相变材料的结构、电/光性质及光电混合信息存储研究
文献类型：学位论文
| 作者 | 孙华军 |
| 学位类别 | 博士 |
| 答辩日期 | 2009 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 侯立松 |
| 关键词 | 相变材料 信息存储 非晶态 结构变化 电/光性质 开关效应 |
| 其他题名 | Structure, electrical and optical properties of phase change materials and optical-electrical hybrid data storage |
| 中文摘要 | 本论文全面综述了相变存储材料的结构、电/光性质和应用研究现状和发展趋势，以及目前相变材料和相变信息存储研究中存在的主要问题。在此基础上，分别研究了衬底加热条件下沉积的Ge2Sb2Te5薄膜的结构、电/光性质。和激光辐照引起Ge2Sb2Te5非晶态薄膜的电/光性质变化规律；研究并发现了Ge2Sb2Te5，Ge1Sb4Te7，Sb2Te3和Ag7.1In13.3Sb55.3Te24.3非晶态薄膜具有与极性相关的可逆开关特性，而这一过程中没有发生相变，据此提出了同相变开关相结合的三阶信息存储方案；为了提高Ge1Sb4Te7相变材料的热稳定性和高低阻状态之间的电阻对比度，研究了Ag搀杂Ge1Sb4Te7相变薄膜的电/光性质，发现6.5%的搀杂薄膜具有晶化温度高、热稳定性好、高低阻状态电阻对比大的特点；在研究激光诱导相变薄膜的电学性质变化的基础上，提出光-电相结合的光电混合信息存储方案，拓宽了相变信息存储材料的应用。具体研究内容和结果如下： 研究了衬底（Si）加热条件下用磁控溅射法制备的Ge2Sb2Te5薄膜的结构及其光/电性质变化，探索可能产生的相分离对电/光性质的影响。结果表明：在25℃（室温）沉积态薄膜为非晶态结构；温度为60℃时，非晶态Ge2Sb2Te5薄膜中出现少量晶态Ge；70℃时，形成GeTe0.97、Sb2Te3、γ-(Sb,Te)晶态相；80℃时，形成GeTe0.97、Sb2Te3、γ-(Sb,Te)、Sb晶态相；100℃时, 形成Sb2Te3、γ-(Sb,Te)、Si2Te3晶态相；110℃时, 形成Te、Sb2Te3、Ge4Sb6晶态相；130℃时, 形成(Sb,Te)、Sb2Te3、Ge2Sb2Te5fcc(200)晶态相；140-200℃时，形成面心立方的Ge2Sb2Te5晶态相；300℃时，除了面心立方相还形成少量六方相Ge2Sb2Te5；400℃时，形成大量Ge2Sb2Te5六方相与少量面心立方相共存。这说明：在衬底加热条件下容易形成晶态薄膜。在140℃～150℃之间易形成均一的、表面光滑的面心立方结构晶态薄膜。不同衬底温度下薄膜的电阻在80℃左右出现拐点。60-80℃时，由于分相的产生，薄膜的反射率高于100℃的沉积薄膜，其余温度段薄膜的反射率随温度升高而增大。 用磁控溅射法在聚碳酸酯（PC）盘基上制备了结构为盘基/Ge2Sb2Te5 (100nm)/SiO2(20nm)的膜层，用POP120-5型初始化仪对Ge2Sb2Te5非晶薄膜进行辐照处理，用范德堡四探针法测定不同功率激光辐照后薄膜的电阻。结果表明：随着激光功率的增大，在功率580mW时薄膜的电阻出现四个数量级的突变（由107Ω/□到103Ω/□），电阻发生突变前、中、后三个样品的XRD结构测试表明此过程为非晶态经中间态到晶态的结构有序化过程，从而引起Ge2Sb2Te5薄膜中扩展态的导电方式取代定域态的导电方式致使电阻突变，薄膜的光学常数也发生相应变化，在可见光范围（λ＜750nm）内薄膜的光学常数（在波长相同情况下）有：n非晶态＞n中间态＞n晶态，k晶态＞k中间态＞k非晶态，α晶态＞α中间态＞α非晶态。三种相态的薄膜折射率均随波长增大而增大，晶态和中间态的消光系数k随波长增大而增大，而反射率变化为R晶态＞R中间态＞R非晶态。 在导电原子力显微镜探针/相变薄膜/金属薄膜的样品结构，发现Ge2Sb2Te5、Ge1Sb4Te7、Sb2Te3和Ag7.1In13.3Sb55.3Te24.3具有极性相关的可逆开关效应。在一定阈值电压下通过电压极性间的切换能够使相变材料在高阻和低阻态之间可逆转换，高低阻值相差约两个数量级。这些相变材料具有不同的开关机理：Ag7.1In13.3Sb55.3 Te24.3 可逆极性开关机理是固态电解产生的，在固态电解过程中高导电离子获得电子形成丝状导电通道，这种丝状导电通道的通断完成了开关过程;而Ge2Sb2Te5，Ge1Sb4Te7，Sb2Te3的开关机理是由于结构中变价对形成的缺陷造成的。缺陷构成的特殊的能带结构在正反向电压作用下的可逆转变引起导电性的可逆转变，从而产生这种开关特性。基于以上研究结果，提出了同相变开关效应相结合的三阶存储方案。 在掺杂Ag的Ge1Sb4Te7薄膜中，掺杂量为25.7 wt%时Ge1Sb4Te7中产生大量的AgTe、Ag2Te、 GeTe、 Sb2Te3、Te分相，悬空键被饱和使表面粗糙度较小。薄膜的变温电阻没有明显的突变，原位变温反射率没有很大的变化，熔点约为309℃。短波长范围吸收几乎不变，只在局部有突变点。在波长922nm处吸收急剧减小到约1（a.u.）。掺杂量为9.7 wt%和6.5 wt%的薄膜仍然保持着Ge1Sb4Te7的非晶态结构，它们的变温电阻在沉积态和退火到600K以后相差四个数量级，高/低阻态电阻对比度比Ge1Sb4Te7薄膜大。非晶态薄膜的电阻随掺杂量增大而减小。晶化温度和熔点随掺杂量增大而增高，晶化温度升高可提高材料的热稳定性。在短波长区低掺杂的薄膜吸收较大，在长波区搀杂量影响不大。因此，掺杂量在6.5 wt%左右的薄膜高/低阻态电阻对比度大，晶化温度高，可提高相变材料的热稳定性，有可能用于相变存储器件中。 磁控溅射制备的Ge2Sb2Te5薄膜为非晶态，其电阻为107Ω/□数量级，在5000纳秒脉宽聚焦激光作用下薄膜电阻转变为104Ω/□数量级，并且其电流形貌可采用导电原子显微镜很好的的识别出，因此采用光写入、电读出的光电混合的信息存储是可行的。在记录薄膜上加上SiO2保护层，使记录点的导电性更高，电流信号更强。同样，采用非晶态AgInSbTe薄膜作为记录介质也可实现光写入、电读出的光电混合信息存储。 |
| 英文摘要 | Current status and development trend of the structure, electrical/optical properties and applications of phase change materials, especially the main problems encountered in their R & D for data storage are reviewed and analyzed. On the basis of this analysis, our research was focused in the following areas: influence of substrate temperature on the structure，electrical and optical properties of Ge2Sb2Te5 thin films; laser-induced change in the electrical and optical properties of amorphous Ge2Sb2Te5 thin films; reversible resistance switching effect in amorphous Ge2Sb2Te5, Ge1Sb4Te7, Sb2Te3 and Ag7.1In13.3Sb55.3Te24.3 thin films without phase transformation , and a three -level data storage scheme is proposed integrating the phase change switching effect. In order to improve the thermal stability and resistance contrast of Ge1Sb4Te7 phase change films, the electrical and optical properties of Ag-doped Ge1Sb4Te7 films were investigated, and it was found that the 6.5 wt.