采用ALD方法制備TiO2/Al2O3布拉格反射鏡并配合金屬反射鏡來增強背鍍結(jié)構(gòu)的反射效率*

陳洪鈞，郭 浩，張 雄，崔一平

(東南大學(xué)電子科學(xué)與工程學(xué)院先進光子學(xué)中心，南京210018)

采用ALD方法制備TiO2/Al2O3布拉格反射鏡并配合金屬反射鏡來增強背鍍結(jié)構(gòu)的反射效率*

陳洪鈞，郭 浩，張 雄*，崔一平

(東南大學(xué)電子科學(xué)與工程學(xué)院先進光子學(xué)中心，南京210018)

首次采用原子層沉積法制備TiO2/Al2O3布拉格反射鏡并配合金屬反射鏡來制備了高反射率的背反射鏡。制備的多層布拉格反射鏡加Al鏡和多層布拉格反射鏡加Ag鏡有很好的平整度和厚度的精確性，并且反射率高于96%。此外，TiO2/Al2O3布拉格反射鏡和Al與藍寶石襯底都有良好的粘合性，這樣可以節(jié)省制備步驟并且可以得到高質(zhì)量的背反射鏡。利用原子層沉積技術(shù)和TiO2/Al2O3布拉格反射鏡，我們得到了高反射率，角度依賴性小，更加穩(wěn)定以及均一性更好的背反射鏡，可以滿足高亮度LED的需求。

背反射鏡;布拉格反射鏡;金屬反射鏡;原子層沉積

In recent years，the availability of higher reflectivity，better film uniformity and more stabilized backside reflector for high brightness light-emitting diode(LED) has attracted much attention.As we all know，photons generated from multiple quantum wells(MQWs)could be emitted in any direction，so a large number of photons are lost and converted into heat，especially for those photons being emitted downward to substrate.Thus，ifwe add a backside reflector beneath the substrate to effectively reflect those photons upward，we should be able to enhance the light extraction efficiency of the LED［1-3］.

Now，most backside reflectors are made of metallic mirror，distributed Bragg reflector(DBR)or combination of both them.TiO2and SiO2are common materials for DBR structure and they are usually prepared by electron beam (EB)evaporation［4-5］，but limited by poor film uniformity and thickness control.H.W.Huang and his colleagues proposed a 14-pair-TiO2/SiO2structure to achieve high reflectivity［6］.For metallic mirrors［7］，both Al and Ag have relatively high reflectivities in the visiblewavelength region.S.J.Chang et al.reported TiO2/SiO2DBR with Al mirror backside reflector using EB evaporation［8］.However，the reflectivity of DBR has strong dependency on film thickness and film uniformity.It is necessary to control the thickness of each layer precisely and choose two different materials with a large refractive index difference［9-10］.Furthermore，in order to grow Al mirror on conventional TiO2/SiO2DBR by traditional method，an Al2O3adhesive layer is required to improve the adhesion between Al and TiO2/SiO2DBR.

In this study，we introduce a novel TiO2/Al2O3DBR using atomic layer deposition(ALD)for high reflectivity backside reflectors applications.ALD offersmany advantages over other deposition methods，such as excellent thickness uniformity over large diameter substrates aswell as an atomic level thickness control［11］.These features make ALD a potential depositionmethod to fabricate high quality backside reflectors.Moreover，we also usemetallic mirror(Al and Ag)to promote the reflectivity of the backside reflector.Meanwhile，although common TiO2/SiO2DBR has high reflectivity as well，its poor adhesion with sapphire substrate and metallicmirror is the disadvantage in LED application.So an adhesive layer is required to improve the adhesion.In this case，due to the good adhesion between TiO2/Al2O3DBR and Almirror，the fabrication process is simplified andmore stabilized backside reflector is achieved.Moreover，since Ag exhibits higher reflectivity than that of Al in the bluewavelength region，so TiO2/Al2O3DBR with Agmirror is also investigated.Furthermore，we achieved 96.5%and 98%reflectivities at 460 nm by Al+3DBR and Ag+3DBR，respectively.

