The unique MR finding of Rathke cleft cysts--high signal intensity on T1-weighted images and low signal intensity on T2-weighted images--might depend mainly on protein concentration, not on cholesterol.

We report a systematic investigation of the transport properties of highly degenerate electrons in Ge-doped and Si-doped GaN epilayers prepared using the pulsed sputtering deposition (PSD) technique. Secondary-ion mass spectrometry and Hall-effect measurements revealed that the doping efficiency of PSD n-type GaN is close to unity at electron concentrations as high as 5.1 × 1020 cm−3. A record low resistivity for n-type GaN of 0.16 mΩ cm was achieved with an electron mobility of 100 cm2 V−1 s−1 at a carrier concentration of 3.9 × 1020 cm−3. We explain this unusually high electron mobility of PSD n-type GaN within the framework of conventional scattering theory by modifying a parameter related to nonparabolicity of the conduction band. The Ge-doped GaN films show a slightly lower electron mobility compared with Si-doped films with the same carrier concentrations, which is likely a consequence of the formation of a small number of compensation centers. The excellent electrical properties presented in this letter clearly demonstrate the striking advantages of the low-temperature PSD technique for growing high-quality and highly conductive n-type GaN.

Highly conductive Ge-doped GaN epitaxial layers were grown by low-temperature pulsed sputtering, and their fundamental structural and electrical properties were investigated. The room-temperature (RT) electron concentration was increased to 5.1 × 1020 cm−3 by the Ge doping, and the atomically flat stepped and terraced surface and the crystalline quality of the layers were maintained. Consequently, the RT resistivity was reduced to 0.20 mΩ·cm, which is comparable to that for typical transparent conductive oxides such as indium tin oxide. The contact resistance of Ge-doped GaN with a Ti/Al/Ti/Au metal stack prepared without annealing was as low as 0.087 Ω·mm. Furthermore, the selective formation of a Ge-doped region using an SiO2 mask was demonstrated. The results clearly indicate the strong potential of pulsed sputtering Ge-doped GaN growth for forming low-parasitic-resistance contact layers of various electrical and optical devices.

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