bluuebunny/crossref_metadata_2025 huggingface dataset:
Power-gated spectral holeburning in MgS:Eu2+, Eu3+: A case for high-density persistent spectral holeburning
Authors: Hasan Z. , Biyikli L. , Macfarlane P. I.
Published: July 2002

We present the case of photoionization-induced holeburning in rare-earth-doped II–VI compounds for high-density persistent holeburning. In this case, the photoproduct of holeburning is distributed across the entire zero-phonon line. This maximizes the total number of possible spectral holes that can be burned into an inhomogeneous line as well as produces holes that are photoerasable. Experimental data on photon-gated holeburning in MgS:Eu2+, Eu3+ are presented. With the proper choice of the host electronic band structure, the optically active rare-earth ion and its electronic transitions involved in the holeburning process, to the best of our knowledge we have observed the highest number of photon-gated holes ever burned in a single electronic transition. The features of these holes are that they suffer no detectable erasure after several thousands of read cycles, they survive thermal cycling to ∼150 K, and they are completely photoerasable. A special case of photon-gated holeburning, power-gated holeburning, was employed to demonstrate that, in such systems, a single laser can be used for burning, reading, and erasing of the spectral holes.
Stimulated emission in ridge quantum wire laser structures measured with optical pumping and microscopic imaging methods
Authors: Watanabe Shinichi , Koshiba Shyun , Yoshita Masahiro , Sakaki Hiroyuki , Baba Motoyoshi , Akiyama Hidefumi
Published: July 2002

We report the observation of stimulated emission in ridge quantum wire (QWR) structures at temperatures from 4.7 to 290 K. To examine the origin of the stimulated emission, the spatially and spectrally resolved microscopic images of the emission were measured. It was most likely attributed to the optical transition between the excited states in QWRs.
Permittivity of GaAs epilayers containing arsenic precipitates
Authors: Vasudevan A. , Carin S. , Melloch M. R. , Harmon E. S.
Published: July 2002

The real part of the permittivity of annealed low temperature grown gallium arsenide (LTG GaAs) has been measured via capacitance measurements taken on p-i-n devices. The intrinsic region of the devices contained LTG GaAs annealed at 700, 800, and 900 °C for 30 s. The capacitance trends as a function of frequency for the annealed LTG GaAs samples were compared to that of GaAs grown at a standard substrate temperature. An increased screening of the electric field was observed for the LTG samples as the test frequency was lowered. The capacitance measurements were taken at various test temperatures, enabling the computation of an activation energy of the electric field screening in the annealed LTG GaAs from Arrhenius plots.
Magnetotransport study on the two-dimensional electron gas in AlGaN/GaN heterostructures
Authors: Wong L. W. , Cai S. J. , Li R. , Wang Kang , Jiang H. W. , Chen Mary
Published: July 2002

We have performed magnetotransport studies on AlGaN/GaN heterostructures at low temperature and magnetic field up to 30 T. The integer quantum Hall effect is observed in two-dimensional electron gas at the AlGaN/GaN interface. From the temperature dependence of the low-field Shubnikov–de Hass oscillations, a carrier effective mass of m*=0.228me is obtained. Dingle plots of our resistivity data show inhomogeneity in the two-dimensional electron gas. Finally, we found that for electronic density as high as 5.47×1012 cm−2, only the lowest subband in the quantum well is occupied.
Modeling of the ion mass effect on transient enhanced diffusion: Deviation from the “+1” model
Authors: Pelaz Lourdes , Gilmer G. H. , Jaraiz M. , Herner S. B. , Gossmann H.-J. , Eaglesham D. J. , Hobler G. , Rafferty C. S. , Barbolla J.
Published: July 2002

The influence of ion mass on transient enhanced diffusion (TED) and defect evolution after ion implantation in Si has been studied by atomistic simulation and compared with experiments. We have analyzed the TED induced by B, P, and As implants with equal range and energy: TED increases with ion mass for equal range implants, and species of different mass but equal energy cause approximately the same amount of TED. Heavier ions produce a larger redistribution of the Si atoms in the crystal, leading to a larger excess of interstitials deeper in the bulk and an excess of vacancies closer to the surface. For high-mass ions more interstitials escape recombination with vacancies, are stored in clusters, and then contribute to TED. TED can be described in terms of an effective “+n” or “plus factor” that increases with the implanted ion mass.
Time-resolved spectroscopy of InxGa1−xN/GaN multiple quantum wells at room temperature
Authors: Pophristic M. , Long F. H. , Tran C. , Karlicek R. F. , Feng Z. C. , Ferguson I. T.
Published: July 2002

We have measured the time-resolved photoluminescence (PL) from a series of InxGa1−xN/GaN (x=0.22) multiple quantum well structures at room temperature. Lifetimes longer than 1 ns (1.87±0.02 ns) were measured at room temperature. The emission lifetime was found to lengthen with increasing excitation power, this is attributed to the saturation of recombination centers. The PL decay kinetics were found to be quite sensitive to the emission wavelength. The energy dependence of the emission lifetime is attributed to nanoscale fluctuations in the indium concentration.
Dynamic self-organization of strained islands during SiGe epitaxial growth
Authors: Floro J. A. , Chason E. , Sinclair M. B. , Freund L. B. , Lucadamo G. A.
Published: July 2002

Dynamic self-organization of coherently strained islands during SiGe molecular beam epitaxy on Si(001) is measured in real time using a novel spectroscopic light scattering technique. We show that an array of hut clusters self-orders on a square mesh with increasing areal coverage. Ordering occurs to minimize the repulsive elastic interactions between neighboring islands. Self-organization breaks down when islands coalesce during deposition or during static coarsening.
Growth and characterization of InGaAs/InGaP quantum dots for midinfrared photoconductive detector
Authors: Kim S. , Mohseni H. , Erdtmann M. , Michel E. , Jelen C. , Razeghi M.
Published: July 2002

