Acceptor Impurities Research Articles

Theoretical study on the interaction of p-type impurities with hydrogen in ZnO

In this work we systematically review our theoretical results about the interaction of important p-type impurities with hydrogen in ZnO. Our investigations indicated that a H atom in ZnO might migrate through different pathways to reach the sites nearby acceptor impurities (N, Li and Ag), forming stable complexes of NO-H, LiZn-H and AgZn-H. Moreover, we found that in the formation processes of these complexes, the H atom overcame energy barriers of no more than 0.5 eV. Whereas the dissociation of the NO-H, LiZn-H and AgZn-H complexes require active energies of 1.25-1.48 eV, 1.10-1.35 eV and 1.10-1.30 eV, respectively. These aspects showed that these complexes are stable at room temperature. In addition, the electronic and vibrational properties of the concerned complexes were introduced.

Off-central acceptor impurity in a spherical quantum dot

The hole energy spectrum with the ion of an acceptor impurity in the quantum dot has been calculated using the spherical approximation of the multiband Luttinger model. The dependence of the hole energy levels on the impurity location in the quantum dot has been studied. The effect of the impurity location on the dipole momentum and the oscillator strength has been analyzed. The hole interlevel absorption coefficient has been calculated.

Features of the conduction mechanisms of the n-HfNiSn semiconductor heavily doped with the Co acceptor impurity

The crystalline structure, electron density distribution, energy and kinetic parameters of a HfNi1 − xCoxSn semiconductor heavily doped with a Co acceptor impurity are studied in the ranges T = 80–1620 K and NACo from 9.5 × 1020 cm−3 (at x = 0.05) to 7.6 × 1021 cm−3 (at x = 0.40). It is shown that variations in the activation energy of hopping conduction ɛ3ρ(x) and the modulation amplitudes of continuous energy bands ɛ3α(x) are caused by the appearance of a donor source in the n-type HfNi1 − xCoxSn semiconductor. It is shown that the doping of n-HfNiSn with a Co acceptor impurity is accompanied by a change in the degree of compensation of the semiconductor due to the simultaneous generation of both structural acceptor-type defects during Ni atom substitution with Co atoms and structural donor-type defects during the partial occupation of Ni sites by Sn atoms. The results are discussed within the Shklovskii-Efros model for a heavily doped and compensated semiconductor.

Light emission lifetimes in p-type δ-doped GaAs/AlAs multiple quantum wells near the Mott transition

The time resolved photoluminescence of beryllium δ-doped GaAs/AlAs multiple quantum wells have been studied over a range of doping concentrations, in order to investigate possible mechanisms for the carrier radiative recombination, both above and below the Mott metal-insulator transition. It was found that at doping concentrations near the Mott transition (NBe ∼ 3 × 1012 cm−2), the radiative recombination of excitons-bound-to-acceptor impurities as well as free electrons with acceptor impurities, dominated in the Be δ-doped GaAs/AlAs MQWs (LW = 15 nm) that were used in this study. Above the Mott transition, the major contribution was from radiative recombination of free electrons with a two-dimensional hole gas. The radiative lifetime would therefore exhibit different behavior with doping. In lightly doped GaAs/AlAs MQWs, this changed from 0.3–1 ns at 3.6 K to 8 ns at 300 K, whilst in quantum wells above the Mott transition, it changed from ∼0.36 ns at 3.6 K to ∼1 ns at 300 K, and was also weakly dependent on the concentrations of acceptor doping.

