Tuning of electrical properties in InAlN/GaN HFETs and Ba0.5Sr0.5TiO3/YIG Phase Shifters

Engineers know well from an early point in their training the trials and tribulations of having to make design tradeoffs in order to optimize one performance parameter for another. Discovering tradeoff conditions that result in the elimination of a loss associated with the enhancement of some other parameter (an improvement over a typical tradeoff), therefore, ushers in a new paradigm of design in which the constraints which are typical of the task at hand are alleviated. We call such a design paradigm “tuning” as opposed to “trading off”, and this is the central theme of this work. We investigate two types of microwave electronic devices, namely GaN-based heterostructure field effect transistors (HFETs) and tunable ferroelectric-ferrite-based microwave phase shifters. The “tuning” associated with these types of devices arises from the notion of an optimal 2DEG density, capable of achieving higher performance in terms of electron velocity and enhanced reliability in the case of the HFET, and the coupling of ferroelectric and ferrite materials in tunable microwave phase shifters, capable of achieving high differential phase shifts while at the same time mitigating the losses associated with impedance mismatching which typically arise when the phase is tuned. Promises and problems associated with HFET devices based on the intriguing InAlN/GaN material system will be described. We focus on the fundamental problem associated with the induction of the large density of carriers at the interface, namely the disintegration of an excess of longitudinal optical phonons (hot phonons) in the channel. We use microwave measurements in conjunction with stress tests to evidence the existence of an optimal 2DEG density wherein the hot phonon effect can be “tuned,” which allows for enhanced high frequency performance as well as device reliability. Next, we focus on the design, fabrication, and measurement of tunable phase shifters consisting of thin films of BaxSr1-xTiO3 (BST), which has the advantage of having high dielectric tunability as well as relatively low microwave loss. We discuss the design, fabrication, and measurement of a simple coplanar waveguide (CPW) type of phase shifter as well as a more complicated “hybrid” phase shifter consisting of a ferrite (YIG) in addition to BST. The use of such a bilayer allows one to “tune” the impedance of the phase shifters independently of the phase velocity through careful selection of the DC biasing magnetic fields, or alternatively through the use of an additional piezoelectric layer, bonded to YIG whose permeability can then be tuned through magnetostriction

Ti/Al/Ni/Au Ohmic contacts for AlInN/AlN/GaN-based heterojunction field-effect transistors

The microstructure of AuNiAlTi/Al0.84In0.16N/AlN/GaNOhmic contacts annealed from 700 to 900 °C has been determined using transmission electron microscopy and associated analytical techniques. Intermixing and phase separation of the metal contact layers was observed to degrade the surface roughness. An optimal contact performance was obtained for contacts annealed at 800 °C and was attributed to the formation of TiN contact inclusions that had penetrated through the AlInN layers into the GaN layers underneath. These TiN contact inclusions had an inverted mushroom shape with a density of ∼108 cm−2, and they were invariably located at the positions of mixed-type threading dislocations. These inclusion defects would act as a conduction path between the metal contacts and the two-dimensional electron gas of heterojunction field-effect transistor devices. The AlInN layer remained intact in dislocation-free areas of all samples

High electron mobility in nearly lattice-matched AlInN∕AlN∕GaN heterostructure field effect transistors

High electron mobility was achieved in Al1−xInxN∕AlN∕GaN (x=0.20–0.12)heterostructurefield effect transistors(HFETs) grown by metal-organic chemical vapor deposition. Reduction of In composition from 20% to 12% increased the room temperature equivalent two-dimensional-electron-gas density from 0.90×1013to1.64×1013cm−2 with corresponding electron mobilities of 1600 and 1410cm2/Vs, respectively. The 10Kmobility reached 17600cm2/Vs for the nearly lattice-matched Al0.82In0.18N∕AlN∕GaNheterostructure with a sheet carrier density of 9.6×1012cm−2. For comparison, the AlInN∕GaNheterostructure without the AlN spacer exhibited a high sheet carrier density(2.42×1013cm−2) with low mobility(120cm2/Vs) at room temperature. The high mobility in our samples is in part attributed to ∼1nm AlN spacer which significantly reduces the alloy scattering as well as provides a smooth interface. The HFETs having gate dimensions of 1.5×40μm2 and a 5μm source-drain separation exhibited a maximum transconductance of ∼200mS∕mm with good pinch-off characteristics and over 10GHz current gain cutoff frequency

