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Epidemiologic data regarding health care acquired pneumonia (HAP) and ventilator-associated pneumonia (VAP) from Nepal are negligible. We conducted a prospective observational cohort study in the intensive care unit (ICU) of a major tertiary hospital in Nepal between April 2016 and March 2018, to calculate the incidence of VAP, and to describe clinical variables, microbiological etiology, and outcomes. Four hundred and thirty-eight patients were enrolled in the study. Demographic data, medical history, antimicrobial administration record, chest X-ray, biochemical, microbiological and haematological results, acute physiology and chronic health evaluation II score and the sequential organ failure assessment scores were recorded. Categorical variables were expressed as count and percentage and analyzed using the Fisher’s exact test. Continuous variables were expressed as median and interquartile range and analyzed using Kruskal-Wallis rank sum test and the pairwise Wilcoxon rank—sum test. 46.8% (205/438) of the patients required intubation. Pneumonia was common in both intubated (94.14%; 193/205) and non-intubated (52.36%; 122/233) patients. Pneumonia developed among intubated patients in the ICU had longer days of stay in the ICU (median of 10, IQR 5–15, P< 0.001) when compared to non-intubated patients with pneumonia (median of 4, IQR 3–6, P< 0.001). The incidence rate of VAP was 20% (41/205) and incidence density was 16.45 cases per 1,000ventilator days. Mortality was significantly higher in patients with pneumonia requiring intubation (44.6%, 86/193) than patients with pneumonia not requiring intubation (10.7%, 13/122, pKlebsiella and Acinetobacter species were the dominant organisms from both VAP and non-VAP categories. Multi-drug resistance was highly prevalent in bacterial isolates associated with VAP (90%; 99/110) and non-VAP categories (81.5%; 106/130). HAP including VAP remains to be the most prevalent hospital-acquired infections (HAIs) at Patan hospital. A local study of etiological agents and outcomes of HAP and VAP are required for setting more appropriate guidelines for management of such diseases.

Compliant substrates allow a new approach to the growth of strained epitaxial layers, in which part of the strain is accommodated in the substrate. In this article, compliant substrates are discussed and a new compliant substrate technology based on bonded thin film substrates is introduced. This technology has several advantages over previously published methods, including the ability to pattern both the top and bottom of the material. A new concept enabled by this compliant substrate technology,strain-modulated epitaxy, will be introduced. Using this technique, the properties of the semiconductor material can be controlled laterally across a substrate. Results of two experiments are presented in which low composition InxGa1−xAs was grown by molecular beam epitaxy on GaAs compliant substrates at thicknesses both greater than and less than the conventional critical thickness. It was found that for t > tc, there was an inhibition of defect production in the epitaxial films grown on the compliant substrates as compared to those grown on conventional reference substrates. For t < tc, photoluminescence and x-ray diffraction show the compliant substrates to be of excellent quality and uniformity as compared to conventional substrates.

Three-dimensionally confined GaAs/AlGaAs and InAs/GaAs structures on 〈100〉 oriented square mesas patterned onto GaAs(001) substrates are realized, in-situ, via size-reducing molecular beam epitaxy. Two stages of mesa top pinch-off involving ∼ {103} and subsequently {101} side facets are revealed. GaAs and InAs quantum boxes with lateral linear dimensions down to 40 nm and confined by AlGaAs and GaAs, respectively, are reported. For InAs, the strain relief in mesas is found to enhance the well known ∼ 2 ML thickness for three-dimensional island formation on unpatterned substrates to, remarkably, > 5 ML for mesa size ∼ 75 nm. Cathodoluminescence emission from the InAs on the mesa top attests to its good optical quality.

The relation between the exciton linewidth and the quantum well width is studied for various In compositions in InGaAs/GaAs quantum wells. A region of minimum exciton linewidth is observed. Thick multiple quantum wells, grown in this minimum linewidth region, show sharp excitonic features when grown under RHEED determined optimized growth conditions. Ultrathick strained multiple quantum wells with good optical quality are realized by exploiting strain relief via growth on pre-patterned substrates.

