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THE ELECTROCHEMISTRY OF SEMICONDUCTORS

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(Translated into Russian), Moscow, 1957, p. 196.8. R.C .G ore, R.B.Barn es, and E.P etersen, Analyt.Chem.,21, 382 (1949).9. G .F .Svatos, C.Curran, and J.V.Quagliano, J.Amer. Ch em.

Soc, 77, 6159 (1955).10. D.Segnini, C.Curran , and J.V.Quagliano, Spectrochim.Acta,16_, 540 (1960).1 1 . J.Lecomte, T.Poberguin, and J.Wyart, J.Phys.(U.S.S.R.),6_, 22 (1945).

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22, 1478 (1950).18. T.Shimanoushi, M.Tsuboi, T.T akenischi, and N.Iwata,Spectrochim.Acta, 16, 1328 (1960).

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(1953).26. J.V.Quagliano, G .F. Svatos, and B.C.C urran , Analyt.Chem., 26, 429 (1954).27. B.C.C urran , D.N .Sen, S.Mizuschima, and J.V.Quagliano,Analyt.Chem., 26, 429 (1954).28. C.Duval and J.Lec omt e, Bull. Soc chim. F rance, 1, 180

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(1960).50. J.D .S.G oulden, Spectrochim.Acta, 15, 657(1959).5 1 . J.D .S.G oulden, Spectrochim.Acta, 16, 715(1960).52. J.D .S.G oulden , Chem.and Ind., 721(1960).53. F .S .P ar ker , Appl.Spectroscopy, 12, 163 (1958).54. A.E.M art in , Nature, 166, 474 (1950).55. J.M.H unsberger, J.Amer.C hem. Soc, 72, 5626 (1950).56. R. S. Rassmussen and R. R. Brattain, J . Amer. Chem. Soc.,71 , 1053 (1949).57. R.S. Rassmussen, D.D.Tuncliff, and R. R. Brat ta in,J.Amer.Chem.Soc, 71, 1068 (1949).58. H.Yamada, Bull.Chem.Soc .Japan, 32, 1051(1959).59. A.Yamamote and Sh.Kambara, J.C hem.Soc .Japan, Pu reChem. Sec, 80, 97A, 1239 (1959).60. L.J.Bellamy and R.F .Branc h, J. Ch em .So c, 4491(1954).6 1 . B.E.Bryant, J.P ariaud, and W.C.F ernelius, J.Org.Chem19, 1889 (1954).62. O.E.Ayers and J. E. Lan d, J. Ph ys.Ch em., 65, 145 (1961).63. R.Cardinaud, Bull. Soc chim. Fran ce, 634(1960).64. B.L.Shaw and T.H.Simpson, J. Ch em .So c, 5027(1952);

655 (1955).Institute of Gen eral andInorganic Chemistry,Academy of Sciencesof the Ukrainian SSR

TH E ELECTROCHEMISTRY OF SEMI-CONDUCTORSV.A.Myamlin and Yu.V.Pleskov

C ONTENTSI . Introduction 2

I I . The semiconductorelectrolyte system at equilibrium 1. Potential distribution at the phase boundary 22. Differential capacity 2

. Kinetics of electrode reactions 1. Rate limitation by the electrochemical stage2. Rate limitation by minority ca rr ie r diffusion 3. Generation of minority ca rr ier s in the space-

- charge region 4. The role of free and valence electrons inelectrochemical reaction s 25. Format ion of oxide layers on the electrode

surface 2


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Vol.32 No.4 RUSSIAN CHEMICAL REVIEWS Apr i l 19IV. Chemical etching 218V. Application of electrochem ical methods of surfacetreatment in physicochemical researc h and in semi-conductor technology 220

I. INTRODUCTIONRecent advances in semiconductor physics, and the wide

application of semiconductor devices in radio circuits havecreated a new branch of electro che mistry: the electro -chem istry of semicon duct ors. Over 200 pap ers prim arilyconcerned with the electrochemical properties of siliconand germaniu m have been published since 1955. In mo rerecent year s, electro chem ists have become interested innew semiconducting ma ter ials, such as inter met allic com -pounds, silicon ca rbide, and various oxides and sulphides.

The development of semiconductor electrochemistryowes its origin to the experiments of Brattain and Garre t t .In the Soviet Union, Efimov and Erusalimchik were the firstto study semiconductor electrochemistry, but it was notlong before the re sea rc he s of worker s at the In stitute ofElectr och emistry and the Institute of Physical Che mistryof the Academy of Sciences of the USSR, and at the KarpovPh ysicoch emical In stitute began to show an aware ne ss ofsemiconductor pro blems. Theo retical studies of semi-conductor elect roc he mistr y in the USSR were star ted underthe initiative of V.G .Levich.

Gerischer, Green, and Dewald are among those whohave mad e significant con tribution to the development ofbasic ideas in semiconductor electrochemistry.

I I . THE SEMICON DU CTOR- ELECTROLYTE SYSTEMAT EQUILIBRIUM1, Po tent ial D istribution at the Ph ase Boundary

The equilibrium poten tial of a semicond uctor is thepotential differencet in the equilibrium system :

metal | semiconductor Ielectrolyte I metal M.This pot ential does not depend on the position of the F er m ilevel in the semiconductor 12.

The Galvani potential which appears at the phase bound-ar y between semicon ducto r and electr olyte is distribut edover three separ ate regions: the diffuse part of the doublelayer in the electro lyte, the Helmho ltz double layer, andth e semiconductor%. Its magnitude is a function of theposition of the F er mi level in the semiconduc tor 1 '28. I tcan be shown ( see below) th at in the absen ce of surfacestates the potential drop across the first two regionscan be made negligibly small in c omp arison with the change

t Equilibrium potentials for reactions at germanium andsilicon electrodes in aqueous solutions have been calculated ~ 5.[More recent values can be found in H olm es160 [Ed. of Trans-lation) .]

$ C ontributions to the Galvani potential arising from adsorptionof polar molecules of the solvent (or solute) on the electrode willnot be considered. The term surface state T8 refers to an electronic energylevel localised at the semiconductor surface. These levelsappear when the crystal lattice is cleaved to generate a surface(Tamm states), when impurities are adsorbed on the surface, andso on.

in poten tial within the semicon duc tor. We shall also calclate the th ickness of the spac e- cha rge layer in the semconductor.Consider first Poisson's equation in the form

dx*

where ( ) is t he potent ial at the point , ( ) is the spac- char ge density at the same point, and e is the dielec trperm ittivity of the medium . The axis is chosen normto the pha se boundary. Let the semicond uctor occupy tregion > 0, and the electrolyte occupy the region


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Vol.32 No.4 RUSSIAN CHEMICAL REVIEWS April 1fo l l o ws : the p o t e n t i a l d r o p 1 in the s e m i c o n d u c t o r is con -c e n t r a t e d in a r e g i o n of t h i c k n e s s d1 (the D e b y e l e n g t h ) s u c ht h a t the r a t i o of 1 to dx m e a s u r e s the i n t e n s i t y of the e l e c -t r i c f ie ld at the s u r f a c e of the s e m i c o n d u c t o r , EJ.

