Shallow Acceptor States Research Articles

Selenium-alloyed tellurium oxide for amorphous p-channel transistors.

Compared to polycrystalline semiconductors, amorphous semiconductors offer inherent cost-effective, simple and uniform manufacturing. Traditional amorphous hydrogenated Si falls short in electrical properties, necessitating the exploration of new materials. The creation of high-mobility amorphous n-type metal oxides, such as a-InGaZnO (ref. 1), and their integration into thin-film transistors (TFTs) have propelled advancements in modern large-area electronics and new-generation displays2-8. However, finding comparable p-type counterparts poses notable challenges, impeding the progress of complementary metal-oxide-semiconductor technology and integrated circuits9-11. Here we introduce a pioneering design strategy for amorphous p-type semiconductors, incorporating high-mobility tellurium within an amorphous tellurium suboxide matrix, and demonstrate its use in high-performance, stable p-channel TFTs and complementary circuits. Theoretical analysis unveils a delocalized valence band from tellurium 5p bands with shallow acceptor states, enabling excess hole doping and transport. Selenium alloying suppresses hole concentrations and facilitates the p-orbital connectivity, realizing high-performance p-channel TFTs with an average field-effect hole mobility of around 15 cm2 V-1 s-1 and on/off current ratios of 106-107, along with wafer-scale uniformity and long-term stabilities under bias stress and ambient ageing. This study represents a crucial stride towards establishing commercially viable amorphous p-channel TFT technology and complementary electronics in a low-cost and industry-compatible manner.

Density functional theory study of adsorption of organic molecules on ZnO monolayers: Implications for conduction type and electrical characteristics

Graphene-like ZnO monolayer, a typical two-dimensional material, has garnered enormous research attention due to its desirable optoelectronic properties and potential broad applications. However, the difficult in fabricating p-type ZnO material is still undergoing. Herein, the atomic and electronic properties of the adsorption of the selected organic molecules including electrophilic molecule tetracyano-p-quinodimethane (TCNQ) and nucleophilic molecule tetrathiafulvalene (TTF) on ZnO monolayers are carried out by the first-principles calculations. The results indicate that the adsorption of TCNQ on the ZnO monolayer leads to typical p-type doping, with a small gap (Ep) between the lowest unoccupied molecular and the valance band maximum with shallow acceptor states in the middle gap. However, the adsorption of TTF can obtain n-type doping of ZnO monolayer with the large gap (En) between the highest occupied molecular and the conduction band minimum with deep donor states. For TCNQ-ZnO monolayer, it exhibits the enhancement absorb in solar energy. There is a considerable charge transfer and strong non-covalent interaction between TCNQ and ZnO monolayer. The work function of TCNQ-ZnO ML is higher than that of pristine ZnO ML. However, the adsorption of TTF can significantly reduce the work function of ZnO ML, turning it into an n-type conduction semiconductor. Additionally, upon the adsorption of organic molecules, the layer thickness and the interlayer coupling as well as the adsorption concentration can modify the band gap of ZnO, and further tune the conduction type of ZnO. Remarkably, applying electric field can alter the number of hole and further greatly tune the electrical characteristic of TCNQ-ZnO ML. The current results can promote the applications of two-dimensional ZnO in nanoelectronics and optoelectronics.

Ferromagnetism in two-dimensional metal dibromides induced by hole-doping

Using spin-polarized first-principles calculations based on density functional theory, we study the stability, electronic properties and magnetic behavior induced by hole-doping of two-dimensional (2D) PbBr2 and HgBr2. Although inherently nonmagnetic, these materials can exhibit stable ferromagnetic order when hole-doped at densities above a few 1013 cm-2. We also examined the impact of intrinsic and extrinsic defects on inducing hole-doping and subsequent ferromagnetism. Our findings suggest that p-type doping can be achieved by Pb and Hg vacancies and Br antisites, but the latter behaves as deep acceptors. Among the possible dopants we considered, Li substituting Pb or Hg, and S replacing Br in 2D HgBr2, can produce shallow acceptor states near the valence band edges and potentially result in a stable ferromagnetic order in these 2D dibromides.

