Seung Hui Hong

Doping- and size-dependent photovoltaic properties of p-type Si-quantum-dot heterojunction solar cells: correlation with photoluminescence

Boron-doped SiOx/SiO2 superlattices have been prepared on n-type Si (100) wafers by ion beam sputtering and subsequently annealed to form p-type Si quantum dots (QDs)/n-type Si-wafer heterojunction solar cells. Systematic studies on photoluminescence (PL) and photovoltaic effects show that optimum formation of Si QDs, proper doping concentration (nB), and minimization of defects are crucial factors for enhancing energy-conversion efficiency of the solar cells. Highest efficiency of 9.5% is obtained under the conditions of x=1.0 (QD size: ∼5 nm) and nB=6.3×1020 cm−3. Possible physical mechanisms are discussed to explain the correlation of the photovoltaic parameters and the QD-/defect-PL intensities. The demonstration of the photovoltaic effects in the Si-QD heterojunction solar cells is promising for the development of next-generation all-Si-QD solar cells.

Active doping of B in silicon nanostructures and development of a Si quantum dot solar cell

Active doping of B was observed in nanometer silicon layers confined in SiO(2) layers by secondary ion mass spectrometry (SIMS) depth profiling analysis and confirmed by Hall effect measurements. The uniformly distributed boron atoms in the B-doped silicon layers of [SiO(2) (8 nm)/B-doped Si(10 nm)](5) films turned out to be segregated into the Si/SiO(2) interfaces and the Si bulk, forming a distinct bimodal distribution by annealing at high temperature. B atoms in the Si layers were found to preferentially substitute inactive three-fold Si atoms in the grain boundaries and then substitute the four-fold Si atoms to achieve electrically active doping. As a result, active doping of B is initiated at high doping concentrations above 1.1 × 10(20) atoms cm( - 3) and high active doping of 3 × 10(20) atoms cm( - 3) could be achieved. The active doping in ultra-thin Si layers was implemented for silicon quantum dots (QDs) to realize a Si QD solar cell. A high energy-conversion efficiency of 13.4% was realized from a p-type Si QD solar cell with B concentration of 4 × 10(20) atoms cm( - 3).

High-efficient ultraviolet emission in phonon-reduced ZnO films: The role of germanium

Photoluminescence (PL) properties have been studied for Ge-doped ZnO films grown on Si wafers by RF-magnetron sputtering. A PL line, named as G line, appears at 3.324eV by Ge doping and is attributed to Ge suboxide states including GeO color centers. As Ge concentration (nGe) increases, the intensities of free-exciton-, and neutral-donor-bound-exciton-, two-electron-satellite-, and G‐PL lines increase, while those of their phonon replicas decrease. By Ge doping, no-phonon line deconvoluted from the near-band-edge (NBE) PL at 300K is enhanced, but its LO phonon replicas are reduced, resulting in the enhancement of the NBE PL with its reduced bandwidth. It is suggested that these results are due to the increase of the Ge suboxide states with increasing nGe, which is also confirmed by the analysis of the Ge 3d core-level spectra by x-ray photoelectron spectroscopy.

Nonvolatile memories of Ge nanodots self-assembled by depositing ultrasmall amount Ge on SiO2 at room temperature

Ge nanodots (NDs) for nonvolatile memories (NVMs) have been self-assembled at room temperature (RT) by ion beam sputtering deposition of ultrasmall amount Ge (<72 ML) on SiO2 without postannealing. High-resolution transmission electron microscopy demonstrates the existence of well-defined Ge ND layers with respect to the SiO2∕Si interface. As Ge amount increases, the size of NDs increases, while their density decreases. A possible mechanism is proposed to explain the formation of Ge NDs at RT based on simple model calculations. The memory window that is estimated by capacitance-voltage hysteresis increases up to 18.7V with increasing Ge amount up to 54 ML. The program speed is enhanced by increasing Ge amount and the charge-loss speed in the programed state is slower for larger Ge amount. These NVM properties are very promising in view of device application.

Self-assembled growth and luminescence of crystalline Si/SiOx core?shell nanowires

Crystalline Si/SiOx core/shell nanowires (NWs) are self-assembled by annealing Ni-coated hydrogenated Si-rich SiOx (SRO:H) films at 1100 degrees C in the presence of Si powder. Plasma-enhanced chemical vapor deposition is used to grow 100 nm SRO:H thin films with varying silicon concentration (n(Si)). The NWs vary from SiOx nanowires to Si/SiOx core/shell structures depending on the composition of the SRO:H substrate, with the fraction of core/shell structures increasing with increasing Si concentration. As n(Si) increases from 37 to 43 at.%, the average diameter of the NWs also increases from 48 to 157 nm. A growth model based on the diffusion-assisted vapor-liquid-solid mechanism is proposed to explain how the core/shell structures are self-assembled. Photoluminescence (PL) spectra of the individual NWs have two major emission bands in the near UV (381 nm) and blue (423 nm) ranges at n(Si) = 43 at.%, named as UV and BL PL bands, respectively. In contrast, only the BL PL band is observed at n(Si) < or = 39 at.%. These results suggest that the BL and UV PL bands can be attributed to the defect states in the SiOx shell and at the Si core/SiOx shell interface, respectively, and that the BL band is closely related to the growth process of the NWs.

