M. Androulidaki

Strained GaAs/InGaAs Core-Shell Nanowires for Photovoltaic Applications

We report on the successful growth of strained core-shell GaAs/InGaAs nanowires on Si (111) substrates by molecular beam epitaxy. The as-grown nanowires have a density in the order of 108 cm−2, length between 3 and 3.5 μm, and diameter between 60 and 160 nm, depending on the shell growth duration. By applying a range of characterization techniques, we conclude that the In incorporation in the nanowires is on average significantly smaller than what is nominally expected based on two-dimensional growth calibrations and exhibits a gradient along the nanowire axis. On the other hand, the observation of sharp dot-like emission features in the micro-photoluminescence spectra of single nanowires in the 900–1000-nm spectral range highlights the co-existence of In-rich enclosures with In content locally exceeding 30 %.

Piezoelectric InAs (211)B quantum dots grown by molecular beam epitaxy: Structural and optical properties

The structural and optical properties of piezoelectric (211)B InAs nanostructures grown by molecular beam epitaxy are systematically investigated as a function of the various growth parameters. Depending on the specific growth conditions, we show that the InAs nanostructures take the form of a quantum dot (QD) or a quantum dash, their height ranges between 2 and 20 nm, and their density varies from a few times 108 cm−2 all the way up to a few times 1010 cm−2. The (211)B QDs are characterized by large aspect ratios, which are compatible with a truncated pyramid morphology. By analyzing the QD emission spectrum, we conclude that only small size QDs, with heights less than 3 nm, are optically active. This is consistent with high resolution transmission electron microscopy observations showing that large QDs contain misfit dislocations, whereas small QDs are dislocation-free. The formation of a two-dimensional wetting layer is observed optically, and its thickness is determined to be between 0.30 and 0.39 nm....

GaN membrane metal-semiconductor-metal ultraviolet photodetector.

GaN is a wide-bandgap semiconductor with still unexplored capabilities for ultraviolet detection. To exploit GaN properties better for ultraviolet detection, a metal-semiconductor-metal-type photodetector structure was designed and manufactured on a 2.2 microm thin GaN membrane fabricated by micromachining techniques. As a result, a very low dark current (30 pA at 3 V) and a maximum responsivity of 14 mA/W at a wavelength of 370 nm were obtained.

Extraction of absorption coefficients from as-grown GaN nanowires on opaque substrates using all-optical method

We demonstrate a new all-optical method to measure absorption coefficients in any family of as-grown nanowires, provided they are grown on a substrate having considerable difference in permittivity with the nanowire-air matrix. In the case of high crystal quality, strain-free GaN nanowires, grown on Si (111) substrates, the extracted absorption coefficients do not exhibit any enhancement compared to bulk GaN values, unlike relevant claims in the literature. This could be attributed to the relatively small diameters, short heights, and high densities of our nanowire arrays.

GaN membrane-supported UV photodetectors manufactured using nanolithographic processes

Membrane GaN metal-semiconductor-metal (MSM) photodetector structures using nanolithographic techniques have been manufactured for the first time. Very low dark currents and unexpected high values for the responsivity have been obtained. It seems that the membrane together with the (submicronic) MSM structure increase the gain of the structure and responsivities in the range of 50-100A/W can be obtained.

Fabrication of GaAs laser diodes on Si using low-temperature bonding of MBE-grown GaAs wafers with Si wafers

The conventional heteroepitaxial GaAs-on-Si growth suffers from poor III-V material quality (dislocation density of /spl sim/10/sup 8/ cm/sup -2/ and a residual thermal stress of /spl sim/10/sup 9/ dyn/cm/sup 2/) and some process incompatibilities between the CMOS and III-V technologies. For these reasons, we have developed a heterogeneous integration scheme, which has the wafer-scale characteristics of monolithic integration and at the same time is compatible with commercial fully processed CMOS or bipolar-CMOS (BiCMOS) technology. We report on the basic process flow the properties of the GaAs/Si material and the processed laser diodes. MBE was used to grow GaAs/AlGaAs heterostructures with an inversed epitaxial structure, on 3inch GaAs substrates, after the inclusion of an AlAs etch stop layer. The GaAs wafer was then bonded at room temperature face-to-face with a 4inch Si wafer covered by spin-on-glass. Backside thinning of the GaAs substrate was used to leave the active thin III-V heterostructure on Si. The optoelectronic quality and the residual stress in the bonded III-V layers were assessed by photoluminescence and photoreflectance spectroscopies.

Selective-area growth of GaN nanowires on SiO2-masked Si (111) substrates by molecular beam epitaxy

We analyze a method to selectively grow straight, vertical gallium nitride nanowires by plasma-assisted molecular beam epitaxy (MBE) at sites specified by a silicon oxide mask, which is thermally grown on silicon (111) substrates and patterned by electron-beam lithography and reactive-ion etching. The investigated method requires only one single molecular beam epitaxy MBE growth process, i.e., the SiO2 mask is formed on silicon instead of on a previously grown GaN or AlN buffer layer. We present a systematic and analytical study involving various mask patterns, characterization by scanning electron microscopy, transmission electron microscopy, and photoluminescence spectroscopy, as well as numerical simulations, to evaluate how the dimensions (window diameter and spacing) of the mask affect the distribution of the nanowires, their morphology, and alignment, as well as their photonic properties. Capabilities and limitations for this method of selective-area growth of nanowires have been identified. A windo...

Ultraviolet MSM photodetector based on GaN micromachining

This paper presents the manufacturing and the characterization of GaN membrane supported MSM photodetector structures obtained by means of nanolithographic techniques. Two different runs of MSM photodetectors, with different dimensions of the MSM structures and different GaN membrane thickness, have been performed and the detectors performances are annalised. Very low dark currents and unexpected high values, in the range of 50-100 A/W for the UV detectors responsivity have been obtained.

III-V material and device aspects for the monolithic integration of GaAs devices on Si using GaAs/Si low temperature wafer bonding

A new process for wafer scale integration of GaAs optoelectronic devices with Si integrated circuits has been investigated, based on low temperature bonding of epitaxial GaAs wafers onto planarized fully processed CMOS/BiCMOS wafers. The basic process flow and the most important aspects of the work concerning the III-V material and devices are presented.

3D micro-structured arrays of ZnΟ nanorods

The fabrication of nanostructures with controlled assembly and architecture is very important for the development of novel nanomaterial-based devices. We demonstrate that laser techniques coupled with low-temperature hydrothermal growth enable complex three-dimensional ZnO nanorod patterning on various types of substrates and geometries. This methodology is based on a procedure involving the 3D scaffold fabrication using Multi-Photon Lithography of a photosensitive material, followed by Zn seeded Aqueous Chemical Growth of ZnO nanorods. 3D, uniformly aligned ZnO nanorods are produced. The increase in active surface area, up to 4.4 times in the cases presented here, provides a dramatic increase in photocatalytic performance, while other applications are also proposed.The fabrication of nanostructures with controlled assembly and architecture is significant for the development of novel nanomaterials-based devices. In this work we demonstrate that laser techniques coupled with low-temperature hydrothermal growth enable complex three-dimensional ZnO nanorods patterning on various types of substrates and geometries. The suggested methodology is based on a procedure involving the 3D scaffold fabrication using Multi-Photon Lithography of a photosensitive material, followed by Zn seeded Aqueous Chemical Growth of ZnO nanorods. 3D, uniformly aligned ZnO nanorods are produced, exhibiting electrical conductance and highly efficient photocatalytic performance, providing a path to applications in a diverse field of technologies.