Plasma stimulated MOCVD of GaAs

Journal of Crystal Growth 77 (1986) 241—249 North-Holland, Amsterdam

241

PLASMA STIMULATED MOCVD OF GaAs H. HEINECKE, A. BRAUERS

~‘,

H. LUTH

*

and P. BALK

Institute of Semiconductor Electronics and SFB 202, Aachen Technical University, D-5100 Aachen, Fed. Rep. of Germany

A critical examination of the literature on the epitaxial deposition of GaAs from TMG and AsH

3 indicates that at kinetically limited conditions the growth rate is not only determined by the dissociation of the metal—alkyl compound but — at low total pressures — also by that of AsH3. A study of dc plasma stimulation of the growth process shows that the latter limitation may be removed by this method. Epitaxial growth could be achieved down to 530 K at 500 Pa total pressure. Below 700 K the process is purely plasma controlled. Between 700 and 920 K the use of a plasma reduces the activation energy for growth from 1.0 to 0.5 eV. The effect of plasma stimulation on the electrical properties of the epitaxial films is discussed.

1. Introduction Lowering of the deposition temperature of semiconductor films is an important objective in the technology of solid state devices since it permits sharper doping and compositional transitions in layered structures. However, at reduced temperatures the rates of deposition may become prohibitively low due to kinetic limitations. To correct for this problem external stimulation, for example by UV irradiation or by a plasma, may be supplied, Plasma stimulation of the epitaxial growth of Si from SiH4 at low pressures has been studied for several years, which has led to a process for the deposition of single crystal films at temperatures as low as 920 K [1].The plasma stimulated growth of GaAs from evaporated elemental Ga and As between 620 and 770 Kgallium was reported Hariu al. [2]. Using trimethyl (TMG)byand AsHet Segui et al. [3] deposited amorphous GaAs using 3a low frequency power discharge. Pande and Seabough using the same materials reported in 1984 for the first time the dc plasma enhanced epitaxy of monocrystalline GaAs films between 570 and 770 K [4]. Even though these studies indicate the basic *

II. Physikalisches Institut and SFB 202, Aachen Technical University, D-5100 Aachen, Fed. Rep. of Germany.

feasibility of plasma stimulated growth of GaAs in the MOCVD system they leave most questions regarding the basic mechanism of the process unanswered. After discussing some general aspects of the kinetics of MOCVD the present paper will deal with a number of these open questions.

2. Kinetic limitations in MOCVD Kinetic limitation of the growth process will become apparent when increasing the rate of mass transfer in the gas phase (for example, by reducing the overall pressure of the H2 carrier) or decreasing the rate of the deposition reaction (by lowering the temperature). In the MOCVD of GaAs, performed in a cold wall system, the latter case is demonstrated by de drop-off of the rate (fig. 1)4 which 5takes place870 forK.total between step i0 Pa near Herepressures the rate limiting and i0 the dissociation of the metalorganic cominvolves pound, as follows from the fact that the rate increases with the TMG pressure but is essentially independent of the AsH 3 pressure [5]. This conclusion is further supported by the observation that the region of kinetic control is shifted to lower temperatures when replacing TMG by the less stable ethyl compound TEG [6,7]. Also, UV irradiation, which primarily affects the dissociation of the metalorganic compound, leads to an

0022-0248/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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Plasma stimulated MOCVD of GaAs

T (K)

1000

1100

goo

E 50

1MG ASH3 V

20

750

800

~ 7 Pa 126 Pa

5.8cm/s

~

of the deposition rate on substrate orientation [8] which is not observed at atmospheric pressure [8,9]. This dependence of the rate on the nature of the substrate is also the cause for selective growth in MOCVD at reduced pressures [10—12].The increasing role of kinetics at lower overall pressure is further shown by the fact that when reducing the total pressure from iO~ to 5 >< 102 Pa the strong decrease of the rate in fig. 1 starts already .

