Residual Acceptors Research Articles

Comparative photoluminescence study on p-type and n-type ZnO films codoped by nitrogen and aluminium

We report on the temperature-dependent photoluminescence (PL) properties of n-type and p-type ZnO films codoped with N and Al. For the n-type film, the dominant emission at low temperature is exciton bound to neutral donors, while for the p-type film it is exciton bound to neutral acceptor at ∼3.33 eV. Four defect or impurity levels, including N acceptor, residual acceptor, and two doping-induced unknown deep acceptors, were identified. The energy level of the N acceptor was determined to be ∼0.23 eV. Excitation energy dependence of the PL was also investigated. It was found that at high excitation energy, the formation of exciton was suppressed by the formation of D +A −eh complexes.

Metastability of the UV luminescence in Mg-doped GaN layers grown by MOVPE on quasi-bulk GaN templates

Optical metastability in Mg-doped GaN layers grown by metal–organic vapor phase epitaxy on thick GaN templates grown by halide vapor phase epitaxy has been studied by photoluminescence (PL) and cathodoluminescence (CL). The total Mg concentration varies from 1×10 19 to 1×10 20 cm −3. Both PL and CL spectra change the initial shape within a few minutes exposure to the excitation source. The effect is permanent at low temperatures but the emission spectrum can be recovered to its initial shape after heating to room temperature. The difference in PL and CL spectra is explained by activation of different regions in the sample by laser and electrons, respectively. In CL the increase of the defect-related luminescence seems to be connected with an enhanced structural defect reaction under electron irradiation, which competes with the regular radiative recombination, while in PL the main effect is related to dissociation of Mg–H complexes with consequent passivation of the residual shallow acceptors.

Deuterium passivation of electrically active defects in nonintentionally doped n-GaN

Deuterium diffusion was achieved in nonintentionally doped n-GaN layers, grown by metal organic chemical vapor deposition, at 460°C and a power density of 1.0Wcm−2. A deuterium diffusion mechanism was observed yielding concentrations around 1018cm−3 in the surface region following the (Dt)1∕2 diffusion law. A second mechanism displaying an abnormally high diffusion coefficient, probably occurring along threading dislocations, was observed. The fast diffusing deuterium traps electrons and passivates residual acceptors, decreasing the electron concentration from 1016to1015cm−3 and increasing the electron mobility from 10to320cm2V−1s−1, producing better material for electronic devices.

An investigation of control mechanisms of the excitonic behavior in reactively sputtered ZnO on (0001) Al2O3

Above-band-edge absorption spectra of reactively sputtered Zn- and O-rich samples exhibit free exciton and neutral acceptor bound exciton (A0X) features. It is shown that the residual acceptors which bind excitons with an energy of 75meV reside about 312meV above the valence band, according to effective mass theory. An intra-band-gap absorption feature peaking at 2.5eV shows correlation with the characteristically narrow A-free exciton peak intensity, suggesting a compensation mechanism of the centers involving oxygen vacancy (VO) related donors. In order to enhance free exciton concentration relative to competing neutral bound exciton density, relevant annealing processes are performed without disturbing the residual shallow acceptor profile which is necessary for at least background p-type conductivity.

Deep Level Point Defects in Semi-Insulating SiC

The high resistivity of SiC required for many device applications is achieved by compensating residual donors or acceptors with vanadium or intrinsic defects. This work addresses the defect levels of substitutional vanadium and the positively charged carbon vacancy (VC +) in semiinsulating (SI) SiC. After reviewing the earlier studies related to both defects, the paper focuses on temperature-dependent Hall measurements and photo-induced electron paramagnetic resonance (EPR) experiments of 4H and 6H SI SiC. In vanadium-doped samples, a V3+/4+ level near Ec-1.1 eV (4H) and Ec-0.85 eV (6H) is estimated by a comparison of dark EPR spectra and the activation energy determined from the Hall data, assuming that vanadium controls the Fermi level. In high purity semiinsulating substrates, analysis of time-dependent and steady-state photo-EPR data suggests that the plus-to-neutral transition of the carbon vacancy involves a structural relaxation of about 0.6 eV.

Can one-dimensional electron gas be generated in vicinal steps of GaAs?

In this work we study the electronic structure of doped multi-quantum wires formed bysteps in the vicinal surfaces of GaAs. It is verified that multi one-dimensional electronicsystems can exist, which contrasts with a previous conclusion in the case of a single wirewhere it was demonstrated that the presence of residual acceptors inhibited the formationof such a type of structure. The effects of dopant diffusion, residual density of acceptorsand exchange–correlations contributions have been addressed by the self-consistentsolution of the Schrödinger equation treated in the effective mass approximation.

