Low-temperature Molecular Beam Epitaxy Research Articles

Atomically Self-Healing of Structural Defects in Monolayer WSe2

Abstract Minimizing disorder and defects is crucial for realizing the full potential of two-dimensional transition metal dichalcogenides (TMDs) materials and improving device performance to desired properties. However, the methods in defect control currently face challenges with overly large operational areas and lack of precision in targeting specific defects. Therefore, we propose a new method for the precise and universal defect healing of TMD materials, integrating real-time imaging with scanning transmission electron microscopy (STEM). This method employs electron beam irradiation to stimulate the diffusion migration of surface-adsorbed adatoms on TMD materials grown by low-temperature molecular beam epitaxy (MBE), and heal defects within the diffusion range. This approach covers defect repairs ranging from zero-dimensional vacancy defects to two-dimensional grain orientation alignment, demonstrating its universality in terms of the types of samples and defects. These findings offer insights into the use of atomic-level focused electron beams at appropriate voltages in STEM for defect healing, providing valuable experience for achieving atomic-level precise fabrication of TMD materials.

Strong Covalent Coupling in Vertically Layered SnSe2/PTAA Heterojunctions Enabled High Performance Inorganic-Organic Hybrid Photodetectors.

Controllable large-scale integration of two-dimensional (2D) materials with organic semiconductors and the realization of strong coupling between them still remain challenging. Herein, we demonstrate a wafer-scale, vertically layered SnSe2/PTAA heterojunction array with high light-trapping ability via a low-temperature molecular beam epitaxy method and a facile spin-coating process. Conductive probe atomic force microscopy (CP-AFM) measurements reveal strong rectification and photoresponse behavior in the individual SnSe2 nanosheet/PTAA heterojunction. Theoretical analysis demonstrates that vertically layered SnSe2/PTAA heterojunctions exhibit stronger C-Se covalent coupling than that of the conventional tiled type, which could facilitate more efficient charge transfer. Benefiting from these advantages, the SnSe2/PTAA heterojunction photodetectors with an optimized PTAA concentration show high performance, including a responsivity of 41.02 A/W, an external quantum efficiency of 1.31 × 104%, and high uniformity. The proposed approach for constructing large-scale 2D inorganic-organic heterostructures represents an effective route to fabricate high-performance broadband photodetectors for integrated optoelectronic systems.

Plotting philosophy picture of uniaxial magnetic anisotropy in arsenide and phosphide dilute ferromagnetic semiconductors

In this work, we performed a systematic investigation on the comparison of magnetic anisotropy between three III-Mn-V dilute ferromagnetic semiconductors (Ga,Mn)As, (In,Mn)As and (Ga,Mn)P prepared by ion implantation and pulsed laser melting. Compressive strain induces in-plane magnetic anisotropy in (Ga,Mn)As and (Ga,Mn)P, while out-of-plane magnetic anisotropy is present in (In,Mn)As due to tensile strain introduced by Mn substitution. Interestingly, all materials prepared herein does not present strong in-plane uniaxial anisotropy, between [110] and [11¯0] directions, which always exhibits in low temperature molecular beam epitaxy (LT-MBE) grown (Ga,Mn)As samples. The reason is ascribed to the fact that the ultra-fast recrystallization induced by pulsed laser melting weakens the formation of Mn-Mn dimers along the [11¯0] direction which presents in LT-MBE grown (Ga,Mn)As.

Ge1−xSnx layers with x∼0.25 on InP(001) substrate grown by low-temperature molecular beam epitaxy reaching 70 °C and in-situ Sb doping

