Dependence Of Photoluminescence Efficiency Research Articles

Influence of Undoped‐AlGaN Final Barrier of MQWs on the Performance of Lateral‐Type UVB LEDs

Aluminium gallium nitride (AlGaN)‐based ultraviolet‐B light‐emitting diodes (UVB LEDs) are expected to offer smart size, wider choice of UVB light emission in the wavelengths range of 280 nm > λ > 320 nm, and low cost as well as low power consumption compared with other UV light sources including toxic mercury UV‐lamps. The hole‐tunneling from p‐AlGaN side of UVB LED into the multi‐quantum‐wells (MQWs) is strongly dependent on the thickness (TFB) and Al‐contents of undoped (ud)‐AlGaN final barrier (FB). Herein, the photoluminescence (PL) efficiency from MQWs of the UVB LED devices is investigated and compared with the electroluminescence (EL) spectra as a function of TFB. Subsequently, the dependence of PL efficiency, external quantum efficiency (EQE), and light output power on the TFB of UVB LEDs is attempted, using the same growth condition for all samples except variation in TFB. When TFB is set to 6–7 nm, improvements in the EQE and light output power, respectively, from 4.3% and 7 mW to the high values of 5.6% and 17 mW in emission band of 295–300 nm under continuous‐wave (cw) at room temperature (RT) are achieved.

Internal quantum efficiency of InGaN/GaN multiple quantum well

The InGaN/GaN multiple quantum wells are grown on a (0001)-oriented sapphire by using metalorganic chemical vapor deposition. Dependences of the photoluminescence (PL) peak energy and PL efficiency on injected carrier density and temperature are studied. The results show that the temperature-dependent behavior of the peak energy is in the manner of decrease-increase-decrease (S-shaped), and the maximum of the PL efficiency is observed at about 50 K. The former is attributed to the potential inhomogeneity and local characteristics of the carrier recombination in the InGaN matrix. The latter indicates that the traditional method that the internal quantum efficiency (IQE) is considered to be 100% at low temperature, should be corrected. Furthermore, it is found that the IQE depends on not only temperature but also injected carrier density. Based on the above discussion, an improved method of setting the IQE, i.e., measuring the dependence of PL efficiency is proposed.

Dependence of photoluminescence efficiency on size in silicon nanostructures: phenomenological investigations

The discovery of efficient photoluminescence (PL) in porous and nanocrystalline silicon has generated considerable interest over the last decade. The present work makes the following simple proposition: the dependence of the PL efficiency on size is non-monotonic and hence there exists an optimum size for PL efficiency in nanocrystalline silicon. We present simple phenomenological formulae which delineate the dependence of the efficiency on size. Besides the existence of an optimum size they also indicate that there is a nanometer range over which the efficiency is substantial. This assertion is not made on the basis of a theoretical calculation but by a wide study and careful analyses of extant literature, largely experimental. We present two sets of compelling reasons for our assertion, one empirical, and the other semi-empirical. We indicate how the assertion of the optimum size may be verified by experimental studies.

Dependence of photoluminescence efficiency on size in silicon nanostructures: phenomenological investigations

The discovery of efficient photoluminescence (PL) in porous and nanocrystalline silicon has generated considerable interest over the last decade. The present work makes the following simple proposition: the dependence of the PL efficiency on size is non-monotonic and hence there exists an optimum size for PL efficiency in nanocrystalline silicon. We present simple phenomenological formulae which delineate the dependence of the efficiency on size. Besides the existence of an optimum size they also indicate that there is a nanometer range over which the efficiency is substantial. This assertion is not made on the basis of a theoretical calculation but by a wide study and careful analyses of extant literature, largely experimental. We present two sets of compelling reasons for our assertion, one empirical, and the other semi-empirical. We indicate how the assertion of the optimum size may be verified by experimental studies.

