- Email: [email protected]
The chemical characterization of silicon-germanium thermoelectric alloys
475
During the 195C)‘s. the thcrmoclcctric propcrtics of semiconductors rcccivcd wide attention. A number of materials with outstanding thcrmoclcctric propcrtics wcrc considcrcd. among them PbTc, BiTc. SiMo. GcTc. SbTc. and SiGc. Although cuch of those has its advantages. the silicon..-germanium alloys cmcrgc as Lading candidutcs bccausc they opcratc in air as well as vacuum. they have outstanding mcchanical propcrtics. and they arc uscfuI at high tcmpcrnturcs’. Silicon.-ycrmnnium alloys containing bctwccn 60 and 80 at.-‘!,, silicon have roccivcd
;L major
share
of the attcntioii.
Phosphorus
is the most
common
Copant
(w.
in n-type SiGc. and boron (CW.0.1 wt.- I’,,) is the most common p-type dopant; in both casts. the sclcction of the dopant is lnrgcly dcpcndcnt on the solid solubility of the clcmcnt in the alloy. Production of homogcncous material is difficult, and the chemical cornposition is scnsitivc to process variables. Bccausc material pcrformuncc is strongly affcctcd by thcsc compositional variations, and. in addition. is highly dcpcndent on dopant concentrations. definition of the chemical compesition is important. This paper dcscritjcs analytical methods for dotcrmining silicon. germanium. phosphorus. and boron in SiGc thcrmoclcctrics. Other tcchniqucs for making thcsc dctcrminations have been reported in the litcraturc; the methods dcscribcd hcrc rcprcscnt original adaptions of standard or classical analytical methods to this problem.
0.3 wt.- :‘;,) used
CiERMANIlJM
Most chemical methods used for the nccuratc dctcrminotion of germanium in SiGc alloys require separation of germanium from silicon and from most dopants and impurities. Cheng and Goydish’ dctcrmincd germanium in pure SiGe alloys by volatilizing the silicon as the tetrafluoride in the presence of citric acid. evaporating the sample to dryness, and igniting to the oxide at SSO-900”. This technique is obviously limited by the impurity level of the material. Other separations, such as precipitation of gcrmanium as the sulfideand volatilization ofgermanium as the tctrachloride, arc suitable but require extreme care in order to achieve loo’%, separation and recovery. Current ion-exchange procedures for germanium involve adsorption of the chloride orlluoridc complexes from strong acid solutions; this requires ion-exchange appara.tus which is resistant to hydrofluoric acid. and presents problems such as loss of germanium by volatilization of the halide.
I’. .I. <‘ONl.
476
I<. Cj. IIOs(‘li.
l<. M. MI:liI
I). I:. WANNl:l<
A new analytical proccdurc for the dctcrmination ofgcrmanium in SiGc alloys. based on the titriltion of ;L gcrmunium munnitol complex-3 with sodium hydroxide Lifter au ion-cxchungc scpuration of the germanium. is proposed hcrc. The SiGc is dissolved hy alkali fusion. the solution is diluted to give a CYI. 1 M sodium hydroxide solution. and gcrniariiuni is clu;~ntit;~tivcly rctaincd on ;I column containing Anibcrlitc is subscXl.i-243 ion-cxchangc resin. Silicon is not held by thu resin. The gcrnianium qucntly clutcd with 1 M nitric acid and dctcrmincci by the mannitolsodium hydroxide titration. Although boron behaves in exactly the same mannaas the germanium. it is error of the dctcrminnn~>rnially prcscnt at 01. 0. I ‘I,, which is within the cxpcrimcntal Lion.
Kcrrgjlc~tlts. Ambcrlitc X It-243 ion-cxchangc rain (Kohm ancl I iaas C‘onipany. Ptiiladclphia) and gcrnianium nictal powder ( - 325 mesh. 99.999”,, purity. AIf InHcvcrly. Mass.) wcrc uscd. All other rcaycnts and chcmiculs wcrc of A. Ii. organics, quality. A/~j~ccr~f~,s. The borosilicatc glass ion-cxchangc columns wcrc 30 x I cm (i.d.) ;lnc1 each contained ;L co;1’rsc frittcd glass disc to retain the resin. am1 stopcocks to control the Ilow rate. Tlic g.crni~tniun~ -ni;innitol titrations wcru clone with a Sargent-Welch Rccording Titrator. Model DC;. with ;I Surgcnt combination glass calomcl clcctrodc to monitor the plH during thu titration. I’rcpowtio~~
drul
r.c~llcrler-trliorl
(jf’ iorl-c~.\-c.lrtrrl~j~l
co1irrm.s.
lkcausc
gas
cvolvcs
during lhc initial contact of the resin with conccntratcd acitl. ttic resin was cquitibratcd with 3 ILI hydrochloric acid bcforc being put intocolumns. The r&n was back-washed with water in the column and ailowccl to scttlc uncicr ;I slight back-pressure of wutcr. 1 lydrochloric acid (50 ml of 3 h/f) wus pusscct through the column. followed by water until the cfflucnt was neutral. An equal amount of 3 M ammonia solution was then passc~l through thu column. followed by flushing with water until the cfflucnt was ncut rul. The same proccdurc was used in rcgcncrating the columns after cnch soparat ion proccd urc. The coliiiiins wcrc run at n7;ixiniuni flow rate (2 -3 ml niin ‘) during the prctrcatnicnt und rcgcncration steps.
Weigh ;I sample containing 30..60 mg of germanium into ~1zirconium crucible along with 2 g of sodium hydroxide pellets. Cover the crucible with u nickel lid. put on ;I hot plate -at room tcnipcraturc, and heat to the maximum temperature of the hot plate (550”) until all visible reaction c~ascs. (Caution : The fusion of SiGc alloy rclcascs hydrogen gas which may ignite violently if the initial fusion is done over ;I flame.) Then heat the crucible over ~1 Mckcr burner until ;L clear melt is obtained. Dissolve the product ofthc fusion in water. dilute to CU. 50 ml. und transfer to the ion-exchange co!umn. Pass the snmplc throu& the column at u flow ri.ltc of l-2 ml min _- I. Wash the column with 50 ml of 0.1 b2 sodium hydroxide at the same flow rate to clutc the remaining silicon. and then wash with water until the cfflucnt is ncutrat. Elutc the germanium with 100 ml of (1 + 9) nitric acid at the same flow rate. f~llowcd by 50 nil of water. Neutralize the cftlucnt with 50”,, sodium hydroxide. :lr1rt/. (‘Ilirtl. ./101,/.hl ( 1971)
. ANAI_YSIS
OF
SiCic
SEMIC’ONI~~_IC’TOKS
477
acidify to a methyl red end-point with the nitric acid. and boil for Z--3 min to rcmovc any dissolved carbon dioxide. T’itratc the germanium with ;I 0.02 RI sodium hydroxide solution. standardized by titration of pure germanium metal dissolved in the same manner as the sample. C’ontinuc the titration past the first end-point (neutralization of strong acid). add a large cxccss of munnitol (w. IO g), and continue the titration to a second end-point. The volurnc of tiirant required botwccn the first Rand second end-points is used to calcuIutc the amount of gcrrnanium in the sample.
Ambcrlitc X t:-243 was first synthcsizcd by Lyman ami Prcussd by aminating a chlororncthylat~tI styrcnc divinylbcnzcnc copolymer with N-mcthylglucamitic for USC us a boron-specific ion-cxcharigc resin. The boron specificity was based on the reaction bctwcen boric acid and the N-mcthylglucaminc group. ~1polyhydric alcohol. A well-known analytical reaction of boric acid with a polyhydric alcohol is that of boric acid with mannitol. resulting in ;I stronger acid which cm1 bc titrated with sodium have chuructcrizcd Ambcrlitc XE-243 with respect hydroxide Suvcral invcstigutors’*” huvc used the resin in the separation and to its intoructions with boron. and others-*’ dctcrmination of boron. No study us to the actual boron spccilicity of the XE-243 resin was avuilnhic. Since the chcmlstry of germanium with rcspcct to reactions with polyhydric alcohols is very similar to that of boron. the applicability of X1-243 to gcrmanium chemistry is not surprising. Although this work was not intcndcd to provide a comprchensivc chaructcrization of an ion-cxchangc rain. an effort was made to dctcrminc the resin capacity and the column breakthrough capacity for the system being used. Table i contains SOIIX data obtained by batch cquilihration methods in I M sodium hydroxide solutions. The utt~ounts of germanium indicated wcrc shukcn with I g of X k-243 resin in 50 ml of I M sodium hydroxide for 4 h. An aliquot was rcmovcd and the gcrmunium remaining in solution was dctcrmincd by the gcrmaniuni--munnitol titration; the amount of gcrmanium on the rain was determined by diffcr’cncc. The batch cxpcrimcnts wcrc done with : (LI) the resin as rcccivcd. (h) resin which had been prctrcutcd by equilibration with both 3 M hydrochloric acid and 3 M ammonia solution. as described previously. and air-dried. (c.) resin prctreatcd as described in (h). and then vacuum-dried at room tcmpcruturc for 18 h. From the data in Table I. it is apparent that the resin capacity is an TABLI:
I
(;I:l
(‘AI’,\C_‘ITY
01
7x.4 136.5 73.7 5.3.’ 91.5 63.X Sl.5
,\hll3I:.HLITli
XI:-243
IN
I AI SOIIIl:M
t l\‘l)HO)r;ll-)li
in solution. the grcatcr the equilibrium property, i.e.. the grcatcr the ion concentration quantity rctaincd by the resin. I-Iowcvcr. the data do indicate an approximatccapacity of 0.142 mmolcs ol’gcrmanium per gram of resin for the conditions used in the anulytical proccdurc. Similar cxpcriments done in 0.1 M sodium hydroxide show that the capacity of the resin for germanium almost doubles. Also of intcrcst is the indication that the prctrcatmcnt of the resin with strong acid and base solutions dots not significantly change the capacity of the rain with rcspcct to germanium. The brcakthrouyh capacity of the columns used in this work was dctcrmincd by passing a 1 M sodium hydroxide solution containing I mg ml I of gcrmunium through the column and chocking the cfflucnt for gcrmunium by atomic absorption tcchniyuus. For the 30 x 1 cm (i.d.) columns at LI flow rate of 2 ml min ‘, the brcakthrough capacity was found to bc 75 mg of germanium. * Thcaccuracy ofthc method is limit4 by the gernianiunimannitol titration and the manipulation of the sample. The ion-cxchungc scparution of germanium and silicon has been shown to bc quantitative by atomic absorption tcchniqucs for qualitative unulyscs of the cfilucnts. The pcrccntagc rccovcry of germanium from the column. for put-c germanium standards. was found to be I00 + 0.3 I,;;‘, which is the sarnc order of rccovcry obtained by fusing germanium in sodium hydroxide and titrating the sumplc directly. Typical rcsuits obtained on a sample of SiGc alloy with a nominal germanium content of 59 76 (by weight) wcrc 58.X6+0.09 ‘,!<,with a maximum spread of 0.27 0,; fat three dctcrminations. iiitcrfcrcnccs in the systems of intcrcst. Si--C&-P and Si---Gc-H. arc limited to boron which is rctaincd on the ion-cxchungc column and clutcd with the gcrmunium. The nominal boron content of the Si-Gc--B system is (*(I.0.1 ‘x,; the intcrfcrcncc caused is within the cxpcrimcntal error of the germanium dctcrmination and. in most instances, would bc ignored. In work demanding highest possible accuracy or involving samples containing higher boron contents. the boron may bc dctcrmit1c.d us described on p. 481 and the germanium results adjusted appropriately. SILIC’ON
The gravimctric dctormination of silicon by dehydration of silicil with pcrchloric acid and subscqucnt volatilization of the silicon with hydrofluoric acid is a Howcvcr. the application of this classic4 method time-honored analyticit method”. to silicon-germanium alloy is not mcntioncd in the litcraturc. A variation of the method. which climinatcs any possible intcrfcrcncc by germanium. is reported hcrc.
