In this work, we used the chemical vapor transport (CVT) method to grow PbI₂ crystals using iodine as a self-transporting agent. The crystals’ structure, composition, and uniformity were confirmed by X-ray diffraction (XRD) and electron probe microanalysis (EPMA) measurements. We investigated the band gap energy using absorption spectroscopy measurements. Furthermore, we explored the temperature dependence of the band gap energy, which shifts from 2.346 eV at 300 K to 2.487 eV at 20 K, and extracted the temperature coefficients. A prototype photodetector with a lateral metal–semiconductor–metal (MSM) configuration was fabricated to evaluate its photoelectric properties using a photoconductivity spectrum (PC) and persistent photoconductivity (PPC) experiments. The resonance-like PC peak indicates the excitonic transition in absorption. The photoresponse I_Light/I_Dark-1 is up to 200%.

Using room-temperature photoreflectance (PR) and photoluminescence (PL) we have characterized four pseudomorphic AlGaAs/InGaAs/GaAs high electron mobility transistor structures with varied quantum well compositional profiles. Several features from the InGaAs modulation doped quantum well portion of the samples have been observed in addition to signals from the AlGaAs, GaAs, and GaAs/AlGaAs superlattice (SL) buffer layer. The PR spectra from the InGaAs quantum well channel can be accounted for by a line shape function which is the first derivative of a step-like two-dimensional density of states and a Fermi level filling factor. A detailed line shape fit makes it possible to evaluate the Fermi energy, and hence the concentration of two-dimensional electron gas in addition to the energies of the intersubband transitions. The lowest lying intersubband transition has been confirmed by a comparison of the PR and PL spectra. From the difference of intersubband transition energies, the surface segregation effects of indium atoms are demonstrated. In addition, other important parameters of the system such as built-in electric field, Al composition, as well as the properties of the GaAs/AlGaAs SL buffer layer are evaluated.

Piezoreflectance (PzR) and contactless electroreflectance (CER) measurements of an In_0.86Ga_0.14As_0.3P_0.7/InP quantum well heterostructure as a function of temperature in the range of 20–300 K have been carried out. A careful analysis of the PzR and CER spectra has led to the identification of various excitonic transitions, mnH(L), between the mth conduction band state and the nth heavy (light)-hole band state. The parameters that describe the temperature dependence of $E_{\mathrm{mn}\mathrm{H(L)}}$ are evaluated. A detailed study of the temperature variation of excitonic transition energies indicates that the main influence of temperature on quantized transitions is through the temperature dependence of the band gap of the constituent material in the well. The temperature dependence of the linewidth of 11H exciton is evaluated and compared with that of the bulk material.

In this paper, the synthesis and characterization of CuIn_1−xGa_xSe₂ (0 ≤ x ≤ 1) nanocrystals are reported with the influences of x value on the structural, morphological, and optical properties of the nanocrystals. The X-ray diffraction (XRD) results showed that the nanocrystals were of chalcopyrite structure with particle size in the range of 11.5–17.4 nm. Their lattice constants decreased with increasing Ga content. Thus, the x value of the CuIn_1−xGa_xSe₂ nanocrystals was estimated by Vegard’s law. Transmission electron microscopy (TEM) analysis revealed that the average particle size of the nanocrystals agreed with the results of XRD. Well-defined lattice fringes were shown in the TEM images. An analysis of the absorption spectra indicated that the band gap energy of these CuIn_1−xGa_xSe₂ nanocrystals was tuned from 1.11 to 1.72 eV by varying the x value from 0 to 1. The Raman spectra indicated that the A₁ optical vibrational mode of the nanocrystals gradually shifted to higher wavenumber with increasing x value. A simple theoretical equation for the A₁ mode frequency was proposed. The plot of this equation showed the same trend as the experimental data.

Co-doped and undoped layered MoS₂ crystals were grown by the chemical vapor transport method using iodine as the transport agent. Both reflectance and piezoreflectance measurements reveal two exciton transitions of the direct band edge around 1.86 and 2.06 eV for undoped MoS₂ and 1.84 and 2.03 eV for Co-doped MoS₂. Hall effect measurements show that the Co-doped MoS₂ sample has a lower carrier concentration and mobility than the undoped sample. These differences between undoped and Co-doped MoS₂ were attributed to the effect of cobalt atoms causing a small lattice distortion, lattice imperfections and/or impurity states that form trap states between the conduction band and valence band. Furthermore, photoconductivity (PC) and persistent PC results show that Co-doped MoS₂ has a longer time constant and better responsivity than undoped MoS₂. This work discusses the advantages of Co-doped MoS₂ for photodetector applications.

${\mathrm{Ga}}_{0.69}{\mathrm{In}}_{0.31}{\mathrm{N}}_x{\mathrm{As}}_{1-x}∕\mathrm{GaAs}$ single quantum well (SQW) structures with three different nitrogen compositions ( $x=0\%$ , 0.6%, and 0.9%) have been characterized, as functions of temperature in the range $10–300\mathrm{K}$ , by the techniques of photoreflectance (PR) and photoluminescence (PL). In PR spectra, clear Franz-Keldysh oscillations (FKOs) above the $\mathrm{GaAs}$ band edge and the various excitonic transitions originating from the QW region have been observed. The built-in electric field in the SQW has been determined from FKOs and found to increase with N concentration. The PR signal has been found to decrease for nitrogen incorporated samples when the temperature was lowered due to a weakening of the modulation efficiency induced by carrier localization. A careful analysis of PR and PL spectra has led to the identification of various excitonic transitions, mnH(L), between the $m$ th conduction band state and the $n$ th heavy (light)-hole band state. The anomalous temperature dependent 11H transition energy and linewidth observed in the PL spectra have been explained as originating from the localized states as a result of nitrogen incorporation. The temperature dependence analysis yields information on the parameters that describe the temperature variations of the interband transitions.