Comparative study of vertical gradient freeze grown CdTe with variable Sn concentration

Four CdTe ingots with gradually increased concentration of the Sn impurity have been grown by the vertical gradient freeze method and were characterized with glow discharge mass spectroscopy, photoinduced current transient spectroscopy, resistivity, photoconductivity, and photoluminescence techniques. It was shown that the Sn impurity strongly influences resistivity and photoconductivity of the material. Concentration of Sn must be higher than the total concentration of residual acceptors to reach strong compensation. The middle-gap donor level pins the Fermi-level. Photoconductive high resistivity material can be prepared with Sn concentrations in the melt in the range 1018–1019 cm−3. In total, 6 electron traps and 3 hole traps were identified in the band gap by several complementary techniques.

Electronic states of germanium grown under micro-gravity condition

Magneto-optical absorption measurements of Sb-doped germaniums grown under micro-gravity condition were carried out to investigate the influence of the gravity on crystal growth, using far-infrared laser and microwave. For comparison, we prepared two germanium crystals grown in the same conditions except the gravity conditions. In spite of the quite short growth period, the germanium grown under micro-gravity has a quite good quality. The lineshape analysis of Zeeman absorption peaks due to donor electrons indicates the existence of residual thermal acceptors.

Liquid encapsulated Czochralski grown undoped p-type gallium antimonide studied by temperature-dependent Hall measurement

Liquid encapsulated Czochralski grown undoped p-type GaSb samples were studied by temperature-dependent Hall measurement. A shallow acceptor having an ionization energy of 32 meV was found to be the important residual acceptor associated with the p-type conduction of the undoped GaSb material. The carrier mobility data were fitted well by the relaxation time approximation model involving various carrier scattering processes. Other than the typical important scattering processes in limiting the carrier mobility in III–V semiconductors (i.e. ionized impurity, acoustic phonon, polar optical phonon and non-polar optical phonon), vacancy scattering has to be included so as to give a good fit to the experimental data. The concentrations of the vacancy and the 32 meV acceptor in the electron irradiated samples were found to increase significantly upon annealing, but such increase was not found in the non-irradiated sample.

Hydrochemical and isotopic effects associated with petroleum fuel biodegradation pathways in a chalk aquifer

Hydrochemical data, compound specific carbon isotope analysis and isotopic enrichment trends in dissolved hydrocarbons and residual electron acceptors have been used to deduce BTEX and MTBE degradation pathways in a fractured chalk aquifer. BTEX compounds are mineralised sequentially within specific redox environments, with changes in electron acceptor utilisation being defined by the exhaustion of specific BTEX components. A zone of oxygen and nitrate exhaustion extends approximately 100 m downstream from the plume source, with residual sulphate, toluene, ethylbenzene and xylene. Within this zone complete removal of the TEX components occurs by bacterial sulphate reduction, with sulphur and oxygen isotopic enrichment of residual sulphate ( ε s=− 14.4‰ to − 16.0‰). Towards the plume margins and at greater distance along the plume flow path nitrate concentrations increase with δ 15N values of up to + 40‰ indicating extensive denitrification. Benzene and MTBE persist into the denitrification zone, with carbon isotope enrichment of benzene indicating biodegradation along the flow path. A Rayleigh kinetic isotope enrichment model for 13C-enrichment of residual benzene gives an apparent ε value of − 0.66‰. MTBE shows no significant isotopic enrichment ( δ 13C = − 29.3‰ to − 30.7‰) and is isotopically similar to a refinery sample ( δ 13C = − 30.1‰). No significant isotopic variation in dissolved MTBE implies that either the magnitude of any biodegradation-induced isotopic fractionation is small, or that relatively little degradation has taken place in the presence of BTEX hydrocarbons. It is possible, however, that MTBE degradation occurs under aerobic conditions in the absence of BTEX since no groundwater samples were taken with co-existing MTBE and oxygen. Low benzene δ 13C values are correlated with high sulphate δ 34S, indicating that little benzene degradation has occurred in the sulphate reduction zone. Benzene degradation may be associated with denitrification since increased benzene δ 13C is associated with increased δ 15N in residual nitrate. Re-supply of electron acceptors by diffusion from the matrix into fractures and dispersive mixing is an important constraint on degradation rates and natural attenuation capacity in this dual-porosity aquifer.

