Effects of back-side ITO layer on the optical and electrical properties of mirror-substrate AlGaInP light-emitting diodes
Date Issued
2010
Date
2010
Author(s)
Chang, Wen-Chi
Abstract
Abstract
Sandwiching an ITO layer in between the metal reflector and the dispersive electrode of mirror-substrate (MS) type AlGaInP LEDs has become a standard procedure in industry to enhance the light power. The mechanism of this improvement, which is not well understood yet, is the research subject of this study.
We first study the effect of ITO layer on the reflectivity of the reflector by comparing reflectors with and without ITO layer. The result shows that the reflectors with ITO layer have lower reflectivity, which is due to the absorption of extra ITO layer. Our measurement shows that an ITO layer with a thickness of 268 nm has a transmission of 88%, which lowers the reflectivity by 16%, when the incident light is at 631 nm.
We then study the effect of ITO layer on the performance of the dispersive electrodes. We compare LEDs with different structures in between the GaP window layer and the Au mirror. Four structures, GaP/Au, GaP/ITO/Au, GaP/p+-GaAs/ITO/Au, and GaP/p+-GaAs/Au, were investigated. Results from transmission-line measurement show that GaP/ITO/Au and GaP/p+-GaAs/ITO/Au has the highest and lowest contact resistance, respectively. In addition, LED with higher contact resistance has better current spreading which leads to higher quantum efficiency at small current region. However, higher contact resistance also results in severer Joule heating, leading to output saturation at lower injection current. Based on this knowledge, a patterned dispersive electrode structure consisting of GaP/ITO/Au structure for the better lateral current spreading and GaP/p+-GaAs/ITO/Au structure for lower resistance along the vertical current path is the best combination to achieve the best light output. Experimental result shows that such device outperforms a control device without ITO layer in saturation output power by 15%.
In summary, this work shows that ITO layer slightly degrades the reflectivity of the back-side reflector but significantly improve the function of the dispersive electrode in MS-type AlGaInP LEDs.
Subjects
ITO
LED
mirror substrate
dispersive electrodes
metal bonding
Type
thesis
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