K.H. Chae

Photoresponse of Si detector based on n-ZnO/p-Si and n-ZnO/n-Si structures

Abstract The ZnO/Si photodiodes have been fabricated depositing n-ZnO films on n- and p-Si by rf sputtering method. All the n-ZnO/p-Si diodes show strong rectifying behavior characterized by the current–voltage ( I – V ) measurement under a dark condition while the n-ZnO/n-Si diodes showed weak rectifying behaviors. Photoelectric effects have been exhibited under an illuminated condition using a red light of 670 nm. High photocurrent or responsivities are obtained under a reverse bias when the crystalline quality of n-ZnO film is good enough to transmit the light into p-Si.

Valence band electronic redistribution in ion-beam-mixed PdAg alloys

Abstract The core-level binding energy shift and L2,3 absorption-edges for pure Pd, Ag, and ion-beam-mixed Pd1−xAgx (x = 0.5-0.9) alloys are measured to investigate the charge redistribution in ion-beam-mixed PdAg alloys. It is found that, in ion-beam-mixed alloys, there is a substantial decrease in the area of the Pd L2,3 white lines compared with that of pure Pd. The observed decrease in white line area was attributed to an increase in the number of d-electrons at the Pd site upon alloy formation. Using the observed core-level binding energy shifts and white line area changes, the net charge transfer at the Pd site of ion-beam-mixed PdAg alloys has been estimated on the basis of a charge compensation model. We find that the net charge transfer is very small owing to back donation of sp-like conduction electron from the Pd site. These results are compared with the self-consistent electronic structure calculation.

Enhancing defect-related photoluminescence by hot implantation into SiO2 layers

Visible photoluminescence around an orange band of 580 nm wavelength are observed from 300 nm thin SiO2 layers implanted by Si or Ge ions at both substrate temperatures of 25 °C [room temperature (RT)] and 400 °C (hot). Si implantations at an energy of 30 keV were performed with doses of 5×1015, 3×1016, and 1×1017 cm−2 while Ge implantations were done at 100 keV with a dose of 5×1015 cm−2. Samples implanted at 400 °C always show much higher intensities of luminescence than those implanted at room temperature. Electron spin resonance signals of the hot-implanted samples indicate relatively smaller amounts of nonradiative defects than those of RT-implanted samples. It is concluded that the hot-implantation effectively enhances the intensity of defect-related photoluminescence by reducing the density of the nonradiative defects and introducing the radiative defects, which contribute to the luminescence in SiO2 layers.

Violet and orange luminescence from Ge-implanted SiO2 layers

Abstract Ge ions of 100 keV were implanted into a 120-nm thick SiO 2 layer at room temperature (RT), 300, and 500°C. The employed doses of Ge ion were 5×10 15 , 1×10 16 , 5×10 16 , and 1×10 17 cm −2 . Maximum intensity of sharp violet photoluminescence (PL) from the sample implanted at room temperature with a dose of 1×10 16 cm −2 is observed after the sample has been annealed at 500°C for 2 h. Broad orange luminescence is also shown in hot-implanted samples besides the violet. Both are known as defect-related luminescences. As observed by current-voltage (I–V) characteristics, the defect-related samples exhibit large leakage currents with electoluminescence (EL) at only reverse bias region while a nanocrystal-related sample obtained by an annealing at 1100°C for 4 h shows the leakages at both the reverse and the forward region. The carrier-transport and EL mechanisms are explained from the PL and I–V results.

Dynamic Monte Carlo simulation for cascade interfacial mixing

Abstract A dynamic Monte Carlo simulation (MCS) program was developed to discribe the processes of interfacial modification of materials by ion beam mixing and applied to the bilayer and multilayer Al/Pd system. For the bilayer system, the MCS results for the dependence of the mixing rate on the film thickness of top layer (Al) show that the optimum film thickness for ion beam mixing corresponding to the mean damage depth in the Al layer. The dynamic MCS results show that the number of moved Pd atoms is larger than that of Al for small Al overlayer thickness due to collisional nature. In the case of multilayer system, the mixing rate depends on the structure of the multilayered system, and differs from the bilayered system. The mixing rate has a maximum value at the second interface with no dependence on structure. The maximum mixing rate for the multilayer system is larger than that of the bilayer system by a factor of 2. The energy deposition due to the nuclear collisions at the interface and the total number of intermixed atoms across the interface are found to play an important role for these interfacial mixing characteristics.

