bluuebunny/crossref_metadata_2025 huggingface dataset:
Semiconductor quantum point contact fabricated by lithography with an atomic force microscope
Authors: Held R. , Heinzel T. , Studerus P. , Ensslin K. , Holland M.
Published: July 2002

We report on the experimental realization of a quantum point contact in a semiconductor heterostructure by lithography with an atomic force microscope (AFM). A thin, homogeneous titanium film on top of the chip surface was patterned by local anodic oxidation, induced by a current applied to an n-doped AFM tip. We demonstrate that self-aligned gate structures in the sub-micron regime can be fabricated with this technique.
Bias effects in high performance GaAs homojunction far-infrared detectors
Authors: Shen W. Z. , Perera A. G. U. , Liu H. C. , Buchanan M. , Schaff W. J.
Published: July 2002

A high performance, bias tunable, p-GaAs homojunction interfacial workfunction internal photoemission far-infrared detector is demonstrated. A responsivity of 3.10±0.05 A/W, a quantum efficiency of 12.5%, and a detectivity D* of 5.9×1010 cmHz/W were obtained at 4.2 K for cutoff wavelengths from 80 to 100 μm. The bias dependences of the quantum efficiency, detectivity, and cutoff wavelength were measured and are well explained by the theoretical model. The effect of the layer number on detector performance and the uniformity of the detectors are discussed. A comparison with Ge:Ga photoconductive detectors suggests that similar or even better performance may be obtainable with a far-infrared detector.
A novel dual-gate high electron mobility transistor using a split-gate structure
Authors: Collier N. J. , Cleaver J. R. A.
Published: July 2002

The split-gate concept has been applied to dual-gate high electron mobility structures for room-temperature operation. The gates are configured so that the second gate is in close proximity to the gap defined by the split-gate electrodes. This allows both gates to control the carrier density in the same region of the device, so that it is possible to control the threshold voltage for either gate by altering the bias at which the other gate is held. The effect of changing the gate configuration is demonstrated.
Effect of buffer layer and substrate surface polarity on the growth by molecular beam epitaxy of GaN on ZnO
Authors: Hamdani F. , Botchkarev A. E. , Tang H. , Kim W. , Morkoç H.
Published: July 2002

We present results on the effect of substrate surface polarity, oxygen and zinc faces, on the quality of GaN epitaxial layers grown on ZnO(0001) substrates by reactive ammonia molecular beam epitaxy. The possible effects dealing with the disparity in surface preparation of the two faces have been eliminated. Photoluminescence and reflectivity measurements demonstrate that the oxygen face leads to higher quality GaN on ZnO compared to the zinc face. We also present optical data obtained by using different low-temperature AlN, GaN, and InxGa1−xN buffer layers. The best result has been obtained with lattice-matched In0.20Ga0.80N buffer layer.
Relaxed Si0.7Ge0.3 layers grown on low-temperature Si buffers with low threading dislocation density
Authors: Li J. H. , Peng C. S. , Wu Y. , Dai D. Y. , Zhou J. M. , Mai Z. H.
Published: July 2002

Si 0.7 Ge 0.3 epilayers with low threading dislocation density have been grown on Si (001) substrates by introducing a low temperature Si buffer. Such a structure can be used as the buffer for the growth of device structures. In comparison with the conventional compositionally graded buffer system, it has the advantages of having lower threading dislocation density, smaller thickness for required degree of relaxation, and smoother surface. Experimental evidence suggests that an anomalous relaxation mechanism has been involved.
Auger recombination in 4H-SiC: Unusual temperature behavior
Authors: Galeckas A. , Linnros J. , Grivickas V. , Lindefelt U. , Hallin C.
Published: July 2002

The band-to-band Auger recombination in 4H-SiC material is studied using a time-resolved photoinduced absorption technique. The Auger recombination coefficient is derived from the kinetics of electron-hole plasma in heavily doped n-type 4H-SiC and in low-doped 4H-SiC epitaxial layers in the temperature interval 300–565 K. Within this range, its value decreases from γ3=(7±1)×10−31 cm6 s−1 to γ3=(4±1)×10−32 cm6 s−1. The observed pronounced reduction of Auger recombination rate with temperature is correlated to temperature dependent threshold energy of Auger process.
High-temperature type-II superlattice diode laser at λ=2.9 μm
Authors: Bewley W. W. , Aifer E. H. , Felix C. L. , Vurgaftman I. , Meyer J. R. , Lin C.-H. , Murry S. J. , Zhang D. , Pei S. S.
Published: July 2002

A 2.9 μm diode laser with InAs/GaSb/Ga0.75In0.25Sb/GaSb superlattice active region displays a maximum operating temperature of 260 K. At 200 K, the threshold current density is 1.1 kA/cm2 and the quantum efficiency is >15%. The peak output power per facet exceeds 800 mW at 100 K and 200 mW at 200 K for a 0.05% duty cycle.
Sensitivity of optimization of mid-infrared InAs/InGaSb laser active regions to temperature and composition variations
Authors: Flatté Michael E. , Grein C. H. , Ehrenreich H.
Published: July 2002

