Halogen transport of molybdenum arsenides and other transition metal pnictides

Journal of Crystal

Growth 15 (1972) 231-239

HALOGEN OTHER

0 North-Holland

TRANSPORT TRANSITION

J. J. MURRAY,

of Chemistry,

Received

1 February

National

Co.

OF MOLYBDENUM METAL

J. B. TAYLOR

Division

Publishing

AND

PNICTIDES

and

L. USNER

Research

1972; revised

ARSENIDES

Council of Canada,

manuscript

received

Ottawa,

4 March

Canada KIA

OR9

1972

Crystals of MoAs,, MozAs3, and Mo,As4 have been grown by chemical transport using halogens as the transport agents. The variation of the transport rate with halogen, halogen concentration, temperature and temperature gradient has been studied. Total pressure, optical/UV absorption, and vapour analysis measurements have been employed to study the equilibrium vapours of the Mo/As/Br system up to 1175 “K. The arsenic trihalide is a major vapour component in this system and probably in other transition metal pnictides/halogen systems. The transport mechanism differs from that found in previously studied systems.

1. Introduction

quired for the use of the rules. Schgfer’s recent review’) illustrates the wide variety of transport systems possible. Simple equilibria of the type of eqs. (1), (2) and (3) alone do not account for many transport results, as the following examples will illustrate: Rare earth compounds can be transported at temperatures at which the equilibrium pressure of the halide is a few torr or less and at which significant dissociation of the halide is not expected 15316). Transport of platinum metal compounds has been observed’ 7), even though halides of these metals are quite unstable, some, in fact, decomposing upon heating without heteronuclear vapours being formed (e.g. PtBr,, PtI,). The transport of molybdenum by iodine does not proceed, yet that of molybdenum sulphides does2). We have chosen to make a detailed study of the MO/As/halogen system. For reasons which will become apparent in this communication, this system is typical of the transition metal arsenide/halogen systems in general and differs markedly from the III/V/halogen and II/VI/halogen systems which have already received detailed study. The binary system MO/AS has been investigated by crystallographic and phase analytical techniques. Three intermediate phases, MoAs,, Mo,As, and Mo,As, are easily prepared and have been well characterized’ ‘,l 9). Two additional phases Mo,,,As,~~) and MoAs”) have been prepared and characterized crystallographically. Taylor et al.’ *) and Jensen et al.‘“) did not observe these phases and the conditions for their stability are obscure.

Chemical vapour transport reactions”‘) are used to prepare epitaxial layers or single crystals for semiconductor applications and for crystallographic or physical property studies. Many of the systems of direct interest to the semiconductor industry, e.g. GaAs/I,3,4), GaAs/Cl, 5,6), GaAs,P, _,/H,/Cl, 7), GaP/Clz6), In As/I,*,~), InAs/InP/I,‘O), CdS/I,“), ZnS/I,l’), and ZnSe/I,13) have been studied in detail, the transport equilibrium identified and the thermodynamic parameters of the equilibrium directly determined. Transport equilibria found in these systems indicate a simple type of mechanism, viz. 2 GaAs(s) + GaCl,(g) CWs) + I,(g)

= 3 GaCl(g) + 3 As,(g),

= Cdbk)

2 GaAs(s) + 2 HCl(g)

+t

f%(g),

(1) (2)

= 2 GaCl(g) + H,(g) + $As,(g) .

(3) This knowledge is vital in the production of materials for semiconductor applications but more generally, the technique is applied purely empirically in that the agent, temperatures and pressures for both transport and good quality crystals are determined by trial and error. A set of rules put forward by Schgfer’), and modified to some extent by Jeffes14), together with thermochemical data are useful in guiding the choice of agent and conditions. Quantitative application of these rules is often precluded by the paucity of reliable thermochemical data, particularly for the halides. At least some knowledge of the probable transport equilibrium is re231

J. J. MURRAY, J. B. TAYLOR AND L. USNER

232 2. Experimental 2.1.

2.2.

