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Optical property of self-assembled GaInNAs quantum dots grown by solid source molecular beam epitaxy
Journal of Crystal Growth 247 (2003) 279–283
Optical property of self-assembled GaInNAs quantum dots grown by solid source molecular beam epitaxy K.C. Yewa, S.F. Yoona,*, Z.Z. Suna, S.Z. Wangb a
School of Electrical and Electronic Engineering (Block S1), Nanyang Technological University, Nanyang Avenue, Block S1, Singapore 639798, Singapore b Singapore–Massachusetts Institute of Technology (MIT) Alliance, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore Received 1 September 2002; accepted 12 September 2002 Communicated by M. Schieber
Abstract Self-assembled GaInNAs/GaAs quantum dots (QDs) are promising structures for extending the emission wavelength of GaInNAs/GaAs quantum wells from 1.3 to 1.55 mm and beyond. We report herein the growth of GaInNAs/GaAs quantum dot samples of different deposited thickness by solid source molecular beam epitaxy using active nitrogen radicals generated by a radio frequency nitrogen plasma source. Images from atomic force spectroscopy reveal the increase of non-uniformity QDs size following the increase in the deposited thickness. Temperature-dependent photoluminescence (PL) measurements show PL line width shrinkage at low temperature suggests the relaxation of carriers into neighboring QD local-energy minimum. The low thermal activation energy of B72 meV, estimated from the temperature-dependent integrated PL intensity curve suggests the existence of non-radiative recombination centers that quench the luminescence intensity at higher temperature. r 2002 Elsevier Science B.V. All rights reserved. PACS: 81.05.Ea; 81.15.Np; 81.40.z Keywords: A3. Self-assembled quantum dots; A3. Solid source molecular beam epitaxy GaInNAs/GaAs
1. Introduction Quantum dot (QD) structures have novel physical properties [1,2] that can overcome the emission wavelength limitation of quantum wells (QWs). In the case of the GaInNAs/GaAs material *Corresponding author. Tel.: +65-7911744; fax: +65-7912687. E-mail address: [email protected] (S.F. Yoon).
system used in semiconductor laser, the practical emission of wavelength has so far been limited to around 1300 nm. Although photoluminescence (PL) emission from GaInNAs QW beyond 1550 nm has been reported [3,4], there is no detailed description of the crystal quality, and the emission intensity is generally low for practical application. The presence of a three-dimensional structure [5,6,7] in quantum dots opens the possibility of its usage for long wavelength
0022-0248/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 2 ) 0 1 9 8 7 - 5
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K.C. Yew et al. / Journal of Crystal Growth 247 (2003) 279–283
emission, hence overcoming the practical limitation imposed by QW structures in optoelectronic devices. So far there have been relatively few reports on the growth of GaInNAs QDs on GaAs substrates. Recent reports [5,6] on GaInNAs QDs grown by gas source molecular beam epitaxy (GSMBE) showed PL peak emission at 1.52 mm [5,6], indicating good prospects for fabricating 1.55 mm lasers using the GaInNAs QD material system. The three-dimensional (3-D) carrier confinement behavior of QDs will enable significant improvement in optical performance, such as low threshold current and high quantum efficiency compared to the QW system [1,2]. Our previous work [8] has shown that the PL efficiency tends to degrade significantly following increase in the indium (In) composition due to increase in non-radiative recombination centers. Hence for this reason, a high In composition should as much as possible be avoided in the case of GaInNAs QD growth. Our previous work [8] has also established the possibility of obtaining PL at around 1.4 mm with In composition of 50%. Therefore, our work on GaInNAs QDs reported herein will be based on materials with In composition of 50%. In this paper, we report the growth of selfassembled GaInNAs QDs with different deposited thickness on GaAs substrate. Low-temperature (5 K) PL measurement showed two luminescence peaks; one centered at around 1.3 mm, and another centered at around 1.5 mm. The effect of deposited thickness is exhibited by the reduction in the 1.3 mm emission intensity and increase in the 1.5 mm emission intensity following increase in deposited thickness. This effect, which is due to QD coalescence, will be discussed. An indication of the QD thermal stability was deduced from the temperature dependence (30–225 K) of the integrated PL intensity. The thermal activation energy of B72 meV is consistent with that of good-quality GaInNAs QWs [9,10].
