利用雷射剝離技術製作氮化鎵發光元件之研究

Abstract

寬能隙氮化鎵半導體已經成功的實現在藍綠光發光二極體、藍光雷射二極體以及紫外光源等。由於氮化鎵基板成長困難，常用的氮化鎵發光元件，其磊晶生長通常在不同基板上磊晶成長，例如藍寶石(sapphire)基板。由於藍寶石基板本身的電導率與熱傳導率的特性不佳，使得在氮化鎵發光元件製作程序相對於其他化合物半導體製作較為複雜並進而影響到氮化鎵發光元件的特性。因此，將氮化鎵元件磊晶層從藍寶石基板剝離並轉換到具有高電導率、高熱傳導效率的基板來製作氮化鎵發光元件是目前主要的研究課題之一。在轉移基板製程技術層次，包含了金屬電極製作、晶片接合技術、剝離基板技術以及磊晶層轉移技術等。本論文係針對這個主要課題進行一系列的研究：建立雷射剝離氮化鎵薄膜製程技術的相關條件，將藍寶石基板剝離後獲得freestanding的氮化鎵薄膜、製作新型p型氮化鎵電極、晶片接合技術以及探索剝離後具有不同結構型態（p型上與p型下）的氮化鎵發光二極體轉移到具有高導電、高熱傳導效率的銅基板上之特性表現。 首先，我們建立雷射操作之條件來達成將未摻雜的氮化鎵薄膜從藍寶石基板剝離之技術。我們利用波長為248奈米、脈衝寬度25奈秒之KrF準分子雷射作用在氮化鎵薄膜進行GaN®Ga+N2的分解，研究其對氮化鎵的蝕刻率。對於雷射蝕刻氮化鎵薄膜，實驗所得到的臨界雷射能量密度值約為 0.3 J/cm^2, 與以雷射致熱於氮化鎵材料之熱傳導模擬公式所計算的臨界之雷射值0.3 J/cm^2相符合。又，由於考慮到藍寶石基板與氮化鎵薄膜的吸收衰減與反射率約為20-30 % 和20 %，製作雷射剝離藍寶石基板時將雷射能量密度調整到0.6 J/cm^2。當雷射光直接射入透明的藍寶石基板時，藍寶石/氮化鎵介面之氮化鎵會分離成氮氣與鎵，樣品經過雷射全面掃射後，我們成功地將未摻雜的氮化鎵薄膜從藍寶石基板剝離。之後，進行SEM、AFM、X-ray、PL spectrum的特性研究。 研發製作freestanding之氮化鎵發光二極體時，幾個主要的課題必須先進行探索。第一、由於準分子雷射於每一個單一脈衝時的出光能量不均勻，因此雷射出光的能量密度需調整為0.8 J/cm^2，其能量密度相對應於藍寶石與氮化鎵介面為0.5 J/cm^2 ，以確保藍寶石/氮化鎵介面的氮化鎵材料完全被雷射所分解，才能達成發光元件磊晶層在剝離時保持完整。第二、新型鎳/鈀/金合金層之p型歐姆接觸電極製作技術研發。摻雜鎂之p型氮化鎵薄膜是由有機金屬器相沈積法(MOCVD)生長在藍寶石基板上，再將樣品於外部爐管加熱退火。樣品依序蒸鍍鎳、鈀、金合金層後，樣品在不同溫度(350℃~650℃)與不同氣氛(空氣、氮氣、氧氣)下進行合金層退火以獲得最佳退火條件，達成低特性電阻歐姆接觸的特點。我們發現在550℃、氧氣氣氛下退火5分鐘可以獲得最佳之低特性電阻值為1.1x10^-4 ohm-cm^2 。此外，我們也進行in-situ 在MOCVD腔體活化之p型氮化鎵薄膜(in-situ樣品)於外部爐管進行於氮氣氣氛下再退火的研究。研究發現經由再次退火處理後，其電洞濃度與鎂相關之PL spectrum強度有增加的趨勢。經過再退火後的in-situ樣品，蒸鍍上鎳、鈀、金合金層後，於相同合金層退火條件下(O2, 550℃, 5min)發現其特性電阻值由10^-2 ohm-cm2 降低到 10^-4 ohm-cm^2。發現此一特性電阻值的降低是由於除了電洞濃度的提升以外，鎵空缺的形成亦可幫助特性電阻值的降低。更進一步地，將一般氮化鎵發光二極體晶片進行再退火並製作此新p型電極後，發現二極體啟動電壓及平均動態電阻值皆有下降的趨勢。第三、選擇具有高導熱與高導電效率的銅作為轉移氮化鎵發光二極體的基板。第四、同時亦研究軟性銦金屬接合與固硬性鎳金屬接合的晶片接合技術對製作雷射剝離之氮化鎵發光二極體的影響。最後，探討利用不同的轉移技術製作二種結構型態（p型上與p型下）之雷射剝離氮化鎵發光二極體接合於銅基板。比較二種結構型態之雷射剝離氮化鎵發光二極體與一般於藍寶石基板製作之氮化鎵發光二極體之特性。 針對製作p型上之雷射剝離氮化鎵發光二極體接合於銅基板上，一般氮化鎵發光二極體元件於藍寶石基板上先製作完成，利用雷射剝離技術一次將已製作的氮化鎵發光二極體元件先轉移到玻璃基板上後，再利用軟性銦金屬接合技術將已製作的發光二極體元件薄膜再次轉移到銅基板上。我們發現氮化鎵發光二極體經過轉移後，其 I-V 的特性並無顯著的變化，而其發光強度與轉移前相比較卻降低了約50%，此結果與雷射剝離後所造成晶片的表面粗糙與接合面不平整有關。然而，轉移到銅基板之氮化鎵發光二極體具有高操作電流(400 mA)的特性，顯示出銅基板具有較佳的散熱效果，與轉移前相較，氮化鎵發光二極體轉移至銅基板後具有增加至1.5倍的熱散能力。 針對製作p型下之雷射剝離氮化鎵發光二極體接合於銅基板上，氮化鎵發光二極體磊晶晶片利用軟性銦金屬接合技術與雷射剝離技術將其轉移至銅基板後，再於銅基板上製作發光二極體元件。p型下之雷射剝離氮化鎵發光二極體接合於銅基板上具有n型氮化鎵層在上的結構型態，可有效提供良好的電流散佈而不需製作透明電極的優點。雖然在 I-V 的特性表現上，其操作電壓較一般氮化鎵發光二極體於藍寶石基板高，這是由於p型金屬在此一製程中，同時亦作為金屬接合所使用。然而，由於p型下之雷射剝離氮化鎵發光二極體接合於銅基板上比一般製作p型上之氮化鎵發光二極體，具有發光面積增大、反射率提升以及無需製作透明電極等優點，本實驗發現p型下之雷射剝離氮化鎵發光二極體接合於銅基板的發光強度比一般製作的p型上氮化鎵發光二極體增加了4倍。p型下之雷射剝離氮化鎵發光二極體接合於銅基板上也具有高操作電流(400 mA)的特性表現。與一般氮化鎵發光二極體於藍寶石基板比較，其更具有高達4倍的熱散能力。利用硬固性鎳金屬接合技術可以改善軟性銦金屬接合技術所遭遇的問題。作為金屬接合之鎳金屬的蒸鍍同時具有可以保護p型金屬電極與有效作為mesa阻擋乾蝕刻作用以製作元件， 硬固性鎳金屬接合技術製作同時具有提供純淨之氬氣環境，將接合金屬層於400℃退火時產生氧化的可能性降到最低。我們也發現了利用硬固性鎳金屬接合技術製作p型下之雷射剝離氮化鎵發光二極體接合於銅基板上之元件比利用軟性銦金屬接合技術製作之元件具有更佳的L-I-V 特性表現。此外，亦製作出更大面積800 um× 800 um 之p型下之雷射剝離氮化鎵發光二極體接合於銅基板上。發現其有效發光面積會隨著電流的增加而增加。並且得到了大面積的樣品比一般面積300um ×300um樣品的發光強度增加了1.8-2.8倍。 雷射剝離技術可以提供未來製作於其他氮化鎵的發光元件，尤其是對於增加光強度、提高操作電流、增加熱傳導效率與製作大面積發光，在提升發光功率上，將具有實質上的應用價值。

