相变材料Ge2Sb2Te5与Si3.5Sb2Te3的带隙与局部态密度研究

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"该研究论文深入探讨了Ge2Sb2Te5 (GST)和Si3.5Sb2Te3 (SST)材料在相变过程中带隙和局部态密度的变化，及其对材料电阻率的影响。" 在相变过程中，GST和SST的电阻率变化被发现与它们的带隙和局部态密度的变化紧密相关。带隙是衡量半导体材料能隙大小的关键参数，它影响了电子在材料中的移动性。当材料相变时，带隙的改变会直接影响其电性能，例如电阻率。 GST和SST在非晶态时，SST的局部态密度稍高于GST，这意味着SST的电子更倾向于在特定区域形成局部化状态。然而，尽管如此，SST材料的较大带隙使得其相变过程相对困难。带隙越大，电子从价带跃迁到导带所需的能量越高，因此相变需要更大的能量输入。这一特性在相变存储器(PCM)的应用中得到了体现。基于SST薄膜的PCM单元在设定和复位操作时表现出比基于GST的PCM单元更高的阈值电压。这表明，为了实现相变，需要在SST材料上施加更大的电场。相变过程中，SST薄膜中形成了非晶态硅富集区，增加了相变的随机性，这可能会影响PCM单元的可预测性和稳定性。此外，这些发现对于优化PCM设备的设计至关重要，因为理解和控制这些材料的相变性质对于提高存储器的性能、可靠性和耐久性具有重要意义。通过调整材料组成或采用不同的处理方法，可能可以改善相变过程的可控性，从而提升基于GST和SST的存储技术。这项研究为理解GST和SST的相变行为提供了新的见解，并为未来开发高性能的非易失性存储解决方案提供了理论基础。通过更深入地探究带隙和局部态密度变化如何影响材料的物理性质，科学家们能够设计出更高效、更可靠的相变存储技术。

Investigation of changes in band gap and density of localized states

on phase transition for Ge

and Si

3.5

materials

Feng Rao

⇑

, Zhitang Song, Yan Cheng, Mengjiao Xia, Kun Ren, Liangcai Wu, Bo Liu,

Songlin Feng

State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Micro-System and Information Technology,

Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China

Received 10 March 2011; received in revised form 8 September 2011; accepted 8 September 2011

Abstract

Upon phase transition, the resistivity changes of Ge

(GST) and Si

3.5

(SST) are proved to be closely related to the vari-

ations of band gap and density of localized states. Amorphous SST has a slightly more localized state than amorphous GST; however,

the larger band gap of SST material causes relatively diﬃcult phase transition processes. Therefore, the phase change memory (PCM) cell

based on the SST ﬁlm shows larger threshold voltages for both set and reset operations than that of the GST-based PCM cell. The for-

mation of amorphous Si-rich segregated areas in the SST ﬁlm during phase transitions increases the randomness of the whole ﬁlm micro-

structure, which leads to a diﬀerent Urbach tail absorption result to that of the GST ﬁlm.

Keywords: Phase change; Band gap; Density of localized states

1. Introduction

Phase change memory (PCM) has been internationally

recognized as the next generation non-volatile memory. It

is able to achieve low power consumption, high storage

density and high operating speed by scaling down the cell

dimension to the 10 nm level with a 10 ns storage time

[1,2]. The usual phase change mate rials utilized in PCM

are chalcogenide alloy s. These chalcogenide alloys, espe-

cially the Te-based ones, e.g. Ge

(GST) and

3.5

(SST) [3,4], show a pronounced change in elec-

trical properties accompanied by reversible structure

change between amorphous (high resistivity state/reset

state) and crystalline (low resistivity state/set state) phases.

To reset data, electrical pulses of high magni tude an d short

duration are applied to the phase change ﬁlm in PCM to

melt a local active region via Joule heating. The fast

cooling speed (>10

1

) quenches the melting area into

an amorphous phase. Correspondingly, to set data,

electrical pulses of lower magnitude and longer period

can heat the amorphous region above its cryst allization

temperature.

This electrically induced structural change in chalcogen-

ide materials has been named Ovonic memory switching

(OMS) [5,6]. It should be noted that OMS usuall y occurs

after another electrically driven switching eﬀect – Ovonic

threshold switching (OTS) [5,6]. OTS also corresponds to a

reversible change between low and high conductive states,

but there is no obvious change in microstructure of the mate-

rial. Once the phase transition temperature is reached after

OTS, the amorphous chalcogenide material can be changed

into a stable crystalline phase with high conductivity. To

explain the OTS mechanism, Adler et al. [7] argued that

the OTS eﬀect is supposed to be a pure electronic phenome-

non without any thermal breakdown. The proposed elec-

tronic phenomeno n describes a balance between a strong

Shockley Hall Reed recombination through trap levels and

doi:10.1016/j.actamat.2011.09.015

⇑

Corresponding author.

E-mail address: fengrao@mail.sim.ac.cn (F. Rao).

www.elsevier.com/locate/actamat

Available online at www.sciencedirect.com

Acta Materialia 60 (2012) 323–328

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