Hot-wall MOCVD of N-polar group-III nitride materials and high electron mobility transistor structures
Sammanfattning: Group III-Nitride semiconductors: indium nitride (InN), gallium nitride (GaN), aluminum nitride (AlN) and their alloys continue to attract significant scientific interest due to their unique properties and diverse applications in photonic and electronic applications. Group-III nitrides have direct bandgaps which cover the entire spectral range from the infrared (InN) to the ultraviolet (GaN) and to the deep ultraviolet (AlN). This makes III-nitride materials suitable for high-efficient and energy-saving optoelectronic devices, such as light-emitting diodes (LEDs) and laser diodes (LDs). The Nobel Prize in Physics 2014 was awarded for the invention of efficient GaN blue LEDs, which further accelerated the research in the field of group III-nitride materials. GaN and related alloys are also suitable for high-temperature, high-power and high-frequency electronic devices with performance that cannot be delivered by other semiconductor technologies such as silicon (Si) and gallium arsenide (GaAs). For example, GaN-based high electron mobility transistors (HEMTs) have been widely adopted for radio frequency (RF) communication and power amplifiers, high-voltage power switches in radars, satellites, and wireless base stations for 5G.Recently, nitrogen (N)-polar group-III nitrides have drawn much attention due to their advantages over their metal-polar counterparts in e.g. HEMTs. These include feasibility to fabricate ohmic contacts with low resistance, an enhanced carrier confinement with a natural back barrier, and improved device scalability. Despite intensive research, the growth of micrometer-thick high-quality N-polar GaN based materials remains challenging. One of the major problems to develop device-quality N-polar nitrides is the high surface roughness, which results from the formation of hexagonal hillocks or step-bunching. Another significant hurdle is the unintentional polarity inversion, which reduces the crystalline quality and prohibits device fabrication. This thesis focuses on the development of N-polar AlN and GaN heterostructures on SiC substrates for HEMT RF applications. The overall aim is to exploit the advantages of the hot-wall MOCVD concept to grow high-quality N-polar HEMT structures for higher operational frequencies and improved device performance. In order to achieve this goal, special effort is dedicated to understanding the effects of growth conditions and substrate orientation on the structural properties and polarity of AlN, GaN and AlGaN grown by hot-wall MOCVD. N-polar AlN nucleation layers (NLs) with layer by layer growth mode and step-flow growth mode can be achieved on on-axis and 4 off-axis SiC (000¯1), respectively, by carefully controlling V/III ratio and growth temperature. Utilizing scanning transmission electron microscopy (STEM) we have established a comprehensive picture of the atomic arrangements, local polarity and polarity evolution in AlN, GaN/AlN and AlGaN/GaN/AlN in the cases of low-temperature and high-temperature AlN NLs both for on-axis and off-axis substrates. We have shown that the typically employed methods for polarity determination using potassium hydroxide wet etching could not provide conclusive results in the case of mixed-polar AlN as Al-polar domains may be easily over-etched and remain undetected. Atomic scale electron microscopy is therefore needed to accurately determine the polarity. A polarity control strategy has been developed by variation of thermodynamic Al supersaturation and substrates offcut angles in order to achieve desired growth mode and polarity. We further have developed growth strategy and have optimized the epitaxial process for N-polar GaN by multiple-step temperature processes on three types of C-face SiC substrates with different offcut angles. A highquality N-polar AlGaN/GaN/AlN heterostructure has been demonstrated. A 2DEG carrier density up to 1013 cm−2 have been demonstrated for N-polar HEMT structure.
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