contact metals, diodes with rectifi cation suffi cient for material characterization were reported. [ 33 ] Nevertheless, rectifi cation has to be improved considerably to exploit In 2 O 3 within detectors or transistors. The realization of pn-heterodiodes is an alternative approach toward rectifying devices and was successful for the case of ZnO as n-type layer as mentioned above. As recently demonstrated, ZnCo 2 O 4 and NiO are especially interesting as p-type layer since they can be deposited at room temperature on the n-type oxide and offer very high rectifi cation up to ten orders of magnitude. [ 9 ] We report on the electric properties of p-ZnCo 2 O 4 /n-In 2 O 3 (ZCO/InO) and p-NiO/n-In 2 O 3 (NiO/InO) heterodiodes investigated at room temperature (RT) by current-voltage (IV), capacitance-voltage (CV) measurements and admittance spectroscopy. Additionally thermal admittance spectroscopy (TAS) was performed on a selected diode. Results and DiscussionA wide-angle X-ray diffraction pattern of a nominally undoped-In 2 O 3 thin fi lm is depicted in Figure 1 . On the (001)YSZ substrates, the In 2 O 3 thin fi lm grows expectedly in [001] direction. This is in-line with investigations of MBE-grown In 2 O 3 layers on (001)YSZ. [ 34 ] An infl uence of the In 2 O 3 :Mg surface layers on the XRD pattern was not observed. Atomic force microscopy showed that all samples are compact, without voids and without faceting. The surface morphology depicted in Figure 2 for a sample without In 2 O 3 :Mg surface layer is similar to that reported by Bierwagen et al. for a thin fi lm (labeled S7) grown by MBE under In-rich conditions at 650 °C (with nucleation layer grown at 600 °C). [ 35 ] For our PLD-grown thin fi lm of Figure 2 the root mean square roughness is about 4 nm and suffi ciently low for growth of pn-heterodiodes.The free electron concentration of our nominally undoped-In 2 O 3 thin fi lms was obtained from Hall effect measurements and is with 3 × 10 18 cm −3 rather high for creating diodes thereon. For such high doping, the extension of the space charge region is small and tunneling currents are likely contributing to current transport across the interface. A reduction of the doping density close to the surface should increase the space charge layer width making tunneling less likely. This should result in smaller leakage currents but may also increase the series resistance of the diodes. Sample series with compensated In 2 O 3 :Mg layers with thickness d of 0, 10, and 100 nm pn-Heterodiodes comprising the wide bandgap semiconducting oxide In 2 O 3 and amorphous p-conducting NiO or ZnCo 2 O 4 are realized. In 2 O 3 is grown at 600 °C and the amorphous p-type oxides at room temperature by pulsedlaser deposition. Highest rectifi cation of about four orders of magnitude is observed for structures with Mg-doped In 2 O 3 layers having lower carrier density than undoped layers. The p-ZnCo 2 O 4 /n-In 2 O 3 diodes do not show degradation at elevated temperatures of 150 °C, whereas the p-NiO/n-In 2 O 3 diodes degrade irreversi...
We report on Schottky barrier diodes on the amorphous oxide semiconductor zinc tin oxide (ZTO). The ZTO thin films were grown by pulsed laser deposition at room temperature. Reactively sputtered gold, nickel, platinum, palladium, and silver layers were used to realize the Schottky contacts. To enhance rectification, a thin semi‐insulating zinc tin oxide layer was introduced at the metal–semiconductor interface. The forward current–voltage characteristics of the platinum Schottky barrier diodes were modeled using thermionic emission theory. From temperature dependent measurements, the mean barrier heights of the diodes were determined to be 0.8 and 1.3 eV for diodes without and with a semi‐insulating layer, respectively. Further, the Pt–ZTO diodes were investigated by capacitance–voltage measurements and thermal admittance spectroscopy (TAS). We found that the obtained net doping density agrees with the free electron density determined from Hall‐effect measurements. TAS revealed two defect levels: one deep defect with an activation energy of 220 meV and one shallow defect connected to the carrier freeze‐out. Assuming scattering at potential barriers within the conduction band minimum being the dominant scattering mechanism in multi‐cation semiconductors at freeze‐out temperature, an activation energy of about 15–27 meV was derived.
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