at higher temperatures. The transition from transport to kinetically

10

•~\ \ \~ \\ \\

5

2

P

•

500 iO’

Pa Pa

limited behavior is again illustrated by the growth rate data in fig. 2 for growth at 923 K, where these two regimes may be recognized from the slopes 1/2 (as expected for diffusion) and 0, respectively. At thisa total temperature takes place around pressure the of ~ transition Pa; the slope —

o

0 for the low pressure range implies that at this

10

0.9

1.0

1.1

1.2

1.3

3/K)

Fig. 1. Temperature dependence of growth rate1/T(1o~ at two total pressures: ~

Pa and 500 Pa.

increase of the growth rate at low temperatures [5,6]. In the temperature region of 870 to 1050 K the rate is constant since the growth process is mainly mass transport limited. Still, when reducing the total pressure by reducing the pressure of the carrier gas already at i0~Pa kinetic effects demonstrate themselves in the form of a dependence

temperature the H2 carrier does not play a dominant role in the overall process. In fact, replacement of H 2 by He as a carrier does not have any significant effect on the growth rate over the entire range of experimental conditions investigated, indicating that H2 does not participate in the reaction [13]. Interestingly, as may be seen from fig. I at temperatures below 870 K, i.e. in the distinctly kinetically limited region, the rate decreases with carrier pressure, which suggests that an additional kinetic limitation occurs for these conditions. In contrast to the rate at iO~Pa, the rate at 5 >< 102 Pa depends on the pressure of AsH3 (see fig. 6).

—

~20 I

+

+

+ ~

~

-

~10

-1/2

5

~AsH3

126 Pa

~TMG

117 Pa 923K 58cm/s

T v

=

2 I

1

3

5

10

I

30

100

I

300

1000 p (102 Pa) tot

Fig. 2. Dependence of growth rate on total reactor pressure.

H. Heinecke et al.

/

Plasma stimulated MOCVD of GaAs

243

When carrying out the process in “vacuum MOCVD” [14], i.e. at very low pressure or using a molecular beam system with the same reactive components (MOMBE), [15] the presence of a hot

would take place exclusively at the hot substrate surface and requires the presence of elemental As. It has been known since 1880 that using a gas discharge complete dissociation of AsH3 is real-

wall or, in MOMBE, the precracking of AsH3 on a hot filament in the injection capillary is an absolute condition for growth. The growth rate in MOMBE is directly proportional to the flux of elemental As. This indicates that at low pressures not only the dissociation of TMG but also that of AsH3 plays a critical role in limiting the rate of growth. Since the nature of the carrier gas does not affect the growth rate but its total pressure does this suggests that, except for experiments performed at very low carrier pressures or under vacuum conditions, direct interaction between TMG and AsH3 occurs in the gas phase. In agreement with Schlyer and Ring’s model [16], this would lead to the successive split-off of three CH4 molecules per pair of reactant molecules. At least one of these steps would take place in the gas phase. Further support for this proposal is derived from the observation by Nishizawa and Kurabayashi [17] that the decomposition of AsH3 is strongly enhanced by the presence of TMG in the reaction chamber. The carrier would only be instrumental in transfering the thermal activation energy the abovetransition mentionedzone gas phase reaction in the for temperature near the substrate in the cold wall system. At very low carrier pressures or in systems without carrier gas the reaction producing GaAs

ized [18]. The method has been used accordingly to prepare hydrogen-free amorphous As films [19]. In the following sections of this paper we describe the results of an experimental study of the low temperature deposition of GaAs from TMG and AsH3 in a low pressure flow reactor utilizing a plasma to produce elemental As from AsH3.

