Dr Igor Marko

Successful manipulation of halide perovskite surfaces is typically achieved via the interactions between modulators and perovskites. Herein, it is demonstrated that a strong-interaction surface modulator is beneficial to reduce interfacial recombination losses in inverted (p-i-n) perovskite solar cells (IPSCs). Two organic ammonium salts are investigated, consisting of 4-hydroxyphenethylammonium iodide and 2-thiopheneethylammonium iodide (2-TEAI). Without thermal annealing, these two modulators can recover the photoluminescence quantum yield of the neat perovskite film in contact with fullerene electron transport layer (ETL). Compared to the hydroxyl-functionalized phenethylammonium moiety, the thienylammonium facilitates the formation of a quasi-2D structure onto the perovskite. Density functional theory and quasi-Fermi level splitting calculations reveal that the 2-TEAI has a stronger interaction with the perovskite surface, contributing to more suppressed non-radiative recombination at the perovskite/ETL interface and improved open-circuit voltage (V-OC) of the fabricated IPSCs. As a result, the V-OC increases from 1.11 to 1.20 V (based on a perovskite bandgap of 1.63 eV), yielding a power conversion efficiency (PCE) from approximate to 20% to 21.9% (stabilized PCE of 21.3%, the highest reported PCEs for IPSCs employing poly[N,N ''-bis(4-butylphenyl)-N,N ''-bis(phenyl)benzidine] as the hole transport layer, alongside the enhanced operational and shelf-life stability for unencapsulated devices.

Over the last decade, 2,2 '',7,7 ''-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9 '-spirobifluorene (spiro-OMeTAD) has remained the hole transporting layer (HTL) of choice for producing high efficiency perovskite solar cells (PSCs). However, PSCs incorporating spiro-OMeTAD suffer significantly from dopant induced instability and non-ideal band alignments. Herein, a new approach is presented for tackling these issues using the functionality of organometallocenes to bind to Li+ dopant ions, rendering them immobile and reducing their impact on the degradation of PSCs. Consequently, significant improvements are observed in device stability under elevated temperature and humidity, conditions in which ion migration occurs most readily. Remarkably, PSCs prepared with ferrocene retain 70% of the initial power conversion efficiency (PCE) after a period of 1250 h as compared to only 8% in the control. Synergistically, it is also identified that ferrocene improves the hole extraction yield at the HTL interface and reduces interfacial recombination enabling PCEs to reach 23.45%. This work offers a pathway for producing highly efficient spiro-OMeTAD devices with conventional dopants via addressing the key challenge of dopant induced instability in leading PSCs.

Opto-stimulation of semiconductor-biointerfaces provides efficient pathways towards eliciting neural activity through selective spectral excitation. In visual prosthesis, tri-colour stimulation capability is the key to restoring full-colour vision. Here we report on investigation of organic photoactive π-conjugated donor–acceptor small molecules based on triphenylamine whose absorption spectra are similar to those of the photoreceptors of the human eye. Photoactive device fabrication and characterisation towards full colour, pixelated retinal prosthesis based on inkjet printing of these molecules is demonstrated, with round pixels reaching 25 microns in diameter. Photo-response is studied via interfacing with biological electrolyte solution and using long-pulse, narrow-band excitation. Both photo-voltage and photo-current responses in the devices with a ZnO hole-blocking interlayer show clear signatures of capacitive charging at the electrolyte/device interface, also demonstrating spectral selectivity comparable to that of human eye’ cones and rods.

A key component for the realization of silicon-photonics is an integrated laser operating in the important communication band near 1.55 μm. One approach is through the use of GaSb-based alloys, which may be grown directly on silicon. In this study, silicon-compatible strained Ga0.8In0.2Sb/Al0.68In0.32Sb composite quantum well (CQW) lasers grown on GaSb substrates emitting at 1.55 μm have been developed and investigated in terms of their thermal performance. Variable temperature and high-pressure techniques were used to investigate the influence of device design on performance. These measurements show that the temperature dependence of the devices is dominated by carrier leakage from the QW region to the Xb minima of the Al0.35Ga0.65As0.03Sb0.97 barrier layers accounting for up to 43% of the threshold current at room temperature. Improvement in device performance may be possible through refinements in the CQW design, while carrier confinement may be improved by optimization of the barrier layer composition. This investigation provides valuable design insights for the monolithic integration of GaSb-based lasers on silicon.

