Self-heating in mid-infrared QCLs leads to beam instabilities and facet related failures. Single-element 4.6 μm-emitting BH QCLs were fabricated, where a tapered region scales the output power and, ahead of the emitting aperture, a narrow section provides mode filtering for suppressing high-order spatial modes. Beam-stability measurements indicate a small degree of collimated-beam centroid motion (< 0.25 mrad) can be achieved at >1.5W QCW output powers. Comparisons between short-pulse current and QCW operation reveal the impact of thermal lensing on the beam properties, while full 3D modeling provides insights into influence of device geometry on mode selection.
Buried heterostructure quantum cascade lasers (BH-QCLs) operating at high temperature in mid-infrared (MIR) to THz spectral range are desired for chemical sensing and free-space optical communication (FOC). In this work, Fe doped semi-insulating InP (SI-InP) regrowth is demonstrated in a hydride vapor phase epitaxy (HVPE) reactor for advanced MIR and THz BH-QCLs grown by MBE and MOCVD. SI-InP regrowth is implemented in THz QCL pillar arrays and narrow width and reverse-taper MIR BH-QCLs for efficient heat dissipation. By exploiting SI-InP regrowth, the parasitic capacitance in MIR distributed feedback BH-QCL can be suppressed, which is exploited for high speed FOC application.
In this report, we summarize our recent achievements in free-space communications in the mid-infrared (MIR) region enabled by directly modulated quantum cascaded laser (QCL) at 4.65 µm (~65 THz). We have experimentally demonstrated a multigigabit free-space transmission link in the lab environment with the QCL operating at room temperature. The QCL chip is mounted on a commercial QCL mount with a water-cooled Peltier element. Multilevel modulation formats at different baud rates are generated and combined with the laser driving current at a custom-made bias-tee to drive and modulate the QCL. A commercial mercury cadmium telluride (MCT, HgCdTe) photovoltaic (PV) MIR detector with a built-in trans-impedance amplifier was used to receive the MIR free-space signal. With the receiver to be the bottleneck of the system bandwidth, the end-to-end 3-dB bandwidth was measured to be around 320 MHz, and the 6-dB bandwidth was around 450 MHz. We have successfully demonstrated up to 6 Gbps free space transmission with multilevel modulation formats, assisted with effective digital equalization techniques despite the limited bandwidth.
Scaling the coherent power of mid-infrared (IR)-emitting quantum cascade lasers (QCLs) to the multi-watt range remains an important objective for applications where the laser beam needs to travel through air to remote targets, such as freespace communication links. For such applications requiring long-range pointing accuracy, measurements of beam stability are also important. We present beam-quality measurement results of narrow-ridge (4-5 μm), 4.6 μm-emitting buriedheterostructure (BH) QCLs. A 40-stage, step-tapered active-region (STA) structure was grown by MOCVD, and ICP etching was used to make deep ridges. InP:Fe was preferentially regrown in the field regions by using an SiO2 mask for ridge etching and Hydride Vapor Phase Epitaxy (HVPE). The HVPE process is attractive for selective regrowth, since high growth rates (0.2-0.3 μm/min) can be utilized, and highly planar top surfaces can readily be obtained. HVPE regrowth has been previously employed for BH devices of MBE-grown QCL ridges, but beam-stability measurements were not reported. HR-coated, 7.5 mm-long devices were measured under QCW operation (100 μsec pulse width, 0.5%-10% duty cycle) – very good beam quality factors, M2 < 1.2, were observed for both 4 μm and 5 μm ridge widths, but the narrower ridge exhibited better pointing stability. Collimated 5 μm-wide BH devices displayed some small degree of centroid motion with increasing power (< 0.125 mrad). This corresponds to a targeting error of ~1.25 cm over a distance of 100 m. Significantly improved lateral-beam stability was observed for narrower ridge width, although at the expense of reduced output power.
