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GaAs structures grown by MBE
Materials Science and Engineering B69 – 70 (2000) 514 – 518 www.elsevier.com/locate/mseb
Optical characterisation of InAs/GaAs structures grown by MBE M. Hjiri *, F. Hassen, H. Maaref De´partement de Physique, Laboratoire de Physique des Semiconducteurs, Faculte´ des Sciences de Monastir, 5000 Monastir, Tunisia
Abstract In the present work we have investigated the optical properties of the strained InAs/GaAs submonolayers. The InAs thickness varies from 0.5 to 2 atomic monolayers (ML). Samples are grown by molecular beam epitaxy (MBE) on (001) GaAs substrates and characterised by photoluminescence spectroscopy (PL). The PL spectrum of 2ML’s sample exhibits only one broad line which comes from 3D structures and shows the presence of self-organised quantum dots. Thinner sample shows responses of two PL bands. The first one, which shows a red shift for increasing the In amount, is associated to the luminescence from 2D structure. The second one at 1.36 eV, not sensitive to the InAs amount, is accompanied by a shoulder at 1.32 eV. These two lines appear when the excitation energy is near the carbon absorption (e-A°) in GaAs. Their exact natures are not yet known and are associated to the radiative recombination on Cu impurity in semi-insulating GaAs substrate and its satellite phonon replica. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Optical properties; InAs/GaAs structures; Molecular beam epitaxy
1. Introduction
2. Experimental results and discussion
In recent years, quantum dot structures have attracted increasing interest in basic and applied physics. Introducing quantum dots (QDs) in laser structures induces a decrease in threshold current, an increase in differential efficiency and better stability than the quantum well (QW) lasers versus temperature [1,2]. Many works have been published, including theoretical studies, optical and electrical characterisations [3– 5], on the self-organised QDs grown by molecular beam epitaxy (MBE) using the Stranski – Krastanov process. It was shown that, in InAs/GaAs structures on (001) GaAs substrate, the 2D 3D growth mode transition appears after a deposition of 1.7 atomic monolayer (ML) of InAs [6– 9]. In this paper, we report results obtained by photoluminescence (PL) on InAs/GaAs structures. The emission from InAs submonolayers thicknesses in the infrared region will be associated to radiative transitions on localised states on the copper centre which acts as an acceptor in the GaAs substrate and not to the quantum dashes as it has been reported [10], nor to the localised states in the InAs/GaAs interface [11].
Samples used in this study are grown by solid source MBE on GaAs semi-insulating (001) substrate. A buffer layer of undoped GaAs was grown at 580°C. The substrate temperature is then reduced to 520°C over 2 min for growth of the InAs layer which is grown between two 200-A, thick layers of GaAs. The temperature is then increased to 580°C and 300 A, GaAs is grown. Samples A, B, C and D have, respectively, 0.5, 1, 1.5 and 2 atomic monolayers of InAs thickness. The photoluminescence experiments have been performed using Ar + pumped sapphir–titanium laser for the sample excitation. The sample sits into a gas flow cryostat operating in a controlled temperature range between 12 and 300 K. The PL emission is detected by S1 photo-multiplier using conventional lock-in technique. In Fig. 1, we show the PL lines from samples A, B and C. Those emissions are associated to the radiative recombinations in 2D regions (QW) and present a red shift by increasing the InAs layer thickness. The shoulder in the low energy side of the PL line from 0.5 ML sample is attributed to the radiative recombination from carbon centre in GaAs barrier. The theoretical calculations [12] assuming the InAs layer as one dimensional finite potential well are just used as an argument
* Corresponding author.
