Development of Ultrahigh Efficiency Compound Semiconductor Quantum Wire Light-Emitting Diodes

高效半导体发光二极管（led）作为21世纪的一种节能、长寿命的通用照明器件，引起了人们的广泛关注。然而，传统的led通常生长在平面衬底上，由于半导体-空气界面存在全内反射，不透明金属电极阻挡光线，以及衬底吸收光线，因此很难将半导体中产生的光高效率地提取到外部自由空间。本项目的目标是实现半导体led的光提取效率远高于传统器件，采用基于选择性生长的自发发射控制技术，在预图像化，非平面衬底上。主要研究成果总结如下：“电流注入和发光区分离的LED”的提出和演示：我们提出并演示了一种新型的半导体LED，具有电流注入和发光区分离，旨在极大地抑制不透明金属电极的光阻挡，利用了在图案衬底上生长的化合物半导体层的带隙能量通常随晶体面而变化的事实。这种新器件的基本思想是在外延层的高带隙能量面上选择性地形成欧姆电极。在该装置中，从高带隙能量面注入的载流子将首先通过势垒和/或量子阱（QWL）有源层扩散到邻近的低带隙能量面，然后在那里重新组合。这样，我们就可以在空间上将电流注入区与发光区分开，从而大大抑制了金属电极对光的阻挡。发现一种新的高效自发发射控制现象：我们发现在v型GaAs衬底的窄脊顶区（横向宽度< 1μm）上生长的（001）平面QWL的光提取效率和内部量子效率都可以通过在（001）QWL的生长方向和横向上包裹高al成分层而得到显著提高。例如，我们通过光致发光研究表明，在Al_<0.65>Ga_<0.35> AS势垒层包围上述QWL结构，可以将Al_<0.3>Ga_<0.7>AS/GaAS/Al_<0.3>Ga_<0.7> AS (001) QWL中60%以上的自发辐射提取到外部空间。此外，Al_<0.3>Ga_<0.7>AS/GaAS/Al_<0.3>Ga_<0.7> AS (001) QWL结构的内部量子效率在室温下也能达到100%，与没有Al_<0.65>Ga_<0.35> AS阻挡层的样品相比，提高了10倍以上。利用这种独特的自发发光控制技术，目前正在开发高效率的led。少

High-efficiency semiconductor light-emitting diodes(LEDs) have attracted great current interests as an energy-efficient and long-lifetime general lighting device of the 21st century. However, in conventional LEDs which are usually grown on flat substrates, it's very difficult to extract light generated in semiconductors to the outside free space with a high efficiency due to the existence of total internal reflection at the semiconductor-air interface, blocking of light by opaque metal electrodes, and light absorption by substrates. The objective of the present project is to realize semiconductor LEDs with light-extraction efficiencies much higher than those of conventional devices using spontaneous emission control technique based on selective growth on pre-patterned, nonplanar substrates. The main achievements are summarized as following.1. Proposal and demonstration of "LED with separated current-injection and light-emitting areas" : We proposed and demonstrated a new type of semico … More nductor LED with separated current-injection and light-emitting areas, aimed at greatly suppressing light blocking by opaque metal electrodes, utilizing the fact that bandgap energies of compound semiconductor layers grown on patterned substrates usually change with crystalline facets. The basic idea of this new device is to form the Ohmic electrodes selectively on the high-bandgap-energy facets of the epitaxial layers. In this device, carriers injected from the high-bandgap-energy facets will first diffuse, through barrier and/or quantum well(QWL) active layers, to the adjacent low-bandgap-energy facets and then recombine there. In this way, we can spatially separate the current-injection area from the light-emitting area and thus greatly suppress the blocking of light by metal electrodes.2. Discovery of a new high-efficiency spontaneous emission control phenomenon: We found that both the light-extraction efficiency and the internal quantum efficiency of the (001) flat QWL grown on the narrow ridge-top region (lateral width < 1μm) of a V-grooved GaAs substrate can be dramatically enhanced just by surrounding the (001) QWL with high Al-composition layers in both the growth and the lateral directions. For example, we have shown by photoluminescence study that more than 60% of the spontaneous emission from an Al_<0.3>Ga_<0.7>AS/GaAS/Al_<0.3>Ga_<0.7>As(001) QWL can be extracted to the outside space by surrounding the above QWL structure with Al_<0.65>Ga_<0.35>As barrier layers. Moreover, the internal quantum efficiency of the above Al_<0.3>Ga_<0.7>AS/GaAS/Al_<0.3>Ga_<0.7>As(001) QWL structure was found to be 100% even at room temperature, which is more than 10 times higher compared with that of sample without the Al_<0.65>Ga_<0.35>As barrier layers. High-efficiency LEDs are being developed currently using this unique spontaneous emission control technique. Less