bluuebunny/crossref_metadata_2025 huggingface dataset:
Water adsorption structures on flat and stepped Ru(0001) surfaces
Authors: Hoffmann Wolfgang , Benndorf Carsten
Published: July 2002

Water adsorption and desorption (H2O and D2O) were studied on vicinal Ru(0001) surfaces with nominal (101̄l) (l=8, 12, and 22) orientations employing low-energy electron diffraction (LEED), thermal desorption spectroscopy, and work-function change measurements (ΔΦ). LEED measurements and theoretical calculations from clean Ru(101̄l) confirmed the existence of terraces with average regular spacing and steps of one atomic height. Due to the hcp structure of Ru the terrace width and the atomic arrangement of the step alternate from one terrace to the next. This is confirmed by LEED measurements and calculations showing a triple-spot splitting of the reflexes. For water adsorption, due to the presence of steps, new desorption states with higher desorption temperature (220–350 K) are observed, which are missing on the flat Ru(0001) surface. LEED experiments demonstrate that the ordered water overlayers found on Ru(0001) are absent on the stepped surfaces. The isotope effect for D2O (the so-called A1 desorption state found for H2O at 212 K is missing for D2O) is also observed on the stepped surfaces indicating that this effect is due to a short-range interaction rather than a longer-range interaction in the water bilayer.
New field-emission device with improved vacuum features
Authors: Mammana V. P. , Degasperi F. T. , Monteiro O. R. , Vuolo J. H. , Salvadori M. C. , Brown I. G.
Published: July 2002

We introduce in this article a novel geometry that can be used in the manufacturing of field-emission displays. The geometry proposed combines superior vacuum conductance and a good field-enhancement factor. A theoretical model is developed for the calculation of the upper limit to the electrostatic field at the emitting regions, and these values are compared to those calculated for the actual geometry. The vacuum conductance of the proposed geometry is also calculated, and we show that conductances up to an order of magnitude higher than other schemes are readily achievable. Finally, we present field-emission results obtained from a prototype, and compare them with other emission schemes.
Increase of electrical conductivity in p-GaN by immersion in H2O2 solution
Authors: Liu B. , Ahonen M. H. , Holloway P. H.
Published: July 2002

Immersion in a peroxide solution was found to increase electrical conductivity in p-GaN. Auger electron spectroscopy, current–voltage, and Hall measurements were used for characterization. All samples in this study were grown on the c plane of sapphire substrates by molecular beam epitaxy using solid Ga and ratio frequency plasma-activated N2 source with a hole concentration of 1.1×1017 cm−3. The samples were first cleaned in boiling aqua regia (3HCl+1HNO3) for 10 min, rinsed in de-ionized (DI) water, and blown dry with N2. Metal contacts (2 nm Ni/100 nm Ti/100 nm Au) were deposited with an electron beam evaporator (<10−5 Torr). After metal deposition, samples were immersed in a H2O2 solution (1H2O2:5H2O or 1H2O2:1H2O) for different time (30–300 s). This was followed by a DI water rinse, a N2 blow dry and a short dry in 80 °C air. For a treatment of 1H2O2:1H2O for 300 s, a ∼100% increase of conductivity was observed. Hall data showed that the carrier concentration increased by 70% after these treatments. X-ray photoelectron spectroscopy data showed Ga 3d and O 1s bond energies were increased by H2O2:H2O treatments. The increased conductivity was speculated to result from reduction of compensation of Mg acceptors.
Elementary excitations in semiconductors with n–i–p–i doping superlattices
Authors: Ruden P.
Published: July 2002

Collective electronic excitations within the subband structure of semiconductors with doping superlattices are discussed. We find that heterodoping superlattices with a layered electron-hole system are good candidates for the observation of acoustic plasmons. Using the self-consistently calculated subband structure we investigate collective electron intersubband transitions and their coupling to LO phonons.
Interface formation at PbTe(100) surfaces: Ge, Al, and In overlayers
Authors: Cerrina F. , Daniels R. R. , Zhao Te-Xiu , Fano V.
Published: July 2002

Adsorption processes were investigated for the first time on a IV–VI substrate, PbTe, by photoemission spectroscopy with synchrotron radiation Ge, Al, and In overlayers on cleaved PbTe(100) all create an inversion layer at the PbTe side of the interface. Substantial Pb and Te outdiffusion is observed even at room temperature, resulting in nonabrupt interfaces. The band discontinuities of the PbTe–Ge interface are much different from those theoretically predicted by the electron affinity rule.
An image processing approach to fast, efficient proximity correction for electron beam lithography
Authors: Chow D. G. L. , McDonald J. F. , King D. C. , Smith W. , Molnar K. , Steckl A. J.
Published: July 2002

A limitation on the quality of electron beam lithography is the proximity effect. This produces exposure of the resist at locations remote from the point of incidence of the electron beam. One of the techniques used to mitigate this problem is to precompensate the applied beam dose. Traditional approaches to this problem have required extensive calculations which occasionally fail to produce satisfactory results. However, the proximity correction problem is quite similar to the edge enhancement problem which arises in pattern recognition. Furthermore, the issue of data base compaction for the precompensated lithography is quite similar to bandwidth compression in image transmission. In this paper, we examine the application of image processing methods to the proximity correction problem. We find that, while the match between these disciplines is not perfect, the idea appears quite promising.
Maskless laser writing of silicon dioxide
Authors: Krchnavek R. R. , Gilgen H. H. , Osgood R. M.
Published: July 2002

