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Study of novel electronic materials by mid-infrared and terahertz optical Hall effect

機(jī)譯：中紅外和太赫茲光學(xué)霍爾效應(yīng)研究新型電子材料

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Development of silicon based electronics have revolutionized our every day life during the last three decades. Nowadays Si based devices operate close to their theoretical limits that is becoming a bottleneck for further progress. In particular, for the growing field of high frequency and high power electronics, Si cannot offer the required properties. Development of materials capable of providing high current densities, carrier mobilities and high breakdown fields is crucial for a progress in state of the art electronics. Epitaxial graphene grown on semi-insulating silicon carbide substrates has a high potential to be integrated in the current planar device technologies. High electron mobilities and sheet carrier densities make graphene extremely attractive for high frequency analog applications. One of the remaining challenges is the interaction of epitaxial graphene with the substrate. Typically, much lower free charge carrier mobilities, compared to free standing graphene, and doping, due to charge transfer from the substrate, is reported. Thus, a good understanding of the intrinsic free charge carriers properties and the factors affecting them is very important for further development of epitaxial graphene. III-group nitrides have been extensively studied and already have proven their high efficiency as light sources for short wavelengths. High carrier mobilities and breakdown electric fields were demonstrated for III-group nitrides, making them attractive for high frequency and high power applications. Currently, In-rich InGaN alloys and AlGaN/GaN high electron mobility structures are of high interest for the research community due to open fundamental questions. Electrical characterization techniques, commonly used for the determination of free charge carrier properties, require good ohmic and Schottky contacts, which in certain cases can be difficult to achieve. Access to electrical properties of buried conductive channels in multilayered structures requires modification of samples and good knowledge of the electrical properties of all electrical contact within the structure. Moreover, the use of electrical contacts to electrically characterize two-dimensional electronic materials, such as graphene, can alter their intrinsic properties. Furthermore, the determination of effective mass parameters commonly employs cyclotron resonance and Shubnikov-de Haas oscillations measurements, which require long scattering times of free charge carriers, high magnetic fields and low temperatures. The optical Hall effect is an external magnetic field induced optical anisotropy in? conductive layers due to the motion of the free charge carriers under the influence of the Lorentz force, and is equivalent to the electrical Hall effect at optical frequencies. The optical Hall effect can be measured by generalized ellipsometry and provides a powerful method for the determination of free charge carrier properties in a non-destructive and contactless manner. In principle, a single optical Hall effect measurement can provide quantitative information about free charge carrier types, concentrations, mobilities and effective mass parameters at temperatures