Charge retention characteristics in Si nanocrystal doped SiO 2 films
4.1 Introduction
In chapter 3, the electronic properties of the tunnel oxide layer were addressed by C- AFM. The lack of a denuded zone with high quality and enough thickness was suggested to be the origin of the overall high conductance of this layer, which largely increased the chances of charges tunneling back into the substrate. To improve charge retention, more efforts should be made to better control the Si nanocrystal distribution.
Indeed, Si nanocrystal memory fabricated through ion implantation has been criticized for its inferior charge retention characteristics in comparison to the 10 year requirement of electrically erasable programmable read only memory (EEPROM), which makes it limited to applications such as dynamic random access memory (DRAM) at its current stage. For longer charge storage, the nanocrystals must be better isolated. With the limitations of the current ion implantation technique, such as poor control over nanocrystal sizes and spatial distribution as well as defect amount, the simplest way out is by increasing the tunnel oxide thickness in the vertical direction and the spacing between nanocrystals in the lateral direction, which sacrifices the benefits from tunnel oxide scaling and large threshold voltage offset. Actually, the ion-implantation energy and dose are selected with consideration of all of these factors.
While C-AFM provides information regarding electron transport through the tunnel oxide layer, it is just an indirect approach for investigating charge retention characteristics. Transistor measurements and C-V measurements give information about device operation, but lack the ability to monitor a few localized charges for the study of charge dissipation dynamics and mechanisms, which are important in guiding device fabrication and bringing up innovation. Conductive-tip nc-AFM98 [refs] and EFM99,100,101 are very sensitive to the electrostatic force produced by a small amount of charges down to a single electron,98 which makes them good tools for microscopic charge analysis. In addition, the conductive AFM tip can be conveniently used to inject charges into the surface layer of a sample surface by applying an appropriate bias. For all these reasons, conductive-tip nc-AFM and EFM have been widely employed for injection and probing of localized charges in silicon nanocrystals.102,103,104,105,106 Ng et al.107 investigated the influence of Si nanocrystal distribution on charge decay rate, and Krishnan et al.108 observed the effect of oxidation on charge localization and transport in a Si nanocrystal layer, both using lift mode scanning with detection of frequency shift. The vertical and lateral charge dissipations studied by conductive-tip nc-AFM and EFM may help to determine whether Si nanocrystal memory can be a viable way for further device scaling into the few tens of nanometers regime.
In this chapter we show from charge injection and imaging experiments in a UHV chamber that holes have a much longer retention time than electrons in the Si nanocrystal floating gate fabricated through ion-implantation. Using an electrostatic model for nc- AFM operation, the charge retention characteristics are analyzed quantitatively and found to have an approximately logarithmic dependence on time. All results from microscopic
charge analysis are consistent with previous capacitance decay measurements. The small dissipation rate of holes in both lateral and vertical directions makes it an interesting choice as the working charge in Si nanocrystal memory. Based on this idea, we suggest p-channel Si nanocrystal memory as a possible candidate for further device scaling.
4.2 Charge injection and imaging by conductive-tip noncontact AFM Charge injection and imaging experiments were performed with an UHV VT STM/AFM (Omicron Nanotechnology). Figure 4.1 shows the schematics of the experiments. An n+-doped silicon cantilever with a resonant frequency of 284192 Hz was used for charge injection and subsequent charge imaging. The pressure inside the UHV chamber is around 10-9 Torr, which not only excludes the influence of surface water and other contamination as charge storage media and dissipation pathways, but also dramatically increases the Q factor and detection sensitivity of the nc-AFM. The scanning height and oscillation amplitude were observed to be around 10 nm and 5 nm, respectively, confirming that the working range is indeed “noncontact mode” rather than
“tapping mode”. After a topographic image was obtained successfully with ∆f = -20 to - 30 Hz [Fig. 4.1(b)], the AFM tip was brought to the center of the area and the feedback was disabled. Then the tip was moved manually toward the sample surface, with the oscillation monitored by an oscilloscope, until the oscillation was fully quenched to indicate complete contact between the Si tip and the sample surface. A bias of typically +10 V or -10 V was applied to the tip with respect to the grounded sample substrate for around 10 s to inject holes or electrons into the sample surface [Fig. 4.1(a)]. To restore
Figure 4.1. (a) Charge injection by lowering the AFM tip to touch the sample surface and applying a potential to the tip for a short period. (b) Noncontact mode imaging of surface morphology or surface morphology with injected charges.
scanning, the tip was retracted 50 nm from the sample surface, the tip bias was reset to zero, and the feedback was enabled again. A minor adjustment called “auto ∆f” was performed to adapt to the possible tip changes that happened in the contact period. After that, the tip can be auto-approached to the working distance of about 10 nm to begin continuous scans in non-contact mode and monitor the charge dissipation process in real time [Fig. 4.1(b)].
