Electrochemical and Etching Behavior of InP Single Crystals in Iodic Acid Solutions

Electrochemical and Etching Behavior of InP Single Crystals in Iodic Acid Solutions

Io E. Vermeir, 1 H. H. Goossens,' F. Vanden Kerchove, and W. P. Gomes*

Universiteit Gent, Laboratorium voor Fysische Scheikunde, Kri jgslaan 281, B-9000 Gent, Belgium

ABSTRACT

The etch rate of the (100)-face of n- and p-type InP and that of the (111)- and (111)-faces of n-InP in aqueous iodic acid solutions was measured as a function of several variables using a flow-cell setup. The electrochemical behavior of HIO3 at the n- and p-InP electrode was investigated cathodically and anodically, in darkness and under illumination. The limiting photocurrent, both cathodically for p-type as anodically for n-type InP, was found to be enhanced when adding HIO3 to the indifferent electrolyte solution; possible electrode reaction mechanisms are discussed. By combining the etch and electrochemical results, the stoichiometry of the etch reaction was deduced, and it was established that the etch process occurs by a chemical mechanism, involving the formation of radical-type decomposition intermediates, chemical etching was found to compete with anodic etching when holes are available at the surface. A difference in surface morphology after etching was observed between the (111)-face, on one hand, and the (111)- and (100)-faces, on the other hand, al though the etch rate was in all cases diffusion-limited. A tentative explanation for this phenomenon is proposed.

Wet etching of III-V semiconductors plays an essential role in various steps of electronic device fabrication and is a commonly used method for removing damage or controlled amounts of material from the crystal surface and for revealing lattice defects. Recent studies have shown that dark etching processes on GaAs (for a review, see Ref. (1)) and GaP (2, 3) may occur either through a so-called electroless mechanism, in which two partial electrical cur- rents involving free charge carriers in the semiconductor cancel each other electrically, but not chemically, or through a purely chemical mechanism in which no electronic charge carriers participate. From the study of the resulting morphology, it was further shown that a diffusion- controlled chemical reaction (in the case of chemical etching) or a diffusion-limited anodic partial current (in the case of electroless etching) leads to a more or less homoge- neously etched surface, whereas a kinetically controlled reaction rate mostly gives rise to etch pits (4).

Although InP constitutes an important material from the technological point of view, the dark etching behavior of this semiconductor has been hitherto less thoroughly studied. According to Notten et al. (1), InP does not un- dergo electroless dissolution in etchants involving oxidizing agents, due to the presence of a thin oxide layer on the semiconductor surface, preventing hole injection. Most of the commonly used etchants for InP are based upon concentrated solutions of HC1 or HBr or upon a combination of one of these with other acids; bromine solutions are also used. These etchants operate according to a chemical mechanism (1). Most of them produce corroding and toxic vapors, or do not fulfill all of the criteria which are nor- mally required from wet etchants. An etching solution based upon NaC10~ in combination with HC1, producing C12, which is in fact the etching reactant, has been proposed in Ref. (5). The present paper pertains to a funda- mental study on the etching of InP in KIO3 + HC1 + H20. Unlike in the case of chlorate, no C12 is formed, so that the etchant may be considered to be an iodic acid (HIO3) solution. In a recent, more technically oriented paper, the ad- vantages of using HIO3 when etching InP for device applications have been demonstrated (6).

In the present work, data on the electrochemical and etching kinetics of HIO~ at the InP surface were mutually compared in order to elucidate the etching mechanism. The resulting morphology was also studied, and the correlation between the etching kinetics and the morphology was investigated.

Experimental The experiments were performed on single crystal n-

and p-type InP samples with (100)-orientation (obtained from MCP Electronics, U.K.: donor (S) density: 6.7 x 10 ~7 cm -3 for n-type and acceptor (Zn) density: 4.5-4.8 • 10 TM

cm -~ for p-type, respectively) and on single crystal n-type * Electrochemical Society Active Member. Research Assistant of the N.F.W.O. ( National Fund for Scien-

tific Research, Belgium).

InP samples with ( l l l ) - and ( l l l ) -or ienta t ion (obtained from M~taux Sp~ciaux, France; donor (S) density: 7.4 x 101~ cm-3). The InP wafers were mounted either as flow- cell electrodes or as rotating disk electrodes (RDE) (for details, see Ref. (7)).

Etch rates were measured by using a flow-cell, i.e., a mi- croelectrochemical cell (cell volume 0.37 cm 3) with a con- t inuous electrolyte flow, in which the electrolyte was collected at the cell exit. The flow rate u could be varied by using a peristaltic pump. The collected electrolyte was an- alyzed for In(III) by differential pulse polarography (7) (EG&G Polarographic Analyzer Model 384B). The etch rate is then defined as the amount of In(III) dissolved per unit of t ime and of InP surface area. Il lumination of the flow-cell electrode was possible by using a hologen lamp, connected to an optical glass fiber (incident optical power 18 mW �9 cm-~).

The RDE setup consisted of an electrochemical cell, made of Perspex, in which the electrode also could be illu- minated by a halogen lamp.

In all of the experiments, a saturated calomel electrode (SCE) was used as the reference electrode; all potentials are given relative to the SCE. The counterelectrode was a Pt electrode. The potential of the InP electrode was controlled by a potentiostat (Tacussel). For the measurement of the current-potential curves, a programmable sweep generator was used. In some experiments, the photocurrent was measured by using chopped light. Impedance measurements were made using an automatic LF-impedance analyzer (Hewlett-Packard 4192A).

The morphology of the surface after etching was studied using a microscope-interferometer (for details, see Ref. (2)).

The solutions were prepared by dissolving analytical grade chemicals in deionized water and were purged with high purity nitrogen gas.

Results Flow-cell experiments at open-circuit potential (V = Vo~).--

Etch rate measurements were carried out in darkness and at room temperature on n- and p-InP in aqueous KIO3 + HC1 at pH = 0. Judged by the redox potentials (8), IO~ is not able to produce C12 in an aqueous HC1 solution, in contrast to C10~. Hence, chlorine can be excluded as a possible etching reactant in the given medium. Since the disso- ciation constant of iodic acid HIO3 is relatively low (K, = 0.169 at 298 K), iodate is largely present as iodic acid at pH = 0 (8). The use of KIO~ + HC1 instead of pure HIO3 solutions allowed us to work at constant pH for different HIO~ concentrations.

