United States

Extreme bandgap (EBG) semiconductors such as AlxGa1-xN (x&gt;0.6, bandgap energy EG &gt; 4.88 eV) are ideal for high power, high voltage, and high temperature power electronics. Their critical breakdown field (Ec) in lateral devices scales as EG2.5, leads to an estimated value of the Baliga’s Figure of Merit (BFOM) significantly higher than that for GaN[1],[2],[3],[4] For achieving high BFOM, in addition to a large breakdown voltage (VBR), a simultaneous low device specific ON-resistance(RON,SP) is also required. For the EBG AlxGa1-xN channel HEMTs to have a low RON,SP , a 2-dimensional electron gas (2DEG) concentration &gt;1x1013 cm-2 is needed, requiring a barrier AlxGa1-xN layer with x &gt; 0.80. At such high value of “x”, it is challenging to make linear ohmic contacts due to large Schottky barrier height between metal-AlGaN interface. To solve these issues, we demonstrate EBG n-Al0.87Ga0.13N-Al0.64Ga0.36N heterostructures over bulk AlN substrate grown by Low Pressure-MOCVD. To facilitate ohmic contact formation, highly doped reverse-graded AlGaN layer(x=0.87 to 0.40) was grown in-situ. Reciprocal Space Map(105) and AFM analysis revealed pseudomorphic epilayers with step-flow morphology and RMS roughness ~0.18 nm. X-Ray Topography estimation of control sample gave us a TDD ~ 7 x 103 cm-2, indicating the high quality of our pseudomorphic epilayers. A planar Zr-based metal stack was deposited and subsequent removal of the reverse graded n-AlxGa1-xN layer in the access region gave linear ohmic contacts, RC~4.3 Ω-mm. Additionally, planar metal-oxide-semiconductor (MOS) diodes were fabricated and 2-terminal gate-source breakdown measurement was performed. A VBR of 739 V was measured at metal flashover point for 0.65 µm gate-source spacing, giving an electric field value ~ 11.37 MV/cm. From the breakdown data and the RON,SP (~2.4 × 10-4 Ω-cm2) value, we estimate the BFOM for our EBG n-Al0.87Ga0.13N-A0.64Ga0.36N heterostructures to be as high as 2.27 GW/cm2. This, to the best of our knowledge is the highest reported value to date for an extreme bandgap AlxGa1-xN (x&gt;0.6) channel heterostructure.[5] References [1] B. J. Baliga, IEEE Electron Dev. Lett., 10, 455 (1989). [2] R. J. Kaplar, A. A. Allerman, A. M. Armstrong, M. H. Crawford, J. Ray Dickerson, A. J. Fischer, A. G. Baca, and E. A. Douglas, ECS J. Solid State Sci., 6, Q3061 (2016). [3] T. Nanjo, A. Imai, Y. Suzuki, Y. Abe, T. Oishi, M. Suita, E. Yagyu, and Y. Tokuda, IEEE Trans. Electron Devices, 60, 1046 (2013). [4] N. Yafune, S. Hashimoto, K. Akita, Y. Yamamoto, H. Tokuda, and M. Kuzuhara, Electronics Lett., 50, 211 (2014) [5] K. Hussain et al., Appl. Phys. Express (2023)

EMC 2023

High Figure of Merit Extreme Bandgap Al0.87Ga0.13N-Al0.64Ga0.36N Heterostructures Over Bulk AlN Substrates

pseudomorphic

breakdown

figure of merit

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. 

