Both n‐type and p‐type ZnO will be required for development of homojunction light‐emitting diodes (LEDs) and laser diodes (LDs). It is easy to obtain strong n‐type ZnO, but very difficult to create consistent, reliable, high‐conductivity p‐type material. The most natural choice of an acceptor dopant is N, substituting for O, and indeed several groups have been able to obtain p‐type material by such doping. Surprisingly, however, other groups have also been successful with P and As, elements with much larger ionic radii than that of O. Although ZnO substrates are now available, most of the epitaxial p‐type layers so far have been grown on sapphire, or other poorly‐matched materials. The lowest p‐type resistivity obtained up to now is about 0.5 Ω‐cm, which should be sufficient for LED fabrication. In spite of the present availability of p‐type ZnO, very few homojunction LEDs have been reported so far, to our knowledge; however, several good heterojunction LEDs have been demonstrated, fabricated with p‐type layers composed of other materials. One such structure, with fairly strong 389‐nm emission at 300 K, involves n‐type ZnO and p‐type AlGaN, grown on an SiC substrate. Also, an N+‐ion implanted ZnO layer, deposited by chemical vapor deposition on Al2O3, exhibits 388‐nm emission at 300 K and could be economical to produce. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Compact, solid‐state UV emitters have many potential applications, and ZnO‐based materials are ideal for the wavelength range 390 nm and lower. However, the most efficient solid‐state emitters are p–n junctions, and p‐type ZnO is difficult to make. Thus, the future of ZnO light emitters depends on either producing low‐resistivity p‐type ZnO, or in mating n‐type ZnO with a p‐type hole injector. Perhaps the best device so far involves an n‐ZnO/p‐AlGaN/n‐SiC structure, which produces intense 390 ± 1 nm emission at both 300 K and 500 K. However, development of p‐ZnO is proceeding at a rapid pace, and a p–n homojunction should be available soon. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Temperature dependent magneto-transport measurements in magnetic fields of up to 12 Tesla were performed on thin film vanadium dioxide (VO 2 ) across the metal-insulator transition (MIT). The Hall carrier density increases by 4 orders of magnitude at the MIT and accounts almost entirely for the resistance change. The Hall mobility varies little across the MIT and remains low, ~0.1cm 2 /V sec. Electrons are found to be the major carriers on both sides of the MIT. Small positive magnetoresistance in the semiconducting phase is measured. Published in PHYSICAL REVIEW B 79, 153107 (2009) 2/16 Vanadium dioxide is being actively investigated due to its potential in switching devices as well as fundamental scientific interest in understanding correlated electron systems. This compound undergoes a metal-insulator transition (MIT) upon temperature decrease through T MIT =67˚C, as well as a sharp change in optical properties [1] and crystal lattice transformation near T MIT . The importance of the contribution of electron correlations to the phase transition has been demonstrated [2], and mechanisms responsible for the MIT are being actively researched. In the Peierls model, the atomic distortion due to the lattice transformation at MIT from rutile metallic phase to monoclinic insulating phase causes the band gap opening [3]. In the Mott-Hubbard model, electron correlations alone can induce an insulator [4]. A correlation-assisted Peierls model where atomic structure aspects are considered on equal footing with intra-dimer V-V correlations has been suggested as well [5]. Recently, experiments by Cavalleri et al. based on ultrafast spectroscopy provided

The phenomenon of persistent photoconductivity is elusive and has not been addressed to an extent to attract attention both in micro and nanoscale devices due to unavailability of clear material systems and device configurations capable of providing comprehensive information. In this work, we have employed a nanostructured (nanowire diameter 30–65 nm and 5 μm in length) ZnO-based metal–semiconductor–metal photoconductor device in order to study the origin of persistent photoconductivity. The current–voltage measurements were carried with and without UV illumination under different oxygen levels. The photoresponse measurements indicated a persistent conductivity trend for depleted oxygen conditions. The persistent conductivity phenomenon is explained on the theoretical model that proposes the change of a neutral anion vacancy to a charged state.

No abstract

No abstract

Electrical properties, including leakage currents, threshold voltages, and deep traps, of AlGaN/GaN heterostructure wafers with different concentrations of carbon in the GaN buffer layer, have been investigated by temperature dependent current-voltage and capacitance-voltage measurements and deep level transient spectroscopy ͑DLTS͒, using Schottky barrier diodes ͑SBDs͒. It is found that ͑i͒ SBDs fabricated on the wafers with GaN buffer layers containing a low concentration of carbon ͑low-͓C͔ SBD͒ or a high concentration of carbon ͑high-͓C͔ SBD͒ have similar low leakage currents even at 500 K; and ͑ii͒ the low-͓C͔ SBD exhibits a larger ͑negative͒ threshold voltage than the high-͓C͔ SBD. Detailed DLTS measurements on the two SBDs show that ͑i͒ different trap species are seen in the two SBDs: electron traps A x ͑0.9 eV͒, A 1 ͑0.99 eV͒, and A 2 ͑1.2 eV͒, and a holelike trap H 1 ͑1.24 eV͒ in the low-͓C͔ SBD; and electron traps A 1 , A 2 , and A 3 ͑ϳ1.3 eV͒, and a holelike trap H 2 ͑Ͼ1.3 eV͒ in the high-͓C͔ SBD; ͑ii͒ for both SDBs, in the region close to GaN buffer layer, only electron traps can be detected, while in the AlGaN/GaN interface region, significant holelike traps appear; and iii͒ all of the deep traps show a strong dependence of the DLTS signal on filling pulse width, which indicates they are associated with extended defects, such as threading dislocations. However, the overall density of electron traps is lower in the low-͓C͔ SBD than in the high-͓C͔ SBD. The different traps observed in the two SBDs are thought to be mainly related to differences in microstructure ͑grain size and threading dislocation density͒ of GaN buffer layers grown at different pressures.

No abstract