Reaction mechanisms of mono-molecular layer growth using chemical adsorption

applied surface science ELSEVIER

Applied Surface Science 106 (1996) 11-21

Reaction mechanisms of mono-molecular layer growth using chemical adsorption Jun-ichi

Nishizawa

a,b,*, Toru Kurabayashi b,c

a Tohoku Unit,ersiO', 2-1-1 Katahira, Aoba-ku, Sendai 980, Japan b Semiconductor Research Institute, Kawauchi, Aoba-ku, Sendai 980, Japan c Faculty of Engineering, Tohoku Unit,ersit3,, Aramaki-Aza-Aoba, Aoba-ku, Sendai 980, Japan

Received 17 September 1995; accepted 17 October 1995

Abstract The surface reaction mechanism of GaAs monolayer after monolayer growth using chemical adsorption of Ga(CH3) 3 (trimethylgallium (TMG)) and arsine (AsH 3) was investigated. The monolayer growth on GaAs(100) was investigated as a function of injection duration, injection pressure, evacuation duration of TMG and AsH 3 and photo-irradiation in an ultra-high vacuum system. It was found that the growth rate per cycle was strongly influenced by the surface stoichiometry of arsenic during the growth. Furthermore, in-situ analysis of the monolayer growth by using mass spectroscopy and the measurement of surface reflectivity during monolayer growth was performed. As the result, a model for the surface reaction and the adsorbed species of mono-molecular layer growth was obtained.

1. Introduction

The photo-exited process, proposed by Nishizawa [1] in 1961 has been used for enhancing and controlling the processing steps of chemical reactions individually, using monochromatic light as an exciting source. The purpose of the present work is to clarify the elemental process steps and to obtain in future perfect low temperature crystal growth using photoexcitation. Mono-molecular layer single crystal growth was first accomplished in 1984 as molecular layer epitaxy (MLE). MLE can produce single crystalline films, monolayer after monolayer by alternate injection of component gases of the materials onto the

* Corresponding author. Tel.: + 81-22-2237287; fax: + 81-222237289.

substrate in an ultra-high vacuum (UHV) chamber. MLE is based on the idea of atomic layer epitaxy (ALE) which was applied to grow II-VI polycrystalline films [2,3]. Until now MLE of GaAs [4-6], Si [7,8], and AlxGal_,As [9] have been developed in combination with the doping methods [10]. Photo-excited MLE was also performed to control the individual process steps [6], because the photo-excited process has a potential to enhance specific reactions selectively, without excitation of undesired processes which occur in plasma and thermal excitation. From the application point of view, MLE is a fascinating technology for the fabrication of mesoscopic devices such as the ideal static induction transistor (ISIT) proposed by Nishizawa in 1979, which has an extremely short gate length, less than the mean free path of the electron [11]. From the scientific point of view also, we believe that the reaction mechanism of

0169-4332/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. Pll S01 69-43 32(96)00428-X

12

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J. Nishizawa, T. Kurabayashi / Applied Surface Science 106 (1996) 11-21

MLE will attract a lot of interest from surface scientists. Since the study of the MLE process using QMS (quadrupole mass spectroscopy) in 1987 by Nishizawa et al. [12], much work has been done on the adsorption species [ 13-16]. In this work, the latest and detailed growth experimental results under various growth conditions, e.g. dosage of gas materials, duration of supply and evacuation time, substrate temperature and photoirradiation effect will be presented. In addition, the experimental results of the surface reaction mechanism measured by mass spectroscopy during growth will be presented. According to these results, the surface properties of the monolayer growth of GaAs, the surface reaction mechanism, and the adsorption species of mono-molecular layer growth will be discussed.

