This paper describes our development efforts at Northwestern University regarding dual-section sampled
grating distributed feedback (SGDFB) QCLs. These devices are the same size, but have much wider electrical tuning,
than a traditional DFB laser. In this paper, I will show how we have dramatically extended the monolithic tuning range
of high power quantum cascade lasers with high side mode suppression. This includes individual laser element tuning of
up to 50 cm-1 and 24 dB average side mode suppression. These lasers are capable of room temperature continuous
operation with high power (<100 mW) output. Additionally, we have demonstrated a broad spectral coverage of over
350 cm-1 on a single chip, which is equivalent to 87.5% of the gain bandwidth. The eventual goal is to realize an
extended array of such laser modules in order to continuously cover a similar or broader spectral range, similar to an
external cavity device without any external components.
Quantum Cascade Lasers (QCLs), operating in continuous wave (cw) at room temperature
(rt) in 3-3.5μm spectral range, which overlaps the spectral fingerprint region of many hydrocarbons,
is essential in spectroscopic trace gas detection, environment monitoring, and pollution control. A
3μm QCL, operating in cw at rt is demonstrated. This initial result makes it possible, for the most
popular material system (AlInAs/GaInAs on InP) used in QCLs in mid-infrared and long-infrared, to
cover the entire spectral range of mid-infrared atmospheric window (3-5μm).
In0.79Ga0.21As/In0.11Al0.89As strain balanced superlattice, which has a large conduction band
offset, was grown. The strain was balanced with composite barriers (In0.11Al0.89As /In0.4Al0.6As) in
the injector region, to eliminate the need of extremely high compressively strained GaInAs, whose
pseudomorphic growth is very difficult.
We demonstrate room temperature, continuous wave operation of quantum cascade ring lasers around 5 μm with single
mode operation up to 0.51 W output power. Single mode operation persists up to 0.4 W. Light is coupled out of the ring
cavity through the substrate with a second order distributed feedback grating. The substrate emission scheme allows for
epilayer-down bonding, which leads to room temperature continuous wave operation. The far field analysis indicates that
the device operates in a high order mode.
We demonstrate widely tunable high power distributed feedback quantum cascade laser array chips that span 190 nm
and 200 nm from 4.4 um to 4.59 um and 4.5 um to 4.7 um respectively. The lasers emit single mode with a very narrow
linewidth and side mode suppression ratio of 25 dB. Under pulsed operation power outputs up to 1.85 W was obtained
from arrays with 3 mm cavity length and up to 0.95 W from arrays with 2 mm cavity length at room temperature.
Continuous wave operation was also observed from both chips with 2 mm and 3 mm long cavity arrays up to 150 mW.
The cleaved size of the array chip with 3 mm long cavities was around 4 mm x 5 mm and does not require sensitive
external optical components to achieve wide tunability. With their small size and high portability, monolithically
integrated DFB QCL Arrays are prominent candidates of widely tunable, compact, efficient and high power sources of
mid-infrared radiation for gas sensing.
For many practical applications that need bright sources of mid-infrared radiation, single mode operation and good beam
quality are also required. Quantum cascade lasers are prominent candidates as compact sources of mid-infrared radiation
capable of delivering very high power both CW and under pulsed operation. While 1D photonic crystal distributed
feedback structures can be used to get single mode operation from quantum cascade lasers with narrow ridge widths,
novel 2D photonic crystal cavity designs can be used to improve spectral and spatial purity of broad area quantum
cascade lasers. In this paper, we demonstrate high power, spatially and spectrally pure operation at room temperature
from narrow ridge and broad area quantum cascade lasers with buried 1D and 2D photonic crystal structures. Single
mode continuous wave emission at λ = 4.8 μm up to 700 mW in epi-up configuration at room temperature was observed
from a 11 μm wide 5 mm long distributed feedback quantum cascade laser with buried 1D gratings. High peak powers
up to 34 W was obtained from a 3mm long 400 μm wide 2D photonic crystal distributed feedback laser at room
temperature under pulsed operation. The far field profile had a single peak normal to the laser facet and the M2 figure of
merit was as low as 2.5. Emission spectrum had a dominating single mode at λ = 4.36 μm.
Quantum dot (QD) devices are a promising technology for high operating temperature detectors. We have studied
InAs QDs embedded in an InGaAs/InAlAs quantum well structure on InP substrate for middle wavelength infrared
detectors and focal plane arrays (FPAs). This combined dot-well structure has weak dot confinement of carriers, and
as a result, the device behavior differs significantly from that in more common dot systems with stronger
confinement. We report on our studies of the energy levels in the QDWIP devices and on QD-based detectors
operating at high temperature with D* over 1010 cmHz1/2/W at 150 K operating temperature and high quantum
efficiency over 50%. FPAs have been demonstrated operating at up to 200 K. We also studied two methods of
adapting the QDWIP device to better accommodate FPA readout circuit limitations.
