We demonstrate a GaAs based Quantum Dot (QD) optoelectronic integration platform with results for surface grating couplers (SGC) and edge emitting lasers. SGCs usually perform poorly in systems without a large refractive index contrast and here we utilize thin oxidized Al0.98Ga0.02As layers to overcome this issue. The results show an increase from 10% to 30% in surface grating coupling efficiency, for the unoxidized compared to oxidized, and substrate-loss decrease from 70% to 20%. Electrically pumped edge-emitting lasers on the same platform exhibit comparable performance to more standard 1300nm QD designs. We describe the design, fabrication and characterization of these devices.
We present a postgrowth selective-area-intermixing approach for on-chip III-V based monolithically integrated laser-waveguide structures for photonic integrated circuits. Implanting selective areas with an energy of 300 KeV and dose of 5 ×1012 cm-2 induced crystal defects in the InAs quantum dot gain material, results in a shifted absorption edge and complete quenching of optical emission. We successfully recovered the optical quality of the gain material through optimized rapid thermal annealing at 635 OC and achieved enhanced intermixing in the implanted region thus causing a relative blueshift of 20 nm in the passive waveguides, mitigating absorption at the laser emission wavelength.
We introduce direct n-doping of quantum dots together with modulation p-doping as a technique to reduce both the threshold current density and the temperature dependence of threshold current density in 1.3um emitting quantum dot lasers. Threshold current density in 1mm long QD lasers with cleaved and uncoated facets is effectively halved at both 27°C and at 97°C when using co-doping as compared to the undoped case. Results indicate that modulation p-doping can improve the threshold current temperature dependence and direct n-doping reduces the magnitude of threshold current density and that the benefits of each is maintained when used together.
The performance of O-band InAs/GaAs quantum-dot (QD) lasers grown by molecular beam epitaxy with three different doping strategies are investigated in a temperature range 17 °C – 97 °C. We demonstrate lasers with a reduced threshold current using direct n-doping (during the dot formation) in the active region compared lasers with a nominally undoped active region. We explain results using calculations of the dot and wetting layer potentials and the electron and hole energy levels.
Monolithic integration of III–V materials and devices on CMOS compatible on‐axis Si (001) substrates enables a route of low‐cost and high‐density Si‐based photonic integrated circuits. Inversion boundaries (IBs) are defects that arise from the interface between III–V materials and Si, which significantly lowers the quality of III–V materials on Si. Here, a novel technique to achieve IB‐free GaAs monolithically grown on on‐axis Si (001) substrates by realizing the alternating straight and meandering single atomic steps on Si surface has been introduced via all-molecular beam epitaxy approach without the use of double Si atomic steps, which was previously believed to be the key for IB‐free III–V growth on Si. The periodic straight and meandering single atomic steps on Si surface are results of high‐temperature annealing of Si buffer layer. As a demonstration, an electrically pumped InAs quantum‐dot laser has been fabricated based on this IB‐free GaAs/Si platform with a maximum operating temperature of 120 °C. These results can be a major step towards monolithic integration of III–V materials and devices with the mature CMOS technology.
A method of hybrid integration of quantum dot microdisk lasers with silicon wafer is proposed and realized. In addition to the possibility of combining microlasers with various silicon-based electronic and photonic devices, this makes it possible to significantly improve heat removal from the active region of the microlaser. The thermal resistance normalized to the mesa area reaches the level of about 0.002 (K/W)*cm2, which is significantly lower than the corresponding values of QD microlasers on GaAs substrate and monolithically grown on Si. As a result, the threshold current as well as current-induced shift of emission wavelength are reduced in continuous-wave regime.
III-V semiconductor nanowires allow easy hetero-integration of optoelectronic components onto silicon due to efficient strain relaxation, well-understood design approaches and scalability. However continuous room temperature lasing has proven elusive. A key challenge is performing repeatable single-wire characterization { each wire can be different due to local growth conditions present during bottom-up growth. Here, we describe an approach using large-scale population studies which exploit inherent inhomogeneity to understand the complex interplay of geometric design, crystal structure, and material quality. By correlating nanowire length with threshold for hundreds of nanowire lasers, this technique reveals core-reabsorption as the critical limiting process in multiple-quantum-well nanowire lasers. By incorporating higher band-gap nanowire core, this effect is eliminated, providing reflectivity dominated behavior.