For both semiconductor types, an average etch rate r of 2.1 x 10 -9 tool - cm -2 �9 s -~ was found in 0.01 mol . 1 -~ KIO8 + HC1 (pH = 0) at a flow rate u of 25 ml �9 h -~. In a previous paper (7), we reported the results of etch rate experiments in solutions containing HC1 only at pH = 0 under similar

J. Electrochem. Soc., Vol. 139, No. 5, May 1992 �9 The Electrochemical Society, Inc. 1389

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1390 J. Electrochem. Soc., Vol. 139, No. 5, May 1992 �9 The Electrochemical Society, Inc.

r / 1 0 - 9 m o t . c r n - 2 , s - 1

2 �9 '*

1

0 I i I 0 1 2 3

ul/31 m11/3, h-1/3

Fig. 1. Average etch rate r as e function of the cube root of the flow rate u. n-lnP in 0.01 tool �9 I _ _ -2 KI03 + HCI, pH = O, V = Vow. �9 (111)- face, �9 ( lO0)-face, �9 (111)-face.

c i rcumstances : the e tch rate was found to be negl ig ib le (about 10 -n m o l . cm -2 �9 s-I). There fore HIO3 appears to be an eff icient e t chan t for InP.

The d e p e n d e n c e of the dark e tch rate r at n - InP on the flow rate was inves t iga ted for the (100)-, (111)-, and ( I l l ) - face in aqueous 0.01 mol �9 1 1 KIO3 + HCI (all fur ther data per ta in to p H = 0). Fo r the th ree faces, a l inear re la t ionship b e t w e e n r and the cubic root o f u was found (see Fig. 1). As shown in Ref. (9), such a re la t ionship is e x p e c t e d to hold for d i f fus ion-cont ro l led react ion rates in a flow-cell. Re- pea t ing the m e a s u r e m e n t at the (100)-face of p - InP led to the same re la t ionship.

T h e e tch rate was found to be propor t iona l to the concen t ra t ion c of KIO3 added to the solut ion up to 10 2 tool - 1-1.

The m o r p h o l o g y of the s emiconduc to r surface after e tch ing was s tud ied for t he three faces of n-InP. It was found tha t on the ( l l l ) - face , e tch pits wi th a t r iangular s y m m e t r y were formed. F igure 2 shows an in te r fe rence p h o tog raph of the n - InP ( l l l ) - f a ce after e tch ing for 10 rain in aqueous 0.01 mol �9 1-1 K ~ + HC1 at a f low rate u = 2.5 ml �9 h -1. At the (100)- and (111)-faces, no e tch pits were obse rved e v e n af ter 4 h e t ch ing unde r the same__conditions as u s e d for t he (111)-face, i.e., t he (100)- and (111)-faces are al- ways e t ched m o r e or less homogeneous ly .

Flow-cell exper iments at V ~ Vo~.--n-InP.--The e tch rate r and the e lectr ical cur ren t dens i ty j were measu red poten- t iostat ical ly as a func t ion of the potent ia l V at the (100)-face of n - InP in aqueous solut ions of 0.01 mol �9 1-1 KIO3 + HC1.

r a t e / m A . c m - 2

2.O

t .o 1,5

1.0

0.5

0

- 0.5

- 1,0 u

2 -~ -1.5 o

~ - 2 . 1

-2. o .. i i

- 0.5

i , r i ,

I i I i i i - 0 A - 0 2 , 0

V vs 5 C E I V

0.2

Fig. 3. Combined etch rate and current density vs. potential plots at (100) n-lnP in 0.01 tool �9 1-1 KIO3 + HCI, pH = 0; u = 50 ml �9 h - I . �9 etch rate, �9 current density.

In order to c o m p a r e bo th types of resul ts quant i ta t ively, the va lues of r were conve r t ed into equ iva len t e tch cur ren t densi t ies , a s suming that t he e tch ing of InP in acid solu- t ions is a s ix -equiva len t process , analogously to the anodic d issolu t ion of I n P (7). In Fig. 3, the c o m b i n e d r vs. V and j vs. V curves are shown. At ca thodic potent ials , a flow-rate d e p e n d e n t r e d u c t i o n cu r ren t is observed , whereas r t ends towards zero. At more pos i t ive potent ials , the ca thodic cur ren t dens i ty gradual ly decreases and eventua l ly changes into a small, po ten t i a l - independen t anodic current. In parallel~ the e tch ra te increases and even tua l ly reaches a f low-rate l imi ted value. The s u m of the e tch cur- ren t dens i ty and the abso lu te va lue of the ca thodic cur ren t dens i ty is cons tan t ove r the ent i re potent ia l region.

p-InP.- -Figure 4 shows the c o m b i n e d r vs. V and j vs. V curves for the (100)-face of p - InP in aqueous 0.01 mol �9 1-1 KIO~ + HC1 at a f low rate u = 50 m l - h -1. In t he m o r e nega- t ive potent ia l region, on ly a ve ry s low reverse current flows, whereas r is i n d e p e n d e n t of V and is comparab le to the va lues ob ta ined at Vow. When vary ing the p - InP elec- t rode potent ia l in the pos i t ive sense, an exponent ia l ly in- creas ing anodic cur ren t is obse rved as wel l as a s imilar increase in e tch rate. S ince we were unab le to obta in a s teady-state s i tuat ion wi th in a reasonable per iod of t i m e in potent ios ta t ic m e a s u r e m e n t s u n d e r anodic polarizat ion, r and j were m e a s u r e d galvanostat ical ly.

Fig. 2. Interference micrograph of (111) n-lnP after etching for 10 rain in 0.01 tool �9 1-1 KIO~ + HCI, pH = O; V = Vo,, u = 2.5 ml . h -1.

