Paramagnetic GaN:Fe and ferromagnetic (Ga,Fe)N:: The relationship between structural, electronic, and magnetic properties
Paramagnetic GaN:Fe and ferromagnetic (Ga,Fe)N:: The relationship between structural, electronic, and magnetic properties
复制标题
DOI：
10.1103/physrevb.75.125210
复制
发表时间：
2007-03-01
影响因子：
3.7
通讯作者：
Dietl, Tomasz
中科院分区：
文献类型：
--
作者：
Bonanni, Alberta;Kiecana, Michal;Dietl, Tomasz
我们报告的金属有机化学气相沉积的GaN：Fe和（Ga，Fe）N层的c-蓝宝石衬底和他们的全面表征，通过高分辨率的X射线衍射，透射电子显微镜（TEM），空间分辨能量色散X射线光谱（EDS），二次离子质谱（西姆斯），光致发光（PL），霍尔效应，电子顺磁共振（EPR），并采用超导量子干涉仪（SQUID）的磁力计。TEM和EDS的组合揭示了相干纳米晶体的存在下，大概FexN的组合物和晶格参数所施加的主机。从TEM和西姆斯的研究，它是说，纳米晶体的密度，从而Fe浓度增加朝向表面。根据霍尔效应测量，在低铁含量x小于或接近0.4%的极限下，来自残余施主的电子被中间带隙Fe受主态捕获，表明Fe 2+离子的浓度以3+电荷态的Fe离子为代价而增加。这种效果是见证PL测量的Fe 3+相关的离子内跃迁，这可以通过与Si施主和Mg受主共掺杂控制的强度的变化。在这一制度下，EPR的Fe 3+离子和居里类磁化率进行了观察。由于自旋-轨道相互作用，Fe 2+不产生任何EPR响应。然而，在2+电荷状态的Fe离子的存在下，可以占一个温度无关的货车Vleck型顺磁信号，我们观察到的SQUID磁强计。令人惊讶的是，在更高的Fe浓度，电子密度被发现与Fe含量大幅增加。导带中的电子和3+电荷态中的Fe的共存与Fe浓度的梯度有关。在铁含量x大于或接近0.4%的层中，已经检测到铁磁特征的存在，例如磁化滞后和自发磁化。已采取一系列预防措施，以排除可能的杂散铁磁贡献源。在这些条件下，铁磁类响应被示出从（Ga，Fe）N外延层产生，其随着铁浓度增加，其持续到室温，并且其是各向异性的，即，对于面内磁场，磁化强度的饱和值较高。我们链接的铁磁签名的存在下，形成富铁纳米晶体，证明了TEM和EDS研究。这种解释是支持磁化测量冷却后，在和没有外部磁场，指向系统的超顺磁性。有人认为，高温铁磁响应由于旋节分解成区域的磁性成分的小和大的浓度是一个通用的属性，稀磁半导体和稀磁氧化物表现出高的表观居里温度。
We report on the metalorganic chemical vapor deposition of GaN:Fe and (Ga,Fe)N layers on c-sapphire substrates and their thorough characterization via high-resolution x-ray diffraction, transmission electron microscopy (TEM), spatially resolved energy dispersive x-ray spectroscopy (EDS), secondary-ion mass spectroscopy (SIMS), photoluminescence (PL), Hall-effect, electron-paramagnetic resonance (EPR), and magnetometry employing a superconducting quantum interference device (SQUID). A combination of TEM and EDS reveals the presence of coherent nanocrystals presumably FexN with the composition and lattice parameter imposed by the host. From both TEM and SIMS studies, it is stated that the density of nanocrystals and, thus the Fe concentration increases towards the surface. According to Hall effect measurements, electrons from residual donors are trapped by midgap Fe acceptor states in the limit of low iron content x less than or similar to 0.4%, indicating that the concentration of Fe2+ ions increases at the expense of Fe ions in the 3+ charge state. This effect is witnessed by PL measurements as changes in the intensity of the Fe3+-related intraionic transition, which can be controlled by codoping with Si donors and Mg acceptors. In this regime, EPR of Fe3+ ions and Curie-like magnetic susceptibility are observed. As a result of the spin-orbit interaction, Fe2+ does not produce any EPR response. However, the presence of Fe ions in the 2+ charge state may account for a temperature-independent Van Vleck-type paramagnetic signal that we observe by SQUID magnetometry. Surprisingly, at higher Fe concentrations, the electron density is found to increase substantially with the Fe content. The coexistence of electrons in the conduction band and Fe in the 3+ charge state is linked to the gradient in the Fe concentration. In layers with iron content x greater than or similar to 0.4% the presence of ferromagnetic signatures, such as magnetization hysteresis and spontaneous magnetization, have been detected. A set of precautions has been undertaken in order to rule out possible sources of spurious ferromagnetic contributions. Under these conditions, a ferromagneticlike response is shown to arise from the (Ga,Fe)N epilayers, it increases with the iron concentration, it persists up to room temperature, and it is anisotropic-i.e., the saturation value of the magnetization is higher for in-plane magnetic field. We link the presence of ferromagnetic signatures to the formation of Fe-rich nanocrystals, as evidenced by TEM and EDS studies. This interpretation is supported by magnetization measurements after cooling in and without an external magnetic field, pointing to superparamagnetic properties of the system. It is argued that the high temperature ferromagnetic response due to spinodal decomposition into regions with small and large concentration of the magnetic component is a generic property of diluted magnetic semiconductors and diluted magnetic oxides showing high apparent Curie temperature.