Vacancy Defects in Si-doped β-(Al,Ga)₂O₃

While n-type doping is routinely achieved for β-Ga₂O₃, it is not a straightforward question for its wider band gap alloys. Cation vacancies are archetypical compensating defects for n-type dopants in compound semiconductors. In this work, we study their formation in Si-doped single crystals of β-(Al,Ga)₂O₃ [1] with positron annihilation spectroscopy [2]. We present results obtained in 8 different (16 pieces) Czochralski-grown β-(Al,Ga)₂O₃ single crystals with Al content 0, 10, 20, and 25%, in both undoped and n-type doped form [1]. The unintentional Si content in the undoped crystals is 2-3×10¹⁷ cm^–3; in the doped crystals, the Si content is 3-6×10¹⁸ cm^–3 as measured by SIMS. Hall measurements show that the undoped crystals are n-type for all Al mole fractions with the carrier concentration roughly matching the unintentional Si content, but in the Si-doped crystals the doping efficiency is reduced at higher Al content, and the doped 25% alloy is electrically insulating [2].

We present results obtained with 4D Doppler broadening spectroscopy [3] and positron lifetime spectroscopy, both performed as a function of temperature. The undoped samples show the unusually high anisotropy observed in β-Ga₂O₃ for all Al contents. In contrast, the Si-doped samples exhibit significantly smaller anisotropy irrespective of the Al content. There is a similar clear distinction between the two groups of samples (undoped vs. Si-doped) in the average positron lifetime in the temperature range of 40-600 K. The positron lifetime is significantly longer (190-220 ps) in the Si-doped samples than in the undoped samples. In the undoped samples, the positron lifetimes are in the same range (170-195 ps) as those obtained earlier in Sn-doped, Fe-doped, and Mg-doped crystals [3,5]. Importantly, there is no difference between the Si-doped samples with Al content of 10-25 %. The presence of Al in all Si-doped crystals causes the emergence of negatively charged defects with no or very little open volume, as can be seen in the low-temperature range from both Doppler and lifetime data.

The results indicate that Si doping generates unrelaxed Ga vacancy-related defects in all the samples, as opposed to the split Ga vacancies that typically dominate the positron annihilation signals in β-Ga₂O₃ [3-6]. However, these unrelaxed Ga vacancies do not appear to be the key to understanding the sudden strong passivation of the Si dopants at higher Al content, as there appear to be no significant differences between the Si-doped samples across the Al content range in our experiments. Similarly, the negative ion-like defects are also independent of the Al content. Further work is required to resolve this issue.

[1] Z. Galazka, et al., J. Appl. Phys. 133, 035702 (2023).
[2] F. Tuomisto, I. Makkonen, Rev. Mod. Phys. 85, 1583 (2013).
[3] I. Zhelezova, et al., J. Appl. Phys. 136, 065702 (2024).
[4] A. Karjalainen, et al., Phys. Rev. B. 102, 195207 (2020).
[5] A. Karjalainen, et al., Appl. Phys. Lett. 118, 072104 (2021).
[6] A. Karjalainen, et al., J. Appl. Phys. 129, 165702 (2021).