Eih-Zhe Liang

Use of SiO2 nanoparticles as etch mask to generate Si nanorods by reactive ion etch

Silicon nanorods 20nm in diameter are fabricated by reactive ion etch (RIE) to study anisotropy and damage profile in decananometer scale. RIE of gas mixture of SF6∕O2 and SF6∕CHF3 is tuned to achieve high anisotropy. The gas specie of SF6∕O2 can reach 90% anisotropy, 84° taper angle, and 10:1 selectivity when SiO2 is used as the etching mask. The gas species of SF6∕CHF3 can reach 95% anisotropy, 87° taper angle, and 10:1 selectivity with Cr as the mask. The fabrication technique of nanorods uses a monolayer of silicon dioxide nanoparticle as the etching mask. The nanorods uniformly cover up the entire 2in. wafers with high density of 2×1011cm−2. Surface damageafter the etching process of nanostructures is monitored using the microwave-reflectance photoconductance decay with KOH removal-and-probe technique. Highly damaged silicon is found within a depth of 30nm and the lightly damaged part extends more than 100nm.

Silicon Waveguide Sidewall Smoothing by KrF Excimer Laser Reformation

A novel laser-reformation technique is presented for sidewall smoothing of silicon waveguides. A KrF excimer laser is used to melt and reform the sidewalls to reduce the surface roughness. Atomic-force-microscopy measurement shows that the root-mean-square (rms) roughness is reduced from 14 to 0.24 nm. The calculated scattering loss is reduced to 0.033 dB/cm. The waveguide profile after laser illumination at an incident angle of 75deg transforms to a shape of arch. The crystal quality of laser-illuminated silicon wafer characterized by microwave reflection photoconductance-decay carrier lifetimes shows 94% less damage than the furnace-treated wafer.

Model for band-edge electroluminescence from metal–oxide–semiconductor silicon tunneling diodes

A detailed model is proposed to explain the electroluminescence spectrum from metal–oxide–silicon tunneling diodes. This model includes phonon-assisted processes and exciton involvement. According to this model, the main peak and the low-energy tail of the electroluminescence spectrum are attributed to the transverse optical phonon and the two-phonon assisted recombination, respectively. With very few fitting parameters, the model accurately predicts the measured electroluminescence spectra.

Effective energy densities in KrF excimer laser reformation as a sidewall smoothing technique

Profile transformation of sidewalls and roughness reduction by KrF excimer laser reformation at different illumination conditions are presented. The effective energy density derived from a finite element model of heat transfer is used to characterize the molten depth of Si during high energy laser illumination at normal and oblique incidence. The operation range of laser reformation as a sidewall smoothing technique, including incident angles and energy densities, is determined by the effective energy density. Using an energy density higher than 1.4 J/cm2 at the incident angle of 75° is recommended for a minimal residual roughness, acceptable sidewall profile transformation, and a practical interval-height ratio. The upper bound of scattering loss after laser reformation at different effective energy densities is also calculated.

Electroluminescence and photoluminescence studies on carrier radiative and nonradiative recombinations in metal-oxide-silicon tunneling diodes

Electroluminescence (EL) and photoluminescence (PL) measurements at silicon band gap energy from metal-oxide-semiconductor tunneling diodes on silicon have been carried out to study the carrier radiative recombination and nonradiative recombination near the silicon/silicon dioxide (Si/SiO2) interface. The temporal EL response indicates that the radiative recombination coefficient involved is as much as ten times greater than that of the bulk silicon. However, the nonradiative recombination is still the dominant carrier recombination process. Voltage-dependent PL shows that PL intensity increases with the bias voltage. This voltage dependence is attributed to the variations of nonradiative recombination rates due to the change of Fermi level with the bias voltage. The intensity of EL is found to be less temperature dependent than that of PL. The near-band-edge Fermi level for EL leads to the reduced and less thermal-active nonradiative recombination as compared to PL. This study shows that Fermi level near...

High-density one-dimensional well-aligned germanium quantum dots on a nanoridge array

The selective growth of high-density one-dimensional well-aligned Ge quantum dots (QDs) on the top of nanoridges patterned on Si substrate is reported. The period of ridge array is 150 nm, the width of each ridge is 80 nm, and the depth of the trench is 20 nm. The areal density of QDs is about 5.4×109 cm−2. Simulations of the chemical potential show that a proper distribution of the surface curvature may give rise to a suitable chemical potential minimum helping positioning the QDs. These ridges can also be used to control the shape and the uniformity of QDs.

Significance of Surface Properties of CdS Nanoparticles

This work shows that the surface properties of nanoparticles are very important for a particle size near 5 nm in diameter or less. A simple theoretical formula is derived to quantitatively predict the size of nanoparticles for which the surface properties become significant. Experimental evidence using electroluminescence from CdS nanoparticles of 5 nm in diameter is also provided to verify the significance of the surface properties. With different processing procedures or capping layers, the surface or surface states dominate the optical characteristics of the CdS nanoparticles and sometimes play even more important roles than internal atoms.

Rigorous carrier dynamic model of electroluminescent metal-oxide-semiconductor silicon tunneling diodes

The carrier dynamics of electroluminescent metal-oxide-semiconductor silicon tunneling diodes is rigorously modeled in this study. Various tunneling and recombination current densities are formulated without using the Maxwell-Boltzmann approximation for the carrier concentrations. This model satisfactorily explains the current-voltage characteristics in strong accumulation using a self-consistent formulation. It also relates light emission efficiency to the interface-state density and the bulk-trap density in a straightforward manner. The internal radiative recombination efficiency in the bulk Si substrate is estimated to be in tens of percents. The model also explains clearly the strongly influenced small-signal light-current response time with respect to the injection current density. It enables the small-signal method to be useful in extracting the parameters of the interface-state density and the bulk-trap density.

CdS-nanoparticle light-emitting diode on Si

The fabrication of CdS-nanoparticle light emitting diodes (LEDs) on Si and their properties at room temperature and variant temperatures are reported. Due to passivation of p- hydroxyl thiophenol group around nanoparticles, 86-meV spectral shift of free exciton transition at room temperature is observed. Controlled conditions for the preparation of CdS-nanoparticle LED such as heat treatment and/or with oxygen-rich environment are found to have significant influences on emission spectra. Radiative recombination of carriers trapped in oxygen-impurity level of 273 meV presents in samples prepared in oxygen-rich environment. Coalescence of nanoparticles into bulk form also occurs to contribute to increased magnitude of luminescence. Spectral behaviors of electroluminescence with varied temperature are studied.

Reduced temperature dependence of luminescence from silicon due to field-induced carrier confinement

Electroluminescence from metal–oxide–semiconductor structures on Si was experimentally found to be much less temperature dependent than photoluminescence of Si. The physical reason is attributed to the field-induced carrier confinement in a small region, which contains much less impurity states, compared to the unconfined region. Thus, electron–hole recombination by radiation emission instead of through highly temperature-dependent impurity states is increased. A proposed model well explains the reduced temperature dependence with the field-induced carrier confinement.