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Low temperature epitaxial growth of germanium islands in active regions of silicon interband tunneling diodes
Materials Science and Engineering B89 (2002) 106– 110
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Low temperature epitaxial growth of germanium islands in active regions of silicon interband tunneling diodes M.W. Dashiell *, C. Mu¨ller, N.Y. Jin-Phillipp, U. Denker, O.G. Schmidt, K. Eberl Max-Planck-Institut fu¨r Festko¨rperforschung, Heisenbergstraße 1, D-70569 Stuttgart, Germany
Abstract We incorporate self-assembling Ge islands into Si-based interband tunneling diodes grown by molecular beam epitaxy. The kinetic limitations of low-temperature Ge island formation have been overcome in the growth windows that are required to retain sharp delta-doping profiles in the diode‘s active region. Ge quantum dots are observed for growth temperatures of 360 °C and grown at a rate of 0.125 monolayers per minute. Photoluminescence spectroscopy and annealing experiments indicate three-dimensional carrier localization and phononless radiative recombination, which confirms a dot-like electronic structure. The Ge quantum dots have been incorporated into the active region of delta-doped Si interband tunneling diodes. Room temperature negative-differential-resistance is observed and the electrical characteristics may be tuned by post-growth rapid thermal annealing. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Germanium quantum dots; Self-assembly; Islands; Tunneling diodes; Photoluminescence; Annealing; Negative differential resistance
1. Introduction Stranski-Krastanov growth of Ge islands on Si [1,2] have received great attention over the past decade for self-organized quasi-zero-dimensional structures, formed without sophisticated lithographic techniques. Three-dimensional (3D) carrier localization within the Ge islands is a pathway to obtaining enhanced optical [3 –5] performance compared to conventional 2D Si1 − xGex /Si heterostructures. Although significant research has been published on the theory and fundamental properties of self-organized Ge islands, there are only few results to date which actually exploit Ge quantum dots in an electronic device [6,7]. Recently, closely stacked self-assembled Ge islands were used for Sibased resonant tunneling diodes and negative differential resistance (NDR) was observed at low temperatures [8]. In this contribution we present an approach to incorporate Ge quantum dots (QDs) into Si based resonant interband tunneling diodes(RITD) [7,9]. Currently, the Si/Si1 − x Gex RITD achieves the highest performance of any Si-based tunnel diode, with a record peak-to-valley * Corresponding author.
current ratio (PVCR) of six [10]. Optimization studies demonstrated that RITD performance increases with higher Ge concentrations in the intrinsic spacer, due to Ge’s greater tunneling transmission probability, which results in higher peak current and PVCR. For a given peak current density, a lower capacitance can be accomplished by adding Ge since the tunneling barrier width need not be as great as for pure Si. Low diode capacitance is an important parameter for high speed tunnel diode circuitry [11]. Unfortunately, increasing the Ge concentration above 50% in 2D spacer layers will generate strain induced misfit-dislocations of the two dimensional Si1 − x Gex layer which degrades the performance. Our approach to increase the Ge concentration to nearly 100% during coherent-epitaxialgrowth is to incorporate pure Ge islands into the intrinsic region of the RITD. One obstacle in achieving this goal is that, due to requirements of high-concentration and abrupt doping profiles of the RITD, the active region must be formed at low temperature (360 °C) for good performance. At low temperatures such as these, the tendency for Ge island formation is suppressed and misfit dislocations are more likely to be introduced during growth [7]. Previous attempts to incorporate self-assembled Ge islands into Si-based in-
0921-5107/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 1 ) 0 0 8 1 1 - X
M.W. Dashiell et al. / Materials Science and Engineering B89 (2002) 106–110
terband tunneling diodes [6,7] have not resulted in a device showing room temperature negative differential resistance, possibly a result of the active region being grown at temperatures \ 500 °C. The RITD device structure studied in this work are based upon the device structure and band-alignment proposed in Ref. [7]. The main difference is that the active region, including Ge islands, is formed entirely at 360 °C. Atomic force microscopy (AFM), photoluminescence (PL) spectroscopy, and transmission electron microscopy (TEM) are used to quantify the structural and optical properties of the Ge islands formed at low temperatures. Finally the electrical characteristics and demonstration of room temperature NDR in a Ge-dot interband tunneling diode are presented.
