A. G. Unil Perera

InAs/GaAs p-type quantum dot infrared photodetector with higher efficiency

An InAs/GaAs quantum dot infrared photodetector (QDIP) based on p-type valence-band intersublevel hole transitions as opposed to conventional electron transitions is reported. Two response bands observed at 1.5–3 and 3–10 μm are due to transitions from the heavy-hole to spin-orbit split-off QD level and from the heavy-hole to heavy-hole level, respectively. Without employing optimized structures (e.g., the dark current blocking layer), the demonstrated QDIP displays promising characteristics, including a specific detectivity of 1.8×109 cm·Hz1/2/W and a quantum efficiency of 17%, which is about 5% higher than that of present n-type QDIPs. This study shows the promise of utilizing hole transitions for developing QDIPs.

A multicolor, broadband (5–20 μm), quaternary-capped InAs/GaAs quantum dot infrared photodetector

An InAs/GaAs quantum dot infrared photodetector with strong, multicolor, broadband (5–20 μm) photoresponse is reported. Using a combined quaternary In0.21Al0.21Ga0.58As and GaAs capping that relieves strain and maintains strong carrier confinement, we demonstrate a four color infrared response with peaks in the midwave- (5.7 μm), longwave- (9.0 and 14.5 μm), and far- (17 μm) infrared regions. Narrow spectral widths (7% to 9%) are noted at each of these wavelengths including responsivity value ∼95.3 mA/W at 14.5 μm. Using strain field and multi-band k⋅p theory, we map specific bound-to-bound and bound-to-quasibound transitions to the longwave and midwave responses, respectively.

Dielectric function model for p-type semiconductor inter-valence band transitions

The contributions of inter-valence band (IVB) transitions to the dielectric function (DF) by free holes among the split-off (so), light-hole (lh) and heavy-hole (hh) bands were investigated. A model was developed to determine the DF of two p-type semiconductors, GaAs and Ge1−y Sny with the Zinc-blend and Diamond crystal structures, respectively. The IVB transitions dominate the spectral range between 0.1–1eV with respect to the spin-orbit splittings between so-hh and lh-hh bands. In conjunction with inter-band transitions, free-carrier and lattice absorption, a complete DF model allows the determination of optical constants with improved accuracy in the spectral range covering both ultraviolet and infrared regions. The model should be applicable to most of the group III-V and IV materials since their valence band structures resemble the ones under investigation.

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.

Effect of emitter layer concentration on the performance of GaAs p/sup +/-i homojunction far-infrared detectors: a comparison of theory and experiment

The performance of GaAs multilayer (p/sup +/-i-p/sup +/-i-...) homojunction interfacial workfunction internal photoemission (HIWIP) far-infrared (FIR) detectors as a function of emitter layer (p/sup +/) concentration is reported. The dark current characteristics have been investigated and compared with a model which includes the space charge, tunneling, and multiple-image-force effects. The experimentally determined detector cutoff wavelength is found to be in reasonable agreement with the high density (HD) theory. The detector responsivity follows well the quantum efficiency predicted by scaling the free carrier absorption coefficient linearly with the doping concentration. All these comparisons are necessary to design and optimize GaAs HIWIP FIR detectors.

Study of valence-band intersublevel transitions in InAs/GaAs quantum dots-in-well infrared photodetectors

The n-type quantum dot (QD) and dots-in-well (DWELL) infrared photodetectors, in general, display bias-dependent multiple-band response as a result of optical transitions between different quantum levels. Here, we present a unique characteristic of the p-type hole response, a well-preserved spectral profile, due to the much reduced tunneling probability of holes compared to electrons. This feature remains in a DWELL detector, with the dominant transition contributing to the response occurring between the QD ground state and the quantum-well states. The bias-independent response will benefit applications where single-color detection is desired and also allows achieving optimum performance by optimizing the bias.

Heterojunction plasmonic midinfrared detectors

In view of the emergence of wide ranging applications in the areas such as environmental monitoring, medical diagnostics, defense, security and sensing etc., it is indispensable to develop resourceful mid-infrared photodetectors. In this article, we present potential design considerations exploiting plasmonic effects in the conventional heterojunction mid-infrared detectors, optimized for their operation in 8–14 μm spectral range. Design issues concerning GaAs-AlGaAs based plasmonic photodetectors are investigated using modal expansion method (MEM) in conjunction with Rayleigh expansion. Simple but insightful fitting expressions useful for designing practical photodetectors are presented. The effects of crucial design parameters on the photodetector performance are discussed in detail. Using metallic grating based plasmonic element, about 20 fold absorption enhancement is predicted, which is comparable or greater than that recently reported for InAs (Quantum Dots) and GaInNAs (Quantum Well) detectors. Pho...

Effect of quantum dot size and size distribution on the intersublevel transitions and absorption coefficients of III-V semiconductor quantum dot

The intersublevel absorption peak energy and absorption coefficient of non-uniform quantum dot (QD) ensembles are calculated analytically. The effect of size variations and size distribution of QDs on their energy states is analyzed. The dots are considered as a quantum box with finite potential at the barriers and the size distribution described by a Gaussian function. The influence of the aspect ratio (base to height ratio) of the QDs on the optical transitions is studied. Our model predicts the dot size (height and base) accurately to determine the absorption peaks and corresponding absorption coefficient. We also compute the absorption coefficient of the QD with different size distributions to verify the results calculated using this model with the reported experimental and other theoretical results.

Band offsets and carrier dynamics of type-II InAs/GaSb superlattice photodetectors studied by internal photoemission spectroscopy

We use internal photoemission spectroscopy to determine the conduction band offset of a type-II InAs/GaSb superlattice (T2SL) pBp photodetector to be 0.004 (±0.004) eV at 78 K, confirming its unipolar operation. It is also found that phonon-assisted hole transport through the B-region disables its two-color detection mode around 140 K. In addition, photoemission yield shows a reduction at about an energy of longitudinal-optical phonon above the threshold, confirming carrier-phonon scattering degradation on the photoresponse. These results may indicate a pathway for optimizing T2SL detectors in addition to current efforts in material growth, processing, substrate preparation, and device passivation.

Early detection of cell activation events by means of attenuated total reflection Fourier transform infrared spectroscopy

Activation of Jurkat T-cells in culture following treatment with anti-CD3 (Cluster of Differentiation 3) antibody is detectable by interrogating the treated T-cells using the Attenuated Total Reflection–Fourier Transform Infrared (ATR-FTIR) Spectroscopy technique. Cell activation was detected within 75 min after the cells encountered specific immunoglobulin molecules. Spectral markers noted following ligation of the CD3 receptor with anti CD3 antibody provides proof-of-concept that ATR-FTIR spectroscopy is a sensitive measure of molecular events subsequent to cells interacting with anti-CD3 Immunoglobulin G. The resultant ligation of the CD3 receptor results in the initiation of well defined, specific signaling pathways that parallel the measurable molecular events detected using ATR-FTIR. Paired t-test with post-hoc Bonferroni corrections for multiple comparisons has resulted in the identification of statistically significant spectral markers (p < 0.02) at 1367 and 1358 cm−1. Together, these data demonst...