Magnetotransport properties of (Ge,Mn) ferromagnetic semiconductor grown on GaAs(001)

利用低溫分子束磊晶的方法，在兩種砷化鎵基板上成長具奈米微結構之鍺錳鐵磁性半導體：磊晶型砷化鎵基板(Ga-GeMn)及非晶質砷覆蓋之砷化鎵基板(As-GeMn)。 在Ga-GeMn試片，鍺錳薄膜中含有自組性高錳奈米圓柱微結構，其成長會垂直表面，故平整面度不同會使奈米圓柱成長呈現平行或交錯狀。磁性量測含兩個相：居里溫度150K之鐵磁高錳奈米圓柱及低濃度錳原子在鍺母體之順磁相。由磁性傳輸特性量測，此P型薄膜具有異常性霍爾效應，並存在不同的磁阻特性：低溫的巨磁阻特性，高溫的磁場二次方之常態磁阻及低磁場具有額外正磁阻。藉由此P型半導體與高錳奈米圓柱之能帶結構，我們也提出一磁性傳輸特性模擬計算之方法，定性地解釋實驗上薄膜之異常性霍爾效應來自高錳奈米圓柱及低磁場具有額外正磁阻(稱為霍爾磁阻)。 在As-GeMn試片，砷原子擴散改變薄膜之成長機制，由二維變成三維成長：居里溫度50K之奈米析出相與鐵磁性之Ge3Mn5聚集相。施體錳與受體砷主導薄膜之電子傳輸特性並發現其補償效應，磁性傳輸量測發現其高非等向性磁阻特性來自於弱局部化現像。此N型鐵磁半導體與金屬性奈米析出物形成蕭基位能障，此位能障始電子傳輸在等向性量測時具穿隧磁阻效應。By low temperature Molecular Beam Epitaxy (MBE), (Ge,Mn) thin films with ferromagnetic nanostructures are grown on two kind of GaAs substrates: epiready GaAs(001) (Ga-GeMn) and GaAs(001) with amorphous As capping layer (As-GeMn). In the Ga-GeMn samples, we observe self-organized Mn-rich nanocolumns which, depending on the initial surface morphology, are either parallel or entangled. Magnetic measurements evidence two magnetic phases: ferromagnetic nanocolumns with Curie temperature around 150K, and the Ge matrix made paramagnetic by diluted Mn. From measurements of magnetotransport, this p-type layer exhibits anomalous Hall effect (AHE) and various magnetoresistivity (MR): negative Giant MR at low temperature, parabolic ordinary MR, and an additional positive MR at low field. The simulation of magnetotransport properties has been initiated, with some assumptions on the energy-band diagram of p-type Ge semiconductor and Mn-rich nanocolumns, and shows the way that AHE in the inclusions induces AHE in the whole layer, and a mechanism of MR which accounts for this contribution (which we call Hall MR). In the As-GeMn samples, the outdiffusion of As atoms changes the growth mechanism, from a 2D spinodal decomposition to a 3D one with the formation of nanoprecipitates with Tc=50K and of ferromagnetic Ge3Mn5 clusters. Compensation effect between Mn (donor) and As (acceptor) dominates the transport behaviors. A highly anisotropic MR is observed in n-type (Ge,Mn) films and shown to be due to weak localization effects. Another contribution is tentatively attributed to tunneling MR due to the Schottky barrier, which forms at the interface between the metallic Mn-rich inclusions and the n-type Ge semiconductor