2. Experimental procedure The self-assembled GaInNAs QD samples were grown on (0 0 1) GaAs substrates in a
solid source molecular beam epitaxy (SSMBE) system equipped with a radio frequency (RF) nitrogen plasma source. The plasma power was kept constant at 600 W while maintaining the growth chamber background pressure at B3.5 106 Torr throughout the GaInNAs QDs growth process. Prior to growth, the GaAs substrates were thermally heated to 5801C for 10 min under arsenic-rich condition to remove the surface oxide. The growth structure consists of a 150-nm-thick GaAs buffer layer grown at 5801C, followed by the GaInNAs QD layer grown at 4801C. Next, a 200nm-thick GaAs layer was grown at 5801C and finally, the structure was terminated by a layer of GaInNAs QDs grown at 4801C for the purpose of surface morphology examination using atomic force microscopy (AFM). The growth process was monitored closely by in situ reflection highenergy electron diffraction (RHEED). The growth rate was kept at 1 mm/h for all layers except for the QD layer, in which the growth rate was reduced to 0.8 monolayer/s (ML/s). The nominal thickness of the GaInNAs QD layer was varied from 7.5 to 22 ML for different samples, by varying the growth duration. Following common practice, the nominal N composition was determined by fitting the X-ray diffraction (XRD) data from GaNAs QWs samples using the dynamical theory, since the growth rate of GaInNAs was adjusted to be the same as that of GaNAs, while neglecting the effect of the In adatom. The surface morphology of the samples was inspected ex situ using atomic force microscopy (AFM). The PL spectrum was taken using excitation from a 514.5 nm Ar ion laser and detected using a liquid N2 cooled Ge detector in conjunction with a standard lock-in technique.
3. Results and discussion Based on our previous work [8], which showed the possibility of PL emission approaching 1.4 mm, QD samples with In composition of 50% and N composition of 0.8% were prepared with 7.5, 15 and 22 ML of deposited thickness. Fig. 1 shows the low-temperature (5 K) PL spectra of the three
K.C. Yew et al. / Journal of Crystal Growth 247 (2003) 279–283
281
600 x1
PL Intensity (a.u)
500
peak 1
GaIn0.5N0.008 As 7.5 ML GaIn0.5N0.008 As 15 ML GaIn0.5N0.008 As 22 ML
400 x3
200
11.82 [nm]
peak2
300
x3
100
0
(a) 1250 1300 1350 1400 1450 1500 1550 1600 1650 1700
Wavelength (nm)
500.00 nm
1.00 x 1.00 um
0.00
Fig. 1. PL spectra of the GaIn0.5N0.008As QDs with different deposited thickness of (a) 7.5 ML, (b) 15 ML and (c) 22 ML.
samples. It can be seen that the spectra contains two photoluminescence peaks (peak 1 and peak 2); one centered at B1.33 mm (peak 1) and another peak of weaker intensity centered at B1.55 mm (peak 2). The intensity of the lower wavelength peak (peak 1) decreases as the deposition thickness was increased from 7.5 to 22 ML. Following this, the intensity of the higher wavelength peak (peak 2) increases in relation to the lower wavelength peak (peak 1). Such behavior is attributed to coalescence of the QDs following increase in deposited thickness. At the initial state, where the deposited thickness is 7.5 ML, most of the QDs are small (average lateral size B30 nm, average height B4 nm) and uniform in size (Fig. 2(a)). When the deposited thickness increases, the QD started to coalesce with each other to form bigger dots with lower confinement energy. This contributes to increase in intensity of the higher wavelength peak (peak 2) compared to the lower wavelength peak (peak 1). Furthermore, the increased broadening of the higher wavelength peak (peak 2) following increase in deposited thickness indicates the QD size distribution is progressively becoming more non-uniform resulting from the QD coalescence. This is confirmed by AFM measurement of the QD samples, as shown in Figs. 2(a)–(c). These results show that overgrowth of the material will degrade the optical quality of the samples even though longer lumi-
20.85 [nm]
(b) 500.00 nm
1.00 x 1.00 um
0.00
21.77 [nm]
(c) 500.00 nm
1.00 x 1.00 um
0.00
Fig. 2. AFM images of GaIn0.5N0.008As QDs with deposited thickness of (a) 7.5 ML, (b) 15 ML and (c) 22 ML.
