Layer Superlattices Research Articles

Threading Dislocation Reduction of Ge by Introducing a SiGe/Ge Superlattice

The influence of introducing a SiGe / Ge superlattice (SL) between Ge layers and Si substrate for the sake of the reduction of the threading dislocation density (TDD) without additional annealing is investigated. In the case of 2.8 μm thick Ge directly grown on Si, the TDD at the surface is 7.6×10-8 cm-2. A slight TDD reduction is observed by introducing a Si0.2Ge0.8 / Ge SL between the Si substrate and the Ge layer. By inserting 5, 10 and 20 cycles of Si0.2Ge0.8 / Ge, the TDD is reduced to 7.1×10-8, 5.9×10,-8 and 5.3×10-8 cm-2, respectively. The lateral lattice parameters of these SLs are ~5.656Å, which is a smaller value compared to that of bulk Ge, indicating plastic relaxation by misfit dislocation (MD) formation. Further TDD reduction is realized with increasing Si concentration in the SiGe / Ge SL without changing the cycle of the SL. However, surface roughening due to pit formation occurs if the Si concentration in the SL is higher than 50% because of increased strain at the interfaces between SiGe and Ge. With increasing SiGe and Ge thickness ratio in the SL layer and maintaining periodicity and cycles, the TDD is reduced to 2.8×10-8 cm-2 without degrading the surface roughness. This improvement is related to a relaxation of the SiGe/Ge SL by plastic deformation.

Effectiveness of InGaAs/GaAs superlattice dislocation filter layers epitaxially grown on 200 mm Si wafers with and without Ge buffers

Low threading dislocation density (TDD) in GaAs epitaxial layers grown on silicon substrate (e.g. TDD < 106 cm−2) is critical for GaAs-based optoelectronic and high-performance electronic devices on silicon. Ge buffers and InxGa1-xAs/GaAs superlattice layers (SLs) are commonly used to reduce the TDD in GaAs epitaxial layers grown on Si wafers. In previous reports, TDD of ~107 cm−2 in GaAs and InGaP layers grown on 200 mm Si wafers was achieved by using Ge buffers only (e.g. Wang et al 2017 Semicond. Sci. Technol. 32 125013) and TDD of ~106 cm−2 in GaAs epi-layers for quantum dot lasers on Si was achieved by using SL insertion layers (e.g. Shang et al 2019 IEEE J. Sel. Top. Quantum Electron. 25 1502207). In this work, the effectiveness of the dislocation filtering effect of InxGa1-xAs/GaAs SLs combined with Ge buffers is investigated. The results are compared with wafers where InxGa1-xAs/GaAs SLs were directly grown on GaAs-on-Si buffers without Ge buffers. The InxGa1-xAs/GaAs dislocation filter layers (DFLs) consisted of 10 nm InxGa1-xAs/10 nm GaAs layers superlattice spaced by GaAs layer. Then, 1000 nm thick InGaP layers were grown on top of the DFLs to characterize the effect. Transmission electron microscopy, etch-pit density, x-ray diffraction, and photoluminescence were performed to characterize the quality of the materials. Our results indicate that for the growth with Ge buffers there was no significant difference in the final TDDs in the InGaP layers compared with a reference wafer that did not have DFL. For the growth without Ge buffers, the dislocation filtering effect was indeed observed, although it was not as significant as in other reports. For all wafers with DFLs, large wafer bow and dense film cracks appeared, which hinders the practical application of this method.

Superlattice hole injection layers for UV LEDs grown on SiC

AlGaN-based germicidal UV LEDs show promise in fighting the COVID-19 pandemic through disinfection of air, water, and surfaces. We report UV LEDs grown by MOCVD on SiC substrates, fabricated into thin-film flip chip devices. Replacing the uniform p-Al x Ga1-xN layer (x = 0.2) with a short-period-superlattice of alternating (x = 0.1 and 0.8) Al-composition improved EQE from 1.3% to 2.7% (3.2% with encapsulation) at 20 A/cm2. Peak EQE and WPE values of 4.8% and 2.8% (287 nm) were measured at current densities below 2 A/cm2, and maximum output power of 7.4 mW (76 mW/mm2) was achieved at 284 nm. Further WPE improvements are expected with both superlattice and uniform layer optimization, improved p-contact metallization, and active region optimization.

