High-quality SiGe films grown with compositionally graded buffer layers for solar cell applications

In this study, we fabricated strain-relaxed Si0.58Ge0.42 thin films on Si substrates by molecular beam epitaxy (MBE) for heterojunction solar cell applications. A combined set of stepwise Si1−xGex buffer layers and a Si0.51Ge0.49 strain-inverted layer, in conjunction with rapid thermal annealing (RTA), were employed to confine the dislocations within the buffer layers. Structural characterization of the samples revealed a low dislocation density of less than 105cm−2 and a relatively smooth surface (0.903nm in root mean square roughness). The results of solar cell characterization showed both an extended absorption edge (1200nm) and improved diode characteristics. For these cells, the short-circuit current density, open-circuit voltage, fill factor, and efficiency were 8.25mA cm−2, 233mV, 0.509, and 0.98%, respectively. ► We fabricated strain-relaxed Si0.58Ge0.42 thin films grown with graded buffer layer by MBE. ► We successfully fabricated low dislocation density (105cm−2) SiGe films with a smooth surface. ► We evaluated the performance of single-crystal Si0.58Ge0.42 heterojunction solar cells. ► SiGe solar cell showed both an extended absorption edge and improved diode characteristics.