The etching of InP in HCl solutions : a chemical mechanism Notten, P.H.L.

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1 The etching f np in HCl slutins : a chemical mechanism Ntten, P.H.L. Published in: Jurnal f the lectrchemical Sciety DO: Published: Dcument Versin Publisher s PDF, als knwn as Versin f Recrd (includes final page, issue and vlume numbers) Please check the dcument versin f this publicatin: A submitted manuscript is the authr's versin f the article upn submissin and befre peer-review. There can be imprtant differences between the submitted versin and the fficial published versin f recrd. Peple interested in the research are advised t cntact the authr fr the final versin f the publicatin, r visit the DO t the publisher's website. The final authr versin and the galley prf are versins f the publicatin after peer review. The final published versin features the final layut f the paper including the vlume, issue and page numbers. Link t publicatin Citatin fr published versin (APA): Ntten, P. H. L. (1984). The etching f np in HCl slutins : a chemical mechanism. Jurnal f the lectrchemical Sciety, 131(11), DO: General rights Cpyright and mral rights fr the publicatins made accessible in the public prtal are retained by the authrs andr ther cpyright wners and it is a cnditin f accessing publicatins that users recgnise and abide by the legal requirements assciated with these rights. Users may dwnlad and print ne cpy f any publicatin frm the public prtal fr the purpse f private study r research. Yu may nt further distribute the material r use it fr any prfit-making activity r cmmercial gain Yu may freely distribute the URL identifying the publicatin in the public prtal? Take dwn plicy f yu believe that this dcument breaches cpyright please cntact us prviding details, and we will remve access t the wrk immediately and investigate yur claim. Dwnlad date: 22. Apr. 218

2 Dwnladed 2 Jan 211 t Redistributin subject t CS license r cpyright; see Vl. 131,N. 11 VAPORATD WO~ FLM R. J. Cttn and J. W. Rabalais, nrg. Chem., 15, K. T. Ng and D. M. Hercules, J. Phys. Chem., 8, D. R. Renn, J. lectrn. Spectr. Relat. Phenm., 9, J-P. Gaban, "Lithium Balleries," p. 27, Academic Press, New Yrk (1983). The tching f np in HC Slutins: A Chemical Mechanism P. H. L. Ntten Philips Research Labratries, 56 JA indhven, The Netherlands ABSTRACT The etch rate f np in slutins f high HC] cncentratin was shwn t be independent f the applied ptential in a wide ptential range negative with respect t the flatband value. Disslutin f the slid led t the frmatin f PHi. The etch rate, which was nt mass-transprt cntrlled, was first rder in mlecular HC1 cncentratin. The results lead us t cnclude that, in HC1 etchants, np is disslved by a purely chemical mechanism. The influence f chemical etching n the andic behavir f np in these electrlytes is described. Aqueus etching f -V materials is ften an imprtant step in device technlgy (1, 2). While andic etching is smetimes used (3), the necessity f making electrical cntact t slices f small dimensins is invariably a disadvantage. Methds nt invlving an external current r vltage surce are therefre favred (1, 2). Such methds can be divided int tw classes: "electrless" and "chemical" disslutin. lectrless disslutin invlves tw separate ptential-dependent electrchemical reactins: the xidatin f the slid and the reductin f an xidizing agent. The principle is illustrated in Fig. 1 fr the etching f a p-type -V semicnductr (4). Fr these materials, hles in the valence band are required at the surface fr bnd breaking. As a result, the andic disslutin current increases at ptentials clse t the flatband value (VrB) as shwn in curve. Fr electrless disslutin, hles must be supplied by an xidizing agent in slutin. f the distributin functin f the xidizing agent verlaps with the valence band f the semicnductr, then reductin via hle injectin int the valence band is expected [curve ]. Here, we have assumed that the reductin reactin is diffusin cntrlled, i.e., the cathdic partial current is independent f ptential. Curve (c) represents the ttal measured current-ptential curve in the electrlyte cntaining the xidizing agent. At the