A laser direct writing technique for forming insulating layers of silicon dioxide from an organosilicate film on various substrate materials is shown. The process resolution is a function of the thermal properties of the substrate and is shown to be 1 μm. The technique allows for a local variation in the oxide thickness by changing process parameters. The quality of the laser written layers is compared to similar films formed by conventional organosilicate processing.
Selective and anisotropic reactive ion etch of LPCVD silicon nitride with CHF3 based gases
Authors: Mele T. C. , Nulman J. , Krusius J. P.
Published: July 2002

The reactive ion etching (RIE) of silicon nitride (Si3N4) films formed by low pressure chemical vapor deposition (LPCVD), has been investigated. Studies were done using plasmas of CHF3, CHF3+O2, and CHF3+CO2 gases. Anisotropic profiles with vertical sidewalls and no undercut have been achieved with all three plasmas. The etch rate of the Si3N4 films in a pure CHF3 plasma was found to decrease as a function of time. This effect is explained via a mass transport limiting mechanism. Constant etch rates were observed with plasmas of CHF3+O2 and CHF3+CO2. A selectivity of 10:1 for the etching of Si3N4 to the underlying Si substrate has been achieved with a CHF3+CO2 plasma.
Recent models of Schottky barrier formation
Authors: Tersoff J.
Published: July 2002

Two recent models of Schottky barrier formation are discussed. These invoke ‘‘metal-induced gap states’’ or native defects to explain Fermi-level pinning. Available experimental data can be satisfactorily explained by states intrinsic to the surface and interface, without postulating a defect pinning mechanism. In contrast, recent theoretical and experimental work appears to contradict the proposed defect mechanism. The connection between Schottky barriers and semiconductor heterojunction band lineups is proposed as a possible test of theory.
Raman spectroscopy of surface layers on crystalline silicon
Authors: Tsang J. C. , Thompson W. A. , Oehrlein G. S.
Published: July 2002

We demonstrate the use of a multichannel Raman scattering system to characterize semiconductor surfaces and interfaces. The introduction of lattice displacements and hydrogen bonded to the Si atoms due to reactive ion etching of Si(100) is observed by Raman spectroscopy. The formation of subnanometer thicknesses of Pd2Si following the deposition of Pd onto Si(111) in ultrahigh vacuum has been detected in situ. These measurements are performed on arbitrary crystalline substrates, without any special sample preparation and can be made in vacuum or in any ambient atmosphere.
Proximity effect correction calculations by the integral equation approximate solution method
Authors: Pavkovich J. M.
Published: July 2002

The task of successfully dealing with the proximity effect problem involves many aspects, all of which must be dealt with in a reasonably satisfactory manner. If one chooses to correct for the proximity effect by doing dose compensation, one major task is the solution of the integral equation which describes the resultant exposure in terms of the incident flux of electrons. One well known method is the self-consistent method. This paper describes a method which provides a relatively accurate approximate solution to the integral equation which is easy to calculate and which provides information on where features should be fractured to obtain good dose compensation. Although the relationship between the incident flux and the resultant exposure is linear, the development process itself is not. This means that the usual integral equation should be modified slightly so that the resultant exposure is defined in a manner which more closely matches the real problem. This paper will attempt to describe how information from the development process can be used to define the exposure problem in a manner which provides more desirable solutions.
The growth of high mobility InGaAs and InAlAs layers by molecular beam epitaxy
Authors: Lee Wai , Fonstad Clifton G.
Published: July 2002

InxGa1−xAs (x≊0.53) and InyAl1−yAs (y≊0.52) epitaxial layers have been grown on (100) InP substrates by molecular beam epitaxy (MBE) and characterized using x-ray diffraction and Hall measurements. Electron mobilities of 11 400 cm2/ V s at 300 K and 47 500 cm2/ V s at 77 K were measured on 1.5 μm thick undoped InGaAs grown directly on InP substrates. The background carrier concentrations of these unintentionally doped layers were typically in the low 1015 cm−3 range. Undoped InAlAs layers grown at 530 °C were n type. A typical 2 μm thick layer had a sheet carrier concentration of 7.5×1011 cm−2 and an electron mobility of 1350 cm2/ V s at room temperature. The Hall mobilities of both undoped InGaAs and InAlAs layers obtained in this work are among the highest ever obtained by MBE. The good quality of these layers is attributed to the slow growth rate employed and the cleanliness of the growth environment.
Barrier effect of selective chemical vapor deposited tungsten films
Authors: Shioya Yoshimi , Maeda Mamoru , Yanagida Kimio
Published: July 2002

Thin selective chemical vapor deposited tungsten films are evaluated as a barrier metal for reaction between Al–1%Si electrode and Si substrate. Change in contact resistance and junction leakage current after annealing at 500 °C in N2 are measured. It is found that an increase of contact resistance at a very small contact hole of 2.25 μm2 is very slight. On the other hand, contact resistance in Al–1%Si electrode increases to one or two magnitudes with annealing of 210 min. Junction leakage current in a tungsten electrode is almost the same as that in Al–1%Si electrode. And it is also found that a thin tungsten layer protects the diffusion of aluminum into the Si substrate. From these results, it is concluded that thin selective chemical vapor deposited tungsten films have a good barrier effect for the reaction between Al–1%Si electrode and Si substrate.
Electronic structure of GaAs–Ga1−xAlxAs quantum wires
Authors: Wong K. B. , Jaros M. , Hagon J. P.
Published: July 2002

