Sakari Sintonen

Diamond-like carbon (DLC) thin film bioelectrodes: effect of thermal post-treatments and the use of Ti adhesion layer.

The effect of thermal post-treatments and the use of Ti adhesion layer on the performance of thin film diamond like carbon bioelectrodes (DLC) have been investigated in this work. The following results were obtained: (i) The microstructure of the DLC layer after the deposition was amorphous and thermal annealing had no marked effect on the structure, (ii) formation of oxygen containing SiOx and Ti[O,C] layers were detected at the Si/Ti and Ti/DLC interfaces with the help of transmission electron microscope (TEM), (iii) thermal post-treatments increased the polar fraction of the surface energy, (iv) cyclic voltammetry (CV) measurements showed that the DLC films had wide water windows and were stable in contact with dilute sulphuric acid and phosphate buffered saline (PBS) solutions, (v) use of Ti interlayer between Pt(Ir) microwire and DLC layer was crucial for the electrodes to survive the electrochemical measurements without the loss of adhesion of the DLC layer, (vi) DLC electrodes with small exposed Pt areas were an order of magnitude more sensitive towards dopamine than Pt electrodes and (vii) thermal post-treatments did not markedly change the electrochemical behavior of the electrodes despite the significant increase in the polar nature of the surfaces. It can be concluded that thin DLC bioelectrodes are stable under physiological conditions and can detect dopamine in micro molar range, but their sensitivity must be further improved.

Analysis of Dislocations Generated during Metal–Organic Vapor Phase Epitaxy of GaN on Patterned Templates

We report on patterning and subsequent metal–organic vapor phase epitaxy overgrowth of GaN films on patterned GaN/sapphire templates. Templates with a hexagonal hole pattern were prepared by photolithography and dry etching. After GaN overgrowth voids were formed at the GaN/sapphire interface. Threading dislocations were found to bend and terminate at void sidewalls during the overgrowth resulting in improved material quality. The dislocations were analyzed by transmission electron microscopy combined with energy dispersive X-ray spectroscopy. Areas with increased Ga concentration were found at the tips of coalesced voids that introduced additional dislocations to the overgrown films.

Microscratch testing method for systematic evaluation of the adhesion of atomic layer deposited thin films on silicon

The scratch test method is widely used for adhesion evaluation of thin films and coatings. Usual critical load criteria designed for scratch testing of coatings were not applicable to thin atomic layer deposition (ALD) films on silicon wafers. Thus, the bases for critical load evaluation were established and the critical loads suitable for ALD coating adhesion evaluation on silicon wafers were determined in this paper as LCSi1, LCSi2, LCALD1, and LCALD2, representing the failure points of the silicon substrate and the coating delamination points of the ALD coating. The adhesion performance of the ALD Al2O3, TiO2, TiN, and TaCN+Ru coatings with a thickness range between 20 and 600 nm and deposition temperature between 30 and 410 °C on silicon wafers was investigated. In addition, the impact of the annealing process after deposition on adhesion was evaluated for selected cases. The tests carried out using scratch and Scotch tape test showed that the coating deposition and annealing temperature, thickness of...

Synchrotron radiation x-ray topography and defect selective etching analysis of threading dislocations in GaN

The crystal quality of bulk GaN crystals is continuously improving due to advances in GaN growth techniques. Defect characterization of the GaN substrates by conventional methods is impeded by the very low dislocation density and a large scale defect analysis method is needed. White beam synchrotron radiation x-ray topography (SR-XRT) is a rapid and non-destructive technique for dislocation analysis on a large scale. In this study, the defect structure of an ammonothermal c-plane GaN substrate was recorded using SR-XRT and the image contrast caused by the dislocation induced microstrain was simulated. The simulations and experimental observations agree excellently and the SR-XRT image contrasts of mixed and screw dislocations were determined. Apart from a few exceptions, defect selective etching measurements were shown to correspond one to one with the SR-XRT results.

X-ray reflectivity characterization of atomic layer deposition Al2O3/TiO2 nanolaminates with ultrathin bilayers

Nanolaminate structures have many prospective uses in mechanical, electrical, and optical applications due to the wide selection of materials and precise control over layer thicknesses. In this work, ultrathin Al2O3/TiO2 nanolaminate structures deposited by atomic layer deposition from Me3Al, TiCl4, and H2O precursors with intended bilayer thicknesses ranging from 0.1 to 50 nm were characterized by x-ray reflectivity (XRR) measurements. The measurements were simulated to obtain values for thickness, density, and roughness of constituting layers. XRR analysis shows that the individual layers within the nanolaminate remain discrete for bilayers as thin as 0.8 nm. Further reduction in bilayer thickness produces a composite of the two materials.

Thermal conductivity of amorphous Al2O3/TiO2 nanolaminates deposited by atomic layer deposition

The thermophysical properties of Al2O3/TiO2 nanolaminates deposited by atomic layer deposition (ALD) are studied as a function of bilayer thickness and relative TiO2 content (0%-100%) while the total nominal thickness of the nanolaminates was kept at 100 nm. Cross-plane thermal conductivity of the nanolaminates is measured at room temperature using the nanosecond transient thermoreflectance method. Based on the measurements, the nanolaminates have reduced thermal conductivity as compared to the pure amorphous thin films, suggesting that interfaces have a non-negligible effect on thermal transport in amorphous nanolaminates. For a fixed number of interfaces, we find that approximately equal material content of Al2O3 and TiO2 produces the lowest value of thermal conductivity. The thermal conductivity reduces with increasing interface density up to 0.4 nm(-1), above which the thermal conductivity is found to be constant. The value of thermal interface resistance approximated by the use of diffuse mismatch model was found to be 0.45 m(2) K GW(-1), and a comparative study employing this value supports the interpretation of non-negligible interface resistance affecting the overall thermal conductivity also in the amorphous limit. Finally, no clear trend in thermal conductivity values was found for nanolaminates grown at different deposition temperatures, suggesting that the temperature in the ALD process has a non-trivial while modest effect on the overall thermal conductivity in amorphous nanolaminates.

