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2'd International Conference on Nanotechnologies and Biomedical Engineering, Chisinau, Repuhtic of Moldova, April I g-20, 201

TheRole ofAlternating Cuffent in Photo-Assisted lectrochemicalorosification f GaNAinorkhilahMahmood*r'',NaserM. Ahmedr, on Tiginyanu3, ushamdanYusofr,yam Fong,Nano-optoe,ctronicft:,';i;.7.Yi:;;:;:;{rn*W'##:;":;)rr,,s,(rniversitiains,2"0,orApptiedciences,,,I:';!';:;ii,ili fr;#3;{#r;::**ang pauh,enang,ataysiaInstitute of Electronic Engineering and Nanotechnologies, Academy of Science of Moldova, andNCMST, Technicol University of Moldova, Republic of MoldovaaPhysicsSection, School of Distance Education, (Jniversiti Sains Malaysia, 11800penang, Malaysia*ainor khilah@yahoo.com

Abstract-- In this paper, we report the formation of porous GaN films under a novel alternating current(sine-wavea.c. (50 Hz)) photo-assistedelectrochemical(ACPEC) etching conditions. The ACpEC formedporous GaN with excellentstructural and surfacemorphology. Field emissionscanning electron microscope(FESEM)' atomic force microscopy (AFM) and high resolution X-ray diffraction (HR-XRD) phi-scan androcking curves measurementsevidenced mportant features of the pore morphology and nanostructures.According to the FESEM micrographs' the spatial nanoarchitectureof the porous structures exhibits poreswith perfecthexagonalshape.The AFM measurementsevealedan increase n the surface roughness nducedby porosification. X-ray diffraction phi-scan showed that porous GaN sample maintained the epitaxialfeatures.Index Tetms-- Porous GaN, Alternating current photo-assisted lectrochemicaletching (ACPEC), FESEM,AFM. HR-XRD.

. I,INTRODUCTIONThe wide band gap semiconductorGaN and relatedmaterialshave received increasing attention in recentyears ue o their potentialapplicationsor optoelectronicdevicesoperating n the spectral egion from the blue tonear-UV and in electronic devices such as hightemperature, igh power and high frequency ransistors[1-3]. Note that GaN devicesare capable o operate nhostile and harsh environments l4l, while GaNnanostructuringnduces an increase in the radiationhardnessof the material [5]. Additionally, GaN hasemerged s mportantmaterial or high power electronicsdevices wing to its high breakdown ield.Since he unearthingof porous Si shows augmented

luminescence fficiency in 1990 [6], initial efforts tomakeporousGaN were motivatedby the desire o realizesimilareffectwith an ulkaviolet (UV) band gap material.PorousGaN exhibitshigh surfacearea,shift ofband gap,luminescencentensityenhancement, s well as efficientphotoresponse scompared o bulk. Thus, the expectationis that porous GaN can be tailored to fabricatenovelsensing evices.Porous GaN can be prepared through dry-etchingtechniques, uch as ion milling, chemical-assistedonbeam etching, reactive ion etching, and inductivelycoupled plasma reactive ion etching. However, thesemethods could induce surface damage; moreover, theylack the desired selectivity for the morphology, dopant,andcomposition[7]. The most feasibleand cost-effectivemethod to prepareporous GaN is the direct current (dc)

photo-assisted lectrochemicaletching. To gain a highporosify ayer, the most common technique s to use dcconditionswith a constantand relatively high currentdensity.Although dramatic esearchhas beenconductedto understand he formation of porous GaN preparedbythe common echnique, ubstantialundamental ropertiesarestill not well understood[8-14].The goal of this study s to prepareporousGaN by anovel technique,namely by alternatingcurrent photo-assistedelectrochemical tching (ACPEC) [15]. Resultsof systematic morphological, structural and surfacestudiesof porousGaN samplesare reported.II. EXPERIMENTAL METHODS

The commercialunintentionally doped(UID) n-type GaNfilm grown by metalorganicchemical vapor deposition(MOCVD) on a two inch diameter sapphire (0001)substrate as used n the formationof porousGaN by theACPEC etching echniques. he thicknessof GaN film is3 pm with carrierconcentration f - 6.05 x l0l? cm-3asdeterminedby Hall effect measurements.The ac etchingprocess was performed with a current density of 25mNcm2 in 4 %o oncentration f KOH electrolyteunderilluminationof 500 W ultra-violet or 45 and 90 minutesetchtime. The porousGaN sampleswere characterizedbyusingFESEM, AI'M and HR-XRD.

III. RESULTSAND DISCUSSIONFESEM imagesof the porous GaN samplesgeneratedunderdifferent etching durationsare shown in Fig. l. TheFESEM images n Fig. I show well-definednetwork of

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the qualityof thestartingGaN epilayers'

pores with different sizes grown in the monocrystallineepilayerof GaN.The average ore size-for45 and99 1* tl l,u-pt.t wereof about35-40and 55-60nm, respectively'Th"'uu..ug. pore sizevariessignificantly as a function of

REFERENCESS. C. Jain, et al., "[I--nitrides: Growth,characterization,and properties,"Journal of AppliedPhysics, ol . 87,PP .965-1006, 000'D."Feiler,et al.,I'Pulsed aserdepositionof epitaxialAlN, GaN, and InN thin films on sapphire(0001),"Jouinat of Crystal Growth, vol' 171, pp' 12-20'1997.

