Lattice-matched Heterostructures Research Articles

Nanostructured AlGaAsSb Materials for Thermophotovoltaic Solar Cells Applications.

Thermophotovoltaic conversion using heat to generate electricity in photovoltaic cells based on the detraction of thermal radiation suffers from many engineering challenges. The focus of this paper is to study the nanostructure of AlGaAsSb for thermophotovoltaic energy conversion using lattice-matched heterostructures of GaSb-based materials in order to overcome the current challenges. The XAFS spectroscopy technique was used to analyze electronic structures and optical properties of GaSb, (Al, In) GaSbAs. The XAFS spectroscopy analysis showed a powerful decay at peak intensity that reveals to be related to a loss in Sb amount and light As atoms replaced in Sb atoms by 25%. Moreover, it was found that Al/In doped samples have highly symmetric data features (same atomic species substitution). The narrow direct bandgap energy, Eg of Al0.125Ga0.875Sb0.75As0.25 material raised (0.4–0.6 eV) compared to conventional photovoltaic cell bandgap energy (which is generally less than 0.4 eV) with weak absorption coefficients. The thermoelectric properties of AlGaAsSb computed via Botlztrap code showed that the electrons made up the majority of the charge carriers in AlGaAsSb. This nanostructure material exhibited a higher and acceptable figure of merit and demonstrated a promising thermoelectric material for solar thermophotovoltaic applications.

Layer-resolved release of epitaxial layers in III-V heterostructure via a buffer-free mechanical separation technique.

Layer-release techniques for producing freestanding III-V epitaxial layers have been actively developed for heterointegration of single-crystalline compound semiconductors with Si platforms. However, for the release of target epitaxial layers from III-V heterostructures, it is required to embed a mechanically or chemically weak sacrificial buffer beneath the target layers. This requirement severely limits the scope of processable materials and their epi-structures and makes the growth and layer-release process complicated. Here, we report that epitaxial layers in commonly used III-V heterostructures can be precisely released with an atomic-scale surface flatness via a buffer-free separation technique. This result shows that heteroepitaxial interfaces of a normal lattice-matched III-V heterostructure can be mechanically separated without a sacrificial buffer and the target interface for separation can be selectively determined by adjusting process conditions. This technique of selective release of epitaxial layers in III-V heterostructures will provide high fabrication flexibility in compound semiconductor technology.

Lattice-matched III-nitride structures comprising BAlN, BGaN, and AlGaN for ultraviolet applications

The optical properties of BAlN, BGaN and AlGaN ternary alloys are investigated using hybrid density functional for the design of lattice-matched optical structures in the ultraviolet spectrum. The calculated AlGaN properties agree well with previous reports, validating the model. A peculiar non-monotonic behavior of the refractive index as a function of the boron composition is found. The results of this calculation are interpolated to generate a three-dimensional dataset, which can be employed for designing a countless number of lattice-matched and –mismatched heterostructures. These heterostructures could span a range of operating wavelengths well into the deep ultraviolet with refractive indices ranging from 1.98 to 2.41 for AlN at 0 eV and GaN near the GaN bandgap, respectively. An example is shown where a lattice-matched heterostructure, AlN/B0.108Ga0.892N, is applied for DBR applications with a large index difference. A DBR comprising the AlN/B0.108Ga0.892N heterostructure at the UV wavelength of 375 nm is found to exceed 93% peak reflectivity with only 10 pairs and reaches 100% reflectivity with 35 pairs. For a chosen design with 25 pairs, the DBR has a peak reflectivity of 99.8% and a bandwidth of 26 nm fulfilling the requirements of most devices especially ultraviolet vertical-cavity surface emitting lasers.

Numerically efficient density-matrix technique for modeling electronic transport in mid-infrared quantum cascade lasers

We present a numerically efficient density-matrix model applicable to mid-infrared quantum cascade lasers. The model is based on a Markovian master equation for the density matrix that includes in-plane dynamics, preserves positivity of the density matrix and does not rely on phenomenologically introduced dephasing times. Nonparabolicity in the band structure is accounted for with a three-band $${\bf k}\cdot {\bf p}$$ model, which includes the conduction, light-hole, and spin-orbit split-off bands. We compare the model to experimental results for QCLs based on lattice-matched as well as strain-balanced InGaAs/InAlAs heterostructures grown on InP. We find that our density-matrix model is in quantitative agreement with experiment up to threshold and is capable of reproducing results obtained using the more computationally expensive nonequilibrium Green’s function formalism. We compare our density-matrix model to a semiclassical model where off-diagonal elements of the density matrix are ignored. We find that the semiclassical model overestimates the threshold current density by 29% for a 8.5- $$\upmu $$ m-QCL based on a lattice-matched heterostructure and 40% for a 4.6- $$\upmu $$ m-QCL based on a strain-balanced heterostructure, demonstrating the need to include off-diagonal density-matrix elements for accurate description of mid-infrared QCLs.

