Molecular layer epitaxy and its fundaments

Journal of Crystal Growth 115 (1991) 12—IS North-Holland

ou~~

CRYSTAL GROWTH

Invited paper

Molecular layer epitaxy and its fundaments Jun-ichi Nishizawa Semiconductor Research Institute, Kass’auchi, Aoba-ku. Sendai 980, Japan

Molecular layer epitaxy (MLE) is a crystal growth method which is able to produce thin films by alternately injecting TMG and AsH~.At first, a self-limiting monolayer growth on a (100) surface was successfully obtained. In addition, we succeeded to obtain self-limiting monolayer crystal growth on a (11 l)B surface. No growth occurs on a (11 l)A surface. Furthermore, in-situ observation of the reaction of MLE on each substrate surface was performed by using a light-reflectance measurement. Here, a He—Ne laser beam penetrates through a window from outside to the surface of the substrate, and the reflected intensity is measured. By measuring the reflection on (100) and (l1l)B surfaces, the adsorption of Ga compounds during TMG supply, the reaction of the adsorhates during the evacuation period, and the surface reaction of the adsorbates with AsH 3 may he distinguished. The characteristics of the reflection change during MLE were quite different on (11 l)A and on (Ill )B or (100) surfaces. A saturation phenomenon of the reflection intensity change may correspond to self-limiting growth in MLE.

1. Introduction Today’s sophisticated semiconductor devices are based on the development of the fabrication processes, and especially on perfect crystal growth technologies. The technology of crystal growth is aimed at achieving a lower temperature process and an extreme precision of thickness control. The photo-excited processes which I have proposed in 1961 [11have the potential to reduce the process temperature and to improve the quality of the films even today. The research has been continued, and the improvement of VPE (vapor phase epitaxy) of Si [21and GaAs [31was obtamed. Using this technique, photo-excited molecular layer epitaxy (MLE) was accomplished on GaAs [3—51, Si [6] and AI5Ga1_5As. In GaAs MLE, TMG (trimethylgallium) and AsH3 are injected alternately on the substrate of GaAs, which is located and heated in a vacuum chamber. This method is based on Suntola’s idea of atomic layer epitaxy (ALE) [7,81.The growth rate at 500 C is decided by the number of gas injection cycles, and the growth rate per cycle saturates at the value of the thickness of a monolayer in a wide range of TMG and AsH3 pressures in the case of a (100) surface. Therefore, no other monitoring °

0022-0248/91/$03.50 © 1991

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of the process is need. The same effect is obtained by Si MLE [61. This paper is concerned with the growth of the (111)A and (111)B surfaces. In addition, the reaction mechanism is discussed as a result of the measurement of the reflected intensity on the growing surface of the substrate. In MLE, the reaction mechanism has been investigated using a mass analyzer which was located just above the surface of the substrate, and some byproducts of the surface reaction were detected [91.But a direct observation of adsorption layers is needed, and therefore an XPS (X-ray photoelectron spectroscopy) measurement and a reflected intensity measurement were added. The results of the XPS study in MLE will be presented in other papers. The method of reflected intensity measurement was developed by Aspnes et al. for monitoring molecular beam epitaxy (MBE) [101,and was also applied for the growth of GaAs migration-enhanced epitaxy (MEE), in which Ga atoms and As4 molecules are alternately deposited on the growing surface [11]. The reflectance measurement is also effective to investigate the reaction mechanism of MLE. We discuss the results of the reflectance measurements of the growing surface in an MLE chamber.

Elsevier Science Publishers By. All rights reserved

J. Nishizawa

/ Molecular layer epitaxy and its fundamentals

2. Experimental setup The experimental setup of the MLE and the reflected intensity measurement system is shown in fig. 1, as a schematic diagram. The details for the MLE system were mentioned in other papers. The of a(400 He—Ne 632.8 nm), system a lightconsists chopper Hz) laser with (Aa lock-in amplifier, a A/2 wave plate, mirrors, and a light detector (Si photodiode). Here, the A/2 wave plate was used to measure the light polarized along the (110) and Kilo) axes in the (100) plane. The noise (N/S) of the whole system is about 0.1% at a time constant of 0.3 s. In this system, the laser beam penetrates through the window of the growth chamber to the surface of =

the substrate, and detector the reflected sured by the light duringintensity MLE. is meaSi-doped GaAs wafers which are oriented to (100), (11l)A and (111)B, respectively, are etched chemically at first. After mounting them in the vacuum chamber they are heated up to 550 C under an AsH 3 flow in order to clean the surface. InMLE,pureAsH3andpureTMG(without carrier gases) are used. For a normal growth °

sequence, AsH3 injection for 20 s, AsH3 exhaust for 4 s, TMG injection for 4 s, and TMG evacuation for 4 s was used, 3. Results

