United States

The IV-VI semiconductor family of PbSe-SnSe alloys spans narrow direct bandgap materials in the Pb-rich cubic rocksalt phase, and indirect bandgap semiconductors in the Sn-rich layered orthorhombic phase. In recent years, there is growing interest in harnessing electronic and photonic properties of these orthorhombic phases that merit their study as epitaxial thin films. Specifically, there is excitement in harnessing in-plane ferroelectricity in SnSe demonstrated in the single or odd number bilayer limit [1]. Additionally, research groups are showing a large contrast in electronic and optical properties for phase-change devices across the orthorhombic to rocksalt structural transition that can not only be driven by temperature but also most by direct application of intense light-fields [2,3]. The development of growth recipes to fabricate high quality thin films of not only SnSe but also Pb1-xSnxSe offers new compositional routes to tune these properties. In this work, we synthesize Pb1-xSnxSe epitaxial films in the orthorhombic phase across a large composition range by molecular beam epitaxy, on technologically relevant III-V GaAs substrates. Importantly, we find it possible to stabilize metastable single phase compositions beyond what is expected in the bulk. A Riber Compact 21 MBE system with compound PbSe and SnSe cells is used to grow films. (001)-oriented semi-insulating GaAs substrates are prepared by desorbing the surface oxide under a Se overpressure, followed by PbSe surface preparation steps. We find that smooth, out-of-plane single oriented SnSe films result both from samples grown with a PbSe dose, and samples grown with a PbSe dose and buffer layer. We expand on the successful growth of SnSe on GaAs and use the identified surface preparation methods to grow orthorhombic Pnma space group Pb1-xSnxSe alloys on GaAs [4]. The Pb1-xSnxSe alloy composition is determined by the relative fluxes of the PbSe and SnSe compound cells. XRD results are used to extract orthorhombic lattice constants as a function of alloy composition, at room temperature. The difference in unit cell volume between alloys around XSn=0.5 and the predicted unit cell volume of similar composition Pb1-xSnxSe alloys in the cubic phase is small, implying a small energy cost to a phase transition at these intermediate compositions. In addition to structural properties, we will be reporting on the optical bandgap as a function of alloy composition. Based on the bulk phase diagram of the PbSe-SnSe system, we expect alloy compositions from XSn=0.4-0.8 to be two phase; both cubic and orthorhombic. The XSn=0.64 sample is phase separated. However, we have managed to grow thin film samples at XSn=0.55 and XSn=0.74 which are purely orthorhombic. Moreover, these are not the only metastable sample that we have produced in this materials system. We have synthesized room-temperature stable orthorhombic Cmcm phase of Pb1-xSnxSe , a phase typically only stable above 700 K. We will be exploring the structural properties of these samples further, including temperature cycling experiments to better understand the stability of these phases in the thin film limit. [1] C. Kai, et al. …S.S. Parkin, Nano letters, 20, 6590,(2020). [2] T. Katase et al. ... T. Kamiya, Science advances, 7(12), (2021). [3] Y. Huang, ... M. Trigo, Physical Review X, 12(1), (2022). [4]B. Haidet et al. ... K. Mukherjee, Phys. Rev. Mater., 4, 033402 (2020).

EMC 2023

Single-Phase Orthorhombic SnSe-PbSe Alloy Thin Films Stabilized on GaAs by Molecular Beam Epitaxy

epitaxial thin film

chalcogenides

After two years of virtual conferences, we're excited to host EMC 2022 in person at The Ohio State University in Columbus, Ohio. In addition to cutting-edge technical presentations given in multiple parallel sessions, opportunities to network with colleagues will be present throughout the conference. The size and format of the meeting makes networking easy, whether you have been coming to the conference for many years or are a first-time attendee.

EMC is coordinated with the Device Research Conference (DRC), which will be held the same week, from June 26-29. This recognizes the strong interaction between device and electronic materials research and provides fruitful exchange of information between attendees of both Conferences.

For information on the 80th Device Research Conference, please visit the DRC 2022 website.



