Matthew Bruce Johnson

Indium Distribution in Ingaas Quantum Wires Observed with the Scanning Tunneling Microscope

The incorporation of In in the growth of crescent‐shaped In0.12Ga0.88As quantum wires embedded in (AlAs)4(GaAs)8 superlattice barriers is studied in atomic detail using cross‐sectional scanning tunneling microscopy. It is found that the In distribution in both the surface and the first subsurface layer can be atomically resolved in the empty‐ and filled‐state images, respectively. Strong In segregation is seen at the InGaAs/GaAs interfaces, but neither an expected enhancement of the In concentration at the center of the quantum wire compared to the planar quantum well nor In clustering beyond the statistical expectation is observed.

Atomic structure and luminescence excitation of GaAs/(AlAs)n(GaAs)m quantum wires with the scanning tunneling microscope

We report on the imaging of a molecular beam epitaxially grown GaAs/(AlAs)n(GaAs)m quantum well‐wire array by means of cross‐sectional scanning tunneling microscopy (XSTM) and scanning tunneling‐induced luminescence (STL). XSTM provides atomically resolved cross‐sectional images of sets of quantum well wires with chemical sensitivity within the group III species and electrical sensitivity to single dopant atoms. This permits the precise observation of growth mechanisms and the identification of defects responsible for inhomogeneities in the growth morphology, as well as the determination of dopant incorporation throughout the structure. STL permits the relative quantum efficiency of individual quantum wires to be quantified.

Semiconductor characterization with the scanning surface harmonic microscope

The scanning surface harmonic microscope, in which a microwave signal is applied across a tip‐sample tunneling gap and higher harmonics are detected, is sensitive to the capacitance/voltage characteristics of semiconductor samples on a nanometer scale. We demonstrate its sensitivity to a wide range of dopant concentrations on Si, and its applications as a dopant profiler. Depletion regions are delineated with remarkable sensitivity, and variations in dopant concentration over a 35‐nm scale are discussed. Indications of a 5 nm resolution have been obtained.

Surface and subsurface imaging of indium in InGaAs by scanning tunneling microscopy

Abstract Investigating the ternary InxGa1−xAs alloy (x ∼ 12%) by cross-sectional scanning tunneling microscopy, we find that on the UHV-cleaved (110) surface the In distribution in both the surface and the first subsurface layer can be atomically resolved in the empty- and filled-state images, respectively. This is found to be mostly a geometric effect due to the larger size of the In. We apply this method to study the incorporation of In during the growth of In0.12Ga0.88As quantum wires on nonplanar substrates. Strong In segregation in the growth direction is seen in the structure, and we compare the incorporation profiles across the quantum wire and a planar quantum well. No In clustering beyond the statistical expectation is observed.

Doping profiling with scanning surface harmonic microscopy

The scanning surface harmonic microscope in which a microwave signal is applied across a tip-sample gap is sensitive to the capacitance/voltage characteristics of semiconductor samples. Its ability to distinguish among a wide range of dopant concentrations with high lateral resolution allow applications as a dopant profiler. Lateral dimensions of electronic features and the ultimate resolution limit of this technique are discussed using atomic resolution data and sizes of domain boundaries on highly oriented pyrolytic graphite and using depletion regions on doping gratings similar to real device structures. The technological applicability of the instrument has been tested on polished surfaces of trench capacitor arrays.

Atomic-scale analysis of quantum nanostructures with the STM

Important features of semiconductor quantum structures can be observed by cross-sectional scanning tunneling microscopy down to the atomic scale. The MBE-grown III-V multilayers are cleaved in UHV to expose an atomically flat cross-sectional plane, which is then imaged with the STM. Chemically selective imaging enables us to distinguish between atoms within the group III and analyze the AlGaAs composition in atomic detail: we find Al clustering on the scale of a few nanometers in many MBE-grown materials. The heterojunction interfaces and their roughness can be analyzed down to the unit-cell lattice period. The electronic signature of active dopants such as Be is observed and quantified. Recent work shows that all these possibilities can be used to study the MBE growth of AlAs/GaAs superlattices on V-grooved substrates with unprecedented detail.