Silicon doping induced increment of quantum dot density

Low-indium-content self-assembled InGaAs/GaAs quantum dots (SAQD) were grown using solid-source molecular beam epitaxy (MBE) and investigated by atomic force microscopy and photoluminescence (PL) spectroscopy. Silicon, which was doped at different quantum dot (QD) growth stages, markedly increased the density of QD. We obtained high density In0.35Ga0.65As/GaAs(001) quantum dots of 10(11)/cm(2) at a growth temperature of 520degreesC. PL spectra and distribution statistics show the high quality and uniformity of our silicon-doped samples. The density increment can be explained using the lattice-hardening mechanism due to silicon doping.

Influence of growth pressure of a GaN buffer layer on the properties of MOCVD GaN

The influence of growth pressure of GaN buffer layer on the properties of MOCVD GaN on alpha-Al2O3 has been investigated with the aid of a home-made in situ laser reflectometry measurement system. The results obtained with in situ measurements and scanning electron microscope show that with the increase in deposition pressure of buffer layer, the nuclei increase in size, which roughens the surface, and delays the coalescence of GaN nuclei. The optical and crystalline quality of GaN epilayer was improved when buffer layer was deposited at high pressure.

Microdefects and electrical uniformity of InP annealed in phosphorus and iron phosphide ambiances

Microdefects originating from impurity-dislocation interactions in undoped InP that had been annealed in phosphorus and iron phosphide ambiances have been studied using optical microscopy. The electrical uniformity of the annealed wafer is improved by removing impurity aggregation around dislocations and by eliminating impurity striations in the annealing process. Compared to as-grown Fe-doped semi-insulating (SI) material, SI wafers obtained by annealing undoped InP in iron phosphide ambiances have better uniformity. This is attributed to the avoidance of Fe aggregation around dislocations and dislocation clusters, Fe precipitation and impurity striations, and is related to the use of a low concentration of Fe in the annealed material. The influence of Fe diffusion on the migration of dislocations in the annealing process has been studied and reviewed. (C) 2003 Elsevier B.V. All rights reserved.

Photoluminescence of Mg-doped GaN grown by metalorganic chemical vapor deposition

Two Mg-doped GaN films with different doping concentrations were grown by a metalorganic chemical vapor deposition technique. Photoluminescence (PL) experiments were carried out to investigate the optical properties of these films. For highly Mg-doped GaN, the PL spectra at 10 K are composed of a blue luminescence (BL) band at 2.857 eV and two excitonic luminescence lines at 3.342 eV and 3.282 eV, in addition to a L2 phonon replica at 3.212 eV. The intensity of the L1 line decreases monotonously with an increase,in temperature. However, the intensity of the L2 line first slowly increases at first, and then decreases quickly with an increase in temperature. The two lines are attributed to bound excitonic emissions at extended defects. The BL band is most likely due to the transition from deep donor Mg-V-N complex to Mg shallow acceptor. From the temperature dependence of the luminescence peak intensity of the BL band, the activation energy of acceptor Mg was found to be 290 meV. (C) 2003 American Vacuum Society.

Time-resolved photoluminescence studies of AlInGaN alloys

We study the two samples of AIInGaN, i.e., 1-mum GaN grown at 1030degreesC on the buffer and followed by a 0.6-mum-thick epilayer of AIInGaN under the low pressure of 76 Torr and the AIInGaN layer deposited directly on the buffer layer without the high-temperature GaN layer, by temperature-dependent photoluminescence (PL) spectroscopy and picosecond time-resolved photoluminescence (TRPL) spectroscopy. The TRPL signals of both the samples were fitted well as a stretched exponential decay at all temperatures, indicating significant disorder in the material. We attribute the disorder to nanoscale quantum dots or discs of high indium concentration. Temperature dependence of dispersive exponent beta shows that the stretched exponential decay of the two samples comes from different mechanisms. The different depths of the localization potential account for the difference, which is illustrated by the results of temperature dependence of radiative recombination lifetime and PL peak energy.

Optical investigation on the annealing effect and its mechanism of GaInAs/GaNAs quantum wells

Thermal annealing of GaInAs/GaNAs quantum wells (QWs) as well as other nitrogen- and indium-contained QW structures grown by molecular beam epitaxy and its effect on optical properties are investigated. The photoluminescence (PL) and photovoltaic (PV) spectra of annealed GaInAs/GaNAs QWs show that the luminescence properties become degraded due to the N diffusion from the GaNAs barrier layers to the GaInAs well layer. Meantime, the annealing-induced blueshift of the PL peak in this QW system is mainly induced by the change of In distribution, suggesting that the In reorganization is greatly assisted by the N-induced defects. The elucidation of annealing effect in GaInAs/GaNAs QW samples is helpful for a better understanding to the annealing effect in the GaInNAs/GaAs QWs. (C) 2003 Elsevier Science B.V. All rights reserved.

