Substitutional Donor Research Articles

Influence of deposition conditions on the thermal stability of ZnO:Al films grown by rf magnetron sputtering

The resistivity and the thermal stability of transparent conducting ZnO layers doped with aluminum have been correlated with the conditions of the sputtering process. Layers deposited at low rf power density (∼1.3 to 2.6 W/cm2) exhibit a low resistivity of 9×10−4 Ω cm, predominantly due to a high concentration of intrinsic donor type defects. These donors are compensated during annealing at high temperature in a vacuum; the low resistivity increases and the layers are not thermally stable. At rf power densities of ∼3.2 W/cm2 and more, the deposition conditions yield a high growth rate and the extrinsic aluminum dopant is incorporated on vacant cation sites. These substitutional donors are thermally stable therefore a low resistivity is retained after annealing at 550 °C.

Nature of the 2.8-eV photoluminescence band in Si-doped GaN

Investigation on the defect-related luminescence centered around 2.8 eV in Si-doped GaN epifilms is presented. It is found that the mechanism of this blue emission (BL) is quite different from that of the yellow luminescence (YL). By comparing the photoluminescence (PL) spectra obtained from either front-side or backside excitation and combining with the results of the BL and YL-related PL excitation and the secondary-ion-mass spectroscopy measurements, we propose that the 2.8-eV emission is the transition from the substitutional oxygen donor $({\mathrm{O}}_{\mathrm{N}})$ level at 0.25 eV above the conduction band to the ${\mathrm{V}}_{\mathrm{Ga}}{\ensuremath{-}\mathrm{O}}_{\mathrm{N}}$ complex acceptor. The energy level of the ${\mathrm{V}}_{\mathrm{Ga}}{\ensuremath{-}\mathrm{O}}_{\mathrm{N}}$ complex is found to be at 0.8 eV above the valence-band edge which is consistent with the theoretical calculation. In addition, we show that the ${\mathrm{V}}_{\mathrm{Ga}}$ defect plays a key role in the link between the time evolution of the YL and BL spectra in GaN samples.

Calculation of shallow donor levels in GaN

The energy levels of shallow substitutional donors in GaN are calculated in an effective-mass theory that includes the effects of mass anisotropy, central-cell potential correction, and the host conduction band edge wave function. The impurities studied include C, Si, and Ge on the Ga sites and O, S, and Se on the N sites of GaN in both zincblende and wurtzite structures. We present the results for the ground state energies and the transition energies from the ground state to the first few excited states. While a number of measured donor levels can be identified with the isolated substitutional donors treated here, there are also a number of experimental results that are substantially different from the present calculations. We discuss these cases and suggest possible explanations.

Study of site change of Li impurities in ZnSe by co-doping with iodine

Co-doping of lithium (Li) and iodine (I) in ZnSe was studied with metalorganic vapor-phase epitaxy. Since Li has the well-known metastability of substitutional acceptor sites and interstitial donor sites, the examination of the stability of Li sites will be effective to study the co-doping effect. Although the ZnSe films doped with Li showed photomuninescence (PL) spectra dominated by neutral-donor-bound excitons at 13 K, the PL spectra showed the systematic shift to the neutral-acceptor-bound excitons by co-doping with I doping. The corresponding turn-over of the electrical properties from n- to p-type was also observed.

Surface micro-roughness and transport properties of Si delta-doped GaAs/InxGa1−xAs/GaAs (0.1⩽×⩽0.25) quantum wells grown by molecular-beam epitaxy on GaAs (001) and GaAs (111)B

We investigated the surface quality and electron transport properties of 200 Å GaAs/InxGa1−xAs/GaAs, 0.1⩽x⩽0.25, quantum well structures grown by molecular-beam epitaxy on GaAs (001) and (111)B, center delta-doped with Si to 1×1013 cm−2. For GaAs barriers grown above 600 °C, room temperature atomic force microscopy of the (111)B samples showed a surface micro roughness below 2 Å for x⩽0.15 but reaching 60 Å for x=0.25. In addition, analysis of Shubnikov–de Haas and Hall effects at 4.2 K showed a strong deterioration of the electrical properties. In contrast, the (001) structures had surface micro roughness below 2 Å and superior transport properties even at x=0.25. At lower growth temperatures the surface morphology roughened, but without a commensurate deterioration of the electrical properties. For the (001) layers self-consistent Poisson–Schrödinger calculations give quantum well subband densities in good agreement with those measured, assuming full incorporation of Si substitutional donors. However, a 20% loss of carriers was observed in the (111) layers, and the excited state subband densities were a smaller fraction of the ground subband occupancy compared to the (001). Amphoteric (Si-on-As sites) behavior is believed responsible for this behavior.

