Effect of Deposition Parameters on the Characteristics

Journal of The Electrochemical Society, 148 共10兲 C685-C689 共2001兲

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0013-4651/2001/148共10兲/C685/5/$7.00 © The Electrochemical Society, Inc.

Effect of Deposition Parameters on the Characteristics of Low-Pressure Chemical Vapor Deposited SiGe Films Grown from Si2 H6 and GeH4 J. Olivares, J. Sangrador,z A. Rodrı´guez, and T. Rodrı´guez Departamento Tecnologıá Electrońica, ETSIT, Universidad Politećnica de Madrid, 28040 Madrid, Spain The growth rate, composition, and crystallinity of SiGe layers deposited in a hot-wall low-pressure chemical vapor deposition system, using disilane and germane as source gases, have been analyzed as functions of the deposition parameters, i.e., temperature, precursors, and carrier gas partial pressures. SiGe films with Ge fraction in the 0 to 0.38 range were deposited at temperatures of between 450 and 600°C. The growth rate increases with the partial pressure of the precursors and with the Ge content of the film, except for depositions made with high precursor partial pressure and low Ge fraction. The Ge content of the films increases with the germane fraction in the gas and the partial pressure of the precursors following a nonlinear dependence. The growth process has been analyzed assuming that the controlling reactions are the dissociation of the precursors at the SiGe surface, which are limited by the existence of free sites on the surface, originated by hydrogen desorption from it. The dissociation rate constants of disilane and germane depend on the surface composition and on the respective precursor partial pressure. Films with a Ge fraction lower than 0.5 deposited at temperatures below 500°C are amorphous, but films grown at higher temperatures or with higher Ge fraction are polycrystalline. © 2001 The Electrochemical Society. 关DOI: 10.1149/1.1399277兴 All rights reserved. Manuscript submitted July 12, 2000; revised manuscript received May 23, 2001. Available electronically September 11, 2001.

The use of SiGe films as a semiconductor material is receiving considerable attention for a large number of different possible applications, which include heterojunction bipolar and metal oxide semiconductor transistors, superlattice structures for optoelectronic devices, infrared photodetectors, and high efficiency solar cells.1-3 One of these applications is the manufacture of polycrystalline thin-film transistors 共TFT兲 for their use in active matrix liquid crystal displays 共AMLCD兲 as pixel drive transistors and as the elements of the integrated driver circuit.4,5 In this application the SiGe alloy can be an advantageous alternative for Si, the material that is used in commercial AMLCD. The polycrystalline Si is obtained by the crystallization of amorphous silicon deposited by plasma-enhanced chemical vapor deposition 共PECVD兲 or low-pressure chemical vapor deposition 共LPCVD兲.6,7 The use of SiGe alloys instead of Si has the technological advantages of a higher growth rate at low deposition temperatures, which is convenient when using low cost glass as the AMLCD substrate and shorter crystallization times for the same temperatures. In addition, SiGe also has the potential advantages of lower defect density and higher free carrier mobility in the crystallized material,8-10 and the possibility of adjusting its bandgap, which makes it an interesting material for forming low-resistance contacts to Si and replacing the poly-Si gate in complementary metal oxide semiconductors 共CMOS兲 technologies.11,12 The deposition of SiGe in a CVD system can be achieved simply by adding germane (GeH4 ) as a Ge source gas to the silane (SiH4 ) used as Si source gas, but the use of disilane (Si2 H6 ) instead of SiH4 allows for a further reduction in the deposition temperature while the deposition rate is maintained at relatively high values.1,13 In this paper we present results on the SiGe deposition in a hot-wall LPCVD system, using disilane as the Si source, germane as the Ge source, and hydrogen as the carrier gas. The effect of the gas composition, the partial pressure of the elements, and the temperature on the growth rate, composition, and crystallinity of the film have been investigated. Experimental Deposition was carried out in a Tempress hot wall LPCVD system with a 135 mm i.d. by 1200 mm long quartz tube. The heating furnace creates a flat temperature zone in the center of the tube 300 mm long, where the samples were placed. The temperature was kept constant during deposition, with values ranging between 450 and