3. Experimental aspects of plasma stimulation Fig. 3 shows a schematic of the low pressure reactor used in our study. The graphite substrate holder was heated by means of a quartz—halogen lamp. An AsH3—H2 mixture was injected upstream of the discharge zone, TMG between this zone and the substrate. Injection of TMG into the plasma zone will lead to Ga depletion of the gas phase [20]; in fact, even in the present configuration at very low flow rates a Ga deposit was observed on the electrodes. To avoid the latter effect the plasma experiments were carried out at a relatively large flow velocity (20 cm s’). The quartz2 reactor a rectangular cross-seeand washad provided with a liner tube. tion of 18 cm Gas mixing, exhaust and pumping system were similar to those described in ref. [8]. An additional capacitance manometer connected directly to the reactor allowed the correct measurement of the

[j~] pressure TMO,H 2

L1J

I

discharge zone

gauge

As~,H2 :

graphite substrate h~der reflector

quartz halogen lamp Fig. 3. Schematic of growth reactor (side view).

244

H. Heinecke et al.

/

Plasma stimulated MOCVD of GaAs

T

11 cm, substrate floating or biased. TMG, H2 ( AsH3) I

We did not make a detailed study of the AsH3 plasma as yet. However, in both electrode configurations a continuous As film was deposited over

hId

I

the entire length of the reactor downstream of the plasma zone at all temperatures (including room temperature) and pressures investigated. The films were always more substantial then those obtained without a plasma even at the highest substrate

I AsH3 H

[_~

2

I

a

______

I

k

ci

temperatures. As expected, optical inspection showed that in configuration (a) the plasma was confined to the area between the electrodes

•~

whereas in (b) the canal rays extended towards the discharge zone

substrate. The I—V plot obtained at 293 K for configuration (b) in fig. 5 demonstrates the expected increase of the ion current with negative

TMG H 2

su~te

substrate bias and a bistable point at zero bias at finite current values.

AsH3

Substrates were semi-insulating GaAs wafers (Wacker-Chemitronic) oriented 2° off (100) to-

L....÷

wards the next (110) plane. The reactants were

H2

b

AsH3 (Matheson, Phoenix Plus) and TMG (Alfa Ventron); H2 (Matheson UHP) was used as a carrier gas. Film thicknesses were measured microscopically after staining and were checked by

__________________________________

bias Fig. 4. Schematic of plasma generation (top view): (a) discharge perpendicular to gas flow direction; (b) discharge parale to gas flow.

weight measurements. The accuracy of the measured rates was at least 5%. The films were characterized by means of Hall measurements using Van der Pauw structures and by low temperature photoluminescence (PL).

pressure in the chamber even at high gas velocities. The apparatus could be operated the 5 Pa;over it thus entire pressure of 102oftoplasma i0 permitted direct range comparison deposition

p~ 0,

with conventional growth over a wide range of parameters. The plasma was generated by means of a DC discharge (500—1000 V; power: 0.1—4 W) using two basic configurations: (a) discharge perpendicular to direction of gasbetween flow (fig. 4a), electrode 2, distance electrodes 5 cm, area 7.5 dcmbetween electrodes and substrate holder distance variable, substrate holder floating; (b) discharge parallel to gas flow, i.e. directed towards substrate (fig. 4b), anode with circular opening and area 12 cm2, cathode with rectangular opening and area 8

cm2, distance between electrodes 6 cm, distance 2) between cathode and substrate holder (area 5 cm

=

500 Pa

PA~H3 =

72 Pc

v

1

=

20crr~s

E

20 1.5

293K

/

X~

~

y /0

7~

.~-~-°°

-100

100

500

~

)V)

Fig. 5. Ion beam characteristics in AsH 3 plasma; plasma power 5 W, electrode voltage 1000 V.

H. Heinecke et aL

/ Plasma stimulated MOCVD of GaAs

4. Growth with discharge perpendicular to flow

1100

1000

245

900

800

750

7 (K) 650

700

Fig. 6 demonstrates that at low pressures and

~

P

0

500 P0

temperatures the growth rate exhibits a linear

—

~TMG

~ Pa 72 P0

:—.

