- Email: [email protected]
GaN quantum wells with low growth temperature GaN cap layers
ARTICLE IN PRESS
Journal of Crystal Growth 307 (2007) 363–366 www.elsevier.com/locate/jcrysgro
InGaN/GaN quantum wells with low growth temperature GaN cap layers S.T. Pendleburya, P.J. Parbrooka,, D.J. Mowbrayb, D.A. Wooda,1, K.B. Leea a
Department of Electronic and Electrical Engineering, University of Sheffield, Mappin Building, Mappin Street S1 3JD, UK b Department of Physics, University of Sheffield, Hicks Building, Sheffield S3 7RH, UK Received 6 June 2007; received in revised form 9 July 2007; accepted 10 July 2007 Communicated by R. Bhat Available online 14 July 2007
Abstract The use of a low growth temperature, thin (0–30 A˚) GaN cap is investigated as a means to reduce indium desorption following the growth of InGaN quantum wells. The cap is grown at the same low temperature as the quantum well, following which the temperature is ramped up for the growth of the remainder of the barrier. Increasing the cap layer thickness results in a red shift of the emission, consistent with decreased indium loss, and a decrease in the room temperature optical efficiency, attributed to the formation of defects in the GaN cap. r 2007 Elsevier B.V. All rights reserved. PACS: 78.55.Cr; 78.67.De; 81.10.Bk; 81.15.Kk Keywords: A1. Photoluminescence; A3. Metalorganic vapor phase epitaxy; A3. Quantum wells; B1. InGaN
1. Introduction The development of wide bandgap nitride semiconductors over the last 10 years has opened up a number of applications, including blue lasers for next generation data storage, white light emitters and solar blind UV photodiodes [1–3]. InGaN is used as the active layer in the vast majority of blue and near-UV nitride-based emitters and hence the optimization of InGaN quantum wells (QW) is very important. The growth of high-quality InGaN is made difficult by the weak bond strength of InN which, as a consequence, exhibits poor thermal stability, subliming around 550 1C. The key issue when growing InGaN is hence the incorporation of indium. This incorporation is very sensitive to the growth temperature, which provides the standard method of controlling the indium composition [4]. Corresponding author. Tel.: +44 114 22 25366; fax: +44 114 22 25143.
E-mail address: p.parbrook@sheffield.ac.uk (P.J. Parbrook). Current address: Filtronic Compound Semiconductors Ltd., Heighington Lane Business Park, Newton Aycliffe, County Durham DL5 6JW, UK. 1
0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2007.07.018
The main measure used to increase indium incorporation is therefore to lower the growth temperature from around 1000 1C, as used for GaN or AlGaN, to less than 800 1C, which results in reduced indium desorption during growth. However, at these reduced temperatures there is inefficient ammonia cracking and, as a consequence, the crystal quality is degraded. Hence during the growth of an InGaN/GaN QW, the temperature is often ramped up after the growth of the InGaN QW, before growth of the GaN barrier, in an attempt to increase the quality of the barrier. However, indium may be lost from the well during this temperature ramp [5]. In previous work, it was found that increasing the barrier growth temperature resulted in a blue shift of the QW emission, consistent with increased indium desorption [6]. In addition the well–barrier interface and emission intensity improved, whilst the emission line width, and hence degree of carrier localization, increased. These effects were attributed to a reordering of the indium during the barrier temperature ramp [6]. Given the improvements which result from an increased barrier growth temperature, it is desirable to develop a technique which allows the preservation of a high QW indium content for high-temperature growth.
ARTICLE IN PRESS S.T. Pendlebury et al. / Journal of Crystal Growth 307 (2007) 363–366
This paper describes the investigation of structures in which the InGaN QW is immediately followed by a thin GaN cap grown at the same temperature. This in turn is followed by the temperature ramp and growth of the remainder of the barrier. The low-temperature GaN cap is designed to prevent indium desorption during the subsequent temperature ramp, allowing the use of higher growth temperatures for a given QW composition. The higher growth temperature should result in an improved material quality. It should be noted that these ‘caps’, which are overgrown by GaN at higher growth temperature, are dissimilar to GaN thin ‘cap’ layers grown on InGaN near surface QW, as reported for example by Tan et al. [7], whereby the effect of the sample surface plays a critical role, rather than the bulk defects in the sample.
