To realize three-dimensional vertical phase-change random-access memory (PCRAM) devices, it is essential to fabricate highly conformal GeSbTe (GST) films inside high-aspect-ratio holes using atomic layer deposition (ALD). Different compositions of GST films are favored for various applications of PCRAM devices. In the present work, the [Ge]/[Sb] ratio of GST thin films prepared by tellurization annealing of ALD Ge–Sb films was controlled through the supercycle process of ALD GeSb and ALD Sb. By adjusting the cycle number ratio of ALD Sb to ALD GeSb between 0 and 0.5, we could change the Ge content of the Ge–Sb film from 0.33 to 0.66 and the Sb content from 0.26 to 0.63. After tellurization annealing at 230 °C, the Ge, Sb, and Te concentrations were 0.11–0.25, 0.19–0.37, and 0.52–0.60, respectively. The phase transition temperature was tunable between 90 and 137 °C depending on the Sb concentration. A crystalline GST film with a uniform composition close to Ge2Sb2Te5 was formed, with a step coverage of higher than 85% on a trench pattern with an aspect ratio of 24.