mo-cvd growth of ingaas using me3ga , asme3, ash 3 and · mo-cvd growth of ingaas using me3ga ,...

Journal of Electronic Materials, Vol. 13, No. 4, 1984

MO-CVD GROWTH OF InGaAs USING Me3Ga , AsMe3, AsH 3 AND

Me3In OR Et3In AND ANALYSES OF ADDUCTS FORMED DURING THE

GROWTH PROCESS

C.H. Cheng* and K.A. Jones** Electrical Engineering Department

K.M. Motyl Chemistry Department

Colorado State University Fort Collins, CO 80523

(Received August 24, 1983; revised April 17, 1984)

High quality InGaAs films have been grown using the adduct Me3In.AsMe~ to block the room temperature reaction between Me, In and ~sH~ and by using a cover piece to prevent the preferential ~vaporation of phosphorus from the InP substrate during the warm up. Infrared spectroscopy shows that Me.Ga + AsMe and Me_Ga + AsH_ form stable adducts at room te~perature,3Me~In +3AsMe^ probably form a stable adduct, while Et In + A~Me_ probably do not form a stable Lewis acid/bas~ adduct. JPoorer quality films are grown with Et3In than with MeqIn because the AsMeq is unable to prevent by adduct formation the room temperature reaction between Et~In and AsH~. Pyrolysis studies show that the individual alk~is are sta~le to -4OO~ and mixtures are stable to -350~ The problems associated with the lower vapor pressure of the adducts, adduct dissociation at room temperature and the growth temperature, and using an adduct as the starting material are discussed.

Key Words: InGaAs, adducts, MO-CVD, electrical characterization, IR absorption spectra, gas chromatography.

*Present address:

**Present address:

IBM T.J. Watson Research Labs., Yorktown Heights, NY 10598 Electrical and Computer Engineering Department, UMass, Amherst, MA 01003

703

704 Cheng, Jones and Motyl

Introduction

The MO-CVD growth technique is attractive because it can be used for growth over large areas and the thickness control is excellent. Moreover, for the AIGaAs system the composition of the solid is proportional to the composition of the vapor (1,2).

The likely growth mechanism for GaAs is that the adduct Me Ga.AsH_, is formed via a Lewis acid-base reaction at roo~ temperature, and then it decomposes at higher temperatures when the three hydrogens combine with the three methyl groups via methane elimination reactions (3). The reactions can occur quickly due to the fact that the activation energy is small. It is small because the hydrogen atom combines directly with the methyl group without first being removed from the arsenic atom. However, if the reaction temperature is not high enough, not all of the methyl groups are removed, and a polymer, (-CH.GaAsH-) , (4) is formed. This reaction channel is chosen both nbecause it becomes increasingly difficult to remove the next methyl group, (5) and the radical -CH3GaAsH- is highly reactive towards polymerization.

Indium containing compounds differ from the gallium containing compounds in that methane elimination reactions occur at room temperature. Didchenko et al. (6) showed that two of the methyls are removed from the Me~In.PH R adduct and a polymer (-CH~InPH-) n is formed. The ~olymef has a low vapor pressure ~o it precipitates out of the gas stream leading to an indium deficiency. It also can precipitate out on the substrate leading to a poor morphology (7). Similar problems occur during the growth of InGaAs from Me3In or Et3In and arsine (8,9).

It is not clear why the indium alkyls are more reactive at room temperature, but it is thought that it is, in part, due to the greater difficulty in removing the second and third alkyl as the atomic number of the metal increases (5), and the strong tendency for indium to form a 4-coordinate bond. This tendency is well illustrated in the crystal structure of Me_In, and its low vapor pressure has also been attributed to'he relatively stronger molecu-

lar bonding (10). Another likely contributing factor is that the negative charge on the alkyl groups is larger for

MO-CVD Growth of InGaAs 705

the indium alkyls than it is for the gallium alkyls (11). Thus, the hydrogen atom on the hydride section of the adduct is more strongly attracted to it.