1.2 15Å
7.5Å
3.75Å
1
Intensity (au)
364
0Å
30Å
0.8 0.6 0.4 0.2 0 2.5
2.7
2.9
3.1
Energy (eV)
2. Experimental procedure
3. Results and discussion Fig. 1 shows normalized room temperature PL spectra of samples with different cap layer thicknesses. Fig. 2 summarizes the optical properties (room temperature PL line width, emission energy and relative intensity) as a function of the cap layer thickness. Both Figs. 1 and 2(b) clearly demonstrate a red shift of the emission with increasing cap thickness, consistent with the retention of more indium in the well. We estimate that the In content in the well will be around 20% for the thickest cap sample. Hence the cap layer appears to be effective in reducing indium desorption from the well during the growth temperature ramp. Fig. 2(c) indicates that the PL efficiency decreases with increasing cap thickness. However, the emission energy is
PL Line Width (meV)
160
140
120
2.9
2.8
PL Energy (eV)
3.0
2.7 PL Intensity (arb units)
The structures were grown by MOVPE on (0 0 0 1) sapphire substrates with a standard hydrogen carrier gas, two-step, low-temperature nucleation layer (500 1C) followed by a high-temperature (1000 1C) buffer layer. The hydrogen carrier gas was then switched to nitrogen to avoid compromising the indium incorporation [8]. Five nominal 30 A˚ InGaN QW were grown at 740 1C with 120 A˚ thick GaN barriers grown at 892 1C. X-ray diffraction (XRD) studies estimate the In fraction of the sample with no cap to be around 10%. The structure was terminated with a 2000 A˚ GaN layer. Following the growth of each QW, a thin (0–30 A˚) GaN cap was grown at the same temperature as the well (740 1C). The growth temperature was then ramped up to 892 1C over a period of 44 s during which the growth was paused. The growth rate of GaN for the low-temperature cap layer was 1 A˚/s at 740 1C and a little higher for the barrier at 892 1C. Photoluminescence (PL) in this work was excited using a He–Cd laser operating at 325 nm, with a 25 mW output power and laser spot area of 1.5 mm2. The resulting luminescence was dispersed using a 0.5 m monochromator equipped with a photomultiplier tube.
Fig. 1. Normalized room temperature PL spectra of InGaN/GaN MQWs with different cap layer thicknesses.
1.0
0.5
0.0 0
5
10 15 20 Cap Thickness (Å)
25
30
Fig. 2. (a) PL line width, (b) emission energy and (c) intensity as a function of GaN cap layer thickness.
also shifted and this is also known to strongly affect the emission efficiency [1]. To allow a more meaningful comparison of the efficiency variation, Fig. 3(b) plots the
ARTICLE IN PRESS S.T. Pendlebury et al. / Journal of Crystal Growth 307 (2007) 363–366
180
PL Line width (meV)
170 160 150 140 130 120
No cap 3.75Å
7.5Å
15Å
1.5 30Å
Intensity (arb units)
2.0
1.0
0.5
0.0 2.65
2.70
2.75 2.80 2.85 PL Emission Eenergy (eV)
2.90
2.95
Fig. 3. (a) PL line width and (b) emission intensity as a function of PL emission energy. The capping layer samples (squares) are compared with reference samples (circles) that have no GaN cap layer.
