Light-Induced Peroxide Formation in ZnO: Origin of Persistent Photoconductivity

Native point defects in antiperovskite Ba3SbN: a promising semiconductor for photovoltaics

We present a theoretical investigation on intrinsic defects of hexagonal antiperovskite Ba₃SbN, a promising lead-free semiconductor for photovoltaics. Our hybrid functional calculations reveal that Ba, Sb and N vacancies, and N interstitials become major point defects in Ba₃SbN. Conversely, other interstitials and antisites have large formation energies and their concentrations are controllable. Herein, defect levels and configuration coordinate diagrams for the major defects are analyzed, thereby revealing that defect-assisted carrier recombination is ineffective. Thus, Ba₃SbN can be a defect-tolerant semiconductor that retains excellent optoelectronic properties despite the presence of point defects. By elucidating the stability of the intrinsic defects of Ba₃SbN and their impacts on the carrier capture process, this work will pave the way for the development of a new class of high-performance solar cells based on antiperovskite semiconductors.

Ultrashort and metastable doping of the ZnO surface by photoexcited defects

Shallow donors in semiconductors are known to form impurity bands that induce metallic conduction at sufficient doping densities. The perhaps most direct analogy to such doping in optically excited semiconductors...

Tail state mediated conduction in zinc tin oxide thinfilm phototransistors under below bandgap optical excitation

We report on the appearance of a strong persistent photoconductivity (PPC) and conductor-like behaviour in zinc tin oxide (ZTO) thinfilm phototransistors. The active ZTO channel layer was prepared by remote plasma reactive sputtering and possesses an amorphous structure. Under sub-bandgap excitation of ZTO with UV light, the photocurrent reaches as high as ~ 10^-4 A (a photo-to-dark current ratio of ~ 10⁷) and remains close to this high value after switching off the light. During this time, the ZTO TFT exhibits strong PPC with long-lasting recovery time, which leads the appearance of the conductor-like behaviour in ZTO semiconductor. In the present case, the conductivity changes over six orders of magnitude, from ~ 10^-7 to 0.92/Ω/cm. After UV exposure, the ZTO compound can potentially remain in the conducting state for up to a month. The underlying physics of the observed PPC effect is investigated by studying defects (deep states and tail states) by employing a discharge current analysis (DCA) technique. Findings from the DCA study reveal direct evidence for the involvement of sub-bandgap tail states of the ZTO in the strong PPC, while deep states contribute to mild PPC.

Fundamental Limit of the Emission Linewidths of Quantum Dots: An Ab Initio Study of CdSe Nanocrystals

The emission linewidth of a semiconducting nanocrystal (NC) significantly affects its performance in light-emitting applications, but its fundamental limit is still elusive. Herein, we analyze the exciton-phonon coupling (EPC) from Huang-Rhys (HR) factors using ab initio calculations and compute emission line shapes of CdSe NCs. When surface traps are absent, acoustic modes are found to dominate EPC. The computed linewidths are mainly determined by the size of NCs, being largely insensitive to the shape and crystal structure. Linewidths obtained in this work are much smaller than most measurements on homogeneous linewidths, but they are consistent with a CdSe/Cd_xZn_1-xSe (core/shell) NC [Park, Y.-S.; Lim, J.; Klimov, V. I. Nat. Mater. 2019 18, 249-255]. Based on this comparison, it is concluded that the large linewidths in most experiments originated from internal fields by surface (or interface) traps or quasi-type II band alignment that amplifies EPC. Thus, the present results on NCs with ideal passivation provide the fundamental minimum of homogeneous linewidths, indicating that only the CdSe/Cd_xZn_1-xSe NC has achieved this limit through well-controlled synthesis of shell structures. To further verify the role of internal fields, we model NCs with charged surface defects. We find that the internal field significantly increases HR factors and linewidths, in reasonable agreement with experiments on single cores. By revealing the fundamental limit of the emission linewidths of quantum dots, this work will pave the way for engineering quantum dots with an ultrasharp spectrum.