Control of amorphous silica nanowire growth by oxygen content of Si-rich oxide

Ni-coated Si-rich oxide (SRO, SiO(x)) on a p-type Si wafer has been annealed with Si powder to grow silica nanowires (NWs), which have a composition of stoichiometric SiO(2), irrespective of x. The diameters of the NWs are well controlled from 82 to 23 nm by increasing x from 0.4 to 1.2 and they have a uniform distribution at a fixed x. These results suggest that the oxygen content (x) plays a crucial role in determining the diameter of the NWs at the early stage of the NW formation. The growth behaviors of the NWs are explained well based on a modified vapor-liquid-solid mechanism.

Size- and doping-dependent time-resolved photoluminescence of doped Si nanocrystals

Time-resolved photoluminescence (PL) has been studied for B- and Sb-doped Si nanocrystals (NCs) fabricated by ion beam sputtering and annealing. For B-doped Si NCs, the PL intensity as well as the PL lifetime (τPL) increases as NC size (d) varies from 1.5 to 2.6 nm, similar to the case for undoped Si NCs, but with further increase of d, they decrease, possibly resulting from the increase of optically less active NCs with the increase of NCs containing more dopants. The PL intensity and τPL monotonically decrease with increasing doping concentration (nD), irrespective of doping element. Si NCs show smaller τPL in B doping than in Sb doping over the full range of nD. The sharp decrease in PL intensity, accompanied by the gradual decrease in τPL for the higher nD of Sb, may be attributed to Auger recombination due to the presence of Sb inside Si NCs. The higher PL quench rate by Sb compared to B could be attributed to better ionization of Sb dopants in Si NCs.

Nonvolatile memories using deep traps formed in Al2O3 by metal ion implantation

This work was supported by the Korea Science and Engineering Foundation KOSEF grant funded by the Korea government MOST Grant No. R01-2007-000-20142-0. M.C.K. acknowledges a support from the Kyung Hee University Graduate School Scholarship for Outstanding Research Papers in the second semester, 2007.

Nonvolatile memories by using charge traps in silicon-rich oxides

The nonvolatile memory characteristics of silicon-rich oxide (SRO, SiOx) grown at room temperature for charge-trapping layer are first reported and shown to exhibit a strong dependence on oxygen content (x). The memory window that is estimated by capacitance-voltage curves monotonically decreases with increasing x from 1.0 to 1.8, possibly resulting from the x-dependent variation in the Si suboxide states responsible for the charge traps, as evidenced by x-ray photoelectron spectroscopy. The density of the charge traps is estimated to be (3.9–8.8)×1012 cm−2 for x=1.0–1.4. The charge-loss rate sharply decreases at x=1.2, but by further increase in x above 1.2, it gradually increases, which can be explained by the lowered SRO/SiO2 barrier due to the increased optical band gap of SRO at larger x

Effect of doping-induced defect concentration on the characteristics of Si-quantum-dot solar cells

A promising method to overcome the efficiency limit of the crystalline Si solar cell is to utilize the visible light in the solar-energy spectrum. Silicon quantum dots (QDs) have been proposed as a source of strong light emission in the visible range based on the quantum confinement effect [1–3]. Light emission in the full visible range of wavelength has been reported by controlling the size and the density of Si QDs embedded within various types of insulating matrix [2, 4]. The increased bandgap energy of Si QDs enables the light absorption in the visible range of the terrestrial solar spectrum. [5]. Formation of p-n junction by doping of n- or p-type impurities into Si QDs is a key process to form all-Si-QD active layers in photovoltaic devices. A Si-QD solar cell with an energy-conversion efficiency of 10.6 % has been developed by forming phosphorus-doped Si-QD superlattices (SLs) on a p-type crystalline silicon substrate for an active heterojunction layer [6, 7]. In this study, we fabricate B-, P-, and Sb-doped p- and n-type Si QDs/SiO2 superlattices (SLs) for solar cells. We characterize the variations of defect concentrations due to doping by photoluminescence (PL) and thermally-stimulated current, and compare them with the photovoltaic characteristics of the solar cells.