We calculate the temperature dependence of the threshold current density Jth in optimized (minimal Jth) and unoptimized InAs/InGaSb superlattices. We find that the threshold current density of the unoptimized superlattice is well described by Jth∝eT/T0, with T0∼32 K from 25 to 275 K. This is the first microscopic calculation for these superlattices which indicates that Jth is well described by an empirical exponential form. In contrast, the threshold current density of the optimized superlattice is not well parametrized by a characteristic temperature T0. This superlattice is only optimized between 250 and 350 K, due to the sharp structure of the intersubband absorption spectrum. We also consider the effect on Jth of uncertainties in layer thicknesses.
Measurement of the zero-bias electron transmittance as a function of energy for half- and quarter-electron-wavelength semiconductor quantum-interference filters
Authors: Guthrie D. K. , First P. N. , Gaylord T. K. , Glytsis E. N. , Leibenguth R. E.
Published: July 2002

Ballistic electron emission spectroscopy has been used to measure the electron transmittance function of both half- and quarter-electron-wavelength (using optics terminology) quantum-interference filters under zero applied voltage bias. At the design energy, these devices exhibit constructive and destructive interference, respectively. Second-derivative spectra from current versus voltage measurements clearly show both tunneling and above-barrier quasibound energy states. The spectra accurately reproduce the transmittance functions of the designed structures, attaining nearly the temperature-limited resolution at 77 K and 300 K. The presence of the above-barrier resonances has been confirmed conclusively by measurements on these complementary half- and quarter-wavelength device structures.
Ultrafast excitonic saturable absorption in ion-implanted InGaAs/InAlAs multiple quantum wells
Authors: Lugagne Delpon E. , Oudar J. L. , Bouché N. , Raj R. , Shen A. , Stelmakh N. , Lourtioz J. M.
Published: July 2002

We report on ultrafast excitonic nonlinearities in ion-implanted InGaAs/InAlAs multiple quantum wells. We find that irradiation with energetic O+ and Ni+ ions can reduce the carrier lifetime from 1.6 ns down to 1.7 ps without significantly altering the excitonic absorption properties, making efficient fast saturable absorbers in the 1.3–1.5 μm wavelength range.
Seeded self-assembled GaAs quantum dots grown in two-dimensional V grooves by selective metal–organic chemical-vapor deposition
Authors: Ishida S. , Arakawa Y. , Wada K.
Published: July 2002

We propose and demonstrate a seeded self-assembling growth technique for the fabrication of quantum dot structures by metal–organic chemical-vapor deposition. GaAs quantum dots were grown at the bottom of two-dimensional V-groove (2DVG) structures, which were composed of (111)A and (111)B facets on a GaAs(100) substrate. The 2DVG was grown by selective growth on a SiO2 patterned substrate. It was found that vertical GaAs quantum wires were also simultaneously formed in the 2DVGs. Using this technique, the stacking of GaAs quantum dots was realized. Photoluminescence and cathodoluminescence from each structure evidenced the formation of both the quantum wires and the quantum dots.
Efficient all-optical light modulation in a piezoelectric heterostructure at room temperature
Authors: Ortiz V. , Pelekanos N. T. , Mula Guido
Published: July 2002

We demonstrate a novel piezoelectric-barrier heterostructure designed for efficient all-optical light modulation. The modulation mechanism relies upon drastic photocarrier separation by the piezoelectric field in the barrier layers. We present room temperature results showing that an optical “control” power of 70 W/cm2 creates in the heart of the structure a space-charge field of about 30 kV/cm, which induces large spectral shifts (≈100 nm) in the photoluminescence spectra of a CdHgTe quantum well in the 1.5 μm range.
Determination of the carrier-type at III-nitride semiconductor surfaces/interfaces using contactless electroreflectance
Authors: Krystek Wojciech , Pollak Fred H. , Feng Z. C. , Schurman M. , Stall R. A.
Published: July 2002

Using contactless electroreflectance at room temperature, we have nondestructively evaluated the band bending (carrier-type) at the surface of epitaxial n- and p-type GaN/sapphire samples as well as at both the InGaN surface and the InGaN/GaN interface of samples of epitaxial InGaN, having average n- and p-type character, grown on top of thick GaN epilayers/sapphire.
Characteristics of scanning-probe lithography with a current-controlled exposure system
Authors: Ishibashi Masayoshi , Heike Seiji , Kajiyama Hiroshi , Wada Yasuo , Hashizume Tomihiro
Published: July 2002

Characteristics of atomic force microscopy lithography using a current-controlled exposure feedback system are investigated by fabricating line-and-space patterns on the negative-type electron beam resist RD2100N. We find that the cross-sectional shape of the developed resist pattern depends on the amount of exposure. The resolution depends on the resist thickness,and a minimum line width of 27 nm is obtained for a 15-nm-thick resist. The proximity effect is evaluated by comparing a resist pattern with a model calculation. Electric-field mapping inside the resist is calculated,and an exposure mechanism is proposed to explain the characteristics.
Experimental study of spontaneous electric field generated by a laser plasma
Authors: Kabashin A. V. , Nikitin P. I. , Marine W. , Sentis M.
Published: July 2002

We report investigations of a space-time structure of the electric field of laser plasma from a solid target. An ArF excimer laser with an intensity of I≅108 W/cm2 was used to produce the plasma on various targets placed in air at atmospheric pressure. A strong difference in both the amplitude (by more than 1–2 orders of magnitude) and the structure of the electric field for conductive and dielectric targets has been observed. The field distribution for a conductive target was found to correspond to a dipole configuration of charges in the laser plasma, while for the dielectric target a quadrupole configuration was revealed. Possible explanations and applications of the observed effect are discussed.
The effect of ultrathin oxides on luminescent silicon nanocrystallites
Authors: Thompson W. Howard , Yamani Zain , AbuHassan Laila , Gurdal Osman , Nayfeh Munir
Published: July 2002

