Session L11: Dopants and Defects in Semiconductors - Complex Oxides and Oxide Interfaces

Doping and interfaces in complex oxide heterostructures and superlattices from first principles

Charge transfer between unit-cell-scale layers of different materials in superlattices and heterostructures can produce modulation doping with high carrier concentrations. In complex oxides, changes in carrier concentration can lead to dramatic changes in the structure and properties, including transitions to distinct phases. The degree of the charge transfer, the character of the electronic states occupied by the transferred charge, and the resulting changes in structure, phonons, transport and optical properties can be readily determined from first principles calculations. In this talk, I will discuss recent results on a range of systems, including materials design of high mobility ferroelectrics for ferroelectric field effect transistors, modulation doping and quantum confinement in BaSnO₃ layers in superlattices, charge-order-driven ferroelectricity in transition-metal perovskite superlattices, and new doping-induced phases in nickelates and nickelate/iridate superlattices, in which the added carriers localize on the Ni and Ir ions to produce novel charge, orbital and spin ordering.   

The effects of excess electrons in BiVO4

BiVO₄ is a promising material as a photoanode for water splitting in photoelectrochemical cells. Despite the extensive research on BiVO_4, basic electronic properties, such as exact value of the band gap, and the impact of point defects on the materials performance have been widely debated. The spread in the experimental and theoretical results is due, in large part, to the existance of competing phases with slightly different crystal structures. Using density functional calculations with the meta-GGA SCAN and hybrid functionals, we study the stability of the different phases, the effects of excess electrons in the conduction band, and the possibility of carrier localization in the form of small polarons. The results are compared to previous calculations and available experimental data.   

Local structure about Er and Hf defects in LiNbO3: possibility of meta-stable distributions

The non-linear optical properties of LiNbO₃ (LNO) are often tuned by doping on the Li or Nb sites. Zn and In suppress the photorefractive response, important for second harmonic generation applications, while Fe and Hf enhance the photorefractive response. LNO can also be grown in two forms, congruent (cLNO, slight Nb excess) and stoichiometric (sLNO), which have somewhat different properties. An important question still under debate is the substitution site for various dopants. EXAFS data for Er(3+) (cLNO and sLNO) and Hf(4+) (sLNO) are reported and compared. A signature for occupation on the Li site is a large peak in the EXAFS r-space plot near 2.9 A, whereas the largest peak above 2 A for Nb site substitution is near 3.5 A. We show that Er is mostly on the Li site with about 30 % on the Nb site, while Hf substitutes roughly equally on both the Li and Nb sites. In contrast another 3+ defect In(3+) is primarily on the Li site. Some limited theoretical calculations differ from these results but most are carried out for low temperatures. Since LNO crystallizes near 1500 K, are the dopants and their charge compensating centers quenched in place at very high T and not in thermal equilibrium? A more difficult question is whether these meta-stable distributions can be controlled.   

Boosting Small Polaron Hopping Mobility of Bismuth Vanadate by Doping from First-Principles Calculations

Bismuth vanadate (BiVO₄) is one of the most promising photoanode materials in water-splitting photoelectrochemical cell. Its extremely low carrier mobility fundamentally limits its effeciency, while recent experimental work shows atomic doping can improve its carrier mobility, which shines light on the possibility of overcoming this limitation. However, its mechanism at the atomic level remains unclear. In this work, we explored the small polaron hopping mechanism in pristine and Mo doped BiVO₄. We compared several first-principles methods to obtain polaron hopping barriers, and investigated the mobility as a function of dopant-polaron distances and dopant concentrations. We found that atomic doping in the stable monoclinic phase leads to a phase transition to the tetragonal phase, which may reduce the barrier. Also, the electrostatic attraction between the polaron and the dopant combined with the energies raised from the lattice expansion on both sites may further reduce the barrier. The understanding of the mechanism provides us the guidance on the rational design of dopants for a better carrier conductivity in BiVO₄ and other similar polaronic metal oxides.
  

