THzsubTHz direct detector challenges rectification and thermal detectors

THz/sub-THz direct detector challenges: rectification and thermal detectors for active imaging F. Sizov, V. Reva, O. Golenkov, V. Petriakov, A. Shevchik-Shekera, S. Korinets, M. Sakhno, I. Lysiuk, V. Zabudsky, S. Bunchuk, S. Dvoretskii Institutes of Semiconductor Physics Kiev (Ukraine), Novosibirsk (Russia) MIKON-2014, Gdansk, 18 June, 2014

THz technologies starting to be important for some applications and they can be added to existing X-ray and IR technologies e. g. in: Possible THz imaging applications. - Security applications (detection of threats and weapons), - Nondestructrive testing (electronics industry, corrosion analysis, agro-food control, …), - Medicine and Biology (e. g. pharmaceutical quality control, skin cancer, …), - Telecommunications. But one of the drawbacks of THz vision technologies now is large acquisition time (up several minutes for systems with single detetor). To increase the acquisition speed but be cost-effective uncooled detector arrays are needed.

THz imaging technologies (a, b): Time domain spectroscopy (TDS), (c) Direct (passive) imaging, and (d) Heterodyne imaging.

Current status of the “uncooled” THz imaging technology a) - Active and Passive Coherent Millimeter and THz Wave Imaging; b) - Pre-amplified Direct Detection Imaging; c) - Incoherent Un-amplified Direct Detection Imaging. Simplified schematic of heterodyne receiver architecture. Can be passive and active. Simplified schematic of a preamplified direct detection receiver (as a rule limited to W-band). Can be passive and active. Simplified schematic of un-preamplified direct detection receiver. Low-temperature can be passive. Un-cooled – active.

Optical system sketch. To estimate NEP needed for a passive system For 1 0. 85 mm, / 0. 3, =1 and NEDT 0. 1 K NEP 1 1. 3 10 -12 W/Hz 1/2. For 1 3. 0 mm, NEP 2 4 10 -13 W/Hz 1/2. For frame rate fr =10 Hz, the integration time for detector tint 10 -1 s and the noise equivalent bandwidth fe =(2 tint)-1 5 Hz. Then for pixel NEP 1 6 10 -13 W/Hz 1/2 and NEP 2 2 10 -13 W/Hz 1/2. If = 0. 3 then these values should be multiplied by 1/2 0. 55.

Earth atmosphere transparency from visible to radiofrequency band regions [A. H. Lettington, et. al. , Proc. SPIE 4719, 327 -340 (2002)]. Also spectral radiances of blackbodies with temperature T 6000 K (Sun) and T 300 K (Earth) are shown. THz gap with respect to source technology: ( ) quantum cascade lasers (QCL) are progressing downward from high frequencies, the lowest = 1. 2 THz, T = 110 K – CW, T = 163 K – pulsed; ( ) frequency multipliers dominate other electronic devices ( ) above about 150 GHz (after T. W. Crowe, et. al. , IEEE J. Solid-State Circuits 40, 2104– 2110 (2005))

Low-temperature bolometers NEP improve a factor of two every two years Curves that define BLIP performance are calculated for diffraction limited beams taking into account that AТ is an invariant of optical system with coherent (heterodyne) detectors. Effective receiving of diffraction limited beams at the entrance of the optical system is defined by AТ = 2, where АТ is a circle aperture area and (sr) is a solid angle. For system with direct detection detectors it is possible AT > 2 and as a rule it is, and that is a benefit of direct detection systems in a case of broad-band radiation (e. g. in vision systems).

Operating temperatures for low-background detectors. Longer wavelength detection - lower operating temperature for (After A. Rogalski, in: THz and Security Applications, Springer, 2014). But for THz/sub-THz applications it is desirable use of cost-effective uncooled detectors.

NEP ~ (Adif f)1/2 ~ ldif Microbolometer NEP spectral dependence for THz FPAs (reprinted from A. Rogalski, in: “THz and Security Applications, ” Springer, 2014).