CRYSTAL GROWTH EXPERIMENTS

Samples of the MO/AS the elements. Molybdenum Fansteel, was reduced in move oxygen. W, Fe, Ni, purities (between 40 and supplied by Cominco, was

phases were prepared from powder, donated by U.S. hydrogen at 1175 “K to reand Cu were the major im100 ppm each). 59 arsenic, fractionally sublimed in va-

cue at 725 “K to remove oxide. Weighed quantities of the elements corresponding to the desired stoichiometric ratio were sealed under vacuum (1.3 x 10e3 N m-‘; lo- ’ torr) in fused silica ampoules. Reaction was achieved by 1 “K/min programmed heating up to 650 “K. The material was subsequently homogenized by repeated anneals at 1000 “K. Guinier focussing camera X-ray powder diffraction patterns were used to identify the various phases. A weighed charge of the polycrystalline MO/AS phase, contained in a fused silica ampoule stoppered with a loosely packed clean quartz wool plug was placed in a 9 mm diameter by 10 cm long fused silica growth tube. The interior surface of the growth tube had been smoothed by heating to the softening point. The quartz wool plug in the charge ampoule eliminated the proliferation of nucleation sites caused by stray powder adhering to the tube walls. A side arm contained a weighed quantity of platinum dihalide. After evacuation and flame degassing of the growth tube, halogen was quantitatively generated by thermally decomposing the platinum dihalide’ 6). The sealed off tube was placed in a multiwound controlled gradient furnace. The overall tube composition was (MoAs,),Br. After l-4 days the tube was air quenched. Care was taken to ensure that complete transport of the charge did not occur. The following parameters were varied: (i) halogen - I, Br or Cl, (ii) concentration of halogen, (iii) average temperature, (iv) temperature gradient, (v) stoichiometry of the MO/AS charge - MoAs,, MoAs,,,, In addition, the variation in technique sugMoAs,,,. gested by Kershaw et a1.22) was used in attempts to obtain large single crystals. Considering the transport of sulphides by sulphur and selenides by selenium which have been observed23,24), attempts were made to transport MoAs, and Mo,As, using 7-12x 10’ N mm2 (7-12 atm) of As,(g) at an average temperature (T) of 1150 “K.

TOTAL PRESSUREMEASUREMENTS

Total pressures were measured to 1300 “K by the usual technique4-6,‘1), using a vertical multizone furnace which had a heated horizontal viewport, a fused silica spoon gauge as a null detector, manually adjusted argon back pressure and a Hass Instrument Corporation A-l manometer to measure back pressures to 17 N m-’ (0.05 torr). Temperatures were measured with Pt/Pt-13 “/, Rh thermocouples and system volumes by expansion from standard volumes. Halogen was introduced as for the crystal growth tubes. After the MoAs, sample was introduced, the system was baked at 700-800 “K before the halogen was introduced. Initially, fused silica spiral gauges were used as null detectors. They were annealed as suggested by the manufacturer at 1275 “K for 1 hr. The zero exhibited gross instability. Longer (25-30 hr) anneals at higher temperatures (1325 OK) improved the zero stability but not to an acceptable degree. Sag in the spiral which eventually caused contact between the turns presumably contributed to the persistent instability. Kirwan‘j) found that such long anneals did lead to a stable zero. Our spirals, although from the same manufacturer, differed in that thin silica fibers supported the spiral on the vertical axis. For the reported measurements less sensitive (65 N mm2 ; 0.5 torr), more fragile (+ 7 x lo3 N m-’ maximum) but much more stable spoon gauges were used as null detectors. 2.3.

OPTICAL AND UV ABSORPTION

A Beckman DU spectrophotometer was modified so that the cell could be heated to 1125 “K, in essentially the same manner used before25,26). Similar work on vapour transport systems has been done using a Cary 14 spectrophotometer3,’ ‘). A vertical multiwinding furnace was fitted with two exactly co-linear and diametrically opposed horizontal side tubes which passed through the walls of the furnace core at about its midpoint. These side tubes were connected by water-cooled light-tight fittings with the monochromator/source and photomultiplier assemblies of the spectrophotometer. The reverse optics geometry of Bruner and Corbett25) was not used. The sample and evacuated reference cells had a 40 mm path length and 22 mm O.D. suprasil end windows. The two cells, one above the other, were connected through a light trap at the furnace top to a

HALOGEN

mechanism

which permitted

quick

TRANSPORT

and highly

OF MOLYBDENUM

repro-

ducible vertical movements of the pair of cells. Thus either cell could be positioned in the beam path and movement of the entire furnace was avoided. By appropriate adjustment of the power applied to the windings of the furnace and by the use of additional heaters on the side tubes, the temperature range in the sample region was less than 3 “K, the maximum temperature occurring at the viewports. By including the thermal emission in the dark current, using graphite light stops at the cold ends of the side tubes to restrict the amount of hot emitting surfaces seen by the photomultiplier, and by using Corning band pass filters, quantitative absorption studies were possible up to 1025 “K. Between 1025 “K and 1125 “K detector flooding by the thermal radiation reduced the accuracy but the results were still quantitatively useful. 2.4.

