Dots-in-a-well Research Articles

Quantum Dot Based Infrared Focal Plane Arrays

In the past decade, there has been active research on infrared detectors based on intersubband transitions in self-assembled quantum dots (QDs). In the past two years, at least four research groups have independently demonstrated focal plane arrays based on this technology. In this paper, the progress from the first raster scanned image obtained with a QD detector to the demonstration of a 640 512 imager based on self-assembled QDs is reviewed. In particular, emphasis will be placed on a novel quantum dots-in-a-well (DWELL) design, which represents a hybrid between a conventional quantum-well infrared photodetector (QWIP) and a quantum-dot infrared photodetector (QDIP). In the DWELL detectors, the active region consists of InAs quantum dots embedded in an InGaAs quantum well. Like QDIPs, the DWELL detectors have 3-D confinement and display normal incidence operation while demonstrating reproducible ldquodial-in recipesrdquo for control over the operating wavelength, like QWIPs. Moreover, the DWELL detectors also have demonstrated bias-tunability and multicolor operation in the midwave infrared (MWIR, 3-5 ), long-wave infrared (LWIR, 8-12 ), and very long wave infrared (VLWIR, ) regimes. Recently midformat 320 256 and 640 512 focal plane arrays (FPAs) with an NETD of 40 mK at have been reported. The paper will conclude with a perspective on the future directions on the research on QDIP FPA including enhanced functionality and higher operating temperatures.

Single bump, two-color quantum dot camera

The authors report a two-color, colocated quantum dot based imaging system used to take multicolor images using a single focal plane array (FPA). The dots-in-a-well (DWELL) detectors consist of an active region composed of InAs quantum dots embedded in In.15Ga.85As quantum wells. DWELL samples were grown using molecular beam epitaxy and fabricated into 320×256 focal plane arrays with indium bumps. The FPA was then hybridized to an Indigo ISC9705 readout circuit and tested. Calibrated blackbody measurements at a device temperature of 77K yield midwave infrared and long wave infrared noise equivalent difference in temperature of ∼55 and 70mK.

640$\,\times\,$512 Pixels Long-Wavelength Infrared (LWIR) Quantum-Dot Infrared Photodetector (QDIP) Imaging Focal Plane Array

Epitaxially grown self-assembled InAs-InGaAs-GaAs quantum dots (QDs) are exploited for the development of large-format long-wavelength infrared focal plane arrays (FPAs). The dot-in-a-well (DWELL) structures were experimentally shown to absorb both 45deg and normal incident light, therefore, a reflection grating structure was used to enhance the quantum efficiency. The devices exhibit peak responsivity out to 8.1 mum, with peak detectivity reaching ~1times1010 Jones at 77 K. The devices were fabricated into the first long-wavelength 640times512 pixel QD infrared photodetector imaging FPA, which has produced excellent infrared imagery with noise equivalent temperature difference of 40 mK at 60-K operating temperature

Thermal activation of excitons in asymmetric InAs dots-in-a-well InxGa1−xAs∕GaAs structures

Photoluminescence, its temperature dependence, and photoluminescence excitation spectra of InAs quantum dots embedded in asymmetric InxGa1−xAs∕GaAs quantum wells [dots in a well (DWELL)] have been investigated as a function of the indium content x (x=0.10–0.25) in the capping InxGa1−xAs layer. The asymmetric DWELL structures were created with the aim to investigate the influence of different barrier values at the quantum dot (QD)/quantum well interface on the photoluminescence thermal quenching process. The set of rate equations for the two stage model for the capture and thermal escape of excitons in QDs are solved to analyze the nature of thermal activation energies for the QD photoluminescence quenching process. The two stage model for exciton thermal activation was confirmed experimentally in the investigated QD structures as well. The localization of nonradiative defects in InAs∕InGaAs DWELL structures is discussed on the base of comparison of theoretical and numerically calculated (fitting) results.

