Quantum Dot Barrier Research Articles

Panning ideal In(Ga)As(P)/InGaP quantum dot structures for intermediate band solar cells

The In(Ga)As(P)/InGaP quantum dot (QD)system has been investigated for QD intermediate band solar cells. In order to obtain optical transition energies compatible with the ideal ones required for the device record performance, namely: 1.95 eV, 1.24 eV and 0.7 eV, first disordered InGaP with a bandgap of 1.91 eV as the barrier material has been obtained. Then InAs QDs nucleated at 490 °C, 1.32 MLs thick, covered by a 3–4 nm GaAs capping layer and In-flushed provided radiation emission in the energy interval between 1.15 eV and 1.35 eV, fully compatible with the ideal 1.24 eV. The 4 nm capped structures have been proven to exhibit a stronger PL emission at 1.24 eV. Nominal InAs QDs suffer a pronounced incorporation of Ga, a consequence of In/Ga intermixing at their capping layer and barrier interfaces. An As/P intermixing also takes place at the quantum dot—barrier interface. The encouraging results herald further improvement of QD intermediate band solar cell’s performance.

Intraband Quantum Dot Barrier Devices - Optimization of Energy Level Alignment

Colloidal quantum dots have been a spotlight of nanoscience and technology due to their exquisite physical properties that can be harnessed for next generation nanoelectronics and optoelectronics. Particularly, semiconductor quantum dots that are active in the thermal infrared provide a new path for fabricating infrared devices with significantly reduced cost and enable capabilities beyond that offered by conventional semiconductor technologies. In this presentation, we report on recent advances in materials design and device structure for silver selenide mid-infrared quantum dots, building on our recent demonstration of uncooled photoconductive devices. We highlight two key approaches to reduce the high dark current associated with self-doped, intraband quantum dots used in these devices with detailed analyses of the device characteristics.

Intraband Quantum Dot Barrier Devices - Optimization of Energy Level Alignment

Mid-infrared barrier detectors constructed from intraband colloidal quantum dots brings the advantage of greatly simplified device fabrication procedure with the potential for high temperature operation. Here, we report the fabrication and characterization of vertically-stacked Ag2Se / PbS / Ag2Se quantum dot barrier devices with improved infrared responsivity. Through optimizing the energy level alignment, 4.5 μm responsivity now reaches 84.1 mA/W which results in 570% improvement in specific detectivity compared to the first generation quantum dot barrier devices.

Light sensitive memristor with bi-directional and wavelength-dependent conductance control

We report the optical control of localized charge on positioned quantum dots in an electro-photo-sensitive memristor. Interband absorption processes in the quantum dot barrier matrix lead to photo-generated electron-hole-pairs that, depending on the applied bias voltage, charge or discharge the quantum dots and hence decrease or increase the conductance. Wavelength-dependent conductance control is observed by illumination with red and infrared light, which leads to charging via interband and discharging via intraband absorption. The presented memristor enables optical conductance control and may thus be considered for sensory applications in artificial neural networks as light-sensitive synapses or optically tunable memories.

Effects of ZnS layer on the performance improvement of the photosensitive ZnO nanowire arrays solar cells

The impact of ZnS layer as an interface modification on the photosensitive ZnO nanowire arrays solar cells was studied. CdS, CdSe and ZnS were deposited on ZnO nanowire arrays by SILAR method. When a ZnS layer was deposited, the quantum dot barrier was indirectly become in contact with the electrolyte, which thus restrained the flow of electrons. The CdS sensitized solar cells has an efficiency of 0.55% with the deposition of the ZnS(3) layer, that is, with a deposition of three times, whereas the CdS/CdSe co-sensitized solar cells has an efficiency of 2.03% with the deposition of the ZnS(1) layer. It was also noted that as the thickness of the of ZnS layer was increased, Voc, Isc and efficiencies of both the solar cells were first increased and then decreased. In addition, the CdS/N719 solar cells has an efficiency of 0.75% with the deposition of the ZnS(2) layer.

