M. J. Steer

Enhancing the dot density in quantum dot infrared photodetectors via the incorporation of antimony

The authors combine optical spectroscopic studies and infrared photodetector development to demonstrate the potential of antimony-mediated InAs quantum dot growth for the production of high performance dot-based devices. By depositing 1 ML of gallium antimonide prior to dot growth, the dot density is increased from ∼3×1010 for conventional InAs dots, to ∼6×1010cm−2. Detailed intra- and interband spectroscopic studies show no significant differences in the electron energy level configuration compared with standard InAs∕GaAs dots, while intraband absorption strength is increased. Furthermore, they have implemented this growth technique to produce a quantum dot infrared photodetector with a detectivity of ∼5×1010cmHz1∕2W−1 at 7.5μm (T=77K).

λ∼4-5.3 μm intersubband emission from InGaAs-AlAsSb quantum cascade structures

The In0.53Ga0.47As–AlAs0.56Sb0.44 materials system, lattice matched to InP, is an attractive candidate for short wavelength quantum cascade lasers due to the very large conduction band discontinuity (∼1.6 eV) and compatibility with well established quantum cascade laser waveguide design and fabrication technology. In this letter we report the operation of In0.53Ga0.47As–AlAs0.56Sb0.44 quantum cascade structures emitting in the wavelength range λ∼4–5.3 μm. Clear intersubband electroluminescence peaks are observed close to the design wavelengths, with full widths at half maximum in the range of ∼30–40 meV.

Intersublevel polaron dephasing in self-assembled quantum dots

Polaron dephasing processes are investigated in InAs/GaAs dots using far-infrared transient four wave mixing (FWM) spectroscopy. We observe an oscillatory behaviour in the FWM signal shortly (< 5 ps) after resonant excitation of the lowest energy conduction band transition due to coherent acoustic phonon generation. The subsequent single exponential decay yields long intraband dephasing times of 90 ps. We find excellent agreement between our measured and calculated FWM dynamics, and show that both real and virtual acoustic phonon processes are necessary to explain the temperature dependence of the polarization decay. PACS numbers: 78.67.Hc, 78.47.+p, 42.50.Md, 71.38.-k The strong spatial confinement of carriers in semiconductor quantum dots (QDs) leads to striking differences in the carrier-phonon interaction compared with systems of higher dimensionality. In particular, the discrete energy level s tructure in QDs results in long exciton and electron dephasing times [1, 2, 3, 4], making these semiconductor nanostructures highly attractive for implementation in quantum information processing applications. The study of dephasing mechanisms in QDs is commonly carried out using transient four wave mixing (FWM) spectroscopy. Using resonant interband excitation, FWM measurements have revealed the absorption lineshape of single QDs to consist of a narrow zero phonon line (ZPL) and an acoustic phonon-related broadband centred at the same energy. The only intraband FWM study [5] involved resonant excitation of high energy transitions in th e valence band of p-doped QDs yielding dephasing times ∼ 15 ps. However it was not possible to determine the dephasing mechanisms in this case. Intraband studies of the well-resolved lowest energy conduction band electron transitions in InAs/GaAs QDs have provided deep insight into the electron-phonon interaction and carrier relaxation processes in n-doped samples. Clear evidence of strong coupling between electrons and phonons, resulting in polaron formation, has been demonstrated using magneto-transmission measurements [6]. Ultrafast studies [7, 8] of polaron decay have shown that the previously assumed ’phonon bottleneck’ picture is not valid. Compared with semiconductor quantum wells, the intraband population relaxation time in QDs is long (∼ 50 ps) suggesting relatively long dephasing times. However there have been no reports of direct dephasing measurements to date. In the present letter we present the first investigations of i ntraband dephasing in n-doped QDs using degenerate FWM. Our calculations of the absorption lineshape in this case sh ow marked differences in comparison with the interband absorption [9]. The intraband lineshape consists of peaked acoustic phonon sidebands separated by ∼ 1.5 meV from the ZPL, which corresponds to phonons with wavelength close to the dot size, and is reminiscent of the lineshape associated wit h impurity-bound electron transitions [10]. Using pulse durations short enough to excite both the ZPL and acoustic phonon sidebands we find damped oscillations in the FWM signal, indicative of coherent acoustic phonon generation, followed by a single exponential decay. In contrast with the interband case, where the origin of the strong temperature dependence of the excitonic linewidth is still subject to debate, the simple 3 -level structure of the lowest energy conduction band states in InAs QDs permits an accurate simulation of the temperature dependence of the FWM signal. The excellent agreement found between experiment and theory, shows that virtual transitions between the p-states is the dominant dephasing mechanism at high temperature. At low temperature, we have measured an intersublevel dephasing time of T2 ∼ 90 ps. It is also interesting to compare our results with previous intraband dephasing measurements in higher dimensional (quantum well) systems [11]. Here phonon-mediated processes are not significant with the intraband dephasing instead determined by electronelectron interactions, yielding typical dephasing times ∼ 0.3 ps which are approximately 2 orders of magnitude faster than for the QD samples studied here. The relatively long intraband dephasing time in QDs is key to the efficient operation of new types of mid-infrared QD-based devices, such as intersublevel polaron lasers [12] and may be relevant for potential device applications such as qubits for quantum information processors [13]. The investigated samples were grown on (100) GaAs substrates by molecular beam epitaxy in the Stranski-Krastranow mode. They comprise 80 layers of InAs self-assembled QDs separated by 50 nm wide GaAs barriers, thus preventing both structural and electronic coupling between QD layers. The polaron transitions were studied between s-like ground (s) and p-like first excited ( p) states within the conduction band. To populate the s state, the samples were delta-doped with Si 2 nm below each QD layer. The doping density was controlled in such a way that the average doping did not exceed 1 electron per dot (see Ref. [14] for more details). Absorp

