Chien-Jui Yeh

Characterization of Porous Silicon-on Insulator Films Prepared by Anodic Oxidation

In this paper, techniques for controlling the growth of the bottom oxide formed by simple electrochemical oxidation of porous Si films were explored and the resulting porous silicon-on-oxide structures were characterized. The thickness, uniformity, and density of the bottom oxide layer can be adjusted by selecting the porous silicon morphology and controlling the conditions of oxide formation. In particular, very uniform interfaces between the oxide layer and a porous silicon overlayer were obtained in multi-texture porous silicon samples.

Study on the photoconductivity characteristics of porous Si

After the report of the visible light emission out of porous Si,’ this material regained a lot of attention in the hope that optoelectronics in group IV materials can someday be mature. However, many essential subjects about it, such as the formation mechanism, the structure, and the origin of visible light emission, remain controversial. While most work has focused on the optical properties of porous Si, not much effort was paid to the electrical aspects. Bilenko et al. ’ measured the electrical conductivity and the Hall coefficient of porous Si. Beale et aZ. 3 used two and four terminal measurements to obtain the resistivity of porous Si. Both works did not present current-voltage (I-V) characteristics. Some recent workG reported the I-V curves of metal-porous Si junctions. In these works, the current flowed through the junction and the porous layer along the direction of electrochemical etching. Results showed that metal-p-type porous Si junctions have Schottky junctionlike behavior while metal-n-type porous Si junctions tend to be ohmic. Despite these, detailed study on the I-V characteristics and the transport mechanism of carriers in porous Si is lacking. As a matter of fact, better understanding in the electrical transport properties of porous Si is not only important to electroluminescence and other photosensitive devices design but also helpful to understanding the structure of porous Si itself. In this work, we present a study on the electrical transport property of porous Si. Detailed I-V characteristics for currents flowing perpendicular to the etching direction are shown. Obvious oscillations in the I-V curves due to photoconductivity have been observed. p-type layers were formed on n-type Si wafers with resistivities of 4-7 L! cm by using low energy boron implantation (dose= 10’5/cm2, energy=25 keV, 50 keV), and thermal annealing at 950 “C for 20 min. The junction depths were determined by the spreading resistance method as 0.52-0.61 pm, and the average value of the resulting doping level was about 1019/cm3. Gold with 500 pm separations were evaporated on the surface. Porous Si was formed by electrochemical anodization without illumination in HF solutions in the region between contacts. Porosity was difficult to be determined accurately in our case. The concentration of HF solution was 10%. The etching

A lateral injection porous silicon device structure for light-emitting diodes

Abstract Lateral injection porous silicon (PS) diodes and metal-PS-metal (MPM) structures incorporated with PS-on-oxide structures are fabricated and characterized. The porous Si and the bottom oxide are formed in the same electrochemical cell but with different chemical solutions. The bottom oxide provides good isolation for the current flowing in the PS region and the current density level is higher compared with the counterpart without bottom oxide. Strong red-orange photoluminescence is observed under daylight, but the electroluminescence is weak owing to the lack of efficient hole injectors and the too thick gold contact films of the samples. A charging effect during electrical measurements and a punch-through-like behaviour at higher voltages are observed in the characteristics of MPM samples, suggesting a large number of traps and the ease of depletion in PS. The diode with bottom oxide is well behaved except for the large series resistance. The PS-on-oxide technique is shown to be beneficial for lateral porous Si devices.

Enhancement of plasma illumination characteristics of few-layer graphene-diamond nanorods hybrid

Few-layer graphene (FLG) was catalytically formed on vertically aligned diamond nanorods (DNRs) by a high temperature annealing process. The presence of 4-5 layers of FLG on DNRs was confirmed by transmission electron microscopic studies. It enhances the field electron emission (FEE) behavior of the DNRs. The FLG-DNRs show excellent FEE characteristics with a low turn-on field of 4.21 V μm-1 and a large field enhancement factor of 3480. Moreover, using FLG-DNRs as cathode markedly enhances the plasma illumination behavior of a microplasma device, viz not only the plasma current density is increased, but also the robustness of the devices is improved.