Ground state of the holes localized in II-VI quantum dots with Gaussian potential profiles

We report on the theoretical study of the hole states in II-IV quantum dots of a spherical and ellipsoidal shape, described by a smooth potential confinement profiles, that can be modelled by a Gaussian functions in all three dimensions. The universal dependencies of the hole energy, $g$-factor and localization length on a quantum dot barrier height, as well as the ratio of effective masses of the light and heavy holes are presented for the spherical quantum dots. The splitting of the four-fold degenerate ground state into two doublets is derived for anisotropic (oblate or prolate) quantum dots. Variational calculations are combined with numerical ones in the framework of the Luttinger Hamiltonian. Constructed trial functions are optimized by comparison with the numerical results. The effective hole $g$-factor is found to be independent on the quantum dot size and barrier height and is approximated by simple universal expression depending only on the effective mass parameters. The results can be used for interpreting and analyzing experimental spectra measured in various structures with the quantum dots of different semiconductor materials.

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.

Tunable zero-field Kondo splitting in a quantum dot

We present detailed low-temperature transport measurements of a single quantum dot formed in an InGaAs/InP heterostructure with a strong tunnel coupling to the source and drain leads. The conventional spin-1/2 Kondo effect is observed for the quantum dot in the $N=9$ charge state. By changing the voltages applied to the quantum dot barrier gates, we find a zero-field splitting of the Kondo resonance and a zero-bias differential conductance, which shows a nonmonotonic in-plane magnetic field and temperature dependence. Using a two-site Hubbard model, we show that the main observed features can be explained in terms of Kondo correlation effects resulting from the exchange interaction between two localized spins.

Superlattice and Quantum Dot Unipolar Barrier Infrared Detectors

We report device performance and provide theoretical analysis of a modified long-wavelength complementary barrier infrared detector structure that incorporates a double tunnel junction contact designed for simpler material growth while retaining the robustness for processing. We also provide analysis of a mid-wavelength quantum dot barrier infrared detector and explain its spectral shape and turn-on characteristics.

Complementary barrier infrared detector (CBIRD) with double tunnel junction contact and quantum dot barrier infrared detector (QD-BIRD)

The InAs/GaSb type-II superlattice based complementary barrier infrared detector (CBIRD) has already demonstrated very good performance in long-wavelength infrared (LWIR) detection. In this work, we describe results on a modified CBIRD device that incorporates a double tunnel junction contact designed for robust device and focal plane array processing. The new device also exhibited reduced turn-on voltage. We also report results on the quantum dot barrier infrared detector (QD-BIRD). By incorporating self-assembled InSb quantum dots into the InAsSb absorber of the standard nBn detector structure, the QD-BIRD extend the detector cutoff wavelength from ∼4.2μm to 6μm, allowing the coverage of the mid-wavelength infrared (MWIR) transmission window. The device has been observed to show infrared response at 225K.

Recombination dynamics of excitons and exciton complexes in single quantum dots

We have studied the recombination dynamics of excitons and excitonic complexes confined in single GaAs quantum dots, embedded in a type-II GaAs/AlAs bilayer, formed at unintended growth imperfections. The small density of defects leads to the spatial isolation of the quantum dots, allowing to address individual specimens without any further sample processing. Any influence of carrier diffusion on the recombination dynamics is avoided by using quasi-resonant excitation, below the quantum dot barrier. Under these excitation conditions, the recombination occurs within a 2 nanosecond time window since relaxation takes place only inside the quantum dot. At low powers, the photoluminescence spectra are dominated by very sharp lines attributed to the exciton and the bi-exciton/charged-exciton transitions, while at large powers it is possible to observe the emission from higher-order exciton complexes. We have found a retardation of the emission increasing the pump power and interpreted it as an evidence for a sequential decay of the different excitonic species.