Extreme bandgap (EBG) semiconductors such as AlxGa1-xN (x>0.6, bandgap energy EG > 4.88 eV) are ideal for high power, high voltage, and high temperature power electronics. Their critical breakdown field (Ec) in lateral devices scales as EG2.5, leads to an estimated value of the Baliga’s Figure of Merit (BFOM) significantly higher than that for GaN[1],[2],[3],[4] For achieving high BFOM, in addition to a large breakdown voltage (VBR), a simultaneous low device specific ON-resistance(RON,SP) is also required. For the EBG AlxGa1-xN channel HEMTs to have a low RON,SP , a 2-dimensional electron gas (2DEG) concentration >1x1013 cm-2 is needed, requiring a barrier AlxGa1-xN layer with x > 0.80. At such high value of “x”, it is challenging to make linear ohmic contacts due to large Schottky barrier height between metal-AlGaN interface. To solve these issues, we demonstrate EBG n-Al0.87Ga0.13N-Al0.64Ga0.36N heterostructures over bulk AlN substrate grown by Low Pressure-MOCVD. To facilitate ohmic contact formation, highly doped reverse-graded AlGaN layer(x=0.87 to 0.40) was grown in-situ. Reciprocal Space Map(105) and AFM analysis revealed pseudomorphic epilayers with step-flow morphology and RMS roughness ~0.18 nm. X-Ray Topography estimation of control sample gave us a TDD ~ 7 x 103 cm-2, indicating the high quality of our pseudomorphic epilayers. A planar Zr-based metal stack was deposited and subsequent removal of the reverse graded n-AlxGa1-xN layer in the access region gave linear ohmic contacts, RC~4.3 Ω-mm. Additionally, planar metal-oxide-semiconductor (MOS) diodes were fabricated and 2-terminal gate-source breakdown measurement was performed. A VBR of 739 V was measured at metal flashover point for 0.65 µm gate-source spacing, giving an electric field value ~ 11.37 MV/cm. From the breakdown data and the RON,SP (~2.4 × 10-4 Ω-cm2) value, we estimate the BFOM for our EBG n-Al0.87Ga0.13N-A0.64Ga0.36N heterostructures to be as high as 2.27 GW/cm2. This, to the best of our knowledge is the highest reported value to date for an extreme bandgap AlxGa1-xN (x>0.6) channel heterostructure.[5] References [1] B. J. Baliga, IEEE Electron Dev. Lett., 10, 455 (1989). [2] R. J. Kaplar, A. A. Allerman, A. M. Armstrong, M. H. Crawford, J. Ray Dickerson, A. J. Fischer, A. G. Baca, and E. A. Douglas, ECS J. Solid State Sci., 6, Q3061 (2016). [3] T. Nanjo, A. Imai, Y. Suzuki, Y. Abe, T. Oishi, M. Suita, E. Yagyu, and Y. Tokuda, IEEE Trans. Electron Devices, 60, 1046 (2013). [4] N. Yafune, S. Hashimoto, K. Akita, Y. Yamamoto, H. Tokuda, and M. Kuzuhara, Electronics Lett., 50, 211 (2014) [5] K. Hussain et al., Appl. Phys. Express (2023)

technical paper

In this work, epitaxial wurtzite phase BAlN films were grown by nitrogen plasma molecular beam epitaxy on different wurtzite phase substrates (e. g. nitrided sapphire, β-Nb2N film, GaN template). Effect of different growth parameters on composition of BAlN films was explored. B incorporation was found to increase with decreasing substrate temperature and with increasing B flux. In-situ electron diffraction and ex-situ x-ray diffraction measurements confirm the growth of wurtzite BAlN films at 300 in plane orientation with sapphire substrate and at 00 orientation with bottom Nb2N films. With increasing boron content in BAlN films, the c-parameter decreases while the a-parameter shows a non-monotonic trend. Ferroelectricity (with remnant polarization of ~16 μC.cm–2 and coercive field of ~1.45 MV.cm-1) and high dielectric constant (of ~18 at 500 kHz) were also observed in these BAlN films. A slight reduction in bandgap is seen with increasing boron content in BAlN films (from ~6.12eV for AlN films to ~5.95eV for B0.15Al0.85N film). This work also reports the first demonstration of polarization-induced two-dimensional electron gas (2DEG) in the epitaxial BAlN/GaN heterojunctions. Observation of ferroelectricity with low coercive field, remnant polarization (lower than the sputtered BAlN [1] and comparable to MBE grown AlScN films [2]) and high-k, makes BAlN an attractive barrier in GaN high electron mobility transistors (HEMTs). Therefore, BAlN/GaN heterostructures were grown on 2” and 3” wafers; showing polarization-induced 2DEG with good sheet resistance of ~650 ohm/sq, ~400-500 cm2/V.s mobility (400-500 cm2/V.s), and 1.4-1.8 e13 cm-2 charge. The results presented in this paper demonstrates MBE growth of BAlN films and heterostructures with interesting properties that would enable new electronic devices. These films could be useful for combining memory and logic functionalities in digital electronics and to mingle epitaxial ferroelectric and high-k barrier technology into RF and mm-wave power electronics. For instance, a new class of ferroelectric HEMTs devices can be formed by integrating epitaxial ferroelectric, high-k nitride barrier with III-V nitride semiconductors [3].