0921-5107/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 9 ) 0 0 2 4 8 - 2
M. Hjiri et al. / Materials Science and Engineering B69–70 (2000) 514–518
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for the identification of the luminescence lines at 1.5, 1.46 and 1.44 eV from 0.5, 1 and 1.5 ML samples, respectively. In the mentioned positions we have used a band offset Qc =0.67 [13]. In Fig. 1c, the PL line is broader than those of Fig. 1a and b. This behaviour can be explained as follows: 1.5 ML of InAs is very close to the critical thickness (1.7 ML) for the 2D 3D growth mode transition and by a random fluctuation of the InAs amount we can allow the QD’s to nucleate in any region of the sample. So the InAs surface layer is not smooth and it presents some ondulations which affect the FWHM of the PL line. In the 2 ML InAs sample, the PL line is a broad peak (Fig. 2), with full width at half maximum (FWHM) about 112 meV, shifted to the low energy side compared to the theoretically predicted position assuming a 2D structure (1.43 eV). In this sample, the InAs layer
Fig. 2. Photoluminescence spectra of the sample D for different density of excitation. Experimental measurements (straight line) and Gaussians fit (dotted line), lines denoted G1 (E1) and G2 (E2) are associated to the luminescence from ground (first excited) states of the two size distributions, respectively.
Fig. 1. Photoluminescence spectra from sample A (0.5 ML), B (1 ML) and C (1.5 ML) excited at 1.517 eV and T=12 K.
width exceeds critical layer thickness associated to the 2D 3D growth mode transition [6–9]. This line can be associated to the radiative transitions in InAs quantum dots. The same results have been reported in QDs grown under the same conditions and MOCVD [14– 17]. The QD’s PL excitation density dependence has been studied, Fig. 2, and shows that the low energy side of the PL band does not shift versus the density of excitation. The FWHM does not show any drastic
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change (1129 4) meV. The high energy side shows a sliding and some shoulders appear by increasing the excitation density. The same results had been reported on CdSe QDs [18], on InGaAs and InAlAs QDs [19] where the PL versus the density of excitation has been reported and the new peaks growing up in the high energy side are attributed to the excited states in such structures. To understand the origin of the luminescence bands (from the ground and excited states of one dominant size distribution or from ground states in multimodal sizes of QDs) in the sample D, a deconvolution of the luminescence spectra for each power density is used and shows that: (i) for the low density, the spectrum can be excellently fitted by two Gaussians separated by 50 meV; (ii) by increasing the power excitation, the number of Gaussians used to fit the experimental results increases from 2 to 4 Gaussians. Those results show that for low-density excitation the luminescence comes from the ground states of two dominant size distributions of QDs. These results are in agreement with those obtained by STM [20] on the
Fig. 4. PL intensity versus excitation density in sample B: square, from 2D region ‘QW’, circle: from peak ‘L1’ and triangle: from peak ‘L2’.
Fig. 3. PL spectra of sample A, B and C depicted in the infrared region excited at 1.501 eV and T= 12 K.
same kind of samples. For high excitation, we can conclude that the luminescence results from the radiative recombination on the ground states and the first excited states [21] of the two QD’s size distributions. From this result, we can deduce that the first excited states are (7895) meV above the ground state for each distribution. This value is very close to that measured and estimated theoretically, 70 [22] and 74 meV [23] in the same kind of samples. In addition to the PL band from QW structure discussed above, spectra of sample A, B and C, where the nominal InAs amount is less than the 2D3D growth mode transition thickness, different PL lines have been observed in the infrared region (in the low energy side of the line associated to the 2D structure). Those emissions (Fig. 3) are composed of two PL lines. Their positions are not affected by the InAs amount. It is important to note that those peaks are well pronounced when the excitation energy is fixed lower than the barrier energy band gap and near the energy of the carbon absorption (1.49 eV). In Fig. 4, the PL intensity versus the density of excitation for the three peaks is depicted, 2D region response one at 1.46 eV and those labelled ‘L1’ and ‘L2’ at 1.36 and 1.32 eV, respectively, obtained in sample B.
