Chai, An-Ti.

K-State Research Exchange

A SINGLE-SOURCE TECHNIQUEFOR VACUUM DEPOSITION OF ALLOY FILMSbyAN-TI CHAIB. S,, National Taiwan University, I96Isubmitted in partial fulfillment of therequirements for the degreeMASTER OF SCIENCEDepartment of PhysicsKANSAS STATE UNIVERSITYManhattan, KansasA MASTER'S THESIS1966Approved by:Major ProfessortHCONTENTS^—'OOLUrr^.f-^^ PageINTRODUCTION 1REVIEW OF PREVIOUS WAYS OF PREPARING NEAR-STOICHIOFiETRIC ANDNON-STRATIFIED THIN FILMS OF InSb BY VACUUM EVAPORATION 1SINGLE SOURCE 2TWO SOURCES 2THE SINGLE SOURCE METHOD 3DESCRIPTION 3THEORY l^.RESULT AND INTERPRETATION 8CONCLUSION 11INTRODUCTIONIn preparing alloy films by vacuum evaporation, great difficulty is ex-perienced in controlling the composition of the film. This arises from thedifferent vapor pressures of the constituents. In the case of indium antimoni-de, for example, at 1200 *C, the vapor pressure of Sb in equilibritim vdth thestoichiometric liquid compound is about one hundred times greater than thatof In. Consequently, Sb is evaporated at a faster rate than In during the•evaporation; this affects the composition of the vapor, so that the film isof composition different from that of the starting material. If all of thestarting material is used up in the evaporation, the film will be stratified,consisting of a layer of Sb overlaid with In-Sb alloy, and finally almost pureIn on the surface. Various techniques for producing stoichiometric, non-strtified thin films of InSb by vacuum evaporation have been attempted, andwhile some of these are promising in principle, none has proved to be ex-perimentally feasible. The method suggested in the present thesis is boththeoretically and practically workable, and it is applicable to the vacuumdeposition of any binary or multi-component mixture.REVIEW OF PREVIOUS WAYS; OF PREPARING NEAR-STOICHIOMETRIC AND NON-STRATIFIEDTHIN FILMS OF InSb BY VACUUM EVAPORATIONTechniques which have been used to deposit films of InSb (and other com-pounds and alloys) in a vacuum can be divided into two catagories accordingthe number of vapor sources being used, namely, single source and two sources.2SINGLE SOURCE(1) Finely powdered InSb is supplied continuously by a mechanical devicesuch as a moving belt, to heated surface, from which it evaporates as it issupplied.This method has the disadvantage that the vapor is not of uniform com-position and that there is no way to reduce the rate of evaporation of a givenparticle. Also, there is danger of "spattering" of partially evaporated parti-cles from the boat, resulting in a deficiency of the less volatile component.Finally there is danger of introducing impurities in the process of grindingthe charge to powder.(2) A whole piece of InSb is evaporated in one evaporation and the filmis annealed after the evaporation.The films obtained by this method have layers owing to the changing con-stitution of the melt, hence of the vapor. Although the effect of annealingcauses recombination between In and Sb atoms, it is by no means certain thatthis recombination is complete. Moreover, the addition of another step (an-nealing) to the process is not particularly desirable.TWO SOURCESSeparate crucibles are used for In and Sb. The temperatures are adjustedto vaporize In and Sb at the same rate.This method should work in principle, but in practice, it is difficultto control the composition within the required accuracy. This is particularlytrue for semiconducting compounds, such as InSb, where the amount of the twocomponents must be equal within perhaps one part in 10^.3THE SINGLE-SOURCE METHODDESCRIPTIONThe method to be described here is essentially a single-source type eva-poration. The vapor source is composed of three parts: crucible, crucibleheater, and boat. Material to be evaporated is sealed in the crucible, and itis kept in liquid form by the crucible heater. The melt in turn is led to theboat by a capillary and is evaporated there. One type of vapor source can bespecified as the following;(A) CrucibleThe crucible is made of a material, such as tantalum or molybdenum,that is vetted by the melt. The top of the crucible is sealed by a screwed-onlid and the bottom terminates in a capillary leading to the high temperaturesurface from which evaporation takes place.(B) Crucible heaterIdeally, a heat shielded type crucible heater should be used to pro-vide uniform temperature throughout the cmicible, including the capillary partup to the opening.