Grazing Incidence Cross-Sectioning of Thin-Film Solar Cells via Cryogenic Focused Ion Beam: A Case Study on CIGSe

Cryogenic focused ion beam (Cryo-FIB) milling at near-grazing angles is employed to fabricate cross-sections on thin Cu­(In,Ga)­Se₂ with >8x expansion in thickness. Kelvin probe force microscopy (KPFM) on sloped cross sections showed reduction in grain boundaries potential deeper into the film. Cryo Fib-KPFM enabled the first determination of the electronic structure of the Mo/CIGSe back contact, where a sub 100 nm thick MoSe_<i>y</i> assists hole extraction due to 45 meV higher work function. This demonstrates that CryoFIB-KPFM combination can reveal new targets of opportunity for improvement in thin-films photovoltaics such as high-work-function contacts to facilitate hole extraction through the back interface of CIGS

Density-Functional Theory Molecular Dynamics Simulations and Experimental Characterization of a‑Al₂O₃/SiGe Interfaces

Density-functional theory molecular dynamics simulations were employed to investigate direct interfaces between a-Al₂O₃ and Si_0.50Ge_0.50 with Si- and Ge-terminations. The simulated stacks revealed mixed interfacial bonding. While Si–O and Ge–O bonds are unlikely to be problematic, bonding between Al and Si or Ge could result in metallic bond formation; however, the internal bonds of a-Al₂O₃ are sufficiently strong to allow just weak Al bonding to the SiGe surface thereby preventing formation of metallic-like states but leave dangling bonds. The oxide/SiGe band gaps were unpinned and close to the SiGe bulk band gap. The interfaces had SiGe dangling bonds, but they were sufficiently filled that they did not produce midgap states. Capacitance–voltage (C–V) spectroscopy and angle-resolved X-ray photoelectron spectroscopy experimentally confirmed formation of interfaces with low interface trap density via direct bonding between a-Al₂O₃ and SiGe

Passivation of InGaAs(001)-(2 × 4) by Self-Limiting Chemical Vapor Deposition of a Silicon Hydride Control Layer

A saturated Si–H_<i>x</i> seed layer for gate oxide or contact conductor ALD has been deposited via two separate self-limiting and saturating CVD processes on InGaAs(001)-(2 × 4) at substrate temperatures of 250 and 350 °C. For the first self-limiting process, a single silicon precursor, Si₃H₈, was dosed at a substrate temperature of 250 °C, and XPS results show the deposited silicon hydride layer saturated at about 4 monolayers of silicon coverage with hydrogen termination. STS results show the surface Fermi level remains unpinned following the deposition of the saturated silicon hydride layer, indicating the InGaAs surface dangling bonds are electrically passivated by Si–H_<i>x</i>. For the second self-limiting process, Si₂Cl₆ was dosed at a substrate temperature of 350 °C, and XPS results show the deposited silicon chloride layer saturated at about 2.5 monolayers of silicon coverage with chlorine termination. Atomic hydrogen produced by a thermal gas cracker was subsequently dosed at 350 °C to remove the Si–Cl termination by replacing with Si–H termination as confirmed by XPS, and STS results confirm the saturated Si–H_<i>x</i> bilayer leaves the InGaAs(001)-(2 × 4) surface Fermi level unpinned. Density function theory modeling of silicon hydride surface passivation shows an Si–H_<i>x</i> monolayer can remove all the dangling bonds and leave a charge balanced surface on InGaAs

Nanoscale Characterization of Back Surfaces and Interfaces in Thin-Film Kesterite Solar Cells

