Silicon doping dependence of highly conductive n-type Al _0.7Ga _0.3N

Highly conductive Si-doped n-type Al _0.7Ga _0.3N alloys were grown by metalorganic chemical vapor deposition on sapphire substrates. Variable temperature Hall-effect measurements have been employed to study the electrical properties for samples with nominal Si dopant concentration (N _si) from 2.6 to 6.8 × 10 ¹⁹ cm ^-3. For the sample with N _si=6.0 × 10 ¹⁹ cm ^-3, we have achieved n-type resistivity of 0.0075 Ω cm with an electron concentration of 3.3 × 10 ¹⁹ cm ^-3 and mobility of 25 cm ²/V s at room temperature. For the same sample, the effective donor (Si) activation energy E ₀ was determined to be as low as 10 meV. E ₀ increases to 25 meV as N _si is reduced to 2.6 × 10 ¹⁹ cm ^-3, which can be explained by the bandgap renormalization effect. This implies that heavy doping is necessary in high-Al-content AlGaN alloys to bring down the donor activation energy, therefore a higher conductivity.