13] G. Landwehr,el al., "Blue emitting heterostructurelaser diodes," Physica E: Low-dimensional SystemsandNanostructures,ol . 3, pp' 158-168,1998'14] J.-Y. Duboz and M' A. Khan, "Transistorsanddetectors asedon GaN-relatedmaterials,"n GroupIII Nitrides SemiconductorsCompounds,B' Gil, Ed''ed:Clarendon ress,Oxford,1998, p' 343-390't5 l V. V. Ursaki, I. M. Tiginyanu,O' Volciuc, V' Popa'V. A. Skuratov and H. Morkog, "Nanostructuringinduced enhancementof radiation hardness n GaNepilayers",AppliedPhysicsLetters,Vol' 90, 161908

(2007).t6] L. T. Canham, "Silicon quantum wlre arrayfabrication by electrochemical and chemicaldissolution of wafers," Applied PhysicsLetters' vol'57 ,pP. 1046-1048, 990.t7l C.'Youtsey,el a/., "Smoothn-type GaN surfaces yphotoenhanced wet etching," Applied PhysicsLetters, ol ' 72,PP.560-562, 998't8 ] M. Mynbaeva,et al., "Photoconductivityn PorousGaN Layers,"physica statussolidi (b), vol' 228' pp'589-592,2001.t9l A. Mahmoo4 et al., "Enhanced Properties OfPorous GaN PrePared BY UV Assisted

Electrochemical Etching " Advanced MaterialsResearch, ol . 364,PP.90-94,2012'tlO] A. Mahmood et al., "Characteristicsof undopedporous GaN PrePared bY UV assistedelectrochemicaletching," Optoelectron'Adv' Mater'RapidComm., ol . 4, pp' l3l6-20' 2010'[l ] A. P. Vaipeyi, et al., "High optical qualitynanoporousGaN preparedby photoelectrochemicaletching,"ElectrochemicalSolid StateLett', vol' 8'pp.G85-G88,2005.t12] i\4. Mynbaeva, et al., "structural characterizationand strain relaxation in porous GaN layers,"AppliedPhysics etters,vol.76, pp' I I 13-1 15,2000't13] A. Mahmood, et al., "Effect

of Porosity on theCharacteristics f GaN Grown on Sapphire," 1PConf.Proc.,vol. 1341, p.45-47,2011'

[14] A. Mahmood,el a/., "structuraland SurfaceStudiesof Undoped Porous GaN Grown on Sapphire"'AdvancedMaterialsResearch, ol ' 620, pp' 4549,ll5l A. Mahmood, et al.,"A Novel AC TechniqueHigh Quality Porous GaN," Int' JournalElectrochentical cience, ol . 8, 2013'A. Nahhas,et al.,"Epiraxial growth of ZnO hlms onSi substrates sing an epitaxial GaN buffer," AppliedPhysics et ters, ol .78,pp. 15l l -1513,2001'

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elg. 1. FESENamageof the as grown andporousGaNformedunderdifferent etchingdurations:(a) asgrown;(b) 45 minutes, c) 45 minutesfor high magnification' and(d) 90 minutes 15]'The AFM measurements revealed that the surfaceroughnessover a 5 i{ m x 5 .# m scan area increased ntho porous GaN samples. These results were further,npplrt"d by FESEM images n Fig' 1' The crystallineq*iitv uttd lafiice parameters were assessed anddetermined by using HR-XRD' The Phi-scan onasymmetricGaN (10 I 2) reflectionplane showed ix-foldaz"imuthal ymmetry for both as-grown and porous GaNsamples. Six-fold symmetric is consistent with thewurtzite (hexagonal)crystal structure[16]'

IV. CONCLUSIONIn summary,a novel, simple and cost-effectivealternatingcurrentPEb (ACPEC) etching was demonstratedo be aneffective technique to form nano-porous GaN withexcellent properties. According to FESEM images, theetching durati,onhas signihcant impact on the size of

thepores. . AFM measurements evidenced that surfaceioughness increased in porous samples' The obtained,"rJtr hittt at the possibility to preparehigh quality nano-porous GaN layers with tuneable stress' We stronglyteteve that further refinements of the sine-wave acelectrochemicalprocessingtechnologies will enhancetheir role in semiconductor nanotechnology andnanoelectronicsn the nearfuture.

ACKNOWLEDGMENTSSupport from Universiti Sains Malaysia, 600-nrvrvdifpncS5l3Fst(2212011) and UiTM Dana

Kecemerlangan Research Grant 600-RMVSTIDANA5/3/Dst(134 2011) ate gratefullyacknowledged.

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