The GaAsN alloy has been receiving increasing attention owing to its drastic band gap reduction property in a dilute nitrogen composition ([N]) [1]. Therefore, a heavily doped p-type GaAsN system alloy such as (In)GaAsN is expected to be applicable for a tunnel diode inserted to a multi-junction solar cell with GaAs system alloys [2]. In this multi-junction solar cell, the recombination mechanism is needed to be revealed for increasing its efficiency. For undoped GaAsN and lightly Si-doped n-type GaAsN, the recombination mechanisms were systematically discussed using both continuous and time-resolved photoluminescence (PL) measurements [3, 4]. However, it was only partially discussed for heavily doped p-type GaAsN using the continuous PL measurement [5]. Therefore, in this study, temperature dependence of PL lifetime is evaluated using the time-resolved PL measurement to reveal the recombination mechanism for heavily Be-doped p-type GaAsN.

Be-doped GaAsN layers were grown by radio-frequency plasma-assisted molecular beam epitaxy on semi-insulating GaAs (001) substrates using nitrogen RF plasma. The Be impurity concentrations ([Be]) were designed at 1×10¹⁹, 6×10¹⁹, and 3×10²⁰ cm^-3, and [N] was fixed at about 0.8 %. The time-resolved PL measurement unit consisted of a picosecond Ti-sapphire laser with a photon energy of 1.80 eV and time-correlated photon counting module using superconducting single photon detector.

PL decay curves are well fitted by the bi-exponential decay function for Be-doped GaAsN independent of [Be] and temperature, meaning that two distinct PL lifetimes coexist as well as undoped GaAsN and lightly Si-doped GaAsN [3, 4]. Especially, the slow PL lifetimes are slightly less than 1 ns for Be-doped GaAsN independent of [Be] and temperature, which correspond to the optical transition from inherent localized levels in dilute GaAsN [3, 4, 6]. This result also shows that these localized levels are formed for ultra-heavily Be-doped GaAsN with extremely high [Be] of 3×10²⁰ cm^-3. In contrast, the fast PL lifetimes tend to decrease with increasing [Be] for Be-doped GaAsN. This is due to increasing density of nonradiative recombination centers with increasing [Be] for Be-doped GaAsN. On the basis of these discussions of two distinct PL lifetimes, the recombination mechanism is revealed for heavily Be-doped GaAsN.

References

[1] J.A.H. Coaquira et al., JAP, 102 (2007) 073716.

[2] D.E. Mars et al., APL, 84 (2004) 2560.

[3] C. Chen et al., JAC, 699 (2017) 297.

[4] Z. Zaaboub et al., SSC, 314 (2020) 113913.

[5] K. Umeno et al., JCG, 312 (2010) 231.

Actinide-based materials possess unique physics due to the presence of 5f electrons, yet their study has been primarily focused on nuclear fuel applications, leaving aspects of their fundamental physics largely open for investigation. Effective examination of the unique quantum phenomena in these materials requires high purity monocrystalline samples. However, thin film synthesis of actinide compounds is particularly underexplored relative to other material systems because of limited source availability and safety regulations due to radioactivity. We will discuss the promises, challenges, and synthesis routes for these actinide-bearing heterostructures. Molecular beam epitaxy (MBE) presents an attractive avenue for the study of actinide heterostructures because of the high degree of control over dimensionality, strain, and interfaces. Idaho National Laboratory has recently installed an MBE chamber with the specific goal of studying uranium, cesium, and thorium containing compounds. To facilitate deposition of these low vapor-pressure elements, the chamber is outfitted with a quad-pocket electron beam source, several high temperature cells, and a nitrogen plasma source. Introductory studies on transition metals with complex oxidation states (Zr, Nb, Mn, Ni, and Cr) will function as early-stage surrogates for actinide-based nitride compounds. These new capabilities will create unrivaled opportunities for the exploration of functional and energy materials with complex electron correlations and provide important experimental validation for computational models of these systems.