Procvtltrre. Samples in cithcr powder or chunk form may bc used in this analysis. Howcvcr. powder allows the fusion step to bc complctcd more rapidly. A sample is taken which will yield CU. 0.2 g of silica. Weigh the prcparcd sample into a Zircaloy crucible and add CU. 10 g of sodium hydroxide pellets. Cover the crucible with a nickel or other suitublc cover and fuse as dcscribcd in the procedure for germanium. Cool the melt and dissolve in distilled water. The final volume of the solution, following washing. is CCL200 ml. Add 100 ml of conccntratcd hydrochloric acid (AR) carcfully to the solution. and boil to rcmovc the germanium as the chloride. When the solution is rcduccd in volume to CU.200 ml. add 70 ml of 70 yj,:pcrchloric acid and cvaporatc to strong fumes.
ANALYSIS
OF
SiGc
SEMICONDUC’TOKS
479
(Caution: The final strong hcnting of the acid must bc done in a special pcrchloric hood and cart taken to avoid spattering of the salts.) After cooling. dissolve the salts in distilled water. Filter the dchydrutcd silica. fire in a wcighcd platinum crucible. cool. and rcwciph to obtain the weight of silica plus impurities. Then volatilize the silica by adding X ml of 4X’:;, hydrofluoric acid plus 3 drops of conccntratcd sulfuric acid to the crucible. and evaporating to dryness. Fire the crucible. cool and rcwcigh. The diffcrcntial weight is the weight of the SiO,.
Standards. as such, for SLGc alloy arc noncxistcnt. Howcvcr. alloys made to have identical concentrations of silicon wcrc analyzed by the above method. and the results indicated that the errors in duplicate analysis wcrc far less than the rcproducibility of the manufacturing process. A scrics of duplicate analysts wcrc run on 14 samples of Si-Gc alloys formulthe alloy can bc produdcd atcd to contain (*(I.61 wt.- “,,‘,,silicon. The results indicated within a limit of 60.89kO.52 wt.-‘?;, silicon. Howcvcr. the maximum deviation of duplicate analysts was only t 0.1 X and the mean deviation for the duplicate runs was 1_0.10. The classical pcrchloric acid method for silicon is gcncrally acccptcd to bc more accurate than is indicated by the analytical results shown above. The limiting factor in the accuracy of SiGc analysts is probably inhomogcncity within a particular sample. Dctailcd studies of homogcncity. by this mqdificd proccdurc. showed widely variable compositions in slices taken from a single zone lcvclcd rod or from a sin@ hot-prcsscd piccc.
After a brief litcraturc survey rclutivc to the dctcrmination of phosphorus used as a dopant in semiconductor and thcrmoclcctric mutcrials. it was apparent that most investigators (eg. O’Connor’“) had used neutron activation tcchniqucs. Polarographic’ ’ and spcctrophotomctric methods based on molybdenum blue” arc dcscribcd for materials such as CdSiP, and CdGcP, which contain phosphorus as a mzi_jor constituent. III attempting to adapt thcsc tcchniqucs to the SiGc system. problems such as time rcquircd for analysis and the chemical behavior of the SiGc alloys bccamc apparcnt. and a diffcrcnt approach was sought. Hcrc. a spcctrophotomctric method for the dctcrmination of phosphorus based on the reaction of molybdatc with a solution containing vanadatc and phosphate, for use in dctcrmining phosphorus dopant first reported by Misson “. is proposed lcvcls in SiGe alloys. Commonly dcscribcd as the molybdovanadophosphatc method. the method has been applied to phosphorus dctcrmination in materials such as fcrtilizcrs’J. iron ore and plain carbon steels’“, copper-base alloys’6. and titanium and dcscribcd by zirconium and their alloys I’. The method hcrcin is basically a procedure Elwell and Wood” for determination of phosphorus in titanium alloys which has been modified for USCin SiGc systems. It has been shown to bc rapid and of sufficient accuracy for phosphorus dopant levels ranging from 0.01 to 0.55’;/, by weight in a SiGe matrix.
rcagcnls and chemicals used in this work wcrc 0fA.R. quality and 110 fitrfhcr purification. Apptr1wf14s. Absorbance tmxtsurctncnts wcrc made in 5-cm &ckman Neat lnfrarcd Silica cells with a l~cckman OK-2A lialio Recording Spcctrophotornctcr using ;L tungsten or dcutcrium source and a photomultiplicr dctcction system. P,*eptr,.trf ioil o/‘t~ltrrk trrut srtrritl~rrti c14r13~‘. Add 12 ml of ( I + I) sulfuric acid to ;i 250-ml volumetric Ilask and dilute to (~1. lo0 ml. Add 3 ml of aqueous 0.5 I’,, (w/v) potussiuni pcrnianganatc solution, followed by sufficient aclu~ous 0.3?;, (w/v) sodium nitrilc solution to rcducc the pcrmunganatc. Add 10 ml of LO’,!;,(w/v) ammonium tartratc solutiot1 and dilute rhc solution to 250 ml. To IO0 ml of this solution (pipcltcd) add 5 ml of 0.25 ‘;:, ;immoniun~ vanadatc solution in 2 I>;,nitric acid, followed by 5 ml of ill1 ~lc~llcolls 20 ‘.:,,, (w/v) solution of (NI-1J),,Mo,013.4H20. Allow to slami for I5 tnin and then USC 11s the blank for the absorbance mcasurCmcn& Obtain the standard phosphorus curve from solulions prcpurcd in an identical m;~nncr with the cxccption of the addition of known an1ounIs of phosphorus. Add the phosphorus ~1s ;I standard solulion of potassium dihydrogcn phosphate hcforc the addition of pcrmanganatc. Mcasurc tt1c absorbanccs of thcsc solutions rulativc to the blank solution at 400 nm in 5-cm cells and ;L color dcvclopmctlt time of 15 min. ‘I’wrtnrrt~f of’strr~~ple. USC sufficient sample to provide 0.2. 0.4 mg of phosphorus. Dissolve the sample in a 70-ml plutitium dish by addirion of 04. 4 ml of (I .-t I) nitric acid, followed by a dropwisc addition of 48 ‘,!,;;hydrofluoric acid until dissolulion is complctc. Add 2 ml of 48 y,‘, hydrofluoric acid in cxccss, followed by 12 ml of (1 -t I ) sulfuric acid and cvuporatc to fumes of sulfur trioxidc. Add 5 ml of wutcr and transfct the solution to ;L bcakcr with 20 ml of (I -t- I) hydrochloric acid. Evaporate to fumes 01 sulfur trioxidc, cool. dilulc with 25 ml of water and heat to boiling. Add a~. 3 ml of aqueous 0.5 ‘f;, (w/v) potassium pcrmang:anatc and boil for I --2 min ; aid sodium nitrite solution dropwisc until the pink color disappears. Transfer the solutions to 250-ml volumetric flasks containing 100 ml of water and 10 ml of the above ammonium tartralc solution and dilute to volume Pipctlc IO-ml aliyuols into plastic bcukcrs. and add 5 nil cuch of the above-mcntioncd vanaciatc and molybdatc solutions. Aftcr IS min. mL’asurc the absorbance at 400 nm OS.a blank prepared at the same time as the unknown solutions. Establist1 the amount of phosphorus from the standard phosphorus curve. 1Zw~g~~f1t.s.All
wcrc used with
In it study of possible intcrfcruncc it1 the tnolybdov~~nadopt~~~sphatc: tncthod. K itson and Mellon ’ ’ found that small amounts of silicate do not intcrfcrc. In the present work. it was shown that germanium causes high results for phosphorus. The rcrnov:Jl of the silicon-gcrm~~nium matrix by distillation of the tctrafluoride and tetrachloride as dcscribcd in fhc Sample Trcatmcnt scclion was shown to bc quantitative in the cast of gcrrnanium; however. tract amounts of silicon remained in solution, probably because of a slight attack on the glassware used after the distillation of silicon tctrafluoridc. The color dcvctopmcnt time of 15 min was suggcslcd by Elwcll and Wood ” and is consistent with minimum time of analysis. In this work. a decrease in pcrccnt transmittance (T) of the solution with time was obsurvcd. Solutions in thu range of 40-
50 ‘,‘j,,, T showed a dccrcasc of (*(I.2.2’,‘.;; T over the period 15-75 min after color dcvclopmcnt. 011 a standard phosphorus curve prcparcd by making mcasurcmcnts 15 min after color dcvelopmcnt, the chnngc corrcspondcd to an incrcasc of 0.015 mg phosphorus. At this rate of change, the change occurring during the absorbance mcasurcmcnts of a scrics of four solutions is ncgzligiblc. The phosphorus blank resulting from rcagcnts and glassware was dctcrmincd by carrying a blank dctcrmination through the cntirc procedure atid measuring ths abprcparcd as dcscribcd above. sorbancc of the resulting solution VS. a blank solution The phosphorus blank was found to bc 0.01-0.02 mg of phosphorus. The accuracy of the method as applied to SiGc alloys is somcwhut difficult to dctcrminc since true standards arc not available. Secondary standards can bc prcparti by addition of known amounts of phosphate to undopcd SiGC alloys. or by addition of phosphate to mixture of silicon and germanium oxides. This can only approximate the actual matrix ~1s SiGc alloys contain phosphorus in the cIcmcnta1 form.
