Analysis of the vacancy-interstitial model ofDX centers

I describe the vacancy-interstitial model ofDX centers, in which the substitutional donor atoms relax toward interstitial sites when they capture electrons. This appears to be the only known model ofDX centers which can account for the structure observed by Mooneyet al. in DLTS spectra of dilute Al _x Ga_1-x As alloys. By comparing its predictions with the DLTS and Hall data from Al _x Ga_1-x As alloys of low Al content one can establish values for many of the relevant parameters, such as the enthalpies and entropies (degeneracies) associated with sites surrounded by differing numbers of Al atoms. The data also show that the distribution of donor configurations among the four available neighboring interstitial sites does not attain thermal equilibrium among states of differing energies before being released, although transitions (through tunneling) among states of equal energy may be possible. The model is seen to be consistent with those treated theoretically by Chadi and Chang forDX and by Dabrowski and Scheffler for EL2 centers.     

Confirmation of large lattice relaxation of theDX center by extended photo-ionization cross-section measurements

Although the large lattice relaxation model (LLR) for electron capture at the donor relatedDX center in Al_xGa_1-xAs has seen wide acceptance over the last 12 years, there have been some recent proposals which have attempted to explain the experimental data with models that only require small lattice relaxation (SLR). One key piece of evidence supporting LLR is the large observed difference (in the case of Si-doped Al_xGa_1-xAs) between the optical (∼1.4 eV) and thermal (∼0.2 eV) ionization energies. The SLR model proposed that the lowest energy optical ionization was a very weak process, and that the optical transition which had been observed previously is a transition to a higher band. These arguments were supported by photoconductivity data showing a finite photo-ionization rate at energies as low as 200 meV. To resolve this question we have measured the photo-ionization cross section over 7 to 8 orders of magnitude using a tunable infrared laser as a source. A consistent optical ionization energy of about 1.4 eV was observed for 4 samples of differing alloy compositions and doping levels.In no case was there any detectable photo- ionization below 0.8 eV. A detailed discussion of these experiments examines the difficulty in obtaining such a large dynamic range optical spectrum. Of particular relevance are the issues of ionization detection, and the brightness of purity of the optical source. A thorough review of these issues and their impact on previous studies of theDX center is presented.     

Electron paramagnetic resonance and optically-detected magnetic resonance of donors in AlxGa1-xAs

The Electron Paramagnetic Resonance (EPR) and Optically-Detected Magnetic Resonance (ODMR) work on Si-donors in Al _{x Ga1- x} As is reviewed in the context of the shallow-deep bistability (DX) problem. Three donor states are important. Little work has been published on donors tied to theT-minimum. However, there are many results forX-donors. In AlAs/GaAs heterostructures, well-resolved spectra reveal a donor state comprised of independentX _x andX _y valleys with theXz valley unpopulated due to the hetero-epitaxial strain. As Al mole fraction decreases, intervalley coupling is evident from the line positions and linewidths. The published attempts to observe and identify the deep (relaxed) state are inconclusive. Some suggestions for future work are presented.     

Atomic structure ofDX centers: Theory

A structural model forDX centers in Al _x Ga_1-x As alloys which explains their unusual properties in terms of two distinct bonding configurations for donor impurities is proposed. The results of ourab initio self-consistent pseudopotential calculations showDX to be a “negative-U” defect center. It results from a large lattice relaxation which is different for a Group IV than a Group VI donor. The proposed structural model provides a satisfactory explanation of the properties ofDX centers.     

Magnetic properties of donors in GaAsP

The magnetic properties of hydrogenic Te donors and bistable,DX-like S donors in GaAS _x P_1-x have been investigated. Te shows an EPR signal after cooling down, both in the dark. The EPR signal from S appears only after illumination. S does not show an EPR signal after cooling down in the dark, but after illumination when the hydrogenic state of this donor is populated. Static magnetic susceptibility measurements show however that the ground state of S in GaAs _x P_1-x is paramagnetic even though it does not produce an EPR signal. This is in agreement with a one-electron ground state of this bistable donor.     

DX centers in AlxGa1-xAs bulk alloy,AlAs/GaAs ordered,and disordered superlattices

Properties of theDX centers in Al_0.5Ga_0.5As bulk alloy (b-AL), (AlAs)₂ (GaSa)₂ ordered superlattice (o-SL) and (AlAs) _m (GaAs) _n disordered superlattice (d-SL) (m = 1, 2, 3,n = 1, 2, 3) with the same macroscopic composition were measured and compared. By deconvolution of deep level transient spectroscopy (DLTS) spectrum due to theDX center, we have found a decrease in the number of separate peaks in DLTS spectrum in an intentionally atomic ordered arrangement. Visiting Scholar of the Japan Society for the Promotion of Science. On leave from Department of Electrical Engineering, San Jose State University, San Jose, California 95192-0084, USA.     

