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The bandgap of germanium-tin (Ge1-xSnx) alloys can vary from ~0.8 eV into the far-IR as the Sn composition increases, and transition from indirect to direct bandgaps between 6 and 17 at% Sn depending on strain. Consequently they are being developed for a range of thermal and multijunction photovoltaic (PV) cells, thermal imaging, and photonic data transmission applications. However, at useful Sn contents these alloys are metastable and require low epitaxial growth temperatures and can only be grown to limited thickness; otherwise films may experience Sn segregation and defect formation. Growing below ideal temperatures for Ge can also induce defects, reducing electrical conductivity. PV cell design is harder to optimize for performance if the absorbing layer is limited in thickness by material considerations. One mitigation strategy for non-optimal growth conditions is to deposit and maintain on the growth surface a thin coverage of an additional atomic species, which acts as a surfactant to modify adatom interactions. Surfactants change the surface energy of the growth front, which controls the mobility of adatoms and affects the rate of positional exchange between buried and surface layers. A surfactant that preferentially moves to the growth surface could thus increase the incorporation of Sn into deeper layers and reduce defects due to growth at low temperatures. Additionally, the filling of partial growth layers by surfactant atoms tends to reduce islanding and therefore suppresses surface roughness. In this work, antimony is applied as an n-type dopant and surfactant in the growth of GeSn alloys to potentially improve the crystallinity of Ge and GeSn films grown by molecular beam epitaxy. Sb has a high vapor pressure at low temperatures, which is ideal for minimizing radiative heating of the substrate during low-temperature growths. Pairs of GeSn samples are deposited without and with the codeposition of Sb. The growth temperature, film thickness, and Sn content are varied to investigate the impact of Sb on Sn incorporation and film critical thickness. Depending on growth conditions, varying amounts of Sb will be incorporated into the growing film, so this study also provide data on the retention of Sb as a dopant during low-temperature GeSn growth. Sn content and film quality are characterized by X-ray diffraction, photoluminescence spectroscopy, Raman spectroscopy, spectroscopic ellipsometry, and optical microscopy. Electrical properties are determined by Hall effect measurement. Results will be discussed in terms of how the Sb surfactant has modified the kinetics of the growth process and considered with design of a GeSn based photodiodes and thermophotovoltaic (TPV) cells in mind.

EMC 2023

Doping and Surfactant Behavior of Antimony in Molecular Beam Epitaxy Grown Germanium-Tin

surfactant

sigesn

optical

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), which will be held during the same week, from June 20-23. This recognizes the strong interaction between device and electronic materials research and provides fruitful exchange of information between attendees of both Conferences.

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EMC 2021

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. 