To eliminate, or at least reduce, the room temperature polymer formation of the indium containing compounds, investigators have I) kept the indium alkyl and the PH_ or AsHR separated until they are immediately above the ~us- cep~or; (7,12) 2) combined low pressure growth with the pyrolysis of the PH_; (13,14) 3) used the adduct, Me In. PMe^; (15) and34) formed the adduct Me^In. PMe_, Me.~n.NMe_~ Me3In.AsMe 3 o r Et3In.AsMe 3 (17-19) i~ the g~s phase. 3

Keeping the constituents separate has not worked well because the reaction occurs fast enough for the polymer to form before the constituents reach the substrate. As a result, the growth rate is slow and the morphology is poor.

The low pressure approach has produced excellent results. The only problems with it are the system is more complicated, and toxic chemicals accumulate in the pump.

The results using the MeRIn. PMe 3 adduct were dis- appointing. It did decompose, but the decomposition products were both indium and InP. An effort was made to eliminate the indium by introducing PCIR along with the adduct. However, the success was limite~ and using PCIR introduced the problems associated with chloride reaction~ with the growth system.

Using NMe~ or PMe_ to form the adduct, Me~In.XMe^, and mixing the ad~uct wit~ PH. to form InP or ~sH~ t~ form InGaAs, Moss and Evans (16~ were able to grow g~od films. Also, Ludowise and his associates (17-19) have grown good InGaAs films using the adduct Me^In.AsMe_ with and without additional arsenic in the form o~ arsine~

We have grown InGaAs films using this latter technique and have investigated the problems associated with using adducts formed by separately mixing the constituents. These include I) the lower vapor pressure of the adduct, 2) the dissociation of the adduct, 3) the reactions of the adduct in H_ at elevated temperatures, and 4) exposing the InP substra~e to a phosphorus deficient atmosphere.


Theoretical Background

The adduct has a lower vapor pressure than the constituents and will precipitate out if two saturated vapors are mixed together. This problem can easily be overcome by bubbling H~ through the alkyls held at a temperature sufficiently below room temperature and then mixing them at room temperature. This is the procedure followed for Me~Ga (20) and AsMe~ (21) and their adduct (22) since they h~ve relatively hi~ vapor pressures. Another way this problem can be overcome is by diluting the saturated vapors at room temperature with a sufficiently large amount of hydrogen before the metal and nonmetal alkyls are mixed. This is what is done for Me,In (23), Et~In (24), and Et3Ga (25) since they have relatively small Vapor pressures.

Adduct dissociation is not a problem that can be easily circumvented. It is a serious problem if, for example, the Me~In.AsMe_ adduct dissociates in the presence

of form ASHa3preci~itate.. Me_I~would t~en be free to react with AsH 3 and

The degree of dissociation for the reaction

R3M + XR' 3 + R3M.XR' 3

A+B § C

can be computed from the equilbrium constant, Kp, where

K = exp(-AG~ . P

Because the adduct bond is relatively weak when compared to the metal and nonmetal carbon bonds and the C-H bonds, AG ~

AH ~ (26). From

Kp = Pc/PAPB = Pc/(PA~176 )

where o designates the partial pressure before mixing,

PC = I/2{(PA~176176176176 PB ~ �9

Thus, as K gets large, PC ~ p o, and as K gets small, P. O, as i~ should, since in a~l applications PB ~ > p o

the nonmetal alkyl pressure is greater than the metal a~kyl pressure.


~=.O025 o T=300K . . . . . . P~=~015 �9 T=600K

- - ' - - ~ : , 0 0 0 5 �9 T : 9 0 0 K

1.0_

:'I / / / /,, '" / �9 8 / /I /

/// /' 1 / " , / / / / / /

0 - ' " I .... I

o 1o 15

- A ~( ~.al/MoJe)

] 2O

Fig. I. The fraction of the adduct dissociated, PcIPA ~ plotted as a function of the enthalpy of formation of the adduct for T = 300, 600, and 900K and PB = 0.0005, 0.0015, and 0.0025 arm.