EMC 2022

The Electronic Materials Conference (EMC) is the premier annual forum on the preparation, characterization and use of electronic materials. This year’s event will feature a plenary session, topical sessions, a poster session and more.

technical paper

The Electronic Materials Conference (EMC) is the premier annual forum on the preparation, characterization and use of electronic materials. This year’s event will feature a plenary session, topical sessions, a poster session and more.

EMC is coordinated with the Device Research Conference (DRC). This recognizes the strong interaction between device and electronic materials research and provides fruitful exchange of information between attendees of both Conferences.

The Electronic Materials Conference (EMC) is the premier annual forum on the preparation, characterization and use of electronic materials. 

III-V materials nowadays are becoming a promising building block for nonlinear optics thanks to their significant nonlinear properties. MBE and MOCVD have been widely used to grow epitaxial films on various substrates. However, the difficulty in lattice matching, the constraint in substrate selection, the issue in epitaxial transfer aligning, and the cost in fabricating, become the barrier to applications. Here, we introduce a new thin-film epitaxial growth method, templated liquid phase (TLP) growth, to directly grow III-V semiconductor film on any substrate with single crystallinity, high uniformity, and high carrier mobility. As a demonstration, we choose lithium niobate (LiNbO3), a promising material widely used in nonlinear optics and acoustoelectric fields, as the substrate to grow patterned InAs layer. With this technology, we grow InAs in line-shape patterns with widths varying from 3 micron to 10 micron and lengths varying from 10 micron to 100 micron. Based on our measurements, the electron mobility reaches ~3500 cm2/Vs at 100K and ~2400 cm2/Vs at 300K. In prospect, TLP growth method can construct a platform to fabricate any desired patterns on any substrates in applications of nonlinear optics profit from the superior nonlinear properties of III-V semiconductors.

Monolithic III-V on LiNbO3 for Nonlinear Optics Application

Over the last few years, tremendous improvements of device performance and reliability have been demonstrated in devices grown on blanket Si substrates. Recently, electrically pumped QD lasers deposited in narrow oxide pockets in a butt-coupled configuration and on-chip coupling have been realized on patterned Si photonic wafers. However, the device yield and reliability, which ultimately determines the scalability of such technology, are limited by the material uniformity. Here, we perform the detailed analysis on the material anisotropy induced by the pocket geometry. Both the QD morphology and defect configuration anisotropy suggest that all pockets should be aligned to the [1 -1 0] direction of the III-V crystal. The 300 mm growth template consists of rectangular oxide pockets both “parallel” and “orthogonal” with respect to the Si substrate step edges, shown in Figure 1(a) and (b). The room temperature photoluminescent (PL) spectra of the as-grown in-pocket QD laser material, shown in Figure 1(d), indicate that QD quality is higher in the “orthogonal” and wider pockets. The corresponding laser performance, shown in Figure 1(e), suggesting the gain from the QDs in the narrower “parallel” pockets is not high enough for lasing. Representative PL intensity maps of a “half-laser” structure and the corresponding atomic force microscopy (AFM) image of the exposed active region dots are shown in Figure 1(f). The drastic discrepancy of the emission uniformity and QD density between the materials in the wider “orthogonal” pocket and the narrower “parallel” pockets are attributed to asymmetric indium adatom diffusion, with [1 -1 0] being the fast diffusion direction. To map the crystal orientation onto the pocket orientation, faceted trenches (FTs), which would form when a GaAs layer experience abrupt tensile relaxation, were introduced in the pocket with the test structure shown in Figure 2(a). Nomarski images show that FTs elongate along the “orthogonal” pocket, confirming that “orthogonal” pockets are in the [] orientation with respect to the III-V crystal, shown in Figure 2(b) and (c). Thus, aligning the pocket to the fast diffusion orientation would prevent indium adatoms from diffusing onto the edges. Since the edge of the III-V strip is stress free, it is also expected that the residual thermal stress state of the in-pocket materials would also be highly anisotropic with the direction along the short edge of the strip experiencing lower residual tension. It has previously been confirmed that the misfit dislocations (MDs) generated from the residual thermal stress is responsible for device degradation. Such stress anisotropy introduced by the pock geometry could then be utilized to reduce the MD density. Two TEM foils were prepared parallel (“‖”) and perpendicular (“⊥”) to a 20 µm [1 -1 0] oriented pocket, shown in Figure 2(d). Comparing to Figure 2(e) where foil oriented parallel to the pocket, the foil perpendicular to the pocket, shown in Figure 2(f), has a significantly lower density of misfit dislocations, even much lower than those measured in blanket substrate materials. It has previously been found that the MDs in the [1 -1 0] direction formed via tensile relaxation have a Ga-core structure, which resides closer to the middle of the GaAs/AlGaAs bandgap than the MDs in the [1 1 0] direction Though the MDs along the long edge of the pocket would be reduced regardless of the alignment of the pocket with respect to the III-V crystal, all pockets should be aligned to the [1 -1 0] direction to effectively lower the density of the most detrimental defects. The in-pocket devices would then potentially surpass the performance of the state-of-the-art devices grown on blanket Si substrate.