The effect of ultrathin oxides on nanocrystallites of luminescent porous silicon is studied using infrared, optical, and Auger spectroscopy. Room-temperature oxidation is performed using H2O2 immersion and UV ozone interactions, producing oxides of ∼5 and ∼10 Å, respectively. The H2O2 oxidized sample is optically active, while the ozone oxidized sample is not active. UV–ozone produces a transverse optical Si–O–Si mode blueshifted by ∼90 cm−1 from bulk oxide, which H2O2 does not produce. Auger Si LVV spectra show an oxidelike signal for UV/ozone samples and a Si-like signal for H2O2 samples. We discuss this in terms of different oxidation behaviors that either preserve or break Si–Si dimers that may be responsible for the optical behavior.
Raman study of silicon nanocrystals formed in SiNx films by excimer laser or thermal annealing
Authors: Volodin V. A. , Efremov M. D. , Gritsenko V. A. , Kochubei S. A.
Published: July 2002

Silicon nitride films of different stoichiometric composition were studied using Raman spectroscopy. A Raman signal due to Si–Si, Si–N bond vibrations in silicon nanoclusters was detected in as-deposited films. The appearance of Raman peaks in the range 493–514 cm−1 after thermal and pulse laser treatments was interpreted as formation of silicon nanocrystals with sizes from 1.3 up to 5 nm depending on treatment parameters. Thermal treatment at 1200 °C allowed Si atom diffusion and its gathering in Si nanocrystals, meanwhile 5 ns pulse laser irradiation leads to crystallization of preexisting silicon nanoclusters inside the as-deposited SiNx films.
Nanometer-scale imaging of domains in ferroelectric thin films using apertureless near-field scanning optical microscopy
Authors: Hubert Charles , Levy Jeremy
Published: July 2002

Images of nanometer-scale ferroelectric domains in BaxSr1−xTiO3 thin films are obtained with 30 Å spatial resolution using apertureless near-field scanning optical microscopy (ANSOM). The images exhibit inhomogeneities in the ferroelectric polarization over the smallest scales that can be observed, and are largely uncorrelated with topographic features. The application of an in-plane static electric field causes domain reorientation and domain-wall motion over distances as small as 40 Å. These results demonstrate the promise of ANSOM for imaging near-atomic-scale polarization fluctuations in ferroelectric materials.
Lasing from InGaAs/GaAs quantum dots with extended wavelength and well-defined harmonic-oscillator energy levels
Authors: Park G. , Shchekin O. B. , Huffaker D. L. , Deppe D. G.
Published: July 2002

Data are presented on the lasing transitions from InGaAs/GaAs quantum dots that exhibit four well-resolved energy transitions, with the electronic state density of a two-dimensional harmonic oscillator. Lasing has been obtained on the second and third transitions, depending on the cavity (gain) length, with the longest lasing wavelength measured to be 1.19 μm. The temperature dependence of threshold is studied and regions of nearly temperature-independent threshold are found. Interesting aspects of the unique electronic state density for lasers are discussed.
Silica capping for Al0.3Ga0.7As/GaAs and In0.2Ga0.8As/GaAs quantum well intermixing
Authors: Li G. , Chua S. J. , Xu S. J. , Wang X. C. , Saher Helmy A. , Ke Mao-Long , Marsh J. H.
Published: July 2002

Spin-on silica capping has been demonstrated to be an effective dielectric encapsulant layer for quantum well (QW) intermixing at temperatures significantly lower than for conventionally deposited silica. A blueshift of up to 125 meV was observed in the photoluminescence (PL) peak energy of both GaAs and InGaAs QWs after annealing for less than 60 s at 850 °C, without noticeable degradation in the PL emission intensity. A threshold temperature was identified below which no significant QW disordering took place. The activation energy for Al diffusion in Al0.3Ga0.7As/GaAs QWs was about 2.55 eV. Broadly similar effects were seen for In0.2Ga0.8As/GaAs QWs but, in addition, strain effects appear to enhance disordering during the early stages of the anneal.
Intrinsic charge transport properties of an organic single crystal determined using a multiterminal thin-film transistor
Authors: Schoonveld W. A. , Vrijmoeth J. , Klapwijk T. M.
Published: July 2002

We present an experimental approach to determine the intrinsic field-effect mobility in an organic single crystal as a function of the in-plane crystal-axis direction. Using a multiterminal geometry the experiment also excludes the effects of the contact resistances on the transport properties. We have applied the method to quaterthiophene thin films. At gate voltages above the threshold voltage, the channel conductance varies linearly with the applied gate voltage. The resulting charge carrier mobility is 1.2×10−3 cm2/V s. This value is constant over a period of weeks of continuous operation and does not vary from sample to sample, in contrast to the typically observed lower mobilities of polycrystalline quaterthiophene thin-film transistor devices. These results emphasize the need for well-characterized model systems to study the intrinsic transport properties of organic materials.
Observation of negative-U centers in 6H silicon carbide
Authors: Hemmingsson C. G. , Son N. T. , Janzén E.
Published: July 2002

Two negative-U centers in 6H SiC have been observed and characterized using capacitance transient techniques. These two defects give rise to one acceptor level (−/0) and one donor level (+/0) each in the band gap. The donor and the acceptor level have inverted ordering, i.e., the thermal ionization energy of the acceptor level is larger than that of the donor level. Direct evidence for the inverted ordering of the acceptor and donor levels and temperature dependence studies of the electron capture cross sections of the acceptor levels are presented.
Observation of quantum dot-like behavior of GaInNAs in GaInNAs/GaAs quantum wells
Authors: Xin H. P. , Kavanagh K. L. , Zhu Z. Q. , Tu C. W.
Published: July 2002

