Showing papers by "Samuel Graham published in 2016"

Elastomer-Polymer Semiconductor Blends for High-Performance Stretchable Charge Transport Networks

Abstract: An inverse relationship between mechanical ductility and mobility/molecular ordering in conjugated polymer systems was determined definitively through systematic interrogation of poly(3-hexylthiophene) (P3HT) films with varied degrees of molecular ordering and associated charge transport performance. The dilemma, whereby molecular ordering required for efficient charge transport conclusively undermines the applicability of these materials for stretchable, flexible device applications, was resolved using a polymer blend approach. Specifically, the molecular interactions between dissimilar polymer materials advantageously induced semiconducting polymer ordering into efficient π–π stacked fibrillar networks. Changes in the molecular environment surrounding the conjugated polymer during the elastomer curing process further facilitated development of high mobility networked semiconductor pathways. A processed P3HT: poly(dimethylsiloxane) (PDMS) composite afforded a semiconducting film that exhibits superior du...

Thermal Boundary Resistance in GaN Films Measured by Time Domain Thermoreflectance with Robust Monte Carlo Uncertainty Estimation

Abstract: In this work, we investigate the thermal boundary resistance and thermal conductivity of GaN layers grown on Si with 100 nm AlN transition layers using time domain thermoreflectance (TDTR). The GaN layers ranged from 0.31 to 1.27 μm. Due to the challenges in determining the thermal boundary resistance of the buried interfaces found in this architecture, a new data reduction scheme for TDTR that utilizes a Monte Carlo fitting method is introduced and found to dramatically reduce the uncertainty in certain model parameters. The results show that the GaN thermal conductivity does not change significantly with layer thickness, whereas the resistance of the AlN layer decreases slightly with GaN thickness.

Field-effect transistors based on wafer-scale, highly uniform few-layer p-type WSe2

Abstract: The synthesis of few-layer tungsten diselenide (WSe2) via chemical vapor deposition typically results in highly non-uniform thickness due to nucleation initiated growth of triangular domains. In this work, few-layer p-type WSe2 with wafer-scale thickness and electrical uniformity is synthesized through direct selenization of thin films of e-beam evaporated W on SiO2 substrates. Raman maps over a large area of the substrate show small variations in the main peak position, indicating excellent thickness uniformity across several square centimeters. Additionally, field-effect transistors fabricated from the wafer-scale WSe2 films demonstrate uniform electrical performance across the substrate. The intrinsic field-effect mobility of the films at a carrier concentration of 3 × 10(12) cm(-2) is 10 cm(2) V(-1) s(-1). The unprecedented uniformity of the WSe2 on wafer-scale substrates provides a substantial step towards producing manufacturable materials that are compatible with conventional semiconductor fabrication processes.

Characterization of the Thermal Conductivity of CVD Diamond for GaN-on-Diamond Devices

Abstract: Diamond films grown by chemical vapor deposition have the potential to improve the thermal management and reliability of AlGaN/GaN high electron mobility transistors. The integration of CVD diamond with GaN involves the nucleation and growth of diamond films on GaN which induces a vertical gradient in thermal conductivity of the diamond and can result in bulk properties that depend greatly on growth conditions. Thus accurate characterization of the thermal conductivity of CVD diamond, especially the lower conductivity near the growth interface is needed to assess the impact on AlGaN/GaN HEMTs. In this work, we present measurements of the thickness dependence of CVD diamond with thicknesses ranging from 5 to 13.8 µ m in addition to bulk diamond substrates using time domain thermoreflectance. Measurements were made on the same samples in two different laboratories which showed excellent correlation between the measurements. The diamond properties were then utilized in a thermal model of a 10 finger AlGaN/GaN HEMT to predict the impact of device junction temperature. Compared to a device made on SiC operating at 5 W/mm, a junction temperature reduction of 30-40% was seen when using CVD diamond and the same device size

Ultraviolet micro-Raman spectroscopy stress mapping of a 75-mm GaN-on-diamond wafer