GaAsP nanowires (NWs) containing a range of different heterostructures are shown to be a highly promising system for the fabrication of efficient and novel ultra-small light emitters. NWs containing GaAs radial quantum wells (QWs) have emission with high thermal stability, due to both large electron and hole confinement potentials. A structure containing three QWs exhibits very low threshold lasing at low temperatures. Within the GaAsP central region of the same NW, the formation of quantum wires (QWRs) on three of the six vertices is observed, these QWRs are aligned parallel to the NW axis. The presence of twins causes a 180° rotation of the crystal about the growth axis, breaking the QWRs into short sections which may act as quantum dots (QDs). Optical studies of the NWs support the formation of optically active QWRs and QDs. In a second type of NW, during growth of the GaAsP NW core the introduction of a short GaAs section forms a QD. The inclusion of up to 50 QDs with high structural and optical quality is shown to be possible; indicating the potential for the fabrication of QD lasers. A structure with only one QD exhibits a single sharp emission line and behavior consistent with single exciton recombination. The addition of passivation layers, grown as a shell on the NW core, is shown to be essential in obtaining good optical properties. Our studies hence demonstrate that GaAsP-GaAs NWs containing heterostructures have significant potential for a range of novel light emitting applications.
The growth of reliable III-V quantum well (QW) lasers on silicon remains a challenge as yet unmastered due to the issue of carrier migration into dislocations. We have recently compared the functionality of quantum dots (QDs) and QWs in the presence of high dislocation densities using rate equation travelling-wave simulations, which were based on 10-μm large spatial steps, and thus only allowed the use of effective laser parameters to model the performance degradation resulting from dislocation-induced carrier loss. Here we increase the resolution to the sub-micrometer level to enable the spatially resolved simulation of individual dislocations placed along the longitudinal cavity direction in order to study the physical mechanisms behind the characteristics of monolithic 980 nm In(Ga)As/GaAs QW and 1.3 μm QD lasers on silicon. Our simulations point out the role of diffusion-assisted carrier loss, which enables carrier migration into defect states resulting in highly absorptive regions over several micrometers in QW structures, whereas QD active regions with their efficient carrier capture and hence naturally reduced diffusion length show a higher immunity to defects. An additional interesting finding not accessible in a lower-resolution approach is that areas of locally reduced gain need to be compensated for in dislocation-free regions, which may lead to increased gain compression effects in silicon-based QD lasers with limited modal gain.
Monolithic integration of III–V semiconductor materials on planar on-axis silicon (001) is one of the most promising method for low-cost and scalable photonic integrated circuits. Here, we present InAs/GaAs quantum dots microdisk lasers monolithically grown on on-axis Si (001) substrate with ultra-low lasing threshold under room-temperature continues-wave optical pumping. The promising lasing characteristics of the microdisk lasers with ultra-low threshold and small footprint represent a major advance towards large-scale, low-cost integration of laser sources on silicon-based platform.
A combination of high operation temperatures and small sizes of diode lasers directly grown on silicon substrates is essential for their application in future photonic integrated circuits. In this work, we report on electrically-pumped III-V microdisk lasers monolithically grown on Si substrates with active regions of two kinds: either an InGaAs/GaAs quantum well (QW) or InAs/InGaAs/GaAs quantum dots (QDs). Microdisk resonators were defined using photolithography and plasma chemical etching. The active region diameter was varied from 11 to 31 µm. Microlasers were tested without external cooling at room and elevated temperatures.
The QW laser structure was epitaxially grown by MOCVD on silicon (100) with an intermediate MBE-grown Ge buffer. Under pulsed injection (0.5-µs-long injection pulses with 150 Hz repetition rate), lasing is achieved in QW microlasers with diameters of 23-31 µm with a minimal threshold current density of 28 kA/cm^2. Quasi-single mode lasing (SMSR is up to 20 dB) is observed with emission wavelength around 988 nm. To the best of our knowledge, this is the first quantum well electrically-pumped microdisk lasers monolithically deposited on (001)-oriented Si substrate. Quantum wells are typically characterized by high optical gain and high direct modulation bandwidth, which can be important in view of further miniaturization of microlasers and their future application. The sidewall passivation can be helpful to reduce the threshold current.
