Miguel García-Rocha

Thermal quenching of the self-activated band of ZnSe:Cl thin films grown by molecular beam epitaxy

Abstract We studied the thermal quenching of the self-activated (SA) band of molecular beam epitaxy (MBE) grown ZnSe:Cl thin films by means of temperature dependent photoluminescence (PL) experiments. We analyzed the spectra of the self-activated (SA) band as a function of temperature and Cl concentration. All studied samples presented the emission of this band, however, the excitonic emission was observed only for those samples with lower Cl concentration. A different activation energy ( E a ) associated to quenching of the SA band was obtained for each sample. These values suggest that different electron and hole levels, which depend on Cl concentration, are associated to the mechanism of quenching of the SA band.

Thermal Properties of Calcium Phosphate for Biomedical Use

The possibilities that offer the photo thermal techniques to determine the thermal properties widely have been demonstrated [1,2]. From these techniques, photo acoustic and photo pyrelectric has turned out to be very useful in studies of biological materials, which frequently present many difficulties due to a high dispersion to the light and structures that vary with the depth. In this work we used the photo acoustic technique in the configuration of open cell and the photo pyrelectric technique, to determine the thermal properties of natural and synthetic calcium phosphate for biomedical use. We report the determination of the values obtained for the diffusivity (a), effusivity (e), thermal conductivity (k) and heat capacity (ρc), of 6 synthetic calcium phosphate samples and 5 natural calcium phosphate samples. The values obtained for the thermal properties of synthetic calcium phosphates turn out to be 82% similar to the values obtained for calcium phosphates of natural origin. This behavior allows us ...

Study of the recombination around the excitonic region of MBE ZnSe: Cl thin films

The recombination processes around the excitonic region of undoped ZnSe and chlorine doped ZnSe thin films were studied by continuous-wave photoluminescence (cw-PL) and time-resolved photoluminescence (TRPL) spectroscopies. Samples with different chlorine concentration were obtained by varying the temperature of the Cl source. The evolution of the PL signal and its decay time were analyzed as a function of temperature. Activation energy (Ea) values associated to the quenching of the D0X and band-to-band emission were obtained from the temperature dependent cw-PL experiments. The activation energy was lower for the film with higher Cl content. The characteristic exponential decay time (TPL) of the PL signal decreased with increasing Cl concentration.

Deep Level Emission And Electrical Activity Of Chlorine In Epitaxial ZnSe Thin Films

We present a study of the deep level emission of n‐type doped ZnSe films as a function of Cl concentration with the purpose to identify features related to the incorporation of Cl at non‐electrically active sites. The initial incorporation of Cl improves the electrical properties of the ZnSe film but further doping degrades the excitonic emission and induces the presence of the SA band. The degradation of the electrical properties appears to be related to deeper energy levels with emission below 2 eV.

Red to Blue Excitonic Emission with Ultra‐Thin Quantum Wells and Fractional Monolayer Quantum Dots of II–VI Semiconductors

Ultra‐thin quantum wells of CdTe and CdSe and fractional monolayer quantum dots of CdSe present very strong excitonic emission covering the red‐blue spectral range. They are grown by atomic layer epitaxy and the thickness fluctuations can be completely avoided in many cases by choosing the appropriate substrate temperature. These systems appear as promising candidates for the elaboration of red to blue light emission devices.

Applications of photoluminescence with continuous and pulsed excitation

Photoluminescence (PL) belongs to the group of non-destructive characterization techniques, which consists basically in causing to impact light in a region of the material under study. Then, the material absorbs part of the energy of the incident light and, as a consequence, emits light. This technique is widely employed in different areas of the Industry and Basic Research; for example, in Medicine, Chemistry, Biology, Physics and in several areas of Engineering. As excitation source it can be used lamps, lasers and even solar light. On the other hand, there are processes that can occur to extremely short time scales. In this case new techniques have been developed. Among them, we can mention Time Resolved Photoluminescence. In this technique ultra-short laser pulses are employed which have a temporal width from a few nanoseconds (x10-9 sec) until femtoseconds (x10-15 sec). The use of pulsed lasers has served for the development of new methods of investigation and technologies, as has been the case of Femto-Chemistry, new Surgery methods in Medicine, Micro- and Nano- machining of materials, etc. In our case, we utilize Photoluminescence and the Time Resolved Photoluminescence for the characterization of semiconductors materials that have potential applications in Optoelectronics.

Ultra-thin quantum wells and fractional monolayer quantum dots of II-VI semiconductors for optoelectronic applications

We will present the results of the combined growth by molecular beam epitaxy (MBE) and atomic layer epitaxy (ALE) of CdSe/ZnSe and CdTe/ZnTe ultra-thin quantum wells (UTQWs), characterized by means of photoluminescence spectroscopy (PL). In spite of its reduced thickness, a single QW, few monolayers (ML) thick, is able to produce bright emission even under low power excitation. These nanostructures are of great interest for optoelectronic applications such as LEDs and diode lasers. In-situ, reflection high energy electron diffraction (RHEED) experiments indicated the 2D growth of the UTQWs and the real-time temporal analysis of the intensity of RHEED pattern features demonstrated the absence of degradation of their structural quality during the ALE growth. Fractional monolayer quantum dots (FMQDs) produced by the deposition of /spl sim/0.5 ML of CdSe present very intense and narrow deep blue excitonic emission. The sharp luminescence of the FMQDs is indicative of their highly uniform size and shape. Even though the 1ML QWs are very thin, the calculated transitions obtained by means of the envelope function in the effective mass approximation are in quite good agreement with the observed excitonic transitions.

Photoluminescence study of CdTe/ZnTe ultra-thin quantum wells grown by pulsed beam epitaxy

CdTe ultra-thin quantum wells (UTQWs) within ZnTe barriers were grown by pulsed beam epitaxy (PBE) on GaAs(001) substrates. In-situ reflection high energy electron diffraction (RHEED) patterns and real-time spot intensity measurements indicated a high structural quality of the QWs. Low temperature photoluminescence (PL) experiments indicated a clear influence of the growth temperature on the structural properties of the samples. The 2 monolayer (ML) thick UTQW grown at Ts = 270°C exhibited an intense and sharp peak at 2.26 eV whereas the 4 ML thick UTQW (Ts = 290°C) presented an intense peak at 2.13 eV and a weak one around 2.04 eV. This behavior is discussed in terms of Cd re-evaporation at the higher Ts.