Study on the growth rate in VPE of GaN

Journal of Crystal Growth 52 (1981) 257-262 © North-Holland Publishing Company

STUDY ON THE GROWTH

RATE IN VPE OF GaN

W . S E I F E R T , G . F I T Z L a n d E. B U T I a E R Sektion Chemie, Karl-Marx-Universitiit Leipzig, Liebigstrasse 18, DDR-7010 Leipzig, DDR

Growth experiments of GaN in a hydrogen as well as in an inert gas ambient show that the growth rate is kinetically controlled over a wide range of temperature. In contradiction to thermodynamic considerations the growth rate in hydrogen is higher than in inert gas for the temperature interval 800 to 1150°C. This points to the importance of a growth limiting surface reaction step involving hydrogen to form HC1. At low temperatures a second mechanism becomes important in which GaC13.NH3 is formed. The occurrence of either the first or the second mechanism in the low temperature range of growth is mainly determined by kinetic reasons. The deciding factor seems to be the competing adsorption of either H2 or GaCl at the C1 surface atom to form either HCl or GaCl3 as the reaction product.

1. Introduction

2. Experimental

For the widely investigated Ga/HCI/AsH3/H2, Ga/AsC13/H2 as well as G a / H C 1 / A s H 3 , PH3/H2 systems important arguments support the a s s u m p t i o n of r a t e - l i m i t i n g s u r f a c e g r o w t h p r o cesses c o n n e c t e d with t h e d e s o r p t i o n of t h e c h l o r i n e a w a y f r o m t h e g r o w i n g s u r f a c e [1-4]. In this p r o c e s s an a c t i v a t e d s u r f a c e c o m p l e x involving h y d r o g e n h a s b e e n c o n s i d e r e d [2, 3]. In VPE of GaN in the analogous Ga/HCI/NH3/H2, He, Ar system the experimental d a t a a r e m u c h p o o r e r . A l t h o u g h a few p a p e r s a r e d e a l i n g with t h e d e p e n d e n c e of t h e g r o w t h r a t e on t e m p e r a t u r e , p a r t i a l p r e s s u r e , a n d s u b s t r a t e o r i e n t a t i o n [ 5 - 1 1 ] , i n f o r m a t i o n o b t a i n e d is n o t sufficient to d e c i d e a b o u t t h e b a s i c g r o w t h m e c h a n i s m in G a N V P E . M o r e o v e r , t h e r e p o r t e d r e s u l t s a r e p a r t l y in c o n t r a d i c t i o n t o e a c h other. N e w e x p e r i m e n t a l results c o n c e r n i n g t h e g r o w t h r a t e of G a N o b t a i n e d f r o m g r o w t h e x p e r i m e n t s in a h y d r o g e n as well as in an i n e r t gas a m b i e n t a r e r e p o r t e d in t h e p r e s e n t p a p e r . T h e results w e r e o b t a i n e d u n d e r c a r e f u l l y cont r o l l e d e x p e r i m e n t a l c o n d i t i o n s with p r a c t i c a l l y no e x t r a n e o u s d e p o s i t i o n on t h e walls s u r r o u n d ing t h e s u b s t r a t e .

2.1. Growth conditions T h e g r o w t h a p p a r a t u s is a h o t wall o p e n flow r e a c t o r (fig. 1). D u e to t h e c o a x i a l a r r a n g e m e n t of t h e gas i n l e t t u b e s a g o o d m i x i n g of t h e r e a c t a n t g a s e s p r i o r to d e p o s i t i o n is a c h i e v e d . T h e s u b s t r a t e s w e r e a r r a n g e d on a special subs t r a t e h o l d e r . T h i s a r r a n g e m e n t c o r r e s p o n d s to conditions where the deposition takes place only on t h e s u b s t r a t e s , w h e r e a s e x t r a n e o u s d e p o s i tions on t h e q u a r t z wall set in f u r t h e r d o w n s t r e a m in t h e r e a c t o r . T h e r e a c t o r is h e a t e d b y a t w o - z o n e r e s i s t a n c e f u r n a c e . T h e G a s o u r c e is h e l d at a c o n s t a n t t e m p e r a t u r e b e t w e e n 850 a n d 900°C. T i t r a t i o n of the unreacted HCI yields the conversion efficiency to b e b e t w e e n 94 a n d 96% for t h e reaction Gao) + HCltg ) ~ GaCl(g) + ~H2tg). T y p i c a l flow r a t e s The substrates single crystals 3 ° t a t i o n (1012), a n d misoriented from 257