1 Experimental

The TiO2/Al2O3DBR stacks were deposited by ALD system.Trimethylaluminium(TMA)and water vapor(H2O)were used as the chemical precursors to provide Al-and O-to the growing surface，respectively［12］. For TiO2，we used TiCl4and H2O as precursors［13-14］. The Al2O3and TiO2deposition mechanism can be summarized by the reactions:

Both materialswere grown at250℃，it is not necessary to change temperature during the growing process so thatwe saved the fabricating time and achieved high quality thin films.The precursors were alternatively fed into the reactor，using pure N2as a carrier gas.In order to avoid gas phase reactions caused by intermixing the precursors，the reactor was also purged with pure N2gas after each precursor pulse.

The actual structure of LEDwith a backside reflector is shown in Fig.1(a).Since surface reflection on the thin film may cause a deviation during themeasurement，so in order tomeasure the reflectivity of the backside reflector accurately，we used the structure in Fig.1(b)tomeasure the reflectivity of backside reflector.Firstof all，the thicknesses of Al2O3and TiO2layers were fixed at 67 and 49 nm，respectively.This is an optimized condition to achieve the highest reflectivity at normal incidence theoretically. The actual thicknesses of each DBR layers were confirmed by SEM(depicted in Fig.1(c))［15-16］.Since adhesive layer is no longer needed in our structure，themetallicmirror Al and Ag were deposited directly on the DBR by EB evaporation.Reflectors combining various DBR layerswith Al or Agmirrorwere deposited onto the backside surface of the sapphire substrate.SiO2and Al2O3double anti-reflection(AR)layers were grown on the reflector to eliminate surface reflection of DBRs，therefore most photonswill be reflected inside the backside reflector，not on the surface of the backside reflector.This AR structure is also beneficial to the extraction of reflective light because of its gradient refractive index.In this case，accurate reflectivity of the backside reflector ismeasured.

Fig.1 (a)Schematic of LED with a backside reflector combining TiO2/Al2 O3 DBR withmetallicmirror on the backside of sapphire substrate.(b)Structure used for measuring and simulating the reflectivity of backside reflector.(c)SEM image of the TiO2/Al2O3 DBR.

Spectroscopic ellipsometry(SE)［HORIBA-JY UVISEL］was used to measure the reflectivities of the fabricated reflectors with incident angles of 30°，60°，and 85°(the angle between normal and incident light).Tomeasure the reflectivity of5°，the 7-SC spectralmeasurement system(SMS)was applied.SE was also used tomeasure the refractive indices of TiO2(n= 2.5)and Al2O3(n=1.6)at460 nm wavelength.

For an infinite periodic structure(N→∞)，according to Bloch’s theorem，the dispersion at any incident angle follows the relation:

Where a and b are the thicknesses of each DBR layer，which are corresponding to the refractive indices of n1and n2．d=a+b.Equation(1)is for a periodic structure with an infinite number of layers.In contrast，our approach was developed for finite structures.We note，however，that our structure is also fit into the equation because of its high reflectivity.

Fig.2 shows the photonic band diagram for TiO2/ Al2O3DBR.The light wave can be separated into TM and TEmode.The green and white regions distinguish between the allowed and forbidden photon states，respectively.There is a complete photonic bandgap(CPBG)between 440 nm and 470 nm，in which lightwaves don’t transmit through the DBR stack in all incident angles［17-18］.So high reflectivity is obtained in this wavelength region theoretically，and it meets the requirements of backside reflector designed for high brightness blue-ray LEDs.

Fig.2 Photonic band diagram for TiO2/Al2 O3 DBR.The green and white regions distinguish between the allowed and forbidden photon states，respectively.