2. Properties of Ge quantum dots grown by molecular beam epitaxy at low temperatures Samples were grown by solid source molecular beam epitaxy (MBE). After de-oxidation of Si substrates, 60 nm Si buffers were grown at 500 °C at a rate of 1 A, s − 1. Prior to Ge deposition, the substrate temperature was ramped during Si growth to values Tg which varied
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from 360 to 500 °C. The Ge was then deposited and subsequently capped with 100 nm Si for PL studies. Reflection high energy electron diffraction (RHEED) indicated a transition from 2D to 3D Ge growth mode, and a recovery of the 2D surface after Si capping for all values of Tg. The Ge layer sequence was then repeated at the surface for AFM analysis. All PL spectra were recorded at 8K under excitation from the 488 nm Ar+ laser line with excitation power of 25 mW and with spot size of approximately 3 mm2 using standard lockin technique and a liquid N2 cooled Ge detector. Fig. 1 shows AFM images of uncapped Ge islands after 5.4 monolayers (ML) nominal Ge deposition for Tg ranging from 360 to 500 °C. For Tg values of 450 and 500 °C the well known Ge hut-clusters [2], which are elongated in the [001] directions and having approximately 20 nm base width are observed in the AFM images. The height of the clusters was determined by line scans to be approximately 1.2 and 0.8 nm for the hut clusters grown at 500 and 450 °C respectively. At temperatures of 400 and 360 °C, the shape of the 3D features drastically changes, and we observe very small three dimensional Ge structures which have not aligned in any discernable crystal directions. AFM line scans indicate the height of these structures to be approximately 0.5 nm with a base width of less than 20 nm.
Fig. 1. 1 × 1mm2 AFM images of uncapped Ge islands after 5.4 ML Ge deposition at 500, 450, 400 and 360 °C.
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Fig. 2. 8K PL spectra of single Ge island layers (5.4 ML Ge deposition) for the four different growth temperatures. The PL signal observed at 1.035eV originates from the Si substrate.
Fig. 2 shows the 8K PL spectra of the single Ge island layers. The Ge hut clusters emit a PL signal centered around 0.8 eV [12] which was recently shown to originate from spatially-indirect, phononless carrier recombination between holes confined in the Ge huts and electrons confined in surrounding tensile-strained Si [13]. For Tg = 360 °C, an emission peak at 765 meV, having a 44 meV full width at half maximum (FWHM) is detected from the single layer. A systematic decrease in PL intensity as Tg is reduced is attributed to point defects generated during low temperature epitaxy. The point defects act as competing non-radiative recombination channels, however we will illustrate in the next section that post-growth annealing can restore some of the PL intensity. Only sparse literature exists [14] that demonstrates quantum-dot like PL from 3D Ge struc-
tures grown at such low temperature, and the previous reports did not unambiguously determine that the luminescence originated from quantum confined energy levels in the dots. A systematic approach must be undertaken to eliminate the possibility that the PL originates from defect bands which lie in similar energy ranges [15,16]. We varied the total deposited Ge thickness, for Tg = 360 °C, from 1.7 ML to over 7.0 ML [17]. The PL spectra revealed a gradual transition from quantum well-like PL emission from flat Ge wetting layers (confirmed by AFM) to quantum dot-like PL emission (the broad PL emission peak observed in Fig. 2). With increasing deposition, the PL emission redshifted due to the increase in size and resultant confinement shifts of the 2D wetting layers and 3D quantum dots. Above 6ML, the PL intensity quenches, presumably due to nucleation of misfit dislocations during growth.
3. Post-growth annealing of the Ge quantum dots Si-based Esaki and interband tunneling diodes grown by MBE require post-growth thermal anneals between 600 and 800 °C to reduce point defect densities associated with low temperature growth (to reduce the undesirable excess current) [9,18]. We investigated the electronic structure and thermal stability of the Ge quantum dots grown at 360 °C after 60 s post growth rapid thermal anneals (RTA) using PL spectroscopy. Fig. 3a (left-hand figure) depicts the post-growth annealing PL spectra (solid lines) and, for reference, the pre-annealing PL spectra (dashed lines). Evident is a systematic blue shift of the Ge related PL band, an
Fig. 3. (a) 8K PL spectra of Ge islands (5.4 ML deposited at 360 °C) after 60 s RTA annealings. The dotted spectra indicate the pre-annealing (as-grown) PL spectra. (b) Summary of the peak PL emission energies (top) and integrated Ge related PL intensity taken from Fig. 3a as a function of RTA annealing temperature.
