High quality (AlGa)0.8In0.2As material with very low threading dislocation density grown on Ge through compositionally graded buffer integrated with strained-layer superlattices

Abstract

Strained-layer superlattices (SLSs) can be used to filter the threading dislocations (TDs) since the misfit strain can annihilate some TDs and suppress the threading dislocation density (TDD) in the followed films. In this work, the In_0.1Ga_0.9As/GaAs SLSs were introduced into the compositionally graded buffer (CGB) structure aiming to grow In_0.2Ga_0.8As layer with low TDD. Epitaxial wafers with (AlGa)_0.8In_0.2As target layers were grown on Ge substrates by using modified CGB structures integrating various In_0.1Ga_0.9As/GaAs SLSs to replace the bottom In_0.05Ga_0.95As layer of CGB. According to cathodoluminescence tests, lower TDDs were clearly observed in the wafers with SLSs. A very low average TDD of 3.5 × 10⁵ cm⁻² was obtained with the structure integrating 10 periods SLSs with individual layer thickness of 12.5 nm, implying that the critical thickness of In_0.1Ga_0.9As layer in SLSs was close to and below 12.5 nm. This point was also verified by the examinations of cross-sectional transmission electron microscopy. The surface morphologies of epitaxial films were studied by atomic force microscopy, and all samples with SLSs exhibited the lower root mean square surface roughness, indicating the better relaxation of target layers. In_0.2Ga_0.8As single-junction solar cells were prepared on Ge substrates by employing six-step CGB layers with and without 10 periods SLSs. The open-circuit voltage (V_oc) has been significantly improved by integrating 10 periods SLSs. The increase in V_oc is attributed to the ultra-low TDD based on 10-QW SLSs, which can effectively reduce non-radiative recombination rate in solar cell. The bandgap-voltage offset (W_oc = E_g/q - V_oc) value of In_0.2Ga_0.8As cell with 10 periods SLSs is 384 mV, which is only a bit higher than 369 mV of the lattice-matched 1.4-eV In_0.01Ga_0.99As cell grown on Ge (R. R. King et al., Prog. Photovolt., 2011, 19: 797–812), representing excellent crystalline quality of In_0.2Ga_0.8As material.