Initial stages of molecular beam epitaxial (MBE) growth of highly mismatched InxGa1-xAs/GaAs(100) have been studied by planar and cross-sectional transmission electron microscopy. For In0.5Ga0.5As growth, we find drastic differences in morphology obtained by reducing the growth temperature form 475 to 420°C. We also observe differences in morphology between alloy growth and short period superlattice ((InAs)n/(GaAs)m (m = 1 monolayer, n = 2 monolayers) growth of equivalent effective composition. In the case of growth by formation of large islands, we present direct evidence of strain relief at the island edges and discuss defect formation in these islands.

A transmission electron microscope study of the twin structure in thin GaAs films grown by molecular‐beam epitaxy on (111)B GaAs reveals the twinned region to be composed of three distinct crystallites which are single‐ and double‐twin variations of the bulk crystal. A three‐dimensional structure of the twin region involving variants of these crystallites, as required by the threefold symmetry of the (111)B plane, is proposed.

We report the first realization on nonplanar patterned substrates of optically active three‐dimensionally confined semiconductor volumes created in situ via a one‐step molecular beam epitaxial growth. Growth is carried out on pyramidal mesas on (111)B substrates and in a regime that results in the emergence of three equivalent {110} side facets which overtake the as‐patterned {100} side facets and lead to mesa pinch‐off. Transmission electron microscopy along with spatially and spectrally resolved cathodoluminescence provide evidence for emission from laterally confined regions with lateral linear dimensions ≲100 nm.

The defect distribution in a highly strained In(0.2)Ga(0.8)As/GaAs multiple-quantum-well (MQW) structure grown on a patterned GaAs substrate is examined with cathodoluminescence imaging and spectroscopy in the near IR. By spatially correlating the luminescence arising from the MQW exciton recombination (950 nm) with the longer wavelength (1000-1200 nm) luminescence arising from the defect-induced recombination, it is demonstrated that it is possible to determine the regions of highest film quality in both the mesa and valley regions. The present approach enables a judicious determination of the optimal regions to be used for active pixels in InGaAs/GaAs spatial light modulators.

Mesas defined on GaAs(111)B substrates by photolithography followed by wet chemical etching are found to be triangular pyramids with truncated tops. The areal dimensions of the unetched tops of the pyramidal mesas diminish with etch time and can be reduced to an apex on continued etching. Transmission electron microscopy is used to examine a post‐growth etched GaAs/Al0.3Ga0.7As multilayer structure grown on GaAs(111)B. A three‐dimensionally confined GaAs volume of the order of 105 atoms is observed.

We report the realization of both high contrast ratio (66:1) and dynamic range (30%) at room temperature in a strained GaAs/InGaAs(100) multiple quantum well based asymmetric Fabry–Perot reflection modulator. The p‐i‐n configuration modulator also acts as a photodetector and exhibits a high quantum efficiency (∼80%).

Growth control of GaAs epilayers with specular surface, free of superficial pyramid‐shape features and bulk twins, is achieved on a nonmisoriented GaAs(111)B substrate via in situ, real time measurement of specular beam intensity of reflection high‐energy electron diffraction (RHEED). Regimes of growth conditions are identified in terms of the static surface phase diagram and the temporal RHEED intensity behavior during growth, thus affording the possibility to realize a reproducible control independent of growth systems.

We report on the optical absorption and modulation characteristics of strained InxGa1−xAs/GaAs(0.1≤x≤0.25) multiple quantum well (MQW) structures grown on GaAs (100) substrates which also included regions of prepatterned mesas. Sharp excitonic features were realized in samples optimally grown by employing reflection high‐energy electron diffraction. High optical quality MQW structures as thick as 1.5 μm are realized. To date, the highest modulation per unit field at low fields (

Growth of low defect density highly strained InxGa1−xAs/GaAs multiple quantum well (MQW) structures of thicknesses suited for application in spatial light modulator (SLM) devices operating in infrared has been thwarted by the occurrence of strain‐induced defects. Exploiting the notion of strain relief at mesa edges, we report here the first realization of very low defect density MQW structures of thickness as high as 2.38 μm at x=0.20. This has opened up the possibility of realizing a variety of reflective and transmissive SLM structures which also fruitfully exploit the transparent nature of the substrate.

A transmission electron microscope study of GaAs grown on Si tilted 4° off (100) has for the first time revealed that of the two twin variants propagating to the GaAs film surface, a substrate tilt about a 〈011〉‐type direction favors one over the other.

