Shallow Donor Levels Research Articles

Synthesis and photoluminescence of ZnS:Cu nanoparticles

The room-temperature photoluminescence (PL) of copper doped zinc sulfide (ZnS:Cu) nanoparticles were investigated. These ZnS:Cu nanoparticles were synthesized by a facile wet chemical method, with the copper concentration varying from 0 to 2 mol%. By Gaussian fitting, the PL spectrum of the undoped ZnS nanoparticles was deconvoluted into two blue luminescence peaks (centered at 411 nm and 455 nm, respectively), which both can be attributed to the recombination of the defect sates of ZnS. But for the doped samples, a third peak at about 500 nm was also identified. This green luminescence originates from the recombination between the shallow donor level (sulfur vacancy) and the t(2) level of Cu2+. With the increase of the CU2+ concentration, the green emission peak is systematically shifted to longer wavelength. In addition, it was found that the overall photoluminescence intensity is decreased at the Cu2+ concentration of 2%. The concentration quenching of the luminescence may be caused by the formation of CuS compound. (c) 2005 Elsevier B.V. All rights reserved.

Silicon Doped with Sulfur as a Detector Material for High Speed Infrared Image Converters

The work aims at approaching the solution of the problem of developing sensitive silicon detectors for high speed IR imaging devices which are semiconductor – gas discharge systems. Among the requirements to detectors is their operation at the temperature which is somewhat higher than that of liquid nitrogen. To meet this requirement, a set of deep impurities is analyzed. It is emphasized that silicon doped with sulphur is a good choice to reach the aim. The doping of silicon with sulphur is done by the technique of high temperature diffusion. Data of the Hall measurements indicate the presence of large density of shallow donor levels in the material obtained. To compensate them, acceptors are introduced afterwards with using radiation doping techniques. Testing of the fabricated detectors in the converter setup shows that they provide imaging of IR fields of temperature T ~ 270 0C with the temporal resolution in the order of 3 µs.

Impurity levels in the layered semiconductor p‐GaSe doped with group V elements As, Bi and Sb

AbstractThe radiative and non radiative recombination mechanisms in the As, Sb and Bi‐doped GaSe have been investigated on the basis of photoluminescence (PL) and Hall effect measurements. The PL features (at 77 K) related to the impurity levels coming from the As, Sb and Bi atoms are dominated by a broad emission band at about 1.7 eV. From the temperature dependences of the peak energy and PL intensity and the dependence of excitation intensity of peak energy, it was found that the 1.7 eV emission band is due to the transition from the shallow donor level at about 0.08 eV below the conduction band to the deep acceptor. In addition it was found, from the temperature dependence of hole concentration, that a deep acceptor level at about 0.6 eV above the valence band is formed by the doping atoms. It is associated with defects or defect complexes. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The origin of n-type conductivity in undoped In2O3

This study explores the origin of the native donor in undoped In2O3. The electronic structure of various point defects in In2O3 clusters is studied using the first-principles molecular orbital calculation. The results show that an oxygen vacancy cannot act as a native donor, because the defect level formed is much lower than the bottom of the conduction band. However, interstitial indium can generate a shallow donor level, close to the conduction band, and an even shallower donor level is formed when it associates with an oxygen vacancy. It is concluded that the origin of the native donor in undoped In2O3 is interstitial indium, but also that the existence of an oxygen vacancy is absolutely essential for carrier generation.

Hydrogen-containing donors in silicon: Centers with negative effective correlation energy

The reconstruction of shallow-level hydrogen-containing donors in Si is studied. The donors are formed by implantation of low-energy (300 keV) hydrogen ions into the experimental samples and subsequent heat treatment at 450°C. The experiments are carried out for Ag-Mo-Si Schottky diodes and diodes with a shallow (∼1 μm) p+-n junction. The concentration and distribution of the donors are determined by applying the method of C–V characteristics at a frequency of 1.2 MHz. An analysis of the temperature dependence of the equilibrium electron concentration shows that the reconstruction of the hydrogen-containing donors can be described under the assumption of recharging of a center with negative effective correlation energy (U < 0). The transformation between two equilibrium configurations of a double hydrogen donor (D B ++ ↔ D A 0 ) proceeds with the Fermi level position EF = E c − 0.30 eV. The reconstruction of the donors from a neutral to a doubly charged state (D A 0 → D B ++ ) which is stimulated by the capture of minority carriers, is observed at room temperature.