EFFECTS OF ON-CENTER IMPURITY ON ENERGY LEVELS OF LOW-LYING STATES IN CONCENTRIC DOUBLE QUANTUM RINGS

In this paper, the electronic eigenstates and energy spectra of single and two-interacting electrons confined in a concentric double quantum rings with a perpendicular magnetic field in the presence of on-center donor and acceptor impurities are calculated using the exact diagonalization method. For a single electron case, the binding energy of on-center donor and acceptor impurities are also calculated. The effects of centrifugal, confinement and diamagnetic potentials on the binding energy are investigated. It is found that the binding energy decreases by increasing the centrifugal or confinement potential. Also, it is shown that the binding energy increases by increasing the magnetic field. The effects of on-center impurity on the energy spectrum and angular momentum transition of the lowest states are investigated for the both single and two-interacting electrons. It is found that the on-center donor impurity increases the fractional Aharonov-Bohm oscillation period while the acceptor impurity acts inversely and decreases the fractional AharonovBohm oscillation period.

Effects of N-doping concentration on the electronic structure and optical properties of N-doped β-Ga2O3

The electronic structures and the optical properties of N-doped β-Ga2O3 with different N-doping concentrations are studied using the first-principles method. We find that the N substituting O(1) atom is the most stable structure for the smallest formation energy. After N-doping, the charge density distribution significantly changes, and the acceptor impurity level is introduced above the valence band and intersects with the Fermi level. The impurity absorption edges appear to shift toward longer wavelengths with an increase in N-doping concentration. The complex refractive index shows metallic characteristics in the N-doped β-Ga2O3.

Effect of zinc doping and temperature on the properties of sprayed CuInS2 thin films

Copper indium disulfide (CuInS2) is an efficient absorber material for photovoltaic and solar cell applications. The structural, optical, photoluminescence properties and electrical conductivities could be controlled and modified by suitably doping CuInS2 thin films with dopants such as Zn, Sn, Bi, Cd, Na, N, O, P and As. In this work Zn (0.01M) doped CuInS2 thin films are (Cu/In=1.25) deposited on to glass substrates in the temperature range 300–400°C. It is observed that the film growth temperature, ion ratio (Cu/In=1.25) and Zn-doping affect structural, optical, photoluminescence and electrical properties of sprayed CuInS2 thin films. As the XRD patterns depict, Zn-doping facilitates the growth of CuInS2 thin films along (112) preferred plane and in other characteristic planes. The EDAX results confirm the presence of Cu, In, S and Zn in the films. The optical studies show, about 90% of light transmission occurs in the IR regions; hence Zn-doped CuInS2 can be used as an IR transmitter. The absorption coefficient (α) in the UV–visible region is found to be in the order of 104–105cm−1 which is the optimum value for an efficient absorber. The optical band gap energies increase with increase of temperatures (1.66–1.78eV). SEM photographs reveal crystalline and amorphous nature of the films at various temperature ranges. Photoluminescence study shows that well defined broad Blue and Green band emissions are exhibited by Zn-doped CuInS2 thin films. All the films present low resistivity (ρ) values and exhibit semiconducting nature. An evolution of p-type to n-type conductivity is obtained in the temperature range 325–350°C. Hence, Zn species can be used as a donor and acceptor impurity in CuInS2 thin films to fabricate efficient solar cells, photovoltaic devices and good IR Transmitters.

Thermodynamics of Defect Formation and Hydration of Y<sub>2</sub>O<sub>3</sub>

Defect formation in yttria with a small content of acceptor impurities in equilibrium with a hydrogen-containing gas phase is studied theoretically. A statistical-thermodynamic description of the yttriagas equilibrium is based on the approach developed for compounds with a complex electronic structure [Phys. Stat. Sol. B (1991) Vol. 168, p. 233]. The considered model of electronic structure for Y2O3 includes, besides valence and conduction bands, acceptor and F-center states. The energy of F-centers was calculated in the framework of the variational quantum-mechanical approach combined with the molecular statics method. It is shown that acceptor states appreciably affect the thermodynamics of defect formation, while the F-centers contribution in a wide range of external parameters is small. The concentrations of defects (protons, oxygen vacancies, electronic defects) and the Fermi level position are determined as functions of temperature and gas phase parameters.

