【作者】 陈江博；

【导师】 王丽；

【作者基本信息】 北京工业大学 ， 光学工程， 2012， 博士

【摘要】 近年来，半导体材料由于其独特的性质和广泛的应用，引起了世界范围的关注并得到了蓬勃发展。InGaZnO（IGZO）是一种透明氧化物半导体材料，导电类型为n型。由于其具有高迁移率、较强稳定性和制备的工艺相对简单等优点，备受科研工作者的关注。由于在元素周期表中，In、Ga和Zn等金属元素阳离子具有这样的电子结构：（n-1）d¹⁰ns⁰（其中n≥5），而透明非晶氧化物半导体（TAOS）的导带底（CBM）主要由于金属元素的ns轨道的未被占据态组成。金属阳离子s电子轨道呈球对称分布，并具有较大的半径，所以s轨道相互交叠形成电子的导通路径，这样就会非常有利于电子的传输。国内外研究者已经对IGZO材料进行了相关的理论和实验分析，但仍有一些问题尚不明确，需要逐步地深入探索。该薄膜的制备方法有多种，如脉冲激光沉积（PLD）、等离子体增强化学气相沉积（PECVD）、磁控溅射（Magnetron sputtering）、溶胶-凝胶法（Sol-Gel）等，本文采用的薄膜制备技术为PLD的方法。本文以IGZO的第一性原理计算为理论依据，以IGZO靶材和薄膜的制备原理与方法为主线，兼具靶材和薄膜的测试与表征，对IGZO体系材料进行较为系统的理论分析与实验测试。本文主要的主要研究内容及结果如下：（1）以理想ZnO的第一性原理计算为基础，进行理想ZnO能带结构、态密度和电荷密度等计算，并通过设置氧空位缺陷研究O缺陷对ZnO电子结构的影响；采用第一性原理计算不同构型下的InGaZnO₄材料的电子结构，从理论上分析IGZO材料应当具备的优势与特点。本文采用平面波超软赝势方法，通过广义梯度近似（GGA）中的PBE为交换相关势，分别计算出理想ZnO和氧空位型ZnO的电子结构。计算结果表明，理想ZnO是一种离子性较强、共价键较弱的混合键金属氧化物，并且为直接带隙宽禁带半导体材料；ZnO的禁带宽度为0.805eV，由于过高地估计了Zn3d态的贡献，造成Zn3d态与O2p态的相互作用增大，使得理论值低于实验值。通过氧空位模型ZnO的构建，氧空位的存在使Zn-O相互作用增强，从而导致价带的主要组成部分Zn3d态与O2p态向低能方向移动，而导带的主要组成部分Zn4s态向低能方向的移动相对较小，导致禁带宽度增大；带隙类型由理想ZnO的直接带隙变为间接带隙，氧空位形成了深施主能级。我们通过分别建立列交替型和原子交替型的两种不同单晶（sc-）InGaZnO₄，计算结果表明sc-InGaZnO₄为直接带隙半导体；导带底主要由金属阳离子的s轨道贡献，特别是In-s轨道贡献最大，Ga、Zn原子的随机排列不会对导带内的电荷传输特性产生较大影响；同时使用虚拟晶体近似（VCA）方法计算了InGaZnO₄的电子结构，计算结果具有一定的合理性。（2）调节制靶时In₂O₃、Ga₂O₃和ZnO粉末的摩尔比例，采用固相反应烧结法制备不同组分的IGZO陶瓷靶材，并对靶材进行物相分析、光致发光光谱分析以及拉曼光谱分析等，研究所制备出靶材质量的优劣。随着In₂O₃在起始原料成分比例的不断增大，烧结完成的靶材径向收缩率增大，较高的径向收缩率也显示出固相反应完全的程度与孔隙率的降低。通过对不同组成成分的IGZO陶瓷靶材进行XRD测试，表明各靶材几乎不含原始In₂O₃、Ga₂O₃和ZnO的物相，说明固相反应的进行具有完全性。T₁靶材具有InGaZn₄O₇、InGaZn₅O₈和InGaZn₂O₅不同物相，产生该现象的原因是固相反应中发生相分离和烧结过程中元素的质量损失等。随着ZnO含量的减少和In₂O₃含量的增加，主物相从InGaZn₂O₅逐渐过渡到斜六面体相的InGaZnO₄；当In₂O₃摩尔比例x≥0.6时，物相构成不能确定。通过Raman光谱测试，观察到各靶材几乎不含ZnO块体材料E_2H-E_2L的振动模式，也没有发现具有Raman活性436cm^-1的特征峰，并且只观察到Ga-O、In-O、Zn-O键的振动模式。但随着In₂O₃含量的不断增加，当In₂O₃的含量处于明显的优势地位时，In₂O₃成分的进一步增加不再对晶格的振动特性产生影响，说明虽然物相改变，In₂O₃仍保持着原有的内部结构。然后，对不同靶材采用波长为325nm的He-Cd激光器进行激发，测试靶材的荧光光谱。仅观察到以614～639nm范围内为发光中心的350～900nm宽发光带，荧光峰值强度随着In₂O₃升高而逐渐减弱，荧光中心波长的改变很可能是由于氧空位缺陷的变化引起带隙结构变化导致的，但具体的发光机制需要进一步研究。（3）采用PLD技术，利用不同成分IGZO陶瓷靶材，改变沉积参数（如衬底温度、氧分压等）制备不同成分IGZO薄膜。通过表面轮廓仪、X射线衍射仪（XRD）、原子力显微镜（AFM）、Hall效应测试和X射线光电子能谱（XPS）等表征手段，分别对IGZO薄膜的薄膜厚度、晶体结构、表面形貌、元素成分和光学、电学性质等进行测试与分析，通过薄膜的测试结果分析，反馈并优化薄膜制备技术参数与工艺。采用T₁靶材样品[（Ga₂O₃）0.1（In₂O₃）_0.1（ZnO）_0.8]，在真空室的氧分压为0.5～44.0Pa变化的条件下进行薄膜制备实验。通过XRD测试确认所制备薄膜为非晶结构；通过AFM测试薄膜的表面形貌，表明薄膜均方根粗糙度（RMS）值随着氧分压的增大而增大，在相对较高氧压下RMS值呈现出饱和的趋势；透射光谱测试表明氧空位缺陷或者金属间隙缺陷会占据导带底，而导致带隙的减小；在0.5Pa氧分压下制备的IGZO薄膜载流子浓度为4.3×10¹⁹cm^-3，而其他研究者报道的在氧分压1.0Pa下制备（In₂O₃）0.1（Ga₂O₃）_0.1（ZnO）_0.4薄膜载流子浓度为5.0×10¹⁹cm^-3；从XPS测试结果推断薄膜中的金属元素是以各自氧化态形式存在于薄膜表面，薄膜中In、Ga和Zn的金属间隙的影响可以忽略。采用T8靶材样品[（Ga₂O₃）_0.1（In₂O₃）_0.1（ZnO）_0.8]在室温、氧分压为1.0～15.0Pa变化的的条件下，制备的IGZO薄膜与T₁靶材相关结果极其类似，但当氧分压增大到10.0Pa的时候，薄膜的电学性质极佳，特别是载流子迁移率均取得最大值28.6cm²/（V·s）。比文献报道的载流子浓度1.0×10²⁰cm^-3和迁移率26.0cm²/（V·s）还要高，而且也比我们小组在前期工作中报道的高Zn含量的IGZO薄膜载流子浓度和迁移率高，通过In含量的增加可以明显提高薄膜的载流子浓度和迁移率。我们认为氧空位缺陷在调制薄膜的光学、电学性质上起了非常重要的作用。采用T1靶材样品，在氧分压保持5.0Pa下，在RT和800°C范围内改变衬底温度制备IGZO薄膜。薄膜即使在200°C条件下生长仍为非晶结构；随着基片温度的提升，粒子可以通过较高的基片温度获得更高的能量，最终移动到晶格的适当位置，形成C轴的择优取向，并获得较高的结晶度；制备的IGZO薄膜在可见光光谱范围内都具有比较高的透射率，最高能达到90%；采用van der Pauw法进行Hall效应测试,衬底温度从RT升至400°C，电阻率从0.148Ω·cm降至3.96×10^-3Ω·cm，同时载流子浓度和载流子迁移率分别都有不同程度的提升，这应归因于晶体质量的优化，同时晶粒尺寸的增大减少了薄膜中的晶界，减少了对载流子的散射或捕获。所以基片温度也会对IGZO薄膜的物理性质产生较大影响。通过对IGZO材料理论计算、靶材制备和薄膜制备三方面的研究，探索IGZO或与之相关材料的研究方法，得出的靶材制备和薄膜制备的优化条件可以最终应用于生产与生活当中，特别是对于IGZO薄膜作为沟道层的薄膜晶体管，乃至平板显示与柔性显示技术均具有一定的现实意义与实用价值。更多还原