Blue and red luminescence from Si ion-irradiated SiO2/Si/SiO2 layers

Abstract Photoluminescence (PL) from the Si ion-irradiated SiO2/Si/SiO2 layers on Si substrate at room temperature has been studied to elucidate the origins of the blue and red luminescence. A luminescence band around 450 nm was observed from as-irradiated sample, which was found to be originated from the diamagnetic defect known as B2 band generated by Si ion irradiation. The intensity of this band increases with the increase of annealing temperatures up to a critical temperature. After annealing at 1100°C, the defect-related PL peaks around 450 and 600 nm disappear and a new PL peak appears around 700 nm. This luminescence band is attributed to ∼5 nm-sized Si nanocrystals formed along the Si layer between SiO2 layers.

Visible photoluminescence in ion beam mixed SiO2/Si/SiO2 layers

Abstract Visible photoluminescence from silicon nanocrystals embedded in SiO2 matrix by ion beam mixing was investigated. Photoluminescence spectra of ion beam mixed SiO2/Si/SiO2 films excited by an Ar-laser (457.9 nm) showed more intense luminescence with a peak centered at 720 nm than that prepared by the conventional ion implantation method. The formation of nanocrystals in SiO2 matrix was confirmed by cross-sectional high resolution transmission electron microscopy. The red luminescence is attributed to the silicon nanocrystals produced by ion beam mixing.

Atomic transport in collisional atomic mixing in bilayer structure

Abstract A dynamic Monte Carlo simulation (MCS) program, containing not only collisional mixing but also sputtering effects, has been used to elucidate the dynamic mixing processes and the atomic transport of constituents in Al-Pd bilayer systems. MCS results reveal that the preferential inward displacement of the top layer element dominates, and that there is an enhancement of the inward displacement when the heavier element is in the top layer. The inward displacement is controlled by both of an anisotropic and isotropic atomic transport. The anisotropic term is caused by the primary recoil of atoms, which has a characteristic of φ (ion dose) dependence, and the isotropic term is associated with the random cascade motion, which has a √φ dependence. However the outward displacement is governed by only the isotropic motion, thus the inward displacement always dominates over the outward motion, which leads to a preferential displacement of top layer element to the bottom layer.

Atomic transport in thermal spike induced ion mixing

A simple relationship between the ratio of atomic transport induced by ion mixing and the activation energies for the impurity diffusion of constituents in a bilayer is presented to describe quantitatively the symmetric and asymmetric atomic transport in the thermal spike induced ion mixing. The model predicts fairly satisfactorily the trend of experimental observations in the bilayer systems which have near zero heats of mixing and relatively high spike activation energies. For instance, the Pd/Co bilayer system shows nearly symmetric atomic transport, since its constituents have similar activation energies for the impurity diffusion.

Photoluminescence from Si ion irradiated SiO2/Si/SiO2 films with elevated substrate temperature

Abstract Photoluminescence (PL) from the Si ion irradiated SiO2/Si/SiO2 layers on Si substrate at room temperature and elevated substrate temperatures has been studied to elucidate the luminescence origins. The irradiation of Si ions into SiO2/Si/SiO2 layers instead of SiO2 films was performed to improve the PL intensity by increasing the number of proper-sized Si nanocrystals. Before annealing at high temperature, a luminescence band around 450 nm is observed. This luminescence band was found to originate from the diamagnetic defect known as B2 band generated by Si ion irradiation. The intensity of this band increases when ion irradiation is carried out at high substrate temperature. After annealing at high temperature, the PL peaks originating from the B2 band disappear and a new PL peak appears around 700 nm. This luminescence band is associated with ∼5-nm sized Si nanocrystals. Also it can be found that the PL peak intensity around 700 nm is significantly increased with the high substrate temperature during ion irradiation. Therefore, it is concluded that ion irradiation into SiO2/Si/SiO2 layers is more effective than ion implantation into SiO2 films to obtain an intensive PL peak originating from Si nanocrystals.