rest ptential, the andic and cathdic partial currents are equal. f the ptential is changed frm this value using an external surce, then the etch rate f the semicnductr will, f curse, als change. Gerischer and c-wrkers (5, 6) have shwn that semicnductrs can be disslved by a purely chemical mechanism, which is characterized by the absence f any influence f the surface carrier cncentratin n the etch rate. Such behavir is bserved with bifunctinal agents, such as halgen r H~O.., mlecules, which are capable f frming tw new bnds with the semicnductr surface mre r less simultaneusly. Fr GaAs disslutin in brmine slutin, fr example, they suggest a crdinated reactin sequence invlving the breaking f Ga-As and Br-Br bnds and the simultaneus frmatin f Ga-Br and As-Br bnds. tchants based n HC1 are widely used fr np semicnductr devices (7, 8). The 'presence f ther acids in the HC1 slutin has a significant influence n the etch rate. Hwever, np des nt disslve in cnventinal etchants invlving simple xidizing agents. n rder t reslve the questin f the disslutin mechanism, we studied bth the etching and electrchemistry f p-np in varius HC1 slutins. xperimental The p-type np slices used in this wrk were made frm liquid-encapsulated Czchralski material with a carrier density in the range is cm -3. The (1) face was expsed t the slutin. The diameter f the electrdes was 3 ram, with the exceptin f the np rtating disk, which had a diameter f 4 mm. The current-ptential measurements were carried ut under ptentistatic cntrl in a cnventinal cell using a Pt cunterelectrde and a saturated calmel electrde (SC) as reference. All ptentials are given with respect t this SC. A Slartrn 1172 Frequency Respnse Analyser was emplyed fr determining the flatband ptentials. All impedance measurements were carried ut at a frequency f 1 khz. The ttal disslutin rate f np at varius ptentials was determined analytically by measuring the indium cncentratin in the etching slutin by inducedcupled-plasma (CP) emissin spectrmetry. The etehant was passed ver the np electrde, which was munted in a glass micrelectrchemical flw cell as described by Harutiunian et al. (9). ndium cncentratins as lw as.5 ppm culd be determined with a relative accuracy f abut 5%. An LKB Variperpex peristaltic pump was used t pump the slutin thrugh the flw cell. The flw rate f the slutin, mainly deter- u - t- _u "(3 J 8 l (c) V(SC) Fig. 1. Curve represents the theretical partial andic disslutin current f a p-lnp electrde in the dark, and curve the diffusinlimited reductin current f an xidizing agent as a functin f the ptential. Curve (c) gives the ttal current-ptential curve.

3 Dwnladed 2 Jan 211 t Redistributin subject t CS license r cpyright; see J. lectrchem. Sc.: SOLD-STAT SCNC AND TCHNOLOGY Nvember 1984 mined by the lwer limit f the indium detectin, was in the range frm.2 t 1. m]min. The gas was analyzed qualitatively using the clrdetectr tube methd ("Dr~ger" tube), which in ur case gave a clr reactin specific fr phsphine. A gas burette was used fr the quantitative gas analysis. Results Flw-cell experiments.--n Fig. 2, results are given fr the ptentistatic etching f p-np in the dark in HC1 slutins. Figure 2a shws the current-ptential curves f this electrde in 3M and 6M HC1. At negative ptentials the current is very lw in bth cases. n the vicinity f the flatband ptential (VFB =.73V vs. SC) the andic current increases, as expected. t shuld be nted that the andic curve fr the 6M HC1 slutin is shifted slightly in the cathdic directin with respect t that fr the 3M slutin. The ttal disslutin rate, accrding t the CP analysis, is shwn in Fig. 2b as a functin f the ptential. Fr the 3M HC1 slutin, the disslutin rate fllws the current clsely. At negative ptentials the etch rate (rt) is very lw and increases cnsiderably near the flatband ptential. f we assume that in this case etching is due slely t andic disslutin, then it can be easily shwn that six hles are required t disslve ne np entity. The ttal etch rate ptential curve fr the 6M HC1 case differs markedly frm that f the crrespnding currentptential curve. This difference is mst bvius at negative ptentials, at which the np disslves at a rate essentially