GaAs-Based Metamorphic Long-Wavelength InAs Quantum Dots Grown by Molecular Beam Epitaxy

A bilayer stacked InAs/GaAs quantum dot structure grown by molecular beam epitaxy on an In0.05Ga0.95As metamorphic buffer is investigated. By introducing a InGaAs Sb cover layer on the upper InAs quantum dots (QDs) layers, the emission wavelength of the QDs is extended successfully to 1.533 mu m at room temperature, and the density of the QDs is in the range of 4 x 10(9) -8 x 10(9) cm(-2). Strong photoluminescence (PL) intensity with a full width at half maximum of 28.6 meV of the PL spectrum shows good optical quality of the bilayer QDs. The growth of bilayer QDs on metamorphic buffers offers a useful way to extend the wavelengths of GaAs-based materials for potential applications in optoelectronic and quantum functional devices.

Time-Resolved Photoluminescence of Metamorphic InGaAs Quantum Wells

We investigate the temperature dependence of photoluminescence (PL) and time-resolved PL on the metamorphic InGaAs quantum wells (QWs) with an emission wavelength of 1.55 mu m at room temperature. Time-resolved PL measurements reveal that the optical properties can be partly improved by introducing antimony (Sb) as a surfactant during the sample growth. The temperature dependence of the radiative lifetime is measured, showing that for QWs grown with Sb assistance, the intrinsic exciton emission is dominated when the temperature is below 60 K, while the nonradiative process becomes activated with further increases in temperature. However, without Sb assistance, the nonradiative centers are activated when the temperature is higher than 20 K.

Photoluminescence characteristics of GaAsSbN/GaAs epilayers lattice-matched to GaAs substrates

The photoluminescence (PL) characteristics of GaAsSbN/GaAs epilayers grown by molecular beam epitaxy (MBE) are carefully investigated. The results show that antimony (Sb) incorporation into GaNAs material has less influence on the N-induced localization states. For the same N concentration, GaAsSbN material can reach an emission wavelength near 1.3 mum more easily than GaInNAs material. The rapid thermal annealing (RTA) experiment shows that the annealing induced rearrangement of atoms and related blueshift in GaAsSbN epilayers are smaller than those in GaNAs and GaInNAs epilayers. The GaAsSbN material can keep a longer emission wavelength near 1.3 mum-emission even after the annealing treatment. Raman spectroscopy analysis gives further insight into the structure stability of GaAsSbN material after annealing. (C) 2004 Elsevier Ltd. All rights reserved.

Gallium antisite defect and residual acceptors in undoped GaSb

Undoped Ga-Sb samples were investigated by positron lifetime spectroscopy (PAS) and the coincident Doppler broadening (CDB) technique. PAS measurement indicated that there were monovacancy-type defects in undoped Ga-Sb samples, which were identified to be predominantly Ca vacancy (V-Ga) related defects by combining the CDB measurements. After annealing of these samples at 520 C, positron shallow trapping have been observed and should be due to Ga-Sb defects. Undoped Ga-Sb is intrinsically p-type having a residual carrier density of 10(16)-10(17) cm(-3). And the Ga-Sb antisite defects are stable in the (0), (1-) and (2-) charge states and act as a double acceptor. Thus, we infer that Ga-Sb antisite defects are the acceptor contributing to the p-type conduction for undoped samples. (C) 2004 Elsevier B.V All rights reserved.

Structural and optical properties of GaAsSb/GaAs heterostructure quantum wells

The structural and optical properties of MBE-grown GaAsSb/GaAs multiple quantum wells (MQWs) as well as strain-compensated GaAsSb/GaAs/GaAsP MQWs are investigated. The results of double crystal X-ray diffraction and reciprocal space mapping show that when strain-compensated layers are introduced, the interface quality of QW structure is remarkably improved, and the MQW structure containing GaAsSb layers with a high Sb composition can be coherently grown. Due to the influence of inserted GaAsP layers on the energy band and carrier distribution of QWs, the optical properties of GaAsSb/GaAs/GaAsP MQWs display a lot of features mainly characteristic of type-I QWs despite the type-II GaAsSb/GaAs interfaces exist in the structure. (C) 2004 Elsevier B.V. All rights reserved.

