The growth and characterization of Si1 − xGex multiple quantum wells on Si(110) and Si(111)

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

ELSEVIER

Journal of Crystal Growth 157 (1995) 21-26

The growth and characterization of Si l_xGex multiple quantum wells on Si(110) and Si(111) P.E. T h o m p s o n a, *, T . L . K r e i f e l s b M. G r e g g b, R . L . H e n g e h o l d b, Y.K. Y e o b D.S S i m o n s c, M . E . T w i g g a, M. F a t e m i a, K. H o b a r t d Naval Research Laboratory, Washington, DC 20375-5347, USA b Air Force Institute of Technology, Wright Patterson Air Force Base, Ohio 45433, USA National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA d SFA, 1401 McCormick Drive, Landover, Maryland 20785, USA

Abstract Undoped and boron-doped multiple quantum well heterostructures composed of Sil_xGe x have been grown on Si(110) and S i ( l l l ) substrates. Photoluminescence (PL), secondary ion mass spectrometry (SIMS), transmission electron microscopy, and X-ray diffraction have been used to characterize the structures. The structures with the highest quality, as defined by quantum confined PL, were grown at 710°C, however SIMS revealed that substantial boron migration out of the wells occurred at this growth temperature.

1. Introduction There is current interest in investigating Si-based materials for infrared detector applications. Extensive research has already been carried out on the application of p-doped Si~_xGe~ multiple quantum wells (MQW) grown on Si(100) [1-7]. Through the use of a waveguide structure, the absorption of incident light, both normal and parallel to the surface, was measured. However, in the case of Si(100), the absorption coefficient for normal incidence is small. We have been investigating the growth of MQWs on Si(110) and Si(111) in order to take advantage of the off-diagonal components of the effective mass tensors which should lead to stronger absorption of

* Corresponding author. Fax: +1 202 404 7194; E-mail: [email protected].

normal incident IR light [8]. In this paper we report on our investigation of the growth of MQWs on Si(110) and S i ( l l l ) using solid source molecular beam epitaxy (MBE). There have been studies on the growth of these structures by other groups [9-12]. We have extended this knowledge base to include growth temperatures from 550 to 800°C and boron doping in the quantum wells.

2. Experimental procedure The SiGe MQWs were grown on 76 mm Si(110) (20-70 f l - c m , p-type (boron)) and S i ( l l l ) (5-20 ~ - c m , p-type (boron)) wafers using MBE. Details of the growth system have been reported earlier [13]. The Si and Ge molecular beams were obtained from elemental sources in electron beam evaporators. Temperature during growth was monitored and con-

0022-0248/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD1 0 0 2 2 - 0 2 4 8 ( 9 5 ) 0 0 3 6 8 - 1

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P.E. Thompson et al. /Journal of Crystal Growth 157 (1995) 21-26

trolled with an optical pyrometer, which was calibrated with respect to eutectic transitions of Au on Si (363°C) and A1 on Si (577°C) by means of a thermocouple embedded in the heater assembly. Throughout this work the MQW structures fabricated were five to twenty QWs composed of Sil_xGe x, x ranging from 0.2 to 0.5. The wells were 3 to 4 nm wide, separated by 30 nm of Si. Boron doping was accomplished using elemental boron in a specially designed high temperature Knudsen cell. Primary growth parameters explored were substrate temperature and doping concentration. A Si buffer layer of thickness > 50 nm was grown on each sample at either the growth temperature specified for the MQWs or at 650°C, whichever was higher. We have employed low energy electron diffraction (LEED), photoluminescence (PL), secondary ion mass spectrometry (SIMS), transmission electron microscopy (TEM), and X-ray diffraction (XRD) in the characterization of the heterostructures. The PL was performed with the sample at pumped liquid He temperature (1.6 K) using either the 647 nm line of a Kr ÷ laser or the 514 or 488 nm line of an Ar + laser. The SIMS was performed with a high performance magnetic sector secondary ion mass spectrometer. The primary ion beam species was O~', with an impact energy of 3 keV, which should result in minimal ion mixing. The primary beam current was 300 nA in a 50 /zm spot which was rastered over a 250 × 250 /zm area. The secondary ions were monitored from a 60 /zm circle centered in the raster square. Depth scales were obtained from stylus profilometry (_+ 3% accuracy). X-ray rocking curves were obtained on a doublecrystal diffractometer, using CuKc~l radiation and the 004 reflection from a Si(100) as the beam conditioner.