(C) BoatAn isolated-hot-zone boat is suitable. The boat is situated justbeneath the opening of the crucible. The crucible can be protected from theradiation from the hot boat by putting a radiation shield between them.The temperatures of the crucible and boat can be adjusted separately. Theboat temperature can be adjusted to a suitable temperature for evaporation.The rate of evaporation can be controlled by adjusting the temperature of thecrucible, since the rate at which the melt runs out through the capillary is4determined by the vapor pressure inside the crucible. The temperature of theboat is independently adjusted to a temperature sufficiently high that thesteady-state volume of the evaporant in the boat is much smaller than the vo-lume of the crucible.The following section will show that the constitution of the vapor thusobtained is exactly stoichiometric to the charge inside the crucible.THEORY '. H -The method under discussion is similar to the first method ("flash eva-poration" of powder) mentioned above, in that the feed to the boat is conti-nuous. A very small stream of melted InSb runs out continuously and uniformlyfrom the opening and is completedly evaporated from the hot region just beneaththe opening. Strictly speaking, however, the liqxiid is not flash evaporatedwhen it reaches the boat, but rather reaches a steady-state volvime such thatthe rate of evaporation is equal to the feed rate from the crucible.The evaporation rates E^^ and Eg^ of the two components In and Sb respec-tively, are related by the expression.• ^Sb ^Sb ^Sb K jWhere W's are atomic concentrations; P's, partial vapor pressixres; M's molecu-lar meights. In the low evaporation temperature range from lOOO'C to 1200*0,is lO"-'- to 1mm Hg; while P^^ is 10 to lO^mm Hg. Kj.^= 114.82, 121.76.Since the initial condition is W^^ = Wg^ = 50^, Eg^ is about a hundred timesgreater than E^^. The much faster evaporation rate of Sb causes the mixture onthe evaporating surface to become increasingly In rich; hence W^^ increasesand correspodingly decreases at the surface. But since the furnished melt5is always stoichiometric InSb, a steady state vrill eventually be reached suchthat Ej.^ = Eg^ = 1 . The rate of approach to steady state depends on the volumeon the evaporation surface and the rate of flow through the capillary. Denotethe concentrations of In and Sb by n^ and atoms /cm , with initial valuesn^Q and n^^ respectively. The evaporation rates are n^P^^ and"2^Sb*"^^^ ratioof P's is constant at constant temperature, hence we can write^In /^Sb = "10 "2s / "20 "iswhere n^Q+ ^20" "' ^"*^"is ^"^ "2sconstants. Since only one constantis required to define the ratio Pj^/^sb "® liberty to choose n^^ + n^^ = n.Then it can be shown thatJ'"10-"is) Vwhere V is the volume of the evaporation surface, v is the linear flow ratethrough the capillary, and A is the cross-sectional area of the capillary.Thus, it is seen that n^ and n^ approach the constant values n^^^ and n^^ astime t approaches infinity, and we have"is^In/n^^Pg^ = "10/^20 = ^as the ratio of the steady state evaporation rates as expected. The rate ofapproach to this steady state is an exponential function of vAt/V, namely, ofthe ratio of the total volume evaporated to the volume on the evaporationsurface. i . ^(1) E. B. Dale and A. T. Chai,'Bulletin of A. P. S., Series 11, vol. 10, niunber 3, 364 (I965)6Two effects that might effect the stoichiometry of the feed to the boator of the evaporant must be considered, namely, the effect of evaporationwithin the crucible, and the effect of diffusion from the boat back into thecrucible.Since the top is sealed, no vapor is lost except by evaporation from theboat at the end of the capillary. Thus the constitution of the melt can bedisturbed only by possible non-stoichiometric condensation inside the crucible.This does not happen, hov;ever, if the temperature of the crucible is uniform,because the composition of each phase in a closed system is constant at ther-mal equilibrium.As the concentration of In builds up on the evaporating surface. In beginsto diffusfe . back through the capillary toward the reservoir, and Sb begins todiffuse from the reservoir toward the evaporating surface. This diffusion flowis superimposed on the forced flow through the capillary and, if important,would prevent the establishment of a steady state condition. Let us consider aconcentrated layer with constant concentration at the end of the opening ofthe capillary. This is essentially a problem of forced diffusion from a cons-tant source into a semi -infinite body. The constant flow v of the melt in thecapillary can be considered as a drift velocity externally imposed on the dif-fusing In atoms, which diffuse from the region of high In concentration (name-ly, the evaporating surface) into the medium of stoichiometric InSb.Let N(x,t) be the number of diffusing In atoms (excess to 50/^ concentra-tion) per unit colume at the position x measured from the opening along thecapillary