Terahertz and infrared optoelectronics: from materials to devices

Resume : Quantum dots (QDs) have been topic of intense research in the field of optoelectronic devices such as lasers, LEDs and infrared photodetectors. Thin spacer layer is used to enhance the coupling to align the dots vertically leading to increase in dots sizes. Here we are studying the optical and spectral characterization of MBE grown InAs/GaAs strain coupled quantum dot infrared photodetectors with the combined capping thickness of thin In(Ga,Al)As and GaAs spacers varying from 12 nm to 21 nm. Photoluminescence spectra of all the samples spread in between 0.9 µm to 1.22 µm exhibiting multimodal distribution of dot families. For 3 nm InGaAs spacer, we observe red-shift in all the dot families with decreasing GaAs thickness from 18 nm to 12 nm. By decreasing the spacer thickness, the buildup of strain field along the vertical column increases the QD size. By using InAlGaAs capping also, we have observed multimodal broad PL spectra laying in between 1.0 µm to 1.2 µm. With proper optimization, these structures can be used in broad-band detector applications. The photoresponse peak of the fabricated QDIPs was observed at 6 µm. Two order enhancements in peak detectivity are observed if we decrease the GaAs spacer from 18 nm to 9 nm. Improved detector performance is obtained by using quaternary InAlGaAs capping layer. InAlGaAs capped layer also shows better thermal stability in peak emission wavelength compared to InGaAs capped samples. DST, India and Riber, France are acknowledged.

Resume : This paper presents a study of design and optimisation of GaAs-based mid-infrared quantum cascade lasers (QCLs). The GaAs/AlGaAs quantum cascade lasers were developed shortly after demonstration of first cascade lasers based on InP [1,2]. They have been however abandoned due to their inferior performance comparing to InP-based QCLs. Our results show that GaAs/AlGaAs lasers can be developed to the stage of practically useful devices [3-5]. A substantial progress has been achieved regarding the values of all exploitation parameters The threshold current density has been lowered from 5 kA/cm2 to 3 kA/cm2 at 77 K and from 15 kA/cm2 to 10 kA/cm2 at 300 K. This has been achieved by improving carrier confinement in active region through deepening the quantum wells. Better device thermal stability (characteristic temperature T0 ~ 130 K comparing to T0 ~ 70 K - 90 K for standard design) resulted in increasing the maximum operating temperature to 320 K and room-temperature power to 100 mW. Bearing in mind that GaAs/AlGaAs technology is by far the most elaborated and cheapest than InP-based technology the GaAs-based QCLs might still have a commercial potential. [1] C. Sirtori, P. Kruck, S. Barbieri, P. Collot, J. Nagle, M. Beck, M. Faist, U. Oesterle, Appl. Phys. Lett., 73, 3486-3488 (1998) [2] H. Page, C. Becker, A. Robertson, G. Glastre, V. Ortiz, C. Sirtori, Appl. Phys. Lett., 78, 3529 (2001) [3] M. Bugajski, K. Kosiel, A. Szerling, P. Karbownik, K. Pierściński, D. Pierścińska, G.Hałdaś, A. K,olek, Proc. of SPIE Vol. 8432, 84320 (2012) [4] K. Pierściński, D. Pierścińska, M. Iwińska, K. Kosiel, A. Szerling, P. Karbownik, M. Bugajski, J. Appl. Phys. 112, 043112 (2012) [5] P. Gutowski, M. Bugajski, K. Pierściński, P. Karbownik, Appl. Phys. Lett., to be published (2014)

Resume : Excitation of plasmon resonanses in high-electron mobility heterostructures is one of physical mechanisms which can be applied to construction of low-temperature detectors of THz radiation. In this paper we describe results of a detailed investigation of such detectors fabricated on a high quality GaAs/AlGaAs heterostructure. A number of samples was investigated with different dimensions of mesa and different geometries of the gate metallization. Magnetospectroscopy experiments were carried out with monochromatic sources covering the frequency range of 0.1 THz - 2.5 THz. An analysis of the photocurrent and photovoltage spectra allowed to determine dispersion relations of plasmons excited in the samples. It occured that the plasmon wavevectors were defined either by the geometry of the gate or the geometry of the mesa. Also, discrimination between gated and ungated plasmons was shown to be dependent on the thickness of the gate meallization. Results of this study lead to a deeper understanding of mechanisms of plasmon generation in investigated samples which is the basis for further developments of resonant plasmonic detectored tuned by the magnetic field.