Quantum Dot Morphology and Defect Configuration Anisotropy for III-V Laser Material Grown on Patterned Si Photonic Wafers

Recent research efforts have unlocked substantial improvements in the performance and reliability of InAs quantum dot lasers on silicon by mitigating the effects of active-region-adjacent misfit dislocations through the introduction of defect trapping layers (TLs). These TLs enable lifetimes of lasers on silicon comparable with those grown on native GaAs substrates. Even so, current state-of-the-art lifetimes still fall somewhat short of requirements for more widespread adoption as both types of lasers exhibit gradual degradation in performance with aging. In this work, we demonstrate that this gradual degradation is a result of the generation of point defect populations during aging, which, over time, coalesce into dislocation microloops which we characterize using scanning transmission electron microscopy (STEM). We selected a single high-performing laser from each of three large aging sets of QD lasers (1) on silicon with TLs, (2) on silicon without TLs (no-TL)), and (3) on native GaAs. Each selected device was aged for between 8700 and 12,000 h at 80 °C at a current density between 1050 and 1900 A cm-2, depending on the initial threshold current and output power. After 2700 h, the no-TL device had doubled its initial threshold current (a common metric for device failure), and after 5700 h it failed to lase entirely. The TL device and the GaAs device, however, show threshold current increases of just 35% and 46% respectively after 8700 h with extrapolated lifetimes of 45,000 h and 75,000 h. Note, however, that these numbers are not directly comparable due to somewhat harsher aging conditions for the GaAs device. After aging, the devices were imaged using backside electroluminescence to check for signs of inhomogeneous degradation. For all lasers, degradation appeared largely homogeneous aside from dark line defects associated with misfit dislocations in the devices on silicon. There are also regions of decreased luminescence throughout the structure for the two devices on silicon, which we attribute to misfit dislocations lying parallel to the laser ridge for the no-TL device and to misfit dislocations escaped from the TL for the TL device, as confirmed by plan-view STEM of these regions. With sufficient thinning of the plan-view foils, dislocation microloops with diameters ranging from 5 to 20 nm are visible both in the [001] zone imaging condition overlaid with the QD contrast and more clearly in various g-3g STEM weak beam imaging modes where competing QD contrast is minimized. In the no-TL device in regions within 100-150 nm of the heavily climbed misfit dislocations, there is a dramatic increase in loop density on the order of 1010 cm-2. We believe this to be the result of a large density of point defects expelled by the misfit dislocations as they undergo climb. Thus, degradation caused by climbing misfit dislocations is multifold: first an increase in total misfit length and second the generation of a high density of point defects which coalesce into microloop defects surrounding the misfit dislocation, all of which substantially enhance non-radiative recombination processes. Meanwhile, the TL and GaAs device active regions, though largely or entirely free of climbed misfit dislocations, are still observed to contain microloop densities on the order of 108–109 cm-2, which likely accounts for the gradual, homogeneous degradation observed in these devices. These microloops coalesce from elevated point defect populations in the active region, and while there are many remaining questions, such as whether these point defects are pre-existing or are generated during aging, this work points the way for future routes to improve device and material quality.