As compared to QWs, quantum dots demonstrate reduced sensitivity to threading dislocations and other crystalline defects as well as to sidewall recombination owing to a suppressed lateral transport of charge carriers which prevents their diffusion towards non-radiate recombination centers. The QD laser structure was directly grown by MBE on Si (001) substrate with 4° offcut to the [011] plane. QD microlasers were tested at room temperature in CW regime with a DC current varied from 0 to 50 mA and at elevated temperatures under CW and pulsed excitation (0.5-µs-long injection pulses with 10 kHz repetition rate). The InAs/InGaAs QDs active region provides the wavelengths in the 1.32–1.35 µm spectral interval. At room temperature, lasing is achieved in microlasers with diameters of 14-30 µm with a minimal threshold current density of 600 A/cm2 (compare with that of 427 A/cm2 in edge-emitting laser). The threshold current density and specific thermal resistance of 0.004 °C×cm^2/mW are comparable to those of high-quality QD microdisk lasers on GaAs substrates. Lasing wavelength demonstrates low sensitivity to current-induced self-heating. Lasing is single mode (SMSR 20 dB) with a dominant mode linewidth as narrow as 30 pm. Under CW excitation lasing sustains up to 60 °C in microlasers with diameter of 30 µm. Because of self-heating, an actual temperature of the active region is close to 100°C. Under pulsed excitation, the maximal lasing temperature is 110°C. To our best knowledge, these are the smallest microlasers on silicon operating at such elevated temperatures ever reported. Up to 90°C lasing proceeds on the ground state optical transition of QDs with wavelength about 1.35 µm. At higher temperatures, lasing wavelength jumps to the excited state transition.
Foreign-catalyzed assisted grown nanowires deteriorate the performance of optoelectronic devices because of incorporation of foreign impurities into the nanowires. We report on growth self-catalyzed p-type GaAs nanowires (NWs) with pure zinc blende (ZB) structure, and the fabrication of single-NW-based photodetector. The carrier mobility and concentration of GaAs field effect transistor are characterized both at room temperature and at low temperature respectively. Due to the absence of stacking faults and single crystal phase of the NWs, the photodetector exhibits room-temperature high photo-responsivity over than 105 A W-1 and great specific detectivity, which outperforms previously reported NW-based photodetectors. These results demonstrate that these self-catalyzed pure-ZB GaAs NWs to be promising candidates for optoelectronics applications. In addition, optoelectronic properties of single quantum dots embedded in single nanowires will be represented with an applied magnetic field.
We propose and demonstrate a scattering-type near-field probe, designed to increase the sensitivity of high-resolution scattering probe microscopy at terahertz (THz) frequencies. For efficient scattering of THz radiation, the probe, fabricated from indium, is designed to resonate like a dipole antenna. Efficient excitation is achieved by integrating the probe with a radially-polarized THz source. Using time-domain spectroscopy (TDS), we observe resonant enhancement of the scattered fields, and using aperture-type near-field microscopy, we see high field confinement at the scattering probe apex.
In this work, the effect of Si doping on InAs/GaAs quantum dot solar cells with AlAs cap layers is studied. The AlAs cap layers suppress the formation of the wetting layer during quantum dot growth. This helps achieve quantum dot state filling, which is one of the requirements for strong sub-bandgap photon absorption in the quantum dot intermediate band solar cell, at lower Si doping density. Furthermore, the passivation of defect states in the quantum dots with moderate Si doping is demonstrated, which leads to an enhancement of the carrier lifetime in the quantum dots, and hence the open-circuit voltage.
Nanowires (NWs) have better functionality and superior performance as compared with the traditional thin film counterparts. However, NW growth is highly complicated and the growth mechanism is far from clear, especially when it is grown by vapor-liquid-solid mode. In this work, the influences of droplet size on the growth of self-catalyzed ternary NWs were studied using GaAsP NWs. The size-induced Gibbs−Thomson (GT) effect is observed for the first time in the self-catalyzed growth mode, which can make the smaller catalytic droplets have lower effective supersaturations. Thus, the droplet size can significantly influence the uniformity and composition of NWs. By carefully control the droplet size, the growth of highly uniform NW arrays are demonstrated. These results provide useful information for understanding the mechanisms of self-catalyzed III−V NW nucleation and growth with the important ternary III−V material systems.
The realization of efficient III-V lasers directly grown on Si substrates is highly desirable for large-scale and low-cost silicon based optoelectronic integrated circuits. However, it has been hindered by the high threading dislocation (TD) density generated at the interface between III-V compounds and Si substrates. Introducing dislocation filter layers (DFLs) to suppress the TD propagation into the active region becomes a key factor for realising lasers with advanced performance. In this paper, optimization of InGaAs/GaAs DFLs in III-V quantum dot (QD) lasers on Si is demonstrated. Based on these optimized DFLs and other strategies, we have achieved a high performance electrically pumped QD laser on a Si substrate with threshold current density of 62.5 A cm-2, over 105 mW output power, maximum operation temperature of 120 °C and over 100,158 h of extrapolated lifetime.
