K. Pond

Atom‐resolved imaging and spectroscopy on the GaAs(001) surface using tunneling microscopy

Scanning tunneling microscopy and scanning tunneling spectroscopy have been used to investigate the structure and current–voltage [I(V)] characteristics of the molecular‐beam epitaxially grown, As‐rich GaAs(001)‐(2×4) surface. High‐resolution images reveal a modulation in the topography of the individual arsenic dimers measured with the tunneling microscope in the constant current mode. The observed features are attributed to an increased tunneling probability out of the occupied electronic lone pair states of the As dimers. The I(V) spectroscopy performed on the (001) surface of low doped n‐type GaAs samples differs considerably from the results obtained on the (110) surface of this semiconductor. This is attributed to band bending that is due to a lower doping concentration below the surface. The electrostatics involved in imaging with a tunneling microscope are described in a simple model, based on the depletion approximation, that accounts for the experimental results.

Tunneling spectroscopy on the GaAs(110) surface: Effect of dopant concentration

Abstract Scanning tunneling spectroscopy has been used to investigate the effect of doping concentration on the current-voltage characteristics of the GaAs(110) surface. For a fixed tip-sample separation, the conductivity gap is found to increase as the doping concentration is reduced. The results are compared with the predictions of a one-dimensional planar tunneling model which takes the tip-induced band-bending into account. Good agreement between the experiment and the calculations is achieved in the high-doping regime. The disagreement at lower doping levels suggests the absence of a complete equilibrium between the minority carriers at the surface and the majority carriers in the bulk, as well as the importance of dimensionality in describing the tip-sample interaction.

Step bunching and step equalization on vicinal GaAs(001) surfaces

Terrace width distributions have been calculated from scanning tunneling microscopy (STM) images of molecular‐beam epitaxy (MBE)‐grown GaAs(001) surfaces misoriented by both 1° and 2° towards the (111)A direction. This analysis reveals a peak in the terrace width distribution at approximately 40–50 A, regardless of the original miscut, with larger terraces forming in order to preserve the angle of vicinality. Growth of a tilted superlattice (TSL) improves the periodicity of the surface. A statistical analysis of the STM image of a 1° TSL capped with three monolayers of GaAs reveals a bell‐shaped distribution of terrace widths with a peak at the average terrace width. These results suggest that MBE growth of vicinal GaAs(001) does not result in equalized steps but that the growth of a TSL does tend towards step equalization. The differences between these two growth regimes are discussed.

Scanning tunneling microscopy of the filled and empty arsenic states on the GaAs(001)-(2×4) surface

Scanning tunneling microscopy and current spectroscopy have been used to investigate the p-type arsenic-terminated GaAs(001)-(2 × 4) surface. Images were collected at both positive and negative voltages applied to the sample with respect to the tip. The main features in the images taken at positive sample bias do not differ from those taken at negative bias. This is in contrast to the scanning tunneling microscopy results on the GaAs(110) surface, where, at negative and positive sample bias, the As and Ga atomic orbitals are imaged, respectively, and are, thus, complementary to each other. The different behavior observed on the GaAs(001) surface is attributed to the fact that, on this surface, the Ga atoms are in the second layer. Hence, at positive sample bias tunneling occurs predominantly into the empty As molecular orbital. This conjecture is supported by a calculation of the tunneling current spectrum that contains the contributions of tunneling into the valence band as well as into the conduction band.

Scanning tunneling microscopy of flat and vicinal molecular‐beam epitaxy grown GaAs(001)‐(2×4) surfaces: The effect of growth rate

Scanning tunneling microscopy (STM) has been used to investigate the effect of the deposition rate on the resulting morphology of nominally flat and vicinal GaAs(001)‐(2×4) surfaces grown by molecular‐beam epitaxy. On the nominally flat surfaces, low deposition rates are found to create smooth surfaces and lead to anisotropic islanding with an A‐type step (Ga terminated, parallel to the [110] direction) to B‐type step (As terminated, parallel to the [110] direction) average aspect ratio which is larger than that produced by standard deposition rates. The results suggest that the increase in the island anisotropy at low growth rates reflects the energetics of the step edges. In contrast, the growth rates investigated are found not to have any obvious effect on the resultant morphology of the 2° A‐type vicinal surfaces. In particular, no islanding on the terraces is observed at either deposition rate. However, detailed statistical analyses of the STM images indicate that there is a larger spread in the ter...

