Growth mechanisms of III-nitride epilayers and nanodots by two-heater MOVPE reactor
|關鍵字:||氮化銦鎵;金屬有機化學氣相磊晶;成長機制;雙加熱系統;InGaN;MOVPE;growth mechanism;two-heater MOVPE|
|摘要:||本論文主要分為三個部分，包括氮化鎵、氮化銦鎵薄膜磊晶機制之探討及氮化銦奈米點的成核機制研究。首先，我們利用雙加熱MOVPE 系統成長氮化鎵薄膜，固定上加熱溫度於1050℃ 改變基板溫度，並與傳統系統成長之樣品做比較。由實驗的結果顯示無論是用傳統方法或是雙加熱MOVPE 系統在質傳區內所求得的活化能皆為3.7 kcal/mol，與MOMBE 的數值接近。而MOMBE 由於在超高真空的環境，並無氣相反應，其成長模式係為自由基表面反應。相同的活化能似乎說明了MOVPE GaN 磊晶成長亦是遵循自由基的表面反應途徑。
以GaN 磊晶機制的研究基礎，我們進一步研究InGaN 薄膜的磊晶機制。為了避免高溫效應，我們選擇在質傳區內基板溫度為600℃ 的條件下，調變上加熱板溫度由600℃至950℃。若磊晶存在副反應，反應氣體通過雙加熱反應腔高溫梯度區， 應會產生更為嚴重之氣相副反應。我們的確發現副反應的存在，藉由氣相溫度梯度的計算，可求得副反應的活化能為31.4 kcal/mol。此一低的活化能值顯示InGaN 薄膜成長中的InN 副反應是藉由加合物的反應途徑形成。
副反應通常會導致磊晶速率沿氣流方向呈現非線性的下降，然而實驗結果顯示InN 的副反應並非十分嚴重。就以雙加熱磊晶系統成長的x=0.4 的InGaN 樣品為例，其18K 光激螢光波長及半高全寬在兩吋基板的均勻度分別是808±6 nm 和229±18 meV，顯示優越的樣品均勻性，同時也驗證了副反應不嚴重的現象。
最後，我們在成長溫度650℃探討氮化銦奈米點的磊晶機制。奈米點的密度五三比的變化符合標準成核模型N∝(R/D0)p exp(E*/kT)的描述，其中p =i/(i+2)而i為臨界成核原子數，擬合數值為i=8。同時，五三比為12000時奈米點密度與形貌產生不連續變化，可歸因主導表面移動能力的吸附原子改變所致，由銦充足條件下的氮原子轉變為氮充足的銦原子。|
For III-nitride growth, the metal alkyls are generally used as source precursors for group III elements while ammonia (NH3) is used as the source precursor of nitrogen. Growth and parasitic reactions of nitride semiconductor can be characterized into two reaction pathways, adduct reaction and radical reaction pathway. Adduct reaction start from TMIII:NH3 and followed by a CH4 elimination, while radical reaction through precursors decomposition individually. Each pathway is responsible for producing a group of chemical species that may eventually dissociate to participate the epitaxial growth or associate to become parasitic reaction. In this dissertation, GaN epitaxial layer has been grown by conventional MOVPE and two-heater MOVPE with ceiling temperature fixed at 1050℃. Experimental results indicate that there exhibits an activation of 3.7 kcal/mol at mass transport region for both growth methods, which is agreed with the value in MOMBE GaN growth. It is known the MBE growth is governed by surface kinetics, the similarity in value of activation energy seems to imply the MOVPE GaN growth is also controlled by surface kinetics, suggesting the radical pathway dominate the growth. Based on the results of GaN growth mechanism, we further extended our study toInGaN growth. We tentatively grew InGaN film at 600℃ in mass transport region to avoid the presence of high temperature effects and with ceiling temperature varied from 600 to 950℃, which could enhance parasitic reactions, if any, in the gas phase. We indeed find parasitic reactions take place in the hot temperature zone above the substrate. The calculated activation energy is 31.4 kcal/mol. Such a low value in activation energy implies that InN parasitic reaction is dominant by adduct pathway. Normally, the presence of parasitic reactions could result in a non-linear growth rate along the flow direction. Nevertheless, we do not find such event occurred. For instance, the sample of x=0.40 InGaN film grown at the substrate temperature of 650℃ and ceiling temperature of 900℃ appears to have mean PL (at 18K) peak wavelength and FWHM are 808±6nm and 229±18meV, respectively. Finally, growth mechanism of InN nanodots by conventional MOVPE was also investigated. It is found that the dot growth at 650oC follows well with standard nucleation theory and the dot density can be express by N∝(R/D0)p exp(E*/kT), where p =i/(i+2) and i is the critical cluster size, which is found to be 8 for InN dot growth. Moreover, there appears sharp transition at V/III=12000 in terms of dot density and morphological size which we ascribe it to the change of diffusivity of decisive adatoms from N adatoms under In-rich conditions to In adatoms under N-rich conditions.