Dragos Seghete

Nucleation and growth of Pt atomic layer deposition on Al2O3 substrates using (methylcyclopentadienyl)-trimethyl platinum and O2 plasma

The nucleation and growth of Pt atomic layer deposition (ALD) on Al2O3 substrates was studied using (methylcyclopentadienyl)-trimethyl platinum (MeCpPtMe3) and O2 plasma as the reactants. The nucleation of Pt ALD was examined on Al2O3 ALD substrates at 300 °C using a variety of techniques including spectroscopic ellipsometry, x-ray reflectivity, x-ray photoelectron spectroscopy, and scanning electron microscopy. These techniques revealed that Pt ALD does not nucleate and grow immediately on the Al2O3 ALD substrates. There was negligible Pt ALD during the first 38 ALD cycles. The Pt ALD growth rate then increased substantially during the next 12 ALD cycles. Subsequently, the Pt ALD growth rate reached a steady state linear growth regime for >50 ALD cycles. These measurements suggest that the Pt ALD first forms a number of nanoclusters that grow slowly during the first 38 ALD cycles. These islands then merge during the next 12 cycles and yield a steady state Pt ALD growth rate of ∼0.05 nm/cycle for >50 ALD ...

The mechanical robustness of atomic-layer- and molecular-layer-deposited coatings on polymer substrates

films was determined to be KIC= 1.89 0.10 and 0.17 0.02 MPa m 0.5 , respectively. From measurements of the saturated crack density, the critical interfacial shear stress was estimated to be c = 39.5 8.3 and 66.6 6.1 MPa, respectively. The toughness of nanometer-scale alumina was comparable to that of alumina thin films grown using other techniques, whereas alucone was quite brittle. The use of alucone as a spacer layer between alumina films was not found to increase the critical strain at fracture for the composite films. This performance is attributed to the low toughness of alucone. The experimental results were supported by companion simulations using fracture mechanics formalism for multilayer films. To aid future development, the modeling method was used to study the increase in the toughness and elastic modulus of the spacer layer required to render improved critical strain at fracture. These results may be applied to a broad variety of multilayer material systems composed of ceramic and spacer layers to yield robust coatings for use in chemical barrier and other applications.

Importance of trimethylaluminum diffusion in three-step ABC molecular layer deposition using trimethylaluminum, ethanolamine, and maleic anhydride.

Hybrid organic-inorganic films were grown by molecular layer deposition (MLD) with a three-step ABC reaction sequence using (A) trimethylaluminum (TMA), (B) ethanolamine (EA), and (C) maleic anhydride (MA) at 90 °C. Very large steady state mass gains of 1854-4220 ng/(cm(2) cycle) were measured depending on reaction conditions. These mass gains are much larger than typical mass gains for surface reactions. The quartz crystal microbalance (QCM) mass profiles during the TMA reaction were consistent with TMA diffusion into and out of the ABC films. The ABC mass gains per cycle also displayed a strong dependence on the TMA dose and purge times that was consistent with the effects of TMA diffusion. Multiple dose experiments conducted at 130 °C revealed that the ABC reactions were self-limiting for thin ABC films. For thicker ABC films, increased TMA diffusion into the ABC film led to non-self-limiting behavior. Numerical modeling assuming Fickian diffusion for TMA diffusing into and out of the ABC film could fit the QCM mass profiles. The results all indicate that TMA diffusion into the ABC MLD film plays a key role in the thin film growth. In addition, X-ray reflectivity (XRR) measurements revealed that the ABC films were exceptionally smooth.

MOSFETs Made From GaN Nanowires With Fully Conformal Cylindrical Gates

We report novel metal-oxide-semiconductor field effect transistors (MOSFETs) based on individual gallium nitride (GaN) nanowires with fully conformal cylindrical gates. The W/Al2O3 gates were deposited by atomic layer deposition. Reverse-bias breakdown voltages exceeded the largest gate voltage tested (-35 V). The nanowire MOSFETs showed complete pinchoff, with threshold voltages between -4 and -12 V. Maximum transconductances exceeded 10 μS, and ON/OFF current ratios higher than 10 8 were measured. Significant gating hysteresis and memory effects were also present, indicative of charge traps. Although further optimization is needed, these results represent a promising step forward in the development of efficient GaN nanowire-based FETs.

