NATO Advanced Research Workshop
UV Solid-State Light Emitters and Detectors
ABSTRACTS
June 17 – 21, 2003
Vilnius, Lithuania
http://www.natoarw-uv.ff.vu.lt
Sponsored by NATO
Cosponsored by:
Defense Advanced Research Projects Agency, USA
Ministry of National Defense of the Republic of Lithuania
Ministry of Education and Science of the Republic of Lithuania
Lithuanian State Foundation of Science and Studies
Vilnius University
Sensor Electronic Technology, Inc.
EKSPLA Ltd.
Roadmap for Solid State UV Emitters for Detection of Hazardous
Biological Agents
LTC John C. Carrano, PhD
Program Manager, DARPA/MTO, USA
jcarrano@darpa.mil
In this talk we will provide a roadmap for developing a revolutionary class of new ultraviolet light sources
capable of performing optical spectroscopy on biological agents. A well known technique for detecting
biological agents is through laser-induced auto-fluorescence. Unfortunately, these bio-detectors are large, power
hungry systems owing in large part to the conventional UV light source used for fluorescence excitation of the
sampled particles. The new optical sources being developed in the DARPA program are semiconductor devices
fabricated in the AlInGaN (aluminum, indium, gallium nitride) material system, which are capable of producing
emissions as short as 280 nm, and are chip-scale in size. We will describe specific device innovations in the area
of laser diodes, and light emitting diodes (LED’s) that operate in either pulsed mode or continuous wave. The
primary focus of the talk will be on the recent development of new, compact, and potential inexpensive
biological agent detection systems enabled by these novel UV sources. We see these new bio-sensing systems as
one critical component in an overall bio-terrorism early warning architecture designed to address the demanding
requirements of homeland security.
AlGaN Based UV Light Emitting Diodes and Photodetectors
Steve P. DenBaars, A. Hanlon,T. M. Katona, M. Pattison, J. Kaeding, R. Sharma,
M. C. Schmidt, T. Margalith, C. Moe , S. Keller, P. Cantu, T. Li*, and S. Nakamura
Solid-State Lighting and Display Center, Materials Department, University of California
Santa Barbara, CA, USA
*Cree Lighting Co., Goleta, CA, USA
denbaars@engineering.ucsb.edu
Ultraviolet (UV) emitters have recently become the focus of intense research due to the number of potential
applications, such as solid-state white lighting via phosphor excitation, biochemical detection, and enhanced
density of optical data storage. Low resistivity n-type AlxGa1-xN alloys with high AlN mole fraction (x) are
necessary for the fabrication of deep ultraviolet (DUV) optoelectronic devices. For growth at 920 oC we obtained
Al0.65Ga0.35N:(Si, In) films with a maximum n-type carrier concentration of 2.5 × 1019 cm-3, and electron mobility
of 30 cm2/Vs, using the In co-doping technique [1]. Ultra-violet light emitting diodes with of peak EL
wavelengths of 272-340 nm were fabricated using high AL containing alloys. The output power of UV LEDs is
strongly dependent on the Al content of the quantum wells. We believe that non-radiative processes and piezoelectric induced separation of carriers dominate the AlGaN based LEDs as the wavelength is decreased from
400nm to 290nm. Using a unique Inverted Heterostructure Photodiode (IHP) design which avoids need for ptype AlGaN we obtained solar-blind photodiodes with excellent device performance of 46% external quantum
efficiency at 274 nm.
The authors gratefully acknowledge the support of the DARPA SUVOS program monitored by
Dr. John C. Carrano.
[1] Adivarahan et al., Appl. Phys. Lett. 79, 1903 (2001).
Basic Device Issues in UV Solid-State Emitters and Detectors
Michael S. Shur1 and Artūras Žukauskas2
1
Broadband Center and ECSE Department, RPI, Troy, NY, USA
shurm@rpi.edu
2
Institute of Materials Science and Applied Research, Vilnius University, Vilnius, Lithuania
arturas.zukauskas@ff.vu.lt
When Monsanto introduced the first commercial LEDs in 1968, they produced only red light with the intensity
of approximately 10–3 lumen barely visible under ambient light. In 1970s and 1980s, much brighter LEDs with
colors ranging from read to green and yellow have been developed. Pioneering work of Pankove, Akasaki, and
Nakamura has led to the development of bright green, blue and UV nitride-based LEDs. UV light emitting
diodes and lasers are expected to find numerous applications in biotechnology, medicine, dentistry, home
security, food and air safety technology, short-range covert communications, industry, and solid state lighting,
where they hold promise of getting two photons for the price of one. 340–400-nm LEDs are already available
commercially and milliwatt power 285 nm LEDs have been demonstrated in a laboratory. AlGaN alloys with a
molar fraction of Al in excess of 0.3 for UV solar blind Schottky barrier, p–n junction and MSM detectors have
been demonstrated. Recent work on surface-acoustic-wave (SAW) UV detectors revealed their potential for
remote solar blind detection applications. However, UV LEDs powers are dropping and challenges becoming
more formidable with decreasing wavelengths. These challenges include difficulties in growing high quality
nitride heterostructures with a high aluminum molar fraction for having a wide band gap, solving problems with
poor photon recycling, with p-doping, with large series resistances linked to low doping efficiency and, hence, to
a large spreading resistance, with self heating, with degradation of packaging, and (for solid lighting
applications) with degradation of phosphors. The solutions to device problems lie in using better substrates (with
bulk AlN substrates in non-polar orientations being especially promising), using better epitaxial growth
techniques (such as atomic layer epitaxy possibly combined with lateral epitaxial overgrowth, cantilever epitaxy,
or pendeo-epitaxy), improving device design for reduced spreading resistance and improving quantum efficiency
via optimizing multiple quantum well structures, and better contact technology and design. The development of
UV-resistant plastics encapsulating materials is an important issue for UV LED packaging.
HVPE-Grown AlN-GaN Based Structures for UV Spectral Region
Alexander Usikov
A. F. Ioffe Physico-Technical Institute of the RAS, St. Petersburg, Russia
usikov@tdii.com
Historically, hydride vapor phase epitaxy (HVPE) was the first technique to produce high quality thick GaN
epitaxial layers. However, device applications of this epitaxial technology were not considered for long time.