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600°C. The substrates were 4 in. Si wafers with a 200 nm layer of SiO2 grown by dry thermal oxidation. Ten wafers, loaded at the center of the furnace flat zone with 9 mm spacing between them, were used in each deposition run. Once the samples were loaded, the system was evacuated using a dual-stage rotary vane vacuum pump to a base pressure of less than 5 mTorr, as measured with a capacitance gauge manometer. In all the deposition runs the pressure was adjusted to 300 mTorr through a controlled N2 purge to the vacuum pump intake port. Pure disilane (Si2 H6 ) and germane (GeH4 ) were used as precursor gases, with flow rates ranging from 0 to 20 sccm. H2 was used as carrier gas, with a flow rate up to 120 sccm. After deposition, the films were patterned and selectively etched with a solution of HNO3 and HF,14 and their thickness was measured with a mechanical profile analyzer. The uniformity was better than 1% on the entire wafer surface except for an external ring less than 1 cm wide, where deviations up to 7% were measured. The uniformity was also excellent from wafer to wafer in all the runs, except for those placed at both ends of the wafer-carrier and depositions made at temperatures higher than 500°C. Energy dispersive X-ray analysis 共EDX兲 and Rutherford backscattering spectrometry 共RBS兲 were used to measure the Ge fraction x of the Si1⫺x Gex layers. The homogeneity of the film composition was also excellent 共better than 2%兲 within the wafer, except for the external ring, and between wafers deposited in the same run, except for those placed at the ends of the wafer carrier. The results presented in the sections that follow were measured in the central area of the wafers placed in the middle of the wafer carrier. The crystallinity of the films was determined by X-ray diffraction 共XRD兲. The preferred grain orientations were obtained by comparing the intensities of the 共111兲, 共220兲, and 共311兲 peaks to their values in a randomly oriented powder.15

Results and Discussion Figure 1 shows the growth rate as a function of the gas composition for SiGe films deposited at 450°C and 300 mTorr in two different experimental conditions, i.e., with and without dilution in a carrier gas (H2 ). When H2 dilution is used, the disilane flow rate is kept constant at a value of 20 sccm, the germane flow rate is varied to obtain the desired germane-to-disilane ratio in every run, and the H2 flow rate is accordingly adjusted for a total flow of 140 sccm. In this way, the disilane partial pressure is the same in all the deposition runs 共43 mTorr兲, while the germane partial pressure varies from run to run 共between 0 and 43 mTorr兲. In this case, the SiGe film

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Figure 1. Deposition rate as a function of the germane-to-disilane ratio in the gas, for SiGe films deposited at 450°C and 300 mTorr. Closed symbols: pure precursors atmosphere; open symbols: precursors diluted in H2 carrier gas.

Figure 2. Ge/Si atomic ratio in the SiGe films as a function of the germaneto-disilane ratio in the gas, for the same processes as in Fig. 1.

kH

2H* ——→ H2 ⫹ 共 * 兲 2 growth rate shows an almost linear increase with the germane content of the gas within the reactor tube. When no dilution is used, a total flow rate of 10 sccm is maintained in all the processes, so both the germane and disilane flow rates change from run to run. Consequently, both partial pressures change 共from 0 to 174 mTorr for the germane and from 300 to 126 mTorr for the disilane兲. In this case, the values of the growth rate are higher than for dilution, as a consequence of the higher partial pressures of both germane and disilane, but a very different behavior can be observed; the SiGe film growth rate shows an initial decrease in values of the germane to disilane ratio lower than 0.5 and an increase for values beyond that minimum. The Ge-to-Si ratios on films deposited in diluted and nondiluted processes are shown in Fig. 2 as a function of the germane-todisilane flow ratio. The Ge content in the film increases with the germane fraction in the gas mixture, and only slightly different values are obtained for the same gas composition depending on the type of process, with a higher Ge content on the films deposited in nondilution processes. To obtain an explanation for the different results obtained from the two types of processes, the deposition process can be analyzed assuming that it occurs through the surface dissociation of disilane and germane following the reactions reported by several authors1,13,16 k Si H 2 6

