GaSb Layer Thickness Research Articles

Improved device performance of interband cascade lasers with hybrid cladding layers operating in the 3–4 µm wavelength region

In this work, we briefly review the development and status of interband cascade lasers (ICLs) as related to long-standing issues due to the InAs/AlSb superlattice cladding. By focusing on a hybrid cladding approach to alleviate these issues, we demonstrate substantially improved device performance of ICLs compared to earlier reported ICLs of a similar design in the 3–4 µm wavelength region. These improvements include a threshold current density for broad-area devices as low as 134 A/cm2 at 300 K and reduced threshold voltage with a peak voltage efficiency of 80%, which is more than 10% higher than that obtained from previously reported ICLs. Moreover, we have demonstrated continuous wave (cw) operation of a broad-area device up to 278 K, the highest cw operating temperature among epi-side up mounted broad-area type-II ICLs, implying improved thermal dissipation with the hybrid cladding approach. Additionally, by conducting a comparative study of ICLs with different GaSb layer thicknesses in the hole injector, we reveal and discuss an interesting correlation between the carrier transport, threshold voltage, and hole-induced absorption loss, which may help to guide device optimization for operation in a targeted temperature range.

High resolution X-ray diffraction study in InAs/GaSb superlattice

In our experiment, the high-resolution X-ray diffraction rocking curves and AFM morphology of InAs/GaSb superlattices with different periods, thicknesses and soaked time are studied, which were grown by molecular beam epitaxy. We get the structural parameters of high-quality InAs/GaSb superlattices. During the growth process, the short-period InAs/GaSb superlattice has larger root mean square roughness than the long period, poor surface uniformity and weak stress release ability. The use of multi-period structure can better overcome stress and mismatch, the X-ray diffraction peak is modulated due to the change of InAs layer thickness and GaSb layer thickness. Through the position and distribution of diffraction peak, the strain characteristics of superlattice structure can be obtained, and then the regulation of energy band structure and shape of high-quality InAs/GaSb superlattice can be realized, which indicates good perspective for multi-period structure devices.

Tunable energy gaps and mid-infrared optical properties in InAs/GaSb type-II superlattices

We investigate on the infrared (IR) optoelectronic properties in short-period InAs/GaSb type-II superlattices (SLs) by a modified eight-band [Formula: see text] model. The electronic mini-band structures for such SLs are evaluated by the modified eight-band [Formula: see text] model, incorporating the microscopic interface effect. We find that with varying the values around 20/25 Å for the InAs/GaSb layer widths, the tunable mid-IR bandgaps can be achieved effectively. The SL bandgap from 275 to 346 meV can be achieved by decreasing the InAs layer thickness from 23 to 17 Å at a fixed GaSb layer thickness of 24 Å, or from 254 to 313 meV by increasing the GaSb layer thickness from 18 to 27 Å at a fixed InAs layer thickness of 21 Å. Correspondingly the optical absorptions in such systems can be tuned evidently. Our theoretical results are in good agreement with experimental data over a series of SL samples. This study confirms further that short-period InAs/GaSb type-II SLs are of great importance for IR applications.

Structural and optical characterization of InAs/GaSb type-II superlattices: Influence of the change in InAs and GaSb layer thicknesses for fixed InSb-like interfaces

In this article, we report on the molecular beam epitaxy growth and characterization of a 140 period InAs/GaSb type-II superlattice structure designed for mid infrared detection. Thickness of a period was systematically altered in each sample by changing the thickness of InAs (GaSb) layers from 9 to 7 monolayers (ML) for a fixed GaSb (InAs) layer at 9 ML (7 ML). The same InSb-like strain compensation interface was used for all samples. High resolution X-ray diffraction analysis, spectral responsivity and external quantum efficiency (QE) measurements were performed to express the effects of layer thickness variations on both structural and photodetector features. The decrease in the InAs thickness resulted in the increased mismatch from 0 to +1626ppm and the blue shift in the 50% cut-off wavelength (λc) from 5.41 to 4.36μm at 77K. The additional decrease in GaSb thickness caused further increase in the mismatch up to +1791ppm. The steepness of the photoresponse at the absorption band edge was quantified and presented comparatively with different photodetector parameters and material properties for a complete picture. The highest optical response was obtained from sample having 8 ML InAs and 9 ML GaSb with λc=4.76μm and QE=23.7% at 4μm.