Recently, significant progress has been achieved on HVPE growth of multi-layer submicron AlGaN-based
structures.
In this report presentation we describe first ultraviolet light emitting diodes (LED) emitting in the
wavelength range from 305 to 340 nm based on HVPE-grown AlGaN/AlGaN heterostructures. Characteristics of
the violet AlGaN/GaN LEDs emitting at 420 nm are also presented. HVPE possesses unique aspects such as
ability to (i) combine deposition of thick low-defect layers and thin device multi-layer structures in the same
growth run and (ii) easily grow high-quality AlGaN layers in the whole composition range. Moreover, HVPE
can provide low-impurity films, as it is a carbon-free process producing free HCl gas in a reactor, which getters
metallic impurities. The growth and characterization of AlGaN-based structures on sapphire substrates are
discussed. At a wavelength of 341 nm, 2 mW of optical output power was achieved at pulsed injection currents
of 110 mA The obtained results proved that the HVPE technique has the significant potential for mass
production of AlGaN-based device epitaxial wafers.
GaN-Based Laser Diodes: Materials Issues and Device Design
Sven Einfeldt, S. Frigge, T. Böttcher, J. Dennemarck, R. Chierchia, and D. Hommel
Institute of Solid State Physics, University of Bremen, Bremen, Germany
einfeldt@physik.uni-bremen.de
Gain-guided laser diodes emitting around 401 nm under pulsed current injection were fabricated based on
heterostructures of group-III nitrides grown by metalorganic vapor phase epitaxy. When operated in LED mode,
the output intensity was measured in dependence on the temperature. This data was used to extract the evolution
of the temperature during one single laser pulse. In conjunction with theoretical simulations it is shown that
heating of the device is critical in achieving continuous-wave operation. Several issues were investigated which
are believed to improve the performance of UV light emitters: (1) Layers of SiN were introduced in the GaN
buffer layer to reduce the dislocation density in the material. (2) The pretreatment of interfaces in the
multiquantum well region was investigated with regard to the luminescence efficiency and alloy fluctuations.
(3) The growth of AlGaN buffer layers and/or InGaN waveguides was studied in view of the increase of the
optical confinement of the guided mode.
Quaternary AlInGaN Materials System for UV Optoelectronics
Edmundas Kuokštis1 and Gintautas Tamulaitis2
1
University of South Carolina, Columbia, SC, USA
koukstis@engr.sc.edu
2
Institute of Materials Science and Applied Research, Vilnius University, Vilnius, Lithuania
gintautas.tamulaitis@ff.vu.lt
Quaternary AlInGaN materials represent a novel class of semiconductor compounds, which can be used in UV
optoelectronics demonstrating a number of advantages. After a short introduction to peculiarities of structure and
growth of AlInGaN layers and multilayered structures, we review the results on investigation and analysis of the
radiative recombination properties of MOCVD-grown AlInGaN-based layers and multiple quantum wells
(MQWs) in a wide temperature range and under different excitation conditions. The discussion is focused on
carrier localization and the influence of indium on mobility and recombination of photoexcited carriers.
Additional influence of built-in electrostatic fields due to spontaneous and piezoelectric effects in MQWs on the
photoluminescence dynamics is also addressed. Recent results on photoluminescence kinetics, study of indium
spatial distribution, and simulation of exciton hopping in AlInGaN are summarized. Different mechanisms of
optical transitions in AlInGaN systems are analyzed. The prospects for practical application of these quaternary
materials for UV emitters are discussed.
Epitaxial AlGaN Layers for UV LEDs on Sapphire and Bulk AlN
Remis Gaska1, Asif Khan2, and Michael S. Shur3
1
Sensor Electronics Technology, Columbia, SC, USA
University of South Carolina, Columbia, SC, USA
3
Rensselaer Polytechnic Institute, Troy, NY, USA
gaska@s-et.com
2
The development of deep UV LEDs requires growing high quality AlGaN epitaxial layers with high molar
fractions of Al. Our Strain Energy Band Engineering approach has considerably improved the quality of such
layers grown on sapphire substrates [1, 2]. Recent improvements in our epitaxial Pulsed Atomic Layer Epitaxy
(PALE) technology have allowed us to grow very uniform high quality AlGaN heterostructures over four inch
sapphire wafers. However, for deep UV emitters, a more promising approach might be to use bulk AlN
substrates that improve lattice matching to high molar fraction AlGaN compounds, have a much better match for
a thermal expansion coefficient, and have a superior thermal conductivity. We recently reported on high Alcontent AlGaN-based deep UV emitter structures grown over single crystal, slightly off c-axis (5.8 degrees) bulk
AlN substrates. AlN/AlGaN multiple quantum well (MQW) structures with up to 50% of Al in the well material
were grown by using low-pressure MOCVD and characterized by using X-ray, AFM, SEM and
photoluminescence techniques. We observed stimulated emission at wavelength as short as 258 nm in
Al0.5Ga0.5N/AlN MQWs grown on bulk AlN single crystals [3]. These results confirm a high potential of
epitaxial structures grown on bulk AlN for scaling our 285 nm UV LEDs on sapphire technology for developing
UV LEDs and laser diodes at 260 nm.
[1] M. Asif Khan, J. W. Yang, G. Simin, R. Gaska, M. S. Shur, H.-C. zur Loye, G. Tamulaitis, A. Zukauskas, D. J. Smith,
D.Chandrasekhar, and R. Bicknell-Tassius, Appl. Phys. Lett. 76, 1161 (2000).
[2] J. P. Zhang, H. M. Wang, M. E. Gaevski, C. Q. Chen, Q. Fareed, J. W. Yang, G. Simin, and M. Asif Khan, Appl. Phys.
Lett. 80, 3542 (2002).
[3] R. Gaska, C. Chen, J. Yang, E. Kuokstis, A. Khan, G. Tamulaitis, I. Yilmaz, M. S. Shur, J. C. Rojo, L. Schowalter, Appl.
Phys. Lett. 81, 4658 (2002).
