Cross-sectional Scanning Tunneling Microscopy Research Articles

Atomic Arrangement in a Triple-period A-type Ordered GaInP Layer Grown with Sb Addition during Metal Organic Vapor Phase Epitaxy Observed by Cross Sectional Scanning Tunneling Microscope

We observed the atomic arrangement of a triple-period (TP-) A-type ordered GaInP layer with Sb added during metalorganic vapor-phase epitaxy, by using an ultra-high-vacuum cross-sectional scanning tunneling microscope (STM). STM imaging of the ordered GaInP region revealed the arrangement of the TP-A-type ordered GaInP layer, and that the layer was composed of two InP-like planes and one GaP-like plane.

Direct Evidence for Shallow Acceptor States with Nonspherical Symmetry in GaAs

We investigate the energy and symmetry of Zn and Be dopant-induced acceptor states in GaAs using cross-sectional scanning tunnelling microscopy (STM) and spectroscopy at low temperatures. The ground and first excited states are found to have a nonspherical symmetry. In particular, the first excited acceptor state has a T(d) symmetry. Its major contribution to the STM empty-state images allows us to explain the puzzling triangular shaped contrast observed in the empty-state STM images of acceptor impurities in III-V semiconductors.

Nanometer-scale studies of point defect distributions in GaMnAs alloys

We have investigated the concentrations and distributions of point defects in GaMnAs alloys grown by low-temperature molecular-beam epitaxy, using ultrahigh-vacuum cross-sectional scanning tunneling microscopy (XSTM). High-resolution constant-current XSTM reveals “A,” “M,” and “V” defects, associated with AsGa, MnGa, and VAs, respectively. A and V defects are present in all low-temperature-grown layers, while M defects are predominantly located within the GaMnAs alloy layers. In the GaMnAs layers, the concentration of V defects ([V]) increases with the concentration of M defects ([M]), consistent with a Fermi-level-dependent vacancy formation energy. Furthermore, [M] is typically two to three times [A] and [V], suggesting significant compensation of the free carriers associated with MnGa. A quantitative defect pair correlation analysis reveals clustering of nearest V–V pairs and anti-clustering of nearest M–M, M–V, and M–A pairs. For all pair separations greater than 2nm, random distributions of defects are apparent.

Structure and intermixing of GaSb∕GaAs quantum dots

We present cross-sectional scanning tunneling microscopy results of GaSb quantum dots in GaAs, grown by metalorganic chemical vapor deposition. The size of the optically active quantum dots with base lengths of 4–8 nm and heights of about 2 nm is considerably smaller than previously published data obtained by other characterization methods. The local stoichiometry, obtained from atomically resolved images, shows a strong intermixing in the partly discontinuous wetting layer with an average GaSb content below 50%, while the GaSb content of the partly intermixed quantum dots is between 60% and 100%.

Capping process of InAs∕GaAs quantum dots studied by cross-sectional scanning tunneling microscopy

The capping process of self-assembled InAs quantum dots (QDs) grown on GaAs(100) substrates by molecular-beam epitaxy is studied by cross-sectional scanning tunneling microscopy. GaAs capping at 500°C causes leveling of the QDs which is completely suppressed by decreasing the growth temperature to 300°C. At elevated temperature the QD leveling is driven in the initial stage of the GaAs capping process while it is quenched during continued overgrowth when the QDs become buried. For common GaAs growth rates, both phenomena take place on a similar time scale. Therefore, the size and shape of buried InAs QDs are determined by a delicate interplay between driving and quenching of the QD leveling during capping which is controlled by the GaAs growth rate and growth temperature.

Formation and atomic structure of GaSb nanostructures in GaAs studied by cross-sectional scanning tunneling microscopy

GaSb nanostructures in GaAs, grown by metalorganic chemical vapor deposition, were studied with cross-sectional scanning tunneling microscopy. Three different samples were examined, containing a thin quantum well, a quantum well near the critical thickness for dot formation, and finally self-organized quantum dots with base lengths of 5–8 nm and heights of about 2 nm. The dots are intermixed with a GaSb content between 60% and 100%. Also small 3D and 2D islands were observed, possibly representing quantum dots in an early growth stage and quantum dot precursors. All GaSb layers exhibit gaps, which are indications of an island-like growth mode during epitaxy.