On the structural characterization of InAs/GaSb type-II superlattices: The effect of interfaces for fixed layer thicknesses

We report on the detailed epitaxial growth conditions for type-II InAs/GaSb superlattice (SL) structures designed for mid-wave infrared detection. The mismatch between the GaSb buffer and the SL is precisely controlled by engineering the InSb-like interfaces for fixed InAs and GaSb layer thicknesses in the SL. High resolution X-ray diffraction and cross-sectional high resolution transmission electron microscopy analyses were performed to determine the mismatch and the thicknesses. It was shown that the in-plane lattice mismatch to the GaSb buffer is inversely proportional to the interface thickness for the samples presented here. The effect of the interfaces on the structure was also confirmed by room temperature transmission measurements: features were red-shifted as the interface thickness is increased.

Band-Tailored InAs/GaSb Superlattice in Infrared Application

The energy band structure of InAs/GaSb superlattice can be tailored by changing the thickness of InAs or GaSb layer thickness. Different superlattice as InAs/GaSb (4ML/8ML, 8ML/8ML and 16ML/7ML) for cut-off wavelengths from short to long IR wavelength were designed and deposited on GaSb substrates by melocular beam epitaxy system. The lattice mismatch was below 1000ppm under the careful Ga1-xInx- type interface control. The responsitivity revealed the band structure in InAs/GaSb superlattice can be tailored and the cutoff wavelength was from 2.5μm to 11μm. The current-voltage relation and the differential resistance in infrared photodiode were also measured and studied.

Electronic structure and optical properties of InAs/GaSb/AlSb/GaSb superlattice

The electronic structure and optical properties of InAs/GaSb/AlSb/GaSb superlattice (M-structure) are investigated in the framework of eight-band effective-mass theory, with the interface potential considered. The calculated energy gaps agree excellently with the experimental results. Via calculations, we find that the electronic structure of M-structure strongly depends on the geometrical structure of superlattice. The electron effective mass increases notably with the thickness of GaSb and AlSb layers, and AlSb layer is more favorable to obtain large electron effective mass than GaSb layer. Increased thickness of AlSb layer also leads to larger variation range of valence band maximum (VBM) and so M-structure has more tunable VBM than InAs/GaSb superlattice. Also the VBM of M-structure rises considerably with the increment of GaSb layer thickness and is almost independent of InAs layer thickness. We further find that M-structure has no remarkable superior optical absorption coefficient over InAs/GaSb superlattice. However, with larger electron effective mass and more tunable valence band maximum compared with InAs/GaSb superlattice, M-structure can be used as barrier in InAs/GaSb superlattice infrared detector to reduce the dark current. And the quantum efficiency of infrared photodiodes will not depend on the bias voltage when the M-structure is appropriately doped and carefully designed based on the dependence of its electronic structure on the superlattice geometry.

How to Use Type II InAs/GaSb Superlattice Structure to Reach Detection Wavelength of 2–3 $\mu{\rm m}$

The photoluminescence (PL) peak of an InAs/GaSb superlattice (SL) structure is found to be shifted from 5.8 to 4.0 $\mu{\rm m}$ at 77 K, when the growth temperature is lowered from 380 to 340 $^{\circ}{\rm C}$ . The PL peak shift is related to In intermixing, thus some SL structures cannot reach a detection wavelength ${ . Increasing the GaSb layer thickness in the SL structure is an effective way to reach a detection wavelength of 2–3 $\mu{\rm m}$ . A p-i-n detector with a 50% cutoff wavelength at 2.56 $\mu{\rm m}$ at 77 K is demonstrated.

Structural Defects in InSb Quantum Wells Grown on GaAs (001) Substrates via Al0.09In0.91Sb/GaSb-AlSb Strained Layer Superlattice/AlSb/GaSb Buffer Layers

ABSTRACTStructural defects in InSb quantum well (QW) samples have been investigated by transmission electron microscopy (TEM). Using molecular beam epitaxy, an InSb QW with remotely-doped Al0.09In0.91Sb barriers was grown on a GaAs (001) substrate with buffer layers consisting of, in order from the substrate: 1 μm of GaSb, 1 μm of AlSb, 50 nm of GaSb-AlSb strained layer superlattice (SLS), and 3 μm of Al0.09In0.91Sb. Cross-sectional TEM analysis indicates that high densities of threading dislocations (TDs) are created at the two highly lattice-mismatched interfaces, the Al0.09In0.91Sb/GaSb-AlSb SLS and the GaSb/GaAs interfaces. Pairs of stereo images taken from plan-view TEM (PV-TEM) specimens show that TDs propagate through the InSb QW layer. The densities of TDs and micro-twin (MT) defects measured by PV-TEM are 9×108/cm2 and 4×103/cm, respectively. These values are worse than those in an InSb QW layer grown with a different buffer layer by a factor of ∼4. The different buffer layer contains an InSb interlayer that effectively filters out both TDs and MTs. Adopting an interlayer structure and reducing the GaSb and AlSb layer thickness may make it possible to fabricate a lower-defect-density yet thinner InSb QW sample with the type of buffer layer examined in this study.

