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近年来, 紫外探测器因为在导弹预警、火焰探测、臭氧层空洞监测, 以及紫外消毒剂量检测等方面的广泛应用而受到全世界研究人员的关注[1-4]. 传统的紫外探测器制备材料主要以第一代半导体和第二代半导体为主, 这些半导体禁带宽度小, 截止波长较大, 用于紫外探测时往往需要加上一层滤光片, 已不能满足目前的需求.
氮化镓(GaN)作为第三代半导体, 具有3.4 eV的带隙, 对应的吸收边为365 nm, 是一种天然的紫外探测材料[5,6]. 同时优异的物理和化学性能使得它制备的器件具有较高的稳定性. 近年来对基于GaN材料的紫外探测器的研究主要有以下几种结构, 如金属-半导体-金属(metal-semiconductor-metal, MSM)型[7]、肖特基结型[8], 以及p-n结型[9-11]等. 对于MSM型探测器, 主要的问题来自于光熄灭后持续的光电导效应[12], 这主要归因于固有缺陷的亚稳态, 如Ga空位和缺陷[13]. 近年来, 研究人员进行了不断的优化, 制备了具有出色光响应性的探测器, 但是这些探测器通常需要外加偏压工作, 这无形中增大了探测器的尺寸不利于小型化的发展趋势, 同时响应速度也较慢. 在p-n结和肖特基结的结接触区由于不同材料之间的电势差会产生内建电场, 它使得光生载流子可以自发且快速的分离, 不仅能够提高响应速度同时还具有自供电的效果. 肖特基结型探测器由于表面的金属电极阻碍了紫外光的入射, 使得探测器的光响应度不佳, 从而限制了它的发展. 相比较而言, 具有自供电、快速光响应的p-n结探测器无疑具有巨大优势, 有利于未来小型化、高效化、智能化的集成网络发展.
最近对基于GaN材料的p-n结探测器也有一些研究, 例如Su等[14]利用金属有机物化学气相淀积(metal-organic chemical vapor deposition, MOCVD)和分子束外延的方法分别沉积p-GaN和n-ZnO构成p-n结器件, 在0 V下对358 nm的光显示出0.68 mA/W的响应度; Zhu等[15]通过MOCVD法在p-GaN衬底上制备n-ZnMgO构成p-n结器件, 0 V下对362 nm的光具有196 mA/W的响应. 前者制备方法简单, 但是器件的光响应性不佳. 后者光响应性有所提升但n-ZnMgO掺杂制备过程较为困难. NiO作为一种天然的p型半导体材料, 由于其较高的空穴迁移率而常被用作空穴传输材料[16]. 优良热稳定性和高透明度, 以及低成本制备的方法(磁控、旋涂)使得NiO成为一种合适的材料用于构筑p-n结器件.
之前的研究表明, NiO与GaN之间具有良好的外延关系以及匹配的能带结构[17-19]. Li等[20]曾报道了NiO/GaN p-n结在探测器方面的应用, 但是没有研究自供电性能, 另外利用热氧化将Ni氧化为NiO的两步制备法, 不仅工艺复杂而且无法保证Ni完全被氧化为NiO.
本文通过磁控溅射的方法, 在500 ℃下制备了NiO薄膜. GaN和NiO薄膜良好的结晶性使得器件暗电流仅为10–10 A, 匹配的能带结构使得制备的GaN/NiO p-n结具有明显的二极管整流特性. 在没有外加偏压的情况下, 探测器对365 nm的紫外光显示出272.3 mA/W的响应度以及高达2.83 × 1014 Jones的探测率.
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利用磁控溅射的方法制备p-NiO薄膜, 靶材选用纯度为99.99%的NiO陶瓷靶, 溅射条件为500 ℃, Ar流量为20 sccm (1 sccm = 1 mL/min), O2流量为20 sccm, 压强为2.0 Pa, 溅射功率为50 W. 衬底选用的是苏州纳维科技有限公司所购买的n-GaN厚膜自支撑片, 采用MOCVD法在2英寸的(0001)面蓝宝石衬底上制备, 厚度约为4.5 μm, 载流子浓度为3.2 × 1018. 在沉积NiO薄膜之前, 将GaN切成1 cm × 1 cm的小片. 为了进行对比, 同样选取了(0001)面蓝宝石作为衬底沉积NiO薄膜.