Tuning of intraband absorption and photoresponse in self-assembled InAs∕GaAs quantum dots by thermal annealing

The effects of the thermal annealing on the conduction band states in self-assembled quantum dots (QDs) have been investigated using midinfrared absorption and photocurrent spectroscopies. We demonstrate a significant tuning of the intraband electron transition energies (∼15%) upon annealing due to the reduction in the confinement energy, observed both in intraband absorption and photocurrent spectra. More uniform QD size distribution in the growth direction after annealing results in narrowing of the absorption peak. We also observe additional intersublevel transitions arising after QD annealing.

Singlet and triplet polaron relaxation in doubly charged self-assembled quantum dots

Polaron relaxation in self-assembled InAs/GaAs quantum dot samples containing 2 electrons per dot is studied using far-infrared, time-resolved pump–probe measurements for transitions between the s-like ground and p-like first excited conduction band states. Spin–flip transitions between singlet and triplet states are observed experimentally in the decay of the absorption bleaching, which shows a clear biexponential dependence. The initial fast decay (~30 ps) is associated with the singlet polaron decay, while the decay component with the longer time constant (~5 ns) corresponds to the excited state triplet lifetime. The results are explained by considering the intrinsic Dresselhaus spin–orbit interaction, which induces spin–flip transitions by acoustic phonon emission or phonon anharmonicity. We have calculated the spin–flip decay times, and good agreement is obtained between the experiment and the simulation of the pump–probe signal. Our results demonstrate the importance of spin-mixing effects for intraband energy relaxation in InAs/GaAs quantum dots.

Polaron relaxation channel in InAs/GaAs self-assembled quantum dots

Polaron relaxation in n-type InAs quantum dots has been studied by picosecond time-resolved pump–probe spectroscopy. Due to inhomogeneous broadening of the absorption features associated with transitions between s-like ground state and p-like first excited state, the energy dependence of the polaron decay time has been measured over a wide spectral region from 40 to 60 meV. The polaron lifetime increases continuously from 20 ps at 40 meV to 65 ps at 54 meV. By analysing the temperature dependence of the polaron lifetime the main polaron decay channel has been identified as the cubic overtone (2LA) decay channel. By fitting the experimental data we extract the LO-phonon lifetime of 10 ± 2 ps (at 33 meV) for InAs quantum dots and electron–phonon coupling strength of 6.2 ± 0.5 meV.

High performance 2.2 μm optically-pumped vertical external-cavity surface-emitting laser

We report the operation of an optically-pumped vertical-external-cavity surface-emitting laser (OP-VECSEL) oscillating at wavelengths up to 2.2 µm and at output powers greater than 200 mW. This versatile platform provides a broad gain bandwidth, and may be tuned and/or controlled by the addition of elements into the external cavity. Moreover, the nature of the semiconductor structure permits precise engineering of the operating wavelength---not possible with traditional solid state crystalline lasers---and the thin (few microns) pump absorption region, coupled with the external cavity control, permits the mode conversion of a low brightness pump into a high quality TEM00 output.

Control of strain in GaSbAs/InAs/GaAs quantum dots

We discuss strain simulations of quantum dot structures covered with a GaSbAs strain reducing capping layer in the presence of Sb segregation. Cross Sectional Scanning Tunneling Microscopy shows strong Sb and In segregation in the material surrounding the quantum dot. Using the three layer model originally proposed for the SiGe system by D. J. Godbey, M. G. Ancona, J. Vac. Sci. Technol. A 15, 976 (1997) we accurately calculate the segregation profile and include a non uniform composition to our models. Using atomistic modeling, we present strain maps of the quantum dot structures that show the propagation of the strain into the GaAs region is strongly affected by the shape and composition of the strain reduction layer.

Electronic spatial distribution of In0.53Ga0.47As∕AlAs0.56Sb0.44 quantum-cascade lasers

We have investigated the band-to-band photoluminescence of In0.53Ga0.47As∕AlAs0.56Sb0.44 quantum-cascade devices operating at λ∼4.3μm, based either on normally grown or nominally As-terminated interfaces. This technique allowed us to probe the spatial distribution of conduction electrons as a function of the applied voltage and to correlate the quantum design of devices with their thermal performance.

InGaAs-AlAsSb-InP quantum cascade lasers: performance and prospects

We report the first demonstration of InGaAs/AlAsSb/InP quantum cascade lasers. Laser characteristics and structural investigations demonstrate the significant potential of this materials system for extending the short wavelength operating limit of quantum cascade lasers.