Intrinsic catalytic process by capillary condensation of Ga atoms into nanotrenches, formed among impinging islands during the wurzite-GaN thin film deposition, is shown to be an effective path to growing GaN nanorods without metal catalysts. The nanocapillary brings within it a huge imbalance in equilibrium partial pressure of Ga relative to the growth ambient. GaN nanorods thus always grow out of a holding nanotrench and conform to the boundaries of surrounding islands. The nanorods are epitaxially orientated with < 0001 >(GaN)parallel to < 111 >(Si) and < 211_0 >(GaN)parallel to < 110 >(Si) similar to the matrix. Concaved geometry is essential and is a condition that limits the axial dimension of the nanorods protruding above the base (matrix) material region. Revelation of the growth mechanism in the current context suggests that fabrication of nanoquantum structures with controlled patterns is enabling for any attainable dimensions.

Abstract : Symposium Q on High Temperature Superconductors-Materials Challenges, covered a variety of HTS Materials issues. The symposium lasted three and half days with two evening poster sessions. A total of 135 papers were delivered, and 80 of those were oral presentations. The attendance was usually around 150 while a few keynote speakers drew larger audiences of over 200. A good fraction of the participants were from Japan and Europe. Overall, the authors honored the title of the symposium. All the material-sensitive issues were discussed, and were appropriate for a MRS meeting. New techniques were reported to fine-tuned HTS properties, particularly concerning boundaries and interfaces, and improving stability of the HTS materials in ambient and operating condition.

We report on the fabrication of AlGaAs/GaAs split-gate electron waveguide devices of lengths between 0.1 and 2 μm using x-ray lithography, and the measurements of these devices at liquid-helium temperatures and up to 15 K. An x-ray mask (parent mask) was fabricated using e-beam lithography and replicated using proximity x-ray lithography (λ=1.32 nm) to generate a replica (daughter) mask. The daughter mask was then aligned to patterns on a high-mobility AlGaAs/GaAs sample and x ray exposed using a conformable mask fixture. The conductance of the electron waveguides was measured as a function of the split-gate bias. Sharp 2e2/h conductance steps were observed in devices up to 0.75 μm long at T=2 K. The features in the conductance remain visible up to 15 K.

We report on the fabrication of quasi-one-dimensional wires on modulation-doped GaAs/AlGaAs using a novel conformable x-ray mask technology which allows us to expose arbitrary sized samples, including samples much smaller than the membrane area, using our laboratory's standard 31 mm-diam silicon-nitride x-ray mask. After optical alignment, the sample and mask are brought into contact electrically, and then loaded into a specially designed cartridge which allows a vacuum to be pulled between mask and substrate. The vacuum causes the x-ray mask to conform around the sample. We find that a vacuum hold down is necessary to allow easy separation of the sample from the mask with minimal risk to both.

We present a method for obtaining electrodeposited Au films of uniform thickness, low stress, and fine grain using a safe, commercially-available plating solution. In our electroplating set-up, the plating geometry is fixed by inserting an annular spacer ring between the Pt anode and the sample to be plated. Plating using this electrode configuration and a stagnant solution yields uniform-thickness Au films with a simple linear relationship between film stress and plating current density. Such films are well-suited as mask absorbers for x-ray nanolithography (sub-100-nm feature sizes).

Using a 50 keV, fine diameter electron beam lithography system, and a substrate consisting of a 10 nm-thick gold plating base on a 1 μm-thick, 2 cm-diam SiNx x-ray mask membrane, we have successfully exposed interdigitated electrode patterns for quantum-effect devices having lines and spaces of 50 nm. The resist is a single layer of polymethyl methacrylate (PMMA), 496 K molecular weight, 250 nm thick. That such fine features are achievable in a single layer of thick resist is attributed to: (1) reduced backscattering from the very thin plating base (10 nm Au versus the standard 30 nm) and thin substrate (1 μm-thick SiNx); (2) a well focused beam; (3) proximity-effect correction; and (4) precise exposure and development control. Once developed and ‘‘de-scummed,’’ 200 nm-thick gold is electroplated into the PMMA mold, yielding high contrast (∼12 dB) x-ray masks suitable for the CuL lines at 1.3 nm. To avoid problems of distortion and peeling of the 50 nm-wide lines, the plating was done under current–density and pH conditions that produce zero stress. The x-ray masks were replicated onto substrates and ‘‘daughter’’ x-ray mask membranes and electroplated, yielding ‘‘polarity reversal.’’ These polarity reversed masks, with 50 nm line-and-space features, can then be exposed onto device substrates using either contact or proximity x-ray lithography. This overall process takes advantage of the best aspects of electron-beam and x-ray nanolithographies, i.e., the capabilities of the former to create patterns of arbitrary geometry, and the robustness and high process latitude of the latter.

