Fabrication of wet-etched patterned sapphire substrates for

Fabrication of Patterned Sapphire Substrate by Wet Chemical Etching for Maskless Lateral Overgrowth of GaNJing Wang,z L.W.Guo,H.Q.Jia,Y.Wang,Z.G.Xing,W.Li,H.Chen,and J.M.ZhouBeijing National Laboratory for Condensed Matter Physics,Institute of Physics,Chinese Academy of Sciences,Beijing100080,ChinaThe grooved c-plane sapphire substrates werefirst systematically investigated by wet chemical etching,with H2SO4and a 3H2SO4:1H3PO4mixture as the etchants.The structural and morphological characteristics of the grooved sapphire with mask stripes along the͗112¯0͘and͗11¯00͘directions,respectively,were studied under different etching time and temperatures by scanning electron microscopy͑SEM͒.Two kinds of groove shapes are obtained.One is a V-groove whose two sidewalls are both formed by a single facet.The other is a U-groove whose one sidewall consists of two or three facets,and the other sidewall is composed of a single facet.SEM cross-sectional images show symmetrical sidewall facets with stripes along the sapphire͗11¯00͘direction and asymmetrical sidewall facets with stripes along the sapphire͗112¯0͘direction.The etching depth is linear with the etching time.The activation energies of etching reaction are evaluated in the temperature range340–400°C.It is confirmed that sapphire with stripes along the͗112¯0͘direction is suitable for lateral epitaxial overgrowth of low-threading-dislocation GaNfilms.©2006The Electrochemical Society.͓DOI:10.1149/1.2163813͔All rights reserved.Manuscript submitted October8,2005;revised manuscript received November17,2005.Available electronically January26,2006.Gallium nitride͑GaN͒is an important material for optoelectronic devices including blue light-emitting diodes͑LEDs͒and laser diodes ͑LDs͒.1GaN is typically grown on substrates like sapphire͑␣-Al2O3͒,2SiC,3and Si,4which exhibit a large difference in lattice constant and thermal-expansion coefficients compared to GaN.This leads to a high density of threading dislocations͑TDs͒which limit the performance of optoelectronic devices.5Epitaxial growth tech-niques like lateral epitaxial overgrowth͑LEO͒6and other modified LEO techniques such as pendoepitaxy͑PE͒7and facet-controlled epitaxial lateral overgrowth͑FACELO͒8are promising techniques to overcome this problem and to reduce the dislocation density in GaN layers grown by metallorganic chemical vapor deposition ͑MOCVD͒.However,LEO techniques typically require multiple MOCVD growths coupled with intermediate processing steps,and often introduce autodoping from the mask.In addition,to ensure a low threading-dislocation-density͑TDD͒and aflatfilm surface to fabricate the high-performance optoelectronic devices,more than ten-micrometer-thick GaN layers have to be grown by MOCVD. Inspiringly,these shortcomings were overcome later by cantilever epitaxy͑CE͒,9which was achieved as a maskless means by which GaNfilms are grown from substrates with periodic grooves prepared by dry etching.10,11Though CE cannot reduce TDD to the order of magnitude that those conventional LEO techniques can achieve,the simplified technique is greatly effective to fabricate high-performance devices with only several-micrometer-thick GaN to form aflatfilm surface.12Nevertheless,dry etching cannot avoid damage and strain to the substrate surface,13which limits the further improvement of epitaxial crystal quality.Especially,it is unavoid-able that TDs propagate into the top epitaxialfilms from GaN layers deposited on the sidewalls of the grooves whose surfaces have been damaged severely in dry etching.For the further improvement of GaN quality,developing wet chemical etching is necessary to make patterned sapphire substrates.So,the wet etching is revealed in this paper.In the subsequent lateral epitaxial overgrowth of GaN,14it is found that GaN layers are deposited not on the grooves but only on the mesas.Consequently,the selective growth avoids TDs from GaN deposited on the groove sidewalls,typically present in CE.The new developed technique may help light up the world with brighter green,blue,and even white semiconductor LED,solid-state lighting.For improving the quality of GaN in LEO,wet chemical etching of grooved sapphire is the key technique.Up to now,there has been no report about the etching means.Thus,in this paper,we present and discuss a systematic study of wet chemical etching of sapphire substrates with stripes oriented along the͗11¯00͘and͗112¯0͘direc-tions,respectively.The different etching profiles of the patterned substrates were investigated at different etching time and tempera-tures;the etching of sapphire substrates using H2SO4,and a3:1by volume mixture of H2SO4:H3PO4as a function of time and tem-perature was studied;finally,the suitable structure of grooved sap-phire for laterally overgrown GaN is demonstrated.ExperimentIn this study,the used2in.