects. Published titration and density/lattice parameter measurements on GaAs show conclusively that melt-grown crystals contain ∼10 19 cm −3 As-vacancies and As-interstitials at the melting point. The arsenic vacancies form a relatively shallow (∼ E c−0.1 eV) donor state. The concentration of these ionised vacancies at the melting point of GaAs, and under the conditions used for LPE growth from Ga-solution, exceeds the intrinsic electron–hole concentration. Hence, these ionised vacancies control the electro-neutrality of the crystal (and hence the position of the Fermi level) during growth under these conditions. The As vacancy concentration obtained from titration measurements, when inserted into a comprehensive equilibrium thermodynamic description of the system, accurately predicts the observed range of linear dopant solubility obtained for both Groups IV and VI donor dopants. The currently accepted hypothesis that this linear range is determined by a non-equilibrium incorporation process controlled by a Schottky barrier at the crystal/melt interface is therefore unnecessary. It is shown that donor and acceptor dopant incorporation, EL2 formation and annealing behaviour in GaAs can all be fully explained by the thermodynamic model. The differing doping behaviour exhibited by the various other zinc-blende III–V compounds is shown to be related to the differing relative numbers of native point defects on the two sub-lattices.

A Comparative Study of the Near Band Edge Luminescence of Epitaxial ZnSe:N Grown by MBE and MOVPE

A comparative study of the photoluminescence, reflection, and photoluminescence excitation spectra of MBE- and MOVPE-grown nitrogen-doped ZnSe samples has been carried out. MOVPE-grown samples demonstrate impurity recombination via shallow donor states while MBE-grown samples show mainly recombination via deep donor states, with significantly lower donor concentrations. Thus, the shallow donors are the main cause of compensation in MOVPE-grown samples. In MBE-grown samples, the excitation transfer to impurity states takes place mainly via free excitonic states and acceptor-bound excitons, while in MOVPE-grown ZnSe : N the excitation transfer occurs mainly via donor-bound excitons. A simultaneous presence of a broad 2.6 eV band and a narrower 2.7 eV band was observed in time-integrated spectra of MOVPE-grown ZnSe : N.

X-ray induced persistent photoconductivity in Si-doped Al0.35Ga0.65As

We demonstrate that x-ray irradiation can be used to induce an insulator–metal transition in Si-doped Al0.35Ga0.65As, a semiconductor with DX centers. The excitation mechanism of the DX centers into their shallow donor state was revealed by studying the photoconductance along with fluorescence. The photoconductance as a function of incident x-ray energy exhibits an edge both at the Ga and As K edge, implying that core–hole excitation of Ga and As are efficient primary steps for the excitation of DX centers. A high quantum yield (≫1) suggests that the excitation is indirect and nonlocal, due to secondary electrons, holes, and fluorescence photons.

Probing the shallow-donor muonium wave function in ZnO and CdS via transferred hyperfine interactions

The assignment of muon spin rotation spectra to muonium counterparts of hydrogen shallow–donor states is reviewed in four II–VI widegap semiconductors, CdS, CdSe, CdTe and ZnO. The existence of extended electronic orbitals is argued from the muon–electron hyperfine parameters and supported by the new muon spin repolarization data for CdS and ZnO, characterizing the superhyperfine parameters on the sparse Cd and Zn dipolar nuclei. The possibility of a more tightly bound electron occupying a compact orbital is reasonably excluded in these materials, contrasting with the muonium state in HgO.

Light-induced shifts in the electronic and shallow-donor states in GaAs-(Ga,Al)As quantum dots

We treat the interaction of light with a spherical GaAs-(Ga,Al)As quantum dot within a dressed-band approach. The Kane band-structure scheme is used to model the GaAs bulk semiconductor and the interaction with the laser field is treated through the renormalization of the semiconductor energy gap and conduction/valence effective masses. This approach, valid far from resonances, is used to investigate the light shifts induced in the electronic and shallow on-center donor states in semiconductor quantum dots, which are shown to be quite considerable. This model calculation may be extended to include magnetic-field effects, and it is suggested that the strong localization of the electronic and impurity states due to the quantum dot and enhanced by laser confinement may prove useful for manipulation of electronic and donor states in some proposed solid-state-based quantum computers.

Effect of PECVD of SiO 2 passivation layers on GaN and InGaP

Thin (200 Å) layers of SiO 2 were deposited by plasma enhanced chemical vapor deposition onto GaN and InGaP patterned with transmission line measurement contact pads. The sheet resistance of n-GaN and n- and p-InGaP was measured as a function of N 2O/SiH 4 ratio, rf chuck power, pressure and substrate temperature during the SiO 2 deposition. The sheet resistance ratio before and after the deposition varied from 0.7 to 1.2, reflecting a competition between mechanisms that decrease doping (hydrogen passivation, ion-induced deep traps) and those that increase it (creation of shallow donor states in n-type material through preferential ion of the group V elements, gettering of hydrogen in p-type material). Under most conditions, SiO 2 deposition creates minimal changes to the electrical properties of GaN and InGaP.