Valence-Band Electronic Structure of ZnO and ZnO:N: Experimental and Theoretical Evidence of Defect Complexes

Undoped and N-doped $\mathrm{Zn}\mathrm{O}$ films are grown by atomic layer deposition under O-rich conditions. Scanning photoelectron microscopy studies carried out on the films cross sections with a state-of-the-art resolution of 130 nm, allowed the study of the electronic structure of individual crystallites. In has been found that crystallites can be divided into two types, that differ in the electronic structure of the valence band. This finding, together with the cathodoluminescence images showing clustering the acceptor- and donor-related emission, unveiled that acceptors and donors are grouped in separate regions of the $\mathrm{Zn}\mathrm{O}$ and $\mathrm{Zn}\mathrm{O}$:$\mathrm{N}$ films. Density-functional-theory (DFT) calculations reveal that the complexes involving zinc vacancy, hydrogen, and nitrogen (in the case of $\mathrm{Zn}\mathrm{O}$:$\mathrm{N}$) modify the density of states in the valence-band region, so the experimentally observed differences in photoelectron spectra between crystallites evidence the grouping of acceptor complexes in some crystallites. DFT calculations also suggest that shallow acceptor states might be of $n\ensuremath{\cdot}{V}_{\mathrm{Zn}}$ origin. Nitrogen willingly joins such complexes and facilitates their formation. The separation of donors and acceptors and its tendency to group along the growth columns is of great relevance for future applications, as it indicates that eventual electronic devices should rather be constructed in a vertical architecture.

Theoretical calculations of the effect of nitrogen substitution on the structural, vibrational, and electronic properties of wolframite-type ScTaO4 at ambient conditions.

In this study, the effect of nitrogen substitution in wolframite-type ScTaO4 was investigated using density-functional theory calculations. First, structural and mechanical properties, as well as the dynamical stability of ScTaO4 were examined deeply for the ambient-pressure structure. Subsequently, we studied how lattice vibrations are affected by hydrostatic pressure and determined the elastic moduli of ScTaO4. The results of our study show that the monoclinic structure of ScTaO4 is rigid and non-compressible. In addition, band-structure calculations show that ScTaO4 has a wide direct band-gap of 4.04 eV, which in turn leads to a possible tuning of electronic properties. We have found that this task can be conducted by partially substituting oxygen atoms in the unit cell with nitrogen atoms. Both band-structure calculations and charge-density analyses revealed a narrowing in the band gap caused by the presence of nitrogen atoms, which act as a shallow acceptor state, resulting in weak repulsive interactions and structural distortions in both Sc and Ta coordination polyhedra; reducing the crystal symmetry from monoclinic to triclinic.

A DFT study on the electronic structure, magnetic and optical properties of Er doped ZnO: Effect of Er concentration and native defects

We report on first-principles study of the effect of Er concentration and intrinsic defects (VO and VZn) on the electronic structure and optical properties of Er-doped ZnO using the GGA + U method. The lattice constants and band gap of ZnO calculated in this work are in agreement with experimental values. The results showed that the band gap of Er-doped ZnO decreases with increasing Er concentration, while it increases with the presence of O or Zn vacancies. Furthermore, the existence of VO causes the formation of a deep donor level in the band gap. Er–ZnO + VZn is a degenerate p-type semiconductor, and shallow acceptor states are created near the Fermi level. The magnetic moment of doped ZnO increases and Er 4f electrons are responsible for the induced magnetic moments. The absorption coefficient enhances in the visible range in Er doped ZnO. In the Er–ZnO + VO model, both absorption and reflection are relatively enhanced in the visible range, leading to a decrease in light transmittance. Hence, the magneto-optoelectronic properties of ZnO could be improved by an optimal concentration of Er as well as with the presence of oxygen vacancies.

