GaN single quantum well

GaN single quantum well

Solid State Communications 129 (2004) 31–35 www.elsevier.com/locate/ssc Radiative carrier recombination dependent on temperature and well width of In...

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Solid State Communications 129 (2004) 31–35 www.elsevier.com/locate/ssc

Radiative carrier recombination dependent on temperature and well width of InGaN/GaN single quantum well A. Sasakia,*, K. Nishizukaa, T. Wangb, S. Sakaic, A. Kanetad, Y. Kawakamid, Sg. Fujitad a

Department of Electronics, Osaka Electro-Communication University, Neyagawa 572-8530, Japan b Nitride Semiconductors Co. Ltd., Tokushima 770-8506, Japan c Department of Electrical and Electronic Engineering and Satellite Venture Business Laboratory, The University of Tokushima, Tokushima 770-8506, Japan d Department of Electronic Science and Engineering, Kyoto University, Kyoto 606-8501, Japan Accepted 2 September 2003 by M. Grynberg

Abstract Photoluminescence (PL) spectra and time-resolved PL are measured from around 10 to 300 K for the InGaN/GaN single quantum wells (SQWs) with well widths of 1.5, 2.5, 4 and 5 nm. For the SQWs with the well widths of 1.5 and 2.5 nm, the peak position of PL exhibits an S-shaped shift with increasing temperature. The radiative recombination time tRAD begins to increase at the temperature for the position to change from the red-shift to the blue-shift. The steep increase of tRAD is observed beyond the temperature from the blue-shift to the red-shift. For the SQWs with the well widths of 4 and 5 nm, the peak position of PL exhibits a monotonic red-shift. tRAD decreases at first and then increases with temperature. It is about 100-times longer in the low temperature region and about 10-times longer at room temperature as compared with those of the SQWs with narrower widths. q 2003 Elsevier Ltd. All rights reserved. PACS: 78.47. þ p; 78.55. 2 m; 78.66Fd; 78.66. 2 w Keywords: A. Quantum wells; D. Optical properties, excitons; E. Luminescence

1. Introduction The InGaN/GaN quantum well (QW) has been much interested in the active layer of blue and green light-emitting devices. However, the carrier recombination process of the InGaN/GaN QW has not been fully understood. Because of the large mismatching of the lattice constants between GaN and InN, the compositional inhomogeneity is presented in the InGaN alloy layer and causes local nanometer-scaled potential minima acting like quantum dots [1 – 6]. The strain is caused in the InGaN well layer due to the lattice constant difference between GaN and InGaN, * Corresponding author. Tel.: þ81-72-824-1131; fax: þ 81-72824-0014. E-mail address: [email protected] (A. Sasaki). 0038-1098/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2003.09.018

and then the InGaN well layer is subjected to the piezoelectric field [7– 11]. The blue-shift of the peak position of luminescence spectra with increasing temperature was observed and theoretically described with the recombination of excitons localized in potential minima produced by the compositional inhomogeneity [12]. Such blue-shift emissions were observed in many cases of nitride semiconductor alloys [13– 20]. They have been interpreted with the experimental data of the photoluminescence (PL) excitation [21] and the time-resolved photoluminescence (TRPL) [22– 28]. However, carrier recombination processes dependent on the well width and on temperature have not been revealed. In this study, we measured the PL spectra and the TRPL for four different well widths of InGaN/GaN single quantum wells (SQWs) with changing temperature from 7 to 300 K. Carrier recombination processes are described

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dependent on the well width and on temperature in terms of (carrier) radiative recombination lifetimes. The lifetime variations with the temperature increase are interpreted in connection with the peak shift of the PL spectra.