On-chip lasers are a key component for the realization of silicon photonics. The performance of silicon-based quantum dot (QD) devices is approaching equivalent QDs on native substrates. To drive forward design optimization we investigated the temperature and pressure dependence of intrinsic and modulation p-doped 1.3 μm InAs dot-in-well (DWELL) laser diodes on on-axis silicon substrates for comparison with devices on GaAs substrates. The silicon-based devices demonstrated low room temperature (RT) threshold current densities ( Jth ) of 192 Acm−2 (538 Acm−2 ) intrinsic (p-doped). Intrinsic devices exhibited temperature stable operation from 170-200 K. Above this, Jth increased more rapidly due to increased non-radiative recombination. P-doping increased the temperature at which Jth(T) started to increase to 300 K with a temperature insensitive region close to RT, but with a higher Jth . A strong correlation was found between the temperature dependence of gain spectrum broadening and the radiative component of threshold Jrad(T) . At low temperature this is consistent with strong inhomogeneous broadening of the carrier distribution, which is more pronounced in the p-doped devices. At higher temperatures Jth increases due to homogeneous thermal broadening coupled with non-radiative recombination. Hydrostatic pressure investigations indicate that while defect-related recombination dominates, radiative and Auger recombination also contribute to Jth .

GaAsBi QWs have the potential to remove inherent recombination losses thereby increasing the efficiency and reducing the temperature sensitivity of near-infrared telecommunications lasers. GaAsBi QW lasers are reported and prospects for 1550nm operation are discussed.

We find that non-radiative recombination plays an important role in p-doped quantum-dot lasers. Along with carrier thermalisation effects, this is responsible for the temperature insensitive operation as observed around room temperature in these lasers.

This paper proposes and demonstrates a new multiquantum well (MQW) laser structure with a temperature-insensitive threshold current and output power. Normally, the mechanisms that cause the threshold current (Ith) of semiconductor lasers to increase with increasing temperature T (thermal broadening of the gain spectrum, thermally activated carrier escape, Auger recombination, and intervalence band absorption) act together to cause Ith to increase as T increases. However, in the design presented here, carriers thermally released from some of the QWs are fed to the other QWs so that these mechanisms compensate rather than augment one another. The idea is in principle applicable to a range of materials systems, structures, and operating wavelengths. We have demonstrated the effect for the first time in 1.5 μm GaInAsP/InP Fabry-Perot cavity edge-emitting lasers. The results showed that it is possible to keep the threshold current constant over a temperature range of about 100 K and that the absolute temperature over which the plateau occurred could be adjusted easily by redesigning the quantum wells and the barriers between them. TEM studies of the structures combined with measurements of the electroluminescent intensities from the wells are presented and explain well the observed effects.

GaInAsSb/GaSb based quantum well vertical cavity surface emitting lasers (VCSELs) operating in mid-infrared spectral range between 2 and 3 micrometres are of great importance for low cost gas monitoring applications. This paper discusses the efficiency and temperature sensitivity of the VCSELs emitting at 2.6 μm and the processes that must be controlled to provide temperature stable operation. We show that non-radiative Auger recombination dominates the threshold current and limits the device performance at room temperature. Critically, we demonstrate that the combined influence of non-radiative recombination and gain peak – cavity mode de-tuning determines the overall temperature sensitivity of the VCSELs. The results show that improved temperature stable operation around room temperature can only be achieved with a larger gain peak – cavity mode de-tuning, offsetting the significant effect of increasing non-radiative recombination with increasing temperature, a physical effect which must be accounted for in mid-infrared VCSEL design.

In this work, we used a combination of photoluminescence (PL), high resolution X-ray diffraction (XRD), and Rutherford backscattering spectrometry (RBS) techniques to investigate material quality and structural properties of MBE-grown InGaAsBi samples (with and without an InGaAs cap layer) with targeted bismuth composition in the 3%–4% range. XRD data showed that the InGaAsBi layers are more homogeneous in the uncapped samples. For the capped samples, the growth of the InGaAs capped layer at higher temperature affects the quality of the InGaAsBi layer and bismuth distribution in the growth direction. Low-temperature PL exhibited multiple emission peaks; the peak energies, widths, and relative intensities were used for comparative analysis of the data in line with the XRD and RBS results. RBS data at a random orientation together with channeled measurements allowed both an estimation of the bismuth composition and analysis of the structural properties. The RBS channeling showed evidence of higher strain due to possible antisite defects in the capped samples grown at a higher temperature. It is also suggested that the growth of the capped layer at high temperature causes deterioration of the bismuth-layer quality. The RBS analysis demonstrated evidence of a reduction of homogeneity of uncapped InGaAsBi layers with increasing bismuth concentration. The uncapped higher bismuth concentration sample showed less defined channeling dips suggesting poorer crystal quality and clustering of bismuth on the sample surface.

