氮化鎵相關材料發光元件製程技術之研究

Abstract

III族氮化物半導體，尤其是氮化鎵(GaN)已經成功實現藍綠光發光二極體、藍光雷射二極體、紫外光源及偵測器。然而，由於p型氮化鎵在需低歐姆接觸上遭遇許多問題，使得這些發光二極體及雷射效率仍然存在許多限制。此外，由於氮化鎵有極佳的化學穩定性及熱穩定性，在光電元件製作上為了高速率蝕刻及高品質的鏡面，蝕刻及製程技術仍待開發。本論文針對此二主要課題進行研究：探討鈹離子佈植p型氮化鎵材料進而提升電洞載子濃度以解決歐姆接觸問題，及研究乾式蝕刻機制並應用此技術於氮化鎵光電元件製作，同時亦利用此蝕刻技術製作奈米結構，而研發成為一嶄新之奈米柱或奈米結構製造方法。 我們研究鈹離子佈植鎂摻雜氮化鎵材料之電性與光性。鎂摻雜之氮化鎵薄膜是由有機金屬化學氣相沈積法(MOCVD)成長在氧化鋁(Al2O3)基板上，而鈹離子是以50keV、150keV的能量及1013cm-2、1014cm-2的劑量佈植。佈植後試片在RTA以900oC, 1000oC, 1100oC及不同時間下退火，得到比未佈植試片電洞載子濃度5.5x1016 cm-3增加三倍，達到8.1x1019cm-3之高電洞濃度。同時這些高電洞載子濃度試片亦得到10-3Ωcm2 及10-6Ωcm2超低特性電阻，其使用金屬分別為Ni/Au and Ni/Pd/Au。從變溫PL光性量測，得到鈹佈植試片中鎂的活化能為170meV，其比未佈植試片中鎂的活化能為250meV，低了將近30%。從x-ray及AFM量測，鈹佈植試片其晶格品質及表面形態沒有明顯的變壞。 我們進行鈹離子佈植in-situ活化之p型氮化鎵快速熱退火效應之研究，研究RTA過程中之升溫速率及高溫停留時間。我們比較試片在1100oC 退火15s 四次(MSA)及1100oC 退火60s一次(SSA)之特性，發現MSA能夠修復鈹離子佈植相關缺陷，一次長時間高溫停留時間造成更多缺陷產生。 在乾式蝕刻研究方面，固定蝕刻條件為Cl2/Ar = 10/25 sccm，ICP/bias power = 300/ 100 W 及腔體壓力(chamber pressure) = 5 mTorr 蝕刻undoped-GaN 試片得到0.2nm之表面粗糙度。在Cl2/Ar = 10/25 sccm，ICP/bias power = 300/ 100 W及腔體壓力 = 30 mTorr 得到n-GaN高的蝕刻速率達12000 Å/min。bias power與腔體壓力影響試片表面粗糙度，在bias power = 50 W時，得到n-及p-GaN低表面粗糙度約1nm。在蝕刻InGaN雷射結構，我們使用高Cl2流量(Cl2/Ar = 50/20 sccm) 及低腔體壓力5 mTorr，得到平滑如鏡之晶面。 利用ICP-RIE系統，我們建立一新式製作氮化鎵奈米柱(GaN nanorods)方法，可控制其大小及密度。在Cl2/Ar = 10/25 sccm，ICP/bias power = 200/ 200 W及腔體壓力10~ 30 mTorr得到密度約108 ~ 1010 cm-2 大小約20~100 nm之p型氮化鎵奈米柱。從micro-PL光譜可以看到在奈米柱區p型氮化鎵訊號約有5~10 nm藍移，此藍移可歸因於量子侷限效應與次能階填充效應。此外，我們亦製作InGaN雷射結構奈米柱，蝕刻條件為Cl2/Ar = 10/25 sccm，ICP/bias power = 200/ 100 W及腔體壓力30 mTorr並得到0.6 □m高之InGaN雷射結構奈米柱。此技術提供了一直接且可控制之氮化鎵奈米結構製作方法，可應用於未來氮化鎵奈米元件之製作。

