Enhanced carrier localization in near-ultraviolet multiple quantum wells using quaternary AlInGaN as the well layers

Abstract

The structural and optical properties of near-ultraviolet (UV) multiple quantum well (MQW) structures using quaternary AlInGaN as the well layers have been investigated. The composition of the barrier layers is determined by three In_0.08Ga_0.92N/Al_xIn_yGa_1−x−yN multiple quantum well samples with varying Al content in the barrier layers. The compositions of the well and barrier layers are estimated from the high-resolution X-ray diffraction (HRXRD) results. In spite of the larger lattice mismatch, the remarkable enhancement of the photoluminescence (PL) intensity of the MQWs sample with AlInGaN as the well layers is attributed to the increase in the carrier localized states induced by the increase in the compositional fluctuation in the AlInGaN well layers. The S-shaped temperature-dependence of the PL peak energy indicates the existence of localized states induced by the potential fluctuations. The magnitude of the carrier localization, which is estimated by the fitting results, is significantly increased in the Al_0.11In_0.13Ga_0.76N/Al_0.16In_0.045Ga_0.795N MQWs due to the improvement of the spatial potential fluctuations using quaternary AlInGaN as the well layers.

---

Introduction

III-Nitride-based ultraviolet multiple quantum well light emitting diodes (LEDs) and laser diodes (LDs) have attracted great attention in recent years for their extensive potential applications in photocatalyst¹ and high-density optical storage systems,² particularly in solid-state lighting using near-UV LEDs light, owing to the high conversion efficiency of the phosphors.³ High-efficiency LED devices can be achieved by the existence of localized exciton states in the active regions.⁴ In conventional LEDs, InGaN/GaN MQW structures have been used as active region for the green, blue and near-UV emission and have led to successful commercialization of the devices. Due to the formation of potential minima at the In-segregation regions in InGaN well layers, the carrier capture is more efficient than the non-radiative recombination centers.⁵ However, the internal quantum efficiency is still limited by the weaker carrier confinement effect in the InGaN/GaN near-UV LEDs because of the small band offset in the quantum wells (QWs). In order to obtain a larger carrier confinement, AlGaN is also used as barrier layers, which have larger band offsets in the QWs. Nevertheless, the existence of biaxial strain and poor material quality severely limit the use of the InGaN/AlGaN MQWs.⁶

Recently, it has been suggested that the optical properties and carrier confinement were enhanced when quaternary AlInGaN was used in place of GaN barrier layers in near-UV LEDs,⁷ as the lattice constant and band gap of the quaternary AlInGaN can be independently controlled by adjusting the composition of AlN, InN and GaN.^8–11 We have reported that the carrier localization can be improved by optimizing the Al composition of the barrier layers in blue In_0.20Ga_0.80N/AlInGaN MQWs with an emission wavelength around 450 nm.⁹

In recent years, several research groups have reported AlInGaN/AlInGaN near-UV MQWs LEDs with greater carrier localization^12,13 due to a uniform growth condition and small lattice mismatch between the well and barrier layers. Mee-Yi Ryu et al. suggested that the PL properties dominated by localized carriers in the AlInGaN MQWs agree well with those of InGaN/GaN MQWs with a high density of localized states.^14,15 P. Lefebvre et al. reported that the in-plane localization of carriers induced by local potential fluctuations could be enhanced by the optimum well width.¹⁶ However, the mechanism of the great carrier localization in the quaternary well layers is not yet clear. In this work, we study the carrier localization effect of the Al_x′In_y′Ga_{1−x′−y′}N/Al_xIn_yGa_1−x−yN MQW structures. As the carrier localization can also be influenced by the composition of the barrier layers, three In_0.08Ga_0.92N/Al_xIn_yGa_1−x−yN MQWs samples were first grown with a similar emission energy to optimize the barrier composition.

