One-dimensional microcavities are optical resonators with coplanar reflectors separated by a distance on the order of the optical wavelength. Such structures quantize the energy of photons propagating along the optical axis of the cavity and thereby strongly modify the spontaneous emission properties of a photon-emitting medium inside a microcavity. This report concerns semiconductor light-emitting diodes with the photon-emitting active region of the light-emitting diodes placed inside a microcavity. These devices are shown to have strongly modified emission properties including experimental emission efficiencies that are higher by more than a factor of 5 and theoretical emission efficiencies that are higher by more than a factor of 10 than the emission efficiencies in conventional light-emitting diodes.
We report a detailed experimental study of the superlattice structures formed in dense binary mixtures of hard-sphere colloids. The phase diagrams observed depend sensitively on the ratio alpha=R(S)/R(L) of the radii of the small (S) and large (L) components. Mixtures of size ratio alpha=0.72, 0.52, 0.42, and 0.39 are studied. The structures of the colloidal phases formed were identified using a combination of light-scattering techniques and confocal fluorescent microscopy. At alpha=0.39, ordered binary crystals are formed in suspensions containing an equal number of large and small spheres which microscopy shows have a three-dimensional structure similar to either NaCl or NiAs. At the larger size ratio, alpha=0.52, we observe LS2 and LS13 superlattices, isostructural to the molecular compounds AlB2 and NaZn13, while at alpha=0.72 the two components are immiscible in the solid state and no superlattice structures are found. These experimental observations are compared with the predictions of Monte Carlo simulations and cell model theories.
We have fabricated high quality, dielectric Ga2O3 thin films. The films with thicknesses between 40 and 4000 Å were deposited by electron-beam evaporation using a single-crystal high purity Gd3Ga5O12 source. Metal-insulator-semiconductor (MIS) and metal-insulator-metal structures (MIM) were fabricated in order to determine dielectric properties, which were found to depend strongly on deposition conditions such as substrate temperature and oxygen pressure. We obtained excellent dielectric properties for films deposited at substrate temperatures of 40 °C with no excess oxygen and at 125 °C with an oxygen partial pressure of 2×10−4 Torr. Specific resistivities ρ and dc breakdown fields Em of up to 6×1013 Ω cm and 2.1 MV/cm, respectively, were measured. Static dielectric constants between 9.93 and 10.2 were determined for these films. Like in other dielectrics, the current transport mechanisms are found to be bulk rather than electrode controlled.
The spontaneous emission lifetime and intensity of Er implanted into SiC>2 is modified by a Si/SiC>2 planar microcavity. The cavity strongly affects the coupling of the optically excited Er 3+ to the radiative and waveguiding modes of the system, resulting in a spontaneous lifetime of 14.8 ms for a cavity offresonance with the Er 3+ versus a lifetime of 9.1 ms for an on-resonance cavity. The observed spontaneous emission lifetime changes agree well with calculated values based on a computation method which is also presented. 32.70.Fw, 32.70.Jz, 78.66.Jg There has been much recent attention regarding the use of planar microcavities to demonstrate fundamental changes in the spontaneous emission rates and spectra of emitting media. After cavity effects at radio frequencies were first proposed by Purcell [1] in 1946, other authors [2] have demonstrated that confocal resonators indeed can drastically modify the spontaneous emission lifetime of an atom within a resonator of millimeter wavelengths. This concept has been extended to short planar cavities at optical wavelengths, where experiments using flowing dye [3], dye-containing films [4], and semiconductors [5] as the active media have demonstrated significant lifetime changes. None of these experimental results in planar cavities, however, was quantitatively compared to theoretical models. Nonradiative energy loss processes, pump-dependent spectra, and the effects of selfabsorption of emitted light by the active media [6] can affect the measured lifetimes. In our experiment, we measure the spontaneous lifetime changes of the A J\V2~^ 4^1 5/2 transition of Er 3+ ions (emission wavelength -1535 nm) implanted at low concentration into a thin (half-wavelength) SiC>2 film, surrounded by highreflectivity Si/SiC>2 planar distributed Bragg reflectors. The Er 3+ ions exhibit atomic, intra-4/, electronic transitions which are effectively shielded by the outlying 5s 2 and 5/7 6 electrons, and the probed 4 713/2 -*^ 4^ 15/2 transition has a nearly 100% quantum efficiency [7]. This system is thus ideal for measuring cavity-induced lifetime changes because of the small self-absorption of the Er 3+ ions, and the narrow atomic emission spectrum. This enables us to compare, for the first time, active-media lifetimes for various thickness planar resonators with calculated values. These result from a general computation method which is also presented.The Er-doped SiC>2 active region (X/2) is -540 nm thick and is surrounded by distributed Bragg reflectors (DBRs), which are composed [8] of 4 (bottom) and 2 j pairs (top) of Si (115 nm) and SiC>2 (270 nm) quarterwave layers, e-beam deposited on a Si substrate. The calculated bottom and top mirror reflectivities (Rbot and /?top) are 99.8% and 98.5%, respectively. Implantation of 800 keV, 1 x 10 15 Er + /cm 2 into the active region was performed prior to deposition of the top DBR, in order to yield a sharp concentration profile (full width at half maximum -260 nm) just below the center plane of the active region. The Er concentration...
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