nescence wavelength can be achieved. In order to increase the quality of the PL spectra, smaller and uniform QDs are preferred, and more closely
K.C. Yew et al. / Journal of Crystal Growth 247 (2003) 279–283 30K 60K 100K 125K 150K 170K 225K
5000 x1
4500
PL Intensity (a.u)
4000
x1
3500 x 1.5
3000 2500
x2
2000
x 2.5 x 2.5
1500
144 142 140
FWHM (meV)
282
138 136 134 132 130
1000 x5
500
128 126
0 1240
1280
1320
1360
1400
1440
0
1480
20
40
60
Wavelength (nm)
stacked columnar QDs might be needed [1] to increase the luminescence wavelength. Fig. 3 plots the temperature dependence from 30 to 225 K of the QD sample with 7.5 ML deposited thickness. This sample was selected for temperature-dependent PL measurement because of its better PL quality compared to other samples. No PL peaks from other excited energy state were observed following increase in temperature. This might be due to the inhomogeneous characteristic of the QDs and coupling effect arising from its high-density nature and short distance between the dots. The photogenerated carriers have the tendency to relax into neighboring QDs that have the lowest energy rather than going into excited states as the temperature increases [11]. The observed redshift of the PL wavelength following increases in temperature is due to the effect of increased interactions from phonon scattering. Further increase in temperature degrades the PL intensity is caused by the recombination of the thermally activated carriers with non-radiative centers in the GaInNAs wetting layer. Hence, closely packed QDs are not favorable due to its greater tendency of coalescence amongst the dots; hence resulting in overall non-uniformity in the material. Fig. 4 shows the temperature dependence of the PL full-width at half-maximum (FWHM). An anomalous decrease in the FWHM to a minimum of 126 meV at 60 K was observed. Such an
100
120
140
160
180
Fig. 4. Plot of PL FWHM as function of temperature for the sample with 7.5 ML deposited thickness.
10
Integrated Intensity (a.u)
Fig. 3. Temperature dependence of GaIn0.5N0.008As QD PL spectrum from 30 to 225 K.
80
Temperature (K)
10
6
5
72 meV
GaIn 0.5 N0.008 As 7.5ML
10
4
0
4
8
12
16
20
24
28
32
36
-1
1000/T (K )
Fig. 5. Plot of PL integrated intensity as function of temperature. The activation energy is derived from the slope of the straight line fitted to the high temperature part of the graph.
anomalous temperature-dependent behavior has been reported for InAs QDs and InGaAs QDs [9,11,12,13], and is attributed to the relaxation of thermalized carriers into neighboring QDs to find a lower localized-energy minimum. As a result, QDs with the lowest energy will likely to be filled by the photogenerated carriers first, resulting in shrinkage of the line width. Beyond 60 K, electron–phonon scattering and thermal distribution of the electrons become increasingly important to a point where such effects become dominant at higher temperature, resulting in broadening of the line width.
K.C. Yew et al. / Journal of Crystal Growth 247 (2003) 279–283
Fig. 5 shows the plot of the PL integrated intensity vs. temperature for the sample with 7.5 ML deposited thickness. Using the following equation [14]: Ið0Þ IðTÞ ¼ ð1Þ 1 þ B expðEa =kTÞ where I is the integrated PL intensity, B a constant, Ea ; the thermal activation energy and Ið0Þ the integrated PL intensity at 0 K. From the Arrhenius plot, the thermal activation energy estimated from the high temperature part (170– 245 K) of the graph is B72 meV. It is believed that the observed thermal quenching of the PL intensity is mainly due to the thermionic emission of carriers into the barriers. Furthermore, it is known for a fact that the wetting layer will serve as a barrier for the thermionic emission of carriers in QDs [9,11]. In our sample, the wetting layer shows a PL peak at B1150 nm, while the QD PL peak is at B1330 nm. Hence, the energy difference is B150 meV. The thermal activation energy value somehow is only about half the energy difference between the QDs and wetting layer (B150 meV). Hence, it is possible that the presence of defects and/or dislocations in the wetting layer may have contributed to the observed quenching of PL luminescence and the reduced thermal activation energy value. This effect warrants further investigation and experimentation [14,15,16].