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QD’s phononless PL intensity [13]. The PL intensity saturates after the transition to quantum-well like PL characteristics. No dislocation related PL bands are observed for any values of TRTA.
4. Ge quantum dots in the intrinsic region of Si based RITDs
Fig. 4. Room-temperature current –voltage characteristics of the Si interband tunnel diode with Ge dots embedded in the active region for several RTA annealing temperatures.
increase of the Ge related PL intensity relative to the pre-annealing intensity, and a gradual evolution of the single broad quantum dot-like luminescence into two well resolved PL peaks. Fig. 3b summarizes the annealing experiments of the Ge quantum dots grown at 360 °C. The top half of Fig. 3b shows the peak PL energies observed after RTA annealing. Above 685 °C, two peaks separated by 55 meV (the energy of the momentum conserving transverse optical phonon in Si/Si1 − x Gex /Si quantum wells) continue to blueshift due to Si– Ge interdiffusion, and are attributed to quantum well-like luminesence resulting from recombination in the intermixed wetting layer. A transisition from quantum dot-like PL (single broad emission) to quantum well-like PL occurs when sufficient material intermixing smears out the 3D confinement potentials of the QD. The gradual material-interdiffusion driven blueshifts and transition, as well as the absence of dislocation related PL lines, indicates the single broad Ge-quantum dot PL originates from phononless radiative recombination within 3D confined energy levels [13,17]. The appearance of the phonon assisting replica of the quantum well-like PL at higher TRTA values indicate that excitons are no longer strongly localized around the Ge quantum dot. We note that a broad luminescence peak is observed from islands grown at 700 °C [19], however those islands are much greater in size than the dots decribed here, and thus are probably less sensitive to slight intermixing across their interfaces. The bottom half of Fig. 3b depicts the integrated Ge-related PL intensity, normalized to its pre-annealing value as a function of annealing temperature. Initially we observe an increase in intensity, which is attributed to reduction of point defects associated with low-temperature growth [20]. At higher temperatures, a decrease in intensity is observed which is attributed to the effect of intermixing on the 3D localization and the
The Ge quantum dot RITD was fabricated by MBE on p+ substrates. The doping profile of the highly doped Si layers were chosen from the optimized RITD structure of Ref. [10]. Dopants B and P were thermally evaporated from effusion cells for p- and n-type doping. The intrinsic spacer was grown at 360 °C consisting of undoped Si (i-Si) and the Ge quantum dots (Ge deposition rate=0.125 ML min − 1). The investigated structure consists of the following layers, starting from the p+ Si substrate: a 50 nm Si p+ (3×1019 cm − 3) buffer layer, followed by a p-type delta-doping layer (5× 1013 cm − 2), a 1 nm i-Si/5.4 ML-Ge/3 nm i-Si intrinsic spacer, a n-type delta doping layer (1×1014 cm − 2) and a 100 nm Si n+ (5×1019 cm − 3) cap. A simplified cross-sectional schematic and band-alignment for the embedded Ge quantum dot RITD can be found in Ref. [7]. Diodes were fabricated using standard photolithography, Ti–Au metallization, and HNO3:HF (50:1) etching to form mesa diodes of approximately 40–50 mm diameter. Fig. 4 displays room temperature current–voltage characteristics of the Ge quantum dot RITD. Negative differential resistance was observed for samples after 60 s post-growth RTA annealing between 700 and 820 °C, where the anneal temperature, TRTA, is indicated in the figure. As for the case of the planar Si/Si1 − x Gex /Si RITDs, the current density decreases with TRTA and an optimal temperature is required to maximize PVCR (A modest 1.035 in this non-optimized Ge-dot RITD structure for TRTA = 775 °C). NDR was observed with peak current densities varying from approximately 50– 500 A cm − 2. To our knowledge, this is the first demonstration of room temperature NDR from a Si based tunnel diode with embedded quantum dots in the active region. Fig. 5 displays TEM images of the active region of the as-grown Ge-QD RITD cross-section (Fig. 5a) and the cross-section after a 60 s RTA anneal at TRTA = 700 °C (Fig. 5b). Very small Ge dots are observed with diameters between 10 and 20 nm, the height of the capped dots (estimated from the contrast of the TEM image) is approximately 1–3 nm. The difference in QD height determined from AFM (uncapped samples) and that determined from the TEM measurements (dots embedded in a Si matrix) could be due to intermixing during growth or the effect of strain fields propagating above the dot into the Si matrix, as well as the fact that
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growth processing (including the high temperature anneal). Room temperature negative-differentialresistance was observed in the Ge-dot RITD and TEM measurements reveal that the Ge QD retains its structure after the required post-growth annealing procedure. No evidence of dislocation generation was observed during high-temperature processing required for RITD fabrication. Careful layer structure optimization, however, is necessary to improve device performance for future applications.
