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The photonic properties of two classes of wavelength-sized cavities are reported for deeply etched InP/InGaAsP/InP planar photonic crystals. The high aspect, deeply etched structures are studied as potential building blocks for nonmembrane type photonic devices in standard InP photonic integrated circuits. The first class consists of cavities of one unit cell in one direction and varying size in the other planar direction. The studied class includes a Fabry-Perot type cavity with one row of missing holes, a simple single missing hole defect cavity, and a cavity consisting of two holes which have been slightly shifted and reduced in hole radius. The best observed quality factor of 65 in this class is obtained for a single hole defect cavity. The second class is comprised of cavities which are derived from a three missing row defect in one direction and varying size in the other direction. This includes a Fabry-Perot type cavity with three rows of missing holes, a point defect cavity consisting of seven unetched holes and a six hole ring cavity. The best observed quality factor of 300 is obtained for the ring cavity in this second class of structures, which is adequate for applications. © 2009 American Institute of Physics.

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 1550 nm. 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 0.25 µm. This makes them very suitable for high-quality photonic crystal patterns.

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.

A novel technique is developed to follow the energetic position of the conduction and valence bands with respect to the Fermi level as a function of the lateral position on semiconductor surfaces. By combining high-resolution scanning-tunneling spectroscopy measurements with model calculations it is possible to relate the apparent change in conduction and valence band position to their real counterparts. This method allows one to determine the charge on surface artifacts like steps or vacancies. For a single step on p-type GaAs we find a charge of 0.9+or-0.3q nm/sup -1/

We report on the fabrication by electron-beam lithography of Bragg gratings, which serve as wavelength-selective elements of a Mach–Zehnder-type add-drop filter. To meet telecommunication network requirements, the gratings must have about 20 000 lines, corresponding to a length on the order of 10 mm. Furthermore, the coupling strength of the gratings has to be varied according to a bell-shaped taper function to suppress unwanted side lobes in the gratings’ reflection spectra. We have realised coupling coefficient variation at constant etch depth by changing the duty cycle along the grating. The employed pattern reversal process provides good line-width control of the grating ridges down to 50 nm, and good coverage by the subsequent PECVD overgrowth. Optical measurements are presented that demonstrate the excellent performance of the grating couplers.

This work focuses on the effect of resist thickness on resolution performance of hydrogen silsesquioxane (HSQ) electron beam resist. Contrast, sensitivity, surface morphology and resolution of the formed structures were found to be substantially dependent on film thickness. Method of resist drying in vacuum at room temperature was found to reduce surface roughness of ultra thin HSQ films compared to hot plate drying at 90 ^oC in air. Results of Monte Carlo simulations of the exposure process are in good agreement with proposed mechanism of sensitivity loss and structure linewidth broadening with increase of resist thickness.

To explore the possibilities of the Orion plus helium ion microscope (HIM) as a nanofabrication tool, the HIM at the TNO VLL is equipped with a pattern generator and a gas injection system. The nanofabrication research focuses on three areas: lithography, direct write and Helium Ion Beam Induced Processing (HIBIP) Lithography of helium ions with negative and positive resists is being studied to obtain ultimate resolution combined with dense patterns. The helium ions interact very locally with the resist. As a result, we observe (almost) no proximity effects, not even at pattern densities of 50% and line widths below 10 nm. For a 5 nm thin HSQ resist layer, 6 nm lines with 15 nm pitch have been achieved as shown in Figure 1 [1]. One of the most simple direct write options is to sputter directly material with helium ions [2]. Auand Pt-rods have been sliced to obtain small incisions. Figure 2 shows an 8 nm incision in Au-rods imaged with the HIM and TEM. The incision is slightly curved on the outside of the Au due to the surface tension. A limitation of direct write is the bubble formation in the substrate ate extremely high ion dose, as investigated by Livengood et al. [3]. HIBIP is performed using the following precursors: Pt, TEOS and XeF2. The Pt precursor is used to illustrate the possibilities of creating conducting 3D nanostructures. Figure 3 shows lines with dimensions of approximately 200, 15 and 30 nm (length, width and height). The same objectives are being investigated for TEOS precursors to fabricate non-conducting objects. XeF2 has been tested for etching materials with high precision. In conclusion, the helium ion microscope equipped with a gas injection system and a pattern generator is a new nanofabrication tool. It allows the user to modify and inspect samples on a nanometer scale with the same instrument.