We report a quantum dot-like behavior of GaInNAs due to composition nonuniformity of N and In in GaInNAs/GaAs quantum wells (QWs). Images of cross-sectional transmission electron microscopy show that the wells of both Ga0.7In0.3As/GaAs and Ga0.7In0.3N0.02As0.98/GaAs are undulated due to lateral variations in strain. This effect is more pronounced in the N-containing QWs due to nonuniform In and N concentrations. Rapid thermal annealing causes a blueshift of the photoluminescence (PL) peak, and results in a splitting of the as-grown broad PL emission into two peaks. The In and N composition fluctuation after annealing becomes predominantly bimodal. The low-energy PL peak is attributed to excitons localized at deep levels, which are originated from In- and N-rich regions in the wells acting as quantum dots (QD). The high-energy peak PH is likely due to the excitons of the 2D QWs. To reduce the local strain, N atoms are preferentially localized in the In-rich regions, so the separation between these two peaks increases with increasing N concentration. Increasing excitation intensity results in a significant blueshift for PL and little blueshift for PH, due to the easier band filling of the localized QD states than QW states.
Negative photoconductivity in SiO2 films containing Si nanocrystals
Authors: Choi Suk-Ho , Elliman R. G.
Published: July 2002

Negative photoconductivity is reported in metal–insulator–semiconductor structures containing Si nanocrystals produced by ion implantation and annealing. Under forward bias, the current is reduced when the sample is illuminated with ultraviolet light whilst under reverse bias it is increased. No such change is observed in structures that have not been ion implanted. This effect is explained by charging of nanocrystals in the oxide layer by photoionization of electrons. The positively charged nanocrystals screen the applied bias voltage leading to a reduction in current at a given voltage. This effect, if larger than the current increase caused by the photoexcitation of carriers in the accumulation layer, gives rise to the observed negative photoconductivity.
Applications of an In0.53Ga0.25Al0.22As/InP continuous-conduction-band structure for ultralow current operation transistors
Authors: Liu Wen-Chau , Pan Hsi-Jen , Cheng Shiou-Ying , Wang Wei-Chou , Chen Jing-Yuh , Feng Shun-Ching , Yu Kuo-Hui
Published: July 2002

Based on the compositional dependence on the conduction band discontinuity, a significant heterojunction bipolar transistor (HBT) with a continuous conduction band heterointerface between the InP emitter and the In0.53Ga0.25Al0.22As base is fabricated. Experimentally, due to the elimination of the potential spike, a very low offset voltage of 50 mV is observed. Also, the studied device exhibits better breakdown characteristics and lower output conductance as compared with other InP/InGaAs or AlInAs/InGaAs HBTs. Furthermore, attributed to the enhancement of the hole confinement by an inserted δ-doped sheet at the emitter–base interface, a dramatic current gain about 4 is found even under an ultralow current operation regime (IC⩽5 nA).
Optical transitions in Pr-implanted GaN
Authors: Zavada J. M. , Mair R. A. , Ellis C. J. , Lin J. Y. , Jiang H. X. , Wilson R. G. , Grudowski P. A. , Dupuis R. D.
Published: July 2002

Photoluminescence (PL) spectroscopy has been used to investigate praseodymium (Pr) related transitions in Pr-implanted GaN. Wurtzite GaN epilayers were grown by metalorganic chemical vapor deposition on sapphire substrates and subsequently ion implanted with Pr to a dose of 5.7×1013/cm2. The implanted samples were annealed in nitrogen to facilitate recovery from implantation related damage. Narrow PL emission bands related to 4f intrashell transitions of the trivalent Pr ion were observed near 650, 950, 1100, and 1300 nm. The dependence of PL emission on sample temperature, excitation intensity, oxygen incorporation, and annealing temperature was systematically studied. We find that the PL efficiency increases exponentially with annealing temperature up to the maximum temperature of 1050 °C applied in the current study. Furthermore, the PL emission shows no evidence of significant thermal quenching over the sample temperature range of 10–300 K. This thermal stability will have particular advantages for applications in high temperature optoelectronic devices.
Ultrafast response of nonlinear refractive index of silver nanocrystals embedded in glass
Authors: Hamanaka Y. , Nakamura A. , Omi S. , Del Fatti N. , Vallée F. , Flytzanis C.
Published: July 2002

Ultrafast Kerr-type nonlinearities and relaxation dynamics of photoexcited electrons in silver nanocrystals embedded in glass have been investigated by means of femtosecond pump and probe spectroscopy. The transient absorption spectrum induced by the surface plasmon excitation shows a redshift and broadening of the surface plasmon band. The additional broadening is ascribed to the increase of surface plasmon damping and the redshift originates from a change in the real part of the dielectric function of the silver nanocrystals due to nonequilibrium electron heating. The observed redshift yields the nonlinear refractive index n2 of +2.4×10−10 esu and its time response is 1.9 ps.
Structural models of amorphous carbon surfaces
Authors: Haerle Rainer , Galli Giulia , Baldereschi Alfonso
Published: July 2002

Using tight-binding molecular dynamics, we have constructed structural models of amorphous carbon surfaces, by imposing tensile strain on computer generated networks containing 512 carbon atoms until fracture is produced and two surfaces are formed. Different tensile strains are applied along different directions, in order to mimic the effect of preparation conditions. The surfaces generated with different strains from networks with a given bulk atomic density, exhibit similar structures and formation energies differ by a few 0.01 eV/Å2. Surface roughness increases with the amount of the graphitic component in the bulk sample. The calculated surface thicknesses are consistent with recent experimental data.
Charge-trapping properties of gate oxide grown on nitrogen-implanted silicon substrate
Authors: Misra D.
Published: July 2002

