Embedded InAs Quantum Dots Research Articles

Emission and elastic strain in InGaAs/GaAs quantum wells with embedded InAs quantum dots

AbstractPhotoluminescence (PL) spectra have been studied in the symmetric GaAs/In0.15Ga0.85As/GaAs quantum wells (QWs) with embedded InAs quantum dots (QDs), grown at different temperatures from the range 470‐535 °C. The increase of QD growth temperature is accompanied by decreasing the QD surface density and the enlargement of QD lateral diameters. Simultaneously the variation of the PL intensity and PL peak positions none monotonously have been detected. To understand the reason of the variation of PL intensity and peak positions the PL temperature dependences and the X‐ray diffraction (XRD) at low angles (1.75‐1.92°) have been studied. The fitting procedure is applied to analysis the temperature shift of PL peak positions. Fitting has been done on the base of empirical expression for the band gap shrinkage that uses the Einstein temperature parameter. The character non monotonous for the Ga/In inter diffusion versus QD growth temperatures has been revealed. The XRD study has detected the high intensity peaks that corresponds to the diffraction of X‐ray beam from the (311) crystal planes in GaAs QWs. The position of XRD peaks in the structures with QD grown at 490‐510 °C is very close to the angles related to the diffraction from (311) planes in the bulk GaAs. In QD structures with QD grown at 470 and 525‐535 °C the (311) XRD peaks shift to the higher diffraction angles that testifies on the essential compressive strains in these structures. The reason of the variation non monotonously of elastic strain versus QD densities has been discussed (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Linearly Polarized, Single-Mode Spontaneous Emission in a Photonic Nanowire

We introduce dielectric elliptical photonic nanowires to funnel efficiently the spontaneous emission of an embedded emitter into a single optical mode. Inside a wire with a moderate lateral aspect ratio, the electromagnetic environment is largely dominated by a single guided mode, with a linear polarization oriented along the ellipse major axis. The resulting monomode spontaneous emission is maintained over a broad wavelength range, a key asset of this 1D photonic structure. Our theoretical analysis is completed by an experimental study of GaAs elliptical photonic wires with embedded InAs quantum dots. In particular, the fraction of collected photons with the desired linear polarization can exceed 95%.

Comparison of Quantum Dots-in-a-Double-Well and Quantum Dots-in-a-Well Focal Plane Arrays in the Long-Wave Infrared

Our previous research has reported on the development of the first generation of quantum dots-in-a-well (DWELL) focal plane arrays (FPAs), which are based on InAs quantum dots (QDs) embedded in an InGaAs well having GaAs barriers, which have demonstrated spectral tunability via an externally applied bias voltage. More recently, technologies in DWELL devices have been further advanced by embedding InAs QDs in InGaAs and GaAs double wells with AlGaAs barriers, leading to a less strained InAs/InGaAs/GaAs/AlGaAs heterostructure. These lower strain quantum dots-in-a-double-well devices exhibit lower dark current than the previous generation DWELL devices while still demonstrating spectral tunability. This paper compares two different configurations of double DWELL (DDWELL) FPAs to a previous generation DWELL detector and to a commercially available quantum well infrared photodetector (QWIP). All four devices are 320 × 256 pixel FPAs that have been fabricated and hybridized with an Indigo 9705 read-out integrated circuit. Radiometric characterization, average array responsivity, array uniformity and measured noise equivalent temperature difference for all four devices is computed and compared at 60 K. Overall, the DDWELL devices had lower noise equivalent temperature difference and higher uniformity than the first-generation DWELL devices, although the commercially available QWIP has demonstrated the best performance.

Photonic crystal slot nanobeam slow light waveguides for refractive index sensing

We present the design, fabrication, and photoluminescence experiment of InGaAsP photonic crystal slot nanobeam slow light waveguides with embedded InAs quantum dots. The strong confinement of electric field in the slot region is confirmed by the measured record high sensitivity of 7×102 nm per refractive index unit (RIU) to the refractive index change of the environment. A cavity, formed by locally deflecting the two beams toward each other, gives an even higher sensitivity of about 9×102 nm/RIU.