4. Conclusion In this paper, we report the growth of selfassembled GaIn0.5N0.008As QDs at different deposited thickness from 7.5 to 22 ML using solid source molecular beam epitaxy. Atomic force microscopy observation showed evidence of QD coalescence following increase in the deposited layer thickness. This leads to degradation of the QD uniformity and reduction in the PL intensity possibly due to increase in dislocation density. The non-uniform QD size will cause relaxation of
283
carriers into neighboring QDs with local-energy minimum, hence decreasing the FWHM following increase in temperature until the phonon scattering effect becomes dominant, beyond which the FWHM increases significantly. The thermal excitation energy of the GaInNAs QDs was derived from the temperature dependence of the integrated PL intensity. The low activation energy of B72 meV compared to the PL energy difference between the wetting layer and QD of B150 meV suggests the presence of non-radiative recombination centers and defects in the wetting layer.
References [1] M. Sugawara, Self-assemble InGaAs/GaAs quantum dots, Vol. 60, Academic Press, New York (Chapter 1). [2] D. Leonard, M. Krishnamurthy, S. Fafard, J.L. Merz, P.M. Petroff, J. Vac. Sci. Techhnol. B 12 (1994) 1063. [3] F. Hohnsdorf, J. Koch, C. Agert, W. Stolz, J. Crystal Growth 195 (1998) 391. [4] E. Tournie, M.A. Pinault, S. Vezian, J. Massies, O. Tottereau, Appl. Phys. Lett. 77 (2000) 2189. [5] S. Makino, T. Miyamoto, T. Kageyama, N. Nishiyama, F. Koyama, K. Iga, J. Crystal growth 221 (2000) 561. [6] M. Sopanen, H.P. Xin, C.W. Tu, Appl. Phys. Lett. 76 (2000) 994. [7] J. Tatebayashi, M. Nishioka, Y. Arakawa, Appl. Phys. Lett. 78 (2001) 3469. [8] K.C. Yew, S.F. Yoon, Z.Z. Sun, T.K. Ng, W.K. Loke, S.Z. Wang, W.J. Fan, 14th IPRM Conference Proceedings, Stockholm 2002, pp. 577–580. [9] Z.Y. Xu, Z.D. Lu, Z.L. Yuan, X.P. Yang, B.Z. Zheng, J.Z. Xu, Phys. Rev. B 54 (1996) 11528. [10] S. Shirakata, M. Kondow, T. Kitatani, Appl. Phys. Lett. 80 (2002) 2087. [11] Z.Y. Xu, Z.D. Lu, Z.L. Yuan, X.P. Yang, B.Z. Zheng, J.Z. Xu, Superlattices Microstruct. 23 (2) (1998) 381. [12] A. Stintz, G.T. Liu, A.L. Gray, R. Spillers, S.M. Delgado, K.J. Malloy, J. Vac. Sci. Technol. B 18 (2000) 1496. [13] B.Q. Sun, D.S. Jiang, Z. Pan, L.H. Li, R.H. Wu, Appl. Phys. Lett. 77 (2000) 4148. [14] K. Kim, A. Zunger, Phys. Rev. Lett. 86 (2001) 2609. [15] J.C. Harmand, G. Ungaro, L. Largeau, G. Le Roux, Appl. Phys. Lett. 77 (2000) 2482. [16] Z. Pan, L.H. Li, W. Zhang, Y.W. Lin, R.H. Wu, Appl. Phys. Lett. 77 (2000) 214.
