Silicon Nanocrystals
2.2 Fabrication via Ion Implantation
Silicon nanocrystals can be fabricated through a variety of techniques including ion implan- tation [23–27], aerosol synthesis [28, 29], ion beam co-sputtering [30, 31], chemical vapor deposition [33, 34], and reactive evaporation of silicon-rich oxides [35, 36]. All of these methods rely on the low mobility of silicon in silicon dioxide [33] and the equilibrium phase separation of Si from SiO2 in silicon-rich oxide layers at high temperatures [81].
Among these processes, we use ion implantation and thermal annealing to create silicon nanocrystals for our experiments and devices. This technique was selected primarily for compatibility with cmos processing; ion implantation is already commonly used in silicon microelectronics to create doped regions in circuits. In the ion implantation procedure, ions are extracted from a plasma and accelerated by an electric field to the sample. The ions
Implant Sciences Corporation in Wakefield, Massachusetts. We have also had some samples implanted at Intel Corporation’s D1-C and RP-1 fab lines in Hillsboro, Oregon, during the course of our collaboration.
Sample preparation begins with the thermal oxidation of a silicon substrate. The re- sulting oxide layer is then implanted with 28Si+ ions to create a silicon-rich zone within the oxide. The implantation dose is typically designed to increase the peak atomic percent excess silicon concentration by 5% to 25% at the intended position for the nanocrystal layer (corresponding to peak stoichiometries of Si1.15−2O2). High implantation doses can be cor- related to larger nanocrystals, but several other factors are important, including oxidation effects, substrate proximity, and annealing conditions [82–84]. When implanting silicon into thin oxide films, losses due to sputtering during the implantation process should also be considered.
The distribution of the implanted silicon ions can be calculated using the SRIM code developed by Ziegler [85]. This code uses universal stopping potentials that can predict the implantation distribution with an average accuracy of about 5% [86]. As shown in figure 2.1, there is a strong correlation between the implantation depth and the width, or
“straggle”, of the ion distribution. In order to have a well defined silicon nanocrystal layer for device applications, it is desirable to implant silicon ions at low energy to achieve a narrow implantation zone [87]. However, the ion beam current that can be extracted from the source plasma decreases rapidly at low ion beam energies, proportionately increasing the implantation time and the cost of implanting the desired stoichiometric excess of silicon in the oxide layer. Most of the samples prepared for this thesis were implanted with 5 keV silicon ions, an energy selected to balance implantation depth, straggle in the depth distri- bution, and sample preparation cost. Figure 2.2 shows the implanted distribution of 5 keV Si ions as predicted by Monte-Carlo simulation with the SRIM code. A 20% peak atomic percentage excess corresponding to a peak stoichiometry of Si1.75O2 can be reached with an implantation fluence of 1.27 ×1016 ions/cm2. The sputtering rate is difficult to predict
Figure 2.1. The calculated implantation range of Si ions into SiO2 as a function of impact energy.
accurately, but may be expected to increase the effective dose by up to a few percent atomic excess by preferentially removing oxygen from the SiO2 layer.
After the silicon-rich layer is formed, the samples are annealed at high temperature to phase separate silicon from the supersaturated solid solution. The redistribution rate of the silicon depends exponentially on the annealing temperature and linearly on the annealing time. This can provide another limited method for controlling the size distribution of the silicon nanocrystals that precipitate. Annealing for longer times at higher temperatures tends to result in larger nanocrystals. This is usually attributed to the Ostwald ripening mechanism, in which a constant probability of escaping from an interface favors the growth of larger nanocrystals at the expense of small conglomerates [88]. Typically silicon nano- crystal samples are annealed between 900 ◦C and 1100 ◦C for 10 to 30 minutes in a tube furnace. It is important to control the ambient oxygen partial pressure during the anneal- ing step in order to avoid consuming the implanted silicon in oxide growth, but a slight background oxygen pressure can be used to suppress preferential oxygen desorption [89].
