Distribution Of Defect Levels Research Articles

Metal-induced growth of crystal Si for low-cost Al:ZnO/Si heterojunction thin film photodetectors

Quality crystal silicon (Si) thin films were grown via the metal-induced growth (MIG) method, which is a low-cost and metal-silicide assisted technique. The metal catalysts of Ni, Pd and Co were first deposited onto substrates for silicide template layer formation. Then, crystal Si thin films with a thickness of ~ 5 µm were epitaxially deposited on the silicide seeds via DC magnetron sputtering. The crystallinity of the Si films was confirmed and investigated by X-ray diffraction (XRD). These Si thin films were then used to fabricate Al doped ZnO (AZO)/MIG Si heterojunction photodetectors. The device achieved a saturation photocurrent of 23.2 mA/cm2 at − 5 V. This photocurrent level is comparable with that of previous reported AZO/bulk-Si devices. Current transport mechanisms and defect distributions were also studied. By analyzing the dark current-voltage (I – V) characteristics, the space charge limited (SCL) current dominated junction behaviors with an exponential defect level distribution were determined. In the bias range of 0.25 V < V < 1.0 V, SCL current transported in the mobility regime based on I ∝ Vβ, where β > 3. For 0 < V < 0.25 V, tunneling and SCL current in ballistic regime were implied due to I ∝ V1.5. In the reverse bias, the device exhibited SCL current in saturation regime or tunneling, suggested by the unity I-V relation. The fabricated devices with the theoretical understanding of the defect and carrier transport mechanisms provided new design insights for low-cost ZnO/MIG-Si thin film optoelectronics.

On the distribution of oxide defect levels in Al2O3 and HfO2 high-k dielectrics deposited on InGaAs metal-oxide-semiconductor devices studied by capacitance-voltage hysteresis

In this work, we study oxide defects in various III-V/high-k metal-oxide-semiconductor (MOS) stacks. We show that the choice of a given starting measurement voltage with respect to the MOS flat-band voltage affects the observed capacitance-voltage hysteresis. We discuss how this behavior can be used to study the distribution of oxide defect levels. With the help of comprehensive experimental data, we show that Al2O3 and HfO2 have different hysteresis characteristics related to different oxide defect distributions. In case of an Al2O3/HfO2 bilayer stack with Al2O3 on the channel side (interfacial layer, IL), as the IL thickness reduces from 3 nm to 0 nm, the hysteresis behavior switches from the typical Al2O3 behavior to the one corresponding to HfO2. We link the characteristic behavior of two dielectrics to the defect level distributions inside their respective band-gaps through a simple energy-driven charging model. Based on the experimental data and simulation results, we show that Al2O3, despite having a lower peak defect density as compared to HfO2, shows a very wide, almost continuous distribution of defect levels across and around the InGaAs channel energy band gap. These results explain the often reported poor reliability of III-V devices with Al2O3-based gate stacks.

Semiconductor steady state defect effective Fermi level and deep level transient spectroscopy depth profiling

The widely used deep level transient spectroscopy (DLTS) theory and data analysis usually assume that the defect level distribution is uniform through the depth of the depletion region of the n—p junction. In this work we introduce the concept of effective Fermi level of the steady state of semiconductor, by using which deep level transient spectroscopy depth profiling (DLTSDP) is proposed. Based on the relationship of its transition free energy level (TFEL) and the effective Fermi level, the rules of detectivity of the defect levels are listed. Computer simulation of DLTSDP is presented and compared with experimental data. The experimental DLTS data are compared with what the DLTSDP selection rules predicted. The agreement is satisfactory.

