Atomic layer deposition of III-nitrides and metal oxides : their application in area selective ALD

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Bilkent University
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III-nitride compound semiconductor materials (GaN, AlN, and InN) and their alloys have generated significant interest in both basic research and commercial applications mainly in the field of photonics, energy storage, nano-sensors, and nano-(opto)electronics. Wurtzite type III-nitrides exhibit direct band gaps, which extend from the ultra-violet (UV) to the mid-IR spectrum with values of 6.2, 3.4 and 0.64 eV for AlN, GaN, and InN, respectively. This feature allows the band gap of III-nitride alloys to be conveniently tuned by precisely controlling the composition for a particular application. Metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) are the most common successful techniques for achieving high-quality epitaxial III-nitride layers with low impurity concentrations and decent electrical properties. However, both of these methods employ high growth temperatures, which is neither compatible with the existing CMOS technology nor suitable for temperature-sensitive device layers (e.g. In-rich InxGa1-xN) and substrates (e.g. glass, flexible polymers, etc.). These limitations are the main driving source for a continuous exploration of alternative low temperature processes for the growth of III-nitride layers and their alloys. High aspect ratio III-nitride nanostructures in the form of nanowires and nanorods have been synthesized using different techniques including vapor-liquid-solid crystal growth, electrospinning, template based synthesis, and etching. Critical breakthroughs in fabrication of III-nitrides nanostructures have been achieved by above mentioned techniques but suffer from limited control over properties of nanostructures (shape, orientation, and size) and in some cases high growth-temperature requirement. A recent flurry of interest in developing high quality I-D III-nitride nanostructures derives from the desire to obtain flexible optoelectronic devices having wider applications. Template-assisted growth technique is one of the most promising approach to fabricate III-nitride nanostructures with precise control over shape, size, position, and distribution. In the first part of thesis, we have deposited InN and III-nitride alloys using hollow-cathode plasma assisted atomic layer deposition (HCPA-ALD) at low growth temperatures. The aim was to deposit III-nitride materials at lowest growth temperatures with decent crystalline quality and minimum impurity content. Depositions were carried out using group III organometallic precursors along with N2/H2 or N2 plasma as metal and nitrogen source, respectively. Process parameters including precursor pulse time, plasma flow duration, purge time, and deposition temperature are investigated and correlations were developed between process parameters and material properties. Refractive index of the InN film deposited at 200 C was found to be 2.66 at 650 nm. 48 nm-thick InN films exhibited relatively smooth surfaces with RMS surface roughness values of 0.98 nm, while the film density was extracted as 6.30 g/cm3. The effect of In content on structural, optical, and morphological properties of InxGa1-xN thin films was investigated. Grazing incidence X-ray diffraction (GIXRD) and transmission electron microscope (TEM) showed that InN and InxGa1-xN thin films were polycrystalline with hexagonal wurtzite structure. Spectral absorption measurements exhibited an optical band edge of InN around 1.9 eV. X-ray photoelectron spectroscopy (XPS) confirmed the deposition of InN and alloy thin films and revealed the presence of low impurity contents. Higher In concentrations resulted in an increase of refractive indices of InxGa1-xN ternary alloys from 2.28 to 2.42 at a wavelength of 650 nm. Optical band edge of InxGa1-xN films red-shifted with increasing In content, confirming the tunability of the band edge with alloy composition. Photoluminescence measurements of InxGa1-xN exhibited broad spectral features with an In concentration dependent wavelength shift. We have also studied the compositional dependence of structural, optical, and morphological properties of BxGa1-xN and BxIn1-xN ternary thin film alloys grown using sequential pulsed CVD. GIXRD measurements showed that boron incorporation in wurtzite lattice of GaN and InN diminishes the