Sudden stress relaxation in compound semiconductor thin films triggered by secondary phase segregation

Abstract

In polycrystalline compound semiconductor thin lms, structural defects such as grain boundaries as well as lateral stress can form during lm growth, which may deteriorate their electronic performance and mechanical stability. In Cu-based chalcogenide semiconductors such as Cu(In, Ga)Se2 or Cu2ZnSn(S, Se)4, temporary Cu excess during lm growth leads to improved microstructure such as a reduced grain boundary density, a strategy that has been used for decades for high-eciency chalcopyrite thin lm solar cells. However, the mechanisms responsible for the benecial eect of Cu-excess are yet not fully claried. Here, we investigate the evolution of lateral stress, grain growth and Cu-Se segregation during Cu-Se deposition onto Cu-poor CuInSe2. Real-time x-ray diraction and uorescence analysis with a double-detector setup reveals that sudden stress relaxation occurs shortly prior to Cu-Se segregation at the surface and precisely coincides with domain growth and change of texture. Numerical reaction-diusion modeling provides an explanation for the observed delay of Cu-Se segregation. Our results show that partial recrystallization of the lm can be already reached without the necessity of an overall Cu-rich lm composition and thus suggest a new synthesis route for the fabrication of high-quality chalcopyrite absorber lms.Co-evaporation of Cu(In, Ga)Se 2 (CIGSe) lms -used as absorber layer in thin lm solar cells with world record energy conversion eciencies 1,2 -features a puzzling peculiarity: For the nal lm, a Cu-poor composi-is required to obtain highest eciencies; however, during deposition, the lm composition is changed from an initially Cu-poor composition to an intermediate Cu-rich composition and nally changed back to a Cu-poor composition. The composition modications during lm deposition are realized by varying the Cu and In+Ga evaporation uxes. Two crucial ndings point out the importance of the Cu-poor → Cu-rich transition: First, highest eciencies are only achieved if an intermediate Cu-rich lm composition was reached during lm deposition. 3 Second, a three-stage process with a Cu-poor → Cu-rich → Cu-poor sequence leads to higher eciencies than a two-stage process with only a Cu-rich → Cu-poor sequence. 46 Thus, it seems that the key challenge in understanding the success of the three-stage process over the two stage-process is -besides the adjustment of an ideal Ga gradient -the identication of the reactions and their driving forces acting during the Cu-poor → Cu-rich transition.While the eect of the Cu-poor → Cu-rich transition on structural and morphological changes in CIGSe lms such as grain growth 710 as well as on electronic properties of the material 3,11 has been thoroughly investigated in the past decade, the physical mechanisms and driving forces of these changes are not fully understood. Reduction of grain boundary (GB) energies or defect densities were proposed as possible driving forces for grain growth; 79 however, no attention has so far been paid to the potential role of stress energy for the mic...

Study and optimization of alternative MBE‐deposited metallic precursors for highly efficient kesterite CZTSe:Ge solar cells

“…This likely indicates a higher porosity of the MBE‐deposited precursors, allowing a fast diffusion of the Se vapors towards the back contact, which could be controlled to some extent by the Se vapor pressure as will be shown below. A reasonable explanation for the different grain growth could be the different nature of the MBE and sputtered precursors; indeed, compared to the former, sputtered films are typically more compact with reasonably higher compressive stress, which is known to act as a driven force for the grain growth during the annealing treatment …”

Evolution of opto-electronic properties during film formation of complex semiconductors

“…It was shown in ref. 27 that relaxation of lateral stress occurs by re-crystallization during the transition from Cu-poor to Cu-rich. However, it should be noted, that this stress relaxation occurs slightly prior to the Cu-saturated regime and that we cannot observe any influence of the relaxation on the optical band gap.…”

“…27. The accessible X-ray energy range was 6 to about 80 keV and the diffraction angle was fixed at 2 θ  = 6.301° ± 0.002° for the first detector and 9.722°° ± 0.002° for the second detector.…”

“…The information depth as defined in the supporting information of ref. 27 is 0.69 μm at the energy of the 112 reflex of CIGS in the second detector, which was used for the calculation of the lattice plain distance in Fig. 4a.…”

Chalcopyrite compound semiconductors for thin film solar cells

“…[67]. It has also been shown that the phase transition between Cu-poor and Cu-rich material is essential for the stress release in the films during growth [68]. It has been long understood that the lower efficiency of Cu-rich solar cells is due to recombination at or near the interface, which limits the open-circuit voltage [64].…”