Toward Understanding Positive Bias Temperature Instability in Fully Recessed-Gate GaN MISFETs

In this paper, fully recessed-gate GaN MISFETs with two different gate dielectrics, i.e., plasma-enhanced atomic layer deposition (PEALD) SiN and ALD Al2O3 gate dielectric, are used to study the origin of positive bias temperature instability (PBTI). By employing a set of dedicated stress-recovery tests, we study PBTI during the stress and relaxation. Hence, a defect band model with different distributions of defect levels inside the gate dielectric is proposed, which can excellently reproduce the experimental data and provide insightful information about the origin of PBTI in GaN MISFETs. The results indicate that the serious PBTI in the device with PEALD SiN is mainly due to a wide distribution of defect levels ( $\sigma \sim 0.67$ eV), centered below the conduction band of GaN ( $E_{C} -0.05$ eV), and can be easily accessed by the channel carriers already at a low-gate voltage. On the other hand, ALD Al2O3 gate dielectric shows a narrower distribution of defects ( $\sigma \sim 0.42$ eV), which are far from the conduction band of GaN ( $E_{C} +1.15$ eV). This observations explain the improved PBTI reliability observed in devices with ALD Al2O3.

Impact of starting measurement voltage relative to flat-band voltage position on the capacitance-voltage hysteresis and on the defect characterization of InGaAs/high-k metal-oxide-semiconductor stacks

In this work, we discuss how the position of the flat band voltage with respect to the starting voltage of the C-V measurement sweep can influence the estimation of the hysteresis in high-k/InGaAs MOS devices. We show that, with the support of experimental data and conceptual oxide defect band calculations, the interpretation and subsequent parameter extraction from flat-band voltage shifts observed in III-V MOS devices is more complex as compared to Si gate stacks. It is demonstrated that such complication arises due to the wider distribution of defect levels in the dielectric band gap in the case of InGaAs/high-k stack as compared to standard Si/SiO2/HfO2 MOS. In particular, for Al2O3 deposited on InGaAs, two wide, partially overlapping oxide defect bands are identified, centered ∼1.5 eV and ∼0.5 eV above and below the channel conduction band, respectively. Such defect levels are expected to affect the device operation and reliability.

First-principle study of the structural, electronic and optical properties of defected amorphous silica

The oxygen-excess structures include oxygen dangling bond, peroxy linkage (POL), peroxy radical (POR) and interstitial oxygen molecule or ozone molecule defects, while the oxygen-deficient structures include neutral oxygen vacancy, E′ center and oxygen double-bond. For both the undefected and the various defected structures, the electronic and optical properties are calculated by plane-wave pseudo potential density functional theory. The oxygen-deficient defected structures lead to the remarkable increases in the density of states (DOS) at the top of valence band and near the bottom of the conduction band. The oxygen-excess defected structures change the distribution of defect levels and there appear new levels between the valence band and conduction band. All the defects lead to the increase of static dielectric constants and the enhancing absorption in the low energy range. Absorption peaks can be observed below 2eV for NBOHC defected structures. Though the energy loss is generated at the lower energy region, the loss strength below 2eV for oxygen-excess defected structures is stronger than oxygen-deficient defected structures. During the calculation the dangling bond in the structures are neutralized by hydrogen atoms. This work may give insights into the laser induced damage towards to vitreous silica.

Influence of deep level defects on carrier lifetime in CdZnTe:In

The defect levels and carrier lifetime in CdZnTe:In crystal were characterized with photoluminescence, thermally stimulated current measurements, as well as contactless microwave photoconductivity decay (MWPCD) technique. An evaluation equation to extract the recombination lifetime and the reemission time from MWPCD signal is developed based on Hornbeck-Haynes trapping model. An excellent agreement between defect level distribution and carrier reemission time in MWPCD signal reveals the tail of the photoconductivity decay is controlled by the defect level reemission effect. Combining 241Am gamma ray radiation response measurement and laser beam induced transient current measurement, it predicted that defect level with the reemission time shorter than the collection time could lead to better charge collection efficiency of CdZnTe detector.