Experimental

All the samples were grown on c-plane sapphire substrates using a Thomas Swan 19 × 2′′ metal organic chemical vapor deposition (MOCVD) system. Trimethylaluminum (TMAl), trimethylindium (TMIn), triethylgallium (TEGa) and ammonia (NH₃) were used as the source precursors for Al, In, Ga and N, respectively. The sapphire substrate was first heated to 1100 °C in a hydrogen atmosphere for the thermal cleaning treatment. Then, a 25 nm-thick low temperature GaN buffer layer was grown, followed by a 1.5 μm-thick undoped GaN template layer. Afterwards, a ten-period MQW structure was deposited with 2.5 nm-thick In_0.08Ga_0.92N well layers and 15 nm-thick Al_xIn_yGa_1−x−yN barrier layers. During the growth process of the barrier layers, the flow rates of TEGa, TMIn and NH₃ were maintained at a constant flow rate and the flow rate of TMAl was set to be 3.5, 4.7 and 5.7 sccm and labeled as samples 1, 2 and 3, respectively. Moreover, a MQW structure with Al_x′In_y′Ga_{1−x′−y′}N well layers was fabricated using the same barrier composition as sample 2 and labeled as sample 4. The detailed growth conditions have been published elsewhere.^8–10

High-resolution X-ray diffraction was employed to analyze the structural parameters. Raman measurements were carried out to verify the existence of the In-rich clusters. Temperature-dependent photoluminescence spectra were measured to investigate the carrier localization using an iHR320 spectrometer with a He–Cd laser at 325 nm as the exciting source.

Results and discussion

In order to obtain the optimized alloy composition in the well and barrier layers, three near-UV In_0.08Ga_0.92N/Al_xIn_yGa_1−x−yN MQWs samples were prepared. Fig. 1 shows the HRXRD (0002) ω-2θ scans of the MQWs samples. In order to determine the composition of the Al_xIn_yGa_1−x−yN barrier layers, some AlInGaN epilayers and similar MQW structures were fabricated under the same growth conditions.^9,10 The detailed growth and structural parameters of the samples are listed in Table 1. The satellite peaks can be observed up to the fourth order, indicating a better interface quality and periodicity in the QWs. In the In_0.08Ga_0.92N/Al_xIn_yGa_1−x−yN MQW samples, the Al content of the barrier layers increases with the increasing TMAl flow rate. The smooth satellite peaks of sample 2 indicate a better lattice-match between the barrier and GaN template layer. Then, sample 4 was grown with the same composition of the barrier layer as sample 2. In addition, the composition of the AlInGaN well layer was chosen to have a theoretically similar emission energy, which is calculated from the following equations:¹⁷

E_g^u(Al_uIn_1−uN) = uE_g(AlN) + (1 − u)E_g(InN) − u(1 − u)b(AlInN) (2)

E_g^v(In_vGa_1−vN) = vE_g(InN) + (1 − v)E_g(GaN) − v(1 − v)b(InGaN) (3)

E_g^w(Al_wGa_1−wN) = wE_g(AlN) + (1 − w)E_g(GaN) − w(1 − w)b(AlGaN) (4)

where

u = (1 − x + y)/2, v = (1 − y + z)/2, w = (1 − x + z)/2 (5)

where x, y and z = 1 − x − y represent the contents of Al, In and Ga in the quaternary Al_xIn_yGa_1−x−yN, respectively. b(AlInN), b(InGaN) and b(AlGaN) are the band gap bowing parameters of the AlInN, InGaN and AlGaN, respectively.

Fig. 1 HRXRD (0002) ω-2θ scans of the MQW samples. Samples 1–3 are the MQWs with In_0.08Ga_0.92N well layers and sample 4 is the MQWs with Al_0.11In_0.13Ga_0.76N well layers.

Table 1 Structural parameters and optical properties of the MQW samples

Sample	XRD				PL
	Well		Barrier		10 K		300 K
	Al (%)	In (%)	Al (%)	In (%)	Energy (eV)	FWHM (meV)	Energy (eV)	FWHM (meV)
1	0	8	15	5.5	3.194	52	3.173	123
2	0	8	16	4.5	3.232	52	3.198	123
3	0	8	17	4	3.199	54	3.169	126
4	11	13	16	4.5	3.171	61	3.150	134

---

The lattice mismatch f between the well and barrier layers were numerically evaluated to be 0.65%, 0.78%, 0.84% and 1% for the MQW samples 1–4, respectively, using the following expression:¹⁸

f = (a_W − a_B)/a_B (6)

where a_W and a_B denote the lattice constants of the well and barrier materials, respectively.

