C. L. Heng

Observation of memory effect in germanium nanocrystals embedded in an amorphous silicon oxide matrix of a metal–insulator– semiconductor structure

The memory effect of a trilayer structure (rapid thermal oxide/Ge nanocrystals in SiO2/sputtered SiO2) was investigated via capacitance versus voltage (C–V) measurements. The Ge nanocrystals were synthesized by rapid thermal annealing of the cosputtered Ge+SiO2 films. The memory effect was manifested by the hysteresis in the C–V curve. Transmission electron microscope and C–V results indicated that the hysteresis was due to Ge nanocrystals in the middle layer of the trilayer structure.

Size control and charge storage mechanism of germanium nanocrystals in a metal-insulator-semiconductor structure

The size of germanium (Ge) nanocrystals in a trilayer metal-insulator-semiconductor memory device was controlled by varying the thickness of the middle (co-sputtered Ge+SiO2) layer. From analyses using transmission electron microscopy and capacitance–voltage measurements, we deduced that both electrons and holes are most likely stored within the nanocrystals in the middle layer of the trilayer structure rather than at the interfaces of the nanocrystals with the oxide matrix.

The 1.54-μm photoluminescence from an (Er, Ge) co-doped SiO2 film deposited on Si by rf magnetron sputtering

In this work, we report on quite strong 1.54‐μm photoluminescence (PL) from an (Er, Ge) co-doped SiO2 film deposited by rf magnetron sputtering. The PL intensity reaches a maximum value after the film is annealed at 700°C for 30min in N2. High-resolution transmission electron microscopy observation, together with energy dispersive x-ray spectroscopy analysis, indicates that amorphous Ge-rich nanoclusters precipitate in the film after 700°C annealing. X-ray diffraction shows the presence of Ge nanocrystals after 900°C annealing, and increasing Ge nanocrystal size with increasing annealing temperature up to 1100°C. The results suggest that the amorphous Ge-rich nanoclusters are more effective than Ge nanocrystals in exciting the Er3+ PL.

The formation of light emitting cerium silicates in cerium-doped silicon oxides

Cerium-doped silicon oxides with cerium concentrations of up to 0.9 at. % were deposited by electron cyclotron resonance plasma enhanced chemical vapor deposition. Bright cerium related photoluminescence, easily seen even under room lighting conditions, was observed from the films and found to be sensitive to film composition and annealing temperature. The film containing 0.9 at. % Ce subjected to anneal in N2 at 1200 °C for 3 h showed the most intense cerium-related emission, easily visible under bright room lighting conditions. This is attributed to the formation of cerium silicate [Ce2Si2O7 or Ce4.667 (SiO4)3O], the presence of which was confirmed by high resolution transmission electron microscopy.

Precipitation and crystallization of nanometer Si clusters in annealed Si-rich SiO2 films

Si-rich SiO2 films with three different degrees of Si-richness were deposited by RF magnetron sputtering using Si-SiO2 composite targets. X-ray photoelectron spectroscopy measurements indicate that Si clusters were present in the as-deposited films. The precipitation and crystallization of nanometer Si clusters in SiO2 films annealed at high temperatures have been studied using high-resolution transmission electron microscopy and electron diffraction. Si nanocrystallites were observed in the sample deposited using a Si-SiO2 composite target having a 30% area of Si and which had been annealed at 900 degrees C. The average size and density of Si nanocrystallites in the films increased notably as the annealing temperature was increased from 900 to 1100 degrees C, Thus, using a 1100 degrees C anneal and increasing the area percentage of Si in the composite target from 20 to 30%, the average size of Si nanocrystallites increased about 15%, and the density of Si nanocrystallites increased by a factor of about 2.5