3. R e s u l t s

3.1. Undoped MQWs on Si(llO) and S i ( l l l ) Using band-edge, quantum confined PL as the signature of high quality material, we initiated our investigation with the goal of determining the growth parameters for Si(110) and Si(l 1 l) that would result in this property. It has been shown that the critical growth parameter to obtain band-edge, quantum con-

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Energy (eV) Fig. 1. PL of MQWs composed of 5 wells, Si0.sGe0. 2 grown at 600, 710, and 800°C on Si(ll0). The wells grown at 600 and 800°C were 4 nm wide. The wells grown at 710°C were 3 nm wide.

fined PL from MQWs grown on Si(ll0) is the temperature during growth [14]. We grew MQWs composed of five wells, x = 0.2, at 600, 710, and 800°C and obtained the 1.6 K PL. The wells grown at 600 and 800°C were 4 nm wide and those at 710°C were 3 nm. The PL results are presented in Figs. 1 and 2. Qualitatively, the spectra are very similar. For each substrate orientation, the growth at 600°C results in the broad PL, at ~ 0.94 eV, which has been observed in low temperature MQW growths on Si(100) [15]. The growths at 710 and 800°C all result in phonon resolved quantum confined PL of the no-phonon bound exciton emission and its phonon replica. For both Si(110) and Si(111), the narrowest emission lines are obtained with the growth at 710°C, implying that the crystalline quality is optimum at that growth temperature. The shift of the emission lines of ~ 30 meV between the samples grown at 710 and 800°C is consistent with the increased width of the well hut could also be caused by a variation in

P.E. Thompson et al. / Journal of Crystal Growth 157 (1995) 21-26 Wavelength (lam)

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23

from 550 to 800°C all revealed narrow, well-defined wells, the interfaces of the wells grown at 710°C appeared to be sharpest. The SIMS profiles of Ge for 3 nm, x = 0.2, MQWs grown at 550°C on Si(111) and at 800°C on Si(110) and Si(111) reveal substantial differences in the Ge distribution due to substrate orientation and growth temperature. The derivatives of the Ge concentration profiles with respect to depth, normalized by the Ge concentration in the well, are presented in Fig. 3. The dominant feature of this figure is that the derivative (slope) of the Ge wells grown on S i ( l l l )

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the growth rate calibration, since the pairs of samples were grown almost a year apart. Comparing the widths of the no-phonon (NP) lines at a given temperature for the two orientations, we observe that the NP line is narrower for the Si(110) orientation compared to the S i ( l l l ) . For the 710°C growths the widths of the NP lines are 5 meV versus 11 meV and for the 800°C growth the widths are 9 meV versus 22 meV, implying that the quality of the crystalline structure is better on the Si(110) oriented substrates. The characterization results by LEED and TEM are in agreement with the PL, in that the growth at 710°C or greater results in the best heterostructures. The LEED pattern is much sharper with less background on the samples grown at high temperatures, where we observe a 2 × n reconstruction on the Si(ll0) surface and a 7 X 7 reconstruction on the S i ( l l l ) surface. While the cross-sectional TEM of the Si(110) undoped MQWs grown at temperatures

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Fig. 3. The derivative of the Ge concentration profiles obtained with SIMS, normalized with respect to the Ge concentration in the wells.

24

P.E. Thompson et al./ Journal of Crystal Growth 157 (1995) 21-26

1022

at 800°C are substantially less than the wells grown on S i ( l l l ) at 550°C or on Si(ll0) at 800°C. The spreading of the Ge wells is consistent with the 22 meV width of NP line observed by PL on this sample. It is seen that the structure with the best defined wells, as determined by the portion of the slope which is near zero, is the sample grown on Si(110) at 800°C. On this sample the magnitude of the derivative of the Ge concentration is greater in the positive direction than the negative (5.2 × 10 -2 versus 3.8 × 10 -2 nm-~), implying preferential diffusion back into the sample. If this was due to knock-on effects during SIMS profiling, a similar asymmetry would be observed in the Si(111) sample grown at 550°C. The sharpness of the Ge profiles with the diffusion of the Ge is consistent with the 9 meV width of the NP line measured on this sample.

3.2. Boron-doped MQWs on Si(110) and Si(lll)

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Since most device applications, optical or electrical, require doping of the heterostructure, the remainder of this paper will discuss boron doping of the Si]_~Ge~ MQWs. The characterization of undoped MQWs clearly demonstrate that the best structural quality MQWs were grown at 710°C. Is this the best temperature to grow a boron-doped MQW? MQWs, composed of 4 nm Sio.sGe0. 2 wells with the center 3 nm doped with 1 × 10 ]9 B//cm 3 and 30 nm Si spacers, were fabricated on Si(110) and Si(111) at a growth temperature of 710°C. The NP and TOsi_s i phonon replica were observed on both doped sampies, but the intensities of the emission peaks have decreased and the width have increased, compared to those of undoped MQW samples. Doped MQWs grown at 550°C do not exhibit any PL emission except the substrate related peaks. This may imply that 710°C is indeed the ideal growth temperature for the doped MQW, but SIMS characterization of these structures leads to a different conclusion. SIMS profiles of boron-doped MQW structures are presented in Fig. 4. The structure grown at 550°C has 10 wells, 4 nm Si0.sGe0. 2 with the center 3 nm doped with 5 × 10 ]9 B//cm 3. The structures grown at 710°C have 5 wells, 4 nm Si0.sGe0. 2 with the center 3 nm doped with 1 × I019 B / c m 3. On both the Si(110) and Si(111) substrates there is substantial segregation of the boron when the growth temperature is