Indcpondcnt
analysts
done by neutron
activation
sumplc which. Table II show
by the proposed method, was found the accuracy obtainable m standards
I’tlOSI~tIOl
C’ON’I‘t:N’I’S
Si Si Si Si Si Si
Ck CA Cic Cit. P CA2 I’ Cic 1’
0.00 0.00 0.00 0.50 0.30 0.x
IX ANALY’I’IC’AL
t
yicldcd a value of 0.42 7’;;on a to contain 0.49’:;,. The data in prcparcd by addition of known
ON
SiCic ALLOYS
0.100
0. IO 1
O.‘OO
0.207 0.307 0.480 + 0.003” 0.31~_+0.0I I” 0.50X + 0.006”
0.300
amounts of potassium dihydrogcn phosphate to undopcd SiGc alloys. the precision obtainable using the method. and the accuracy of the method relative to the nominal phosphorus content of the material being analyzed. In the range 0.2-0.5 ‘x, phosphorus. the general spread in the data obtained by multiple analysts was 0.01 ‘x, or Icss. The pcrccnt rccovcry for phosphorus added as phosphate to undopcd SiGc alloy was 100 +- 5’,‘;, within the range O.l---0.3 mg of phosphorus.
for the determination of boron in a variety of matrices. The analysis of steel for boron has been well documented lt( - 20. Boron has been dctcrmincd in silicon by d.c:arc excitation”. Zil’bcrshtcinzz conccntratcd boron from high-purity silicon dioxide with hydrofluoric acid. and retained the boron with mannitol for a.c. arc analysis of the dried rcsiduc on graphite ekctrodcs. The boron content of carbon and graphite has been reported by Feldman and ElknThere
arc many
spcctrochcmical
methods
_.
1:. J.
482
C’ONl(A.I).‘lI.
oSC’H.
I<. M. M1:I~Iil1.1_.
II. I:. WANNt:I<
burglA who used ;Ld.c. arc with carbon clcctrodcs or a siftcr clcctrodc system with hiphvoltugc spark excitation. A.S.T.M. emission methods for the dctcrmination of boron in 14 dif!‘crcnt mutriccs arc available Ix . Howcvcr. thcrc arc no reports on the applicution of’ this tcchniyuc to silicon--germanium alloys. Methods which fill this nc:cd arc j>rcscntcd hcrc. Intcrcst was divided bctwccn two’ lcvcls of boron doping which dcmundcci somewhat difl‘crcnt spcctrogruphic tcchniqucs. Thcrcforc, two analytical methods. one of which is ;I solution tcchniquo applicable to doping lcvcls bctwccn 0.1 and 0.2 wt.- ‘I,, applicable to lowcr boron concentrations. boron, and the other, ~1powder tcchniquc, wcrc dcvclopcd. No spectrographic standards for the SiCiclS system wcrc available and, bccuusc most quuntitativc spcctrochcmical methods arc based on standards, it was ncccssary to use ;L method whcrc synthetic standards could provide rcliablc analytical results for boron. A solution proccdurc with a carbon rotating tiisk clcctrodc was chosen liar highly dopod tnatcrial bccausc: (tl) this laboratory has cxtcnsivcly used rotating disk solution proccdurcs for many matrices; (/I) synthetic standards could bc prcparcd in ;I suitublcconccntration range; (L’)an added internal standard could bc used since ncithcr silicon nor germanium would bc at ;Lconstant pcrccntagc; (tl) the inhcrcnt higher prccision of the high-voltage spark source should result in a butter ovcrull method. For the lower doping Icvcls. it was apparent that the sensitivity of the rotating disk spark proccdurc was inadcquutc, hcncc &fort was dircctcd towards ;I powder tcchniquc which used an ignited ;I.c. arc I’or excitation. It was considered impracticut to make synthetic standards for the pow&r method by mixing powders of silicon, germanium and boron ; thcrcforc. a boron-doped SiGc alloy charactcrizcd by the solution method. was “diluted” by additions ol’ n-type (boron-free) SiCic semiconductor material to produce powders with ;I range of boron contents. Carbon was added to these standards to make thcni cIcctricitlly conductive. ’
A/J~LIIYI~IHattti ~~~rgclc’rlts. ‘I’hc spectrograph used for both of thcsc boron methods Ebcrt witha gratingof 15.0(M)lines/inch. Elcctrodcs wcrc National C.‘arbon : L4072 (ASTM D-2) rotating disk and L4006 cupped Iowa clcctrodc. The photographic plates wcrc Eastman Kodak 111-Ofor the solution proccdurc and SA # I for the powdc~ method. D-19 Dcvclopcr was used in proccssing. Carbon for the powder method was obtuincd from the Ultra Carbon Corporation (UCPZ 100). A .larrcll-Ash Model 23100 Microphotomctcr was used for obtaining line transmittances. All chemicals used wcrc of A. R. grade or bcttcr. liottrthg tlislc sohtior~ .Spl/ri
wasa
3.4-m
ANALYSIS TABLE
OF
SiGc
483
SEMICONDUCTORS
;I1
COMPOSITION
OF
SOLUTION
STANDAKDS
Si
.-
(4
FOR
BORON
M (O.OOI !, ,?I/
‘I;, GP
Gr
7x.01 7l.YS 65.05 62. I7 69.Xx
’)
“,, f3
(Id)
(H)
0.07805 0.07 195 0.06505 0.06205 O.OhYYS
ANALYSES
. 3.0 3.5 6.0 X.0 I .o
21.71) 27.7 34.35 37.02 30.02
0.02 I 8 0.0’77 0.3435 0.3695 0.03005
0.20 0.35 0.60 O.HO 0. IO
_
acid. Place a tight fitting 2 ml of conccntratcd nitric acid and 1 ml of 4X Y/i,hydrofluoric plastic lid on each beaker and heat the contents over a steam bath’. Add more nitric and hydrofluoric acid as required to dissolve the silicon and germanium standards. Add an aliquot of a standardized boric acid solution to each boakcr (xc Table III). and transfer the sample to a 100-ml plastic volumetric flask for final volume adjustmcnt. Dissolve 0.1-g snmplcs of the SiGc alloys using the same procedure. Transfer 20 ml ofcach standard or sample to 1-CMdropping bottles to which 1 ml of nickel intcrnul standard solution (($05 g Ni ml - ‘) was added. (Nickel was chosen as an internal standard since it was not a normal impurity in this material, and a nickel lint was within 10.0 nm of the boron lint of intcrcst.) Record the spectra in triplicate for each standard and sample using the conditions summarized in Table IV. lJsc the TAl3LE
IV
1NSTRUML:NT
PARAMETERS
EMISSION
SPI:,C”rROSCOPY
(Spcctrogruph 3.4-m Ibert. lS.OW l/in. (2nd order): slit width. 335.0 nni: filter ‘-step. pcrccnt transmission. 100;12)
-_
._
_
.___..
_
II V spark Capacitance. /IIInductance. jt1.l~ Rosistuncc, ohms Voltage. primary. V Hrcaks per half cycle Power setting, spark Power setting. urc RF current, A Electrode. upper lower Prchurn. sh Exposure. sh Emulsion type Dcvclopcr D- 19. min’ Stop bath (acetic ucid). s Fix. Kod Rapid. min .-_...._-_- ..-..-. ---------.-...._.. ” National h Controlled ’ Nitrogen
O.W75 IS5 0 220 6 6 7 6” x 4” ‘l ASTM D2 8 5x 111-O 4 20 2 . _._.____.._____._____ ._
Carbon Rod. SPK L3828, by stopwatch. burst, 3 s on. IO s off.
rounded
end.
0.005 625 0 220 7 6 6 17.5 L3955 L4W6 21 3x SA#I 4 20 2
40 pm. . spectral
range
(2nd order).
210.0-
I:. J. C’ONKAID.
