Gustaaf Borghs

Electrical spin injection in a ferromagnet/tunnel barrier/semiconductor heterostructure

We demonstrate experimentally the electrical electron spin injection from a ferromagnetic metal/tunnel barrier contact into a III–V semiconductor light emitting diode (LED). The injected electrons have in-plane spin orientation. By applying a relatively small oblique external magnetic field this spin orientation within the semiconductor can be manipulated to have a nonzero out-of-plane component. By measuring the resulting circular polarization of the emitted light, we observe injected spin polarization in excess of 9% at 80 K in a CoFe/AlOx/(Al,Ga)As/GaAs surface-emitting spin-LED.

Specific Cell Targeting with Nanobody Conjugated Branched Gold Nanoparticles for Photothermal Therapy

Branched gold nanoparticles are potential photothermal therapy agents because of their large absorption cross section in the near-infrared window. Upon laser irradiation they produce enough heat to destroy tumor cells. In this work, branched gold nanoparticles are biofunctionalized with nanobodies, the smallest fully functional antigen-binding fragments evolved from the variable domain, the VHH, of a camel heavy chain-only antibody. These nanobodies bind to the HER2 antigen which is highly expressed on breast and ovarian cancer cells. Flow cytometric analysis and dark field images of HER2 positive SKOV3 cells incubated with anti-HER2 conjugated branched gold nanoparticles show specific cell targeting. Laser irradiation studies reveal that HER2 positive SKOV3 cells exposed to the anti-HER2 targeted branched gold nanoparticles are destroyed after five minutes of laser treatment at 38 W/cm(2) using a 690 nm continuous wave laser. Starting from a nanoparticle optical density of 4, cell death is observed, whereas the control samples, nanoparticles with anti-PSA nanobodies, nanoparticles only, and laser only, do not show any cell death. These results suggest that this new type of bioconjugated branched gold nanoparticles are effective antigen-targeted photothermal therapeutic agents for cancer treatment.

Nanometer‐scale magnetic MnAs particles in GaAs grown by molecular beam epitaxy

Spherical MnAs ferromagnetic particles with controllable diameters (5–30 nm) are embedded in a high quality GaAs matrix. The particles are formed in a two step process consisting of the epitaxy of a homogeneous Ga1−xMnxAs layer at low temperatures using molecular beam epitaxy followed by phase separation upon annealing. During the annealing step, the excess arsenic in the as‐grown film forms magnetic MnAs precipitates with the Mn from the Ga1−xMnxAs lattice. Structural and room‐temperature magnetic properties of the heterogeneous GaAs:MnAs films are described. The magnetic MnAs rich layers can be incorporated into semiconductor heterostructures as demonstrated by growing (GaAs/AlAs) multiple quantum well structures in combination with GaAs:MnAs layers.

Impact of texture-enhanced transmission on high-efficiency surface-textured light-emitting diodes

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.

Improvement of AlGaN∕GaN high electron mobility transistor structures by in situ deposition of a Si3N4 surface layer

We have made AlGaN∕GaN high electron mobility transistors with a Si3N4 passivation layer that was deposited in situ in our metal-organic chemical-vapor deposition reactor in the same growth sequence as the rest of the layer stack. The Si3N4 is shown to be of high quality and stoichiometric in composition. It reduces the relaxation, cracking, and surface roughness of the AlGaN layer. It also neutralizes the charges at the top AlGaN interface, which leads to a higher two-dimensional electron-gas density. Moreover, it protects the surface during processing and improves the Ohmic source and drain contacts. This leads to devices with greatly improved characteristics.

40% efficient thin-film surface-textured light-emitting diodes by optimization of natural lithography

In conventional light-emitting diodes (LEDs), the external efficiency is limited by total internal reflection at the semiconductor-air interface. The problem can be overcome by the concept of the nonresonant cavity LED, which is an LED with a textured top surface and a rear reflector. The surface is textured using natural lithography. A monolayer of randomly positioned polystyrene spheres acts as a mask for dry etching. We present details about the optimization of the parameters of the texturing process for GaAs/AlGaAs LEDs. The studied parameters are the size of the spheres, the distribution of the spheres on the surface and the etching depth. Using optimized texturing conditions, we have realized un-encapsulated top-emitting oxide-confined GaAs/AlGaAs nonresonant cavity LEDs with an external quantum efficiency of 40%.

Experimental Separation of Rashba and Dresselhaus Spin Splittings in Semiconductor Quantum Wells

The relative strengths of Rashba and Dresselhaus terms describing the spin-orbit coupling in semiconductor quantum well (QW) structures are extracted from photocurrent measurements on n-type InAs QWs containing a two-dimensional electron gas (2DEG). This novel technique makes use of the angular distribution of the spin-galvanic effect at certain directions of spin orientation in the plane of a QW. The ratio of the relevant Rashba and Dresselhaus coefficients can be deduced directly from experiment and does not relay on theoretically obtained quantities. Thus our experiments open a new way to determine the different contributions to spin-orbit coupling.

Light-emitting diodes with 31% external quantum efficiency by outcoupling of lateral waveguide modes

The external quantum efficiency of light-emitting diodes (LEDs) 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 (NRC) 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%.

Franz–Keldysh oscillations originating from a well‐controlled electric field in the GaAs depletion region

The Franz–Keldysh oscillations induced by the electric field in the depleted zone below the GaAs surface are studied by photoreflectance spectroscopy. The electric field is precisely controlled by a molecular beam epitaxy grown buried highly doped layer and the pinned position of the Fermi level at the surface. It is shown that the electric field value as derived from theory is in disagreement with the value derived from electrostatic calculations. Consequently a determination of the Fermi level pinning is only possible from a measurement of both n‐ and p‐doped samples.

Recycling of guided mode light emission in planar microcavity light emitting diodes

Results are presented on planar microcavity light emitting diodes with different device diameters. A record external quantum efficiency of 20% is achieved for a 1.5 mm light emitting diode. The strong dependence of the quantum efficiency on current density and device size are compared with theoretical results. A good correspondence is obtained when spectral broadening and photon recycling are taken into account.