Ge1−xSnx with x∼25%, group-IV alloy semiconductor, is highly attracted to mid-infrared (MIR) photodetector applications because of its narrow bandgap with ∼0.25 eV, the compatibility to the Si integrated circuit platform, and lattice matching of Ge0.75Sn0.25 to InP substrate. Although the heteroepitaxial growth of Ge0.75Sn0.25 on InP has been reported, the basic physical properties of Ge0.75Sn0.25 have not yet been clarified. In this study, we discuss three topics: (1) understanding the crystalline growth features of Ge0.75Sn0.25 on InP, (2) revealing the thermal stability of Ge0.75Sn0.25, and (3) developing a carrier-control technique. First, we found that a low-temperature molecular beam epitaxy could achieve a Ge0.75Sn0.25 layer with superior crystallinity without dislocations. However, low Sn-content Ge1−xSnx regions are locally formed during the Ge0.75Sn0.25 heteroepitaxy; the origins of the low Sn-content Ge1−xSnx regions were discussed by transmission electron microscopy analysis. In addition, we found that lowering the growth temperature from 100 °C to 70 °C also effectively reduces the area of the low Sn-content Ge1−xSnx regions. Next, we found that Ge0.75Sn0.25 layers grown at 70 °C sustain thermal stability up to 200 °C without causing crystalline degradations. Finally, we demonstrated the in-situ Sb doping to Ge0.75Sn0.25. We found that the undoped and in-situ Sb-doped Ge0.75Sn0.25 layers exhibited p-type and n-type conduction, respectively, with carrier concentrations of order 1019 cm−3 by Hall effect measurement. This study suggests the MIR photodetector application is practically possible using low-temperature processes with process temperatures lower than 200 °C.

Composition and strain effects on Raman vibrational modes of GeSn alloys with Sn contents up to 31 % grown by low-temperature molecular beam epitaxy

We study the behavior of Ge–Ge, Ge–Sn, and Sn–Sn vibrational modes in GeSn semiconductors with Raman Spectroscopy. Raman spectroscopy is a rapid, nanoscale spatial resolution and non-destructive approach to accurately determine the composition and strain information of GeSn, and it is thus crucial for the material investigation and device application of GeSn alloys. By using several excitation wavelengths at 532, 633 and 785 nm on a set of fully strained and fully relaxed Ge1-xSnx layers with the Sn composition in the range 2.3 % < xSn < 31 %, all modes are identified and their evolution as a function of strain and Sn content is determined. The Raman shifts of all vibrational modes are found to exhibit the same function versus the composition xSn and in-plane strain ε//, Δω = ωGeSn - ωGe = axSn + bε//, where a is the Sn composition factor and b is the strain shift factor. In addition, the Ge–Sn mode intensity increases with Sn content. It is discovered for the first time that the Sn composition determined from the plot of the intensity ratio of the Ge–Sn mode over the Ge–Ge mode as a function of Sn composition at 785 nm excitation agrees well with that the X-ray Diffraction (XRD) Reciprocal Space Mapping (RSM), offering a novel approach for determining Sn content by Raman spectroscopy.

Band Engineering of Magnetic (Ga,Mn)As Semiconductors by Phosphorus Doping

Impact of P and Mn incorporation into GaAs layers on their electronic and band structures as well as magnetic and structural properties has been studied. A set of the homogenous (Ga,Mn)(P,As) layers with 8% of Mn and 0-27% of P contents of high structural perfection have been grown by low-temperature molecular-beam epitaxy. Embedding P ions into the GaAs crystal lattice leads to an increase in the band gap, while Mn impurities lead to its decrease. A significant impact of Mn interstitial impurities on the structural and magnetic properties of the films was observed. We observe an enhancement of charge density in the presence of Mn. Higher energy interband transitions involving energy band structure that includes the split-off valence band and the L-point bands remained nearly unchanged.

Valence Band Dispersion in Bi-Doped (Ga,Mn)As Epitaxial Layers

The impact of incorporating Bi into (Ga,Mn)As layers on their electronic, band structure, magnetic, and structural properties has been studied. The low-temperature molecular beam epitaxy technique was employed to grow homogeneous (Ga,Mn)(Bi,As) layers with high structural perfection. Post-growth annealing treatment resulted in improved structural and magnetic properties, as well as an increase in the hole concentration in the layers. Modulation photoreflectance spectroscopy confirmed modifications to the valence and split-off band in the (Ga,Mn)(Bi,As) layers. The experimental results are consistent with the valence band model of hole-mediated ferromagnetism in the layers. The (Ga,Mn)(Bi,As) compound combines the properties of (Ga,Mn)As and Ga(Bi,As) ternary compounds, offering the possibility of tailoring the bandgap structure to the requirements of novel device functionalities for future spintronic and photonic applications.

