The 25th International Conference on Amorphous and Nano-crystalline Semiconductors
August 18–23, 2013 Toronto, Ontario Canada
Amorphous Oxides I
Chair: Gurinder K. Ahluwalia, College of the North Atlantic
Fr-A1.1 (invited) 8:10–8:40
Band Edge and Mid-band-gap Electronic States: Chemical Bonding and Ligand Field Splittings
North Carolina State University, Raleigh, NC, USA
This paper focuses on pre-existing defects (PEDs) in non-crystalline SiO2 and GeO2 that occur during film deposition and/or after low temperature thermal annealing. These mid-band defects are detected by (i) electron spin resonance (ESR), Spectroscopic Ellipsometry (SE), and soft-X-ray Absorption Spectroscopy (XAS). These defects are associate with vacated O-atom sites. They are qualitative different than defects created by X-ray or Gamma-Ray irradiation which are associated with broken Si-O bonds. The energies of PEDs span the spectral range of about 5–7 eV, their energies relative to the valence band edge are obtained from the SE and XAS measurements. Defects occur as singlet and triplet four-vectors (X, Y, Z and Spin: up or down). Qualitatively similar defects occur in Transition Metals oxides such as TiO2 (Ti4+) and Ti2O3 (3+). The energy difference between single and triplet PEDs is described by a Ligand Field Splitting (ΔLF), which is correlated with the formal electron ionic charge is different for SiO2 and GeO2, the Ti oxides identified above. The energies of the PEDs are described by Tanable-Sugano energy levels and scale with ΔLF.
The defects are contained with 1-nm-scale clusters: these are a-periodic in nc-SiO2 and GeO2 and are space filling, in TM oxides they are contained with nano-grains typically 2–3 nm in size. These defects can act as electron or hole traps, and initial and final states for luminescence. As such they are important for device applications; e.g., as gate oxides for MOS devices, and memory cells. Limiting defect densities in Si(Ge)O2 are of order 5x1015 cm–3, and in TM oxides 2–5 times greater.
Similar defects occur in hydrogenated amorphous Si with limiting values similar to SiO2. In SiO2 films with low densities of O-atoms, defect states can be stabilized with respect to light-induced processes, thereby reducing or completely eliminated the Stabler-Wronski effects. These reductions have been defects I ESR studies.
Signal to noise ratios in Soft-X-ray Absorption (XAS) measurements are typically greater than 105:1 so that defects states about 103 times reduced in amplitude are easily detected. These provide a basis for measurement DLF and making correlations with atomic radii that occur in SiO2 and GeO2 and their respective alloys. These comparisons are radially extended the Ti and other TM oxides.
Electrical Characteristics of a Non-volatile MIM Based Memory (Al/Al2O3/Al Fabricated on Glass at 300°C for BEOL Processing
National Institute of Astrophysics, Optics and Electronics (INAOE), Luis Enrique Erro #1, Tonantzintla, Puebla 72000, Mexico
In this work, we present the electrical characteristics of Metal-Insulator-Metal (MIM) Non-Volatile Memory devices based on amorphous-Al2O3 and which is deposited on glass at low temperature by Atomic-Layer Deposition (ALD). The maximum processing temperature for this memory device is 300°C (used for a thermal treatment in N2) therefore, making it ideal for Back-End Of Line (BEOL) processing which requires very low-thermal budgets. Given that for integrated circuit fabrication, BEOL generally starts with the first layer of metal being deposited on the wafer, we have used this approach by depositing an Al/Al2O3/Al stack on Corning glass and annealing the whole stack in N2 at 300°C. Both aluminum layers are 500 nm in thickness and they are deposited by E-beam evaporation under ultra-high vacuum conditions whereas the a-Al2O3 layer is 10 nm in thickness and is deposited by ALD using H2O and Trimethyl-Aluminum (TMA) as precursors at 250°C. During deposition of all layers, the Al/Al2O3/Al stacked structure was sequentially deposited on glass taking care to minimize the transit time specially when transferring the glass samples from the E-beam evaporator to the ALD system and vice versa, so that exposure of the Al2O3 layer to humidity from the atmosphere was reduced. After gate pattern definition and annealing in N2, all the samples were electrically characterized using a Keihtley Model 82-DOS Simultaneous C–V system (at 100 kHz) and an HP 4156B Semiconductor Parameter Analyzer for C-V and I-V characterization respectively. Even though some variations in the forming, set and reset voltages (VSET,form, VSET, VRES) are obtained for many of the measured MIM devices (mainly due to roughness variations of the MIM interfaces as observed after Atomic-Force Microscopy), the memristor effect (switch between ON/OFF states) has been clearly obtained after cyclic I-V measurements for these devices. These transitions between the HIGH/LOW conduction states show a conductivity (or resistivity) window of around 5–6 orders of magnitude and is formed at gate voltages of Vg < 4 V. Also, from the first cycle of operation and afterwards, the conduction mechanisms from the OFF condition up to VSET are highly dependent on the current compliance (CC) of the I-V system. For the larger CC = 100 mA, abrupt transitions from the OFF to ON conditions are observed and which are related to hard breakdown mechanisms of the oxide layer. On the other hand, when CC = 50 μA, soft-breakdown characteristics are observed for the MIM device in which the slope for the OFF/ON transition is increased with respect to the first I-V measurement cycle. This is important since by limiting the electron flux through the stacked Al/Al2O3/Al structure, whether an increase in the diameter of a single conductive filament or generation of additional conductive filaments (connecting both top and bottom aluminum layers) can be expected. In conclusion, low-thermal budget MIM based memories using Al/Al2O3/Al structures have been fabricated and the memristor effect has been observed by showing different conduction mechanisms before breakdown which are dependent on the amount of electrons tunneling through the device.
