- Email: [email protected]
SiGe quantum wells
Physica B 308–310 (2001) 554–557
High resolution minority carrier transient spectroscopy of defects in Si and Si/SiGe quantum wells J.H. Evans-Freeman*, M.A. Gad Centre for Electronic Materials, Department of Electrical Engineering and Electronics, UMIST, P.O. Box 88, Sackville St. Manchester, M60 1QD, UK
Abstract High resolution Laplace minority carrier transient spectroscopy (LMCTS) has been used to study defects in gas source molecular beam epitaxy grown Si/Si0.855Ge0.145 strained quantum wells. In LMCTS, the minority carrier emission transient is analysed and detailed emission properties of minority carrier traps which have similar energy can be characterised. The technique was evaluated by comparing LMCTS of a Au : H hole trap, G3, in n-type silicon with Laplace deep level transient spectroscopy of the same trap in p-type silicon. Both techniques confirm that this level consists of two states, as previously suggested in the literature. MCTS was then applied to the n-type Si/SiGe quantum well sample, and five-hole traps were observed. A level at 95 K in the MCTS spectrum was identified as a possible candidate for hole emission from the quantum wells. LMCTS showed that the emission rate of this hole trap exhibited only a slight temperature dependence compared to that exhibited by hole states associated with isolated point defects. This is attributed to holes tunnelling out of the quantum well assisted by the electric field present in the experiment. r 2001 Elsevier Science B.V. All rights reserved. PACS: 73.29.Dx; 73.40.Gk Keywords: Laplace MCTS; SiGe; Quantum wells
1. Introduction Deep level transient spectroscopy (DLTS) is a commonly used technique to characterise the electrical activity of defects in semiconductors [1]. It yields a universal signature of the enthalpy of deep levels, but has some drawbacks. Only defect states in one half of the band gap can be easily characterised, and closely spaced energy levels are not resolvable because of the poor time constant resolution; both these problems have been addressed in recent years. To characterise deep states throughout the whole band gap, a complementary technique, minority carrier transient spectroscopy *Corresponding author. Tel.: +44-161-200-4796; fax: +44161-200-4796. E-mail address: [email protected] (J.H. Evans-Freeman).
(MCTS) [2] was developed, in which thermally stimulated minority carrier emission is measured and processed in the same way as DLTS. As an example of its use, it was reported [3] that Au : H complexes in silicon produce four deep levels, two in the upper half of the band gap and two in the lower, established by examining n- and p-type silicon [3]. The application of MCTS meant that all four new levels were observed in n-type Si [4]. MCTS can also be used to measure activation energies and capture cross sections, and can be combined with DLTS to examine carrier dynamics at recombination centres [5] but is difficult to use for depth profiling [6]. The problem of closely spaced levels has been addressed by the development of algorithms for analysing the components in the capacitance transient [7,8], and these have been used with considerable success to resolve some long standing defect problems in silicon
0921-4526/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 8 2 5 - 0
J.H. Evans-Freeman, M.A. Gad / Physica B 308–310 (2001) 554–557
[9–16]. This technique is termed Laplace DLTS (LDLTS) and produces an output of intensity as a function of emission rate. In this paper, we report on application of the same algorithms to minority carrier emission transients, which we call Laplace MCTS (LMCTS). The technique is verified by applying it to minority carrier traps in n-type Si containing Au : H complexes. Earlier work had suggested that one of the Au : H states, G3, consisted of two closely spaced levels [3,4], based on detailed capture cross-section measurements [4]. G3 is a hole trap, therefore we have compared LMCTS of this trap in n-type Si with LDLTS of the trap in p-type Si. We have then applied the technique for the examination of hole emission from strained Si/SiGe/Si quantum wells (QWs). The majority of the band offset is in the valence band [17] and we have used MCTS and LMCTS to detect thermally stimulated hole emission from the wells. The results are compared with PL quenching experiments which yield the hole confinement energy.
