Advancing GaN power devices with ammonia MBE

Operating in the kilovolt regime, GaN vertical p-n high-power diodes produced by ammonia MBE combine a fast growth rate with an ultra-clean thick drift layer and a smooth surface morphology.

BY ESMAT FARZANA,* JIANFENG WANG, KAI SHEK QWAH, ASHLEY WISSEL-GARCIA, KELSEY JORGENSEN, TAKEKI ITOH, ZACHARY BIEGLER, MORTEZA MONAVARIAN AND JAMES SPECK FROM THE UNIVERSITY OF CALIFORNIA, SANTA BARBARA

THE LAST FEW YEARS have seen a surge in demand for efficient power devices. It’s a ramp that’s been spurred on by the rise of a number of next-generation technologies, including fast chargers, data centres, smart grids, high-speed telecommunication, and electric transportation.

For these high-power switching applications, today’s silicon-based technology is not up to the task, with concerns surrounding its poor efficiency, excessive voltage levels, bulky system volume, extensive heating, and power loss. This has driven interest in alternatives based on newer materials systems, such as wide bandgap semiconductors, which are attracting much attention thanks to their capability to enhance power-handling while shrinking system size. Amongst these promising candidates GaN has much appeal, thanks to its excellent combination of a high material breakdown field of around 3.3 MV cm^-1, a high mobility, good thermal conductivity, and the availability of both p-type and n-type material.

With such attractive properties, it’s of little surprise that GaN has already been adopted in a wide variety of commercial electronics, including RF transistors, power amplifiers, and LEDs. However, for the case of power electronics, commercial GaN devices tend to be limited to 650 V, typically employing lateral topologies.

Switching to a vertical device geometry enables an increase in the operating range beyond 650 V, thanks to a superior electric field confinement and enhanced current capability, supported by a large-area backside contact. However, to enter the multi-kilovolt range, these vertical power devices need to have thick GaN drift layers with very low and well-controlled doping alongside minimal compensation.

Historically, it has been difficult to address these challenges, due to a lack of freestanding high-quality native substrates. This limitation explains why much of the development of conventional GaN devices has taken place on foreign substrates, such as sapphire and silicon. One downside of these heteroepitaxial growth platforms is that they lead to a high dislocation-density in the epilayers – it is about 10⁸ cm^-2. Such a high dislocation density may still be tolerable in lateral RF transistors, because dislocations remain perpendicular to the lateral channel. However, that’s not the case in vertical devices, where the threading dislocation core directly appears along the vertical channel. For these devices, dislocations severely degrade device performance, creating a high leakage, reducing breakdown voltage, and impairing current transport.

Figure 1. Unintentional background doping in ammonia MBE GaN epilayers as a function of growth rate. The GaN-on-GaN epilayers provided the lowest doping compared with the GaN-on-sapphire ones. For more details, see APL Materials 9 081118 (2021).

In recent years this state-of-affairs has shifted, due to rapid advances in freestanding GaN substrates, which have significantly lower dislocation densities, ranging from 10⁴ cm^-2 to 10⁶ cm^-2. These far lower dislocation densities are opening new doors and starting to redefine the landscape of GaN power devices. Within the GaN community, efforts are now being directed at the development of efficient strategies for growing thick epitaxial layers with low impurities that will ensure the desired high-voltage rating with vertical device structures. However, growing thick epilayers is challenging. It demands fast growth rates, comparable or more than 1 µm hr^-1, to ensure timely growth, a requirement that leads to increases in impurity concentration and surface roughness.

A significant research effort over many years has involved improving the quality of thick GaN epilayers on native substrates. However, these reports tend to focus on MOCVD-growth of GaN. It’s an epitaxial technology that offers a fast growth rate, but due to the use of a chemical-vapour-based synthesis process involving precursors, such as trimethylgallium, it often introduces a high concentration of carbon and hydrogen impurities in the GaN epilayer. These impurities are a menace, dragging down doping efficiency in GaN, due to a combination of donor compensation effects by carbon, and magnesium acceptor passivation effects by hydrogen. Due to these issues, it is far from easy to realise controllable doping over a wide range in n-type and p-type GaN with conventional MOCVD.

Figure 2. Atomic force microscopy scans of 2 µm by 2 µm area for unintentionally doped ammonia MBE GaN epilayers (a) on GaN-on-sapphire template using an indium flux beam-equivalent pressures of 0 Torr (b) on GaN-on-sapphire template using an indium flux 5 × 10⁻⁸ Torr (c) on a Mitsubishi Chemical Corporation free-standing GaN substrate using indium flux beam-equivalent pressures of 5 × 10⁻⁸ Torr. Homoepitaxial GaN growth on a free-standing GaN substrate shows the smoothest surface morphology with very low root-mean-square roughness of 0.21 nm. For more details, see APL Materials 10 081107 (2022).

Ammonia MBE for thick GaN
To overcome these challenges, our team that is based at the University of California, Santa Barabara (note that some of us have recently moved on, taking up academic positions elsewhere) have developed an alternative approach to growing thick GaN epilayers that employs ammonia MBE.

Compared with MOCVD, all forms of MBE offer four key advantages: a clean growth environment in an ultra-high vacuum, which allows the formation of high-purity GaN layers with a background doping as low as 10¹⁵ cm^-3, as well as minimal compensating impurities, such as carbon and hydrogen; a controlled doping profile over a wide range of concentrations, spanning the mid 10¹⁵ cm^-3 to 10²⁰ cm^-3, thanks to minimal compensating impurities; fully activated as-grown p-GaN, due to an absence of hydrogen passivation effects; and the opportunity to form an abrupt p⁺-n junction, due to absence of the magnesium memory effect, a well-known challenge in MOCVD GaN growth.

Within the family of MBE, ammonia MBE offers additional advantages over its plasma-enhanced MBE counterpart. The latter utilises gallium-rich growth conditions, while ammonia MBE allows the growth of GaN deep within the nitrogen-rich region. Due to this, ammonia MBE eliminates gallium droplets, a problem that plagues plasma-enhanced MBE and causes a poor surface morphology.

In recent years, part of our focus has been directed at advancing ammonia MBE growth for the development of high-voltage vertical GaN power switches. Through our pursuit of a synergistic effort that extends from ammonia MBE growth optimisation to device development, we have been able to demonstrate kilovolt-range vertical GaN-on-GaN p-n diodes.