Influence of Bi doping on the electronic structure of (Ga,Mn)As epitaxial layers

The influence of the addition of Bi to the dilute ferromagnetic semiconductor (Ga,Mn)As on its electronic structure as well as on its magnetic and structural properties has been studied. Epitaxial (Ga,Mn)(Bi,As) layers of high structural perfection have been grown using low-temperature molecular-beam epitaxy. Post-growth annealing of the samples improves their structural and magnetic properties and increases the hole concentration in the layers. Hard X-ray angle-resolved photoemission spectroscopy reveals a strongly dispersing band in the Mn-doped layers, which crosses the Fermi energy and is caused by the high concentration of Mn-induced itinerant holes located in the valence band. An increased density of states near the Fermi level is attributed to additional localized Mn states. In addition to a decrease in the chemical potential with increasing Mn doping, we find significant changes in the valence band caused by the incorporation of a small atomic fraction of Bi atoms. The spin–orbit split-off band is shifted to higher binding energies, which is inconsistent with the impurity band model of the band structure in (Ga,Mn)As. Spectroscopic ellipsometry and modulation photoreflectance spectroscopy results confirm the valence band modifications in the investigated layers.

Epitaxial growth of Ge1-xSnx on c – Plane sapphire substrate by molecular beam epitaxy

In this paper, we have demonstrated the first epitaxial growth of Ge1-xSnx on c – plane Sapphire. This has been achieved by growing Ge1-xSnx on Ge(111)/AlAs/Al2O3(0001) engineered substrates by low-temperature molecular beam epitaxy. The growth starts with three-dimensional islands growth, characteristics of the Volmer – Weber growth mode. The presence of type A and type B oriented domains of Ge1-xSnx epilayers was shown by reflection higher energy electron diffraction analysis. High-resolution X-rays diffraction measurement shows that the grown crystals are tetragonally strained. Further, from the recorded pole figure measurement of the two grown samples, the additional presence of reflection micro-twins was also revealed. The presence of twining has been significantly increased for the case of the sample grown with a higher Sn percentage of 5.2%, as compared to that of the one grown with 3.6% of Sn. However, the presence of defects in the epilayers may be suppressed by growing significantly a thicker layer.

The transition of growth behaviors of moderate Sn fraction Ge1-xSnx (8 % < x < 15 %) epilayers with low temperature molecular beam epitaxy

Epitaxial breakdown phenomenon for Ge1-xSnx thin film with Sn fraction of less than 6 % during low-temperature molecular beam epitaxy has been reported [J. Appl. Phys. 97, 044904, 2005]. However more Sn incorporation is required for direct bandgap transition of GeSn alloy, which has much attractive for GeSn optoelectronic devices. In this work, moderate Sn fraction Ge1-xSnx (8 %∼15.3 %), as well as composition-stepped multiple GeSn layers, grown on Ge (001) substrate by molecular beam epitaxy at 120–150 ℃ are investigated. Epitaxy breakdown occurs for the Ge1-xSnx epilayer with Sn fraction of about 8 % and 10 %, even with low Sn fraction buffer layers, in which the film delaminates into a fully compressively strained bottom layer with dislocation free and a fully relaxed top layer spontaneously. Once the epitaxy breakdown occurs, the amorphous columnar structures extend into the following deposited GeSn epilayer, regardless of Sn fraction. While, the strain in the whole Ge1-xSnx epilayer with Sn fraction of 15.3 % is directly released during growth by generation of misfit and threading dislocations or agglomeration of islands. Those results indicate that there are three growth modes including fully compressive strained thin GeSn epilayer, epitaxial breakdown induced self-delamination of GeSn layer with moderate Sn fraction, and strain relaxed Ge1-xSnx epilayer with high Sn fraction of above 15 %. The complex behavior can be attributed to the competition between kinetic roughening and strain- driven GeSn agglomeration or generation of dislocations. The understanding of the growth behavior is helpful for controlling crystal quality of GeSn thin films for their application in optoelectronic devices.