484
K. Ci. l>OSC.‘tl.
K. M. Ml:.KI
I>. Ii. WANNI:K
two-step iron n~clhod li)r emulsion calibration with II prclitninary cclrvc’ H*‘J. (>)bt:lin the transmittances of the boron 249.67%nm lint’“, adjacent background. and the nickel 259.590-nm lint and detcrtninc the backgroltnd-corrcctcd boron-to-nickel rclativc intensity ratios from the ctnulsion calibration curve. Plot thcsc ratios against boron concentration on log- log graph paper to product Ihc analytical curve. C)btain the boron concentrations for the unknown samples by applying the ratios to the analytical curve. I(gcl,liietl U.C. III’C powder tnefhotl. A spark-ignited 1 10-V a.c. arc is used to cxcitc powdercd standards and samples packed in the lower cupped graphite clcctrodc countcrcd with a rounded end yraphitc clcctrodc. Spectra of standards and samples arc obtained on photographic plates. Transmittances of the boron lint. acl.jaccnt background, and the itluminutn internal standard lint arc used in establishing the analytical curve. Slarttlard trrd scmple prcpwutiorl (powder mctlroti): USC two SiCic powders (200 mesh), one boron-free. the olhcr containing 0.14’%, boron, to prcparc standards. Mix each powder with 9 parts of carbon (containing I ‘x, alumina for use as an internal standard). Mixtures of thcsc two base powders result in a set of standards for boron in the matrix of intcrcst (Table V). Powder solid samples by crushing in a Plattncr mortar and pcstlc, sicvc through 200-mesh nylon screen, and mix I part ol’sumplc to 9 parts of carbon (containing I ‘A’,alumina). All mixing was done in 2-in. high. 0.5-in. diamctcr lucitc vials with a 0.375-in. diumctcr lucitc ball. Mixing time was 2 min in a Wig-L-Bug. Pack the clcctrodcs by pressing the cupped portion uf the clcctrodc into the powder. Use graphite with a rounded end as the counter clcctrodc. &cord spectra of standards and samples in triplicutc, following the conditions outlined in Table IV. Culibratc the emulsion as outlined for the rotating disk. Obtain transmittances of the boron 249.67% ntn lint, adjaccnl background. and the aluminum 256.799-ntn lint for subscyucnt conversion to corrcctcd rclativc intensity ratio of boron to aluminum. Conversion of rclativc intensity ratios to boron concentration is similar to that ouflincd above. l’Al3LI?
SrtrtdtrtYl
v
110.
Borvtl (pctr’fs)
I3 I4UO w30 13700 13J60 l32xo I3140 I370 I328 B B:1sc _~ .
Rrstrlts
I 2 I I I 1 I I 0
hsc
UOWl-jkl~ hsc
0 I 1 2 4 9 I ‘1 49 I
I:,, 13
(pcuvs)
0. I4 0.09.1 0.070 0.046 o.o,g 0.0 I4 0.007 0.007x 0.000
~uul tiiscmsic~r~
Table VI cotnparcs the intended concentration of boron and the results by both tnethods. From this comparison and the pcrccntagc differences. it may bc concluded that the rotating disk sp::rk proccdurc yields results within +_5 ‘x, in the range O.l-O.S’x,. For the ignited a.~. arc powder method, the reproducibility is cu. + 10(x, in the
ANALYSIS
OF
SiCic
SkMICONDUCTORS
485
range 0.002--0.09 ‘.t;,.The accuracy of the mclhods could not bc cvaluatcd owing to the lack of primary standards. The solution spcctrochemical proccdurc yicldcd adequate precision and scnsitivily for boron in the range O.l-0.8’:;, (by weight). The solution standards were mudc to contain varying amounts of silicon and germanium which scrvcd two purposts: to dctcrmine if the boron response was influcnccd by the matrix. and to make possible simultaneous analysts for silicon and germanium. if dcsircd. When the boron concentration was lowered. it was necessary to USCa higher sensitivity source to retain rcasonablc precision. A moving plate study was made to compare the ignited a.~. arc burnout of aluminum and boron with time. The relative intensity ratio of boron to aluminum was csscntially constant from 20 s-to 60 s; thcrcforc. a 20-s prcburn was chosen. Aluminum was substituted for nickel for internal standardization in the more scnsitivc method bccausc boron and aluminum have evaporation rates more similar than those of nickel and boron (this is not significant in solution analysis). SUMMARY
The pcrformancc of thcrmoclcctric devices is sensitive to the chemical composition. the dopant concentration and the uniformity of the semiconductor materials used in their fabrication. Analytical methods for the determination of silicon. germanium. boron, and phosphorus in SiGc thermoelectric materials are described. These methods were dcvclopcd bcca’use of the fragmentary data available in the literature regarding their chemical characterization. The methods are rapid, accurate, and. with the exception of boron. rcquirc a minimum of cxponsive apparatus. Germanium was determined acidimetrically by means of the mannitol complex after ion-exchange separation on Ambcrlitc XE-243, silica by the conventional gravimctric method, phosphorus calorimetrically as the vanadomolybdophosphate, and boron by emission spectroscopy.
On dkcrit dcs. mL;thodcs d’analyse du bore et du phosphore dans Ic materic
pour Ic dosage du silicium. du germanium. thermo&.lcctriyuc SiGe. Ellcs sont rapidcs,
F. J. CONRAD.
486
R. G. DOSCH,
R. M. MERRILL.
D. E. WANNER
du bore, elles ne nCcessitcnt pas un appareillagc coQteux: pr@ciscs ; et, ti l’cxccption Lc germanium est do& acidimktriquement, en prCsence de mannitol, apres skparation sur Ambcrlitc XE-243; la silice par mCthode gravimCtrique conventionnelle; le phosphore colorimCtriquement comme vanado-molybdophosphatc et le bore par spcctroscopie d’bmission. ZUSAMMENFASSUNG
Die Leistungsftihigkcit thermoelektrischcr Bauelcmcntc htingt von der chemischcn Zusammensctzung, der Dotierungskonzcntration und der Gleichfiirmigkeit des Halbleitermaterials ab. dass bei der Herstellung vcrwendet wird. Analytische Methoden fiir die &stimmung von Silicium, Germanium, Bor und Phosphor in thermoclcktrischen SiGc-Materialicn werden bcschriebcn. Dicse Methoden wurden wegen der unvollstiindigcn Angabcn entwickelt, die in der Litcratur beziiglich der chemischcn Charakterisierung zu linden sind. Die Methoden sind schnell. genau und erfordern, van Bor abgcsehen, tin Minimum kostspieliger Apparaturen. Germanium wurde mit Hilfe der Mannitkomplexe nach Ionenaustausch-Abtrcnnung an Amberlite XE-243 acidimetrisch bestimmt, Siliciumdioxid nach dcr herkiimmlichen gravimctrischcn Methode, Phosphor kolorimetrisch als Vanadatomolybdatophosphat und Bor durch Emissionsspektroskopic. REFERENC:ES
1 F. D. Rossi;~hc~moclcctricity and Thermoclcctric J’owcr Gcncration, So/iti .S~rrrclElccrrorrics. Vol. I 1, Pcrgamon J%ss, 1968, pp. 833-868. 2 K. L. Chcng and J3. L. Goydish, hd. Chern.. 35 (9) (1963) 1273. 3 H. J. Clulcy, A~~ctlyst. 76 (1951) 517. 4 W. Lyman and A. Prcuss, U.S. Put. 2. 813, 838. Nov. 19. 1957. 5 R. Kunin and A. F. Prcuss, 111rl. EII~. C/IWI. Prod. Kcs. Dewlop., 3 (1964) 304. 6 F. J’inon. J. Dcson and R. Rosset. Rltll. Sot. Chim. Ft... 8 (1968) 3454. 7 R. M. Carlson and J. L. Paul, Amd. Chem.. 40 (1968) 1292. 8 R. M. Carlson and J. L. Paul, Soil Sci., 188 (4) (1969) 266. 9 H. Furman. Stcmhrtl Metlturls oj’Cltettu’cct1 Attttlpsis. Vol. 1. 6th Ed.. 1962. p. 955. IO J. O’Connor, Sci. Twlr. &rasp. Rep.. 2 (6) (1964) 737. 11 R. Yu. Lyalikovu. Ztruotl. Lob.. 55 (4) (1969) 431. 12 Ji. Yu. Lyalikova. Zh. Amrl. Khittt.. 23 (81) (1968) 1240. 13 G. Misson, C/I~~~I.Ztg.. 32 (1908) 633. 14 W. C. l-l:~nson. J. Sci. Food Agr.. I (1950) 172. 15 I<. E. Kikmn and M. G. Mellon. Id. Ettg. C’ltm. Atutl. Ed.. 16 (1944) 370. 16 W. T. Elwcll and t-1. N. Wilson. Atrdyst. 81 (lY56) 136. 17 W. T. I~lwcll ;1nl1 D. I;. Wood. The Atttrlysis o/ Tifcrttiuttr. %ircrmitrttt cttttl T/tpir Alloys. Wiley. iY61. 18 AS-I’M. r\~lrt/tot/s,/i~r Etttissiott S/‘c’ctroc/tctttic,~t/ Attct/>wis. 6111 Ed., 107 I, pp. 40-l 63. 19 C. 1~. Hines and J. K. Hurwitz. Appl. Specrrosc.. 21 (1967) 277. 20 J. Goryczka and W. Klimccki. Pr. Inst. Htcttt.. 18 (1966) 45. 21 A. N. Shtcinbcrg. Tr. Inst. Met. Akutl. Nmrk SSSR. 11 (1962) 22Y. 22 K. I. Zil’bershtein, 0. N. Nikitina and M. P. Semov. Poltrch. Atrctl. Veshc/tes~t~ Osohoi C/list. M&w. Vsrs. Kottj:, Gorky, USSR, 1963. pp. 139-141. 23 C. Feldman and J. Y. Ellenburg, Ad. Cltettt.. 27 (1955) 1914. 24 J. R. Churchill, ltd. Atut/. Cheer.. 16 (1944) 653. 25 MIT Wutxlettgtlt Tulles. The M.I.T. Press. 1969. Awl.
