Nelson Y. Garces

Assessment of GaN Surface Pretreatment for Atomic Layer Deposited High-k Dielectrics

We report the effects of GaN surface pretreatments on the material and electrical properties of Al2O3 dielectric deposited by atomic layer deposition (ALD). A layer of Al2O3 was deposited at different temperatures on metal organic chemical vapor deposition grown n-GaN that was treated with either H2O2:H2SO4 (1:5, piranha), HCl:H2O (1:1, HCl), or HF:H2O (1:1, HF) prior to Al2O3 deposition. The Al2O3 layers on piranha- and HF-treated GaN were observed to be uniformly smooth. The piranha pretreatment resulted in the lowest hysteresis. Pretreatment of the GaN surface with piranha removes carbon and hydroxylates the surface, resulting in better quality ALD Al2O3.

Graphene functionalization and seeding for dielectric deposition and device integration

Graphene has recently attracted wide-spread attention because of its unique transport and physical properties that are appealing for a wide range of electronic applications. Integration with scalable high-κ dielectrics is important for the realization of graphene-based top-gated electronic devices, including next generation THz applications. Atomic layer deposition (ALD), a low temperature deposition method based on two separate self-limiting surface reactions, is a preferred technique to achieve high-quality, conformal, ultrathin dielectric films with precise control of thickness and chemical composition at the atomic scale. Unfortunately, ALD of oxides on graphene is hindered by the inertness of the graphene surface. To alleviate this graphene-oxide incompatibility, several different functionalization and seeding methods have recently been developed to render the graphene more susceptible to the ALD process of high-κ dielectrics including: ozone, wet chemical and fluorine pretreatments, low-k polymer se...

Epitaxial graphene surface preparation for atomic layer deposition of Al2O3

Atomic layer deposition was employed to deposit relatively thick (∼30 nm) aluminum oxide (Al2O3) using trimethylaluminum and triply-distilled H2O precursors onto epitaxial graphene grown on the Si-face of silicon carbide. Ex situ surface conditioning by a simple wet chemistry treatment was used to render the otherwise chemically inert graphene surface more amenable to dielectric deposition. The obtained films show excellent morphology and uniformity over large (∼64 mm2) areas (i.e., the entire sample area), as determined by atomic force microscopy and scanning electron microscopy. X-ray photoelectron spectroscopy revealed a nearly stoichiometric film with reduced impurity content. Moreover, from capacitance-voltage measurements a dielectric constant of ∼7.6 was extracted and a positive Dirac voltage shift of ∼1.0 V was observed. The graphene mobility, as determined by van der Pauw Hall measurements, was not affected by the sequence of surface pretreatment and dielectric deposition.

Plasma-assisted atomic layer deposition of nanolaminates for gate dielectric applications

Thin [(x)Al2O3 + (y)TiO2] nanolaminates (NLs) films of various TiO2 and Al2O3 volume fractions were deposited on n-Si substrates at 250 °C using remote plasma-assisted atomic layer deposition. While the overall thickness of the dielectric was held relatively constant at ∼16 nm, the relative ratio of Al2O3 to TiO2 in the NL was varied by changing the number of deposition cycles of each component. This permitted the evaluation of changes in the dielectric constant κ, index of refraction Nf, optical band gap, Eg, and the electrical performance of the resulting oxides. Capacitance–voltage and current–voltage results on 100 μm diameter circular capacitors were obtained. The data reveals that the high-content TiO2 films show limited evidence of oxide charge trapping and relatively large dielectric constants (k ∼ 15) with reduced reverse-biased leakage current, whereas the high-content Al2O3 films offer a larger optical band-gap and excellent insulating character with reduced leakage currents. In addition, the a...

Effect of GaN surface treatment on Al2O3/n-GaN MOS capacitors

The surface preparation for depositing Al2O3 for fabricating Au/Ni/Al2O3/n-GaN (0001) metal oxide semiconductor (MOS) capacitors was optimized as a step toward realization of high performance GaN MOSFETs. The GaN surface treatments studied included cleaning with piranha (H2O2:H2SO4 = 1:5), (NH4)2S, and 30% HF etches. By several metrics, the MOS capacitor with the piranha-etched GaN had the best characteristics. It had the lowest capacitance–voltage hysteresis, the smoothest Al2O3 surface as determined by atomic force microscopy (0.2 nm surface roughness), the lowest carbon concentration (∼0.78%) at the Al2O3/n-GaN interface (from x-ray photoelectron spectroscopy), and the lowest oxide-trap charge (QT = 1.6 × 1011 cm−2eV−1). Its interface trap density (Dit = 3.7 × 1012 cm−2eV−1), as measured with photon-assisted capacitance– voltage method, was the lowest from conduction band-edge to midgap.

