Quantum cascade light emitting diodes based on type-II quantum wells Page: 3 of 9
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output power (>1 mW) because of the low thermal conductivity of lead-salt alloys, and their
susceptibility to damage. For narrow bandgap HNI semiconductors such as HgCdTe alloys, the first
reported laser diodes operated under pulse injection up to 90 K at the wavelength of 2.86 pm. It is thus
very desirable to develop semiconductor lasers using I-V alloys, because these materials offer good
metallurgical and thermal properties and because better-quality substrates are readily available.
Recently, MIR lasers utilizing Sb-based type-I structures are showing a stead progress [3-9]. Room-
temperature operation of laser diodes at wavelengths shorter than 2.8 pm has been realized [3-5].
However, due to the intrinsic nature of type-I interband transitions, the lasing wavelength is limited to the
material bandgap, and the device performance suffers from non-radiative Auger recombination. For
wavelength longer than 4 pm, MIR lasers based on type-I interband structure has proven to be quite
MI.R lasers based on InAs/bnGaSb type-U superlattices (SLs)  have achieved similar performance
as those based on Sb-based type-I structures. A maximum operating temperature T, = 255 K was
achieved at 3.2 pm . By simply changing the InAs and InGaSb layer thicknesses, the losing
wavelength of type-1i InAs/InGaSb lasers could vary from MIR to long IR. This is an important
advantage of lnAs/InGaSb type-U lasers compared to the Sb-based type-I devices in terms of epitaxy
growth, since the composition control, composition unformity, and growth reproducibility of InGaAsSb,
InAlAsSb, and AIGaAsSb are very difficult. Additionally, due to the unique feature of type-U structures,
the Auger recombination could be dearratically suppressed through careful bandgap engineering. For the
gain spectrum, InAsf/nGaSb type-U structures can display strong oscillator strength as long as the the
layers are thin enough to allow enough interpenetration of the electron and hole wavefimctions, and
hence large gain should be attainable from type-U structures. Recently, we have demonstrated above
room-temperature operations of optically-pumped MIR. lasers based on InAs/InGaSb/AISb type-I
quantum wells (QWs) with record maximum operating temperature T. and chairacteristic temperature
T [11-15]. Laser emission at 4.2 to 4.5 pm has been observed at temperatures up to 310 K, and a peak
output power exceeding 2 W/facet was observed at 200 K .
Type-I quantum cascade (QC) lasers  which utilize sequential multiple photon emissions between
subbands in a staircase of coupled type-I InGaAs/InAIAs QW structure promises significantly improved
performance of MIR lasers. The recent demonstrations of a high power room-temperature QC laser at 5
pm  and a long wavelength IR (-11 pm) QC laser operating up to 200 K [171 indicate a great
potential for the QC configuration. The QC lasers are especially suitable for obtaining high output
powers. However, the type-I QC lasers still have a relatively low radiative efficiency (<1x13) due to a
fast non-radiative phonon relaxation between subbands, which leads to a high threshold current density
and substantial heating.
In contrary, the type-il QC lasers, as originally proposed in Ref. 18, are based on interband
transitions where the phonon relaxation is intrinsically suppressed due to the opposite curvatures of
conduction and valence band structures. Figure I shows the schematic drawing of a n-type QC laser
based on type-H QWs [19, 20]. Under a forward bias, electrons are injected from an injection region into
the InAs well. The InGaSb, AlSb, and GaSb layers are thick enough to efficiently block the direct
tunneling of electrons out of the InAs well, resuling in a reduced leakage current.
SPACE UACUUM EPITAXY CENTER UH + 5058448985 O0
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Lin, C. H.; Yang, R. Q.; Zhang, D.; Murry, S. J.; Pei, S. S.; Allerman, A. A. et al. Quantum cascade light emitting diodes based on type-II quantum wells, article, January 21, 1997; Albuquerque, New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc674274/m1/3/: accessed April 23, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.