We present and compare the existing methods of heteroepitaxy of III-Vs on silicon and their trends. We focus on the epitaxial lateral overgrowth (ELOG) method as a means of achieving good quality III-Vs on silicon. Initially conducted primarily by near-equilibrium epitaxial methods such as liquid phase epitaxy and hydride vapour phase epitaxy, nowadays ELOG is being carried out even by non-equilibrium methods such as metal organic vapour phase epitaxy. In the ELOG method, the intermediate defective seed and the mask layers still exist between the laterally grown purer III-V layer and silicon. In a modified ELOG method called corrugated epitaxial lateral overgrowth (CELOG) method, it is possible to obtain direct interface between the III-V layer and silicon. In this presentation we exemplify some recent results obtained by these techniques. We assess the potentials of these methods along with the other existing methods for realizing truly monolithic photonic integration on silicon and III-V/Si heterojunction solar cells.
Buried heterostructure (BH) lasers are routinely fabricated for telecom applications. Development of quantum cascade
lasers (QCL) for sensing applications has largely benefited from the technological achievements established for telecom
lasers. However, new demands are to be met with when fabricating BH-QCLs. For example, hetero-cascade and multistack
QCLs, with several different active regions stacked on top of each other, are used to obtain a broad composite gain
or increased peak output power. Such structures have thick etch ridges which puts severe demand in carrying out
regrowth of semi-insulating layer around very deeply etched (< 10 μm) ridges in short time to realize BH-QCL. For
comparison, telecom laser ridges are normally only <5 μm deep. We demonstrate here that hydride vapour phase epitaxy
(HVPE) is capable of meeting this new demand adequately through the fabrication of BH-QCLs in less than 45 minutes
for burying ridges etched down to 10-15 μm deep. This has to be compared with the normally used regrowth time of
several hours, e.g., in a metal organic vapour phase epitaxy (MOVPE) reactor. This includes also micro-stripe lasers
resembling grating-like ridges for enhanced thermal dissipation in the lateral direction. In addition, we also demonstrate
HVPE capability to realize buried heterostructure photonic crystal QCLs for the first time. These buried lasers offer
flexibility in collecting light from the surface and relatively facile device characterization feasibility of QCLs in general;
but the more important benefits of such lasers are enhanced light matter interaction leading to ultra-high cavity Q-factors,
tight optical confinement, possibility to control the emitted mode pattern and beam shape and substantial reduction in
laser threshold.
Together with the optimal basic design, buried heterostructure quantum cascade laser (BH-QCL) with semi-insulating regrowth offers a unique possibility to achieve an effective thermal dissipation and lateral single mode. We demonstrate here the realization of BH-QCLs with a single-step regrowth of highly resistive (>1×108 ohm·cm) semi-insulating InP:Fe in <45 min for the first time in a flexible hydride vapor phase epitaxy process for burying ridges etched down to 10 to 15 μm depth, both with and without mask overhang. The fabricated BH-QCLs emitting at ∼4.7 and ∼5.5 μm were characterized. 2-mm-long 5.5-μm lasers with a ridge width of 17 to 22 μm, regrown with mask overhang, exhibited no leakage current. Large width and high doping in the structure did not permit high current density for continuous wave (CW) operation. 5-mm-long 4.7-μm BH-QCLs of ridge widths varying from 6 to 14 μm regrown without mask overhang, besides being spatially monomode, TM00, exhibited wall plug efficiency (WPE) of ∼8 to 9% with an output power of 1.5 to 2.5 W at room temperature and under CW operation. Thus, we demonstrate a quick, flexible, and single-step regrowth process with good planarization for realizing buried QCLs leading to monomode, high power, and high WPE.
Together with the optimal basic design, buried heterostructure quantum cascade laser (BH-QCL) with semi-insulating regrowth offers unique possibility to achieve an effective thermal dissipation and lateral single mode. We demonstrate here for the first time realization of BH-QCLs with a single step regrowth of highly resistive (<1x108 ohm•cm) semiinsulating InP:Fe in less than 45 minutes in a flexible hydride vapour phase epitaxy process for burying ridges etched down to 10-15 μm deep both with and without mask overhang. The fabricated BH-QCLs emitting at ~4.7 μm and ~5.5 μm were characterized. 2 mm long 5.5 μm lasers with ridge width 17-22 μm, regrown with mask overhang, exhibited no leakage current. Large width and high doping in the structure did not permit high current density for CW operation. 5 mm long 4.7 μm BH-QCLs of ridge widths varying from 6-14 μm regrown without mask overhang, besides being spatially monomode, TM00, exhibited WPE of ~8-9% with an output power of 1.5 – 2.5 W at room temperature and under CW operation. Thus, we demonstrate a simple, flexible, quick, stable and single-step regrowth process with extremely good planarization for realizing buried QCLs leading to monomode, high power and high WPE.