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分别在GaN与NiO膜上方磁控溅射Ti/Au作为复合电极. 通过X射线衍射(X-ray diffraction, XRD, D8Discovery)、紫外-可见分光光度计(UV-2600)、场发射扫描电子显微镜(scanning electron microscope, SEM, HITACHI S-4800)分别对NiO膜和GaN膜进行表征. 利用半导体测量系统(4200-SCS)对器件的光电性能进行测试, 使用的光源波长分别为254和365 nm.
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在Al2O3衬底上生长的不同时间的NiO薄膜的XRD结果如图1(a)所示, 在36.5°附近显示出明显的特征衍射峰并且除此之外再无其他衍射峰, 说明制备的NiO薄膜具有良好的结晶性并且沿着(111)晶面择优生长. 随着溅射时间的增加, 薄膜的厚度增大, 晶体的衍射峰强度升高, 半高峰宽减小, 相应的结晶性变好. 图1(a)中其余3个位置的衍射峰均来源于蓝宝石衬底, 其中41.7°对应Al2O3的(0001)面, 37.5°和40°位置的小峰则对应Al2O3的(004)和(200)面. 图1(b)是NiO薄膜紫外-可见吸收谱, 可以看到NiO薄膜对紫外光有着强烈的吸收, 利用Tauc等提出的公式[21]
图 1 生长在蓝宝石衬底上的NiO薄膜的XRD图谱(a)和紫外-可见吸收图谱(b)以及NiO光学带隙(插图); 生长在GaN膜上的NiO薄膜的XRD图谱(c)和紫外-可见吸收图谱(d)以及GaN的光学带隙(插图)
Figure 1. (a) XRD patterns and (b) UV-vis absorption spectra of the NiO film deposited on sapphire substrate (0001) plane. (panel (b) insert) Plots of (αhν)2 versus photon energy of the NiO film; (c) XRD patterns and (d) UV-vis absorption spectra of the NiO film deposited on GaN film. (panel (d) insert) Plots of (αhν)2 versus photon energy of the GaN film.
$ {\left(\alpha hv\right)}^{2}=A\left(hv-{E}_{\mathrm{g}}\right) $ 可以计算得出NiO薄膜的带隙为
$ {E}_{\mathrm{g}\mathrm{N}\mathrm{i}\mathrm{O}} $ = 3.24 eV. 图1(c)为在GaN衬底上生长的NiO薄膜的XRD图, 由于GaN的衍射峰太强和NiO膜较薄, 所以只能观察到溅射2 h的NiO薄膜的(111)晶面的衍射峰, 可看到在GaN上生长的NiO和在Al2O3上生长的NiO具有相同的择优生长方向. 图1(d)为GaN薄膜和GaN/NiO复合薄膜的吸收光谱, 可看到GaN薄膜对365 nm附近的紫外光具有强烈的吸收, 并且复合了NiO薄膜之后, 其对波长大于365 nm的光没有明显变化, 但对小于365 nm的紫外光吸收有明显的增强. 说明覆盖的NiO薄膜具有良好的可见光透过性, 不仅没有阻碍光的透过反而增强了光的吸收, 有利于制备p-n结器件. 图1(d)插图显示GaN的光学带隙${E}_{\mathrm{g}\mathrm{G}\mathrm{a}\mathrm{N}} \!=\!3.36\;{\rm eV}$ .之后对NiO/GaN p-n结的电流-电压(I-V)特性进行了测试, 如图2(c)所示, 在黑暗条件下显示出了明显的整流特性, 插图为器件的简单示意图. 为了验证这个整流效应是否来源于GaN与NiO构成的p-n结, 分别对单层NiO MSM结构和单层GaN MSM结构在相同条件下进行了I-V测试, 结果如图2(a)和图2(b)所示. 其中NiO显示出了良好的欧姆接触, GaN显示出了准欧姆接触. 插图中分别显示了两个器件在0 V偏压下对365 nm紫外光的电流-时间(I -T )光响应特性曲线, 可以看到此时两个器件在不外加电压的情况下几乎没有光电流产生. 以上结果表明图2(c)所观察到的整流特性来源于GaN与NiO形成的p-n结, 同时± 0.5 V下整流比大于102. 图2(d)显示出了不同光强的365 nm紫外光照射下NiO/GaN p-n结器件的I -V特性, 可以观察到在0 V下器件具有明显的光响应, 并且随着光强的增大光电流值增加.
