Special methods were developed to grow Si/ZnS/Si heterostructures for the purpose of developing silicon-based quantum devices. An arsenic passivation technique was used on silicon surfaces for the growth of ZnS. The effectiveness of this surface passivation was demonstrated by the improved crystal quality and decreased heterostructural interface-state density. A two-step growth method was developed for the epitaxial growth of silicon on ZnS surfaces. In this method the normal silicon epitaxial growth was preceded by a solid phase epitaxy process. Single crystal films of silicon were successfully grown on ZnS surfaces. Our discussion addresses two key issues: crystal defects and the density of interface states. The results of RHEED (reflection high energy electron diffraction), C-V (capacitance voltage), and SIMS (secondary ion mass spectroscopy) measurements are presented.

Si growth by molecular beam epitaxy on nonplanar patterned Si substrates is studied as a function of growth parameters. The substrates consist of a truncated pyramid template with {111} sides and (100) tops formed by anisotropic etching of Si(100). For growth temperatures ≤ 550°C no qualitative changes in the morphology of the template are observed. At growth temperatures between 650–700°C {113} facets begin to form on the (100) surface and reduce the lateral dimensions of the (100) facet to < 20 nm. At high temperatures (∼800°C) {113} facets remain stable and {111} facets no longer exist. The small (100) mesa formed at medium temperatures by facet reduction is exploited through the growth of Si/Si 1-xGex multiple quantum wells leading to low-dimensional structures. Observations are quantified by scanning electron and transmission electron microscopies.

Substrate encoded size-reducing epitaxy (SESRE) is utilized to fabricate, in-situ, threedimensionally confined InAs volumes on <100> oriented square mesas patterned onto GaAs(001) substrates. As a function of InAs deposition thickness two remarkable results are found: (i) the strain relief available in mesas of linear size ≤ 100 nm allows the mesa top InAs layers up to thickness ∼ 11 ML to remain coherent and maintain 2D morphology even though on unpatterned substrates onset of 3D island formation at ∼ 2 ML InAs is well documented. (ii) With increasing deposition, once the InAs thickness on mesa tops of linear size ∼ 75 nrn reaches ∼ 11 ML, no further growth occurs even for deposition amounts in excess of twice this value. This suggests complete migration of In away from the mesa top to the sidewalls once the InAs thickness on the mesa top reaches ∼ 11 ML even though, at the early stages of deposition, In migrates from the sidewalls to the mesa top. A strain-induced “self-limiting” growth behavior on sub 100nm mesas is thus indicated for the first time.

Central to nanofabrication is the ability to transfer a pattern from an imaging layer to a device or structure. At the smallest dimensions (<20 nm), thin resists or imaging layers have been used exclusively. The transfer of a pattern that is formed in a thin layer resist presents severe technological challenges to resist materials development. A novel approach based on self-assembling monomolecular layer resists is demonstrated with two organosilane films, formed from (aminoethylaminomethyl)phenethyltrimethoxysilane (PEDA) and 4-chloromethylphenyltrichlorosilane (CMPTS). The molecules have separate chemical functionalities for binding to a Si substrate and for promoting chemistry leading to catalysis and the growth of an electroless plated metal film. STM lithographic exposure destroys the ability of the molecule to bind to a catalyst, which initiates an electroless metallization. This forms the basis for a selective imaging and the pattern transfer process. A 25 nm thick Ni layer acts as a very robust etch mask, even as the unmasked regions of Si are etched as deep as 5 μm by reactive ion etching with SF6. With our process 15 nm lines with 3.3 nm edge roughness have been fabricated in the plated Ni and etched into the underlying Si. The development of the resist process and the STM lithography will be described and the resolution of the approach will be discussed.

Etch rates for binary nitrides in ECR Cl2/CH4/H2/Ar are reported as a function of temperature, rf-bias, microwave power, pressure and relative gas proportions. GaN etch rates remain relatively constant from 30 to 125 °C and then increase to a maximum of 2340 Å-min−1 at 170 °C. The AIN etch rate decreases throughout the temperature range studied with a maximum of 960 Å-min−1 at 30 °C. When CH4 is removed from the plasma chemistry, the GaN and InN etch rates are slightly lower, with less dramatic changes with temperature. The surface composition of the III–V nitrides remains unchanged over the temperatures studied. The GaN and InN rates increase significantly with rf power, and the fastest rates for all three binaries are obtained at 2 mTorr. Surface morphology is smooth for GaN over a wide range of conditions, whereas InN surfaces are more sensitive to plasma parameters.

Laser-induced pyrolytic etching of silicon by chlorine has been used to process structures with nanoscale resolution. A direct write system has been designed, based on an Ar ion laser with a UV-option, on-axis optics, acousto-optic beam modulation, and high resolution translation stages for sample displacement and for focusing.

The resolution of the process is determined by the size of the melt generated by the focused laser beam. The melt size is a function of optical power, and resolutions better than the focused laser spot diameter are achieved by reducing the power onto the silicon sample. In this way we have etched trenches with a resolution better than 150 nm. We demonstrate feature sizes of about 40 nm by closely spacing etched trenches. The minimum width of these features is determined by the etch selectivity. The feature width depends on the trench geometry, line spacing, and on scan speed, due to heat flow confinement.