In addition to being a traditional thermoelectric material, Sb₂Se₃ has recently been investigated as a candidate for next generation photovoltaic devices, such as solar cells. Under ambient conditions, Sb₂Se₃ has orthorhombic crystal structure. It has recently been shown that rhombohedral Sb₂Se₃ can be stable up to five quintuple layers (5QL≈5nm) when grown on top of Bi₂Se₃, a topological insulator. In rhombohedral Sb₂Se₃, a quintuple layer refers to vertical stacking of atomic layers of ‘Se-Sb-Se-Sb-Se’ which make up one monolayer. Angle-resolved photoemission spectroscopy measurements on the Sb₂Se₃-Bi₂Se₃ heterostructure indicate that the rhombohedral Sb₂Se₃ is a topologically trivial material at ambient pressure. Beyond 5QL thickness, Sb₂Se₃ films tend to form in the thermodynamically stable orthorhombic phase.

To grow heterostructures of Sb₂Se₃ and Bi₂Se₃ that take advantage of the Sb₂Se₃ photovoltaic properties and the topological properties of Bi₂Se₃, we have investigated growth of 50nm Sb₂Se₃ on a 10nm Bi₂Se₃ seed layer on sapphire via direct co-deposition in a Veeco GENxplor R&D molecular beam epitaxy system. Instead of the ‘worm-like’ structure that is normally obtained when Sb₂Se₃ is directly grown on sapphire, we see a hexagonal mesh of orthorhombic Sb₂Se₃ nanoneedle structure. The surface roughness of the Sb₂Se₃ film decreases as the substrate temperature decreases from 300°C to 150°C. From the x-ray diffraction (XRD) pattern, we conclude that the Sb₂Se₃ growth plane is (021), and its crystallinity is enhanced by adding a Bi₂Se₃ seed layer instead of growing directly on sapphire. A high index plane of Bi₂Se₃ is obtained when we grow it on the mesh of Sb₂Se₃ nanoneedles rather than the standard (001) orientation. This research has paved the way for investigations of Sb₂Se₃ heterostructures with novel quantum materials like Dirac/Weyl semimetals and topological insulators.

⁺Author for correspondence: wzty@udel.edu, slaw@udel.edu

Epitaxial growth on (111) substrates, using molecular beam epitaxy (MBE), was mainly studied in the 1970s, 1980s and 1990s with the focus on GaAs [1-3]. However, since most of the growth conditions resulted in a defective and rough surface, McFee et al. [4] concluded that growth on (111) substrates is too difficult; therefore, efforts should focus on (001) surfaces for reproducible device quality epitaxial layers. In recent years, growth on the polar (111) surfaces has generated renewed interest due to the emergence of new applications. Prime examples are devices based on spin transport [5] and novel tensely strained quantum dots for quantum computing applications [6].

InGaAs/InAlAs grown on (001) InP substrates has application in optoelectronics. Also, MBE-grown InGaAs/InAlAs superlattices (SLs) on (001) InP have been successfully used for terahertz (THz) transmitters (Tx) and receivers (Rx) in photoconductive antennas for THz time-domain spectroscopy systems [7]. Unlike the more established MBE-grown low-temperature GaAs (LT-GaAs), InGaAs can be excited over fiberoptics at a wavelength of 1.55 μm using compact and relatively inexpensive telecom lasers. However, MBE growth conditions must be separately optimized for Tx and Rx structures, adding to cost and complexity. Recently, InGaAs/InAlAs SLs grown on InP(111) substrates were proposed as a next-generation material system, combining advantages of LT-GaAs and non-polar InGaAs/AlGaAs material systems. However, attempts resulted in defective growth [8]. Additional tailoring of strain in (111) InGaAs and InAlAs SL layers can embed strong piezoelectric fields in the material, bringing further enhancement of the Tx and Rx antennas performance without the need for individual MBE growth optimization.

With the use of density functional theory calculations and extensive scanning transmission electron microscopy (STEM) analysis along with the STEM Moiré Geometrical Phase analysis, we showed the first defect-free epitaxial growth of InGaAs/InAlAs heterostructures on InP (111)B substrates. Our work sets the stage for developing InGaAs/InAlAs/InP(111) photoconductive antennas for THz-TDS system with properties tuned by the embedded piezoelectric fields.

We present on the growth of low-density, site-controlled InAs quantum dot columns (QDCs) for templating high optical quality, self-assembled InAs quantum dots (QDs) and quantum dot molecules (QDM). The epitaxial formation of InAs QDs has been extensively studied[1] and have largely been considered as possible qubits[2]. However, a scalable platform to produce large arrays of identical QDs for integration into semiconductor devices is still being refined[3]. It is a challenge to produce high-optical quality site-templated QDs due to the presence of defects at the regrowth interface. We have previously shown a molecular beam epitaxy (MBE) grown low-density site-templated InAs QD platform[4]. Using this initial site-controlled growth, we are exploring the growth and morphology of InAs QDCs to serve as templated buffer layers for self-assembled QDs. This method maintains the spatial location of our patterns while obtaining QDs with optical qualities of self-assembled structures.

