MOCVD growth and study of thin films of Indium Nitride

Open Access
Author:
Jain, Abhishek
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 30, 2006
Committee Members:
  • Joan Marie Redwing, Committee Chair
  • Darrell G Schlom, Committee Member
  • Theresa Stellwag Mayer, Committee Member
  • Joseph R Flemish, Committee Member
Keywords:
  • MOCVD
  • InN
  • polarity
  • graded buffer
  • in-situ stress analysis
  • growth mechanism
Abstract:
InN is the least studied group III nitride semiconductor, because of challenges associated with growth, including a low dissociation temperature (~500 °C) and the lack of a lattice matched substrate. With the development of epitaxial growth techniques, molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) and the discovery that the band-gap energy of InN is smaller (~0.7 eV) than the previously established value (~1.9 eV), there has been renewed interest in growth and development of InN thin films for potential applications in solar cells and light emitting diodes. The majority of InN growth studies thus far have utilized MBE, due to its abilities to produce active nitrogen species via plasma-enhanced methods, which is essential for low temperature (~500 °C) growth, and to precisely control In/N ratio. These studies provided valuable information on the electrical and optical properties of InN and insight into the effect of crystalline polarity on film growth. MOCVD growth of InN has proven more challenging due to the thermal stability of NH3 which is used as the nitrogen source in this process and difficulties controlling both the polarity and local In/N ratio during growth. However, there is significant interest in developing an MOCVD process for InN growth since it is the epitaxial growth technology currently in use for commercial fabrication of group III-nitride thin films. This thesis is therefore focused on a study of MOCVD growth of InN with the goal of providing new information on the effects of growth conditions and buffer/substrate materials on InN film properties. Initial studies, using both (111) Si and (0001) sapphire substrates, identified an optimum growth temperature window of 540-560 °C for the formation of stable InN films. Thin AlN buffer layers, grown at low temperature (~500 °C) were used to improve the structural properties of InN grown on sapphire. An AlN buffer layer thickness of 30 Å was determined to be optimum for growth of continuous InN films with reduced surface roughness. InN film was also observed to be well-oriented with the c-plane sapphire substrate, for optimum LT-AlN buffer thickness of 30 Å, with rocking curve FWHM value of 0.5° for (0002) InN peak. When attempting to grow InN films on sapphire with thicknesses greater than approximately 150 nanometers using an AlN buffer layer, the InN films were observed to delaminate from the buffer/substrate at growth temperature. In-situ wafer curvature measurements were then carried out to study the evolution of film stress during MOCVD growth to better understand the origin of the film delamination problem. The InN films were observed to grow under a ~0.2 GPa tensile stress which is believed to originate from island coalescence and the three-dimensional growth mode of the film. The combined effect of compressive stress due to high lattice mismatch between InN and AlN (~14%) and tensile stress due to grain coalescence along with the relatively weak bond strength of InN compared to GaN and AlN, is believed to cause the InN film to crack along the interface and delaminate. To further investigate the effect of the buffer layer on InN growth, studies were carried out using GaN films grown on sapphire as the growth template. Recent MBE results had indicated a significant difference in the thermal stability and growth mode of In-polar and N-polar InN, with improved properties reported for N-polar material grown on N-polar GaN. MOCVD growth of N-polar GaN is very difficult; consequently, all of the results reported in the literature for InN growth on GaN were likely carried out on Ga-polar material resulting in films with a high surface roughness. In order to study the effect of film polarity in MOCVD growth, N-polar GaN films on sapphire, produced by MBE, were obtained and used as templates for InN growth for comparison to growth on Ga-polar GaN grown by MOCVD and HVPE. By utilizing N-polar and Ga-polar GaN films, it was possible to produce N-polar and In-polar InN films by MOCVD, as determined by convergent beam electron diffraction (CBED) analysis. Furthermore, the polarity was found to dramatically alter the surface roughness and growth mode of the InN films with enhanced lateral growth and reduced surface roughness obtained for N-polar InN. A qualitative model was proposed to explain the different growth mechanisms observed for In-polar and N-polar InN. In spite of the improvements in surface morphology obtained with growth of N-polar InN, delamination at the InN/GaN interface was still observed in these films, and was also present in In-polar InN samples. Attempts were made to further reduce the lattice mismatch and improve the adhesion between InN and GaN by using a compositionally graded InGaN buffer layer. The fabrication of InGaN over its entire composition range is challenging since the optimal growth parameters window for InGaN varies with composition and film quality is strongly dependent on temperature and precursor flow rates. Initial graded InGaN buffer layers were produced by using optimum growth conditions for low-In fraction InGaN (800 °C) and InN (~540 °C) as the initial and final growth set points of the buffer layer and continuously varying growth conditions between these two extremes. Cross-section samples were prepared for transmission electron microscopy and chemical analysis of the graded layers was carried out using x-ray energy dispersive spectroscopy (XEDS) in the scanning TEM mode. The In fraction was observed to vary over the entire composition range in the buffer layer, however, a ~40 nm thick region of approximately constant composition (~In0.5Ga0.5N) was observed at the center of the graded layer. The region of constant composition also had a much higher defect density than the rest of the graded layer possibly due to phase segregation which is predicted to occur in InGaN. Consequently, the structural properties of the InN films grown on the graded InGaN layers were comparable to films grown directly on GaN, however, the film adhesion was significantly improved with no evidence of interfacial cracks between the InN and GaN. These preliminary results indicate that graded InGaN layers can be used to improve the adhesion of InN on both Ga-polar and N-polar GaN, however, further work is needed to develop graded InGaN buffer layers or constant composition InGaN interlayers with improved structural properties for InN growth.