Charge-trapping properties of ultrathin gate oxide grown on a nitrogen-implanted silicon substrate were investigated using high-field Fowler–Nordheim injection. By applying an empirical model and monitoring threshold voltage shift due to current stress, it was found that both hole trapping and electron trapping are suppressed in the nitrogen-implanted oxide. Smaller trap-generation rate compared to pure SiO2 film was also noticed. Our results indicate that nitrogen implantation into silicon substrate before gate oxide growth is an alternate way to incorporate nitrogen into the Si/SiO2 interface.
Polarization fields determination in AlGaN/GaN heterostructure field-effect transistors from charge control analysis
Authors: Garrido J. A. , Sánchez-Rojas J. L. , Jiménez A. , Muñoz E. , Omnes F. , Gibart P.
Published: July 2002

Al x Ga 1−x N/GaN heterostructure field-effect transistors with different Al concentrations (0.15<x<0.25) and barrier widths (150 Å<WB<350 Å) have been fabricated and characterized. Experimental results were analyzed by using a self-consistent solution of the Schrödinger and Poisson equations with the proper boundary conditions. The total (piezoelectric and spontaneous) polarization has been included as a fitting parameter in the self-consistent calculations. From the analysis of the transistor charge-control experimental data, a linear increase of the polarization field with the Al concentration has been found. Our results indicate that the slope of such dependence, and the magnitude of the total polarization field are lower than the predicted ones using the usually accepted values of the piezoelectric and spontaneous polarization coefficients.
Normal-incident intersubband photocurrent spectroscopy on InAs/GaAs quantum dots
Authors: Chu L. , Zrenner A. , Böhm G. , Abstreiter G.
Published: July 2002

We report on intersubband photocurrent spectroscopy of self-assembled InAs/GaAs quantum dots (QDs) both in normal incidence and in multipass waveguide geometry. The bound-to-continuum transition energy in the conduction band lies in the 200–500 meV spectral range. Polarization dependent photocurrent spectroscopy shows that the intersubband transitions in the InAs-QDs are nearly independent of the polarization of the incoming radiation.
Effect of reactive ion etching on the yellow luminescence of GaN
Authors: Basak D. , Lachab M. , Nakanishi T. , Sakai S.
Published: July 2002

Photoluminescence spectra of GaN grown by metalorganic chemical-vapor deposition on sapphire show that by reactive ion etching, the intensity of the yellow luminescence (YL) band decreases compared to that of the as-grown GaN, due to nonradiative recombination at the damage-induced defect centers. The intensity of the YL in dry-etched GaN has been found to be dependent on rf power and postetch annealing. No change in intensity is observed with further etching indicating a uniform spread of yellow emitters in the epilayer which supports the view that point defects like VGa are the origin of the YL.
Effects of NiO on electrical properties of NiAu-based ohmic contacts for p-type GaN
Authors: Maeda T. , Koide Yasuo , Murakami Masanori
Published: July 2002

Effects of a NiO layer on the electrical properties of NiAu-based ohmic contacts for p-GaN were studied by depositing a p-type NiO layer on the p-GaN using a sputter-deposition technique. NiO layers doped with Li[NiO(Li)] had a p-type conduction with sheet resistivity of around 1 Ω cm after annealing at temperatures lower than 500 °C. A variety of the NiAu-based contacts with the NiO layers such as NiO/Au, NiO(Li)/Au, Ni/NiO(Li)/Au, Ni/Li2O/NiO/Au, and Ni/Li2O/Ni/NiO/Au contacts were prepared by depositing on the p-GaN, where a slash “/” indicates the deposition sequence. However, these contacts did not provide specific contact resistances (ρc) lower than that (ρc∼10−2 Ω cm2) of the conventional Ni/Au contacts prepared by annealing in N2 ambient. From the present results, it was believed that the p-NiO layer did not act as an intermediate semiconductor layer to reduce the Schottky barrier height at the p-GaN/Au interface.
p-
GaN
surface treatments for metal contacts
Authors: Sun Jingxi , Rickert K. A. , Redwing J. M. , Ellis A. B. , Himpsel F. J. , Kuech T. F.
Published: July 2002

The chemical bonding and electronic properties of wet, chemically treated p-GaN surfaces were studied using synchrotron radiation photoemission spectroscopy. Chlorine-based chemical bonding was identified on the conventional HCl-treated p-GaN surface, which is associated with a shift of the surface Fermi level toward the conduction band edge by ∼0.9 eV with respect to the thermally cleaned surface. Compared to the HCl-treated surface, the surface Fermi level on the KOH-treated surface lies about ∼1.0 eV closer to the valence band edge, resulting in a much smaller surface barrier height to p-type materials than the HCl-treated surface. The smaller surface barrier height to p-GaN after KOH treatment can lead to a lower contact resistivity and can play an important role in lowering the metal contact resistivity to p-GaN.
Tuning the energy levels of self-assembled InAs quantum dots by rapid thermal annealing
Authors: Hsu T. M. , Lan Y. S. , Chang W.-H. , Yeh N. T. , Chyi J.-I.
Published: July 2002

