- Email: [email protected]
Al)As quantum dots on (100) and (N11) B InP substrates
Physica E 8 (2000) 164–169
www.elsevier.nl/locate/physe
Optical properties of self-assembled ternary In(GA= Al)As quantum dots on (1 0 0) and (N 1 1)B InP substrates Sun Zhong-Zhe ∗ , Liu Feng-Qi, Wu Ju, Ye Xiao-Ling, Ding Ding, Xu Bo, Liang Ji-Ben, Wang Zhan-Guo Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, People’s Republic of China
Abstract In this paper, we investigated the self-assembled quantum dots formed on (1 0 0) and (N 1 1)B (N = 2; 3; 4; 5) InP substrates by molecular beam epitaxy (MBE). Two kinds of ternary QDs (In0:9 Ga0:1 As and In0:9 Al0:1 As QDs) are grown on the above substrates; Transmission electron microscopy (TEM) and photoluminescence (PL) results con rm QDs formation for all samples. The PL spectra reveal obvious di erences in integral luminescence, peak position, full-width at half-maximum and peak shape between di erent oriented surfaces. Highest PL integral intensity is observed from QDs on (4 1 1)B surfaces, which shows a potential for improving the optical properties of QDs by using high-index surface. ? 2000 Elsevier Science B.V. All rights reserved. PACS: 81.05.Ea; 78.55.Cr; 78.66.Fd Keywords: Self-assembled quantum dots; InP substrate; High index; In(Ga; Al)As=InAlAs=InP; MBE
1. Introduction During the last years, the fabrication of selfassembled quantum dots (QDs) by Stranski– Krastanow (S–K) mode growth [1] has attracted much attention both for basic physics and for optoelectronic device applications. Self-assembled quantum dots have been successfully fabricated with many di erent material combinations; such as, GaAs-based QD: AlInAs=AlGaAs [2] of visible emission and In(Ga)As=GaAs [3,4], GaSb=GaAs [5], ∗
Corresponding author. E-mail address: [email protected] (Sun Zhong-Zhe).
InP=InGaP [6] of near-infrared emission. InP-based QD: In(Ga=Al)As=InP [7–9] of 1.3–2 m emission; quantum dots laser based on GaAs [10,11] and InP substrates [12] have been realized. Besides extensive studies concentrated on selfassembled QDs on (1 0 0) oriented substrates, some works focused on QDs formation on non-(1 0 0) surfaces. It is well known that substrate orientation has a large impact on the optical and structural properties of the self-assembled QDs formed on these surfaces because the di erent substrate surfaces provide different chemical potentials for the deposited species thus a ecting the kinetics of adsorption, migration and desorption processes, and the particular substrate
1386-9477/00/$ - see front matter ? 2000 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 9 4 7 7 ( 0 0 ) 0 0 1 3 4 - X
Z.-Z. Sun et al. / Physica E 8 (2000) 164–169
165 ◦
orientation and reconstruction determine the strain relaxation mechanism. Studying QDs formation on non-(1 0 0) surface is expected to provide a better understanding of self-assemble growth kinetics and improve the structural and optical properties of QDs. Today, QD formation on many high-index surface have been studied [13–20], and, QDs formed on some surfaces exhibit good structural and=or optical properties that are comparable or superior to dots on (1 0 0) surface. For example, high-quality ordering InGaAs quantum disks are fabricated on GaAs (3 1 1)B substrates by MOVPE [15,16] and narrow line width (13 meV at 10 K), strong photoluminescence emission is observed in these strained quantum disks [16]; Fortina et al. [17] report the InAs QDs formation on (N 1 1)A=B (N = 2; 3; 4; 5) GaAs substrates and PL eciency of those low-coverage samples is found to be comparable to that of QDs grown on (1 0 0) surface. In another study [18] of InGaAs=GaAs QDs formed on (0 0 1) and (N 1 1)B (N = 1; 2; 3; 5; 7) surfaces, the highest quantum eciency is observed from QDs on (7 1 1)B substrate. As far as QDs on high-index InP surfaces are concerned, Notzel et al. [19] investigated In0:8 Ga0:2 As QDs formed on InP(3 1 1)A=B substrates by MOCVD. Well-ordered QDs array is observed on (3 1 1)B surface similar to the results from the GaAs substrate [15]. Nishi et al. report the achievement of lasing from InAs QDs on InP(3 1 1)B substrate [20]. In this paper, we investigated the self-assembled quantum dots formed on (1 0 0) and (N 1 1)B (N = 2; 3; 4; 5) InP substrates by molecular beam epitaxy (MBE). Two kinds of ternary QDs (In0:9 Ga0:1 As and In0:9 Al0:1 As QDs) are grown on the above substrates; TEM and PL results con rm QDs formation for all samples with di erent orientation. The PL spectra reveal obvious di erences in peak position, line width, intensity and peak shape between di erent oriented surfaces. In addition, QDs formed on (1 0 0) and (4 1 1)B surfaces give strong narrow luminescence, and, to our interest, both In0:9 Ga0:1 As and In0:9 Al0:1 As dots on (4 1 1)B surface is found to give stronger luminescence than dots on (1 0 0) surface.
