Optoelectronics Research Centre
The Optoelectronics Research Centre is led by Prof. Mircea Guina. The group conducts a comprehensive chain of research activities targeting synthesis of novel III-V photonics materials, development of advanced nanotechnology tools for the fabrication of optoelectronics devices, and the development of application tailored optoelectronics devices. The focus areas are epitaxy highly mismatched III-V semiconductor alloys, such as GaInNAsSb and GaAsBi, site-controlled epitaxy of quantum dots and nanowires, high efficiency multi-junction solar cells, high power disk lasers and edge-emitting lasers, and active chips for photonic integrated circuits.
The core technology of ORC is Molecular Beam Epitaxy (MBE); with 5 operational MBE reactors we can fabricate a wide range of material systems based on GaAs, InP and GaSb. We pool our efforts to maintain a leading position in MBE by developing new techniques enabling breakthroughs in the science of new materials and photonic devices. The combined expertise in epitaxy and processing of optoelectronics devices has enabled us to achieve important milestones in the development of optoelectronic devices covering a spectral range from 630 nm to 2.5 µm. The technology related activities are supported by state-of-the-art materials and device characterization tools as well as by unique expertise in device theory and simulation.
- Epitaxy of dilue-nitride alloys for advanced lasers and photovoltaics
- Novel epitaxial technologies for III-V nanostructures
- Device processing
- Device simulation and theory
- Characterization laboratory
- Application laboratory
- Research highlights and selected publications
Main Areas of Research and Expertise
Important parts of our MBE activities are focused on advancing the epitaxy of dilute-nitride (GaInNAs/GaAs) compounds, a material system which holds many opportunities for developing advanced lasers, ultrafast nonlinear devices, and high efficiency solar cells. The dilute-nitrides are part of a class of matterials that are highly mismatched in terms of electronegativity, the so called highly mismatched alloys. These alloys are metastable in terms of epitaxy and they do not combine easily using conventional epitaxial techniques. Owing to anticrossing interaction between localized states of N and the extended states of the GaAs lattice, their conduction bands are strongly modified. These modifications enable, for example, to fabricate 1eV-band gap materials latticed matched to GaAs, which are essential for a more efficient absorption of the solar spectrum using multijunction solar cells.
ORC has gained a wide recognition for its inovative approaches to exploit the unique advantages offered by dilute-nitrides. The importance of this research area is reflected by a relatively high number of on-going project supporting the epitaxy of dilute-nitride heterostructures.The research is funded by: FP7 (RAMPLAS, RAPIDO), ESA, Academy of Finland, and TEKES (ReLASE).
The newest research direction related to highly mismatched alloys (funded by HIGHMAT, Academy of Finland) concerns the development of epitaxy for Bi-containing materials (III-V-Bi). Used in connection with dilute-nitrides, Bi can be used both as a surfactant to promote N incorporation, and for band gap engineering of GaAs based optoelectronics devices, enabling to engineer also the valence band offset. Recent results are published in [Nanotechnology 25, 205605 (2014)] and [J. Appl. Phys. 114, 243504 (2013)].
|Left: Countour map of the maximum solar cell output power as a function of growth temperature and nominal nitrogen composition of a GaInNAs p-i-n structure [SOLMAT 124, 150 (2014)]. Right: A RHEED picture revealing a (1x3) reconstruction of GaInNAs surface [J. Appl. Phys. 112, 023504 (2012)].|
The development of future photonic devices, such as single-photon emitters or nanophotonics waveguides, requires fabrication of quantum-dot (QD) systems on pre-determined locations. Site-controlled epitaxy provides new opportunities for the miniaturization of optoelectronic devices and circuits and would open new avenues for reducing the energy consumption, increasing the device functionality, and reducing the manufacturing costs of nanophotonic devices. Ultimately, the convergence of photonic and electronic technologies along with photonic integration technologies would become possible.
The traditional methods used so far for positioning QDs include the use of patterning by e-beam lithography or selective area growth. As an alternative method for the fabrication of site-controlled QDs, we combine UV-nanoimprint lithography (UV-NIL) and MBE. We have shown that this technique enables the simultaneous growth of high optical quality QD chains with different orientations [ Appl. Phys. Lett. 97, 173107, 2010] as well as site-controlled epitaxy of single QDs [Nanotechnology 24, 235204 (2013)]. Recently we have demonstrated cavity-enhanced single-photon emission from ordered InAs QDs integrated in micropillar cavities [Appl. Phys. Lett. 104, 213104 (2014)].
Left: InAs QD chains grown on a UV-NIL prepared groove patter. Right: Single site-controlled InAs QDs integrated in micropillar cavities.
During the last 15 years we have gained broad expertise in micro- and nanonofabrication of optoelectronics devices utilizing III-V semiconductors, silicon, metals and dielectrics. We have two fully equipped clean rooms dedicated to processing and packaging of optoelectronic components. We have developed fabrication and packaging processes for high power laser diodes and bars, vertical cavity surface emitting lasers (VCSELs), resonance cavity – LED (RC-LED), super luminescence diodes (SLDs), semiconductor optical amplifiers (SOAs), solar cells, and many other important components for different unique applications.