The effect of neutron irradiation on the properties of n-InSb whisker microcrystals

The results of studying the effect of irradiation with fast neutrons in an IBR-2 reactor on the characteristics of magnetic-field sensors based on n+-InSb whisker microcrystals are reported (the measurements were carried out in the course of the irradiation). The optimum concentration of free electrons n for providing the highest possible radiation resistance of the InSb sensors is estimated (n ≈ (6–7) × 1017 cm−3). The contributions of two competing processes to variation in the electrical properties of InSb under neutron irradiation (transmutation-related doping of InSb with a Sn shallow-level donor impurity and compensation of the initial n+-InSb conductivity as a result of generation of deep-level acceptor-type radiation defects) are determined separately.

Low-temperature instabilities of the electrical properties of Cd0.96Zn0.04Te:Cl semi-insulating crystals

The electrical properties in the temperature range 295–430 K and low-temperature (4.2 K) photoluminescence of Cd1−xZnxTe:Cl semi-insulating crystals grown from melts with a variable impurity content (CCl0 = 5 × 1017–1 × 1019 cm−3) are investigated. Nonequilibrium processes leading to a decrease in carrier concentration are observed in all the samples at low temperatures (T = 330–385 K). These changes are reversible. The activation energy of these processes Ea is found to be 0.88 eV. As with semi-insulating CdTe:Cl, the observed phenomena can be explained by a change in the charge state of background copper atoms: CuCd ↔ Cui. The introduction of Zn changes the ratio of the concentrations of shallow-level donors Cui and ClTe from their levels in the initial material.

Unintentionally doped n-type Al0.67Ga0.33N epilayers

Unintentionally doped Al0.67Ga0.33N epilayers were grown on AlN∕sapphire templates by metalorganic chemical vapor deposition. Optimized undoped Al0.67Ga0.33N epilayers exhibited an n-type conductivity as confirmed by Hall-effect measurement with a room-temperature resistivity of about 85Ωcm. Variable temperature Hall-effect measurement revealed the existence of a shallow donor level with activation energy of about 90meV. The photoluminescence (PL) spectra exhibited an emission peak at 4.13eV (4.06eV) related to an impurity transition at 10K (300K). Temperature dependent PL measurement also confirmed the existence of a shallow donor with comparable activation energy as that obtained by Hall-effect measurement. Isolated oxygen impurities are believed to be a strong candidate of the donor that remains as a shallow state in AlxGa1−xN up to x∼0.7. Compensating defects and the nature of the O donor in Al0.67Ga0.33N epilayers are also discussed.

Preparation and photoluminescence of ZnO nanostructures by thermal evaporation growth without catalysts

ZnO nanostructures have been grown on p-type Si(1 0 0) and glass substrates by thermal evaporating pure zinc powder without any metal catalysts at 450 or 480 °C. The microstructures of the samples under SEM show nanowires, nanorods and feathers-like morphologies. XRD reveals a preferred orientation of the well-aligned nanorods grown on silicon substrates at 480 °C. And all kinds of grown nanostructural samples are hexagonal ZnO crystal. The growth mechanism of synthesized nanostructures is discussed. Temperature and substrate are critical experimental parameters for the formation of different morphologies of ZnO nanostructures. Room-temperature photoluminescence spectrum of the well-aligned nanorods shows a blue emission band at 420 and 444 nm. The blue luminescence is due to the electrons transition from the shallow donor levels of oxygen vacancies to the valence band.

Electron spin resonance investigation of impurity and intrinsic defects in Nb-doped BaTiO3 single crystal and ceramics

Electron spin resonance (ESR) spectra of impurity and intrinsic defects have been measured at 4.2–295 K in Nb-doped BaTiO3 single crystals and ceramics to clarify their role in the conductivity and positive temperature coefficient of resistivity (PTCR effect). The measurements revealed a small amount of Fe3+, Cr3+, and Mn2+ impurities, which change their valence state (for example, Cr5+→Cr3+, Mn4+→Mn2+) with increased Nb concentration due to the compensation of the excess charge of Nb5+ ions. Besides the 3d-metal impurities, several types of Ti3+ polaronic and possible fluctuon states, where electrons can be localized near ferroelectric domain boundaries, have been revealed as well. All of them are associated with Ti3+ lattice ions and not with Nb5+∕4+ impurity, which apparently represents rather a very shallow donor level. The data obtained strongly support the polaronic origin of Nb-doped BaTiO3 conductivity at T&amp;lt;300K. Comparative investigations of ESR spectra in single crystals and ceramics of the same kind of BaTiO3 together with computer simulation allowed us unambiguously to ascribe complex ESR signals observed in ceramic samples to Cr3+, Mn2+, and Fe3+ ions and Ti3+ polarons and/or fluctuons. The role of manganese ions at grain boundaries in the PTCR effect is discussed as well.

