Electron mobility in InAs, InSb
and related alloys is nearly two decades higher than that of holes which results in
domination of electron injection in a forward biased p-n junction. That’s why most
homojunction III-V light sources in the mid-IR spectral region (3-7 mm) exhibit recombination in
p-side of a junction. This is one of the reasons for relatively low conversion efficiency
since materials with p-type conductivity are characterized by high nonradiative Auger
rates due to the energy gap and split-off energy “resonance”.
The situation could be positively changed if carrier injection will be
performed via optical pumping of n-type material in which high quantum efficiency is
expected. GaAs based LEDs are good candidates for the pumping source since they
already exhibit high external efficiency. Moreover, such disadvantages of narrow band p-n
junctions as high tunnel and leakage currents are easily eliminated in a construction with
broad band injector and narrow band recombination area been separated.
We have proposed LEDs based on optical pumping of an active area
fabricated from n-type semiconductor with high quantum efficiency. Such source consists of
separated injection and recombination areas made out of two different semiconductors,
joined together by a chalcogenic glass (“glue”) as shown in Fig.1.
First optically pumped III-V mid-IR surface emitting LED
prototypes with no antireflection coating and incorporating narrow gap layers of
InSb (l =6.5 mm), In(Ga)As(Sb) (l =3.1-3.7 mm), and graded band
gap InAsSb(P) (l =4.3- 5.5 mm)
compositions have been fabricated.
Because of biasing broad band p-n junction the OP LEDs consume more
electrical power than conventional mid-IR LEDs and thus they may suffer from Joule heating
effects. Due to the absence of several routines in LED fabrication such as p-n junction
formation and photolithography processes the above disadvantage is, however, balanced by
low device price. Moreover, the proposed LED design is easy to use in fibre optical
applications by “gluing” the flat fibre end directly onto an OP LED surface. The
design is advantageous for the immersion lens package providing narrow far field (Fig.1,
left) and ~ 10-fold increase of the output power in comparison
with the surface emitting LEDs. The immersion lens OP LEDs exhibited record outputs as
shown in Fig.2 and can thus find numerous applications in gas and liquid analysis,
dynamic scene simulation systems, target finding, free space communication, fibre optic
systems etc.
