Improvement of GaN Deep Etched Surface State by Fluorination Dedicated to Power Devices

Gallium nitride (GaN) is a III-V semiconductor with a large and direct bandgap (3.4 eV). Due to this bandgap, GaN is already used in optoelectronic applications for blue Light Emitting Diodes (LED), white LED and Blu-ray disc. Furthermore, GaN has a high electron mobility and strong chemical bonds (8.92 eV). These physical properties make GaN very interesting for microelectronics and open new prospects. Indeed, GaN-based devices, compared to silicon devices, can operate under high temperature, high power and high frequency.

        For GaN-based power devices, an etched depth as high as 6 to 10 µm is typically required. This is considered as deep etching compared with the usual etched depth for light emitter devices (a few hundred nanometers) [1]. It was shown that wet etching of GaN c-plan (plan where etching is generally needed) is limited due to its chemical inertness [2]. Therefore, GaN deep etching is achieved by plasma etching. Chlorine-based chemistries are commonly used because GaCl₃is the most volatile Ga etching product.

        GaN (8 µm thick epilayer grown on Si) MESA etching was studied in Cl₂/Ar based plasmas using two industrial Inductively Coupled Plasma (ICP) reactors (with and without diffusion chamber) and Ion Beam Etching (IBE). Our studies were carried out on samples mounted on a carrier wafer. After etching, regimes of defects were observed. Etch pits are linked to nanopipes and dislocations created during epitaxial growth of GaN [3]. Preferential oxidation of dislocations leads to columns appearance. Oxygen based species come from either the SiO₂ carrier wafer or the alumina/quartz tube of ICP sources. In the case of surface over-oxidation, a very rough surface, called “White GaN”, is obtained. For industrial applications, both defects and roughness must be limited. Using these 3 reactors and different carrier wafer materials, etched GaN surfaces were compared in order to point out the best etching condition. In ICP reactors, usual Cl₂/Ar based-chemistry was modified. Addition of SiCl_4, CHF₃ or SF₆ combined with the use of a Si carrier wafer results in a decrease of the defect density and, in some cases, in their elimination. Plasma and surface investigations by Langmuir probe, OES, mass spectrometry, XPS, SEM and AFM were performed to better understand GaN defects formation and Ga/N surface ratio modification. It was observed that fluorine species are able to protect GaN surface with the formation of a Ga_xF_y -like “passivation layer”. This result led to the development of a time-multiplexed etching process consisting in alternating etching and passivation steps. After etching, ripples were clearly visible on the sidewalls. This process appears to be a way to etch GaN without defects even if the carrier wafer is made of SiO₂.

        Defect free GaN etched surface can also be obtained using IBE technique. An optimum angle of 30° between the Ar⁺ion beam and the GaN surface was find to etch dislocations at the same rate as c-plan.

Finally, we studied the selectivity of GaN with different mask materials for IBE and ICP chlorine plasmas. A selectivity as high as 6 using ICP etching with a SiO₂ mask was obtained, with a GaN etch rate of 1 µm.min^-1. By using IBE, a selectivity of 5 was obtained with a TiN mask.

References

1. P. Waltereit, O. Brandt, A. Trampert, H. T. Grahn, J. Menniger, M. Ramsteiner, M. Reiche and R. H. Ploog, Nature, 406 (2000) 865-868

2. D. A. Stocker, E. F. Schubert and J. M. Redwing. Applied Physics Letters, 73 (1998) 2654-2656

3. J. Ladroue, A. Meritan, M. Boufnichel, P. Lefaucheux, P. Ranson and R. Dussart, J. Vac. Sci. Technol. A, 28 (2010) 1226