Hydrogen–rhodium complexes in silicon

New hydrogen-induced deep levels in rhodium-doped n- and p-type silicon were observed after hydrogenation by wet-chemical etching. The levels were studied by DLTS measurements on Schottky diodes. We have found in n-type samples the levels E(150) at E C −0.33 eV; E(90) at E C −0.16 eV; and E(70) at E C −0.14 eV. Levels E(150) and E(270) belong to the substitutional rhodium donor and acceptor. Evidence is presented that the level E(70), which was formerly ascribed to isolated rhodium, is due to a hydrogen–rhodium complex. In p-type samples two levels were detected: H(280) at E V +0.50 eV and H(200) at E V +0.37 eV. Two different hydrogen–rhodium complexes are assigned to these levels. The thermal stability of the levels was investigated up to temperatures of 600 K.

Electric field gradients of acceptor–donor pairs in semiconductors

The interaction between substitutional and interstitial donors and single or double acceptors in Si, GaAs, InP, and InAs has been studied by perturbed angular correlation spectroscopy (PAC). For the case of Si, complex formation between substitutional donors (As, P) and different radioactive acceptors (111In, 111Cd, 117Cd) has been observed. The formation of Cd–hydrogen pairs using either 111Cd or 117Cd is discussed for GaAs, InP, and InAs.

Role of the Substitutional Oxygen Donor in the Residual N-Type Conductivity in GaN

A detailed photoluminescence (PL) study reveals a striking similarity in local vibrational properties of a defect center in GaN as compared to that for the substitutional OP donor in GaP. This observation could be interpreted as if the center is in fact related to the substitutional oxygen donor in GaN. The deep-level nature experimentally determined for the defect center calls for caution of a commonly referred model that the substitutional oxygen donor is responsible for the residual n-type conductivity in GaN.

Effect of Local Lattice Relaxation on the Electronic Structure of Substitutional Donors in Silicon

The electronic structure of substitutional group V (P, As and Sb) and group VI (S, Se and Te. both neutral and singly ionized) impurities in silicon has been investigated using self-consistent Multiple Scattering Xα (MSXα) method within the framework of the standard muffin-tin potential approximation, incorporating the effect of lattice relaxation surrounding the impurity. A systematic stud y of the effect of local lattice relaxation on the electronic structure of group V (shallow) and group VI (deep) donors in Si is attempted. It is interesting to note that the size difference between the impurity atom and that of the host Si atom seems to be shared almost equally by the impurity atom (either expansion or squeezing) and the nearest neighbours (inward or outward movement). The effect of size difference on charge transfer and the bonding characteristics between the impurity atom and Si atoms are also analyzed.

Laser spectroscopy of donor-acceptor complexes inβ-CdP2

In p and n type CdP2 samples (tetragonal modification) narrow symmetric absorption bands with energy maxima at 1.77, 1.69, 1.60, and 1.50 eV were observed using laser spectroscopy. They depend strongly on the mutual orientation of the electric vector of the radiation and the optical c axis of the crystal. The results found experimentally can easily be explained as complexes consisting of deep acceptors which are doubly-charged cadmium vacancies and shallow substitutional donor atoms S, occupying phosphorus sites in the 1-st, 2-nd, 3-rd, and 4-th coordination spheres relative to the cadmium vacancies.

Characterizing the local nitrogen environment at platelets in type IaA/B diamond

The nitrogen K‐electron energy‐loss near‐edge structure from {100} platelets of type IaA/B diamond has been modelled using multiple scattering theory. It is demonstrated that the available experimental results are consistent with nitrogen present as a substitutional donor impurity in a diamond‐like environment. The most plausible model is the dinitrogen substituted pair (A centre). Detailed comparison between the fine structure in electron energy‐loss spectra and theory confirms the previously published A centre model. A less plausible model is the singly substitutional nitrogen with one of the C–N bonds elongated by 12.5–15%. However, this site is paramagnetic, in apparent contradiction to experiment.

Role of the Substitutional Oxygen Donor in the Residual N-Type Conductivity in GaN

AbstractA detailed photoluminescence (PL) study reveals a striking similarity in local vibrational properties of a defect center in GaN as compared to that for the substitutional Op donor in GaP. This observation could be interpreted as if the center is in fact related to the substitutional oxygen donor in GaN. The deep-level nature experimentally determined for the defect center calls for caution of a commonly referred model that the substitutional oxygen donor is responsible for the residual n-type conductivity in GaN.

