Abstract
Native and hydrogen-plasma induced shallow traps in hydrothermally grown ZnO crystals have been investigated by charge-based deep level transient spectroscopy, photoluminescence and cathodoluminescence microanalysis. The as-grown ZnO exhibits a trap state at 23 meV, while H-doped ZnO produced by plasma doping shows two levels at 22 meV and 11 meV below the conduction band. As-grown ZnO displays the expected thermal decay of bound excitons with increasing temperature from 7 K, while we observed an anomalous behaviour of the excitonic emission in H-doped ZnO, in which its intensity increases with increasing temperature in the range 140–300 K. Based on a multitude of optical results, a qualitative model is developed which explains the Y line structural defects, which act as an electron trap with an activation energy of 11 meV, being responsible for the anomalous temperature-dependent cathodoluminescence of H-doped ZnO.
Highlights
Calculations based on density-functional theory (DFT) by Van de Walle et al [3, 4] indicated that interstitial hydrogen (Hi) and hydrogen trapped at an oxygen vacancy (HO) can act as two shallow donors, which can be a cause of n-type conductivity in ZnO
Shallow carrier traps investigated by Q-DLTS A typical I–V characteristic of the ZnO/Au junction, showing a rectifying behaviour, is presented in figure 1
Q-DLTS has been applied successfully to ZnO crystals to reveal shallow traps at 11 and 23 meV below the conduction band edge, which have not been previously detected by capacitance-based deep level transient spectroscopy (C-DLTS)
Summary
Calculations based on density-functional theory (DFT) by Van de Walle et al [3, 4] indicated that interstitial hydrogen (Hi) and hydrogen trapped at an oxygen vacancy (HO) can act as two shallow donors, which can be a cause of n-type conductivity in ZnO. The charge-based deep level transient spectroscopy (Q-DLTS) used in this work has been developed to facilitate probing of shallow trapping states and has been applied to the investigation of these defect centers in ZnO crystals. The thermal emission rate en, according to Maxwell–Boltzmann statistics, can be expressed as [21, 22]: en
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