$Sn_2P_2S_6$ ferroelectric material was prepared by chemical vapor transport method and its photovoltaic properties was investigated. stoichiometric amounts of the constituting elements(sulfur, phosphorus and tin) and iodine as transport agent were sealed into quartz ampoules and brought to reaction at 700℃. after 2 days, upon completion of the reaction, the ampoule was placed into a horizontal temperature gradient. in the cold end, crystals started to grow by chemical transport. also, single crystal well grown was obtained from Ukraine. $Sn_2P_2S_6$ ceramic was prepared by the mechanical milling of single crystal and sintering with atmospheric powder(sulfur, phosphorus). photovoltaic current was measured from intercept of Y-axis in IV curve under illumination and photovoltage was obtained from intercept of X-axis in same condition.
Large $Sn_2P_2S_6$ single crystals were not grown in quartz ampoule. It results from constituting elements, rough surface of quartz tube wall, transport agent concentration and temperature gradient condition, etc.
From XRD figure measurement and It is found that the single crystal from Ukraine has [002] direction perpendicular to parallel planes in crystal.
Photovoltaic current and voltage was measurend after cutting and electroding to the [002] direction. Photovoltage of $Sn_2P_2S_6$ single crystal is 0.2(V/cm). however, further research about the domain structure of single crystal must be done because photovoltaic current was measured without poling procedure.
$Sn_2P_2S_6$ ceramic sintered with atmospheric powder(sulfur, phosphorus) at 500℃ during 2hours was obtained. the dark conductivity of ceramic according to atmospheric powder weight shows minimum value at optimum condition(0.0269g). also the highest relative density is obtained in ceramic sintered at the same condition. it is assumed that small Pr in PE curve is due to small grain size or nonstoichiometric effect. after 3kV/cm poling at room temp during 30min, photovoltaic current and voltage was measured. photoconductivity in ceramic is low than that in single crystal because of potential barrier formed by charge carriers trapped at grain boundary and photovoltage of ceramic is about 6(V/cm).
In conclusion $Sn_2P_2S_6$ single crystal has high photoconductivity and low photovoltage. $Sn_2P_2S_6$ ceramic has low photoconductivity and high photovoltage. however, current density in single crystal is higher than that in ceramic because photoconductivity difference between single crystal and ceramic is higher than photovoltage difference.'
화학기상이동법으로 합성된 $Sn_2P_2S_6$를 다결정으로 제조하였을 때의 강유전특성과 빛을 조사하였을 경우 나타나는 광전압특성에 관해 조사하였고, 그 결과를 단결정과 비교해 보았다. 구성원소(tin,phosphorus,sulfur)들을 화학양론적으로 혼합한 다음, transprot agent로서 iodine을 첨가하여 quartz tube에 진공봉합후 700℃에서 반응시키고 온도 구배를 통하여 cold zone에서 결정들을 성장시켰으며, Ukraine로부터 잘 성장된 단결정을 입수하였다. 다결정 시편은 단결정을 기계적방법으로 분쇄한 후, 분위기 분말 (sulfur,phosphorus)과 함께 quartz tube에 진공봉합하여 소결하였다. 결정성장 실험결과 큰 단결정들이 성장하지 못하였고, 이것은 구성원소의 quality와 quartz tube표면상태, transport agent의 농도, 온도구배등이 영향을 미친 것으로 생각된다. Ukraine로부터 입수한 단결정들은 평형한 면이 [002]방향 으로 성장되어 있었으며, 광전압특성 측정을 위해 [002]방향이 전극면이 되도록 cutting후 전극처리하였다. 빛조사와 동시에 IV curve로부터 단결정은 약0.2(V/cm)의 광전압을 얻을 수 있었으나, poling 과정없이도 광전압(전류)가 측정 가능하였고 따라서 $Sn_2P_2S_6$의 domain구조에 관한 연구가 필요하다. 분위기 분말을 이용하여 소결한 다결정 시편의 경우 분위기 분말양이 0.0269g인 최적조건에서 가장 낮은 전기전도도와 가장 높은 밀도를 가지는 다결정을 얻을 수 있었다. 다결정의 PE curve에서 측정된 낮은 Pr값은 grain size나 비화학양론적 효과에 의한 것으로 생각된다. 다결정은 상온에서 3kV/cm로 30분간 poling한 후, 측정된 광전압은 약 6(V/cm)이며, 입계에 trap된 전하들에 의해 형성된 potential barrier에 의해 단결정 보다 낮은 광전도도를 나타내었다. 다결정이 단결정보다 높은 광전압을 가지고 있으나, 광전도도가 낮고 그 차이가 광전압의 차이보다 훨씬 크므로 단결정이 상대적으로 높은 전류밀도를 나타내었다.
