K. Kamath

Far-infrared photoconductivity in self-organized InAs quantum dots

We report far-infrared photoconductivity in self-organized InAs/GaAs quantum dots grown by molecular beam epitaxy. Through use of a Fourier transform infrared spectrometer, a photoconductivity signal peaked at 17 μm is observed from a n–i–n detector structure with doped InAs quantum dots in the intrinsic region. Comparison of photoluminescence and band-to-band photocurrent absorption spectra suggests the far-infrared response is due to intersubband transitions in the quantum dots.

In(Ga)As/GaAs self-organized quantum dot lasers: DC and small-signal modulation properties

Self-organized growth of InGaAs/GaAs strained epitaxial layers gives rise to an ordered array of islands via the Stranski-Krastanow growth mode, for misfits >1.8%. These islands are pyramidal in shape with a base diagonal of /spl sim/20 nm and height of /spl sim/6-7 nm, depending of growth parameters. They therefore exhibit electronic properties of zero-dimensional systems, or quantum dots. One or more layers of such quantum dots can be stacked and vertically coupled to form the gain region of lasers. We have investigated the properties of such single-layer quantum dot (SLQD) and multilayer quantum dot (MLQD) lasers with a variety of measurements, including some at cryogenic temperatures. The experiments have been complemented with theoretical calculations of the electronic properties and carrier scattering phenomena in the dots. Our objective has been to elucidate the intrinsic behavior of these devices. The lasers exhibit temperature independent threshold currents up to 85 K, with T/sub 0//spl les/670 K. Typical threshold currents of 200-/spl mu/m long room temperature lasers vary from 6 to 20 mA. The small-signal modulation bandwidths of ridge waveguide lasers are 5-7.5 GHz at 300 K and increased to >20 GHz at 80 K. These bandwidths agree well with electron capture times of /spl sim/30 ps determined from high-frequency laser impedance measurements at 300 K and relaxation times of /spl sim/8 ps measured at 18 K by differential transmission pump-probe experiments. From the calculated results we believe that electron-hole scattering intrinsically limits the high-speed performance of these devices, in spite of differential gains as high as /spl sim/7/spl times/10/sup -14/ cm/sup 2/ at room temperature.

SMALL-SIGNAL MODULATION AND DIFFERENTIAL GAIN OF SINGLE-MODE SELF-ORGANIZED IN0.4GA0.6AS/GAAS QUANTUM DOT LASERS

We report small-signal modulation bandwidth and differential gain measurements of a single-layer self-organized In0.4Ga0.6As/GaAs quantum dot laser grown by molecular beam epitaxy. The 3 dB bandwidth of single-mode ridge waveguide lasers was measured to be 7.5 GHz at 100 mA under pulsed measurements, demonstrating the possibility of high speed operation of these devices. The differential gain was measured to be 1.7×10−14 cm2.

Photoluminescence and far-infrared absorption in Si-doped self-organized InAs quantum dots

We report far-infrared absorption in directly doped self-organized InAs quantum dots. Photoluminescence spectra demonstrate a blue shift in peak intensity for increasing doping in the quantum dots. Far-infrared absorption measurements using a Fourier transform infrared spectrometer show absorption in the range of 13–18 μm for quantum dots with Al0.15Ga0.85As and GaAs as the barrier material.

Quantum capture times at room temperature in high-speed In/sub 0.4/Ga/sub 0.6/As-GaAs self-organized quantum-dot lasers

The quantum capture times in high-speed single-mode self-organized quantum-dot (QD) lasers with I/sub th/=15-30 mA, and small-signal modulation bandwidth f/sub -3 dB/=4.5 GHz, have been estimated from high frequency electrical impedance measurements. The effective carrier capture times, determined from this relatively simple measurement technique, vary in the range of 20-40 ps, depending on bias current, and are in excellent agreement with theoretical predictions. The results suggest that carrier capture, not damping, may prove to be the limiting factor in the modulation bandwidths of QD lasers.

Photoluminescence and time-resolved photoluminescence characteristics of InxGa(1−x)As/GaAs self-organized single- and multiple-layer quantum dot laser structures

The characteristics of ground and excited state luminescent transitions in In0.4Ga0.6As/GaAs and In0.35Ga0.65As/GaAs self-organized single- and multiple-layer quantum dots forming the active regions of lasers have been studied as a function of incident excitation intensity, temperature and number of dot layers. The results have been correlated with molecular beam epitaxial growth conditions. The threshold excitation density for the saturation of the ground state increases with the number of dot layers and no saturation is observed in samples with more than six dot layers up to an excitation power density of 2 kW/cm2. The luminescent decay times for the ground and excited states are around 700 and 250 ps, respectively, almost independent of the number of dot layers.

Lateral oxidation of InAlAs in InP-based heterostructures for long wavelength vertical cavity surface emitting laser applications

We have studied the wet thermal oxidation of In0.52Al0.48As and its potential application in current and optical confinement in vertical cavity surface emitting lasers (VCSELs). Two types of InP-based heterostructures were used to study the effect of adjacent layer compositions on the lateral oxidation behavior of the InAlAs. It was found that the oxidation of In0.52Al0.48As with InP adjacent layers, compared with In0.53Ga0.47As adjacent layers, proceeded faster, more uniformly and with minimal degradation of the surrounding layers making it ideal for optoelectronic applications.

Enhanced modulation bandwidth (20 GHz) of In/sub 0.4/Ga/sub 0.6/As-GaAs self-organized quantum-dot lasers at cryogenic temperatures: role of carrier relaxation and differential gain

Measurements of the threshold current, slope efficiency and optical modulation characteristics of self-assembled InGaAs-GaAs quantum-dot lasers have been made in the temperature range of 20-200 K in order to understand the carrier dynamics in these devices. The dc characteristics of these devices showed a region of almost temperature independent threshold current up to 85 K (T/sub 0/=670 K) with a maximum slope efficiency at 150 K. The maximum measured bandwidth increased from 5 GHz at room temperature to 20 GHz at 80 K. This is consistent with the bandwidth being limited by carrier relaxation time through electron-hole scattering.

Characteristics of InAs/AlGaAs self-organized quantum dot modulation doped field effect transistors

We have investigated the dc characteristics of InGaAs/AlGaAs modulation doped field effect transistors in which a layer of self-organized InAs quantum dots is inserted adjacent to the pseudomorphic quantum well channel. Distinct steps and a negative differential resistance are observed in the current–voltage characteristics at room temperature and lower temperatures. These are attributed to conduction through the bound states in the quantum dots.

LINEAR AND QUADRATIC ELECTRO-OPTIC COEFFICIENTS OF SELF-ORGANIZED IN0.4GA0.6AS/GAAS QUANTUM DOTS

The electro-optic properties of self-organized In0.4Ga0.6As/GaAs quantum dots have been studied experimentally. Single-mode ridge waveguide structures were grown by molecular beam epitaxy with self-organized In0.4Ga0.6As/GaAs quantum dots in the guiding region. The measured linear and quadratic electro-optic coefficients are 2.58×10−11 m/V and 6.25×10−17 m2/V2, respectively, which are much higher than those obtained for bulk GaAs or quantum well structures. The measured transmission characteristics indicate that low-voltage amplitude modulators can be realized with quantum dot active regions.