We report InGaAs quantum dot intersubband infrared photodetectors grown by low-pressure metalorganic chemical vapor deposition on semi-insulating GaAs substrates. The optimum growth conditions were studied to obtain uniform InGaAs quantum dots constructed in an InGaP matrix. Normal incidence photoconductivity was observed at a peak wavelength of 5.5 μm with a high responsivity of 130 mA/W and a detectivity of 4.74×107 cm H1/2/W at 77 K.
Luminescence of porous silicon in a weak confinement regime
Authors: Polisski G. , Heckler H. , Kovalev D. , Schwartzkopff M. , Koch F.
Published: July 2002

We report on luminescence properties of porous silicon emitting efficient light only a few tens of meV above the band gap of bulk Si. This emission band has a well-defined low-energy limit coincident with the position of the lowest possible luminescing exciton state of bulk silicon. The resonant photoluminescence spectrum exhibits all possible combinations of zero-phonon and momentum-conserving TA- and TO-phonon assisted absorption and exciton emission processes known for bulk silicon. We found that TO-phonon assisted processes give a major contribution to the light emission. We discuss the implications of these studies for the understanding of the origin of porous silicon photoluminescence.
The infrared optical functions of AlxGa1−xN determined by reflectance spectroscopy
Authors: Yu G. , Ishikawa H. , Umeno M. , Egawa T. , Watanabe J. , Soga T. , Jimbo T.
Published: July 2002

Infrared reflectivity measurements have been carried out on samples with structures of GaN/sapphire and AlxGa1−xN/GaN/sapphire as well as sapphire substrates. Analyses of the reflectance data of sapphire using the Kramers–Kronig technique and fitting of the reflectance spectra of GaN and AlxGa1−xN samples using analytical expressions have been made. The high-frequency dielectric constant ε∞ and the transverse phonon frequency ωTO, are found to vary from 5.15 to 4.2 and from 559.7 to 586.4 cm−1, respectively, when the composition x is varied from 0 to 0.35 at room temperature. The E2 mode, which arises from the disordered state of the alloys, has been observed in the reflectivity spectrum of AlxGa1−xN, and the intensity of the peak is enhanced by increasing the Al content.
Exploiting the properties of carbon nanotubes for nanolithography
Authors: Dai Hongjie , Franklin Nathan , Han Jie
Published: July 2002

Carbon nanotube tips are explored in fabricating oxide nanostructures on silicon surfaces with an atomic force microscope. Nanotubes can write nanostructures at speeds up to 0.5 mm/s over large surface areas, and present a solution to the long-standing tip-wear problem. Experimental and theoretical work find that nanotube tips are impervious to high compressive and lateral forces and breakdown in high electric fields. A “cleaving” method is developed to reproducibly obtain dome-closed multi-walled nanotube tips with suitable length. Nanotube materials could become key elements for future miniaturization applications.
Deposition of highly crosslinked fluorinated amorphous carbon film and structural evolution during thermal annealing
Authors: Yang Hongning , Tweet Douglas J. , Ma Yanjun , Nguyen Tue
Published: July 2002

Highly crosslinked fluorinated amorphous carbon films are obtained by increasing deposition temperature, enhancing ion bombardment, and fine tuning the fluorine-to-carbon ratio. The as-deposited films undergo significant mechanical, chemical, and electrical changes after thermal annealing. Most importantly, these changes occur only at the initial stage of annealing. After the thermal treatment, the films tend to be thermally stable and retain reasonably good electrical properties for use as a low-k dielectric.
Site control of self-organized InAs dots on GaAs substrates by in situ electron-beam lithography and molecular-beam epitaxy
Authors: Ishikawa Tomonori , Kohmoto Shigeru , Asakawa Kiyoshi
Published: July 2002

We studied a site-control method for self-organized InAs quantum dots on GaAs substrates by a combination of in situ electron-beam (EB) lithography and molecular-beam epitaxy (MBE) using an ultrahigh-vacuum multichamber system. Small and shallow holes were patterned on a MBE-grown GaAs (001) surface by in situ EB writing and Cl2-gas etching. When more than a 1.4 monolayer of InAs was applied to the patterned surface, In(Ga)As dots were preferentially self-organized in the holes while dot formation around the holes was sufficiently suppressed. When we further increased the amount of InAs, the dots enlarged remarkably, presumably due to a stress-relaxation effect.
Temperature dependence of avalanche breakdown for epitaxial diodes in 4H silicon carbide
Authors: Konstantinov A. O. , Nordell N. , Wahab Q. , Lindefelt U.
Published: July 2002

The temperature dependence of avalanche breakdown is investigated for uniform and microplasma-related breakdown in epitaxial 4H SiC p-n junctions. P-n mesa diodes fabricated with positive angle beveling and oxide passivation can withstand temperatures of up to 300–400 °C in the breakdown regime. Uniform avalanche breakdown in 4H silicon carbide appears to have a positive temperature coefficient, in contrast to the 6H polytype, where the temperature coefficient is negative. The influence of deep levels on avalanche breakdown in epitaxial diodes is of minor importance for uniform breakdown, but appears to be significant for breakdown through microplasmas. A negative temperature coefficient for the avalanche breakdown voltage can be observed even for 4H SiC if the breakdown is dominated by microplasmas.
Influence of HCl on the chemical vapor deposition and etching of Ge islands on Si(001)
Authors: Kamins T. I. , Briggs G. A. D. , Williams R. Stanley
Published: July 2002