Topologically protected spin diffusion and spin generator using chalcogenide superlattices

Spintronics is expected to be the basis for future ultra-low-energy nanoelectronic devices. To operate such devices at room temperature, amplifiers, batteries, capacitors, as well as spin current sources are required. Here we report a chalcogenide superlattice composed of GeTe and Sb2Te3 layers that have a topologically protected spin diffusion length exceeding 100 μm at room temperature. A spin generator is demonstrated by combining magnetic injectors (TbFeCo) with this superlattice. The spin current was found to increase exponentially with the number of superlattice periods. We used this effect to demonstrate a 15-fold increase in the spin current. In addition, spin rectification is possible by growing the superlattice layers with atomic-level thickness accuracy. The reported chalcogenide superlattice spin generators and rectifiers open new opportunities to design low-energy spintronic integrated circuits and quantum computers.

Robust Bilayer Charge Pumping for Spin- and Density-Resolved Quantum Gas Microscopy.

Quantum gas microscopy has emerged as a powerful new way to probe quantum many-body systems at the microscopic level. However, layered or efficient spin-resolved readout methods have remained scarce as they impose strong demands on the specific atomic species and constrain the simulated lattice geometry and size. Here we present a novel high-fidelity bilayer readout, which can be used for full spin- and density-resolved quantum gas microscopy of two-dimensional systems with arbitrary geometry. Our technique makes use of an initial Stern-Gerlach splitting into adjacent layers of a highly stable vertical superlattice and subsequent charge pumping to separate the layers by 21 μm. This separation enables independent high-resolution images of each layer. We benchmark our method by spin- and density-resolving two-dimensional Fermi-Hubbard systems. Our technique furthermore enables the access to advanced entropy engineering schemes, spectroscopic methods, or the realization of tunable bilayer systems.

The growth and characteristics of In2Se3/(Bi1−xInx)2Se3 superlattices with asymmetric graded interfaces by molecular beam epitaxy

The growth of In2Se3/Bi2Se3 superlattices (SLs) by molecular beam epitaxy at an elevated temperature is explored. The crystalline phase structure of In2Se3 layers in the as-grown SLs is determined to be α-In2Se3. The diffusion of In from In2Se3 to Bi2Se3 is significantly promoted, while Bi diffusion into In2Se3 layers is insignificant as manifested by the in situ lattice evolution analysis, so that the achieved SL structure is of graded (Bi1−xInx)2Se3 solid-solution layers periodically separated by α-In2Se3 layers. The lattice vibration characteristics due to phonon confinement in the achieved SLs are also exhibited.

Effect of the Optical Pumping and Magnetic Field on the States of Phase Separation Domains in Eu0.8Ce0.2Mn2O5

The effect of optical pumping and applied magnetic field on the characteristics of ferromagnetic layers in one-dimensional superlattices is studied. At low enough temperatures, these layers correspond to phase separation domains in RMn_2O_5 and R_{0.8}Ce_{0.2}Mn_2O_5 multiferroics. The formation of such domains occurs owing to the charge ordering of Mn^{3+} and Mn^{4+} ions and to the finite probability for e_g electrons to tunnel between these pairs of ions. The volume occupied by such superlattices is rather small, and they can be treated as isolated ferromagnetic semiconductor heterostructures, spontaneously formed in the host crystal. The sequences of ferromagnetic resonances related to the superlattice layers in Eu_{0.8}Ce_{0.2}Mn_2O_5 are studied. The characteristics of these resonances give information on the properties of such layers. For the first time, it is demonstrated that the optical pumping gives rise to a new metastable state of superlattices, which can be recovered by the magnetic field cycling to the state existing before the optical pumping. It is found that the superlattices recovered by the magnetic field exist up to temperatures higher than those in as-grown crystals.

