194PHILlPS TECHNICAL REVIEW

VOLUME 26

Doping methods for the epitaxial growthof silicon and germanium layers

J. Goorissen and H. G. Bruijning

548.4: 669.782/.783

it has long been known that single crystals of silicon and germanium can be grown from thevapour as well as from the liquid. The term "epitaxial growth" is used when the crystalgrows ill the form of a layer on a crystal platelet (the substrate), from which it derives itscrystal orientation. The growth from the gas phase call take place at relatively loll' tem-:peratures, and the crystals thus obtained are comparable ill quality to crystals grown fromthe liquid phase. Epitaxy has become an indispensable aid in the fabrication of semicoilductordevices. By the addition of suitable impurities during the process (doping) P_ and Ni-typelayers can be formed in any sequence and with any desired concentratten gradient. Thisart iele describes various doping techniques by means of which multilayers of silicon andgermanium with sharp junctions can be achieved.

Introduction

Epitaxial growth methods

It has long been known that single-crystal germa-nium or silicon can be grown from the liquid phase.By sawing and grinding, pieces of crystal are thenobtained of the size needed for the fabrication of semi-conductor devices. After a surface treatment, the P-Njunctions and contacts required for a particular tran-sistor or diode are applied by alloying or diffusiontechniques.

Single crystals of this kind can also be grown fromthe vapour phase, particularly in the form of a layeror film deposited on a crystal substrate. When the lat-tice planes of the substrate continue in the layer, theprocess is referred to as "epitaxial" growth. Substrateand layer do not necessarily have to be of the samematerial; it is known, for example, that gallium arsenidecan be grown on germanium, and there are numerousother combinations. By the addition of suitable impu-rities to the vapour phase, P-and N-type layers havingthe desired concentration gradient can be grown. In thisway certain characteristics, e.g. electrical conductivity,can be varied more or less at will.

Since about 1959 the technique of producing single-crystallayers from the gas phase has rapidly establisheditself, on the one hand because already existing typesof semiconductor devices can be fabricated moreeasily in this way, and also because it opened up thepossibility of producing novel and more complicatedcircuit devices.

J. Goorissen and Ir. H. G. Bruijnlng are research workers at PhilipsResearch Laboratories, Eindhoven,

Many methods are known by means ofwhich single-crystal layers can be grown epitaxially from the gasphase and the required doping effected. Although theydiffer in many respects, a feature common to thesemethods is that the vapour flows continuously to thesubstrate and deposits the element there in one way oranother. There are three different procedures.1) A closed reaction vessel contains the starting ma-terial, the substrate, the impurity to be added, and fgas which takes part in the reaction and effects thetransport. A thermal gradient is brought about in thevessel, and, since this method depends on an equi-librium state, the establishment of that equilibriumwill differ at the two ends of the vessel. Apartfrom the material to be deposited, only gaseous re-action partners occur, so that if the conditions areproperly chosen, this material will be transported bythe chemical reaction via the gas phase from one endto the other, and there deposited. This transport takesplace by diffusion or convection. The parameters in thisprocess are the temperature at which the growth occursand the thermal gradient. Schäfer and his associates .have investigated many of these transport reactions [1].

Marinace has grown germanium epitaxially by meansof the iodide equilibrium [2]: .

Î

,?

400 'C

2 Geh ~ Ge + GeI4 + Q.600 'C

If an impurity has to be incorporated, the conditionthen is that its transport, e.g. with an analogous re-action, must take place in the same direction.2) The second procedure makes use of an open system.A gas mixture is produced which consists of a com-

)

No.7, 1965DOPING OF EPITAX1AL Si AND ye

195

pound of the semiconductor, a compound of the im-purity and a gaseous carrier, and this mixture is con-ducted to the space where the epi taxi al layer grows.There it flows over the substrate, the temperature ofwhich ensures that the semiconductor is deposited andgrows on the substrate epitaxially. The gaseous reactionproducts are expelled. In this method the parameters arethe concentration of the compounds of semiconductorand impurity, the rate of flow of the gas and the growthtemperature. Theuerer [31,Marck and others have pro-duced epitaxial layers of silicon on silicon by the re-duetion of silicon tetrachloride with hydrogen inaccordance with the equations:

SiCI4 + Si ~ 2 SiCI2,

SiCI2 + H2 -:r ~i + 2 HCI.

