A thermophotovoltaic (TPV) device design based on multiple quantum well (MQW) materials composed of Pb0.81Sn0.19Se wells and Pb0.80Sr0.20Se barriers, which are nanostructured materials that can be gown on low-cost silicon, was modeled to predict electrical power generation densities. MQW materials with intersubband energy gaps of 343 meV and 450 meV in a dual junction configuration were studied. For a thermal radiator at a temperature of 1364ºC the short circuit current density was estimated to be 12.1 A/cm2 for each junction. Open circuit voltages for each junction ranged from 130 mV to 262 mV depending on bandgap and temperature. Power generation densities for this dual junction device increased from 2.7 W/cm2 to 3.4 W/cm2 as temperature decreased from 50ºC to 7ºC. Using a conservative value of $1/cm2 for the manufacturing of this silicon-based TPV device technology, the costs for this novel electrical power generation technology are projected to be less than $0.30/W.
A potential materials system that may play a key role in IR spectroscopy applications is PbSe/Pb0.934 Sr0.066Se quantum
well structure. In this work, the characteristic temperature (T0) relationship with laser cavity length was studied for this
material system at three temperature ranges 77<T<150 K, 150<T<300 K, and 77<T<300 K. It was noticed that T0 is
high for short cavity lengths then decreases to an almost constant value after some critical length. The data were best
fitted to a second degree polynomials which can be used to determine these critical values. Also, we analyzed the effects
of quantum efficiency on the characteristic temperature values. Inclusion of theoretical values for the quantum efficiency
due to Auger recombination and leakage current reduces the characteristic temperature T0 in these ranges. It was found
that inclusion of the quantum efficiency decreases the characteristic temperature by 60% for a wide range of cavity
The characteristic temperature calculations and dependency on cavity length was analyzed for Pb0.934Sr0.066 Se multiple
Quantum well Structure at three temperature ranges 77<T<150 K, 150<T<300 K, and 77<T<300 K. In this work, we
show the behavior of the characteristic temperature as a function of cavity length and were able to best fit the data to a
second degree polynomial. Inclusion of theoretical values for the quantum efficiency due to Auger recombination
reduces the characteristic temperature T0 in these ranges. It was found that inclusion of the quantum efficiency decreases
the characteristic temperature by a factor of 0.6 for a wide range of cavity lengths. When results were compared to
experimental data, it was concluded that there is a leakage current above the barrier due to thermionic emission. The
leakage current density was estimated to be around 5423 A/cm2 at room temperature. With this high value more work is
needed to understand the thermionic emission process to improve on the performance of this material system and similar
The effects of quantum efficiency on PbSe/Pb0.934Sr0.066 Se multiple Quantum well Structure were analyzed. We calculate and identify the critical design parameters required to optimize and study the MQW system as a function of five temperatures assuming the quantum efficiency is not equal to one and hence we include the effects of nonradiative recombination due to Auger recombination and carrier leakage over the barrier into the confinement layers. Inclusion of quantum efficiency in addition to temperature dependence increased the threshold current values by almost 10 times. Also, it was noticed that the threshold current density values dropped fast at small cavities and remained constant after some critical cavity length around 100 μm. When experimental quantum efficiency values were used, the threshold current values were higher than those found using the theoretical quantum efficiency values due to leakage current over the barrier.
Temperature dependence is a key parameter in designing quantum well lasers. In this work, we calculated the effects of
temperature on the energy levels and emitted wavelength for PbSe/PbSrSe Single Quantum Well Laser at four different
temperatures: 77K, 150K, 200K, 250K, and 300K. This material system is currently being used in Tunable Laser
Spectroscopy which plays a key role in detecting biomarker molecules in exhaled breath at wavelengths in the infrared
region. We determined the system design parameters to obtain the desired emitted wavelengths associated with certain
disease biomarkers as a function of temperature. Our calculated emitted wave lengths are in excellent agreement with
experimental data assuming parabolic and nonparabolic energy band structures. Moreover, we calculated the effects of
temperature on the confinement factor, gain and current density. The modal gain versus current density curved showed
that the nominal current density and the saturation level increases with temperature similar to other material systems.
