Исследование аномально высокого времени релаксации фототока в диодах Шоттки на основе a-Ga2O3

Ga2O3 is an ultra-wideband material with excellent optical characteristics. It is a promising material for power applications and optoelectronics because of its high electrical breakdown voltage and radiation hardness. It is optically transparent for visible light and UVA but UVC-sensitive. One of the main disadvantages of this material is the anomalous slow photoeffect: photoconductivity rise and decay characteristic times can be more than hundreds of seconds long. This "slow" photoconductivity effect severely limits the utilisation of the Ga2O3-based devices. The aim of this work is the investigation of the nature of this effect. The results of the photoinduced current rise and decay under 530 nm and 259 nm LED are measured in the HVPE-grown α-Ga2O3-based Schottky diode. Upon UV-illumination the photocurrent rise consists of three parallel processes: fast signal growth, slow growth and very slow decay with characteristic times near 70 ms, 40 s and 300 s respectively. Subsequent 530 nm LED illumination resulted in photoinduced current rise consisting of two mechanisms with characterisatic times 130 ms and 40 s on which a very slow decrease of the photocurrent amplitude with characteristic time of 1500 s was superimposed. 530 nm illumination stimulates this process. Protoinduced current relaxation analysis shows the presence of the deep levels with energies (EC - 0.17 eV). It is suggested that extremely slow relaxations can be associated with potential fluctuations near the Schottky barrier.Ga2O3 — широкозонный материал с рядом уникальных характеристик, которые делают его перспективным материалом фотоники: он оптически прозрачен для оптического и ближнего ультрафиолетового излучения, обладает высокими значениями пробивных напряжений и высокой радиационной стойкостью. Одним из недостатков, которые в настоящее время препятствуют использованию данного материала в солнечно-слепых фотодетекторах, является аномально большое время нарастания и спада фотопроводимости, которое может достигать сотен секунд. Такая «замедленная» фотопроводимость существенно ограничивает область применения этих материалов. Проведены исследования природы этого эффекта. Выполнены измерения времени нарастания и спада фотоиндуцированного тока в диодах Шотки на основе α-Ga2O3, выращенных методом HVPE на сапфире, при засветке светодиодами с длиной волны 259 и 530 нм. При засветке ультрафиолетовым излучением рост тока через фоточувствительную структуру из двух встречных диодов происходил в три этапа: достаточно быстрое нарастание с характерным временем 70 мс, медленный рост с характерным временем 40 с и затянутый спад с характерным временем порядка 300 с. При последующей засветке излучением зеленого цвета рост тока с характерным временем 130 мс и 40 с накладывался на стимулируемый засветкой медленный спад амплитуды максимального тока с характерным временем порядка 1500 с. Анализ релаксации тока показал наличие глубоких центров с энергией (EC – 0,17 эВ). Существенное замедление релаксации фотоиндуцированного тока можно связать с флуктуациями потенциала вблизи барьера Шотки

Electrical properties, structural properties, and deep trap spectra of thin α-Ga2O3 films grown by halide vapor phase epitaxy on basal plane sapphire substrates

Undoped epitaxial films of α-Ga2O3 were grown on basal plane sapphire substrates by halide vapor phase epitaxy (HVPE) in three different modes: standard HVPE, HVPE with constant flow of Ga and pulsed supply of O2 (O2-control growth regime), and with constant flow of O2 and pulsed delivery of Ga (Ga-control growth fashion). The best crystalline quality as judged by x-ray symmetric and asymmetric reflection half-widths and by atomic force microscopy morphology profiling was obtained with the O2-control deposition, and these results appear to be the best so far reported for α-Ga2O3 films. All grown α-Ga2O3 epilayers were high-resistivity n-type, with the Fermi level pinned near Ec − 1 eV deep traps. Photoinduced current transient spectra also showed the existence in standard HVPE samples and samples grown under the O2-control pulsed growth conditions of deep hole traps with levels near Ev + 1.4 eV whose density was suppressed in the Ga-control pulsed HVPE samples. The levels of the dominant deep traps in these α-Ga2O3 samples are close to the position of dominant electron and hole traps in well documented β-Ga2O3 crystals and films

