中红外Ge20As20Se15Te45硫系玻璃光纤光栅光开关

利用熔融淬冷法制备了Ge20As20Se15Te45硫系玻璃,测试结果表明该玻璃在中红外波段具有良好的红外透过特性和极高的非线性,采用Z-扫描技术测试其中红外三阶非线性系数高达n2=6.7210-18 m2/W @ 4 m。以此材料为基质,设计了两种工作在中红外波段的硫系玻璃光纤光栅光开关。利用分布时域传递矩阵法(TD-TMM)和非线性耦合方程模拟计算了光栅光开关的反射谱和波长偏移特性。研究结果表明,通过调节输入光强可以灵活调节光纤光栅的中心波长向短波或长波方向移动,进而可以获得光开关。在调制深度为110-3的均匀硫系光纤光栅,获得光开关的功率阈值约为1.6 GW/cm2,当引入相移光栅后,调制深度可以降低为310-4,并且其光开关的功率阈值也降低到210 MW/cm2。     

Fabrication of long lengths of low-loss IR transmitting As40S(60-x)Sex glass fibers

Teflon clad and As₄₀S₆₀ glass clad As₄₀ S₅₅Se₅ fibers transmitting in the 1-6 μm wavelength region have been fabricated in lengths of about 50 m and with minimum losses of 0.098 and 0.65 dB/m, respectively. Short lengths of the Teflon clad fiber possessed a minimum loss of 0.047 dB/m. While current fiber losses are dominated by extrinsic scattering and absorption, the calculated theoretical minimum loss is estimated to be 3.6 dB/km at 5.3 μm and is limited by the contribution from the weak absorption tail. Improvements in the purification and processing of the glasses into the optical fibers are required to reduce the losses further     

硫系玻璃Ge28Sb12Se60薄膜光学非线性增强色散特性

采用真空热蒸发以及退火工艺制备了支持局域表面等离激元的微纳结构薄膜,在此薄膜上蒸镀了硫系玻璃Ge₂₈Sb₁₂Se₆₀薄膜。应用Z-扫描技术,在飞秒激光脉冲激发下研究其光学非线性增强的色散特性,在650 nm和850 nm波段观察到了非线性吸收增强;非线性折射率随着波长的增加由负变正。通过扫描电子显微镜和透过光谱表征和分析了硫系玻璃Ge₂₈Sb₁₂Se₆₀薄膜非线性吸收增强的原理,非线性吸收随着波长的增加由单光子吸收为主逐渐转变为双光子吸收为主;银膜的微纳结构导致硫系玻璃薄膜的共振中心频率发生了偏移。实验制备的用于增强硫系玻璃非线性的微纳结构制作简单,无需复杂光刻工艺,为非线性光子学器件的设计提供了新的思路。     

Amorphous-fcc transition in Ge2Sb2Te5

In this paper we discuss some major aspects on the physics of the phase transition from the amorphous to the face-centered-cubic (fcc) polycrystal in Ge₂Sb₂Te₅ at low temperature. We follow the phase transformation by using structural techniques such as TEM, XRD, and electrical resistivity measurements by using the 4-point-probe technique. The results are interpreted in the framework of a quantitative model.     

Patterning of Ge2Sb2Te5 phase change material using UV nano-imprint lithography

DRAM is the most commonly used memory due to many advantages such as high speed and easy manufacturability owing to its simple structure, but is volatile. On the other hand, flash memory is non-volatile, but has other disadvantages such as slow speed, short lifetime, and low endurance for repetitive data writing. Compared to DRAM and flash memory, PRAM (Phase-change Random Access Memory), which is a non-volatile memory using a reversible phase change between amorphous and crystalline state, has many advantages such as high speed, high sensing margin, low operating voltage, and is being pursed as a next generation memory. Being able to pattern and etch phase change memory in nanometer scale is essential for the integration of PRAM. This study uses the Nano-Imprint Lithography (NIL) for patterning the PRAM in nanometer scale which is believed to be a future lithography technology that will replace the conventional Photo Lithography. Si wafers coated with SiO₂ were used as substrates, and Ge₂Sb₂Te₅ (GST) films with the thicknesses of 100 nm were deposited by RF sputtering. Poly-benzylmethacrylate based polymer patterns were formed using NIL on the surface of GST films, and the GST films were etched using Cl₂/Ar plasma in an Oxford ICP (inductively coupled plasma) etcher.     

