Table of contents

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.

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.

I am grateful to Professors M J Adams, X Marie and Dr H Riechert for their help and contributions to the preparation of the editorial.

The epitaxial growth and characterization of dilute Ga_1−xIn_xAs_1−yN_y films and quantum wells are presented. Starting with the epitaxy on GaAs, recent results on the local bonding of nitrogen in Ga_1−xIn_xAs_1−yN_y are reviewed, revealing that bonding of nitrogen is controlled by an interplay between bond cohesive energy and reduction of local strain. Thus, III–N bonding in Ga_1−xIn_xAs_1−yN_y can be changed from Ga–N to In–N by post-growth thermal annealing. Nitrogen loss due to the annealing process is not observed. We then adopted this concept for the epitaxy on InP substrates, which is equivalent to a drastic increase in indium content of the Ga_1−xIn_xAs_1−yN_y system and thus an extension to longer wavelengths. The low-energy shift of quaternary pseudomorphically strained Ga_0.4In_0.6As_1−yN_y double quantum wells upon nitrogen incorporation is reported. The deterioration of the photoluminescence characteristics in terms of reduced peak intensity and increased linewidth with increasing nitrogen incorporation can be partially compensated by rapid thermal annealing, which is accompanied by a blueshift with respect to the as-grown samples. Within the resolution limits of our secondary ion mass spectrometry experiments, no annealing-induced loss of nitrogen was observed even for the high indium content Ga_1−xIn_xAs_1−yN_y quantum well structures. The results on indium-rich highly strained Ga_0.22In_0.78As_0.99N_0.01 quantum wells on InP substrate are reported, showing room temperature photoluminescence at wavelengths up to 2.3 µm. We finally conclude with the demonstration of multi quantum well lasers emitting at wavelengths beyond 2 µm.

Structural and optical properties of GaInNAs/GaAs quantum structures grown by metalorganic vapour phase epitaxy (MOVPE) are studied. The growth of arsenide–nitrides by MOVPE is reviewed and compared to the other major growth technique, molecular beam epitaxy. Post-growth thermal annealing and laser irradiation are employed and found to affect GaInNAs quantum wells differently: with laser treatment no undesired blue-shift of the photoluminescence peak is observed. The critical thickness for misfit dislocation formation of GaAsN on GaAs is found to be about twice as large as the theoretical prediction. GaAsN epilayers are also found to contain Ga vacancies in defect complexes. The current status of experimental research on GaInNAs quantum dot structures is reviewed. Self-organized and strain-induced GaInNAs quantum dots are grown and their formation and optical properties are studied. The emission wavelength of InGaAs quantum dots is extended by using InGaAs and GaInNAs barrier layers.

An overview of our present knowledge and understanding of defects in dilute nitrides will be provided and their important roles in determining the success of dilute nitrides for optoelectronic device applications will be underlined. A brief summary of experimental results of defects by various techniques reported so far in the literature will be given. Our recent results from optically detected magnetic resonance studies of grown-in non-radiative defects in Ga(In)NAs and Ga(Al, In)NP will be discussed in some detail, in an effort to provide chemical identification and experimental signatures of defects. Among them, intrinsic defects such as antisites and self-interstitials have been positively identified, and the effects of growth conditions, chemical compositions and post-growth processing on the formation of the defects were studied. The information retrieved from the experimental findings is expected to provide useful guidance for designing strategies to eliminate defects that are harmful to device performance.

This paper reviews recent experimental progress in the investigation of nitrogen in GaAs and GaAs-based dilute nitrides by Fourier transform infrared absorption spectroscopy. Isolated substitutional nitrogen on the anion site gives rise to a local vibrational mode at a wavenumber of 472 cm⁻¹, which can be detected at low temperatures as a sharp absorption band. Based on a comparison with secondary ion mass spectroscopy and x-ray diffraction analysis, a calibration factor is derived for the integrated absorption of the band. Quantitative determination of substitutional nitrogen is possible in epitaxial layers of only 10 nm thickness. The influence of indium–nitrogen bonding on the local mode frequency is discussed. A mode at 490 cm⁻¹ is tentatively attributed to the InGa₃N configuration. An additional local vibrational mode occurs at 638 cm⁻¹. A valence-force model is presented, which reproduces quantitatively the experimentally observed host-isotope fine structure. The results are consistent with an assignment to a nitrogen–gallium vacancy complex.