Effect of fluorine doping on structural, electrical and optical properties of ZnO thin films

We studied effect of fluorine doping on ZnO thin films prepared using chemical spray pyrolysis technique. Structural, electrical and optical characterization techniques were used for the study. Both doped and undoped films showed preferential orientation along (0 0 2) plane. Optical absorption studies indicated a decrease of energy band gap and optical transmission increased with increase in fluorine doping. PL spectrum of undoped sample has a single broad peak at 517 nm while three additional peaks appeared for fluorine-doped film. Electrical resistivity and photosensitivity decreased considerably on doping. Temperature dependent conductivity studies revealed that prominent shallow donor levels are Zn in interstitial and regular lattice positions.

Photoluminescence investigations on a native donor in ZnO

ABSTRACTThe shallow donor impurities in ZnO with binding energies between 46 and 56 meV have been studied in great detail in the recent years. They give rise to neutral donor bound exciton recombinations with the A- and B-valence bands, show rotator states and two-electron-satellite transitions. These properties allowed to establish the excited state splittings of the donors as well as confirming Hayne's rule in ZnO. So far they all seem to be of extrinsic origin, hydrogen, aluminum, gallium and indium in order of increasing binding energy. For many years it was common sense that intrinsic defects would dominate the n-type conductivity of ZnO. Interstitial zinc as well as oxygen vacancies should be double donors, and in order to contribute to the n-type-conduction they should have shallow levels, and low formation energies to be abundant. In PL-measurements at T∼100 K on various ZnO samples, single crystals as well as thin films, a luminescence around 3.31 eV was detected. Due to its line shape and temperature behaviour it is identified as bound-to-free recombination. If we assume that the 3.31 eV band with its level at EC ∼ 130 meV is the ++/+ level of the zinc interstitial we calculate for the binding energy of the +/0 level ∼ 130 meV, i.e. around 33 meV. Undoped Zn-rich epitaxial films grown by CVD show a dominant I3 recombination at 3.367 eV which according to Haynes rule is consistent with a shallow donor level at 33 meV. Moreover, they have free n-type carrier densities of 2×1019 cm−3 and as revealed by SIMS the common donor impurities (Al, Ga, In) cannot account for the high carrier densities.

Interaction of hydrogen with boron, phosphorus, and sulfur in diamond

The production of $n$-type doped diamond has proved very difficult. Phosphorus, and possibly sulfur, when in substitutional sites in the lattice, forms a donor which could be used in electronic devices. Boron, which is a relatively shallow acceptor, can be passivated by hydrogen, and it is possible that some of the difficulties in producing electrically active donors could be due to their passivation by the hydrogen which is present during the chemical vapor deposition growth of diamond. We report ab initio modeling of these dopants and their complexes with hydrogen in diamond and show that it is energetically favorable for hydrogen to be trapped and to passivate boron and phosphorus. We predict that sulfur with one hydrogen atom produces shallow donor levels in the band gap of diamond with a previously unconsidered configuration being the most stable and producing the shallowest level. We show that the S-H pair is stable under conditions of limited H availability. Further, we show that it is energetically favorable for both P-H and $\mathrm{P}\text{\ensuremath{-}}{\mathrm{H}}_{2}$ to dissociate forming $\mathrm{H}_{2}{}^{*}$. H diffusion in $n$-type P-doped diamond is inhibited by this formation of immobile $\mathrm{H}_{2}{}^{*}$. This is in contrast to B-doped diamond, where we predict that $\mathrm{H}_{2}{}^{*}$ will dissociate in the presence of substitutional B atoms, forming B-H complexes. We demonstrate that the recently observed shallow $n$-type conductivity is unlikely to arise from $\mathrm{B}\text{\ensuremath{-}}{\mathrm{D}}_{2}$ complexes, because these complexes would dissociate into B-D plus a distant deuterium interstitial. We also predict that they would induce deep levels in the band gap.

Preparation and photoluminescence of surface N-doped ZnO nanocrystal

ZnO nanoparticles doped with nitrogen on surface were prepared by calcinating pure ZnO nanoparticles at 550 and 600 °C in NH 3 atmosphere. Uniform N-doped ZnO nanocrystal was characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and XPS. A Strong violet photoluminescence (PL) at 400nm was observed at room temperature when excited with 300 nm light, and the emission peak increases with the increase of nitrogen atoms concentration. The violet PL originated from the electron transition from shallow donor levels of oxygen vacancies and doping nitrogen atoms to the top of valence band level.

The role of trapping levels of nonequilibrium electrons during the formation of pinning centers for domain walls in the magnetic semiconductor CdCr2Se4

A complex study of electrical conductivity and photoconductivity in constant and variable electric fields, thermally stimulated conductivity, and the photoferromagnetic effect are carried out in the temperature range 10–300 K for a CdCr2Se4 magnetic semiconductor with various concentrations of Ga impurity and Se vacancies (V Se). The phenomenon of hopping photoinduced conduction is observed for the first time. It is shown that shallow donor levels that exchange photoelectrons trapped by them with Cr3+ magnetic ions may be responsible for the photoferromagnetic effect. As a result of this exchange, the Cr3+ ions acquire valence and spin instabilities, which lead to nonequilibrium pinning of the domain wall by these ions.

