Characteristics of Silicon Nanocrystals and Silicon Carbide Nanocrystals Embedded Amorphous Silicon-rich Silicon Carbide Based PIN Junction Light Emitting Diode
Date Issued
2011
Date
2011
Author(s)
Tai, Hung-Yu
Abstract
In this thesis, the synthesis of a-SixC1-x films embedded with Si-ncs and SiC-ncs by fluence-ratio detuned PECVD at high-temperature growth is investigated to modify its luminescent property and to enrich the crystallinity after thermal annealing at 1100oC. With changing the deposition temperature from 450oC to 650oC, the Si concentration increases from 64.7% to 71.6%. However, the carbon and oxygen contents decrease from 27.7% to 22.8% and 7.6% to 5.5% and the O/Si ratio is reduced from 0.12 to 0.07 from the XPS analysis due to growth of better crystallinity to prevent the oxygen invasion in a-SixC1-x films. A significant signal at 510 cm-1 is shown to confirm the existence of Si-ncs after post-annealing. The other two intensive peaks at 744 and 933 cm-1 are red-shifted than bulk 3C-SiC Raman peaks at 796 (TO) and 972 cm-1 (LO) and ascribed to reduced nanograin size of SiC-ncs, respectively. From the results of XRD spectra, the average crystallite sizes of Si and 3C-SiC nanocrystals are around 4.2±0.5 nm and 2.4±0.3 nm, respectively. On the basis of FTIR analysis, the Si-H3 stretching mode is transformed into Si-H stretching mode after annealing since the hydrogen bond is broken up to diffuse out. Accordingly, Si-ncs can be easily aggregated by dehydrogenation in Si-H3 radical. A distinct band at 792-806 cm-1 is ascribed to Si-C stretching mode and the blue-shifted peak from 792 to 802 cm-1 is due to enhanced strength of bonds between Si and C atoms. The intense visible PL centered at 485 nm is found in annealed sample of g=60% and it attributed to the luminescence of SiC-ncs due to the self-trapped excitons at the surface states between SiC-ncs and surrounding. In addition, the PL peak at 580 nm is also observed from the contribution of Si-ncs in view of quantum confinement effect. Composition ratio x in SixC1-x is detuned from 0.74 to 0.62 with increasing fluence ratio from 40 to 70% by XPS spectra.
The resistivity of P type a-SixC1-x network at g of 50% reduces to 2.2×101 Ω-cm when B2H6 doping mole fraction increases to 2% since the appropriate amounts of boron atoms occupy the position in tetrahedral SiC network to release enough holes to form the electrically active dopant after thermal process of 650oC. The dopant density is also increased to 1.35×1016 cm-3 when doping mole fraction is at 2% corresponding to activation energy of 0.17 eV. On the other hand, the resistivity is reduced from 22 to 0.72 Ω-cm and dopant density is increased from 1.35×1016 to 4.35×1017 cm-3 due to the minimization of the overdoping phenomenon to reduce the influence on excess impurity atoms scattering and collision between released carriers when enlarged triply doping gaseous fluence and it is equivalent to gaseous dilution owing to various dissociation energy for different process gas. The resistivity of N-SiC is decreased abruptly to 11.3 Ω-cm when RF power changes to 80 W corresponding PH3 dopant density of 1.46×1015 cm-3.
The PIN thin film light emitting diode with intrinsic layer embedded with Si-ncs and SiC-ncs is fabricated to enlarge optical power, reduce turn on voltage and enhance carrier injection efficiency. Carrier injection and transport properties can be improved with the higher doping concentration P-SiC layer. The thicker intrinsic layer caused the larger series resistance thus whether turn on voltage or injection current is larger than the thinner I-layer and the optical power emitted from PIN TFLED with I layer thickness of 50 nm is triple than I layer thickness of 25 nm at g of 60%. With increasing thickness of intrinsic layer from 25 nm to 100 nm at g of 50% since the carrier tunneling probability is decreased with enlarging the intrinsic layer thickness since insufficient electric field across the I-SiC film is to reduce the carrier injection and the luminescent centers are more plentiful owing to the Si-rich SiC matrix at g of 50% with embedded more quantity of Si-ncs. The optical power from PIN LED with g of 50% at I layer thickness of 50 nm is three point five times than I layer at 25 nm. The PCR increasing trend is due to the production of more luminescent centers at thickness of 50 nm but subsequently decreased is owing to too thicker I-layer caused the carrier tunneling probability reduction. The EQE is larger twice than others with increasing intrinsic layer thickness from 25 to 50 nm at g of 60% and the P-I slope is four times than others since the trade off relation between carrier transport and tunneling into active layer and the more luminescent centers in active layer are observed thus the optimized thickness of intrinsic layer is 50 nm. The EQE is four times than others with increasing g of 50% intrinsic layer thickness from 25 to 50 nm and the P-I slope is six times than others since the appropriate intrinsic thickness at 50 nm is needed to enhance light emission and preserve the carrier injection ability. Carrier transport via band to band tunneling is confirmed owing to high electric field and then radiative transition is also occurred due to the carrier tunneling into intrinsic region and its neighborhood. The principal EL peak at 495 nm with narrower shape is assigned to self-trapped excitons at the surface states between SiC-ncs and surrounding at g of 60% corresponding to blue-white EL emission pattern. Moreover, the main EL wavelength centered at 570 nm with broader shape is attributed to nearly direct band to band transition by Si-ncs at g of 50% and consistent with orange-yellow EL emission pattern. The injection efficiency is six times than N-SiC with 1015 cm-3 when dopant density increased to 1016 cm-3. The EQE is enhanced nearly six times when injection efficiency is increased from 7.84% to 46 %.
Subjects
Si nanocrystals
SiC nanocrystals
Si-rich SiC
Resistivity
Dopant density
PIN junction thin film light emitting diode
Injection efficiency
Type
thesis
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