P Peter Offermans

Gas Detection with Vertical InAs Nanowire Arrays

Nanowire-based devices show great promise for next generation (bio)chemical sensors as evidenced by the large volume of high-quality publications. Here, a nanoscale gas sensing device is presented, based on gold-free grown vertical InAs nanowire arrays. The nanowires are contacted Ohmically in their as-grown locations using an air bridge construction, leaving the nanowire surface free for gas adsorption. Noise measurements were performed to determine the measurement resolution for gas detection. These devices are sensitive to NO(2) concentrations well below 100 ppb at room temperature. NO(2) exposure leads to both a reduction in carrier density and electron mobility.

Atomic-scale structure of self-assembled In(Ga)As quantum rings in GaAs

We present an atomic-scale analysis of the indium distribution of self-assembled In(Ga)As quantum rings (QRs) which are formed from InAs quantum dots by capping with a thin layer of GaAs and subsequent annealing. We find that the size and shape of QRs as observed by cross-sectional scanning tunneling microscopy (X-STM) deviate substantially from the ring-shaped islands as observed by atomic force microscopy on the surface of uncapped QR structures. We show unambiguously that X-STM images the remaining quantum dot material whereas the AFM images the erupted quantum dot material. The remaining dot material shows an asymmetric indium-rich crater-like shape with a depression rather than an opening at the center and is responsible for the observed electronic properties of QR structures. These quantum craters have an indium concentration of about 55% and a diameter of about 20nm which is consistent with the observed electronic radius of QR structures.

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.

Active Control of the Strong Coupling Regime between Porphyrin Excitons and Surface Plasmon Polaritons

We experimentally demonstrate the active control of the coupling strength between porphyrin dyes and surface plasmon polaritons supported by a thin gold layer. This control is externally exerted by a gas flow and is reversible. The hybridized exciton-polariton branches resulting from the exciton-plasmon coupling display a splitting proportional to the coupling strength of the light-matter interaction. The coupled system changes from the weak (no splitting) to the strong coupling regime (splitting of 130 meV) by controlling the effective oscillator strength in the dye layer, via exposure to nitrogen dioxide. The modification of the coupling strength of the system allows tailoring of the dispersion of the hybridized modes as well as of their group velocity.

Formation of InAs wetting layers studied by cross-sectional scanning tunneling microscopy

We show that the composition of (segregated) InAs wetting layers (WLs) can be determined by either direct counting of the indium atoms or by analysis of the outward displacement of the cleaved surface as measured by cross-sectional scanning tunneling microscopy. We use this approach to study the effects of the deposited amount of indium, the InAs growth rate, and the host material on the formation of the WLs. We conclude that the formation of (segregated) WLs is a delicate interplay between surface migration, strain-driven segregation, and the dissolution of quantum dots during overgrowth.

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.

Digital alloy interface grading of an InAlAs/InGaAs quantum cascade laser structure studied by cross-sectional scanning tunneling microscopy

We have studied an InGaAs/InAlAs quantum cascade laser structure with cross-sectional scanning tunneling microscopy. In the quantum cascade laser structure digital alloy grading was used to soften the barriers of the active region. We show that due to alloy fluctuations, softening of the barriers occurs even without the digital grading.

High sensitive quasi freestanding epitaxial graphene gas sensor on 6H-SiC

We have measured the electrical response to NO2, N2, NH3, and CO for epitaxial graphene and quasi freestanding epitaxial graphene on 6H-SiC substrates. Quasi freestanding epitaxial graphene shows a 6 fold increase in NO2 sensitivity compared to epitaxial graphene. Both samples show a sensitivity better than the experimentally limited 1 ppb. The strong increase in sensitivity of quasi freestanding epitaxial graphene can be explained by a Fermi-energy close to the Dirac point, leading to a strongly surface doping dependent sample resistance. Both sensors show a negligible sensitivity to N2, NH3, and CO.

(In,Ga)As sidewall quantum wires on shallow-patterned InP (311)A

(In,Ga)As sidewall quantum wires (QWires) are realized by chemical beam epitaxy along [01-1] mesa stripes on shallow-patterned InP (311)A substrates. The QWires exhibit strong lateral carrier confinement due to larger thickness and In composition compared to the adjacent quantum wells, as determined by cross-sectional scanning-tunneling microscopy and microphotoluminescence (micro-PL) spectroscopy. The PL of the (In,Ga)As QWires with InP and quaternary (Ga,In)(As,P) barriers reveals narrow linewidth, high efficiency, and large lateral carrier confinement energies of 60–70meV. The QWires are stacked in growth direction with identical PL peak emission energy. The PL emission energy is not only controlled by the (In,Ga)As layer thickness but also by the patterned mesa height. Stacked (In,Ga)As QWires with quaternary barriers exhibit room temperature PL emission at 1.55μm in the technologically important wavelength region for telecommunication applications.

InAs Quantum Dot Formation Studied at the Atomic Scale by Cross-sectional Scanning Tunnelling Microscopy

Publisher Summary This chapter reviews that self-assembled quantum dots (QDs) have attracted much attention in the earlier years. These nanostructures are very interesting from a scientific point of view because they form nearly ideal zero-dimensional systems in which quantum confinement effects become very important. These unique properties also make them very interesting from a technological point of view. It explains that X-STM is used to study at the atomic scale the formation of self-assembled QDs. This technique allows determining the size, shape, and composition of the buried nanostructures and provides vital information to understand the QD formation process. The chapter also discusses the capping process, which is found to be a critical step in QD formation because it strongly modifies the QD structure. In its initial stage, GaAs capping induces leveling of the QDs to drastically decrease their height. During continuous capping, the QD leveling is quenched when the QDs become buried.