A simple method of growing large areas of InP on Si through Epitaxial Lateral Overgrowth (ELOG) is
presented. Isolated areas of high quality InP suitable for photonic integration are grown in deeply etched SiO2
mask fabricated using conventional optical lithography and reactive ion etching. This method is particularly
attractive for monolithically integrating laser sources grown on InP with Si/SiO2 waveguide structure as the
mask. The high optical quality of multi quantum well (MQW) layers grown on the ELOG layer is promisingly
supportive of the feasibility of this method for mass production.
Development in photonics for communications and interconnects pose increasing requirements on reduction of footprint,
power dissipation and cost, as well as increased bandwidth. Nanophotonics integrated photonics has been viewed as a
solution to this, capitalizing on development in nanotechnology and an increased understanding of light matter
interaction on the nanoscale. The latter can be exemplified by plasmonics and low dimensional semiconductors such as
quantum dots (QDs). In this scenario the development of improved electrooptic materials is of great importance, the
electrooptic polymers being an example, since they potentially offer superior properties for optical phase modulators in
terms of power and integratability. Phase modulators are essential for e.g. the rapidly developing advanced modulation
formats, since phase modulation basically can generate any type of modulation. The electrooptic polymers, in
combination with plasmonics nanoparticle array waveguides or nanostructured hybrid plasmonic media can give
extremely compact and low power dissipation modulators. Low-dimensional semiconductors, e.g. in the shape of QDs,
can be employed for modulation or switching functions, offering possibilities for scaling to 2 or 3 dimensions for
advanced switching functions. In both the high field confinement plasmonics and QDs, the nanosizing is due to nearfield
interactions, albeit being of different physical origin in the two cases. Epitaxial integration of III-V structures on Si
plays an important role in developing high-performance light sources on silicon, eventually integrated with silicon
electronics. A brief remark on all-optical vs. electronically controlled optical switching systems is also given.
Silicon photonics is an emerging technology offering novel solutions in different areas requiring highly integrated
communication systems for optical networking, sensing, bio-applications and computer interconnects. Silicon photonicsbased
communication has many advantages over electric wires for multiprocessor and multicore macro-chip
architectures including high bandwidth data transmission, high speed and low power consumption. Following the
INTEL's concept to "siliconize" photonics, silicon device technologies should be able to solve the fabrication problems
for six main building blocks for realization of optical interconnects: light generation, guiding of light including
wavelength selectivity, light modulation for signal encoding, detection, low cost assembly including optical connecting
of the devices to the real world and finally the electronic control systems.
Epitaxial Lateral Overgrowth has been proposed as a key technology of a novel hybrid integration platform
for active silicon photonic components. By fabricating silicon oxide mask on top of a thin InP seed layer, we
can use the so called defect necking effect to filter out the threading dislocations propagating from the seed
layer. By optimizing the process, thin dislocation free InP layers have been successfully obtained on top of
silicon wafer. The obtained characterization results show that the grown InP layer has very high quality,
which can be used as the base for further process of active photonic components on top of silicon.