crystallinity of BxGa1-xN and BxIn1-xN sample. Refractive index decreased from 2.24 to 1.65 as the B concentration of BxGa1-xN increased from 35 to 88 %. Similarly, refractive index of BxIn1-xN changed from 1.98 to 1.74 for increase in B concentration value from 32 to 87 %, respectively. Optical transmission band edge values of the BxGa1-xN and BxIn1-xN films shifted to lower wavelengths with increasing boron content, indicating the tunability of energy band gap with alloy composition. Atomic force microscopy measurements revealed an increase in surface roughness with boron concentration of BxGa1-xN, while an opposite trend was observed for BxIn1-xN thin films. Moreover, we demonstrate vertical GaN, AlN, and InN hollow nano-cylindrical arrays (HNCs) integrated to Si(100) substrates using anodized aluminum oxide (AAO) membrane templated PA-ALD. Our fabrication and Si-integration strategy consists of the following process steps: (i) reactive ion etching (RIE) of Si using AAO membrane as hard mask material to achieve nanoporous Si substrate, (ii) conformal growth of III-nitride films on nanoporous Si via low-temperature PA-ALD, (iii) removal of PA-ALD coated III-nitride material from top surface of Si via plasma etching, and (iv) isotropic dry etching of surrounding Si to attain long-range ordered vertical III-nitride HNCs. The material properties of nanostructured III-nitride materials have been compared with the thin-film counterparts which were also grown using low-temperature PA-ALD. SEM images revealed that long-range ordered arrays of III nitride HNCs were successfully integrated in Si(100) substrates. TEM, GIXRD, and selected area electron diffraction (SAED) cumulatively confirmed that III-nitride HNCs possess hexagonal wurtzite crystalline structure. XPS survey and high-resolution scans detected presence of different elements and peaks at specific binding energies which confirmed the formation of III-nitride HNCs. The second part of the thesis deals with self-aligned thin film patterning of metal oxides using area selective atomic layer deposition (AS-ALD). Nanoscale process integration demands novel nano-patterning techniques in compliance with the requirements of next generation devices. Conventionally, top-down subtractive (etch) or additive (deposition/lift-off) processes in conjunction with various lithography techniques is employed to achieve film patterning, which become increasingly challenging due to the ever-shrinking misalignment requirements. To reduce the complexity burden of lithographic alignment in critical fabrication steps, self-aligned processes such as selective deposition and selective etching might provide attractive solutions. We demonstrate a methodology to achieve AS-ALD by using inductively couple plasma (ICP) grown fluorocarbon polymer film as growth inhibition layer. The fluorocarbon layer was grown using C4F8 feed gas in a conventional ICP-etch reactor. Our approach has been tested for metal-oxides including ZnO, Al2O3, TiO2, and HfO2. Additionally, we investigate the poly(methyl methacrylate) (PMMA) and polyvinylpyrrolidone (PVP) as growth inhibition layers for AS-ALD of TiO2. Contact angle, XPS, spectroscopic ellipsometer, energy dispersive X-ray spectroscopy (EDX), and scanning electron microscopy (SEM) measurements were performed to investigate the blocking ability of polymer layers against ALD-grown films. Characterizations carried out revealed that effective blocking on fluorocarbon layer is achieved for ZnO film upto 136 growth cycles. On the other hand, a rather slow nucleation has been observed for HfO2 growth on fluorocarbon coated surfaces, while TiO2 and Al2O3 growth showed almost no delay with a growth rate equal to the ones on conventional substrate surfaces. For TiO2, PMMA revealed successful growth inhibition upto the maximum inspected growth cycles while PVP was able to block TiO2 growth upto 300 growth cycles. By exploiting this inhibition feature, thin film patterning has been demonstrated by growing ZnO films on photo lithographically patterned fluorocarbon/Si samples. We also demonstrate nanoscale patterned deposition of TiO2 using a PMMA masking layer that has been patterned using e-beam lithography.

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Plasma-Enhanced Atomic Layer Deposition, III-Nitrides, Area-Selective Atomic Layer Deposition, Metal Oxides, Thin Films, Nano-Cylinders, Thin-Film Patterning
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