Gradual Degradation via Dislocation Microloop Formation in InAs Quantum Dot Lasers on Si and GaAs

InGaAs quantum wells (QW) grown by MOVPE are used to extend the absorption edge of GaAs photovoltaics into the infrared, increasing the current density with minimal loss in voltage as compared to a bulk GaAs device. GaAsP strain compensation is used to provide a tensile offset to the compressive strain in the wells, allowing growth of 50 pairs of strain balanced wells which did not exhibit detrimental effects of lattice relaxation/defect formation on photovoltaic figures of merit. Significant work was performed to optimize the growth and interfaces of these films to prevent step bunching and poor surface morphology and device performance when grown on offcut substrates, which are necessary for e.g. growth on non-polar germanium for a bottom junction or atomic ordering of InGaP for a top junction. A distributed Bragg reflector (DBR) is incorporated below the device, effectively increasing the optical path through the QWs and nearly doubling their sub-GaAs-bandgap collection per well. Dual-junction InGaP/GaAs devices were designed using a drift-diffusion model incorporating the optical effects of each subcell, connecting tunnel junction, and anti-reflective coating (ARC) along with the expected contribution of a varying number of QWs. These parameters were co-optimized to design control and QW-enhanced designs which were each nominally current-matched under AM0. Dual-junction devices were grown with an InGaP top junction coupled to either a control GaAs bottom cell without quantum wells or to the previously developed 50-QW enhanced GaAs bottom cell. Once fabricated, the QW-enhanced device provided an extra 2.6 mA/cm2 from sub-gap absorption with a minimal loss in voltage (11 mV) compared to a control device without QWs or DBR, providing an absolute efficiency increase of 3.6% under AM0.

InGaAs/GaAsP Multiple Quantum Well Enhanced Multijunction Photovoltaics

Traditional lock-in amplifiers employ a simple IIR filter which can have long measurement times to produce acceptable noise reduction. Using AC field Hall Effect measurements, we demonstrate that a combination of digital lock-in output stage filters, FIR and IIR low pass filters, can produce improved noise reduction and measurement time over the traditional approach of independent IIR filters or FIR filters. We will present step response, equivalent noise bandwidth and autocorrelation times for a n pole IIR filter (n <=4) followed by a filter response of any finite length. We will demonstrate that the performance of the hybrid output filter, with time measured in units of FIR time, depend only on the ratio of the IIR time constant to the FIR time. Finally, we will present criteria for statistically independent samples for the various filter configurations. AC field Hall Effect characterization is carried out with very low AC field frequency, typically 100 mHz, to avoid inductive pick-up in the measurement. Measurement times for a single voltage reading are as long as 60 seconds. The standard protocol for the Hall effect measurement includes current reversal and two different geometries which increases the number of voltage readings to 6 and results in a 6-minute total observation time to measure a single Hall voltage. Multiple Hall voltage readings are needed to calculate the of standard error of the Hall voltage and increases the measurement time in excess of 1 hour. An effective hybrid filter approach for AC field Hall effect measurement promises shorter measurement times or lower noise in the same observation window. Because it is a well understood material, an InAs sample was chosen as a model system for performance testing of the hybrid output filter in an AC field Hall effect measurement. Using currents of 10 microamps, the Hall voltage will be on the order of a few microvolts for this InAs sample depending on the field magnitude. In this work, an InAs sample with 6.52e16 cm-3 carrier density was first measured in a Lake Shore Cryotronics 8407 using the AC field option of the 8400 system. Using a 10 microamp excitation current and 0.644 T AC field amplitude, the magnitude of the measured Hall voltage was 2.44 microvolts with a 119 nanovolt standard deviation. The measurement electronics were subsequently replaced with the Lake Shore Cryotronics Model M81 Synchronous Source Measure platform equipped with a balanced current source (BCS-10) and voltmeter (VM-10) and the Hall effect measurement repeated with the same magnet and power supply. The M81 lock-in was configured with a hybrid output filter consisting of an IIR filter with a 4 second time constant and 12 dB roll-off combined with a FIR filter with 40 second averaging time which results in a total settle time of the filter of 60 seconds. The measured Hall voltage was 2.47 microvolts with a standard deviation of the Hall voltage of 37 nanovolt – a factor 3.2 improvement in measurement uncertainty over the 8400 system.

