AlSb Barriers Research Articles

Theoretical analysis on the energy band properties of N- and M-structure type-II superlattices

Abstract The electronic band structure of conventional InAs/GaSb superlattices (SLs), N- and M-structure were investigated by 8-band k·p method. A good agreement between the theoretical calculation and experimental results was obtained. Compared with the conventional structure, the band gap and effective mass of N- and M-structure both increased with the thickness of AlSb layers raising. The splitting energy between first heavy hole band (HH1) and first light hole band (LH1) increased in N-structure, but firstly raised and then reduced in M-structure. The impact of changing the position of AlSb barrier in M-structure is also studied. By altering the position of the barrier layer, the wave function overlaps can be controlled at the InAs/GaSb or GaSb/InAs interface in PN junction. By comparing the three type SLs, we found that N- and M-structure can obviously suppress dark current and enhance wave function overlap. Besides, M-structure can be more alternative in structure design.

High-operating temperature InAsSb/AlSb heterostructure infrared detectors grown on GaAs substrates by molecular beam epitaxy

InAsSb/AlSb barrier detectors were grown on (100) semi-insulating GaAs substrates by a molecular beam epitaxy. We compare the performance of two detectors with different active layers denoted as p + BppBpN + and p + Bpnn + . InAs0.81Sb0.19 absorber allows to operate up to 5.3-μm cut-off wavelengths at 230 K. p + Bpnn + detector (n-type absorber) exhibits diffusion-limited dark currents above 200 K. AlSb barrier provides low dark currents and suppresses surface leakage currents. With a value of 0.13 A / cm2 at 230 K, the current is of about an order of magnitude larger than determined by the “Rule 07.” Dark currents of p + BppBpN + detector (p-type absorber) are much higher due to a contribution of Shockley–Read–Hall mechanisms. On the other hand, a device with a p-type absorber exhibits the highest value of current responsivity, up to 2.5 A / W, pointing out that there is a tradeoff between dark current performance and quantum efficiency.

Influence of an in-plane magnetic field on the electronic structure of an inverted InAs/GaSb quantum well

The electronic structure of an inverted InAs/GaSb quantum well embedded in AlSb barriers is studied theoretically. The influence of an in-plane magnetic field is examined within the 14-band k⋅p approach. The spin-dependent subband energy dispersion curves are strongly modified by the in-plane magnetic field and by the conduction-valence band hybridization. The dispersion curves in the direction parallel to the magnetic field become quite different from that in the perpendicular direction. At strong magnetic fields, one observes the interplay between the confinement induced by the magnetic field and the confinement due to the quantum well, and the interplay between the strong intrinsic spin-orbit interaction and the spin alignment induced by the magnetic field. The well-known two-dimensional topological insulator model is generalized to take into account the influence of the in-plane magnetic field. The bulk-like state conduction channels become available in addition to the edge state conduction channels for a moderate magnetic field.

Anti phase boundary free GaSb layer grown on 300 mm (001)-Si substrate by metal organic chemical vapor deposition

Antiphase boundaries free GaSb epitaxial layers with low surface roughness (<0.5nm) have been synthesized on standard microelectronic 300mm nominal (001)-Si substrates by metal organic chemical vapor deposition using a two-step growth process. By adjusting the growth temperature and the thickness of the nucleation layer, antiphase boundary free GaSb layers as thin as 250nm are obtained. The 12% lattice mismatch between GaSb and Si is accommodated by both the formation of threading dislocations and a periodic array of 90° misfit dislocations at the interface. A GaSb layer inserted between AlSb barriers has been grown on an optimized GaSb/(001)-Si buffer layer and exhibits room temperature photoluminescence.

Partial hybridisation of electron-hole states in an InAs/GaSb double quantum well heterostructure

InAs/GaSb coupled quantum well heterostructures are important semiconductor systems with applications ranging from spintronics to photonics. Most recently, InAs/GaSb heterostructures have been identified as candidate two-dimensional topological insulators, predicted to exhibit helical edge conduction via fully spin-polarised carriers. We study an InAs/GaSb double quantum well heterostructure with an AlSb barrier to decouple partially the 2D electrons and holes, and find conduction consistent with a 2D hole gas, with an effective mass of 0.235 ± 0.005 m0, existing simultaneously with hybridised carriers with an effective mass of 0.070 ± 0.005 m0, where m0 is the bare electron mass.