3.1. Monolayer growth Fig. 2a shows the growth thickness per cycle on the (100) surface of GaAs [41as a function of

Quartz Plate

~Si Photo Diode Mirror LamP/ ‘ 1.amp House / ~“~‘\ Ron/(

He Ne Laser

400Hz

7~/2 Wave Plate

~ Chamber \i~atevalve IMP -

Vacuum Gauge

(Pa) 1O~ iO~ ~ 8 (1MG-AsH3 )System ~‘

102 ,_—.

/

~

U--(111)B~5OO~ 6OO~ a111)A1,d,,y////~1o0)

~

-

.~

~

(ill) Monolayer (100)Moooiayer

4T

c 3 ~ 2

•-.~

1x10

9’ -

c lO~

(a)

iO~

i0~

~TMG (Torr)

(Pa)

b-3 ~ ~ ~8 6

102 I

(TMG -AsH 3) System 4T0rr Tsubn490°C PASH3O2x1O 0

(111 )B

hh1~.

u ~

ll1~x~_3

~ 2 ~~Cl” 10-s (b)

26.~

A,

~TMG

A 10-a

~

AA-ET1 w339 381 m495 381 lSBT

(Torr)

Fig. 2. Dependence of growth thickness per cycle on TMG pressure.

the TMG pressure. Fig. 2b shows the thickness on (111)A and (111)B surfaces. In the case of growth on the (100) surface, the growth thickness per cycle increases monotonically when the ternperature is over 500 C, (e.g. 600 C). However, the growth thickness per cycle saturates at the value of one monolayer of (100) (2.83 A) in a pressure range of over 1 X 1O~Torr at 500 C. Namely, under such conditions, Ga compounds or Gaadsorption. adsorbates On automatically undergo layer the other hand, the monoTMG may decompose to Ga, and the Ga forms a multi0

0

0

_______

Chopper

13

-

Fig. 1. Experimental setup of molecular layer epitaxy and reflected intensity measurement system.

layer deposition at the temperature range over 500°C. The growth thickness on the (111)B surface is large, while the thickness on the (111)A surface is very small, under monolayer growth conditions on the (100) surface. Fig. 2b shows the growth thickness per cycle on (111)B and on (111)A, as a function of the

14

J. Nishjzayva

/ Molecular layer epitaxy and

its fundamentals

TMG pressure, at 490 C. The growth thickness per cycle on (111)B saturates at a value of one monolayer of (ill) (3.26 A) in a pressure range of 2.4 x l0~ to I x iO~ Torr. However, it increases monotonically in the pressure range over 1 x 10~Torr, since monolayer adsorption of the Ga compound may be formed automatically under the conditions mentioned previously. Under the latter condition (over I x i0’ Torr), multilayer adsorption of Ga or Ga compounds may occur. On the other hand, growth on (llIl)A is always very small; therefore, it seems that an adsorption layer of Ga or Ga compounds is not formed, or desorbs quickly. °

3.2. Decomposition of TMG and of AsH3 As a fundamental experiment, the decomposition of TMG [121and of AsH3 [13] was measured on substrates of various orientations. Fig. 3 shows the surfaces of the substrates after TMG supply. Some Ga droplets were observed by a scanning electron microscope (SEM) on (111)B and (100) surfaces. The Ga deposition was performed by changing the temperature of the substrates and the pressure of TMG. On (111)B, the deposition is detected at a temperature above 400 C. Q~ (100), it occurs at a temperature above 500°C, and the droplets of Ga are very small, as shown in fig. 3. The Ga deposition on (111)B and (100) are quite different from each other, as can be seen in the figure. However, no Ga deposition occurs on (111)A even at a temperature of 650°C. (11 1)B is the arsenic stable surface, (11 1)A is the gallium stable surface, and (100) has no preferential surface layer. From these results, we condude that a TMG or Ga compound may adsorb on an arsenic atom, and the adsorbate undergoes thermal decomposition on the surface of (111 )B and (100). On the other hand, TMG or Ga compound cannot adsorb on gallium atoms, and therefore, no decomposition occurs on a stable gallium (111)A surface. is in compliance with the results of the growthThis by MLE that is shown in fig. 2a. A precise explanation of discussion about the adsorbation on each surface is given by the 0

results of the reflected intensity measurement.