Rotat ing-disk vo l tammetry . - -n- InP. - -T .he j vs. V curves, reg is te red at the (100)-, (111)-, and ( l l l ) - f a c e s of n- InP RDE-e lec t rodes in aqueous HC1 at pH = 0 e i ther wi thou t or wi th 10 -3 mo l �9 1-1 KIO3, are shown in Fig. 5. When KIO3 is added to the HC1 solution, the onse t of the ca thodic cur- ren t is shif ted in the pos i t ive d i rec t ion over 0.6-0.8 V depend ing on the sample, ind ica t ing that HIO3 is be ing reduced. At t he three faces cons idered , the reduc t ion w a v e exhib i t s a p la teau reg ion in wh ich j is p ropor t iona l to the square root of the ro ta t ion rate w (see inset of Fig. 5), wh ich shows that the r educ t ion reac t ion is diffusion-control led.

When KIO3 is added to the ind i f fe ren t HC1 solution, t he anodic da rk l imi t ing current increases by an a m o u n t wh ich is, for the (100)- and (11 D-faces, of t he order of about 2% of the co r re spond ing r educ t ion current , whereas for



J. Electrochem. Sac., Vol . 139, No. 5, May 1992 �9 The Electrochemical Society, Inc. 1391

r a t e l m A.cm -2

5

0 & - -0.5 0 0.5 1.0

V v s S C E I V

Fig. 4. Combined etch rate and curernt density vs. potential plots at (100) p-lnP in 0.01 mol �9 1-1 KIO3 + HCI, pH = 0; u = 50 ml �9 h -1. �9 etch rate, O current density.

the__(ll 1)-face, the increase is much lower. At the (100)- and (111)-faces, the values o f j in the anodic plateau are linearly dependent on w u=. In Fig. 6, j , is plotted vs. ~1 for the (100)- face of n-InP in an aqueous solution ofHC1 + 10 -~ mol- 1 -~ KIO3 (pH = 0); a linearly proportional relationship is found to hold. The slope of the j~ vs. [J~l curve was not influenced by the concentration between 5 - 10 -4 and 10 -2 mol - 1-1 KIO3 nor by the temperature between 280 and 310 K. Ap- proximately the same slope was found at the (11 D-face. At the (11 D-face, the enhancement of the anodic current is too small to make similar measurements.

In view of the following discussion, it is important to stress that in a vol tammetr ic experiment, performed at the

-2.5

~o

I -5.0

i /mA.cm -2

0

/,/ i'.!

(11

I! ..p

(2)

" ~ J lciml /rnA.cm-2 , ': / l, , , �9

OV J ~ , 0 2 4 8

~ii0 . . . . . 01s . . . . . . . . . - 0 0.5

v vs SCEI V Fig. 5. Current density vs. potential curves at the n-lnP electrode in

darkness. Scan rate- - 5 mV �9 s-l; rotation rate w = 20 s -1. (1) HCI, pH = 0; (111 )-, (111 )- and (100)-faces; (2) HCI + 10 -3 mol �9 I-1 K~03, pH = 0; (100)- and (1 ! 1)-faces; . . . . . (111)-face; Inset: IJ~'l vs. ~v/w measured at V = - 0 . 5 V vs. SCE �9 (100)-face, A { ~ ) - f a c e , x (1 ! 1 )-face.

j / I J , A.cm -2

80. , , , , , ,

0 ~" i i , i i i 0 1 2 3

Fig. 6. Anodic vs. cathodic limiting current density at the (100) n-lnP electrode. Vo = +0 .3 V vs. SCE; V~ = - 0 . S V vs. SCE; electrolyte composition: HCI + 10 -3 mol �9 I - I KIO3, pH = 0.

(100)-face of n-InP in 7 mol �9 1-1 HC1, no increase of the anodic dark limiting current with respect to the current in 1.2 mol . 1-1 HC1 (pH = 0) was observed, while the chemical etch rate at Vo~ in 7 mol �9 1 -] HC1 amounted to about 1.5 • 10 -9 mol- cm -2- s -~, which is comparable to the etch rate in aqueous HC1 + KIO3 solution at pH = 0.

Figure 7 shows current-density vs. potential curves, recorded at the (100)-face of n-InP under chopped illumination in aqueous HC1 at pH = 0, either in the absence or in the presence of KIO3. When adding KIO3 to the solution, an enhancement of the anodic limiting photocurrent was observed. The current enhancement was found to depend upon the concentration of KIO3 added and upon the incident light intensity (in Fig. 7, the increase amounts to a factor 1.5). As a measure for the incident light intensity, the limiting photocurrent resulting from anodic dissolution in indifferent electrolyte solution was taken. Enhanced light

j /mA.cm -2

d

|

g " I ph / ." |

Jp h

/ / / z

/ / / , q /

/

-0.2 0 0.2 0.4 ~6

V vs SCEI V

Fig. 7. Current-density vs. potential curves recorded at the (100) n-lnP electrode under chopped illumination. Scan rate = 5 mV �9 s-I; w = 20 s - I . .C)current under illumination in HCI, pH = 0; (~)dark current in 10 -2 real �9 1-1 KIOs + HCI, pH = O; (~) current under illumination in 10 -2 real �9 1-1 KIO3 + HCl, pH = 0. The dark current curve in HCI, pH = O, coincides with the V-axis. j~ photocurrent in indifferent electrolyte. J~h photocurrent in the presence of iodate.