Electronic structure and optical properties of N-Zn co-doped β-Ga2O3

The electronic structure and optical properties of N-doped β-Ga2O3 and N-Zn co-doped β-Ga2O3 are investigated by the first-principles calculation. In the N-Zn co-doped β-Ga2O3 system, the lattice parameters of a, b, c, V decrease and the total energy Etotal increases in comparison with N-doped β-Ga2O3. The calculated ionization energy of N-Zn co-doped β-Ga2O3 is smaller than that of N-doped β-Ga2O3. Two shallower acceptor impurity levels are introduced in N-Zn co-doped β-Ga2O3. Compared with N-doped β-Ga2O3, the major absorption peak is red-shifted and the impurity absorption edge is blue-shifted for N-Zn co-doped β-Ga2O3. The results show that the N-Zn co-doped β-Ga2O3 is found to be a better method to push p-type conductivity in β-Ga2O3.

Impedance spectrometry of optimized standard and inverted P3HT-PCBM organic solar cells

A study of the optical and impedance behavior of optimized standard and inverted photovoltaic solar cells based on P3HT:PCBM active nano composites is presented. The standard cells sequence is ITO/HTL1/P3HT:PCBM/Ca/Al and the inverted cells one is ITO/ZnO/P3HT:PCBM/HTL2/Ag where HTL1 and HTL2 are Hole Transport Layers. Absorption and action spectra, together with I–V characteristics, are shown to be quite similar and lead to 4.05% and 3.90% energy conversions, for standard and inverted cells, respectively. Built-in potentials of 0.82–0.89V and acceptor impurities concentrations of 1.6–2.4 1015cm−3 are found through capacitance measurements. Impedance spectrometry shows the classical two-circle complex plan curves, one being related to the effective lifetime of charge carriers before recombination at low frequency, and the other one to the diffusion time of these carriers at high frequency. The shape of the curves is identical, showing the ohmic role of the ZnO layer. It is shown that overall resistances in the dark are higher for inverted cells as compared to standard ones, and that this feature is inverted under illumination, with a thousand-time decrease. Global mobilities are in the range 3.8–4.6 10−3cm2V−1s−1, which is slightly higher as compared to the literature. Four different equivalent circuit models are tested on experimental results, and it is concluded that the classical RCPE model (or its Garcia–Belmonte variant) is suitable for this kind of cells.

Polarization-Induced pn Diodes in Wide-Band-Gap Nanowires with Ultraviolet Electroluminescence

Almost all electronic devices utilize a pn junction formed by random doping of donor and acceptor impurity atoms. We developed a fundamentally new type of pn junction not formed by impurity-doping, but rather by grading the composition of a semiconductor nanowire resulting in alternating p and n conducting regions due to polarization charge. By linearly grading AlGaN nanowires from 0% to 100% and back to 0% Al, we show the formation of a polarization-induced pn junction even in the absence of any impurity doping. Since electrons and holes are injected from AlN barriers into quantum disk active regions, graded nanowires allow deep ultraviolet LEDs across the AlGaN band-gap range with electroluminescence observed from 3.4 to 5 eV. Polarization-induced p-type conductivity in nanowires is shown to be possible even without supplemental acceptor doping, demonstrating the advantage of polarization engineering in nanowires compared with planar films and providing a strategy for improving conductivity in wide-band-gap semiconductors. As polarization charge is uniform within each unit cell, polarization-induced conductivity without impurity doping provides a solution to the problem of conductivity uniformity in nanowires and nanoelectronics and opens a new field of polarization engineering in nanostructures that may be applied to other polar semiconductors.