The efficiency limiting mechanisms in type-I GaInAsSb-based quantum well (QW) lasers, emitting at 2.3 μm, 2.6 μm and 2.9 μm, are investigated. Temperature characterization techniques and measurements under hydrostatic pressure identify an Auger process as the dominant non-radiative recombination mechanism in these devices. The results are supplemented with hydrostatic pressure measurements from three additional type-I GaInAsSb lasers, extending the wavelength range under investigation from 1.85-2.90 μm. Under hydrostatic pressure, contributions from the CHCC and CHSH Auger mechanisms to the threshold current density can be investigated separately. A simple model is used to fit the non-radiative component of the threshold current density, identifying the dominance of the different Auger losses across the wavelength range of operation. The CHCC mechanism is shown to be the dominant non-radiative process at longer wavelengths (> 2 μm). At shorter wavelengths (< 2 μm) the CHSH mechanism begins to dominate the threshold current, as the bandgap approaches resonance with the spin-orbit split-off band.

We investigate the optical and electrical characteristics of GaInNAs/GaAs long-wavelength photodiodes grown under varying conditions by molecular beam epitaxy and subjected to postgrowth rapid thermal annealing (RTA) at a series of temperatures. It is found that the device performance of the nonoptimally grown GaInNAs p-i-n structures, with nominal compositions of 10% In and 3.8% N, can be improved significantly by the RTA treatment to match that of optimally grown structures. The optimally annealed devices exhibit overall improvement in optical and electrical characteristics, including increased photoluminescence brightness, reduced density of deep-level traps, reduced series resistance resulting from the GaAs/GaInNAs heterointerface, lower dark current, and significantly lower background doping density, all of which can be attributed to the reduced structural disorder in the GaInNAs alloy.© 2012 TMS.

Replacing small amounts of As with Bi in InGaBiAs/InP induces large decreases and increases in the bandgap, E, and spin-orbit splitting, Δ, respectively. The possibility of achieving Δ > E and a reduced temperature (T) dependence for E are significant for suppressing recombination losses and improving performance in mid-infrared photonic devices. We measure E (x, T) and Δ (x, T) in InGa BiAs/InP samples for 0 x 0.039 by various complementary optical spectroscopic techniques. While we find no clear evidence of a decreased dE/dT (≈0.34 ± 0.06 meV/K in all samples) we find Δ > E for x > 3.3-4.3. The predictions of a valence band anti-crossing model agree well with the measurements. © 2012 American Institute of Physics.

The potential to extend the emission wavelength of photonic devices further into the near- and midinfrared via pseudomorphic growth on conventional GaAs substrates is appealing for a number of communications and sensing applications. We present a new class of GaAs-based quantum well (QW) heterostructure that exploits the unusual impact of Bi and N on the GaAs band structure to produce type-II QWs having long emission wavelengths with little or no net strain relative to GaAs, while also providing control over important laser loss processes. We theoretically and experimentally demonstrate the potential of GaAs1−xBix/GaNyAs1−y type-II QWs on GaAs and show that this approach offers optical emission and absorption at wavelengths up to ~3 μm utilising strain-balanced structures, a first for GaAs-based QWs. Experimental measurements on a prototype GaAs0.967Bi0.033/GaN0.062As0.938 structure, grown via metal-organic vapour phase epitaxy, indicate good structural quality and exhibit both photoluminescence and absorption at room temperature. The measured photoluminescence peak wavelength of 1.72 μm is in good agreement with theoretical calculations and is one of the longest emission wavelengths achieved on GaAs to date using a pseudomorphically grown heterostructure. These results demonstrate the significant potential of this new class of III-V heterostructure for longwavelength applications.

