Sensitivity of ZnO Film-Based X-ray Sensors: A Comprehensive Study on the Effect of Thickness of ZnO Layer.

ABSTRACT

For all types of photosensors, efficient absorption of photons of particular interest is very essential. We report the effect of thickness of the ZnO layer in ZnO film-based X-ray sensors. A set of five samples Z1, Z2, Z3, Z4, and Z5 is developed by varying the thickness of the ZnO layer between 10 and 73 µm. The dark I-V characteristics of the sensors show a "pseudorectifying" type nature. A quantitative analysis of the dark currents reveals that the dark I-V characteristics are affected by space charge limited current (SCLC) due to intrinsic defects present in the ZnO films. The effect of the SCLC is prominent in the thicker films in comparison to the thinner ones. The sensors show high signal-to-noise (S/N) ratio below 5.0 V bias voltage. The S/N ratio is found to increase with the thickness of the ZnO layer due to efficient absorption of X-ray photons. The photoresponse characteristics of the sensors against dose rate are sublinear between 0.015 and 0.234 Gy/s. The photoresponse time of the sensors are found to be nearly 1 s. The sensitivities of Z1, Z2, Z3, Z4, and Z5 sensors at 4.5 V bias voltage for 0.234 Gy/s dose rate are estimated to be 55.51, 337.08, 312.01, 152.81, and 103.52 µC/Gy cm3, respectively. The sensitivity of the device is found to increase with increase in thickness of the ZnO layer and reaches an optimum level for the thickness of about 19-26 µm. Beyond this range, the sensitivity is found to decrease due to the Schubweg effect.