Abstract
With the high potential of mass production by MOCVD process and theprobadility of material and device hybridization for hateroepitaxy, thegrowth of CdTe/GaAs heteroepitaxial films by MOCVD process was studied inthis work. We hope that it can thus increase our understanding of thefundamental mechanisms of this epitaxial process and also the developmentof high quality CdTe/GaAs epilayers.In. thermodynamic calculations, the principle of the Gibbs free energyminimization was used to predict the relationship between the growthcondition and the composition in HgCdTe epilayer as grown by the MOCVDprocess. We had also successfully extended this technique to evaluate theequilibrium point defect concentrations in HgDdTd. This method is morefavorable in the case of multiphase and multicomponent (including opintdefects) equilibrium calculations and it can be easily generalized into acomputer program.The growth of DdTe epitaxial layers on both (100) and (111) GaAs wasstudied experimentally. Our results indicate that the growth rate on (111)GaAs is about twice of that on corresponding (100) GaAs substrates underthe same growth conditions. This is due to the difference of atomic layerdistances and atomic bonding patterns between the (100) and (111)surfaces.In the case of (100)CdTe epitaxial growth, the growth rates remained aboutthe same when DMCd was increased, but it increased to about twice thevalue when DETe was increased instead. However, this increase seemed tostop when the molar ratio of DETe to DMCd went beyond around 3. This isprobably due to the smaller diffusion coefficient and adsorptionequilibrium constant of DETe.The growth rate will in general increase with increasing layer thickness,because the subgrain boundaries and dislocation density will decrease withincreasing layer thickness.There are two growth modes observed in the (111) CdTe/(111) GaAs epitaxialgrowth, which occurs at different growth temperatures. The evidence ofthis change in growth mode can be found in the surface morphology, FWHM ofthe DCRC measurement and the d-spacing variation in these epitaxiallayers. From DCRC measurements, we know that the dislocation density andthe lattice strain in the epitaxial layers caused by the lattice mismatchwill decrease with increasing layer thickness.由於MOCVE 製程具有可量產之潛力，及異質磊晶成長的易於達到材料及元件混成的目的。因此吾人針對MOCVD方法生長CdTd/GaAs磊晶，進行完整性的研究，期望能瞭解CdTe成長機構，俾能對日後磊晶技術的發展提供更深一層的瞭解與幫助。在理論計算方面，吾人利用傳統熱力學的自由能最小化原理，可以預估於一個複雜的MOCVD 反應系統內，生長HgDdTe時，在熱力學平衡的狀況下，必須以何種進料濃度才能獲得預期的組成。吾人並且成功地將傳統化工熱力學的計算方法，推展至缺陷熱力學的計算，並可獲得多相，多成份（包括缺陷物質）在各個平衡狀態下的濃度。在實驗方面，吾人以二甲基鎘及二乙基碲為反應物在(100) GaAs及(111) GaAs基板上成長磊晶，並且觀察到CdTe磊晶在(111) GaAs上的生長速率比在(100) GaAs上快約１倍，此一生長速率的差距，主要是由於不同晶面的層間距不同，以及二者表面原子鍵結方式不同，致使表面反應程序不同所造成。在(100)CdTe 的生長系統中，當吾人增加DMCd的進料量時，生長速率仍保持不變，但是若反過來增加DETe的進料量時，生長速率會隨著增加，直到DETe/DMCd>3 時，才達到一個飽和的狀態，這是由於DETe的擴散速率及吸附力較DMCd小所造成的。另一方面生長速率也隨著磊晶厚度的增加而增加，這是由於磊晶內部由界面上晶格不匹配漸漸消失，另一方面次晶粒擴大，次晶粒界減少也使差排密度減少，而促成生長速率的增加。在(111)DdTe(111)GaAs的成長系統中，吾人可觀察到兩種不同的生長機構。主要的原因是由於生長溫度不同，造成提供堆疊原子的量及表面擴散速率不同所致。這種生長機構的變化，並且可以在表面型態，雙晶X-光繞射的半高寬及磊晶本身面間距的變化獲得印證。由DCRC測量，並可獲知磊晶內部的差排密度及界面晶格不匹配所引起的應變，將隨著磊晶厚度的增加而減少。