1392 J. E lec t rochem. Soc,, Vol. 139, No. 5, May 1992 �9 The Electrochemical Society, Inc.

T u

.u_.

u

jtrnA~cm-2 4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

-0.5

J

(5) (4} (3) (2) (1)

j, -0.5 -o16 -o14 -0'2 v-vrb/v

Fig. 8. Dark current density vs. V-VFB curves at the (100) p/InP electrode. Scan rate = 5 mV �9 s-l; w = 20 s -1. Electrolyte composition: (1) HCI, pH = 0; (2) 1 • 10 -3 mol �9 1-1 KIO3 + HCI, pH = 0; (3) 5.5 • 10 -3 mol �9 I - I KIO3 + HCI, pH = 0; (4) 9.5 • 10 -3 mol �9 1-1 KIO3 + HCI, pH = 0; (5) 1.1 • 10 -2 mol �9 I - I KIO3 + HCI, pH = O.

absorption due to roughening of the surface by etching can be excluded as the reason for the photocurrent increase, as after the exper iment in HIO3, the original value of the photocurrent in indifferent electrolyte was again measured. In the flow-cell, a photocurrent increase by a factor of 1.2 was found in aqueous HC1 + 0.01 mol . 1-1 KIO3 (pH = 0) and with a light intensity corresponding to a limiting photocurrent density in HC1 of 7.5 mA- cm -~. When increasing the incident light intensity, resulting in a limiting photocurrent density at which the photocurrent density and the corresponding etch current density equal each other (of the order of 10 mA �9 cm-2), no photocurrent enhancement is observed (since etch rates and hence etch current densities can be measured in the flow-cell only, the light-intensity dependence of the photocurrent enhancement due to HIO3 was investigated in the flow-ceU).

p - I n P . - - A t the (100)-face of p-InP, current-potential curves in darkness were recorded either in the absence or in the presence of KIO~. The results at different KIO~ concentrations have been plotted in Fig. 8 as current densities vs. V - V ~ , VFB representing the flatband potential. To that purpose, V ~ was determined by means of impedance measurements at the dark p-InP electrode at different fre- quencies, in which the potential was scanned from negative towards more positive values. Extrapolation of the lin- 2 ear Mott-Schottky plots to C- = 0 led to coinciding values

�9 P .

for frequencms between 5 and 40 kHz, allowing the flatband potential to be determined. A shift of V ~ towards more positive potentials was observed with increasing KIO3 concentration, saturating to about 0.25 V for c = 0.01 mol - 1-1.

The cathodic dark current in KIO3-containing solutions is negligibly low (see Fig. 8). Since, in view of the lack of

j/mA.cm -2 o,

'~

0 , 0

(2

0.2 0.4 0.6 0.8 V vs SCEI V

Fig. 9. Dark current density vs. potential curves at the (100) p-lnP electrode. Scan rate = 5 mV �9 s-l; w = 20 s -1. (1) HCI, pH = 0; (2) 5 • 10 -4 mol �9 I -~ KI03 + HCI, pH = O.

conduction band electrons in dark p-InP, cathodic current flow is only possible via a hole injection mechanism, it can be hence concluded that the first step in the HIOa reduction is not a hole injection step. As at n-type electrodes a reduction wave due to HIO3 is observed, this first step must involve the capture of a conduction band electron.

The anodic dark current depends strongly upon the KIO~ concentration added. A pronounced negative shift of the anodic current density vs. V -Vrs (hence vs. band bending) curve is observed with increasing iodate concentration, saturating at hV = -0.4 V for c = 0.01 mol . 1-1KIO3. At sufficiently large current densities, the j-V curve in a iodate-containing solution eventually coincides with that in the solution without iodate (Fig. 9). Since in the potential range corresponding to high anodic current densities, the band-edge position at the surface cannot be deduced with certainty from capacitance measurements, the results of Fig. 9 have been plotted as current densities vs. potential.

The cathodic current at p-InP was also studied under chopped illumination with and without KIO3 added. It was found that the l imting cathodic photocurrent in the presence of iodate was from 1.5 up to 2 times larger than that in the absence of iodate (see Fig. 10).

D i s c u s s i o n

C a t h o d i c behav io r o f I n P in H I 0 3 s o l u t i o n s . - - T h e diffusion-limited cathodic plateau at dark n-InP (Fig. 5) indicates that iodate, predominantly present as HIO3 mole- cules at pH = 0, can be reduced electrochemically at the InP electrode. The overall reduction reaction is six- equivalent, as can be deduced as follows. When diffusion- limited, the current density Jura depends on the rotation rate w according to the Levich equation (10)

L~ml = 0.155. n . F . D m . v -'16. w m . c [1]

with n the number of elementary charges involved in the electrochemical reaction, F the Faraday constant, D the diffusion coefficient of the electroactive species (in m 2 �9 s-l), v the kinematic viscosity of the med ium (in m 2 �9 s-l), c the concentration of the electroactive species (in mol �9



J. Electrochem. Sac., Vol. 139, No. 5, May 1992 �9 The Electrochemical Society, Inc.

j ]mA.crn -2

T ._u

|174

l

b Jph

i i J ~ i i i i i

-0.4 -0.2 0 0.2 0./,

V vs S CE/V

Fig. 10. Current-density vs. potential, curves recorded at the (100) p-lnP electrode under chopped illumination. Scan rate = 5 mV �9 s-l; w = 20 s-'. (~)Dark current in HCI, pH = 0; (~)]current under illumination in HCI, pH = O;l(~)dark current in iO -3 mol �9 1-1 KI03 + HCI, pH = O ; Q c u r r e n t under illumination in 10 -3 real �9 1-1 KIO~ + HCI, pH =

.a 0. Iph photocurrent in indifferent electrolyte; I'bph photocurrent in the presence of iodate.

1-1), and in which Jnm is expressed in A - cm -~ and w in s -1 (number of revolutions per second). Experimental RDE results, inserted in Eq. [1], may either serve to determine n, provided that D is known, or vice-versa. The diffusion coefficient of iodic acid DHIOa in the medium of our experiments was determined from the Levich plot which we obtained at a Pt disk, an electrode at which the reduction of HIO3 is known to be six-equivMent. Inserting the obtained value (DHIo3 = 0.75 x 10 -9 m 2 . s -1) and the RDE results for the HIO~ reduction on n-InP into Eq. [1], we find that also here, n = 6.

Comparing the dark cathodic current behavior of n- vs. p-InP, it can be concluded that the first step of the reduction reaction occurs over the conduction band.