emission intensity against emission energy. Also included in this plot is data from a set of nominally identical samples (reference samples), which, however, lack the low-temperature GaN cap, and use the growth temperature of the quantum well to vary the indium content and thus the emission energy. Growth temperatures of 680 and 718 1C were used for the samples emitting at 2.655 and 2.831 eV, respectively. Fig. 3(b) reveals that for a given emission energy the intensity of the samples with the thin GaN cap is significantly lower than the corresponding reference samples. For a cap thickness \7.5 A˚ this difference is approximately one order of magnitude. The intensities of the 15 and 30 A˚ cap samples are very similar, indicating that the effect causing the decrease in intensity has saturated. Fig. 3(a) plots the emission line widths of both the cap layer samples and reference samples against emission energy. The line width of the sample with a 3.75 A˚ cap is increased by about 15% with respect to that of the sample with no cap layer but still falls slightly below the trend line for the reference samples. This would indicate that the main reason for this line width increase in this
365
sample is the increased retention of In content in the QW and that the behavior in general for such a thin cap is similar to that of a sample where no cap is used. There is also evidence of weak structure in the peak of the sample with no cap. The reason for this is not clear, although some modulation due to interference effects cause by the total layer thickness is possible. With further increase of the cap thickness, the line width decreases to values significantly less than those obtained for the reference structures. To summarize the experimental findings, as the cap layer thickness increases there are significant decreases in the emission energy and intensity, with a weaker decrease of the emission line width. The decrease in emission energy demonstrates that the cap layers are effective in preventing indium desorbing from the quantum wells during growth, justifying the initial aim of using the cap layers. The change in emission energy is relatively rapid as the thickness is increased to 3.75 A˚ but is less rapid for further increase. Given that one monolayer of GaN corresponds to a thickness of 2.6 A˚ it appears that a single monolayer is effective in reducing indium desorption from the well. The decrease in PL intensity with increasing cap thickness is most likely due to an increased number of defects in the low-temperature grown GaN cap layer, which is situated immediately next to the quantum well. A similar behavior has been observed in studies where the barrier growth temperature is varied [6]. Defects close to the quantum well are particularly significant as the electron and hole wavefunctions penetrate into the barriers. In addition, decreased carrier localization due to a modification of the InGaN structure (as observed in the decrease of the emission line width and discussed further below) may also contribute to the reduced optical efficiency [9,10]. The present finding is in contrast to the report of Kim et al. [11] who found a slight increase (1.5x) in the PL intensity after growing a cap layer of two to four monolayers. This was attributed to a decrease in the density of pits formed in the layers during the barrier temperature ramp. However, the In composition of the wells in the work of Kim et al. [11] is a little higher (28%), and was achieved using a pre-cracking of the ammonia before the growth region, making a direct comparison with the samples studied in the present work difficult. Also the emission energy of their capped layer was different to that of the uncapped layer but no account was made of this when they compared intensities. As the In content increased, stronger electron–hole separation and reduced wavefunction overlap due to the piezoelectrically induced quantum confined stark effect (QCSE) is in principle expected. However, the wells here are thin so we expect this effect to be reduced. Furthermore, we to some degree, account for this by comparing the intensities of the reference samples grown using different methods, but emitting at same wavelength which for similar well widths will have similar electric fields
ARTICLE IN PRESS 366
S.T. Pendlebury et al. / Journal of Crystal Growth 307 (2007) 363–366
present. Finally, we note that Bai et al. [12] have reported that the QCSE is not significant in their InGaN SQW samples with an In fraction of 23% and a well thickness of 2.5 nm. Based on this, we believe that the influence of varying QCSE on the PL intensity in our samples with different cap thickness is not significant. Apart from the 3.75 A˚ cap sample, the line widths of the samples with the GaN caps are slightly narrower in comparison to the sample with no cap (Fig. 2(a)) and as a function of emission energy the cap samples exhibit significantly narrower emission in comparison to the reference samples (Fig. 3(a)). This behavior is consistent with previous observations for samples in which the narrowest emission line width was found when the well and barrier were grown at the same temperature [6]. The reason for this behavior is unclear. One possibility is that indium desorption is a random process and hence decreasing the total desorption results in greater uniformity of the InGaN. Work on growth interruptions post quantum well growth by Cheong et al. [13] and Cho et al. [14] found that there was a decrease in localization as the length of the growth interruption increased. This seems contrary to our results; however, their samples had high (30%) indium fractions and the wells studied were very narrow, the thickest being 15 A˚. As their samples had no capping layer, then as the growth pauses increased the wells would also have become even narrower. This is significant as Bai et al. [12] showed that for their growth conditions localization was a maximum at 25 A˚ but decreased as the wells became narrower. Uchida et al. [15] also studied the effects of growth interruptions but their PL data showed no clear trends. However, they do suggest that growth interruptions cause an increase in pit formation, which induce fluctuations of indium around the pits. Whatever the actual reason, the present results indicate that the microscopic structure of the InGaN is very sensitive to its post-deposition treatment. Further studies will involve the growth of samples with different cap layers but exhibiting a constant emission energy (achieved by varying the quantum well growth temperature) to isolate the effects of the cap layer. However, the present results indicate that a cap thickness of 4–7 A˚ significantly reduces indium loss from the quantum wells, whilst retaining a reasonable emission intensity, and giving a line width smaller than a comparable reference structure.