M. Hjiri et al. / Materials Science and Engineering B69–70 (2000) 514–518
For the excitation energy of 1.502 eV, the QW PL peak does not show any saturation with the increasing of the power density. We take this behaviour as a fingerprint of the infinite number of states in the 2D structure. The PL bands denoted ‘L1’ and ‘L2’, which keep the same positions in the three samples, show opposite behaviours versus the density of excitation. The ‘L1’ band PL intensity increases by increasing the density of excitation but that of ‘L2’ band seems to be saturated. The integrated PL intensity of those two bands vanishes rapidly when the temperature increases and they disappear above 45 K. The nature of those lines are not yet well known. They are associated: (i) to the luminescence from the InAs aggregated structures if we believe that the InAs QDs nucleation is a continuous phenomenon versus InAs deposited amount as reported by PL measurements on the thin InAs/GaAs submonolayer [10] or by AFM imaging [24] where they show the presence of InAs wire-like island structures for 1 and 1.5 ML of InAs; (ii) to the localised states in the InAs/GaAs interface induced by the strain as reported on InP/GaP structures [11], the interfacial localised states can only exist on coherently strained systems like InAs/GaAs, InP/GaP; (iii) to the radiative transition on the copper centre, which acts as an acceptor in semi-insulating GaAs substrate. In Refs. [25,26] the Cu centre is shown, by DLTS spectroscopy and photoluminescence, to suggest bound states at 0.15 eV from the GaAs valence band. PL measurements performed on the substrates, which are used to grow the InAs samples used in this study, for the same excitation-energy, near the carbon absorption in GaAs, show the same peaks ‘L1’ and ‘L2’ observed in sample A, B and C. The PL intensity, in the GaAs substrate response, versus temperature and the density of excitation have the same behaviour as reported above. We can conclude that these lines, ‘L1’ and ‘L2’, are not related to the InAs layer and submonolayers in our samples, they are not associated to the localised states in the InAs/GaAs interfaces as reported [11] and not to the nucleated QDs in InAs submonolayers [10] because their energetic positions are not affected by the InAs amount. Line ‘L1’ could be attributed to the radiative recombination on the Cu centre in GaAs substrate [25]. ‘L2’, at 1.32 eV, is 040 meV below the ‘L1’ transition and can be identified as the satellite phonon LO replica. The contamination of Cu may affect device characteristics on GaAs MESFETs [25] by causing a change of carrier concentration in the channel. On another hand, it induces some confusions on the intrinsic optical properties of InAs/ GaAs self-organised quantum dots. It can affect the FWHM and the position of QD’s PL bands. When the carbon is excited, at low temperature, electrons localised on Cu centre relax their energy by going down to the carbon level and give rise to a
517
radiative transition from the GaAs conduction band to the empty states bound to the Cu level. As the temperature increases, the relaxation of electrons from the Cu level to the carbon level becomes less important and leads to decrease in the PL intensity observed in sample A, B and C. The PL intensity of line ‘L1’ increases when we increase the density of excitation, because the relaxation of electrons from Cu level down to that of the carbon centre is a continuous phenomenon at fixed temperature and leads to an increase in the density of the empty states on the Cu centre. If the excitation energy is not resonant with the carbon absorption, the bound states density created on the carbon centre is very weak and lead a slight electron relaxation from Cu to carbon level resulting in the remarkable reduction or the absence of the PL intensity at 1.36 and 1.32eV.
3. Conclusion In conclusion, InAs/GaAs monolayers and submonolayers are studied by photoluminescence. QDs appear only when the InAs amount exceeds the critical thickness of the 2D 3D growth mode transition. In submonolayer samples, we have observed two new luminescence lines, ‘L1’ and ‘L2’, at 1.36 and 1.32 eV, respectively, when the excitation energy is fixed near the carbon absorption in GaAs substrate. These lines are attributed to the electronic radiative transition from the GaAs conduction band to the bound states localised on the Cu level, which acts as an acceptor, in GaAs substrate. By increasing the temperature or exciting the sample far from the carbon absorption the electronic relaxation from Cu level to carbon level will be very weak and the PL intensity of these lines decrease drastically. Those two lines are not associated to the luminescence from the localised states in the InAs/GaAs interface induced by the coherent strain and not attributed to the transition in the nucleated QDs in thin InAs/GaAs submonolayers because they have been observed in GaAs substrate used for the growth of our samples. It is very important for the studies of the intrinsic optical properties of InAs/GaAs QDs to avoid the luminescence of copper in GaAs substrate. This can be reached by exciting the structure less than the carbon absorption or by reducing the Cu concentration in the substrate.
References [1] Y. Arkawa, H. Sakaki, Appl. Phys. Lett. 40 (1982) 939. [2] A. Moritz, R. Wirth, A. Hangleiter, A. Kurtenbach, K. Eberl, Appl. Phys. Lett. 69 (1996) 212. [3] U. Bockelmann, Phys. Rev. B 48 (1993) 17637.