The association parameter, p_/p.O, is plotted as a function of -AM ~ in Fig. I for T=~0~600 and 90OK, p o = .0005, .0015, and .0025 atm., and p o = 8p.O. One w~uld like a value of AH ~ for which there ~s essenStially no dissociation at room temperature and almost complete dissociation at the growth temperature of 900K. This would prevent room temperature precipiation reactions when AsH~ or PH~ is present, and would allow the AsH~ or PH~ to react directly with the metal alkyl at the grbwth t~mperature provided


that the dissociation kinetics are fast enough. This ideal is approached for p_/p.o when A H ~ = -8 kcal/mole. At 300K p~/p.O > 0.99 for a~l ~hree values of p o whereas at 900K p~/p~o : 0.0418, 0.1147, and 0 1765 fo~ p o : 0.005,

" o 0.0015 an~ 0.0025 respectively. These P_/P. values can be altered by changing the PR~ ~ ratio,~bu~ the effect is not significant. For example, the three 900K values of p_/p u are decreased to 0.0416, 0.1133, and 0.1733 when the p~O/~.o ratio is 4, and they are decreased to 0.0412, 0~110~, and 0.1672 when this ratio is 2.

If AH ~ is too large, the adduct will not sufficiently dissociate at the growth temperature, and much of the metal bearing adduct will be transported out of the system if the adduct itself does not decompose. This, of course, will reduce the rate of incorporation of the metal from this particular adduct -- a problem which has appeared when Me_In.PMe^ (19L was used. This is consistent with our pl~t in F~g. 1as the dissociation enthalpy for this adduct is 17 kcal/mole (26).

Experimental Procedure

The films were grown in a vertical reactor at atmospheric pressure using an rf heating source. Purified hydrogen was bubbled through Et3In at 50-200 ccm or passed over solid Me,In at 25-100 ccm at room temperature. The vapor was dil~ted with H~ flowing at 1500-2000 ccm, and then it was mixed with As~e_ to form the adduct. The AsMe~ vapor was obtained by bub~llng H~ at 25-50 ccm throug~ AsMe~ held at OuC. Downstream ~e_Ga was added to the mixtu~re by bubbling H_ at 2-20 ccm ~hrough Me^Ga held at -15~ This mixture e~ptied into the growth chamber where it was mixed with a I0~ AsH~ in hydrogen mixture flowing at 100-200 ccm that was int~bduced through a second inlet line. The gas flow rates were monitored with Tylan flow controllers, and the organometallics were purchased from Alpha.

The SiC coated graphite susceptor had a notch cut into it where the substrate could be placed and then covered by a quartz flat that could be pulled back using a vacuum feed through remote control device. The susceptor was mounted on a quartz thermocouple tube, and the assembly could be

MO-CVD Growth of lnGaAs 709

rotated while the temperature was being continuously monitored.

The (100) InP substrate oriented 3 ~ towards the [110] direction (Crysta Comm) was chemically-mechanically polished with a I% bromine/methanol solution and then soaked in a KOH solution to remove oxides from the surface. It was then cleaned using a procedure described by Clawson et al (28). This includes a rinse in deionized H20, dip into HoSO a for 3 min with ultrasonic agitation, rrnse in hot methanol, dip in H^SO~ for 3 min, rinse in hot methanol, and blow dry wit~ ultra-pure N 2.

After the quartz growth chamber was baked out, the substrate was mounted in the susceptor. The system was then purged for 15-20 min with N2, and this was followed by a purge in purified Hp for 30 min. The substrate was covered, the rf genera%or was turned on, and the gases began to flow. After the substrata reached the growth temperature (630-750~ and the gas flows stsblized, the cover piece was removed and films were deposited for 20-60 min. At the end of the growth cycle the H 2 to the metal alkyls and the rf generator were tuned off, and the H 2 to the AsMe~ and the AsH^ were turned off when the temperature dropped ~elow 400~ j

The films were characterized by examining their morphology with a Nomarski interference microscope and an SEM, determining the lattice mismatch using a Lang x-ray camera, measuring their carrier concentrations and mobilities at room and LN temperatures using the van der Pauw technique, and measuring the growth rate by measuring the film thickness of a cleaved and etched sample in an SEM. The (400) reflections of CUKa radiation from a fine focus tube with a beam extender were used to determine the mismatch. For the Hall effect measurements 12% Ge in Au contacts annealed for -5 min at 460~ were used. An A-B etch was used to delineate the film-substrate interface.