On the Fabrication and Study of Lattice-Matched Heterostructures for Quantum Cascade Lasers

The fabrication and study of the characteristics of a lattice-matched quantum cascade laser structure on an indium-phosphide substrate, designed for a wavelength of ~4.8 μm corresponding to one of the atmospheric windows are described. The heterostructure grown by molecular-beam epitaxy consisted of thirty cascades. Lasing was experimentally observed at temperatures up to 200 K at a wavelength coinciding with the calculated one, which confirms the high heterointerface quality and high precision of the layer thicknesses and active-region doping levels.

Engineering quantum spin Hall insulators by strained-layer heterostructures

Quantum spin Hall insulators (QSHIs), also known as two-dimensional topological insulators, have emerged as an unconventional class of quantum states with insulating bulk and conducting edges originating from nontrivial inverted band structures and have been proposed as a platform for exploring spintronics applications and exotic quasiparticles related to the spin-helical edge modes. Despite theoretical proposals for various materials, however, experimental demonstrations of QSHIs have so far been limited to two systems—HgTe/CdTe and InAs/GaSb—both of which are lattice-matched semiconductor heterostructures. Here, we report transport measurements in yet another realization of a band-inverted heterostructure as a QSHI candidate—InAs/InxGa1−xSb with lattice mismatch. We show that the compressive strain in the InxGa1−xSb layer enhances the band overlap and energy gap. Consequently, high bulk resistivity, two orders of magnitude higher than for InAs/GaSb, is obtained deep in the band-inverted regime. The strain also enhances bulk Rashba spin-orbit splitting, leading to an unusual situation where the Fermi level crosses only one spin branch for electronlike and holelike bands over a wide density range. These properties make this system a promising platform for robust QSHIs with unique spin properties and demonstrate the strain to be an important ingredient for tuning spin-orbit interaction.

Inline electron holography and VEELS for the measurement of strain in ternary and quaternary (In,Al,Ga)N alloyed thin films and its effect on bandgap energy.

We present the use of (1) dark-field inline electron holography for measuring the structural strain, and indirectly obtaining the composition, in a wurtzite, 4-nm-thick InAlGaN epilayer on a AlN/GaN/AlN/GaN multinano-layer heterosystem, and (2) valence electron energy-loss spectroscopy to study the bandgap value of five different, also hexagonal, 20-50-nm-thick InAlGaN layers. The measured strain values were almost identical to the ones obtained by other techniques for similarly grown materials. We found that the biaxial strain in the III-N alloys lowers the bandgap energy as compared to the value calculated with different known expressions and bowing parameters for unstrained layers. By contrast, calculated and experimental values agreed in the case of lattice-matched (almost unstrained) heterostructures.

Importance of growth temperature on achieving lattice-matched and strained InAlN/GaN heterostructure by plasma-assisted molecular beam epitaxy

We investigate the role of growth temperature on the optimization of lattice-matched In0.17Al0.83N/GaN heterostructure and its structural evolutions along with electrical transport studies. The indium content gradually reduces with the increase of growth temperature and approaches lattice-matched with GaN having very smooth and high structural quality at 450ºC. The InAlN layers grown at high growth temperature (480ºC) retain very low Indium content of ∼ 4 % in which cracks are mushroomed due to tensile strain while above lattice matched (>17%) layers maintain crack-free compressive strain nature. The near lattice-matched heterostructure demonstrate a strong carrier confinement with very high two-dimensional sheet carrier density of ∼2.9 × 1013 cm−2 with the sheet resistance of ∼450 Ω/□ at room temperature as due to the manifestation of spontaneous polarization charge differences between InAlN and GaN layers.

Analysis of interface trap states in InAlN/AlN/GaN heterostructures

Gate-source frequency-dependent capacitance and conductance measurements were performed on the In0.17Al0.83N/AlN/GaN heterostructure field-effect transistor with side-Ohmic contacts to study the characteristics of trap states at the interface between InAlN and GaN. The values of interface trap state density and time constant were determined to be (0.96–3.36) × 1013 cm−2 eV−1 and (0.29–1.61) μs, respectively. We calculated the strain in the InAlN barrier layer under the gate and found that the InAlN barrier layer was compressively strained with the in-plane strain of 1.31%. This is a possible reason for such a high interface trap state density which is not as low as supposed in the lattice-matched heterostructures.