Resume : This paper presents research results of time constant in LWIR HgCdTe photodiodes, operating without cryogenic cooling. Heterostructures Hg1-xCdxTe are grown by MOCVD technics in common research-production laboratory VIGO System S.A. and Military University of Technology. The basic construction element of photovoltaic device is the absorbing layer, made in narrow-gap HgCdTe. One of the most important functional parameter of photodiode, is its response speed. This paper discuss research results of time constant as function of reverse bias voltage for detectors working at temperature range from 190 to 293 K. Measurements show, that usage of nonequilibrium mode of detector work, results in significant decrease of time constant, with respect to equilibrium mode of work. It is consequence of occurrence exclusion and extraction effect of charge carriers in absorber, which lead to depletion of this layer. In the best operating point, presented devices achieved time constant below 200 ps. This performance is synonymous with upper 3dB cutoff frequency approximately equal to 1 GHz.

Resume : Photodetectors designed for the mid-wave infrared region is presented. Mercury cadmium telluride (MCT) thin films grown by MBE methods onto various substrates (Si, GaAs) were investigated as a piezoelectric heterostructure for infrared (IR) detection. Mechanical stresses at the layer-substrate interface were analyzed. It was determined that for [310] oriented MCT-based structures under the anisotropic restriction of the deformation the nonzero shear components of the strain tensor arise and stress induced piezoelectric polarization is generated. Existence of the built-in electric field in the strained MCT-based heterostructure results in the spatial separation of the nonequilibrium carriers and the possibility of the room temperature detection of the IR radiation is realized. A prototype of IR detector was fabricated on the base of HgCdTe/Si heterostructure. Investigated detector operates in the middle and long wavelength spectral range. The measured value of responsivity was from ~0.5 V/W to ~4.3 V/W at 0.5 mW incident power in mid-wavelength infrared region. It was also found out that the prototype of photovoltaic detector is sensitive to CO2 laser radiation on the level of ~0.04 V/W at 1 mW laser beam power in focal spot. The detailed analysis of the detector’s performance such as spectral responsivity, detectivity versus operating temperature and frequency, absorber doping and absorber composition were performed pointing out optimal working condition.

Resume : The SiGe thin-film is considered as promising candidate to meet the outstanding need for photovoltaic applications. This is mainly due to its excellent optic properties. The high absorption performance provided by the SiGe alloy leads to photocurrent gain; however reduction of the material bandgap causes voltage losses, which tends to cancel out the improved electrical performance of the thin-film solar cell. Therefore, in this paper we present a new approach based on multi-trench technique to improve the electrical performances, which are the fill factor and the electrical efficiency. Based on 2-D numerical investigation and optimization of amorphous SiGe double-junction (a-Si:H/a-SiGe:H) thin film solar cells, in the present paper a numerical models of electrical and optical parameters are developed to explain the impact of the multi-trench technique on the improvement of the double-junction solar cell electrical behavior for high performance photovoltaic applications. In this context, electrical characteristics of the proposed design are analyzed and compared with conventional amorphous silicon double-junction thin film solar cells.

Resume : Photosensitivity at room temperature in the 1.0 - 2.0 micron semiconductor heterostructures based on group A3V6 In4Se3 and In4Te3 are promising to create photosensitive elements for near-infrared region of the spectrum, as well as primary transducers radiation with a wavelength λ = 1.550 microns to modern telecommunications systems. Thin film heterostructures In4Se - In4Te, In4Se3 - In4 (Se3)(1-x)Te(3x) and In4Se3 - Ge, obtained by liquid phase epitaxy. To improve the structural perfection and photosensitivity characteristics of these structures using their pulse laser processing. In addition, crystals substrates In4Se3 and In4(Se3)(1-x)Te(3x) used doping impurities Ge ta Bi during their growing Czochralski method and their annealing in selenium and mercury vapor. The temperature dependence of the signal / noise ratio for the heterojunction, made application to the p-In4(Se3)0,98(Te3)0,02 layer n-In4Se3: Bi. The method of SEM revealed that after laser irradiation with intensity of 15-25 kW/cm2 been improved structural perfection of the epitaxial structure heteroboundary In4Se3 - Ge. Thus the spectral range of photosensitivity expanding into more short-wave region 0.7 ÷ 0.8 microns compared with the curve before irradiation. Value fotosignal after laser treatment increased almost twice. This result can be attributed to structural improvements at heteroboundary In4Se3 layer and increase its photosensitivity due to impurity diffusion layer of Ge and them stabilization by laser irradiation.