C/tint. Acta.
61 (1972)
During the 195C)‘s. the thcrmoclcctric propcrtics of semiconductors rcccivcd wide attention. A number of materials with outstanding thcrmoclcctric propcrtics wcrc considcrcd. among them PbTc, BiTc. SiMo. GcTc. SbTc. and SiGc. Although cuch of those has its advantages. the silicon..-germanium alloys cmcrgc as Lading candidutcs bccausc they opcratc in air as well as vacuum. they have outstanding mcchanical propcrtics. and they arc uscfuI at high tcmpcrnturcs’. Silicon.-ycrmnnium alloys containing bctwccn 60 and 80 at.-‘!,, silicon have roccivcd
;L major
share
of the attcntioii.
Phosphorus
is the most
common
Copant
(w.
in n-type SiGc. and boron (CW.0.1 wt.- I’,,) is the most common p-type dopant; in both casts. the sclcction of the dopant is lnrgcly dcpcndcnt on the solid solubility of the clcmcnt in the alloy. Production of homogcncous material is difficult, and the chemical cornposition is scnsitivc to process variables. Bccausc material pcrformuncc is strongly affcctcd by thcsc compositional variations, and. in addition. is highly dcpcndent on dopant concentrations. definition of the chemical compesition is important. This paper dcscritjcs analytical methods for dotcrmining silicon. germanium. phosphorus. and boron in SiGc thcrmoclcctrics. Other tcchniqucs for making thcsc dctcrminations have been reported in the litcraturc; the methods dcscribcd hcrc rcprcscnt original adaptions of standard or classical analytical methods to this problem.
0.3 wt.- :‘;,) used
CiERMANIlJM
Most chemical methods used for the nccuratc dctcrminotion of germanium in SiGc alloys require separation of germanium from silicon and from most dopants and impurities. Cheng and Goydish’ dctcrmincd germanium in pure SiGe alloys by volatilizing the silicon as the tetrafluoride in the presence of citric acid. evaporating the sample to dryness, and igniting to the oxide at SSO-900”. This technique is obviously limited by the impurity level of the material. Other separations, such as precipitation of gcrmanium as the sulfideand volatilization ofgermanium as the tctrachloride, arc suitable but require extreme care in order to achieve loo’%, separation and recovery. Current ion-exchange procedures for germanium involve adsorption of the chloride orlluoridc complexes from strong acid solutions; this requires ion-exchange appara.tus which is resistant to hydrofluoric acid. and presents problems such as loss of germanium by volatilization of the halide.
I’. .I. <‘ONl
476
I<. Cj. IIOs(‘li.
l<. M. MI:liI
I). I:. WANNl:l<
A new analytical proccdurc for the dctcrmination ofgcrmanium in SiGc alloys. based on the titriltion of ;L gcrmunium munnitol complex-3 with sodium hydroxide Lifter au ion-cxchungc scpuration of the germanium. is proposed hcrc. The SiGc is dissolved hy alkali fusion. the solution is diluted to give a CYI. 1 M sodium hydroxide solution. and gcrniariiuni is clu;~ntit;~tivcly rctaincd on ;I column containing Anibcrlitc is subscXl.i-243 ion-cxchangc resin. Silicon is not held by thu resin. The gcrnianium qucntly clutcd with 1 M nitric acid and dctcrmincci by the mannitolsodium hydroxide titration. Although boron behaves in exactly the same mannaas the germanium. it is error of the dctcrminnn~>rnially prcscnt at 01. 0. I ‘I,, which is within the cxpcrimcntal Lion.
Kcrrgjlc~tlts. Ambcrlitc X It-243 ion-cxchangc rain (Kohm ancl I iaas C‘onipany. Ptiiladclphia) and gcrnianium nictal powder ( - 325 mesh. 99.999”,, purity. AIf InHcvcrly. Mass.) wcrc uscd. All other rcaycnts and chcmiculs wcrc of A. Ii. organics, quality. A/~j~ccr~f~,s. The borosilicatc glass ion-cxchangc columns wcrc 30 x I cm (i.d.) ;lnc1 each contained ;L co;1’rsc frittcd glass disc to retain the resin. am1 stopcocks to control the Ilow rate. Tlic g.crni~tniun~ -ni;innitol titrations wcru clone with a Sargent-Welch Rccording Titrator. Model DC;. with ;I Surgcnt combination glass calomcl clcctrodc to monitor the plH during thu titration. I’rcpowtio~~
drul
r.c~llcrler-trliorl
(jf’ iorl-c~.\-c.lrtrrl~j~l
co1irrm.s.
lkcausc
gas
cvolvcs
during lhc initial contact of the resin with conccntratcd acitl. ttic resin was cquitibratcd with 3 ILI hydrochloric acid bcforc being put intocolumns. The r&n was back-washed with water in the column and ailowccl to scttlc uncicr ;I slight back-pressure of wutcr. 1 lydrochloric acid (50 ml of 3 h/f) wus pusscct through the column. followed by water until the cfflucnt was neutral. An equal amount of 3 M ammonia solution was then passc~l through thu column. followed by flushing with water until the cfflucnt was ncut rul. The same proccdurc was used in rcgcncrating the columns after cnch soparat ion proccd urc. The coliiiiins wcrc run at n7;ixiniuni flow rate (2 -3 ml niin ‘) during the prctrcatnicnt und rcgcncration steps.
Weigh ;I sample containing 30..60 mg of germanium into ~1zirconium crucible along with 2 g of sodium hydroxide pellets. Cover the crucible with u nickel lid. put on ;I hot plate -at room tcnipcraturc, and heat to the maximum temperature of the hot plate (550”) until all visible reaction c~ascs. (Caution : The fusion of SiGc alloy rclcascs hydrogen gas which may ignite violently if the initial fusion is done over ;I flame.) Then heat the crucible over ~1 Mckcr burner until ;L clear melt is obtained. Dissolve the product ofthc fusion in water. dilute to CU. 50 ml. und transfer to the ion-exchange co!umn. Pass the snmplc throu& the column at u flow ri.ltc of l-2 ml min _- I. Wash the column with 50 ml of 0.1 b2 sodium hydroxide at the same flow rate to clutc the remaining silicon. and then wash with water until the cfflucnt is ncutrat. Elutc the germanium with 100 ml of (1 + 9) nitric acid at the same flow rate. f~llowcd by 50 nil of water. Neutralize the cftlucnt with 50”,, sodium hydroxide. :lr1rt/. (‘Ilirtl. ./101,/.hl ( 1971)
. ANAI_YSIS
OF
SiCic
SEMIC’ONI~~_IC’TOKS
477
acidify to a methyl red end-point with the nitric acid. and boil for Z--3 min to rcmovc any dissolved carbon dioxide. T’itratc the germanium with ;I 0.02 RI sodium hydroxide solution. standardized by titration of pure germanium metal dissolved in the same manner as the sample. C’ontinuc the titration past the first end-point (neutralization of strong acid). add a large cxccss of munnitol (w. IO g), and continue the titration to a second end-point. The volurnc of tiirant required botwccn the first Rand second end-points is used to calcuIutc the amount of gcrrnanium in the sample.
Ambcrlitc X t:-243 was first synthcsizcd by Lyman ami Prcussd by aminating a chlororncthylat~tI styrcnc divinylbcnzcnc copolymer with N-mcthylglucamitic for USC us a boron-specific ion-cxcharigc resin. The boron specificity was based on the reaction bctwcen boric acid and the N-mcthylglucaminc group. ~1polyhydric alcohol. A well-known analytical reaction of boric acid with a polyhydric alcohol is that of boric acid with mannitol. resulting in ;I stronger acid which cm1 bc titrated with sodium have chuructcrizcd Ambcrlitc XE-243 with respect hydroxide Suvcral invcstigutors’*” huvc used the resin in the separation and to its intoructions with boron. and others-*’ dctcrmination of boron. No study us to the actual boron spccilicity of the XE-243 resin was avuilnhic. Since the chcmlstry of germanium with rcspcct to reactions with polyhydric alcohols is very similar to that of boron. the applicability of X1-243 to gcrmanium chemistry is not surprising. Although this work was not intcndcd to provide a comprchensivc chaructcrization of an ion-cxchangc rain. an effort was made to dctcrminc the resin capacity and the column breakthrough capacity for the system being used. Table i contains SOIIX data obtained by batch cquilihration methods in I M sodium hydroxide solutions. The utt~ounts of germanium indicated wcrc shukcn with I g of X k-243 resin in 50 ml of I M sodium hydroxide for 4 h. An aliquot was rcmovcd and the gcrmunium remaining in solution was dctcrmincd by the gcrmaniuni--munnitol titration; the amount of gcrmanium on the rain was determined by diffcr’cncc. The batch cxpcrimcnts wcrc done with : (LI) the resin as rcccivcd. (h) resin which had been prctrcutcd by equilibration with both 3 M hydrochloric acid and 3 M ammonia solution. as described previously. and air-dried. (c.) resin prctreatcd as described in (h). and then vacuum-dried at room tcmpcruturc for 18 h. From the data in Table I. it is apparent that the resin capacity is an TABLI:
I
(;I:l
(‘AI’,\C_‘ITY
01
7x.4 136.5 73.7 5.3.’ 91.5 63.X Sl.5
,\hll3I:.HLITli
XI:-243
IN
I AI SOIIIl:M
t l\‘l)HO)r;ll-)li
in solution. the grcatcr the equilibrium property, i.e.. the grcatcr the ion concentration quantity rctaincd by the resin. I-Iowcvcr. the data do indicate an approximatccapacity of 0.142 mmolcs ol’gcrmanium per gram of resin for the conditions used in the anulytical proccdurc. Similar cxpcriments done in 0.1 M sodium hydroxide show that the capacity of the resin for germanium almost doubles. Also of intcrcst is the indication that the prctrcatmcnt of the resin with strong acid and base solutions dots not significantly change the capacity of the rain with rcspcct to germanium. The brcakthrouyh capacity of the columns used in this work was dctcrmincd by passing a 1 M sodium hydroxide solution containing I mg ml I of gcrmunium through the column and chocking the cfflucnt for gcrmunium by atomic absorption tcchniyuus. For the 30 x 1 cm (i.d.) columns at LI flow rate of 2 ml min ‘, the brcakthrough capacity was found to bc 75 mg of germanium. * Thcaccuracy ofthc method is limit4 by the gernianiunimannitol titration and the manipulation of the sample. The ion-cxchungc scparution of germanium and silicon has been shown to bc quantitative by atomic absorption tcchniqucs for qualitative unulyscs of the cfilucnts. The pcrccntagc rccovcry of germanium from the column. for put-c germanium standards. was found to be I00 + 0.3 I,;;‘, which is the sarnc order of rccovcry obtained by fusing germanium in sodium hydroxide and titrating the sumplc directly. Typical rcsuits obtained on a sample of SiGc alloy with a nominal germanium content of 59 76 (by weight) wcrc 58.X6+0.09 ‘,!<,with a maximum spread of 0.27 0,; fat three dctcrminations. iiitcrfcrcnccs in the systems of intcrcst. Si--C&-P and Si---Gc-H. arc limited to boron which is rctaincd on the ion-cxchungc column and clutcd with the gcrmunium. The nominal boron content of the Si-Gc--B system is (*(I.0.1 ‘x,; the intcrfcrcncc caused is within the cxpcrimcntal error of the germanium dctcrmination and. in most instances, would bc ignored. In work demanding highest possible accuracy or involving samples containing higher boron contents. the boron may bc dctcrmit1c.d us described on p. 481 and the germanium results adjusted appropriately. SILIC’ON
The gravimctric dctormination of silicon by dehydration of silicil with pcrchloric acid and subscqucnt volatilization of the silicon with hydrofluoric acid is a Howcvcr. the application of this classic4 method time-honored analyticit method”. to silicon-germanium alloy is not mcntioncd in the litcraturc. A variation of the method. which climinatcs any possible intcrfcrcncc by germanium. is reported hcrc.
