Chapter 6 - Lattice matched III–V materials on Si via Si1−xGex buffer layer

Abstract

In this chapter we review some of the research works on III–V on Si solar cells while focusing on cells grown via Si1−xGex graded buffers. Furthermore, we review effects of threading dislocation densities (TDD) on solar cell performance and provide optical properties of GaAs1−yPy alloys grown on lattice matched Si1−xGex/Si substrates. Recently there have been a number of research groups that have reported GaAs/Si or GaAsP/Si1−xGex monolithic two-terminal dual-junction solar cells with low TDD. However, results presented here show that the performance of monolithic III–V/Si cells is lower compared to tandem cells when III–V cells are lifted off from their lattice matched substrates and mechanically bonded to separately fabricated Si solar cells. This is mostly because monolithic tandem cells suffer from some level of unavoidable optical losses in graded buffer. Although bonded tandem cells have better reported performance, Si1−xGex graded buffers on Si substrates are shown to be an excellent low-cost lattice matched alternative for III–V growth. Moreover, using low-cost substrates instead of GaAs or Ge carriers has the potential to lower costs of multi-junction solar cells and increase commercialization of III–V based solar cells.