软抛光垫条件下非晶态Ge2Sb2Te5的化学机械抛光

Chemical mechanical planarization（CMP） of amorphous Ge₂Sb₂Te₅（a-GST） is investigated using two typical soft pads（politex REG and AT） in acidic slurry.After CMP,it is found that the removal rate（RR） of a-GST increases with an increase of runs number for both pads.However,it achieves the higher RR and better surface quality of a-GST for an AT pad.The in-situ sheet resistance（R_s） measure shows the higher R_s of a-GST polishing can be gained after CMP using both pads and the high R_s is beneficial to lower the reset current for the PCM cells. In order to find the root cause of the different RR of a-GST polishing with different pads,the surface morphology and characteristics of both new and used pads are analyzed,it shows that the AT pad has smaller porosity size and more pore counts than that of the REG pad,and thus the AT pad can transport more fresh slurry to the reaction interface between the pad and a-GST,which results in the high RR of a-GST due to enhanced chemical reaction.     

Fabrication of Ge2Sb2Te5 based PRAM device at 60 nm scale by using UV nanoimprint lithography

Phase change memory is one of the most promising non-volatile memory for the next generation memory media due to its simplicity, wide dynamic range, fast switching speed and possibly low power consumption. Low power consuming operation of phase change random access memory (PRAM) can be achieved by confining the switching volume of phase change media into nanometer scale. Nanoimprint lithography is an emerging lithographic technique in which surface protrusions of a mold such as sub-100 nm patterns are transferred into a resin layer easily. In this study, crossbar structures of phase change device array based on Ge₂Sb₂Te₅ were successfully fabricated at 60 nm scale by two consecutive UV nanoimprint lithography and metal lift-off process, which showed on/off resistance ratio up to 3000.     

溅射参数为Ge2Sb2Te5薄膜光学常数的影响

研究了溅射参数对Ge2Sb2Te5薄膜的光学常数随波长变化关系的影响。结果表明：（1）当浅射功率一定时,随溅射氩气气压的增加Ge2Sb2Te5薄膜的折射率先增大后减小,而消光系数先减小后增大。（2）当溅射氩气压一定时,对于非晶态薄膜样品,在500nm波长以下,折射率随溅射功率的增加先增加后减小,消光系数则逐渐减小;在500nm以上,折射率随溅射功率的增加逐渐减少,消光系数先减小后增加。 对于晶态薄膜样品,在整个波长范围折射率随溅射功率的增加先减小后增加,消光系数则逐渐减少。（3）薄膜样品的光学常数都随波长的变化而变化,在长波长范围变化较大,短波长范围变化较小,探讨了影响Ge2Sb2Te5薄膜光学常数的机理。     

Reactive-ion etching of Ge2Sb2Te5 in CF4/Ar plasma for non-volatile phase-change memories

Etching of Ge₂Sb₂Te₅ (GST) is a critical step in the fabrication of chalcogenide random access memories. In this paper, the etch characteristics of GST films were studied with a CF₄/Ar gas mixture using a reactive-ion etching system. We observed a monotonic decrease in etch rate with decreasing CF₄ concentration indicating its importance in defining the material removal rate. Argon, on the other hand, plays an important role in defining the smoothness of the etched surface and sidewall edge acuity. We have studied the importance of gas mixture and RF power on the quality of the etched film. The smoothest surfaces and most vertical sidewalls were achieved using a CF₄/Ar gas mixture ratio of 10/40, a background pressure of 80 mTorr, and power of 200 W.     

