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.