Ternary GaAsP nanowires (NWs) have gained great attention due to their structure-induced novel properties and band gap that can cover the working wavelength from green to infrared. However, the growth and hence applications of selfcatalyzed GaAsP NWs are troubled by the difficulties in controlling P and the complexities in growing ternary NWs. In this work, self-catalyzed core-shell GaAsP NWs were successfully grown and demonstrated almost stacking-fault-free zinc blend crystal structure. By using these core-shell GaAsP NWs, single NW solar cells have been fabricated and a single NW world record efficiency of 10.2% has been achieved. Those NWs also demonstrated their potential application in water splitting. A wafer-scale solar-to-hydrogen conversion efficiency of 0.5% has been achieved despite the low surface coverage. These results open up new perspectives for integrating III−V nanowire photovoltaics on a silicon platform by using self-catalyzed GaAsP core−shell nanowires.
We report on high quality GaAs-on-Si layers with low threading dislocations obtained by a combination of nucleation layer and dislocation filter layers using the molecular beam epitaxy (MBE) growth method. As a result, we achieved a Si-based electrically pumped 1.3 μm InAs/GaAs quantum dot (QD) laser that lases up to 111°C with a lasing threshold of 200 A/cm2, and a single facet output power exceeding 100 mW at room temperature. In addition to Si-based lasers, we also demonstrated the first Si-based InAs/GaAs QD superluminescent light-emitting diode (SLD), from which a close-to-Gaussian emission with a full width at half maximum (FWHM) of ~114 nm centered at ~1258 nm and maximum output power of 2.6 mW has been achieved.
Three InAs quantum dot (QD) samples with dislocation filter layers (DFLs) are grown on Si substrates with and without in-situ annealing. Comparison is made to a similar structure grown on a GaAs substrate. The three Si grown samples have different dislocation densities in their active region as revealed by structural studies. By determining the integrated emission as a function of laser power it is possible to determine the power dependence of the radiative efficiency and compare this across the four samples. The radiative efficiency increases with decreasing dislocation density; this also results in a decrease in the temperature quenching of the PL. A laser structures grown on Si and implementing the same optimum DFL and annealing procedure exhibits a greater than 3 fold reduction in threshold current as well as a two fold increase in slope efficiency in comparison to a device in which no annealing is applied.
Self-catalyzed GaAsP nanowires (NWs) have a band gap that is capable of covering the working wavelengths from green to infrared. However, the difficulties in controlling P and the complexities of the growth of ternary NWs make it challenging to fabricate them. In this work, self-catalyzed GaAsP NWs were successfully grown on Si substrates by solid-source molecular beam epitaxy and demonstrated almost stacking fault free zinc blend crystal structure, Growth of high-quality shell has been realized on the core NWs. In the shell, a quasi-3-fold composition symmetry has been observed for the first time. Moreover, these growth techniques have been successfully applied for growth on patterned Si substrates after some creative modifications such as high-temperature substrate cleaning and Ga pre-deposition. These results open up new perspectives for integrating III−V nanowire photovoltaics and visible light emitters on the silicon platform using self-catalyzed GaAsP core−shell nanowires.
KEYWORDS: Group III-V semiconductors, Luminescence, Tandem solar cells, Photovoltaics, Silicon solar cells, Silicon, Solar cells, Photons, Absorption, Diffusion, Performance modeling, Solar energy, Computer architecture
A model, derived from the detailed balance model from Shockley and Queisser, has been adapted to monolithically grown GaAsP/Si tandem dual junction solar cells. In this architecture, due to the difference of lattice parameters between the silicon bottom cell – acting as the substrate – and the GaAsP top cell, threading dislocations (TDs) arise at the IIIV/ Si interface and propagate in the top cell. These TDs act as non-radiative recombination centers, degrading the performances of the tandem cell. Our model takes into account the impact of TDs by integrating the NTT model developed by Yamaguchi et. al.. Two surface geometries have been investigated: flat and ideally textured. Finally the model considers the luminescent coupling (LC) between the cells due to reemitted photons from the top cell cascading to the bottom cell. Without dislocations, LC allows a greater flexibility in the cell design by rebalancing the currents between the two cells when the top cell presents a higher short-circuit current. However we show that, as the TD density (TDD) increases, nonradiative recombinations take over radiative recombinations in the top cell and the LC is quenched. As a result, nonoptimized tandem cells with higher short-circuit current in the top cell experience a very fast degradation of efficiency for TDDs over 104cm-2. On the other hand optimized cells with matching currents only experience a small efficiency drop for TDDs up to 105cm-2. High TDD cells therefore need to be current-matched for optimal performances as the flexibility due to LC is lost.
