Atomic layer etching of wide bandgap semiconductors for RF power transistors

Leuven - PhD
More than two weeks ago

Explore fundamental properties of next-generation semiconductors to impact the future of RF power technologies


Topic description

In many application areas, for instance those related to the aerospace, telecommunication, automotive and petroleum industries, there is a strong need for electronic devices capable of operating at high frequencies, high-power levels, high temperatures and that are resistant to caustic environments. This motivates the very significant interest in wide-bandgap materials. Of special importance for high-power and high-temperature microwave devices is a large bandgap Eg (i.e. a large value of Eg/kT), a high breakdown field UB, a high saturated drift velocity vd and a high thermal conductivity κ. Group III-nitrides such as GaN, AlGaN, InAlN are very promising because their properties are superior to the classical semiconductors such as Si and GaAs. The excellent thermal conductivities of the group III-nitrides, the large breakdown fields together with the large bandgaps and the high saturated drift velocity are responsible for the considerable interest in these materials for the development of future device technology nodes.

Etching processes (e.g. for layer selective etching, trimming, surface and interface passivation, contamination removal) are an essential part of semiconductor device fabrication. Due to the very small transistor dimensions selectivity and control at the (sub)atomic-layer-scale is required. Dry etching techniques can be used. These include reactive ion etching (RIE), electron cyclotron RIE, inductively coupled plasma RIE, magnetron RIE and chemically assisted ion-beam etching. Besides the complexity of the equipment used, one important disadvantage is that dry etching may give rise to damaged surfaces and bulk electronic states which are detrimental to device performance. Furthermore, etching selectivity and (sub)atomic-layer-scale control is difficult to achieve. Wet etching methods offer a simple and attractive alternative which can avoid these problems.

There are two forms of wet-chemical "open-circuit" etching. In electroless etching, an oxidizing agent in solution removes electrons from the valence-band of the semiconductor. This generates mobile charge carriers (holes) which, when localized in surface bonds, cause oxidation and dissolution of the solid. In the case of chemical etching, a local exchange of electrons occurs between surface bonds and the active etching agent in solution. The extreme chemical stability and the energetically low-lying valence-band edge of the group III-nitrides pose a problem for open-circuit etching. There are no stable oxidizing agents in solution capable of "injecting holes" into the valence band of these semiconductors. Consequently, conventional electroless etching is not possible. For this reason, one needs to resort to processes based on radicals such as OH· or SO4-· in solution. These highly reactive species have energy levels that are sufficiently low to allow charge transfer across the semiconductor solution interface. Due to their short lifetime, methods need to be developed to produce radicals close to the surface. Alternatively, III-N can be etched using (photo)electrochemical techniques. Utilization of formed oxides as surface activation agents is of high interest.

Development of new processes requires a thorough insight in the physical and chemical interactions between the semiconductor surface and the solution. For atomic layer etching, usually a kinetically controlled and self-limiting process is preferred to exclude hydrodynamic effects and defect selective etching. The learning obtained from the semiconductor/electrolyte interface studies will be used as a starting point for dielectric interface formation. An important question, key to improved device performance, is how surface chemistry relates to interface quality.

Type of work

As a PhD student, you will learn to work in a highly dynamic environment and be exposed to a large variety of analytical techniques and experimental methods. Electrochemistry in combination with inductively coupled plasma mass spectrometry will be used to obtain insight in oxidation/dissolution kinetics and mechanisms, passivation, surface stoichiometry. Physics and chemistry of surfaces and interfaces will be studied both ex situ and post operando using x-ray photoelectron spectroscopy (XPS), hard x-ray photoemissions spectroscopy (HAXPES), synchrotron radiation photoemission spectroscopy (SRPES). Other complementary techniques like elastic recoil detection analysis, photoluminescence spectroscopy, capacitance-voltage measurements, atomic force microscopy, scanning and transmission electron microscopy are available to complement your studies. 

Required background: Materials Science, Physics or Chemistry

Type of work: 60% experimental work and data analysis, 40% model development and writing.

Supervisor: Stefan De Gendt

Daily advisor: Dennis van Dorp

The reference code for this position is 1812-34. Mention this reference code on your application form.


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