Monolithic integration of a laser diode with a modulator usually requires complicated and potentially defect-generating processing steps such as epitaxial regrowth or bonding. Alternatively, the same medium grown in a single epitaxial run can be used in both gain and electro-absorption section. We demonstrate a universal active medium with tunnel coupled InGaAs short period superlattice (SPSL) quantum well grown on InAs quantum dots (QW-on-QDs) in an AlGaAs waveguide. In this configuration, the QW serves as a tunnel injector of electrons into the QDs in the forward-biased gain section, while in the reverse-biased modulator section the InGaAs QW acts as a wavelength matched quantum Stark effect electro-absorber. The optimum QW and QD ground state separation is close to 30meV for efficient tunnel injection into QDs to increase the gain. The 3x(QW-on-QDs) double heterostructure with separate 0.3μm waveguide was grown by molecular beam epitaxy on n-type GaAs substrate. The device was processed into two-section dies with an intermediate distributed Bragg reflector (DBR) between the sections. Focused ion beam air-gap DBR fabrication with low electrical leakage and required (~30%) optical mode reflection was employed. The laser-modulator performance with slope efficiency over 50%, 6dB power modulation, linear dynamic range>24dB at 2GHz was shown with a 200μm long modulator section and low optical feedback from the modulator to laser.
Quantum dot infrared photodetectors (QDIPs) have attracted significant attention due to selective photoresponse, high photoconductive gain, and numerous possibilities for nanoscale engineering of photoelectron processes, which control the detector characteristics. Our approach to improving QDIP performance is based on optimization of three dimensional nanoscale potential profile created by charged quantum dots (QDs). Nanoscale profile around QDs allows us to control photoelectron capture processes, which determines the photoelectron lifetime, detector operating speed, responsivity, the spectral density of noise, noise bandwidth, and the detector dynamic range. The nanoscale potential profile is determined by doping of QDs and inter-dot space. In this work, we study various ways of selective doping and its effects on characteristics of photodetectors. We investigate and compare intra-dot doping, inter-dot doping, and complex bipolar doping. To investigate effects of selective doping, we fabricated AlGaAs/InAs QD structures with n-doping of QD layers, structures with n-doping of barriers, and structures with p-doping of QD layers and n-doping of interdot space. We measured dark current, spectral photoresponse, voltage dependence of responsivity, and noise characteristic. The photoresponse is improved due to photon-electron coupling, which increases with QD filling by electrons. However, the noise current also increases due to increase in QD filling. Therefore, possibilities for improvement of QDIP structures with unipolar doping are very limited. Our results show that spectral photoresponse, responsivity, and detector sensitivity are substantially improved due to bipolar doping, which provides decoupled control of electron filling of QDs and the potential barriers around QDs.
Quantum well infrared photodetectors are widely used in focal plane arrays operating at liquid nitrogen temperatures. Compared to quantum-well structures, quantum dot (QD) nanomaterials are more flexible to control photoelectron processes by engineering of the nanoscale potential profiles formed by charged quantum dots. Quantum dots with builtin charge (Q-BIC) suppress capture of photoelectrons by QDs and provide strong coupling to infrared radiation. We review design approaches, fabrication and characterization of photodetectors based on Q-BIC media with strong selective doping to increase the built-in dot charge. Characterization of Q-BIC media includes the structural, spectral (photoluminescence measurements) and electrical characterization (dark current, I-V measurements). After several design-growth-characterization cycles we reached relatively high density of quantum dots, small concentration of defects related to quantum dot growth, and suppressed carrier capture by QDs. Optimized Q-BIC media were used for fabrication of Q-BIC IR photodetectors. We studied spectral and temperature dependences of photoresponse and also its dependences on bias voltage and parameters of Q-BIC medium.