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elevated to 710°C. In the region from the last well grown to the surface, the boron profiles show an exponential decay in the growth direction n(z) = n0 e x p ( - z / A )

(1)

in the same manner of the boron delta-doped layers investigated by Jorke and Kibbel [16]. The measured values of A for the two structures grown at 550°C were 9 nm for Si(110) and 3 nm for the S i ( l l l ) (not shown). There are two distinct regimes for the structures grown at 710°C. There is an initial fast decrease, lasting less than a decade of concentration and an extended slow decay. The fast mode has A

P.E. Thompson et al. /Journal of Crystal Growth 157 (1995) 21-26

values of 5 nm for Si(110) and 9 nm for Si(111). The A values for the slow decay are 54 nm for Si(ll0) and 58 nm for S i ( l l l ) . Jorke and Kibbel [16] reported values of A of 4.t ___0.3 nm at 550°C and 29.0 + 6 nm at 700°C for Si(100). The existence of the fast mode, which is in or near the Ge well, is consistent with the reported, reduced A values for B in Si0.sGe0. 2 [16]. By taking the derivative of the boron profiles with respect to depth, normalized by the anticipated boron concentration, a measure of the abruptness within wells can be obtained. For the 550°C growths, the a v e r a g e m a x i m u m (positive/negative) slopes are (8 X 1 0 - 2 / 6 . 4 X 10 -2 nm -1) for Si(ll0) and ( 7 . 2 × 1 0 - 2 / 6 . 0 X 10 -2 n m - 1 ) for Si(111). For the 710°C growths, the average maximum (positive/negative) slopes are (7 X 1 0 - 2 / 5 X 1 0 -2 nm 1) for Si(ll0) and (5X 10 3 / 5 x 10 _3 nm -1) for S i ( l l l ) . It is seen that the boron doping profiles are more abrupt within the MQWs for the 550°C growths, especially for the case of Si(111), than the structures grown at 710°C. The structural properties of boron-doped MQWs grown on Si(110) at 550°C have been investigated with cross-sectional TEM and XRD. The sample investigated with TEM had 15 wells of 4 nm Si0.8Ge0. 2 with the center 3 nm doped at 2 x 1019 B / c m 3. There were 30 nm Si spacers between the wells. It was observed that the individual wells had a rougher interface ( ~ 1 nm) than observed in the undoped MQWs and there appeared to be a graininess in the distribution of the Ge. Since this was the only doped MQW structure investigated, no general statement can be made at this time. Undoped MQWs grown on Si(110) at temperatures from 550 to 800°C all had smooth interfaces and the Ge concentration appeared to be uniform. The Si(110) sample investigated with XRD had 20 wells of 2 nm Si0.7Ge0. 3 doped at 1 X 1019 B / c m 3. The results were compared to a similar structure (3 nm wells) grown on Si(100). Multiple satellite reflection peaks were measured indicating that the layers were crystalline. The full width half maximum (fwhm) for the substrate peak was 21 arcsec compared to a theoretical value of 6.4 arcsec. Similarly, most satellite peaks had fwhm between 31 and 34 arcsec, compared to the theoretical values of 20-24 arcsec. In comparison, the Si(100) structure had a fwhm for the substrate of 5,1 arcsec and 24.1 arcsec for the first satellite,

25

compared to the theoretical values of 5.1 and 23.7 arcsec, respectively. The lower resolution of X-ray results for the Si(ll0) sample is due to several factors, including the quality of the Si(ll0) substrates and the lower angle of diffraction (23 °) using the 220 reflection compared to that for the 004 reflection (34.5 °) used for the Si(100) substrate.

4. Summary High quality epitaxial growth has been achieved on both Si(110) and Si(111) substrates, as indicated by the quantum confined, band-edge PL lines. It was observed that these lines are sharpest for the growth temperature of 710°C. If it is desirable to dope the quantum wells with boron, then it is necessary to reduce the growth temperature, in order to minimize segregation or diffusion.

Acknowledgements The authors wish to recognize the support of AFSOR and ONR for this work and the efforts of L. Ardis in TEM sample preparation and MBE assistance.

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