The metal and nonmetal alkyls and their adducts were characterized by their IR spectra, and their decomposition products at elevated temperatures were analyzed by gas chromatography.


The procedure for recording and comparing the IR absorption spectra is as follows. A nonmetal alkyl gas is collected from the HO-CVD system in a pyrex mantle, and the alkyl is condensed at 77K and the hydrogen is removed in vacuo. The mantle is warmed, and the alkyl is transferred in vacuo to the gas cell by condensation in a liquid nitro- gen cooled cold finger. The gas phase IR spectra f~ these compounds are recorded in the range 400-4000 cm , the longer wavelength limit being determined by the absorption edge of the KBr windows. The nonmetal alkyl is transferred out of the gas cell into a storage flask by the reverse of the above procedure. The same procedure is followed for transferring and recording the IR spectra of the metal alkyl. Finally, the two alkyls are mixed in the gas cell and the IR spectrum of the mixture is recorded. When AsH_ or PH_ is used, the gas cell is filled directly from th~ MO-CV~ system.

For analyzing the decomposition products of the individual molecules and their adducts, we again must first col- lect a sample in a pyrex container from the HO-CVD system. A sand filled heating mantle with a pyrex sample holder is brought to a predetermined temperature between 100 and 400-C, and the pyrex container is inserted into the sample holder. After a predetermined time the bulb is removed and cooled to room temperature. The hydrocarbons generated at this temperature are transferred through a -120~ trap using a Toepler pump. The -120 ~ volatile components are collected and analyzed by gas chromatography employing direct injection from the Toepler pump collection chamber. The components are identified by their retention times and the quantities are determined by comparison to the inte- grated intensity of a CO 2 internal standard.

Experimental Results

Micrographs of films grown with and without the cover piece are shown in Fig. 2. Note that the film grown with a cover piece is much smoother. Also, the micrographs in Fig. 3 illustrate that films grown with He3In are smoother than those grown with Et3In.

The room temperature and LN mobilities are plotted as a function of the net carrier .~oncentration in Fig. 4. The lattice mismatch is _2.5 x 10 -J. As one would expect, the


(a)

Fig. 2. Morphology of a film grown a.) with (500 x) and b.) without (500 x) a cover piece.

(b)

712 Cheng, Jones and Motyi

(a)

Fig. 3. Morphology of a film grown with a.) Me31n (500 x) and b.) Et31n (500 x).

(b)

M O - C V D G r o w t h o f I n G a A s 713

>

>.

.J

O

Fig. 4.

lo'

�9 :T= 77~

= 3 0 0 ~ K

(•01 I ~ I f f I I I I 1

N d - N a ( c m - 3 )

The room temperature and LN mobilities plotted as a function of the net carrier concentration.

ro~m temperature mobility decreases from 5200 to 2800 cm~V-sec ~ the~carrier concentration increases from 2 x 10 to 10 ' cm-~,^while the LN mobilities decrease from 22,000 to 6,900 cmZ/V-sec. In Fig. 5 the mobilities are plotted as a function of the lattice mismatch. Be room temperature mobility decreases from 58~0 to 2_~0 cm /V-s as the mismatch increases from 1.5 x I0 -J to I~ , and the LN mobility decreases from 25,000 to 6,100 cm /V-see. There is some variation due to a variation in the background carrier concentration since it was not possible to keep this parameter constant. The room temperature mobility and carrier concentration are plotted as a function of the growth temperature in ~g.cm_36 The carri~ concentration increases from 1.5 x 10 -v to 1.2 x 10 ~ cm -~, and the mobility decreases from 5800 to 2500 cm /V-see as the growth temperature increases from 630 to 750~ The growth rates varied between 1.4 and 2.5 um/hr.