UV Metal Semiconductor Metal Detectors
Jean-Luc Reverchon
Thales Research Technology, France
jean-luc.reverchon@thalesgroup.com
UV detection is interesting for combustion optimization, air contamination control, fire and solar blind rocket
launching detection. Most of these applications require that UV detectors have a huge dynamic response between
UV and the visible, and a very low dark current in the range of the UV flux measured. (Al,Ga)N alloys present a
large direct bandgap in this range and therefore can be used as an active region in such detectors. To take
advantage of the large Schottky barrier, the good alloy quality, and to avoid doping problems, we have
developed MSM photodetectors. High quality material has been grown with MOCVD and MBE on sapphire
substrates. Stress is controlled for aluminum contents up to 65% which reduces crack density, correlated dark
current, sub-bandgap response and non-linearity. The spectral selectivity between UV and visible reaches five
orders of magnitudes and the peak response is optimized with a geometry of inter-digitized fingers. An
optimized Schottky barrier and dielectric passivation result in dark currents lower than 1 fA up to 30 V for a
100×100 µm² pixel. Consequently, detectivity is mainly limited by shot noise and corresponds to a detectable
flux limit of 500 photons/s.
Bulk GaN Crystals Grown at High Pressure as Substrates for UV-Emitters
Technology
M. Leszczynski et al.
High Pressure Research Center UNIPRESS, Warsaw, Poland
mike@unipress.waw.pl
GaN single crystals exhibiting the lowest dislocation density (below 100/cm2) are grown at high hydrostatic N2
pressures of 10-20 kbars. Despite small dimensions of such crystals (up to 1/2”) they offer a unique chance to
construct efficient UV-emitters. The paper will report on the structural, optical and electrooptical properties of
LED and LD structures grown on the bulk GaN crystals using a combined method of MOVPE and MBE.
Characterization of Advanced Materials for Optoelectronics by Using
Tunable UV Lasers and Four-Wave Mixing Techniques
Kęstutis Jarašiūnas
Institute of Materials Science and Applied Research, Vilnius University, Vilnius, Lithuania
kestutis.jarasiunas@ff.vu.lt
The recent development and applications of light-induced dynamic grating technique for optical characterization
of bulk crystals, layers, and structures are reviewed. Studies of carrier generation, transport, and recombination
processes in different semiconducting compounds, varying from UV-materials and structures for optoelectronics
to IR bulk crystals for telecommunications demonstrate possibility for direct control of semiconductor material
fabrication technology.
Time-resolved studies of carrier dynamics in bulk and epitaxial GaN as well as in InGaN/GaN structures
with different In content have been carried out using ps duration pulses at fixed harmonics (355 or 532 nm) or
tunable wavelengths up to 420 nm. The studies provided number of carrier parameters, as lifetime, surface
recombination velocity, and diffusion coefficient. Exposure characteristics of FWM allowed monitoring of
carrier generation and/or recombination rate in light-emitting structures and comparison of various InGaN and
GaN structures. In heavily doped p-GaAs double heterostructures, injection of carriers through a thin buffer layer
was used for direct determination of minority carrier diffusion coefficients and electron mobilities. In bulk
crystals of CdZnTe and GaAs, the transport of carriers, generated from deep traps and affected by space-charge
field allowed determination of a photogenerated carrier type, electrical activity of defects, and control of wafer
homogeneity.
Commercial wave-mixing holographic devices for control of semiconductor parameters are on the verge of
becoming a reality; we demonstrate the first example.
State of the Art of White and UV LEDs Studies in Russia
Alexander E. Yunovich
Department of Physics, M. V. Lomonosov Moscow State University, Moscow, Russia
yunovich@scon175.phys.msu.su
Introduction. History of GaN– studies in USSR and Russia. GaN crystallographic structure (G.S.Zhdanov,
1936); growth of GaN crystals and films (Leningrad Institutes, 60-70th); luminescence studies (Moscow
University, 70-80th); white LEDs.
Research and development of GaN- based LEDs in 90th. Studies of LEDs from various laboratories (Nichia
Chemical, Toyoda Gosei, Hewlett-Packard, Ulm University, CRHEA). Mechanisms of radiative recombination
in InGaN/AlGaN/GaN heterostructures with quantum wells. Tunnel radiative recombination. Piezoelectric and
spontaneous polarization. Modulated doped structures.
All-Russian Workshops and Conferences “Nitrides of Gallium, Indium and Aluminum: Structures and
devices” in 1997-2003. Papers of the last Conference (St. Petersburg, February 2003) on UV luminescence.
Studies of AlGaN/GaN luminescence in Moscow University.
Problems of white LEDs development. Various methods to get white light emitting diodes. Development of
white LEDs on the base of blue InGaN/GaN heterostructures coated by yellow-green phosphors. Power and
color problems. Studies of white LEDs from various laboratories (Corvette Lights, Optel). White LEDs based on
ultraviolet AlGaN/InGaN/GaN structures coated by three phosphors: merits and deficiencies.
Quantum Phosphors: Observation of the Photon Cascade Emission Process
for Pr3+-doped Phosphors under Vacuum Ultraviolet (VUV) and X-Ray
Excitation
Arjan P. Vink1,2, E. van der Kolk1, P. Dorenbos1, and C. W. E. van Eijk1
1
Radiation Technology Group, Interfaculty Reactor Institute, Delft University of Technology, Delft,
The Netherlands
2
Chemical Sciences, Netherlands Organisation for Scientific Research, The Hague, The Netherlands
vink@nwo.nl
In tube lighting (TL), mercury is used to excite (λmax=254 nm) three phosphors, resulting in white light. The use
of mercury however gives environmental problems and causes an undesired delay in startup. This delay limits
the number of applications for TL lighting and is caused by the presence of mercury as a liquid. When the tube is
switched on, the mercury first has to evaporate.
If the mercury is replaced by xenon, which is already in the gaseous state and is harmless to the
environment, both problems are absent. The emission from xenon is however at higher energy (λmax= 172 nm)
and the phosphors, used in mercury-based tube lighting, show a less efficient absorption to this vacuum ultra
violet (VUV) radiation. Therefore, much effort is put in developing new phosphors.