Samples fabricated at Intel were annealed in a Rapid Thermal Annealing (RTA) furnace, which uses intense illumination to heat the surface layer of a sample rapidly. This tool is
Figure 2.2. The simulated depth distribution of Si ions implanted into SiO2 at 5 keV.
typically used in a silicon fab to activate dopants by short duration “spike” annealing treat- ments. Because of tool safety concerns, our samples could be annealed for only 5 minutes at a temperature of 1080◦C in the RTA. However, we found that this short annealing time was sufficient to form a dense layer of optically active silicon nanocrystals. It is apparent that ion implantation can be used to create nanocrystals across a wide processing window.
Unfortunately the robustness of the fabrication method also tends to limit the changes that processing conditions can make in the nanocrystal size distribution or the density of the nanocrystals. While the ion implantation fabrication method has proven sufficient for our experiments and proof of concept devices, it is likely that improvements could be made us- ing an alternative fabrication method that offers more precise control over the nanocrystal geometry.
When the implanted Si ions enter the oxide layer, they impart momentum to atoms in the silica matrix. These atoms recoil and are reincorporated at new locations in the SiO2
matrix. This mixing process results in significant damage in the form of dangling bonds and vacancies within the amorphous oxide matrix in the vicinity of the silicon nanocrystals.
These defects can be observed in photoluminescence measurements [90] and can reduce the internal radiative quantum efficiency of the silicon nanocrystals by introducing non-
radiative recombination relaxation pathways. Some of the damage is repaired during the high temperature nanocrystal formation anneal, but it is common to additionally anneal silicon nanocrystal samples in a hydrogen-rich atmosphere in an attempt to passivate any remaining dangling bonds. The photoluminescence intensity typically increases by a factor between 2 and 10 following hydrogen passivation. Many of our samples are annealed in a
“forming gas” ambient of 10% H2:N2 at 450 ◦C for∼30 minutes for this purpose.
Figure 2.3. The size distribution of Si nanocrystals fabricated by our fullycmoscompatible ion implantation process is determined using vacuum noncontact AFM measurements (a).
(b) and (c) are the histogram and distribution, respectively, of Si nanocrystal sizes based on the measurements of the 83 Si nanocrystals in (a) [95].
thesis work [95, 96] using a combination of scanning probe measurements and reflection high energy electron diffraction (rheed). His experiments were conducted using the same sample set we use in the optoelectronic experiments described in this thesis, allowing us to make direct use of his results. These samples were fabricated during our collaboration with Intel on 300 mm substrates by implantation with 5 keV Si+ ions to a total fluence of 1.27 × 1016 cm−2 and were annealed in an RTA furnace at 1080 ◦C for 5 minutes in an atmosphere containing 2% oxygen. His measurements show that the nanocrystals fab- ricated by this procedure are crystalline and that they are distributed in an approximate monolayer within the oxide layer. He estimates the areal density of nanocrystals in the samples to be ∼4 ×1012 cm−2. Figure 2.3 is reproduced from Feng’s thesis, and shows a measurement of the distribution of silicon nanocrystal diameters that suggests a mean di- ameter of∼2.5 nm. This size is somewhat too small to correspond to the photoluminescence spectra we measure, and probably implies a decrease in diameter during the etching proce- dure used to separate the nanocrystals from the oxide matrix prior to the vacuum atomic force microscopy measurement. A distribution of nanocrystal diameters centered at 2.5 nm would imply that ∼26% of the implanted silicon contributes to nanocrystal formation. If the etching procedure has reduced the average diameter from∼4 nm, as may be more con- sistent with the range of observed photoluminescence wavelengths, ∼82% of the implanted silicon contributes to nanocrystal formation. The remainder of the implanted silicon could be incorporated into the substrate during the nanocrystal formation process, adding∼1 nm of silicon to the interface. Alternatively, the implanted silicon could be incorporated in the oxide or present in agglomerates that are too small to observe using vacuum AFM.