2. Experimental procedure The samples were grown by an atmospheric pressure metalorganic vapor phase epitaxy. The layer structure is 60nm-thick GaN cap layer/undoped In0.23Ga0.77N well layer/undoped 1.8-mm-thick GaN layer/25-nm-thick low temperature GaN buffer layer/(0001)-oriented sapphire substrate. The InGaN well widths are 1.5, 2.5, 4 and 5 nm, respectively. The indium fraction and the well width were determined by X-ray diffraction and transmission electron microscopy measurements. The samples investigated here are same as in the previous paper [29]. In that paper, about 23% of the In composition was estimated based on the bowing parameter of 1 eV. The value of the bowing parameter spreads in a wide range and still from 1.4 to 2.3 eV even in the recent results [30,31]. If we take these values, the 23% of the In composition would be overestimated. However, we cite the previous estimation for the consistency. Detailed description about the sample preparation can be seen in Ref. [29]. The PL spectra were measured with the excitation of the 325-nm He– Cd laser for all samples from 7 to 300 K. The excited power at the front of the cryostat window is 8 mW. The TRPL was measured with the excitation of 353-nm, generated with the second harmonics of the Titan:Sapphire laser output, for all samples from 12 to 300 K. The pulse width is 1.5 ps and the excited energy density is 1.29 mJ/cm2.

3. Results and discussion The PL spectra are very similar to those reported in Ref. [29]. For the SQWs with the well widths of 1.5 and 2.5 nm, the PL peak positions exhibit the S-shaped shift: the red-, the blue- and the red-shifts in succession with increasing temperature. For the SQWs with the well widths of 4 and 5 nm, they exhibit a monotonous red-shift except that a very small blue-shift was observed for the SQW with the well width of 4 nm in the low temperature region. They are shown in Fig. 1. The TRB and TBR in the figure indicate the temperature at which the shift of the PL peak position changes from the red-shift to the blue-shit and vice versa. The TRB and TBR are 100 and 150 K for the well width 1.5 nm, and 60 and 140 K for the well width 2.5 nm. The ranges of the blue-shift are 9 and 17 meV, respectively. The change of the shift occurs at the lower temperature for the wider well width. In general, with increasing temperature the band gap

shrinks due to the temperature-dependent dilation of the lattice [32,33] and electron – lattice interaction. Thus, the peak position of PL spectrum exhibits only the red-shift. The empirical equation for this characteristic has been given by Varshni [34]. It is 1g ¼ 10 2 aT 2 =ðT þ bÞ where 1g denotes the energy gap at temperature T; 10 at 0 K, and a and b constants. This band gap shrinkage with temperature increase was expressed also by a Bose – Einstein-type expression taken the electron – phonon interaction into account [35 – 37]. The constants a and b are considered in principle dependent on the sample compositions, but independent of the growth method and the well width. Assuming the In composition in the well layers of all samples being same, we used the same values of the constants a and b to all samples. The energy gap 1g variations with temperature are shown with the thin line together with the experimental data in Fig. 1. The values of a and b were evaluated from the linear interpolation from the values for GaN and InN. The values of a and b are 0.77 meV and 600 K for GaN [38] and 0.245 meV and 624 K for InN [39], respectively. The deviation of the PL peak position from the Varshni’s characteristics is considered due to the thermal broadening of carrier distribution, the delocalization of carriers, the dissociation of the excitons, and the piezoelectric field intensity with temperature change. The carrier recombination lifetime tPL was obtained from the measurement of the time-resolved PL. From this tPL ; the radiative carrier lifetimes tRAD and the non-radiative carrier lifetime tNON-RAD were derived by the equation IðTÞ ¼ Ið0ÞtNON-RAD ðTÞ={tRAD ðTÞ þ tNON-RAD ðTÞ} and 1=tPL ðTÞ ¼ {1=tRAD ðTÞ} þ {1=tNON-RAD ðTÞ} where IðTÞ and Ið0Þ denote the PL integrated intensity at temperature T and zero. The lowest temperature in the experiment was about 10 K. To evaluate the tRAD ðTÞ and tNON-RAD ðTÞ from the tPL ðTÞ; we assumed tNON-RAD ðTÞ q tRAD ðTÞ and Ið0Þ 6 Ið10Þ: The PL spectra were reduced to several Gaussian distributions. The Gaussian distribution centered at the PL peak position at the temperature T was taken as the PL integrated intensity IðTÞ: The carrier recombination lifetimes tPL ðTÞ; tRAD ðTÞ; tNON-RAD ðTÞ for the four samples are shown in Fig. 2. With the increase of temperature, the radiative carrier lifetimes and the PL peak positions are changed. Fig. 3 shows the radiative carrier lifetimes with respect to the PL peak positions. It can be seen in Fig. 3(a) and (b) for the narrower wells that (1) the radiative carrier lifetimes are not much increased in the red-shift region at the low temperature region, (2) begin to increase gradually in the blue-shit range, and (3) steeply increase beyond the red-shift range. Their values are summarized in Table 1. As seen in Fig. 4(a) and (b) for the wider wells, the radiative carrier lifetimes decrease at first in the low temperature region and then increase to the fairly large values. The increasing rates with the temperature region are listed in Table 2. The carrier recombination processes can be suspected based on the results. The excitons localized in the potential

