Submonolayer Quantum Dots Research Articles

Development of quantum well, quantum dot, and type II superlattice infrared photodetectors

We present an overview of III-V semiconductor-based infrared detector and focal plane array development at the NASA Jet Propulsion Laboratory in recent years. Topics discussed include: (1) the development of long-wavelength quantum well infrared photodetector for imaging spectrometer applications, (2) the concept and realization of the submonolayer quantum dot infrared photodetector (SML-QDIP) as an alternative to the standard QDIP-based on Stranski-Krastanov (SK) quantum dots, (3) the mid-wavelength infrared quantum dot barrier infrared detector with extended cutoff wavelength, and (4) high-performance type-II superlattice long-wavelength infrared detectors based on the complementary barrier infrared detector architecture.

Effect of barrier on the performance of sub-monolayer quantum dot infrared photodetectors

We report on the effect of confinement barriers on the performance of InAs/InGaAs sub-monolayer quantum dot infrared photodetectors. Two samples with different AlxGa1-xAs barrier compositions (x = 0.07 for sample A and x = 0.20 for sample B) were grown with four-stacks of sub-monolayer quantum dot. Sample A had a peak response at ~7.8 μm, whereas sample B demonstrated three peaks at ~3.5, ~5, and ~7.0 μm with the intensity of the peaks strongly dependent on the applied bias. At 77 K, sample A and B had a detectivity of 1.2 × 1011 cm.Hz1/2/W (Vb = −0.4 V bias) and 5.4 × 1011 cm.Hz1/2/W (Vb = −1.5 V bias), respectively.

Tuning Between Quantum-Dot- and Quantum-Well-Like Behaviors in Type II ZnTe Submonolayer Quantum Dots by Controlling Tellurium Flux During MBE Growth

We report tuning of properties of type II nanostructures between quantum dot (QD)-like and quantum well (QW)-like behaviors in ZnSe layers with ZnTe submonolayer insertions, grown by migration-enhanced epitaxy. The sizes of QDs are estimated from magneto-photoluminescence (PL) measurements, which showed no significant change in the QD lateral size with increasing Te flux, indicating increase in QD density instead. The area density of QDs is estimated from the results of secondary-ion mass spectrometry measurements. It is determined that, in the sample grown using the highest Te flux, the electronic wavefunctions begin to overlap, leading to QW-like behavior before the formation of a full QW layer. This is also confirmed via temperature-dependent time-resolved PL, which showed significant change of excitonic lifetimes and binding energies of type II excitons.

Comprehensive study on molecular beam epitaxy-grown InAs sub-monolayer quantum dots with different capping combinations

Here the authors report a comprehensive study on InAs sub-monolayer quantum dots with different capping layers. After performing systematic optimization of InAs deposition and GaAs thickness, they grew three samples, namely A, B and C, using solid-state molecular beam epitaxy with identical architecture but different capping materials (2 nm of GaAs, InGaAs-GaAs, and InAlGaAs-GaAs, respectively). Photoluminescence emission peaks due to the ground state transition from the dots were observed at 898, 917, and 867 nm for samples A, B, and C, respectively. Narrow full-width half-maxima (19–32 meV) of the emission peaks indicates high uniformity of dot size distribution. Using the conventional Arrhenius plot, the authors calculated the thermal activation energies from temperature-dependent photoluminescence experiment for samples A, B, and C as 49, 112, and 109 meV, respectively. To complete the study, single-pixel photodetectors were fabricated from samples A, B, and C and temperature-dependent dark current variation with applied bias voltage was measured. Dark current was calculated to be in the range of 10−5–10−4 A/cm2 at a 0.5 V applied bias at 77 K. The activation energies calculated from temperature-dependent dark current measurement for samples A, B, and C were 75, 160, and 155 meV, respectively, and followed the trend observed in temperature-dependent photoluminescence measurements.

