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GaAs superlattices near quantum dot formation
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Physica E 17 (2003) 300 – 302 www.elsevier.com/locate/physe
Peculiarities of the electron transport in very short period InAs/GaAs superlattices near quantum dot formation Vladimir A. Kulbachinskiia;∗ , Roman A. Lunina , Vasili A. Rogozina , Nicolai B. Brandta , Vladimir G. Mokerovb , Yury V. Fedorovb , Yury V. Khabarovb a Low
Temperature Physics Department, Moscow State University, 119992, GSP-2 Moscow, Russia b Institute of Radioengineering and Electronics, RAS, Moscow, Russia
Abstract The photoluminescence, magnetoresistance, Shubnikov-de Haas and Hall e6ect have been investigated in short period InAs/GaAs superlattices with di6erent numbers of periods (3 6 N 6 24) and a total thickness of 14 nm as a function of InAs layer thickness Q in the range 0:33 6 Q 6 2:7 monolayer (ML). These superlattices represent a quantum well with average composition In0:16 Ga0:84 As. Photoluminescence intensity and electron mobility enhancement occur when the InAs layer thickness Q is equal to 0.33 or 2:0 ML. When Q ¿ 2:7 ML, quantum dots are formed. The mobility of electrons and the anisotropy of resistivity do not depend monotonically on the thickness Q of InAs layers. ? 2002 Elsevier Science B.V. All rights reserved. PACS: 73.21.Cd; 73.21.La; 73.21.Fg Keywords: Short period superlattice; Quantum dots; Photoluminescence
1. Introduction While the mechanism of formation of quantum dots and optical properties of undoped structures with quantum dots have been widely studied [1], structures with InAs layer thickness below the threshold for quantum dot formation are less studied. Here we report on the optical and electrical properties of doped structures at the initial stage of quantum dot formation. We measured the photoluminescence at a temperature 77 K, the Shubnikov-de Haas (SdH) e6ect and the Hall e6ect in short period InAs/GaAs ∗ Corresponding author. Tel.: +7-095-939-1147; fax: +7-095932-8876. E-mail address: [email protected] (V.A. Kulbachinskii).
superlattices in magnetic Felds up to 8 T in the temperature range 0.4 –4:2 K.
2. Samples The structures were grown by MBE at 490◦ C on semi-insulating (0 0 1)GaAs substrates. The samples consisted of a 1 m thick GaAs bu6er layer, a InAs/GaAs superlattice (with di6erent periods (3 6 N 6 24) and a total thickness equal to 14 nm), a 10 nm thick spacer layer Al0:2 Ga0:8 As, a delta-Si layer for doping, a 35 nm thick layer Al0:2 Ga0:8 As and a 6 nm thick GaAs cap layer. In the investigated structures the e6ective thickness Q of InAs layers was changed from 0.33 up to 2:7 ML. The thickness
1386-9477/03/$ - see front matter ? 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S1386-9477(02)00812-3
V.A. Kulbachinskii et al. / Physica E 17 (2003) 300 – 302
301
Table 1 The InAs layer thickness Q in monolayers (ML), the energy hmax of the maximum of the photoluminescence spectra at T = 77 K, the Hall density n, and the Hall mobility at T = 4:2 K
Sample 1 2 3 4 5 6 7 8
Q (InAs, ML)
Number of periods N
hmax (eV)
QW
—
1.434, 1.375
0.33 0.67 1.00 1.33 1.58 2.00 2.70
24 12 8 6 5 4 3
1.419, 1.411, 1.411, 1.418, 1.404, 1.406, 1.265
In0:16 Ga0:84 As
1.367 1.369 1.370 1.374 1.368 1.356
n (1011 cm−2 )
(cm2 = V s)
8.7
8100
11.5 5.98 15.2 8.66 5.60 10.4 1.52
9400 2060 2450 4220 4910 7060 120
of GaAs layers in superlattice was also proportionally changed from 1.7 to 13:5 ML to keep the mean composition of the superlattice (number of periods varied from 24 in samples 2 to 3 in sample 8) equivalent to a solid solution In0:16 Ga0:84 As. A sample with a 14 nm wide quantum well (QW) In0:16 Ga0:84 As was also grown. The relevant parameters of the structures are listed in Table 1. 3. Results and discussion Photoluminescence and AFM data clearly showed that for Q ¡ 2:7 ML single monolayer islands of InAs were formed, while for Q = 2:7 ML InAs quantum dots were formed (Fig. 1). The photoluminescence spectra of samples with thickness less than 2:7 ML showed two peaks (see Table 1). These two peaks correspond to optical transitions from the two electronic subbands to the hole subband. (Only the lowest subband is occupied by electrons according to SdH e6ect data.) The calculated band diagram for sample 4 is shown in Fig. 2. The wave functions of the superlattices are almost identical to those of a single quantum well. However, when the e6ective thickness of InAs layers equals to 2:7 ML a new broad and intensive photoluminescence peak with a maximum at 1:265 eV appears due to the change from 2D layer-by-layer growth to the formation of vertically stacked quantum dots. In sample 8 the photoluminescence peak at ≈ 1:36 eV was not observed. Dots formation for Q = 2:7 ML was directly observed by AFM (see
Fig. 1. AFM image of quantum dots in sample 8 after selective etching.
