分子束磊晶低溫成長III-V族稀磁半導體之結構與性質

Abstract

III-V族半導體廣為應用在光電及快速元件。然而，同時具有半導體傳導特性與磁性之奈米薄膜，因具有降低元件膜層間自旋電子散射之特性，而成為可應用在自旋電子元件的重要關鍵技術之一。在傳統的半導體材料中加入少量磁性元素，形成所謂稀磁半導體(DMS)，例如將Mn原子加入傳統的III-V族半導體，即屬於此類應用而新發展的材料。本實驗成功的在GaAs與InP基材上，生長系列稀磁半導體多層膜，並研究其結構與性質。不同於傳統磊晶的生長方法，利用分子束磊晶低溫(＜300℃)成長，降低沉積時原子間互相擴散而形成介穩態，較傳統方法可加入較高含量的磁性原子以增加其磁性。利用TEM、DXRD、EPMA與SQUID研究其結構與性質，實驗結果略分為三類： 1、 LT-GaAs磊晶層中過量砷析出行為的研究，發現不同方向的GaAs基板之磊晶層中過量砷濃度在GaAs(311)B＞ (311)A＞ (001)。可能因晶面配位數因子與鍵結位置的差異而形成。比較表面空懸雙鍵與單鍵的鍵結位置，後者配位數因子較高。此外另設計一LT-GaAs 包含6層區域(i-n1-i, i-n2-i, i-n3-i, i-p1-i, i-p2-i, 與i-p3-i多層膜) 結構，調變Si與Be的摻雜濃度(1016 ~ 1018 ㎝-3)，發現一有趣結果，當摻雜Si濃度≦1017㎝-3，As析出在n層形成空乏區，此現象在文獻中從未被報導。此現象可由i-n上、下二界面的德拜長度重疊區超越n層膜厚度之關係解釋。 2、有關生長摻雜7 ﹪Mn，後退火處理和DMS厚度對的LT-(Ga, Mn)As稀磁半導體，(LT-(Ga, Mn)As / LT-GaAs / GaAs)三層結構磁性之研究。結果顯示：DMS的居里溫度(Tc)隨著膜厚度下降與退火處理，而大幅的上升，並且在GaAs(001)方向的基材具最高Tc。退火處理基本上是將晶格內多出的插入型MnI原子移出，因而減少其類施者型缺陷，使電洞載子濃度提高而增加Tc 。換句話說，當膜厚度下降，Mn擴散的路徑縮短，在退火時更有效地將插入型MnI原子移出，並使Tc增加。不同GaAs基材方向之效應，基本上是影響多餘As原子之析出行為。因為過量的砷反置缺陷在GaAs(311)A晶面基板上，比在(001)面基板上多，所以會中和較多電洞載子，使電洞濃度下降，所以Tc的下降在GaAs(311)A面較多。 3、有關在InP基板上變化緩衝層晶格常數效應對摻雜Mn的LT-(In, Al, Mn)As DMS之影響研究。緩衝層晶格變化包括：與基板晶格常數幾乎吻合的單層膜晶格，以及3層膜梯度結構的晶格。磁性量測結果顯示，DMS皆顯示順磁行為，但當Mn的成分≧6%，LT-(In, Al, Mn)As DMS由順磁態變為鐵磁態。利用梯度緩衝層調變晶格常數，Mn濃度可提高到18 ﹪，且沒有第二相產生。當Mn濃度由11 ﹪增加到18 ﹪， Tc從25K變為40K。

The III-V compound semiconductors have been widely used for high-speed electronic devices as well as for optoelectronic devices. However, new magnetic semiconducting nanosized film materials to minimize the scattering of electron spins between layers in the devices have been one of the key issues in spintronic device applications. The semiconductor with lower concentration of magnetic material (also called “diluted magnetic semiconductor (DMS)”) is one of the new developing materials for such applications, which can be formed by incorporating magnetic atoms, e.g. Mn, into conventional III-V semiconductors. In this work, series of processes for fabricating DMS multilayer materials based on GaAs and InP semiconductors were successfully developed to examine their structures and properties. The processes include using the low temperature-molecular beam epitaxial (LT-MBE) (＜300℃) techniques to minimize the mutual diffusion between elements during the deposition period, so the metastable phases can be obtained with a relative higher concentration of magnetic material than the conventional method to improve their magnetic properties. The structures and propertied were characterized by TEM, DXRD (double crystal XRD), EPMA and SQUID. The experimental results can be roughly divided into three categories: (1) On examining excess arsenic precipitation behaviors in LT-GaAs deposits, the results indicate that the excess arsenic content (Asex) is depending on crystallographic orientation of the GaAs substrate, i.e., Asex on (311)B > (311)A > (001); this may be due to a combination effect of the accommodation factors of the crystallographic plane and bonding site difference. By comparing the double- and single-dangling-bond sites, the latter sites preferentially possess a greater accommodation factor. Furthermore, the results of annealing experiments of the LT-GaAs structure containing six active regions (i-n1-i, i-n2-i, i-n3-i, i-p1-i, i-p2-i, and i-p3-i multilayers) find an interesting result: the excess arsenic depletion zone or distribution in i-n-i structures after annealing is depending on the doping concentration. For Si-doped concentration ≦ 1017 cm-3, the arsenic depletion zone in n-layer can be formed in the present cases, which had not reported in the literature. This can be explained by the overlapping the n-layer thickness with the Debye length of the substrate at both sides of i-n interfaces. (2) As to effects of 7 % Mn addition, post annealing and DMS layer thickness of Mn-doped LT-GaAs on their magnetic properties in three-layers structure (LT-(Ga, Mn)As /LT-GaAs/GaAs), the results show that the Curie temperature (Tc) of DMS can be greatly increased by a decrease in thickness and via annealing treatment, and indicates the greatest Tc for (001) GaAs substrate orientation. Annealing treatment is essentially to remove excess MnI from the interstitial sites in the lattice to decrease the donor-like defects, which may cause an increase in hole concentration and Tc. In other words, the diffusion path of Mn for the thinner DMS thickness is much shorter, which may result in a more effective removal of excess MnI from the lattice and a greater increase in Tc after annealing. Effect of substrate orientation is basically to affect the excess arsenic precipitation behavior. Therefore, a greater excess arsenic antisite defects in (311)A substrate orientation than in (001) orientation may neutralize more holes in the lattice to decrease Tc more in (311)A orientation. (3) On (001) InP substrate, effects of the lattice constant variations of buffer layers on Mn doping of LT-(In, Al, Mn)As DMS materials were examined. The buffer layers of LT-(In, Al)As include nearly matched one-layer and the graded three-layers lattice structures. The results show that the LT-(In, Al, Mn)As DMS materials are paramagnetic, and become ferromagnetic for Mn % ≧ 6 %. By using the graded buffer structures, the Mn concentration of DMS can reach 18 % without 2nd phase precipitation. The Tc of the DMS changes from 25K to 40K, when Mn concentration varies from 11 to 18 %.