Search

The carrier lifetime control over 150 μm thick 4H-SiC epitaxial layers via thermal generation and annihilation of carbon vacancy (VC) related Z1/2 lifetime killer sites is reported. The defect developments upon typical SiC processing steps, such as high- and moderate-temperature anneals in the presence of a carbon cap, are monitored by combining electrical characterization techniques capable of VC depth-profiling, capacitance–voltage (CV) and deep-level transient spectroscopy (DLTS), with a novel all-optical approach of cross-sectional carrier lifetime profiling across 4H-SiC epilayer/substrate based on imaging time-resolved photoluminescence (TRPL) spectroscopy in orthogonal pump-probe geometry, which readily exposes in-depth efficacy of defect reduction and surface recombination effects. The lifetime control is realized by initial high-temperature treatment (1800 °C) to increase VC concentration to ≈1013 cm−3 level followed by a moderate-temperature (1500 °C) post-annealing of variable duration under C-rich thermodynamic equilibrium conditions. The post-annealing carried out for 5 h in effect eliminates VC throughout the entire ultra-thick epilayer. The reduction of VC-related Z1/2 sites is proven by a significant lifetime increase from 0.8 to 2.5 μs. The upper limit of lifetimes in terms of carrier surface leakage and the presence of other nonradiative recombination centers besides Z1/2, possibly related to residual impurities such as boron are discussed., Physica Status Solidi A, 220 (10), ISSN:1862-6300, ISSN:1862-6319, ISSN:0031-8965, ISSN:1521-396X

Electrical properties of point defects in 4$H$-SiC have been studied extensively, but those related to carbon interstitials (C$_{i}$) have remained surprisingly elusive until now. Indeed, when introduced via ion irradiation or implantation, signatures related to C$_{i}$ observed by deep level transient spectroscopy (DLTS) tend to overlap with those of other primary defects, making the direct identification of C$_{i}$-related levels difficult. Recent literature has suggested to assign the so-called M center, often found in as-irradiated 4H-SiC, to charge state transitions of the C$_{i}$ defect in different configurations. In this work, we have introduced excess carbon into low-doped n-type 150 {\mu}m thick 4$H$-SiC epilayers by thermal annealing, with a pyrolyzed carbon cap on the sample surface acting as a carbon source. Because the layers exhibited initially low concentrations of carbon vacancies ([V$_{C}$] = 10$^{11}$ cm$^{-3}$), this enabled us to study the case of complete V$_{C}$ annihilation, and formation of defects due to excess carbon, i.e. carbon interstitials C$_{i}$ and their higher-order complexes. We report on the occurrence of several new levels upon C injection which are likely C$_{i}$-related. Their properties are different from those found for the M center, which points towards a different structural identity of the detected levels. This suggests the existence of a rich variety of C$_{i}$-related defects. The study will also help generating new insights into the microscopic process of V$_{C}$ annihilation during carbon injection processes.

Silicon carbide (SiC) is a wide band-gap semiconductor of great technological importance, showing promise for application areas ranging from quantum computing and communication to power devices. Vital in both the contexts of power devices and quantum technology is the understanding of intrinsic defects that are introduced during various device processing steps, both immediately after their formation and over the course of defect evolution with temperature. Here we monitor the formation and evolution of intrinsic point defects in n-type 4H-SiC after proton irradiation at room temperature and subsequent annealing in the temperature range 300-1000 °C, and discuss the nature and origin of the EH4 and EH5 deep level defects observed by deep level transient spectroscopy around 400-500 K. In particular, the controversy on the nature of the EH5 trap in particular is addressed, where we propose the presence of two overlapping defect peaks: one metastable level that appears after low energy electron irradiation below the silicon displacement limit, and one more stable level that gradually decreases in concentration until an annealing temperature of 1000°C. We argue that the former is likely related to carbon interstitials, while the latter was recently tentatively attributed to the carbon antisite-vacancy pair., Materials Science Forum, 1062, ISSN:0255-5476, ISSN:1662-9752

