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2010 Estudios estructurales y morfológicos de mezclas basadas en ZnO Structural and morphological studies of compounds based on ZnO Diana Rueda1, Mauricio Barrios1, Julio Mass1, Tomás Rada1, David Landinez2 1Universidad del Norte: Grupo de Física de la Materia Condensada, A.A.1569, Barranquilla Universidad Nacional de Colombia: Grupo de Física de Nuevos Materiales, A.A.146001, Bogotá 2 Recibido XXXX; Aceptado XXXX; Publicado en línea XXXX Resumen En el siguiente trabajo han sido estudiadas las propiedades estructurales y morfológicas del óxido de zinc (ZnO) y se observa la influencia, en sus propiedades, de la adición del óxido de magnesio (MgO). Se presentan los resultados referentes a la morfología de los compuestos, mediante imágenes obtenidas con el microscopio electrónico de barrido (SEM) y su análisis. En éstas imágenes se detallan algunas características morfológicas como la forma y tamaño de los granos del ZnO y MgO en las combinaciones molares 1:1, 2:1y 4:1. De las imágenes de ZnO se distinguen un carácter granular casi esférico, mientras que las imágenes SEM del MgO se muestran un comportamiento laminar, sin embargo las imágenes de los polvos combinados no muestra el carácter laminar del MgO, lo que hace suponer inicialmente la ocurrencia de una interacción entre esos dos compuestos. También se realiza la caracterización estructural de las mezclas mediante difracción de rayos X (XRD) y se comparan los efectos de las combinaciones de dichos compuestos debido a la preparación de éstas mediante la ruta de polvos secos. Palabras claves: ZnO/MgO, XRD, oxido de zinc Surface Photovoltage technique 1. Introduction The surface photovoltage (SPV) is a well established non-contact technique for the characterization of semiconductors based on analyzing the changes induced in the surface voltage under illumination. It has been used, but not extensively, since the middle of last century for studies of surface and bulk of many semiconductors and semiconductors interfaces. However, in the 90´s appeared with renewed developments in SPV related techniques, such as Kelvin Probe Force Microscopy (KPFM), Kronik and Shapira [i] have an excellent report on this technique. 2. Basic concepts The photovoltaic effect consists of an induced change in the potential distribution in a given structure under illumination and is typically the consequence of some charge transfer and/or redistribution within the device [i, ii]. The surface photovoltage (SPV), which is a variant of photovoltaic effect, method is a non-destructive and contactless characterization technique that has been successfully used to study the electronic properties of semiconductor materials [13, iii, iv] The ideal crystalline semiconductor has a periodic structure and ending in a free surface that may form surface-localized electronic states within semiconductor energy bandgap and /or double layer of charge, known as a surface dipole [i]. The appearance of surface-localized states induces charge transfer between bulk and surface in order to establish thermodynamic equilibrium between the two. The charge transfer results in a non-neutral region (with a non-zero electric field) at the semiconductor surface, usually referred to as surface space charge region (SCR), along with the electron or hole depletions regions and forming a surface band bending upward (downward) for n-type (p-type) semiconductor (See Figure 1). In the SCR a potential drop is observed that is manifested by the bending of the semiconductor bands, where electrons are repelled from the surface and holes are attracted to it, due to the trapped surface electrons. In this situation, when the energy band edge is lower the electrical potential is higher, so that a negative (positive) Vs corresponds to upward bent (downward-bent) bands [ii]. Thus, even under equilibrium conditions the surface potential Vs is different from the bulk. The thickness of the SCR is usually the order of 1-103 nm, depending on the carrier density and dielectric constant of the semiconductor. Surface space charge region a) b) VSO VS EC EC Eg EF Eg E VS ΔVS VSO ΔVS hv ≥ Eg EF EV hv ≥ Eg EV Fig 1. Scheme of band bending for semiconductor: a) n–type, and b) p–type. The SPV mechanism in a semiconductor depends strongly on photon incident that can induce the formation of free charge carriers by creating electron-hole pairs, via band-to-band transitions in the vicinity of the surface (super-bandgap transitions), and/ or by realising captured charge carriers at the surface states, via trap-to-band transitions (sub-bandgap transitios) [iii]. In the former case, a significant amount of charge may transfer in opposite directions under the built–in electric field (SCR), i.e. the electric field in the SCR causes excess electrons to diffuse from the surface to the bulk and excess holes diffuse from the bulk to the surface. This charges the surface and therefore modifies the surface potential, decreasing the bending of the surface band. The latter mechanism involves either electrons or holes trapping at surface defects with changes in the net surface charge. In other words, the surface potential barrier, Vs, changes. Thus the difference (Vs) between the surface potential under illumination (Vs) and in the dark (Vso), which is expressed as: Vs= Vs-Vso, is defined as the SPV signal. The process mentioned above are the most common mechanism, however there are present sometimes other effects, which we do not consider here, such as Franz-Keldish effect [v] CPD (mV) 0,0 ZnO -0,1 -0,2 Dark Relaxation Dark Illumination vacuum Relaxation -0,3 0 500 Time (s) 1 2 3 4 0 Photon Energy(eV) 500 1000 Time (s) Reference ++++ - - - Dark Under Illumination Figure 2 scheme of a surface in the dark (left) and under illumination (right), note that light induced charge of the work function which depends on the charge of the surface dipole. The corresponding spectrum of ZnO under dark and illumination can be observed. In a simple manner, the SPV measured correspond to a voltage of a parallel plate capacitor arrangement (for illustration see Figure 2), which is SPV Q (2a) d o It should be notice that all variables are functions of time and coordinate, however to show the sensitivity of this technique, we should write eq. (2a) in more details: SPV Q( x, t , , R, L, D , , n, N s ( E ),...) (2b) d ( x, t , , R, L, D , , n, N s ( E ),...) o ( x, t ,...) where x represents the position in one dimension, t is time, is the wavelength, R is the electron (hole) recombination rate per unit volume, L is the diffusion length, D is the electron (hole) diffusion coefficient, n is the intrinsic electron (hole) carrier density, N is the surface state density, is the minority carrier life time (prior to recombination). In resume, this technique can be applied in different ambience, with a large experimental variability, and can explore the dependence on any slow or fast process leading to charge separation. One of the elementary applications of SPV is the determination of the semiconductor type, which is based on the large increase in the absorption coefficient near the bandgap energy Eg. This increase means an important change of the SPV signal that correspond a sharp change in slope of the SPV curve. [i] It should be notice that SPV = – CPD