Si2 H6 共 g兲 ⫹ 共 * 兲 2 ——→ 2SiH* 3 k GeH

关1兴

4

GeH4 共 g兲 ⫹ 共 * 兲 2 ——→ GeH3* ⫹ H*

关2兴

where ( * ) 2 denotes a dual free site in the surface 共i.e., two neighboring sites not bonded to H兲, k Si2H6 and k GeH4 are the adsorption rate constants for the two precursors, and the* superscript indicates that the species is bonded to a surface site. The surface-free sites are generated by the desorption of atomic hydrogen from the surface through the reaction

关3兴

where k H is the desorption rate constant of hydrogen from the SiGe surface. The dissociation rates of Si2 H6 and GeH4 , r Si2H6, and r GeH4, respectively, are given by r Si2 H6 ⫽ k Si2 H6 • p Si2 H6 • ␪ 2 *

关4兴

r GeH4 ⫽ k GeH4 • p GeH4 • ␪ 2 *

关5兴

where ␪ is the fraction of free sites in the surface, and p Si2H6 and * p GeH4 are the partial pressures of the precursors. The desorption rate of hydrogen from the surface will equate, in a steady state, the hydrogen generation rate as a consequence of the dissociation of the hydrides 2 r H ⫽ 6r Si2 H6 ⫹ 4r GeH4 ⫽ k H␪ H ⫽ k H共 1 ⫺ ␪ 兲 2 *

关6兴

where ␪ H is the fraction of surface sites occupied by the hydrogen. The values of r Si2H6 and r GeH4, as well as the film growth rate and composition, can be obtained from Eq. 4 to 6 if the adsorption and desorption rate constants k Si2H6, k GeH4, and k H were known for the particular deposition conditions. These constants are dependent not only on the temperature but also on the composition of the SiGe surface, as sites over Si or Ge atoms are energetically different. No clear model exists for this type of dependence, although different proposals, mostly empirical, have been made.3,17 All of them consider that an increase on the number of the Ge sites reduces the adsorption rate of precursors. If the dependence of the adsorption rate constants on x is assumed to be the same for the two precursors, as in the aforementioned models, the atomic ratio in the film, which can be obtained from Eq. 4 and 5 as twice the ratio between the dissociation rates of GeH4 and Si2 H6 , should be proportional to the germane-to-disilane ratio in the gas. The proportionality constant will be the ratio between the adsorption rate constants. Although the experimental results shown in Fig. 2 approximately follow this behavior, a deviation exists from the exact linear dependence in two ways: 共i兲 the films deposited at higher GeH4 /Si2 H6 ratio have a


Figure 3. Ratio of dissociation rate to partial pressure 共left兲 of disilane 共〫⽧兲 and germane 共䊐䊏兲, and hydrogen desorption rate 共䊊䊉 right兲, vs. the Ge fraction 共x兲 on the film. Open symbols: H2 dilution process; closed symbols: nondilution process. Values obtained from experimental data in Fig. 1 and 2.