VAPOUR

233

ARSENIDES

PYROPHYLLITE PLUG

SUPPORT

BREAK

SILICA ROD

SEAL

AMPOULE

GRAPHITE

ROD

GRAPHITE ELECTRODES

GRAPHITE SUPPORT

ROD

CONSTRICTION

\

SILICA ENVELOPE

ANALYSIS

A sample of one of the MO/AS phases was placed in the smaller volume of a silica glass ampoule consisting of an approximately 40 cm3 volume joined by a constriction to an approximately 5 cm3 volume. The ampoule and a sidearm containing PtBr, were evacuated to 1.3 x 10m3 N rnp2 and bromine was quantitatively generated and transferred to the ampoule (vide supra) before the ampoule was sealed off. The bromine concentration was generally about 12 g-atom me3 (1 mg cmm3) and the overall composition was (MoAs,),Br with x = 2, 1.5 or 0.8. The ampoule was placed in the sealing apparatus as shown in fig. 1. Care was taken not to allow the solid charge to enter the large volume at any time. The graphite electrodes were positioned as shown in fig. lb. Equilibration at temperature was for at least 12 hr with a 30 “K linear temperature difference along the ampoule, the top being the hotter. A dry nitrogen flow eliminated oxidation of the graphite rods and electrodes. The constriction was heated to the sealing temperature by striking a dc arc between the electrodes using a standard arc welding power supply. This technique permitted quick (about l-2 set) seals without appreciable change in the overall ampoule temperature. A successful seal was detected by observing the small volume section to fall onto a mineral wool stop pad just below the furnace - usually within 3-6 set of electrode power being applied. Upon cooling to room temperature, the condensate

TOP GRAPHITE

VIEW OF ELECTRODES

b

DRY

N

,‘I’ a

Fig. 1.

Apparatus

for sealing vapour analysis ampoules.

in the remaining large volume was usually AsBr, with traces of MO, As and Mo/Br phases. The ampoules were broken open under 5 ‘A aqueous NaOH to avoid the loss of As and Br due to the volatility of AsBr, and its hydrolysis products; standing for 24 hr ensured complete solution of AsBr, and the Mo/Br phases. The residue, consisting of elemental MO and As was taken into solution using aqua regia. Molybdenum and arsenic were determined by atomic absorption spectroscopy and bromine by titration with aqueous AgNO, using a Ag/Ag’ electrode to determine the end point. The accuracy of the MO analysis was severely limited by the low levels present and the high dilution involved in the initial solution process. In some experiments, only MO was analysed by first crushing the ampoule, then dissolving the sample in the minimum volumes of 5% NaOH and aqua regia. In bromine-free samples, arsenic was analysed similarly using aqua regia as solvent,

234

J. .I. MURRAY,

J. B. TAYLOR

AND L. USNER

3. Results

3.1.

TABLE 1

Transport rates of MoAs, CRYSTAL GROWTH EXPERIMENTS

Acicular crystals of MoAs, which had a metallic lustre and maximum dimensions of 1 x 1 x 5 mm were readily grown by transport of a MoAs, charge down a 50 “K degree difference at mean temperature (T) in the range 1025-1275 “K using either chlorine or bromine as the transport agent. The halogen concentration n, was varied in the range 3 to 50 g-atom mm3 (0.1 x 1.7 mg cmP3 Cl, 0.25 to 4.0 mg cme3 Br). The crystals obtained were identified by X-ray diffraction. Similar work using iodine gave either no transport or small yields of crystals having a metallic lustre and a multifaceted, equiaxed habit. For a MoAs, charge, these crystals were usually Mo2As, although MoAs, crystals occasionally resulted. Experiments with halogen omitted did not give any transport, nor did the experiments using 7-12 x 10’ N mm2 of arsenic and no halogen. MO metal was easily transported up the thermal gradient using bromine, nBr = 10 g-atom mP3, T = 1250 “K, AT = 70 “K. With T - 1050 “K and n, as the only variable, n, - 10 g-atom m-3 gave the largest yield and the best crystals. n, - 50 g-atom mP3 gave a 50”/) lower yield and smaller more intergrown crystals. The charge was extensively mineralized under such conditions. % - 3.0 g-atom mm3 gave marginally smaller yields of good quality crystals. A gradient of 5 “K cm-l gave the optimum yields and quality. Neither yield nor quality were markedly dependent on the magnitude of the gradient for gradients up to 10 “K cm-‘. Using n, = 10 g-atom rnm3, AT = 50 “K, and keeping all geometric variables fixed, the transport rate of MoAs, by Cl, Br or I was measured as a function of the average temperature T. The reproducibility of the results was only _t 25 “/o presumably because of variation in the nucleation conditions, i.e. a seed or substrate was not used. The transport rates given in table 1 are valid within these limits. Within the uncertainty, the transport rate for both chlorine and bromine was temperature independent for T = 1025-1275 “K. The behaviour using iodine was too erratic to establish optimum transport conditions. Larger, up to 2 x 2 x 5 mm, and less intergrown crystals of MoAs, were obtained by using the technique of Kershaw, Vlasse and Weld”). After some initial trans-