Combinations of sub 1 μm absorber films with high-work-function back surface contact layers are expected to induce large enough internal fields to overcome adverse effects of bulk defects on thin-film photovoltaic performance, particularly in earth-abundant kesterites. However, there are numerous experimental challenges involving back surface engineering, which includes exfoliation, thinning, and contact layer optimization. In the present study, a unique combination of nanocharacterization tools, including nano-Auger, Kelvin probe force microscopy (KPFM), and cryogenic focused ion beam measurements, are employed to gauge the possibility of surface potential modification in the absorber back surface via direct deposition of high-work-function metal oxides on exfoliated surfaces. Nano-Auger measurements showed large compositional nonuniformities on the exfoliated surfaces, which can be minimized by a brief bromine–methanol etching step. Cross-sectional nano-Auger and KPFM measurements on Au/MoO₃/Cu₂­ZnSn­(S,Se)₄ (CZTSSe) showed an upward band bending as large as 400 meV within the CZTSSe layer, consistent with the high work function of MoO₃, despite Au incorporation into the oxide layer. Density functional theory simulations of the atomic structure for bulk amorphous MoO₃ demonstrated the presence of large voids within MoO₃ enabling Au in-diffusion. With a less diffusive metal electrode such as Pt or Pd, upward band bending beyond this level is expected to be achieved

A Versatile Thin-Film Deposition Method for Multidimensional Semiconducting Bismuth Halides

Despite the significant progress in fabricating hybrid organic–inorganic lead halide perovskite solar cells, their toxicity and low stability remain as major drawbacks, thereby hindering large-scale commercialization. Given the isoelectronic nature of lead­(II) and bismuth­(III) ions, potentially stable and nontoxic alternatives for efficient light absorption in thin-film photovoltaic (PV) devices may be found among bismuth-based halide semiconductors. However, high-quality polycrystalline films of many of these systems have not been demonstrated. Here we present a versatile and facile two-step coevaporation approach to fabricate A₃Bi₂I₉ (A = Cs, Rb) and AgBi₂I₇ polycrystalline films with smooth, pinhole-free morphology and average grain size of >200 nm. The process involves an initial two-source evaporation step (involving CsI, RbI or AgI, and BiI₃ sources), followed by an annealing step under BiI₃ vapor. The structural, optical, and electrical characteristics of the resulting thin films are studied by X-ray diffraction, optical spectroscopy, X-ray/UV photoelectron spectroscopy, and scanning electron microscopy

<i>In Situ</i> Observation of Initial Stage in Dielectric Growth and Deposition of Ultrahigh Nucleation Density Dielectric on Two-Dimensional Surfaces

Several proposed beyond-CMOS devices based on two-dimensional (2D) heterostructures require the deposition of thin dielectrics between 2D layers. However, the direct deposition of dielectrics on 2D materials is challenging due to their inert surface chemistry. To deposit high-quality, thin dielectrics on 2D materials, a flat lying titanyl phthalocyanine (TiOPc) monolayer, deposited via the molecular beam epitaxy, was employed to create a seed layer for atomic layer deposition (ALD) on 2D materials, and the initial stage of growth was probed using <i>in situ</i> STM. ALD pulses of trimethyl aluminum (TMA) and H₂O resulted in the uniform deposition of AlO_<i>x</i> on the TiOPc/HOPG. The uniformity of the dielectric is consistent with DFT calculations showing multiple reaction sites are available on the TiOPc molecule for reaction with TMA. Capacitors prepared with 50 cycles of AlO_<i>x</i> on TiOPc/graphene display a capacitance greater than 1000 nF/cm², and dual-gated devices have current densities of 10^–7A/cm² with 40 cycles

Atomic Layer Deposition of Al₂O₃ on WSe₂ Functionalized by Titanyl Phthalocyanine

To deposit an ultrathin dielectric onto WSe₂, monolayer titanyl phthalocyanine (TiOPc) is deposited by molecular beam epitaxy as a seed layer for atomic layer deposition (ALD) of Al₂O₃ on WSe₂. TiOPc molecules are arranged in a flat monolayer with 4-fold symmetry as measured by scanning tunneling microscopy. ALD pulses of trimethyl aluminum and H₂O nucleate on the TiOPc, resulting in a uniform deposition of Al₂O₃, as confirmed by atomic force microscopy and cross-sectional transmission electron microscopy. The field-effect transistors (FETs) formed using this process have a leakage current of 0.046 pA/μm² at 1 V gate bias with 3.0 nm equivalent oxide thickness, which is a lower leakage current than prior reports. The n-branch of the FET yielded a subthreshold swing of 80 mV/decade