Characterization Of Gallium Oxide With A Novel Non-Contact Electrical Metrology, CnCV, For Wide Bandgap Semiconductors
Authors: Wilson Marshall , Marinskiy Dmitriy , Savtchouk Alexandre , Almeida Carlos , Schrayer Bret , Lagowski Jacek 
Published: July 2022

Purpose of Work

Cost savings and short feedback time are significant advantages of the corona non-contact capacitance voltage (CnCV) technique1, recently introduced for wide bandgap SiC, GaN, and AlGaN/GaN as a replacement for C-V measurements on fabricated electrical test structures. The goal of this work is to verify the basic applicability and effectiveness of CnCV in the characterization of Gallium Oxide. The results demonstrate excellent measurement conditions on Ga2O3 epitaxial and bulk grown wafers, enabling the determination of the entire set of electrical properties in Table 1 and in Figures 1 to 5.

Results and Discussion

Non-contact electrical measurements with the CnCV technique involve electric charge dosing, ΔQ, by corona discharge in air. The response of the semiconductor is monitored as a surface voltage change, ΔV, measured with a vibrating Kelvin probe. A unique combination of charge, voltage, capacitance (C=ΔQ/ΔV), and leakage current, J=CdV/dt, measured in depletion enables the determination of the whole set of electrical properties presented in Table 1. The state-of-the-art CnCV tool by Semilab SDI was modified to include a 255nm UV light source, which provided excellent corona charge removal, restoring the Ga2O3 surface to a pre-charged condition needed for repeated measurements. An absolute Kelvin-probe calibration with a Ag/AgCl reference half-cell allowed for the quantification of the work function, WF, and electron affinity, X.
A critical requirement for CnCV applicability is the stability of deposited corona charge with respect to the magnitude of charge under the Kelvin probe and the aerial charge distribution. In this work, the assessment of charge stability on Ga2O3 surfaces was performed using surface voltage profiling after charging. An excellent stability condition was established with a thermal desorption treatment commonly used in semiconductor measurements. A constant and stable charging profile was maintained up to a maximum depletion field of about 3.5MV/cm for epitaxial surfaces and polished surfaces of bulk grown Ga2O3 wafers.
In the CnCV metrology, surface voltage mapping is used for identification of the electrical activity of defects. Such mapping of Ga2O3 after full wafer charging revealed the absence of high leakage spots which is probably the most significant difference compared to SiC. This difference is evident from the corresponding maps in Fig. 1 and Fig. 2 for Ga2O3 and 4H-SiC, respectively. The red spots in the 4H-SiC map are defects with the largest electrical activity, such as 3C-SiC related micro-structures2,3. No defect spots were observed in Ga2O3 maps. This significant result may be a consequence of the low probability of the poly-type inclusions in Ga2O3.
Examples of non-contact C-V and corresponding 1/C2 characteristics are shown in Fig. 3 for epi-Ga2O3. These results give a constant dopant depth profile in Fig. 4. According to the initial capacitance, C0, the Ga2O3 surfaces are initially in depletion with negative surface charges as given in Table 1. The presently determined electron affinity, X, was uniform across the sample (see Fig. 5). The value 3.6 ± 0.1 is close to the literature range.
In summary, we conclude that CnCV is a powerful alternative to standard C-V and mercury C-V and shall prove very useful in the investigation of important fundamental properties of Ga2O3 and the practical development and process control of emerging Ga2O3 power devices.

References

[1] M. Wilson, et al., ECS J. Solid State Sci. Technol., Vol. 6, issue 11 (2017), S3129-S3140.
[2] J. Guo et al., Journal of Crystal Growth, 480 (2017) 119-125.
[3] M Wilson, et al., CS Mantech 2021 Conference Proceedings, Orlando, FL.

Figure 1