The charge injection and imaging experiments were first tried on the Si nanocrystal sample [Fig. 2.2(a)] which is most similar to the real device geometry, but no charging effect was observed. The 7-s-etched Si nanocrystal sample [Fig. 3.4(b)] was also tested and did not show the existence of local charges. We attribute the fail to a leaky SiO2 layer which caused insufficient charging of Si nanocrystals. The majority of charges tunneled between the tip and the substrate rather than flowing into the nanocrystal layer during the charge injection, so the charged nanocrystals were limited to a very small area.
Furthermore, the quantity of charges in the nanocrystal layer may be too little to be detected. In addition, because the nanocrystals are very close to the substrate, polarized charges at the interface further weakened the electric field produced by injected charges and made charge detection even harder.
It should be noted that we generally prefer intact Si nanocrystal samples rather than etched samples in the charging experiments. The latter contains mobile nanocrystals adhering to the surface which may touch each other and form fast charge dissipation paths. Even if the situation can be improved by additional oxidation109 or by removal of the adhered nanocrystals using ultrasonic bath after etching, the charge dissipation dynamics still deviate from what would really happen in the original SiO2 matrix.
In order to remove the influence of the leaky oxide on the charging and discharging processes, we chose to significantly increase the tunnel oxide thickness. The samples were fabricated by ion-implantation into 100 nm SiO2 films followed by thermal annealing with similar conditions to that for device fabrication [Fig. 4.2]. The peak concentration of the implanted silicon is at a depth of about 10 nm, so the tunnel oxide
Figure 4.2. Synthesis of SiO2 films (100 nm) containing Si nanocrystals which are distributed at a depth of around 10 nm. The Si+ ion implantation doses are 0.95×1016 cm-2 (low dose sample) and 1.27×1016 cm-2 (high dose sample), respectively.
thickness is around 90 nm, which almost completely forbids charge tunneling into the substrate during experiments. In addition to the same ion-implantation dose of 1.27×1016 cm-2 (“high dose”) that was used to fabricate the nanocrystal memory device in Chapter 2, a lower dose of 0.95×1016 cm-2 was also selected to evaluate the nanocrystal density dependence of the charge retention characteristics.
For these samples the anticipated mechanism of charge injection was described in Fig. 4.3. Since charge tunneling between the nanocrystal layer and the substrate can be neglected, the charge injection process is similar to the charging process of a parallel capacitor, in which the silicon substrate is the bottom plate and the nanocrystal layer is the top plate. In a classical parallel capacitor, charges flow into both conductive plates and distribute homogeneously until arriving at a stable state in which the electric field is limited to region between two plates. But in this case, such a state can not be reached since the charge diffusion was limited by the separation between nanocrystals. The field- enhanced charge diffusion stopped after a distance, resulting in a charged disk in the nanocrystal layer.
After the charge injection, the stored charges in Si nanocrystals may dissipate both laterally and vertically (Fig 4.4) through several possible mechanisms including direct tunneling, Fowler-Nordheim tunneling, thermionic emission, and hopping conduction.
Charges on nanocrystals close to a sample surface may dissipate back onto the surface
Figure 4.3. Schematic of the mechanism of charge injection with a biased AFM tip.
Figure 4.4. Schematic of the charge dissipation mechanisms.
driven by the electric field produced by other charges. Similarly, charges at the lower part of the nanocrystal layer may be trapped into nanocrystals and defects at larger depths, or even dissipate into the substrate through some high conductance paths. The lateral dissipation includes the dissipation on the sample surface (for surface charges) and the dissipation within the nanocrystal layer (for those charges still in nanocrystals). The latter is believed to have a strong dependence on the density of Si nanocrystals. During charge dissipation, both the loss of charges and the evolution of charge distribution change the electric field, which can be detected by the sensitive UHV nc-AFM. By analyzing time- dependent nc-AFM signals, abundant information regarding charge dissipation dynamics can be obtained.