Enhanced photoluminescence of GeSn by strain relaxation and spontaneous carrier confinement through rapid thermal annealing

In this work, the structural evolution and photoluminescence (PL) of 200 nm pseudomorphic Ge 0.9338 Sn 0.0662 on Ge (001) substrate grown by low-temperature molecular beam epitaxy (MBE) after rapid thermal annealing (RTA) is studied. Under RTA at 350 °C or lower, the GeSn film is coherently strained on Ge substrate. As RTA temperature further increases, gradual strain relaxation of GeSn is enabled by generation of misfit dislocations and threading dislocations. As RTA temperature reaches 550 °C or beyond, Sn segregation occurs along with strain relaxation. The PL intensity of annealed samples is enhanced compared to that of as-grown sample probably due to improved crystal quality and strain relaxation (for RTA at> 350 °C) of GeSn. The sample annealed at 500 °C exhibits highest PL intensity due to formation of a Sn-component-graded (SCG) heterojunction with highest Sn content in surface region resulted from interdiffusion of Ge and Sn. The formation of SCG heterojunction renders spontaneous confinement of optically pumped carriers in the surface region and enlarges occupation probability of carriers in Γ valley. Additionally, the carrier confinement in the surface region reduces self-absorption of GeSn and suppresses nonradiative recombination near the GeSn/Ge interface. The results manifest that RTA is an appropriate approach to improve the light emitting property of GeSn grown by low-temperature MBE. • Strain relaxation of pseudomorphic GeSn on Ge is realized by rapid thermal annealing. • A Sn-component-graded heterojunction is formed after rapid thermal annealing. • The light emitting property of GeSn is dramatically enhanced.

Spin transport in fully ferromagnetic p–n junctions

We systematically investigate the spin-dependent transport properties of fully ferromagnetic (Ga,Fe)Sb/GaSb/(In,Fe)Sb and (Ga,Fe)Sb/(In,Fe)As p–n junctions grown by low-temperature molecular beam epitaxy. The (Ga,Fe)Sb/GaSb/(In,Fe)Sb p–n junctions show high Curie temperature (170–310 K) and rectifying characteristics. The polarity and magnitude of magnetoresistance of the (Ga,Fe)Sb/GaSb/(In,Fe)Sb junctions strongly depend on the GaSb spacer thickness (t). Large positive magnetoresistance (MR, 58%) and negative MR (−1.6%) were observed at 3.7 K for the samples with t = 2 and 4 nm, respectively. When the n-type (In,Fe)Sb layer was replaced by the n-type (In,Fe)As, giant MR over 500% was observed, which can be explained by spin-valve and spin-splitting effects. Our results shed light on rich spin-transport physics observed in fully ferromagnetic p–n junctions.

Secondary Epitaxy of High Sn Fraction Gesn Layer on Annealing-Induced Strain Relaxation Gesn Virtue Substrate by Low Temperature Molecular Beam Epitaxy

Secondary Epitaxy of High Sn Fraction Gesn Layer on Annealing-Induced Strain Relaxation Gesn Virtue Substrate by Low Temperature Molecular Beam Epitaxy

Growth and characterization of quaternary-alloy ferromagnetic semiconductor (In,Ga,Fe)Sb

We study the growth and properties of the quaternary-alloy ferromagnetic semiconductor (In0.94−x,Gax,Fe0.06)Sb (x = 5%–30%; the Fe concentration is fixed at 6%) grown by low-temperature molecular beam epitaxy. Reflection high-energy electron diffraction patterns, scanning transmission electron microscopy lattice images, and x-ray diffraction spectra indicate that the (In0.94−x,Gax,Fe0.06)Sb layers have a zinc blende crystal structure without any other second phase. The lattice constant of the (In0.94−x,Gax,Fe0.06)Sb films changes linearly with the Ga concentration x, indicating that Ga atoms substitute In atoms in the zinc-blende structure. We found that the carrier type of (In0.94−x,Gax,Fe0.06)Sb can be systematically controlled by varying x, being n-type when x ≤ 10% and p-type when x ≥ 20%. Characterization studies using magnetic circular dichroism spectroscopy indicate that the (In0.94−x,Gax,Fe0.06)Sb layers have intrinsic ferromagnetism with relatively high Curie temperatures (TC = 40–120 K). The ability to widely control the fundamental material properties (lattice constant, bandgap, carrier type, and magnetic property) of (In0.94−x,Gax,Fe0.06)Sb demonstrated in this work is essential for spintronic device applications.

