been widely studied for optoelectronic applications, due to their low direct energy

bandgap, piezoelectricity, and long charge carrier diffusion length [1].

The II−VI compounds can crystallize in wurtzite (WZ), zinc blende (ZB), rock salt (RS), or

nickeline (NC) structures (Figure 13.1) with typical space groups of P63mc, F4̅ 3m, Fm3̅ m,

and P63/mmc, respectively [1]. The WZ and NC will have a hexagonal close-packed

structure, while the cubic closed packing is observed in ZB and RS. These structures will

also have different cation coordination, which is tetrahedral in WZ and ZB, and octahedral

in RS and NC. Most II−VI compounds are either WZ or ZB, and the structures are asso­

ciated with attractive piezoelectric properties due to central asymmetry [2,3].

Although commonly an sp³ covalent bonding is observed in structures with tetrahedral

coordination, these WBG semiconductors have substantial ionic character [3]. The p va­

lence energies tend to increase going down group VI (O – 2p, S – 3p, Se – 4p, Te – 5p),

while the ionicity tends to decrease. From the molecular orbital theory, when the orbitals

have similar energies they are more likely to hybridize. Additionally, the tetrahedral

coordination observed for most II−VI compounds allows the hybridization of the orbitals

p and d [5]. Therefore, going down group VI, there is a greater p-d orbital overlap and

the hybridization becomes stronger, increasing the covalency of the M-X bond and the

delocalization of valence bands [1]. As a consequence of the p-d hybridization, there is

the reduction of the direct bandgap (see below Section 13.3 – Table 13.1) turning these

materials strong candidates to optoelectronic devices [5].

FIGURE 13.1

Crystal structures found in common WBG materials. Structures drawn using VESTA software [ 4].

Wide Bandgap Semiconductors

205