Session K05: Defects and Doping in Oxide Semiconductors

Comparative energetics of nanovoid growth in wide- and ultrawide-band gap semiconductors

The emergence of wide- and ultra-wide-bandgap semiconductors has helped expand the field of power devices. The wide band gaps enable these materials to handle large applied voltages, but their reliability in radiation-rich environments is still under investigation. In this project, we examine the comparative energetics of void growth, nucleated at either pre-existing vacancies or vacancies generated by ion beams, in SiC, GaN, AlN, AlGaN alloys, and β–Ga₂O₃. We employ density functional theory to calculate the energies to sequentially remove atoms neighboring the initial cation or anion vacancy to distant interstitial sites. Low atom-removal energies, the order of the material’s band gap, suggest that the material would be vulnerable to nanovoid growth, with the necessary energy provided either by the high voltages, Joule heating, recombination of excess electron-hole pairs, or combinations thereof. The potential roles of nanovoid formation in the degradation and failure of electronic devices will be discussed.   

Telecom wavelength single-photon emitters in β-Ga2O3

Solid-state defects, essentially artificial atoms trapped in solids, in wide band-gap semiconductors exhibit promising characteristics for applications in quantum computing, communication, and sensing. In this study, we report first principles calculations of transition metal impurities, such as chromium (Cr), manganese (Mn), iron (Fe), and cobalt (Co), at gallium (Ga) sites during the synthesis of β-Ga₂O₃. These impurities are found to be more stable when they form clusters in octahedral and tetrahedral positions within the Ga lattice. Thermodynamic charge transition level analysis suggests these impurities act as deep acceptors and donors within the band gap of β-Ga₂O₃. Notably, these transition metal clusters provide a two-level system within the band gap in the +1 charge state, making them suitable single-photon emission sources. The zero-phonon lines of these charged clusters are situated around 1 eV, a proximity to the telecom band, enabling their operation at telecom wavelengths for low-loss fiber transmission. Our findings propose that transition-metal clusters in β-Ga₂O₃ holds promise for quantum information science.   

Different concentrations of Ti4+ as a donor and thermoelectric/Electrical Properties of Bi2-xTixO3

Bi(2-x)TixO3 (x = 0, 0.01, 0.03. & 0.05) (BO-xT) ceramics are prepared by

conventional solid-state route followed by low sintering temperatures. X-ray

diffraction analyses show the presence of the monoclinic phase of Bi2O3. The

electrical conductivities at room temperature concerning the frequency (ranging

from 25 kHz to 5 MHz) and Seebeck Coefficient ranging from 50°C to 400°C were

measured. With an increase in Ti (dopant) content, the conductivity and Seebeck

Coefficient increased with the temperature increment. The BO-0.03T has the

highest Seebeck value (47 μV/°C), which shows a higher carrier concentration. In

terms of electrical conductivities, the BO-0.05T ceramic shows the maximum

electrical conductivity, i.e. 2.0 × 10−9 μS/m as compared to other samples, which

exhibit the presence of free electrons. Moreover, relative permittivity (dielectric

constant) and dielectric loss are also measured concerning the frequency at room

temperature to investigate the dielectric behaviour of the ceramics. This lowtemperature

sintering ceramics will open new applications in the domain of

electronic materials.   

What Simulated NMR Chemical Shifts can teach us about Hydrogen Species in Perovskite BaTiO3

Numerous studies investigating hydrogen incorporation in titanate ATiO₃ perovskites find species including protons as interstitials and on A-site positions, hydride ions on oxygen sites, surface OH groups, and trapped H₂ molecules on Ti positions depending on the chemical potential of oxygen. Which species dominate depends on synthesis and processing conditions; yet, fingerprinting the density and distribution of species experimentally remains challenging. Nuclear Magnetic Resonance (NMR) spectroscopy is sensitive to the local chemical environment and chemical shifts arise from the electron density surrounding the hydrogen nuclei. This makes NMR a well-suited probe for determining the hydrogen species present; however, it does not directly provide the charge state of the nuclei, but a relative shift which must be interpreted. In recent literature on BaTiO₃-derived oxyhydrides, the hydride peak is assigned to chemical shifts ranging from 4.4 to -60 ppm, while the additional peaks between 6.5 and 1 ppm are attributed to surface OH or residual Ca(OH)₂ in the samples from fabrication; protons have not been identified. To that end, we present results from density functional theory simulations of NMR chemical shifts for the BaTiO₃-derived oxyhydrides, with a focus on testing each possible hydrogen species to unambiguously identify each peak. Our work shows how solid-state NMR can be used to identify the nature of the hydrogen present in titanate perovskites and determine their relative concentrations.   