Optical property of self-assembled GaInNAs quantum dots grown by solid source molecular beam epitaxy K.C. Yewa, S.F. Yoona,*, Z.Z. Suna, S.Z. Wangb a
School of Electrical and Electronic Engineering (Block S1), Nanyang Technological University, Nanyang Avenue, Block S1, Singapore 639798, Singapore b Singapore–Massachusetts Institute of Technology (MIT) Alliance, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore Received 1 September 2002; accepted 12 September 2002 Communicated by M. Schieber
Abstract Self-assembled GaInNAs/GaAs quantum dots (QDs) are promising structures for extending the emission wavelength of GaInNAs/GaAs quantum wells from 1.3 to 1.55 mm and beyond. We report herein the growth of GaInNAs/GaAs quantum dot samples of different deposited thickness by solid source molecular beam epitaxy using active nitrogen radicals generated by a radio frequency nitrogen plasma source. Images from atomic force spectroscopy reveal the increase of non-uniformity QDs size following the increase in the deposited thickness. Temperature-dependent photoluminescence (PL) measurements show PL line width shrinkage at low temperature suggests the relaxation of carriers into neighboring QD local-energy minimum. The low thermal activation energy of B72 meV, estimated from the temperature-dependent integrated PL intensity curve suggests the existence of non-radiative recombination centers that quench the luminescence intensity at higher temperature. r 2002 Elsevier Science B.V. All rights reserved. PACS: 81.05.Ea; 81.15.Np; 81.40.z Keywords: A3. Self-assembled quantum dots; A3. Solid source molecular beam epitaxy GaInNAs/GaAs
1. Introduction Quantum dot (QD) structures have novel physical properties [1,2] that can overcome the emission wavelength limitation of quantum wells (QWs). In the case of the GaInNAs/GaAs material *Corresponding author. Tel.: +65-7911744; fax: +65-7912687. E-mail address: [email protected] (S.F. Yoon).
system used in semiconductor laser, the practical emission of wavelength has so far been limited to around 1300 nm. Although photoluminescence (PL) emission from GaInNAs QW beyond 1550 nm has been reported [3,4], there is no detailed description of the crystal quality, and the emission intensity is generally low for practical application. The presence of a three-dimensional structure [5,6,7] in quantum dots opens the possibility of its usage for long wavelength
0022-0248/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 2 ) 0 1 9 8 7 - 5
280
K.C. Yew et al. / Journal of Crystal Growth 247 (2003) 279–283
emission, hence overcoming the practical limitation imposed by QW structures in optoelectronic devices. So far there have been relatively few reports on the growth of GaInNAs QDs on GaAs substrates. Recent reports [5,6] on GaInNAs QDs grown by gas source molecular beam epitaxy (GSMBE) showed PL peak emission at 1.52 mm [5,6], indicating good prospects for fabricating 1.55 mm lasers using the GaInNAs QD material system. The three-dimensional (3-D) carrier confinement behavior of QDs will enable significant improvement in optical performance, such as low threshold current and high quantum efficiency compared to the QW system [1,2]. Our previous work [8] has shown that the PL efficiency tends to degrade significantly following increase in the indium (In) composition due to increase in non-radiative recombination centers. Hence for this reason, a high In composition should as much as possible be avoided in the case of GaInNAs QD growth. Our previous work [8] has also established the possibility of obtaining PL at around 1.4 mm with In composition of 50%. Therefore, our work on GaInNAs QDs reported herein will be based on materials with In composition of 50%. In this paper, we report the growth of selfassembled GaInNAs QDs with different deposited thickness on GaAs substrate. Low-temperature (5 K) PL measurement showed two luminescence peaks; one centered at around 1.3 mm, and another centered at around 1.5 mm. The effect of deposited thickness is exhibited by the reduction in the 1.3 mm emission intensity and increase in the 1.5 mm emission intensity following increase in deposited thickness. This effect, which is due to QD coalescence, will be discussed. An indication of the QD thermal stability was deduced from the temperature dependence (30–225 K) of the integrated PL intensity. The thermal activation energy of B72 meV is consistent with that of good-quality GaInNAs QWs [9,10].
2. Experimental procedure The self-assembled GaInNAs QD samples were grown on (0 0 1) GaAs substrates in a
solid source molecular beam epitaxy (SSMBE) system equipped with a radio frequency (RF) nitrogen plasma source. The plasma power was kept constant at 600 W while maintaining the growth chamber background pressure at B3.5 106 Torr throughout the GaInNAs QDs growth process. Prior to growth, the GaAs substrates were thermally heated to 5801C for 10 min under arsenic-rich condition to remove the surface oxide. The growth structure consists of a 150-nm-thick GaAs buffer layer grown at 5801C, followed by the GaInNAs QD layer grown at 4801C. Next, a 200nm-thick GaAs layer was grown at 5801C and finally, the structure was terminated by a layer of GaInNAs QDs grown at 4801C for the purpose of surface morphology examination using atomic force microscopy (AFM). The growth process was monitored closely by in situ reflection highenergy electron diffraction (RHEED). The growth rate was kept at 1 mm/h for all layers except for the QD layer, in which the growth rate was reduced to 0.8 monolayer/s (ML/s). The nominal thickness of the GaInNAs QD layer was varied from 7.5 to 22 ML for different samples, by varying the growth duration. Following common practice, the nominal N composition was determined by fitting the X-ray diffraction (XRD) data from GaNAs QWs samples using the dynamical theory, since the growth rate of GaInNAs was adjusted to be the same as that of GaNAs, while neglecting the effect of the In adatom. The surface morphology of the samples was inspected ex situ using atomic force microscopy (AFM). The PL spectrum was taken using excitation from a 514.5 nm Ar ion laser and detected using a liquid N2 cooled Ge detector in conjunction with a standard lock-in technique.
