• TCAD Examples

ledex05.in : Optical Output Coupling Efficiency of a III-V LED Device

Requires: BLAZE/LED Minimum Versions: Atlas 5.28.1.R

This example demonstrates how to extract the optical output coupling efficiency of an LED/OLED device by using various techniques including reverse ray-tracing, the source term method and finite-difference time-domain (FDTD).

Reverse ray-tracing is a technique that allows us to obtain optical output characteristics of an active optoelectronic device based on material properties and device geometry. Ray-tracing is most commonly used for the modeling of passive optoelectronic components such as photo-detectors. Rays originating at the external light source are traced into the device and are absorbed to form electron/hole pairs, which are subsequently detected. Use of ray-tracing method in LUMINOUS is shown in examples optoex08 - optoex15.

Contrary to direct ray-tracing, the rays in the reverse method originate inside the active region and are traced until they exit the device. One or multiple origin points (user-specified) for rays are considered. Interference effects for rays originating at the common source point can be taken into account. When interference is enabled, the spectral selectivity of the device structure can be analyzed by performing ray-tracing at multiple wavelengths.

The source term method works in a similar fashion but directly accounts for interference effects by applying an algorithm akin to the transfer matrix method which only applies to truely 1D devices.

The finite-difference time-domain method works by direct solution of Maxwell's equations in the time domain and thus explicitly accounts for interference and diffraction in 1, 2 or 3 dimensions.

Both the source term method and FDTD use the same method as for ray tracing in that the emission from a set of dipoles located at user specified locations is integrated/averaged over the device.

The device structure and settings in this example are identical to those of optoex06, where LED luminous efficiency calculation is demonstrated. See the description of example optoex06 for more detailed information.

An additional functionality of LUMINOUS to assess light extraction from the device is implemented in the SAVE statements (after biasing). Five different sets of parameters for reverse ray-tracing are used in successive SAVE statements to demonstrate the features available for the modeling of LEDs. The parameter sets are given in the order of increasing complexity of simulation. Computation times also vary. It takes approximately 200 times longer to run the last SAVE line compared to the first one.

The parameter angpower should be set in the SAVE statement in order to start the reverse ray-tracing algorithm. The name of the output file containing the angular power density vs. output angle dependence is specified as a value of the angpower parameter. The information in the angpower file includes the angular power density for TE- and TM- polarized light, as well as total angular power density and total flux angular density vs. output angle. Note: In TonyPlot polar charts, the y-axis is directed upward (in the opposite direction to the internal coordinate system used in Atlas). Therefore, the plots appear to be flipped around the x-axis (top of the structure is at the bottom of the chart).

The parameter rayplot specifies the name of the output file containing the information on each ray exiting the device. This file is only created when a single origin for all rays is assumed. The information includes ray output angle, relative ray power (TE-, TM-polarization, and total), and initial internal angle at the origin (only if the interfere parameter is not specified). 0-degree angle corresponds to the rays in the X axis direction, 90-degree angle corresponds to the rays in the Y axis direction.

To start ray-tracing from one point of origin the user has to specify the following parameters: coordinates of the origin of rays X and Y and wavelength L.WAVE . It is important that the origin be chosen in the active region of the device. Rays are not traced if the radiative recombination is zero at the ray origin. All remaining parameters, outlined below, are optional and their default values are assumed if the parameters are not specified.

Reflects specifies a number of reflections to be traced for each ray originating at the point X , Y (similar to reflects parameter in the BEAM statement). The default value ( REFLECTS =0) provides for a quick estimate of the coupling efficiency. REFLECTS >0 should be used to obtain a more accurate result, especially if MIR.TOP or MIR.BOTTOM are specified. The choice of this parameter is based on a compromise between calculation time and accuracy. The maximum allowed value is REFLECTS =10. the number of reflections set to 3 or 4 is often a good choice. The parameter set #1 produces a simple ray-tracing analysis of an LED considered in this example.

The MIR.TOP parameter specifies that the top surface of the device be treated as an ideal mirror.

The MIR.BOTTOM parameter specifies that the bottom surface of the device be treated as an ideal mirror.

SIDE specifies that the rays reaching the sides of the device are terminated there and do not contribute to the total light output. This is often a good assumption for realistic LEDs as these rays tend to be either absorbed internally or blocked by the casing of the device.

TEMPER is the temperature needed for using appropriate refractive indexes of the materials. A default setting of 300 K will be used if not specified.

