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Electrical Properties of Materials Fall 2007/Bill Knowlton
Optoelectronics ā Photonics Lab Objectives :
- To gain familiarity with characterizing light emitting devices and relating electrical properties to test and measurement data
- To determine the energy, wavelength and frequency range of optical power
- To determine the turn-on voltage of the device from the I V characteristics.
- To examine the relationship of Plankās constant with regard to the data
- To use energy band diagrams to conceptualize device structure, properties and performance as related to Bandgap engineering
Introduction :
n-Si i-Ge p-Si
ohmic contact
ohmic contact
Cladding layer
Cladding layer
h Ī½ Active region
Figure 1a : Schematic of a double heterostructure, single quantum well LED where the cladding layer is the larger bandgap semiconductor and the active region is the smaller bandgap semiconductor. The semiconductors chosen in this example are not used in LEDs because they are not very efficient light producers. Why is this the case?
Cladding layerActive Region Cladding layer
Ee-
E (^) f,p
E (^) f,n E^ c
E (^) v
Figure 1b : Energy band diagram at flatband condition of the device structure shown in figure 1a.
Light emitting diodes (LEDs) are solid state devices consisting of two (or more) different semiconductor materials. Some common materials for LEDs include InGaN (near UV and blue), GaN (blue and green), AlGaP (green), AlGaInP (green, yellow, and orange), AlAs & GaAs (red and near infrared), InGaAsP (near infrared).
Bandgap engineering toward a given device performance parameter is the goal for devices such as LEDs. For heterostructure LEDs, the performance parameter is light of a nearly specific wavelength (usually a range) that is produced in response to the flow of the current at the heterojunctions between these materials. The LED can be made by a p - n heterostructure junction. LEDs can also be fabricated using semiconductor quantum wells (figure 1) and are much more efficient. In this case, the active region (i.e., where e--h+^ recombination occurs and the light is produced) is sandwiched between the cladding (i.e., outside) layers which then have ohmic contacts for current injection (figure 1). The materials of the cladding layer have a wider energy bandgap than the active region (figure 1b). In both cases, the applied voltage must exceed a minimum voltage in order for the current to flow. It is approximately equal to the magnitude of the energy bandgap of the active region semiconductor. The magnitude of the energy bandgap
is related to the energy of the emitted photon by E=h Ī½
where h is Planckās constant and Ī½ is the frequency of
the emitted photon.
Equipment and Materials :
- Optical spectrum analyzer (make, model and number?)
Electrical Properties of Materials Fall 2007/Bill Knowlton
- Optics table
- Lens and translation stages
- Manipulator probes
Procedure and Data Analysis : Dr. Wan Kuang and Cory Sparks will set up the experiment in which several light emitting devices are to be characterized. Each group is responsible for measuring the output spectrum and the measured current versus input voltage (I-V curve) characteristics. From each set of measurements, you are to calculate the following: (1) Determine the energy, wavelength and frequency range and maximum of the optical power of the LEDs measured. (2) Determine the turn-on voltage of the device from the I-V characteristics. (3) Determine Planckās constant (mean and standard deviation) from the turn-on voltage and the peak emission wavelength. Perform a statistical analysis (i.e., % difference) to show the error involved in the measurement. (4) Determine the bandgap engineered structure of the device. That is, theorize the semiconductor material for each region of the device structure. Draw the energy band diagram of the cladding layer and the active region for the following conditions:
Voltage (V)
Current (A)
Turn-on Voltage
data fit
Figure 2 : Current versus voltage plot of an LED showing the turn-on voltage where the fit intersects with the voltage axis (red = data, dashed blue = linear fit).
Wavelength (nm)
Optical Power (dBm)
Frequency (Hz)
Energy (eV)
Figure 3 : Optical power spectrum versus wavelength, frequency and energy of two LEDs of different colors.
Peak Frequency (Hz)
e*V
Turn-on
at Peak Max (eV) Slope~eV/s
data fit
Figure 4 : The data of Vturn-on as a function of maximum Peak Frequency. Vturn-on is obtained from figure 2. The slope of the fit provides a number that should be close in value to a very famous constant.
a. Flatband condition b. Equilibrium conditions.
The minimum number of data plots for the report is 3 and the report should include (at least) the following plots:
- Current versus voltage with data fit (see figure 2)
- Optical Power (dBm) versus wavelength, energy and frequency (see figure 3)
Electrical Properties of Materials Fall 2007/Bill Knowlton
Title of Report
Lab Report for MSE 310: Electrical Properties of Materials
U.R. Dirac^1 , E.T. Compton^2 , and O.U. Moseley1, (^1) Department of Materials Science and Engineering (^2) Department of Electrical and Computer Engineering Boise State University
Summary : (1/4-1/2 page; max. Ā½ page) Summarize your findings. The audience for your summary is a manager (e.g., your manager or your bossesā boss) that might only read the summary.
Purpose : (1/4-1/2 page; max. 1 page) Describe the purpose and objectives of the lab as related to you the student. Have the objectives been met? Why might this lab be useful to you? What insight have you gained that supplements/compliments the course lectures by performing the lab?
Experimental : (1/4-1/2 page; max. Ā½ page) Describe the instrumentation, devices and methods used for each measurement. Data Analysis Approach : (1/4-1/2 page; max. 1/2 page) Describe the data analysis approach. Include software used, statistics & error analysis used, fitting method (e.g., linear regression & how it is done) etc. Equations should be numbered. E = h Ī½ (1)
Data Analysis Results and Discussion : (minimum 1 page; max. 2 pages) Thoroughly describe and discuss your results. Include data, data regression, energy band diagrams (equilibrium & and biased conditions), sketches of LED materials structure (semiconductors, substrate & contacts), etc. Describe your results and state or theorize the reasons behind the results (i.e., discussion) using your knowledge of bandgap engineering, band theory, SP^3 (structure, properties, processing and performance) and other aspects of Materials Science and Engineering. Figures should have captions and figure numbers (e.g., see figures used in this document).
Conclusions and Summary : (minimum: 1/4 page; max. 1/2 page)
References : List Journal and Conference papers, books, and course notes. Refrain from using websites. If you really feel that you want to use a website as a reference, please talk to me about it. Use more than just a few references. References should be cited within the body of your report.
Formatting Examples: [1] Author list, Title of journal paper , Volume (Year) page numbers. [2] Author list, Title of conference paper , Conference Name (Year) page numbers. [3] Author list, Title of Book , Edition (Publisher, Year) pages used.
Please use fonts between 10 ā 12 (Title can be 14). Please use 1 inch margins and page numbers. If you want to take a journal publication approach, you may use formatting approaches used in the journals of the IEEE called IEEE Transactions. The template for the journals include nearly
Electrical Properties of Materials Fall 2007/Bill Knowlton
all aspects of formatting for you and also contains examples. The word file template can be downloaded at the following website: http://www.ieee.org/portal/pages/pubs/transactions/stylesheets.html or on the course website.