Strain-driven alloy decomposition of In0.15Ga0.85As well layers in InAs/In0.15Ga0.85As dots-in-a-well structure

Two InAs/In0.15Ga0.85As quantum dots-in-a-well (DWELL) samples have been grown by solid source molecular beam epitaxy (MBE). The increased size of InAs dots and more homogeneous dot-size distribution have been found in one InAs DWELL sample with growth optimized by changing both the growth temperature and the thickness in In0.15Ga0.85As well layers. The improved optical properties of this sample have also been confirmed by photoluminescence (PL) and piezomodulated reflectance (PzR) spectra. The numerical calculations based on effective mass approximation indicate that the increase of dot size and the improved optical properties are dominantly due to the strain driven alloy decomposition of In0.15Ga0.85As well layers.

Demonstration of 640 × 512 pixels long-wavelength infrared (LWIR) quantum dot infrared photodetector (QDIP) imaging focal plane array

We have exploited the artificial atom-like properties of epitaxially grown self-assembled quantum dots (QDs) for the development of high operating temperature long wavelength infrared (LWIR) focal plane arrays (FPAs). QD infrared photodetectors (QDIPs) are expected to outperform quantum well infrared detectors (QWIPs) and are expected to offer significant advantages over II–VI material based FPAs. We have used molecular beam epitaxy (MBE) technology to grow multi-layer LWIR dot-in-a-well (DWELL) structures based on the InAs/InGaAs/GaAs material system. This hybrid quantum dot/quantum well device offers additional control in wavelength tuning via control of dot-size and/or quantum well sizes. DWELL QDIPs were also experimentally shown to absorb both 45° and normally incident light. Thus we have employed a reflection grating structure to further enhance the quantum efficiency. The most recent devices exhibit peak responsivity out to 8.1μm. Peak detectivity of the 8.1μm devices has reached ∼1×1010Jones at 77K. Furthermore, we have fabricated the first long-wavelength 640×512pixels QDIP imaging FPA. This QDIP FPA has produced excellent infrared imagery with noise equivalent temperature difference of 40mK at 60K operating temperature.

OPTICAL AND ELECTRICAL INVESTIGATION OF LOW DIMENSIONAL SELF-ASSEMBLEDInAsQUANTUM DOT FIELD EFFECT TRANSISTORS

Self-assembled InAs QD dot-in-a-well (DWELL) structures were grown on GaAs substrate by MBE system, and heterojunction modulation-doped field effect transistor (MODFET) was fabricated. The optical properties of the samples show that the photoluminescence of InAs/GaAs self-assembled quantum dot (SAQD) is at 1.265 μm at 300 K. The temperature-dependence of the abnormal redshift of InAs SAQD wavelength with the increasing temperature was observed, which is closely related with the inhomogeneous size distribution of the InAs quantum dot. According to the electrical measurement, high electric field current–voltage characteristic of the MODFET device were obtained. The embedded InAs QD of the samples can be regard as scattering centers to the vicinity of the channel electrons. The transport property of the electrons in GaAs channel will be modulated by the QD due to the Coulomb interaction. It has been proposed that a MODFET embedded with InAs QDs presents a novel type of field effect photon detector.

Rate Equations for 1.3-<tex>$mu$</tex>m Dots-Under-a-Well and Dots-in-a-Well Self-Assembled InAs–GaAs Quantum-Dot Lasers

A rate-equation model, in which three discrete quantum-dot (QD) energy levels are assumed and all possible relaxation paths and carrier transport in the GaAs barrier are considered, is presented to analyze the steady-state performance of 1.3 mum undoped and doped dots-under-a-well (DUW) and dots-in-a-well (DWELL) InAs-GaAs QD lasers. DWELL QD lasers have higher saturation value of QD level occupation probabilities and characteristic temperature (T0) than that of DUW QD lasers due to the improvement of hole confinement. The p-doped QD laser shows lower threshold current density than n-doped QD laser at the same threshold condition, and the T0 of n-doped DWELL laser is higher than that of p-doped DWELL laser at room temperature. Optimized QD layer number of DUW and DWELL QD lasers with different QD density is discussed

Spectral function of InAs∕InGaAs quantum dots in a well detector using Green’s function

Theoretical modeling of an InAs∕InGaAs quantum dot-in-a well (DWELL) detector is reported. The DWELL structure consists of pyramidal-shaped InAs quantum dots with dimensions of 11nm (base) and 6.5nm (height) placed on the top half of an InGaAs quantum well of 11nm width, which is buried in a GaAs matrix. The Green’s function method is used to calculate the spectral function and the density of states of the DWELL. The kinetic equation that governs Green’s functions is solved numerically using the method of finite differences. From the information obtained from the density of states, the responsivity of the DWELL can be estimated. The calculated energy eigenvalues are compared with the experimentally measured responsivity of a DWELL detector.

