Yun-Xiang Lu

Mechanism of Na-doped p-type ZnO films: Suppressing Na interstitials by codoping with H and Na of appropriate concentrations

Hydrogen is codoped with sodium into ZnO films. X-ray photoelectron spectroscopy and secondary ion mass spectroscopy indicate that the Na concentration decreases as the substrate temperature increases. Hall-effect tests reveal a transition from n-type to p-type conduction when the growth temperature increases, which is explained by the suppression of Na interstitials by codoping with H and Na of appropriate concentrations. An insulating intended Na–H codoped sample shows reduced resistivity and p-type conductivity after annealing at 550 °C, which may be due to dissociation of NaZn–H complexes. The realization of p-type ZnO by Na–H codoping may explain the discrepancies in behavior of Na in ZnO and suggests the potential of Na–H codoping method [E.-C. Lee and K. J. Chang, Phys. Rev. B 70, 115210 (2004)].

Plasma-free nitrogen doping and homojunction light-emitting diodes based on ZnO

The authors develop a plasma-free metalorganic chemical vapour deposition method to grow N-doped p-type ZnO films. The incorporation of the N acceptor and the corresponding change in the Fermi level are well confirmed by x-ray photoelectron spectroscopy. Temperature-dependent photoluminescence reveals the acceptor-related emissions, namely, neutral acceptor-bound exciton and probably donor–acceptor pair transition. In addition, typical rectifying I–V characteristics and room-temperature electroluminescence from ZnO homojunction light-emitting diodes are demonstrated.

ZnMgO quantum dots grown by low-pressure metal organic chemical vapor deposition

The authors report on reproducible growth of ZnMgO quantum dots (QDs) by a metal organic chemical vapor deposition method. Mg is introduced into ZnO QDs, as confirmed by x-ray photoelectron spectroscopy and photoluminescence measurements. The size, density, and optical band gap of ZnMgO QDs can be well controlled by simply modulating the growth parameters.

Ni/Au Ohmic Contacts to p-Type N-Doped ZnO

The authors fabricated low-resistivity Ni/Au ohmic contacts on N-doped p-type ZnO films using electron-beam evaporation and the transmission line model technique. Both as-deposited and thermally annealed contacts showed ohmic conduction, and the specific contact resistivity decreased with the increase of annealing temperature. The lowest specific contact resistivity of 2.06 X 10 -4 Ω cm 2 was achieved after annealing at 600°C. Secondary ion mass spectroscopy indicated that activation of nitrogen acceptors and the interdiffusion at the metal/ZnO interface after annealing were accounted for by the improvement of ohmic conduction. The results also showed that the fabricated contacts were thermally stable.

Zero-dimensional field emitter based on ZnO quantum dots

ZnO quantum dots (QDs) with diameter around 6 nm were grown by a catalyst-free metalorganic chemical vapor deposition method and applied as electron field emitters. The Ga doping resulted in a considerable improvement of the field emission properties. Macroscopic field emission measurements revealed that the Ga-doped ZnO QDs have a low turn-on field of 1.3 V/μm and a large field enhancement factor up to 104. Probe force microscopy and scanning field emission probe microscopy were used for mapping at submicrometer scale of the spatial distribution of the work function and the field emission, respectively. The local measurements indicated a spatial correlation between both properties that could be linked to spatial variations of the Ga doping level.

Formation Mechanisms of Low-Resistivity Ni/Pt Ohmic Contacts to Li-Doped p-Type ZnO

Low-resistivity Ni/Pt ohmic contacts were fabricated on Li-doped, p-type ZnO films using electron-beam evaporation, which wereconfirmed by the transmission line model technique. The current transport and formation mechanisms of the ohmic contacts wereinvestigated by X-ray photoelectron spectroscopy and temperature-dependent contact resistivity measurements. Activation ofacceptors in the ZnO films, formation of Pt–Ni solid solution near the metal surface, and forming a Zn-deficient region near theZnO surface were considered to result in the improvement of the ohmic contacts. The relatively low barrier height confirmed thatthe surface states played an important role in the formation of ohmic contacts.© 2008 The Electrochemical Society. DOI: 10.1149/1.3055337 All rights reserved.Manuscript submitted October 20, 2008; revised manuscript received December 1, 2008. Published December 18, 2008.