In situ control and monitoring of doped and compositionally graded SiGe films using spectroscopic ellipsometry and second harmonic generation

Applied Surface Science 154–155 Ž2000. 229–237 www.elsevier.nlrlocaterapsusc

In situ control and monitoring of doped and compositionally graded SiGe films using spectroscopic ellipsometry and second harmonic generation L. Mantese

a,)

, K. Selinidis b, P.T. Wilson a , D. Lim a , Y.Y. Jiang a , J.G. Ekerdt b, M.C. Downer a

a

b

Department of Physics, UniÕersity of Texas, Mail Code C1600, Austin, TX 78712, USA Department of Chemical Engineering, UniÕersity of Texas, Mail Code C0400, Austin, TX 78712, USA Received 1 June 1999; accepted 18 July 1999

Abstract We have implemented linear and nonlinear optical spectroscopies to monitor and control the growth of Si xGe1yx films. Using spectroscopic ellipsometry and the virtual substrate approximation ŽVSA., controlled growth of compositionally graded SiGe films deposited by chemical vapor deposition ŽCVD. is achieved by adjustment of disilane flow based on feedback from ellipsometric inputs. Stepped and linear growth profiles are investigated. Using spectroscopic, surface second harmonic generation ŽSHG. by a tunable, unamplified Ti:sapphire 100 femtosecond Žfs. laser, shifts of the SH spectral feature near the Si E 1 critical point with varying Ge composition are observed. A comparison is made to linear spectroscopy and related qualitatively to surface composition. Data acquisition time is then reduced to a few seconds by substituting a 10 fs laser and spectrally dispersing generated SH radiation onto an array detector, thus enabling real-time spectroscopic SHG. The reflected SH spectrum near the E 1 region is also highly sensitive to bulk boron doping of SiGe. Definite trends in the peak positions and amplitudes as a function of boron incorporation are observed and interpreted qualitatively in terms of dc-electric-field-induced SHG in the depletion region. The results demonstrate the feasibility of SE and spectroscopic SHG operating as complementary, in situ sensors of SiGe CVD. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Surface second harmonic generation; Spectroscopic ellipsometry

1. Introduction The SiGe materials system has become of interest for novel device applications where characteristics such as lattice mismatch and strain within the alloy suggest opportunities for bandgap engineering w1x. In ) Corresponding author. Tel.: q1-512-471-7251; fax: q1-512471-9637. E-mail address: [email protected] ŽL. Mantese..

addition, p-type Si xGe1yx quantum well structures have been investigated as infrared ŽIR. photodetectors operating in the 8–13 mm range w2x and as bipolar transistors containing p-SiGe base layers w3x. For such devices to be realized, however, control over Ge composition and p-type doping Žmost commonly B. must be achieved. Recipe-driven growth fails under spurious growth conditions and requires frequent calibration runs. However, by maintaining a constant update of the composition and dopant levels

0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 3 8 6 - 4

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during growth, routine calibrations become obsolete and composition correction during film growth is possible. Because optical techniques are nondestructive and can be used in any growth environment, they have emerged as probes capable of controlling and monitoring growth during film deposition. Examples include ellipsometry where growth control of compositionally graded Al xGa 1yx As was first demonstrated by Aspnes et al. w4x using this technique. Pickering et al. have extensively analyzed the effects of composition and strain variation in the Si xGe1yx materials system on spectroscopic ellipsometry ŽSE. both under static w5x and monitored w6x growth conditions. Optical probes, however, that are sensitive to in situ monitoring of SiŽ001. doping levels are not readily available. Probes capable of monitoring the chemical vapor deposition ŽCVD. of dopants during growth would be particularly attractive for the B:SiGer SiŽ001. complex where effects such as Ge surface segregation complicate the growth process w7x. More recently, with the availability of commercial, femtosecond Žfs. laser systems in situ, real-time monitoring of the optical second harmonic ŽSH. responses of SiŽ001. and SiGeŽ001. growth surfaces has become possible w8x. Spectroscopic SHG has demonstrated high absorbate-specific sensitivity to submonolayer ŽML. coverages of H and Ge w9,10x and in situ, single-wavelength SHG has accurately monitored CVD growth kinetics at SiŽ001. in real-time w11x. The prospect of using this technique for in situ dopant monitoring is suggested by the high sensitivity of SHG to subsurface dc-electric fields demonstrated in MOS structures w12x and interconnects w13x and is qualitatively investigated here. In this paper, we demonstrate the parallel use of SE and surface SH to analyze compositionally graded and B-doped SiGe films. We use SE and the virtual substrate approximation ŽVSA. developed by Aspnes w14x to determine outerlayer compositions during ultrahigh vacuum ŽUHV. CVD. Feedback control is achieved by comparing the Ge composition of the outerdeposited layer determined from ellipsometric inputs to the setpoint value. The flow of disilane is then automatically adjusted to reach the Ge setpoint composition. Stepped and linear growth profiles are analyzed. The SHG responses of SiGe samples of varying Ge composition are then presented and com-