Most analyses of the effects of diffraction and source coherence on image quality in proximity x-ray lithography have used Kirchhoff boundary conditions and scalar diffraction theory. In this article we treat the x-ray absorber as a lossy dielectric and employ the vector form of Maxwell's equations to calculate image intensity as a function of position for lines, spaces, and gratings, at 100 nm linewidths and below. We show that vector and scalar theories give different results. Simulations are done for two point sources (Cu(L) and an Fe-plasma) so that calculations could be compared with our experimental results. Agreement was excellent for 80 and 50 nm features at relatively large gaps. We define image contrast and show that, contrary to common intuition, it is enhanced (and spurious ringing is suppressed) as penumbral blurring is increased, reaching a maximum when the extent of blurring is somewhat larger than the minimum feature size.

To realize a technology for x-ray nanolithography (<100 nm features), which is compatible with manufacturing, a number of mask design requirements must be met that are unrelated to patterning, repair, and alignment. These include high-flatness membranes and support structures so that mask-wafer gaps less than 10 μm can be achieved without risk of damage, and a rigid mask frame to avoid problems of distortion during handling. The membrane material should be compatible with semiconductor-processing, possess high strength, be radiation hard, and be transparent to light for alignment purposes. Details of a mask architecture that meets these requirements will be described.

We demonstrate the replication of device patterns consisting of interdigital lines ∼50 nm wide on 100 nm pitch using 1.32 nm x rays at a mask-to-substrate gap of 2.72 μm. The exposure latitude exceeds the factor 2.3 at this gap. From the expression that relates gap, G, linewidth, W, and wavelength λ (i.e., G=αW2/λ), we obtain α=1.44, which is well beyond the predictions of previous theoretical analyses based on the Kirchhoff boundary conditions. We attribute this disparity to the fact that an x-ray absorber is a lossy dielectric, hundreds of wavelengths thick, and hence the Kirchhoff boundary conditions are not applicable.

We report the fabrication and replication of x-ray masks with large-area (∼50 mm2) 100nm-period gratings. Achromatic holographic lithography was used to generate 100nm-period surface gratings in PMMA resist. Subsequent dry processing formed high-aspect-ratio grating lines down to the base of the resist. The x-ray absorber was defined by either: (i) reactive-ion etching low-stress sputter-deposited tungsten, using the resist lines as an etch mask, or (ii) electroplating gold using the resist lines as a mold. The absorber patterns were fabricated on silicon substrates coated with 1 μm-thick polyimide as the membrane material. X-ray masks were formed by etching away the silicon substrate, leaving the x-ray absorber pattern supported by the polyimide membrane. Results of x-ray exposures of PMMA, using the CK line (λ=4.5 nm), are presented.

The Submicron Structures Laboratory at MIT develops techniques for fabricating surface structures with feature sizes in the range from nanometers to micrometers and uses these structures in a variety of research projects. These projects, described briefly below, fall into four major categories: (1) development of submicron and nanometer fabrication technology; (2) nanometer and quantumeffect electronics; (3) crystalline films on non-latticematching substrates; and (4) periodic structures for x-ray optics, spectroscopy and atomic interferometry.

A technology has been developed for the printing of 0.1-μm-linewidth patterns using a ‘‘microgap,’’ out-of-contact scheme for x-ray nanolithography (as opposed to zero-gap electrostatic contact). Mask-to-wafer gaps of ∼5 μm are maintained by the use of gap-setting aluminum studs fabricated onto the front surface of the mask mesa rim. X-ray mask blanks are fabricated from silicon wafers coated with low-stress, silicon-rich low-pressure chemical vapor deposition SiNx. The resulting 1–2-μm-thick 6×108 dyn/cm2 stress membranes exhibit extreme strength. A novel aluminum-stencil etching procedure—which includes three CF4 RIE and two KOH etching steps—is used to define the mask membrane and ‘‘mesa’’ structure. Mask absorber pattern fabrication is performed by focused-ion-beam lithography (FIBL) and gold electroplating. We present details of the mask fabrication procedure and the results of testing these masks with x rays from a laboratory CuL (λ=1.34 nm) electron bombardment source and a commercial pulsed laser-plasma x-ray source. This marks the first time that FIBL-generated masks have been used with a pulsed laser-plasma source, and also the finest lines that have been printed using a laser plasma. The results of the evaluation of the x-ray resists PMMA, RAY-PF, RAY-PN, and SAL-601 are presented.