Void formation and failure in InGaN/AlGaN double heterostructures

Condensed clusters of point defects within an InGaN/AlGaN double heterostructure grown by metal-organic vapor phase epitaxy on sapphire substrate have been observed using transmission electron microscopy. The existence of voids results in failure of the heterostructure in electroluminescence. The voids are 50-100 nm in diameter and are distributed inhomogeneously within In0.25Ga0.75N/AlGaN active layers. The density of the voids was measured as 10(15) cm(-3), which corresponds to a density of dangling bonds of 10(20) cm(-3). These dangling bonds may fully deplete free carriers in this double heterostructure and result in the heterostructure having high resistivity as confirmed by electrical measurement. (C) 2003 Elsevier Science B.V. All rights reserved.

Localized exciton dynamics in AlInGaN alloy

Carrier recombination dynamics in AlInGaN alloy has been studied by photoluminescence (PL) and time-resolved PL (TRPL) at various temperatures. The fast red-shift of PL peak energy is observed and well fitted by a physical model considering the thermal activation and transfer processes. This result provides evidence for the exciton localization in the quantum dot (QD)-like potentials in our AlInGaN alloy. The TRPL signals are found to be described by a stretched exponential function of exp[(-t/,tau)13], indicating the presence of a significant disorder in the material. The disorder is attributed to a randomly distributed QDs or clusters caused by indium fluctuations. By studying the dependence of the dispersive exponent beta on temperature and emission energy, we suggest that the exciton hopping dominate the diffusion of carriers localized in the disordered QDs. Furthermore, the localized states are found to have 0D density of states up to 250 K, since the radiative lifetime remains almost unchanged with increasing temperature. (C) 2003 Elsevier Science Ltd. All rights reserved.

Selective area growth of GaN on GaAs(001) substrates by metalorganic vapor-phase epitaxy

Selective area growth (SAG) of GaN on SiO2 stripe-patterned GaN/GaAs(001) substrates was carried out by metalorganic vapor-phase epitaxy. The SAG samples were investigated by using X-ray diffraction (XRD) and scanning electron microscopy (SEM). SEM observations showed that the morphology of SAG GaN is strongly dependent on the window stripe orientation and slightly affected by the orientation relationship between the window stripes and the gas flow. The (I 1 1)B sidewalls formed on the SAG GaN stripes are found to be stable. XRD measurements indicated the full-widths at half-maximum (FWHMs) of cubic GaN (0 0 2) rocking curves are reduced after SAG. The measured FWHMs with omega-axis parallel to [1(1) over bar 0] are always larger than the FWHM values obtained with omega-axis parallel to [I 10], regardless of the orientation relationship between the w-axis and the GaN stripes. (C) 2003 Elsevier Science B.V. All rights reserved.

Nano-layer structure of silicon-on-insulator materials

Silicon-on-insulator (SOI) has been recognized as a promising semiconductor starting material for ICs where high speed and low power consumption are desirable, in addition to its unique applications in radiation-hardened circuits. In the present paper, three novel SOI nano-layer structures have been demonstrated. ULTRA-THIN SOI has been fabricated by separation by implantation of oxygen (SIMOX) technique at low oxygen ion energy of 45 keV and implantation dosage of 1.81017/cm2. The formed SOI layer is uniform with thickness of only 60 nm. This layer is of crystalline quality. and the interface between this layer and the buried oxide layer is very sharp, PATTERNED SOI nanostructure is illustrated by source and drain on insulator (DSOI) MOSFETs. The DSOI structure has been formed by selective oxygen ion implantation in SIMOX process. With the patterned SOI technology, the floating-body effect and self-heating effect, which occur in the conventional SOI devices, are significantly suppressed. In order to improve the total-dose irradiation hardness of SOI devices, SILICON ON INSULATING MULTILAYERS (SOIM) nano-structure is proposed. The buried insulating multilayers, which are composed of SiOx and SiNy layers, have been realized by implantation of nitride and oxygen ions into silicon in turn at different ion energies, followed by two steps of high temperature annealing process, respectively, Electric property investigation shows that the hardness to the total-dose irradiation of SOIM is remarkably superior to those of the conventional SIMOX SOI and the Bond-and-Etch-Back SOI.

