【Abstract】In this work, effect of precondition on porous silicon formation was investigated by performing electrochemical polarization measurements. The surface morphologies and optical properties of the samples were also studied by scanning electron microscopy (SEM) and Raman
spectrometer. It was demonstrated that precondition enhanced the chemical/electrochemical reactions occurred at Si/solution interface and the growth of porous silicon, which ultimately resulted in a red shift in photoluminescence. However, the thickness of porous silicon decreased with the increase of precondition time. More broad bands were observed with prolonged precondition.【中文摘要】本工作初步探讨了开路电位下对硅片进行预处理时多孔硅的形成过程 .电化学极化实验、扫描电镜和拉曼谱学的研究表明 ,预处理可以加速硅 /溶液界面上的化学或电化学反应 ,从而加快多孔硅的生长过程 ,最终导致光致发光的光谱红移 .多孔硅的厚度随预处理时间的增长而减

CHENG Xuan

LUO Guang-feng

程璇

罗广丰

Xiamen University Institutional Repository

第 8 卷 第 2 期2002 年 5 月电化学ELECTROCHEMISTRYVol. 8 No. 2May 2002Article ID: 1006_3471( 2002) 02_0134_05Effect of Precondition on Porous Silicon Formation*CHENG Xuan* , LUO Guang_feng( Dep t . of Chem . , State K ey L ab . f or Phys . Chem . of Solid Surf aces ,X iamen Univ . , X iamen 361005, China)Abstract: In this w ork, ef fect of precondit ion on porous silicon format ion w as investigated byperforming electrochemical polarization measurements. T he surface morpholog ies and opt icalpropert ies of the samples were also studied by scanning elect ron microscopy ( SEM ) and Ramanspect rometer. It w as demonst rated that precondition enhanced the chemical/ electrochemical reac-t ions occurred at Si/ solut ion interface and the grow th of porous silicon, w hich ult imately resultedin a red shift in photoluminescence. How ever, the thickness of porous silicon decreased w ith theincrease of precondit ion t ime. M ore broad bands w ere observed w ith prolonged precondit ion.Key words: Porous silicon, Photolum inescence, Precondit ionCLC Number: O 646 Document Code: A1 IntroductionT he phenomenon of strong v isible photoluminescence from porous silicon at room temperaturehas at t racted w idespread attention. And the possibility of making light- emit ting porous silicon asphotolum inescence dev ices to extend the functionality of silicon technology from microelect ronicsinto optoelect ronics has mot ivated considerable interest worldw ide. Although porous silicon isreadily formed by anodizing silicon w afers in HF_based solut ions, its application in silicon_basedoptoelectronic devices is great ly lim ited due to its poor stability and low luminescence y ield.Previous related studies[ 1, 2] revealed that the pulse current and periodic potential applicat ionssignif icantly accelerate porous silicon g row th as compared w ith conventional cont inuous current /potent ial applicat ions. The improvement in surface uniformity and st ructural propert ies of poroussilicon might be contributed by that surface_related species generated during silicon etching can beact ively removed from the pore t ips[ 2]and a regenerat ion of the HF concentrat ion at the pore t ipsReceived date: 2001_12_08* To whom corr espondence should be addressed, T el: 0592_2187701, E_mail: xcheng@ xmu. edu. cnFoundation item: The National Science Foundation of China under Grant Number 20073036can be induced during each step [ 3] .T o further understand the nature of porous silicon formation and to better control the surfaceand st ructural properties of porous silicon, it is necessary to gain a complete picture by collect ingmore data. T his work w as, therefore, carried out to study the ef fect of precondit ion on porous si-licon formation. Electrochemical polarizat ion measurements, SEM and Raman analyses w ere per-formed to characterize electrochemical responses, surface and optical propert ies of porous siliconunder controlled condit ions.2 ExperimentalT he silicon w afers used in this w ork w ere p- type, ( 100) oriented single crystal w ith resis-t ivit ies of 15~ 50 . cm and they w ere diced into 1. 