2 Results and discussion

Firstly we used optical software(The Essential Macleod，demo version)to simulate the reflectivity of DBR. Measured refractive indices of TiO2and Al2O3were used to improve the accuracy of simulation.As shown in Fig.3 (a)，themeasured reflectivity data for 3DBR，Al+3DBR and Ag+3DBR coincide with the simulated data.The reflectivity of each reflector wasmeasured at four different locations to verify the film uniformity.As shown in Fig.3 (c)，we took 3DBR，6DBR and Al+3DBR reflectors as examples，similar reflectivities were measured from four points of the same reflector，which indicate ALD deposited TiO2/Al2O3reflectors with excellent film uniformity is a practical and promising technology for high brightness LED application.

Since photons could be emitted in any direction，so the angle dependence behavior has also been investigated.Fig.4(a)and Fig.4(b)show the reflection spectra of 3DBR and 6DBR reflectors，both spectra show a concave region from 40°to 90°，since DBR structure has angle and wavelength selectivity，the thickness of each DBR layer is designed for particular incident angle and wavelength.For 60°incidence case，the effective optical path becomes longer than that in 0°case，resulting in a lower reflectivity.However，with the increase of incident angle，its reflectivity goes upward when the incident angle increases，as the light energy is divided into horizontal and vertical direction，the light energy along the vertical direction is very smallwhen the incidentangle goes up to90°，thereforemost photonswill be reflected.In or-der to enhance the reflectivity over large incident angles，Al and Agmetallicmirrorswere used on the backside of DBR.As shown in Fig.4(c)～Fig.4(e)，all reflectors except Al+6DBR reflector demonstrate a reflectivity over 96%and the concave region in the reflection spectra is nearly annihilated，because Al+6DBR reflector is more affected by the reflectivity of DBR.Therefore，by adding the metallic mirror，backside reflectors with high reflectivity in wide range of incident angles are achieved.

Fig.4 Reflection spectra of(a)3DBR，(b)6DBR，(c)Al+ 3DBR，(d)Al+6DBR and(e)Ag+3DBR simulated and measured with different incident angles at460 nm.

Fig.5 Reflection spectra of 3DBR，Al+3DBR and Ag+3DBRmeasured with an incident angle of(a)5°，(b)30°，(c)60°，and(d)85°.

As demonstrated clearly in Fig.5，we compared 3 pairs of DBR with differentmetallic mirror and the one without metallic mirror structure.Compared with the DBR-only structures，the ones with metallic mirrors display much less wavelength dependency in reflectivity. Fig.5(a)shows reflection spectra of different reflectors measured with an incident angle of 5°.It is observed that reflectivity of Ag+3DBR is around 96% ～98%in the wavelength region between 450 to 550 nm whereas the reflectivity of Al+3DBR is around 93%～96.5%in the same wavelength region.With themetallic mirror，it is found that the reflectivity of3DBR is significantly enhanced from 75%to 96.5%(with Almirror)and 98% (with Agmirror).Fig.5(b)，F(xiàn)ig.5(c)and Fig.5(d) show reflection spectra for the reflectorsmeasured at incident angles of 30°，60°and 85°，respectively.From these three figureswe found that the reflectivity of DBR depends strongly on the wavelength while adding a metallic mirror is better to reflect the photons at large wavelength.Moreover，due to the bandgap and reflection characteristics，the highly reflective bands became narrower aswe increased the incidentangle.In addition，compared with Almirror，Ag+3DBR reflector shows a better reflectivity than Al+3DBR reflector over large wavelength.But poor adhesive property of Ag on TiO2/Al2O3DBR hampers its further application.Besides，protective coatingmust be applied to prevent the formation of sil-ver sulfide［19］.In order to realize the application of Ag mirror，more investigation is obviously needed.

