E. Kapon

Molecular beam epitaxy of GaAs/AlGaAs superlattice heterostructures on nonplanar substrates

GaAs/AlGaAs superlattice heterostructures with layer thicknesses ≲100 A were grown by molecular beam epitaxy on nonplanar GaAs substrates. The resulting superlattices exhibit different periods, depending on the crystal plane on which they grow. Period variation of more than 50%, from 180 to 80 A, was obtained for adjacent superlattice sections. The transition between regions of different periodicity was mostly smooth and occurred within lateral dimensions ≲100 A. Our results suggest that molecular beam epitaxy of superlattice heterostructures on patterned substrates provides a method for obtaining controllable lateral variations in physical properties which depend on the superlattice period. In particular, by growing quantum well heterostructures on nonplanar substrates, it might be possible to utilize the strong dependence of the carrier confinement energy on the well thickness in order to achieve lateral carrier confinement.

Single quantum wire semiconductor lasers

Single quantum wire GaAs/AlGaAs injection lasers were fabricated using organometallic chemical vapor deposition on V‐grooved GaAs substrates. The quantum wire active region has a crescent‐shaped cross section ∼100 A thick and less than 1000 A wide. Amplified spontaneous emission and lasing spectra of the quantum wire lasers exhibit effects due to transitions between quasi‐one‐dimensional subbands separated by ∼10 meV. Single quantum wire laser structures with tight optical confinement exhibited threshold currents as low as 3.5 mA for uncoated devices at room temperature.

Few-Particle Effects in Semiconductor Quantum Dots: Observation of Multicharged Excitons

We investigate experimentally and theoretically few-particle effects in the optical spectra of single quantum dots (QDs). Photodepletion of the QD together with the slow hopping transport of impurity-bound electrons back to the QD are employed to efficiently control the number of electrons present in the QD. By investigating structurally identical QDs, we show that the spectral evolutions observed can be attributed to intrinsic, multi-particle-related effects, as opposed to extrinsic QD-impurity environment-related interactions. From our theoretical calculations we identify the distinct transitions related to excitons and excitons charged with up to five additional electrons, as well as neutral and charged biexcitons.

Patterned quantum well heterostructures grown by OMCVD on non-planar substrates: Applications to extremely narrow SQW lasers

Abstract We have studied the OMCVD growth of GaAs/AlGaAs quantum well heterostructures on non-planar substrates in the temperature range of 625 to 750°C. The lateral variation in layer thickness and other growth features were found to depend not only on the growth temperature but also on the aluminum content of the layer. An example of the application of this technique of producing lateral thickness variations in quantum well heterostructures to a quantum well semiconductor laser is given. A unique feature of this laser is the formation of a quantum-wire-like crescent shaped active region at the center of a two-dimensional optical waveguide.

Focused electron beam induced deposition of gold

Codeposition of hydrocarbons is a severe problem during focused electron beam writing of pure metal nanostructures. When using organometallic precursors, a low metal content carbonaceous matrix embedding and separating numerous nanosized metal clusters is formed. In this work, we present a new and easy approach to obtain high purity gold lines: the use of inorganic PF3AuCl as a precursor. Electrical resistivities as low as 22 µOhms cm at 295 K (ten times the bulk Au value) were obtained. This is to our knowledge the best value for focused electron beam deposition obtained from the vapor phase so far. No special care was taken to prevent hydrocarbon contamination. The deposited nanostructure consists of gold grains varying in size and percolation with beam parameters.

Long-wavelength VCSELs: Power-efficient answer

The commercialization of long-wavelength vertical-cavity surface-emitting lasers (VCSELs) is gaining new momentum as the telecoms market shifts from long-haul applications to local and access networks. These small, power-efficient devices offer several advantages over traditional edge-emitters.

Two‐dimensional phase‐locked arrays of vertical‐cavity semiconductor lasers by mirror reflectivity modulation

Coupling of two‐dimensional (2D) vertical‐cavity surface‐emitting lasers (VCSELs) to give a coherent supermode is described. The top metal layer of a stained‐layer InGaAs quantum well VCSEL structure was patterned laterally by depositing various metals with different optical reflectivities. This lateral reflectivity patterning defined a 2D laser array sharing the same ‘‘supercavity’’. It is shown that these 2D arrays oscillate in a stable single, coherent 2D supermode. This was achieved with a simple planar process and without significant deterioration of threshold current and efficiency relative to an equivalent broad‐area VCSEL.

Double heterostructure GaAs/AlGaAs thin film diode lasers on glass substrates

The epitaxial liftoff approach has been attracting increasing interest as an alternative to lattice-mismatched heteroepitaxy. A thin-film GaAs double heterostructure injection diode laser fabricated on a glass substrate by the epitaxial liftoff technique is reported. This presages the integration of the two major optical communication materials, III-V semiconductor crystals with SiO/sub 2/ glass.<<ETX>>

Large two‐dimensional arrays of phase‐locked vertical cavity surface emitting lasers

The phase‐locking of two‐dimensional (2D) arrays incorporating a large number of electrically pumped, vertical cavity surface emitting lasers (VCSELs) is described. The InGaAs/GaAs quantum well VCSELs are phase locked by patterning the reflectivity of the laser back mirrors. Structures with both weak and strong modulation of the mirror reflectivity have been studied. The strongly modulated (weakly coupled) structures exhibit superior coherence and beam patterns, with up to 27×27 lasers oscillating in virtually a single (highest order) supermode. The weakly modulated (strongly coupled) arrays also emit highly coherent beams, but with lower threshold currents and higher differential efficiencies. Weakly modulated arrays of 20×20 elements (120×120 μm2) exhibited threshold currents of <1 mA per laser and ≳300 mW pulsed output powers. These coherent 2D arrays should be useful for optical signal processing and high optical power applications.

Probing carrier dynamics in nanostructures by picosecond cathodoluminescence

Picosecond and femtosecond spectroscopy allow the detailed study of carrier dynamics in nanostructured materials. In such experiments, a laser pulse normally excites several nanostructures at once. However, spectroscopic information may also be acquired using pulses from an electron beam in a modern electron microscope, exploiting a phenomenon called cathodoluminescence. This approach offers several advantages. The multimode imaging capabilities of the electron microscope enable the correlation of optical properties (via cathodoluminescence) with surface morphology (secondary electron mode) at the nanometre scale. The broad energy range of the electrons can excite wide-bandgap materials, such as diamond- or gallium-nitride-based structures that are not easily excited by conventional optical means. But perhaps most intriguingly, the small beam can probe a single selected nanostructure. Here we apply an original time-resolved cathodoluminescence set-up to describe carrier dynamics within single gallium-arsenide-based pyramidal nanostructures with a time resolution of 10 picoseconds and a spatial resolution of 50 nanometres. The behaviour of such charge carriers could be useful for evaluating elementary components in quantum computers, optical quantum gates or single photon sources for quantum cryptography.