Improved AC Field Hall Measurements Using Hybrid Filters

Blue and green InGaN/GaN light-emitting diodes (LEDs) are crucial for solid-state lighting and display applications. Violet–blue LEDs have achieved a high EQE of ~80%. However, InGaN/GaN LEDs still suffer from strong efficiency loss in the green region, which is usually referred to as the “green gap.” The green gap issue has been related to Auger recombination, the quantum-confined Stark effect (QCSE), carrier leakage, random alloy fluctuations, and defects and carrier localization, among other causes. To achieve longer emission wavelengths, the indium composition must be increased, leading to several problems. First, high indium content increases lattice mismatch and introduces additional point defects during metalorganic chemical vapor deposition (MOCVD). High indium content also leads to strong polarization-related electric fields in the quantum wells (QWs), lowering the wave function overlap and decreasing the radiative and non-radiative recombination rates (i.e., from the QCSE). An important question is how much of the change in efficiency reduction from blue to green is due to intrinsic effects (i.e., wave-function overlap and phase-space filling (PSF)) vs. extrinsic effects (i.e., material degradation). In this work, we use small-signal electroluminescence (SSEL) to study the carrier dynamics in commercial c-plane LEDs with various emission wavelengths ranging from blue (~470 nm), to cyan (~500 nm), to green (~530 nm). The LED series consists of three wafers grown by Lumileds with identical active region designs (3X, 3 nm QWs) but different indium compositions. LEDs with mesa diameters of 100 µm and ground-signal-ground electrodes were fabricated. SSEL was then used to study the carrier dynamics of the LEDs. Each element in an equivalent circuit model was acquired via simultaneous fitting of the impedance and modulation responses, leading to the injection efficiency and total carrier recombination lifetime. Combining the total carrier lifetime with internal quantum efficiency (IQE) enables the separation of the radiative and non-radiative recombination lifetimes and rates as a function of carrier density (n) and current density (J). The results reveal that efficiency loss at longer wavelength is related to reductions in injection efficiency and radiative recombination, and an increase in Auger recombination. The interplay between the QCSE and carrier-current density relationship is shown to determine the relative importance of the radiative (B(n)*n2) and non-radiative recombination rates (A(n) and C(n)*n3) as the indium content is increased. Both the SSEL experiment and Schrödinger-Poisson calculations showed C(n)/B(n) is similar for the three wafers at high n, indicating that intrinsic material degradation is not significant. In addition, compared with QCSE, PSF, and material degradation, the increase of the carrier density at a given J for longer wavelengths is the dominant reason for the green gap. Thus, methods to decrease the carrier density in each QW are suggested to increase the EQE of green LEDs.

Carrier Dynamics in Blue, Cyan and Green Commercial InGaN/GaN LEDs Measured by Small-Signal Electroluminescence

