- Email: [email protected]
Excitation power dependent photoluminescence of In0.7Ga0.3As1−xNx quantum dots grown on GaAs (0 0 1)
ARTICLE IN PRESS
Journal of Crystal Growth 278 (2005) 244–248 www.elsevier.com/locate/jcrysgro
Excitation power dependent photoluminescence of In0.7Ga0.3As1xNx quantum dots grown on GaAs (0 0 1) A. Nishikawaa,, R. Katayamaa, K. Onabea, Y.G. Hongb, C.W. Tub a
Departement of Advanced Materials Science, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8561, Japan b Department of Electrical and Computer Science Engineering, University of California, San Diego, 9500 Gilman Dr. La Jolla, CA 92093-0407, USA Available online 23 January 2005
Abstract The excitation power dependent photoluminescence (PL) of self-assembled In0.7Ga0.3As1xNx (x ¼ 0; 0:02) quantum dots (QDs) has been investigated in the excitation power density ranged from 3.0 kW/cm2 to 50 mW/cm2. As the excitation power increases, the emission peak of In0.7Ga0.3As0.98N0.02 QDs shifts to shorter wavelengths, while the peak of In0.7Ga0.3As QDs remains at the same wavelength. In the low-excitation cases, the PL peak of the In0.7Ga0.3As0.98N0.02 QDs has a tail on the lower energy side, on the other hand, that of the In0.7Ga0.3As QDs shows a symmetrical shape. Since the dot size distribution of the In0.7Ga0.3As0.98N0.02 QDs is similar to that of the In0.7Ga0.3As QDs, this blue-shift and the lower energy tail of the PL peak of the In0.7Ga0.3As0.98N0.02 QDs are attributed to the nitrogen-related states below the conduction band edge as are observed in the case of InGaAsN quantum wells (QWs). r 2005 Elsevier B.V. All rights reserved. PACS: 78.67.Hc; 81.15.Hi; 81.05.Ea Keyword: A1. Low-dimensional structures; A3. Molecular beam eitaxy; B2. Semiconducting III–V materials
1. Introduction Self-assembled In(Ga)As quantum dots (QDs) grown on a GaAs substrate compose an attractive material for long wavelength laser devices for Corresponding author. Tel./fax: +81 4 7136 3775.
E-mail address: [email protected] (A. Nishikawa).
optical fiber communications because of the possibility of low threshold current density and temperature-insensitive lasing [1]. Typically, the emission wavelength of In(Ga)As QDs is around 1.1 mm due to the quantum-size effect. To push the wavelength to the longer regime, much effort or optimizing the layer structure is required, such as alternating deposition [2,3] or inserting InxGa1xAs strain-reduced barrier layers [4,5].
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.12.067
ARTICLE IN PRESS A. Nishikawa et al. / Journal of Crystal Growth 278 (2005) 244–248
Besides these kinds of techniques, we have proposed an incorporation of a small amount of nitrogen to QDs in order to extend the emission wavelength. In the previous studies, we have shown 1.3 and near 1.55 mm emission from In0.7Ga0.3As1xNx QDs at room temperature (RT) [6], and also shown an improvement of the crystal quality of In0.7Ga0.3As1xNx QDs, which is comparable to that of In0.7Ga0.3As QDs in terms of the photoluminescence (PL) intensity and full-width at half-maximum (FWHM) [7]. Since the crystal quality of In0.7Ga0.3As1xNx QDs is improved to make it as high as that of the In0.7Ga0.3As QDs, the effect of nitrogen on the optical properties of QDs could be investigated in some detail. In this study, the excitation power-dependent PL of In0.7Ga0.3As1xNx QDs has been investigated in order to obtain further information of the effect of nitrogen incorporation on QDs.
