Simple Contact-Less Photo-Electrochemical (CL-PEC) Etching for AlGaN/GaN Heterostructures
Authors: Sato Taketomo , Oki Yugo , Shiozawa Naoki 
Published: December 2024

AlGaN/GaN high electron mobility transistors (HEMTs) are expected to become high-frequency power devices required for next-generation communication systems because of the high breakdown voltage characteristics of GaN-based materials and the high channel density and high electron mobility of the two-dimensional electron gas (2DEG) induced at the AlGaN/GaN hetero-interface. Etching is an important process for transistors using 2DEGs at the AlGaN/GaN hetero-interface as a channel, as it provides isolation to electrically separate the neighboring devices by deep etching from the AlGaN layer to the GaN layer, and also controls the threshold voltage by the thickness of the AlGaN layer. Contactless photo-electrochemical (CL-PEC) etching is performed by immersing a sample with a cathode pad in a solution containing an oxidant and irradiating it with ultra-violet (UV) light. It is very simple set-up and is one of the most promising wet etching techniques for nitride semiconductors such as GaN [1] and AlGaN [2]. In this study, we have demonstrated the deep and shallow etching of AlGaN/GaN structures by optimizing the CL-PEC conditions.
The AlGaN/GaN samples with a cathode pad were immersed into the K2S2O8-based solution, where the UV-light was irradiated to the top surface. In the CL-PEC etching, the hole concentration at the anode area to be etched and the efficiency of electron extraction at the cathode area are key parameters to control the etching rate. To demonstrate to realize both the deep and shallow etching by CL-PEC technique, the UV-light power, P
in, and the ratio of the cathode area to the anode area, C/A, was changed.
We have compared the etching rate of the samples having different cathode pads. The etching depth was linearly controlled by the time for the sample having a Ti/Al/Ti/Au ohmic cathode. On the other hand, the etching rate was very slow, and the etching was saturated for the sample having a Ti Schottky cathode. The results indicate that the deep and shallow etching can be switched by changing the cathode pad, where their conductivity plays an important role.
The effect of changes in P
in and C/A ratio on etching rate was investigated for the case of deep etching with Ti/Al/Ti/Au ohmic cathodes. The etching rate was found to be strongly dependent on P
in and independent of C/A. These results suggest that the hole concentration produced by light irradiation is the dominant factor in determining the etching rate for the sample having a Ti/Al/Ti/Au ohmic cathode. We have demonstrated that such deep CL-PEC etching is useful for separating devices fabricated on AlGaN/GaN heterostructures. Evaluation of the current-voltage characteristics between two ohmic electrodes placed across the insulating region formed by CL-PEC etching showed that deep CL-PEC etching increased the resistance between the electrodes by a factor of 108. Despite being a very simple method, this is comparable to device separation by dry etching or ion implantation.
The Shallow etching with a Ti Schottky cathode was then evaluated by AFM for etching depth and surface morphology after CL-PEC etching. Under optimum conditions, the etching rate of the AlGaN layer was very slow (<1.0 nm/min) and the residual thickness of the AlGaN layer could be accurately controlled. The etched surface was very smooth, with a root mean square (RMS) value of 0.4 nm for roughness. This shallow etching was applied to the gate recess fabrication of AlGaN/GaN HEMTs. The drain current-voltage (I
DS-V
DS) characteristics of the recess-gate HEMTs showed good pinch-off behavior, with a decrease in drain leakage current under the reverse bias and a positive shift in Vth from -3.4V (planar gate device) to 0V. The subthreshold slope (SS) values obtained from the transfer characteristics were smaller than those obtained for the planar-gate HEMTs.
The CL-PEC etching enabled device separation and threshold voltage control of AlGaN/GaN HEMTs by optimizing its conditions. We believe that the CL-PEC etching is a promising alternative to dry etching and other processes that cause processing damage.
Acknowledgements: This work was supported by JSPS KAKENHI- JP23K26131.

References

[1] F. Horikiri et al., Applied Physics Express, 12, 031003 (2019).
[2] M. Toguchi et al., Journal of Applied Physics, 130, 024501 (2021).

Figure 1