Letter pubs.acs.org/JPCL
Realizing Direct Gap, Polytype, Group IIIA Delafossite: Ab Initio Forecast and Experimental Validation Considering Prototype CuAlO2 Nilesh Mazumder,†,§ Dipayan Sen,†,§ Uttam K. Ghorai,‡ Rajarshi Roy,† Subhajit Saha,‡ Nirmalya S. Das,‡ and Kalyan K. Chattopadhyay*,†,‡ †
Physics Department and ‡School of Materials Science and Nanotechnology, Jadavpur University, Kolkata 700032, India S Supporting Information *
ABSTRACT: Modification of orbital interactions by suitable “orbital alloying” is a necessity to obtain direct gap group IIIA delafossite with shallow acceptor states, which unfortunately have never been realized. This Letter promotes an optimistic trend regarding the oxide delafossite family to obtain significant hole delocalization in the vicinity of the (Cu 3d + O 2p) valence band with an unprecedented direct gap by chalcogen (Ch = S, Se) doping-induced polytype formation (R3̅m → P3m1). Polytypism and a modulated valence band in prototype CuAlO2 are “observed” upon sulfur incorporation at the O site. From the exhibition of better transport properties, weak local σ symmetry between Cu 3dz2 and O 2pz states are reflected in the doped material. Dispersion of Cu 3d character by incorporating shallower acceptor (Ch 3p) states is concluded upon analyzing the photoemission spectra of the valence band. SECTION: Energy Conversion and Storage; Energy and Charge Transport
P
pz states. Doping of a less electronegative element than oxygen at the O site can serve two-fold, (I) to reduce the overall “hole capacity” of the valence band (VB) edge by reducing the effective electronegativity and (II) to modify the band structure consisting of an increased shallower antibonding density of states (DOS) with lower effective mass for major charge carriers. For this to arrange, one of the dopant’s dominant orbitals must participate in the “reconstruction” of the VB by hybridizing with Cu 3dz2 or O 2pz or with both of these. Copper aluminate, the prototype p-type delafossite, was chosen as host to verify the proposition. As Ch 3pz is found to be situated near Cu 3dz2 and O 2pz states with effective p−p and p−d coupling, we have considered them as a proper dopant choice. Therefore, the aim of this Letter becomes to examine a proposition regarding generation of a shallower acceptor level in the vicinity of antibonding Cu 3d states as to facilitate higher conductivity as well as to disturb strong σ-symmetry between Cu 3dz2 and O 2pz states in group IIIA delafossites. Our first-principles finding regarding the intrinsic indirectness of the CAO band structure (Figure 1a) is in accordance with previous reports of HF and self-consistent calculations.16,20 The minimum-energy point of the conduction band (CBM) and the maximum-energy point of the VB (VBM) are found to be situated near Γ and F points, respectively, along the Γ−F direction, consistent also with the FLAPW scenario.20,21 The calculated direct and indirect gaps
hase-transition-induced polymorph or polytype formation is nonetheless significant for extracting new potential applications out of engineered materials and the chemical processes involved behind the physical transformation of the matrix. Contemporary literature is embedded with many such engineered phase transformations in semiconductor nano or bulk structures exploiting strain,1 doping,2,3 and external influence4,5 induced band structure modification for promising applications, keeping the main focus in elemental6 or binary semiconductor oxides.7 To enlarge the window of opportunity, these relentless efforts should also meet strategically important ternary or quaternary semiconductors, which are rather scarce. Here, considering CuAlO2, a prototype group IIIA delafossite as an example, we demonstrate geometrical and band structure engineering both theoretically and experimentally to remove its inherent indirectness of the band gap with additional potential functionality. The delafossite family of oxides, first discovered back in 1873,8 usually crystallizes with rhombohedral and hexagonal symmetry.9,10 Since the path breaking work of Hosono and his co-workers,11 CuMO2 delafossites, where M represents metals, rare earths, and lanthanides,12,13 have been identified as the potential p-TCOs, heralding a new hope in transparent electronics.14,15 Band structures of mostly cuprous delafossites have been investigated rigorously,10,16−19 but direct gap IIIA delafossite has not been reported in the literature to date. The equilibrium band structure of, say, CuAlO2 is featured with an intrinsic indirect gap. The flat band from Γ to Z is due to π antibonding between Cu and O, and dispersion around F is due to hybridization between Cu 3dz2−s and between dz2−s and O © 2013 American Chemical Society
Received: September 2, 2013 Accepted: October 4, 2013 Published: October 4, 2013 3539
dx.doi.org/10.1021/jz4018656 | J. Phys. Chem. Lett. 2013, 4, 3539−3543
The Journal of Physical Chemistry Letters
Letter
electron localization function (ELF) (η = 0.1) around O (red), S (yellow), and Se (brown) atoms, respectively. The value of η represents the regions with fairly higher Pauli repulsion compared to that of the uniform electron gas.23 As the “attractor capacity” weakens gradually from CAO to CAO/Se, the tendency of delocalization would be increased for the electron cloud, prompting the bound holes also to be delocalized. It is observed that Pauli repulsion is more effective along the Ch−Al bond length than along Ch−Cu, irrespective of the choice of element for the anionic site. As the value of
