negative-U Defects Research Articles

Theory of the Thermal Stability of Silicon Vacancies and Interstitials in 4H–SiC

This paper presents a theoretical study of the electronic and dynamic properties of silicon vacancies and self-interstitials in 4H–SiC using hybrid density functional methods. Several pending issues, mostly related to the thermal stability of this defect, are addressed. The silicon site vacancy and the carbon-related antisite-vacancy (CAV) pair are interpreted as a unique and bistable defect. It possesses a metastable negative-U neutral state, which “disproportionates” into VSi+ or VSi−, depending on the location of the Fermi level. The vacancy introduces a (−/+) transition, calculated at Ec−1.25 eV, which determines a temperature threshold for the annealing of VSi into CAV in n-type material due to a Fermi level crossing effect. Analysis of a configuration coordinate diagram allows us to conclude that VSi anneals out in two stages—at low temperatures (T≲600 °C) via capture of a mobile species (e.g., self-interstitials) and at higher temperatures (T≳1200 °C) via dissociation into VC and CSi defects. The Si interstitial (Sii) is also a negative-U defect, with metastable q=+1 and q=+3 states. These are the only paramagnetic states of the defect, and maybe that explains why it escaped detection, even in p-type material where the migration barriers are at least 2.7 eV high.

Light-induced-degradation defect independent of the boron concentration: Towards unifying admittance spectroscopy, photoluminescence and photoconductance lifetime spectroscopy results

Recent photoluminescence and admittance spectroscopy measurements point towards the defect responsible for Light-Induced Degradation (LID) being a shallow level. Dopant dependent lifetime spectroscopy and temperature dependent lifetime spectroscopy on the other hand reveal the existence of a deep level driving the recombination activity of the LID defects. There is an apparent disagreement between admittance spectroscopy results and lifetime spectroscopy results. Here we overcome this apparent disagreement and we show that a negative-U defect with one shallow level can explain both the doping dependent lifetime data and injection dependent data. The critical consequence of this model is that it readily explains the dependence of the LID defect on the hole concentration rather than the Boron concentration.

Kinetics of the persistent photocurrent in semiconductors: a case example for amorphous chalcogenides

Although the persistent photocurrent or the residual photocurrent decay (RPD) in photoconductive semiconductors is known to be dominated by dispersive recombination kinetics which produce the stretched exponential functional decay, exp(-Ctβ), where β (<1.0) is the dispersion parameter, the reason for the occurrence of the RPD is still not clear. We discuss the origin of the dispersive nature of long-term photocurrent decay in amorphous chalcogenides (a-Chs) as a case example. The slow recombination path of excess photocarriers in a-Chs can be attributed to their characteristic feature of negative-U defects at which lattice relaxation with multiphonon process may play a role.

Deep-Level Defects in High-Dose Proton Implanted and High-Temperature Annealed Silicon

We review the principal mechanisms of deep-level defect formation after proton irradiation and subsequent annealing of float zone grown crystalline silicon. Various reaction paths commonly related to vacancy-complexes, interstitial oxygen and hydrogen are discussed. In the experimental part DLTS results of proton-irradiated pn-diode structures are presented. It appears that some detected defects show a thermal stability differing from that reported in literature. Furthermore the metastability of two defects at 300 meV and 418 meV below the conduction band is examined. This behavior commonly designated to a negative-U defect is for the first time reported in a sample annealed at 350° C.

Deep-Level Defects in High-Dose Proton Implanted and High-Temperature Annealed Silicon