Tin-containing group IV alloys are being developed for infrared photonic devices. Si_1-x-y­Ge_xSn_y alloys grown by molecular beam epitaxy, particularly high tin content films with direct bandgaps, require low substrate temperatures during growth. The resulting layers are metastable, so additional heat exposure for subsequent layer growth or device processing may induce strain relaxation and tin segregation within or to the surface of the film. A complex set of material and temperature factors contribute to the relative degrees of defect formation and tin segregation. Each mechanism has different implications for device design, processing methods, and working lifetime at elevated temperatures such as those experienced by thermophotovoltaic cells.

In this work, we show studies of the thermal stability and annealing behaviors of Si_1-x-y­Ge_xSn_y films performed by temperature varying spectroscopic ellipsometry (SE). Heating was carried out on a Linkam heating stage mounted to a JA Woollam VASE instrument under a nitrogen atmosphere. Changes in strain state and material uniformity present as changes in the location and shape of the bandgap absorption edge and critical points in the dielectric function, and overall shifts in optical properties of the films. Findings and additional evidence of film changes are confirmed by Nomarski optical microscopy, high-resolution x-ray diffraction, and atomic force microscopy. We collect data at room temperature after in-situ rapid thermal anneals to approximate thermal effects from device processing, and take continuous SE measurements at elevated temperatures for lifetime assessment. Tin content, film thickness, and strain state are varied to examine the effects of these properties on breakdown behaviors and to demonstrate the flexibility of the SE technique.

Quantum dots (QDs) based devices often requires for their functionality to successfully stack several layers barrier/QD/barrier structures. Nevertheless, the growth gets more challenging as each layer of QDs is added, since diffusion, segregation, alloy intermixing, and strain effects intrinsically appear along the process. In this work, the strain distribution, and the electronic band structure of InAs QDs embedded in (Al)GaAs matrixes are investigated by numerical analysis based on finite element method.As input data, experimental parameters from MBE grown samples such as the QDs morphology, wetting layer (WL) thickness and the composition of the materials were employed. The biaxial (ε_xx) and hydrostatic (ε_hydro) strain tensors were calculated for pyramidal and truncated pyramidal QDs shapes, considering that capping usually flattens the apex of the islands [1]. The results revealed that ε_xx resulted more affected with the variations of InAs QDs geometry, which is correlated with changes in the heavy hole (HH) and light hole (LH) band structure behavior of the simulated heterostructures. Differences of the maximum and minimum values ​​of ε_xx and ε_hydro tensors above islands, which carry information about the vertical pairing probability, for each type of heterostructure geometry were observed. The electronic confinement states and eigenfunctions probability distributions were calculated for the QD well. Higher energy states and lower number of eigenfunctions can be confined in truncated pyramids, as compared with non-modified shape QDs. This work shows a clear picture of the strain fields and their consequences on the band structure of InAs/(Al)GaAs multi-stacked heterojunctions found in actual QDs devices.

[1] C.A.Mercado-Ornelas, et.al., Physica E: Low-dimensional Systems and Nanostructures. 2020. Vol. 124.

Direct wide bandgap (WBG) III-Nitride (III-N) semiconductor material is promising alternative to silicon (Si) technology for high-power density, high-frequency, and high-voltage applications. However, native substrate material for most WBG semiconductors is not yet commercially available at low cost.One inexpensive alternative is to use standard Si wafers as substrates due to their extremely mature development.This has been relatively well developed for GaN on Si, with several commercial examples of well-developed devices. Here, we investigate the growth of the ultra-WBG semiconductor, AlN, on Si(111) in a high-temperature regime by plasma-enhanced MBE. For thick films, the low surface roughness and monocrystalline quality is characterized by the AFM and XRD respectively. Additionally, metal Al (111) signal is seen in the XRD, which corresponds to Al droplets resulting from the metal rich growth. Optical microscopy, however, reveals the presence of two distinct regions on the surface. Further investigation using micro-Raman spectroscopy identifies one region as high-quality, relaxed, crystalline Si and the other region as relaxed AlN.Analysis of the Raman spectra reveals week AlN peaks alongside the Si peaks, indicating that the Si exists as a film on the surface. AFM on the AlN regions is extremely smooth with surface roughness of less than 0.4 nm, while the Si regions are slightly rougher. In addition, it is found that small, three-dimensional crystals of Si have started to nucleate out of the Al droplets.All elemental conclusions are confirmed by electron dispersive x-ray (EDX) spectroscopy performed in a scanning electron microscope. Piezoresponse force microscopy has been performed on the free AlN regions and demonstrates that the AlN is single domain and metal polar. Thinner films, in stark contrast to the thick films, reveal only AlN domains with no areas of Si apparent.These results along with the findings from cross-sectional material analysis demonstrate a complicated diffusion effect between Al, Si, and AlN, occurring during the material growth, which will be discussed.