A s s t a t e d a b o v e , the v a l u e of the G a lv a n i p o t e n t i a l canb e c o n s i d e r e d as the sum of t h r e e t e r m s : = 0 + + ',w h e r e 0, } and ' d e n o t e the p o t e n t i a l d r o p in the H e l m -h o l t z l a y e r , in the s e m i c o n d u c t o r , and in the diffuse p a r t oft h e e l e c t r o l y t e d o u b l e l a y e r r e s p e c t i v e l y .

T h e c h a r g e d e n s it y in a u n i - u n i v a l e n t e l e c t r o l y t e is g ivenby(6 )

w h e r e c+ an d c_ d e n o t e the n u m b e r of c a t i o n s and of a n i o n sp e r cm 3 . By s i m u l t a n e o u s l y s o l v i n g P o i s s o n ' s e q u a t i o n (1)a n d Eqn . (6) for th e e l e c t r o l y t e we o b t a i n an e x p r e s s i o n oft h e s a m e fo r m a s E qn. (5) for th e ' - p o t e n t i a l 1 0 . H e n c ew e c o n c l u d e t h a t the diffuse c h a r g e d i s t r i b u t i o n s in thes e m i c o n d u c t o r and in th e e l e c t r o l y t e a r e v e r y s i m i l a r .S i n c e the e l e c t r i c f ie ld in the H e l m h o l t z d o u b l e l a y e r isa s s u m e d to be c o n s t a n t , i.e.

The thickness of the Helmholtz layer is approximatel10"8 cm. Therefore, using Eqns. (9), (10), and (11), wc o n c l u d e that in sufficiently concentrated solutions themajor par t of the G alvani potential drop occu rs in the sec o n d u c t o r . Thus we are justified in considering the sec o n d u c t o r - s o l u t i o n interface as a capacito r in which thelayers of charge have a diffuse structu re. The propertof the interface are determined mainly by the distributespace charge in the semiconductor.

The importan ce of the assumed absence of surface ston the semiconductor needs to be stre ssed. In the pr e-sence of a large density of surface states the potential dference across the Helmholtz layer can account for a lafraction of the drop in Galvani potent ial. We shall calclate the poten tial drop in the Helmholtz region for the cof a density of surface states Ns = 1014 cm"2, assumingdegree of ionisation of 0.1 . F rom the formula for a plaparallel- plate condenser, the electric field in the Helmh o l t z layer is given by the following expression (in whico = 3):

zz. IO7 V cm .

w e can e x p r e s s the G a l v a n i p o t e n t i a l in the f o r m = Exdy . +


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Vol.32 No.4 RUSSIAN CHEMICAL REVIEWS Apr i l 19space charge in the semiconductor. The electrode potential(measured against some reference electrode) which corre-sponds to this state of affairs is called the "flat band poten-tial" because there is no electric field to distort the edgesof the energy bands near the semiconductor surface.Measurements of photopotential are therefore a very con-venient way of obtaining the value of the flat band poten -tial2122 . The influence of free electron concentration ont h e flat band potential of germanium provides further sup-port for the conclusion that in this material the potentialdrop in the space- charge region of the semiconductor iscomparable in magnitude with the potential drop in theHelmholtz layer 2 3.

The flat band potential of germanium is affected by themethod of surface preparat ion and also by the nature of theelectrolyte23 . In germanium with int rinsic conductivity,for example, the flat band potential is 0.5 V in 1 and0.13 V in IN H2SO4, showing that even in the absence ofspace charge within the semiconductor, the Helmholtzpotential drop (and therefore also the surface charge) isquite appreciable.The appearance of surface charge on germanium hasbeen discussed23 25 in terms of oxidation of the surface.Measurements of charging curves have shown 26- 30 that ger-manium surfaces ar e normally covered with a monolayerof oxide in non- oxidising solut ions. Evidently the surfaceoxide partly dissociates (e.g. by forming GeOOH" groups inan alkaline medium), the degree of dissociation dependingon the pH of the solution. The surface charge thus gener-ated is retained even when there is no space charge in thesemiconductor (i.e. at the flat band potential) and is balancedby the charge of the ions electrostat ically adsorbed fromt h e solution. The double layer present on a germaniumelectrode at it s flat band potent ial has an obvious similarityt o the structure of a mercury surface at the point of zerocharge under conditions of specific adsorption of, say,bromide ionsiF. The flat band potential should not be con-fused, however, with the zerocharge poten tial: the latt ercorresponds to a situation such that both the space chargeand the surface charge must vanish.The surface recombination velocityt, an important pro-perty of semiconductor surfaces, is related to the potentialdrop across the Debye region of the semiconductor7. Ithas been shown that the surface recombination velocity at asemicon ductor- electrolyte in terface is a function of poten-tial3 9"43 , solution composition 44"46, and surface p repara -t i o n . A characteristic , bell- shaped curve is obtained whent h e surface recombination velocity is plotted against poten-tia l: the situation is therefore analogous to the semicon-ductor- gas interface47 . The param eters of the curve canbe used to characterise the recombination centres by evaluat-

ing the ratio of the capture cross- sections for electronsand for holes39 . Mild anodic etching of the germaniumreduces the ra te of surface recombinat ion; strong cathodicpolarisation, on the other hand, sharply increases it 394849.I t would appear that additional recombination centres arecreated, under these conditions, as a result of adsorptionof the evolved hydrogen or penetration of the hydrogen intot h e germanium crystal lattice}:. See also Refs.16 and 24.IF The adsorption of various ions on german ium and silicon hasbeen investigated31"38.t See p.215.t It has been suggested50 that adsorbed hydrogen can give riseto acceptor levels as well as recombination centres.