Resume : We report on room-temperature plasmonic detection of the thermal emission from a black body in the terahertz and mid-infrared domains by asymmetric dual-grating-gate InAlAs/InGaAs/InP high electron mobility transistors (A-DGG HEMTs). In such detectors, the asymmetric grating gate of a large area acts as an effective antenna that improves the performance in the two spectral domains. A-DGG HEMTs, consist of two gate fingers, G1 and G2 where asymmetric factor, the ratio of the interfinger spaces, d1/d2 was set to be 0.5. All detection experiments were done, as a function of Vg2 by fixing Vg1 value to 0 V. The MIR and THz radiation was generated by a thermal emitter with a radiating element at 1200°C. At the working point Vg2 = -0.85 V and Id = 7.5 µA, the photoresponse was enhanced by a factor of 30. The use of complementary spectral filters shows that the A-DGG HEMT detects in the range 2.65 - 16 THz with a mean responsivity of 5 kV/W for applied drain current of 7.5 µA. Part of this work was supported by the ANR-JST “WITH, the COST Action MP1204 "TERA-MIR radiation: materials, generation, detection and applications" and by the PHC Polonium "Investigation of FET Transistors as THz detectors and its development in imaging applications” and by the Polish National Centre for Research and Development with grant no PBS1/A9/11/2012.

Resume : InP double heterojunction bipolar transistors (DHBTs) designed with a 0.7-μm emitter have been studied as room-temperature detectors of terahertz (THz) radiation in the frequency range 0.265-0.375 THz. The photoresponse was found to depend on the base-emitter bias. The values of responsivity, 200 V/W and 4 kV/W obtained under unbiased and biased conditions, respectively, show that these DHBT detectors without antennas and/or integrated amplifier circuits can already compete with conventional THz detectors or with field effect transistor-based detectors [1]. In order to demonstrate the imaging capability of the InP DHBT detectors, we took raster scanned transmission mode THz images of different objects (incandescent light bulb in its box, tree leaves). At 10 ms integration time, the resulting dynamic range was higher than 5.5x103 (38 dB) and the details of the objects appear clearly in the THz image. Moreover, InP DHBTs have the advantages of being compatible with InP technology yielding medium-scale integrated circuits operating at over 100 Gbit/s and potentially exhibiting much higher operating speed [2]. This work was supported by the ANR P2N “NADIA” project, and by the french scientific interest group GIS “Teralab”. [1]. W. Knap, S. Rumyantsev, M. S. Vitiello, D. Coquillat, S. Blin, N. Dyakonova, M. Shur, F. Teppe, A. Tredicucci, and T. Nagatsuma, Nanotechnology 24, 214002, (2013). [2]. J-Y. Dupuy et al., IEEE Transactions on Microwave Theory and Techniques 61, 517 (2013).

Resume : One of the factors which limits performance of a mid wavelength infrared (MWIR) photodiodes made by Antimonide-based type II superlattices (T2SL) is tunneling current with the deep energy levels contribution. If one want to improve the design of the photodiodes, the thorough knowledge about the nature of these deep levels is essentially needed. Currently, there are very few reports, which contain any experimental results in this matter. In this paper we describe results of our findings, obtained by two different measurement techniques: low frequency noise spectroscopy and Fourier transformed photoluminescence (FTPL). In the first case thermal analysis (temperature has been changed from 80 to 300 K) of the spectral power density SI in the frequency (f) range from 1 Hz to 10 kHz, have been used. From noise spectrum measured at low bias voltage (50 mV), few re-scaled Lorentz type curves and traced their maxima in all temperature range. The analysis of the spectra obtained for our InAs10ML/GaSb10ML samples, showed existence of a few activation energies, including energies lower than energy of the fundamental transition in SL. These results have been well agreed with data achieved by using temperature dependence of FTPL measurement realized within temperature range from 20 to 260 K. This paper has been done under financial support of The National Science Center - grant DEC-2012/05/N/ST7/01057 and the European Union funds by the European Social Fund.