The incorporation of small size boron into conventional III-V alloys provides a method for strain engineering of near-infrared, direct-bandgap alloys that can be lattice-matched to silicon1 and other2,3 substrates. In other highly-mismatched alloys (HMAs) such as dilute-nitrides and dilute-bismides, dilute amount of nitrogen or bismuth changes the lattice constant and perturbs the host band structure, reducing the bandgap though often at the expense of degraded optical properties. The advantage of incorporating boron is a higher predicted solid solubility, which suggests that less degradation of material properties can be expected than for other HMAs.4 However, B-III-V alloys remain underexplored in both theory and experiment. Thermal treatment is often employed to recover the optical properties of HMAs, but is typically accompanied by an undesirable blueshift in emission wavelength from changes in the nearest-neighbor bonding configuration.5 A blueshift in emission wavelength and modest improvement in photoluminescence (PL) has been observed with in situ and ex situ thermal annealing of BGaInAs films,2 while the converse is observed from BGaAs.6 Here we report the effects of atomic rearrangement on the band structure and optical properties of B-III-V alloys to understand this dichotomy. BGa(In)As films were grown by solid-source molecular beam epitaxy on (100) semi-insulating GaAs substrates. Samples were progressively annealed by rapid thermal annealing for 1 minute in 50°C steps from 550°C to 900°C and then characterized by room-temperature PL. Experimental results were then compared to ab initio simulations obtained with the density functional theory (DFT) package VASP using HSE06 hybrid functionals. A set of BGaInAs structures were simulated varying numbers of In atoms in the first group-III nearest neighbor positions around B atom, as well as BGaAs structures with varying distances between B atoms. We found an increase in bandgap with increasing numbers of B-In bonds and the most energy-favorable B0.037Ga0.663In0.3As structure was that with the highest number of B-In bonds. This suggests In atoms tend to be in the first nearest neighbor position of B under thermally stable conditions, contributing to a blueshift during annealing. We used an elevated In concentration for BGaInAs calculations compared with experimental data to both sweep a larger number of B-In bonds and investigate bandgap energies closer to the technologically significant 1.3 and 1.55 µm emission wavelengths. Further studies of rearrangement in BGaInAs with low In concentration are in progress. In contrast, the calculated bandgap differences, and energetic driving force due to different B-B distances in B0.03Ga0.97As were much smaller than for In-containing alloys, suggesting BGaAs alloys are not likely to show a strong blueshift upon annealing. This is consistent with the minimal, ~5 meV shift in peak PL energy observed experimentally from B0.03Ga0.97As with annealing. While the intrinsic matrix element strength of BGaInAs increased by about 30% from no B-In bonds to most B-In bonds in our calculations, it can explain a portion of the improvement in optical quality observed with annealing. Since improvement in PL intensity was observed from both BGaAs and BGaInAs, we suspect the removal of the defects played a significant role. Studies of the In concentration on the magnitude of blueshifting in BGaInAs, specifically targeting 1.3 and 1.55µm emitters, are in progress and will be reported at the conference. This work was supported by the National Science Foundation (Award Nos. ECCS-1933836, DMR-1508646, and CBET-1438608). The authors also acknowledge the LEAP cluster at Texas State University for computing resources. 1 McNicholas et al. Cryst. Growth. Des. (2021) 2 El-Jaroudi et al. Appl. Phys. Lett. (2020) 3 El-Jaroudi et al. Cryst. Growth. Des. (2022) 4 Hart et al. Phys. Rev. B. (2000) 5 Lordi et al. Phys. Rev. B. (2003) 6 White et al. 62nd EMC (2020)

Atom Rearrangement in BGa(In)As Alloys Under Annealing

Exceptional properties of ultra-wide band gap (UWBG) materials in chemical/thermal stability, spontaneous electric polarization, low dielectric permittivity, high sound velocity, high biocompatibility, and high mechanical flexibility cause increasing attention to UWBG materials as great candidates for wide range of piezoelectric applications. However, compared to those of currently dominant piezoelectric materials such as lead zirconate titanate (PZT), the piezoelectric coefficients of III-N materials are relatively low. Nevertheless, these lead-free materials have a great potential to replace the PZT because of their operational performance in high temperature and pressure conditions. Also, the biocompatibility of III-N materials makes them a promising candidate to overcome the critical limitation of PZT in epidermic and implantable sensors and electronics. Therefore, we introduce a hybrid chemical vapor deposition (HybCVD) as a new thin-film growth technique for Group-IIIa-Group-IIIb-Nitride alloy to significantly improve the piezoelectric properties in group-IIIa-N (III-N) materials such as wurtzite AlN, by group-IIIb transition metals including scandium (Sc) and yttrium (Y). In this study, we focus on the thermodynamic analysis of precursor reaction chemistry and solid-phase formation, including growth temperatures, input V/III ratios, and input carrier gas ratios in different parts of source zones and mixing/growth zone. The main reason for our calculations comes from the necessity of control on the alloy composition depending on various precursors and deposition conditions. For YGaN alloys, there is not a linear relationship between the input cation precursor ratio, RY, and composition, xY: the xY is always higher than RY, indicating significantly higher driving force for the formation of YN in the ternary phase. Also, higher growth temperature (700 → 900 ○C), more H2 in the carrier gas (0 → 100 Pa), and lower V/III ratio (200 → 100) result in higher xY by further decrease in driving force of GaN deposition. In contrast, RY vs. xY is nearly linear for YAlN alloys. When the effects of temperature, carrier gas mixture, and V/III ratio on xY are marginal, ScAlN showed the same trend as YAlN in different growth parameters’ changes. The new growth technique offers the potential to produce well-controlled, uniform, and high-quality piezoelectric films based on transition-metal-alloyed III-N materials which is critical to obtain high performance piezoelectric devices based on this material.