following orientations: (1 0 0) ± 0:5 ; (N 1 1)B ± ◦ 0:5 (N = 2; 3; 4; 5). The epilayer structure consists of ∼300 nm lattice-matched In0:52 Al0:48 As bu er layer, the desired dot layer and ∼50 nm In0:52 Al0:48 As cap layer. Two kinds of ternary QDs (In0:9 Ga0:1 As and In0:9 Al0:1 As QDs) are grown on the above substrates. Each growth run is carried out simultaneously on ve di erent InP substrates mounted on the same Mo block side by side and substrate rotation is used to improve uniformity. During the growth, As4 partial pressure is kept at 1 × 10−7 T; substrate ◦ temperature is 500 C for In0:9 Al0:1 As deposition ◦ and 490 C for In0:9 Ga0:1 As deposition. The growth progress was in situ monitored by 30 kV re ection high-energy electron di raction (RHEED); during the deposition of strained dot layer, 2D–3D growth transition is indicated by RHEED change from a streaky to a spotty pattern. The critical thickness for the transition is found to be of 8–10 monolayers (ML) for both In0:9 Al0:1 As and In0:9 Ga0:1 As dot layer (∼ 2:6% lattice mis t). The 2D–3D growth transition for (InGa)As dot layer is observed slightly latter than that for (InAl)As dot layer, which may be caused by slight composition uctuation of dot layer and=or due to di erent di usion lengths for Ga and Al atoms. The nominal deposition thickness for In0:9 Ga0:1 As and In0:9 Al0:1 As dot layers are 16.5 and 13 ML, respectively. (The above monolayer values refer to the equivalent thickness on (1 0 0) surfaces.) After epitaxy growth, (1 0 0) samples are characterized by TEM measurement and low-temperature PL measurement is used for all samples. TEM observation is performed using a JEOL JEM 1000 microscope. PL experiment is performed on an IF120HR Fourier transform infrared spectrometer with the 514.5 nm excitation line of an Ar + ion laser source. The samples are cooled in a closed-cycle He cryostat; a cooled InSb detector and a CaF2 beam splitter are employed to record the spectra.
2. Experimental procedure
Fig. 1(a) and (b) shows the plan-view TEM images for the (0 0 1) samples with In0:9 Ga0:1 As (a) and In0:9 Al0:1 As (b) deposition. The coherent dot formation is evident through the characteristic dark=bright
Samples are grown by a Riber 32P MBE system on semi-insulating Fe-doped InP substrates with the
3. Results and discussion
166
Z.-Z. Sun et al. / Physica E 8 (2000) 164–169
Fig. 1. The plan-view TEM images for the (0 0 1) samples with In0:9 Ga0:1 As (a) and In0:9 Al0:1 As (b) deposition.
contrast, which re ects an inhomogeneous strain eld caused by the lattice deformation in a 3D island; regions of dark contrast correspond to In0:9 Ga0:1 As (or In0:9 Ga0:1 As) dots. It can be seen from the gure that most of the In0:9 Al0:1 As dots appear approximately round shaped; while In0:9 Ga0:1 As dots do not seem very regular; and are interconnecting due to higher dot layer coverage. We think this is the initial stage of QDs coalescence. Using the plan-view TEM results, the average lateral size of In0:9 Ga0:1 As dot is about 40 nm, while that of In0:9 Al0:1 As dot is about 80 nm; areal densities of In0:9 Ga0:1 As and In0:9 Al0:1 As dots are 2:2 × 1010 and 2:6 × 109 cm−2 , respectively.
Fig. 2(a) and (b) shows the low-temperature (15 K) PL spectra for all samples. Each spectrum is normalized to its maximum. It can be seen that all the spectra show a PL emission peak at lower energy (0.55 – 0.65 eV) and the spectra from (1 0 0) and (5 1 1)B samples give another obvious luminescence peak at higher energy site (around 0.9 –1.1 eV). Higher energy peaks in the spectra originate from wetting layer (WL) formed in S–K growth. Large full-width at half-maximum (FWHM) of them should be attributed to the un at wetting layer, which are composed of quantum wells with several monolayers thickness
uctuation. According to our calculation, the QW thickness deviation range corresponding to the peak site is 8–12 ML for (1 0 0) sample, which agrees with in situ RHEED observation for critical thickness mentioned above. (Moreover, calculation results also show luminescence from 2D wetting layer for our sample structures will not be less than ∼ 0:8 eV.) Then, luminescence peaks at lower energy (0.55 – 0.65 eV) should be attributed to QDs structure formed on each orientation; these PL results con rm QD formation for all samples with di erent orientation. It is listed in Table 1 that the peak positions (Ep ), FWHM, the integral intensity (IPL) of QDs peak for all samples. The integral intensity is normalized relative to those on (1 0 0) substrate. Fig. 3 shows the variation of peak positions and FWHM with di erent substrate orientation. It can be seen from Fig. 3 and Table 1 that, for both