III-Nitride Epitaxy for Electronic and Optoelectronic Devices
Authors: Shahedipour-Sandvik F. Shadi , Mahaboob Isra , Marini Jonathan , Hogan Kasey , Rocco Emma 
Published: February 2020

High quality AlxGa1-xN material is sought after for a large range of applications including emitters, detectors, and power electronic devices. To realize deep ultraviolet (UV) LEDs and Lasers, highly conductive films with high crystalline quality and high Al content AlGaN are required. Epitaxially grown AlN and AlGaN serve as buffer layers for GaN-based devices on Silicon. Moreover, AlGaN layers form the cornerstone of III-N high electron-mobility transistors. Given the importance of this material, significant research effort has been devoted toward the development of epitaxial techniques for the growth of AlxGa1-xN, with many challenges remaining. For the growth of high quality, higher composition AlGaN, GaN substrates and templates are unsuitable due to the higher lattice constant of GaN compared to AlGaN which causes tensile strain in the overgrown AlGaN leading to cracks and defects. Native substrates for AlGaN are non-existent, and AlGaN-based layers and device structures are commonly heteroepitaxially grown on non-native substrates such as sapphire, SiC, and Si.

Due to its lack of centerosymmetry along (0001) direction, III-Nitrides can be grown polar (along 0001 direction for Ga-polar and 000-1 direction for N-polar), non-polar (along a- and m- directions) and in between to allow semi-polar growth. The past two decades have shown tremendous progress in possibilities for growth and device design based on these polarities. Here we present our progress in developing a number of epitaxial growth techniques for devices their function is inspired by specific polarity direction, including AlGaN/GaN

When grown along (0001) direction, AlGaN/GaN HEMT structures demonstrate normally-on characteristics with 2DEG density in the 1013 cm-2 range and mobility in excess of 1200 Vsec/cm2. Growth on Si substrate offers opportunity for integration with the rest of the circuitry as well as for development of stretchable electronics where liftoff from substrate is essential. The past decade has seen great progress in managing stress that is developed due to the large thermal expansion coefficient mismatch between the Si substrate and III-Nitride layers. In our research, we employ MOCVD growth technique to develop stress-engineered/high-quality device structure-on-Si. Here we describe two methodologies to obtain thick buffer layers with reduced stress levels which are desirable for obtaining high breakdown voltage structures for power devices: Implementation of in-situ stress-monitoring to monitor stress evolution to achieve sufficient compressive stress in the AlGaN/AlGaN buffer layers; and use of selective area epitaxy to develop as-grown HEMT structures in areas patterned in active device geometric designs (e.g. horseshoe for stretchable geometry).

The N-polar orientation has potential for improved and expanded device design space due to the reversal of the built-in and stress-induced polarization fields. Achieving high-conductivity p-type material is crucial for many device applications, but has traditionally been a challenge in III-Nitrides. Previous work has found comparable levels of intentional impurity incorporation and carrier concentration between Ga- and N-polar GaN grown by conventional MOCVD. Recently, pulsed MOCVD growth has emerged as a promising way to achieve higher carrier concentration in Ga-polar p-type GaN, and especially as an effective means of increasing conductivity in p-type AlGaN films . The effect of pulsed growth in the N-polar orientation on dopant incorporation and conductivity is so far unstudied, and has the potential for improved conductivity for p-type N-polar films. Our studies of p-GaN:Mg show a strong effect on incorporation due to polarity when using the pulsed growth mode, with a factor of 3 or more reduction in Mg incorporation for N-polar films grown as compared to traditional continuous growth, from 3x1019 cm-3 to 1x1019 cm-3.

Here, we examine the effects of growth mode and polarity on dopant incorporation and activation in single layer and device structures using in-situ stress monitoring, secondary ion mass spectroscopy (SIMS), AFM, XPS, Hall Effect measurements and internal photoemission (IPE).