nBn based infrared detectors using type-II InAs/(In,Ga)Sb superlattices

Proposed by Smith and Mailhiot 1 in the 1980s, detectors based on InAs/GaSb superlattices (SLs) have attracted
a lot of attention over the past few years as a possible alternative to present-day IR detection systems. The InAs/GaSb
SLs consist of alternating layers of nanoscale materials whose thicknesses vary from 4-20 nm. These heterostructures are
characterized by a broken-gap type-II alignment as illustrated on the Figure 1. The overlap of electron (hole) wave
functions between adjacent InAs (GaSb) layers results in the formation of an electron (hole) minibands in the conduction
(valence) band. Optical transitions between holes localized in GaSb layers and electrons confined in InAs layers are
employed for the detection of infrared radiation. The effective bandgap of the InAs/InGaSb SLs can be tailored from
3µm to 30 µm by varying thickness of constituent layers, thus allowing fabrication of devices with operating
wavelengths spanning the entire IR region
The effective mass of the superlattice is not dependent on the bandgap, unlike the case of bulk semiconductor
material. In contrast with QWIPs, normal incidence absorption is permitted in SLs, resulting in high quantum efficiency.
The larger effective mass in SLs leads to a reduction of tunneling currents. Large splitting between heavy-hole and lighthole
valence subbands due to strain in the SLs contributes to the suppression of Auger recombination. Commercial
availability of low defect density substrates and a high degree of uniformity for III-V processing over a large area also
offers technological advantages for the SLs. This makes detectors based on SLs an attractive technology for the
realization of high performance single element detectors and FPAs.
Presently all SLs detectors are based on a photodiode (p-on-n or n-on-p) design. In this case, the optically active
area of the photodiode is defined by an etched mesa as shown on the Figure 2 (a). During the mesa isolation process, the
periodic nature of the idealized crystal structure ends abruptly at the mesa lateral surface. Disturbance of the periodic
potential function due to a broken crystal lattice leads to allowed electronic quantum states within the energy band gap of
SLs, resulting in large surface leakage currents. The suppression of these currents is the most demanding challenge for
present day SLs technology, especially for LWIR and VLWIR spectral regions, since the dimensions of SLS pixels have
to be scaled to ~ 20 µm in FPAs.
In order to overcome the limitation imposed by surface leakage currents, a stable surface passivation layer is
needed. So far, various approaches have been proposed such as the deposition of polyimide layer 2 , overgrowth of wide
band gap material 3 , deposition of passivation sulphur coating electrochemically 4 and post etch treatment in chemical
solutions 5 . However, these passivation methods are either sensitive to the cut-off wavelength of the device or complicates
the fabrication process of detectors and FPAs.
InAs
InAs
Electron
miniband E
Cl
Hole —
miniband
Yl
iHHn1n1r
— I— I — I —
__________________
$1
I I
Absorption
GaSb
Figure 1. Schematic bandgap alignment of Type II InAs/GaSb superlattices (SLs).
Recently 6 , a new heterostructure design with extremely low surface currents has been proposed. This socalled
nBn structure consists of a n-type narrow band-gap contact and absorber layers separated by a 50-100 nm thick
wide band-gap barrier layer. A 100K increase in the background limited infrared photodetector (BLIP) temperature has
been demonstrated. Implementation of the nBn design for InAs/GaSb MWIR SLs single element detectors has been
reported 7,8 . Unlike a conventional photodiode processing, the size of the device with nBn design is not defined by the
Proc. of SPIE Vol. 6940 69400E-2