(Invited) Recent Advances in III-Nitride Devices Using Ultrawide Bandgap AlxGa1-Xn Active Layers

Abstract

III-Nitride devices with high-Al AlxGa1-xN (x>0.4) active layers have the potential of addressing several key application areas. For these high Al-compositions the direct bandgap leads to a cut-off wavelength<290 nm which makes the material ideal for solar blind detectors. Similarly, AlxGa1-xN (x>0.4) multiple quantum wells (MQWs) are used in the active region of the deep ultraviolet (DUV) light emitting diodes. More recently, to take advantage of the higher breakdown field for high voltage devices, several researchers including us have explored Heterojunction Field-Effect Transistors (HFETs) and metal-oxide-semiconductor (MOS) HFETs with high-Al AlxGa1-xN (x>0.4) channel layers. Currently nearly all the (UWBG) AlGaN devices are fabricated on 3-5 µm thick MOCVD grown high quality AlN/sapphire templates. This choice is dictated by the deep UV optical transparency, the easy availability and the low-cost of sapphire. Increasing the AlN template thickness to larger than 5 µm leads to stress related cracking. The low thermal conductivity of sapphire and this AlN growth thickness limitation leads to device performance limitations especially for high power operation. To avoid this, recently using a pulsed MOCVD approach, we have for the first time succeeded in growing 16 µm thick, crack-free, low-defect AlN template layers on basal plane sapphire substrates. These were then used to deposit epitaxial structures for the AlGaN channel HFET/MOSHFETs. In this presentation we will present studies to establish the role that the high conductivity thick AlN buffers play in improving the devices’ electrical and thermal performance. Recently we have also for the first time conducted optical waveguiding studies at UVC wavelengths. For these studies we fabricated monolithically integrated UVC LEDs, detectors and waveguides on an AlN/sapphire template platform. Optical attenuation coefficient as low as 4.8 cm-1 was measured for Al0.5Ga0.5N waveguide with an AlN clad layer. Details of our integrated device fabrication and optical losses measurements scheme will also be discussed. Acknowledgement: This work was partially supported by Defense Advanced Research Projects Agency, Army Research Office and Office of Naval Research (Monitors: Dr. M. Gerhold. Dr. Paul Maki and Dr. Lynn Petersen).