pared to reported SE responses w15x from similar surfaces. Shifts in the spectral feature near the E 1 critical point of Si with increasing Ge composition are observed in both spectroscopies. The SHG acquisition time is then reduced to seconds by using a broadband 10 fs laser where data from this source are compared to that taken from the tuned Ti:sapphire output. The SH response near the E 1 region of Si is also highly sensitive to varying concentrations of bulk B doping of SiGe. A qualitative overview of the basic phenomenology and sensitivity range is presented with a quantitative analysis given by Lim et al. w16x. As SE and SHG diagnose complementary features of the SiGe film our results motivate the development of a fused optical instrument that incorporates both sensors.

2. Spectroscopic ellipsometry 2.1. Experimental set-up SiGe films were grown in a custom built UHV CVD chamber w17x with disilane ŽSi 2 H 6 . and He-diluted 20% germane ŽGeH 4 . as gas sources. The samples were heated by five quartz halogen lamps located at the backside of the wafer with the sample freely resting on three pins and facing downward in the reactor. Prior to transfer into the deposition chamber, the SiŽ001. wafers were prepared by an initial, modified RCA clean with a final HF dip Ž10:1, H 2 O:HF. leaving the surfaces H-terminated. The chamber is equipped with two fused quartz strain-free BOMCO windows w18x with optical access to the sample at approximately an angle of incidence of 798. During ellipsometric alignment, 20 sccm of disilane was continuously flowed. Spectroscopic C , D data were taken with a commercial Verity Instruments ellipsometer similar to the polarizer-modulator-sample-analyzer ŽPMSA. type described by Duncan et al. w19x The main components are a Xe arc lamp source in a lamp house with collimating lenses and a fiber optic input. The polarizer and analyzer are of calcite and the photoelastic modulator replaces rotating elements. The detector assembly is a photodiode array ŽPDA. consisting of 76 photodiodes. No fiber optic is used at the output end of the ellipsometer. The polarizer, analyzer, and

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ethernet line to a second computer where in-house C q q code was written w20x to run VSA analysis of the data to determine outlayer compositions with LabView w21x as the software user interface. A proportional-integral-derivative ŽPID. controller Žsee, for example, Ref. w22x., also written into the LabView interface, was used to establish feedback control of the disilane mass-flow controller ŽMFC..

2.2. Calibration

Fig. 1. SIMS data for the SE calibration run. The Ge composition was stepped from 5% to 50%.

modulator are set at q458, y458, and 08, respectively with respect to the plane of incidence. The relative positions are determined and manually adjusted at the beginning of each run. C , D data and angle of incidence are measured using the commercial software Verifilm 3. The data shown here were averaged over five readings from each detector and the growth control data were taken with an acquisition time of approximately 5 seconds per data point. The first control run, however, was taken at twice this acquisition time since we allowed for a second measurement to subtract out the background noise. This did not appear to affect the noise quality of the data, however. C , D data were transferred across an

It was necessary to first establish a database of Ge composition as a function of outerlayer dielectric function ´ 1 q i ´ 2 at the growth temperature. This was achieved by an open loop control run designed to step the Ge composition from 5% to 50% in 5% increments with the disilane flow manually adjusted based on previous chamber calibration measurements. The germane flow was held constant at 50% and the sample temperature held at 5508C. Secondary ion mass spectrometry ŽSIMS. data for this calibration run are shown in Fig. 1 with the corresponding ellipsometric C , D data taken during growth shown in Fig. 2a,b. The VSA algorithm uses growth rate as an input. This was calibrated as well from the SIMS data and determined as a function of germanerdisilane flow rate ratio ranging from ap˚ per second. Fig. 3 shows ´ 2 proximately 1 to 2 A data after applying the c , D data from Fig. 2a and b to the VSA algorithm.