2. Experimental The samples containing Au : H complexes were either n-type CZ Si doped with 2 1015 cm3 phosphorus or p-type Si doped with 2 1015 cm3 boron. The gold diffusion and introduction of hydrogen (by wet chemical etching), is described elsewhere [4]. Gold Schottky diodes were evaporated onto the n-type Si, and titanium Schottky diodes were sputtered onto the p-type Si. The SiGe/Si QW structure was grown by gas source molecular beam epitaxy and consisted of 10 strained Si0.855Ge0.145/Si QWs grown on a Si substrate. The Si cap layer was 199.8 nm, the wells were 5.7 nm thick, the barriers were 55 nm thick, and the layer was n-type because of a residual phosphorus level of about 1 1016 cm3. For the MCTS and LMCTS measurements, thin (20 nm) Au diodes were evaporated. Electrical measurements were carried out in a closed cycle cryostat between temperatures of 300 and 20 K. In MCTS, above band gap light is incident on the semitransparent Schottky diode, and creates electron-hole pairs. The wavelength of the light ensures that the extinction depth is greater than the depletion region width. Majority carriers are rejected from the depletion region, and it is predominantly minority carriers that are available for capture in the region. In the LMCTS experiment, the capacitance transient due to minority carrier emission was recorded up to 10,000 times at a fixed temperature in a high stability cryostat. For the photoluminescence (PL) measurements, the sample was attached to a cold finger inside a closed cycle cryostat and excited by using the 514.2 nm line of an argon ion laser. The signal was
555
detected using a liquid nitrogen cooled Ge detector and analysed by conventional lock-in techniques.
3. Results and discussion When silicon containing Au : H complexes was characterised by DLTS and MCTS (not shown) the expected traps G1–G4 [3] due to Au : H complexes, and the Au acceptor and donor states were observed. In earlier work [3,4], it was noted that G3, at Ev þ 0:47 eV, exhibited different behaviour from the levels G1, G2 and G4. The annealing behaviour was different [3] and it exhibited a two-stage capture process [4]. The existence of two closely spaced levels in G3 is confirmed here by carrying out LDLTS and LMCTS. Fig. 1(a) shows LMCTS carried out at 225 K and Fig. 1(b) shows the LDLTS recorded at the same temperature in p-type Si. In both spectra two emission rates are observed. The relative intensities of the two emission rates are different between the n- and p-type samples, but this may reflect the differences observed and reported previously regarding G3 when it is in n- or p-type Si. These spectra confirm that LMCTS yields the same information at LDLTS. Turning to the strained QW layer, PL at 5 K layer showed very clear zero phonon, transverse optical and transverse acoustic phonon peaks [18,19]. The hole confinement energy can be found by examining the PL quenching as a function of temperature [20]. This yielded an activation energy for thermal emission out of the wells of 110 meV, which is the depth above the Si valence band of the n ¼ 1 heavy hole confined state.
Fig. 1. (a) LMCTS carried out at 225 K of the trap G3 associated with Au : H complexes in n-type silicon, and (b) LDLTS of the same defect recorded at the same temperature in p-type Si.
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J.H. Evans-Freeman, M.A. Gad / Physica B 308–310 (2001) 554–557
In MCTS and LMCTS, the QWs capture holes during the fill pulse, the fill pulse creates a photocurrent through the diode, and the hole quasi-Fermi level varies with photon flux [6]. The calculation is further complicated by the presence of the QWs, because the quasi-Fermi level in them will depend upon the fraction of injected holes that are captured and reside in the wells. Difficulties in the calculation of the emission rates can easily be overcome if a few simple assumptions are made [21]. It can be assumed that all captured carriers will reside in the lowest confined energy state. This means that the experimental conditions are the same as during the PL experiment. It can also be assumed that the quasi-Fermi level in the QWs lies several kT above the confined heavy hole state, and will not affect the carrier dynamics during the emission process [22]. Hence, the emission rate for the holes out of the wells depends only upon the difference between the confined hole state energy and the valence band in the Si barrier. It has been reported that at low temperatures (p100 K) and relatively modest electric fields, X30 kVcm1, a plateau can be observed in the DLTS due to a constant emission rate of carriers out of QWs. This is attributed to tunnelling through a triangular potential barrier between the QW and the relevant carrier band [22]. Fig. 2 shows the MCTS spectrum of the Si/SiGe QW, in two halves for clarity. As an example of a point defect in this system, Fig. 3 shows the LMCTS spectra measured between 30 and 38 K, and shows that two emission rates are present. An Arrhenius plot of these two states, taken from the LDLTS data, yields activation energies of 5073 and 3573 meV. Both defects exhibit exponential filling, hence can be attributed to point defects. No Poole–Frenkel effect was observed. The broad feature in Fig. 2 between 85 and 100 K is a strong candidate for the QW signal. The feature is
Fig. 2. MCTS spectrum of the Si0.855Ge0.145/Si MQW sample, spectrum in two halves for clarity, at a rate window of 200 s1. The feature proposed to be due to hole tunnelling out of the wells is indicated.
