2001 nanotechnology nanotechnology V 1505 -254Room-Temperature Ultraviolet Nanowire Nanolasers.-(0001)oriented ZnO nanowires are grown on sapphire substrates by a simple vapor transport and condensation process using Au thin films as catalysts. These nanowires form natural laser cavities with diameters varying from 20 to 150 nm and lengths up to 10 µm. Under optical excitation, surface-emitting lasing action is observed at 385 nm, with an emission linewidth less than 0.3 nm. The chemical flexibility and the one-dimensionality of the nanowires make them ideal miniaturized laser light sources. These short-wave-length nanolasers could have many applications, including optical computing, information storage, and

The enhancement of the emission rate of charge carriers from deep-level defects in electric field is routinely used to determine the charge state of the defects. However, only a limited number of defects can be satisfactorily described by the Poole-Frenkel theory. An electric field dependence different from that expected from the Poole-Frenkel theory has been repeatedly reported in the literature, and no unambiguous identification of the charge state of the defect could be made. In this article, the electric field dependencies of emission of carriers from DX centers in Al x Ga 1Ϫx As:Te, Cu pairs in silicon, and Ge:Hg have been studied applying static and terahertz electric fields, and analyzed by using the models of Poole-Frenkel and phonon assisted tunneling. It is shown that phonon assisted tunneling and Poole-Frenkel emission are two competitive mechanisms of enhancement of emission of carriers, and their relative contribution is determined by the charge state of the defect and by the electric-field strength. At high-electric field strengths carrier emission is dominated by tunneling independently of the charge state of the impurity. For neutral impurities, where Poole-Frenkel lowering of the emission barrier does not occur, the phonon assisted tunneling model describes well the experimental data also in the low-field region. For charged impurities the transition from phonon assisted tunneling at high fields to Poole-Frenkel effect at low fields can be traced back. It is suggested that the Poole-Frenkel and tunneling models can be distinguished by plotting logarithm of the emission rate against the square root or against the square of the electric field, respectively. This analysis enables one to unambiguously determine the charge state of a deep-level defect.

O and Si donors in GaN are studied by Raman spectroscopy under hydrostatic pressure p. The ground state of O is found to transfer from a shallow level to a deep gap state at p . 20 GPa reminiscent of DX centers in GaAs. Transferred to Al x Ga 12x N we predict that O induces a deep gap state for x . 0.40. In GaN:Si no such state is induced up to the highest pressure obtained ͑p 25 GPa͒ equivalent to x 0.56 in Al x Ga 12x N and possibly higher. We attribute this distinction to the lattice sites of the dopants. O substituting for N is found to be the origin of high free electron concentration in bulk GaN crystals. [S0031-9007(97)03179-7] PACS numbers: 71.55. Eq, 62.50.+ p, 72.20.Jv, 78.30.Fs Donors in III-V compound semiconductors are of special interest because they can assume both extended or localized states, i.e., they can be metastable [1][2][3][4]. In the most thoroughly studied system, GaAs, either a high free carrier concentration n, alloying with AlAs, or application of hydrostatic pressure p can induce a transition from a shallow hydrogenic state of the dopant to a strongly localized one of the same impurity. Many different donor species, e.g., Si Ga (group-IV element on group-III site) and S As (group-VI on group-V site), transform into the nonhydrogenic configuration at very similar characteristic transition pressures [3,4]. In addition, metastability effects, such as a limited free electron concentration and persistent photoconductivity, have been found and interpreted with activation barriers between the different configurations of the donor. All of these effects have been associated with a so-called DX center [2].In GaN we find that the O donor dopant shows characteristic features of a DX defect when hydrostatic pressure is applied. Si, in contrast, behaves like a hydrogenic donor. This distinction is attributed to the actual lattice site of the impurity, and this effect is extremely pronounced in this compound semiconductor system. The pressure experiments can directly be transferred from GaN to the Al x Ga 12x N system predicting a strongly localized gap state of O for higher Al concentrations.Dopant impurities typically can induce both resonant and hydrogenic defect levels in the electronic band structure. In many cases only the hydrogenic level is relevant. Under certain conditions, however, a charge transfer from a quasihydrogenic state to a strongly localized neutral charge state ͑D 0 ͒ can occur [5]. In addition, as proposed by Chadi and Chang [1], a structural relaxation of the donor impurity in the vicinity of the transition conditions can lead to an activation barrier between the two states. This widely accepted model of a structural relaxation explains the metastability and the activation barrier between the different states of DX centers. Promoted by the transfer of electrons, this new strongly localized state (DX) can be the ground state [6]. Ab initio calculations reproduce the experimental observations in GaAs, including the very similar transition conditions found for all substituti...

Combined voltage-dependent scanning tunneling microscopy ͑STM͒ images with atomic resolution, local scanning tunneling spectroscopy, and simulations of the potential distribution in the interface-STM tip system are used to extract the physical imaging mechanisms of GaAs p-n interfaces in STM images. It is shown that ͑i͒ the tip-induced changes of the potential near the interface result in the tunneling characteristics of the p-type (n-type͒ layer being dragged into the interfaces' depletion region at positive ͑negative͒ sample voltage. ͑ii͒ This leads to a considerable reduction of the apparent width of the image of the depletion zone in STM images. ͑iii͒ At small negative sample voltages, a pronounced depression line appears. The depression is directly correlated with the electronic interface. It arises from the interplay of competing current contributions from the valence and conduction bands. This understanding of the imaging process allows us to develop methods on how to extract accurate physical data about the properties of the electronic interfaces from scanning tunneling microscopy images.