and the time t, D, the coefficient of diffusion. The initial condi-tion is simply N(x,0) = 0, and the boundary consition at the opening x = isN(0,t) = N*. The value of N* can be found by requiring a stoichiometxdc eva-7poration, that is,In ^In Sb,^n^ ^Sb' ^In^' ^Sb' ^^In^ ^Sbhence N'=5: - = 99/^ N*in bbwhere N* is the number of atoms per vmit volume in stoichiometric InSb. Thediffusion flow - D 8N/3x, together with the forced flow J^= - Nv which isin the negative X-axis direction, give the combined flowJ = - D aN/dx - NvThe equation o^ continuity offers a time dependent relation here, that isaN/at + aj/ax = oor aN/at = D a^N/ax^ + v aN/axThe boundary conditions areN(x,0) = 0, N(0,t) = N'(2)The solution is found to beN(x,t) = iN' exp(-v/D x)where erf(y) = 2^ jQexp(-z ) dzSince erf(-y) = - erf(y), erf(O) = 0, erf(oo) = 1, the foregoing equationsyield, as a limiting value,N(x,t) ^ lim N(x,t) = iN' exp(- v/D x) f(l+l) + exp(v/D x) (l+l)J., .= N' exp(- v/D x)As a typical esse, consider the evaporation of 300 mg InSb over a peirLodof 50 minutes. If the dimensions of the capillary are: Diameter = 0,0^^ cm,(2) B. I.'Boltaks, "Diffusion in Semiconductors", ,. • . •8Length =0.4 cm, then v is approximately 0.02 cm/sec. For the temperaturerange not too higher than the melting point of InSb (i.e., in the normal range2 Iof crucible temperatures), D is of the order of 10 cm /sec. Upon substitu-tion of all these numerical values into the expression N(x,t) = N* exp(-v/D x),it is seen thatK(x=0,'^ cm) ^ N* exp(-80) < N* 10"^^This indicates that with reasonably attainable parameters diffusion flow hasa negligible effect on the flow.RESULT Am INTERPRETATIONIn early experiments a quartz crucible with the top completely sealedwas used to determine whether or not the vapor pressure inside the crucibleis sufficient to push the melt out through the capillary. It was found thatmolten InSb does not wet quartz, and further that when the temperature of thecrucible is raised sufficiently so that the liquid is forced through the ca-pillary, the flow is always too fast for practical use. This is owing to theeffects of low friction and large angle of contact between the melt and thequartz. This discontinuity of flow can be completely eliminated by using atantalum crucible, which is wetted to some extent by In-Sb mixture.A crude crucible was made of tantalvira with the following characteristics(Two-electrode model): ' /1.The crucible V7as made of tantalum tubing with a wall thickness of about0.005".The capillary part was made by squeezing the bottom part of the crucible.3. The evaporating boat was formed by extending the squeezed part of the cru-cible and narrowing the section near the opening of the capillary.9k. A graphite lid was made to fit the top of the crucible .( The resulting sealwas not perfect.)In use, the crucible was maintained at uniform temperature of about 800*Cby passing current through it. The same current passed through the narrowpart near the opening of the capillary (i.e. the boat), maintaining it at asufficiently high temperature that the effluent evaporated rapidly.Using the above mentioned tantalum crucible as a constant vapor source,a set of five successive films was produced without changing the temperature.The total amount of InSb in the crucible was 270 mg. The following measure-ments of the films were obtained after they were annealed for a few hours at200'C:—1 3Film Type Thickness(A) Conductivity( ohm-cm) ~ Hall Coefficient(cm /coul)I.Not measurableII n 2600 3.8 11III n 1200 2.8 • . 36IV p 900 ' 2860 0.0095V Not measurableIt is normally expected that the first material evaporated will be Sbrich, because some time is renuired for the evaporating liquid outside thecapillary to come to equilibrium. It is also expected that the last part tobe evaporated will be In rich, because it contains the In-rich residue thatis left after the supply in the boat is exhausted. These effects are enhancedby use of a small crucible charge. This accounts for the radical non-stoichiometry of the first and fifth films. The measurements of the otherfilms, particularly the presence of a measurable Hall coefficient, indicates10that they are nearly stoichiometric. Thus, even though the crucible was notcompletely sealed, so that there was a chance for escape of non-stoichiometricvapor and that non-stoichiometric condensation might occur in the upper partof the crucible (which was a little cooler than the bottom) , and that somecontamination was probably introduced by handling the home-made crucible, themeasurements of these films indicated that the present method is practicallyworkable. The relatively low conductivities and high Hall coefficients of then-type Film II and