Resistive random access memory (ReRAM) devices based on HfO2, in which a conducting filament (CF) that acts as a circuit breaker/switch between the electrodes, are studied intensively because of its high compatibility with CMOS process beyond the 22 nm node [1]. The breaking and forming of a CF occurs over a nanometer-scale region in a timespan of nanoseconds [2]. In order to explore their potential applications, it is imperative to investigate the cryogenic performance of these devices [3]. In this work, the electrical characterization of Ni/HfO2/Si ReRAM devices is carried out. Unipolar resistive switching (RS) loops are reported in the whole temperature range studied (77 - 473 K). The MOS devices were fabricated on (100) n-type CZ silicon wafers with resistivity in the range (7-13) mµÙ·cm. The 20 nm-thick HfO2 layers were deposited by atomic layer deposition (ALD). The top Ni electrode was deposited by magnetron sputtering. A complete electrical characterization with focus on the oxide and oxide-semiconductor quality assessment was performed in a cryogenic system with liquid nitrogen. Fig. 1 shows the Poole-Frenkel fitting of I-V curves at several temperatures. Data fit well especially in a limited high E-field range [4, 5], and indicates that the main conduction is bulk limited, associated with the field enhanced thermal excitation of charge carriers from traps. The theoretical value of field-lowering coefficient, βPF, of HfO2 is 1.5∙10-5 eV m1/2V-1/2 [6]. The experimental values are reasonably in agreement with the theoretical one. In Fig. 2 current-temperature dependence for several bias voltages is shown. In the 150-300 K temperature range the relationship between ln(I) and 1/kBT is clearly lineal, and the slopes of the lines do not vary appreciably with the applied voltage, indicating that the conduction takes place through an activated process having a single activation energy, ΔEσ. The activation energy values obtained are similar to those previously reported for HfO2[7]. As the temperature decreases below 155 K the current becomes temperature independent. This can be attributed either to the presence of the competing emission mechanisms or to the device self-heating [8]. After the study of pristine samples, the MIS devices were electroformed by DC reverse bias sweeping from 0 to 13 V with a current compliance of 0.1 mA. Electroforming causes a current-limited oxide breakdown, and a metallic filament is created [9]. So, devices are taken to the low resistance state (LRS). A cycle is completed when the CF is broken by applying a reset voltage and samples switch to the high resistance state (HRS). I-V cycles were recorded using an HP4155B semiconductor parameter analyzer in the voltage sweep mode with current compliance of 0.1 mA for the HRS state and 100 mA for the LRS state (Fig. 3). A whole discussion of all the experimental results will be given at the conference. [1] S. M. Yu, X. M. Guan, and H. S. P. Wong, Appl. Phys. Lett. 99, 063507 (2011). [2] D. S. Jeong, R. Thomas, R. S. Katiyar, J. F. Scott, H. Kohlstedt, A. Petraru, and C. S. Hwang, Reports on Progress in Physics, 75, 076502 (2012). [3] R. Fang, W. Chen, L. Gao, W. Yu, and S. Yu, IEEE Electron Dev. Lett. 36, 567 (2015). [4] C. Walczyk, D. Walczyk, T. Schroeder, et al, IEEE Trans on Electron Dev. 58, 3124 (2011). [5] S. Dueñas, H. Castán, H. García, et al, J. Vac. Sci. Technol. B, 27, 389 (2009). [6] D. S. Jeong, H. B. Park, and C. S. Hwang, Appl. Phys. Lett. 86, 072903 (2005). [7] S. Dueñas, H. Castán, H. García, et al, Semicond. Sci. Technol. 22, 1344 (2007). [8] O. Mitrofanov, and M. Manfra, J. Appl. Phys. 95, 6414 (2004). [9] C. Nauenheim, C. Kuegeler, A. Ruediger, and R. Waser, Appl. Phys. Lett. 96, 122902 (2010). Figure 1

A complete electrical characterization of hydrogenated amorphous silicon layers (a-Si:H) deposited on crystalline silicon (c-Si) substrates by electron cyclotron resonance chemical vapor deposition (ECR-CVD) was carried out. These structures are of interest for photovoltaic applications. Different growth temperatures between 30 and 200 °C were used. A rapid thermal annealing in forming gas atmosphere at 200 °C during 10 min was applied after the metallization process. The evolution of interfacial state density with the deposition temperature indicates a better interface passivation at higher growth temperatures. However, in these cases, an important contribution of slow states is detected as well. Thus, using intermediate growth temperatures (100–150 °C) might be the best choice.