We give an overview of the current knowledge of the relationship between the structural and electronic properties of heterostructures containing the metastable (Ga,In)(N,As) alloy. We focus on the effects of different growth methods (i.e. molecular beam epitaxy and metal-organic vapour-phase epitaxy) and varying annealing conditions. In as-grown samples, detailed TEM studies of the morphological structural phase transition occurring in Ga_1−yIn_yN_xAs_1−x above a critical N content x_c as well as of interface roughness and strain fields in Ga_1−yIn_yN_xAs_1−x/GaAs multiple quantum wells are presented. In a second part, we address the effects of thermal annealing on (Ga,In)(N,As)/GaAs quantum wells, in particular, focusing on phenomena related to the commonly observed large blueshifts of the (Ga,In)(N,As) band gap upon annealing. Extrinsic as well as intrinsic mechanisms contributing to the blueshift are described. Special attention is given to the intrinsic effect due to the rearrangement of the nearest-neighbour environment which changes from Ga-rich to In-rich during the annealing procedure. Our results suggest that by careful procedures (Ga,In)(N,As) material of high structural quality can be obtained and can be driven into a stable electronic configuration by annealing. The existence of such a stable electronic configuration and the reproducibility of the fabrication process are essential for employing metastable semiconductor alloys in devices.

In this paper, we present the application of photoreflectance (PR) spectroscopy to investigate the energy level structure of GaInNAs-based quantum wells (QWs). Series of single GaInNAs/GaAs QWs with different nitrogen and indium contents are analysed. The electron effective mass (m_e^*) and conduction band offset (Q_C) are determined and compared with the literature data. The Q_C in GaInNAs/GaAs system in the range of investigated GaInNAs content (28–41% of In, 0.3–5.3% of N) has been found to be almost the same as for GaInAs/GaAs system, i.e. . In addition, the energy level structure for the step-like GaInNAs/Ga(In)NAs/GaAs QWs tailored at 1.3 and 1.55 µm and the Sb-containing Ga(In)NAs/GaAs QWs is investigated. Also, the character of PR transitions, the influence of rapid thermal annealing (RTA) on the energy level structure, and the influence of the carrier localization effect on the efficiency of PR photomodulation are discussed.

The gain and α-factor spectra of a molecular-beam-epitaxy-grown 1.3 µm GaInNAs laser are experimentally determined using the method of Hakki and Paoli and a transmission method. The gain in our structure is due to inhomogeneously broadened band–band transitions but, in general, critically depends on the growth process. The value 2.5 for α at the laser emission wavelength clamps with increasing injection current.

GaAs-based dilute nitride semiconductor saturable absorber mirrors, SESAMs, have been developed and employed as cavity end mirrors of fibre lasers to produce ultra-short optical pulses over a wide spectral range. These devices are shown to be able to mode-lock diode-pumped erbium-doped and ytterbium-doped fibre lasers at around 1.55 and 1 µm, respectively, and produce reliable, self-starting picosecond pulse operation. The SESAM-equipped fibre lasers represent a promising route to designing a portable (sub-)picosecond light sources for low-cost maintenance turnkey operation for a variety of applications.