Tuning the carrier type and density of monolayer tin selenide via organic molecular doping

Utilizing first-principles calculations, charge transfer doping process of single layer tin selenide (SL-SnSe) via the surface adsorption of various organic molecules was investigated. Effective p-type SnSe, with carrier concentration exceeding 3.59 × 1013 cm−2, was obtained upon adsorption of tetracyanoquinodimethane or 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane on SL-SnSe due to their lowest unoccupied molecular orbitals acting as shallow acceptor states. While we could not obtain effective n-type SnSe through adsorption of tetrathiafulvalene (TTF) or 1,4,5,8-tetrathianaphthalene on pristine SnSe due to their highest occupied molecular orbitals (HOMO) being far from the conduction band edge of SnSe, this disadvantageous situation can be amended by the introduction of an external electric field perpendicular to the monolayer surface. It is found that Snvac will facilitate charge transfer from TTF to SnSe through introducing an unoccupied gap state just above the HOMO of TTF, thereby partially compensating for the p-type doping effect of Snvac. Our results show that both effective p-type and n-type SnSe can be obtained and tuned by charge transfer doping, which is necessary to promote its applications in nanoelectronics, thermoelectrics and optoelectronics.

The influence of shallow donor and acceptor states on carriers' lifetime in long wavelength HgCdTe infrared detectors

The influence of various recombination mechanisms in long-wavelength n-type HgCdTe structures on the lifetime of optically excited minority and majority carriers in a wide temperature range was analyzed. It has been shown that considering recombination involving shallow impurity states makes it possible to explain the dependence of minority carriers' lifetime on temperature without assigning too much importance to the SRH processes related to mercury vacancies.

Investigation of p-type doping in β- and κ-Ga2O3

We have systematically investigated the effects of all possible combinations of vacancies and silicon substitutions on the electronic structure of the β and κ phases of Ga2O3 using plane-wave density functional theory (DFT) methods. It was found that VGa defects are associated with a sufficient shift of the Fermi level to lower energy to induce p-type behavior, with formation energies in the range of 9.0 ± 0.2 eV. Calculations with single atom substitutions in the κ phase, including nitrogen, phosphorous, and silicon, did not show p-type character, although NO substitutions may lead to shallow acceptor states. In the pursuit of elucidating how MOCVD growth of Ga2O3 can result in p-type behavior, as indicated by experimental results in the literature, we examined the role of combining hydrogen and silicon substitutions. The results showed that p-type behavior is observable when gallium atoms are substituted for hydrogen within the coordination sphere of SiO substitutions. This shows that silicon can act as an amphoteric dopant for p-type Ga2O3 semiconducting materials when hydrogen is included with formation energies<6.0 eV.

Chlorine termination of selenium dangling bonds decreases tellurium-diffusion-induced acceptor states in hexagonal selenium

In this study, we examined the effects of doping chlorine (Cl) into crystalline selenium (c-Se) on diminishing the tellurium-diffusion-induced shallow acceptor states associated with the dangling bonds that appear at the ends of Se chains by theoretical analysis with the first-principles calculations. In this model, the negatively charged dangling bonds can be neutralized by inserting monovalent Cl atoms. Furthermore, the defect termination by doping Cl into fabricated Se thin films was experimentally demonstrated. Low-temperature cathodoluminescence measurements were implemented to show that acceptor-related emissions from the c-Se films were significantly decreased by Cl termination of the dangling bonds, experimentally confirming the results of the simulations.