In this study, we report the first full scale calculation of the effect of confinement in GaAs–Ga1−xAlxAs 〈001〉 quantum wire structures. Our calculation is based on a pseudopotential scheme which has recently been applied successfully to model conventional superlattices. For thick wires our results support the infinite well approximation as far as the ground state is concerned. All confined states in thin wires (<70×70Å2) are significantly affected by additional reflections which can be identified from the charge density contour plots and tables of the momentum wave function coefficients. We find that in quantum wires the mixing of Γ and X-like momentum components is greatly diminished compared to that in conventional superlattices. We have also evaluated the oscillator strengths of the optical transitions across the forbidden gap and the conduction band nonparabolicity in the directions parallel and perpendicular to the quantum wire axis. These results are compared with the corresponding values obtained for bulk GaAs and for conventional GaAs–Ga1−xAlxAs superlattices.
Techniques for realizing linearly graded composition or doping profiles in molecular-beam epitaxy
Authors: Sauer N. J. , Chang T. Y. , Dayem A. H. , Westerwick E. H.
Published: July 2002

Linear grading of alloy composition in molecular-beam epitaxy (MBE) materials is more complex than it first appears because of the effect of changing beam flux on the growth rate and the nonlinear dependence of beam flux on the effusion-cell temperature. In this paper, mathematical relations between the effusion-cell temperature and time that must be followed during the MBE growth to produce a linearly graded depth profile of alloy composition or doping concentration are derived. Implementation of this relation in a programmable controller using a novel dual timer technique for effusion-cell temperature control is also described. The validity of this technique is demonstrated experimentally for compositional grading as well as doping concentration grading.
Electron beam source molecular-beam epitaxy of III–V compounds
Authors: Malik Roger J.
Published: July 2002

A new technique for the growth of III–V compounds by molecular-beam epitaxy has been developed which uses electron beam heating for evaporating the group III liquid metals. The growth chamber is vertical and incorporates three large capacity 40 cc hearths for Ga, Al, and In. The metal beam fluxes are measured and feedback controlled using a chopped ion gauge signal detection scheme. The beam signal is input to a Eurotherm 820 digital controller which regulates the e-beam power supply and the control system is interfaced with an AT&T 3B2 computer. The dynamic response of the group III metal beam fluxes is very fast and well-controlled. The beam fluxes can be changed by two orders of magnitude in <10 s without oscillation. The beam flux can be precisely tailored through the digital controller over a very wide range. This enables the growth of variable band gap III–V alloys with almost arbitrary composition. In addition, the system geometry provides excellent uniformity over large diameter wafers. This technique appears very attractive for production of III–V epilayers.
The interaction of Ag and Al overlayers with InP (110): Surface and diode studies of the effect of Sb interlayers
Authors: Ismail A. , Ben Brahim A. , Dumas M. , Lassabatere L.
Published: July 2002

We study the properties of the Sb–InP (110) interface by contact potential difference, surface photovoltage, and Auger electron spectroscopy, and then the effect of the Sb interlayer on Al and Ag–InP (110) Schottky diode properties. For Sb coverage <1 ML, the surface Fermi level EFs is strongly pinned near the conduction band and the electron affinity decreases. The Sb interlayer effect on the Schottky contacts Al–InP and Ag–InP is already noticeable for a submonolayer of Sb. The EFs position moves to a common position situated at about 1 eV above the valence band maximum and the abruptness of the interface is changed. The interaction of the metal with the semiconductor is partially or completely screened. In the case of Al–InP contact In or P diffusion into the metal is stopped. The interface is more abrupt in Al–InP contact, less abrupt in Ag–InP contacts. This is due to a high density of interface states in the gap, the origin of which could be found in a possible modification of the surface relaxation inducing displacements into the gap of intrinsic surface states or in the formation of new states, corresponding for example to new bonds between Sb and InP or to metal-induced defects.
Lead strontium telluride and lead barium telluride grown by molecular-beam epitaxy
Authors: Partin D. L. , Thrush C. M. , Clemens B. M.
Published: July 2002

Long wavelength diode lasers operating in the 3–10-μm wavelength range are of interest as optical sources for future fiber optics communications and sensor systems. PbEuSeTe large optical cavity single quantum well diode lasers have so far attained the highest operating temperatures (174 K cw, 280 K pulsed) in this wavelength range. Two alternative materials systems PbSrTe and PbBaTe have now been grown for the first time by molecular-beam epitaxy. The choice of these ternaries was motivated by the fact that SrTe and BaTe have the same face-centered-cubic crystal structure as PbTe and roughly comparable lattice constants, but much larger energy band gaps. In the Pb1−xSrxTe system, x-ray diffraction studies show clear evidence of phase segregation for x>0.15. The carrier mobilities decrease monotonically with strontium concentration in the single phase region, which suggests that disorder-related scattering is dominant. The energy band gap increases approximately as dEg/dx=3.2 eV for small x, and the index of refraction decreases monotonically with increasing x. In the Pb1−xSrxTe system, a combination of x-ray diffraction, energy band gap and other data clearly demonstrate that the solubility of BaTe in PbTe is limited to about 4%, possibly because of the relatively large difference in lattice constants between PbTe (6.460 Å) and BaTe (7.001 Å). These results imply that PbSrSeTe grown lattice matched to PbTe is a promising material system for long wavelength diode lasers and light emitting diodes.
Local electronic structure and surface geometry of Ag on Si(111)
Authors: Demuth J. E. , Von Lenen E. J. , Tromp R. M. , Hamers R. J.
Published: July 2002

