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1. Role of tin clustering in band structure and thermodynamic stability of GeSn by atomistic modeling

   https://doi.org/10.1116/6.0003563  

   Karthikeyan, Sengunthar; Hudait, Mantu K (May 2024, Journal of Vacuum Science & Technology B)

   Synthesis of device-quality GeSn materials with higher Sn compositions is hindered by various factors, such as Sn segregation, clustering, and short-range ordering effects. In the present work, the impact of the clustering of Sn atoms in a GeSn semiconductor alloy was studied by density functional theory using SG15 pseudopotentials in a Synopsys QuantumATK tool, where the thermodynamic stability, effective band structure, indirect and direct bandgaps, and density of states (DOS) were computed to highlight the difference between a cluster-free random GeSn alloy and a GeSn alloy with Sn–Sn clusters. A 54-atom bulk Ge1–xSnx (x = 3.71%–27.77%) supercell was constructed with cluster-free and a first nearest neighbor Sn–Sn clustered GeSn alloy at each composition for this work. Computation using the generalized gradient approximation exchange-correlation functional showed that the thermodynamic stability of GeSn was reduced due to the clustering of Sn, which increased the formation energy of the GeSn alloys by increasing the Hartree potential energy and exchange-correlation energy. Moreover, with the effective band structure of the GeSn material at a Sn composition of ∼22%, both direct (Eg,Γ) and indirect (Eg,L) bandgaps decreased by a large margin of 40.76 and 120.17 meV, respectively, due to Sn–Sn clustering. On the other hand, Eg,Γ and Eg,L decrease is limited to 0.5 and 12.8 meV, respectively, for Sn composition of ∼5.6%. Similar impacts were observed on DOS, in an independent computation without deducing from the electronic band structure, where the width of the forbidden band reduces due to the clustering of Sn atoms in GeSn. Moreover, using the energy bandgaps of GeSn computed with the assumption of it being a random alloy having well-dispersed Sn atoms needs revision by incorporating clustering to align with the experimentally determined bandgap. This necessitates incorporating the effect of Sn atoms clustered together at varying distributions based on experimental characterization techniques such as atom probe tomography or extended x-ray absorption fine structure to substantiate the energy bandgap of the GeSn alloy at a particular composition with precision. Hence, considering the effect of Sn clusters during material characterization, beginning with the accurate energy bandgap characterization of GeSn would help in mitigating the effect of process variations on the performance characteristics of GeSn-based group IV electronic and photonic devices such as varying leakage currents in transistors and photodiodes as well as the deviation from the targeted wavelength of operation in lasers and photodetectors. 

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2. Experimental determination of the valence band offsets of ZnGeN2 and (ZnGe)0.94Ga0.12N2 with GaN

   https://doi.org/https://doi.org/10.1088/1361-6463/abee45  

   Karim, Md Rezaul; Noesges, Brent A.; Jayatung, Benthara Hewage; Zhu, Menglin; Hwang, Jinwoo; Lambrecht, Walter R.; Brillson, Leonard J.; Kash, Kathleen; Zhao, Hongping (March 2021, Journal of physics)

   null (Ed.)

   A predicted type-II staggered band alignment with an approximately 1.4 eV valence band offset at the ZnGeN2/GaN heterointerface has inspired novel band-engineered III-N/ZnGeN2 heterostructure-based device designs for applications in high performance optoelectronics. We report on the determination of the valence band offset between metalorganic chemical vapor deposition grown (ZnGe)1−xGa2xN2, for x = 0 and 0.06, and GaN using x-ray photoemission spectroscopy. The valence band of ZnGeN2 was found to lie 1.45–1.65 eV above that of GaN. This result agrees well with the value predicted by first-principles density functional theory calculations using the local density approximation for the potential profile and quasiparticle self-consistent GW calculations of the band edge states relative to the potential. For (ZnGe)0.94Ga0.12N2 the value was determined to be 1.29 eV, ∼10%–20% lower than that of ZnGeN2. The experimental determination of the large band offset between ZnGeN2 and GaN provides promising alternative solutions to address challenges faced with pure III-nitride-based structures and devices. 