Reflectance measurement of two-dimensional photonic crystal nanocavities with embedded quantum dots

The spectra of two-dimensional photonic crystal slab nanocavities with embedded InAs quantum dots are measured by photoluminescence and reflectance. In comparing the spectra taken by these two different methods, consistency with the nanocavities' resonant wavelengths is found. Furthermore, it is shown that the reflectance method can measure both active and passive cavities. Q-factors of nanocavities, whose resonant wavelengths range from 1280 to 1620 nm, are measured by the reflectance method in cross polarization. Experimentally, Q-factors decrease for longer wavelengths and the intensity, reflected by the nanocavities on resonance, becomes minimal around 1370 nm. The trend of the Q-factors is explained by the change of the slab thickness relative to the resonant wavelength, showing a good agreement between theory and experiment. The trend of reflected intensity by the nanocavities on resonance can be understood as effects that originate from the PC slab and the underlying air cladding thickness. In addition to three dimensional finite-difference time-domain calculations, an analytical model is introduced that is able to reproduce the wavelength dependence of the reflected intensity observed in the experiment.

Controlling mode degeneracy in a photonic crystal nanocavity with infiltrated liquid crystal

We demonstrate the control of the mode degeneracy when a liquid crystal (LC) is infiltrated into an InGaAsP membrane photonic crystal nanocavity with embedded InAs quantum dots. Mode splitting exists in the anisotropic nematic LC state, and not in the unfilled or isotropic LC state. The degeneracy lifting of the quadrupole mode is attributed to the different interactions of the two orthogonal basis modes of the degenerate mode with the two components of the refractive index of the LC. The interpretation is supported by the quantitative agreement between the experimental results and the three-dimensional finite-difference time-domain computations.

Some aspects of emission variation in InAs quantum dots coupled with symmetric quantum wells

The photoluminescence (PL), its temperature and power dependences, as well as PL inhomogeneity and x ray diffraction (XRD) has been studied in the symmetric In0.15Ga1−0.15As/GaAs quantum wells with embedded InAs quantum dots (QDs) (dot-in-a-well, DWELL) with different QD densities, obtained by the variation in QD growth temperatures. It is shown that four reasons are responsible for the difference in emission intensities, PL peak positions and PL inhomogeneity in studied QD structures: (i) the high concentration of nonradiative (NR) recombination centers in the capping In0.15Ga1−0.15As layer at low QD growth temperatures (470 °C), (ii) the QD density and size distributions for the structures with QD grown at 490–535 °C, (iii) the high concentration of NR recombination centers in the GaAs barrier at high QD growth temperatures (535 °C), and (iv) the variation nonmonotonous of elastic strain versus QD density. XRD study confirms that with decreasing density of QDs in DWELLs from 1.1×1011 cm−2 down to 1.3×1010 cm−2 at the rise of QD growth temperatures the level of compressive strain in DWELLs varies nonmonotonously. The reasons of compressive strain variation and the impact of this variation on emission parameters of DWELLs have been discussed as well.

Nanobeam photonic crystal cavity quantum dot laser

The lasing behavior of one dimensional GaAs nanobeam cavities with embedded InAs quantum dots is studied at room temperature. Lasing is observed throughout the quantum dot PL spectrum, and the wavelength dependence of the threshold is calculated. We study the cavity lasers under both 780 nm and 980 nm pump, finding thresholds as low as 0.3 microW and 19 microW for the two pump wavelengths, respectively. Finally, the nanobeam cavity laser wavelengths are tuned by up to 7 nm by employing a fiber taper in near proximity to the cavities. The fiber taper is used both to efficiently pump the cavity and collect the cavity emission.