TCAD‐based DLTS simulation for analysis of extended defects

The fabrication technology of extra‐functionality CMOS devices involves process steps which lead to high damage in the silicon lattice. Amorphizing implants and simultaneous reduction of thermal budgets to gain better control of the formation of ultra‐shallow junctions render the presence of extended defects in active regions unavoidable. In particular, dislocation loops (DLs) have proven to be stable under thermal treatment. To better understand the electrical properties of DLs and their impact on the leakage current we developed an analytical tool to extract defect parameters from measured Deep Level Transient Spectroscopy (DLTS) signals and capacitance transients. Commercial process and device simulators are used to test the plausibility of applied defect models and the basic assumptions about the electrical activity of DLs.Simulation of DLTS peak broadening caused by the broadening of a defect level distribution in the band gap of silicon.

Excitation-dependent recombination and diffusion near an isolated dislocation in GaAs

In low-magnification, plan-view photoluminescence images of a nominally lattice-matched, undoped GaAs/GaInP heterostructure, we observe a random distribution of isolated dark spots. We attribute the dark spots to crystal dislocations, where nonradiative recombination is augmented by transitions utilizing defect-related energy levels between the conduction and valence bands. We note that, when the laser excitation intensity is reduced, the darkened regions expand. At lower excitation, the density of photogenerated electrons and holes is reduced, and they are more likely to reach the defective region before encountering a partner for radiative recombination. When we model the behavior with a simulation that allows for Laplacian diffusion and defect-related recombination only through mid-bandgap energy levels, we do not obtain good agreement between experimental and simulated images. But if we allow for an arbitrary distribution of defect levels, such that the occupation of the levels and bands can change independently, we have more flexibility for fitting the density-dependent recombination rates. The more sophisticated model produces results that are more consistent with experimental images.

Identifying Interface States in AlInN/GaN Heterostructure by Photocurrent Method

In order to assess the residual leakage current mechanisms in electronic devices based on AlInN/GaN heterostructures, defect states were investigated by photocurrent method based on a metal-semiconductor-metal structure device. As a result, a continuous distribution of defect levels from 56 to 110 meV below the conduction band of GaN was identified at the AlInN/GaN interface by analyzing photocurrent spectra under different bias voltages and photogenerated carrier transport based on the simulation of electric field distribution. These interface states filled with electrons at zero bias can release electrons at reverse bias and provide a path of gate leakage.

Using the excitation-dependent radiative efficiency to assess asymmetry in the defect-related density of states

Measurements of steady-state radiative efficiency versus photoexcitation rate probe the carrier-density-dependent competition between nonradiative and radiative mechanisms in semiconductors. Nonradiative recombination through defect levels is proportional to the product of defect level occupation and carrier density in the opposing band. Band-to-band radiative recombination scales with the product of band densities. The excitation rate required for defect level saturation establishes the effective density of participating defects. More subtle features in the changeover from defect-related to radiative-dominated recombination, and its temperature dependence, provide additional insight into the distribution of defect levels. In this letter, the authors consider the effect of asymmetry about the midgap.

Deep donor-acceptor pair recombination in InGaAs-based heterostructures grown on InP substrates

We are investigating a series of lattice-matched InxGa1−xAs∕InAsyP1−y double heterostructures with indium concentrations ranging between x=0.53 and x=0.78. The double heterostructures incorporating indium-rich alloys (x&amp;gt;0.53) experience lattice mismatch relative to the InP substrate. Previous work has produced convincing but indirect evidence that the distribution of defect levels in the InxGa1−xAs changes dramatically when the epistructure deviates from the lattice-matched condition. In particular, deep midgap states appear to give way to shallower near-band-edge states with increasing mismatch. Here, we report sub-band-gap photoluminescence measurements that explore these changes directly. We observe a broad low-energy peak in the spectra of the lattice-matched and nearly lattice-matched epistructures that is not present in the more mismatched case. The sub-band-gap emission blueshifts and grows superlinearly with photoexcitation up to and exceeding 1000W∕cm2. This unusual behavior is attributed to transitions between ordinary acceptor levels and deep, defect-related donorlike states. We find no evidence for the shallower defect states that we expected to arise with increasing lattice mismatch.