Composition Control of Ternary Group-IIIa-IIIb-Nitride Alloy by Hybrid Chemical Vapor Deposition—A Thermodynamic Analysis

Silicon Carbide (SiC) is a wide bandgap semiconductor, which has been widely applied to power electronics due to its excellent properties of wide band gap, high break down voltage and thermal stability. However, defects, such as threading edge dislocations (TEDs), threading screw dislocations (TSDs) and basal plane dislocations (BPDs), can limit the performance of the devices [1,2], so characterization of those defects becomes a critical aspect to improve the quality of 4H-SiC wafers. Synchrotron monochromatic beam X-ray topography (SMBXT) is a powerful technique to characterize the defects with the advantages of large field of view and clear contrast of the defects. SMBXT in 11-28 reflection has been widely used, however, the penetration depth, around 17um [3], appears to be shallow to characterize device with deep PN junction. Therefore, a new reflection with deeper X-ray penetration depth and clear contrast on the defects has been sought for. 22-4 16 reflection, the harmonic reflection of 11-28, shows better contrast on the defects, higher strain sensitivity and deeper X-ray penetration. Contrasts from the same defect of 4H-SiC with 24 um epilayer in 11-28 and 22-4 16 reflections are compared as shown in Fig. 1. From 11-28 SMBXT, BPD segments near the surface of the wafer can be observed, as shown in the blue arrows in Fig 1(a), but contrasts of the segments lying deeper in the crystal are blurred as shown in the green boxes. On the other hand, in 22-4 16 SMBXT (Fig 1(b)), the contrasts of the full BPDs can be clearly seen and the contrasts of BPD segments lying deeper in the crystal are highlighted in green boxes to compare with contrasts in 11-28 reflection. This result also proves that the penetration depth of 22-4 16 SMBXT is deeper than that of 11-28 SMBXT. Previously we have simulated images of defects in 11-28, 0008 and 000 12 using the principle of ray tracing, showing good correlation with the topograph[4,5]. In this paper, we apply the ray tracing principle to simulate defect images in the 22-4 16 topograph and compare and discuss the contrast for the different defect types. Reference: [1] K. Konishi, R. Fujita, Y. Mori and A. Shima, Semiconductor Science and Technology 2018 Vol. 33 Issue 12 [2] W. Gao, G. Yang, Y. Qian, X. Han, C. Cui, Xi. Pi, D. Yang and R. Wang, Frontiers in Materials, Jan 2023, DOI:10.3389/fmats.2023.1022878 [3] Q. Cheng, H. Peng, S. Hu, Z. Chen, Y. Liu, B. Raghothamachar and M. Dudley, Materials Science Forum 1062, 366-370 [4] H. Peng, Z. Chen, Y. Liu, B. Raghothamachar, X. Huang, L. Assoufid and M. Dudley, J. Appl. Crystallogr. 55 (2022) 544-550. [5] H. Peng, Z. Chen, Y. Liu, Q. Cheng, S. Hu, X. Huang, L. Assoufid, B. Raghothamachar and M. Dudley

Ray Tracing Simulation of Defects of 4H-SiC in 22-4 16 Reflection of Synchrotron Monochromatic Beam X-Ray Topography in Grazing Incident Geometry

Nano Fourier Transform InfraRed (nanoFTIR) spectroscopy is a powerful non-destructive characterization technique offering nanoscale spatial resolution combined with spectroscopic information. In this work we apply the combination of nanoFTIR and finite dipole modeling to characterize and identify the polytype of in-grown stacking faults (IGSF) in 4H silicon carbide. The results of stacking fault identification are verified using the established techniques of confocal Raman spectroscopy and ultraviolet photoluminescence spectroscopy. Results from one IGSF consisting of a double layered 3C inclusion will be reported. Results from additional IGSF exhibiting other polytypes are expected as the work continues in upcoming months, with the goal of identifying an IGSF with 8H polytype and extracting the dielectric function using nanoFTIR. 8H stacking faults are common, but the dielectric function of 8H is yet unmeasured due to the lack of commercially available 8H wafers.