【Abstract】 Recently, since the semiconductor materials have unique properties and widelyapplications, it attracted more attention all over the world, and obtained greatdevelopment. InGaZnO （IGZO） is a kind of transparent oxide semiconductor, with ntype conductivity. It gradually drew the attentions of many scientists due to highcarrier mobility, better stability, and relatively simple fabrication processes, etc. Inperiodic table, In, Ga, Zn and other metal cations have the electronic structure:（n-1）d¹⁰ns⁰（n≥5）. The conduction band maximum （CBM） were consisted ofunoccupied state in ns-orbital of metal elements for transparent amorphous oxidesemiconductor （TAOS）. The s-orbital of cation performed a spherical symmetry, withlarge radius, so that the overlap among the s-orbital forms of the conductive path, andthis will do good to the transportation of electrons. Although many scientists fromdomestic and abroad investigated in the relative theoretical and experimental analysis,still, there are some questions not clear, needs to be solved. There are many methodsto prepare in the IGZO thin films, such as pulsed laser deposition （PLD）, plasmaenhanced chemical vapor deposition （PECVD）, magnetron sputtering, sol-gel and etc.In the present research, the PLD method was preformed.In this thesis, the research was based on the first principle calculation of IGZO,while the theory and method in the fabrication of IGZO ceramic targets and thin filmsacted as a clue. We did a relatively systematic theoretical analysis and experimentalexaminations. The contents and results of this thesis are as followed:（1） Based on the first principle calculation of ideal ZnO, the band structure,density of state and electron density were calculated out, and by means of introduceoxygen vacancy, we evaluated the effect on the electronic structure of ZnO. TheInGaZnO₄in different model was calculated, we analyzed the advantages andcharacters that IGZO should have.The plane wave pseudo potential method was applied in the electron structurecalculation of ideal ZnO and ZnO with oxygen vacancy. We use thePerdew-Burke-Ernzerhof exchange correlation functional （PBE） approach utilizingthe generalized gradient approximation （GGA） scheme. It indicates that ideal ZnO is akind of mix bonds metal oxide, with stronger electrovalent bond and weaker covalentbond. ZnO is a semiconductor with direct bandgap and wide forbidden band. Theideal ZnO was evaluated a bandgap of0.805eV. Because of overrating thecontribution of Zn3d state, the correlation between Zn3d state and O2p state, thetheoretical value of bandgap is lower than the experimental one. Furthermore, webuilt a model of ZnO with oxygen vacancy. The existence of oxygen vacancy makesthe interaction between Zn and O increase, which will induce the Zn3d state and O2p state shifts into lower energy in the valence band, while the Zn4s state shift to lowerenergy too, but the change is relatively small. This will make the bandgap increase.The direct band changed into indirect band, and finally the oxygen vacancy forms thedeep donor state.We built two different single crystalline （sc-） InGaZnO₄,line