Impurity centers of tin in glassy arsenic chalcogenides

¹¹⁹Sn atoms produced by radioactive decay of ¹¹⁹Sb impurity atoms in the structure of As _x S_{1 − x} and As _x Se_{1 − x} glasses are stabilized in the form of Sn²⁺ and Sn⁴⁺ ions at arsenic sites and correspond to ionized states of the amphoteric two-electron center with negative correlation energy (Sn²⁺ is an ionized acceptor, and Sn⁴⁺ is an ionized donor), whereas the neutral state of the Sn³⁺ center is unstable. The fraction of Sn⁴⁺ states increases with chalcogen content in glass. ¹¹⁹Sn atoms produced by radioactive decay of ^119m Te impurity atoms in the structure of As _x S_{1 − x} and As _x Se_{1 − x} glasses are stabilized at chalcogen sites (they are electrically inactive) and arsenic sites, and the fraction of arsenic atoms decreases with the chalcogen content in glass.     

Effect of local alloy disorder on the emission kinetics of deep donors (DX centers) in AlxGa1-xAs

In this paper we summarize our recent studies of the effects of local alloy disorder on the properties ofDX levels. A single emission rate is observed in GaAs where all Si-donors have identical local environments. In contrast, three discrete emission rates are observed in dilute AlGaAs alloys, suggesting that the group IV donor moves towards the interstitial site, thereby “selecting” three of the twelve surrounding group III atoms. We present evidence for an ordering of theDX levels consistent with Morgan’s model of a deepening potential well for theDX level as Al atoms are subsequently substituted for Ga atoms near the relaxed donor. These conclusions are consistent with earlier calculations of Chadi and Chang.     

Tin impurity centers in glassy germanium chalcogenides

Tin atoms produced by radioactive decay of ^119mm Sn and ¹¹⁹Sn impurity atoms in the structure of Ge _x S_{1 − x} and Ge _x Se_{1 − x} glasses are stabilized in the form of Sn²⁺ and Sn⁴⁺ ions and correspond to ionized states of the amphoteric two-electron center with negative correlation energy (Sn²⁺ is an ionized acceptor, and Sn⁴⁺ is an ionized donor), whereas the neutral state of the Sn³⁺ center appears to be unstable. ¹¹⁹Sn atoms produced by radioactive decay of ^119m Te impurity atoms in the structure of Ge _x S_{1 − x} and Ge _x Se_{1 − x} glasses are stabilized at both chalcogen sites (they are electrically inactive) and germanium sites.     

Electronic states on silicon surface after deposition and annealing of SiO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> films

The spectrum of the photoconductivity induced by the polarization field of charges at surface states and traps in the film bulk has been analyzed to determine the energy band diagram at the c-Si-SiO _x interface and the changes in the electronic states after the film annealing. It is found that the energy bands are bent at the Si-SiO _x interface and the Si surface is enriched in electrons. In equilibrium the photocurrent peak at 1.1 eV is due to the band-to-band transitions in the silicon part of the interface. Annealing shifts the peak to higher energies; this shift increases with an increase in the annealing temperature from 650 to 1000°C. This effect is accompanied by a decrease in the photocurrent at ≤1.1 eV and weakening of the band-edge photoluminescence near the Si surface. The changes revealed are explained by the formation of an oxide layer with Si nanoclusters at the Si-SiO _x interface upon annealing. This process is caused by oxygen diffusion from the SiO _x film, which occurs mainly via defects on the Si wafer surface. The photoconductivity spectrum of the samples charged by short-term application of a negative potential to silicon exhibits electronic transitions in the SiO _x film, both from the matrix electronic states and from the states of the defects and Si nanoclusters in the film.     

