There is a strong interest in developing sensitive Short Wavelength Infrared (SWIR) avalanche photodiodes (APDs) for
applications like eye safe laser ranging and robotic vision. The excess noise associated with the avalanche process is
critical in dictating the sensitivity of APDs. InGaAs APDs that are commonly used in the SWIR region have either InP
or InAlAs as an avalanche layer and these materials have excess noise factor of 0.5 and 0.22, respectively. Earlier,
Spectrolab had developed APDs with impact ionization engineering (I2E) structures based on InAlAs and InGaAlAs
heterostructures as avalanche layers. These I2E APDs showed an excess noise factor of 0.15. A photoreceiver based on
the I2E APD exhibited an noise equivalent power (NEP) of 150 fW/rt(Hz) over 1 GHz bandwidth at 1.06 μm. In this
paper, a new multiplier structure based on multiple stages of I2E is studied. The APDs show optical gains over 100
before device breakdown. The increased gain and low excess noise will improve the sensitivity of InGaAs APDs based
photoreceivers.
Future NASA light detection and ranging (LIDAR) mapping systems require multi-channel receivers with high
sensitivity and bandwidth operating at 1-1.5 μm wavelengths. One of the ways to improve the system performance is to
improve the sensitivity of photoreceiver. InGaAs avalanche photodiode (APD) sensor technology is considered for this
wavelength region because of high reliability. However, commercially available InGaAs APDs have low sensitivity due
to the high excess-noise of InP material. Spectrolab has been developing low excess noise InGaAs avalanche
photodiodes (APDs) with impact ionization engineering (I2E) structures and recently, APDs with excess noise factor of
0.15 have been demonstrated using an I2E design. Single channel photoreceivers built using low noise I2E APDs show a
noise equivalent power (NEP) of 150 fW/rt(Hz) over a bandwidth of 1 GHz, a record for InGaAs based APDs. A 16
channel GHz SWIR photoreceiver was designed and built at Spectrolab. The photoreceiver was designed to work with a
custom fiber bundle which couples the light from telescope to detectors. The photoreceiver shows a system level NEP
less than 300 fW/rt(Hz) with 1 GHz bandwidth.
The development of low cost and compact biological agent identification and detection systems, which can
be employed in place-and-forget applications or on unmanned vehicles, is constrained by the photodetector currently
available. The commonly used photomultiplier tube has significant disadvantages that include high cost, fragility,
high voltage operation and poor quantum efficiency in the deep ultraviolet (240-260nm) necessary for methods such
as fluorescence-free Raman spectroscopy. A III-Nitride/ SiC separate absorption and multiplication avalanche
photodiode (SAM-APD) offers a novel approach for fabricating high gain photodetectors with tunable absorption
over a wide spectrum from the visible to deep ultraviolet. However, unlike conventional heterojunction SAM APDs,
the performance of these devices are affected by the presence of defects and polarization induced charge at the
heterointerface arising from the lattice mismatch and difference in spontaneous polarization between the GaN
absorption and the SiC multiplication regions. In this paper we report on the role of defect density and interface
charge on the performance of GaN/SiC SAM APDs through simulations of the electric field profile within this
device structure and experimental results on fabricated APDs. These devices exhibit a low dark current below 0.1
nA before avalanche breakdown and high avalanche gain in excess of 1000 with active areas 25x larger than that of
state of the art GaN APDs. A responsivity of 4 A/W was measured at 365 nm when biased near avalanche
breakdown.
Next generation LIDAR mapping systems require multiple channels of sensitive photoreceivers that operate in the
wavelength region of 1.06 to 1.55 microns, with GHz bandwidth and sensitivity less than 300 fW/√Hz. Spectrolab has
been developing high sensitivity photoreceivers using InAlAs impact ionization engineering (I2E) avalanche photodiodes
(APDs) structures for this application. APD structures were grown using metal organic vapor epitaxy (MOVPE) and
mesa devices were fabricated using these structures. We have achieved low excess noise at high gain in these APD
devices; an impact ionization parameter, k, of about 0.15 has been achieved at gains >20 using InAlAs/InGaAlAs as a
multiplier layer. Electrical characterization data of these devices show dark current less than 2 nA at a gain of 20 at room
temperature; and capacitance of 0.4 pF for a typical 75 micron diameter APD. Photoreceivers were built by integrating
I2E APDs with a low noise GHz transimpedance amplifier (TIA). The photoreceivers showed a bandwidth of 1 GHz and
a noise equivalent power (NEP) of 150 fW/rt(Hz) at room temperature.
There is strong interest in developing Short Wavelength Infrared (SWIR) photo receivers for applications like laser
ranging and robotic vision. Recently, Spectrolab has developed a first generation low noise receiver for NASA. The
receiver shows a bandwidth of 180 MHz, presently limited by the transimpedance amplifier (TIA). The first generation
photoreceiver has InP avalanche photodiode (APD). The overall photoreceiver noise equivalent power (NEP) is less than
300 fW/√Hz.
Furthermore, Spectrolab is developing low excess noise APDs with Impact Ionization Engineering (I2E). The I2E low
noise APDs were built from baseline InAlAs APDs with a keff value of 0.22. A thin layer of InGaAlAs alloy was
incorporated into the InAlAs multiplication layer in these devices. All the I2E APDs show lower keff-value than InAlAs
and very low dark currents. Values as low as keff<0.1 have been demonstrated. These I2E APDs will be used in
Spectrolab's second generation photoreceiver. A Noise Equivalent Power (NEP) of 300 fW/√Hz is expected over a
1GHz response bandwidth.
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