(1)

a r e s h o w n in t a b l e 1. used were polished sapphire misoriented from the orienM g / A l - s p i n e l single crystals 3 ° t h e o r i e n t a t i o n (111). T h e

258

W. Seifert et al. / Study on growth rate in V P E of G a N

diluent NH 3

GaCI

e i-~'-!e°u's" "de P° s i t s

~

exhaust

/ substrate

Fig. 1. Part of the reactor of the G a N growth apparatus. Table 1 Typical growth conditions (all the flow rates correspond to r o o m temperature and pressure of 1 atm) Total flow rate

(ml/min)

Flow rate HCI Flow rate NH3

(ml/min) (ml/min)

3000 15 1000

low temperature region the growth rate is higher in an inert gas carrier, whereas in the high temperature region the higher growth rate can be observed in the hydrogen growth system. The different behaviour is also reflected in the different slope of the log r = f(1/T) curve in the low temperature region. From this slope follows that the apparent activation energies are 155 and 1200

resulting growth directions are (1120) and (0001), respectively. The growth rate was determined by direct thickness m e a s u r e m e n t s by fracturing the sample and viewing the layer in cross section using optical microscopy. Since the thickness of the sample varies from the lower to the upper end of the substrate we t o o k the thickness at 5 m m from the lower end as a reference.

2.2. Effect of growth temperatures Figs. 2 and 3 show the effects of substrate temperature upon the growth rate. The following characteristics are obvious: (i) In analogy to the growth rate in V P E of other I I I - V compounds, there is a growth rate m a x i m u m at a mean growth temperature. At lower temperatures the growth rate decreases exponentially with decreasing temperature. At higher temperatures the thermodynamically predicted behaviour is observed an.d the rate decreases as the temperature increases. This is especially evident for the growth rate in a hydrogen ambient. (ii) There is a clearly different growth rate behaviour for different carrier gases used. In the

1100

1000

i

i

900

800

700°C

10 O ~o x 5.

L -r I-

1,

PHCI

O nO

: 0.005 arm

p~lH 3 : 0.33

Q5

o carrier

atm

gas

hydrogen helium

•

0.1

'~

I

n

I

0.7

0.8

0.9

R EC I P R O C A L

I

1.0 x 10 - 3

T E M P E R A T U R E ( K-1 )

Fig. 2. Variation of the growth rate as a function of reciprocal temperature. Growth direction (1120).

W. Seifert et al. / Study on growth rate in V P E of G a N 1200

1100

1000

900

800

I

I

I

I

700°C

E

It is reasonable to rule out that boundary layer effects are responsible for the different growth behaviour in hydrogen and inert gas carder. The diffusion controlled growth rate r is given by

E

& i,i I-.,,< n,-

I-.

of the GaC1 in the pressure range used. A weak tendency to saturation could be observed in the hydrogen system. H e r e further experiments are in progress.

3. Discussion

S.

'1"

259

1.

PI~CI : 0.005 atm

~

\

k/o

o

,

(2)

r = -R--~ (pi - P i ),

O i,r t3

p~ll..13: 0.33 arm

0.5

where p0 and p* are the input partial pressure and the partial pressure immediately at the surface, respectively. The mass transfer coefficient kid for diffusion transport of a molecule i through a boundary layer is given by [12]:

carrier gas h e l i u m

•

0.1

I 0,7

(115o)

,

(ooo,)

I 0,8

I 0,9

I 1,0 x 10"3

REC I P R O C A L TEMPERATURE ( K "1 ) Fig. 3. V a r i a t i o n of t h e g r o w t h r a t e as a f u n c t i o n of r e c i p r o cal t e m p e r a t u r e f o r d i f f e r e n t c r y s t a l l o g r a p h i c o r i e n t a t i o n s .

88 k J/mole in hydrogen and inert gas, respectively. Experiments with A r as a carrier result in the same temperature dependence as it was observed with He. (iii) The growth rate in the two crystallographic orientations (1120) and (0001) is significantly different in the range of low growth temperatures. With increasing temperature these differences become smaller and vanish at the highest growth temperatures here applied.

k o = a (ff,/d)l/2(ylrl)1/6D2/3,

(3)

where a is a proportionality factor, ff is the mean linear velocity of the gas flow, d is the diameter of the tubular reactor, y and r/ are the density and the viscosity, respectively, of the gas mixture, and Di is the diffusion coefficient. The rate limiting reactant i in the growth system is GaCl. Consequently, the ratios of the mass transfer coefficients in the different ambients should be D . D 16 3 . 1/6 2 3 kH2. kH~ = (T/n)~2D~.cl-.2. (T/n)mDgac,-H~.