Formation of InAs quantum dots and wetting layers in GaAs and AlAs analyzed by cross-sectional scanning tunneling microscopy

We have used cross-sectional scanning-tunneling microscopy (X-STM) to compare the formation of self-assembled InAs quantum dots (QDs) and wetting layers on AlAs (1 0 0) and GaAs (1 0 0) surfaces. On AlAs we find a larger QD density and smaller QD size than for QDs grown on GaAs under the same growth conditions (500 °C substrate temperature and 1.9 ML indium deposition). The QDs grown on GaAs show both a normal and a lateral gradient in the indium distribution whereas the QDs grown on AlAs show only a normal gradient. The wetting layers on GaAs and AlAs do not show significant differences in their composition profiles. We suggest that the segregation of the wetting layer is mainly strain-driven, whereas the formation of the QDs is also determined by growth kinetics. We have determined the indium composition of the QDs by fitting it to the measured outward relaxation and lattice constant profile of the cleaved surface using a three-dimensional finite element calculation based on elasticity theory.

Mn diffusion in Ga1−xMnxAs∕GaAs superlattices

Ga 1 − x Mn x As ∕ GaAs superlattices with Mn concentrations of 1% and 5% in the Ga1−xMnxAs layers and a GaAs spacer thickness of 4 and 60 GaAs monolayers have been studied by cross-sectional scanning tunneling microscopy. By achieving atomic resolution of the superlattices, we observe individual Mn atoms in the Ga1−xMnxAs layers and in the GaAs spacer. We find that about 20% of the total amount of Mn diffuses from the GaMnAs layers into the GaAs spacer layers. Our results can be related to previous measurements of the magnetic properties of short period Ga1−xMnxAs∕GaAs superlattices.

Stacked InAs quantum dots in InP studied by cross-sectional scanning tunnellingmicroscopy

We report on cross-sectional scanning tunnelling microscopy (XSTM) measurements onvertically stacked InAs quantum dots in InP barriers. We have investigated two-, five- andtenfold stacked quantum dot structures grown by low-pressure metal–organic vapour phaseepitaxy. The XSTM images reveal that the quantum dots are generally verticallywell aligned, and have a truncated pyramidal shape in agreement with similarstudies of InAs dots in GaAs. STM images displaying atomic resolution indicatethat the dots have a pure InAs stoichiometry, with intermixing only occurring inthe top and bottom dot rows. Further, we have investigated various anomalies(considered as defects) as observed in the quantum dot stacks. The origins of theseanomalies are discussed and compared to theoretical predictions available so far.

Nanovoids in InGaAs∕GaAs quantum dots observed by cross-sectional scanning tunneling microscopy

We present cross-sectional scanning tunneling microscopy data of a type of InGaAs∕GaAs quantum-dot structure characterized by a hollow center. This void structure develops during a long growth interruption applied after deposition of a quantum dot layer and a thin cap layer, resulting in an eruption of indium-rich material. Subsequent fast overgrowth does not fill the void completely. This growth behavior demonstrates limitations of current strategies to grow large quantum dots.

Cross-sectional scanning tunneling microscopy observation of atomic arrangement in triple period-A type ordered AlInAs alloy

Atomic arrangements in a triple-period (TP)-A type ordered AlInAs layer were investigated by a cross-sectional scanning tunneling microscope (XSTM) for the first time. The distributions of cation atoms in the ordered layer were distinguished using the tunneling spectroscopy method. The XSTM image on the cleaved surface of the ordered AlInAs layer revealed the presence of ragged short-range ordering domains comprising periodic structures aligned along the [ 1 1 2 ¯ ] direction on the whole surface, separated by three-fold periodicity along the [1 1 1] direction. This three-fold periodic structure comprises units of In–In–Al and/or In–Al–Al. That is, the TP-A type ordered AlInAs layer comprises two kinds of three-fold periodic planes; one is composed of two InAs-like planes and one AlAs-like plane, and the other is composed of one InAs-like plane and two AlAs-like planes. In addition, a local two-fold ordered structure having alternating InAs-like and AlAs-like planes can be observed in the STM image.

