Investigation of Threshold Carrier Densities in the Optically Pumped Amplified Spontaneous Emission of Formamidinium Lead Bromide Perovskite Using Different Excitation Wavelengths

The high crystal quality of formamidium lead bromide perovskite (CH(NH2)2PbBr3 = FAPbBr3) was infiltrated in a mesoporous TiO2 network. Then, high-quality FAPbBr3 films were evaluated as active lasing media, and were irradiated with a picosecond pulsed laser to demonstrate amplified spontaneous emission (ASE), which is a better benchmark of its intrinsic suitability for gain applications. The behavior was investigated using two excitation wavelengths of 440 nm and 500 nm. Due to the wavelength-dependent absorbance spectrum and the presence of a surface adsorption layer that could be reduced using the shorter 440 nm wavelength, the ASE power dependence was strongly reliant on the excitation wavelength. The ASE state was achieved with a threshold energy density of ~200 µJ/cm2 under 440 nm excitation. Excitation at 500 nm, on the other hand, needed a higher threshold energy density of ~255 µJ/cm2. The ASE threshold carrier density, on the other hand, was expected to be ~4.5 × 1018 cm−3 for both excitations. A redshift of the ASE peak was detected as bandgap renormalization (BGR), and a BGR constant of ~5–7 × 10−9 eV cm was obtained.

Structural and Electronic Properties of Laser Crystallized Silicon Films

Fundamental properties of silicon films crystallized by a 30-ns-pulsed XeCI excimer laser were discussed. Although crystallization of 50-nm thick silicon films formed on quartz substrates occurred through laser hearing at the crystalline threshold energy density of 160 mJ/cm2, a higher laser energy density at 360 mJ/cm2 was necessary to crystallize silicon films completely. Analyses of free carrier optical absorption revealed that phosphorus-doped silicon films with a carrier density about 2×1020 cm−3 had a high carrier mobility of 20 cm2/Vs for irradiation at the crystallization threshold energy density, while Hall effect measurements gave a carrier mobility of electrical current traversing grain boundaries of 3 cm2/Vs. This suggested that the crystalline grains had good electrical properties. As the laser energy density increased to 360 mJ/cm2 and laser pulse number increased to 5, the carrier mobility obtained by the Hall effect measurements markedly increased to 28 cm2/Vs because of improvement of grain boundary properties, while the carrier mobility obtained by analysis of free carrier absorption increased to 40 cm2/Vs. A post annealing method at 190°C with high-pressure H2O vapor was developed to reduce the density of defect states. Increase of carrier mobility to 500 cm2/Vs was demonstrated in the polycrystalline silicon thin film transistors fabricated in laser crystallized silicon films.

Excimer Laser Planarization of Diamond Films

ABSTRACTThe planarization of rough polycrystalline diamond films synthesized by DC arc discharge plasma jet CVD was attempted using KrF excimer laser pulses. The effects of laser incidence angle and reaction gases (ozone and oxygen) on etching rate were studied. The temperature change of diamond and graphite with different laser fluences was calculated by computer simulation to explain the etching behavior of diamond films. The calculated threshold energy density for etching of pure crystalline diamond was about 1.7 J/cm2. However, the threshold energy density was affected by the angle of laser incidence. Preferential etching of a particular crystallographic plane was observed through scanning electron microscopy. The etching rate of diamond with ozone was lower than that with oxygen. Also, the etching rate of diamond films at normal laser incidence was lower than that of films tilted at 45° for laser fluences above 2.3 J/cm2. When the angle of incidence was 80° to the diamond surface normal, the peak-to-valley surface roughness was significantly reduced, from 30 μm to 0.5μm.

Threshold Energy Density for Pulsed Laser Annealing of Silicon

ABSTRACTWe have made a systematic investigation of the threshold energy density for recrystallization of ion-implanted silicon by Q-switched laser irradiation as function of thickness of the disordered layer, temperature during implantation, type and dose of implanted impurity, laser wavelength, and substrate orientation. Most results have been obtained with a Q-switched ruby laser. A linear dependence of the threshold on layer thickness (in the region of 60–300 nm) was found for arsneic-implanted silicon, but not for silicon-implanted silicon. For an amorphous layer thickness of 200 nm we found very little dependence of the threshold on type of dopant. In the case of the Nd:YAG laser, however, the lowest threshold was observed for column VI elements, the highest for column IV elements and intermediate and equal thresholds for the elements from column III and B. The influence of temperature during implantation was found to be small, but the threshold appeared to be different for (100)- and (111)- oriented substrates.