4. Conclusions In conclusion, the effect of different thickness GaN cap layers on the optical properties of InGaN/GaN multiple quantum well structures has been investigated. A red shift of the PL emission energy and a decrease in PL intensity is observed with increased cap layer thickness. This behavior is consistent with a reduction in indium desorption during the barrier temperature ramp but an increase in the defect density in the low-temperature grown GaN cap, which is immediately adjacent to the wells. The emission line width indicates that the microscopic structure of the InGaN is sensitive to its post-deposition treatment. Acknowledgments This work was supported by the EPSRC. S.H.P. wishes to acknowledge student support from Thomas Swan Scientific Ltd. K.B.L. wishes to acknowledge student support from QinetiQ Ltd. References [1] S. Nakamura, S.J. Pearton, G. Fasol, The Blue Laser Diode: The Complete Story, Springer, ISBN 3540665056, 2000. [2] E. Monroy, F. Omnes, F. Calle, Semicond. Sci. Technol. 18 (2003) 33. [3] A. Hangleiter, MRS Bull. 28 (2003) 350. [4] T. Matsuoka, N. Yoshimoto, T. Sasaki, A. Katsui, J. Electron. Mater. 21 (1992) 157. [5] Y. Wang, X.J. Pei, Z.G. Xing, L.W. Gou, H.Q. Jia, H. Chen, J.M. Chou, J. Appl. Phys. 101 (2007) 033509. [6] S.M. Olaizola, S.T. Pendlebury, J.P. O’Neill, D.J. Mowbray, A.G. Cullis, M.S. Skolnick, P.J. Parbrook, A.M. Fox, J. Phys. D 35 (2002) 599. [7] L.T. Tan, R.W. Martin, K.P. O’Donnell, I.M. Watson, Appl. Phys. Lett. 89 (2006) 101910. [8] R.D. Dupuis, J. Crystal Growth 178 (1997) 56. [9] Y. Narukawa, Y. Kawakami, M. Funato, S. Fujita, S. Fujita, S. Nakamura, Appl. Phys. Lett. 70 (1997) 981. [10] Y. Sun, Y.H. Cho, E.-K. Suh, H.J. Lee, R.J. Choi, Y.B. Hahn, Appl. Phys. Lett. 84 (2004) 49. [11] S. Kim, K. Lee, K. Park, C.-S. Kim, J. Crystal Growth 247 (2003) 62. [12] J. Bai, T. Wang, S. Sakai, J. Appl. Phys. 88 (2000) 4729. [13] M.G. Cheong, H.S. Yoon, R.J. Choi, C.S. Kim, S.W. Yu, C.H. Hong, E.K. Suh, H.J. Lee, J. Appl. Phys. 90 (2001) 5642. [14] H.K. Cho, J.Y. Lee, N. Sharma, C.J. Humphreys, G.M. Yang, C.S. Kim, J.H. Song, P.W. Yu, Appl. Phys. Lett. 79 (2001) 2594. [15] K. Uchida, M. Kawata, T. Yang, A. Miwa, J. Gotoh, Jpn. J. Appl. Phys. 37 (1998) 571.
