Implementation of the step-taper active-region (STA) design to 8-9 μm-emitting quantum cascade lasers (QCLs) has resulted in both high T0 and T1 values: 220 K and 665 K, and short lower-level lifetimes: 0.12 ps. In turn, the internal differential efficiency ηid, which is the product of the injection efficiency and the differential laser-transition efficiency, reaches values as high as 86 % for both 8.4 μm- and 8.8 μm-emitting QCLs. Such ηid values are 30-50% higher than those obtained from conventional QCLs emitting in the 7-11 μm wavelength range. Achieving both carrier-leakage suppression and miniband-like carrier extraction in mid-infrared (IR) QCLs leads to ηid values close to the fundamental limit of ~ 90 %. In turn, the currently employed fundamental wallplug-efficiency limits over the mid-IR wavelength range have to be increased by ~ 34 % (e.g., the wallplug-efficiency limit at λ= 4.6 μm increases from 29 % to 39 %). Preliminary results from STA-type 4.8-5.0 μm-emitting QCLs include 1.5 W CW operation, and 77 % internal differential efficiency; that is, 30-50% higher than the ηid values obtained from conventional 4.0-6.5μm-emitting QCLs.
By stepwise tapering both the barrier heights and quantum-well depths in the active regions of 8.7 μm- and 8.4 μm-emitting
quantum cascade lasers (QCLs) virtually complete carrier-leakage suppression is achieved, as evidenced by
high values for both the threshold-current characteristic temperature coefficient T0 (283 K and 242 K) and the slope-efficiency
characteristic temperature coefficient T1 (561 K and 279 K), over the 20–60 °C heatsink-temperature range, for
low- and high-doped devices, respectively. Such high values are obtained while the threshold-current density is kept
relatively low for 35-period, low- and high-doped devices: 1.58 kA/cm2 and 1.88 kA/cm2, respectively. In addition, due
to resonant extraction from the lower laser level, high differential-transition-efficiency values (89-90%) are obtained. In
turn, the slope-efficiency for 3 mm-long, 35-period high-reflectivity (HR)-coated devices are: 1.15-1.23 W/A; that is, 30-
40 % higher than for same-geometry and similar-doping conventional 8-9 μm-emitting QCLs. As a result of both
efficient carrier-leakage suppression as well as fast and efficient carrier extraction, the values for the internal differential
efficiency are found to be ≈ 86%, by comparison to typical values in the 58-67 % range for conventional QCLs emitting
in the 7-11 μm wavelength range.
On-chip resonant leaky-wave coupling of quantum cascade lasers (QCLs) emitting at 8.36 μm has been realized by selective regrowth of interelement layers in curved trenches, defined by dry and wet etching. The fabricated structure provides large index steps (Δn = 0.10) between antiguided-array element and interelement regions. In-phase-mode operation to 5.5 W front-facet emitted power in a near-diffraction-limited far-field beam pattern, with 4.5 W in the main lobe, is demonstrated. A refined fabrication process has been developed to produce phased-locked antiguided arrays of QCLs with planar geometry. The main fabrication steps in this process include non-selective regrowth of Fe:InP in interelement trenches, defined by inductive-coupled plasma (ICP) etching, a chemical polishing (CP) step to planarize the surface, non-selective regrowth of interelement layers, ICP selective etching of interelement layers, and non-selective regrowth of InP cladding layer followed by another CP step to form the element regions. This new process results in planar InGaAs/InP interelement regions, which allows for significantly improved control over the array geometry and the dimensions of element and interelement regions. Such a planar process is highly desirable to realize shorter emitting wavelength (4.6 μm) arrays, where fabrication tolerance for single-mode operation are tighter compared to 8 μm-emitting devices.