The GaN-based wide band gap semiconductors have been employed for blue light emitting diodes (LEDs) and laser diodes. These devices were grown heteroepitaxially onto dissimilar substrates such as sapphire and SiC because of difficulties in the growth of bulk GaN. However, due to the poor electrical and thermal conductivity of sapphire substrate, the device process steps are relatively complicated compared with other compound semiconductor optoelectronic devices. Therefore, fabrication of GaN-based light emitting devices on electrically and thermally conducting substrate by separating sapphire substrate is most desirable. Several techniques were used to achieve this process including, metallization and wafer bonding, lift-off, and layer transfer. In this thesis, we report the research results on the fabrication of free standing GaN LEDs on conductivity substrate. The establishment of laser lift-off (LLO) conditions for freestanding GaN thin film was presented. By combing the LLO process, new p-type ohmic contact metallization, and wafer bonding techniques, the performance of freestanding LLO-LEDs on copper substrate with p-side up and p-side down configuration, and a large-area-emission LEDs were demonstrated. We established the LLO conditions using the undoped GaN. The rate of removable of GaN or etching rate as a function of laser fluence was investigated using the KrF excimer laser at wavelength of l=248 nm with pulse width of 25 ns. The experiment value of threshold laser fluence for removable of GaN layer was estimated of about 0.3 J/cm^2, which is in agreement with the simulated threshold laser fluence of 0.3 J/cm^2 for GaN material. Successful separation of undoped GaN thin film from sapphire substrate was achieved by using the pulsed laser directly passing through the transparent sapphire substrate. The incident laser fluence was set at 0.6 J/cm^2 by taking account into the attenuation and reflection of sapphire and GaN. Characterization of LLO GaN sample using scanning electro microscopy, atomic force microscopy, x-ray rocking curve, and photoluminescence showed that a no major degradation after LLO. For the fabrication of freestanding GaN LEDs, several major considerations and technical approaches were discussed and described. First, the incident laser fluence was modified to a value of 0.8 J/cm^2, corresponding to a laser fluence of about 0.5 J/cm^2 at the interface of GaN/sapphire to complete interfacial decomposition for obtaining the accomplished device thin film. Second, electrical