Light Emission from Rare-Earth Doped Silicon Nanostructures

Rare earth (Tb or Ce)-doped silicon oxides were deposited by electron cyclotron resonance plasma-enhanced chemical vapour deposition (ECR-PECVD). Silicon nanocrystals (Si-ncs) were formed in the silicon-rich films during certain annealing processes. Photoluminescence (PL) properties of the films were found to be highly dependent on the deposition parameters and annealing conditions. We propose that the presence of a novel sensitizer in the Tb-doped oxygen-rich films is responsible for the indirect excitation of the Tb emission, while in the Tb-doped silicon-rich films the Tb emission is excited by the Si-ncs through an exciton-mediated energy transfer. In the Ce-doped oxygen-rich films, an abrupt increase of the Ce emission intensity was observed after annealing at 1200∘C. This effect is tentatively attributed to the formation of Ce silicate. In the Ce-doped silicon-rich films, the Ce emission was absent at annealing temperatures lower than 1100∘C due to the strong absorption of Si-ncs. Optimal film compositions and annealing conditions for maximizing the PL intensities of the rare earths in the films have been determined. The light emissions from these films were very bright and can be easily observed even under room lighting conditions.

Effects of rapid thermal annealing time and ambient temperature on the charge storage capability of SiO 2 /pure Ge/rapid thermal oxide memory structure

A metal-insulator-semiconductor device with a tri-layer structure consisting of sputtered silicon dioxide (SiO2) (∼ 50 nm)-evaporated pure germanium (Ge) (2.3 nm)-rapid thermal oxidation (RTO) oxide (5 nm) was fabricated on a p-type silicon (Si) substrate. This structure was rapid thermal annealed at 1000 °C in argon. For the as-prepared structure and those that were annealed from 10 to 400 s, it was observed that the hysteresis of the capacitance versus voltage (C-V) curves increased from ∼ 1.5 to 10 V. This indicated that the charge storage capability of the structure improved with increasing annealing time. From our transmission electron microscope results, we observed that as the annealing time increased, more Ge nanocrystals were formed. When the ambient temperature was increased from 25 to 150 °C, the width of the hysteresis of our devices reduced. The charge storage mechanism of the Ge nanocrystals was explained in terms of charging/discharging from traps at the internal/surface of Ge nanocrystals and tunneling of charges to the interface states at the Si-RTO oxide interface.

Enhanced Shubnikov–De Haas Oscillation in Nitrogen-Doped Graphene

N-doped graphene displays many interesting properties compared with pristine graphene, which makes it a potential candidate in many applications. Here, we report that the Shubnikov-de Haas (SdH) oscillation effect in graphene can be enhanced by N-doping. We show that the amplitude of the SdH oscillation increases with N-doping and reaches around 5k Ω under a field of 14 T at 10 K for highly N-doped graphene, which is over 1 order of magnitude larger than the value found for pristine graphene devices with the same geometry. Moreover, in contrast to the well-established standard Lifshitz-Kosevich theory, the amplitude of the SdH oscillation decreases linearly with increasing temperature and persists up to a temperature of 150 K. Our results also show that the magnetoresistance (MR) in N-doped graphene increases with increasing temperature. Our results may be useful for the application of N-doped graphene in magnetic devices.

On the effects of double-step anneal treatments on light emission from Er-doped Si-rich silicon oxide

We have studied photoluminescence (PL) from an Er-doped Si-rich Si oxide (SRSO) film thermally annealed under different conditions. Compared to the case of annealing in N2 alone, double-step annealing the film at 875°C in N2 and then at ∼850°C in O2 or vice versa increases Er PL intensities by 10%–15%; while double-step annealing in N2+5%H2 (FG) and then in O2 or vice versa yields significant enhancements of the PL from the SRSO matrix and the Er PL intensity decreases differently by exchanging the processing order. Fourier transform infrared spectroscopy indicates that silicon oxynitride forms after annealing in FG ambient, and for the samples initially oxidized, the increase of Er PL intensity after secondary annealing in N2 (or FG) is due to more Si nanoclusters being formed. The PL spectra exhibit different annealing behavior with increasing the FG annealing temperature and the processing order. Weak oxygen bonds and silicon oxynitrides are believed to form upon O2 and FG annealing, respectively, and ...

Electrical characterization of a trilayer germanium nanocrystal memory device

In this paper, we report the effects of the thickness of the middle layer and the rapid thermal oxide (RTO) layer on the charge storage capability of the trilayer devices. The capacitance versus voltage (C-V) measurements showed that devices with a thinner middle layer and the same thickness for the RTO layer have better charge storage capability (i.e., larger C-V hysteresis). For devices with the same middle layer thickness, a larger C-V hysteresis was observed from devices with a thinner RTO layer.