William E. Packard

Scanning tunneling microscope tip structures

Studies of electrochemically etched tungsten scanning tunneling microscope tips, using scanning electron microscopy, show that (i) the tips are often convolved or bent if the mass of the tungsten wire submerged in the etchant is large (an effect ascribed to surface plastic flow), (ii) bent tips nevertheless often produce good quality scanning tunneling microscopy images of Au films in air, but (iii) tips, once crashed clumsily into the Au films, no longer produce images.

Nano-machining of gold and semiconductor surfaces

Using a scanning tunnelling microscope tip formed by cutting a platinum wire, we have modified the surfaces of gold and Hg1‐xCdxTe on a nanometre scale by mechanical contact between the tip and the surface. By using the same tip to form images, we have been able to gain ‘before’ and ‘after’ pictures of surfaces that have been selectively ‘sanded’, controllably ‘chiselled’, and ‘swept’.

Externally strained Si(100) observed with scanning tunneling microscopy

Applying a uniaxial strain to Si(100) lifts the orientational degeneracy of the surface energy for 2×1 and 1×2 domains, with a corresponding population increase for the favored domains. An external uniaxial strain was applied to Si(100) by pushing or pulling the free end of a long thin cantilevered sample and the resulting domain structure was observed with a scanning tunneling microscope (STM). On externally unstrained Si(100) there were equal populations of 2×1 and 1×2 domains. The STM images show that under strain individual domains of  2×1 grew in size at the expense of individual domains of 1×2, and the pattern on the strained surface was a large majority terrace, a single‐atom‐high step to a thin minority terrace, followed by a single‐atom‐high step back to a large majority terrace. The minority domains formed thin terraces which could have meandering directions. Regions of 2×2 reconstruction were frequently observed on the strained surface.

Scanning tunneling microscopy study of the cleaved InSb(110) surface

The clean InSb(110) surface was imaged in ultrahigh vacuum with scanning tunneling microscopy. A 1×1 surface structure was observed. A super‐periodicity consistent with a c(4×6) reconstruction was also observed on some regions of some cleaved surfaces, and appears to be cleavage dependent. The InSb(110) surface can easily be altered by the tunneling process to produce nanoscopic dots on the surface as small as 9 A radius.

Deep Levels in Type-II Superlattices

Quantum confinement in superlattices affects shallow levels and band edges considerably (length scale of order 100 A), but not deep levels (length scale of order 5 A). Thus by band-gap engineering, one can move a band edge through a deep level, causing the defect responsible for the level to change its doping character . For example, the cation-on-anion-site defect in Al x Ga 1−x Sb alloys is predicted to change from a shallow acceptor to a deep acceptor-like trap as the valence band edge passes through its T 2 deep level with increasing At alloy content x. In a, Type-II superlattice, such as InAs/Al x Ga 1−x Sb for x>0.2, where the conduction band minimum of the InAs should lie energetically below the antisite defects T 2 level in bulk Al x Ga 1−x Sb, the electrons normally trapped in this deep level (when the defect is neutral) remotely dope the InAs n-type in the superlattice, leaving the defect positively charged. Thus a native defect that is thought of as an acceptor can actually be a donor and control the n-type doping of InAs quantum wells. The physics of such deep levels in superlattices and in quantum wells is summarized, and related to high-speed devices.