almost plateau-like, in accordance with Ref. [22], and no other features in the MCTS spectrum have an activation energy of 110 meV. Fig. 4 shows the emission rate of this deep state between 100 and 112.3 K, and it is nearly constant. The best fit of the emission rate to temperature was by assuming a square root dependence upon the temperature, rather than a Boltzmann relationship. Other defects in the LMCTS spectrum of this sample do not exhibit this behaviour, and we therefore conclude that we are observing direct evidence of holes tunnelling out of SiGe QWs into the cladding layers.
Fig. 3. Shows the LMCTS spectra of the Si0.855Ge0.145/Si MQW sample measured between 30 and 38 K.
Fig. 4. LMCTS of the QW-related state, showing the nearly constant emission rate between 100 and 112.3 K.
J.H. Evans-Freeman, M.A. Gad / Physica B 308–310 (2001) 554–557
4. Conclusions A technique which uses the same analytical approach to the capacitance transient as LDLTS but examines the detailed emission properties of minority carrier traps, LMCTS, has been described. In LMCTS, minority carriers are injected into a depletion region by the application of a light pulse of suitable wavelength. LDLTS and LMCTS are complementary because traps in the whole of the band gap can be accessed in semiconductors of one polarity. To verify the technique, LMCTS in n-type Si containing Au : H complexes was compared to LDLTS of p-type Si containing the same complexes, and found to yield the same results. The LMCTS technique was then applied to n-type Si which contains 10 Si0.855Ge0.145 strained QWs. Several hole traps were observed, but it was possible to identify a candidate for hole emission out of the QWs. When examined by LMCTS over a temperature range of 102–116 K, it was found that the emission rate for holes out of the QWs was only weakly dependent upon temperature. This indicates that this defect is not pointdefect-like, and it is suggested that the nearly constant emission rate is due to holes tunnelling out of the QW. This is the first direct observation of this effect, which up till now could only be inferred from conventional DLTS spectra.
Acknowledgements We acknowledge the EPSRC, UK for funding, and J Zhang at Imperial College for growing the MBE layer.
References [1] D.V. Lang, J. Appl. Phys. 45 (1974) 3023. [2] B. Hamilton, A.R. Peaker, D.R. Wight, J. Appl. Phys. 50 (1979) 6373. . Sveinbjornsson, . . Phys. Rev. B 52 (1995) [3] E.O. O. Engstrom, 4884.