Procvtltrre. Samples in cithcr powder or chunk form may bc used in this analysis. Howcvcr. powder allows the fusion step to bc complctcd more rapidly. A sample is taken which will yield CU. 0.2 g of silica. Weigh the prcparcd sample into a Zircaloy crucible and add CU. 10 g of sodium hydroxide pellets. Cover the crucible with a nickel or other suitublc cover and fuse as dcscribcd in the procedure for germanium. Cool the melt and dissolve in distilled water. The final volume of the solution, following washing. is CCL200 ml. Add 100 ml of conccntratcd hydrochloric acid (AR) carcfully to the solution. and boil to rcmovc the germanium as the chloride. When the solution is rcduccd in volume to CU.200 ml. add 70 ml of 70 yj,:pcrchloric acid and cvaporatc to strong fumes.
ANALYSIS
OF
SiGc
SEMICONDUC’TOKS
479
(Caution: The final strong hcnting of the acid must bc done in a special pcrchloric hood and cart taken to avoid spattering of the salts.) After cooling. dissolve the salts in distilled water. Filter the dchydrutcd silica. fire in a wcighcd platinum crucible. cool. and rcwciph to obtain the weight of silica plus impurities. Then volatilize the silica by adding X ml of 4X’:;, hydrofluoric acid plus 3 drops of conccntratcd sulfuric acid to the crucible. and evaporating to dryness. Fire the crucible. cool and rcwcigh. The diffcrcntial weight is the weight of the SiO,.
Standards. as such, for SLGc alloy arc noncxistcnt. Howcvcr. alloys made to have identical concentrations of silicon wcrc analyzed by the above method. and the results indicated that the errors in duplicate analysis wcrc far less than the rcproducibility of the manufacturing process. A scrics of duplicate analysts wcrc run on 14 samples of Si-Gc alloys formulthe alloy can bc produdcd atcd to contain (*(I.61 wt.- “,,‘,,silicon. The results indicated within a limit of 60.89kO.52 wt.-‘?;, silicon. Howcvcr. the maximum deviation of duplicate analysts was only t 0.1 X and the mean deviation for the duplicate runs was 1_0.10. The classical pcrchloric acid method for silicon is gcncrally acccptcd to bc more accurate than is indicated by the analytical results shown above. The limiting factor in the accuracy of SiGc analysts is probably inhomogcncity within a particular sample. Dctailcd studies of homogcncity. by this mqdificd proccdurc. showed widely variable compositions in slices taken from a single zone lcvclcd rod or from a sin@ hot-prcsscd piccc.
After a brief litcraturc survey rclutivc to the dctcrmination of phosphorus used as a dopant in semiconductor and thcrmoclcctric mutcrials. it was apparent that most investigators (eg. O’Connor’“) had used neutron activation tcchniqucs. Polarographic’ ’ and spcctrophotomctric methods based on molybdenum blue” arc dcscribcd for materials such as CdSiP, and CdGcP, which contain phosphorus as a mzi_jor constituent. III attempting to adapt thcsc tcchniqucs to the SiGc system. problems such as time rcquircd for analysis and the chemical behavior of the SiGc alloys bccamc apparcnt. and a diffcrcnt approach was sought. Hcrc. a spcctrophotomctric method for the dctcrmination of phosphorus based on the reaction of molybdatc with a solution containing vanadatc and phosphate, for use in dctcrmining phosphorus dopant first reported by Misson “. is proposed lcvcls in SiGe alloys. Commonly dcscribcd as the molybdovanadophosphatc method. the method has been applied to phosphorus dctcrmination in materials such as fcrtilizcrs’J. iron ore and plain carbon steels’“, copper-base alloys’6. and titanium and dcscribcd by zirconium and their alloys I’. The method hcrcin is basically a procedure Elwell and Wood” for determination of phosphorus in titanium alloys which has been modified for USCin SiGc systems. It has been shown to bc rapid and of sufficient accuracy for phosphorus dopant levels ranging from 0.01 to 0.55’;/, by weight in a SiGe matrix.
rcagcnls and chemicals used in this work wcrc 0fA.R. quality and 110 fitrfhcr purification. Apptr1wf14s. Absorbance tmxtsurctncnts wcrc made in 5-cm &ckman Neat lnfrarcd Silica cells with a l~cckman OK-2A lialio Recording Spcctrophotornctcr using ;L tungsten or dcutcrium source and a photomultiplicr dctcction system. P,*eptr,.trf ioil o/‘t~ltrrk trrut srtrritl~rrti c14r13~‘. Add 12 ml of ( I + I) sulfuric acid to ;i 250-ml volumetric Ilask and dilute to (~1. lo0 ml. Add 3 ml of aqueous 0.5 I’,, (w/v) potussiuni pcrnianganatc solution, followed by sufficient aclu~ous 0.3?;, (w/v) sodium nitrilc solution to rcducc the pcrmunganatc. Add 10 ml of LO’,!;,(w/v) ammonium tartratc solutiot1 and dilute rhc solution to 250 ml. To IO0 ml of this solution (pipcltcd) add 5 ml of 0.25 ‘;:, ;immoniun~ vanadatc solution in 2 I>;,nitric acid, followed by 5 ml of ill1 ~lc~llcolls 20 ‘.:,,, (w/v) solution of (NI-1J),,Mo,013.4H20. Allow to slami for I5 tnin and then USC 11s the blank for the absorbance mcasurCmcn& Obtain the standard phosphorus curve from solulions prcpurcd in an identical m;~nncr with the cxccption of the addition of known an1ounIs of phosphorus. Add the phosphorus ~1s ;I standard solulion of potassium dihydrogcn phosphate hcforc the addition of pcrmanganatc. Mcasurc tt1c absorbanccs of thcsc solutions rulativc to the blank solution at 400 nm in 5-cm cells and ;L color dcvclopmctlt time of 15 min. ‘I’wrtnrrt~f of’strr~~ple. USC sufficient sample to provide 0.2. 0.4 mg of phosphorus. Dissolve the sample in a 70-ml plutitium dish by addirion of 04. 4 ml of (I .-t I) nitric acid, followed by a dropwisc addition of 48 ‘,!,;;hydrofluoric acid until dissolulion is complctc. Add 2 ml of 48 y,‘, hydrofluoric acid in cxccss, followed by 12 ml of (1 -t I ) sulfuric acid and cvuporatc to fumes of sulfur trioxidc. Add 5 ml of wutcr and transfct the solution to ;L bcakcr with 20 ml of (I -t- I) hydrochloric acid. Evaporate to fumes 01 sulfur trioxidc, cool. dilulc with 25 ml of water and heat to boiling. Add a~. 3 ml of aqueous 0.5 ‘f;, (w/v) potassium pcrmang:anatc and boil for I --2 min ; aid sodium nitrite solution dropwisc until the pink color disappears. Transfer the solutions to 250-ml volumetric flasks containing 100 ml of water and 10 ml of the above ammonium tartralc solution and dilute to volume Pipctlc IO-ml aliyuols into plastic bcukcrs. and add 5 nil cuch of the above-mcntioncd vanaciatc and molybdatc solutions. Aftcr IS min. mL’asurc the absorbance at 400 nm OS.a blank prepared at the same time as the unknown solutions. Establist1 the amount of phosphorus from the standard phosphorus curve. 1Zw~g~~f1t.s.All
wcrc used with
In it study of possible intcrfcruncc it1 the tnolybdov~~nadopt~~~sphatc: tncthod. K itson and Mellon ’ ’ found that small amounts of silicate do not intcrfcrc. In the present work. it was shown that germanium causes high results for phosphorus. The rcrnov:Jl of the silicon-gcrm~~nium matrix by distillation of the tctrafluoride and tetrachloride as dcscribcd in fhc Sample Trcatmcnt scclion was shown to bc quantitative in the cast of gcrrnanium; however. tract amounts of silicon remained in solution, probably because of a slight attack on the glassware used after the distillation of silicon tctrafluoridc. The color dcvctopmcnt time of 15 min was suggcslcd by Elwcll and Wood ” and is consistent with minimum time of analysis. In this work. a decrease in pcrccnt transmittance (T) of the solution with time was obsurvcd. Solutions in thu range of 40-
50 ‘,‘j,,, T showed a dccrcasc of (*(I.2.2’,‘.;; T over the period 15-75 min after color dcvclopmcnt. 011 a standard phosphorus curve prcparcd by making mcasurcmcnts 15 min after color dcvelopmcnt, the chnngc corrcspondcd to an incrcasc of 0.015 mg phosphorus. At this rate of change, the change occurring during the absorbance mcasurcmcnts of a scrics of four solutions is ncgzligiblc. The phosphorus blank resulting from rcagcnts and glassware was dctcrmincd by carrying a blank dctcrmination through the cntirc procedure atid measuring ths abprcparcd as dcscribcd above. sorbancc of the resulting solution VS. a blank solution The phosphorus blank was found to bc 0.01-0.02 mg of phosphorus. The accuracy of the method as applied to SiGc alloys is somcwhut difficult to dctcrminc since true standards arc not available. Secondary standards can bc prcparti by addition of known amounts of phosphate to undopcd SiGC alloys. or by addition of phosphate to mixture of silicon and germanium oxides. This can only approximate the actual matrix ~1s SiGc alloys contain phosphorus in the cIcmcnta1 form.