图 2 (a) 在365 nm光照下和黑暗中的NiO MSM结构的I -V曲线, 插图NiO MSM结构示意图和0 V下的I -T曲线; (b) 在365 nm光照下和黑暗中的GaN MSM结构的I -V曲线, 插图为GaN MSM结构示意图和0 V下的I -T曲线; (c) 黑暗中NiO/GaN p-n结的I -V特性, 插图为NiO/GaN p-n结器件结构示意图; (d) 不同强度的365 nm光照下NiO/GaN p-n结的I -V特性
Figure 2. (a) I-V curves of the NiO MSM structure in dark and under 365 nm light illumination, (insert) diagram of the NiO MSM structure and I -T curve under zero bias; (b) I -V curves of the GaN MSM structure in dark and under 365 nm light illumination, (insert) diagram of the GaN MSM structure and I -T curve under zero bias; (c) I -V curve of the NiO/GaN p-n junction in dark, (insert) diagram of the device based on NiO/GaN p-n junction; (d) I -V curves of the NiO/GaN p-n junction under 365 nm light with various light intensities.
基于NiO/GaN p-n结的光电探测器的结构示意图如图3(a)所示, 下方为Al2O3衬底, 中间的GaN层约4.5 μm厚, 上方的NiO层约70 nm厚(图3(b)), Ti/Au电极约70 nm厚(如插图所示), 不同层之间具有清晰的边界.
图 3 (a) 基于NiO/GaN p-n结的光电探测器结构示意图; (b) NiO/GaN p-n结的截面SEM图, 插图为镀有电极的p-n结截面SEM放大图
Figure 3. (a) Schematic illustration of the fabricated prototype NiO/GaN p-n junction photodetector; (b) cross-sectional SEM image of the NiO/GaN p-n junction, where the insert is the enlargement cross-sectional SEM image of p-n junction with electrode plating.
在没有外加偏压的情况下, 探测器对紫外光具有明显响应, 例如在700 μW/cm2的365 nm光照射下, 电流值从黑暗条件下的0.17 nA迅速上升至275 nA, 在1300 μW/cm2的254 nm光照下, 光电流值从0.17 nA迅速上升至223 nA. 关闭光照后, 探测器的电流值迅速下降到初始水平(图4(a)). 其中对于365 nm和254 nm光的开关比(
$ {I}_{\mathrm{o}\mathrm{n}}/{I}_{\mathrm{o}\mathrm{f}\mathrm{f}} $ )分别达到1617和1311. 之后对探测器的光响应速度进行了测试, 结果如图4(b)所示, 其中$ {\tau }_{\mathrm{r}}/{\tau }_{\mathrm{d}} $ 分别为37 ms/31 ms. 为了进一步了解NiO/GaN p-n结内部载流子的输运情况, 图4(c)给出了NiO/GaN p-n的能带结构. 其中GaN和NiO的电子亲和能($ \chi $ )分别为4.2 eV和1.8 eV, 上面测得$ {E}_{\mathrm{g}\mathrm{N}\mathrm{i}\mathrm{O}} $ = 3.24 eV,$ {E}_{\mathrm{g}\mathrm{G}\mathrm{a}\mathrm{N}} $ = 3.36 eV, 由此可以计算得出导带差($ {\Delta }{E}_{\mathrm{C}} $ )和价带差($ {\Delta }{E}_{\mathrm{V}} $ ):
图 4 (a) 0 V电压下探测器对254和365 nm光照的I -T响应; (b) 对365 nm的光响应速度拟合; (c) NiO/GaN p-n结的能带图; (d) 不同偏压下探测器对365 nm光照的I -T响应
Figure 4. (a) I -T curves of the photodetector under a zero bias at 254 and 365 nm illumination; (b) enlarged view of the rise/decay edges and the corresponding exponential fitting; (c) energy band diagrams of NiO/GaN p-n junction; (d) I -T curves of the photodetector under various biases with a 365 nm light illumination.