The Pr3+ (4f2) ion shows strong absorption in the VUV range, which is assigned to the 4f2→4f15d1
transition. Another interesting effect is that in some hosts the Pr3+ ion can, after excitation into the 4f15d1 bands,
show a two-step relaxation to the ground state. This process is called photon cascade emission or quantum
cutting and could result in a quantum efficiency larger than 100%. The emitted photons are typically in the
ultraviolet (UV) and green spectral range.
In this talk the method to select host crystals in which Pr3+ shows quantum cutting from data available on
the Ce3+ (4f1) ion is discussed. Furthermore several Pr3+-doped hosts are presented, which show the photon
cascade emission process both under VUV and host excitation.
Novel Heterostructures for UV-LEDs and Biosensors
Martin Stutzmann
Walter Schottky Institut, Technische Universitaet Muenchen, Garching, Germany
stutz@wsi.tum.de
The AlGaN-alloy system would be ideally suited for UV light emitting diodes as well as UV detectors due to the
continuously variable direct bandgap between 3.4 and 6.2 eV. The versatility of AlGaN for UV detection will be
briefly demonstrated on the basis of simple heteroepitaxial structures designed for selective narrow-band UV
detection (e.g. for combustion control). Much more challenging is the development of efficient LEDs in the UVB or UV-C spectral range. Here, the main problem is the inefficient p-type doping of AlGaN with Al
concentrations above 30 %. To overcome this problem, the heteroepitaxy of n-type AlGaN (doped with Si) on
p-type diamond substrates (boron-doped) is an interesting approach.
The current status of AlGaN/diamond heterostructures and possible improvements for the near future will
be discussed. Finally, I will address the use of AlGaN/GaN heterostructures for future biosensors. An interesting
issue of such sensors is the possibility to combine or even integrate sensitive electronic detection of ionic
processes in cells or cell models with the well established fluorescence techniques widely used for the
investigation of biological or physiological processes.
Nitride Photodetectors in UV Biological Effects Studies
Elias Muñoz, José L. Pau, and Carlos Rivera
Institute for Systems based on Optoelectronics and Microtechnology and DIE ETSI Telecomunicación,
Univ. Politécnica de Madrid, 28040-Madrid, Spain
elias@die.upm.es
The role of the solar UV radiation in our biosphere will be briefly reviewed. Stratospheric ozone absorption,
UVA+UVB radiation and its action spectrum in various biological and medical processes, will be summarized.
DNA absorption (damage), chloroplast activity (photosynthesis inhibition), and erythema (sunburn) are key
cases of the UV bioaction.
Some applications of the UV radiation in recent biophotonics techniques will be shortly described.
Fluorescence has become one of the most important techniques in present biological and medical studies. The
use of dyes and micro spheres, and specially the recent discovery of fluorescent proteins, have open new
possibilities.
Finally, requirements and potential applications of AlInGaN-based UV detectors will be described in the
above framework. Long-term stability and reproducibility are two main issues. Suitable photodetector structures
will be described. AlGaN detectors fitting very well the erythema action spectrum are available. Problems in
present nitride-III devices, related to traps, slow time-domain effects, reproducibility and photoconductive gain
will be presented. QW photodetectors that use present InGaN technology are suggested as adequate solutions in
biophotonics applications. Multifunctional, integrated biochips, where optical excitation and detection is based
only on nitrides seem to be quite feasible today.
Biosensors
Valdas Laurinavičius
Institute of Biochemistry, Vilnius, Lithuania
valdasL@bchi.lt
Biosensors are analytical devices consisting of biological component responsible for the recognition of the target
compound, and electrical or electrochemical transducer, responsible for the conversion of the reaction of the
biological component into electrical signal. Two points of view will be discussed: (i) nature of biological
material in biosensors; (ii) methods of signal processing. Enzyme-based biosensor will be presented and
discussed in details. Amperometric oxidases containing biosensors are historically first designed and mostly
widespread. Wiring of the oxidases by redox active molecules and redox polymers will be discussed. Behavior
of multienzymatic systems, kinetics of multienzymatic systems also will be presented. Main parameters of the
biosensors on the basis of cells, tissue and organelles will be discussed.
Electrochemical, electrical and optical methods of signal transduction will be reviewed and compared. Main
fields of application of biosensors in medicine, food quality control and environmental monitoring will be
discussed.
Detection of Hazardous Biological Agents Using UV Diode Lasers
Richard DeFreez
Principal Scientist, Photonics R&D Manager, Pacific Scientific Instruments, Grants Pass, OR, USA
rick.defreez@pacsciinst.com
Ultraviolet laser-induced auto-fluorescence (UV-LIF) of key biological substances provides a real-time
technique for detecting airborne pathogens such as aerosolized bacterial spores and viruses. To date, most LIF
bio-aerosol sensors have relied on harmonic generation of UV using pulsed solid-state lasers. Such lasers are
costly, bulky, inefficient, and fragile. A less obvious, but significant, drawback is that self-Q-switched diodepumped solid-state lasers used in leading bio-aerosol sensors (BAWS-III and BARTS) preclude the use of the
excitation beam for particle sizing (particle respirability determination). CW Ultraviolet diode lasers (UVDLs)
have long been recognized as a potential solution to this predicament, but, until recently, such devices didn’t
exist. In mid-2001, Nichia Corporation delivered the first CW, room temperature UVDLs to Pacific Scientific
Instruments (PSI). Within two months, PSI prototyped Bioni, a LIF bio-aerosol sensor based on these devices.
Bioni employs the CW diode laser to induce auto-fluorescence while simultaneously determining particle
respirability through direct detection of elastic scatter. PSI has demonstrated that Bioni can detect, within
seconds, the dispersal of threat organisms in postal sorting facilities and many other settings, including airports
and subways. However, because of its single excitation and single fluorescence wavelengths Bioni can be prone
to false positive alarms (FPAs). Beginning in 2002, PSI began developing BioLert which employs multiple
diode laser excitation wavelengths and/or multiple detection channels to reduce the probability of FPAs. The
Semiconductor Ultraviolet Semiconductor Optical Source (SUVOS) Program, MTO, DARPA, DOD, USA. has
fully sponsored and supplied the laser sources for the BioLert development.
