M. Delmas

Influence of the period thickness and composition on the electro-optical properties of type-II InAs/GaSb midwave infrared superlattice photodetectors

In this paper, the electro-optical properties of InAs/GaSb superlattice (SL) midwave infrared photodiodes with different periods were investigated. Three devices with different SL periods, but the same cut-off wavelength at 5??m at 77?K, were grown by molecular beam epitaxy on p-type GaSb substrates. The optical and electrical behaviours were characterized and analysed. Our investigations show strong influence of the SL composition on both the material properties and photodetector performances, such as the background doping concentration, shape of the response spectra and the dark current behaviours.

Electrical modeling of InAs/GaSb superlattice mid-wavelength infrared pin photodiode to analyze experimental dark current characteristics

Dark current characteristics of 7 Monolayers (ML) InAs/ 4 ML GaSb SL pin photodiodes are simulated using ATLAS software. Using appropriate models and material parameters, we obtain good agreement between the simulated and the experimental dark current curves of photodiodes grown by molecular beam epitaxy. The n-type non-intentionally-doped (nid) SL samples exhibit a dependence of the lifetime with temperature following the T−12 law, signature of Shockley-Read-Hall (SRH) Generation-Recombination current. We also studied the dependence of the dark current with the absorber doping level. It appears that the absorber doping level must not exceed a value of 2 × 1015 cm−3, above this value the dark current is increasing with increased doping level. However for this doping value, a dark current as low as 5 × 10−9 A/cm2, at 50 mV reverse bias at 77 K can be obtained.

Noise measurements for the performance analysis of infrared photodetectors

Evaluating the performances of infrared photodetectors requires both electrical and optical measurements, including a detailed analysis of the noise behavior. Indeed, in such detectors, noise can arise from the absorbing layer (Schottky noise due to photonic and dark currents, thermal noise, low frequency noise due to technological imperfections) as well as from the read-out integrated circuit (ROIC) or from the analog to digital converter (ADC). Therefore, it is of great importance to carry out noise measurements on single elements (without ROIC and ADC) as well as on focal plane arrays. In this paper, we present noise measurements on single detectors and on a large format focal plane array, both suitable to address the 3-5μm spectral range. The single detectors are InAs/GaSb superlattice pin photodiodes which already proved to be a very promising emerging technology, whereas the focal plane array relies on the well established, high performance InSb technology.

Radiometric characterization of type-II InAs/GaSb superlattice (t2sl) midwave infrared photodetectors and focal plane arrays

In recent years, Type-II InAs/GaSb superlattice (T2SL) has emerged as a new material technology suitable for high performance infrared (IR) detectors operating from Near InfraRed (NIR, 2-3μm) to Very Long Wavelength InfraRed (LWIR, λ > 15μm) wavelength domains. To compare their performances with well-established IR technologies such as MCT, InSb or QWIP cooled detectors, specific electrical and radiometric characterizations are needed: dark current, spectral response, quantum efficiency, temporal and spatial noises, stability… In this paper, we first present quantum efficiency measurements performed on T2SL MWIR (3-5μm) photodiodes and on one focal plane array (320x256 pixels with 30μm pitch, realized in the scope of a french collaboration ). Different T2SL structures (InAs-rich versus GaSb-rich) with the same cutoff wavelength (λc= 5μm at 80K) were studied. Results are analysed in term of carrier diffusion length in order to define the optimum thickness and type of doping of the absorbing zone. We then focus on the stability over time of a commercial T2SL FPA (320x256 pixels with 30μm pitch), measuring the commonly used residual fixed pattern noise (RFPN) figure of merit. Results are excellent, with a very stable behaviour over more than 3 weeks, and less than 10 flickering pixels, possibly giving access to long-term stability of IR absolute calibration.

Flexibility properties of type-II InAs/GaSb SL to design MWIR pin photodiodes

InAs/GaSb superlattice (SL) is a peculiar quantum system for infrared detection, where electrical and optical properties are directly governed by the composition and the periodicity of the InAs/GaSb cell. Indeed, several structures with different InAs to GaSb thickness ratios in each SL period, can target the same cut-off wavelength. Likewise, the type of conductivity of the non-intentionally doped SL structure is also linked to the InAs/GaSb SL period. The objective of this communication is to use the flexibility properties of InAs/GaSb SL to design and then to fabricate by MBE a pin photodiode where the active zone is made of different SL periods. Electrical and electro-optical characterizations are reported. The results show that SL structure for the MWIR domain can be designed by combining the best of each SL periods.

