Anup L. Dadlani

Quantifying Geometric Strain at the PbS QD-TiO2 Anode Interface and Its Effect on Electronic Structures

Quantum dots (QDs) show promise as the absorber in nanostructured thin film solar cells, but achieving high device efficiencies requires surface treatments to minimize interfacial recombination. In this work, lead sulfide (PbS) QDs are grown on a mesoporous TiO2 film with a crystalline TiO2 surface, versus one coated with an amorphous TiO2 layer by atomic layer deposition (ALD). These mesoporous TiO2 films sensitized with PbS QDs are characterized by X-ray and electron diffraction, as well as X-ray absorption spectroscopy (XAS) in order to link XAS features with structural distortions in the PbS QDs. The XAS features are further analyzed with quantum simulations to probe the geometric and electronic structure of the PbS QD-TiO2 interface. We show that the anatase TiO2 surface structure induces PbS bond angle distortions, which increases the energy gap of the PbS QDs at the interface.

Exploring the local electronic structure and geometric arrangement of ALD Zn(O,S) buffer layers using X-ray absorption spectroscopy

The growing interest in zinc oxysulfide (Zn(O,S)) thin films as buffer layers has been motivated by higher efficiencies achieved in solar cells. In this work we present insights into the electronic-geometric structure relationship of varying compositions of Zn(O,S) grown by atomic layer deposition (ALD). The X-ray absorption near edge structure (XANES), a local bonding-sensitive spectroscopic tool, with quantum simulations helps link the atomic structure to the unoccupied density of states (DOS) of the films. The infiltration of sulfur into a ZnO matrix results in the formation of S 3p–Zn 4sp–O 2p hybridized orbitals in the near edge X-ray absorption fine structure (NEXAFS) region of both the O and S K-edges. The extent of sulfur incorporation affects the ionicity of Zn, which in turn alters the bond lengths of Zn–O within the structure and its resulting bandgap. Knowing Zn(O,S)s electronic-geometric structure interplay allows one to predict, tailor, and optimize its buffer layer performance.

Self-limiting atomic layer deposition of barium oxide and barium titanate thin films using a novel pyrrole based precursor

Barium oxide (BaO) is a critical component for a number of materials offering high dielectric constants, high proton conductivity as well as potential applicability in superconductivity. For these properties to keep pace with continuous device miniaturization, it is necessary to study thin film deposition of BaO. Atomic layer deposition (ALD) enables single atomic layer thickness control, conformality on complex shaped substrates, and the ability to precisely tune stoichiometry. Depositing multicomponent BaO containing ALD films in a self-limiting manner at low temperatures may extend the favorable bulk properties of these materials into the ultrathin film regime. Here we report the first temperature and dose independent thermal BaO deposition using a novel pyrrole based Ba precursor (py-Ba) and water (H2O) as the co-reactant. The growth per cycle (GPC) is constant at 0.45 A with excellent self-terminating behavior. The films are smooth (root mean squared (RMS) roughness 2.1 A) and contain minimal impurities at the lowest reported deposition temperatures for Ba containing films (180–210 °C). We further show conformal coating of non-planar substrates (aspect ratio ∼ 1:2.5) at step coverages above 90%. Intermixing TiO2 ALD layers, we deposited amorphous barium titanate with a dielectric constant of 35. The presented approach for infusing self-terminating BaO in multicomponent oxide films may facilitate tuning electrical and ionic properties in next-generation ultrathin devices.

Relating electronic and geometric structure of atomic layer deposited BaTiO3 to its electrical properties

Atomic layer deposition allows the fabrication of BaTiO3 (BTO) ultrathin films with tunable dielectric properties, which is a promising material for electronic and optical technology. Industrial applicability necessitates a better understanding of their atomic structure and corresponding properties. Through the use of element-specific X-ray absorption near edge structure (XANES) analysis, O K-edge of BTO as a function of cation composition and underlying substrate (RuO2 and SiO2) is revealed. By employing density functional theory and multiple scattering simulations, we analyze the distortions in BTO’s bonding environment captured by the XANES spectra. The spectral weight shifts to lower energy with increasing Ti content and provides an atomic scale (microscopic) explanation for the increase in leakage current density. Differences in film morphologies in the first few layers near substrate–film interfaces reveal BTO’s homogeneous growth on RuO2 and its distorted growth on SiO2. This work links structural changes to BTO thin-film properties and provides insight necessary for optimizing future BTO and other ternary metal oxide-based thin-film devices.