Capacitance vs. voltage (C(V)) measurements represent a common way to trace levels of free charge carriers in semiconductors and semiconductor heterostructures. Concentration profiles of free charges obtained from C(V) measurements are widely accepted to reflect residual impurity background levels in GaN/AlGaN layer stacks. Particularly when the material is grown by MBE on highly compensated (insulating) GaN substrates, free carrier concentrations reaching levels below 10¹⁵ cm^-3 are often reported for GaN buffer layers [1]. The growth of GaN/AlGaN stacks, hosting a 2-dimensional electron gas (2DEG), on insulating substrates is essential to probe the intrinsic 2DEG transport properties and to enable lateral HEMT functionality.

We realized a strong discrepancy in such GaN/AlGaN layer stacks - grown by MBE on highly compensated GaN substrates - between the concentrations of free charges extracted from C(V) data and the elemental donor background observed in secondary ion mass spectroscopy (SIMS) runs [2]. The conversion of the C(V) data into depth profiles relies on a model assuming a parallel plate capacitor geometry with defined top and bottom electrodes. The low level of free charges <10¹⁵ cm^-3 compared to the unintentional oxygen donor background of >2·10¹⁶ cm^-3 is attributed to the experimental details of the measurement. A gate metal layer serves as the top electrode, while a defined bottom electrode is missing once the 2DEG is completely depleted and the C(V) data become unreliable for charge profiling.

Such a defined bottom electrode can be introduced by either growing GaN/AlGaN stacks on conductive substrates or – as implemented here - by making use of atmospheric silicon adhesion at the surface of unintentionally-doped (uid) substrates, which leads to parasitic conductivity at the substrate/MBE regrowth interface [3]. This parasitic channel is detrimental for investigating the intrinsic 2DEG transport properties and for HEMT functionality. On the other hand, the free charge carrier and donor background concentrations both agree well at a level of 4·10¹⁶ cm^-3. Here, the bottom electrode is preserved after the 2DEG is depleted.

In summary, a parasitic channel in GaN/AlGaN heterostructures is a building block for a parallel plate capacitor, as it represents a defined bottom electrode for C(V) measurements. Consequently, free charge carrier concentrations extracted from C(V) data agree well with the elemental donor concentrations determined by SIMS.

[1] M.J. Manfra et al., J. Appl. Phys. 92, 338 (2002)

[2] S. Schmult et al., J. Vac. Sci. Technol. B 35 (2), 02B104 (2017)

[3] S. Schmult et al., accepted, J. Crys. Growth (2022)

The growth and structure of the cubic manganese nitride, namely θ - MnN, η- Mn₃N₂(010), η- Mn₃N₂(001), and ε- Mn₄N have already been investigated intensively on MgO (001) substrate [1,2]. However, the hexagonal ζ- Mn₂N has remained unexplored. The thin films of hexagonalζphase Mn₂N were grown successfully on MgO (001) using molecular beam epitaxy (MBE) under manganese-rich conditions. Multiple samples were grown by varying the Mn: N flux ratio from 0.6:1 to 0.9:1, and the ζphase is observed at the flux ratio of 0.9:1 at 455 ± 30 ⁰C growth temperature. The sample growth process was monitored by in-situ reflection high energy electron diffraction (RHEED). During the sample growth, the streaky RHEED pattern with reduced streak spacing compared to MgO substrate was observed. The chemical composition of the samples was determined by in-situ Auger electron spectroscopy (AES) at different locations of the sample. The stoichiometric ratio of Mn: N on the film is nearly 2:1 which is consistent with the ζ- phase Mn₂N. The experimentally measured in-plane lattice constant, based on the RHEED, is 2.86 ± 0.02 Å, and the out-of-plane lattice constant, measured using X-ray diffraction (XRD), is 4.56 ± 0.02 Å, which agrees with the a (2.82 Å)and c (4.54 Å) value of Mn₂N reported by Aoki et al. (2004) [3]. Furthermore, the room temperature scanning tunneling microscopy (STM) studies show some steps with a step height of 4.50 Å and atomic resolution of hexagonal arrays. This research was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-FG02-06ER46317.