Atomic Layer Deposition (ALD) Tungsten NEMS Devices via a Novel Top-Down Approach

In this paper we present a novel low temperature, CMOS compatible, direct top-down nano-fabrication process employing ALD tungsten (WALD) as a structural material for nano-electro-mechanical systems (NEMS). Using this process doubly clamped suspended NEMS devices have been successfully fabricated and demonstrated. The devices have been observed to operate comparably to 2-terminal electrostatic carbon nanotube (CNT) switches, and MEMS tunneling devices. A lifetime in excess of 660,500 cycles has been observed under low-current-limited operating conditions. Under these conditions the device behavior is stable, reproducible and hysterisis free, resembling that of MEMS tunneling devices.

Packaging and interconnect technologies for the development of GaN nanowire-based light emitting diodes

c-axis oriented GaN nanowires (NWs) grown on Si(111) using nitrogen plasma assisted molecular beam epitaxy (MBE) offer promising new approaches for realizing efficient LED technology. The nanowires grow remarkably free of defects, suggesting that eventual LED structures may operate with high quantum efficiencies. Furthermore, the dense NW morphology offers LED configurations with high light extraction efficiency compared to conventional planar LED structures. Interconnecting these vertically aligned nanowires is challenging because of their small diameters, extremely high aspect ratios, and random distributions on the substrate. We have developed novel packaging and interconnection methods by atomic layer deposition (ALD) multilayer encapsulation of the nanowires with 50 nm thick alumina layers and 40 nm thick tungsten layers. This nano-scaled multilayer also provides efficient thermal connections that are mechanically reliable. These encapsulated NWs were used as thermal test structures and the temperature dependence of the photoluminescence peak position as recorded from the portions of the nanowire protruding from the encapsulation was used to monitor the sample temperature. In principle, the internal junction-to-substrate thermal resistance of eventual NW LED structures could be reduced by 400 times by electroplating copper into the interstitial regions between the NWs. These novel packaging and interconnect technologies developed for GaN nanowire-based LEDs can be applied to many other nanowire-based microsystems.

Thermal management of vertical gallium nitride nanowire arrays: Cooling design and tip temperature measurement

This study reports a new thermal management scheme for vertical Gallium Nitride (GaN) nanowire (NW) arrays. A new cooling design for vertical NWs is developed by encapsulating NWs with electroplated copper. Numerical simulations show that the thermal performance of NW array could be significantly enhanced when encapsulated with high thermal conductivity materials. We have also developed a tip temperature measurement technique to characterize the tip temperature of vertical GaN NWs by using steady-state photoluminescence (PL) system. The cooling design and tip temperature measurement technique can be applied to various NW-based nanoelectronic and nanophotonic applications.

Atomic layer deposition enabled interconnect technology for vertical nanowire array devices

In this study, we have demonstrated atomic layer deposition (ALD) enabled interconnection technology for vertical, as-grown c-axis oriented GaN nanowire (NW) arrays encapsulated by Benzocyclobutene (BCB). The nano-scaled ALD multilayer is essential to provide conformal dielectric/conductor coverage and precise thickness control for NW interconnects. Cross-sectional images taken in a focused ion beam (FIB) tool and resistance measurement performed on the NW devices confirms the conformality of ALD-W films. This interconnect technology can be applied to different vertical nanowire array devices, such as nanowire light emitting diodes (LEDs), metal semiconductor field effect transistor (MESFET), resonator or solid state super-capacitors.

High-Q, gallium nitride nanowire-ALD composite mechanical resonators

We are investigating high quality-factor, Q, gallium nitride (GaN) nanowire (NW) - atomic layer deposition (ALD) composite nano-electromechanical resonators. GaN-NWs of typical 100 nm radius and a few microns length are single crystal structures grown by molecular beam epitaxy. These NWs are singly-clamped cantilevers, resonate near 1 MHz, and have typical Q in the range 104–105 under high vacuum. We are exploring the potential for surface functionalization and resonance frequency tuning of these high-Q resonators through ALD coatings. We report that thin coatings (≪ 10 nm) of ALD Al2O3 lead to a broadening of the GaN-NW fundamental mode resonance peak and a consistent increase in resonance frequency of ~15 kHz per 5 nm ALD.