Carrier quantum capture and escape mechanisms in semiconductor quantum structures

为了探究InGaN量子阱发光二极管（QW LEDs）高发光效率背后的有趣机制，研究了其电致发光（EL）和光致发光（PL）效率随外加偏压（电场）的变化关系。主要研究结果如下：（1）研究了具有和不具有n型InGaN电子储存层（ERL）的蓝光多量子阱LED的电致发光效率与温度和电流的关系。随着ERL的加入，由于减小了正向偏压，EL效率大大提高。在低于100 K的高注入下观察到的主EL带的降低的EL效率在低注入下通过减少伴随着短波长卫星带的消失的正向偏压而显著改善，这表明载流子逃逸的重要作用。(2)研究了单量子阱绿色发光二极管的发光效率随偏压的变化。在直接激发下，当正向偏压超过+2V时，PL强度可以降低，这表明由于载流子逃逸过程导致的PL降低。此外，在间接激发下，其中载流子主要在势垒中光生，由于有效的载流子捕获，PL效率的增强可以在+2.3V下发生，但是超过+3.25V，PL效率急剧降低。该结果解释了二极管的EL效率在较高正向偏压条件下降低的原因。(3)在不同的激发功率下，研究了有和没有ERL的蓝色量子阱二极管的PL效率与偏压的关系。通过对阱和ERL的PL效率的比较，我们观察到在弱功率和浅激发深度下，由反向偏压引起的PL猝灭更强。这可归因于空穴逃逸过程限制阱区内的辐射复合效率的事实。

In order to pursue the interesting mechanism behind the high luminescence efficiency of InGaN quantum-well (QW) light-emitting diodes (LEDs) the electroluminescence (EL) and photoluminescence (PL) efficiencies have been investigated as a function of external bias voltage (field). Following results are obtained:(1) EL efficiency for blue multiple QW LEDs with and without an n-type InGaN electron reservoir layer (ERL) has been studied as a function of temperature and current. With an addition of the ERL the EL efficiency is greatly improved due to decreased forward bias. The reduced EL efficiency of the main EL band observed under high injection below 100 K is significantly improved under low injection by decreasing forward bias accompanying disappearance of a short-wavelength satellite band, indicating important roles of carrier escape.(2) PL efficiency for green single QW LED has been studied as a function of bias. Under direct excitation the PL intensity can be decreased when the forward bias exceeds over +2V, indicating the PL reduction due to carrier escape processes. Furthermore, under indirect excitation where carriers are mostly photogenerated in the barriers enhancement of the PL efficiency due to efficient carrier capture can occur at +2〜3V, but over +3.25V the PL efficiency drastically decreases. This result explains the reason why the EL efficiency of the diodes is decreased under the higher forward bias conditions.(3) PL efficiency for the blue QW diodes with and without the ERL has been studied as a function of bias under indirect excitation with various excitation powers. Comparison of the PL efficiency for the well and the ERL allows us to observe that PL quenching by the reverse bias is stronger under weak power and shallow excitation depth. This can be attributed to the fact that hole escape processes limit the radiative recombination efficiency within the well regions.