Influence of semi-insulating InP substrates on InAlAs epilayers grown by molecular beam epitaxy

Tensile-strained InAlAs layers have been grown by solid-source molecular beam epitaxy on as-grown Fe-doped semi-insulating (SI) InP substrates and undoped SI InP substrates obtained by annealing undoped conductive InP wafers (wafer-annealed InP). The effect of the two substrates on InAlAs epilayers and InAlAs/InP type II heterostructures has been studied by using a variety of characterization techniques. Our calculation data proved that the out-diffusion of Fe atoms in InP substrate may not take place due to their low diffusion, coefficient. Double-crystal X-ray diffraction measurements show that the lattice mismatch between the InAlAs layers and the two substrates is different, which is originated from their different Fe concentrations. Furthermore, photoluminescence results indicate that the type II heterostructure grown on the wafer-annealed InP substrate exhibits better optical and interface properties than that grown on the as-grown Fe-doped substrate. We have also given a physically coherent explanation on the basis of these investigations. (C) 2003 Elsevier Science B.V. All rights reserved.

Structural and optical properties of InAlGaN films grown directly on low-temperature buffer layer with (0001)sapphire substrate

Quaternary InAlGaN film has been grown directly on top of low-temperature-deposited GaN buffer layer by low-pressure metalorganic vapor phase epitaxy. High-resolution X-ray diffraction and photoluminescence (PL) results show that the film has good crystal quality and optical property. Temperature-dependent PL and time-resolved PL (TRPL) have been employed to study the carriers recombination dynamics in the film. The TRPL signals can be well fitted as a stretched exponential function exp[-(t/tau)(beta)] from 14 to 250 K, indicating that the emission is attributed to the radiative recombination of excitons localized in disorder quantum nanostructures such as quantum disks originating from indium (In) clusters or In composition fluctuation. The cross-sectional high-resolution electron microscopy measurement further proves that there exist the disorder quantum nanostructures in the quaternary. By investigating the dependence of the exponential parameter beta on the temperature, it is shown that the multiple trapping-detrapping mechanism dominates the diffusion among the localized states. The localized states are considered to have two-dimensional density of states (DOS) at 250 K, since radiative recombination lifetime tau(r) increases linearly with increasing temperature. (C) 2002 Elsevier Science B.V. All rights reserved.

Growth temperature effect on the optical and material properties of AlxInyGa1-x-yN epilayers grown by MOCVD

AlxInyGa1-x-yN epilayers have been grown by metalorganic chemical vapor deposition (MOCVD) at different temperatures from 800 to 870degreesC. The incorporation of indium is found to increase with decreasing growth temperature, while the incorporation of Al remains nearly constant. The optical properties of the samples have been investigated by photoluminescence (PL) and time-resolved photoluminescence (TRPL) at different temperatures. The results show that the sample grown at 820 C exhibits the best optical quality for its large PL intensity and the absence of the yellow luminescence. Furthermore the temperature-dependent PL and TRPL of the sample reveals its less exciton localization effect caused by alloy fluctuations. In the scanning electron microscopy measurement, much uniform surface morphology is found for the sample grown at 820degreesC, in good agreement with the PL results, The improvement of AlxInyGa1-x-yN quality is well correlated with the incorporation of indium into AlGaN and the possible mechanism is discussed. (C) 2002 Elsevier Science B.V. All rights reserved.

The growth morphologies of GaN layer on Si(111) substrate

The growth morphologies of metalorganic chemical vapor deposition (MOCVD) grown GaN layer on Si(111) substrate were studied using atomic force microscopy and transmission electron microscopy. It was found that the growth process of GaN/Si(111) consisted of two cycles of island growth and coalescence. These two cycles process differs markedly from that of one cycle process reported. The stress of evolving GaN layers on Si(111) was characterized by measuring the lattice constant c of GaN using X-ray diffraction (XRD) technique. It was proposed that the large tensile stress within the film during growth initiated this second island growth cycle, and the interaction between the GaN islands with high orientational fluctuation on the buffer layer induced this large tensile growth stress when coalescence occurred. (C) 2002 Elsevier Science B.V. All rights reserved.

Luminescence study of (11(2)over-bar0) GaN film grown by metalorganic chemical-vapor deposition

Investigations on photoluminescence properties of (11 (2) over bar0) GaN grown on (1 (1) over bar 02) Al2O3 substrate by metalorganic chemical-vapor deposition are reported. Several emission lines not reported before are observed at low temperature. The sharp peak at 3.359 eV is attributed to the exciton bound to the neutral acceptor. Another peak at 3.310 eV represents a free-to-bound, probably a free electron-to-acceptor, transition. The 3.241 and 3.170 eV lines are interpreted as phonon replica lines of the 3.310 eV line. The phonon energy is 70 meV, consistent with the energy of transverse optical E-1 phonon. The optical properties of the lines are analyzed. (C) 2003 American Institute of Physics.