【作者】 梁康；

【导师】 刘胜； 甘志银；

【作者基本信息】 武汉大学 ， 机械电子工程， 2021， 博士

【摘要】 源炉是分子束外延设备中的核心部件之一,对于分子束外延设备及其成膜性质具有决定性的影响,源炉的性能对于外延薄膜的厚度均匀性、外延薄膜晶体的表面粗糙度和晶体质量以及晶体的界面质量的具有十分重要的影响。在分子束外延微观尺度模拟方面,还需要更加真实的模拟分子束外延工艺的特点;对于薄膜厚度均匀性研究,目前还没有建立起包括分子束外延设备几何结构配置和源炉束流角度分布的外延薄膜厚度均匀性计算模型,也没有运用多参数优化方法对外延薄膜厚度均匀性进行整体优化。本文对分子束外延源炉沉积外延薄膜的微观尺度模拟、源炉沉积外延薄膜的厚度分布均匀性、源炉的温度特性和源炉制造性及可靠性进行了系统的研究。主要的研究内容以及研究结论如下:（1）运用分子动力学模拟粒子从源炉发射并沉积到衬底上形成外延薄膜的过程,计算了粒子的入射角度、衬底旋转速度、衬底的温度和N:Ga束流比对外延薄膜的表面形貌、表面粗糙度和晶体组分的影响。结果表明,在源炉粒子入射角度从15°增加到45°时,外延薄膜的表面粗糙度小幅下降,但是随着入射角度的进一步增加,薄膜的表面粗糙度迅速上升,在源炉粒子入射角度为45°时,薄膜的表面粗糙度最优。外延薄膜中纤锌矿组分随源炉粒子入射角度的增加而有所提升。在N:Ga束流比为0.8时,外延薄膜的表面粗糙度和表面形貌优于N:Ga束流比为1.2时,当N:Ga束流比为1时,随着衬底温度的增加,外延薄膜的表面粗糙度会出现明显的下降。总体而言,在N:Ga束流比为1时,晶体的结晶质量要优于束流比为0.8和1.2时。衬底的旋转能有效的降低外延薄膜的表面粗糙度,在衬底转速从0增加到2 r/ML时,外延Ga N薄膜表面粗糙度迅速的降低,随着衬底旋转速度的进一步增加,外延薄膜表面粗糙度开始增加,外延薄膜的表面粗糙度在衬底转速为2 r/ML时达到最优。外延薄膜晶体中纤锌矿的组分随着衬底旋转速度的增加而有所提升。（2）源炉的束流角度分布是影响分子束外延薄膜厚度分布的主要影响因素之一。对源炉的束流角度分布采用以余弦函数为基函数的多项式来描述,建立了外延薄膜的厚度均匀性计算模型,得到了各参数对于外延薄膜厚度的影响规律。采用粒子群优化算法对上述计算模型中的多参数问题进行了优化,计算得到了实现最优厚度均匀性时所需要的源炉目标束流角度分布。采用蒙特卡洛方法计算了源炉坩埚内壁形状与源炉的束流角度分布之间的关系,运用人工神经网络优化方法根据目标束流角度分布计算了最优的坩埚内壁形状。最后根据模拟计算的结果,采用优化的坩埚内壁形状,进行了蒸发模拟实验,实验中源炉-衬底的相对几何位置关系与仿真中保持一致,对4英寸衬底上沉积的外延薄膜厚度进行测量,瞬时蒸发外延薄膜厚度不均匀度约为2.5%,累积蒸发外延薄膜厚度不均匀度约为3.75%。此外还通过坩埚垂直于衬底蒸发验证了蒙特卡洛计算的有效性。（3）设计了一款热蒸发源,采用分子流模块计算了源炉的蒸发面温度与衬底表面分子入射通量的关系。以Al N生长计算,在外延薄膜生长速率为1μm/h时,分子入射通量约为1.33×10¹⁹/m²s,对应的蒸发材料表面温度约为1421 K（1148℃）。此外还利用有限元分析软件中的动网格模拟技术研究了外延过程中的衬底的旋转对入射分子通量的影响,得到了旋转衬底上外延薄膜的不均匀度为2.89%。采用饱和蒸气压公式和软件中的自由分子流模块对脉冲温度扰动进行了计算,得到了源炉温度波动对束流强度稳定性的影响,在蒸发温度为1421 K时,当温度变化幅值为1 K,变化率为0.07%时,衬底表面分子入射通量的变化率为1.77%,即对于Al元素的蒸发,束流量变化率约为温度变化率的25倍。确定了源炉各零件材料,建立了源炉温度场方程并求解源炉在不同温度下的温度动态调节响应时间常数。（4）对于常见的外延薄膜生长速率（1μm/h）,为避免粒子碰撞散射影响束流角度分布,根据源炉口到衬底距离计算,残余气体的压强需低于10^-3Pa;为保持外延薄膜中杂质元素的浓度小于10¹⁴cm^-3,残余气体的压强需要保持在3.4×10^-8Pa,即要求生长腔体处于超高真空水平。根据Al材料和PBN坩埚从室温加热到873 K（600℃）的仿真计算结果,在源炉中坩埚锥角大于1°时,升温过程中源炉内的蒸发材料存在明显滑移现象,坩埚应力显著降低。对Al源炉在蒸发面温度为1421 K时的源炉关键零部件进行仿真分析,得到了各零件的温度分布和应力分布,仿真结果表明,多层钽箔构成的屏蔽罩具有良好的辐射隔热功能,源炉结构设计可靠,选材合理。通过在生长腔体上设计源炉安装水冷法兰和在生长腔体内放置液氮冷屏可有效的将生长腔体的壁温控制在约343 K（70℃）。更多还原