Point defects in semiconductors are promising single-photon emitters (SPEs) for quantum computing, communication, and sensing applications. However, factors such as emission brightness, purity. and indistinguishability are limited by interactions between localized defect states and the surrounding environment. Therefore, it is important to map the full emission spectrum from each SPE, to understand the complex interplay between the different defect configurations, their surroundings, and external perturbations. Herein, we investigate a family of regularly spaced sharp luminescence peaks appearing in the near-infrared portion of photoluminescence (PL) spectra from n-type 4H-SiC samples after irradiation. This periodic emitter family, labeled the L lines, is only observed when the zero-phonon line signatures of the negatively charged Si vacancy (so-called V lines) are present. The L lines appear with 1.45 meV and 1.59 meV energy spacing after H and He irradiation and increase linearly in intensity with fluence-reminiscent of the intrinsic defect trend. Furthermore, we monitor the dependence of the L-line emission energy and intensity on heat treatments, electric field strength, and PL collection temperature, discussing these data in the context of the L lines. Based on the strong similarity between the irradiation, electric field, and thermal responses of the L and V lines, the L lines are attributed to the Si vacancy in 4H-SiC. The regular and periodic appearance of the L lines provides strong arguments for a vibronic origin explaining the oscillatory multipeak spectrum. To account for the small energy separation of the L lines, we propose a model based on rotations of distortion surrounding the Si vacancy driven by a dynamic Jahn-Teller effect. ISSN:1098-0121 ISSN:0163-1829 ISSN:1550-235X ISSN:0556-2805 ISSN:2469-9969 ISSN:1095-3795 ISSN:2469-9950

The development of defect populations after proton irradiation of $n$-type $4H$-SiC and subsequent annealing experiments is studied by means of deep level transient (DLTS) and photoluminescence spectroscopy. A comprehensive model is suggested describing the evolution and interconversion of irradiation-induced point defects during annealing below $1000^{\ensuremath{\circ}}\mathrm{C}$. The model proposes the ${\mathrm{EH}}_{4}$ and ${\mathrm{EH}}_{5}$ traps frequently found by DLTS to originate from the ($+$/0) charge transition level belonging to different configurations of the carbon antisite-carbon vacancy (CAV) complex. Furthermore, we show that the transformation channel between the silicon vacancy (${\mathrm{V}}_{\mathrm{Si}}$) and CAV is effectively blocked under $n$-type conditions, but becomes available in samples where the Fermi level has moved towards the center of the band gap due to irradiation-induced donor compensation. The annealing of ${\mathrm{V}}_{\mathrm{Si}}$ and the carbon vacancy (${\mathrm{V}}_{\mathrm{C}}$) is shown to be dominated by recombination with residual self-interstitials at temperatures of up to $400^{\ensuremath{\circ}}\mathrm{C}$. Going to higher temperatures, a decay of the CAV pair density is reported which is closely correlated to a renewed increase of ${\mathrm{V}}_{\mathrm{C}}$ concentration. A conceivable explanation for this process is the dissociation of the CAV pair into separate carbon anitisites and ${\mathrm{V}}_{\mathrm{C}}$ defects. Lastly, the presented data supports the claim that the removal of free carriers in irradiated SiC is due to introduced compensating defects and not passivation of shallow nitrogen donors.

This work reports on temperature-induced out-diffusion and concentration decay of the prominent intrinsic point defect ${\mathrm{V}}_{\mathrm{Ni}}$ (nickel vacancy) in the wide-gap $p$-type semiconductor nickel oxide (NiO). ${\mathrm{V}}_{\mathrm{Ni}}$ can easily be introduced into NiO thin films by offering high oxygen reactivity during film growth, rendering nonstoichiometric semiconducting material. However, exposure to lower oxygen reactivity after growth, e.g., in a standard atmosphere, usually leads to a gradual decrease of film conductivity, because the vacancy concentration returns to its thermodynamic equilibrium value. In this study we observe this process in situ by performing temperature-dependent measurements of the electrical conductivity on a room temperature-grown NiO film. At a temperature of 420 K under exclusion of oxygen, the doping level decreases by a factor of 8 while the associated room temperature DC conductivity drops by six orders of magnitude. At the same time, out-diffusion of the mobile ${\mathrm{V}}_{\mathrm{Ni}}$ species can be indirectly observed through the occurrence of electrode polarization characteristics.

