Radiant: GUI-based design software for performing simulations of optoelectronic thin film devices such as LED and OLED

 

Introduction

Device simulation helps users understand and depict the physical processes in a device and to make reliable predictions of the behavior of the next device generation. Device simulations with properly selected, calibrated models and an appropriate mesh structure are very useful for predictive analysis of novel device structures. This helps provide improved reliability and scalability, while also helping to increase development speed and reduce risks and uncertainties.

In order to be even more efficient Silvaco releases Radiant an interactive tool to simulate LED and OLED devices. Radiant consists of a very easy to use and intuitive graphical user interface allowing the user to input key simulation parameters without having to know the syntax of the device simulator. Behind the scene and automatically an input deck is created and run. Results are displayed using TonyPlot.

In this introduction to Radiant a one layer diode, with anode and cathode contacts, is created and simulated. Please see the Atlas manual for more information about Atlas commands.

 

Creating the Device

Radiant is started by typing

radiant

at the command prompt. The initial window appears (Figure 1).

Figure 1. The initial main window of Radiant.

The two Air layers are the external medium the device will exist in. The device parameters are defined in the right hand panel. An image of the current device is shown in the left hand panel. The Name of the device, “oled” in this example, is used as the root of the files generated when simulating the device.

To add a layer, ensure the Structure tab is selected in the right hand panel. and click on the Add button on the bottom right. The Add Layer window appears as show in Figure 2.

Figure 2. The initial Add Layer window.

The top layer is to be the anode contact, select the Layer Type drop-down list and choose Schottky Contact (Figure 3).

Figure 3. Select Schottky Contact for the Layer Type.

Enter

anode in the Name line
ITO in the Material line
25 in the y Del line
5 in the y NStep line

The Name is the name of the layer in the Atlas deck, a name is required for a contact, but is optional for the other types of layer. The Material is the material of the layer. y Del is the thickness of the layer, in nanometers. y NStep is the minimum number of mesh points Atlas will add to the layer.

Figure 4. The definition of the anode.

Click Continue. This will add the layer to the device and leave the Add Layer window open. The main window will show the anode has been added to the device as shown in Figure 5.

Figure 5. The main window when the anode has been added to the device.

Now add a doped Alq3 layer, which is the active layer of the diode. Choose a Semiconductor Layer Type, leave the Name blank, define a Material of Alq3, a y Del of 100 and a y NStep of 20. In the doping line select n-type from the drop-down list and set a concentration of 1e14.

Figure 6. The definition of the Alq3 layer.

And click Continue.

Now add an aluminum cathode to the bottom of the device

Figure 7. The definition of the cathode.

 

And click on OK. OK will add the layer to the device and close the Add Layer window.

The main window will show the device as shown in Figure 8.

Figure 8. The main window with the complete device.

To define material parameters select the Material tab in the left hand panel (Figure 9).

Figure 9. The main window with the Material tab activated.

Select the ITO line, click on Edit and the Edit Material window opens.

There are three columns next to each label. A Temporary column, a User Defined column, and a Default column. The data in the Default column is built into Radiant and cannot be changed by the user. Any data entered into the User Defined column will be automatically saved when exiting Radiant (on a per-user basis) and will be available the next time Radiant is run. Any data entered into the Temporary column are not automatically saved, but will be saved in the File->Save deck. The value used in a simulation is the right-most value in a particular row: Temporary data will be used in preference to User Defined data, which will be used in preference to the Default data.

Enter 5.4 in the Temporary column of the Work Function line (Figure 10), and click OK.

Figure 10. Defining the Work Function of the anode.

Select the Alq3 line, click Edit, and enter 3.4 to the EG300 line, and 2.2 to the Affinity line, and click OK (Figure 11.)

Figure 11. Defining the band alignment of the Alq3 layer.

Finally set the work function of the aluminum cathode to 3 as shown in Figure 12.

Figure 12. Defining the Work Function of the cathode.


