Development of Pump-Probe Photoreflectance Setup

`The primary goal of the project is to establish a pump-probe photoreflectance setup, for the optical characterization of semiconductors. It is a non-destructive technique for the optical characterization of materials. For this, the designed setup includes white light from a xenon arc lamp is conver

Project Title

Development of Pump-Probe Photoreflectance Setup

Project Area of Specialization

Electrical/Electronic Engineering

Project Summary

`The primary goal of the project is to establish a pump-probe photoreflectance setup, for the optical characterization of semiconductors. It is a non-destructive technique for the optical characterization of materials. For this, the designed setup includes white light from a xenon arc lamp is converted to a single light of wavelength, after passing through a monochromator. Monochromatic light is then focused onto the sample, using convex lenses. Then a beam of light, usually a laser, falls on the sample, in the form of a cut. The reflected light contains a DC component and a modulated component, which is collected by a convex lens and focused onto a detector. The detector output is amplified and fed to the lock-in amplifier. The lock-in amplifier is tuned to the frequency of the modulation source. Then the change in reflectance ?R is processed by the computer program to evaluate the relative change in reflectance ?R/R, as a function of wavelength or energy. Experimental control, data acquisition and analysis of PR spectra were performed using the U12 Labjack and recorded in the Labview VI.

Project Objectives

Photoreflectance is a pump-probe, non-destructive optical technique that is employed to extract useful information about bulk semiconductors, as well as semiconductor interfaces. Some of the parameters that can be obtained using this technique include determining the doping concentration, band structures, band gap and internal electric fields of semiconductors. It also helps in determining other properties like physical strain, crystallinity, composition of different materials. Photoreflectance is a type of modulation spectroscopy. In general, modulation spectroscopy is any optical technique, which is based on measuring the amount of light reflected, refracted transmitted or absorbed by a sample at a given wavelength, in response to a strong pump light source, modulated at a particular high modulation frequency. The setup makes use of an electro-optical modulation scheme, employing a Thorlabs optical chopper and Stanford Instrument SR-830 lock-in amplifier. The electric field at the surface or at the interface of a semiconductor material is modulated, using this method and the ratio of change in reflection ?R (modulated AC signal) to the reflection R (DC signal), as a function of the wavelength is measured.

There are three types of reflection spectroscopies, which are external reflection spectroscopy, also known as specular reflectance, internal reflection spectroscopy (IRS) and diffused reflection spectroscopy. In this project, specular or external reflectance is used in which light is reflected from a smooth (mirror-like) sample to record its spectrum. External reflectance is a non-contact, non-destructive technique. It is in particular beneficial for film thickness and refractive index measurements. Over the years, owing to its characteristics of being a non-destructive and contact less technique, it has proven itself to be a highly efficient tool to obtain insight into the physics behind various semiconducting epitaxial structures. The pump probe technique tool is of choice for measuring dynamic processes that take place on ultrashort time scales, time-resolved type of measurement. Here, one promotes the system in a highly non-equilibrium state by optical excitation with an intense pump pulse and monitors the subsequent transient dynamics by a time-delayed, weaker probe pulse. In such a technique, as it is very sensitive to every direct optical transition in semiconducting quantum structures and allows as well to optically measure internal electric fields in space charge layers, through Franz–Keldysh oscillation (FKO) analysis.

Project Implementation Method

Fig 1: Basic schematic diagram for the pump-probe photoreflectance setup.

The above diagram shows the experimental setup for the project. A Xenon lamp (12V, 55W) will be employed as the white light source for the setup. An Optometric (SDMC-02) monochromator will be used for wavelength selection and scanning. The NEMA stepper motor is controlled through a Labview VI interfaced to National Instruments U12 Labjack. The output light from the monochromator will be collimated and focused onto the sample using appropriate converging (convex) lenses. The pump light beam is provided by a 2 mW laser source (He-Ne laser), which is externally modulated using a Thorlabs optical chopper. The pump light is also focused on the sample and aligned such that the pump and probe spots completely overlap each other. The test materials for the project are Gallium arsenide (GaAs), silicon (Si) and silicon dioxide (SiO2). The reflected probe signal is collected by a convex lens and focused onto a Si photodetector. The output contains both the DC as well as the modulated signal. A Thorlabs PD200 current amplifier is used to amplify the electrical signal from the photodiode and give out a voltage signal. This DC signal R from this output is fed directly to an analog input port of the U12 Labjack and recorded in the Labview VI. ?R is measured by demodulating the detector output signal using the SR-830 lock-in amplifier. The demodulated signal ?R collected at the output of the lock-in is then also recorded using th Labview interface. of detector is amplified and is fed to lock-in amplifier. The Labview program is then used to calculate and process the relative change in the reflectance ?R/R, as a function of wavelength or energy.

Benefits of the Project

The utility of photo-reflectance for characterization of semiconductor samples has been recognized since the late 1960s. Over the years, owing to its characteristics of being a non-destructive and contact less technique, it has proven itself to be a highly efficient tool to obtain insight into the physics behind various semiconducting epitaxial structures. The pump-probe technique is useful for measuring dynamic processes that take place on ultrashort time scales, time-resolved type of measurement. It is in particular beneficial for measurement thin film thicknesses, to calculate the bandgaps and doping concentrations of semiconductors and for refractive index measurements. This makes it an invaluable tool for semiconductor fabrication industry.

Technical Details of Final Deliverable

Fig. 1: Detailed setup diagram for the pump-probe photoreflectance setup.

The figure below provides a detailed diagram for the stepper motor connection to the TB6560 stepper motor controller. This will be employed to control the scanning process of the SDMC-02 monochromator and it will be interfaced to a Labview programme. The 'Enable' pin allows for the signal to be sent to the stepper motor. A high logic state on this pin enables the motor operation. A The 'Step' pin is responsible for moving the motor in the forward or the reverse directions which is controlled by the 'Direction' pin. A low to high pulse on this pin moves the stepper motor by one step, in the forward or the reverse direction. A logic low (0 V) on the 'Direction' pin sets the motor direction in the forward direction, whereas a logic high (5 V) sets the motor in the reverse direction. These pins will be provided the appropriate signals, using the U12 Labjack which is interfaced to a Labview programme. 

Fig 2: Schematic diagram for the TB560 stepper motor controller.

Final Deliverable of the Project

HW/SW integrated system

Core Industry

Manufacturing

Other Industries

Core Technology

Shared Economy

Other Technologies

Sustainable Development Goals

Industry, Innovation and Infrastructure

Required Resources

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