For patterning we use UV-lithography instruments that can structure features down to 500 nm linewidth and a UV nanoimprint lithography (NIL) system that enables the fabrication of 2D and 3D nanostructures down to 20 nm linewidth. We have capabilities to etch GaAs, GaSb and InP-based compounds using dry and wet etching. For thin film deposition we have at our disposal three evaporation systems and a plasma enhanced CVD. Our device packaging line contains automated dicing and wire bonding equipment, pick and place system, reflow oven and dedicated system for mounting and soldering of laser bars and laser diodes to heat sinks.
Very recently we have pioneered a cost effective process for fabricating DFB laser diodes based on NIL and surface gratings. We have demonstrated NIL-based DFBs operating at 894 nm, 980 nm [Microelectron. Eng. 86, 321, 2009], 1550 nm [Electron. Lett. 47, 400, 2011], and 1950 nm [Electron. Lett. 46, 1146, 2010]. We are currently developing processes for DFBs operating at 1310 nm and 2330 nm. Such lasers can reach up to 20 mW of output power, while exhibiting a 60 dB side mode suppression ration and a linewidth as narrow as 100 kHz.
Left: UV-NIL fabricated nanocomb structure of a distributed feedback laser. Middle: Laser chip mounted on a metal heatsink. Right: UV-NIL fabricated moth eye antireflective coating for a multi-junction solar cell [Progress in Photovoltaics 21, 1158 (2013)].
The device simulation and theory group of ORC has several goals:
- Advance the understanding of the physical phenomena governing the devices developed in ORC
- Develop the device modeling (e.g. advanced k•p method, mode-solver, transfer matrix and modified rate equation models)
- Create and develop new device concepts (e.g. resonant-cavity light-emitting diodes exploited as resonant-cavity photodetectors, distributed feedback and distributed Bragg reflector lasers with photon-photon-resonance-enhanced modulation bandwidths)
- Sustain device development by extensive simulation studies
These goals are supported by a wide range of tools, both in-house-developed and commercial software packages. The in-house-developed tools cover a broad range of particular phenomena, operation characteristics (from energy band profiles to photon re-cycling, from optical mode profiles to side-mode-suppression ratios, from emission linewidth to modulation bandwidth) and devices (from edge-emitting to vertical cavity light emitters and from solar cells to single and entangled photon sources). On the other hand, the commercial software packages (like LASTIP and PICS3D from CrossLight Inc.) provide the capability to comprehensibly integrate the optical, electrical and thermal perspectives in complex device simulations, based on the particular characteristics derived by in-house-developed tools.
|Left: Differential gain variation with wavelength and electron concentration in a 7 nm GaInasN/GaAs QW. Right: Simulated I-V performance for a current-matched GaInP/GaAs/GaInNAs/Ge four-junction devices [Nanoscale Research Letters 9, 61 (2014)].|
Multidisciplinary semiconductor characterization methods are needed for thorough investigations of III-V semiconductor devices and structures.
We have a high-resolution five-crystal X-ray diffractometer which is used for studying crystalline quality, material compositions, residual strain and unit cell distortions in bulk materials and multilayer III-V semiconductor structures. Surface characterization is done using optical microscopy, scanning electron microscopy (SEM) and atomic force microscopy (AFM). Special AFM techniques, including Kelvin probe force microscopy (KPFM), are used and developed for probing the spatial potential variations inside high-efficiency III-V solar cell structures. Capabilities in measuring advanced capacitance transient and admittance spectroscopy allow us to study the material quality and defect densities with very high accuracy.
For optical characterization we employ a large variety of measurement systems, including photoluminescence measurements, reflectivity and transmission measurements, and spectroscopic ellipsometry. Our cryostat-based micro-photoluminescence equipment enables time-resolved and steady-state photoluminescence measurements at the spectral range from visible to 1700 nm with spatial resolution of 500 nm. A new area in our research portfolio is electro-optical studies of III-V structures and materials at high magnetic fields. These experiments are carried out in customized low-temperature cryostat that has magnetic fields up to seven Tesla oriented perpendicular or parallel to the sample axis.
The application laboratory of ORC was set up for carrying out application specific research and development projects driven by the needs of industrial partners. The laboratory has the capability to design, build and characterize several types of high-power and frequency converted laser systems. The ongoing work is focused on development of semiconductor disk lasers (SDLs) operating at new wavelengths, such as 1120 nm, 1156 nm, 1178 nm and 2–3 µm, exploiting in-house epitaxial technology for gain mirror fabrication. These developments are financed via industrial collaborative projects and target applications in medical, astronomy, sensing, and scientific fields. Main achievements include demonstration of the highest power frequency double yellow laser [Opt. Express 18, 25633, 2010] (589 nm) and the first demonstration of a 2-µm femtosecond disk laser [Electron. Lett. 47, 454, 2011], and the first demonstration of a mode-locked red-emitting semiconductor disk laser [Optics Letters 38, 2289 (2013)].
