摘要
具有半导体-金属态相变性质的二氧化钒材料可用于光电探测器的激光致盲防护。本文报道了基于磁控溅射法制备二氧化钒薄膜材料的结构、形貌特性,以及在不同温度下的光学性质。使用椭偏光谱法测量了下可见-近红外波段二氧化钒材料的椭偏参数,利用Gaussian、Lorentz模型获取了薄膜在相变前的光学性质,结合Drude模型拟合获取了材料在相变后的光学特性,获取了材料在300~1 700 nm之间的变温折射率和消光系数等参数。变功率下1 550 nm红外激光透射率的实验测试研究表明,VO2薄膜样品的相变阈值功率为12 W/c
关键词
激光武器作为一种新型高科技武器,具备杀伤力强、攻击速度快、射击精度高等优势特
相变材料作为激光致盲防护光学材料的一种,在外界光热激励作用下能发生可逆相变。而相变防护技术正是利用相变材料的这一特性来进行激光防护,该技术恰好能满足现代激光防护中对防护材料兼具红外信号接收与针对激光致盲武器防护功能的要
因此,本文利用椭圆偏振光谱法测量了不同温度条件下白宝石衬底的VO2薄膜材料的光学性质,基于Lorentz、Gaussian获取了薄膜在相变前的光学性质,结合Drude模型拟合获取了材料在相变后的光学特性,报道了材料在300~1 700 nm之间的折射率和消光系数等特性参数,并测定了薄膜在1 550 nm激光照射时产生相变的阈值功率和开关率。
本实验中,VO2薄膜制备利用美国Kurt J. Lesker/LAB Line SPUTTER 5物理气相薄膜沉积系统完成。磁控溅射靶材选用纯度为99.99%的VO2陶瓷靶,直径为50.8 mm,厚度为3 mm,附带3 mm厚的紫铜背板。磁控溅射前背景真空度为4×1

图1 测试示意图,(a) VO2薄膜样品变温椭圆偏振测试示意图;(b)1 550 nm变功率照射下VO2薄膜透射率测试系统
Fig. 1 Test diagram, (a) schematic diagram of variable temperature ellipsometric tests for VO2 thin film samples;(b) 1 550 nm transmission testing system for VO2 thin film under variable irradiation powers
光学显微镜观察表明,420 下后退火的薄膜材料的表面颜色由退火前的蓝黑色变成了退火后的深棕色,而更低温度退火的样品颜色未明显改变。

图2 AFM照片及测试结果,(a) 380后退火制备VO2薄膜的AFM照片;(b) 420后退火制备VO2薄膜的AFM照片;(c) 不同后退火温度下X射线衍射测试结果,扫描电子显微镜薄膜样品截面图样;(d) 420后退火制备VO2薄膜四探针变温电阻率测试结果
Fig. 2 AFM photos and test results, (a) AFM morphology of VO2 film prepared by annealing at 380;(b) AFM morphology of VO2 film prepared by annealing at 420 ;(c) X-ray diffraction test results at different annealing temperatures, scanning electron microscope cross-section of thin film sample;(d) test results of four-probe variable temperature resistivity of VO2 film prepared by annealing at 420
样品 | 物相 | (h k l) | |||
---|---|---|---|---|---|
340 °C | VO2 | () | 64.736° | 0.902 | 11.6 |
380 °C | VO2 | () | 64.437° | 0.910 | 9.7 |
420 °C | VO2 | () | 64.858° | 1.00 | 10.5 |
使用变角度椭圆偏振光谱仪测试了薄膜材料的光学性质参数。依次设定测试温度为20 、30、40、50、60、70、80、90、100。由于变温椭圆偏振光谱测试的温度点较多,所测椭偏参数的数据量较大,因此在本文中一方面给出了30和80下测得的Psi值和Delta值,如

图3 Psi值和Delta值,(a-b)30和80下的Psi值和Delta值测试结果;(c-d) 不同温度(20~100 ℃)下测试角度为65°时的Psi值和Delta的值
Fig. 3 Psi and Delta values, (a-b) Test results of Psi and Delta values at 30 and 80;(c-d) test results of Psi and Delta values under varied temperatures ranging between 20-100 at a fixed incident degree of 65°
材料介电函数变化与材料晶体结构和能带变化密切相关。低温下的VO2材料呈对称性较低的单斜金红石结构(Monoclinic rutile),π*轨道能级高于费米能级,且π*轨道与d||轨道分离,两者之间形成一个0.7 eV的禁带,使得VO2呈现出半导体特性。对于高温相变后的四方金红石结构(rutile)VO2材料而言,半满的d轨道和π*轨道部分重叠,而费米能级介于两者之间,能带部分重合,构成了一个未满的导带,因而容易导电,使得VO2材料呈现金属
本文对于相变发生前的样品选择设置了Gaussian模型与多个Lorentz相结合的椭偏模型;而对于发生相变后的样品的椭偏参数,则设置为Gaussian+Lorentz+Drude模型。材料的光学介电函数主要用于描述材料对光波的响应,介电函数的实部和虚部体现了材料对光的折射和吸收作用。描述半导体态及金属态光学性质的介电常数的色散模型数学表达式如公式(
. | (2) |
. | (3) |
拟合得到20~100 ℃不同温度下材料的n、k值,如

图4 不同温度下VO2薄膜材料的n、k值的变化规律
Fig. 4 Changes of n and k values of VO2 thin films at different temperatures
在变激光功率照射下,测量了VO2薄膜材料透射率变化曲线。入射激光经过一面凹面聚焦镜反射会聚到焦平面的样品位置。通过光路的调节与设置,使得光斑聚焦的位置与CCD相机的成像焦平面位置相重合。此处样品置换为感光检测卡时,检测卡上的亮斑将被成像系统的CCD相机检测到并成像,通过测量亮斑的大小即可大致确定光斑的尺寸。如

图5 功率及透射率,(a) 会聚到样品表面的不同功率激光光斑图样;(b) 不同工作电流下测得有/无VO2样品条件下的功率值;(c) 不同红外激光功率密度下VO2的透射率值;(d) 二氧化钒薄膜高低温条件下相变前后的透射率谱
Fig. 5 Power and transmittance, (a) the pattern of light spots with different powers converged on the sample surface;(b) the measured laser powers with/without VO2 samples at different working currents;(c) the transmittance values of VO2 under different infrared laser power densities; (d) the transmittance spectra of VO2 films before and after the phase transition at 30 ℃ and 80 ℃
如
. | (4) |
如
, | (5) |
, | (6) |
, | (7) |
其中,T为透射率,Rij为第i层介质与第j层介质交界面处的反射率,分别表示空气、VO2薄膜和热压氧化铝衬底,、d和n分别表示不同介质的吸收系数、厚度和折射率。
. | (8) |
将3和的折射率与消光系数、材料厚度等参数代入
本文利用椭圆偏振光谱法测量了不同温度条件下VO2薄膜材料的光学性质。实验结果表明,随着温度的升高,材料在相变前1 550 nm处的折射率为3.2~3.6,消光系数为0.5~1.7,相变后1 550 nm处的折射率降低为1.7~2.0,消光系数增大至3.2~3.6。变功率激光透射率测试结果表明,55 nm厚度氧化钒薄膜材料的相变阈值功率为12 W/c
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