Ni/HfO2/Si ReRAM devices were extensively characterized. In the pristine state, they show adequate performance with low leakage currents and moderate interfacial state density. Leakage current is dominated by Poole-Frenkel mechanism. Activation energies of conduction processes and soft-optical phonons in the insulator bulk are 80 and 50 meV, respectively. Both are usual values in high-k dielectrics. Devices show unipolar resistive switching behavior, with two well-defined resistance states. They can switch properly at temperatures as low as 77 K. Transitions between both resistance states are electric field and temperature dependent.

Resistive switching conduction in Ni/HfO 2 /Si capacitors is studied at temperatures ranging from 77 to 473 K. A model for the low-resistance state (LRS) consistent with the experimental data is proposed. The LRS current-voltage (I-V) curves show a maximum resistance, R 0 , at zero bias and a minimum value, R ∞ , at voltages close to reset, which indicates a departure from linearity. A three-parameter model for the I-V curves is reported and its temperature dependence analyzed.

The effects of 2 MeV electron irradiation on the electrical properties of high-k dielectric based metal–insulator–semiconductor capacitors have been studied. Samples consist of 5.9 nm-thick films of aluminum oxide and hafnium oxide deposited by atomic layer deposition on silicon substrates. Deep-level transient spectroscopy (DLTS) and admittance measurements reveal that electron irradiation modifies the defect density of both surface states at the dielectric–semiconductor interface and border traps existing inside the dielectric. The experimental results indicate that irradiation has a double effect. The incident electrons transfer their energy and generate additional surface states, leading to a degradation of the interface. On the other hand, irradiation generates electron–hole pairs inside the dielectric. Some of the holes are trapped by border traps located inside the dielectric at locations close to the interface. As a result, border traps capturing holes are neutralized and become inactive after irradiation. Moreover, interface state profiles, as measured by DLTS, are affected by the presence of border traps and yield overestimated interface state densities. Admittance spectroscopy is used to distinguish among border traps and interface traps. A detailed study of the conductance signal as a function of voltage, temperature and frequency for samples with different irradiation doses (nonirradiated, 2.5, 25, and 250 Mrad) is presented here. The influence of the irradiation dose has been analyzed in order to compare the defect distribution before and after irradiation.

Dysprosium-doped zirconium oxide thin films grown by atomic layer deposition (ALD) were studied in order to assess its suitability as dielectric in metal-insulator-metal (MIM) electronic devices. The films show high stability and linearity. The film quality clearly improves after annealing at 700 degrees C in O-2 during 30 min. All films crystallize in as-deposited state and contained cubic and/or tetragonal ZrO2 phases. Current and charge measurements show hysteresis when varying the applied voltage. Structures with the highest Dy content in the dielectric showed the widest hysteresis cycles. Scanning electron microscopy reveals that the crystallite grain size increases with Dy content. A correlation between crystal grain size and I-V, and Q-V hysteresis exist, thus indicating that a charging process at the grain boundaries takes place. (C) 2015 Elsevier B.V. All rights reserved.

Holmium titanium oxide (HoTiOx) thin films of variable chemical composition grown by atomic layer deposition are studied in order to assess their suitability as dielectric materials in metal–insulator–metal electronic devices. The correlation between thermal and electrical stabilities as well as the potential usefulness of HoTiOx as a resistive switching oxide are also explored. It is shown that the layer thickness and the relative holmium content play important roles in the switching behavior of the devices. Cycled current–voltage measurements showed that the resistive switching is bipolar with a resistance window of up to five orders of magnitude. In addition, it is demonstrated that the post-breakdown current–voltage characteristics in HoTiOx are well described by a power-law model in a wide voltage and current range which extends from the soft to the hard breakdown regimes.

In the attempt to form an intermediate band in the bandgap of silicon substrates to give it the capability to absorb infrared radiation, we studied the deep levels in supersaturated silicon with titanium. The technique used to characterize the energy levels was the thermal admittance spectroscopy. Our experimental results showed that in samples with titanium concentration just under Mott limit there was a relationship among the activation energy value and the capture cross section value. This relationship obeys to the well known Meyer-Neldel rule, which typically appears in processes involving multiple excitations, like carrier capture/emission in deep levels, and it is generally observed in disordered systems. The obtained characteristic Meyer-Neldel parameters were Tmn = 176 K and kTmn = 15 meV. The energy value could be associated to the typical energy of the phonons in the substrate. The almost perfect adjust of all experimental data to the same straight line provides further evidence of the validity of the Meyer Neldel rule, and may contribute to obtain a deeper insight on the ultimate meaning of this phenomenon.