Fig. 1). Enhancement of the intensity of photoluminescence in samples with Q = 0:33 and 2:0 ML was observed. The maximal electron mobility corresponded to the same samples (see Table 1). The relatively high mobilities in these samples are possibly explained by the weakness of the elastic strain Luctuations and a near-perfect crystal lattice as compared to the solid solution In0:16 Ga0:84 As. It is worth noting that for these two samples also the intensity of the photoluminescence peak is much higher. In all superlattice samples (except QW sample 1) the anisotropy of resistance is observed which depends on the thickness Q and attains a value up to 1.3.
302
V.A. Kulbachinskii et al. / Physica E 17 (2003) 300 – 302
The anisotropy of the resistance is typical for structures with preferential growth of deposited material in one direction. When Q = 2:7 ML the metallic type of conductivity changes to variable range hopping conductivity.
0.2 Ec
E1 U (eV)
0.0
E0
-0.2
Acknowledgements
-0.4 -0.6
40
60
80
z (nm) Fig. 2. Band diagram for sample 4. The lowest two subbands E0 (dashed line) and E1 (solid line) are indicated together with corresponding wave functions. The Fermi level is represented by the dash–dotted line.
The work was supported by the RFBR (Grant N 00-02-17493). References [1] D. Bimberg, V.A. Shchukin, N.N. Ledentsov, et al., Appl. Surf. Sci. 130 –132 (1998) 71.
Physica E 17 (2003) 300 – 302 www.elsevier.com/locate/physe
Peculiarities of the electron transport in very short period InAs/GaAs superlattices near quantum dot formation Vladimir A. Kulbachinskiia;∗ , Roman A. Lunina , Vasili A. Rogozina , Nicolai B. Brandta , Vladimir G. Mokerovb , Yury V. Fedorovb , Yury V. Khabarovb a Low
Temperature Physics Department, Moscow State University, 119992, GSP-2 Moscow, Russia b Institute of Radioengineering and Electronics, RAS, Moscow, Russia
Abstract The photoluminescence, magnetoresistance, Shubnikov-de Haas and Hall e6ect have been investigated in short period InAs/GaAs superlattices with di6erent numbers of periods (3 6 N 6 24) and a total thickness of 14 nm as a function of InAs layer thickness Q in the range 0:33 6 Q 6 2:7 monolayer (ML). These superlattices represent a quantum well with average composition In0:16 Ga0:84 As. Photoluminescence intensity and electron mobility enhancement occur when the InAs layer thickness Q is equal to 0.33 or 2:0 ML. When Q ¿ 2:7 ML, quantum dots are formed. The mobility of electrons and the anisotropy of resistivity do not depend monotonically on the thickness Q of InAs layers. ? 2002 Elsevier Science B.V. All rights reserved. PACS: 73.21.Cd; 73.21.La; 73.21.Fg Keywords: Short period superlattice; Quantum dots; Photoluminescence
1. Introduction While the mechanism of formation of quantum dots and optical properties of undoped structures with quantum dots have been widely studied [1], structures with InAs layer thickness below the threshold for quantum dot formation are less studied. Here we report on the optical and electrical properties of doped structures at the initial stage of quantum dot formation. We measured the photoluminescence at a temperature 77 K, the Shubnikov-de Haas (SdH) e6ect and the Hall e6ect in short period InAs/GaAs ∗ Corresponding author. Tel.: +7-095-939-1147; fax: +7-095932-8876. E-mail address: [email protected] (V.A. Kulbachinskii).
superlattices in magnetic Felds up to 8 T in the temperature range 0.4 –4:2 K.
