Electroluminescence from Si/SiGe quantum cascade emitters

Intersubband electroluminescence results are presented from Si/SiGe quantum cascade emitters at 3.2 THz and at temperatures up to 150 K. The effect of adding doping into the active quantum wells was studied in addition to reduced barrier widths from previous measurements. While the current through the sample is increased by the addition of doping, the emitted power is reduced through additional free carrier absorption and Coulombic scattering. Free electron laser measurements confirm the intersubband transitions in the quantum wells of the cascade devices and produce non-radiative lifetimes of 20 ps between 4 and 150 K.     

Enhanced infrared absorption of spatially ordered quantum dot arrays

We have investigated the intersubband absorption for spatially ordered and non-ordered quantum dots (QDs). It is found that the intersubband absorption of spatially ordered QDs is much stronger than that of non-ordered QDs. The enhanced absorption is attributed to the improved size uniformity concurrent with the spatial ordering for the growth condition employed. For the FTIR measurement under normal incidence geometry, using a undoped sample as reference can remove the interference effect due to multiple reflections.     

Strain compensated Si/SiGe quantum well and quantum cascade on Si0.5Ge0.5 pseudosubstrate

This study follows up our previous investigation of the valence band (VB) intersubband emission from quantum cascade structures grown lattice matched on Si substrates. Here, Si/Si_1−xGe_x (x=80%) heterostructures are investigated which are deposited by MBE on a virtual substrate of relaxed SiGe containing 50% of Ge. TEM analysis reveal flat and abrupt interfaces for structures grown at temperatures T_growth≈300°C. Intersubband absorption and photoluminescence emission manifest well-defined interfaces and good material quality. The observed intersubband line positions are found to be in good agreement with k·p model calculations for the VB. This is in contrast to the observed type II no phonons recombination which is found at consistently lower energy than expected. Finally, electrically excited intersubband emission from a strain compensated cascade structure containing three periods is presented.     

InAs/GaAs quantum dot infrared photodetectors with different growth temperatures

InAs/GaAs quantum dot infrared photodetectors were fabricated with quantum dots grown at three different temperatures. Large detection wavelength shift (5–14.5 μm) was demonstrated by changing 40 degrees of the epitaxy temperature. The smaller quantum dots grown at lower temperature generate 14.5 μm responses. The detectivity of the normal incident 15 μm QDIP at 77 K is 3 × 10⁸ cm Hz^1/2/W. A three-color detector was also demonstrated with quantum dots grown at medium temperature. The three-color detection comes from two groups of different sizes of dots within one QD layer. This new type of multicolor detector shows unique temperature tuning behavior that was never reported before.     

Type II GaSb/GaAs quantum dot/ring stacks with extended photoresponse for efficient solar cells

We report on the fabrication of GaAs based p–i–n solar cells containing 5 and 10 layers of type II GaSb quantum rings grown by molecular beam epitaxy. Solar cells containing quantum rings show improved efficiency at longer wavelengths into the near-IR extending up to 1500 nm and show enhanced short-circuit current under 1 sun illumination compared to a GaAs control cell. A reduction in the open-circuit voltage is observed due to the build-up of internal strain. The MBE growth, formation and photoluminescence of single and stacked layers of GaSb/GaAs quantum rings are also presented.     

Temperature dependent responsivity of quantum dot infrared photodetectors

Temperature dependent behavior of the responsivity of InAs/GaAs quantum dot infrared photodetectors was investigated with detailed measurement of the current gain. The current gain varied about two orders of magnitude with 100 K temperature change. Meanwhile, the change in quantum efficiency is within a factor of 10. The dramatic change of the current gain is explained by the repulsive coulomb potential of the extra carriers in the QDs. With the measured current gain, the extra carrier number in QDs was calculated. More than one electron per QD could be captured as the dark current increases at 150 K. The extra electrons in the QDs elevated the Fermi level and changed the quantum efficiency of the QDIPs. The temperature dependence of the responsivity was qualitatively explained with the extra electrons.     