2. Differential CapacityWe shall now calculate the capacity of a semiconductoelectrode. The differential capacity is defined asC = \dQ/d


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Vol .32 4 RUSSIAN CHEMICAL REVIEWS Apr i l 19where Cv C o, and C 2 are the capacities of the semiconduc-t o r (with allowance for surface states) , of the Helmholtzlayer, and of the diffuse part of the double layer in theelectrolyte, respect ively. The capacity of the semiconduc-t o r , C,= \dQ/d(


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Vol.32 No.4 RUSSIAN CHEMICAL REVIEWS April 19Efimov and Erusalimchik found a minimum in the capa-city of the germanium elect rode at a quite different poten-tial: 0.6 V in 0.1 HC1. They call this potential the zero -- charge potent ial of german ium 80. Their high specificcapacity (10-20 " c m " 2) , unaffected by the properties oft h e semiconductor, suggests that these workers were mea-suring the capacity of the Helmholtz layer. Before eachmeasurement the electrodes were subjected to prolonged

cathodic polarisation which, as stated above, can ra ise thesurface recombination velocity very considerably. Itwould appear that this treatment also makes the semicon-d u c t o r surface "metal- like". The full significance ofEfimov and Erusalimchik's measurements80 , however, isfar from clear. These remarks also apply to Efimov andErusalimchik's 81 anomalously high value for the capacityof a silicon electrode. . K I N E T I C S OF E L E C T R O D E R E A C T I O N S1. R a t e L i m i t a t i o n by the E l e c t r o c h e m i c a l S t a g e

T o give a c l e a r e r u n d e r s t a n d i n g of the b e h a v i o u r of th es e m i c o n d u c t o r - e l e c t r o l y t e i n t e r f a c e we s h a l l d i s c u s s ane l e c t r o c h e m i c a l r e d o x r e a c t io n of the t y p eA+ +


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Vol.32 N o. 4 RUSSIAN CHEMICAL REVIEWS A p r i l 19were able toshow78 that the current isproportional to theintensity ofillumination and therefore that the slow step iskinetically ofthe first order with respect to holes.

The anodic oxidation ofgermanium in alkaline solutionsconsumes hydroxide ions, as shown bythe appearance of alimiting current inthe polarisation curves obtainedindilute NaOH solutions. The value ofthe limiting currentis determined81 bythe rate ofdiffusion ofOH" ions to theelectrode surface. Atstill higher potentials the currentbegins to rise again because anew reaction sets in: thedissolution ofgermanium, which involves water molecules.Beck and Gerisch er25 studied the effect ofalkali concen-t r a t i o n on the overvoltage (at cur ren ts well below the limit-ing diffusion curren t ofOH" ions) and concluded that theslow step inthe anodic dissolution ofgermanium isuni-molecular with respect to hydroxide ions. The cu rr en t-potential relationship isdescribed by a Tafel equation with = 0.7-0.81 (F ig. 2, curve 1).

Beck and G erischer 25 have put forward a reaction mech-anism forthe anodic dissolution ofgermanium whichaccounts forthese experimental facts. Germanium atomsat the electrode surface become oxidised with formationofG e O O H " groups, aprocess requiring one hydroxide ion andone positive hole. The slow stage isassociated with thepassage ofthe GeOOH" into solution, togive H 2Ge03.

In the derivation ofthe kinetic equation (25) we used theBoltzmann distribution to give the electron concentration int h e semiconductor during the passage ofcurrent. Thisdistribution, however, isstrictly applicable only to equili-brium situations, i.e. inthe absence ofexternal currentf.We will now discuss the limitations ofthis procedure.Consider the expression forthe current ofelectrons in an- type semiconductor:= eu_E(x) ( ) -feD- (29)

H e r e M_ and D_ ar e the mobility and the diffusion coefficientof electrons respectively. The term euJEn describes thedrift current ofelectrons due to the presence ofan electricfield; the second term, eD_(dn/ dx), isthe diffusion curren t.

tO1 1O'i,k cm *Fig. 2. Polarisation curves for the anodicdissolution ofgermanium: l)P- type; 2)n- type.

f Turner finds26 / = 0.12, whence = 0 . 5 .t We are not considering the case ofver y high charge on the

electrode, when the Boltzmann distribution ceases toapply evenunder equilibrium conditions.

F o r i =0, integration ofEqn. (29) leads to the Boltz-m a n n distribution. Even if i 0, however, it ispossibt o haveeu_ En dx

so that inEqn. (29) i becomes negligible incomparison wt h e much larger te rms on the right- hand side. Underthese conditions, integration ofEqn. (29) again gives theBoltzmann distribution.

I t follows that we can legitimately apply Boltzmannexpressions provided that the total curr ent ismuch smallert h a n both the drift and the diffusion curr en ts.But what is the magnitude ofexpressions ofthe typeeu.En? Consider, forexample, an anodic pro cess at agermanium electrode. We shall assume that the pr e-sence ofthe electrolyte incr eases the steady- state concet r a t i o n ofholes atthe surface, resulting in a Galvanipotential of0.1 V.The electric field is ofthe orderofdx = 10~4 cm, we find = 103 Vcm

pjd. PuttingIn germaniumof1 cm resistivity the concentration offree electrons, nis 2 x1015 cm "3, and the concentration ofholes, />>, is3 x 1011 cm "3. Under these conditions the drift cur ren t holes takes the value

eii+fp lO-'Acin1. (3

Consequently, hole den sities will be accurately describedby Boltzmann expressions aslong asthe anodic curren t isignificantly smaller than the above valuet .These results sometimes prove incorrect when applieto electrodes forwhich the surface density ofcarriers isvery much less than the carrier density inthe uniform buof the semiconductor, farfrom the space- charge region.

The concentration ofelectrons atthe interface isgivenby = nooexpie^/fcT), and can be very small if the exponenhas a low enough value. The Boltzmann distribution ceat o apply when the surface layer ofthe semiconductor isdepleted offree ca rr ie rs . Two recent ly published papehave dealt with the calculation ofpolarisation curves withallowance for anon- equilibrium distribution of carriers.One ofthese 8 2 discusses the current- voltage charact eistic ofthe interface between an electrolyte and an - typsemiconductor. Strong electron depletion atthe equilibriupotential isassumed. This produces a surface layer wita low concentration offree carriers, ofhigh electrical rsistance, and having rectifying propert iest . When curreis passed through such a contact in the forward directionelectrons are injected into it bythe field and the resistan

decreases. Reverse cur rent , on the other hand, furtherdepletes the contact ofelectrons and makes its resistancstill higher. The donor levels are assumed82 to be weaionised. The rectification atthe contact isdescribed byt h e ratio AF(+ t)/ AV(- t), where 7(+ ) and AF(- i) refer%Arectification effect isobserved also atmetal-solutioncontacts, e.g.when the anodic and the cathodic reaction havwidely different overvoltages, orwhen the rate ofone ofthesreactions islimited by concentration polarisation. We are notconcerned here with this type ofrectification (althoughitcan ocat semiconducting as well as metallic elect rodes), but only withrectification effects specific to the semiconductor - electrolyteinterface.