$ {\Delta }{E}_{\mathrm{C}}={\chi }_{\mathrm{G}\mathrm{a}\mathrm{N}}-{\chi }_{\mathrm{N}\mathrm{i}\mathrm{O}}=2.4\;\mathrm{e}\mathrm{V}, $ $ {\Delta }{{E}}_{\mathrm{V}}={E}_{\mathrm{g}\mathrm{G}\mathrm{a}\mathrm{N}}-{E}_{\mathrm{g}\mathrm{N}\mathrm{i}\mathrm{O}}+{\Delta }{E}_{\mathrm{C}}=2.52\;\mathrm{e}\mathrm{V}, $ 其中较大的势垒差有利于光生载流子的分离并抑制复合, 从而增大了光电流并降低了暗电流. 另外势垒差还有助于载流子的输运, 加快了光响应速度. 不同偏压下探测器对365 nm的I -T响应如图4(d)所示, 随着反向偏压的增大, 暗电流首先增大, 这是因为在电场的作用下, 释放出了氧空位所捕获的载流子, 光电流增大的更加明显, 是因为施加的电场促进了光生载流子的有效分离. 在外加偏压的条件下, 探测器依然显示出良好的稳定性.
随着365 nm光照强度的增加, 探测器的光电流明显增加, 从50 μW/cm2强度下的82 nA增加到700 μW/cm2强度下的275 nA (如图5(a)和图5(b)所示). 同时可以看到, 随着光强的增加, 光响应度(R)逐渐降低(图5(b)), 计算公式为
$R= $ $ {I}_{\mathrm{p}\mathrm{h}}/\left(PS\right)$ , 其中Iph为光电流, P为光强, S为有效面积. 光强为50 μW/cm2时光响应度(R)达到最大值(273.2 mA/W). 探测率(D)是评价器件灵敏度的一项重要指标, 计算公式为$D= R{S}^{\tfrac{1}{2}}/{\left(2\mathrm{e}{I}_{\mathrm{d}\mathrm{a}\mathrm{r}\mathrm{k}}\right)}^{\tfrac{1}{2}}$ , 图5(c)显示了探测率随光强变化的函数关系图像. 随着光强的增大, 探测率逐渐降低, 在50 μW/cm2时达到最大值2.83 × 1014 Jones. 将本文制备的NiO/GaN p-n紫外探测器与最近报道的其他在0 V下工作的探测器进行比较(表1), 结果表明本文制备的探测器具有优异的性能.
图 5 (a) 0 V偏压下探测器对不同光强的365 nm光照的I-T响应; (b) 光电流与响应度随光强的变化; (c) 探测率随光强的变化
Figure 5. (a) Time-dependent photoresponse of the photodetector under zero bias and a 365 nm light with various light intensities; (b) photocurrent and responsivity as a function of light intensity; (c) detectivity as a function of light intensity.
Photodetector Wavelength Responsivity/(mA·W–1) Detectivity/Jones Rise/Decay time/ms Ref. GaN/ZnO 350 nm 95.8 2.9 × 1012 730/50 [22] GaN/r-GO:Ag NPs 360 nm 266 2.62 × 1011 680/700 [23] GaN/NiO 365 nm 150 — — [20] GaN/Ga2O3 365 nm 54.49 — — [24] r-GO/GaN 350 nm 1.54 1.45 × 1011 60/267 [25] ZnO nanoarrays/CdS/GaN 300 nm 176 2.5 × 1012 350 [26] NiO/GaN 365 nm 272.3 2.83 × 1014 31/37 This work 表 1 基于GaN的自供电探测器件性能参数比较
Table 1. Self-powered device parameters comparison of photodetectors based on GaN from previous works and this work.