Keywords: MIM memory, Al2O3, ALD, memristor, BEOL
Optical and Electrical Properties of Nanocrystalline Si Doped SiOx Thin Films Formed by Co-sputtering
Department of Electronics and Bioimformatics, Meiji University, Kawasaki 214-8571, Japan
The quantum size effect of nanocrystalline (nc)-Si domains results in a widening of the electronic band-gap and a luminescence from visible to near-infrared area, which allow us to use nc-Si as light emitters [1,2]. Moreover, the nc-Si particles can be used as a floating gate in nonvolatile MOS memories . So far , electronic properties of nc-Si doped SiOx (nc-Si:SiOx) thin films and their correlation with optical properties such as photoluminescence (PL), cathodoluminescence (CL) and optical absorption (OA) have not been well understood. In this work, we focus on the leakage current-voltage (I-V) and impedance-frequency (Z-f) characteristics of nc-Si:SiOx films. The results were modeled through parallel resistor (R) - capacitor (C) equivalent circuits.
Thin films with a thickness of ~100 nm were deposited on p-type (100) Si substrates (3–5 Ωcm) by RF magnetron co-sputtering in Ar using a SiO2 target (φ 4 inch) and Si chips concentrically on the SiO2 target . The ratio of target area (rSi/SiO2) was varied from 0.00 to 0.19. Then, they were subjected to thermal annealing at 1000°C for 1 hour in Ar. Finally, Al gate-electrodes were deposited on thin films. Chemical composition of films was measured by EDS. PL and CL spectra were evaluated at 300K with a He-Cd laser (325 nm) and an electron beam energy of 15 kV, respectively. OA spectra were also evaluated at 300K. I-V and Z-f (20 Hz–1 MHz) curves were measured at 300K.
OA measurements revealed that the indirect band gap of nc-Si decreased from 4.87 eV to 2.32 eV with increasing rSi/SiO2, whereas no noticeable absorption was observed for the samples with rSi/SiO2 = 0.00. PL measurements showed that considerably weak emissions due to oxygen deficient center in SiOx films were observed at 514–539 nm for samples with lower rSi/SiO2 of 0.00–0.09, while the strong emissions due to interfacial Si layer between the nc-Si core and the a-SiO2 surface layer were observed at 746–808 nm for samples with higher rSi/SiO2 of 0.13–0.19. On the other hand, CL peaks were observed at 455 nm and 465nm for the samples with 0.04 and 0.13, respectively, in which both peaks were assigned to originate from nc-Si core. Impedance analysis showed that the quantity of nc-Si acting as R increases with increasing x in SiOx. Furthermore, C values increase with increasing x in SiOx, which indicates an increase in dielectric constant of SiOx films.
 C. Chen et al., Thin Solid Films, 517, 6659 (2009)
 A. N. Mikhaylov, et al., Physics of the Solid State, 54, 368 (2012)
 M. Wang, et al., Physica E, 41, 912 (2009)
 K. Hirata, et al., Digest of the 25th International Microprocesses and Nanotechnology Conference, Kobe, Japan, 2C-9-3 (2012)
Keywords: nanocrystalline-Si, nc-Si , photoluminescence, band-gap, impedance
The Role of Biasing Electric Field in Intrinsic Resistive Switching Characteristics of Silicon Highly Rich a-SiOx (x=0.73) Films
State Key Laboratory of Solid State Microstructures, School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, People's Republic of China
Recently, amorphous silicon oxide (a-SiOx) material has attracted much more attention because of its resistive switching characteristics which can be used in the next generation resistive random access memory (RRAM) devices. Until now, three kinds of models have been proposed to explain the resistive switching in a-SiOx materials. The first is the metal filament based on the electrochemical redox reaction of active metal electrode . The second is the forming and rupture of nc-Si related filament in a-SiOx matrix . While in our study, the silicon dangling bonds percolation path should be responsible for the conductive filament in a-SiOx films . In this work, we report the more experimental evidences for supporting our model and the role of biasing electric field in the resistive switching processes.
