Z. R. Wasilewski

Terahertz quantum cascade lasers operating up to ~200 K with optimized oscillator strength and improved injection tunneling

A new temperature performance record of 199.5 K for terahertz quantum cascade lasers is achieved by optimizing the lasing transition oscillator strength of the resonant phonon based three-well design. The optimum oscillator strength of 0.58 was found to be larger than that of the previous record (0.41) by Kumar et al. [Appl. Phys. Lett. 94, 131105 (2009)]. The choice of tunneling barrier thicknesses was determined with a simplified density matrix model, which converged towards higher tunneling coupling strengths than previously explored and nearly perfect alignment of the states across the injection and extraction barriers at the design electric field. At 8 K, the device showed a threshold current density of 1 kA/cm2, with a peak output power of ∼ 38 mW, and lasing frequency blue-shifting from 2.6 THz to 2.85 THz with increasing bias. The wavelength blue-shifted to 3.22 THz closer to the maximum operating temperature of 199.5 K, which corresponds to ∼ 1.28ħω/κB. The voltage dependence of laser frequency is related to the Stark effect of two intersubband transitions and is compared with the simulated gain spectra obtained by a Monte Carlo approach.

Size and shape engineering of vertically stacked self-assembled quantum dots

A new procedure for the growth of stacked self-assembled quantum dot layers is described. The main effect of the procedure is to convert the quantum dot population into a population of quantum disks of approximately equal height. Proposed model of the overgrowth process for highly strained 3D islands invokes mechanisms which may lead to a variety of dot shape modification and opens up new ways of influencing the dot population by deliberate control of the overgrowth process. Demonstrated very good performance of test devices indicate that the proposed procedure may have positive impact on the further development of quantum dot lasers.

Negative capacitance effect in semiconductor devices

Nontrivial capacitance behavior, including a negative capacitance (NC) effect, observed in a variety of semiconductor devices, is discussed emphasizing the physical mechanism and the theoretical interpretation of experimental data. The correct interpretation of NC can be based on the analysis of the time-domain transient current in response to a small voltage step or impulse, involving a self-consistent treatment of all relevant physical effects (carrier transport, injection, recharging, etc.). NC appears in the case of the nonmonotonic or positive-valued behavior of the time-derivative of the transient current in response to a small voltage step. The time-domain transient current approach is illustrated by simulation results and experimental studies of quantum well infrared photodetectors (QWIPs). The NC effect in QWIPs has been predicted theoretically and confirmed experimentally. The huge NC phenomenon in QWIPs is due to the nonequilibrium transient injection from the emitter caused by the properties of the injection barrier and the inertia of the QW recharging.Nontrivial capacitance behavior, including a negative capacitance (NC) effect, observed in a variety of semiconductor devices, is discussed emphasizing the physical mechanism and the theoretical interpretation of experimental data. The correct interpretation of NC can be based on the analysis of the time-domain transient current in response to a small voltage step or impulse, involving a self-consistent treatment of all relevant physical effects (carrier transport, injection, recharging etc.). NC appears in the case of the non-monotonic or positive-valued behavior of the time-derivative of the transient current in response to a small voltage step. The time-domain transient current approach is illustrated by simulation results and experimental studies of quantum well infrared photodetectors (QWIPs). The NC effect in QWIPs has been predicted theoretically and confirmed experimentally. The huge NC phenomenon in QWIPs is due to the non-equilibrium transient injection from the emitter caused by the properties of the injection barrier and the inertia of the QW recharging.

Addition spectrum of a lateral dot from Coulomb and spin-blockade spectroscopy

Transport measurements are presented on a class of electrostatically defined lateral dots within a high mobility two dimensional electron gas (2DEG). The new design allows Coulomb Blockade(CB) measurements to be performed on a single lateral dot containing 0, 1 to over 50 electrons. The CB measurements are enhanced by the spin polarized injection from and into 2DEG magnetic edge states. This combines the measurement of charge with the measurement of spin through spin blockade spectroscopy. The results of Coulomb and spin blockade spectroscopy for first 45 electrons enable us to construct the addition spectrum of a lateral device. We also demonstrate that a lateral dot containing a single electron is an effective local probe of a 2DEG edge.

