Low-temperature Molecular Beam Epitaxy Research Articles

Structure and magnetism in strained Ge1−x−ySnxMny films grown on Ge(001) by low temperature molecular beam epitaxy

In this letter, we study the structural and magnetic properties of Ge1−x−ySnxMny films grown on Ge(001) by low temperature molecular beam epitaxy using X-ray diffraction, high resolution transmission electron microscopy, and superconducting quantum interference device. Like in Mn doped Ge films, Mn atoms diffuse during the growth and aggregate into vertically aligned Mn-rich nanocolumns of a few nanometers in diameter. Transmission electron microscopy observations in plane view clearly indicate that the Sn incorporation is not uniform with concentration in Mn rich vertical nanocolumns lower than the detection limit of electron energy loss spectroscopy. The matrix exhibits a GeSn solid solution while there is a Sn-rich GeSn shell around GeMn nanocolumns. The magnetization in Ge1−x−ySnxMny layers is higher than in Ge1−xMnx films. This magnetic moment enhancement in Ge1−x−ySnxMny is probably related to the modification of the electronic structure of Mn atoms in the nanocolumns by the Sn-rich shell, which is formed around the nanocolumns.

Local structure of amorphous GaN1−xAsx semiconductor alloys across the composition range

Typically only dilute (up to ∼10%) highly mismatched alloys can be grown due to the large differences in atomic size and electronegativity of the host and the alloying elements. We have overcome the miscibility gap of the GaN1−xAsx system using low temperature molecular beam epitaxy. In the intermediate composition range (0.10 < x < 0.75), the resulting alloys are amorphous. To gain a better understanding of the amorphous structure, the local environment of the As and Ga atoms was investigated using extended x-ray absorption fine structure (EXAFS). The EXAFS analysis shows a high concentration of dangling bonds compared to the crystalline binary endpoint compounds of the alloy system. The disorder parameter was larger for amorphous films compared to crystalline references, but comparable with other amorphous semiconductors. By examining the Ga local environment, the dangling bond density and disorder associated with As-related and N-related bonds could be decoupled. The N-related bonds had a lower dangling bond density and lower disorder.

Angle-dependent spin-wave resonance spectroscopy of (Ga,Mn)As films

A modeling approach for standing spin-wave resonances based on a finite-difference formulation of the Landau-Lifshitz-Gilbert equation is presented. In contrast to a previous study [C. Bihler et al., Phys. Rev. B 79, 045205 (2009)], this formalism accounts for elliptical magnetization precession and magnetic properties arbitrarily varying across the layer thickness, including the magnetic anisotropy parameters, the exchange stiffness, the Gilbert damping, and the saturation magnetization. To demonstrate the usefulness of our modeling approach, we experimentally study a set of (Ga,Mn)As samples grown by low-temperature molecular-beam epitaxy by means of angle-dependent standing spin-wave resonance spectroscopy and electrochemical capacitance-voltage measurements. By applying our modeling approach, the angle dependence of the spin-wave resonance data can be reproduced in a simulation with one set of simulation parameters for all external field orientations. We find that the approximately linear gradient in the out-of-plane magnetic anisotropy is related to a linear gradient in the hole concentrations of the samples.

The microstructural and optical properties of Ge/Si heterostructures grown by low-temperature molecular beam epitaxy

Abstract

Mid-infrared electroluminescence from a Ge/Ge0.922Sn0.078/Ge double heterostructure p-i-n diode on a Si substrate

We report the observation of mid-infrared room-temperature electroluminescence from a p-i-n Ge/Ge0.922Sn0.078/Ge double heterostructure diode. The device structure is grown using low-temperature molecular beam epitaxy. Emission spectra under various injection current densities in the range of 318 A/cm2–490 A/cm2 show two distinct profiles peaked at 0.545 eV (2.275 μm) and 0.573 eV (2.164 μm), corresponding to indirect and direct bandgaps of the Ge0.922Sn0.078 active layer, respectively. This work represents a step forward towards the goal of an efficient direct-bandgap GeSn light-emitting device on a Si substrate by incorporating higher Sn content of 7.8% in a diode structure that operates at lower current densities.

