面射型雷射製程技術之研究

Abstract

本論文旨在研究面射型雷射之製程技術以及特性改良相關研究。因面射型雷射具有圓形輸出光束、低製作成本、單一縱膜操作、以及整合二維陣列的潛在特性，因此在光纖通信及中、短距數據通信上，成為極具潛力的發光源，另外亦可應用在光儲存、光感測、顯示系統上等等。面射型雷射元件的製程方法主要有蝕刻、橫向電流或光場的侷限、歐姆電極蒸鍍、介電簿膜蒸鍍。其中，最重要的即是橫向電流或光場侷限的技術。一個好的光場和電流的侷限可以提昇元件的電光轉換效率。基本上， 使橫向光場被侷限在面射型雷射中的結構有三種，為增益侷限（gain-guiding）、折射率侷限（index-guiding）和同時使用增益侷限與折射率侷限的混合侷限（hybrid-guiding）。本論文主要在研究面射型雷射不同侷限構造的製程技術和元件特性。我們嘗試改善傳統gain-guiding結構的元件特性、發展新型index-guiding技術，並研發能結合上述兩種侷限之長處的新型hybrid-guiding結構。我們大部分所使用的晶片為長晶技術成熟、品質穩定又易購得的850nm面射型雷射晶片。另外，我們也發展了適用於氮化鎵系列元件的侷限技術—光化學氧化（photoelectrochemical oxidation）技術。 在面射型雷射元件的製作上，gain-guiding的結構是非常簡單而且是平面化的製程技術，一般常用質子佈植為gain-guiding，但其元件之輸出特性（電流對輸出光強，L-I）常會出現扭曲（kink）且輸出光強的不穩定和其中的雜訊則會導致調變速率被侷限在1.25 Gb/s附近。我們提出使用新的p型電極組合，利用Ti和透明電極（ITO）蒸鍍在一般的p型電極上，成功的改善了質子佈植的面射型雷射之kink現象，且元件亦能成功的操作在2.125 Gb/s。 由於gain-guiding缺乏強的光場侷限能力，故其操作特性表現不如index-guiding。我們使用矽離子佈植取代氫離子佈植，得到了臨界電流約為2.2 mA，光孔徑為13□13 □m2的面射型雷射元件，且其L-I曲線沒有kink出現，而此元件操作在2.125 Gb/s之眼圖（eye diagram）非常清晰；且其橫向橫態的表現和氧化侷限的元件非常相似，可能是因矽離子佈植後會造成晶格插排導致元件有較氫離子佈植強之index-guide的效果。 由於在光通訊中，單橫模輸出的面射型雷射有較好的傳輸特性，因此我們提出了兩種hybrid-guided的結構。一是結構是將氧離子佈植在氧化侷限的面射型雷射中，另一是在質子佈植的面射型雷射上製作光子晶體的結構。在將氧離子佈植在氧化侷限的面射型雷射中，此元件在整個輸出特性中皆為單橫模，且其在光孔徑為8 □m下之臨界電流1.5 mA，且其最高的輸出光強為3.8 mW，此外，此元件的高頻操作特性可超過10 Gb/s。 在光子晶體的這個結構中，光子晶體提供元件光場侷限（即index-guiding），而質子佈植提供電流侷限（即gain-guiding）。此元件的單橫模輸出特性非常優異，其側模壓抑率（side-mode suppression ratio）大於40 dB，且在質子佈植孔徑為10 □m下，其臨界電流只有1.25 mA，而其遠場發散角約為7o。我們亦將此種侷限結構應用到1.3 □m InAs 量子點的面射型雷射，同樣得到單橫模的輸出。 我們相信上述的製程技術都將能用在製作長波長面射型雷射上。但，對於以氮化鎵系列為材列的藍紫外光，傳統用於GaAs的溼式選擇性氧化是不適用的。我們成功的架設了光化學氧化的製程，並在氮化鎵材料上穩定成長出氧化物。我們亦利用此方法在藍光LED的p型氧化鎵表面成長氧化物而增加了LED的輸出光強。我們相信此製程技術將能應用於氮化鎵系列面射型雷射之製程上。總而言之，我們在本論文中研究了面射型雷射橫向侷限的結構，並提出幾種不同的侷限製程技術，且研究其元件操作特性和不同侷限機制的現象，希望這些經驗未來能對長波長和藍紫外光的面射型雷射之製程技術有所裨益。