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Vol.32 4 RUSSIAN CHEMICAL REVIEWS April

t h e total potential drop (in the semiconductor and in theHelm holt layer) under conditions of reverse and forwardbias respectively. It s value is given by

(31)

where i0

is the exchange current of the reaction, I is thethickness of the semiconductor. Inserting I = 10~3 cm, dx( the Debye length) = 10"4 cm, d0 = 10"8 cm, and In (i/i0) == 10, we find:A V ( + I) .2x10. (32)

Clearly, this system has rectifying properties.The above treatment 82 has been extended83 to the caseof an - type semiconductor with current ca rr ie rs of bothtypes. The authors consider a react ion of type (21),involving carriers of a single type (free elect ron s). Theequilibrium concen trations of elect ron s and holes in thesurface layer are assumed to be much smaller t h a n thec o n c e n t r a t i o n of majority car rie rs (free electrons) in the

bulk of the semiconductor. The calculated current- volt-age characteristic shows an interesting feature: rectifica-t ion is present only over a limited range of potentials (oft h e order of the width of the energy gap) as long as the sur-face remains depleted of electron s and holes. As soon ast h e applied field raises the surface density of holes to thelevel of the bulk electron density a distinctive breakdownp h e n o m e n o n sets in.

2. Rate Limitation by Minority Carrier DiffusionWe have so far considered systems in which the electro-chemical reaction is the slowest step . This happens atlow current densities (less t h a n 10"* A cm"2 on germanium)o r when majority carriers (holes in p-type semiconductors,electrons in - type semiconductors) ar e involved in thereaction. If the reaction , however, consumes minorityca rr ie rs , which ar e present at low concentration, it is notsurprising to find that the current is limited (beyond a cer-t a in potential) by the rat e of arrival of minority car ri er s a tt h e electrode surface. We shall discuss the kinetics ofreac tion s of this type, taking the anodic dissolution of ger-manium as a concrete example8485.As stat ed above, holes must be supplied to the interfacefrom the bulk of the semiconductor in order that germaniumshall dissolve. Pa rt of the reaction current is car riedac ross the phase boundary by free electron s, however, andwe may write the ratio of hole to electron current in the

form!& = . (33)

where r and m ar e integers,ium dissolution is given by Thetotal current of german-

0) + L (x = 0) = kpa(i (34)where is a constant. The germanium surface at itsequilibrium potential is assumed to have a high density ofexcess holes. The current- voltage curve for - type ger-manium now takes the form

where the term % includes constants and functions whivary much more slowly t h a n ln i . The value of ium isby

where / is the concentrat ion of free electrons in intrigermanium, D+ is the hole diffusion constant , is the sistivity of the germanium, and L is the hole diffusionlength.Let us examine Eqn. (35) in detail. At currentsi iiim the potential varies with current according to

RT, I

and Tafel's law is obeyed. At currents close to the limcurrent the th ird term of Eqn. (35) becomes predominan

When the potential is raised still further a sharply defilimiting cur ren t is observed.An analysis has been given

85of the distribution ofca rr ie rs close to the surface. Fig. 3 shows the distribt ion of holes for currents not very different from the ling current. In the Debye region (0


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Vol.32 No.4 RUSSIAN CHEMICAL REVIEWS Apr i l 1

F i g . 3. Hole concentration profile neart h e surface of - type germanium dissolvinganodically under limiting current conditions.

We will now reconsider our assumption that recombina-tion can be neglected, because th e limiting d issolutioncurrent of germanium can be increased by hole generationin the Debye region. To allow for this effect we use theconcept of surface recombination velocity7, vg We shalln ot attempt to discuss the actual mechanism of hole gener-ation at surface state s or in the space- charge region.Following Shockley, we assume a hole generat ion c ur ren tis proportional to the departure of the hole concentrationa t the edge of the space- charge region, plf from its equili-brium value, poo'.

- ) (42)Under limiting current conditions, px = 0 and

(43)Therefore the limiting cur ren t of positive holes is given by

(44)T h e hole diffusion length L and the hole lifetime ar e r e -lated by the expression L = ( D T ) * , SO that finally

+ lim = (45)Vdovin et al.85 have discussed one par ticu lar generationmechanism, and calculated curren t- voltage ch arac teristic san d limiting current densities on the assumption that holegeneration oc cur s entirely at surface stat es, and not withint h e space- charge region.A limiting cur ren t is not observed in the anodic dissolu-tion of />-type germanium, because the supply of holes toth e surface cannot be diffusion- cont rolled when holes ar et h e majority carriers.Curve 2 of F ig. 2 shows an experim ent al pola risatio ncurve for the anodic dissolution of - type germanium. Forcurrent densities up to 10~* A cm*2 the current - potentialrelat ion ship is of the Tafel form (as in the case of />-typegermanium). At higher potentials the current plateauappears: Flynn 86 states that the value of the limiting cur-rent is pro portio nal to the resistivity of the german ium (p)an d inversely pro por tion al to the hole diffusion length (L),as required by Eqn. (36). It seem s clear, ther efore, thatt h e diffusion of ho les from the bulk of the spec imen to it ssurface constitutes the slow step, and that surface reco m-bination occ urs at a negligible ra te . The limiting cur ren tc an be incr eased by providing m ore h oles at the surface, e.g.by illumination of the electrode 78 , by hole injection from a

p- n junction located close to the electrode surface 87 , analso by raising the temperature (which raises the bulk cocentration of holes in the semic on du cto r) 88 .Thus in the anodic dissolution of w- type germanium thsemiconduct or- electrolyte boundary behaves very muchlike a reverse- biased p- n junction when - type germaniu m is subjected to anodic dissolution . Cu rren t is ca rriac ro ss the boundary by minority ca rr ie rs (holes), and th

rate at which holes can diffuse from the bulk to the surfof the semiconductor determines the value of the currentT h e blocking curre nt is therefor e diffusional, in con tra st h e type of rectification discussed on p.213- 214. P ract ict h e whole of the potent ial difference applied to the electr oapp ears a cr oss the blocking layer of the semiconductorwhen limit ing cu rr en t is flowing: only a very small fractof the potential is dropped across the Helmholtz layerSince the thickness of the blocking layer is very small (ot h e same order as the Debye length), the field intensitywithin the blocking layer can exceed 105 V cm"1 for a potial of a few volts. Under the se conditions ho le- elect ronpa ir generation can occur by an avalanche p ro cess, whicprodu ces breakdown in this rectifier- like stru ctu re (seecurve 2 of F ig. 2). The value of the breakdown voltagedepends on the resistivity of the german ium, and can amout o several ten s of volts in the case of high- resistivity spemens.