Th~ complete IR absorption spectra between 4000 and 400 cm- for Me3Ga (29,30), AsMe 3 (31,32) and Me3Ga.AsMe 3


10 4

O >

10 16 3

Fig. 5.

Fig. 6.

e : T = 77~

& : T = 3 O O ~

I I I I l l t , t I

l d 2 A a Q

The r o o m t e m p e r a t u r e a n d LN m o b i l i t i e s p l o t t e d a s a f u n c t i o n o f t h e l a t t i c e m i s m a t c h .

The r o o m t e m p e r a t u r e m o b i l i t y a n d n e t c a r r i e r c o n - c e n t r a t i o n p l o t t e d a s a f u n c t i o n o f t h e g r o w t h t e m p e r a t u r e s .

u

=,

1o

I I I I I 1 I I

I I I I I I I I ~ 0 640 660 680 7 ~ ~ 0 740 760

1017

' E r

r z

" 0 z

16 I0

G R O W T H T E M P E R A T U R E I~ ]

M O - C V D G r o w t h o f I n G a A s 7 1 5

4-"

* '4

4.)

0 " 0 ~ . '4

!

0

(D -'4

e"

�9 0 ! '~" ~ - '4

0

4~

Z "

0

�9 "4 H~

0 0-,

0 ~- o'3 ~- cO oo

oo L~

~ ~ r ~ 0 4 L ~ ~

~:~,1 ~ ~ ~ ~ ~

�9 . ~ ~ , ~

Od ,~0 CO L~ -..-r o r

t ' N 0 4 ~

*'4

q~

t-- �9 "4 .~ "U o

"u

0

0 , , ~

L r . ~ �9 "4 c,~ r ~.,

" o

e . ~-- o ~

�9 , 4 , ~

e t .

o c . . ~

r r m ~

(1~ s . e., o

[ - , ~.q

,..-t

r - t


are shown in Fig. 7, and the peak positions and intensities are listed in Table I. Note that the adduct peaks are less intense. Similar spectra were recorded for Me3Ga, AsH_ (33) and Me.Ga.AsH_, and again the intensities for th~ adduct peaks~Were l~ss. We also observed a yellow-gold precipitate on the cell wall. For MeqIn (30), EtqIn and their AsMe~ adduets we were unable to obtain a compl~te spectrum( only ~ the CH~ symmetric deformation peaks near 1150 cm-" were obtaine~ and the Me_In.AsMe^ peaks were less intense than the Me.In peaks, b~t the ~t^In and the Et^In.AsMe^ peaks had s~milar intensities. T~e Et~In peaks ~ere mor~ intense when the NaCl windows were u~ed, and the Me3In peaks were absent when the KBr windows were used. In addition, we noted that it took a much longer time to evacuate the gas cell when it contained Meqln and EtqIn. Finally, when AsH~ was used, no absorption peaks gere observed, and a br~wn precipitate formed on the cylinder walls.

B0

M e O l + A S M e 3 3

I00- ' = , i _ J = , , L = ~ , , , , , , , , , , , , ,

Me As IO0 ~ ~ i 1 I 1 ~ i J i �9 ~ I , , , , , , i , 1 , , ,

M e G ~ r . . . . . . . . . . . . . . . . . . . . . . . , I 4000 3000 2000 1500 1000 500

C M - I

Fig. 7. -I

Complete IR system between 4000 and 400 cm a.) Me3Ga , b.) AsMe 3, and c.) Me3Ga.AsMe 3.

for


The metal CH R symmetric deformation peaks are expanded to approximately the same height and are compared in Fig. 8. Note in Fig. 8a that the rotational fine structure PQR peaks are distinguishable for Me^Ga, but they are not distinguishable in the Me~Ga.AsH 3 j and Me~Ga.AsMe 3 adducts. There is no apparent fi~e structure in tNe Me,In and Et3In peaks.