independent f applied ptential. Since n cathdic reactin ccurs in this range, we must cnclude that np is chemically disslved by HC1. f we assume that andic disslutin in 6M HC1 here als requires six hles per np, then the chemical etch rate can be calculated frm the ttal etch rate and the measured andic current. Figure 2c shws that the chemical etch rate (rchem) remains 15 O M HCL x 6M HC cnstant, even at ptentials at which the electrde disslves andically. A further example is shwn in Fig. 3 fr 1.5M HC1 in cncentrated acetic acid slutin. tchants based n HC1 and acetic acid are ften used in np technlgy (7). Figure 3a shws the current-ptential curve, and Fig. 3b the calculated chemical etch rate as a functin f the electrde ptential. The results in the cathdic regin are similar t thse fund with 6M HC1 slutin; the current is lw and a high chemical etch rate is fund. The slw increase f the chemical etch rate with ptential is prbably due t a rughening f the electrde surface during the experiment. The increase in the andic current at ptentials near VFB is cnsiderably lwer than that fund in a HC1 slutin f the same cncentratin but withut acetic acid. A dramatic decrease f the chemical disslutin rate is bserved when the andic disslutin starts. n this ptential range, we bserved the frmatin f an rangeclred film at the electrde surface. Phsphrus was fund in this film by DAX analysis. Obviusly, a passivating film is frmed during andic disslutin. This film inhibits bth the andic and the chemical disslutin reactins. t shuld be nted that the chemical disslutin rate depends markedly n the surface cnditin f the np electrde. Since inductin effects are bserved, care must be taken t ensure a cnstant disslutin rate befre measurements are made. This can be dne by pre-etching the electrde in the same etching slutin. The results shwn s far suggest that the chemical etch rate f np is strngly dependent n HC1 cncentratin. This dependence was studied by varying the HC1 cncentratin ver a wide range fr tw different systems. An np crystal was chemically etched in these slutins in the flw cell (the measured current was zer), and the chemical disslutin rate was again analytically determined by CP emissin spectrmetry. The results are shwn in Fig. 4. Fr curve, the cncentratin was varied by diluting cncentrated HC1 with water. At 9M HC1, the chemical disslutin rate is high and decreases rapidly as the HC1 cncentratin is lwered. Fr a 5M HC % 1 r-.25 im.4,-, X.6 & 4, m m ; ~ C = '2 f x _ e J (c).~,2 j O )q ){ x ~ 9 A ~--'C_ V(SC) Fig. 2. Ptentistaticnlly measured current-ptential curves, ttal etch rate as a functin f the electrde ptential, and chemical etch rate as a functin f the electrde ptential (c) fr a p-lnp electrde in the dark in a 3M HC () and 6M HC (x) slutin in water. i i V(SC) Fig. 3. Ptentistatically measured current-ptential curve and chemical etch rate as a functin f the electrde ptential in the dark fr p-lnp electrde in a slutin f 1.SM HC in cncentrated acetic acid.

4 Dwnladed 2 Jan 211 t Redistributin subject t CS license r cpyright; see Vl. 131, N. 11 TCHNG OF np N HC1 SOLUTONS 2643 r- c. "l ~. J O [HCL] (Mit) Fig. 4. Chemical etch rate as a functin f the HC cncentratin fr a p-lnp crystal in the dark. The HC cncentratin was varied by diluting with water [curve ] and cncentrated acetic acid [curve ]. cncentratin, the etch rate is less than 4 ~,min and even lwer fr mre dilute slutins. Fr curve, cncentrated HC1 was diluted with cncentrated acetic acid. t is striking that here a linear relatinship between etch rate and HC1 cncentratin is bserved and that the etch rate is significantly higher than in the crrespnding HC1-H2 slutins. T decide whether the disslutin is a diffusin r a kinetically cntrlled prcess, the etch rate was measured as a functin f the rtatin rate using a p-np rtating disk electrde at pen circuit. Figure 5 shws that the chemical etch rate measured in a 3M HC1 in cncentrated acetic acid slutin is essentially independent f the rtatin rate (N). This means that the rate f the disslutin reactin is kinetically cntrlled. Gas analysis.