Low power resistive switching memory using Cu metallic filament in Ge0.2Se0.8 solid-electrolyte

Bipolar resistive switching memory device using Cu metallic filament in Au/Cu/Ge_0.2Se_0.8/W memory device structure has been investigated. This resistive memory device has the suitable threshold voltage of V_th > 0.18 V, good resistance ratio (R_High/R_Low) of 2.6 × 10³, good endurance of >10⁴ cycles with a programming current of 0.3 mA/0.8 mA, and 5 h of retention time at low compliance current of 10 nA. The low resistance state (R_Low) of the memory device decreases with increasing the compliance current from 1 nA to 500 μA for different device sizes from 0.2 μm to 4 μm. The memory device can work at very low compliance current of 1 nA, which can be applicable for extremely low power-consuming memory devices.     

Characterization of Ge2Sb2Te5 thin film transistor and its application in non-volatile memory

With the increasing requirement of high density memory technology, a new cell structure—1TR has received much attention. It consists of a single thin film transistor (TFT) with chalcogenide Ge₂Sb₂Te₅ as the channel material. In order to evaluate the feasibility of its application in the field of non-volatile memory, we take a further step in researching on the characteristics of GST-TFT. We fabricated a back-gate GST-TFT and investigated the output and transfer characteristics of its two states. The experimental results show that gate voltage can modulate the GST channel currents in both the amorphous and the crystalline states. Based on the experiments, we can expect that this novel device can ultimately lead to a new nonvolatile memory technology with even higher storage density.     

Fabrication and characterization of a depletion-mode ZnS0.07Se0.93 MESFET

We report the fabrication and characterization of a depletion-mode n-channel ZnS_0.07Se_0.93 metal-semiconductor field effect transistor (MESFET). A ZnSSe FET could be a key element in opto-electronic integration consisting of light emitters, light receivers and MESFET pre-amplifiers. Mesa isolation, recess etching and self-alignment techniques were adopted to optimize the MESFET performance. Source and drain (S/D) ohmic contacts and gate Schottky contact were formed by Cr/In/Cr and Au deposition, respectively. Depletion mode FET's with varying gate width-to-length ratio of W/L=200 μm/20 μm, 200 μm/4 μm and 200 μm/2 μm were fabricated. A 2 μm FET was characterized as follows: the turn-on voltage, V_on≈1.75 V, the pinch-off voltage, V_p≈-13 V, the unit transconductance, g_m≈8.73 mS/mm, and the breakdown voltage with zero gate-source bias, BV≈28 V     

As2Se3硫系玻璃非球面镜片的精密模压成型

硫系玻璃镜片是新型温度自适应红外光学系统的重要组件之一。随着红外热成像民用市场的日益成熟,对硫系玻璃镜片的产业化技术需求越来越迫切。文中进行了As2Se3硫系玻璃非球面镜片的精密模压实验,研究并优化出口径21 mm硫系镜片的模压工艺参数。通过模具的补偿修正,获得了完全满足镜片设计要求（PV值小于0.7 m）的模压镜片。此外,研究了模压处理对As2Se3玻璃物理性质的影响,发现模压后As2Se3玻璃的密度、硬度和玻璃转变温度Tg降低,最大透过率提高。通过拉曼光谱测试,分析并讨论了造成这种反常现象的微观结构原因。为今后较大口径非球面硫系镜片的批量制造提供了科学数据和参考。     

Effect of annealing on the structure of Bi2Te3-Bi2Se3 films

The crystallization dynamics of Bi₂Te₃-Bi₂Se₃ polycrystalline films annealed at 200–230°C has been investigated. The formation of ordered blocks 70—150 nm in size, depending on the annealing time and temperature, is observed.     

表面增强硫系玻璃Ge28Sb12Se60薄膜非线性吸收（特邀）

文中利用热蒸发以及退火等工艺制备了支持局域表面等离子体激元（LSP）的微纳结构,来增强硫系玻璃Ge₂₈Sb₁₂Se₆₀ (GSS)薄膜的非线性吸收效应;搭建了Z-scan光路,实现了对样品非线性折射与吸收的测量;通过对样品透射光谱的分析,揭示了GSS非线性吸收增强效应的原理。并研究了该微纳结构对不同厚度GSS非线性吸收的增强规律。文中用到的LSP微纳结构制作简单,无需复杂光刻工艺,可为增强材料光学非线性研究提供重要参考。     