Gate length scaling effect on high-electron mobility transistors devices using AlGaN/GaN and AlInN/AlN/GaN heterostructures.

Compared to AlGaN/GaN HEMT with 0.15 μm T-gate length, the AlInN/AlN/GaN one exhibits much higher current density and transconductance of 1558 mA/mm at Vd = 2 V and 330 mS/mm, respectively. The high extrinsic ft and fmax of 82 GHz and 70 GHz are extracted from AlInN/AlN/GaN HEMT. Besides, we find that the transconductance roll-off is significant in AlGaN/GaN, but largely improved in AlInN/AlN/GaN HEMT, suggesting that the high carrier density and lattice-matched epitaxial heterostructure is important to reach both large RF output power and high operation frequency, especially for an aggressively gate length scaling.

Design of multijunction GaPNAs/Si heterostructure solar cells by computer simulation

The designs of two- and three-junction solar cells based on GaPNAs/Si lattice-matched hetero-structures are calculated. It is shown that the efficiency of two-junction solar cells constituted by a junction based on a GaPNAs solid solution with a band gap E g of 1.78 eV and a junction based on Si may reach a value of 30.3% under AM1.5 D, 100 mW/cm2, and 35.4% under AM1.5D, 20 W/cm2. The maximum values of the efficiency of the three-junction solar cell constituted by top and middle junctions based on GaPNAs with E g of 2 and 1.5 eV, respectively, and a Si-based bottom junction are 39.2% under AM1.5 D, 100 mW/cm2, and 44.5% under AM1.5D, 20 W/cm2. It is shown that the thickness and minority carrier lifetime of the photoactive layers affect the efficiency of solar-light conversion by the heterostructures being developed.

Electronic Structural Moiré Pattern Effects on MoS2/MoSe2 2D Heterostructures

The structural and electronic properties of MoS2/MoSe2 bilayers are calculated using first-principles methods. It is found that the interlayer van der Waals interaction is not strong enough to form a lattice-matched coherent heterostructure. Instead, a nanometer-scale Moiré pattern structure will be formed. By analyzing the electronic structures of different stacking configurations, we predict that the valence-band maximum (VBM) state will come from the Γ point due to interlayer electronic coupling. This is confirmed by a direct calculation of a Moiré pattern supercell containing 6630 atoms using the linear scaling three-dimensional fragment method. The VBM state is found to be strongly localized, while the conduction band minimum (CBM) state is only weakly localized, and it comes from the MoS2 layer at the K point. We predict such wave function localization can be a general feature for many two-dimensional (2D) van der Waals heterostructures and can have major impacts on the carrier mobility and other electronic and optical properties.

Magnetization Studies of Antiferromagnetic Interlayer Coupling in EuS-SrS Semiconductor Multilayers

Temperature and magnetic eld dependence of magnetization of EuS SrS multilayers grown epitaxially on KCl (001) substrate is experimentally studied by superconducting magnetometry technique. In these lattice-matched semiconductor heterostructures EuS layers are ferromagnetic quantum wells whereas SrS layers are nonmagnetic spacer barriers. The multilayers composed of EuS layers with thickness 3.5 5 nm and SrS layers (thickness 0.5 10 nm) exhibit ferromagnetic transition at 17 K. In the multilayers with ultrathin SrS spacers (0.5 1 nm) a nonmonotonic temperature dependence of magnetization as well as a characteristic switching in magnetic hysteresis loops is observed. These experimental ndings are explained considering antiferromagnetic interlayer coupling between ferromagnetic EuS layers via nonmagnetic SrS spacers. The strength of this coupling is determined based on model magnetization calculations.

The SiGeSn approach towards Si-based lasers

Approaches using lattice-matched heterostructures consisting of group-IV elements – Si, Ge, Sn and their alloys, are proposed for the development of Si-based lasers that can be monolithically integrated on Si substrate. All proposed laser structures can be deposited strain free on Si substrates with either Ge or GeSn buffer layers. Designs of double heterostructure, multiple quantum well, and quantum cascade lasers are presented.

Ⅲ族氮化物InAlN半导体异质结研究进展

Due to the great potential applications in high-frequency and high-power solid state microwave electronic device, GaN-based high electron mobility transistors (HEMTs) have attracted much attention in the past two decades, and tremendous progress has been achieved toward realizing the commercialization in the market. To further improve the device performance, especially for high operation frequency and device reliability under high voltage, advanced materials and device fabrication process, novel device structures and design are proposed. Among the proposed approaches, InAlN-based lattice-matched heterostructures may become the preferred material for the next HEMT owing to its unique superior material properties. In this paper, in combination with the relative research work in the eld of group-Ⅲ compound semiconductor materials and devices, we present a brief review on the progress in growth of state of the art of InAlN-based heterostructures semiconductor materials. Based on the analysis of epitaxial growth of InAlN-based heterostructures, we discuss the advantages and achievements of proposed pulsed (surface reaction-enhanced) metal organic chemical vapor deposition (MOCVD) for InAlN/GaN heterostructures epitaxy.