contacts for GaN LEDs by using the new p-type ohmic contact metallization of Ni/Pd/Au were studied. The ‘as-grown p-type GaN’ samples were grown by metalorganic chemical vapor deposition (MOCVD) and the p-GaN layer was annealed in external fluence. The ‘as-grown p-type GaN’ samples were deposited with Ni (20 nm)/Pd (20 nm)/Au (100 nm) and then alloyed in air, nitrogen and oxygen ambient conditions at different annealing temperatures ranging from 350℃ to 650℃ to optimize the alloy condition. Linear I-V ohmic characteristics were observed with the specific contact resistance as low as 1.1x10^-4 ohm-cm^2 for the samples alloyed at 550℃ in oxygen atmosphere for 5 min. Conventionally, the p-GaN layer of GaN-based light emitting devices was in-situ annealed in MOCVD reactor namely the ‘in-situ p-type GaN’. The in-situ annealed Mg-doped p-GaN samples were re-annealed in external furnace with and without cap in higher pressure N2-ambient. Both the hole carrier concentration and the intensity of the Mg-related p-GaN spectrum was increased after the re-annealing process. The specific contact resistance of the re-annealed samples was reduced by two orders of magnitude from 10^-2 ohm-cm^2 to 10^-4 ohm-cm^2. The increasing in hole carrier concentration and the creation of Ga vacancies could be responsible for the reduction of the contact resistance. The GaN light emitting diodes (LEDs) fabricated with re-annealed GaN LED wafer also show the improvement of the turn on voltage at 20 mA and the reducing of average dynamic resistances. Third, the Cu metal was selected for the fabrication of GaN-LLO-LEDs due to it has good electrical and thermal conductivity. Fourth, both soft In bonding process and strong Ni bonding process were investigated and compared for the fabrication of GaN LLO-LEDs. For the soft In bonding, In has relatively soft and low melting temperature (Tm=156℃) that can adherent with different metals and GaN easily to simplify the fabrication process steps of GaN LLO-LEDs. For the strong Ni bonding process, Ni is a good bonding substance compared with In to limit the electrical properties after the bonding process. Ni is a very useful metal to prevent the GaN layer during the inductively coupled plasma reactive ion etching. Finally, two types of GaN LLO-LEDs can be fabricated. We fabricated the InGaN LED structures on sapphire substrate and transferred to Cu substrate by LLO process into two different configurations, namely p-side up LLO-LEDs on Cu and p-side down LLO-LEDs on Cu. The GaN LLO-LEDs on Cu with p-side up and p-side down configurations were fabricated by two different transfer processes and their performance were compared. For fabrication of p-side up