(4)

Using Gilliland's equation [13], molar volumes for I-I2, He, and NH3 tabulated in [14], and a molar volume of GaCl, Voao--35.9cm3/mole [15], one obtains:

2.3. Effect of partial pressures Doaci-H2 = 1.05 x 10-4T 3/2cm2/s; The input partial pressure of NH3 does not significantly influence the growth rate in our growth system with input pressures generally above 0.1 atm. However, the growth rate depends approximately linearly on the input partial pressure

Doao-m = 8.68

x

10-5T 3/2 cm2/s,

DGaC1-NH3= 2.84 X 10-5T 3/2cm2/s. Since always an appreciable amount of NH3 is present (0.33 atm), the diffusion coefficients as

260

W. Seifert et al. / Study on growth rate in VPE of G a N

well as density and viscosity must be calculated for the gas mixture with NH3. This was done for DGaCI-H2-NH3and DGaCI-He-NH3 using the formula by Wilke [16]. The density and viscosity values were approximated linearly from the values of the pure gases [14]. In this way one obtains the ratio of mass transfer coefficients (950°C) to be: D

.

D

k H 2 - n n 3 - kHe-NH3

=

1.06: 1.00.

Hence, the nearly threefold higher growth rate found experimentally in hydrogen can hardly be explained by mass transport effects through a boundary layer. From a thermodynamic point of view the supersaturation in the inert gas system is higher, though above 800°C the growth rate is lower than in hydrogen. This points to the existence of a rate-limiting surface reaction step. The supersaturation derived from the overall reaction

1100 1

-2 10

1000 I

900 1

800 I

GaCltg) + NH~) ~ GaN(s) + HCltg) + H2~) (5) cannot alone determine the growth rate. A thermodynamic calculation of the equilibrium partial pressures including the six species HCI, GaC1, NH3, GaC13"NH3, HE, and He (fig. 4) shows that in the high temperature region the only Ga species GaC1 should be of importance, whereas with decreasing temperature molecules of GaCIa.NH3 must also be taken into consideration. For a qualitative understanding of the surface kinetics we propose now the following model: For the high temperature range we assume that rate determining step is the formation of an activated complex containing Ga, C1 and H:

+

Oa"Cl"n=H'l* ,...,. (6)

700°C I

1100 I

-1

1° I

HCl

I0

[

100o I

900 I

800 I

700°C !

HCI

-2_ 10

-3

Ga

///

GaCl

"". -3

-4

10

10

/i'

E

E

// /

m o.

~.

-5

-4

/

to

10

/ GaCI~,NH 3 !

10

10

-6

-7

- -

inHe

---

in

I 0.7

H 2

/ //l't ///lIe / t// / ¢' 0.8

//

PHCI

0,05

at m

PNH3

0.33

a t rn

/ -5 10

/

/I i/I

GaCI3~ NH 3

I 0.9

I

a

10 x 10 - 3

RECIPROCAL TEMPERATURE (K -1 )

I

-6

10

0.7

I 0.8

~

n

H e

---

n

H2 I 0.9

RECIPROCAL TEMPERATURE

b I 1.0 x 10 -3

( K "1)

Fig. 4. Calculated equilibrium partial pressures for low (a) and high (b) input pressures of HCI. The calculation was done for a hydrogen system (pOe = 0) a n d a helium system (p0H2 = 0).

W. Seifert et al. / Study on growth rate in VPE of GaN

The growth rate of GaN is then given by the number of activated surface complex molecules decomposing irreversibly per unit time [~Ga-CI-H-H]*tI,~,,.f) ~ [~/Ga] (surf)+ HCI + H-

(7) N_ The [-~Ga](surf) is now available for further adsorption of N species. Thus the deposition on the surface will continue. This adsorption process should be fast and not be a rate determining step because of high excess of NH3 in the vapour phase. Due to the activation equilibrium given in eq. (6), the hydrogen partial pressure determines the concentration of the activated complex molecules on the growing surface. From this follows that a reduced hydrogen partial pressure reduces the growth rate as was also observed in GaAs VPE [3, 17]. For the low temperature range and especially in inert gas ambient a further mechanism with a lower apparent activation energy is to be taken into consideration, which is responsible for the observed higher growth rate in the inert gas carder. This mechanism seems to be analogously to the so called "disproportionation" mechanism, already mentioned in the earlier literature (see, e.g., refs. [1,2]). Interpreted as a heterogen-catalytic surface process, this mechanism could be described as follows: The C1 on the surface reacts with GaC1, which is adsorbed on a neighbor site: [ ~/Ga-CI + GaCl(s~rf) --~/Ga-CI

~

I .