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本文通过磁控溅射的方法在蓝宝石衬底以及n-GaN厚膜衬底上沉积了NiO薄膜, XRD结果显示制备的薄膜具有良好的结晶性能, SEM测得薄膜厚度约为70 nm. 在n-GaN厚膜衬底上成功制备了NiO/GaN p-n结并以此构建了紫外探测器. p-n结在黑暗中表现出典型的整流特性. 由于内建电场的存在, 探测器可以在没有外加偏压的条件下工作. 在0 V偏压下, 探测器对365 nm紫外光显示出高达2.83 × 1014 Jones的探测率, 同时光响应度达到272.3 mA/W, 响应速度达到31 ms. 本文研究结果表明, NiO/GaN p-n结在紫外探测器领域有着广阔的应用前景, 为自供电探测技术的发展提供了新的思路.
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紫外探测器在火灾预警、导弹跟踪以及紫外线杀菌消毒的剂量检测方面有着很重要的作用, 与人类生活息息相关. 随着探测系统集成化的发展, 对探测器的尺寸、能耗等方面的要求越来越严格, 需要外加电源工作的传统探测器已经不能满足这样的要求. 于是本文提出了一种基于NiO/GaN p-n结的紫外探测器. 利用磁控溅射的方法, 在高质量的n-GaN膜上(由金属有机化学气相沉积生长在蓝宝石衬底上)沉积一层p-NiO, 构建了NiO/GaN p-n结, 在 ± 0.5 V下显示出明显的二极管整流特性. 利用结区产生的内建电场, 器件可以在没有外加偏压的条件下工作. 0 V下对365 nm的紫外光显示出272.3 mA/W的响应度以及高达2.83 × 1014 Jones的探测率. 得益于薄膜良好的结晶性, 暗电流低至10–10 A, 开关比 > 103, 同时响应速度达到31 ms. 这些优异的性能显示出了基于NiO/GaN p-n结的器件在紫外探测领域广阔的应用前景, 为未来智能化集成发展提供了新的思路.
Ultraviolet photodetector plays an important role in fire warning, missile tracking and dose detecting of ultraviolet sterilization and disinfection, which is closely related to human lives. With the development of integrated detection system, the requirements for the size and energy consumption of the detector are becoming more and more stringent. Traditional detector that requires an external power supply can no longer meet these requirements. Moreover, a traditional ultraviolet detector is mainly composed of first-generation semiconductors and second-generation semiconductors. These semiconductors have small band gaps and large cut-off wavelengths, and are more suitable for infrared detection. When used for implementing the ultraviolet detection, an additional layer is often required, which increases not only the volume but also the cost. Gallium nitride (GaN), as a third-generation semiconductor, has a band gap of 3.4 eV and a corresponding absorption edge of 365 nm. It is a natural ultraviolet detection material. At the same time, the excellent physical and chemical properties make the devices prepared by GaN have high stability. In recent years, some studies have shown that the GaN-based ultraviolet photodetectors have excellent responsiveness, but each of these detectors usually requires an external bias and has a slow response speed. Here, we propose a high responsivity, fast response speed and self-powered ultraviolet photodetector based on NiO/GaN p-n junction. By using the magnetron sputtering, a layer of 70 nm thick p-NiO film is deposited on a high-quality n-GaN film that has been grown on a sapphire substrate by the metal-organic chemical vapor deposition. The fabricated p-n junction shows obvious rectification characteristics at ± 0.5 V. Due to the existence of the built-in electric field, the device can work without externally applied bias. Under zero bias, the detector shows a responsivity of 272.3 mA/W for 365 nm ultraviolet light while the intensity is 50 μW/cm2, and has a detectivity as high as 2.83 × 1014 Jones. This indicates that the detector has a high sensitivity even for very weak light. Owing to the good crystallinity of the film, the dark current is as low as 10–10 A, the switching ratio is > 103, and the response speed reaches 31 ms. These excellent properties show the broad application prospects of the devices based on NiO/GaN p-n junctions in the field of self-powered ultraviolet detection, and thus providing new ideas for the future development of intelligent integration. -
Keywords:
- ultraviolet photodetector /
- self-powered technology /
- GaN /
- NiO
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图 1 生长在蓝宝石衬底上的NiO薄膜的XRD图谱(a)和紫外-可见吸收图谱(b)以及NiO光学带隙(插图); 生长在GaN膜上的NiO薄膜的XRD图谱(c)和紫外-可见吸收图谱(d)以及GaN的光学带隙(插图)
Fig. 1. (a) XRD patterns and (b) UV-vis absorption spectra of the NiO film deposited on sapphire substrate (0001) plane. (panel (b) insert) Plots of (αhν)2 versus photon energy of the NiO film; (c) XRD patterns and (d) UV-vis absorption spectra of the NiO film deposited on GaN film. (panel (d) insert) Plots of (αhν)2 versus photon energy of the GaN film.
