J. Schneir

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.

Pattern generation on semiconductor surfaces by a scanning tunneling microscope operating in air

Recent results employing scanning tunneling microscope‐based techniques for the generation of nanometer‐scale patterns on passivated semiconductor surfaces are presented. Preparation and characterization of hydrogen‐passivated silicon and sulfur‐passivated gallium arsenide surfaces are described and the determination of the chemical and morphological properties of the patterned regions by scanning electron microscopy and time‐of‐flight secondary ion mass spectrometry are discussed. Our recent demonstration that ultrashallow, oxide features written by scanning tunneling microscope (STM) can serve as an effective mask for selective‐area GaAs heteroepitaxy on silicon is used to illustrate key requirements necessary for the realization of a unique, STM‐based nanotechnology.

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.

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.

Influence of data analysis and other factors on the short-term stability of vertical scanning-probe microscope calibration measurements

We report a study of a fundamental limit to the accuracy of vertical measurements made using scanning-probe microscopes (SPM): the short-term stability of a vertical calibration using a waffle-pattern artifact. To test the instrumental component of this stability, we acquired three data series, at different humidity levels. We compare the variations in waffle-pattern depth in these three data series with the differences in depth estimates using several different analysis methods. The three methods tested are: a histogram method, the scanning-probe image processor, and the polynomial step-function fit. To clarify the importance of the analysis method, a discussion of the different leveling, averaging, and depth-estimation aspects of the various methods is presented. To understand the true repeatability limit of SPM calibration, it is necessary to treat imaging artifacts such as tilt, nonlinearities, and image bow carefully. We find that, when such care is taken, the dependence of the average waffle-cell de...

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. ...