全域組成氮化銦鎵薄膜之磊晶成長與光學特性分析

Abstract

在本論文中，我們利用在有機金屬化學氣相磊晶 (metalorganic chemical vapor deposition，MOCVD)系統上另行安裝一種氨預熱的裝置以增加熱分解效率，藉由改變成長溫度與改變三甲基銦 (Trimethylindium，TMIn)之莫耳流量，成功地成長全域組成氮化銦鎵(InxGa1-xN)薄膜。首先，薄膜成長溫度由750降低至650 oC，固相銦組成可由14 %增加至40 %，由光激螢光光譜顯示，我們成功地調變氮化銦鎵薄膜發光波長，並深入深紅光738 奈米(nm)的波段，其譜線寬度為180 meV。另外，當氣相比增加至73 %時能將固相銦組成提高至44 %，其發光波長可進一步延伸至950 nm，譜線寬度為235 meV。我們認為以另行安裝一種五族預熱裝置以提高氨熱分解效率的方式，已能突破目前以MOCVD成長氮化銦鎵薄膜無法達到全域組成皆能發光的瓶頸，特別是650-1100奈米波段，遠超過目前商業MOCVD的極限。 此外，透過光激發光光譜分析結果顯示，高溫成長的樣品(> 700 oC)會出現高能量與低能量兩個發光譜峰，隨著成長溫度從750-700 oC，高能量從2.94至2.58 eV，低能量從2.44至2.07 eV，高低發光譜峰能量差約500 meV。由倒置空間圖譜(reciprocal space map，RSM)與陰極螢激發螢光譜(cathodoluminescence spectroscopy，CL)顯示高低發光譜峰分別來自薄膜下、上應力層(strained)與鬆弛層(relaxed)所致。700 oC以下成長之樣品，由於銦組成較高與底層氮化鎵緩衝層不匹配程度亦較大，所以並無應力層存在，故發光光譜只存在一譜峰。 為了進一步獲得更長波長之氮化銦鎵薄膜，我們並在650 oC固定成長溫度下，改變三甲基銦 (Trimethylindium，TMIn)之莫耳流量，探討固、氣相In/III族比值相互關係。實驗結果顯示，過高的氣相In/III族比會產生大量金屬銦顆粒析出於表面，直徑約3-5微米不等，同時不利於高銦組成氮化銦鎵薄膜之成長。此外，透過低溫14-K光激發光光譜分析結果顯示，氮化銦鎵薄膜發光峰值可由2.75 eV至1.29 eV，所涵蓋波長由藍光，綠光，紅光至近紅外波段。在這些樣品中，發光之光子能量相對於吸收能隙其能量差(稱史塔克位移，Stokes shift)，由120 meV增加至570 meV隨著銦組成的增加由16 至44 %，我們認為較大的史塔克位移的現象歸因於銦含量分佈不均勻所造成。

In this dissertation, the use of preheating ammonia installation can be used to increase the thermal decomposition efficiency of ammonia, we have demonstrated that the entire composition of InxGa1-xN epilayers prepared by MOCVD can be achieved merely by varying the growth temperature and In vapor mole fraction. First, the In solid composition, as anticipated, was increased from 0.14 to 0.40 as the growth temperature decreased from 750 to 650 oC, which corresponds to a wavelength can be extends to the 738 nm in the deep red region with linewidth 180 meV. Further, as input In vapor mole fraction raise to 73 %, we could estimate that the In solid composition of the InxG1-xN films increases to 0.44, which corresponds to a wavelength range extend to near infrared 950 nm with linewidth 235 meV. Therefore, we have surmounted the technique bottleneck of the absence of entire composition of InGaN epilayers, especially the emission wavelength 650-1100 nm. For samples grown at temperature > 700 oC, separated by about 500 meV, two emission peaks are observable. The corresponding emission peaks, namely high and low emission peaks, are shift from 2.94 to 2.58 eV and from 2.44 to 2.07 as the growth temperature decrease from 750 to 700 oC. The high peak energy originates from strained layer closer to GaN buffer and low energy from relaxed layer near the surface, as revealed by the results of high-resolution x-ray reciprocal space mapping (RSM) and cathodoluminescence (CL) measurements. For samples grown at temperature < 700 oC, high In content epilayers having a large lattice mismatch with under GaN buffer layer, the nearly absence of strained layer resulting in the feature of single emission peaks in both PL and CL spectra. In order to further extend the wavelength of the InGaN epilayer, we grew the InGaN sample at a growth temperature of 650 oC, in an attempt to investigate the dependence of InGaN solid composition on input In reactant flow rate. For the In solid composition, we thought that too high the TMIn flow rate will lead to decrease of In concentration solid, unfavorable to the high In content InGaN growth. Besides, we also reveals that the 14-K photoluminescence peak energy of InGaN epilayers exhibit a wide emission tunability from 2.75 to 1.29 eV, covering a wavelength ranges from blue, green, red and even reaches infrared 960 nm spectrum region. It shown that the bowing parameter of b ~ 2.3 eV for our results and the literature data for the band gap of InxGa1-xN over the entire composition. Our observation provides conclusive evidence that the InxGa1-xN epilayers exhibits a larger Stokes shift, showing that the alloy’s inhomogeneity.