Improving the emission efficiency of MBE-grown GaN/AlN QDs by strain control
© Niu et al; licensee Springer. 2011
Received: 6 September 2011
Accepted: 2 December 2011
Published: 2 December 2011
The quantum-confined stark effect induced by polarization has significant effects on the optical properties of nitride heterostructures. In order to improve the emission efficiency of GaN/AlN quantum dots [QDs], a novel epitaxial structure is proposed: a partially relaxed GaN layer followed by an AlN spacer layer is inserted before the growth of GaN QDs. GaN/AlN QD samples with the proposed structure are grown by molecular beam epitaxy. The results show that by choosing a proper AlN spacer thickness to control the strain in GaN QDs, the internal quantum efficiencies have been improved from 30.7% to 66.5% and from 5.8% to 13.5% for QDs emitting violet and green lights, respectively.
Recently, with progress in the growth of high-quality bulk AlN [1, 2], a lot of efforts have been devoted to GaN/AlN quantum dots [QDs] because of their unique properties such as broad emission wavelength range covering the whole visible light, which provides a promising way to achieve white light-emitting diodes [LEDs] . Besides, the large conduction band offset (approximately 2 eV for GaN/AlN) offers a prospect to cover the fiber optical telecommunication wavelength range (1.3 to 1.55 μm) by intersubband transition [4, 5].
By controlling the growth conditions, the sizes and densities of the GaN/AlN QDs can be varied, and the photoluminescence [PL] wavelength can also be tuned. However, the large lattice mismatch between GaN and AlN and their polarization properties induce a strong built-in electric field, causing a remarkable quantum-confined stark effect [QCSE] which reduces the internal quantum efficiency [IQE] of the QDs. The reason is that the built-in electric field leads to energy band decline and separation of electron and hole wave functions, resulting in the decrease of recombination efficiency as well as the red shift of emission wavelength. Furthermore, the emission peak shifts to a shorter wavelength with increasing injection current, which is caused by Coulomb screening of the internal electric field . This phenomenon also exists in InGaN/GaN materials. In order to suppress the influence of QCSE, the compressive strain in the QD structures should be decreased, whereas, on the other hand, a certain degree of strain is required to perform the Stranski-Krastanov [S-K] mode growth of QDs. Therefore, it is a crucial issue to control the strain distribution in order to improve the IQE of GaN/AlN QD emission.
Nowadays, some work has been done to avoid the QCSE. Adelmann et al. grew self- assembled cubic GaN QDs by using plasma-assisted molecular-beam epitaxy [PA-MBE] on cubic AlN . However, due to the very narrow growth window, it was difficult to grow high-quality GaN bulk and QDs. Cros et al. reported GaN/AlN QD growth on a-plane 6H-SiC [8, 9]. This method suffered from an extremely expensive substrate, and compared with the GaN and AlN bulks grown on c-plane, the crystal quality still needed to be improved. Furthermore, an AlGaN buffer layer has been used instead of AlN to reduce the polarization effect . However, a certain surfactant was required in order to achieve the two-dimensional to three-dimensional [2D-to-3D] growth transition . Also, the bandgap of AlGaN is smaller than that of AlN; thus, the strong confinement in GaN QDs is weakened.
In our previous experiments, GaN/AlN QDs with varied morphologies have been obtained by properly choosing the growth parameters. The emission peaks of the QDs vary from 400 to 670 nm, and the QDs with a larger average height exhibit a longer emission wavelength but with a lower efficiency, due to the influence of QCSE. In this work, the morphologies and emission properties of GaN QDs grown on a partially relaxed AlN layer are investigated. The emission efficiencies of GaN QDs have been obviously improved by controlling the strain status of the underneath AlN layer.
Structural parameters of the GaN/AlN QD samples and the measured morphology characteristics
AlN spacer thickness (nm)
GaN insertion layer thickness (nm)
QD density (cm-2)
Mean QD height (nm)
Mean QD diameter (nm)
4.4 × 1011
4.0 × 1011
2.2 × 1011
9.6 × 1010
The samples' surface morphologies were measured by scanning electron microscopy [SEM] and atomic force microscopy [AFM]. The crystalline properties were examined by transmission electron microscopy [TEM] and X-ray diffraction [XRD]. To evaluate the samples' optical properties, temperature-dependent PL measurements were carried out using a 325-nm laser as excitation.
Results and discussion
There are two factors when considering the influence of QCSE on the IQE of the QDs: one is the strain-induced internal electric field, and the other is the QD morphology. A careful simulation is required to fully understand the influence of QD morphology. Ngo et al. reported that the emission wavelength of QDs exhibits a red shift with the increasing QD height, base, volume, or aspect ratio [AR] at a fixed volume . For our samples, as seen in Table 1, the QD diameter, aspect ratio, and volume increase with the QD height. Therefore, in order to simplify the discussion, we only consider the QD height in the following part.
This mechanism also accounts for the improvement of the samples emitting green light. The IQE of the sample without the GaN insertion layer is only 5.8%, while for the sample with the proposed epitaxial structure, the IQE has been improved to 13.5%. These results imply a promising way to optimize the performance of QD LEDs.
GaN/AlN QD samples with a partially relaxed GaN insertion layer followed by an AlN spacer layer have been grown by PA-MBE. The proposed structure can control the strain in GaN QDs and thus the QCSE induced by polarization. As a result, the IQEs for GaN QDs emitting violet and green lights have been improved from 30.7% to 66.5% and from 5.8% to 13.5%, respectively. And for the samples with the AlN spacer of a certain range of thickness, the emission wavelength keeps nearly unchanged when the IQE increases.
This work was supported by the National Basic Research Program of China (grant nos. 2011CB301902 and 2011CB301903), the High Technology Research and Development Program of China (grant nos. 2011AA03A112, 2011AA03A106, and 2011AA03A105), the National Natural Science Foundation of China (grant nos. 61176015, 60723002, 50706022, 60977022, and 51002085), and the Beijing Natural Science Foundation (grant no. 4091001).
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