硒化鎵 (Gallium Selenide, GaSe) 為 III-VI 族層狀半導體，不同層之間，由弱凡得瓦爾力 (Van der Waals force) 鍵結，使得層與層之間可以較容易使用機械剝離法分離。本次研究透過變化 Ga 和 Se 的比例或摻雜 Cd、Si、Ge 和 Sn 來成長晶體，觀察其光電性質與載子濃度變化，期許後續製成同質接面 (Homo-junction)，發展成發光二極體 (Light-emitting diode, LED)。
GaSe 系列半導體材料是使用布里茲曼法 (Bridgman method) 成長，晶體在能量色散 X 射線光譜 (EDS)、X 射線晶格繞射 (XRD)、拉曼散射光譜 (Raman) 的結構分析結果均與預想相符，表示改變 Ga 和 Se 的比例或摻雜並不會對 GaSe 的結構和振動模態造成變化。
光學量測上，變溫光激發螢光光譜 (PL) 和變溫熱調制反射光譜 (TR) 實驗中，可以發現隨著溫度降低，能隙位置逐漸藍移，且訊號逐漸增強。在 300 K 下能係位置皆在 1.96 eV ~ 2.00 eV 之間，20 K 下則是 2.08 eV ~ 2.13 eV 之間，螢光也轉為束縛激子發光為主。此外，在光激發螢光與時間解析光激螢光 (TRPL) 的區域螢光波長與時間常數映射中，皆顯示材料的螢光強度、螢光波長和螢光生命區分布均勻，印證了材料的高均勻度與優異的螢光性能。綜合 TRPL 結果，缺 Ga 或 Se 所生長之樣品的螢光強度與 GaSe 相近，且螢光生命期變長，此結果可以推斷在這些條件下所生長的 GaSe 會造成淺層缺陷 (Shallow-level defects) 的產生。然而，當摻雜第四族元素 (Si, Ge, Sn) 之後，螢光則大幅下降，且螢光生命期縮短，同時在電性量測上，電阻也大幅增加，從此可以推測出摻雜將會造成深層缺陷 (Deep-level defects) 的產生。
在 X 射線光電子光譜和霍爾量測 (Hall effect) 中，GaSe:Cd 1% 皆得出較 GaSe 高的電洞濃度。同時也利用凱爾文探針為進行功函數的量測，分析費米能階至真空能階的能量，了解在各種不同摻雜下的費米能階分布情形。量測結果可得知，GaSe:Cd 1% 擁有比 GaSe 更高的功函數，為 p 型半導體，擁有大電阻的 GaSe:Sn 1% 功函數比 GaSe 低，接近本質半導體。所以在應用層面上，我們利用摻雜 Cd 1% (p 型半導體) 與摻雜 Sn 1% (接近本質半導體) 兩材料之間費米能階差所形成的勢壘，來進行同質接面二極體元件堆疊，並且成功利用電壓-電流量測出二極體曲線，其截止電壓為 0.8 eV。也在電致發光光譜 (EL) 上測得 1.95 eV (636 nm) 的螢光光譜，期許未來可發展全新的顯示技術或光電元件。

This study examines the modification of gallium selenide (GaSe), a two-dimensional material with outstanding luminescent and electrical properties. GaSe is III-VI layered semiconductor composed of layers of gallium (Ga) and selenium (Se), producing a hexagonal structure. Covalent bonds exist between Ga and Se atoms within each layer and stack by weak van der Waals forces, allowing easy exfoliation between layers. Reducing the amounts of Ga or Se throughout doping the material with cadmium (Cd), silicon (Si), germanium (Ge), and tin (Sn) allowed it to exhibit either p-type or n-type semiconductor behavior and contribute to different optoelectronic applications. The main objective of this study is to develop GaSe-based homo-junction devies for use in the fabrication of light-emitting diodes (LEDs).
The GaSe semiconductor series were synthesized using the Bridgman method, and their crystal structures were confirmed and validated the stoichiometric composition through scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM/EDS). X-ray diffraction (XRD) and Raman spectroscopy analysis revealed that vacancies and doping did not induce structural changes in GaSe, as all samples exhibited identical signals and wavenumbers.
Temperature-dependent photoluminescence measurements and modulation spectroscopy experiments were performed to investigate changes in bandgap and defects. As the temperature decreased, it revealed non-radiative recombination, which was dominant in defect emissions and indicated a blue shift in bandgap position and an increase in PL intensity. These results were consistent with the findings from modulation spectroscopy at 20 K and 300 K comparison. At 300 K, the bandgap energies of all samples ranged from 1.96 eV to 2.00 eV, while at 20 K, the range was 2.08 eV to 2.13 eV. Furthermore, PL intensity and lifetime measurements from fluorescence excitation and time-resolved photoluminescence experiments, samples doped with group IV elements (Si, Ge, Sn) showed a significant decrease in fluorescence intensity and shorter fluorescence lifetimes, accompanied by a substantial increase in electrical resistance in the electrical characterization, indicating the introduction of deep-level defects through doping.
Hall effect measurements revealed higher hole concentrations in GaSe:Cd 1% compared to GaSe. Kelvin probe measurements of the Fermi level positions for each material indicated that the GaSe:Sn series samples tended towards n-type behavior compared to other materials. Therefore, for future applications, we selected GaSe doped with Cd (favoring p-type behavior) and Sn (favoring n-type behavior) for the stacking of diode components. Successful diode characteristics and electroluminescence spectra at 1.95 eV (636 nm) were achieved, suggesting the potential for advancing novel display technologies and optoelectronic devices.

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