High pressure electroluminescence (EL) measurements were carried out on blue and green emitting InGaN-based light emitting diodes (LEDs). The weak pressure coefficient of the peak emission energy of the LEDs is found to increase with increasing injection current. Such behaviour is consistent with an enhancement of the piezoelectric fields under high pressure which become increasingly screened at high currents. A subsequent increase in the quantum confined Stark effect (QCSE) is expected to cause a reduction of the light output power as pressure is applied at a fixed low current density (∼10Acm). A similar proportional reduction of light output power as pressure is applied at a fixed high current density (260Acm) suggests that there is a non-radiative loss process in these devices which is relatively insensitive to pressure. Such behaviour is shown to be consistent with a defect-related recombination process which increases with increasing injection. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Quantum well lasers have been extremely successful in a wide range of applications, with optical fibre communications being of particular importance. However, in spite of their success, their performance is not ideal, for example, the threshold current of semiconductor lasers is often very sensitive to temperature. This has led to the need for thermoelectric coolers and associated control electronics to stabilize the laser temperature, however, such coolers often consume more energy than the laser they are controlling and also add to the overall heat dissipation of the system. Such coolers also tend to have far less long-term reliability than the laser diode itself. There are consequently many circumstances where it would be advantageous and far cheaper to simply compensate for temperature variations by mechanisms built into the epitaxial structure of the laser chip itself. This paper focuses on a new design and demonstration of a MQW laser structure which can overcome the intrinsic temperature sensitivity of the laser.

In this chapter we consider the important optical and electronic processes which influence the properties of semiconductor photonic devices. Focussing on a number of material systems, we describe semiconductor materials and structures used for light-emitting applications (lasers and LEDs) operating in a wide spectral range from visible to mid-infrared. The main carrier recombination mechanisms in semiconductor devices are discussed and experimental methodologies for measuring and analysing these mechanisms are introduced. Near infra-red (IR) quantum well (QW) lasers are discussed in depth considering several new approaches to overcome fundamental performance issues. Different approaches for the longer wavelength (mid-IR) semiconductor devices are reviewed showing the benefits of different approaches to material and device design where energy efficiency and high temperature operation are the principal concerns. Finally, semiconductor lasers and LEDs for the visible spectral range are briefly introduced in terms of the most important issues related to their performance.

In this work we study the nature of the band gap in GeSn alloys for use in silicon-based lasers. Special attention is paid to Sn-induced band mixing effects. We demonstrate from both experiment and ab-initio theory that the (direct) Γ- character of the GeSn band gap changes continuously with alloy composition and has significant Γ-character even at low (6%) Sn concentrations. The evolution of the Γ-character is due to Sn-induced conduction band mixing effects, in contrast to the sharp indirect-to-direct band gap transition obtained in conventional alloys such as Al1-xGaxAs. Understanding the band mixing effects is critical not only from a fundamental and basic properties viewpoint but also for designing photonic devices with enhanced capabilities utilizing GeSn and related material systems.

Gain saturation increases the radiative component, J(rad), of the threshold current density, J(th), and its contribution to the thermal sensitivity of J(th) in short cavity or low QD density devices. However, the main cause of their thermal sensitivity is a strong non-radiative recombination.

We have investigated the threshold current Ith and differential quantum efficiency as the function of temperature in InGaAlAs/InP multiple quantum well (MQWs) buried heterostructure (BH) lasers. We find that the temperature sensitivity of Ith is due to non-radiative recombination which accounts for up to ~80% of Jth at room temperature. Analysis of spontaneous emission emitted from the devices show that the dominant non-radiative recombination process is consistent with Auger recombination. We further show that the above threshold differential internal quantum efficiency, ηi, is ~80% at 20°C remaining stable up to 80°C. In contrast, the internal optical loss, αi, increases from 15 cm-1 at 20°C to 22 cm-1 at 80°C, consistent with inter-valence band absorption (IVBA). This suggests that the decrease in power output at elevated temperatures is associated with both Auger recombination and IVBA.

InAs quantum-dot (QD) lasers were investigated in the temperature range 20-300 K and under hydrostatic pressure in the range of 0-12 kbar at room temperature. The results indicate that Auger recombination is very important in 1.3-mum QD lasers at room temperature and it is, therefore, the possible cause of the relatively low characteristic temperature observed, of T-0 = 41 K. In the 980-mn QD lasers where T-0 = 110-130 K, radiative recombination dominates. The laser emission photon energy E-las increases linearly with pressure p at 10.1 and 8.3 meV/kbar for 980 nm and 1.3-mum QD lasers, respectively. For the 980-mn QD lasers the threshold current increases with pressure at a rate proportional to the square of the photon energy E-las(2). However, la the threshold current of the 1.3-mum QD laser decreases. by 26% over a 12-kbar pressure range. This demonstrates the presence of a nonradiative recombination contribution to the threshold current, which decreases with increasing pressure. The authors show that this nonradiative contribution is Auger recombination. The results are discussed in the framework of a theoretical model based on the electronic structure and radiative recombination calculations carried out using an 8 x 8 k(.)p Hamiltonian.

