GaSb Layer Width Research Articles

Evaluation of the optical characteristics of a type-II InAs/GaSb superlattice infrared p–i–n photodetector

We investigated the characteristics of a type-II InAs/GaSb superlattice (SL) with various composition periods based on the k·p band calculation. Considering the application to the infrared photodetectors, the essential material parameters are calculated and evaluated from the electronic band structure as a function of the width of InAs and GaSb layers. Under dark conditions, the smaller effective mass in a short period SL (especially with small GaSb layer width) could result in the reduction of the Shockley–Read–Hall (SRH) leakage current which is proportional to the intrinsic carrier density. However, this merit is also concerned with the increase of the dark current due to the enhanced band-to-band tunneling in high electric field conditions. Moreover, in terms of the photo-absorption property, the smaller effective mass results in the smaller joint density of states, which compensates the contribution of the large optical matrix element to the absorption coefficient in short period SL.

Magneto-optics of InAs/GaSb superlattices

We investigate the optical and electrical properties of a series of InAs/GaSb superlattices (SLs) as a function of InAs layer width d, from 21 to 55 Å, with a fixed GaSb layer width of 24 Å, corresponding to SLs with the cutoff wavelengths between 4 and 19 μm. Since the higher electron mass in InAs/GaSb SLs than in mercury cadmium telluride should lead to lower photodiode tunneling currents, we also measured the cyclotron effective mass for a very long wavelength infrared design SLs. For d<40 Å, the SLs were p-type, with hole mobilities of approximately 8 000 cm2/V s. For a high mobility p-type sample no hole cyclotron resonance signal was detected. However, the SLs with d≥40 Å were n-type, with electron mobilities increasing from 865 to 6126 cm2/V s. Cyclotron resonance data on an n-type SL sample yielded an electron cyclotron mass of 0.068 m0, which is three times the InAs bulk value of 0.023 m0. The mass enhancement was only partially accounted for by conduction band nonparabolicity, based on our 8×8 envelope function calculation.

Optimizing residual carriers in undoped InAs/GaSb superlattices for high operating temperature mid-infrared detectors

The mid-infrared 21 Å InAs/24 Å GaSb superlattices (SLs) designed for the 4 μm cutoff wavelength were grown by molecular beam epitaxy at growth temperatures between 370 and 430 °C in order to reduce residual background carriers. The lowest density of 1.8×10 11 cm −2 was obtained from the SLs grown at 400 °C. With increasing growth temperature, in-plane hole mobility decreased from 8740 to 1400 cm 2/V s due to increased interfacial roughness, while the photoluminescence (PL) intensity increased due to a decrease in the number of nonstoichiometric nonradiative defects. Further reduction of carrier density to 1×10 11 cm −2 was achieved by increasing barrier width. As GaSb layer width increases from 24 to 48 Å, the cutoff wavelength decreased from 4.1 to 3.4 μm, which is still in the mid-infrared detection window. More importantly, a dramatic improvement on the PL intensity and the full width at half maximum was achieved from the SL samples with the wider GaSb widths. All mid-infrared SL samples investigated in our studies were residually p-type.

Multiband [formula omitted] Riccati equation for electronic structure and transport in type-II heterostructures

An alternative method is proposed and implemented to calculate electronic structure and quantum transport properties of type-II heterojunctions. By deriving a multiband k·p Riccati equation for the envelope function matrix, it is shown how to obtain the reflection matrix through a simple numerical integration of the Riccati equation. Numerical instability, which is usually associated with type-II systems due to the simultaneous presence propagating and evanescent states, is avoided by working with the logarithmic derivative of the envelope function matrix.The theory is implemented numerically for a 6-band k·p matrix Hamiltonian in which the electron spin components have been decoupled. Preliminary results are presented for InAs/GaSb/InAs quantum wells. First, the calculated transmission versus energy curves are compared to those obtained from the microscopic empirical pseudo-potential calculation of Edwards and Inkson [Semicond. Sci. Technol. 9 (1994) 178]. For GaSb layer widths of 18, 55 and 152Å, the transmission spectra agree almost exactly. Minor discrepancies are discussed. Second, the net current density is calculated at room temperature as a function of applied voltage. For a GaSb layer width of 74Å, the self-consistently calculated net current density and peak-to-valley ratios are found to be in semi-quantitative agreement with the experiment of Yu et al. [Appl. Phys. Lett. 57 (1990) 2675].Other than its applications to electron tunneling, which can include spin-dependent tunneling (spintronics), the theory may also be applied to emerging field of multi-channel inverse scattering; in which desired transmission or reflection properties are used as input for designing heterostructure potential profiles.

Demonstration of interface-scattering-limited electron mobilities in InAs∕GaSb superlattices

The in-plane transport in InAs∕GaSb type-II superlattices (SLs) is a sensitive indicator of SL growth quality and of the eventual performance of devices made from these materials. The in-plane mobility of electrons that move predominantly in the InAs layer is affected by a number of intrinsic and extrinsic scattering mechanisms, including interface roughness scattering (IRS). The hallmark of classic IRS-limited transport in SLs and quantum wells is the sixth power dependence of mobility on layer width. While IRS-limited transport was demonstrated in a number of SL and quantum well systems, it has never been demonstrated in the important InAs∕GaSb SL material. In this paper, we perform temperature dependent Hall effect measurements on a series of InAs∕GaSb SLs with a fixed GaSb layer width and a variable InAs layer width d. The low temperature (10K) in-plane electron mobilities μ as a function of d behave as μ∝d6.20, which follows the classic sixth power dependence expected from theory. At the same time, the dominance of the IRS-limited transport indicates that our samples are less affected by other scattering mechanisms, so that mobility measurements are another indicator of sample quality.