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.

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.

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=215 nm d=129 nm has been fabricated at an etch rate of 320 nm/min and an etch depth of 650 nm. For comparison, an etch rate of 530 nm/min has been obtained in etching trench lines down to 1.61 µm deep with a width of 500 nm. The developed process has been used to fabricate GaN photonic crystal (PC) waveguides for 1.55 µm wavelength. Transmission measurements reveal the M stop band in hole type PC and illustrate the feasibility of the fabrication process.

Scanning-tunneling spectroscopy experiments performed on conjugated polymer films are compared with three-dimensional numerical model calculations for charge injection and transport. It is found that if a sufficiently sharp tip is used, the field enhancement near the tip apex leads to a significant increase in the injected current, which can amount to more than an order of magnitude and can even change the polarity of the predominant charge carrier. We show that when charge injection from the tip into the organic material predominates, it is possible to probe the electronic properties of the interface between the organic material and a metallic electrode directly by means of tip height versus bias voltage measurements. Thus, one can determine the alignment of the molecular orbital energy levels at the buried interface, as well as the single-particle band gap of the organic material. By comparing the single-particle energy gap and the optical absorption threshold, it is possible to obtain an estimate of the exciton binding energy. In addition, our calculations show that by using a one-dimensional model, reasonable parameters can only be extracted from $z\text{\ensuremath{-}}V$ and $I\text{\ensuremath{-}}V$ curves if the tip apex radius is much larger than the tip height. In all other cases, the full three-dimensional problem needs to be considered.

For the first time, we directly measured the potential distribution inside organic semiconductors. Combined spectroscopic measurements are performed on MDMO-PPV layers on Au and Yb substrates, using a scanning-tunneling microscope. The results are analyzed with a model that treats both current injection and bulk transport in detail. It is found that tip height-bias curves, which are taken by following the height of the STM tip as a function of bias, while the STM feedback system is active, reflect the potential distribution between tip and sample electrode.

We report for the first time the use of the relatively low index material SiON (n=1.56) for photonic crystal applications. With this system it is possible to enlarge the complete TE-band gap of a 2D-photonic crystal by properly designing the unit cell filling. This may be practically realised because of the larger dimensions of photonic crystals in low index materials, which allows less stringent etch conditions when compared to traditional InP based materials. The optimal distribution of the refractive index is found by placing the displacement field in high or in low dielectric material. The structure is defined by two figures of merit, the width of the band gap and the smallest feature size, which is the key factor in etching the structure. In addition, we suggest the use of a graded-index profile to reduce out of plane scattering. This graded index profile can be realised by controlling the nitrogen flow during deposition of the SiON layer.

We demonstrate that the soft nature of organic semiconductors can be exploited to directly measure the potential distribution inside such an organic layer by scanning-tunneling microscope (STM) based spectroscopy. Keeping the STM feedback system active while reducing the tip-sample bias forces the tip to penetrate the organic layer. From an analysis of the injection and bulk transport processes it follows that the tip height versus bias trace obtained in this way directly reflects the potential distribution in the organic layer.

We have studied epitaxially grown III/V materials with cross-sectional scanning tunnelling microscopy, Using this method we can identify individual Al and Ga atoms in the ternary AlGaAs layers. Through this identification we observed clustering Al atoms in AlGaAs alloys. Furthermore cross-sectional scanning tunnelling microscopy can be used to study the growth history of quantum wells and quantum wires. As well, using this cross-sectional technique, one can observe individual electrically active dopant atoms. Through this sensitivity delta doped layers have been studied with atomic resolution where we observe Coulomb repulsion between active Be-doping atoms at high doping concentrations.

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.

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. ©2008 American Institute of Physics

The manipulation of antimony dimers, Sb2, on the silicon (001) surface by means of a scanning tunneling microscope (STM) has been experimentally investigated. Directed hopping of the Sb2 dimers due the STM tip can dominate over the thermal motion at temperatures between 300 and 500 K. Statistics on the enhanced hopping are reported and possible tip–adsorbate models are discussed focusing on a field-based interaction. The low yield of directed hopping is believed to be due to the low gradient in the interaction energy intrinsic to a field-based mechanism. Ultimate resolution and limiting factors of this manipulation technique are discussed.