The temperature dependence of the radiative and nonradiative components of the threshold current density of 1.3 mu m InAs/GaAs quantum-dot lasers have been analyzed both experimentally and theoretically. It is shown that the weak temperature variation measured for the radiative current density arises because the optical matrix element for excited state transitions is significantly smaller than for the ground state transition. In contrast, nonradiative Auger recombination can have a similar probability for transitions involving excited states as for those involving ground state carriers. The sharp increase in the threshold current density at high temperatures follows the temperature variation of the cubed threshold carrier density confirming that Auger recombination is the dominant recombination mechanism in these devices at room temperature.

Interface-mediated recombination losses between perovskite and charge transport layers are one of the main reasons that limit the device performance, in particular for the open-circuit voltage (VOC) of perovskite solar cells (PSCs). Here, functional molecular interface engineering (FMIE) is employed to retard the interfacial recombination losses. The FMIE is a facile solution-processed means that introducing functional molecules, the fluorene-based conjugated polyelectrolyte (CPE) and organic halide salt (OHS) on both contacts of the perovskite absorber layer. Through the FMIE, the champion PSCs with an inverted planar heterojunction structure show a remarkable high VOC of 1.18 V whilst maintaining a fill factor (FF) of 0.83, both of which result in improved power conversion efficiencies (PCEs) of 21.33% (with stabilized PCEs of 21.01%). In addition to achieving one of the highest PCEs in the inverted PSCs, the results also highlight the synergistic effect of these two molecules in improving device performance. Therefore, the study provides a straightforward avenue to fabricate highly efficient inverted PSCs.

We used high hydrostatic pressure techniques to understand the deteriorating temperature performance with decreasing wavelength of short wavelength quantum cascade lasers. Influence of inter-valley scattering and distribution of the electron wave functions will be discussed.

We show that even in quantum-dot (QD) lasers with very low threshold current densities (J(th) = 40-50 A/cm(2) at 300 K), the temperature sensitivity of the threshold current arises from nonradiative recombination that comprises similar to 60% to 70% of J(th) at 300 K, whereas the radiative part of J(th) is almost temperature insensitive. The influence of the nonradiative recombination mechanism decreases with increasing hydrostatic pressure and increasing band gap, which leads to a decrease of the threshold current. We also studied, for the first time, the band gap dependence of the radiative part Of Jth, which in contrast increases strongly with increasing band gap. These results suggest that Auger recombination is an important intrinsic recombination mechanism for 1.3-mu m lasers, even in a very low threshold QD device, and that it is responsible for the temperature sensitivity of the threshold current.

We show that even in quantum dot lasers with very low threshold current density (Jth=740-50 A/cm(2) at 300 K) the temperature sensitivity of the threshold current arises from nonradiative recombination which comprises similar to60-70% of Jth at 300 K.

The radiative and nonradiative components of the threshold current in 1.3 mu m, p-doped and undoped quantum-dot semiconductor lasers were studied between 20 and 370 K. The complex behavior can be explained by simply assuming that the radiative recombination and nonradiative Auger recombination rates are strongly modified by thermal redistribution of carriers between the dots. The large differences between the devices arise due to the trapped holes in the p-doped devices. These both greatly increase Auger recombination involving hole excitation at low temperatures and decrease electron thermal escape due to their Coulombic attraction. The model explains the high T-0 values observed near room temperature. (c) 2005 American Institute of Physics.

Low-cost, continuous-wave GaSb-based vertical cavity surface emitting lasers (VCSELs) operating at ~ 2.4 mum up to 50degC have been demonstrated recently. In this work we have used high pressure techniques to investigate ways to improve their performance and extend their working temperature range. Since the band-gap and energy of the gain peak (Ep) increase with pressure at 0.126 meV/MPa at constant temperature, when applied to edge emitting lasers (EEL) we can use pressure to determine the radiative and non-radiative recombination processes occurring. In the VCSEL the pressure dependence of the threshold current, is much more complicated. At the higher temperature the decreasing Auger recombination initially dominates. Therefore we predict that either increasing the band gap or increasing the operating wavelength will allow an improved temperature performance of these GaSb-based VCSELs.