Electron paramagnetic resonance of conduction-band electrons in silicon

The g value of conduction-band electrons in silicon was properly determined by using electron paramagnetic resonance. A linear empirical relationship was first found between the g values and the thermal ionization energies of several well-known group-V substitutional shallow donors in silicon. An extrapolation of the empirical relation to zero ionization energy predicted the g value of conduction-band (CB) electrons, ${\mathrm{g}}_{\mathrm{CB}}$=1.9995, which is slightly but definitely different from that of conduction electrons in the donor-impurity band of degenerate n-type silicon; although both g values have been tacitly accepted to be identical for nearly four decades. The prediction was directly verified by measuring the g value of CB electrons created either by thermal emission from shallow donors in phosphorus-doped silicon at T=125 K and by above-band-gap optical excitation in high-purity p-type silicon at T=3.5 K; the measured g value in both experiments was precisely ${\mathrm{g}}_{\mathrm{CB}}$=1.9995(1). The empirical relation is still not theoretically explained.

DX-like center generated by uniaxial strains of screw dislocations in CdS

A center in plastically deformed CdS, labeled herein as the SDX center, was investigated by means of deep level transient spectroscopy, thermally stimulated capacitance, photocapacitance, and Hall effect measurements. The essential features of the center are: thermally activated electron capture with an activation energy (0.30 eV), comparable with the electron emission activation energy (0.40 eV); an optical ionization energy of the SDX center (1.47 eV) significantly exceeding the equilibrium electron binding energy (0.1 eV); a persistent photoconductivity at low temperatures; dependence of electron emission from SDX centers in an electric field is significantly weaker than expected from the Pool–Frenkel effect. The concentration of SDX centers, as measured by methods of capacitance spectroscopy and conductivity, increases with increasing density of screw dislocations. Exponential electron thermo-emission from the SDX center is accompanied by nonexponentiality of the electron capture. The capture kinetics is fitted using several models. The best agreement with the data is obtained for the model of randomly distributed barrier heights. A model of the origin of SDX centers is proposed and discussed. According to the model, SDX center is a noneffective-mass state (DX level) of a shallow substitutional donor (possibly Al). In as-grown samples, it cannot be occupied because it lies significantly above the Fermi level. In plastically deformed samples the DX level moves down to the Fermi level under the elastic lattice distortions around dislocations and becomes detectable.

Coexistence of the DX center with nonmetastable states of the donor impurity in Si-doped AlxGa1-xAs: Effects on the low-temperature electron mobility.

The coexistence of the DX center with nonmetastable bound states of the ordinary substitutional configuration of the donor impurity is extensively investigated in Si-doped ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As samples having different x AlAs molar fractions in the direct gap region. The occupation of nonmetastable states is evidenced by comparing the ${\mathit{n}}_{\mathit{CV}}$ ``electron density,'' as derived from capacitance-voltage measurements in Schottky diodes, with the ${\mathit{n}}_{\mathit{H}}$ Hall density. In samples of compositions not far from the direct-to-indirect gap transition and with doping levels in the ${10}^{18}$-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ range, a nonmetastable state SX, degenerate in energy with the conduction band, can reach a significant occupancy when the saturated persistent photoconductivity condition is approached during low-temperature photoionization of DX centers. On the other hand, when the free-electron density is smaller than a critical density of a few ${10}^{16}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$, electrons freeze out into a localized S\ensuremath{\Gamma} state, or into a low mobility impurity band, linked to the \ensuremath{\Gamma} conduction-band edge. A finite occupancy of the SX or S\ensuremath{\Gamma} state gives rise to a significant ${\mathit{N}}_{{\mathit{D}}^{0}}$ density of substitutional donors in the neutral ${\mathit{D}}^{0}$ charge state having a strongly correlated spatial distribution. Electron capture into donor states, DX or not, has a spatially selective character, as can be evidenced by low-temperature mobility data under conditions where impurity scattering dominates. For DX centers, this is demonstrated by the hysteretic behavior of low temperature (${\mathit{T}}_{0}$) ${\mathrm{\ensuremath{\mu}}}_{\mathit{H}}$ vs ${\mathit{n}}_{\mathit{H}}$ data, where any given ${\mathit{n}}_{\mathit{H}}$ value is reached through DX center photoionization steps or through electron capture via a proper thermal cycling. When ${\mathit{N}}_{{\mathit{D}}^{0}}$ is negligible the hysteresis amplitude is maximum. However, whenever ${\mathit{N}}_{{\mathit{D}}^{0}}$ reaches significant values, either at the ${\mathit{T}}_{0}$ temperature or during the thermal cycle, the hysteresis amplitude vanishes. This is systematically observed in all the cases where the occupation of SX or S\ensuremath{\Gamma} states is independently demonstrated through the analysis of ${\mathit{n}}_{\mathit{CV}}$ and ${\mathit{n}}_{\mathit{H}}$ data. Two diverse complementary effects are proposed to explain the observed vanishing of the hysteresis amplitude. \textcopyright{} 1996 The American Physical Society.