͑0001͒sapphire wafers are misori-ented0.25°off toward the͗11¯00͘direction.SiO2was used to form the etch mask defining the grooves.Accordingly,first,300-nm-thick SiO2layers were deposited on the whole surface of underlying sap-phire wafers at300°C by plasma-enhanced chemical vapor deposi-tion͑PECVD͒.Subsequently,standard photolithography with reac-tive ion etching͑RIE͒was used to fabricate SiO2mask stripes patterned with nominally3.5-␮m-wide openings with a period of 6.5␮m oriented in the͗11¯00͘and͗112¯0͘directions,respectively, of the underlying sapphire.After achieving the striped mask,the 2in.substrates were cut into1ϫ1cm slices as the investigated samples.Two kinds of acid solutions were chosen:98wt%H2SO4 and68wt%H3PO4.Two different etching solutions were studied: H2SO4and a3:1by volume mixture of H2SO4:H3PO4.With the beginning of etching,the slices were immersed in the etching solu-tion in a quartz beaker.The solution was heated on a thermostatic stove to a different temperature from340°C up to410°C.Every etching experiment was repeated at least three times using the fresh solution every time.After etching sapphire,SiO2mask was removed in dilute HF and at last sapphire slices were rinsed in deionized͑DI͒water for5min and blown dry under nitrogen.Scanning electron microscopy͑SEM͒was performed to characterize the etching pro-files and determine the etching depths of patterned sapphires.Results and DiscussionFigure1shows SEM plan-view micrographs of sapphire sub-strates with stripes along the͗11¯00͘direction after30min etching at410°C in͑a͒a3H2SO4:1H3PO4mixture and͑b͒H2SO4.Image in Fig.1a displays the typical plan-view etched profile of grooved sapphire and is similar to that of other samples at the lower tem-peratures in the two etchants.The edge of the mesas is straight andz E-mail:wangjing6900@ Journal of The Electrochemical Society,153͑3͒C182-C185͑2006͒0013-4651/2006/153͑3͒/C182/4/$20.00©The Electrochemical SocietyC182groove sidewalls are smooth with preferential etching facets.But with the temperature increasing to 410°C in H 2SO 4,insoluble large reaction products appear in the grooves,as visible in Fig.1b.It is known that the etching reaction involves a series of steps:the trans-port of reactant to the interface,the reaction at the interface,and the transport of the products away from the interface.The rate-limiting step in H 2SO 4at 410°C is probably the transport of reaction prod-ucts away from the sapphire surface and the solubility of the reac-tion products.Dwikusuma et al.15reported that the insoluble products on the sapphire surface are identified as a mixture of poly-crystalline aluminum sulfates,Al 2͑SO 4͒3and Al 2͑SO 4͒3·17H 2O,in the study on sapphire surface preparation.The insoluble products form a surface layer,acting as a barrier impeding the reactant trans-port to the sapphire surface.Nevertheless for the 3H 2SO 4:1H 3PO 4mixture,H 3PO 4acts as a buffer agent,decreasing the etching rate discussed below,so there are no insoluble reaction products visible in Fig.1a.In addition,the residue in the grooves is likely to be imperfect initial patterning of the SiO 2mask.As mentioned above,the sapphire substrates were patterned with SiO 2mask stripes along two orientations of the underlying sapphire.Figure 2shows the resulting cross-sectional SEM micrographs of the substrates after etching in H 2SO 4at 400°C at different times.Sample A etched for 1h ͑Fig.2a ͒and sample B etched for 1.5h͑Fig.2b ͒were both patterned along the ͗112¯0͘orientation.Samples C ͑Fig.2c ͒and D ͑Fig.2d ͒etched for 1and 2h,respectively,wereboth patterned along the ͗11¯00͘orientation.As shown in Fig.2a and c,sapphire ͑0001͒planes still remain in the trenches,and the cross sections of the trenches display U-shape,whereas with increasingthe etching time,the ͑0001͒planes are eaten off finally,and the cross