245
an argon ion laser and a near-infrared photomultiplier tube (NIR-PMT R5509-71; Hamamatsu Photonics). The excitation power density varied from 50 mW/cm2 to 0.25 kW/cm2. For the high power excitation, micro-PL measurement was carried out at 4 K using a 532 nm-YAG laser and an InGaAs detector, in which excitation power density ranged from 7 W/cm2 to 3.0 kW/cm2. The spot diameter was typically 1 mm.
3. Results and discussion Fig. 1 shows the lateral size distribution of (a) In0.7Ga0.3As, and (b) In0.7Ga0.3As0.98N0.02 QDs. Incorporating nitrogen tends to disturb the 80 InGaAs1-xNx QDs
70
N=0%
The In0.7Ga0.3As1xNx (x ¼ 0; 0:02) QDs were grown on GaAs (0 0 1) substrates by gas-source molecular beam epitaxy (GSMBE) using solid Ga and In, arsine cracked at 980 1C, and an RF plasma nitrogen source. The nominal N concentration was 0% and 2%, determined by X-ray diffraction of In0.3Ga0.7As1xNx/GaAs QW samples grown with the same RF power and N2 flow rate. The nominal thickness and growth temperature of QDs were 4 monolayers (MLs) and 450 1C, respectively. The sample structure consists of a 50nm-thick GaAs buffer, a bottom In0.7Ga0.3 As1xNx QD layer, a 50-nm-thick GaAs barrier, and a top In0.7Ga0.3As1xNx QD layer. The top QD layer is for atomic force microscopy (AFM) measurements, and the bottom QD layer is for PL measurements. Both of the QD layers were grown under the same growth condition. According to the AFM images, the average lateral dot size and height of QDs are estimated to be typically 15 and 5 nm, respectively, and the dot density ranges from 1.1 1011 to 7.6 1010 cm2, which decreases with increasing N concentration [7]. Photoluminescence spectroscopy was carried out at 8 K and RT, using
50 40
Peak 16 nm FWHM 4 nm
30 20 10 0 0
10
20
(a)
30 40 Dot size (nm)
50
60
40 InGaAs1-xNx QDs
35
N=2% 30 Island count
2. Experimental procedure
Island count
60
25 20
Peak 15 nm FWHM 5 nm
15 10 5 0 0
(b)
10
20
30 40 Dot size (nm)
50
60
Fig. 1. The lateral dot size distribution of (a) In0.7Ga0.3As and (b) In0.7Ga0.3As0.98N0.02 QDs. The scan size of AFM images is 0.5 0.5 mm2.
ARTICLE IN PRESS A. Nishikawa et al. / Journal of Crystal Growth 278 (2005) 244–248
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In0.7Ga0.3As0.98N0.02QDs µ-PL 4 K macro-PL 8 K
PL intensity (a.u.)
3.0 kW/cm2 1.0 kW/cm2 0.24 kW/cm2 24 W/cm2
0.25 kW/cm
2.5 W/cm2
2
25 W/cm2
PL intensity (a.u.)
In0.7Ga0.3 As QDs µ-PL 4 K macro PL 8 K
50 mW/cm2
1000 1050 1100 Wavelength (nm)
24 W/cm
0.25 kW/cm2 2
2.5 W/cm2
25 W/cm2
0.25 W/cm2
0.25 W/cm2
950
1.0 kW/cm2 0.23 kW/cm2
2.5 W/cm2
2.5 W/cm2
(a)
3.0 kW/cm2
50 mW/cm2
1150 (b)
1000 1050 1100 1150 1200 1250 Wavelength (nm)
Fig. 2. The Excitation power dependent PL spectra of (a) In0.7Ga0.3As and (b) In0.7Ga0.3As0.98N0.02 QDs measured in a conventional PL system at 8 K for low excitations (broken line) and in a micro-PL system at 4 K for high excitations (solid line).
