Silicon-rich Oxide Layers Research Articles

On the photoluminescence of multilayer arrays of silicon rich oxide with high silicon content prepared by low pressure chemical vapor deposition

The photoluminescence emission of multilayer structures composed of layers of silicon rich oxide with high silicon content and layers of silicon rich oxide with low silicon content obtained by low pressure chemical vapor deposition is here presented. Different parameters for the preparation of the multilayers have been varied such as the Si concentration and the thicknesses of the layers. Additionally, the samples were oxidized at different temperatures. For all samples the photoluminescence seems to have the same origin: defects in the oxide matrix and defects at the interfaces between the Si nanocrystals. The structural and compositional properties of the multilayer structures are discussed.

Single layer of silicon quantum dots in silicon oxide matrix: Investigation of forming gas hydrogenation on photoluminescence properties and study of the composition of silicon rich oxide layers

Structures consisting of a single layer of silicon quantum dots in a SiO 2 matrix show interesting optoelectronic properties and potential use as energy selective filters, which are a crucial component for the realization of the hot carrier solar cell. In this work single layer silicon quantum dots in SiO 2 have been realized using a magnetron sputtering technique. Quantum dots are formed by annealing of a silicon rich oxide layer deposited between a thermally grown SiO 2 layer and a sputtered SiO 2 layer. The effects of a forming gas post-hydrogenation process on the photoluminescence of the single layer of quantum dots have been investigated in order to understand the photoluminescence mechanism. It was found that for sputtered silicon quantum dots in SiO 2 matrix the photoemission mechanisms are primarily due to quantum confinement and does not strongly rely on matrix defects. In addition, physical and optical properties of several thick silicon rich oxide layers, with different chemical compositions, have been investigated in order to optimize the stoichiometry of silicon rich oxide in the single layers.

Photoelectric Properties of MOS-like Structures with Twofold SRO Films

The optical properties of silicon rich oxide (SRO) have been deeply studied because, between other reasons, they emit an intense photoluminescence (PL) from visible to the near infrared range when excited with UV light. MOS-like structures with SRO film as the active layer have shown an enhanced conductivity under different illumination conditions. In this paper, MOS-like structures with double SRO layer were fabricated in order to have a barrier to isolate the silicon substrate from the active SRO layer. Results show that all structures have a higher current when light shines on them than that obtained under dark conditions. A possible application of this photo-effect can be used to increase the response of photodetectors and silicon solar cells.

Silicon quantum dot based solar cells: addressing the issues of doping, voltage and current transport

AbstractSilicon quantum dot (Si QD) solar cells offer the potential to tune the effective band gap through quantum confinement and hence allow fabrication of optimised tandem devices in one growth run in a thin film process. Previous work in our group has shown how such cells can be fabricated by sputtering of thin layers of silicon rich oxide sandwiched between a stoichiometric oxide that on annealing crystallise to form Si QDs of uniform and controllable size. Doping multilayers with P and B allows formation of a rectifying junction with an effective band gap of 1.8 eV, which can give an open circuit voltage (VOC) of almost 500 mV. However, the doping behaviour of P and B in these QD materials is not well understood. In addition P and B have big but opposite effects on QD crystallisation, with P(B) forming larger (smaller) QDs than for undoped material. Two possible models for the doping mechanisms in these materials are explored: one relying on doping of a sub‐oxide region around the Si QDs and the other based on the differing nucleation effects of P and B. In addition initial results on hetero‐interfaces in the QD superstructure are assessed as a means to improve vertical current transport, and the effects of hydrogenation on improving VOC by passivating defects. Routes to incorporating these explanations for doping and other structural improvements are discussed as means of optimising the performance of these Si QD cells. Copyright © 2010 John Wiley & Sons, Ltd.