Precipitation of highly luminescent phases from PECVD Si suboxides

ABSTRACTSi nanoclusters (Si-nc) embedded in SiO2 present outstanding luminescent emission in the visible and are the material of choice for the realization of efficient light sources integrated with Si technology. PECVD is an attractive preparation route but there is still the need to understand how Si excess and matrix composition affect the precipitation of Si-nc and their photoluminescence (PL) efficiency. The SiOx PECVD layers studied here have a Si excess up to 50% and a thickness between 50 and 100 nm. The phase separation, precipitation and growth of the Si-nc have been achieved by annealing at 1250 °C. For reference, the same study has been performed in Si-nc/SiO2 materials synthesized by ion implantation and annealing. Refractive index and thickness measured by ellipsometry show a densification of the layers after the H release during annealing. A detailed composition profile has been determined by XPS and FTIR analyses and shows almost complete phase separation except for the interfaces, where a depletion of Si-nc is found. EFTEM demonstrates that isolated Si-nc are formed for Si excess up to 25% while for higher Si excess a continuous Si phase is observed. The PL efficiency in PECVD samples is maximized for a Si excess of 17% which is the same Si excess than that for the most emitting implanted samples. No dependence of PL efficiency has been found on the presence of Nitrogen in the matrix (up to the 10%).

Radiative and nonradiative recombination in polymerlikea−C:Hfilms

Localized electronic states corresponding to \ensuremath{\pi} electrons in hydrogenated amorphous carbon have been investigated using photoluminescence (PL) and optical-absorption spectroscopies. Carbon films contain \ensuremath{\pi}-bonded ``grains'' with different configurations which spatially confine photogenerated electron-hole pairs. A model of PL in $a\ensuremath{-}\mathrm{C}:\mathrm{H},$ including excitonlike radiative recombination and electron tunneling to nonradiative acceptor centers at constant energy, is proposed to explain the following results. The broad PL emission spectra are composed of three bands (at 2.20--2.40 eV, 2.65 eV, and 2.95 eV, independent of the excitation energy) arising from different structural units containing $\mathrm{C}\mathrm{=}\mathrm{C}$ double bonds. Resonance features have been evidenced in the PL excitation spectra and attributed to excitation of confined electron-hole pairs similar to confined exciton optical transitions. Excitonic behavior is consistent with the redshift of peak (b) excitation resonance observed as a function of increasing gap (decreasing \ensuremath{\varepsilon}). Relative values of the radiative rates deduced from PL efficiency indicate that the exciton radius has the same order of magnitude for all three emissive centers. Nonradiative recombination cannot be explained by multiphonon emission processes. The dependence of PL efficiency on the density of acceptor sites (deduced from $\ensuremath{\pi}\ensuremath{-}{\ensuremath{\pi}}^{*}$ optical transitions) can rather be described by electron tunneling from a confined exciton toward nonradiative localized ${\ensuremath{\pi}}^{*}$ states arising from distorted \ensuremath{\pi}-bonded sites. This mechanism accounts for the anticorrelation of the PL efficiency with the optical gap, the decrease of the decay time as a function of emission energy, and the relative enhancement of the blue emission (2.95-eV peak) as a function of decreasing optical gap. In contrast, the band-tail model proposed earlier leads to unrealistically low values of the electron Bohr radius.

Characterization of interfaces between a-Si: H and a-SiN: H layers by photoluminescence measurement

Abstract In order to elucidate the difference in interface quality between a-Si: H and a-SiN:H layers arising from the order of their deposition, the dependence of photoluminescence efficiency on the thickness of the a-Si: H layer has been studied for three types of specimen structure: a-Si: H/fused silica, a-SiN: H/a-Si: H/fused silica and a-SiN: H/a-Si: H/a-SiN: H/fused silica. Since each type of structure contains a single a-Si: H layer and a combination of different kinds of interface adjacent to it, the defect densities at individual interfaces adjoining the a-Si: H layer can be evaluated separately. The interface between a-SiN: H and a-Si: H when the former is deposited on the latter is of good quality, having a defect density less than 1011 cm−2, whereas interface of the reverse structure contains a higher defect density by about an order of magnitude.