Download Estudios estructurales y morfológicos de mezclas basadas en ZnO

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