Cyclotron resonance of dirac fermions in InAs/GaSb/InAs quantum wells

The band structure of three-layer symmetric InAs/GaSb/InAs quantum wells confined between AlSb barriers is analyzed theoretically. It is shown that, depending on the thicknesses of the InAs and GaSb layers, a normal band structure, a gapless state with a Dirac cone at the center of the Brillouin zone, or inverted band structure (two-dimensional topological insulator) can be realized in this system. Measurements of the cyclotron resonance in structures with gapless band spectra carried out for different electron concentrations confirm the existence of massless Dirac fermions in InAs/GaSb/InAs quantum wells.

InAs/GaSb/AlSb composite quantum well structure preparation with help of reflectance anisotropy spectroscopy

Magnetotransport, optical, spin-dependent and topological properties of the composite InAs/GaSb/AlSb quantum wells based on the broken-gap heterojunctions have been actively studied during the last two decades as promising materials for spintronic and nanoelectronic applications.Mostly the structures were prepared by MBE, since preparing heterostructures of this materials system by MOVPE presents several challenges. We have successfully prepared by MOVPE structures with broken gap InAs/GaSb composite quantum wells surrounded by AlSb barriers on InAs and GaSb substrates. The whole process of structure preparation was optimized with help of in situ LayTec Epiras measurements. Suitable interfaces, switching sequences, growth rates and suitable precursors for the growth of the structure are discussed. The quality of the structure was checked by electron paramagnetic resonance for differently oriented magnetic field up to 14kOe at temperatures from 2.7K to 20K. Intense Shubnikov de Haas oscillations appeared at low temperatures and helped us characterize the properties of the structure.

Bulk InAsxSb1-x nBn photodetectors with greater than 5μm cutoff on GaSb

Mid-wavelength infrared nBn photodetectors based on bulk InAsxSb1-x absorbers with a greater than 5 μm cutoff grown on GaSb substrates are demonstrated. The extended cutoff was achieved by increasing the lattice constant of the substrate from 6.09 to 6.13 Å using a 1.5 μm thick AlSb buffer layer to enable the growth of bulk InAs0.81Sb0.19 absorber material. Transitioning the lattice to 6.13 Å also enables the use of a simple binary AlSb layer as a unipolar barrier to block majority carrier electrons and reduce dark current noise. Individual test devices with 4 μm thick absorbers displayed 150 K dark current density, cutoff wavelength, and quantum efficiency of 3 × 10−5 A/cm2, 5.31 μm, and 44% at 3.4 μm, respectively. The instantaneous dark current activation energy at a given bias and temperature is determined via Arrhenius analysis from the Dark current vs. temperature and bias data, and a discussion of valence band alignment between the InAsxSb1-x absorber and AlSb barrier layers is presented.

Narrow-Band Type II Superlattice Photodetector With Detection Wavelength Shorter Than &lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;$2~\mu \text{m}$ &lt;/tex-math&gt;&lt;/inline-formula&gt;

We demonstrate that type II InAs/GaSb superlattice photodetector can reach the detection wavelength shorter than $2~\mu \text{m}$ by inserting AlSb barriers. The detector exhibits a narrow-band photoresponse feature ( $\delta \lambda /\lambda =10.3\%$ ). At 77 K, the photoresponse maximum is at 1.84 $\mu \text{m}$ and the Johnson noise limited detectivity is $1.1\times 10^{13}$ Jones. We show that unlike a typical broad-band response feature using the interband transitions, for a p-i-n detector in the short wavelength range, a narrow-band response feature is the case, which is ascribed to the stronger absorption for the higher energy photons from the n region.

Landau level transitions in InAs/AlSb/GaSb quantum wells**Project supported by the National Natural Science Foundation of China (Grant Nos. 61076092 and 61290303).

The electronic structure of InAs/AlSb/GaSb quantum wells embedded in AlSb barriers and in the presence of a perpendicular magnetic field is studied theoretically within the 14-band k·p approach without making the axial approximation. At zero magnetic field, for a quantum well with a wide InAs layer and a wide GaSb layer, the energy of an electron-like subband can be lower than the energy of hole-like subbands. As the strength of the magnetic field increases, the Landau levels of this electron-like subband grow in energy and intersect the Landau levels of the hole-like subbands. The electron–hole hybridization leads to a series of anti-crossing splittings of the Landau levels. The magnetic field dependence of some dominant transitions is shown with their corresponding initial-states and final-states indicated. The dominant transitions at high fields can be roughly viewed as two spin-split Landau level transitions with many electron–hole hybridization-induced splittings. When the magnetic field is tilted, the electron-like Landau level transitions show additional anti-crossing splittings due to the subband-Landau level coupling.

