Deposit conditions in LPCVD system for different
Off stoichiometric silicon oxide, also known as silicon-rich oxide (SRO), is a light-emitting material that is compatible with silicon technology; therefore, it is a good candidate to be used as a light source in all-silicon optoelectronic circuits. The SRO obtained by low-pressure chemical vapor deposition (LPCVD) has shown the best luminescent properties compared to other techniques. In spite of LPCVD being a simple technique, it is not a simple task to obtain SRO with exact silicon excess in a reliable and repetitive way. In this work, the expertise obtained in our group to obtain SRO by LPCVD with precise variation is presented. Also, the characteristics of this SRO obtained in our group are revised and discussed. It is demonstrated that LPCVD is an excellent technique to obtain single layers and multilayers of nanometric single layers with good characteristics.
Chemical vapor deposition (CVD) is a versatile and economical technique used to deposit different materials. In the microelectronics industry, it has found a main place and it is a standard process. Currently, many efforts are being done to produce optoelectronic circuits using the mature technology of integrated circuits. A major drawback to integrate a whole silicon circuit that manages both electronic and optical signals is that silicon does not emit light efficiently. There are serious restrictions to integrate a light source in such a system . Basically, two approaches have been under study to solve the problem of the light source: One of them uses a reverse-biased
Off stoichiometric silicon oxide (with empirical formula SiO
Dong et al. showed that for
In order to have intense luminescent SiO
SRO obtained using LPCVD is perhaps the most luminescent compared with SRO obtained by other methods ; however, in this technique, it is difficult to control the silicon excess with some precision, and to have films with controllable properties required of personal with expertise on this type of technique.
In this paper, details of our LPCVD deposition processes to obtain SRO single layers (SLs) and multilayers (MLs) with different
2. Our system
We have two homemade LPCVD reactors, one for two- and another for four-inch wafers. Both reactors have the same layout, thus we will describe only one of them in a general way. Our laboratory is a teaching and research facility, therefore every day different materials have to be deposited and the equipment has to be very versatile. For this reason, we found that controlling it manually produces better results than using automatic parts, then the control of the gas flux using rotameter (ABB model 10A6131NB1B1X00) give us enough functionality. High throughput is not required and our main concern is to have good films with repetitive characteristics. Normally, polysilicon, silicon nitride, silicon oxynitride, and SROs are deposited in the reactor; however, we are not limited to only those materials. Perhaps, obtaining SROs with good characteristics is the most demanding, that is because small differences in silicon excess produce big changes in its characteristics. In the following paragraphs, we will concentrate on describing the details to obtain SRO in a controllable manner.
As shown in Figure 1, our LPCVD system is hot wall type that allows having a uniform temperature in the whole deposition chamber area. The heating element is a three-zone furnace, and a flat zone of ±2°C can be obtained. In the past, an analysis using multivariated experiment was carried out to study different parameters involved in the deposition process . Based on that, we decide to maintain the wafer horizontally on a flat quartz wafer holder. The working temperature profile was chosen with an increasing slop to obtain lesser thickness variation, as shown in Figure 2. The increasing temperature compensates the changes of the boundary layer and produces a more uniform deposition through the flat wafer holder . The deposition temperature allows to deposit SRO from
To obtain SRO, the reactive gases are N2O and SiH4 at 5%, the silane is diluted in N2. The high dilution of silane is a restriction of the system in order to increase the versatility of the reactor. Thereby, we have no possibilities to vary the chamber pressure varying a gas carrier. Figure 3 shows a calibration graph of the pressure of silane and nitrous oxide as a function of the gas flux. The flux of the N2O is controlled by two rotameters as shown in the schematic of Figure 1. Double control of nitrous oxide allows for an efficient way to produce nanometric layers in multilayers with different
The characteristics of the SRO depend strongly on the silicon excess; the flux ratio
Considering that only 0.05 parts of gas corresponds to silane,
In the everyday procedure to obtain always the same conditions, we fixed the
|Flux (slpm)||Flux (slpm)|
|10||0.30||2.0||5.20||1.74 ± 0.05||13|
|20||0.60||3.4||3.80||1.64 ± 0.03||3|
|25||0.74||4.5||3.10||1.57 ± 0.01||2|
|30||0.88||5.2||3.05||1.52 ± 0.01||2|
|50||1.5||9.8||6.50||1.44 ± 0.01||12|
There are many partial pressures combinations that fulfill Eq. (1). However, depending on each laboratory conditions, it is recommendable to set a linear relationship between the flux ratio and the partial pressure ratio of each
To deposit a multilayer with different
As it is expected in a low-pressure system, deposition in our system works under surface reaction kinetics limited [9, 10]. It implies that the deposit rate is low and good step coverage is obtained, and also by-products are trapped in the film, and that is why off stoichiometric silicon oxide is obtained. The LPCVD also has shown good step coverage, and in order to corroborate our step coverage results, the sticking factor and the gas arrival dates are estimated.