Impact of Hydrogen Plasma on Electrical Properties and Deep Trap Spectra in Ga₂O₃ Polymorphs

In this study, the results of hydrogen plasma treatments of β-Ga2O3, α-Ga2O3, κ-Ga2O3 and γ-Ga2O3 polymorphs are analyzed. For all polymorphs, the results strongly suggest an interplay between donor-like hydrogen configurations and acceptor complexes formed by hydrogen with gallium vacancies. A strong anisotropy of hydrogen plasma effects in the most thermodynamically stable β-Ga2O3 are explained by its low-symmetry monoclinic crystal structure. For the metastable, α-, κ- and γ-polymorphs, it is shown that the net result of hydrogenation is often a strong increase in the density of centers supplying electrons in the near-surface regions. These centers are responsible for prominent, persistent photocapacitance and photocurrent effects

Deep traps in InGaN/GaN single quantum well structures grown with and without InGaN underlayers

The electrical properties and deep trap spectra were compared for near-UV GaN/InGaN quantum well (QW) structures grown on free-standing GaN substrates. The structures differed by the presence or absence of a thin (110 nm) InGaN layer inserted between the high temperature GaN buffer and the QW region. Capacitance-voltage profiling with monochromatic illumination showed that in the InGaN underlayer (UL), the density of deep traps with optical threshold near 1.5 eV was much higher than in the QW and higher than for structures without InGaN. Irradiation with 5 MeV electrons strongly increased the concentration of these 1.5 eV traps in the QWs, with the increase more pronounced for samples without InGaN ULs. The observations are interpreted using the earlier proposed model explaining the impact of In-containing underlayers by segregation of native defects formed during growth of GaN near the surface and trapping of these surface defects by In atoms of the InGaN UL, thus preventing them from infiltrating the InGaN QW region. Deep level transient spectroscopy (DLTS) also revealed major differences in deep trap spectra in the QWs and underlying layers of the samples with and without InGaN ULs. Specifically, the introduction of the InGaN UL stimulates changing the dominant type of deep traps. Irradiation increases the densities of these traps, with the increase being more pronounced for samples without the InGaN UL. It is argued that light emitting diodes (LEDs) with InGaN UL should demonstrate a higher radiation tolerance than LEDs without InGaN UL. (C) 2020 Elsevier B.V. All rights reserved

Effects of 5 MeV electron irradiation on deep traps and electroluminescence from near-UV InGaN/GaN single quantum well light-emitting diodes with and without InAlN superlattice underlayer

The electrical properties, electroluminescence (EL) power output and deep trap spectra were studied before and after 5 MeV electron irradiation of near-UV single-quantum-well (SQW) light-emitting diodes (LED) structures differing by the presence or absence of InAlN superlattice underlayers (InAlN SL UL). The presence of the underlayer is found to remarkably increase the EL output power and the radiation tolerance of LEDs, which correlates with a much lower and more slowly changing density of deep traps in the QW region with radiation dose, and the higher lifetime of charge carriers, manifested by higher short-circuit current and open-circuit voltage in current-voltage characteristics under illumination. The observed phenomena are explained by the capture of native defects segregated at the growing surface by In atoms in the underlayer which traps them in the underlayer and prevents their penetration into the QW region

Effects of 5 MeV electron irradiation on deep traps and electroluminescence from near-UV InGaN/GaN single quantum well light-emitting diodes with and without InAlN superlattice underlayer

The electrical properties, electroluminescence (EL) power output and deep trap spectra were studied before and after 5 MeV electron irradiation of near-UV single-quantum-well (SQW) light-emitting diodes (LED) structures differing by the presence or absence of InAlN superlattice underlayers (InAlN SL UL). The presence of the underlayer is found to remarkably increase the EL output power and the radiation tolerance of LEDs, which correlates with a much lower and more slowly changing density of deep traps in the QW region with radiation dose, and the higher lifetime of charge carriers, manifested by higher short-circuit current and open-circuit voltage in current-voltage characteristics under illumination. The observed phenomena are explained by the capture of native defects segregated at the growing surface by In atoms in the underlayer which traps them in the underlayer and prevents their penetration into the QW region