Nanoscale Spectroscopy of Extended Defects in 4H Silicon Carbide

Ion implantation is the most useful technology for doping of 4H silicon carbide to fabricate power devices. For n-type doping, nitrogen is the most common element used in device fabrication due to its low atomic mass and proven applicability in epitaxial n-type SiC. Phosphorus can be an alternative to nitrogen (N) for providing a lower sheet resistance, although the research on phosphorus as a dopant is limited [1-3] in range of doping and activation temperatures. A rigorous study comprising different doping concentrations would help to verify the activation percentage and ionization energies and will lead to a better understanding of the properties of dopants other than N, to take advantage of any underlying benefits. In this work, phosphorus is implanted in different concentrations on 150 mm Si-face 4H SiC (0001) of 4 degree off-axis wafers and activated above 1600 oC. To study the electrical characteristics, circular van der Pauw structures were made. Deep p-well implants were performed to isolate the phosphorus doped region from the n-type substrate. Nickel contacts were deposited using evaporation and post-annealed at high temperature to get ohmic contacts. The samples were then placed on a 1cm x 1cm PCB board with 4 contacts at the corners, and wire bonding from sample to board was performed. Following that, 4-point probe resistance and Hall measurements were conducted to obtain resistivity, carrier concentration, and Hall mobility for different doping levels. Thereafter, dopant activation percentage and ionization energy levels are calculated. Figure 1 shows typical IV characteristics of the fabricated van der Pauw samples that indicate linear or ohmic behavior of the contacts. Figure 2 shows the temperature-dependent variation of resistivity for different doping levels. From 150 to 300 K the resistivity drops as temperature increases, while above room temperature a small increase of resistivity can be observed. An increase in the carrier concentration versus temperature is shown in Fig. 3, for doping less than 9x1019 cm-3. The sample with a doping concentration of 9x1019 cm-3 behaves like a semimetal for which the carrier concentration is almost constant with changing temperature. A small upshoot in carrier concentration at very low temperature (~150 K) might be caused either by a change of effective implant thickness with temperature, or p-well dopant freeze out effects underneath the n-wells. Figure 4 shows mobility versus temperature for different Phosphorus implanted concentrations. At lower temperatures mobility drops due to increased ionized impurity scattering. Mobility reaches a peak value near room temperature and drops again as temperature increases due to increased phonon scattering owing to enhanced lattice vibrations. Figure 5 shows the carrier concentrations as modeled in hexagonal and cubic lattice sites (50:50 distribution assumed) with assigned specific ionization energies. For example, for the sample with implanted doping concentration of 4x1018 cm-3, the ionization energies are extracted to be 48 and 110 meV for hexagonal and cubic lattice positions from the model fit. The values of activated dopants are also calculated here from this analysis and plotted as a function of doping concentrations as shown in figure 6. It is clear that the phosphorus activation is well below 100%. The extracted activation percentages and dopant ionization levels will be shown for all samples, and compared to literature values, in the full manuscript. The activation percentages and ionization energies, extracted from the data, will be useful for calibration of Technology Computer Aided Design (TCAD) models for electrical simulations of 4H-SiC based devices. [1] M.A. Capano et al., J. Electron. Mater. 29, 210 (2000). [2] J. Gardner et al., J. Appl. Phys. 83, 5118 (1998). [3] T. Kimoto, J.A. Cooper, Fundamentals of Silicon Carbide Technology (2014).