It was confirmed that the In-segregation effect taking place in the well layers of InGaN-based blue LEDs is the source of the high radiative recombination efficiency.^19,20 Raman scattering measurements were carried out at room temperature to study the In-rich clusters in these near-UV MQW samples. The inset of Fig. 2 shows the Raman scattering of sample 4 over the measurement range of 100–1000 cm⁻¹. The scale was enlarged to get a clear picture of the Raman vibration of the In-rich clusters, as shown in Fig. 2. Two vibration modes, which can be clearly seen in these four samples at around 432 cm⁻¹ and 453 cm⁻¹, were considered to be from the vibration of InN⁹ and indicate the existence of nearly pure InN regions in the quantum wells. Therefore, local potential fluctuations could be formed around the In-rich regions. Evidently, the intensities of the two vibration modes from sample 4 become stronger as induced by the higher In content in the well layers. Moreover, the shift to lower frequency of the vibration peak positions in sample 4 is considered to be also affected by the increase in the In content.²¹

Fig. 2 Raman scattering of the MQWs samples at room temperature. The vertical dashed lines indicate the positions of the In-cluster related Raman vibrations. The inset is the Raman scattering of sample 4 over the measurement range of 100–1000 cm⁻¹.

The PL spectra measurements of the MQW samples were performed for 10 K (Fig. 3(a)) and 300 K (Fig. 3(b)). At 10 K and 300 K, the PL spectra of the MQW samples are dominated by a sharp emission peak. In the blue In_0.20Ga_0.80N/Al_xIn_yGa_1−x−yN MQWs, the PL emission peak energy exhibits a clear redshift with increasing Al content in the barrier layers due to the increase in the quantum-confined Stark effect (QCSE).⁹ In this work, however, the PL emission peak energies of samples 1–3 with increasing Al content in barrier layers do not present this redshift. We speculate that the reduced band offsets in the QWs may be responsible for this phenomenon, due to the larger band gap of the In_0.08Ga_0.92N well layers compared to that of the blue MQWs. It is worth noting that the PL intensity of sample 4 is nearly twice as strong as that of the samples with In_0.08Ga_0.92N well layers, in spite of the larger lattice mismatch of up to 1% in the QWs. Moreover, the FWHM of the PL curve from sample 4 is larger than that of the other samples at either 10 or 300 K, as shown in Table 1. Therefore, we speculate that the intense emission of sample 4 is attributed to the increase in the local compositional variation by introducing Al atoms into the InGaN well layers, leading to the enhancement of the spatial potential fluctuation and carrier localization effect, and thus, the radiative recombination efficiency is significantly improved.

Fig. 3 PL spectra of the MQW samples measured at (a) 10 K and (b) 300 K.

The temperature-dependent PL spectra were measured in the temperature range from 10 to 300 K to support our speculation that the carrier localization effect is enhanced by increasing the compositional fluctuation. The S-shaped temperature-dependence of the PL peak energy in these MQW samples, that is, the redshift–blueshift–redshift, can be clearly observed in Fig. 4. This behavior is a well-known manifestation of the existence of localized carriers in the QWs.^22,23 To estimate the magnitude of the carrier localization effect in these MQW samples, the measurement results of the emission energy were fitted by the modified Varshni empirical equation:²⁴

E(T) = E(0) − αT²/(T + β) − σ²/K_BT (7)

Fig. 4 PL emission peak energy as a function of temperature in the samples with MQWs of (a) In_0.08Ga_0.92N/Al_0.15In_0.055Ga_0.795N, (b) In_0.08Ga_0.92N/Al_0.16In_0.045Ga_0.795N, (c) In_0.08Ga_0.92N/Al_0.17In_0.04Ga_0.79N and (d) Al_0.11In_0.13Ga_0.76N/Al_0.16In_0.045Ga_0.795N. The solid dots stand for the measurement data. The solid line shows the best fitting results with the Varshni empirical equation. The fitting parameters are also given in the figures.