When HCl is added during the growth of Ge islands on Si(001) by chemical vapor deposition, the reduced Ge surface diffusion impedes island development. There is a shift in the relative populations of different island types even when other conditions such as temperature, coverage, and growth rate, are unchanged. The effect of HCl on the net rate of deposition is proportional to the square of the HCl partial pressure, suggesting a surface reaction with the Ge. When larger islands are etched with HCl at high enough temperature, they revert to a shape characteristic of smaller islands, confirming the reversibility of transformations from one island type to another. It has not proved possible to use etching to produce smaller and more uniform islands.
High minority-carrier lifetimes in GaAs grown on low-defect-density Ge/GeSi/Si substrates
Authors: Sieg R. M. , Carlin J. A. , Boeckl J. J. , Ringel S. A. , Currie M. T. , Ting S. M. , Langdo T. A. , Taraschi G. , Fitzgerald E. A. , Keyes B. M.
Published: July 2002

A high bulk minority-carrier lifetime in GaAs grown on Si-based substrates is demonstrated. This was achieved by utilizing a step-graded Ge/GeSi buffer (threading dislocation density 2×106 cm−2) grown on an offcut (001) Si wafer, coupled with monolayer-scale control of the GaAs nucleation to suppress antiphase domains. Bulk minority-carrier lifetimes (τp) were measured using room-temperature time-resolved photoluminescence applied to a series of Al0.3Ga0.7As/GaAs/Al0.3Ga0.7As double-heterojunction structures doped n=1.1×1017 cm−3 with GaAs thicknesses of 0.5, 1.0, and 1.5 μm. A lifetime τp=7.7 ns was determined for GaAs grown on Si. The extracted interface recombination velocity of 3.9×103 cm/s is comparable to recombination velocities found for Al0.3Ga0.7As/GaAs interfaces grown on both GaAs and Ge wafers, indicating that the crosshatch surface morphology characteristic of strain-relaxed Ge/GeSi surfaces does not impede the formation of high-electronic-quality interfaces. These results hold great promise for future integration of III–V minority-carrier devices with Si wafer technologies.
Dependence on the microwave field of the surface resistance for YBa2Cu3O7−δ films fabricated on copper substrates
Authors: Liu Jian-Fei , Asano Kiyomitsu , Ezura Eizi , Inagaki Shigemi , Isagawa Shigeru , Nakanishi Hiroshi , Fukutomi Masao , Komori Kazunori , Saito Masakazu
Published: July 2002

The microwave field dependence of the surface resistance for YBa2Cu3O7−δ films was studied at field levels up to 400 A/m. The c-axis normal YBa2Cu3O7−δ films were deposited onto copper disks 36 mm in diameter by a laser ablation method. The surface resistance increased linearly below 50 K as the microwave magnetic field increased, which is explained better by the critical-state model than by the coupled-grain model. The critical current density decreased almost linearly with increasing temperature from 2.8×105 A/cm2 at 20 K to 4×104 A/cm2 at 77 K.
Effect of oxygen on the growth of (101̄0) GaN surfaces: The formation of nanopipes
Authors: Elsner J. , Jones R. , Haugk M. , Gutierrez R. , Frauenheim Th. , Heggie M. I. , Öberg S. , Briddon P. R.
Published: July 2002

Local density–functional methods are used to examine the behavior of O and O-related defect complexes on the walls of nanopipes in GaN. We find that O has a tendency to segregate to the (101̄0) surface and identify the gallium vacancy surrounded by three oxygen impurities [VGa–(ON)3] to be a particularly stable and electrically inert complex. We suggest that during Stranski–Krastanow growth, when interisland spaces shrink, these defects reach a critical concentration beyond which further growth is prevented and nanopipes are formed.
Metallic resistively coupled single-electron transistor
Authors: Pashkin Yu. A. , Nakamura Y. , Tsai J. S.
Published: July 2002

We have fabricated and measured transport properties of resistively coupled single-electron transistors (R-SETs). In our version, a chromium thin-film resistive strip served as a gate and was connected directly to a mesoscopic island formed between two ultrasmall Al/AlOx/Al tunnel junctions. In current–voltage-gate voltage dependences of the R-SETs, a characteristic Coulomb blockade pattern was observed. Our simulations based on the orthodox theory of single-electron tunneling are in good qualitative agreement with the experimental data and indicate excessive electron temperature of the devices.
Birefringent thin-film polarizers for use at normal incidence and with planar technologies
Authors: Hodgkinson Ian , Wu Qi Hong
Published: July 2002

We discuss the design and fabrication of an all-dielectric thin-film polarizer that is compatible with existing planar technologies. This polarizer consists of a stack of quarter-wave biaxial layers. Each quarter-wave layer is formed by reactive electron-beam evaporation, using a bideposition technique that causes a columnar structure to grow perpendicular to the substrate, produces large normal-incidence linear birefringence, and avoids thickness wedging that is inherent in tilted-columnar biaxial layers. p-polarized light that is incident on the polarizer encounters an index-matched stack and is transmitted, whereas s-polarized light is rejected by a coexisting high-reflectance stack. A fabrication figure-of-merit of ten film periods per decade in the extinction ratio has been achieved in practice for a titanium oxide/tantalum oxide polarizer.
Nonuniform carrier distribution in asymmetric multiple-quantum-well InGaAsP laser structures with different numbers of quantum wells
Authors: Hamp Michael J. , Cassidy Daniel T. , Robinson B. J. , Zhao Q. C. , Thompson D. A.
Published: July 2002

A method for quantifying the degree to which the uneven carrier distribution affects the operation of multiple quantum well (MQW) lasers is developed by comparing the net gains of wells in mirror image asymmetric MQW structures. The uneven carrier distribution is found to affect the performance of devices with as few as two quantum wells and decreases the net gain for wells on the n side of a ten quantum well structure by more than a factor of two.
Strain effects and microstructure of epitaxial manganite thin films and heterostructures
Authors: Wiedenhorst B. , Höfener C. , Lu Yafeng , Klein J. , Alff L. , Gross R. , Freitag B. H. , Mader W.
Published: July 2002

