Observation of antisite domain boundaries in Cu₂ZnSnS₄ by atomic-resolution transmission electron microscopy

Abstract

Atomic resolution transmission electron microscopy has been used to examine antisite defects in Cu2ZnSnS4 (CZTS) kesterite crystals grown by a hot injection method. High angle annular dark field (HAADF) imaging at sub-0.1 nm resolution, and lower magnification dark field imaging using reflections sensitive to cation ordering, are used to reveal antisite domain boundaries (ADBs). These boundaries, typically 5-20 nm apart, and extending distances of 100 nm or more into the crystals, lie on a variety of planes and have displacements of the type ½[110] or ¼[201], which translate Sn, Cu and Zn cations into antisite positions. It is shown that some ADBs describe a change in the local stoichiometry by removing planes of S and either Cu or Zn atoms, implying that these boundaries can be electrically charged. The observations also showed a marked increase in cation disorder in regions within 1-2 nm of the grain surfaces suggesting that growth of the ordered crystal takes place at the interface with a disordered shell. It is estimated that the ADBs contribute on average ∼0.1 antisite defect pairs per unit cell. Although this is up to an order of magnitude less than the highest antisite defect densities reported, the presence of high densities of ADBs that may be charged suggests these defects may have a significant influence on the efficiency of CZTS solar cells.

Point defects in Cu2ZnSnSe4 (CZTSe): Resonant X‐ray diffraction study of the low‐temperature order/disorder transition

“…Antiphase domain boundaries are well established in bimetallic alloys (with the classic case of Cu 3 Au), although the effect of the diffraction is much different . Somewhat similar antiphase boundaries (stacking faults) have been reported but involve Sn site swapping with much larger domains .…”

Opposing effects of stacking faults and antisite domain boundaries on the conduction band edge in kesterite quaternary semiconductors

“…Formation of SFs was found in CZTS grown on single-crystal Si (111) wafers [21] and CZTS nanoparticles [22]. Another recent experimental study has shown that ADBs are formed abundantly in CZTS nanocrystals [23], possibly due to the low formation energy of antisite defect complexes in multicomponent semiconductors [2]. A density functional theory (DFT) calculation also shows that pre-existing defect complexes can lower the energy cost to form another defect complex in close configuration [24], providing a hint that point defects can be gathered and form a spatially extended defect.…”

“…Experimental evidence of faults in the layer with Cu and Sn has been provided [23], however, Cu and Zn are difficult to distinguish by transmission electron microscopy (TEM). Due to the larger chemical (size and charge) mismatch, the layer with Cu and Sn should be more rigid than the layer with Cu and Zn.…”

“…Other extended defects like stacking faults (SFs) and antisite domain boundaries (ADBs) have been less documented as compared to the GBs, but since SFs in CdTe act as electron barriers and reduce the efficiency [17][18][19][20], SFs in CZTS should be investigated. There is also growing evidence that the materials have extended defects [21][22][23]. Formation of SFs was found in CZTS grown on single-crystal Si (111) wafers [21] and CZTS nanoparticles [22].…”

Doping and alloying of kesterites

“…Zhang et al showed by DFT Sb Sn exhibits smaller lattice relaxations and has lower formation energy than native CZTS defects [131]. It should be mentioned that Cu Sn and Zn Sn antisite domain boundaries have been experimentally observed by atomic resolution transmission electron microscopy in CZTS crystallites [132]. Furthermore, DFT supercell calculations by Chen et al concluded that such antisites give rise to mid-gap traps which could be highly detrimental to device performance [6].…”