Metalorganic chemical vapor deposition of high mobility AlGaN/GaN heterostructures

In this article, we discuss parameters influencing (a) the properties of thin ${\mathrm{Al}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N}$ layers grown by metalorganic chemical vapor deposition and (b) the electrical properties of the two-dimensional electron gas (2DEG) forming at the ${\mathrm{Al}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N/GaN}$ heterojunction. For $x_{\mathrm{Al}}\text{>0.3,}$ the ${\mathrm{Al}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N}$ layers showed a strong tendency towards defect formation and transition into an island growth mode. Atomically smooth, coherently strained ${\mathrm{Al}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N}$ layers were obtained under conditions that ensured a high surface mobility of adsorbed metal species during growth. The electron mobility of the 2DEG formed at the ${\mathrm{Al}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N/GaN}$ interface strongly decreased with increasing aluminum mole fraction in the ${\mathrm{Al}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N}$ layer and increasing interface roughness, as evaluated by atomic force microscopy of the surfaces prior to ${\mathrm{Al}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N}$ deposition. In the case of modulation doped structures $({\mathrm{GaN/Al}}_x{\mathrm{Ga}}_{\text{1−x}}{\mathrm{N/Al}}_x{\mathrm{Ga}}_{\text{1−x}}{\mathrm{N:Si/Al}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N}\mathrm{),}$ the electron mobility decreased with decreasing thickness of the undoped spacer layer and increasing silicon doping. The electron mobility was only moderately affected by the dislocation density in the films and independent of the growth temperature of the ${\mathrm{Al}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{N}$ layers at $x_{\mathrm{Al}}\text{=0.3.}$ For ${\mathrm{Al}}_{\mathrm{0.3}}{\mathrm{Ga}}_{\mathrm{0.7}}\mathrm{N/GaN}$ heterojunctions, electron mobility values up to 1650 and 4400 ${\mathrm{cm}}^{\mathrm{2}}\mathrm{/V s}$ were measured at 300 and 15 K, respectively.