I t should be noted that free electro ns (as well as holesc an take part in the anodic dissolution of german ium,making the limiting anodic dissolution cur ren t larger that h e limiting hole diffusion cu rren t [see Eqn. (36)]. Theratio ( r + m)/ r = a' is called the cu rre nt multiplicatiocoefficient by analogy with the cu rre nt m ultiplication whitakes place at a tra nsistor collector. Brattain and G ar re tdevised th e following meth od of measurin g a ', which waadopted in more recent work8789 . The electrode con sisof a german ium wafer which is made much thinn er than thole diffusion length (F ig. 4). An annular ohmic con tactan d a. p- n junction (prep ared , for example, by the indiumalloy process) are placed on one side of the wafer; th eopposite side is imm ersed in the electrolyte and act s a st h e anode. During polarisation of the p- n junction in thtransmitting direction it serves as a source of holes, whdiffuse through the germanium plate and reach the germaium- solution in terface, where they part icipat e in the anodissolution reac tio n. The limitin g dissolution and hole fusion cur ren ts can be measured simultaneo usly, and ust o calculate ot'. M easurement s by th is (and by oth er)methods all give 7889 ot' values of the order of 1.65, point o the following stoic hio metric equation for the anodic disolution of germanium in alkaline solutions:G e + 2.4e+ + 6OH" -> GeOr + 1 -6e" + 3HaO.

where e* and e~ denote holes and electro ns respect ively.

I I 0

F i g . 4. Electrode for studying the role offree carriers in electrochemical reactions ongermanium:l )P -n junction; 2)n- type germanium, 3) solu-tion.


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Vol.32 No.4 RUSSIAN CHEMICAL REVIEWS Apr i l 19The value of a' varies with dissolution conditions (forexample, with the current of injected holes) 89 "92 and cannot,therefore, be identified with any particular molecularscheme for the reac tion. It would appear that when ger-manium goes into solution electrons have to enter both thevalence band (a process which consumes holes) and the con-duc t ion band: the ratio of hole curren t to electron currentis a stat istical quantity depending on the concentration ofholes at the interface, and can be alte red by photo- injectionof holes or by other means8990.If the thickness of the electrode shown in F ig. 4 is notm u c h greater than the thickness of the space- charge region(~20, say), the latter can be made to grow and "flood" theentire specimen if a sufficiently high reverse voltage isapplied to the p-n junction 9394. Under these conditions thehole (and electron) concentrations fall much below theirequilibrium values, and the germanium ceases to dissolveanodically as soon as the edge of the depletion region comesi n t o contact with the germanium- electrolyte interfaced. 93The limiting currents reported in the anodic dissolutionof selenium98 and of - type cadmium sulph ide 99 "10 2 suggestt h a t positive holes may be involved also in these reactions.The exact mechanism whereby holes participate in theanodic dissolution of the semiconductors is still not clear.Possibly, the rupture of a covalent bond between lat ticeatoms becomes easier when a hole is localised at a surfacea t o m .The cathodic evolution of hydrogen at a - type german-ium surface also shows features indicative of minorityca rrie r diffusion from the bulk to the surface of the semi -conductor. Brattain and Garrett 78 established that freeelectrons take part in the reaction, which reaches a limit-ing rate (~ 1 mA cm"2) at a definite potential. This con-clusion was subsequently confirmed by G reen 2, Gerischer 4 9,and Dewald21. The limiting diffusion current of electronsis usually less clearly defined than the limiting hole dif-fusion current: this is because the surface condition of thegermanium electrode is gradually altered by the adsorptionand penetration of atomic hydrogen into the germaniumlatt ice during the evolution. These phenomena ar e known(see p.210) to increase the rat e of surface generation andrecombination of hole- electron pairs48>8e>103104. The exactsimilarity, even at very high current densities, betweent h e polarisation curves for hydrogen evolution from - and/>- type germanium electrodes previously saturated withhydrogen 105 can be explained in the same way. Such elec-trodes behave like metallic electrodes and effectively losetheir semiconductor pro per ties. If the polarisation curve,on the other hand, is determined rapidly, the hydrogenovervoltage on - type germanium is found to be smallert h a n on / >- type106.The observation of Paleolog et al.107 that the rate ofhydrogen evolution on />-type germanium, though smallert h a n on - type, decreases with increasing germaniumresistivity, cannot be reconciled with the views outlinedabove, which require that the limiting electron currentshould increase with increase in resistivity see Eqn. (36).The authors suggest 10 7 that valence electrons rather thanfree electrons are concerned in the hydrogen evolution r e-action on />-type germanium.

All the instan ces of diffusional control so far outlinedapply to electrodes of thickness many times larger than minority carr ier diffusion length, L. Calculations havealso been made10 8 of the voltage- current characteristic of the limiting current value for the case of a germaniumwafer of thickness W < L, in contact with the electrolyteon one side and provided with an ohmic metal contact (whdetermin es the surface recombination velocity vs) on theo t h e r side.U n d e r these conditions the limiting anodic dissolutioncurrent takes the value

>+ cosh -f vsLsinh (

Efimov and Erusalimchik109110 have observed, at leaqualitatively, a relationship of th is type between the limiing current and the resistivity and hole diffusion length overy thin germanium electrodes.

3. Generation of Minority Car rier s in the Space-ChargeRegionWe will now discuss the anodic dissolution of - typesilicon. Eqn. (36) predict s a much smaller limiting disslution current for silicon than for germanium of compararesistivity if the dissolution of silicon, like that of germium, requires holes. The quantity t- which appears inE q n . (36) is related to the width of the energy gap, thus:

I t follows that ni, and therefore also the limiting diffusiocurrent of minority ca rr ier s, must decrease very rapidlif the width of the forbidden band is increased. Experi-mentally, however, the dissolution of silicon is found tosupport currents which are comparable in magnitude tothose of germanium anodes. F lynn 88 believes that genet ion in the space- charge region can provide the holesneeded for the dissolution of silicon. One of us m hasderived a quantitative model for the anodic dissolution ofsilicon in which allowance is made for generation in theDebye region. The theory of Shockley and Read11 2 is ust o describe the recombination process. The number ofca rr ier s recombining per unit volume and unit time worko u t to

(4where

= / exp kT Pi.= n, exp B,-Etk T

A method of producing very t h in germanium wafers 95> 9 bythis technique has been described (see also Albers and Thom as97).

0 and 0 ar e, respectively, the hole lifetime in a stron- type sample and the electron lifetime in a strongly/-tsample containing the same recombination centres as thexperimental material; Et is the energy level of the recobination centres; is the Fermi level of the intrinsicsemiconductor. The polarisation curve for silicon dissot i on now has the form(4

21 6


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Vol.32 No.4 RUSSIAN CHEMICAL REVIEWS A p r i l 19where 0 is a constant, i0 is the exchange current of thesilicon dissolution reaction, and the current iy is given by

(49)dx being the Debye length of the semiconductor.