Fig. 2. Ža. C data taken during the calibration growth run with the corresponding SIMS data shown in Fig. 1. Žb. D data for the same growth run as Ža..

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Fig. 3. ´ 2 data using the VSA and the C , D data of Fig. 2.

We were able to establish Ge composition in terms of dielectric function and obtained the analytic expression %Ge s Al ´ 2 Ž l. 3 q Bl ´ 2 Ž l. 2 q Cl ´ 2 Ž l. q Dl where l is wavelength. The coefficients A through D are constants that are a function of wavelength l. To determine composition for feedback control we averaged ´ 2 over the wavelengths shown in Table 1. As a self-consistency check, we verified that we were able to reconstruct the SIMS profile from Fig. 1 using the c , D data we collected during the calibration growth run. 2.3. Controlled growth results and discussion Closed-loop feedback control was achieved by using a PID controller to adjust the flow of disilane based on a comparison of setpoint and ellipsometrically measured compositions. Fig. 4a shows the result of a stepped growth profile at 5508C with Ge composition setpoint values shown as the thick, solid line. The inset is a plot of the valve positions of the mass flow controllers during growth. Ellipsometrically measured composition fluctuates by approximately 2 at.% above 20% with fluctuations as large as 6 at.% occurring for the lower Ge compositions. Because the growth rate is slower at the lower Ge compositions the derivative of ´ with respect to thickness used in the VSA algorithm is enhanced and consequently the sensitivity to noise fluctuations in the c , D inputs is increased. The delay of the

ellipsometrically determined composition in following the sharp discontinuities of the setpoint values, evident in Fig. 4a, results from our choice of PID control constants intended to avoid ringing at the step edges. Our simulations show that with less conservatively chosen values a ten-fold faster step response is possible and should be able to follow such a sharp, stepped profile, as will be investigated in future work. SIMS data for this growth run are shown in Fig. 4b. The compositions determined from SIMS analysis give Ge compositions between 1 and 2 at.% greater than our targeted values. Simulations show that an error of approximately q0.258 in the angle of incidence can generate a change in composition greater by ; 2 at.%. It is not unreasonable that the current mode of determining angle of incidence, a comparison of c , D inputs with library values, could generate errors of "0.18. In addition, although the angle of incidence was determined after reaching growth temperature, the sample freely rests on three pins so that a minor sample movement may introduce a corresponding change in the angle of incidence. Further, because the discrepancy appears to be constant for each setpoint rather than random, it suggests a source such as angle of incidence. Either one or a combination of both effects may introduce sources of error. The discrepancy in final film thickness determined from SIMS Ž; 2.25 mm. and the monitored thickness Ž2 mm. may be due in part to sample temperature. Because of the current heater assembly we are unable to measure directly and feedback on the growth temperature. We speculate that the growth rate determined from the calibration run may have been taken at a lower temperature than the actual growth runs so that the overall thickness is

Table 1 Coefficients of the analytic expression %Ge s Al ´ 2 Ž l. 3 q Bl ´ 2 Ž l. 2 qCl ´ 2 Ž l.q Dl relating ´ 2 to Ge composition Composition correlation %Ges AX 3 q BX 2 qCX q D; X s ´ 2

l Žnm.

A

B

C

D

R2

505 496 487 477

0.0499 0.0416 0.0301 0.0281

y1.246 y1.063 y0.828 y0.7636

13.329 12.119 10.343 9.5483

y9.2075 y9.2807 y9.7923 y10.997

0.9998 0.9998 0.9998 0.9997

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Fig. 4. Ža. Stepped growth profile using feedback control of the disilane valve. The inset corresponds to the disilane and germane valve positions during the growth run. Žb. Corresponding SIMS data showing Ge composition as a function of depth.

underestimated. These results illustrate the necessity of thickness monitoring during growth and will be addressed in future work. Fig. 5 shows the results of a linear growth profile at 5508C using feedback control for a Ge composition range of 15% to 30%. The ellipsometrically measured compositions in Fig. 5a remain slightly

below the setpoint values. This is a result of the PID controller since as the setpoint of the composition continuously increases the controller lags as it adjusts itself with respect to the previous setpoint which is less than that of the current value. For the growth of 2.25 mm thick SiGe films the critical thickness may have been reached and strain

Fig. 5. Ža. Linear growth profile from 15% to 35% Ge composition using feedback control of the disilane valve. The inset corresponds to the disilane and germane valve positions during growth. Žb. Corresponding SIMS data.