We present research results on the electronic transport in heterostructures based on p-type nickel oxide (NiO) with the n-type oxide semiconductors zinc oxide (ZnO) and cadmium oxide (CdO). NiO is a desirable candidate for application in (opto-)electronic devices. However, because of its small electron affinity, heterojunctions with most n-type oxide semiconductors exhibit conduction and valence band offsets at the heterointerface in excess of 1 eV. ZnO/NiO junctions exhibit a so called type-II band alignment, making electron-hole recombination the only process by which a current can vertically flow through the structure. These heterojunctions are nevertheless shown to be of practical use in efficient optoelectronic devices, as exemplified here by our UV-converting transparent solar cells. These devices, although exhibiting high conversion efficiencies, suffer from two light-activated recombination channels connected to the type-II interface, one of which we identify and analyse in more detail here. Furthermore, CdO/NiO contacts were studied - a heterostructure with even larger band offsets such that a type-III band alignment is achieved. This situation theoretically enables the development of a 2-dimensional electronic system consisting of topologically protected states. We present experiments demonstrating that the CdO/NiO heterostructure indeed hosts a conductive layer absent in both materials when studied separately.

In this work, we revisit the issue of the nature of electronic transport in nickel oxide (NiO) and show that the widely used model of free small polaron hopping, initially raised to characterize transport in high-purity samples, is not appropriate for modeling intrinsically doped NiO. Instead, we present extensive evidence, collected by means of temperature- and frequency-dependent measurements of the electrical conductivity $\sigma$, that the model of polaronic inter-acceptor hopping can be used to consistently explain the electronic conduction process. In this framework, holes are localized to acceptors (Ni vacancies), forming a strongly bound, polaron-like state. They can only move through the film by hopping to a neighboring, at least partially unoccupied, acceptor. This renders the spatial overlap between neighboring polaronic wave functions a highly critical parameter. The signature of this process is the occurrence of two temperature regions of the DC conductivity, separated by about half the Debye temperature $\theta_D/2 \approx 200 K$. For $T > \theta_D/2$, holes are transferred by phonon-assisted hopping over the potential barrier between two sites, whereas phonon-assisted tunneling through the barrier dominates below that temperature. We also show that the degree of structural and electronic disorder plays a vital role in determining the characteristics of the transport process: high disorder leads to strong energetic broadening of the acceptor states such that hopping to more distant sites may be favored over transfer to nearest neighbors (variable range hopping). The assumption of high binding energies of the charge carriers at VNi is in accordance with the recent paradigm shift regarding the understanding of the electronic structure of NiO: holes doped into NiO couple to Ni 3d spins, thereby occupying deep polaron-like states within the band gap (Zhang-Rice bound doublets).

The fabrication of all-oxide junction-FETs consisting of n-type ZnO channels and p-type NiO gate electrodes with high optical transmission in the visible spectral range is reported. The influence of the channel layer thickness on the transfer characteristics in terms of ON-voltage, current ON/OFF ratio, and subthreshold slope was investigated. Best devices showed channel mobilities of around 4 cm $^{2}\text{V}^{-1}\text{s}^{-1}$ , ON/OFF ratios of up to $3\times 10^{7}$ , and subthreshold slopes close to the thermal limit. The ON-voltage can be shifted from −3.5 to $\sim 0$ V when the channel thickness is decreased from 80 to 10 nm. At elevated temperatures of up to 100 °C, the devices remain fully operational. After the annealing cycle, the OFF-current is only slightly increased irreversibly. The frequency dependence of the switching behavior was investigated (gate-lag effect), and a high-frequency cutoff frequency at $f_{c} =123$ kHz was determined.