High Phonon-Drag Thermoelectric Efficiency of SiC at Low Temperatures

Results confirming the high values of Seebeck coefficient phonon-drag component Sph in SiC are discussed and the relevant consequences to the thermoelectric figure-of-merit ZT=T S 2  / are studied. The fact that Sph increases at low temperatures as T 2.4 similarly for low-doped p-6H- SiC, Si and Ge is demonstrated. Materials comparison criterion Sph  / meff, based on heat conductivity, lattice mobility and carrier effective mass is offered and Sph strength proportions 0.1 : 1 : 5 : 10 for Ge, Si, SiC and C (diamond) are proposed. Related to that high T500K phonon- drag domination limit is obtained for SiC. It is shown that the proportionality Sph , yielding ZT , could reverse some concepts accepted in thermoelectric material development. The best-case calculations predict good (>0.1) and excellent (>1) ZT values with steep temperature-dependence T 3…4 for low temperatures 50200K if sufficiently high electrical conductivity  could be ensured. The last condition may be feasible for n-SiC with relatively shallow donor levels but difficult to achieve in the case of p-SiC with relatively deep acceptor levels.

Observation of a Be-correlated donor state in GaN

A Be-related donor level was identified in the band gap of GaN. Thermal admittance spectroscopy (TAS) was combined with the radiotracer principle by applying the radioactive isotope Be7 which was implanted into n-type and p-type GaN. TAS spectra of n-type GaN recorded, repeatedly, during the elemental transmutation of 7Be to 7Li reveal one shallow donor level undergoing concentration changes correlated to the radioactive decay 7Be→7Li;T1/2=53.3d). From this, a relation is deduced between Be and this level at 390 meV below the conduction band edge of GaN. Furthermore, the implantation of 7Be was observed to enhance the hole concentration significantly in Mg-doped p-type GaN. A Be-correlated acceptor state was not detected.

Majority-Carrier Lifetime in the Potential Barrier of an MIS Capacitor under Monopolar Conditions

A theoretical study is reported of the lifetime–potential relationship for nonequilibrium, photogenerated carriers in the space-charge region of a semiconductor. The problem is addressed in the context of an MIS capacitor operated under monopolar conditions, the semiconductor being doped with deep-level acceptor impurity and partly compensated with shallow-level donor impurity. The temperature is taken to be so low that impurity atoms are weakly ionized. The magnitude and depth profile of carrier lifetime in the potential barrier are examined as functions of constant capacitor voltage. The major findings are the following: (i) The lifetime is voltage-selective in that the lifetime–surface-potential characteristic exhibits a relatively sharp peak when the capacitor is operated near the flat-band condition. (ii) The lifetime is maximal when the surface potential is negative (forward bias), the maximum lifetime being independent of the compensating-impurity concentration. (iii) With fixed impurity concentrations, the point of maximum lifetime moves away from the surface as the surface potential increases in absolute value.

Optical and electric properties of Nb-doped anatase TiO 2 single crystal

Nb-doped anatase TiO 2 single crystal has been grown by chemical vapour transport method. Raman spectra shows that the obtained crystal with Nb of 0.08 wt% has typical anatase structure. An absorption band was observed at around 2.2 eV, which seems to be due to the d–d transition in the conduction band. The electron paramagnetic resonance and electric resistivity measurements show that the doped niobium makes quite shallow donor level whose orbital is d xy -like centered at the titanium position of anatase.

Shallow donor formation in phosphorus-doped ZnO thin films

ZnO is a direct band gap semiconductor that is attractive for a number of applications, including high-temperature electronics, transparent electronics, and blue to UV optoelectronics. The most significant impediment to the exploitation of ZnO is the difficulty in achieving effective p-type carrier doping. Attractive acceptor-state candidates for ZnO are the group-V elements substituted on the oxygen site. In this paper, the doping behavior of phosphorus in as-deposited ZnO crystalline thin films grown by pulsed-laser deposition is reported. The experimental work reported here indicates that phosphorus doping significantly increases the electron carrier density in as-deposited films, making the material heavily n-type. These results indicate the formation of a shallow donor level associated with the phosphorus impurity, resulting from either substitution on the Zn site or formation of a compensating complex. Based on X-ray photoelectron spectroscopy measurements, the phosphorus-dopant ions in the films occupy multiple sites yielding oxidation states of -3 and +5. This would explain the compensated n-type behavior for group-V doping in the II-VI host material.