2. E x p e r i m e n t a l s y s t e m

Fig. 1 shows the schematic diagram of the molecular layer epitaxy (MLE) system and gas injection sequence. This apparatus consists of the growth chamber, the gas control system of AsH 3 and TMG, the heating lamp system, the temperature control unit with a pyrometer, the loading chamber and a pumping system. The growth chamber was evacuated by a turbo-molecular and rotary pump. The net pumping speed was about 200 / / s , and the background preslight

..... QMS ..... QMS e_~~rtz

plate ---LN shroud

heating lamp Fig. 2. Setup for the QMS measurement. sure was about l × 10 -9 Torr. The arsenic source was pure AsH 3 (100%), and the gallium source was pure TMG. The 15 mm square substrate was (100) GaAs doped with Si (n = 2 × 10 ]8 cm-3). Before g r o w t h , the substrate was etched with H z S O 4 : H 2 0 2 : H 2 0 (10: 1: 1) for 1 rain and about 60 islands of SiN x (800 X 1000 /xm 2 in size) were formed on the substrate. Prior to the growing process, the substrate was heated to 580°C for 3 min under an AsH 3 pressure of 5 × 10 -4 Torr to remove the surface oxide. After that, the substrate temperature was set at the growth temperature. For molecular layer growth, TMG and AsH~ were introduced alternately after being evacuated respectively. The total number of cycles of the growing sequence was

heating

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GASINJECTIONSEQUENCEIN MLEGROWTH Fig. 1. Schematic construction of the MLE chamber.

J. Nishizawa, T. Kurabayashi / Applied Surface Science 106 (1996) 11-21

150-200. The growth occurred selectively and no deposition was observed on the SiN X films. After removing SiN x by H F solution, the growth thickness was measured by a stylus profiling equipment (TENCOR Inst. alpha step)• The analysis of the reaction products by a quadrupole mass spectrometer (QMS) was performed in almost the same apparatus as used in the growth experiments, except that the QMS was located on top, as shown in Fig. 2. The entrance of the QMS was located at 2 - 3 cm above the substrate. The QMS chamber was evacuated by another turbo molecular pump (200 f / s ) . The substrate was heated by an IR lamp located below the substrate. The QMS port and the heating lamp were surrounded by a liquid nitrogen shroud• Moreover, a thin quartz masking plate of 0.3 mm thick was used as a mask of the substrate. The substrate was first masked, then the gas was introduced onto the masking plate to measure the background signal. After that the mask was removed to expose the substrate to the gas flux, and the transition of the signal was measured. This method has been applied by Taylor et al. [17] and Smentkowski et al. [18]. 3. Reactions in M L E

3.1. AsH 3 supply Fig. 3 shows the growth rate per cycle at 513°C versus AsH 3 dosage, i.e., the product of the pressure and the duration of AsH 3 in each cycle. In the 3.5

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P ( A S H 3 ) x t ( A S H 3 ) (Torr sec) :X Fig. 3. G r o w t h rate per cycle as a function o f A s H 3 dosage. A s H 3 dosage m e a n s the product o f the pressure and the duration of A s H 3 injection per cycle. The T M G pressure was 5 X 10 -5 Tort.

13

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figure, the closed squares were obtained for a constant AsH 3 pressure of 1 × 10 -3 Torr, and the injection duration was changed in the constant pressure mode. The open circles in the figure pertain to a constant AsH 3 injection duration of 10 s and the AsH 3 pressure was changed as in the constant duration mode, to examine a lower AsH 3 dosage. In addition, the growth rate for each mode at the temperature of 475°C is also plotted in the figure• The results are as follows. (1) There was no difference in the growth rate per cycle between the constant pressure mode and the constant duration mode. The dominant factor of monolayer growth would be the AsH 3 dosage. (2) There were three different characteristics of the growth rate versus AsH 3 dosage. The first region under the AsH 3 dosage of 2 × 10 -3 Torr s showed a linear dependence of the growth rate ( y ) on AsH 3 dosage ( x ) given by y = 1000x. (Please note that the scale of the y axis is logarithmic.) The second region from 2 × 10 -3 to 7 × 10 -2 Torr s showed a linear dependence with some offset given by y = 12.5x + 2.0. Probably it means the existence of a 2.0 thick offset which corresponds to about 70% coverage of arsenic on the growing surface. Finally, the third region of the AsH 3 dosage above 7 x 10 -2 Torr s showed a constant growth rate at the monolayer value (2.83 A). It is likely that there are two kinds of stable states of arsenic on the growing surface. One is near 70% coverage, which is found at a lower AsH 3 dosage, and the other is at almost 100% coverage with an excess of AsH 3 dosage. This

