Koji Shinohara

Properties of PbTe/EuTe short period superlattices and their application to laser diodes

Abstract Short period superlattices (SPSLs) consisting of PbTe (E g =0.3eV) and EuTe (E g -2.0eV) were prepared on KCl (100) substrates by alternate growth of the PbTe (∼100 A) and Eu (less than a few atomic layers) using a hot-wall technique. The structural properties were studied through x-ray diffraction and electron-probe micro analysis. The band gap of the SPSL, depending on average Eu (EuTe) composition, was determined by optical transmission measurement. It was found that the band gap of the SPSL of a small EuTe composition increases very sensitively with the EuTe composition, while that of a large EuTe composition depends only on the period length, resulting from the quantum size effect due to large band gap of EuTe (barrier). Results of magneto resistance measurements of the SPSL are also presented. A multiple quantum-well (MQW) laser was fabricated using the SPSL for barrier layers in the MQW active region and cladding layers. The laser operated up to 160 K (pulsed).

PbSnTe multiple quantum well lasers for pulsed operation at 6 μm up to 204 K

Lattice‐matched PbSnTe‐PbTeSe multiple quantum well lasers fabricated by hot wall epitaxy have been developed. Though the PbSnTe‐PbTeSe system is a type I’ superlattice, by using band bending due to doping, laser operation like type I superlattices has been achieved for the first time. The laser worked well in pulsed operation up to 204 K with 6 μm radiation (cw operation at 130 K with 6.6 μm radiation).

Misfit dislocations in PbTe-PbSnTe heterojunction

Abstract Misfit dislocations at the heterojunction of PbTe-PbSnTe layers grown by liquid phase epitaxy have been investigated by etch pit studies. The linear density of misfit dislocations along the 〈100〉 direction parallel to the heterojunction was in good agreement with the predicted value. This agreement indicates that the strain due to lattice mismatch between PbTe and PbSnTe is completely relieved by the introduction of misfit dislocations. It has been shown from the distribution of etch pits, electron probe analysis, and calculations, that the misfit dislocations spread around the heterojunction in the region determined by Sn self-diffusion during epitaxial growth.

Lasing mechanism of type-I' PbSnTe-PbTeSe multiquantum well laser with doping structure

Even though PbSnTe–PbTeSe superlattice has a type‐I’ structure, its multiquantum well (MQW) laser has been realized by introducing band bending in the MQW active region. The MQW laser showed anomalous temperature dependence in the threshold current and the corresponding output photon energy. The lasing mechanism and anomalous behavior of the laser is qualitatively well explained by taking into account the variation of band bending due to large temperature dependence of the dielectric constants, assuming the barrier height ΔEc=−40 meV.

Lead-europium-chalcogenide films and superlattices for 3 μm laser

Abstract Pb 1−x Eu x Te multiple quantum well and double heterostructure lasers were prepared by hot wall epitaxy. Pulsed laser operations were observed for the multiple quantum well laser up to 230K (3.8 μm), and 205K (3.4 μm) for the double heterostrucsture laser. Pb 1−x Eu x S films and superlattices were also prepared. Step-like absorptions corresponding to interband electron transitions between valence and conduction subbands were observed in the optical transmission spectra of the superlattices. Electrical properties of doped Pb 1−x Eu x S films were also measured. High carrier concentration n-type films were obtained by doping bismuth, but films doped by acceptor impurities had always small carrier concentration, owing to the deep donor levels due to europium.

High Sensitivity Short-Path Monitoring of Trace Gases Employing PbSnTe Tunable Diode Laser

A system for atmospheric trace gas monitoring is described. The absorption spectrum in the infrared region of a specific gas is involved. A lead-salt diode laser is employed for the tunable light source and the short optical path of less than one meter allows the equipment to be portable. Fast measurement can be performed thanks to be built-in microcomputer and specially developed electronic system. Reliability is achieved because a correlational algorithm is calculated by the computer avoiding the ambiguity of the absolute laser frequency. Gas-density data is obtained every few seconds. Experiments were executed for methane and a sensitivity of 0.3 ppmm with S/N=1 was attained being limited by parasitic etalon fringes. A higher sensitivity down to 0.01 ppmm is possible provided that a transversally mono-modal laser is employed.

Single-mode PbSnTe laser with a buried heterostructure

A Pb(1-x)Sn(x)Te single-mode laser with a buried heterostructure has been developed by use of a two-step liquid-phase epitaxial growth. The active-layer size was calculated for fundamental transverse-mode operation. Tin diffusion was simulated in the active layer during the second growth step, and the results were used to select the optimum growth temperature. Analysis of the crystal-growth mechanism in the buried-heterostructure formation enabled us to develop a new growth method. The laser we developed showed single-mode operation with a wavelength of 8 µm and a power of 1.7 mW at 50 K. At 80 K, the threshold current for continuous-wave (cw) operation is 60 mA, which is nearly equal to the threshold current for pulsed operation. Maximum operating temperatures are 105 K for cw operation and 144 K for pulsed operation.

Heterointerface morphology of Pb1-YSnYTe-Pb1-XSnXTe (X, Y ⪅ 0.3) double heterostructure (DH) grown by LPE

The heterointerface morphology of Pb1-YSnYTe-Pb1-XSnXTe (X, Y ⪅ 0.3) DH grown by LPE has been investigated. The heterointerfaces of Pb0.93Sn0.07Te-Pb0.85Sn0.15Te DH are flat, and those of Pb0.87Sn0.13Te-Pb0.75Sn0.25Te DH are rough under the usual equilibrium cooling growth conditions. It has been found that this roughening phenomenon occurs because the melt and the epitaxial layer at the interface deviated much from equilibrium when the melt was slid on the substrate. There are two ways to get flat interfaces with an active layer composition about X = 0.25. One is to grow layers under supercooled conditions. The other is to reduce the compositional difference between the active and the two confining layers.

Utilization of the tunability of the lead-salt diode laser for a fast and local measurement of specific gas density

In the optical gas monitoring method using a lead-salt diode laser, a spectral scanning operation of the diode brings an immunity against spectral interference which has limited the sensitivity of prevailing methods. Local, sensitive, and noncontacting features as well as quickness are attained simultaneously. Experiments for atmospheric methane showed a sensitivity of 0.3 ppm over a path length of only 1 m. Density data were obtained at every 4 s and the immunity for spectral interference was demonstrated. Theoretically, a sensitivity of 0.015 ppm for 1 m is expected.

Pb1-xSnxTe Lasers With High Efficiencies

Internal quantum efficiencies of Pb Sn Te lasers with double-hetero geometries have been estimated. The efficiencies of conventional PbTe/PbSnTe/PbTe lasers are very low and far from the efficiencies which are limited by Auger-recombination mechanism. The main cause of low efficiency is thought to be due to the misfit dislocations at the PbTe-PbSnTe hetero-interface. To reduce the misfit dislocations, lattice-matched PbTeSe/PbSnTe/PbTeSe lasers are prepared and they show considerable improvements. However, due to the difficulty of cleaving of PbTeSe layers, the efficiencies are not sufficient. By designing a one-side-lattice-matched PbTeSe/PbSnTe/PbTe structure, the lasers whose efficiencies are very close to the theoretical limit can be prepared.