Strain Energy Band Engineering and Innovative Growth Techniques for
Nitride-Based UV LEDs and Solar Blind Photodetectors
Remis Gaska1, Asif Khan2, and Michael S. Shur3
1
Sensor Electronics Technology, Columbia, SC (USA)
University of South Carolina, Columbia, SC (USA)
3
Rensselaer Polytechnic institute, Troy, NY, (USA)
gaska@s-et.com
2
Deep UV LEDs on sapphire substrates involve active regions comprised of pulsed atomic layer epitaxial (PALE)
deposited quaternary AlInGaN or ternary AlGaN multiple quantum wells (MQWs). The design of these layers
should account for strain and strain-induced and spontaneous polarization effects that we control using Strain
Energy Band Engineering approach based on strain compensation using varying indium composition in
quaternary AlGaInN compounds and using growth on non polar substrates. Device characterization of Deep UV
LEDs indicated a key role-played by current crowding, thermal effects, and active region defects originating
from buffer AlGaN layers in controlling the emitted powers. Using a unique AlGaN/AlN superlattice to control
strain we have deposited Si-doped high Al-content n+-AlGaN layers over sapphire with thickness in excess of 2
microns. These layers and a new active layer design have yielded high power deep UV LEDs with emission
wavelengths from 280-340 nm. Under pulsed pumping, the external quantum efficiencies of about 0.1 %
(278 nm), 0.45 % (325 nm) and 0.55 % (340 nm) of our UV LEDs represent record high values for the shortest
LED emission wavelengths to date. These devices exhibited pulsed powers as high as 3 mW, 10 mW and 13
mW for 1 A of pumping current.
Promising Results of Plasma Assisted MBE for Optoelectronic Applications
Alexandros Georgakilas
FORTH-IESL and University of Crete, Physics Dept., Microelectronics Research Group (MRG),
Heraklion, Greece
alexandr@physics.uoc.gr
Plasma-assisted MBE (PAMBE) is not considered yet as an epitaxial growth method that could produce device
quality material for optoelectronic applications. However, several results suggest that PAMBE can grow high
quality heterostructures on GaN templates and even directly on sapphire substrates. High quality N-face material
can be probably grown on sapphire only by PAMBE. Following the right treatment of sapphire nitridation, a
GaN layer-by-layer growth seems to be followed immediately from the sapphire surface. Recently, high quality
In-containing alloys, such as quaternary InAlGaN alloy thin films and quantum well heterostructures, have also
been grown by PAMBE with good control of the In incorporation. The layers exhibited strong PL up to room
temperature and lasing under optical pumping.
Low Dislocations Density GaN/Sapphire for Optoelectronics Devices
B. Beaumont, J-P. Faurie, E. Frayssinet, E. Aujol, and P. Gibart
Lumilog, Vallauris, France
piere.gibart@lumilog.com
It is nowadays well established that threading dislocations (TDs) are degrading the performances and the
operating lifetime of optoelectronics GaN based devices (LDs and UV-LEDs). GaN/sapphire layers have been
grown by Metal Organic Vapour Phase Epitaxy (MOVPE). An amorphous silicon nitride layer is deposited using
a SiH4/NH3 mixture prior to the growth of the low temperature GaN buffer layer. Such a process induces a 3D
nucleation at the early beginning of the growth, resulting in a kind of maskless ELO process with random
opening sizes. This produces a significant decrease of the TDs density compared to the best GaN/sapphire
templates. GaN layers with TD density as low as 7×107cm–2 were obtained (as measured by atomic force
microscopy (AFM), cathodoluminescence and transmission electron microscopy (TEM). The two-step epitaxial
lateral overgrowth technology (2S-ELO) allows decreasing the TDs around 107cm-2. These templates are suitable
for fabricating LDs. Regrowth by HVPE on these ELO GaN/sapphire further decreases the TDs density below
5×106cm–2.
Stimulated Emission and Gain in GaN Epilayers Grown on Si
Alexander L. Gurskii1, E. V. Lutsenko1, V. Z. Zubialevich1, V. N. Pavlovskii1, and
G. P. Yablonskii1
K. Kazlauskas 2, G. Tamulaitis 2, S. Juršėnas 2, and A. Žukauskas 2
Y. Dikme3, H. Kalisch3, A. Szymakowski3, and R. H. Jansen3
B. Schineller4, and M. Heuken4
1
Stepanov Institute of Physics of NAS Belarus, F. Skaryna Ave. 68, 220072 Minsk, Belarus
Institute of Materials Science and Applied Research, Vilnius University, Lithuania
3
Institut für Theoretische Elektrotechnik, RWTH, Aachen, Germany
4
AIXTRON AG, Aachen, Germany
gurskii@dragon.bas-net.by
2
GaN based heterostructures grown on silicon substrates are promising for creation of powerful and cheap LEDs
and LDs. Silicon substrates allow integrating GaN based light emitting devices with Si optoelectronics. Optical
and laser properties of GaN/Si and GaN/Si/SiO/Si with strain reducing AlGaN/AlN layer stack were investigated
in this work in wide intervals of optical excitation intensities and temperatures of 10-290 K. Both lasers with and
without SiO layers had the same laser threshold (700 kW/cm2) at room temperature. However, lasers with SiO
interlayer had higher (2-3 times) PL and laser efficiency. The net-gain spectra of all samples were measured at
room temperature. The peculiarities of the PL, laser and gain spectra are discussed from viewpoint of differences
in the carrier lifetime leading to an essential difference in the excited and lasing volume of the epitaxial layers as
well as of the role of absorption in the passive regions and in the substrates.
Materials Characterization of Group-III Nitrides under High-Power
Photoexcitation
Saulius Juršėnas, G. Kurilčik, S. Miasojedovas, and A. Žukauskas
Institute of Materials Science and Applied Research, Vilnius University, Vilnius, Lithuania
saulius.jursenas@ff.vu.lt
Group-III nitride materials are widely used for production of UV-blue light emitting diodes and lasers.