Epitaxial growth of ferromagnetic semiconductor Ga1 - xMnxAs film on Ge(001) substrate

We have grown a high-quality epitaxial Ga1-xMnxAs (x=0.06) film on a Ge(001) substrate without a buffer layer by using low-temperature molecular beam epitaxy. The transmission electron microscope image revealed an atomically flat Ga1-xMnxAs(001)/Ge(001) interface as well as the absence of precipitates such as MnAs. The film exhibited clear hysteresis in the magnetization curves at low temperatures, indicating a ferromagnetic Ga1-xMnxAs film. The Curie temperature of the Ga1-xMnxAs/Ge sample was strongly enhanced by post-growth annealing, which was similar to the Ga1-xMnxAs/GaAs reference sample. Such results will accelerate the integration of III–Mn-V ferromagnetic semiconductors into Ge-based spintronics devices.

All Zinc-Blende GaAs/(Ga,Mn)As Core–Shell Nanowires with Ferromagnetic Ordering

Combining self-catalyzed vapor-liquid-solid growth of GaAs nanowires and low-temperature molecular-beam epitaxy of (Ga,Mn)As, we successfully synthesized all zinc-blende (ZB) GaAs/(Ga,Mn)As core-shell nanowires on Si(111) substrates. The ZB GaAs nanowire cores are first fabricated at high temperature by utilizing the Ga droplets as the catalyst and controlling the triple phase line nucleation, then the (Ga,Mn)As shells are epitaxially grown on the side facets of the GaAs core at low temperature. The growth window for the pure phase GaAs/(Ga,Mn)As core-shell nanowires is found to be very narrow. Both high-resolution transmission electron microscopy and scanning electron microscopy observations confirm that all-ZB GaAs/(Ga,Mn)As core-shell nanowires with smooth side surface are obtained when the Mn concentration is not more than 2% and the growth temperature is 245 °C or below. Magnetic measurements with different applied field directions provide strong evidence for ferromagnetic ordering in the all-ZB GaAs/(Ga,Mn)As nanowires. The hybrid nanowires offer an attractive platform to explore spin transport and device concepts in fully epitaxial all-semiconductor nanospintronic structures.

An ultra-thin buffer layer for Ge epitaxial layers on Si

Using an Fe3Si insertion layer, we study epitaxial growth of Ge layers on a Si substrate by a low-temperature molecular beam epitaxy technique. When we insert only a 10-nm-thick Fe3Si layer in between Si and Ge, epitaxial Ge layers can be obtained on Si. The detailed structural characterizations reveal that a large lattice mismatch of ∼4% is completely relaxed in the Fe3Si layer. This means that the Fe3Si layers can become ultra-thin buffer layers for Ge on Si. This method will give a way to realize a universal buffer layer for Ge, GaAs, and related devices on a Si platform.

Molecular Beam Epitaxy of Co2MnSi Films on Group-IV Semiconductors

We explore epitaxial growth of Co2MnSi films on Si(111) or Ge(111) by means of low-temperature molecular beam epitaxy. We find that as-grown Co2MnSi films consist of mixed phases with L21-ordered structures and microcrystalline ones. As a result, the magnetic moment, which is nearly half of the ideal value, can be obtained even at very low growth temperature. Post-growth annealing was effective to crystallize the microcrystalline phases observed in the as-grown layer, leading to a further enhancement in the magnetic moment. We discuss a difference in growth mechanism between Co2MnSi and other Heusler alloys examined in our previous works.

Growth and Characterization of Heteroepitaxial Layers of GeSiSn Ternary Alloy

We have examined the epitaxial growth of Ge1-x-ySixSny layers on Ge substrates with a Sn content of 3~15% with low temperature molecular beam epitaxy method. The Ge1-x-ySixSny layers are psuedomorphically grown on Ge substrates with high crystalline quality. The surface morphology of the Ge1-x-ySixSny layers with a Sn content below 7% shows very flat and uniform, although surface roughening occurs in the sample with a Sn content as high as 15% provably due to the Sn precipitation. We also roughly estimated the energy band structure with liner approximation calculation. The energy bandgap of Ge1-x-ySixSny alloy prepared in this study is expected to be 0.8~1.0 eV at the L or X point.

