Lithuanian Journal of Physics article / Lietuvos fizikos žurnalo straipsnis

TERAHERTZ BOW-TIE DIODE BASED ON ASYMMETRICALLY SHAPED AlGaN/GaN HETEROSTRUCTURES
Justinas Jorudas^a, Dalius Seliuta^a, Linas Minkevičius^a, Vytautas Janonis^a, Liudvikas Subačius^a, Daniil Pashnev^a, Sandra Pralgauskaite^a, Jonas Matukas^a, Kęstutis Ikamas^b, Alvydas Lisauskas^b, Emilis Šermukšnis^c, Artūr Šimukovič^c, Juozapas Liberis^c, Vitalij Kovalevskij^d, and Irmantas Kašalynas^a,b
^aTerahertz Photonics Laboratory, Center for Physical Sciences and Technology, Saulėtekio 3, 10257 Vilnius, Lithuania
^bInstitute of Applied Electrodynamics and Telecommunications, Vilnius University, Saulėtekio 3, 10257 Vilnius, Lithuania
^cFluctuation Research Laboratory, Center for Physical Sciences and Technology, Saulėtekio 3, 10257 Vilnius, Lithuania
^dExperimental Nuclear Physics Laboratory, Center for Physical Sciences and Technology, Saulėtekio 3, 10257 Vilnius, Lithuania
Email: irmantas.kasalynas@ftmc.lt

Received 3 October 2023; accepted 3 October 2023

Asymmetrical shaping of AlGaN/GaN heterostructures containing a conductive layer of two-dimensional electron gas (2DEG) was used for the development of bow-tie (BT) diodes for room temperature terahertz (THz) detection. Considering operation of the THz BT diode in the unbiased mode as preferable for practical applications, we investigated the diodes with an obvious asymmetry of IV characteristics, which was found to be more pronounced with the decrease of an apex width, resulting in the sensitive THz detection. A nonuniform heating of carriers in a metalized leaf of the BT diode was attributed as the main mechanism that caused the rectification of THz waves. The responsivity and noise-equivalent power (NEP) at the fundamental antenna frequency of 150 GHz were up to 4 V/W and 2 nW/√Hz, respectively. Such high sensitivity of BT diodes allowed us to measure for the first time the response spectrum of the asymmetric BT antenna demonstrating fundamental and higher order resonances in good agreement with finite-difference time-domain simulation data in a broad spectrum range. The detailed investigation of the low- and high-frequency noise characteristics of AlGaN/GaN BT diodes revealed that only thermal noise needs to be considered for the unbiased operation, the value of which was relatively low due to a high density of 2DEG enabling low resistivity values. Moreover, we observed that the responsivity of BT diode scales with its resistance, revealing that tapering of the diode apex below a few microns could be ineffective in applications which require low NEP values.
Keywords: THz bow-tie diode, AlGaN/GaN heterostructure, 2DEG, asymmetrical BT antenna

ASIMETRIŠKAI SUSIAURINTŲ AlGaN/GaN HETEROSTRUKTŪRŲ PETELIŠKĖS TIPO DIODAI TERAHERCINIAM DAŽNIŲ RUOŽUI
Justinas Jorudas^a, Dalius Seliuta^a, Linas Minkevičius^a, Vytautas Janonis^a, Liudvikas Subačius^a, Daniil Pashnev^a, Sandra Pralgauskaite^a, Jonas Matukas^a, Kęstutis Ikamas^b, Alvydas Lisauskas^b, Emilis Šermukšnis^c, Artūr Šimukovič^c, Juozapas Liberis^c, Vitalij Kovalevskij^d, Irmantas Kašalynas^a,b

^aFizinių ir technologijos mokslų centro Terahercų fotonikos laboratorija, Vilnius, Lietuva
^bVilniaus universiteto Taikomosios elektrodinamikos ir telekomunikacijų institutas, Vilnius, Lietuva
^cFizinių ir technologijos mokslų centro Fliuktuacinių reiškinių laboratorija, Vilnius, Lietuva
^dFizinių ir technologijos mokslų centro Eksperimentinės branduolio fizikos laboratorija, Vilnius, Lietuva

Asimetriškai susiaurintos AlGaN/GaN heterostruktūros su laidžiu dvimačių elektronų dujų (2DEG) sluoksniu panaudotos sukurti peteliškės tipo (BT) diodus, tinkančius terahercinio dažnių ruožo (THz) bangoms registruoti kambario temperatūroje. Atsižvelgiant į tai, kad be išorinės postūmio įtampos THz BT diodo veikimas yra labiau tinkamas praktiniams taikymams, ištyrėme diodus su didele srovės ir įtampos (IV) charakteristikų asimetrija, kuri buvo tuo ryškesnė, kuo mažesnis kakliuko plotis, parodant didelį diodų jautrumą THz bangoms. Krūvininkų kaitinimas metalizuotame diodo lapelyje buvo išskirtas kaip pagrindinis mechanizmas, lemiantis THz bangų lyginimą BT diode. Jautris ir triukšmo ekvivalentinė galia (NEP) ties pagrindiniu antenos dažniu 150 GHz siekė iki 4 V/W ir atitinkamai 2 nW/√Hz. Toks didelis BT diodų jautrumas leido mums pirmą kartą išmatuoti asimetrinės BT antenos dažninę charakteristiką, stebint pagrindinį ir aukštesnės eilės antenos rezonansus, parodant gerą sutapimą su baigtinių skirtumų laiko skalėje atliktais modeliavimo rezultatais plačiame dažnių ruože. Išsamus AlGaN/GaN BT diodų žemo ir aukšto dažnio triukšmo charakteristikų tyrimas atskleidė, kad nenaudojant įtampos postūmio reikia atsižvelgti tik į šiluminį triukšmą, kurio vertė buvo santykinai maža dėl didelio 2DEG tankio, lemiančio nedidelę diodo varžą. Taip pat nustatėme, kad BT diodo jautrumas netiesiniu dėsniu priklauso nuo jo varžos, dėl to diodo kakliuko siaurinimas žemiau kelių mikronų vertės gali būti mažai veiksmingas, kai tikimasi mažų NEP verčių.

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