--when an np crystal was disslved at the rest ptential in a cncentrated HC1 slutin, gas evlutin was bserved at the slid surface. Using the clrdetectr methd, we shwed that this gas was phsphine. A clear clr change was indeed bserved even with the mst insensitive tubes. We determined the gas 1.5 quantitatively with a gas burette and shwed that the phsphrus is cnverted fr 1% ( 1%) as PHi. lectrchemical measurements.--the current-ptential curves f p-np in 1N H2SO4 and in different HC1 slutins in the dark are shwn in Fig. 6. n all cases, the blcking current in the cathdic regin was very lw (<.2 macm2). The nset f the andic current in the case f 1N H~SO~ [curve ] ccurs near the flatband ptential (VFB =.73V), as expected. The andie current fr 1M HC1 [curve ] is shifted sme 15 mv in the negative ptential directin with respect t the H~SO~ case. With an increase in the HC1 cncentratin, this effect becmes mre prnunced. Fr the 9M HC1 slutin, the shift amunts t abut 35 mv. When the curve was measured again in 1N H2SO4 after the HC1 experiments, exactly the same result was btained as in the first measurement [curve _]. We als measured the Mtt-Schttky plts fr each f the slutins used in Fig. 6. The flatband ptential did nt depend n the HC1 cncentratin: VrB = V in all cases. The slpe f the Mtt-Schttky plts decreased smewhat as the HC1 cncentratin was increased. This is prbably due t an increase in the surface area f the electrde due t rughening as a result f etching. A similar effect can als be seen in Fig. 3b. Discussin Frm Fig. 2 and 4, it is bvius that chemical disslutin ccurs in aqueus slutin when the HC1 cncentratin exceeds a certain critical value. The rate is strngly dependent n the HC1 cncentratin and becmes very lw at values lwer than 5M [see Fig. 4, curve ]. This suggests that the etch rate depends n the degree f dissciatin f HC1 mlecules. Althugh it is clear that at lw cncentratins the disslutin f HC1 is cmplete, there is a cnsiderable discrepancy in the literature with respect t higher cncentratins (1). Calculatins based n vapr pressure measurements and n Hammett functins shw that the cncentratin f undissciated HC1 begins t increase significantly abve 5 ml1 (11, 12). n rder t avid the uncertainty invlved when HC1 is diluted with water, we studied the disslutin rate in HC1- acetic acid slutins. The disssciatin cnstant f HC1 in acetic acid (Ka = 1 -s~5) is much lwer than in water (Ka = 1 +3) (13). Cnsequently, the degree f dissciatin f HC1 in acetic acid is negligible, even at lw HC1 cncentratins. The chemical etch rate culd therefre be studied as a functin f the mlecular HCt cncentratin. The linear dependence f the chemical disslutin rate n the HC1 cncentratin in cncentrated acetic acid [Fig. 4, curve ] indeed cnfirms that chemical disslutin is determined by the mlecular HC1 cncentratin. The " (e){d)(c) v ~.5.5 L 2 z.()o N (r.p.m.) Fig. 5. Chemical etch rate in the dark as a functin f the rtatin speed f a p-lnp electrde in a slutin f 3M HC in cncentrated acetic acid. V (S} Fig. 6. Current-ptential curves fr a p-lnp electrde in the dark in N H2SO4, 1M HC, 5M HC (c), 7M HC.(d), and 9M HC (e) in water. Scan rate: 1 mvs.

5 Dwnladed 2 Jan 211 t Redistributin subject t CS license r cpyright; see J. lectrchem. Sc.: SOLD-STAT SCNC AND TCHNOLOGY Nvember 1984 high etch rate f HC1 slutins diluted with cncentrated acetic acid cmpared t that fr aqueus slutins, als fund by Adachi (7), can be understd in this way. Figure 5 shws that the etch rate is independent f the rtatin rate f an np electrde meaning that the chemical etch rate is kinetically determined by these HC1 mlecules. The mechanism presented by Gerischer and Wallem- Mattes fr the chemical disslutin f semicnductrs invlves symmetrical bifunctinal etching agents such as H~O~ and halgen mlecules (5, 6). Althugh we are, in the present wrk, dealing with an asymmetrical HC1 mlecule, we prpse a reactin scheme similar t that fr symmetrical agents. The first step invlves a synchrnus exchange f bnds: n-c1 and P-H bnds replace the riginal H-C1 and n-p bnds. This is very likely the ratedetermining step C -- H C H z > --n -- P-- --n P -- \ \ Since the indium and phsphrus atms at the surface are triply bnded t neighbring atms, tw further bnds must be brken in an analgus manner t remve