We have grown epitaxial La2/3Sr1/3MnO3 (LSMO) and La2/3Ba1/3MnO3 (LBMO) thin films as well as La2/3Ba1/3MnO3/SrTiO3 heterostructures by pulsed-laser deposition. The microstructure of the films was analyzed by x-ray diffraction and transmission electron microscopy. A significant effect of strain due to lattice mismatch was found. Whereas the thick LBMO films show perfect epitaxy and grow coherently strained over the full film thickness, the LSMO films were found to be composed of two layers separated by an intrinsic interface region containing a high density of defects. The approximately 60 nm thick bottom layer grows coherently on the SrTiO3 (STO) substrate and is highly strained, whereas the top layer is almost strain free. The LBMO/STO heterostructures are coherently strained and show a very low density of defects and sharp interfaces.
Polyaniline and poly(N-vinylcarbazole) blends as anode for blue light-emitting diodes
Authors: Chung Jinkoo , Choi Beomrak , Lee Hong H.
Published: July 2002

Polyaniline and poly(N-vinylcarbazole) (PANI:PVK) blends are introduced as an anode, and the advantages are demonstrated for organic light-emitting diodes (LEDs). Compared to the usual PANI network electrodes, use of the blend allows for simpler fabrication and provides a better planarized surface, especially for the device with vapor-deposited emitting layer. PANI is not usually used for blue LEDs because of its strong light absorption in the deep-blue spectral region. This problem is practically solved by the use of the blend.
Thermal stability of Ta2O5 in metal–oxide–metal capacitor structures
Authors: Chang J. P. , Steigerwald M. L. , Fleming R. M. , Opila R. L. , Alers G. B.
Published: July 2002

Using x-ray photoelectron spectroscopy and Rutherford backscattering spectrometry, we have studied structures used in metal–oxide–metal capacitors including Ta2O5/TiN/Ti, Ta2O5/Ti, Ta2O5/TaN/Ti, Ta2O5/WN/Ti, and Ta2O5/M, where M=Ta, Pt, W, Al, and Si. We find that Ti and Al are able to reduce the Ta2O5 to Ta, forming oxides of Ti and Al, respectively. The diffusion barriers TiN, TaN, and WN hamper the diffusion of oxygen and therefore postpone the reduction of Ta2O5 to higher temperatures. As judged by the temperatures at which the reduction of Ta2O5 occurs, TaN and WN are more effective oxygen-diffusion barriers than TiN. We observe no oxygen remaining in the diffusion barrier when a Ti layer is present underneath. We observe no reduction of Ta2O5 when M=Pt, W, or Si.
Self-organized GaAs quantum-wire lasers grown on (775)B-oriented GaAs substrates by molecular beam epitaxy
Authors: Higashiwaki Masataka , Shimomura Satoshi , Hiyamizu Satoshi , Ikawa Seiji
Published: July 2002

Self-organized GaAs/(GaAs)4(AlAs)2 quantum-wire (QWR) lasers were grown on (775)B-oriented GaAs substrates by molecular-beam epitaxy. The QWRs were naturally formed at thick parts in the GaAs/(GaAs)4(AlAs)2 quantum well with a corrugated AlAs-on-GaAs upper interface and a flat GaAs-on-AlAs lower interface. The density of the QWRs was as high as 8×106 cm−1. Stripe-geometry lasers with the self-organized (775)B GaAs/(GaAs)4(AlAs)2 QWRs as an active region oscillated at 20 °C with threshold current densities of about 3 kA/cm2 for uncoated mirrors.
Epitaxial ferroelectric/giant magnetoresistive heterostructures for magnetosensitive memory cell
Authors: Grishin A. M. , Khartsev S. I. , Johnsson P.
Published: July 2002

Epitaxial ferroelectric/giant magnetoresistive Pb(Zr0.52Ti0.48)O3/La0.67Ca0.33MnO3 (PZT/LCMO) heterostructures have been grown onto LaAlO3 (001) single crystal by KrF pulsed laser deposition. Main processing parameters have been optimized to preserve giant magnetoresitivity in the LCMO film after deposition of the top ferroelectric layer. High degree of c-axis orientation and strong in-plane texture, coherent with the substrate, both in template LCMO and PZT layers, high dielectric permittivity of 720, remnant polarization of 17 μC/cm2 of PZT, and magnetoresistivity in LCMO of 28% at H=0.5 T indicate excellent characteristics of coexisting magnetoresistive and ferroelectric properties. We compare the performance of magnetosensitive memory cells with La0.67Ca0.33MnO3 and La0.75Sr0.25MnO3 giant magnetoresistive electrodes.
Low-pressure metal organic chemical vapor deposition of GaN on silicon(111) substrates using an AlAs nucleation layer
Authors: Strittmatter A. , Krost A. , Straßburg M. , Türck V. , Bimberg D. , Bläsing J. , Christen J.
Published: July 2002

We have investigated the growth of GaN on silicon by low-pressure metal organic chemical vapor deposition. Good quality GaN layers are grown on silicon(111) using an AlAs nucleation layer. AlAs is thermally stable even at 1050 °C and, unlike GaN and AlN buffer layers, the formation of SiNx on the Si surface is prevented. Single crystalline GaN films are obtained by introducing a thin low-temperature GaN buffer layer grown on the AlAs nucleation layer. The GaN layers are characterized by x-ray diffraction, atomic force microscopy, secondary ion mass spectroscopy, photoluminescence, and cathodoluminescence.
Bi: Perfect surfactant for Ge growth on Si(111)?
Authors: Schmidt T. , Falta J. , Materlik G. , Zeysing J. , Falkenberg G. , Johnson R. L.
Published: July 2002