The temperatures at which the individual alkyls and their adducts begin to decompose are shown in Table 2 along with the ratio of the partial pressures of the methane evolved and the partial pressure of the alkyl. For the double alkyl adducts the denominator is the partial pressure of the metal alkyl.

- - Me30a

. . . . . . . . . Me3Oa + AsH 3

__ - - Me3Ga + As~ te 3

12 i 00 [/

,I i: i :

I; �9 i / j . ~ ?k. ~i

i ' \

- - Me31n

. . . . . . . . . I~e31n + AsH

- - - - - Me 3 § AsMe

115q

x\

/

Et31n

. . . . . . . . . Et31n 4- AsH 3

- - - - - Et31n 4- AsMe 3

1150

' , " 4' /

J

/

Fig. 8. Enlargements of the CH~ symmetric deformation absor tion peak for a ~ Me^Ga, Me^Ga AsMe~, and P " 3 " J Me Ga AsH , b ) Me In, Me ~n AsMe~, and Me~In.AsH~ 3 " " 3 " J J and c.) E~3In, Et3~n-AsMe 3, and E~3In.AsH 3.


I Pyrolysis PCH4/V.P of

Substance Temperature (~ Constituent

~e3Ga 400 -0.001

0.O01 %sMe 3

~e3In

400

400 Too Little to be

Detected

Et3In 400 Too Little to be

Detected

Me3Ga " AsMe 3 400 0.0093

Me3In " AsMe 3 375 0.24

Et3In " AsMe 3 360 0.33

Table 2. The temperature at which noticeable decomposition occurs for the alkyls and their adducts,.and the ratio of the partial pressures of the methane evolved and alkyl partial pressures.

Discussion

The morphology is much improved using a cover piece. This was also found to be the case for the hydride growth of InGaAs and has been attributed to the reduction of the preferential evaporation of phosphorus. (34,35)

Our carrier concentrations are similar to those obtained by Dietze et al. (18), and for the same composition our room temperature mobilities are similar. We, however, grew some material that contained more indium and this material had a higher mobility. The investigators observed similar increases in the carrier concentration and decreases in the mobility as the growth temperature was in- creased.

Whitely and Ghandhi who grew InGaAs films using AsH~, Et~In, and Me~Ga (36) or Et~Ga (37) also had similar mobi~- itfes and car~ier concentrations. However, our LN mobili-


ties were a bit higher. They also determined that their carrier concentrations decreased with increasing growth temperatures up to 700~

The best MO-CVD InGaAs has been grown by Duchemin's (14) group. Using a low pressure apparatus, they have grg~n fi~ms with carrier concentrations as low as 1.8 x I02- cm -~, room temperature mobilities as high a~ 11,900 cm /V.sec, and LN mobilities as high as 54,600 cm /V-sec. In addition to using low pressure, these outstanding results were obtianed using a growth temperature of 550UC and the films were more closely lattice matched than ours.

Because we were able to grow InGaAs films at relatively low flow rates, it is likely that the Me3In.AsMe_ adduct did form and inhibited the room temperature reactio~ between Me.In and AsH~. Other evidence is" that the CH 3 symmetric ~eformation ~eak of Me,In was reduced in intensity when AsMe~ was introduced. The intensity was presume- ably reduced b~y the condensation of the lower volatility adduct.

The IR absorption evidence is not strong, however, due to the fact only the single peak could be observed and then only when a NaCI window was used. A possible explanation for this is that the NaCI and KBr windows provide Lewis base sites which bind Me^In. This is consistent with the fact that KBr is a stronger base than NaCI, and it took longer to outgas the gas cell when it contained indium alkyls. Others (30) have observed all of the Me,In peaks using NaCI and KBr windows, but they heated th~ cell to 100UC. They did this to increase the vapor pressure; it could also have decreased the amount of adsorption.