~

(surf)

Ga"CI~. .Ga-CI Ga._-CI.Y

l

(8)

(surf)

The growth is realized decomposition reaction:

by the

irreversible

261

-•Ga"CI•

. ~Ga-C1 -~Ga"CI'"7

I

(sea) 1 *

Oa]

The formed GaCI3 is stabilized by complex formation with the NH3. The real existence of the complex GaCI3.NH3 in the vapour phase during the epitaxy process has been shown by mass spectrometric measurements even at temperatures of 900°C [181. Surprising is the fact that the second mechanism (presented by eq. (8)) is clearly preferential in the inert gas ambient (also in Ban's measurements), though the equilibrium pressure of GaC13.NH3 in the temperature range of interest is nearly the same both in hydrogen and in inert gas atmosphere. This points to competing adsorption processes at the Cl-surface atom. H2 competes with GaC1 to form either the activated complex according to eq. (6) or according to eq. (8). Thus it is reasonable to conclude that the mechanism given by eq. (8) is favoured in systems with a low hydrogen partial pressure and a high input pressure of GaCI. A further conclusion concerns the different apparent activation energies, obtained from the log r = f(1/T) plot shown in fig. 2. Since in He the dominating reaction mechanism changes from the machanism via the GaCI3 formation at low temperatures into the mechanism via the HCI formation at high temperatures, the measured activation energy is a mean value. In the HE system, the measured activation energy (155 kJ/mole) seems to be close to the real value for the HC1 formation mechanism. Both mechanisms, according to eqs. (6), (7) and (8), (9), respectively, are competing processes. This explains why the hydrogen in the low temperature range has a blocking effect on the growth. From this qualitative consideration, the assumption of atomic hydrogen as surface reactant [2] does not seem to be a necessary condition.

262

W. Seifert et al. / Study on growth rate in V P E of G a N

Acknowledgements The authors wish to thank Dr. G. Knobloch and Dr. P. Peuschel for their collaboration in computer calculations, Dr. K. Jacobs for helpful comments, and Miss M. Uhlig for technical assistance.

References [1] D.W. Shaw, J. Crystal Growth 31 (1975) 130. [2] R. Cadoret and M. Cadoret, J. Crystal Growth 31 (1975) 142. [3] J.B. Theeten, L. Hollan and R. Cadoret, in: Crystal Growth and Materials, Eds. E. Kaldis and K.J. Scheel (North-Holland, Amsterdam, 1977). [4] C.H. Wu, R. Solomon, W.L. Snyder and T.L. Larsen, J. Electron. Mater. 7 (1978) 791. [5] A. Shintani and S. Minagawa, J. Crystal Growth 22 (1974) 1.

[6] A. Tempel, W. Seifert, J. Hammer and E. Butter, Kristall Tech. 10 (1975) 747. [7] M. Sano and M. Aoki, Japan. J. Appl. Phys. 15 (1976) 1943. [8] S.S. Liu and D.A. Stevenson, J. Electrochem. Soc. 125 (1978) 1161. [9] I.G. Pichugin and M. Tlachala, Neorg. Mater. 12 (1976) 2051. [10] G. Jacob, M. Boulou and D. Bois, J. Luminescence 17 (1978) 263. [11] R. Madar, D. Michel, G. Jacob and M. Boulou, J. Crystal Growth 40 (1977) 239. [12] C.O. Bennett and J.E. Myers, Momentum, Mass and Heat Transfer (McGraw Hill, New York, 1962). [13] E.R. Gilliland, Ind. Eng. Chem. 26 (1934) 681. [14] H.H. Landolt and R. B6rnstein, Physikalisch-Chemische Tabellen, Vol. 4 (Springer, Berlin, 1961) p. 544. [15] K. Jacobs and W. Seifert, J. Crystal Growth 50 (1980) 701. [16] C.R. Wilke, Chem. Eng. Progr. 46 (1950) 95. [17] D. Mizuno and H. Watanabe, J. Crystal Growth 30 (1975) 240. [18] V.S. Ban, J. Electrochem. Soc. 119 (1972) 761.