Ab initio study of polaron dynamics in transition metal oxides

In the highly insulating materials like transition metal oxides, electrons strongly couple with neighboring lattice sites, severely deforming the surrounding lattice and leading to localized states, and they are self-trapped by self-induced local lattice distortions. As a result, electronic transport is mostly dominated by the small polaron hopping from lattice site to lattice site rather than the drift motion. Here, using the first-principles density functional theory calculations, we explore the polaron dynamics in the insulating Fe₂O₃. We find that the polaron hopping occurs only in the two dimensional FeO plane and its hopping barrier is highly susceptible to the lattice deformation induced by intrinsic oxygen vacancy and extrinsic Sn dopant. We expect our results to provide important information for development of highly efficient Fe₂O₃-based electrochemical device.   

Reduced Spin-Phonon Coupling and Increased Hole Mobility of CuO by Li doping from First-principles Calculations

Cupric oxide (CuO) is a promising material with many desirable characteristics as a photocathode in photoelectrochemical cells and owns exotic physical properties due to its similarity to high T_C superconductors. Unfortunately, it suffers from low carrier mobilities that is characterized by an activated polaronic hopping conduction. Therefore, it is believed that CuO, similar to other well-studied transition metal oxides (e.g. Fe₂O₃, BiVO₄) exhibits strong electron-phonon coupling leading to formation of small polarons and low mobility. However, we find that electron-phonon coupling in this system is uncharacteristically low and well-established first principles methods of computing polaron hopping barriers based in Marcus theory, severely underestimate the hopping barrier in the pristine system. We propose instead that limitations in the transport of carriers in this system is dominated by strong spin-phonon coupling due to carriers forming localized spin states which must hop through the magnetic lattice. Furthermore, we investigate Li doped CuO (known from experiment to introduce more carriers and improve carrier mobility) and how Li leads to changes in the magnetic couplings in this system that may enhance the carrier mobility in CuO.   

Charge Localization Near Acceptors in Transition-Metal Oxides

Controlling the conductivity in transition-metal (TM) oxides is a great challenge to make them usable in electronic and optoelectronic devices. P-type doping, in particular, is hindered by the high ionization potential of these materials; their valence bands (VB) are derived from O 2p orbitals, lying very low in energy with respect to the vacuum. Adding holes to the VB, either by introducing acceptors or by photon excitation, invariably leads to the localization of the carriers, in the form of small polarons, precluding p-type conductivity. In this presentation, we use density functional theory with the HSE06 hybrid functional to explore the impact of acceptor impurities on the electronic and optical properties of TM oxides, such as TiO₂ and SrTiO₃, paying attention to charge localization, i.e., the interaction between the acceptors and small hole polarons. We first discuss hole localization in the absence of any impurity, and then analyze the formation of small polarons and their stability near acceptor impurities through the calculation of binding energies. We address the effects of size and chemical differences between the acceptors and the host atoms on binding energies and ionization energies. Finally, we discuss possible cases where p-type conductivity could be observed.   

Ab initio study on the effectiveness of Mg-doping on improving conductivity in the transparent conducting oxides CuAlO2, AgAlO2, and CuCrO2

Identifying effective p-type transparent conducting oxides (TCOs) is an important factor in increasing the efficiency of technologies like flat panel displays and photovoltaic cells and requires overcoming the intrinsically high hole effective masses in pure TCOs caused by highly localized valence states with predominantly O-2p character. A first principles study, within density functional theory, using projector-augmented plane-waves (PAW) as implemented in the VASP code is presented on the effects of Mg-doping (replacing Al) on the conductivity and optical properties of 2H- CuAlO₂, AgAlO₂, and CuCrO₂. Exchange and correlation potentials are treated using the generalized gradient approximation (GGA) with a Hubbard correction, U, for the 3d(4d) orbitals of Cu/Cr(Ag), and the hybrid functional HSE06 for the bulk. GGA is used to obtain optical properties. The bulk is modelled using an 8-atom hexagonal primitive cell and a 64-atom hexagonal supercell, which is also used for the Mg-doped system. Obtained electronic and optical properties and calculated hole effective masses are presented in the context of results from literature and the effectiveness of Mg-doping on these TCOs is discussed.   