IR and THz vision technologies are different in many aspects (i) IR technologies now are passive and THz technologies can be passive only with sensitive detectors in some applications. (ii) The sizes of IR sensitive elements, as a rule, are larger or comparable with the wavelength but the sizes of THz/sub-THz sensitive elements are smaller (at ≤ 3 THz) the wavelength and, as a rule, they require antennas use. (iii) Differences in physics of signal registration processes and constraints, especially when integrated in large arrays (systems), and many details not important when constructing IR arrays are crucial when making up THz arrays (e. g. antennas, substrate permittivities, their thickness, lenses etc. ). Different physical phenomena are present that calls for multidisciplinary special knowledge. Three detector types were considered: MCT narrow-gap bolometers, SBDs, and FETs (HEMTs).

141 GHz, without Si lens, d=400 m. 141 GHz, without Si lens, d=350 m.

Equivalent circuits for electrical matching with antenna Si - FET SBD Long channel, LCH>Leff, ZA ~ 50 ÷ 200 Zero-bias MCT- HEB RS ~ 200 ÷ 500 RS ~ 20 ÷ 100 ZIN ~ 103 ÷ 104 ZIN ~ 1000 ÷ 2000 ZIN ~ 100 ÷ 1000 Simplified schematic representations with basic parasitic components. ZA - antenna impedance; VA - antenna voltage amplitude; RS = RG + Rsource in FET is the active series (parasitic) resistance of FET, where RG is the gate active resistance; RS in SBD and MCT-HEB is series parasitic active resistance; RD is SBD differential active resistance; CP is the parasitic reactance (usually capacitive), ZGS, int is internal source-gate impedance.

MCT bolometers MCT hot electron bolometer Electrons in MCT bolometer are heated by electromagnetic wave field changing the bolometer resistance Three free carrier effects are responsible for MCT bolometer response: -Dember effect (photodiffusion effect) contribution; -Thermoelectromotive contribution; -Free carriers concentration changes. They are differently temperature dependent that may cause the change of the response sign on temperature. (V. Dobrovolski, F. Sizov, Opto-Electr. Rev. , 18, 250 (2010))

Example of linear hybrid array of hot electron MCT bolometers on Ga. As substrate with antennas on quartz substrate for radiation frequency ~ 125 – 145 GHz. Quartz substrate (e ~ 4. 8) thickness d=200 m Linear hybrid array of hot electron bolometers with antennas on quartz substrate for radiation frequency ~ 125 – 145 GHz. Quartz substrate (e ~ 4. 8) thickness is 200 m. F. Sizov, V. Petriakov, et. al. , Appl. Phys. Lett. 101, 082108 (2012) Signal profile dependence at detector displacement

a) b) Schematic of glass fibre laminate wafer with microlens and sensitive element (a), microlenses and sensitive elements (MCT microbolometers on the back side of microlenses) with antennas on Ga. As substrate and fiber glass wafer (b). Signal frequency dependences for 3 MCT microbolometers with Si lenses immersed into glass fibre laminate wafer by 1 mm. S/N ~> 3 104, Ibias=3 m. A, with lock-in.

MCT THz/sub-THz detector IR responsivity spectra a) T = 78 K, b) T = 300 K a) b)

FET Long Channel detectors

Dependencies of drain-source currents and effective coefficient fz on FET channel dimensions and radiation frequency

Antennas, 265 -375 GHz. (Exp. data of W. Knap, D. But, et. al. ).

MOSFET and SBD as mm-wave/THz detector Vdet = Pant, max RV, int a L Power transmission coefficient is ratio of power absorbed in internal part of transistor Pin, int to power Pin that is absorbed in transistor as a whole. a is an antenna transfer coefficient, L is loading matching coefficient. a~0. 2 at Zant~(100 - j 100) , 77 GHz, L ~1 at voltmeter Rinput~10 M is in the range of ~10 -40 A/W for almost every transistor. For SBD at T=300 K and n = 1, RI, int=19. 3 A/W is a max figure. RVmeas ~Vdet ~ -2 if wide aperture antennas are used (or, for example, in experiments lenses are used), RVmeas ~Vdet ~ -4 in other cases.