Lanthanum Doping Enabling High Drain Current Modulation in a p-Type Tin Monoxide Thin-Film Transistor

Effects of lanthanum (La) loading on the structural, optical, and electrical properties of tin monoxide (SnO) films were examined as a p-type semiconducting layer. La loading up to 1.9 atom % caused the texturing of the tetragonal SnO phase with a preferential orientation of (101), which was accompanied by the smoother surface morphology. Simultaneously, the incorporated La cation suppressed the formation of n-type SnO₂ in the La-doped SnO film and widened its optical band gap. These variations allowed the 1.9 atom % La-loaded SnO film to have a high hole mobility and carrier density, compared with the La-free control SnO film. The superior semiconducting property was reflected in the p-type thin-film transistor (TFT). The control SnO TFTs exhibited the field-effect mobility (μ_SAT) and I_ON/OFF ratio of 0.29 cm² V^-1 s^-1 and 5.4 × 10², respectively. Enhancement in the μ_SAT value and I_ON/OFF ratio was observed for the TFTs with the 1.9 atom % La-loaded SnO channel layer: they were improved to 1.2 cm² V^-1 s^-1 and 7.3 × 10³, respectively. The reason for this superior performance was discussed on the basis of smoother morphology, suppression of disproportionation conversion from Sn²⁺ to Sn + Sn⁴⁺, and reduced gap-state density.

Temperature-dependent thermal behavior of impurity hydrogen trapped in vacancy-type defects in single crystal ZnO

Interacting nature between impurity hydrogen atoms and vacancy-type defects in single crystal ZnO was investigated by means of positron annihilation lifetime spectroscopy. In order to clarify the observation of their thermal behavior, the sample was implanted with ¹H⁺ using an electrostatic accelerator. After the implantation, the positron lifetime became shorter, which suggests that the hydrogen atoms were captured by zinc vacancies (V_Zn) to form vacancy-hydrogen complexes (V_Zn + nH). The complexes decompose by heat treatment: most of the hydrogen atoms gradually dissociate from V_Zn + nH in the temperature range 393-773 K. It was also suggested that large vacancy clusters were formed by the agglomeration of smaller clusters during the process of stepwise isochronal annealings at temperatures from 773 to 1073 K, and their decomposition took place at 1173-1373 K. Temperature-dependent thermal behaviors of hydrogen atoms and vacancy-type defects in ZnO are discussed.

Transformation from Film to Nanorod via a Sacrifical Layer: Pulsed Laser Deposition of ZnO for Enhancing Photodetector Performance

A novel fabrication method for single crystalline ZnO nanorods by pulsed laser deposition (PLD) using a chemical-bath-deposited ZnS seed layer is proposed. For the substrate temperature (T_s) lower than 700 °C, the PLD-ZnO showed a polycrystalline phase and film-type morphology, resulting from the ZnS seed layer with a cubic phase. However, the ZnS film became a sacrifical layer and single crystalline ZnO(002) nanorods can be achieved at T_s of 900 °C, where ZnS was decomposed to zinc metals and sulfur fumes. The transformation from ZnO film to nanorod microstructure was demonstrated with the change of ZnS layer into Zn grains. Enhanced performance of the metal-semiconductor-metal photodetectors were fabricated with ZnO/ZnS samples grown at T_s of 500, 700, and 900 °C. The responsivities (@1 V and 370 nm) of these three devices were 1.71, 6.35, and 98.67 A/W, while their UV-to-visible discrimination ratios were 7.2, 16.5, and 439.1, respectively. Obviously, a higher light-capturing efficiency was obtained in the 900 °C-grown ZnO/ZnS device owing to its one-dimensional nanostructure with high crystal quality. The results indicate PLD combined with a sacrifical nanostructure is a promising method for obtaining high-quality ZnO nanorods, which paves the way for the fabrication of high performance ZnO-based devices.