【作者】 杨帆；

【导师】 马瑾；

【作者基本信息】 山东大学 ， 微电子学与固体电子学， 2009， 博士

【摘要】 近几年来,由于透明氧化物半导体材料在发光二极管、激光器、紫外探测器、透明薄膜晶体管、薄膜太阳能电池及平面显示等方面具有广阔的应用前景,因而引起了人们的广泛关注。In₂O₃和Ga₂O₃是具有直接带隙的透明氧化物半导体材料,带隙宽度分别为3.6eV和4.9eV。In₂O₃作为一种重要的n型半导体材料,特别是锡掺杂的氧化铟（ITO）薄膜,因其优良的光电性能而被广泛应用于太阳能电池及平面显示等领域。ITO及目前大量使用的透明氧化物半导体材料均存在着一个共同的问题,那就是:光学带隙一般小于3.7eV。随着透明光电子学和光电子器件的不断发展,要求氧化物半导体材料的透明区域向紫外扩展。β-Ga₂O₃是一种很有潜力的深紫外透明半导体材料,但是其过高的带隙宽度使得β-Ga₂O₃作为深紫外透明导电薄膜材料存在一定的困难。因此,有必要研究一种新型的可调制带隙宽度的透明半导体薄膜材料。镓铟氧化物(Ga_2-2xIn_2xO₃)材料可以认为是Ga₂O₃和In₂O₃两种材料的合金,其带隙宽度可以通过改变样品中Ga与In的比例,在3.6eV到4.9 eV之间调制,因此是一种很有希望的紫外光电材料。目前,国际国内对Ga_2-2xIn_2xO₃（0.1≤x≤0.9）薄膜材料的研究工作尚缺乏深入系统的研究。通过研究制备不同组分的Ga_2-2xIn_2xO₃薄膜,并对制备薄膜的生长、组分、晶格结构、结构相变、电学和光学性质进行较为系统的研究,将会为该材料在透明电子器件及紫外电子器件上的应用奠定基础。在这样的背景下,本论文开展了镓铟氧化物薄膜的MOCVD法制备及其性质的研究。本论文分为三大部分。第一部分采用MOCVD法制备了Ga₂O₃薄膜,研究了薄膜的结构及光学性质;第二部分制备出了高质量的In₂O₃单晶薄膜,研究了薄膜的结构、光学及电学性质,并首次观察到α-Al₂O₃衬底上制备的In₂O₃薄膜的带间跃迁光致发光现象;第三部分采用MOCVD法制备了Ga_2-2xIn_2xO₃（0.1≤x≤0.9）薄膜和Sn掺杂的Ga_1.4In_0.6O₃薄膜,较为系统的研究了薄膜的结构、光学以及电学性质。第一部分主要的研究工作及结果如下:采用MOCVD法,以高纯Ga（CH₃）₃作为镓源,高纯O₂作为氧源,高纯N₂作为载气,在600℃衬底温度下成功地在蓝宝石（0001）衬底上制备出了Ga₂O₃薄膜,研究了退火处理对制备薄膜结构和光学性质的影响。XRD与AFM测试结果表明,制备的Ga₂O₃薄膜呈现非晶结构;经退火处理后薄膜结晶程度得到明显改善,由非晶转变为了具有β-Ga₂O₃结构的多晶薄膜;且随着退火温度的升高,薄膜的择优取向性增强,晶粒增大,薄膜结晶质量变好。样品的透射谱测量结果表明,退火前后薄膜在可见光区的透过率均超过95%;退火处理后,薄膜样品在近紫外区的透过率得到显著提高,样品的吸收边向短波长方向略有移动,未退火及800℃退火后样品的光学带隙分别为4.71eV和4.89eV。第二部分主要的研究工作及结果如下:采用MOCVD方法,以高纯In（CH₃）₃作为铟源,高纯O₂作为氧源,高纯N₂作为载气,首次成功地在蓝宝石（0001）衬底上制备出了高质量的In₂O₃单晶外延薄膜。对不同衬底温度下制备薄膜的晶格结构、光学和电学性质进行了研究,并对In₂O₃单晶薄膜的外延生长机理进行了分析。XRD测试结果表明,制备薄膜具有氧化铟的立方结构,且具有沿（111）的单一取向。650℃衬底温度下制备的薄膜具有最好的结晶质量。HRXRD和HRTEM测试结果表明在650℃衬底温度下制备的样品为具有立方结构的单晶薄膜,且薄膜与蓝宝石（0001）衬底存在In₂O₃（111）//Al₂O₃（0001）,In₂O₃[（?）0]//Al₂O₃[11（?）0]的外延关系。薄膜（222）面的ω摇摆曲线半高宽仅为0.14°,这一结果显示制备样品具有很好的单晶质量。样品的透射谱测量结果表明,不同衬底温度下制备的薄膜在可见光范围的透过率均超过90%;650℃衬底温度下制备薄膜的光学带隙为3.66eV。样品的霍尔测试结果表明,制备In₂O₃薄膜的最低电阻率为5.10×10^-3Ω·cm、霍尔迁移率最高达35.2cm²·v^-1·s^-1,载流子浓度为1.61×10¹⁹ cm^-3-4.38×10¹⁹ cm^-3。室温下对650℃衬底温度下制备的In₂O₃薄膜进行光致发光测量,首次在337nm附近观测到一个强而尖锐的紫外发光峰。低温测量时,337 nm附近发光峰结构没有出现明显变化,但发光峰的位置出现不同程度蓝移。337 nm附近的紫外发光峰被归因于电子从导带底到价带顶的跃迁。第三部分主要的研究工作及结果如下:1.首次采用MOCVD法,以高纯Ga（CH₃）₃作为镓源,高纯In（CH₃）₃作为铟源,在550℃衬底温度下蓝宝石（0001）衬底上制备出了Ga_2-2xln_2xO₃（0.1≤x≤0.9）薄膜。对制备薄膜的晶格结构、结构相变、组分、光学和电学性质进行了系统研究,并研究了高温退火处理对生长Ga_2-2xIn_2xO₃薄膜的结构和光电性能的影响。XRD测试结果表明,随着样品中In含量的降低,Ga_2-2xIn_2xO₃薄膜的结构先由立方In₂O₃结构转变为非晶,进而转变为多晶β-Ga₂O₃结构;700℃退火处理后薄膜结构得到明显改善,对于富In及Ga与In比例相同的样品其结晶质量得到了明显的提高,而对于富Ga的样品其择优取向发生明显的改变。对样品的XPS和RBS的分析结果表明,采用MOCVD方法制备的Ga_2-2xIn_2xO₃薄膜的组分与我们的实验初始设定值是一致的。样品的透射谱测量结果表明,制备薄膜样品在可见光区的透过率在扣除了衬底的影响后均达到了85%以上,带隙宽度随样品中Ga含量的改变在3.76 eV到4.43eV之间变化;经过退火处理后,薄膜样品在近紫外区的透过率得到显著提高,样品的带隙宽度明显增大。对薄膜的电学性能测试表明,Ga_2-2xIn_2xO₃薄膜的电阻率随Ga含量的增大由3.414×10^-3Ω·cm单调增加到6.71×10⁴Ω·cm,且经退火处理后薄膜的电阻率进一步升高。2.采用MOCVD法,以高纯Ga（CH₃）₃作为镓源,高纯In（CH₃）₃作为铟源,在700℃衬底温度下蓝宝石（0001）衬底上制备出了Ga_2-2xIn_2xO₃（0.1≤x≤0.9）薄膜。对制备薄膜的晶格结构、组分、光学和光致发光性质进行了系统研究。