Extended Defect Control in Stannate Perovskite Oxide Thin Films Using Nanoscale Patterning

Stannate perovskite oxides like BaSnO₃ and SrSnO₃ are known to have a wide-range of extended defects in their structures, introducing unique and desirable properties for application in devices. Hence, understanding the origins of their formation and methods to control defect concentration are important. In this work, we use nanoscale patterning of the substrate surface, SrTiO₃ using a Ga ion beam in a focused ion beam instrument, to create localized atomic-scale roughness for controlling defect nucleation. By adjusting the Ga ion-beam doses, different surface structures are created onto the substrate, which is utilized in the growth of SrSnO₃ and La: BaSnO₃ thin films by hybrid molecular beam epitaxy (MBE).  X-ray diffraction and scanning electron microscopy (SEM) of the films after growth show in both cases, films are epitaxial, high-quality and grow over the ion-beam modified areas of the substrate. Atomic structure of the films in the patterned areas are studied using analytical scanning transmission electron microscopy (STEM), which reveals: (i) planar Ruddlesden Popper type of faults present in the SrSnO₃ growing in the patterned channels with a five-fold higher density when compared to the bulk defect free film, and (ii) four-fold higher concentration of line defects, namely single and dissociated dislocations in the case for La: BaSnO₃, thus demonstrating the use of this method to control defect growth.   

Oral: Room Temperature Persistent Photoconductivity of KTaO3 single crystals.

Persistent photoconductivity is a phenomenon in which the conductivity of a material increases upon light exposure and remains without the source of exciting illumination. Potassium tantalate, KTaO₃ (KTO), is a cubic perovskite semiconductor with a wide bandgap of 3.6 eV. This novel material is a subject of interest due to its possible applications in electronic and storage devices.

In this work, KTO crystals were annealed in a silica ampoule filled with hydrogen gas and a piece of tantalum wire, creating an oxygen-poor environment and thus oxygen vacancies. IR spectroscopy showed a transmission threshold at 340 nm, free carrier absorption, and a reduction of the hydrogen peaks post-anneal treatment. After exposing the crystals to a 340 nm light-emitting diode (LED), evidence of persistent photoconductivity (PPC) was observed. Hall effect measurements indicate an increase in n-type conductivity of three orders of magnitude that lasts for several weeks. The PPC effect can be reversed by an open-air anneal and re-induced by repeating the irradiation.   

Hydrogen-related defects in KTaO3

Hydrogen is a ubiquitous impurity in semiconductors, in most cases sitting at interstitial sites and passivating other defects/impurities, changing their charge states. In some semiconductors, such as in wide-band-gap oxides, H can also be found in very unusual sites, such as replacing an oxygen atom (H_O), acting as a source of n-type conductivity. I.e., in these cases, H_O is a shallow donor. Directly probing the presence of H_O has been quite challenging, requiring a combination of experimental techniques. Recent experiments and calculations in SrTiO₃ indicate that H_O can also lead to long-lasting persistent photoconductivity (PPC) at room temperature, an effect that is also quite unusual since PPC is commonly only observed at low temperatures and reported to last for short periods. In this presentation, we report the results of first-principles calculations based on hybrid density functional theory for hydrogen-related defects in KTaO₃, exploring the stability of both interstitial (H_i) and the substitutional H_O configurations, and their interactions with other defects, especially the Ta and K vacancies. We also discuss the effects of optical excitation of the defect stability and present results for the frequencies of the local vibration mode frequencies, aiming at the experimental identification of the different hydrogen impurity configurations.   

