Margaret Buchanan

Correlation of proton radiation damage in InGaAs-GaAs quantum-well light-emitting diodes

The effect of proton irradiation of InGaAs/GaAs quantum-well (QW) light-emitting diodes (LEDs) has been studied at energies ranging from 1 to 500 MeV in order to determine device damage mechanisms. The data are analyzed in terms of the theory of Rose and Barnes (1982), and complete correlation of the data over the entire proton energy range was achieved. This degradation data, along with data from other GaAs-based optoelectronic devices, are discussed in terms of the nonionizing energy loss (NIEL). The energy dependences of the various damage coefficients for proton energies greater than about 10 MeV are bounded by the total NIEL and the elastic NIEL.

Simultaneous detection of ultraviolet and infrared radiation in a single GaN/GaAlN heterojunction

Results are presented for a dual-band detector that simultaneously detects UV radiation in the 250-360 nm and IR radiation in the 5-14 microm regions with near zero spectral cross talk. In this detector having separate UV- and IR-active regions with three contacts (one common contact for both regions) allows the separation of the UV and IR generated photocurrent components, identifying the relative strength of each component. This will be an important development in UV-IR dual-band applications such as fire-flame detection, solar astronomy, and military sensing, eliminating the difficulties of employing several individual detectors with separate electronics-cooling mechanisms.

n-Type GaAs/AlGaAs heterostructure detector with a 3.2 THz threshold frequency.

Terahertz detection using the free-carrier absorption requires a small internal work function of the order of a few millielectron volts. A threshold frequency of 3.2 THz (93 microm or approximately 13 meV work function) is demonstrated by using a 1 x 10(18) cm(-3) Si-doped GaAs emitter and an undoped Al(0.04)Ga(0.96)As barrier structure. The peak responsivity of 6.5 A/W, detectivity of 5.5 x 10(8) Jones, and quantum efficiency of 19% were obtained at 7.1 THz under a bias field of 0.7 kV/cm at 6 K, while the detector spectral response range spans from 3.2 to 30 THz.

Pixelless 1.5-/spl mu/m up-conversion imaging device fabricated by wafer fusion

We designed and fabricated a first-ever pixelless optical up-conversion imaging device using wafer-fusion technology. The device consists of an In/sub 0.53/Ga/sub 0.47/As-InP p-i-n detector and a GaAs-AlGaAs light-emitting diode (LED), which were grown on an InP and a GaAs substrate, respectively, and wafer-bonded together. The layer structures and doping profiles of the common region linking the detector and LED were designed such that lateral carrier diffusion was successfully suppressed while effective electrical connection was well preserved. Pixelless up-conversion imaging from 1.5 to 0.87 /spl mu/m was demonstrated. Moreover, an internal electrical gain of over 100 was observed for the detector part of the integrated device. The internal up-conversion quantum efficiency was measured to be /spl sim/50% at room temperature.

Near-room-temperature mid-infrared quantum well photodetector.

We demonstrate InGaAs mid-infrared quantum well infrared photodetectors (MIR PV-QWIPs) that enable cost-effective mature GaAs-based detection and imaging technologies, with exceptional material uniformity, reproducibility, and yield, over a large area, with high spectral selectivity, innate polarization sensitivity, radiation hardness, high detectivity, and high speed operation at TEC temperatures without bias.

High-resolution surface-emitting spectrometer and deformation sensors with nonlinear waveguides.

We investigate the limits of frequency resolution attainable in a nonlinear waveguide optical spectrometer, including the effects that are due to surface distortions and waveguide inhomogeneities, and demonstrate that the frequency-resolving capability is directly scalable with the radiating aperture length. The resolution of the waveguide is diffraction limited, and therefore the far-field radiation pattern can be used to characterize the phase variations along the waveguide that are due to surface distortions. The use of this device as a highly sensitive deformation sensor is demonstrated by application of a distortion to the waveguide and confirmation of the far-field diffraction pattern generated.