557
[4] J.A. Davidson, J.H. Evans, Semicond. Sci. Technol. 11 (1996) 1704. [5] R.H. Wu, A.R. Peaker, Solid State Electron. 25 (1982) 643. [6] J.A. Davidson, J.H. Evans, J. Appl. Phys. 81 (1997) 251. [7] L. Dobaczewski, P. Kaczor, I.D. Hawkins, A.R. Peaker, J. Appl. Phys. 76 (1994) 194. [8] J. Kang, H. Zhan, Q. Huang, J. Crystal Growth 210 (2000) 247. [9] L. Dobaczewski, P. Kaczor, M. Missous, A.R. Peaker, Z.R. Zytkiewicz, Phys. Rev. Lett. 68 (1992) 2508. [10] P. Deixler, J. Terry, I.D. Hawkins, J.H. Evans-Freeman, A.R. Peaker, L. Rubaldo, D.K. Maude, J.-C. Portal, L. Dobaczewski, K. Bonde Nielsen, A. Nylandsted Larsen, A. Mesli, Appl. Phys. Lett. 73 (1998) 3126. [11] L. Rubaldo, P. Deixler, I.D. Hawkins, J. Terry, D.K. Maude, J.-C. Portal, J.H. Evans-Freeman, L. Dobaczewski, A.R. Peaker, Mat. Sci. Eng. B 58 (1999) 126. [12] A.R. Peaker, J.H. Evans-Freeman, L. Rubaldo, I.D. Hawkins, L. Dobaczewski, K. Vernon-Parry, Physica B 273–274 (1999) 243. [13] L. Dobaczewski, K. Goscinski, K. Bonde Nielsen, A.N. Larsen, J.L. Hansen, A.R. Peaker, Phys. Rev. Lett. 83 (1999) 4582. [14] K.B. Nielsen, L. Dobaczewski, K. Goscinski, R. Bendesen, O. Andersen, B. Bech Nielsen, Physica B 273–274 (1999) 167. [15] L. Dobaczewski, K.B. Nielsen, K. Goscinski, A.R. Peaker, A. Nylandsted Larsen, Physica B 273–274 (1999) 620. [16] L. Dobaczewski, K.B. Nielsen, K. Goscinski, O. Andersen, Acta Phys. Pol. A 98 (2000) 231. [17] R. People, J.C. Bean, Appl. Phys. Lett. 48 (1986) 538. [18] J.C. Sturm, H. Manoharan, L.C. Lechyshyn, M.L.W. Thewalt, N.L. Rowell, J.-P. No.el, D.C. Houghton, Phys. Rev. Lett. 66 (1991) 1362. [19] E.F. Gross, N.S. Sokolov, A.N. Titkov, Sov. Phys. Solid State 14 (1973) 1732. [20] D.C. Houghton, N.L. Rowell, J.-P. No.el, G. Aers, M. Davies, A. Wang, D.D. Perovic, Silicon-Based Optoelectronic Materials Symposium, Mater. Res. Soc. Vol. 298, 1993, p. 3. [21] L. Vescan, R. Apetz, H. Luth, . J. Appl. Phys. 73 (1993) 7427. [22] X. Letartre, D. Sti!evenard, M. Lannoo, E. Barbier, J. Appl. Phys. 69 (1991) 7336.
High resolution minority carrier transient spectroscopy of defects in Si and Si/SiGe quantum wells J.H. Evans-Freeman*, M.A. Gad Centre for Electronic Materials, Department of Electrical Engineering and Electronics, UMIST, P.O. Box 88, Sackville St. Manchester, M60 1QD, UK
Abstract High resolution Laplace minority carrier transient spectroscopy (LMCTS) has been used to study defects in gas source molecular beam epitaxy grown Si/Si0.855Ge0.145 strained quantum wells. In LMCTS, the minority carrier emission transient is analysed and detailed emission properties of minority carrier traps which have similar energy can be characterised. The technique was evaluated by comparing LMCTS of a Au : H hole trap, G3, in n-type silicon with Laplace deep level transient spectroscopy of the same trap in p-type silicon. Both techniques confirm that this level consists of two states, as previously suggested in the literature. MCTS was then applied to the n-type Si/SiGe quantum well sample, and five-hole traps were observed. A level at 95 K in the MCTS spectrum was identified as a possible candidate for hole emission from the quantum wells. LMCTS showed that the emission rate of this hole trap exhibited only a slight temperature dependence compared to that exhibited by hole states associated with isolated point defects. This is attributed to holes tunnelling out of the quantum well assisted by the electric field present in the experiment. r 2001 Elsevier Science B.V. All rights reserved. PACS: 73.29.Dx; 73.40.Gk Keywords: Laplace MCTS; SiGe; Quantum wells
1. Introduction Deep level transient spectroscopy (DLTS) is a commonly used technique to characterise the electrical activity of defects in semiconductors [1]. It yields a universal signature of the enthalpy of deep levels, but has some drawbacks. Only defect states in one half of the band gap can be easily characterised, and closely spaced energy levels are not resolvable because of the poor time constant resolution; both these problems have been addressed in recent years. To characterise deep states throughout the whole band gap, a complementary technique, minority carrier transient spectroscopy *Corresponding author. Tel.: +44-161-200-4796; fax: +44161-200-4796. E-mail address: [email protected] (J.H. Evans-Freeman).
