雙加熱金屬氣相沈積系統成長氮化鎵磊晶機制與特性研究

Abstract

本篇論文著重於使用雙加熱以及傳統有機金屬氣相沈積系統成長氮化鎵，關於磊晶前驅物氣相反應以及表面反應與腔體內熱力學、動力學之比較。雙加熱系統由於具備上加熱源，在相同的基板溫度的條件下，可針對氣相溫度做調整。由於使用於磊晶之前驅物於腔體中的裂解以及前驅物間的交互反應深受氣相溫度影響，雙加熱系統可以藉由調整上加熱源，以達到適合前驅物裂解之最佳氣象溫度，方能達到更佳的磊晶品質。實驗結果發現使用雙加熱系統成長氮化鎵，於低成長溫度區段，即基板設定溫度700℃至900℃區間，能有效提升氮化鎵的薄膜結晶品質以及光學特性，比較傳統系列樣品於相同溫度區間，雙加熱系列樣品使用X射線繞射分析，其(102)面半高寬降低一半，並且光激發螢光譜強度有著兩次方數量級的提升，同時，代表缺陷發光的黃光亦能被有效的抑制。此溫度區段之氮化鎵薄膜亦為商用藍光、綠光InGaN主動層發光元件所常用之氮化鎵位障層以及P型氮化鎵成長溫度此外，於雙加熱系列樣品，基板溫度750oC至950℃區間，得到了一段有別於傳統系列樣品的真正質傳區，我們推測由於雙加熱系統所營造的高溫環境，使得前驅物於氣相得到充分裂解，進而使得前驅物到達表面形成氮化鎵不需要額外的裂解能量，此段質傳區延伸了氮化鎵薄膜之成長窗口至低溫750℃。從變上加熱源溫度系列氮化鎵樣品，我們發現雙加熱系列樣品有著比傳統系列樣品更高的成長效率。有鑒於此，我們建立了一個數值化模型，用以分析雙加熱與傳統系統間，反應腔溫度造成之影響以及成長效率之差異。我們發現由於雙加熱系統所營造之高溫腔體環境，造成腔體氣相中各個位置的反應前驅物之擴散係數增加，進而造成到達表面之反應前驅物增加。除此之外，由於傳統系統只有基板為唯一熱源，因此營造由下到上之溫度梯度，造成一由下而上之熱泳力，熱泳力將反應前驅物推離成長表面。雙加熱系統則與之相反，上加熱源所營造之高溫，造成由上熱源指向基板方向之熱泳力，將前驅物推向成長表面。我們推測此一由溫度梯度所造成的熱泳力亦為雙加熱系統有著較高成長效率的原因。

We studied the fundamental issues which includes thermodynamic, hydrodynamic, and kinetic aspects of the growth process in MOCVD, both in the gas phase and on the surface. Two-heater MOCVD totally changed the gas phase phenomenon and the chemical reactions in the gas phase when compared with conventional MOCVD. By using gas phase thermal treatment with the ceiling temperature in two-heater MOCVD, the better optical quality with IYL/INBE reduced remarkably to 1/1000 and the crystalline quality with lower dislocation about 5x109 cm-2 of low temperature GaN films had been obtained. In order to find the reasons why two-heater MOCVD had the better low temperature GaN quality and higher growth efficiency, two series of sample had been used, substrate series and ceiling temperature series. From the analytical kinetic model for two-heater MOCVD and the experimental results of ceiling temperature series, we could found that both the enhancement of diffusivities and the thermophoretic force affected the mass transfer of the reactants in the reactor. As the result, the growth efficiencies would further increase when ceiling temperature were added. Base on the experiment results of the Arrhenius plot of growth efficiencies in conventional and two heater MOCVD. It seems that ceiling temperature supplied enough energy for the precursors in the gas phase made them to conquer the surface formation barriers, thus we could found that the growth efficiency reached the mass transport region when ceiling temperature had been used. When it comes to the development of long wavelength light emitting diodes (LEDs), such as green, amber and red LEDs, the growth temperature of GaN barrier should set below 900oC. In such low temperature the GaN films quality are worse. By using two heater MOCVD, high quality low temperature GaN films from 700℃ to 900℃ had been achieved, extending the growth window for the high quality GaN barrier of InGaN green LED and amber LED.