At currents i < ix the second term in Eqn. (48) variesmore slowly t h an the third, and the curren t- voltage curveapproximates to Tafel *s law:

= In + const.When the cur ren ts i and ix ar e of similar magnitude theparabolic relation ship between cur ren t and potent ial becomespredominant, and the potential increa ses much faster t h anwith the logarithm of the curr ent. The cur ren t ix charac-te rises the tran sition to a region of strongly cur ren t-- dependent potential. F r o m Eqn. (49), ix is proportional tot h e Debye length, and therefore also to the square root oft h e silicon resistivity:

ii~V7 (50)E q n . (49) can be obtained by the following semiquantita-tive argument . We seen from Eqn. (47) that effectivegeneration can occur where np ?;


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Vol.32 No.4 RUSSIAN CHEMICAL REVIEWS Apr i l 19t h e transfer coefficients a and ar e either 1 Eqn. (51) o r 0 Eqn. (52). The overall react ion curren t is thesum of these partial currents:

Fr\

We are particularly interested in the ratio of hole currentto electron cur ren t, which can be measured experimentally.At not too high over voltages this ratio is given by:

The quantity - 1 is simply the potential of the electrode(except for an unknown additive constant).The more positive the potential, the refore, the higher ist h e proportion of electrons from the valence band in re-duc t i o n reactions and the proportion of holes in oxidationreactions.At very high overvoltages the concentration of freecarriers at the semiconductor surface becomes very large,an d the surface tends towards metallic pro per ties. In

part icular , the potential drop at the interface becomesc o n c e n t r a t e d mostly in the Helmholtz layer, and the abovecon sideration s cease to apply. G erischer has discussedthis system 12.In their study126 of the anodic dissolution of - type ger-m a n i u m in solutions of oxidants (alkaline K 3F e ( C N )6;F e C l3) , G erisch er and Beck found that the limiting german-ium dissolution current increases with the concentrationof the oxidising agent. They explain this phenomenon bypostulating that the dissolution reaction is accompanied bysome reduction of the oxidising agent, in which elect ron sfrom the valence band ar e involved. Thus the reductionreac tion injects holes into the valence band of the semicon-d u c t o r which can be used by the dissolution r eac tion. Holeinjection has also been observed 127 in alkaline solutions of

oxygen, in KMnO4, and in Ce(SO4)3.D i r e c t experimental evidence103 , and indirect evidencebased on polarisation curves 12 8, 12 9, favours the hypothesist h a t valency electrons take part in a number of reductions.Even the formation of G eH 4 on a germanium cathode hasbeen shown 130 to involve electrons from the valence band.D i r e c t measurements of the relative numbers of freean d valence- band electrons taking par t in reduction rea c-tions have been carried out 13 1 with the electrode shown inFig.4. The reverse- biased p- n junction was used as ahole collec tor . Any reduction react ion at the germanium -solution inter face which int eracts preferen tially with thevalence band will inject holes into the semiconductor.These are able to diffuse across the thin germanium wafer,

an d are collected by the p- n junction. The ra tio of theincrease in collector curr ent to the reduction current iseffectively equal to , the fraction of valence- band elec-t r o n s in the reduction current.The value of can vary from close to unity (KMnO40.8-0.9, K 3F e ( C N ) 6 0.6- 0.8) to zero (K2Cr 2O7 0.03-0.08,H 2O 2 0). KI3 and quinone occupy an intermediate position(y ~ 0.4). In qualitative agreem ent with the theoreticalcalculation, the cont ribution of the valence band to ther e d u c t i o n current is found 10 4'13 2 to increase at more posi-tive electrode potentials.Oxidation reactions at a germanium electrode have notbeen studied in det ail. The oxidation of bivalent vanadium

ions13 3, of oxalate ions, and probably also of iodide ions 13 4> 1does not involve positive holes. The oxidation currentK 4F e ( C N )e in acid solution 127 and of ethylide ions in a Zieler electrolyte13 8, on the other hand, passes directly intot h e valence band.5. Form ation of Oxide Layers on the Electrode Surfac

If the products of anodic oxidation are insoluble (orsparingly soluble) the anode surface becomes covered witan adsorbed layer or with a relat ively thick oxide film,which often has a very high resi stanc e. Silicon elect rodform an oxide film in most aqueous solutions (except HFan d KOH), and in some non- aqueous solutions such a smethylacetamide137 - 13 9, liquid ammonia, and sulphurdioxide 14 0 '14 1.Surface oxide can bring about passivation of the silicona n o d e . Electrical breakdown of the film is observed at high enough poten tial (~ 15 V). Beyond this point the anodc u r r e n t is determined by the semiconductor properties ofsilicon: th us, on a />- type anode the curren t inc reasesexponentially with poten tial, whereas - type anodes show

limiting current 73 . Under cert ain conditions a layer ofamorphous silicon can form on the surface of a siliconsingle crystal 14 2.G e rm a n iu m oxide is readily soluble in water , but oxilayers can be produced on germanium anodes by usingsolutions of nitrates in methylacetamide 138 '14 3. When indium antimonide is oxidised in KOH solution s, the ra te offormation and the proper ties of the oxide film depend on tcrystallographic orientation of the electrode sur face 14 4"1On the whole, the pro per ties of oxide layers on silicongermanium, and indium antimonide ar e relatively unaffecteby the semiconducting propert ies of the underlying e lec-trode material. The electrochemical behaviour of thesefilms (rectifying pro per ties, photo-effect, etc.) is very

simila r to that of anodic oxide film son aluminium, tant alumniobium, and other metals.

IV. CHEMICAL ETCHINGChemical etching of semiconductor sur faces is a stan -dard operation in the radio industry and in laboratory pratice (for metallographic, elect ron ic, and other investiga-t i o n s ) . Etchants contain, as a rule, an oxidising agent,an d also a complexing agent whose function is to combinewith the semiconductor ions in solution 148 . The view thachemical etching involves chemical oxidation of the semic o n d u c t o r followed by dissolut ion of the resulting oxides iwidely held 14 9"15 2. However, careful studies of the etchi

behaviour of germanium and silicon in solutions of the mi m p o r t a n t oxidising agents (HNO 3, H 2O2, etc.) have demostrated that the pro cess is electroch emical in nat ure. Twcomplementary reactions take place on the semiconductorsurface at identical ra te s and at a common poten tial: ananodic half- react ion (dissolution of the semiconductor) ana cathodic half- react ion (reduction of the oxidising agent)The param ete rs for these two react ions determine thesteady- state potential and the corrosion ra te 11 915 3"15 6.In some cases {e.g. silicon in alkali solu tio ns 73 ' 15 7 '15 9)etching can take place by a purely chemical mechanismiiIT Various aspects of chemical etching are discussed in a recmonograph160.