Long-wavelength (λ ≈ 12 - 16 μm) Quantum Cascade (QC) lasers are crucial devices for improving the detection sensitivity of QC-laser based sensing for important gases including BTEX (benzene, toluene, ethylbenzene, and xylenes) or uranium hexafluoride. A high-performance QC laser emitting at ~ 14 μm is reviewed, optimized by employing a diagonal optical transition and a “two-phonon-continuum” depletion scheme. It shows a low threshold current density of 2.0 kA/cm2, a peak power of 336 mW, all at 300 K, as well as a high characteristic temperature ~ 310 K over a wide temperature range around room temperature (240- 390 K). Single-mode operation is demonstrated with short cavities, with a mode-hop-free continuous tuning range of ~ 5.5 cm-1. The ridge-width dependence of threshold of ~ 14 μm QC lasers by both wet etching and dry etching is studied. The main challenge for narrowing wet-etched ridges is the high loss caused by mode coupling to surface plasmon modes at the insulator/metal interface of sloped sidewalls. Conversely, dryetched ridges avoid surface plasmon mode coupling due to the absence of transverse magnetic polarization for the vertical insulator and metal layers. To further improve the efficiency of QC lasers, a same-wavelength cascaded transition approach is developed, with two sequential cascaded transitions at the same wavelength ~ 14.2 μm in each stage. This same-wavelength cascaded-transition QC gain medium was inserted between two conventional QC stacks at the same wavelength. Slope efficiency is increased by 46% when laser operation changes from the single-transition region to the cascaded-transition region.
Resistance to temperature and ionizing radiation of space optoelectronic devices can be improved through control of carrier kinetics in nanoscale systems. Recent results in the science and technology of self-assembled heteroepitaxial InAs quantum dot (QD) medium related to photonic applications are discussed. Focus is placed on management of carrier kinetics via nanoengineering of electronic spectrum and potential profiles in the QD ensemble using modeling and controlled fabrication of QDs with molecular beam epitaxy. Shape-engineered QD sheets embedded into GaAs quantum wells were found to withstand two orders of magnitude higher proton dose than QWs and to account for high luminescence efficiency and thermally stable laser diodes. Built-in charge in QDs is responsible for improvement of both near and mid-IR optical absorption, but also control photoelectron lifetime in the structures. The negatively charged QD medium was the first QD material that has recently shown credible improvement of solar cell efficiency. It has resulted from IR energy harvesting and suppressed fast electron capture processes. It is thus expected that QD InAs/GaAs photovoltaics will overcome the efficiency and lifespan of multi-junction solar cells. Potentials due to QD built-in charge are also responsible for improved photoelectron lifetime in QD infrared photodetectors. QD correlated clusters provide even higher collective potential barriers around clusters and constitute the novel approach to the optoelectronic materials combining manageable photoelectron lifetime, high mobility, and tunable localized and conducting states.
Structures with tunnel-coupled pairs consisting of InGaAs quantum wells (QWs) grown on top of self-assembled InAs quantum dots (QDs) were used previously as a gain medium for vertical cavity surface emitting lasers (VCSELs) to eliminate problems with QD-limited maximum saturated gain. Conventional molecular beam epitaxy of tunnel-coupled QDs with slow InAs growth rate and InGaAs solid solution QW injector with high InAs growth rate required a long delay in growth process for changing indium source temperature/flux. This leads to non-intentional doping of tunnel barrier and reproducibility issues. To overcome these problems, structures of tunnel-coupled QDs-QW pairs consisting of InAs/InGaAs short period superlattice (SPSL) QW injector with compatible slow InAs growth rate (QDs-SPSL) were
developed and compared with traditional InAs-InGaAs (QDs-InGaAs). Photoluminescence (PL) and electroluminescence were used to study the properties of the "well-on-dots" active medium with InAs/InGaAs SPSL
QW and with InGaAs QW. The optimized tunnel triple pair QDs-SPSL structure with 2x reduction of growth time has demonstrated a 2x enhanced PL efficiency as compared with traditional QDs-InGaAs structures. A novel tunnel-coupled triple QDs InAs-SPSL was successfully employed as a gain medium of VCSELs with doped all-epitaxial distributed Bragg reflectors (DBRs). Room temperature CW lasing wavelengths in the range from 1100 nm to 1150 nm were
measured in VCSELs with attuned DBRs. These QDs-SPSL VCSELs demonstrated minimum threshold current value Ith = 0.85 mA and maximum differential efficiency of 0.16 W/A.
