We demonstrated the ion-sensitive field-effect transistors (IS-FETs) based on nanowires (NWs) with different diameters and doping concentrations to obtain the high sensitivity and various applications. The growth of the catalyst-free InAs NWs was carried out using a horizontal reactor MOCVD system (AIXTRON Inc.). A p-type Si (111) wafer (ρ = 1 -10 Ω-cm) was prepared for the NW growth. Here, NWs with diameters of around 50 ~ 150 nm were grown and the doping concentration also was changed around x±1016~18 /cm2. IS-FETs with the grown InAs NWs were fabricated using the photolithography and the lift-off process. The gas sensing characteristics have been investigated through studying the gate response of the NW conductance in different ambient conditions.
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.
We analyzed and demonstrated the double layered metallic nano-structures using polystyrene lift-off process on the
conventional surface plasmon resonance (SPR) sensor to enhance the sensitivity of an SPR surface. The double layered
plasmonic structures are optimized using the three-dimensional finite-difference time-domain method for the width,
thickness, and period of the polystyrene beads. The thickness of the metal film and the metallic nano-hole is 20 and 20
nm in the 305 nm wide nano-hole size, respectively. The double layered metallic nano-structures are fabricated with
monolayer polystyrene beads of chloromethyl latex 4% w/v 0.4 μm. The sensitivities of the conventional SPR sensor
and the double layered plasmonic sensor are obtained to 42.2 and 60 degree/RIU, respectively. The SPR devices are also
applied to the lead ion sensor. The resonance shifts of SPR sensors with and without a poly(vinyl chloride) membrane
are 1328 RU and 788 RU from 10-5 M to 10-2 M concentration, respectively.
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.
The equations for threshold-current density Jth and external differential quantum efficiency d of quantum cascade lasers (QCLs) are modified to include electron leakage and the electron-backfilling term corrected to take into account hot electrons in the injector. We show that by introducing both deep quantum wells and tall barriers in the active regions of 4.8-µm-emitting QCLs, and by tapering the conduction-band edge of both injector and extractor regions, one can significantly reduce electron leakage. The characteristic temperatures for Jth and d, denoted by T0 and T1, respectively, are found to reach values as high as 278 and 285 K over the 20 to 90°C temperature range, which means that Jth and d display 2.3 slower variation than conventional 4.5- to 5.0-µm-emitting, high-performance QCLs over the same temperature range. A model for the thermal excitation of hot injected electrons from the upper laser level to the upper active-region energy states, wherefrom some relax to the lower active-region states and some are scattered to the upper miniband, is used to estimate the leakage current. Estimated T0 values are in good agreement with experiment for both conventional QCLs and deep-well QCLs. The T1 values are justified by increases in both electron leakage and waveguide loss with temperature
In this work we show that by using both deep quantum wells and tall barriers in the active regions of quantum cascade
(QC)-laser structures and by tapering the conduction-band edge of both injector an extractor regions one can
significantly reduce the leakage of the injected carriers. Threshold-current, Jth and differential-quantum efficiency, ηd
characteristic temperatures, T0 and T1, values as high as 278 K and 285 K are obtained to 90 °C heatsink temperature,
which means that Jth and ηd vary ~ 2.5 slower over the 20-90 °C temperature range than in conventional QC devices.
Modified equations for Jth and ηd are derived. In particular, the equation for ηd includes, for the first time, its dependence
on heatsink temperature. A model for the thermal excitation of injected carriers from the upper lasing level to upper
active-region energy states from where they relax to lower active-region energy states or get scattered to the upper Γ
miniband is employed to estimate carrier leakage. Good agreement with experiment is obtained for both conventional
QC lasers and deep-well (DW)-QC lasers.
In this work, a novel self-aligned process utilizing non-selective, O2-enhanced wet thermal oxidation is presented for
fabricating InP-based, ridge waveguide mid-infrared (λ=5.4 μm) quantum cascade lasers (QCLs) with a straincompensated,
30-stage (1.53 μm thick) InGaAs/AlInAs active region, grown via metal organic chemical vapor
deposition. This process, previously used in GaAs-based diode lasers containing low-Al content AlGaAs or even Alfree
III-As alloys, forms a highly-insulating native oxide layer while simultaneously smoothing and passivating the etchexposed
active region, resulting in low-loss, strongly-confining waveguides. Here we report the first application of this
process for directly oxidizing the deeply-etched QCL InGaAs/AlInAs active region ridge waveguide sidewalls and field
(outside the ridge), eliminating the need for a deposited dielectric for electrical isolation, thus allowing self-aligned
device fabrication. An 8 hour, 500 °C wet oxidation with 7000 ppm added O2 (relative to N2 carrier gas) yields a
uniform oxide of ~350 nm in the field outside the ridge to ~500 nm on the ridge sidewall. Laser devices tested under
room temperature, pulsed excitation exhibit a threshold current density of Jth~3.2 kA/cm2 for a 19.5 μm wide x 3 mm
long stripe width.
In this work we present the characteristics of a novel type of quantum-cascade (QC) laser: the deep-well (DW) QC
device, which, unlike conventional QC lasers, contains a superlattice of quantum wells and barriers of different
composition, respectively. The fabrication of DW-QC devices is made possible by the use of metal-organic chemical
vapor deposition (MOCVD), a crystal growth technique which allows one to easily vary the composition of wells and
barriers within QC structures, thus providing significantly increased flexibility in optimizing the device design. We have
designed such varying-composition QC structures to have deep quantum wells in and tall barriers in and around the
active region. DW- QC laser structures have fabricated into 19 μm-wide ridges and 3 mm-long chips. Threshold-current
densities as low as 1.5 kA/cm2 are obtained at room temperature in the 4.6-4.8 μm wavelength region. In conventional
QC lasers emitting in the 4.5-5.5μm range there is substantial thermionic carrier leakage from the upper laser level to the
continuum, as evidenced by a significant decrease in the slope efficiency above 250 K, which is understandable given
the relatively small (i.e., ~ 200 meV) energy differential, δE, between the upper lasing level and the top of the exit
barrier. For the DW design carrier leakage is suppressed due to deep active wells and tall barriers, such that δE reaches
values in excess of 400 meV. Preliminary results include a threshold-current characteristic temperature, T0, value of 218
K over the temperature range: 250- 340 K.
Intersubband Quantum-Box (IQB) lasers; that is, devices consisting of 2-D arrays of ministacks (i.e., 2-4 stages) intersubband QB emitters are proposed, as an alternative to 30-stage quantum-cascade (QC) devices, as sources for efficient room-temperature (RT) emission in the mid-infrared (4-6 µm) wavelength range. Preliminary results include: 1) the design of devices for operation with 50% wallplug efficiency at RT; 2) realization of a novel type of QC device: the deep-well (DW) QC laser, that has demonstrated at =4.7µm low temperature sensitivity of the threshold current, a clear indication of suppressed carrier leakage; 3) the formation of 2-D arrays at nanopoles by employing nanopatterning and dry etching; 4) the formation of 40nm-diameter, one-stage IQB structures on 100nm centers by preferential regrowth via metal-organic vapor phase epitaxy (MOVPE).
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