References:

[1] H. Yang, H. Al-Brithen, E. Trifan, D. C. Ingram, and A. R. Smith, J. Appl. Phys. 91, 3 (2002).

[2] A. Foley, J. Corbett, A. L. Richard, K. Alam, D. C. Ingram, and A. R. Smith, J. Crystal growth 446, 60 (2016).

[3] M. Aoki, H. Yamane, M. Shimada, and T. Kajiwara, Material Research Bulletin 39, 827 (2004).

Ultra-wide bandgap (UWBG) semiconductors with a bandgap greater than 3.4 eV, such as c-BN, AlN, high Al content AlGaN, β-Ga₂O_3, anddiamond, have higher figures of merit values than GaN and SiC for power and rf devices making them candidates for next generation high-power/high-temperature electronic materials [1-4]. Higher operating frequency and power density enable smaller transformers and better power quality filter components, which mitigate Navy architecture space/weight/power (SWaP) issues, and also allow smaller size, higher-speed for future ship payloads and electric propulsion systems [1].

The availability of inexpensive large-area bulk substrates synthesized by melt growth techniques at atmospheric pressure provides a scaling advantage for β-Ga₂O₃ over other UWBG semiconductors [2]. In addition, homoepitaxial growth on bulk substrates offers the potential for low defect density films for vertical power devices. Further, controlled n-type doping with a shallow donor level (15-50meV [5]) is another advantage of β-Ga₂O₃ compared to AlN and high Al-content AlGaN. For these reasons, homoepitaxial growth of unintentionally- and impurity-doped Ga₂O₃ films and their electrical and structural properties is of great interest.

In this talk, we will present advances in MBE growth homoepitaxial of β-Ga₂O₃ thin films. First, the growth rate was increased approximately from 1 to 3 nm/min by optimizing the growth conditions such as Ga flux, plasma conditions and growth temperature (T_g). At optimal conditions with a T_g of 725 ^oC, surface roughness and X-ray rocking curve full-width at half maximum were 0.36 nm and 20 arc-sec, respectively, for 390 nm thick films. Optimal growth conditions that resulted in high structural and surface quality were used to explore doping parameter space using Sn and Si impurity. Hall effect measurements were carried out doped layers. For Sn-doped layers, the free carrier density can be controlled in the range 1x10¹⁶ to 3x10¹⁹ cm^-3. A mobility of 49 cm²/V-s with free carrier density of 3x10¹⁹ cm^-3 was measured which is comparable to the previously reported values for Sn-doped β-Ga₂O₃ [6]. We will also present the data on Si doped homoepitaxial β-Ga₂O₃ thin films.

This work was funded by the Office of Naval Research and the Office of the Secretary of Defense.

References

[1]Naval Power Systems Technology Development Roadmap PMS 320

[2]H.H. Tippins, Physical Review 140, A316 (1965).

[3]K. Akito et al., Jpn. J. Appl. Phys. 55, 1202A2 (2016).

[4]J.Y. Tsao, Adv. Electron. Mater. 4, 1600501 (2018).

[5]Neal et al., Appl. Phys. Lett. 113, 062101 (2018).

[6]A.J. Green et al., APL Materials 10, 029201 (2022).

Combination of broken inversion symmetry and spin-orbit coupling gives rise to a wide range of exotic superconducting states, such as mixed-parity superconductivity, superconducting Weyl state, and superconducting diode effect. Incipient ferroelectrics e.g., PbTe, are near a polar instability. The emergence of superconductivity has been reported in some of these incipient ferroelectrics upon doping. Many unconventional superconductors, such as the cuprates, pnictides, and heavy fermion systems, occur in close proximity to magnetic fluctuations or magnetic orders, suggesting that these are important ingredients in the superconductivity in these materials. Here, ferroelectricity and superconductivity could be connected or accidental neighbors. Furthermore, the intersection of superconductivity and topologically nontrivial states is a fertile landscape for exciting quantum phenomena, including non-abelian excitations.