An integrated optoelectronic device, comprising VCSEL and intracavity electro-absorption modulator within the same
epitaxial structure, has been previously developed by several research groups. Such a combination device, despite having
relatively weak DC modulation, exhibits strong optical feedback, resulting in strong optoelectronic resonance feature in
small-signal modulation response characteristic . At large modulation amplitude, device demonstrates pulsed response.
Similar to Q-switching operation, energy accumulated in the gain medium over full modulation cycle is released in a
single short pulse once cavity Q-factor is increased. As a result, traditional NRZ amplitude modulation becomes
ineffective. We are proposing a phase-pulse modulation approach to drive this device, when strong optical feedback is
used for obtaining very fast rise and fall times of short pulses. Such transient times can be on the order of few photon
lifetimes, e.g. few picoseconds. Gain medium depletion can be avoided by variation of Q-factor both above and below
steady-state value and keeping total emitted energy per cycle at a constant level. Data showing modulation properties
(pulse energy >100 fJ, FWHM 40 ps non-controlled pulse length at 4 GHz,) and device characteristics, along with
numerical analysis of such device for different modulation waveforms is presented.
Quantum dot (QD) -based vertical cavity surface emitting lasers (VCSELs) are predicted to have faster modulation
response and better thermal stability as compared with quantum well (QW) VCSELs. QD size distribution, limited
carrier capture and thermalization rates affect the maximum saturated gain of QD-based lasers. To address these
problems, structures of tunnel coupled pairs consisting of InGaAs QW grown on top of self-assembled InAs QDs (QWon-
QDs) were employed as a gain medium for VCSELs. Photoluminescence and transmission electron microscopy were
used to study the properties of the "well-on-dots" active medium. We have developed a triple-pair tunnel QW-on-QDs
structure with a QD transition which is red-shifted ~ 32 meV relative to QW ground state (GS). This optimized energy
separation ▵E = EQW - EQDs was found to be close to the energy of the LO phonon. All-epitaxial tunnel-coupled QD
VCSELs demonstrated continuous wave (CW) mode lasing in a wide temperature range from T = - 20°C to above
150°C. The room temperature lasing wavelength λ = 1131 nm corresponds to the QD GS transition. A minimum
threshold current value Ith = 0.7 mA was measured in a 9 μm oxide aperture VCSEL. The maximum power from a single
device was 2.5 mW and maximum differential efficiency was 0.16 W/A. Small signal modulation responses of these
VCSELs showed a maximum resonance frequency of about 9 GHz. The damping-limited cut-off frequency for these
tunnel QW-on-QDs VCSELs was estimated at 34 GHz from the dependence of damping factor and resonance frequency
on driving current.
We have proposed and demonstrated the principle of optical decoupling of the AC modulation component in a lossmodulated
Vertical Cavity Surface Emitting Laser (VCSEL) using a detuned duo-cavity device. This approach allows
the VCSEL power to be modulated without changing the photon density in the active region. Analysis of reflectivity
spectra of a Fabri-Perot cavity with absorber shows that at a certain detuning from the resonance wavelength, reflectivity
is almost independent of absorption magnitude. At this spectral detuning between the active region cavity and modulator
cavity, a feedback-free transmission modulation of the VCSEL output is possible. The use a multiple-double-QW
(MDQW) electroabsorption modulator allows absorption swing between 0.2% and 2% per pass. Optical power
modulation of transmission with contrasts up to 40% and chirp of less than 0.05 nm at 930 nm was demonstrated with
our design. Initial cavity resonance detuning is controlled through growth and was determined to be ideally ~0.7 nm
from analysis of stand-alone absorber cavities. Resonance coupling (splitting) was calculated to be less than 0.3 nm in
case of matching resonances. Applying bias at the MDQW modulator section allows adjustment of detuning between
cavities by changing the top cavity resonance wavelength mainly via Kramers-Kronig relations. The high frequency
modulation characteristics can be tuned in this manner to show little or no sign of resonance, in which case the high
frequency roll-off of the modulation response is entirely determined by parasitics of the modulator section. We have
demonstrated a flat (+/-3db) response up to 20 GHz.