25 1. 25 cm squares. Before conduct ing theelect rochemical experiments, the silicon samples w ere cleaned in a H2SO4: H 2O2= 4: 1 ( by vo-lume) solut ion for 10 m in to remove organic contaminants, and then rinsed w ith DI w ater. Thesamples were then dipped in a 2 % dilute HF solut ion for 30 s, rinsed w ith DI w ater again, andfinally dried in the air.Porous silicon m icrost ructures w ere formed when a constant potent ial of 6 V w as continuous-ly applied to the wafer surfaces[ 2] . Precondit ion w as introduced by let t ing the samples exposed to20 % HF and EtOH solutions in the elect rochemical cell and stayed at open- circuit potent ial fordifferent periods of t ime before applying 6 V. Polarizat ion curves were measured by scanning froma cathodic to an anodic direct ion at a scan rate of 1 mV/ s using an EG& G Model 273A Potentio-stat / Galvanostat . The w afer samples w ere precondit ioned for 0. 5, 2 and 4 h, respect ively , beforeeach elect rochemical measurement. The surface and cross_sect ion morpholog ies of porous siliconformed w ere then exam ined by scanning elect ron microscope ( SEM ) [ 1] . The optical propert iesw ere studied by Raman spect rometer[ 1]. The values of corrosion current density ( i corr) w ere ca-lculated based on the experimental results. The thickness of porous silicon formed under variouscondit ions w as evaluated f rom the cross_sectional microg raphs.3 Results and DiscussionSurface and cross_sect ional morphologies of porous silicon formed w ithout precondition andunder dif ferent periods of precondition t ime are compared in Fig. 1. Without precondit ion treat-ment , the pore st ructure appeared in a well_def ined w ay w ith the thickness being less than 5 m .With precondit ioned for half an hour, the pore sizes enlarg ed and the thickness remarkably in-creased. How ever, it becomes apparent that the thickness decreased w ith an increase of precond-it ioning t ime. More uniformed pore structure w as observed when the sample was preconditionedfor 2 h as evident in Fig. 1( c) . Some reg ions appeared to have severe bubble attachment and pref-erent ial etching pits when the sample was preconditioned for 4 h. Shorter ( 0. 5 h) precondit ionproduced rather thick layer but non_uniformed pores as seen in Fig. 1( b) , w hile prolonged pre-condition ( 4 h) led to thinner layer w ith the pore sizes being ranged more w idely as show n in Fig .135第 2 期 程 璇等: 预处理对多孔硅形成过程的影响1( d) . Some small pores whose pore sizes w ere less than 0. 5 m were formed w ithin the w all oflarge pores, and several pit s indicated that some part of the silicon subst rate w as more severelyetched. Thereby, prolonged precondit ioning led to poor uniformity.F ig. 1 Surface and cr oss_sect ional morphologies of porous silicon layers formed on the p_type siliconin 20 % HF+ EtOH solutions at constantly applied 6. 0 V w hen preconditioned for ( a) 0 h( b) 0. 5 h ( c) 2 h ( d) 4 hF ig. 2 A comparison of polarization curves obtainedbefore and upon constantly applying 6. 0 V tothe p_type silicon without preconditioningand w ith preconditioning for differ ent per iodsof time indicated on each curvePolarizat ion curves obtained in the absence( indicated as 0 h) and presence of v arious pre-condition t imes indicated on each curve areshown in Fig. 2. For a comparison, the polar-ization curve measured at open_circuit potential( indicated as blank ) is also included in thefigure. It w as observed that the open_circuit po-tential ( E ocp) shifted to a more negat ive direc-t ion w hen 6. 0 V w as applied to the silicon sur-face w ithout precondit ion t reatment and thecurve became less symmetric as compared w iththe blank curve. How ever, w hen the samplesw ere precondit ioned for 0. 