Physical analysis of normally-off ALD Al2O3/GaN MOSFET with different substrates using self-terminating thermal oxidation-assisted wet etching technique

Based on the self-terminating thermal oxidation-assisted wet etching technique, two kinds of enhancement mode Al$_{2}$O$_{3}$/GaN MOSFETs (metal-oxide-semiconductor field-effect transistors) separately with sapphire substrate and Si substrate are prepared. It is found that the performance of sapphire substrate device is better than that of silicon substrate. Comparing these two devices, the maximum drain current of sapphire substrate device (401 mA/mm) is 1.76 times that of silicon substrate device (228 mA/mm), and the field-effect mobility ($\mu_{\rm FEmax}$) of sapphire substrate device (176 cm$^{2}$/V$\cdot$s) is 1.83 times that of silicon substrate device (96 cm$^{2}$/V$\cdot$s). The conductive resistance of silicon substrate device is 21.2 $\Omega {\cdot }$mm, while that of sapphire substrate device is only 15.2 $\Omega {\cdot }$mm, which is 61% that of silicon substrate device. The significant difference in performance between sapphire substrate and Si substrate is related to the differences in interface and border trap near Al$_{2}$O$_{3}$/GaN interface. Experimental studies show that (i) interface/border trap density in the sapphire substrate device is one order of magnitude lower than in the Si substrate device, (ii) Both the border traps in Al$_{2}$O$_{3}$ dielectric near Al$_{2}$O$_{3}$/GaN and the interface traps in Al$_{2}$O$_{3}$/GaN interface have a significantly effect on device channel mobility, and (iii) the properties of gallium nitride materials on different substrates are different due to wet etching. The research results in this work provide a reference for further optimizing the performances of silicon substrate devices.     

Effects of Dislocation on High Temperature Transport Characteristics of Unintentionally Doped GaN

High temperature transport characteristics of unintentionally doped GaN have been investigated by means of high temperature Hall measurements from room temperature to 500^o C. The increment of electron concentration from room temperature to 500^o C is found to vary largely for different samples. The dispersion of temperature dependence of electron concentration is found to be directly proportional to the density of dislocations in GaN layers calculated by fitting the FWHM of the rocking curves in x-ray diffraction measurements （XRD）. The buildup levels in persistent photoconductivity （PPC） are also shown to be directly proportionM to the density of dislocations. The correlation of XRD, Hall and PPC results indicate that the high temperature dependence of electron density in unintentional doped GaN is directly dislocation related.