Square pulses for DLTS measurements on the MFIA

Producing square voltage pulses and capturing capacitance transients

This blog post shows how to produce square voltage pulses and capture the resulting capacitance transients on a short timescale (20 us) for example, for Deep-level Transient Spectroscopy (DLTS) measurements. Using the LabOne Threshold Unit (which comes as standard with the MFIA) it’s possible to produce square pulses with a definable voltage offset and amplitude of up to 10 V. This means the MFIA (or MFLI with MF-IA Option) can replace many of the numerous instruments required for DLTS, such as the capacitance meter DAC card and pulse generator, as described in further detail in this application note.

DLTS is a powerful and commonly used technique to investigate both the concentration and carrier binding energy of defects in semiconductors. The technique essentially involves measuring capacitance transients at different temperatures. It is commonly used to characterize devices and materials such as MOSFETS, solar cells, OLEDs, Schottky diodes and TFTs.

Setting up the square voltage pulses

Figure 1: Setting up the Threshold Unit and Aux Tab of LabOne to produce square voltage pulses on the MFIA.

 

Figure 1 shows the required steps to configure the square voltage pulses. Each step is explained in detail as follows:

  1. Select the signal input of one of the four threshold units to be “TU Output Value”. Invert this signal in the logic unit sub-tab to create a ring oscillator whose output will be zero or one.
  2. Set the delay in the enable and disable field as highlighted in step 2 in figure 1. We have selected 300 ms and 1 s for the enable and disable respectively.
  3. Now we move to the Aux tab, and set the output of Aux Output 1 to be defined by the output of the threshold unit defined in step 1. On the front panel of the MFIA, connect Aux Ouput 1 to Aux Input 1 with a BNC cable (as shown in figure 2).
  4. The scaling and offset can be set to the desired voltage. Using a negative scaling factor allows the pulses to be inverted. Take care to check the limits match the expected output, otherwise clipping may occur.
  5. Open the plotter tab and add the trace “Signal 1 Aux in 1” to view your pulses and adjust the offset and amplitude as required.

 

In the lock-in tab, enable the Add checkbox in the Signal Output field. This will add the voltage at Aux Input 1 to the AC test signal. Also ensure that the output range of the MFIA is larger than your desired pulse voltage. This can be set in the advanced mode of the Impedance Analyzer tab as shown in Figure 3.

 

Figure 2: MFIA with the MFITF fixture and Aux Ouput 1 connected to Aux Input 1 via a short BNC cable. A commercially available monocrystalline solar cell is mounted on an MFITF sample carrier inserted into the MFITF. 

 

Measuring capacitance (or other impedance parameters)  during the application of the voltage pulses

Open and enable the LabOne Impedance Analyzer Tab. Configure the settings to suit your sample or use the settings shown in figure 3 as a starting point. We are measuring a monocrystalline solar cell (Digikey KXOB22-04X3L-ND, pictured in figure 2) in 4-terminal mode with an AC test signal of 100 mV at 1 MHz. Here are a few tips when measuring capacitance transients, referring to the highlighted fields in figure 3:

  1. Increase the Max Bandwidth to be fast enough to capture the capacitance transient.
  2. Increase the data transfer rate to be fast enough to get the desired time resolution (107 kSa is the maximum).
  3. Ensure that the voltage signal output range is large enough to be able to generate both the pulse and the AC test signal.
  4. Switch to manual mode when it comes to the current input range. This will avoid any unwanted range changes.

 

Figure 3: LabOne Impedance Analyzer Tab, showing settings used for measuring the capacitance of the solar cell sample. The advanced mode must be selected to show all the options.

 Once the impedance measurement settings are set, open the LabOne Plotter Tab, add the signal to be measured (for example, Capacitance) to the vertical axis group along with the “Signal 1 Aux in 1” trace. Now you can see both the DC bias voltage pulses and the capacitance simultaneously. Figure 4 shows two traces; capacitance in red and the corresponding square pulse of the DC voltage bias in blue. The measured capacitance at zero DC bias is 1.6 nF and 2.3 nF at a bias of 1 V.

 

Figure 4: LabOne Plotter module showing capacitance (red trace) and corresponding DC bias voltage (blue trace). The voltage pulses are 1 V in amplitude and offset + 0.5 V. The width of the 1 V pulse is 1 s. Oscilloscope measurements (not shown)  reveal the time required for the voltage step to be 18 ns.

For faster measurements which require advanced triggering, the LabOne DAQ module should be used. The DAQ module allows you to define the required number of points, and its flexible triggering settings allow you to reliably capture the full transient including the pre-step steady state.  Figure 5a shows data acquired with the DAQ module for a voltage pulse width of just 1 ms.  Figure 5b shows a timebase-zoom at the point where the voltage is stepped from 1 V to 0 V. It shows that the value of capacitance is accurately measured before (2.3 nF) and after (1.6 nF) the reset. The time required for the MFIA to capture these accurate capacitance measurements is just 20 us!

 

Figure 5: LabOne DAQ module showing capacitance (red trace) and corresponding DC bias voltage (blue trace). Figure 5a shows the width of the pulses to be 1 ms with a delay of 3 ms. Figure 5b shows the same trace but with the timebase zoomed to show the short time required to measure the capacitance accurately before and after the reset of the DC bias voltage from 1 V back to 0 V. This time is just 20 us. NB The voltage step is faster than 20 us, but the data point density does not capture this. Oscilloscope measurements (not shown)  reveal the time required for the voltage step to be 18 ns.

Conclusion
This blog post shows the how the MFIA can produce square voltage pulses and reliably measure the resulting capacitance transients. The device under test use for this demonstration was a monocrystalline solar cell, measured with an AC test signal of 100 mV at 1 MHz. The MFIA is able to measure the capacitance before (2.3 nF) and after (1.6 nF) the voltage pulse is applied, with a time resolution of just 20 us.

This functionality of the MFIA is of great help for users looking to increase the performance and flexibility of their DLTS setups while simultaneously decreasing complexity. The MFIA can replace the capacitance meter, DAC card and pulse generator in a single small-footprint instrument. The increased capacitance range of the MFIA compared with existing instruments such as the Boonton 7200, and the ability to set the test signal frequency freely between 1 mHz and 5 MHz allow the user to push the envelope of DLTS measurements.

For a demo, or for further information on using the MFIA for DLTS measurements, such as temperature control using its analog or digital outputs, get in touch.