H. Ehrenreich

Auger lifetime enhancement in InAs–Ga1−xInxSb superlattices

We have experimentally and theoretically investigated the Auger recombination lifetime in InAs–Ga1−xInxSb superlattices. Data were obtained by analyzing the steady‐state photoconductive response to frequency‐doubled CO2 radiation, at intensities varying by over four orders of magnitude. Theoretical Auger rates were derived, based on a k⋅p calculation of the superlattice band structure in a model which employs no adjustable parameters. At 77 K, both experiment and theory yield Auger lifetimes which are approximately two orders of magnitude longer than those in Hg1−xCdxTe with the same energy gap. This finding has highly favorable implications for the application of InAs–Ga1−xInxSb superlattices to infrared detector and nonlinear optical devices.

Long wavelength InAs/InGaSb infrared detectors: Optimization of carrier lifetimes

The performance characteristics of type‐II InAs/InxGa1−xSb superlattices for long and very long‐wave infrared detection are discussed. This system promises benefits in this wavelength range over conventional technology based on Hg1−xCdxTe, in part because of suppressed band‐to‐band Auger recombination rates which lead to improved values of detectivity. The formalism for calculating Auger rates in superlattices is developed and the physical origin of Auger suppression in these systems is discussed. Accurate K⋅p band structures are used to obtain radiative, electron–electron, hole–hole, and band‐to‐band Auger rules, as well as shallow trap level assisted Auger recombination rates for photodiodes. Theoretical limits for high temperature operation of ideal photovoltaic detectors are presented and compared with HgCdTe.

Minority carrier lifetimes in ideal InGaSb/InAs superlattices

Calculations of band‐to‐band Auger and radiative recombination lifetimes of the recently proposed InxGa1−xSb/InAs superlattices (SL) show them to be promising infrared detectors. Several superlattices with energy gaps in the 5–11 μm range exhibit suppressed p‐type Auger recombination rates due to a large light hole–heavy hole splitting. The p‐type Auger lifetime at 77 K of an 11 μm InxGa1−xSb/InAs SL is found to be, respectively, three and five orders of magnitude longer than those of bulk and superlattice HgCdTe with the same energy gap. The n‐type lifetimes are comparable.

Mechanism for dislocation density reduction in GaAs crystals by indium addition

The strengthening of GaAs single crystals by the substitution of a few percent In on Ga sites is analyzed on the basis of a solid solution hardening model. The hardening entity is an In atom and its four nearest As neighbors. The predicted hardening is substantial and may account for the reduction in dislocation density in as‐grown GaAs crystals containing In.

Theoretical performance of InAs/ InxGa1−xSb superlattice‐based midwave infrared lasers

We show that for appropriate layer widths the performance of ideal InAs/InxGa1−xSb superlattice‐based midwave injection lasers can be limited by radiative rather than Auger recombination. The threshold carrier densities and lifetimes are calculated over the 77–300 K temperature range at 3.5 μm. Lifetimes are obtained from detailed calculations of band‐to‐band Auger and radiative recombination rates based on realistic nonparabolic band structures. This system is therefore a promising new laser candidate.  We show that for appropriate layer widths the performance of ideal InAs/InxGa1−xSb superlattice‐based midwave injection lasers can be limited by radiative rather than Auger recombination. The threshold carrier densities and lifetimes are calculated over the 77–300 K temperature range at 3.5 μm. Lifetimes are obtained from detailed calculations of band‐to‐band Auger and radiative recombination rates based on realistic nonparabolic band structures. This system is therefore a promising new laser candidate.

Theoretical performance limits of 2.1–4.1 μm InAs/InGaSb, HgCdTe, and InGaAsSb lasers

Ideal threshold current densities of 2.1–4.1 μm IR lasers are calculated for active layers composed of InAs/InGaSb superlattices, InGaAsSb quantum well quaternaries, InAsSb bulk ternaries, and HgCdTe superlattices. The fully K‐dependent band structure and momentum matrix elements, obtained from a superlattice K⋅p calculation, are used to calculate the limiting Auger and radiative recombination rates and the threshold current density. InGaAsSb quantum wells and InAs/InGaSb superlattices are found to be more promising laser candidates than HgCdTe superlattices and InAsSb bulk ternaries. The calculated threshold current densities of InAs/InGaSb superlattices are similar to those of InGaAsSb active layers at 2.1 μm, but are significantly lower at longer wavelengths. Comparison with experiment indicates that the threshold current densities of InGaAsSb‐based devices are about three times greater than those calculated for 25 cm−1 gain. The threshold current densities of present InAs/InGaSb superlattices are abou...

Theoretical performance of very long wavelength InAs/InxGa1−xSb superlattice based infrared detectors

Optimal detectivities of very long wavelength (11–19 μm) photovoltaic infrared detectors based on ideal InAs/InGaSb superlattices are calculated. Accurate K⋅p band structures are used to obtain radiative, electron–electron and hole–hole band‐to‐band Auger, and for the first time shallow acceptor level assisted Auger recombination rates for n‐on‐p photodiodes. The suppression of band‐to‐band Auger by ‘‘band gap engineering’’ is predicted to lead to improved background‐limited operating temperatures just as it does in long‐wave InAs/InGaSb infrared detectors.

Phonon dispersion effects and the thermal conductivity reduction in GaAs/AlAs superlattices

The experimentally observed order-of-magnitude reduction in the thermal conductivity along the growth axis of (GaAs)n/(AlAs)n (or n×n) superlattices is investigated theoretically for (2×2), (3×3) and (6×6) structures using an accurate model of the lattice dynamics. The modification of the phonon dispersion relation due to the superlattice geometry leads to flattening of the phonon branches and hence to lower phonon velocities. This effect is shown to account for a factor-of-three reduction in the thermal conductivity with respect to bulk GaAs along the growth direction; the remainder is attributable to a reduction in the phonon lifetime. The dispersion-related reduction is relatively insensitive to temperature (100<T<300 K) and n. The phonon lifetime reduction is largest for the 2×2 structures and consistent with greater interface scattering. The thermal conductivity reduction is shown to be appreciably more sensitive to GaAs/AlAs force constant differences than to those associated with molecular masses.

Multilayer thermoelectric refrigeration in Hg1−xCdxTe superlattices

The thermoelectric figure of merit ZT of Hg1−xCdxTe superlattices (SLs) having currents along the growth axis is computed using a realistic SL band structure and the multisubband Boltzmann equation. For a narrow well and wide barrier, a heavy C1 and higher-lying light C2 subband combine to form a (nonoptimal) carrier-energy filter, enhancing the thermopower. The multilayer thermionic emission model accounts for this effect qualitatively but not quantitatively. However, for a narrow well and narrow-barrier system, ZT is 20% larger than that in the wide-barrier structure, indicating that devices based on carrier-energy filter/thermionic processes are not necessarily advantageous. ZT is almost three times larger than that in Bi2Te3 and is four times larger than that in an alloy with the average composition of the SL. This effect is associated with reduced lattice thermal conductivity in the SL rather than improved electronic transport.

Optical Constants in the X‐Ray Range

In conjunction with x‐ray transmission data, the integral relationship between the refraction index and the absorption coefficient can be employed to extend the range over which optical constants have been determined by Kramers‐Kronig analysis of reflectance data. The procedure is applied to measurements on aluminum and the optical constants are determined for the energy range 10−3 to 105 eV. The results are useful in testing some general sum rules. They also permit a comparison of the behavior of the dielectric constant for energies above the K absorption edge with the expected asymptotic behavior.