Halogen

Transport rate (mg hr-‘)

&) Br

875-915 1025-1275 1300 875-975 1025-1275 1075-1275

Cl I

0.05 0.7 0.2 (0.05 0.5 SO.05

Halogen concentration n, = 10 g-atom mV3; AT = 50 “K; ampoule length = 10 cm, ampoule cross-section = 0.60 cm2.

port, a reverse gradient was used to reduce the number of nuclei; the re-establishment of the transport gradient was gradual. Moving the growth zone away from the extreme end of the tube had no observable effect on the crystal quality. Crystals of other MO/AS phases were obtained with bromine as the agent by changing the stoichiometry of the charge as shown in table 2. The charge was placed TABLE 2

Transport conditions for various MO/AS phases Charge

T (“K)

Crystals

Direction

MoAs, MozAsa Mo,Asd

102551275 1250-1350 1250

MoAs, MoZAsB MogAs4

Down* Down Down

Bromine concentration nB, N 10 g-atom rnm3; AT = 50 “K. * Charge temperature > growth zone temperature.

at the midpoint of a 20 cm long tube with a gradient of 5 “K cm-‘, so that transport both up and down the gradient could occur. No simultaneous growth of two phases was observed. Mo,As, transported at rates comparable to those of MoAs, and an intergrown mass of acicular crystals with metallic lustre resulted. The transport of Mo,As, was much slower and the crystals were very small and intergrown. For T < 1200 “K, there was no observable transport. All crystals were identified by X-ray diffraction. The other phases which have been reported by some workers20,21) were not observed. Jensen et al.’ “) grew crystals of all these phases by halogen chemical transport; they did not specify which halogen or conditions gave success. Using chlorine, nc, = 3.7 x lo* g-atom mm3 (13 mg cme3), Taylor et al.‘“) grew MoAs, by down gradient transport at

HALOGEN

T = 1200“K. (The impression

TRANSPORT

of up-gradient

OF MOLYBDENUM

transport

in ref. 18 is erroneous.) Qualitative observations of the phases which quench from the vapour phase are useful in establishing the transport

mechanism.

For

table

TABLE

Yield of vapour Charge

quench

Mo,As4,

charges

Yield of vapour

and transport

Br

agents

which

readily

which

readily

identified by their colour, state, melting point, volatility, etc. The black needles were not identifiable but were probably a Mo/Br phase, possibly MoBr, which is dark greenz7). It is noteworthy that Mo/Br vapour phases were apparent only for the metal-rich charges. Because the elemental arsenic occurred as a mirror or as a secondary growth on the crystals and charge, it was difficult to estimate its amount; it was, however, significantly less than the amount of AsBr, or AsCl,. 3.2.

TOTAL

PRESSURE

MEASUREMENTS

Typical PT-l versus T curves are shown in fig. 2. Either bromine or chlorine, plus excess molybdenum equilibrated by cycling to 1200 “K showed a vapour pressure <7 x 10’ N mV2 (5 torr) up to 900°K. Above

60-

ner = IO g-atom

m

500

1000

T (OK) Fig.

2.

Typical

235

900 “K the observed pressure rise was presumably that of the disproportionation of MoBr, or MoCl,“). Bromine or chlorine plus excess MoAs, similarly equilibrated by a slow cycle to 1200 “K showed AsX, vaporization at 300-450 “K followed by a gradual and minor increase in PT- ’ up to ca. 1000 “K. Above 1000 “K, PT-' increased rapidly; at our measurement limit, 1275 “K,

quench

AsCl, + As AsBr, +As A&+MoI-J?) AsBr, + black needles hydrolyse AsBrs + black needles hydrolyse

MoAsz, Cl MoAs,, Br MoAsZ, I Mo,As3, Br

were

3

for various

and agent

3, the phases

ARSENIDES

PT- ' versus T plots.

All MO/AS/X and MO/X samples exhibited slow equilibration (several hours) in the temperature region near 1000 “K. At higher temperatures the equilibration occurred in less than 1 hr. At lower temperatures, for MoAs, +Br,, equilibration was fast, there being no further pressure changes after thermal equilibration. In all of 8 experiments, an unexpectedly high room temperature pressure - ca. 2 x IO3 N me2 - was observed after cycling to 1275 “K. These vapours could not be quenched at dry ice temperature (195 OK) but could be quenched at liquid nitrogen temperature (77 OK)*. 3.3.