We have studied the modulation properties of VCSEL with intracavity multiple quantum well (MQW) electroabsorption
modulator integrated into the top distributed Bragg reflector (DBR) . Small signal analysis of rate equations for loss
modulation shows an intrinsic high-frequency roll-off slope of 1/&ohgr; instead of 1/&ohgr;2 in directly modulated laser diodes, and
consequently bandwidths in excess of 40 GHz are obtainable with this configuration . Possible limiting factors to high
bandwidth were examined by fitting high frequency characteristics to a multi-pole transfer function, and include RC
delay and carrier drift-limited time of flight (TOF) in the modulator intrinsic region. Intracavity loss modulation shows a
strong (+20dB) relaxation oscillation resonant feature in both theory and experiment. As demonstrated, this feature can
be significantly reduced in amplitude using parasitics. We have extracted relative contribution of TOF and parasitic
capacitance by varying the modulator intrinsic region width (105 and 210 nm) and lateral size of the modulator (18 and
12&mgr;m). It was estimated that the small size modulator exhibits parasitics f-3dB at 8GHz. To estimate the carrier TOF
contribution to bandwidth limits, low temperature growth of a 210 nm absorber i-region and MQW was employed to
reduce photogenerated carrier lifetime. Bandwidth limitations were found to be mostly due to diode and metallization
capacitances, in addition to one pole set by the optoelectronic resonance frequency. We have used p-modulation doping
of the gain region to increase the relaxation frequency. Pronounced active Q-switching was observed, yielding pulse
widths of 40 ps at a 4 GHz rate.
Quantum dot (QD) size distribution and limitations in carrier capture and thermalization rates are still limiting the
maximum saturation gain in QD-based laser diodes and the utilization of QD-medium in all-epitaxial vertical cavity
surface emitting lasers (VCSELs). To overcome these problems structures of tunnel coupled pairs consisting of InGaAs
quantum wells grown on top of self-assembled InAs QDs (QW-on-QDs) were employed as a gain medium for VCSELs.
Photoluminescence, transmission electron microscopy and electroluminescence were used to study the properties of the
multiple-layer QW-on-QDs active medium. QW-on-QDs tunnel structures with 3 - 5 nm tunnel barrier thicknesses and
with different ground state (GS) relative separations were grown with varying InGaAs QW while the QD growth process
parameters were kept constant. We have developed a tunnel QW-on-QDs structure with a QD PL line red-shifted by 32
meV relative to QW GS line. The narrow linewidth (22 meV) of this QD transition likely indicates an efficient LOphonon
assisted tunneling of carriers from QW into QD ensemble states. Optimized tunnel (with 3 nm barrier thickness)
QW-on-QDs structures were evaluated in VCSELs. All-epitaxial VCSELs with triple-pair tunnel QW-on-QDs as active
medium demonstrated continuous wave mode lasing. These QD-based VCSELs with n-doped AlGaAs/GaAs mirrors
and tunnel n-p junction exhibited 1.8 mA (Jth ~ 800 A/cm2) minimum threshold current at QD GS emission wavelength,
1135 nm, with 0.7mW optical power and 12% slope efficiency.
Structures of tunnel coupled pairs consisting of InGaAs quantum wells grown on top of self-assembled InAs quantum dots (QW-on-QDs) were employed to improve the gain medium in laser diodes. Photoluminescence, transmission electron microscopy and electroluminescence were used to study the properties of multiple-layer QW-on-QDs active medium. QW-on-QDs tunnel structures with 4.5 nm tunnel barrier thickness and with different ground state (GS) relative separations were grown by variation of InGaAs QW while the QD growth process was kept constant. We have developed a tunnel QW-on-QDs structure with a resonance transition which is red-shifted ~35 meV relative to QW GS. This transition with narrow linewidth, 21.6 meV at T=77K, likely indicates an efficient LO-phonon assisted tunneling of carriers from QW into QD ensemble states. The highest gain was achieved with a QW-on-QDs active medium with GS relative separation of close to 35-40 meV. Optimized triple-pair tunnel QW-on-QDs laser diodes with cleaved mirrors emitting at 1145 nm (corresponding to QD GS) exhibited a saturated modal gain exceeding 80 cm-1 with minimum cavity length of 0.14 mm. Small signal modulation characteristics of these lasers were measured. From the damping factor and resonance frequency dependence on driving current, the damping-limited cut-off frequency for this QW-on-QDs medium can be estimated as exceeding 30 GHz. All-epitaxial vertical cavity surface emitting lasers with triple-pair tunnel QW-on-QDs as active medium demonstrated continuous wave mode lasing with 5.7 mA minimum threshold current at QD GS emission wavelength, 1131 nm.