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relaxation could have occurred during deposition w23x. In the VSA analysis, however, the dielectric function of the substrate is continually re-evaluated since each set of measurements defines a new threelayer model with all of the undergrowth as a new virtual substrate. The possibility of strain w23x and Ge segregation w7x in the overlayer region at the growth surface, however, introduce more fundamental complications which our present VSA analysis does not yet address. Aspnes estimated that outerlayer surface effects, such as surface reconstructions, on the Al xGa 1yx As system would introduce a potential error of 3.5% in x w14x. The influence of overlayer strain can be taken into account by refining the calibration database to differentiate between the dielectric properties of strained and unstrained overlayers as reported, for example, by Carline et al. w15x The ´ database for a given composition is no longer single-valued, but depends on film thickness and

accumulated strain. Additional modelling would be further required to incorporate Ge segregation into the VSA for the Si xGe1yx complex. Alternatively, both of these subtleties can be addressed in principle by augmenting SE with a surface-specific probe such as spectroscopic SHG which ˚ is sensitive to material properties of the top ; 10 A, the thickness of the VSA overlayer region. The next section describes our initial efforts to develop such an overlayer-specific probe.

3. Surface second harmonic generation (SHG) 3.1. SH responses from Si x Ge1y x r Si(001) The thin-solid lines of Fig. 6a show the SH responses from SiGerSiŽ001. surfaces for samples of varying Ge composition. Each of the spectra was

Fig. 6. Ža. SH responses of SiGe with increasing Ge composition. The thin-solid lineshapes were measured using a tuned laser output and the heavy-dashed line shapes measured using a broadband laser source. Žb. E1 critical point energy as a function of Ge composition as measured by SE w15x compared to the SH spectral peak position.

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normalized against a standard quartz reference and obtained by tuning the output of a Coherent Mira laser system that produces 120 fs unamplified Ti:sapphire laser pulses at a repetition rate of 76 ˚ or less, MHz. The sample thicknesses are 3000 A below the critical layer thickness for strain relaxation w23x. Following a simple Lorentz fit to the spectra, the peak positions are plotted as a function of Ge composition in Fig. 6b as the solid-square lineshape. Extensive SE analysis has also been made of similar surfaces to extract critical point energy shifts as a function of Ge composition w15,24x. The result of such an analysis for strained SiGe layers w15x is shown as the solid-triangular lineshape in Fig. 6b. The two plots show similar trends, namely that the E 1 resonance redshifts with increasing Ge content. The discrepancy between the two curves may have several sources. First, SHG probes only material ˚ of the interface, which may differ in within ; 10 A composition and strain from the deeper bulk region probed by SE. Secondly, the E 1-like SHG peak position differs from the corresponding bulk E 1 critical point for reasons unrelated to either alloy composition or strain w10,25x. For example, E 1-like SHG peaks from pure Si are shifted 0.1 to 0.15 eV from the bulk E 1 energy Ž; 3.4 eV.. The underlying theory of these shifts is still being developed w26,27x. Thirdly, differences in the samples used in the comparison may also have played a role. Unlike those of the SH analysis which were measured after air oxida˚ . for several days, the surfaces from the tion Ž; 15 A SE analysis were grown by low pressure vapor phase expitaxy ŽLPVPE. and quickly measured ex situ with a minimum in native oxide growth w28x. If we compare previous SH data taken under UHV conditions, where the native oxide has been removed w29x, we find that the slope of the energy versus composition plot is effectively increased. These data demonstrate the sensitivity of SH to Ge composition, similar in effect to that of SE analysis. The thin-solid lineshapes of Fig. 6a were taken on the order of several Ž; 10. min, unacceptable for real-time growth applications. To overcome this problem, Wilson et al. w30x have developed a means of acquiring broadband SH spectra in a few seconds by using a ; 10 fs fundamental pulse and spectrally dispersing the generated SH radiation in a grating spectrometer onto an array detector. Real-time