Luminescence spectroscopy is one of most powerful tolls used for material characterization. Here we report on
new phenomena, which can be observed in group-III nitrides by luminescence spectroscopy at high-density laser
excitation.
First we describe a high-density electron hole plasma in a GaN epilayer. We demonstrate carrier and
phonon system heating, temperature dependent band-gap renormalization and enhanced recombination dynamics
of nonthermalized degenerated carrier system.
Second we introduce material quality characterization method, based on luminescence transient studies
under the deep-trap saturation regime. Application of the method for various GaN epilayers grown on sapphire,
HVPE grown GaN and high-quality homoepitaxial GaN layers, will be demonstrated.
Third, advances of applications of high-density excitation for characterisation of various InGaN/GaN
multiple quantum wells will be shown. High excitation regime enables us to reveal In-segregation related
disorder under conditions of screening of built-in electric field by a high-density carrier system.
Small Internal Fields in Quaternary InAlGaN Heterostructures
Sławomir P. Lepkowski
High Pressure Research Center UNIPRESS, Warsaw, Poland
slawek@uipress.waw.pl
Built-in electric fields usually present in nitride ternary quantum wells reduce significantly the intensity of light
emission from these structures. Quaternary AlInGaN quantum heterostructures offer possibility of employing
independent bandgap and lattice-constant engineering leading to minimization of the magnitude of the built-in
electric fields. In this work we concentrated on the determination of the built-in electric field in two sets of
quaternary nitride multi-quantum wells with different quantum well widths. The samples used exhibit very
efficient photoluminescence in the near ultraviolet range, i.e., between 350 and 400 nm. One series of samples
consists of structures with lower Al-content barriers. The second set is based on high concentration of Al in
AlInGaN barriers. A significant contribution of the spontaneous and piezoelectric polarizations to the properties
is expected in these series of quantum wells. Measurements of the photoluminescence decay time as a function
of the quantum well width and a hydrostatic pressure shift of the emitted light energy vs. quantum well width are
used as sensors of the internal electric field. We have found surprisingly low magnitude of the built-in electric
field in the studied structures. This result is not confirmed by calculations based on available data concerning
nonlinear piezoelectric properties of the used nitride alloys. In the performed analysis we discuss possible
sources of that discrepancy.
MOCVD Growth of AlGaN Epilayers and AlGaN/GaN SLs in a Wide
Composition Range
Wsevolod V. Lundin
A. F. Ioffe Physico-Technical Institute of the RAS, St. Petersburg, Russia
lundin.vpegroup@mail.ioffe.ru
Peculiarities of AlGaN and AlGaN/GaN SLs growth were investigated using R&D-scale (Epiquip VP 50, 1x2
inch) and production-scale (AIX2000HT, 6x2 inch) MOCVD reactors. Structures with AlGaN in a whole
composition range were grown.
For both reactors it was revealed that dependence of AlGaN composition on TMAl flow has a strong trend
for saturation (more pronounced effect for the larger reactor).
The saturation seems to be a manifestation of parasitic reactions and a critical parameter responsible for it
is TMAl partial pressure in the reactor. Besides reduction of reactor pressure AlN mole fraction in AlGaN layers
may be increased by magnification of total flow in the reactor and reduction of TMGa flow. Up to 20% AlN
mole fraction was reached in AIX2000HT at 400 mbar reactor pressure using this practice. For growth of AlGaN
with AlN mole fraction above 60-70% (smaller reactor) ammonia flow also should be reduced to suppress
parasitic reactions.
Native Aluminum Nitride Substrates for UV Emitters
Leo J. Schowalter
Physics Dept., Rensselaer Polytechnic Institute, Troy, NY, USA
schowl@rpi.edu
Single-crystal aluminum nitride (AlN) substrates have the potential to dramatically improve nitride
semiconductor materials for deep UV devices. Advantages include the identical crystal structure and close lattice
match to high Al-content nitride alloys. In addition, AlN and GaN have closely matched thermal expansions over
typical growth temperatures. The AlN has a band-gap energy of 6.2 eV with an index of refraction 2.2 which is
attractive for the extraction of UV light.
We use a sublimation-recondensation approach to produce large bulk AlN single crystals. Single crystal
boules 15 mm in diameter and several cm in length have been grown and substrates with dislocation densities
below 103 cm–2 and with a thermal conductivity exceeding 3 W/cm⋅K, have been obtained from those boules.
Crystal IS is currently developing larger diameter substrates.
Optical transmission measurements typically show a large and rather broad absorption peak centered at
4.5 eV. This peak is related to oxygen impurities and varies from approximately 25 to over several hundred cm–1
in our single-crystal material. The yellow color typically observed in our substrates is due to absorption at
2.8 eV. The origin of this absorption is not currently understood although we speculate that it is due to nitrogen
vacancies.
The current state-of-the-art and key issues regarding the fabrication of single-crystal AlN substrates from
bulk AlN will be reviewed.
Optical and Electrical Characteristics of III-Nitrides Device Correlated
with the Degree of Order of Mosaic Structure
Natalia M. Shmidt
A. F. Ioffe Physico-Technical Institute of the RAS, St. Petersburg, Russia
natalia.shmidt@mail.ioffe.ru
A mosaic (domain) structure (MS) is typical of III-nitrides of the hexagonal modification. A new
characterization method of MS peculiarities has been used in this study. It is based on multifractal analysis of
AFM data and it quantitatively characterizes such MS peculiarities as the degree of order of MS (DOMS).
The correlation between DOMS and the properties of III-nitrides (GaN, AlGaN) such as charge carrier
mobility, the main photoluminescence peak position, as well as the device parameters such as the main
electroluminescence peak position and the efficiency of InGaN/GaN light-emitting diodes, the persistence
photoconductivity, the leakage currents of photodetectors has been established.
The difference in behavior of epilayers with well (DOMS<0.35) and poor (DOMS>0.35) structural ordering
under irradiation is discussed.
Thermoactivation and Spectroscopy of Charge Localization States in
InGaN/GaN Quantum Wells
Maris Springis, P. Kulis, and I. Tale
Institute of Solid State Physics, University of Latvia, Riga, Latvia
sprigis@cfi.lu.lv
It can be expected that like other structure defects and impurities the QD will act as deep traps for charge
carriers. By injection or photo-generation of free electrons and holes subsequent capture of charge carriers in QD
can be important formation mechanism of excitons.