50

dependence the process on is the As-limited. AsH3 pressure, The effect indicating of plasma that stimulation, here in the configuration of fig. 4a, is evident, but its relative importance decreases at higher AsH3 pressures where more elemental As is available from thermal cracking of the AsH3. It is interesting to note that even without the use of a plasma and at low AsH3 pressures mirror-like surfaces are obtained. Thus, the As limitation of the the growth process not lead deterioration of surface. Thisdoes behavior was toalso found for MOMBE [15] and a similar explanation may hold in the present case: The breakdown of the TMG molecule appears to take place at the surface and requires the presence of elemental As. Thus the detrimental effect of excess elemental Ga on the surface is avoided. Even though the growth process at 500 Pa is kinetically controlled over the entire temperature region below 1050 K (fig. 7) it may be seen that a distinct but modest plasma effect on the rate only occurs below 800 K. However, the effect of the plasma on the p-type background doping is more pronounced. Both with and without plasma the background doping is strongly temperature dependent. The use of a plasma lowers the carrier concentration, above 800 K by at least a factor of

20

a ,,—

F’403_ lb

v

~

plasma 20cm/s

haul plasma

10

2 1

0,2 0.9

1.0

1.1

1.2

1.3

1.4

1.5

3/K)

1/1 )10 Fig. 7. Temperature dependence of the growth rate at a gas velocity of 20 cm/s with and without plasma; plasma power

1.4 W, electrode

voltage 800 V.

two for the growth conditions used in this experiment. At lower temperatures plasma stimulation reduces the carrier concentration by as much as

1100

1000

900

800

I (KI 700

750

—

E a

I 1017

1016

=500Pa

7

C

lot

—

p1MG =6.7Pa

: ~

v

=

20cm/s

1

=783K

o

/

/

ia’5 a

6.7 Pa

P 101

__ ~__—1~~ : with plasma withoal plasma

_______________________________________ 20 40 60 80 100 120 P AsH lPa) 3

Fig. 6. Dependence of growth rate on AsH3 pressure, showing plasma effect; plasma power 1.4 W, electrode voltage 800 V.

1014

500 Pa

V a05H30 WIth a pIasa~a 72plasma 20cm/s Pa mithoat

0.9

1.0

1.1

1.2

N’ 1.3

1.4 3IKl

i/1Ii0 Fig. 8. Temperature dependence of the background doping with and without plasma; plasma power 1.4 W, electrode voltage 800 V.

246

/

H. Hejnecke et al.

T

Plasma stimulated MOC VD of GaAs

~783K

P,

0,

~TMG

=

DASH3 V ~ with 0

2

6

4

=

500 Pa

33 Pa

6.7 Pa

a

37 Pa 37cm/s

PA~l= 72 Pa V = 20cm/s

plasma

0

WIth

withOut plasma

+

wIthout plasma

8

10

12

plasma

1/. d )cm)

Fig. 9. Dependence of growth rate on distance d between plasma zone and substrate using H~plasma; plasma power 4 W, electrode voltage 450 V (for v = 37 cm/s) independent of d; increasing from 720 to (180 V (for 1 = 20 cm) for d increasing from 3 to 15 cm.

three orders of magnitude. In addition, particularly at these lower temperatures the epilayers obtained without a plasma are rather strongly compensated, those grown with a plasma not. For example, at 780 K films grown without plasma have the following electrical characteristics: P300 3, ~s 2 V~ 51 with 1,2 x 1O~ cm 300 70 cm plasma: P300 2 x 1015 cm3, ~30O 400 cm2 V1 ~ Most likely the high acceptor concentrations obtained in the present experiments without

plot of the growth rate in dependence on the distance d between cathode and substrate holder (at fixed distance between AsH3 inlet and substrate holder) shows that the rate is independent of the application of the plasma for any distance investigated the higher flow rate (37 cm An effect onat the background doping wass not observed either in this case. The increase of the rate with plasma for smaller d values at the lower flow rate (20 cm s1) is caused by back diffusion