Influence of etching processes on electronic transport in mesoscopic InAs/GaSb quantum well devices

We report the electronic characterization of mesoscopic Hall bar devices fabricated from coupled InAs/GaSb quantum wells sandwiched between AlSb barriers, an emerging candidate for two-dimensional topological insulators. The electronic width of the etched structures was determined from the low field magneto-resistance peak, a characteristic signature of partially diffusive boundary scattering in the ballistic limit. In case of dry-etching the electronic width was found to decrease with electron density. In contrast, for wet etched devices it stayed constant with density. Moreover, the boundary scattering was found to be more specular for wet-etched devices, which may be relevant for studying topological edge states.

Characterization of thin AlSb/AlAs barriers on InAs by mid-infrared intersubband absorption measurements

We present mid-infrared intersubband absorption measurements on InAs/AlSb coupled-quantum-wells systems with thin AlSb barriers. The AlSb barrier width in our samples is varied between 1 and 4 monolayers, with a strained AlAs boundary layer used for strain compensation. The optical absorption energy difference between the 1–4 and 2–3 transitions is used as a measure of the barrier coupling strength and modeled by a one-band Schroedinger solver. Our results let us conclude that the composite AlSb/AlAs barrier behaves like an effective AlSb barrier with an effective thickness that does not include the last As layer. This observation must be taken into account when designing complex heterostructures relying on very thin AlSb, like in InAs/AlSb quantum cascade lasers.

Features of the persistent photoconductivity in InAs/AlSb heterostructures with double quantum wells and a tunneling-transparent barrier

The spectra of persistent photoconductivity for InAs/AlSb heterostructures with double quantum wells and a separation AlSb barrier with varying thickness between 0.6–1.8 nm are measured at T = 4.2 K. The electron concentrations in the wells at various illumination wavelengths are determined from the Fourier analysis of Shubnikov-de Haas oscillations. The features associated with the tunneling transparency of a separation barrier 0.6 nm thick (two monolayers) are revealed. The performed self-consistent calculations of the energy profile of a double quantum well showed that a symmetric profile is established in the structures in the region of negative residual photoconductivity, while the region of positive persistent photoconductivity has an asymmetric potential profile, which leads to Rashba spin splitting (>2 meV at the Fermi level). It is shown that the introduction of the tunneling-transparent separation barrier increases the Rashba splitting.

Spin-related phenomena in InAs/GaSb quantum wells

We have studied theoretically the influence of symmetry breaking mechanisms: structural inversion asymmetry, bulk inversion asymmetry, relativistic and non-relativistic interface Hamiltonian and warping on spin split of levels Delta E and optical absorption of linearly polarized light in asymmetrical quantum wells made from zincblende materials grown on [001] direction. The AlSb/InAs/GaSb/AlSb broken-gap quantum wells with hybridized electron-hole states sandwiched by the AlSb barriers have been considered. We have obtained substantial contributions of these effects into the absolute values of spin split of electron and hole states and spinflip optical transitions for the initial state in-plane wave vectors along low symmetry directions such as [12].

Magneto-infrared modes in InAs-AlSb-GaSb coupled quantum wells

We have studied a series of InAs/GaSb coupled quantum wells using magneto-infrared spectroscopy for high magnetic fields up to 33 T within temperatures ranging from 4 to 45 K in both Faraday and tilted field geometries. This type of coupled quantum wells consists of an electron layer in the InAs quantum well and a hole layer in the GaSb quantum well, forming the so-called two-dimensional electron-hole bilayer system. Unlike the samples studied in the past, the hybridization of the electron and hole subbands in our samples is largely reduced by having narrower wells and an AlSb barrier layer interposed between the InAs and the GaSb quantum wells, rendering them weakly hybridized. Previous studies have revealed multiple absorption modes near the electron cyclotron resonance of the InAs layer in moderately and strongly hybridized samples while only a single absorption mode was observed in the weakly hybridized samples. We have observed a pair of absorption modes occurring only at magnetic fields higher than 14 T, which exhibited several interesting phenomena. Among which we found two unique types of behavior that distinguish this work from the ones reported in the literature. This pair of modes is very robust against rising thermal excitations and increasing magnetic fields aligned parallel to the heterostructures. While the previous results were aptly explained by the antilevel crossing due to the hybridization of the electron and hole wave functions, i.e., conduction-valence Landau-level mixing, the unique features reported in this paper cannot be explained within the same concept. The unusual properties found in this study and their connection to the known models for InAs/GaSb heterostructures will be discussed; in addition, several alternative ideas will be proposed in this paper and it appears that a spontaneous phase separation can account for most of the observed features.