For further analysis，we increased the DBR pairs to 6，as shown in Fig.6.Since Ag+3DBR reflector shows a higher reflectivity than Al+6DBR reflector and its reflectivity is close to 100%，so we only discuss the Ag+ 3DBR case regardless the Ag+6DBR one.From Fig.6，it is found 6DBR reflector has a 95%reflectivity at 5° incident angle，which improves the 75%reflectivity of 3DBR reflector.Albeit Al+6DBR reflector has a little higher reflectivity than Al+3DBR reflector，Al+6DBR reflector ismore wavelength dependent than Al+3DBR reflector.In general，we can obtain backside reflector with higher reflectivity by increasing DBR pairs，but it also results in a reflection efficiency drop in larger wavelength and higher incident angle.

Fig.6 Reflection spectra of6DBR and Al+6DBRmeasured with an incident angle of(a)5°，(b)30°，(c)60°，and(d)85°.

3 Conclusions

In summary，high reflectivity backside reflectors combining TiO2/Al2O3distributed Bragg reflector by atomic layer deposition with metallic mirror have been demonstrated for the first time.With the help of ALD technology，we have fabricated backside reflectorwith better film uniformity andmore thickness accuracy than other depositionmethods.Both Al and Ag can further enhance the reflectivity of backside reflectors.Since TiO2/Al2O3DBR has good adhesion with both sapphire substrate and Al mirror，we simplified the fabrication process and achieved more stabilized backside reflector.By combining ALD deposition with TiO2/Al2O3DBR，high reflectivity，less angle dependency，more stabilized and excellent film uniformity backside reflector is achieved to meet the requirements of high brightness LEDs.

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High Reflectance of Backside Reflector w ith a Hybrid M etallic M irror and ALD-TiO2/Al2O3DBR*

CHEN Hongjun，GUO Hao，ZHANG Xiong*，CUIYiping

(Advanced Photonics Center，School of Electronic Science and Engineering，Southeast University，Nanjing210018，China)

High reflectivity backside reflectors combining TiO2/Al2O3distributed Bragg reflector(DBR)by atomic layer deposition(ALD)with metallic mirror have been demonstrated for the first time.Multi-pair-DBRs/Al and multi-pair-DBRs/Ag stackswith excellentuniformity and thickness accuracy have exhibited high reflectivities above 96%.Besides，TiO2/Al2O3DBR has good adhesion with both sapphire substrate and Almirror，we simplified the fabrication process and achieved more stabilized backside reflector.By combining ALD deposition with TiO2/Al2O3DBR，high reflectivity，less angle dependency，more stabilized and excellent film uniformity backside reflector is achieved tomeet the requirements of high brightness light-emitting diodes(LEDs).

backside reflector;distributed Bragg reflector(DBR);metallic mirror;atomic layer deposition(ALD)

10．3969/j．issn．1005-9490．2013．04．001

O782;O435．1;O48231 文獻標識碼:A 文章編號:1005-9490(2013)04-0431-06

項目來源:東南大學(xué)科研啟動費(4006001034)

2013-02-28修改日期:2013-03-15

EEACC:0520;4260D

陳洪鈞(1988-)，現(xiàn)為東南大學(xué)電子科學(xué)與工程學(xué)院先進光子學(xué)中心碩士研究生，主要研究方向是高亮度LED光提取效率的優(yōu)化，主要是從LED背鍍，表面粗化，ITO優(yōu)化以及鈍化層等方面來提高LED的出光強度;

張 雄(1962-)，男，1985年本科畢業(yè)于中國科技大學(xué)物理系，1992年獲得日本東京大學(xué)工學(xué)博士學(xué)位，現(xiàn)為東南大學(xué)電子科學(xué)與工程學(xué)院教授、博士生導(dǎo)師。主要研究方向為寬禁帶半導(dǎo)體光電子材料與器件，特別是高亮度LED的外延生長和芯片制備工藝的研究。出版有英文SCI論文38篇，英文學(xué)術(shù)專著2部，擁有國內(nèi)外專利20余項。