Dielectric properties of amorphous phase-change materials
Dielectric properties of amorphous phase-change materials
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DOI：
10.1103/physrevb.95.094111
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发表时间：
2017-03-14
影响因子：
3.7
通讯作者：
Wuttig, M.
中科院分区：
文献类型：
--
作者：
Chen, C.;Jost, P.;Wuttig, M.
采用阻抗谱（9 kHz-3 GHz）和从远红外（20 cm（-1），0.6 THz）到近红外（12 000 cm（-1），360 THz）的光谱相结合的方法测定了几种非晶相变材料的介电函数，从DC极限到第一带间跃迁。虽然相变材料在结晶时经历从共价键合到共振键合的变化，但普通硫族化物半导体的非晶相和结晶相都由几乎相同的共价键控制。在这里，我们研究的非晶相变材料的介电性能的赝二元线GeTe和Sb 2 Te 3之间。这些数据提供了重要的见解的电荷传输和非晶相变材料中的键合的性质。在阻抗谱测量中没有可辨别的介电常数和电导率的频率依赖性。因此，不存在介电弛豫。与频率无关的电导率与通过扩展态的电荷传输一致。静态介电常数显著超过光学介电常数。这一观察结果得到了远红外透射率测量的证实，该测量显示了光学声子。从这些声子模式的强度，一个大的玻恩有效电荷的推导。然而，已知结晶相变材料（诸如GeTe）具有甚至显著更大的玻恩有效电荷。因此，结晶伴随着玻恩有效电荷的巨大增加，这揭示了结晶后键合的显著变化。此外，一个明确的化学计量的趋势，在静态介电常数沿着之间的GeTe和Sb 2 Te 3的伪二元线已被确定。
The dielectric function of several amorphous phase-change materials has been determined by employing a combination of impedance spectroscopy (9 kHz-3 GHz) and optical spectroscopy from the far-(20 cm(-1), 0.6 THz) to the near- (12 000 cm(-1), 360 THz) infrared, i.e., from the DC limit to the first interband transition. While phase-change materials undergo a change from covalent bonding to resonant bonding on crystallization, the amorphous and crystalline phases of ordinary chalcogenide semiconductors are both governed by virtually the same covalent bonds. Here, we study the dielectric properties of amorphous phase-change materials on the pseudobinary line between GeTe and Sb2Te3. These data provide important insights into the charge transport and the nature of bonding in amorphous phase-change materials. No frequency dependence of permittivity and conductivity is discernible in the impedance spectroscopy measurements. Consequently, there are no dielectric relaxations. The frequency-independent conductivity is in line with charge transport via extended states. The static dielectric constant significantly exceeds the optical dielectric constant. This observation is corroborated by transmittance measurements in the far infrared, which show optical phonons. From the intensity of these phonon modes, a large Born effective charge is derived. Nevertheless, it is known that crystalline phase-change materials such as GeTe possess even significantly larger Born effective charges. Crystallization is hence accompanied by a huge increase in the Born effective charge, which reveals a significant change of bonding upon crystallization. In addition, a clear stoichiometry trend in the static dielectric constant along the pseudobinary line between GeTe and Sb2Te3 has been identified.