GaN-based materials find a wide range of applications in optical and electronic devices. The recent technological breakthroughs have enabled the fabrication of high-purity epitaxial GaN on freestanding GaN substrates, making GaN materials attractive for potential power devices applications. GaN-based power devices exhibit a range of advantages, including low loss, high frequency and ability to operate at higher breakthrough voltages compared to their counterparts like silicon power devices. Notably, fabricating high-performance power devices requires precise doping control and accurate dopant concentration quantification. However, quantifying the concentration of Mg acceptors in p-type GaN is extremely challenging. This can be attributed to the difficulty in forming the required p-type ohmic contacts due to the lack of metals with higher work functions than p-type GaN. This also hinders accurate measurements of capacitance-voltage and Hall effect characteristics.
Photoluminescence measurements have been successfully used in the optical quantification of dopant concentration in silicon. This simple, contactless and non-destructive approach is typically based on the intensity ratio of the free exciton line to the impurity-bound exciton line. Its main advantage is its ability to independently quantify multiple impurities, even for samples containing both acceptors and donors. Concerning the application of photoluminescence in quantifying impurities in GaN, no quantification method based on photoluminescence has been developed for magnesium (Mg) acceptor concentration in GaN. On the other hand, various available conventional electrical methods are limited and fail to evaluate Mg acceptor concentrations in GaN effectively.
On this account, Oita University researchers: Associate Professor Masato Omori, Mr. Taisei Miyazaki, Mr. Kenta Watanabe, Mr. Maito Shiraishi, Mr. Ryusei Wada and Mr. Takashi Okawa investigated the applicability of photoluminescence-based method to evaluate Mg acceptor concentration in GaN quantitatively. In their approach, the proposed method was the first to apply the intensity ratio between free exciton emission and acceptor bound emission in the photoluminescence spectra of GaN. Metal-organic chemical vapor deposition was adopted to grow Mg-doped GaN epitaxial layers on freestanding GaN substrates. A total of five different wafers with different concentrations were prepared. The net acceptor concentration and Mg dopant concentrations were evaluated using C-V measurements and secondary-ion mass spectroscopy, respectively. The aim was to determine the calibration curves associated with Mg acceptor concentration in GaN. Their research work is currently published in the journal, Applied Physics Express.
The authors findings showed that for photoluminescence spectra at 40 K, a calibration curve ranging from to cm-3 was reported for Mg acceptor concentrations. The Mg acceptor bound exciton (ABE) intensity exhibited a linear relationship with Mg acceptor concentration relative to the free exciton intensity. With the calibration curves, the unknown Mg acceptor concentration was easily estimated. And the resulting calibration value was universal and independent of the crystal defects. In addition, the Mg acceptor concentration detection limit was found to be about 1010 cm-3. Furthermore, the emission line ratios were influenced by the changes in temperature, and the optimal temperature was identified to be 40K.
In summary, the research team developed a new method based on photoluminescence measurement for the quantitative evaluation of Mg acceptor concentration in GaN. No significant power dependence for ABE/free exciton ratio was reported within the 2.4 – 240 W cm-2 power range. Using the calibration curves and ABE/free exciton ratio plots, it was easy to obtain the Mg acceptor concentration. This method allowed for simple, unambiguous, low-cost and non-destructive Mg acceptor concentration quantification in p-type GaN. In a statement to Advances in Engineering, the authors noted that the proposed methodology is a promising candidate for high-performance power-device applications.
Omori, M., Miyazaki, T., Watanabe, K., Shiraishi, M., Wada, R., & Okawa, T. (2021). Determination of Mg acceptor concentration in GaN through photoluminescence. Applied Physics Express, 14(5), 051002.