Germanium alloyed with α-tin (GeSn) transitions to a direct bandgap semiconductor of significance for optoelectronics. It is essential to localize the carriers within the active region for improving the quantum efficiency in a GeSn based laser. In this work, epitaxial GeSn heterostructure material systems were analyzed to determine the band offsets for carrier confinement: (i) a 0.53% compressively strained Ge 0.97 Sn 0.03 /AlAs; (ii) a 0.81% compressively strained Ge 0.94 Sn 0.06 /Ge; and (iii) a lattice matched Ge 0.94 Sn 0.06 /In 0.12 Al 0.88 As. The phonon modes in GeSn alloys were studied using Raman spectroscopy as a function of Sn composition, that showed Sn induced red shifts in wavenumbers of the Ge–Ge longitudinal optical phonon mode peaks. The material parameter b representing strain contribution to Raman shifts of a Ge 0.94 Sn 0.06 alloy was determined as b = 314.81 ± 14 cm −1 . Low temperature photoluminescence measurements were performed at 79 K to determine direct and indirect energy bandgaps of E g,Γ = 0.72 eV and E g,L = 0.66 eV for 0.81% compressively strained Ge 0.94 Sn 0.06 , and E g,Γ = 0.73 eV and E g,L = 0.68 eV for lattice matched Ge 0.94 Sn 0.06 epilayers. Chemical effects of Sn atomic species were analyzed using X-ray photoelectron spectroscopy (XPS), revealing a shift in Ge 3d core level (CL) spectra towards the lower binding energy affecting the bonding environment. Large valence band offset of Δ E V = 0.91 ± 0.1 eV and conduction band offset of Δ E C,Γ–X = 0.64 ± 0.1 eV were determined from the Ge 0.94 Sn 0.06 /In 0.12 Al 0.88 As heterostructure using CL spectra by XPS measurements. The evaluated band offset was found to be of type-I configuration, needed for carrier confinement in a laser. In addition, these band offset values were compared with the first-principles-based calculated Ge/InAlAs band alignment, and it was found to have arsenic up-diffusion limited to 1 monolayer of epitaxial GeSn overlayer, ruling out the possibility of defects induced modification of band alignment. Furthermore, this lattice matched GeSn/InAlAs heterostructure band offset values were significantly higher than GeSn grown on group IV buffer/substrates. Therefore, a lattice matched GeSn/InAlAs material system has large band offsets offering superior carrier confinement to realize a highly efficient GeSn based photonic device.
more »
« less
This content will become publicly available on May 1, 2025
Role of tin clustering in band structure and thermodynamic stability of GeSn by atomistic modeling
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.
more » « less- Award ID(s):
- 2042079
- NSF-PAR ID:
- 10522368
- Publisher / Repository:
- AIP
- Date Published:
- Journal Name:
- Journal of Vacuum Science & Technology B
- Edition / Version:
- Accepted Version
- Volume:
- 42
- Issue:
- 3
- ISSN:
- 2166-2746
- Page Range / eLocation ID:
- 032211
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
more » « less
Abstract In this work we study the nature of the band gap in GeSn alloys for use in silicon-based lasers. Special attention is paid to Sn-induced band mixing effects. We demonstrate from both experiment and ab-initio theory that the (direct) Γ-character of the GeSn band gap changes continuously with alloy composition and has significant Γ-character even at low (6%) Sn concentrations. The evolution of the Γ-character is due to Sn-induced conduction band mixing effects, in contrast to the sharp indirect-to-direct band gap transition obtained in conventional alloys such as Al1−xGaxAs. Understanding the band mixing effects is critical not only from a fundamental and basic properties viewpoint but also for designing photonic devices with enhanced capabilities utilizing GeSn and related material systems.
-
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.more » « less
-
Designing new two-dimensional (2D) materials, exploring their unique properties and diverse potential applications are of paramount importance to condensed matter physics and materials science. In this work, we predicted a novel 2D SN 2 monolayer ( S -SN 2 ) by means of density functional theory (DFT) computations. In the S -SN 2 monolayer, each S atom is tetracoordinated with four N atoms, and each N atom bridges two S atoms, thus forming a tri-sublayer structure with square lattice. The monolayer exhibits good stability, as demonstrated by the moderate cohesive energy, all positive phonon modes, and the structural integrity maintained through 10 ps molecular dynamics simulations up to 1000 K. It is an indirect-bandgap semiconductor with high hole mobility, and the bandgap can be tuned by changing the thickness and external strains (the indirect-bandgap to direct-bandgap transition occurs when the biaxial tensile strain reaches 4%). Significantly, it has large Young's modulus and three-dimensional auxetic properties (both in-plane and out-of-plane negative Poisson's ratios). Therefore, the S -SN 2 monolayer holds great potential applications in electronics, photoelectronics and mechanics.more » « less
-
In this article, the electronic band structure of a β−(AlxGa1−x)2O3 alloy system is calculated, with β−Ga2O3 as the bulk crystal. The technique of band unfolding is implemented to obtain an effective band structure for aluminum fractions varying between 12.5% and 62.5% with respect to gallium atoms. A 160-atom supercell is used to model the disordered system that is generated using the technique of special quasi-random structures, which mimics the site correlation of a truly random alloy by reducing the number of candidate structures that arise due to the large number of permutations possible for alloy occupation sites. The impact of the disorder is then evaluated on the electron effective mass and bandgap, which is calculated under the generalized gradient approximation.more » « less