Imprinting Ferromagnetism and Superconductivity in Single Atomic Layers of Molecular Superlattices

Ferromagnetism and superconductivity are two antagonistic phenomena since ferromagnetic exchange fields tend to destroy singlet Cooper pairs. Reconciliation of these two competing phases has been achieved in vertically stacked heterostructures where these two orders are confined in different layers. However, controllable integration of these two phases in one atomic layer is a longstanding challenge. Here, an interlayer-space-confined chemical design (ICCD) is reported for the synthesis of dilute single-atom-doped TaS2 molecular superlattice, whereby ferromagnetism is observed in the superconducting TaS2 layers. The intercalation of 2H-TaS2 crystal with bulky organic ammonium molecule expands its van der Waals gap for single-atom doping via co-intercalated cobalt ions, resulting in the formation of quasi-monolayer Co-doped TaS2 superlattices. Isolated Co atoms are decorated in the basal plane of the TaS2 via substituting the Ta atom or anchoring at a hollow site, wherein the orbital-selected p-d hybridization between Co and neighboring Ta and S atoms induces local magnetic moments with strong ferromagnetic coupling. This ICCD approach can be applied to various metal ions, enabling the synthesis of a series of crystal-size TaS2 molecular superlattices.

Performance enhancement of AlGaN-based deep ultraviolet light-emitting diodes by using stepped and super-lattice n-type confinement layer

Abstract In this paper, a new n-type confinement layer that uses the stepped and super-lattice structure to replace conventional n-type AlGaN layer of deep ultraviolet light-emitting diode (DUV LED) is investigated. The simulation results indicate that the new structure significantly enhances the light output power (LOP) and internal quantum efficiency (IQE) of DUV LED. This is because the capability of carrier confinement in the quantum wells (QWs) is enhanced; the carrier concentrations and the radiative recombination rate in the active region of DUV LED are improved.

Effect of InGaN/GaN Superlattice on Lattice Curvature of GaN Layers Grown on Sapphire Substrates

The lattice curvature due to the difference in the thermal expansion coefficients between sapphire and GaN decreases the device performance and production yield. There is room to improve the lattice curvature and radius of curvature of GaN layers grown on sapphire substrates. Herein, an InGaN/GaN superlattice (SL) is grown on a low‐temperature GaN buffer layer through metalorganic vapor‐phase epitaxy to improve the lattice curvature. The lattice curvature of the GaN layer is investigated in terms of In content of the InGaN layer, the number of SL layers, cap layer, and GaN thickness. The radius of curvature increases with the increase in the number of SL layers and In content. The radius of curvature is 45 m when a 10‐period SL structure is used and the In content is 23.4%. When using a 20‐period SL structure, the GaN layer partially separates despite its very low thickness (2 μm). The improvement in the lattice curvature can be attributed to the thermal decomposition of InGaN during the annealing process from the growth temperature of InGaN to that of GaN.

Superlattice period dependence on nonradiative recombination centers in the n-AlGaN layer of UV-B region revealed by below-gap excitation light

Nonradiative recombination (NRR) centers in n-AlGaN layers of UV-B AlGaN samples with different numbers of superlattice (SL) periods (SLPs), grown on the c-plane sapphire substrate at 1150 °C by the metalorganic chemical vapor deposition technique, have been studied by using below-gap-excitation (BGE) light in photoluminescence (PL) spectroscopy at 30 K. The SLP affects the lattice relaxation of the SL and n-AlGaN layer. The PL intensity decreased by the superposition of BGE light of energies from 0.93 eV to 1.46 eV over the above-gap-excitation light of energy 4.66 eV, which has been explained by a two-level model based on the Shockley–Read–Hall statistics. The degree of PL quenching from n-AlGaN layers of the sample with SLP 100 is lower than those of other samples with SLP 50, 150, and 200. By a qualitative simulation with the dominant BGE energy of 1.27 eV, the density ratio of NRR centers in n-AlGaN layers of 50:100:150:200 SLP samples is obtained as 1.7:1.0:6.5:3.4. This result implies that the number of SLP changes lattice relaxation and determines the density of NRR centers in the n-AlGaN layer, which affects the performance of LEDs.

Near-Strain-Free GaN/AlGaN Narrow Line Width UV Light Emission with Very Stable Wavelength on Excitation Power by Using Superlattices

Because of the strong strain in nitrides, superlattice layers have been used to release the strain in the QW and reduce the quantum confined Stark effect. However, few reports discuss comprehensively the strain relaxation behavior and optical performance of a GaN/AlGaN single quantum well (QW) with inserted GaN/AlGaN superlattices (SLs). In this work, we examined a group of graded Al content GaN/AlxGa1–xN SL layers under the GaN/Al0.3Ga0.7N single QW grown on c-plane sapphire. Both the excitation power and temperature dependence of the time-integrated micro-photoluminescence (μ-PL) and time-resolved μ-PL were measured. The samples exhibited very narrow UV emission and had almost unchanged emission wavelength and stable line width behavior with excitation power as well as “S-shape” and weak “W-shape” characteristics with temperature due to the localization. The temperature-dependent PL lifetime was measured from 5 to 300 K, and the relatively fast recombination lifetime of the two samples was examined. Mic...