In0:9 Al0:1 As and In0:9 Ga0:1 As QDs samples, with changing the substrate orientation from (1 0 0) to (2 1 1)B, on an average (non-monotonic), the peak position red shift slightly and the FWHM increases obviously. Since FWHM of luminescence peak generally re ects the unavoidable size uctuations in the QD ensemble (inhomogeneous broadening), the increase of FWHM means a deterioration of QD size uniformity with changing substrate orientation from (1 0 0) to (2 1 1)B; while, for the red shift of QD peak, one possible explanation is that the substrate orientation a ects the shape and average size of QDs by means of surface strain elds and of the chemical potentials. Besides, some other e ects, such as buildup of piezoelectric eld, e ective mass anisotropy and In segregation could also be responsible for the peak shift of QDs formed on high-index surface. Such trend of peak shift is not similar to the published results from InAs QD on high-index
Z.-Z. Sun et al. / Physica E 8 (2000) 164–169
167
Fig. 2. The low-temperature (15 K) PL spectra for all samples with (a) In0:9 Ga0:1 As and (b) In0:9 Al0:1 As dot layer on each surface. Each spectrum is normalized to its maximum. Table 1 The peak positions (Ep ), FWHM, the integral intensity (IPL) of QDs peak for all samples. The integral intensity was normalized relative to those on (1 0 0) substrate Orientation
(1 0 0) (5 1 1)B (4 1 1)B (3 1 1)B (2 1 1)B
Angle
0 15.8 19.5 25.2 35.3
In0:9 Ga0:1 As dot
In0:9 Al0:1 As dot
Ep (eV)
FWHM (meV)
IPL
Ep (eV)
FWHM (meV)
IPL
0.633 0.639 0.617 0.616 0.607
40 115 80 119 144
1.000 0.123 1.906 0.136 0.258
0.636 0.574 & 0.648 0.603 0.611 0.587
64 128 75 120 177
1.000 1.253 4.866 0.145 0.030
GaAs surfaces [17], where blue shift is observed for (N 1 1)B surfaces whereas red shift is observed for (N 1 1)A ones. We think such di erence is attributed to the di erent material system or respective growth condition. In addition, QDs size distribution can also be re ected by the line shape of QDs luminescence peaks. It can be seen from Fig. 2 that most of the samples display asymmetric dot luminescence peaks, as a typical case, double-peaks structure of QD luminescence is observed for In0:9 Al0:1 As dots on (5 1 1)B surface. We measure the PL spectra of this sample under di erent excitation power density (Fig. 4), and a linear excita-
tion density dependence of two peaks is observed. So we believed that such double-peaks structure is caused by distinct dual-model of size distribution other than emission from excited state in the same QD. Such effect from multi-modal size distribution has been reported for QDs on both GaAs [13,21] and InP [22] substrate system. Fig. 5(a) and (b) gives the variation of PL integral intensity for In0:9 Ga0:1 As (a) and In0:9 Al0:1 As (b) dots with di erent orientation surfaces. In0:9 Ga0:1 As QDs formed on (4 1 1)B and (1 0 0) surfaces and In0:9 Al0:1 As QDs formed on (4 1 1)B, (5 1 1)B and (1 0 0) surfaces give relatively strong narrow lumi-
168
Z.-Z. Sun et al. / Physica E 8 (2000) 164–169
Fig. 3. Luminescence peak energy and FWHM of (a) In0:9 Ga0:1 As dots and (b) In0:9 Al0:1 As dots as a function of the tilt angle with respect to (1 0 0). Peak site is represented by circles (or squares) and the FWHM of each peak is indicated by the error bar. For In0:9 Al0:1 As dots on (5 1 1)B surface, the peak position is represented by the average of the two peak energy values (triangle).
nescence; while weaker PL emissions are observed on (2 1 1)B and (3 1 1)B orientations. Such di erence re ects di erent QD structure quality a ected by substrate orientation under our growth condition. It is worth noting that, for both In0:9 Ga0:1 As and In0:9 Al0:1 As dots, the highest integral intensity is from quantum dots grown on (4 1 1)B substrate, which is about 2–5 times higher than QDs grown on (1 0 0) plane or 1–2 orders of magnitude higher than that of QD on other high-index planes. In general, higher structure quality (defect-free QD density, size homogeneity and alignment ordering, etc.) is accounted for better optical property, just as the study on QDs on GaAs (3 1 1)B substrate [15,16]. Nevertheless, we cannot conclude that a higher size uniformity is achieved for (4 1 1)B samples because the luminescence peak of dots on (4 1 1)B surface is slightly wider than that of (1 0 0) samples; so, higher density of defect-free QDs on (4 1 1)B surface may be the main reason for the improvement of PL eciency. Therefore, higher density of defect-fee QDs may be
Fig. 4. Photoluminescence spectra with various excitation power densities for the sample with In0:9 Al0:1 As dots on (5 1 1)B substrate.
the main reason for high PL intensity from (4 1 1)B samples.