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[4] J.Y. Marzin, G. Bastard, Solid State Commun. 92 (1994) 437. [5] M. Sopanen, H. Lipsanen, J. Ahopelto, Appl. Phys. Lett. 66 (1995) 2364. [6] L. Bursaferri, S. Sanguiretti, E. Grilli, M. Guzzi, A. Bignazzi, F. Bogani, L. Carraresi, M. Collocci, Appl. Phys. Lett. 69 (1996) 3354. [7] F. Bogani, L. Carraresi, R. Mattolini, M. Collocci, A. Bosacchi, S. Franchi, Solid State Electron. 40 (1996) 363. [8] D. Leonard, K. Pond, P.M. Petroff, Phys. Rev. B 50 (1994) 11687. [9] J.M. Ge´rard, Appl. Phys. Lett. 61 (1996) 2096. [10] A. Polimeni, A. Patane´, M. Capizzi, F. Marterilli, L. Nasi, G. Salviati, Phys. Rev. B 53 (1996) R4213. [11] A.J. Williamson, A. Zunger, A. Canning, Phys. Rev. B 57 (1998) R4253. [12] C. Cohen-Tannoudji, B. Diu, F. Laloe¨, Me´canique Quantique, Hermann Editions, Paris, 1992. [13] K.J. Moor, G. Duggan, K. Woodbridge, C. Roberts, Phys. Rev. B 41 (1990) 1095. [14] D. Leonard, M. Krishnamurthy, S. Farad, J.L. Merz, P.M. Petroff, J. Vac. Sci. Technol. B12 (1994) 1063. [15] G.E. Tsyrlin, V.N. Petrov, M.V. Maksimov, N.N. Ledentsov, Semiconductors 31 (1997) 777. [16] M. Grundmann, J. Christin, N.N. Ledentsov, J. Bo¨hrer, D. Bimberg, S.S. Ruvimov, P. Werner, U. Richter, U. Go¨sele, S.
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[24]
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Heydenreich, V.M. Ustinov, A.Yu. Egorov, A.E. Zhukov, P.S. Kopiev, Zh.I. Alferov, Phys. Rev. Lett. 74 (1995) 4043. J.Y. Marzin, J.M. Ge´rard, A. Izrae¨l, D. Berrier, G. Bastard, Phys. Rev. Lett. 73 (1994) 716. O. Wind, F. Gindele, U. Woggon, C. Klingshirn, Crystal Growth 159 (1996) 567. S. Farfad, R. Leon, D. Leonard, J.L. Merz, P.M. Petroff, Phys. Rev. B 57 (1995) 5752. M. Henini, S. sanguinetti, L. Brusaferri, E. Grilli, M. Guzzi, M.D. Upward, P. Moriarty, P.H. Beton, Microelectron. J. 28 (1997) 933. L. Hao, L.W. Roger, J.J. Thomas, Y. Weidong, C.S. Peter, Appl. Phys. Lett. 24 (1998) 3556. M. Kohki, O. Nobuyuki, S. Hajime, S. Mutsuru, Appl. Phys. Lett. 68 (1996) 3013. M.J. Steer, D.J. Mowbray, W.R. Tribe, M.S. Skolnick, M.D. Strge, M. Hopkinson, A.G. Cullis, C.R. Whitehouse, R. Murray, Phys. Rev. B 54 (1996) 17738. G.E. Cirlin, G.M. Guryanov, A.O. Golubok, S.Ya. Tipissev, N.N. Ledentsov, P.S. Kop’ev, M. Grundmann, D. Bimberg, Appl. Phys. Lett. 67 (1995) 97. L. Montelius, L.S. Nilsson, L. Samuelson, in: G. Grossmann, L. Ledebo (Eds.), Semi-Insulating III – V Materials, Adam Hilger, Malmo, Sweden, 1988, p. 361. T. Hiramoto, T Ikoma, in: G. Grossmann, L. Ledebo (Eds.), Semi-Insulating III – V Materials, Adam Hilger, Malmo, Sweden, 1988, p. 337.