DC Simulation

Select the Analysis tab, and click on Add DC. The Add DC window opens. Leave anode in the Contact drop-down box, this is the contact the bias will be applied to. Add a VStep of 0.1 and a VFinal of 10 (Figure 13).

Figure 13. The Add DC window.

And click OK. A single trial has been added to the Analysis table as shown in Figure 14.

Figure 14. The main window with the Analysis tab selected.


Click on the Models button, and ensure the pf.mob option is unchecked. If it were activated then the Poole-Frenkel mobility model would be used, but that model is not required for this simulation (Figure 15).

Figure 15. The Models window.


Click on OK.

Open the Run menu, and select Run. A dialog box will pop up to confirm whether to run the simulation. It will look similar to Figure 16.

Figure 16. A confirmation dialog.

The identity of the simulation will be different though. The first part is the date (YYYYMMDD), and the second part is the time (HHMMSS). Click on Yes to run the simulation.

Radiant creates a unique directory and runs the simulation in that directory, so that any output files from this simulation do not overwrite the files from any other simulation. A DeckBuild is created to run the simulation, this will initially be minimized. Once DeckBuild has finished open the View menu, and select Current.

Figure 17. The View->View window.

Click on View to see a list of the files generated by the simulation. Change the File type filter to All files (Figure 18).

Figure 18. The files generated by a DC trial.

The oled.in file is the Atlas input deck generated by Radiant, the oled.in.out is the run-time output generated by Atlas when running the oled.in deck. oled_init.str is the structure of the device after the initial SOLVE INIT. These files are generated whenever Radiant runs.

Each trial also generates files, the types of files generated depend upon the trial. oled_0.log is the log file generated during this DC trial and oled_0.str is the structure file at the end of the trial. The contact current at the final bias is also extracted in the input deck with the lines

extract init inf=”oled_0.log”
extract name=”i_tr0” y.val from curve(v.”anode”,i.”anode”) where x.val=10.

The result of this extract can be used by other tools.

Double clicking on oled_0.log will open the file in TonyPlot (Figure 19).

Figure 19. The DC-IV curve of the forward biased diode.

The anode workfunction was 5.4 eV and the cathode workfunction was 3.0 eV and the diode turn-on is at the expected 2.4 V.

 

Saving and Loading Devices

To save the current state of a device, its structure and analysis, open the File menu and select Save as. Enter the required file name in the File name box and click Save. The file that is saved is an Atlas deck (that can be run in Atlas) with some extra commands at the end that store additional information required by Radiant.

Figure 20. The File->Save As window.

To close down Radiant open the File menu and select Exit. If there are any unsaved changes to the device a dialog box will open to check if these changes should be saved or discarded.

To start Radiant with an existing device, the file can be given as a command line option

radiant oled.in

A deck can also be read in by opening the File menu and selecting Open.

 

Optical Simulation

Select the Analysis tab, and click on Add TM. The Add TM window opens (Figure 21).

Figure 21. The Add TM window.

The Alq3 layer is the active layer in this diode, so select Alq3 from the Layer drop-down list.
The position of a dipole is defined relative to a layer. Define a dipole in the middle of the Alq3 layer by adding 0.5 to the Min column in the X and Y lines.

Calculate over the visible spectrum in 1 nm steps by adding 390, 700, and 311 to the boxes in the Lambda line (the step is automatically calculated and shown in the final column).

Select emit.top, dipole, and norm.ang. Horizontal, Num Rays, and Angle Output can be left blank (they will take their default values). Set Angle to 0. The Angle is the angle used when extracting results from this trial (Figure 22).

Figure 22. The Add TM window with the parameters for the trial.

Click on OK and the trial has been added to the analysis list. Once a dipole has been defined the Dipole On button is activated on the left-hand panel. Clicking on this will show the location of the dipoles in the device (Figure 23).

Figure 23. The main window showing the position of the dipole.

