Amorphous hydrogenated carbon-germanium films for hard multilayer IR optical coatings.

Diamondlike amorphous hydogenated carbon (a-C:H) films have found widespread use as hard coatings for IR optical applications. The refractive index of these films (n=2) allows the deposition of quarter-wave antireflection coating on germanium. More complex optical designs, however, require the extension of the range of the optical constants. For this reason we studied amorphous hydrogenated carbon-germanium alloys. The corresponding films with 0 smaller than X smaller than 1 were grown by rf-plasma deposition using mixtures of a hydrocarbon gas and germane or tetramethylgermanium (TMG) as precursors. Film composition, refractive index and IR absorption spectra were measured. This material system offers the possibility to adjust the refractive index between 1:8 and 4:1 and thus allows the deposition of multilayer thin film systems for IR optical applications. As a result of numerical design calculations, a three layer structure with a hard a-C:H top layer is propsed, which acts as a bro adband antireflection coating on germanium substrates for 8-12 mym. We report on the growth and performance of these hard multilayer AR coatings

Influence of the recharging process on the dark current noise in quantum-well infrared photodetectors

We show that the dark current noise spectrum of an In(0.30)Ga(0.70)As/GaAs quantum-well infrared photodetector (QWIP) is characterized by two plateau-like frequency regions. At high frequencies, the observed noise current is due to the generation-recombination noise of carriers, which have been emitted thermionically from the quantum wells into the continuum. In the low-frequency regime, an additional contribution to the noise current is caused by the redistribution of space charges, that occurs on a time scale similar to the dielectric relaxation time

Compositional and structural analysis of AlSb (As) tunneling barriers in InAs/AlSb (As) / GaSb resonant interband-tunneling structures

InAs/AlSb/GaSb/AlSb/InAs interband-tunneling structures have been analyzed with respect to the composition and structural quality of the AlSb tunneling barriers. The addition of AlAs monolayers at the interfaces between the AlSb barriers and the InAs and GaSb layers was found to result in the expected reduction in the valley current density of the resonant interband-tunneling diode. Vibrational mode Raman spectroscopy showed that the introduction of AlAs monolayers led to the formation of pseudoternary Al(SbAs) barriers, which cause the observed reduction of the valley current density. Ellipsometric measurements indicate that the structural quality of both types of barrier layers, with and without AlAs monolayers added to the interfaces, is inferior to that of thick bulklike AlSb layers. The observation of Raman scattering from a coupled hole plasmon-phonon mode indicates the formation of a hole gas in the GaSb quantum well at the center of the tunneling structure

Ultrasensitive femtosecond two-photon detector with resonantly enhanced nonlinear absorption

NRC publication: Ye

Two-photon QWIPs for quadratic detection of weak mid-infrared laser pulses

NRC publication: Ye

High-speed photocurrent in quantum well infrared photodetectors

Quantum well infrared photodetectors (QWIPs) are very promising for ultrafast photodetection in the 8- 12 mu m infrared regime. We report on time-resolved studies of the intersubband photocurrent in GaAs/AlGaAs (QWIPs). The photocurrent is excited by sub-picosecond infrared pulses, which are obtained by difference frequency mixing of the signal and idler waves of an optical parametric oscillator. Using a particular detector geometry with < 900 mu m (exp 2) device area, the measured electrical pulses have a full-width at half-maximum of only 16 ps and a 10-90 per cent rise time of 13 ps. For practical applications, in particular for heterodyne detection, it is desirable to reduce the noise floor by using a cooled low-noise preamplifier. We have performed experiments with a low-temperature hybrid circuit consisting of a QWIP in connection with a GaAs transimpedance amplifier based on HEMT technology. This configuration yields a rise time of 19 ps at an amplifier gain of approximately 35 dB