IV-VI compounds show a wide range of electronic and polar instabilities. Pb_1-xSn_xTe demosntrates a highly tunable superconductivity and ferroelectricity which are dependent on carrier density and Sn(Pb) concentration. Furthermore, a topological phase transition is expected at x~0.4, which inverts the conduction and valence bands for Sn rich compounds. I will report on our recent growth efforts of high quality Pb_1-xSn_xTe heterostructures,using a chalcogenide molecular beam epitaxy (Veeco 930) near the topological phase transition (x~0.4). The cross-sectionhigh-angle annular dark-field (HAADF) imaging in scanning transmission electron microscopy (STEM) shows abrupt interfaces. X-ray diffraction demonstrates single oriented films. The sheet resistancev.s. temperature demosntrates a metallic-like behavior, dR/dT >0, in doped samples extending to 2 K. The Hall carrier density was measured at various temperatures, resolving the carrier mobility. The carrier mobility is inversely proportional to doping concentration. We report on superconductivity in electron and hole doped Pb_1-xSn_xTe heterostructures and its relation to neighboring ferroelectric and topological phase transitions.

The growth of InN quantum dots (QDs) on c-plane sapphire by droplet epitaxy (DE) using radiofrequency plasma-assisted molecular beam epitaxy (MBE) was reported. The QD growth process from liquid In droplets to the InN QDs was described with a focus on the effect of RF-plasma on the formation of In droplets size and density as a function of substrate temperatures. Two nitridation procedures were used to investigate the crystallization of In droplets in order to understand the process. The variation in the shape and size of InN QDs was explained in terms of the In atom migration of droplets and surface diffusion. The growth of InN QDs using the DE method has many advantages over the classical Stranski-Krastinow technique, including the ability to control a wide range of QD shapes, sizes, and densities.

Recently, we proposed the momentum(k)-space carrier separation (k-SCS) concept that combines the advantages of both direct and indirect bandgaps for light detection/conversion devices. The basic principle is to have a direct bandgap in a semiconductor that has a slightly larger bandgap than the indirect fundamental bandgap, giving a thermalization barrier, D_G-L, such as 3k_BT, for electrons at the conduction band edge. In this example, the Γ-valley minimum is 3k_BT higher in energy than the L-valley minimum. The sharp absorption edge of the direct bandgap energy, E_g,Γ, appears at just slightly (3k_BT) higher in energy than the slow onset of the absorption edge at the indirect bandgap, E_g,L,. Under light illumination, electrons are excited to the direct Γ-valley in the conduction band while leaving holes in the valence band edge at the symmetry point of Γ. The large majority of photogenerated electrons in the direct Γ-valley will quickly thermalize at a sub-picosecond time scale to the lower energy indirect L-valley. These electrons in the L-valley will recombine with the holes in the valence band at a time scale of tens of microseconds to milliseconds. Both carriers, electrons and holes, are transported in real space but with different momentums, i.e. separately in k-space, to their corresponding contacts with negligible recombination. This clever design not only improves photogenerated carrier lifetime, similar to indirect bandgap semiconductors, but also offers a large absorption coefficient, similar to direct bandgap semiconductors.

Group-IV alloys such as GeSn are a model material system to demonstrate the novel idea as photodetectors with Sn compositions near the indirect-to-direct bandgap transition are predicted to have greater detectivity than conventional IV-VI and III-V compound photodetectors at room temperature, and comparable detectivity to InAs detectors operating at 77 K. GeSn samples with D_G-L between 0.4k_BT and 3k_BT were grown by MBE on Ge substrates for MWIR (2 to 5 µm). Substrate surfaces were first cleaned using HF and HCl solution prior to a UHV outgas at 550 ˚C. A Ge buffer was grown at a substrate temperature of 500 ˚C before cooling down to 200 ˚C for GeSn growth. Ge effusion cell was held constant while Sn effusion cell was varied between 825 to 900 ˚C to obtain designed composition. RHEED showed a streaky (2x1) surface reconstruction pattern that transitions to a (1x1) with increasing Sn. Introducing Si expands the wavelength coverage range to 2 ~ 22 µm, making it ideal for MWIR, LWIR and VLWIR applications. More details of the theory and experiments will be reported at the conference.