Using a scanning microwave microscope, we investigated the local electrical properties of epitaxial CaRuO3 thin films. The films showed a metal–insulator transition depending on the growth temperature and their thickness. We observed spatially separated highly conducting and poorly conducting regions in the films grown at a high temperature of 800 °C, which showed insulating behavior. The conduction in the CaRuO3 thin films with insulating behavior is percolative through the highly conducting regions and is closely related to this two-phase behavior.

Rapid progress in the development of the AlInGaN material system during the past decade has resulted in high-quantum-efficiency and high-flux light-emitting solid state devices covering the near-ultra-violet (-380 nm) to green (-550 nm) wavelength range. Applications for single-color emitters @rincipally signaling and decorative lighting) are numerous and important, and the combination of short-wavelength nitride devices with phosphors for white-light generation is well-established and results in efficiencies as high as 30 lumens per Watt, which exceeds the efficiency of all incandescent light sources. Nevertheless, significant improvement in flux generation per unit area are required for nitride-based optoelectronic devices to compete with conventional light sources in many applications, such as general illumination and projection displays.The flux emitted by state-of-the-art 111-nitride LEDs approaches 200 mW at 450 nm at a drive current density of 50 A/crnZ, corresponding to a power-conversion efficiency of -20% (see Figure 1). These devices are fabricated with a large active area (-1 x 1 mm2) which is mounted epi-side-down in a special high-power package (see Figure 2). In addition to flux improvements, the high-power package improves the reliability of the LEDs, compared to conventional LEDs mounted in wire l e a d -b e s .There are numerous materials challenges involved in the production of high-efficiency III-nitride lasers and LEDs, some of which can be mitigated by epitaxy and device physics. The lack of a suitable lattice-matched substrate for epitaxy of AlInGaN films results in high dislocation densities (-108 -1010 cm-2) and a large amount of residual strain in the deposited films. The role of the dislocations is not well-understood, although there is clear evidence that laser reliability is improved by reducing their density. Acceptor dopants AlInGaN have large ionization energies (> 200 mew, resulting in poor p-type conductivity and therefore poor hole injection efficiency for lasers and LEDs. The hole injection efficiency can be improved by including a heterojunction and by optimizing the dopant profile. Improvement in the power-conversion efficiency is achieved by doping compensation of the spontaneous and piezoelectric polarization fields which are present in the active regions of conventional IJI-nitride devices which are grown with a wurtzite crystal structure.Finally, the internal quantum efficiency of InGaN quantum wells must be improved, particularly at high injection levels. The drop in quantum efficiency which occurs at drive current densities exceeding 10 A/& results in an unacceptable trade-off between flux and efficiency in light-emitting diodes, and has a negative impact on the efficiency of 111-nitride lasers. Current status and opportunities for improvement in these areas will be discussed. 0-7803-7500-9/02/$17.~002 IEEE 641