Fig. 3. SEM photograph of deposited Ga by the surface reaction of TMG. TMG was injected for a few minutes at 6.9x 10 Torr. (a) On (111)B surface. T,~5= 54()0 C; (h) on (100) surface, ‘~uh= 540°C. (c) on (lll)A surface, ~ = 6000 C. Marker represents 5 p.m.

The decomposition of AsH3 on variously onented surfaces was measured in a normal pressure MOCVD reactor by means of infrared absorption (IR) spectroscopy [131.The total area of 2, each the reactor was 9 about 9 cm and asubstrate Ga melt inwhich had about cm2 surface was used. These samples were located in the reactor and were heated, AsH 3 was injected and the decomposition rate was measured. For quan-

J. Nishizawa

/ Molecular layer epitaxy and

its fundamentals

titative analysis, the intensity of the largest IR

AsH44” 3.a..~j—

absorption of AsH3 at 2122in cm sured. The peak results are shown fig. 4.was Onmeathe vertical axis the deviation of the absorbance with

TMG—D—~-—————— 4’:

and the surface, without and sample is notable plotted. in theThis temperature deviation describes thethe amount ofis decomposed AsH 3 on range of 500 to 650°C. The (lIl)A surface has the largest effect for the decomposition of AsH3, followed by the Ga melt and lastly the (100) surface. The (111)B surface has no effect for the decomposition of AsH3. Each curve has a maximum at a temperature near 600 C, depending on the pyrolytic decomposition of AsH3 itself in the vapor phase and such, and therefore, a decrease of the deviation does not mean a decrease of the decomposition of AsH3 on the surface. In the caseofofpolycrystalline a Ga melt, the GaAs surface is covered with a crust during the injection of AsH 3, and the effect of the Ga melt on the decomposition of AsH3 may be due to this crust. From these results, we conclude that AsH3 may adsorb on the gallium atoms on the GaAs substrate, and the adsorbate of AsH3 causes a catalytic decomposition on the surface of (lll)A

15 4’~ 4”

1—~-—————— TMG—~

:

5.Ox1~ ~

2

400

~

450 ~ 425

2X1a54~,

__‘N-~..... 2

~

500 525~

3.0 x)0~

°

i.0xiO~

Jr’\.~,,_

550

~

~ ~Tsub

1.0x1~TOrr

500°C

T9~g~°C). Eli (11 0) 4Torr R~=1.0x1O’ ~MG=~0xl0T0rr

Fig. 5. Reflectance difference during MLE on (100) surface. Dependence of injected pressure of TMG and substrate temperature.

and (100). On the other hand, no catalytic effect occurs on the stable arsenic (Ii1)B surface. 3.3. Reflected intensity measurement during MLE

0.15

i

I

I

(AsH3.H2)System(1

AT

I

Pressure)

D---with GaAs (ill )A

•---withGaAsIlll)B 0.1

-

~t1, ~..wiIh

GaAs Ga 1100)

‘

E

e

°

We tried to measure the intensity of the reflectance of the laser beam from the growing surface by MLE. In the experiment, a He—Ne laser beam penetrated through a window from outside the MLE chamber to the surface of the wafers, as shown in fig. 1. The (100), (1 ll)B and

0.05~

0

(111 )A surfaces were used as substrates of GaAs, respectively. It was found that the intensity of the reflectance was synchronized by the period of the alternate injection TMGgrowth and AsH3, and was reproducible under of normal conditions. No

~

~..;;7

----~.-~

•

5/

0

~ .\

-

I

400

I

500 Temperature

I

600

I

700

(°C)

Fig. 4. Temperature dependence of AsH3 decomposition. The versical axis gives the deviation of absorhance between with GaAs and without the substrate,

intensity changes could be measured without the growth process. Fig. 5 shows the reflectance difference during MLE on (100) surfaces. These spectra were measured when the light was polarized along the (110) axis. The reflectance difference on the (100) surface was measured by changing the pressure of TMG at a fixed AsH3 pressure of 1.0 X