Improvement of crystal quality and surface morphology of Ge/Gd2O3/Si(111) epitaxial layers by cyclic annealing and regrowth

The role of post-growth cyclic annealing (PGCA) and subsequent regrowth, on the improvement of crystal quality and surface morphology of (111)-oriented Ge epitaxial layers, grown by low temperature (300 °C) molecular beam epitaxy on epi-Gd2O3/Si(111) substrates, is reported. We demonstrate that PGCA is efficient in suppressing rotational twins, reflection microtwins and stacking faults, the predominant planar defect types in Ge(111) epilayers. Continuing Ge growth after PGCA, both at low (300 °C) and high (500 °C) temperatures, does not degrade the crystal quality any further. By promoting adatom down-climb, PGCA is observed to also heal the surface morphology, which is further improved on Ge re-growth. These results are promising for development of high-quality Ge(111) epitaxial layers for photonic and electronic applications.

Study of strain evolution mechanism in Ge1−xSnx materials grown by low temperature molecular beam epitaxy

Ge1−xSnx materials with constant and step-graded compositions have been successfully grown on Ge/Si(001) substrates by using low temperature molecular beam epitaxy (LT-MBE). It has been observed that both completely strained and partially relaxed GeSn materials with the same composition could be formed within the same sample, without adjusting any growth parameters. The residual in-plane strain in GeSn changes in a specific pattern from the GeSn/Ge interface towards the surface. The lower section of the GeSn material remains fully strained and free of dislocations, while most of the threading dislocations are located in the upper section of the layer, causing considerable strain relaxation within this section while maintaining the composition unchanged. This behavior could be explained by kinetic roughening and dislocation generation mechanisms at low temperatures and is remarkably different from GeSn materials grown by chemical vapor deposition (CVD), which exhibit a gradual strain relaxation process as growth continues. This work contributes to the fundamental understanding of the strain relaxation mechanisms of GeSn materials grown by MBE, which is instructive for improving the material quality in the future.

(Invited) Thermoelectric Properties of Tin-Incorporated Group-IV Thin Films

Integration of a thin-film energy-harvesting device in a silicon (Si) chip strongly desires to realize a standalone sensing network system called the internet-of-things (IoT) society. The thermoelectric generator (TEG), which could convert waste heat into electricity, is one option. Considering many IoT sensors will be distributed worldwide, it is desirable to manufacture them using the same technology as next-generation Si-based integrated circuits, including the material. Tin-incorporated group-IV materials, such as germanium tin (Ge1−x Sn x ) and silicon tin (Si1−x Sn x ), theoretically possess a lower thermal conductivity [1] than those of Ge and Si, respectively, owing to the mass-difference scattering of phonons. Thermoelectric efficiency is inversely proportional to thermal conductivity; hence, the tin-incorporated group-IV thin films are one of the attractive TEG materials. The low thermal conductivity also helps ensure temperature differences within small TEG devices.Against the background, this paper will discuss the thermoelectric properties of the tin-incorporated group-IV thin films grown by low-temperature molecular beam epitaxy. The thermal conductivity of Ge0.95Sn0.05 was reduced down to 2.5 Wm−1K−1 [2] by introducing the stacking faults due to microscopic tilt in the crystals, which value is 4% of bulk-Ge (60 Wm−1K−1 [3]). The stacking faults did not have much adverse effect on carrier conduction, especially for electron conduction. Additionally, relatively high-power factors of ~10 and ~30 μWcm−1K−2 at room temperature (RT) were realized for the p- and n-type Ge1−x Sn x layers, respectively. The power factor for the n-type layers is comparable with the counterpart of the n-type BiTe-based layers (~25 μWcm−1K−2 at RT) [4]. Combined with the recent progress in the polycrystalline layers [5], we will present a guideline for enhancing the power factor in Sn-incorporated group-IV materials. Acknowledgments This work as partly supported by PRESTO (No. JPMJPR15R2) and CREST (No. JPMJCR19Q5) from the JST in Japan, JSPS KAKENHI (Nos. 19K21971, 19H00853, 20H05188, 21H01366), and a research grant (Creation of Life Innovation Materials for Interdisciplinary and International Researcher Development) from the MEXT in Japan.