MBE Growth, Characterization of BAlN Films and 2D Electron Gas in Epitaxial BAlN/GaN Heterojunction

Two-dimensional (2D) transition metal dichalcogenides (MX2, with M being a transition metal and X a chalcogen) have potential application in next-generation optoelectronics, electronics, chemical sensing and quantum technologies. Epitaxial growth of single crystal TMD monolayers at the wafer-scale is of significant interest for device applications. Metalorganic chemical vapor deposition (MOCVD) is a promising approach for TMD growth as it enables the use of high substrate temperatures (900-1000oC) and chalcogen/metal ratios (103-105) which are beneficial for epitaxy. In addition, the flow rate of precursors can be modulated during growth to enhance the surface diffusion of metal-containing species and control the lateral growth rate of TMD domains. Sapphire has emerged as a promising substrate for wafer-scale TMD monolayers and few layer films due to its crystallographic compatibility and commercial availability. Despite the broad interest in wafer-scale 2D TMDs, the fundamental understanding of how the growth conditions, e.g. growth temperature, impacts the epitaxial growth and properties of 2D TMDs has rarely been investigated. In this study, MOCVD growth of wafer scale MoS2 monolayer was carried out in a horizontal cold-wall reactor that includes an induction-heated rotating SiC-coated graphite susceptor and separate gas inlets for the metal and chalcogen precursors. Molybdenum hexacarbonyl (Mo(CO)6) and hydrogen sulfide (H2S) were used as precursors in a H2 carrier gas. A 2” c-plane sapphire substate was used for all MoS2 growths. The growth temperature was varied as 900oC, 950oC, and 1000oC. The flow rate of the molybdenum precursor used in this work ranged from 0.001 to 0.003sccm while the H2S flow rate was maintained at 400 sccm. Growths were carried out with a moderate reactor pressure of 50 Torr using H2 as the carrier gas. Using these conditions, growth of fully coalesced monolayer MoS2 was achieved in 10-30 minutes across the 2” substrate. AFM and FESEM were used to characterize the surface coverage and morphology of the layers and in-plane X-ray diffraction was used to assess the epitaxial quality. Spatially resolved Raman and photoluminescence spectroscopy were carried out to assess the uniformity of the monolayer and along with variable temperature photoluminescence measurements. A coalesced monolayer of MoS2 was achieved on 2” c-plane sapphire over the temperature range from 900oC to 1000oC. The surface coverage (growth rate) was found to be only weakly dependent on the growth temperature and scaled roughly linearly with Mo(CO)6 flow rate indicating that the growth is limited by mass transport rather than reaction kinetics. In-plane XRD phi-scans confirm that the films are epitaxially oriented with respect to the sapphire over the temperature range (900oC to 1000oC) with the (1120) of MoS2 aligned with the (1120) of α-Al2O3. The FWHM of the phi-scan peak of MoS2 decreases with increasing growth temperature, indicating a reduction in in-plane rotational misorientation of the MoS2 domains at higher growth temperature. The MoS2 monolayer sample grown at 1000oC exhibits strong photoluminescence at room temperature centered around 1.9 eV and defect-related emission which dominates at low temperature of 6 K. PL measurements also reveal an increase in the intensity ratio of the A exciton to trion emission with increasing growth temperature indicating a reduction in defects. Additional results correlating the structural and optical properties of the MoS2 monolayers to the transport properties of field-effect devices will be reported. The results of this work provide a deeper understanding of the epitaxial growth of MoS2 monolayers grown by MOCVD and the impact of defects on the optical and electrical properties of the layers.

