Persistent N-Type Photoconductivity in P-Type ZnO

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Research activity on ZnO has increased over the past few years, with particular interest in potential electronic and optical device applications such as transparent field-effect transistors (FETs) and light-emitting diodes (LEDs). High-quality bulk and epitaxial samples have been prepared using a variety of growth techniques; however, progress on ZnO-based devices has been limited by the lack of reliable and reproducible p-type doping. Unintentionally doped ZnO films usually exhibit n-type conduction, generally attributed to interstitial H or Zn, oxygen vacancies, or substitutional impurities such as AlZn serving as shallow donors. Recent efforts have demonstrated p-type conduction using N, P, As and Sb as acceptor dopants, with hole concentrations as high as 1019cm−3. In this work, the electrical properties of N- and P-doped p-type ZnO are characterized by temperature-dependent Hall-effects and photo-Hall-effects. An MBE-grown ZnO:N homoepitaxial layer exhibits weak p-type conduction with an acceptor energy EA≈90meV in the dark and n-type photoconduction with a peak electron mobility at low temperature μn>850cm2/Vs under blue/UV light. This n-type photoconductivity persists for days when the sample is maintained in the dark, under vacuum, at room temperature. A sputtered ZnO:P film shows degenerate p-type conduction withp≈4×1018cm−3 and a hole mobility μp≈3cm2/Vs in the dark at room temperature. Under blue/UV light exposure, this P-doped sample undergoes a classic-mixed conduction transition from p-type to n-type where the carrier concentration exhibits a singularity, the Hall mobility (μH) goes to zero and both change sign as the temperature is increased. However, the n-type photoconductivity persists and no transition from n- back to p-type is observed upon subsequent cooling. Sequential, 400K anneals with the sample in the dark and under vacuum cause the mixed conduction transition to reappear and shift to progressively higher temperatures, ultimately returning the sample to its original p-type state. A surface-layer model provides qualitative agreement with the observed behavior.