InAs quantum dots embedded in InGaAs quantum wells with InAlAs barriers on InP substrate grown by
metalorganic chemical vapor deposition are utilized for high operating temperature detectors and focal plane arrays
in the middle wavelength infrared. This dot-well combination is unique because the small band offset between the
InAs dots and the InGaAs well leads to weak dot confinement of carriers. As a result, the device behavior differs
significantly from that in the more common dot systems that have stronger confinement. Here, we present energy
level modeling of our QD-QW system and apply these results to interpret the detector behavior. Detectors showed
high performance with D* over 1010 cmHz1/2/W at 150 K operating temperature and with high quantum efficiency
over 50%. Focal plane arrays have been demonstrated operating at high temperature due to the low dark current
observed in these devices.
We report a room temperature operating InAs quantum-dot infrared photodetector grown on InP substrate. The self-assembled
InAs quantum dots and the device structure were grown by low-pressure metalorganic chemical vapor
depositon. The detectivity was 2.8 x 1011 cmHz1/2/W at 120 K and a bias of -5 V with a peak detection wavelength
around 4.1 &mgr;m and a quantum efficiency of 35 %. Due to the low dark current and high responsivity, a clear
photoresponse has been observed at room temperature, which gives a detectivity of 6.7 x 107 cmHz1/2/W. A 320 x 256
middle wavelength infrared focal plane array operating at temperatures up to 200 K was also fabricated based on this
kind of a device. The focal plane array had 34 mA/W responsivity, 1.1 % conversion efficiency, and noise equivalent
temperature difference of 344 mK at 120 K operating temperature.
Self-assembled semiconductor quantum dots have attracted much attention because of their novel properties and thus possible practical applications including the lasers, detectors and modulators. Especially the photodetectors which have quantum dots in their active region have been developed and show promising performances such as high operation temperature due to three dimensional confinement of the carriers and normal incidence in contrast to the case of quantum well detectors which require special optical coupling schemes. Here we report our recent results for mid-wavelength infrared quantum dot infrared photodetector grown by low-pressure metalorganic chemical vapor deposition. The material system we have investigated consists of 25 period self-assembled InAs quantum dot layers on InA1As barriers, which are lattice-matched to InP substrates, covered with InGaAs quantum well layers and InA1As barriers. This active region was sandwiched by highly doped InP contact layers. The device operates at 4.1 μm with a peak detectivity of 2.8×1011 cmHz1/2/W at 120 K and a quantum efficiency of 35 %. The photoresponse can be observed even at room temperature resulting in a peak detectivity of 6×107 cmHz1/2/W. A 320×256 focal plane array has been fabricated in this kind of device. Its performance will also be discussed here.
We report our recent results about mid-wavelength infrared quantum-dot infrared photodetectors (QDIPs) grown by low-pressure metalorganic chemical vapor deposition. A very high responsivity and a very low dark current were obtained. A high peak detectivity of the order of 3×1012 Jones was achieved at 77 K. The temperature dependent device performance was also investigated. The improved temperature insensitivity compared to QWIPs was attributed to the properties of quantum dots. The device showed a background limited performance temperature of 220 K with a 45° field of view and 300K background. The current device problems are a low quantum efficiency and a stronger than expected performance degradation as a function of operating temperature. Possible ways to improve the quantum efficiency and operating temperature are discussed.
Quantum-dot infrared photodetectors (QDIPs) have recently been considered as strong candidates for numerous applications such as night vision, space communication, gas analysis and medical diagnosis involving middle and long wavelength infrared (MWIR and LWIR respectively) operation. This is due to their unique properties arising from their 3-dimensional confinement potential that provides a discrete density of states. They are expected to outperform quantum-well infrared photodetectors (QWIPs) as a consequence of their natural sensitivity to normal incident radiation, their higher responsivity and their higher-temperature operation. So far, most of the QDIPs reported in the literature were based on the InAs/GaAs system and were grown by molecular beam epitaxy (MBE). Here, we report on the growth of a high detectivity InGaAs/InGaP QDIP grown on a GaAs substrate using low-pressure metalorganic chemical vapor deposition (LP-MOCVD). The peak photoresponse was around 4.7μm and the peak responsivity had a value of 1.2 A/W at a peak detection bias of -0.9V at 77K. A noise current of 3.3×10-14 A at - 0.9V bias yielded a specific peak detectivity of 1.2×1012cmHz1/2/W at 77K. Peak responsivity and specific peak detectivity of 190.5mA/W and 8.3×1010 cmHz1/2/W were still measured at 120K for a peak detection bias of -0.6V. A BLIP temperature of 200K was determined with a 45° field of view and a 300K background.