This procedure in an open system (that is at I atm)is very widely employed, and is already used on a pro-duction scale. In the doping methods described in thisarticle, both for germanium and for silicon, use is also.made ofthe reduction ofthe tetrachloride by hydrogen.In recent times epitaxial techniques have been deve-10peU based on the thermal decomposition (pyrolysis)of hydrides [41,e.g. silicon hydride: SiH4 --? Si + 2H2.This involves no reaction that first affects the sub-strate itself, as under (1). As a result the concentrationboundary between substrate and layer is sharper [51[61.3) Of an entirely different nature are the methodsin which the element itself isevaporated and condensed.Molten silicon or germanium, to which the impuritymay have already been added, is evaporated in avacuum (10-5 to 10-6 torr) and epitaxial growth is pro-duced by condensation on a heated substrate. Com-pared with chemical methods this process is notable forthe high rate of growth [71.

Doping methodsIn the following we shall confine ourselves to the pro-

duction of silicon and germanium layers by the reduc-tion of the tetrachloride in an open system. Dopingcan be carried out by adding to the gas mixture volatilecompounds of the appropriate impurity, and, if the. concentration of the compound can be varied rapidlyenough in the gas, it is possible to obtain successivelayers of different conductivity so as to give to the epi-taxial structure the electrical characteristics which aredesired.

First of all, these compounds can be dissolved in thesilicon tetrachloride, which is liquid at room tempera-ture. Theuerer [81obtained reproducible results by evap-orating a mixture of this kind in such a way that thevapour had a constant composition: the special con-struction of the apparatus he used for this purposeenabled him to give the vapour the same composition

(I)

(2)

as the liquid - in spite of the generally different vapourpressures of the components. This method is particular-ly suitable for producing large quantities of epitaxialstructures with identical properties.

Secondly, a vaporized doping mixture can be in-jected into the gas mixture, as described by Corrigan [91;

For this purpose the halides of boron and phosphorus,which are liquid at room temperature, have a suitablevapour pressure. The injection is carried out by meansof a diffusion mechanism, so that the injected quantitydepends only on the temperature of the liquid dope.With this technique it is possible to produce numerousstructures consisting of various layers that possessdefined properties. Even better - in particular morequickly variable - is the use of volatile hydrides ofboron and phosphorus (Cave and Czorny [lOl).

In the following sections of this article we shall de-scribe the doping methods which we employ. For thedoping of silicon epitaxiallayers we have developed thespark-doping technique. Between two electrodes con-sisting ofthe element to be added, or which contain thatelement, a spark discharge is generated in the mixtureof silicon tetrachloride and hydrogen. This gives riseduring the discharge to a narrow zone of high energydensity, in which the doping compound is formed by re-action with silicon tetrachloride and hydrogen and trans-ported by the gas mixture. By varying the repe-tition frequency and the energy of the sparks it ispossible to change at any required rate the concentra-tion of the doping compound in the gas. Using thistechnique it is possible to grow highly reproducible mul-tiple as well as single layers.

For germanium layers we use a gas-doping method

(1] H. Schäfer, Chemische Transportreaktionen, Verlag Chemie,Weinheim 1962.

(2] J. C. Marinace, Epitaxial vapor growth of Ge single crystalsin a closed-cycle process, IBM J. Res. Devel. 4, 248-255, 1960.

(3] H. C. Theuerer, Epitaxial silicon films by the hydrogen re-duetion of SiC!.!, J. Electrochem. Soc. 108, 649-653, 1961.

(4] S. E. Mayer and D. E. Shea, Epitaxial deposition of siliconlayers by pyrolysis of silane, J. Electrochem. Soc. 111, 550-556, 1964.

(5) E. A. Roth, H. Gossenberger and J. A. Amick, The growthof germanium epitaxial layers by the pyrolysis of germane,RCA Rev. 24, 499-510, 1963.

(6] B. A. Joyce and R. R. Bradley, Epitaxial growth of siliconfrom the pyrolysis of monosilane on silicon substrates, J.Electrochem. Soc. 110, 1235-1240, 1963.