Effect of laser annealing on the distribution of defect levels in CdSe films

Effects of laser treatment on the modification and/or distribution of defect levels in chemically-deposited CdSe thin films have been discussed. By annealing the CdSe films with laser in air, we could observe a drastic reduction of defect density and a redistribution of defect states in the low quality chemically deposited films. Increase of photoluminescence efficiency by laser treatment demonstrates the effect of recombination enhanced defect reactions at the annealed region. By manipulating the laser power and the time of annealing, we could obtain the films of better optoelectronic quality. The process might be useful for the control of defect levels in chalcogenide thin films for their applications in optoelectronic devices.

Direct Measurement of Surface Defect Level Distribution Associated with GaAs Antiphase Boundaries

Using an electrostatic force microscope, we measure surface contact potential (SCP) variations across antiphase boundaries (APBs) on GaAs films grown on Ge substrates. The SCP at the APBs is consistently and reproducibly measured to be 30 mV higher than that at GaAs domains. This is due to Fermi levels being pinned at different surface states. The identical electrical behavior observed for all APBs indicates that they are the lowest energy ${}\left(110\right)$ orientation. The sign of observed Fermi level shift is consistent with a prevalence of Ga-Ga bonds at real ${}\left(110\right)$ APBs.

The effect of doping density and injection level on minority-carrier lifetime as applied to bifacial dendritic web silicon solar cells

The measured short-circuit current density in bifacial dendritic web silicon solar cells has been found to decrease with decreasing base resistivity, particularly under back illumination. In addition, the ratio of short-circuit current under back illumination to short-circuit current under front illumination was observed to vary with light intensity. These observations reflect the fact that the minority-carrier lifetime in the base of these cells is a function of the base resistivity and the illumination level. The dopant was assumed to play only an indirect role in determining lifetime. This decrease in lifetime is shown to follow from a distribution of defect levels in the bandgap. These levels are a consequence of extended defects that have been observed in the web material, namely oxide precipitates and the dislocation cores that they decorate. The dopant, acts only in the indirect role of moving the Fermi level over an existing background distribution of defect levels that arise from the extended defects. Assuming a parabolic distribution of defect levels in the bandgap, the minority-carrier lifetime was calculated as a function of doping density and excess carrier concentration (illumination level) using the Shockley-Reed-Hall theory. The short-circuit current densities that were calculated using these lifetimes agreed reasonably well with measured values for bifacial dendritic web silicon solar cells. The measurements were made over a range of doping densities (6*10/sup 14/ to 3*10/sup 16/ cm/sup -3/) and illumination levels (0.001 to 1 sun) for both front and back illumination of the bifacial cells. >

Nature of Bombardment Damage and Energy Levels in Semiconductors

The different effects of Co60 gamma ray and fast neutron bombardment on the electrical behavior of germanium are discussed in terms of different local distributions of lattice defects expected for these two types of radiation. For the first of these, which is expected to introduce randomly distributed pairs of interstitials and vacancies, the state at 0.20 ev below the conduction band has been found to increase the concentration of ionized scattering centers on becoming occupied with conduction electrons. This state has been ascribed to a mobile interstitial because of its annealing behavior at moderate temperatures. Energy levels in p-type Ge and their annealing behavior are also discussed. In neutron irradiated n-type germanium the behavior of Hall mobility and the apparent energy distribution of defect levels are discussed in terms of a tentative model for the potential distribution around regions of high local lattice disorder expected to result from neutron bombardment. This model postulates that the disordered region is p type and is surrounded by a positive space charge region in the n-type matrix which insulates it from the matrix. This configuration of electrostatic potential around the disordered region in effect produces an insulating region whose radius is ∼10 times that of the disordered region itself. The behavior of Hall mobility in irradiated n-type specimens is consistent with the predictions of this model.