Nonradiative recombination centers (NRCs) are known to negatively affect the efficiency of light-emitting diodes (LEDs). NRCs contribute not only to Shockley–Read–Hall (SRH) recombination, but also to trap-assisted Auger recombination (TAAR). The relationship between NRCs and the TAAR processes has been predicted theoretically and observed experimentally to a limited degree. However, some details about TAAR are still under debate, including its magnitude relative to intrinsic Auger and SRH recombination, its dependence on carrier density (e.g., ∝n2 or ∝n3), and its role in the green gap, particularly in high-quality commercial-grade LEDs. TAAR is typically assumed to be linearly dependent on trap density, directly tying the magnitude of TAAR to the quality of the sample. Previous studies have examined TAAR using time-resolved photoluminescence and optical differential lifetime measurements, which do not capture certain effects related to electrical injection. In this work, we report a quantitative study of TAAR in c-plane commercial-grade green LEDs grown by metal-organic chemical vapor deposition (MOCVD). We investigate a series of three wafers with identical active regions (3X, 3-nm-thick quantum wells) but different NRC trap densities (4.4e14 cm-3 < Nt < 1.5e15 cm-3). We use small-signal electroluminescence (SSEL) and deep-level optical spectroscopy (DLOS) to study the carrier dynamics and density of NRCs in the samples, respectively. SSEL is used to determine the carrier lifetime, which is used in conjunction with the internal quantum efficiency (IQE) to decouple the radiative and nonradiative recombination rates. The total recombination lifetime in higher quality growth is found to be longer than that in lower quality growth and is attributed to the longer nonradiative recombination time in higher quality samples. On the other hand, the radiative recombination rates for different growth qualities at a given carrier density (n) are identical, as expected from samples with the same active region design. We extract the relationship between the density of NRCs and the corresponding nonradiative recombination rates at a given carrier density (n). The difference in nonradiative recombination between the samples offers information about both SRH and TAAR with respect to growth quality. The linear dependence between the SRH recombination rate and the density of NRCs at low n was confirmed. However, TAAR was found to be sublinear with the density of NRCs, possibly due to the relatively small changes (~3X) in defect density (Nt) in these samples. We find that TAAR is best fit to an n2 dependence on carrier density. This is supported by the observation that all three samples have similar IQE at high carrier density, which would not be consistent with an n3 process. We also show that the TAAR recombination coefficients (D) are on the order of ~1e13 cm3 s-1, which is similar to that reported for violet LEDs and shows the weak dependence of TAAR on bandgap. Using the extracted TAAR coefficients, we calculate that TAAR accounts for about 10% of the total nonradiative recombination rate at moderate carrier densities. Thus, intrinsic Auger dominates the nonradiative recombination and TAAR is not a major contributor to the green gap in these commercial LEDs. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

Trap-Assisted Auger Recombination in Commercial Green InGaN/GaN LEDs

GaN-based light emitting diodes (LEDs) for white light illumination are one of the biggest technological achievements in energy efficiency. However, challenges remain with GaN-based LEDs such as efficiency droop and low efficiency for longer wavelengths. Auger-Meitner recombination, a three-carrier process, has been identified as the dominant mechanism for efficiency droop via electron emission spectroscopy (EES). In EES, LEDs were activated to negative electron affinity (NEA) to allow for the emission of hot electrons from the surface which were quantitatively correlated to Auger-Meitner recombination processes. In recent years, V-defects have been proposed as the solution to the excess driving voltage of long wavelength nitride-based LEDs, one of the sources of their diminished efficiency. One way to image the effects of V-defects is using electron emission microscopy (EEM). EEM is a newly developed technique analogous to EES that allows for spatially resolved microscopy of hot electrons. EEM uses a low energy electron microscope with its illumination column turned off, such that only internally generated electrons escaping out the surface during electrical driving of the device are observed. Commercial blue (λ ≈ 460 nm) and green (λ ≈ 525 nm) LED wafers from Seoul Viosys were acquired and processed into LEDs. The diodes were formed by Pd/Au (20/30 nm) p-contacts with open circular and hexagonal apertures lithographically defined with known dimensions. A C-shaped n-contact partially surrounding the p-contact was formed by reactive ion etching into the n-region and contacted with Ti/Au (30/300 nm). EEM was performed in an ELMITEC LEEM III system modified to allow for electrical biasing of devices from the sample stage, where Cs was deposited onto the sample using a SAES Getters Cs source to activate the sample to NEA. For LEDs, EEM images the non-radiatively-recombined electrons that cross the diode junction and diffuse through the p-GaN and escape out the surface, which can include overflow, Auger-Meitner, and trap-assisted Auger-Meitner electrons. EEM images of blue and green LEDs show non-electron emitting regions (dark spots) with densities of ~2 × 107 cm-2 and ~3 × 108 cm-2, respectively, which we identify as V-defects. This is confirmed through the corresponding dark spots of panchromatic CL measurements of the same samples, as well as SEM and plan-view TEM for V-defect densities. Since the density and dimensions of the non-electro-emitting and non-illuminating regions are identical, we attribute our measured lack of electron emission directly from the V-defect areas to electrons fast traveling through the sidewalls of the V-defects and being injected laterally and efficiently into planar quantum wells without any Auger-Meitner recombination or overflow within the sidewalls. This is supported by the stronger electron emission observed at the ridge of the V-defects due to larger carrier densities there and thus increased hot electrons generation. Our results agree with simulations that electrons travel up the sidewalls of V-defects that have been nucleated prior to the active region and directly inject themselves into the quantum wells in the active region, so the carrier density in these sidewalls is negligible [1]. Support at UCSB was provided by U.S. Department of Energy under the Office of Energy Efficiency & Renewable Energy (EERE) Award No. DE-EE0009691. [1] C.-H. Ho, J. S. Speck, C. Weisbuch, and Y.-R. Wu. Phys. Rev. Appl. 17, 014033 (2022).