alternated modeland atom alternated model. In describing the relationship between valence state andcore state, we utilized pseudopotential method. The valence electron of In, Ga, Zn andO is4d¹⁰5s²5p¹,3d¹⁰4s²4p¹,3d¹⁰4s²and2s²2p⁴, respectively, while the other orbitalelectrons were treated as core electrons. Sc-InGaZnO₄is a direct bandgapsemiconductor, and the CBM is manly contributed by s-orbit of metal ionic, especiallyfor the In-s orbit. The Ga and ZnO randomly arrange will not seriously affect thetransportation of charges in the conduction band. We also use the virtual crystalapproximation （VCA） method to calculate the electron structure of sc-InGaZnO₄, andthe results have certain rationality.（2） The molecular ratio of In₂O₃, Ga₂O₃and ZnO powders were tuned, theIGZO targets with different chemical stoichiometric were prepared by means of solidstate reaction. We did phase analysis, photoluminescence and Raman spectrumdiagnosis to make sure the targets have good quality.The shrinkage in diameter of product IGZO target increased with the In₂O₃content in the starting material increased. The large shrinkage in diameter shows thewell-reacted process and lower amount of porosity. The X-ray diffraction （XRD） wereutilized to measure the samples, and it indicates that all of the ceramic targets almostdon’t contain the starting phase, such as In₂O₃, Ga₂O₃and ZnO. That means theprocess of solid state reaction was thorough. The T1target,（Ga₂O₃）0.1（In₂O₃）_0.1（ZnO）_0.8, has InGaZn₄O₇, InGaZn₅O₈and InGaZn₂O₅phase,which was attributed to phase separation and the loss of each elements in hightemperature, etc. With the increase of In₂O₃content and decrease of ZnO content, themain phase changed from InGaZn₂O₅phase into rhombohedral InGaZnO₄. Whenx≥0.6, the phase can’t be recognize by the present JCPDS cards database. Insuccession, Raman spectrum result shows that we can nearly observe the E_2H-E_2Lvibration mode, which should be obvious in bulk ZnO, also can’t find the symbolpeak at about436cm^-1 which should be related to Raman active. There were onlyGa-O, In-O and Zn-O vibration mode we could observe. With the increasing In₂O₃content, while the In₂O₃content is the main starting material, the further increase willnot affect the vibration mode any more. It indicates that the In₂O₃will maintainprevious internal structure. The photoluminescence （PL） properties of each target wasexcited by the He-Cd laser, with the wavelength of325nm, the board luminescenceband in350～900nm was the only observation, which is centered at614～639nm. The PL intensity became weak while the content of In₂O₃increase, and the centerwavelength altered may be attributed to band structure change due oxygen vacancy,but the luminescence mechanism need further investigating.