Single Donor State Research Articles

Fe‐Related Defects in Si: Laplace Deep‐Level Transient Spectroscopy Studies

Herein, the electrical properties of Fe-related defects in hydrogenated n- and p-type Si doped with iron during the crystal growth in a vacancy mode are investigated. Six deep-level transient spectroscopy (DLTS) peaks labeled Fei, FeB, CH, E125FeH, E145Fe, and E240Fe are observed in as-grown n- and p-type Si after wet chemical etching (WCE). By comparing the electrical properties of the defects with those previously reported in the literature, Fei is correlated with interstitial iron, FeB with an iron–boron complex, and C–H with a carbon–hydrogen complex in Si. No traces of the peak previously assigned to the single donor state of FeiH are observed after reverse bias annealing at 110 °C in hydrogenated Si. By analyzing the depth profiles of the defects in n-type Si, it is shown that E125FeH comprises only one H atom. The electrical and annealing properties of E125FeH, E145Fe, and E240Fe are investigated and their origin is discussed.

Electronic states and optical properties of single donor in GaN conical quantum dot with spherical edge

In this paper we present a theoretical investigation of quantum confinement effects on the electron and single donor states in GaN conical quantum dot with spherical edge. In the framework of the effective mass approximation, the Schrödinger equations of electron and donor have been solved analytically in an infinite potential barrier model. Our calculations show that the energies of electron and donor impurity are affected by the two characteristic parameters of the structure which are the angle Ω and the radial dimension R. We show that, despite the fact that the reduction of the two parameters Ω and R leads to the same confinement effects, the energy remains very sensitive to the variation of the radial part than the variation of the angular part. The analysis of the photoionization cross-section corresponding to optical transitions between the conduction band and the first donor energy level shows clearly that the reduction of the radius R causes a shift in resonance peaks towards the high energies. On the other hand, the optical transitions between 1s−1p, 1p−1d and 1p−2s show that the increment of the conical aperture Ω (or reduction of R) implies a displacement of the excitation energy to higher energies.

Charged grain boundaries and carrier recombination in polycrystalline thin film solar cells.

We present analytical relations for the dark recombination current of a junction with positively charged columnar grain boundaries in the high defect density regime. We consider two defect state configurations relevant for positively charged grain boundaries: a single donor state and a continuum of both acceptors and donors. Compared to a continuum of acceptor+donor states, or to the previously studied single acceptor+donor state, the grain boundary recombination of a single donor state is suppressed by orders of magnitude. We show numerically that superposition holds near the open-circuit voltage , so that our dark relations determine for a given short circuit current . We finally explicitly show how depends on the these two grain boundary defect state configurations.

Identification of carbon‐hydrogen complexes in n‐ and p‐type silicon

AbstractThe origin of several deep traps E42, E90, E262, and H180 in hydrogenated n ‐type and p ‐type FZ and CZ Si is investigated. Comparing the depth profiles of these defects in samples with different C, O, H, and shallow donor (acceptor) concentrations we conclude that they belong to carbon‐hydrogen‐related defects consisting of one C and one H atom. The similar annealing behavior and identical depth profiles of E42 and E262 correlate them with two different charge states of the same defect. From a comparison with earlier calculations we attribute E42 to the double acceptor and E262 to the single acceptor state of the CH1AB complex. In good agreement with the results of previous studies E90 is assigned to the acceptor state of the CH1BC complex. Our preliminary Laplace DLTS studies on SiGe with a Ge content <5% show different local Ge environments for E90 and H180 in the nearest and second‐nearest neighborhood. We interpret this as an indication for a different origin of these defects. After annealing the samples under reverse bias at 320 K another CH‐related defect (CHB) can be detected by Laplace DLTS in n ‐type Si. Previously, this defect was often wrongly assigned to the single donor state of CH1BC. The field dependence of the emission rate of CHB identifies the level as a single donor state. However, the depth distribution of CHB differs clearly from that of E90. Our results on the carbon‐hydrogen complex give a conclusive explanation of previously reported controversial experimental data. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetism and metal-insulator transition in oxygen-deficientSrTiO3