[7] J. C. Courvoisier, W. Haidinger, P. J. W. Jochems and L. J.Tummers, Evaporation-condensation method for makinggermanium layers for transistor purposes, Solid-State Elcc-tronies 6, 265-270, 1963.

[8] H. C. Theuerer, Steady-state evaporation method for corn-position control of thin films prepared by halide reduction,J. Electrochem. Soc. 109, 742-743, 1962.

[D] W. J. Corrigan, Doping of silicon epitaxial layers, Conf."Metallurgy ofsemiconductor materials", Los Angeles 1961,pp 103-111, Interscience Pubi., New York 1962.

(10) E. F. Cave and B. R. Czorny, Epitaxial deposition of siliconand germanium layers by chloride reduction, ReA Rev. 24,523-545, 1963.

196 PHILIPS TECHNICAL REVlEW VOLUME26

spark gap fO~\P-conduction .

H2+!%SiClq+doping

compound

Fig. I. Experimental arrangement for producing epitaxial silicon layers, using the new dopingmethod (spark-doping). The layer grows on the silicon substrate in the reaction vessel. Thesubstrate is raised to the required temperature by the high-frequency-heated silicon carrier.A spark discharge can be produced across one of two spark gaps havingsuitable electrodes fordoping with elements that give rise to N- or P- type conductivity. The repetition frequency ofthe sparks can be varied, and also their energy (by switching-in different capacitances).

The apparatus is in principle the same as that used forsilicon, except that the doping compound is not pro-duced in the spark but is added in a gaseous form tothe mixture of germanium tetrachloride and hydrogen.

Spark-doping of silicon \

Apparatus used

Fig. 1 shows schernatically the experimental set-upfor the spark-doping of silicon. The carrier gas, hy-drogen, is purified with a palladium filter, after whichit is split into two streams; one of which is saturatedwith silicon tetrachloride. In: this way the required hy-

ldrogen/chloride ratio (100: I) is easily obtained. Inall experiments the rate of ga~1flow is I I/min. The sili-con tetrachloride used is the pommercial "very pure"grade. If silicon is made from this without doping, itbecomes N-type with a resistivity of approx. 15 ncm.After the stream of pure hydropen and the stream ofthe hydrogen and silicon tetraèhloride mixture havecombined, the doping compound can be added in thenext section of the equipment with the aid of the sparkdischarge. This being done, the entire gas mixture isthen fed into the reaction vessel. Contained in thisvessel is an inductively heated carrier, consisting ofsilicon with a resistivity (at room temperature) of about0.1 ncm. The induction coil, fed with 400 kc/s, en-closes the reaction vessel. The gaseous reaction pro-ducts leave the system through an absorption tube.Finally the flow rate of the emergent gas is measured.

On the silicon carrier is placed the substrate, whichhas previously been lapped and treated with a polishingetchant [11]. This makes the surface so smooth that nosurface structure is observable under the microscope.

Pd-diffusionfilter

saturation ~rk gap forwith SiCI" / N-conduction

H2+!%SiCI"

During the etching procedure air is passed through thesolution to prevent the adhesion of gas bubbles, whichwould cause non-uniform etching.In the reaction vesselthe substrate is first heated to 1275 °C in pure hydrogenfor about half an hour to reduce residual surface oxi-dation: Immediately afterwards the epitaxial layer isgrown. At a temperature of 1225 °C a layer 11± I (J.mthick grows in 15minutes. During this time the substraterotates at a speed of 50 r.p.m., so that on an average thewhole surface comes into contact in the same way withthe gas stream. "

The doping system

The actual doping system consists of a spark genera-,tor and several pairs of electrodes, one pair for eachdoping element. When the spark discharge is set upacross one of the electrode pairs, the relevant compound!is formed by the reaçtion of the electrode material with'silicon tetrachloride' and hydrogen, and is transported.in the stream of gas. To obtain P-type layers we useboride electrodes (LaB6 or B4C) and for N-type layerswe use antimony, silicon with 0.1% phosphorus, andalso antimony with 1% arsenic. The electrodes are con-tained in a glass tube with metalleads, the whole as-sembly measuring only a few centimetres (seefig. 2).

