T.-C. Shen

Atomic scale desorption through electronic and vibrational excitation mechanisms

The scanning tunneling microscope has been used to desorb hydrogen from hydrogen-terminated silicon (100) surfaces. As a result of control of the dose of incident electrons, a countable number of desorption sites can be created and the yield and cross section are thereby obtained. Two distinct desorption mechanisms are observed: (i) direct electronic excitation of the Si-H bond by field-emitted electrons and (ii) an atomic resolution mechanism that involves multiple-vibrational excitation by tunneling electrons at low applied voltages. This vibrational heating effect offers significant potential for controlling surface reactions involving adsorbed individual atoms and molecules.

Nanoscale patterning and oxidation of H‐passivated Si(100)‐2×1 surfaces with an ultrahigh vacuum scanning tunneling microscope

Nanoscale patterning of the hydrogen terminated Si(100)‐2×1 surface has been achieved with an ultrahigh vacuum scanning tunneling microscope. Patterning occurs when electrons field emitted from the probe locally desorb hydrogen, converting the surface into clean silicon. Linewidths of 1 nm on a 3 nm pitch are achieved by this technique. Local chemistry is also demonstrated by the selective oxidation of the patterned areas. During oxidation, the linewidth is preserved and the surrounding H‐passivated regions remain unaffected, indicating the potential use of this technique in multistep lithography processes.

Recent developments in ohmic contacts for III-V compound semiconductors

Recent advances in the technology and understanding of ohmic contacts for a variety of III–V compound semiconductor material systems are reviewed. Special attention is focused on factors and critical issues involved in making low resistance and reliable ohmic contacts. The solid‐phase regrowth mechanisms of key metallization systems are described. In addition, special techniques to improve the ohmic contacts are discussed. Finally, the reliability issues of ohmic contacts are addressed.

Breaking individual chemical bonds via STM-induced excitations

We present experimental and theoretical results on the STM-induced SiH bond-breaking on the Si(100)-(2 × 1):H surface. First, we examine the character of the STM-induced excitations. Using density functional theory we show that the strength of chemical bonds and their excitation energies can be decreased or increased depending on the strength and direction of the field. By shifting the excitation energy of an adsorbate below the tip, energy transfer away from this excited site can be suppressed, and localized excited state chemistry can take place. Our experiments show that SiH bonds can be broken when the STM electrons have an energy >6 eV, i.e. above the onset of the σ→σ∗ transition of SiH. The desorption yield is ∼2.4 × 10−6 H-atoms/electron and is independent of the current. We also find that D-atom desorption is much less efficient than H-atom desorption. Using the isotope effect and wavepacket dynamics simulations we deduce that a very fast quenching process, ∼1015 s−1, competes with desorption. Most of the desorbing atoms originate from the “hot” ground state produced by the quenching process. Most interestingly, excitation at energies below the electronic excitation threshold can still lead to H atom desorption, albeit with a much lower yield. The yield in this energy range is a strong function of the tunneling current. We propose that desorption is now the result of the multiple-vibration excitation of the SiH bond. Such excitation becomes possible because of the very high current densities in the STM, and the long SiH stretch vibrational lifetime. The most important aspect of this mechanism is that it allows single atom resolution in the bond-breaking process — the ultimate lithographic resolution.

STM-induced H atom desorption from Si(100): isotope effects and site selectivity

Abstract We investigate the scanning tunnelling microscopy-induced H and D atom desorption from Si(100)-(2 × 1):H(D). The desorption of both atoms shows the same energy threshold that corresponds well with the computed σ → σ ∗ excitation energy of the SiH group. The H desorption yield, however, is much higher than the D yield. We ascribe this to the greater influence of quenching processes on the excited state of the SiD species. We use wavepacket dynamics to follow the motion of H and D atoms, and conclude that desorption occurs, for the most part, from the ‘hot’ ground state populated by the quenching process. Site-selective excitation-induced chemistry is found in the desorption of H from Si(100)-(3 × 1):H.

Nanometer scale patterning and oxidation of silicon surfaces with an ultrahigh vacuum scanning tunneling microscope

Nanoscale patterning of the Si(100)‐2×1 monohydride surface has been achieved by using an ultrahigh vacuum (UHV) scanning tunneling microscope (STM) to selectively desorb the hydrogen passivation. Hydrogen passivation on silicon represents one of the simplest possible resist systems for nanolithography experiments. After preparing high quality H‐passivated surfaces in the UHV chamber, patterning is achieved by operating the STM in field emission. The field emitted electrons stimulate the desorption of molecular hydrogen, restoring clean Si(100)‐2×1 in the patterned area. This depassivation mechanism seems to be related to the electron kinetic energy for patterning at higher voltages and the electron current for low voltage patterning. The patterned linewidth varies linearly with the applied tip bias achieving a minimum of <10 A at −4.5 V. The dependence of linewidth on electron dose is also studied. For positive tip biases up to 10 V no patterning occurs. The restoration of clean Si(100)‐2×1 is suggestive...

Nanoscale oxide patterns on Si(100) surfaces

Ultrathin oxide patterns of a linewidth of 50 A have been created on Si(100)‐2×1 surfaces by a scanning tunneling microscope operating in ultrahigh vacuum. The oxide thickness is estimated to be 4–10 A. The morphology and spectroscopy of the oxide region are obtained. Hydrogen passivation is used as an oxidation mask. The defects caused by oxidation in the passivated region before and after the hydrogen desorption are compared and discussed. The multistep silicon processings by an ultrahigh vacuum scanning tunneling micropscope is thus demonstrated.

Electron stimulated desorption induced by the scanning tunneling microscope

The use of the scanning tunneling microscope (STM) as an excitation source and a probe of electron stimulated desorption on the atomic scale is reviewed. The case of H desorption from H-terminated Si(001) is examined in detail. Experimental results on excitation thresholds, desorption cross-sections, isotope effects and site-selectivities are presented. Evidence for mechanisms involving direct electronic and hot ground-state desorption, as well as a novel multiple-vibrational excitation mechanism is discussed. Using the latter mechanism, the ultimate resolution limit of selective single atom desorption is achieved. New results on desorption from Si dihydride, including a proposed mechanism for the STM-induced HSi(001)-3 × 1 to 2 × 1 conversion, are presented. Possible applications of STM-induced desorption in nanofabrication are considered.

Ultradense phosphorous delta layers grown into silicon from PH3 molecular precursors

Phosphorous δ-doping layers were fabricated in silicon by PH3 deposition at room temperature, followed by low-temperature Si epitaxy. Scanning tunneling microscope images indicate large H coverage, and regions of c(2×2) structure. Hall data imply full carrier activation with mobility <40 cm2/V s when the surface coverage is ≲0.2 ML. Conductivity measurements show a ln(T) behavior at low temperatures, characteristic of a high-density two-dimensional conductor. Possible future applications to atom-scale electronics and quantum computation are briefly discussed.

Metal silicide patterning: a new approach to silicon nanoelectronics

The difficulties faced by conventional silicon technology over the next ten years have been widely publicized, along with the possibility for a slow-down and eventual stagnation in the power of integrated circuits. Surmounting this problem will require new initiatives in lithography, materials processing, and device architecture which must be carefully coordinated in order to evolve a manufacturable nanoelectronics. Here we present one long-term strategy which incorporates a number of attractive features, based upon recent research results from several different fields. Our goal is not to propose an alternative roadmap, but to expand discussion of long-term possibilities in future silicon technology.