Abstract: Full-wafer stress mapping is accomplished using visible and ultraviolet (UV) micro-Raman spectroscopy of a 730-nm thick GaN layer integrated with diamond grown by chemical vapor deposition. The UV measurements taken from both sides of the wafer reveal a higher tensile stress of 0.86 ± 0.07 GPa at the free GaN surface compared to 0.23 ± 0.06 GPa from the GaN/diamond interface, each with good cross-wafer uniformity. Factors influencing the overall stress and stress gradient are understood based on relaxation from dislocations in the GaN which vary in density along the growth direction. Simulations incorporating a model for stress relaxation in the GaN elastic modulus adequately describe the observed dependence.

Environmentally Assisted Cracking in Silicon Nitride Barrier Films on Poly(ethylene terephthalate) Substrates

Abstract: A singular critical onset strain value has been used to characterize the strain limits of barrier films used in flexible electronics. However, such metrics do not account for time-dependent or environmentally assisted cracking, which can be critical in determining the overall reliability of these thin-film coatings. In this work, the time-dependent channel crack growth behavior of silicon nitride barrier films on poly(ethylene terephthalate) (PET) substrates is investigated in dry and humid environments by tensile tests with in situ optical microscopy and numerical models. The results reveal the occurrence of environmentally assisted crack growth at strains well below the critical onset crack strain and in the absence of polymer-relaxation-assisted, time-dependent crack growth. The crack growth rates in laboratory air are about 1 order of magnitude larger than those tested in dry environments (dry air or dry nitrogen). In laboratory air, crack growth rates increase from ∼200 nm/s to 60 μm/s for applied st...

Organic Field-Effect Transistors with a Bilayer Gate Dielectric Comprising an Oxide Nanolaminate Grown by Atomic Layer Deposition

Abstract: We report on top-gate OFETs with a bilayer gate dielectric comprising an Al2O3 /HfO2 nanolaminate layer grown by atomic layer deposition and an amorphous fluoro-polymer layer (CYTOP). Top-gate OFETs display average carrier mobility values of 0.9 ± 0.2 cm2/(V s) and threshold voltage values of −1.9 ± 0.5 V and high operational and environmental stability under different environmental conditions such as damp air at 50 °C (80% relative humidity) and prolonged immersion in water at a temperature up to 95 °C.

Experimental investigation of defect-assisted and intrinsic water vapor permeation through ultrabarrier films

Abstract: In the development of ultrabarrier films for packaging electronics, the effective water vapor transmission rate is a combination of permeation through pinhole defects and the intrinsic permeation through the actual barrier film. While it is possible to measure the effective permeation rate through barriers, it is important to develop a better understanding of the contribution from defects to the overall effective barrier performance. Here, we demonstrate a method to investigate independently defect-assisted permeation and intrinsic permeation rates by observing the degradation of a calcium layer encapsulated with a hybrid barrier film, that is, prepared using atomic layer deposition (ALD) and plasma enhanced deposition (PECVD). The results are rationalized using an analytical diffusion model to calculate the permeation rate as a function of spatial position within the barrier. It was observed that a barrier film consisting of a PECVD SiNx layer combined with an ALD Al2O3/HfOx nanolaminate resulted in a de...

Hybridization-Induced Carrier Localization at the C60/ZnO Interface

Abstract: Electronic coupling and ground-state charge transfer at the C60 /ZnO hybrid interface is shown to localize carriers in the C60 phase. This effect, revealed by resonant X-ray photoemission, arises from interfacial hybridization between C60 and ZnO. Such localization at carrier-selective electrodes and interlayers may lead to severely reduced carrier harvesting efficiencies and increased recombination rates in organic electronic devices.

Spectroscopy and control of near-surface defects in conductive thin film ZnO.