Proton-implantations are valued for their ability to modify the properties of semiconductor grade silicon over a wide spatial range spanning four orders of magnitude between a depth of about 100 nm and 1 mm at comparatively low thermal budgets. At moderate proton doses up to about 1×1013 cm-2 and low annealing temperatures at about 300 °C, proton implantations are used to tailor the carrier life-times in bipolar power semiconductor devices. A large number of deep-level defects are known to form in this parameter regime. At higher doses and higher temperatures proton implantations may be used for modifying the doping profile of the silicon substrate by the creation of hydrogen-related thermal donors. While a large number of studies have investigated deep-level defects induced in the parameter regime typical to life-time tailoring, only few are concerned with the deep-level defects remaining in the samples.In this report we give a brief review of the principal mechanisms of deep-level defect formation after proton irradiation and subsequent annealing of float zone-grown crystalline silicon. Various reaction paths commonly related to vacancy-complexes, interstitial oxygen and hydrogen are discussed. In the experimental part of the study, new DLTS measurements of proton-irradiated pn-diode structures are presented. The study focusses on the regime of elevated doses and annealing temperatures, typically used for proton-implantation doping. The results suggest that for the increased doses used, some of the detected defects show a thermal stability differing from that reported in literature. At least for some of these defects, this may be due to their higher initial concentration. In addition, other defects may appear at higher temperatures as decay products of other, thermally more stable, defects induced by the high-dose implantations.For the first time, we also report on two defect levels at 300 meV and 418 meV below the conduction band with a metastable behavior. The observed levels and their behavior show a high similarity to a negative-U defect known from samples implanted at 88 K without any further thermal budget above room temperature. To our knowledge, these levels have not been detected in a sample annealed above room temperature before.The figure depicts possible reaction paths of vacancies in crystalline silicon under the presence of extrinsic hydrogen and oxygen. Many of these defects lead to localized levels in the silicon bandgap and can therefore impact the electrical properties of a device. The actual appearance of a defect is however determined by the activation energy for its formation together with the applied process temperatures. If known from literature, the activation energy for each reaction path is indicated.

Long range coupling between defect centres in inorganic nanostructures: Valence alternation pairs in nanoscale silica

Valence alternation pair (VAP) states are formed by a closed-shell combination of two space- and charge-separated topological defect centres. These pairs of defects, although historically invoked to explain the electronic properties of bulk inorganic glassy materials (e.g., amorphous silicon dioxide) via the concept of negative-U defects, have more recently been found in a number of theoretical studies of silica surfaces and nanoscale silica clusters. Using density functional theory we systematically probe the structure and internal stability of VAPs in a number of silica nanoclusters with respect to the separation of the two constituent defect centres. We find that VAP states in nanosilica are strongly stabilised by the attractive electrostatic interaction between their separated oppositely charged component defects such that VAPs can persist up to an internal separation of a least 1.5 nanometres. Beyond this distance VAPs become unstable with respect to an open-shell combination of topological defects, virtually indistinguishable from two isolated open-shell defect centres. Finally, we theoretically analyse the possibility of experimental observation of VAP states through their infra-red vibrational spectra.

Shallow levels in virgin hydrothermally grown n-type ZnO studied by thermal admittance spectroscopy

Three n-type single crystal hydrothermally grown ZnO samples with resistivities of 5.1±0.6, 15±2 and 220±20 Ω cm, respectively, have been electrically characterized using thermal admittance spectroscopy (TAS). The presence of three main donors: two shallow ones D1 and D2 and a deeper one D3 with activation energies of ∼30, ∼50 and ∼290 meV, respectively, are detected. In addition, the TAS spectra reveal the presence of a fourth level, DX, with a peak amplitude in the conductance spectra that decreases with the temperature occurrence. It is shown that this anomalous behavior is consistent with DX being a negative-U defect of donor-type. An activation energy of ∼80 meV for the ++/+ transition, a capture cross section equal to ∼3×10−17 cm2 and an energy barrier for atomic reconfiguration of ∼0.25 eV, respectively, deduced according to the assignment of DX to a negative-U defect. A tentative assignment of the DX defect with oxygen vacancies is discussed.

Persistent phenomena in the electrical characteristics of solar cells based on Cu(In,Ga)S2

AbstractCurrent‐voltage and capacitance‐voltage characteristics of ZnO/CdS/CuInGaS2 solar cells have been measured in order to investigate persistent changes induced by the reverse bias stress or light soaking. Significant metastable fill factor loss after red illumination under reverse bias condition has been observed. We show a correspondence between relaxation times of the capacitance and photovoltaic parameters after all treatments proving that defects in the absorber are responsible for the changes. Similar phenomena have been observed in Cu(In,Ga)Se2 based devices, therefore we propose similar explanation – negative‐U defects in Cu(In,Ga)S2 which create a highly negatively charged layer close to the interface. They are responsible also for a difference between current transport in the dark and under illumination. Moreover, these defects seem to be a source of an limitation of the open circuit voltage in these devices. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Understanding defect-related issues limiting efficiency of CIGS solar cells