218


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Vol .32 No.4 RUSSIAN CHEMICAL REVIEWS Apr i l 19A more detailed discussion of the electrochemical mech-anism of cor rosion seem s justified. We have alreadystressed the fact that holes ar e required for the anodic dis-solution of semiconductors like silicon and germanium.These holes can be generated in the bulk or at the surfaceof the semiconductor. It follows that the rate of corrosionof an - type semiconductor cannot exceed the limiting r at eof diffusion (or generat ion) of holes. By the same argument,if the cathodic reaction consumes free elect rons the ra te ofcorrosion of a />-type semiconductor is determined by thelimit ing rat e of diffusion or generation of free elect rons.However, an additional source of supply of holes is alsoavailable to the anodic process. We have seen that manyoxidising agents are reduced at a semiconductor by elec-t r o n s from the valence band. Holes ar e thus injected intot h e semiconductor by the cathodic reaction, and are imme-diately available to the anodic reac tion . Under these con-ditions the cathodic and the anodic reaction are no longeri n d e p e n d e n t : the rate of corrosion ceases to be a functionof the semiconductor properties of the sample and islimited by the diffusion of the oxidising agent in solution orby the electrochemical step. In pract ice the rat e of corr o-sion of germanium in concentrated solution s of oxidising

agents is found to be higher than the limiting current ofholes to the electrode surface by several powers of ten.These ideas were first put forward by G erischer and Beck128 ,a n d were further developed by Dewald119 and Turner 1B 3.In th is way, many oxidising agen ts can play a dual rolein the corrosion of semiconductors: they undergo thecathodic reaction, and they also provide holes for theanodic reactio n. When holes ar e generated faster thant h e anodic dissolution can use them up, the excess holes(accompanied by an equal number of electrons) diffusefrom the surface into the bulk of the corroding specimen ^ , ^ 3 .This hole current has been experimentally observed by them e t h o d described on p.218 . w132 This process can raiset h e bulk (as well as th e surface) concen tration of holes con-

siderably, and in - type and in weakly />-type specimenswell above the equilibrium value. Since holes are involvedin the potent ial- deter mining step of the reaction , the steady-- state potent ial (but not the corrosion r ate) should be afunction of the initial hole concentration in the semiconduc-t o r . This effect has been observed by G erischer andBeck16 1 [dissolution of germanium in alkaline K3Fe(CN)6]a n d by Turner 15 3 and Dewald119 (dissolution of germaniuman d silicon in H N O 3).The reduction of nitr ic acid on germanium consumeselectrons from the valence band 119 182 . In concentratedsolutions of H N O 3, therefore, germanium and siliconcorrode at a high rate (up to 6 A c m " 2 ) , independent of thesemiconductor prope rties of these elemen ts. The co rr o-sion rate is determined by the cathodic reaction, and thisis found to be auto catalytic. The slow step is the formationof N 2O 4 (HNO3 + H N O 2 - > N 2O 4 + H 2O), which is further r e -duced at the electro de. On raising the concentration ofHNO3 the rate of corrosion of germanium increases at first,reaches a maximum for a 6 solution, and finally de-cr eases a s a result of the formation of a thick, passivatingfilm of oxide in very concentrated solutions of H N O 3.Addition of HF destroys the passivity18 318 4.The rate of corrosion of germanium in acid solutions ofhydrogen peroxide is decreased by both anodic and cathodicpolarisation, suggesting that an electrochem ical p rocessis involved. Experimentally measured corrosion ra te s,however, are significantly higher than the values expected

from polarisation curves 1 5 4 > 1 8 5 . It is possible that chemcal oxidation of germanium by hydrogen peroxide takesplace in parallel with the electrochemical corrosion pro-cess.When finished germanium devices, containing variousmetallic components, are etched in H 2O2, the anodic andt h e cathodic reac tion s become localised in different pa rt s t h e device: germanium functions as the anode, copper a

t h e cathode. Indium and tin ar e usually strongly polar isa n d have very little effect on the rat e of dissolu tio n 188 .U n d e r factory conditions, alkaline solutions of H 2O2 areused for etching germanium, and the etching rate dependon the concentration of alk ali 18 7.G e r m a n i u m is corroded by solutions of electrolytes cotaining dissolved oxygen, the necessary cathodic reactionbeing provided by the reduction of O 2. Oxygen concentrations ar e usually small, and the etching rat e (1- 10 cmis less than the limiting curr ent of minority ca rr ie rs, anunaffected by semiconductor proper ties. At low O2 concentra tion s the etching ra te i s det ermin ed by the diffusioof oxygen to the reactin g surface; at higher concentratiot h e electrochemical step (reduction of adsorbed oxygen) rate- limitin g. It is important to note that the rat e of corosion depends on the concen tration of indifferent elect rolyte, and reaches a maximum at a cr itical concentration(normally less than 0.01 N) which is characteristic of thepar ticular elect rolyte. Harvey and Gatos have suggestedt h a t the reduct ion of oxygen is catalysed by anions adsorbon the germanium surface: when the adsorption of aniobecomes excessive, the oxygen coverage of the surface(and therefore also the reaction rate) must decrease 18 8"1Even in the absence of oxidising agents the steady- stapoten tial of germanium and silicon is a corro sion potetial 3> 171172. Measurements car ried out under scrupulouconditions of reagent purity have shown that the cathodicprocess is hydrogen evolution, the anodic process is germ a n i u m oxidation to give the brown modification of GeO

between pH 0 and pH 4 and the yellow modification betwepH 6 and pH 12. GeOf" ions may be involved in the potetial- determining reaction at pH > 12.5. The measuresteady- state poten tial is pract ically independent of the gm a n i u m conductivity type3.When the solution contains sa lts of meta ls more nobt h a n silicon or germanium, the cathodic reaction takes tform of electrodeposition of these me tals. The so- calle"elect ro less" n ickel platin gt (followed by heat- trea tmentis used to provide ohmic contacts for silic on 17 3.The corrosion behaviour of A m B v compounds is vesimila r to that of german ium. Thus, GaAs cor rod es by aelectrochemical mechanism 17 4 in solutions containing disolved oxygen or KAu(CN)2. In dilute solutions of oxidis

agents the rat e of corrosion of InSb is det ermined by dif-fusion of the oxidant 175176; in concentra ted solutions, ont h e other hand, the electroch emical step is rate - dete r miing. The compounds of the AI H BV series become passivin concentrated H N O 3.

t Chemical deposition of a nickel phosphide coating by reducof a nickel salt with hypophosphate (Ed. of Translation).