Acknowledgements The author acknowledges financial support from a Max-Planck-Gesellschaft Stipendium.
References
Fig. 5. TEM image of the active region of the Si interband tunnel diode with Ge dots embedded in the spacer region before (top) and after (bottom) RTA annealing at 700 °C.
the AFM dot height does not include the Ge WL. Annealing the samples at 700 °C does not observably alter the quantum dot-structure shown in the TEM image compared to the as-grown sample. This appears to contradict the PL findings-that suggest annealing at 700 °C results in a Ge layer more quantum well-like than dot-like, however 3D exciton localization and the resultant dot-like phononless recombination depend highly on the interfacial characteristics [13]. Annealing at 700 °C results in 8K exciton delocalization within the Ge QD, due to atomic scale intermixing across the Si Ge interface, however, the Ge quantum dot embedded in the Si RITD retains much of its original shape, made evident from the TEM images of Fig. 5.
5. Conclusions In summary we have synthesized very small Ge quantum dots by molecular beam epitaxy at low temperatures. The Ge dots exhibit luminescence properties which indicate that radiative recombination results from 3D quantum confinement. PL emission energies blue-shift with increasing annealing temperature, and at high annealing temperatures 3D exciton localization becomes smeared out due to Is– Ge intermixing. We incorporated these Ge quantum dots into the active region of a Si RITD and demonstrated that the dots are compatable with RITD growth requirements and post-
[1] D.J. Eaglesham, M. Cerullo, Phys. Rev. Lett. 64 (1943) 1990. [2] Y.W. Mo, D.E. Savage, B.S. Schwartzentruber, M.G. Lagally, Phys. Rev. Lett. 65 (1990) 1020. [3] S. Fukatsu, H. Sunamura, Y. Shiraki, S. Komiyama, Appl. Phys. Lett. 71 (1997) 258. [4] O.G. Schmidt, C. Lange, K. Eberl, O. Kienzle, F. Ernst, Appl. Phys. Lett. 71 (1997) 2340. [5] O.G. Schmidt, K. Eberl, J. Auerswald, J. Luminenescence 80 (1999) 491. [6] P. Schittenhelm, C. Engel, F. Findeis, G. Abstreiter, A.A. Darhuber, G. Bauer, A.O. Kosogov, P. Werner, J. Vac. Sci. Technol. B 16 (1998) 1575. [7] K. Eberl, O.G. Schmidt, R. Duschl, O. Kienzle, E. Ernst, Y. Rau, Thin Solid Films 369 (2000) 33. [8] O.G. Schmidt, U. Denker, K. Eberl, O. Kienzle, F. Ernst, R.J. Haug, Appl. Phys. Lett. 77 (2000) 4321. [9] S.L. Rommel, T.E. Dillon, M.W. Dashiell, H. Feng, J. Kolodzey, P.R. Berger, P.E. Thompson, K. Hobart, R. Lake, A.C. Seabaugh, G. Klimeck, D.K. Blanks, Appl. Phys. Lett. 73 (1998) 2191. [10] R. Duschl, O.G. Schmidt, K. Eberl, Appl. Phys. Lett. 76 (2000) 879. [11] S. Sze, Physics of Semiconductors, 2nd ed., Wiley, New York, 1985, pp. 531 – 536. [12] O.G. Schmidt, C. Lange, K. Eberl, Appl. Phys. Lett. 75 (1905) 1999. [13] M.W. Dashiell, U. Denker, O.G. Schmidt, Appl. Phys. Lett. 79 (2001) 2261. [14] V.A. Markov, H.H. Cheng, C. Chia, A.I. Nikiforov, V.A. Cherepanov, O.P. Pchelyakov, K.S. Zhuraylev, A.B. Talochkin, E. McGlynn, M.O. Henry, Thin Solid Films 369 (2000) 