Study of Dopant Activation and Ionization for Phosphorus in 4H-SiC

Motivation To reduce the device cost, body diode is expected as the freewheeling diode for commercial trench SiC power MOSFETs in the high frequency applications. Therefore, it is necessary to investigate the reliability of body diodes. This study compares the body diode degradation of commercial 1.2 kV planar and trench SiC power MOSFETs from several vendors listed in Table 1. Besides, 1.2 kV planar SiC power MOSFETs are fabricated with the nominal and deep JFET. The deep JFET extends down from the nominal JFET into the drift layer, and part of the additional n+ layer is in contact with p-body. We assume that the deep JFET enhances the recombination of the injected minority carriers from p-body. This leads to the suppression of body degradation by reducing the recombination energy provided to expand SFs. These two different JFET structures are presented in Fig. 1. Results Fig. 2 compares the average degradation ratio of VF, RON, and forward leakage current in the blocking mode (IL) on the commercial 1.2 kV planar SiC power MOSFETs from Vendors E, I, and C. Decline in device performance from body diode degradation is at negligible or acceptable levels for all these three vendors. It can be concluded that body diode degradation is not a big issue in most commercial 1.2 kV planar SiC power MOSFETs. In Fig. 3, commercial trench DUTs from Vendor K have more significant average body diode degradation than planar DUTs. Trench DUTs from Vendor D have no obvious degradation. However, the influence of body diode degradation on IL is not reflected as expected. This may be due to the fact that devices are breaking down in the termination area where no SFs exist, as opposed to the active area. As shown in Fig. 4, 7 of 10 DUTs from Vendor K have reached the degradation criterion while there’s no significant body diode degradation in 8 DUTs, apart from the 2 dead devices, for Vendor D. This can be explained in Fig. 5 that the deep p+ contact by the Al implantation with high dose and energy is required to shield the electric-field-crowded corner of the trench gate, which creates many BPDs around the p+ contact in the trench structure from Vendor K. Although the p+ contact with the heavy dose is still needed for Vendor D, it is implanted after the dummy trench formed by RIE. That means the p+ contact can be shallow which significantly reduces the new BPDs. In the planar structure, high density of new BPDs around the implanted regions is hard to create because of light-dose and shallow p-type contact, as well as many improved designs and processes that have already minimized the BPDs arising from the SiC substrate wafer and epitaxial growth. In addition, Vendor D not only has the Gen 3 trench structure mentioned above, but also has the Gen 4 trench structure as shown in Fig. 6. In the Gen 4 trench structure, the Al implantation with the high energy and dose for the deep p+ contact is performed before the RIE for the trench dummy. Therefore, this new trench structure is expected to have significant body diode degradation as that of Vendor K. The body diode degradation test on the Gen 4 DUTs from Vendor D is still in progress. Furthermore, Fig. 7 compares the body diode degradation of the fabricated 1.2 kV SiC power MOSFETs with the nominal and deep JFET. It can be observed that there is increasing average body diode degradation in the DUTs with the nominal JFET during the stress. However, for the DUTs with the deep JFET under the same process, there is no average body diode degradation during the stress. It is also reflected that the breakdown voltage of the DUTs with the nominal JFET decreases due to the increasing leakage current while there is no variation for the DUTs with the deep JFET. This confirms the assumption that body diode degradation in commercial 1.2 kV SiC power MOSFETs, especially in the trench devices, is mainly caused by the new BPDs around the p-type contacts introduced by the deep Al implantation with heavy dose.

A Comparison of Body Diode Degradation in Commercial 1.2 kV SiC Power MOSFETs with the Planar and Trench Structure

Generation of basal plane dislocations (BPDs) in 4H-silicon carbide (SiC) epitaxial layers due to ion implantation was reported by Stahlbush et. al. [1]. These BPDs formed loops that were generated at the epitaxial layer surface within the implanted layer and propagated towards the substrate interface. Such BPD generation was observed when high dosage Al ion implantation was carried out at room temperature (RT) for typical contact formation. With recent advances in device processing, several of these newly generated BPDs have been mitigated either by high temperature (HT) ion implantation or by other processing techniques such as post implant oxidation [2]. It is, however, desirable to perform ion implantation at RT to reduce high temperature fabrication steps, which could affect wafer yield due to less thermal induced stress in the wafer. It is also unclear in device layers with differently doped regions if the BPDs would result in stacking fault (SF) formation and if the typical carrier injection conditions would cause SF expansion. In this work, we investigate SF expansion from potential BPDs generated by ion implantation in SiC epitaxial layers that had both RT and HT ion implantation and compare their robustness using high carrier injection using high intensity ultraviolet (UV) laser excitation. For this work, MOSFET device structure with 30 mm drift layers were fully fabricated on 4 commercial 150 mm diameter SiC wafers at a commercial foundry. Two of the wafers had a RT implant for the high Al dose contact layer and two remaining wafers had a HT implant performed at approximately 600 oC. Several of the device dies were removed from each of the four wafers and their metal layers were stripped to observe the generation of defects. UV photoluminescence (UVPL) imaging using a 355nm laser and a microscopy setup was performed with typical (10-20 W/cm2) excitation power on all the 4 samples. This was followed by high intensity (500 W/cm2) UV carrier injection. Sequence of images were collected to study progression of SF formation. The UVPL images obtained using typical excitation power did not introduce any BPDs in either the RT or the HT implanted samples even after exposure for several minutes. Upon UV excitation at 500 W/cm2, SF expansion from BPDs were observed in the RT implanted samples near the edge of the termination regions and not in the active areas within 8 secs. UV excitation generates carriers with an exponential depth decay uniformly across the device region, which is different from current injection that depends on background doping. Hence, in highly doped regions, UV excitation simulates significantly higher minority carrier generation responsible for SF generation compared to current injection. A comparison of UV excitation power with current injection conditions will be presented. The active device region did not show SF expansion, which is likely due to fast recombination of electrons from the implanted region due to depletion in the p-well. Details of the mechanisms will be presented. In the HT implanted samples, no SFs were observed even after 96 secs of high intensity UV exposure, indicated no generation of new BPDs. Even though SF expansion was not observed in active regions of the RT samples, BPD are likely formed in the contact layers and pose a reliability issue for long term device operation, especially if surge current conditions are expected during switching. [1] R. E. Stahlbush, N. A. Mahadik, Q. Zhang, et. al., Mater. Sci. Forum 821-823, 389 (2015) [2] Y. Bu, H. Yoshimoto, N. Watanabe, et. al., J. Appl. Phys., 122, 244504 (2017) Acknowledgement: Author Kallon’s research was sponsored by the Office of Naval Research NREIP program.