Responsivity RVIO ( ) as a function of radiation frequency in the linear region for pulsed detection measurement. Dots are experimental data of FET detectors (HEMT) at Iir = 10 W/cm 2. Line is fitting with = 2. [D. But, W. Knap, et. al. , JAP, 115 (2014)]. Vdet ~ω-2 if wide aperture antennas are used (or, for example, in experiments lenses are used), Signal ~ ω -4 in other cases (Sakhno M, et. al. J Appl Phys (2013).

FET NEPel improvement performance when going from 1 µm technology, W/L=20/2 ( m) to 0. 35 µm technology, W/L=1/1 ( m). M. Sakhno, A. Golenkov, F. Sizov, JAP, 114, 164503 (2013). NEPopt with antenna impedance Zant=(100–j 100) Ω (taken into account parasitics). Open marks - for Si FET detectors and filled marks - for SBD detectors.

FET THZ/sub-THz detectors (Si-KMOP, 0. 35 m design rules) One-chip eight-element THz/sub. THz linear array with antennas, amplification and information processing circuits. Output signals of eight-element linear array under Gaussian beam.

Ga. N HEMT detectors W/L~ 100 Authors are thankful to K. Zhuravlev and J. Gumenjuk for supplying Ga. N transistors

Parameters of sub/THz detectors investigated Detector MCT , GHz 138 Si-FET 140 without antenna Ga. N HEMT Output Resp. , SV Noise NEP, W/Hz 1/2 11 m. V ~140 V/W ~37 n. V/Hz 1/2 ~2. 6· 10 -10 (Ibias=5 m. A) (G 8 d. Bi) VDS ~ 60 m. V ~200 V/W 140 Conventional 139 SBD (Ga. As) without antenna Zero-biased 150 – 440 SBD Zero-biased 320 SBD (In. Ga. As/In. P) UJN ~ 90 n. V/Hz 1/2 ~10 -10 ~46 58 m. V (Vbias =585 m. V) Vbias= 0 ~5 10 -10 (G = 1, e = 1) ~800 V/W ~360 n. V/Hz 1/2 (Vbias=585 m. V) ~5. 7· 10 -10 (G 1. 05 d. Bi) SV = 3001000 V/W 120÷ 200 V/W - 2· 10 -11÷ 5· 10 -12 - 5· 10 -10÷ 1· 10 -11

Imaging examples obtained with single FET and MCT uncooled bolometer system ~150 GHz Pictures of lighter at 150 GHz by existing incoherent un-amplified direct detection single detector prototype with MCT bolometer. a) – lighter in envelope, b) – lighter in envelope behind the gypsum plasterboard of d = 12 mm, c) – visible region. Two medicine pills in the thick non-transparent envelope of different form and dimensions. In the upper pill the small (~2 x 2 mm) dielectric item (d~0. 3 mm) is imbedded. Dark rings around pills seem arise due to the phase differences between the beams in air and in pills. Right – example of leafs imaging F. Sizov, V. Zabudsky, et. al. , Optic. Eng. , 52, # 3 (2013) Visible ~150 GHz

Lighter (1), electric cable (2) and a bit of metal sheet (3) in opaque envelope in SNR ~ 54 d. B reflection configuration through the gypsum plasterboard with d=12 mm Lighter in envelope behind (radiation passes twice through gypsum plasterboard of plasterboard) at 150 GHz. d = 12 mm, SNR ~ 41 d. B 27

Conclusions -Uncooled MCT, FET and HEMT detectors and arrays can be applied in active THz/sub-THz direct detection systems; - Long channel FET detector performance is mainly limited by parasitic effects; - FET, SBD and MCT detectors performance is proportional to -2 or to -4 in dependence on the antenna type and measurement procedure; - FET detector performance can be improved with the design rules advance due to lowering the parasitic effects; Partly these investigations were supported by NATO contract Sf. P 984544.