Camelback channel for fast decay of LO phonons in GaN heterostructure field-effect transistor at high electron density

Fluctuation technique is used to measure hot-phonon lifetime in dual channel GaN-based configuration proposed to support high-power operation at high frequencies. The channel is formed of a composite Al0.1Ga0.9N/GaN structure situated in an Al0.82In0.18N/AlN/Al0.1Ga0.9N/GaN heterostructure. According to capacitance–voltage measurements and simultaneous treatment of Schrödinger–Poisson equations, the mobile electrons in this dual channel configuration form a camelback density profile at elevated hot-electron temperatures. The hot-phonon lifetime was found to depend on the shape of the electron profile rather than solely on its sheet density. The camelback channel with an electron sheet density of 1.8 × 1013 cm−2 demonstrates ultrafast decay of hot phonons at hot-electron temperatures above 600 K: the hot-phonon lifetime is below ∼60 fs in contrast to ∼600 fs at an electron sheet density of 1.2 × 1013 cm−2 obtained in a reference Al0.82In0.18N/AlN/GaN structure at 600 K. The results suggest a suitable method to increase the electron sheet density without the deleterious effect caused by inefficient hot-phonon decay observed in a standard design at similar electron densities

Stress test measurements of lattice-matched InAlN/AlN/GaN HFET structures

InAlN/GaN heterostructures offer some benefits over existing AlGaN/GaN heterostructures for HFET device applications. In addition to having a larger bandgap than typical AlGaN compounds used in HFET devices (with Al < 30%), which leads to better confinement and subsequent larger power carrying capacity, InAlN can be grown lattice-matched to GaN, resulting in strain-free heterostructures. As such, lattice-matched InAlN provides a unique system wherein the reliability of the devices may exceed that of the strained AlGaN/GaN devices as a result of being able to decouple the hot electron/hot phonon effects on the reliability from the strain related issues. In this work, we subjected lattice-matched InAlN-based HFETs to electrical stress and observed the corresponding degradation in maximum drain current. We found that the degradation rates are lower only for a narrow range of moderate gate biases, corresponding to low field average 2-dimensional electron gas (2DEG) densities of 9–10 × 10 12  cm −2 . We propose that the degradation is attributable to the buildup of hot phonons since the degradation rates as a function of electron density generally follow the hot phonon lifetime versus electron density. This provides evidence that hot phonons have a significant role in device degradation and there exists an optimal 2DEG density to minimize hot phonon related degradation. We did not observe any correlation between the degradation rate and the gate leakage.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/77433/1/1345_ftp.pd

Epitaxial growth of (001)-oriented Ba0.5Sr0.5TiO3 thin films on a-plane sapphire with an MgO/ZnO bridge layer

High quality (001)-oriented Ba0.5Sr0.5TiO3 (BST) thin films have been grown on a-plane sapphire(112¯0) by rf magnetron sputtering using a double bridge layer consisting of (0001)-oriented ZnO (50 nm) and (001)-oriented MgO (10 nm) prepared by plasma-assisted molecular beam epitaxy. X-ray diffraction revealed the formation of three sets of in-plane BST domains, offset from one another by 30°, which is consistent with the in-plane symmetry of the MgO layer observed by in situ reflective high electron energy diffraction. The in-plane epitaxial relationship of BST, MgO, and ZnO has been determined to be BST [110]//MgO [110]//ZnO [112¯0] and BST [110]/MgO [110]//ZnO [11¯00]. Capacitance-voltage measurements performed on BST coplanar interdigitated capacitor structures revealed a high dielectric tunability of up to 84% at 1 MHz

Effect of large strain on dielectric and ferroelectric properties of Ba0.5Sr0.5TiO3 thin films

