C. A. Hoffman

Type‐II quantum‐well lasers for the mid‐wavelength infrared

We discuss an improved mid‐wave infrared diode laser structure based on InAs‐Ga1−xInxSb‐ InAs‐Ga1−xAlxSb Type‐II multiple quantum wells. The proposed design combines strong optical coupling, 2D dispersion for both electrons and holes, suppression of the Auger recombination rate, and excellent electrical and optical confinement.

Off‐resonant third‐order optical nonlinearities of metal‐substituted phthalocyanines

The third‐order optical susceptibility of the Pt, Pb, and metal‐free tetrakis(cumylphenoxy)phthalocyanines was measured by degenerate four‐wave mixing at 1.064 μm, far from strong absorption bands. Metal substitution strongly enhances the off‐resonant χ(3). The χ(3)xxxx for Pt‐phthalocyanine (2×10−10 esu) and Pb phthalocyanine (2×10−11esu) is about 45 times and 5 times that of the metal‐free form (4×10−12 esu), respectively.

Auger lifetime in InAs, InAsSb, and InAsSb‐InAlAsSb quantum wells

The intensity‐dependent photoconductive response to 2.06 μm excitation has been used to determine Shockley‐Read and Auger lifetimes for InAs, InAs0.91Sb0.09, and an InAs0.85Sb0.15‐InAlAsSb multiple quantum well. The Auger rate at 77 K correlates with the proximity to resonance between the energy gap and the split‐off gap. Thus the Auger coefficient in the alloy decreases with decreasing temperature, whereas that in the quantum well increases by nearly a factor of 5 between 300 and 77 K.

Large magnetoresistance in postannealed Bi thin films

We have observed a large increase in the magnetoresistance (MR) of molecular beam epitaxy grown Bi thin films, which were subjected to a postannealing procedure 3 °C below the Bi melting point. We have achieved an increase in the MR by a factor of 2560 at helium temperatures compared with of 343 for an as-grown film. The enhancement of the MR in the annealed films is due to higher electron and hole mobilities (μe≈1×106 cm2/V s at 5 K) relative to those of the as-grown films (μe≈9×104 cm2/V s at 5 K). The enhancement of the mobility in the annealed films is also supported by the observation of Shubnikov–de Haas oscillations.

MBE grown mid-infrared type-II quantum-well lasers

We have employed molecular-beam epitaxy (MBE) for the growth of mid-infrared (MIR) type-II quantum-well laser structures. These lasers consist of strain-balanced InAs/InGasb/InAs/AlSb type-II quantum wells lattice-matched to the AlSb cladding layers. We have demonstrated optically pumped lasers emitting from 3 to 4.3 μm under pulsed operation. For the 3.2-μm lasers, stimulated emission was observed at temperatures up to 350 K. The characteristic temperature T 0 at operation temperatures above ambient was 68 K. Here, we discuss the optimization of the MBE growth of MIR type-II quantum-well lasers, including substrate temperatures, V/III beam-equivalent pressure ratios, and shutter sequencing for better interface control and laser performance.

Thermoelectric power of MBE grown Bi thin films and BI/CDTE superlattices on CdTe substrates

Abstract We have measured the thermoelectric power (TEP) of epitaxial Bi thin films and Bi/CdTe superlattices grown on CdTe substrates as a function of temperature in the range 20–300 K. We have observed that the TEP of a 10 000ABi thin film is in good agreement with the bulk single crystal value and that the TEPs for superlattices with 400Aand 800ABi well thicknesses are enhanced over the bulk values. However, p -type doping effects in both thin films and superlattices lead to a positive TEP in samples with thinner quantum wells and a suppression of the magnitude in all cases. This suggests that the TEP may be enhanced further by altering the growth conditions to reduce the excess hole concentration.

Electron transport in InAs/Ga1-xInxSb superlattices

Abstract We have experimentally investigated the electron transport properties of a series of n -typeInAs/Ga 1- x In x Sb superlattices with InAs thicknesses ( d 1 ) between 25 and 86 A and a fixed Ga 1- x In x Sb thickness ( d 2 ) of 25 A. Interface roughness scattering is found to dominate the electron mobility ( μ n ), but with a much weaker dependence on layer thickness than the conventional d 1 6 . We observe an abrupt decrease in μ n ( d 1 ) at the semiconductor-to-semimetal transition point and the coexistence of two electron species in semimetallic samples. All of these findings can be understood by considering the theoretical band structures for InAs/Ga 1- x In x Sb superlattices with thin d 2 , which differ considerably from those in InAs/GaSb structures with comparable energy gaps and thick d 2 , but are strikingly similar to results for HgTe/CdTe. Since the system is strongly 3D rather than 2D in character, it is surprising that in some samples the quantum oscillations in the Hall conductivity are much larger than those in the diagonal conductivity.

Momentum-space reservoir for enhancement of intersubband second-harmonic generation

We analyze fundamental limits to the second-harmonic conversion efficiency attainable from semiconductor intersubband devices employing asymmetric stepped and double quantum wells. The coupled propagation equations have been solved numerically, accounting for saturation, absorption, and optical heating. It is found that the key figure of merit is the conversion efficiency at the onset of saturation, which has a remarkably simple form depending only on the ratio of broadening time to intersubband relaxation time and on another ratio involving the optical matrix elements. We show that since there are fundamental limits to the values of these ratios, it is unlikely that conversion efficiencies exceeding /spl ap/10% can be attained in devices of the type considered in the previous literature, and for surface incidence even efficiencies approaching that value will require impractically-thick active regions. While detuning from the double resonance condition is often advantageous, net improvements to the optimum performance are relatively modest. However, these limitations ran be transcended by placing the subband system in contact with an optically-inactive momentum-space reservoir, which shunts the intersubband relaxation and delays saturation by refilling the depleted subband states with electrons from the reservoir. We propose a specific device based on /spl Gamma/-valley active states and L-valley reservoir states in InAs-GaSb-AlSb asymmetric double quantum wells, whose energy levels and optical matrix elements are modeled using an 8-band finite-element calculation. It is predicted that a conversion efficiency of 20% can be achieved in an active-layer thickness of less than 10 /spl mu/m. >

HgTe‐CdTe superlattices for infrared detection revisited

Selected properties of HgTe‐CdTe superlattices are re‐examined in light of the new consensus that the valence‐band offset is large. We conclude that while the cutoff wavelength for infrared detectors remains easier to control in superlattices than in the corresponding Hg1−xCdxTe alloy, the advantage is less than was predicted earlier assuming a small offset. The reduction of tunneling noise and minority carrier collection efficiency are discussed on the basis of revised electron and hole masses in the growth direction.

Infrared electro‐optical modulators based on field‐induced Γ‐L intervalley transfer

Electro‐optical modulators based on intersubband transitions induced by normal‐incidence radiation in GaSb‐AlSb‐family asymmetric double quantum wells are proposed. An applied field transfers electrons from Γ‐valley states in one well to L‐valley states in the other well, thereby radically altering the selection rules, resonance energies, and in‐plane effective masses.