Beam Flux Ratio Research Articles

Advancements in InAs/GaSb superlattice infrared detectors: Material innovations and optimization techniques

This paper presents a detailed study of the advancements in InAs/GaSb Superlattice (SL) infrared detectors, highlighting the significant progress made in material development and optimization techniques. The research begins with an overview of the historical developments, particularly the emergence of Tape InAs/GaSb binary II superlattice, which has revolutionized Third-Generation Infrared Focal Plane Arrays. A key focus is the optimization of fabrication processes, including temperature control and the V/III beam flux ratio, which are critical for enhancing the quality and efficiency of SL infrared detectors. The paper also explores the selection of superlattice interfaces, crucial for reducing strain and improving infrared absorption. Looking ahead, the study outlines future directions for SL infrared detectors, such as the exploration of new materials, incorporation of quantum well technology, and advancements in miniaturization and integration techniques. These developments aim to increase the stability, sensitivity, and overall performance of infrared detectors.

MBE growth of SnTe films on GaAs substrates with ZnTe buffer layers

SnTe (100) single-domain thin films have been previously grown on GaAs substrates by molecular beam epitaxy but obtaining excellent crystallinity can be difficult. In this paper, ZnTe buffer layers are introduced to improve the crystallinity of SnTe films. The effects of the molecular beam flux ratio (JTe/JSn) and growth temperature (Tg) on the crystallinity are investigated to achieve high-quality layers. The surface morphology and electrical properties of the SnTe thin films are also evaluated in addition to the conventional X-ray diffraction (XRD)measurements. The crystallographic properties of the grown samples are shown to be significantly improved by introducing a ZnTe buffer layer. XRD (θ–2θ) full width at half maximum of the lowest value obtained so far is about 0.14°. It is also found that lowering JTe/JSn to 0.6 results in a reduced growth rate, and an increased growth temperature (Tg = 220 °C) promotes the migration of atoms at the growth front. Both effects contribute to improvement in the crystallographic quality, including the layer surface smoothness. The cross-sectional transmission electron microscopy observation of the sample indicates the abrupt interface formation with different lattice structures. It is also found that low growth temperatures (below 200 °C) are not favorable for the SnTe layer growth, which is confirmed by the surface morphology.

A comparison of indium arsenide antimonide and mercury cadmium telluride as long wavelength infrared detector materials

The basic material parameters that govern the performance of bulk alloys for long wavelength infrared detection have been calculated for indium arsenide antimonide and mercury cadmium telluride. The numbers show that similar performance could be expected from the two materials provided they can be synthesized at similar degrees of perfection. In order to approach parity, the quality of InAsSb must be optimized, which requires careful variations of molecular beam epitaxy (MBE) growth parameters, namely, substrate temperature and flux magnitudes and ratios. Molecular dynamics-based simulations of MBE growth offer a means to optimize these parameters by providing constants for first-principles or empirical growth models or by directly relating the qualities of the simulation-grown crystals to the MBE parameters.

Molecular beam epitaxy of InAlN alloys in the whole compositional range

The fabrication of high-quality InxAl1−xN alloys over the whole composition is very challenging. Controlling the In/(In + Al) beam flux ratio and the growth temperature, this paper reports the fabrication of single crystalline InxAl1−xN alloys over the whole In composition by radio-frequency plasma-assisted molecular beam epitaxy. A comprehensive systematic study on the structural and vibrational properties of the InxAl1−xN alloys for the whole In composition has been carried out experimentally by Raman spectroscopy and theoretically by the forced vibrational method. The InxAl1−xN alloys show broad Raman peaks in the intermediate range of In composition. The appearance of Raman inactive B1 (High) mode has been confirmed by the experimental and theoretical results. The B1 (High) and A1 (LO) modes show one-mode behavior, whereas the E2 (High) mode shows the two-mode behavior in the whole In composition range. The observed Raman modes from the grown InxAl1−xN alloys well match with the calculated phonon modes of the InxAl1−xN alloys. These results provide an in-depth understanding of the growth of whole compositional InxAl1−xN alloys and the fabrication of optoelectronic devices using these promising materials.

