H. Luo

Efficient electron spin injection in MnAs-based spin-light-emitting-diodes up to room temperature

Studies of ferromagnetic MnAs in recent years have revealed a wide range of properties desirable for spintronic applications. Previously studied MnAs spin-light-emitting-diodes exhibited a low value of spin injection into the device active region. In this work, we have investigated injection of spin polarized electrons from MnAs into AlGaAs(n)/GaAs(i)/AlGaAs(p) n-i-p structures. The band-edge electroluminescence emitted from these devices has a saturation circular polarization of 26% at 7 K and B=2u2002T. Using optical pumping measurements the corresponding electron spin polarization was determined to be 52%. Emission persists up to room temperature, with a saturation circular polarization of 6% at B=2u2002T.

Photoluminescence studies of type-II diluted magnetic semiconductor ZnMnTe/ZnSe quantum dots

Type-II diluted magnetic semiconductor ZnMnTe quantum dots (QDs) in ZnSe matrix grown by molecular beam epitaxy were investigated by conventional and magnetophotoluminescence (PL) spectroscopy. The QD emission exhibits a type-II characteristic in excitation power dependence of PL peak energy. A nonzero circular polarization of PL at the absence of magnetic field was observed. This phenomenon is attributed to the accumulation of interface charges confined in adjacent layers. The magneto-optical measurement demonstrates a magnetic-induced degree of circular polarization in the PL spectra, indicating the Mn incorporation into the QD system.

Type-I and type-II interband transitions in CdSe/ZnTe quantum well structures

We have studied the luminescence intensity associated with type-I and type-II interband transitions in several CdSe/ZnTe quantum well structures as a function of CdSe layer thickness. It was found that as the CdSe layers become wider, the intensity of the type-I transition increases, while that of the type-II transition decreases. These results are analyzed in terms of a variational calculation which takes into account the Coulomb interaction between the electrons and holes that participate in the interband transitions.

NITROGEN ION IMPLANTED ZNSE/GAAS P-I-N PHOTODETECTORS

p-i-n photodiodes were fabricated on nitrogen ion implanted undoped ZnSe/n-type ZnSe epilayers grown on n+GaAs (100) substrates by molecular beam epitaxy. To obtain a quasi-uniform p layer doping profile, nitrogen ions at multiple energies and ion doses were implanted at room temperature. The activation of implanted species was carried out by an optimized post-annealing in a nitrogen ambient. Optical studies were performed on the implanted/annealed devices by photoluminescence spectroscopy at 10 K, which indicated donor–acceptor pairs at an energy of 2.7 eV and its phonon replicas with 30 meV intervals. The circular p-i-n diodes with a 1 mm diam contact area showed a device breakdown voltage to be linearly dependent on the thickness of the undoped ZnSe epilayer. For p-i-n diodes fabricated on an initial 0.5 μm thick undoped ZnSe layer, an ideality factor of 1.19 and a reverse bias breakdown voltage of 12 V was observed. A large photocurrent, good linearity with light intensity, and low dark current were o...

Realizing chemical codoping in TiO2

We demonstrate experimentally a chemical codoping approach that would simultaneously narrow the band gap and control the band edge positions of oxide semiconductors. Using TiO2 as an example, we show that a sequential doping scheme with nitrogen (N) leading the way, followed by phosphorous (P), is crucial for the incorporation of both N and P into the anion sites. Various characterization techniques confirm the formation of the N-P bonds, and as a consequence of the chemical codoping, the band gap of the TiO2 is reduced from 3.0 eV to 1.8 eV. The realization of chemical codoping could be an important step forward in improving the general performance of electronic and optoelectronic materials and devices.

Magnetization reversal in epitaxial MnAs thin films

The magnetization reversal process of MnAs epitaxial films grown on semiconductor substrates was investigated by magneto-optical Kerr effect (MOKE) microscopy, magnetic hysteresis, and magnetoresistance measurements. While the stripe patterns with opposite magnetization in the α-phase of MnAs have been observed previously, we have observed domain dynamics on a vastly different length scale. MOKE images revealed domain nucleation and avalanche domain reversal, which are a collective behavior resulting from interactions among a large number of α-phase regions. The observed reversal processes are reflected in the magnetic hysteresis and magnetotransport measurements. These effects are strongly film thickness dependent.

Transition from island formation to pseudomorphic growth in the submonolayer CdSe/ZnSe multilayer system

Transmission electron microscopy and photoluminescence measurements have been performed on samples with submonolayer (0.3 up to 0.9 ML) CdSe insertions in ZnSe in both single- and multilayer geometries. We observe a significant difference in Cd distribution in the layer plane between these two geometries for the same Cd coverage. While Cd-rich islands can be seen for a single layer of Cd, a layered growth is achieved in the multilayer samples. This effect is attributed to the reduction of lattice mismatch in the multilayer samples due to a high level of diffusion of Cd atoms along the growth direction.

The coupling between quasilocalized states in semiconductor double quantum barriers

The interaction between above-barrier states quasilocalized in barrier layers was studied in a double quantum barrier configuration. Optical transmission experiments were performed on type-II CdSe/ZnTe samples, in which the valence band has a double-barrier structure. Numerical studies reveal a change of symmetry for the coupled quasilocalized states, as a function of the separation between the two barriers. Consequently, the strongest optical transition occurs between different pairs of states as the barrier separation changes. The behavior is illustrated by both experimental and numerical results.

Pattern transfer to GaAs substrates and epitaxial growth of GaAs nanostructures using self-organized porous templates

GaAs nanostructures were grown on patterned GaAs (111) B substrates with molecular beam epitaxy. Nanopatterns were achieved by patterning a thin film of silicon dioxide (SiO2) grown on the substrate surface with a self-organized porous alumina template. Growth of patterned nanostructures took place through the holes in the SiO2 film. The authors obtained two kinds of nanostructures: nanopillars and nanodots. The majority of nanopillars had two kinds of tops, i.e., hexagonal flat top and pyramidal top, as observed with a scanning electron microscope. High resolution transmission electron microscopy studies showed epitaxial relationships between the nanostructures and the substrates. Photoluminescence measurements of nanopillars showed the photoluminescence peak shifted to a higher energy compared to films grown under the same condition.

Fabrication of flexible monocrystalline ZnSe‐based foils and membranes

We report the successful fabrication of flexible single crystal semiconductor structures. A highly selective etching solution allowed us to obtain large area foils and membranes of good structural integrity, using films of indium and silicone as flexible substrates. Photoluminescence and transmission measurements verified that the optical properties of these structures were preserved.