We present what we believe to be the first ever high-pressure and spontaneous emission measurements on quantum dash lasers. The results show that temperature sensitivity of these lasers is caused by nonradiative processes, which depend on the lasing wavelength.

Bismuth-containing III-V alloys open-up a range of possibilities for practical applications in semiconductor lasers, photovoltaics, spintronics, photodiodes and thermoelectrics. Of particular promise for the development of semiconductor lasers is the possibility to grow GaAsBi laser structures such that the spin-orbit-splitting energy (ΔSO) is greater than the bandgap (Eg) in the active region for devices operating around the telecom wavelength of 1.55 μm, thereby suppressing the dominant efficiency-limiting loss processes in such lasers, namely Auger recombination and inter-valence band absorption (IVBA). The ΔSO > Eg band structure is present in GaAsBi alloys containing > 10% Bi, at which composition the alloy band gap is close to 1.55 μm on a GaAs substrate making them an attractive candidate material system for the development of highly efficient, uncooled GaAs-based lasers for telecommunications. Here we discuss progress towards this goal and present a comprehensive set of data on the properties of GaAsBi lasers including optical gain and absorption characteristics and the dominant carrier recombination processes in such systems. Finally, we briefly review the potential of GaAsBiN and InGaAsBi material systems for near- and mid-infrared photonic devices on GaAs and InP platforms, respectively.

The gain of p-doped and intrinsic InAs/GaAs quantum dot lasers is studied at room temperature and at 350 K. Our results show that, although one would theoretically expect a higher gain for a fixed carrier density in p-doped devices, due to the wider nonthermal distribution of carriers amongst the dots at T=293 K, the peak net gain of the p-doped lasers is actually less at low injection than that of the undoped devices. However, at higher current densities, p doping reduces the effect of gain saturation and therefore allows ground-state lasing in shorter cavities and at higher temperatures.

In this study low temperature and high pressure techniques have been used to investigate the recombination processes taking place in InGaN-based quantum well light emitting diodes (LEDs) which have emission across the blue-green region. Despite relatively high peak efficiencies of the GaN-based emitters, there remain issues relating to the strong efficiency reduction at higher currents that are required for normal operation in most applications. It is observed that there is a relative reduction in efficiency as injection current is increased in a phenonmenon which is known as efficiency droop. There are three main arguments for the cause of efficiency droop that are discussed in the literature: non-radiative Auger recombination, carrier leakage and a defect-related loss mechanism. In spite of extensive research to date, there is little agreement on the cause of efficiency droop as most experiments can only measure the overall efficiency behaviour leading to difficulties in determining the individual contributions from the different loss mechanisms. © 2013 IEEE.

This paper reports on progress in the development of GaAsBi/(Al)GaAs based lasers grown using metal-organic vapour phase epitaxy and focuses on the underlying processes governing their efficiency and temperature dependence. Room temperature lasing has been achieved in devices with 2.2% Bi and lasing in devices with 4.4% Bi was observed up to 180 K. We show that the device performance can be improved by optimizing both electrical and optical confinement in the laser structures. Analysis of the temperature dependence of the threshold current together with pure spontaneous emission and high hydrostatic pressure measurements indicate that device performance is currently dominated by non-radiative recombination through defects (>80% of the threshold current at room temperature in 2.2% Bi samples) and that to further improve the device performance and move towards longer wavelengths for optical telecommunications (1.3-1.5 μ m) further effort is required to improve and optimize material quality. © 2014 IOP Publishing Ltd.

The threshold current and its radiative component in 1.5 mu m InAs/InP (311)B quantum dot lasers are measured as a function of the temperature. Despite an almost temperature insensitive radiative current, the threshold current increases steeply with temperature leading to a characteristic temperature T-0 approximate to 55 K around 290 K. Direct observation of spontaneous emission from the wetting layer shows that some leakage from the dots to the wetting layer occurs in these devices. However, a decrease in the threshold current as a function of pressure is also measured suggesting that Auger recombination dominates the nonradiative current and temperature sensitivity of these devices.

