Howard H. Harary

Modification of hydrogen‐passivated silicon by a scanning tunneling microscope operating in air

The chemical modification of hydrogen‐passivated n‐Si (111) surfaces by a scanning tunneling microscope (STM) operating in air is reported. The modified surface regions have been characterized by STM spectroscopy, scanning electron microscopy (SEM), time‐of‐flight secondary‐ion mass spectrometry (TOF SIMS), and chemical etch/Nomarski microscopy. Comparison of STM images with SEM, TOF SIMS, and optical information indicates that the STM contrast mechanism of these features arises entirely from electronic structure effects rather than from topographical differences between the modified and unmodified substrate. No surface modification was observed in a nitrogen ambient. Direct writing of features with 100 nm resolution was demonstrated. The permanence of these features was verified by SEM imaging after three months storage in air. The results suggest that field‐enhanced oxidation/diffusion occurs at the tip‐substrate interface in the presence of oxygen.

P2S5 passivation of GaAs surfaces for scanning tunneling microscopy in air

We report a novel method of GaAs substrate preparation which imparts significantly improved topographical and chemical uniformity to the surface. The procedure, employing an aqueous P2S5/(NH4)2S solution, leaves the surface in a highly ordered state and resistant to air oxidation for periods of a day or more without the presence of foreign chemical layer such as sulfur. Surface quality was determined by scanning tunneling microscopy (STM), time‐of‐flight secondary ion mass spectrometry, reflection high‐energy electron diffraction, and x‐ray photoelectron spectroscopy. The remarkable stability and smoothness of treated III‐V surfaces is illustrated by STM imaging of an Al0.51Ga0.49As/GaAs superlattice in air. The superlattice consisted of periodic alternating AlGaAs/GaAs layers of various thicknesses from 10 to 1000 nm.

Fitting circles and spheres to coordinate measuring machine data

This work addresses the problem of enclosing given data points between two concentric circles (spheres) of minimum distance whose associated annulus measures the out-of-roundness (OOR) tolerance. The problem arises in analyzing coordinate measuring machine (CMM) data taken against circular (spherical) features of manufactured parts. It also can be interpreted as the “geometric” Chebychev problem of fitting a circle (sphere) to data so as to minimize the maximum distance deviation. A related formulation, the “algebraic” Chebychev formula, determines the equation of a circle (sphere) to minimize the maximum violation of the equation by the data points. In this paper, we describe a linear-programming approach for the algebraic Chebychev formula that determines reference circles (spheres) and related annuluses whose widths are very close to the widths of the true geometric Chebychev annuluses. We also compare the algebraic Chebychev formula against the popular algebraic least-squares solutions for various data sets. In most of these examples, the algebraic and geometric Chebychev solutions coincide, which appears to be the case for most real applications. Such solutions yield concentric circles whose separation is less than that of the corresponding least-squares solution. It is suggested that the linear-programming approach be considered as an alternate solution method for determining OOR annuluses for CMM data sets.

Integration of scanning tunneling microscope nanolithography and electronics device processing

The emerging field of nanoelectronics demands innovative methods to fabricate nanometer‐scale structures. Such structures will play a critical role in the quantum‐effect device physics of future highly integrated circuit architectures. An integrated approach to compound semiconductor nanostructure fabrication based on scanning tunneling microscope (STM) nanolithography, molecular‐beam epitaxy, and reactive ion etching techniques is described. The critical elements of this approach, which have been demonstrated recently, are reviewed. Prospects for the coevolutionary development of nanoelectronics and STM‐based fabrication and characterization are considered.

Nanolithography on III-V Semiconductor Surfaces Using a Scanning Tunneling Microscope Operating in Air

Nanometer‐scale pattern generation on III‐V semiconductor substrates using a scanning tunneling microscope (STM) operating in air is demonstrated. The sample substrates, consisting of arsenic‐capped, epitaxial layers of n‐doped GaAs, AlxGa1−xAs and InyGa1−yAs were prepared by molecular beam epitaxy and characterized by time‐of‐flight secondary‐ion mass spectrometry and x‐ray photoelectron spectroscopy. The direct patterning of features of width ≤50 nm on GaAs and In0.2Ga0.8As surfaces is shown to be the result of the formation of a strongly bonded surface oxide induced under high electric field conditions existing between the scan tip and the substrate. The significance of STM pattern generation of nanometer‐scale oxide masks for use in the fabrication of low‐dimensional heterostructures is discussed.

Selective-area epitaxial growth of gallium arsenide on silicon substrates patterned using a scanning tunneling microscope operating in air

Selective‐area epitaxial growth of gallium arsenide on n‐Si(100) substrates is reported, where the oxide (SiOx) mask consists of 1–2 monolayer‐thick features patterned onto a silicon substrate using a scanning tunneling microscope (STM) operating in air. The technique for generating the STM patterns on hydrogen‐passivated silicon was reported recently [J. A. Dagata, J. Schneir, H. H. Harary, C. J. Evans, M. T. Postek, and J. Bennett, Appl. Phys. Lett. 56, 2001 (1990)]. The GaAs epilayer was grown by migration‐enhanced epitaxy at 580 °C and its morphology was investigated by scanning electron microscopy. The chemical selectivity of the STM‐patterned regions was verified by imaging time‐of‐flight secondary‐ion mass spectrometry. The implications of these results for the development of a unique, STM‐based nanostructure fabrication technology are discussed.

Imaging of passivated III–V semiconductor surfaces by a scanning tunneling microscope operating in air

Abstract A procedure is described for preparing stable GaAs and other III–V semiconductor surfaces for scanning tunneling microscope (STM) imaging under ambient conditions. The procedure involves the use of a dilute P 2 S 5 /(NH 4 ) 2 S passivating solution, which produces a highly uniform, ultra-thin surface oxide. STM imaging with nanometer-scale resolution of a P 2 S 5 -passivated, Al x Ga 1 −xAsGaAs, x = 0.1–0.4, compositional superlattice and a variable-period Al 0.51 Ga 0.49 As/GaAs superlattice is used to illustrate some of the properties of this passivation method.

Scanning Tunneling Microscopy Of Optical Surfaces

We have imaged diamond turned gold surfaces and a gold coated silicon surface with the STM. In order to determine the reproducibility of the topographic information obtained with the STM, we imaged the same diamond turned gold surface with different tips. Both mechanically formed and electrochemically etched tips were used. Surface images observed with the STM varied from tip to tip for both methods of preparation. The use of the STM to image optical surfaces hinges on the ability to manufacture stable and reproducible tips.

Scanning tunneling microscope‐based nanostructure fabrication system*

We have designed a novel scanning tunneling microscope (STM) and control electronics system to investigate STM‐based nanostructure fabrication on semiconductor surfaces. Several elements of our design are unique to this application. The STM, which resides in a vacuum chamber, can be positioned anywhere on a 1 cm×1 cm sample. This is accomplished by using three piezoelectric motors (Burleigh Inchworms) and locating the tip position with a long working distance optical microscope (Questar). One piezoelectric motor is used for the coarse Z approach while the other two adjust the X,Y tip position. The tip is attached to a tube‐type piezoelectric scanner. The piezo tube’s x–y scan range is 12 μm×12 μm. The tube’s mechanical and electrical response are linear to 6 kHz and allow rapid scanning. Both tip and sample are attached to the microscope magnetically to facilitate rapid self‐aligned exchange under vacuum. A computer controlled pattern generation system allows arbitrary patterns to be drawn on the sample. ...