We have investigated the growth of Ge on Bi-terminated Bi:Si(111)-∛×∛. In-situ measurements of x-ray standing waves, crystal truncation rods and scanning tunneling microscopy clearly show that, at substrate temperatures around 485 °C, smooth and homogeneous Ge films of thicknesses exceeding 30 bilayers Ge can be grown. For Ge coverages larger than 10 bilayers, the Ge film is completely relaxed. Bi is found to segregate to the surface during Ge deposition, and can be removed from the surface after growth by mild annealing at 520 °C as proven by Auger electron spectroscopy.
One-dimensional to one-dimensional electron resonant tunneling in a double asymmetric quantum-wire structure
Authors: Wang Yongqiang , Huang Qi , Zhou Junming
Published: July 2002

An atomically precise double asymmetric quantum-wire structure was fabricated with the cleaved edge overgrowth method. A resonant tunneling diode for such a structure was successfully achieved. Its voltage–current characteristic was measured at 77 K. A sharp resonant tunneling current peak was observed and its peak-to-valley ratio is about 18:1, which is much larger than that of a double-barrier quantum-well structure, and it is ascribed to one-dimensional to one-dimensional electron resonant tunneling through the double asymmetric quantum-wire structure.
A full vector analysis of near-field luminescence probing of a single quantum dot
Authors: Chavez-Pirson Arturo , Chu Sai Tak
Published: July 2002

We solve numerically the three-dimensional vector form of Maxwell’s equation for the situation of near-field excitation and collection of luminescence from a single quantum dot using a scanning near-field optical fiber probe with sub-wavelength resolution. Applying a finite-difference time-domain method, we calculate the complete vector fields emerging from a realistic probe structure, as well as the near-field luminescence image of the dot captured by the same probe. We show that a collimating effect in the high index semiconductor significantly improves the spatial resolution in excitation/collection mode. We find that the spatial resolution, image shape, and collection efficiency of near-field luminescence strongly depend on the orientation of the radiating dipole in the dot.
Photorefractive multiple quantum well devices approaching diffraction-limited resolution
Authors: Abeeluck A. K. , Garmire E. , Canoglu E. , Jehanno A. , Tayebati P. , Sacks R. N.
Published: July 2002

We demonstrate that a resolution approaching their fundamental drift-free diffraction limit can be achieved in photorefractive multiple quantum well (MQW) devices by proper choice of the growth and anneal conditions. Previously reported devices are compared with four MQW samples grown at low temperature and annealed after growth at 620 °C for over 3 h. A simple analytical model with and without lateral drift of photocarriers is used to explain the observed experimental resolutions. The growth temperature and annealing procedure of the low-temperature-grown MQW are shown to significantly influence the device resolution by reducing lateral drift.
Enhanced optical emission from GaN films grown on a silicon substrate
Authors: Zhang Xiong , Chua Soo-Jin , Li Peng , Chong Kok-Boon , Feng Zhe-Chuan
Published: July 2002

GaN films have been grown on a silicon-(001) substrate with specially designed composite intermediate layers consisting of an ultrathin amorphous silicon layer and a GaN/AlxGa1−xN (x=0.2) multilayered buffer by metal-organic chemical vapor deposition and characterized by photoluminescence and x-ray diffraction spectroscopy. It was found that the GaN films grown on the composite intermediate layers gave comparable or slightly stronger optical emission than those grown on a sapphire substrate under an identical reactor configuration. Moreover, the full-width at half-maximum for the GaN band-edge-related emission is 40 meV at room temperature. This fact indicates that by using the proposed composite intermediate layers, the crystalline quality of GaN-based nitride grown on a silicon substrate can be significantly improved.
Formation of three-dimensional Si islands on Si(111) with a scanning tunneling microscope
Authors: Shklyaev Alexander A. , Shibata Motoshi , Ichikawa Masakazu
Published: July 2002

Silicon islands up to 10 nm in base length and 3 nm in height were grown on a Si(111) surface at room temperature with a scanning tunneling microscope at constant tunneling currents. The islands grew with constant rates at earlier growth stages by accumulating Si atoms from the surface area around the islands. The growth rate decreased when the island height exceeded 3 nm. At negative tip biases above 7 V, the technique produced a highly reproducible formation of the islands whose growth rate increased as the bias voltage increased.
Lattice location of Ca in GaN using ion channeling
Authors: Kobayashi H. , Gibson W. M.
Published: July 2002

The Ca dopant site in the GaN lattice has been investigated using ion channeling. Metal organic chemical vapor deposition grown GaN on c-plane sapphire substrates implanted with Ca40 at a dose of 1×1015 cm−2 with postimplant annealing were studied. Our results indicate that more than 80% of Ca are near Ga sites even in as-implanted samples, however, they are displaced by ∼0.2 Å from the Ga sites and that the Ca goes to the exact Ga sites after annealing at 1100 °C while the annealing did not change the apparent fraction of substitutional Ca. We suggest that the displaced Ca in the as-implanted sample are electrically compensated due to formation of complex defects with donor-like point defects and that CaGa becomes electrically active when these complex defects are broken and the point defects diffuse away with annealing at 1100 °C.
A-site layer terminated perovskite substrate: NdGaO3
Authors: Ohnishi Tsuyoshi , Takahashi Kazuhiro , Nakamura Masashi , Kawasaki Masashi , Yoshimoto Mamoru , Koinuma Hideomi
Published: July 2002