Journal of Crystal Growth 307 (2007) 363–366 www.elsevier.com/locate/jcrysgro
InGaN/GaN quantum wells with low growth temperature GaN cap layers S.T. Pendleburya, P.J. Parbrooka,, D.J. Mowbrayb, D.A. Wooda,1, K.B. Leea a
Department of Electronic and Electrical Engineering, University of Sheffield, Mappin Building, Mappin Street S1 3JD, UK b Department of Physics, University of Sheffield, Hicks Building, Sheffield S3 7RH, UK Received 6 June 2007; received in revised form 9 July 2007; accepted 10 July 2007 Communicated by R. Bhat Available online 14 July 2007
Abstract The use of a low growth temperature, thin (0–30 A˚) GaN cap is investigated as a means to reduce indium desorption following the growth of InGaN quantum wells. The cap is grown at the same low temperature as the quantum well, following which the temperature is ramped up for the growth of the remainder of the barrier. Increasing the cap layer thickness results in a red shift of the emission, consistent with decreased indium loss, and a decrease in the room temperature optical efficiency, attributed to the formation of defects in the GaN cap. r 2007 Elsevier B.V. All rights reserved. PACS: 78.55.Cr; 78.67.De; 81.10.Bk; 81.15.Kk Keywords: A1. Photoluminescence; A3. Metalorganic vapor phase epitaxy; A3. Quantum wells; B1. InGaN
1. Introduction The development of wide bandgap nitride semiconductors over the last 10 years has opened up a number of applications, including blue lasers for next generation data storage, white light emitters and solar blind UV photodiodes [1–3]. InGaN is used as the active layer in the vast majority of blue and near-UV nitride-based emitters and hence the optimization of InGaN quantum wells (QW) is very important. The growth of high-quality InGaN is made difficult by the weak bond strength of InN which, as a consequence, exhibits poor thermal stability, subliming around 550 1C. The key issue when growing InGaN is hence the incorporation of indium. This incorporation is very sensitive to the growth temperature, which provides the standard method of controlling the indium composition [4]. Corresponding author. Tel.: +44 114 22 25366; fax: +44 114 22 25143.
E-mail address: p.parbrook@sheffield.ac.uk (P.J. Parbrook). Current address: Filtronic Compound Semiconductors Ltd., Heighington Lane Business Park, Newton Aycliffe, County Durham DL5 6JW, UK. 1
0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2007.07.018
The main measure used to increase indium incorporation is therefore to lower the growth temperature from around 1000 1C, as used for GaN or AlGaN, to less than 800 1C, which results in reduced indium desorption during growth. However, at these reduced temperatures there is inefficient ammonia cracking and, as a consequence, the crystal quality is degraded. Hence during the growth of an InGaN/GaN QW, the temperature is often ramped up after the growth of the InGaN QW, before growth of the GaN barrier, in an attempt to increase the quality of the barrier. However, indium may be lost from the well during this temperature ramp [5]. In previous work, it was found that increasing the barrier growth temperature resulted in a blue shift of the QW emission, consistent with increased indium desorption [6]. In addition the well–barrier interface and emission intensity improved, whilst the emission line width, and hence degree of carrier localization, increased. These effects were attributed to a reordering of the indium during the barrier temperature ramp [6]. Given the improvements which result from an increased barrier growth temperature, it is desirable to develop a technique which allows the preservation of a high QW indium content for high-temperature growth.
ARTICLE IN PRESS S.T. Pendlebury et al. / Journal of Crystal Growth 307 (2007) 363–366
This paper describes the investigation of structures in which the InGaN QW is immediately followed by a thin GaN cap grown at the same temperature. This in turn is followed by the temperature ramp and growth of the remainder of the barrier. The low-temperature GaN cap is designed to prevent indium desorption during the subsequent temperature ramp, allowing the use of higher growth temperatures for a given QW composition. The higher growth temperature should result in an improved material quality. It should be noted that these ‘caps’, which are overgrown by GaN at higher growth temperature, are dissimilar to GaN thin ‘cap’ layers grown on InGaN near surface QW, as reported for example by Tan et al. [7], whereby the effect of the sample surface plays a critical role, rather than the bulk defects in the sample.