First-Principles Calculations Of Diffusion Of Chlorine Atoms In GaAs

AbstractThe properties of chlorine atoms in crystalline GaAs, such as stable configurations, migration paths, charge-state effects, and interaction with dopant atoms are theoretically investigated. The calculations are based on the local density functional theory using first-principles pseudopotentials in a supercell geometry. We determine the stable charge state of an isolated Cl atom as a function of the Fermi energy. When the Fermi level is situated at the top of the valence band of GaAs, the Cl atom occupies preferentially the bond-center site of a Ga-As bond in the positive charge state. The Cl atom diffuses through the GaAs crystal via a path in the region of high electron density, with a fairly large energy barrier. When the Fermi level is at the bottom of the conduction band, the lowest-energy configuration of the Cl atom is the tetrahedral interstitial site in the negative charge state and the bond center site is very slightly higher in energy. In Si-doped GaAs, the C1 atom occupies the tetrahedral interstitial site with the substitutional Si donor atom as a nearest neighbor, forming a neutral Cl-Si complex. The Cl-Si complex is weak and easily dissociates into the isolated C1 and Si atoms in GaAs. A comparison will be made between the behavior of Cl and F atoms in GaAs.

The pressure-induced transition between shallow and deep states of an Si donor in GaAs

Using the tight-binding approach we calculate the pressure-induced transition between the and (L) substitutional donor states. There are no energy barriers for such transformation and this is in agreement with the experimental findings. The arguments are presented that the DX state of this donor arises from the (L) resonant state as a result of spontaneous tetragonal distortion.

Deep level transient spectroscopy assessment of silicon contamination in AlGaAs layers grown by metalorganic vapor phase epitaxy

A systematic silicon contamination has been detected by deep level transient spectroscopy in undoped and n-type doped (Te, Se, Sn) AlGaAs layers, grown in two different metalorganic vapor phase epitaxy reactors. DX center generation by substitutional donors, with very specific capture and emission thermal barriers (fingerprints), is the key to unambiguously identifying their presence, with detection limits well below the standard secondary ion mass spectroscopy capability. We comment on the potential sources of Si contamination (most common in this epitaxial technique), and on the relevance of such contamination to interpreting correctly experimental data related to the microscopic structure of DX centers.

The DX center and other tin-related defects in AlGaAs semiconductors

AlxGa1-xAs semiconductors doped with both natural and enriched119Sn have been studied by Mossbauer spectroscopy (MS) to help to determine the atomic-scale nature of a deleterious, deep-level defect known as the DX center. Spectra have been acquired in the dark at 76 K and under sub-bandgap illumination at 4 or 10 K to distinguish the DX center from the substitutional shallow donor defect. Although electrical differences are clearly detected in these two states, no difference in the Mossbauer spectra are observed. Unexpected high Sn contents, determined by quantitative MS, demonstrate a large non-electrically active Sn fraction in some samples and this may be obscuring the observation of the DX center by both X-ray absorption spectroscopy and MS. Grinding the single-crystal layers into fine powders leads to an Sn defect that is attributed to a surface-oxidized site.

Identification of phosphorus in diamond thin films using electron paramagnetic-resonance spectroscopy

An electron paramagnetic-resonance study of diamond films doped by implantation of phosphorus during film deposition is reported. Samples with nominal phosphorus concentration between 1016 and 1017 cm−3 exhibit two isotropic lines of equal intensity separated by 27 G. The double-line spectrum is expected for a nuclear-spin 1/2 species such as phosphorus. The symmetry of the center and the temperature dependence of the spin concentration suggest that phosphorus forms a substitutional shallow donor in diamond.