Threshold current is a key parameter in the design and proper operation of quantum well lasers. In this publication,
threshold current analysis and calculations are done on four PbSe/Pb0.934Sr0.066 Se quantum well laser structures: SQW,
SCH-SQW, MQW, and MMQW. The current work is a continuation to previous publications where energy levels, modal
gain, optical confinement, and total losses were published for these four structures assuming the energy bands are non-parabolic.
The threshold current as a function of total losses, cavity length, and cavity end mirror reflectivity was
obtained for these structures. It is shown that the threshold current decreases with a decrease in the cavity length and then
increases at a critical cavity length. The effects of non-parabolicity on the threshold current values are more obvious for
short cavities and decreases with an increase in cavity. Whether the SQW or the MQW is the better structure depends on
the loss level. At low loss, the SQW laser is always better because of its lower current density where only one QW has
to be inverted. At high loss, the MQW is always better because the phenomena of gain saturation can be avoided by
increasing the number of QW's although the injected current to achieve this maximum gain also increases. Owing to this
gain saturation effect, there exists an optimum number of QW's for minimizing the threshold current for a given total
loss. At this typical value, the effects of non-parabolicity on the threshold current values can be neglected without loss of
accuracy. However, there is a 20% shift in the output lasing energy that cannot be neglected.
In order for laser oscillation to occur, the modal gain at the lasing photon energy must equal the total losses. In this work,
we analyze and calculate the total losses due to the free carrier absorption, optical waveguide scattering and the laser
cavity end losses for PbSe/Pb0.934Sr0.066 Se quantum well laser structures. The small confinement factor value causes the free carrier absorption loss to be negligible. The calculated scattering loss values showed a decreasing order for the
MQW, MMQW and SCH-SQW structures, for a surface roughness amplitude of 10nm. Increasing the surface roughness
amplitude increases these scattering losses even further. However, the calculated cavity loss calculations showed that its
values are in an increasing order for the MQW (or MMQW) and SCH-SQW structures. These cavity losses are lowest
for uncoated cavity ends. Coating these ends with a quarter wavelength BaF2 layer increases the total cavity loss. In
addition, coating the cavity ends with alternating quarter wavelength layers of BaF2 and CaF2 also results in an increase in the cavity loss. The increase in cavity loss due to coating is caused by the decrease in the mirrors' reflectivity values. These results show that coating with fluoride layers can best be utilized in applications where high transitivity values are needed.
In this study we present a simulation model to optimize and engineer PbSe/Pb 0.934 Sr0.066 Se quantum well
laser structures which is a promising material system that has been used in IR Tunable Laser Spectroscopy.
Four nanostructures were investigated: Single Quantum Well Lasers (SQW), Separate Confinement
Heterostructure_Single Quantum Well Lasers (SCH_SQW), Multiple Quantum Well Lasers (MQW), and
Modified Multiple Quantum Well Lasers (MMQW). We calculated the emitted wavelengths, the amount of optical
energy that was confined in these structures, modal gain, total losses and the threshold current behavior as a function of
laser cavity length and mirror reflectivity. The results showed that at low modal gain values, there are crossover
points between the gain-current density curves for the four structures investigated. These points are crucial
for determining the structure of choice with optimize and design parameters. The anisotropy in the constant
energy surfaces and effects of non parabolic band structure for this system are included in the calculations. Finally, the
theoretical model used in this research can be used with other material systems as a design and optimization
tool for quantum well laser structures.
Most material systems have parabolic band structures at the band edge, however away from the band edge the bands are strongly non parabolic. Other material systems are strongly parabolic at the band edge such as IV-VI lead salt semiconductors. The effects of this property can be ignored for bulk material and structures. However, we will show that its effect can't be ignored in the nanoscale range and it is important to calculate for and take into consideration the effects of this unique property in any design and analysis. Based on the energy dependent effective mass, a theoretical model was developed to conduct this study on several lead salt (IV-VI) laser nanostructures in the infrared region. The effects of non-parabolicity on the gain versus current density relation are a reduction in the current density needed for any given gain and an increase in the gain saturation level. The non-parabolicity of the bands in the growth direction lowers the values of the confinement factor relative to parabolic bands which in turn lowers the modal gain values. Finally, the effects of non-parabolicity on the threshold current are significant for short cavity lasers and decrease with an increase in the cavity length.