4. Conclusion In summary, we investigated the In0:9 Ga0:1 As and In0:9 Al0:1 As quantum dots formed on (1 0 0) and high-index InP substrates by MBE. TEM and PL results con rm the QDs formation for all samples. PL measurement reveals obvious di erence in optical properties caused by substrate orientation e ect. QDs formed on (4 1 1)B surface give the highest PL intensity, which show the existence of optimal substrate surface for improving QDs emission on InP substrate. More detailed investigations for these structures are still required to optimize growth parameters for device application and fully understand the self-assembling mechanism on non-(1 0 0) substrates.
Z.-Z. Sun et al. / Physica E 8 (2000) 164–169
Fig. 5. The PL eciency (i.e. PL integral intensity) from (a) In0:9 Ga0:1 As dots and (b) In0:9 Al0:1 As dots on di erent orientation surfaces.
Acknowledgements The authors would like to thank the nancial supports from the National Natural Science Foundation of China (NNSFC) and the National Advanced Materials Committee of China (NAMCC). References [1] D.J. Eaglemann, M. Cerullo, Phys. Rev. Lett. 64 (1990) 1943. [2] R. Leon, S. Fafard, D. Leonard, J.L. Merz, P.M. Petro , Appl. Phys. Lett. 67 (1995) 521. [3] D. Leonard, S. Fafard, K. Pond, Y.H. Zhang, J.M. Merz, P.M. Petro , J. Vac. Sci. Technol. B 12 (1994) 2516. [4] J.Y. Marzin, J.M. Gerald, A. Izrael, D. Barrier, G. Bastard, Phys. Rev. Lett. 73 (1994) 716.
169
[5] F. Hatami, N.N. Ledentsov, M. Grundmann, J. Bohrer, F. Heinrichsdor , M. Beer, D. Bimberg, S.S. Ruvimov, P. Werner, U. Gosele, J. Heydenreich, U. Richter, S.V. Ivanov, B.Y. Meltser, P.S. Kop’ev, Z. Alferov, Appl. Phys. Lett. 67 (1995) 656. [6] A. Kurtenbach, K. Eberl, T. Shitara, Appl. Phys. Lett. 66 (1995) 361. [7] H. Marchand, P. Desjardins, D. Guillon, J.-E. Paultre, Z. Bougrioua, R.Y.-F. Yip, R.A. Masut, Appl. Phys. Lett. 71 (1997) 527. [8] S. Fafard, Z. Wasilewski, J. McCa rey, S. Raymond, S. Charbonneau, Appl. Phys. Lett. 68 (1996) 991. [9] H. Li, J. Wu, B. Xu, J. Liang, Z. Wang, Appl. Phys. Lett. 72 (1998) 2123. [10] H. Ishikawa, H. Shoji, Y. Nakata, K. Mukai, M. Sugawara, M. Egawa, N. Otsuka, Y. Sugiyama, T. Futatsugi, N. Yokoyama, J. Vac. Sci. Technol. A 16 (1998) 794. [11] S. Fafard, K. Hinzer, S. Raymond, M. Dion, J. McCa rey, Y. Feng, S. Charbonneau, Science 274 (1996) 1350. [12] V.M. Ustinov, A.E. Zhukov, A.Yu. Egorov, A.R. Kovsh, S.V. Zaitsev, N.Yu. Gordeev, V.I. Kopchatov, N.N. Ledentsov, A.F. Tsatsul’nikov, B.V. Volovik, P.S. Kop’ev, Z.I. Alferov, S.S. Ruvimov, Z. Liliental-Weber, D. Bimberg, Electron. Lett. 34 (1998) 670. [13] M. Henini, S. Sanguinetti, L. Brusaferri, E. Grilli, M. Guzzi, M.D. Upward, P. Moriarty, P.H. Beton, Microelectronic J. 28 (1997) 933. [14] P.P. Gonzalez-Borrero, D.I. Lubyshev, E. Marega Jr., E. Petitprez, P. Basmaji, Superlattices Microstruct. 23 (1998) 365. [15] R. Notzel, T. Fukui, H. Hasegawa, T. Temmyo, T. Tamamura, Appl. Phys. Lett. 65 (1994) 2854. [16] R. Notzel, J. Temmyo, H. Kamada, T. Furuta, Appl. Phys. Lett. 65 (1994) 457. [17] S.C. Fortina, S. Sanguintti, E. Grilli, M. Guzzi, M. Henini, A. Polimeni, L. Eaves, J. Crystal Growth 187 (1998) 126. [18] D.I. Lubyshev, P.P. Gonzalez-Borrero, E. Marega Jr., E. Petitprez, P. Basmaji, J. Vac. Sci. Technol. B 14 (3) (1996) 2212. [19] R. Notzel, J. Temmyo, A. Kozen, T. Tamamura, T. Fukui, H. Hasegawa, Appl. Phys. Lett. 66 (1995) 2525. [20] K. Nishi, M. Yamada, T. Anan, A. Gomyo, S. Sugou, Appl. Phys. Lett. 73 (1998) 526. [21] M. Colocci, F. Bogani, L. Carraresi, R. Mattolini, A. Bosacchi, S. Franchi, P. Frigeri, S. Taddei, M. Rosa-Clot, Superlattices Microstruct. 22 (1997) 81. [22] F.-Q. Liu, Z.-G. Wang, J. Wu, B. Xu, W. Zhou, J.-J. Qian, J. Crystal Growth 197 (1999) 789.