新型长波红外非线性晶体PbIn6Te10的生长

新型长波红外非线性晶体PbIn6Te10具有透光波段宽(1.3~31μm)、非线性系数大(d11=51 pm/V),双折射适宜(~0.05)等优点,在14~25μm乃至25μm以上波段具有较大应用潜力。文中通过相图分析结合具体实验,筛选出较合适的组分配比,并采用高温单温区法合成多晶,布里奇曼法生长出尺寸φ11 mm×55 mm的单晶棒。对生长的PbIn6Te10晶体进行X射线衍射、摇摆曲线、透过率等测试,结果表明,晶体为三方结构,晶格常格为a=b=1.496 1 nm,c=1.825 7 nm,生长出的单晶结晶性较好,半高宽(FHWM)约0.253°,2.5~25μm波段晶体的平均透过率在50%以上,对应收系数处于0.3~0.6 cm-1之间。     

CdS0.2Se0.8纳米晶掺杂玻璃的超快非线性吸收特性

利用飞秒脉冲通过Z扫描技术和泵浦-探测技术研究了CdS0.2Se0.8纳米晶掺杂的硼酸盐玻璃滤波片RG665的非线性吸收特性。研究结果表明在800 nm波长 130 fs脉冲激光作用下,RG665滤波片表现较强的非线性吸收特性。通过理论分析拟合实验结果证明RG665滤波片在800 nm下的非线性吸收包含双光子吸收及双光子吸收诱导的激发态吸收两部分,得到双光子吸收系数为0.05 cm/GW,激发态吸收截面为e=310-23 m2,以及导带中低能态电子和导带底电子的寿命分别为13 ps和210 ps。研究结果表明CdS0.2Se0.8纳米晶体掺杂的玻璃是一种很好的非线性光学材料。     

Deposition of thin Bi2Te3 and Sb2Te3 films by pulsed laser ablation

Bi₂Te₃ and Sb₂Te₃ films were obtained by pulsed laser ablation. The films were deposited in vacuum (1 × 10⁻⁵ Torr) on single crystal substrates of Al₂O₃ (0001), BaF₂ (111), and fresh cleavages of KCl or NaCl (001) heated to 453–523 K. The films were 10–1500 nm thick. The structures of the bulk material of targets and films were studied by X-ray diffractometry and transmission high-energy electron diffraction, respectively. Electrical properties of the films were measured in the temperature range of 77–300 K. It is shown that the films possess semiconductor properties. Several activation portions are observed in the temperature dependences of resistivity; the energies of activation portions depend on the film thickness and crystallite size.     

Thermal stress effects of Ge2Sb2Te5 phase change material: Irreversible modification with Ti adhesion layers and segregation of Te

This paper reviews material properties of chalcogenide phase change material Ge₂Sb₂Te₅ under thermal anneal treatments. Stress evolutions of pure Ge₂Sb₂Te₅ films and stacks of Ge₂Sb₂Te₅ integrating with Ti adhesion layers are investigated. Segregation of Te atoms in the Ge₂Sb₂Te₅ film to the interface drives an interaction between Ti and Te atoms and formation of Ti-Te binary phases. The irreversible phase segregation and modification of Ge₂Sb₂Te₅ change the crystallization process, completely suppress the final transformation into otherwise stable hcp phase, and thus impact the ultimate life-cycle of such a phase change based memory cell. Since the adhesion layer is required in cell applications, the optimization of adhesion layer material and thickness may improve the life-cycles and reliability of devices.     

Fabrication of fluoride glass single-mode fibers

Single-mode optical fibers are obtained using ZrF4-based fluoride glasses. The fibers are drawn from a preform and jacketing tube. The preform with cladding/core ratio of 5.1 is made by using a built-in casting method. The cutoff wavelength of the fiber is experimentally determined to be 2.7 μm by bending loss measurement. Minimum transmission loss of the obtained fiber is 160 dB/km at a wavelength of 3.28 μm. TheV-value at this wavelength is estimated to be 2.03 from the core diameter and the refractive index difference.