Interface Formation and Electrical Transport in SnO2:Eu3+/GaAs Heterojunction Deposited by Sol–Gel Dip-Coating and Resistive Evaporation

Abstract

The natural n-type conduction of tin dioxide (SnO 2 ) may be compensated by trivalent rare-earth doping. In this work, SnO 2 thin films doped with Eu 3+ have been deposited by the sol-gel dip-coating (SGDC) process, topped by a GaAs layer deposited by the resistive evaporation technique. The goal is the combination of a very efficient rare-earth emitting matrix with a high-mobility semiconductor. The x-ray diffraction pattern of SnO 2 :Eu/GaAs heterojunctions showed simultaneously the crystallographic plane characteristics of GaAs as well as cassiterite SnO 2 structure. The electric resistance of the heterojunction device is much lower than the resistance of the SnO 2 :2 at.%Eu and GaAs films considered separately. Micrographs obtained by scanning electron microscopy (SEM) of the cross-section showed that the interface is clearly identified, exhibiting good adherence and uniformity. A possible explanation for the low resistivity of the SnO 2 :2 at.%Eu/GaAs heterojunction is the formation of small channels with two-dimensional electron gas (2DEG) behavior.

Photo-Induced conductivity of heterojunction GaAs/Rare-Earth doped SnO2

“…Although the formation of a two dimensional electron gas (2DEG) is expected in heterojunctions built from single crystals, in the case of the heterojunction SnO 2 :2%Eu/GaAs obtained in this work, the higher conductivity allows thinking of the possible formation of small 2DEG channels, giving better mobility to the conducting electrons, explaining the higher conductivity of the heterojunction compared to the individuals films. Published SEM of the cross section 27 , shows that the interface substrate/SnO 2 as well as the 27 , we may state that the larger particles are grains constituted by smaller crystallite units. Figure 2 shows optical absorption data of GaAs, SnO 2 and SnO 2 :2%Eu/GaAs thin films, and the inset in Figure 2 show clearly that for the thicker GaAs layer the absorption becomes stronger and, besides, the optical absorption of the SnO 2 :2%Eu/GaAs heterojunction is an evident contribution from both layers, even though the GaAs contribution must be stronger because it has lower bandgap energy, and shall absorb the incident light before.…”

“…The formation of the two-dimensional electron gas is expected in single crystals samples, grown by MBE for instance, but in our case, the deposition methods themselves should lead to an uneven surface, with a high density of interfacial defects. However, the quality of the interface in SnO 2 :2.0 at%Eu/GaAs is evident 27 . The same sort of images for SnO 2 :0.5 at%Eu/GaAs sample leads to similar results, with an estimated thickness of about 350 nm and 250 nm, for the film of SnO 2 :0.5 at%Eu and GaAs, respectively, followed by a disorganized layer, due to the used GaAs mass for evaporation, which was three times higher in this case.…”

“…SEM of cross section and surface of SnO 2 :2.0 at%Eu/GaAs sample has been published elsewhere 27 . The interface film/substrate as well as the interface SnO 2 /GaAs present good adherence.…”

“…Thus, there are two possible explanations for the conductivity increase observed in Figure 3: 1) electrons are being excited from the GaAs to the 2DEG. With increasing temperature, these electrons occupy higher levels in the potential well, until they have enough energy to become delocalized at the GaAs conduction band, and recombine with the holes in the GaAs side 27 . Another possibility is that the capture of photoexcited electrons could evolve into some sort of lattice relaxation, with electron trapping becoming a thermally activated process, where the electrons will overcome the capture barrier 31 .…”

Influence of pH of colloidal suspension on the electrical conductivity of SnO2 thin films deposited via Sol-Gel-Dip-Coating

“…When the total of 10 layers is reached, samples are submitted to thermal annealing at 550 °C for 1 hour in the same furnace. Resulting average thickness of fi lms obtained by this procedure is about 350 nm, as evaluated from scanning electron microscopy of cross section 18,19 . X-ray diffraction data of fi lms were obtained with a RIGAKU diffractometer, model D/MAX-2100/PC, with a scanning rate of 1°/minutes in range of 20-80 degrees.…”