Fabrication of Recessed-Gate AlGaN/GaN Hemts Utilizing Contactless Photo-Electrochemical (CL-PEC) Etching
Authors: Toguchi Masachika , Miwa Kazuki , Horikiri Fumimasa , Fukuhara Noboru , Narita Yoshinobu , Ichikawa Osamu , Isono Ryota , Tanaka Takeshi , Sato Taketomo 
Published: December 2020

AlGaN/GaN HEMTs are promising devices to attain both high-power and high-speed switching operation for the use in next-generation power electronics circuits. From the viewpoint of fail-safe and low-power consumption design, however it is necessary to attain normally-off operation with a positive threshold voltage (V
th). One of the promising approaches to attain such operation is adopting a recessed-gate structure, which can be fabricated by thinning the AlGaN layer beneath the gate electrode. We have reported the photo-electrochemical (PEC) etching process for the fabrication of the recessed-gate AlGaN/GaN HEMTs was promising to improvement of transfer characteristics of the device and control of V
th[1]. In this study, we developed the contactless PEC (CL-PEC) etching process[2-5], whose experimental system was simpler than that of conventional PEC etching process, for the fabrication of the recessed-gate structure. Consequently, for the devices fabricated by CL-PEC, the etched surface was very smooth such as the as-grown surface. The device characteristics improved to be compared with those without the recessed-gate fabrication process.

We used an i-Al0.24Ga0.76N (28.4 nm)/i-GaN heterostructure grown by metal organic vapor phase epitaxy on a semi-insulating SiC substrate as a sample. A Ti ohmic contact was formed on the edge of the sample as a cathode pad which was not connected to the outer circuit. The CL-PEC etching process was conducted by only dipped the sample into 0.025 M K2S2O8 aqueous solution (pH = 3.4) under illumination of UVC light, whose center wavelength was 260 nm and the intensity was 4 mW/cm2. The distance between UVC light source and the surface of sample was set to 4 cm, where the optical path in the solution was fixed at 0.5 cm. As an etching mask, the lamination film of SiN (100 nm) and photoresist was formed by sputtering and photolithography process. After the etching, TMAH treatment was conducted in recessed-gate region. An inductive-coupled-plasma reactive-ion-etching (ICP-RIE) was conducted for the device isolation. The source-drain electrodes (Ti/Al/Ti/Au), Schottky-gate and MIS-gate electrodes were fabricated by a standard evaporation process. The various devices with different gate lengths (3, 5 and 10 µm) were fabricated on the same chip. The gate width for all device was designed to be 100 μm.

The etching depth and the surface morphology were evaluated by AFM technique after the CL-PEC etching. An etching rate of AlGaN layer was very slow (< 1.0 nm/min) under the optimal condition, leading to the precise control of the residual thickness of the AlGaN layer. In addition, the self-termination of the etching was observed. The etching was stopped in the AlGaN layer, leaving a thickness of 6 nm. The surface after the etching was very smooth, where the root-mean-square (RMS) value of roughness was 0.4 nm (in 7 × 7 μm2 area). The drain current-voltage (I
DS-V
DS) characteristics of Schottky recessed-gate HEMTs showed good pinch-off behavior. The drain leakage currents with a reverse bias decreased and the V
th was shifted in the positive direction from -3.4V (planar-gate devices) to 0 V. The value of sub-threshold slope (SS), obtained from the transfer characteristics, was smaller than that obtained for the planar-gate HEMTs. In addition, the standard deviation, σ, of the V
th was 5.7 mV which was much smaller than that of the planar-gate devices. These results show that the uniformity of device performance enhanced after the CL-PEC process. The MIS recessed-gate HEMTs also showed good I
DS-V
DS characteristics with constant saturation currents and pinch-off behavior. As similar to the Schottky HEMTs, shift of V
th in the positive direction, and the improved V
th uniformity were observed. In addition to this, the MIS gate structure has the advantage that the device operates under positive bias by reducing gate leakage currents. This feature is promising for increasing the drain current reduced by the recess gate structure. In conclusion, excellent electrical properties such as low-leakage currents and precise control of V
th for recessed-gate AlGaN/GaN HEMTs were achieved by the low-damage CL-PEC etching with the self-terminated nature.

Acknowledgements: This work was supported by JSPS KAKENHI-JP16H06421, 20H02175.

[1] Y.Kumazaki et al., JJAP, 121, 184501 (2017). [2] F.Horikiri et al., APEX, 12, 031003 (2019) [3] M.Toguchi et al., APEX, 12, 066504 (2019). [4] F.Horikiri et al., IEEE Trans. Semicond. Manuf., 32, 489 (2019). [5] K.Miwa et al., APEX, 13, 026508 (2020).

Figure 1