CD SEM (two dimension X-Y scan) is usually calibrated using X- SEM (two dimension X-Z scan). However, CD uncertainty associated with line roughness, location error (large scale), ruler measurement error and X-SEM scale error could be as large as 20 nm. In recent years, AFM (three dimension X-Y-Z scan) started to get people's attention for its non- destructive cross section scans and has been used as a calibration tool for CD SEM. The focus of the study is AFM and X-SEM correlation. Two types of CD AFM tip shape (sharp corner type and smooth corner type) are identified based on SEM photos. For line width measurements in our study, for one AFM tip, 50 nm has to be added to the tip width calibrated from the built-in calibration standard (Silicon Nano-Edge) in order to match X-SEM line widths. A limitation of AFM in resist foot detection was also found for the same tip. An attempt was made to explain the limitation based on sharp corner tip shape. Based on the explanation, a desirable tip shape calibration procedure is proposed for tip type identification. Since AFM tip width is the tip scan length (or time) dependent, AFM tip calibration is critical for CD SEM calibration.

The potential of realizing quantum box structures via in-situ molecular beam epitaxical growth on the (111)B face of GaAs is explored. Growth is carried out on patterned (111)3B substrates with an array of truncated triangular pyramidal mesas. The mesa tops with arbitrarily small areal dimensions are used as templates for quantum box realization. Under appropriate conditions of growth along with the attendant interfacet migration, a mesa top growth profile characterized by monotonically shrinking lateral dimensions resulting in 'pinch-off is achieved.

We report the realization of all-optical photonic switches using strained InGaAs/GaAs multiple quantum well based inverted cavity asymmetric Fabry-Perot reflection modulators monolithically integrated with GaAs/AlGaAs based heterojunction phototransistors. The photonic switches show both bistable and non-bistable switching behavior with a contrast ratio of 12:1 and optical gain of 2 to 4 dB. The design and growth considerations for such an integrated structure are also discussed.

In a thick strained GaAs/InGaAs MQW we recently reported the unusual observation of the exciton linewidth initially narrowing upon application of a reverse bias, before the usually observed broadening set in with further increase in the bias. The phenomena suggested the existence of a spatially varying electric field in the MQW region arising from a depletion of the net charge density with increasing reverse bias. Here we provide an explanation for the unusual observation arrived at through a systematic examination of the sample behavior using electro-transmission, electro-photoluminescence, capacitancevoltage profiling and transmission electron microscopy. We conclude that shallow levels cannot account for the observation and that the presence of strain induced point defect related deep levels (either n of p type) offers a consistent explanation. This is the first clear manifestation of the influence of deep levels on the free exciton electroabsorption behavior and has practical implications for MQW based electroabsorptive / electrorefractive light modulators.

We report the realization of high quality strained InGaAs/GaAs multiple quantum wells (MQW) grown on planar GaAs (100) substrates through optimization of molecular beam epitaxical (MBE) growth conditions and structure. Such MQWs containing ∼ 11% In have lead to the realization of an asymmetric Fabry-Perot (ASFP) reflection modulator with a room temperature contrast ratio of 66:1 and an on-state reflectivity of 30%. For In composition ≥ 0.2, the improved optical quality for very thick (gt;2μm) InGaAs/GaAs MQWs grown on pre-patterned substrates is demonstrated via transmission electron microscopy (TEM) and micro-absorption measurements.