Photovoltaic effect in a photon storage cell

The response of photonic memory effect in I-V characteristics of a specially designed photonic memory cell was reported. When the cell is biased in a storage mode, the optical excitation with the photon's energy larger than the energy gap gives rise to a step-like jump in the current. A set-up was used to measure the transient photocurrent at the biases where the step-like jump showed up. It is proved that the falling transient edge of the photocurrent, as the photoexcitation turns off, mainly maps the decaying of electrons and holes, which were previously stored in the cell during the illumination. Its time constant is a measure of photonic memory time.

Influence of the rapid thermal annealing on the properties of thin a-Si films

The variation of the structure, morphology and the electrical properties of thin amorphous silicon films caused by Rapid Thermal Annealing is studied. The films annealed at 1200degreesC for 2 minutes change their structure to polycrystalline and as a result their resistivity decreases by 4 orders of magnitude. Due to the small thickness of the as deposited amorphous silicon the obtained poly-Si is strongly irregular and has many discontinuities in its texture.

Electronic properties of GaAs/GayIn1-yNxAs1-y-xSby superlattices

Using Keating's semiempirical valence force field model and Monte Carlo simulation, we calculate the bond distributions and atom positions of GaAs/GaInNAsSb superlattices. The electronic structures of the superlattices are calculated using the folded spectrum method combined with an empirical pseudopotential proposed by Williamson The effects of N and Sb on superlattice energy levels are discussed. The deterioration of the optical properties induced by N is explained by the localization of the conduction-band states around the N atom. The electron and hole effective masses of the superlattices are calculated and compared with the effective masses of the GaAs and GaInAs.

Ga1-xMnxSb grown on GaSb substrate by liquid phase epitaxy

The Gal(1-x)Mn(x)Sb epilayer was prepared on the n-type GaSb substrate by liquid phase epitaxy. The structure of the Gal(1-x)Mn(x)Sb epilayer was analyzed by double-crystal X-ray diffraction. From the difference of the lattice constant between the GaSb substrate and the Ga1-xMnxSb epilayer, the Mn content in the Ga1-xMnxSb epilayer were calculated as x = 0.016. The elemental composition of Ga1-xMnxSb epilayer was analyzed by energy dispersive spectrometer. The carrier concentration was obtained by Hall measurement. The hole concentration in the Ga1-xMnxSb epilayer is 4.06 x 10(19)cm(-3). It indicates that most of the Mn atoms in Ga1-xMnxSb take the site of Ga, and play a role of acceptors. The current-voltage curve of the Ga1-xMnxSb/GaSb heterostructure was measured, and the rectifying effect is obvious. (C) 2003 Elsevier B.V. All rights reserved.

GaAs-based room-temperature continuous-wave 1.59 mu m GaInNAsSb single-quantum-well laser diode grown by molecular-beam epitaxy

Starting from the growth of high-quality 1.3 mu m GaInNAs/GaAs quantum well (QW), the QW emission wavelength has been extended up to 1.55 mu m by a combination of lowering growth rate, using GaNAs barriers and incorporating some amount of Sb. The photoluminescence properties of 1.5 mu m range GaInNAsSb/GaNAs QWs are quite comparable to the 1.3 mu m QWs, revealing positive effect of Sb on improving the optical quality of the QWs. A 1.59 mu m lasing of a GaInNAsSb/GaNAs single-QW laser diode is obtained under continuous current injection at room temperature. The threshold current density is 2.6 kA/cm(2) with as-cleaved facet mirrors. (c) 2005 American Institute of Physics.