(MCTS) [2] was developed, in which thermally stimulated minority carrier emission is measured and processed in the same way as DLTS. As an example of its use, it was reported [3] that Au : H complexes in silicon produce four deep levels, two in the upper half of the band gap and two in the lower, established by examining n- and p-type silicon [3]. The application of MCTS meant that all four new levels were observed in n-type Si [4]. MCTS can also be used to measure activation energies and capture cross sections, and can be combined with DLTS to examine carrier dynamics at recombination centres [5] but is difficult to use for depth profiling [6]. The problem of closely spaced levels has been addressed by the development of algorithms for analysing the components in the capacitance transient [7,8], and these have been used with considerable success to resolve some long standing defect problems in silicon
0921-4526/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 8 2 5 - 0
J.H. Evans-Freeman, M.A. Gad / Physica B 308–310 (2001) 554–557
[9–16]. This technique is termed Laplace DLTS (LDLTS) and produces an output of intensity as a function of emission rate. In this paper, we report on application of the same algorithms to minority carrier emission transients, which we call Laplace MCTS (LMCTS). The technique is verified by applying it to minority carrier traps in n-type Si containing Au : H complexes. Earlier work had suggested that one of the Au : H states, G3, consisted of two closely spaced levels [3,4], based on detailed capture cross-section measurements [4]. G3 is a hole trap, therefore we have compared LMCTS of this trap in n-type Si with LDLTS of the trap in p-type Si. We have then applied the technique for the examination of hole emission from strained Si/SiGe/Si quantum wells (QWs). The majority of the band offset is in the valence band [17] and we have used MCTS and LMCTS to detect thermally stimulated hole emission from the wells. The results are compared with PL quenching experiments which yield the hole confinement energy.
2. Experimental The samples containing Au : H complexes were either n-type CZ Si doped with 2 1015 cm3 phosphorus or p-type Si doped with 2 1015 cm3 boron. The gold diffusion and introduction of hydrogen (by wet chemical etching), is described elsewhere [4]. Gold Schottky diodes were evaporated onto the n-type Si, and titanium Schottky diodes were sputtered onto the p-type Si. The SiGe/Si QW structure was grown by gas source molecular beam epitaxy and consisted of 10 strained Si0.855Ge0.145/Si QWs grown on a Si substrate. The Si cap layer was 199.8 nm, the wells were 5.7 nm thick, the barriers were 55 nm thick, and the layer was n-type because of a residual phosphorus level of about 1 1016 cm3. For the MCTS and LMCTS measurements, thin (20 nm) Au diodes were evaporated. Electrical measurements were carried out in a closed cycle cryostat between temperatures of 300 and 20 K. In MCTS, above band gap light is incident on the semitransparent Schottky diode, and creates electron-hole pairs. The wavelength of the light ensures that the extinction depth is greater than the depletion region width. Majority carriers are rejected from the depletion region, and it is predominantly minority carriers that are available for capture in the region. In the LMCTS experiment, the capacitance transient due to minority carrier emission was recorded up to 10,000 times at a fixed temperature in a high stability cryostat. For the photoluminescence (PL) measurements, the sample was attached to a cold finger inside a closed cycle cryostat and excited by using the 514.2 nm line of an argon ion laser. The signal was
555
detected using a liquid nitrogen cooled Ge detector and analysed by conventional lock-in techniques.
3. Results and discussion When silicon containing Au : H complexes was characterised by DLTS and MCTS (not shown) the expected traps G1–G4 [3] due to Au : H complexes, and the Au acceptor and donor states were observed. In earlier work [3,4], it was noted that G3, at Ev þ 0:47 eV, exhibited different behaviour from the levels G1, G2 and G4. The annealing behaviour was different [3] and it exhibited a two-stage capture process [4]. The existence of two closely spaced levels in G3 is confirmed here by carrying out LDLTS and LMCTS. Fig. 1(a) shows LMCTS carried out at 225 K and Fig. 1(b) shows the LDLTS recorded at the same temperature in p-type Si. In both spectra two emission rates are observed. The relative intensities of the two emission rates are different between the n- and p-type samples, but this may reflect the differences observed and reported previously regarding G3 when it is in n- or p-type Si. These spectra confirm that LMCTS yields the same information at LDLTS. Turning to the strained QW layer, PL at 5 K layer showed very clear zero phonon, transverse optical and transverse acoustic phonon peaks [18,19]. The hole confinement energy can be found by examining the PL quenching as a function of temperature [20]. This yielded an activation energy for thermal emission out of the wells of 110 meV, which is the depth above the Si valence band of the n ¼ 1 heavy hole confined state.