Temperature-dependent characteristics of GeSn/Ge multiple-quantum-well photoconductors on silicon.

Temperature-dependent characteristics of GeSn/Ge multiple-quantum-well (MQW) photoconductors (PCs) on silicon substrate were investigated. The high quality GeSn/Ge MQW epitaxial structure was grown on a silicon substrate using low temperature molecular beam epitaxy techniques with atomically precise thickness control. Surface-illuminated GeSn/Ge MQW PCs were fabricated using complementary metal-oxide-semiconductor-compatible processing and characterized in a wide temperature range of 55-320 K. The photodetection range was extended to λ=2235nm at T=320K due to bandgap shrinkage with Sn alloying. Measured spectral responsivity was enhanced at reduced temperatures. These results provide better understanding of GeSn/Ge MQW structures for efficient short-wave infrared photodetection.

Room-temperature two-terminal magnetoresistance ratio reaching 0.1% in semiconductor-based lateral devices with L21-ordered Co2MnSi

We report on the highest two-terminal magnetoresistance (MR) ratio at room temperature in semiconductor-based lateral spin-valve devices. From first-principles calculations, we predict energetically stable ferromagnet–semiconductor heterointerfaces consisting of Co2MnSi (CMS) and Ge(111) upon insertion of Fe atomic layers. Using low-temperature molecular beam epitaxy, we demonstrate L21-ordered CMS epilayers at 80 °C on Ge(111), where the CMS layer can be utilized as a spin injector and detector. Two-terminal MR ratios as high as 0.1% are achieved in n-Ge-based lateral spin-valve devices with CMS/Fe/Ge Schottky tunnel contacts annealed at 200 °C. This study will open a path for semiconductor-based spintronic devices with a large MR ratio at room temperature.

Positive linear magnetoresistance effect in disordered L21B -type Mn2CoAl epitaxial films

The inverse Heusler alloy ${\mathrm{Mn}}_{2}\mathrm{CoAl}$ is known as a spin gapless semiconductor (SGS). It is believed that the positive linear magnetoresistance (PLMR) effect observed at low temperatures indicates the quantum linear MR effect in the gapless electronic band structures near the Fermi level, i.e., evidence of the realization of a SGS [Ouardi et al., Phys. Rev. Lett. 110, 100401 (2013)]. For this reason, one presumes that the observation of the PLMR effect is indirect proof of the demonstration of $XA$-type ${\mathrm{Mn}}_{2}\mathrm{CoAl}$ consisting of the inverse Heusler structure. In this paper, we observe the PLMR effect at 10 K in homogeneous and single-phase ${\mathrm{Mn}}_{2}\mathrm{CoAl}$ epitaxial films grown by low-temperature molecular beam epitaxy at $100{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. From anomalous x-ray diffraction measurements and analyses, we clarify that the $100{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$-grown ${\mathrm{Mn}}_{2}\mathrm{CoAl}$ epitaxial film is not composed of the $XA$-type structure but a disordered $L{2}_{1}B$-type structure including some amount of Mn(A site) $\ensuremath{\Leftrightarrow}\mathrm{Co}$(C site) swapping and Mn(B site) $\ensuremath{\Leftrightarrow}\mathrm{Al}$(D site) swapping. On the basis of first-principles density-functional theory calculations, we discuss the correlation between the PLMR effect observed at 10 K and the possible electronic band structure in the disordered $L{2}_{1}B$-type ${\mathrm{Mn}}_{2}\mathrm{CoAl}$.