A. Sasaki et al. / Solid State Communications 129 (2004) 31–35

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Fig. 1. PL peak position dependent on temperature. The TRB is the temperature at which the PL position changes from the red-shift to the blueshift and the TBR vice versa. The thin curve lines were calculated by Varshni’s equation. (a) For the wells of 1.5 and 2.5 nm widths and (b) for the wells of 4 and 5 nm widths. Table 1 The values of the temperatures, TRB and TBR ; and the radiative lifetimes at their temperatures and room temperature 300 K d (nm)

TRB (K)

tRADRB (ns)

TBR (K)

tRADBR (ns)

tRAD300 (ns)

1.5 2.5

100 60

1.71 1.61

150 140

4.77 11.1

168 242

minima produced by the In inhomogeneity are released with increasing temperature and then the binding energy is reduced. The blue-shift is observed when the reduction in the binding energy overcomes the band shrinkage due to the lattice dilation. The localization relief causes the increase of

non-radiative recombination processes. The excitons are less localized in the potential minima in the red-shift range beyond the blue-shift range and thus the non-radiative recombination processes are steeply increased. The PL peak positions are not much changed in the narrower wells with increasing the excited power in the PL measurement, but clearly increased in the wider wells [29]. It has been understood in terms of the quantum confinement Stark effect [40 – 42]. Since electrons and holes are spatially separated in the wider wells, the radiative recombination lifetimes become relatively greater than those in the narrower wells. The radiative recombination lifetimes decrease in the low temperature region. It is considered at present that thermalization supplies the energy for carriers

Fig. 2. Carrier recombination lifetimes dependent on temperature. The tPL ; tRAD and tNON-RAD denote measured, radiative, and non-radiative lifetimes, respectively.

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Fig. 3. Radiative lifetime tRAD with respect to PL peak position. (a) For the well of 1.5 nm width and (b) for the well of 2.5 nm width. Table 2 The increasing rates of the radiative lifetimes with respect to temperature Well width (nm)

Red-shift (ns/K)

Blue-shift (ns/K)

To 300 K (ns/K)

1.5 2.5 4 5

0.0194 0.0335

0.0612 0.119

1.09 1.44 4.15 2.12

The values in the red-shift and the blue-shift columns are the average rates in their shift ranges. The values in the column ‘to 300 K’ are the average rates from TBR to 300 K for the wells of 1.5 and 2.5 nm widths, respectively, and they are the average rates from 90 to 300 K for the well of 4 nm and from 160–300 K for the well of 5 nm width.

to move away from the interface where non-radiative centers exist. The difference in the PL peak positions from those given by the Varshni’s equation suggests recombination processes

excluding the band gap shrinkage caused by the lattice dilation. The difference increases still beyond the temperature TBR in the narrower wells. The main reason could be considered due to the carrier thermalization. In the wider wells, the PL peak positions are lower than the Varshni’s values. The detail investigation of the values a and b for the quantum well of alloy semiconductors are the future subject.