We present results of fabrication and transport measurements on surface-structured quantum wells. The structures are fabricated on GaAs/AlGaAs modulation-doped layers. Three different devices are examined: the grid-gate lateral-surface-superlattice, the planar-resonant-tunneling field-effect transistor, and the multiple parallel quantum wires. In the first two structures, transport is perpendicular to the field-induced potential barriers. At 4.2 K, we observed evidence for resonant tunneling in both types of devices. In the third type of structure, transport is through isolated quantum wires parallel to the barriers. The presence of one-dimensional energy subbands, and mobility modulation, above and below the two-dimensional value, were observed.

The authors discuss evidence of (1) electron standing waves forming underneath a 2000-AA period Ti/Au Schottky grid or grating incorporated as a gate into a GaAs HEMT (high-electron-mobility transistor) structure, and (2) modulation of the scattering time due to intersubband scattering in an array of approximately 100 parallel wires of 400-AA width formed by laterally patterning a GaAs/GaAlAs hete...

We have fabricated and tested grid-gate lateral-surface-superlattice (LSSL) FET devices on a modulation-doped GaAs/AlGaAs heterostructure. The LSSL consists of a 0.2-mu-m-period Ti/Au grid on top of the AlGaAs layer, which presents a tunable, 2-dimensional periodic potential modulation to the electrons traveling from source to drain. Conductance measurements at 4.2 K as a function of gate bias show a nonlinear behavior above threshold, providing evidence of a superlattice effect (i.e. coherent back-diffraction). In addition to the structures at low field, a negative differential resistance at high field was also observed.

In the past, nearly all x-ray nanolithography (i.e., sub-100-nm linewidths) employed the CK x-ray line at 4.5 nm. This, in turn, necessitated near-zero gaps (to avoid diffraction) and carbonaceous masks (e.g., polyimide, which is subject to distortion). In order to use x-ray replication in the fabrication of multilevel devices and circuits that cover large areas (∼a few cm2) and have feature sizes well below 100 nm, we have turned to the CuL line at 1.3 nm. Masks consist of 1–1.5 μm thick Si or Si3N4 membranes and Au absorber patterns, 200 nm thick, which provide 10 db contrast. Focused-ion-beam-lithography (FIBL) with Be++ ions at 280 keV was used to produce quantum-effect-device patterns with minimum linewidths of ∼50 nm. These were replicated using the CuL line, indicating that photoelectrons are not a serious problem. The FIBL process [exposure of 300 nm-thick polymethylmethacrylate (PMMA), followed by Au electroplating] is high yield and much simpler than a trilevel electron-beam-lithography process designed to give comparable results. This is the first time FIBL has been used to make x-ray masks at sub-100-nm linewidths. Along with the device patterns, linear-zone-plate alignment marks were also written on the masks, to be aligned to corresponding marks on the substrate via an optical alignment scheme.

We have fabricated and tested lateral-surface-superlattice (LSSL) and quasi-one-dimensional (Q1D) devices on a modulation-doped GaAs/GaAlAs heterostructure. The LSSL consists of a 0.2-μm-period (0.1-μm nominal linewidth) Ti/Au grating or grid on top of the GaAlAs layer, forming a Schottky barrier which presents a tunable periodic potential modulation to the electrons traveling from source to drain. The grating gate was fabricated using x-ray lithography to define the grating lines in poly(methylmethacrylate), and deep-UV lithography to expose gate contact pads, followed by lift-off of Ti/Au. Plots of the source–drain current as a function of the grating-gate bias showed distinct plateaulike features at 4.2 K, providing evidence of a superlattice effect, that is, electron backdiffraction. Minor modifications of the fabrication process permitted Q1D and grid-gate devices to be made. These also showed the expected structure in the conductance.

We report transport phenomena exhibited by a two-dimensional electron gas at the interface of a modulation doped GaAs/GaAlAs heterostructure in the presence of a field-effect-controlled periodic potential modulation. By means of x-ray lithography and lift-off, a 0.2-μm-period Schottky barrier grating gate was fabricated in lieu of the common continuous gate in a field-effect transistor configuration. Conductance measurements at 4.2 K provide evidence of a superlattice effect.

Contains table of contents for Part I, table of contents for Section 1, description of Submicron Structures Laboratory research, reports on twelve research project and a list of publications.