Indcpondcnt
analysts
done by neutron
activation
sumplc which. Table II show
by the proposed method, was found the accuracy obtainable m standards
I’tlOSI~tIOl
C’ON’I‘t:N’I’S
Si Si Si Si Si Si
Ck CA Cic Cit. P CA2 I’ Cic 1’
0.00 0.00 0.00 0.50 0.30 0.x
IX ANALY’I’IC’AL
t
yicldcd a value of 0.42 7’;;on a to contain 0.49’:;,. The data in prcparcd by addition of known
ON
SiCic ALLOYS
0.100
0. IO 1
O.‘OO
0.207 0.307 0.480 + 0.003” 0.31~_+0.0I I” 0.50X + 0.006”
0.300
amounts of potassium dihydrogcn phosphate to undopcd SiGc alloys. the precision obtainable using the method. and the accuracy of the method relative to the nominal phosphorus content of the material being analyzed. In the range 0.2-0.5 ‘x, phosphorus. the general spread in the data obtained by multiple analysts was 0.01 ‘x, or Icss. The pcrccnt rccovcry for phosphorus added as phosphate to undopcd SiGc alloy was 100 +- 5’,‘;, within the range O.l---0.3 mg of phosphorus.
for the determination of boron in a variety of matrices. The analysis of steel for boron has been well documented lt( - 20. Boron has been dctcrmincd in silicon by d.c:arc excitation”. Zil’bcrshtcinzz conccntratcd boron from high-purity silicon dioxide with hydrofluoric acid. and retained the boron with mannitol for a.c. arc analysis of the dried rcsiduc on graphite ekctrodcs. The boron content of carbon and graphite has been reported by Feldman and ElknThere
arc many
spcctrochcmical
methods
_.
1:. J.
482
C’ONl(A.I).‘lI.
I<. M. M1:I~Iil1.1_.
II. I:. WANNt:I<
burglA who used ;Ld.c. arc with carbon clcctrodcs or a siftcr clcctrodc system with hiphvoltugc spark excitation. A.S.T.M. emission methods for the dctcrmination of boron in 14 dif!‘crcnt mutriccs arc available Ix . Howcvcr. thcrc arc no reports on the applicution of’ this tcchniyuc to silicon--germanium alloys. Methods which fill this nc:cd arc j>rcscntcd hcrc. Intcrcst was divided bctwccn two’ lcvcls of boron doping which dcmundcci somewhat difl‘crcnt spcctrogruphic tcchniqucs. Thcrcforc, two analytical methods. one of which is ;I solution tcchniquo applicable to doping lcvcls bctwccn 0.1 and 0.2 wt.- ‘I,, applicable to lowcr boron concentrations. boron, and the other, ~1powder tcchniquc, wcrc dcvclopcd. No spectrographic standards for the SiCiclS system wcrc available and, bccuusc most quuntitativc spcctrochcmical methods arc based on standards, it was ncccssary to use ;L method whcrc synthetic standards could provide rcliablc analytical results for boron. A solution proccdurc with a carbon rotating tiisk clcctrodc was chosen liar highly dopod tnatcrial bccausc: (tl) this laboratory has cxtcnsivcly used rotating disk solution proccdurcs for many matrices; (/I) synthetic standards could bc prcparcd in ;I suitublcconccntration range; (L’)an added internal standard could bc used since ncithcr silicon nor germanium would bc at ;Lconstant pcrccntagc; (tl) the inhcrcnt higher prccision of the high-voltage spark source should result in a butter ovcrull method. For the lower doping Icvcls. it was apparent that the sensitivity of the rotating disk spark proccdurc was inadcquutc, hcncc &fort was dircctcd towards ;I powder tcchniquc which used an ignited ;I.c. arc I’or excitation. It was considered impracticut to make synthetic standards for the pow&r method by mixing powders of silicon, germanium and boron ; thcrcforc. a boron-doped SiGc alloy charactcrizcd by the solution method. was “diluted” by additions ol’ n-type (boron-free) SiCic semiconductor material to produce powders with ;I range of boron contents. Carbon was added to these standards to make thcni cIcctricitlly conductive. ’
A/J~LIIYI~IHattti ~~~rgclc’rlts. ‘I’hc spectrograph used for both of thcsc boron methods Ebcrt witha gratingof 15.0(M)lines/inch. Elcctrodcs wcrc National C.‘arbon : L4072 (ASTM D-2) rotating disk and L4006 cupped Iowa clcctrodc. The photographic plates wcrc Eastman Kodak 111-Ofor the solution proccdurc and SA # I for the powdc~ method. D-19 Dcvclopcr was used in proccssing. Carbon for the powder method was obtuincd from the Ultra Carbon Corporation (UCPZ 100). A .larrcll-Ash Model 23100 Microphotomctcr was used for obtaining line transmittances. All chemicals used wcrc of A. R. grade or bcttcr. liottrthg tlislc sohtior~ .Spl/ri
wasa
3.4-m
ANALYSIS TABLE
OF
SiGc
483
SEMICONDUCTORS
;I1
COMPOSITION
OF
SOLUTION
STANDAKDS
Si
.-
(4
FOR
BORON
M (O.OOI !, ,?I/
‘I;, GP
Gr
7x.01 7l.YS 65.05 62. I7 69.Xx
’)
“,, f3
(Id)
(H)
0.07805 0.07 195 0.06505 0.06205 O.OhYYS
ANALYSES
. 3.0 3.5 6.0 X.0 I .o
21.71) 27.7 34.35 37.02 30.02
0.02 I 8 0.0’77 0.3435 0.3695 0.03005
0.20 0.35 0.60 O.HO 0. IO
_
acid. Place a tight fitting 2 ml of conccntratcd nitric acid and 1 ml of 4X Y/i,hydrofluoric plastic lid on each beaker and heat the contents over a steam bath’. Add more nitric and hydrofluoric acid as required to dissolve the silicon and germanium standards. Add an aliquot of a standardized boric acid solution to each boakcr (xc Table III). and transfer the sample to a 100-ml plastic volumetric flask for final volume adjustmcnt. Dissolve 0.1-g snmplcs of the SiGc alloys using the same procedure. Transfer 20 ml ofcach standard or sample to 1-CMdropping bottles to which 1 ml of nickel intcrnul standard solution (($05 g Ni ml - ‘) was added. (Nickel was chosen as an internal standard since it was not a normal impurity in this material, and a nickel lint was within 10.0 nm of the boron lint of intcrcst.) Record the spectra in triplicate for each standard and sample using the conditions summarized in Table IV. lJsc the TAl3LE
IV
1NSTRUML:NT
PARAMETERS
EMISSION
SPI:,C”rROSCOPY
(Spcctrogruph 3.4-m Ibert. lS.OW l/in. (2nd order): slit width. 335.0 nni: filter ‘-step. pcrccnt transmission. 100;12)
-_
._
_
.___..
_
II V spark Capacitance. /IIInductance. jt1.l~ Rosistuncc, ohms Voltage. primary. V Hrcaks per half cycle Power setting, spark Power setting. urc RF current, A Electrode. upper lower Prchurn. sh Exposure. sh Emulsion type Dcvclopcr D- 19. min’ Stop bath (acetic ucid). s Fix. Kod Rapid. min .-_...._-_- ..-..-. ---------.-...._.. ” National h Controlled ’ Nitrogen
O.W75 IS5 0 220 6 6 7 6” x 4” ‘l ASTM D2 8 5x 111-O 4 20 2 . _._.____.._____._____ ._
Carbon Rod. SPK L3828, by stopwatch. burst, 3 s on. IO s off.
rounded
end.
0.005 625 0 220 7 6 6 17.5 L3955 L4W6 21 3x SA#I 4 20 2
40 pm. . spectral
range
(2nd order).
210.0-
I:. J. C’ONKAID.
