3.1. Electrical Transport

Variable temperature Hall effect measurements are commonly employed to study the electrical transport properties of AlGaN. For example, Zhu et al.76 prepared heavily Si-doped n-type Al _0.7Ga _0.3N films by OMVPE on sapphire substrates. SIMS profiling indicates the nominal carrier concentration (N _Si) ranging from 2.6 to 6.8 10 ¹⁹ cm ^-3. The electrical transport measurement results are plotted in Figure 19. T _max in Fig. 19(c) indicates the characteristic temperature at which electron mobility reaches a maximum value. Strong thermal activation process is evident for the sample with the lowest doping level (see Fig. 19(d)). At high doping levels, samples exhibit degenerated behavior. There is a clear trend of decreasing resistivity with increasing Si doping level. For the sample with N _Si=6.0 10 ¹⁹ cm ^-3, n-type resistivity of 0.0075 ? cm was achieved with an electron concentration of 3.3 10 ¹⁹ cm ^-3 and a mobility of 25 cm ²/Vs at room temperature. For the same sample, the effective donor 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 band gap renormalization effect. This implies that heavy doping is necessary in high-Al-content AlGaN alloys to bring down the donor activation energy in order to obtain higher conductivity.

Figure...