In this paper, use of a comprehensive self-consistent optical–electrical–thermal–gain threshold model of oxide-confined GaAs-based (GaIn)(NAs)/GaAs quantum-well vertical-cavity surface-emitting diode lasers (VCSELs) is demonstrated. The model was developed to facilitate better understanding of the physics of operation of the above devices including the full complexity of the many interactions, in the volumes of the devices, between individual physical phenomena. Besides this, the model was applied in designing and optimizing such VCSELs for long-wavelength 1.3 µm second-generation optical fibre communication system use and for examining their anticipated continuous-wave (CW) room-temperature (RT) low-threshold performance. The influences of many construction parameters on device CW RT lasing thresholds and mode selectivity were investigated. Some essential design guidelines were proposed to support efforts of technological centres to produce low-threshold single-mode RT CW devices. In particular, the 1.5λ-cavity VCSELs have been found to exhibit lower RT CW lasing thresholds, whereas the 3λ-cavity ones offer better thermal properties: higher values of the parameter T₀ and single-fundamental-mode operation possible at higher temperatures. As expected, the desired stable RT CW single-fundamental-mode operation is possible in smaller VCSELs (with active-region diameters not exceeding 6 µm in the case of the 1.5λ-cavity VCSELs). A deliberate detuning of the lasing mode from the RT active-region optical peak gain towards higher wavelengths ensures better device operation at higher temperatures.

We describe the use of x-ray absorption spectroscopy (XAS) with synchrotron radiation to study the local structure in dilute nitrides. After a brief description of the advantages of XAS to probe local atomic arrangements in semiconductor alloys we focus our attention on (InGa)(AsN). We discuss data which demonstrate that atomic ordering (in the form of an excess of In–N over Ga–N bonds) is present, but is significantly weaker than predicted; also, we show that the experimental values for the bond lengths are in agreement with recent models which take into account strain due to pseudomorphic growth.

In this paper we derive and discuss the boundary conditions for the electron wavefunction in general Ga_1−xIn_xN_yAs_1−y-based heterostructures described by the band anticrossing model. The use of these boundary conditions greatly simplifies the calculation of, for example, transition energies in quantum wells. We then apply the derived equations to model the temperature-dependent bandgap of Ga_1−xIn_xN_yAs_1−y/GaAs quantum well structures with high indium concentrations. From a fit to our experimental photoreflectance data we find evidence that the effective nitrogen level E_N in the band anticrossing Hamiltonian, measured with respect to the valence band edge, shifts to higher energies with decreasing temperature. This supports and extends similar results reported in the literature for low indium content epilayers.

The influence of nitrogen incorporation on the microstructure of dilute nitride films and quantum wells has been investigated using cross-section scanning transmission electron microscopy. The elemental distributions of Ga, In, N and As in GaInNAs and GaAsN films have been mapped by energy dispersive x-ray analysis. Below the solubility limit for nitrogen, homogeneous GaAsN and GaInNAs compositions are obtained of good crystalline quality near to the epilayer/GaAs interface. In GaAsN films containing nitrogen above the solubility limit a graded transition from arsenide-like to nitride-like material occurs after some 50 nm of growth. GaInNAs films containing nitrogen exceeding the solubility limit exhibit a 'cellular' microstructure consisting of a gallium-depleted InGaAs centre with walls of a GaN-like composition. A similar segregation effect is observed in GaInNAs quantum wells containing nitrogen around and above the solubility limit. The formation of In-rich regions occurs within the wells, which causes three-dimensional growth at higher nitrogen contents, with the consequent initiation of growth defects. The electronic structure of GaInNAs quantum wells has been investigated by mapping the variation in the plasmon frequency using electron energy loss spectrometry on a dedicated scanning transmission electron microscope. Kramers–Kronig analysis of the singly scattered low-loss region yields a measure of the local effective valence electron density.

We use a combination of magneto-tunnelling and photocurrent spectroscopy techniques to explore the admixing of the extended GaAs conduction band states with the localized N impurity states in dilute GaAs_1−yN_y quantum wells (QWs) with y<2%. We show that the current–voltage I(V) characteristics of our resonant tunnelling diodes are profoundly affected by the incorporation of N. For a small N content (y = 0.08%), electron tunnelling into the N-induced E₋ and E₊ hybridized subbands of the GaAs_1−yN_y QW layer leads to two main resonances in I(V). Magneto-tunnelling spectroscopy provides us with a powerful means of probing the energy–wavevector ε(k) dispersion curves of the E₋ and E₊ subbands. We find that E₋ and E₊ have a very well defined character and that the heavy effective mass E₊ states have a significant Γ conduction band character even at k = 0. In the devices with larger y-values (y>0.08%) the resonances are smeared out and the measured currents are much smaller. This is attributed to the disorder in the QW and carrier trapping on localized N levels, which break the momentum conservation condition required for observing sharp resonances in I(V). The current resonances can be partially recovered by optical excitation due to the effect on the current of photocreated holes that recombine with majority electrons tunnelling in resonant states of the QW. This photoinduced current enhancement depends on the optical absorption of the GaAs_1−yN_y QW and provides a means of measuring the energy position of the valence to E₋ subband transition at various y-values. Our study indicates that although the E₋ subband states are significantly energy broadened due to disorder effects, their energy position is well described by a band anticrossing model.

