Work Function Measurements in Single-Crystalline In2O3 for Conduction Modelling

U. WEIMAR; N. BARSAN; F. SCHIPANI; A. PAPADOGIANNI; A. OPREA; O. BIERWAGEN

Ferrara

Workshop; The 8th GOSPEL Workshop. Gas Sensors Based on Semiconducting Metal Oxides: Basic Understanding & Application Fields; 2019

University of Ferrara

To date, there are only a few studies on the gas sensing properties of single crystalline sensors.The preferred study of polycrystalline materials is mainly due to the considerably larger sensorsignals, which are caused by the presence of grain boundaries [1,2]. However, the high quality andcontrolled growth of single-crystalline materials has the promise to help the fundamentalunderstanding of sensing: the electrical measurements of crystals under in- operando temperaturesand different gas atmospheres are an avenue for directly extracting fundamental electronic behaviourof each material that is essential for building an accurate model of sensor behaviour. Due to the recentadvance in the development of in-operando investigation methods, it seems now possible to combinethem with controlled single crystalline model sample.Indium oxide is a wide-bandgap semiconducting material with a direct bandgap of around2.8?2.9 eV. It has been extensively used as a transparent conductive oxide (TCO) in electronics, forphotovoltaic devices, light emitting diodes and chemical sensors [3,4]. Nevertheless, the knowledgeabout sensing with In2O3 based devices is still insufficient.Here, we present results of investigations performed on an approximately 440 nm thickcrystalline In2O3 film grown by plasma-assisted molecular beam epitaxy (PA-MBE) on a YSZsubstrate. Combined DC resistance and work function change measurements performed at anoperation temperature of 300 °C in various atmospheres were used in order to obtain informationabout the conduction mechanisms and electronic properties of the material in the same manner thatwas previously employed for the study of polycrystalline samples [5].The work function and resistance changes are measured with the Kelvin Probe technique, whichis a non-contact, non-destructive method that uses a vibrating reference electrode and measures thechanges of the contact potential difference (CPD) between the sample and the electrode. Variationsin the CPD induced by changes in the gas atmosphere represent relative work function variations ofthe sample [2]. The work function in a semiconductor can be expressed byφ = χ + (EC − EF) + VS, (1)where 𝑉𝑠 is the surface band bending, χ represents the electron affinity, which we assume constantwhen no humidity is present, and (EC − EF) is the difference between the conduction band in the bulkand the Fermi level. In Figure 1 the dependence of the sample conductance on work function changes is presented.Experimental results will be interpreted using two approaches. First, all changes will be attributed to𝑉𝑠 (surface processes), meaning that no bulk electron concentration changes are allowed in the model.Next, all changes will be attributed to bulk-related processes, where only the bulk electronconcentration changes (EC − EF) and no band bending at the surface is present.The total conductance of a compact layer is the sum of the part of the layer influenced by surfaceprocesses and the conductance of the layer that is left unchanged, the bulk.Gtotal = Gs + Gb = e WLμ[Z0ns + (D − Z0)nb] (2)In Equation (2), L is the length of the layer, W its width, D its thickness and 𝑍0 the thickness ofthe surface layer and 𝜇 the mobility.Room temperature experiments indicate that a large downwards band bending is present,which is causing the appearance of a surface electron accumulation layer (SEAL) [6] that cannot bedescribed using Boltzmann statistic. For that reason, a numerical conduction model using Fermi-Dirac statistics, which are valid for all electron densities, has been developed. This was solved withan iterative process to find equilibrium concentration of electrons in the bulk (nb) and surfaceproperties such as band bending and surface density of electrons (ns). The fitting parameters(fitting not shown here) would imply a bulk electron concentration of nb = 5.6 × 1019 m−3, which wouldbe very low and not realistic.On the other hand, if we apply a flat band approach, where all changes in conductivity and workfunction are due to variations of (EC − EF), the results also cannot be fully explained. From theexperimentally measured conductance in pure nitrogen and pure synthetic air (green and blue pointsin Figure 1), the bulk electron concentration found is nb = 4.7 × 1023 m−3 and nb = 4.7 × 1021 m−3respectively. From this, the difference between the Fermi Level and the conduction band can beestimated, using the effective density of states NC = 5.9 × 1024 m−3 and Equation (3):( ) ( B ) ln[ C ]C FbE E k T Ne n− = (3)Here, this difference is (EC − EF) = 0.125 eV in pure nitrogen and (EC − EF) = 0.350 eV in syntheticair. These results indicate that the experimental changes in conductance (a factor 100 fromnitrogen to synthetic air) and work function differences (approximately 0.5 eV) are neither purelydue to bulk changes nor purely surface dominated and implies that the atmosphere changes affectboth bulk and surface electron concentration.