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HgTe量子点合成用油胺作为反应溶剂。在100 ℃条件下,将无机汞盐及单质碲分别溶解于油胺及三辛基膦中。在无水无氧环境中将其混合,通过控制反应时间,量子点尺寸可以得到精确控制,进而实现对响应波长的准确调节。图1所示为笔者实验所用HgTe量子点的透射电子显微镜(transmission electron microscopy, TEM)图片,其直径约8 nm。使用傅里叶红外光谱仪,可以对其吸收截止波长进行精准测量,室温下吸收截止波长约3.8 μm(图1)。图2所示为量子点室温和80 K下的响应光谱。在80 K工作温度下,量子点探测器的响应截止波长达到4.6 μm。
图 1 HgTe量子点的透射电子显微镜图片和室温下的吸收光谱
Figure 1. TEM image and absorption spectra of HgTe quantum dots (CQDs) at room temperature
图 2 HgTe量子点探测器室温和80 K下的响应光谱
Figure 2. Response spectra of HgTe CQDs-based photodetectors at room temperature and 80 K
为了精确控制HgTe量子点的掺杂,可采用混合相配体交换法,包括液相配体交换、无机盐掺杂改性和固相配体交换。在液相配体交换过程中,HgTe量子点中需加入β-巯基乙醇取代原有的油胺配体。同时,HgTe量子点会从非极性的正己烷溶液转移到极性溶液N, N二甲基甲酰胺(N, N-Dimethylformamide, DMF)。然后,在量子点/DMF混合物中加入HgCl2或(NH4)2S,以进行电子或空穴掺杂,进而实现本征、n型、p型HgTe量子点的制备。
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HgTe中波红外探测器采用捕获模式光电探测器结构。器件结构及能带示意图如图3所示。
图 3 (a) 捕获型器件结构示意图;(b)捕获型器件能带示意图
Figure 3. (a) Device structure diagram of trapping-mode photodetectors; (b) Energy band diagram of trapping-mode photodetectors
捕获型器件在本征量子点通道的顶部添加了n型重掺杂HgTe量子点作为电子陷阱层,进而在n型及本征层量子点界面处产生耗尽层,并在垂直方向产生内建电场。当入射光子被吸收后,产生光生电子及空穴。由于内建电场的存在,光生电子被驱动至n型量子点层附近,空穴被驱动至本征量子点层。在外加偏压作用下,空穴横向移动被电极收集,形成光电流。光生电子由于内建电场作用,被短暂捕获于n型量子点层内,使得空穴复合概率降低,产生了光电流增益。探测器中的每个像素包括一对电极,即像素电极及地电极。偏置电压施加在像素电极和地电极之间以产生横向电场来分离和驱动激活的空穴和电子。捕获型光电探测器将外部电场和内部电场结合在一起促进了载流子的移动和传输[15-16]。
HgTe量子点可与读出电路直接耦合,实现晶圆级探测器制备。将8 in的焦平面读出电路晶圆用丙酮进行清洗,旋涂本征型HgTe量子点溶液使其均匀地平铺在晶圆表面。通过多次旋涂,在本征型HgTe量子点薄膜厚度达到450 nm之后,旋涂50 nm厚度的n型HgTe量子点。最终,将8 in晶圆裁切成单独的成像器芯片。
如图4所示,将裁切出的成像芯片粘贴到PCB转接板上,并用铝线键合机进行线绑定引出信号,在液氮杜瓦瓶中进行降温测试,采用F数为2的冷屏屏蔽噪声信号。
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对捕获型红外焦平面探测器的性能进行定量分析,测试参数包括响应非均匀性、噪声电压、比探测率、有效像元率等。使用校准后的黑体作为激发光源,用反馈控制电路稳定黑体的温度。黑体的发射腔直径约为4 cm,成像仪与发射腔之间的距离约为25 cm。黑体辐射源辐照在焦平面探测器上的红外光可保证各个像元受到均匀辐照,图5(a)为探测器的响应分布图和统计直方图,由图中的颜色分布可知各个像素点的响应均匀。响应统计直方图横坐标为像素点的响应电压值,纵坐标为该响应电压值下的像素点个数。直方图的半高宽越窄说明探测器的像素点之间的响应越相近,实验结果表明量子点焦平面阵列器件的响应非均匀性低至3.42%。
图 5 探测器的(a)响应分布图和响应统计直方图、(b)噪声分布图和噪声统计直方图、(c)比探测率分布图和比探测率统计直方图
Figure 5. (a) Response distribution diagram and response statistics histogram, (b) noise distribution diagram and noise statistics histogram, (c) detectivity distribution diagram and detectivity statistics histogram of photodetectors
探测器的噪声是衡量性能的重要指标,由出读出电路本身噪声和探测器像素点薄膜厚度的均匀性决定,晶圆级旋涂工艺提高了单片器件的薄膜厚度均匀性,也降低了器件噪声。由图5(b)可知探测器的噪声整体较低,在积分时间2 ms、器件偏压2.3 V时,平均噪声电压低至0.66 mV。图5(c)为探测器比探测率分布图,平均峰值比探测率约为2×1010 Jones。图6为探测器的盲元图,蓝色代表死像元,红色代表过热像元,有效像元率可达99.99%。
噪声等效温差(NETD)是红外热探测器不同于可见光探测器的主要指标之一,反映了探测器的温度灵敏性。将方形面源黑体采集探测器在20 ℃和35 ℃温度下的响应电压值,代入公式(1)计算探测器的平均噪声等效温差。