Introducing dopants by diffusion to improve the conductivity of silicon quantum dot materials in 3rd generation photovoltaic devices

AbstractAn alternative method to introduce dopants in Silicon rich oxide (SRO) layers was investigated. A silicon dioxide (SiO2) layer containing phosphorous pentoxide (P2O5) or boron (B) was deposited on the quartz substrate. A bilayer structure was subsequently deposited using Si and SiO2. SRO layers were doped via diffusion during the high temperature annealing required to form the Silicon quantum dot (Si‐QD) materials. Two kinds of dopants (phosphorus and boron) were introduced as they are typical dopants for n‐type and p‐type Si respectively. The annealing time was also varied to achieve an optimum conductivity of the quantum dot material. Different Si concentrations in the SRO layers were investigated. A 5% change of Si concentration could change the conductivity of the Si‐QD material by more than one order of magnitude. Using circular transmission line measurement (TLM), the sheet resistivity of the Si‐QD material was extracted. It was demonstrated that this new method of introducing dopants by diffusion is successful. Moreover, it was found that the conductivity of Si‐QD materials decreased with increasing Si concentration in the SRO layers and was also influenced by the type of dopants. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Structural analyses of thermal annealed SRO/SiO2 superlattices

AbstractNanocrystalline silicon embedded in dielectric matrices is currently studied for Si‐photonics, memory devices and solar cells. A common method for the preparation of silicon nanocrystals embedded in oxides is the phase separation of silicon rich oxide (SRO) in SiO2 and Si via thermal annealing. Phase separation, nucleation and crystallization of SRO are known to depend on the thickness of the SRO layer. Here we investigate the structural changes in a sample consisting of alternated nanometer‐thick SRO and SiO2 layers—a so‐called superlattice (SL)—during thermal annealing. Under a thermal treatment the material undergoes a series of modifications due to sintering, phase separation, crystallization and layer mixing. In this work we investigate these transformations in an SL grown by plasma enhanced chemical vapor deposition (PECVD) with several analytical techniques: XPS, variable angle ellipsometric spectroscopy (VASE), SIMS, and X‐ray reflectivity (XRR). Both SIMS and XRR measurements clearly reveal the periodicity of the samples. XPS analysis reveals that phase separation of SRO in silicon and SiO2 occurs in the annealing temperature range 600–925 °C. The process is accompanied by reduction in overall thickness of the samples (ongoing also at higher temperatures) as evidenced from the ellipsometric spectra. A maximum form birefringence is achieved at 925 °C and stays nearly constant until 1100 °C. Eventually, the form birefringence decreases at the highest annealing temperature of 1150 °C, which according to SIMS measurements is caused by a partial oxidation of silicon in the outermost SRO layer. Copyright © 2010 John Wiley & Sons, Ltd.

Photoconduction in silicon rich oxide films obtained by low pressure chemical vapor deposition

Photoconduction properties of silicon rich oxide (SRO) thin films were studied under different illumination conditions. In the past, Al/SRO/Si structures showed a high photocurrent in spite of the fact that an opaque Al layer was on the active area. In order to elucidate this observation, new Al/SRO/Si structures were tested, but this time they were also measured horizontally. SRO thin films were deposited on silicon wafers by low pressure chemical vapor deposition technique using SiH4 (silane) and N2O (nitrous oxide) as reactive gases at 700°C. 1%–12% silicon excess was used. Structures with a single SRO layer and with a double layer were fabricated in order to have a barrier to isolate the silicon from the active SRO layer. The results show that all structures have a higher current when light shines on them than when they are in the dark. It is proposed that the photocurrent is produced in the SRO bulk, and an explanation for these observations is given.