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Vol.32 No.4 RUSSIAN CHEMICAL REVIEWS Apr i l 19V. APPLICATION O F ELECTROC HEM ICAL METHODSO F SURFACE TREATME NT IN PHYSICOCHEMICALRESEARCH AND IN SEMICONDU CTOR TECHNOLOGY

N o practical device based on a semicon duct or- electr o-lyte system h as yet been developed. Th e Bell Teleph oneLaborat ories have described an intern ally powered t ra nsi s-tor 1 7 8 , based on the ce ll

GelKOHiAg2O, Ag,and a field triode " 9 , in which the pr op er ti es of the Z nO -electrolyte inter face are utilised, but neith er device hasyet been put to pra ctic al use}. N evertheless, elec tr o-chemical methods have found wide application in semicon-ductor device technology.

Electrochemical (as well as chemical) etching techniquesare used to remove mechanically damaged layers fromsemicon duct or surfa ces. F or silicon and silicon carbideth e electrolyte is a solution of hydrofluoric acid 18 118 2; forgerman ium , solutions of sodium or potassium hydroxidehave been use d 1 8 3 , and oth er compounds 1 8 4 "1 8 7 . Germaniumcan be electropo lished in aqueous alkali or in non- aqueousso lu t ions1 8 8"1 9 0 . Mixed me lt s of alkal i me ta l hal ides havealso been used for ele ctro polishin g germaniu m 1 9 1 . Siliconis electropo lished in solutions of hydrofluoric a ci d 1 9 2 "1 9 4 ,gallium arsenide and indium antimonide in a mixture ofperchloric acid and acetic anhydride 1 95 . Cur rent densitiesa r e usually between a few hun dre dt hs of an am pe re and afew amperes per cm 2 of specimen surfac e. A very smoothsurfa ce, with a ro ughn ess of less than 50 A, can be obtainedby stirring the electrolyte 19 619 7.

Elec tro lytic polishing of germ aniu m in KOH solutio ns,and also in solut ion s of SbOCl, pro du ces sur face s of e x-tr em ely low surface recom binat ion velocity (of the ord erof 100- 200 cm sec" 1 ) 1 9 8 " 2 0 0 .

The jet- etching met hod 20 1 has been developed for theproduction of shallow depressions a few tens of microns indiameter on germanium wafers in the manufacture ofsurfaces- barrier t ra nsi sto rs. A thin jet of electrolyte isdirect ed from a glass nozzle on to the sample , which is biasedanod ically. The cathode is sealed in the wall of the nozzle.Under facto ry con dition s two jets ar e used, one on eachside of the wafer. The elect rolyt e consist s of potassiumhydroxide 20 2"20 4 or a metallic salt 1 7 3"1 7 91 8 2"1 9 61 9 8"2 0 8 . Theelectrolyte should have a much higher resistivity than thegerman ium . Etching is carr ied out at a curre nt density ofabout 1 A cm "2 , and the operatio n is stopped when the r e -main ing th ickn ess of germa ni um (which is to form t he baseof the tr an sist or ) is no mo re than a few mi cr on s. Themethod is also suitable for gallium arsenide 2 0 9 , but a solu -tion of C d ( C N )

2mu st be used as the elect rolyt e.

Germanium can also be jet- etc hed cathodically (forma-tion of G e H 4) , but very high current densities are needed(up to 160 A c m " 2 ) . 2 1 0 - 2 1 2

D epr ession s of any desir ed shape can be etched intogerman ium by the use of micro cat hod es situat ed very closeto the surface 21 321 4 . Sharp poin ts, with a rad ius of curva-ture of 1 and suitable for field emission experiments, canbe prepared by etching germanium in N a O H . 2 1 5 Lastly, anelectrolytic saw for germanium has been made 2 1 6 in whichthe cathode is a moving, fine tun gsten wir e.

Thick, non- porous oxide films have been grown on gemanium and silicon devices, to pro tec t them from the effof th eir ambien t surro un din gs, by anodising the sem iconductors in a solution of nitrates in methylacetamide 21 7 .

By taking advantage of the rectifying properties of p-junctions and of the dependence of the rate of dissolution germa ni um on the type and magnitud e of its cond uctivity,selective etching of the variou s pa rt s of a t ra nsi sto r(emitter, base, collector) can be ach ieved 21 8 - 22 1 . Varioutechn iques for revealin g the pr esen ce of p-n junctions ingermanium single crystals ar e based on the same p rin -ciple22 222 3. If, for example, a re verse- bia sed p-n junction (n region con nected to the positive ter min al of thebattery, p region conn ected to the negative ter min al) isim me rse d in a dilute solution of alkali, most of the applipotent ial is dropped ac ro ss the junction itself and the tworegions are at a very different pot ent ial. The region,being anodic, will dissolve, whereas the p region will acas a cath ode. If, on the oth er hand, both sides of the jution ar e at the same (anodic) poten tial, and an auxiliarycathode is provided, the p region will dissolve preferen-tially because at a given overpoten tial />- type german iumanodes dissolve faster than - type. The method can alsbe used to re veal bo un dar ies between region s of the sa meconduc tivity t ype, but of widely different donor (or acceptconcentration: the so- called l-h junc t ions2 2 4"2 2 6 .

I t is also possible to delineate p-n junctions in germaium by electro depositio n of copper or gold. A re ver se-- biased p-n junction im me rsed in a solution of a copperor gold salt will be electroplated on the p side, since thiregion is at a cathodic po te n ti al 2 2 7*2 31 . If a solut ion of the Methylene Blue is used instead of the salt solution, the dyis red uced (and decolo rised) on the p side of the junctionSilicon junction s ar e not usually polar ised from an e xternbatt ery, but are flooded with light in ste ad 2 33 : th e injecthole- electron pairs ar e separated by the p-n junction(which behaves in this case as a photodiode), charging th region negatively and en surin g th at me ta l (gold, cop peor silver) is electr odepo sited only on th is r egio n 2 3 4"2 3 7 .

Elect rodep osition is also used to pr ep ar e rectifying anohmic contacts on silicon and germ an ium 2 3 8"2 4 2 . Germaniucoatings on oth er met als can be electr odepo sited from no- aqueous elect rolyte s2 4 3"2 4 6 ; elect rolysis of molten sal t sgives germanium dendrites suitable for use in semicondutor devices2 47 .

% A recent communication180 claims that the Si- H 2SO4 systemcan form the basis of a radiation detector.

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Application to Transistor Electronics" (Translated intoRussian), Inostr.Lit., 1953.

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Institute of Electrochemistry,

electrochemistry of semiconductors

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