www.elsevier.nl/locate/physe
Optical properties of self-assembled ternary In(GA= Al)As quantum dots on (1 0 0) and (N 1 1)B InP substrates Sun Zhong-Zhe ∗ , Liu Feng-Qi, Wu Ju, Ye Xiao-Ling, Ding Ding, Xu Bo, Liang Ji-Ben, Wang Zhan-Guo Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, People’s Republic of China
Abstract In this paper, we investigated the self-assembled quantum dots formed on (1 0 0) and (N 1 1)B (N = 2; 3; 4; 5) InP substrates by molecular beam epitaxy (MBE). Two kinds of ternary QDs (In0:9 Ga0:1 As and In0:9 Al0:1 As QDs) are grown on the above substrates; Transmission electron microscopy (TEM) and photoluminescence (PL) results con rm QDs formation for all samples. The PL spectra reveal obvious di erences in integral luminescence, peak position, full-width at half-maximum and peak shape between di erent oriented surfaces. Highest PL integral intensity is observed from QDs on (4 1 1)B surfaces, which shows a potential for improving the optical properties of QDs by using high-index surface. ? 2000 Elsevier Science B.V. All rights reserved. PACS: 81.05.Ea; 78.55.Cr; 78.66.Fd Keywords: Self-assembled quantum dots; InP substrate; High index; In(Ga; Al)As=InAlAs=InP; MBE
1. Introduction During the last years, the fabrication of selfassembled quantum dots (QDs) by Stranski– Krastanow (S–K) mode growth [1] has attracted much attention both for basic physics and for optoelectronic device applications. Self-assembled quantum dots have been successfully fabricated with many di erent material combinations; such as, GaAs-based QD: AlInAs=AlGaAs [2] of visible emission and In(Ga)As=GaAs [3,4], GaSb=GaAs [5], ∗
Corresponding author. E-mail address: [email protected] (Sun Zhong-Zhe).
InP=InGaP [6] of near-infrared emission. InP-based QD: In(Ga=Al)As=InP [7–9] of 1.3–2 m emission; quantum dots laser based on GaAs [10,11] and InP substrates [12] have been realized. Besides extensive studies concentrated on selfassembled QDs on (1 0 0) oriented substrates, some works focused on QDs formation on non-(1 0 0) surfaces. It is well known that substrate orientation has a large impact on the optical and structural properties of the self-assembled QDs formed on these surfaces because the di erent substrate surfaces provide different chemical potentials for the deposited species thus a ecting the kinetics of adsorption, migration and desorption processes, and the particular substrate
1386-9477/00/$ - see front matter ? 2000 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 9 4 7 7 ( 0 0 ) 0 0 1 3 4 - X
Z.-Z. Sun et al. / Physica E 8 (2000) 164–169
165 ◦
orientation and reconstruction determine the strain relaxation mechanism. Studying QDs formation on non-(1 0 0) surface is expected to provide a better understanding of self-assemble growth kinetics and improve the structural and optical properties of QDs. Today, QD formation on many high-index surface have been studied [13–20], and, QDs formed on some surfaces exhibit good structural and=or optical properties that are comparable or superior to dots on (1 0 0) surface. For example, high-quality ordering InGaAs quantum disks are fabricated on GaAs (3 1 1)B substrates by MOVPE [15,16] and narrow line width (13 meV at 10 K), strong photoluminescence emission is observed in these strained quantum disks [16]; Fortina et al. [17] report the InAs QDs formation on (N 1 1)A=B (N = 2; 3; 4; 5) GaAs substrates and PL eciency of those low-coverage samples is found to be comparable to that of QDs grown on (1 0 0) surface. In another study [18] of InGaAs=GaAs QDs formed on (0 0 1) and (N 1 1)B (N = 1; 2; 3; 5; 7) surfaces, the highest quantum eciency is observed from QDs on (7 1 1)B substrate. As far as QDs on high-index InP surfaces are concerned, Notzel et al. [19] investigated In0:8 Ga0:2 As QDs formed on InP(3 1 1)A=B substrates by MOCVD. Well-ordered QDs array is observed on (3 1 1)B surface similar to the results from the GaAs substrate [15]. Nishi et al. report the achievement of lasing from InAs QDs on InP(3 1 1)B substrate [20]. In this paper, we investigated the self-assembled quantum dots formed on (1 0 0) and (N 1 1)B (N = 2; 3; 4; 5) InP substrates by molecular beam epitaxy (MBE). Two kinds of ternary QDs (In0:9 Ga0:1 As and In0:9 Al0:1 As QDs) are grown on the above substrates; TEM and PL results con rm QDs formation for all samples with di erent orientation. The PL spectra reveal obvious di erences in peak position, line width, intensity and peak shape between di erent oriented surfaces. In addition, QDs formed on (1 0 0) and (4 1 1)B surfaces give strong narrow luminescence, and, to our interest, both In0:9 Ga0:1 As and In0:9 Al0:1 As dots on (4 1 1)B surface is found to give stronger luminescence than dots on (1 0 0) surface.