Towards Si-based electrically injected group-IV lasers

Approaches towards Si-based lasers are discussed using lattice-matched group-IV heterostructures that can be deposited strain free on Si substrates with either Ge or GeSn buffer layers. Several strain-free designs are presented that include Ge/GeSiSn quantum cascade structure, GeSn/GeSiSn double heterostructure, and GeSn/GeSiSn multiple quantum wells.

Performance Improvement of One-Transistor DRAM by Band Engineering

We propose a novel one-transistor (1T) quantum well (QW) DRAM with raised GaP source/drain. This novel device structure shows much better retention time and sense margin than the existing silicon 1T DRAM (with and without QW). Detailed simulation study indicates that the proposed structure is scalable up to 15-nm gate length. The proposed device utilizes nearly lattice-matched heterostructures which have already been realized in the literature.

Band offsets, wells, and barriers at nanoscale semiconductor heterojunctions

Epitaxially-grown semiconductor heterostructures give the possibility to tailor the potential landscape for the carriers in a very controlled way. In planar lattice-matched heterostructures, the potential has indeed a very simple and easily predictable behavior: it is constant everywhere except at the interfaces where there is a step (discontinuity) which only depends on the composition of the semiconductors in contact. In this paper, we show that this universally accepted picture can be invalid in nanoscale heterostructures (e.g., quantum dots, rods, nanowires) which can be presently fabricated in a large variety of forms. Self-consistent tight-binding calculations applied to systems containing up to 75 000 atoms indeed demonstrate that the potential may have a more complex behavior in axial hetero-nanostructures: The band edges can show significant variations far from the interfaces if the nanostructures are not capped with a homogeneous shell. These results suggest new strategies to engineer the electronic properties of nanoscale objects, e.g. for sensors and photovoltaics.

A novel engineered oxide buffer approach for fully lattice-matched SOI heterostructures

Epitaxial (epi) oxides on silicon can be used to integrate novel device concepts on the canonical Si platform, including functional oxides, e.g. multiferroics, as well as alternative semiconductor approaches. For all these applications, the quality of the oxide heterostructure is a key figure of merit. In this paper, it is shown that, by co-evaporating Y2O3 and Pr2O3 powder materials, perfectly lattice-matched PrYO3(111) epilayers with bixbyite structure can be grown on Si(111) substrates. A high-resolution x-ray diffraction analysis demonstrates that the mixed oxide epi-films are single crystalline and type B oriented. Si epitaxial overgrowth of the PrYO3(111)/Si(111) support system results in flat, continuous and fully lattice-matched epi-Si(111)/PrYO3(111)/Si(111) silicon-on-insulator heterostructures. Raman spectroscopy proves the strain-free nature of the epi-Si films. A Williamson–Hall analysis of the mixed oxide layer highlights the existence of structural defects in the buffer, which can be explained by the thermal expansion coefficients of Si and PrYO3.

Exact determination of electrical properties of wurtzite Al1−xInxN/(AlN)/GaN heterostructures (0.07 ≤ x ≤ 0.21) by means of a detailed charge balance equation

This paper discusses the determination of key electrical parameters of AlInN/(AlN)/GaN heterostructures from capacitance–voltage (CV) measurements. These heterostructures gained recently importance since they allow for high electron mobility transistor (HEMT) devices with several remarkable records: densities of the 2D electron gas (2DEG) of 2.6 × 1013 cm−2 for lattice-matched (LM) heterostructures and barrier thickness of 14 nm, beyond 2 A/mm saturation currents, above 100 GHz operation for heterostructures grown on Si (111) with gate length of 0.1 µm. Despite these striking experimental results, a consistent determination of the most important electrical parameters, namely polarization sheet charge density, surface potential, and dielectric constant of the alloy are still missing. By setting up the correct charge balance equation, these parameters can unambiguously be determined. For instance, in the case of nearly LM Al0.85In0.15N these parameters amount to σAl0.85In0.15N/GaN ~ 3.7 × 1017 m−2, eΦS ~ 3 eV and ɛAl0.85In0.15N ~11.2, for the charge density, the surface barrier potential, and the dielectric constant, respectively.