14

J. N i s h i z a w a ,

T. K u r a b a y a s h i

/Applied

may be due to the interaction of AsH 3 with the arsenic on the surface. (3) The dependence of the growth rate per cycle on AsH 3 dosage was not influenced at temperatures of 475°C and 513°C. Chadi [19] predicted two different structures of (2 X 4) As-stabilized surface. One is (2 X 4)y with an arsenic coverage of 100% (monolayer), and the other is (2 X 4)/3 with an As coverage of 75% ( 3 / 4 monolayer). The coverage of 75% may fit our result of 2 A offset (70% coverage of arsenic). However, this is not certain because Ga or Ga compound adsorption by T M G injection may also depend on the As coverage, i.e. the growth rate may be affected not only by arsenic coverage but also by Ga coverage. About the adsorbate of the Ga compound by T M G injection, the reaction product of the surface reaction was analyzed by mass spectroscopy. This is to be discussed later. 3.2. Ec,acuation o f AsH~ To study the possibility of desorption of arsenic atoms from the growing surface the dependence of the growth rate per cycle on the evacuation duration of AsH 3 was investigated. The results are shown in Fig. 4. AsH 3 was introduced for 100 s at a pressure of 1 X 1 0 -3 Torr (dosage 1 X l 0 - l Tort s) and T M G for 16 s at 5 X 10 5 Torr. This condition was the monolayer growth condition as shown in Fig. 3. As a result, an almost complete monolayer growth occurred when the evacuation duration of AsH 3 was

Science

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106 (1996)

l 1-21

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pressure was 1× 10 3 Tort. shorter than 4 s, but the growth rate per cycle decreased gradually with increasing the evacuation duration of AsH 3 and saturated near 2 ,~. This coincides with 70% coverage saturation of arsenic as shown in Fig. 3. All samples exhibited mirror-like surfaces. The growth rates at temperatures of 475°C and 513°C show similar tendency. The effect of AsH 3 evacuation duration on the growth rate was similar at both temperatures. It is suggested that the arsenic coverage changes from 100% to 70% above 30 s the AsH 3 evacuation. The arsenic re-evaporation from the monolayer covered surface might occur in the monolayer growth condition, i.e. monolayer growth would be realized within 4 s of AsH 3 evacuation. 3.3. TMG supply

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T M G D o s a g e ( T o r r sec) Fig, 5. G r o w t h rate p e r c y c l e as a f u n c t i o n o f the T M G d o s a g e . T M G d o s a g e m e a n s the p r o d u c t o f the p r e s s u r e a n d the d u r a t i o n of TMG Tort.

Surface

injection p e r cycle. T h e A s H 3 p r e s s u r e w a s 1 × 10 3

Fig. 5 shows the growth rate dependence on T M G dosage. In the figure the constant pressure mode was 1 0 0 " - 4 " - x " - 4 " ; a 100 s AsH 3 injection, a 4 s evacuation of AsH 3, a x s T M G injection, and a 4 s T M G evacuation. Thus the T M G injection duration is changed. The constant duration mode was 1 0 0 " - 4 " 16"-4", and the pressure of T M G was changed. In the case of AsH 3 dosage as discussed, both modes showed the same characteristic. However, in the case of T M G dosage each mode showed quite different characteristics. The results are summarized as follows. (1) When the T M G dosage was from 2.4 × 10 -5 to 8.0 X 10 -5 Torr s in the constant duration mode in Fig. 5, i.e. a p r e s s u r e from 1 . 5 × 1 0 6 to 5 . 0 ×