XRD和HRTEM测试结果表明,随着样品中In含量的增加,Ga_2-2xIn_2xO₃薄膜的结构先由多晶β-Ga₂O₃结构转变为立方In₂O₃结构多晶薄膜,进而转变为具有In₂O₃（111）单一择优取向的单晶结构薄膜。样品的透射谱测量结果表明,所有制备样品在可见光区的平均透过率均超过90%,薄膜的带隙宽度随Ga含量的增加从3.72 eV单调增加至4.58eV。室温下对不同铟组分的薄膜样品进行光致发光测量,对于In含量较高（x≥0.5）的样品,在338 nm（3.67eV）附近观测到了紫外发光峰,并且紫外发光峰的相对强度随In含量的减小而减弱;而当Ga含量较高（0<0.5）时,在332nm（3.73eV）附近观测到一个较为尖锐的紫外发光峰,并且随着Ga含量的增加发光峰的强度增强。对x=0.3的样品进行低温测量时,332 nm附近发光峰的位置出现不同程度蓝移。尝试性地对不同组分Ga_2-2xIn_2xO₃薄膜的光致发光机制进行了分析,338 nm处发光峰的起源被归因于电子从导带到价带的跃迁;332 nm处发光峰可能起源于Ga_2-2xIn_2xO₃中β-Ga₂O₃晶粒导带到受主的跃迁发光。3.首次采用MOCVD法,以高纯Ga（CH₃）₃作为镓源,高纯In（CH₃）₃作为铟源,高纯Sn（C₂H₅）₄作为锡源,在550℃衬底温度下蓝宝石（0001）衬底上制备出了不同Sn掺杂浓度的Ga_1.4In_0.6O₃(Ga_1.4In_0.6O₃:Sn)薄膜,研究了掺杂浓度对薄膜结构和光电性能的影响。对不同掺杂浓度Ga_1.4In_0.6O₃:Sn薄膜样品的结构性质及形貌分析表明,当掺杂浓度超过3%时,样品由非晶结构转变为具有单斜Ga₂O₃结构的多晶薄膜;并且随着Sn掺杂浓度的增加,Ga_1.4In_0.6O₃:Sn薄膜的晶粒明显增大,结晶质量得到进一步的改善。样品的透射谱和霍尔测试结果表明,制备Ga_1.4In_0.6O₃:Sn薄膜的光学带隙、电阻率、载流子浓度和霍尔迁移率均随掺杂浓度不同而发生变化。其中掺杂比例为5%的Ga_1.4In_0.6O₃:Sn薄膜具有最佳的光电性能,其可见光透过率超过85%,光学带隙为3.85 eV;室温下该样品的电阻率和霍尔迁移率分别达到了4.9×10^-3Ω·cm和21.4 cm²·v^-1·s^-1。更多还原

【Abstract】 In recent years, intense interest has been paid to transparent oxide semiconductors （TOSs） owing to their potential applications in light-emitting diodes, laser diodes, ultraviolet （UV） detectors, transparent thin-film transistors, flat panel displays and thin-film solar cells. Both In₂O₃ and Ga₂O₃ are good TOSs with direct band gap energy of 3.6 and 4.9 eV, respectively. In₂O₃ is a very important n-type transparent semiconductor and has been widely used in many fields such as solar cells and flat-panel displays owing to its excellent optoelectrical properties. The optical bandgap of the conventional transparent oxide semiconductors such as ITO films is usually smaller than 3.7eV. A need for materials which are transparent in the UV region has emerged with the development of transparent optoelectronics and optoelectronic devices. Ga₂O₃ with the monoclinic structure （β-Ga₂O₃） is a promising candidate for deep-ultraviolet （UV） transparent conductive oxides. But Ga₂O₃ as a deep-UV transparent conductive material presents some difficulties due to its over high band gap energy. Hence, it is necessary to investigate novel bandgap tunable TOSs materials. (Ga_2-2xIn_2xO₃) is considered to be an alloy of In₂O₃ and Ga₂O₃. The band gap of (Ga_2-2xIn_2xO₃) could be tuned from 3.6eV to 4.9eV by controlling the composition of the films, and this make (Ga_2-2xIn_2xO₃) a promising UV photoelectric material. Until now, there is still lack of systematical investigation for properties of (Ga_2-2xIn_2xO₃) （0.1≤x≤0.9） films. To prepare Ga_2-2xIn_2xO₃ films and investigate the growth mechanism, film composition, crystal structure, phase transition and optoelectrical properties of the films will lift this material to higher