As far as the subsequent steps are concerned, the observed enhancement of the cathodic phototurrent at p-InP when adding HIO3 has to be taken into account. Such an effect of l imiting photocurrent enhancement over a factor up to 2 (current doubling) was, in the case of p-type electrodes, originally observed with two-equivalent oxidizing agents such as $202- and H~O2 (11), and attributed to a reduction mechansim in which first a conduction band electron is captured, and subsequently a hole is injected by the intermediate formed. As to the current-doubling mechanism of the HIO3 reduction at the p-InP photocathode, several possibilites may be considered. In principle, the reduction may involve six consecutive electrochemical steps, three of which may occur by hole injection. How- ever, in the case of the Pt electrode in HC1 medium, a mechanism has been proposed (12) in which iodic acid is reduced to I- via a catalytic process: some I-, initially formed by direct electrochemical reduction of HIO3, reacts with HIOa to form I2, which is then electroreduced to I-

HIO3 + 5H § + 6 e- --~ I- + 2H20 [2]

5I- + HIO3 + 5H + ---> 3 I2 + 3H20 [3]

I2 + 2 e- ~ 2I- [4]

This scheme may possibly describe the HIO3 reduction at the InP electrode also, in which case reaction [4] might in-

1393

valve a current-doubling mechanism. It cannot be dmtm- guished from the direct reduction pathway mentioned above, since the reduction plateau height at n-InP in a HIO3 solution of given concentration was found to be the same as that in a three times more concentrated acidic I~ solution. It should be mentioned that the diffusion coefficient of iodine (actually I~) in the given medium is 0.80 x 10 -9 m 2- s- ' , as we established from RDE measurements on Pt, and hence nearly the same as that for HIO3.

An alternative mechanism for the HIO3 reduction on Pt has been recently proposed by Wels et al. (13). It essen- tially involves the dissociative adsorption of a HIO~ molecule at the electrode surface in which IO~ and OH' radicals are formed, and the subsequent electrochemical reduction of both radicals. In the case of the InP electrode, this mechanism seems rather improbable however, as we have indications that homolytic cleavage of the HO-IO2 bond does not Occur at the InP surface (see below).

Etching react ion. - -Since no holes are injected in the first step of the electrochemical reduction, electroless etching of InP by HIO3 can be excluded. Hence, the observed etching of InP by HIO3 may be concluded to occur by a chemical mechanism. This is confirmed by the fact that p-InP is etched under cathodic polarization (see Fig. 4); etching via holes can be excluded in this potential region. Figure 3 shows that the sum of the etch current density and the ab- solute value of the cathodic current density is constant over the entire potential range, which means that the HIO~ consumption by electrochemical reduction and by chemical etching of InP are competi t ive reactions. Indeed, as al- ready mentioned earlier at n-InP in the more negative potential region, HIO3 is reduced electrochemically at a diffusion-limited rate and the etching is almost completely suppressed; with increasing potential, the surface concentration of electrons decreases, so that the electrochemical reduction rate is lowered while the chemical etch rate is in- creased; at sufficiently positive potentials, at which HIO3 is no longer reduced electrochemically, the chemical etch rate at the three faces under consideration is also diffusion-controlled. Assuming that the etching of InP in acid solutions is a six-equivalent oxidation reaction, it was found at the (100)-face that both rates expressed in mA �9 cm -2, are equal (see Fig. 3), i.e.

with

and

~J = no. F . J ~

[5]

[6]

Jetth = 6Fr [7]

In Eq. [6], Jd~r is the flux of HIO3, i.e., the number of moles of HIOa reaching the surface per unit of t ime and surface area, and nc is the equivalence of the reduction reaction; in Eq. [7], r stands for the number of moles of InP dissolving per unit of t ime and surface area. Since nr equals six, it follows that the chemical etching of InP consumes one mole of HIOa per mole of InP. Hence the results in Fig. 3 are con- sistent with the assumption made that the oxidative dissolution of InP is six-equivalent.

Chemical etching of InP by I2, which would be formed by reaction of I- with HIO3 (Eq. [3]), can be largely ne- glected, although it may occur to a minor extent. Indeed, at the (11 D-face, a flow-rate independent etch rate of about 0.5 x 10 -9 mol �9 cm -2 �9 s -1 was found in a solution containing 0.03 mol �9 1-1 I~, which is the concentration which the solution would contain if in an originally 0.01 mol - 1-1 iodate solution, all iodate were reduced to iodine. Compar- ing the etch rates in both solutions, it is seen that with stoi- chiometrically corresponding concentrations, the etch rate in I~ is much lower than that in HIO3, which is diffusion- controlled. The surface reaction leading to the chemical dissolution of InP in iodic acid solutions can hence be written as

InP + HIOa + 2H + --> In s+ + H~PO3 + I- [8]

(in acidic solutions below pH = 3, In s+ is the predominant species of trivalent indium (14)).



1394 d. Electrochem. Soc., Vol. 139, No. 5, May 1992 �9 The Electrochemical Society, Inc.

Etching mechanism.--The enhanced anodic dark current at n- InP in the presence of HIO~ indicates the injection of electrons into the conduction band. In analogy to the chemical etching of GaAs and GaP in acid Br2 solution (15, 3) it is reasonable to assume that electrons are injected by surface decomposition intermediates of InP which are formed during the etching process. The difference in electron injection behavior between the (111)-face on one hand and the (111)- and (100)-faces on the other hand may be due to structural differences of these intermediates between these faces. The existence of such intermediates indicates that the decomposition involves consecutive steps instead of a concerted mechanism. The following mechanism is proposed.

In the first step, a HIO3 molecule interacts with an InP surface uni t to form a positively charged decomposition intermediate X1, which corresponds to an electron-defi- cient surface bond (in what follows, the subscripts of X in- dicate the oxidation states of the InP surface entity, the charges of X~ (i = 1, 2 . . . . 5) being omitted; in the symbols I(j) (j = V, I V , . . . - I) , j indicates the oxidation state of iodine)

kl (InP)s~,f + I(V) ~ X1 + I(IV) [9]

In Eq. [9], ( I n P ) ~ stands for an unoxidized InP surface entity. It is assumed that/(IV) is adsorbed at the InP surface.

