High-Power Mid-IR Interband Cascade Lasers Based on Type-II Heterostructures
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Mat. Res. Soc. Symp. Proc. Vol. 607 @2000 Materials Research Society
a forward bias, electrons are injected from the emitter into the Ee electron level, which is in the band-gap region of the adjacent GaInSb layer. Since the electrons at the Ee level are effectively blocked from directly tunneling out by the GaInSb, AlSb, and GaSb layers, they tend to relax to the Eh hole state in the adjacent valence-band QW, which results in the emission of photons. Electrons in the Eh state cross the thin AlSb barrier and GaSb layers by tunneling and scattering into the conduction band of the next injection region because of a strong spatial interband coupling in type-II QWs. They are then ready for the next interband transition, which leads to sequential photon emission with quantum efficiencies exceeding the conventional limit of unity. AISb ' AM~ GaIn~bGab
AM~ Ga~bAM~
hv
GaInSb
electrons,,,. In ..sA( .. ..... nS .. multilayers - ..----------------
Oa[
AISb
hv '-11 In s•
]I ........ I~ /A(n S
• SbI[I .
JniIA(I)S multilmultilayersu------er
InAs/Al(In)sb
-
A--------------
-
multilayers active region
injection region --
AMS
-------------------.
Fig. 1. Schematic band diagram of a type-II interband cascade laser structure. Based on the same IC structure design, three laser samples were grown in a Varian Gen-lI MBE system on GaSb substrates from two different vendors. For all three samples, many sharp satellite peaks in double-crystal X-ray diffraction spectra indicated their high structural quality. After the growth, samples were processed into broad-area gain-guided and mesa-stripe devices with several different widths. Au/AuGe and Au/Ti electrical contacts were deposited onto the top n-type layer and p-type substrate, respectively. Laser bars were cleaved to cavities from -0.5 to 1.5 mm long with both facets left uncoated. The laser bars were glued with silver epoxy, epilayer side up, onto a chip carrier and devices were wire-bonded to the pin-outs. The chip carrier was then mounted on the temperature-controlled cold finger of an optical cryostat. The optical output power was measured with a cooled InSb detector together with a lock-in amplifier calibrated by a thermopile power meter when the average power was high. For most measurements, neutral density filters were used to avoid saturating the detector. Devices made from all three samples, labeled ICLI (vendor A), ICL2 (vendor B), and ICL3 (vendor B), lased in the wavelength range from 3.8 to 4 pm. The lasing wavelengths agree well with the designed wavelength (-3.9 pim at 80 K) considering some uncertainty in the growth rates, the sample uniformity, and Stark shifts in the active regions. Lasers made from different vendor substrates exhibited different general characteristics. Lasers from ICL1 have much lower threshold current densities. They were able to operate at duty cycles as large as 20-30% (20-30 jts at 10 kHz). The threshold current densities of lasers from ICL1 were found to be relatively insensitive to the cavity length. This suggests that the o
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