A comparison of electronic structure and optical properties between N-doped β-Ga2O3 and N–Zn co-doped β-Ga2O3

The electronic structure and optical properties of N-doped β-Ga2O3 and N–Zn co-doped β-Ga2O3 are investigated by the first-principles calculation. In the N–Zn co-doped β-Ga2O3 system, the lattice parameters of a, b, c, V decrease and the formation energy of N–Zn co-doped β-Ga2O3 is smaller in comparison with N-doped β-Ga2O3. There are two shallower acceptor impurity levels in N–Zn co-doped β-Ga2O3. Comparing with N-doped β-Ga2O3, the major absorption peak is red-shifted and the impurity absorption edge is blue-shifted for N–Zn co-doped β-Ga2O3. The results show that the N–Zn co-doped β-Ga2O3 is found to be a better method to push p-type conductivity in β-Ga2O3.

Interlevel transitions in a quantum dot with an acceptor impurity

Interlevel transitions in a quantum dot with an acceptor impurity

Acceptor Dopants in Bulk and Nanoscale ZnO

ABSTRACTZinc oxide (ZnO) is a semiconductor that emits bright UV light, with little wasted heat. This intrinsic feature makes it a promising material for energy-efficient white lighting, nano-lasers, and other optical applications. For devices to be competitive, however, it is necessary to develop reliable p-type doping. Although substitutional nitrogen has been considered as a potential p-type dopant for ZnO, recent theoretical and experimental work suggests that nitrogen is a deep acceptor and will not lead to p-type conductivity. In nitrogen-doped samples, a red photoluminescence (PL) band is correlated with the presence of deep nitrogen acceptors. PL excitation (PLE) measurements show an absorption threshold of 2.26 eV, in good agreement with theory. The results of these studies seem to rule out group-V elements as shallow acceptors in ZnO, contradicting numerous reports in the literature. Optical studies on ZnO nanocrystals show some intriguing leads. At liquid-helium temperatures, a series of sharp IR absorption peaks arise from an unknown acceptor impurity. The data are consistent with a hydrogenic acceptor 0.46 eV above the valence band edge. While this binding energy is still too deep for many practical applications, it represents a significant improvement over the 1.4-1.5 eV binding energy for nitrogen acceptors. Nanocrystals present another twist. Due to their high surface-to-volume ratio, surface states are especially important. In our model, the 0.46 eV level is shallow with respect to the surface valence band, raising the possibility of surface hole conduction.

The effect of group IIIA metal ion dopants on the photocatalytic activities of nanocrystalline Sr0.25H1.5Ta2O6·H2O

A series of group IIIA metal ion electron acceptors doped into Sr(0.25)H(1.5)Ta(2)O(6)·H(2)O (HST) samples have been prepared by an impregnation and calcination method for the first time. The samples are characterized by XRD, TEM, DRS and XPS. The variations in the electronic structure and photoelectric response after metal ion doping are investigated by theoretical calculations and photocurrent experiments, respectively. Results show that the metal ions can be efficiently incorporated into the HST crystal structure, which is reflected in the lattice contraction. Meanwhile, the photoabsorption edges of the metal-doped HST samples are red shifted to a longer wavelength. Taking into account the ionic radii and electronegativities of the dopants, as well as the XRD and XPS results, it is concluded that Ta(5+) ions may be partially substituted by the Al(3+) and Ga(3+) ions in the framework, while In(3+) ions are the favourable substitutes for Sr(2+) sites in the cavity. The first-principles DFT calculations confirm that the variation of the band structure is sensitive to the type of group IIIA metal ion. Introducing the dopant only at the Ta site induces an obvious variation in the band structure and the band gap becomes narrow. Meanwhile, an ''extra step'' appeared in the band gap, which can trap photogenerated electrons from the valance band (VB) and could enhance the charge mobility and the photocurrent. For the photocatalytic degradation of methyl orange in an aqueous solution and in benzene in the gas phase, the doped samples show superior photocatalytic activities compared with both undoped samples and TiO(2). The enhanced photocatalytic activities can be well explained by their electronic structure, photoabsorption performance, photoelectric response, and the concentration of the active species. Due to the fact that Ga ion doping can create an acceptor impurity level and change the electronic band, efficiently narrowing the band gap, the Ga-doped sample shows the highest photocatalytic activity.