First-principles calculations to study the electronic and magnetic properties of bulk, oxygen-deficient SrTiO$_3$ (STO) under different doping conditions and densities have been conducted. The appearance of magnetism in oxygen-deficient STO is not determined solely by the presence of a single oxygen vacancy but by the density of free carriers and the relative proximity of the vacant sites. We find that while an isolated vacancy behaves as a non-magnetic double donor, manipulation of the doping conditions allows the stability of a single donor state, with emergent local moments coupled ferromagnetically by carriers in the conduction band. Strong local lattice distortions enhance the binding of this state. The energy of the in-gap local moment can be further tuned by orthorhombic strain. Consequently we find that the free-carrier density and strain are fundamental components to obtaining trapped spin-polarized electrons in oxygen-deficient STO, which may have important implications in the design of optical devices.

Two different carbon-hydrogen complexes in silicon with closely spaced energy levels

An acceptor and a single donor state of carbon-hydrogen defects (CHA and CHB) are observed by Laplace deep level transient spectroscopy at 90 K. CHA appears directly after hydrogenation by wet chemical etching or hydrogen plasma treatment, whereas CHB can be observed only after a successive annealing under reverse bias at about 320 K. The activation enthalpies of these states are 0.16 eV for CHA and 0.14 eV for CHB. Our results reconcile previous controversial experimental results. We attribute CHA to the configuration where substitutional carbon binds a hydrogen atom on a bond centered position between carbon and the neighboring silicon and CHB to another carbon-hydrogen defect.

Identification of titanium-hydrogen complexes with up to four hydrogen atoms in silicon

Using high-resolution Laplace deep level transient spectroscopy studies, several TiH-related complexes (E40′, E170, E170′, and E260) were observed in wet-chemically etched and H-plasma treated n-type Si. We assign E40′ and E170 to two different configurations of Ti with one H atom. Both of them are shown to behave as single donor states with an activation enthalpy of EC − 0.07 eV (E40′) and EC − 0.34 eV (E170) in the upper half of Si. E170′ with an activation energy of EC − 0.37 eV is correlated with the donor state of the TiiH2 defect, whereas E260 is attributed to the donor state of TiiH3. Besides the TiH defects, the presence of electrically inactive TiH4 is reported. No titanium-hydrogen-related levels were observed in p-type Si.

Electrical levels in nickel doped silicon

Deep Level Transient Spectroscopy (DLTS) reveals three electrical levels of substitutional nickel in silicon at EC – 0.07 eV, EC – 0.45 eV, and EV + 0.16 eV. A number of additional DLTS peaks are observed after hydrogenation of the samples. We identify different NiHx -complexes with x = 1, 2, and 3. NiH introduces a single acceptor and a single donor state at about EC – 0.17 eV and EV + 0.49 eV into the band gap of silicon. NiH2 and NiH3 are shown to have a single acceptor state at EV + 0.58 eV and EV + 0.46 eV, respectively. In addition to the electrically active NiHx-complexes, a total passivation of the electrical activity of nickel by hydrogen is observed.

Density-functional electronic structure calculations for native defects and Cu impurities in CdS

We performed density-functional electronic structure calculations for wurtzite CdS with native defects and Cu impurity. We investigate formation energies and ionization levels of the defects in various charge states. Our results reveal that the S vacancy is a double donor with the strongly localized orbital at the defect site, and could act as an intrinsic compensation defect under $p$-type doping. On the other hand, the interstitial S at the tetrahedral site is a double acceptor which forms into a dumbbell-like atomic configuration with the nearest S atom. The impurity Cu substituting Cd produces a single acceptor state, while the interstitial Cu generates a single donor state. The calculated formation energies imply that the donor state could also cause the compensation in the Cu doped CdS.