Optimization of growth conditions of type-II Zn(Cd)Te/ZnCdSe submonolayer quantum dot superlattices for intermediate band solar cells

Intermediate band solar cells (IBSCs) have been predicted to be significantly more efficient than the conventional solar cells, but have not been realized to their full potential due to the difficulties related to the fabrication of practical devices. The authors report here on growth and characterization of Zn(Cd)Te/ZnCdSe submonolayer quantum dot (QD) superlattices (SLs), grown by migration enhanced epitaxy. These QDs do not exhibit formation of wetting layers, which is one of the culprits for the unsatisfactory performance of IBSCs. The ZnCdSe host bandgap is ∼2.1 eV when lattice matched to InP, while the Zn(Cd)Te-ZnCdSe valence band offset is ∼0.8 eV. These parameters make this material system an excellent candidate for a practical IBSC. The detailed structural analysis demonstrates that the process of desorption of Cd and the preferential incorporation of Zn facilitates the formation of unintentional strained ZnSe-rich layer at the QD-spacer interface. The growth conditions have been then optimized so as to obtain high crystalline quality lattice matched SL, by growing intentionally Cd-rich spacers, which strain balanced the SL. The excitation intensity dependent photoluminescence confirmed the type-II nature of these multilayer QD structures, which is expected to suppress nonradiative Auger recombination, and improve the carrier extraction process when implemented in an actual device.

Multi-stack InAs/InGaAs sub-monolayer quantum dots infrared photodetectors

We report on the design and performance of multi-stack InAs/InGaAs sub-monolayer (SML) quantum dots (QD) based infrared photodetectors (SML-QDIP). SML-QDIPs are grown with the number of stacks varied from 2 to 6. From detailed radiometric characterization, it is determined that the sample with 4 SML stacks has the best performance. The s-to-p (s/p) polarized spectral response ratio of this device is measured to be 21.7%, which is significantly higher than conventional Stranski-Krastanov quantum dots (∼13%) and quantum wells (∼2.8%). This result makes the SML-QDIP an attractive candidate in applications that require normal incidence.

Cavity-Volume Scaling Law of Quantum-Dot Metal-Cavity Surface-Emitting Microlasers

Quantum-dot (QD) metal-cavity surface-emitting microlasers are designed, fabricated, and characterized for various sizes of cavity volume for both lateral and vertical confinements. Microlasers using submonolayer QDs in the active region are fabricated according to our design model optimized for a resonant metal cavity. The cavity-volume scaling law is studied by our theoretical modeling and experimental demonstration. The smallest laser has a diameter of 1 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu\hbox{m}$</tex></formula> with silver metal cladding operating at room temperature with electrical injection in pulsed mode. Our experimental results show significant self-heating effect in the smaller devices with a diameter of a few micrometers due to high series resistance and high threshold gain. With the use of hybrid metal-DBR mirrors, the number of DBR pairs in the top hybrid mirror can be reduced from 19.5 to 5.5 without sacrificing threshold current density.

Temperature-Dependent Characteristics of Single-Mode InAs Submonolayer Quantum-Dot Lasers

Temperature-dependent lasing characteristics of transverse single-mode GaAs-based InAs submonolayer (SML) quantum-dot (QD) lasers are investigated. The SML QD lasers, when operated under continuous wave (cw) conditions up to 65 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{\circ}{\rm C}$</tex> </formula> , exhibit a high characteristic temperature <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$(T_{0})$</tex></formula> of around 101 K. The temperature-dependent net modal gain is also studied using the Hakki–Paoli method, yielding a narrow gain spectrum width of 15.6 meV at 20 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{\circ}{\rm C}$</tex></formula> where the peak gain is 9.6 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm cm}^{-1}$</tex></formula> . The lasers exhibit a low loss of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${-}{6}$</tex></formula> to <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${-}{\rm 9}~{\rm cm}^{-1}$</tex></formula> in the temperature range of 15 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{\circ}{\rm C}$</tex> </formula> to 65 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{\circ}{\rm C}$</tex></formula> .

