Ge composition saturation behavior during low-temperature Si1 − xGex growth by disilane and solid Ge molecular beam epitaxy

j........ C R Y S T A L GROWTH

ELSEVIER

Journal of Crystal Growth 181 (1997)441 445

Letter to the Editors

Ge composition saturation behavior during low-temperature Sil - xGex growth by disilane and solid Ge molecular beam epitaxy J.P. Liu*, X.F. Liu, J.P. Li, D.Z. Sun, M.Y. Kong Materials Science Center, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, People "s Republic of China

Received 22 June 1997

Abstract Ge composition dependence on the Ge cell temperature has been studied during the growth of Six_xGex by disilane and solid Ge molecular beam epitaxy at a substrate temperature of 500°C. It is found that the composition x increases and then saturates when the Ge cell temperature increases, which is different from the composition-dependent behavior in growth at high temperature as well as in growth by molecular beam epitaxy using disilane and germane. The enhanced hydrogen desorption from a Ge site alone cannot account for this abnormal composition-variation behavior. We attribute this behavior to the increase of rate constant of H desorption on a Si site when the Ge cell temperature increases. PACS: 68.55; 68.60 Keywords." Sil_xGex alloys; Low-temperature epitaxy; Composition dependence; Growth kinetics

Recently, gas source molecular beam epitaxy (GSMBE) has been proposed as an alternative way for epitaxial growth of Sil_xGex alloys [1], The growth kinetics of Six-xGex by G S M B E using disilane (Si2H6) and germane (GeH4) have been widely studied [ 2 4 ] and it was found [4] that

*Corresponding author. Fax: + 86 10 6256 2389; e-mail: [email protected].

the Ge composition x in the film grown is determined by the Ge composition in the gas sources, Xg. With the increase of Xg, x can increase from 0 to 1. Six-xGex alloys have also been grown by G S M B E using Si2H 6 and solid Ge [5-8]. Zhang et al. [6] reported Six-xGex growth at 630°C, and found x increases from 0 to 1 when the Ge flux increases, which is very similar to the case in growth with both hydrides. However, there is very few reports on low-temperature growth. In this paper, growth

0022-0248/97/$17.00 (C' 1997 Elsevier Science B.V. All rights reserved PII S 0 0 2 2 - 0 2 4 8 ( 9 7 ) 0 0 4 0 7 - 7

J.P. Liu et al. /Journal of Crystal Growth 181 (1997) 441-445

442

kinetics of Sil_=Gex by disilane and solid Ge molecular beam epitaxy at a substrate temperature of 500°C and different Ge cell temperatures is studied. It is found that Ge composition x first increases with the increase of Ge cell temperature then saturates at about 0.45 with the further increase of Ge cell temperature, which is quite different from previous reports on the composition-dependent behavior. All samples were grown on Si(1 0 0) substrates in a home-made gas source MBE system [8]. The S i z H 6 flow rate was held at 4 sccm and Ge cell temperature ranges from 1160 to 1270°C. Substrate temperature was kept at 500°C. The compositions and thicknesses were determined by Rutherford backscattering spectrometry (RBS) and Auger electron spectrometry (AES), both results are in good agreement. Then the Ge growth rate, Rg ~xR, and the Si growth rate Rs = ( 1 - x)R were obtained [4]. Fig. 1 shows the alloy composition x dependence on Ge cell temperature. Ge cell temperature dependence of Rg a s well as that of vapor pressure of solid Ge is also plotted in this figure. Rg dependence

on Ge cell temperature shows the same trend as vapor pressure of solid Ge dependence, which m e a n s Rg is solely determined by Ge flux. This can be attributed to the large Ge desorption barrier, 2.5 eV [9] from Si surface. When Rg is small, x increases with Rg, similar to those dependences reported before. While it is noted that when the Rg increases to a certain value, in our case 8.2 A/min, the composition x stops increasing and remains at about 0.45. This is different from the behavior in growth with both hydrides [2 4] as well as with Si2H 6 and solid Ge at high substrate temperature [5-7] as mentioned above. Apparently, there is a new mechanism acts. It was suggested [-4] that in Sil_~Gex growth using both hydrides the difference between Rg and Rs is determined by hydrides adsorption, thus, Ge composition in the alloy is related to Ge composition in the gas source, Xg by 1 / x = J/xg + 1 - J,

(l)

where J is the ratio of rate constant of disilane adsorption Asl to that of germane adsorption Ag e. Since GeH4 is replaced by solid Ge, Ge growth rate

Ge cell temperature (°C) 0.50

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IO00/T (K) Fig. 1. Ge composition, Ge growth rate, as well as Ge vapor pressure (10 -4 Torr) dependence on Ge cell temperature. Samples were grown when Si2H 6 flow rate was held at 4 sccm, and substrate temperature was 500°C.