Optimization of the MBE growth of metastable zinc blende MnS on GaAs (1 0 0) substrates using ZnS as sulphur source

MnS has been grown by molecular beam epitaxy in the metastable zinc blende (ZB) crystal structure on GaAs (1 0 0) substrates using ZnS as a sulphur source. MnS layers were grown over a wide range of growth conditions by varying the growth temperature from 220 °C to 300 °C and ZnS:Mn flux ratio from 5 to 20. Double-crystal X-ray rocking curves were used to determine the crystallinity and residual zinc content in the layers. This allowed a phase diagram to be produced for ZB MnS giving zinc incorporation as function of the growth temperature and the ZnS:Mn beam flux ratio. In turn, this allowed an optimum growth region to be identified at a growth temperature 240 °C and ZnS:Mn flux ratio 6 where ZB MnS with less than 1% residual zinc incorporation were obtained. Under these optimum growth conditions the maximum ZB MnS layer thickness obtained was double that obtained in previous studies, with layers up to 250 nm thick produced.

Growth and characterisation of MnSb(0 0 0 1)/InGaAs(1 1 1)A epitaxial films

MnSb layers have been grown on InxGa1-xAs(1 1 1) A virtual substrates using molecular beam epitaxy (MBE). The effects of both substrate temperature (Tsub) and Sb/Mn beam flux ratio (JSb/Mn) were investigated. The surface morphology, layer and interface structural quality, and magnetic properties have been studied for a 3 × 3 grid of Tsub and JSb/Mn values. Compared to known optimal MBE conditions for MnSb/GaAs(1 1 1) [Tsub = 415°C,JSb/Mn = 6.5], a lower substrate temperature is required for sharp interface formation when growing MnSb on In0.48Ga0.52As(1 1 1) A [Tsub = 350°C,JSb/Mn = 6.5]. At high flux ratio (JSb/Mn = 9.5) elemental Sb is readily incorporated into MnSb films. At higher substrate temperatures and lower flux ratios, (In,Ga) Sb inclusions in the MnSb are formed, as well as MnAs inclusions within the substrate. The Sb and (In,Ga) Sb inclusions are epitaxial, while MnAs inclusions are endotaxial, i.e. all have a crytallographic relationship to the substrate and epilayer. MBE optimisation towards different device structures is discussed along with results from a two-stage growth scheme.

Control of Beam Energy and Flux Ratio in an Ion-Beam-Background Plasma System Produced in a Double Plasma Device**supported by National Natural Science Foundation of China (Nos. 11575183, 11175177)

Plasmas containing ion beams have various applications both in plasma technology and in fundamental research. The ion beam energy and flux are the two factors characterizing the beam properties. Previous studies have not achieved the independent adjustment of these two parameters. In this paper, an ion-beam-background-plasma system was produced with hot-cathode discharge in a double plasma device separated by two adjacent grids, with which the beam energy and flux ratio (the ratio between the beam flux and total ion flux) can be controlled independently. It is shown that the discharge voltage (i.e., voltage across the hot-cathode and anode) and the voltage drop between the two separation grids can be used to effectively control the beam energy while the flux ratio is not affected by these voltages. The flux ratio depends sensitively on hot-filaments heating current whose influence on the beam energy is relatively weak, and thus enabling approximate control of the flux ratio

Residual stress in AlN films grown on sapphire substrates by molecular beam epitaxy

Residual stress in AlN films grown by molecular beam epitaxy (MBE) has been studied by Raman scattering spectroscopy. A strain-free Raman frequency and a biaxial stress coefficient for E2(high) mode are experimentally determined to be 657.8 ± 0.3 cm−1 and 2.4 ± 0.2 cm−1/GPa, respectively. By using these parameters, the residual stress of a series of AlN layers grown under different buffer layer conditions has been investigated. The residual compressive stress is found to be obviously decreased by increasing the Al/N beam flux ratio of the buffer layer, indicating the generation of tensile stress due to stronger coalescence of AlN grains, as also confirmed by the in-situ reflection high energy electron diffraction (RHEED) monitoring observation. The stronger coalescence does lead to improved quality of AlN films as expected.