J. Nishizawa, T. Kurabayashi / Applied Surface Science 106 (1996) 11-21 10 - 6 Torr, the growth rate per cycle exceeded the monolayer height, and the surface obtained was rough, whereas when the TMG dosage was above 1.5 × 10 4 T o r r s, monolayer growth occurred and a mirror-like surface was obtained. Those characteristics did not show in the constant pressure mode. Therefore this 'enhanced growth' at rather lower TMG dosage and monolayer growth at a rather higher TMG dosage are connected with the TMG pressure. (2) In the constant pressure mode with a TMG pressure of 5 × 10 5 Torr, the growth rate exceeded the monolayer height when the TMG dosage was above 1.5 × 10 3 Torr s, i.e. the injection duration was longer than 30 s. The constant duration mode did not show such an over-monolayer growth, i.e. 'enhanced growth' in the same region of TMG dosage as shown in Fig. 5. The growth rate dependence on the growth temperature was studied as shown in Fig. 6. The mode was 100"-4"-16"-4", and the AsH 3 pressure was fixed at 1 × 10 3 Torr. The TMG pressures were 4 × 10 6 Torr corresponding to the 'enhanced growth' and 5 × 10 -5 Torr corresponding to the monolayer growth. In the case of TMG pressure of 4 × 10 6 Torr, the growth rate per cycle exceeded the monolayer height at temperatures above 495°C, and a rough surface was obtained in this region. In addition, monolayer growth may not occur in the temperature range below 495°C. In the case of a pressure of 5 × 10 -5 Torr, monolayer growth occurred at temperatures from 445 to 522°C, and a

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Temperature (°C) Fig. 8. Growth rate dependence on growth temperature with and without irradiation. A Hg lamp of l W / c m 2 was used as irradiation source. The TMG pressure was 4;< 10 .6 Torr and 5× 10 5 Torr, respectively. The AsH 3 pressure was 1 × 10 3 Torr.

smooth surface morphology was observed. Therefore, the reaction of TMG would be different between 4 × 10 6 and 5 × 1 0 - 5 Torr of TMG pressure. The complete decomposition or the decomposition enhancement of TMG may occur at a TMG pressure of 4 X 10 - 6 Torr above 495°C. This might be the origin of the 'enhanced growth'. 3.4. Evacuation

of TMG

In Fig. 7, the dependence of the growth rate per cycle on the TMG evacuation duration time is shown as a function of growth temperature. In the case of 513°C, the growth rate per cycle was near the value of a monolayer. Surface roughness was obtained when the duration was longer than 20 s, whereas mirror-like morphology was obtained when the duration was shorter. We assume the surface migration of the Ga or Ga compound species is enhanced during the TMG injection and the TMG evacuation, and the nucleation of Ga occurs when the duration is long enough, such as 20 s evacuation. This nucleation might be an origin of the rough surfaces. On the other hand, mirror-like surfaces were observed even when the TMG evacuation duration was longer than 20 s at temperatures of 475, 445, and 425°C. The growth rate per cycle decreased with increasing evacuation duration of TMG and probably saturated above 40 s. The adsorbate formed by TMG supply might not be Ga, but Ga compounds which may be volatile and re-evaporate during the TMG evacuation.

16

J. Nishizawa, T. Kurabayashi / Applied Surface Science 106 (1996) 11-21 100000

The most important factor is the identification of the Ga species during or after the TMG injection. To clarify these questions the reaction products of the growth process have been studied with a mass spectrometer, as discussed later. 3.5. Photo-excitation in MLE

exposed

ainu 84 GaCH~ X 5

masked

soooo

co -50000

Photo-excited MLE was also applied for the individual excitation of each process step as mentioned in Ref. [20]. As source gases Ga(C2Hs) 3 (triethylgallium (TEG)) and AsH 3 were used, and excimer lasers were used as the irradiation source. The irradiation duration was synchronized with the duration of the AsH 3 supply, the TEG supply, the AsH 3 evacuation, or the TEG evacuation. In the case of 308 and 350 nm laser irradiation, the residual impurities in the grown film decreased when the irradiation was synchronized with AsH 3 a n d / o r TEG supply. Fig. 8 shows the growth rate dependence on the growth temperature in TMG-AsH 3 MLE with and without the irradiation. A high pressure Hg lamp of about 1 W / c m 2 output power was used as the irradiation source. The growth rate was only slightly but always enhanced. The carbon concentration of the growth layer which was measured by SIMS (secondary ion mass spectroscopy) increased also in the case of the irradiation. We will examine the effect of wavelength of the irradiation on the monolayer growth.