Semi-transparent pn-heterojunctions were fabricated from pulsed laser deposited (PLD) n-type ZnO and DC magnetron sputtered p-type NiO, working as UV-active solar cells. The complete cell stack has an average transmission of 46% in the visible spectral range and an optical absorption edge at 380 nm. The diodes exhibit high current rectification of up to eight orders of magnitude at ±2 V. Upon illumination with a solar simulator, the devices show photovoltaic activity with open-circuit voltages of up to 520 mV, short-circuit current densities of 0.5 mAcm2, and a maximum external quantum efficiency of 55%. However, we observed rather low fill factors of the current–voltage characteristics of around 40%, resulting in total power conversion efficiencies of around 0.1% and efficiencies in the UV range of 3.1%. To identify possible loss mechanisms, the voltage-dependent efficiency of carrier collection was calculated. The data were fitted using a model that considers recombination losses at the NiO/ZnO interface as well as within the electric field region, yielding a high hole interface recombination velocity of 1×105cm s−1. We conclude that the carrier collection efficiency is strongly deteriorated by recombination losses at the NiO/ZnO interface, causing a strong bending of the jV characteristics under illumination and thereby low fill factors.

Wide-gap semiconductors are often unipolar and can form type II bipolar heterostructures with large band discontinuities. We present such diodes with very high rectification larger than 1 × 10(10). The current is assumed to be entirely due to interface recombination. We derive the ideality factor for both symmetric and asymmetric diodes and find it close to 2 in agreement with experimental data from NiO/ZnO and CuI/ZnO type II diodes. The comparison with experimental results shows that the actual interface recombination rate is orders of magnitude smaller than its possible maximum value.

We report on advances in the fabrication of high quality bipolar heterodiodes with oxideelectrodes. The highest rectification above 1010 is obtained for a structure from a-ZCO/ZnO/ZnO:Alon Al2O3 (a-ZCO: amorphous ZnCo2O4). Rectification better than 106, a value larger than reportedfor all previous attempts, is obtained for our a-ZCO/a-ZTO (a-ZTO: amorphous zinc tin oxide), a-NiO/ZnO and CuI/ZnO diodes. The ZCO/ZnO has been used as gate in JFETs with ZnO as channel.The bipolar diodes open the field of oxide semiconductor electronics to applications in photovoltaics.

Three different ZnO-based diodes are compared that can be employed as semitransparent ultraviolet (UV)-active solar cells: a Schottky diode using platinum oxide as front contact, a p+n diode with magnetron-sputtered nickel oxide and a pn diode with a pulsed laser deposited NiO front contact. The UV conversion efficiencies are 4.1% for the Schottky diodes and 3.1% for the NiO-based cells. In the NiO-based structures, a strong deformation of the current–voltage characteristics under white light illumination (one sun) is observed, leading to reduced open-circuit voltages. Measurements of the external quantum efficiency with and without simultaneous white-light illumination reveal that also the collected photocurrent in these devices types is significantly reduced under strong illumination. It is shown that the magnitude of both the injected current and the recombination current of photogenerated carriers is increased in this state. A model is proposed that explains both effects within the framework of an opt...

We present the history of, and the latest progress in, the field of bipolar oxide thin film devices. As such we consider primarily pn-junctions in which at least one of the materials is a metal oxide semiconductor. A wide range of n-type and p-type oxides has been explored for the formation of such bipolar diodes. Since most oxide semiconductors are unipolar, challenges and opportunities exist with regard to the formation of heterojunction diodes and band lineups. Recently, various approaches have led to devices with high rectification, namely p-type ZnCo2O4 and NiO on n-type ZnO and amorphous zinc-tin-oxide. Subsequent bipolar devices and applications such as photodetectors, solar cells, junction field-effect transistors and integrated circuits like inverters and ring oscillators are discussed. The tremendous progress shows that bipolar oxide electronics has evolved from the exploration of various materials and heterostructures to the demonstration of functioning integrated circuits. Therefore a viable, facile and high performance technology is ready for further exploitation and performance optimization.

pn-Heterodiodes comprising the wide bandgap semiconducting oxide In2O3 and amorphous p-conducting NiO or ZnCo2O4 are realized. In2O3 is grown at 600 °C and the amorphous p-type oxides at room temperature by pulsed-laser deposition. Highest rectification of about four orders of magnitude is observed for structures with Mg-doped In2O3 layers having lower carrier density than undoped layers. The p-ZnCo2O4/n-In2O3 diodes do not show degradation at elevated temperatures of 150 °C, whereas the p-NiO/n-In2O3 diodes degrade irreversibly for T > 100 °C. Thermal admittance spectroscopy revealed shallow defect levels in the In2O3 layer with activation energy of about 12 and 250 meV, respectively.