homogeneity of the dot size. However, the dot size distribution of the In0.7Ga0.3As0.98N0.02 QDs can be improved to be comparable to that of the In0.7Ga0.3As QDs with decreasing growth temperature to 450 1C [7]. According to the AFM measurements, the heights of the both QDs were also quite similar. Therefore, we are able to compare the optical properties of the In0.7Ga0.3As0.98N0.02 QDs with that of the In0.7Ga0.3As QDs without the dot size and dot size distribution difference. Fig. 2 shows the excitation power dependence of PL spectra of (a) In0.7Ga0.3As and (b) In0.7Ga0.3As0.98N0.02 QDs. The measurements in the low-excitation cases were carried out in a conventional macro-PL system at 8 K and those in the high-excitation cases were carried out in a micro-PL system at 4 K. As the excitation power increases, the emission peak of the In0.7Ga0.3As0.98N0.02 QDs shifts to shorter wavelengths up to 0.25 kW/cm2 and then the blue-shift saturates to some wavelength, while the emission peak of In0.7Ga0.3As QDs almost remains at the same wavelength through all the excitation cases. In the low-excitation cases, the spectra of the In0.7Ga0.3As0.98N0.02 QDs have a tail on the lower energy side. On the other hand, those
of the In0.7Ga0.3As QDs show a symmetrical shape. In the high-excitation cases, the PL peaks of both the In0.7Ga0.3As QDs and the In0.7Ga0.3As0.98N0.02 QDs have a shoulder on the higher energy side. In general, the emission wavelength of QDs is independent of the excitation power due to its delta-function-like density of states, and a higher energy peak would appear instead of the blue-shift of the peak with increasing excitation power [8,9]. Because of this higher energy peak, the shoulder on the higher energy side was observed in the PL peaks of both the In0.7Ga0.3As QDs and the In0.7Ga0.3As0.98N0.02 QDs in the high-excitation cases. On the other hand, since the dot size distributions of the In0.7Ga0.3As0.98N0.02 QDs and the In0.7Ga0.3As QDs are quite similar as shown in Fig. 1, the blue-shift of the peak and the tail on the lower energy side in the low-excitation cases could be attributed to the effect of the incorporated nitrogen in the QDs. Fig. 3 shows the excitation power dependence of the FWHM of the In0.7Ga0.3As QDs and the In0.7Ga0.3As0.98N0.02 QDs. As the excitation power increases, the FWHM of the PL spectra of the In0.7Ga0.3As QDs increases monotonically, but that of the In0.7Ga0.3As0.98N0.02 QDs decreases at
ARTICLE IN PRESS A. Nishikawa et al. / Journal of Crystal Growth 278 (2005) 244–248
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55
50
In0.7Ga0.3As1-xNx QDs
In0.7Ga0.3N0.98As0.02 QDs
8K
RT N=0%
45
PL intensity (a.u.)
FWHM (meV)
N=2%
40
35
0.25 kW/cm2 0.13 kW/cm2
30
63 W/cm2 0.1
1 10 Excitation power density (W/cm2)
25 W/cm2
100
13 W/cm2
Fig. 3. The FWHMs of In0.7Ga0.3As and In0.7Ga0.3As0.98N0.02 QDs as a function of the excitation power measured by macroPL system.
1100
low excitations and then increases. This result indicates that there are some nitrogen-related states below the ground state in the In0.7Ga0.3As0.98N0.02 QDs, as is observed in the case of InGaAsN quantum wells (QWs) [10], which causes the lower energy tail in a PL spectrum. These nitrogen-related states have a less density of states than the ground state of the QDs, so that the emission from the ground state becomes dominant with increasing excitation power. Because of the change of the dominant peak in the PL spectra of the In0.7Ga0.3As0.98N0.02 QDs with an increase of the excitation power, the reduction of FWHM and the blue-shift of the peak of the In0.7Ga0.3As0.98N0.02 QDs were observed in low excitation powers. Fig. 4 shows that the excitation power dependent PL spectra of the In0.7Ga0.3As0.98N0.02 QDs at RT. No peak shift was observed in spite of the increase of the excitation power, indicating that the carriers were thermally excited from the nitrogen-related state to the ground state of QDs. This result is consistent with the previous results that the localized energy is about 20 meV [11,12].