Site-controlled growth of GaAs nanoislands on pre-patterned silicon substrates

GaAs islands have been grown by molecular beam epitaxy on Si (100) substrates patterned with nanoscale holes using only few monolayer deposition thicknesses. The site-controlled growth has been achieved without masking the Si surface with SiO2 between the patterned holes. The nucleation density of GaAs islands between the patterned holes shows a strong dependence on the growth temperature, V/III beam flux ratio and nominal deposition thickness. Through the selection of optimal growth parameters GaAs islands have been grown highly selectively in the patterned holes over a wide range of hole diameters for fixed spacing as well as over a wide range of spacings for fixed diameter of the holes. The influence of pattern features (hole diameter, spacing) on the morphological properties of the site-controlled GaAs islands has been investigated in detail. A (5 × 5) μm2 AFM image of GaAs nanoislands selectively grown in patterned holes with 70 nm diameter and a spacing of 0.5 μm on Si substrate.

Growth of thick InGaN films with entire alloy composition using droplet elimination by radical-beam irradiation

Droplet elimination by radical-beam irradiation (DERI), which is suitable for the growth of InN and periodic InN/InGaN structures, was developed for the growth of thick InGaN films by radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE). In this method, Ga-to-nitrogen radical (N) beam flux ratio was fixed in the entire growth process, and In beam supply was modulated under the condition in which excess In of more than 2–3 monolayer wetting layers always existed on the surface, that is, no precise control of In beam flux during growth was needed. Thick and uniform InGaN films were successfully grown by the developed method. In addition, InGaN films with an entire alloy composition could be simply grown by controlling Ga-to-N beam flux ratio.

Ga-assisted MBE growth of GaAs nanowires using thin HSQ layer

We present detailed results about the molecular beam epitaxy (MBE) growth of GaAs nanowires (NWs) on GaAs (111)B substrates prepared for the growth by a new method using hydrogen silsesquioxane (HSQ). Before the growth, HSQ is converted to SiOx by thermal treatment. The NWs are grown via the vapor–liquid–solid (VLS) mechanism. The influence of five growth parameters are described: SiOx thickness, growth time, substrate temperature and Ga and As4 beam fluxes. It is shown that the nanowire density can be tuned by two orders of magnitude by adjusting the SiOx thickness. Additionally, the results demonstrate that the axial growth is controlled by the As4 beam flux whereas the lateral growth is controlled by the Ga beam flux. The observed NW tapering is mainly determined by the V/III beam flux ratio. Our study gives important information about the VLS growth mechanism, which is extended by considering the secondary adsorption process of Ga adatoms. The nanowires have predominantly zinc blende crystal structure with rotational twins. A wurtzite segment is always found at the top of the NWs being associated with the growth after the Ga shutter has been closed.

Observation of regular defects formed on the surface of PbTe thin films grown by molecular beam epitaxy

Regular shape defects on the surface of PbTe thin films grown by molecular beam epitaxy (MBE) were studied by scanning electron microscope (SEM). Two types of regular shape defects were observed on Te-rich PbTe films grown at substrate temperature T ≥ 235 °C with a beam flux ratio of Te to PbTe ( R f) to be 0.5 and at 280 °C with a R f ≥ 0.4, which include cuboids and triangular pyramids. The formation mechanism of the observed regular shape defects is interpreted as following: They are the outcome of fast growth rate along {1 0 0} crystal planes that have the lowest surface energy and the enclosure of the {1 0 0} crystal planes. The formation of the regular shape defects in the growth of PbTe needs appropriate substrate temperature and Te-rich ambience. However, when R f is decreased low enough to make the films slightly Pb-rich, triangular pits that originate from the insufficient glide of the threading dislocations along the main 〈1 1 0〉 {1 0 0} glide system of PbTe in Cottrell atmosphere, will be the main feature on the film surface.