There is an intense interest on integration of III-V materials on silicon and silicon-on-insulator for realisation of optical
interconnects, optical networking, imaging and disposable photonics for medical applications. Advances in photonic
materials, structures and technologies are the main ingredients of this pursuit. We investigate nano epitaxial lateral
overgrowth (NELOG) of InP material from the nano openings on a seed layer on the silicon wafer, by hydride vapour
phase epitaxy (HVPE). The grown layers were analysed by cathodoluminescence (CL) in situ a scanning electron
microscope, time-resolved photoluminescence (TR-PL), and atomic force microscope (AFM). The quality of the layers
depends on the growth parameters such as the V/III ratio, growth temperature, and layer thickness. CL measurements
reveal that the dislocation density can be as low as 2 - 3·107 cm-2 for a layer thickness of ~6 μm. For comparison, the
seed layer had a dislocation density of ~1·109 cm-2. Since the dislocation density estimated on theoretical grounds from
TRPL measurements is of the same order of magnitude both for NELOG InP on Si and on InP substrate, the dislocation
generation appears to be process related or coalescence related. Pertinent issues for improving the quality of the grown
InP on silicon are avoiding damage in the openings due to plasma etching, pattern design to facilitate coalescence with
minimum defects and choice of mask material compatible with InP to reduce thermal mismatch.
Integration of III-V materials on silicon wafer for active photonic devices have previously been achieved by growing
thick III-V layers on top of silicon or by bonding the III-V stack layers onto a silicon wafer. Another way is the epitaxial
lateral overgrowth (ELOG) of a thin III-V material from a seed layer directly on the silicon wafer, which can be used as
a platform for the growth of active devices. As a prestudy, we have investigated lateral overgrowth of InP by Hydride
Vapor Phase Epitaxy (HVPE) over SiO2 masks of different thickness on InP substrates from openings in the mask.
Openings which varied in direction, width and separation were made with E-beam lithography allowing a good
dimension control even for nano-sized openings (down to 100 nm wide). This mimics overgrowth of InP on top of
SiO2/Si waveguides. By optimizing the growth conditions in terms of growth temperature and partial pressure of the
source gases with respect to the opening direction, separation and width, we show that a thin (~200 nm) layer of InP with
good morphology and crystalline quality can be grown laterally on top of SiO2. Due to the thin grown InP layer,
amplification structures on top of it can be well integrated with the underlying silicon waveguides. The proposed ELOG
technology provides a promising integration platform for hybrid InP/silicon active devices.
Integration of active photonic components on silicon and silicon on insulator (SOI) would be versatile for nanophotonics
since CMOS compatible processes are available for fabricating passive devices on Si/SOI. Selective area growth of III-V
semiconductors is also attractive for realising periodic structures for nanophotonics. Here we report on the recent results
of high quality InP on Si and InP on SOI achieved by means of nanopatterning. MQW structures have been realised on
InP/Si and InP/SOI. We would elaborate routes for monolithic integration of active and passive devices for
nanophotonics.
The characteristics of intersubband transitions in III-nitride quantum wells are promising for detectors and all-optical
switches through a high intrinsic speed (~1 THz), and can also provide a high optical saturation power and a desired
small negative chirp parameter in electroabsorption modulators. The high LO-phonon energy allows to improve the
operating temperature of THz emitters. Recent achievements and prospects for intersubband III-nitride photonic devices,
mainly for λ=1.55 μm, are briefly reviewed. Further, means to enhance material quality by achieving crack-free growth
of GaN/AlN multiple-quantum-well (MQW) structures, and by employing intersubband transitions in multiple-quantum-disk
(MQD) structures incorporated into dislocation free GaN nanocolumns are discussed. We investigate the occurrence
of cracks in MBE-grown GaN/AlN MQWs on GaN MOVPE templates with respect to the buffer layer, the number of
QWs and the temperature reduction rate after growth. Intersubband absorption in GaN/AlN MQDs in the wavelength
range 1.38-1.72 μm is demonstrated in three samples grown on Si(111).
We have used MBE to grow MQW structures on MOVPE GaN/sapphire templates. The MQW devices are intended for high speed intersubband electroabsorption modulator devices operating at 1.55-&mgr;m. The GaN/AlN multiple quantum well material was systematically studied regarding the surface morphology, structural characterization and optical property by atomic force microscopy, X-ray diffraction and Fourier transform infrared spectroscopy, respectively. The intersubband resonance energy was also calculated considering many-body effects in n-type doped structures. The multiple quantum well structure showed superior performance in terms of linewidth when grown on GaN templates as compared on sapphire. GaN quantum well and AlN barriers with a thickness of 3.3 and 4.2 nm respectively resulted in FWHM of the intersubband absorption peak as low as 93 meV at an absorption energy of 700 meV. This is promising for intersubband modulator applications.