Optical characterisation of InAs/GaAs structures grown by MBE M. Hjiri *, F. Hassen, H. Maaref De´partement de Physique, Laboratoire de Physique des Semiconducteurs, Faculte´ des Sciences de Monastir, 5000 Monastir, Tunisia
Abstract In the present work we have investigated the optical properties of the strained InAs/GaAs submonolayers. The InAs thickness varies from 0.5 to 2 atomic monolayers (ML). Samples are grown by molecular beam epitaxy (MBE) on (001) GaAs substrates and characterised by photoluminescence spectroscopy (PL). The PL spectrum of 2ML’s sample exhibits only one broad line which comes from 3D structures and shows the presence of self-organised quantum dots. Thinner sample shows responses of two PL bands. The first one, which shows a red shift for increasing the In amount, is associated to the luminescence from 2D structure. The second one at 1.36 eV, not sensitive to the InAs amount, is accompanied by a shoulder at 1.32 eV. These two lines appear when the excitation energy is near the carbon absorption (e-A°) in GaAs. Their exact natures are not yet known and are associated to the radiative recombination on Cu impurity in semi-insulating GaAs substrate and its satellite phonon replica. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Optical properties; InAs/GaAs structures; Molecular beam epitaxy
1. Introduction
2. Experimental results and discussion
In recent years, quantum dot structures have attracted increasing interest in basic and applied physics. Introducing quantum dots (QDs) in laser structures induces a decrease in threshold current, an increase in differential efficiency and better stability than the quantum well (QW) lasers versus temperature [1,2]. Many works have been published, including theoretical studies, optical and electrical characterisations [3– 5], on the self-organised QDs grown by molecular beam epitaxy (MBE) using the Stranski – Krastanov process. It was shown that, in InAs/GaAs structures on (001) GaAs substrate, the 2D 3D growth mode transition appears after a deposition of 1.7 atomic monolayer (ML) of InAs [6– 9]. In this paper, we report results obtained by photoluminescence (PL) on InAs/GaAs structures. The emission from InAs submonolayers thicknesses in the infrared region will be associated to radiative transitions on localised states on the copper centre which acts as an acceptor in the GaAs substrate and not to the quantum dashes as it has been reported [10], nor to the localised states in the InAs/GaAs interface [11].
Samples used in this study are grown by solid source MBE on GaAs semi-insulating (001) substrate. A buffer layer of undoped GaAs was grown at 580°C. The substrate temperature is then reduced to 520°C over 2 min for growth of the InAs layer which is grown between two 200-A, thick layers of GaAs. The temperature is then increased to 580°C and 300 A, GaAs is grown. Samples A, B, C and D have, respectively, 0.5, 1, 1.5 and 2 atomic monolayers of InAs thickness. The photoluminescence experiments have been performed using Ar + pumped sapphir–titanium laser for the sample excitation. The sample sits into a gas flow cryostat operating in a controlled temperature range between 12 and 300 K. The PL emission is detected by S1 photo-multiplier using conventional lock-in technique. In Fig. 1, we show the PL lines from samples A, B and C. Those emissions are associated to the radiative recombinations in 2D regions (QW) and present a red shift by increasing the InAs layer thickness. The shoulder in the low energy side of the PL line from 0.5 ML sample is attributed to the radiative recombination from carbon centre in GaAs barrier. The theoretical calculations [12] assuming the InAs layer as one dimensional finite potential well are just used as an argument
* Corresponding author.
0921-5107/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 9 ) 0 0 2 4 8 - 2
M. Hjiri et al. / Materials Science and Engineering B69–70 (2000) 514–518
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for the identification of the luminescence lines at 1.5, 1.46 and 1.44 eV from 0.5, 1 and 1.5 ML samples, respectively. In the mentioned positions we have used a band offset Qc =0.67 [13]. In Fig. 1c, the PL line is broader than those of Fig. 1a and b. This behaviour can be explained as follows: 1.5 ML of InAs is very close to the critical thickness (1.7 ML) for the 2D 3D growth mode transition and by a random fluctuation of the InAs amount we can allow the QD’s to nucleate in any region of the sample. So the InAs surface layer is not smooth and it presents some ondulations which affect the FWHM of the PL line. In the 2 ML InAs sample, the PL line is a broad peak (Fig. 2), with full width at half maximum (FWHM) about 112 meV, shifted to the low energy side compared to the theoretically predicted position assuming a 2D structure (1.43 eV). In this sample, the InAs layer
Fig. 2. Photoluminescence spectra of the sample D for different density of excitation. Experimental measurements (straight line) and Gaussians fit (dotted line), lines denoted G1 (E1) and G2 (E2) are associated to the luminescence from ground (first excited) states of the two size distributions, respectively.