J. Nishizawa / Molecular layer epitaxy and its fundamentals

16

l0-~ Torr, and a growth temperature of 5000 C. that is, normal MLE conditions. The reflectance increases and saturates during TMG injection, and it increases and saturates again during TMG evacuation when the pressure of the TMG is of the order of 5.0 X i0~ Torr. During each cycle of AsH3 injection, the reflectance decreases to

during TMG injection is due to monolayer adsorption of Ga or Ga compounds. The increase of the reflectance during TMG evacuation in this temperature range may indicate that the condition of the adsorbate is infected, i.e. surface migration or reaction of the Ga compound adsorbate may occur. In addition, non-saturation of the reflectance during TMG injection may he due to the scattering of light by Ga droplets on the surface at temperatures above 550 C. The reflectance change during MLE on the (111 )B surface was also measured. Fig. 6a shows the reflectance difference under TMG injection conditions of 10 s at a pressure of 4.0 ~ l0~ Torr, for AsH3 injection of 20 s at the pressure of 2.5 X l0—~Torr, and for an evacuation time of each gas of 10 s. When the growth temperature is above 4900 C, the reflectance increases at once and decreases during TMG injection, hut it increases and saturates during TMG evacuation. During AsH3 injection, the reflectance increases rapidly at once, and then decreases up to the initial level, as shown in the temperature curves of 510, 500 and 490°C. On the other hand, the

the initial level, The reflected intensities were also measured by changing the growth temperature. An identical phenomenon of increase and saturation of the reflectance during TMG injection and evacuation was observed in the temperature range from 450 to 5250 C. However, the reflectance does not saturate during TMG injection at a temperature below 4500 C and decreases during a TMG injection at a temperature of 4000 C. But as a temperature above 550°C the reflectance increases rapidly and does not saturate during TMG injection. These phenomena are not affected by the length of the injection period of the TMG, as we get the same results when the injection time of TMG is 16 s. From the result in fig. 2a, it is supposed that the saturation of the reflectance

AsH3., TMG-41-~——~--i—

Hi ~

~UJ~

S 1OI~LJ~

1

~O.6’ 0.4

470

410—1

(a)

“0

Lii.’

,,,.

/~~

‘

TMG—~~——’—

I io~

(!~N-.

.i’it:

i’-4.

~

0.2 ~

.~

_________________

(b)

~

‘

‘

~

~

~

~Jf:riL J~ 1~hL~L .f: :

Y’

1.0’ 08’

11 ~I IA II 480

~-

f

~

~111~B /

4Torr 1.4’ ~=2.5x1O 1.2’ T~490°C

.f~Y

500

0

AsH

I iO’~

~rMG

4Torr

iP 3=2.5xlO

370~+~~”~-\1~=4.0x10”[orr T~~C)

Fig. 6. Reflectance difference during MLE on (111)B surface: (a) temperature dependence: (b) TMG pressure dependence.

J. Nishizawa

/ Molecular layer epita.xy and

reflectance increases and saturates during TMG injection, increases again during TMG evacuation, and finally decreases up to the initial level during AsH3 injection at a temperature range

~I and il’ increase monotonically with increasing TMG pressure up to 1 x iO~ Torr, and decrease at the pressure range over 1 X iO~ Torr. This tendency may correspond to the result of MLE as shown in fig. 2b, that is, the growth thickness per cycle saturates at once, but increases rapidly at a pressure range of over 1.5 x i0~ Torr. Fig. 7a shows the reflectance difference on (11 1)A under the same conditions as that for the (I11)B shown in fig. 6. Except for the result of the highest temperature (600 C), the reflectance always decreases during TMG injection. For example, at 550 and 585 C, the reflectance is restored immediately when TMG is stopped, and it can be seen that the time constant of restoration becomes larger at lower temperatures. In addition, when the temperature is lower than 350 C, the reflectance decreases when TMG is first injected, but after that no change occurs. In addition, the reflectance does not change during AsH3

°

0

°

°

°

°

_

°

_______________________

~ J~J(~,.) k~ L~J

45Q.~Jj,

~ 0.5 ~0.4’

T~p~=490°C

0.2’ I

-

1O~

(b) ~

(a) Fig.

‘

fl

(111)4 ~SH3=Z5x10~T0rr

600’~.: 585~1~,~,ft°°1.,,J~ff1 550-’~

17

the maximum value of the reflectance difference during TMG evacuation. The relation of these values and self-limiting growth is not clear, but

between 450 and 490 C. When the temperature is below 410°C, the reflectance increases during TMG injection, but it does not increase during TMG evacuation. The phenomenon during TMG injection at the temperature range above 490 C may be due to the scattering of light by the formation of Ga droplets as shown in fig. 3. In addition, the reflectance decreases gradually by increasing the number of cycles during growth, at a temperature of 510 and 500°C. This may be due to the roughness of the growing surface. The change of reflectance during TMG evacuation may indicate the rearrangement and surface readtions of the adsorbate, which can be Ga at a temperature above 490 C, and Ga compounds at a temperature below 490 C. Fig. 6b shows the result of the reflectance difference on (111)B at a temperature of 490°C. ~I means the maximum value of the reflectance difference during TMG injection, and ii’ means