Structural characteristics of a multilayer of silicon rich oxide (SRO) with high Si content prepared by LPCVD

AbstractSingle layer films and a multilayer structure of SRO (Silicon Rich Oxide) have been prepared by LPCVD (Low Pressure Chemical Vapour Deposition) and characterized by FTIR, SIMS, XPS, TEM and AFM measurements. The stacked structure is composed of alternating layers of SRO with high Si content and SRO with low Si content. The layered structure is confirmed by SIMS and TEM measurements. The composition of the materials is discussed. Besides Si nanocrystals, the existence of agglomerates of silicon oxide with structure close to fused silica and the existence of oxynitrides is evidenced in the films with high Si content. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Transient charging current measurements and modelling in silicon nanocrystal floating gate devices

Nanocrystals-based memories are promising candidates for the scaling of Flash memories in the next decade. However, tools to specify where the carriers are actually stored are still needed in order to optimize the elaboration processes of the nanocrystals. This article presents a model which may be used to extract the physical parameters of the objects in which the carriers are charged. These parameters are extracted from transient current measurements which may be performed by most conventional static ammeters. The model is based on the study of an equivalent circuit followed by a finite elements analysis of the memory device. This technique has been used to study MOS capacitors containing silicon nanocrystals obtained by annealing LPCVD silicon rich oxide layers. Our model was found to predict all the transient features observed in the current. It quantitatively determined that in average, one electron is charged into 3–5 nm diameter nanocrystals having a density of 0.84–1.5 × 10 12 cm −2. These parameters are very close to the size and the density which have been targeted during the elaboration process.

Application of the phonon confinement model on the optical phonon mode of silicon nanoparticles

The Si-rich oxide (SiO x) thin films are prepared by the physical vapor deposition method (PVD). In order to induce the phase separation to SiO 2 and Si phases the samples were annealed at the temperatures 950–1100 °C. The phonon confinement model of the optical modes and the low frequency spherical acoustic vibration modes model were used to calculate the size of silicon nanoparticles from the recorded Raman spectra. In comparison to the high resolution transmission electron microscopy measurements, the both – acoustic and optical vibrational modes give ∼50% in average larger diameter of silicon nanoparticles. The results are interpreted by the formation of an amorphous silicon rich oxide layer that surrounds the silicon nanoparticle.

Comparative study of Si precipitation in silicon-rich oxide films

Two techniques often used in semiconductor industry, namely, low-pressure chemical vapour deposition (LPCVD) and plasma enhanced chemical vapour deposition (PECVD), were used to deposit silicon-rich oxide layers. Thermal treatments have been performed to promote Si precipitation. The composition of the layers was monitored by X-ray photoelectron spectroscopy showing significant nitrogen content ( ∼ 8 % ) only in PECVD layers. The formation of Si nanoclusters has been evidenced by Raman spectroscopy and photoluminescence measurements. LPCVD layers revealed a precipitation even in the as deposited films, and a phase separation was achieved at 1100 ∘ C , while the clusters remain amorphous for PECVD grown films. Electron images of the films revealed an average size of the Si clusters of 3–5 nm with a density around 5 × 10 17 cm - 3 . The optimum temperature for a strong near-infrared emission from Si precipitates was found around 1100 ∘ C for LPCVD and 1250 ∘ C for PECVD layers.

Light emitting devices based on nanocrystalline-silicon multilayer structure

In this paper we report the visible–near-infrared light emission properties of nanocrystalline silicon (nc-Si) light emitting devices (LEDs) based on nc-Si/SiO 2 multilayer structures. Multilayer structures of silicon-rich oxide (SRO) and SiO 2 were grown by plasma enhanced chemical vapor deposition (PECVD) and studied by transmission electron microscopy (TEM) and ellipsometry. A higher nc-Si density in the multilayer samples than in the homogeneous sample was found by comparing photoluminescence (PL) intensities. The PL band located in the near-infrared region can be tuned by the size of nc-Si, which in the multilayer sample is controlled by the thickness of the SRO layer. The multilayer LED shows a much larger current density under low applied voltages than the LED based on a single thick layer. The significant lower driving voltage and enhanced light emission intensity suggest higher power efficiency in multilayer LED. It is believed that the improvement of the LED characteristics is due to the higher nc-Si density caused by the multilayer structure.