Effects of Growth Temperature on the Properties of Wafer-Scale Epitaxial MoS2 Monolayers Grown by Metalorganic Chemical Vapor Deposition

Single photon emitters (SPEs) in 2D transition metal dichalcogenides (TMDCs) offer a compelling solution to create bright, spectrally narrow quantum emitters with high photon-extraction efficiencies. Quantum emitters in TMDC monolayers are typically associated with point defects or strain in engineered structures. Another potential avenue for SPEs is the formation of lateral heterostructures in which a TMDC nanodot is embedded in a matrix comprised of a wider bandgap TMDC. The nanodot can capture excitons from the surrounding matrix and may also host quantum states. Despite these intriguing prospects, there have been few studies focused on nanodot formation in TMDC monolayer material. In this work, we report on the synthesis of MoSe2 nanodots embedded within a WSe2 monolayer matrix via metal-organic chemical vapor deposition (MOCVD). MOCVD growth was carried out at 800-1000 °C in a cold-wall reactor using Mo(CO)6 and W(CO)6 as metal precursors and H2Se as the chalcogen source. C-plane sapphire was used as the substrate with H2 as the carrier gas. A three-step process was employed whereby a short nucleation step was initially used to form nanoscale MoSe2 domains followed by a ripening step in which the Mo(CO)6 was switched out of the reactor and the sample was annealed under H2Se. W(CO)6 was then introduced resulting in lateral growth of a WSe2 matrix surrounding the MoSe2 domains. The nucleation time was varied to change the size of the MoSe2 nanodots and the duration of ripening was modified to control the interfacial abruptness. Raman spectroscopy of the monolayers confirmed the presence of both the A1g peaks for WSe2 and MoSe2 at 247 cm-1 and 239 cm-1, respectively. High resolution scanning transmission electron microscopy (STEM) and energy dispersive spectroscopy (EDS) revealed the presence of triangular MoSe2 nanodots with edge lengths on the order of 10-100 nm embedded within a continuous WSe2 matrix. Variable temperature photoluminescence (PL) measurements were carried out on monolayer samples removed from the sapphire growth substrate and encapsulated in hBN. At room temperature, PL peaks were observed at 1.65 eV and 1.57 eV which correlate with exciton-related emission from WSe2 and MoSe2, respectively. As the temperature was decreased from 280K to 80K, the MoSe2 PL intensity increased relative to the WSe2 emission which is suggestive of charge transport from the WSe2 matrix to the MoSe2 nanodots.

MOCVD Growth and Characterization of MoSe2 Nanodots within a WSe2 Monolayer Matrix

Defects can occur during the growth of two-dimensional (2D) transition metal dichalcogenide (TMD) crystals. Exposure to these defects in 2D TMD to air and harsh conditions initiates the reaction of exposed transition metal atoms with active species such as O2 and H2O molecules, triggering an oxidation process. Understanding the oxidation behavior of 2D TMDs is crucial for their practical applications in electronics, optoelectronics, and electro-/photocatalysis because any structural deformation under ambient conditions directly affects their unique properties. As 2D semiconductor, 2H-MoTe2 is highly susceptible to air, it has the potential to control the conversion between p- and n-type characteristics. Nevertheless, previous research has been conducted on macroscopic scale oxidation, there is limited understanding of the oxidation behavior at the atomic level. It is imperative to explore the oxidation behavior of 2H-MoTe2 in greater detail to effectively modify its properties. In this study, we provide a detailed investigation on the oxidation of 2H-MoTe2 under various environmental conditions and insights for oxidation control. Exposure of synthesized 2H-MoTe2 flakes to air at room temperature for 17 months led to two striking morphological changes: 1) Hexagonal or triangular etching was observed from defects such as screw dislocations, crystal edges, and grain boundaries, resulting in the preferential oxidation and formation of pits with an average lateral size of several micrometers. 2) After a relatively slow reaction, triangular protruding oxide layer consisting of a stable MoO2 hillocks were formed on the basal plane of MoTe2, as confirmed by surface and structure characterization using scanning electron microscope (SEM), transmission electron microscope (TEM) and Raman. Experiments monitoring at the oxidation process at 230 °C for 3 h and TEM analysis showed that the substrates and active species have a significant effect on oxidation mechanism. Interestingly, under an O2 atmosphere, MoO2 layers was formed on the entire surface of 2H-MoTe2, while in a H2O atmosphere, most of the surface remained intact but oxidation etching occurred at defects in the basal plane. Furthermore, using theoretical calculations, we investigated the reaction of the surface Te atoms of defective 2H-MoTe2 with O2 and H2O molecules, which provide volatile TeO2 and H2Te and create a Te vacancy in the 2H-MoTe2 basal plane. It was found that, although O2 dissociation in the basal plane of 2H-MoTe2 with a Te vacancy is energetically favored over H2O dissociation, the subsequent reaction involving H is a dominant mechanism in forming Te vacancies than reactions involving an O atom. More importantly, when TeO2 volatilizes, the additional adsorbed O atom suppresses the lattice reconstruction around TeO2, resulting in an increased kinetic barrier (∼2.22 eV) to volatile TeO2. This finding is consistent with the TEM analysis results of the monitoring experiment, which demonstrated the formation of a stable oxide film under an O2 atmosphere. Our study provides valuable insight into the oxidation dynamics of 2H-MoTe2 at the atomic level and can inspire a new approach to improve the reliability and durability of TMD-based 2D devices.

