AVS2011 Session TF-FrM: Thin Films: Growth and Characterization II

In order to grow uniform, smooth and pinhole-free films of nm thickness, the nucleation step must occur with high areal density on the substrate and the nuclei must have a narrow size distribution. If the film must be deposited inside of a deep structure of very high aspect ratio, then the growth process must be strictly governed by the surface reaction rate of the precursor species rather than by the rate at which precursor is transported to the growth surface. Here, we present a method of control in chemical vapor deposition that simultaneously meets both of the above criteria.

We previously reported that the steady-state film growth rate is reduced when a molecular species, called the inhibitor, is added to the CVD process. The inhibitor interacts transiently with the growth surface but does not decompose and incorporate its elements into the film, i.e., the inhibitor is not consumed. We also reported that the inhibitor greatly improves the film nucleation step, e.g. for the growth of HfB₂, a high performance diffusion barrier, on SiO₂ substrates. In the absence of the inhibitor the nucleation density is low and the size distribution is broad, such that some islands attain a height of > 10 nm before the film even coalesces. In the presence of the inhibitor, nuclei attain a height of ~ 1 nm, but then grow only very slowly. Additional nuclei continue to form and fill in the bare substrate, such that coalescence occurs at a thickness of ~ 2 nm with a rms surface roughness of ≤ 0.5 nm.

The use of a growth inhibitor allows the nucleation regime of film growth to afford excellent nm-thick coatings in extremely high aspect ratio features. Note that when the film growth rate is low, so also is the consumption of precursor, such that a partial pressure of precursor species persists to the bottom of the feature. The inhibitor further assures that the nucleation process will be uniform. In features of aspect ratio > 200, we show that a film is deposited on all surfaces with a rms roughness of 0.6 nm. We also discuss the mechanisms that can afford the observed results. We suggest that the ability to reduce (homogenize) the size distribution and increase the areal density of nuclei will greatly extend the useful range of CVD precursor-substrate combinations which can afford nm-thick coatings in very high aspect ratio features.

MME is a growth technique applied to molecular beam epitaxy of III-nitrides in which metal and dopant cell shutters are periodically opened and closed while active nitrogen flux remains constant. This technique uses metal-rich fluxes that would accumulate droplets in traditional MBE, taking advantage of the enhanced adatom mobility provided by excess metal. The periodic shuttering of the effusion cells allows the excess metal to be consumed, providing smooth, dry surfaces required for devices that demand abrupt interfaces.

In this study, MME is applied to the growth of InGaN and transient RHEED intensities are monitored for differing metal shutter open times (Fig. 1). It is found that these RHEED transients are the result of a RHEED oscillation associated with the buildup and consumption of the metal adlayer. This allows for observation and control of fractions of adsorbed metal layers. However, there is a drastic difference between the RHEED transients of low and high metal shutter open times. This difference is attributed to indium surface segregation, resulting in leftover In on the surface that does not form InN at these elevated temperatures. A model for adlayer buildup and consumption is developed, and a key RHEED signature is identified as the onset of surface segregation. The thickness of the adsorbed metal at this onset is found to be between 1 and 2 ML for various compositions. Samples grown throughout the miscibility gap using this technique exhibited single phase XRD spectra and < 1 nm RMS roughnesses (Fig. 2). High hole concentrations in p-InGaN are also achieved by MME (Fig. 3).

At the low temperatures required for high indium-content InGaN, an intermediate growth regime in MBE does not exist. Thus, traditional MBE cannot produce metal-rich InGaN without droplets or surface segregation. Because the onset of surface segregation (1<x<2 ML) occurs below droplet accumulation (<2.3 ML), this study confirms that for metal-rich InGaN growth, some form of modulation must be employed.

This work was supported by the Air Force Office of Scientific Research under a basic science grant managed by Kitt Reinhardt.

We will report on two new ternary cubic oxides offering close lattice matching to MgO, ZnMgO and NiMgO, and compare the properties of films grown by both RF Plasma-Assisted MBE and a much more economical sol-gel deposition process. Using both methods we have successfully demonstrated band gap tuning in the UV-C spectral region, with ZnMgO films producing band gaps ranging from ~5eV to 7.8eV and NiMgO films having band gaps ranging from 3.5eV to 7.8 eV. XRD revealed 2θ FWHM values as narrow as 0.19°, and atomic force microscopy of ZnMgO films demonstrated surface roughness of 3.4nm with NiMgO films having roughness below 1Å. Optical, compositional, and morphological results from films produced using both synthesis methods will be presented. We will also report on initial photodetectors with 5µm interdigitated fingers that produced peak responsivities of 12 mA/W at 250 nm.

Nanoparticle and thin film magnetic alloys of 3d-4d/5d metals such as Fe-Pt, Co-Pt, and Fe-Pd are of technological interest due to their ordered L1₀ tetragonal phase which exhibits high magnetocrystalline anisotropy. Hard magnetic properties combined with ductility and corrosion resistance make these alloys ideal for applications including micro-electro-mechanical systems and ultra high-density magnetic storage. These materials have also been shown to possess a unique strain-induced chessboard eutectoid microstructure between their hard L1₀ and soft L1₂ magnetic phases that features exchange coupling effects. Within this class of materials Fe-Pd alloys possess a low order-disorder transition temperature making them a good candidate for ordered structure studies.