Microstructural Evolution of Nanocrystalline Diamond Films Due to CH4/Ar/H2 Plasma Post-Treatment Process

Plasma post-treatment process was observed to markedly enhance the electron field emission (EFE) properties of ultrananocrystalline diamond (UNCD) films. TEM examinations reveal that the prime factor which improves the EFE properties of these films is the coalescence of ultrasmall diamond grains (∼5 nm) forming large diamond grains about hundreds of nanometers accompanied by the formation of nanographitic clusters along the grain boundaries due to the plasma post-treatment process. OES studies reveal the presence of large proportion of atomic hydrogen and C2 (or CH) species, which are the main ingredients that altered the granular structure of the UNCD films. In the post-treatment process, the plasma interacts with the diamond films by a diffusion process. The recrystallization of diamond grains started at the surface region of the material, and the interaction zone increased with the post-treatment period. The entire diamond film can be converted into a nanocrystalline granular structure when post-treated for a sufficient length of time.

Formation of bottom oxides in porous silicon films by anodic oxidation

The formation of bottom oxide by electrochemical oxidation in porous silicon layers is studied. A technique of controlling the oxide layer thickness is developed. It is shown that the design of the anodization current level and of the porous silicon texture is an effective method for oxide formation control.

Improvement of electron field emission properties of nanocrystalline diamond films by a plasma post-treatment process for cathode application in microplasma devices

The electron field emission (EFE) properties of nanocrystalline diamond (NCD) films were markedly enhanced when prepared with a plasma post-treatment on the ultra-small-grain granular-structured diamond films, as compared with conventional NCD films directly grown on Si using CH4/Ar/H2 plasma. Transmission electron microscopy reveals that the primary influence for the improvement of the EFE properties of these films was owing to an induction of the nanographitic phase in the films, while the ultrasmall diamond grains (∼5 nm) coalesced to form large diamond grains (∼hundreds of nanometers) during the plasma post-treatment process. This modification of the granular structure of the NCD films was greatly enhanced when a negative bias voltage (−300 V) was applied during the plasma post-treatment process. Moreover, three-electrode microplasma devices performed overwhelmingly better than two‐electrode devices, exhibiting a higher plasma current density with a longer lifetime stability. These microplasma devices...

Synthesis of ultra-nano-carbon composite materials with extremely high conductivity by plasma post-treatment process of ultrananocrystalline diamond films

Needle-like diamond grains encased in nano-graphitic layers are an ideal granular structure of diamond films to achieve high conductivity and superior electron field emission (EFE) properties. This paper describes the plasma post-treatment (ppt) of ultrananocrystalline diamond (UNCD) films at low substrate temperature to achieve such a unique granular structure. The CH4/N2 plasma ppt-processed films exhibit high conductivity of σ = 1099 S/cm as well as excellent EFE properties with turn-on field of E0 = 2.48 V/μm (Je = 1.0 mA/cm2 at 6.5 V/μm). The ppt of UNCD film is simple and robust process that is especially useful for device applications.

The microstructural evolution of ultrananocrystalline diamond films due to P ion implantation process—the annealing effect

The microstructural evolution of UNCD films which are P-ion implanted and annealed at 600 °C (or 800 °C) is systematically investigated. The difference of interaction that the UNCD content undergoes along the trajectory of the incident P-ions is reflected in the alteration of the granular structure. In regions where the P-ions reside, the “interacting zone,” which is found at about 300 nm beneath the surface of the films, coalescence of diamond grains occurs inducing nano-graphitic clusters. The annealing at 600 °C (or 800 °C) heals the defects and, in some cases, forms interconnected graphitic filaments that result in the decrease in surface resistance. However, the annealing at 600 °C (800 °C) induces marked UNCD-to-Si layers interaction. This interaction due to the annealing processes hinders the electron transport across the interface and degrades the electron field emission properties of the UNCD films. These microstructural evolution processes very well account for the phenomenon elaborating that, in spite of enhanced conductivity of the UNCD films along the films surface due to the P-ion implantation and annealing processes, the electron field emission properties for these UNCD films do not improve.

Microstructural Effect on the Enhancement of Field Electron Emission Properties of Nanocrystalline Diamond Films by Li-Ion Implantation and Annealing Processes

The impact of lithium-ion implantation and postannealing processes on improving the electrical conductivity and field electron emission (FEE) characteristics of nitrogen-doped nanocrystalline diamond (nNCD) films was observed to be distinctly different from those of undoped NCD (uNCD) films. A high-dose Li-ion implantation induced the formation of electron trap centers inside the diamond grains and amorphous carbon (a-C) phases in grain boundaries for both types of NCD films. Postannealing at 1000 °C healed the defects, eliminated the electron trap centers, and converted the a-C into nanographitic phases. The abundant nanographitic phases in the grain boundaries of the nNCD films as compared to the uNCD films made an interconnected path for effectual electron transport and consequently enhanced the FEE characteristics of nNCD films.