Electron drift velocity in wurtzite ZnO at high electric fields: Experiment and simulation

Abstract

The hot-electron effect on electron transport in nominally undoped ZnO epilayers is studied at electric fields up to 430±50 kV/cm applied with 3 ns voltage pulses. The transient measurements do not demonstrate any change in the electron density at the fields up to 320 kV/cm. The deviation from Ohm’s law can be approximated by the linear dependence of the current on the electric field valid at the moderate fields (from 50 to 250 kV/cm). The model calculations based on the Boltzmann kinetic equation are used to demonstrate that the differential mobility at the moderate fields is almost independent of the scattering on the charged point defects. The electron drift velocity is estimated from the experimental values on the differential conductivity and the current density together with the calculated differential mobility as a reference for calibration. The drift velocity reaches 2.7±0.3×107 cm/s at the field of ∼320 kV/cm and approaches the theoretical limit predicted by the known Monte Carlo simulations.

Estimation of the charged defect density from hot-electron transport studies in epitaxial ZnO

“…The current deduced during the rise edge nearly coincides with that available from the falling edge. This indicates that the electron density remains constant in the electric field range up to 320 kV/cm [27].…”

“…The calculations are performed for the nonparabolic Γ 1 valley with the electron effective mass of 0.21 m e and the non-parabolicity coefficient of 0.4 eV [23]. More details on the model are available elsewhere [22,27]. Eight spherical harmonics are found sufficient for an accurate enough approximation of the electron distribution function.…”

“…The electron mobility available from the Hall effect measured in the van der Pauw configuration was substantially lower than that obtained from the magnetoresistance measurements carried out on the TLM samples [27], and a new method based on the hot-electron effect was developed for the electron density and the low-field mobility. This enabled the estimation of the oxygen vacancy and other charged centre densities.…”

“…The dependence of electron drift velocity on the electron density was deduced at room temperature for doped ZnO [26]. High electric fields (430 kV/cm) and velocities (~2.7 × 10 7 cm/s at 320 kV/cm) were reported for nominally undoped ZnO epilayers at room temperature [27].…”

“…The results of Monte Carlo simulations [4,8,23,24] were used for the interpretation of the experimental studies [25][26][27] of high-field electron transport in epitaxial ZnO channels. The dependence of electron drift velocity on the electron density was deduced at room temperature for doped ZnO [26].…”