Dilute nitrides have emerged from conventional III–V semiconductors
such as GaAs or InP by the insertion of nitrogen into the group V
sub-lattice, which has a profound influence on the electronic
properties of these materials and allows widely extended band
structure engineering. This is expected to lead to novel devices,
e.g. for optical data transmission, solar cells, biophotonics or gas
sensing, some of which are already making their way into the
market. Unlike in all other cases, where a reduction in bandgap
energy is achieved by inserting an element that increases the
lattice constant, N accomplishes this and at the same time reduces
the lattice constant. Thus smaller bandgaps can be achieved and the
unusual role of N in the lattice also allows a tailoring of band alignments.
Both of these effects have opened up a new dimension of bandgap
engineering and the rapid progress in the field led to the demonstration
of high quality 1300 nm lasers on GaAs and eventually to the realization
of the first VCSELs that can be mass produced at low cost and emit
at 1300 nm. This in turn will allow extending inexpensive data
transmission through optical fibers from the present range of
about 300 m to a distance of 10 to 20 km and at the same time
increasing the data rate by about a factor of four. Thus it will enable
metro-area data links, which are presently considered to be the
bottleneck for large-scale optical communications. Furthermore,
the fact that GaNP and related alloys can be grown lattice-matched
on Si substrates has offered intriguing new possibilities of OEIC
and integration of efficient III–V optoelectronic devices with
the mainstream microelectronics based on Si.
GaAs structure
Despite their promising applications and the first encouraging
experimental results, very little is known about the physical
properties of such alloys. For instance the difficulty of
incorporating nitrogen into GaInAs while maintaining good
optical quality has provoked much work to establish an
understanding of the underlying factors determining the
optical quality of GaInNAs, such as composition, growth
and annealing conditions. We are still far from establishing
an understanding of the band structure and its dependence
on composition. Fundamental electronic interactions such
as electron–electron and electron–phonon scattering,
dependence of effective mass on composition, strain and
orientation, quantum confinement effects, effects of localized
nitrogen states on high field transport and on galvanometric
properties, and mechanisms for light emission in these materials,
are yet to be fully understood. Nature and formation mechanisms
of grown-in and processing-induced defects that are important
for material quality and device performance are still unknown.
Such knowledge is required in order to design strategies to
efficiently control and eliminate harmful defects. For many
potential applications (such as solar cells, HBTs) it is
essential to get more information on the transport properties
of dilute nitride materials. The mobility of minority carriers is
known to be low in GaInNAs and related material. The
experimental values are far from reaching the theoretical ones,
due to defects and impurities introduced in the material during
the growth. The role of the material inhomogeneities on the
lateral carrier transport also needs further investigation.
From the device's point of view most attention to date has
been focused on the GaInNAs/GaAs system, mainly because
of its potential for optoelectronic devices covering the
1.3–1.55 µm data and telecommunications wavelength bands.
As is now widely appreciated, these GaAs-compatible
structures allow monolithic integration of AlGaAs-based distributed
Bragg reflector mirrors (DBRs) for vertical cavity surface-emitting
lasers with low temperature sensitivity and compatibility with
AlOx-based confinement techniques. In terms of conventional
edge-emitting lasers (EELs), the next step is to extend the
wavelength range for cw room-temperature operation, as well
as improving the spectral purity, modulation speed and peak
power output. Many applications in medicine, environmental
sensing and communications can be addressed with the
achievement of significant improvements in these parameters.
Semiconductor optical amplifiers (SOAs) are also important
devices of interest, since it is widely predicted that the market
for SOAs in photonic access networks will increase dramatically
in the next few years. In addition to EELs and SOAs, vertical
cavity surface-emitting lasers (VCSELs), vertical external cavity
surface-emitting lasers (VECSELs), vertical cavity semiconductor
optical amplifiers (VCSOAs), and semiconductor saturable
absorber mirrors (SESAMs) are of increasing importance.
The VECSELs can potentially incorporate saturable absorbers
for very high repetition rate (~100 GHz) pulsed and potentially
MEMS-tuneable sources. VECSEL devices in the 2–3 µm range
for applications in e.g. free-space optical (FSO) communications,
are possible using InAsN/InGaAs/InP with AlGaAs metamorphic
mirror growth. Semiconductor saturable-absorber mirror structures
(SESAMs) have demonstrated widespread applicability for
self-starting passive mode locking of (diode-pumped) solid-state
lasers, to produce high-performance picosecond and femtosecond
laser sources for scientific, instrumentation and industrial use.
Very recently, these devices have also shown applicability for ultra
short pulse generation at >GHz repetition rates, both in DPSS lasers
and surface-emitting semiconductor lasers. These devices are undoped
monolithic DBR structures incorporating one or more quantum
wells for saturable absorption. Low-loss and high-damage threshold
requirements demand pseudomorphic growth, and have, until very
recently, essentially limited these devices to the 800–1100 nm range,
but extension beyond this range is urgently required by a host of
mode locking applications. In addition to these devices modulators and
photodiodes, including quantum well infrared photodetectors (QWIPs)
and resonant cavity-enhanced photodiodes (RCEPDs) based on dilute
nitrides need to be investigated extensively.
To date, most theoretical attention has been focused on understanding
the band structure of the GaInAsN/GaAs system and on evaluating gain
spectra and threshold conditions for 1.3 µm lasers. However, as our
understanding of band structure and the effects of strain, defects, etc
in dilute nitrides improves we can calculate the electrical and optical
properties, including radiative and non-radiative recombination for the
materials and structures of interest. The spontaneous and stimulated
emission rates have already been calculated for GaInNAs at 1.3 µm by
many authors, but extension to other dilute nitrides and other wavelength
ranges still represents a major challenge. Many-body effects, including
exchange-correlation effects, are essential for accurate models of gain
spectra in lasers and optical amplifiers. The differential gain is a key
parameter for laser modulation and remains an important subject of
study as new materials and structures are explored. Similarly the
differential refractive index and linewidth enhancement factor have
strong influences on laser spectrum (chirp, linewidth), dynamics and
noise, and these must also be studied theoretically. As regards to
non-radiative recombination, in addition to recombination through
defects, the Auger effect is of especial significance for wavelengths
beyond 1 µm and is a worthy subject for theoretical study. The
converse effect, impact ionization, is of key importance for avalanche
photodiodes (APDs) and has yet to be evaluated for the dilute nitride
materials. Inter-valence band absorption (IVBA) is of significance, as
a possible cause of temperature sensitivity in lasers and this must
be investigated theoretically in the dilute nitrides. Third-order
non-linear optical coefficients should be calculated in order to
assess the scope for all-optical signal processing components
within the dilute nitrides. Electro-absorption and electro-refractive
effects—Franz-Keldysh (FK) and quantum-confined Stark effect
(QCSE) need to be studied theoretically in view of their
importance for optical modulators.
The aim of this special issue is to review the recent progress
in theory, growth, characterization and device applications of
dilute nitrides, and to collate what is known and what is not
known in the field and address important fundamental physical
properties and key material and device issues. The issue brings
together a wide selection of papers from over 27 prominent
research groups that have made key contributions to the field
in the areas of research including growth, characterization
and physical properties, devices and device integration, and
theory and modelling. The editor is very grateful to all the
invited authors for their contribution to this issue of Journal
of Physics: Condensed Matter.
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