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.
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.
ROSSANA HERNANDEZ
ESTAD SOLIDO
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