PacSurf2016 Session TF-WeE: Microstructure & Surface Morphological Evolution in Organic & Inorganic Films

The presence of excitons in organic materials offer new opportunities for low-cost photovoltaics and electronic systems and provide prospects for unique energy science and applications. In this talk I will review our understanding of organic quasepitaxy. I will then discuss our demonstration of the growth of ordered organic-organic hetero-quasieptiaxial supperlattices, composed of incommensurate organic semiconductors with sustained registry grown from the bottom up via a new step-edge nucleation driven growth mechanism. By probing a range of molecular pairings with in-situ and real-time diffraction, we further uncover driving forces that can broadly enable this type of growth, which are completely distinct from the requirements of inorganic epitaxy. It is well known that crystalline order, orientation, and quantum confinement of highly anisotropic organic semiconductors can significantly alter the properties and performance of organic electronics. Thus, these demonstrations can enable entirely new photophysical phenomena and provide opportunities for manipulating energy in a variety of excitonic structures.

A focus area of research on III-nitrides attempts to lower the growth temperature of films, because molecular beam epitaxy and metalorganic chemical vapor deposition (MOCVD) must be done at relatively high temperatures to produce a high quality material. As an example, aluminum nitride MOCVD takes place at over 1000°C. Tunable band gaps, high breakdown voltages, and piezoelectric properties are attractive qualities of aluminum nitride films, and thus motivate the search for alternate fabrication methods. Atomic layer epitaxy (ALE) of GaN, AlN, and InN films have been shown at temperatures far below that required for MOCVD. [1] Further exploration of the growth parameters of these films is underway. Research during the actual growth process allows much more information on early stages of film deposition, and eliminates many of the ex-situ issues with surface oxidation upon sample removal from the reactor. The X-ray photons available at some synchrotrons allow insight into the growth process from grazing incidence small angle X-ray scattering (GISAXS), a technique that can be carried out at ALE pressures and is extremely sensitive to surface morphological evolution.

The work presented will describe film growth of aluminum nitride using trimethylaluminum and a nitrogen/argon or hydrogen/nitrogen/argon plasma. Based on ex-situ examinations, aluminum nitride ALE on MOCVD GaN at 500°C was previously seen to exhibit single crystal character, with a full-width at half maximum of 670 arc-seconds. [2] Work presented here describes results on changing the temperature and the plasma conditions for ALE AlN. The substrates were sapphire and the growth took place at Brookhaven National Laboratory and at the Cornell High Energy Synchrotron Source in a custom reactor designed to facilitate GISAXS monitoring during growth.

These results suggest that surface feature distances are related to the final quality of the aluminum nitride films. Samples grown at 350-400°C have higher impurities than samples grown at 450-500°C, and GISAXS shows closer features at the lower temperatures. In addition, holding the flow constant and changing the nitrogen to argon ratio resulted in changes in the atomic species present in the plasma source, and changes in nucleation and growth behavior.

References:

1 N. Nepal et al., Thin Solid Films 589 47 (2015)

2 N. Nepal et al., Appl. Phys. Lett. 103 0 82110 (2013)

We present the effects of the variation oxygen concentration in the thin films of CdS:O deposited by RF-Sputtering. This kind of thin film was studied with the purpose to be used as a window layer in a solar cell. The films were deposited on Corning glass substrate (area 1 in²) from a ceramic target of CdS with 99.99% purity, in an atmosphere of mixture of gases Ar + O2. Deposition was performance at room temperature with a working pressure of 25 mTorr and a sputtered power of 40 Watts. Films with different oxygen content were prepared with a variation of the oxygen concentration in a range from 1% to 1.5% with respect to the total pressure . The structural and morphological properties were characterized by using X-ray Diffraction (XRD) and field emission scanning electron microscopy (FE-SEM), respectively. The diffraction patterns showed that samples prepared with oxygen concentration less to 1.1% are crystalline and present the hexagonal CdS (h-CdS) structure, while samples with oxygen concentrations greater than 1.1% are amorphous, all crystalline films were oriented preferentially (002). Also we observed a decrease in the crystallite size in a range from 5.9 nm to 4.3 nm, which can be correlated in the micrographs. SEM images show that is possible to obtain nanostructure thin films of CdS varying the grain size in a range from 20nm to 70nm, the grain size decrease as the oxygen percentage raises, all samples present high uniformity.