A 60 nm thick a-SiOx (x=0.73) film is prepared by plasma enhanced chemical vapor deposition (PECVD) method to form Pt/SiO0.73/Pt structure. It shows good resistive switching behavior after forming to low resistance state (LRS) at ~10.5 V and 10 mA current compliance (CC). It can be reset to high resistance state (HRS) at ~0.7 V and set back to LRS at ~1.7 V. Furthermore, to eliminate the possible contamination of metal Pt electrode, we fabricate the metal free structure. The SiO0.73 films are sandwiched between n++-Si sub and 200 nm thick P-doped a-Si, which is also deposited by PECVD method to achieve a resistivity of ~20 mΩcm. And it is observed that the a-Si(P-doped)/SiO0.73/n++-Si/Pt structure exhibits almost the same resistive switching behavior as that of Pt/SiO0.73/Pt structure. That is to say, the resistive switching behavior is an intrinsic characteristic of a-SiOx material.
In order to verify that the electric field induced breakage of Si-O related bonds is responsible for silicon dangling bonds percolation path model in a-SiOx films, the temperature dependence of forming voltage is investigated. We found that even if the temperature of the device decreases from room temperature to ~5.5K, the forming voltage shows no obvious changes and keeps at ~10.5 V, which indicates that the current induced hot carrier injection to our a-SiO0.73 films is insignificant during the forming process , where the forming voltage should increase greatly when the temperature decreases . While the dipole-electric field coupling effect is accountable for the formation of silicon dangling bonds percolation path. When the device is applied by high electric field during forming process, the activation energy of Si-O bond breakage will decrease because of the coupling effect between dipole moment of Si-O bonds and the electric field. As a result, a number of Si-O bonds in SiO0.73 films can be gradually broken, creating new silicon dangling bonds. When the concentration of dangling bonds reaches the threshold value the dangling bonds percolation path will be formed and the device is switched to LRS.
 R. Huang et al., Appl. Phys. A. 102, 927 (2011)
 J. Yao et al., Nano Letters. 10, 4105 (2010)
 Yuefei Wang et al., Appl. Phys. Lett. 102, 042103 (2013)
 J. W. Mcpherson et al., J. Appl. Phys. 88, 5351 (2000)
 Yanzhen Wang et al., Appl. Phys. Lett. 100, 083502 (2012)
Keywords: a-SiOx, resistive switching memory, Si dangling bonds percolation
Roles of Hydrogen in Amorphous In-Ga-Zn-O
1. Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
2. Materials Research Center for Element Strategy, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
Amorphous oxide semiconductor represented by a-In-Ga-Zn-O (a-IGZO)  is now used for thin-film transistors (TFT) in mobile devices, 32" 4K2K LCD, and 55" OLED TV . From the initial stage of research, it has been recognized that a-IGZO TFT has an enough performance for the next-generation displays such as large mobility and small subthreshold voltage swing. On the other hand, instability against various modes of ambient and stress tests have been serious issues for commercialization. Although many of them, such as ambient instability and bias/current instability in dark, have been improved remarkably and would now be satisfactory for the above commercial products, improvement in other issues including temperature instability and light-illumination instability are still primarily important issues to be improved.
We have reported that weakly-bonded oxygen  and unintentionally-incorporated hydrogen  are important origins of these instability issues. In this paper, we focus on roles of hydrogens and discuss the following findings.
1. Usual a-IGZO films contain high-density hydrogens at 1020–1021 cm–3.
2. Free electrons generated by the hydrogens are compensated by excess oxygen.
3. Hydrogen-less a-IGZO films are of a low density and have a higher-density electron traps; while incorporation of some hydrogen enhances structural relaxation and assists to produce a good semiconductive a-IGZO.
4. Such low-density film induces structural relaxation from a low temperature of 100°C.
5. Some hydrogens passivate electron traps.
6. Some (maybe other) hydrogens cause extra t1/4-law instability.
7. Some hydrogens enhance desorption of oxygen in a-IGZO.
8. The roles of these hydrogens would be classified by chemical bonding strength such as thermal desorption temperature of H2O.
These results indicate that hydrogens in a-IGZO have double-face features similar to the amorphous silicon case. That is, hydrogen assists to produce good and less-defective a-IGZO, and improve static TFT characteristics. While, it causes poorer instability against temperature-bias-current stress.
 K. Nomura, et al., Nature 432, 488 (2004)
 T. Kamiya and H. Hosono, Handbook of Zinc Oxide and Related Materials (Taylor & Francis) 2012
 K. Ide, et al, Appl. Phys. Lett. 99, 093507 (2011)
 Nomura, et al., ECS J. Sol. Stat. Sci. Technol. 2, 5 (2013)
Keywords: amorphous oxide semiconductor, thin-film transistor, impurity, defect, hydrogen, chemical bond