Fig. 1. Photoluminescence spectra from sample A (0.5 ML), B (1 ML) and C (1.5 ML) excited at 1.517 eV and T=12 K.
width exceeds critical layer thickness associated to the 2D 3D growth mode transition [6–9]. This line can be associated to the radiative transitions in InAs quantum dots. The same results have been reported in QDs grown under the same conditions and MOCVD [14– 17]. The QD’s PL excitation density dependence has been studied, Fig. 2, and shows that the low energy side of the PL band does not shift versus the density of excitation. The FWHM does not show any drastic
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change (1129 4) meV. The high energy side shows a sliding and some shoulders appear by increasing the excitation density. The same results had been reported on CdSe QDs [18], on InGaAs and InAlAs QDs [19] where the PL versus the density of excitation has been reported and the new peaks growing up in the high energy side are attributed to the excited states in such structures. To understand the origin of the luminescence bands (from the ground and excited states of one dominant size distribution or from ground states in multimodal sizes of QDs) in the sample D, a deconvolution of the luminescence spectra for each power density is used and shows that: (i) for the low density, the spectrum can be excellently fitted by two Gaussians separated by 50 meV; (ii) by increasing the power excitation, the number of Gaussians used to fit the experimental results increases from 2 to 4 Gaussians. Those results show that for low-density excitation the luminescence comes from the ground states of two dominant size distributions of QDs. These results are in agreement with those obtained by STM [20] on the
Fig. 4. PL intensity versus excitation density in sample B: square, from 2D region ‘QW’, circle: from peak ‘L1’ and triangle: from peak ‘L2’.
Fig. 3. PL spectra of sample A, B and C depicted in the infrared region excited at 1.501 eV and T= 12 K.
same kind of samples. For high excitation, we can conclude that the luminescence results from the radiative recombination on the ground states and the first excited states [21] of the two QD’s size distributions. From this result, we can deduce that the first excited states are (7895) meV above the ground state for each distribution. This value is very close to that measured and estimated theoretically, 70 [22] and 74 meV [23] in the same kind of samples. In addition to the PL band from QW structure discussed above, spectra of sample A, B and C, where the nominal InAs amount is less than the 2D3D growth mode transition thickness, different PL lines have been observed in the infrared region (in the low energy side of the line associated to the 2D structure). Those emissions (Fig. 3) are composed of two PL lines. Their positions are not affected by the InAs amount. It is important to note that those peaks are well pronounced when the excitation energy is fixed lower than the barrier energy band gap and near the energy of the carbon absorption (1.49 eV). In Fig. 4, the PL intensity versus the density of excitation for the three peaks is depicted, 2D region response one at 1.46 eV and those labelled ‘L1’ and ‘L2’ at 1.36 and 1.32 eV, respectively, obtained in sample B.