ACKNOWLEDGEMENTS: This work has been supported by CONACYT-SENER (México) under projects CeMiESol 207450/P25 and CeMiE-Sol 207450/P26. M. Loeza-Poot acknowledges CONACYT-Mexico for scholarship (number 556332) in the Applied Physics Department of CINVESTAV-Merida. . Measurements were performed at LANNBIO CINVESTAV-Mérida, supported from projects FOMIX-Yucatán 2008-108160, CONACYT LAB-2009-01 No. 123913 and CB2012/ 178947. The authors gratefully acknowledge to D. Huerta and Wiliam Cauich for technical support, and to L. Pinelo as secretarial assistant.

InGaN quantum wells (QWs) have been well established as active layers in LEDs and laser diodes. Recently, there has been interest in the growth of thick InGaN to serve as strain-reducing layers for deep-green and red emitters. The growth of thick layers is, however, challenging. The miscibility gap between InN and GaN leads to InGaN phase separation and indium clustering. In addition to low indium incorporation at high growth temperatures, a high density of V-pits are observed in InGaN films grown in the III-polar direction. Alternatively, the growth of films in the N-polar direction offers advantages that are attractive for the growth of thick InGaN. N-polar growth enables higher indium incorporation and V-pits are less favorable to form. Despite these advantages, limited studies on thick N-polar InGaN films have been reported. This is because N-polar InGaN grown by MOCVD typically suffer from high densities of hexagonal hillocks that originate from low quality N-polar GaN templates. Therefore, in order to assess the quality of thick N-polar InGaN films, it is necessary to probe the local structure and optical properties of high quality grown films.

In this study, we compare the structural and optical properties of thick III-polar and N-polar InGaN. Both III-polar and N-polar InGaN growth was carried out by MOCVD under identical conditions (130 nm, 780°C, 300 Torr, V/III=2450) on high quality GaN base layers. The N-polar InGaN films were free of hexagonal hillocks and exhibited a reduced RMS roughness of 0.66 nm in comparison to the RMS roughness of 2.30 nm for III-polar InGaN. High resolution XRD, SIMS and EDX measurements revealed higher indium incorporation in the N-polar films (13%) when compared to III-polar InGaN (7.5%). From PL measurements, two distinct peak emissions located at different depths of the film were observed for both film polarities, one peak originating from a pseudomorphically strained region and the other from a partially relaxed region. To further investigate the optical properties of InGaN films, we performed both steady state and time-resolved cathodoluminescence mapping of the III-polar and N-polar films at room and liquid helium temperatures. From CL measurements, distinct emission from indium clusters and threading dislocation around V-pits were observed in III-polar InGaN films. In the case of N-polar InGaN, CL emission was homogenous across the film surface. With the combination of spectroscopic techniques, this study elucidates the differences in the luminescence in thick InGaN as a function of film polarity and gives new insights into possible mechanisms of luminescence quenching commonly observed N-polar InGaN.

Normal approach in 3D-SIMS can display the species in layered or complex structure. However, detailed analysis of interfaces between phases, such as concentration change and inhomogeneity of diffused species near interface, attract more attention that need to be explored. For example, diffused interface is expected to be a sign of solid bonding for the safety of coating service life. New 3D imaging approach is developed to separate the diffused species through the interface. In terms of the ion imaging of CsAl+ in diffusion region, diffused Al at the interface between the ceramic coating and Zircaloy substrate were investigated in both as-deposited and annealed states.

The new analytics toolkit to resolve diffused species is a key evaluation to develop coating layer on alloys. The new method based on 3D-SIMS analytics is expected to provide diffusion information between coating and substrate with new view. It will continue to advance and gives more information about the formation of diffusion interface which facilitate the interface analysis from variety of research interest.

References

[1] Erin H. Seeley et al. Anal. Chem., 2012, 84 (5), 2105–2110

[2] James Bailey et al. ACS Appl. Mater. Interfaces 2015, 7, 2654−2659

[3] Alam et al. Nucl. Eng. Des. 241(2011)3658-3677

[4] Pint et al. J. Nucl. Mater. 440(2013)420-427

[5] Houssiau et al. Appl. Surf. Sci. 231-232(2004)585-589