MIS capacitors based on Dy2O3-doped ZrO2 oxide dielectrics were studied. The oxide films were grown by ALD. Defect concentrations at the oxide/semiconductor interface and inside the oxide depended on the film annealing that reduced the interface quality and increased defect densities inside the oxide. The leakage current density decreased at moderate voltages when the amount of dysprosium in the films increased.

Ho2O3-TiO2 based metal-insulator-metal capacitors were grown by ALD, using Ho(thd)3, Ti(OCH(CH3)2)4 and ozone as precursors. The thicknesses of the films were in the range of 7.7 to 25 nm. Some of the films were post-deposited annealed in order to study the treatment effects. The capacitors were electrically characterized. Leakage current decreases as the amount of holmium increased in the films. Resistive switching behavior was obtained in the samples where the leakage current was low. This effect was also observed in Ho2O3 films, where no titanium was present in the films.

Defects on mono-like and polycrystalline silicon solar cells are studied in depth. These defects are in the basis of the higher quantum efficiency of mono-like solar cells (~18%) with respect to polycrystalline ones (~16%). Using the thermal admittance spectroscopy technique we found that both of them have a deep level due to a Fe-B complex. Furthermore, the deep level in the first one (224meV) is shallower than in the second one (345meV). Shallower deep levels degrade less the efficiency on solar cells, so this characteristic of the deep level in the mono-like solar cells leads to a better results in efficiency.

The electrical properties of HfO2-based metal–insulator–semiconductor capacitors have been systematically investigated by means of I–V and C–V characteristics, admittance spectroscopy, deep level transient spectroscopy, conductance transient, and flat band voltage transient techniques. Attention is also given to the study of the temperature dependence of the leakage current. HfO2 films were grown on p-type silicon substrates by atomic layer deposition using hafnium tetrakis(dimethylamide) as hafnium precursor, and ozone or water as oxygen precursors. The growth temperature ranged from 150 to 350 °C. Low growth temperatures prevent decomposition and high growth rate, as well as high contamination levels. As a result, the leakage current is lower for lower deposition temperatures. Some of the deposited samples were submitted to a postdeposition annealing at 650 °C in N2 atmosphere, showing a decrease in the leakage current and an increase in the equivalent oxide thickness (EOT), whereas interfacial state density increases and defect density inside the dielectric bulk decreases. Regarding dielectric reliability, in our experimental conditions, HfO2 layers grown at 150 °C exhibit the largest EOT and breakdown voltage. The electrical behaviour is clearly linked with structural properties, and especially with the formation of an interfacial layer between the HfO2 layer and the silicon substrate, as well as with the presence of several impurities.

2MeV electron irradiation effects on the electrical properties of Al2O3 and HfO2-based metal–insulator–semiconductor capacitors have been studied. High-k dielectrics were directly grown on silicon by atomic layer deposition. Capacitors were exposed to three different electron irradiation doses of 0.025, 0.25 and 2.5MGy. Capacitance–voltage, deep-level transient spectroscopy, conductance transients, flat-band voltage transients and current–voltage techniques were used to characterize the defects induced or activated by irradiation on the dielectric bulk and on the interface with silicon substrate. In all cases, positive charge is trapped in the dielectric bulk after irradiation indicating the existence of hole traps in the dielectric. When the samples are exposed to 2MeV electron beam (e-beam) irradiation, electron–hole pairs are created and holes are then captured by the hole traps. Insulator/semiconductor interface quality slightly improves for low irradiation doses, but it is degraded for high doses. Irradiation always degrades the dielectric layers in terms of gate leakage current: the trapped holes are mobile charge which can contribute to leakage current by hopping from trap to trap.

The electrical properties of ScO-based MIS structures have been electrically studied. The high-k films were deposited by high pressure sputtering (HPS). Aluminum and Ti were used as gate electrodes. Defects inside the oxide seem to be reduced when increasing the chamber pressure. However, leakage current density increases in this case.