Mid-infrared quantum dot barrier photodetectors with extended cutoff wavelengths

A method to extend the cutoff wavelength of mid-infrared barrier photodetectors by incorporating self-assembled InSb quantum dots into the active area of the detector is demonstrated. This approach enables the extension of the cutoff wavelength of barrier photodetectors from 4.2 to 6 m, demonstrating infrared response at temperatures up to 225 K.

Temperature dependence of the photoluminescence of self-assembledInAs∕GaAsquantum dots in pulsed magnetic fields

We have studied the magnetic field $(<50\phantom{\rule{0.3em}{0ex}}\mathrm{T})$ dependence of the photoluminescence (PL) of self-assembled $\mathrm{In}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ quantum dots as a function of temperature $(T)$. As the temperature is raised from 4.2 up to $80\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, thermal redistribution causes the PL to be increasingly dominated by dots with a lower PL energy. Magneto-PL demonstrates that these low-energy dots are larger in size only in the growth direction and not in the plane of the sample. At high temperatures $(T>100\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, a different physical phenomenon emerges: we see an anomalous decrease of the PL shift in magnetic field, which is attributed to field enhancement of the quantum dot barrier potential. This mechanism strongly favors excitons in small dots with a weak PL shift in magnetic field, hence laterally smaller dots increasingly dominate the PL at high temperatures and high fields.

Sensitive detector for a passive terahertz imager

We report progress in developing a sensitive detector for terahertz radiation, based on a semiconductor quantum dot (QD) capacitively coupled to a metallic single electron transistor (SET). A charge polarization of the QD induced by the absorption of individual photons is detected by the voltage-biased SET. We investigate the sensitivity of the detector to broadband radiation, over a range of QD barrier heights, and find that there is a measurable photo-signal over wide range of gate voltages defining the QD. This is an improvement on previous designs of terahertz detector based on the QD/SET principle, and makes the new detector a candidate for use in an imaging device.

A large enhancement of the maximum drift velocity of electrons in the channel of a field-effect heterotransistor

It is shown that the optical-phonon momentum quantization in a GaAs quantum well resulting from the introduction of an InAs quantum-dot barrier layer provides for the elimination of inelastic scattering of electrons by optical phonons and, thus, makes the acceleration of electrons above the saturation drift velocity possible. It is shown experimentally that the maximum drift velocity of electrons in high electric fields in AlGaAs/GaAs heterostructure with InAs quantum-dot barriers introduced into the GaAs quantum well exceeds the saturation drift velocity in bulk GaAs by as much as a factor of 10. Such a rise in the maximum drift velocity of electrons ensures increased maximum current density, transconductance, and cutoff frequency of the heterostructure field-effect transistor with quantum dots.

Shallow-donor states in spherical quantum dots with parabolic confinement

AbstractThe evidence of a parabolic potential well in quantum wires and dots was reported in the literature, and a parabolic potential is often considered to be a good representation of the “barrier” potential in semiconductor quantum dots. In the present work, the variational and fractionaldimensional space approaches are used in a thorough study of the binding energy of on-center shallow donors in spherical GaAs-Ga1-xAlxAs quantum dots with potential barriers taken either as rectangular [Vb (eV) ??1.247 x for r >] or parabolic [Vb (r) ??β2?r2] isotropic barriers. We define the parabolic potential with a β?parameter chosen so that it results in the same E0 groundstate energy as for the spherical quantum dot of radius R and rectangular potential in the absence of the impurity. Calculations using either the variational or fractional-dimensional approaches both for rectangular and parabolic potential result in essentially the same on-center binding energies provided the dot radius is not too small. This indicates that both potentials are alike representations of the quantum-dot barrier potential for a radius R quantum dot provided the parabolic potential is defined with?β?chosen as mentioned above.

Electron-electron interactions and the magnetoconductance of submicron quantum dots

The magnetoconductance of quantum dots at high magnetic fields has been studied experimentally and theoretically as a function of the quantum dot barrier height. It is found that even when the dot is not Coulomb blockaded, electron-electron interactions are required to explain the magnetoconductance oscillations.