NAMBE 2023 Session NM-TuM1: Novel Materials, Optoelectronics

To push into new generations of spintronic devices requires understanding new magnetic phenomena and also how to control both known and emerging material platforms as high-quality epitaxial thin films. Specifically, altermagnets are a new phase that is predicted to exhibit a strong spin splitting of the band structure, which can form the basis for new spintronic applications. MnTe, a candidate altermagnet, is a room-temperature antiferromagnet (TN ≈ 310 K) semiconductor (energy gap of order 1 eV) with a NiAs structure. Here, we present results on the molecular beam epitaxy growth and properties of MnTe/InP(111). Using polarized neutron reflectivity and magnetotransport, we find that there is emergent ferromagnetic behavior likely driven by a combination of charge transfer and strain. The ferromagnetic component is likely a slight canting of the bulk-like A-type antiferromagnetic state, as seen by neutron diffraction. This high level of tunabililty of MnTe opens the door to tailoring interlayer magnetic interactions in this layered material system. Together these results provide a potential mechanism of tuning antiferromagnetic ordering for applications in high-speed, next-generation spintronics.

We present our work on the growth of ErAs:InGaAlBiAs through a digital alloy approach where a superlattice of thin layers of the quaternary InGaBiAs and InAlBiAs, behave as a quinary and allows tuning the bandgap with the gallium and aluminum composition just with layer thickness variation. We target InGaAlBiAs for 1550 nm optical excitation for photoconductive switches (PCS) that can generate and detect terahertz (THz) pulses. Semiconductors implemented in PCS need high dark resistance, subpicosecond carrier lifetime, and a high carrier mobility is desirable. Incorporating erbium above the solubility limit creates ErAs nanoparticles, decreasing the material's carrier lifetime. At the same time, ErAs nanoparticles have a strong pinning effect of the effective Fermi level on the material. In this narrow bandgap semiconductor, the Fermi level is close to the conduction band, i.e., there is a high electron concentration, which decreases the dark resistivity. Other authors have found that the size of ErAs nanoparticles affects the electron concentration in InGaAs, where bigger nanoparticles cause a lower electron concentration. Er can be co-deposited, forming nanoparticles at the same time the matrix is grown, which saves growth time; however, while the usual 490°C growth temperature could give enough adatom mobility to create sizeable nanoparticles, our structure of interest is a dilute bismuthide that requires lower temperature growth (~280°C). At this low temperature, we use interrupt growth and migration-enhanced epitaxy, which allows us to have lower carrier concentration. All these samples are characterized by different techniques to determine relevant properties. Dark resistance is obtained from Hall effect and Van der Pauw measurements, carrier lifetime from optical pump THz probe spectroscopy, optical band gap from spectrophotometry, material quality from high-resolution x-ray diffraction, and composition through Rutherford backscattering spectrometry.

GaSe is an attractive van der Waals (vdW) material due to its intriguing bandgap behavior and is an ideal choice for quantum photonic technology. We will grow GaSe films on patterned substrates by molecular beam epitaxy (MBE) to obtain wafer-scale films with site-controlled localized quantum emitters (QEs). A proper substrate is critical as the substrate pattern is used to funnel electrons and holes to a local minimum to form emission. Unlike traditional epitaxy, vdW materials may grow on substrates with large differences in lattice constant/structure as the weak interlayer bonds make them less affected by substrate. However, it also means less control over the crystal structure and surface morphology of the deposited film. To achieve the goal—GaSe films with strain-localized QEs—we first need flat atomically-thin GaSe films with minimal twin boundaries. Our previous work[1] proved that Al₂O₃ was not suitable for the growth of GaSe films due to the poor wettability of Ga, so we turned to GaAs(111)B, which has the same cation as GaSe and a relatively small lattice mismatch (6%).

Typically, vdW materials cannot grow heteroepitaxially on 3D substrates unless the dangling bonds are terminated. We developed a process—annealing in Se at 680^oC—to deoxidize GaAs(111)B in an As-free MBE while generating a Se-terminated surface smooth enough for subsequent growth (Fig. S1). Preliminary work has resulted in GaSe films with somewhat rough surfaces (Fig. S2a, 2d). Additional annealing of the substrate prior to growth can notably promote film coalescence and reduce roughness (Fig. S2b, 2e), probably because it facilitates a fully Se-terminated GaAs surface. Fig. S2e shows a surface comprised of twinned triangular domains, and the reflection high energy electron diffraction (Fig. S2g) confirms the coexistence of two GaSe orientations. In addition, the 2-step method (low-temperature nucleation followed by high-temperature growth) facilitates film coalescence (Fig. S2c, 2f), since high substrate temperature can enhance adatom mobility. Moreover, fresh GaAs(111)B wafers lead to more directional nucleation of GaSe (Fig. S3), implying that additional oxidation due to air exposure is detrimental to the growth of high-quality GaSe. We also found that the quality of vdW films had an opposite dependence on growth conditions (temperature and rate) in 2D/2D and 2D/3D growth modes. The next challenge is to achieve uniform coverage and unidirectional nucleation. The effects of growth parameters, substrate pretreatment, and uncracked/cracked Se will be examined to better understand the growth mechanics and morphology evolution of GaSe.