InAs/GaSb superlattice pin photodiode: choice of the SL period to enhance the temperature operation in the MWIR domain

In this communication, we studied the influence of the SL period on the electrical performances of MWIR pin photodiodes, fabricated by MBE on p-type GaSb substrate. These SL structures are made of symmetric or asymmetric SL period designs and exhibited cut-off wavelength around 5μm at 77K. Experimental measurements carried out on several SL pin photodiodes show the superiority, in terms of dark current density, of the asymmetric SL structure composed of 7 InAs monolayers (MLs) and 4 GaSb MLs. As a result, the 7/4 SL diode exhibits dark current density values as low as 40nA/cm2 and R0A product greater than 1.7x106 Ohm.cm2 at 77K, one decade larger than the value obtained with equivalent symmetric 10/10 SL diode. This result obtained demonstrates the strong influence of the SL period design on the performances, and then on temperature operation, of MWIR SL photodiodes.

Analysis of electrical and electro-optical characteristics of midwave infrared InAs/GaSb SL pin photodiodes

In this communication, we examine the influence of the SL period of InAs/GaSb superlattice (SL), with diverse InAs to GaSb thickness ratio, on the material and device properties of midwave infrared pin photodiodes. Three SL devices made of three different periods, but exhibiting the same cut-off wavelength at 5 μm at 77K, were grown by molecular beam epitaxy on p-type GaSb substrates. Optical and electrical characterizations (photoluminescence, current-voltage, capacitance-voltage, and photoresponse measurements) were performed and analyzed in order to explain the results obtained. Our investigations show the strong influence of the SL composition on both the material and photodetector properties, such as residual doping concentration, shape of the response spectra and dark current values.

Material and device characterization of Type-II InAs/GaSb superlattice infrared detectors

Abstract This work investigates midwave infrared Type-II InAs/GaSb superlattice (SL) grown by molecular beam epitaxy on GaSb substrate. In order to compensate the natural tensile strain of the InAs layers, two different shutter sequences have been explored during the growth. The first one consists of growing an intentional InSb layer at both interfaces (namely GaSb-on-InAs and InAs-on-GaSb interfaces) by migration enhanced epitaxy while the second uses the antimony-for-arsenic exchange to promote an ‘InSb-like’ interface at the GaSb-on-InAs interface. SLs obtained via both methods are compared in terms of structural, morphological and optical properties by means of high-resolution x-ray diffraction, atomic force microscopy and photoluminescence spectroscopy. By using the second method, we obtained a nearly strain-compensated SL on GaSb with a full width at half maximum of 56 arcsec for the first-order SL satellite peak. Relatively smooth surface has been achieved with a root mean square value of about 0.19 nm on a 2 µm × 2 µm scan area. Finally, a p-i-n device structure having a cut-off wavelength of 5.15 µm at 77 K has been demonstrated with a dark-current level of 8.3 × 10−8 A/cm2 at −50 mV and a residual carrier concentration of 9.7 × 1014 cm−3, comparable to the state-of-the-art.

Identification of a limiting mechanism in GaSb-rich superlattice midwave infrared detector

GaSb-rich superlattice (SL) p-i-n photodiodes grown by molecular beam epitaxy were studied theoretically and experimentally in order to understand the poor dark current characteristics typically obtained. This behavior, independent of the SL-grown material quality, is usually attributed to the presence of defects due to Ga-related bonds, limiting the SL carrier lifetime. By analyzing the photoresponse spectra of reverse-biased photodiodes at 80 K, we have highlighted the presence of an electric field, breaking the minibands into localized Wannier-Stark states. Besides the influence of defects in such GaSb-rich SL structures, this electric field induces a strong tunneling current at low bias which can be the main limiting mechanism explaining the high dark current density of the GaSb-rich SL diode.

Comparison of the electro-optical performances of MWIR InAs/GaSb superlattice pin photodiode and FPA with asymmetrical designs

We first present an electro-optical characterization of the radiometric performances of a type-II InAs/GaSb superlattice (T2SL) pin photodiode operating in the mid-wavelength infrared domain. This photodiode was grown with an InAs-rich structure. We focused our attention on quantum efficiency and responsivity: quantum efficiency of mono-pixel device reaches 23% at λ = 2.1 μm for 1 μm thick SL structure and 77K operating temperature. Then we measured the angular response of this photodiode: the response of the photodiode doesn’t depend on the angle of incidence of the flux. We also report the QE of 2μm-thick InAs-rich T2SL pin 320×256 pixels focal plane array, which reaches 61% at λ = 2.6 μm.