Pressure dependence of the optical properties of wurtzite and rock‐salt Zn_1–xCo_xO thin films

Abstract

In this paper we investigate the electronic structure of Zn 1-x Co x O by means of optical absorption measurements under pressure. Thin films of Zn 1-x Co x O with different Co content (from 1 to 30%) were prepared by pulsed laser deposition on mica substrates. Absorption spectra exhibit three main features that are clearly correlated to the Co content in the films: (i) absorption peaks in the infrared associated to crystal-field-split internal transitions in the Co 3d shell, with very small pressure coefficients due to their atomic character; (ii) a broad absorption band below the fundamental edge associated to charge transfer transitions, that exhibit relatively large pressure coefficients, indicating that the Co 3d final states must be strongly hybridized to the conduction band; and (iii) a blue-shifted fundamental absorption edge associated to band to band transitions with a pressure coefficient close to that of pure ZnO. In the up-stroke the transition pressure from wurtzite to rock-salt phase decrease almost linearly as the Co increases, from 9.5 GPa in pure ZnO to about 6.5 GPa for x = 30%. In the down-stroke pressure we observe a similar behavior, yielding a metastable rock-salt phase at room pressure, after a pressure cycle up to 15 GPa.

Synthesis of rock-salt MeO–ZnO solid solutions (Me=Ni2+, Co2+, Fe2+, Mn2+) at high pressure and high temperature

“…Recently, it has been shown that the pressure of the wurtzite-to-rock-salt transformation decreases for Co-and Mn-doped ZnO as compared to pure ZnO [7][8][9]. The high pressure -high temperature (HPHT) synthesis of ZnO-rich rs solid solutions Mn 0.25 Zn 0.75 O and Co 0.27 Zn 0.73 O in the form of thin films has been reported [8,9], however, in the case of bulk samples the situation may be different.…”

Electrical properties of bulk n‐ZnO single crystals under hydrostatic pressure

“…This effect should be especially pronounced in diamond-like direct-bandgap semiconductors of n-А 3 В 5 and n-А 2 В 4 С 5 2 families, which is caused by, alongside with a χ(Р) dependence, a noticeable m n (P) dependence. As has been mentioned above, a sharp decrease of the resistivity by some orders of magnitude is observed, which testifies to the presence of the phase transition from diamond to NaCltype structure at P > 9 GPa [5,6]. According to formulas derived in the framework of the 'heterophase structureeffective medium' model [12], the relative volume fractions of the phases in the vicinity of the critical region of the polymorphous transformation (Fig.…”

“…[1]) make the studies of the electron spectrum and charge transport of this compound of current importance. Nevertheless, data on electron transport phenomena at atmospheric and hydrostatic pressures obtained for bulk crystals are still not sufficient (see [1][2][3][4][5][6] and references therein). Earlier in [7], the kinetic properties of charge carriers in ZnO bulk crystals had been investigated depending on temperature (77-400 K at atmospheric pressure) and hydrostatic pressure up to P = 7 GPa at 300 K. In the present work these investigations are continued, being supplemented with measurements of the resistivity at quasihydrostatic pressure up to P = 25 GPa at T = 300 K.…”

“…In accordance with formulas derived from the 'heterophase structure -effective medium' model, phase volume fractions in the critical region of the polymorphous transformation have been calculated and the threshold values of normalized effective resistivity have been determined. [6] means that this donor center is a hydrogen-like shallow one [7]. According to Eq.…”

Structural Metastability and Quantum Confinement in Zn1–xCoxO Nanoparticles

“…Both effects are correlated and the pressure beyond which they appear is smaller for larger Co concentrations. When OD ≤ 2, the shape of the CTT and the direct interband absorption edge becomes similar in NPs and thin films, 39 indicating that the effective W− Zn 1−x Co x O contents in the platelets is about the same order as in 150 nm thick thin films.…”