Lattice-mismatched 1.7eV Al0.2Ga0.8As photovoltaic solar cells have been monolithically grown on Si substrates using Solid Source Molecular Beam Epitaxy (SSMBE). As a consequence of the 4%-lattice-mismatch, threading dislocations (TDs) nucleate at the interface between the Si substrate and III-V epilayers and propagate to the active regions of the cell. There they act as recombination centers and degrade the performances of the cell. In our case, direct AlAs/GaAs superlattice growth coupled with InAlAs/AlAs strained layer superlattice (SLS) dislocation filter layers (DFLSs) have been used to reduce the TD density from 1×109cm-2 to 1(±0.2)×107cm-2. Lattice-matched Al0.2Ga0.8As cells have also been grown on GaAs as a reference. The best cell grown on silicon exhibits a Voc of 964mV, compared with a Voc of 1128mV on GaAs. Fill factors of respectively 77.6% and 80.2% have been calculated. Due to the lack of an anti-reflection coating and the non-optimized architecture of the devices, relatively low Jsc have been measured: 7.30mA.cm-2 on Si and 6.74mA.cm-2 on GaAs. The difference in short-circuit currents is believed to be caused by a difference of thickness between the samples due to discrepancies in the calibration of the MBE prior to each growth. The bandgap-voltage offset of the cells, defined as Eg/q-Voc, is relatively high on both substrates with 736mV measured on Si versus 572mV on GaAs. The non-negligible TD density partly explains this result on Si. On GaAs, non-ideal growth conditions are possibly responsible for these suboptimal performances.
We present the design and fabrication of a dual air-bridge waveguide structure integrated with MEMS functionality. The structure is designed to function as a tunable optical buffer for telecommunication application.
The optical buffer structure is based on two parallel waveguides made of high refractive index material with subwavelength dimensions. They are suspended in air, and are separated by a sub-micron air gap. Due to the fact that the size of the waveguides is much smaller than the wavelength of light that propagates in the structure, a significant fraction of the optical mode is in the air gap between the waveguides. By changing the size of the air gap using MEMS techniques, we can vary this fraction and hence the effective refractive index of the waveguide structure, thus generating tunable optical delay.
The optical buffer structure was grown on an InP substrate by molecular beam epitaxy, and the device layer was made of InGaP. An InGaAs layer was sandwiched between the device layer and the substrate to serve as a sacrificial layer. The sub-micron waveguides, their supports in the form of side pillars with tapered shapes in order to minimize optical losses, and the MEMS structures were patterned using electron beam lithography and plasma etching. Electrodes were integrated into the structure to provide electrostatic actuation. After the sample patterning, the waveguide structure was released using HF etch. Our simulations predict that by varying the waveguide separation from 50 nm to 500 nm, we could achieve a change in propagation delay by a factor of two.
We present the design and fabrication of a tunable optical buffer device based on III-V semiconductor platform for
telecommunication applications. The device comprises two indium phosphide suspended parallel waveguides with cross
sectional dimension of 200 nm by 300 nm, separated by an air gap. The gap between the waveguides was designed to be
adjustable by electrostatic force. Our simulation estimated that only 3 V is required to increase the separation distance
from 50 nm to 500 nm; this translates to a change in the propagation delay by a factor of 2. The first generation of the
suspended waveguide structure for optical buffering was fabricated. The sample was grown on an InP substrate by
molecular beam epitaxy. The waveguide pattern is written onto a 300 nm thick InP device layer by electron beam
lithography and plasma etching. Electrodes were incorporated into the structure to apply voltages for MEMS actuation.