BaxSr1−xTiO3 is ideally suited as a tunable medium for radio frequency passive component. In this context we have studied the effect of biaxial strain on the dielectric and ferroelectricproperties of Ba0.5Sr0.5TiO3thin filmsgrown epitaxially on SrTiO3 (001) substrates. The lattice parameters of the films determined by high-resolution x-ray diffraction with the thickness varying from 160 to 1000 nm indicated large biaxial compressive strain which decreased from 2.54% to 1.14% with increasing film thickness. Temperature-dependent measurements of the dielectric constant in our strained Ba0.5Sr0.5TiO3thin films revealed a significant increase in the Curie temperature as the film thickness is below 500 nm. Enhanced ferroelectric behavior was observed for highly strained films with a remanent polarization of 15 μC/cm2 in the 160-nm-thick layer. However, the thick films(≥500 nm) exhibited weak temperature dependence of the dielectric constant without any pronounced peak corresponding to the Curie temperature, which may suggest inhomogeneous strain distribution in the thick films

Cardiometabolic risk factors and quality of life in severely obese children and adolescents in the Netherlands

BACKGROUND: The prevalence of severe obesity in children and adolescents is increasing. However, little is known about cardiometabolic risk factors and quality of life of children with severe obesity.Therefore, the aim of this study was to assess the demographic characteristics and the prevalence of cardiometabolic risk factors and quality of life in severely obese children and adolescents undergoing intensive inpatient treatment for obesity. METHODS: Data were collected between August 2009 and April 2011 on 16 children (8-13y) and 64 adolescents (13-19y) with severe obesity (SDS-BMI >= 3.0 or SDS-BMI >= 2.3 and comorbidity) participating in an RCT evaluating two intensive inpatient treatment programs for obesity. Demographic, anthropometric, clinical characteristics and two components of the EuroQol for the assessment of quality of life are described. RESULTS: Eighty percent of participants in this study had at least one cardiometabolic risk factor in addition to severe obesity. Low HDL-cholesterol and hypertension were most prevalent (65.0% respectively 31.2%). The highest significant correlations were found between SDS-BMI and SDS-waist circumference, fasting plasma insulin and HOMA-IR (correlation coefficients respectively 0.80, 0.49, and 0.48). With regard to quality of life, the mean utility score of the participants was 0.79 on a scale of 0.0 to 1.0 on the EuroQol questionnaire and their mean individual valuation was 69.1 on a scale of 0 to100. CONCLUSION: Cardiometabolic risk factors are already highly prevalent in this group of severely obese children and adolescents. The score of 69.1 found for quality of life in this study suggests that participants experience important limitations in their quality of life. However, quality of life is not associated with the prevalence of cardiometabolic risk factors. TRIAL REGISTRATION: Netherlands Trial Register (NTR1678, registered 20-Feb-2009)

Ibrutinib for Relapsed / Refractory CLL: A UK and Ireland Analysis of Outcomes in 315 patients

In 2014, ibrutinib was made available for relapsed/refractory chronic lymphocytic leukaemia (CLL) patients. The UK CLL Forum collected data from UK/Ireland patients with a minimum of 1 year follow-up with pre-planned primary endpoints; the number of patients still on therapy at 1 year (Discontinuation Free Survival; DFS) and 1 year overall survival (OS). With a median 16 months follow-up, data on 315 patients demonstrated 1 year DFS of 73.7% and 1 year OS of 83.8%. Patients with better pre-treatment performance status (PS 0/1 vs 2+) had superior DFS (77.5% vs 61.3%;p14 days and had OS of 89.7%, while 26% of patients had dose reductions and 13% had temporary treatment breaks >14 days. We could not demonstrate a detrimental effect of dose reductions alone (1 year OS: 91.7%), but patients who had first year treatment breaks > 14 days, particularly permanent cessation of ibrutinib had both reduced 1 year OS (68.5%) and also a statistically significant excess mortality rate beyond one year. Although outcomes appear inferior to the RESONATE trial (1 year OS;90%: PFS;84%), this may partly reflect the inclusion of PS 2+ patients and that 17.5% of patients permanently discontinued ibrutinib due to an event other than disease progression