Oxidation Mechanism of 2H-MoTe2 at the Atomic Scale

Mixed valence manganese oxides (MnOx) have attracted significant interest in recent years due to the easily reversible redox reactions between manganese oxidation states (Mn+2, Mn+3, and Mn+4)1, which has enabled applications in catalysis2, energy storage3, and gas sensing4. Manganese dioxide (MnO2), in particular, has demonstrated tunable structural properties, due to its wide variety of synthesizable structural polymorphs ((α, β) tunnel5, (γ) spinel5, and (δ) layered5) which allow for control over its available surface area and the reactive properties. Among these structural polymorphs, the α and δ phases exhibit strongly mixed-valence character, with Mn+3 defects found throughout the crystal structure5, which is desirable due to it being a highly active site6. Water-stabilized hexagonal δ-MnO2 (HMnO2) contains many Mn+3 ions as the interlayer contains Mn+2/+3 ions to neutralize the layer charge from Mn+3 lattice defects7. However, the direct synthesis of HMnO2 has not been reported extensively in the literature, as most synthesis routes rely on permanganate salts1, which leads to the synthesis of ion intercalated δ-MnO2 and a reduction in Mn+3 defects in the lattice9. In our previous work, we presented the synthesis of high Mn+3 content HMnO2 via electrodeposition on epitaxial graphene/silicon carbide (EG/SiC) substrates4. In this work, we extend upon this by transforming also these as-grown films via intercalating with ions to form alternate birnessites and controlling the growth via patterning. The electrodeposition was carried out by a three-pulse deposition in a three-electrode electrochemical cell, as described previously4. The resulting δ-MnO2 thin films were then characterized through Raman spectroscopy, scanning electron microscopy (SEM), and atomic force microscopy(AFM). Raman spectroscopy confirmed the formation of δ-MnO2 as the peaks were consistent with the literature4,8. The AFM indicated that the surface was made up of nanofibrous nanoparticles, which SEM confirmed and indicated that the deposition on EG/SiC was made up of microplates larger than 40 μm x 40 μm in size. After this characterization, the resulting films were exfoliated mechanically and analyzed with AFM to determine if the films were epitaxial. This exfoliation generated 6-layer MnO2 nanoplates, indicating we can reach the 2D limit and confirm epitaxial growth. The resulting films were intercalated with alkali metals (Na+) via cyclic voltammetry in a similar three-electrode cell to the deposition, using a 2 M metal nitrate solution. Optical Analysis of the film indicated two distinct regions formed in the film, large multicolored regions, and small blue and black islands. Raman analysis of these regions found that the islands were unintercalated MnO2 but the colored regions were Na-type birnessite due to their Raman shifts which are consistent with the literature8. Believing ion diffusion resistance to be impeding the intercalation we patterned δ-MnO2 into 100 μm ribbons and attempted additional intercalation. Optical analysis of the film post patterned intercalation revealed two regions, a brown and blue region. Raman spectroscopy of these two regions indicated that both were Na-type birnessite. However, differing peak intensities indicate the crystallinity of the two regions is different. This demonstrates that we have successfully grown and transformed the H-type birnessite to Na-type birnessite with 100% efficiency on EG/SiC, allowing for additional fine-tuning of its properties and expanding its application in energy storage to Na+ and K+ chemistries. References: [1]L. Spinelle et al. Sensors 17(7), 2017 [2] F. Cheng et al. Chem. Mater 22(3), 2010 [3] Y.J. Huang et al. Electrochim Acta 99, 2013 [4] M. Pedowitz et al. J. Microelectromech Syst 29(5), 2020 [5] Z. Chang et al. Proc Natl Acad Sci USA 115(23), 2018 [6] Julien et al. Spectrochim Acta A 60, 2004 [7] Drits et al. Am Mineralogist 82, 1997 [8] Julien et al. Solid State Ionics 159, 2003