OPTICAL

AND UV

ABSORPTION

The adsorption spectra in the range 250-700 nm up to 1075 “K were obtained for the vapours in equilibrium with the following cell charges; bromine, arsenic, arsenic tribromide, molybdenum plus bromine and molybdenum diarsenide plus bromine. For samples involving bromine, a concentration equivalent to nBr = 2.5 g-atom mm3 (0.2 mg cmP3) was used. In the Mo/Br, and MoAs,/Br, samples, five times as much MO as needed to form MoBr, was used. For the arsenic sample, nAS = 5 g-atom m-3 (PAS4= lo4 N rnm2; 80 torr at 1000 “K). These concentrations correspond to the lower limit of the range for which appreciable transport has been observed. Only for bromine was a well defined absorption peak obtained at all temperatures. The results agreed with those tabulated by Christian2 6). For the arsenic sample at 675 “K (P,,, of As, - 2.5 x lo3 N m-‘) an absorption peak at 260 nm was observed. At higher temper* A mass spectrometric studyz8) has shown that significant quantities of hydrogen are present in thoroughly degassed silica ampoules after cycling to 1300 “K. With bromine present, HBr is the significant room temperature vapour species. This is consistent with the physical behaviour described here.

236

J. J. MURRAY, 100

J. B. TAYLOR

r

AND L. USNER

transport system were abandoned when, for chlorine, the percent transmission above 775 “K increased continuously with time at fixed temperature and wavelength, probably because chlorine reacted slowly with the silica forming either a non-absorbing or involatile species. 3.4.

tMoAs215Br nBr

g-atom

(0.2

mg cmw3)

------

375’ 5750

----

775”

....... ..... . ...

---

m

-3

= 2.5

K K

K 975" K 1075” K

VAPOUR ANALYSIS

The equilibrium vapour over (MoAs,),Br was ana,lysed in the range 775-1175 “K; the results are given in table 4. The errors are f 2 “/;I, f 5 “/, and f 20 % in total Br, As, and MO respectively. The arsenic error is caused by the low sensitivity of the atomic absorption technique for arsenic; the molybdenum error is due to the low molybdenum concentrations. The error in the potential Br quantity, calculated assuming all bromine was in the vapour phase, is +3x, largely due to the uncertainty in estimating the relative volumes of the sections of the ampoule. Except at 775 “K, the measured bromine quantity always exceeded the potential value indicating that the error is systematic. MO results, accurate to ca. 5 “/, were obtained by the small volume solution technique, vide supra; such results are given in table 5. Using these higher accuracy results, it is clear TABLE 4

Vapour analysis for overall composition htnml

Fig. 3.

Optical/UV

absorption

Ratio Measured Br Potential Br

Temperature (“K)

for BrZ and for MoAs2/Br2.

atures only an absorption edge at 360-380 nm was observed. For MoBr,, because of the disproportionation reactions and subsequent slow equilibration at lower temperatures, only results for above 775 “K were valid. This sample exhibited a single absorption edge at 425435 nm. AsBr, also showed only an absorption edge at 320-360 nm. MoAs,/Br, showed only an edge at 330-360 nm. (The wavelength range given is the values of 50 % transmission over the temperature range 475-1075 “K.) The results for Br, and for MoAs,/Br, at selected temperatures are shown in fig. 3. It is apparent that in the system MoAs,/Br,, free bromine is not a significant vapour component up to 1125 “K. The anticipated shift in vapour composition from AsBr 3 to arsenic plus some Mo/Br species was not apparent. Attempts to obtain similar results for the chlorine

0.84 1.04 1.06 1.03 1.05 1.00 1.07 1.05 1.03

71.5 925 975 1025 107.5 1075 1125 1175 1175

Potential bromine concentration

(MoAs&Br

Vapour composition Atomic ratios Br/As MO/AS 2.9 3.3 3.2 3.0 3.0 3.1 3.1 3.0 3.0

0.002 0.008 0.006 0.02 0.01 0.02 0.02 0.02 0.04 ~.____ nBr N 10 g-atom rnm3.

TABLE 5

MO analysis for overall composition Temperature (“K) 715 975 1025 1175 Potential bromine concentration

(MoAs,),Br

MO concentration (mol rnm3) X 10’ < 0.2 2.9 4.6 6.8 nBr N 10 g-atom mm3.