First-principles study of proton migration in indium oxide

Indium oxide is a semiconductor with a band gap of 2.68 eV, widely used as a transparent conducting oxide when doped with Sn. Previous work has shown that hydrogen also acts as a shallow donor [1]. Understanding diffusion of interstitial hydrogen is essential to control device properties such as conductivity and carrier density. As such, we investigate the migration of hydrogen in In₂O₃ using first-principles calculations based on density functional theory. We employ a hybrid functional to precisely describe the three possible charge states of hydrogen in the material and find that H⁺ (i.e., a proton) is most stable. We decompose the possible long-range migration paths of H⁺ into short-range mechanisms. Two mechanisms are involved in the motion of protons: rotations of the hydrogen atom around oxygen atoms and jumps between two oxygen atoms. We calculate the migration barrier heights using the nudged elastic band method. We establish that the jump between oxygen atoms is the rate-limiting step in the long-range migration mechanism, with an energy barrier of 0.8 eV.

[1] S. Limpijumnong et al., Phys. Rev. B 80, 193202 (2009).   

Indium defects and their diffusion in monoclinic (Inx­Ga1-x)2O3

Measurements on thin film (In_xGa_1-x)₂O₃ are compared to hybrid density functional theory calculations to study how defects relate to an increase in n-type conductivity.  XRD shows that the films are monoclinic up to 10% In (x = 0.1), and a lack of broadening indicates that the inclusion of In does not introduce an appreciable amount of dislocation defects.  Theory predicts expansion of the lattice due to the inclusion of indium which results in a reduction of the electronic band gap as a function of In concentration; Tauc plots confirm this as the electronic band gap decreases from 4.9 eV to 4.63 eV as In concentration increases to 10%.  Hall and magnetoresistance measurements reveal an increase in n-type conductivity as the In concentration is increased from x=0 in pure Ga₂O₃ to x=0.1 in In_0.1Ga_0.9O₃, a somewhat anomalous behavior as In and Ga are isoelectronic.  Formation energy calculations reveal, however that a stable In-based defect complex comprised of a substitutional In (In_Ga) and interstitial In (In_i) may form spontaneously in a +2 charge state, resulting in a near conduction band edge, singly occupied band gap level 0.2 eV below the band edge.  There also exists a (+/0) transition state due to this defect 0.5 eV above the conduction band edge, which can be attributed to the increase in n-type conductivity seen in experiment.  Diffusion paths and mechanisms from hybrid density functional theory nudged elastic band calculations will also be discussed.     

Environment Around Sn and Ge in Doped Fe2O3: Substitution Vs. Precipitation

Hematite is predicted to have a high water-splitting efficiency but is limited by a low carrier conductivity. This limitation can be overcome by increasing the carrier concentration through doping; however, the formation of dopant precipitates hinders the free polaron conductivity. Recent theoretical calculations predict that a critical concentration of dopants can be incorporated into the hematite structure at a given annealing temperature, above which a simple dopant-oxide precipitate begins to form. This critical point varies with the type of dopant and for Sn and Ge, synthesized at 800^oC, they are 0.22% and 0.44% respectively. Using EXAFS data analysis, we test these predictions by determining the fraction of dopants going into hematite and the fraction going into a dopant-precipitate for various dopant concentrations. We find that at low concentrations the dopants disperse evenly into the hematite lattice and use these results to model the dopant in hematite, while the dopant in a dopant precipitate is modeled using results from diffraction data. Our results suggest the critical point is between 0.23-0.41% and 0.20-0.53% for Sn and Ge respectively. Above the critical point the fraction of dopants going into the precipitate increases logarithmically with total dopant concentration. We find that a simple dopant-oxide SnO₂ forms in the Sn doped system, whereas a more complex structure Fe₈Ge₃O₁₈ forms in the case of Ge. Our results for the saturation point are consistent with theoretical predictions.    

The ADAQ defect database for quantum applications

A handful of point defects in a few materials are being studied for quantum applications. Could there be undiscovered defects with better properties? To address this, we have developed an online database built using ADAQ [1] which holds the high-throughput results of over 30000 processed defects. Among the hosts are well-studied quantum materials like diamond, where we recently found sodium-related defects to have interesting properties (such as high spin) for quantum applications [2]. There are also novel quantum materials like CaO, where we found the Bi_Ca-Vac_O defect with coherence time beyond seconds at the clock-like transition [3]. These discoveries are due to the high-throughput database ADAQ, and in this presentation, we explore it to highlight some promising systems.

