Emil Manolov

Absorption and transport properties of Si rich oxide layers annealed at various temperatures

Thin films of SiOx (x = 1.15, d = 1 and 2 µm), deposited by thermal vacuum evaporation of SiO on n- and p-type crystalline Si or quartz substrates, and then furnace annealed at 250, 700 and 1000 °C, are studied. Optical and infrared transmission measurements prove phase separation upon annealing at 700 and 1000 °C and growth of amorphous Si nanoparticles upon annealing at 700 °C, whose optical band gap is ~2.6 eV. High-resolution electron microscopy data confirm growth of Si nanocrystals with average size ~5 nm in the films annealed at 1000 °C. Both kinds of transmission data were used to estimate the nanoparticle volume fraction and values of 0.2–0.25 and 0.25–0.30 for the films annealed at 700 and 1000 °C, respectively, are determined. Current–voltage characteristics (at fields >5 × 104 V cm−1) are measured on metal/SiOx/c-Si/metal structures to explore carrier transport mechanisms in all kinds of samples. They are nearly symmetric, which indicates that in all samples carrier transport via structures is dominated by the transport in the SiOx layers. It is concluded that current transport is space-charge-limited for the layers annealed at 250 °C. In the films further annealed at 700 °C containing amorphous nanoparticles, Poole–Frenkel transport mechanism is reported while tunnelling is assumed for the films annealed at 1000 °C.

High energy electron-beam irradiation effects in Si-SiOx structures

Homogeneous SiOx films (x=1.3, 200 nm and 1000 nm thick) and composite a-Si-SiOy films (y ~ 1.80) containing amorphous Si nanoparticles have been prepared on crystalline (c-Si) substrate. A part of the films was irradiated at temperature below 50°C by 20 MeV electrons with two different fluences (7.2x1014 and 1.44x1015 el.cm-2). Atomic force microscopy (AFM), Raman spectroscopy and capacitance (conductance) - voltage (C(G)-V) measurements on Al/c-Si/SiOx/Al or Al/c-Si/(a-Si-SiOy)/Al structures were used to get information about the irradiation induced changes in the surface morphology, the phase composition in the film bulk and at the Si-SiOx interface. The AFM results show that the electron irradiation decreases the film surface roughness of the films annealed at 250°C. The Raman scattering data imply appearance of amorphous silicon phase and some structural changes in the oxide matrix of the homogeneous SiOx films. In the composite films electron beam stimulated decrease of the defects at the a-Si/SiOy interface has been assumed. The initial C(G)-V results speak about electron induced formation of electrically active defects in the SiOy matrix of the composite films.

Application of Metal-Oxide-Semiconductor structures containing silicon nanocrystals in radiation dosimetry

Abstract This article makes a brief review of the most important results obtained by the authors and their collaborators during the last four years in the field of the development of metal-insulator-silicon structures with dielectric film containing silicon nanocrystals, which are suitable for applications in radiation dosimetry. The preparation of SiOx films is briefly discussed and the annealing conditions used for the growth of silicon nanocrystals are presented. A two-step annealing process for preparation of metal-oxide-semiconductor structures with three-layer gate dielectrics is described. Electron Microscopy investigations prove the Si nanocrystals growth, reveal the crystal spatial distribution in the gate dielectric and provide evidences for the formation of a top SiO2 layerwhen applying the two-step annealing. Two types of MOS structures with three region gate dielectricswere fabricated and characterized by high frequency capacitance/conductancevoltage (C/G-V) measurements. The effect of gamma and ultraviolet radiation on the flatband voltage of preliminary charged metal-oxide-semiconductor structures is investigated and discussed.

Radiation Dosimeter Based on Metal-Oxide-Semiconductor Structures Containing Silicon Nanocrystals

MOS structures containing silicon nanocrystals in the gate dielectric have been tested as dosimeters for ionizing radiation. Before irradiation the nanocrystals have been charged with electrons by applying a pulse to the gate electrode. The γ-irradiation with doses in the range 0-100 Gy causes approximately linear variation of the flatband voltage, resulting in sensitivities of ~ 2.5 mV/Gy. At higher doses the sensitivity decreases because of decrease of the oxide electric field.