The concentration of the doping compound in thegas mixture, and hence in the epitaxially grown layer,depends among other things on the nature of the elec-trode material, the composition of the negative elec-trode being in any case the governing factor. Hardlyany change is to be noticed if, for example, platinum orsilicon is used for the positive electrode. The concentra-tion ofthe doping compound in the gas mixture furtherdepends on the repetition frequency of the spark, and

!SkY,2-200 s-'Si-substrate Si-heating element

'(/h.f.-heating, 400 kHz

reaction vessel

197No. 7, 1965

DOPING OF EPITAXIAL Si AND Ge

._

~ ~ (..- I

- # ~ V~, - ) IJ+a b e

;1

Fig. 2. Configuration of the spark gap. The spark channel has a cross-sectional area of 2 mm-(a) or I ern- (b). In (c) a photograph is shown of this part of the equipment (bottom left a partof the capacitors).

on the capacitance of the capacitor which is shuntedacross the spark and governs its energy. These sparkparameters can be varied, the frequency continuouslyfrom 2 to 200 per second, the capacitance in steps of10, 100,300 or 700 pF. In this way it is possible to varythe energy of the spark by a factor of 7 X 103•

Spark gap and spark genera tor

Certain requirements which the spark discharge hadto satisfy led to an unusual construction of the sparkgenerator. Fig. 3a shows the circuit diagram. Thespark generator delivers a current pulse of the formshown in fig. 3b, which is conducted to the parallelconfiguration of spark gap and capacitor. fig. 3e givesthe form of the corresponding capacitor voltage V,which is likewise the potential between the spark elec-tredes. The breakdown occurs at a specific voltage, atwhich an energy -te V2 is generated, causing chemicalconversions to take place at the electrode surface.

This breakdown voltage is not definitely fixed. It isinfluenced by a number of factors, such as variations ingas composition, the spark frequency, the shape oftheelectrodes and the distance between them, so that onecan only say that the breakdown occurs at a meanvoltage Vm- The ionization produced by the breakdown

9G

I

i

It, -------

_I

Fig. 3. a) Charging circuit. G spark generator. C capacitor. Fspark gap. b) Current pulse for charging the capacitor. c) Voltageon the capacitor and across the spark gap. The breakdown takesplace at a mean potential Vm, after which current still flows alongthe spark channel at a potential Vr.

[I1J Saturated KMn04 solution in 50% HF (room ternperature).

198 Pl·HUPS TECHNICAL REVIEW VOLUME 26

enables the rest of the currellt pulse to flow through thedischarge channel thus formed (cross-hatched area infig. 3b). At the pressure and: composition of the gas ashere employed, this current! would flow at the fairlyappreciable voltage Vr•This means, however, that afterthe breakdown, energy will still be supplied to the gas,leading to overheating. This in turn means that anappreciable quantity of silicon tetrachloride will reactwith hydrogen, in accordance with the bulk equation:

SiCI4 + 2 H2 -:+ Si + 4 HCI. .

As a result the electrodes become gradually coated withsilicon and less and less doping compound enters thegas, since the spark no longèr encounters the properelectrode composition. Whatis more, the electrodesgrow towards each other. 'I

l To avoid these effects a circuit is employed which cutsoff the current pulse almost immediately after the break-down: see jig. 4. At the moment of breakdown the. .

Fig. 4. Circuit for shorting the current source immediately afterthe spark discharge. I current pulse. Q transmitting valve typeQB 3.5/750. F spark gap. C capacitor.

sudden change in the potential across the spark gap isused for short-circuiting the current source with the aidof an electronic tube. This occurs about 3 fls after thebreakdown, and as a result the deposition of siliconis. almost completely suppressed.

The voltage across the spark gap at the moment ofbreakdown is between 10 and 15 kV in the experimentsdescribed here. The valve mentioned must be capableof withstanding this voltage while the capacitor isbeing charged up. A small transmitting valve, e.g. typeQB 3.5/750, meets this requirement. Moreover, thevalve must be able to handle the current from the gener-ator circuit at low anode voltage, at which the potentialacross the spark gap becomes so low that the dischargebreaks off. To avoid overloading the valve a generatorcircuit was chosen that supplies a low current; thiscurrent must, of course, flow long enough for the re-quired voltage to be built up across the capacitor.

