InGaN/GaN multilayer quantum dots yellow-green light-emitting diode with optimized GaN barriers
© Lv et al.; licensee Springer. 2012
Received: 19 September 2012
Accepted: 25 October 2012
Published: 7 November 2012
InGaN/GaN multilayer quantum dot (QD) structure is a potential type of active regions for yellow-green light-emitting diodes (LEDs). The surface morphologies and crystalline quality of GaN barriers are critical to the uniformity of InGaN QD layers. While GaN barriers were grown in multi-QD layers, we used improved growth parameters by increasing the growth temperature and switching the carrier gas from N2 to H2 in the metal organic vapor phase epitaxy. As a result, a 10-layer InGaN/GaN QD LED is demonstrated successfully. The transmission electron microscopy image shows the uniform multilayer InGaN QDs clearly. As the injection current increases from 5 to 50 mA, the electroluminescence peak wavelength shifts from 574 to 537 nm.
KeywordsInGaN quantum dots GaN barriers metalorganic vapor phase epitaxy light-emitting diodes
In recent years, white light-emitting diodes (LEDs) have attracted much attention for applications in general lighting and liquid crystal display back-lighting [1, 2]. White LEDs based on red, green, and blue LED chips exhibit higher color rendering index and higher luminous efficiency limit simultaneously, which is theoretically superior to the solution of blue LED plus yellow phosphor . However, the performance of RGB white LEDs fails to meet expectation due to the well-known ‘green gap’ [4–6]. Additionally, in the spectral range of green gap, the efficiency of yellow-green LEDs decreases dramatically compared to that of short-wavelength green LEDs . This is attributed to the low internal quantum efficiency (IQE) of InGaN/GaN multi-quantum wells (MQWs) with high indium composition. The dislocations and defects induced by the large lattice mismatch between InGaN and GaN act as nonradiative recombination centers, thus weakening the IQE . Furthermore, the quantum-confined Stark effect (QCSE) of high-In-content InGaN/GaN MQWs leads to energy band tilting, which decreases the overlap integral of electrons and holes by spatial separation [9–12]. To circumvent the above disadvantages, various approaches are adopted, such as InGaN quantum dots (QDs) grown as the alternative active region [13, 14] and InGaN QWs grown on nonpolar or semipolar planes [6, 15]. In previous reports, growth behaviors of high-In-content InGaN quantum dots using a growth interruption method are intensively investigated [16–20], which paves the way to high-efficiency QD LEDs. As the capping layer of InGaN QDs, GaN barrier is critical to the performance of multilayer InGaN QDs. Previous literature indicates that GaN barrier exhibits a rough surface when grown under the same parameters as the InGaN QDs, which will influence the QD formation and distribution of different layers . Thus, it is necessary to optimize the growth parameters of the GaN barrier layer in multilayer InGaN/GaN QD structure. Some GaN barrier growth approaches are believed to improve the structural properties of InGaN/GaN QD or QW structure, such as the growth of GaN barrier at higher temperature [22, 23], using H2 as carrier gas in GaN barrier growth [22, 24, 25], and growth interruption after QW growth [24, 26]. In this study, samples of InGaN QDs with GaN barrier layer structure have been grown by metal organic vapor phase epitaxy (MOVPE) with different growth parameters. It is shown that the surface morphology of the GaN barrier layer is improved by increasing the growth temperature and switching the carrier gas from N2 to H2. Based on the optimized growth parameters, a yellow-green LED including 10-layer InGaN/GaN QDs is successfully fabricated. Transmission electron microscopy (TEM) image indicates a uniform multilayer InGaN QD structure clearly. The electroluminescence peak of the LED shows a blueshift from 574 to 537 nm as the current increases from 5 to 50 mA.
Growth parameters of samples B, C, and D
GaN barrier growth temperature (°C)
GaN barrier growth ambient
The surface morphologies of samples A, B, C, and D were measured by PSIA X-100 atomic force microscope (AFM). The QD density and average diameter and height were calculated by the Scanning Probe Image Processor software. The cross section of sample E was observed at 200 kV by Tecnai G20 TEM system. For Z-contrast imaging of sample F, JEOL-2010F equipped with a high-angle annular dark-field detector for scanning TEM was used. The photoluminescence (PL) of samples B, C, D, and E was measured using a 325-nm He-Cd laser as the excitation source. The excitation light spot diameter was 1 mm, and the excitation optical power was 27 mW. Sample E was then processed into 300 × 300-μm2 chips. The n-GaN is exposed by etching with inductively coupled Cl2/BCl3/Ar2 plasma. Ni/Au was deposited on the transparent electrodes, and the transparent electrodes were annealed at 600°C for 3 min in oxygen ambient. Cr/Au was deposited on the n-GaN layer and p-GaN layer as n- and p-electrodes.
Results and discussion
The comparison of QD diameter and height between samples A and E
In addition, micro-PL at low temperature shows a sharp peak from single QDs with the same QD growth parameters in our previous reports [16, 36]. Based on the micro-PL measurements, we believe that the optical properties of our samples reflect the quantum confinement effect in the InGaN quantum dots.
In summary, by increasing the growth temperature and switching the carrier gas to H2, the surface morphology and crystalline quality of GaN barrier layer have been optimized. Compared to the different GaN barrier growth parameters, hydrogen eliminating effect is confirmed by PL results. During the multi-InGaN QD growth, the three-dimensional nanostructure was maintained due to the thin GaN protective layer and relative growth temperature of InGaN QDs and GaN barrier. Based on the optimized growth parameters, a 10-layer InGaN/GaN QD yellow-green LED is successfully grown by MOVPE. The EL wavelength shows a blueshift from 574 to 537 nm as injection current increases from 5 to 50 mA.
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. 60723002, 50706022, 60977022, and 51002085), and the Beijing Natural Science Foundation (grant no. 4091001).
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