热原子层沉积钛掺杂氧化镓薄膜的光学性能

Optical Properties of Titanium-doped Gallium Oxide Thin Films by Thermal Atomic Layer Deposition

在250℃的低温下,以三甲基镓、四(二甲氨基)钛为前躯体源,O_(3)为反应气体,采用热原子层沉积制备了Ti掺杂Ga_(2)O_(3)(TGO)薄膜。Ga_(2)O_(3)和TiO_(2)的生长速率分别为0.037 nm/cycle和0.08 nm/cycle,TGO薄膜厚度低于理论计算值。X射线光电子能谱仪测试结果表明膜中Ti浓度随Ga_(2)O_(3)/TiO_(2)循环比减少而增加,O 1s、Ga 2p和Ti 2p的峰位置向较低的结合能移动,这是因为Ti原子取代了Ga原子的某些位点引起了结合能降低,表明Ti元素成功掺杂到Ga_(2)O_(3)薄膜中。TiO_(2)和Ga_(2)O_(3)的芯能级光谱分析表明薄膜中存有Ti^(4+)和Ga^(3+)离子。TGO薄膜的O 1s芯能级光谱中Ga-O键随着Ti-O键含量增加而下降,表明TGO薄膜中形成Ga_(2)O_(3)-TiO_(2)复合材料。掠入射X射线衍射图中没有出现衍射峰,表明沉积的Ga_(2)O_(3)和TGO薄膜为非晶态。原子力显微镜观察到薄膜表面平整光滑,均方根粗糙度为0.377 nm,这得益于原子层沉积逐层生长的优势。TGO薄膜在可见光区表现出较高的透明度,对紫外光强烈吸收。随着Ti掺杂浓度的增加,TGO薄膜的折射率由于化学变化从1.75增加到1.99,紫外光区消光系数增大引起透过率减小,吸收边缘出现了红移,光学带隙从4.9 eV减小到4.3 eV。分光光度法和X射线光电子能谱法测定薄膜光学带隙所得的结果一致。

Gallium oxide(Ga_(2)O_(3))is a wide bandgap(4.8 eV)semiconductor oxide with the advantages of high transparency and excellent chemical and thermal stability.Therefore,Ga_(2)O_(3) thin film has a wide range of applications in metal oxide field effect transistors,photodetectors and so on.However,the large bandgap of Ga_(2)O_(3) is unfavorable to the conductivity,which limits the application of Ga_(2)O_(3) film in optoelectronic devices.The optical and electrical properties of Ga_(2)O_(3) can be significantly improved by elemental doping,thereby enhancing device performance.The lattice deformation of Ti-doped Ga_(2)O_(3)(TGO)is small due to the close matching of the Shannon ion radii(0.0605 nm,0.042 nm)of Ti^(4+)in octahedral and tetrahedral coordination with Ga^(3+)(0.062 nm,0.047 nm).The reported plasma-enhanced atomic layer deposition at 120℃for the preparation of TGO films requires four precursors:triethyl gallium,oxygen plasma,titanium tetraisopropoxide(TTIP)and H2O.Using H2O as an oxidizer requires long purging times after water vapor exposure and brings in hydroxyl(-OH)impurities at deposition temperatures below 150℃.Compared with H2O,O_(3) has stronger oxidability and higher volatility and does not introduce impurities.In order to avoid the problems caused by using H2O as precursor.TiO_(2),Ga_(2)O_(3) and TGO films are prepared by thermal atomic layer deposition using Trimethylgallium(TMG)and Tetrakis-dimethyl-amido Titanium(TDMAT)as precursor sources and O_(3) as reaction gas at 250℃.The Ti-doped Ga_(2)O_(3) concentration is adjusted by designing the Ga_(2)O_(3)/TiO_(2) cycle ratio.TGO thin films form sandwich structure through different cycles(9,6 and 3)of Ga_(2)O_(3) and 1 cycle of TiO_(2).The growth rates of Ga_(2)O_(3) and TiO_(2) measured by spectroscopic ellipsometry are 0.037 nm/cycle and 0.08 nm/cycle,respectively.The growth rate of TGO film is lower than the theoretical calculated value due to the delayed growth of Ga_(2)O_(3) nucleation caused by the decrease of surface reactive site density after TiO_(2) growth.The results of X-ray photoelectron spectroscopy show that the concentration of Ti in the film increases with the decrease of Ga_(2)O_(3)/TiO_(2) cycle ratio,the binding energy of O 1s,Ga 2p and Ti 2p shifts to the lower,which is attributed to the replacement of some sites of Ga by Ti atoms,indicating that Ti elements are successfully doped into Ga_(2)O_(3) films.The core level spectra of TiO_(2) and Ga_(2)O_(3) show the presence of Ti^(4+)and Ga^(3+)ions in the films.In the O 1s core level spectra of TGO films,Ga-O bonding decreases with increasing Ti-O bonding content,indicating the formation of Ga_(2)O_(3)-TiO_(2) composites in the TGO films.The absence of diffraction peaks in the grazing incidence X-ray diffraction spectra indicates that the deposited Ga_(2)O_(3) and TGO films are amorphous.The surface root mean square roughness of 0.377 nm is observed by atomic force microscopy,indicating that the surface of the film is flat and smooth.This is attributed to the layer-by-layer growth of atomic layer deposition with the advantage of atomic-level thickness control.The TGO films exhibit high transparency in the visible region and strongly absorb ultraviolet light.With the increase of Ti doping concentration,the refractive index of TGO films increases from 1.75 to 1.99 due to chemical changes,the transmittance decreases due to the increase of extinction coefficient in the ultraviolet region,the absorption edge appears red-shifted and the optical bandgap decreases from 4.9 eV to 4.3 eV.The reduced band gap of TGO films can extend the sensitive region of optoelectronic devices to longer wavelengths.The optical band gap of thin films measured by spectrophotometric and X-ray photoelectron spectroscopy shows consistent results.The comprehensive analysis shows that Ti doping has a significant impact on the optical properties of Ga_(2)O_(3).