Metal-insulator-metal (MIM) capacitors were grown by atomic layer deposition using (BuN)-Bu-t = Nb(NEt2)(3) and ozone as niobium and oxygen precursors, respectively. Three different deposition temperatures were used and some of the films were postdeposition annealed. The permittivity values obtained reached a value of about 50 for the films crystallized after annealing at temperatures higher than 500 degrees C. However, the leakage current values for the crystalline films were higher than those in the case of amorphous films.

The performance of commercial solar cells is strongly controlled by the impurities and defects present in the substrates. Defects induce deep energy levels in the semiconductor bandgap, which degrade the carrier lifetime and quantum efficiency of solar cells. A comprehensive knowledge of the properties of defects require electrical characterization techniques providing information about the defect concentration, spatial distribution and physical origin. The experimental techniques available in our laboratory are described in this work. In contrast, the efficiency of single junction solar cells can be drastically improved by the formation of an intermediate band in the midgap of a semiconductor. The intermediate band can be created from deep level defects if their concentration is high enough. Experimental results proving the intermediate band formation are also presented in this work.

Intermediate band formation on silicon layers for solar cell applications was achieved by titanium implantation and laser annealing. A two-layer heterogeneous system, formed by the implanted layer and by the un-implanted substrate, was formed. In this work, we present for the first time electrical characterization results which show that recombination is suppressed when the Ti concentration is high enough to overcome the Mott limit, in agreement with the intermediate band theory. Clear differences have been observed between samples implanted with doses under or over the Mott limit. Samples implanted under the Mott limit have capacitance values much lower than the un-implanted ones as corresponds to a highly doped semiconductor Schottky junction. However, when the Mott limit is surpassed, the samples have much higher capacitance, revealing that the intermediate band is formed. The capacitance increasing is due to the big amount of charge trapped at the intermediate band, even at low temperatures. Ti deep levels have been measured by admittance spectroscopy. These deep levels are located at energies which vary from 0.20 to 0.28 eV below the conduction band for implantation doses in the range 10(13)-10(14) at./cm(2). For doses over the Mott limit, the implanted atoms become nonrecombinant. Capacitance voltage transient technique measurements prove that the fabricated devices consist of two-layers, in which the implanted layer and the substrate behave as an n(+)/n junction.

In order to find the regions in solar cells where the efficiency drops an experimental setup is tuned up. Through this equipment a set of samples are characterized checking that its response is the expected. The photocurrent maps obtained allow us to determine the regions with higher defects concentration. These regions will be characterized using electrical techniques which will give us additional information of the nature of these defects.

In this work, the electrical characterization of Gd2O3 and Sc2O3-based metal–insulator–silicon (MIS) structures has been performed using capacitance–voltage, deep level transient spectroscopy, conductance transients, flat-band voltage transients, and current–voltage techniques. High-k films were deposited by high pressure sputtering using Sc and Gd metallic films in a pure Ar plasma and, subsequently, in situ room temperature plasma oxidation in a mixed Ar/O2 atmosphere was performed. Three different metals were used as gate electrodes: aluminium, platinum, and titanium, in order to check electrical differences of the samples and to check the interface scavenging after high-k dielectric deposition. In particular, it was proved that Ti electrode is a well SiO2 interlayer scavenger for both materials. Additionally, the authors observed that the predominant conduction mechanism for these high-k based-MIS structures is Poole–Frenkel emission, as usually reported for high-k dielectrics.

Intermediate band silicon solar cells have been fabricated by Titanium ion implantation and laser annealing. A two-layer heterogeneous system, formed by the implanted layer and by the unimplanted substrate is obtained. In this work we present electrical characterization results which evidence the formation of the intermediate band on silicon when ion implantation dose is beyond the Mott limit. Clear differences have been observed between samples implanted with doses under or over the Mott limit. Samples implanted under the Mott limit have capacitance values much lower than the non-implanted ones as corresponds to a highly doped semiconductor Schottky junction. However, when the Mott limit is surpassed the samples have much higher capacitance, revealing that the intermediate band is formed. The capacitance increase is due to the big amount of charge trapped at the intermediate band, even at low temperatures. Titanium deep levels have been measured by Admittance Spectroscopy. These deep levels are located at energies which vary from 0.20 to 0.28 eV bellow the conduction band for implantation doses in the range 10^13-10^14 at/cm^2. For doses over the Mott limit the implanted atoms become non recombinant. Admittance measurements are the first experimental demonstration the Intermediate Band is formation. Capacitance voltage transient technique measurements prove that the fabricated devices consist of two-layers, in which the implanted layer and the substrate behave as an n^+/n junction.