Twenty five years ago Arakawa suggested that by confining carriers in three dimensions (in quantum dots) a temperature insensitive threshold current (I-th) could be achieved in semiconductor lasers. In this paper we discuss investigations on state-of-the-art 1.3 mu m InAs/GaAs undoped and p-doped quantum dot lasers for telecommunication applications and discuss the extent to which this original hypothesis has been verified. In this study, the threshold current and its radiative component (I-rad) are measured as a function of temperature and pressure. The results show that although the radiative component of the threshold current can be temperature insensitive in undoped quantum dot lasers, a strong contribution from non-radiative Auger recombination makes the threshold current highly temperature sensitive. We find that p-doped devices can have a temperature insensitive I-th over a limited range around room temperature resulting from an interplay between an increasing non-radiative Auger current and decreasing radiative current. The decrease in I-rad, also observed below 200 K in undoped devices, is attributed to an improvement in the carrier transport with increasing temperature. Gain measurements show that even if p-doping is successful in reducing the effect of gain saturation, the modal net gain of p-doped devices is less than in undoped lasers due to increased non-radiative recombination and non-thermal carrier distribution.

We present what we believe to be the first ever high-pressure and spontaneous emission measurements on quantum dash lasers. The results show that temperature sensitivity of these lasers is caused by nonradiative processes, which depend on the lasing wavelength.

In this paper we present preliminary work on group IV photonic waveguides that may be suitable for mid-infrared wavelengths. Fabrication and experimental results for two waveguide structures are given.

In order to identify the performance limitations of InGaAs/AlAs(Sb) quantum cascade lasers, experimental investigations of the temperature and pressure dependencies of the threshold current (I) were undertaken. Using the theoretical optical phonon current (I) and carrier leakage (I) to fit the measured threshold current at various pressures, we show that the electron scattering from the top lasing level to the upper L-minima gives rise to the increase in I with pressure and temperature. It was found that this carrier leakage path accounts for approximately 3% of I at RT and is negligible at 100K. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The Auger recombination in 1.3μm InAs/GalnAs quantum dot lasers were investigated. To analyse the experimental results, theoretical model was used which includes strain, piezoelectric field and electronic structure calculated in the QDs of truncated pyramid shape. It was found that the radiative current increases with pressure, but the Auger recombination current decreases with pressure and is the dominant recombination path at room temperature in 1.3μm QD lasers. © 2003 IEEE.

It has been found that Auger recombination is very important in 1.3 μm quantum dot (QD) lasers at room temperature and is the cause of the low characteristic temperature, while in the 980 nm QD lasers radiative recombination dominates.

In this work we present results from high performance silicon optical modulators produced within the two largest silicon photonics projects in Europe; UK Silicon Photonics (UKSP) and HELIOS. Two conventional MZI based optical modulators featuring novel self-aligned fabrication processes are presented. The first is based in 400nm overlayer SOI and demonstrates 40Gbit/s modulation with the same extinction ratio for both TE and TM polarisations, which relaxes coupling requirements to the device. The second design is based in 220nm SOI and demonstrates 40Gbits/s modulation with a 10dB extinction ratio as well modulation at 50Gbit/s for the first time. A ring resonator based optical modulator, featuring FIB error correction is presented. 40Gbit/s, 32fJ/bit operation is also shown from this device which has a 6um radius. Further to this slow light enhancement of the modulation effect is demonstrated through the use of both convention photonic crystal structures and corrugated waveguides. Fabricated conventional photonic crystal modulators have shown an enhancement factor of 8 over the fast light case. The corrugated waveguide device shows modulation efficiency down to 0.45V.cm compared to 2.2V.cm in the fast light case. 40Gbit/s modulation is demonstrated with a 3dB modulation depth from this device. Novel photonic crystal based cavity modulators are also demonstrated which offer the potential for low fibre to fibre loss. In this case preliminary modulation results at 1Gbit/s are demonstrated. Ge/SiGe Stark effect devices operating at 1300nm are presented. Finally an integrated transmitter featuring a III-V source and MZI modulator operating at 10Gbit/s is presented. © 2012 SPIE.

InGaAsN is a promising material system to enable low-cost GaAs-based detectors to operate in the telecommunication spectrum, despite the problems posed by the low growth temperature required for nitrogen incorporation. We demonstrate that InGaAsN p+-i-n+ structures with nominal In and N fraction of 10% and 3.8%, grown by molecular beam epitaxy (MBE) under non-optimal growth conditions, can be optimized by post growth thermal annealing to match the performance of optimally grown structures. We report the findings of an annealing study by comparing the photoluminescence spectra, dark current and background concentration of the as-grown and annealed samples. The dark current of the optimally annealed sample is approximately 2 μA/cm2 at an electric field of 100 kV/cm, and is the lowest reported to date for InGaAsN photodetectors with a cut-off wavelength of 1.3 μm. Evidence of lower unintentional background concentration after annealing at a sufficiently high temperature, is also presented.