We realise growth of both GaAsP and GaAs core nanowires (NWs), as well as GaAsP core-shell NWs grown on (111) Si substrates using solid source molecular beam epitaxy (MBE). By modifying the growth conditions it is possible to change the dimensions of the GaAsP NWs and optimisation of these conditions yields high crystal quality structures. Scanning electron microscopy (SEM) as well as temperature, power and time resolved photoluminescence (PL) are used to study the optical and structural properties of the NWs. The incorporation of P into the NWs is used to shift the PL emission for ~ 810 nm to ~ 730 nm at 77 K, and also results in enhanced PL and an improved carrier lifetime. The addition of a p-doped GaAsP shell to a GaAsP core NW reduces the nonradiative recombination at surface states, as evidenced by x14 reduction of PL quenching with temperature, enhanced carrier lifetime, as well as a x3.5 increase in 77 K integrated PL intensity.
We introduce the concept of using strained superlattice structures as defect filters, with their purpose to reduce the upwards propagation of dislocations that result from the lattice mismatch which occurs when III-V materials are grown on silicon substrates. Three samples with defect filter layers are grown on Si with and without in situ annealing and are compared to a similar structure grown on a GaAs substrate. Transmission electron microscopy is used to verify the effectiveness of the different designs grown on Si, with the twice-annealed sample reducing the number of defects present in the active region by 99.9%. Optical studies carried out exhibit brighter room temperature emission and reduced photoluminescence quenching with temperature in samples where annealing is performed. Photoluminescence excitation measurements reveal a ~20 meV redshift in the position of the GaAs exciton for the samples grown on Si compared to that of GaAs, indicating a residual inplane tensile strain ~0.35% in the GaAs of the active region for the samples grown on Si.
Quantum dot (QD) lasers incorporating the dot-in-a-well (DWELL) structures offer the prospect of lowcost
and high-performance sources for telecom applications at 1300 nm. A number of significant
advantages have been demonstrated to arise from the 0-D density of states, such as low threshold, low
noise, low chirp and relative temperature insensitivity. However QD lasers suffer from a low modal gain
per dot layer, which is a major factor of limiting high-speed performance. To address this, both a high inplane
dot density and the use of multilayer structure are necessary and this presents a major challenge for
molecular beam epitaxy (MBE) growth. In this work, to increase the gain of 1300-nm quantum-dot (QD)
lasers, we first optimize the MBE growth of InAs/InGaAs QD structure for single-layer epitaxy structure
with In composition within InGaAs well. Then we proposed a growth technique, high-growthtemperature
spacer layer to suppress the dislocation formation for the multilayer QD structure. These
lead to the realization of high-performance multilayer 1300-nm QD lasers with extremely low threshold
current density (Jth) of 17 A/cm2 at room temperature (RT) under continuous-wave (cw) operation and
high output power of over 100 mW. By combining the high-growth-temperature spacer layer technique
with the p-type modulation doping structure, a negative characteristic temperature above RT has been
demonstrated for a 5-layer QD laser structure. Further modification of the high-growth-temperature
spacer layer technique, we realized a very low RT threshold current density of 33 A/cm2 for a 7-layer ptype-
modulated QD laser. The temperature coefficient of ~0.11 nm/K over the temperature range from
20 to 130 °C has also been realized by modifying the strain profile of InGaAs capping layer. These
techniques could find application in lasers designed for optical fiber systems.
Electroluminescence (EL) and its temperature dependence of InAs quantum dots embedded in In0.15Ga0.85As quantum
well [dots in a well (DWELL)] have been investigated as functions of the growth temperature of the GaAs spacer layer.
The EL intensity at room temperature increases as the spacer growth temperature increases. The integrated EL intensity
as a function of injection current at room temperature for all samples shows that at low currents, the gradients are
superlinear but this superlinearity decreases as the spacer growth temperature is increased. From a simple analysis of the
generation-recombination rate equations, it can be shown that the superlinearity stems from the nonradiative
recombination being the dominant recombination process. As the spacer growth temperature is increased, this
nonradiative recombination become less dominant. An Arrhenius plot of the temperature dependence of the EL intensity
gives an activation energy of ~300 ± 15 meV at high temperature. The dominant loss mechanism is therefore concluded
to be the electron escape from the quantum dot ground state to the GaAs barrier.