This equation is only valid for temperatures above 70 K. E(0) is the band gap energy at 0 K; α and β are the Varshni thermal coefficients; K_B is the Boltzmann constant; σ indicates the magnitude of the localization effect, namely, a larger value of σ means a stronger carrier localization effect. The fitting results are plotted by the solid lines and the fitting parameters are also given in Fig. 4(a)–(d). The magnitude of the carrier localization increases significantly in sample 4 compared with the In_0.08Ga_0.92N/Al_xIn_yGa_1−x−yN MQW samples, indicating that more carriers are confined in the localized states in the QWs. Therefore, the radiative recombination efficiency will be improved significantly by introducing Al atoms into the InGaN well layers, due to the increase in the local potential fluctuation.

Alternatively, the transition temperature from the redshift to blueshift of the MQW samples 1–4 is 40, 80, 80 and 100 K, and that from blueshift to redshift is 110, 140, 150 and 170 K, respectively. The blueshift energy between the two transition temperatures is about 4, 3, 1 and 6 meV for these four MQW samples. The expected temperature-reduced band gap shrinkage (redshift) is about 5, 9, 11 and 13 meV for the corresponding temperature regions, which were estimated from GaN.²⁵ Therefore, the actual blueshift energy of the emission peaks with respect to the band edge is about 9, 12, 12 and 19 meV. The larger amount of blueshift energy means a stronger carrier localization, which accords well with the variation tendency of the magnitude of the localization effect (σ).

Conclusions

In summary, near-UV MQW structures using quaternary AlInGaN as the well layers were grown. In spite of the larger lattice mismatch, the PL intensity could be remarkably enhanced by substituting quaternary Al_0.11In_0.13Ga_0.76N for In_0.08Ga_0.92N as the well layers. This can be attributed to the increase in the carrier localized states induced by the increase in the local compositional variation. The S-shaped temperature-dependence of PL peak energy indicated that the magnitude of the carrier localization effect, which is estimated by the fitting results, was enhanced significantly by introducing Al atoms into the InGaN well layers due to the improvement of the spatial potential fluctuations. Therefore, the carrier localization becomes strong enough to prevent their capture by the non-radiative recombination centers, and hence the optical properties can be enhanced using quaternary AlInGaN as the well layers in near-UV MQWs, which may open up an opportunity for the GaN-based high-efficiency LED devices.

Acknowledgements

This work was supported by National Science Foundation (no. 61306014), Harbin Special Fund for Creative Talents in Science and Technology (no. 2012RFLXG029), Research Fund for the Doctoral Program of Higher Education of China (no. 20122302120009), and Open Project Program of Key Laboratory for Photonic and Electric Band gap Materials, Ministry of Education, Harbin Normal University (no. PEBM201302).