3. Results and discussion Based on our previous work [8], which showed the possibility of PL emission approaching 1.4 mm, QD samples with In composition of 50% and N composition of 0.8% were prepared with 7.5, 15 and 22 ML of deposited thickness. Fig. 1 shows the low-temperature (5 K) PL spectra of the three
K.C. Yew et al. / Journal of Crystal Growth 247 (2003) 279–283
281
600 x1
PL Intensity (a.u)
500
peak 1
GaIn0.5N0.008 As 7.5 ML GaIn0.5N0.008 As 15 ML GaIn0.5N0.008 As 22 ML
400 x3
200
11.82 [nm]
peak2
300
x3
100
0
(a) 1250 1300 1350 1400 1450 1500 1550 1600 1650 1700
Wavelength (nm)
500.00 nm
1.00 x 1.00 um
0.00
Fig. 1. PL spectra of the GaIn0.5N0.008As QDs with different deposited thickness of (a) 7.5 ML, (b) 15 ML and (c) 22 ML.
samples. It can be seen that the spectra contains two photoluminescence peaks (peak 1 and peak 2); one centered at B1.33 mm (peak 1) and another peak of weaker intensity centered at B1.55 mm (peak 2). The intensity of the lower wavelength peak (peak 1) decreases as the deposition thickness was increased from 7.5 to 22 ML. Following this, the intensity of the higher wavelength peak (peak 2) increases in relation to the lower wavelength peak (peak 1). Such behavior is attributed to coalescence of the QDs following increase in deposited thickness. At the initial state, where the deposited thickness is 7.5 ML, most of the QDs are small (average lateral size B30 nm, average height B4 nm) and uniform in size (Fig. 2(a)). When the deposited thickness increases, the QD started to coalesce with each other to form bigger dots with lower confinement energy. This contributes to increase in intensity of the higher wavelength peak (peak 2) compared to the lower wavelength peak (peak 1). Furthermore, the increased broadening of the higher wavelength peak (peak 2) following increase in deposited thickness indicates the QD size distribution is progressively becoming more non-uniform resulting from the QD coalescence. This is confirmed by AFM measurement of the QD samples, as shown in Figs. 2(a)–(c). These results show that overgrowth of the material will degrade the optical quality of the samples even though longer lumi-
20.85 [nm]
(b) 500.00 nm
1.00 x 1.00 um
0.00
21.77 [nm]
(c) 500.00 nm
1.00 x 1.00 um
0.00
Fig. 2. AFM images of GaIn0.5N0.008As QDs with deposited thickness of (a) 7.5 ML, (b) 15 ML and (c) 22 ML.