Fig. 1. (a) LMCTS carried out at 225 K of the trap G3 associated with Au : H complexes in n-type silicon, and (b) LDLTS of the same defect recorded at the same temperature in p-type Si.
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J.H. Evans-Freeman, M.A. Gad / Physica B 308–310 (2001) 554–557
In MCTS and LMCTS, the QWs capture holes during the fill pulse, the fill pulse creates a photocurrent through the diode, and the hole quasi-Fermi level varies with photon flux [6]. The calculation is further complicated by the presence of the QWs, because the quasi-Fermi level in them will depend upon the fraction of injected holes that are captured and reside in the wells. Difficulties in the calculation of the emission rates can easily be overcome if a few simple assumptions are made [21]. It can be assumed that all captured carriers will reside in the lowest confined energy state. This means that the experimental conditions are the same as during the PL experiment. It can also be assumed that the quasi-Fermi level in the QWs lies several kT above the confined heavy hole state, and will not affect the carrier dynamics during the emission process [22]. Hence, the emission rate for the holes out of the wells depends only upon the difference between the confined hole state energy and the valence band in the Si barrier. It has been reported that at low temperatures (p100 K) and relatively modest electric fields, X30 kVcm1, a plateau can be observed in the DLTS due to a constant emission rate of carriers out of QWs. This is attributed to tunnelling through a triangular potential barrier between the QW and the relevant carrier band [22]. Fig. 2 shows the MCTS spectrum of the Si/SiGe QW, in two halves for clarity. As an example of a point defect in this system, Fig. 3 shows the LMCTS spectra measured between 30 and 38 K, and shows that two emission rates are present. An Arrhenius plot of these two states, taken from the LDLTS data, yields activation energies of 5073 and 3573 meV. Both defects exhibit exponential filling, hence can be attributed to point defects. No Poole–Frenkel effect was observed. The broad feature in Fig. 2 between 85 and 100 K is a strong candidate for the QW signal. The feature is
Fig. 2. MCTS spectrum of the Si0.855Ge0.145/Si MQW sample, spectrum in two halves for clarity, at a rate window of 200 s1. The feature proposed to be due to hole tunnelling out of the wells is indicated.
almost plateau-like, in accordance with Ref. [22], and no other features in the MCTS spectrum have an activation energy of 110 meV. Fig. 4 shows the emission rate of this deep state between 100 and 112.3 K, and it is nearly constant. The best fit of the emission rate to temperature was by assuming a square root dependence upon the temperature, rather than a Boltzmann relationship. Other defects in the LMCTS spectrum of this sample do not exhibit this behaviour, and we therefore conclude that we are observing direct evidence of holes tunnelling out of SiGe QWs into the cladding layers.
Fig. 3. Shows the LMCTS spectra of the Si0.855Ge0.145/Si MQW sample measured between 30 and 38 K.
Fig. 4. LMCTS of the QW-related state, showing the nearly constant emission rate between 100 and 112.3 K.
J.H. Evans-Freeman, M.A. Gad / Physica B 308–310 (2001) 554–557
4. Conclusions A technique which uses the same analytical approach to the capacitance transient as LDLTS but examines the detailed emission properties of minority carrier traps, LMCTS, has been described. In LMCTS, minority carriers are injected into a depletion region by the application of a light pulse of suitable wavelength. LDLTS and LMCTS are complementary because traps in the whole of the band gap can be accessed in semiconductors of one polarity. To verify the technique, LMCTS in n-type Si containing Au : H complexes was compared to LDLTS of p-type Si containing the same complexes, and found to yield the same results. The LMCTS technique was then applied to n-type Si which contains 10 Si0.855Ge0.145 strained QWs. Several hole traps were observed, but it was possible to identify a candidate for hole emission out of the QWs. When examined by LMCTS over a temperature range of 102–116 K, it was found that the emission rate for holes out of the QWs was only weakly dependent upon temperature. This indicates that this defect is not pointdefect-like, and it is suggested that the nearly constant emission rate is due to holes tunnelling out of the QW. This is the first direct observation of this effect, which up till now could only be inferred from conventional DLTS spectra.
