Sheet resistance under Ohmic contacts to AlGaN/GaN heterostructures

Abstract

For the determination of specific contact resistance in semiconductor devices, it is usually assumed that the sheet resistance under the contact is identical to that between the contacts. This generally does not hold for contacts to AlGaN/GaN structures, where an effective doping under the contact is thought to come from reactions between the contact metals and the AlGaN/GaN. As a consequence, conventional extraction of the specific contact resistance and transfer length leads to erroneous results. In this Letter, the sheet resistance under gold-free Ti/Al-based Ohmic contacts to AlGaN/GaN heterostructures on Si substrates has been investigated by means of electrical measurements, transmission electron microscopy, and technology computer-aided design simulations. It was found to be significantly lower than that outside of the contact area; temperature-dependent electrical characterization showed that it exhibits semiconductor-like behavior. The increase in conduction is attributed to n-type activity of nitrogen vacancies in the AlGaN. They are thought to form during rapid thermal annealing of the metal stack when Ti extracts nitrogen from the underlying semiconductor. The high n-type doping in the region between the metal and the 2-dimensional electron gas pulls the conduction band towards the Fermi level and enhances horizontal electron transport in the AlGaN. Using this improved understanding of the properties of the material underneath the contact, accurate values of transfer length and specific contact resistance have been extracted.

Comparison between AlGaN surfaces etched by carbon tetrafluoride and argon plasmas: Effect of the fluorine impurities incorporated in the surface

“…We investigated the compositional and morphological changes in Al 0.24 Ga 0.76 N surfaces etched by the CF 4 and Ar plasmas at various gas pressures and etching times. The comparison between the results induced by the CF 4 and Ar plasma etchings clarified the contribution of fluorine impurities incorporated in the surface by the CF 4 plasma etching.…”

“…We also checked the etch depths of the surfaces because they are generally relevant to the compositional and morphological changes in the surfaces. We discuss a difference between the surface characteristics induced by the CF 4 and Ar plasma etchings in terms of the incorporated fluorine impurities.…”

“…In particular, the effect of fluorine impurities incorporated in the surface by the CF 4 plasma etching has been examined by the comparison between the results induced by the CF 4 and Ar plasma etchings. Here, it should be noticed that neither of these plasma chemistries is intended for creating deep features in AlGaN because there is no chemical component to the etching and because the CF 4 plasma will create an etch-stop reaction through the formation of AlF 3 [21,22].…”

“…In the present study, we have investigated compositional and morphological changes in AlGaN surfaces etched by CF 4 and Ar plasmas at various gas pressures and etching times by using a radio-frequency (RF) capacitively coupled plasma (CCP) reactor. In particular, the effect of fluorine impurities incorporated in the surface by the CF 4 plasma etching has been examined by the comparison between the results induced by the CF 4 and Ar plasma etchings.…”

Improved Ohmic contact to GaN and AlGaN/GaN two‐dimensional electron gas using trap assisted tunneling by B implantation

“…Si implanted processes are also in use to make the GaN heavily n‐type before the formation of Ohmic contacts . It is generally understood that the dominant mechanism for electron transport is tunneling between TiN and heavily doped GaN formed due to nitrogen vacancy . Here, we have shown that intentional traps created by a boron (B) implantation can significantly reduce the contact resistance for GaN‐based heterostructures.…”