A. J. SpringThorpe

Terahertz quantum-well photodetector

The design and projected performance of quantum-well infrared photodetectors (QWIP) for the terahertz (1–10 THz) or the very-far-infrared region are presented together with our initial demonstration of a GaAs/AlGaAs QWIP working at photon energies below the optical phonons. We point out the problem with this initial device, discuss possible causes, and suggest areas of improvement.

Near-infrared to visible light optical upconversion by direct tandem integration of organic light-emitting diode and inorganic photodetector

The authors report a hybrid organic/inorganic optical upconversion device that converts 1.5μm infrared light to ∼520nm visible light. The device was made by direct tandem integration of an inorganic InGaAs∕InP photodetector with an organic light-emitting diode (OLED). Optical upconversion with an external efficiency of 0.7% W/W at room temperature has been achieved. Interfacial structure at the inorganic-organic interface was found to play a vital role in enabling the integration of the hybrid tandem upconverter. Both sulfur-terminated InP surface and nanocarbon fullerene interlayer were found crucial to form a good interface contact, permitting continuous flow of photocarriers from the inorganic detector into the OLED.

Studies and modeling of growth uniformity in molecular beam epitaxy

We have developed a numerical model for the flux distribution in molecular beam epitaxy over stationary and rotating substrates. The existence of a temperature profile along the crucible and accumulation of material at the crucible orifice is taken into account. The influence of melt level tilt on the flux distribution is discussed in detail, and, in contrast to previous reports, is found to be small for the cases considered. Accurate thickness maps of GaAs wafers grown with two different types of effusion cells on stationary substrates were obtained using a scanning reflectance system. Excellent agreement of these experimental results with predictions of the model is demonstrated. We explain some of the deficiencies of the present arrangement and calculate the optimum system conditions for obtaining uniformity across a wafer to better than ±0.3%.

Two-dimensional profiling of carriers in a buried heterostructure multi-quantum-well laser: Calibrated scanning spreading resistance microscopy and scanning capacitance microscopy

We report results of a scanning spreading resistance microscopy (SSRM) and scanning capacitance microscopy (SCM) study of the distribution of charge carriers inside multi-quantum-well (MQW) buried heterostructure (BH) lasers. We demonstrate that individual quantum-well–barrier layers can be resolved using high-resolution SSRM. Calibrated SSRM and SCM measurements were performed on the MQW BH laser structure, by utilizing known InP dopant staircase samples to calibrate the instrumentation. Doping concentrations derived from SSRM and SCM measurements were compared with the nominal values of both p- and n-doped regions in the MQW BH lasers. For n-type materials, the accuracy was bias dependent with SSRM, while for SCM, excellent quantitative agreement between measured and nominal dopant values was obtained. The SSRM was able to measure the dopant concentration in the p-type materials with ∼30% accuracy, but quantitative measurements could not be obtained with the SCM. Our results demonstrate the utility of c...

Scanning spreading resistance microscopy current transport studies on doped III–V semiconductors

Two-dimensional (2D) carrier concentration profiling using scanning spreading resistance microscopy (SSRM) has been carried out on molecular beam epitaxy-grown GaAs and InP dopant calibration samples. The current transport mechanisms between the diamond-coated SSRM tip and the III–V semiconductor cleaved surface (110) was investigated as a function of semiconductor dopant concentration via current–voltage (I–V) measurement. A positive or negative tip bias was applied while scanning over each dopant concentration region (1016–1019 cm−3). The results were compared to simulated I–V curves based on thermionic emission theory. The best fits to the data obtained under forward bias indicated that the contact barrier heights, φb, were much lower than expected from conventional large area planar contacts to GaAs or InP. The effect increases with increasing doping concentration, as a result of a combination of barrier height lowering mechanisms such as image forces, thermionic field emission and minority carrier in...

Optimized GaAs∕AlGaAs light-emitting diodes and high efficiency wafer-fused optical up-conversion devices

Achieving a high internal quantum efficiency in GaAs∕AlGaAs based light-emitting diodes (LEDs) for room-temperature operation at low current-density injection is crucial for applications such as optical up-converters based on the integration of LEDs and photodetectros. We report the experimental results as well as the theoretical analyses of the internal quantum efficiency of GaAs∕AlGaAs LEDs as a function of the p-doping concentration of the active region for low current injection operation. By optimizing the doping concentration, we have achieved a close to 100% internal quantum efficiency for room-temperature operation of LEDs in the low injection current-density range, i.e., around 0.1A∕cm2. An optical up-converter was fabricated using wafer-fusion technology by integrating the optimized GaAs∕AlGaAs LED with an InGaAs∕InP photodetector. The internal up-conversion quantum efficiency was measured to be 76%.