We studied the photoluminescence spectra of rapid-thermal-annealed self-assembled InAs quantum dots at 10 K. For annealing temperatures ranging from 700 to 950 °C, we observed a blueshift in the interband transition energies, a decrease in the intersublevel spacing energies, and a narrowing of photoluminescence linewidths. In this letter, we demonstrate that the tuning of the InAs quantum dots interband transition and intersublevel spacing energies can be achieved by 30 s of rapid thermal annealing. The relation between interband transition energy changes and the intersublevel spacing energies is found to be linear, with a slope close to the ratio of the dots’ height to their diameter.
Field effect transistors with SrTiO3 gate dielectric on Si
Authors: Eisenbeiser K. , Finder J. M. , Yu Z. , Ramdani J. , Curless J. A. , Hallmark J. A. , Droopad R. , Ooms W. J. , Salem L. , Bradshaw S. , Overgaard C. D.
Published: July 2002

SrTiO 3 has been grown epitaxially by molecular beam epitaxy on Si. The capacitance of this 110 Å dielectric film is electrically equivalent to less than 10 Å of SiO2. This structure has been used to make capacitors and metal oxide semiconductor field effect transistors. The interface trap density between the SrTiO3 and the Si is 6.4×1010 states/cm2 eV and the inversion layer mobility is 221 and 62 cm2/V s for n- and p-channel devices, respectively. The gate leakage in these devices is two orders of magnitude smaller than a similar SiO2 gate dielectric field effect transistor.
Regimented placement of self-assembled Ge dots on selectively grown Si mesas
Authors: Jin G. , Liu J. L. , Wang K. L.
Published: July 2002

The control of positioning of self-assembled dots is critical for the fabrication of regimented arrays in signal processing applications. We report the controllable positioning of self-assembled Ge dots using selectively grown Si mesas as a template. The dependence of the dot arrangement on growth temperature and Ge thickness has been investigated. The experimental results show ability to control the positioning of Ge dots based on energetically preferential nucleation. The Ge dot growth on Si mesas is demonstrated to be a promising way to realize the placement of regimented arrays of self-assembled dots and even a single dot. This technique can be extended to other heterostructure growths.
Near-field microscope probe for far infrared time domain measurements
Authors: Mitrofanov O. , Brener I. , Wanke M. C. , Ruel R. R. , Wynn J. D. , Bruce A. J. , Federici J.
Published: July 2002

A near-field probe fabrication technique for far-infrared frequencies based on photoconducting antennas is developed. A subwavelength-size field source is accomplished by means of an aperture and protruding high refractive index tip. The near-field probe is tested by using free space traveling electromagnetic pulses with a broadband spectrum in the range of 0.3–1.5 THz. A spatial resolution of 60 μm is achieved for a 50 μm aperture. The described probe may be used for near-field transmission microscopy in illumination and collection modes. Resolution may be further improved by means of a smaller aperture.
In-plane anisotropic strain of ZnO closely packed microcrystallites grown on tilted (0001) sapphire
Authors: Siah F. , Yang Z. , Tang Z. K. , Wong G. K. L. , Kawasaki M. , Ohtomo A. , Koinuma H. , Segawa Y.
Published: July 2002

We have found that ZnO films consisting of epitaxially ordered arrays of closely packed hexagonal microcrystallites grown on vicinal (1000) sapphire exhibit biaxial in-plane optical anisotropy. The optical anisotropy resonance occurs near the band gap energy of ZnO. The line shape of the resonance is consistent with that induced by an in-plane anisotropic strain. The direction of the anisotropy coincides with the projection of the miscut direction of the (0001) sapphire substrates onto the sample surface plane. The magnitude of the anisotropy is generally larger for films with high crystalline quality, and on substrates with larger miscut angles. A possible origin of the strain anisotropy due to the miscut angle and the difference in thermal expansion rate of sapphire along its c and a axes is proposed.
Dynamics of solitary water in benzene and hexafluorobenzene: An infrared and Raman study
Authors: Besnard M. , Danten Y. , Tassaing T.
Published: July 2002

Solutions of water very diluted (xw∼10−3 mf) in benzene and hexafluorobenzene have been investigated using vibrational spectroscopy. In these two solvents, it was found that water is essentially in its monomeric form. The band-shape analysis of the infrared and Raman profiles associated with the symmetric v1 and antisymmetric v3 stretching vibrations of the water molecule in benzenic compounds has been performed and compared with the spectral results obtained for water diluted in liquid CCl4, considered here as the standard “inert” solvent. It is found that the reorientational motions of water around its different axes are always more hindered in benzene and hexafluorobenzene than in liquid CCl4. Moreover, the reorientational motion of the main symmetry axis (z) of water is more hindered than that of the y-axis (in the molecular plane). Although the rotational behavior of water appears similar in both aromatic solvents, the shifts in the vibrational frequencies and the change in the vibrational intensities suggest that the solute–solvent interactions in these two solutions are in fact quite different. In benzene, a weak H-bonding type of interaction with water is likely to exist in the liquid state in agreement with previous works. In contrast, in liquid perfluorinated benzene, a Lewis acid–base interaction is more likely to be involved. This conclusion is consistent with our previous ab initio calculations [Y. Danten et al., J. Phys. Chem. 103, 3530 (1999)].
Optical characterization of Mg-doped GaN films grown by metalorganic chemical vapor phase deposition
Authors: Martínez-Criado G. , Cros A. , Cantarero A. , Dimitrov R. , Ambacher O. , Stutzmann M.
Published: July 2002