To meet these conditions the generator must have ahigh impedance and be able to supply current pulsesof relatively long duration. An induction coil of theRuhrnkorff type possesses these properties and is

. therefore eminently suited to our purpose. The primarycurrent in our case is supplied via a transistor amplifier,using a circuit which generates the repetition frequencyrequired for the discharge.

Results of silicon doping

The choice of the electrode material is governed by anumber of factors. For example, there must be no con-tamination of the mixture of silicon tetrachloride andhydrogen if no spark discharge takes place. Volatileor reactive elements or compounds are therefore ruledout as electrode material. This applies, for example, toarsenic and phosphorus, which are frequently used asdoping elements. These elements can, however, be usedin the form of a solid solution in an otherwise inertelectrode material, as for example phosphorus in sili- .con. Good results have also been obtained with 1%arsenic in antimony (seejig. 5, curve A).

In the experiments to which fig. 5 relates, the oppositetype of substrate material was in all cases chosen inorder to form a P-N junction. The conductivity andthe layer thickness, from which the concentration iscalculated, were measured by previously reportedmethods [12][13].Each point indicated in the figure is theaverage of at least three experimental values.P-type layers were obtained using boron. Although

the electrical conductivity of this element.itself is toolow for it to be used as electrode material, it is veryuseful as such in the form of sintered AlB12, LaB6 orB4C. The sintered material must, however, be highlyhomogeneous. Curves Band C in fig. 5 give the boronconcentration obtained in the epitaxiallayer as a func-tion of the spark frequency, with the capacitanceas parameter. The figure shows that the expectedlinear relationship is not found. There are variousreasons for this, some of which are bound up withthe fact that successive discharges have an increasinglystronger influence on each other as the frequency in-creases.

Of the factors already mentioned which govern the

A:Sb+T%As8:La86CILa86D:Si+o.T%PEILa 86(e84C)

TOpF700pF700pF300pF300pF

H2+f;;,;I"H2+SiC/"H2+SiCI"H2 .H2

780/s700

2·

7~----~~----~----~~----~70'5 TOl6 1017 1018 1019/cm3

-NL

Fig. 5. Concentration of the doping element in the epitaxial layeras a function of the spark frequency, using various electrode ma-terials (curves A to E). The appropriate capacitance and the gasin which the spark discharge took place are indicated for eacheurve. In cases A and D an N-type layer is grown, in the othercases a P-type layer.

No. 7,1965 DOPING OF EPITAXIÄ.L Si AND Ge 199

breakdown voltage, and hence the energy generatedin each spark, the most important is the SiCL, concen-tration. If this concentration is increased by a factor of2, the spark capacitance and frequency remaining un-changed, four to five times as much doping elemententers the epitaxial layer, depending on the electrodematerial. The presence of hydrogen, incidentally, isessential to the formation of the compound in thespark. If argon is used as carrier gas for the silicontetrachloride, and the hydrogen is not added untilafter the spark (hydrogen is in any case necessary fordepositing silicon on the substrate in accordance withreactions (1) and (2», then no doping compound isformed. In the spark discharge the following reactionshave apparently taken place:

SiCl4 + 2 Hz --'»- Si + 4 Hel, (3)

6 HCI + 2 0 --'»- 2 DCb + 3 H2 (4)

the reaction (3) occurring in the spark and (4) at thesurface of the electrode (0 = doping element). Afurther argument in support of this statement is theclearly perceptible formation of silicon, which has tobe removed after a few experiments by etching theelectrodes. The above-mentioned necessity for cuttingoff the spark is also understandable in this connection.This cut-off is found to benefit considerably both thereproducibility and the quantity of doping compoundformed.

The dimensions of the spark channel also infiuencethe reproducibility and the amount of compoundformed, so that there must be a direct relation betweenthese two quantities and the spatial extent of the sparkdischarge. This was investigated with the two configu-rations shown in fig. 2. It was found that in the narrowtube the amount formed was smaller (about V but thereproducibility was better than in the wide tube, giventhe same electrode spacing and spark parameters.It is obvious from the foregoing that the use of the

spark-doping method involves a large number of fac-tors; it is not difficult in practice, however, to takethese into account. The reproducibility is the sameas found with other methods (fluctuations of about25%). The great advantage of the method is that thepossibility of fast variation makes it relatively easy toproduce very abrupt junctions in the growing layer.