potential in the field of transparent and UV optoelectronic devices. Under such background, the preparation and characterization of Ga_2-2xIn_2xO₃ films are investigated in this article.The content of this article consists of three parts. In the first part, the Ga₂O₃ films were deposited by MOCVD. The structural and optical properties of the films were investigated. In the second part, high quality In₂O₃ films were prepared by MOCVD. The structural, optical and electrical properties of the films were investigated in detail, and the PL phenomena of band-to-band transition in In₂O₃ films prepared onα-Al₂O₃ were first observed. In the third part, Ga_2-2xIn_2xO₃ （0.1≤x≤0.9） and Sn doped Ga_1.4In_0.6O₃ films were prepared by MOCVD. The structural, optical and electrical properties of the films were investigated systematically.The major research work and results of the first part are as follows:The Ga₂O₃ films were successfully prepared onα-Al₂O₃ （0001） substrates at 600℃by MOCVD. Ultra high purity Ga（CH₃）₃, N₂ and O₂ were used as the organometallic source, carrier gas, and oxidant, respectively. The film deposition method was described in this part and the annealing effect on the structural and optical properties of deposited films were investigated. XRD and AFM results indicated that the deposited film was amorphous and transformed to polycrystalline structure ofβ-Ga₂O₃ film after annealing. The increase of the annealing temperature could enhance the preferred orientation, enlarge the grain size and improve the crystallization of the film. The optical transmittance spectra showed that the average transmittance in the visible range for the as-deposited and annealed films was over 95%. The transmittance of the annealed film in the near UV region was obviously improved and the absorption edge shifted to shorter wavelength. The optical gaps for the as-grown and annealed samples were 4.71 eV and 4.89eV, respectively.The major research work and results of the second part are as follows:1. High-quality In₂O₃ epitaxial films were successfully prepared onα-Al₂O₃ （0001） substrates by MOCVD for the first time. Ultra high purity In（CH₃）₃, N₂ and O₂ were used as the organometallic source, carrier gas, and oxidant, respectively. The structural, optical and electrical properties of the films as a function of substrate temperature were investigated, and the epitaxial growth mechanism of single crystalline In₂O₃ films was analyzed. The XRD results indicated that the prepared samples are pure In₂O₃ films of body-centered cubic （bcc） structure with a single orientation of （111） direction. The sample prepared at 650℃substrate temperature has the best crystalline quality. HRXRD and HRTEM results indicated that