Thermostimulated depolarization and thermostimulated recombination luminescence are utilized to
investigate the relative concentration and the thermal activation energy of traps. The TSL curve monitored after
excitation in the fundamental absorption band is represented by the temperature independent luminescence up to
the main TSL peak at 255 K. The TSDC curve qualitatively coincides with the TSL curve showing peak in the
250–260 K region. Temperature independent afterglow indicates that the recombination process is caused by
tunneling recombination in close donor–acceptor pairs tunneling recombination followed by the thermoactivated
recombination in the donor–acceptor pairs in the 350–375 nm region.
GaN Schottky Barriers and MSM UV Photodetectors
B. Boratynski and M. Tlaczala
Faculty of Microsystem Electronics and Photonics, Wroclaw University of Technology, Wroclaw, Poland
bborat@wemif.pwr.wroc.pl
GaN Schottky diodes and MSM UV photodetectors were fabricated on GaN grown by MOCVD technique
sapphire substrates. Process technology of temperature stable Schottky barrier contacts to GaN and AlGaN
epitaxial layers, was elaborated. Electrical parameters of the Schottky barriers were evaluated in a range of
temperatures up to 260C. The Schottky barrier height, calculated from I-V measurements was 0.77eV; the
ideality factor value was 1.7. The MSM photodetectors of two Pt/Au contact geometry: 2µm/3µm and 1µm/2µm
were used. The response to UV light was measured using mercury lamp, chopper and a lock-in technique.
Output ac signal dependence on the photodetector bias up to 25V was evaluated. The responsivity of 0.3 A/W
was obtained. The detectors were insensitive to wavelengths above 400nm. The MSM photodetector response to
305nm-laser beam was also measured. In a parallel study, test structures of AlGaN/GaN HFETs were fabricated
and their electrical parameters have been evaluated.
III-Nitride Based Ultraviolet Surface Acoustic Wave Sensors
Daumantas Čiplys1,3, A. Sereika1, R. Rimeika1, R. Gaska2, M. Shur3, J. Yang4, and
M. Asif Khan4
1
Dept. of Radiophysics, Vilnius University, Vilnius, Lithuania
Sensor Electronic Technology, Inc., Columbia, SC, USA
3
Dept. of Electrical, Computer, and Systems Engineering, Rensselaer Polytechnic Institute, Troy, NY, USA
4
Dept. of Electrical Engineering, University of South Carolina, Columbia, SC, USA
daumantas.ciplys@ff.vu.lt
2
Large piezoelectric constants of AlN, GaN, and their alloys make these materials attractive for applications
involving surface acoustic waves (SAW). The electrical conductivity and, thus, SAW velocity in these materials
is affected by UV radiation. This allowed us to develop a SAW-based UV GaN sensor by placing a SAW
element into an oscillator feedback loop [1-3]. The output of such sensor is a radio signal with UV radiationdependent frequency, which makes this sensor especially attractive for remote sensing applications. In addition,
the spectrum line width depends on the noise of the UV source and is especially narrow for solar radiation
allowing for a solar-blind detection. We will present the basic principles of the III-nitride based SAW UV
sensors, summarize the results of theoretical simulations and experimental investigations of their properties, and
discuss possible applications.
[1] D. Ciplys, R. Rimeika, A. Sereika, R. Gaska, M. S. Shur, J. W. Yang, and M. A. Khan, Electron. Lett. 37, 545 (2001).
[2] D. Ciplys, R. Rimeika, M. S. Shur, R. Gaska, A. Sereika, J. Yang, and M. Asif Khan, Electron. Lett. 38, 134 (2002).
[3] D. Ciplys, R. Rimeika, M. S. Shur, S. Rumyantsev, R. Gaska, A. Sereika, J. Yang, and M. Asif Khan, Appl. Phys. Lett.
80, 2020 (2002).
Optically Pumped Lasing in InGaN/GaN/AlGaN MQW Laser Structures
Vitalii Yu. Ivanov
Institute of Physics, Polish Academy of Science, Warsaw, Poland
ivanov@ifpan.edu.pl
Optical pumping experiments can be an important tool used, not only for fast characterization of semiconductor
laser structures, but also for basic studies of their parameters. This method allows for measuring optical lasing
threshold and also provides a unique possibility to study optical gain and stimulated emission.
We report ultra low threshold powers for optical pumping of stimulated emission from InGaN/GaN/AlGaN
laser diodes grown on bulk GaN substrates. The threshold powers at room temperature are between 2.4 and 5.8
kW/cm2, depending on a cavity length, i.e., the lowest ever reported. We believe that this is the consequence of a
significant reduction of concentration of nonradiative centers in the active layer of homoepitaxial structures. We
report also that spacing of the longitudinal laser modes exceeds by the factor of almost 10 the value predicted
from the standard calculations.
High-Power LED and its Thermal Management
Adrian Mahlkow
Optotransmitter Umweltschutz Technologie e.V. (OUT e.V.), Berlin, Germany
adrian@mahlkow.com
The research institute OUT e.V. (Berlin) developed a high-power LED (HL-LED) for SMD (Iron and Reflow).
With an outline 1210 (3.2×2.8 mm2) the device can perform chip-size up to ‫ٱ‬1.2 mm, power dissipation up to
1,500 mW (e.g. 400 mA for InGaN chips) in a corresponding thermal ambient. The thermal resistance is
RJD<15 K/W. For high integrated applications (spotlights, general lighting) special PCBs with isolating layers
thinner than 15 µm (commercial solutions: 150 µm) were developed in a second cooperation research project.
Modules were realized on 1 mm copper sheet, area 40×40 mm2 (produced with standard PCB-technologies) with
100 HL-LEDs (Chips: ‫ٱ‬400 µm, AlInGaP, 589 nm, 150 A/cm2) with 250 mA each: Ptot=50 W, Popt=8 W,
thermal resistance RDA=6 K/W (Picture: thermal distribution).