Large-area analysis of dislocations in ammonothermal GaN by synchrotron radiation X-ray topography

The large-area defect structure of high-quality ammonothermal GaN was studied by synchrotron radiation X-ray topography (SR-XRT) and high-resolution X-ray diffraction (XRD). The threading dislocation densities of mixed and screw dislocations were determined separately by SR-XRT and were 3.2 × 104 and 3.1 × 103 cm−2, respectively. Threading edge dislocations were not observed. SR-XRT images show that TMDs are clustered on a large scale and form short dislocation arrays with an average spacing between dislocations of 12 µm. XRD rocking curves were used to measure an average lattice tilt of 10 arcsec caused by the dislocation arrays.

Aluminum oxide/titanium dioxide nanolaminates grown by atomic layer deposition: Growth and mechanical properties

Atomic layer deposition (ALD) is based on self-limiting surface reactions. This and cyclic process enable the growth of conformal thin films with precise thickness control and sharp interfaces. A multilayered thin film, which is nanolaminate, can be grown using ALD with tuneable electrical and optical properties to be exploited, for example, in the microelectromechanical systems. In this work, the tunability of the residual stress, adhesion, and mechanical properties of the ALD nanolaminates composed of aluminum oxide (Al 2O3) and titanium dioxide (TiO2) films on silicon were explored as a function of growth temperature (110–300 °C), film thickness (20–300 nm), bilayer thickness (0.1–100 nm), and TiO2 content (0%–100%). Al 2O3 was grown from Me3 Al and H2O, and TiO2 from TiCl4 and H2O. According to wafer curvature measurements, Al 2O3/TiO2 nanolaminates were under tensile stress; bilayer thickness and growth temperature were the major parameters affecting the stress; the residual stress decreased with increasing bilayer thickness and ALD temperature. Hardness increased with increasing ALD temperature and decreased with increasing TiO2 fraction. Contact modulus remained approximately stable. The adhesion of the nanolaminate film was good on silicon.

Tribological properties of thin films made by atomic layer deposition sliding against silicon

Interfacial phenomena, such as adhesion, friction, and wear, can dominate the performance and reliability of microelectromechanical (MEMS) devices. Here, thin films made by atomic layer deposition (ALD) were tested for their tribological properties. Tribological tests were carried out with silicon counterpart sliding against ALD thin films in order to simulate the contacts occurring in the MEMS devices. The counterpart was sliding in a linear reciprocating motion against the ALD films with the total sliding distances of 5 and 20 m. Al2O3 and TiO2 coatings with different deposition temperatures were investigated in addition to Al2O3-TiO2-nanolaminate, TiN, NbN, TiAlCN, a-C:H [diamondlike carbon (DLC)] coatings, and uncoated Si. The formation of the tribolayer in the contact area was the dominating phenomenon for friction and wear performance. Hardness, elastic modulus, and crystallinity of the materials were also investigated. The nitride coatings had the most favorable friction and wear performance of the...

Substrate-patterning techniques for nitride growth

Although gallium nitride (GaN) and its alloys are widely used in LEDs, there is still room for efficiency improvements. Because of the high cost of GaN substrates, foreign materials such as sapphire are commonly used in deposition of GaN films (epitaxy) onto LEDs. Several problems arise from this mismatch between the epitaxially grown GaN layer and the substrate.1, 2 The mismatch in lattice constants and thermal-expansion coefficients creates strain and dislocations in the grown layer, while the refractive-index difference causes total internal reflection. These factors reduce the internal and light-extraction efficiencies of LEDs. Several approaches have been successfully applied to reduce the dislocation density and improve the light-extraction efficiency of GaN films and LED structures, most importantly epitaxial lateral overgrowth,3 pendeo epitaxy,4 and patterned sapphire substrates (PSSs).5 Substrate-patterning methods have as an advantage that they cancel the effects of mismatch in lattice constant and refractive index between substrate and epitaxially grown film. It is, therefore, possible to improve both the material quality and the light-extraction efficiency using a single process. Two different methods are commonly used, i.e., patterning of either sapphire or sapphire/GaN templates. PSSs have the advantage of single epitaxial growth, while patterning of sapphire/GaN templates can be done using a larger range of techniques and offers more control in dislocation reduction and light-extraction engineering. We have studied growth of GaN films on c-plane (the flat part of a sapphire crystal) PSSs and the effect of substrate patterning on LED performance.6, 7 Patterning consists of hexagonal pit or pillar structures that are wet etched onto the sapphire surface: see Figure 1(a). When overgrown with GaN, the sapphire surface is completely covered with GaN: see Figure 1(b). However, Figure 1. (a) Scanning-electron-microscope (SEM) image of hexagonally patterned sapphire substrate (top view). The hexagonal pillar-top diameter and spacing are 2 and 1 m, respectively. (b) SEM image of overgrown patterned sapphire (cross section). Scale bars: (a) 5 and (b) 1 m.