Scanning electron microscopy, micro-Raman, and photoluminescence (PL) measurements are reported for Mg-doped GaN films grown on (0001) sapphire substrates by low-pressure metalorganic chemical vapor phase deposition. The surface morphology, structural, and optical properties of GaN samples with Mg concentrations ranging from 1019 to 1021 cm−3 have been studied. In the scanning micrographs large triangular pyramids are observed, probably due to stacking fault formation and three-dimensional growth. The density and size of these structures increase with the amount of magnesium incorporated in the samples. In the photoluminescence spectra, intense lines were found at 3.36 and 3.31 eV on the triangular regions, where the presence of cubic inclusions was confirmed by micro-Raman measurements. The excitation dependence and temperature behavior of these lines enable us to identify their excitonic nature. From our study we conclude that the interface region between these defects and the surrounding wurtzite GaN could be responsible for PL lines.
Effect of rapid thermal annealing on GaInNAs/GaAs quantum wells grown by plasma-assisted molecular-beam epitaxy
Authors: Pan Z. , Li L. H. , Zhang W. , Lin Y. W. , Wu R. H. , Ge W.
Published: July 2002

We have studied the effect of rapid thermal annealing (RTA) on GaInNAs/GaAs quantum wells (QWs) grown by molecular-beam epitaxy using a dc plasma as the N source. It was found that RTA at low temperature (LT, 650 °C) and high temperature (HT, 900 °C) could both improve the QW quality significantly. To clarify the mechanism of quality improvement by RTA, a magnetic field perpendicular to the path of the N plasma flux was applied during the growth of the GaInNAs layers for the sake of comparison. It was found that LT-RTA mainly removed dislocations at interfaces related to the ion bombardment, whereas, HT-RTA further removed dislocations originating from the growth. LT-RTA caused only a slight blueshift of photoluminescence peak wavelength, probably due to defect-assisted interdiffusion of In–Ga at the QW interfaces. The blueshift caused by HT-RTA, on the other hand, was much larger. It is suggested that this is due to the fast defect-assisted diffusion of N–As at the QW interfaces. As defects are removed by annealing, the diffusion of In–Ga at interfaces would be predominant.
Tunable piezoelectric semiconductor laser controlled by the carrier injection level
Authors: Ortiz V. , Pelekanos N. T. , Mula Guido , Dang Le Si
Published: July 2002

We propose a tunable laser diode based on a piezoelectric heterostructure. The tuning mechanism consists of modulating the gain spectrum during lasing by the quantum-confined Stark effect. The modulating electric field is produced by carrier separation in the active region, and its amplitude depends on the injected carrier density. In a proof-of-principle photopumped experiment, we were able to generate a space-charge field of 20 kV/cm which shifted the lasing wavelength by more than 3 nm in the 800 nm spectral region.
Relativistic and correlated all-electron calculations on the ground and excited states of AgH and AuH
Authors: Witek Henryk A. , Nakijima Takahito , Hirao Kimihiko
Published: July 2002

We report relativistic all-electron multireference based perturbation calculations on the low-lying excited states of gold and silver hydrides. For AuH, we consider all molecular states dissociating to the Au(2S)+H(2S) and Au(2D)+H(2S) atomic limits, and for AgH, the states corresponding to the Ag(2S)+H(2S), Ag(2P)+H(2S), and Ag(2D)+H(2S) dissociation channels. Spin-free relativistic effects and the correlation effects are treated on the same footing through the relativistic scheme of eliminating small components (RESC). Spin–orbit effects are included perturbatively. The calculated potential energy curves for AgH are the first reported in the literature. The computed spectroscopic properties agree well with experimental findings; however, the assignment of states does not correspond to our calculations. Therefore, we give a reinterpretation of the experimentally observed C 1Π, a 3Π, B 1Σ+, b(3Δ1)1, D 1Π, c13Π1, and c0(3Π0) states. A labeling suggested by us is a1, C0+, b0−, c2, B3Π0+, d3Π1, e1, f1 and g1, respectively. The spin–orbit states corresponding to Ag(2D)+H(2S) have not well defined the Λ and S quantum numbers, and therefore, they probably correspond to Hund’s coupling case c. For AuH, we present a comparison of the calculated potential energy curves and spectroscopic parameters with the previous configuration interaction study and the experiment.
Dome-to-pyramid shape transition in Ge/Si islands due to strain relaxation by interdiffusion
Authors: Henstrom William L. , Liu Chuan-Pu , Gibson J. Murray , Kamins T. I. , Williams R. Stanley
Published: July 2002

Ge islands were grown on Si(001) and then annealed at 650 °C for 0, 20, 40, and 60 min in a chemical-vapor deposition reactor following Ge deposition. This letter confirms the previous observations directly. By combining the ability to quantify strain with the ability to measure island dimensions in a transmission electron microscope, we were able to plot strain versus aspect ratio for the various annealing times. The islands first relax strain because of Si intermixing with the Ge epilayer causes the lattice mismatch to be lowered. Once the mismatch is sufficiently reduced, and thus the strain energy sufficiently reduced, it becomes favorable for the islands to reverse their shape back from domes to pyramids, thus reducing surface energy. This confirms the reversibility of island shape and thus the thermodynamics of the transition.
Effects of polyethylene glycol on protein interactions
Authors: Kulkarni Amit M. , Chatterjee Avik P. , Schweizer Kenneth S. , Zukoski Charles F.
Published: July 2002

