GaAs P-N Junction Infrared LED

This model simulates an LED that emits in the infrared part of the electromagnetic spectrum. The device structure is made up of a single p-n junction formed by a layer of p-type doping near the top surface of an otherwise n-type wafer. This kind of device geometry is simple and cheap to produce and similar LEDs are found in many household applications, for example, the IR emitters in TV remote controls. In this model the Optical Transition feature is used to calculate the electroluminescence from the device. The electronic properties are computed and the efficiency of the light production is assessed. Also, by visualizing the spacial distribution of the radiative recombination it is possible to make design suggestions to maximize the total efficiency of the output light.

Introduction

Infrared LEDs are widely used in many optoelectronic applications, but perhaps the most familiar in a household setting is for use in remote controls. Although more efficient light production can be achieved using complicated device structures, simple p-n junctions are still used for many IR emitters due to their low cost and ease of manufacture.

In this model, a simple p-n junction infrared LED is modeled. The device geometry is cylindrically symmetric, which enables a 3D visualization of the results with a much shorter computation time than would be required for a full 3D model. This model shows an example of how to use the Semiconductor Initialization study to create a refined mesh in cylindrical geometry, this is useful because this study step automatically refines the mesh around p-n junction. A second Stationary study is then used to sweep the bias applied to the device, allowing the current-voltage curve and optical emission characteristics to be calculated.

Model Definition

The device is made using 60 μm diameter circular chip of GaAs. Although, due to the crystalline nature of semiconductor materials, actual LED chips are likely to be fabricated using rectangular chips, only the circular active region formed by the p- and n-type doping is included in this simulation. This is in order to allow cylindrical symmetry, which considerably reduces computation time. Because the r=0 axis is an axis of symmetry, the device geometry is represented by a 30 μm wide rectangle with a thickness of 10 μm. The bottom 7.5 μm of the device are n-doped and the top 2.5 μm are p-doped. A rectangular trench has been etched down to the n-type layer at the right hand side of the domain to allow an electrical contact to be deposited onto the n-type side of the p-n junction, creating a ring of metal around the edge of the device. The p-type contact is deposited directly on the top surface in the center of the device, creating a metal disk. This configuration is shown in Figure 1.

Figure 1: Device geometry. Top: Geometry view showing one half of a 2D cross section through the diameter of the device. The axis of symmetry is shown as a red dashed line. Bottom: The 3D device geometry.

An feature is added to the domain to allow the electroluminescence to be calculated. This is achieved by clearing the check box within the feature, such that only the will be calculated. In the absence of any optical excitement from an incident electromagnetic field the spontaneous emission will be the electroluminescence due to the carrier dynamics of the injected current.

An feature has been added to include a nonradiative recombination mechanism. This is only one possible nonradiative recombination mechanism, in reality a variety of other processes may be present, such as lattice/impurity scattering and phonon interactions. However, for the purpose of this demonstration model Auger recombination is sufficient to capture the important behavior that the efficiency of the radiative emission drastically reduces with increased current densities.

The model uses two studies. The first study is a Semiconductor Initialization study, which is used to automatically refine the mesh around the gradient of the dopant concentration. This is useful because the refined mesh is denser around the p-n junctions, which is where a smaller resolution is required.

The second study sweeps the voltage bias across the device from 0 V to 1.5 V. The current-voltage curve is plotted, to show that the device is indeed a diode, and the emission rate throughout the device is calculated.

Results and Discussion

The default mesh, with the size set to , is shown in Figure 2. The refined mesh, output by the first study, is shown in Figure 3 and the doping is shown in Figure 4. From these two figures it can be seen that the mesh has been refined in region of the p-n junction as desired.

Figure 2: Default physics induced mesh after the size node has been set to Finer.

Figure 3: Refined mesh output by the Semiconductor Initialization study step in the first study. The mesh density is increased in the p-n junction region.

Figure 4: Signed dopant concentration shows the p-type surface layer in blue and the n-type region in red. The doping gradually transitions from n-type to p-type over a range of a few μm to create a wide region in which light emission can occur.

Figure 5: Current as a function of voltage showing diode behavior with a turn-on voltage just above 1.2 V.

Figure 5 shows the current as a function of the voltage bias. The device is clearly a diode, with a turn-on voltage of around 1.2 V.

Figure 6 and Figure 7 show the emission rate throughout the device for a voltage of 1.5 V and 1.2 V. At the larger bias the emission has become concentrated toward the center of the device.

Figure 8 shows the total rate of emission, integrated over the entire domain of the device, as a function of the current. The sublinear curve is characteristic of a LED droop, a phenomenon where the efficiency of LEDs decreases with increased current density. This reduction in efficiency is even more apparent in Figure 9, which shows the Internal Quantum Efficiency (IQE) as a function of the current. The IQE is the ratio of the emission rate to the carrier injection rate, and gives the proportion of injected carriers which radiatively recombine to emit light. Initially the IQE is very low, as no current flows, however as soon as current begins to flow the IQE is initially quite high. As the current increases the IQE drops drastically to plateau at a value of 0.075. In this model, Auger recombination has been included as a nonradiative recombination mechanism. The rate of Auger recombination is proportional to the cube of the carrier density, whilst the radiative emission rate is proportional to the square of the carrier density. This means that as the carrier density is increased by the increasing current a larger proportion of the carriers recombine via the Auger mechanism, leading to the observed drastic reduction in IQE.

