High‐field characteristics of ZnO and ZnO/ZnMgO heterostructures

Abstract

High‐field transport properties of ZnO and ZnO/ZnMgO heterostructures were investigated. Three types of structures, a 500‐nm‐thick ZnO and ZnO/ZnMgO single‐hetero‐ (SH) and single‐quantum‐well (SQW) structures containing a thin (10‐20 nm) ZnO channel layer, were grown on a‐plane sapphire substrates by plasma‐assisted molecular beam epitaxy. The dc and pulsed I ‐V characteristics were measured at room temperature by using two‐terminal devices with the source‐to‐drain spacing of 2‐8 μm. By using the pulsed I ‐V measurements, the velocity‐field (v ‐E) characteristics were observed for electric field up to about 100 kV/cm. The v‐E characteris‐ tics revealed that no velocity saturation was observed until the velocity reached at a value of 7.6 × 106 cm/s for the thick ZnO structure while a rapid reduction of the mobility thus a saturation of the velocity at a relatively low electric field was observed for the SH structure. The reduction of the mobility was even pronounced for SQW structure. The velocity saturation might be due to the interplay between the inhomogeneities and polarization in the heterostructures. To our knowledge, this is the first experimental report on the high‐field characteristics in ZnO and ZnO/ZnMgO heterostructures. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High-field electron transport in doped ZnO

“…The highest value for the drift velocity of ∼ × 1.5 10 7 cm s −1 is obtained at a field of 108 kV cm −1 . This velocity value exceeds the published experimental value of ∼ × 7.6 10 6 cm s −1 at approximately 100 kV cm −1 measured in MBE-grown, nominally undoped ZnO layers with electron density of ∼ × 2.4 10 16 cm −3 (figure 2, dashed blue line, [8]).…”

“…Data on high-field electron transport in epitaxial ZnO are scarce [8]. An electron drift velocity of × 7.6 10 6 cm s −1 has been reported for nominally undoped ZnO at room temperature for an applied electric field of 94 kV cm −1 [9].…”

“…Voltage pulse duration is 2 ns. Dashed blue line: experiment with 300 ns voltage pulses for nominally undoped sample with electron density of × 2.4 10 16 cm −3[8]. Monte Carlo simulations: triangles-no impurities, circles-electron density of × 1.4 10 17 cm −3 and ionized impurity density of × 1.1 10 18 cm −3 .…”

Microwave performance of ZnO/ZnMgO heterostructure field effect transistors

“…We have reported velocity‐field ( v – E ) characteristics of ZnO/ZnMgO heterostructures by performing pulsed current–voltage characterization using two‐terminal configuration where a significant dependence of the velocity on the sample structure was observed 16. Figure 5 illustrates the three types of structures used for the v – E characteristics measurements.…”

Theory of high field carrier transport and impact ionization in ZnO

“…To date very few attempts to measure the high field electron transport properties have been carried out. 4 These experiments have been performed at electric field strengths much below the impact ionization regime. To the authors knowledge, no experimental information is available on the ionization coefficients in ZnO.…”