Layer-resolved release of epitaxial layers in III-V heterostructure via a buffer-free mechanical separation technique.

Layer-release techniques for producing freestanding III-V epitaxial layers have been actively developed for heterointegration of single-crystalline compound semiconductors with Si platforms. However, for the release of target epitaxial layers from III-V heterostructures, it is required to embed a mechanically or chemically weak sacrificial buffer beneath the target layers. This requirement severely limits the scope of processable materials and their epi-structures and makes the growth and layer-release process complicated. Here, we report that epitaxial layers in commonly used III-V heterostructures can be precisely released with an atomic-scale surface flatness via a buffer-free separation technique. This result shows that heteroepitaxial interfaces of a normal lattice-matched III-V heterostructure can be mechanically separated without a sacrificial buffer and the target interface for separation can be selectively determined by adjusting process conditions. This technique of selective release of epitaxial layers in III-V heterostructures will provide high fabrication flexibility in compound semiconductor technology.

Thermal Behavior of an AlGaN/GaN-Based Schottky Barrier Diode on Diamond and Silicon Substrates.

Devices based on AlGaN/GaN heterostructures, for example, Schottky barrier diodes (SBDs) and high electron mobility transistors (HEMTs), have been intensively investigated for applications to high-frequency and high-power areas. Presently, the substrates widely distributed are AlGaN/GaN on SiC for its high performance in radio frequency (RF) applications, for examples high cutoff frequency (fT) or high maximum oscillation frequency (fmax), and AlGaN/GaN on Si for its high power performance, for examples high breakdown voltage or high voltage operation. Chemical vapor deposition (CVD) diamond substrates have a thermal conductivity of 12 W/cm·K, and this is a remarkable point because HEMTs or SBDs on AlGaN/GaN on CVD diamonds are one of the promising alternatives for power and RF applications. In comparison, the thermal conductivity of AlGaN/GaN on a sapphire substrate is 0.33 W/cm·K while that of AlGaN/GaN on a Si substrate is 1.3 W/cm·K and that of AlGaN/GaN on a SiC substrate is 4.9 W/cm·K. In this work, we fabricated SBDs with a 137 mm Schottky channel length on AlGaN/GaN on Si and also on a CVD diamond substrate. We also compared the thermal behaviors of these fabricated large scale SBDs on Si and a CVD diamond substrate.

Effects of Recess Depth Under the Gate Area on the Vth-Shift for Fabricating Normally-Off Field Effect Transistors on AlGaN/GaN Heterostructures.

Fabrication of normally-off field effect transistors (FETs) possessed uniform turn-on threshold voltage (Vth) is of special interests. In this work, they were fabricated using dry etching recess techniques under the gate region, with dry etching conditions of extremely low rate. We report how the recess depth under the gate area induced the Vth shift of normally-off FETs on AlGaN/GaN heterostructure, which were fabricated with a 1.5 nm/min etching rate. Chlorine-based inductively coupled plasma (ICP) was applied to perform the etching process for the AlGaN/GaN heterostructure. Devices were fabricated with different recess depths under the gate area, and examined to determine their performances, particularly the dependence of recess time and recess depth on Vth shift. The applied dry etching conditions resulted in a low-damaged and not-rough morphology on the etched surfaces of AlGaN/GaN. Fine controlled and well defined recess depth of the AlGaN/GaN heterostructure under the gate region was achieved with no etch-stop layers. Conventional fabrication processes were applied with the dry etching conditions of extremely low rate to fabricate normally-off MOSFETs of Al2O3/AlGaN/GaN. The achieved Vth of +5.64 V was high positive and the leakage current of off-state was measured as ~10-6 A/mm.

Thermal Properties of Schottky Barrier Diode on AlGaN/GaN Heterostructures on Chemical Vapor Deposition Diamond.

High electron mobility transistors (HEMTs) and Schottky barrier diodes (SBDs) based on AlGaN/GaN heterostructure have been widely studied for high-frequency and/or high-power application. Widely distributed substrates for the high performance of RF applications are presently AlGaN/GaN on SiC, and those for high power performance are AlGaN/GaN on Si. Because the thermal conductivity of CVD diamond substrates is as high as 12 W/cm · K, devices on AlGaN/GaN on CVD diamond are one of the excellent alternatives for power and RF applications. In comparison, the thermal conductivity of AlGaN/GaN on SiC is 4.9 W/cm K, and that of AlGaN/GaN on Si is 1.3 W/cm · K. In this work, we report the fabrication of SBD devices with 163.8 mm Schottky channel length. We also compared the thermal properties of the fabricated large scale SBD devices on different substrates.