Structures of tunnel pairs consisting of InGaAs quantum well (QW) and self-assembled InAs quantum dots (QDs) were employed to improve gain medium in laser diodes. Photoluminescence, transmission electron microscopy and electroluminescence were used to study the influence of the relative position of ground states (GS) energies of QW and QDs as well as structure design on the properties of tunnel structures and the characteristics of multi-layer lasers. QDs on QW structures with different GS relative separation were grown by variation of In concentration in QWs with fixed growth process of QDs. An 1160 nm edge-emitting lasers with 4 pairs of QDs-on-QW as active medium showed higher (than in a similar multilayer QD lasers) maximum saturated gain, 26 cm-1, with low minimum threshold current density Jth = 95 A/cm2. First attempt of a triple-pair tunnel QW-on-QDs laser emitting at 1125 nm exhibited a saturated modal gain more than 50 cm-1. These lasers demonstrated broad multi-peak emission spectra with minimum threshold current density Jth = 255 A/cm2 and with lasing from intermediate states between QDs and QW GS transitions. RF small signal modulation characteristics of the 3x(QW-on-QDs) lasers were measured. From the damping factor and resonance frequency dependence, maximum possible 3dB cut-off frequency for this QW-on-QD media can be estimated as 13 GHz.
Reduction of the stresses produced in hybrid integrated structures due to thermal expansion coefficient difference requires removal of the substrate as one of the key elements. Commonly used epitaxial lift-off technique can hardly be employed for the fabrication of VCSELs with all-epitaxial DBRs due to the low etching selectivity between AlAs sacrificial layer and DBR layers with high Al contents. Novel method of substrate removal named oxidation lift-off was proposed and demonstrated. This process shows higher selectivity against Al-content than epitaxial lift-off method, that allows for the release of a VCSEL structure with epitaxial DBRs and separate individual components on Si, reduces the number of process steps and eventually reduces the cost of the fabricated/integrated devices. Au-Ge alloy was used for the metal bonding of the test oxidation lift-off structures grown by MBE. 1 μm thick AlAs imbedded sacrificial layer was laterally oxidized to release the partially processed devices from the GaAs substrate. 2D array of separated VCSELs was fabricated on top of the Si substrate. Contact annealing, substrate removal, device separation, bonding and formation of the oxide apertures were completed within a single processing step. Electroluminescent spectra, I-V and P-I characteristics of fabricated devices were measured. Series resistance of fabricated devices was found to be about 100 Ohms. Lasing with threshold current of 8 mA was demonstrated for the device with 25 μm aperture.
Nanoengineering approach was used to develop an efficient active medium based on self-assembled InAs/GaAs quantum dots (QDs) for laser diodes operating at elevated temperatures. Photoluminescence (PL), transmission electron microscopy, and electroluminescence were used to study the influence of an overgrowth procedure on the properties of multiple-layer QDs. Optical properties of QDs were optimized by the adjustment of a GaAs overlayer thickness prior to a heating step, responsible for the truncation of the pyramid-shaped QDs. Triple-layer QD edge-emitting lasers with 1220 nm emitting wavelength exhibited a maximum saturated modal gain of 16 cm-1. To use truncated QD active medium for vertical cavity surface emitting lasers, seven layers of QDs with 20 nm of short period superlattice barriers between layers was developed. A wavelength of 1190 nm edge-emitting lasers with 120 nm total thickness 7xQDs active medium showed almost two times higher maximum saturated gain, 31 cm-1. Unfortunately, these lasers with closer distance between QD layers in active medium demonstrated stronger temperature dependence (with To = 110 K) of threshold current density and lasing wavelength. A record high characteristic temperature for lasing threshold, To = 380 K up to 55 C, was measured for edge-emitting laser diodes, which contained triple-layer truncated QD active medium. We believe that AlAs capping in combination with truncation procedure result in significant suppression of carrier transport between QDs within the layer as well as between QD layers.
