Design of an inverse photoemission spectrometer for studies of quantum wires.

摘要

As microelectronic device and interconnect dimensions become smaller, a knowledge of exactly how electrons behave when confined to reduced dimensions will become crucial to their design. A fundamental limit is reached when the width of a conduction channel, or wire, reaches that of a single atom (1). Such conductors are known as quantum wires, or quasi-one-dimensional systems. An inverse photoemission spectrometer was designed, constructed and used to study the electronic structure of quantum wires.; The electronic structures of two self-assembled quantum wire systems were investigated. Low energy electron diffraction was used to determine the surface periodicities and orientations. Inverse photoemission spectroscopy was used to study the unoccupied electronic structures and determine the metallicities of the systems.; Si(111)-Au(5 x 2) surfaces were prepared by depositing sub-monolayer quantities of Au on vicinal Si(111) samples at {dollar}650spcirc{dollar}C. Prior to deposition, the samples were cleaned by high-temperature flashing in an ultrahigh vacuum to remove the native oxide and produce a well ordered Si(111)-(7 x 7) surface. These Si(111)-Au(5 x 2) samples showed a distinct single-domain (5 x 1) low energy electron diffraction pattern with diffuse half-order streaks, in agreement with previous studies (2, 3). The k-resolved inverse photoemission (KRIPES) studies of this surface found no evidence of bands crossing the Fermi level, in contradiction with the results of Collins et al. (4) who studied the occupied states using photoemission spectroscopy and found a 'metallic' band. The similarity between the electronic structures, both occupied and unoccupied, of this system and the clean Si(111)-(7 x 7) system led to the conclusion that the electronic states in the vicinity of the Fermi level seen by direct and inverse photoemission are derived from Si atoms in adatom geometries in the Au(5 x 2) structure, and not from the Au chains, as assumed previously (4).; Si(111)-In(4 x 1) surfaces were prepared by depositing In atoms on vicinal Si(111)-(7 x 7) surfaces heated to {dollar}395spcirc{dollar}C, and by depositing In atoms on room temperature samples followed by a 5 minute annealing cycle at {dollar}450spcirc{dollar}C. Predominantly single-domain samples were produced using the former procedure, as judged by low energy electron diffraction. The latter procedure, used in earlier studies of this system (5), repeatably produced three-domain samples revealing normal incidence inverse photoemission spectra distinctly different from those collected on the single-domain samples. These spectra contained the sum of all features observed in spectra from the single-domain samples and those seen on the lower coverage In{dollar}(sqrt{lcub}3{rcub}{dollar} x {dollar}sqrt{lcub}3{rcub}){dollar}R30{dollar}spcirc{dollar} surface. It was concluded that these three-domain samples actually contained a mixture of (4 x 1) and {dollar}(sqrt{lcub}3{rcub}{dollar} x {dollar}sqrt{lcub}3{rcub}){dollar}R30{dollar}spcirc{dollar} phases. KRIPES studies of the single-domain samples identified a clear Fermi level crossing at {dollar}approx{dollar}0.5 A{dollar}sp{lcub}-1{rcub}{dollar} parallel to the chains, in excellent agreement with a recent photoemission study (6). This band was found to have a quasi-one-dimensional nature, dispersing strongly parallel to and negligibly perpendicular to the In chains.