Combined XPS, SIMS, and AFM analyses of silicon nanocrystals embedded in silicon oxide layers

AbstractSilicon nanocrystals (Si‐ncs) embedded in a dielectric matrix (e.g. Si3N4, SiO2) attract significant interest for their use in silicon‐based devices such as memories, light amplifiers, and light emitting diodes. Si‐ncs in an SiO2 matrix are generally obtained through thermal annealing of silicon‐rich oxide (SRO) at elevated temperatures (900–1200 °C). In this study, we have investigated the composition of annealed SRO layers close to the interfaces with Si3N4, or c‐Si with XPS, SIMS, and atomic force microscopy (AFM). The analyses indicate an approximately 2‐nm thick Si‐depleted region close to both the Si3N4 and c‐Si interface formed after annealing at 1050 °C. The formation of this Si‐depleted layer is of importance for both electrical transport and charge storage in annealed SRO. Copyright © 2008 John Wiley & Sons, Ltd.

Optical characterization of silicon rich oxide films

Silicon rich oxide (SRO) films with different silicon excess were deposited by low pressure chemical vapor deposition (LPCVD) using SiH 4 and N 2O as the reactant gasses. A set of SRO films was implanted with silicon ions (SI-SRO). After thermal annealing, SRO and SI-SRO films with low Si excess showed a strong visible photoluminescence (PL) in the 1.4–2.1 eV range, characteristic of silicon nanocrystals (Si-nc's) formation. For SRO layers, a redshift of the PL peak was only observed by increasing the silicon excess from 4 to 5.1 at. %, no redshift took place when the silicon excess was 12.7 at. %. The SI-SRO films exhibited a similar behaviour. For implanted and non implanted samples, transmission electron microscopy analysis only showed silicon clusters when the silicon excess was higher than 5 at. %. It has been observed that the Si-clusters size was larger as the silicon excess increased and that the Si-clusters density increased when the SRO films were implanted. Therefore, a stronger PL response was observed in the SI-SRO films. The structural and optical properties of SRO and SI-SRO films have been related, suggesting that the emission could be associated to Si-clusters/defects interaction.

Photoluminescence of Silicon Nanocrystals in Silicon Oxide

Recent results on the photoluminescence properties of silicon nanocrystals embedded in silicon oxide are reviewed and discussed. The attention is focused on Si nanocrystals produced by high-temperature annealing of silicon rich oxide layers deposited by plasma-enhanced chemical vapor deposition. The influence of deposition parameters and layer thickness is analyzed in detail. The nanocrystal size can be roughly controlled by means of Si content and annealing temperature and time. Unfortunately, a technique for independently fine tuning the emission efficiency and the size is still lacking; thus, only middle size nanocrystals have high emission efficiency. Interestingly, the layer thickness affects the nucleation and growth kinetics so changing the luminescence efficiency.

Growth of Highly Luminescent Silicon Nanocrystals by Rapid Thermal Chemical Vapor Deposition

We investigate the growth of highly luminescent silicon nanocrystals by rapid thermal chemical vapor deposition (RTCVD), employing SiH4 and N2O as source gases. For [N2O]/[SiH4] = 7 ∼ 8 and a growth temperature of 650°C, we obtain the optimized deposition condition for silicon rich oxide (SRO) layer having highly luminescent Si nanocrystals after post-deposition annealing. The cross sectional transmission electron microscope investigation reveals the existence of Si nanocrystals in the SRO matrix. Thus, the photoluminescence (PL) from the SRO layer is attributed to the quantum confinement effect of carriers in Si nanocrystals. Based on a single layer growth study, we fabricate ultra-thin SRO/SiO2 superlattice having 25 periods on a 3-inch Si wafer. The superlattice has continuous thickness variation from the center to the edge positions of the Si wafer due to inherent wafer temperature variation during growth. Photoluminescence spectra show a systematic blue-shift from a thicker position (center position) to a thinner position (edge position) which is indicative of nanocrystal size control by SRO layer thickness in the superlattice.