_

HALOGEN TRANSPORT TABLE Vapour

analysis

6

is not observed,

for overall

compositions

(MoAQ.~)~BTand (MoAso.&Br Overall composition

Temp. (“K)

Ratio Measured

Br

Potential

Br

Vapour composition Atomic ratios Br/As

MO/AS

WoAsl.&Br

1075

0.98

3.1

0.04

(MoAs&&r

1175 775 1075

1.03 0.59 0.69

3.0 3.1 3.6

0.04 0.06 0.20

1175

0.81

4.0

0.43

Potential

bromine

concentration

n,,

-

10 g-atom

me3.

TABLE 7 Decomposition

pressure

Charge

of MO/AS

phases

by vapour

analysis

Temp. (“K)

MoAs2 +Mo,Ass

MozAs~ + MosAsL MoSAs,+Mo

1075 1175

‘4.1 x 102 3.7 x 103

1075, 1175 1075, 1175

<40 <40

237

OF MOLYBDENUM ARSENIDES

that the concentration of MO in the vapour increases approximately linearly with temperature between 775 and 1175 “K. The analyses of (MoAs,.,),Br and (MoAs,,,),Br samples are summarized in table 6. It is clear that not all the available bromine was present in the vapour phase for the latter sample, even at 1175 “K. The equilibrium decomposition pressures of the MO/AS phases were estimated by analysis of the total arsenic concentration of the vapour. The results, using two phase compositions to ensure invariant conditions, are given in table 7. Pressure was estimated assuming all the arsenic vapour was As,. 4. Discussion The following data, condensed from the preceding section, are pertinent to a discussion of possible transport mechanisms. For growth conditions, arsenic trihalide is the principal component of the vapour although for the more metal rich charges there is evidence of significant amounts of an additional vapour component. The composition of the transported crystals directly corresponds to the composition of the polycrystalline charge, except for transport by iodine, which, as well, is distinctly inferior to bromine and chlorine as a transport agent. In the absence of halogen, transport

even in the presence

of high arsenic

pressures. For equilibrium in the MoAs,/Br system, AsBr, is the major vapour phase up to at least 1000 “K, elemental bromine is an unobservable vapour species up to at least 1075 “K, and Mo/Br species are minor vapour components, although their concentration increases linearly with temperature, up to at least 1175 “K. Above about 1000 “K, an equilibrium shift occurs as indicated by the rapid increase of vapour density with temperature. The concentration of molybdenum species in the vapour is slightly greater in the Mo,As,/Br equilibrium system than in the MoAs,/Br system. For both these systems the atomic ratio Br/As remains 3.0 up to at least 200 “K above the temperature at which the vapour density begins to increase rapidly. The concentration of molybdenum species in the vapour is significantly greater for the Mo,As,/Br equilibrium system, and for this system only, a low volatility bromide is stable, presumably MoBr,. In the only previous detailed study of the transport of transition metal pnictides by halogens, Schafer and Fuhr”), using iodine as the agent, were able to grow crystals of all the intermediate phases (known at that time) in the Nb/As and NbjSb systems. In these cases the simple relationship between the composition of the charge and of the transported crystals also existed. A mechanism was proposed which was admittedly not strictly valid because of the suspected interference by arsenic iodide. However, the equilibrium was of a known type, type 2 (see introduction), and was consistent with the shift from down to up gradient transport which they observed when the niobium content of the charge was increased. An equilibrium of the same type for the transport of the MoAs, by Br is MoAs,(s)

+ 2 Br,(g)

= MoBr,(g)

++ As,(g) .

(4)

This mechanism is consistent with our observations if it is postulated that it occurs in a vapour consisting primarily of AsBr, which is inert to the transport process. The equilibrium for Mo,As, transport would be Mo,As,(s)

+ Br,(g)

= 2 MoBr,(g)

+ 2 As,(g) .

In this case, there would be a competitive equilibrium

non-transport

(5)

J. J. MURRAY,

238 2 Mo,As,(s)

+ 2 (Br,g) = MoBr,(g)

J. B. TAYLOR

= &As,(g) + 3 Brig),

L.

