Antimony-doped p-type ZnO films epitaxially grown on (0001) sapphire substrates were fabricated by pulsed laser deposition at 400–600°C in 5.0×10−2Torr oxygen without postdeposition annealing. The films grown at 600°C have among the highest reported hole concentration of 1.9×1017cm−3 for antimony doping, Hall mobility of 7.7cm2∕Vs, and resistivity of 4.2Ωcm. Transmission electron microscopy reveals that the p-type conductivity closely correlates to the high density of defects which facilitate the formation of acceptor complexes and the compensation of native shallow donors. The thermal activation energy of the acceptor was found to be 115±5meV and the corresponding optical ionization energy is ∼158±7meV.

The process of particle generation during ultrafast pulsed laser ablation of nickel is investigated. Two types of particles with different sizes depending on the laser fluence are found, indicating different particle generation mechanisms. By limiting the laser fluence below a threshold of strong plasma formation, the large dropletlike particles can be eliminated. In addition, by supplying different background gases, various crystalline structures are obtained for the particles, including Ni∕NiO core/shell spheres and NiO cubes. This study provides evidence that ultrafast laser ablation can be a room temperature physical method for generating nanocrystals with a narrow particle size distribution.

Zinc oxide is a wide bandgap semiconductor with potential applications in optoelectronic devices. The greatest challenge for these applications, however, remains the fabrication of reliable and stable p-type ZnO thin films. Here we report stable phosphorus-doped p-type ZnO thin films grown on (0001) sapphire substrates by pulsed laser ablation. While as-deposited films all show n-type conductivity, films grown at 600°C become p-type after annealing in oxygen atmosphere with a resistivity of 4.9 × 10 1 X cm, a Hall mobility of 1 cm 2 V -1 s -1 , and a hole concentration of 1.3 × 10 17 cm -3 . Such p-type films have been stable under ambient conditions for 16 months so far without apparent degradation. Transmission electron microscopy reveals that the p-type films consist of a high density of dislocations, which enhance both the solubility of phosphorus and the formation of Zn vacancies to facilitate the n-to-p conversion of electrical conductivity. These studies provide microscopic evidence of the amphoteric nature of the phosphorus dopant in ZnO. There has recently been an increasing interest in ZnO for applications in optoelectronics such as light emitting diodes, ultraviolet (UV) lasers, and UV light detectors because of its wide bandgap (3.37 eV). In comparison with GaN, ZnO has some obvious advantages for optoelectronic applications due to the availability of single crystal substrates, relatively low growth temperatures (T G ), and a large exciton binding energy (∼ 60 meV).[1] Optically pumped excitonic lasing of ZnO thin films at room temperature (RT) has been reported. [2,3] Lasing effects in ZnO nanowire arrays have been demonstrated, [4] and electroluminescence (EL) has been observed at room temperature in thin-film ZnO homojunctions. [5][6][7] Although p-type ZnO thin films were reported by several groups, they showed high resistivity and/or poor stability and reproducibility. Thus, the greatest remaining challenge for ZnO optoelectronics is the reproducible fabrication of stable p-type ZnO thin films. Like many other II-VI semiconductors, ZnO has asymmetric doping limits: [8] it can be easily doped n-type, [9] but remains strongly resistant to p-type doping.[10] Though nitrogen is theoretically the most promising acceptor for ZnO, its low solubility and compensation by donors such as hydrogen [11] and Zn interstitials [12] are major obstacles.As alternatives to N, larger-size group V elements such as P, As, Sb and Bi have been widely studied. Puzzling observations of p-type conductivity in such materials have stimulated theoretical investigations into the electronic structure of the defects induced by P, As or Sb in ZnO. Limpijumnong et al. [13] predicted that under oxygen-rich growth conditions, a complex involving a group V antisite and two zinc vacancies (V Zn ) would have a low formation energy, and behave as a shallow acceptor with an ionization energy of 150-160 meV. Lee et al. used the same concept to study phosphorus complexes in ZnO.[14] One of the most important conclusions from these studies ...

We introduce an alternative approach of pulsed laser deposition (PLD) using groups of closely time spaced (20 ns) femtosecond laser pulses, namely burst-mode fs-PLD. This approach enables a broad and continuous tunability over the material morphologies ranging from nanoparticle aggregates to epitaxial thin films with completely droplet-free and atomically smooth surfaces. The tunability of materials is realized by simply tuning laser parameters. An unusual phenomenon of laser-matter interaction is revealed in terms of the breakdown of nanoparticles, the enhancement of plasma ionization, and the decrease of ablation threshold during the burst-mode fs-ablation. A TiO2 film, a wide band gap semiconductor, was deposited with as low as 50 nJ of pulse energy. This approach and the phenomenon are applicable to many other materials.

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The microscopic defects and their effects on the electrical properties of phosphorus-doped ZnO films epitaxially grown on (0 0 0 1) sapphire and ZnO substrates by pulsed laser deposition are studied. While threading dislocations were observed only in heteroepitaxial films, a high density of partial dislocations associated with interstitial dislocation loops was observed in films grown on both substrates. These dislocations provide sinks to quench native donors and favour the injection of zinc vacancies to form acceptor-complex defects, thus leading to p-type conductivity.

Epitaxial ZnO thin films doped uniformly with nitrogen at 10 20 atoms/cm 3 were fabricated by pulsed laser ablation of a Zn-rich Zn 3 N 2 target. The films grown at 300°C and annealed at 600°C in O 2 showed p-type conductivity. Two acceptor levels at 105 and 224 meV were determined by temperature-dependent Hall and photoluminescence measurements of the p-type samples. Transmission electron microscopy studies revealed that the p-type ZnO films consist of 10-20 nm columnar grains with a high density of defects and grain boundaries that may facilitate the annihilation of native donors and the activation of acceptors during postdeposition annealing.