Sub-monolayer quantum dots in confinement enhanced dots-in-a-well heterostructure

We have investigated optical properties and device performance of sub-monolayer quantum dots infrared photodetector with confinement enhancing (CE) barrier and compared with conventional Stranski-Krastanov quantum dots with a similar design. This quantum dots-in-a-well structure with CE barrier enables higher quantum confinement and increased absorption efficiency due to stronger overlap of wavefunctions between the ground state and the excited state. Normal incidence photoresponse peak is obtained at 7.5 μm with a detectivity of 1.2 × 1011 cm Hz1/2 W−1 and responsivity of 0.5 A/W (77 K, 0.4 V, f/2 optics). Using photoluminescence and spectral response measurements, the bandstructure of the samples were deduced semi-empirically.

Evolutional feature of InAs/GaAs quantum dots with coverages from submonolayer to a few monolayers

In this study, we demonstrate InAs/GaAs submonolayer quantum dots (SQDs) with no wetting layer (WL) formed by using the self-assembling technique. An evolutional feature was observed in the photoluminescence spectra taken from QDs whose coverages were in the range from submonolayer to a few monolayer (0.5–2.5 ML). The QD samples revealed that a small number of conventional QDs (CQDs) with a WL coexisted with the majority of SQDs with no WL. Multipeak spectra were gradually changed to single-peak profiles, and an abrupt transition from SQDs to CQDs arose before and after WL formation. A single-mode SQD ensemble was achieved by 1.5 ML, for which the coverage was close to that of the WL (1.7 ML) in which the strain field was nearly uniform over the entire layer. This showed that the sublevel of SQDs (1.32 ± 0.1 eV) was much closer to the GaAs band edge than that of CQDs (∼1.2 eV).

InAs/GaAs submonolayer quantum dot superluminescent diode emitting around 970 nm

According to the InAs/GaAs submonolayer quantum dot active region, we demonstrate a bent-waveguide superluminescent diode emitting at a wavelength of around 970 nm. At a pulsed injection current of 0.5 A, the device exhibits an output power of 24 mW and an emission spectrum centred at 971 nm with a full width at half maximum of 16 nm.

Role of magnesium in band gap engineering of sub-monolayer type-II ZnTe quantum dots embedded in ZnSe

Modification of the bandgap of sub-monolayer type-II ZnTe quantum dots (QDs), by means of direct incorporation of magnesium in the QDs, is reported. Nitrogen co-doped QDs embedded in a ZnSe matrix have been grown by a migration-enhanced molecular beam epitaxy technique. Incorporation of Mg in the ZnTe QDs decreases the valence band discontinuity, leading to reduced localization of the holes, which results in a higher electrical conductivity in the samples as deduced from the Hall effect measurements. The type-II alignment of the bands in the QDs is supported by intensity dependent and time-resolved photoluminescence measurements. Hall effect measurements indicate that the material has p-type conductivity with mid-1015 carriers/cm3 and hole mobilities in the 5–50 cm2/V·s range.

Effect of inhomogeneous broadening of submonolayer quantum dots on the generation of vertical-cavity laser with saturable absorber

The experimental dependences for a laser based on submonolayer quantum dots (QDs) without an absorber were used to calculate the energy characteristics of a semiconductor laser based on QDs and quantum wells with a saturable absorber under bistable lasing conditions; the data obtained make it possible to implement a monolithic version of this laser. All of the mechanisms of carrier relaxation and QD filling, which are important for experiment, are taken into account. The effect of inhomogeneous QD broadening over resonant frequency is considered. The laser mode shift that is caused by an increase in the pump current is found to limit the lasing power. It is shown that the choice of active and passive layers with inhomogeneously broadened QDs makes it possible to obtain bistable lasing in the entire range of available pump currents.

Dots, QWISPs, and BIRDs

We report work on several quantum structure based infrared detectors. We describe the concept of the submonolayer quantum dot based infrared photodetectors, report device results, and present imaging results from a megapixel focal plane array. We describe the concept and experimental progress of the quantum well intra-subband photodetector (QWISP), which is closely related to the quantum well infrared photodetector (QWIP), but uses the dopant-assisted intra-subband absorption mechanism in quantum wells for normal-incidence far infrared/terahertz radiation detection. We discuss aspects of superlattice heterostructure based barrier infrared detectors (BIRDs).