5, 2 and 4 h, Eocpmoved back to the potentials near blank curvew hich was obtained neither applying 6. 0 V norprecondit ion, w hile the reaction rate signif icant-ly increased as the Rp value reduced. With 4 h136 电 化 学 2002 年precondit ion t ime, Eocp moved to more anodic direct ion and the reaction rate further increased asindicated by the smaller R p value obtained.It can be seen from the table that the Rp values slight ly decreased when an anodic potent ial of6 V w as applied to fabricate porous silicon, but signif icant ly decreased w ith the precondit iontreatments, in part icular, w hen the sample w as preconditioned for 4 h.F ig. 3 Effects of precondit ioning time on ( a) t he electrochemical reaction rate and ( b) the thick-ness of por ous siliconF ig. 4 Raman spectra obtained from the porous sil-icon layers formed by constantly applying 6. 0V without and w ith preconditioning. ( )preconditioned for 0 h ( ) preconditionedfor 0. 5 h ( ) precondit ioned for 2 h( )preconditioned for 4 hThe react ion rate in terms of i corr and thethickness of porous silicon formed as a functionof precondit ion t ime are presented in Fig. 3. Ingeneral, precondition treatments accelerated theelect rochemical react ion occurred at Si/ solutioninterface, in part icular w ith 4 h precondition,and enhanced the porous silicon growth. How-ever, the thickness decreased w ith the increaseof precondit ion t ime. T hicker layer w as observedwhen the sample w as preconditioned for 2 h.Raman spect ra obtained under different testcondit ions are provided in Fig. 4. A red shif tfrom 709 nm w ith 2 h pret reatment to 807 nmw ith 4 h pret reatment w as observed. It is ap-parent that the Raman band became broaderw hen the sample w as treated w ith shorter ( 0. 5h) or prolonged ( 4 h ) preconditions. This137第 2 期 程 璇等: 预处理对多孔硅形成过程的影响might suggest that a suitable length of precondition improved the uniform ity and photolum-inescence of porous silicon, while shorter or prolonged precondit ion time adversely affected the sur-face and opt ical properties of porous silicon. T he surface reaction products generated during pre-conditioning could become predominant factor to inf luence the uniform ity and grow th of poroussilicon. Further study is needed to clarify this po int.4 ConclusionIt w as demonst rated that the precondit ion at the open_circuit potent ial enhancedthe chem-ical /electrochemical reactions occurred at Si/ solut ion interface, and decreased the thickness ofporous silicon, w hich ult imately resulted in a red shift . A suitable precondit ion time, for ex ample2 h, could improve the uniform ity of porous silicon.* Authors w ish to thank Dr. L iu Feng- ming and Ms Xue Ru for t he assistances of Raman and SEM charac-terizations on the porous silicon samples.References:[ 1] Cheng X, L in C. J. Fabrications and char acterizations of porous silicon by two_step Techniques, : Pulsecurrent application[ J] , Electrochemistry , 2001, 7( 1) : 78~ 84.[ 2] Cheng X, Wen Z X, Luo G F . Character izations of porous silicon layers formed on modified silicon wafer sur-faces, Quantum confinement : Nanostructured materials and devices[ C] , Pro ceedings of the Six th Interna-tional Symposium on Quantum Confinement, T he Electrochem. Soc. , Penning ton, NJ ( 2001) 169.[ 3] Billa S, Th nissen M , A rens_F ischer R, et al. I nfluence of etch stop on the microstructure of porous siliconlayers[ J] . Thin Solid F ilms, 1997, 297: 22.预处理对多孔硅形成过程的影响程 璇,罗广丰(厦门大学化学系, 固体表面物理化学国家重点实验室,福建 厦门 361005)摘要: 本工作初步探讨了开路电位下对硅片进行预处理时多孔硅的形成过程. 电化学极化实验、扫描电镜和拉曼谱学的研究表明,预处理可以加速硅/溶液界面上的化学或电化学反应, 从而加快多孔硅的生长过程,最终导致光致发光的光谱红移.多孔硅的厚度随预处理时间的增长而减小.关键词: 多孔硅;光致发光; 预处理138 电 化 学 2002 年

预处理对多孔硅形成过程的影响

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预处理对多孔硅形成过程的影响

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