Improvements on the Diamond-V2O5 2DHG Electronic Structure using an Al2O3 Passivation Layer

Vanadium pentoxide as well as other high electron affinity transition metal oxides (MoO₃, WO₃ etc) have been used as the electron accepting material for a stable diamond-oxide 2DHG structures. Holes within a few nanometers in the diamond after electron transfer from diamond to vanadium pentoxide. V₂O₅ outperforms other oxides on diamond because of its high electron affinity up to 5.8 eV. This largest potential enables electron transfer to the V2O5 achieving a high sheet carrier concentration at the diamond-V₂O₅ interface. A disadvantage of the charge transfer process is a reduced carrier mobility . We propose to use a thin layer of Al₂O₃ grown by PEALD to serve as a passivation layer and thus increase the carrier mobility between diamond and V₂O₅. After Al₂O₃ growth on H-terminated diamond with a post hydrogen plasma treatment in situ XPS indicates upward band bending confirming that the Al₂O₃ retains the 2DHG structure. In situ XPS indicates the charge transfer process can be tuned by controlling the oxidation state of vanadium in the V_xO_y mixture. The 2DHG structure is established only when vanadium is in its highest oxidation state (+5), while lower oxidation states show a reduction of the 2DHG density.   

Investigation of 3C-SiC/SiO2 Interfacial Point Defects from First Principles Calculations and Electron Paramagnetic Resonance Measurements

SiC is widely used in high-power, high-frequency electronic devices. It has also been used as a building block in hybrid nanocomposites for photovoltaics. Analogous to Si, SiC features SiO₂ as native oxide that can be used for passivation and insulating layers. However, a significant number of defect states are reported to form at SiC/SiO₂ interfaces, limiting mobility and increasing recombination of free charge carriers. Combining ab initio g-tensor and hyperfine interactions calculations with electron paramagnetic resonance (EPR) measurements, we show that carbon antisite dangling bond (Csi-db) defects explain the measured EPR signatures. Csi-db is found to be strongly stabilized at the interface, because carbon changes its hybridization from sp³ in the SiC-bulk to sp² at the interface, creating a dangling bond inside a porous region of the SiO₂ passivating layer. The calculated energy level of a neutral Csi-db coincides with the barrier height of the interface states from internal photoemission (IPE) of SiC/SiO2 interfaces, indicating a contribution of Csi-db to the measured interface states.   

Analytical Electron Microscopy of Antimony Doped 4H-SiC/SiO2 and 4H-SiC/Boron and Phosphorus Doped SiO2 Interface Structures in MOS Devices

A high density of electronic defects at the SiC/SiO₂ interface adversely affects SiC-based MOS devices. Various treatments are known to improve device performance. Annealing in nitric oxide (NO) passivates electronic defects at the interface and raises the carrier mobility in the active region by an order of magnitude, to 35-40 cm²/Vs, but passivation with phosphorus¹ or boron² improves upon NO by a factor of 3, increasing the mobility to over 120 cm²/Vs.¹ Antimony doping of the SiC in conjunction with NO annealing also increases the mobility over 100 cm²/Vs.³ We investigate the chemical and structural effects of these treatments on the SiC/SiO₂ interface using high-resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy Spectrum Imaging (EELS SI). The latter allows identification of the composition and types of bonding at the interface. Machine learning techniques applied to the EELS data reveal intermediate bonding states within this region as well as inhomogeneous distribution of the P and B dopants. ¹G. Liu et al., Appl Phys Rev. 2, 021307 (2015). ²D. Okamoto et al., IEEE Electron Device Lett. 35, 1176 (2014). ³A. Modic et al., IEEE Electron Device Lett. 35, 894 (2014).   

Optically-pumped 75As NMR Reveals an Electric Field Gradient at an Al2O3-GaAs Interface and Very Low Nuclear Spin Temperatures

We have investigated the interface between bulk GaAs and an 11nm thick layer of atomic layer deposition (ALD) Al₂O₃. Comparing GaAs with a native oxide and that with alumina results in different spectra. Using optical pumping of conduction electrons, nuclear spins enhancements localized near the interface is achieved by tuning the laser to bandedge states that result in a shallow penetration depth.

The ⁷⁵As OPNMR spectra that are recorded in this regime show evidence of unusual quadrupolar splitting for GaAs. The splitting arises in part from biaxial strain, introduced by the different thermal expansion coefficients of the two materials (OPNMR is conducted at temperatures of 6 -10 K). However, strain alone does not account for the quadrupolar satellite shapes. We will discuss contributions to the electric field gradient at the interface that lead to the quadrupolar satellites.

In addition, the satellites themselves are asymmetric and reveal that these depend strongly on the photon energy used for optical pumping. Our models of spin temperature suggest a surprising cooling of the spin system to between 1.5 to 3 mK.