Midgap Defect States Research Articles

Ultrafast Carrier Dynamics of CdTe: Surface Effects

Cadmium telluride has been an important absorber material used in solar cells for decades given its near-optimum band gap and lower cost for device fabrication. However, the overall efficiency has been low compared to the theoretical limit. One major contributor to the problem of the relatively low open-circuit voltage is the high surface recombination of photogenerated carriers. In this contribution, time-resolved pump–probe reflectivity was used to study the carrier dynamics of CdTe(111) under the influence of selected surface conditions. It was found that surfaces after the processes of chemical etching and thermal oxidation for short durations exhibit bulk-like transient reflectivity and a clearly reduced surface recombination velocity, according to a model calculation. The comparative study indicates that the origins of carrier traps include the midgap defect states that are associated with the uncapped Te surface atoms, namely the Te antisites and Te interstitials, and subsurface damages resulting f...

Lifetimes of photogenerated electrons on a GaAs surface affected by nanostructural defects

The recombination dynamics of photogenerated carriers on temporal and spatial scales of 100 fs and 100 nm, respectively, on an inhomogeneous GaAs surface with structural defects was investigated using time-resolved photoemission electron microscopy with femtosecond laser pulses. The observed photocarrier lifetimes (τ) ranged from subpicoseconds to picoseconds at isolated structural defects and were inversely proportional to the photoemission intensity at each defect. We concluded that τ corresponds to the carrier trapping time to midgap defect states and estimated the density of the midgap states at each defect on the basis of the relation between τ and the photoemission intensity.

The Influence of Structure and Processing on the Behavior of TiO2 Protective Layers for Stabilization of n-Si/TiO2/Ni Photoanodes for Water Oxidation

Light absorbers with moderate band gaps (1-2 eV) are required for high-efficiency solar fuels devices, but most semiconducting photoanodes undergo photocorrosion or passivation in aqueous solution. Amorphous TiO2 deposited by atomic-layer deposition (ALD) onto various n-type semiconductors (Si, GaAs, GaP, and CdTe) and coated with thin films or islands of Ni produces efficient, stable photoanodes for water oxidation, with the TiO2 films protecting the underlying semiconductor from photocorrosion in pH = 14 KOH(aq). The links between the electronic properties of the TiO2 in these electrodes and the structure and energetic defect states of the material are not yet well-elucidated. We show herein that TiO2 films with a variety of crystal structures and midgap defect state distributions, deposited using both ALD and sputtering, form rectifying junctions with n-Si and are highly conductive toward photogenerated carriers in n-Si/TiO2/Ni photoanodes. Moreover, the photovoltage of these electrodes can be modified by annealing the TiO2 in reducing or oxidizing environments. All of the polycrystalline TiO2 films with compact grain boundaries investigated herein protected the n-Si photoanodes against photocorrosion in pH = 14 KOH(aq). Hence, in these devices, conduction through the TiO2 layer is neither specific to a particular amorphous or crystalline structure nor determined wholly by a particular extrinsic dopant impurity. The coupled structural and energetic properties of TiO2, and potentially other protective oxides, can therefore be controlled to yield optimized photoelectrode performance.

Ultrafast pre-breakdown dynamics in Al₂O₃SiO₂ reflector by femtosecond UV laser spectroscopy.

Ultrafast carrier dynamics in Al2O3/SiO2 high reflectors has been investigated by UV femtosecond laser. It is identified by laser spectroscopy that, the carrier dynamics contributed from the front few layers of Al2O3 play a dominating role in the initial laser-induced damage of the UV reflector. Time-resolved reflection decrease after the UV excitation is observed, and conduction electrons is found to relaxed to a mid-gap defect state locating about one photon below the conduction band . To interpret the laser induced carrier dynamics further, a theoretical model including electrons relaxation to a mid-gap state is built, and agrees very well with the experimental results.. To the best of our knowledge, this is the first study on the pre-damage dynamics in UV high reflector induced by femtosecond UV laser.