Using a combination of experimental and theoretical techniques we present the dependence of the bandgap Eg and the spin orbit splitting energy so, with Bi concentration in GaAsBi/GaAs samples. We find that the concentration at which so,> Eg occurs at 9%. Both spectroscopic as well as first device results indicate a type I alignment.

The development of laser technology based on silicon continues to be of key importance for the advancement of electronic-photonic integration offering the potential for high data rates and reduced energy consumption. Progress was initially hindered due to the inherent indirect band gap of silicon. However, there has been considerable progress in developing ways of incorporating high gain III-V based direct band gap materials onto silicon, bringing about the advantages of both materials. In this paper, we introduce the need for lasers on silicon and review some of the main approaches for the integration of III-V active regions, including direct epitaxial growth, hybrid integration, defect blocking layers and quantum dots. We then discuss the roles of different carrier recombination processes on the performance of devices formed using both wafer fusion and direct epitaxial approaches.

Quantum Cascade Lasers (QCLs) have been very successful at long wavelengths, >4μm, and there is now considerable effort to develop QCLs for short wavelength (2-3μm) applications. To optimise both interband and QC lasers it is important to understand the role of radiative and non-radiative processes and their variation with wavelength and temperature. We use high hydrostatic pressure to manipulate the band structure of lasers to identify the dominant efficiency limiting processes. We describe how hydrostatic pressure may also be used to vary the separation between the Γ, Χ and L bands, allowing one to investigate the role of inter-valley carrier scattering on the properties of QCLs. We will describe an example of how pressure can be used to investigate the properties of 2.9-3.3μm InAs/AlSb QCLs. We find that while the threshold current of the 3.3μm devices shows little pressure variation even at room temperature, for the 2.9μm devices the threshold current increases by ~20% over 4kbar at 190K consistent with carrier scattering into the L-minima. Based on our high pressure studies, we conclude that the maximum operating temperature of InAs/AlSb QCLs decreases with decreasing wavelength due to increased carrier scattering into the L-minima of InAs.

GaAsBi QWs have the potential to remove inherent recombination losses thereby increasing the efficiency and reducing the temperature sensitivity of near-infrared telecommunications lasers. GaAsBi QW lasers are reported and prospects for 1550nm operation are discussed.

Semiconductor lasers with quantum dot (QD) based active regions have generated a huge amount of interest for applications including communications networks due to their anticipated superior physical properties due to three dimensional carrier confinement. For example, the threshold current of ideal quantum dots is predicted to be temperature insensitive. We have investigated the operating characteristics of 1.55 μm InAs/InP (100) quantum dot lasers focusing on their carrier recombination characteristics using a combination of low temperature and high pressure measurements. By measuring the intrinsic spontaneous emission from a window fabricated in the n-contact of the devices we have measured the radiative component of the threshold current density, Jrad. We find that Jrad is itself relatively temperature insensitive (Fig. 1). However, the total threshold current density, Jth, increases significantly with temperature leading to a characteristic temperature T0~72 K around 220 K-290 K. From this data it is clear that the devices are dominated by a non-radiative recombination process which accounts for up to 94% of the threshold current at room temperature (Fig. 1).

Electrically pumped GaAsBi/GaAs quantum well lasers are a promising new class of near-infrared devices where, by use of the unusual band structure properties of GaAsBi alloys, it is possible to suppress the dominant energy-consuming Auger recombination and inter-valence band absorption loss mechanisms, which greatly impact upon the device performance. Suppression of these loss mechanisms promises to lead to highly efficient, uncooled operation of telecommunications lasers, making GaAsBi system a strong candidate for the development of next-generation semiconductor lasers. In this report we present the first experimentally measured optical gain, absorption and spontaneous emission spectra for GaAsBi-based quantum well laser structures. We determine internal optical losses of 10–15 cm−1 and a peak modal gain of 24 cm−1, corresponding to a material gain of approximately 1500 cm−1 at a current density of 2 kA cm−2. To complement the experimental studies, a theoretical analysis of the spontaneous emission and optical gain spectra is presented, using a model based upon a 12-band k.p Hamiltonian for GaAsBi alloys. The results of our theoretical calculations are in excellent quantitative agreement with the experimental data, and together provide a powerful predictive capability for use in the design and optimisation of high efficiency lasers in the infrared.