% Ag-doped films possess higher crystallization temperature, good thermal stability and high resistance contrast. Based on the experimental results of laser-induced change in the electrical property of amorphous phase change films, optical-electrical hybrid data storage is proposed using phase change materials. The details are given as follows: By using magnetron-sputtering deposition method, Ge2Sb2Te5 films were prepared at different substrate temperatures on Si substrates. Their structure, crystallization temperature, electrical and optical properties were studied for clarifying the influence of phase separation. The results show that the as-deposited film is amorphous at 25℃; a few crystalline Ge in appears at 60℃; Ge1Te0.97, Sb2Te3 and γ-(Sb,Te) at 70℃, Ge1Te0.97, Sb2Te3, γ-(Sb,Te) and Sb at 80℃; Sb2Te3, γ-(Sb,Te) and Si2Te3 at 100℃; Te, Sb2Te3 and Ge4Sb6 at 110℃；(Sb,Te), Sb2Te3, and fccGe2Sb2Te5 at 130℃；fcc Ge2Sb2Te5 at 140-200℃； co-existence of fcc Ge2Sb2Te5 and hex Ge2Sb2Te5 at 300℃ and 400℃. These results indicate that the crystalline films can be easily formed on substrates at higher temperatures, and at 140℃～150℃, homogeneous and smooth crystalline fcc films are more easily formed. The electrical resistance of the films deposited at different temperatures shows an inflexion at 80℃during the temperature increasing process. Reflectivity of the films increase with increasing substrate temperature, except the films deposited at 60～80℃,whose reflectivity is higher than those deposited at 100℃. Stack films Ge2Sb2Te5(50nm)/SiO2(25nm) were deposited on 1.2mm thick polycarbonate (PC) substrates by the magnetron-sputtering method. Laser-irradiation of the stack films was performed using a POP120-5F initializer, and the subsequent quenching can make the high temperature structure maintained. The electrical resistance of the films was measured with a four-point probe following the procedure proposed by van der Pauw. The results show that the sheet resistance decreases with increasing laser power, and undergoes an abrupt change of four orders of magnitude (107→103 Ω/sq) at about 580mW; x-ray diffraction studies of the three samples before, at and after the abrupt point revealed the phase change process of the Ge2Sb2Te5 thin films from disordered structure to ordered structure, the abrupt is caused by the change in the conduction mechanism from localized electron conduction to extended electron conduction. At a same wavelength in the visible region, the optical constants of the three samples exhibit the following relations: na＞ni＞nc and kc＞ki＞ka, where a, i and c represent the amorphous, intermediate and crystalline states of the films, respectively. The refractive indexes (n) of the three states increase with increasing wavelength, the extinction coefficients (k) of the crystalline and intermediate states increase with increasing wavelength, while that of the amorphous state increases at first and decreases later with increasing wavelength. Similarly, the absorption coefficient (α) and reflectivity (R) hold relations as αr＞αi＞αa, and