1.2 15Å
7.5Å
3.75Å
1
Intensity (au)
364
0Å
30Å
0.8 0.6 0.4 0.2 0 2.5
2.7
2.9
3.1
Energy (eV)
2. Experimental procedure
3. Results and discussion Fig. 1 shows normalized room temperature PL spectra of samples with different cap layer thicknesses. Fig. 2 summarizes the optical properties (room temperature PL line width, emission energy and relative intensity) as a function of the cap layer thickness. Both Figs. 1 and 2(b) clearly demonstrate a red shift of the emission with increasing cap thickness, consistent with the retention of more indium in the well. We estimate that the In content in the well will be around 20% for the thickest cap sample. Hence the cap layer appears to be effective in reducing indium desorption from the well during the growth temperature ramp. Fig. 2(c) indicates that the PL efficiency decreases with increasing cap thickness. However, the emission energy is
PL Line Width (meV)
160
140
120
2.9
2.8
PL Energy (eV)
3.0
2.7 PL Intensity (arb units)
The structures were grown by MOVPE on (0 0 0 1) sapphire substrates with a standard hydrogen carrier gas, two-step, low-temperature nucleation layer (500 1C) followed by a high-temperature (1000 1C) buffer layer. The hydrogen carrier gas was then switched to nitrogen to avoid compromising the indium incorporation [8]. Five nominal 30 A˚ InGaN QW were grown at 740 1C with 120 A˚ thick GaN barriers grown at 892 1C. X-ray diffraction (XRD) studies estimate the In fraction of the sample with no cap to be around 10%. The structure was terminated with a 2000 A˚ GaN layer. Following the growth of each QW, a thin (0–30 A˚) GaN cap was grown at the same temperature as the well (740 1C). The growth temperature was then ramped up to 892 1C over a period of 44 s during which the growth was paused. The growth rate of GaN for the low-temperature cap layer was 1 A˚/s at 740 1C and a little higher for the barrier at 892 1C. Photoluminescence (PL) in this work was excited using a He–Cd laser operating at 325 nm, with a 25 mW output power and laser spot area of 1.5 mm2. The resulting luminescence was dispersed using a 0.5 m monochromator equipped with a photomultiplier tube.
Fig. 1. Normalized room temperature PL spectra of InGaN/GaN MQWs with different cap layer thicknesses.
1.0
0.5
0.0 0
5
10 15 20 Cap Thickness (Å)
25
30
Fig. 2. (a) PL line width, (b) emission energy and (c) intensity as a function of GaN cap layer thickness.
also shifted and this is also known to strongly affect the emission efficiency [1]. To allow a more meaningful comparison of the efficiency variation, Fig. 3(b) plots the
ARTICLE IN PRESS S.T. Pendlebury et al. / Journal of Crystal Growth 307 (2007) 363–366
180
PL Line width (meV)
170 160 150 140 130 120
No cap 3.75Å
7.5Å
15Å
1.5 30Å
Intensity (arb units)
2.0
1.0
0.5
0.0 2.65
2.70
2.75 2.80 2.85 PL Emission Eenergy (eV)
2.90
2.95
Fig. 3. (a) PL line width and (b) emission intensity as a function of PL emission energy. The capping layer samples (squares) are compared with reference samples (circles) that have no GaN cap layer.