Nucleation of islands and continuous high-quality In2O3(001) films during plasma-assisted molecular beam epitaxy on Y-stabilized ZrO2(001)

The surface roughness on different length scales of In2O3(001) films is detrimental to potential semiconductor applications of this material. One type of surface roughness consists of square shaped islands with edge lengths on the micron scale. This islanding was observed in In2O3(001) films grown on Y-stabilized ZrO2(001), YSZ(001), by plasma-assisted molecular beam epitaxy under oxygen-rich and under In-rich growth conditions. Our reflection high-energy electron diffraction observations during the initial phase of the growth showed that the islanding is caused by a nucleation of the In2O3 with incomplete wetting of the substrate and not by a strain-induced breakup of the film. By systematic variation in the growth parameters Indium-to-oxygen beam flux ratio (VI/III ratio) and substrate temperature we found that the wetting of YSZ(001) by the In2O3(001) film is thermodynamically not preferred and that kinetic limitations, such as low substrate temperatures or (to a lesser extent) high VI/III ratios, were able to force nucleation of continuous films. A growth diagram was constructed that shows the parameter regions with forced or inhibited wetting. The nucleation with inhibited wetting can be applied to the formation of square shaped In2O3(001) islands whose size increases and areal density decreases with decreasing VI/III ratio. To realize continuous, smooth In2O3(001) films under In-rich conditions, a thin nucleation layer was grown at lower substrate temperature to completely wet the substrate. The bulk of the film was grown on top of this nucleation layer at higher temperature to ensure a high crystalline quality.

Effect of nitrogen plasma power on defect levels in Ni/n-GaN Schottky diodes grown by molecular beam epitaxy

The incorporation of deep level defects in n-type GaN grown by plasma assisted molecular beam epitaxy was studied via systematic adjustment of the nitrogen plasma source power from 150 to 400 W while maintaining a constant V/III beam flux ratio. Deep level optical spectroscopy and conventional thermal deep level transient spectroscopy measurements, which together enable deep level detection throughout the GaN band gap, revealed several deep level concentrations that depend strongly on rf-plasma power. The concentrations of the gallium vacancy deep level at EC-2.60 eV and carbon-related point defects with energy levels at EC-3.28 and EC-1.35 eV are found to be very sensitive to the nitrogen source power, increasing by up to 50 times for a corresponding increase in plasma power from 150 to 400 W. The relation between the concentrations of these traps and plasma power follows an Arrhenius-type behavior and is suggestive of plasma damage associated with the energetics of the constituent active nitrogen species. In contrast, two traps at EC-0.86 and EC-0.59 eV did not exhibit a systematic dependence on plasma power, with this difference a result of the dislocation-related nature of these defects.

Molecular beam epitaxial growth of ferromagnetic Heusler alloys for group-IV semiconductor spintronic devices

Our recent progress in low-temperature molecular beam epitaxy of ferromagnetic Heusler alloys on group-IV-semiconductor is reviewed. By optimizing beam flux ratio (Fe:Si = 3:1) and growth temperature (130 °C), a high-quality hybrid structure, i.e., DO 3-type Fe 3Si on Ge with an atomically flat interface, was achieved. Excellent magnetic properties with a small coercivity (0.9 Oe) and electrical properties with Schottky barrier height of 0.52 eV were obtained. The ratio of the on-current to the off-current of Schottky diode was on the order of 10 4. In addition, heteroepitaxy of half-metallic alloys (Fe 3 − X Mn x Si( X = 0.6–1.4)) on Ge substrates was demonstrated. These results will be a powerful tool to open up group-IV-semiconductor spin-transistors, consisting of Ge channel with high mobility and ferromagnetic source/drain for spin-injection.