【Abstract】 The source furnace is one of the most important key components of molecular beam epitaxy equipment.It has a decisive influence on the equipment and the epitaxial film.The performance of the source furnace and the control of the beam flow are of great significance to the thickness uniformity,the surface roughness,the crystal quality and the interface quality of the epitaxial film.For the micro-scale simulation of molecular beam epitaxy,it is necessary to simulate the characteristics of molecular beam epitaxy more realistically.In terms of the research of film thickness uniformity,the calculation model including the geometric configuration of molecular beam equipment and source beam angular distribution has not been established and the multi-parameter optimization method to optimize overall thickness uniformity is not used.The micro-scale simulation,film thickness uniformity,temperature characteristics and reliability related to the source furnace are studied in this dissertation.The main contents and conclusion are as follows:（1）The molecular dynamics is used to simulate the influence of the incident angle of the source furnace particles,the incident speed of the source furnace particles,the rotation speed of the substrate,the temperature of the substrate and the N:Ga beam ratio on the surface morphology,roughness and crystal quality of the epitaxial film.The results show that the surface roughness reaches the optimum at the source furnace incident angle of 45°and the wurtzite component in the epitaxial film increases with the increase of the incident angle of the source furnace particles.When the N:Ga beam ratio is 0.8,the surface roughness and surface morphology of the epitaxial film are better than N:Ga beam ratio of 1.2,and when the N:Ga beam ratio is 1,as the substrate temperature increases,the surface roughness of the epitaxial film will decrease significantly.When the N:Ga beam ratio is 1,the quality of the crystal is better than the beam ratios of 0.8 and 1.2 in overall trend.The rotation of the substrate can effectively reduce the surface roughness of the epitaxial film.The surface roughness of the epitaxial film reaches the optimum when the substrate rotation speed is 2r/ML and the increasing rotating speed could promote the crystal quality.（2）The angular distribution of the source furnace is one of the main factors affecting the thickness distribution of molecular beam epitaxial films.The angular distribution of the source furnace is described by a polynomial with a cosine function as the basis function.A calculation model for the thickness uniformity of the epitaxial film is established,and the influence of each parameter on the thickness uniformity is obtained.The Monte Carlo method is used to calculate the relationship between the shape of the crucible inner wall of the source furnace and the beam angular distribution of the source furnace.The artificial neural network optimization method is used to calculate the optimal inner wall shape of the crucible according to the target beam angular distribution.Finally,the optimized inner wall of the crucible was used to carry out the evaporation simulation experiment according to the simulation results.In the experiment,the thickness of the epitaxial film deposited on the 4-inch substrate was measured.The epitaxial film thickness uniformity of instantaneous evaporation is about 2.5%,and the epitaxial film thickness uniformity of cumulative evaporation is about 3.75%.In addition,the validity of Monte Carlo calculation is verified by the evaporation of the crucible perpendicular to the substrate.（3）A thermal evaporation source was designed.The molecular flow method is used to calculate the relationship between the evaporation surface temperature of the source furnace and the incident flux of molecules on the substrate.Taking the growth of Al N as an example for calculation,when the epitaxial film growth rate is 1μm/h,the molecular incident flux is about 1.33×10¹⁹/m²s,and the corresponding evaporation material surface temperature is about 1421 K.In addition,the moving mesh is used to simulate the rotation of the substrate during the epitaxy process,and the uniformity of the epitaxial film on the rotating substrate is 2.89%.The influence of the source furnace temperature fluctuation on the source beam intensity stability is studied.The saturated vapor pressure formula and the free molecular flow method are used to calculate the temperature pulse disturbance.At the evaporation temperature of1421 K,when the temperature fluctuation amplitude is 1 K,with the corresponding temperature fluctuation rate of 0.07%,the fluctuation rate of incident flux of molecules on the substrate is 1.77%.That is,for the evaporation of Al element,the change rate of beam flux is about 25 times the rate of temperature change.The materials of the parts of the source furnace are selected.The temperature equation of the source furnace is established,and the temperature dynamic response time constant of the source furnace at different temperatures is solved.（4）For the common epitaxial film growth rate of 1μm/h,in order to avoid particle collision scattering affecting the beam angular distribution,the residual gas pressure should be lower than 10^-3Pa according to the distance from the source furnace to the substrate.In order to keep the concentration of impurity elements less than 10¹⁴cm^-3,the residual gas pressure needs to be maintained at 3.4×10^-8Pa,which means that the growth chamber is required to be at an ultra-high vacuum level.According to the simulation results of source furnace heated from room temperature to 873 K using evaporation material of aluminum and crucible material of PBN,when the cone angle of the crucible is greater than 1°,the evaporation material in the source furnace slips significantly and the crucible stress is significantly reduced.The key parts of the Al source furnace are simulated and analyzed at the evaporation surface temperature of 1421 K,and the temperature distribution and stress distribution of each part are obtained.The simulation results show that the shielding cover composed of multilayer tantalum foil has good radiation heat insulation capacity.Source furnace structure design and material selection are reasonable and reliable.By designing and installing a water-cooled flange on the growth chamber and placing a liquid nitrogen cooling screen in the growth chamber,the wall temperature of the growth chamber can be effectively controlled at about 343 K.更多还原