AsH~ ,.~ TMG-~-430+—

its fundamentals

iO~

I

10~

1O~

PTMG(Torr)

(~.~3=2.5x1O~Torr 4.OX1O orr \~MG=

7. Reflectance difference during MLE on (111)A surface: (a) temperature dependence; (b) TMG pressure dependence.

IS

J. Nishizawa

/ Molecular layer epita.vy and its fundamentals

injection on the (111)A surface for every temperature range. In fig. 7b, the dependence of .~Ion the injected TMG pressure is shown. Here, we see that .~Jsaturates at a certain value with tncreasing pressure. From these results, the rcason that no growth occurs on (1II)A, as shown in fig. 2b, can he the desorption of adsorbate during TMG evacuation. From the results of the reflectance measurement we can conclude that adsorption of Ga compounds may occur during TMG injection. If the adsorbate from TMG was Ga, the Ga should stay on the surface because its

a monolayer growth condition. The phenomenon of reflectance increase during TMG evacuation may he due to the surface reaction or migration of Ga on Ga compound. (4) The reflectance differences were measured on a (lll)A surface. Ga compound adsorption may occur during TMG injection, hut it may desorh immediately during TMG evacuation at temperatunes of 425 ~ T,uh 585 C. On the other hand, the Ga compound may be stable on (Ill )A at lower temperatures (‘.ub 350°C),hut the reaction of the adsorbate with AsH3 may not occur at

vapour pressure is low enough. Therefore the adsorbate must be a gallium compound like Ga (CH3)~(x ±0). When the temperature is lower than 3500 C, the adsorbate may be stable even during TMG evacuation and during AsH3 injection, but the reaction of the adsorbate with AsH2 may not occur, therefore no growth occurs.

such low temperatures.

~

0

References [I] J. Nishlzawa.

J. Metals 25 (1961) 149;

J.

N~sh~zawa. J. Metals 25 (1961) 177. [2] M. Kumagawa, H. Sunami, ~F.Terasaki and J. Nishizawa, Japan. J. AppI. Phys. 7(111(1986) 1332. [3] J. Nishizawa and Y. Kokubun. in: Extended Abstracts

4. Conclusions

16th Intern. Conf. on Solid State Devices and Materials.

Kohe, 1984, p. I.

We have been studying MLE and its fundaments and have performed an in-situ observation of the surface chemistry by measuring the reflected intensity on the growing surfaces which led to the following results: (1) Self-limiting growth conditions were found on (100) and (Ill )B, respectively. (2) The decomposition of TMG and/or Asl-13 strongly depended on the surface orientation of the crystal. The surface might act as a catalyst. The (11 1)A surface has the largest catalytic ahilmty for AsH3 decomposition, but not for TMG decomposition. The (11 I)B surface has the largest efficiency for the decomposition of TMG, hut not for that of AsH The (100) surface has characteristics intermediate between (III )A and (Ill )B. (3) The reflectance differences were measured on (100) and (11 1)B surfaces during MLE. The saturation effect of the reflectance may correspond to ~.

[4] J. Nishlzawa. H. Abe and

r.

trochem. Soc. 132 (1985)1197. Nishizawa. H. Abe and T

Kuraha~ashi, J. Elec-

J

Kurahayashi.J. [Icctroclicni. Soc. 136 (1989) 478. [6] J. Nishizawa, K. Aoki. S. Suzuki and K. Kikuchi. J. Crystal Growth 99 (1990) 502: [5]

J. N~shiza~a.K. Aoki. S. Suzuki and K. Klkuchl. J. Electrochern. Soc. 137 (1990)1898. [7] T. Suntola, US Patent No. 4058430.

181

‘1’. Suntola, in: Extended Abstracts 16th Intern. Conf. on Solid State Devices and Materials, Kohe, 1984, p. 647.

[9] J. Nishlzawa, T. Kurabayashl. H. Abc and A. Nozoe. Surface Sd. 185 (1987) 249.

[Ill] D.E. Aspnes. J.P. Harhison, A.A. Studna and L.T. Florez, Phvs. Rev. Letters 59 (1987) 1687. [Ii]

N. Kohayashi and Y. liorikoshi, Japan.

J. AppI.

Phys. 28

(1989) Lt880. [12] J. Nlshizawa, T. Kurahayashi. II. Abe and N. Sakural. J. Elcctrochem. Soc. 134 (1987) 945. 1131 J. Nishizawa and T. Kurabayashi. J. Crystal Grwoth 99 (1990) 525; J. Nishizawa and T. Kurahayashi, J. Electrochern. Soc. 130 (1983) 413.