'12) G. Backenstoss, Evaluation of the surface concentration ofdiffused layers in silicon, and F. M. Srnits, Measurement ofsheet resistivities with the four-point probe, Bell Syst. tech. J.37,699-710 and 711-718,1958.

[13) S. Mendelson, Stacking fault nucleation in epitaxial siliconon variously oriented silicon substrates, J. appl. Phys. 35,1 570-1581, 1964.

[14) The method used was devised by J. Appels of this labora-tory, A cleavage plane is etched for about 10 seconds witha solution of 0.1 % concentrated nitric acid in 50 % HF.The P-type conducting parts turn a dark colour.

o-N

Fig. 6. Cross-section of an NPN structure grown on a p++ sub-str ate by the spark doping method.

Fig.6 shows a P++NPN structure whose layers weredoped by the spark doping method. On a cleavage facethe various layers can be made visible by etching [14].

The current-voltage characteristic that can be measuredon the etched surface is illustrated in fig. 7.

Finally, it should be mentioned that spark reactionscan also be produced in pure hydrogen in a by-pass armof the system. In that case both with LaB6 and withSi + 0.1 % P the corresponding hydrides are formedinstead of chlorides as in reaction (4). Curves D and Ein fig. 5 give the relevant measured results. Since in thiscase no amorphous silicon is formed and deposited onthe electrodes, this method of doping is even more at-tractive, and is at present the subject of intensive in-vestigation. In this way spark-doping can also be usedfor germanium epitaxy. The experimental set-updescribed above for silicon cannot be used for this pur-pose, because large amounts of amorphous germaniumare deposited, in accordance with the reaction:

GeCl4 + 2 H2 --'»- Ge + 4 HCI.

In!:r--'J 'I r li if l~r r r :~li=:.T t îHn~ ~r ~~ ~~ 11

~~ ~~ l,-T innr- -.. .I ~~ II-'~r-~~r-~~ 1; ~:.-- ~~---

. . ;l t. ~~ l! , .. ' ti-, -r- -, ..-----,.--~~- 1- ..,.- _.~;)!.... fIIJI.t .... ..-i&" JI...;aifllJlawllll.4~ .... ~~r;;:J[OI Jl_r""-,r--,r- ..,~r- ....~r,.J'I~"""''''''-',..'''.....,r ...... ..,-- -

.j Ol J, lr I.. J L__-,~~~!-,..--ï- -~. 1'r " 11~- lf ---- - 1-

H (. _'. j,:.II ~:--~~~~~-"

_1_Jf .t, h

Fig. 7. Current-voltage characteristic of the P++NPN structureshown in fig. 6. Each scale division on the abscissa represents10 V, on the ordinate 5 mA.

200 PHlLfPS TECHNICAL REVIEW VOLUME 26

Gas-doping of germanium

Experimental

As mentioned at the end ofthe last section, complica-tions arise in the doping of germanium by the sparkmethod. For the doping of epitaxial layers of germa-nium we have adopted a gas-doping method, the prin-cip_Ie of which is illustrated in fig. 8. Apart from theactual system of doping, the set-up is basically thesame as used for silicon. The high-frequency-heatedcarrier on which the substrate is placed consists hereof molybdenum with a closely fitting quartz sleevewhich is open at the bottom end. Reaction of themolybdenum with germanium tetrachloride or withreaction products is negligible in this arrangement.

The liquid germanium tetrachloride is purified byextraction for 24 hours with aqua. regia followed byphase .separation and distillation. An epitaxial germa-nium layer grown from this germanium tetrachloride isfound to possess the intrinsic conductivity of Ge, i.e.it contains no foreign atoms, provided the substrateonly comes into contact with quartz or with polytetra-fluorethylene during handling. The pre-treatment ofthe germanium substrate is similar to that described inthe case of silicon, except that the composition of thepolishing etchant is different [15]. The reduction tem-perature is now 875 °C. The gas containing the dopeenters the system via stopcocks A or B in fig. 8. Theepitaxial growth takes place at 840 °C; at a GeCI4 con-centration of 0.2 % a layer of about 10 f-lmthick is thenobtained in 30 minutes.