Figure 6: Emission rate throughout the device at a voltage bias of 1.5 V.

Figure 7: Emission rate throughout the device at a voltage bias of 1.2 V.

Figure 8: Total emission rate as a function of current. The sublinear curve is indicative of LED droop.

Figure 9: Internal Quantum Efficiency as a function of current. The drastic drop in efficiency corresponds with the sublinear increase in total emission.

The IQE is only a measure of the efficiency of the radiative recombination within the device. In order to assess the actual efficiency of the device as a light source other considerations need to be included. Specifically, it is important to consider where within the device the light is emitted from. As the p-contact on the top surface is opaque, emission underneath the p-contact may not contribute to the emitted light intensity from the device. From Figure 6 and Figure 7, which show the spacial distribution of the radiative emission, it can be seen that the emission becomes concentrated toward the center of the device with increasing voltages. This is more apparent in Figure 10, which shows a 3D visualization of the data from Figure 6. The IQE, shown in Figure 9, indicates that efficient operation of the LED would occur with low currents below 5 mA. This would be the desirable range of operation current for a remote control device, where intensity of the emission is not important compared to increased battery lifetime. These currents corresponds to voltages between 1.2 and 1.3 V and so the emission will be distributed similar to that shown in Figure 7, where the emission occurs throughout the entire p-type layer. For these voltages the position of the p-contact does not block most of the light emission from leaving the device. However, if the LED was to be used in a situation where total brightness was more important than efficiency it could be desirable to drive the device with a larger current. This could be the case if the LED was providing illumination for an infrared night-vision security camera, for example. In this case, although increasing the current increases the total rate of emission, the localization of the emission to the region underneath the p-contact could significantly reduce the total light emitted. For such a scenario it would be advantageous to change the material used for the p-contact so that it was transparent to IR light, or perhaps amened the geometry of the p-contact. For example, a ring shaped p-contact, which would allow the brightest emission on the central axis to leave the device, could be considered.

This example highlights the importance of simulation tools when designing optoelectronic semiconductor devices, as it is much cheaper to simulate several designs before committing to manufacture a device.

Figure 10: Emission rate throughout the device for an applied voltage of 1.5 V. This figure is a 3D visualization of the data in Figure 6.

Semiconductor_Module/Photonic_Devices_and_Sensors/gaas_pn_junction_infrared_led_diode

Modeling Instructions

From the menu, choose .

New

In the window, click

Model Wizard

In the window, click

In the tree, select .

Click .

Click

In the tree, select .

Click

Global Definitions

Parameters 1

In the window, under click .

In the window for , locate the section.

In the table, enter the following settings:


Name	Expression	Value	Description
V_n	0[V]	0 V	n voltage
V_p	1.5[V]	1.5 V	p voltage
w_dom	25[um]	2.5E-5 m	Domain width
h_dom	10[um]	1E-5 m	Domain height
w_con_n	w_dom/5	5E-6 m	Width of n contact
h_con_n	h_dom/5	2E-6 m	Height of n contact
w_con_p	w_dom/10	2.5E-6 m	Width of p contact
h_p	h_dom/4	2.5E-6 m	Height of surface p-type layer
r_fill	0.25[um]	2.5E-7 m	Fillet radius

Geometry 1

In the window, under click .

In the window for , locate the section.

From the list, choose .

Rectangle 1 (r1)

In the toolbar, click

In the window for , locate the section.

In the text field, type w_dom.

In the text field, type h_dom.

Rectangle 2 (r2)

In the toolbar, click

In the window for , locate the section.

In the text field, type w_con_n.

In the text field, type h_con_n.

Locate the section. In the text field, type w_dom.

Union 1 (uni1)

In the toolbar, click

and choose .

Click in the window and then press Ctrl+A to select both objects.

In the window for , locate the section.

Clear the check box.

Polygon 1 (pol1)

In the toolbar, click

In the window for , locate the section.

In the table, enter the following settings:


r (µm)	z (µm)
0	h_dom
w_con_p	h_dom

Fillet 1 (fil1)

In the toolbar, click

On the object , select Point 4 only.

It might be easier to select the correct point by using the window. To open this window, in the toolbar click and choose . (If you are running the cross-platform desktop, you find in the main menu.)

In the window for , locate the section.

In the text field, type r_fill.

Click

Click the

button in the toolbar.

Add Material

In the toolbar, click

to open the window.

Go to the window.

In the tree, select .

Click in the window toolbar.

In the toolbar, click

to close the window.

Semiconductor (semi)

In the window, under click .

In the window for , locate the section.

From the list, choose .

p Doping

In the toolbar, click

and choose .

In the window for , type p Doping in the text field.

Locate the section. From the list, choose .