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)

InAs/GaSb type-II superlattices for high performance mid-infrared detectors

The superlattice (SL) design parameters of a 50 period InAs/GaSb SL structure with InSb-like interfaces (IFs) were systematically varied around the 26 Å InAs/ 27 Å GaSb design in order to explore the parameter space for maximum photoresponse in the 3–5 μ m mid-infrared atmospheric window. Using previously optimized growth conditions, the SL structures were grown on p-type GaSb substrates by molecular beam epitaxy with precisely calibrated growth rates. The electrical properties of the SLs were characterized by magnetic field-dependent Hall effect measurements below the carrier freeze-out temperature of the p-type substrate. Multi-carrier analysis at 4.2 K determined an electron sheet carrier concentration of 8.5 × 10 10 cm - 2 with a mobility of 8200 cm 2 /Vs. Two sets of SLs were used in the optimization process: the first set with a fixed InAs width of 26 Å, and the second with a fixed GaSb width of 27 Å . As the GaSb layer width varied from 15 to 27 Å , the photoresponse cut-off wavelength shifted from 6.47 to 5.24 μ m. Similarly, as the InAs width varied from 26 to 13 Å, the cut-off wavelength shifted from 5.08 to 3.05 μ m. The strongest photoresponse in the 3–5 μ m mid-IR window was achieved with the InAs (20 Å)/GaSb (27 Å) SL design.

Optimization of mid-infrared InAs∕GaSb type-II superlattices

The effect of small changes in GaSb layer width on the photoresponse spectrum of 20.5ÅInAs∕InSb-interfaces∕XÅ GaSb type-II superlattice (SL) suitable for mid-infrared detection was investigated. By decreasing the GaSb width X from 27 to 18Å, the cut-off wavelength was increased from 4.03 to 4.55μm. This decrease of the SL band gap and other effects of the design changes on photoresponse spectrum with narrower GaSb layers are explained by a nonperturbative, modified envelope function approximation calculation that includes the interface coupling of heavy, light, and spin–orbit holes resulting from the in-plane asymmetry at InAs∕GaSb interfaces.

Effect of interfaces and the spin-orbit band on the band gaps of InAs/GaSb superlattices beyond the standard envelope-function approximation

We develop a modified $8\ifmmode\times\else\texttimes\fi{}8$ envelope-function approximation (EFA) formalism for the noncommon-atom (NCA) superlattices (SL's), incorporating the effect of anisotropic and other interface (IF) interactions that go beyond the standard EFA. The boundary conditions in the presence of IF interactions are used to set up a secular equation (including a transfer matrix derivation) whose physical transparency makes possible a number of valuable insights (possibility of IF bound states, analytic solutions, indirect gaps, etc.). We show that the heavy-hole--spin-orbit IF coupling is very important due to the IF localization of the SO wave function components and the ability of the IF potential to potentially bind a hole at the IF's, all of which pose convergence problems for perturbative solutions. With two adjustable parameter for the two possible IF's, we find a very good agreement between experiment and theory for the band gaps of several sets of very long-infrared and midinfrared InAs/GaSb SL's grown at several laboratories and by us. The band gaps as a function of GaSb and InAs widths are explained in terms of variations of the HH and conduction (C) band bandwidths. We show that the cut-off wavelengths can be reduced by increasing the GaSb layer width. Thus, a consistent application of the EFA method with the inclusion of well established IF effects can provide useful physical insights and possesses good predictive capacity in the design of NCA SL's.

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.

Demonstration of resonant transmission in InAs/GaSb/InAs interband tunneling devices

We have performed a theoretical and experimental analysis of current transport in InAs/GaSb/InAs interband tunneling devices as a function of GaSb layer width. Our results demonstrate that current transport in these devices occurs not through simple ohmic conduction, as had been previously proposed, but via light-hole-like resonances in the GaSb valence band formed due to the imperfect matching of InAs conduction-band and GaSb valence-band wave functions at the InAs/GaSb interfaces. These resonances produce a strong dependence of the current-voltage characteristics on GaSb layer width that is both predicted theoretically and observed experimentally. Our results also suggest that coupling between InAs conduction-band and GaSb heavy-hole valence-band states is relatively unimportant in these devices. In addition, we have been able to obtain peak current densities of ∼9×104 A/cm2, significantly higher than any previously reported current densities for this structure.

Resonant interband coupling in single-barrier heterostructures of InAs/GaSb/InAs and GaSb/InAs/GaSb

A new mechanism for negative differential resistance due to electron/light hole coupling has been observed using broken gap heterostructures of InAs/GaSb/InAs and GaSb/InAs/GaSb. The best peak-to-valley ratio is about 2:1 (3.7:1 at 77 K) for a GaSb layer width of 10 nm. The peak current density of 4.2 kA cm−2 and the peak voltage of 300 mV are consistent with the interpretation of these experiments as interband coupling between the InAs conduction band and the GaSb valence (light hole) band.