Formation of columnar (In,Ga)As quantum dots on GaAs(100)

Columnar (In,Ga)As quantum dots (QDs) with homogeneous composition and shape in the growth direction are realized by molecular-beam epitaxy on GaAs(100) substrates. The columnar (In,Ga)As QDs are formed on InAs seed QDs by alternating deposition of thin GaAs intermediate layers and monolayers of InAs with extended growth interruptions after each layer. The height of the columnar (In,Ga)As QDs is controlled by varying the number of stacked GaAs∕InAs layers. The structural and optical properties are studied by cross-sectional scanning tunneling microscopy, atomic force microscopy, and photoluminescence spectroscopy. With increase of the aspect ratio of the columnar QDs, the emission wavelength is redshifted and the linewidth is reduced.

Arsenic cross-contamination in GaSb/InAs superlattices

We have investigated the cross-contamination of As in GaSb/InAs superlattices. We demonstrate a method of varying the lattice constant of the superlattice. By controlling the As background pressure in the growth chamber, the strain can be controlled to about 0.01%, corresponding to As cross-incorporation variations of about ±1%. The distribution of As is investigated by X-ray diffraction and cross-sectional scanning tunneling microscopy, and the critical thickness is obtained.

Defect structure ofGa1−xMnxAs: A cross-sectional scanning tunneling microscopy study

We have studied the atomic scale structure of molecular beam epitaxy grown Ga1-xMnxAs compounds with various Mn concentrations by cross-sectional scanning tunneling microscopy and first principles calculations. Only bright protrusions close to the top layer As atoms are directly correlated with the bulk Mn concentration. Atomically resolved filled and empty states images of this defect are compared to images derived from first principles calculations. We identify the Mn related defect as substitutional Mn in the second layer Ga site. Surprisingly no substitutional Mn is observed in the top-most Ga layer. The experimental results are consistent with the energetics of our first principles total energy calculations.

CROSS-SECTIONAL SCANNING TUNNELING MICROSCOPY OF BURIED HETEROSTRUCTURE LASERS

A single-mode buried heterostructure laser has been imaged using Cross-Sectional Scanning Tunneling Microscopy (X-STM). The problem of positioning the tip on the restricted active region on the (110) face has been overcome using combined Scanning Electron Microscopy (SEM). In order to understand the change in the STM scans when biased, particularly the physical change in surface step defects caused by commercial sample preparation, the experimental setup has been modified to allow the sample to be biased. A simpler double quantum well test structure has been biased and it has been demonstrated that it is possible to continue performing STM whilst the device is powered. The change in the relative contrast across the image has been shown to be unaffected by this external bias for the range scanned, as predicted by a fully-coupled Poison drift–diffusion model calculated using Fermi–Dirac statistics.

STM/STS investigation of the interaction of Si with atomic scale vacancies on cleaved GaAs

Cross-sectional scanning tunnelling microscopy (STM) images on a freshly cleaved III–V semiconductor laser facet regularly indicate atomic scale defects, such as steps and vacancies on the non-polar (1 1 0) surface. In this work, studies were made when sub-monolayer quantities of Si were deposited at 280 °C on clean cleaved GaAs(1 1 0). Selective adsorption of Si on atomic defects has been observed with STM imaging of the surface. The simultaneous use of STM and scanning tunnelling spectroscopy (STS) allows the quantification of the Fermi shifts present at atomic steps and vacancies before and after Si adsorption. Localised current–voltage measurements, performed using STS, on the atomic steps and vacancy defect clusters of p-type GaAs(1 1 0) surfaces showed Fermi-level shifts of ∼0.8 and 0.7 eV towards mid-gap position, respectively. However, measurements taken from silicon-coated atomic steps and defect clusters showed that the Fermi-level reverted back towards the ‘ideal’ flat band position by ∼0.4 eV. This behaviour was distinctively different to silicon adsorbed at defect-free surface of GaAs(1 1 0). These results indicated the passivating potential of silicon on the defect sites of GaAs(1 1 0) surface when deposited under these conditions. The ability to measure and control surface properties on the atomic scale is crucial to the success of nano-technology and the STM/STS studies demonstrate the effectiveness of the technique for this role.