emission intensity against emission energy. Also included in this plot is data from a set of nominally identical samples (reference samples), which, however, lack the low-temperature GaN cap, and use the growth temperature of the quantum well to vary the indium content and thus the emission energy. Growth temperatures of 680 and 718 1C were used for the samples emitting at 2.655 and 2.831 eV, respectively. Fig. 3(b) reveals that for a given emission energy the intensity of the samples with the thin GaN cap is significantly lower than the corresponding reference samples. For a cap thickness \7.5 A˚ this difference is approximately one order of magnitude. The intensities of the 15 and 30 A˚ cap samples are very similar, indicating that the effect causing the decrease in intensity has saturated. Fig. 3(a) plots the emission line widths of both the cap layer samples and reference samples against emission energy. The line width of the sample with a 3.75 A˚ cap is increased by about 15% with respect to that of the sample with no cap layer but still falls slightly below the trend line for the reference samples. This would indicate that the main reason for this line width increase in this
365
sample is the increased retention of In content in the QW and that the behavior in general for such a thin cap is similar to that of a sample where no cap is used. There is also evidence of weak structure in the peak of the sample with no cap. The reason for this is not clear, although some modulation due to interference effects cause by the total layer thickness is possible. With further increase of the cap thickness, the line width decreases to values significantly less than those obtained for the reference structures. To summarize the experimental findings, as the cap layer thickness increases there are significant decreases in the emission energy and intensity, with a weaker decrease of the emission line width. The decrease in emission energy demonstrates that the cap layers are effective in preventing indium desorbing from the quantum wells during growth, justifying the initial aim of using the cap layers. The change in emission energy is relatively rapid as the thickness is increased to 3.75 A˚ but is less rapid for further increase. Given that one monolayer of GaN corresponds to a thickness of 2.6 A˚ it appears that a single monolayer is effective in reducing indium desorption from the well. The decrease in PL intensity with increasing cap thickness is most likely due to an increased number of defects in the low-temperature grown GaN cap layer, which is situated immediately next to the quantum well. A similar behavior has been observed in studies where the barrier growth temperature is varied [6]. Defects close to the quantum well are particularly significant as the electron and hole wavefunctions penetrate into the barriers. In addition, decreased carrier localization due to a modification of the InGaN structure (as observed in the decrease of the emission line width and discussed further below) may also contribute to the reduced optical efficiency [9,10]. The present finding is in contrast to the report of Kim et al. [11] who found a slight increase (1.5x) in the PL intensity after growing a cap layer of two to four monolayers. This was attributed to a decrease in the density of pits formed in the layers during the barrier temperature ramp. However, the In composition of the wells in the work of Kim et al. [11] is a little higher (28%), and was achieved using a pre-cracking of the ammonia before the growth region, making a direct comparison with the samples studied in the present work difficult. Also the emission energy of their capped layer was different to that of the uncapped layer but no account was made of this when they compared intensities. As the In content increased, stronger electron–hole separation and reduced wavefunction overlap due to the piezoelectrically induced quantum confined stark effect (QCSE) is in principle expected. However, the wells here are thin so we expect this effect to be reduced. Furthermore, we to some degree, account for this by comparing the intensities of the reference samples grown using different methods, but emitting at same wavelength which for similar well widths will have similar electric fields
ARTICLE IN PRESS 366
S.T. Pendlebury et al. / Journal of Crystal Growth 307 (2007) 363–366
present. Finally, we note that Bai et al. [12] have reported that the QCSE is not significant in their InGaN SQW samples with an In fraction of 23% and a well thickness of 2.5 nm. Based on this, we believe that the influence of varying QCSE on the PL intensity in our samples with different cap thickness is not significant. Apart from the 3.75 A˚ cap sample, the line widths of the samples with the GaN caps are slightly narrower in comparison to the sample with no cap (Fig. 2(a)) and as a function of emission energy the cap samples exhibit significantly narrower emission in comparison to the reference samples (Fig. 3(a)). This behavior is consistent with previous observations for samples in which the narrowest emission line width was found when the well and barrier were grown at the same temperature [6]. The reason for this behavior is unclear. One possibility is that indium desorption is a random process and hence decreasing the total desorption results in greater uniformity of the InGaN. Work on growth interruptions post quantum well growth by Cheong et al. [13] and Cho et al. [14] found that there was a decrease in localization as the length of the growth interruption increased. This seems contrary to our results; however, their samples had high (30%) indium fractions and the wells studied were very narrow, the thickest being 15 A˚. As their samples had no capping layer, then as the growth pauses increased the wells would also have become even narrower. This is significant as Bai et al. [12] showed that for their growth conditions localization was a maximum at 25 A˚ but decreased as the wells became narrower. Uchida et al. [15] also studied the effects of growth interruptions but their PL data showed no clear trends. However, they do suggest that growth interruptions cause an increase in pit formation, which induce fluctuations of indium around the pits. Whatever the actual reason, the present results indicate that the microscopic structure of the InGaN is very sensitive to its post-deposition treatment. Further studies will involve the growth of samples with different cap layers but exhibiting a constant emission energy (achieved by varying the quantum well growth temperature) to isolate the effects of the cap layer. However, the present results indicate that a cap thickness of 4–7 A˚ significantly reduces indium loss from the quantum wells, whilst retaining a reasonable emission intensity, and giving a line width smaller than a comparable reference structure.