79. [15] J.C. Sturm, A. St.Amour, Y. Lacroix, M.L.W. Thewalt, Appl. Phys. Lett. 64 (1994) 2291. [16] R. Sauer, J. Weber, J. Soltz, E.R. Weber, K.H. Ku¨ sters, H. Alexander, Appl. Phys. A 36 (1985) 1. [17] M.W. Dashiell, U. Denker, O.G. Schmidt, Appl. Phys. Lett., accepted for publication. [18] M.W. Dashiell, R.T. Troeger, T.N. Adam, P.R. Berger, J. Kolodzey, A.C. Seabaugh, R. Lake, IEEE Trans. Electron. Devices 47 (2000) 1707. [19] O.G. Schmidt, K. Eberl, Phys. Rev. B 61 (2000) 13721. [20] S. Schieker, O.G. Schmidt, K. Eberl, N.Y. Jin-Phillipp, F. Phillipp, Appl. Phys. Lett. 72 (1998) 3344.
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Low temperature epitaxial growth of germanium islands in active regions of silicon interband tunneling diodes M.W. Dashiell *, C. Mu¨ller, N.Y. Jin-Phillipp, U. Denker, O.G. Schmidt, K. Eberl Max-Planck-Institut fu¨r Festko¨rperforschung, Heisenbergstraße 1, D-70569 Stuttgart, Germany
Abstract We incorporate self-assembling Ge islands into Si-based interband tunneling diodes grown by molecular beam epitaxy. The kinetic limitations of low-temperature Ge island formation have been overcome in the growth windows that are required to retain sharp delta-doping profiles in the diode‘s active region. Ge quantum dots are observed for growth temperatures of 360 °C and grown at a rate of 0.125 monolayers per minute. Photoluminescence spectroscopy and annealing experiments indicate three-dimensional carrier localization and phononless radiative recombination, which confirms a dot-like electronic structure. The Ge quantum dots have been incorporated into the active region of delta-doped Si interband tunneling diodes. Room temperature negative-differential-resistance is observed and the electrical characteristics may be tuned by post-growth rapid thermal annealing. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Germanium quantum dots; Self-assembly; Islands; Tunneling diodes; Photoluminescence; Annealing; Negative differential resistance
1. Introduction Stranski-Krastanov growth of Ge islands on Si [1,2] have received great attention over the past decade for self-organized quasi-zero-dimensional structures, formed without sophisticated lithographic techniques. Three-dimensional (3D) carrier localization within the Ge islands is a pathway to obtaining enhanced optical [3 –5] performance compared to conventional 2D Si1 − xGex /Si heterostructures. Although significant research has been published on the theory and fundamental properties of self-organized Ge islands, there are only few results to date which actually exploit Ge quantum dots in an electronic device [6,7]. Recently, closely stacked self-assembled Ge islands were used for Sibased resonant tunneling diodes and negative differential resistance (NDR) was observed at low temperatures [8]. In this contribution we present an approach to incorporate Ge quantum dots (QDs) into Si based resonant interband tunneling diodes(RITD) [7,9]. Currently, the Si/Si1 − x Gex RITD achieves the highest performance of any Si-based tunnel diode, with a record peak-to-valley * Corresponding author.