Two‐color focal plane arrays based on self assembled quantum dots in a well heterostructure

Novel InAs/InGaAs quantum dots-in-a-well (DWELL) infrared photodetectors are described. These detectors, in which the active region consists of InAs quantum dots embedded in an InGaAs quantum well, represent a hybrid between a conventional quantum well infrared photodetector (QWIP) and a quantum dot infrared photodetector (QDIP). Like QDIPs, the DWELL detectors display normal incidence operation without gratings or optocouplers while demonstrating reproducible “dial-in recipes” for control over the operating wavelength, like QWIPs. We report the first demonstration of a two-color infrared focal plane array based on a voltage-tunable quantum dots-in-well (DWELL) design. The active region consists of multiple layers of InAs quantum dots in an In0.15Ga0.85As quantum well. Spectral response measurements yielded a peak at 5.5 µm for lower biases and at 8–10 µm for higher biases. Using calibrated blackbody measurements, the mid-wavelength and long wavelength specific detectivity (D*) were estimated to be 7.1 × 1010 cmHz1/2 /W (Vb = 1.0 V) and 2.6 × 1010 cmHz1/2 /W (Vb = 2.6 V) at 78 K, respectively. This material was processed into a 320 × 256 array and integrated with an Indigo 9705 readout chip and thermal imaging was achieved at 80 K. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of strain on the characteristics of InGaN–GaN multiple quantum-dot blue light emitting diodes

In this study, Raman spectra were measured in the backscattering geometry at temperatures from 100 K to 298 K. Samples with the InGaN self-assembled quantum dot (SAQD) structures of high strain show a strong compressive stress in InGaN epilayer by Raman measurement. Furthermore, we have applied the dots-in-a-well (DWELL) structure to nitride-based light-emitting diodes (LEDs). It was found that EL peak variation of the LED with DWELL structure is more sensitive to the amount of injection current, as compared with the MQW LEDs.

Quantum structures for multiband photon detection

AbstractThe work describes multiband photon detectors based on semiconductor micro-and nano-structures. The devices considered include quantum dot, homojunction, and heterojunction structures. In the quantum dot structures, transitions are from one state to another, while free carrier absorption and internal photoemission play the dominant role in homo or heterojunction detectors. Quantum dots-in-a-well (DWELL) detectors can tailor the response wavelength by varying the size of the well. A tunnelling quantum dot infrared photodetector (T-QDIP) could operate at room temperature by blocking the dark current except in the case of resonance. Photoexcited carriers are selectively collected from InGaAs quantum dots by resonant tunnelling, while the dark current is blocked by AlGaAs/InGaAs tunnelling barriers placed in the structure. A two-colour infrared detector with photoresponse peaks at ∼6 and ∼17 μm at room temperature will be discussed. A homojunction or heterojunction interfacial workfunction internal photoemission (HIWIP or HEIWIP) infrared detector, formed by a doped emitter layer, and an intrinsic layer acting as the barrier followed by another highly doped contact layer, can detect near infrared (NIR) photons due to interband transitions and mid/far infrared (MIR/FIR) radiation due to intraband transitions. The threshold wavelength of the interband response depends on the band gap of the barrier material, and the MIR/FIR response due to intraband transitions can be tailored by adjusting the band offset between the emitter and the barrier. GaAs/AlGaAs will provide NIR and MIR/FIR dual band response, and with GaN/AlGaN structures the detection capability can be extended into the ultraviolet region. These detectors are useful in numerous applications such as environmental monitoring, medical diagnosis, battlefield-imaging, space astronomy applications, mine detection, and remote-sensing.