Structure of DX-like centers in narrow-band IV–VI semiconductors doped with group-III elements

This paper presents a study of the structure of the IR reflectance spectra in the sub-gap region of lead telluride doped with indium and gallium and the Raman spectra in PbTe(In). In the Raman and reflectance spectra of PbTe(In), features are observed at a frequency of ω ₀⋍120 cm⁻¹, whose amplitude sharply increases at temperatures T below the temperature where delayed photoconductivity appears, T _c⋍25 K. A similar feature at a frequency of ω ₀⋍155 cm⁻¹ is also observed in PbTe(Ga), with the amplitude of the feature sharply increasing for T>T _c⋍80 K. An analysis of the resulting data makes it possible to conclude that, in contrast with classical DX centers in III–V semiconductors, the microscopic structure of the impurity centers in the two-electron (DX-like) ground state does not correspond to an impurity atom shifted from a lattice site, whereas the impurity atom is shifted from a lattice site for the metastable one-electron impurity state. Fiz. Tekh. Poluprovodn. 32, 679–683 (June 1998)     

Two electron transitions of the exciton bound at the Si Donor confined in GaAs/AlxGa1-xAs quantum wells

The exciton bound to the shallow Si-donor confined in a 100A wide GaAs quantum well has been studied in selective photoluminescence (SPL) and photoluminescence excitation (PLE) spectroscopy. The transition from the ground state, ls(Γ₆), to the first excited state, 2s(Γ₆), of the confined Si donor has been observed via two-electron transitions (TETs) of the donor bound exciton observed in SPL for the first time to the best of our knowledge. The interpretation of the TET peaks is confirmed by PLE measurements. Further, from Zeeman measurements, the magnetic field dependence of the donor ls(Γ₆)-2s(Γ₆) transition energy has been determined.     

The E3 Defect in MgxZn1?xO

The authors used highly rectifying PdO _y /Mg _x Zn_1−x O Schottky barriers to determine the apparent capture cross-section and the thermal activation energy of the E3 defect in Mg _x Zn_1−x O thin films by thermal admittance spectroscopy and deep-level transient spectroscopy for x < 0.08. The samples were grown by pulsed-laser deposition. It is observed that both the apparent capture cross-section and the thermal activation energy increase with increasing Mg mole fraction.     

Excitonic effects in the photoconductivity of quantum-well GaxIn1−x As/InP structures

The photoconductivity spectra of quantum-well structures consisting of 50 alternating 7 to 12-mm-thick Ga_xIn_1−x As (x=0.47) layers forming quantum wells and 10 to 15-nm-thick InP barriers have been investigated. Characteristic excitonic peaks 11H, 11L, 13H, 22H, and 22L were observed in the high-quality structures. A strong temperature dependence was found for the 11H exciton, while no such dependence was observed for any of the other excitons. This is explained by the fact that the 11H state of an exciton falls in the range of energies which are forbidden for free carriers, while the remaining states are resonance states and overlap with the continuous spectrum. The thermal activation energy of photoconductivity was found to be 150±30 meV, much greater than the exciton binding energy and close to the depth of the potential well for electrons. This shows that the photosensitivity is due to above-barrier charge-carrier transfer and that the tunneling transfer between the wells is negligible. Fiz. Tekh. Poluprovodn. 31, 848–850 (July 1997)     

Modeling the Metal–Insulator Phase Transition in LixCoO2 for Energy and Information Storage

An electro‐chemomechanical phase‐field model is developed to capture the metal–insulator phase transformation along with the structural and chemical changes that occur in Li_xCoO₂ in the regular operating range of 0.5 < x < 1. Under equilibrium, in the regime of phase coexistence, it is found that transport limitations lead to kinetically arrested states that are not determined by strain‐energy minimization. Further, lithiation profiles are obtained for different discharging rates and the experimentally observed voltage plateau is observed. Finally, a simple model is developed to account for the conductivity changes for a polycrystalline Li_xCoO₂ thin film as it transforms from the metallic phase to the insulating phase and a strategy is outlined for memristor design. The theory can therefore be used for modeling Li_xCoO₂‐electrode batteries as well as low voltage nonvolatile redox transistors for neuromorphic computing architectures.     

Deep levels and a possible d-x-like center in molecular beam epitaxial inxal1−xas

The work reported here was performed in order to establish whether or not complex defects like the D-X center can be present in InAlAs. Dominant deep electron and hole traps in lattice-matched In_0.52Al_0.48As/InP have been identified and characterized. The traps have activation energies ranging from 0.25 to 0.95 eV. The electron traps have very large capture cross-sections,~10⁻¹²-10⁻¹¹ cm², similar to attractive centers. The lattice-matched samples do not show any persistent photoconductivity effects at low temperatures. In strained In_1−xAl_xAs with 0.48 <x ≤ 0.57, an electron trap with a thermal ionization energy of 0.35 eV, a strong dependence of its concentration on donor doping and very small thermal capture cross-section, 10¹⁸ cm², is identified. These samples exhibit persistent photoconductivity. We believe this trap is similar to the D-X center commonly observed in Al_xGa_1−xAs forx ≥ 0.28. He was on leave with the Solid State Electronics Laboratory, Department of Electrical Engineering and Computer Science, The University of Michigan, Ann Arbor, MI 48109. He is with the Institute of Radio Physics and Electronics, The University of Calcutta, Calcutta, India 700 009.     