$$ N E T D=\frac{1}{M \times N-(d+h)} \sum_{i=1}^{M} \sum_{j=1}^{N}\left(\frac{T-T_{0}}{V_{s}(i, j) / V_{N}(i, j)}\right) $$ (1) 式中:M为像元总行数;N为像元总列数;d为死像元;h为过热像元;T为高的黑体温度;T0为低的黑体温度;Vs(i,j)为黑体温差下的响应电压值;VN(i,j)为低温下输出的电压值。图7为探测器积分时间与NETD的关系图,由于增加积分时间探测器的信噪比获得提升,NETD值不断减少,探测器的温度灵敏性增加。当积分时间增加至8 ms时,探测器的NETD为51.26 mK。
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捕获型中波红外探测器可进行红外热成像。图8(a)为50 ℃热水和200 ℃电烙铁的成像图。图8(b)从左到右分别为装有50、0、25 ℃水的水杯热成像图。图8(c)和(d)分别为人体热成像和手持手机的人体热成像图。
图 8 (a) 50 ℃热水和200 ℃电烙铁的成像图;(b)从左到右分别为装有50、0、25 ℃水的成像图;(c)人体热成像;(d)手持手机的人体热成像图
Figure 8. Thermal images of (a) 50 ℃ water and 200 ℃ soldering iron, (b) bottles with water at temperature of 50 ℃, 0 ℃ and 25 ℃ (from left to right); (c) human body and (d) human body with phones
如表1所示,文中所介绍中波HgTe量子点焦平面相比PbS及光导型HgTe中波焦平面,在探测波长及NETD等核心参数指标方面,具有领先优势。
表 1 文中工作与前期量子点焦平面阵列指标对比
Table 1. Device performance comparison between this work and the previous CQDs-based focus plane arrays
640×512 HgTe colloidal quantum-dot mid-wave infrared focal plane array (invited)
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摘要: 中波红外成像在军事侦察、遥感测绘、航天航空等领域发挥了重要作用。现有中波红外焦平面主要采用碲镉汞、二类超晶格、锑化铟等块体半导体材料,其性能优异、稳定性高。然而,其复杂的材料制备及倒装键合工艺限制了块体半导体焦平面阵列的批量化制备及低成本应用。胶体量子点作为一种新兴液态半导体材料,具有光谱调控范围“宽”、合成规模“大”、制备成本“低”、以及加工工艺“易”等优势,为新型红外焦平面阵列研发提供了全新的思路。碲化汞量子点采用“热注法”合成,并通过旋涂方法实现与硅基读出电路的直接电学耦合,阵列规模及像元间距为640×512及15 µm。在80 K工作温度下对焦平面阵列进行了性能测试,碲化汞焦平面阵列响应截止波长达到4.6 μm、比探测率为2×1010 Jones、噪声等效温差51.26 mK(F#=2)、响应非均匀性3.42%且有效像元率高达99.99%,展现了较好的成像性能,为非倒装键合体制中波红外成像焦平面的制备提供了新的方案。Abstract:
Objective Mid-wave infrared imaging plays an important role in various fields including military reconnaissance, remote sensing, and aerospace. The existing mid-wave infrared focal planes mainly use bulk semiconductor materials such as mercury cadmium telluride, type-II superlattices, and indium antimonide, which have excellent performance and high stability. However, the complex material preparation and flip-chip bonding processes limit the production volume and their usage in cost-sensitive application. As an emerging infrared semiconductor material, colloidal quantum dots (CQDs) have the advantages of wide spectral tunability, large-scale synthesis, and low-cost preparation, providing a new route towards high-performance and low-cost infrared focal plane arrays. For this purpose, HgTe CQDs have been investigated and a mid-wave infrared focal plane array imager has been proposed in this paper. Methods Oleylamine was used as the reaction solvent for the synthesis of HgTe CQDs. Inorganic mercury salts and tellurium were dissolved in oleylamine and trioctylphosphine, respectively, at 100 ℃. After mixing them in an anhydrous and oxygen-free environment, the size of the HgTe CQDs can be precisely controlled by the reaction time, thus the response