Hole mobility in pseudomorphic InGaSb quantum well modulation doped with carbon

Carbon-tetrabromide (CBr4) is utilized as the p-type doping source in modulation-doped pseudomorphic In0.3Ga0.7Sb/AlxGa1−xSb quantum well structure. Carbon delta-doping is achieved by switching off group III elements while the flow of CBr4 is on during the growth of AlSb barrier layer. The hole mobility of strained In0.3Ga0.7Sb quantum well decreases monotonically from 600 to 400 cm2/V s while the sheet carrier concentration increases from 7.5×1011 to 4.1×1012 cm−2 with increasing carbon delta-doping.

Spin states in InAs/AlSb/GaSb semiconductor quantum wells

We investigate theoretically the spin states in InAs/AlSb/GaSb broken-gap quantum wells by solving the Kane model and the Poisson equation self-consistently. The spin states in InAs/AlSb/GaSb quantum wells are quite different from those obtained by the single-band Rashba model due to the electron-hole hybridization. The Rashba spin splitting of the lowest conduction subband shows an oscillating behavior. The D'yakonov-Perel' spin-relaxation time shows several peaks with increasing the Fermi wave vector. By inserting an AlSb barrier between the InAs and GaSb layers, the hybridization can be greatly reduced. Consequently, the spin orientation, the spin splitting, and the D'yakonov-Perel' spin-relaxation time can be tuned significantly by changing the thickness of the AlSb barrier.

Influence of substrate doping and point defects on Al and Ga interdiffusion in AlSb/GaSb superlattice structures

Positron annihilation spectroscopy has been used to investigate the role of vacancies in the interdiffusion of Al and Ga in AlSb/GaSb superlattices. The samples were grown by metalorganic vapor-phase epitaxy on undoped and Te doped GaSb and consisted of ten periods of GaSb quantum wells (thickness 13 nm) and AlSb barriers (thickness 2–3 nm) and an approximately 50 nm thick capping layer of GaSb. The superlattices were annealed at 908 K for up to 250 s, resulting in interdiffusion of Al and Ga between well and barrier. A secondary ion mass spectrometry study showed that the Te dopant diffused from the substrate through the superlattice structure in the annealing process. In the positron annihilation study we observe that the vacancy concentration clearly decreases with annealing for the samples grown on undoped substrates, whereas the samples grown on Te doped substrates show a different annealing behavior.

InAlSb/InAs/AlGaSb Quantum Well Heterostructures for High-Electron-Mobility Transistors

Heterostructures for InAs-channel high-electron-mobility transistors (HEMTs) were investigated. Reactive AlSb buffer and barrier layers were replaced by more stable Al0.7Ga0.3Sb and In0.2Al0.8Sb alloys. The distance between the gate and the channel was reduced to 7–13 nm to allow good aspect ratios for very short gate lengths. In addition, n+-InAs caps were successfully deposited on the In0.2Al0.8Sb upper barrier allowing for low sheet resistance with relatively low sheet carrier density in the channel. These advances are expected to result in InAs-channel HEMTs with enhanced microwave performance and better reliability.

Cyclotron masses andg-factors of hybridized electron-hole states inInAs∕GaSbquantum wells

Using the eight-band k center dot p model and the Burt-Foreman envelope function theory to perform self-consistent calculations, we have studied the effect of electron-hole hybridization on the cyclotron masses m(*) and the effective g-factors g(*) of two-dimensional quasiparticles in InAs/GaSb quantum wells under a magnetic field applied perpendicular to the interfaces. We can modify the degree of hybridization by changing the InAs and/or GaSb layer width, or by inserting a thin AlSb barrier. While electron-light-hole hybridization dominates at both low and high fields, due to a sequence of anticrossings between electronlike and heavy-holelike levels, there is also an important contribution from heavy-hole states to the strong hybridization in the intermediate field range. The field-dependence of the hybridized energy eigenstates is manifested in the variations of m(*) and g(*). Characteristic discontinuous changes of both m(*) and g(*) appear at each anticrossing, resulting in a magnetic-field-driven oscillating behavior of these quantities for electronlike states of a given Landau level index. The electron g-factor can change sign when two eigenstates anticross. Hybridization of electron and hole states enhances the electron effective mass, and we have found a complicated dependence of this effect on the interaction strength. Without inserting an AlSb barrier, the strong interaction between the electronlike and the light-holelike states at low magnetic fields produces a large level repulsion, and hence relatively small effective masses and g-factors associated with these states. Intermediate interaction leads to weaker level repulsion and therefore very heavy electron cyclotron masses as well as large g-factors associated with the lowest Landau levels. A weak interaction only enhances the cyclotron masses of the electronlike states slightly. The hole effective masses change with both the magnetic field and the sample structure in a more complicated fashion. (Less)