Modeling the effect of nanosecond laser conditioning on the femtosecond laser-induced damage of optical films.

The effect of nanosecond laser conditioning on the femtosecond laser-induced damage behaviors of Al2O3, HfO2, SiO2 single layers and Al2O3/SiO2 high reflectors (HR) are explored. During femtosecond laser damage test, negative effects on enhancing the femtosecond laser-induced damage threshold (LIDT) of optical films after the nanosecond laser conditioning is found, which is opposite to the LIDT improvement in the nanosecond range. To explain the mechanism after nanosecond laser conditioning, a theoretical model including multiphoton ionization (MPI), avalanche ionization (AI) and decays of electrons with one defect state is built to simulate the evolution of electron density in the conduction band. A permanent mid-gap defect state resulting from the process of laser conditioning is introduced in our model, which is found to contribute seed electrons to conduction band and hence accelerate the final breakdown. Both the experimental result and theoretical calculation agree very well with each other.

Synthesis and characterization of ZnS with controlled amount of S vacancies for photocatalytic H2 production under visible light.

Controlling amount of intrinsic S vacancies was achieved in ZnS spheres which were synthesized by a hydrothermal method using Zn and S powders in concentrated NaOH solution with NaBH4 added as reducing agent. These S vacancies efficiently extend absorption spectra of ZnS to visible region. Their photocatalytic activities for H2 production under visible light were evaluated by gas chromatograph, and the midgap states of ZnS introduced by S vacancies were examined by density functional calculations. Our study reveals that the concentration of S vacancies in the ZnS samples can be controlled by varying the amount of the reducing agent NaBH4 in the synthesis, and the prepared ZnS samples exhibit photocatalytic activity for H2 production under visible-light irradiation without loading noble metal. This photocatalytic activity of ZnS increases steadily with increasing the concentration of S vacancies until the latter reaches an optimum value. Our density functional calculations show that S vacancies generate midgap defect states in ZnS, which lead to visible-light absorption and responded.

Dark current mechanisms in amorphous selenium-based photoconductive detectors: an overview and re-examination

The transient and steady-state dark current behaviors in various amorphous selenium (a-Se) detectors are analyzed by developing mathematical models considering all possible mechanisms (e.g., carrier depletion, thermal generation, and carrier injection from electrodes) and charge carrier transport properties of a-Se. The theoretical models are validated by comparing them with recently published measured transient and steady-state dark currents in various a-Se detectors. The fittings of the models with the experimental results reveal various important material and device properties such as the optimum trap depth and concentrations in the blocking layers for faster stabilization and lower dark current, and the effective barrier height between the metal/a-Se contacts. The thermal generation current is significantly higher in avalanche detectors (at extremely high fields) than that in conventional detectors. The effective blocking layers for both holes and electrons are necessary for the minimum level of the dark current. The minimum dark current is determined by the thermal generation current. The thermal generation current can be lowered by reducing the mid gap defect states.

Electron–Hole Pair Generation of the Visible-Light Plasmonic Photocatalyst Ag@AgCl: Enhanced Optical Transitions Involving Midgap Defect States of AgCl

We explored how the visible-light energy absorbed by noble-metal nanoparticles (NPs) is converted to electrons and holes in the semiconductor in a visible-light plasmonic photocatalyst by studying the representative system Ag@AgCl on the basis of density functional calculations and classical electrodynamics calculations. These calculations suggest that the energy transfer from the Ag NPs to the semiconductor AgCl requires the presence of midgap defect states in the semiconductor and that the surface plasmon resonance (SPR) of the Ag NPs strongly enhances the optical transitions of the semiconductor involving the defect states. We verified this suggestion experimentally by preparing Ag@AgCl samples possessing different degrees of bulk and surface defects and subsequently by carrying out photodegradation experiments using these samples.

