We report a photoluminescence study of excitons localized by interface fluctuations in a narrow GaAs͞AlGaAs quantum well. This type of structure provides a valuable system for the optical study of quantum dots. By reducing the area of the sample studied down to the optical near-field regime, only a few dots are probed. With resonant excitation we measure the excited-state spectra of single quantum dots. Many of the spectral lines are linearly polarized with a fine structure splitting of 20 -50 meV. These optical properties are consistent with the characteristic asymmetry of the interface fluctuations. PACS numbers: 78.55.Cr, 71.35.Cc In this Letter we describe the polarization dependence of the optical spectra of single naturally formed GaAs quantum dots. Most previous optical studies of quantum dots (QDs) have probed large ensembles which have led to inhomogeneous broadening of the spectral features. However, recently several groups have shown that it is possible to study single QDs with photoluminescence (PL) either by reducing the size of the sample, [1] by cathodoluminescence [2,3], or by reducing the size of the laser spot on the sample through microscopic [4,5] or optical near-field techniques [6]. Here we use a similar technique whereby we combine high spatial and spectral resolution optics with excitation spectroscopy to study in detail the spectrum of a single QD [7]. With improved resolution we are able to resolve the spectral lines and to study the polarization dependence of the PL spectrum of an individual QD. We often find that the PL is linearly polarized along the (110) crystal axes and observe a fine structure splitting in each of the spectral lines. These results are analogous to the early days of atomic spectroscopy as improvements in techniques allowed the observation of fine structure splittings in the optical spectra. However, the physical phenomena responsible for the effects presented here are unique to the quantized condensed matter system.The QDs we have studied were formed naturally by interface steps in narrow quantum wells [4][5][6][7]. Specifically, the electrons and holes become localized into QDs in regions of the quantum well that are a monolayer wider than the surrounding region and, therefore, have a slightly smaller confinement energy. These well width fluctuations arise from monolayer-high islands at the interfaces which are randomly formed on the growth-interrupted surface by the migration of the cations to step edges. By interrupting the growth these islands can grow to diameters larger than the exciton Bohr diameter (20 nm). A scanning tunneling microscope image of a growth-interrupted GaAs surface grown under similar conditions as our quantum dot sample is shown in Fig. 1. Large monolayer-high islands of varying lateral sizes are evident, and the islands tend to be elongated along the [110] crystal axis. Thus we intuitively expect that the optical properties associated with the localized excitons will reflect this characteristic interface structure. In fact, as we will...

The homogeneous linewidths in the photoluminescence excitation spectrum of a single, naturally formed gallium arsenide (GaAs) quantum dot have been measured with high spatial and spectral resolution. The energies and linewidths of the homogeneous spectrum provide a new perspective on the dephasing dynamics of the exciton in a quantum-confined, solid-state system. The origins of the linewidths are discussed in terms of the dynamics of the exciton in zero dimensions, in particular, in terms of lifetime broadening through the emission or absorption of phonons and photons.

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We have studied the magnetic and structural properties of epitaxial bcc Fe͑001͒ films grown at 175°C on molecular-beam epitaxy-prepared GaAs͑001͒-2ϫ4 and -c(4ϫ4) reconstructed surfaces, with film thicknesses ranging up to ϳ30 ML ͑ϳ43 Å͒. We present measurements of the thickness-dependent evolution of the magnetic properties of the Fe films as determined by in situ magneto-optic Kerr effect. We find that the magnetic properties and growth mode are similar for both 2ϫ4 and c(4ϫ4) reconstructions, although the initial adsorption sites and island nucleation as measured by scanning tunneling microscopy are clearly dominated by the substrate surface reconstruction. The onset of room-temperature ferromagnetism occurs at 6 ML for growth on both GaAs surface reconstructions. At this coverage, the measured Curie temperature ͑ϳ100°C͒ is significantly reduced from that of bulk ␣-Fe ͑770°C͒. The anisotropy is dominated by a uniaxial component such that the two ͗110͘ axes are inequivalent for all coverages studied. Shape anisotropy does not appear to play a significant role. ͓S0163-1829͑97͒08037-5͔

Low-injection minority carrier lifetimes ͑MCLs͒ and deep trap spectra have been investigated in n − 4H-SiC epilayers of varying layer thicknesses, in order to enable the separation of bulk lifetimes from surface recombination effects. From the linear dependence of the inverse bulk MCL on the concentration of Z1/Z2 defects and from the behavior of the deep trap spectra in 4H-SiC p-i-n diodes under forward bias, we conclude that it is Z1/Z2 alone that controls the MCL in this material.

A novel magnetoelectronic device incorporating a single microstructured ferromagnetic film and a micron scale Hall cross was fabricated and characterized at room temperature. Magnetic fringe fields from the edge of the ferromagnet generate a Hall voltage in a thin film semiconducting Hall bar. The sign of the fringe field, as well as the sign of the output Hall voltage, is switched by reversing the magnetization of the ferromagnet. This new device has excellent output characteristics and scaling properties, and may find application as a magnetic field sensor, nonvolatile storage cell, or logic gate.

High-resolution, variable-temperature photoluminescence studies of recombination processes associated with excitons bound to donors in hydride-vapor-phase epitaxial GaN are presented. Detailed analyses of the two-electron satellite ͑2ES͒ region identify transitions associated with ground and excited states of both the donor-bound exciton complexes and of the donor itself. All of the 2ES transitions observed in this work can be accounted for by the recombination of excitons bound to Si and O substitutional impurities and the line positions are in excellent agreement with the energies of donor intraimpurity transitions measured previously by infrared absorption. Conflicting aspects of donor identification and the binding energies of impurities and excitons are clarified.

Quantum wells of InGaSb clad by AlGaSb were grown by molecular beam epitaxy. The InGaSb is in compressive strain, resulting in a splitting of the heavy- and light-hole valence bands and an enhancement of the mobility. The mobility was found to increase with increasing InSb mole fraction for values of strain up to 2%. Room-temperature mobilities as high as 1500cm2∕Vs were reached for 7.5nm channels of In0.40Ga0.60Sb. These results are an important step toward the goal of high-performance p-channel field-effect transistors for complementary circuits operating at extremely low power.