Effect of InGaN/GaN superlattice as underlayer on characteristics of AlGaN/GaN HEMT

The impact of an InGaN/GaN superlattice (SL) on AlGaN/GaN high electron mobility transistor characteristics was investigated, and two effects were discovered: one is a substantial improvement in the conduction characteristics as a result of the InGaN/GaN channel layer, while the other is the effect of diffusion suppression relating to impurities or point defects from the carbon-doped layer. The InGaN/GaN SL was used as a channel layer to improve the mobility and concentration of the two-dimensional channel electron gas. It was found that by inserting the InGaN/GaN SL just above a C-doped semi-insulating GaN layer as the InGaN underlayer, the conduction current of the SL with five periods (5SL) was observed to be much higher than that of the conventional material with a GaN channel layer of over 2 μm in thickness. The results demonstrated that this SL layer is effective in suppressing the diffusion of impurities or point defects originating from the carbon-doped layer, resulting in the device performance improvement.

A comparative study on GaSb epilayers grown on nominal and vicinal Si(100) substrates by molecular beam epitaxy

The direct growth of GaSb buffer layers on Si substrates is attracting considerable interest in the integration of group III-Sb based device structures on lower-cost Si substrates. Here, we present the effect of various growth steps on the defect types and defect density that are crucial for advancing high crystal quality GaSb buffer layer on nominal/vicinal Si substrate. As a growth step, the applied thermal annealing at an intermediate step provided a decrease in the threading dislocation (TD) density down to 1.72 × 108 cm−2, indicating a more effective method compared to post-growth annealing. Additionally, the importance of period number and position of GaSb/AlSb superlattice layers inserted in GaSb epilayers is demonstrated. In the case of the GaSb epilayers grown on vicinal substrates, the APB density as low as 0.06 µm−1 and TD density of 1.98 × 108 cm−2 were obtained for the sample grown on 4° miscut Si(100) substrate.

Approach to stacking superlattice layers points to high-performance spintronic devices

Alternating polar and nonpolar layers with electrons of different energy levels leads to structures with modifiable magnetic properties.

The Influence of Superlattice Structure on the Dynamic Buffer Response of AlInN/GaN-on-Si HEMTs

In this work, we investigate the dynamic buffer response of AlInN/GaN high electron mobility transistors (HEMTs) grown on silicon substrates. Significant improvements in vertical breakdown voltage, reduced buffer conduction and charge trapping effect are observed on the samples with a superlattice layer (SL) in the buffer. The dynamic response of the buffer layer largely depends on the composition, thickness and position of the SL in the stress-mitigating buffer. The vertical breakdown voltage increases consistently with decreasing edge-type dislocations when the density of screw-type dislocations is low. Negative back-gating measurements show the lowest buffer current hysteresis in the sample with the lowest edge-type dislocation density, indicating least charge trapping in the sample. Four terminal positive back-gating measurements in the on-state, further confirm the observed trend. Based on the experimental observations, we propose that the total density of threading dislocations, (acts as vertical leakage path) and the acceptor like edge-type dislocations should be accounted along with the electron injection from the silicon substrate to the buffer. Therefore, resulting in higher vertical breakdown voltage by alleviating serious dynamic buffer response in GaN-based HEMTs.

Двухканальный электронный транспорт в подвешенных квантовых точечных контактах с боковыми затворами

The conductance of a suspended quantum point contact fabricated from GaAs/AlGaAs heterostructures with a two-dimensional electron gas, equipped with the in-plane side gates separated from the constriction using lithographical trenches, is studied. The conductance as a function of the gate voltages demonstrates unusual double-channel regime with independent channel’s conductance quantization: two side gates can drive the conductance of the separate channels independently. A possible electrostatic mechanism of the double-channel structure formation inside a single constriction is connected with the lateral redistribution of the low-mobility X-valley electrons contained in superlattice layers, resulting in the emergence of the potential barrier in the middle of quantum point contact, separating the conducting electrons into two channels, symmetrically shifted towards the lithographical trenches, defining the nanostructure geometry.