4. In-situ analysis of MLE 4.1. TMG reaction on GaAs

When TMG was injected on the masking plate at room temperature, the parent ion signal of amu 114 Ga(CH3) ~ was detected as the smallest signal, but the signal of amu 99 Ga(CH3) ~- was the largest, and ainu 84 GaCH~, amu 69 Ga +, amu 15 CH~-, and amu 16 CH + were also detected. The signals except for ainu 114 Ga(CH3) ~ should be the fragment signals of TMG formed by the QMS ionizer. The net concentration for each signal due to the surface reaction was obtained by subtracting the count of the fragment signal of TMG from each signal. Fig. 9 shows typical QMS signals during TMG injection at a substrate temperature of 480°C and a TMG pressure of 2.3 × 10 7 Yorr. when the quartz

amu 99 Ga(CH3) 2 -100000

100

200

300

400

Time (sec) Fig. 9. The transient signal of ainu 15 CH~-, amu 99 Ga(CH3 )+ and amu 84 GaCH~ when the substrate was exposed to the TMG flux. The signal level before exposure was subtracted. The fragment of TMG was subtracted from the ainu 84 GaCH~" signal. Substrate: GaAs(100), substrate temperature: 480°C, TMG pressure: 2.3× 10 -7 Torr.

plate was removed the amu 99 Ga(CH3) + signal decreased rapidly and then recovered. This decrease is due to the consumption of TMG on the surface. Amu 15 CH ~ showed a rapid increase followed by a saturation. This change of amu 15 CH~ signal can be due to methyl radical formation. On the other hand the amu 84 GaCH~- increased gradually after the exposure. It appears that at least two reactions occur during TMG injection. One is the reaction corresponding to the rapid increase of ainu 15 CH~and the rapid decrease of amu 99 Ga(CH3)~'. The other is that corresponding to the gradual increase of the amu 84 GaCH~- signal. Fig. 10(a) shows the substrate temperature dependence (an Arrhenius plot) of the QMS signals after the signal intensity became steady (just before the substrate was masked again). During this time, the reaction which corresponds to the formation of ainu 84 GaCH~- at a steady rate is dominant. All the signals in Fig. 10(a) show at least three different reactions: the first at around 400460°C, the second at 510-610°C and the third at 610-660°C. Fig. 10(b) shows the temperature dependence of the ratio of amu 15 CH~ to amu 99 Ga(CH3)~-. The ratio in the steady state remains at about 1-1.2 in the temperature range 500-670°C. In this temperature range GaCH 3 is produced mainly as shown in Fig. 10(a). When the ratio is larger than 1-1.2, additional CH 3 desorption occurs, i.e. Ga would be formed on the growing surface. When the ratio is lower than 1-1.2, a smaller amount of CH 3 would desorb than the GaCH 3 formation, hence

J. Nishizawa, T. Kurabayashi /Applied Surface Science 106 (1996) 11-21

Ga(CH3) = would be formed. In view of the QMS results for the signal in the steady state, the following dominant reaction paths of TMG on the GaAs(100) surface are deduced:

and Ga(CH3) 3 ~ G a 1 " + 3CH 3 $ (steady state: 610-750°C).

(1) Ga(CI43)3 --, Ga(CI%)= 1' + CI-I3 "r (2)

Ga(CH3) 3 ~ GaCH 3 1' + 2CH 3 1' (steady state: 510-750°C),

(4)

The initial state reaction was also considered by subtracting the signal of the steady state from the measured signal, assuming that the desorption of amu 84 GaCH~ is caused only by the steady state reaction. Fig. 11 shows the dependence of the initial surface reaction of TMG on the substrate temperature. At relatively low temperatures, it took a long time to complete the reaction. As the temperature increased, the reaction time became shorter and the

Ga(CH3) 3 ~ no reaction (steady state: < 400°C),

(steady state: 400-460°C),

17

(3)

Substrate Temperature (* C)

Steady state on GaAs (100)

