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本文中提出的SP-LED发光原理如图 1所示。相比于普通的LED,该LED除了辐射复合能够产生光子外,还能通过珀塞尔效应即通过SP与量子阱耦合产生更多的光子。与此同时,SP-LED因为具有光栅结构,所以除了在全反射临界角以内的光子可以逃逸出LED器件外,临界角以外的光子也可从LED器件中被提取出。
因为SP-LED在出光面添加了金属光栅结构,所以可以同时激发SP中的SPP和LSP,当SPP和LSP的谐振频率与量子阱的频率重叠时,SPP和LSP可以与量子阱耦合,从而激发出更多的光子,因此LED的内量子效率将会得到提高。具体也可由下式表示[20]:
$ \eta_{\mathrm{int}}=\frac{k_{\mathrm{rad}}+k_{\mathrm{SP}}}{k_{\mathrm{rad}}+k_{\mathrm{non}}+k_{\mathrm{SP}}} $
(1) $ k_{\mathrm{SP}}=\frac{2 \pi}{h}|d E(\omega)|^{2} \rho(\omega) $
(2) 式中,krad, knon和kSP分别是辐射复合速率、非辐射复合速率和量子阱与SP的耦合速率, d是电子与空穴的偶极矩,h是普朗克常数,ρ(ω)是SP的态密度,E(ω)是SP在介质层中的电场, ω为入射光角频率。当SP被激发后,SP周围可以产生强烈的电场,并且可以提供非常大的ρ(ω),这就会导致kSP急剧增加,可极大地提高内量子效率ηint。添加金属光栅除了可以提高LED的内量子效率,也可以提高LED的光提取效率ηext。SP-LED的光提取效率表达式为[20-21]:
$ \eta_{\mathrm{ext}}=\frac{C_{\mathrm{ext}} k_{\mathrm{rad}}+C_{\mathrm{SP}} k_{\mathrm{SP}}}{k_{\mathrm{non}}+k_{\mathrm{rad}}+k_{\mathrm{SP}}} $
(3) 式中,Cext是光提取效率,CSP是SP光提取效率。为了能够提高CSP,许多课题组选择在金属薄层或光栅下方添加一层介质层,这样可以减少金属薄层或光栅对光子的吸收。
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本文中提出的SP-LED具体结构如图 2a所示。从图 2a中可以看到, SP-LED自下而上分别是蓝宝石衬底、N-GaN层(厚度为400nm)、多量子阱(multiple quantum well,MQW)、P-GaN层、SiO2缓冲层、金属Ag光栅和ITO层。MQW是由5对InGaN/GaN构成。SiO2层的厚度为20nm; Ag光栅的厚度、周期和宽度分别为h2, p和w; h1是ITO层厚度、h3是P-GaN层厚度。图 2b是仅有一层Ag金属薄层的GaN LED结构(Ag-LED)。在利用FDTD软件模拟计算时,GaN, ITO, SiO2的折射率分设为2.5, 2, 1.45。而金属Ag的介电常数则从参考文献[22]中获得,如图 3所示。
对于本文中提出的SP-LED在将来实验制备中工艺也较为简单,首先通过外延设备生长出GaN LED芯片,然后通过等离子体增强化学的气相沉积法在P-GaN上生长一层SiO2层,紧接着利用磁控溅射生长一层Ag薄层,随后利用电子束光刻技术制备出Ag光栅结构,最后通过外延技术在Ag光栅中填充ITO。
利用金属光栅提高LED发光效率的研究
Research on improving LED luminous efficiency by using metal grating
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摘要: 为了提高发光二极管(LED)的发光效率,在LED出光面放置金属光栅,采用时域有限差分法进行了理论分析和模拟计算。结果表明,对光栅优化后,金属光栅对波长460nm的透射率接近1,可提高LED的光提取效率; 在此波长下,可同时激发局域表面等离激元和表面等离极化激元,有助于提高LED内量子效率;且具有金属光栅结构的LED的发光效率是仅在出光面放置一层Ag薄层的LED的30倍。该研究为未来制备高发光效率的LED提供了理论指导。Abstract: In order to improve the luminous efficiency of light-emitting diode (LED), a metal grating was placed on the light emitting surface of LED. The theoretical analysis based on finite difference time domain method shows that the transmittance of metal grating to the wavelength of 460nm is close to 1 after the optimization of the grating, and the light extraction efficiency of LED can be improved. The optimized metal grating can simultaneously excite surface plasmons polariton and the localized surface plasmon at 460nm, which is helpful to improve the internal quantum efficiency of LED. The luminous efficiency of LED with metal grating structure at 460nm is 30 times higher than that of LED with only a thin layer of Ag on light-emitting surface. The research can provide theoretical guidance for the preparation of high luminous efficiency LED in the future.
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