1200
1300
Wavelength (nm) Fig. 4. The excitation power dependent PL spectra of In0.7Ga0.3As0.98N0.02 QDs measured in a macro-PL system at RT.
4. Summary The excitation (power) dependent PL of In0.7Ga0.3As1xNx QDs has been investigated by macro- and micro-PL measurements, in which the excitation power density ranges from 3.0 kW/ cm2 to 50 mW/cm2. With increasing excitation power, the emission peak of In0.7Ga0.3As0.98N0.02 QDs shifts to shorter wavelengths while the peak of In0.7Ga0.3As QDs remains at the same wavelength. The emission peak of In0.7Ga0.3As0.98N0.02 QDs has a tail on the lower energy side in low excitation powers. Since the dot size distributions of the In0.7Ga0.3As0.98N0.02 and the In0.7Ga0.3As QDs are similar, the blue-shift of the peak and the lower energy side tail could be attributed to the nitrogen-related states below the ground state, as is observed in the case of InGaAsN QWs.
ARTICLE IN PRESS 248
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Acknowledgements The authors would like to thank Prof. H. Akiyama and Dr. M. Yoshita, Institue for Solid State Physics, the University of Tokyo, for their help in micro-PL measurements. References [1] Y. Arakawa, H. Sakaki, Appl. Phys. Lett. 73 (1982) 939. [2] K. Mukai, N. Ohtsuka, M. Sugawara, S. Yamazaki, Jpn. J. Appl. Phys. 33 (1994) L1710. [3] R.P. Mirin, J.P. Ibbetson, K. Nishi, A.C. Gossard, J.E. Bowers, Appl. Phys. Lett. 67 (1995) 3795. [4] K. Nishi, H. Saito, S. Sugou, J.-S. Lee, Appl. Phys. Lett. 74 (1999) 1111.
[5] J. Tatebayashi, M. Nishioka, Y. Arakawa, Appl. Phys. Lett. 78 (2001) 3469. [6] M. Sopanen, H.P. Xin, C.W. Tu, Appl. Phys. Lett. 76 (2000) 994. [7] A. Nishikawa, Y.G. Hong, C.W. Tu, Phys. Stat. Sol. (b) 240 (2003) 427. [8] M. Grundmann, N.N. Ledentsov, O. Stier, D. Bimberg, V.M. Ustinov, P.S. Kop’ev, Zh.I. Alferv, Appl. Phys. Lett. 68 (1996) 979. [9] M. Grundmann, N.N. Ledentsov, O. Stier, J. Boehrer, D. Bimberg, V.M. Ustinov, P.S. Kop’ev, Zh.I. Alferov, Phys. Rev. B 53 (1996) R10509. [10] S. Shirakata, M. Kondow, T. Kitatani, Jpn. J. Appl. Phys. 41 (2002) 2082. [11] H.P. Xin, K.L. Kavanagh, C.W. Tu, J. Crystal Growth 208 (2000) 145. [12] M.A. Pinault, E. Tournie, Appl. Phys. Lett. 78 (2001) 1562.