Electrochemical reduction of nitrate on boron-doped diamond electrodes: Effects of surface termination and boron-doping level

This study is among the first to systematically study the electrochemical reduction of nitrate on boron-doped diamond (BDD) films with different surface terminations and boron-doping levels. The highest nitrate reduction efficiency was 48% and the highest selectivity in the production of nitrogen gas was 44.5%, which were achieved using a BDD electrode with a hydrogen-terminated surface and a B/C ratio of 1.0%. C–H bonds served as the anchor points for attracting NO3− anions close to the electrode surface, and thus accelerating the formation of NO3−(ads). Compared to oxygen termination, hydrogen-terminated BDD exhibited higher electrochemical reactivity for reducing nitrate, resulting from the formation of shallow acceptor states and small interfacial band bending. The hydrophobicity of the hydrogen-terminated BDD inhibited water electrolysis and the subsequent adsorption of atomic hydrogen, leading to increased selectivity in the production of nitrogen gas. A BDD electrode with a boron-doping level of 1.0% increased the density of acceptor states, thereby enhancing the conductivity and promoting the formation of C–H bonds after the cathodic reduction pretreatment leading to the direct reduction of nitrate.

Raman investigation of N‐implanted ZnO: Defects, disorder and recovery

AbstractUnderstanding defects, disorder and doping due to N implantation in ZnO is one of the most debated issues for the last few years. In the present work, a comprehensive investigation has been carried out using Raman, photoluminescence (PL) spectroscopy and grazing‐incidence X‐ray diffraction on 50 keV N ions implanted granular ZnO with different fluence (approximately up to 6.5% atomic concentration) along with postimplantation annealing. Raman investigation suggests that 275, 510, 643, and 857 cm−1 modes are directly related to nitrogen. Additionally, VZn may have some role in stabilizing 275 cm−1 mode. The broadening (or tailing) of E2low mode is related to vibration of distorted Zn sublattice, which may be a product of ion implantation generated defect cluster like VZn–VO. The distortion starts to reduce with annealing at elevated temperatures. Direct correlation between 555 cm−1 Raman mode and the tailing of E2low mode has been found. More defect clustering is vivid from the reduced PL of the ZnO samples with increasing implantation fluence. So, tailing of E2low Raman mode, increasing intensity of 555 cm−1 mode and nonradiative defect centers are of common origin. Both the ratios E1(2LO)/E1(LO) and E2high/E1(LO) can be used as parameters to measure the defective nature of ZnO after ion implantation/irradiation. Low temperature PL (selected samples) suggests absence of shallow acceptor states, although negative thermal quenching above 175 K has been observed (implantation fluence 1 × 1016 ions/cm2 and annealed at 500°C) which can be a signature of deep acceptors.

Boron–Oxygen Complex Responsible for Light‐Induced Degradation in Silicon Photovoltaic Cells: A New Insight into the Problem

Results available in the literature on minority carrier trapping and light‐induced degradation (LID) effects in silicon materials containing boron and oxygen atoms are briefly reviewed. Special attention is paid to the phenomena associated with “deep” electron traps (J. A. Hornbeck and J. R. Haynes, Phys. Rev. 1955, 97, 311) and the recently reported results that have linked LID with the transformation of a defect consisting of a substitutional boron atom and an oxygen dimer (BsO2) from a configuration with a deep donor state into a recombination active configuration associated with a shallow acceptor state (M. Vaqueiro‐Contreras et al., J. Appl. Phys. 2019, 125, 185704). It is shown that the BsO2 complex is a defect with negative‐U properties, and it is responsible for minority carrier trapping and persistent photoconductivity in nondegraded Si:B+O samples and solar cells. It is argued that the “deep” electron traps observed by Hornbeck and Haynes are the precursors of the “slow” forming shallow acceptor defects, which are responsible for the dominant LID in boron‐doped Czochralski silicon (Cz‐Si) crystals. Both the deep and shallow defects are BsO2 complexes, transformations between charge states and atomic configurations of which account for the observed electron trapping and LID phenomena.