（3） The PLD technique was applied in IGZO thin film preparation. Throughtuning the deposited parameters （e.g. substrate temperature, oxygen partial pressureand etc.）, the thin film with different chemical stoichiometric were obtained. Thethickness, crystal structure, surface morphology, proportion of each element, opticaland electrical properties were performed by step profiler, X-ray diffraction （XRD）,atomic force microscopy （AFM）, Hall-effect measurement and X-ray photoelectronSpectroscopy （XPS）, and etc, respectively. The optimized technical parameters wereacquired by means of results evaluation.Using target T₁,（Ga₂O₃）_0.1（In₂O₃）_0.1（ZnO）_0.8, we produced a series of IGZO thinfilms at the oxygen partial pressure of0.5～44.0Pa under room temperature. ThroughXRD examination, it was confirmed that all the films were amorphous. AFMobservation results indicate that surface root mean square （RMS） roughness valueincreased with oxygen pressure. Although the RMS changes rapidly at lowerdeposition pressure and seems to be saturated at higher deposition pressure. It indicatefrom transmittance spectrum that a number of oxygen vacancies or metallicinterstitials might occupy the bottom of conduction band, leading to a decreasedbandgap. The carrier concentration for the films with the composition of（In₂O₃）_0.1（Ga₂O₃）_0.1（ZnO）_0.4deposited at1.0Pa oxygen partial pressure is5.0×10¹⁹cm^-3, which is almost same as that for our films deposited at0.5Pa oxygen partialpressure being4.3×10¹⁹cm^-3. Therefore the present XPS results indicated that the In,Ga, Zn elements exist in their oxidized states on the surface of the film, and thenumber of metallic interstitials are negligible in the films. The situation is quitesimilar for the IGZO film fabricated using T₈target,（Ga₂O₃）_0.1（In₂O₃）_0.1（ZnO）_0.8.We acquired good electronic features at an oxygen pressure of10.0Pa, especially forthe maximum carrier mobility of28.6cm²/（V·s）, is higher than the relative literature.The quality is better than our previous work, which contributed to high Zn contentfilms. Controlling In content could sharply increase the carrier concentration andmobility of IGZO thin films. Oxygen vacancies could play an important role indetermining the electrical properties ofthe films.T₁target was adopted to grow IGZO film, and oxygen pressure maintained at5.0Pa, while the substrate temperature altered from room temperature （RT） to800°C.The film grown at substrate temperature of200°C has amorphous structure. With thetemperature increased, species could gain higher energy and finally shifted to properposition of crystal site. In visible spectrum range, the transmittance could achieve at90%. In van der Paw configuration, the hall-effect examination shows that theresistivity dropped from3.96×10^-3Ω·cm to0.148Ω·cm, when the temperature increased from RT to400°C, furthermore, the carrier concentration and mobility haveaugmentation to some extent. This could be attributed to crystal quality optimization,while the grain size increase will reduce the gain boundary, in order to minimize thescattering and capture mechanism. So the substrate temperature could affect thephysical properties of IGZO thin film.We performed three aspects of research, theoretical calculation on IGZO material,target preparation and thin film fabrication, in order to explore the investigationmethods on different subject. The optimized condition in target preparation and thinfilm fabrication will not only be utilized in daily life and production, but also benefitthe thin film transistor fabrication, even in flat panel display and flexiable display,with short and long term interest.更多还原