Transformation to Alkali Birnessites via Simple Intercalation of Electrochemically Grown 2D-Layered H-Type Manganese Dioxide on Epitaxial Graphene

In this talk I will describe a variant of molecular-beam epitaxy (MBE)—“suboxide MBE”—that makes it possible to deposit many oxide materials with excellent structural perfection at relatively low deposition temperature compared with other epitaxial methods. Examples include: -type oxide Semiconductors—b-Ga2O3, a-(Al,Ga)2O3, In2O3, BaSnO3, SrSnO3 -type oxide Semiconductors—SnO, PbO, SnTa2O6, SnWO4, Bi2BaTaO6 proposed high-Tc Superconductors—Ban+1InnO2.5n+1 with n=1, 2, ∞ Of these, b-Ga2O3, In2O3, SnO, PbO, Sr3SnO, and SnWO4 can all be grown epitaxially at back end of line temperatures. In suboxide MBE the molecular beams consist of pre-oxidized elements (suboxides) that help navigate kinetic pathways. For example, supplying a molecular beam of gallium suboxide (Ga2O) eliminates the rate limiting step of conventional MBE to the growth of b-Ga2O3—the oxidation of gallium to its suboxide—and by skipping this step growth rates exceeding 1 μm/hr with excellent crystallinity, surface smoothness, and at a low growth temperature are achieved. In addition to extensive structural characterization, electrical characterization and working transistors will also be shown. *This work was performed in collaboration with coauthors from the groups of: K. Ahadi, K. Chabak, K. Cho, S. Datta , R. Engel-Herbert, C.B. Eom, F. Giustino, V. Gopalan, A.J. Green, C. Guguschev, R.T. Haasch, G. Hautier, F.V.E. Hensling, J.T. Heron, A. Hock, D. Jena, M. Jeong, Y. Kim, D. Klimm, K. Lee, Z.K. Liu, S. Mou, D.A. Muller, H.P. Nair, A.T. Neal, T. Onuma, H. Paik, X.Q. Pan, L.F.J. Piper, N.J. Podraza, S. Salahuddin, D.G. Sangiovanni, K.M. Shen, Y.E. Suyolcu, M.O. Thompson, T.E. Tiwald, R. Uecker, P.A. van Aken, P. Vogt, M.D. Williams, C.H. Winter, H.G. Xing, and Q. Zheng.

Suboxide Molecular-Beam Epitaxy

Integrating III-V semiconductors on silicon has been a long desire to realize the advanced electronics and photonics application for the future ‘extended’ Moore’s Law regime. Transfer methods have been developed to integrate the III-V layer onto silicon, which is limited by the costive epitaxial wafer and alignment issue. Direct growth is, an alternative method to realize heterogeneous integration, which is necessary to use lattice-matching buffer layers. This imposes a significant growth challenge: thick lattice-matching buffer layers for each material must be grown with nanometer spacing, a significant challenge considering. As such, enabling high-quality growth without the use of lattice matching layers would provide a clear path toward direct growth of III-V CMOS. Here, we propose a defect-filtering growth technique to enable high-quality III-V growth on silicon by combining conventional growth with templated liquid phase growth to ensure the quality and uniformity of the grown material. Critically, this approach enables the growth and manufacturing of high-quality crystalline materials on silicon without requiring a nearly lattice-matched substrate, potentially impacting a wide range of fields, including electronics, photonics, and energy devices.

Defect Filtering at the Interface of MOCVD/TLP Heterogenous Epitaxial III-V on Silicon

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.

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

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.

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