Nanolithography has been studied by using electron beam technology. Ten-nm linewidth PMMA resist patterns have been demonstrated by 50 kV scanning electron beam. The self-developing properties of a AlF3 doped LiF inorganic resist under a scanning electron beam irradiation with energy of 20–50 keV have been studied for sub-10 nm lithography. By optimizing the inorganic resist film quality, 5 nm linewidth patterns with 60 nm periodicity were directly delineated under a 30 keV electron beam. Another approach for nanolithography using electron beam holography has been proposed. Line and dot patterns with 100 nm periodicity were exposed on PMMA resist by electron beam holography with thermal field emitter gun and an electron biprism. Subsequent atomic force microscope observation has confirmed that both patterns are successfully fabricated. This technique allows us to produce nanoscale periodic patterns simultaneously. The selective atomic desorption of Cl atoms adsorbed on a Si (111) 7×7 surface has been studied by field evaporation using a scanning tunneling microscope (STM). The STM tip is placed on the adsorbed Cl on the surface, and pulse voltage was applied. This results in selective atomic desorption of Cl.

We have applied high resolution synchrotron X-ray diffraction to two-dimensional (2D) Si gratings with wavelengths of 300 and 400nm. We show that this method is very sensitive to both the geometry (grating wavelength, height, and angle of inclined sidewall) and the state of strain. Morphology changes produced by vacuum annealing can be detected so that the mass transport rates on Si surfaces can be measured. Strain measurements show that grating pillars covered with 1 lnm of thermal oxide were under tension (ε = 3.7 × 10−4). This strain was elastically relaxed by removing the oxide.

We have investigated the technique of implantation enhanced interdiffusion (IEI) for optical nanostructure fabrication in strained InxGal-xAs/GaAs quantum wells. Implantation masks with widths from 40 nm to 40 μm were fabricated on the surface of InxGal-xAs/GaAs (x+0.1, 0.2) 3.5 nm quantum well material which was implanted with 100 kV As+ with doses ranging from 5 × 1012 to 8.5× 1013 ions/cm2. After mask removal and a high temperature anneal, cathodoluminescence (CL) spectroscopy was used to investigate the optical properties of the resulting structures. We have measured electron-heavy hole recombination energy shifts due to quantum well interdiffusion of up to 60 meV for the highest doses used here with broad area implants. However, while quantum well emission under large (40 μm) masks is preserved, smaller masks show an emission blue shift not due to ions penetrating through the mask. A simple model of the width dependence of this shift yields an enhanced lateral diffusion length of approximately 1 μm which is many times larger than the lateral straggle of the implanted As+. We conclude that lateral diffusion effects may impose a limit on nanostructure fabrication in the InxGal-xAs/GaAs system with this technique.

We have investigated several methods to form submicron-size porous silicon regions. Porous silicon can emit light from the violet to past 1.5 μm with high photoluminescence efficiency at room temperature. It is composed of a high density of nanometer-scale crystalline silicon wires or dots. To integrate light-emitting porous silicon (LEPSi) LEDs with conventional Si microelectronics, it is necessary to produce miniature LEPSi regions adjacent to fully protected crystalline silicon regions. These techniques can be divided into two groups. In the first group formation of LEPSi is prevented during electrochemistry. Using optical and electron beam lithography, and a trilayer process with silicon nitride or amorphization by ion-implantation, we have made LEPSi patterns as small as 100 nm. In the second group, the formation of LEPSi during electrochemistry is enhanced by ion-milling or reactive ion-etching which we have found to help the pore nucleation. We have used a variety of mapping techniques, such as photoluminescence, atomic force and electron beam microscopies, to characterize the sharpness of the interface between the porous silicon and crystalline silicon regions.

We have prepared nanoparticles of titania using the sol-gel technique. The samples were characterized by FTIR, DTA, TGA, EPR and XRD. The properties observed with these techniques are correlated in the temperature region between 200 and 800°C. The Ti-OH bonds are basic to interpret this correlation.

The microstructure of an agglomerate can be characterized by the coordination number. The relationship between the fractal dimension and the coordination number of agglomerates of nanometer particles was investigated in experiments and computer simulations. The results for silver agglomerates formed by laser ablation agreed well with the simulations. The coordination number is low for low density fractals because of the large fraction of surface particles which have fewer bonds. The sensitivity of the coordination number to the fractal dimension increases with increasing fractal dimension.

This study shows that reactive ion/surface collisions can provide information on the surface structure of molecular systems. The microscopic structures of perfluorinated polyether (PFPE) liquid surfaces are characterized by ion/surface collision experiments in a mass spectrometer and the results are compared to those obtained with a fluorinated self-assembled monolayer (F-SAM) surface. Low energy (< 100 eV) beams of W+ and W(CO)6+ ions are used for surface-induced dissociation (SID) and ion/surface reactive scattering, while Xe+ and Kr+ beams are used for chemical sputtering experiments. Both the PFPE and F-SAM surfaces show similar SID fragments while reactions with W+ and W(CO)6+ lead to the scattered metal ion with multiple fluorine atoms attached. The main peaks in chemical sputtering spectra are similar for both surfaces as well. Both hydrogen probe beams and chemical sputtering confirm that the liquid PFPE surface is nearly free of impurities. Our results on both surfaces also suggest that the CF3 group of the monomer units is most likely projected outward from the bulk of the liquid surface.