correspondingly lower Ge content, and 共ii兲 films deposited at higher partial pressures are always Ge richer for the same gas composition. This implies that 共i兲 the dependence on the film composition of the two adsorption rate constants is not the same and 共ii兲 each constant also depends on the precursor partial pressure. Several researchers have reported a similar behavior, such as Kim et al.13 for depositions made from GeH4 and Si2 H6 and King and Saraswat8 for depositions made from GeH4 and SiH4 , and some factors were accounted for these nonlinearity results, such as surface reaction and gas-phase kinetics acting as deposition-limiting processes. The initial decrease of the growth rate with the germane-todisilane ratio on the gas, observed in nondilution processes, may be due to the decrease in the disilane partial pressure 共which does not occur in the dilution case兲 or to changes in the other terms of Eq. 4 and 5. To clarify this point, Fig. 3 presents the ratio of dissociation rate 共r兲 to partial pressure 共p兲 for each hydride (r/p curves兲 and the hydrogen desorption rate (r H) as a function of the film Ge fraction 共x兲 calculated from experimental data. These curves indicate that the initial decrease on the growth rate is not due only to the reduction in the disilane partial pressure. The decrease in the adsorption rate constant of both precursors as x increases also causes this decrease in the growth rate for low x values. This reduction in the adsorption rate constant is compensated, in the case of dilution depositions, by a sharp rise of the free sites fraction as x increases, resulting in a continuous increase of the growth rate. For each deposition run, the k GeH4 to k SiH4 ratio can be directly obtained as k GeH4 k Si2 H6

⫽

r GeH4 /p GeH4 r Si2 H6 /p Si2 H6

关7兴

since the fraction of free sites has a unique value in each run. The ratio between the adsorption rate constants decreases from 1.25 to 0.85 as x increases. This suggests that the models for the dependence of these parameters on x should have a different expression for each one. Besides that, this ratio presents a different behavior with x when processes are carried out with and without dilution. This implies again that the adsorption rate constants depend on other deposition parameters besides film composition, such as the precursor partial pressure. The values for the k GeH4 to k Si2H6 ratio obtained in

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our depositions are considerably lower than the values of around 3.1 reported by Holleman et al.3 and Robbins et al.17 for the k GeH4 to k SiH4 ratio, which indicates that disilane has a higher reactivity than silane, similar to that of germane. This similarity is an interesting feature that will produce a more uniform consumption of the species in the gas than in the case of deposition from silane and germane. It will cause a less significant change in the composition of the film, in the case that gas depletion along the reactor tube may occur. The hydrogen desorption rate constant k H is also dependent on the surface composition, as the hydrogen desorption from a site over the Ge atom is energetically more favorable than from a site over Si. Different models have also been proposed for the dependence of k H on x,3,17,18 based on their own experimental data or on those reported by Sinniah19 for the desorption of hydrogen from a crystalline Si共100兲 surface. None of these models can be applied to our case as they give, for a 450°C deposition temperature, k H values of between one and three orders of magnitude lower than our r H experimental results, which should imply, according to Eq. 6, negative values of the free site fraction ␪ . Possible causes for this discrep* ancy may be the consideration usually made3,17 of Reaction 3 as a second-order reaction, while it is proposed in Ref. 19 as being a first-order reaction, or the difference in the surface density of surface sites between amorphous SiGe and crystalline Si共100兲. Regardless of the exact dependence of k H on x, the comparison between r H values for depositions that yield similar x values indicates, considering Eq. 6, that processes made with dilution have free site fractions higher than those of the nondilution process. The same difference can be observed when comparing the ␪ values obtained from * the r/p curves using Eq. 4 and 5: dilution processes have higher values than nondilution processes. However, while the difference in the free site fraction between the dilution and nondilution processes decreases as x increases, when obtained from the r H curves, to almost 0 for x ⬎ 0.3, the difference does not reduce as x increases if the free site fractions are obtained from the r/p curves. The big difference in the r/p values in x ⫽ 0.3 共both for disilane and germane兲 cannot be explained by the almost negligible difference on the free site fractions 共obtained from the r H curves兲, but by a dependence of k GeH4 and k SiH4 on the respective precursor partial pressure 共decreasing with increasing pressure兲. This means that the dissociation rate follows a nonlinear law with the partial pressure, presenting a saturation effect for higher pressures. Figure 4 shows the effect of the temperature on the deposition rate of SiGe films deposited at 300 mTorr, without a carrier gas and with flow rates of 5 sccm of germane and 5 sccm of disilane. Between temperatures of 450 and 500°C the growth rate increases exponentially and can be fitted to an Arrhenius plot, giving a value for the activation energy of 0.94 eV. This is a similar value to that reported by some researchers for the SiGe deposition, of similar composition, from germane and disilane13 or germane and silane2,3,8 mixtures. This result indicates that the mechanism limiting the deposition rate is the same in both cases, i.e., the hydrogen desorption from the surface. For depositions made at temperatures higher than 500°C, a depletion of the gas within the reactor occurs which produces a decreasing growth rate along the tube. The difference in the layer thickness between two wafers 6 cm apart can be as high as 40% for the 600°C deposition. The values plotted in Fig. 4, which correspond to samples in the center of the tube, are considerably lower than those which could be obtained in nondepletion conditions. The films deposited at temperatures higher than 500°C presents a Ge fraction of x ⫽ 0.40, slightly higher than that of films deposited at lower temperatures (x ⫽ 0.35). This change in the composition of the film is due to the depletion of the gas. This was also observed in depositions from germane and silane,3 but its magnitude is lower and the sign contrary to those reported in Ref. 3, as a consequence of the relative values of the adsorption rate constants of germane, silane, and disilane. The crystallinity of SiGe LPCVD grown films is dependent on

