Biju Patnaik Central Library

Thesis (Ph.D)- National Institute of Technology, Rourkela

A theoretical study of the structural, electronic and optical properties of a series of
group I −III −V I2, II −IV −V2, I −III2 −V I4, II −III2 −V I4, I2−III −V I4 and
few substituted chalcopyrite type semiconductors are presented in this thesis. Systems
studied are AgAlM2 (M = S, Se, Te), CuInSe2, ZnSnX2 (X = P, As, Sb), AAl2Se4 (A
= Ag, Cu, Cd, Zn), CuIn2X4 (X = S, Se), CdGa2X4 (S, Se, Te), CdIn2Te4, Cu2InSe4,
ZnXIn2Te4 (X = O, Mn), CdMGa2S4 (X = Ag, Al), CuNaIn2S4, CuLiIn2Se4 and
Cu2InXSe4 (X = Al, Ga) substituted chalcopyrite semiconductors. Our study is density
functional theory (DFT) based first principle calculation within the frame work of tight
binding linear muffin-tin orbital (TB-LMTO) basis.
The structural parameters such as lattice constants, anion displacement, tetragonal distortion
and bond lengths are calculated by proper energy minimization. Bulk modulus
of all the systems except ZnXIn2Te4 (X = O, Mn), are calculated by extended Cohen
formula. Our study shows an inverse proportionality relation between lattice constant and
bulk modulus for these systems. Band structure and total density of states (TDOS) of all
the systems under study show that they are direct band gap semiconductors. AAl2Se4 (A
= Ag, Cu), CuIn2X4 (X = S, Se) and Cu2InSe4 are p-type direct band gap semiconductors
whereas CdMGa2S4 (X = Ag, Al) and ZnXIn2Te4 (X = O, Mn) are n-type direct
band gap semiconductors. Our calculated results agree well with the available experimental
results. Our study of partial density of states (PDOS) reveals that the contribution to
upper valence band comes from the cation d and anion p hybrid orbitals in case of group
I −III −V I2, I −III2 −V I4 , I2 −III −V I4 and their substituted chalcopyrites. This
leads to a strong p-d hybridization. But this is not the case for the group II − IV − V2,
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II − III2 − V I4 and their substituted chalcopyrites. This is because cation d states behaves
like core states and do not participate in p-d hybridization.
A quantitative estimate of effects of p-d hybridization and structural distortion on band
gap and hence on electronic properties are carried out for AgAlM2 (M = S, Se, Te),
CuInSe2 and ZnSnX2 (X = P, As, Sb), AAl2Se4 (A = Ag, Cu, Cd, Zn), CuIn2X4 (X =
S, Se), CuNaIn2S4 and CuLiIn2Se4 compounds. A significant reduction in band gaps
are found for all the above systems due to the former effect. There is an increment of
band gap due to the latter effect in the case of AgAlM2 (M = S, Se, Te) and AAl2Se4
(A = Ag, Cu, Cd, Zn). Where as this effect on band gap is reversed in case of CuInSe2
and ZnSnX2 (X = P, As, Sb), CuIn2X4 (X = S, Se), CuNaIn2S4 and CuLiIn2Se4.
Quantitative effect of cation-electronegativity on band gap of ZnSnX2 (X = P, As, Sb)
compounds is also carried out. Our study shows that there is an increment of band gap in
these three systems with respect to their binary analogs due to this effect.
We calculate real and imaginary parts of the dielectric function, refractive index and absorption
co-efficient in our optical properties study for the systems AAl2Se4 (A = Ag, Cu),
CuIn2S4, CdGa2X4 (S, Se, Te), CdIn2Te4, ZnXIn2Te4 (X = O, Mn) and CuNaIn2S4
compounds. Static dielectric constants and static refractive index are also calculated for
all the systems. We find a propertionality relation between static dielectric constant and
refractive index. Our result agrees well with the available experimental and other theoretical
results for systems studied by others.
We have explicitely caslculated optical matrix elements (OME) and joint density of states
(JDOS) to show their respective contribution in the optical properties. Our result shows
OME has greater contribition in the Infrared and visible region of the spectrum where
as JDOS has greater contribution in UV region of the spectrum. Significant effects of
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structural distortion and p-d hybridization on optical properties, JDOS and OME are also
observed in case of the studied systems AgAl2Se4, CuIn2S4 and CuNaIn2S4. Effect of
Na substitution in CuIn2S4 and Mn, oxygen substitutions in ZnIn2Te4 on optical properties
are also reported. Their substitution significantly alters the optical properties of the
host. Our study shows that chalcopyrites are anisotropic in nature and different optical
properties get enhanced when photon is polarized ⊥ c-axis.