Effect of localized states on the reciprocity between quantum efficiency and electroluminescence in Cu(In,Ga)Se2 and Si thin-film solar cells

We present the electroluminescence and quantum efficiency of three different types of thin-film solar cells based on absorbers made from Cu(In,Ga)Se2 as well as from μc-Si:H and a-Si:H. Simulations of our experimental results show that the main contribution to the electroluminescence spectrum originates from band-to-tail transitions in the case of Cu(In,Ga)Se2, from tail-to-tail transitions in μc-Si:H, and from tail-to-tail as well as from band-to-midgap-defect transitions in a-Si:H. By comparing the electroluminescence with the quantum efficiency, we analyze the effect of localized states on the optoelectronic reciprocity relation in the three material systems. The relatively steep band–tail density of states in Cu(In,Ga)Se2 is compatible with the reciprocity relation at room temperature while the shallower band–tail density of states in μc-Si:H and the deep mid-gap defect states in a-Si:H lead to substantial deviations from the reciprocity.

Luminescence and solar cell from ligand-free colloidal AgInS2 nanocrystals

Ligand-free AgInS2 nanocrystals display significant defect-related sub-band gap photo-absorption in addition to the excitonic absorption. The post-synthesis annealing of the nanocrystal dispersion at 150 °C reduces the intrinsic defect density, and also increases the nanocrystal size (5.8 nm) thereby reducing surface related defects. The AgInS2 nanocrystals exhibit a broad emission spectra, with a large Stokes shift compared to the excitonic absorption, which is attributed to two radiative electron–hole recombination processes: the first one involves the localized defect-state and the delocalized valence/conduction band with a lifetime of ~20 ns, and the second one involves two localized donor and acceptor states with a lifetime >250 ns. The faster decay dominates the higher energy end of the emission spectrum. Quantum dot sensitized solar cells using these ligand-free AgInS2 nanocrystals exhibit a maximum energy conversion efficiency of 0.8% and an open circuit voltage of 0.45 V. The device performance is better compared to previous AgInS2 nanocrystal based solar cells, due to both the use of the ligand-free nanocrystals that improve the charge transport and the post-synthesis annealing which partially removes the mid-gap defect states.

Impact of varied sulfur incorporation on the device performance of sequentially processed Cu(In,Ga)(Se,S)2 thin film solar cells

In order to improve the performance of chalcopyrite Cu(In1 − x,Gax)(Se1 − ySy)2 solar cells, the implementation of a bandgap widening at the absorber/buffer interface via an increase of the [S]/[S + Se] ratio is investigated. In this work we examine industrially processed samples, which were fabricated via the deposition-reaction process with varied H2S pressure during rapid thermal processing (RTP). Precursors which were exposed to a crucial amount of sulfur during the RTP step resulted in samples with significantly improved device performance. The increase of the open circuit voltage by about 150 mV cannot sufficiently be explained by bandgap widening. Observation of a strongly reduced saturation current density and ideality factor in intensively sulfurized samples suggest subdued recombination via mid-gap defect states located in the space charge region. This hypothesis is supported by results of deep-level transient spectroscopy measurements, which show that in both samples a mid-gap minority defect is present albeit its concentration is about one magnitude larger in sulfur-poor samples. These results confirm that sulfur passivates recombination centers in the depletion layer and hence significantly increases the open circuit voltage and the overall device performance of the photovoltaic devices.

Challenge to the deep-trap model of the surface in semiconductor nanocrystals

Temperature-dependent photoluminescence spectra of a range of semiconductor nanocrystals are presented which reveal an unanticipated thermodynamic relationship between the core excitonic states and the surface of the nanocrystal. These results challenge the widely held view of the surface emission arising from an energetically broad distribution of midgap defect states at the surface. We show that a simpler model based on semiclassical electron transfer and invoking only a single surface state uniquely describes all observables.