Scanning tunneling microscopy has been used to evaluate the overall morphology and growth conditions for Ag ordered overlayer and island films on Si(111), and to determine the nature of the √3×√3 Ag/Si(111) structure. Current image tunneling spectroscopy (CITS) is used to examine the energy-dependent tunneling and evaluate possible model structures for the √3×√3 structure. Support is found for a √3×√3 structure composed of Ag trimers embedded below the top layer of threefold coordinated Si atoms. The surface electronic structure for these √3 structures shows a 1.2 eV gap, and exhibits very few intrinsic states near EF. The few defects observed in the √3 areas of this surface introduce deep levels while the most dominant tunneling features near EF appear to be associated with Ag islands that coexist with √3 areas. The implications of the surface perfection and local electronic structure to Schottky barrier formation are discussed.
EL3 system for quarter-micron electron beam lithography
Authors: Groves T. R. , Pfeiffer H. C. , Newman T. H. , Hohn F. J.
Published: July 2002

The introduction of the shaped beam imaging technique has greatly enhanced the exposure efficiency of electron beam (e-beam) lithography systems. The electron beam lithography systems from IBM provide the throughput capability needed for lithography applications in semiconductor fabrication lines. The resolution of these systems has steadily improved over the past 15 years, in support of the semiconductor trend towards submicron dimensions. This paper describes the latest version EL3 system, capable of fabricating 0.25-μm dimension features. The technical challenges of submicron e-beam lithography are discussed. A practical solution is described, with experimental results. The outlook for the future is also highlighted.
Voltage contrast electron beam testing experiments on very large scale integrated chip packaging substrates
Authors: Woodard Ollie C. , Hartnett Fred , Myers Tom , Ross Andrew , Thompson Ronald
Published: July 2002

Test and inspection are key in reducing costs and increasing reliability of multichip hybrid modules. Increasing very large scale integrated chip packaging densities requiring finer linewidths and more substrate interconnection networks are taxing the probing resolution, throughput, and reliability of conventional mechanical probe test systems. Voltage contrast electron beam test technology was seen as an attractive alternative for high-throughput, noncontact testing of unpopulated packaging substrates and high-density printed wiring boards. A conventional scanning electron microscope was modified by adding large-field vector deflection, a large-field secondary analyzer, beam blanking, an electron flood gun, and a computer control system, thereby providing the primary test functions required for opens and shorts testing: net charging, net voltage measurement, and global charge removal. Successful substrate test results are shown along with data which highlight potential failure mechanisms such as substrate material variations, charge leakages, and contaminants.
Self-limiting advancing gates for GaAs metal–semiconductor field effect transistors
Authors: Fernandes M. G. , Han C. C. , Xia W. , Lau S. S. , Kwok S. P.
Published: July 2002

The threshold voltage of GaAs metal–semiconductor field effect transistors (MESFET’s) can be controlled by the use of buried gates. The controlling mechanism is through the alloying reaction between the gate material and the GaAs substrate. Although this can be done by choosing suitable annealing conditions, the alloying reaction may continue to proceed if the operating temperature of the device is high and/or for long-device field service. To overcome this problem a Ni–W alloy is chosen as the gate metallization. The idea here is to utilize the concentration dependence of the interfacial reactivity between the Ni–W alloy and the GaAs to limit the alloying reaction. Below 400 °C Ni leaches out of the Ni–W alloy to react with the GaAs, and W does not participate in the reaction. As Ni leaches out, the composition of W at the interface increases and this appears to choke off the reaction, resulting in a self-limiting reaction for buried gates. We found that a thin-Pd (20 Å) layer placed at the GaAs/NiW interface is helpful in improving the interfacial reaction uniformity. It was found that the threshold voltage of GaAs MESFET’s can be adjusted to any desired value using this approach. After the desired threshold voltage has been obtained, the mean value of the threshold voltage of 30 field effect transistors (FET’s) is found to stabilize to ±5 mV with a standard deviation of ±27 mV at a temperature of 235 °C for a period of at least 90 h. The activation energies for the change in threshold voltage at different stages of anneal have also been estimated.
Behavior of indium on the Si(111)7×7 surface at low-metal coverage
Authors: Nogami J. , Park Sang-il , Quate C. F.
Published: July 2002

We have used scanning tunneling microscopy to study the behavior of indium on the Si(111)7×7 surface at low-metal coverages. At sufficiently low densities, In atoms replace Si in the 7×7 adatom positions, preserving the 7×7 structure. In this case the In atoms have a tendency to occupy positions along the edges of the unit cell rather than adjacent to the corner holes. Agglomeration of higher In densities around atomic steps can lead to local areas of √3×√3 reconstruction. Examination of a phase boundary between 7×7 and √3×√3 allows the identification of the bonding site of the In adatoms in the √3×√3 structure as the threefold symmetric site above a second layer Si atom in the surface Si double layer.
The effect of interface disorder on the eigenvalue spectrum of quantum well heterostructures
Authors: Duggan G. , Hellon C. M.
Published: July 2002