Novel Si structures for photonic applications

Although silicon is the paramount material for the microelectronic industry, bulk Si is of little use for photonic devices owing to its indirect band-gap, which prevents the all-important direct optical transitions. However, a new type of luminescent Si has opened up its future for photonic applications. This new material is light-emitting SiO2/Si superlattices, fabricated in our laboratory. Our theoretical calculations showed that the energy band within the silicon layer has direct bandgap character, a result of strong quantum-confinement caused by the large band-offset at the SiO2/Si interface, so that the direct optical transition is not only possible but also vigorous. For a quantum-confined amorphous silicon, the breakdown of angular momentum will naturally make all optical transition possible. Our experiments have shown that SiO2/Si superlattices can indeed emit bright light. Moreover, the band-gap or the wavelength can be tuned over the visible range by changing the Si layer thickness, in good agreement with quantum confinement theory. The luminescence intensity as a function of Si layer thickness is found to increase, reach a maximum, and then decrease. Theoretical studies show that this phenomena is caused by competition between an increased overlap of electron-hole wave functions in the normal direction to the quantum well and an increased exciton radius in the plane of the quantum well.

Comparative study of laser- and ion implantation-induced quantum well intermixing in GaInAsP/InP microstructures

Laser-induced quantum well intermixing (laser-QWI) and ion implantation-induced quantum well intermixing (II-QWI) techniques have been studied to selectively modify the optical properties of GaInAsP/InP laser microstructures. Following the annealing with a cw Nd:YAG laser, a blue shift in the quantum well photoluminescence of up to 124 nm was observed for samples annealed up to 4 min. A comparison of the laser annealing results with those of II-QWI, which were obtained for the same GaInAsP/InP microstructure, indicates that laser- QWI yields material with comparable, or better optical properties. The one-step processing used in the laser-QWI approach makes it an attractive alternative in fabricating photonic integrated circuits at low cost.

Transparent waveguides for WDM transmitter arrays using quantum well shape modification

A technique for fabricating transparent waveguides on the same wafer as a quantum well (QW) DBR laser array has been developed. High [MeV] energy ion implantation is used to create a large number of vacancies and interstitials throughout the active region of the device. Upon annealing, these entities enhance the intermixing of the QW and barrier materials resulting in a blue shift of the QW bandgap. Energy shifts (measured using low temperature photoluminescence spectroscopy) of greater than 60 meV can be achieved. Room temperature waveguide absorption measurements verify the shift in the bandgap energy and confirm that the waveguide is now effectively transparent in the wavelength range of the QW lasers. This technique is being used in an eight wavelength WDM transmitter array in which the waveguiding region is selectively implanted and blue shifted.

GaAs/AlGaAs p-type multiple quantum wells for infrared detection at normal incidence: model and experiment

The development of devices for mid-, long-, and very long- wavelength IR detection has benefitted greatly from advances in band-gap engineering. Recently, there has been great progress in the development of n-type GaAs/AlGaAs quantum well infrared photoconductor (QWIP) detector arrays in all three technologically important wavelength windows. P-type GaAs.AlGaAs QWIPS represent a viable alternative to n-type GaAs/AlGaAs QWIPs, offering the advantage of normal incidence absorption without the need for grating couplers. The maturity of the MBE of GaAs/AlGaAs layered materials offers the possibility of mass producing low cost, high performance, large size, high uniformity, multicolor, high frequency bandwidth, two-dimensional imaging QWIP arrays. This paper describes progress in optimizing the performance of p- type GaAs/AlGaAs QWIPs through modeling, growth, and characterization. Using the 8x8 envelope-function approximation (EFA), a number of structures were designed and their optical absorption calculated for comparison with experiment. Samples were grown by MBE based on the theoretical designs and their photoresponse measured. P-type QWIPs were optimized with respect to the well and barrier widths, alloy concentration, and dopant concentration; resonant cavity devices were also fabricated and temperature dependent photoresponse was measured. The quantum efficiencies and the background-limited (BLIP) detectivities under BLIP conditions of our own p-QWIPs are comparable to those of n-QWIPs; however, the responsivities are smaller. For our mid-IR p-QWIPs, the 2D doping densities of 1- 2x1012 cm-2 maximized the BLIP temperature and dark current limited detectivity by operating at around 100K. At 80K, the detectivity of the optimum doped sample was (formula available in paper)at 10V bias. Barrier widths greater than 200 A were sufficient to impede the tunneling dark current; resonant cavities enhanced absorption five-fold.