The performance of lasers with self assembled quantum dot active regions is significantly affected
by the presence of the two dimensional wetting layer and the other states necessary for carrier
injection due to the manner in which carriers are distributed amongst the various states. In this work
we describe three approaches to overcome the low value of maximum saturated gain, which has
been observed by many groups worldwide, and explain the approaches in terms of the impact on the distribution of carriers within the available states. We present results of direct measurements of the modal gain and measurements that indicate the form of the carrier distribution within the samples to justify our argument. The structures examined include the use of a high growth temperature to smooth the matrix layer, the use of p-type modulation doping and the use of InAlAs capping layers and all have been grown by solid source molecular beam epitaxy. We demonstrate CW operation at 1.3&mgr;m for 1mm long devices with uncoated facets and very low threshold current density (< 40Acm-2) in longer devices. We also demonstrate that the negative T0 (reducing threshold current density with increasing temperature) obtained around room temperature in our p-doped devices is due to the temperature dependence of the gain.
We have developed 1.3 μm quantum dots (Qdot) using a dot in a well (DWELL) structure based on GaAs and 1.55 μm quantum dash (Qdash) based on InP Fabry-Perot lasers using a ridge waveguide operating in continuous wave at room temperature. The quantum dot lasers have demonstrated high power of 135 mW per facet and 50 mW per facet for the quantum dash devices. We have obtained very low relative intensity noise (RIN) with a nearly flat spectrum, around -159 dB/Hz ± 2 dB/Hz within 0.1-10 GHz range for the quantum dots and -160 dB/Hz ± 2 dB/Hz over a wide bandwidth from 50 MHz to 18 GHz for the quantum dash lasers. Recent experimental results are presented and analysed especially those relating to the noise performances and reliability tests to demonstrate the suitability of these new devices for microwave optical links.
A high-growth-temperature step used for the GaAs spacer layer is shown to significantly improve the performance of 1.3-μm multilayer InAs/GaAs quantum-dot (QD) lasers. The high-growth-temperature spacer layer inhibits threading dislocation formation, resulting in enhanced electrical and optical characteristics and hence improved laser performance. The combination of high-growth-temperature GaAs spacer layers and high-reflectivity (HR) coated facets has been utilized to further reduce the threshold current and threshold current density (Jth) for 1.3-μm InAs/GaAs QD lasers. Very low continuous-wave room-temperature threshold current of 1.5 mA and a threshold current density of 18.8 A/cm2 are achieved for a 3-layer device with a 1-mm long HR/HR cavity. For a 2-mm cavity the continuous-wave threshold current density is as low as 17 A/cm2 at room temperature for an HR/HR device. An output power as high as 100 mW is obtained for a device with HR/cleaved facets. The high-growth-temperature spacer layers have only a relatively small effect on the temperature stability of the threshold current above room temperature. To further increase the characteristic temperature (T0) of the QD lasers, 1.3-μm InAs/GaAs QD lasers incorporating p-type modulation doping have been grown and studied. A negative T0 and Jth of 48 A/cm-2 at room temperature have been obtained by combining the high-growth-temperature GaAs spacer layers with the p-type modulation doping of the QDs.
We report the on the characterisation of 1.3μm emitting GaInNAs quantum well (QW) lasers grown by molecular beam epitaxy using a plasma nitrogen source. Through the optimization of the structural and optical properties as a function of substrate temperature and nitrogen flux conditions, we show that high optical quality structures, which exhibit good room temperature photoluminescence intensity and photoluminescence linewidths <10meV at low temperature, can be routinely achieved. To obtain 1.3μm emission, we employed a structure containing quantum wells with an indium content of 40% and a nitrogen content of 2.5% which have low nitrogen content (1%) lattice matched quaternary GaInNAs barriers, the latter enabling us to grow thick barrier structures without introducing further strain. For unmounted and uncoated 15μm ridge waveguide lasers we have achieved threshold current densities as low as 377Acm-2 for a 3 QW and record low value of 178Acm-2 for a single QW device emitting above 1310nm. The devices show excellent temperature characteristics with characteristic temperatures >90°C observed in several structures. In comparison to GaInAs quantum well lasers, the results show that at this composition (2.5%) there is no appreciable degradation of performance due to the presence of nitrogen in these samples. Increasing the nitrogen content by 1% was observed to shift the wavelength to 1390nm, but with a threshold current density increased by a factor of 2 to 830Acm-2. The results also indicate that although high quality GaInNAs lasers can be achieved at wavelengths suitable for the 1.31μm optical fibre waveband, the performance of devices with higher N content, and therefore with emission at longer wavelength, are degraded.
