Epitaxial designs for maximizing efficiency in resonant tunnelling diode based terahertz emitters

We discuss the modelling of high current density InGaAs/AlAs/InP resonant tunneling diodes to maximize their efficiency as THz emitters. A figure of merit which contributes to the wall plug efficiency, the intrinsic resonator efficiency, is used for the development of epitaxial designs. With the contribution of key parameters identified, we analyze the limitations of accumulated stress to assess the manufacturability of such designs. Optimal epitaxial designs are revealed, utilizing thin barriers, with a wide and shallow quantum well that satisfies the strained layer epitaxy constraint. We then assess the advantages to epitaxial perfection and electrical characteristics provided by devices with a narrow InAs sub-well inside a lattice-matched InGaAs alloy. These new structures will assist in the realization of the next-generation submillimeter emitters

Characterisation of High Current Density Resonant Tunneling Diodes for THz Emission Using Photoluminescence Spectroscopy

We discuss the numerical simulation of high current density InGaAs/AlAs/InP resonant tunneling diodes with a view to their optimization for application as THz emitters. We introduce a figure of merit based upon the ratio of maximum extractable THz power and the electrical power developed in the chip. The aim being to develop high efficiency emitters as output power is presently limited by catastrophic failure. A description of the interplay of key parameters follows, with constraints on strained layer epitaxy introduced. We propose an optimized structure utilizing thin barriers paired with a comparatively wide quantum well that satisfies strained layer epitaxy constraints

Distribution and Endocrine Morphology of Polypeptide YY (PYY) Containing Cells in the Human Gut

Using human materials, the distribution of PYY containing cells was determined by immunocytochemical methods and discussion was made on their morphological endocrinology. PYY cells were fairly numerous in the lower gastrointestinal tract of man, particularly in the colon and rectum. The cells were also present in the pancreas and duodenum but quite rarely. PYY cells were not observed at all in the lower part of the esophagus, stomach and gall bladder. Their peculiar and characteristic shapes as well as distribution suggest that PYY may have some action (probably specific) on the function of the distal gastrointestinal tract

Non-destructive characterization of thin layer resonant tunnelling diodes

We present an advanced nondestructive characterization scheme for high current density AlAs/InGaAs resonant tunneling diodes pseudomorphically grown on InP substrates. We show how low-temperature photoluminescence spectroscopy (LT-PL) and high-resolution X-ray diffractometry (HR-XRD) are complementary techniques to increase the confidence of the characterized structure. The lattice-matched InGaAs is characterized and found to be of high quality. We discuss the inclusion of an undoped “copy” well (C-well) in terms of enhancements to HR-XRD and LT-PL characterization and quantify the improved precision in determining the structure. As a consequence of this enhanced precision in the determination of physical structure, the AlAs barriers and quantum well (QW) system are found to contain nonideal material interfaces. Their roughness is characterized in terms of the full width to half-maximum of the split LT-PL emission peaks, revealing a ±1 atomic sheet variance to the QW width. We show how barrier asymmetry can be detected through fitting of both optical spectra and HR-XRD rocking curves

Fabrication, Characterisation, and Epitaxial Optimisation of MOVPE-Grown Resonant Tunnelling Diode THz Emitters