Generation and Detection of a Pure Spin Current Using Co-Based Heusler-Alloy Spin Injector and Detector: Comparison of Co2MnSi and Co2FeSi

We have demonstrated the electrical generation and detection of giant pure spin currents using lateral spin-valve devices (LSVs) with highly-ordered Co2FeSi (CFS) electrodes, where the CFS films were grown on Si(111) by low-temperature molecular beam epitaxy (LT-MBE). In this paper, we study the performance of LSVs with Co2MnSi (CMS) electrodes. Unlike CFS, we cannot obtain high-quality CMS films on Si(111) even in the same LT-MBE condition. As a result, it is difficult for CMS grown on Si(111) to generate a large pure spin current in LSVs because of a relatively poor crystal quality with low spin polarization compared with CFS grown on Si(111).

Effect of an Atomically Matched Structure on Fermi-level Pinning at Metal/p-Ge Interfaces

Using low-temperature molecular beam epitaxy (LTMBE), we demonstrate high-quality heterointerfaces consisting of CoFe alloys and germanium (Ge) with a good atomic matching at the (111) plane. Electrical properties of the atomically matched CoFe/p-Ge(111) junctions with the area of (S) < ~10 μm2 clearly show the suppression of the Fermi-level pinning (FLP) effect, similar to the features of the atomically matched Fe3Si/p-Ge(111) junctions with S < ~10 μm2 in our previous work. This means that the suppression of FLP is independent on a kind of metals. As a result, we conclude that the formation of the atomically matched metal/Ge interfaces with almost no defects is important to suppress FLP.

Highly mismatched N-rich GaN1−xSbx films grown by low temperature molecular beam epitaxy

We have grown N-rich, dilute Sb GaN1−xSbx alloys by low temperature molecular beam epitaxy. At low growth temperature of &amp;lt;100 °C the material loses crystallinity and becomes primarily amorphous with small crystallites of 2–5 nm at a Sb composition of &amp;gt;4 at. %. Despite the different microstructures found for GaN1−xSbx alloys with different composition, the absorption edge shifts continuously from 3.4 eV (GaN) to close to 1 eV for samples with Sb content &amp;gt;30 at. %. GaN1−xSbx alloys with less than 5 at. % Sb show sufficient bandgap reduction (∼2 eV), making them suitable for photoelectrochemical applications.

Room-temperature electroluminescence from tensile strained double-heterojunction Germanium pin LEDs on Silicon substrates

In this work, electroluminescence from the intrinsic Germanium layer of tensile strained Germanium LEDs is observed at room temperature. The pin LEDs are fabricated by low temperature molecular beam epitaxy with a double-heterojunction process on Silicon substrates. The tensile strain is adjusted at 0.24% with an annealing step at 700°C leading to a lowering of the direct band gap. Electroluminescence spectra show a value of 0.781eV for the direct band gap or an infrared shift of 19meV. Increasing of the intrinsic layer thickness leads to higher electroluminescence intensity.

Effect of antimony nano-scale surface-structures on a GaSb/AlAsSb distributed Bragg reflector

Effects of antimony crystallization on the surface of GaSb during low temperature molecular beam epitaxy growth are investigated. The geometry of these structures is studied via transmission electron and atomic force microscopies, which show the surface metal forms triangular-shaped, elongated nano-wires with a structured orientation composed entirely of crystalline antimony. By depositing antimony on a GaSb/AlAsSb distributed Bragg reflector, the field is localized within the antimony layer. Polarization dependent transmission measurements are carried out on these nano-structures deposited on a GaSb/AlAsSb distributed Bragg reflector. It is shown that the antimony-based structures at the surface favor transmission of light polarized perpendicular to the wires.

