R. Taalat

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.

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.

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.

Comparison of the electro-optical performances of symmetrical and asymmetrical MWIR InAs/GaSb superlattice pin photodiodes

We report the full electrooptical characterization of two MWIR InAs/GaSb superlattice (SL) pin photodiodes. The first one features a symmetrical period with 8 InAs monolayers (MLs) and 8 GaSb MLs, while the second one relies on an asymmetrical period with 7.5 InAs MLs and 3.5 GaSb MLs. This asymmetrical design was recently proposed by IES to both decrease the dark current (since it decreases the intrinsic carrier concentration) and increase the quantum efficiency (since it increases the wavefunctions overlap). We present dark current, noise, spectral response and quantum efficiency measurements. Our results confirm that the asymmetrical design allows to greatly improve the performance of MWIR SL pin photodiodes, with an improvement of more than one decade in terms of dark current and an improvement of a factor 1.5 in terms of quantum efficiency. The noise measurements under dark conditions show that the symmetrical (asymmetrical) sample remains Schottky noise-limited up to a bias voltage of -600mV (resp -800mV) and that 1/f noise remains very low.

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.

Optimization of InAs/GaSb superlattice pin photodiode design for the high temperature operation in the midwave infrared range

InAs/GaSb superlattice pin photodiodes showing asymmetrical period design were fabricated by MBE on ptype GaSb substrate. These SL structures exhibited cut-off wavelength in the midwave infrared domain (MWIR) at 5μm at 80K. Electrical characterizations including dark current and capacitance-voltage measurements were performed on single detectors in the temperature range [77K-300K]. The SL photodiode measurements revealed carrier concentrations of about 6x1014 cm-3 at 77K, dark current densities J= 4x10-8 A/cm2 at 77K, J = 0.19A/cm2 at 200K and J = 10A/cm2 at 300K for Vbias =-50mV. The measured R0A product is higher than 1.5x106Ω.cm2 at 77K and equal to 1x10-2Ω.cm2 at T=300K, for cut-off device equal to 5μm and 6.05μm, respectively. These results are compared with the ones obtained by symmetrical SL structure and show that the differential resistance area product is improved by more than one order of magnitude. These results obtained help us to define the optimized SL pin structure design suitable for high temperature operation in the MWIR domain.

Noise performance analysis of MWIR InAs/GaSb superlattice pin photodiodes

We report the full electrooptical characterization of a MWIR InAs/GaSb superlattice (SL) pin photodiode, including dark current, noise, spectral response and quantum efficiency measurements. The SL structure was made of 8 InAs monolayers (MLs) and 8 GaSb MLs, with a total thickness of 3μm. It exhibits a cut-off wavelength of 4.55 μm at 77K. Dark current measurements reveal a diffusion-limited behavior for temperatures higher than 95K, and a R0A value of 1x106Ωcm2 at 77K. Noise measurements were performed under dark conditions and are interpreted in this paper. The results show that the SL detector remains Schottky noise-limited up to a bias voltage of -600mV and that 1/f noise is not present above 6Hz. Spectral response revealed that the cut-off wavelength increases from 4.48μm to 4.91μm when the temperature increases from 12K to 170K. The quantum efficiency in photovoltaic mode and at 77K is 25% (3μm-thick active zone device, single pass and without any antireflection coating). All these electrooptical performances confirm the high quality of the MWIR SL pin photodiode under test.

Performances analysis of symmetrical and asymmetrical InAs/GaSb superlattice pin photodiode

Symmetric and asymmetric mid-wavelength infrared (MWIR) InAs/GaSb superlattice (SL) pin photodiode were fabricated by Molecular Beam Epitaxy (MBE) on p-type GaSb substrate and characterized as a function of temperature. The symmetric SL structure was made of 8 InAs monolayers (MLs) and 8 GaSb MLs and exhibits at 80K a cut-off wavelength (λc) of 4.5μm, while the asymmetric SL design was composed of 7.5 InAs MLs and 3.5 GaSb MLs for λc = 5.5μm at 80K. Optical characterizations made of photoluminescence as a function of temperature and room temperature absorption spectra were performed on these two kinds of structures. Several electrical characterizations including dark current and capacitance-voltage measurements were also carried out on single detectors in the temperature range [77K-300K]. Results obtained were compared and analyzed in order to define optimized SL structure design for the high performance in the MWIR domain.