M. Hjiri et al. / Materials Science and Engineering B69–70 (2000) 514–518
For the excitation energy of 1.502 eV, the QW PL peak does not show any saturation with the increasing of the power density. We take this behaviour as a fingerprint of the infinite number of states in the 2D structure. The PL bands denoted ‘L1’ and ‘L2’, which keep the same positions in the three samples, show opposite behaviours versus the density of excitation. The ‘L1’ band PL intensity increases by increasing the density of excitation but that of ‘L2’ band seems to be saturated. The integrated PL intensity of those two bands vanishes rapidly when the temperature increases and they disappear above 45 K. The nature of those lines are not yet well known. They are associated: (i) to the luminescence from the InAs aggregated structures if we believe that the InAs QDs nucleation is a continuous phenomenon versus InAs deposited amount as reported by PL measurements on the thin InAs/GaAs submonolayer [10] or by AFM imaging [24] where they show the presence of InAs wire-like island structures for 1 and 1.5 ML of InAs; (ii) to the localised states in the InAs/GaAs interface induced by the strain as reported on InP/GaP structures [11], the interfacial localised states can only exist on coherently strained systems like InAs/GaAs, InP/GaP; (iii) to the radiative transition on the copper centre, which acts as an acceptor in semi-insulating GaAs substrate. In Refs. [25,26] the Cu centre is shown, by DLTS spectroscopy and photoluminescence, to suggest bound states at 0.15 eV from the GaAs valence band. PL measurements performed on the substrates, which are used to grow the InAs samples used in this study, for the same excitation-energy, near the carbon absorption in GaAs, show the same peaks ‘L1’ and ‘L2’ observed in sample A, B and C. The PL intensity, in the GaAs substrate response, versus temperature and the density of excitation have the same behaviour as reported above. We can conclude that these lines, ‘L1’ and ‘L2’, are not related to the InAs layer and submonolayers in our samples, they are not associated to the localised states in the InAs/GaAs interfaces as reported [11] and not to the nucleated QDs in InAs submonolayers [10] because their energetic positions are not affected by the InAs amount. Line ‘L1’ could be attributed to the radiative recombination on the Cu centre in GaAs substrate [25]. ‘L2’, at 1.32 eV, is 040 meV below the ‘L1’ transition and can be identified as the satellite phonon LO replica. The contamination of Cu may affect device characteristics on GaAs MESFETs [25] by causing a change of carrier concentration in the channel. On another hand, it induces some confusions on the intrinsic optical properties of InAs/ GaAs self-organised quantum dots. It can affect the FWHM and the position of QD’s PL bands. When the carbon is excited, at low temperature, electrons localised on Cu centre relax their energy by going down to the carbon level and give rise to a
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radiative transition from the GaAs conduction band to the empty states bound to the Cu level. As the temperature increases, the relaxation of electrons from the Cu level to the carbon level becomes less important and leads to decrease in the PL intensity observed in sample A, B and C. The PL intensity of line ‘L1’ increases when we increase the density of excitation, because the relaxation of electrons from Cu level down to that of the carbon centre is a continuous phenomenon at fixed temperature and leads to an increase in the density of the empty states on the Cu centre. If the excitation energy is not resonant with the carbon absorption, the bound states density created on the carbon centre is very weak and lead a slight electron relaxation from Cu to carbon level resulting in the remarkable reduction or the absence of the PL intensity at 1.36 and 1.32eV.
3. Conclusion In conclusion, InAs/GaAs monolayers and submonolayers are studied by photoluminescence. QDs appear only when the InAs amount exceeds the critical thickness of the 2D 3D growth mode transition. In submonolayer samples, we have observed two new luminescence lines, ‘L1’ and ‘L2’, at 1.36 and 1.32 eV, respectively, when the excitation energy is fixed near the carbon absorption in GaAs substrate. These lines are attributed to the electronic radiative transition from the GaAs conduction band to the bound states localised on the Cu level, which acts as an acceptor, in GaAs substrate. By increasing the temperature or exciting the sample far from the carbon absorption the electronic relaxation from Cu level to carbon level will be very weak and the PL intensity of these lines decrease drastically. Those two lines are not associated to the luminescence from the localised states in the InAs/GaAs interface induced by the coherent strain and not attributed to the transition in the nucleated QDs in thin InAs/GaAs submonolayers because they have been observed in GaAs substrate used for the growth of our samples. It is very important for the studies of the intrinsic optical properties of InAs/GaAs QDs to avoid the luminescence of copper in GaAs substrate. This can be reached by exciting the structure less than the carbon absorption or by reducing the Cu concentration in the substrate.
References [1] Y. Arkawa, H. Sakaki, Appl. Phys. Lett. 40 (1982) 939. [2] A. Moritz, R. Wirth, A. Hangleiter, A. Kurtenbach, K. Eberl, Appl. Phys. Lett. 69 (1996) 212. [3] U. Bockelmann, Phys. Rev. B 48 (1993) 17637.
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