A perovskite single-crystal substrate, NdGaO3 (001), was thermally annealed in air to give an atomically defined surface structure. From analysis with coaxial impact-collision ion scattering spectroscopy, the terminating atomic layer was identified to be NdO1+δ , i.e., the A-site oxide monolayer in perovskite ABO3. This result is contrary to the B-site oxide (BO2−δ) termination observed in other perovskite surfaces, such as wet etched SrTiO3 and LaAlO3 or annealed (LaAlO3)0.3–(Sr2AlTaO6)0.7 (LSAT).
Spin-polarized current in semimagnetic semiconductor heterostructures
Authors: Chitta V. A. , Maialle M. Z. , Leão S. A. , Degani M. H.
Published: July 2002

A semimagnetic semiconductor tunneling device is proposed as a spin filter. This device, which gives spin-polarized electron current, is obtained by choosing different layers of II-VI compounds, with magnetic moments of substitutional ions of Mn2+ in some of the layers. We present a theoretical calculation of the tunneling current for the device, in which electron spin-flip scattering produced by the thermal fluctuations of the magnetic moments is accounted for and found to be inefficient in depolarizing the current. A different system is also investigated to show more clearly the effects of spin-flip scattering on the tunneling current.
Temperature dependence of far-infrared electroluminescence in parabolic quantum wells
Authors: Ulrich J. , Zobl R. , Unterrainer K. , Strasser G. , Gornik E. , Maranowski K. D. , Gossard A. C.
Published: July 2002

We have measured the far-infrared emission from parabolically graded quantum wells driven by an in-plane electric field in the temperature range from 20 to 240 K. The peak emission corresponds to the intersubband plasmon in the parabolic potential. Its photon energy (6.6/9.8 meV) remains rather unaffected by temperature variations, the full-width at half-maximum ranges from 1 (T=20 K) to 2 meV (T=240 K). The reduction of emission efficiency with increasing temperature is attributed to the change in the nonradiative lifetime.
Boride-enhanced diffusion in silicon: Bulk and surface layers
Authors: Cowern N. E. B. , Theunissen M. J. J. , Roozeboom F. , van Berkum J. G. M.
Published: July 2002

Epitaxial silicon boride layers, located at the surface or within the bulk of single-crystal silicon, give rise to enhanced diffusion of B during annealing. A submonolayer buried boride layer releases ≈0.4 interstitials per B atom in the layer, generating a transient diffusion enhancement in the range of 10–100 for several minutes at 900 °C. The resulting profile broadening is comparable to that caused by ion implantation damage. At the same temperature, surface boride layers generate a diffusion enhancement of ∼6, part of which arises from the B diffusion flux and part from the chemical influence of the boride layer.
Contribution of atomic and molecular ions to dry-etch damage
Authors: Deng L. G. , Rahman M. , van den Berg J. A. , Wilkinson C. D. W.
Published: July 2002

Using ions of low energy is accepted as an essential requirement in achieving low damage when dry-etching III–V semiconductors. SiCl4 is widely used to make GaAs electron devices. We have studied the effect of a SiCl4 reactive ion etching environment as well as the effect of the bombardment by the separate constituent ions from a SiCl4 discharge in a low-energy implanter. Photoluminescence intensity measurements from GaAs/AlGaAs quantum well probe structures have been used to study the damage inflicted. We find that molecular ions contribute less to deep damage than do atomic ions. Thus, low damage may be promoted by selecting reactive etch chemistries with low ion energies and small atomic to molecular ion fractions.
Study of strain in partially relaxed Ge epilayers on Si(100) substrate
Authors: Jiang Z. M. , Pei C. W. , Zhou X. F. , Jiang W. R. , Shi B. , Liu X. H. , Wang Xun , Jia Q. J. , Zheng W. L. , Jiang X. M.
Published: July 2002

Ge epilayers of different thicknesses are grown by molecular-beam epitaxy with Sb as a surfactant on Si(100) substrates. X-ray diffraction illustrates that these Ge thin films are partially strained, and the strains decrease gradually with increasing epilayer thickness. Raman spectra reveal a downward shift of the Ge–Ge mode peak as the epilayer thickness increases. In the regions of high strain, the relationship between the Raman shift of this mode and the strain in the partially relaxed samples is considerably different from the linear relationship reported before, which is mainly attributed to the spatial confinement effect of phonons in a nanocrystal.
High-quality AlN grown on Si(111) by gas-source molecular-beam epitaxy with ammonia
Authors: Nikishin S. A. , Antipov V. G. , Francoeur S. , Faleev N. N. , Seryogin G. A. , Elyukhin V. A. , Temkin H. , Prokofyeva T. I. , Holtz M. , Konkar A. , Zollner S.
Published: July 2002

Hexagonal AlN layers were grown on Si(111) by gas-source molecular-beam epitaxy with ammonia. The transition between the (7×7) and (1×1) silicon surface reconstructions, at 1100 K, was used for in situ calibration of the substrate temperature. The initial deposition of Al, at 1130–1190 K, produced an effective nucleation layer for the growth of AlN. The Al layer also reduced islands of SiNx that might be formed due to background NH3 on the silicon surface prior to the onset of epitaxial growth. The transition to two-dimensional growth mode, under optimum conditions, was obtained after the initial AlN thickness of ∼7 nm.
Boron-related minority-carrier trapping centers in p-type silicon
Authors: Macdonald Daniel , Kerr Mark , Cuevas Andrés
Published: July 2002