sections of the trenches show V-shape,as shown in Fig.2b and d.The adjacent sidewalls of samples A and B are asymmetric,while those for samples C and D are symmetric.Carefully observed,the one sidewall of the U-grooves comprises multifacets,while the other sidewall is composed of a single facet.For sample A,the shortinclined sidewall on the left is a sapphire ͕11¯02͖facet,and the right long sidewall is composed of two facets:one is the ͕11¯04͖facetat Figure 1.SEM plan-view micrographs of sapphire substrates with stripesalong the ͗11¯00͘direction after 30min etching at 410°C in different etchants:͑a ͒3H 2SO 4:1H 3PO 4mixture and ͑b ͒H 2SO 4.Figure 2.SEM cross-section micrographs of sapphire etched in H 2SO 4at 400°C at different times.Sample A etched for 1h ͑a ͒and sample B etchedfor 1.5h ͑b ͒are both patterned along the ͗112¯0͘direction;sample C etched for 1h ͑c ͒and sample D etched for 2h ͑d ͒are both patterned along the ͗11¯00͘direction.The straight lines in micrographs ͑a ͒and ͑c ͒show the different facets composed of the sidewalls.C183Journal of The Electrochemical Society ,153͑3͒C182-C185͑2006͒the near mesa region,and the other is the ͕11¯05͖facet at the near bottom region.Similarly,for sample C in Fig.2c,adjacent sidewallsare composed of three facets from the top down,namely ͕112¯4͖,͕112¯5͖,and ͕112¯6͖,respectively.In contrast with the U-groove,the two sidewalls of the V-groove are both composed of a single facet.The corresponding facets are ͕11¯02͖facets in the short sidewall and ͕11¯04͖facets in the long sidewall for sample B,and the ͕112¯4͖facets for sample D.In addition,at a temperature range of 340–400°C,the observed etched profiles of sapphire in H 2SO 4and 3H 2SO 4:1H 3PO 4are similar to those in Fig.2,and are summarizedin Table I.Intentionally,͕11¯02͖and ͕112¯4͖facets are always re-tained on one sidewall for stripe directions along ͗112¯0͘and ͗11¯00͘,respectively.For V-grooves with stripes along the ͗112¯0͘direction,the ͕11¯04͖facets can be replaced by the ͕11¯05͖or ͕11¯06͖facets.Groove etching is attributed to crystallographic structure and de-pends on the configuration of surface atoms and the bond structure.16Asymmetrical sidewalls are likely to be attributed to thesapphire surface with 0.25°misorientation toward the ͗11¯00͘orien-tation.As known,those crystal planes with the slowest etch rate determine the etch profile.In the U-grooves in Fig.2a and c,the sidewall facets are not unique.The stable sidewall facets are not formed until the trenches are etched to V-shape shown in Fig.2b and d.The groove profiles obtained suggest that the etching rates of stable sidewall facets are all slower than that of the ͑0001͒plane.As is well known,the etching rate in an acidic solution is deter-mined by temperature,time,solubility of the etching product,and acidity of the solution.The acidity of the solution is affected by temperature as both phosphoric and sulfuric acids are known to dehydrate to diacids ͑pyro acids ͒upon heating.Figure 3shows plotsof the etching depth of sapphire with stripes along the ͗11¯00͘direc-tion and the ͗112¯0͘direction,respectively,as a function of time at 400°C in H 2SO 4and a 3H 2SO 4:1H 3PO 4mixture.As shown,theetching depth is linear with time,from which the change of average etching rate can be deduced approximately.It is obvious that at 400°C the etching rate in H 2SO 4is faster than that in a 3:1acid mixture,and the etching rate of sapphire with stripes along the ͗11¯00͘direction is close to that along the ͗112¯0͘direction in one etchant according to Fig.3.At temperatures lower than 400°C,the etching depths are also linear with times,similar to Fig.3.The differing etching behaviors exhibited by H 2SO 4and H 3PO 4can be traced to inherent differences in the chemical nature of the two acids.15Figure 4shows Arrhenius plots of the etching rate of sapphirewith stripes along the ͗11¯00͘direction and the ͗112¯0͘direction,respectively,in H 2SO 4and 3H 2SO 4:1H 3PO 4mixture solutions.At temperatures above 400°C in H 2SO 4,the etching depth was not measured due to heavy deposits of insoluble reaction products in the grooves.The activation energies of the etching reactions were 10.6and 10.4kcal/mol in H 2SO 4,and 9.4and 9.3kcal/mol in a3H 2SO 4:1H 3PO 4mixture.The activation energy was close for two samples patterned along the different directions in one etchant.Hav-ing the