Ka bant radar uygulamaları için GaN / algan HEMT yapılarında alaşımlı ve alaşımsız ohmik kontakların karşılaştırılması

Gallium Nitride (GaN) based High Electron Mobility Transistor (HEMT) is the most powerful alternative for high-power and high-frequency applications due to their unique material properties, such as high breakdown field, high electron drift velocity, high operation temperatures, high radiation resistance, and so on. A low ohmic contact resistance (Rc) is critical to enhancing the device performance at high frequency and high output power. Various metal stacks with different metal types and individual metal layer thicknesses have been reported. Electron beam evaporated Ti/Al/Ni/Au metal stacks have been widely used for ohmic contacts for GaN-based HEMT devices. In general, Ti-based ohmic contacts require higher annealing temperatures above 800 °C. Annealing metal stacks at high temperatures (above 800 °C) lead to a deterioration in the metal surface morphology and metal edge acuity which makes it difficult to carry out further operations to form the gate region. High annealing temperatures may also result in thermal degradation of the epitaxial heterostructure. It is essential to decrease the transistor dimensions including the gate length (Lg) and the source-drain spacing to operate the device at Ka-band (26.5GHz-40GHz) and higher frequencies. To realize these potentials, the transistor should have a proper epitaxial design along with lower on-resistance and smoother ohmic contact morphology. Not only ohmic contact resistance but also ohmic contact morphology and metal edge acuities of the down-scaled HEMT are very crucial since it affects the next coming device processing steps including gate alignment. Low-temperature annealing (<600oC) of metal stacks with recess etching is an alternative way of forming ohmic contacts to down-scaled GaN-based HEMTs. Annealing at lower temperatures is good for keeping metal surface morphology reasonable but this method may result in higher contact resistances which is not good for HEMT devices. Implantation could be another way of forming ohmic contacts for down-scaled HEMTs. Implanted species need to be annealed at high temperature (>1200oC) which may results in deteriorated epitaxial quality since the growth temperature of epitaxial layers is generally in the range of 900oC-1200oC. Regrown of GaN-based materials gathering great interest for forming an ohmic contact to HEMT which does not require annealing of metals which results in much better metal surface morphology compared to alloyed ohmic contacts. Both Molecular Beam Epitaxy (MBE) and Metal-Organic Chemical Vapor Deposition (MOCVD) methods are used to grown non-alloyed GaN-based material for forming ohmic contacts. To this end, this work describes the development of MOCVD regrown InGaN non-alloyed ohmic contacts for GaN-based HEMT devices designated for Ka-band application. To do the best of our knowledge, we are the first group in the world to successfully implement MOCVD regrown of InGaN layers for forming non-alloyed ohmic contacts to GaN-based HEMT devices for Ka-band applications. Entire microfabrication processes were performed at Coatings and Thin Films Laboratory at Middle East Technical University (METU) and AB MicroNano Company. MOCVD regrown InGaN material which was grown on patterned wafer exhibited high doping concentration above 1020cm-3 which is crucial for forming ohmic contact and the corresponding surface morphology of metal contacts was excellent. HEMT with alloyed ohmic contacts were also fabricated to do a performance comparison. The highlights of this study include improved contacts resistance of non-alloyed regrown InGaN ohmic contacts down to 0.3 W.mm compared to HEMT with alloyed ohmic contacts. An almost 7% improvement was observed in both drain-source current (Ids), transconductance (gm), and small-signal performance. Large-signal measurements showed that the output power of the HEMT with non-alloyed regrown InGaN ohmic contact was 3.07 W/mm which was 9% higher compared to HEMT with alloyed ohmic contacts. In this work, Atomic Layer Deposition (ALD) of Aluminum doped Zinc Oxide (AZO) films were also characterized. Al composition of AZO films was varying in 2-4% with varying growth conditions. AZO films exhibited degenerate doping which corresponds to the sheet resistance of about 10-3 .cm. Having similar electronic properties with MOCVD of InGaN thin films, ALD of AZO could be an alternative for non-alloyed ohmic contacts to GaN-based HEMT devices. In this respect, an investigation of material properties of ALD of AZO films performed. Initial trials did not yield expected results due to not optimized recess etching and post-plasma cleaning conditions.