M. Gao

GaAs/InGaAs quantum well infrared photodetector with a cutoff wavelength at 35 μm

GaAs/InGaAs far-infrared quantum well photodetectors based on a bound-to-continuum intersubband transition with a (zero response) cutoff wavelength of 35 μm are reported. A peak responsivity of 0.45 A/W and detectivity of 6.0×109 cmHz/W at a wavelength of 31 μm and a temperature of 4.2 K have been experimentally achieved. Infrared response was observed at temperatures up to 18 K. A calculated responsivity spectrum using a bound-to-continuum line shape corrected for phonon absorption is fitted to the experimental response. The calculated line shape without absorption gives a cutoff wavelength of 38 μm with a peak responsivity of 0.50 A/W and a detectivity of 6.6×109 cmHz/W at 32 μm.

GaAs/AlGaAs quantum-well photodetector for visible and middle infrared dual-band detection

We present experimental results on quantum-well photodetectors for visible and infrared dual-band detection. Large band gap top contacts were used on a standard GaAs/AlGaAs quantum-well infrared photodetector so that visible light could reach the quantum-well region and be absorbed via interband transitions. Two designs were investigated, using a high Al fraction AlGaAs and a short period GaAs/AlAs superlattice contact layer. The dual-band response spectral regions are 0.55–0.7 and 7–10 μm. Measured responsivities are about 0.7 A/W at 8.3 μm and 0.1 A/W at 0.63 μm under −6 V bias voltage.

Heterojunction wavelength-tailorable far-infrared photodetectors with response out to 70 μm

Results are presented on the performance of a heterojunction interfacial workfunction internal photoemission (HEIWIP) wavelength-tailorable detector. The detection mechanism is based on free-carrier absorption in the heavily doped emitter regions and internal emission across a workfunction barrier caused by the band gap offset at the heterojunction. The HEIWIP detectors have the high responsivity of free-carrier absorption detectors and the low dark current of quantum well infrared photodector type detectors. For a 70±2 cutoff wavelength detector, a responsivity of 11 A/W and a D*=1×1013 cmHz/W with a photocurrent efficiency of 24% was observed at 20 μm. From the 300 K background photocurrent, the background limited performance (BLIP) temperature for this HEIWIP detector was estimated to be 15 K. This HEIWIP detector provides an exciting approach to far-infrared detection.

Efficient GaAs light-emitting diodes by photon recycling

Heterostructure AlGaAs/GaAs light-emitting diodes (LEDs) with a thick active region have shown high external efficiencies, thanks to reabsorption in the active region. For high injection currents and low temperature, we report a 22% efficiency which corresponds to a 98% efficiency internally. We discuss the application of such LED when integrated with a quantum-well infrared photodetector for pixelless thermal imaging systems.

Resonant-cavity-enhanced p-type GaAs/AlGaAs quantum-well infrared photodetectors

Resonant cavities are used to enhance the absorption efficiency in p-type GaAs/AlGaAs quantum-well infrared photodetectors. The cavities are fabricated by applying thick gold films on the detector bottom sides after substrate removal via selective wet etching. The observed peak enhancement and spectral shape are in good agreement with model predictions. Peak absorption of about 25% is obtained for the device studied.

Integration of n-type and p-type quantum-well infrared photodetectors for sequential multicolor operation

A multicolor infrared photodetector based on the epitaxial integration of an n-type with a p-type GaAs/AlGaAs quantum-well stack is experimentally demonstrated. Additionally, a quantum-well GaAs light-emitting diode is inserted between the stacks to achieve up-conversion of mid-infrared radiation to near-infrared signal. This device shows a remarkable selectivity on wavelength: depending on the bias voltage the peak wavelength detection can be switched on and off between 9.1 and 4.85 μm.

Progress on optimization of p-type GaAs/AlGaAs quantum well infrared photodetectors

We report the optimization of barrier thickness and well doping density for GaAs/AlGaAs p-type quantum well infrared photodetectors covering the 3–5 μm wavelength region. We investigated a series of samples with barrier widths varying from 10 to 50 nm and found that the optimum barrier thickness is about 20 nm. For devices operating at about 100 K, the optimum two-dimensional doping density is found to be in the range 1–2×1012 cm−2, which maximizes the background limited infrared performance temperature and dark current limited detectivity.

Optimizing well doping density for GaAs/AlGaAs p-type quantum well infrared photodetectors

Effects of well doping density on the performance of GaAs/AlGaAs p-type quantum well infrared photodetectors are systematically studied. We find that for devices covering the 3–5 μm wavelength region and operating at about 100 K, the optimum two-dimensional doping density is in the range 1–2×1012 cm−2, which maximizes the background limited infrared performance temperature and dark current limited detectivity. Increasing the doping density not only enhances the peak absorption but also broadens the linewidth pronouncedly.

Dual-band photodetectors based on interband and intersubband transitions

We present experimental results on quantum-well photodetectors for visible or near-infrared and middle- or far-infrared dual-band detection. We report on two types of devices based on (1) InGaAs/InP and (2) GaAs/AlGaAs quantum wells. In the first case, InGaAs/InP quantum-well infrared photodetectors (QWIPs) for both near and middle infrared spectra are shown. In the second case, large bandgap top contacts were used on standard GaAs/AlGaAs QWIPs so that visible light could reach the quantum-well region and be absorbed via interband transitions. Two large band gap top contacts were investigated, using a high Al fraction AlGaAs and a short period GaAs/AlAs superlattice. We evaluate and analyze the detector performance. We find that such devices are potentially useful for applications involving dual-band simultaneous detection and imaging.

Electron field emission from diamond-like carbon, a correlation with surface modifications

We report a series of experiments characterizing the emission obtained from near-amorphous carbon deposited by excimer laser ablation. We found that vacuum arc discharge and material transfer are responsible for morphology modifications that greatly enhance emission. When the morphology of the materials are well controlled, we find that our carbon has a work function one half that of silicon.