Reliability of Room Temperature vs High Temperature Implantation in 3.3kV SiC MOSFETs

Molecular electronics are being developed as materials for light-harvesters and -emitters, sensors, and quantum technology. The themes for optimizing these systems are miniaturization and improved precision, which increasingly moves these materials into the quantum-mechanical regime. Natural systems like photosynthetic pigment-protein complexes have shown that vast improvements to functional bottlenecks like energy-transport can be achieved, if the structural and energetic properties are finely controlled. In that spirit, we are developing modular molecular networks whose structures are controlled by DNA scaffolds. This approach controls the network’s structural, energetic, and dynamic properties. However, this increased control presents new needs and challenges. We must know what the ideal parameters are for optimal function. In turn, that optimization requires a better understanding of the molecular-scale characteristics of the systems. Another challenge is that these characteristics often include many degrees of freedom, especially for large chemical networks, so an approach must be developed to treat large amounts of data efficiently, yet thoroughly. Here, we describe approaches to address these challenges. By combining machine-learning tools with experimental spectroscopies and computational models (Figure 1), we can simultaneously understand and map the sample characteristics on the molecular scale, the energy transport characteristics on the mesoscale, and the material performance on the bulk scale. We will discuss how we have used these tools to predicted optimal system configurations for energy-transport, and how they reveal the interchromophore effects in chromophore arrays.

Understanding Electronic Energy Transport in DNA-Scaffolded Molecular Networks Using Machine-Learning Methods

Organic molecules, such as pentacene, rubrene and phthalocyanine are at the forefront of materials research, due to their desired electric, excitonic and mechanical properties. They represent the feasible alternative to traditional silicon-based semiconductor applications for flexible electronics platforms, where materials in flexible devices will experience complex strain environment. Octabutoxy phthalocyanine (H2-OBPc) is a soluble derivative of the Phthalocyanine (Pc). It has π-conjugated systems and large directional intermolecular interactions, which allow the formation of molecular crystal with well-defined structure like J-aggregate, where molecules are stacked in a head-to-tail fashion (Fig.1a). 1, 2 In this quasi one-dimension system, the Frenkel exciton can couple with lattice and intramolecular vibrations to form exciton-polarons which are delocalized along the molecular aggregates. The formation of these one-dimensional delocalized exciton-polaron states grants H2-OBPc the potential for efficient excitation energy transport in the coherent exciton regime. 3-6 In this work, H2-OBPc polycrystalline thin films were prepared using a solution-based pen-writing deposition technique. These ordered thin films with macroscopic grain size (Fig.1b, millimeter scale) uniquely offer the opportunity to spatially explore optical and excitonic properties within a single crystalline grain, free from the influence of grain boundaries or static disorder (Fig.1b). Temperature-dependent absorption and photoluminescence (PL) experiment points to the existence of red-shifted bandgap when temperature T < 175 K (Fig.2c and d). This new bandgap at low temperature is the result of the formation of exciton-polaron states, where the molecular Frenkel excitons couple with intermolecular low energy phonon modes (i.e. dynamic disorder fluctuations ), leading to the wavefunction spreading over the π-stacked molecular chain (Fig.1a). These delocalized exciton-polaron states have robust emission properties and they are potentially coherent. 6 The critical role played by low-energy phonons (25-300 cm-1) 7 in the formation of delocalized, 1D coherent exciton-polaron states is explored using an externally applied uniaxial mechanical strain (example of the tensile strain geometry is shown in Fig.2a). Strain-dependent absorption (Fig.2b), PL microscopy (Fig.2c), and confocal Raman measurements (Fig2.e) reveal the strain-induced softening (red shift) of the phonon modes favors the survival of the coherent exciton-polaron states at room temperature (A2 peak in Fig.2b and P2 peak in Fig.2c). Remarkably, an 80% enhancement in PL intensity is observed with a uniaxial tensile strain of 4.7% applied at room temperature (Fig.2c), which is equivalent to lowering the temperature to 200K for the unstrained film. 8 Time-resolved transient absorption (TA) spectroscopy confirms the behavior of the exciton-polaron state as a function of strain. Reference (1) Rawat, N.; Pan, Z.; Manning, L. W.; Lamarche, C. J.; Cour, I.; Headrick, R. L.; Waterman, R.; Woll, A. R.; Furis, M. I. J. Phys. Chem. lett. 2015, 6, (10), 1834-1840. (2) Manning, L. W.; Rawat, N.; Lamarche, C.; Waterman, R.; Headrick, R. L.; Furis, M. J. Phys. Chem. C 2016, 120, (22), 11966-11976. (3) Ghosh, R.; Spano, F. C. Acc. Chem. Res. 2020, 53, (10), 2201-2211. (4) Spano, F. C. Acc. Chem. Res. 2010, 43, (3), 429-439. (5) Yamagata, H.; Spano, F. C. J. Phys. Chem. lett. 2014, 5, (3), 622-632. (6) Liang, L.; Burrill, K.; Furis, M. I. J. Phys. Chem. C 2021, 125, (51), 27966-27974. (7) Fornari, R. P.; Aragó, J.; Troisi, A. J. Phys. Chem. C 2016, 120, (15), 7987-7996. (8) Liang, L.; Czar, K.; Furis, M. I. J. Phys. Chem. C 2022, 126, (20), 8889-8896.