Selective switching of GaN polarity on Ga-polar GaN using atomic layer deposited Al2O3

Patterned layers of Al2O3 prepared by atomic layer deposition (ALD) are used to produce structures of alternating polarity (c-plane orientation) of GaN on Ga-polar GaN epilayers. Annealing prior to epitaxial growth allows the amorphous ALD layers to attain sufficient crystallinity, which enables N-polar GaN growth over the annealed ALD Al2O3 and Ga-polar growth over the bare substrate. Transmission electron microscopy and electron channeling contrast imaging were carried out to characterize the structural nature of the material and confirm crystallinity. ALD Al2O3 may be suitable for fabricating novel variable-polarity devices, particularly where growth on native, Ga-polar GaN substrates is beneficial.

Impact of surface treatments on high-κ dielectric integration with Ga-polar and N-polar GaN

Gallium- and nitrogen-polar GaN surfaces are subjected to a variety of pretreatments, including oxidation, before the application of high-κ dielectrics by atomic layer deposition (ALD) in order to assess the “best” preparation of smooth, clean, and electrically high-performing dielectric semiconductor interfaces. In terms of topographical and chemical cleanliness, a pretreatment with a wet chemical piranha etch (H2SO4:H2O2) was found to be optimum for both surfaces, and additionally, (NH4)2S is effective for N-polar surfaces. Both thermal and plasma oxidations were employed for controlled growth of native oxides. For Ga-polar surfaces, all native oxides were as smooth as pretreated surfaces, while for N-polar surfaces, all native oxides are much rougher except for very short, high temperature oxidations. ALD Al2O3 films on Ga-polar surfaces are smoother for pretreated surfaces than for as-received surfaces, whereas for N-polar surfaces the opposite is true. In general, ALD HfO2 films on Ga-polar surfaces ...

Mode-locked 2-μm wavelength fiber laser using a graphene-saturable absorber

Abstract. Soliton-like pulses with a 1984-nm center wavelength are produced from a Tm-doped mode-locked fiber laser. The linear cavity has a graphene saturable absorber mirror at one end and a fiber Bragg grating as the output coupler. The laser operates without dispersion compensation, and the repetition rate was tuned from 20 to 5 MHz by the addition of SMF-28 fiber. The dry transfer process used to place the graphene on a mirror could be extended to any optical substrate. This enables integration of graphene with optics such as an optical window coated with a graphene filter or a graphene-saturable absorber placed directly on a semiconductor laser facet.

Positive Temperature Coefficient SiC PiN Diodes

Integration of patterned ballast resistance into the anode of SiC PiNs is a solution to the dilemma of negative dVf /dT for such diodes. In fabricated 4H-SiC PiN diodes, we demonstrate a cross-over from negative to positive temperature coefficient for current densities as low as 80 A/cm2. Adjusting the percentage of the patterned anode area, the positive or neutral dVf /dT can be achieved over a wide current-density range without substantial penalty in the forward voltage drop. This characteristic is crucial for high-power SiC packages with ganged-parallel rectifier arrays.

Cysteamine coated Ag and Au nanorods for improved surface enhanced Raman scattering from dinitrotoluene and trinitrotoluene

Surface-enhanced Raman scattering (SERS) from trinitrotoluene and other nitro-based explosives is important for the development of a reliable detection scheme exhibiting low false-positive rates. However, the interaction of these compounds with Ag and Au causes the molecules to orient in ways such that the primary vibrations of the nitro groups, the main identifying Raman marker of these compounds, are inhibited in addition to causing a reduction in the SERS response. It has recently been shown that cysteamine, which contains amine functional end groups, will electrostatically attract the nitro groups of TNT. Therefore, as the thiol functional group of cysteamine chemically bonds this molecule to the plasmonically-active Au and Ag nanoparticles studied, SERS of TNT can be obtained following the nitro-amine functional group complex formation. It is observed that the cysteamine adsorbs in one of two configurations on the metal surface, with the trans configuration consisting of bonding at the S end of the molecule and the cysteamine is perpendicular to the metal surface, while in the Gauche configuration S bonding occurs, but the molecule bends over towards the metal film surface, approaching the parallel configuration allowing the amine groups interact with the surface. We find that the trans configuration is best for the detection of SERS from TNT. Experiments compare well with DFT calculations of the cysteamine and TNT complex and their adsorption on Ag.