Impurity-induced disordering in AlGaInP superlattices studied using cross-sectional scanning tunneling microscopy

The effect of doping an (Al0.3Ga0.7)0.5In0.5P/Al0.5In0.5P superlattice was investigated using cross-sectional scanning tunneling microscopy (XSTM). Two superlattices of different doping concentration were studied; namely, a superlattice where both well and barriers were doped, and a superlattice with only the barriers doped. Interdiffusion and impurity-induced layer disordering at the well and barrier interfaces is reported for both superlattice structures. However, the extent of interdiffusion across the interfaces, when only the barrier was doped, was measured to be at least 2.5 times more than the superlattice with both well and barriers doped. The considerable extent of Al–Ga interdiffusion in the barrier doped superlattice is thought to be caused by Zn diffusion at the (AlxGa1−x)0.5In0.5P heterostructures during Zn activation at 690 °C. This activation process encourages the segregation of the acceptor Zn out from the AlInP barrier layers and into AlGaInP well layers. In addition, this process is also known to enhance Al and Ga diffusion rates. XSTM images also show alloy clustering at both differently doped superlattices. Interdiffusion, alloy clustering, and interface roughness observed at the AlGaInP superlattices would have a detrimental effect on the suitability of a multiquantum barrier structure, based on this material system, to improve quantum confinement in red laser devices.

Comparison of strain field in cleaved XSTM specimen with un-cleaved InAs/GaAs quantum dot nanostructure

Abstract A three-dimensional finite element analysis has been carried out to investigate the strain field at the surface of cleaved cross-sectional scanning tunnelling microscopy (XSTM) specimen of InAs quantum dots embedded in GaAs matrix. As a comparison standard, the strain field in un-cleaved state has also been analysed. As compared with the bulk state, the tensile ezz strain component increases significantly at the cleaved surface, especially at the central region of the dot, where the experimental determination of the strain is usually performed. On the other hand, magnitudes of the compressive exx and eyy components decrease with the cleavage of specimen. Development of shear strain, ezx, with the specimen cleavage is most apparent at the base and top surface of the truncated dot. Therefore, care has to be taken when referring to experimentally determined strain information based on XSTM specimen, since the artefact due to the cleavage of the specimen is involved in the experimental data.

Atomic-scale observation of interfacial roughness and As–P exchange in InGaAs/InP multiple quantum wells

Cross-sectional scanning tunneling microscopy (XSTM) has been used to study interfacial properties of InP-on-InGaAs interfaces in InGaAs/InP multiple quantum wells grown by metalorganic vapor phase epitaxy with a growth interruption. XSTM has enabled us to separately identify step-like roughness and distributions of As atoms incorporated in the InP layer near the interface. The As composition profile along the growth direction analyzed from distributions of As atoms in XSTM images shows an exponential variation with distance from the InP-on-InGaAs interface. It is found that the growth interruption of 30 s reduces considerably the roughness amplitude to 0.45 nm from 1.1 nm and increases the coherent length from 22 to 27 nm.

Migration-enhanced epitaxy (MEE) growth of five-layer asymmetric coupled quantum well (FACQW) and its cross-sectional STM observation

The five-layer asymmetric coupled quantum well (FACQW) is a new potential-tailored quantum well that is promising for ultra-fast and ultra-low-voltage optical modulators and switches. FACQW samples were grown by the migration-enhanced epitaxy (MEE) and the conventional molecular beam epitaxy methods with steep and flat heterointerfaces in the monolayer accuracy. They were characterized with the cross-sectional scanning tunneling microscopy (STM). In the cross-sectional STM image, double-stripe structures with different contrast were observed. The stripe area corresponds to the FACQW (about 10 nm wide), sandwiched with the AlGaAs barrier layers ( 15 nm wide). A dark line observed at the middle of the FACQW stripe area corresponds to the 3-monolayer-thick AlAs layer. The cross-sectional STM images of the high-quality heterointerface FACQW structures were successfully observed for the samples grown by the MEE method. More detailed studies of this kind of cross-sectional STM observations will be very effective to obtain the optimized growth conditions for fine and complicated ultra-thin structures.