plasma are caused by the incorporation of carbon,

of the AsH

as was also found for layers grown by MOMBE [15] (fig. 8). It should be mentioned that the plasma not only produces elemental arsenic but also hydrogen radicals. In principle the enhancement of the rate could also be caused by the reaction of these hydrogen radicals with TMG to form CH4. A similar process could lead to removal of carbon or CH3 groups from the surface, thereby reducing the acceptor concentration in the material. However, the latter explanation is unlikely, as follows from the data in fig. 9. In this experiment a pure H2 plasma was used during growth by injecting the AsH3 downstream of the discharge area. The

confirmed by the significant change in the burning voltage of the plasma upon switching on the AsH3 in the latter case. Unless in an AsH3 plasma a distribution of hydrogen species is obtained different from that in a H2 plasma these observations indicate that reactions involving H species do not play a significant role in plasma stimulated growth.

=

=

=

=

3 into the discharge area. This was

5. Growth in the canal ray configuration The canal ray configuration (fig. 4b) permits direction of a beam of arsenic containing ions at the substrate surface. The kinetic energy of these

H. Heinecke et al.

1100

1000

800

900

/

Plasma stimulated MOCVD of GaAs

750

1(K) 600

650

700

247

P,

0~ =500 Pa 6.7Pa

PTMGO

50

PA5H0 72

+ ,,-._--.-—--,--.-.-..---.---=.--

v

20

~

a

+

10

Pa

20 cm/s

mIlhout plasma

Aw~hp~sma

0.5 0.2

0:9

1.0

1.1

1:2

1:3

1.4

1.5

1.6 1.7 3/K) 1/T(10

Fig. 10. Temperature dependence of growth rate showing plasma effect of ion beam stimulation; plasma power 2.1 W. electrode voltage 950 V, substrate bias —400 V.

species can be controlled by means of a substrate bias. The plot of the temperature dependence of the rate (fig. 10) shows that the activation energy of the growth process is reduced from 1.0 to 0.5

eV by the use of a plasma combined with a negative accelerating potential at the substrate. Fig. 11 shows that upon decreasing the TMG pressure at otherwise identical conditions in the T (K)

900

700

800

650

600

550

p, 01 _~100

PASR3

=

500 Pa 72 Pa

A

~TMG

=

6.7 Pa

+

0

~TMG

=

30 Pa

°

with plasma

~TMG

=

6.7 Pa

3.0 Pa without plasma ~TMG =

1:

l/T (l0~’K) Fig. 11. Temperature dependence of growth rate showing plasma controlled growth in temperature region below 700 K (dashed lines); for comparison, data from fig. 10 are also shown (solid lines); plasma data as in fig. 10.

H. Hejnecke et al.

248

/

Plasma stimulated MOCVD of GaAs

low temperature region of fig. 10 a reduction of the rate, but by a smaller fraction, occurs. However, the activation energy remains unchanged. Below 700 K growth is only possible by plasma stimulation. The temperature dependence of the rate becomes very weak and the process appears to be fully plasma controlled. Relatively smooth monocrystalline GaAs films could be grown down to 530 K. Below this temperature the sample resistance appears to increase; the bias current through the substrate is suppressed and inhomogenous growth is obtained. Growth between 530 and 700 K is, in addition to the availability of AsH3 cracking products, probably promoted by the energy of the ions reaching the surface. However, the possibility that UV radiation, produced in the plasma, also plays a role in dissociating TMG cannot be excluded at the present time. The layers from figs. 10 and 11 prepared with plasma stimulation and 400 V substrate bias were all high resistivity material and strongly compensated. By reducing the bias to 0 V or by keeping the substrate floating samples from the temperature region where the growth exhibits an 0.5 eV activation energy (for example, at 783 K) still show high breakdown voltages (> 1000 V). However, low temperature PL data indicate that the the signal is comparable to that of samples grown under the same conditions in the configuration with a discharge perpendicular to the direcdon of flow (see data in figs. 7 and 8), so that the background doping must be rather low. The growth rate is still that obtained without plasma. On the other hand, films grown at higher bias voltages show no PL signal. Most likely these materials have been radiation damaged by the ion bombardment. The optimum conditions for plasma stimulated growth are presently being studied.