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measurement and analysis thus become feasible. The same SiGe samples as measured by laser tuning were measured with the broadband source from 3.2 to 3.4 eV and yield similar spectra, as shown in Fig. 6a as the heavy-dashed line shapes. Due to complications from using quartz as a reference w31x for a broadband source, the data were normalized against GaAs which has a resonance at 3.1 eV and may generate its own broad background near the lower end of the spectra. This may introduce the discrepancies that are evident in Fig. 6a between the two sets of SH data. Nevertheless, these results demonstrate that as broadband SH spectroscopy is refined and once a database of Ge composition as a function of E 1 peak position has been established, such a highly surface and interface-specific probe could be used not only for growth monitoring but also for feedback control of composition. This may be envisioned as an alternative approach to the more extensive analysis that is required in order to extract surface-composition information from bulk optical probes such as SE. 3.2. SH responses from bulk B doping The optical responses of doped materials have been investigated using the linear spectroscopies, SE and reflectance-difference spectroscopy ŽRDS.. Aspnes et al. have extensively analyzed the effect of doping on the dielectric responses of heavily doped SiŽ001. using SE. The effects, however, were subtle and required fairly extensive modelling w32,33x. Reflectance-difference spectroscopy ŽRDS. has been used to monitor doping levels in GaAs w34x. In particular, the RDS response of doped GaAs shows a linear electro-optic structure near 3 eV that has been used to determine the type and concentration of free carriers w35x. This technique, however, would not be suitable for flat SiŽ001. surfaces. Because RDS measures a difference in reflectance, Drrr, the overall signal of a structurally isotropic material, such as flat SiŽ001. yields a null response. Thus, there is a need for a noncontact, sensitive method of monitoring dopant deposition during growth. Electric-field-induced second-harmonic ŽEFISH. generation shows promise for fulfilling this need. The SH responses of bulk B-doped samples have been investigated using the tuned output of a Coherent Mira laser system. Details of the sample prepara-

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tion are given in w16x. Briefly, Ž2 = 1.SiŽ001. surfaces were prepared in a CVD UHV chamber by resistively heating the samples to 10008C followed by deposition at 6008C of an epilayer of Si with disilane as the precursor. Bulk B-doped samples were then grown at 6008C using diborane and disilane. SH measurements were taken after growth, in situ at room temperature. The open-square lineshape in Fig. 7a shows the SH response from undoped Ž2 = 1.SiŽ001.. The amplitude of the SH signal is quenched with H termination, as shown by the filled squares of Fig. 7a, consistent with previous results for this surface w9x. Upon in situ B doping to ; 5 = 10 18 rcm3, the SH signal increases by about five times and the peak blue shifts toward 3.4 eV, shown by the open circles of Fig. 7a. Again, with H exposure the signal becomes quenched Žfilled circles.. The effect of various doping concentrations on the SH response is shown in Fig. 7b. The SH signal increases monotonically with doping to ; 5 = 10 18 rcm3 and then begins to decrease. These trends are explained qualitatively in terms of EFISH generation w12x in the space charge region. At low doping concentrations Ž- 10 18 rcm3 .

the surface Fermi level remains pinned and the band bending and the bulk electric field both increase with B doping. However, in the region of heavy B the surface Fermi level becomes unpinned in order to achieve equilibrium with the bulk so that band bending and the associated EFISH decrease. This general trend is described in the inset of Fig. 7b. A quantitative discussion of these results will be presented elsewhere w16x.

4. Summary and conclusions The use of linear and nonlinear optical spectroscopies has been investigated as probes to monitor and control the growth of compositionally graded and doped SiGe films. By adjustment of disilane flow based on compositional feedback from ellipsometric inputs we have demonstrated the controlled growth of stepped and linear profiles. There remain, however, several issues to be addressed including the simultaneous monitoring of thickness. This topic has been discussed in the literature w36–38x although the feasibilty of using existing methods in our applica-

Fig. 7. Ža. SH responses of clean Ž2 = 1.SiŽ001. and highly B doped Ž; 5 = 10 18 rcm3 . surfaces. Žb. SH responses as a function of B doping.

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tion is not clear. The SH responses of Si xGe1yxr SiŽ001. have been shown to have a sensitivity to Ge composition similar to that of SE and to be unique in its sensitivity to B doping. As data acquistion times have been reduced to ; 1 s w30x, the use of SH spectroscopy for real-time applications of surface monitoring and control becomes apparent.

Acknowledgements Special thanks to E. Quinones and A. Bala for their assistance with the UHV CVD chamber for the ellipsometric growth control. L.M. would like to thank Professor D.E. Aspnes for a critical reading of the manuscript. This work was supported by an AFOSRrDARPA MURI Grant F49620-95-1-0475, the Robert Welch Foundation ŽGrant 003658-178., the Science and Technology Centers Program of the National Science Foundation ŽGrant CHE8920120., the Texas Advanced Technology Program ŽGrant 003658-178., and a grant from Advanced MicroDevices.

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