PhD Thesis

On Friday the 5th of July, 2009, I defended my PhD at the University of Oslo.


PhD Thesis:  TEM and XPS studies of nanocrystals and clusters in nanostructured materials used for memory storage applications (PDF).

 

 

Summary: Nanoscaled electronic devices have attracted much attention due to their optical and electronic properties, especially related to MOS (Metal-Oxide Semiconductor) devices used for memory storage applications. Improved electrical properties, longer retention, lower gate voltage and lower power consumption are assumed to be possible when replacing bulk floating gate in flash memory devices with nanocrystals. Multilayer samples with Si, Ge, Er oxide, and Pd nanocrystals and clusters were studied in detail. The nucleation, distribution, defects, composition, and atomic and electronic structure are important factors to understand in order to improve performance of memory storage devices. These parameters were studied by high resolution transmission electron microscopy, energy filtered transmission electron microscopy, electron energy loss spectroscopy, X-ray photoelectron spectroscopy, energy dispersive spectroscopy, and secondary ion-mass spectrometry.

Trial lecture: Electrical properties of inorganic nanocrystals

Supervisors: Arne Olsen, Bengt Svensson, and Terje Finstad.

Publications from the PhD work and related work:

  • Annett Thøgersen, Jeyanthinath Mayandi, Lasse Vines, Martin F. Sunding, Arne Olsen, Spyros Diplas, Masanori Mitome and Yoshio Bando. Composition and structure of Pd nanoclusters. Journal of Applied Physics 109, 084329 (2011).

    Abstract: The nucleation, distribution, composition, and structure of Pd nanocrystals in SiO2 multilayers containing Ge, Si, and Pd are studied using high resolution transmission electron microscopy (HRTEM) and x-ray photoelectron spectroscopy (XPS), before and after heat treatment. The Pd nanocrystals in the as deposited sample (sample ASD) seem to be capped by a layer of PdOx. A 1–2 eV shift in binding energy was found for the Pd-3d XPS peak, due to initial state Pd to O charge transfer in this layer. The heat treatment results in a decomposition of PdO and Pd into pure Pd nanocrystals and SiOx. 

     

  • Annett Thøgersen, Jeyanthinath Mayandi, Terje Finstad, Arne Olsen, Spyros Diplas, Masanori Mitome and Yoshio Bando, The formation of Er-oxide nanoclusters in SiO2 thin films with excess Si. Journal of Applied Physics 106, 014305 (2009).

    Abstract: The nucleation, distribution, and composition of erbium embedded in a SiO2–Si layer were studied with high resolution transmission electron microscopy (TEM), electron energy loss spectroscopy, energy filtered TEM, scanning transmission electron microscopy, and x-ray photoelectron spectroscopy. When SiO2 layer contains small amounts of Si and Er, nanoclusters of Er oxide are formed throughout the whole layer. The exposure of oxide to an electron beam with 1.56 × 106 electrons nm2 s causes nanocluster growth. Initially this growth matches the Ostwald ripening model, but eventually it stagnates at a constant nanocluster radius of 2.39 nm. 

     

  • Annett Thøgersen, Jeyanthinath Mayandi, Terje G. Finstad, Arne Olsen, Jens Sherman Christensen, Masanori Mitome and Yoshio Bando. Characterization of amorphous and crystalline silicon nanoclusters in ultra thin silica layers. Journal of Applied Physics. 104, page: 094315 (2008). 

    Abstract: The nucleation and structure of silicon nanocrystals formed by different preparation conditions and silicon concentrations (28 – 70 area %) have been studied using transmission electron microscopy (TEM), energy filtered TEM, and secondary ion mass spectroscopy. The nanocrystals were formed after heat treatment at high temperature of a sputtered 10 nm thick silicon rich oxide on 3 nm SiO2 layer made by rapid thermal oxidation (RTO) of silicon. Nanocrystals precipitate when the excess silicon concentration exceeds 50 area %. Below this percentage amorphous silicon nanoclusters were found. In situ heat treatment of the samples in the TEM showed that the crystallization requires a temperature above 800 °C. The nanocrystals precipitate in a 4 nm band, 5 nm from the Si substrate, and 4 nm from the SiO2 sample surface. The silicon nucleates where the excess Si concentration is the highest. The top surface has less excess Si due to reaction with oxygen from the ambient during annealing. The SiO2-RTO layer is more Si rich due to Si diffusion from the SiO2–Si layer into RTO. Twinning and stacking faults were found in nanocrystals with 4 – 10 nm in diameter. These types of defects may have large effects on the usability of the material in electronic devices. Both single and double twin boundaries have been found in the nanocrystals by high resolution TEM. Image simulations were carried out in order to obtain more information about the defects and nanocrystals. The stacking faults are extrinsic and located in the twin boundaries. 

     

  • Annett Thøgersen, Spyros Diplas, Jeyanthinath, Mayandi, Terje Finstad, Arne Olsen, John F. Watts, Masanori Mitome and Yoshio Bando. An experimental study of charge distribution in crystalline and amorphous Si nanoclusters in thin silica films. Journal of Applied Physics. 103, page: 024308 (2008).

