Electrical and Computer Engineering ETDs

Publication Date

Summer 7-2017


Measurement of recombination mechanisms provides critical feedback on the material quality of semiconductors. Strained layer type-II superlattices (T2SLs) have seen a recent increase in interest as they possess intriguing properties making them prime candidates for use as infrared detectors. As T2SL-based detectors approach the performance of industry-standard Hg1-xCdxTe photodetectors, measurement of the carrier lifetime is becoming increasingly important. A comparison of the lifetime measurement techniques time-resolved photoluminescence, frequency-modulated photoluminescence, time-resolved microwave reflectance, and frequency-modulated conductance is made. Although photoluminescence-based measurement techniques are more common in literature, it is shown that the microwave reflectance-based measurement technique is able to probe lower carrier densities and, therefore, more accurately measure minority carrier lifetime in low-doped samples.

In addition to the lifetime measurement comparison, using a multiple harmonic approach based on the frequency-modulated photoconductivity method, the recombination mechanisms are measured in an InAs/InAs1-xSbx T2SL at a temperature of 100 K. The second harmonic of the generated carrier density is dependent on the high-injection recombination mechanisms and not the minority carrier recombination, enabling accurate extraction of parameters governing high-injection recombination. From these measurements, it is found that the Shockley-Read-Hall lifetime is 3.47 microseconds, the radiative recombination is 10-10 cm3s-1, and the Auger recombination coefficient is 2.29 10-26 cm6s-1, agreeing well with the more-readily used time-resolved microwave reflectance measurement. With this approach, characterization can be performed using basic laboratory equipment without the need of high-end laser systems or fast electronics.

The minority carrier lifetime and equilibrium electron concentration (i.e. the doping level, n0) are both important values that directly determine diffusion current in infrared photodetectors utilizing n-type absorbing regions. Here, time-resolved microwave reflectance measurements are used to non-destructively measure both of these values in mid-wave infrared InAs/InAs1-xSbx type-II superlattices with varying n-type doping levels between 2 1014 cm-3 and 2 1016 cm-3. The measured data are analyzed using carrier recombination theory to determine the doping level ranges where Shockley-Read-Hall, radiative, and Auger recombination limit the minority carrier lifetime. The optimal doping level, which minimizes dark current, is experimentally determined and corresponds to the electron density at which the minority carrier lifetime switches from Shockley-Read-Hall limited to Auger limited behavior. A comparison of two InAs/InAs1-xSbx photodetectors of different equilibrium electron densities demonstrates a decrease in dark current for a doping level near the optimal doping density minority carrier lifetime product.


carrier lifetime, type-II superlattice, semiconductors, T2SL lifetime, T2SL characterization

Document Type




Degree Name

Electrical Engineering

Level of Degree


Department Name

Electrical and Computer Engineering

First Committee Member (Chair)

Dr. Charles Fleddermann

Second Committee Member

Dr. Eric Shaner

Third Committee Member

Dr. Sanjay Krishna

Third Advisor

Dr. Mansoor Sheik-Bahae

Fourth Committee Member

Dr. Ganesh Balarkishnan