[1] https://httk.org/adaq/

[2] Davidsson, J., Stenlund, W., Parackal, A. S., Armiento, R., & Abrikosov, I. A. (2023). Na in Diamond: High Spin Defects Revealed by the ADAQ High-Throughput Computational Database. arXiv preprint arXiv:2306.11116. https://arxiv.org/abs/2306.11116

[3] Davidsson, J., Onizhuk, M., Vorwerk, C., & Galli, G. (2023). Discovery of Atomic Clock-Like Spin Defects in Simple Oxides from First Principles. arXiv preprint arXiv:2302.07523. https://arxiv.org/abs/2302.07523   

Ultraviolet Persistent Emitting Phosphors and Upconversion Charging

We have developed ultraviolet (UV) persistent emitting phosphors utilizing upconversion charging, with blue or white LEDs as excitation sources.  Unlike conventional phosphors that require a dark environment, these UV phosphors can glow even in broad daylight. The upconversion charging mechanism, facilitated by readily available LEDs, eliminates the need for high-energy radiation sources and bulky instruments, opening doors to innovative and practical applications. 

In this talk, we will use various typical phosphor systems to demonstrate and analyze the dynamical processes of upconversion changing and UV emitting, including energy transfer, competition between trap filling and photo-stimulated detraping, and the effects of ambient conditions. Furthermore, we will explore several potential applications that have become feasible due to these pioneering advancements.   

Hybrid functional calculation of intrinsic defects in SbSeI

In recent years, the quasi-one-dimensional semiconductor Sb₂Se₃ has achieved a remarkable efficiency breakthrough over 10% [1] as a solar absorber. Due to their similar structures, SbSeI has also received significant attention. Particularly, the lone pair electrons in the valence band cause strong anti-bonding states formed by the hybridization of Sb 5s-Se 4p, as well as Sb 5s-I 5p orbitals, so the electronic properties can be more benign. However, its efficiency still remains at 4.1% [2]. Considering the crucial role of deep-level defects in limiting the photovoltaic efficiency of Sb₂Se₃, it is essential to investigate the defect properties of SbSeI to explore strategies for enhancing its photovoltaic performance. First-principles calculation reveals that SbSeI is, in fact, a semiconductor with high defect tolerance. Defects with high concentrations (10¹⁷-10¹⁸ cm^-3) in SbSeI are shallow (Se_I and I_Se), thus eliminating non-radiative recombination centers. However, these defects induce severe donor-acceptor compensation effect, resulting in carrier concentrations below 10¹¹ regardless of the growth condition and thus limiting the photovoltaic efficiency. To overcome the deficiency, we propose that p-type doping is favorable under Se-rich conditions, whereas under Sb-poor conditions, n-type doping is favored. Given the high defect tolerance of this material, it is expected that proper doping can significantly enhance the efficiency of photovoltaic devices.   

Cryogenic radiation-hard fully-depleted silicon-on-insulator field-effect transistors

The radiation-hard transistors for space electronics need to operate at low temperatures and under irradiation by charged particles, x-rays, and gamma rays [1]. Fully-depleted silicon-on-insulator (FD-SOI) technologies, thanks to the small field-effect transistor (FET) volume, superior electrostatics, undoped channel, and threshold voltage V_TH adjustability with back-biasing (BB), are more suitable for radiation hardness and low-temperature operations as compared to bulk CMOS FETs [2].

In this work, we irradiated our custom FD-SOI FETs [3] with 7.5 MeV protons for the total irradiation dose of 30 Mrad(Si), at the radiation-hard technology threshold [1]. Studying transfer/output characteristics and low-frequency current noise of irradiated FETs with different dimensions, we observed that introduced defects and impact of irradiation, which degrades FET's electrostatics, e.g. in terms of subthreshold swing and V_TH, can be effectively mitigated through reverse BB (pushing the channel toward the top Si/SiO₂ interface) from 293 K down to 4 K.

In summary, we demonstrate that our FD-SOI technology is a promising candidate for room temperature and cryogenic radiation-hard applications.

[1] IEEE Trans Nucl Sci, vol. 34, no.6, pp.1621-1628 (1987).

[2] IEEE Int. SOI Conf., Tempe, AZ, USA, pp.1-2 (2011).

[3] arXiv:2208.12131 (2022).