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Journal of The Electrochemical Society, 148 共10兲 C685-C689 共2001兲 material exists in the film. This amorphous fraction disappears in films deposited at 600°C, which have a 共220兲 preferential orientation. The XRD diffractograms of films deposited at 450°C with different Ge fractions indicate that the material is amorphous for x lower than 0.5, changing to a 共220兲 preferentially oriented polycrystalline structure for x above that value. Although the XRD does not detect any difference in the crystallinity of material deposited at 450°C in the x ⫽ 0 to x ⫽ 0.5 range, a certain difference should exist between samples of composition above and below x ⫽ 0.3 as a different behavior in the solid-phase crystallization of these samples has been observed.10 The higher crystallization rate and the smaller in-plane grain size observed for x ⬎ 0.3 suggest the existence of a certain crystalline order in localized sites in the volume of these samples, which do not exist for x ⬍ 0.3, acting as nucleation centers.

Conclusions

Figure 4. Growth rate of SiGe films as a function of the deposition temperature.

The use of disilane and germane as source gases allows for the deposition by LPCVD of amorphous silicon-germanium alloys of controlled composition at low temperatures with a relatively high growth rate. In the range of pressures and temperatures investigated, the process can be analyzed assuming that the reactions that control the deposition are the surface dissociation of the precursors, limited by the existence of free sites in the surface, similar to the deposition from silane and germane. The adsorption rate constants of disilane and germane have similar values, with a ratio around one. This produces a more uniform consumption of the species in the gas than in the case of deposition from silane and germane. Both adsorption rate constants decrease as the Ge content on the surface increases, but also depend on each precursor partial pressure. This means that the dissociation rate follows a nonlinear law with the partial pressure, presenting a saturation effect for higher pressures. The published models for the hydrogen desorption give nonconsistent results when the reported values of the parameters are applied to this case. The crystallinity of the films is dependent on the composition and the deposition temperature. Depositions at temperatures lower than 500°C and a Ge fraction lower than 0.5 produce amorphous material, while films grown at temperatures or compositions higher than these values are polycrystalline. The 500°C temperature also sets an upper limit to depositions with no depletion of the gas, for our LPCVD system.

Acknowledgment This work was supported by CICYT Project MAT 99-1214. Universidad Politećnica de Madrid assisted in meeting the publication costs of this article.

References

Figure 5. X-ray diffractograms of SiGe films with x ⬇ 0.35 deposited at various temperatures.

the composition and the deposition temperature. Figure 5 shows the XRD diffractograms of samples deposited at several temperatures with a composition of around x ⫽ 0.35. For temperatures lower than 500°C the diffractograms show only the characteristic halo of the amorphous material, while the films deposited at higher temperatures present peak characteristics of the diffraction from the 共111兲, 共220兲, and 共311兲 planes. The films deposited at 550°C have a 共311兲 slightly preferential orientation and their diffraction curves still have the amorphous halo, which indicates that a fraction of amorphous

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