We have studied the influence of overgrowth procedure and a few monolayer-thick AlAs capping layers on the properties of self-assembled InAs quantum dots (QDs) using transmission electron microscopy (TEM), scanning electron microscopy, and photoluminescence (PL). PL spectroscopy was used to study and optimize optical properties of the QDs by shape engineering (QD truncation) through adjustment of the thickness of overlayers and temperature of the subsequent heating. QDs with 6 nm-thick overlayer with heating step at 560°C was found to have the highest PL intensity at room temperature and the lowest FWHM, 29 meV. Ground state energy of the truncated QDs is very stable against variations of growth parameters. TEM measurements show that the capping AlAs layer covers the QDs entirely even though the dots are truncated by the heating step. 1.22 μm edge-emitting laser with triple-layer truncated QD gain medium demonstrated room temperature minimum threshold current density, 56 A/cm2, and high saturated modal gain, 16 cm-1. Extremely high characteristic temperature, To = 304 K in the 20 - 60°C interval, and maximum lasing temperature of 219°C were measured for this laser diode.
Inadequate performance of interconnects in emerging integrated circuitry has generated a need for alternative signal transmission solutions. Integration of dense arrays of high frequency III-V photoemitters and photodetectors with Si platform is one of the challenging tasks. Comparison of monolithic and hybrid integration technologies highlights the advantages of hybrid approaches at least for emitters highly sensitive to growth defects. A novel protocol for fabrication of III-V optoelectronic components such as LEDs, VCSELs and photodetectors on Si platform is proposed. The simulations of thermal behavior and mechanical stresses of this integration scheme was performed using finite element analysis and revealed adequate heat dissipation. Simulations show that this protocol allows to reduce overheating and mechanical stresses to enhance the optoelectronic devices performance and increase their lifetime. The III-V structures are grown homoepitaxialy on GaAs substrate, then bonded to a Si wafer using low-temperature polymer followed by wet etching of the substrate. The scheme involves VCSEL processing with coplanar metallization on Si with PMGI reflow planarization. MBE-grown reversed VCSEL structure was used for manufacturing of the test devices using this novel protocol. An AlAs etch stop layer was imbedded into the structure. 10 um thick VCSEL structure was bonded on Si using BCB (CycloteneTM). Substrate was completely removed by selective etching to reduce thermal stresses to enhance the optoelectronic devices performance and increase their lifetime. The array of the 3D devices was fabricated using wet etching. A 10 um-thick high frequency VCSEL with coplanar metallization is processed on Si with PMGI reflow planarization. Electro-luminescence spectrum, I-V and P-T characteristics were measured and compared with a reference structure. It was found that measured thermal impedance is about five times higher than for devices on a host GaAs wafer. Simulation of thermal behavior was done for bonded and non-bonded structure. It was found that measured values of thermal impedance are in good agreement with simulation results.
The influence of two monolayer (ML)-thick AlAs under- and overlayers on the formation and properties of self-assembled InAs quantum dots (QDs) has been studied using transmission electron microscopy, photoluminescence (PL) and electroluminescence. The main purpose of this work was to achieve high internal quantum efficiency of the active medium and temperature stability of the laser diodes. Single and multiple layers of 2.0-2.4ML InAs QDs with various combinations of under- and overlayers were grown on GaAs (001) substrate by molecular beam epitaxy inside a AlAs/GaAs short-period superlattice. It was found that a 2.4-ML InAs QD layer with GaAs underlayer and 2-ML AlAs overlayer exhibited the lowest QD surface density of 4.2x1010 cm-2 and the largest QD lateral size of about 19 nm as compared to the other combinations of cladding layers. This InAs QD ensemble has also shown the highest room temperature PL intensity with a peak at 1210 nm and the narrowest linewidth, 34 meV. Fabricated edge-emitting lasers using triple layers of 2.2-ML InAs QDs with AlAs overlayer demonstrated 120 A/cm2 threshold current density and 1230 nm emission wavelength at room temperature. Excited state QD lasers have shown high thermal stability of threshold current up to 130 degree(s)C.