Remarkable progress made in quantum cascade lasers (QCLs) has led them to find an increasing number of applications in remote sensing, chemical sensing, and free space communications, in addition to potential space applications. However, little has been reported on reliability and failure modes of QCLs although it is crucial to understand failure modes and underlying degradation mechanisms in developing QCLs that meet lifetime requirements for space missions. Focused ion beam (FIB) techniques have been employed to investigate failure modes in various types of laser diodes. Our group has also used FIB to study failure modes in single-mode and multi-mode InGaAs-AlGaAs strained QW lasers, but few groups have used this technique to investigate failure modes in QCLs. In our study, we report on destructive physical analysis (DPA) of degraded InGaAs-InAlAs QCLs using FIB and high-resolution TEM techniques. The active region of QCLs that we studied consisted of two-23 stage layers of InGaAs-InAlAs separated by a 0.5 μm thick InP spacer layer for 8.4μm QCLs and 30-stage layers of lattice-matched InGaAs-InAlAs heterostructure for 4.7μm QCLs. The MOVPE-grown laser structures were fabricated into deep-etched ridge waveguide QCLs. L-I-V-spectral characteristics were measured at RT under pulsed operation. Our 8.4μm QCLs with as-cleaved and HR-coated facets showed a laser threshold of 1.7 A and a threshold voltage of 13 V at RT, whereas our 4.7μm QCLs without facet coating showed threshold currents of 320 - 400 mA and threshold voltages of 13 - 13.5V. Failures were generated via short-term tests of QCLs. FIB systems were used to study the damage area on the front facet and also to prepare TEM cross sections at different locations along the waveguide for defect and chemical analyses using a HR-TEM. In contrast to the COMD damaged area showing as a blister on the front facet of QW lasers, the damaged area of QCLs was significantly extended into the InP substrate due to a much less absorption of lasing photons in QCLs. Our detailed destructive physical analysis results are reported including defect, structural, and chemical analysis results from degraded QCLs.
A new deep-well (DW) quantum-cascade laser (QCL) design: Tapered Active-Region (TA), for which the barrier layers
in each active region are tapered such that their conduction band edges increase in energy from the injection barrier to
the exit barrier, causes a significant increase in the energy difference between the upper laser level and the next higher
energy level, E54; thus, resulting in further carrier-leakage suppression compared to DW QCLs. High E54 values (80 -100
meV) are primarily obtained because the energy separation between the first excited states of a pair of coupled QWs
(CQWs) is larger when the CQWs are asymmetric than when they are symmetric. Then, we reach an optimized TA-QCL
design (λ= 4.7 μm) for which E54 values as high as 99 meV are obtained, while insuring good carrier depopulation of the
lower laser level (i.e., τ3 = 0.2 ps) via the double-phonon-resonance scheme. In addition, the upper-laser-level lifetime
increases by ~ 15 % compared to that for conventional QCLs. As a result, the relative carrier leakage decreases to values
≤ 1% and the room-temperature (RT) threshold-current density decreases by ~ 25 % compared to that for conventional
QCLs. Then, we estimate that single-facet, continuous-wave (CW) RT wallplug-efficiency values as high as 27 % are
possible. Preliminary results from TA QCLs include T0 and T1 values as high as 231 K and 797 K, respectively, over the
20-60 °C heatsink-temperature range.
The equations for the threshold-current density Jth, differential quantum efficiency ηd and maximum wallplug efficiency
ηwp,max for quantum-cascade lasers (QCLs) have been modified for electron leakage and backfilling. We used a thermalexcitation
model of "hot" injected electrons from the upper laser state to upper active-region energy states to calculate
leakage currents. Then the calculated characteristic temperature T0 for Jth was found to agree well with experiment for
both conventional and deep-well QCLs. The characteristic temperature T1 for ηd was deduced to be due to both electron
leakage and an increase in the waveguide-loss coefficient. For conventional mid-infrared QCLs ηwp,max is found to be
strongly temperature dependent which explains experimental data. By using a new concept: tapered active-region (TA),
deep-well QCLs have been optimized for virtual suppression of the electron-leakage currents. In turn, at room
temperature, for continuous-wave (CW)-operating, 4.5-5.0 μm-emitting TA QCLs we estimate the threshold current to
decrease by ~ 25 %, the active-region temperature rise at the ηwp,max point to decrease by ~ 30 %, and the single-ended,
ηwp,max value to become at least 22 %. Preliminary results from TA QCLs include T1 values as high as 454 K, over the
20-60 oC heatsink-temperature range.
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