| Abstract |
In the present PhD thesis the epitaxial growth and the properties of advanced III-Nitride heterostructures for High Electron Mobility Transistors (HEMT), with higher output power compared to “conventional” AlGaN/GaN heterostructures, was studied. The epitaxial growth experiments were carried out using the molecular beam epitaxy technique, using an RF plasma source for nitrogen. Initially, the epitaxial growth of GaN (0001) on Si(111) substrates was investigated. Then, the growth and properties of InAlN alloys, spanning the entire composition range, from AlN to InN, as well as the growth of InAlN/GaN HEMTs was investigated.
It was found that the proper choice of growth temperature and III/V flux ratio, greater than unity, makes possible the growth of GaN(0001) with atomically smooth surfaces without metallic Ga droplets. The dependence of the excited molecular and atomic nitrogen as a function of the RF plasma source operating conditions was specified and it was found that the GaN growth rate is directly connected to the flux of excited molecular nitrogen.
The investigation of the heteroepitaxial growth of GaN(0001) on Si(111) substrates was focused on the study of the mechanism for the reduction of the tensile stress in the GaN thin film, by introduction of a thin AlN interlayer. It was found that the AlN interlayer was relaxed only in local areas where threading dislocations from the GaN layer end up, while it is elastically strained in all other areas.
Finally, the epitaxial growth of AlN on GaN was studied and AlN/GaN HEMT heterostructures were optimized. Control of the surface potential permitted to obtain a two dimensional electron gas (2DEG) up to 6.1x1013 cm-2, which is the highest value for all known semiconductors up to date.
Then, the epitaxial growth of thin InxA11-xN films on GaN/A12O3 and A1N/A13O2 spanning the entire composition range was studied. The role of the growth parameters (growth temperature, constituent metal fluxes, excess nitrogen flux) in the indium incorporation in InAlN was analyzed and thin InAlN films with different alloy compositions were obtained. The morphological, optical, electrical and structural properties as a function of the In composition were determined by SEM, AFM, HR-XRD, I-V, C-V and Hall effect measurements. The energy bandgap as a function of the alloy composition was, also, determined, by spectroscopic ellipsometry measurements. InAlN alloys with In content near 18% were extensively studied due to the fact that they were lattice matched to GaN (same a-lattice constant).
InAlN/GaN HEMT heterostructures were grown, with In content around 18% and InAlN layer thicknesses up to 15 nm, in order to study the 2DEG formation. Hall effect and C-V measurements revealed good results, but smaller than expected , due to electrical defects in the InAlN layers, grown at relatively low temperatures and, also, due to lateral inhomogeneity of the alloys. HEMT transistors with gate length of 1 μm were fabricated with photolithography and current densities up to ~700 mA/mm were obtained.
In order to overcome the difficulties of InAlN growth on GaN the AlN/GaN heterostructure was chosen, using very thin AlN layers. The maximization of the polarization difference between the two materials was expected to uplift the 2DEG concentration to a maximum, as long as the formation of the AlN/GaN heterostructure could avoid the insertion of crystal defects. The relaxation of the elastic stress of AlN on GaN was studied as a function of the AlN layer thickness and the degradation of the electrical characteristics of HEMTs was observed due to the formation of microcracks, when the critical AlN layer thickness was exceeded. A critical AlN layer thickness of 5 nm was determined. AlN/GaN HEMT heterostructures with sheet resistance of 129 Ω/sq were grown and transistors with gate length of 1 μm were fabricated with photolithography. A maximum drain current density of 1.8 A/mm was achieved, with a transconductance as high as 400 mS/mm.
Based on these results the research focused on the optimization of the AlN/GaN HEMTs, based on the 4.5 nm AlN optimal thickness. It was found that the 2DEG density is dependent on the Fermi level of the surface, which may not be pinned due to surface states. This can be achieved by proper coverage of the surface and prevention of possible AlN oxidation. Surface coverage with 2 nm Al resulted to Rs= 129 Ω/sq, Ns=6.1x1013 cm-2 and μ= 793 cm2/Vs. This Ns value is the highest ever reported in the semiconductor systems for a 2DEG.
In conclusion, the present PhD thesis has lead to the physical understanding of the epitaxial growth and properties of advanced III-Nitride heterostructures for the formation of a two dimensional electron gas (2DEG), with very high values of electron concentration and carrier mobility. This material is suitable for the fabrication of HEMT transistors for microwave applications with power densities that greatly surpass those of the “conventional” GaAs and GaN transistors.
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Επιταξιακή ανάπτυξη με μοριακές δέσμες ετεροδομών (In) AIN/GAN για τρανζίστορ υψηλής ευκινησίας
