Content of TOPICAL REVIEW—III-nitride optoelectronic materials and devices in our journal

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    Metal-organic-vapor phase epitaxy of InGaN quantum dots and their applications in light-emitting diodes
    Wang Lai (汪莱), Yang Di (杨迪), Hao Zhi-Biao (郝智彪), Luo Yi (罗毅)
    Chin. Phys. B, 2015, 24 (6): 067303.   DOI: 10.1088/1674-1056/24/6/067303
    Abstract687)   HTML    PDF (1149KB)(755)      

    InGaN quantum dot is a promising optoelectronic material, which combines the advantages of low-dimensional and wide-gap semiconductors. The growth of InGaN quantum dots is still not mature, especially the growth by metal–organic–vapor phase epitaxy (MOVPE), which is challenge due to the lack of 、itin-situ monitoring tool. In this paper, we reviewed the development of InGaN quantum dot growth by MOVPE, including our work on growth of near-UV, green, and red InGaN quantum dots. In addition, we also introduced the applications of InGaN quantum dots on visible light emitting diodes.

    Status of GaN-based green light-emitting diodes
    Liu Jun-Lin (刘军林), Zhang Jian-Li (张建立), Wang Guang-Xu (王光绪), Mo Chun-Lan (莫春兰), Xu Long-Quan (徐龙权), Ding Jie (丁杰), Quan Zhi-Jue (全知觉), Wang Xiao-Lan (王小兰), Pan Shuan (潘拴), Zheng Chang-Da (郑畅达), Wu Xiao-Ming (吴小明), Fang Wen-Qing (方文卿), Jiang Feng-Yi (江风益)
    Chin. Phys. B, 2015, 24 (6): 067804.   DOI: 10.1088/1674-1056/24/6/067804
    Abstract604)   HTML    PDF (1200KB)(2171)      

    GaN-based blue light emitting diodes (LEDs) have undergone great development in recent years, but the improvement of green LEDs is still in progress. Currently, the external quantum efficiency (EQE) of GaN-based green LEDs is typically 30%, which is much lower than that of top-level blue LEDs. The current challenge with regard to GaN-based green LEDs is to grow a high quality InGaN quantum well (QW) with low strain. Many techniques of improving efficiency are discussed, such as inserting AlGaN between the QW and the barrier, employing prestrained layers beneath the QW and growing semipolar QW. The recent progress of GaN-based green LEDs on Si substrate is also reported: high efficiency, high power green LEDs on Si substrate with 45.2% IQE at 35 A/cm2, and the relevant techniques are detailed.

    Progress in bulk GaN growth
    Xu Ke (徐科), Wang Jian-Feng (王建峰), Ren Guo-Qiang (任国强)
    Chin. Phys. B, 2015, 24 (6): 066105.   DOI: 10.1088/1674-1056/24/6/066105
    Abstract691)   HTML    PDF (938KB)(1904)      

    Three main technologies for bulk GaN growth, i.e., hydride vapor phase epitaxy (HVPE), Na-flux method, and ammonothermal method, are discussed. We report our recent work in HVPE growth of GaN substrate, including dislocation reduction, strain control, separation, and doping of GaN film. The growth mechanisms of GaN by Na-flux and ammonothermal methods are compared with those of HVPE. The mechanical behaviors of dislocation in bulk GaN are investigated through nano-indentation and high-space resolution surface photo-voltage spectroscopy. In the last part, the progress in growing some devices on GaN substrate by homo-epitaxy is introduced.

    Design of patterned sapphire substrates for GaN-based light-emitting diodes
    Wang Hai-Yan (王海燕), Lin Zhi-Ting (林志霆), Han Jing-Lei (韩晶磊), Zhong Li-Yi (钟立义), Li Guo-Qiang (李国强)
    Chin. Phys. B, 2015, 24 (6): 067103.   DOI: 10.1088/1674-1056/24/6/067103
    Abstract557)   HTML    PDF (4312KB)(1335)      

    A new method for patterned sapphire substrate (PSS) design is developed and proven to be reliable and cost-effective. As progress is made with LEDs' luminous efficiency, the pattern units of PSS become more complicated, and the effect of complicated geometrical features is almost impossible to study systematically by experiments only. By employing our new method, the influence of pattern parameters can be systematically studied, and various novel patterns are designed and optimized within a reasonable time span, with great improvement in LEDs' light extraction efficiency (LEE). Clearly, PSS pattern design with such a method deserves particular attention. We foresee that GaN-based LEDs on these newly designed PSSs will achieve more progress in the coming years.

    Progress in research of GaN-based LEDs fabricated on SiC substrate
    Xu Hua-Yong (徐化勇), Chen Xiu-Fang (陈秀芳), Peng Yan (彭燕), Xu Ming-Sheng (徐明升), Shen Yan (沈燕), Hu Xiao-Bo (胡小波), Xu Xian-Gang (徐现刚)
    Chin. Phys. B, 2015, 24 (6): 067305.   DOI: 10.1088/1674-1056/24/6/067305
    Abstract660)   HTML    PDF (2886KB)(801)      

    The influence of buffer layer growth conditions on the crystal quality and residual stress of GaN film grown on silicon carbide substrate is investigated. It is found that the AlGaN nucleation layer with high growth temperature can efficiently decrease the dislocation density and stress of the GaN film compared with AlN buffer layer. To increase the light extraction efficiency of GaN-based LEDs on SiC substrate, flip-chip structure and thin film flip-chip structure were designed and optimized. The fabricated blue LED had a maximum wall-plug efficiency of 72% at 80 mA. At 350 mA, the output power, the Vf, the dominant wavelength, and the wall-plug efficiency of the blue LED were 644 mW, 2.95 V, 460 nm, and 63%, respectively.

