Hwm Huub Salemink

Determination of the shape and indium distribution of low-growth-rate InAs quantum dots by cross-sectional scanning tunneling microscopy

We present a cross-sectional scanning-tunneling microscopy investigation of the shape, size, and composition of InAs quantum dots in a GaAs matrix, grown by molecular beam epitaxy at low growth rate. From the dimensional analysis we conclude that the investigated quantum dots have an average height of 5 nm, a square base of 18 nm oriented along [010] and [100] and the shape of a truncated pyramid. From outward relaxation and lattice constant profiles we conclude that the dots consist of an InGaAs alloy and that the indium concentration increases linearly in the growth direction. Our results justify the predictions obtained from previous photocurrent measurements on similar structures and the used theoretical model.

Cl2∕O2-inductively coupled plasma etching of deep hole-type photonic crystals in InP

We have developed an inductively coupled plasma etching process for fabrication of high-aspect-ratio hole-type photonic crystals in InP, which are of interest for optical devices involving the telecommunication wavelength of 1550nm. The etching was performed at 250°C using Cl2∕O2 chemistry for sidewall passivation. The process yields nearly cylindrical features with an aspect ratio larger than 10 for hole diameters near 0.25μm. This makes them very suitable for high-quality photonic crystal patterns.

Determination of the outward relaxation of cleaved strained InAs structures by scanning tunneling microscopy

When a semi-conductor structure containing strained layers such as quantum wells (QWs) or quantum dot layers is cleaved, the surface will relax outward in order to release built-in strain. This outward relaxation is directly linked to the composition of the strained layers, and can thus provide accurate information about the local composition of these layers. By using cross-sectional scanning tunneling microscopy (X-STM) it is possible to measure this outward relaxation. The measured height profiles, however, are also dependent on the chemical composition of the measured surface, resulting in an extra height contrast in the images. In order to analyze only the outward relaxation, it is necessary to suppress this latter chemical component in the STM measurements. This can be achieved by choosing the proper tunnel conditions.

Compact Mach-Zehnder interferometer based on self-collimation of light in a silicon photonic crystal

We demonstrate a compact silicon photonic crystal Mach-Zehnder interferometer operating in the self-collimation regime. By tailoring the photonic band structure such as to produce self-collimated beams, it is possible to design beam splitters and mirrors and combine these to a 20 x 20 microm(2) format. With transmission spectroscopy we find a pronounced unidirectional optical output, the output ratio being as high as 25 at the self-collimation wavelength. Furthermore, the self-collimated beams and the unidirectionality are clearly observed in real space using near-field and far-field optical microscopy. Interpretation of the optical data is strongly supported by different types of simulations.

Wavelength tuning of planar photonic crystals by local processing of individual holes

Tuning of the resonant wavelength of a single hole defect cavity in planar photonic crystals was demonstrated using transmission spectroscopy. Local post-production processing of single holes in a planar photonic crystal is carried out after selectively opening a masking layer by focused ion beam milling. The resonance was blue-shifted by enlargement of selected holes using local wet chemical etching and red-shifted by infiltration with liquid crystals. This method can be applied to precisely control the resonant frequency, and can also be used for mode selective tuning.

Fabrication of two dimensional GaN nanophotonic crystals (31)

The authors have investigated chlorine based inductively coupled plasma etching of GaN by using different gas mixtures of Ar, Cl2, and N2. The etch mechanism and N2 role have been studied. We found that both ion energy and ion current density are important. The N2 plays a multiple role in etching GaN, chemical reaction, and ion bombardment. A reliable process to fabricate GaN nanophotonic crystals has been developed. Plasma conditions have been optimized toward a balance of ion current density, ion energy, and chemical species density. As a result, flat bottom, anisotropic photonic crystal with a=215nm d=129nm has been fabricated at an etch rate of 320nm∕min and an etch depth of 650nm. For comparison, an etch rate of 530nm∕min has been obtained in etching trench lines down to 1.61μm deep with a width of 500nm. The developed process has been used to fabricate GaN photonic crystal (PC) waveguides for 1.55μm wavelength. Transmission measurements reveal the ΓM stop band in hole type PC and illustrate the feas...

Spatially resolved scanning tunneling luminescence on self-assembled InGaAs/GaAs quantum dots

Scanning-tunneling microscope induced luminescence at low temperature has been used to study the carrier injection into single self-assembled InGaAs/GaAs quantum dots. Electrons are injected from the tip into the dots, which are located in the intrinsic region of a p-i-n junction, and contain excess holes under typical operational conditions. Only a fraction (∼4%) of the dots is found to be optically active under local electrical excitation. Spatial dependent measurements indicate a highly nonhomogeneous electron diffusion towards the dots. By analyzing the spatial dependence of individual peaks in the measured spectra, the contributions of individual dots to the total, multidot spectrum can be disentangled.

Comparative study of Cl2, Cl2∕O2, and Cl2∕N2 inductively coupled plasma processes for etching of high-aspect-ratio photonic-crystal holes in InP

An extensive investigation has been performed on inductively coupled plasma etching of InP. An important motivation for this work is the fabrication of high-aspect-ratio holes for photonic crystals. The essential chemistry is based on Cl2 with the addition of N2 or O2 for sidewall passivation. The influence of different process parameters such as gas flows, temperature, pressure, ion energy, and inductively coupled plasma power on the hole geometry is presented. It is concluded that photonic crystals can be etched with Cl2 only; however, temperature and pressure control is critical. Adding passivation gases largely broadens the window in the parameter space for hole etching. Most importantly, etching of narrow holes can be carried out at higher temperatures where the etching is mass limited and spontaneous etching of InP by Cl2 occurs.

Transmission measurement of the photonic band gap of GaN photonic crystal slabs

A high-contrast-ratio (30 dB) photonic band gap in the near-infrared transmission of hole-type GaN two-dimensional photonic crystals (PhCs) is reported. These crystals are deeply etched in a 650 nm thick GaN layer grown on sapphire. A comparison of the measured spectrum with finite difference time domain simulations gives quantitative agreement for the dielectric band and qualitative agreement for the air band. The particular behavior of the air band arises from the relatively low index contrast between the GaN layer and the sapphire substrate. Our results call for extension of the operation of GaN PhCs to the visible range.

Wavelength-sized, tunable nanocavity in deeply etched InP /InGaAsP /InP photonic crystals

Wavelength-sized point defect cavities coupled to access waveguides are reported for deeply etched InP/InGaAsP/InP two-dimensional photonic crystals. The observed quality factor of 60 is comparable to those found for one-row defect Fabry-Perot cavities and for simple point defect cavities in membranes. The quality factor was changed by varying the number of rows of holes. Upon infiltration of the holes with liquid crystal, frequency tuning was demonstrated.