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700

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600

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Initial state

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Steady state

700

800

Substrate Temperature ( ° C) Fig. 10. (a) Temperature dependence of the net QMS signals at the steady state. (b) Temperature dependence of the ratio of CH~ to Ga(CH3) ~- at the steady state and initial states. The signal of CH~ is obtained as intensity increase• The signal of Ga(CH3) ~ is obtained as intensity decrease as shown in "-amu 99 Ga(CH3 )+ "

18

J. Nishizawa, T. Kurabayashi / Applied Surface Science 106 (1996) 11-21

C/)

~0

x\

8 450"C 430"C

5100C 480"C

60000 570"C

650"C

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Fig. 11. Temperature dependence of the QMS signal of the initial surface reaction of TMG on GaAs(100). Substrate was exposed at 0 min and masked at 6 rain• peak of the signal became higher in the case of the amu 15 CH~- (or lower in case of the amu 99 Ga(CH3)~-). The time constants obtained by analyzing the delay of the signals in Fig. 11 are shown in Fig. 12. The time constants decreased with increasing the substrate temperature. The self-limiting monolayer growth was achieved in the temperature range 450-510°C from the experiments as shown in

Fig. 6. The time constants of the delay shown in Fig. 12 were 36 s for CH~- and 25 s for Ga(CH3) f at 450°C, and 15 s for CH~ and 10 s for Ga(CH3) f at 510°C. In Fig. 11, the area surrounded by the signal curve and the base line in Ga(CH3) ~- and C H f correspond to the amount of consumed T M G and the amount of methyl radical released by the initial surface reaction, respectively. Fig. 13 shows the 120

200

Area

100

,00

A

. . - - [ ]I1~4~/~' ~ • signal *""-base

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line

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

50

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t~

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r~

40

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20

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o •

•

• a m u 15 CH 3

60

-~

~-

© a m u 9 9 Ga(CH3) ~

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00

99Ga(CH3)2 i

450

.......................... i

560

550

0

.oo i

600

Temperature(°C) Fig. 12. The time constants of CH~ and Ga(CH3)+ obtained by exponential fitting of each signal in Fig. 11.

O

&o

&o

I

I

600

650

700

Substrate Temperature (°C)

Fig. 13. Temperature dependence of the area surrounded by the signal of Fig. 11 and the base line as shown in the hatching area. This value would be proportional to the net amount of adsorption species on the surface and the released methyl radical.

J. Nishizawa, T. Kurabayashi /Applied Surface Science 106 (1996) 11-21

dependence of these areas on the substrate temperature. The net amounts of these areas, i.e. the amount of consumed TMG and the amount of released methyl, are almost the same in the specific temperature range of 420-510°C. The ratio of the amount of the amu 15 CH~- signal to that of the amu 99 Ga(CH3) ] signal was about 1.6-1.8 in this temperature range. The final remaining adsorbed species may be Ga, because the ratio of about 1-1.2 corresponds to GaCH 3 as already mentioned. However, the time constant of the reaction of GaCH 3 to produce Ga and CH 3 would be of the order of 15-36 s in the temperature range for monolayer growth condition. We have deduced also the following surface reaction process of TMG: at 420-510°C the monolayer of GaCH 3 is formed on the GaAs(100) surface by the introduction of TMG temporarily, from which adsorbed Ga and desorbed CH 3 with a time constant of 15-36 s are formed. The time constant of formation for the Ga adsorbed layer is longer if the substrate temperature is lower. Temporary adsorption of species such as Ga(CH3) 2 should also be present on the GaAs surface at lower temperatures. The further decomposition of TMG occurs on this Ga adsorbed layer. The products of this reaction may be

19

volatile molecules such as GaCH 3 or Ga(CH3) 2. From steady state considerations, the volatile species would be Ga(CH3) 2 below 460°C and GaCH 3 at 510-610°C. Finally, Ga is formed at higher temperatures and the atoms are concentrated to form Ga globules on the surface. The main reaction of TMG on GaAs(100) are hence as follows: Ga(CH3)3 --* GaCH3(ad) + 2CH3 1' (initial state),

(5) GaCH3(ad ) ~ Ga(ad) + CH 3 1" (transient state),

(6) and Ga(CH3) 3 --* GaCH 3 + 2CH 3 1' (steady state) or Ga(CH3) 3 ~ a a ( C U 3 ) 2 + CH 3 1' (steady state).