The sticking coefficient (
3. Experimental procedure
SRO films were deposited on <100> and low resistivity (5–10 Ω cm) silicon substrates by LPCVD at 736°C. The ratio between reactive gases nitrous oxide (N2O) and silane (SiH4) was varied to obtain films with different silicon excess. Single layers with
Thickness and refractive index of all samples, including multilayer structure, were determined using a null ellipsometer Gaertner L117 with a laser He–Ne of 632.8 nm wavelength. The PL emission spectra were obtained with a Fluoromax-3 spectrometer; all the films were excited with UV radiation (300 nm) and the luminescence was measured from 370 to 1000 nm with a resolution of 1 nm. Optical filters were used in order to guarantee the wavelength of excitation beam. CL measurements were performed using a luminoscope equipment model ELM2-144, 0.3-mA current and 5 kV were used. The luminescence spectra (PL and CL) were measured at room temperature.
For electrical and electroluminescent studies, Metal-Insulator-Semiconductor (MIS) devices were fabricated, and we refer to them as light emitting capacitor (LEC). A ~250-nm-thick semitransparent n+ polycrystalline silicon (Poly) gate was deposited onto the SRO film surface by LPCVD. After a photolithography process step, square-shaped gates of 4-mm2 area were defined. The backside contact was formed with 1-μm thick aluminum layer by evaporation. Finally, the devices were thermally annealed at 480°C in forming gas.
A source meter Keithley model 2400 was used to obtain current versus voltage (I-V) curves. EL spectra were obtained by biasing the device with a constant DC voltage. The light emitted was collected with an optical fiber located facing the Poly gate and connected to the Fluoromax 3 spectrometer.
4. Composition of SRO by LPCVD
SRO is a multiphase material composed of silicon oxides of different stoichiometry and Si nanocrystals (Si-ncs). In XPS spectra of this material, each Si 2p core level band is composed of bands originated in Si at different oxidation states (Si0, Si1+, Si2+, Si3+, Si4+), which manifest themselves at different energies. The position of the peaks corresponding to Si0 and Si4+ (SiO2) is well known and is easily distinguishable , but the peaks related with silicon suboxides cannot be distinguished unequivocally in a complex spectrum composed of different Si oxide species; they have been usually studied at Si/SiO2 interfaces [13–16]. In this way, a quantitative analysis of such highly convoluted spectra is not straightforward. The material can be conveniently considered as composed of SiO2, elemental Si and SiO
Table 3 presents the compositions of SRO with different
From Table 3, it is possible to make a fit of the monotonically varying data (Si and SiO2). For the fit is considered that
For R0’s below 8, the amount of SiO
For our CVD system, one can write the chemical reaction as
It is worthy to mention that certain amount of nitrogen is incorporated in SiO
It is also important to know the form how elemental Si is present in the samples. Through transmission electron microscopy (TEM) studies, it has been possible to evidence Si-ncs in samples with
5. Electrical characteristics
Figure 8 shows the current density (