Current metal/metal oxide patterning techniques typically involve photolithography, an expensive, time-consuming process, and/or a vacuum system, which also adds significantly to the cost. In this study, fabrication of metal oxide upper electrodes for Pb(ZrxTi1-x)O3 (PZT) memories by metallorganic decomposition (MOD) was coupled with laser beam patterning. Metal oxide organic precursors were treated with an absorption-enhancing dye and were spun on to ferroelectric PZT films followed by selective area laser beam exposure. This technique was employed to pattern RuO2 and (La,Sr)CoO3 electrodes. A variety of patterns (dot, line, and circular) were prepared which demonstrates the suitability of laser patterning for a variety of applications. The minimum power density required to generate a continuous conductive oxide pattern was found to be approximately 2.5 kW/cm2 at a scan speed of 25 mm/min. Resistivity values for the patterned lines were 660 Ω-cm for RuO2 and 59 mΩ-cm for LSCO.

A fabrication process for sub-100 nm Ge wires on Si substrates is reported for the first time. Wires with a cross section of 6 × 57 nm2 are demonstrated. The wire structures are analyzed by atomic force (AFM), scanning electron (SEM), and transmission electron microscopy (TEM). Sample preparation for TEM is performed using a novel technique using both pre and in situ deposition of multiple protection layers using a Focused Ion Beam (FIB) micromachining system.

Scanning tunneling spectroscopy of the high frequency response of photoexcited carriers in the layered structure semiconductors n–type MoS2 and p–type Wse2 is demonstrated using the beat frequency of the longitudinal modes of a HeNe laser at the tunneling junction. We analyze the optical response taking into account the effects of tip-induced band bending and surface states.

An ambient scanning tunneling microscope (STM) was used to excite luminescence in ptype epitaxial GaAs with four separate surface preparations: bare GaAs, Au layer, sulfurmonochloride layer, and one monolayer of octa-decyl-thiol. The STM with tungsten tip was operated at a constant tunnel current of 5 nA during a +1 V bias applied to the sample and the resulting tip to sample distance was fixed during a higher voltage bias pulse which excited luminescence. The luminescence intensity increased rapidly with increasing bias voltage for all passivation types with the octa-decyl-thiol passivation achieving the highest STM excited luminescence (STMEL) of 3500 photons/sec at 4 V bias. Above about 4 V the luminescence from the octa-decyl-thiol and sulfur-monochloride passivated samples fell off irreversibly, indicating that the sample surface had been modified. The Au passivated and unpassivated samples showed no such luminescence drop up to 4.8 V, the highest bias employed. Photoluminescence (PL) studies of the samples showed that PL intensities exhibited a weaker dependence upon passivation type than did STMEL intensities, a result consistent with the assertion that STMEL is more sensitive to the surface properties of the sample than is PL.

Anodization of Ti by high electric fields at the tip of a scanned probe can be used to produce nanoscale features consisting of oxides of Ti. In this manner, Ti can be used as a sacrificial resist for nanoscale lithography by exploiting the etching selectivity differences between Ti and anodized Ti. The anodization was accomplished with an atomic force microscope using Ticoated silicon nitride cantilevers. The anodizing bias voltage is applied to the tip and is independent of the feedback, unlike the scanning tunneling microscope. With this setup we were able to fabricate sub-40 nm lines by direct anodization of Ti. It is also shown that once tip and sample are brought into hard contact, subsequent bending of the cantilever has little effect on the linewidth or thickness of the anodized material.

An AFM-based nanolithography process is described. We employ the local electric field of a metal-coated AFM tip which is operated in air to selectively oxidize regions of a H-passivated Si surface. The resulting oxide, ∼ 3 nm thick, is used as a mask for selective etching of the unoxidized regions of Si. This AFM-based fabrication process is fast, reliable, simple to perform and is well suited for device fabrication. We apply this technique to the fabrication of Si and GaAs nanostructures, as well as to the fabrication of a nanometer-scale Si side-gated transistor. In addition, we discuss the ultimate resolution limits of the technique.

The ability to create atomic-scale structures with the scanning tunneling microscope (STM) plays an important role in the development of a future nanoscale technology. I briefly review the various modes of STM-based fabrication and atomic manipulation. I focus on using a UHV-STM to directly pattern the Si(001) surface by atomic manipulation at room temperature. By carefully adjusting the tip morphology and pulse voltage, a single atomic layer can be removed from the sample surface to define features one atom deep. Segments of individual dimer rows can be removed to create structures with atomically straight edges and with lateral features as small as one dimer wide. Trenches ∼3 nm wide and 2–3 atomic layers deep can be created with less stringent control of patterning parameters. Direct patterning provides a straightforward route to the fabrication of nanoscale test structures under UHV conditions of cleanliness.