本實驗目的是將鐵酸鉍這種室溫下電阻極高的鐵電材料變成半導體, 提高其導電率, 以用作後續光伏性質之應用, 藉由溶膠凝膠法製成富鐵相鐵酸鉍塊材及薄膜, 並將塊材在兩種氣氛在450(oC)下煆燒6小時及成燒12小時; 薄膜樣品則是在氬氣下燒結1小時以穩定吾人旋轉塗佈後的在基板上的薄膜其物理性質。目的再現前人所發現之四氧化三鐵析出於鐵酸鉍晶粒表面的現象, 且藉由不同燒結氣氛比較其介電性質, 電導率變化以及光伏性質的影響, 且不同於前人合成之手段, 吾人在合成鐵酸鉍塊材時, 在其外部通以固定電場, 使樣品內部極化方向能在合成結束時便自發排列整齊。

由XRD圖顯示吾人在混合氣氛下合成之樣品隨鐵濃度增加其雜相便增加; 在大氣下合成之樣品則是隨著鐵濃度增加, 內部出現越來越明顯的Bi25FeO40雜相。由SEM及EDX結果可以知道即使在XRD圖中可以看見較多富鉍的雜相生成, 推估吾人樣品中應具有相當數量的結晶性差的四氧化三鐵相, 但成分比例依舊符合吾人的名義成分, 且其晶粒大小也隨著鐵濃度增加而增加, 符合吾人估算之結果。

介電性質及阻抗分析的結果可知吾人樣品的導電主要由遠程傳導組成, 較不同的地方是在混合氣氛下合成之樣品在室溫下為介電鬆弛主導的導電過程; 且吾人可將整體材料的導電過程等效為一個RC並聯的電路, 其中電阻是由鐵酸鉍晶粒及覆蓋於表面的四氧化三鐵所貢獻。
XPS的結果可以知道吾人在兩種氣氛下合成的塊材樣品內皆含有一定量的二價鐵陽離子, 再與VSM的成果交叉比對, 可見吾人樣品具有明顯的磁滯迴圈與較大的磁矩可間接說明吾人成功合成在晶粒表面析出的四氧化三鐵。

在電性導電率的量測證明摻雜稍微過量的鐵使整體導電性提高, 基於XPS和VSM的結果, 這個導電率提高主要是由四氧化三鐵中的二三價鐵中的電子跳動造成的, 但要是鐵濃度達到1:1.05其導電率並不會隨著鐵濃度增加而增加, 反而在高溫時稍微降低。由電流電壓量測結果可知, 吾人在外加電場下合成之塊材樣品其內部具有偏壓, 因為其電流電壓曲線與常見的二極體P-N介面相似。在光伏性質方面, 1:1.01的樣品其光伏性質比鐵未過量的試片好, 在20(V)的電壓下, 1:1.01試片產生的光電流相較鐵未過量樣品高出3~4個數量級, 而當鐵過量太多，如1:1.05時, 試片的光伏性質反而變差。

關鍵字: 鐵酸鉍;鐵電光伏打效應;四氧化三鐵;電導率

Our previous work showed that there was a thin layer of Fe3O4 coating at grain surface in the polycrystalline BiFeO3 (BFO) samples sintered at 400~500 C. Such a Fe3O4 segregation at grain boundary makes BFO have a much higher DC conductivity than its stoichiometric counterpart, which is desired in terms of a potential photovoltaic application. In this study, attempts were made to synthesize such Fe3O4 segregated BFO samples by intentionally adding more Fe in the BFO samples and sintering under reduced atmosphere. Although Fe3O4 was not observed by X-ray diffraction, its existence was supported by the magnetic measurement and the X-ray photoelectron spectroscopy. The former showed an increased magnetization of 0.4~1.5 emu/g, while the latter indicated the presence of Fe2+ in the samples. The Fe excess samples indeed exhibited a much higher DC conductivity than stoichiometric BFO. However, too large a Fe excess did not further increase the conductivity of the samples. The IV curves of the Bi/Fe=1/1.01 samples showed a clear rectification effect similar to that observed in the conventional semiconductor pn junctions. Under light illumination, the forward current was increased by nearly an order of magnitude, from 3.1410-5 A to 2.5210-4 A at 20 V in a sample with the area of about 55 mm2.

Keywords: BiFeO3, electronic conductivity, photovoltaic, ferroelectric

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