The effect of the lateral dimension of interface islands on the subband eigenvalue spectrum of quantum well heterostructures is investigated by direct numerical solution of the two-dimensional Schrödinger equation. We find a decrease in the lowest subband energy equivalent to a half-monolayer change in well width for an island size of two to three times the nominal well thickness. Changes in the lowest subband eigenvalue equivalent to a monolayer change in well width are not anticipated for island sizes <∼1000 Å for quantum well widths of ∼100 Å.
Elimination of the flux transients from molecular-beam epitaxy source cells following shutter operation
Authors: Chilton P. A. , Truscott W. S. , Wen Y. F.
Published: July 2002

We present an analysis, based on control theory, of the transients in flux from a molecular-beam epitaxy source cell which occur after opening the shutter and also after changing the set point of the cell temperature controller. This analysis permits the calculation of the variation of this set point with time which produces a transient equal and opposite to that following shutter operation. Experimental results show that use of this set point variation results in a beam flux constant to within 1%, and that the parameters in this analysis do not change significantly over at least 100 h of cell operation. In addition we show that simple manual control of the set point based on this analysis can reduce the flux transient by a factor of 5.
Molecular-beam epitaxy of CrSi2 on Si(111)
Authors: Fathauer R. W. , Grunthaner P. J. , Lin T. L. , Chang K. T. , Mazur J. H. , Jamieson D. N.
Published: July 2002

Chromium disilicide layers have been grown on Si(111) in a commercial molecular-beam epitaxy machine. Thin layers (10 nm) exhibit two epitaxial relationships, which have been identified as CrSi2(0001)//Si(111) with CrSi2[101̄0]//Si[101̄], and CrSi2(0001)//Si(111) with CrSi2[112̄0]//Si[101̄]. The latter case represents a 30° rotation of the CrSi2 layer about the Si surface normal relative to the former case. Thick (210 nm) layers were grown by four different techniques, and the best-quality layer was obtained by codeposition of Cr and Si at an elevated temperature. These layers are not single crystal; the largest grains are observed in a layer grown at 825 °C and are 1–2 μm across.
Optical reflectance measurements of transients during molecular-beam epitaxial growth on (001) GaAs
Authors: Harbison J. P. , Aspnes D. E. , Studna A. A. , Florez L. T.
Published: July 2002

We report measurements of the reflectance difference (RD) of light polarized along the [110] and [1̄10] directions of a growing (001) GaAs crystal in a molecular-beam epitaxy (MBE) chamber. Observations of RD transients occurring due to changes in surface stabilization resulting from interruption of the As beam are compared with simultaneous observations of reflection high-energy electron diffraction intensities and are shown to give both chemical and structural information about the growing surface. Spectral dependencies of the RD data show distinct responses for Ga and Al surface coverages, suggesting optimal wavelengths and polarizations for selective modification of surface dimer bonding during growth.
Ultralow threshold graded-index separate-confinement heterostructure single quantum well (Al,Ga)As lasers
Authors: Derry P. L. , Chen H. Z. , Morkoç H. , Yariv A. , Lau K. Y. , Bar-Chaim N. , Lee K. , Rosenberg J.
Published: July 2002

Broad area graded-index separate-confinement heterostructure single quantum well lasers grown by molecular-beam epitaxy (MBE) with threshold current density as low as 93 A/cm2 (520 μm long) have been fabricated. Buried lasers formed from similarly structured MBE material with liquid phase epitaxy regrowth had threshold currents at submilliampere levels when high reflectivity coatings were applied to the end facets. A cw threshold current of 0.55 mA was obtained for a laser with facet reflectivities of ∼80%, a cavity length of 120 μm, and an active region stripe width of 1 μm. These devices driven directly with logic level signals have switch-on delays <50 ps without any current prebias. Such lasers permit fully on–off switching while at the same time obviating the need for bias monitoring and feedback control.
Quantum size effect in δ-doped AlGaAs heterostructures
Authors: Cunningham J. E. , Tsang W. T. , Schubert E. F. , Timp G. , Chiu T. H. , Chang A. , Agyekum E. , Ditzenberger J. A.
Published: July 2002

We report a new structure formed by δ doping the barrier of an AlGaAs heterostructure. Our structure contains a pair of electronically coupled quantum systems that develop from the quantum size effect occurring in the barrier and at the heterointerface. Compared to conventional homogeneous doping, we find enhancements in interface density resulting in δ-doped structures for all spacer thicknesses. A mobility of 1.9×106 cm2/V s is observed at a spacer layer thickness of 360 Å. Systematic dependences of interface mobility and density on spacer thickness are deduced from the quantum Hall effect and variable temperature Hall measurements.
Growth kinetics, impurity incorporation, defect generation, and interface quality of molecular-beam epitaxy grown AlGaAs/GaAs quantum wells: Role of group III and group V fluxes
Authors: Munnix S. , Bauer R. K. , Bimberg D. , Harris J. S. , Köhrbrück R. , Larkins E. C. , Maierhofer Ch. , Mars D. E. , Miller J. N.
Published: July 2002