In order to theoretically predict whether the adduct is stable at room temperature, we must know the enthalpy of formation. Drago and Wayland (38,39) developed a four parameter equation

-AHAB = EAE B + CAC B

for predicting acid-base reaction enthalpies for the formation of adducts in the gas phase.


For MeRIn , C A : 0.654 and E A : 15.3. Unfortunately, we were unaDle to find C B and E_ values for AsMe_, but we can still determine the mlnimum ~H value for Me3I~.AsMe ~ by using the known -AH = 10 kcal/mole value for MeRGa.ASMeR (23) and the fact that the Me3In.AsMe^ bond is weaMer. Foff Me3Ga, C A = 0.881 and E A = 13.3. Thu~

-AH = 0.881 x - 13.3Y = 10 or

x = 11.35 - 15.1y

Therefore, for Me3In.AsMe 3

-AH = 0.654 (11.35 - 15.1y) + 15.3y = 7.42 + 5.43y

so the minimum adduct bond energy is 7.4 kcal/mole.

From Fig. I it is seen that for -AH = 7.4, the adduct should be almost completely associated (Pc/P. ~ = I) at room temperature. This is near the point where t~ere is a steep rise in the room temperature dissociation curve so it is probably important to keep the AsMe~ pressure relatively high to reduce the tendency to dissocfate.

This theoretical analysis differs substantially from the experimental results of Coates and Whitcombe (27) who determined that the Me_In.AsMe^ adduct is substantially dissociated at I00~ ~heir results for the dissociation of the Me~Ga.SMe^ adduct differ in a similar way. (22) A possible ~xplana~ion is that they were observing the dissociation of dimers such as (Me~In.AsMe^)^ (4). However, we did see some black precipitate w~ere the AsH_ was introduced when the Me_In concentration was higher. ~his, of course, indicates t~at either there is more dissociation than is theoretically predicted, the Me.In.AsMe. adduct has not had a sufficient time to form ~efore t~e AsH 3 is introduced, or the AsH_ is able to replace AsMe_ in the undissociated adduct. ~ 5

Coates and Whitcombe (27) have also determined the vapor pressure curve for Me3In.AsMe 3 to be

l o g P (mm) = -2590 /T + 8 .925


This yields a room temperature (295K) vapor pressure of 1.40 mm which is about one third that of Me3In. Thus, by diluting the MeRIn with H 2 by at least 10:1 before mixing it with AsMe3, Ve should avoid condensation of the adduct.

The work of Ludowise et al. (17) also indicates that an adduct forms, and the adduct has a vapor pressure similar to that predicted by Coates and Whitcombe (27). They found that the composition of their films could best be explained by an adduct vapor pressure of 0.21 mm at 0~ This compares with the predicted value of 0.27 mm. The slight discrepancy could be due to experimental error, or it could be due to the incorporation rate of indium being less than that of gallium. Oishi and Kuroiwa (41) did find that the solid contained less indium than the vapor, but they attributed it to parasitic reactions between the indium alkyl and AsH 3.

The IR absorption measurements confirm that there is a room temperature reaction between Me^In and AsH~. The Me_In absorption peak disappears because Me_In is j removed from the vapor when it reacts with AsH~ to form the precipitate.

Our results indicate that Et3In and AsMe^ do not form a strongly associated adduct at room temperature. The poorer quality films grown using Et_In shown in Fig. 3 can be attributed to precipitates fO~med by the reaction between Et~In and AsH~. This reaction would occur if the EtRIn is n~t protected from the AsHR by a blocking agent. Our IR absorption studies support tffis hypothesis in that there was no noticeable reduction in intensity in the Et3In peaks when it was mixed with AsMe 3.

The work of Drago et al. (38,39) support this idea. They found that, in general, the triethyl derivatives form weaker adduct bonds than the corresponding trimethyl derivatives do. Thus, since our results indicate that the Me_In.AsMe_ adduct bond is near the minimum acceptable va~ue, it ~s likely that the bond energy of the Et_In.AsMe_ adduct is less than this value, and, therefore, t~e adduc~ is largely dissociated. Ludowise et al. (17) also found that growing films with Et_In and AsMe~ produced poorer quality films than those gro~n with Me~IK. The IR results also confirm that there is a room ~emperature reaction between Et3In and AsH 3.