Detection of Blue Light by Self-Assembled Monolayer of Dipolar Molecules
O. Neilands1, Inta Muzikante2, E. Fonavs2, L. Gerca2, S. Jursenas3, R. Valiokas4,
R. Karpicz5, L. Valkunas5
1
Riga Technical University, Riga, Latvia
Institute of Physical Energetics, Riga, Latvia
3
Institute of Materials Science and Applied Research, Vililnius University, LT-2040 Vilnius, Lithuania
4
Institute of Technology, Linköpings University, Linköping, Sweden
5
Institute of Physics, Vilnius, Lithuania
intam@edi.lv
2
Dipolar donor-acceptor molecules show pronounced changes in the dipole moment upon photoexcitation, resulting in
transformation of their optical and electrical properties. A monolayer of 4’-(3H-1,2,5-dithiazepan-5-yl)benzylidene
indan-1,3-dione (DMABI-2S) compound, possessing high electric dipole moment is self-assembled (SAM) on
Au layer. Analysis of IR vibrational and fluorescence spectra of SAMs supports the conclusion about the specific
tight packing of the molecules in the SAM structure.
The changes of the surface potential on irradiation with blue-light is measured by Kelvin probe technique.
The reversible changes of surface potential of the order several tenths of milivolts on irradiation take place. The
relationship between absorption spectra and spectral dependence of surface potential is observed.
Such SAM structures can be applied for design of molecular sensors and 2D recognition devices.
Atomic and Molecular Spectroscopy with UV and Visible Superbright
LEDs
Goran Pichler
Institute of Physics, Zagreb, Croatia
pichler@ifs.hr
Our most recent absorption and emission experiments with heavy alkali vapor using superbright light emitting
diodes from NICHIA, LUMILEDS and OSRAM will be presented. UV LEDs together with LEDs in the visible
spectral region present invaluable narrow band continuum light source for very precise absorption
measurements. We used all-sapphire cells in a usual absorption arrangement high resolution scanning
monochromators. Beside self-broadened alkali resonance lines we investigated molecular bands and diffuse
bands of alkali dimers similar to experiment in ref. (1). A number of applications, especially in dentistry will be
also presented.
Towards the Hybrid Biosensors Based on Biocompatible π-π Conjugated
Polymers
A. Ramanavičienė, A. Finkelšteinas, and Arūnas Ramanavičius
Faculty of Chemistry, Vilnius University, Vilnius, Lithuania
arunas@imi.lt; arunas.r@bchi.lt
The effective combination of biological and physical methods in analytical device could provide the basis for
direct detection of wide range of analytes with great sensitivity and specificity. The main aim of here presented
work is to construct affinity biosensors based on dual detection (optical and electrochemical) system. Current
state of research and future perspectives of π-π conjugated polymer polypyrrole application in hybrid biosensors
will be presented. Investigations of polypyrrole shows that florescence of this π-π conjugated polymer is
undetectable. It can be successfully exploited as the immobilization matrix in fluorescence based biosensors. The
biological recognition parts of affinity biosensors were based on polypyrrole doped by bovine leukemia virus
(very similar to human immunodeficit virus) proteins able to bind specific antibodies or single-stranded DNA
able specifically recognize and bind provirus DNA. The electrochemical investigations of affinity biosensor will
be presented.
UV Absorption Spectroscopy Applied for the Environmental Mercury
Detection
Andrius Urba and K. Kvietkus
Institute of Physics, Vilnius, Lithuania
urba@ktl.mii.lt
Mercury (Hg) is considered as important environmental pollutant worldwide. On the other hand, it is the only
metal which is found in the form of monoatomic gas under the natural conditions in the atmosphere, and thus it
can be detected by the atomic absorption UV spectrometry without using an atomization step.
In the Institute of Physics, a mercury analyzer based on a two-beam, light modulation and phase-sensitive
detection atomic absorption photometry is built. The analyzer features a superior detection limit of 0.5 ppt Hg
that is among the best of portable mercury analyzers worldwide.
To be able to build a stable operating, compact and sensitive UV photometers for Hg detection, it is a
crucial point to possess stable, sensitive and long-living UV radiation sources and detectors. Up to now, the
conventional low pressure Hg EDL lamps have been used as the radiation sources and UV-range vacuum
phototubes as the detectors. The main disadvantage of the components is that Hg EDL lamps are not stable
enough against temperature changes and are aging rather fast. The conventional phototubes feature rather high
dimensions and low aperture, thus causing certain limitations on the instrument design and its compactness. The
use of solid-state UV radiation emitters and detectors is now being considered for the UV absorption mercury
detection technique.
Optically Pumped UV-Blue Lasers Based on InGaN/GaN/Al2O3 and on
InGaN/GaN/Si Heterostructures
Gennadii P. Yablonskii1, A. L. Gurskii1, E. V. Lutsenko1, V. Z. Zubialevich1,
V. N. Pavlovskii1, A. S. Anufryuk1, Y. Dikme2, H. Kalisch2, R. H. Jansen2, B. Schineller3,
and M. Heuken3
1
Stepanov Institute of Physics of NAS Belarus, Minsk, Belarus
Institut für Theoretische Elektrotechnik, RWTH, Aachen, Germany
3
AIXTRON AG, Aachen, Germany
yablon@dragon.bas-net.by
2
Optically pumped lasers based on GaN epitaxial layers and InGaN single, multiple and electroluminescence test
quantum well heterostructures grown on sapphire and silicon substrates are investigated as functions of
temperature (80–650 K), excitation intensity of the nitrogen and dye laser radiation (10–100 kW/cm2), excitation
and operation wavelengths, MOVPE growth conditions. Laser action was achieved in all types of the
heterostructures from the near ultraviolet (370 nm) up to the blue spectral region (470 nm). The lowest laser
threshold at room temperature was 35 kW/cm2, the maximal laser power was 80 W, and the half width of the
laser line was 0.04 nm. The maximal operating temperature of 630 K was for InGaN/GaN/Si lasers. On the base
of investigation of temperature dependence of laser threshold and photoluminescence spectra,
photoluminescence and laser excitation spectra, conclusions about the role of the localized states in the gain
mechanisms were made.