Plasma-assisted atomic layer deposition of III-nitride thin films

Abstract

III-nitride compound semiconductors and their alloys have emerged as versatile and high-performance materials for a wide range of electronic and optoelectronic device applications. Besides possessing very unique material properties individually, members of the III-nitride family with wurtzite (hexagonal) crystal structure also exhibit direct band gaps, which cover a wide range with values of 6.2, 3.4 and 0.64 eV for AlN, GaN and InN, respectively. In this respect, ternary and quaternary alloys of this family are particularly important since their bandgaps can easily be tuned by adjusting the alloy composition. Although high quality IIInitride thin lms can be grown at high temperatures (>1000 XC) with signi cant rates, deposition of these lms on temperature-sensitive device layers and substrates necessitates the adaptation of low-temperature methods such as atomic layer deposition (ALD). ALD is a special type of chemical vapor deposition, in which the substrate surface is exposed to sequential pulses of two or more precursors separated by purging periods. When compared to other low-temperature thin lm deposition techniques, ALD stands out with its self-limiting growth mechanism, which enables the deposition of highly uniform and conformal thin lms with sub-angstrom thickness control. Moreover, alloy thin lms can be easily deposited by ALD, where lm composition is digitally controlled by the relative number of subcycles. In this thesis, we report on the development of plasma-assisted ALD (PAALD) processes for III-nitrides, and present detailed characterization results for the deposited thin lms and fabricated nanostructures. PA-ALD of polycrystalline wurtzite AlN thin lms was realized at temperatures ranging from 100- 500 XC using trimethylaluminum (AlMe3) as the Al precursor. Films deposited at temperatures within the ALD window (100-200 XC for both ammonia (NH3) and N2/H2 plasma processes) were C-free and had relatively low O concentrations (<3 at.%). We also demonstrated the conformality of AlMe3-NH3 plasma process by fabricating high surface area AlN hollow nano bers using electrospun nylon nano ber mats as sacri cial templates. Our initial e orts for depositing GaN and InN resulted in thin lms with high O concentrations. Although - at rst - the most probable source of this contamination was presumed as the O-containing impurities in the unpuri ed 5N-grade NH3 gas, subsequent experiments revealed the true source as the quartz tube of inductively coupled RF-plasma (ICP) source itself. In view of these circumstances, the choice of N-containing plasma gas (NH3, N2/H2 or N2) determined the severity of O incorporation into AlN and GaN lms deposited by PA-ALD. As an e ort to completely avoid this plasma-related oxygen contamination problem, we replaced the original quartz-based ICP source of the ALD system with a stainless steel hollow cathode plasma (HCP) source. Thereby we demonstrated the low-temperature hollow cathode PA-ALD (HCPAALD) of crystalline AlN, GaN and AlxGa1−xN thin lms with low impurity concentrations (O, C <1 at.%) using AlMe3 and trimethylgallium (GaMe3) as the Al and Ga precursors, respectively. Optical band edge values of the AlxGa1−xN lms shifted to lower wavelengths with the increasing Al content, indicating the tunability of band edge values with alloy composition. HCPA-ALD of InN was also investigated within the scope of this study. Initial results revealed the possibility to obtain single-phase wurtzite InN thin lms using cyclopentadienyl indium (CpIn) as the In precursor.