A systematic knowledge of the influence of molecular-beam epitaxy growth parameters on the properties of AlGaAs/GaAs quantum wells grown under realistic growth conditions is important in order to obtain optimal performance of modern electronic and optoelectronic devices. Photoluminescence (PL) was used to investigate the influence of the Ga-controlled growth rate in the range below standard growth rates of 1 μm/h down to 0.1 μm/h, and of the As:Ga beam equivalent pressure ratio in the range of 10 to 60, on the growth kinetics, the interface quality, and the impurity incorporation, at a substrate temperature Ts =620 °C. As compared to in situ reflection high-energy electron diffraction (RHEED) measurements, where no sample rotation is possible, PL has the advantage that realistic growth conditions can be used. A careful line shape analysis, together with infrared and time-resolved PL measurements gives information on the interface roughness, the impurity incorporation, and the deep trap concentration. Non-negligible desorption of Ga during growth is found for the range of conditions under study. The desorption is found to increase upon a decrease of As:Ga ratio. The interface roughness as well as the impurity and trap incorporation are found to decrease with decreasing growth rate, an optimum interface quality being obtained below 0.5 μm/h. At this optimal growth rate, increasing the As4:Ga ratio leads to a decrease of shallow impurity concentration and thus to a narrower line width, but to a simultaneous increase of defect generation. Optimal growth conditions are found at a beam equivalent pressure ratio of 15. The observed desorption kinetics and interface properties can be explained in accordance with existing theoretical simulations. Finally, growth interruption was found to lead to optimal formation of flat growth islands when the overall growth rate is lowered.
Nonradiative damage measured by cathodoluminescence in etched multiple quantum well GaAs/AlGaAs quantum dots
Authors: Clausen E. M. , Craighead H. G. , Harbison J. P. , Scherer A. , Schiavone L. M. , Van der Gaag B. , Florez L. T.
Published: July 2002

We report a study of nonradiative surface recombination in etched GaAs quantum well structures. Low-temperature cathodoluminescence was used to measure the relative luminescence efficiencies of etched quantum dots as a function of size, etch depth and etching conditions, and quantum well width. The relationship between etching damage and quantum well width was determined by using three samples, each consisting of three quantum wells of 2, 4, and 9 nm thickness, with the placement of the wells relative to the surface varied systematically. Arrays of quantum dots which ranged in size from 5 μm down to 40 nm were produced by electron beam lithography and reactive ion etching or ion beam assisted etching. The nonradiative surface damage produced by the etching process degrades the luminescence efficiency in quantum dots smaller than 1 μm in diameter. We have determined that etching processes which use argon gas increase the nonradiative surface layer thickness compared to etching processes which use xenon. We have also found that the lowest confinement energy quantum well is most strongly affected by the sidewall damage and the highest confinement energy quantum well is affected the least by the damage.
Cation incorporation rate limitations in molecular-beam epitaxy: Effects of strain and surface composition
Authors: Evans K. R. , Stutz C. E. , Lorance D. K. , Jones R. L.
Published: July 2002

The effects of strain and surface composition on cation incorporation rates in III–V molecular-beam epitaxy are examined for three growth systems. In initial stages of strained InAs on GaAs(001) heteroepitaxy, lattice-mismatch strain is found to reduce the cation incorporation rate (relative to that of InAs homoepitaxy) by lowering the energy barrier associated with cation desorption. The magnitude of this energy barrier is found to depend on surface reconstruction. In unstrained InAs homoepitaxy, the energy barrier associated with In desorption is found to be 4.0 eV; this value is found to be in excellent agreement with predictions based on simple bond energy scaling arguments applied to results obtained for GaAs homoepitaxy. In unstrained growth of AlxGa1−xAs on GaAs(001), surface chemical composition is found to affect the Ga incorporation rate in a complicated manner: the presence of Al results in an increase in the Ga incorporation coefficient even though the energy barrier associated with Ga loss is reduced. These and related results are shown to be consistent with a model in which: (i) surface Ga and Al atoms compete for cation desorption sites, and (ii) surface Al atoms undergo exchange reactions with underyling Ga atoms.
Growth of quasi-three-dimensional modulation-doped semiconductor structures by molecular-beam epitaxy
Authors: Sajoto T. , Jo J. , Wei H. P. , Santos M. , Shayegan M.
Published: July 2002

We report the realization of a modulation-doped quasi-three-dimensional electron system. The structure consists of a wide undoped AlxGa1−xAs well bounded by undoped (spacer) and doped layers of AlyGa1−yAs (y>x) on both sides. The alloy composition in the well (x) is varied quadratically so that the combined potentials due to the AlxGa1−xAs and the electric charge in the well produce a square potential well with a nearly uniform carrier density. Magnetotransport data, and self-consistent calculations of the subband structure, reveal that the electrons in the well occupy four electric subbands. The electron wavefunction is nearly constant over the central 1000 Å of the well. A three-dimensional electron density of ≂2.5×1016 cm−3 with a low-temperature mobility in excess of 1×105 cm2/V s is deduced, indicating the high quality of the electron system.
Defect generation by Schottky contacts on n-GaAs
Authors: Meyer Eckhard , Heymann Günter
Published: July 2002