The doping system

The doping of the germanium layers is based on agas mixture of hydrogen and 0.015% PH3 or B2H6.This mixture is contained in a normal gas cylinder at aninitial pressure of 120 atm; the supply is regulated witha reduction valve.The arrangement of the actual doping system is

shown infig. 9. The pressures PI and P2 on the capilla-ries Cl and C2 make it possible to adjust accurately aprescribed concentration of the doping compound inthe gas. This is done as follows. The mixture of hydro-gen and hydride can be fed in either through Tl or T2.Let us first assume that the mixture is supplied via Tl.The mixture then flows only through capillary Cl. Thedimensions of the latter are such that, by adjusting thepressure PI, which can be 100 g/cm2 maximum, the flowthrough Cl is varied from about 5 to 100 cmë/min. Theamount of doping compound supplied varies corre-spondingly. Now this amount can be reduced by ad-mitting the mixture via T2 while introducing pure hy-drogen through Tl. This decreases the concentration ofthe doping compound in the hydrogen-hydride mixture

A B

Fig. 8. Experimental arrangement for growing epitaxial germa-nium layers, using a gas-doping method. Only those parts areshown in which the set-up differs essentially from that in fig. I.The layer grows on the germanium substrate Ge, which in thiscase is raised to the appropriate temperature by an indirectly heat-ed carrier Mo of molybdenum, which is enclosed in a quartzsleeve.

in a ratio which can be calculated as follows. Uponvariation of the pressurep-, which can also be 100 g/cm2maximum, the capillary C2 delivers a gas stream of0.5 to 10 cmê/min, Ifwe now pass the smallest possiblestream through the capillary C2 (0.5 cm3/min) and allowhydrogen to pass via TI until the flow meter S givesa reading of 150 cm3/min, at the same time making thegas flow through Cl as small as possible (5 cmê/min),

s

Fig. 9. One of the two doping systems for Ge, which, in the ar-rangement in fig. 8, are connected at A and B.

No. 7,1965DOPING OF EPITAXIAL Si AND Ge

201

then we have brought down the lower limit of thedoping compound supplied to 5xO.5/150 = 0.016cm3/min. We can thus change the supply of dopingcompound from 100 to 0.016 cm3/min, that is to say bya factor of 5x 103• For most purposes this is amplysufficient. The dual system illustrated here makes it.possible to give the epitaxial layer almost any required .resistivity,' with P- and N-type layers following one an-other in any arbitrary sequence.

In order to obtain a sufficiently sharp junction be-tween two layers, the gas mixture must previously havebeen set to the appropriate value. This is done asfollows (see fig. 9). Before the actual experiment, thehydrogen-hydride mixture is allowed to flow without itbeing able to reach capillary Cl. For this purpose stop-cock K is set in a position a't which the opening 0 isturned through 180° with respect to the position shownin the figure. After a few minutes the gas mixture has

I

.reached the appropriate composition inside the stop-cock, and it can then be admitted into capillary Cl bysimply turning the stopcock through 180°. The mixtureof germanium tetrachloride and hydrogen now. imme-diately takes up the doping compound with the 're-quired concentration.

Results of measurement for germanium

The concentration of the doping element in the epi-taxiallayer can be calculated from the measured resisti-vity. Infig. JO this concentration NL(number of chargecarriers per cm3 in the layer) is plotted versus the quo-tient [D] [Ge]/[GeCI4], where [D] and [GeCI4] arethe respective concentrations of the doping compoundand ofGeCI4 in the gas, and [Ge] represents thenumberof germanium atoms per erna (4.4X 1022) [161. It canbe seen from the graph that a linear relation. exists,

so that:

NL = K [D][Ge] [GeCI4]

If the doping element could be incorporated in thesame ratio as the germanium in the layer, the proportion-ality factor K would be equal to unity. In fact, how-ever, one finds K = 36, both for boron from B2H6 andfor phosphorus from PH3. Plainly, then, K is unexpect-edly large. If we add these elements with the aid ofother compounds (BBr3 and PCI3), then Khas the samevalues as if the hydrides had been used [171.

(15) 15 cm3 fuming nitric acid and 1 cm3 50% HF (at boilingpoint).