J.P. Liu et al. / Journal o f Crystal Growth 181 (1997) 441 445

seen that Rs increases with Rg even when x become saturated. It is noted that there is no Ge blocking effect as reported in Refs. [3, 6]. Thus, it is further confirmed that SizH6 adsorption is not a ratelimiting process. We assume the newly deposited Ge atoms during a unit time are the only effective H desorption centers, during this period, if Si2H6 flow rate is Fsi, there is 2Fsi Si atoms come to the surface, thus, 0 = Rg/2F~i. From Eqs. (1) and (2) we obtain

can be controlled independently by changing Ge cell temperature, which differs from the growth with both hydrides. Moreover, Si2H6 adsorption was found little dependent on a Ge site or a Si site [10]. As a result, Ge composition cannot be determined by Eq. (1). To determine Ge compositions of Sil_xG% alloys grown using Ge instead of GeH4 quantitatively, Rg and Rs should be known first. It is well established that in Si epitaxial growth using Si2H6 the balance of the dissociative adsorption and hydrogen desorption determines Si growth rate. At low growth temperature, Rs is determined by different H desorption for SizH6 on a Si or Ge site. Following Zaimai et al. [11] Rs can be written as R~ = (1

--

O)Rsi/si -Jr ORsi/ge,

1/x = N~gsi/Rg + Ns(Kge - KsO/2Fs~ + 1.

where 0 is the surface coverage of Ge atoms as effective H desorption centers, Rs~/~i and Rsi/g~ are the Si growth rates on a Si and Ge site, respectively. They are approximately equal to Ns x K~i and N~ x K g e [12, 13], the limits for the case of low dangling bonds density. N~ is the Si(1 0 0) surface site number density. K~, Kg~ are the rate constants of H desorption for Si2H6 on Si and Ge, respectively. In Fig. 2, Rs is plotted against Rg and x. It can be

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In Fig. 3, 1/x is plotted against 1/Rg, we find when Rg < 8.2 ~,/min, 1/x shows ideally linear dependence on 1/Rg: Fitting these points we get K~ = 0 . 0 8 s -1, Kge = 11.1 s -1 which agrees well with the published rate constant of H desorption for Si2H6 on Si [12, 13] and Ge [14]. This may justify our initial assumption on 0. By comparing Eqs. (1) and (3) one can see the different composition determining mechanisms in Sil-xGex growth using different sources. In S i l - x G % growth using SizH6 and GeH4, hydrides adsorption determined the difference of R~ and Rg, thus x, and since J > 0, 1 - J is no more than 1, so from Eq.(1) x can increase from 0 to 1 when

(2)

24

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,

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t

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Ge growth rate (Angstrom/min) Fig. 2. Si g r o w t h rate, Rs plotted a g a i n s t G e g r o w t h rate, R~ a n d Ge c o m p o s i t i o n , x.

444

,~P. Liu et aL /Journal of Crystal Growth 181 (]997) 441 445 4.0 Linear Regression Y = A + B * X ' Parameter Value

Error

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1.42737 6.58567

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reciprocal of G e growth rate (min/Angstrom) Fig. 3. 1Ix plotted against 1/Rg, x is Ge composition and Rg is Ge growth rate. The solid line is the linear fitting of the points when Rg < 8.2 A,/min.

Xg increases to 00. Similar composition dependence behavior is justified in Sil-xGex growth using Si2H6 and Ge at high temperature since R~ is also determined by Si2H6 adsorption. While in Sil-xGex growth using Si2H6 and Ge at low temperature, H desorption determined R~. Since Ns(Kge -- Ksi)/2Fsi + 1 is fitted to be about 1.50, far more than 1, even when Rg increases to oo, x cannot increase to 1 but turns to saturate to be 0.67. It is interesting to point out that saturation comes earlier in our experimental results and the saturated composition is about 0.45 (Fig. 1). F r o m Fig. 3 it can be seen that 1Ix deviates the linear dependence on 1/Rg when Rg increases. The deviation indicates that the catalytic effect of Ge alone cannot account for this abnormal composition variation behavior. We tentatively attribute it to the increase of Ksi when Rg increases. In this aspect, Ning and Crowell [15] investigated the desorption of deuterium from clean and Ge-covered Si(1 0 0) surfaces and found a 2.3 kcal/mol decrease of activation energy in Ksl when the Ge atoms surface coverage increases from 0 to 9.6%. If the same amount of activation energy decrease occurs

when Rg = 14A/min, then from Eq. (3), we get x = 0.455, which agrees with our experimental results well. In conclusion, Si, ~Gex/Si(1 0 0) alloys with Ge composition, 0.2 < x < 0.5, have been grown by disilane and solid-Ge molecular beam epitaxy at a substrate temperature of 500°C. The disilane flow rate was held at 4 sccm and the Ge cell temperature varied in the range of 1160-1270°C. It is found that the composition x increases and then saturates when the Ge cell temperature increases, which is different from the composition dependent behavior in growth at high temperature as well as in growth by G S M B E using disilane and germane. A simple model based on different H desorption on a Si and Ge site has been proposed to determine the composition of the alloys grown. Following the model, it is found that the rate constant of H desorption for disilane on Si and Ge are 0.08 and 11.1 s-1, which are consistent with the published values. The Ge composition saturation behavior is then attributed to the increase of rate constant of H desorption on a Si site when the Ge cell temperature increases.

J.P. Liu et al. /Journal of C~'stal Growth 181 (1997) 441 445

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