Electron Emission Microscopy of an Electrically Driven III-Nitride-Based LED: Evidence of Lateral Electron Injection at V-Defect Sidewalls

Rapid progress has been achieved in perovskite solar cells, improving the efficiency from 3.8 % to 25.7 % in less than a decade. However, the stability of perovskites still need to be improved before commercialization. This study reports the thermal stability of perovskites exposed to an ion beam irradiation. Such combined stressors are seen in atomic/nanoscale microscopy, where a perovskite lamella is characterized using a controlled heating/cooling stage. Focused ion beams (FIBs) are frequently used to section perovskites of interest. Previous studies proposed that high-energy electron beams could cause unexpectedly fast thermal degradation. Alternatively, the perovskite surface may be already altered during FIB processes, accelerating the deterioration. We use a grazing angle argon ion (Ar+) beam directly irradiated on methyl-ammonium lead iodide (MAPbI3) to test the impact of ion beams to degradation mechanisms. This perovskite film was spin-coated on a stack of glass / FTO (fluorine-doped tin oxide; 200 nm) / SnO2 (35 nm). A 4 kV Ar+ ion beam was injected into a part of the MAPbI3 sample at an angle of 3° for 10 minutes. A strong photoluminescence (PL) peak was observed near 765 nm (1.62 eV) for both samples, attributed to the bandgap emission. However, the PL intensity decreased by 40 % after the Ar+ polishing. A finite difference time domain (FDTD) simulation estimates an approximately 40 % of the PL excitation (244 nm laser) can be absorbed within ≈15 nm of the MAPbI3 subsurface. This result agrees with the ion-beam penetration depth, calculated by Monte Carlo simulations. The PL analysis and the numerical simulations indicate that the 4 kV ion beam irradiation at an incident angle of 3° introduces an optically inactive “dead layer (≈15 nm)” on MAPbI3. We performed x-ray photoemission spectroscopy (XPS) to investigate possible surface compositional changes of MAPbI3 under Ar+ polishing. Quantitative XPS analysis shows an apparent reduction of the organic components (e.g., C, O, and N) after the ion beam irradiation. In contrast, XPS signals of the inorganic species (e.g., Pb, I) significantly increase with introducing the doublet metallic Pb (0) near the bonding energy of 137 eV and 142 eV. Overall, the I/Pb ratio decreases from 2.6 (pristine) to 2.1 (polished), suggesting the ion beam irradiation modifies the subsurface of MAPbI3 to be a lead-rich, iodine-deficient layer (< 10 nm). The impact of the modified subsurface of MAPbI3 on thermal deterioration was qualitatively investigated using a heat lamp at 150 °C. Owing to the partial surface decomposition, we observed the ion-beam exposed MAPbI3 shows a faster decay than the control under the heating. Apparent deterioration of MAPbI3 (yellow color) on the Ar+ polished section after applying the heating stressor for 20 minutes. At this stage, the color of the pristine area remains unchanged (dark-brown color). The deterioration of the pristine area began at around 40 minutes. Both sample sections fully deteriorated at 70 minutes. Our experiment confirms that subsurface modification (≈15 nm) can significantly influence the deterioration mechanisms of MAPbI3. This talk will discuss a new method of monitoring the deterioration of perovskites in real-time. A prototype design consists of a patch antenna (≈ 3 cm × 4 cm) and an array of four MAPbI3 solar cells (1 cm × 1 cm). The distance of the adjacent solar cells is set to 1.4 cm in width and 0.6 cm in length. Computer Simulation Technology (CST) simulations display the resonance frequency of the patch antenna changes from 7.13 GHz to 7.07 GHz as the MAPbI3 film decomposes to volatile species and PbI2. While the shift is small, our pilot design demonstrates the feasibility of wireless monitoring the deterioration of perovskite solar cells. Tailoring antenna design and experimental validations are in progress.

