MOCVD growth of group III nitrides for high power, high frequency applications



This paper reports on the LP-MOCVD growth optimisation of bulk GaN, GaAlN materials and GaAlN/GaN heterostructures grown on Sapphire and Silicon Carbide substrates for MESFET and HEMT applications, and on the device performances obtained with these structures. High purity and high resistivity GaN grown on sapphire and Silicon Carbide has been obtained. GaN/Al2O3 MESFETs based on such high purity GaN buffer layers, have exhibited very promising static and microwave performances: high beakdown voltage ∼200 V, ft = 12 GHz, fmax = 25 GHz and CW output power in excess of 2.2 W/mm at 2 GHz.. They have shown the best low frequency noise performances, with the lowest Hooge's parameter as compared to previous values reported in the literature for different GaN-based FETs (HFETs and HEMTs). The main mechanisms involved in the growth of GaAlN/GaN alloys have been studied and their impact on the physical properties of these materials determined. A parasitic reaction was clearly identified to occur in the gas phase between TMA and NH3, with a strong influence on the growth rate and the aluminium incorporation. The SiC substrate surface preparation (both ex-situ and in-situ) and the nucleation layer growth conditions (growth temperature, thickness, composition and strain) have been found to be key steps of the GaAlN/GaN/SiC growth process. Static characteristics of the devices such as maximum drain current Idss or pinch-off voltage have been correlated with the nucleation layer composition of the HEMT structure and the defect density of the SiC substrate. The devices performances related to our first GaAlN/GaN HEMT structures grown on Sapphire and Silicon Carbide have confirmed the high potentiality of GaN and related alloys for high power microwave transistors. Load-Pull measurements performed at 2 GHz on devices related to GaAlN/GaN/Al2O3HEMT structures, have shown a remarkably high output power density (4.4 W/mm) and absolute power level (3.2 W for 1 mm devices). These power level results for devices on sapphire substrates measured on wafer, are well in agreement with the international state of the art. Nevertheless, the thermal effects, which lead to a significant reduction of the power density with the increase of the transistor size, have been found to be more pronounced for devices on sapphire. In order to improve the thermal properties of HEMT's material, the growth process of GaAlN/GaN HEMTs was transferred onto Silicon Carbide substrates. Devices related to GaAlN/GaN/SiC HEMT structures have been measured at 10 GHz using a load pull system. They exhibited a CW output power in excess of 6.2 W/mm for a gate length of 0.25 µm. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)