As X1 is positively charged, it is reasonable to assume that it may react with water, resulting in a neutral and im- mobile X1-OH intermediate analogously to the case of the chemical etching of GaP in acidic bromine solutions (3) and to that of the anodic dissolution of III-V semiconductors (16, 17)

k'c X I + H 2 0 ~ X 1 - O H + H § [10]

Then the neutral species is further oxidized by I(IV)

k2 X1 - OH + I(IV) --~ X~ + I(III) [11]

As mentioned before, the enhanced current observed under anodic bias leads us to suppose that under these circumstances, electron injection by decomposition intermediates into the conduction band takes place; for simplicity it is assumed that only one type of intermediate, i.e. X1, can inject an electron. It is hence proposed that the following reaction occurs anodically in parallel to reaction (10)

~ X1 ~ X2 + e- [12]

The chemical nature of X2 in Eq. [12] is obviously different from that in reaction [11], but further chemical reactions are not taken into account since they do not compete with the electron injection step. The further oxidation of X2 to the final decomposition products is assumed to proceed as follows

X2 + s --~ X3 + I(III) [13]

X5 + I(O) -* dissolution products + (InP)~rf + I ( - I )

[14]

In reaction [9], X1 and I(IV) are formed in equal amounts whereas in reaction [12], X1 but not I(IV) is consumed, and X2 but not I(III) is formed; since furthermore in Eq. [13], X2 and I(III) are consumed in equal amounts, the consumption of the excess X2 and I(IV) can be assumed to proceed as follows

X2 + I(IV) ~ X3 + I(III) [15]

Although this parallel reaction path slightly influences the reaction stoichiometry, it does not influence the ensu- ing kinetics. In what follows, square brackets will be used to denote surface concentrations (mol- cm-2), and the sym-

bols kc = k'c[H20] and k_c = k'_c[H +] will be used in the ki- netic equations. In what follows, it will be assumed that the chemical equil ibr ium of reaction [10] lies far to the right. From the reaction scheme (9-15) and under steady- state conditions, it follows that the anodic current density resulting from electron injection is given by

Ja = F . k~ "j [X,] [16]

The etch rate under anodic conditions can be measured by the diffusion-limited cathodic current density, as can be seen in Fig. 3. Hence, by combining Eq. [5] and Eq. [7] with the rate equation for reaction [9], one obtains

[lc[ = 6. f . kl [I(V)] [17]

From the steady-state conditions for X1 it follows that

~ol = 6 - r - [~o + k2~} IX1] [18]

Hence, the relationship between ja and jr is given by

ja = [k~r162 + k~)] " ~Jd [19]

Equation [19] predicts Ja to be proportional to ~r as confirmed experimentally (see Fig. 6). The slope of the experimental plot is found to be temperature-independent between 280 and 310 K. Considering Eq. [19], this result suggests that neither kc nor .k~ "J is significantly temperature-dependent. As far as k~ is concerned, this would imply that the relevant decomposition intermediates have an energetic position either right underneath or above the conduction bandedge, so that practically no thermal excitation is required for electron injection.

Chemical etching of I nP by HIOa can thus be concluded to proceed through a mechanism involving consecutive steps, in which decomposition intermediates are formed by interaction with the oxidizing agent, and not by a concerted, i.e., a simple bond-exchange, mechanism, in which HIO3 would be homolytically cleaved, e.g., into an IO~ and a hydroxyl radical. An analogous mechanism has been proposed for the etching of GaAs (15) GaP (3) in acid Br2 solutions. An additional proof for the operation of such a mechanism is offered by the fact that in contrast, in a non- oxidizing chemical etchant (7 mol- 1-1 HC1) no electron injection is observed. In the latter case, a synchronous exchange of bonds is more likely, since the etching process is not a redox reaction.

Interaction between chemical and anodic etching.-- From the shift of the onset of the anodic dark current curve in the negative direction with increasing HIO3 concentration (Fig. 8) and from the coincidence of the j vs. V curves in solutions with and without HIOz at high current densities (Fig. 9), it is obvious that the chemical and electrochemical dissolution strongly influence each other. The anodic dissolution of InP in acid aqueous medium is six- equivalent. Since in the preceding section the chemical equil ibrium (10) was found to be shifted to the right, it is reasonable to assume that holes rather than mobile X1 intermediates are involved in the consecutive steps of the anodic etching reaction. This is confirmed by results on stabilization kinetics, obtained by Vanmaekelbergh on the n-InP photoanode in strongly acidic Fe 2+ solutions (18). A reaction scheme analogous to Eq. [9] through Eq. [14] can be proposed

(InP)s~ + h § --> X1 [20]

X1 + H20 --~ X1 - OH + H § [21]

(with in parallel to a small extent, reaction [12])

X1 - OH + h + --> X2 [22]

X2 + 4 h § --~ products + (InP)suu [23]

In Ref. (19), Etcheberry et al. mention the occurrence of three electron excitation steps in the anodic dissolution of InP, induced by hole-injecting species at low injection current densities. Hence in Eq. [23], some of the hole-capture steps may be replaced by electron injection steps.

According to Notten (20), the attack of the first In-P bond (Eq. [20]) is the slowest and hence the rate-determining



J. Electrochem. Soc., Vol. 139, No. 5, May 1992 �9 The Electrochemical Society, Inc. 1395

step in the anodic dissolution reaction. If this In-P bond is attacked by HIO3, which can be supposed to be a faster reaction than step [20] at low hole concentrations, then the remaining bonds may be more easily attacked by holes and the anodic dissolution may start at lower band bending, as observed experimentally. With increasing band bending, more holes are available at the surface and the competition between the chemical step (9) and the electrochemical step [20] will change in favor of the latter. Eventu- ally, at sufficiently high hole concentrations, InP will be predominant ly etched electrochemically. This is confirmed experimentally by the good correlation between j and the etch rate at high values of j (Fig. 4), and by the absence of a shift of the j vs. V curve under the same conditions (Fig. 9).

An interesting result emerging from this work is the ob- servation of anodic photocurrent enhancement when adding HIO3 to the HC1 solution. In analogy to the cathodic current-doubling mechanism, an oxidation mechanism can be proposed in which first a valence band hole is captured and subsequently an electron is injected by the intermediate formed (21). However, from the thermodynamic point of view, it seems unlikely for HIO3 to be oxidized to HIO4, since the standard redox Fermi level of the HIO4/ HIO3 couple (E~,redox = -1.38 eV vs. SCE) lies far beneath the valence bandedge of InP (Ev = -0.8 eV vs. SCE).

