R. M. Feenstra

Tunneling spectroscopy of the Si(111)2 × 1 surface

Abstract Using a scanning tunneling microscope, the tunneling current versus voltage is measured at fixed values of separation between a tungsten probe-tip and a Si(111)2 × 1 surface. Rectification is observed in the I - V curves and is quantitatively accounted for by an electric-field enhancement due to the finite radius-of-curvature of the probe-tip. The parallel wave-vector of certain states is obtained from the decay length of the tunneling current. A rich spectrum is obtained in the ratio of differential to total conductivity, yielding a direct measure of the Si surface density-of-states. Small shifts are observed in the spectrum as a function of doping, and are attributed to shifts in the position of the surface Fermi level.

Determination of wurtzite GaN lattice polarity based on surface reconstruction

We identify two categories of reconstructions occurring on wurtzite GaN surfaces, the first associated with the N face, (0001), and the second associated with the Ga face, (0001). Not only do these two categories of reconstructions have completely different symmetries, but they also have different temperature dependence. It is thus demonstrated that surface reconstructions can be used to identify lattice polarity. Confirmation of the polarity assignment is provided by polarity-selective wet chemical etching of these surfaces.

Tunneling spectroscopy of the GaAs(110) surface

The scanning tunneling microscope is used to study the spectroscopy of p‐type, n‐type, and oxygen‐covered GaAs(110) surfaces. On the clean surface, three components of the current are identified—tunneling out of valence‐band states, tunneling into conduction‐band states, and tunneling through dopant‐induced states in the semiconductor. The results are compared with a theoretical computation of the tunneling current, including band bending in the semiconductor. Good agreement between theory and experiment is obtained only when tunneling through the space‐charge region of the semiconductor is included. On the oxygen‐covered surface, the spectroscopic results show evidence of band bending due to the oxygen adsorbates.

Asymmetries in dislocation densities, surface morphology, and strain of GaInAs/GaAs single heterolayers

The dislocation densities, surface morphology, and strain of Ga1−xInxAs/GaAs epitaxial interfaces as a function of indium composition and layer thickness have been investigated by transmission electron microscopy, medium energy ion blocking, and double‐crystal x‐ray diffractometry. The electron microscopy shows that in the thinnest dislocated films (90 and 160 nm, x=0.07) 60° α dislocations form first in one 〈110〉 direction at the interface. Surprisingly, however, an asymmetry in residual layer strain is not detected in these samples, suggesting that the dislocations have the same Burgers vector or are evenly distributed between two Burgers vectors. Orthogonal arrays of dislocations are observed in films thicker than 300 nm (60° and edge‐type, x=0.07). In this case, dislocation densities in each 〈110〉 direction are equal to within experimental error while an asymmetry in in‐plane strain is measured (18% and 30% for x=0.07, 300, and 580 nm thick, respectively). An unequal distribution of Burgers vectors of...

Reconstructions of GaN(0001) and (0001̄) surfaces: Ga-rich metallic structures

Reconstructions of GaN(0001) and (0001) surfaces are studied by scanning tunneling microscopy and spectroscopy, by electron diffraction, by Auger electron spectroscopy, and using first-principles theory. Attention is focused on Ga-rich reconstructions for each surface, which are found to have a metallic character involving significant overlap between Ga valence electrons. The electron counting rule is thus violated for these surfaces, but they nonetheless form minimum energy structures.

Inversion of wurtzite GaN(0001) by exposure to magnesium

Magnesium incorporation during the molecular-beam epitaxy growth of wurtzite GaN is found to invert the Ga-polar (0001) face to the N-polar face. The polarity is identified based on the two different sets of reconstructions seen on the film prior to and after about 1 monolayer Mg exposure. The inversion boundary is seen to lie on the (0001) plane from transmission electron microscopy images, and a structural model is presented for the inversion. On the Ga-polar face, Mg is also seen to stabilize growth in the N-rich regime.

Influence of misfit dislocations on the surface morphology of Si1−xGex films

The influence of misfit dislocations on the surface morphology of partially strain relaxed Si1−xGex films is studied by atomic force microscopy and transmission electron microscopy. Surface steps arising from the formation of single and multiple 60° dislocations are identified. The role of such steps in the development of a cross‐hatch pattern in surface morphology is discussed.

Field effect in epitaxial graphene on a silicon carbide substrate

The authors report a strong field effect observed at room temperature in epitaxially synthesized, as opposed to exfoliated, graphene. The graphene formed on the silicon face of a 4H silicon carbide substrate was photolithographically patterned into isolated active regions for the semimetal graphene-based transistors. Gold electrodes and a polymer dielectric were used in the top-gate transistors. The demonstration of a field effect mobility of 535cm2∕Vs was attributed to the transistor geometry that maximizes conductance modulation, although the mobility is lower than observed in exfoliated graphene possibly due to grain boundaries caused by the rough morphology of the substrate surface.

Scanning tunneling spectroscopy

Abstract The development of the field of spectroscopic measurement with the scanning tunneling microscope (STM) is discussed. A historical review of early experimental results in this field is presented, with emphasis on the techniques for data acquisition and interpretation. The applicability of STM spectroscopic measurement to surface structural determination is addressed. The role of geometric versus electronic contributions to STM images is discussed, with reference to studies of Si(111)7 × 7, Si(111)2 × 1, and Ge(111)c(2 × 8) surfaces. It is concluded that, for semiconductor surfaces, the observed corrugations are dominated by electronic effects. Issues of dynamic range in spectroscopic measurement, and interpretation of spectroscopic images, are examined.

Single-particle tunneling in doped graphene-insulator-graphene junctions

The characteristics of tunnel junctions formed between n- and p-doped graphene are investigated theoretically. The single-particle tunnel current that flows between the twodimensional electronic states of the graphene (2D-2D tunneling) is evaluated. At a voltage bias such that the Dirac points of the two electrodes are aligned, a large resonant current peak is produced. The magnitude and width of this peak are computed, and its use for devices is discussed. The influences of both rotational alignment of the graphene electrodes and structural perfection of the graphene are also discussed.