Characterization of structure and defects in dot-in-well laser structures

Recent progress in the development of 1.3 μm InAs/InGaAs dots-in-a-well (DWELL) laser structures has led to efficient CW room temperature laser operation with low current thresholds. However, present devices suffer from non-ideal temperature characteristics due to gain saturation, a consequence of the finite dot density, and to carrier escape due to the small energy separation between the quantum dot (QD) ground and first-excited states. In order to improve device performance, we have examined methods to increase the QD quality and density. In these studies, we have examined the effect of using thin InAlAs capping layers and high temperature buffer layers. Both effects are observed to strongly modify the structure of the QDs producing significant improvements in the InAs QDs optical properties at room temperature. Initial attempts at multilayer QD structures showed substantial degradation in optical and electrical properties compared to single layer structures. Analysis by Transmission Electron Microscopy (TEM) has identified the presence of defects arising from the complex interaction of QDs, which propagate through the QD layers into the upper regions of the structure as being the primary cause of the poor electronic device characteristics. The use of high growth temperature spacing layers (HGTSLs) have recently allowed us the fabrication of a defect free five layer-stacked structure with record low threshold current density.

Intersubband optical absorption in quantum dots-in-a-well heterostructures

The theoretical analysis of intersubband optical transitions for InAs∕InGaAs quantum dots-in-a-well (DWELL) detectors are performed in the framework of effective-mass envelope-function theory. In contrast to InAs∕GaAs quantum dot (QD) structures, the calculated band structure of DWELL quantitatively confirms that an additional InGaAs quantum well effectively lowers the ground state of InAs QDs relative to the conduction-band edge of GaAs and enhances the confinement of electrons. By changing the doping level, the dominant optical transition can occur either between the bound states in the dots or from the ground state in the dots to bound states in the well, which corresponds to the far-infrared and long-wave infrared (LWIR) peaks in the absorption spectra, respectively. Our calculated results also show that it is convenient to tailor the operating wavelength in the LWIR atmospheric window (8–12μm) by adjusting the thickness of the InGaAs layer while keeping the size of the quantum dots fixed. Theoretical predictions agree well with the available experimental data.

InGaN/GaN MQD p–n junction photodiodes

The p–n junction photodiodes with InGaN/GaN MQD have been prepared by metal-organic chemical vapor deposition (MOCVD) growth; we achieved nanoscale InGaN self-assembled QDs in the well layers of the active region. The RT PL spectrum peak position for the fabricated InGaN/GaN MQD p–n Junction PDs is located at 464.6 nm and FWHM is 24.2 nm. After finishing device process, it was fond that the turn on voltage in forward bias and the break down voltage in reverse bias are about 3 and −13.5 V, respectively. Furthermore, with 1, 2, and 3 V applied bias, the maximum responsivity of the fabricated MQD p–n junction PD was observed at 350 nm, and the minimum of spectral response was measured at 465 nm. It was also found that the responsivity was nearly a constant from 390 to 440 nm. It seems to suggest that the spectral response in the range of 390–440 nm is due to the effect of the InGaN dots-in a-well active layers.

Optimizing the growth of 1.3-μm InAs/InGaAs dots-in-a-well structure: Achievement of high-performance laser

The structural and optical properties of single- and multi-layer GaAs-based 1.3-μm InAs/InGaAs dots-in-a-well (DWELL) structures have been optimized to improve the performance of quantum dot (QD) lasers. A strong dependence of the InAs QD density and the QD emission wavelength on the In composition of InGaAs well has been demonstrated for single-layer samples. The optimum In composition in the well for 1.3-μm applications is obtained to be 15%. The effects of growth temperature of the spacer layers (SPLs) are investigated using a 5-layer structure. It is found that the growth temperature of GaAs SPLs has a pronounced effect on both the structural and optical properties of the InAs QDs. Dislocations are introduced in the second and subsequent QD layers for samples where the GaAs SPLs are deposited at a low temperature of 510 °C. By incorporating a high-temperature (580 °C) growth step for the GaAs SPLs, dislocation formation is inhibited and a high level of inter-layer dot uniformity is achieved. An extremely low continuous wave room-temperature threshold current density of 39 A/cm 2 is demonstrated for an as-cleaved 5-layer device with emission at 1.306 μm and ground-state operation up to 100 °C.