Film III indicated they have a little excess of Sb whichserved as donor-type impurities. The high conductivity and low Hall coeffi-cient of the p-type Film IV indicated that near the end of the evaporation,the melt in the crucible was becoming non-stoichiometric with an excess of Indue to the Sb-rich vapor lost and the non-stoichiometric condensation at thetop of the crucible. , •It was expected that, if no non-stoichiometric vapor were lost and ifthere were no non-stoichiometric condensation at the top of the crucible, amuch better set of films could be produced. An improved four-electrode modelcrucible xcas designed for this purpose. It was made of tantalum, and was simi-lar to the two-electrode one except for the following differences:1. The top was sealed completely with a spot welder so that there was no vaporlost at the top.2. In extending the top of the crucible to the electrode, the part near thetop was narrowed such that the temperature of the top of the crucible wasnearly the same as that of the bottom. This reduced the non-stoichiometriccondensation inside the crucible greatly.3. Two more electrodes were connected to the narrow part at the opening of thecapillary. This provided an independent control of the temperatures of the11crucible and of the hot evaporating region.A set of eight films was produced with the four-electrode model crucible.The total amount of InSb was about 800 mg. The following measurements weremade after the films had been annealed for 12 hours at about 200° C.Film Type Thickness(A) Conductivity ( ohm-cm) ~ Hall Coefficient(cm-^/coul)I n 740 6.14 0.216II n 1550 1.98 137III n 960 6.3 I / 43IV* P 2600 39.8 1.8V n 2500 9.5 68VI n 1940 1.31 71VII n 2260 0.237 : 26VIII'Not measurable fEvidently, the above results offer strong support to the method suggestedhere.CONCLUSIONSince the whole argument is not limited to any particular compound oralloy, it is concluded that it is both theoretically and practically true thatthe present method can be used to provide a constant stoichiometric vapor sour-ce for any compound and alloy. It is also evident that a large amount of mate.*rial can be used at one evaporation and the rate and the temperature of eva-poration can be changed with little effect on the stoichiometry of the vaporexcept in the short duration of changing temperatvire.* Evaporation was stopped for a short time between films III and IV. This nodoubt explains the deviation of the properties of Film IV.ACKNOWLEDG£}^SI wish to thank Dr. E. Brook Dale, ay aajor professor, for his patiantgttldanoo and stimulating encouragement. Thanks aire also extended to J. W.Calvert, for his friendly help.A SINGLE-SOURCE TECHNIQUEFOR VACUUM DEPOSITION OF ALLOY FILI-ISbyAN-TI CHAIB. S., National Taiwan University, 196IAN ABSTRACT OF A MASTER'S THESISsubmitted in partial fulfillment of therequirements for the degreeMASTER OF SCIENCEDepartment of PhysicsKANSAS STATE UNIVERSITYManhattan, Kansas1966 ', ;It is well known that fractional distillation ofthe evaporant is thegreatest difficulty in making alloy films by vacuumevaporation. A single-source technique that avoids this difficulty is describedhere, and the resultof a successful trial of the technique is presented.The vapor source designed for this purpose consistsof three parts: thecrucible, the crucible heater, and the boat. The chargein the crucible iskept in the liouid state by the crucible heater. Thecrucible is completelysealed except for a capillaiy at the bottom which extendsto a small openingat the hot evaporating boat. The pressure inside thecrucible, which is con- .trolled by the crucible heater, can be adjusted to give an appropriateflowof charge through the opening. As the material in theboat evaporates, it isconstantly replenished by stoichiometric mixture from the crucible.It approa-ches a steady state composition such that the compositionof the materialleaving the boat, i.e., the vapor, is the same as that ofthe material flovdnginto it.Care is taken to keep the crucible at a uniform temperatureso that thereis no non-stoichiometric condensation inside the crucible. Thediffusionproblem is considered theoretically. It is shown that with practicallyattain-able capillary dimensions and flow rates, the effect of diffusionis negligib-ly small.The suggested technique has been tested by using a crude cruciblewhichhas the main characteristics proposed here. The results support thepresentmethod strongly.

A single-source technique for vacuum deposition of alloy films

http://krex.k-state.edu/dspace/bitstream/2097/26016/1/LD2668T41966C434.pdf

A single-source technique for vacuum deposition of alloy films

Abstract

Similar works

Full text

Available Versions

K-State Research Exchange