Xinwei Zhao

Nanocrystalline silicon electron emitter with a high efficiency enhanced by a planarization technique

A cold electron emitter has been fabricated based on nanocrystalline silicon (nc-Si) quantum dots formed in the gas phase by very-high-frequency plasma decomposition of SiH4. A small size of less than 10 nm and the spherical shape of the nc-Si dots facilitated the generation of hot electrons. Electrons with kinetic energies higher than the work function of the top electrode were extracted into vacuum through the electrode. A planarization process of the nc-Si layer by annealing enhanced the electron emission efficiency to 5%. Efficiency was optimized by varying the thicknesses of the nc-Si layer, the SiO2 layer, and the top electrode film.

Highly erbium-doped zinc–oxide thin film prepared by laser ablation and its 1.54 μm emission dynamics

Erbium-doped ZnO (ZnO:Er) thin films were fabricated by the KrF excimer laser ablation technique, which is a useful and simple technique to dope Er atoms on the order of 1020 cm−3 into a host material. As-prepared ZnO:Er films showing strong c-axis orientation with a hexagonal crystalline structure indicate a low electrical resistivity of 6.4×10−3 Ω cm. The sharp and intense photoluminescence (PL) at 1.54 μm originating from the intra-4f transition in the Er3+ ions as well as PL in UV region from the ZnO host were observed even at room temperature. Significant distinction arising from the different Er emission centers responsible for the 1.54 μm emission cannot be found in the temperature dependence between the ZnO:Er and Si:Er film as a reference, except for the PL spectrum feature and main PL peak position. This result suggests the existence of Er emission centers in ZnO:Er and Si:Er films that are different from each other. The details of Er-related 1.54 μm emission dynamics of ZnO:Er films have been i...

1.54 μm emission dynamics of erbium-doped zinc-oxide thin films

Erbium-related 1.54 μm emission dynamics of Er-doped ZnO thin films has been investigated for the different excitation conditions. The excitation was achieved either by exciting indirectly Er3+ ions due to an electron–hole-mediated process or exciting directly discrete energy levels of Er3+ ions. There is no change in the 1.54 μm emission spectrum feature in spite of the different excitation conditions, whereas dramatic change can be seen in the rise time of 1.54 μm emission. The shorter rise time of 1.54 μm emission observed for indirect excitation implies an excitation efficiency superior to direct excitation of Er3+ ions.

Fabrication and stimulated emission of Er-doped nanocrystalline Si waveguides formed on Si substrates by laser ablation

Er-doped nanocrystalline Si (nc-Si) waveguides were fabricated on Si substrates and investigated by optical pumping. A stimulated emission at 1540 nm was demonstrated at room temperature. The sizes of the fabricated Er-doped nc-Si waveguides were 5000 nm×200 nm×L, where L is the cavity length and is changed from 1 to 10 mm. Superlinear optical outputs at 1540 nm were observed for the waveguides longer than 3 mm. The threshold of the optical output where the stimulated emission occurs is in the order of 10 MW/cm2, and is demonstrated to depend on the cavity length of the waveguides. A large reduction of decay lifetimes of the light output from a cleavage facet of the Er-doped nc-Si waveguides was observed when the pumping power density exceeded the thresholds indicating an increase of transition probabilities in intra-4f electrons in Er3+ ions caused by the stimulated emission. Better 1540 nm laser performance and lower pumping power density should be obtained by optimizing the device structure and increas...

Violet and Blue Light Emissions from Nanocrystalline Silicon Thin Films

Nanocrystalline silicon thin films with a grain diameter from three to seven nanometers were fabricated on silicon substrates. It is demonstrated for the first time that the thin films show intense violet and blue luminescence at room temperature. The luminescence spectra include three peaks at wavelengths of 415 nm, 437 nm and 465 nm. Anodizations of these thin films introduce additional green and red luminescence in the spectra. Fourier transform infrared spectroscopy indicates no hydrogen- or oxygen-related absorptions in the nanocrystalline silicon thin films. Only the anodized thin films show Si–Hx absorptions. The violet luminescence should be evidence of light emission from zero-dimensionally confined silicon structures.

Change in photoluminescence from Er-doped TiO2 thin films induced by optically assisted reduction

Erbium-doped TiO2 (TiO2:Er) thin films with the anatase structure have been prepared on Si substrate by laser ablation. Sharp and intense Er-related emission in the visible region as well as in the IR region has been observed under over-band-gap excitation. The broad photoluminescence (PL) peaking at about 530 nm newly appears at low temperature. It has been understood that the broad PL is induced by an optically assisted reduction effect that is caused by both the H2O adsorption and the reduction process of TiO2 to Ti2O3 by UV illumination. In the IR region, Er-related emission consisted of one main peak located at 1.534 μm and many subpeaks located at around 1.54 μm can be observed even at room temperature. The drastic thermal quenching of the Er-related 1.54 μm emission is also considered due to the optically assisted reduction effect.

Time response of 1.54 μm emission from highly Er-doped nanocrystalline Si thin films prepared by laser ablation

Er-doped nanocrystalline Si thin films have been controllably prepared over the Er density of 1019–1021 cm−3 using a prescribed amount of Er in a bulk target by laser ablation. Intense photoluminescence at 1.54 μm originating from intra-4f shell transitions in Er3+ ions has been observed. The increase of Er density cannot immediately result in a linear increase in Er3+-emission intensity. The time response measurement indicated that the change in the rise time of the Er3+ emission directly shows that Er3+ ions are excited by the energy transfer associated with the recombination of electron–hole pairs generated optically in the Si host. We found that the decrease of the excitation efficiency of Er3+ ions was responsible for the suppression of the Er3+-emission intensity in highly Er-doped nanocrystalline Si thin films.

Violet luminescence from anodized microcrystalline silicon

Microcrystalline silicon (μ‐Si) thin films were anodized in dilute HF solutions in the same manner as forming porous materials. It is demonstrated for the first time that the anodized μ‐Si thin films show strong violet luminescence (415 nm) at room temperature. Visible green and red emissions were also observed accompanying the violet luminescence. Structural investigations with scanning electron microscopy indicate that any formation of micrometer‐sized pores which is typical for porous silicon does not exist in the anodized μ‐Si thin films as reported here. This fact is useful for device applications of silicon‐based materials.

Room‐temperature luminescence from erbium‐doped silicon thin films prepared by laser ablation

We present a useful and simple technique to prepare controllable Er‐doped Si thin films using KrF excimer laser ablation. The sharp intense photoluminescence (PL) at 1.54 μm originating from the intra‐4f shell transition in Er3+ ions was observed from 18 K up to room temperature. Characteristics of PL thermal quenching and time decay of prepared Er‐doped Si thin films are very similar to those of Er‐doped porous Si and/or Er‐doped amorphous Si. Furthermore, observation of Er3+ emission from as‐prepared thin films without thermal annealing suggests that the Er doping in the form of Er atomic radical species produced by laser ablation is essential in activation of Er3+ ions. Moreover, incorporating a prescribed amount of Er in the bulk target enables us to control the Er doping level in thin films prepared by laser ablation.

Nanocrystalline Si : a material constructed by Si quantum dots

Abstract Intense blue luminescence with decay lifetimes of 100–500 ps is observed from nanocrystalline silicon thin films at room temperature. The grain size reduction of the silicon crystallites to 3–5 nm leads to the generation of the luminescence. It is demonstrated that the blue emission is due to direct transitions in silicon nanocrystallites which are caused by a quantum confinement effect. The contribution of silicon dioxide to the blue emission is excluded. A model is proposed to understand the enhancement of the direct transitions in silicon nanocrystallites.