    Transient thermal analysis as measurement method for IC package structural integrity
    Alexander Hanß, Maximilian Schmid, E Liu, Gordon Elger
    Chin. Phys. B, 2015, 24 (6): 068105.   DOI: 10.1088/1674-1056/24/6/068105
    Abstract547)   HTML    PDF (6145KB)(2244)      

    Practices of IC package reliability testing are reviewed briefly, and the application of transient thermal analysis is examined in great depth. For the design of light sources based on light emitting diode (LED) efficient and accurate reliability testing is required to realize the potential lifetimes of 10^5 h. Transient thermal analysis is a standard method to determine the transient thermal impedance of semiconductor devices, e.g. power electronics and LEDs. The temperature of the semiconductor junctions is assessed by time-resolved measurement of their forward voltage (Vf). The thermal path in the IC package is resolved by the transient technique in the time domain. This enables analyzing the structural integrity of the semiconductor package. However, to evaluate thermal resistance, one must also measure the dissipated energy of the device (i.e., the thermal load) and the k-factor. This is time consuming, and measurement errors reduce the accuracy. To overcome these limitations, an innovative approach, the relative thermal resistance method, was developed to reduce the measurement effort, increase accuracy and enable automatic data evaluation. This new way of evaluating data simplifies the thermal transient analysis by eliminating measurement of the k-factor and thermal load, i.e. measurement of the lumen flux for LEDs, by normalizing the transient Vf data. This is especially advantageous for reliability testing where changes in the thermal path, like cracks and delaminations, can be determined without measuring the k-factor and thermal load. Different failure modes can be separated in the time domain. The sensitivity of the method is demonstrated by its application to high-power white InGaN LEDs. For detailed analysis and identification of the failure mode of the LED packages, the transient signals are simulated by time-resolved finite element (FE) simulations. Using the new approach, the transient thermal analysis is enhanced to a powerful tool for reliability investigation of semiconductor packages in accelerated lifetime tests and for inline inspection. This enables automatic data analysis of the transient thermal data required for processing a large amount of data in production and reliability testing. Based on the method, the integrity of LED packages can be tested by inline, outgoing inspection and the lifetime prediction of the products is improved.

    GaN substrate and GaN homo-epitaxy for LEDs: Progress and challenges
    Wu Jie-Jun (吴洁君), Wang Kun (王昆), Yu Tong-Jun (于彤军), Zhang Guo-Yi (张国义)
    Chin. Phys. B, 2015, 24 (6): 068106.   DOI: 10.1088/1674-1056/24/6/068106
    Abstract913)   HTML    PDF (960KB)(773)      

    After a brief review on the progresses in GaN substrates by ammonothermal method and Na-flux method and hydride vapor phase epitaxy (HVPE) technology, our research results of growing GaN thick layer by a gas flow-modulated HVPE, removing the GaN layer through an efficient self-separation process from sapphire substrate, and modifying the uniformity of multiple wafer growth are presented. The effects of surface morphology and defect behaviors on the GaN homo-epitaxial growth on free standing substrate are also discussed, and followed by the advances of LEDs on GaN substrates and prospects of their applications in solid state lighting.

    Phosphor-free white light-emitting diodes
    Guo Xia (郭霞), Liu Qiao-Li (刘巧莉), Li Chong (李冲), Liu Bai (刘白), Dong Jian (董建), Shen Guang-Di (沈光地)
    Chin. Phys. B, 2015, 24 (6): 068505.   DOI: 10.1088/1674-1056/24/6/068505
    Abstract550)   HTML    PDF (469KB)(937)      

    The multiple color-matching schemes that could improve the color rendering index for phosphor-free white LEDs are discussed. Then we review a few of the recent research directions for phosphor-free white LEDs, which include the development of monolithic GaN-based white LEDs and hybrid integrated GaN-based and AlGaInP-based white LEDs. These development paths will pave the way toward commercial application of phosphor-free white LEDs in the coming years.

    Progress and prospects of GaN-based LEDs using nanostructures
    Zhao Li-Xia (赵丽霞), Yu Zhi-Guo (于治国), Sun Bo (孙波), Zhu Shi-Chao (朱石超), An Ping-Bo (安平博), Yang Chao (杨超), Liu Lei (刘磊), Wang Jun-Xi (王军喜), Li Jin-Min (李晋闽)
    Chin. Phys. B, 2015, 24 (6): 068506.   DOI: 10.1088/1674-1056/24/6/068506
    Abstract778)   HTML    PDF (1406KB)(3761)      

    Progress with GaN-based light emitting diodes (LEDs) that incorporate nanostructures is reviewed, especially the recent achievements in our research group. Nano-patterned sapphire substrates have been used to grow an AlN template layer for deep-ultraviolet (DUV) LEDs. One efficient surface nano-texturing technology, hemisphere-cones-hybrid nanostructures, was employed to enhance the extraction efficiency of InGaN flip-chip LEDs. Hexagonal nanopyramid GaN-based LEDs have been fabricated and show electrically driven color modification and phosphor-free white light emission because of the linearly increased quantum well width and indium incorporation from the shell to the core. Based on the nanostructures, we have also fabricated surface plasmon-enhanced nanoporous GaN-based green LEDs using AAO membrane as a mask. Benefitting from the strong lateral SP coupling as well as good electrical protection by a passivation layer, the EL intensity of an SP-enhanced nanoporous LED was significantly enhanced by 380%. Furthermore, nanostructures have been used for the growth of GaN LEDs on amorphous substrates, the fabrication of stretchable LEDs, and for increasing the 3-dB modulation bandwidth for visible light communication.

ISSN 1674-1056   CN 11-5639/O4

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