Photoluminescence (PL) under a magnetic field (B = 0–12 T) has been employed to determine the electron effective mass, m_e^*, exciton radius, r_exc, and degree of localization of carriers in In_xGa_1−xAs_1−yN_y/GaAs heterostructures. This study concerns nitrogen concentration values spanning from a very dilute (y<0.1%) to a full alloy (y = 5%) limit and indium concentrations x from 0% to . For indium-free samples, we find that m_e^* and r_exc undergo a sudden increase and squeezing, respectively, for . For indium-containing samples (), m_e^* and r_exc show a sudden variation similar to that for x = 0 but shifted to much higher N concentration (). In addition, in the N alloy limit the temperature dependence of the B-induced shift of the PL peak position reveals that radiative recombination at low temperature is not excitonic.

The application of electron spectroscopies in dilute nitride semiconductor research for both chemical analysis and the determination of electronic and lattice vibrational properties is described. X-ray photoelectron spectroscopy of the nitrogen bonding configurations in dilute InN_xSb_1−x and InN_xAs_1−x alloys is presented. High resolution electron-energy-loss spectroscopy (HREELS) of the plasmon excitations in InN_xSb_1−x is shown to provide information on the electronic properties of the material, before and after annealing. HREELS is also used to investigate the GaN-like phonon modes in GaN_xAs_1−x alloys.

We have calculated the band structure of InGaAsN/GaAs(N)/GaAs compressively strained quantum wells (QW) emitting at 1.3 µm using the band anticrossing model and an eight-band Hamiltonian. The calculated interband optical transition energies have been compared to the experimental ones deduced from photocurrent, photoluminescence and excitation of photoluminescence spectroscopy experiments and measured laser characteristics extracted from the recent literature. Because of the high compressive strain in the QW, strain-compensated structures may be required in order to grow stable multiple QWs; in view of this we have studied the band structure of InGaAsN/GaAsP/GaAs QWs emitting at 1.3 µm. Dilute nitride structures also offer the possibility of growing tensile strained QW lasers on InP substrate emitting in the 1.55 µm emission wavelength range. In order to evaluate the potentialities of such structures we have determined the band characteristics of InGaAsN/InGaAsP/InP heterostructures with a TM polarized fundamental transition.

The effect of post-growth thermal annealing on crystallinity and bandgap in GaInNAs systems is systematically investigated. Annealing greatly improves the crystallinity of GaInNAs and also blue shifts the bandgap. This improved crystallinity is due to the elimination of non-radiative centres, and is not directly related to the blue shift in the bandgap. The blue shift may be closely related to the red shift in the bandgap seen when N is added to GaInNAs; if so, this would provide a clue to the reasons for the red shift in the bandgap of III–N–V alloys. The unexpected behaviour of the bandgap in GaInNAs is investigated by examining the bonds through optical phonons. We have found that the bandgap varies with the strength or length of the GaN bonds in GaInNAs. Annealing changes atomic positions in a unit cell of GaInNAs and thus blue shifts its bandgap.