(7) From these results the adsorbate thickness is illustrated in Fig. 14 as a function of the temperature and the injection time duration of TMG. 4.2. A s H 3 reaction on G a A s

The QMS signal during an MLE growth sequence, i.e. alternative injection of TMG and AsH 3,

Fig. 14. Schematic illustration of the adsorbates as parameters of the temperature and the injection duration (time) on (100) by TMG injection.

J. Nishizawa, 72. Kurabayashi / Applied Surfilce Science 106 (1996) l 1-21

20

TMG

AsH 3

800000

Expos~ ~

700000

7

~

. . . . . . . .amu2 ......

~

H2 +

600000

/, A ....M's,

500000

v

¢-

.....ll

400000 Difference xl0

300000

/

200000 O9

O

100000 0

-100000 -200000

250000

amu15

Difference x l 0

200000

CH 3

v

ttm of)

150000

/

100000

CO

O

50000

B

0

0

50

100

150

Time(sec) Fig. 15. One cycle of each exposed and masked cycle is picked up from each cycle of the growth. 'Exposed' means the exposed cycle spectrum, 'masked' the masked cycle spectrum, 'difference' the differencebetweenthem. The pressure of TMG was 5.6 × 10 -6 Torr, and the pressure of AsH3 was 2.6 × 10 5 Torr. The gas injectionmode was 30 s injectionof TMG, 20 s evacuationof TMG, 100 s injectionof AsH3 and 20 s evacuationof AsH3. The substrate temperature was 490°C. were measured. In this measurement, the substrate was masked at every other cycle. The difference between the exposed and the masked cycles is shown in Fig. 15. During the TMG injection, there was a large difference in the amu 15 CH 3 signal, which agrees with the results shown in the previous section. During the AsH 3 injection the differences between amu 2 H~- and amu 15 CH~- were observed respectively, due to the surface reaction of AsH 3 with the adsorption species by TMG. The signal transients during AsH s injection were detected as amu 1 H +, amu 2 H~, amu 15 CH~ and ainu 16 CH~-. The amu 15 CH;- and amu 16 CH2 signals increased just after the exposure and then decreased. The ratio of

amu 16 CH~ to amu 15 CH~ would indicate the CH 4 formation from the ionization efficiency measurements. The ainu 15 CH~ transient peak (difference spectrum of 'exposed' and 'masked' at the exposure during the AsH s injection was about 2000 at maximum, and it rose to 23000 at the initial state of TMG injection. Thus the intensity of the 'difference × 10' of amu 15 CH~ during the TMG injection was 50 times larger than that during the AsH 3 injection. But the amount of the product should be integrated by the reacting time. The area surrounded by the signal of amu 15 CH~ 'exposed' and the base line 'masked' corresponds to the amount of the produced 15 CH~. In the figure, the ratio of

J. Nishizaw'a, T. Kurabayashi / Applied Surface Science 106 (1996) 11-21

initially desorbed amu 15 CH~ (A) and the produced amu 15 CH3~ (B) during the AsH 3 injection is about 6: 1. When we consider that C H 4 produces fragments of 15 CH~ and 16 CH~ of nearly the same intensity, the amount of C H 4 produced during the AsH 3 injection and the amount of CH 3 which desorbed at the initial state of TMG injection was roughly 1/3. This means that about 30% of the Ga adsorbates have CH3; the species on the growing surface consists of 30% GaCH 3 and 70% Ga. GaCH 3 and Ga on the surface would react with AsH 3 and produce GaAs growth, C H 4 and H 2 as the byproducts: GaCH3(adsorb ) + AsH 3 -~ GaAs + CH 4 + H 2 ,

(8) and 3

Ga(adsorb) + AsH 3 ~ GaAs + 7H2.