Abstract: The electronic structure of inorganic semiconductor interfaces functionalized with extended π-conjugated organic molecules can be strongly influenced by localized gap states or point defects, often present at low concentrations and hard to identify spectroscopically. At the same time, in transparent conductive oxides such as ZnO, the presence of these gap states conveys the desirable high conductivity necessary for function as electron-selective interlayer or electron collection electrode in organic optoelectronic devices. Here, we report on the direct spectroscopic detection of a donor state within the band gap of highly conductive zinc oxide by two-photon photoemission spectroscopy. We show that adsorption of the prototypical organic acceptor C60 quenches this state by ground-state charge transfer, with immediate consequences on the interfacial energy level alignment. Comparison with computational results suggests the identity of the gap state as a near-surface-confined oxygen vacancy.

Near-ultraviolet micro-Raman study of diamond grown on GaN

Abstract: Ultraviolet (UV) micro-Raman measurements are reported of diamond grown on GaN using chemical vapor deposition. UV excitation permits simultaneous investigation of the diamond (D) and disordered carbon (DC) comprising the polycrystalline layer. From line scans of a cross-section along the diamond growth direction, the DC component of the diamond layer is found to be highest near the GaN-on-diamond interface and diminish with characteristic length scale of ∼3.5 μm. Transmission electron microscopy (TEM) of the diamond near the interface confirms the presence of DC. Combined micro-Raman and TEM are used to develop an optical method for estimating the DC volume fraction.

The impact of ultra-thin titania interlayers on open circuit voltage and carrier lifetime in thin film solar cells

Abstract: We study the effects of modifying indium tin oxide electrodes with ultrathin titania (TiO2) layers grown via plasma-enhanced atomic layer deposition (PE-ALD). We find an optimal thickness of PE-ALD-grown titania by tracking performance, which initially increases, peaks, and eventually decreases with increasing TiO2 thickness. We use scanning Kelvin probe microscopy (SKPM) to measure both the local work function and its distribution as a function of TiO2 thickness. We find that the variance in contact potential difference across the surface of the film is related to either the amorphous or anatase TiO2 form. Finally, we use local SKPM recombination rate experiments, supported by bulk transient photovoltage and charge extraction measurements. We show that the optimum TiO2 thickness is the one for which the carrier lifetime is the longest and the charge carrier density is the highest, when the TiO2 is amorphous, in agreement with the device measurements.

The thermal effects of substrate removal on GaN HEMTs using Raman Thermometry

Abstract: The ability to fabricate AlGaN/GaN high electron mobility transistors (HEMTs) on Si substrates has enabled the production of low cost high power electronics. To further enhance the performance of GaN electronics for high power conversion, the ability to maintain high off-state breakdown voltages with large electron densities is necessary. The use of Si substrates, however, limits the device's capabilities due to its weak electrical field strength. This limitation has been identified as the main cause for breakdown in HEMTs when the high electric field reaches the silicon substrate underneath the region between the gate and drain. To overcome this obstacle, removal of the Si substrate between the gate and drain region has shown to increase the device's breakdown voltage up to 3000 V. While removing the Si substrate extends the capabilities of GaN HEMTs for high voltage applications, the effects of the Si removal on the thermal performance during operation has not yet been investigated. Raman Thermometry, a well-developed technique, is used to compare the maximum temperature rise between a Local Substrate Removed (LSR) device and a non-LSR device. The application of nanoparticles (TiO2 and ZnO) for measuring surface temperatures via Raman spectroscopy is also investigated and applied to determine a more accurate temperature of the gate junction temperature. The LSR device was found to have a much higher thermal resistance than its non-LSR device counterpart limiting the maximum power dissipation the LSR device can achieve before severe degradation. Volumetric averaged residual stress mapping was also measured via Raman Spectroscopy and suggests the removal of the Si relaxes the stress in the GaN buffer layer and AlGaN barrier which can be exploited in designs to improve reliability. Methods to improve the thermal reliability of LSR devices are key to implementing such devices as future power switches.