The groups III-nitride semiconductors, especially Gallium nitride (GaN), have been successfully employed to realize blue-green light-emitting diodes, blue laser diodes, UV light sources and detectors. However, the performance of such light-emitting diodes and lasers remains limited by several problems related to the formation of low-resistance ohmic contact to the P-type GaN. Moreover, due to their excellent chemical and thermal stabilities, the etching technology and processing are need developing to fabricate these optoelectronic devices for the fastest etching rate and high quality mirror-like facet. In this thesis, we concentrate on these two topics. To investigate beryllium implanted p-type GaN to enhance hole carrier concentration and resolve the ohmic contact problem. Additionally, we also study the dry etching mechanism and technology to fabricate GaN optoelectronic devices and nanostructures, and develop to become an alternate new method to form nanorods or nanostructures. We investigated the electrical and optical characteristics of beryllium implanted Mg-doped GaN materials. The Mg-doped GaN samples were grown by metalorganic chemical vapor deposition system and implanted with Be ions at two different energies of 50, 150 keV and two different doses of about 1013, 1014 cm-2. The implanted samples were subsequently rapidly thermal annealed at 900oC, 1000oC, 1100oC for various periods. The annealed samples showed an increase of hole concentration by three-order of magnitude from non-implanted value of 5.5x1016 to 8.1x1019cm-3 as obtained by Hall measurement. The high hole concentration samples also showed low specific resistance ohmic contact of about 10-3Ωcm2 and 10-6Ωcm2 using Ni/Au and Ni/Pd/Au metallization respectively without any further annealing process. It is also found from the temperature dependent photoluminescence that the activation energy of Mg dopants of the Be implanted samples has an estimated value of about 170meV, which is nearly 30% lower than the as-grown samples of about 250meV. The crystal quality and surface morphology of the Be implanted samples measured by x-ray diffraction and atomic force microscopy show no obvious degradation in the crystal quality and surface morphology. We studied rapid thermal annealing effect on beryllium implanted into in-situ activated p-type GaN samples, and investigate the ramping and the isothermal annealing effect of RTA process. We compared the multiple steps annealing at 1100oC for 15s of four periods with single step annealing for 60s of one period at the same annealing temperature, and observed that the ramping effect with MSA could repair Be-related complex defect and one time, long period isothermal annealing effect with SSA seems to be induced much more defect. It seems that the multiple step annealing is more effective and induces fewer defects than single step annealing for Be-implanted in-situ activated p-type GaN samples. Dry etching of un-doped, n-, p-GaN and InGaN laser structure has been carried out in Cl2/Ar inductively coupled plasmas using Ni mask. As Cl2/Ar gas flow rates were fixed at 10/25 sccm, the etched surface roughness has the lowest value of 0.2 nm at constant ICP/bias power = 300/ 100 W and 5 mTorr chamber pressure for un-doped GaN. The highest etching rate of 12000 Å/min for n-GaN was achieved at 30mTorr, 300W ICP, 100W bias power using low Cl2 flow rate (Cl2/Ar = 10/25 sccm) gas mixtures. The surface roughness was dependent of bais power and chamber pressure, and shows a low RMS roughness value of about 1nm at 50 W of bias power for n- and p-GaN. For etching of InGaN laser structure using high Cl2 flow rate (Cl2/Ar = 50/20 sccm) and low chamber pressure 5 mTorr, a smooth mirror-like facet of InGaN laser diode structure was obtained by ICP system. We established a novel method of fabricating gallium nitride (GaN) nanorods of controllable dimension and density from GaN epitaxial film using inductively coupled plasma reactive ion etching (ICP-RIE). The GaN epitaxial film was grown on a sapphire substrate by metal-organic chemical vapor deposition. Under the fixed Cl2/Ar flow rate of 10/25 sccm and ICP/bias power of 200/200 W, the p-GaN nanorods and array were fabricated with a density of 108 ~ 1010 cm-2 and dimension of 20~100 nm by varying the chamber pressure from 10 to 30 mTorr. From the micro-PL spectra, it can be seen that there are 5~10nm blue-shift for the p-type peaks at the nanorods area. It is suggested that quantum confined effect and band filling effect induce the blue shift of p-type PL peaks. Moreover, we also fabricated InGaN laser structure nanorods with the etching condition of the chamber pressure = 30mTorr, mixed gas Cl2/Ar flow rate = 10/25sccm, and ICP/bias power = 200/100 W for a 1 min etching time, and obtain the InGaN nanorods of about 0.6 □m height. The technique offers one-step, controllable method for the fabrication of GaN nanostructures and should be applicable for the fabrication of GaN-based nano-optoelectronic devices.