ALD Zn(O,S) Thin Films’ Interfacial Chemical and Structural Configuration Probed by XAS

The ability to precisely control interfaces of atomic layer deposited (ALD) zinc oxysulfide (Zn(O,S)) buffer layers to other layers allows precise tuning of solar cell performance. The O K- and S K-edge X-ray absorption near edge structure (XANES) of ∼2–4 nm thin Zn(O,S) films reveals the chemical and structural influences of their interface with ZnO, a common electrode material and diffusion barrier in solar cells. We observe that sulfate formation at oxide/sulfide interfaces is independent of film composition, a result of sulfur diffusion toward interfaces. Leveraging sulfur’s diffusivity, we propose an alternative ALD process in which the zinc precursor pulse is bypassed during H2S exposure. Such a process yields similar results to the nanolaminate deposition method and highlights mechanistic differences between ALD sulfides and oxides. By identifying chemical species and structural evolution at sulfide/oxide interfaces, this work provides insights into increasing thin film solar cell efficiencies.

Plasma-Enhanced Atomic Layer Deposition of SiN–AlN Composites for Ultra Low Wet Etch Rates in Hydrofluoric Acid

The continued scaling in transistors and memory elements has necessitated the development of atomic layer deposited (ALD) of hydrofluoric acid (HF) etch resistant and electrically insulating films for sidewall spacer processing. Silicon nitride (SiN) has been the prototypical material for this need and extensive work has been conducted into realizing sufficiently lower wet etch rates (WERs) as well as leakage currents to meet industry needs. In this work, we report on the development of plasma-enhanced atomic layer deposition (PEALD) composites of SiN and AlN to minimize WER and leakage current density. In particular, the role of aluminum and the optimum amount of Al contained in the composite structures have been explored. Films with near zero WER in dilute HF and leakage currents density similar to pure PEALD SiN films could be simultaneously realized through composites which incorporate ≥13 at. % Al, with a maximum thermal budget of 350 °C.

Revealing the Bonding Environment of Zn in ALD Zn(O,S) Buffer Layers through X-ray Absorption Spectroscopy

Zn(O,S) buffer layer electronic configuration is determined by its composition and thickness, tunable through atomic layer deposition. The Zn K and L-edges in the X-ray absorption near edge structure verify ionicity and covalency changes with S content. A high intensity shoulder in the Zn K-edge indicates strong Zn 4s hybridized states and a preferred c-axis orientation. 2–3 nm thick films with low S content show a subdued shoulder showing less contribution from Zn 4s hybridization. A lower energy shift with film thickness suggests a decreasing bandgap. Further, ZnSO4 forms at substrate interfaces, which may be detrimental for device performance.

ETCH “sandbox”: Controlled release dimensions through atomic layer deposition etch stop with trench refill and polish

We report on the demonstration of a microfabrication process which allows the release of suspended films or structures of varying sizes in which the release volume is predefined by i) lithographic patterning, ii) etch-stop deposition by atomic layer deposition (ALD), iii) refilling sacrificial layer into the empty volume, and iv) subsequent chemical mechanical planarization (CMP). We refer to this as an “etch sandbox,” which allows the user to process devices on a planarized substrate after completion of the sandbox. We demonstrate micrometer scale Al2O3 suspended membranes utilizing Al2O3 as the etch-stop and polysilicon as the refill/sacrificial etch material. Within a single substrate and single release, defined etch volumes up to differing by up to a factor of 104 can be realized. Considering the flexibility of this process, suspended membranes with more complicated structure and composition could be developed in the future.