Investigation on the p-type formation mechanism of nitrogen ion implanted ZnO thin films induced by rapid thermal annealing

N-doped p-type zinc oxide (ZnO) thin films were prepared by rapid thermal annealing (RTA) of nitrogen ion implanted high quality ZnO epitaxial layers. Annealing at 900 °C in a nitrogen atmosphere leads to the conversion of conductivity from n to p-type with a hole concentration of 9.60×1017 cm-3, which is reflected in photoluminescence spectra. To reveal the thermal activation and doping mechanism of this film, the samples were also analyzed by Raman scattering and X-ray photoelectron spectroscopy. The results indicate that elimination of the Zni related shallow donors and the formation of shallow acceptor complex No-VZn account for the stable p-type conductivity of N-doped ZnO. The shallow acceptor state is calculated at 0.161 eV above the valence band edge.

Formation of defects and impurities in MoSx and their effect on electronic properties

The authors investigate the thermodynamic and electronic properties of few- and monolayer MoS2 using density functional theory. They calculate the formation energies of S vacancies and the doping effect of C and O under various chemical environments which are commonly observed during MoS2 deposition. The results show that in the oxidizing condition, O readily gets incorporated into the MoS2 layer and assists in C doping. They analyze the bandgap and density of states of MoS2 at different concentrations of defect. They find that the bandgap decreases as the defect concentration increases, wherein the ordering of S vacancies has a more significant influence than that of the other two defects. The S vacancy and C dopants generate deep acceptor gap states within the bandgap of MoS2, while O dopant states are delocalized. They also observe that the carbon dopants form penetrative dimers, which lead to shallow acceptor states. This work contributes to the understanding of defect/impurity incorporation during deposition and prediction of electronic behavior of deposited low-quality MoS2 materials.

Direct evidence of causality between chemical purity and band-edge potential fluctuations in nanoparticle ink-based Cu2ZnSn(S,Se)4 solar cells

Kesterite solar cells based on chalcogenide Cu2ZnSn(S,Se)4 (CZTSSe) are a viable approach to thin film photovoltaics, utilising Earth-abundant, non-toxic elements. CZTSSe films produced from nanoparticle inks offer a cost-effective solution-based method of fabrication. However, improving efficiency in these devices has proved challenging, in part due to the presence of detrimental complex defects within the bulk of the CZTSSe absorber. In this study, the behaviour of nanoparticle-based CZTSSe absorbers and solar cells made from relatively low and high quality grade chemicals is investigated with a view to improving cost-effectiveness of the ink-based fabrication process. Photoluminescence spectroscopy revealed the presence of similar shallow acceptor plus shallow donor states in both low and high purity precursor absorbers. We demonstrate a relationship between the average depth of energy band-edge potential fluctuations and absorber quality where the higher grade chemical precursor-based absorber outperforms the lower purity version. In addition, the low purity precursor absorber had a higher total defect density resulting in a 10 meV increase in the average electrostatic potential fluctuations. Deep level transient spectroscopy in solar devices indicated the presence of detrimental deep defect states in both types of absorber. Notwithstanding the high purity precursor absorber with lower defect density, the power conversion efficiencies of both types of CZTSSe solar cells were similar (∼5%), implying an issue other than defects in the absorber bulk inhibits device performance as evidenced by quantum efficiency analysis and current–voltage measurements.

Synthesis and properties of tellurium-nitrogen co-doped ZnO micro-/nano-rods

Vertically aligned tellurium-nitrogen co-doped ZnO micro-/nano-rods with a hexagonal symmetry were fabricated via the chemical vapor transport method. The as-grown samples exhibited excellent crystalline. The incorporation of tellurium and nitrogen was confirmed by X-ray photoelectron spectra, and we found tellurium is beneficial to reinforce the nitrogen doping efficiency. Combining with Raman and variable temperature photoluminescence characterizations, we have demonstrated the suppression of zinc interstitial related shallow donor defects due to tellurium incorporation. Meanwhile, the intensity of the emission at 3.311 eV, which was ascribed to the radiative recombination of the free electron to the zinc vacancy related shallow acceptor states, was enhanced in the photoluminescence spectra of the sample with higher tellurium and nitrogen concentration. Our results show that tellurium-nitrogen co-doping might be a possible path for realizing reliable p-type one-dimensional ZnO nanostructures.