Ordered InAs quantum dots with controllable periods grown on stripe-patterned GaAs substrates

GaAs (001) substrates are patterned by electron beam lithography and wet chemical etching to control the nucleation of InAs quantum dots (QDs). InAs dots are grown on the stripe-patterned substrates by solid source molecular beam epitaxy, A thick buffer layer is deposited on the strip pattern before the deposition of InAs. To enhance the surface diffusion length of the In atoms, InAs is deposited with low growth rate and low As pressure. The AFM images show that distinct one-dimensionally ordered InAs QDs with homogeneous size distribution are created, and the QDs preferentially nucleate along the trench. With the increasing amount of deposited InAs and the spacing of the trenches, a number of QDs are formed beside the trenches. The distribution of additional QDs is long-range ordered, always along the trenchs rather than across the spacing regions.

Deposition of p-type nc-SiC : H thin films with subtle carbon incorporation for applications in p-i-n solar cells

This paper presents a detailed study on the effects of carbon incorporation and substrate temperature on structural, optical, and electrical properties of p-type nanocrystalline amorphous silicon films. A p-nc-SiC: H thin film with optical gap of 1.92 eV and activation energy of 0.06 eV is obtained through optimizing the plasma parameters. By using this p-type window layer, single junction diphasic nc-SiC : H/a-Si : H solar cells have been successfully prepared with a V-oc of 0.94 eV.

Hydrogenated p-type nanocrystalline silicon in amorphous silicon solar cells

A wide bandgap and highly conductive p-type hydrogenated nanocrystalline silicon (nc-Si:H) window layer was prepared with a conventional RF-PECVD system under large H dilution condition, moderate power density, high pressure and low substrate temperature. The optoelectrical and structural properties of this novel material have been investigated by Raman and UV-VIS transmission spectroscopy measurements indicating that these films are composed of nanocrystallites embedded in amorphous SiHx matrix and with a widened bandgap. The observed downshift of the optical phonon Raman spectra (514.4 cm(-1)) from crystalline Si peak (521 cm(-1)) and the widening of the bandgap indicate a quantum confinement effect from the Si nanocrystallites. By using this kind of p-layer, a-Si:H solar cells on bare stainless steel foil in nip sequence have been successfully prepared with a V c of 0.90 V, a fill factor of 0.70 and an efficiency of 9.0%, respectively. (c) 2006 Elsevier B.V. All rights reserved.

Heterojunction solar cells with n-type nanocrystalline silicon emitters on p-type c-Si wafers

Hydrogenated nanocrystalline silicon (nc-Si:H) n-layers have been used to prepare heterojunction solar cells on flat p-type crystalline silicon (c-Si) wafers. The nc-Si:H n-layers were deposited by radio-frequency (RF) plasma enhanced chemical vapor deposition (PECVD), and characterized using Raman spectroscopy, optical transmittance and activation energy of dark-conductivity. The nc-Si:H n-layers obtained comprise fine grained nanocrystallites embedded in amorphous matrix, which have a wider bandgap and a smaller activation energy. Heterojunction solar cells incorporated with the nc-Si n-layer were fabricated using configuration of Ag (100 nm)/1T0 (80 nm)/n-nc-Si:H (15 nm)/buffer a-Si:H/p-c-Si (300 mu m)/Al (200 nm), where a very thin intrinsic a-Si:H buffer layer was used to passivate the p-c-Si surface, followed by a hydrogen plasma treatment prior to the deposition of the thin nanocrystalline layer. The results show that heterojunction solar cells subjected to these surface treatments exhibit a remarkable increase in the efficiency, up to 14.1% on an area of 2.43 cm(2). (c) 2006 Elsevier B.V. All rights reserved.

Magnetron sputtering growth of InAs0.3Sb0.7 films on (100) GaAs substrates: Strong effect of growth conditions on film structure

The growth of highly lattice-mismatched InAs0.3Sb0.7 films on (100) GaAs Substrates by magnetron Sputtering has been investigated and even epitaxial lnAs(0.3)Sb(0.7) films have been successfully obtained. A strong effect of the growth conditions on the film structure was observed, revealing that there was a growth mechanism transition from three-dimensional nucleation growth to epitaxial layer-by-layer growth mode when increasing the substrate temperature. A qualitative explanation for that transition was proposed and the critical conditions for the epitaxial layer-by-layer growth mode were also discussed. (c) 2005 Elsevier B.V. All rights reserved.