Locate the section. In the NA0 text field, type 1e18[1/cm^3].

Locate the section. Specify the r0 vector as


0[um]	R
h_dom-h_p	Z

In the W text field, type w_dom.

In the H text field, type h_p.

Locate the section. From the list, choose .

In the ld text field, type 2[um].

n Doping

In the toolbar, click

and choose .

In the window for , type n Doping in the text field.

Locate the section. From the list, choose .

In the ND0 text field, type 1e18[1/cm^3].

Locate the section. In the W text field, type w_dom+w_con_n.

In the H text field, type h_dom-h_p.

Locate the section. From the list, choose .

In the ld text field, type 2[um].

p Contact

In the toolbar, click

and choose .

In the window for , type p Contact in the text field.

Select Boundary 3 only.

Locate the section. In the V0 text field, type V_p.

n Contact

In the toolbar, click

and choose .

In the window for , type n Contact in the text field.

Select Boundary 7 only.

Locate the section. In the V0 text field, type V_n.

Optical Transitions 1

In the toolbar, click

and choose .

Select Domain 1 only.

In the window for , locate the section.

Clear the check box.

Auger Recombination 1

In the toolbar, click

and choose .

Select Domain 1 only.

Materials

GaAs - Gallium Arsenide (mat1)

In the window, under click .

In the window for , locate the section.

In the table, enter the following settings:


Property	Variable	Value	Unit	Property group
Auger recombination factor, electrons	Cn	1e-31[cm^6/s]	m6/s	Auger recombination
Auger recombination factor, holes	Cp	1e-31[cm^6/s]	m6/s	Auger recombination

Add a nonlocal integration coupling for use in postprocessing the results.

Definitions (comp1)

Integration 1 (intop1)

In the toolbar, click

and choose .

Select Domain 1 only.

Use a semiconductor initialization study to refine the mesh.

Study 1: Mesh Refinement

In the window, click .

In the window for , type Study 1: Mesh Refinement in the text field.

Locate the section. Clear the check box.

In the toolbar, click

Component 1 (comp1)

Compare the two meshes, note that the refined mesh is denser around the p-n junction.

Mesh 1

In the window, expand the node.

Right-click and choose .

Click the

button in the toolbar.

Level 1 Adapted Mesh 1

Click the

button in the toolbar.

Before adding and configuring the main voltage sweep study, confirm the doping profile using a 2D surface plot.

Results

Doping

In the toolbar, click

and choose .

In the window for , type Doping in the text field.

Surface 1

Right-click and choose .

In the window for , locate the section.

In the text field, type semi.Nd-semi.Na.

In the toolbar, click

The doping should look like Figure 4.

Add a study to perform the voltage sweep.

Add Study

In the toolbar, click

to open the window.

Go to the window.

Find the subsection. In the tree, select .

Click in the window toolbar.

In the toolbar, click

to close the window.

Study 2: Voltage Sweep

In the window, click .

In the window for , locate the section.

Clear the check box.

In the text field, type Study 2: Voltage Sweep.

Step 1: Stationary

In the window, under click .

In the window for , click to expand the section.

Select the check box.

Click

In the table, enter the following settings:


Parameter name	Parameter value list	Parameter unit
V_p (p voltage)	log(range(1,0.25,3.5)) log(range(3.6,0.1,4.5))	V

Solution 5 (sol5)

In the toolbar, click

In the window, expand the node.

Right-click and choose .

In the window, expand the node, then click .

In the window for , click to expand the section.

From the list, choose .

In the window, under right-click and choose .

In the window for , locate the section.

In the text field, type comp1.Ne 1 comp1.Ph 1.

In the toolbar, click

Plot the current-voltage curve to see that the device behaves as a diode.

Results

Current vs. Voltage

In the toolbar, click

and choose .

In the window for , type Current vs. Voltage in the text field.

Locate the section. From the list, choose .

Locate the section. Select the check box.

In the associated text field, type Voltage (V).

Select the check box.

In the associated text field, type Current (mA).

Click to expand the section. From the list, choose .

In the text area, type Current as a function of voltage.

Global 1

Right-click and choose .

In the window for , locate the section.

In the table, enter the following settings:


Expression	Unit	Description
semi.I0_1	mA	Terminal current

Click to expand the section. Clear the check box.

In the toolbar, click

Plot the electroluminescence emission rate throughout the device, the effects of varying the voltage can be seen by changing the value of for which the results are plotted. At voltages just above the turn on voltage the emission is spread out radially but it becomes more concentrated under the central p-contact as the voltage is increased.

Emission Rate

In the toolbar, click

and choose .

In the window for , type Emission Rate in the text field.

Locate the section. From the list, choose .

Click to expand the section. From the list, choose .

In the text area, type Emission Rate.

Surface 1

Right-click and choose .

In the window for , locate the section.

In the text field, type semi.ot1.R_spon.

In the field, type 1/(cm^3*s).

In the toolbar, click

The default plot shows the case for . However, cycling through several values of allows data for other values to be viewed. This is done by selecting the desired value from the list, for example:

Emission Rate

In the window, click .