Photoconductivity-based measurements of recombination lifetimes in multicrystalline silicon are often hampered by carrier trapping effects, which cause a characteristically large relative increase in the photoconductance. Single-crystal p-type float-zone wafers of varying resistivities were cross contaminated with multicrystalline wafers that exhibited such trapping. A proportion of the impurities present in the multicrystalline samples was found to effuse into the float-zone wafers, where they act as both recombination centers and trapping centers. By the application of a simple theoretical model, the trap density in the float-zone samples was determined, and found to be directly proportional to the boron-dopant concentration. These results suggest that the trapping centers are caused by boron-impurity pairs.
Dual integrated actuators for extended range high speed atomic force microscopy
Authors: Sulchek T. , Minne S. C. , Adams J. D. , Fletcher D. A. , Atalar A. , Quate C. F. , Adderton D. M.
Published: July 2002

A flexible system for increasing the throughput of the atomic force microscope without sacrificing imaging range is presented. The system is based on a nested feedback loop which controls a micromachined cantilever that contains both an integrated piezoelectric actuator and an integrated thermal actuator. This combination enables high speed imaging (2 mm/s) over an extended range by utilizing the piezoelectric actuator’s high bandwidth (15 kHz) and thermal actuator’s large response (300 nm/V). A constant force image, where the sample topography exceeds the range of the piezoelectric actuator alone, is presented. It has also been demonstrated that the deflection response of the thermal actuator can be linearized and controlled with an integrated diode.
Spectral analysis of InGaAs/GaAs quantum-dot lasers
Authors: Smowton P. M. , Johnston E. J. , Dewar S. V. , Hulyer P. J. , Summers H. D. , Patanè A. , Polimeni A. , Henini M.
Published: July 2002

The cause of the unusual spectral distribution, often observed in InGaAs/GaAs quantum-dot lasers, is investigated by analyzing the spectra from devices fabricated with different substrate thickness (100–400 μm). Using a Fourier transform analysis to determine the optical path length, it is found that the measured modulation period correlates with the device thickness. Such a result provides evidence for spectral modulation mediated by the device structure rather than the quantum-dot material itself and is consistent with the idea that the modulation is due to a mode propagating in the transparent substrate.
Microscopy of electronic states contributing to lasing in ridge quantum-wire laser structure
Authors: Watanabe Shinichi , Koshiba Shyun , Yoshita Masahiro , Sakaki Hiroyuki , Baba Motoyoshi , Akiyama Hidefumi
Published: July 2002

Distribution of electronic states inside ridge quantum-wire (QWR) laser cavity was investigated using spatially and spectrally resolved top-view photoluminescence (PL) imaging method. PL inhomogeneity in QWR has shown that the electronic states were perturbed by the fluctuation of vertical thickness in the scale of ∼nm, while the optical waveguide was not. The PL images of QWR were traced up to the lasing condition to examine the lasing origin.
Enhanced magnetoresistance effect in layered systems
Authors: Ferreira M. S. , d’Albuquerque e Castro J. , Muniz R. B. , Villeret Murielle
Published: July 2002

Magnetoresistance ratios several orders of magnitude higher than those of conventional multilayers may be obtained with much smaller saturation fields in magnetic layers separated by a periodically modulated structure. Conditions for the occurrence of such effect, as well as the possible use of these systems as spin-filter devices and magnetic logical gates, are discussed.
Radiative recombination of two-dimensional electrons in a modulation-doped Al0.37Ga0.63N/GaN single heterostructure
Authors: Kwon Ho Ki , Eiting C. J. , Lambert D. J. H. , Shelton B. S. , Wong M. M. , Zhu T.-G. , Dupuis R. D.
Published: July 2002

We have studied the low-temperature (4 K) photoluminescence (PL) of a modulation-doped Al0.37Ga0.63N/GaN single heterostructure. Radiative recombination is observed between electrons in the two-dimensional quantum states at the heterointerface and holes in either the flatband region or bound to residual acceptors. This PL emission disappears when the top AlGaN layer is removed by reactive ion etching, indicating that the luminescence is from the two-dimensional electron states. From the peak separation of the luminescence related to the two-dimensional electrons, the maximum electric field at the heterointerface is estimated to be 3.7×104 V/cm.
Optical spectra of nitride quantum dots: Quantum confinement and electron–hole coupling
Authors: Hohenester Ulrich , Di Felice Rosa , Molinari Elisa , Rossi Fausto
Published: July 2002

We calculate the optical properties of nitride-based quantum dots by taking into account quantum confinement as well as electron–hole interaction. We analyze model structures simulating InxGa1−xN dots in InyGa1−yN layers with different alloy compositions. We discuss the trends with the dot size and show that quantum confined excitations exist for a broad range of sizes down to the smallest observed dots. Our results allow us to identify the strong role of Coulomb correlations in the optical spectra and to predict a strong influence of photoexcitation power on selection rules in polarized samples. The signature of quantum confinement can be utilized for a critical interpretation of measured optical spectra.
Cyclotron resonance in modulation-doped ZnSe/Zn1−xCdxSe and ZnTe/CdSe single quantum wells
Authors: Ng H. K. , Leem Y. A. , Knobel R. , Smorchkova I. P. , Sirenko A. A. , Samarth N.
Published: July 2002

We report low-temperature (4.2 K) cyclotron resonance measurements on high-mobility, two-dimensional electron gases in modulation-doped ZnSe/Zn1−xCdxSe (x=0.06, 0.12, and 0.24) single quantum wells, as well as in a modulation-doped ZnTe/CdSe single quantum well. These experiments carried out in magnetic fields ranging up to 17 T yield reliable measurements of the effective mass m* of conduction-band electrons in Zn1−xCdxSe alloys, including the measurement of m* in cubic CdSe.
Surface treatment of indium tin oxide by SF6 plasma for organic light-emitting diodes
Authors: Choi Beomrak , Yoon Hyunsik , Lee Hong H.
Published: July 2002

