A threshold-shifting, single transistor memory structure with fast read and write times and long retention time is described. The structure consists of a silicon field-effect transistor with nano-crystals of germanium or silicon placed in the gate oxide in close proximity of the inversion surface. Electron charge is stored in these isolated 2-5 nm size nano-crystals which are separated from each other by greater than 5 nm of SiO/sub 2/ and from the inversion layer of the substrate surface by less than 5 nm of SiO/sub 2/. Direct tunneling of charge from the inversion layer and its storage in the nano-crystal causes a shift in the threshold voltage which is detected via current sensing. The nano-crystals are formed using implantation and annealing or using direct deposition of the distributed floating gate region. Threshold shift of 0.3 V is obtained in Ge-implanted devices with 2 nm of SiO/sub 2/ injection layer by a 4 V write pulse of 300 ns duration. The nano-crystal memories achieve improved programming characteristics as a nonvolatile memory as well as simplicity of the single poly-Si-gate process. The V/sub T/ window is scarcely degraded after greater than 10/sup 9/ write/erase cycles or greater than 10/sup 5/ s retention time. Nano-crystal memories are promising for nonvolatile memory applications.

Fast and long retention-time nano-crystal memory

Strong room-temperature photoluminescence (PL) in the wavelength range 650–950 nm has been observed in high temperature annealed (1000–1300 °C) substoichiometric silicon oxide (SiOx) thin films prepared by plasma enhanced chemical vapor deposition. A marked redshift of the luminescence peak has been detected by increasing the Si concentration of the SiOx films, as well as the annealing temperature. The integrated intensity of the PL peaks spans along two orders of magnitude, and, as a general trend, increases with the annealing temperature up to 1250 °C. Transmission electron microscopy analyses have demonstrated that Si nanocrystals (nc), having a mean radius ranging between 0.7 and 2.1 nm, are present in the annealed samples. Each sample is characterized by a peculiar Si nc size distribution that can be fitted with a Gaussian curve; by increasing the Si content and/or the annealing temperature of the SiOx samples, the distributions become wider and their mean value increases. The strong correlation betw...

Correlation between luminescence and structural properties of Si nanocrystals

Si nanoclusters embedded in SiO2 have been produced by thermal annealing of SiOx films prepared by plasma enhanced chemical vapor deposition. The structural properties of the system have been investigated by energy filtered transmission electron microscopy (EFTEM). EFTEM has evidenced the presence of a relevant contribution of amorphous nanostructures, not detectable by using the more conventional dark field transmission electron microscopy technique. By also taking into account this contribution, an accurate quantitative description of the evolution of the samples upon thermal annealing has been accomplished. In particular, the temperatures at which the nucleation of amorphous and crystalline Si nanoclusters starts have been determined. Furthermore, the nanocluster mean radius and density have been determined as a function of the annealing temperature. Finally, the optical and the structural properties of the system have been compared, to demonstrate that the photoluminescence properties of the system depend on both the amorphous and crystalline clusters.

Formation and evolution of luminescent Si nanoclusters produced by thermal annealing of SiOx films

Time resolved photoluminescence (PL) decays have been measured for Si nanocrystals embedded in silicon dioxide. The nanocrystals were formed by implanting 40 keV Si ions into a 1000 A thick film of thermally grown SiO2, followed by thermal annealing at 1000–1200 °C. The observed luminescence, peaking at 700–850 nm, is compared to similar measurements performed on porous Si emitting in the same wavelength range. The results show that the PL from the nanocrystals exhibits a stretched exponential decay with characteristic decay time τ in the range 10–150 μs and dispersion factor β in the range 0.7–0.8. Both parameters are, however, higher for the nanocrystals compared to those of porous Si indicating superior passivation of the nanocrystals in the SiO2 matrix. Evidence is also presented for a single exponential behavior at the decay end suggesting a remaining fraction of excitons in isolated nanocrystals. We attribute the highly nonlinear dose dependence of the PL yield to a nucleation process for the nanocr...

Analysis of the stretched exponential photoluminescence decay from nanometer-sized silicon crystals in SiO2

In this article the luminescence properties of Si nanocrystals (nc) formed by plasma enhanced chemical vapor deposition and their interaction with Er ions introduced by ion implantation are investigated in detail. Si nc with different size distributions and densities were produced and all show quite intense room temperature luminescence (PL) in the range 700–1100 nm. It is shown that the time-decay of the luminescence follows a stretched exponential function whose shape tends towards a single exponential for almost isolated nc. This suggests that stretched exponential decays are related to the energy transfer from smaller towards larger nc. Indeed, by comparing samples with similar nc size distributions, but with very different nc densities, it is demonstrated that the PL has a quite strong redshift in the high density case, demonstrating a clear energy redistribution within the sample. Excitation cross sections have been measured in all samples yielding a value of ∼1.8×10−16 cm2 for isolated nc excited w...