    Abstract: Crystalline and amorphous nanoparticles of silicon in thin silica layers were examined by transmission electron microscopy, electron energy loss spectroscopy, and x-ray photoelectron spectroscopy (XPS). We used XPS data in the form of the Auger parameter to separate initial and final state contributions to the Si2p energy shift. The electrostatic charging and electron screening issues as well as initial state effects were also addressed. We show that the chemical shift in the nanocrystals is determined by initial state rather than final state effects, and that the electron screening of silicon core holes in nanocrystals dispersed in SiO2 is inferior to that in pure bulk Si. 

     

  • J. Mayandi, T.G. Finstad, S. Foss, A. Thøgersen, U. Serincan and R. Turan. Ion beam synthesized luminescent Si nanocrystals embedded in SiO2 films and the role of damage on nucleation during annealing. Surface & Coatings Technology 201, page: 84828485 (2007). 

    Abstract: Si nanocrystals in thermal oxide films (∼ 250 nm) were fabricated by 100 keV Si ion implantation followed by high temperature annealing. Two different doses were compared after annealing at 1050 °C for 2 h. A sample implanted with a dose of 1 × 1017 cm−2 shows a broad photo luminescence peak centered around 880 nm after annealing. A dose of 5 × 1016 cm−2 yields a considerable blue shift of about 100 nm relative to the higher dose as well as a reduction in intensity. Transmission electron microscopy study reveals a difference in the microstructure of the SiO2 films. Nanocrystals are clearly identified in the middle of the film for the highest dose, but not for the lower dose. The difference is discussed in terms of concentration dependent nucleation rate and differences in defect concentration. It is argued that the latter effect has a strong effect on the depth distribution of nanocrystals. 

     

  • J. Mayandi, T.G. Finstad, A. Tøgersen, S. Foss, U. Serincan and R. Turan. Scanning probe measurements on luminescent Si nanoclusters in SiO2 films. Thin Solid Films 515, page: 63756380 (2007).

    Abstract: Embedded Si nanocrystals in SiO2 have a large current interest due to the prospects for practical applications. For most of these it is essential to characterize and ultimately control the nanocrystal size, size distribution and spatial distribution. Here we present a study of Si nanocrystals and clusters in SiO2 studied by atomic force microscopy (AFM). Since it is an indirect method, it requires several other methods to establish a reliable description of the structure of the samples. We here compare the AFM results with photoluminescence (PL) and transmission electron microscopy (TEM). Si nanocrystals in thermal oxide films (∼ 250 nm) were fabricated by 100 keV Si ion implantation at a dose of 1 × 1017 cm−2 followed by high temperature annealing. AFM micrographs were taken after different etching times of the oxide and compared to TEM measuerements of the nanocrystal size and distribution. The correlations observed strongly indicate AFM signatures connected to the nanocrystals. We have analyzed and modeled the etch sectioning technique. Comparisons with the experiments let us conclude that the sectioning technique has some memory effect, but yields a distribution of nanocrystals with depth. A dose of 5 × 1016 cm−2 yields a PL blue shift of about 100 nm relative to the higher dose. No nanocrystals are observed with TEM in this latter case. However distinct signatures can be observed with AFM and is tentatively attributed to the presence of non-crystalline Si-rich nanoclusters

     

  • J. Mayandi, T. G. Finstad, C. L. Heng, Y. J. Li, A. Thøgersen, S. Foss and H. Klette. A comparison between 1.5 μm photoluminescence from Er-doped Si-rich SiO2 films and (Er,Ge) co-doped SiO2 films. ENS06 Paris, France, 14-15 December 2006. 

    Abstract: We have studied the 1.5 µm photoluminescence (PL) from Er ions after annealing two different sample sets in the temperature range 500 °C to 1100 °C. The different sample sets were made by magnetron sputtering from composite targets of Si+SiO2+Er and Ge+SiO2+Er respectively for the different sample sets. The annealing induces Si – and Ge-nanoclusters respectively in the different film sets. The PL peak reaches its maximum intensity after annealing at 700 °C for samples with Ge nanoclusters and after annealing at 800 °C for samples with Si. No luminescence from nanoclusters was detected in neither sample sets. This is interpreted as an energy transfer from the nanocluster to Er atoms. Transmission electron microscopy shows that after annealing to the respective temperature yielding the maximum PL intensity both the Ge and Si clusters are non-crystalline. Here we mainly compare the spectral shape of Er luminescence emitted in these different nanostructured matrixes. The PL spectral shapes are clearly different and witness a different local environment for the Er ions. 

     

  • J Mayandi, T G Finstad, S Foss, A Thøgersen, U Serincan and R Turan. Luminescence from silicon nanoparticles in SiO2: atomic force microscopy and transmission electron microscopy studies. Physica Scripta T126, page: 7780 (2006).

    Abstract: Si nanocrystals in thermal oxide films (~250 nm) were fabricated by 100 keV Si ion implantation at various doses followed by high temperature annealing. After annealing a sample implanted with a dose of 1 × 1017 cm−2 at 1050 °C for 2 h, a broad photoluminescence peak centred around 880 nm was observed. A dose of 5 × 1016 cm−2 yields a considerable blue shift of about 100 nm relative to the higher dose. Transmission electron microscopy and atomic force microscopy (AFM) are used to characterize the microstructures in the SiO2 film. The limitations of these techniques for the study of the nanostructures are addressed in this paper and it is suggested that AFM combined with etching can yield a structural spectroscopy with very good sensitivity. 

     

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