Submonolayer quantum dot infrared photodetector

We describe the concept of the submonolayer quantum dot infrared photodetector (SML QDIP) and report experimental device results on long-wavelength infrared detection. An SML QDIP structure was fabricated into megapixel focal plane arrays, which produced clear infrared images up to 80 K. Detectors in the focal plane showed a responsivity peak at 7.8 μm and noise equivalent temperature difference of 33 mK at 70 K.

Room temperature midinfrared electroluminescence from InSb/InAs quantum dot light emitting diodes

Self-assembled InSb submonolayer quantum dots (QDs) in an InAs matrix have been grown by molecular beam epitaxy using Sb2 and As2 fluxes. The structures exhibit bright midinfrared photoluminescence up to room temperature. Intense room temperature electroluminescence with a peak at wavelength near 3.8 μm was observed from p-i-n light emitting diode structures containing ten InSb submonolayer QD sheets inserted within the InAs active region.

Characteristics of InGaAs Submonolayer Quantum-Dot and InAs Quantum-Dot Photonic-Crystal Vertical-Cavity Surface-Emitting Lasers

We have made InGaAs submonolayer (SML) quantum-dot (QD) and InAs QD photonic-crystal vertical-cavity surface-emitting lasers (PhC-VCSELs) for fiber-optic communications in the 990 and 1300 nm ranges, respectively. The active region of the InGaAs SML QD PhC-VCSEL contains three InGaAs SML QD layers, with each of the SML QD layer formed by alternating depositions of InAs and GaAs. The active region of the InAs QD PhC-VCSEL contains 17 undoped InAs-InGaAs QD layers. Both types of QD PhC-VCSELs exhibit single-mode characteristics throughout the current range, with side-mode suppression ratio (SMSR) larger than 35 dB. A maximum output power of 5.7 mW has been achieved for the InGaAs SML QD PhC-VCSEL. The near-field image study of the QD PhC-VCSELs indicates that the laser beam is well confined by the photonic-crystal structure of the device.

Characteristics of single-mode InGaAs sub-monolayer quantum-dot photonic-crystal VCSELs emitting in the 990 nm range

We have made InGaAs sub-monolayer (SML) quantum dot photonic crystal vertical-cavity surface-emitting lasers (QD PhC-VCSELs) for fiber-optic applications. The active region of the device contains three InGaAs SML QD layers. Each of the InGaAs SML QD layer is formed by alternate deposition of InAs (<1 ML) and GaAs. Single fundamental mode CW output power of 3.8 mW at 28 mA has been achieved in the 990 nm range, with a threshold current of 0.9 mA. Side-mode suppression ratio (SMSR) larger than 35 dB has been demonstrated over the entire current operation range. Near-field images of the PhC-VCSELs were also measured and studied.

High-power semiconductor disk laser based on InAs∕GaAs submonolayer quantum dots

An optically pumped semiconductor disk laser using submonolayer quantum dots (SML QDs) as gain medium is demonstrated. High-power operation is achieved with stacked InAs∕GaAs SML QDs grown by metal-organic vapor-phase epitaxy. Each SML-QD layer is formed from tenfold alternate depositions of nominally 0.5 ML InAs and 2.3 ML GaAs. Resonant periodic gain from a 13-fold nonuniform stack design of SML QDs allows to produce 1.4W cw at 1034nm. The disk laser demonstrates the promising potential of SML-QD structures combining properties of QD and quantum-well gain media for high-power applications.

Short exciton radiative lifetime in submonolayer InGaAs∕GaAs quantum dots

The exciton radiative lifetime in submonolayer (SML) InGaAs∕GaAs quantum dots (QDs) grown at 500°C was measured by using time-resolved photoluminescence from 10to260K. The radiative lifetime is around 90ps and is independent of temperature below 50K. The observed short radiative lifetime is a key reason for the high performance of SML QD devices and can be explained by the theory of Andreani et al. [Phys. Rev. B 60, 13276 (1999)] calculating the radiative lifetime of QDs formed at the interface fluctuations of a quantum well, as the SML QDs are 20–30nm in diameter and embedded within the lateral InGaAs QW.