We studied the influence of different fabrication techniques on the electrical properties of n-GaAs Schottky contacts. In comparison to the conventional technology for Schottky metallizations represented by wet chemical etching and thermal evaporation the pretreatment of the semiconductor surface was performed either by wet chemical or Ar+ etching and the metallization step by evaporating, sputtering or by ion beam contacting. Diodes prepared by such procedures show very different electrical gehavior but no monotonic dependence of the barrier height from preparation parameters, especially from the ion energy. Whereas ion cleaning at 500 V results in barrier heights of ∼0.3 eV, cleaning at 1000 V gives a barrier of 0.4 eV. The implantation of the contact metal results in a barrier of 0.65 eV. DLTS measurements reveal a tendency of relationships between the barrier height and the dominating deep levels, thus supporting the model of Fermi levels pinning by defect states. DLTS further indicates a sensitive reaction of the EL 2 center on the metallization procedure.
Thin-film inorganic resists for submicron fabrication
Authors: Khaleque F. , Green Mino
Published: July 2002

Submicron features have been fabricated in silicon dioxide and chromium using cadmium fluoride (CdF2) in a lift-off mask scheme. High-resolution lines are patterned by the electron beam on PMMA on a SiO2 substrate and a thin film of CdF2 is then evaporated. Following a lift-off stage in chloroform, a pattern reversal of the PMMA image is obtained in CdF2, which is then etched in the RIE mode, providing a high selectivity with SiO2 (>30) and giving a highly anisotropic etch in SiO2. In this way, high-resolution submicron features in silicon dioxide with dimensions of 0.4 μm (wide) and 1 μm (deep) have been obtained. A similar procedure was used for chromium on glass.
Degradation behavior of 0.5-μm p-channel metal–oxide–semiconductor transistors fabricated by means of x-ray and optical lithography
Authors: Friedrich D. , Bernt H. , Schmidt L. , Windbracke W.
Published: July 2002

Partially scaled p-channel metal–oxide–semiconductor surface channel transistors with effective channel lengths down to 0.20 μm have been successfully fabricated with x-ray and optical lithography at all four levels, respectively. The submicrometer feature sizes in the optical process were realized by the isotropic O2 plasma etch of the resist structures at the gate level. In order to determine the influence of x-ray radiation on the device parameters, a comparison of x-ray and optically processed transistors before and after hot-carrier stress was performed. The initial threshold voltage deviation before hot-carrier stress between both types of transistors was ΔVT=9 mV due to the hole trapping within the gate oxide of the x-ray exposed transistors. An absorbed dose of 2.5 Mrad within the 10-nm gate oxide was calculated by the simulation program xmas. The interface state density Dit measured by charge pumping reveals no difference and is in the order of 1.5×1010 1/eV cm2. After hot-carrier stress with a drain voltage VD=−6 V, a maximal VT shift of −18 mV was observed for 0.5-μm transistors. The ΔVT difference between optically and x-ray-processed 0.5-μm devices was 5 mV due to charge buildup of x-ray-induced neutral traps within the gate oxide. The tendency of higher negative VT shift of the x-ray-exposed wafers depends strongly on decreasing channel length and stress conditions. In the sub-half-micrometer range hot-carrier stress was performed with a reduced drain voltage VD=−3 V, which is a reasonable supply voltage, expected for the quarter-micrometer regime. Under these stress conditions the maximal VT shift was 7 mV for the x-ray transistors, which is within the technological process bandwidth.
A high quantum efficiency i
n s
i
t
u doped mid-wavelength infrared p-on-n homojunction superlattice detector grown by photoassisted molecular-beam epitaxy
Authors: Harris K. A. , Myers T. H. , Yanka R. W. , Mohnkern L. M. , Otsuka N.
Published: July 2002

HgTe/CdTe superlattices in infrared (IR) detector structures have been theoretically shown to allow for better control over cutoff wavelength, minimize diffusion currents, and greatly reduce band-to-band tunneling currents as compared with the corresponding HgCdTe alloy. However, the few HgTe/CdTe superlattice detectors that have been fabricated exhibit little or no quantum efficiency. In this paper, we report the first high quantum efficiency mid-wavelength infrared (MWIR) detectors based on HgTe/CdTe superlattices. This result is significant because it represents the first experimental verification that IR detectors with useful characteristics can in fact be fabricated from HgTe/CdTe superlattices. The MWIR detectors were fabricated from an in situ doped p-on-n MWIR homojunction superlattice epilayer grown by photoassisted molecular-beam epitaxy (PAMBE). This growth technique produces low defect growth of superlattice material, as is described in this paper. Our development of an extrinsic doping technology using indium and arsenic as the n-type and p-type dopants, respectively, led to the successful doping of the superlattice and is also discussed.
400 kHz radio-frequency biased electron cyclotron resonance plasma etching for Al–Si–Cu patterning
Authors: Samukawa Seiji , Toyosato Tomohiko , Wani Etsuo
Published: July 2002

A radio-frequency (rf) biased electron cyclotron resonance (ECR) plasma etching technology with efficient ion acceleration in high density and uniform ECR plasma for accurate Al–Si–Cu alloy film etching has been developed. Substrate is located at the ECR position (875 G position) and etching is carried out with 400 kHz rf bias power. This technology achieves high etching rate (more than 5000 Å/min), excellent uniformity (±5%), highly anisotropic and Cu residue-free etching using only Cl2 and Cl2/BCl3 gas plasma at a low 100 °C substrate temperature. These characteristics are achieved by the combination of dense and uniform ECR plasma generation and efficient accelerated ion flux at the ECR position by using 400 kHz rf bias.
Scanning tunneling microscope with novel coarse sample positioning technique
Authors: Park Chan , Park Kap-Suk , Huh Yun-Sung , Jeon Il Cheol , Kim Sehun
Published: July 2002

