Semiconductor Technology Group

The Semiconductor Technology Group 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 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.

Contents
 

• 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

Epitaxy of dilue-nitride alloys for advanced lasers and photovoltaics

Figure 1. A bright RHEED picture revealing a (2x4) reconstruction of InGaAsN surface.

Important parts of our MBE activities are focused on advancing the epitaxy of dilute-nitride (InGaAsN/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: FP7 RAMPLAS, FP7 DeLight, TEKES Solar III-V, and TEKES Flip-SOI.

The newest research direction related to highly mismatched alloys 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.

Novel epitaxial technologies for III-V nanostructures

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 [ Appl. Phys. Lett. 97, 173107, 2010]. The next development steps targets the epitaxial fabrication of stacked layers of QDs on pre-defined position toward demonstrating functional nanophotonic devices.

Device processing

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.

Device simulation and theory

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.

Figure 2. Differential gain variation with wavelength and electron concentration in a 7 nm GaInasN/GaAs QW.

Characterization laboratory

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 polarization resolved photoluminescence measurements, reflectivity and transmission measurements, and spectroscopic ellipsometry. 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.

Applications laboratory

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].

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.

Research highlights and selected publications

• The first demonstration of a 2 µm semiconductor disk laser generating sub-picosecond pulses has been recently published in Electronics Letter [Electron. Lett. 47, 454, 2011]. The paper was also selected as a highlight article for the March last issue of the IET Electronics Letter Journal [Electron. Lett. (feat.), 47(7), 2011].
• The first demonstration of an InP DFB laser fabricating using nanoimprint litography (NIL) has been reported [Electron. Lett. 47, 400, 2011]. The DFB exhibited a very pure single-mode spectrum with intrinsic linewidth of ~200 kHz and side-mode suppression ratio of ~50 dB. The paper was well received by the optoelectronics community and will be soon accompanied by a highlight article in Compound Semiconductors Magazine.
• Postgraduate student Teemu Hakkarainen has received the prize for the best student presentation at the 16th Euro-MBE Workshop. The awarded presentation concerned the epitaxy of quantum nanostructures on pre-determined locations [Appl. Phys. Lett. 97, 173107, 2010]. In his work Teemu Hakkarainen used an innovative approach combining MBE and nanoimprint technology techniques studied at ORC.