From radical-enhanced to pure thermal ald of gallium and aluminium nitrides
Sourish Banerjee is a PhD student in the department of Integrated Devices and Systems (IDS). His supervisor prof.dr. D.J. Gravensteijn and his co-supervisor is dr. A.Y. Kovalgin are both from the faculty Electrical Engineering, Mathematics and Computer Science (EEMCS).
In order to continue the ever-increasing miniaturization trend of Silicon (Si)-based microelectronic components (such as microprocessor, memories and logic that are used for high-performance digital computing), in an era when we have almost fully-exploited the physical capabilities of Si, various other semiconductors that can potentially complement Si are being at present intensively researched.
Gallium nitride (GaN) and aluminium nitride (AlN) are so-called compound semiconductors which, in their monocrystalline form, have certain material properties superior to Si; e.g., direct and wider bandgap, high electron mobility and high breakdown field. However, the application of these semiconductors has been restricted to the opto-electronics and communication domains, such as light emitting diodes (LED) and high electron mobility transistors (HEMT). This is because, the progress of Si-based microelectronics (and associated process technology) has been propelled by the easy availability / ease-of-manufacturing of monocrystalline Si substrates, and the superb insulating property and interface qualities of silicon dioxide. These properties are exploited to fabricate transistors: the building block of digital electronics. In comparison, the substrates suitable for (Al)GaN-based electronics, such as bulk Al(GaN) crystals, SiC and Sapphire, have been prohibitively expensive to allow their mass-scale production, and also posed several challenges to achieve good material and electronic properties of the over-grown (Al)GaN layers. Therefore, whereas the Si-based microelectronic devices have become ubiquitous in every-day applications (e.g., smart-phones, internet-of-things, etc.), their (Al)GaN-counterparts have been limited mainly to opto-electronic applications, and the two technologies have barely crossed paths.
Combining the power of the mature Si-based process technology with the superior material properties of (Al)GaN semiconductors on one platform potentially enables future microelectronic devices in accordance with the ‘More-than-Moore’ philosophy. In parallel, exploring the polycrystalline and thin film (i.e., thicknesses below a micron) versions of (Al)GaN is also attractive, since that helps to diversify their growth requirements and process conditions. For example, polycrystalline-(Al)GaN layers can be deposited on a large range of substrates, beyond Si (such as glass and plastic) and by several physical and chemical deposition techniques. Further, they can be deposited under a wide range of temperatures and using a variety of precursors. Finally, polycrystalline-(Al)GaN layers also enable the addition of foreign elements for tuning their material and electronic properties. All these above-mentioned features allow for the realization of a wide array of micro- and opto-electronic devices, beyond the traditional LED and HEMT applications.
Atomic layer deposition (ALD) is a highly relevant technique for poly-(Al)GaN layers in view of their usage in current and future device applications. This is because of the unprecedented level of control over the layer thicknesses (almost reaching atomic level precision) that the technique offers. In addition, ALD ensures a high-degree of spatial uniformity of thickness and material properties, useful in the currently used large-sized (e.g., 300 mm in diameter) wafers, as well as offers superb conformality, which is useful for fabricating three-dimensional devices with high-aspect-ratios.
On this premise, this thesis addressed the ALD of polycrystalline (Al)GaN thin films on Si and Si-compatible substrates. A variety of activation techniques, ranging from thermal, plasma to the novel hot-wire ALD, were explored. Reports of ALD of (Al)GaN is appearing only recently in the literature, which suggests that the field is gaining prominence. Keeping in mind the possible utilization of the research results in the industry, standard industrial precursors were used. Several novel results were obtained which are recapitulated below.
The research started with developing recipes for (Al)GaN layers with plasma-enhanced ALD (Chapter 2). The changes to the layer polycrystallinity were explored by varying the plasma composition, substrates, etc. Diagnosis of the plasma provided important insights into the underlying causes behind changes to the polycrystallinity and the associated layer optical properties. The deposition of (Al)GaN was subsequently explored by the plasma-free radical-assisted hot-wire ALD technique (Chapter 3). The experiments provided critical insights into the generation and delivery of various radicals, obtained from the dissociation of the nitrogen-containing precursor, from the hot-wire to the substrate. Moreover, the results showed the relative influence of these radicals in the layer growth mechanism, and showed their changing effects with changes to the reactor geometry and deposition conditions. Such insights paved the way towards realising a novel purely-thermal, radical-free ALD process for GaN and GaCN layers (Chapter 4 and 5). The composition of such layers were charted under a wide range of deposition temperatures and reactor pressures. It was shown that, carbon, obtained from the dissociation of the Ga-containing precursor, had a profound influence on the optical properties of the layers. A conceptual optical device was proposed based on this observation. Moreover, the atypical observation of the pressure-dependent thermal ALD of GaN was reported, along with a possible explanation behind the growth mechanism. This also provided the means to selectively deposit GaN layers (Chapter 6), which is highly relevant field of industrial ALD research. Finally, the aspect of coalescence (or closure) of thin sub-10 nm AlN films, formed during the initial stages of ALD, were investigated using electrical and optical techniques (Chapter 7). The pre-coalesced cluster-like state of AlN was also shown.
In conclusion, the results obtained from this thesis, and the possible implementation of the suggested future work, are expected to advance the state-of-the-art in (Al)GaN and ALD research.