4. Summary Radiative carrier recombination lifetimes tRAD dependent on temperature from 10 to 300 K and well widths, 1.5, 2.5, 4 and 5 nm of In0.23Ga0.77N/GaN SQWs have been investigated. The tRAD begins to increase at the temperature for the PL peak position to change from the red-shift to the blue-shift and it increases steeply beyond the temperature from the blue-shift to the red-shift in the narrower wells. In the wider wells, the tRAD decreases in the low temperature region and then increases monotonically to room temperature. The value becomes about 100-times longer at low

Fig. 4. Radiative lifetime tRAD with respect to PL peak position. (a) For the well of 4 nm width and (b) for the well of 5 nm width.

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temperature and about 10-times longer at room temperature relative to those of the narrower wells.

Acknowledgements The subject was in part performed in the Academic Frontier Promotion Project of Osaka Electro-Communication University.

References [1] Y. Narukawa, Y. Kawakami, M. Funato, Sz. Fujita, Sg. Fujita, S. Nakamura, Appl. Phys. Lett. 70 (1997) 981. [2] S. Chichibu, K. Wada, S. Nakamura, Appl. Phys. Lett. 71 (1997) 2346. [3] Y. Narukawa, Y. Kawakami, M. Funato, Sz. Fujita, Sg. Fujita, S. Nakamura, Phys. Rev. B 55 (1997) R1938. [4] P.A. Crowell, D.K. Young, S. Keller, E.L. Hu, D.D. Awschalom, Appl. Phys. Lett. 72 (1998) 927. [5] D. Behr, J. Wagner, A. Ramakrishnan, H. Obloh, K.-H. Bachem, Appl. Phys. Lett. 73 (1998) 241. [6] A. Vertikov, I. Ozden, A.V. Nurmikko, Appl. Phys. Lett. 74 (1999) 850. [7] S. Chichibu, A.C. Abare, M.S. Minsky, S. Keller, S.B. Fleisher, J.E. Bowere, E.L. Hu, U.K. Mishra, L.A. Colden, S.P. DenBaars, T. Sota, Appl. Phys. Lett. 73 (1998) 2006. [8] T. Takeuchi, C. Wetzel, S. Yamaguchi, H. Sakai, H. Amano, I. Akasaki, Y. Kaneko, S. Nakagawa, Y. Yamaoka, N. Yamada, Appl. Phys. Lett. 73 (1998) 1691. [9] C. Wetzel, T. Takeuchi, H. Amano, I. Akasaki, Jpn. J. Appl. Phys., Part 2 38 (1999) L163. [10] C. Wetzel, T. Takeuchi, H. Amano, I. Akasaki, J. Appl. Phys. 85 (1999) 3786. [11] H. Kollmer, J.S. Im, S. Heppel, J. Off, F. Scholz, A. Hangleiter, Appl. Phys. Lett. 74 (1999) 82. [12] P.G. Eliseev, P. Perlin, J. Lee, M. Osinski, Appl. Phys. Lett. 71 (1997) 569. [13] W. Shan, B.D. Little, J.J. Song, Z.C. Feng, M. Schuman, R.A. Stall, Appl. Phys. Lett. 69 (1996) 3315. [14] K.L. Teo, J.S. Colton, P.Y. Yu, E.R. Weber, M.F. Li, W. Liu, K. Uchida, H. Tokunaga, N. Akutsu, K. Matsumoto, Appl. Phys. Lett. 73 (1998) 1697. [15] W. Shan, W. Walukiewicz, E.E. Haller, B.D. Little, J.J. Song, M.D. mcCluskey, N.M. Johnson, Z.C. Feng, M. Schuman, R.A. Stall, J. Appl. Phys. 84 (1998) 4452. [16] Y.-H. Cho, G.H. Gainer, A.J. Fischer, J.J. Song, S. Keller, U.K. Mishra, S.P. DenBaars, Appl. Phys. Lett. 73 (1998) 1370. [17] Y. Narukawa, S. Saijou, Y. Kawakami, Sg. Fujita, T. Mukai, S. Nakamura, Appl. Phys. Lett. 74 (1999) 558.