Electrical properties of PbTe doped with BaF2

We study here the p-type doping of PbTe with BaF2. For the investigation, PbTe layers were grown on (111) BaF2 substrates by molecular beam epitaxy. The beam flux ratio between BaF2 and PbTe, defined as the nominal doping level, was varied from 0.02% to 1%. The hole density increases from 5×1017 to 1×1019 cm−3 as the doping level rises from 0.02% to 0.4% and saturates at p∼1019 cm−3 for higher levels. The saturation effect was attributed to self-compensation. The carrier concentration of all samples remained almost constant as the temperature was varied from 10 to 350 K, indicating that no thermal activation is present in the whole doping range. It suggests that the impurity level in PbTe doped with BaF2 remains resonant with the valence band, similar to the native defects behavior. The low-temperature mobility showed a pronounced reduction from 50 000 to 2 000 cm2/V s as the doping level rises from 0.02% to 1%, mainly due to the substantial increase in the hole concentration. For temperatures higher than 80 K, the mobility was essentially limited by phonon scattering. Our results demonstrate that a controlled p-type doping of PbTe with BaF2 can be obtained up to 1019 cm−3.

Observation of triangle pits in PbSe grown by molecular beam epitaxy

PbSe thin films on BaF 2 (1 1 1) were grown by molecular beam epitaxy with different selenium beam flux. Evolution of PbSe surface morphologies with Se/PbSe beam flux ratio ( R f) has been studied by atomic force microscopy and high-resolution X-ray diffraction. Growth spirals with monolayer steps on PbSe surface are obtained using high beam flux ratio, R f ≥ 0.6. As R f decreases to 0.3, nano-scale triangle pits are formed on the surface and the surface of PbSe film changes to 3D islands when R f = 0. Glide of threading dislocations in 〈1 1 0〉{1 0 0}-glide system and Pb-rich atom agglomerations are the formation mechanism of spiral steps and triangle pits. The nano-scale triangle pits formed on PbSe surface may render potential applications in nano technology.

Preparation of ferromagnetic (In,Mn)As with a high Curie temperature of 90K

The authors found high Curie temperatures of up to 90K in annealed (In,Mn)As epilayers grown by molecular beam epitaxy using a relatively high V/III beam flux ratio. Magnetization data suggest that increasing the As beam flux results in an increase in the effective Mn content, and the carrier transport data indicate an increase in the hole concentration from a 1019to1020cm−3 range with low-temperature annealing after growth. The highest Curie temperature was obtained in lattice matched (In,Mn)As∕Al(As,Sb) samples. Both Hall effect and magneto-optical polar Kerr rotation measurements confirm that the high Curie temperature is due to hole-mediated ferromagnetism.

Low-resistance p-type polycrystalline GaSb grown by molecular beam epitaxy

We report the growth and electrical properties of low resistance carbon-doped polycrystalline GaSb (poly-GaSb) by molecular beam epitaxy using CBr 4. The resistivity of poly-GaSb is found to have strong dependence on film thickness and grain size of the polycrystalline film, particularly when the film thickness is comparable with the grain size. It is observed that grain size is mainly determined by growth temperature, while hole concentration is significantly affected by antimony to gallium beam flux ratio. With same doping level, grain size, and similar film thickness, the resistivity of carbon-doped poly-GaSb is more than one order of magnitude lower than that of carbon-doped poly-GaAs. This result is attributed to GaSb's favorable surface Fermi-level pinning in the valence band and higher hole mobility. It is proposed that carbon-doped poly-GaSb be used as extrinsic base material in InP heterojunction bipolar transistor to improve the device's high-frequency performance.

MBE Growth and Characterization of Hexagonal ZnCdSe Layers on GaAs(111)-A and -B Substrates

Zn 0.5 Cd 0.5 Se epilayers were grown on GaAs(111)-A and -B substrates by MBE. We investigated how the polarity of the substrate affects both the epilayer crystalline structure and quality. It was found that the epilayers consisted of hexagonal and cubic phases independent of the substrate polarity. The hexagonal phase content in the epilayers tended to be minimal at a growth temperature around 300°C; it increased with decreasing growth temperature in the range of 200-300°C and increased again above 300°C. The hexagonal phase content in the epilayers grown on the (111)-B face was relatively high compared with that on the (111)-A face. However, the crystalline quality of the epilayers grown on the (111)-A face was much better. Furthermore, the hexagonal phase content increased for both (111)-A and -B faces with increasing VI/II beam flux ratio. In that case, the epilayer quality was improved on (111)-A face, though it tended to become poor on (111)-B face.