Tuning quantum-dot based photonic devices with liquid crystals

Microdisks made from GaAs with embedded InAs quantum dots are immersed in the liquid crystal 4-cyano-4'-pentylbiphenyl (5CB). The quantum dots serve as emitters feeding the optical modes of the photonic cavity. By changing temperature, the liquid crystal undergoes a phase transition from the isotropic to the nematic state, which can be used as an effective tuning mechanism of the photonic modes of the cavity. In the nematic state, the uniaxial electrical anisotropy of the liquid crystal molecules can be exploited for orienting the material in an electric field, thus externally controlling the birefringence of the material. Using this effect, an electric field induced tuning of the modes is achieved. Numerical simulations using the finite-differences time-domain (FDTD) technique employing an anisotropic dielectric medium allow to understand the alignment of the liquid crystal molecules on the surface of the microdisk resonator.

Sensitivities of InGaAsP photonic crystal membrane nanocavities to hole refractive index

The sensitivities of resonant wavelengths of photonic crystal (PhC) membrane nanocavities with embedded InAs quantum dots to the ambient refractive index are reported for use in (bio) chemical sensing. The resonances for the different modes of several point-defect type cavities are obtained by photoluminescence measurements. Systematic trends of the variation of sensitivity with increase of the overlap of the modes with the PhC holes are observed for varying cavity type as well as for a given mode within a cavity type. A maximum sensitivity of approximately 300 nm/RIU (refractive index unit) is observed, corresponding to approximately 25% mode overlap with the holes and complete infiltration with the aqueous solution.

Whispering gallery mode lasing in high quality GaAs/AlAs pillar microcavities

We report whispering gallery mode (WGM) lasing from high quality GaAs/AlAs micropillars with embedded InAs quantum dots, under continuous optical pumping. For temperatures ranging from 5 to 100 K, simultaneous lasing from TE1,1,m WGMs is observed for pillar diameters in the 3–4 μm range. Spectral linewidths and energy shifts of the lasing modes are analyzed as a function of the pump power. Thanks to the efficient heat sinking provided by the micropillar geometry, a clear line narrowing is observed above threshold. Moreover, the lasing mode energy remains stable for pump power as large as six times the lasing threshold.

Demonstration of a bias tunable quantum dots-in-a-well focal plane array

Infrared detectors based on quantum wells and quantum dots have attracted a lot of attention in the past few years. Our previous research has reported on the development of the first generation of quantum dots-in-a-well (DWELL) focal plane arrays, which are based on InAs quantum dots embedded in an InGaAs well having GaAs barriers. This focal plane array has successfully generated a two-color imagery in the mid-wave infrared (i.e. 3–5 μm) and the long-wave infrared (i.e. 8–12 μm) at a fixed bias voltage. Recently, the DWELL device has been further modified by embedding InAs quantum dots in InGaAs and GaAs double wells with AlGaAs barriers, leading to a less strained InAs/InGaAs/GaAs/AlGaAs heterostructure. This is expected to improve the operating temperature while maintaining a low dark current level. This paper examines 320 × 256 double DWELL based focal plane arrays that have been fabricated and hybridized with an Indigo 9705 read-out integrated circuit using Indium-bump (flip-chip) technology. The spectral tunability is quantified by examining images and determining the transmittance ratio (equivalent to the photocurrent ratio) between mid-wave and long-way infrared filter targets. Calculations were performed for a bias range from 0.3 to 1.0 V. The results demonstrate that the mid-wave transmittance dominates at these low bias voltages, and the transmittance ratio continuously varies over different applied biases. Additionally, radiometric characterization, including array uniformity and measured noise equivalent temperature difference for the double DWELL devices is computed and compared to the same results from the original first generation DWELL. Finally, higher temperature operation is explored. Overall, the double DWELL devices had lower noise equivalent temperature difference and higher uniformity, and worked at higher temperature (70 K and 80 K) than the first generation DWELL device.

