Excelling in the etching of gallium oxide

The etching of Ga₂O₃ works best when using an MOCVD chamber to direct gallium-containing organic molecules onto epilayers.

BY FIKADU ALEMA, AARON FINE, WILLIAM BRAND AND ANDREI OSINSKY FROM AGNITRON AND ABISHEK KATTA AND NIDHIN KURIAN KALARICKAL FROM ARIZONA STATE UNIVERSITY

By taking rapid strides Ga₂O₃ is now poised to shape the market for power devices. This ultra-wide bandgap semiconductor is equipped with the characteristics to enable superior devices, realised at a very competitive cost, thanks to the opportunity to make high-quality bulk substrates from the melt.

During the last decade, the performance of this class of power device has come on in leaps and bounds. Key breakthroughs have included the fabrication of devices with breakdown voltages of more than 8 kV and breakdown field strengths greater than 5 MV/cm, which is beyond the theoretical limits of SiC and GaN. However, Ga₂O₃-based devices are yet to produce a level of performance that’s close to the theoretical limit. Why? Well, in part it is due to the lack of appropriate, highly controllable, damage-free etching processes.

Controllable etching and the removal of material are essential steps in the fabrication of many different types of semiconductor devices. The capability to remove material is just as important as the ability to deposit it – this pair of complementary processes enables the shaping of semiconductor materials into well-defined device architectures.

Prior to etching, the active region of many devices tends to be protected by dielectrics, such as SiO_x or SiN_x. That’s also the case with Ga₂O₃-based devices. However, it’s not easy to find a damage-free etch process that avoids compromising the performance or the reliability of the device.

A wide range of etch processes have been employed during the fabrication of numerous β-Ga₂O₃ based devices. These processes include dry etching, wet etching, photochemical etching, and metal-assisted chemical etching. Unfortunately, all these forms of etching exhibit limitations, such as surface damage, the introduction of angled sidewalls, and anisotropic etching along certain crystal planes. Due to these downsides, none of these approaches are suitable for producing high-performance devices.

Offering far more promise are vapour-based in-situ etching techniques undertaken in the chambers of growth tools, such as those used for MBE and MOCVD. By their very nature, such approaches to epitaxial growth enable etching and re-growth of epilayers and dielectrics without breaking the vacuum, resulting in cleaner interfaces. As the quality of β-Ga₂O₃ epilayers is highest when grown by MOCVD, this type of reactor is the best choice for integrating growth and etching processes.

At Agnitron, we have a well-earned reputation as the leading provider of MOCVD reactors for the growth of β-Ga₂O₃ epilayers, with our tools now installed in a number of world-class labs. What may be less well known, however, is that our reactors can be used for the in-situ etching of Ga₂O₃ films, substrates, and related alloys. That’s the topic for the remainder of this article, where we discuss the etching of β-Ga₂O₃ using metal organic sources containing gallium and chlorine.

Etching with MBE…
The growth of Ga₂O₃ by MOCVD involves the reaction of gallium and oxygen precursors, with the rate of deposition typically governed by the concentration of gallium that’s introduced into the reactor. However, when the growth chamber is deprived of active oxygen, the gallium precursor performs a different role, etching the surface of the substrate or the epitaxial film.

In MBE, this etching process is well-known. As the growth of Ga₂O₃ progresses, this competes with the formation of Ga₂O₃ and its volatile suboxide Ga₂O – and when higher gallium fluxes land on the substrate in an oxygen deficient environment, the rate of growth of Ga₂O₃ decreases (what’s actually happening is that material is etched from the Ga₂O₃ surface, rather than growing on it).

When these conditions occur, involving oxygen deficiency, they favour formation of the extremely volatile suboxide Ga₂O, which desorbs from the surface rather than contributing to the growth of Ga₂O₃. Due to these conditions, it’s impractical to accelerate the growth rate of Ga₂O₃ with standard MBE growth processes. Drawing on this phenomenon, many researchers have turned to gallium metal to etch the surface of Ga₂O₃ in an MBE chamber. According to reports, this is a low-damage etch technique, with gallium reacting with Ga₂O₃.

....and MOCVD
We have investigated using MOCVD to etch Ga₂O₃ in collaboration with Nidhin Kalarickal’s group at ASU, who have experience of gallium etching of Ga₂O₃ in an MBE chamber. Working together, we have shown that in the absence of oxygen, metalorganic gallium-containing precursors, such as triethylgallium (TEGa), enable in-situ etching of Ga₂O₃ in an MOCVD chamber. Again, etching comes from the reaction of gallium with Ga₂O₃ to produce the volatile Ga₂O suboxide, but this time the process begins with the homogeneous decomposition of TEGa. Pyrolysis takes place when TEGa is exposed to a temperature of around 350 °C or more, which it gets from the substrate temperature, creating gallium adatoms that move over the surface. The stable organic ethylene by-product produced during pyrolysis is removed from the reactor through the exhaust, following minimal interaction with the substrate’s surface.

To monitor the etch rate in-situ, we take advantage of the significant refractive index contrast between the β-Ga₂O₃ film and the underlying sapphire substrate. We measure the etch rate by installing a fibreoptic reflectometer, operating at 470 nm, in our Agilis 100.

Figure 1. An Agnitron MOCVD reactor with a blue LED shining at the center of a 2-inch wafer (left). (Right) The red trace is the measured reflectance data, and the blue trace fits the reflectance data from which the etch rate is estimated. An etch rate of 9 µm/hr is estimated.

With this set-up, we have investigated the effect of various process conditions on the etch rate of (2.01) oriented β-Ga₂O₃ epitaxial films grown on c-plane sapphire. In particular, we have studied the TEGa molar flow rate, substrate temperature, and chamber pressure. These experiments reveal that we can vary the etch rate from around 0.3 µm/hr to 9 µm/hr (see Figure 2).

We have found that for low TEGa molar flow rates – that is, values below 100 µmol/min – the etch rate increases linearly with increasing flow rate (see Figure 2(a)). However, at higher flow rates, the etching rate only increases slightly before saturating. When the etching conditions are in the linear regime, the suboxide reaction rate is high enough to consume all the gallium adatoms that are supplied from reaching the substrate surface. Due to this, the etch rate is limited by the supplied TEGa molar flow rate, resulting in the observed linear relationship. In contrast, when the number of supplied gallium adatoms is increased to the extent that they exceed what can be consumed by the suboxide reaction, the etch rate increases sub-linearly, and then saturates.