Nanoparticles and thin films of eutectoid (61.5 at% Pd) Fe-Pd were deposited by pulsed laser deposition. Additionally ordered nanoparticles were deposited utilizing a novel technique involving the matrix assisted decomposition of metal-organic precursors. Nanoparticles and films were subsequently characterized by x-ray diffraction, vibrating sample magnetometry, high resolution transmission electron microscopy, and scanning electron microscopy.

***PLEASE NOTE YOU MUST IDENTIFY A DIFFERENT PRESENTER FOR THIS ABSTRACT. YOU MAY PRESENT ONE PAPER ONLY (ORAL OR POSTER) AT THE CONFERENCE. YOU ARE ALREADY LISTED AS PRESENTER OF ABSTRACT #554***Surface Plasmon Resonance (SPR) excitation in metallic thin films and nanostructures has been widely applied to study biomolecules absorption, bio-imaging, bio-sensing, and sensitivity enhanced Raman spectroscopy [1,2,3]. However, although the theoretical principles underlying SPR excitation are applicable to any conductive material, only metals, and in particular Au and Ag, have been considered for practical applications. Here we investigate SPR excitation in a conducting metal oxide such as RuO₂. Due to the optical properties frequency dependence, SPR excitation is more intense in the infrared region for this material. The RuO₂ thin films investigated were grown using reactive magnetron sputtering on two different substrates resulting on amorphous RuO₂ films when deposited on glass and crystalline RuO₂ films when epitaxially deposited on TiO₂ (001) substrates. We have used Atomic Force Microscopy (AFM) and Reflection High-Energy Electron Diffraction (RHEED) to characterize the surface morphology and microstructure of these samples. Four-point probe was used to investigate the electrical conductivity properties and ellipsometry was used to characterize the optical properties of the films. We will show a comparison of the physical properties including the SPR excitation between these two kinds of thin film RuO₂ samples. The investigation of SPR in conducting metal oxide materials provides a significant advancement for thin film characterizations as well as opens new venues for photonic and plasmonic applications.

This work was financially supported by NSF (DMR-1006013).

[1] Amanda J. Haes and Richard P. Van Duyne, J. Am. Chem. Soc., 2002, 124 (35), pp 10596–10604.

[2] Lin He, Michael D. Musick, Sheila R. Nicewarner, Frank G. Salinas, Stephen J. Benkovic, Michael J. Natan, and Christine D. Keating, J. Am. Chem. Soc., 2000, 122 (38), pp 9071–9077.

[3] Lehui Lu, Atsuko Kobayashi, Keiko Tawa, and Yukihiro Ozaki, Chem. Mater., 2006, 18 (20), pp 4894–4901

Physical vapour deposition (PVD) methods, which are characterized by highly ionized deposition fluxes of the film forming species, provide added means for the synthesis of tailor-made materials. They can, for instance, facilitate the growth of meta-stable phases, nanostructures as well as selective deposition on complex-shaped substrates. In such methods, the generation of highly ionized deposition fluxes stems from high electron (plasma) densities. Cathodic arc and pulsed laser deposition are examples of such discharges where electron densities in the order of 10²¹ m^-3 can be obtained. These techniques, while providing as high as 100% degree of ionization of the deposition flux, exhibit several drawbacks, such as macroparticle ejection from the target, lack of lateral film uniformity, and in some cases are difficult to scale up. Magnetron sputtering based techniques are technologically more relevant, owing to their inherent advantages of conceptual simplicity, upscalability, and film uniformity. However, electron densities in magnetron discharges are significantly smaller, in the range of 10¹⁴-10¹⁶ m^-3 and therefore generation of a highly ionized deposition flux is often difficult. This difficulty is overcome by high power impulse magnetron sputtering (HiPIMS), where plasma densities on the order of 10¹⁹ m^-3 are achieved. HiPIMS has been successful in enhancing the ionization for most common metals (Cu, Al, Ta, Ti), but it is challenged when non-metals such as carbon is considered. Previous investigations have shown that C⁺/C ratio in HiPIMS does not exceed 5%, which does not provide efficient control over the physical properties and synthesis of carbon in various technologically relevant forms, e.g. tetrahedral amorphous carbon. In the present study we address the low degree of ionization of carbon in magnetron discharges. We have developed a new HiPIMS based process, which provides a plasma characterized by high electron temperature and plasma density as determined by time-resolved Langmuir probe measurements. The C⁺ ion energy distribution functions (IEDFs) determined by time-averaged energy resolved mass spectrometry demonstrate an energetic C⁺ ion population and an overall five-fold increase of the C⁺ ion fraction as compared to standard HiPIMS methods. The enhanced ionized fraction of carbon facilitates the growth of carbon films with mass densities as high as approx. 2.8 g/cm³ as determined by high resolution x-ray reflectively measurements. Determination of the D-peak to G-peak ratio (I(D)/I(G)) and full width at half maximum of the G-peak in Raman spectra indicate that the films contain a large fraction of diamond-like bonded (sp³) carbon.