We present results for lattice matched (AlxGai..xAs, x=0.5 and x=0.0) and lattice mismatched (InxGai..xAs, x□0.25) growth on patterned GaAs (100) substrates. For the AlGai..xAs structures, the GaAs substrates were patterned in the form of elongated mesas parallel to [011 II with widths of approximately 3 tim. Interfacet migration effects observed on the nesas via cross-section transmission electron microscope studies are explained in terms of a ledge-ledge interaction on the vicinal surfaces formed due to growth on the mesas. InxGai..xAs (x□0.25) structures were grown on GaAs (100) substrates patterned in the form of elongated mesas parallel to [01 11 with widths of approximately 1 tim. This patterning direction was chosen since under cutting in the [0 1 1] direction eliminates inter-facet migration effects so that compositional change induced strain effects can be minimised. For x □0.15, we find a reduction in misfit dislocation densities in films upto five times the nominal critical thickness for growths on the patterned mesas as compared to the growths on the corresponding non patterned regions. For x=0.25 no such difference is observed and a large number ofthreading dislocations ( around iO cm2) are found in both the patterned and the non patterned regions. This is believed to be a consequence of the onset ofa 3-D island growth mode. Finally we present some results for the growth of InO5Gaj"75As I AlAs resonant tunneling diode (RTD) structures and a 100 period InjUGaU8As (80 A) IGaAs (160 A) Multiple Quantum Well (MQW) such as suited for spatial light modulator (SLM) structures on GaAs (100) substrates patterned in both directions on a length scale of 12 to 20 tm. For the RTD structures we conclude that benefits from patterning are expected for x□0.25 provided the growth kinetics are appropriately adjusted to prevent 3D island growth mode. For the MQW -SLMstructure we demonstrate superior optical properties for the growth in the patterned region and a corresponding absence of threading dislocations in the central region of the mesas.© (1990) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In conclusion, we have demonstrated that the problem of macroscopic surface features with GaAs films grown on GaAs (111)B substrates is tied to the surface phase region employed. It is possible to define a region in phase space wherein mirror-smooth films can be grown. There is direct correlation between the nature of the RHEED specular spot intensity variation during growth and the macroscopic quality of the surface morphology. Dynamic RHEED can be used to monitor, predict and control the macroscopic morphology of a growing film in real time. A combination of using the correct phase regime and exploiting the in situ capabilities of dynamic RHEED has allowed us to achieve the long sought objective of growing specular-surfaced GaAs films on GaAs (111)B reproducibly.

The fabrication and characterization of struc- migration also ensures that the (211) layers are tures with their electronic states confined in three thinner than the (111)B mesa top layers. The dimensions (i.e. quantum dots) is an area of ac- (11 1)B GaAs layers are thus surrounded from all tive research. The maskless one-step in-situ ap- lateral directions by the thinner and thus higher proach that exploits growth kinetics to create band gap GaAs quantum wells growing on the suitable size templates upon which nanostruc- (211) side facets. This difference in band gap tures can be realized via growth alone offers provides the lateral electronic confinement. Verpotential advantages [1] over ex-situ approaches. tical confinement is provided by the AlGaAs layPhotolithography followed by wet chemical ers. The continuous shrinkage of the mesa top etching of the (111)B face of GaAs yields trun- layers results in the mesa pinching off at the 8th

The realization of three‐dimensionally confined GaAs/AlGaAs structures on GaAs (111)B and GaAs (001) substrates via one step in situ molecular beam epitaxy is reported. Growth is carried out on nonplanar patterned substrates with crystallographically equivalent mesa top edges. Equivalent side facets evolve during growth and completely surround the mesa top. Adatom migration from these facets to the mesa top result in shrinkage of the mesa top area leading to mesa pinch‐off. Scanning and transmission electron microscopy provide evidence for the realization of structures with lateral linear dimensions ≲500 A. Cathodoluminescence images from the (111)B structures attest to their high optical quality.

The spatial distribution of the long‐wavelength luminescence in thick In0.2Ga0.8As/GaAs multiple quantum wells (MQWs) grown on patterned and unpatterned substrates has been investigated using cathodoluminescence (CL) imaging and spectroscopy. By spatially correlating the luminescence arising from the MQW exciton recombination (λ≊950 nm) with the longer wavelength (1000≲λ≲1200 nm) luminescence arising from the defect‐induced recombination, we demonstrate that it is possible to determine the regions of highest film quality in both the mesa and valley regions for growth on patterned GaAs substrates. The present approach enables a judicious determination of the optimal regions to be used for active pixels in InGaAs/GaAs devices. For growth on unpatterned substrates, the CL spectra show defect‐induced broad bands between 1000≲λ≲1600 nm. These bands exhibit spatial variations which correlate with the dark line defects (DLDs) observed in the λ=950 nm exciton luminescence imaging. Transmission electron microscopy...

The intensity behavior of the specular beam in reflection high‐energy electron diffraction (RHEED) from GaAs(111)B grown by molecular‐beam epitaxy (MBE) is investigated for various growth and diffraction conditions. The temporal behavior during the initial growth of a buffer layer is examined at a fixed diffraction condition. Intensity increase is observed during and after the initial stages of buffer layer growth and found to saturate after about 80 monolayer growth. Intensity oscillations are seen starting at different moments of initial growth, the earliest observed after only 14 monolayer growth. These results are used to guide and control GaAs(111)B growths with mirrorlike surface morphology.