Gallium antisite defect and residual acceptors in undoped GaSb

Undoped GaSb samples were investigated by positron lifetime spectroscopy (PAS) and the coincident Doppler broadening (CDB) technique. PAS measurement indicated that there were monovacancy-type defects in undoped GaSb samples, which were identified to be predominantly Ga vacancy ( V Ga ) related defects by combining the CDB measurements. After annealing of these samples at 520 °C, positron shallow trapping have been observed and should be due to Ga Sb defects. Undoped GaSb is intrinsically p-type having a residual carrier density of 10 16– 10 17 cm − 3 . And the Ga Sb antisite defects are stable in the (0), (1-) and (2-) charge states and act as a double acceptor. Thus, we infer that Ga Sb antisite defects are the acceptor contributing to the p-type conduction for undoped samples.

Advances in the crystal growth and device fabrication technology of CdZnTe room temperature radiation detectors

The performance of CdZnTe room-temperature X-ray and gamma-ray detectors is determined by material and device defects that govern carrier transport trough the device. In this contribution, we review common bulk, interface, and surface defects and their effects on charge transport, charge transport uniformity, and device performance. We note that pure CdZnTe grown under Te-rich conditions has an excess of Cd-vacancies and other Te-related native defects and must be electrically compensated in order to obtain high resistivity material. Through the critical analysis of the various compensation schemes it is shown that deep level defects must be introduced with donor doping elements in order to achieve a practical compensation technique. The role of carrier trapping and limitations on detector performance with increasing crystal size are discussed. Based on typical measured carrier lifetimes and the available literature data on carrier capture cross sections, we estimate that the residual acceptor concentration in CdZnTe detector crystals is much lower than widely thought, about 10/sup 11/ cm/sup -3/ instead of 10/sup 15/ cm/sup -3/. The deleterious effects of structural defects within single crystals are also discussed. We also provide a brief overview of the progress in CdZnTe crystal growth and device fabrication technologies aiming at reducing the concentration of the detrimental defects and improving CdZnTe detector performance.

Influence of epitaxy on the surface conduction of diamond film

The influence of the crystalline and surface properties of diamond homoepitaxial layers on device properties in H-terminated surface-channel diamond field effect transistors (FETs) is investigated. Crystalline defects inside the layers result in gate current leakage in FET DC operation. We confirmed incorporation of boron acceptors inside the layers. Their residual acceptors result in buffer leakage in DC operation, but do not affect RF characteristics much. Adsorbates from the environment change the concentration of the surface holes generated by the H-termination.

Shallow-impurity-assisted transitions in the course of submillimeter magnetoabsorption of strained Ge/GeSi(111) quantum-well heterostructures

The submillimeter (f=130–1250 GHz) magnetoabsorption spectra of strained Ge/GeSi(111) multilayer heterostructures with quantum wells are investigated at T=4.2 K upon band-gap optical excitation. It is found that the magnetoabsorption spectra contain lines associated with the excitation of residual shallow acceptors. The resonance absorption observed can be initiated by optical transitions between the impurity states belonging to two pairs of Landau levels of holes in germanium quantum-well layers.

Oxygen related shallow acceptor in GaN

ABSTRACTWe report on deliberate O doping of GaN epitaxial layers during MOCVD growth, using H2O and CO2 as precursors. The photoluminescence spectra show a dominant 3.27 eV emission at 2 K known to be a donor-acceptor pair (DAP) transition. In our samples the intensity of this DAP spectrum correlates with the commonly observed 3.466 eV acceptor bound exciton (ABE) peak, suggesting these spectra are related to the same acceptor. The general correlation of these acceptor spectra with O concentration (as established with SIMS data) suggest that the acceptor is O-related, most likely a VGa-O complex. The concentration was measured with positron annihilation spectroscopy and found to be in the 1016 cm−3 -1017 cm−3 range in different samples. Considering previous results the identity of this residual acceptor is suggested to be a VGa-O-H complex. Previous suggestions that this acceptor is related to Mg, Si or C are discussed and found to be less likely.

Effects of oxygen on anoxic biodegradation of benzoate during continuous culture.

