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Dielectrics & Conductors

Dielectrics
Stimulated by the explosive growth in the electronics and telecommunications industries, there is an increasing need for capacitors that operate at high frequencies while enabling a reduction in size and cost through greater integration. To fill this need, new concepts in ferroelectric-based thin film decoupling/broadband capacitors on sapphire substrates are emerging for specialized high frequency opto-electronic and digital systems that represent a substantial market opportunity. In addition, long-term opportunities exist for these ferroelectrics in a wide range of electronic applications.

These opportunities mandate that cost effective, scalable technologies for depositing various dielectrics including ferroelectric-based materials be developed on sapphire and/or silicon substrates. Current vacuum-based thin film technologies, such as pulsed laser deposition (PLD), sputtering, and metalorganic chemical vapor deposition (MOCVD), have difficulty depositing these ferroelectrics on sapphire substrates. These methods suffer from disadvantages including variable properties (stoichiometry), high capital cost, high production cost, scalability issue, low throughput, and long development cycles for new compositions. nGimat is utilizing its low cost, open atmosphere CCVD process to address these needs by developing a core competency in the deposition of high-k dielectric oxides, such as barium strontium titanate (BST), lead zirconium titanate (PZT) and new materials such as CuxCa1-xTiO3, and grows these with nanolayer structures to optimize performance. These and similar compositions are currently being evaluated for application as decoupling capacitors, uncooled IR sensors, optical filters, and electro-optic waveguides.


SEM Cross-section and x-ray diffraction pole figure demonstrating dense,
epitaxial growth of BST on a sapphire substrate.




BST parallel plate capacitor structure demonstrating a dielectric constant of 500
and a dielectric tunability of 17% at 2v.


Conductors
Transparent conducting oxide (TCO) films, exhibiting high optical transmission and electrical conductivity, play an important role in various thin film solar cell and flat panel display technologies. Doped SnO2 and indium tin oxide (ITO) are the dominant TCO materials for these applications. However, for solar cell applications, absorption losses in the standard (iron-containing) float-glass and insufficient light scattering properties of the TCO layer still limit the efficiency of a-Si:H solar modules. nGimat is pioneering the development of high refractive index, low cost, highly transparent conducting doped ZnO films with nanolayers and nanostructures using its proprietary CCVD Process to overcome this limitation.

nGimat has successfully produced highly transparent and conductive doped ZnO (n ~ 1.95) films on glass substrates via the CCVD Process. The fabricated films were found to be smooth (RMS < 2 nm), uniform (5%) and have high adhesion to the substrate. A sheet resistance of 10 _/sq with an optical transmission >90% in the visible region and a haze of <5% has been achieved for Ga and Al doped ZnO (ZnO:Ga and ZnO:Al) films. In addition, nGimat continues to build on this capability through the development of new materials systems (Ti, Zr, Ce, and Al doped ZnO) with superior electro-optical properties and large area deposition. See Figure 3 below for representative ZnO transmission spectra and SEM cross-section micrograph, which shows an intentionally roughened surface for optimal device performance.


SEM cross-section micrograph (left)
and transmission data (right) from a representative ZnO thin film.


In addition to transparent conductive oxides, nGimat has developed the ability to deposit other conductive materials, such as platinum, copper, and conductive oxides. Many of these materials are complimentary to the development of dielectric materials and are necessary for the production of high performance capacitor architectures. For example, nGimat's platinum thin films can be deposited on a variety of substrates and exhibit excellent crystalline texture and temperature stability. These properties make it an exceptional template for the subsequent deposition of high quality dielectric layers. nGimat also has the ability to deposit nanolayers of conductive oxides, such as lanthanum strontium cobalt oxide and strontium ruthenium oxide, between the conductor and dielectric in order to improve the electrical interface and reduce film stresses, and therefore improve electrical performance. The ease in which nGimat's deposition process fabricates these complex nanostructures allows for rapid optimization of device structures and high performance electronic devices.

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