following orientations: (1 0 0) ± 0:5 ; (N 1 1)B ± ◦ 0:5 (N = 2; 3; 4; 5). The epilayer structure consists of ∼300 nm lattice-matched In0:52 Al0:48 As bu er layer, the desired dot layer and ∼50 nm In0:52 Al0:48 As cap layer. Two kinds of ternary QDs (In0:9 Ga0:1 As and In0:9 Al0:1 As QDs) are grown on the above substrates. Each growth run is carried out simultaneously on ve di erent InP substrates mounted on the same Mo block side by side and substrate rotation is used to improve uniformity. During the growth, As4 partial pressure is kept at 1 × 10−7 T; substrate ◦ temperature is 500 C for In0:9 Al0:1 As deposition ◦ and 490 C for In0:9 Ga0:1 As deposition. The growth progress was in situ monitored by 30 kV re ection high-energy electron di raction (RHEED); during the deposition of strained dot layer, 2D–3D growth transition is indicated by RHEED change from a streaky to a spotty pattern. The critical thickness for the transition is found to be of 8–10 monolayers (ML) for both In0:9 Al0:1 As and In0:9 Ga0:1 As dot layer (∼ 2:6% lattice mis t). The 2D–3D growth transition for (InGa)As dot layer is observed slightly latter than that for (InAl)As dot layer, which may be caused by slight composition uctuation of dot layer and=or due to di erent di usion lengths for Ga and Al atoms. The nominal deposition thickness for In0:9 Ga0:1 As and In0:9 Al0:1 As dot layers are 16.5 and 13 ML, respectively. (The above monolayer values refer to the equivalent thickness on (1 0 0) surfaces.) After epitaxy growth, (1 0 0) samples are characterized by TEM measurement and low-temperature PL measurement is used for all samples. TEM observation is performed using a JEOL JEM 1000 microscope. PL experiment is performed on an IF120HR Fourier transform infrared spectrometer with the 514.5 nm excitation line of an Ar + ion laser source. The samples are cooled in a closed-cycle He cryostat; a cooled InSb detector and a CaF2 beam splitter are employed to record the spectra.
2. Experimental procedure
Fig. 1(a) and (b) shows the plan-view TEM images for the (0 0 1) samples with In0:9 Ga0:1 As (a) and In0:9 Al0:1 As (b) deposition. The coherent dot formation is evident through the characteristic dark=bright
Samples are grown by a Riber 32P MBE system on semi-insulating Fe-doped InP substrates with the
3. Results and discussion
166
Z.-Z. Sun et al. / Physica E 8 (2000) 164–169
Fig. 1. The plan-view TEM images for the (0 0 1) samples with In0:9 Ga0:1 As (a) and In0:9 Al0:1 As (b) deposition.
contrast, which re ects an inhomogeneous strain eld caused by the lattice deformation in a 3D island; regions of dark contrast correspond to In0:9 Ga0:1 As (or In0:9 Ga0:1 As) dots. It can be seen from the gure that most of the In0:9 Al0:1 As dots appear approximately round shaped; while In0:9 Ga0:1 As dots do not seem very regular; and are interconnecting due to higher dot layer coverage. We think this is the initial stage of QDs coalescence. Using the plan-view TEM results, the average lateral size of In0:9 Ga0:1 As dot is about 40 nm, while that of In0:9 Al0:1 As dot is about 80 nm; areal densities of In0:9 Ga0:1 As and In0:9 Al0:1 As dots are 2:2 × 1010 and 2:6 × 109 cm−2 , respectively.
Fig. 2(a) and (b) shows the low-temperature (15 K) PL spectra for all samples. Each spectrum is normalized to its maximum. It can be seen that all the spectra show a PL emission peak at lower energy (0.55 – 0.65 eV) and the spectra from (1 0 0) and (5 1 1)B samples give another obvious luminescence peak at higher energy site (around 0.9 –1.1 eV). Higher energy peaks in the spectra originate from wetting layer (WL) formed in S–K growth. Large full-width at half-maximum (FWHM) of them should be attributed to the un at wetting layer, which are composed of quantum wells with several monolayers thickness
uctuation. According to our calculation, the QW thickness deviation range corresponding to the peak site is 8–12 ML for (1 0 0) sample, which agrees with in situ RHEED observation for critical thickness mentioned above. (Moreover, calculation results also show luminescence from 2D wetting layer for our sample structures will not be less than ∼ 0:8 eV.) Then, luminescence peaks at lower energy (0.55 – 0.65 eV) should be attributed to QDs structure formed on each orientation; these PL results con rm QD formation for all samples with di erent orientation. It is listed in Table 1 that the peak positions (Ep ), FWHM, the integral intensity (IPL) of QDs peak for all samples. The integral intensity is normalized relative to those on (1 0 0) substrate. Fig. 3 shows the variation of peak positions and FWHM with di erent substrate orientation. It can be seen from Fig. 3 and Table 1 that, for both In0:9 Al0:1 As and In0:9 Ga0:1 As QDs samples, with changing the substrate orientation from (1 0 0) to (2 1 1)B, on an average (non-monotonic), the peak position red shift slightly and the FWHM increases obviously. Since FWHM of luminescence peak generally re ects the unavoidable size uctuations in the QD ensemble (inhomogeneous broadening), the increase of FWHM means a deterioration of QD size uniformity with changing substrate orientation from (1 0 0) to (2 1 1)B; while, for the red shift of QD peak, one possible explanation is that the substrate orientation a ects the shape and average size of QDs by means of surface strain elds and of the chemical potentials. Besides, some other e ects, such as buildup of piezoelectric eld, e ective mass anisotropy and In segregation could also be responsible for the peak shift of QDs formed on high-index surface. Such trend of peak shift is not similar to the published results from InAs QD on high-index