Monitoring Stability of Metal-Halide Perovskites Under Combined Stressors of Ion-Beam and Heat

A large portion of energy consumption (50%) in buildings arises from heat transfer to buildings, specifically through windows due to solar illumination. Energy-efficient thermochromic smart windows, which are composed of special materials that enable them to adjust light transmission dynamically based on the ambient temperature, can allow energy demand for building cooling to be reduced. Hybrid halide perovskites are promising candidates for thermochromic smart windows owing to their capability to undergo reversible color and phase transitions between low and high temperatures. Furthermore, a new concept for smart photovoltaic windows has been realized based on inorganic perovskite CsPbI3-xBrx thin films, which possess both photovoltaic and thermochromic features. However, the perovskite-based thermochromic smart windows require relatively high transition temperatures (>50°C), and this hinders their practical application in buildings since in most places the window temperatures do not reach 50°C in direct sunlight. Therefore, a configuration based on a thermochromic perovskite layer integrated with an external heating source is needed for the activation of thermochromism. Transparent heaters are a key component in thermochromic smart window devices that employ transparent conductive electrodes (TCEs) to provide electric heating. Low resistance and high optical transparency of TCEs are two essential prerequisites for the fabrication of thermochromic smart windows. Among various TCEs, indium tin oxide (ITO) is most commonly used; but the brittle nature limits its application in flexible optoelectronics, and the scarcity of indium results in high material cost. Transparent and conductive metal nanomesh (NM) films, which are composed of periodic nanostructured metal networks, have gained considerable attention as high-performance TCEs offering several advantages, including excellent mechanical flexibility, high conductivity, tunable transmittance, and low fabrication cost. In this study, we introduce a new class of dielectric/metal/dielectric nanomesh films based on ZnO/Au/Al2O3 with periodicities of 200 nm and 370 nm which are fabricated using nanosphere lithography. For the metal mesh film design, an Au (15nm) thin film is chosen as the electrode material due to its malleability and high electrical conductivity. Thin layers of Al2O3 film (20 nm) and ZnO film (20 nm) are used as high refractive index dielectric layers to induce antireflective effect and suppress surface plasmons. We use relatively smaller metal mesh periodicities of 200 nm and 370 nm as compared to previously reported nanomesh films to create a potential pathway to optoelectronic applications, such as smart windows. As a result, the ZnO/Au/Al2O3 NM film with a 370-nm periodicity exhibits a transmittance over 90% at 550 nm and sheet resistance lower than 20 Ω/sq. Based on a standard figure of merit, this structure outperforms current state-of-the-art nanomesh films. Moreover, we integrate the ZnO/Au/Al2O3 NM film into thermochromic perovskite smart windows that operate by inducing a thermochromic transition from transparent to opaque state via electric heating of ZnO/Au/Al2O3 NM film at 50°C, which can be then maintained solely by 1-sun AM 1.5G solar illumination. This climate-adaptive, low power-activated, and fast-switching smart window structure opens new pathways toward its practical application in the real world.

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