Interaction Rates of Group-III and Group-V Impurities with Intrinsic Point Defects in Irradiated Si and Ge

A comparative study of interactions of shallow impurities with primary defects in oxygen- and carbon-lean moderately doped Si and Ge subjected to irradiation with 0.9 MeV electrons, 60Co gamma-rays, and 15 MeV protons at room temperature is presented and discussed. For the quantitative characterization of such interactions, changes in the total concentration of the original shallow group-V donor or group-III acceptor impurities in the irradiated materials are determined by Hall effect measurements over a wide temperature range. Losses of the shallow donor or acceptor states in the irradiated Si and Ge are indicative of their removal rates that can be used for estimation of production rates of primary defects interacting with the dopants. Some important factors affecting the interactions between primary defects and shallow impurities in Si and Ge are highlighted.

P-type conductivity in hydrogenated radio frequency sputtered tin oxide thin films

P-type oxide semiconductors are being intensively investigated for their potential use in optoelectronic and microelectronic applications, such as active electronics and transparent transistors. In this work, the p-type conductivity of tin oxide thin films was examined. The films were deposited using radio frequency magnetron sputtering in an atmosphere containing a mixture of hydrogen and argon. The structural, chemical, optical, and electrical properties of the films were analyzed as the hydrogen-to-argon concentration was varied from 0% to 20%. The films were polycrystalline except those deposited under the highest hydrogen concentration which became amorphous. Chemical analysis of the films revealed a substantial concentration of oxygen vacancies. Optically, the films were transparent and their band gaps decreased as the hydrogen concentration increased. The pure tin oxide films had n-type conductivity, which was attributed to oxygen vacancies. However, the conductivity was converted to p-type as the hydrogen concentration increased. This was attributed to the formation of hydrogen shallow acceptor states, which was supported by the observation of a red-shift of the band gap. The optimum hydrogen concentration for p-type conductivity was found to be 10%. The p-type character of the films was demonstrated by fabricating a hetero p-n junction based on SnO2/Si, which exhibited the expected non-linear diode rectification behavior.

Influence of Antimony doping on the electronic, optical and luminescent properties of ZnO microrods

We synthesized Sb-doped ZnO (ZnO:Sb) microrods with varying Sb content and carried out a systematic study on their structural, optical and photoluminescent properties. Scanning electron microscopy revealed a hexagonal morphology of the as grown microrods, while the x-ray photoelectron spectroscopy (XPS) and Ultra Violet-Visible spectroscopy results indicated the incorporation of Sb dopants into the ZnO lattice. XPS and x-ray diffraction analysis revealed that all ZnO:Sb microrods with different Sb doping possessed typical wurtzite structure and had no other impurity phases. Furthermore, the XPS results showed that Sb ions are in an oxidation state between 3 + and 5+, indicating the existence of an acceptor complex in the ZnO:Sb microrods. In addition, another deep acceptor originated form 2 + oxygen vacancies was identified. Photoluminescence (PL) measurements confirmed the formation of the (SbZn-2VZn) shallow acceptor states in the ZnO:Sb microrods. PL measurements at low temperature showed strong violet luminescence, which is originated from free-electron to acceptor level (FA) transitions. The FA emission showed a slight blue shift with the increase of the temperature. As a result of Sb incorporation into the ZnO lattice, we observed a red shift in the ZnO:Sb microrods' energy gap with the increase of Sb doping. This red shift is attributed to the formation of acceptor levels inside the ZnO band gap. The identification of this acceptor level in these homogeneous single-phase ZnO:Sb microrods provides strong promise of p-type conductivity of ZnO by Sb doping.