The development of wide band gap semiconductors extends their applications in optoelectronics devices to
the UV domain. Compact lasers and high sensitivity APD detectors in UV range are currently needed for
different applications such as, purification, covert communication and real time detection of airborne
pathogens. Until now, the full exploitation of these potential materials has been limited by the lack of
suitable GaN substrates. Recently, a novel class of materials has been reported based on BGaN and BAlN,
potentially reducing the crystal defect densities by orders of magnitude compared to existing wide band gap
heterostructures. Characteristics of these new alloys are similar to those of AlGaN materials with the
advantage that these can be lattice matched to AlN and SiC substrates. In addition, these materials offer the
possibility of using quaternary BAlGaN alloys at Ultra Violet (UV) wavelengths and hence lead to more
degrees of freedom in designing sophisticated device structures.
In this paper we describe the MOVPE growth conditions used to incorporate boron in GaN and AlGaN.
Detailed characterization and analysis in terms of structural and electrical properties are discussed.
Epitaxial lateral overgrowth of (ELO) InP on (001) InP/Si substrate is explored in a low pressure hydride vapor phase epitaxy system under various growth conditions. The effect of gas phase supersaturation on boundary plane formation of ELO and the behavior of dislocations in the grown layers are investigated. We found that the growth rate on (1 1 1)A boundary plane is determined by Burton-Cabrera-Frank model, which predicts a parabolic relationship between gas phase supersaturation and growth rate. Formation of (1 1 1)A plane will cause stacking faults in the grown InP layer. They will interact and annihilate each other and introduce fresh dislocations during the growth. Gas phase supersaturation can also be changed by varying opening separation distance. Low gas phase supersaturation is obtained by decreasing the distance between two openings. It gives rise to a lower staking fault density due to the suppression of nucleation of { 1 1 1 } facet plane at the edge of ELO. Etch pit density (EPD) and X-ray diffraction (XRD) techniques are used to estimate the dislocation density. Full width at half maximum of rocking curve at (004), (115) and (117) reflections were used to calculate the dislocation density in ELO InPISi. Experimentally measured etch pit density is smaller than the dislocation density derived from XRD data. In general the dislocation density is dependent on gas phase supersaturation. In this work, we demonstrate that it is possible to grow high quality InP layer on silicon substrate by epitaxial lateral overgrowth technique under optimized growth conditions.
We present an approach for systematic high-speed characterization of VCSELs and discuss both its potential benefits and problems. We show how the VCSEL dynamics, under certain conditions, can be well described by a small number of key parameters that can be extracted from measurements and used for further optimization. The calibrated small signal modulation responses of the laser are measured and fitted to an analytical transfer function allowing the estimation of the resonance frequency, damping factor and parasitic cut-off at different bias points. From this data the relative importance of different bandwidth limiting effects due to damping, thermal heating and parasitics can be deducted. We illustrate the approach on 850nm datacom VCSELs using either ion implantation, selective oxidation or semi-insulating regrowth for current confinement. The bandwidth ofthe implanted device appears to be limited by parasitics effects to 3.3GHz. Due to a much smaller injection diameter, the oxidized VCSEL reaches 10GHz, being mainly limited by the high damping. Finally the regrown VCSEL operates up to 5GHz, limited by the parasitics
Results from modulation measurements of 40 high-speed multi quantum well (MQW) lasers ((lambda) equals 1.55 micrometer) of various designs are presented. By fitting the careful calibrated measurements, both magnitude and phase, to an analytical transfer function we were able to determine if a certain laser was limited by thermal effects, parasitic-like effects, or nonlinear gain effects. We found that most of the devices in the study were limited by thermal effects and/or contact parasitics. The parasitics were found to be determined by the width of the high-doped contact layer and cladding layers below the metallic contact. It was also found that a high doping of the separate confinement heterostructure (SCH) layers decreases the damping of the relaxation peak since it facilitates the carrier transport. Improved contact design and high doped SCH-layers resulted in modulation bandwidths of around 24 GHz.
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