Metal Oxides for Optoelectronic and Resistive Switching Applications

Abstract

This Thesis has been focused on the fabrication and characterization of different CMOS- compatible materials in order to determine both their electro-optical and resistive switching properties. Basically, two materials have been explored, silicon-aluminum oxynitride (SiAlON) and zinc oxide (ZnO). The first material under study, SiAlON, has been fabricated using three techniques, namely RF-sputtering, pulsed-laser deposition and electron beam evaporation. In this case, different stoichiometries were analyzed in order to obtain excellent optical and electrical properties. The incorporation of different rare earths (REs) was also carried out, using Ce and Eu, which exhibited photoluminescence (PL) emission under laser excitation. The electro-optical characterization was performed after fabricating device structures onto p-type silicon substrates. The employed top electrode was selected depending on the characterization technique. To collect the electroluminescence (EL) from the devices a transparent conductive oxide (TCO) was required, using indium tin oxide (ITO) because of its excellent electrical and optical properties. Light emission was obtained from both devices, containing Ce and Eu, suggesting that SiAlON is a great candidate to be employed as RE host matrix. In addition, the resistive switching properties of these devices were analyzed as well, using Al as top electrical contact.

Similar fabrication processes were carried out towards attaining rare earth (RE)-doped SiAlON. This was achieved by depositing a multilayered structure of Tb-Al/SiO2, which allowed determining the RE ions inclusion effectivity of the delta-doping approach. The optical characterization demonstrated PL emission from trivalent Tb3+ ions. Different (Al/Tb/SiO2) multilayer configurations were tested to optimize the number of active luminescent centers. Finally, the resistive switching properties of RE-doped SiO2 were also analyzed and the role of the RE ions within was explored as well.

The second studied material was ZnO. In this case, the material was deposited onto p- type silicon via either RF-sputtering or atomic layer deposition (ALD) depending on the role of the deposited layer. While the first one was used to deposit the ZnO as RE host matrix, the second one was employed to attain a ZnO layer acting as TCO top electrode. In the first case, different REs (Tb and Eu) were tested. A clear PL emission of both REs was obtained. The narrow peak-like features in the spectra indicate the optical activation of the trivalent RE ions, being the ZnO an optimum host matrix for this kind of luminescent centers. To carry out the electrical characterization, device structures were attained using ITO as top TCO electrode. The EL from these devices was obtained, achieving similar spectra than the ones observed via PL. However, the luminescent degradation with time suggests the formation of conductive paths which effectively quench the EL emission. Taking into account this behavior, the resistive switching properties of these devices were analyzed, obtaining different cycles. The role of the REs in the resistive switching properties of ZnO was studied as well, allowing for a reduction of the current compliance in the electroforming process, but increasing the required voltages to induce the resistive switching phenomenon. Moreover, the incorporation of the REs into the ZnO host matrix permitted obtaining more stable Reset processes, which suggests that the REs near the conductive paths could trap part of the out-diffused oxygen ions and, consequently, the re-oxidation of these conductive paths becomes easier.

Finally, when using a ZnO layer as top electrical contact, a multilayered SiOx/SiO2 structure was employed. After deposition, this structure was annealed at high temperature in order to induce the precipitation and crystallization of the silicon excess in the form of silicon nanocrystals (Si-NCs). The optical and electrical properties of these nanostructures are well known and have reported in previous works and doctoral theses of the research group. Therefore, the incorporation of the ZnO as TCO was implemented to determine the EL of the Si-NCs when current is injected under different electrical polarizations. Studies in DC and AC have been carried out, obtaining interesting results related to the modulation of the light emission from ZnO defects and enhancing the EL emission from the Si-NCs. The incorporation of a thin Si3N4 inversion layer, between the Si substrate and the multilayered Si-NCs, allowed modifying the injected current, thus obtaining an enhancement of the EL emission. These measurements confirmed the good electrical and optical properties of the ZnO working as TCO and permitted to understand the physical mechanisms involved in the EL process of the luminescent centers. In addition, the resistive switching properties of these devices were determined. In this case, devices presented some cycles with well defined resistive states. Under these resistive switching conditions, devices exhibit EL emission, being the intensity and the threshold voltage dependent on the resistive state.

In conclusion, the results presented in this Thesis accomplish the initial pursued objectives, demonstrating the correlation between the EL emission and the resistive switching properties. Using these characteristics, the resistive state can be read not only electrically, but also optically from the emission of the luminescent centers through the TCO top electrode contact. Overall, these results pave the way to a new set of memory devices that can be, in a near future, integrated into the Photonics field, dominated by faster interconnections and less dependence to material transmission media.

Aquesta Tesi es basa en la fabricació i caracterització de diferents materials compatibles amb al tecnologia CMOS, per tal de poder determinar les seves propietats electró-òptiques i de commutació resistiva. Bàsicament, s’han estudiat dos materials, oxinitrur de silici i alumini (SiAlON) i òxid de zinc (ZnO). En el cas del SiAlON, s’han emprat diferents tècniques de fabricació i s’han tingut en compte diferents estequiometries per tal d’optimitzar-ne les propietats òptiques i elèctriques. Tanmateix, s’ha dut a terme la incorporació de terres rares (Ce i Eu) en aquestes matrius, les quals han presentat emissió de fotoluminescència (PL) sota excitació làser. Les propietats electro-òptiques de dit material s’han pogut estudiar a partir de dispositius bàsics obtinguts després de dipositar òxid d’indi i estany (ITO) com a elèctrode transparent, els quals han mostrat emissió d’electroluminescència (EL) de les terres rares. Dits dispositius també han presentat propietats de commutació resistiva (emprant Al com a contacte superior).

L’ZnO es va utilitzar com a matriu contenidora de terres rares (Tb i Eu), les quals han demostrat ésser òpticament actives tot presentant emissió de PL i d’EL. No obstant, dita luminescència mostra una degradació, possiblement deguda a la formació de camins conductius. Aquest fet va promoure l’estudi de les propietats de commutació resistiva, obtenint resultats interessants depenent de l’existència o no de la terra rara. Per altra banda, l’ZnO es va emprar com a contacte transparent sobre una estructura de multicapes de nanocristalls de Si (Si-NCs). Mitjançant estudis elèctrics en AC, es va poder modular l’emissió d’EL de dit sistema. A més, la incorporació d’una capa fina de nitrur de silici a la interfície entre el substrat de Si i els Si-NCs va permetre també millorar l’eficiència d’emissió EL d’aquestes nanoestructures. Finalment, el sistema ZnO/Si-NCs ha mostrat propietats de commutació resistiva i emissió EL de forma simultània, aquesta última depenent de l’estat resistiu del dispositiu.

En resum, la relació entre les propietats electró-òptiques i les de commutació resistiva en els materials estudiats permet obrir les portes a nous dispositius amb dos estats resistius que puguin ser llegits tant elèctricament com òpticament.