Journal of Crystal Growth 278 (2005) 244–248 www.elsevier.com/locate/jcrysgro
Excitation power dependent photoluminescence of In0.7Ga0.3As1xNx quantum dots grown on GaAs (0 0 1) A. Nishikawaa,, R. Katayamaa, K. Onabea, Y.G. Hongb, C.W. Tub a
Departement of Advanced Materials Science, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8561, Japan b Department of Electrical and Computer Science Engineering, University of California, San Diego, 9500 Gilman Dr. La Jolla, CA 92093-0407, USA Available online 23 January 2005
Abstract The excitation power dependent photoluminescence (PL) of self-assembled In0.7Ga0.3As1xNx (x ¼ 0; 0:02) quantum dots (QDs) has been investigated in the excitation power density ranged from 3.0 kW/cm2 to 50 mW/cm2. As the excitation power increases, the emission peak of In0.7Ga0.3As0.98N0.02 QDs shifts to shorter wavelengths, while the peak of In0.7Ga0.3As QDs remains at the same wavelength. In the low-excitation cases, the PL peak of the In0.7Ga0.3As0.98N0.02 QDs has a tail on the lower energy side, on the other hand, that of the In0.7Ga0.3As QDs shows a symmetrical shape. Since the dot size distribution of the In0.7Ga0.3As0.98N0.02 QDs is similar to that of the In0.7Ga0.3As QDs, this blue-shift and the lower energy tail of the PL peak of the In0.7Ga0.3As0.98N0.02 QDs are attributed to the nitrogen-related states below the conduction band edge as are observed in the case of InGaAsN quantum wells (QWs). r 2005 Elsevier B.V. All rights reserved. PACS: 78.67.Hc; 81.15.Hi; 81.05.Ea Keyword: A1. Low-dimensional structures; A3. Molecular beam eitaxy; B2. Semiconducting III–V materials
1. Introduction Self-assembled In(Ga)As quantum dots (QDs) grown on a GaAs substrate compose an attractive material for long wavelength laser devices for Corresponding author. Tel./fax: +81 4 7136 3775.
E-mail address: [email protected] (A. Nishikawa).
optical fiber communications because of the possibility of low threshold current density and temperature-insensitive lasing [1]. Typically, the emission wavelength of In(Ga)As QDs is around 1.1 mm due to the quantum-size effect. To push the wavelength to the longer regime, much effort or optimizing the layer structure is required, such as alternating deposition [2,3] or inserting InxGa1xAs strain-reduced barrier layers [4,5].
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.12.067
ARTICLE IN PRESS A. Nishikawa et al. / Journal of Crystal Growth 278 (2005) 244–248
Besides these kinds of techniques, we have proposed an incorporation of a small amount of nitrogen to QDs in order to extend the emission wavelength. In the previous studies, we have shown 1.3 and near 1.55 mm emission from In0.7Ga0.3As1xNx QDs at room temperature (RT) [6], and also shown an improvement of the crystal quality of In0.7Ga0.3As1xNx QDs, which is comparable to that of In0.7Ga0.3As QDs in terms of the photoluminescence (PL) intensity and full-width at half-maximum (FWHM) [7]. Since the crystal quality of In0.7Ga0.3As1xNx QDs is improved to make it as high as that of the In0.7Ga0.3As QDs, the effect of nitrogen on the optical properties of QDs could be investigated in some detail. In this study, the excitation power-dependent PL of In0.7Ga0.3As1xNx QDs has been investigated in order to obtain further information of the effect of nitrogen incorporation on QDs.
245
an argon ion laser and a near-infrared photomultiplier tube (NIR-PMT R5509-71; Hamamatsu Photonics). The excitation power density varied from 50 mW/cm2 to 0.25 kW/cm2. For the high power excitation, micro-PL measurement was carried out at 4 K using a 532 nm-YAG laser and an InGaAs detector, in which excitation power density ranged from 7 W/cm2 to 3.0 kW/cm2. The spot diameter was typically 1 mm.