484
K. Ci. l>OSC.‘tl.
K. M. Ml:.KI
I>. Ii. WANNI:K
two-step iron n~clhod li)r emulsion calibration with II prclitninary cclrvc’ H*‘J. (>)bt:lin the transmittances of the boron 249.67%nm lint’“, adjacent background. and the nickel 259.590-nm lint and detcrtninc the backgroltnd-corrcctcd boron-to-nickel rclativc intensity ratios from the ctnulsion calibration curve. Plot thcsc ratios against boron concentration on log- log graph paper to product Ihc analytical curve. C)btain the boron concentrations for the unknown samples by applying the ratios to the analytical curve. I(gcl,liietl U.C. III’C powder tnefhotl. A spark-ignited 1 10-V a.c. arc is used to cxcitc powdercd standards and samples packed in the lower cupped graphite clcctrodc countcrcd with a rounded end yraphitc clcctrodc. Spectra of standards and samples arc obtained on photographic plates. Transmittances of the boron lint. acl.jaccnt background, and the itluminutn internal standard lint arc used in establishing the analytical curve. Slarttlard trrd scmple prcpwutiorl (powder mctlroti): USC two SiCic powders (200 mesh), one boron-free. the olhcr containing 0.14’%, boron, to prcparc standards. Mix each powder with 9 parts of carbon (containing I ‘x, alumina for use as an internal standard). Mixtures of thcsc two base powders result in a set of standards for boron in the matrix of intcrcst (Table V). Powder solid samples by crushing in a Plattncr mortar and pcstlc, sicvc through 200-mesh nylon screen, and mix I part ol’sumplc to 9 parts of carbon (containing I ‘A’,alumina). All mixing was done in 2-in. high. 0.5-in. diamctcr lucitc vials with a 0.375-in. diumctcr lucitc ball. Mixing time was 2 min in a Wig-L-Bug. Pack the clcctrodcs by pressing the cupped portion uf the clcctrodc into the powder. Use graphite with a rounded end as the counter clcctrodc. &cord spectra of standards and samples in triplicutc, following the conditions outlined in Table IV. Culibratc the emulsion as outlined for the rotating disk. Obtain transmittances of the boron 249.67% ntn lint, adjaccnl background. and the aluminum 256.799-ntn lint for subscyucnt conversion to corrcctcd rclativc intensity ratio of boron to aluminum. Conversion of rclativc intensity ratios to boron concentration is similar to that ouflincd above. l’Al3LI?
SrtrtdtrtYl
v
110.
Borvtl (pctr’fs)
I3 I4UO w30 13700 13J60 l32xo I3140 I370 I328 B B:1sc _~ .
Rrstrlts
I 2 I I I 1 I I 0
hsc
UOWl-jkl~ hsc
0 I 1 2 4 9 I ‘1 49 I
I:,, 13
(pcuvs)
0. I4 0.09.1 0.070 0.046 o.o,g 0.0 I4 0.007 0.007x 0.000
~uul tiiscmsic~r~
Table VI cotnparcs the intended concentration of boron and the results by both tnethods. From this comparison and the pcrccntagc differences. it may bc concluded that the rotating disk sp::rk proccdurc yields results within +_5 ‘x, in the range O.l-O.S’x,. For the ignited a.~. arc powder method, the reproducibility is cu. + 10(x, in the
ANALYSIS
OF
SiCic
SkMICONDUCTORS
485
range 0.002--0.09 ‘.t;,.The accuracy of the mclhods could not bc cvaluatcd owing to the lack of primary standards. The solution spcctrochemical proccdurc yicldcd adequate precision and scnsitivily for boron in the range O.l-0.8’:;, (by weight). The solution standards were mudc to contain varying amounts of silicon and germanium which scrvcd two purposts: to dctcrmine if the boron response was influcnccd by the matrix. and to make possible simultaneous analysts for silicon and germanium. if dcsircd. When the boron concentration was lowered. it was necessary to USCa higher sensitivity source to retain rcasonablc precision. A moving plate study was made to compare the ignited a.~. arc burnout of aluminum and boron with time. The relative intensity ratio of boron to aluminum was csscntially constant from 20 s-to 60 s; thcrcforc. a 20-s prcburn was chosen. Aluminum was substituted for nickel for internal standardization in the more scnsitivc method bccausc boron and aluminum have evaporation rates more similar than those of nickel and boron (this is not significant in solution analysis). SUMMARY
The pcrformancc of thcrmoclcctric devices is sensitive to the chemical composition. the dopant concentration and the uniformity of the semiconductor materials used in their fabrication. Analytical methods for the determination of silicon. germanium. boron, and phosphorus in SiGc thermoelectric materials are described. These methods were dcvclopcd bcca’use of the fragmentary data available in the literature regarding their chemical characterization. The methods are rapid, accurate, and. with the exception of boron. rcquirc a minimum of cxponsive apparatus. Germanium was determined acidimetrically by means of the mannitol complex after ion-exchange separation on Ambcrlitc XE-243, silica by the conventional gravimctric method, phosphorus calorimetrically as the vanadomolybdophosphate, and boron by emission spectroscopy.
On dkcrit dcs. mL;thodcs d’analyse du bore et du phosphore dans Ic materic
pour Ic dosage du silicium. du germanium. thermo&.lcctriyuc SiGe. Ellcs sont rapidcs,
F. J. CONRAD.
486
R. G. DOSCH,
R. M. MERRILL.
D. E. WANNER
du bore, elles ne nCcessitcnt pas un appareillagc coQteux: pr@ciscs ; et, ti l’cxccption Lc germanium est do& acidimktriquement, en prCsence de mannitol, apres skparation sur Ambcrlitc XE-243; la silice par mCthode gravimCtrique conventionnelle; le phosphore colorimCtriquement comme vanado-molybdophosphatc et le bore par spcctroscopie d’bmission. ZUSAMMENFASSUNG
Die Leistungsftihigkcit thermoelektrischcr Bauelcmcntc htingt von der chemischcn Zusammensctzung, der Dotierungskonzcntration und der Gleichfiirmigkeit des Halbleitermaterials ab. dass bei der Herstellung vcrwendet wird. Analytische Methoden fiir die &stimmung von Silicium, Germanium, Bor und Phosphor in thermoclcktrischen SiGc-Materialicn werden bcschriebcn. Dicse Methoden wurden wegen der unvollstiindigcn Angabcn entwickelt, die in der Litcratur beziiglich der chemischcn Charakterisierung zu linden sind. Die Methoden sind schnell. genau und erfordern, van Bor abgcsehen, tin Minimum kostspieliger Apparaturen. Germanium wurde mit Hilfe der Mannitkomplexe nach Ionenaustausch-Abtrcnnung an Amberlite XE-243 acidimetrisch bestimmt, Siliciumdioxid nach dcr herkiimmlichen gravimctrischcn Methode, Phosphor kolorimetrisch als Vanadatomolybdatophosphat und Bor durch Emissionsspektroskopic. REFERENC:ES
1 F. D. Rossi;~hc~moclcctricity and Thermoclcctric J’owcr Gcncration, So/iti .S~rrrclElccrrorrics. Vol. I 1, Pcrgamon J%ss, 1968, pp. 833-868. 2 K. L. Chcng and J3. L. Goydish, hd. Chern.. 35 (9) (1963) 1273. 3 H. J. Clulcy, A~~ctlyst. 76 (1951) 517. 4 W. Lyman and A. Prcuss, U.S. Put. 2. 813, 838. Nov. 19. 1957. 5 R. Kunin and A. F. Prcuss, 111rl. EII~. C/IWI. Prod. Kcs. Dewlop., 3 (1964) 304. 6 F. J’inon. J. Dcson and R. Rosset. Rltll. Sot. Chim. Ft... 8 (1968) 3454. 7 R. M. Carlson and J. L. Paul, Amd. Chem.. 40 (1968) 1292. 8 R. M. Carlson and J. L. Paul, Soil Sci., 188 (4) (1969) 266. 9 H. Furman. Stcmhrtl Metlturls oj’Cltettu’cct1 Attttlpsis. Vol. 1. 6th Ed.. 1962. p. 955. IO J. O’Connor, Sci. Twlr. &rasp. Rep.. 2 (6) (1964) 737. 11 R. Yu. Lyalikovu. Ztruotl. Lob.. 55 (4) (1969) 431. 12 Ji. Yu. Lyalikova. Zh. Amrl. Khittt.. 23 (81) (1968) 1240. 13 G. Misson, C/I~~~I.Ztg.. 32 (1908) 633. 14 W. C. l-l:~nson. J. Sci. Food Agr.. I (1950) 172. 15 I<. E. Kikmn and M. G. Mellon. Id. Ettg. C’ltm. Atutl. Ed.. 16 (1944) 370. 16 W. T. Elwcll and t-1. N. Wilson. Atrdyst. 81 (lY56) 136. 17 W. T. I~lwcll ;1nl1 D. I;. Wood. The Atttrlysis o/ Tifcrttiuttr. %ircrmitrttt cttttl T/tpir Alloys. Wiley. iY61. 18 AS-I’M. r\~lrt/tot/s,/i~r Etttissiott S/‘c’ctroc/tctttic,~t/ Attct/>wis. 6111 Ed., 107 I, pp. 40-l 63. 19 C. 1~. Hines and J. K. Hurwitz. Appl. Specrrosc.. 21 (1967) 277. 20 J. Goryczka and W. Klimccki. Pr. Inst. Htcttt.. 18 (1966) 45. 21 A. N. Shtcinbcrg. Tr. Inst. Met. Akutl. Nmrk SSSR. 11 (1962) 22Y. 22 K. I. Zil’bershtein, 0. N. Nikitina and M. P. Semov. Poltrch. Atrctl. Veshc/tes~t~ Osohoi C/list. M&w. Vsrs. Kottj:, Gorky, USSR, 1963. pp. 139-141. 23 C. Feldman and J. Y. Ellenburg, Ad. Cltettt.. 27 (1955) 1914. 24 J. R. Churchill, ltd. Atut/. Cheer.. 16 (1944) 653. 25 MIT Wutxlettgtlt Tulles. The M.I.T. Press. 1969. Awl.
C/tint. Acta.
61 (1972)