Resonant tunnelling diodes (RTDs) are a strong candidate for future wireless communications in the THz region, offering compact, room-temperature operation with Gb/s transfer rates. We employ the InGaAs/AlAs/InP material system, offering advantages due to high electron mobility, suitable band-offsets, and low resistance contacts. We describe an RTD emitter operating at 353GHz, radiating in this atmospheric transmittance window through a slot antenna. The fabrication scheme uses a dual-pass technique to achieve reproducible, very low resistivity, ohmic contacts, followed by accurate control of the etched device area. The top contact connects the device via the means of an air bridge. We then proceed to model ways to increase the resonator efficiency, in turn improving the radiative efficiency, by changing the epitaxial design. The optimization takes into account the accumulated stress limitations and realities of reactor growth. Due to the absence of useful in-situ monitoring in commercially-scalable metal-organic vapour phase epitaxy (MOVPE), we have developed a robust non-destructive epitaxial characterisation scheme to verify the quality of these mechanically shallow and atomically thin devices. A dummy copy of the active region element is grown to assist with low temperature photoluminescence spectroscopy (LTPL) characterisation. The resulting linewidths limits the number of possible solutions of quantum well (QW) width and depth pairs. In addition, the doping levels can be estimated with a sufficient degree of accuracy by measuring the Moss-Burstein shift of the bulk material. This analysis can then be combined with high resolution X-ray diffractometry (HRXRD) to increase its accuracy

Resonant Tunnelling Diodes for Next Generation THz Systems

Resonant tunnelling diodes (RTDs) are a strong candidate for future wireless communications in the THz spectrum (sub-millimetre waves), offering compact, roomtemperature operation with the potential to exceed the bit transfer rate mandated by the 12G-SDI standard, using a single wireless link. A free-space RTD emitter operating at 353GHz is described. The fabrication process consists of a dual-pass I-line photolithography & etch technique using an air bridge, allowing low resistivity ohmic contacts, and accurate control of desired device area. With extrinsic circuit elements taken into account, the intrinsic semiconductor efficiency is analysed to investigate structural improvements for radiative efficiency. Such optimised structures are presented, and then characterised after being epitaxially grown with commercially viable metal-organic vapour phase epitaxy (MOVPE) reactors. A combination of low temperature photoluminescence spectroscopy, X-Ray diffractometry, and transmission electron microscopy attest the quality of the new material. We end with a suggestion for the next steps to exceed technological readiness levels of 8, and use monolithic RTD emitters as components in new systems

Fitting of photoluminescence spectra for structural characterisation of high current density resonant tunnelling diodes for THz applications

High-resolution X-ray diffraction (HR-XRD), and low-temperature photoluminescence spectroscopy (LT-PL) are used to investigate the structural properties and inhomogeneities of high current density InGaAs/AlAs/InP resonant tunnelling diode (RTD) wafer structures. The non-destructive assessment of these structures is challenging, with structural variables: well and barriers thickness and the well indium molar fraction, in addition to electronic variables such as the band-offsets being functions of strain, growth sequence, etc.. Experimental PL data are compared with simulations allowing the deconvolution of the PL spectra, that includes Type I and Type II transitions broadened by interface fluctuations on length scales smaller and much larger than the exciton. This method provides details of the non-uniformity of the epitaxial material nondestructively

Clinical Statistics for Dysphagia Patients ≦ 18 Years of Age in the Center of Special Needs Dentistry, April 2012-March 2013

In April 2012, the Center of Special Needs Dentistry (SND) was established at Showa University Dental Hospital to provide function training for children with eating and swallowing disorders. A statistical clinical assessment was performed on new patients ≤18 years of age who visited the Center over a 1-year period (April 2012–March 2013) to assess the conditions present at the initial visit. In all, 60 patients (29 boys, 31 girls, mean (± SD) age 4.2±4.1 years, range 0-18 years of age) were included in the study. Most patients were <1 year of age (32%) and most came from one of four cities in the Johnan area (Shinagawa City, Meguro City, Ota City and Setagaya City). The most common primary diseases at the initial visit were cerebral palsy and cleft lip and palate. The third largest patient group was of healthy children with oral function problem. Over 60% of patients attended the Center of SND because of an eating-related complaint. More than 50% of patients were obtaining nutrients via oral intake; the remaining patients were obtaining nutrients via non-oral or a combination of oral and non-oral intake. Because of the young age of the patients and the fact that most were from neighboring areas, it can be inferred that effective community health care is being provided. It is necessary for the Center of SND to continue to provide professional treatment for dysphagia and to contribute to community medicine