Photocurrent spectroscopy of indirect transitions in Ge/Si multilayer quantum dots at room temperature

Lateral photoconductivity spectra of multilayer Ge/Si heterostructures with Ge quantum dots were studied in the work proposed at room temperature. The photocurrent with minimal energy 0.48-0.56 eV that is smaller than Ge band gap was observed from such structures at the geometry of waveguide excitation. Generation of the photocurrent with the limit energy 0.48-0.56 eV was explained by spatially indirect electron transitions from heavy hole states of SiGe valence band into Δ₂-valley of the conduction band of Si surrounding. It was found out that the limit energy of such transitions decreased, as the number of SiGe quantum dot layers increased.     

AC-hopping conductance of self-organized Ge/Si quantum dot arrays

Dense (n=4×10¹¹ cm^-2) arrays of Ge quantum dots in a Si host were studied using attenuation of surface acoustic waves (SAWs) propagating along the surface of a piezoelectric crystal located near the sample. The SAW magneto-attenuation coefficient, ΔΓ=Γ(ω,H)-Γ(ω,0), and change of velocity of SAW, ΔV/V=(V(H)-V(0))/V(0), were measured in the temperature interval T=1.5–4.2 K as a function of magnetic field H up to 6 T for the waves in the frequency range f=30–300 MHz. Based on the dependences of ΔΓ on H, T and ω, as well as on its sign, we believe that the AC conduction mechanism is a combination of diffusion at the mobility edge with hopping between localized states at the Fermi level. The measured magnetic field dependence of the SAW attenuation is discussed based on existing theoretical concepts.     

Two-electron absorption in a biased quantum well

Linear light absorption of 2D electrons confined within a biased quantum well is studied theoretically. We demonstrate that for light polarization perpendicular to the 2D plane, in addition to conventional absorption peak at frequency ω≈Δ, where Δ is the intersubband energy distance, there exists a peak around a double frequency ω≈2Δ. This additional peak is entirely due to electron–electron interactions, and corresponds to excitation of two electrons by one photon. The magnitude of two-electron absorption is proportional to U², where U is the applied bias.     

Strain-induced quantum dot superlattice

Coupled double quantum dots and quantum dot superlattices are formed by utilizing the strain of an InP island on top of a near-surface multi-quantum-well structure. The number and composition of the quantum wells together with the thickness of the barrier separating the quantum wells are varied to investigate the coupling of the wave functions of the carriers confined in separate vertically stacked dots. Photoluminescence studies show that the reduction of the barrier thickness and the increase of the number of wells enhance the coupling, which is observed as red shift and narrowing of the quantum dot peak. The calculated shifts of the peak positions agree closely with the experimental values.     

Energy levels in doped SiGe quantum well studied by admittance spectroscopy

SiGe/Si quantum wells (QWs) with different Boron doping concentrations were grown by molecular beam epitaxy (MBE) on p-type Si(1 0 0) substrate. The activation energies of the heavily holes in ground states of QWs, which correspond to the energy differences between the heavy hole ground states and Si valence band, were measured by admittance spectroscopy. It is found that the activation energy in a heavily doped QW increases with doping concentration, which can be understood by the band alignment changes due to the doping in the QWs. Also, it is found that the activation energy in a QW with a doping concentration of 2 × 10²⁰ cm⁻³ becomes larger after annealing at a temperature of 685 °C, which is attributed to more Boron atoms activation in the QW by annealing.     

Insulating state in open quantum dots and quantum dot arrays

We describe the observation of novel localization in mesoscopic quantum dots and quantum dot arrays, which are realized in high mobility GaAs/AlGaAs heterojunctions using the split‐gate technique. With a sufficient gate voltage applied to form the devices, their resistance diverges as the temperature is lowered below a degree Kelvin, behavior which we attribute to localization. Evidence for the localization is found over the entire range of gate voltage for which the dots are defined, persisting to conductances higher than 50e²/h.     

Magnetoconductance of a few-electron open quantum dot

Magnetoconductance of a small open lateral dot is studied both theoretically and experimentally for the conditions when the dot contains down to 15 electrons. We confirm the existence of a new regime for open dots in which the transport through the structure occurs through individual eigenstates of the corresponding closed dot. In particular, at low magnetic fields the characteristic features in the conductance are related to the underlying eigenspectrum shells. When the number of modes in the leads is reduced more detailed structures within the shells due to single eigenlevels becomes discernible. At higher fields Landau level condensation is evident as well as the crossing of levels collapsing to the different Landau levels.     