The emission spectrum of dipoles in a layer can be specified on the Material tab. In this example the Alq3.spc file has been copied from the Atlas common/sspeclib directory. Select the Material tab, select the Alq3 line, and click on Edit (Figure 24).

Figure 24. Select a spectrum file for the Alq3 layer.

Once a file has been selected the data can be viewed by selecting the bottom option in the drop-down list.

Figure 25. Once the file has been selected the data can be viewed.

 

Figure 26.The data in the Alq3 file.

Click on Cancel to close this window.

Open the Run menu and select Run. The trials are run in the order they are defined in the list on the Analysis tab, here the DC sweep will be performed first followed by the transfer-matrix optical simulation. To see the files generated by this simulation open the View menu, select Current, and click on View.

Figure 27. The files generated by the DC and TM trials.

The oled.in, oled.in.out, and oled_init.str are the standard files generated by any simulation. oled_0.log and oled_0.str were generated by the initial DC trial.

The files oled_1_ap.log, oled_1_os.log, and oled_1_sa.log have been generated by the TM trial. The _ap file is from the ANGPOWER parameter on the Atlas SAVE command, the _os file is from the OUT.SPEC parameter, and the _sa file is from the SPECT.ANGLE parameter. The TM trial extracts CIE x, CIE y, the luminosity, the intensity, and the luminance at the given angle. The wavelength of the peak of the spectral power density is also extracted (Figure 27).

Figure 28. The results of the TM trial.

 

Sweep

The Run->Sweep option can be used to map the changes in the outputs to changes in the structure of the device.

Open the Run menu and select Sweep and the sweep window appears. This is used to specify the parameter that will vary and the range they will vary over.

Figure 29. The Sweep variables window.

Click on Add and the Add Variable window opens as shown in Figure 30.

Figure 30. The Add Variable window.

Independent variables are associated with layers, select Alq3 from the Layer drop-down box. Within each layer the variables are grouped. The Alq3 layer has Layer and Trial Dipole groups. The Layer variables are parameters associated with the layer as a whole; the thickness and the doping. The Trial Dipole variables are parameters associated with the dipoles defined on the layer; the position, the output angle, the orientation of the dipoles. Select the Layer (Independent) Group and the Donors Variable. Enter 1e14 to Start, 1e17 to End, 7 to NPoint and choose the Logarithmic Scale. Linear variables sweep between the Start and End values in an arithmetic progression, Logarithmic variables sweep between the Start and End values in a geometric progression.

Figure 31. Sweep the doping in the Alq3 layer between 1e14 and 1e17.

Click on OK to add the variable to the trial.

Figure 32. The Sweep variables window with the doping variable added.

Click on OK and a dialog appears asking it the simulation should be run, click on Yes. A DeckBuild will open and use DBInternal to run the set of simulations. Open the View menu and select Current. The current simulation may have a status of Running.

Figure 33. The View->Current window while the sweep is running.

Currently Radiant doesn’t allow the files of a running simulation to be viewed. Wait until the simulation has finished and click Refresh (Figure 34).

Figure 34. The View->Current window when the sweep has finished.


The files can now be viewed. A sweep generates more files than a single simulation. The files generated by each trial of each simulation are saved. Their names have “_bd$’trial_id’” appended before the file extension to distinguish between them. The DC IV curves from the seven trials are shown in Figure 35.

Figure 35. The DC-IV curves for the different trials.

DBInternal saves the results of the sweep, this contains the variables that have been swept, and the results of any EXTRACT statements in the trial deck. These results have been saved in the “oled_db.log” file. The anode current at Vanode=10V as a function of donor doping is shown.

Figure 36. The anode current as a function of donor concentration.

 

Conclusion

Radiant is a GUI-based powerful design software for performing accurate simulations of optoelectronic thin film devices such as LED and OLED.

Radiant provides a fully integrated and user-friendly environment including coupled electrical and optical device simulations, optimization and visualization based on state of the art Silvaco TCAD softwares.

Radiant enables faster time to market for the novel lighting devices.