Many electrical characteristics of Cu(In,Ga)Se 2-based solar cells, including current–voltage characteristics, are affected by specific properties of negative-U defects in the absorber. We present these characteristics and discuss them in the framework of a V Se– V Cu defect model proposed by Lany and Zunger. We show how these defects influence photocarrier transport and the dominant recombination mechanism, and hence also the photovoltaic parameters of the cells. Numerical simulations validating our approach will also be presented.

Theoretical Investigations of Complexes of p-Type Dopants and Carbon Interstitial in SiC: Bistable, Negative-U Defects

Interaction of boron and aluminum with interstitial carbon is studied using first principles calculations. It is shown that carbon can form very stable complexes with Al and B, forming a family of negative-U bistable defects with deep levels. The influence of this effect on the activation rate of p-type implants is discussed.

Defect levels in the band gap of amorphous selenium

Energy locations in the band gap have been determined for the thermally accessible levels of the negative-U defect centers in amorphous selenium. Both the temperature dependence of the steady-state photocurrents, and an analysis of emission currents in the post-transit regime of a time-of-flight transient photoconductivity experiment on the same samples, agree on the presence of defect levels at (0.42 ± 0.04) eV above the valence band mobility edge and (0.53 ± 0.06) eV below the conduction band. Both measured current levels and the resolved energy positions of the defects are subject to the Poole–Frenkel effect.

Distribution of gap states in amorphous selenium thin films

Post-transit photocurrent analysis, based on the time-of-flight transient photoconductivity technique, was successfully carried out for a series of amorphous selenium $(a\ensuremath{-}\mathrm{Se})$ thin films. The method allows a determination of the density of gap states beyond the shallow tail states from the emission-dominated post-transit currents. Prominent hole and electron traps were resolved some 0.4--0.5 eV above the valence-band edge and 0.55--0.65 eV below the conduction-band edge. These two traps represent the thermally accessible levels of the ${D}^{+}$ and ${D}^{\ensuremath{-}}$ intrinsic negative-U defects in $a\ensuremath{-}\mathrm{Se}.$ The tail end of the transient currents and an apparent temperature dependence below 0 \ifmmode^\circ\else\textdegree\fi{}C of the resolved densities can be interpreted as indications for a further set of hole and electron traps close to the Fermi level, but experimental uncertainties and a simulation of the temperature dependence show that such assignment remains questionable.

Some Peculiarities During Creation and Destruction of the Native Defects with the Negative Correlation Energy in Semiconductors

AbstractThe creation of negative-U defects (NUD) is a spin-dependent process, that could be used for getting information about NUD themselves; and the NUD destruction is accompanied by the Coulomb’s repulsion between charge carriers.

Electron paramagnetic resonance investigation of charge trapping centers in amorphous silicon nitride films

We have explored the nature of the silicon dangling-bond center in amorphous hydrogenated silicon nitride (a-SiNx:H) thin films, and its relationship to the charge trapping centers using electron paramagnetic resonance (EPR) and capacitance-voltage (C-V) measurements. We have investigated the quantitative relationship between the concentration of silicon dangling bonds using EPR and the concentration of charge traps, measured by C-V measurements, for both UV-illuminated and unilluminated a-SiNx:H thin films subjected to both electron and hole injection sequences. A theoretical framework for our results is also discussed. These results continue to support a model in which the Si dangling bond is a negative-U defect in silicon nitride, and that a change in charge state of preexisting positively and negatively charged Si sites is responsible for the trapping phenomena observed in these thin film dielectrics.

Atomic and molecular hydrogen in gallium arsenide: A theoretical study.