Role of steps in epitaxial growth

Abstract The role of steps in the epitaxial growth of quantum structures is discussed. We present experimental results and theoretical predictions of growth on stepped surfaces. Scanning tunneling microscopy (STM) images of molecular beam epitaxy grown GaAs(001) surfaces misoriented by 1° and 2° towards the (111) A direction show non-uniform terraces with a peak in the terrace width distribution at 40 A. Simple models of atoms landing on a step and attaching at the ascending step edge, however, predict an equalization of terrace widths. A thermodynamic model which allows the steps to move freely with the constraint that it costs energy to form a kink predicts step bunching for high kink energies. Steps on vicinal surfaces have been utilized for growing quantum wire structures using a technique where fractional monolayers of different materials are deposited on a stepped surface, leading to the creation of a lateral superlattice (LSL). The terrace width uniformity is observed by STM to improve dramatically with the growth of an AlAsGaAs LSL. Cross-sectional transmission electron microscopy of LSLs shows good segregation of the composite layers.

Effect of growth rate on the surface morphology of MBE-grown GaAs(001)-(2×4)

Abstract Scanning tunneling microscopy (STM) has been used to investigate the effect of the deposition rate on the resulting morphology of GaAs(001)-(2 × 4) surfaces grown by molecular beam epitaxy. Low deposition rates (0.2 microm/h) are found to create smooth surfaces and lead to anisotropic islanding with average length ratios of A steps to B steps which are measurably larger than those produced by standard deposition rates (0.7 microm/h). For each growth rate, the mean-square-height fluctuation function has been calculated in order to characterize the surface roughness observed with STM and the anisotropy associated with the roughness.

A two-dimensional ultrahigh vacuum positioner for scanning tunneling microscopy

A two-dimensional ultrahigh vacuum compatible positioner is presented. The positioner uses two piezoelectric inchworms which allow for motions of up to 1 cm with a precision of 4 nm mounted at right angles to each other in order to give two dimensions of motion. Images of three-dimensional In0.3Ga0.7As islands in cross section are presented to demonstrate the functionality of the positioner. It is found that motion towards the tip is smooth, while motion in the perpendicular direction is less smooth.

Step-edge energetics of the Ge/GaAs(001)-(1 × 2) superstructure

Abstract For the first time the step-edge energies have been measured for a superstructure resulting from nonpolar/polar heteroepitaxy. Using scanning tunneling microscopy, we have investigated the morphology of the Ge/GaAs(001)-(1 × 2) reconstructed surface using substrates miscut by 1° and 2° toward the [110] direction to expose the A-type steps. The superstructure has well-defined step edges and smooth terraces, even when the initial substrate is rough. The primary defect structures are antiphase domain boundaries remaining from the formation of the (1 × 2) reconstruction. The kink energies are similar to those for the A-type steps on both the GaAs(001)-(2 × 4) and Si(100)-(2 × 1) surfaces. In addition, there is an apparent kink-kink attraction of about 120 meV between nearest-neighbor kinks. Using the model incorporating nearest-neighbor interactions between kinks developed for the Ge/GaAs(001)-(1 × 2) superstructure, the kink-kink repulsion observed on GaAs(001)-(2 × 4) is discussed quantitatively and shown to involve energies significantly higher than those which could be reasonably attributed to dipole-dipole interactions.

A scanning tunneling microscopy study of low-temperature grown GaAs

Scanning tunneling microscopy (STM) has been used to investigate the effect of low-temperature (LT) growth of GaAs by molecular beam epitaxy on the morphology of the resulting surface. We present STM images of a GaAs(001) surface that was grown at ∼300°C and subsequently annealed at 600°C and show that there is a recovery of the (2×4) reconstruction. We also report images of a surface grown on top of a buried LT GaAs layer and show that the LT layer has little effect on the resulting surface morphology. In addition, scanning tunneling spectroscopy spectra are presented which demonstrate that the current-voltage characteristics of annealed and unannealed LT grown GaAs are significantly different.