the sample was single crystalline film having the bcc structure of pure In₂O₃ with a clear epitaxial relationship of In₂O₃ （111）//A1₂O₃ （0001） with In₂O₃ [（?）0]//Al₂O₃[11（?）0]. The fullwidth at half maximum of the to-rocking curve of （222） reflection is only 0.14°, indicating a high quality crystalline structure. The optical transmittance spectra showed that the average transmittance in the visible range for all films was over 90%. The optical band gap for the 650℃prepared sample was 3.66eV. The Hall measurements indicated that the lowest resistivity and the highest Hall mobility for the obtained films were 5.10×10^-3Ω·cm and 35.2 cm²·v^-1·s^-1, respectively. The carrier concentration varied between 1.61×10¹⁹ cm^-3 and 4.38×10¹⁹ cm^-3.The PL spectra of the In₂O₃ film deposited at 650℃on sapphire （0001） were measured. An intense and sharp UV PL peak near 337 nm was observed at room temperature for the first time. While measured at low temperature, there were no obvious change for the structure of this UV PL peak, but the peak position shifted towards higher energy. The origin of the UV PL peak near 337 nm was ascribed to the electron transition from the conduction band to the valence band.The major research work and results of the third part are as follows:1. The Ga_2-2xln_2xO₃（0.1≤x≤0.9） films were prepared onα-Al₂O₃ （0001） substrates at 550℃by MOCVD for the first time. High purity Ga（CH₃）₃ and In（CH₃）₃ were used as the organometallic sources. The crystal structures, structural phase transitions, optical and electrical properties as well as compositions of these films were investigated in detail. In addition, the annealing effect on the structural and optical properties of deposited films was discussed. The XRD results indicated that, as the In content decreases, the structure of the films changed from bcc to amorphous, and finally to monoclinic structure ofβ-Ga₂O₃. After annealing at 700℃, the crystalline quality of all the films was improved and the preferred orientation for the Ga-rich film changed obviously. The compositions of Ga_2-2xIn_2xO₃ films calculated from the XPS and RBS spectra were consist with the experiment enactment values. The optical transmittance spectra showed that the average transmittance in the visible range for all prepared films was over 85%. The Eg of the Ga_2-2xIn_2xO₃ films could be tuned from 3.76 eV to 4.43eV by controlling the Ga content. The transmittance of the films in the near UV region was obviously improved and the band gap energy increased after annealing at 700℃. The resistivity of Ga_2-2xIn_2xO₃ films increased monotonously from 3.414×10^-3Ω·cm to 6.71×10⁴Ω·cm and increased further after the annealing treatment.2. The Ga_2-2xIn_2xO₃（0.1≤x≤0.9） films