Acknowledgements We acknowledge the EPSRC, UK for funding, and J Zhang at Imperial College for growing the MBE layer.
References [1] D.V. Lang, J. Appl. Phys. 45 (1974) 3023. [2] B. Hamilton, A.R. Peaker, D.R. Wight, J. Appl. Phys. 50 (1979) 6373. . Sveinbjornsson, . . Phys. Rev. B 52 (1995) [3] E.O. O. Engstrom, 4884.
557
[4] J.A. Davidson, J.H. Evans, Semicond. Sci. Technol. 11 (1996) 1704. [5] R.H. Wu, A.R. Peaker, Solid State Electron. 25 (1982) 643. [6] J.A. Davidson, J.H. Evans, J. Appl. Phys. 81 (1997) 251. [7] L. Dobaczewski, P. Kaczor, I.D. Hawkins, A.R. Peaker, J. Appl. Phys. 76 (1994) 194. [8] J. Kang, H. Zhan, Q. Huang, J. Crystal Growth 210 (2000) 247. [9] L. Dobaczewski, P. Kaczor, M. Missous, A.R. Peaker, Z.R. Zytkiewicz, Phys. Rev. Lett. 68 (1992) 2508. [10] P. Deixler, J. Terry, I.D. Hawkins, J.H. Evans-Freeman, A.R. Peaker, L. Rubaldo, D.K. Maude, J.-C. Portal, L. Dobaczewski, K. Bonde Nielsen, A. Nylandsted Larsen, A. Mesli, Appl. Phys. Lett. 73 (1998) 3126. [11] L. Rubaldo, P. Deixler, I.D. Hawkins, J. Terry, D.K. Maude, J.-C. Portal, J.H. Evans-Freeman, L. Dobaczewski, A.R. Peaker, Mat. Sci. Eng. B 58 (1999) 126. [12] A.R. Peaker, J.H. Evans-Freeman, L. Rubaldo, I.D. Hawkins, L. Dobaczewski, K. Vernon-Parry, Physica B 273–274 (1999) 243. [13] L. Dobaczewski, K. Goscinski, K. Bonde Nielsen, A.N. Larsen, J.L. Hansen, A.R. Peaker, Phys. Rev. Lett. 83 (1999) 4582. [14] K.B. Nielsen, L. Dobaczewski, K. Goscinski, R. Bendesen, O. Andersen, B. Bech Nielsen, Physica B 273–274 (1999) 167. [15] L. Dobaczewski, K.B. Nielsen, K. Goscinski, A.R. Peaker, A. Nylandsted Larsen, Physica B 273–274 (1999) 620. [16] L. Dobaczewski, K.B. Nielsen, K. Goscinski, O. Andersen, Acta Phys. Pol. A 98 (2000) 231. [17] R. People, J.C. Bean, Appl. Phys. Lett. 48 (1986) 538. [18] J.C. Sturm, H. Manoharan, L.C. Lechyshyn, M.L.W. Thewalt, N.L. Rowell, J.-P. No.el, D.C. Houghton, Phys. Rev. Lett. 66 (1991) 1362. [19] E.F. Gross, N.S. Sokolov, A.N. Titkov, Sov. Phys. Solid State 14 (1973) 1732. [20] D.C. Houghton, N.L. Rowell, J.-P. No.el, G. Aers, M. Davies, A. Wang, D.D. Perovic, Silicon-Based Optoelectronic Materials Symposium, Mater. Res. Soc. Vol. 298, 1993, p. 3. [21] L. Vescan, R. Apetz, H. Luth, . J. Appl. Phys. 73 (1993) 7427. [22] X. Letartre, D. Sti!evenard, M. Lannoo, E. Barbier, J. Appl. Phys. 69 (1991) 7336.