Influence of band folding in InAs/GaSb superlattices

We demonstrate the importance of accurate full zone fits for the band diagrams of constituent materials in the superlattice empirical pseudopotential method (SEPM) modeling of InAs/GaSb type-II strained superlattices. In particular, we show that inaccurate fits to X-valley energies in the constituent GaSb conduction band can lead to significant errors in the prediction of the primary band gaps of these superlattices. Specifically, we examine a set of five samples, each with a fixed InAs layer thickness of six monolayers. These samples show a strong blueshift of the band gap as the GaSb layer thickness is systematically increased. We demonstrate that the superlattice-induced band folding of the GaSb conduction band X valley significantly influences our theoretically predicted results for the shorter-period superlattice samples. This result helps shed light on the proper domain of application of heterostructure theories, such as effective mass theories, that only involve zone center properties of the constituent materials.

The Influence of GaSb Layer Thickness on the Band Gap of InAs/GaSb Type-II Superlattices for Mid-Infrared Detection

ABSTRACTThe effect of small changes in GaSb layer thickness on the photoresponse spectrum of InAs/GaSb superlattices (SLs) designed for mid-infrared detection was systematically investigated. The samples were grown by molecular beam epitaxy with precisely calibrated growth rates. The basic SL used for this study consisted of 40 periods of InAs (20.5 Å)/GaSb (X Å), where the nominal value for X was adjusted from 18 to 27 Å in four different samples. An InSb-like interface (IF) was inserted between the layers to balance the SL strain. By decreasing the GaSb width, the photoresponse cut-off wavelength (λc) was adjusted from 4.03 μm to 4.55 μm, i.e., the SL energy band gap is being decreased. This decrease in the energy separation between the first heavy hole band (HH1) and the first conduction band (C1) as the GaSb layer is narrowed is counter intuitive. However, this experimental trend can be explained by a modified envelope function approximation (EFA) calculation that includes the effect of in-plane asymmetry at InAs/GaSb interfaces. As expected, the HH band is pushed away from the top of the GaSb valence band as the GaSb layer width becomes narrower. However, at the same time the C1 band is significantly broadened by the increased wave function overlap of the electron states in the InAs layer. The trend to smaller band gap with narrower GaSb layers and other effects of the design changes on the photoresponse spectrum are discussed.

Spectral blueshift and improved luminescent properties with increasing GaSb layer thickness in InAs–GaSb type-II superlattices

We describe the photoluminescence spectroscopy (PL) and Fourier transform infrared absorbance spectroscopy characterization of a large set of InAs/GaSb type-II strained layer superlattice (SLS) samples. The samples are designed to probe the effect of GaSb layer thickness on the optical properties of the SLS, while the InAs-layer thickness is held fixed. As the GaSb layer thickness is increased, we observe a spectral blue shift of the PL peaks that is accompanied by an increase in intensity, narrower linewidths, and a large reduction in the temperature sensitivity of the luminescence. These effects occur despite a significant reduction in the electron-hole wave function overlap as the GaSb layer thickness is increased. In addition, we compare the results of empirical pseudopotential model (EPM) calculations to the observed blueshift of the primary band gap. The EPM calculations are found to be in very good agreement with the observed data.

Linewidth analysis of the photoluminescence from InAs/GaSb/InAs/AlSb type-II superlattices

We present photoluminescent (PL) linewidth measurements on InAs:GaSb, type-II superlattices as a function of temperature and power. The observed PL linewidth for the samples, studied at 80 K, was 40–60 meV, which is significantly larger than a thermally broadened line of width 2 kT. The larger linewidth is well explained by a combination of homogeneous and inhomogeneous broadening. The data suggest that the inhomogeneous broadening is dominated by interface roughness and that the roughness amplitude at the InAs–GaSb interface is on the order of 1 ML. A significant fraction of the broadening can be accounted for by the presence of interfacial regions which show a 1 ML decrease in the GaSb layer thickness. To account for homogeneous broadening, a Lorentzian function of width δ (full width at half maximum) is employed to smooth and broaden the synthetic spectrum which is calculated from a simple model of the spontaneous emission rate. A δ=10 meV was found to give the best fit of the synthetic spectra to the observed spectra. The spectral distribution was also found to be dependent on excitation power, and the data indicate that the photogenerated carriers preferentially occupy and fill the low energy regions of the heterointerface.