Rc＞Ri＞Ra , respectively. Polarity-dependent reversible resistance switching effects were discovered of the amorphous Ge2Sb2Te5, Ge1Sb4Te7, Sb2Te3 and Ag7.1In13.3Sb55.3Te24.3 thin films in the cell structure of CAFM probe / phase change film / metal film. Switching between the high and low resistance states can be realized by modulating polarity of the electric field over the voltage-threshold. The resistance difference between the two states is two orders of magnitude. The switching effect of Ag7.1In13.3Sb55.3Te24.3 is due to the formation of conductive filamentary pathways resulted from solid-state electrochemical reactions in the films: formation and rupture of the conductive filaments leads to the switching effect; whereas those of Ge2Sb2Te5, Ge1Sb4Te7 and Sb2Te3 films stem from the valence-alternation induced defects which produce reversible changes in the energy bands and thereby the conductivity under the action of polarity change of electric voltage. Based on these results, a three-level data storage scheme integrating the switching effect is proposed. In Ag (25.7 wt %)-doped Ge1Sb4Te7 films, lots of crystalline AgTe, Ag2Te, GeTe, Sb2Te3, and Te phases are produced and the dangling bonds are saturated, so that the surface roughness is better, and the melting point is about 309℃. No abrupt change in the resistance-temperature and reflectivity-temperature curves was found In the short wavelength range, the absorption is almost the same (ca. 6.6 a.u.), while at 922 nm, it abruptly comes down to about 1 (a.u.) and then stands with little change. In the Ag-doped Ge1Sb4Te7 films of 9.7 wt% and 6.5 wt % dosages, the amorphous structure is maintained and their resistance-temperature curves undergo an abrupt change of four orders of magnitude when annealing temperature is over 600K, and the resistance contrast is higher than that of the Ge1Sb4Te7 films. The electrical resistance of the amorphous films decreases with increasing Ag dosage, while the crystallization temperature and melting point increase with increasing Ag dosage making the films more stable. The optical absorption decreases with increasing Ag dosage in the short wavelength range, while keeps nearly unchanged with the Ag dosage in the long wavelength range. Thus, we consider that the 6.5 wt % Ag-doped films can be used for electrical memory as a recording medium. The resistance change of the as-deposited Ge2Sb2Te5 films is four orders of magnitude (from 107 Ω/sq. to 104 Ω/sq) due to irradiation by focused pulse laser of 5000 nS. This can be used for electrical reading of the recorded dots by CAFM. That is to say that our experimental results pave the way for optical-electrical hybrid data storage by means of optical recording and electrical reading using Ge2Sb2Te5 films an the recording media. Moreover, as the protective layer, SiO2 film deposited on the phase change films makes the recording films crystallize more easily. leading to higher conductivity of the recorded dots and stronger read signals. Similarly, optical-electrical hybrid data storage using AgInSbTe films as the recording media is also possible. |
| 语种 | 中文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15250]  |
| 专题 | 上海光学精密机械研究所_学位论文 |
| 推荐引用方式 GB/T 7714 | 孙华军. 相变材料的结构、电/光性质及光电混合信息存储研究[D]. 中国科学院上海光学精密机械研究所. 2009. |
入库方式： OAI收割
来源：上海光学精密机械研究所
浏览0
下载0
收藏0
其他版本
除非特别说明，本系统中所有内容都受版权保护，并保留所有权利。