氮化銦及氮化鎵之磊晶成長與特性分析

Abstract

在本論文中，我們使用有機金屬化學氣相磊晶法來成長氮化銦、氮化鎵等相關寬能隙材料，並利用X光繞射、拉曼散射、霍爾效應、冷光激光光譜及同步輻射之吸收光譜等量測方法，來探討薄膜之光電物理特性。 實驗結果顯示，六角晶型結構的氮化銦的薄膜成長與III族之TMIn反應源流率及長晶溫度有極大之關聯。而氮化銦結構似乎含有兩種成分：一種是馬賽克晶體結構；另外一種則是纖維狀長條微晶結構。拉曼光譜證據顯示在高溫下成長之氮化銦薄膜由較穩定的六角晶體結構來主導，但在450℃下薄膜就出現立方與六角晶型的混和結構。此外，我們也發現基板的氮化處理對氮化銦的電性品質有極大的影響。在適當的磊晶條件下，我們以自製的磊晶機台製備出相當不錯的氮化銦材料，其X光及拉曼光譜的半高寬及霍爾電性分別為96 arcsec 、 4.5 cm-1 、 270 cm2/V.s及5×1019 cm-3等，是目前所有文獻報導中有關此類材料所能獲得之最佳薄膜品質。 在氮化鎵的研究方面，我們發現其薄膜特性不僅與低溫緩衝層製備有關，其成長溫度亦非常敏感。另外，磊晶速率、基板氮化處理、反應源的五三比與混合載流氣體的比例對高品質氮化鎵的成長也有決定性的影響。以霍爾電性而言，最佳化之成長參數分別為：緩衝層厚度375A、熱處理升溫速率100℃/min、成長溫度1,075℃、磊晶速率0.8 mm/hr、1,050℃基板氮化溫度維持2分鐘、反應源的五三比為3,000和純氮氣載流氣體保持在2,000 sccm的流率。在此條件下，薄膜呈現鏡面般的平坦表面，電子移動率可達300 cm2/V.s、背景濃度亦降低至1×1017 cm-3的水準，初步達成了製備高濃度p型鎂摻雜氮化物的需求。 在氮化鎵的摻鎂雜質之研究上，我們發現鎂雜質極容易融入氮化鎵薄膜且迅速達到高飽和濃度的狀態。在高摻雜樣品中，其拉曼光譜的A1(LO)振動模有藍移現象並出現新的TO振動模，由此可知，氮化鎵原屬的單純的六角晶體結構會因鎂雜質的融入增加而混有立方晶相，這與精細的X光近邊緣吸收光譜觀察完全一致。除此之外，延伸X光吸收光譜分析又進一步發現，鎂原子的摻雜還會引發晶格空缺、原子置換和空隙填擠等缺陷產生。這些缺陷在長晶方向上與其垂直方向上呈現異向分佈的特質。上述混合晶相及缺陷的產生，相信是與鎂摻雜所導致的氮化鎵德拜溫度降低有關，不僅造成了吸收光譜振動函數破壞性干涉，同時也被認為是氮化鎵產生高補償p型效應的最主要原因之一。所有證據顯示，雖然鎂原子的摻雜，有助于氮化鎵材料電洞濃度的提昇，但稍有過量的摻雜卻易于造成晶體結構的破壞，導致大量缺陷的產生，反而不利於p型材料的製作。在適度的鎂濃度摻雜之下，p型氮化鎵樣品在經700℃退火45分鐘，其電洞濃度可高達3×1018 cm-3，電阻也下降至0.2 W-cm的元件製備水準，而百分之十的p型電性活化率，據我們所知，已可稱得上世界級的水平。

We have carried out systematic studies on epitaxial growth of both InN and GaN compounds using metalorganic vapor phase epitaxy technique (MOVPE). Regarding InN, experimental data indicated that its crystalline properties depend strongly on TMIn flow rate and the growth temperature. The deposited hexagonal InN film seems to comprise two major components: one is mosaic crystallites and the other is fiber-like structures. Raman data reflect that the hexagonal structure is predominant for InN film prepared at high growth temperatures, whereas cubic/hexagonal mixing prevailing at temperatures below 450℃. Besides, pregrowth treatments such as nitridation temperature and nitridation duration also have profound effects on InN film quality. By optimizing the growth parameters, we can obtain InN films with X-ray linewidth, Raman E2 mode bandwidth, Hall mobility, and carrier concentration of 96 arcsec, 4.5 cm-1, 270 cm2/V.s, and 5×1019 cm-3, respectively. These values, to our knowledge, are among the best ones ever reported for such type of film. As far as GaN is concerned, we have found that the film quality is very sensitive to the procedures adapted in low-temperature nucleation layer growth, including growth temperature and thickness. Besides, growth rate, nitridation conduction, V/III ratio as well as mixed carrier gas also have great influences on GaN deposition. The optimum growth parameters in our study are as follows: nucleation layer thickness＝375A (520℃), ramping rate＝100℃/min, growth temperature＝1,075℃, growth rate＝0.8 mm/hr, 1,050℃ nitridation temperature = 2 min, V/III ratio＝3,000, and pure N2 carrier gas of flow rate = 2,000 sccm. As a consequence, GaN film with good Hall properties of 300 cm2/V.s and 1×1017 cm-3 can be realized by our reactor, which conceivably is a prerequisite for further p-type GaN growth study. For GaN:Mg growth, results indicate that the Mg atoms can easily incorporate into GaN film and saturate at a value of about 1020 cm-3. When saturation occurs, blue shift of A1(LO) Raman mode presents, concurrently with the appearance of TO modes, which strongly suggesting a mixing of cubic phase in the hexagonal structure. This assertion is also confirmed by Ga K-edge and N K-edge X-ray absorption measurements. Extended X-ray absorption region of the spectra (EXAFS) from Ga K-edge further reveals the induced defects comprising primarily vacancies, substitutions, and interstitial occupations. They are formed anisotropically in the crystal either along c-axis direction or on its perpendicular plane and lead to disorderliness of the doped GaN films. This feature is believed to be one of the essential factors responsible for the high compensation, hence poor Hall properties of Mg-doped p-type GaN film. Nevertheless, extraordinary high hole concentration ( ~3×1018 cm-3) and low resistivity (~0.2 W-cm) are obtained for a GaN:Mg sample annealed at 700℃ for 45 min. The corresponding 10% activation efficiency, which is well above a typical value of ~1%, to our best knowledge is among the best records ever reported for such type of film.