The wide-band-gap semiconductor thallium gallium selenide (TlGaSe2) is promising for X-ray and γ-ray detection. In this study, the synthesis and crystal growth of semiconducting TlGaSe2 was accomplished using a stoichiometric combination of TlSe, Ga, and Se and a modified Bridgman method. These large detector-grade crystals can be synthesized and cut to dimensions appropriate for a detector. The crystals have mirror-like cleaved surfaces and are transparent red, in agreement with a band gap of 1.95 eV observed in absorption measurements. Single-crystal X-ray diffraction refinements confirm that TlGaSe2 crystallizes in the monoclinic C2/c space group with a layered crystal structure consisting of planes of GaSe4 corner-sharing tetrahedra connected by weak Tl–Se bonds. Electronic band structure calculations made using the full-potential linearized augmented plane wave method with the screened-exchange local density approximation, including spin orbit coupling, indicate the unusual characteristic of the hole effective mass being lower than that of the electrons. Photoconductivity measurements on the grown TlGaSe2 crystals show mobility–lifetime (μτ) products of electrons and holes approaching the values of the state-of-the-art commercial material Cd0.9Zn0.1Te. The promising properties of this material system are confirmed by the ability of a TlGaSe2-based detector to show good signal response to X-rays and resolve Ag K radiation energetically.

The heavy element semiconductor compound Cs2Hg6S7 is of interest as a potential wide gap semiconductor for gamma ray detection. To determine electrically active defects and their energy levels, photoconductivity (PC) spectroscopy was carried out over the temperature range of 90-295 K. The low temperature spectrum exhibits photoconductive transitions at 1.495, 1.61, 1.66, and 1.68 eV. The optical transitions are tentatively attributed to defects with levels located at energies of 50, 70, 120, and 240 meV from the band edge. A superlinear dependence of photocurrent on illumination intensity is observed that is attributed to a two-center recombination process that involves shallow traps and recombination centers. Near band edge photoluminescence (PL) was observed over the temperature range of 24–80 K. The spectrum revealed three defect related emission bands located at 1.68, 1.66, and 1.62 eV, whose ionization energies are 57 meV, 78 meV, and 115 meV, respectively. From the temperature and excitation dependencies of the observed peak intensities and energies, the radiative recombination mechanisms of the bands were attributed to transitions involving excitons bound to neutral and ionized acceptors. Good agreement of the defect level energies determined by PL and PC were noted, indicating that they were of the same origin. The defects were tentatively attributed to metal vacancies that form shallow acceptor levels.

We report the first demonstration of room-temperature (RT) lasing at 1.3 µm from the ground state of three-stacked InAs quantum dots (QDs) in an In0.15Ga0.85As quantum well, which was grown by atomic layer epitaxy (ALE). For an as-cleaved device with a 2000-µm-long × 15-µm-wide ridge structure, the threshold current density (J th) at RT is 155 A/cm2 with the ground state lasing at 1310 nm under pulsed operation. The thermal coefficient of a lasing wavelength shift is 0.53 nm/K and the characteristic temperature is 103 K near RT. The lasing wavelength of the QD laser diodes (LDs) shows simultaneous lasing and the state switching from the ground state at 1310 nm and to the first excited state at 1232 nm with increasing injection current owing to the gain saturation of the ground state. The performance of ALE QD-LD is comparable to that of the conventional Stranski–Krastanov QD-LD.

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The Photoluminescence spectra (PL), their temperature and power dependence were investigated for the ground state in InAs quantum dots (QDs) embedded in InGaAs asymmetric quantum well (Asym. QW). In-atom segregation is well known phenomena in such structures, which result in altering the inter-atomic distances; as a consequence the thermo-dynamical parameters change as well, namely Debye temperature. The bigger value of Debye temperature for the studied sample with respect to the corresponding bulk value is attributed to In/Ga inter-diffusion during growth. The inter-diffusion process causes non-radiative defects in the sample. As a consequence, rapid decrease in the QDs integrated emission intensity as the temperature increases was occurred.