Growth and Characterization of Ultra-wide Bandgap III-N by Plasma-Assisted Molecular Beam Epitaxy

Abstract

N-type doping of ultra-wide bandgap aluminum nitride (AlN) is pivotal for electronic and optoelectronic applications as it enhances the material's conductivity, enabling the fabrication of high-performance devices such as transistors, diodes, and sensors. The versatility and compatibility of n-type doped AlN with existing semiconductor technologies make it suitable for integration into various electronic and optoelectronic systems, while its thermal stability ensures reliability under extreme operating conditions. Recent advancements in ultra-wide bandgap materials, such as AlScN and InAlN, have shown significant potential for high-frequency RF and ferroelectric applications, further expanding their utility in advanced electronic and optoelectronic applications. In the first part of this dissertation, the demonstration of controllable Si doping in N-polar AlN films grown on single crystal AlN substrates by plasma assisted molecular beam epitaxy (PAMBE) is presented. Through optimization of growth conditions, high quality N-polar AlN films at 950°C is obtained. However, our studies revealed that Si incorporation dramatically decreases at such high growth temperature. To enable higher Si incorporation, a hybrid low-temperature and high-temperature growth condition was developed by using Ga as a surfactant at low-temperature growth. By lowering the growth temperature of AlN to 750°C, I was able to incorporate Si with concentrations as high as 2×〖10〗^20 〖cm〗^(-3) and demonstrated an electron concentration as high as 1.25×〖10〗^19 cm^(-3) at room temperature. In the second part, the investigation of polarization doping of N-polar graded Al_x Ga_(1-x) N films grown on single crystal AlN substrates by PAMBE, is presented. Room temperature hall measurement confirms the presence of three-dimensional electron gas (3DEG) in the N-polar graded Al_x Ga_(1-x) N films, and a sheet electron concentration of 6.9×〖10〗^12 cm^(-2) as well as a mobility of 37 cm^2/(Vs) have been obtained. Temperature dependent Hall measurement reveals that electron concentration remains almost constant in the temperature range of 50K - 400K. After performing extrinsic doping of the graded Al_x Ga_(1-x) N films with 1×〖10〗^20 cm^(-3) Silicon (Si), I obtained an electron concentration of 1.1×〖10〗^20 cm^(-3) and a mobility of 39 cm^2/Vs at room temperature. A minimum specific contact resistivity of 4.5×〖10〗^(-5) Ωcm^2 has been obtained from the 1×〖10〗^20 cm^(-3) Si-doped N-polar graded Al_x Ga_(1-x) N films. In the third part, high temperature growth of N-polar InAlN films on N-polar single crystal GaN substrate have been demonstrated by PAMBE. Through optimization of the growth conditions, high quality In0.18AlN0.82N films, that is lattice matched to GaN, were grown, which enabled to grow N-polar GaN HEMT with In0.18AlN0.82N backbarrier. Through varying the growth temperatures of InAlN and the back barrier thickness, a 2DEG sheet charge density 3×〖10〗^13 cm^(-2) and an electron mobility of 1320 cm^2/Vs have been obtained in N-polar InAlN/GaN HEMTs. The devices demonstrated a small signal current gain cut-off frequency of 33 GHz and power gain cut-off frequenct of 50 GHz. In the last part, I present the demonstration of the epitaxial growth of ultra-wide band gap wurtzite AlScN on 4° miscut N-polar GaN-on-Sapphire substrate, bulk N-polar GaN substrate and Ga-polar GaN-on-Sapphire template, using the PAMBE. The samples yield very smooth surface morphology with RMS roughness ~1nm. The wurtzite phase of the AlScN samples The wurtzite structure could be maintained at temperatures as high as 800°C.