We present first-principles calculations of the properties of atomic and molecular hydrogen in pure bulk GaAs. Our results indicate that H penetrates into GaAs in atomic form. Inside GaAs, atomic H tends to form ${\mathrm{H}}_{2}$ molecules in tetrahedral sites, which are deep energy wells for ${\mathrm{H}}_{2}$. The ${\mathrm{H}}_{2}^{\mathrm{*}}$ defect, formed by one H in a bond-center site and one H in an adjacent tetrahedral position, has higher energy than ${\mathrm{H}}_{2}$ but lower-energy barriers for diffusion. Isolated H could be present as a metastable species. We compute the stable charge state of isolated H as a function of the Fermi energy. Our results suggest that H behaves as a negative-U defect. As a consequence, isolated H is expected to be present only as a charged species (positively charged in p-doped samples, negatively charged in undoped and n-doped samples). Our conclusions are compared with experimental results and with the results of calculations for H in other semiconductors. The main features of H in GaAs are quite similar to what has been found in Si.

The Impact of a Long-Ranged Random Potential on the Transport Properties of Amorphous Semiconductors

ABSTRACTWe discuss electrostatic random potentials in doped and in compensated amorphous semiconductors. These potentials are caused by the residual inhomogeneity of a random distribution of charged dopants and their compensating charges. Random potentials are also present in undoped material with negative-U defects. A high density of positive-U defects can also give rise to a random potential in undoped material.We demonstrate with the help of detailed model calculations the effect of such random electrostatic potentials on the transport properties. For transport in extended states the random potential does not give rise to a mere shift of the mobility edge. Instead several new features are observed: the activation energy of the resulting Ohmie dc conductivity is virtually unaffected by the random potential in contrast to the activation energy of the thermoelectric power and that of the Hall effect, respectively. The Ohmie dc current changes at high fields into a superlinear current. The random potential contributes to the dispersion of the transients in time-of-flight experiments but leaves the field dependence of the TOF mobility unaltered. Comparing our results with experimental data we discuss under which circumstances the effect of random potentials can be identified.

Ionization and capture kinetics of DX centres in AlGaAs and GaSb: approach for a negative-U defect

In this paper the consequences of an idea in which DX centres in semiconductors form a negative-U system for the approach to commonly used DLTS and photoionization measurements are discussed. For the negative-U model of DX centres to be valid the carrier exchange between the DX states and the conduction band must occur via an intermediate one-electron D0 state. In such a system the neutral D0 state must be thermodynamically unstable, but obviously should play a role in all carrier capture and emission processes (D- from or to D0+e- from or to D++2e-). The experimental evidence for the existence of such an intermediate state is presented. It is based upon the detailed observations of the temperature evolution of the photoionization transients of the DX centres in AlxGa1-xAS:Te as well as the capacitance transients of the thermal emission process from DX centres in GaSb:S. Rate equations for photoionization and isothermal DLTS experiments on a defect forming a negative-U system are presented.

Possible negative-U properties of the cation vacancy in HgCdTe

The cation vacancy in HgCdTe is commonly used for p-type doping, but the energy levels of the vacancy acceptor are not known. We propose that the vacancy is a negative-U center, with inverted energy levels. Comparison with the vacancy in silicon,which is a well-characterized negative-U defect, indicates that the HgCdTe vacancy is likely to be a negative-U center also. The minority carrier recombination rate involving gap levels of the vacancy will show a quadratic dependence on the vacancy concentration, and the deep levels of a negative-U vacancy may be particularly suitable as recombination centers due to their location near the middle of the gap.

Charge carrier statistics of disordered semiconductors with negative-u defects and electrically active impurity atoms

It is shown that the temperature dependences of conductivity of disordered semiconductors: As 2Se 3 doped by Ni and Bi, and a-Si:H doped by E u, may be explained by statistical interaction between approximately equal numbers of negative-U intrinsic defects and electrically active impurity atoms.

Kinetics of the metastable optically induced ESR in a-Si:H.

The kinetics of a metastable, optically induced increase in the ESR in a-Si:H have been examined between 77 and \ensuremath{\sim}500 K. Isothermal annealing (decay) and inducing (growth) curves indicate that the decay is bimolecular and thermally activated with an activation energy of \ensuremath{\sim}1 eV, and the growth is relatively temperature independent. These kinetics are more easily explained through the optical rearrangement of charge in existing dimagnetic (negative-U) defects and less easily in terms of the optical creation of additional, paramagnetic localized states in the gap.