We have constructed a scanning tunneling microscope with coarse positioning device which enables adjustment of the sample to tip distance into operating range of z-piezo element without stretching for each step of sample approach. This new convenient technique is achieved by monitoring sample current during coarse sample approach. The sample to tip distance of several thousand angstroms could be detected directly on oscilloscope. It is considered that sample current is generated by field emission mode of tip operation or field ionization of air molecules due to a potential induced by sawtooth voltage used for driving inertial piezotranslator. In addition, the topographic images of clusters and ultrathin films of carbon on cleaved graphite will be presented.
Electric field coupling to quantum dot diodes
Authors: Randall J. N. , Seabaugh A. C. , Kao Y.-C. , Luscombe J. H. , Newell B. L.
Published: July 2002

We have fabricated two different structures in the GaAs/AlGaAs heterojunction system to quantify the transfer characteristics of the quantum dot subjected to external, local electric fields. The first structure is a single quantum dot diode with an annular field electrode placed adjacent to the double-barrier structure by a self-aligned fabrication process. A second structure consists of a pair of independently contacted quantum dot diodes separated by several hundred angstroms. The fabrication processes and transport properties for both of these structures are described. We have also attempted for the first time to form quantum dots in InGaAs/AlAs system lattice matched to InP and have observed that strong conductance fluctuations not related to lateral size quantization can occur. These fluctuations arise from the formation of rotation-induced finite superlattices.
Experimental evaluation of a scanning tunneling microscope-microlens system
Authors: Muray L. P. , Staufer U. , Bassous E. , Kern D. P. , Chang T. H. P.
Published: July 2002

This paper presents the results of the first successfully fabricated scanning tunneling microscope (STM) aligned field emission (SAFE) microsource. SAFE sources have been shown to produce 2–3 orders of magnitude improvement in brightness over conventional field-emission sources. Lens electrodes were fabricated from 1-μm thick silicon membranes by electron-beam lithography and reactive-ion-beam etching. Two-element microlenses were tested in an ultrahigh vacuum (UHV) chamber with a piezo mounted tungsten tip and a dual feedback system. The sources demonstrated stable emission for periods of hours at beam energies of up to 1 kV. Measurement of the virtual source position showed good agreement with calculated values.
Influence of thermal annealing on the electron mobility in modulation doped Si/SiGe heterostructures
Authors: Schäffler F. , Herzog H.-J. , Jorke H. , Kasper E.
Published: July 2002

We grew n-type modulation doped Si/SiGe multiple quantum well structures with the highest electron mobilities reported so far for this heterosystem. The samples were annealed at temperatures between 750 and 950 °C for 1000 s and subsequently characterized by x-ray rocking analysis, secondary ion mass spectroscopy, and temperature dependent Hall measurements. A moderate decrease in room temperature Hall mobility is observed up to annealing temperatures of 900 °C. Above 900 °C the samples become homogeneously doped and show strong Si/Ge interdiffusion at the heteroboundaries. The annealing effects are discussed in terms of dopant and Ge diffusion, and of metastability of the SiGe layers.
Imaging of an organic superconductor (BEDT-TTF)2(NH4)Hg(SCN)4 surface structure by scanning tunneling microscopy
Authors: Kawazu Akira , Yoshimura Masamichi , Shigekawa Hidemi , Mori Hatsumi , Saito Gunzi
Published: July 2002

The STM image of the two-dimensional organic superconductor (BEDT-TTF)2 (NH4)Hg(SCN)4 [BEDT-TTF: bis(ethylenedithio)tetrathiafulvalene] crystal showed a periodic structure, which is in good agreement with the molecular arrangement with the conductive BEDT-TTF layer in the crystal. The influence of the top anion layer on the STM image was observed, as was previously shown for the (BEDT-TTF)2KHg(SCN)4 crystal. The high stability of this surface has also been revealed.
Etching of GaAs and InP using a hybrid microwave and radio-frequency system
Authors: Pang S. W. , Liu Y. , Sung K. T.
Published: July 2002

A hybrid system consisting of a multipolar electron cyclotron resonance source driven by a microwave power supply at 2.45 GHz and a radio-frequency (rf) powered electrode at 13.56 MHz is used for the etching of GaAs and InP. Both CCl2F2 and Cl2 are used as the gas sources, and the effect of rf and microwave power, pressure, as well as flow rate on etch rate and surface morphology are evaluated. Fast etch rate up to 3.4 μm/min is obtained for GaAs. Etch rate increases with rf power and decreases with microwave power when CCl2F2 is used. Optical emission and x-ray photoelectron measurements indicate that more C-related compounds are generated when microwave power is on. With Cl2 addition, etch rate increases with microwave power and smoother morphology is obtained. Etching for InP yields smooth morphology at a typical rate of 50 nm/min.
Alignment and registration schemes for projection electron lithography
Authors: Farrow R. C. , Berger S. D. , Gibson J. M. , Liddle J. A. , Kraus J. S. , Camarda R. M. , Huggins H. A.
Published: July 2002

Techniques for doing alignment and registration in projection electron lithography are reviewed. The issues associated with extending these techniques to the case of high incident electron energy exposure are discussed. Measurements of backscatter electron emission contrast from W markers on Si wafers at 200 kV show that mark detection within the expected range of voltages is feasible.