higher activation energy,H 2SO 4has the faster etching rate all through the temperature range of 340–400°C.The etching is reaction-rate-limited,as judged by both an activation energy of about 10kcal/mol and the linear dependence of etch depth on time shown in Fig.3.17In order to confirm if sapphire wafers patterned along the two directions are both suitable for lateral epitaxial overgrowth of GaN,GaN films are deposited on the patterned maskless substrates by MOCVD,as shown in Fig.5.The detailed growth processes have been reported in a previous paper.14The U-grooved sapphire in Fig.5a and V-grooved sapphire in Fig.5b are both patterned along the ͗112¯0͘orientations.As shown in Fig.5a,GaN is deposited both on the mesas and on the grooves.The laterally overgrown GaN from the adjacent mesas spans the grooves and coalesces above the grooves,but the top overgrown GaN is not in contact with GaN on the grooves.Even in Fig.5b,no GaN layers are deposited on the grooves.14Lateral epitaxial overgrowth of GaN occurs on such sub-strates,which are beneficial to reduce the TDD in GaN due to de-creasing the interface between GaN and sapphire compared to pla-Table I.Observed etching facets composed of the sidewalls of sapphire grooves in H 2SO 4and 3H 2SO 4:1H 3PO 4at the tempera-ture range of 340–400°C,with the stripes patterned along the Š11¯00‹and Š112¯0‹directions,respectively.Sample ͗11¯00͘direction ͑symmetric ͒͗112¯0͘direction ͑asymmetric ͒U-groove ͕112¯4͖,͕112¯5͖,͕112¯6͖͕11¯04͖,͕11¯05͖,͕11¯06͖͑two-sides ͕͒112¯4͖,͕112¯5͖,͕112¯6͖͕11¯02͖V-groove ͕112¯4͖͕11¯04͖,͕11¯05͖,͕11¯06͖͑two-sides ͕͒112¯4͖͕11¯02͖Figure 3.The sapphire etching depth dependence of etching time in H 2SO 4and 3H 2SO 4:1H 3PO 4.Solid and hollow symbols represent sapphire withstripes along ͗11¯00͘and ͗112¯0͘,respectively.Figure 4.Arrhenius plot of sapphire etching rates in H 2SO 4and 3H 2SO 4:1H 3PO 4.Solid and hollow symbols represent sapphire with stripesalong ͗11¯00͘and ͗112¯0͘,respectively.C184Journal of The Electrochemical Society ,153͑3͒C182-C185͑2006͒nar sapphire.Moreover,as shown in Fig.5a,the U-groove must be deep enough to avoid the touch of GaN on the grooves with top lateral overgrowth of GaN,while the V-groove is very reliable be-cause there is no GaN grown on the grooves.The U-grooved sap-phire in Fig.5c is patterned along the ͗11¯00͘orientation.As visible,GaN is grown not only on the mesas but also on the sidewalls and the bottoms of the grooves.The GaN on the sidewalls grows rapidly and coalesces with the GaN on the mesas,which is not grown lat-erally toward the ͗112¯0͘direction.The coalescence fronts are clearly shown over the edges of the mesas.As a result,the TDD in GaN films cannot be reduced.Therefore,it is confirmed that c -planesapphire patterned along the ͗112¯0͘sapphire or ͗11¯00͘GaN orientation,not along the ͗11¯00͘sapphireorientation,is suited as a substrate forlateral epitaxial overgrowth of low-TDD GaN.ConclusionIn this paper,promising wet chemical etching of c -plane sapphiresubstrates patterned along the ͗11¯00͘and ͗112¯0͘orientations,re-spectively,was systematically studied as a function of time and tem-perature in H 2SO 4and a 3:1H 2SO 4:H 3PO 4mixture.The resulting etching profiles of sapphire performed by SEM show that the adja-cent sidewalls of grooves are symmetric with stripes along the ͗11¯00͘orientation,while they are asymmetric with stripes along the ͗112¯0͘orientation.With the higher activation energy,H 2SO 4has the faster etch rate.H 3PO 4as a buffer agent reduces the etch rate and avoids the insoluble deposits produced in the grooves.Sapphire pat-terned along the ͗112¯0͘orientation is suited as a substrate for lateral epitaxial overgrowth of low-TDD GaN.AcknowledgmentsThe authors wish to acknowledge funding support from Chinese High Technology Plan ͑863no:2001AA313120͒,Basic Research Plan ͑973no:2002CB311900͒from Chinese Science and Technol-ogy Ministry,and the National Science Foundation of China ͑no.10474126͒.The Chinese Academy of Sciences assisted in meeting the publication costs of this article.References1.S.Nakamura and G.Fasol,The Blue Laser ,Springer,Berlin ͑1997͒.2.H.Amano,N.Sawaki,I.Akasaki,and Y .Touoda,Appl.Phys.Lett.,48,353͑1986͒.3.T.W.Weeks,Jr.,M.Bremser,K.Ailey, E.Carlson,W.Perry, E.Piner,M.El Masry,and R.F.Davis,J.Mater.Res.,11,1011͑1996͒.4. 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