wavelength can be accurately adjusted. The transmission electron microscopy (TEM) image of the HgTe quantum dots used in this experiment is shown (Fig.1), with a diameter of about 8 nm. The response spectra of quantum dots at room temperature and 80 K are shown (Fig.2). The response cut-off wavelength of the quantum dot detector reaches 4.6 μm at 80 K. The HgTe CQDs mid-wave infrared detector uses a trapping-mode photodetector configuration. The device structure and energy band diagram are shown (Fig.3). Results and Discussions The diagram of signal extraction and dewar test package is shown (Fig.4). The performance of the trapping-mode infrared focal plane detector is quantitatively analyzed by testing parameters including photoresponse non-uniformity, noise voltage, specific detectivity, and operable pixel rate. A calibrated blackbody is used as the excitation light source, and the temperature of the blackbody is stabilized with a feedback control circuit. The blackbody emitting cavity is about 4 cm in diameter and the distance between the imager and the emitting cavity is about 25 cm. The experimental results show that the non-uniformity of the photoresponse of the focal plane array device is as low as 3.42% (Fig.5(a)). The noise of the detector is an important indicator of performance, which is determined by the noise of the readout circuit itself and the uniformity of the film thickness of the detector pixel points. The overall noise of the detector is low, and the average noise voltage is as low as 0.66 mV at an integration time of 2 ms and a device bias of 2.3 V (Fig.5(b)). The distribution of the specific detectivity, and the average peak specific detectivity is about 2 × 1010 Jones (Fig.5(c)). The operable pixel rate can reach 99.99% (Fig.6). Conclusions In this paper, we report a CMOS-compatible trapping-mode HgTe CQDs mid-wave infrared focal plane and demonstrate the infrared thermal imaging capability. With a noise equivalent temperature difference of 51.26 mK (F#=2), a low photoresponse nonuniformity of 3.42%, an operable pixel rate of 99.99%, a response cutoff wavelength of 4.6 μm, and a peak specific detectivity of 2×1010 Jones at 80 K, the HgTe CQDs-based focal plane array is expected to potentially solve the bottlenecks faced by traditional bulk semiconductors. In the future, HgTe CQDs will be combined with 3D nanostructure embossing and other processing technologies to develop multi-functional and multi-mode infrared detectors. -
表 1 文中工作与前期量子点焦平面阵列指标对比
Table 1. Device performance comparison between this work and the previous CQDs-based focus plane arrays
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