J. Bennett

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.

Patterning of self‐assembled alkanethiol monolayers on silver by microfocus ion and electron beam bombardment

Decanethiol [CH3(CH2)9SH] self‐assembled monolayer films on silver substrates have been irradiated in selected areas by focused ion or electron bombardment. Subsequent immersion of the irradiated sample in a solution of a fluoromercaptan [CF3(CF2)2(CH2)2SH] results in attachment of this molecule to the silver surface in the ion or electron‐exposed regions, producing a micrometer spatial‐scale pattern of two chemically distinct alkanethiol monolayers. The coverage of the fluoromercaptan on the bombarded areas was found to reach maximum levels of 70% at ion doses of 6×1013 ions/cm2 and 50% at electron doses of 2×1017 electrons/cm2 as determined by secondary ion mass spectrometry. These methods of maskless patterning may be useful for semiconductor or biosensor device fabrication.

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.

Formation and emission of tetraalkylammonium salt molecular ions sputtered from a gelatin matrix

A gelatin matrix was simultaneously doped with nine equimolar, homologous, tetraalkylammonium salts ranging in mass from 210 to 770 Da. Bombardment of the sample with kiloelectronvolt ions resulted in a nonidentical distribution of relative cation intensities with a maximum at m/z 242 for samples with a total salt concentration of 0.004 g of salt/g of gelatin. A rapid increase in relative intensities with increasing mass is observed for the low mass salts and is believed to be linked to changes in the ionization efficiencies. The changes in ionization efficiencies are likely related to decreasing coulombic attractive forces between the organic cation and the counterion. Disappearance cross sections, determined from decay curves, indicate that sputter-induced damage increases with increasing mass of the cation. Fragment-to-intact cation ratios also suggest that damage accumulates fastest in the heaviest salts. These observations indicate that desorption yields of the organic salts in a gelatin matrix decrease with increasing mass. In addition, suppression of lower mass tetraalkylammonium salt intact cation intensities was observed for salt-in-gelatin concentrations greater than 10−3 g/g.

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.

Ultra‐shallow depth profiling with time‐of‐flight secondary ion mass spectrometry

Time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS) is an efficient, sensitive method for characterizing semiconductor surfaces. In addition, TOF‐SIMS can be applied in a depth profiling mode allowing qualitative characterization of the top 20 nm of material. The utility of TOF‐SIMS ultra‐shallow depth profiling is demonstrated on GaAs substrates that were passivated with P2S5 solutions and oxidized by exposure to an UV/ozone treatment.

Low temperature plasma for the preparation of crater walls for compositional depth profiling of thin inorganic multilayers

An indirect, compositional depth profiling of an inorganic multilayer system using a helium low temperature plasma (LTP) containing 0.2% (v/v) SF6 was evaluated. A model multilayer system consisting of four 10 nm layers of silicon separated by four 50 nm layers of tungsten was plasma-etched for (10, 20, and 30) s at substrate temperatures of (50, 75, and 100) °C to obtain crater walls with exposed silicon layers that were then visualized using time-of-flight secondary ion mass spectrometry (ToF-SIMS) to determine plasma-etching conditions that produced optimum depth resolutions. At a substrate temperature of 100 °C and an etch time of 10 s, the FWHM of the 2nd, 3rd, and 4th Si layers were (6.4, 10.9, and 12.5) nm, respectively, while the 1/e decay lengths were (2.5, 3.7, and 3.9) nm, matching those obtained from a SIMS depth profile. Though artifacts remain that contribute to degraded depth resolutions, a few experimental parameters have been identified that could be used to reduce their contributions. Further studies are needed, but as long as the artifacts can be controlled, plasma etching was found to be an effective method for preparing samples for compositional depth profiling of both organic and inorganic films, which could pave the way for an indirect depth profile analysis of inorganic-organic hybrid structures that have recently evolved into innovative next-generation materials.