Z.-Z. Sun et al. / Physica E 8 (2000) 164–169
167
Fig. 2. The low-temperature (15 K) PL spectra for all samples with (a) In0:9 Ga0:1 As and (b) In0:9 Al0:1 As dot layer on each surface. Each spectrum is normalized to its maximum. Table 1 The peak positions (Ep ), FWHM, the integral intensity (IPL) of QDs peak for all samples. The integral intensity was normalized relative to those on (1 0 0) substrate Orientation
(1 0 0) (5 1 1)B (4 1 1)B (3 1 1)B (2 1 1)B
Angle
0 15.8 19.5 25.2 35.3
In0:9 Ga0:1 As dot
In0:9 Al0:1 As dot
Ep (eV)
FWHM (meV)
IPL
Ep (eV)
FWHM (meV)
IPL
0.633 0.639 0.617 0.616 0.607
40 115 80 119 144
1.000 0.123 1.906 0.136 0.258
0.636 0.574 & 0.648 0.603 0.611 0.587
64 128 75 120 177
1.000 1.253 4.866 0.145 0.030
GaAs surfaces [17], where blue shift is observed for (N 1 1)B surfaces whereas red shift is observed for (N 1 1)A ones. We think such di erence is attributed to the di erent material system or respective growth condition. In addition, QDs size distribution can also be re ected by the line shape of QDs luminescence peaks. It can be seen from Fig. 2 that most of the samples display asymmetric dot luminescence peaks, as a typical case, double-peaks structure of QD luminescence is observed for In0:9 Al0:1 As dots on (5 1 1)B surface. We measure the PL spectra of this sample under di erent excitation power density (Fig. 4), and a linear excita-
tion density dependence of two peaks is observed. So we believed that such double-peaks structure is caused by distinct dual-model of size distribution other than emission from excited state in the same QD. Such effect from multi-modal size distribution has been reported for QDs on both GaAs [13,21] and InP [22] substrate system. Fig. 5(a) and (b) gives the variation of PL integral intensity for In0:9 Ga0:1 As (a) and In0:9 Al0:1 As (b) dots with di erent orientation surfaces. In0:9 Ga0:1 As QDs formed on (4 1 1)B and (1 0 0) surfaces and In0:9 Al0:1 As QDs formed on (4 1 1)B, (5 1 1)B and (1 0 0) surfaces give relatively strong narrow lumi-
168
Z.-Z. Sun et al. / Physica E 8 (2000) 164–169
Fig. 3. Luminescence peak energy and FWHM of (a) In0:9 Ga0:1 As dots and (b) In0:9 Al0:1 As dots as a function of the tilt angle with respect to (1 0 0). Peak site is represented by circles (or squares) and the FWHM of each peak is indicated by the error bar. For In0:9 Al0:1 As dots on (5 1 1)B surface, the peak position is represented by the average of the two peak energy values (triangle).
nescence; while weaker PL emissions are observed on (2 1 1)B and (3 1 1)B orientations. Such di erence re ects di erent QD structure quality a ected by substrate orientation under our growth condition. It is worth noting that, for both In0:9 Ga0:1 As and In0:9 Al0:1 As dots, the highest integral intensity is from quantum dots grown on (4 1 1)B substrate, which is about 2–5 times higher than QDs grown on (1 0 0) plane or 1–2 orders of magnitude higher than that of QD on other high-index planes. In general, higher structure quality (defect-free QD density, size homogeneity and alignment ordering, etc.) is accounted for better optical property, just as the study on QDs on GaAs (3 1 1)B substrate [15,16]. Nevertheless, we cannot conclude that a higher size uniformity is achieved for (4 1 1)B samples because the luminescence peak of dots on (4 1 1)B surface is slightly wider than that of (1 0 0) samples; so, higher density of defect-free QDs on (4 1 1)B surface may be the main reason for the improvement of PL eciency. Therefore, higher density of defect-fee QDs may be
Fig. 4. Photoluminescence spectra with various excitation power densities for the sample with In0:9 Al0:1 As dots on (5 1 1)B substrate.
the main reason for high PL intensity from (4 1 1)B samples.
4. Conclusion In summary, we investigated the In0:9 Ga0:1 As and In0:9 Al0:1 As quantum dots formed on (1 0 0) and high-index InP substrates by MBE. TEM and PL results con rm the QDs formation for all samples. PL measurement reveals obvious di erence in optical properties caused by substrate orientation e ect. QDs formed on (4 1 1)B surface give the highest PL intensity, which show the existence of optimal substrate surface for improving QDs emission on InP substrate. More detailed investigations for these structures are still required to optimize growth parameters for device application and fully understand the self-assembling mechanism on non-(1 0 0) substrates.