On the other hand, the anodic photocurrent increase may also be explained by assuming that the interaction with HIO3 reduction intermediates may change the electron energy level of certain decomposition intermediates, so that the electrons involved can be excited into the conduction band by light. With increasing anodic photocurrent density, the contribution of chemical etching will become less important so that the photocurrent enhancement will decrease.

Etch morphology.--It has been shown previously (see Ref. (22)) that the morphology of both the (111)- and the ( l l l ) - face of GaP after wet etching depends on the rate- determining step of the etch reaction. Diffusion-limited etch reactions give rise to flat surfaces, regardless of the etching mechanism, the semiconductor type and the crystal orientation, whereas in all other cases etch figures are formed.

However, it was ment ioned above that the (l l l)-face of InP, which is etched by HIO3 under diffusion control, shows etch pits on the surface. This apparent discrepancy may be interpreted as follows. The rate of the first step in the chemical decomposition reaction, in which an I n - - P bond is broken by a HIO3 molecule, leading to the formation of a first intermediate X~, is determined by the supply of HIO3 at the surface. Hence, at all surface sites X1 is formed at the same rate. If one of the subsequent steps is sensitive to local variations in the reaction rate, then the reaction between an intermediate of the I(V) reduction and a decomposition intermediate will be slow at sites with a low intrinsic reaction rate. The intermediates of the iodate reduction I(j) (j < V) will have the opportunity to move along the surface and hence to react selectively at places with a higher intrinsic reaction rate, so that at these sites, etch pits will develop. Such variations in reaction rate are assumed to occur at dislocation sites and damaged surface sites (23).

The reason why no pits are observed at the (111)- and (100)-faces may be that the local variations in reaction rate are less pronounced. This lower selectivity at these faces in comparison to the (l l l)-face can be attributed to a higher reactivity of intact sites at the (111)- and (100)-faces, assuming that the reactivity of the defect sites is independent of the crystal orientation. The difference in reactivity at intact sites between (111)- and (ll l)-faces can be attributed to the difference in the In-P surface dipole orientation which causes the electron density to be higher at the (111)- face and hence also the reactivity with respect to the oxidizing agent. The similarity in selectivity of the (111)- and the (100)-faces can be explained by a similar reactivity of intact sites. For a discussion on this point, see Ref. (4).

Conclusion In the present work, the electrochemistry and the etch-

ing of InP in aqueous solutions of KIO3 + HC1 were studied. From combined electrochemical and etch experiments on p- and n-InP in these solutions, it has been shown that the etching follows a chemical mechanism. A simple bond exchange mechanism, as it is often assumed for chemical etching, was shown to be incompatible in the given case with the electron injection effect observed with n-type electrodes: the mechanism appears to involve consecutive steps and decomposition intermediates. It was demonstrated that chemical and electrochemical etching compete when holes are available at the surface. It was furthermore observed that diffusion-limited etching at the (l l l)-face gives rise to etch pits. This phenomenon may be connected with the nonsynchronous character of the etching mechanism and can be explained by assuming that at least one of the decomposition steps, but not the first one, is sensitive to local variations in reaction rate.

From the practical point of view, an aqueous solution of KIO3 in HC1 at pH = 0 appears to be a fast etching reactant for InP. Unlike the etchants frequently mentioned in the literature, no aggressive or toxic vapors are involved. A good reproducibility of the etch rate is found and the solution is very easy in use. The etching of the (100)-face, important in view of practical applications, leads to an homo- geneously etched surface and yields etch rates which can, under the given circumstances (10 .2 mol �9 1-1 KIO3; u = 50 ml- h-l), amount to 3 • 10 -9 mol . cm -~- s -1, corresponding to an average depth change of about 1 nm�9 s -1.

Acknowledgments The I. I. K. W. (Inter-University Insti tute for Nuclear Sci-

ences, Belgium) is acknowledged for financial support.

Manuscript submitted Oct. 31, 1991; revised manuscript received Feb. 3, 1992.

The Universiteit Gent assisted in meeting the publication costs o f this article.

REFERENCES 1. P. H. L. Notten, J. E. A. M. van den Meerakker, and

J. J. Kelly, in "Etching of III-V Semiconductors: An Electrochemical Approach," Chapter 6, Elsevier Ad- vanced Technology, Oxford (1991).

2. H. H. Goossens, I. E. Vermeir, F. Vanden Kerchove, and W. P. Gomes, Electrochim. Acta, 35, 1351 (1990).

3. H. H. Goossens, K. Strubbe, and W. P. Gomes, J. Elec- troanal. Chem.. ~86, 133 (1990).

4. H. H. Goossens and W. P. Gomes, Electrochim. Acta, 37,811 (1992).

5. Y. Zhu, P. Van Daele, I. Moerman, and P. Demester, in "SOTAPOCS XII," D. D'avanzo, R. Enstrom, A. Macrander, and D. DeCoster, Editors, PV 90-15, p. 254, The Electrochemical Society Softbound Pro- ceedings Series, Pennington, NJ (1990).

6. C. S. Sundararaman, A. Mout6n, and J. F. Currie, In- d ium Phosphide Relat. Mater. Conf. Proc., IEEE, New York (1990).

7. I. E. Vermeir, F. Vanden Kerchove, and W. P. Gomes, J. Electroanal.Chem., 313, 141 (1991).

8. R.C. Weast, C. R. C. Handbook of Chemistry and Phys- ics, 58th. ed., p. D-141, D-142, D-151, The Chemical Rubber Publishing Co., Cleveland, Ohio (1977).

9. H. Gerischer, I. Mattes, and R. Braun, J. Electroanal. Chem., 10, 553 (1965).

10. W. J. Albery and M. L. Hitchman, "Ring-disc Elec- trodes," Clarendon Press, Oxford (1971).

11. R. Memming, This Journal, 116, 785 (1969). 12. P. G. Desideri, J. Electroanal. Chem., 9, 218 (1965). 13. B. R. Wels, D. S. Austin-Harrison, and D. C. Johnson,

Langmuir, 7, 559 (1991). 14. M. Pourbaix, "Atlas of Electrochemical Equilibria in

Aqueous Solutions," p. 436, Cebelcor, Brussels (1966).