Recombination pathways and hole leakage behavior in InGaN/GaN multiple quantum wells with V-shaped pits

Abstract Hole transport in InGaN/GaN multiple quantum wells (MQWs) LEDs has been proposed to be insufficient due to relatively large effective mass and low mobility. Here, for the first time, we have direct observed hole overflow from MQWs and enter into InGaN/GaN superlattices (SLs) layer in V-pits-containing GaN-based green LEDs at room temperature. To suppress the hole leakage, an n-AlGaN hole blocking layer (HBL) is inserted between SLs layer and MQWs. The experimental show an interesting and baffling result is that the HBL-containing sample not only exhibits severe hole leakage in high current density range, but also has higher external quantum efficiency (EQE). Numerical simulation reveals that the HBL can change the transportation path of carriers, forcing more electrons injecting from c-plane into the MQWs instead of from V-pit sidewalls. This will increase the carrier concentration in the MQWs, and eventually lead to higher EQE. Meanwhile, less electrons transported through V-pit allowed more holes injecting into deeper layers via V-pit sidewalls, and electroluminescence emission from the SLs layer became stronger when HBL was inserted.

A Numerical Study of the V/III Ratio in the MOCVD Process with Pulsed Injection Method

AlN layer grabs lots of attention as a great super lattice layer between Si or sapphire substrate and GaN-based material. V/III ratio in the AlN MOCVD process is regard as an important parameter which causes different AlN growth modes on the substrate surface. The growth mode changes from 3D nucleation to 2D growth while the V/III ratio decreases. On the other hand, the pulsed injection method is applied in the MOCVD process to depress parasitic reaction and improve the quality of the AlN film. However, the growth conditions at the wafer surface are strongly affected by the mass transport in the reactor chamber which depends on the flow pattern and gas phase reaction. The momentum, heat and mass transfers are complicated by the pulse action. Different types of pulse sequences and pulse times are reported for finding a better process condition. Until now, it still depends on the empirical data very much and induces enormous time consumption and financial cost in the chamber design process. A suitable numerical simulation is needed to be developed for overcoming this difficulty. In this study, a time-dependent numerical model is developed to analysis the flow motion and the heat and mass transfer caused by different pulse conditions. For the case with the TMAl gas mixture maintained a constant and a pulsed NH3 gas mixture, the V/III ratio at the surface of substrate increases with the process time after the pulse of NH3 mixture ejected into the chamber. It approaches to a constant when the time is greater than a certain value. The V/III ratio decreases to zero after the pulse of NH3 mixture ends. The V/III changes quickly in a little time when the pulse of NH3 mixture is turned on or off. When the NH3 and TMAl are pulsed into the chamber alternatively, the V/III ratio at the surface of the substrate keeps increasing after the pulse of NH3 mixture turned on. The V/III ratio decreases to zero after the pulse of NH3 mixture turned off. The V/III ratio does not stay at a certain value like the cases which has the constant supply of TMAl mixture. Based on the variation of V/III ratio at the surface of the substrate, we can predict the history of the growth mode at the surface of the substrate during the process.

Magnetoresistance of CoFeNi/Cu Superlattices Differing in the Ferromagnetic Alloy Composition

Abstract—The structure and magnetoresistive properties of Ta(5 nm)/NiFeCr(5 nm)/[FM(tFM)/Cu(2.2 nm)]n/ Ta(5 nm) superlattices prepared by magnetron sputtering are studied in this work. The ferromagnetic layer thickness tFM is varied in a 1.1–1.5 nm range. The Co90Fe10, Co85Fe12Ni3, Co77Fe17Ni6, and Co70Fe20Ni10 alloys with an fcc structure and close saturation magnetization values are used as the ferromagnetic (FM) materials. It is shown that, as the Ni and Fe contents in the CoFeNi alloy increase, the magnetoresistance of superlattices containing eight bilayers remains high and equal to 24–28%; in this case, the twofold decrease in the hysteresis occurs. As the number of bilayers in the superlattices, which contain CoFeNi alloys differing in composition increases to 12, the close values of magnetoresistance 30–31% are obtained. The correlation between the saturation magnetization of the ferromagnetic material of superlattice layers and maximum value of magnetoresistance is discussed.