LLO-LEDs on Cu, the regular p-side up GaN LEDs on sapphire substrate was first fabricated and then transferred to a supported glass carrier by LLO process. The lift-off film with LED devices was then double transferred to Cu substrate by soft In metal bonding. Although the I-V characteristics showed the same behavior before and after LLO, the light output power of p-side up LLO-LEDs on Cu was reduced to be about 50% compared with the regular GaN LEDs on sapphire. The relatively uneven and rough surface of the bonding interface could affect the light output. However, a high operation current up to 400 mA for the p-side up LLO-LEDs on Cu and about 1.5 times larger heat dissipation capacity for the p-side up LLO-LEDs on Cu compared to the regular LEDs on sapphire was demonstrated. For the fabrication of the p-side down LLO-LEDs on Cu, the LED wafer sample was transferred to Cu substrate by soft In bonding and LLO process. The LEDs was fabricated on Cu substrate. For the p-side down LLO-LEDs on Cu, an n-side up configuration without semitransparent metal contact providing a better current spreading in n-GaN layer was demonstrated. The I-V characteristics showed higher operation voltage; nevertheless, a high operation current up to 400 mA for the p-side down LLO-LEDs on Cu was also obtained. The light output power of p-side down LLO-LEDs on Cu showed a nearly 4-fold increase over the regular LLO-LEDs on sapphire. About 4 times larger heat dissipation capacity for the p-side down LLO-LEDs on Cu compared to the regular LEDs on sapphire was obtained. Several issues of soft indium bonding were improved using strong Ni bonding process. The bonding substance metal of Ni protects the Ni/Pd/Au p-contact metallization scheme for metal bonding process and acts as a mesa for ICP/RIE etching. The strong metal bonding process can provide a pure Ar atmosphere to minimize oxidation of the bonding metals during 400℃ annealing. The p-side down LLO-LEDs on Cu fabricated by strong Ni bonding process showed better L-I-V characteristics compared with the soft In bonding process. In order to scale up the light output power, a large sample size with 800 um× 800 um of p-side down LLO-LEDs on Cu was also fabricated by strong Ni bonding process. The active light emission area of the large size sample increased with increasing current. About 1.8-2.8 times higher light output power for the large size sample compared to the regular sample size with 300um× 300um was demonstrated. The LLO process should be also applicable to other GaN-based optoelectronics devices and in particular suitable for high light output power, high operation current, high heat dissipation capacity, and large area emission.