Shallowversus deep hydrogen states in ZnO and HgO

The muonium states mimicking interstitial hydrogen in ZnO and HgOare compared. Whereas in ZnO a theoretically predicted shallowdonor state is confirmed, in HgO we find a considerably deeperstate. The respective ionization temperatures are around 40 K and150 K and the donor ionization energies are 19±1 and136±3 meV, deduced from the temperature dependence of theµSR (muon spin-rotation) signal amplitudes. The µSR spectraprovide a comprehensive characterization of the undissociatedparamagnetic states: the hyperfine parameters, which measure theelectron spin density on and near the muon, differ by a factor of~30. These define a hydrogenic radius of 1.1 nm in ZnO butindicate a much more compact electronic wavefunction in HgO, moreakin to those of Mu* and the AA9 centre in Si. These data should largelycarry over to hydrogen as a guide to its electrical activity inthese materials.

The fractional-dimensional space approach to MBE-grown quantum-sized semiconductor low-dimensional systems

Shallow impurities and excitons in MBE-grown quantum-sized semiconductor low-dimensional systems are studied within the fractional-dimensional space approach. We present calculations for shallow-donor states in GaAs–(Ga,Al)As quantum wells and superlattices and for excitons in GaAs–(Ga,Al)As quantum wells and symmetric-coupled double quantum wells. Effects of growth-direction applied magnetic fields are also considered. Results are shown to be in good agreement with previous variational calculations and available experimental measurements.

Mechanothermal Effects on the Defect Structure in ZnO Powders Subjected to Hydrostatic Pressure

Using electron paramagnetic resonance (EPR) spectroscopy method the mechanism of defect formation has been studied in ZnO powders, put to a bottoming (a hydrostatic pressing) up to P=8 GPa. The initiation of 6 paramagnetic centers by a pressing has been shown (V Zn - :Zn i 0 -I, V Zn - -II, (V Zn - ) 2 - -III, V o + -IV, SDS (shallow donor states) - V and SDS (SDS associated with impurities) - VI). The dependence of the signal intensity (I) on pressure P has been studied. The maxima on curves I = f(P) for EPR signals from centres I and II are attributed to the development of mechanothermal processes promoting an annealing of the created defects during pressing. Comparison with the data on annealing of paramagnetic centers has allowed to construct the dependence of an average sample temperature as a function of hydrostatic pressure, and to propose the model diagram of the development of mechanothermal processes under pressure.

Central-cell corrections and shallow donor states in strong magnetic fields

Ionization energies and the central-cell corrections have been calculated for a few shallow donors in Si, GaP, and GaAs. We have assumed a short range potential with two parameters for the strength and the range for each donor, representing the central-cell effects. These parameters are fixed using the experimentally available ionization energies for each donor in a semiconductor. In the presence of a magnetic field the donor ionization energies are estimated using a variational procedure. Our results show that the ionization energies and the central-cell corrections increase with magnetic field. Our results are compared for GaAs with the recent work by Heron et al. [R. J. Heron, R. A. Lewis, P. E. Simmonds, R. P. Starret, A. V. Skougarevsky, R. G. Clark, and C. R. Stanley, J. Appl. Phys. 85, 893 (1999)].

Physical origin of negative persistent photoconductivity in a GaAs–AlAs/GaAs single heterojunction

When a Si-doped GaAs–AlAs/GaAs heterostructure is illuminated with 1.98 eV light at 4.2 K, a reduction in the two-dimensional (2D) electron density (n2D) is observed. This reduction is followed at times by a small increase in n2D even after the illumination has been switched off. This change is observed on the time scale of minutes and can be explained based on the band bending that results after a reduction in n2D. The negative persistent photoconductivity (NPPC) effect characterized by a persistent reduction in n2D, a postillumination change in n2D, and a long persistence time for T⩽40 K has been investigated. I have used Shubnikov–de Haas oscillations and time-resolved, as well as temperature-dependent, Hall-effect measurements to investigate the origin of this phenomenon. The illumination generates electron–hole (e–h) pairs in the superlattice, where the electrons are trapped into the shallow donor state (SDS) of Si and the holes drift to the two-dimensional channel to recombine with the 2D electrons. All the trapped electrons can be recovered by heating the sample to 60 K. The temperature dependence of the NPPC effect is determined only by the binding energy of the SDS of Si, which is found to be about 5 meV. The e–h recombination in the 2D channel is caused by negatively charged defects, which temporarily bind the holes. This fact is manifested also in the optical quenching of this effect by photons with 1.41 eV or larger energy. The saturation values of n2D(n2Dsat) obtained for 0.8, 1.41, or 1.98 eV illumination at 4.2 K have been investigated and the results confirm the presence of these fixed negative charges (FNCs) near the 2D channel. The change in n2Dsat for 0.8 eV illumination, caused by 1.98 eV illumination, also confirms the presence of FNCs.

Experimental confirmation of the predicted shallow donor hydrogen state in zinc oxide.

We confirm the recent prediction that interstitial protium may act as a shallow donor in zinc oxide, by direct spectroscopic observation of its muonium counterpart. On implantation into ZnO, positive muons--chemically analogous to protons in this context--form paramagnetic centers below about 40 K. The muon-electron contact hyperfine interaction, as well as the temperature and activation energy for ionization, imply a shallow level. Similar results for the cadmium chalcogenides suggest that such shallow donor states are generic to the II-VI compounds. The donor level depths should serve as a guide for the electrical activity of interstitial hydrogen.