Resonant tunnelling through single donor states in GaAs/AlAs/GaAs devices

Our paper is a first step towards tunnelling spectroscopy of individual X-minimum-related shallow donors. We investigated I– V characteristics of submicrometer mesa structures made on GaAs/AlAs/GaAs wafers δ-doped with silicon in the middle of AlAs layer. At 4.2 K and magnetic field up to 6 T we resolved well-separated peaks attributed to resonant tunnelling via individual donors.

Mesoscopic effects in resonant tunnelling diodes

We have investigated resonant tunnelling in GaAs/(AlGa)As heterostructures which have been fabricated into square mesas 6 × 6 μm. A δ-layer of donors ( n ∼ 2 × 10 9 cm −2) has been incorporated at the centre of the quantum well which is 9 nm wide. The I( V) characteristics show a feature at ∼70 mV, which is below the threshold for the main resonance and is due to resonant tunnelling through single donor states in the well. This feature is also present in large area mesas. AT lower biases and at low temperatures we see a new set of resonances which, although they occur in all small area mesas, differ in detail between devices with regard to their strength and bias position. The form of the low-bias structure is strongly dependent on temperature, T, below 4 K where several very sharp steps appear, becoming sharper as T is decreased. We have also investigated the dependence of the new structure on magnetic field, B, parallel to the current direction. We attribute the new features to tunnelling through potential fluctuations on the mesoscopic scale due to donor clustering.

Transport in sub-micron resonant tunnelling devices

We have investigated the I( V) characteristics of a sub-micron gated GaAs/ (AlGa)As resonant tunnelling diode. We find that, relative to a large area device, there is additional structure at both high and low source-drain bias. The low bias features are shown to be due to the presence of impurities in the quantum well. This is confirmed by experiments on δ-doped large area devices. Using the gate on the sub-micron devices we are able to control the current through a single donor state. The high voltage structure occurs within the main resonance and only in forward bias where the peak-to-valley ratio (PVR) is poor. We ascribe the poor PVR to a lateral variation of the resonance condition and discuss the high voltage structure in these devices, and in similar two-terminal devices, in the light of this hypothesis.

Zero dimensional resonant tunneling through single donor states

We have observed remarkable new structure in the source-drain I(V) characteristics of a symmetric double barrier resonant tunneling device in which the cross sectional area may be varied from ≅ 1μm 2 to ≅0.1 μm 2 by applying a voltage to a gate. In the source-drain voltage range close to, but slightly below, the threshold for current flow, peaks are observed with a peak value of ≅ 20 pA, and peak/valley ratio up to 10 1 . The structure in I(V) is independent of temperature between 35 mK and 10 K, and of gate voltage between 0 and −2 V. The first peak in I(V) in each polarity is unaffected by a magnetic field applied parallel to the current flow for fields up to 6 T. This structure cannot be explained by lateral quantisation or Coulomb blockade arising from the confinement. We propose that it is due to resonant tunneling between zero-dimensional states formed in the active region by the Coulombic potential of a single ionised donor, combined with the barrier potential.

Electric field effect on electron emission from the DX centre in GaAlAs

The effect of the electric field on the emission rate of electrons from the DX centre has been studied in Te-doped, liquid phase epitaxy Ga1-xAlxAs materials of different alloy composition x. The results obtained-a decrease of the ionization energy or an increase in the emission rate-show that this emission is sensitive to the Poole-Frenkel effect. This clearly demonstrates first that the potential which binds the electron on the DX centre is not the short-range one expected if the DX centre is a deep defect, and second that the DX centre is a single donor state.

Electronic state of A C-ls core exciton in diamond

Abstract The carbon-ls core exciton formation is diamond has been investigated within the framework of the local density theory associated to a molecular cluster model. By using the multiple-scattering technique to solve the one-electron Schroedinger equation it has been shown that the electron state in the exciton is rather delocalized and quite similar to the single donor state induced by a substitucional on center nitrogen impurity. Therefore a hydrogenic-effective-mass theory description of a carbon-ls core exciton in diamond is expected to be highly appropriate.