Nanometer-Scale Strain Measurements in AlGaN/GaN High-Electron Mobility Transistors During Pulsed Operation

Abstract: Electric, thermal, and mechanical strain fields drive the degradation of AlGaN/GaN high-electron mobility transistors (HEMTs). The resulting mechanical strains within the devices are particularly important. However, a lack of high-resolution measurements of device deformation has limited progress in understanding the related phenomena. This paper presents the atomic force microscope measurements of thermomechanical deformation of AlGaN/GaN HEMT devices during pulsed operation. We investigate the devices with various operating conditions: drain–source voltage, $V_{\mathrm {\mathrm {DS}}}$ , of 0–50 V; drain–source power of 0–6 W/mm; and operating frequency of 55–400 kHz. As $V_{\mathrm {\mathrm {DS}}}$ increases, thermomechanical deformation decreases, especially in the region above the gate. An electrothermomechanical model closely matches with and helps to explain the measurements. According to the model, the maximum periodic tensile thermal stress, which occurs at the drain-side edge of the gate footprint, is 55% larger for $V_{\mathrm {\mathrm {DS}}} = 10$ V than for $V_{\mathrm {\mathrm {DS}}} = 48$ V for the same device power. The maximum tensile thermal stress in the device depends on the gate temperature and not the maximum device temperature. As $V_{\mathrm {\mathrm {DS}}}$ increases, the hotspot moves away from the gate, leading to lower gate temperature rise and lower tensile thermal stress.

Pool boiling enhancement through hierarchical texturing of surfaces

Abstract: Pool boiling is an important technology that can be used for the thermal management of microelectronics. Previous studies have shown that pool boiling enhancement can be obtained by controlling the surface wettability, by patterning surface features, and by controlling micro/nano porosity characteristics of the heater surface. While many of these studies have focused on the increase in critical heat flux (CHF), both an increase in CHF and heat transfer coefficient are desired for application to thermal management. In this study pool boiling experiments are conducted on hierarchical copper microporous structures in order to maximize the heat transfer coefficient and critical heat flux (CHF). This was accomplished by investigating the boiling curve for DI water on flat and structured microporous copper surfaces fabricated with spherical copper powder. The surfaces were tested as-fabricated and by machining channels into the coated surfaces to control vapor and liquid flow paths. Unpatterned (as-fabricated) microporous structures showed that the CHF can be increased over 220% compared to that of flat surfaces, both with a significant rise (>100°C) in the surface superheat. The large increase in superheat was found to arise from the existence of partial dry-out in the microporous copper. By patterning the microporous copper, vapor was allowed to escape and resulted in a 412% increase in CHF with significant decrease in surface superheat (<49°C) over those of flat surfaces. This large enhancement is believed to be attributed to the effective removal of the partial dry-out within the patterned porous copper as well as the separation of liquid and vapor flow over the surface. Moreover, it was found that both the thickness of the microporous layer and depth of the channel played an important role in both CHF and heat transfer coefficient enhancement. Both can be easily controlled during packaging to improve thermal management of devices cooled through pool boiling.

Thermal raman and IR measurement of heterogeneous integration stacks

Abstract: Thermal management and planning is important for heterogeneous integration due to the introduction of a complex thermal path. Thermal measurement of operating devices provides necessary data points for future design as well as validation of models. In this paper, two methods for measuring thermal performance of DAHI (Diverse Accessible Heterogeneous Integration) GaN HEMTs are presented and contrasted: IR microscopy and micro Raman spectroscopy. The QFI IR system uses a per-pixel material emissivity flat temperature calibration when the device is in an off-state, and then calculates operating temperatures by CCD exposure. Two separate QFI systems with differing CCD resolutions were used to collect thermal data and are compared. Raman Thermometry by contrast, is a laser point measurement of the frequency shift in scattered photons due to phonon vibrational modes whose frequencies are temperature dependent. Differences in measurements between the two methods arising from the stack of materials used in the DAHI process and their transparency are discussed. A method for measuring the surface temperature of the devices through Raman by the use of TiO2 nanoparticles is also presented in conjunction with a profile of the HEMT. Measurements are presented alongside thermal simulation results using prototype software Mentor GraphicsTM Calibre®.