Direct bandgap narrowing in Ge LED’s on Si substrates

In this paper we investigate the influence of n-type doping in Ge light emitting diodes on Si substrates on the room temperature emission spectrum. The layer structures are grown with a special low temperature molecular beam epitaxy process resulting in a slight tensile strain of 0.13%. The Ge LED's show a dominant direct bandgap emission with shrinking bandgap at the Γ point in dependence of n-type doping level. The emission shift (38 meV at 10²⁰cm⁻³) is mainly assigned to bandgap narrowing at high doping. The electroluminescence intensity increases with doping concentrations up to 3x10¹⁹cm⁻³ and decreases sharply at higher doping levels. The integrated direct gap emission intensity increases superlinear with electrical current density. Power exponents vary from about 2 at low doping densities up to 3.6 at 10²⁰cm⁻³ doping density.

Achieve high-quality GaN films on La0.3Sr1.7AlTaO6 (LSAT) substrates by low-temperature molecular beam epitaxy

The growth of c-plane GaN films on La0.3Sr1.7AlTaO6 (LSAT) (111) substrates has been carried out by molecular beam epitaxy (MBE) at various substrate temperatures. GaN grown at 750 °C shows poor crystallinity with an up to 8 nm thick interim layer between LSAT and GaN, mainly due to the severe interfacial reaction between the substrate and the film. On the contrary, an atomically abrupt interface has been observed when GaN was grown at a lower temperature of 500 °C. This low-temperature grown GaN exhibits high crystallinity with a very smooth surface and strong band-edge emission, which we attribute to the well-controlled oxygen atom evaporation from LSAT substrate surface thanks to the low temperature growth. On the one hand, it avoids the severe interfacial reaction that takes place in the case of high-temperature epitaxial growth; on the other hand, it conserves the crystalline structure of the substrate surface, and consequently takes good advantage of the small lattice mismatch between GaN and LSAT. Considering the advantages of LSAT over sapphire, and the device-ready film quality of GaN achieved on LSAT, this work exhibits a promising future for high-performance light emitting diodes on this unconventional substrate.

Microstructure of GaN1−x Bi x

In this paper we describe detailed transmission electron microscopy studies of GaN1−xBix with 0.05 < x < 0.18 grown by low-temperature molecular beam epitaxy under Ga-rich conditions. Microstructural transformation from columnar growth separated by thin amorphous areas in the films with lowest Bi content (5%) to pseudo-amorphous structure with crystalline grains embedded in the amorphous matrix in the samples with higher Bi content (13% to 18%) was observed. In addition, metallic Bi segregation occurred in the samples with the highest Bi concentration. An abrupt decrease in absorption edge energy is found in samples with higher Bi content.

Growth and characterization of n-type electron-induced ferromagnetic semiconductor (In,Fe)As

We show that by introducing isoelectronic iron (Fe) magnetic impurities and Beryllium (Be) double-donor atoms into InAs, it is possible to grow an n-type ferromagnetic semiconductor (FMS) with the ability to control ferromagnetism by both Fe and independent carrier doping by low-temperature molecular-beam epitaxy. We demonstrate that (In,Fe)As doped with electrons behaves as an n-type electron-induced FMS. This achievement opens the way to realize spin-devices such as spin light-emitting diodes or spin field-effect transistors, as well as helps to understand the mechanism of carrier-mediated ferromagnetism in FMSs.

GeSn p-i-n detectors integrated on Si with up to 4% Sn

GeSn heterojunction photodetectors on Si substrates were grown with Sn concentration up to 4%, fabricated for vertical light incidence, and characterized. The complete layer structure was grown by means of ultra low temperature (100 °C) molecular beam epitaxy. The Sn content shifts the responsivity into the infrared, about 310 nm for the 4% Sn sample. An increase of the optical responsivity for wavelengths higher than 1550 nm can be observed with increasing Sn content. At 1600 nm, the optical responsivity is increased by more than a factor of 10 for the GeSn diode with 4% Sn in comparison to the Ge reference diode.