Electron-trap centers in ZnO layers grown by molecular-beam epitaxy

Abstract

We have investigated electron-trap centers in ZnO layers grown under different Zn∕O flux ratios by molecular-beam epitaxy. Frequency-dependent capacitance measurements show that ZnO layers grown under Zn-rich and stoichiometric flux conditions suffer from larger dispersion than a ZnO layer grown under an O-rich flux condition. Temperature-dependent capacitance measurements reveal that all the ZnO layers have shallow electron-trap centers ET1 and deep electron-trap centers ET2, while the Zn-rich ZnO layer has another shallow electron-trap center ET3 besides ET1 and ET2: the thermal activation energies of ET1, ET2, and ET3 are estimated to be 0.033–0.046, 0.12–0.15, and 0.065 eV, respectively. Moreover, it is exhibited that the trap density of ET2 is larger than those of ET1 or ET3 in all the cases and increases as the Zn∕O flux ratio increases. Consequently, it is suggested that the large dispersion effect observed in the Zn-rich and stoichiometric ZnO layers is ascribed to the large density of deep electron-trap center ET2.

Electrical properties of ZnO/GaN heterostructures and photoresponsivity of ZnO layers

“…We have reported on the characteristics of Schottky contacts to ZnO:N layers elsewhere [7,8]. It was found that the low growth temperature and Zn-polar direction are favoured for N incorporation in the growth of ZnO:N layers, which contributes to the increased resistivity in ZnO:N layers and results in good Schottky characteristics.…”

Fabrication of AZO TCO Films by RF-sputtering and Their Physical Properties

“…The annealing temperature dependence of electrical properties in the AZO films is opposite to the phenomena found in high-quality single-crystalline ZnO films, reported previously. Those change of electrical properties by the thermal annealing has direct relationship with the formation and annihilation of the native defects: the increase of the electron concentration in the lowtemperature region is due to the annihilation of the native defects, which weaken the effect of carrier compensation, and the decrease of the electron concentration in the hightemperature region is due to the formation of the native defects, which strengthen the effect of carrier compensation [4,5,15,16]. On the other hand, the relationship between electrical properties and working gas ratio in the AZO films were excluded in this discussion, because the electrical resistance was significantly increased as much as any current does not flow in the AZO films fabricated under the working gases of the mixture of Ar and O2.…”

“…Thin film ZnO has the background electron concentration of high 10 16 -high 10 18 cm -3 due to native defects such as H, Zn i , and V O [4,5]. The inappropriate fabrication conditions of the low growth temperature that is not enough to supply the formation energy of ZnO bonds and the heterosubstrate with a large lattice mismatch of ZnO unit cells, are known to generate the various donor-type native defects unintentionally, though the background electron concentration level of the ZnO films depends on their growth techniques [4,5]. Moreover, the n-type conductivity of ZnO can be effectively controlled by doping group III elements such as Al, Ga, and In.…”

“…AZO, Al-doped ZnO has been studied as a promising material for transparent conductive oxides (TCOs) for a long time due to the wide bandgap of 3.37 eV and the high n-type conductivity by native donor-type defects [1][2][3]. Thin film ZnO has the background electron concentration of high 10 16 -high 10 18 cm -3 due to native defects such as H, Zn i , and V O [4,5]. The inappropriate fabrication conditions of the low growth temperature that is not enough to supply the formation energy of ZnO bonds and the heterosubstrate with a large lattice mismatch of ZnO unit cells, are known to generate the various donor-type native defects unintentionally, though the background electron concentration level of the ZnO films depends on their growth techniques [4,5].…”

Origin of Ultraviolet Luminescence from Bulk ZnO Thin Films Grown by Molecular Beam Epitaxy

“…Under Zn vapor rich environments Zn interstitials are known to be dominant intrinsic donor defects in bulk ZnO [17]. The thermal activation energy of the Zn i lies in the range of 0.02 eV -0.2 eV below the minimum of conduction band edge as determined by electrical techniques as well as theoretical calculations [18,19]. Zn i being ionized donor sites below the conduction band at room temperature, act as effective electron traps.…”