ranging from few kelvins to room temperature and above. Further, it was demonstrated that for transparent samples, a backside cavity can be employed to enhance the optical Hall effect. Measurement of the optical Hall effect by generalized ellipsometry is an indirect technique requiring subsequent data analysis. Parameterized optical models are fitted to match experimentally measured ellipsometric data by varying physically significant parameters. Analysis of the optical response of samples, containing free charge carriers, employing optical models based on the classical Drude model, which is augmented with an external magnetic field contribution, provide access to the free charge carrier properties. The main research results of the graduate studies presented in this licentiate thesis are summarized in the five scientific papers. Paper I. Description of the custom-built terahertz frequency-domain spectroscopic ellipsometer at Link?ping University. The terahertz ellipsometer capabilities are demonstrated by an accurate determination of the isotropic and anisotropic refractive indices of silicon and m-plane sapphire, respectively. Further, terahertz optical Hall effect measurements of an AlGaN/GaN high electron mobility structures were employed to extract the two-dimensional electron gas sheet density, mobility and effective mass parameters. Last, in-situ optical Hall effect measurement on epitaxial graphene in a gas cell with controllable environment, were used to study the effects of environmental doping on the mobility and carrier concentration. Paper II. Presents terahertz cavity-enhanced optical Hall measurements of the monolayer and multilayer epitaxial graphene on semi-insulating 4H-SiC (0001) substrates. The data analysis revealed p-type doping for monolayer graphene with a carrier density in the low 1012 cm?2 range and a carrier mobility of 1550 cm2/V·s. For the multilayer epitaxial graphene, n-type doping with a carrier density in the low 1013 cm?2 range, a mobility of 470 cm2/V·s and an effective mass of (0.14 ± 0.03) m0 were extracted. The measurements demonstrate that cavity-enhanced optical Hall effect measurements can be applied to study electronic properties of two-dimensional materials. Paper III. Terahertz cavity-enhanced optical Hall effect measurements are employed to study anisotropic transport in as-grown monolayer, quasi free-standing monolayer and quasi free-standing bilayer epitaxial graphene on semi-insulating 4H-SiC (0001) substrates. The data analysis revealed a strong anisotropy in the carrier mobilities of the quasi freestanding bilayer graphene. The anisotropy is demonstrated to be induced by carriers scattering at the step edges of the SiC, by showing that the mobility is higher along the step than across them. The scattering mechanism is discussed based on the results of the optical Hall effect, low-energy electron microscopy, low-energy electron diffraction and Raman measurements. Paper IV. Mid-infrared spectroscopic ellipsometry and mid-infrared optical Hall effect measurements are employed to determine the electron effective mass in an In0.33Ga0.67N epitaxial layer. The data analysis reveals slightly anisotropic effective mass and carrier mobility parameters together with the optical phonon frequencies and broadenings. Paper V. Terahertz cavity-enhanced optical Hall measurements are employed to study the free charge carrier properties in a set of AlGaN/GaN high electron mobility structures with modified interfaces. The results show that the interface structure has a significant effect on the free charge carrier mobility and that the sample with a sharp interface between an AlGaN barrier and a GaN buffer layers exhibits a record mobility of 2332±73 cm2/V·s. The determined effective mass parameters showed an increase compared to the GaN value, that is attributed the the penetration of the electron wavefunction into the AlGaN barrier layer.