nescence wavelength can be achieved. In order to increase the quality of the PL spectra, smaller and uniform QDs are preferred, and more closely
K.C. Yew et al. / Journal of Crystal Growth 247 (2003) 279–283 30K 60K 100K 125K 150K 170K 225K
5000 x1
4500
PL Intensity (a.u)
4000
x1
3500 x 1.5
3000 2500
x2
2000
x 2.5 x 2.5
1500
144 142 140
FWHM (meV)
282
138 136 134 132 130
1000 x5
500
128 126
0 1240
1280
1320
1360
1400
1440
0
1480
20
40
60
Wavelength (nm)
stacked columnar QDs might be needed [1] to increase the luminescence wavelength. Fig. 3 plots the temperature dependence from 30 to 225 K of the QD sample with 7.5 ML deposited thickness. This sample was selected for temperature-dependent PL measurement because of its better PL quality compared to other samples. No PL peaks from other excited energy state were observed following increase in temperature. This might be due to the inhomogeneous characteristic of the QDs and coupling effect arising from its high-density nature and short distance between the dots. The photogenerated carriers have the tendency to relax into neighboring QDs that have the lowest energy rather than going into excited states as the temperature increases [11]. The observed redshift of the PL wavelength following increases in temperature is due to the effect of increased interactions from phonon scattering. Further increase in temperature degrades the PL intensity is caused by the recombination of the thermally activated carriers with non-radiative centers in the GaInNAs wetting layer. Hence, closely packed QDs are not favorable due to its greater tendency of coalescence amongst the dots; hence resulting in overall non-uniformity in the material. Fig. 4 shows the temperature dependence of the PL full-width at half-maximum (FWHM). An anomalous decrease in the FWHM to a minimum of 126 meV at 60 K was observed. Such an
100
120
140
160
180
Fig. 4. Plot of PL FWHM as function of temperature for the sample with 7.5 ML deposited thickness.
10
Integrated Intensity (a.u)
Fig. 3. Temperature dependence of GaIn0.5N0.008As QD PL spectrum from 30 to 225 K.
80
Temperature (K)
10
6
5
72 meV
GaIn 0.5 N0.008 As 7.5ML
10
4
0
4
8
12
16
20
24
28
32
36
-1
1000/T (K )
Fig. 5. Plot of PL integrated intensity as function of temperature. The activation energy is derived from the slope of the straight line fitted to the high temperature part of the graph.
anomalous temperature-dependent behavior has been reported for InAs QDs and InGaAs QDs [9,11,12,13], and is attributed to the relaxation of thermalized carriers into neighboring QDs to find a lower localized-energy minimum. As a result, QDs with the lowest energy will likely to be filled by the photogenerated carriers first, resulting in shrinkage of the line width. Beyond 60 K, electron–phonon scattering and thermal distribution of the electrons become increasingly important to a point where such effects become dominant at higher temperature, resulting in broadening of the line width.
K.C. Yew et al. / Journal of Crystal Growth 247 (2003) 279–283
Fig. 5 shows the plot of the PL integrated intensity vs. temperature for the sample with 7.5 ML deposited thickness. Using the following equation [14]: Ið0Þ IðTÞ ¼ ð1Þ 1 þ B expðEa =kTÞ where I is the integrated PL intensity, B a constant, Ea ; the thermal activation energy and Ið0Þ the integrated PL intensity at 0 K. From the Arrhenius plot, the thermal activation energy estimated from the high temperature part (170– 245 K) of the graph is B72 meV. It is believed that the observed thermal quenching of the PL intensity is mainly due to the thermionic emission of carriers into the barriers. Furthermore, it is known for a fact that the wetting layer will serve as a barrier for the thermionic emission of carriers in QDs [9,11]. In our sample, the wetting layer shows a PL peak at B1150 nm, while the QD PL peak is at B1330 nm. Hence, the energy difference is B150 meV. The thermal activation energy value somehow is only about half the energy difference between the QDs and wetting layer (B150 meV). Hence, it is possible that the presence of defects and/or dislocations in the wetting layer may have contributed to the observed quenching of PL luminescence and the reduced thermal activation energy value. This effect warrants further investigation and experimentation [14,15,16].
4. Conclusion In this paper, we report the growth of selfassembled GaIn0.5N0.008As QDs at different deposited thickness from 7.5 to 22 ML using solid source molecular beam epitaxy. Atomic force microscopy observation showed evidence of QD coalescence following increase in the deposited layer thickness. This leads to degradation of the QD uniformity and reduction in the PL intensity possibly due to increase in dislocation density. The non-uniform QD size will cause relaxation of
283
carriers into neighboring QDs with local-energy minimum, hence decreasing the FWHM following increase in temperature until the phonon scattering effect becomes dominant, beyond which the FWHM increases significantly. The thermal excitation energy of the GaInNAs QDs was derived from the temperature dependence of the integrated PL intensity. The low activation energy of B72 meV compared to the PL energy difference between the wetting layer and QD of B150 meV suggests the presence of non-radiative recombination centers and defects in the wetting layer.
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