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3. Substrate Orientation Mediated Epitaxial GeSn Optoelectronic Materials

   Hudait, M K; Karthikeyan, S; Johnston, S; Joshi, R; Priya, D; Laha, A; Mahapatra, S (October 2023, Conference presentation (6th Tri-Service GeSn Conference))

   Researchers have been aggressively investigating group-IV (Ge, SiGeSn, GeSn) optoelectronic materials to realize tunable wavelength lasers, photodetectors, and transistors. By exploiting strain and bandgap engineering of these materials via choice of substrate orientation and intelligent buffer engineering as well as precise control of Sn alloy composition during material synthesis, it will offer widespread device applications. There is an opportunity to improve the device-level quality of GeSn material systems along with higher Sn incorporation that face growth challenges during epitaxy. The current research work presents the substrate orientation and misorientation (100)/2˚, (100)/6˚, (110), (111) mediated epitaxial GeSn and Ge optoelectronic materials synthesized via MBE and analyzed using several analytical tools. X-ray analysis demonstrated high quality GeSn materials with less broadening and good symmetricity on (100) compared to (110) GeSn materials. Minority carrier lifetimes of these GeSn epilayers were extracted as > 400 ns for the (100) substrate misoriented by 6˚ towards [110] direction. Raman spectroscopy measurements were performed to study the vibrational properties, where the LO phonon wavenumber shifts at ωLO = 301.11 ± 0.8 cm¬–1 from (100)/2˚, (100)/6˚ and (110) oriented GeSn epilayers that were synthesized in equivalent growth conditions. Cross-sectional TEM of (100)/2˚ GeSn sample was performed that revealed good quality GeSn material on GaAs. Elimination of the interfaced electronic dipole charge effects, that destabilize the group-IV/group III-V heterointerface and further layer growths, is attributed to aid in achieving superior quality GeSn epitaxial materials over a (100) substrate that is misoriented by 6˚ towards the [110] direction. This substrate offcut will enable to annihilate antiphase domains due to polar-on-non-polar epitaxial growth, which further reduce non-radiative recombination centers in GeSn material. Hence, growth of GeSn material on misoriented (100) substrate offers two-fold benefits: (i) reduced active defects at the GeSn/III-V heterointerface, and (ii) self-annihilation of the antiphase domain boundaries for enhancing the efficiency of optical devices. 

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4. Bending and precipitate formation mechanisms in epitaxial Ge-core/GeSn-shell nanowires

   https://doi.org/10.1039/d1nr04220c  

   Meng, Andrew C.; Wang, Yanming; Braun, Michael R.; Lentz, J. Zach; Peng, Siying; Cheng, Huikai; Marshall, Ann F.; Cai, Wei; McIntyre, Paul C. (October 2021, Nanoscale)

   Core–shell Ge/GeSn nanowires provide a route to dislocation-free single crystal germanium-tin alloys with desirable light emission properties because the Ge core acts as an elastically compliant substrate during misfitting GeSn shell growth. However, the uniformity of tin incorporation during reduced pressure chemical vapor deposition may be limited by the kinetics of mass transfer to the shell during GeSn growth. The balance between Sn precursor flux and available surfaces for GeSn nucleation and growth determines whether defects are formed and their type. On the one hand, when the Sn precursor delivery is insufficient, local variations in Sn arrival rate at the nanowire surfaces during GeSn growth produce asymmetries in shell growth that induce wire bending. This inhomogeneous elastic dilatation due to the varying composition occurs via deposition of Sn-poor regions on some of the {112} sidewall facets of the nanowires. On the other hand, when the available nanowire surface area is insufficient to accommodate the arriving Sn precursor flux, Sn-rich precipitate formation results. Between these two extremes, there exists a regime of growth conditions and nanowire densities that permits defect-free GeSn shell growth. 

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5. Photovoltage spectroscopy of direct and indirect bandgaps of strained Ge1-xSnx thin films on a Ge/Si(001) substrate

   S.V. Kondratenko Yu.V. Hyrka, Yu.I. Mazur (April 2019, Acta materialia)

   The near-bandgap optical properties of Ge1-xSnx alloys were characterized by photovoltage spectroscopy and spectral ellipsometry measurements. Contributions of Urbach tailing as well as direct and indirect optical transitions were observed. The compositional dependence of direct bandgaps of strained GeSn films grown on a Ge buffered Si substrate was studied for up to 15% Sn content. The contribution to the photovoltage spectra of Ge1-xSnx alloys (x < 6%) from indirect optical transitions was observed at lower energies than from direct bandgaps. Using bowing parameters, a correlation was detected between calculated and measured indirect and direct bandgaps at 82 K. As the Sn content was increased, the difference between the energies of the indirect and direct bandgaps decreased, resulting in a smaller contribution of the indirect transitions due to competition with direct transitions and Urbach tails. Two sublayers with different Sn content, strain values and bandgaps were observed for samples with x ~12%. The results indicated that strain relaxation in films with thicknesses exceeding a critical value occurs via formation of a Sn-rich top layer with higher direct bandgap. These findings have important implications when designing IR photodetectors or solar cells. 

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