Lithographic and optical tuning of InGaAsP membrane photonic crystal nanocavities with embedded InAs quantum dots

Hexagonal symmetry InGaAsP membrane type cavities with embedded InAs quantum dots as active emitters were investigated by room temperature photoluminescence experiments at wavelengths near 1.50 µm. Cavities consisting of simple defects of just removing one or seven air holes were studied as well as modified cavities with additional holes decreased in size and shifted in position. The latter include the H0 cavity, in which only two adjacent holes were modified, but none removed. Low-Q cavity modes were observed for the simple cavities while high-Q modes were observed after modification of the surrounding holes. The resonant frequencies were varied over a large range of lithographic parameters both by changing the lattice spacing or the size of the modified holes. More than 15 nm reversible dynamic optical tuning of the resonance modes was observed by changing the applied laser power up to 5 mW. For thermo-optic tuning, this corresponds to a heating of up to 200 °C.

Rapid thermal annealing temperature dependence of noise properties in Au/n‐GaAs Schottky diodes with embedded InAs quantum dots in asymmetric In0.2Ga0.8As wells

AbstractThe effect of rapid thermal annealing temperature on the trap properties of Au/n‐GaAs Schottky diodes with embedded InAs quantum dots in asymmetric In0.2Ga0.8As wells have been investigated by capacitance–voltage (C –V) and low frequency noise (LFN) measurements in both reverse and forward bias regimes. The current noise spectra show 1/f behaviour and generation–recombination (g–r) noise, attributed to uniformly distributed traps in energy and to a discrete trap level in the energy band‐gap of the GaAs capping layer, respectively. The experimental results show that the annealing temperature is closely related with the level of these noise sources. The apparent doping concentrations, calculated from the C –V characteristics, indicate that the density of trapping states near the buffer layer interface is increased as the annealing temperature increases. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ideality factor dependence of capacitance and reverse current noise in Au/n‐GaAs Schottky diodes with embedded self‐assembled InAs quantum dots

AbstractThe trap properties of Au/n‐GaAs Schottky diodes with embedded InAs quantum dots (QDs) and different ideality factors were studied by capacitance‐voltage (C‐V) and low‐frequency noise (LFN) measurements in the reverse bias regime. The reverse current noise spectra show 1/f behaviour and g‐r noise, attributed to uniformly distributed traps in energy or to a discrete trap level in the energy band‐gap of the GaAs capping layer, respectively. The Schottky contact performance or characteristics is closely related with the level of these noise sources. The C‐V characteristics indicate the existence of traps with Gaussian energy distribution in the GaAs capping layer and in the InAs QDs layer. From analysis of the C‐V characteristics, the density and the activation energy of these trap distributions are determined. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electron dephasing scattering rate in two-dimensional GaAs/InGaAs heterostructures with embedded InAs quantum dots

We report a comprehensive study of weak-localization and electron-electron interaction effects in a GaAs/InGaAs two-dimensional electron system with nearby InAs quantum dots, using measurements of the electrical conductivity with and without magnetic field. Although both the effects introduce temperature dependent corrections to the zero magnetic field conductivity at low temperatures, the magnetic field dependence of conductivity is dominated by the weak-localization correction. We observed that the electron dephasing scattering rate τφ−1, obtained from the magnetoconductivity data, is enhanced by introducing quantum dots in the structure, as expected, and obeys a linear dependence on the temperature and elastic mean free path, which is against the Fermi-liquid model.

Energy Spectrum of InAs Quantum Dots in GaAs/AlAs Superlattices

Photoand contactless electroreflectance spectroscopies were applied to study optical properties and electronic structure of GaAs/AlAs superlattice systems with embedded InAs quantum dots. The observed interband transitions related to the quantum dot ground and excited states, as well as optical transitions in the combined system formed by the InAs wetting layer and GaAs/AlAs superlattice are discussed.