Valency configuration of transition metal impurities in ZnO

We use the self-interaction corrected local spin-density approximation to investigate the ground state valency configuration of transition metal (TM=Mn, Co) impurities in n- and p-type ZnO. We find that in pure Zn_1−xTM_xO, the localized TM²⁺ configuration is energetically favored over the itinerant d-electron configuration of the local spin density (LSD) picture. Our calculations indicate furthermore that the (+/0) donor level is situated in the ZnO gap. Consequently, for n-type conditions, with the Fermi energy ε_F close to the conduction band minimum, TM remains in the 2+ charge state, while for p-type conditions, with ε_F close to the valence band maximum, the 3+ charge state is energetically preferred. In the latter scenario, modeled here by co-doping with N, the additional delocalized d-electron charge transfers into the entire states at the top of the valence band, and hole carriers will only exist, if the N concentration exceeds the TM impurity concentration.     

Nonmonotonic behavior of magnetophotoconductivity in p-CdHgTe

Magnetophotoconductivity of p-Cd_xHg_1?x Te, measured in the Hall configuration with illumination along the magnetic field (k ∥ B ⊥ E), exhibits nonmonotonic behavior with magnetic field in the temperature range in which the material has a mixed conduction. The effect manifests itself as a peak at B ≠ 0; it is caused by a significant magnetoresistance associated with equilibrium charge carriers. The peak appears if the conductivity of equilibrium electrons is greater than or equal to half the conductivity of equilibrium heavy holes; in Cd _x Hg_1?x Te with x = 0.22, this condition is satisfied at temperatures of 140–150 K.     

Charging d-Orbital Electron of ReS2+x Cocatalyst Enables Splendid Alkaline Photocatalytic H2 Evolution

Rhenium disulfide (ReS₂) holds expansive perspective in photocatalytic water-splitting field, but its H₂-production rate is severely impeded by the strong hydroxyl (OH_ad) adsorption on catalytic Re atoms. Herein, an ingenious strategy about charging d-orbital electrons of ReS_2+x cocatalyst by integrating metallic Au is explicitly clarified to effectively accelerate OH_ad desorption for promoting alkaline photocatalytic H₂-evolution activity. To this end, core-shell Au@ReS_2+x nanostructures as H₂-production cocatalysts are skillfully fabricated onto TiO₂ by a directional assembly pathway. Experimental and theoretical data validate an free-electron transfer from metallic Au core to S-rich ReS_2+x shell, thus essentially charging electrons to the d-orbital of Re atoms to construct active Re^(4-δ)+ sites. The charged d-orbital electron state of Re^(4-δ)+ atoms raises antibonding occupancy of the Re^(4-δ)+ OH_ad bonds, thereby accelerating OH_ad desorption and endowing core-shell Au@ReS_2+x cocatalysts an efficient H₂ production from alkaline water splitting. Moreover, the core-shell Au@ReS_2+x cocatalysts can effectively capture photogenerated electrons from TiO₂ as unveiled by operando Kelvin probe force microscopy. Consequently, the optimized TiO₂/Au@ReS_2+x photocatalyst achieves an exceptional H₂-production rate of 6013.45 µmol h⁻¹ g⁻¹ with releasing visual H₂ bubbles in alkaline media. This research furnishes original insights for charging orbital electrons to optimize the adsorption strength between intermediates and catalytic atoms.     

Multiphonon capture of charge carriers by deep-level centers in a depletion region of a semiconductor

Multiphonon field-assisted thermal capture of thermally equilibrium charge carriers by deep-level centers located in a depletion region of a semiconductor is analyzed. It is shown that, in the case of strong electron-phonon coupling (SEPC), the multiphonon capture with preliminary tunneling of an electron through a potential barrier in the depletion region occurs with a lower rate as compared to the direct multiphonon capture in the electrically neutral bulk of the semiconductor, whereas, in the case of weak electron-phonon coupling (WEPC), the capture rate in the depletion region of a semiconductor may exceed that in the electrically neutral bulk by several orders of magnitude. The results of experimental study of capture processes in AlGaAs doped with silicon indicate that electron-phonon coupling is strong in DX centers.