As we can see, the presence of defects including the Si-nps, either crystalline or amorphous, and their density in the SRO films affect clearly the current transport when they are used in MIS devices. LEC with SRO30 films show a high current state (HCS) at low electric fields, and then after the applied voltage increases, the current is switched to a low conduction state (LCS), as shown in Figure 8. The switching from the HCS to LCS shown by SRO30-based devices was observed by our group before and for both forward and reverse bias [24–28]. That effect was related to the annihilation of conductive paths created by adjacent stable Si-nps and unstable silicon nanoclusters (Si-ncls) through structural changes and by the possible creation of defects (breaking off Si-Si bonds) [24, 25, 27]. Recent studies regarding the same electrical switching in SRO films was observed and related with a conductive filament [29–32]. The conductance switching behavior observed in that SRO films was explained also by structural changes through an electroforming process. In fact, the structural changes in the conductive filament was analyzed by in situ imaging TEM analysis, showing that the conductance switch is related with a crystallization and an amorphization process of Si-nps that creates the conductive filament . These observations are in agreement with our asseverations about the conductance switching observed in our SRO30-based LECs .
In the HCS regime, current jumps and drops, which are observed independently of the temperature of annealing, have been related to the creation and annihilation of the preferential conductive paths and with the appearing or disappearing of electroluminescent spots (EL dots) on the LEC surface [24, 25, 27, 28]. A clear correlation between current jumps/drops and EL dots appearing/disappearing was observed . Once the current fluctuations disappear, through an electrical annealing, the current behavior stabilizes, as reported in [24, 28].
On the other hand, the electrical behavior of most of LECs with SRO20 films does not show current fluctuations. This effect has been related with the presence of well-separated and crystalline silicon nanoparticles (or Si-ncs) and mainly on the density of Si-nps . The Si-nps density estimated from energy-filtered transmission electron microscopy (EFTEM) images of SRO20 films thermally annealed at 1100°C is ~2.46 × 1012 cm−2, about twice the Si-nps density in SRO30 with ~1.1 × 1012 cm−2 . Therefore, a uniform network of conductive paths becomes possible as the Si-nps density increases, allowing a uniform charge flow through the whole capacitor area. Meanwhile, as the Si-nps density decreases (SRO30 films), the distance between them increases reducing the amount of available paths, with a resulting set of discrete and preferential conductive paths within the oxide.
Basically, there are four main mechanisms known to contribute in the carrier transport through a Si-rich oxide layer, including the direct tunneling, Fowler Nordheim tunneling (F-N), Poole-Frenkel (P-F) and the trap-assisted tunneling (TAT) [33–37]. It has been found that the TAT conduction mechanism predominates in our SRO30-based LECs, where the trap energy (
6. Electro-optical characteristics
6.1. Single layer
PL spectra of annealed films from
As can be observed, the PL emission exhibits a shape dependence on the silicon excess, which could indicate different emission mechanism. Because of this, the multi-Gaussian deconvolution of PL spectra was performed for some annealed samples, and the set of band positions have been determined (Figure 10). Each spectrum can be well fitted to a superposition of three Gaussian distributions: a main band (1) and two shoulders (2 and 3). Fit peaks are centered at (1) 710–730, (2) 780–790, and (3) 820 nm with FWHM of (1) 50–60, (2) 20–29, and (3) 18 nm, respectively.