Our IR measurements establish that Me~Ga and AsMe~ form an adduct at room temperature. This i~ indicated b~ the loss of the fine rotational structure and the decrease in the absorption peak intensities when they are mixed. We attribute the loss of the fine rotational structure to the formation of a larger more bulky molecule, and the decrease in the absorption peak heights to the condensation of the adduct.

That the Me^Ga.AsMe^ adduct does form is confirmed by Coates (22) and ~eib et ~I. (42) They found that -Ag = 10 Kcal/mole and Coates (22) found that the vapor pressure equation is

log P(mm) : -2458/T + 9.114.

At room temperature P = 6.05 mm. This is substantially less than the 0~ vapor pressure of Me^Ga (67.9 mm) and AsMe^ (97.0 mm), and one would, therefore, expect the addu~ to condense. For collecting the IR and gas chromatograph samples the Me~Ga and AsMe~ canisters were cooled to 0vC, but for the ~rowth process, Me_Ga was cooled further to -15~ At this temperature its jvapor pressure is 29.5 mm which is still larger than the room temperature vapor pressure of the adduct. Thus, it is important to make certain that the AsMe^ has been diluted in the line before it is mixed with theJMe3Ga and/or the line from the Me~Ga is heated. Our dilution ratio of > 10:1 should have be~n sufficient.

The results of our IR studies also indicate that the Me~Ga.AsH~ adduct forms. The peak heights are reduced and th~ fine ~otational structure is eliminated when the two gases are mixed. In addition, a yellow-gold condensate forms. Again one sees the importance of H 2 dilution before the Lewis acid and base are mixed.

3chlyer and Ring (4) found that _I equivalent methane o

evolved from a Me_Ga/AsH^ mixture at 200 C. This suggests that a Me~Ga.AsH~dduct~s formed and is stable to ~200~ one methyl grou~ is removed, probably through a methane elimination reaction when a hydrogen reacts with a methyl group, and at 260~ a second methyl group is removed by a similar reaction. This is similar to what happens in a Me3In/PH 3 mixture at room temperature (6). It also sug-

MO-CVD Growth of lnGaAs 723

gests that the Me_Ga/AsH_ mixture should not be held at 200uC for any period of ~ime or problems similar to those experienced with room temperature mixtures of indium alkyls and nonmetal hydrides could occur.

The results from the gas chromatograph studies indicate that the individual alkyls and their mixtures are more stable than the metal alkyl/nonmetal hydride mixtures. The individual alkyls are stable in Hp to at least 400~ Asycough and Emeleus (43) found tha% AsMe. homogeneously decomposes between 400 and 500~ and Jacko ~ Price (44) found that Me_Ga in toluene did not begin to decompose until T:400~ j

Moss and Evans (16) used PEt~ as the blocking agent instead of AsMe~. The advantage of doing this is that the adduct bond energy is larger (15.8 kcal/mole as determined from Drago's (38,39) data) so there is less dissociation. The disadvantages are that the vapor pressure of the Me_In.PEt 3 is smaller (27,45), it requires a much larger ~ fl~w rate through the adduct to obtain the same amount material in the vapor phase achieved by a smaller flow rate through the alkyl and the subsequent dilution with Hp prior to the formation of the adduct, there is less dissociation of the adduct at the growth temperature and therefore more indium will pass out of the system if the adduct does not decompose, and phosphorus from the PEt^ could contaminate the sample. However, Moss and Evans (I~) did not find any phosphorus in their InGaAs films down to 100 ppm.

acknowledgements

We would like to acknowledge the many helpful discus- sions we had with Professor Jack Norton, the encouragement of Professor David Ferry, and the support of the Office of Naval Research.

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mo-cvd growth of ingaas using me3ga , asme3, ash 3 and · mo-cvd growth of ingaas using me3ga ,...

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