Absorbance spectroscopy and identification of valence subband transitions in type-II InAs/GaSb superlattices

We describe the molecular-beam epitaxy growth, as well as both the structural and optical characterization of a set of InAs/GaSb type-II strained-layer superlattice samples, in which the GaSb layer thickness is systematically increased. Absorbance spectroscopy measurements show well-defined features associated with transitions from the various valence subbands to the lowest conduction subband, and also a significant blueshift of the band edge when the GaSb layers thickness is increased. Empirical pseudopotential method calculations are shown to successfully predict the blueshift and help identify the higher-energy transitions.

Effect of interfacial bonding on the structural and vibrational properties of InAs/GaSb superlattices.

We have studied InAs/GaSb superlattices (SL's) grown with either InSb-like or GaAs-like interfaces (IF's) on top of a GaSb buffer layer on (100) GaAs substrates. The InAs layer thickness was varied from 4 to 14 monolayers (ML) while the GaSb layer thickness was kept fixed at 10 ML. The type of IF bonds realized was verified by Raman scattering from mechanical IF modes. High-resolution x-ray diffraction using one- and two-dimensional mapping of symmetric and asymmetric reflections allowed us to determine independently the lattice parameters parallel and perpendicular to the growth direction. The GaSb buffer layer was found to be fully relaxed whereas the SL's with InSb-like IF's were coherently strained to the in-plane lattice parameter of the GaSb buffer for InAs layer thicknesses exceeding 6 ML. The strain distribution within the SL's with GaAs-like IF's was obtained from simulations of the x-ray reflection profiles. The SL's were found to be coherently strained close to the GaSb buffer and showed increasing strain relaxation with increasing distance from the buffer layer. In addition, these simulations provide an accurate determination of the SL periods. Well-resolved Raman spectra of backfolded longitudinal acoustic (LA) phonons were observed showing for SL's with InSb-like IF's folded LA phonon lines up to the seventh order. The spectrum of quasiconfined optical SL phonons was examined by Raman spectroscopy and by IR reflection. A detailed analysis of the IR reflection spectra allowed an independent determination of the individual layer widths within the SL stack, including the spatial extent of the GaAs-like IF mode. \textcopyright{} 1996 The American Physical Society.

Electronic Structure of Type-II InAs/GaSb Misaligned Superlattice

The Energy band edges of type-II InAs/GaSb (001) misaligned superlattices as functions of InAs and GaSb layer thicknesses are calculated by a pseudopotential method with quick convergency. The results show the trend of deep-shallow transitions of impurities in the superlattices. The regime in which the superlattice changes from semiconductor to semimetal is also showed.

Growth of InAs-AlSb quantum wells having both high mobilities and high concentrations

Low-temperature mobilities in InAs-AlSb quantum wells depend sensitively on the buffer layer structures. Reflection high energy electron diffraction and x-ray diffraction show that the highest crystalline quality and best InAs transport properties are obtained by a buffer layer sequence GaAs → AlAs → AlSb → GaSb, with a final GaSb layer thickness of at least 1 μm. Using the improved buffer scheme, mobilities exceeding 600,000 cm2/Vs at 10 K are routinely obtained. Modulation δ-doping with tellurium has yielded electron sheet concentrations up to 8 × 1012 cm−2 while maintaining mobilities approaching 100,000 cm2/Vs at low temperatures.

Modeling of novel heterojunction tunnel structures

We have implemented a simple model that allows realistic yet rapid simulation of conventional as well as interband resonant tunneling devices. Using this model we have studied GaAs/AlAs asymmetric triple barrier structures and found that coherence between the quasibound states in the two quantum wells should be observable in the I–V characteristics of the devices. We have also examined InAs–GaSb–InAs broken-gap interband tunnel devices and found that, despite the absence of classically forbidden barrier regions, a resonant tunneling process is involved in producing the observed negative differential resistance. Furthermore, we have found that maximum peak current densities should be found in devices with GaSb layer thicknesses corresponding to a single, rather than a multiple transmission resonance peak in the broken-gap region.