3. Results and discussion Fig. 1 shows the lateral size distribution of (a) In0.7Ga0.3As, and (b) In0.7Ga0.3As0.98N0.02 QDs. Incorporating nitrogen tends to disturb the 80 InGaAs1-xNx QDs
70
N=0%
The In0.7Ga0.3As1xNx (x ¼ 0; 0:02) QDs were grown on GaAs (0 0 1) substrates by gas-source molecular beam epitaxy (GSMBE) using solid Ga and In, arsine cracked at 980 1C, and an RF plasma nitrogen source. The nominal N concentration was 0% and 2%, determined by X-ray diffraction of In0.3Ga0.7As1xNx/GaAs QW samples grown with the same RF power and N2 flow rate. The nominal thickness and growth temperature of QDs were 4 monolayers (MLs) and 450 1C, respectively. The sample structure consists of a 50nm-thick GaAs buffer, a bottom In0.7Ga0.3 As1xNx QD layer, a 50-nm-thick GaAs barrier, and a top In0.7Ga0.3As1xNx QD layer. The top QD layer is for atomic force microscopy (AFM) measurements, and the bottom QD layer is for PL measurements. Both of the QD layers were grown under the same growth condition. According to the AFM images, the average lateral dot size and height of QDs are estimated to be typically 15 and 5 nm, respectively, and the dot density ranges from 1.1 1011 to 7.6 1010 cm2, which decreases with increasing N concentration [7]. Photoluminescence spectroscopy was carried out at 8 K and RT, using
50 40
Peak 16 nm FWHM 4 nm
30 20 10 0 0
10
20
(a)
30 40 Dot size (nm)
50
60
40 InGaAs1-xNx QDs
35
N=2% 30 Island count
2. Experimental procedure
Island count
60
25 20
Peak 15 nm FWHM 5 nm
15 10 5 0 0
(b)
10
20
30 40 Dot size (nm)
50
60
Fig. 1. The lateral dot size distribution of (a) In0.7Ga0.3As and (b) In0.7Ga0.3As0.98N0.02 QDs. The scan size of AFM images is 0.5 0.5 mm2.
ARTICLE IN PRESS A. Nishikawa et al. / Journal of Crystal Growth 278 (2005) 244–248
246
In0.7Ga0.3As0.98N0.02QDs µ-PL 4 K macro-PL 8 K
PL intensity (a.u.)
3.0 kW/cm2 1.0 kW/cm2 0.24 kW/cm2 24 W/cm2
0.25 kW/cm
2.5 W/cm2
2
25 W/cm2
PL intensity (a.u.)
In0.7Ga0.3 As QDs µ-PL 4 K macro PL 8 K
50 mW/cm2
1000 1050 1100 Wavelength (nm)
24 W/cm
0.25 kW/cm2 2
2.5 W/cm2
25 W/cm2
0.25 W/cm2
0.25 W/cm2
950
1.0 kW/cm2 0.23 kW/cm2
2.5 W/cm2
2.5 W/cm2
(a)
3.0 kW/cm2
50 mW/cm2
1150 (b)
1000 1050 1100 1150 1200 1250 Wavelength (nm)
Fig. 2. The Excitation power dependent PL spectra of (a) In0.7Ga0.3As and (b) In0.7Ga0.3As0.98N0.02 QDs measured in a conventional PL system at 8 K for low excitations (broken line) and in a micro-PL system at 4 K for high excitations (solid line).
homogeneity of the dot size. However, the dot size distribution of the In0.7Ga0.3As0.98N0.02 QDs can be improved to be comparable to that of the In0.7Ga0.3As QDs with decreasing growth temperature to 450 1C [7]. According to the AFM measurements, the heights of the both QDs were also quite similar. Therefore, we are able to compare the optical properties of the In0.7Ga0.3As0.98N0.02 QDs with that of the In0.7Ga0.3As QDs without the dot size and dot size distribution difference. Fig. 2 shows the excitation power dependence of PL spectra of (a) In0.7Ga0.3As and (b) In0.7Ga0.3As0.98N0.02 QDs. The measurements in the low-excitation cases were carried out in a conventional macro-PL system at 8 K and those in the high-excitation cases were carried out in a micro-PL system at 4 K. As the excitation power increases, the emission peak of the In0.7Ga0.3As0.98N0.02 QDs shifts to shorter wavelengths up to 0.25 kW/cm2 and then the blue-shift saturates to some wavelength, while the emission peak of In0.7Ga0.3As QDs almost remains at the same wavelength through all the excitation cases. In the low-excitation cases, the spectra of the In0.7Ga0.3As0.98N0.02 QDs have a tail on the lower energy side. On the other hand, those