Self-assembled In(Ga)As quantum dot (QD) lasers incorporating p-type modulation doping have generated much interest recently due to reports of a temperature insensitive threshold current and increased modulation bandwidth. The mechanism by which p-type doping improves the performance of QD lasers is thought to be similar to that envisaged for quantum well lasers, where increased gain is expected for a given quasi-Fermi level separation due to a shift in both quasi-Fermi levels towards the valence states. However, the benefits may be much more pronounced in quantum dot structures since the population of the smaller number of dot states can be dramatically affected using relatively low doping levels, which may incur less penalty with regard to increased non-radiative recombination and internal optical mode loss. We present results of direct measurements of the modal gain measured as a function of the quasi-Fermi level separation for samples with different degrees of doping, which demonstrate unambiguously the increased gain that can be obtained at a fixed quasi-Fermi level separation. In addition, we have measured the internal optical mode loss and radiative and non-radiative recombination currents for samples containing 0, 15 and 50 dopant atoms per dot and show that, although the internal optical mode loss is similar for all three samples, the non-radiative recombination current increases for samples containing p-doping. We show that our experimental results are consistent with a simple computer simulation of the operation of our structures.
We discuss a technique for tailoring the emission bandwidth of a quantum dot (QD) superluminescent light emitting diode (SLED). We utilize a multi-dot-in-well (DWELL) structure with different indium compositions within each well which we term dots in compositionally modulated well (DCMWELL) structures. One key aspect of our design is the overlap of the ground and excited state emission of different DWELL layers. Such SLED devices operate CW at room temperature with powers in excess of 2.5mW per facet, and exhibit a single peak almost 85 nm wide, which is almost flat topped.
Recent progress in the development of 1.3 mm InAs/InGaAs/GaAs dots-in-a-well (DWELL) laser structures has led to efficient CW room temperature laser operation with low current thresholds. However, present devices suffer from non-ideal temperature characteristics due to gain saturation, consequence of the finite dot density and carrier escape due to the small energy separation between the quantum dot (QD) ground and first-excited states. In order to improve device performance, we have examined methods to increase the QD quality and density. In these studies, we have examined the effect of different growth parameters which strongly modify the InAs QDs structure such as temperature and thickness of barrier layers and thickness and composition of the well. Analysis by Transmission Electron Microscopy (TEM), Photoluminescence (PL) and atomic force microscopy (AFM) have identified the presence of defects arising from the complex interaction of QDs, which propagate through the structure into the upper regions being the primary cause of the poor electronic device characteristics. The use of optimized growth has allowed, however, the fabrication of a defect free five layer-stacked structure with record low threshold current density.
Time-resolved photoluminescence decay measurements have been performed on samples with varying sized self-assembled InAs/GaAs quantum dot ensembles, formed by substrate mis-orientation alone, but otherwise under identical growth conditions. Ground-state radiative recombination lifetimes from 0.8 to 5.3 ns in the incident energy density range of 0.79 pJcm-2 - 40 nJcm-2 at a temperature of 77 K were obtained. It was found that a reduction of the quantum dot size led to a corresponding reduction of the radiative lifetime. The evident bi-exponential decay was obtained for the ground state emission of the quantum dot array, with the slower second component attributed to a carrier re-capturing and indirect radiative recombination processes. Also experimental evidence of the effect of the AlGaAs barrier in InAs QDs emitting in the wavelength range 1200-1300nm is presented. Time-resolved photoluminescence measurements have been performed on samples with different compositions of Al in the barrier. A full discussion of the lifetimes of these near infra-red emitting dots will be presented.
Quantum dots have demonstrated improved performance relative to quantum wells in lasers and amplifiers for structures where the total optical loss, and hence the gain required from the dot active material, has been kept low. In many applications higher gain and/or high differential gain are required and high gain structures must be routinely produced if quantum dots are to replace quantum wells in more than a few niche applications. The obvious approach is to use multiple layers of quantum dots in the active region of the laser or amplifier. However, stacking multiple quantum dot layers modifies the growth of subsequent layers and in the extreme case leads to defect formation.
In this work we study an approach where the negative effects caused by the introduction of multiple layers of quantum dots are minimised using a high growth temperature spacer layer (HGTSL) to planarize the surface before deposition of the subsequent layer of dots. We show that this has a dramatic affect on the threshold current of our 1.3μm emitting lasers and by use of detailed characterisation show that this is due to 4 physical effects. Samples containing the HGTSL exhibit less inhomogenous broadening, have an increased dot density, a lower internal optical mode loss and contain fewer defects than samples containing a conventional spacer layer. Our results demonstrate the importance of going beyond an approach based on defect reduction alone.
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