Terahertz Quantum Well Photodetectors

The terahertz (THz) part of the electromagnetic spectrum promises a wide range of new and novel, some may be disruptive, applications. However, the development of technologies in the THz spectrum or the very far infrared region has been slow mainly because of the lack of convenient detectors and lasers. There are a few competing new approaches for better detectors, and here, we concentrate on one based on quantum wells. We report on the design and simulated performance of quantum-well photodetectors for the terahertz (1-10 THz). Quantum well, barrier, and doping parameters are optimized in terms of operating temperature, absorption, and detectivity. We also report on our experimental demonstration of GaAs/AlGaAs photodetectors with background limited infrared performance. These devices are suited for a variety of applications, especially in conjunction with the newly developed THz quantum cascade lasers. Examples include THz sensing and imaging and free space communication.

Enhanced efficiency in near-infrared inorganic/organic hybrid optical upconverter with an embedded mirror

We report a hybrid organic-inorganic optical upconverter with an embedded mirror, which converts 1.5μm infrared light to visible light. The device was fabricated through direct tandem integration of an organic light-emitting diode with an inorganic InGaAs∕InP photodetector. It was found that the device with an embedded mirror exhibited a low turn-on voltage (∼3.2V) and an enhanced efficiency. The ratio of photocurrent-induced light with an input power density of 0.67mW∕mm2 versus dark-current-induced visible light was over 500 at a device bias of 6V at room temperature. The results show that the embedded mirror at the inorganic-organic interface plays a vital role in the performance enhancement of a hybrid upconverter.

Quantum and transport mobilities in an AlGaAs∕GaAs parabolic quantum-well structure

We study quantum and transport mobilities in a parabolic quantum-well structure when one or more subbands are occupied. We developed an original analytical method to extract the quantum mobility from the multiple occupied subband transport characteristics at low temperature. We tune the carrier density and hence the subband structure of the parabolic quantum well over a wide range by illumination with a red light-emitting diode. In order to obtain the quantum mobilities, Fourier transforms of the first differential of the experimental magnetoresistance traces (ρxx versus magnetic field) are taken and fitted by a conductivity tensor model in the same magnetic field range. We find that both the quantum and transport mobilities increase nonlinearly with increasing carrier density for both the first and second subbands and conclude that the intersubband scattering is predominantly large angle.

Widely tunable self-assembled quantum dot lasers

Quantum dot laser diodes with up to five well-defined electronic shells are fabricated using self-assembled quantum dots (QDs) grown by molecular beam epitaxy. At 77 K, we tune the lasers from the first to the fourth excited state of the QDs by varying the cavity length, this covers a wavelength range from 869 to 963 nm. At room temperature, we obtain lasing from the second to the fourth excited state covering the 938 to 984 nm wavelength range. For high injection currents, a large part of the QD ensemble contributes at once to the stimulated emission yielding a lasing emission linewidth having a full width at half maximum of 25 nm. By increasing the energy spacing between the QD energy level contributing to the lasing and the wetting layer energy levels, improved thresholds at higher temperatures are observed, leading to lasing below 100 A/cm2 at room temperature.Quantum dot laser diodes with up to five well-defined electronic shells are fabricated using self-assembled quantum dots (QDs) grown by molecular beam epitaxy. At 77 K, we tune the lasers from the first to the fourth excited state of the QDs by varying the cavity length, this covers a wavelength range from 869 to 963 nm. At room temperature, we obtain lasing from the second to the fourth excited state covering the 938 to 984 nm wavelength range. For high injection currents, a large part of the QD ensemble contributes at once to the stimulated emission yielding a lasing emission linewidth having a full width at half maximum of 25 nm. By increasing the energy spacing between the QD energy level contributing to the lasing and the wetting layer energy levels, improved thresholds at higher temperatures are observed, leading to lasing below 100 A/cm2 at room temperature.