of the In0.7Ga0.3As QDs show a symmetrical shape. In the high-excitation cases, the PL peaks of both the In0.7Ga0.3As QDs and the In0.7Ga0.3As0.98N0.02 QDs have a shoulder on the higher energy side. In general, the emission wavelength of QDs is independent of the excitation power due to its delta-function-like density of states, and a higher energy peak would appear instead of the blue-shift of the peak with increasing excitation power [8,9]. Because of this higher energy peak, the shoulder on the higher energy side was observed in the PL peaks of both the In0.7Ga0.3As QDs and the In0.7Ga0.3As0.98N0.02 QDs in the high-excitation cases. On the other hand, since the dot size distributions of the In0.7Ga0.3As0.98N0.02 QDs and the In0.7Ga0.3As QDs are quite similar as shown in Fig. 1, the blue-shift of the peak and the tail on the lower energy side in the low-excitation cases could be attributed to the effect of the incorporated nitrogen in the QDs. Fig. 3 shows the excitation power dependence of the FWHM of the In0.7Ga0.3As QDs and the In0.7Ga0.3As0.98N0.02 QDs. As the excitation power increases, the FWHM of the PL spectra of the In0.7Ga0.3As QDs increases monotonically, but that of the In0.7Ga0.3As0.98N0.02 QDs decreases at
ARTICLE IN PRESS A. Nishikawa et al. / Journal of Crystal Growth 278 (2005) 244–248
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55
50
In0.7Ga0.3As1-xNx QDs
In0.7Ga0.3N0.98As0.02 QDs
8K
RT N=0%
45
PL intensity (a.u.)
FWHM (meV)
N=2%
40
35
0.25 kW/cm2 0.13 kW/cm2
30
63 W/cm2 0.1
1 10 Excitation power density (W/cm2)
25 W/cm2
100
13 W/cm2
Fig. 3. The FWHMs of In0.7Ga0.3As and In0.7Ga0.3As0.98N0.02 QDs as a function of the excitation power measured by macroPL system.
1100
low excitations and then increases. This result indicates that there are some nitrogen-related states below the ground state in the In0.7Ga0.3As0.98N0.02 QDs, as is observed in the case of InGaAsN quantum wells (QWs) [10], which causes the lower energy tail in a PL spectrum. These nitrogen-related states have a less density of states than the ground state of the QDs, so that the emission from the ground state becomes dominant with increasing excitation power. Because of the change of the dominant peak in the PL spectra of the In0.7Ga0.3As0.98N0.02 QDs with an increase of the excitation power, the reduction of FWHM and the blue-shift of the peak of the In0.7Ga0.3As0.98N0.02 QDs were observed in low excitation powers. Fig. 4 shows that the excitation power dependent PL spectra of the In0.7Ga0.3As0.98N0.02 QDs at RT. No peak shift was observed in spite of the increase of the excitation power, indicating that the carriers were thermally excited from the nitrogen-related state to the ground state of QDs. This result is consistent with the previous results that the localized energy is about 20 meV [11,12].
1200
1300
Wavelength (nm) Fig. 4. The excitation power dependent PL spectra of In0.7Ga0.3As0.98N0.02 QDs measured in a macro-PL system at RT.
4. Summary The excitation (power) dependent PL of In0.7Ga0.3As1xNx QDs has been investigated by macro- and micro-PL measurements, in which the excitation power density ranges from 3.0 kW/ cm2 to 50 mW/cm2. With increasing excitation power, the emission peak of In0.7Ga0.3As0.98N0.02 QDs shifts to shorter wavelengths while the peak of In0.7Ga0.3As QDs remains at the same wavelength. The emission peak of In0.7Ga0.3As0.98N0.02 QDs has a tail on the lower energy side in low excitation powers. Since the dot size distributions of the In0.7Ga0.3As0.98N0.02 and the In0.7Ga0.3As QDs are similar, the blue-shift of the peak and the lower energy side tail could be attributed to the nitrogen-related states below the ground state, as is observed in the case of InGaAsN QWs.
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