Notes and references

1. P. M. Tu, C. Y. Chang, S. C. Huang, C. H. Chiu, J. R. Chang, W. T. Chang, D. S. Wuu, H. W. Zan, C. C. Lin, H. C. Kuo and C. P. Hsu, Appl. Phys. Lett., 2011, 98, 211107.
2. S. N. Lee, S. Y. Cho, H. Y. Ryu, J. K. Son, H. S. Paek, T. Sakong, T. Jang, K. K. Choi, K. H. Ha, M. H. Yang, O. H. Nam, Y. Park and E. Yoon, Appl. Phys. Lett., 2006, 88, 111101.
3. M. Shatalov, A. Chitnis, V. Adivarahan, A. Lunev, J. Zhang, J. W. Yang, Q. Fareed, G. Simin, A. Zakheim, M. Asif Khan, R. Gaska and M. S. Shur, Appl. Phys. Lett., 2001, 78, 817.
4. H. Zhu, C. X. Shan, B. H. Li, Z. Z. Zhang, J. Y. Zhang, B. Yao, D. Z. Shen and X. W. Fan, J. Appl. Phys., 2009, 105, 103508.
5. J. Zhang, J. Yang, G. Simin, M. Shatalov, M. Asif Khan, M. S. Shur and R. Gaska, Appl. Phys. Lett., 2000, 77, 2668.
6. D. Zhu, M. J. Kappers, P. M. F. J. Costa, C. McAleese, F. D. G. Rayment, G. R. Chabrol, D. M. Graham, P. Dawson, E. J. Thrush, J. T. Mullins and C. J. Humphreys, Phys. Status Solidi A, 2006, 203, 1819.
7. S. H. Baek, J. O. Kim, M. K. Kwon, I. K. Park, S. I. Na, J. Y. Kim, B. Kim and S. J. Park, IEEE Photonics Technol. Lett., 2006, 18, 1276.
8. T. Liu, S. J. Jiao, D. B. Wang, L. C. Zhao, T. P. Yang and Z. G. Xiao, Appl. Surf. Sci., 2014, 301, 178.
9. T. Liu, S. J. Jiao, D. B. Wang, S. Y. Gao, T. P. Yang, H. W. Liang and L. C. Zhao, J. Alloys Compd., 2015, 621, 12.
10. D. B. Wang, S. J. Jiao, L. C. Zhao, T. Liu, S. Y. Gao, H. T. Li, J. Z. Wang, Q. J. Yu and F. Y. Guo, J. Phys. Chem. C, 2013, 117, 543.
11. S. Suihkonen, O. Svensk, P. T. Törmä, M. Ali, M. Sopanen, H. Lipsanen, M. A. Odnoblyudo and V. E. Bougrov, J. Cryst. Growth, 2008, 310, 1777.
12. T. Wang, G. Raviprakash, F. Ranalli, C. N. Harrison, J. Bai, J. P. R. David, P. J. Parbrook, J. P. Ao and Y. Ohno, J. Appl. Phys., 2005, 97, 083104.
13. Y. Liu, T. Egawa, H. Ishikawa, H. Jiang, B. Zhang, M. Hao and T. Jimbo, J. Cryst. Growth, 2004, 264, 159.
14. M. Y. Ryu, C. Q. Chen, E. Kuokstis, J. W. Yang, G. Simin and M. Asif Khan, Appl. Phys. Lett., 2002, 80, 3730.
15. M. Y. Ryu, C. Q. Chen, E. Kuokstis, J. W. Yang, G. Simin, M. Asif Khan, G. G. Sim and P. W. Yu, Appl. Phys. Lett., 2002, 80, 3943.
16. P. Lefebvre, S. Anceau, P. Valvin, T. Taliercio, L. Konczewicz, T. Suski, S. P. Łepkowski, H. Teisseyre, H. Hirayama and Y. Aoyagi, Phys. Rev. B: Condens. Matter Mater. Phys., 2002, 66, 195330.
17. A. J. Ghazai, S. M. Thahab, H. Abu Hassan and Z. Hassan, Optik, 2012, 123, 856.
18. A. Shalimov, J. Bąk-Misiuk, V. M. Kaganer, M. Calamiotou and A. Georgakilas, J. Appl. Phys., 2007, 101, 013517.
19. Y. J. Sun, O. Brandt, B. Jenichen and K. H. Ploog, Appl. Phys. Lett., 2003, 83, 5178.
20. S. Y. Karpov, R. A. Talalaev, I. Y. Evstratov and Y. N. Makarov, Phys. Status Solidi A, 2002, 192, 417.
21. M. R. Correia, S. Pereira, E. Pereira, J. Frandon and E. Alves, Appl. Phys. Lett., 2003, 83, 4761.
22. X. A. Cao, S. F. LeBoeuf, L. B. Rowland, C. H. Yan and H. Liu, Appl. Phys. Lett., 2003, 82, 3614.
23. Y. H. Cho, G. H. Gainer, A. J. Fischer, J. J. Song, S. Keller, U. K. Mishra and S. P. DenBaars, Appl. Phys. Lett., 1998, 73, 1370.
24. P. G. Eliseev, P. Perlin, J. Lee and M. Osiński, Appl. Phys. Lett., 1997, 71, 569.
25. B. Monemar, Phys. Rev. B: Solid State, 1974, 10, 676.

---