Strain-Enhanced Formation of 1D Coherent Exciton-Polaron States in Small Molecule Semiconductors

There have been studies of understanding and controlling the charge transport through molecular junction wince it gives fundamental grounds that helps to gain a novel idea to realize numerous potential molecular functional devices. [1,2] There are some key factors considered in molecular charge transport, such as molecular barrier which is directly related to molecular orbital levels, coupling strength, asymmetric factor, and molecular configuration and conformation. Much research has been reported to obtain deeper insights of such key factors under certain controlled conductions with the effect of the external stimuli applied on the molecular junction such as light irradiation [3], electrochemical gating [4], electrical field [5], and mechanical stress such by tip loading force [6]. Molecular heterojunction with two-dimensional (2D) semiconductor provides various key factors for the charge transport through the heterojunction; however, the effect of molecular tilt configuration on the molecular heterojunction have never been studied. Here, we present the effect of molecular tilt configuration on the molecule/mono-layer (1L) 2D semiconductor heterojunction by tip loading force with conductive atomic force microscopy (C-AFM) technique. With various tip loading force (1-30 nN), the molecular tilt configuration affects differently for the three different interfaces in Au/molecule/1L-2D semiconductor/Au heterojunction that the transition voltage spectroscopy (TVS), which is a conventional tool to analyze charge transport through molecular junction, rectification ratio (RR), non-linearity (NL) and possible array size are demonstrated. Molecule/2D semiconductor heterojunction system has been reported as molecular diode (with RR) and selector (with NL) and it is a novel platform owning so much various potential in, that understanding the charge transport by the effect of external stress on the heterojunction will help developing such future molecular scale functional devices. [1] L. A. Bumm, J. J. Arnold, M. T. Cygan, T. D. Dunbar, T. P. Burgin, L. Jones, D. L. Allara, J. M. Tour, P. S. Weiss, Science 271, 1705 (1996),. [2] H. Song, Y. Kim, Y. H. Jang, H. Jeong, M. A. Reed, T. Lee, Nature 462, 1039 (2009). [3] G. Reecgt, C. Lotze, D. Sysoiev, T. Huhn, K. J. Franke, ACS Nano 10, 10555 (2016). [4] R. A. Wong, Y. Yokta, M. Wakisaka, J. Inukai, Y. Kim, Nat. Commun. 11, 4194 (2020). [5] J. Shin, S. Yang, Y. Jang, J. S. Eo, T.-W. Kim, T. Lee, C.-H. Lee, G. Wang, Nat. Commun. 11, 1 (2020). [6] G. Wang, T. -W. Kim, G. Jo, T. Lee, J. Am. Chem. Soc. 131, 5980 (2009).

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