The main driving force for High Operating Temperature (HOT) detectors is the strong need for low cost, compact IR imaging solution capable of supporting a wide range of military and civilian applications. In the HOT regime where imagers can be cooled with multi-stage thermoelectric coolers, the major portion of the cost is due to the die-level back-end process, from the chip hybridization to final packaging. We present here an approach to achieve significant cost reduction of MWIR imagers by monolithically integrating III-V devices directly on Silicon substrates for wafer-scale fabrication and packaging of focal plane arrays (FPAs). High quality InAs films can be grown on a blanket Silicon wafer by metal-organic chemical vapor deposition (MOCVD) in a low growth temperature regime that complies with the thermal budget of the Si-electronics. High Resolution Transmission Electron Microscopy reveals predominantly oriented, single-crystal-like InAs films, with Σ3(111) twin boundaries, which our band structure calculations predict to be electrically benign. More intriguingly, selective-area growth on SiO2-masked ROIC-like templates is demonstrated with single-crystal-like InAs film nucleation at small Si(001) openings, together with the suppression of unwanted deposition on the dielectric mask. High crystallinity lateral epitaxial overgrowth of the InAs islands and film coalescence is achieved, enabling the potential to fully cover the entire patterned substrate. MBE-grown MWIR devices (λcut-off = 4.1 μm) on blanket InAs/Si templates exhibit a dark current of 2x10-5 A/cm2 , a specific detectivity of 6x1011 Jones and a quantum efficiency (QE) above 60% at 100K. The QE remains constant at high temperatures (<200K) where the dark current approaches that of baseline single-crystal HOT devices grown on native substrates At 230K, it is 6x10-2 A/cm2, yielding a specific detectivity of 1010 Jones.
We have observed that the temperature of the electrons drifting under a relatively-high electric field in an
AlN/GaN-based high-electron-mobility transistor is significantly higher than the lattice temperature (i.e. the
hot electrons are generated). These hot electrons are produced through the Fröhlich interaction between the
drifting electrons and long-lived longitudinal-optical phonons. By fitting electric field vs. electron
temperature deduced from the measurements of photoluminescence spectra to a theoretical model, we have
deduced the longitudinal-optical-phonon emission time for each electron is to be on the order of 100 fs. This
value is consistent with the value measured previously from Raman scattering.
GaN and AlN compounds have been proven useful in wide bandgap microelectronics and optoelectronics. Also
properties of bulk GaN and AlN have been studied extensively. However, many characteristics of AlGaN/GaN
superlattices are not well known. In particular, the properties of phonons have not been determined. In order
to determine phonon properties, this study measured infrared reflectivity spectra on short period superlattices,
which were grown by high quality molecular beam epitaxy. The superlattices consisted of 300 periods of alternating
layers of GaN and AlGaN, each containing between 1 and 8 monolayers. Next, the reflectivity of each sample
was measured using a Bruker IFS-66V spectrometer. From these experimental spectra the dielectric function,
and hence the optical phonon properties (namely phonon frequency and phonon damping), were determined.
Mapping the experimental spectra with theoretical calculations determined the longitudinal and transverse optical
phonon energies present in the AlGaN/GaN superlattices. Through the examination of different AlGaN/GaN
superlattice combinations, plots of phonon energies versus material composition were obtained. Furthermore,
new phonons, that were not present in bulk AlN and GaN, were discovered. Finally, phonon characteristics were
measured as a function of temperature, confirming that phonon energies decrease with increasing temperature.
A conventional p-i-n photodetector designed for absorption in the C-band region has been integrated with a low-cost all-dielectric based filter for single wavelength detection at 1542nm. The dielectric based filter of SiO2 cavity layer sandwiched between two pairs of highly reflecting Si3N4/SiO2 mirrors are deposited by PECVD. The full width at half maximum (FWHM) of the reflectance spectra for the filter is measured to be 9.2nm. Reflectance measurement indicates a transmittivity of 83.6% at 1542nm, while reflecting all other wavelengths from 1400 to 1730nm. Dark-current measurement of the photodetector is in the range of 10-8A at a reverse bias voltage of -2V. A photocurrent enhancement of 4 orders of magnitude, at an incident wavelength of 1542nm for a reverse-bias voltage of -2V is observed.
X-crossing vertical coupler filters may be used as optical add/drop multiplexers (OADM’s) in wavelength-division multiplexing (WDM) systems to select and route different channels. The basic requirement is to effectively suppress the side-lobes and enhance wavelength selectivity, which is a complex function of structure geometry and material dispersion. To understand the mechanisms and optimize X-crossed OADM, we have studied the relationship between the performance of the coupling structure and some of their geometrical parameters such as the width of drop channel, the cladding layer thickness and the angle between two vertical waveguides. In our designed structure, the cladding layer is undoped InP and the input/through channel and drop/add channel are both in the guiding layer of InGaAsP material. The field-coupling phenomena of the coupling in the layered structures are simulated by three-dimension beam propagation method (3-D BPM). Four distinct parameters have been identified to characterize the device performance, namely the peak wavelength, 3dB bandwidth of the main peak (drop channel), side-lobe level, and crosstalk, which is defined as the relation between drop efficiency and the throughput attenuation at resonance. The results show that to obtain a decreasing trend of the peak wavelength, increasing the width of drop channel is the only efficient way. Additionally, we observe the 3dB bandwidth shrinks because of the above variation. Increasing the angle contributes more to the side-lobe level suppression, whereas the cladding layer thickness improves the crosstalk, maximizing it up to 27.22dB. The simulation gives optimization ranges for each geometrical parameter, outside which the intensity profile tends to split into two. This work provides some useful optimization results for designing the X-crossing vertical coupler based OADM.