Role of the energy transfer in the optical properties of undoped and Er-doped interacting Si nanocrystals

Two sources of room temperature visible luminescence are identified from SiO2 films containing ion beam synthesized Si nanocrystals. From a comparison of luminescence spectra and photoluminescence decay lifetime measurements between Xe + -implanted SiO2 films and SiO2 films containing Si nanocrystals, a luminescence feature attributable to defects in the SiO2 matrix is unambiguously identified. Hydrogen passivation of the films selectively quenches the matrix defect luminescence, after which luminescence attributable to Si nanocrystals is evident, with a lifetime on the order of milliseconds. The peak energy of the remaining luminescence attributable to Si nanocrystals ``redshifts'' as a function of different processing parameters that might lead to increased nanocrystal size and the intensity is directly correlated to the formation of Si nanocrystals. Upon further annealing hydrogen-passivated samples at low temperatures (< 500 °C), the intensity of nanocrystal luminescence increases by more than a factor of 10.

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Defect‐related versus excitonic visible light emission from ion beam synthesized Si nanocrystals in SiO2

Synthesis of Ge nanocrystals in SiO2 films is carried out by precipitation from a supersaturated solid solution of Ge in SiO2 made by Ge ion implantation. The films exhibit strong room-temperature visible photoluminescence. The measured photoluminescence peak energy and lifetimes show poor correlations with nanocrystal size compared to calculations involving radiative recombination of quantum-confined excitons in Ge quantum dots. In addition, the photoluminescence spectra and lifetime measurements show only a weak temperature dependence. These observations strongly suggest that the observed visible luminescence in our samples is not due to the radiative recombination of quantum-confined excitons in Ge nanocrystals. Instead, observations of similar luminescence in Xe+ -implanted samples and reversible PL quenching by hydrogen or deuterium suggest that radiative defect centers in the SiO2 matrix are responsible for the observed luminescence.

/pdf/the-role-of-quantum-confined-excitons-vs-defects-in-the-1faheapnac.pdf

The role of quantum-confined excitons vs defects in the visible luminescence of sio2 films containing ge nanocrystals

Electroluminescent devices were fabricated in SiO_2 films containing Ge nanocrystals formed by ion implantation and precipitation during annealing at 900 °C, and the visible room‐temperature electroluminescence and photoluminescence spectra were found to be broadly similar. The electroluminescent devices have an onset for emission in reverse bias of approximately −10 V, suggesting that the mechanism for carrier excitation may be an avalanche breakdown caused by injection of hot carriers into the oxide. The electroluminescent emission was stable for periods exceeding 6 h.

/pdf/electroluminescence-and-photoluminescence-of-ge-implanted-si-2fhw1bnity.pdf

Electroluminescence and photoluminescence of Ge‐implanted Si/SiO2/Si structures

Ion beam synthesis of Si and Ge nanocrystals in an SiO 2 matrix is performed by precipitation from supersaturated solid solutions created by ion implantation. Films of SiO 2 on (100) Si substrates are implanted with Si and Ge at doses 1 × 10 16 /cm 2 - 5 × 10 16 /cm 2 . Implanted samples are subsequently annealed to induce precipitation of Si and Ge nanocrystals. Raman spectroscopy and high-resolution transmission electron microscopy indicate a correlation between visible room-temperature photoluminescence and the formation of diamond cubic nanocrystals approximately 2–5 nm in diameter in annealed samples. As-implanted but unannealed samples do not exhibit luminescence. Rutherford backscattering spectra indicate a steepening of implanted Ge profiles upon annealing. Photoluminescence spectra are correlated with annealing temperatures, and compared with theoretical predictions for various possible luminescence mechanisms, such as radiative recombination of quantum-confined excitons, as well as possible localized state luminescence related to structural defects in SiO 2 . Potential optoelectronic device applications are also discussed.

Ion Beam Synthesis of Luminescent Si and Ge Nanocrystals in a Silicon Dioxide Matrix

Synthesis and size-dependent photoluminescence has been performed for Ge nanocrystals in SiO2 matrices with average diameters between 2 and 9 nm, formed by room-temperature ion implantation into SiO2 followed by precipitation during vacuum thermal anneals. Nanocrystal size distributions obtained from electron microscopy data were used in conjunction with a quantum-confined exciton recombination model [T. Takagahara and K. Takeda, Phys. Rev. B 46 (1992) 15578] to generate calculated photoluminescence spectra, which were compared with experimental spectra. Qualitative agreement between calculated and observed spectra could be obtained when: (i) the non-radiative and radiative carrier recombination rates were approximately equal for nanocrystal sizes corresponding to visible emission, (ii) the spatial extent of the nanocrystal confinement potential is assumed to be slightly smaller than the measured size of the nanocrystal, and (iii) coalescence of individual nanocrystals produces electronic states corresponding to single nanocrystals with larger volume.

K. V. Shcheglov

Papers

Defect‐related versus excitonic visible light emission from ion beam synthesized Si nanocrystals in SiO2

The role of quantum-confined excitons vs defects in the visible luminescence of sio2 films containing ge nanocrystals

Electroluminescence and photoluminescence of Ge‐implanted Si/SiO2/Si structures

Ion Beam Synthesis of Luminescent Si and Ge Nanocrystals in a Silicon Dioxide Matrix

Correlation of size and photoluminescence for Ge nanocrystals in SiO2 matrices