Formation of Si Nanocrystals in Thin SiO2 Films for Memory Device Applications

X-ray Diffraction and Reflectivity, Transmission Electron Microscopy and Atomic Force Microscopy were applied to study the effect of thermal annealing on the properties of thin SiOx films (~ 15 nm) prepared by thermal evaporation of SiO in vacuum. It has been shown that furnace annealing at 1000 oC causes phase separation and formation of uniformly distributed Si nanocrystals into a SiO2 matrix. Clockwise hysteresis has been observed in the C-V curves measured and explained by assuming charging and discharging of the NCs with carriers, which tunnel from the Si substrate.

Memory effect in MOS structures containing amorphous or crystalline silicon nanoparticles

Metal-oxide-silicon structures are fabricated by sequential physical vapor deposition of SiOx (x=1.15) and RF sputtering of SiO2 on n-type crystalline silicon. High temperature annealing in an inert gas ambient at 700degC or 1000degC is used to grow amorphous or crystalline silicon nanoparticles in the SiOx layer. The nanoparticle formation is proven by infrared transmission and Raman scattering measurements. The annealing is also used to form a dielectric layer with tunneling thickness at the silicon wafer/insulator interface. High frequency C-V measurements show that both types of structures can be charged negatively/positively by applying a positive/negative voltage on the gate. The structures with amorphous silicon nanoparticles show lower defect density at the interface between the wafer and the tunnel silicon oxide, lack of fluctuations of the C-V characteristics over the wafer and better reliability when compared to the nanocrystal ones. The most essential advantage is their better retention characteristics; upon negative charging; they retain about 60% of the negative charge trapped in the nanoparticles after 96 h while the structures with nanocrystals retain about 50% after 22 hours.

MOS Structures Containing Si Nanocrystals for Applications in UV Dosimeters

Metal-Oxide-Semiconductor structures with semitransparent Au top electrode and containing Si nanocrystals in the gate dielectric are fabricated and studied. The structures can be charged negatively or positively by injecting or extracting electrons from the top electrode. Illumination with 395-400 nm, 10.4 mW UV light source causes discharge of previously charged structures with rate which varies between 2 mV/s and 12 mV/s. The discharge rate depends on the sign of the trapped charge, as well on the internal electric field in the gate dielectric.

Resistive switching behavior of SiOx layers with Si nanoparticles

First results on resistive switching in SiOx film containing crystalline silicon nanoparticles are reported. SiOx layers (x = 1.15) with thickness of 50 nm were deposited on n-Si crystalline substrates and annealed for 60 min at 1000oC to grow crystalline nanoparticles. Part of the samples were annealed in an inert atmosphere, while the rest were subjected to a two-step (O2+N2/N2) annealing process. Current-voltage (I-V) characteristics were by applying positive or negative voltage to the top contact. For both types of samples the I-V characteristics were asymmetric with lower currents measured at negative voltage, especially in the case of two-step annealed samples. In most of the N2 annealed structures switching behavior high-low/low-high resistance state was observed in both polarities at voltages with amplitudes in the range (2 - 4) V. Uncontrolled switching low/high resistance was also seen, more frequently at positive voltages. In contrast, the two-step annealed samples showed stable behavior. The transition high-low resistance state was achieved by negative voltages in the (-2, -5) V range leading to an increase of the current by more than three orders of magnitude. The structures were reset to the high resistive state, by positive voltage in the range (3 - 4) V. Uncontrolled switching was not observed in the two-step annealed samples for both polarities and they showed higher reliability regarding the number of switching cycles.

Visible Light Sensor Based on Metal-Oxide-Semiconductor Structure

We present results for the effect of visible and UV light on the capacitance-voltage characteristics of a p-type Si based Metal-Oxide-Semiconductor ring-dot structure. It is shown that the structure can be used as a low cost sensor that is simple and easy to manufacture. The sensor can be used to measure visible and UV light intensity or as a light switch. The output signal is change of the inversion capacitance under light illumination at a given bias. The device power consumption is very low and the output signal is independent of the temperature.

Electrical Characterization of MOS Structures with Silicon Nanocrystals Suitable for X-Ray Detection

Metal-Oxide-Semiconductor structures with silicon nanocrystals in the oxide layer are prepared and characterized by Transmission Electron Microscopy and electrical measurements. High temperature annealing of SiO1.15 films at 1000 °C for 30 or 60 min leads to formation of silicon nanocrystals with diameters of 2-3 or 4-6 nm. The processes used to obtain the multilayer gate dielectric and to grow nanocrystals do not deteriorate the properties of the cSi wafer/thermal SiO2 interface. For the interface defect density and the fixed oxide charge values 1010 cm-2 eV-1 and ~ 1010 cm-2 were obtained.