Applications for stand-alone systems with small form factor, long lifetime and high energy density have arisen. Nuclear batteries have the potential to achieve specific powers of 1-50mW/g and devices that utilize the beta emitter titanium tritide (TiT2) have the most potential. We present our progress in making textured TiT2 based devices and their performance as compared to planar devices. In planar devices a 200nm 4H-SiC P+N junction is used for charge collection. Dark current measured less than 6.1 pA/cm2. Samples measured with tritium foil produced an open circuit voltage of 2.09V, short circuit current of 75.47nA/cm2, fill factor of 0.86 and power efficiency of 18.6%. This device is near the practical limit efficiency. In order to increase the power output the effective area of the beta source is increased by patterning the topmost absorption layer into channels 10 micron in height. Textured devices achieved open circuit voltage of 1.91V and short circuit current of 11.3nA.
Metal free direct growth of graphene on h-BN using a high temperature (~1550°C) chemical vapor deposition technique was done under Ar environment. Growth temperature, methane partial pressure, hydrogen/methane flow ratio, and growth time were varied and optimized. Raman spectroscopy clearly showed the signature of graphene with G- (~1580cm-1) and 2D-mode (~2700cm-1). The smallest width of G- and 2D-peak was 30 and 55cm-1, respectively, and the Raman I2D/IG ratio varied between 0.7 and 1.8. Raman D-peak (~1350cm-1) shows a strong dependence on growth temperature with the smallest ID/IG value of 0.15 at 1550°C. In the case of long growth, nitrogen and boron doping were detected by x-ray photoelectron spectroscopy with a small Raman D’-peak. A continuous graphene film with the rms roughness (1×1 μm2 area) of 0.32nm was shown by atomic force microscopy. Early stage of growth revealed circular shaped nucleation islands, the density and heights of which are ~15/μm2 and 1-2 graphene monolayer (ML), respectively. The hydrogen/methane flow ratio was found to be a critical parameter to obtain smooth 2D growth. Growth of h-BN is performed with ammonia borane, hydrogen and Ar. The growth is found to be critically dependent on the conditions of the ammonia boran precursor. Reproducible continuous films of h-BN are reported.
A preliminary study on reduced temperature chemical vapor deposition of graphene on copper substrates was
performed. Graphene's exceptional mechanical strength, very high electrical and thermal conductivity, and
stability at atomic layer thicknesses, generates potential for a broad range of applications, from nanodevices to
transparent conductor to chemical sensor. Of the techniques demonstrated for graphene formation, chemical
vapor deposition is the sole process suitable for manufacturing large area films. While large area film deposition
of graphene has been shown on metal substrates, this process has been limited to high temperatures, 900-1000C,
which increases the cost of production and limits methods of integrating the graphene with other material
structures. In this work, CVD of graphene on copper foil was attempted over a range of temperatures (650
- 950C) on substrates as large as 5 x 15 cm in a horizontal tube reactor. Depositions were performed using
both CVD and upstream Plasma-Enhanced CVD (PECVD), and the results are compared for both techniques.
Quality of graphene films deposited with and without plasma enhancement was characterized by micro Raman
spectroscopy.
Aluminum nitride (AlN) thin film is a wide bandgap semiconductor which shows great potential for opto- electronic applications in blue and ultraviolet regions. A prism-thin film coupling technique, and an automatic temperature controlling device have been used to study the thermo-optic properties of single crystalline aluminum nitride thin films grown on sapphire substrates by low pressure metal organic chemical vapor deposition. The temperature coefficient of refractive index ne for AlN is determined to be 6 X 10-5/ degree(s)C at a wavelength of 632.8 nm.
We have investigated an optical waveguide formed by aluminum nitride (AlN) thin film on sapphire. A good quality AlN thin film on sapphire substrate was prepared by metal organic chemical vapor deposition in this laboratory. A rutile prism coupler was employed to display the waveguide modes with the wavelength of 632.8, 532.1, 514.5 and 488.0nm. The refractive index and thickness of the waveguide material is obtained by prism-coupler measurement. The dispersion curve of AlN film is given and the dispersion equation is derived. The attenuation in waveguide is evaluated using scattering loss measurements by using a fiber probe in this experiment. The attenuation coefficient alpha is 1.5-2.1 cm-1 various with different sample and the different modes of waveguide. The accuracy of the measurement is discussed.
The cathodoluminescence (CL) measurements of carbon doped and undoped aluminum nitride (AlN) thin films near the band- edge region were performed at temperatures of 300, 77 and 4.2 K. These films were grown on three different substrates sapphire, 6H-SiC and 4H-SiC. The typical thickness of these films was in the range of 0.35 - 0.4 micrometer. Two distinct peaks 'A' and 'B' were observed in undoped samples around 6.1 and 5.9 eV, respectively. Also, there is an expected peak within the low energy side of peak 'B.' The absorption spectra of different carbon doped AlN on sapphire substrate were carried out to study the Urbach tail's parameters which play an important role near the band-edge transitions.
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