Caneau, C.

Ersen, A.

Gmitter, T. J.

Scherer, A.

Schnitzer, I.

Yablonovitch, E.

Caltech Authors - Main

M4.4 4:30pm - 4:50pm 
30% EXTERNAL QUANTUM EFFICIENCY FROM SURFACE TEXTURED, 
THIN-FILM LIGHT-EMI’ITING-DIODE ARRAYS 
I. Schnitzer, E. Yablonovitch, A. Ersen, A. Scherer,t C. Caneau,V and T. J. Gmittefl 
The Department of Electrical Engineering, UCLA, 405 Hilgard Ave., Los Angeles, CA 90024-1594 
Bellcore, 331 Newman Springs Rd., Red Bank, NJ 07701-7040 t Caitech, Pasadena, CA 91125, 
High efficiency light-emitting-diodes (LED’s) are desired for many applications such as displays, 
printers, short-haul communication, and opto-electronic computer interconnects. However, there is an 
enormous gap between the theoretical efficiency of LED’s and their actual efficiency. The internal quantum 
yield, v, of good quality double heterostructures can exceed 99%, as we have demonstrated recently? On the 
other hand run-of-the-mill commercial LED’s are usually only a few percent efficient. The reason for this 
long-standing shortfall is the difficulty for light to escape from high refractive index semiconductor. A mere 2% 
of the internally generated light is coupled into free space through the 16” escape cone, the rest suffering total 
internal reflection and risking re-absorption. The present commercial state-of-the-art, - 20% external 
efficiency in AlGaAs-based LED’s, is achieved by growing a thick transparent semiconductor superstrate, and 
total substrate removal in a particularly clean, low-loss, optical design which can add greatly to the cost. 
The key to increasing the escape probability is to give the photons multiple opportunities to find the 
escape cone. This requires angular randomization or scrambling of the light rays. One way to do this is by 
photon high self-absorption, and a very clean, non-dissipative optical design. In ref. 1 (see Fig. la)  we used the 
epitaxial liftoff (ELO) technique2 to mount thin film heterostructures on high reflectivity surfaces. It was found 
that while the photon re-cycling can yield an ultrahigh external quantum efficiency (72%)’ it is very susceptible 
to parasitic loss mechanisms (free carrier absorption in doped layers, for example) or slight degradation in the 
internal quantum efficiency. A more practical approach, and the one which we propose here is the angular 
randomization by elastic scattering of the photons from a textured semiconductor surface. 
Our approach has two components: i) Separation of thin-film heterojunctions from the growth substrate 
using the E L 0  technique, and ii) Nano-texturing of the thin-film semiconductor interface by natural 
lithography? The implementation of these two components leads to a thin film (< lpm) geometry shown in 
Fig. lb. This geometry employs the statistical mechanical concept of phase-space filling by angular 
randomization. We invoke a statistical approach4 to model the planar and textured film goemetries. It is 
concluded that the textured film geometry can boost the external efficiency to 50% or more, while relieving the 
demand for the utmost material quality, high self-absorption, and low parasitic losses, as required by the planar 
film geometry of Ref. 1 (Fig. la). 
The actual LED structure is a conventional n+ -AlGaAs/p-GaAs/p+ -AlGaAs double heterostructure, 
grown over a 0.05pm thick AlAs release layer by organo-metallic chemical vapor deposition. The LED 
structure is shown in Fig 2. We have used the concept of current crowding to eliminate parasitic light absorption 
at the electrodes, a significant problem in all LED’s. Edge passivation, another common problem in low 
injection photonics, is effectively achieved by the diffusion barrier to the exposed step as shown in Fig. 2. 
Texturing of the light emitting region is the final processing step, following device fabrication, testing, ELO, and 
Van der Waals bonding by surface tension forces onto dielectric coated Au mirrors. The surface of the LED is 
coated by a randomly close-packed array of polystyrene spheres, 0.2pm diameter, using the surface forces 
between the charged spheres and the semiconductor, as shown in Fig. 3. The spheres then act as an etch mask 
for a Cl2 assisted Xe+ ion beam etching, about 0.17pm deep. 
The light versus current characteristics of these LED’s have been measured and modeled (see Fig. 4). 
We have observed 9% external quantum efficiency from untextured LED’s array transforming into 30% 
external quantum efficiency following the surface texturing treatment. We conclude that by employing the 
principle of phase-space-filling in an improved device geometry, 56% efficient LED’s arrays can be expected. 
Such LED’s are simpler and more reliable than lasers. Unlike lasers they are thresholdless, yet they offer 
comparable external efficiency. Moreover, the principles and the device geometry that we are proposing can be 
applied to other semiconductor material systems to obtain very bright visible thin film, top processed, LED’s 
arrays as well. It is clear that the availability of 30%-50% efficient, low-cost, reliable, visible LED’s would 
change dramatically the appearance of our homes, offices, and streets. 
1. I. Schnitzer, E. Yablonovitch, C. Caneau, and T. J. Gmitter, Appl. Phys. Lett. 62, 131 (1993). 
2. E. Yablonovitch, T. J. Gmitter, J. P. Harbison, and R. Bhat, Appl. Phys. Lett. 51,2222 (1987). 
3. H. W. Deckman and J. H. Dunsmuir, Appl. Phys. Lett. 41,377 (1982). 
4. E. Yablonovitch, J. Opt. Soc. Am. 72,899 (1982). 
31 
Figure 1: Typical photon trajectory in 
the thin epi-liftoff heterostructure: a. In 
the smooth structure the photons 
trajectories are randomized by the self- 
absorption/re-emission events, 
represented by dots. b. In the textured 
film, angular randomization is achieved 
by strong surface scattering. 
Figure 2 Schematic cross section of an 
epi-liftoff, textured LED, Van der Waals 
bonded onto an Si02 coated metallic 
reflector. The spatial distribution of 
injection current is represented by the 
curve on the graph j(x) versus x. Current 
crowding is obtained due to the different 
conductivity of the n+ and p+ contact 
layers, in order to minimize absorption 
losses at the metallic electrodes. The 
width of the textured area is about 50 
times the film thickness. 
a. Smooth 
b. Textured \ 
n-contact 
‘h + 
rbfi-dcir p -Ab.F%& 
Natural Lithography 
0.03pm 
0.4pm 
0.04pm 
0.2pm 
0.3pm 
Figure 3: Natural Lithography - 
Polystyrene spheres, 0.2 pm diameter, 
coat the surface of the LED in a 
randomly close-packed array. The 
spheres then act as an etch mask for a 
Cl2 assisted Xe+ ion beam etching, 
about 0.17pm deep. The spheres can be 
easily washed away to leave a clean high 
refractive index textured surface which 
enhances the outcoupling of light. 
Poly-spheres ‘ Semiconductor ‘ 
Figure 4 Log-Log plot of the measured 
total light output versus injection current, 
for typical textured and untextured 
(smooth) LED’s, as indicated. The linear 
fits have slopes 1, and their intercepts 
suggest the indicated external quantum 
efficiency. 
10 ’1 1 
TEX. 
30% 4 
1 1 
I 1 
.oi I 
.1 1 10 100 
Injection Cunent [mA] 
32 
. 


30% External Quantum Efficiency From Surface Textured, Thin-film Light-emitting-diode Arrays

https://authors.library.caltech.edu/93950/1/00696877.pdf

30% External Quantum Efficiency From Surface Textured, Thin-film Light-emitting-diode Arrays

Abstract

Similar works

Full text

Available Versions

Caltech Authors - Main