We report a detailed structural and optical characterization of Ga_0.46In_0.54N_xP_1−x () films grown by gas-source molecular beam epitaxy on GaAs(001) substrates. The polarized high resolution x-ray rocking curves (HXRC) and contactless electroreflectance (CER) and piezoreflectance (PzR) spectra at room temperature show anisotropic character along the [110] and directions. Ordering-induced superlattice-like microstructure observed in high resolution transmission electron microscope (HTEM) images confirms the spontaneous ordering in Ga_0.46In_0.54N_xP_1−x layers. In addition, the temperature dependent optical properties are characterized via polarized PzR measurements in the range between 15 and 300 K. The PzR spectra obtained are fitted using the first derivative of a Lorentzian line-shape functional form. The valence band maximum, crystal field/strain splitting and spin–orbit splitting to conduction band transition energies, denoted respectively as E_g, E_g+Δ₁₂ and E_g+Δ₁₃, are accurately determined. The temperature dependences of these near band edge critical point transition energies are analysed using the Varshni expression and an expression containing the Bose–Einstein occupation factor for phonons. The parameters that describe the temperature variation of the transition energies are evaluated and discussed.

We use an sp³s^* tight-binding Hamiltonian to investigate the band-anti-crossing (BAC) model for dilute GaN_xAs_1−x alloys. The BAC model describes the strong band-gap bowing at low N composition x in terms of an interaction between the conduction band edge (E₋) and a higher-lying band of localized nitrogen resonant states (E₊). We demonstrate that the E₋ level can be described very accurately by the BAC model, in which we treat the nitrogen levels explicitly using a linear combination of isolated nitrogen resonant states (LCINS). We also use the LCINS results to identify E₊ in the full tight-binding calculations, showing that at low N composition E₊ forms a sharp resonance in the conduction band Γ-related density of states, which broadens rapidly at higher N composition when the E₊ level rises in energy to become degenerate with the larger L-related density of states. We then turn to the conduction band dispersion, showing that the two-level BAC model must be modified to give a quantitative understanding of the dispersion. We demonstrate that the unexpectedly large electron effective mass values observed in some GaNAs samples are due to hybridization between the conduction band edge and nitrogen states close to the band edge. Finally we show that there is a fundamental connection between the strong composition-dependence of the conduction-band-edge energy and the n-type carrier scattering cross-section in Ga(In)N_xAs_1−x alloys, imposing general limits on the carrier mobility, comparable to the highest measured mobility in such alloys.

Here we present the physics and device characteristics of high performance strain-compensated MOCVD-grown 1200 nm InGaAs and 1300–1400 nm InGaAsN quantum well (QW) lasers. Utilizing the GaAsP barriers surrounding the highly strained InGaAsN QW active regions, high performance QW lasers have been realized from 1170 nm up to 1400 nm wavelength regions. The design of the InGaAsN QW active region utilizes an In content of approximately 40%, which requires only approximately 0.5–1% N content to realize emission wavelengths up to 1300–1410 nm. Threshold current densities of only 65–90 A cm⁻² were realized for InGaAs QW lasers, with emission wavelengths of 1170–1233 nm. Room temperature threshold and transparency current densities of 210 and 75–80 A cm⁻², respectively, have been realized for 1300 nm InGaAsN QW lasers. Despite the utilization of the highly strained InGaAsN QW, multiple-QW lasers have been realized with excellent lasing performance. Methods for extending the lasing emission wavelength up to 1400 nm with InGaAsN QW lasers are also presented. Theoretical analysis and experiments also show suppression of thermionic carrier leakages in InGaAsN QW systems leading to high performance lasers operating at high temperature.

We carried out magneto-absorption and Faraday rotation experiments to investigate the Zeeman splitting in GaInNAs alloys. We determined the effective g^*-factors for the fundamental transition , as well as for higher energy transitions and . The following experimental values of the electron and hole g-factors in Ga_0.96In_0.04N_0.01As_0.99 were obtained: g_E−^* = −0.7 ± 0.2, g_E+^* = +2.8 ± 0.8, g_Γ8^* = −1.6 ± 0.2 and g_Γ7^* = −4.9 ± 0.6 for the E₋, E₊, Γ₈ and Γ₇ band, respectively. The electron effective g-factor for the E₋ band was studied in detail as a function of N and In content. Oscillations of the Faraday rotation angle allowed us to determine the electron effective mass at the bottom of the conduction band for Ga_0.96In_0.04N_0.01As_0.99, m_e^* = 0.086 m₀.