Defects Energetics, Electronic Structure and Optical Properties of Cu-Doping and Zn Vacancy Impurities in ZnSe

Based on the first-principles within the density functional theory, the geometric structures of perfect zinc blend ZnSe, that with Zn vacancies (Zn0.875Se) and Cu-doped ZnSe (Zn0.875Cu0.125Se) were optimized using the plane-wave ultrasoft pseudopotential method. The defect formation energy, band structure, density of states, Mulliken charges, and optical spectra were calculated and discussed in detail. The results demonstrated that in Zn0.875Se and Zn0.875Cu0.125Se systems, because of the introduction of the vacancy acceptor level or acceptor impurity level, the band gap is reduced, and the absorption peaks show a remarkable redshift. Cu doping into the ZnSe system was found to be relatively stable, while the monovacancy system was not.

X-ray photoelectron spectroscopy study of cubic boron nitride single crystals grown under high pressure and high temperature

The defects, impurities and their bonding states of unintentionally doped cubic boron nitride (cBN) single crystals were investigated by X-ray photoelectron spectroscopy (XPS). The results indicate that nitrogen vacancy (VN) is the main native defect of the cBN crystals since the atomic ratio of B:N is always larger than 1 before Ar ion sputtering. After sputter cleaning, around 6at% carbon, which probably comes from the growth chamber, remains in the samples as the main impurity. Carbon can substitute nitrogen lattice site and form the bonding states of CBN or CB, which can be verified by the XPS spectra of C1s, B1s and N1s. The C impurity (acceptor) and N vacancy (donor) can compose the donor–acceptor complex to affect the electrical and optical properties of cBN crystals.

First-principles study on electronic structure and optical properties of N-doped P-type β-Ga2O3

The band structure, density of states, electron density difference and optical properties of intrinsic β-Ga2O3 and N-doped β-Ga2O3 were calculated using first-principles based on density functional theory. After N doping, the band gap decreases, shallow acceptor impurity levels are introduced over the top of the valence band and the absorption band edge is slightly red-shifted compared to that of the intrinsic one. The anisotropic optical properties are investigated by means of the complex dielectric function, which are explained by the selection rule of the band-to-band transitions. All calculation results indicate that N-doping is a very promising method to get P-type β-Ga2O3.

The nonlinear optical rectification of an ellipsoidal quantum dot with impurity in the presence of an electric field

We have performed theoretical calculation of second-order nonlinear optical rectification (OR) coefficient in a typical GaAs/AlGaAs QD with ellipsoidal confinement potential in the presence of an impurity and an applied electric field. Using an appropriate coordinate transformation and the perturbation theory, we have investigated the OR coefficient as a function of incident photon energy. Calculation results show that the values of OR coefficient increase with an increase of applied electric field. However, the values decrease with increases in confinement strength and ellipticity constant. Additionally, the presence of a donor impurity shifts the OR coefficient peak positions to higher energies (blueshift), contrary to that of an acceptor impurity.

MOS Capacitance—Voltage Characteristics II. Sensitivity of Electronic Trapping at Dopant Impurity from Parameter Variations

Low-frequency and high-frequency Capacitance—Voltage (C—V) curves of Metal—Oxide—Semiconductor Capacitors (MOSC), including electron and hole trapping at the dopant donor and acceptor impurities, are presented to illustrate giant trapping capacitances, from > 0.01COX to > 10COX. Five device and materials parameters are varied for fundamental trapping parameter characterization, and electrical and optical signal processing applications. Parameters include spatially constant concentration of the dopant-donor-impurity electron trap, NDD, the ground state electron trapping energy level depth measured from the conduction band edge, EC–ED, the degeneracy of the trapped electron at the ground state, gD, the device temperature, T, and the gate oxide thickness, xOX.