Mid-wave infrared (MWIR) detectors are widely used in various fields, such as medical devices, remote sensing, and spectroscopy. InSb-based infrared focal plane arrays (FPAs) have emerged as a popular choice for their affordability, scalability, and temporal stability, as well as their spatial uniformity. However, achieving fully relaxed tunable absorber to cover MWIR and LWIR is challenging due to lack of binary substrates. Thus, type-II superlattices and metamorphic buffers are employed to cover the MWIR spectrum and extend into the long-wave infrared (LWIR) region [1], [2]. In this study, we propose using interfacial misfit dislocations to grow fully relaxed InSb on an InAs substrate, combined with direct growth of Al_xIn_1-xSb buffer layers on InAs, to develop tunable InAsSb absorbers for MWIR and LWIR applications. This approach is expected to lead to the production of high-performance MWIR and LWIR detectors, potentially opening new avenues for future applications.

We will discuss using interfacial misfit dislocations to form instantly relaxed buffer layers on InAs substrates, and analyze the directly grown AlInSb and InSb epilayers using HR-XRD ω-2θ scans and reciprocal space mapping. TEM analysis showed misfit dislocation arrays at the AlInSb/InAs interface. We investigated InSb quantum wells grown with different AlxIn1-xSb barrier layers using PL and present a detailed analysis of the InSb quantum wells based on PL and TRPL results. These findings provide a better understanding of the properties of these materials and their potential for MWIR and LWIR applications.

[1] W. L. Sarney, S. P. Svensson, Y. Xu, D. Donetsky, and G. Belenky, “Bulk InAsSb with 0.1 eV bandgap on GaAs,” J. Appl. Phys., vol. 122, no. 2, p. 025705, Jul. 2017.

[2] A. Rogalski, “Next decade in infrared detectors,” in Electro-Optical and Infrared Systems: Technology and Applications XIV, Warsaw, Poland, 2017.

⁺Author for correspondence: incef@unm.edu

There have been very few studies of antiperovskite structure Mn₃GaN in general and it was seen in MBE growth mainly as a second-phase precipitate when growing MnGaN [4]. However, this material is very interesting since it can support the Kagome antiferromagnetic spin structure [3]. And so, we grow thin films of Mn₃GaN on cubic MgO(001) substrates using rf N–Plasma MBE. In our work, Mn₃GaN is deposited at 250 ±10°C with a Mn:Ga:N flux ratio of 3:1:1. We keep the Ga:N ratio fixed using an RF plasma nitrogen source. The sample surface is continuously monitored throughout the growth using reflection high energy electron diffraction. During the growth, the RHEED pattern was observed to be highly streaky, indicating an atomically smooth surface. In addition, we observed half-order fractional streaks (2x pattern) in the [100] direction. The calculated in-plane lattice constant based on RHEED is 3.89 ±0.06 Å. This value is very close to the lattice constant a of Mn₃GaN according to theory (3.898 Å) [1] and with the in-plane experimental value for sample growth by sputtering (3.896 Å); and in that work, the authors also observed a 2x pattern [2]. We also measure the out-of-plane lattice constant using X-ray diffraction. For the major 002 peak, the value calculated is 3.84 ±0.06 Å which also agrees well with the theoretical value (3.898 Å) [1] and with the experimental reported c value [2] (3.881 Å). Since we did not observe significant second-phase peaks, the phase purity of the sample is quite high, and Rutherford backscattering confirms a stoichiometry of 3:1:1.

This work is 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] E. F. Bertaut, D. Fruchart, J. P. Bouchad, and R. Fruchart, (1968). Diffraction Neutronique de Mn₃GaN. Solid State Commun.6, 251–256.

[2]T. Hajiri, K. Matsuura, K. Sonoda, E. Tanaka, K. Ueda, & H. Asano, (2021). Spin–Orbit–Torque Switching of Noncollinear Antiferromagnetic Antiperovskite Manganese Nitride Mn 3 Ga N. Physical Review Applied, 16(2), 024003.

[3]T. Nan, C.X. Quintela, J. Irwin, G. Gurung, D.F. Shao, J. Gibbons, N. Campbell, K. Song, S.Y. Choi, L. Guo, and R.D. Johnson, (2020). Controlling spin current polarization through non–collinear antiferromagnetism. Nature communications, 11(1), p.4671.

[4] KH. Kim, KJ. Lee, HS. Kang, FC. Yu, JA. Kim, DJ. Kim, KH. Baik, SH. Yoo, CG. Kim, YS. Kim, (2004).Molecular beam epitaxial growth of gan and gamnn using a single precursor. physica status solidi (b), 241(7):1458–1461