USNER

+ 3 MoAs,(s) . (6)

The relative occurrence of (5) and (6) is dictated by the decomposition pressure of MoAs,; if this pressure is low, equilibrium (6) would predominate, As, would then be a minor vapour species, even relative to MoBr,, and the transport rate would be limited by the rate of diffusion of arsenic in the vapour. Using the decomposition pressures of MoAs, derived from vapour analysis, we estimate that for T = 1200 “K and AT = 50 “K, that AP,,, would be about 1.3 x lo3 N mm2. Using this T and AP, Ptota, = 4.0 x IO4 N mm2 (300 torr), a diffusion path of 10 cm and a binary diffusion constant D = 0.40 cm2 set-‘, the diffusion rate can be calculated from Fick’s law. D was estimated from the usual equation3’) assuming the mean molecular diameter to be 6 A, which is consistent with the other gas being either AsBr, or MoBr,. The result is equivalent to a Mo,As, transport rate of 6 mg hr-I, which is comparable with the observed transport rate. If Mo,As, were to be transported by a similar mechanism, then the decomposition pressure of Mo,As, would limit the arsenic pressure in the transport system. At 1175 “K this pressure is <40 N mW2. A AP,,, of 40 N mm2 would just yield the slow transport (ca. 0.05 mg hr- ‘) which we observed. However both PAS4 and AP,,, are probably significantly less than 40 N mm2 and this is incompatible with the observed rate of transport. Because of the demonstrated existence of AsBr, in the vapour, the co-transport of MoAs, (x = 2, 1.5,0.8) by MoBr, and AsBr, is the logical modification of this mechanism which eliminates the above incompatibility. Less probable mechanisms could be advanced, involving either a kinetic preference for reactions of type (5) relative to type (6), or ternaries, oxyhalides, or hydrogen halides in the vapour phase. In any case, the transport process is a minor phenomenon in a vapour which is predominately AsBr, so that detailed consideration of total pressure measurements is unlikely to be of value in elucidating a transport mechanism. Even the major shift in equilibrium above 1000 “K (which, in any case, is above the minimum temperature for observable transport) is inexplicable in terms of any reasonable conceivable As/Br and Mo/Br vapour species. A shift of the equilibrium AsBr,(g)

AND

(7)

TABLE 8

Observed

transport of transition metal (see also tables 1 and 2)

pnictides

Charge

Agent

Phase transported

Transport direction

T

NbAsz NbAs NbSbz Nb$bd Nb,Sb NbP WAS, MoAsz Ni,As, PdzAs

1 I I 1 I I Br Cl I, Br Cl Cl Cl, Br Cl, Br Br I I I I, Br I I, Br 1

NbAs, NbAs NbSbz Nb5Sb4 Nb,Sb NbP

Down?

1190 1250 1220 1180 1180 1235 1260 1200 1075 125 850 1075 1075 1200 975

Pdz.TAs NbAs, TaAs, NbAsz FeSbz FeP

UP* Down UP UP Down Down Down Down ?

WzAs3 MoAsz Ni,Asz PdsAs Pd,As NbAs, TaAs, NbAs FeSbz FeP CoAsz Re,As,

UP Down Down Down Down Down

CrAs

FeP, Nb,(Sb,

Te)

Ref.

29 29 29 29 29 29 18 18 32 17 17 33 33 33 34 35 36 37 38 39 40

In addition, Kjekshus and co-workers have obtained crystals of MoAsZ, MozAss, MogAsq, WzAs,, WAs2, Mo3Sb,19), CoAs2, RhAsz, TrAsZ4’). Neither charge composition, halogen nor thermal conditions were specified. t Charge temperature > growth zone temperature. * Charge temperature < growth zone temperature.

would be compatible with the observations of density, As/Br ratio and MO content of the vapour but it is known to be strongly shifted towards the left under the conditions of our experiments31). Table 8 lists the transports of transition metal pnictides of which we are aware. A stability of the metal halides per g-atom of halogen which is comparable to or less than that of AsX, or SbX, is anticipated for metals (Me) of groups VB through IB inclusive, and particularly for the metals of the second and third series42). Thus the behaviour of the system Mo/As/Br should be typical of the systems Me/Y/X were X is I, Br or Cl and Y is As or Sb. Possible contribution by an oxyhalide and/or hydrogen halide transport mechanism cannot be neglected. Oxyhalides, as proposed by Schlifer2), formed by reaction of halides with water desorbed from the container walls, may explain why some chalicides (and, as tabulated here, some pnictides) of MO and certain other transition metals are transported with I whereas the

HALOGEN

TRANSPORT

OF

pure metals cannot be transported with I. The situation for transport by Br or Cl is different. Transport of MO with Br was easily attained and transport of MO with chlorine has also been observed43). As well, the inferiority of I relative to Br and Cl as a transport agent for MoAs, suggests different mechanisms. That is, the I transport may well be trace oxyiodide transport whereas the Br and Cl transport are probably true halide transports. Acknowledgements The contributions of the following persons are gratefully acknowledged : N. Goodhue who did the chemical analyses; A. Fabris and D. Cochrane who assisted in the experimental work.