SF 6 plasma treatment of indium tin oxide highly improves the power efficiency and the stability of the organic light-emitting diode based on poly[2-methoxy-5-(2-ethylhexyloxy)-1,4phenylenevinylene]. The treatment leads to a slight reduction in the surface roughness and a decrease in the surface content of Sn. The major effect, however, has to do with the surface incorporation of fluorine. This fluorinated surface improves the hole injection and thus the device performance.
Desorption of positive ions from ionic crystals accompanying 248 nm laser irradiation
Authors: Bandis C. , Langford S. C. , Dickinson J. T.
Published: July 2002

We present a study of the energy distributions of positive ions (Na+, Li+, Ca+, and Mg+) photodesorbed from cleaved NaCl, LiF, MgO, NaNO3, and CaCO3 surfaces during 248 nm excimer laser irradiation at fluences well below the damage thresholds. The observed ion energies are significantly higher than those predicted by already existing models that allow ion rearrangement and relaxation during the electrostatic ion repulsion by the nearby photoionized sites. In contrast to what one would expect, we find that treating the ions as fixed charges and neglecting any ion rearrangement during the emission of the adions describes best the experimentally observed ion energies from all five ionic crystals.
First-principles study of NH3 exposed Si(001)2×1: Relation between N 1s core-level shifts and atomic structure
Authors: Rignanese G.-M. , Pasquarello Alfredo
Published: July 2002

Using a first-principles approach, we assign N 1s core-level shifts at ammonium exposed Si(001)2×1 surfaces to definite bonding configurations of N atoms. Model structures are obtained by fully relaxing the atomic positions of N atoms in different bonding configurations. Calculated values of N 1s core-level shifts of N-Si3, N-Si2H, and N-SiH2 structural units show a linear dependence on the number of nearest-neighbor H atoms, in good agreement with data from photoemission experiments. Our results support the picture in which NH3 is adsorbed dissociatively as NH2 and H.
Valence band structure of cubic AlGaN/GaN superlattices
Authors: Rodrigues S. C. P. , Scolfaro L. M. R. , Leite J. R. , Sipahi G. M.
Published: July 2002

The influence of different material parameters, spin-orbit interaction, and strain effects on the valence band structure of cubic AlGaN/GaN superlattices is investigated. One-particle hole state calculations are carried out within the k⋅p theory by means of a full six-band Luttinger-Kohn Hamiltonian in a plane-wave representation. It is shown that the use of distinct values for the Luttinger parameters for the barrier and well regions leads to significant changes in the hole levels, particularly for large Al content. Spin-orbit interaction effects are responsible for strong nonparabolicities due to the light- and split-off-hole bands mixing. Besides, the hole levels are very sensitive to strain effects. Thus, it is demonstrated that these effects cannot be neglected in a realistic description of the valence band structure of these materials.
Temperature-dependent tunneling through thermally grown SiO2 on n-type 4H– and 6H–SiC
Authors: Waters Richard , Van Zeghbroeck Bart
Published: July 2002

The temperature dependence of field emission through thermally grown silicon dioxide (SiO2) on n-type 4H and 6H silicon carbide (SiC) substrates is reported. Room-temperature SiO2/SiC barrier heights, ΦB, of 1.92 and 2.12 V are extracted for the 4H– and 6H–SiC samples, respectively, using a Fowler–Nordheim analysis. Barrier heights of 2.2 and 2.4 V along with a linear temperature-dependent barrier height lowering, ΔΦB/ΔT, of 2.4 and 2.0 mV/K for 4H– and 6H–SiC are extracted using an alternative analytical expression for tunneling from semiconducting substrates derived previously. In both analyses, the temperature-dependent flatband voltage, using the measured room-temperature value, was included.
Velocity overshoot onset in nitride semiconductors
Authors: Rodrigues C. G. , Freire V. N. , Vasconcellos A. R. , Luzzi R.
Published: July 2002

A theoretical study on the electron drift velocity and some nonequilibrium thermodynamic characteristics of wurtzite GaN, AlN, and InN is presented. It is based on a nonlinear quantum kinetic theory which provides a description of the dissipative phenomena developing in the system. The ultrafast time evolution of the electron drift velocity and quasitemperature is obtained, and overshoot effects are evidenced on both. The overshoot onsets are shown to occur at 20 kV/cm in GaN, 60 kV/cm in AlN, and 10 kV/cm in InN, electric field intensities which are considerably smaller than those that have been recently derived resorting to Monte Carlo simulations.
Metastable phases of cobalt-ironsilicide formed by sequential implantation of Co and Fe in Si (111)
Authors: Dézsi I. , Fetzer Cs. , Kiss M. , Pattyn H. , Vantomme A. , Langouche G.
Published: July 2002

By implanting Co and Fe in sequence into Si (111), metastable ternary Co1−xFexSi2 phases were formed. Mössbauer effect measurements showed three resonance line components in the spectrum. Comparison of the central shift (CS) values of the components with those appearing in the stable ternary phases indicated that iron atoms are positioned in the substitutional Co site, in the empty cube of the fluorite-type lattice and in CsCl-like B2 structures. It was found that the CS values of two components are in the velocity range of the values obtained for the metastable γ-FeSi2 synthesized using various methods. This result suggests the existence of a similar structure.
Coherent control of precessional dynamics in thin film permalloy
Authors: Crawford T. M. , Kabos P. , Silva T. J.
Published: July 2002

We demonstrate a method of eliminating overshoot and ringing in magnetization dynamics when the system bandwidth includes the intrinsic ferromagnetic resonance (FMR). The method employs staggered step excitation for the cancellation of FMR oscillations while maximizing the risetime of the magnetization response. The second-harmonic magneto-optic Kerr effect is used to measure the magnetic response at a localized spot on the sample. The measured response is adequately modeled with the Landau–Lifshitz–Gilbert differential equation. We explain the observed behavior in terms of destructive interference.