Current crowding effect on light extraction efficiency of thin-film LEDs

Transcription

1 1 8 th International Conference on Nitride Semiconductors, October 18-23, 2009, Jeju, Korea Current crowding effect on light extraction efficiency of thin-film LEDs M. V. Bogdanov, K. F. Bulashevich, O. V. Khokhlev, I. Yu. Evstratov, M. S. Ramm, and S. Yu. Karpov STR Group Soft-Impact, Ltd.

2 2 Content current crowding and operation of thin-film high-power blue LED effect of the current crowding on light extraction efficiency a new mechanism contributing to EP (%) LED efficiency droop 85 possible ways to improve the light extraction efficiency Total LEE (%) conventional with blocking layer under pad refined electrode BL & refined electrode Forward current (ma)

3 3 Basic design of µm 2 blue LED die V. Härle et al., Proc. SPIE 4996 (2003) 133 / phys. stat. solidi (a) 201 (2004) 2736 Γ-shaped n-electrodes n-contact layer textured surface n-contact layer n-pad active region p-contact layer highly reflective p-electrode Micrograph of the die by MuAnalysis, Inc., 2008

4 4 Key elements of simulation model SimuLED TM package is used for modeling: 3D coupled simulation of electrical, thermal, and optical processes in the LED die Auger recombination is considered is the main non-thermal mechanism of the IQE droop at high current densities textured surface is modeled by closely packed hexagonal pyramids with the aspect ratio h/c ~ 4 optical properties of Au and Ag are used for n- and high-reflective p-electrodes, respectively c Internal quantum efficiency T = 300 K T = 340 K T = 380 K T = 420 K Current density (A/cm 2 ) no electron leakage is predicted for InGaN MQW LED structure h incident light

5 5 Current crowding near/under n-electrodes Current I = 700 ma temperature, current density, and IQE distributions in the active region IQE T (K) local IQE reduction in highcurrent density area B A J (A/cm 2 ) Normalized current density ma 130 ma 700 ma Distance (µm) AB crosssection current density nonuniformity depends on the total operating current

6 6 Light extraction from the LED B A EP (%) probability of light extraction falls down under and next to n-electrode n-electrode Extraction probability (%) distribution of extraction probability is nearly independent of current Current I = 700 ma AB cross-section Distance (µm) light generated under n-pad is not practically extracted from the die because of incomplete multiple reflection from metallic electrode IQE (%)

7 7 Dependence of light extraction efficiency on current variation of n-contact layer parameters affects weakly the current crowding and, hence, the LEE at ~ ma alternative approaches are required Approach 1: insertion of an insulating layer under the n-pad to avoid parasitic current flow in this region Total LEE (%) n-contact layer: 3 µm, 5x10 18 cm -3 3 µm, 5x10 19 cm -3 6 µm, 5x10 19 cm -3 strong dependence of LEE on forward current Approach 2: use of narrower Г-shaped electrodes with reduced spacing between them Forward current (ma)

8 8 Current spreading in LED dice of modified designs Total current through the diode I = 700 ma J (A/cm 2 ) J (A/cm 2 ) parasitic current flow under the n-pad is partly suppressed both approaches are found to work well reduction of the current density contrast in the active region

9 9 Probability of light extraction from the dice of modified designs Total current through the diode I = 700 ma EP (%) EP (%) probability of light extraction is comparable with that of the basic die design considerable enlarging of the area with high probability of light extraction

10 10 Assessment of performance improvement due to variation of LED die design 85 Performance improvements at the current of 700 ma: Total LEE (%) conventional with blocking layer under pad refined electrode BL & refined electrode Current (ma) 1000 conventional BL under pad 800 refined electrode BL + refined 600 electrode LEE from 60 to 70% V f remains the same optical power from 530 to 635 mw (by ~20%) WPE from 23 to 28% (by ~22%) WPE (%) Forward current (ma) 40 conventional BL under pad refined electrode 35 BL + refined electrode Current (ma) Top output power (mw) Forward voltage (V) conventional 200 BL under pad refined electrode 100 BL + refined electrode Forward current (ma)

11 11 Conclusions current crowding may result in a strong dependence of light extraction efficiency (LEE) on operating current in vertical thin-film LEDs because of incomplete emitted light reflection from metallic n-electrodes predicted decay of LEE with current enhances the droop of total LED efficiency; this actually represent one more nonthermal mechanism limiting the LED performance the undesirable LEE dependence on current may be remarkably suppressed by appropriate modifications of the chip design; in particular, inserting of insulating film under the n-pad and the use of narrow electrodes with reduced spacing are found to be promising for improvement of LED performance