||Marko Tadjer, Travis J. Anderson, Karl Hobart, Tatyana Feygelson, Joshua Caldwell, Charles Eddy, Jr., Fritz Kub, James Butler, John Melngailis
High electron mobility transistors (HEMTs) based on the AlGaN/GaN heterostructure are excellent candidates for high temperature power switching applications due to their wide band gap. However, the reduction of drain current that occurs at high drain bias, referred to as self-heating, can lead to reduced switching efficiency [1-2]. Capping the HEMT with CVD diamond has been explored in the past, but the HEMT gate temperature sensitivity have limited the diamond growth temperature . In this work, we present HEMTs fabricated on 2 nm/17.5 nm/2 μm thick GaN/Al<SUB>0.26</SUB>Ga<SUB>0.74</SUB>N/GaN structures on (111) Si substrates with integrated nanocrystalline diamond (NCD) heat spreading films. The HEMTs were grown by metal-organic CVD with sheet resistance R<SUB>SH</SUB>=557 Ω/sq. A 50 nm thick layer of SiO<SUB>2</SUB> was deposited by PECVD to serve as surface passivation and NCD nucleation layer. The 500 nm thick NCD film was grown at 750°C, 100 nm per hour, and was unintentionally B-doped for p-type conductivity. Hall measurements on a van der Pauw geometry performed before NCD deposition yielded R<SUB>SH</SUB>=563 Ω/sq., a sheet carrier concentration n<SUB>s</SUB>=8x10<SUP>12</SUP> cm<SUP>-2</SUP>, and Hall mobility μ<SUB>h</SUB>=1507 cm<SUP>2</SUP>/V-s at room temperature. Following NCD growth, the measurements resulted in n<SUB>s</SUB>=5.7x10<SUP>12</SUP> cm<SUP>-2</SUP>, μ<SUB>h</SUB>=399 cm<SUP>2</SUP>/V-s, R<SUB>SH</SUB>=2860 Ω/sq., and ρ<SUB>c</SUB>=9.5x10<SUP>-3</SUP> Ω-cm<SUP>2</SUP> (31 Ω-mm). The isolation current between devices increased significantly, from the nA to the mA range. This current did not decrease when undoped NCD was deposited, and was attributed to surface accumulation on the diamond film due to the negative electron affinity of H-terminated NCD . A negative shift in threshold voltage indicated that positive charge was accumulated in the diamond layer, in addition to the SiO<SUB>2</SUB> charge. Device temperature profiling was performed using Raman thermography . This spectroscopic technique measures the shift in the Raman transverse optical (TO) phonon mode peak position caused by temperature-dependent phonon scattering. The measurement was calibrated by placing the samples on a heated chuck and determining the Raman shift at a given temperature. The shifts in the GaN TO (568 cm<SUP>-1</SUP>) and Si (521 cm<SUP>-1</SUP>) Raman modes were recorded under DC bias and the Raman peak position was used to determine the corresponding device temperature. The sample with NCD heat spreading layers exhibited about 20% lower temperature than the reference HEMT. The authors acknowledge Dr. Edwin L. Piner (Nitronex Inc.) for HEMT growth.  S. C. Binari et al., IEEE Trans. Electr. Dev., vol. 48, no. 3, pp. 465, 2001.  H. I. Fujishiro et al., Phys. Stat. Sol. (c) 2, no. 7, 2696-2699 (2005).  M. Seelman-Eggebert et al., Diamond and Relat. Mater. 10 (2001), 744-749.  J. Cui et al., Phys. Rev. Lett. 81, 429 (1998).  R. J. T. Simms et al., IEEE Trans. Electr. Dev., vol. 55, no. 2, 2008.