图 2 (a) 在365 nm光照下和黑暗中的NiO MSM结构的I -V曲线, 插图NiO MSM结构示意图和0 V下的I -T曲线; (b) 在365 nm光照下和黑暗中的GaN MSM结构的I -V曲线, 插图为GaN MSM结构示意图和0 V下的I -T曲线; (c) 黑暗中NiO/GaN p-n结的I -V特性, 插图为NiO/GaN p-n结器件结构示意图; (d) 不同强度的365 nm光照下NiO/GaN p-n结的I -V特性
Fig. 2. (a) I-V curves of the NiO MSM structure in dark and under 365 nm light illumination, (insert) diagram of the NiO MSM structure and I -T curve under zero bias; (b) I -V curves of the GaN MSM structure in dark and under 365 nm light illumination, (insert) diagram of the GaN MSM structure and I -T curve under zero bias; (c) I -V curve of the NiO/GaN p-n junction in dark, (insert) diagram of the device based on NiO/GaN p-n junction; (d) I -V curves of the NiO/GaN p-n junction under 365 nm light with various light intensities.
图 3 (a) 基于NiO/GaN p-n结的光电探测器结构示意图; (b) NiO/GaN p-n结的截面SEM图, 插图为镀有电极的p-n结截面SEM放大图
Fig. 3. (a) Schematic illustration of the fabricated prototype NiO/GaN p-n junction photodetector; (b) cross-sectional SEM image of the NiO/GaN p-n junction, where the insert is the enlargement cross-sectional SEM image of p-n junction with electrode plating.
图 4 (a) 0 V电压下探测器对254和365 nm光照的I -T响应; (b) 对365 nm的光响应速度拟合; (c) NiO/GaN p-n结的能带图; (d) 不同偏压下探测器对365 nm光照的I -T响应
Fig. 4. (a) I -T curves of the photodetector under a zero bias at 254 and 365 nm illumination; (b) enlarged view of the rise/decay edges and the corresponding exponential fitting; (c) energy band diagrams of NiO/GaN p-n junction; (d) I -T curves of the photodetector under various biases with a 365 nm light illumination.
图 5 (a) 0 V偏压下探测器对不同光强的365 nm光照的I-T响应; (b) 光电流与响应度随光强的变化; (c) 探测率随光强的变化
Fig. 5. (a) Time-dependent photoresponse of the photodetector under zero bias and a 365 nm light with various light intensities; (b) photocurrent and responsivity as a function of light intensity; (c) detectivity as a function of light intensity.
表 1 基于GaN的自供电探测器件性能参数比较
Table 1. Self-powered device parameters comparison of photodetectors based on GaN from previous works and this work.
Photodetector Wavelength Responsivity/(mA·W–1) Detectivity/Jones Rise/Decay time/ms Ref. GaN/ZnO 350 nm 95.8 2.9 × 1012 730/50 [22] GaN/r-GO:Ag NPs 360 nm 266 2.62 × 1011 680/700 [23] GaN/NiO 365 nm 150 — — [20] GaN/Ga2O3 365 nm 54.49 — — [24] r-GO/GaN 350 nm 1.54 1.45 × 1011 60/267 [25] ZnO nanoarrays/CdS/GaN 300 nm 176 2.5 × 1012 350 [26] NiO/GaN 365 nm 272.3 2.83 × 1014 31/37 This work 
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