4. Conclusions In conclusion, the effect of different thickness GaN cap layers on the optical properties of InGaN/GaN multiple quantum well structures has been investigated. A red shift of the PL emission energy and a decrease in PL intensity is observed with increased cap layer thickness. This behavior is consistent with a reduction in indium desorption during the barrier temperature ramp but an increase in the defect density in the low-temperature grown GaN cap, which is immediately adjacent to the wells. The emission line width indicates that the microscopic structure of the InGaN is sensitive to its post-deposition treatment. Acknowledgments This work was supported by the EPSRC. S.H.P. wishes to acknowledge student support from Thomas Swan Scientific Ltd. K.B.L. wishes to acknowledge student support from QinetiQ Ltd. References [1] S. Nakamura, S.J. Pearton, G. Fasol, The Blue Laser Diode: The Complete Story, Springer, ISBN 3540665056, 2000. [2] E. Monroy, F. Omnes, F. Calle, Semicond. Sci. Technol. 18 (2003) 33. [3] A. Hangleiter, MRS Bull. 28 (2003) 350. [4] T. Matsuoka, N. Yoshimoto, T. Sasaki, A. Katsui, J. Electron. Mater. 21 (1992) 157. [5] Y. Wang, X.J. Pei, Z.G. Xing, L.W. Gou, H.Q. Jia, H. Chen, J.M. Chou, J. Appl. Phys. 101 (2007) 033509. [6] S.M. Olaizola, S.T. Pendlebury, J.P. O’Neill, D.J. Mowbray, A.G. Cullis, M.S. Skolnick, P.J. Parbrook, A.M. Fox, J. Phys. D 35 (2002) 599. [7] L.T. Tan, R.W. Martin, K.P. O’Donnell, I.M. Watson, Appl. Phys. Lett. 89 (2006) 101910. [8] R.D. Dupuis, J. Crystal Growth 178 (1997) 56. [9] Y. Narukawa, Y. Kawakami, M. Funato, S. Fujita, S. Fujita, S. Nakamura, Appl. Phys. Lett. 70 (1997) 981. [10] Y. Sun, Y.H. Cho, E.-K. Suh, H.J. Lee, R.J. Choi, Y.B. Hahn, Appl. Phys. Lett. 84 (2004) 49. [11] S. Kim, K. Lee, K. Park, C.-S. Kim, J. Crystal Growth 247 (2003) 62. [12] J. Bai, T. Wang, S. Sakai, J. Appl. Phys. 88 (2000) 4729. [13] M.G. Cheong, H.S. Yoon, R.J. Choi, C.S. Kim, S.W. Yu, C.H. Hong, E.K. Suh, H.J. Lee, J. Appl. Phys. 90 (2001) 5642. [14] H.K. Cho, J.Y. Lee, N. Sharma, C.J. Humphreys, G.M. Yang, C.S. Kim, J.H. Song, P.W. Yu, Appl. Phys. Lett. 79 (2001) 2594. [15] K. Uchida, M. Kawata, T. Yang, A. Miwa, J. Gotoh, Jpn. J. Appl. Phys. 37 (1998) 571.