V. Bressler-Hill

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.

Characterization of MOCVD-grown InP on InGaPGaAs(001)

Abstract Atomic force microscopy and transmission electron microscopy have been used to investigate the metalorganic chemical vapor deposition of InP on nominally flat InGaP GaAs (001) substrates. From a detailed structural analysis of the deposited InP, we observe coherent Stranski-Krastanov growth, i.e., after the initial formation of a two-dimensional layer, two types of coherently strained islands appear above a defined thickness. At higher coverages a third type of larger, incoherent and defected island is observed. The two types of coherently strained islands can be characterized by their distinct sizes. For low coverages the coherently strained islands possess irregular dimensions in the plane of the substrate and have a maximum height of 6 nm. At higher coverages the second type of coherently strained islands appears. These are cap-shaped and have a rather narrow distribution of base diameter (120 ± 10 nm) and height (22.5 ± 2.5 nm). An appreciable density of incoherent and defected relaxed islands also becomes evident at increased coverages. These have a hexagonal shape exposing clear facets and are elongated along the [110] crystallographic direction of the substrate.

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.

InP islands on InGaP/GaAs(001): island separation distributions

Abstract Heteroepitaxy of InP on InGaP/GaAs(001) has been studied using atomic force microscopy (AFM) as a function of substrate temperature and misorientation. Stranski-Krastanov growth of InP on GaAs results in three types and sizes of islands. The smallest and medium-sized islands are coherently strained to the substrate, whereas dislocations are seen in the largest islands. Growth, morphology and island separation distributions have been studied for the small and medium islands. Scaling in the radial distribution function indicates a uniformly random growth of medium-sized InP dots on all the substrates. However, on substrates grown at 650°C and misoriented by 2°, clustering of islands is also observed. We suggest that is due to the preferential growth of mediumsized islands at the step edges of the substrate. Separation distributions along and perpendicular to the step edges of this surface reveal that the step edges are decorated by clusters of 6–8 islands having an average period of 2 μm between them. These clusters exhibit long-range ordering and have a potential for device applications. Radial separation distribution studies indicate that the substrate morphology and topography are crucial for achieving a desirable distribution of islands. Moreover, the small InP islands and the dislocated islands are uniformly distributed on all the substrates irrespective of the growth temperature or misorientation, suggesting a different nucleation mechanism than for the medium-sized InP islands.

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.

Surface scienceCharacterization of MOCVD-grown InP on InGaPGaAs(001)

Abstract Atomic force microscopy and transmission electron microscopy have been used to investigate the metalorganic chemical vapor deposition of InP on nominally flat InGaP GaAs (001) substrates. From a detailed structural analysis of the deposited InP, we observe coherent Stranski-Krastanov growth, i.e., after the initial formation of a two-dimensional layer, two types of coherently strained islands appear above a defined thickness. At higher coverages a third type of larger, incoherent and defected island is observed. The two types of coherently strained islands can be characterized by their distinct sizes. For low coverages the coherently strained islands possess irregular dimensions in the plane of the substrate and have a maximum height of 6 nm. At higher coverages the second type of coherently strained islands appears. These are cap-shaped and have a rather narrow distribution of base diameter (120 ± 10 nm) and height (22.5 ± 2.5 nm). An appreciable density of incoherent and defected relaxed islands also becomes evident at increased coverages. These have a hexagonal shape exposing clear facets and are elongated along the [110] crystallographic direction of the substrate.

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...

Effects of deposition rate on the size of self-assembled InP islands formed on GaInP/GaAs(100) surfaces

Utilizing the Stranski-Krastanov growth mode, three-dimensional InP islands are formed on a GalnP/GaAs surface using metalorganic chemical vapor deposition. The islands have been investigated with atomic force microscopy, and the effect of the deposition rate on the shape of the islands has been quantified. The height of the islands varies with deposition rate, whereas the basediameters are nearly constant around 1260 ± 35Å. The island height is 290 ± 12 Å at a high (2.6 ML/s) deposition rate and decreases to approximately 250 ± 16 Å for low (0.1 ML/ s) and moderate (0.8 ML/s) deposition rates.

Performance of an ultrahigh-vacuum sample transfer system for investigation of molecular-beam epitaxy grown semiconductor surfaces

This article describes the performance of a sample transfer system which allows direct coupling between a molecular‐beam epitaxy (MBE) chamber and several ultrahigh vacuum surface characterization chambers. The transfer system is simple, compact, and easily adaptable to any MBE or surface analysis chamber. By using this transfer system, high quality III–V compound semiconductor samples are routinely achieved, as judged by the scanning tunneling microscopy images obtained on these surfaces.