current ratio (PVCR) of six [10]. Optimization studies demonstrated that RITD performance increases with higher Ge concentrations in the intrinsic spacer, due to Ge’s greater tunneling transmission probability, which results in higher peak current and PVCR. For a given peak current density, a lower capacitance can be accomplished by adding Ge since the tunneling barrier width need not be as great as for pure Si. Low diode capacitance is an important parameter for high speed tunnel diode circuitry [11]. Unfortunately, increasing the Ge concentration above 50% in 2D spacer layers will generate strain induced misfit-dislocations of the two dimensional Si1 − x Gex layer which degrades the performance. Our approach to increase the Ge concentration to nearly 100% during coherent-epitaxialgrowth is to incorporate pure Ge islands into the intrinsic region of the RITD. One obstacle in achieving this goal is that, due to requirements of high-concentration and abrupt doping profiles of the RITD, the active region must be formed at low temperature (360 °C) for good performance. At low temperatures such as these, the tendency for Ge island formation is suppressed and misfit dislocations are more likely to be introduced during growth [7]. Previous attempts to incorporate self-assembled Ge islands into Si-based in-
0921-5107/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 1 ) 0 0 8 1 1 - X
M.W. Dashiell et al. / Materials Science and Engineering B89 (2002) 106–110
terband tunneling diodes [6,7] have not resulted in a device showing room temperature negative differential resistance, possibly a result of the active region being grown at temperatures \ 500 °C. The RITD device structure studied in this work are based upon the device structure and band-alignment proposed in Ref. [7]. The main difference is that the active region, including Ge islands, is formed entirely at 360 °C. Atomic force microscopy (AFM), photoluminescence (PL) spectroscopy, and transmission electron microscopy (TEM) are used to quantify the structural and optical properties of the Ge islands formed at low temperatures. Finally the electrical characteristics and demonstration of room temperature NDR in a Ge-dot interband tunneling diode are presented.
2. Properties of Ge quantum dots grown by molecular beam epitaxy at low temperatures Samples were grown by solid source molecular beam epitaxy (MBE). After de-oxidation of Si substrates, 60 nm Si buffers were grown at 500 °C at a rate of 1 A, s − 1. Prior to Ge deposition, the substrate temperature was ramped during Si growth to values Tg which varied
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from 360 to 500 °C. The Ge was then deposited and subsequently capped with 100 nm Si for PL studies. Reflection high energy electron diffraction (RHEED) indicated a transition from 2D to 3D Ge growth mode, and a recovery of the 2D surface after Si capping for all values of Tg. The Ge layer sequence was then repeated at the surface for AFM analysis. All PL spectra were recorded at 8K under excitation from the 488 nm Ar+ laser line with excitation power of 25 mW and with spot size of approximately 3 mm2 using standard lockin technique and a liquid N2 cooled Ge detector. Fig. 1 shows AFM images of uncapped Ge islands after 5.4 monolayers (ML) nominal Ge deposition for Tg ranging from 360 to 500 °C. For Tg values of 450 and 500 °C the well known Ge hut-clusters [2], which are elongated in the [001] directions and having approximately 20 nm base width are observed in the AFM images. The height of the clusters was determined by line scans to be approximately 1.2 and 0.8 nm for the hut clusters grown at 500 and 450 °C respectively. At temperatures of 400 and 360 °C, the shape of the 3D features drastically changes, and we observe very small three dimensional Ge structures which have not aligned in any discernable crystal directions. AFM line scans indicate the height of these structures to be approximately 0.5 nm with a base width of less than 20 nm.
Fig. 1. 1 × 1mm2 AFM images of uncapped Ge islands after 5.4 ML Ge deposition at 500, 450, 400 and 360 °C.
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Fig. 2. 8K PL spectra of single Ge island layers (5.4 ML Ge deposition) for the four different growth temperatures. The PL signal observed at 1.035eV originates from the Si substrate.