In this study, various amounts of oxygen were added to denitrifying chemostats receiving benzoate to mimic the input of oxygen to anoxic zones of biological nutrient removal systems. The effect of oxygen on the biodegradative capability of the mixed-microbial culture for benzoate was investigated. The anoxic benzoate biodegradative capability of the culture was not significantly changed as the mass flowrate of oxygen was increased to 40% of the input benzoate chemical oxygen demand (COD) mass flowrate, but was decreased approximately 70% when the mass flowrate of oxygen was increased to 70% of the input benzoate COD mass flowrate. The decrease in the anoxic benzoate biodegradative capability was due primarily to the loss of the denitrifying enzymes (measured by the anoxic pyruvate-degrading ability) and not to the loss of the key anoxic catabolic enzyme (benzoyl-coenzyme A reductase). The proportional increase in the concentration of nitrate as the residual terminal electron acceptor and the lack of synthesis of aerobic ring-cleavage enzymes as the oxygen input to the chemostat was increased suggest that the mixed microbial culture preferred oxygen to nitrate as the terminal electron acceptor, but degraded benzoate using the anoxic metabolic pathway. The concentration of the mixed microbial culture increased as the oxygen input to the chemostat was increased, suggesting that the oxygen was used by cytochrome cbb3 rather than quinol oxidase because the energetic yield of cytochrome cbb3 is higher than that of quinol oxidase or the nitrogen oxide reductases.

Native defects and self-diffusion in GaSb

The recent results for the self-diffusivities, D(Ga) and D(Sb), of Ga and Sb in GaSb obtained by Bracht et al (Bracht H, Nicols S P, Walukjewicz W, Silveira J P, Briones F and Haller E E 2000 Nature 408 69 and Bracht H, Nicols S P, Haller E E, Silveira J P and Briones F 2001 J. Appl. Phys. 89 5393) are compared and related to the earlier measurements by Weiler and Mehrer (Weiler D and Mehrer H 1984 Phil. Mag. A 49 309). It is proposed that the differences between the two sets of data are due to higher concentrations of hydrogen impurity in the samples of Weiler and Mehrer. The experimental evidence indicates that the diffusion mechanisms associated with D(Ga) and D(Sb) both have two parallel mechanisms. For D(Ga) the native defects involved are the Frenkel pair, GaiVGa, and the Ga vacancy, VGa. For D(Sb) one mechanism is due to the defect pair SbiVGa and the second to either the vacancy pair VGaVSb or the triple defect VGaGaSbVGa. It is proposed that the mobilities of all these defects, excepting GaiVGa, are enhanced in the presence of hydrogen as an impurity in the GaSb lattice. On this basis the differences in the data obtained by Bracht et al and Weiler and Mehrer can be reconciled. It is also shown that measured free hole concentrations identify Ga2−Sb as the residual acceptor in GaSb and that undoped GaSb is intrinsic at diffusion anneal temperatures.

Investigation of electronic properties of space-charge quantum wires

We investigated the electronic structure of space-charge quantum wires produced through attachment of Si donors in misorientation steps of semiconductor vicinal surfaces of GaAs, by means of density functional theory, taking into account of residual acceptors. The coupled Schrödinger and Poisson equations were self-consistently solved in the frame of Thomas–Fermi, Hartree and local density approximation, respectively. We found that the one-dimensional electron gas strongly depends upon the dopant concentration and residual acceptors. A presence of residual acceptors in GaAs makes it impossible to grow δ-doped quantum wires.

Effect of residual acceptors on electron mobility in single asymmetric quantum wells

We present the results of the calculation of the low-temperature electron mobility in one-side modulation-doped single asymmetric quantum wells, taking into account the presence of residual impurities. The effects of ionized acceptors were included in the variational calculation within the depletion approximation. All relevant scattering rates were taken into account, namely acoustic phonons, ionized impurities (remote and background), alloy disorder and interface roughness. The mobility dependencies on the electron gas density and residual acceptor concentration were found and we show that the background impurity scattering rate is the main scattering mechanism for acceptor concentrations above ∼3×10 15 cm −3 , in good agreement to recent experimental works involving high-quality GaAs/AlGaAs modulation-doped quantum wells.

Origin of the residual acceptor ground-state splitting in silicon.

The residual ground-state splitting of acceptors in high-quality silicon has been studied intensely by different experimental techniques for several decades. Recently, photoluminescence studies of isotopically pure silicon revealed the ground-state splitting to result from the random distribution of isotopes in natural silicon. Here we present a new model that explains these surprising experimental results, and discuss the implications for acceptor ground-state splittings observed in other isotopically mixed semiconductors, as well as for the acceptor ground state in semiconductor alloys.