each atm frm the lattice C H n P HC ~ nci 3 + PH3~ \ ndium is therefre disslved as hydrlyzed ncl~ and PH:~ is evlved as a gas. Such a mechanism can accunt fr the etching results bserved. The chemical etch rate f np in HC1 slutin is independent f ptential. At ptentials near the flatband value (Fig. 2 and 3) the p-np electrde disslves andieally. The rate f the andie etching increases as the surface hle cncentratin is increased, i.e., as the ptential is made mre psitive. n aqueus HC1 slutins, we have shwn that six hles are required t disslve ne n-p entity. This means that bth n and P are xidized t the trivalent state, as is cmmn fr LV materials (3-6). With a cnsiderable cncentratin f acetic acid in the HC1 slutin, hwever, a film is frmed n the electrde which inhibits bth the chemical and andic disslutin (Fig. 3). DAX measurements have shwn that this layer cntains phsphrus but n indium r chlrine. We suspeet that, in this case, phsphrus is nt xidized directly during andic disslutin np + 3h + --> n n 4- P A similar three-hle mechanism has been shwn fr GaP (14). t is bvius frm Fig. 6 that the chemical disslutin rate strngly influences the electrchemical behavir f np. Andic disslutin starts at a mre negative ptential as the HC1 cncentratin is increased, althugh the flatband ptential des nt change. This must mean that the activatin energy fr electrchemical disslutin is lwered by chemical etching. A similar effect has been demnstrated by Gerischer and Wallem-Mattes (6) fr the disslutin f GaAs in brmine slutin. As in the GaAs case, this result can be explained if we assume that rup- ture f the first n-p surface bnd is rate determining fr andic disslutin \ \ n : P + 2 h X- ~ n P \ \ f this bnd is brken during a chemical attack by HC1, then the remaining bnds are mre easily attacked andically and the nset ptential fr andic disslutin is cnsequently lwered, indicating that it is, indeed, likely that the first step is the rate-determining disslutin step. Cnclusins The disslutin f np in cncentrated HC1 slutins fllws a chemical mechanism in which undissciated HC1 mlecules play a decisive rle. t seems likely that ther etchants fr np, such as HBr and Br2 (7, 8), are based n a similar mechanism. The reasn why np des nt underg electrless disslutin in cnventinal etchants cntaining xidizing agents is prbably related t the presence f a thin, highly resistant xide layer n the semicnductr (15). Such a layer, which can inhibit either the disslutin f the slid r hle injectin frm the xidizing agent, is unlikely t be present at the high HC1 cncentratins used here. Acknwledgment The authr wishes t thank Dr. J. J. Kelly, Mr. J.. A. M. van den Meerakker, and Dr. R. Mernming (Philips Frschungslabr, Hamburg, Germany) fr helpful discussins, and the Analytical Department f r. P. J. Rmmers fr the indium analyses. Manuscript submitted March 19, 1984; revised manuscript received June 11, Philips Research Labratries assisted in meeting the publicatin csts f this article. RFRNCS 1. W. Kera, RCA Rev., 39, 278 (1978). 2. R.P. Tijburg and T. van Dngen, This Jurnal, 123, M. M. Faktr, T. Ambridge, C. R. llitt, and J. C. Regnault, Curr. Tp. Mater. Sci., 6, 1 (198). 4. H. Gerischer and. Mattes, Z. Phys. Chem. N. F., 49, 112 (1966). 5. H. Gerischer and W. Mindt, lectrchim. Acta, 13, 1329 (1968). 6. H. Gerischer and. Wallem-Mattes, Z. Phys. Chem. N.F., 64, 187 (1969). 7. S. Adachi and H. Kawaguchi, This Jurnal, 128, 1342 (1981). 8. S. Adachi, Y. Nguchi, and H. Kawaguchi, ibid., 129, 153 (1982). 9.. Harutiunian,J. Sandin, P. Clechet, D. Lamuche, and J. Martin, ibid., 131, 27 (1984). 1. "Gmelins Handbuch der Anrganischen Chemic," Chlr (6), rg~nzungsband, Teil B - Lieferung 1, p. 223, Verlag Chemic, GmbH, Weinheim, Germany (1968). 11. A. ucken, Z. lektrchem., 52, 255 (1948). 12. W. Kangr, Z. Phys. Chem., 32, 273 (1962). 13. W. Huber, "Titratins in Nnaqueus Slvents," pp. 215, 226, Academic Press, New Yrk (1967). 14. R. Memming and G. Schwandt, lectrchim. Acta, 13, 1299 (1968). 15. A. Heller, R. Vadimsky, W. Jhnstn, Jr., K. Strege, H. Leamy, and B. Miller, in "Prceedings f the 15th Phtvltaics Specialists Cnference, Kissimmee, Flrida, May 198," p. 1422,, New Yrk (1981).