The laboratory equipment includes tools for standard laser characterization, such as power meters, spectral measurements, beam profile measurements, and high-power laser bar testers. In addition we have some specialized characterization tools, such as spectral linewidth measurements based on a delayed self-homodyne technique; with this set-up we have been able to measure linewidths below 100 kHz for DFB laser diodes.
Left: Mode-locked red laser emitting picosecond pulses [Optics Letters 38, 2289 (2013)]. Right: Frequency-doubled, optically-pumped disc laser based on GaInNAs gain mirror.
For more information, contact Prof. Mircea Guina.
- 2016 - Quantum dot-plasmon coupling demonstrated in a hybrid structure consisting of site-controlled InAs quantum dot chains in the proximity of an Ag film [OSA Optica 3(2), 139 (2016)].
- 2016 - Influence of As/group-III flux ratio on defects formation and photovoltaic performance of GaInNAs solar cells, Polojärvi, V., Aho, A., Tukiainen, A., Raappana, M., Aho, T., Schramm, A. & Guina, M. 1 May 2016 In : Solar Energy Materials and Solar Cells. 149, p. 213-220 8 p.
- 2016 - A new method is presented for determining the composition of GaInNAsSb layers and for calculating the the corresponding bandgap [J. Crystal Growth 438, 49 (2016)].
- 2015 - Optical properties and thermionic emission in solar cells with InAs quantum dots embedded within GaNAs and GaInNAs, Polojärvi, V., Pavelescu, E-M., Schramm, A., Tukiainen, A., Aho, A., Puustinen, J. & Guina, M. 19 Jun 2015 In : Scripta Materialia. 108, p. 122-125 4 p.
- 2015 - GaInNAs Solar Cell with Back Surface Reflector, Aho, T., Aho, A., Tukiainen, A., Polojärvi, V., Penttinen, J-P., Raappana, M. & Guina, M. 14 Jun 2015 42nd IEEE Photovoltaic Specialists Conference (PVSC), 2015. IEEE, 4 p. (Conference record of the IEEE photovoltaic specialists conference)
- 2015 - Temperature coefficients for GaInP/GaAs/GaInNAsSb solar cells, Aho, A., Isoaho, R., Tukiainen, A., Polojärvi, V., Aho, T., Raappana, M. & Guina, M. 2015 In : AIP Conference Proceedings. 1679, p. 1-5 5 p., 050001
- 2015 - MBE GROWN GaInNAsSb MULTIJUNCTION SOLAR CELLS: PATH TOWARDS 50% EFFICIENCY, Aho, A. J., Polojärvi, V. V., Aho, T. A., Raappana, M. J. S., Tukiainen, A. K. & Guina, M. D. 2015 18th European Molecular Beam Epitaxy Workshop. Canazei, Italy.
- 2015 - Detecting lateral composition modulation in dilute Ga(As,Bi) epilayers [Nanotechnology 26, 425701 2015].
- 2015 – Arto Aho wins the best oral presentation award at the 31st European Photovoltaic Solar Energy Conference for his work concerning high quality dilute nitride materials for multi-junction solar cells.
- 2015 - Te-doped self-catalyzed GaAs nanowires grown by molecular beam epitaxy [Appl. Phys. Lett. 107, 012101 (2015)].
- 2015 - Lithography-free patterning method developed for growth of highly uniform GaAs nanowires on Si(111) [Nanotechnology 26, 275301 (2015)]
- 2014 - Teemu Hakkarainen's PhD Thesis "Site-controlled epitaxy and fundamental properties of InAs quantum dot chains" selected as the best Thesis of 2014 by Photonics Finland. His teammate Juha Tommila was granted the same award in 2013.
- 2014 - Performance assessment of multijunction solar cells incorporating GaInNAsSb, Aho, A., Tukiainen, A., Polojärvi, V. & Guina, M. 2014 In : Nanoscale Research Letters. 9, p. 1-7 7 p., 61
- 2014 - Microscopy of annealed GaAsBi material is revealed in collaboration with Paul Drude Institute (Berlin) [Nanotechnology 25, 205605 (2014)] and [J. Appl. Phys. 114, 243504 (2013)].
- 2014 – Growth of GaInNAs bulk material for solar cells perfected [SOLMAT 124, 150 (2014)].
- 2014 - Emmi Kantola wins the best student award at Photonics Europe 2014 for her work concerning 20 W pulsed yellow semiconductor disk lasers. [Opt. Express 22, 6372 (2014)]
- 2014 - Cavity-enhanced single-photon emission from site-controlled InAs QDs integrated in micropillar cavities [Appl. Phys. Lett. 104, 213104 (2014)].
- 2013 – First demonstration of a mode-locked red-emitting semiconductor disk laser [Optics Letters 38, 2289 (2013)].
- 2012 – Demonstration of uniform large arrays of single InAs QDs grown on UV-NIL patterned substrates [Nanotechnology 237, 175701 (2012)].
- 2011 - Demonstration of a mode-locked semiconductor disk laser emitting sub-400 fs pulses at 2 µm [Electron. Lett. 47, 454, 2011].