Fig. 2 shows the 8K PL spectra of the single Ge island layers. The Ge hut clusters emit a PL signal centered around 0.8 eV [12] which was recently shown to originate from spatially-indirect, phononless carrier recombination between holes confined in the Ge huts and electrons confined in surrounding tensile-strained Si [13]. For Tg = 360 °C, an emission peak at 765 meV, having a 44 meV full width at half maximum (FWHM) is detected from the single layer. A systematic decrease in PL intensity as Tg is reduced is attributed to point defects generated during low temperature epitaxy. The point defects act as competing non-radiative recombination channels, however we will illustrate in the next section that post-growth annealing can restore some of the PL intensity. Only sparse literature exists [14] that demonstrates quantum-dot like PL from 3D Ge struc-
tures grown at such low temperature, and the previous reports did not unambiguously determine that the luminescence originated from quantum confined energy levels in the dots. A systematic approach must be undertaken to eliminate the possibility that the PL originates from defect bands which lie in similar energy ranges [15,16]. We varied the total deposited Ge thickness, for Tg = 360 °C, from 1.7 ML to over 7.0 ML [17]. The PL spectra revealed a gradual transition from quantum well-like PL emission from flat Ge wetting layers (confirmed by AFM) to quantum dot-like PL emission (the broad PL emission peak observed in Fig. 2). With increasing deposition, the PL emission redshifted due to the increase in size and resultant confinement shifts of the 2D wetting layers and 3D quantum dots. Above 6ML, the PL intensity quenches, presumably due to nucleation of misfit dislocations during growth.
3. Post-growth annealing of the Ge quantum dots Si-based Esaki and interband tunneling diodes grown by MBE require post-growth thermal anneals between 600 and 800 °C to reduce point defect densities associated with low temperature growth (to reduce the undesirable excess current) [9,18]. We investigated the electronic structure and thermal stability of the Ge quantum dots grown at 360 °C after 60 s post growth rapid thermal anneals (RTA) using PL spectroscopy. Fig. 3a (left-hand figure) depicts the post-growth annealing PL spectra (solid lines) and, for reference, the pre-annealing PL spectra (dashed lines). Evident is a systematic blue shift of the Ge related PL band, an
Fig. 3. (a) 8K PL spectra of Ge islands (5.4 ML deposited at 360 °C) after 60 s RTA annealings. The dotted spectra indicate the pre-annealing (as-grown) PL spectra. (b) Summary of the peak PL emission energies (top) and integrated Ge related PL intensity taken from Fig. 3a as a function of RTA annealing temperature.
M.W. Dashiell et al. / Materials Science and Engineering B89 (2002) 106–110
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QD’s phononless PL intensity [13]. The PL intensity saturates after the transition to quantum-well like PL characteristics. No dislocation related PL bands are observed for any values of TRTA.
4. Ge quantum dots in the intrinsic region of Si based RITDs
Fig. 4. Room-temperature current –voltage characteristics of the Si interband tunnel diode with Ge dots embedded in the active region for several RTA annealing temperatures.
increase of the Ge related PL intensity relative to the pre-annealing intensity, and a gradual evolution of the single broad quantum dot-like luminescence into two well resolved PL peaks. Fig. 3b summarizes the annealing experiments of the Ge quantum dots grown at 360 °C. The top half of Fig. 3b shows the peak PL energies observed after RTA annealing. Above 685 °C, two peaks separated by 55 meV (the energy of the momentum conserving transverse optical phonon in Si/Si1 − x Gex /Si quantum wells) continue to blueshift due to Si– Ge interdiffusion, and are attributed to quantum well-like luminesence resulting from recombination in the intermixed wetting layer. A transisition from quantum dot-like PL (single broad emission) to quantum well-like PL occurs when sufficient material intermixing smears out the 3D confinement potentials of the QD. The gradual material-interdiffusion driven blueshifts and transition, as well as the absence of dislocation related PL lines, indicates the single broad Ge-quantum dot PL originates from phononless radiative recombination within 3D confined energy levels [13,17]. The appearance of the phonon assisting replica of the quantum well-like PL at higher TRTA values indicate that excitons are no longer strongly localized around the Ge quantum dot. We note that a broad luminescence peak is observed from islands grown at 700 °C [19], however those islands are much greater in size than the dots decribed here, and thus are probably less sensitive to slight intermixing across their interfaces. The bottom half of Fig. 3b depicts the integrated Ge-related PL intensity, normalized to its pre-annealing value as a function of annealing temperature. Initially we observe an increase in intensity, which is attributed to reduction of point defects associated with low-temperature growth [20]. At higher temperatures, a decrease in intensity is observed which is attributed to the effect of intermixing on the 3D localization and the