Here we report the first demonstrations of infrared focal plane array (FPA) based on GaAs and InP based quantum dot infrared photodetectors (QDIPs). QDIPs are extension of quantum well infrared photodetectors (QWIPs) and are predicted to outperform QWIPs due to their potential advantages including normally incident absorption, higher responsivity and high temperature operation. Two material systems have been studied: InGaAs/InGaP QDIPs on GaAs substrates and InAs QDIP on InP substrates.
An InGaAs/InGaP QDIP has been grown on GaAs substrate by LP-MOCVD. Photoresponse was observed at temperatures up to 200 K with a peak wavelength of 4.7 μm and cutoff wavelength of 5.2 μm. A detectivity of 1.2x1011 cmHz1/2/W was obtained at T=77 K and bias of -0.9 V, which is the highest for QDIPs grown by MOCVD.
An InAs QDIP structure has also been grown on InP substrate by LP-MOCVD. Photoresponse of normal incidence was observed at temperature up to 160K with a peak wavelength of 6.4 μm and cutoff wavelength of 6.6 μm. A detectivity of 1.0x1010 cmHz1/2/W was obtained at 77K at biases of -1.1 V, which is the first and highest detectivity reported for QDIP on InP substrate.
256×256 detector arrays were fabricated first time in the world for both the GaAs and InP based QDIPs. Dry etching and indium bump bonding were used to hybridize the arrays to a Litton readout integrated circuit. For the InGaAs/InGaP QDIP FPA, thermal imaging was achieved at temperatures up to 120 K. At T=77K, the noise equivalent temperature difference (NEDT) was measured as 0.509K with a 300K background and f/2.3 optics. For the InP based QDIPs, thermal imaging was achieved at 77 K.
Inter-subband detectors such as quantum well infrared photodetectors (QWIP) have been widely used in infrared detection. Quantum dot infrared photodetectors (QDIPs) have been predicted to have better performance than QWIPs including higher operation temperature and normal incidence detection. Here we report our recent results of InAs QDIP grown on InP substrate by low-pressure metalorganic chemical vapor deposition (MOCVD). The device structures consist of multiple stacks of InAs quantum dots with InP barriers. High detectivities in the range of 1010cmHz1/2/W were obtained at 77K. The measurements at higher temperatures show better temperature dependent performance than QWIP. However, the performances of QDIPs are still far from the expected. One of the reasons is the low quantum efficiency due to the low fill factor of quantum dots layer. Resonant cavity enhanced QDIP has been studied to increase the quantum efficiency. Different schemes of mirrors using free carrier plasma and distributed Bragg reflector are discussed.
We report an InGaAs/InGaP/GaAs quantum dot infrared photodetector grown by metalorganic chemical vapor deposition with detectivity of 1.3x1011 cmHz1/2/W at 77K and 1.2x1010 cmHz1/2/W at 120K. Modeling of the Quantum dot energy levels showed us that increased photoresponse could be obtained by doping the quantum dots to 4 electrons per dot instead of the usual 2 electrons per dot. This happens because the primary photocurrent transition is from the first excited state to a higher excited state. Increasing the quantum doping in our device yielded significant responsivity improvement and much higher detectivity as a result. This paper discusses the performance of this higher doping device and compares it to our previously reported device with lower doping.
Inter-subband detectors such as quantum well infrared photodetectors (QWIP) have been widely used in infrared remote sensing. Quantum dot infrared photodetectors (QDIPS) have been predicted to have better performanc than QWIPs due to the novel properties of quantum dots caused by the extra confinement. Here we report our recent results of InAs QDIP grown on InP substrate by low-pressure metalorganic chemical vapor deposition. The device structure consists of multiple stacks of InAs quantum dots with GaAs/AlInAs/InP barrier. 400μx400μm test mesas were fabricated for device characterizations. Photoresponse was observed with a peak wavelength of 6.4 μm and a cutoff wavelength of 6.6 μm at both 77K and 100K. A detectivity of 1.0x1010 cmHz1/2/W was obtained at 77K at a bias of -1.1. V. To the best of our knowledge, this is the highest detectivity reported for InAs QDIP grown on InP substrate. At 100K, the detectivity only drops to 2.3x109cmHz1/2/W.
InGaAs/InGaP quantum-dots have been grown by low-pressure metalorganic chemical vapor deposition technique on GaAs substrate. The important growth parameters, such as growth temperature, V/III ratio, etc, have been optimized. A 10-stack quantum-dot infrared photodetector based on these InGaAs dots showed a detectivity of 3.6x1010 cmHz1/2/W at 95K. The peak photoresponse was 4.7 μm with a cutoff at 5.2μm. A 256x256 middle-wavelength infrared focal plane array based on our quantum-dot detectors was fabricated via dry etching technique. The detector array was bonded to a silicon readout integrated circuit via flip chip bonding with indium bumps. A noise equivalent temperature difference of 509 mK was achieved for this array at 120K. With the goal of improving array uniformity, exploratory work into nanopillar structure IR detectors was also performed. Experimental methods and characterization results are presented here.
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