were prepared onα-Al₂O₃ （0001） substrates at 700℃by MOCVD. High purity Ga（CH₃）₃ and In（CH₃）₃ were used as the organometallic sources. The structural, optical and PL properties as well as compositions of these films were investigated in detail. XRD and HRTEM results indicated that, as the In content increases, the films changed from monoclinic structure ofβ-Ga₂O₃ to bcc structure of polycrystalline In₂O₃, and finally to single crystalline In₂O₃ films with a single orientation of （111） direction. The optical transmittance spectra showed that the average transmittance in the visible range for all prepared films was over 90%, and the band gap of the Ga_2-2xIn_2xO₃ films was tuned from 3.72 eV to 4.58eV by controlling the Ga content.The PL spectra of the Ga_2-2xIn_2xO₃ films were measured at room temperature. For the higher In content （x≥0.5）, an UV PL peak near 338nm （3.67eV） was observed. As decreasing the In content, the relative intensity of the UV emission decreased. And for the lower In content （x<0.5）, only a sharp UV PL peak located at 332nm （3.73eV） was observed, and the relative intensity of the peak increased with decreasing the In content. For x=0.3, while measured at low temperature, the peak position shifted towards higher energy. The UV PL mechanisms of Ga_2-2xIn_2xO₃ thin films were discussed. The origin of the UV PL peak near 338 nm was ascribed to the electron transition from the conduction band to the valence band. The peak near 332nm was originated fromβ-Ga₂O₃ grains, and the PL mechanisms can be attributed to the electron transition from the conduction band to the acceptor level.3. Sn-doped Ga_1.4In_0.6O₃ films were prepared onα-Al₂O₃ （0001） substrates at 550℃by MOCVD for the first time. High purity Ga（CH₃）₃, In（CH₃）₃ and Sn（C₂H₅）₄ were used as the organometallic sources. The structural, optical and electrical properties of the films dependent on doping level were investigated in detail. The structural and morphological analysis revealed that, when the Sn-doping level exceeded 3%, the films changed from amorphous structure to polycrystalline ofβ-Ga₂O₃. The grain size of the films increased obviously on elevating the Sn concentration, which indicated an improvement of crystallization. The optical transmittance spectra and Hall measurements indicated that the optical band gap, resistivity, carrier concentration and Hall mobility for the films changed with the doping level. The 5% Sn-doped Ga_1.4In_0.6O₃ film exhibited the best optical and electrical properties with average transmittance over 85% in the visible range and optical band gap of 3.85 eV. The resistivity and Hall mobility for the 5% Sn-doped film were 4.9×10^-3Ω·cm and 21.4 cm²·v^-1·s^-1, respectively.更多还原