6. Conclusions Our study shows that in the MOCVD of GaAs not only the dissociation of the metalorganic Ga compound but at very low pressures also the break up of the AsH3 molecule may play a role in the rate determining step. These low pressure conditions are favorable for plasma stimulation. —

—

Stimulation by means of a DC plasma is feasible at 900 K and below. A further increase of the growth rate is obtained by ion acceleration in a canal ray configuration. This method permits growth at conditions where otherwise the rate has essentially dropped to zero. For higher acceleration voltages radiation damage is observed. The method thus requires careful optimization. It may he expected that the same approach can be used to stimulate the depostion of phosphorus-containing compounds using PHI.

Acknowledgement The authors are indebted to J. Geurts for a number of low temperature PL measurements.

References [1] T.J. Donahue and R. Reif, J. AppI. Phys. 57 (1985) 2757. [2] T. Hariu, K. Takenaka, S. Shibaya, Y. Komabu and Y.

Shibeta, Thin Solid Films 80 (1981) 235. [3] Y. Segui, F. Carrere and A. Bui, Thin Solid Films 92 (1982) 303.

[4] K.P. Pande and AC. Seabough, J. Electrochcm. Soc. 131 (1984) 1357. [5) N. Pfltz, H. Heinecke. E. Veuhoff. G. Arens, M. Heyen. H. LOth and P. Balk, J. Crystal Growth 68 (1984) 194. [6) P. Balk, H. Heinecke, C. Plass. N. POtz and H LOth, J Vacuum Sci. Technol., in press. [7] M. Yoshida, H. Watanabe and F. Uesugi, J. Electrochem. Soc. 132 (1985) 677. [8] H. Heinecke, E. Veuhoff, N. POtz. M. Heyen and P. Balk. J. Electron. Mater. 13 (1984) 815. [9] R.R. Saxena, C.B. Cooper III, M J. Ludowise. S Hikido. V.M. Sardi and PG. Borden. J. Crystal Growth 55(1981) 58. [10] P. Balk and H. Heinecke. in: Physical Problems in Microelectronics, Ed. J. Kassabov (World Scientific PubI. Co.. Singapore, 1985) p. 190.. [11] K. Kamon, S. Takagishi and H. Mon. J. Crystal Growth 73 (1985) 73. 112] H. Heinecke. A. Brauers. F. Grafahrend. C. Plass. N. PUtz, K. Werner. M. Weyers, H. Luith and P. Balk, J. Crystal Growth 77 (1986) 303. [13] G. Arens, Fl. Heinecke. N. Pijtz, H Lcith and P. Balk, J. Crystal Growth, in press. 114] L.M. Fraas, P.S. McLeod, iA. Cape and L.D. Partain, J. Crystal Growth 68 (1984) 490.

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Plasma stimulated MOCVD of GaAs

[15] N. POtz, E. Veuhoff, H. Heinecke, M. Heyen. H. LUth and P. Balk, J. Vacuum Sci. Technol. (1985) 671. [16] DJ. Schlyer and MA. Ring, J. Onganometallic Chem. 114 (1976) 9. [17] J. Nishizawa and T. Kurabayashi, J. Electnochem. Soc. 130 (1983) 413. [18] Gmelins Handbuch den Anorganischen Chemie, Arsen.

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System No. 17, 8. AufI. (Verlag Chemie. Weinheim, 1952) p. 219. [19] J.C. Knights and J.E. Mahan. Solid State Commun. 21 (1977) 983. [20] F. Wiherg and Tb. Johannson, Naturwissenschaften 29 (1941) 320.