In this dissertation, we have studied the process technology and characteristics improvement of vertical-cavity surface-emitting lasers (VCSELs). VCSELs possess circular-output beam, low production-cost, single longitudinal-mode operation, and possible integration of two-dimensional array. Hence, VCSELs are potentially suitable for light sources in fiber communication systems, medium and short distance data transmission systems. Other applications include optical storage, optical sensing, and display systems etc. In the processes of fabricating VCSEL devices, there are several main techniques, including mesa etching, transverse confinement of electrical current or optical fields, ohmic contact metal deposition and dielectric film deposition. The most important fabricating process of VCSEL device is transverse confinement technique. A well design of transverse optical and electrical confinement will enhance the electrical-to-optical conversion efficiency or the wallplug efficiency of the devices. Basically, the transverse optical field can be confined inside the VCSELs using gain-guiding, index-guiding, or hybrid-guiding of gain-guiding and index-guiding mechanisms. This dissertation will focus on study of the processing techniques and devices performances of various transverse guiding methods of VCSEL devices. We tried to improve the characteristics of the VCSEL with traditional gain guiding structure, develop new type of index-guiding technique and combine the merits of gain-guiding and index-guiding technique to obtain the high modulation speed VCSEL. The wafers used in this study are essentially GaAs-based 850 nm VCSEL wafers especially most on 850 nm GaAs VCSELs, because the mature epitaxial growth techniques and easy to purchase the VCSEL wafers. In addition, we also develop new guiding technique—photoelectrochemical oxidation for GaN-based materials. Gain-guiding mechanism is a simple and planar process for fabricating VCSELs. However, due to the gain-guided nature of the proton-implanted VCSELs, the kinks usually occur in light output power versus current (L-I) curve, and the laser power output jitters and noise also tends to limit the modulated speed around 1.25 Gb/s. We proposed a new p-contact scheme using Ti and ITO transparent overcoating on the regular p-contact of the proton-implanted VCSEL and obtain substantial improvement in the kink characteristics and the modulation response of the proton-implanted VCSEL shows a more open clear eye and lower jitter of 35 ps operating at 2.125 Gb/s under 10 mA bias and 9 dB extinction ratio compared to the device without overcoated. Because of gain-guiding mechanism nature of lacking ability of strong optical confinement, gain-guided VCSELs could not exhibit performance as good as index-guided VCSEL. We tried to find a new VCSEL structure with virtue of gain-guiding, simple planar process, and possessing index-guide. We utilize silicon implantation to replace proton implantation. The VCSELs with aperture of 13□13 □m2 exhibit kink-free current-light output performance with threshold currents about 2.2 mA, and the eye diagram operating at 2.125 Gb/s with 7 mA bias and 10 dB extinction ratio shows very clean eye with jitter less than 30 ps. The mode patterns of silicon-implanted VCSEL showed similar stable transverse mode patterns of oxide-confined VCSELs suggested that the silicon-implantation induce disordering of the implanted regions, so that silicon-implanted VCSELs have better index-guide than other ion-implanted VCSEL. For high side mode suppression ratio (SMSR) single-mode VCSEL purposed for fiber communication, we suggested two hybrid-guided structures. One is oxygen implantation cooperated selective oxide-confined, another is using photonic crystal on proton-implanted VCSEL. In selective oxide-confined VCSEL with oxygen implantation, the oxygen implantation is for gain-guiding and the selective oxidation is for index-guiding. This approach showed single transverse mode emission within the full operational range with large emission aperture of 8 □m. It also exhibited good performance with a threshold current of 1.5 mA, and a maximum output power of 3.8 mW. Moreover, the single mode VCSELs also demonstrate superior high speed performance up to 10 Gb/s. In proton-implanted VCSELs with photonic crystal structures, the 2-D photonic crystal is for gain-guiding and proton implantation is for index-guiding. This approach showed single-output transverse mode with high SMSR over 40 dB and ultra-low divergent angle about 7o. Moreover, this device also had a ultra-low threshold current about 1.25 mA with proton-implanted aperture of 10 □m. We also applied this technique on the long-wavelength 1.3 □m InAs quantum-dots VCSELs and the VCSELs also show the single-output transverse mode in whole operation range. All of the developed techniques studied in this dissertation showed promising applied on the long-wavelength (LW) VCSELs for the optical communication. But in short-wavelength of nitride-based blue and UV devices, wet selective-oxide technique not suitable for forming oxide-confined region in the GaN-based structure devices. For solving this problem, we developed photoelectrochemical (PEC) oxidation techniques and obtain stable growing oxide film on GaN material. We also applied the PEC oxidation on the p-GaN surface to enhance light output of GaN-based LEDs. We believe that the PEC oxidation could apply on fabricating GaN-based VCSEL. All in all, basic transverse confinement mechanisms are studied and several types of transverse confinement techniques applied on fabricating VCSELs are proposed. The characteristics of device operation and phenomenon of different confine mechanism were also investigated. We hope those all will turn into useful information in fabricating LW-VCSELs and blue/UV VCSELs in the future.