Magnetoplasmon excitations in quantum dot arrays

Motivated by the far-infrared transmission experiments of Demel et al., we have investigated the magnetoplasmon excitations in an array of quantum dots within the Thomas–Fermi–Dirac–von Weizsäcker (TFDW) approximation. Detailed calculations of the magnetic dispersion and power absorption from a uniform radiation field unambiguously demonstrates that the noncircular symmetry of the individual dots is responsible for the anticrossing behaviour observed in the experiments. The interdot Coulomb interaction is unimportant at the interdot separation of the samples studied.     

Tuning photoresponse through size distribution control of silicon quantum dots

We report a detailed experimental and theoretical investigation on the photocurrent characteristics of nanocrystalline Si thin films, with the emphasis on the effect of Si dot size distribution. Broader photocurrent response has been observed in Si quantum dots with larger size dispersion due to the improvement of light harvest. As a result of tunneling loss in the expanded energy distribution, we have demonstrated that there is a tradeoff between the absorption enhancement and reduced transport for the photocurrent intensity. The present work opens new strategy to maximize the photoresponse through size distribution control for quantum dot solar cell application.     

Single-photon detection mechanism in a quantum dot transistor

We study the transport mechanisms in a quantum dot MODFET by tuning the localization induced by charge stored on the quantum dots with light. The temperature dependence of the resistivity of a macroscopic sample reveals a hopping transport when the dots contain an excess of electrons. The resistance of a mesoscopic sample however, which is capable of detecting single photons, exhibits a much weaker dependence upon temperature. This points towards source-drain tunnelling as a transport mechanism and is confirmed by a statistical analysis of the single-photon-induced conductance steps. The complexity of the conducting paths increases as the average hopping length reduces.     

Control of the entanglement between triple quantum dot molecule and its spontaneous emission fields via quantum entropy

The time evolution of the quantum entropy in a coherently driven triple quantum dot molecule is investigated. The entanglement of the quantum dot molecule and its spontaneous emission field is coherently controlled by the gate voltage and the rate of an incoherent pump field. The degree of entanglement between a triple quantum dot molecule and its spontaneous emission fields is decreased by increasing the tunneling parameter.     

Quantum dots in quantum well structures

Recent progress toward fabricating and characterizing quantum dots in III–V quantum well structures is reviewed. Quantum dots made by use of lithography and etching, including deep-etched, barrier-modulated, strain-induced and interdiffused quantum dots, are described. Quantum dots fabricated by growth, including natural quantum dots, dots on patterned substrates, and self-assembled dots, are discussed. Dot sizes and uniformity, energy-level splittings, and luminescence efficiencies that are now being achieved are discussed. The status of key issues, such as the energy relaxation in quantum dots, is mentioned.     

Damping of coherent oscillations in a quantum dot photodiode

In this letter, we develop a model to describe the Rabi oscillations observed in a quantum-dot photodiode. Using a multi-level density matrix formulation, which includes multi-exciton and single particle states, we show that the damping observed in recent experiments is the result of a non-resonant excitation from or to the continuum of the wetting layer states.     

Theoretical investigation of intersubband transition in AlxGa1−xN/GaN/AlyGa1−yN step quantum well

A modified self-consistent method is introduced for the design of Al_xGa_1−xN/GaN step quantum well (SQW) with the position and energy-dependent effective mass. The effects of nonparabolicity are included. It is shown that the nonparabolicity effect is minute for the lowest subband energy level and grows in size for the higher subband states. The effects of nonparabolicicty have significant influence on the transition energies and the oscillator strengths and should be taken into account in the investigation of the optical transitions. The strong asymmetric property introduced by the step quantum well magnifies the weak intersubband transition from the ground state to the third state (1→3). It is shown that in an appropriate scope, the intersubband transition (1→3) has the comparable oscillator strength with transition from the ground state to the second one (1→2), which suggests the possible application of the two-color photodetectors. The results of this work should provide useful guidance for the design of optically pumped asymmetric quantum well lasers and quantum well infrared photodetectors (QWIPs).