The effect of interface processing on the distribution of interfacial defect states and the C-V characteristics of III-V metal-oxide-semiconductor field effect transistors

We have investigated the effect of interface formation and processing conditions of Al2O3 on GaAs on the density and distribution of interface state charge in the band gap. We have formed the insulator/semiconductor interface using both atomic layer deposition (ALD) and chemical vapor deposition (CVD). In situ ALD, ex situ ALD, and in situ CVD of aluminum oxide (Al2O3) on GaAs were employed using metal-organic CVD. Isopropanol (IPA) was chosen as the oxygen source for Al2O3 deposition. No arsenic or gallium oxide was detected at the in situ ALD Al2O3/GaAs interface, while gallium oxide was observed at the in situ CVD Al2O3/GaAs interface. The entire distributions of interfacial defects from different processes were determined by conductance frequency method with temperature-variation capacitance-voltage (C-V) measurements. The existence of Ga2O3 at the interface was found to be a possible method to lower the density of midgap defect states. From the C-V simulation, the midgap defect states are acceptorlike, which may originate from gallium vacancies near the interface. These states may also result in high frequency dispersion observed in the C-V curves of n-type metal-oxide-semiconductor field effect transistors. We correlate the interfacial defect states with the processes used to form the insulator-semiconductor interface.

Epitaxial Growth of High-κ Dielectrics for GaN MOSFETs

ABSTRACTHigh-dielectric constant (high-κ) oxide growth on hexagonal-GaN (on sapphire) is examined for potential use in enhancement-mode metal oxide semiconductor field effect transistor (MOSFET). Enhancement-mode MOSFET devices (ns > 4×1013 cm−2) offer significant performance advantages, such as greater efficiency and scalability, as compared to heterojunction field effect transistor (HFET) devices for use in high power and high frequency GaN-based devices. High leakage current and current collapse at high drive conditions suggests that the use of a high-κ insulating layer is principle for enhancement-mode MOSFET development. In this work, rare earth oxides (Sc, La, etc.) are explored due to their ideal combination of permittivity and high band gap energy. However, a substantial lattice mismatch (9-21%) between the rare earth oxides and the GaN substrate results in mid-gap defect state densities and growth dislocations. The epitaxial growth of the rare earth oxides by molecular beam epitaxy (MBE) on native oxide passivated-GaN is examined in an effort to minimize these growth related defects and other growth-related limitations. Growth of the oxide on GaN is characterized analytically by RHEED, XRD, and XPS. Preliminary MOS electrical analysis of a 50 Å La2O3 on GaN shows superior leakage performance as compared to significantly thicker Si3N4 dielectric.

Effect of growth temperature on defect states of GaAsSbN intrinsic layer in GaAs∕GaAsSbN∕GaAs photodiode for 1.3μm application

A GaAsSbN layer closely lattice matched to GaAs was used as an intrinsic layer (i layer) in a GaAs∕GaAsSbN∕GaAs p-i-n photodiode with response up to 1.3μm. Deep level transient spectroscopy measurement on the GaAs∕GaAsSbN∕GaAs reveals two types of hole traps (HTs) in the GaAsSbN i layer; (i) HT1: a shallow N-related defect state (Ea∼0.10–0.12eV) and (ii) HT2: an AsGa point defect-related midgap defect state with Ea∼0.42–0.43eV. Reduction in growth temperature from 480to420°C reduces the HT2 trap concentration from 4×1015to1×1015cm−3, while increases the HT1 trap concentration from 1×1014to7×1014cm−3. Reduction in the HT2 trap concentration following growth temperature reduction was attributed to the suppression of AsGa point defect formation. Evidence of possible change of the AsGa midgap state to a shallow level defect due to the formation of (AsGa–NAs) pairs was also suggested to have increased the HT1 trap concentration and reduced the HT2 trap concentration. An ∼4dBm improvement in photoresponse under 1.3μm laser excitation and approximately eight times reduction in dark current at −8V reverse bias were attributed to the reduction in the overall trap concentration and mainly the reduction of the AsGa-related midgap trap concentration in the sample grown at 420°C.