The Ge quantum dot RITD was fabricated by MBE on p+ substrates. The doping profile of the highly doped Si layers were chosen from the optimized RITD structure of Ref. [10]. Dopants B and P were thermally evaporated from effusion cells for p- and n-type doping. The intrinsic spacer was grown at 360 °C consisting of undoped Si (i-Si) and the Ge quantum dots (Ge deposition rate=0.125 ML min − 1). The investigated structure consists of the following layers, starting from the p+ Si substrate: a 50 nm Si p+ (3×1019 cm − 3) buffer layer, followed by a p-type delta-doping layer (5× 1013 cm − 2), a 1 nm i-Si/5.4 ML-Ge/3 nm i-Si intrinsic spacer, a n-type delta doping layer (1×1014 cm − 2) and a 100 nm Si n+ (5×1019 cm − 3) cap. A simplified cross-sectional schematic and band-alignment for the embedded Ge quantum dot RITD can be found in Ref. [7]. Diodes were fabricated using standard photolithography, Ti–Au metallization, and HNO3:HF (50:1) etching to form mesa diodes of approximately 40–50 mm diameter. Fig. 4 displays room temperature current–voltage characteristics of the Ge quantum dot RITD. Negative differential resistance was observed for samples after 60 s post-growth RTA annealing between 700 and 820 °C, where the anneal temperature, TRTA, is indicated in the figure. As for the case of the planar Si/Si1 − x Gex /Si RITDs, the current density decreases with TRTA and an optimal temperature is required to maximize PVCR (A modest 1.035 in this non-optimized Ge-dot RITD structure for TRTA = 775 °C). NDR was observed with peak current densities varying from approximately 50– 500 A cm − 2. To our knowledge, this is the first demonstration of room temperature NDR from a Si based tunnel diode with embedded quantum dots in the active region. Fig. 5 displays TEM images of the active region of the as-grown Ge-QD RITD cross-section (Fig. 5a) and the cross-section after a 60 s RTA anneal at TRTA = 700 °C (Fig. 5b). Very small Ge dots are observed with diameters between 10 and 20 nm, the height of the capped dots (estimated from the contrast of the TEM image) is approximately 1–3 nm. The difference in QD height determined from AFM (uncapped samples) and that determined from the TEM measurements (dots embedded in a Si matrix) could be due to intermixing during growth or the effect of strain fields propagating above the dot into the Si matrix, as well as the fact that
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growth processing (including the high temperature anneal). Room temperature negative-differentialresistance was observed in the Ge-dot RITD and TEM measurements reveal that the Ge QD retains its structure after the required post-growth annealing procedure. No evidence of dislocation generation was observed during high-temperature processing required for RITD fabrication. Careful layer structure optimization, however, is necessary to improve device performance for future applications.
Acknowledgements The author acknowledges financial support from a Max-Planck-Gesellschaft Stipendium.
References
Fig. 5. TEM image of the active region of the Si interband tunnel diode with Ge dots embedded in the spacer region before (top) and after (bottom) RTA annealing at 700 °C.
the AFM dot height does not include the Ge WL. Annealing the samples at 700 °C does not observably alter the quantum dot-structure shown in the TEM image compared to the as-grown sample. This appears to contradict the PL findings-that suggest annealing at 700 °C results in a Ge layer more quantum well-like than dot-like, however 3D exciton localization and the resultant dot-like phononless recombination depend highly on the interfacial characteristics [13]. Annealing at 700 °C results in 8K exciton delocalization within the Ge QD, due to atomic scale intermixing across the Si Ge interface, however, the Ge quantum dot embedded in the Si RITD retains much of its original shape, made evident from the TEM images of Fig. 5.
5. Conclusions In summary we have synthesized very small Ge quantum dots by molecular beam epitaxy at low temperatures. The Ge dots exhibit luminescence properties which indicate that radiative recombination results from 3D quantum confinement. PL emission energies blue-shift with increasing annealing temperature, and at high annealing temperatures 3D exciton localization becomes smeared out due to Is– Ge intermixing. We incorporated these Ge quantum dots into the active region of a Si RITD and demonstrated that the dots are compatable with RITD growth requirements and post-
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