The interactions between globular proteins in the presence of poly (ethylene glycol) (PEG) are probed through the measurement of the protein solution second virial coefficient (B2). The solution properties of PEG are characterized for four molecular weights (400, 1000, 6000, and 12 000), providing an opportunity for quantitative comparison of measurements and theoretical predictions of B2. PEG displays a buffer and molecular weight-dependent lower critical solution temperature. As the polymer solution approaches phase separation, the consequences of depletion attractions increase significantly. For lysozyme and bovine serum albumin in sulfate buffers with PEG, B2 is not well described by standard depletion models. This failure is accentuated in acetate buffers where B2 is a nonmonotonic function of polymer concentration. The attractive minima in B2 are closely associated with the proximity of the heating-induced phase separation of aqueous PEG solutions. The experimental data for both proteins in the presence of PEG are well captured by the thermal polymer reference interaction site model for depletion interactions where the polymer density fluctuation correlation length is treated as a function of temperature, polymer concentration, and molecular weight.
Excited state localization in organic molecules consisting of conjugated and nonconjugated segments
Authors: Zojer E. , Buchacher P. , Wudl F. , Cornil J. , Calbert J. Ph. , Brédas J. L. , Leising G.
Published: July 2002

We investigate, both experimentally and theoretically, a series of novel molecules consisting of conjugated segments (such as stilbene, naphthylene, and anthrylene) that are separated from each other by nonconjugated bridges. Excited state localization effects are studied theoretically by post-Hartree–Fock calculations—taking into account electron correlation effects. In this context, we compute the electron-hole two-particle wave functions for the prominent excited states and discuss the nature of the molecular orbitals involved in their description. We also investigate geometry relaxation effects following the electronic excitations in order to locate the regions where the strongest rearrangement of the electron density occurs. These conceptionally different approaches (relying also on different semiempirical Hamilton operators and configuration interaction techniques) yield consistent results regarding the localization of the excitations and thus prove helpful to determine the nature of the lowest excited states in such multichromophoric systems. Knowing the exact nature of the different states observed in the experimental absorption and luminescence excitation spectra allows for selective excitations of the different segments of the molecules. When performing site-selective spectroscopy, we find that in all the materials the emission originates from the S1→S0 transition, independent of the excitation wavelengths. This points to an efficient intramolecular energy transfer that occurs in spite of the broken conjugation between the molecular building blocks.
Estimating global and local scaling exponents in turbulent flows using discrete wavelet transformations
Authors: Katul Gabriel , Vidakovic Brani , Albertson John
Published: July 2002

High frequency longitudinal velocity (u) measurements were performed in the atmospheric surface layer to investigate the inertial subrange structure of turbulence. The u measurements, collected over a wide range of atmospheric stability conditions, were used to investigate local and global intermittency buildup in the inertial subrange. Global scaling exponents and other statistical properties were derived using nondecimated (NDWT) and critically sampled orthonormal (OWT) wavelet transformations. These statistical measures were contrasted to similar statistical measures derived by applying NDWT and OWT to an ensemble of fractional Brownian motion (fBm) time series with Hurst exponent of 1/3. Such comparisons permit direct assessment as to whether discrepancies in observed intermittency corrections are artifacts of wavelet transformations or limitations in sample size. This study demonstrated that both NDWT and OWT were able to resolve intermittency-based departures from global power laws observed in higher-order structure functions of turbulence time series. Particularly, global power laws inferred from OWT and NDWT were consistent with new intermittency correction results derived from the dynamics of the fourth order structure functions. This study is the first to report on the ensemble behavior of such a power law for a wide range of surface boundary conditions (e.g., variable surface heating and friction velocity). The wavelet computed global intermittency departures from the classical Kolmogorov theory (or K41) were marginally smaller than those computed by the traditional structure function approach. In terms of local exponents, we found that the application of NDWT to fBm time series resulted in a wide empirical frequency distribution of local scaling exponents (α). The latter finding clearly demonstrates that previous and present α determined by wavelet analysis cannot be used as evidence for multifractality in turbulence. We also demonstrated that the classical local regression estimation of α is theoretically impaired by heteroscedascity when the local scale is finite. While the spread in α does not reflect any multifractal signatures, the modes of the local α frequency distribution support findings from global exponent analysis. We found that the modes of the local α distribution are well reproduced by global intermittency models for u and by K41 for the fBm.
Path-integral Monte Carlo study of the structural and mechanical properties of quantum fcc and bcc hard-sphere solids
Authors: Sesé Luis M.
Published: July 2002

Path-integral Monte Carlo simulations involving the Cao–Berne’s hard-sphere propagator and aimed at exploring the high-density region (ρ*=0.8, 0.9) of the quantum hard-sphere (QHS) system are reported. By starting from single cubic (sc), body-centered cubic (bcc), and face-centered cubic (fcc) lattices, the following range of temperatures defined by the reduced de Broglie’s wavelengths 0.116⩽λB*⩽0.5 is studied. The r-space structural quantities computed are pair radial correlation functions (instantaneous, linear response, and necklace center of mass) and necklace radii of gyration. In addition, the following quantities related to the necklace centers of mass are calculated: maximal structure factor values, Steinhardt et al.’s orientational order parameters, and Lindemann’s index. The thermodynamic properties evaluated are energies and pressures. Comparison with Scheraga et al.’s results available in the literature [J. Chem. Phys. 96, 7005 (1992)] is made wherever possible. As shown, only the fcc lattice maintains its features under the strong QHS repulsions, whereas bcc and sc cannot cope with these effects transforming into striking partially crystalline [bcc(q)] and fluid phases, respectively. Conclusions on the features of the resulting phases which can help to explain the stages of the partial crystallization of the QHS fluid are also drawn.