Picosecond Raman spectroscopy has been used to study non-equilibrium electron distributions and energy loss rate in a metal–organic-chemical-vapour-deposition-grown In_xGa_1−xAs_1−yN_y (x = 0.03 and y = 0.01) epilayer grown on GaAs substrate. It is demonstrated that for the photoexcited electron–hole pair density of electron distributions can be described very well by Fermi–Dirac distributions with effective electron temperatures substantially higher than the lattice temperature. From the measurement of electron temperature as a function of the pulse width of the excitation laser, the energy loss rate in In_xGa_1−xAs_1−yN_y is estimated to be about 64 meV ps⁻¹. These experimental results are compared with those of GaAs and important implications are given.

We explore the potential of GaInNAs-based vertical cavity semiconductor optical amplifiers (VCSOAs) operating at 1.3 µm. The obvious advantage of structural compatibility of GaInNAs active regions to GaAs/AlAs DBRs is accompanied by good predicted performance characteristics, comparable to published results on other material systems. We examine the potential of GaInNAs-VCSOAs to be operated as modulators both in transmission and reflection, and optimization schemes are discussed.

Alloying III–V compounds with small amounts of nitrogen leads to a dramatic reduction of the fundamental band-gap energy in the resulting dilute nitride alloys. The effect originates from an anticrossing interaction between the extended conduction-band states and localized N states. The interaction splits the conduction band into two non-parabolic subbands. The downward shift of the lower conduction subband edge is responsible for the N-induced reduction of the fundamental band-gap energy. The changes in the conduction band structure result in a significant increase in electron effective mass and a decrease in the electron mobility, and lead to a large enhancement of the maximum doping level in GaInNAs doped with group VI donors. In addition, a striking asymmetry in the electrical activation of group IV and group VI donors can be attributed to mutual passivation process through formation of the nearest neighbour group-IV donor nitrogen pairs.

We have developed an InGaP/GaInAsN/GaAs double heterojunction bipolar transistor technology that substantially improves upon existing GaAs-based HBTs. Band-gap engineering with dilute nitride GaInAsN alloys is utilized to enhance a variety of key device characteristics, including lower operating voltages, improved temperature stability and increased RF performance. Furthermore, GaInAsN-based HBTs are fully compatible with existing high-volume MOVPE and IC fabrication processes. While poor lifetimes have limited the applicability of dilute nitride materials in photovoltaic applications, we achieve minority carrier characteristics that approach those of conventional GaAs HBTs. We have found that a combination of growth algorithm optimization and compositional grading are critical for improving minority carrier properties in GaInAsN. In this work, we characterize the impact of both carbon and nitrogen doping on minority carrier lifetimes in GaInAsN base layers. Minority carrier lifetimes are extracted from direct measurements on bipolar transistor device structures. Specifically, lifetime is derived from the DC current gain, or β, taken in the bias regime dominated by neutral base recombination. Lifetimes extracted using this technique are observed to be inversely proportional to both carbon and nitrogen doping. As with conventional C-doped GaAs HBTs, current soaking (i.e. burn-in) is found to have a significant impact on GaInAsN HBTs. While we can replicate poor as-grown lifetimes consistent with those reported in photovoltaic dilute nitride materials, our best material to date exhibits nearly 30 × higher lifetime after current soaking.

We present an overview of our optical characterization work on dilute nitride quantum well (QW) samples. A simple model for calculating interband transition energies is constructed, tested against published results and used to model experimental data. Steady state photoluminescence (PL), time-resolved PL and photomodulated reflectance measurements are utilized to characterize GaNAs/GaAs, GaInNAs/GaAs and InGaAs/GaAs QWs. The effects of carrier localization, hot-carrier relaxation, non-radiative recombination and the reduced bandgap temperature dependence of dilute nitrides are investigated. Emission from recombining hot carriers in a GaInNAs/GaAs QW is recorded and used to estimate the LO-phonon scattering energy. The addition of small fractions of N is found to have little effect on phonon energy, which is found to be  meV.