(9)

When we consider the time constant of 15 CH~ (40 s at 450°C and 20 s at 510°C) at the initial state of TMG injection as mentioned above, the coverage of GaCH 3 during or after the TMG injection might be considerably higher than 30%.

5. Conclusions We conclude that the surface reaction process of TMG proceeds in the following way. At 420-510°C, a monolayer of GaCH 3 is formed on the GaAs(100) surface by sufficient TMG introduction. The adsorbed GaCH 3 would form Ga and CH3 with a time constant of 15-36 s (transient state). The reaction between AsH 3 and the adsorbed GaCH 3 produces C H 4 and H 2 as byproducts with a long time constant ( > 30 s). Although GaCH 3 is volatile and not a stable molecule, the monolayer self-limiting growth in MLE was successfully achieved within 4 s evacuation duration of TMG after sufficient TMG injection. To actualize the monolayer growth not only monolayer adsorption of GaCH 3 but also monolayer

21

arsenic formation is needed. The arsenic on the growing surface of the monolayer has two kinds of stable coverage. One is 70% coverage of arsenic and the other is 100% i.e. monolayer coverage. The 100% coverage is achieved in the AsH 3 dosage range above 7 × 10 -2 Torr s, and the AsH 3 evacuation duration is within 4 s.

References [1] J. Nishizawa, J. Jpn. Inst. Met. 25 (1961) 149, 177; M. Kumagawa, H. Sunami, T. Terasaki and J. Nishizawa, Jpn. J. Appl. Phys. 7 (1968) 1332; J. Electrochem. Soc. 117 (1970) 907. [2] T. Suntola, U.S. Pat. No. 4058430 (1977). [3] M. Ahonen, M. Pessa and T. Suntola, Thin Solid Films 65 (1980) 301. [4] J. Nishizawa and Y. Kokubun, Extended Abstr. 16th Int. Conf. on Solid State Devices and Materials, 1984, p. 1. [5] J. Nishizawa, H. Abe and T. Kurabayashi, J. Electrochem. Soc. 132 (1985) 1197. [6] J. Nishizawa and H. Abe, American Vacuum Society, 32nd Natl. Symp., Final Program 109, Houston, TX, Nov. 1985; J. Nishizawa, H. Abe, T. Kurabayashi and N. Sakurai, J. Vac. Sci. Technol. A 4 (1986) 706. [7] J. Nishizawa, K. Aoki, S. Suzuki and K. Kikuchi, J. Electrochem. Soc. 137 (1990) 1898. [8] J. Nishizawa, K. Aoki, S. Suzuki and K. Kikuchi, J. Cryst. Growth 99 (1990) 502. [9] J. Nishizawa and T. Kurabayashi, Densi Tokyo, IEEE Jpn. 26 (1987) 120. [10] J. Nishizawa, H. Abe and T. Kurabayashi, J. Electrochem. Soc. 136 (1989) 478. [11] J. Nishizawa, Proc. l lth Conf. on Solid State Devices, 1979 (unpublished); Jpn. J. Appl. Phys. Suppl. 19 (1980) 3. [12] J. Nishizawa, T. Kurabayashi, H. Abe and A. Nozoe, Surf. Sci. 185 (1987) 249. [13] M.L. Yu, U. Memmert and T.F. Kuech, Appl. Phys. Lett. 55 (1989) 1011. [14] U. Memmert and M.L. Yu, Appl. Phys. Lett. 58 (1990) 1883. [15] J.R. Creighton, Surf. Sci. 234 (1990) 287. [16] K. Kodama, M. Ozeki, K. Mochizuki and N. Ohotsuka, Appl. Phys. Lett. 54 (1989) 666. [17] P.A. Taylor, R.M. Wallace, WJ. Choyke, M.J. Dresser and J.T. Yams, Jr., Surf. Sci. 215 (1989) 1286. [18] V.S. Smentkowski and J.T. Yates, Jr., J. Vac. Sci. Technol. A 7 (1989) 3325. [19] D.J. Chadi, J. Vac. Sci. Technol. A 5 (1987) 834. [20] J. Nishizawa, Appl. Surf. Sci. 79/80 (1994) 1.