2. Samples The structures were grown by MBE at 490◦ C on semi-insulating (0 0 1)GaAs substrates. The samples consisted of a 1 m thick GaAs bu6er layer, a InAs/GaAs superlattice (with di6erent periods (3 6 N 6 24) and a total thickness equal to 14 nm), a 10 nm thick spacer layer Al0:2 Ga0:8 As, a delta-Si layer for doping, a 35 nm thick layer Al0:2 Ga0:8 As and a 6 nm thick GaAs cap layer. In the investigated structures the e6ective thickness Q of InAs layers was changed from 0.33 up to 2:7 ML. The thickness
1386-9477/03/$ - see front matter ? 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S1386-9477(02)00812-3
V.A. Kulbachinskii et al. / Physica E 17 (2003) 300 – 302
301
Table 1 The InAs layer thickness Q in monolayers (ML), the energy hmax of the maximum of the photoluminescence spectra at T = 77 K, the Hall density n, and the Hall mobility at T = 4:2 K
Sample 1 2 3 4 5 6 7 8
Q (InAs, ML)
Number of periods N
hmax (eV)
QW
—
1.434, 1.375
0.33 0.67 1.00 1.33 1.58 2.00 2.70
24 12 8 6 5 4 3
1.419, 1.411, 1.411, 1.418, 1.404, 1.406, 1.265
In0:16 Ga0:84 As
1.367 1.369 1.370 1.374 1.368 1.356
n (1011 cm−2 )
(cm2 = V s)
8.7
8100
11.5 5.98 15.2 8.66 5.60 10.4 1.52
9400 2060 2450 4220 4910 7060 120
of GaAs layers in superlattice was also proportionally changed from 1.7 to 13:5 ML to keep the mean composition of the superlattice (number of periods varied from 24 in samples 2 to 3 in sample 8) equivalent to a solid solution In0:16 Ga0:84 As. A sample with a 14 nm wide quantum well (QW) In0:16 Ga0:84 As was also grown. The relevant parameters of the structures are listed in Table 1. 3. Results and discussion Photoluminescence and AFM data clearly showed that for Q ¡ 2:7 ML single monolayer islands of InAs were formed, while for Q = 2:7 ML InAs quantum dots were formed (Fig. 1). The photoluminescence spectra of samples with thickness less than 2:7 ML showed two peaks (see Table 1). These two peaks correspond to optical transitions from the two electronic subbands to the hole subband. (Only the lowest subband is occupied by electrons according to SdH e6ect data.) The calculated band diagram for sample 4 is shown in Fig. 2. The wave functions of the superlattices are almost identical to those of a single quantum well. However, when the e6ective thickness of InAs layers equals to 2:7 ML a new broad and intensive photoluminescence peak with a maximum at 1:265 eV appears due to the change from 2D layer-by-layer growth to the formation of vertically stacked quantum dots. In sample 8 the photoluminescence peak at ≈ 1:36 eV was not observed. Dots formation for Q = 2:7 ML was directly observed by AFM (see
Fig. 1. AFM image of quantum dots in sample 8 after selective etching.
Fig. 1). Enhancement of the intensity of photoluminescence in samples with Q = 0:33 and 2:0 ML was observed. The maximal electron mobility corresponded to the same samples (see Table 1). The relatively high mobilities in these samples are possibly explained by the weakness of the elastic strain Luctuations and a near-perfect crystal lattice as compared to the solid solution In0:16 Ga0:84 As. It is worth noting that for these two samples also the intensity of the photoluminescence peak is much higher. In all superlattice samples (except QW sample 1) the anisotropy of resistance is observed which depends on the thickness Q and attains a value up to 1.3.
302
V.A. Kulbachinskii et al. / Physica E 17 (2003) 300 – 302
The anisotropy of the resistance is typical for structures with preferential growth of deposited material in one direction. When Q = 2:7 ML the metallic type of conductivity changes to variable range hopping conductivity.
0.2 Ec
E1 U (eV)
0.0
E0
-0.2
Acknowledgements
-0.4 -0.6
40
60
80
z (nm) Fig. 2. Band diagram for sample 4. The lowest two subbands E0 (dashed line) and E1 (solid line) are indicated together with corresponding wave functions. The Fermi level is represented by the dash–dotted line.
The work was supported by the RFBR (Grant N 00-02-17493). References [1] D. Bimberg, V.A. Shchukin, N.N. Ledentsov, et al., Appl. Surf. Sci. 130 –132 (1998) 71.