References 1) H. Schafer, 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16)

Chemical Transport Reactions (Academic Press New York, 1964). H. Schafer, J. Crystal Growth 9 (1971) 17. D. Richman, RCA Rev. 24 (1963) 596. V. J. Silvestri and V. J. Lyons, J. Electrochem. Sot. 109 (1962) 963. M. A. Zuegel, J. Electrochem. Sot. 112 (1965) 1153. D. J. Kirwan, J. Electrochem. Sot. 117 (1970) 1572. V. S. Ban, J. Electrochem. Sot. 118 (1971) 1473. L. A. Egorov and V. N. Kochnev, Inorg. Mater. 5 (1969) 1172. A. V. Sandulova, V. A. Voronin and V. A. Prokhorov, Zh. Fiz. Khim. 45 (1971) 2165. L. A. Egorov, V. N. Doronin and Z. S. Medvedeva, Inorg. Mater. 6 (1970) 1707. R. Nitsche and D. Richman, Z. Elektrochem. 66 (1962) 709. I. V. Gordev and V. V. Karelin, Inorg. Mater. 5 (1969) 1012. T. 0. Sedgwick and B. J. Aqule, J. Electrochem. Sot. 113 (1966) 54. J. H. E. Jeffes, J. Crystal Growth 3,4 (1968) 13. F. Hulliger, U.S. Patent 3,403,002 (1968). J. J. Murray and J. B. Taylor, J. Less-Common Metals 21 (1970) 159.

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ARSENIDES

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17) G. S. Saini, L. D. Calvert, R. D. Heyding and J. B. Taylor, Can. J. Chem. 42 (1964) 620. 18) J. B. Taylor, L. D. Calvert and M. R. Hunt, Can. J. Chem. 43 (1965) 3045. 19) P. Jensen, A. Kjekshus and T. Skansen, Acta Chem. Stand. 20 (1966) 403. 20) A. Brown, Nature 206 (1965) 502. Monatsh. Chem. 95 (1964) 1272. 21) H. Boiler and H. Nowotny, M. Vlasse and A. Wold, Inorg. Chem. 6 (1967) 22) R. Kershaw, 1599. 23) H. Schafer, F. H. Wehmeier and M. Trenkel, J. Less-Common Metals 6 (1968) 290. E. T. Keve and S. C. Abrahams, Inorg. 24) F. H. Wehmeier, Chem. 9 (1970) 2125. J. Phys. Chem. 68 (1964) 25) B. L. Bruner and J. D. Corbett, 1115. Ph. D. Thesis, University of Washington 26) J. D. Christian, (1965). in: Halogen Chemistry, Vol. 3, Ed. V. Gut27) J. E. Fergusson, mann (Academic Press, New York, 1967) p. 227. R. F. Pottie and R. Sander, to be published. 28) J. J. Murray, Metals 8 (1965) 29) H. Schafer and W. Fuhr, J. Less-Common 375. New 30) R. D. Present, Kinetic Theory of Gases (McGraw-Hill, York, 1958). Thermodynamic Properties, 31) Selected Values of Chemical NBS Technical Note 270-3 (1968). 32) G. S. Saini, L. D. Calvert and J. B. Taylor, Can. J. Chem. 42 (1964) 150. 33) G. S. Saini, L. D. Calvert and J. B. Taylor, Can. J. Chem. 42 (1964) 630. Acta Chem. Stand. 23 (1969) 34) H. Holseth and A. Kjekshus, 3043. 35) A. Wold, Intern. Congr. Pure and Appl. Chem., Sydney, 1969, see ref. 2. and M. Wintenberger, Bull. Sot. Fr. Mineral. 36) R. Darmon Crist. 89 (1966) 213. and T. Skansen, J. Less-Common 37) P. Jensen, A. Kjekshus Metals 17 (1969) 455. W. E. Jamison, A. F. Andresen and 38) K. Selte, A. Kjekshus, J. E. Engebretsen, Acta Chem. Stand. 25 (1971) 1703. 39) E. Dahl, Acta Chem. Stand. 23 (1969) 2677. and A. Kjekshus, Acta Chem. Stand. 20 (1966) 40) S. Furuseth 245. Acta Chem. Stand. 25 (1971) 411. 41) A. Kjekshus, 42) A. W. Searcy, in: Progress in Inorganic Chemistry, Vol. 3 (1962) p. 49; in: Survey of Progress in Chemistry, Vol. 1 (1963) p. 35. 13 (1934) 405. 43) A. E. van Arkel, Metallwirt.