Z.-Z. Sun et al. / Physica E 8 (2000) 164–169
Fig. 5. The PL eciency (i.e. PL integral intensity) from (a) In0:9 Ga0:1 As dots and (b) In0:9 Al0:1 As dots on di erent orientation surfaces.
Acknowledgements The authors would like to thank the nancial supports from the National Natural Science Foundation of China (NNSFC) and the National Advanced Materials Committee of China (NAMCC). References [1] D.J. Eaglemann, M. Cerullo, Phys. Rev. Lett. 64 (1990) 1943. [2] R. Leon, S. Fafard, D. Leonard, J.L. Merz, P.M. Petro , Appl. Phys. Lett. 67 (1995) 521. [3] D. Leonard, S. Fafard, K. Pond, Y.H. Zhang, J.M. Merz, P.M. Petro , J. Vac. Sci. Technol. B 12 (1994) 2516. [4] J.Y. Marzin, J.M. Gerald, A. Izrael, D. Barrier, G. Bastard, Phys. Rev. Lett. 73 (1994) 716.
169
[5] F. Hatami, N.N. Ledentsov, M. Grundmann, J. Bohrer, F. Heinrichsdor , M. Beer, D. Bimberg, S.S. Ruvimov, P. Werner, U. Gosele, J. Heydenreich, U. Richter, S.V. Ivanov, B.Y. Meltser, P.S. Kop’ev, Z. Alferov, Appl. Phys. Lett. 67 (1995) 656. [6] A. Kurtenbach, K. Eberl, T. Shitara, Appl. Phys. Lett. 66 (1995) 361. [7] H. Marchand, P. Desjardins, D. Guillon, J.-E. Paultre, Z. Bougrioua, R.Y.-F. Yip, R.A. Masut, Appl. Phys. Lett. 71 (1997) 527. [8] S. Fafard, Z. Wasilewski, J. McCa rey, S. Raymond, S. Charbonneau, Appl. Phys. Lett. 68 (1996) 991. [9] H. Li, J. Wu, B. Xu, J. Liang, Z. Wang, Appl. Phys. Lett. 72 (1998) 2123. [10] H. Ishikawa, H. Shoji, Y. Nakata, K. Mukai, M. Sugawara, M. Egawa, N. Otsuka, Y. Sugiyama, T. Futatsugi, N. Yokoyama, J. Vac. Sci. Technol. A 16 (1998) 794. [11] S. Fafard, K. Hinzer, S. Raymond, M. Dion, J. McCa rey, Y. Feng, S. Charbonneau, Science 274 (1996) 1350. [12] V.M. Ustinov, A.E. Zhukov, A.Yu. Egorov, A.R. Kovsh, S.V. Zaitsev, N.Yu. Gordeev, V.I. Kopchatov, N.N. Ledentsov, A.F. Tsatsul’nikov, B.V. Volovik, P.S. Kop’ev, Z.I. Alferov, S.S. Ruvimov, Z. Liliental-Weber, D. Bimberg, Electron. Lett. 34 (1998) 670. [13] M. Henini, S. Sanguinetti, L. Brusaferri, E. Grilli, M. Guzzi, M.D. Upward, P. Moriarty, P.H. Beton, Microelectronic J. 28 (1997) 933. [14] P.P. Gonzalez-Borrero, D.I. Lubyshev, E. Marega Jr., E. Petitprez, P. Basmaji, Superlattices Microstruct. 23 (1998) 365. [15] R. Notzel, T. Fukui, H. Hasegawa, T. Temmyo, T. Tamamura, Appl. Phys. Lett. 65 (1994) 2854. [16] R. Notzel, J. Temmyo, H. Kamada, T. Furuta, Appl. Phys. Lett. 65 (1994) 457. [17] S.C. Fortina, S. Sanguintti, E. Grilli, M. Guzzi, M. Henini, A. Polimeni, L. Eaves, J. Crystal Growth 187 (1998) 126. [18] D.I. Lubyshev, P.P. Gonzalez-Borrero, E. Marega Jr., E. Petitprez, P. Basmaji, J. Vac. Sci. Technol. B 14 (3) (1996) 2212. [19] R. Notzel, J. Temmyo, A. Kozen, T. Tamamura, T. Fukui, H. Hasegawa, Appl. Phys. Lett. 66 (1995) 2525. [20] K. Nishi, M. Yamada, T. Anan, A. Gomyo, S. Sugou, Appl. Phys. Lett. 73 (1998) 526. [21] M. Colocci, F. Bogani, L. Carraresi, R. Mattolini, A. Bosacchi, S. Franchi, P. Frigeri, S. Taddei, M. Rosa-Clot, Superlattices Microstruct. 22 (1997) 81. [22] F.-Q. Liu, Z.-G. Wang, J. Wu, B. Xu, W. Zhou, J.-J. Qian, J. Crystal Growth 197 (1999) 789.