MOCVD growth of AlN/GaN DBR structures under various ambient conditions

doi:10.1016/j.jcrysgro.2003.10.062

Journal of Crystal Growth

Volume 262, Issues 1–4, 15 February 2004, Pages 151-156

https://doi.org/10.1016/j.jcrysgro.2003.10.062 Get rights and content

Abstract

The high-reflectivity AlN/GaN distributed Bragg reflector (DBR) structures were realized by metal organic chemical vapor deposition (MOCVD) growth under pure N₂ ambient for AlN epilayer growth. The highest peak reflectivity of about 94.5% with a stopband width of 18 nm at a center wavelength of 442 nm was obtained. For the DBR structure with AlN layer grown under mixture of N₂/H₂ and pure H₂ conditions, the center wavelength was blue-shifted to 418 and 371 nm and the peak reflectivity also showed a reduction to 92% and 79%, respectively. The stopband width also decreases with increasing H₂ contents. The surface roughness and the grain size of the grown DBR structures showed an increase with increasing the H₂ ambient gas ratio. For realization of a high reflectivity and broad bandwidth of AlN/GaN DBR by using the MOCVD growth method, the pure N₂ ambient gas for growth of AlN layer should be preferable and optimal condition.

Introduction

Gallium nitride is a direct wide band-gap semiconductor which has attracted great attention because of applications in fabrication of light sources of short wavelengths. In particular, GaN-based semiconductor laser diodes (LDs) and light-emitting diodes (LEDs) have applications in displays, traffic signals, and high-density digital versatile disks (HD-DVD). The blue GaN-based LEDs and edge emitting LDs have been developed in recent years. The research interest has gradually shifted to the development and demonstration of GaN-based vertical cavity surface emitting lasers (VCSELs) [1], [2], [3], [4] and resonant-cavity light-emitting diodes (RCLEDs) [5], [6], [7], [8], [9], [10]. An important requirement for the operation of such devices is the need of high reflectance mirrors, usually in the form of distributed Bragg reflectors (DBRs). The VCSELs require highly reflective DBR mirrors on both sides of the active region to form the laser cavity, while for the RCLEDs the high reflectance DBRs can improve the output power and emission spectrum.

The DBR structures are particular important for GaN VCSELs in two aspects. First, according to Honda et al. [11], the threshold current density of a GaN VCSEL can be reduced by an order of magnitude with an increase of the DBR peak reflectance from 90% to 99%. Therefore, the DBRs with high reflectivity are necessary for VCSELs. The second aspect is that the DBRs should have large stopband width. This is important because the active region of the GaN-based VCSEL is typical made of InGaN multiple quantum wells (MQWs), and the emission peak of the InGaN MQWs tends to fluctuate with small variations in either the growth conditions or the process parameters [12], [13], [14]. The DBRs with wide stopband can provide sufficient coverage of such spectral variation in emission wavelength.

The main difficulty in fabrication of GaN-based DBRs with high reflectivity and large stopband width is the small index of refraction contrast that can be obtained within the entire AlGaN alloy compositions. As a result, a large number of pairs are required to achieve the high reflectivity. In addition, the large lattice mismatch between GaN and AlGaN tends to create a lot of tensile stress induced cracks during the growth of the DBR structure resulting in reduced reflectivity and increase in scattering loss. Several GaN/AlGaN-based DBR structures have been reported previously. These DBRs were either grown by molecular beam epitaxy (MBE) [15], [16], [17] or by metal organic chemical vapor deposition (MOCVD) [18], [19], [20]. Among these, Figiel and co-workers [19] reported the control and elimination of the tensile growth stress by insertion of multiple AlN interlayers in the GaN/AlGaN DBR structures and demonstrated the crack-free growth of 60 pairs of Al_0.20Ga_0.80N/GaN DBR mirrors over the entire 2 in wafer with a maximum reflectivity of at least 99%, but the stopband width was only 13 nm.

In order to reduce the number of pairs and increase the mirror stopband, high index of refraction contrast material pairs using AlN/GaN were also reported using the MBE [21], [22], [23] and MOCVD system [24]. However, the lattice mismatch between AlN and GaN (∼2.4%) and the large different of thermal expansion coefficients between GaN (5.59×10⁻⁶/K) and AlN (4.2×10⁻⁶/K) could lead to the crack formation. Using the plasma-assisted MBE, Ng et al. [22] reported the network of cracks on AlN/GaN DBR can be reduced or complete eliminated by the asymmetric DBR structures and obtained a peak reflectance of up to 99% centered at 467 nm with a wide bandwidth of 45 nm. However, it is relatively difficult for the MOCVD system to grow good structural quality and high-reflectivity AlN/GaN DBR structures, because of the difficulty in the control of the epitaxial layer thickness due to much complex chemical processes and changes on the surface conditions. For example, the onset temperature for GaN decomposition is lower in H₂ ambient compared to other ambient conditions such as N₂, Ar and vacuum [25]. Yamaguchi et al. [26] reported that using N₂ as a carrier gas, AlN can release the strain energy between the GaN interface by the generation of misfit dislocation, not by that of a network of cracks. Hence, using the MOCVD system to grow AlN/GaN DBR structures is relatively difficult and the peak reflectance obtained so far was only about 88% [24].

In this paper, we report the study of the ambient gas effect during the growth of AlN in AlN/GaN DBR structures by the MOCVD system. The optimal growth conditions of AlN/GaN DBRs for obtaining high optical and structural quality with high reflectivity and wide stopband width were established.

Section snippets

Experiments

The GaN/AlN DBRs are grown on the polished optical-grade C-face (0 0 0 1) 2′′ diameter sapphire substrates by the MOCVD system (EMCORE D-75). Trimthylgallium (TMGa) and trimthylaluminum (TMAl) were used as Ga and Al sources, respectively, and ammonia (NH₃) was used as N source. A thermal cleaning process was carried out at 1080°C for 10 min in a stream of hydrogen ambient before the growth of epitaxial layers. After depositing of a 30-nm-thick GaN nucleation layer at 530°C, the temperature was

Results and discussion

The surface images of the grown AlN/GaN DBR samples observed by the OM with 200× magnification are shown in Fig. 2. All these samples showed a crack-free surface over the whole 2 in wafer. We also measured the surface morphology of three samples by AFM and the measurement results are shown in Table 1. The surface roughness of these DBR samples increased from 8.1, 10.7, to 12.4 with increasing the H₂ gas content, and the grain size also increased from 2, 8, to 10 μm for samples A, B, and C,

Conclusions

In conclusion, the high-reflectivity AlN/GaN DBR structures were obtained by MOCVD growth under pure N₂ ambient gas for growth of AlN epilayers. The highest peak reflectivity of about 94.5% with a stopband width of 18 nm at center wavelength of 442 nm was obtained. For the DBR structure with AlN layer grown under mixture N₂/H₂ and H₂ conditions, both the peak reflectivity and the stopband width were decreased and the center wavelengths of the DBR structures were blue-shifted. Therefore for

Acknowledgements

This research was supported by the National Science Council of Taiwan, ROC, under Contract No. NSC 90-2215-E-009-102 and by the Ministry of Education of Taiwan, ROC under Contract No. 88-FA06-AB. The authors would like to thank Y.C. Hsien of Department of Materials Science and Engineering, National Chiao Tung University for the TEM experiment supporting.

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MOCVD growth of AlN/GaN DBR structures under various ambient conditions

Abstract

Introduction

Section snippets

Experiments

Results and discussion

Conclusions

Acknowledgements

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