Dan Hessman

Monolithic GaAs/InGaP nanowire light emitting diodes on silicon

Vertical light emitting diodes (LEDs) based on GaAs/InGaP core/shell nanowires, epitaxially grown on GaP and Si substrates, have been fabricated. The devices can be fabricated over large areas and can be precisely positioned on the substrates, by the use of standard lithography techniques, enabling applications such as on-chip optical communication. LED functionality was established on both kinds of substrate, and the devices were evaluated in terms of temperature-dependent photoluminescence and electroluminescence.

Assembling strained InAs islands on patterned GaAs substrates with chemical beam epitaxy

The assembly of strained InAs islands was manipulated through growth on patterned GaAs substrates with chemical beam epitaxy. Conditions were found to selectively place the islands in patterns features but not on surrounding unpatterned fields. Chains of islands having 33 nm minimum periods were formed in trenches, and single or few islands were grown in arrays of holes. When capped with GaAs, the islands behave as quantum dots and are optically active.

Excited states of individual quantum dots studied by photoluminescence spectroscopy

The photoluminescence from individual InP quantum dots embedded in a matrix of GaInP has been studied. In addition to the ground state emission that consists of several peaks, we observe excited states of the dot. These states are observed either via state filling or with photoluminescence excitation spectroscopy. We observe a fast relaxation to the set of states with lowest energy but no relaxation between these states.

Fifteen-piconewton force detection from neural growth cones using nanowire arrays.

We used epitaxially grown monodisperse nanowire arrays to measure cellular forces with a spatial resolution of 1 mum. Nerve cells were cultured on the array and cellular forces were calculated from the displacement of the nanowire tips. The measurements were done in situ on live cells using confocal microscopy. Forces down to 15 pN were measured on neural growth cones, showing that this method can be used to study the fine details of growth-cone dynamics.

Measurements of the band gap of wurtzite InAs1−xPx nanowires using photocurrent spectroscopy

We report measurements of the band gap of InAs1−xPx nanowires having wurtzite crystal structure as a function of the composition for 0.14<x<0.48. The band gap is measured by photocurrent spectroscopy on single InAs nanowires with a centrally placed InAs1−xPx segment. The photocurrent measurements are performed at a temperature of 5 K. The data fit well with a quadratic dependence of the band gap on the composition. Using a bowing parameter of 0.2 eV the extracted values for the band gaps are 0.54 eV for InAs and 1.65 eV for InP. These values are larger than the corresponding zinc blende band gaps. We attribute this increase to the fact that the crystal structure is wurtzite rather than zinc blende.

Formation of interface layers in GaxIn1−xAs/InP heterostructures: A re‐evaluation using ultrathin quantum wells as a probe

We have studied the metalorganic vapor phase epitaxy growth of ultrathin GaInAs/InP and GaAs/InP quantum well (QW) structures using photoluminescence (PL) spectra as a probe for interface effects. In parallel we have also studied InAsxP1−x ‘‘interface QWs’’ formed by simply exposing InP to AsH3. We see a correlation between QW properties (PL peak position, effective thickness, PL half‐width) and the surface phase during the growth of the QW material. For GaAs QWs grown under conditions where typically the As‐excess c(4×4)/d(4×4)‐ or (1×2)‐like (with As double layers) surface reconstructions, we find a strong red‐shift of the PL peak positions. The red‐shift becomes smaller the closer the growth conditions come toward the border to the (2×4) reconstruction (with only one As‐termination layer). We thus conclude that the surface itself is one source for As carryover. For GaInAs QWs a boundary between an As‐excess/no As‐excess surface reconstruction seems to exist at higher AsH3/lower T values. Near to this b...

Band filling at low optical power density in semiconductor dots

We have studied band filling in strained InP dots grown on GaInP. Compared to quantum wells, the dots show band filling at two orders of magnitude lower optical excitation power density. We show that the emission attributed to band filling originates from the dots by using spatially resolved photoluminescence recording emission from single dots with very high spectral resolution. With time‐resolved photoluminescence spectra we follow the dynamic relaxation of the charge carriers in the dots.

Optical determination of Young's modulus of InAs nanowires

We present a study of Young’s modulus of epitaxially grown InAs nanowires with diameters from 40 to 95 nm. The dynamic behavior of the nanowires is investigated using optical stroboscopic imaging. The Young’s modulus, evaluated using the eigenfrequencies of the fundamental and the first excited modes in air, decreases for smaller diameters. To avoid the influence of the electric field on the resonance frequency, we use the free ring-down response to a voltage step rather than driving with a harmonic voltage.

Nano-Optical Studies of Individual Nanostructures

Optical techniques play a significant role in studies of nano-structures. The electronic structures of quantum dots, for example, vary with the geometric sizes in an ensemble, resulting in broadened spectral lines. Recently, different forms of local spectroscopic techniques have been applied to investigate such inhomogeneously broadened emission lines. In this paper we report on three methods for local spectroscopy : cathodo-luminescence, luminescence induced by a scanning tunnel microscope and microphotoluminescence. Each of these techniques is shown to have the capacity to investigate single quantum dots, with linewidths in the range 40-1000 μev. Besides demonstrating the possibility of imaging and spectroscopically studing individual dots, we also demonstrate the possibility of investigating single impurity atoms, in imaging as well as in emission spectroscopy modes.

Nanoimprint lithography for the fabrication of interdigitated cantilever arrays

We report on the realization of a novel interdigitated cantilever array with electrostatic control of the shape of the interdigitated array. It consists of an array of SiO2/metal double-finger cantilevers in a grating configuration together with an electrical connection part. The complete grating structure is fabricated with nanoimprint lithography, UV lithography and reactive ion etching. The patterns of the cantilever arrays are defined by nanoimprint lithography. The electrical contact pads are defined and aligned with the imprinted grating pattern by UV lithography. The two steps of reactive ion etching are optimized to get vertical sidewalls of the SiO2 cantilevers and finally to release them from the Si substrate. By applying a bias, the shape of the cantilever array can be altered due to the electrostatic force. The dimensions of the cantilevers and the spacing between them are optimized to achieve the desired functional operating characteristics of the structures. Since the fabrication scheme is based on nanoimprint lithography, such electrostatically controlled periodic structures may be relatively easily and non-expensively realized in various configurations, allowing them to function as optical switching elements, electrical filters, mass sensors, etc.