Effect of “buffer layers” on the optical properties of silicon nanocrystal superlattices

Abstract Silicon nanocrystal superlattices consisting of alternating layers of silicon-rich oxide (SRO) and silica (SiO2) were grown by electron beam evaporation, followed by thermal processing. All of the SRO depositions and the processing steps were done under conditions that were as similar as possible so as not to vary the average nanocrystal size. Three sets of samples were synthesized. In the first set, the SRO layers were grown with a different thickness in the different samples while the buffer layers were always 20 nm thick, and in the second set, the SRO layers were 20 nm thick while the SiO2 buffer layers were varied. There were pronounced differences in the photoluminescence spectrum and decay dynamics due to the differences in the thickness of the various layers. The results are interpreted in terms of the migration of charge carriers between nanocrystals, and illustrate that the peak of the PL spectrum can vary between 750 and 950 nm without changing the average particle size. Finally, a third set of samples in which the buffer layers were erbium oxide instead of silica were grown, and the results were interpreted in terms of efficient carrier migration through the nanocrystal-containing layers.

Study of the conduction properties of silicon-rich oxide under illumination

Carrier transport in silicon-rich oxide (SRO) was studied under illumination using the standard current versus voltage and capacitance versus voltage measurements. Al/SRO/Si metal-oxide-semiconductor (MOS)-like structures were used, and SRO layers with different thickness and excess Si concentration were deposited by low-pressure chemical vapor deposition (CVD) technique. In reverse bias condition, two conduction regimes were observed. The photocurrent in low-voltage regime is limited by the transport mechanism of carriers; while in the high-voltage regime, it is limited by the density of photogenerated carriers. The Poole–Frenkel conduction mechanism was found to dominate the carrier transport in SRO under illumination. It was also demonstrated that high photosensitivity was achieved in this structure that is a potential visible optical sensor.

New experimental observations of single electron trapping properties of Si nanoclusters in SRO obtained by LPCVD

It has been proposed that silicon nanoclusters embedded in silicon-rich oxide (SRO) can trap one electron, or one hole. Also, it has been suggested that embedded nanoclusters can be classified into interface and bulk nanoclusters. The possibility of using the characteristic of one electron trapping property in new devices is now under intensive research. In this paper, the charging and discharging properties of silicon-rich oxide layer were studied using an Al/SRO/Si MOS-like structure. The SRO layer was deposited by low pressure chemical vapor deposition, and half of the samples were thermally annealed at 1100°C in N 2 ambient. The measured I–V characteristics show clear current peaks and valleys in low voltage regime. This extraordinary behavior is ascribed to the charge exchange between the embedded nanoclusters in SRO layer and the Si substrate. The valence band of the nanoclusters is supposed to play an important role during the charging and discharging process. C–V measurements were also performed in order to clarify the results. The effect of thermal annealing was explored.

Formation of luminescent Si nanocrystals by high-temperature rapid thermal chemical vapor deposition

We observe a completely different growth regime of silicon-rich oxide (SRO) layers by rapid thermal chemical vapor deposition for the formation of luminescent nanocrystals. The growth regime is characterized by low [N2O]/[SiH4] ratios (<1) and high growth temperatures (>700 °C). High-resolution cross-sectional transmission electron microscopy (XTEM) shows the bimodal distribution of large polycrystals and nanocrystals after post-deposition annealing. The luminescence is attributed to the nanocrystals. Fourier transform infrared spectroscopy in conjunction with XTEM and energy-dispersive x-ray studies show the phase separation and bonding reconfiguration in as-deposited SRO layers. The effectively increased oxygen content in the oxide matrix by phase separation and bonding reconfiguration reduces the diffusion coefficient of Si in the matrix, resulting in the formation of nanocrystals during post-deposition annealing.