InAs self‐assembled quantum dots on InP by molecular beam epitaxy

We present results of room temperature photoluminescence (PL) emission from a 0‐dimensional system in the ∼1.4 to ∼1.7 μm spectral region. Molecular beam epitaxy was used to grow InAs self‐assembled quantum dots in AlInAs on an InP substrate. Preliminary characterizations have been performed using PL and transmission electron microscopy. The low temperatures PL spectra also display excited state emission and state filling as the excitation intensity is increased.We present results of room temperature photoluminescence (PL) emission from a 0‐dimensional system in the ∼1.4 to ∼1.7 μm spectral region. Molecular beam epitaxy was used to grow InAs self‐assembled quantum dots in AlInAs on an InP substrate. Preliminary characterizations have been performed using PL and transmission electron microscopy. The low temperatures PL spectra also display excited state emission and state filling as the excitation intensity is increased.

Terahertz quantum-cascade lasers based on a three-well active module

The authors report on a design of terahertz quantum-cascade lasers based on three-well active modules. Each module consists of two tunnel-coupled wells for the two lasing states and another well for both resonant-phonon depopulation and carrier injection. This design is the simplest so far among the various published working devices. The test device has a lasing frequency of 3.4THz and maximum operating temperature of 142K.

Background-limited terahertz quantum-well photodetector

We report terahertz quantum-well photodetectors with background-limited infrared performance (BLIP). The device dark current characteristics were improved by employing thick barriers to reduce interwell tunneling. BLIP operations were observed for all samples (three in total) designed for different wavelengths. BLIP temperatures of 17, 13, and 12K were achieved for peak detection frequencies at 9.7THz (31μm), 5.4THz (56μm), and 3.2THz (93μm), respectively.

Mode-Locked Pulses from Mid-Infrared Quantum Cascade Lasers

In this study, we report the unequivocal demonstration of midinfrared mode-locked pulses from quantum cascade lasers. The train of short pulses was generated by actively modulating the current and hence the gain of an edge-emitting quantum cascade laser (QCL). Pulses with duration of about 3 ps at full-width-at-half-maxima and energy of 0.5 pJ were characterized using a second-order interferometric autocorrelation technique based on a nonlinear quantum well infrared photodetector. The mode-locking dynamics in the QCLs was modeled based on the Maxwell-Bloch equations in an open two-level system. Our model reproduces the overall shape of the measured autocorrelation traces and predicts that the short pulses are accompanied by substantial wings as a result of strong spatial hole burning. The range of parameters where short mode-locked pulses can be formed is found.

How good is the polarization selection rule for intersubband transitions

Using GaAs based quantum well infrared photodetectors (QWIPs) with either GaAs or InGaAs wells, we experimentally investigate the accuracy of the polarization selection rule for conduction band intersubband transitions. We employ a device structure and a light coupling geometry where the parasitic light scattering is negligible. The experiments imply that the selection rule is followed to an accuracy of 0.2% for a 8.1 μm QWIP with GaAs wells; this degrades to 3% for a 4.6 μm QWIP with In0.1Ga0.9As wells.

Segregation of Si δ doping in GaAs‐AlGaAs quantum wells and the cause of the asymmetry in the current‐voltage characteristics of intersubband infrared detectors

Dopant segregation in the well region of a multiple quantum well intersubband photodetector can cause an asymmetry in the observed forward and reverse current‐voltage characteristics. We compensate for the segregation by shifting the position of the Si δ doping in the well and model the effect with good agreement for a range of shift values. For samples grown at a substrate temperature of 605 °C, we find that the observed behavior is best described by assuming that the Si δ‐doping profile smears in the growth direction resulting in an asymmetric broadening of about 27 A.