Improvement of Infrared Lights Sensitivity on PZT EMITER Daisuke Takamuro, Hidekuni Takao, Kazuaki Sawada and Makoto Ishida

Abstract (1) Electron emission type infrared light sensor and imager using PZT thin plate. Electron emission current incident infrared light energy. 30% of the pyroelectric current was emitted to the vacuum. Increment of emission efficiency is important to increase its sensitivity. Relation of emission efficiency and anode voltage.  Emission current increased as the anode voltage increased.

Abstract (2) To applied electric field whole PZT surface, the surface of the PZT was polished flat.  Emission efficiency increased two times from unpolished one. Pulsed infrared light irradiation electron emission phenomena from the PZT thin plate. Emission current increased rapidly and decreased. Emission electrons can be multiplied 40 times using MCP device. Emission electron is observed under weak irradiation conditions.

Introduction (1) Simple and high sensitive infrared sensors. security systems and intelligent transportation system. ① Quantum type infrared sensor.  High sensitivity. But expensive, requires a cooling system. ② Thermal type infrared sensor.  Work at room temperature without cooling system and low fabrication cost. PMT(photo multiplier tube) and I/I(image intensifier).  Quantum type, Can not detect on mid-far infrared region because photo detection part(photocathode) is not sensible on this region.

Introduction (2) Quantum type photocathode with Al/InP/InGaAS/InP.  Detects near infrared lights about 1.7, Quantum efficiency is less than 0.1%  If photocathode can detect the mid-far infrared light Infrared image sensor with more highly sensitivity at room temperature Thermal type photocathode is not reported, because electron emission phenomena from thermal type sensor is not studied.

Induction (3) ① Electron emission phenomena by an infrared light irradiation and Electron emission current was proportional to the incident infrared light energy. ② Improvement of infrared lights sensitivity and the electron emission characteristic under a pulsed infrared light irradiation.

Structure and Principle Electron was emitted from ferroelectric materials to the vacuum to invert the spontaneous polarization by an infrared light irradiation Thermal equilibrium state -> Electrons and holes are existed on the both side of the ferroelectric material to compensate the spontaneous polarization Infrared light irradiation -> Quantity of polarization is change by an infrared light irradiation -> compensated electrons were emitted from a ferroelectric material surface

Characteristics of PZT pyroelectric coefficient : Thickness : (not polished) Single polarization precess : 2kv/mm 100nm thick chromium electrode : on one side of the PZT The surface SEM image : fig. 2 PZT gain size : 1~3 Of PZT thin plate was measured in various temperatures : Fig.3 -> 33 at room temperature.

Characteristics of PZT

Electron emission characteristic of polished PZT 10 groove in the center of the quartz substrate. 100nm Al film was used as an anode electrode -> 0~1000V Sensor was placed in vacuum chamber. Infrared light was irradiated through Cr electrode ( -> grounded). Small infrared light source of the filament type ->DC 1V. (distance from sensor is 15mm) 5 height emission plane -> compare emission characteristic of unpolished and polished plane

Multiplying emission electron Sensor ( vacuum chamber) ① PZT thin plate ② Micro Channel Plate (MCP) -> diameter:12, thickness:0.48mm 0 ~ 1000V ③ Fluorescent substrate -> 3500V Operation condition of the infrared light source : temperature of, chopping frequency of 0.1Hz

Electron emission characteristics by irradiation a pulsed infrared light Si substrate : anode electrode -> 500V (in order to extract electrons) Mica : 100 air gap PZT : electron emission plane -> Cr electrode on one side of PZT (ground) Infrared light was irradiated into the PZT thin plate from the Cr electrode side through a sapphire window on the chamber Pyroelectric material type infrared sensor responds to the variation of the infrared light -> using image sensor Pyroelectric imager needs mechanical light chopper distance (light source and sensor) was 10mm Operation condition :, 1Hz

Result(Improvement by polishing emission plane) Polished surface of emission plane by AFM in fig. 7 Anode dependence of emission current with unpolished and polished emission plane in fig. 8 Emission efficiency increases 2 times from unpolished one Unpolished one : electric field is concentrated the rough surface apex -> valleys are hard to emit.

Multiplying emission electron Multiply the emission electrons using MCP device Emission current of fluorescent substrate in Fig. 9 Emission electrons can be multiplied 40 times

Electron emission response by a pulsed infrared light irradiation Electron emission from PZT thin plate by a pulsed infrared light irradiation in Fig. 10 Chopping frequency of infrared light : 1Hz Irradiation and Shielding time : 0.3s, 0.7s Emission current rapidly increased and decreased. Electron emission type infrared light sensor is useful to detect pulsed inputs of an infrared light irradiation

Conclusions Improvement of infrared lights sensitivity by polishing emission plane and multiplying electrons. Electron emission phenomena from the PZT thin plate (pyroelectric material) caused by a pulsed infrared light irradiation. Emission current increased rapidly and decreased slowly. Viability of the electron emission type infrared light sensor or infrared image sensor for mid-far infrared region using pyroelectric material.