The external quantum efficiency of light-emitting diodes ͑LEDs͒ is usually limited by total internal reflection at the semiconductor-air interface. This problem can be overcome by a combination of light scattering at a textured top surface and reflection on a backside mirror. With this design, we achieve 22% external quantum efficiency. One of the main loss mechanisms in such nonresonant cavity ͑NRC͒ light-emitting diodes is coupling into an internal waveguide. Texturing the surface of this waveguide allows the partial extraction of the confined light. In this way, we demonstrate an increase in the external quantum efficiency of NRC-LEDs to 31%.

We study the transport of spin polarized electrons in n-GaAs using spatially resolved continuous wave Faraday rotation. From the measured steady state distribution, we determine spin relaxation times under drift conditions and, in the presence of strain, the induced spin splitting from the observed spin precession. Controlled variation of strain along [110] allows us to deduce the deformation potential causing this effect, while strain along [100] has no effect. The electric field dependence of the spin lifetime is explained quantitatively in terms of an increase of the electron temperature.Sufficiently long spin lifetimes and the possibility to manipulate the spin orientation are required for the development of spintronics devices [1]. Electron spin lifetimes τ s in semiconductors have been measured by means of the Hanle effect (depolarization of photoluminescence in a magnetic field) [2], time resolved photoluminescence [3], and time resolved measurements of magneto-optical effects (Faraday / Kerr rotation) [4], which are suitable to measure τ s in the absence of holes. Although future spintronics applications are likely to depend on spin transport, very little attention has been paid to the influence of an electric drift field F on τ s . While the possibility to transport the electron spin over substantial distances in fields up to F = 100 V/cm has been demonstrated several years ago [5], first measurements of τ s in this field range have only been reported recently [7]. Under the influence of strain, spin precession of the drifting electrons [5,6,8] has been observed and even the possibility to generate spin polarized currents without magnetic materials or optical excitation has been demonstrated [7]. However, no theoretical interpretation of the observed spin lifetimes and no quantitative analysis of the influence of strain on the observed spin splitting has been given.In this Letter, we present a method to study the lateral spin transport in thin films of n-doped GaAs under steady state conditions, similar to the experiments reported recently in [8], but with the possibility to determine spin lifetimes and to quantify the influence of strain. By the absorption of circularly polarized light we locally generate a steady-state electron spin polarization and determine the spin drift length L s from the spatial decay of spin polarization along the drift direction, which results from the combined process of electron diffusion, drift and spin relaxation. As this signal can be traced over several 100 µm while the signal varies by up to three orders of magnitude, this cw method allows for accurate measurements of the electric field, doping density and temperature dependence of L s . Knowing the drift velocity v dr , the spin relaxation time τ s can be determined from L s . In the presence of strain, we observe spin precession in addition to the spatial decay. As realistic theoretical pre-0.001 0.01 0.1 1 0.0 -50 0 50 100 150 -1 -0.1 -0.01 -0.001 0.01 0.1 1 0.0 0.5 -50 0 50 100 150 0.01 0.1 1 e F [V/cm] 40 60 80 80 ...

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The transmission properties of semiconductor surfaces can be changed by surface texturing. We investigate these changes experimentally and find that an enhancement of the angle-averaged transmission by a factor of 2 can be achieved with optimum texturing parameters. This enhanced transmission provides an additional light extraction mechanism for high-efficiency surface-textured light-emitting diodes. External quantum efficiencies of 46% and 54% are demonstrated before and after encapsulation, respectively.

Tunneling transport through the depletion layer under a GaAs {110} surface is studied with a low temperature scanning tunneling microscope (STM). The observed negative differential conductivity is due to a resonant enhancement of the tunneling probability through the depletion layer mediated by individual shallow acceptors. The STM experiment probes, for appropriate bias voltages, evanescent states in the GaAs band gap. Energetically and spatially resolved spectra show that the pronounced anisotropic contrast pattern of shallow acceptors occurs exclusively for this specific transport channel. Our findings suggest that the complex band structure causes the observed anisotropies connected with the zinc blende symmetry.