Materials Science Forum Vols. 615-617

Abstract: The influence of surface preparation of 4H-SiC substrates on structural properties of GaN grown by low pressure metalorganic vapour phase epitaxy was studied. Substrate etching has an impact on the crystallographic structure of epilayers and improves its crystal quality. The GaN layers were characterized by RBS/channelling and HRXRD measurements. It was observed that on-axis 4H-SiC is most suitable for GaN epitaxy and that substrate etching improves the crystal quality of epilayer.

Abstract: Cubic gallium nitride epitaxial layers grown on differently carbonized silicon substrates were studied by high resolution X-ray diffraction. In the case of cubic GaN layers with equal layer thickness an improvement of the layer quality in terms of full width of the half maximum can be achieved by using higher carbonization temperatures. The higher crystalline quality led to an in¬crease of the residual stress in the grown layer. An increase in the thickness of the cubic Gallium Nitride allows to improve the crystallinity and to reduce the residual stress.

Abstract: Presented are the results of studies on Ti-Al-N MAX phase formation in thin film multilayers of Ti, Al and TiN deposited on n-type GaN by magnetron sputtering. Two approaches to phase formation are shown, annealing Ti-Al-TiN multilayers at 600oC in argon and annealing Ti/Al multilayers at 600oC in nitrogen. Samples are characterized by means of High Resolution X-Ray Diffraction and Secondary Ion Mass Spectrometry profiling. As MAX phases are very stable at high temperatures the potential of their application as ohmic contacts to n-GaN devices is discussed.

Abstract: Electrical, thermal and chemical properties of Ti/Al/Ti/Au ohmic contacts with different former Ti-Al ratio are investigated for application in GaN HEMTs. Lowest resistivity of 4.22x10-5 Ω.cm2 has been obtained to the channel of the HEMT structure. It is found out that the initial Ti/Al ratio influences the optimal annealing temperature at which the lowest resistivity is obtained and the element distribution and interface chemistry of the annealed contacts. XPS analysis revealed two compounds contributing to ohmic properties: an intermetal compound AlAu2 in the contact layer and a semimetal TiN at the interface with GaN.

Abstract: In this work, Ti/Al bilayer sputtered ohmic contacts on n-type Gallium Nitride films were studied as a function of process parameters such as Ti thickness, surface cleaning procedure or annealing temperature. Epilayers, with doping concentration of 5.8x1018 at.cm-3, were grown on sapphire using AlN buffer layer. Electrical characterizations were made using circular Transfer Length Method (cTLM) patterns with a four probes equipment. Specific Contact Resistance (SCR) was then extracted for all the process conditions. Our results show that surface treatment is not a critical step in the ohmic contact process while annealing temperature has a larger impact. Finally, SCR values of 1x10-5 Ω.cm2 can be reproducibly achieved, which is of high interest for future devices fabrication using this material.

Abstract: The recent improvement of GaN material quality launched new perspective for its application in power devices. However, ion-implanted guard-ring edge terminations, necessary to improve the breakdown voltage, are not well developed as in SiC technology. Indeed, the effects of high-temperature annealing, required for the electrical activation of the implanted species in GaN, on the electrical behaviour of Schottky contact was not reported. In this work, the influence of high temperature annealing (1150-1200°C) on the surface morphology of GaN and on the electrical behaviour of Schottky contact was studied. Although the morphology of GaN surface did not substantially change after annealing, a worsening of the electrical behaviour of Schottky contact was observed. This latter was ascribed to the formation of a high density of interface states after annealing.

Abstract: Gallium nitride (GaN) vertical Schottky barrier diodes (SBDs) with a SiNx field plate (FP) structure on low-dislocation-density GaN substrates have been designed and fabricated. We have successfully achieved the SBD breakdown voltage (Vb) of 680V with the FP structure, in contrast to that of 400V without the FP structure. There was no difference in the forward current-voltage characteristics with a specific on-resistance (Ron) of 1.1mcm2. The figure of merit V2b/Ron of the SBD with the FP structure was 420MWcm-2. The FP structure and the high quality drift layers grown on the GaN substrates with low dislocation densities have greatly contributed to the obtained results.

Abstract: Gallium nitride (GaN) and related alloys (AlxGa1-xN) are promising semiconductors for high-frequency and high-power devices applications. In particular, the growth of AlGaN/GaN heterostructures on SiC substrates, due to the high thermal conductivity of SiC, can lead to significant improvement in the device performances. An important issue in the fabrication of AlGaN/GaN devices is the influence of the required thermal budget (700-900°C) on the properties of Ohmic contacts and device isolation. In this work, the influence of thermal annealing on the fabrication of AlGaN/GaN devices was studied. Ti/Al/Ni/Au multilayers led to an Ohmic behavior with a specific resistance c in the 10-5 cm2 range upon annealing between 750°C and 850°C. The electrical behavior of Ohmic contacts was correlated with the evolution of the formed phases and with the temperature behavior of c. The stability of the inter-device isolation obtained by nitrogen-implantation during annealing at these temperatures was also monitored.

Abstract: AlGaN/GaN HEMTs employing a tapered field plate, which decreases the gate leakage current and improves the breakdown voltage without any sacrifice of the forward electrical characteristics were proposed and fabricated. The tapered structure was fabricated by the wet etching process. The depletion region of the proposed structure expands toward the slope of the tapered field plate and the electric field concentration at the gate edge can be successfully suppressed. The gate leakage current of proposed device at VGS= -5 V and VDS= 100 V was decreased by 2-3 orders compared with that of the conventional one. The breakdown voltage of proposed device was 880 V while that of conventional one was 548 V.

Abstract: This paper describes and evaluates two MMIC broadband high power amplifiers in the frequency band 2-6 GHz in microstrip technology. These amplifiers have scalable output-stage periphery of 4 and 8 mm. The amplifiers are based on 1 mm AlGaN/GaN high electron mobility transistor (HEMT) technology on SiC substrate. They were fabricated in the European foundry SELEX Sistemi Integrati, which has a gate process technology of 0.5 μm. The 4 mm amplifier has exhibited an output power of 15 W and the 8 mm of 28 W at Vds=25 V in pulsed conditions. The best power performance in continuous wave are 10.5 W and 15 W for 4 mm and 8 mm, respectively. Better than 20% PAE over the 2-6 GHz frequency range is achieved in CW.