Optical properties of ZnMgO films grown by spray pyrolysis and their application to UV photodetection

Abstract

This work presents a comprehensive optical characterization of Zn 1 _ x Mg x O thin films grown by spray pyrolysis (SP). Absorption measurements show the high potential of this technique to tune the bandgap from 3.30 to 4.11 eV by changing the Mg acetate content in the precursor solution, leading to a change of the Mg-content ranging from 0 up to 35%, as measured by transmission electron microscopy-energy dispersive x-ray spectroscopy. The optical emission of the films obtained by cathodoluminescence and photoluminescence spectroscopy shows a blue shift of the peak position from 3.26 to 3.89 eV with increasing Mg incorporation, with a clear excitonic contribution even at high Mg contents. The linewidth broadening of the absorption and emission spectra as well as the magnitude of the observed Stokes shift are found to significantly increase with the Mg content. This is shown to be related to both potential fluctuations induced by pure statistical alloy disorder and the presence of a tail of band states, the latter dominating for medium Mg contents. Finally, metal-semiconductor-metal photodiodes were fabricated showing a high sensitivity and a blue shift in the cut-off energy from 3.32 to 4.02 eV, i.e., down to 308 nm. The photodiodes present large UV/dark contrast ratios (10 -10 7 ), indicating the viability of SP as a growth technique to fabricate low cost (Zn, Mg)0-based UV photodetectors reaching short wavelengths.

ZnMgO-based UV photodiodes: a comparison of films grown by spray pyrolysis and MBE

“…The large absorption tail observed below bandgap correlates quite well with the very large Stokes shift in these films, which increases with the Mg content up to 275 meV for 35 % Mg [8]. However, as it was mentioned above, the extremely large illumination/dark current ratios above bandgap can be quite attractive for applications that require very high sensitivity but limited selectivity and response time, for a fraction of the price that is required to make an MBE-based UV photodiode.…”

“…The samples were exposed to a H 2 O 2 pre-treatment to passivate the surface, which has been previously shown to lead to very high quality Schottky contacts on (Zn,Mg)O/ZnO PDs [7]. In the SP-grown material, metal-semiconductor-metal (MSM) contacts were realized with 800 Å-thick Au layers [8]. In this case, no H 2 O 2 pre-treatment was used because of the high porosity of the films.…”

“…First, increasing the Mg content typically yields a lower carrier concentration in the ZnMgO film, producing more resistive layers [10]. Second, and most important in this case, the grain size decreases with increasing Mg content, yielding larger grain boundaries respect to the volume of the crystal [8]. This effect also directly affects the total carrier concentration and mobility of the films, such that films with small grains show much larger resistivities due to carrier trapping/scattering at the boundaries.…”

Phase segregation in MgxZn1–xO probed by optical absorption and photoluminescence at high pressure

“…As commented before, previous optical absorption spectroscopy studies [15,17] tration a low-energy absorption tail appears overlapping to the main absorption edge of the sample. In Fig.…”

“…as explained in the work of López-Ponce et al [17], and on c-oriented ScAlMgO 4 substrates in the case of x = 0.3 for studies under high pressure. The samples were characterized by scanning electron microscopy (SEM) to see their morphology and by XRD.…”