15. B. P. Minks, D. Vanmaekelbergh, and J. J. Kelly, J. Electroanal. Chem., 273, 133 (1989).

16. S. Lingier, D. Vanmaekelbergh, and W. P. Gomes, ibid., 228, 77 (1987).



1396 J. Electrochem. Soc., Vol. 139, No. 5, May 1992 �9 The Electrochemical Society, Inc.

17. D. Vanmaekelbergh and J. J. Kelly, This Journal, 136, 108 (1989).

18. D. Vanmaekelbergh, Ph.D. Thesis, Univ. of Gent (1984).

19. A. Etcheberry, J. Gautron, E. M. Khoumri, and J. L. Sculfort, J. Electroanal. Chem., 283, 177 (1990).

20. P. H. L. Notten, This Journal, 131, 2641 (1984).

21. S. R. Morrison and T. Freund, J. Chem. Phys., 47, 1543 (1967).

22. H. H. Goossens and W. P. Gomes, This Journal, 138, 1696 (1991).

23. H. C. Gatos and M. Lavine, in "Progress in Semicon- ductors," Vol. 9, A. F. Gibson and R. E. Burgess, Edi- tor, Temple Press, London (1965).

Theoretical and Experimental Investigations of Chlorine RF Glow Discharges

I. Theoretical

Eray S. Aydil' and Demetre J. Economou Department of Chemical Engineering, University of Houston, Houston, Texas 77204-4792

ABSTRACT

A comprehensive model of chlorine plasma etching of polysilicon in a parallel plate reactor was developed. The Boltz- mann transport equation and a bulk plasma model were used to calculate the rate coefficients of electron impact reactions. These coefficients were then used in a transport and reaction model to calculate the atomic chlorine concentration distribution. The same methodology may be applied to other plasma systems as well. Theoretical results for an empty reactor (no polysilicon film) are presented in this paper. Unified plots were developed which relate the electron density, self-sustained electric field, and electron impact coefficients in the bulk plasma to pressure, power, and reactor geometry. The cal- culated atomic chlorine concentration showed similar dependence on pressure and electrode spacing for either first or second order surface recombination kinetics. Experimental verification of the model predictions is presented in the ac- companying paper.

Chemically reactive gas plasmas are used widely for manufacturing of integrated circuits, especially for etching and deposition of thin films (1). For example, chlorine and chlorine-containing plasmas are used to etch polysilicon, a luminum, a n d compound semiconductors (2). De- spite the wide use of plasma processes, plasma reactor design continues to rely heavily upon trial-and-error procedures. The operating conditions, such as pressure, power, gas flow rate, and gas composition are varied in an empirical fashion unti l the process performance is satisfactory. This empirical approach is taken because of the complexity of the plasma and the intricate coupling of different physicochemical phenomena. When a satisfactory design is thus identified, however, it is invariably difficult to scale to new processes or levels of performance. Mathe- matical models can provide more rational methods for reactor design, and can be used for process optimization and control. The models must be tested with experimental data obtained under well-characterized conditions in order to improve accuracy and extend the range of validity.

There are a variety of phenomena affecting the outcome of a plasma process (3,4). Both charged and neutral particle transport and reaction, potential distribution in the plasma and in the sheath, and plasma-surface interactions strongly influence reactor behavior. Recently there has been emphasis on plasma reactor modeling and diagnostics in order to improve our understanding of reactive gas plasmas, and to develop more rational procedures for plasma reactor design. Several investigators have con- ducted modeling and/or experimental studies of low- pressure plasma systems (5-23). In each of these investigations, different aspects of the problem were emphasized. Graves (8) simulated an electropositive discharge using a cont inuum model with emphasis on the discharge struc- ture and physics. Boeuf (9) studied an electronegative RF discharge, whereas Tsai and Wu (10) modeled two-dimen- sional RF discharges in nitrogen and sulfur hexafluoride. Kushner (11) and Cteland and Hess (12) emphasized the discharge chemistry, while Dalvie et al. (13-14), Folta and Alkire (15), Kao and Stenger (16), Kobayashi et al. (17), and Venkatesan et al. (18) emphasized neutral transport and re-

1 Present address: AT&T Bell Laboratories, Murray Hill, NJ 07974-2070.

action phenomena. Other investigators focused on understanding the plasma sheath and ion bombardment of surfaces (6, 19, 20). Kline and Kushner (21), and Graves (22) reviewed recent experimental and theoretical efforts in plasma processing.

Recently, Economou and Alkire (23), and Economou et al. (24) combined submodels describing the different aspects of the plasma reactor into an overall engineering model. Similar approaches have been taken by other re- searchers (25). However, a detailed analysis of the bulk plasma was not included. In the present study, a detailed description of the bulk plasma is included in an overall plasma reactor model.

Besides being a system of industrial importance, chlorine plasmas have been studied to some extent and there is a reasonable amount of information in the literature con- cerning the dominant species and reactions (26-30). Fur- thermore, chlorine plasmas involve a simple gas where po- lymerization does not occur. The chlorine plasma was chosen to elucidate the coupling between electron kinetics and neutral transport and reaction in plasma reactors. However, the general method of approach followed in this study may be applied to other plasma systems as well.

A model of the CI~ plasma etching of polysilicon was developed from first principles and was tested experimentally using various plasma diagnostics. The experimental and modeling approach is shown in Fig. i. The overall model can be conceptually separated into four different submodels: the Boltzmann transport model, the bulk plasma model, the sheath model, and the etchant transport and reaction model. The electron energy distribution function, electron impact reaction rate coefficients, and electron transport properties, were determined by solving the Boltzmann transport equation. The t ime-dependent electron density and self-sustained electric field were calcu- lated using an extension of the bulk plasma model developed by Rogoff et al. (26). The time-averaged reaction rate coefficients and electron density were subsequently used in a neutral transport and reaction model to determine the etchant concentration profiles, etch rate of polysilicon, and uniformity of etching. The predictions of the submodels, and of the overall model, were compared with experimental measurements. In general, the four sub-



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Electrochemical and Etching Behavior of InP Single Crystals in Iodic Acid Solutions