Quantum dots-in-a-well infrared photodetectors

Novel InAs/InGaAs quantum dots-in-a-well (DWELL) infrared photodetectors are reviewed. These detectors, in which the active region consists of InAs quantum dots (QDs) embedded in an InGaAs quantum well, represent a hybrid between a conventional quantum well infrared photodetector (QWIP) and a QD infrared photodetector (QDIP). Like QDIPs, DWELL detectors display normal incidence operation without gratings or optocouplers while demonstrating reproducible ‘dial-in recipes’ for control over the operating wavelength, like QWIPs. Using femtosecond spectroscopy, long carrier lifetimes have been observed in DWELL heterostructures, suggesting their potential for high temperature operation. Moreover, DWELL detectors have also demonstrated bias-tunability and multicolour operation in the mid-wave infrared (3–5 µm), long-wave infrared (LWIR, 8–12 µm) and very long-wave infrared (>14 µm) regimes. We have recently developed LWIR 320 × 256 focal plane arrays operating at liquid nitrogen temperatures. One of the potential problems with these detectors is the low quantum efficiency, which translates into low responsivity and detectivity. Some solutions for mitigating these problems are suggested at the end of this paper.

Effect of well width on three-color quantum dots-in-a-well infrared detectors

Three-color InAs-InGaAs quantum dots-in-a-well (DWELL) detectors having different well sizes are presented. Three DWELL detectors (1388, 1373, and 1299) with different quantum well (QW) widths (120, 110, and 90/spl Aring/, respectively) have been characterized showing response peaks at three distinct wavelengths. The detector 1388 has peak wavelengths at /spl sim/6.25, /spl sim/10.5, and /spl sim/23.3 /spl mu/m. The two peaks at 6.25 and 10.5 /spl mu/m are believed to be due to bound-to-bound transitions from the ground state in the dot to a state in the well, whereas the longer wavelength peak (/spl sim/23.3 /spl mu/m) which has a detectivity of 7.9/spl times/10/sup 10/ cm/spl middot//spl radic/Hz/W at 4.6 K under -2.2-V bias is due to an intersubband transition between the dot levels. The operating wavelength of these detectors in the short wavelength region can be tailored by changing the width of the QW. Spectral responsivity curves of 1373 and 1299 show a shift of the short wavelength peaks toward decreasing wavelength while the long wavelength peak remains around /spl sim/23.3 /spl mu/m confirming that the particular transition is due to the quantum dot.

Quantum dots-in-a-well infrared photodetectors

Novel InAs/InGaAs quantum dots-in-a-well (DWELL) infrared photodetectors are reported. These detectors, in which the active region consists of InAs quantum dots embedded in an InGaAs well quantum well, represent a hybrid between a conventional quantum well infrared photodetector (QWIP) and a quantum dot infrared photodetector (QDIP). Like QDIPs, the DWELL detectors display normal incidence operation without gratings or optocouplers while demonstrating reproducible “dial-in recipes” for control over the operating wavelength, like QWIPs. Using femtosecond spectroscopy, long carrier lifetimes have been observed in DWELL heterostructures suggesting their potential for high temperature operation. Moreover, the DWELL detectors also have demonstrated bias-tunability and multi-color operation in the mid wave infrared (MWIR, 3–5 μm), long wave infrared (LWIR, 8–12 μm) and very long wave infrared (VLWIR, >14 μm) regimes. We have recently realized LWIR 320 × 256 focal plane arrays (FPAs) operating at liquid nitrogen temperatures. One of the potential problems with these detectors is the low quantum efficiency, which translates into low responsivity and detectivity. Some solutions to mitigate these problems are suggested at the end of the paper.

Optical loss and lasing characteristics of high-quality-factor AlGaAs microdisk resonators with embedded quantum dots

Optical characterization of AlGaAs microdisk resonant cavities with a quantum dot active region is presented. Direct passive measurement of the optical loss within AlGaAs microdisk resonant structures embedded with InAs∕InGaAs dots-in-a-well (DWELL) is performed using an optical-fiber-based probing technique at a wavelength (λ∼1.4μm) that is red detuned from the dot emission wavelength (λ∼1.2μm). Measurements in the 1.4μm wavelength band on microdisks of diameter D=4.5μm show that these structures support modes with cold-cavity quality factors as high as 3.6×105. DWELL-containing microdisks are then studied through optical pumping at room temperature. Pulsed lasing at λ∼1.2μm is seen for cavities containing a single layer of InAs dots, with threshold values of ∼17μW, approaching the estimated material transparency level. Room-temperature continuous-wave operation is also observed.