Peak position and intensity vary according to the silicon excess, as shown in Figure 11. There is a blue wavelength shift for all components when the silicon excess decreases (except for
CL spectra from SRO films with different silicon excess are depicted in Figure 12(a). The CL spectra of SRO with thermal treatment consist of a broad emission in the visible and NIR from ~400 to 850 nm. After annealing, intensity of the blue band at ~460 nm increases with increasing the
As CL emission has asymmetrical shape for all SRO samples, it can be assumed that CL emission is also due to different luminescent centers. Hence, multi-Gaussian deconvolution of CL spectra was also obtained, shown in Figure 12(b). The best fit of CL spectra requires four and six components for
Depending on the emission wavelength, multiple luminescence centers have been reported in SiO2 films. Luminescent emission at 460 nm (2.7 eV), 520 nm (2.4 eV) and 650 nm (1.9 eV) nm are mainly related to defects such as oxygen deficiency-related centers (ODC) or oxygen vacancies [39–41],
Figure 13 shows the electroluminescence spectra from the SRO-based LECs. Blue electroluminescence is observed in the SRO30 film, as observed in Figure 13(a). Nevertheless, this blue EL in whole area of LECs is obtained only after the current drop. The main EL peak remains at 468 nm even for different thermal annealing temperatures . A long spectral shift, blue shift, of almost ~227 nm has been observed between the EL and PL band of the SRO30 films. Devices with SRO20 films emit a broad EL spectrum in the red region (713 nm), as observed in Figure 13(b). An additional EL peak of low intensity is also observed at 468 nm. There exists also a blue shift of the EL spectra with respect to PL spectrum in SRO20 films. Nevertheless, both EL and PL spectra in SRO20 films appear in the red region, which could indicate that the same luminescent centers are involved. Images of the blue and red LECs are shown in the insets of the Figure 13. As we can see, intense EL is emitted in the whole area of the LEC devices.
The spectral shift between PL and EL has been reported before and it has been explained according to three different mechanisms [35, 44, 45]. Our experimental results have suggested that the red EL observed in SRO20 films can be related with the combination of some surface defect on the Si-ncs, while the blue EL in SRO30 devices is consistent with the defect emission which could be intrinsically present or generated by electric field within the SRO matrix .
Multilayer structures were fabricated in order to improve the optical properties of the SRO films. Two samples were obtained, one of those is a combination of low silicon excess (
PL spectra of annealed multilayers and single layer are shown in Figure 14(a). As can be seen the intensity emission is improved in the multilayer samples, where multilayer SRO10/SRO25 (ML-10/25) is the most intense. In order to obtain the components of every layer, the multi-Gaussian deconvolution of PL spectra was also obtained. Figure 14(b) and (c) shows the position and the intensity of the three peaks obtained from the Gaussian deconvolution. There is a blue-shift wavelength for the three peaks that can be due to the participation of high silicon excess (
Figure 15 shows a scheme of the multilayered structure fabricated with its dimensional characteristics (left side), and a TEM image of the structure, which exposes the layers composing the SRO multilayer (right side). The goal of this structure is to improve the electro-optical properties of the LECs. In this multilayer, the luminescent properties of three layers with low silicon excess (SRO25) are combined with four conducting layers (SRO5).
The electroluminescence in this ML-SRO-based LEC is observed at forwardly bias considering the substrate as reference. A broad band with the main peak at about 600 nm is observed with
However, the electric field needed to turn on the emission on an ML-SRO-based LEC is lower than an LEC of single layer (see Figures 13 and 16). This proves that the electro-optical properties of a ML-SRO-based LEC are improved, thereby the conductivity of the structure is increased by layers of high silicon excess, and luminescence response is conserved using layers of high
In this chapter, details of a homemade hot-wall LPCVD system were presented. Also, important aspects of how to obtain SRO in a reliable and repetitive form were addressed. We show that in our system, it is possible to obtain single layers with variable silicon excess, and also good quality multilayered structures of nanometric layers. The structural, electrical, and luminescent characteristics of single- and multilayered structures were reviewed and discussed.
Authors recognize the financial support of the CONACYT, particularly, J. Alarcón-Salazar for his grant with number 353251. Also, authors thank the microelectronic laboratory technicians Pablo Alarcon, Victor Aca, and Armando Hernández for their help during the fabrication process.