A design strategy is proposed to optimize the structure of photonic crystal (PC) slabs in order to achieve high reflection on its interface. A broad band (Δλ/λ > 19%) and ultrahigh (>95%) reflection spectrum can be realized for one-dimensional (1D) and two-dimensional (2D) PC slabs. Tolerance analysis of design parameters reveals distinct merits for different PC structures: as to maintain high reflection, 1D PC slabs show better structural tolerance, while 2D PC ones have a much greater tolerance for the material parameter. Such a difference may provide a criterion for the design of PCs-assist mirror structure in optoelectronic devices.
High power GaN-based LEDs with nano-structured Ga-doped ZnO (GZO) transparent conductive layer (TCL) were
fabricated by using metal-organic chemical vapor deposition (MOCVD) method. Compared with the conventional LED
with Ni/Au or ITO process, the saturation current in the LEDs with GZO TCL approximately increased up to more than
14 % and 13 %, and the light output intensity up to 57.5 % and 30.1 %, respectively. This improvement was attributed to
the high carrier concentration of GZO TCL and the planar structure at the TCL bottom, which improved the electrical
conductivity, and therefore promoted current spreading. The refractive index of GZO is similar to GaN (n ≈ 2) and
thereby results in the reduction of the reflection loss between GaN and TCL interface. In addition, the nano-structure of
GZO TCL increased the light output critical angle and enhanced surface light emitting while reducing the lateral light
loss and consequently improved light extraction efficiency of LEDs.
We showed a detailed thermal simulation of an Epi-down flip-chip packaged LED. Simulation results show that chip attachment defects can cause significant thermal gradients across the active layer of chip, leading to premature failures.
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