Redistribution of native defects and photoconductivity in ZnO under pressure

Abstract

Control and design of native defects in semiconductors are extremely important for industrial applications.Here, we investigated the effect of external hydrostatic pressure on the redistribution of native defects and their impact on structural phase transitions and photoconductivity in ZnO. We investigated morphologically distinct rod-(ZnO-R) and flower-like (ZnO-F) ZnO microstructures where the latter contains several native defects namely, oxygen vacancies, zinc interstitials and oxygen interstitials. Synchrotron X-ray diffraction reveals pressure-induced irreversible phase transformation of ZnO-F with the emergence of a hexagonal metallic Zn phase due to enhanced diffusion of interstitial Zn during decompression. In contrast, ZnO-R undergoes a reversible structural phase transition displaying a large hysteresis during decompression. We evidenced that the pressure-induced strain and inhomogeneous distribution of defects play crucial roles at structural phase transition. Raman spectroscopy and emission studies further confirm that the recovered ZnO-R appears less defective than ZnO-F. It resulted in lower photocurrent gain and slower photoresponse during time-dependent transient photoresponse with the synergistic application of pressure and illumination (ultra-violet). While successive pressure treatments improved the photoconductivity in ZnO-R, ZnO-F failed to recover even its ambient photoresponse. Pressure-induced redistribution of native defects and the optoelectronic response in ZnO might provide new opportunities in promising semiconductors. † Electronic supplementary information (ESI) available: Pressure-dependent X-ray diffraction patterns, pressure-dependent Raman spectra of ZnO-R and ZnO-F, whole prole tting of the X-ray diffraction patterns of both ZnO micro-structures, pressure-dependent ratio of the d-spacing of (100) and (002) planes of wurtzite, (200) and (220) planes of rocksalt phase of both the ZnO micro-structures; pressure-dependent I-V characteristics and resistivity of ZnO-R and ZnO-F. See

High-Pressure Phase Transitions of Morphologically Distinct Zn2SnO4 Nanostructures

“…24,33 In this context, the impacts of various microstructural features, for instance, the distinct shape or morphology, dimension, and homogeneity, of the materials are often found to coincide with the size effects of the nanocrystals in many studies. 9,14,33,36−39 The microstructure-induced strains appearing at the contact points of the grains of the materials may cause significant structural distortions, which also contribute to the transition pressure and phase stability. 8,14…”

“…Over the past few decades, semiconducting nanomaterials of different types and structures have greatly contributed to significant progress in nanoscience and technology due to their salient and flexible opto-electronic, photonic, magnetic, and mechanical features. − As far as the exploration of semiconducting nanomaterials under high pressure is concerned, studies on a variety of materials, namely, the group (IV) elements C, Si, and their compound SiC; group (II–VI) compounds such as ZnS, ZnSe, CdS, CdSe, and CdTe; group (IV–VI) PbS; group (III–V) GaN and AlN; the superhard material, BC 2 N; binary oxides such as TiO 2 , ZnO, SnO 2 , Fe 2 O 3 , and CeO 2 ; the rare-earth oxide, Ho 2 O 3 ; wide band-gap oxides such as β-Ga 2 O 3 and Y 2 O 3 ; p-type compounds including CuO, CoO, and MnS; n-type BaTiO 3 ; and narrow band-gap layered group (V–VI) semiconductors such as Bi 2 Te 3 , have been performed. ,,,,− Most of these studies have shed light on the interesting kinetics of pressure-induced first-order, solid–solid structural transformations, compressibilities, bulk moduli, and stiffness or hardness of the materials. − The thermodynamics of phase transformations and relative stabilities of the phases have also been noted in several studies. − ,− Although there are conflicting trends in the reported transition pressures relating to the Hall–Petch effect that is found as bulk materials are reduced to smaller crystallites, a significant influence from nanosized particles or grains has been commonly suggested as the cause for the dissimilar types of nucleation, growth dynamics, phase transition pathways, and even sequences of the phase transitions or amorphizations of semiconducting materials under high pressure. ,,, A specific size, at which the typical nanoscale effects start to occur in materials, has also been defined as their respective “critical size” in several cases. ,, The contributions of the nanoscale-induced differences in the surface energies of the relevant phases mainly account for the stabilities of the corresponding structures. …”

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“…Indeed, such an effect caused by hydrostatic pressure was observed in ZnO microstructures. [22] The Zn accumulation on the hexagonal grain boundaries is in accordance with the decrease of the intensity of the green PL band in ZnO ceramics sintered from the milled powder, which is explained by the decrease of Zn i interstitials.…”

Pressure-Dependent Colossal Resistivity and Anomalous Optical Signatures in FeSbO4

“…In a report, Petrera et al observed the spin− lattice relaxation in FeSbO 4 with particle size of ∼250 to 650 Å after calcining a sample with particle sizes of 50 Å above 700 °C. 18 In our study, the observed pressure-induced reduction of crystallite size from ∼58 to ∼53 nm and the associated microscopic strain derived from the Williamson−Hall plot analysis 37 shown in Figure S8b might affect this irreversible phase transition. However, essentially, the lattice distortion at moderate pressures turns semiconducting FeSbO 4 into a highly insulating phase.…”