POLAR specifies polarization of the emitted photons in degrees (linearly polarized light is assumed). Parallel (TM-mode, POLAR =0.0) and perpendicular (TE-mode, POLAR =90.0) polarizations result in significantly different output coupling values. The default value ( POLAR =45.0) should be used if there is no preferred direction of polarization of emitted photons (unpolarized light emission).

MIN.POWER specifies the minimum relative power of a ray (similar to min.power parameter in the BEAM statement). The ray is not traced after its power falls below min.power value. This is useful for limiting the number of rays traced, and the default value is 1e-4.

NUMRAYS is used to specify the number of rays starting from the origin. The default is 180, the acceptable range 36 - 3600.

Parameter set #2 shows how some of the parameters described above are used for modeling a realistic LED. The angular distribution of light power obtained in optoex18ang_2.log exhibits a Lambertian pattern, which is what we expect for a simple LED considered in this example. Note that the optical coupling coefficient produced in this calculation reflects the 2D nature of the example (the light origin is not a point but rather an infinite line in z-direction). Simultaneously, it is possible to calculate the optical coupling efficiency for an axially symmetric 3D device (normally this is the value to be compared with experimental results). This calculation is enabled if coupling3D parameter is specified (while side is also set). For this calculation the light source is assumed to be a point located on the axis of symmetry.

The rays are assummed to be incoherent by default. This is a good approximation if the thickness of the active layer of the device is on the order of a wavelength/index. For layer thicknesses << wavelength coherent effects might be important. When the interfere parameter is set, the rays originating at the common source point are taken to be 100% coherent. In this case the phase information upon propagation is preserved. Phase change upon reflection is also considered. Thus, interference of rays exiting the device at the same angle is taken into account. The internal angle information is not written to the output rayfile in this case. The parameter set #3 takes interference into account.

Absorption of rays can be taken into account when the absorb parameter is set. The absorption is assumed to be constant specified for each material by the imaginary part of the refractive index. Carrier density dependent absorption and photon-recycling are not considered at this point.

Although ray-tracing from one point of origin can give a reasonable estimate of optical output coupling and angular distribution of light power, it is often desirable to consider multiple points within the active layer of the device to obtain more accurate results. Multiple origin points can be set using the following parameters:

XMIN , XMAX , YMIN , YMAX define a rectangular area containing all origin points. XNUM and YNUM specify the number of points along x- and y- axes within the rectangular area. If XNUM is equal to 1, the x-coordinate of all origin points is set to XMIN so that the points are chosen along the line x = XMIN . Similarly, points along the specific y-line can be chosen. Ray-tracing from multiple origins is realized by repeating a single origin algorithm for each point and by adding up the normalized angular power density values thus obtained. The luminous power assigned to each source (origin) is proportional to the radiative recombination at that point. Luminous power of all sources adds up to the value obtained by integration of radiative recombination over the entire device. Rays originating at different source points are completely incoherent (even when interfere is set), which is consistent with the spontanuous character of the radiation produced by an LED. The rayplot file is not written for multiple origins (even if parameter rayplot is specified). The parameter set #4 shows how multiple origin simulation can be done.

Spectral selectivity of optical output coupling for LEDs can also be taken into account. Reverse ray-tracing at multiple wavelengths is considered if the spectrum parameter specifies a filename for spectral selectivity output, while the angpower parameter is also set. EMIN and EMAX , or LMIN and LMAX specify the energy or wavelength range respectively. NSAMP specifies the number of spectral components to be considered. It is suggested that the SPECTRUM file is specified only when interfere is set (a warning is issued otherwise, while ray-tracing proceeds). When ANGPOWER and SPECTRUM parameters are set in the SAVE statement, the resulting optical output coupling is averaged over the entire energy (wavelength) range from EMIN to EMAX (from LMIN to LMAX ). The same applies to the quantities in the output angular distribution file. The spectrum file only shows how output coupling changes with wavelength. At the moment the shape of the gain curve is not taken into account (flat gain). The set of parameters #5 shows how multiple spectral components can be considered. The results obtained after averaging over spectral components and multiple origin points while taking interference into account are remarkably similar to the single point source model, where interference and multiple spectral content has been ignored (parameter set #2). This demonstrates the applicability of a simpler model in this case.

To load and run this example, select the Load button in DeckBuild > Examples. This will copy the input file and any support files to your current working directory. Select the Run button in DeckBuild to execute the example.

Additional Info:

Input Files
Output Results
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