35

[18] S. Chu, T. Saisho, K. Fujimura, S. Sakakibara, F. Tanoue, K. Ishino, A. Ishida, H. Harima, Y. Oka, K. Takahiro, Y. Chen, H. Fujiyasu, Jpn. J. Appl. Phys., Part 1 38 (1999) 4973. [19] P. Riblet, H. Hirayama, A. Kinoshita, A. Hirata, T. Sugano, Y. Aoyagi, Appl. Phys. Lett. 75 (1999) 2241. [20] H.P.D. Schenk, P. de Mierry, F. Omnes, P. Gibart, Phys. Status Solidi A 176 (1999) 303. [21] S. Chichibu, T. Azuhata, T. Sota, S. Nakamura, Appl. Phys. Lett. 70 (1997) 2822. [22] M. Smith, G.D. Chen, J.Y. Lin, H.X. Jiang, M.A. Khan, Q. Chen, Appl. Phys. Lett. 69 (1996) 2837. [23] T.J. Schmit, Y.-H. Cho, G.H. Gainer, J.J. Song, S. Keller, U.K. Mishra, S.P. DenBaars, Appl. Phys. Lett. 73 (1998) 560. [24] M. Pophristic, H.H. Long, C. Tran, R.F. Karlicek Jr., Z.C. Feng, I.T. Ferguson, Appl. Phys. Lett. 73 (1998) 815. [25] T.J. Schmit, Y.-H. Cho, G.H. Gainer, J.J. Song, S. Keller, U.K. Mishra, S.P. DenBaars, Appl. Phys. Lett. 73 (1998) 1982. [26] M. Pophristic, H.H. Long, C. Tran, I.T. Ferguson, R.F. Karlicek Jr., Appl. Phys. Lett. 73 (1998) 3550. [27] M. Pophristic, H. H. Long, C. Tran, I. T. Ferguson, R. F. Karlicek Jr., J. Appl. Phys. 86 (1999) 1114. [28] Y. Narukawa, Y. Kawakami, Sg. Fujita, S. Nakamura, Phys. Rev. B 59 (1999) 10283. [29] J. Bai, T. Wang, S. Sakai, J. Appl. Phys. 88 (2000) 2729. [30] J. Wu, W. Walukiewcz, K.M. Yu, J.W. Ager III, E.E. Haller, H. Lu, W.J. Schaff, Appl. Phys. Lett. 80 (2002) 4741. [31] M. Hori, K. Kano, T. Yamaguchi, Y. Saito, T. Araki, Y. Nanishi, N. Teraguchi, A. Suzuki, Phys. Status Solidi B 234 (2002) 750. [32] R. Mo¨glich, R. Rompe, Z. Phys. 119 (1942) 492. [33] J. Bardeen, W. Shockley, Phys. Rev. 80 (1950) 72. [34] Y.P. Varshni, Physica 34 (1967) 149. [35] L. Vin˜a, S. Logothetidis, M. Cardona, Phys. Rev. B 30 (1984) 1979. [36] P. Lautenschlager, M. Garriga, S. Logothetidis, M. Cardona, Phys. Rev. B 35 (1987) 9174. [37] C.F. Li, Y.S. Huang, L. Malikova, F.H. Pollak, Phys. Rev. B 55 (1997) 9251. [38] M.E. Levinshtein, S.L. Rumyantsev, M.S. Shur, Properties of Advancd Semiconductor Materials, Wiley, New York, 2001, pp. 4. [39] Q. Guo, A. Yoshida, Jpn. J. Appl. Phys. 33 (1994) 2453. [40] D.A.B. Miller, D.S. Chelma, T.C. Damen, A.C. Gossard, W. Wiegman, T.H. Wood, A. Burus, Phys. Rev. B 32 (1985) 1043. [41] S. Schmitt-Rink, D.S. Chelma, D.A.B. Miller, Adv. Phys. 38 (1989) 89. [42] G. Bastard, Wave Mechanics Applied to Semiconductor Heterostructures, Halsted Press, 1988, Chapt VIII; and herein references.