Quantum Dot Spectral Tuning of Multijunction III-V Solar Cells

AbstractImproving the production of photocurrent in the middle junction of a InGaP/Ga(In)As/Ge triple-junction solar cells (TJSC) can improve the overall conversion efficiency of cell. One possible method to improve the middle junction photocurrent is inserting a quantum dot (QD) superlattice (SL) into the stack. It has been predicted that QD-SL enhanced TJSCs have an efficiency ceiling of 47% under a one-sun AM0 illumination spectrum. Additionally, QD array enhanced GaAs cells have the added benefit of possible intermediate band effects, anisotropic absorption and enhanced radiation tolerance. Embedding InAs quantum dots (QDs) in a single junction GaAs solar cell can increase sub-GaAs bandgap photocurrent generation. This method has been shown to improve the short circuit current density (Jsc) of single junction cells under simulated 1 sun air mass zero (AM0) illumination. However, the increase in strain due to the InAs QD self-assembly may cause defects that reduce the minority carrier lifetime resulting in losses in the cell open circuit voltage (Voc) on the order of 300-500 mV. The introduction of strain compensation (SC) layers into the superlattice (SL) structure of a QD solar cell has previously been shown to improve the device performance, including the partial recovery of Voc. Strain compensation can be used effectively to balance the residual strain, impede dislocation formation, and improve the solar cell characteristics. The effect of GaP strain compensation on the solar cell electrical and material properties was investigated. High resolution X-ray diffraction (HRXRD) scans along the symmetric (004) Bragg peak show weak intensity and wide FWHM at the zero order SL peak (SL0) for non-SC samples. Optimum SC thickness was theoretically determined using a zero in plane stress method and experimentally verified using HRXRD. Optimal strain compensation was then used to increase the QD SL stacking from 5x to 10x and 20x. Use of SC resulted in shifting of the SL0 peak toward the substrate peak as well as reduced FWHM and improved SL peak definition. Kinematical diffraction modeling of the QD structures using numerical simulation indicated this peak shift resulted from reduced overall strain in the SL stack up to 5ML of SC. The material quality improvement in the SC QD solar cells was manifested in an improved spectral response and Jsc. The optoelectronic results for GaAs solar cells with QD SL’s demonstrate a strong dependence on GaP SC layer thickness. In addition, comparison of multi junction (MJ) solar cells which incorporate the SC QD SL’s demonstrate the utility of additional sub-GaAs bandgap current contribution as a tool for additional current-matching optimization in MJ solar cells.

Current–voltage and noise characteristics of reverse-biased Au/n-GaAs Schottky diodes with embedded InAs quantum dots

Schottky contacts on n-type GaAs with embedded InAs quantum dots (QDs) were studied by current–voltage (I–V) and low-frequency noise measurements. For comparison, diodes not containing QDs were investigated as reference devices. A wide distribution of the ideality factor was observed, correlated with the level of the leakage current. Reverse I–V characteristics on the logarithmic scale indicate that the space-charge limited current dominates the carrier transport in these diodes. In all diodes, the reverse current noise spectra show 1/f behaviour, attributed to traps uniformly distributed in energy within the band-gap of the GaAs capping layer. Depth profiling measurements of the 1/f noise power spectral density demonstrate the impact of the QDs on these traps. In diodes containing QDs, in addition to the 1/f noise, a generation–recombination noise is found originating from a deep trap level localized in the vicinity of the QD plane.

Drain-Current Deep Level Transient Spectroscopy Study of Carrier Emission Process from InAs Quantum Dots in GaAs Narrow-Wire Field Effect Transistors

We studied the states working in the memory operation of GaAs narrow-wire field-effect transistors (FETs) with embedded InAs quantum dots (QDs). The GaAs narrow-wire FETs with a few InAs QDs above the channel were fabricated by selective-area metal organic vapor-phase epitaxy (SA-MOVPE). The drain current, measured by sweeping the gate voltage forward and backward, exhibited clear clockwise hysteresis due to the charging of electrons into the states induced by InAs QDs with a threshold voltage shift (ΔVth) of 30 mV at 20 K. To better understand the mechanism of this memory operation, we studied the traps concerning the InAs QDs by drain-current deep-level transient spectroscopy (DLTS). Peaks representing three kinds of electron traps concerning InAs QDs were observed in the DLTS spectra. These peaks exhibited different dependences on the applied gate pulsed voltage during the DLTS measurement. In comparison with the temperature dependence of ΔVth, we found that our memory operation was attributed to one localized state introduced by InAs QDs.