,I';: This graphical method, which might seem somewhat com-plicated, takes account of the fact that the germanium pres-ent in the gas arrives only as a minute fraction in the grownlayer.

(17) Communication from J. Bloem, Philips SemiconductorWorks, Nijmegen.

D

- :- 1I~/

V

rfV

" D

° [7'

IfV

~/0

/V'°V 0

'7 1° PH3l=-

" I D B2H6 -

5

2

1018

5NL

t 2

1017

5

2

1016

5

2

1015

5

2

101~016 2 5 1017 2 5 1018 2 5 1019 2 5 1020cm-3[0] [ 1

- [GeCl,] Ge .

Fig. 10. Concentration NL of the doping element (i.e. of the chargecarriers) in the layer, as a function of the concentration of thedoping compound in the GeCI4, expressed as the number of PH3molecules per 4.4 X 1022 molecules of GeCI4.

The proportionality factor also differs from 1 in thecase of silicon. Corrigan [91 reports K = 5 for PCl3 andK = 10 when BBr3 is used. In the circumstancesdescribed here, values of 5 and 10 are also obtainedusing PH3 and B2H6..

From these observations it may be concluded thatthe take-up of foreign substances involves unexpectedfactors. These do not depend on the equilibrium statefor the corresponding chlorides, because this differsconsiderably for phosphorus and boron. The fact thatthe value of K differs from 1 is attributed by Corriganto circumstances of geometry. At the present stage,however, attempts to explain this effect would seem to

be premature.

Summary. By the reduction of silicon and germanium tetrachlo-ride with hydrogen in an open system, epitaxial layers can begrown and the doping element incorporated during growth. Forthe doping of silicon layers the spark-dopillg method has been de-veloped in the Philips Research Laboratories, Eindhoven. Bymeans of a spark discharge between two electrodes that containthe doping element, accurately defined quantities of the dopecan be added to the mixture of SiCI4 and hydrogen. By changingthe spark parameters (capacitance and frequency) it is possible tochange with immediate effect the concentration of the dopingelement in the growing layer. In this way one can obtain sharplydefined layers with the required values of resistivity, or structureswith very accurately controlled concentration gradients. Thedoping of germanium layers is done by a gas method, the dopebeing added in a gaseous state to the mixture of GeCl4 and hy-drogen. This addition can be accurately regulated within widelimits. The concentration of the doping element in the layer canbe calculated from the measured resistivity of the epitaxiallayer.In the case of silicon the authors examine the dependence of thisconcentration on the spark capacitance and frequency, and in thecase of germanium its dependence on the concentration of thedoping compound in the gas.

N.V. PH\l\PS' GLOE\LAMPEPFABR\EKER

TECHNICAL REVIEWPH I LI PSVOLUME 26, 1965, No. 8/9

Research and production in telecommunication formthe business of many departments of the Philips Groupin various countries. The centre of these activities, sofar as they concern actual telecommunication equipment,is N. V. Philips' Telecommunicatie Industrie, with itsmain establishments at Hilversum and Huizen and othersat The Hague, Hoorn and Amersfoort. Work on defencesystems, which are closely related to telecommuni-cation, is chiefly concentrated in the N. V. HollandseSignaalapparaten plant at Hengelo.

The present number of Philips Technical Review pre-sents a collection of articles giving a survey of the workdone at these establishments. Such a survey must nat-urally be incomplete. A more complete picture is given inthe columns of the quarterly "Philips TelecommunicationReview", published by N. V.Philips' TelecommunicatieIndustrie. We have chosenfor this number the followingrepresentative subjects: the development of carrier tele-

phone systems; the design of automatic telegraph switch-ing centres and an interesting aid for such centres,a magnetic tape store with static read-out; the problemof traffic control at large airports; and the design oftelevision transmitters for the new bands IV and V.Appended to these articles is an account of the funda-mental research being done at the Research Labora-tories, Eindhoven, on companders for the better adap-tation of speech-level variations to the telecommunicationsystem.

These articles have been prepared for publication byIr. F. Westerveld, farmer editor of "Philips Telecom-munication Review", whom we were pleased to have onour editorial board for the occasion.

The reader wil! also find in this issue a historicalintroduetion appropriate to the general theme, whichdescribes some important episodes in the early days ofmodern telecommunication engineering.