機(jī)譯：在過去的三十年中，基于硅的電子產(chǎn)品的發(fā)展徹底改變了我們的日常生活。如今，硅基器件的工作接近其理論極限，這正成為進(jìn)一步發(fā)展的瓶頸。特別地，對于高頻和高功率電子學(xué)的增長領(lǐng)域，Si不能提供所需的性能。能夠提供高電流密度，載流子遷移率和高擊穿電場的材料的開發(fā)對于電子技術(shù)的發(fā)展至關(guān)重要。在半絕緣碳化硅襯底上生長的外延石墨烯具有集成在當(dāng)前平面器件技術(shù)中的巨大潛力。高電子遷移率和薄片載流子密度使石墨烯對于高頻模擬應(yīng)用極為有吸引力。剩下的挑戰(zhàn)之一是外延石墨烯與襯底的相互作用。通常，據(jù)報(bào)道，與自支撐石墨烯相比，自由電荷載流子遷移率要低得多，并且由于電荷從基底轉(zhuǎn)移而摻雜。因此，對內(nèi)在自由電荷載流子特性及其影響因素的良好理解對于進(jìn)一步發(fā)展外延石墨烯非常重要。 III族氮化物已經(jīng)被廣泛研究，并且已經(jīng)證明了其作為短波長光源的高效率。 Ⅲ族氮化物具有高載流子遷移率和擊穿電場，對高頻和高功率應(yīng)用具有吸引力。當(dāng)前，由于存在開放的基本問題，富In的InGaN合金和AlGaN / GaN高電子遷移率結(jié)構(gòu)引起了研究界的廣泛關(guān)注。通常用于確定自由電荷載流子特性的電氣表征技術(shù)需要良好的歐姆接觸和肖特基接觸，在某些情況下可能難以實(shí)現(xiàn)。要訪問多層結(jié)構(gòu)中的埋入式導(dǎo)電通道的電特性，需要修改樣品并充分了解結(jié)構(gòu)中所有電觸點(diǎn)的電特性。此外，使用電觸點(diǎn)來電氣表征二維電子材料（例如石墨烯）可以改變其固有特性。此外，有效質(zhì)量參數(shù)的確定通常采用回旋共振和Shubnikov-de Haas振蕩測量，這要求自由電荷載流子的散射時(shí)間長，高磁場和低溫。光學(xué)霍爾效應(yīng)是由于洛倫茲力的作用下自由載流子的運(yùn)動(dòng)而在導(dǎo)電層中由外部磁場引起的光學(xué)各向異性，并且等效于光學(xué)頻率下的電霍爾效應(yīng)。光學(xué)霍爾效應(yīng)可以通過廣義橢圓偏振法進(jìn)行測量，并提供了一種以非破壞性和非接觸方式確定自由電荷載流子性質(zhì)的有力方法。原則上，單個(gè)光學(xué)霍耳效應(yīng)測量可以在幾開爾文到室溫及以上的溫度范圍內(nèi)提供有關(guān)自由電荷載流子類型，濃度，遷移率和有效質(zhì)量參數(shù)的定量信息。此外，已證明對于透明樣品，可以采用背面腔來增強(qiáng)光學(xué)霍爾效應(yīng)。通過廣義橢圓偏振法測量光學(xué)霍爾效應(yīng)是一項(xiàng)間接技術(shù)，需要后續(xù)的數(shù)據(jù)分析。通過改變物理上重要的參數(shù)，可以對參數(shù)化的光學(xué)模型進(jìn)行擬合，以匹配實(shí)驗(yàn)測量的橢偏數(shù)據(jù)。使用基于經(jīng)典Drude模型的光學(xué)模型對包含自由載流子的樣品的光學(xué)響應(yīng)進(jìn)行分析，并通過外部磁場的作用來增強(qiáng)對自由載流子屬性的訪問。五篇科學(xué)論文總結(jié)了本許可論文中研究生研究的主要研究成果。論文I.Link?ping大學(xué)定制的太赫茲頻域光譜橢圓儀的描述。太赫茲橢偏儀的功能通過分別分別確定硅和m面藍(lán)寶石的各向同性和各向異性折射率來證明。此外，采用AlGaN / GaN高電子遷移率結(jié)構(gòu)的太赫茲光學(xué)霍爾效應(yīng)測量來提取二維電子氣片密度，遷移率和有效質(zhì)量參數(shù)。最后，在可控環(huán)境中對氣室中外延石墨烯進(jìn)行原位光學(xué)霍爾效應(yīng)測量用來研究環(huán)境摻雜對遷移率和載流子濃度的影響。論文二。提供了半絕緣4H-SiC（0001）基板上單層和多層外延石墨烯的太赫茲腔增強(qiáng)光學(xué)霍爾測量。數(shù)據(jù)分析表明，單層石墨烯的p型摻雜具有低1012 cm-2范圍的載流子密度和1550 cm2 / V·s的載流子遷移率。對于多層外延石墨烯，提取了n型摻雜，其載流子密度在1013 cm-2的較低范圍內(nèi)，遷移率為470 cm2 / V·s，有效質(zhì)量為（0.14±0.03）m0。這些測量表明，腔增強(qiáng)的光學(xué)霍爾效應(yīng)測量可用于研究二維材料的電子特性。論文三。太赫茲腔增強(qiáng)光學(xué)霍爾效應(yīng)測量用于研究在半絕緣4H-SiC（0001）襯底上生長的單層，準(zhǔn)自支撐單層和準(zhǔn)自支撐雙層外延石墨烯中的各向異性傳輸。數(shù)據(jù)分析表明，準(zhǔn)獨(dú)立式雙層石墨烯的載流子遷移率具有很強(qiáng)的各向異性。各向異性是由載流子在SiC臺階邊緣處的散射引起的，這表明沿著臺階的遷移率高于穿過臺階的遷移率?；诠鈱W(xué)霍爾效應(yīng)，低能電子顯微鏡，低能電子衍射和拉曼測量的結(jié)果討論了散射機(jī)理。論文四。采用中紅外光譜橢圓儀和中紅外光學(xué)霍爾效應(yīng)測量來確定In0.33Ga0.67N外延層中的電子有效質(zhì)量。數(shù)據(jù)分析揭示了各向異性的有效質(zhì)量和載流子遷移率參數(shù)，以及光學(xué)聲子頻率和展寬。論文V.太赫茲腔增強(qiáng)光學(xué)霍爾測量用于研究一組具有修飾界面的AlGaN / GaN高電子遷移率結(jié)構(gòu)中的自由載流子特性。結(jié)果表明，界面結(jié)構(gòu)對自由電荷載流子遷移率具有顯著影響，并且在AlGaN勢壘和GaN緩沖層之間具有清晰界面的樣品表現(xiàn)出記錄的遷移率2332±73 cm2 / V·s。所確定的有效質(zhì)量參數(shù)顯示出與GaN值相比有所增加，這歸因于電子波功能滲透到AlGaN勢壘層中。

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    Armakavicius, Nerijus;

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  • 年度 2017
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  • 正文語種 eng
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