Ab initiomodels of amorphousSi1−xGex:H

We study the structural, dynamical, and electronic properties of amorphous ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}:\mathrm{H}$ alloys using first-principles local basis molecular dynamics techniques. The network topology and defects in the amorphous network have been analyzed. Structural changes, particularly an increase in number of defects and strained bond angles, have been found as the Ge content increases from $x=0.1$ to 0.5. The electronic density of states exhibits a decreasing band gap and additional midgap and band-tail defect states as Ge concentration increases. We report the network structures which are responsible for midgap and band-tail states. The band tails show an exponential (Urbach) behavior. The mobility gap is coarsely estimated as a function of Ge concentration.

Thermal enhancement of the antiferromagnetic exchange coupling between Fe epilayersseparated by a crystalline ZnSe spacer

We provide evidence for the existence of an antiferromagnetic coupling betweeniron epilayers separated by a wedge-like ZnSe crystalline semiconductor bymagnetometric and ferromagnetic resonance experiments. The coupling strength of46 µJ m−2 for tZnSe = 25 Å is strongly reduced as the barrier thickness is increased. Thecoupling increases linearly with temperature from 5 to 300 K, with a5.5 × 10−9 J m−2 K−1 rate. Thermally induced effective exchange coupling mediated by spin-dependent tunnellingof electrons via localized mid-gap defect states in the ZnSe spacer layer appearsto be the most plausible mechanism to induce the antiferromagnetic coupling.

Effect of contact properties on current transport in metal/molecule/GaAs devices

Previous reports on metal/molecule/semiconductor (MMS) devices have investigated the effects of molecular species, including dipoles, but have not considered the semiconductor contact properties in detail. In this paper we report on a study of the effects of variations in the semiconductor contact on the conduction properties of MMS devices. Metal/molecule/gallium arsenide (GaAs) devices were fabricated using various semiconductor contact layers, electrically characterized versus temperature and analyzed using an electrostatic model. The various semiconductor contacts included heavily doped n-type and p-type GaAs layers, as well as n-doped and p-doped surface layers of low-temperature-grown GaAs (LTG:GaAs), which provide a high density of midgap defect states near the semiconductor surface. The impact of changing the work function of the top metal contact has also been studied. An electrostatic model that incorporates information on the molecular dipole moment, defect states in GaAs surface layers, and the work function of the metal contact, has been developed in order to understand the band diagrams corresponding to the various device types, and to explain the current-voltage behavior observed in the devices. It is shown by controlling the properties of the semiconductor contact that the device characteristics can be tuned from being dominated by the GaAs barrier (heavily doped n-type GaAs) to being dominated by the molecular states that are strongly coupled to the GaAs contact (LTG:GaAs and heavily doped p-type GaAs).

Chiral optical absorption bands in [ ]CdGa2(1−x)In2xS4 semiconducting compound

Spectroscopic investigation of chiral absorption bands in [ ]CdGa2(1−x)In2xS4 has been carried out at 55K. The origin of these bands is attributed to both the complex structure of the midgap defect states created by inhomogeneity in as-grown samples and the presence of ordered vacancies at cadmium sites. The appearance of chiral bands are associated with dangling bonds in ordered vacancies which help to bring the component of the magnetic dipole moment parallel to the electric dipole moment. The cause for rotation of electric vector of the field is also discussed.

Distribution of radiation induced defects and modification of optical constants of GaAs implanted with high energy 56Fe ions

Single crystal GaAs substrates implanted with high-energy 56 Fe ions have been optically investigated before and after annealing, over the photon energy range 0.1–1.4 eV. The spatial distribution of the damage has been probed by etching the sample and shows that defect density peaks at a depth of about 10 μm. Annealing studies show that the mid gap defect states are annealed out more rapidly than the near band edge defect states during annealing up to 350 °C whereas the near band edge defect states are annealed out more rapidly than mid gap defect states during annealing between 350–600 °C. The fringe patterns observed in the transmission spectra have been analyzed to investigate quantitatively changes in the refractive index of the radiation-damaged region. Kramers–Kronig relation has been used to correlate the absorption due to damage with changes in the refractive index. The results show that the high-energy ion-implanted layer contained amorphous pockets.