Persistent Photocurrent Effects Research Articles

Fully transparent InZnSnO/β-Ga2O3/InSnO solar-blind photodetectors with high schottky barrier height and low-defect interfaces

Ultra-wide bandgap β-Ga2O3 photodiodes have received great attention due to transparent solar-blind deep ultraviolet photodetectors. We demonstrated an all oxide-based single-crystal β-Ga2O3 photodiode incorporated with transparent conductive InZnSnO and InSnO as Schottky and Ohmic contacts, respectively. High work function and low resistivity of InZnSnO allow for clear rectifying characteristics with a low dark current (<0.1 nA) of up to −100 V of reverse bias. The Schottky barrier height and junction defects at the Schottky interface were modified using post-annealing treatment, thereby influencing the deep ultraviolet photoresponse. The responsivity of the annealed device was 9.6 mA/W, decreasing the Schottky barrier height engineering, while the photo responding speed was degraded and caused a persistent photocurrent effect due to the generation of defect states.

A 2DEG back-gated graphene/AlGaN deep-ultraviolet photodetector with ultrahigh responsivity* *Project supported by the Research Innovation Fund for College Students of Beijing University of Posts and Telecommunications (Grant No. 202002046) and the National Natural Science Foundation of China (Grant No. 61804012).

A graphene/AlGaN deep-ultraviolet (UV) photodetector is presented with ultrahigh responsivity of 3.4 × 105 A/W at 261 nm incident wavelength and 149 pW light power. A gain mechanism based on electron trapping at the potential well is proposed to be responsible for the high responsivity. To optimize the trade-off between responsivity and response speed, a back-gate electrode is designed at the AlGaN/GaN two-dimensional electron gas (2DEG) area which eliminates the persistent photocurrent effect and shortens the recovery time from several hours to milliseconds. The 2DEG gate is proposed as an alternative way to apply the back gate electrode on AlGaN based devices on insulating substrates. This work sheds light on a possible way for weak deep-UV light detection.

Ultrahigh and Broad Spectral Photodetectivity of an Organic-Inorganic Hybrid Phototransistor for Flexible Electronics.

The creation of new organic-inorganic phototransistors with high and broad spectral photosensitivity is reported. The extended charge transport and photoconductivity between the layers in the bilayer structure results in a notable detectivity of over 10(12) Jones and a linear dynamic range of over 100 dB at a broad spectral bandwidth across the UV-NIR range. Furthermore, the considerably reduced persistent photocurrent effect of In-Ga-Zn-O (IGZO)-based hybrid phototransistors is first demonstrated via an organic-inorganic bilayer approach.

Photocurrent Phenomena in Nanoribbon InAlN/GaN High Electron Mobility Transistors

High electron mobility transistors (HEMTs) based on InAlN/GaN nanoribbon (NR) structures appear as a new technology which can deliver record current densities. Ultraviolet (UV) photodetectors based on ungated InAlN/GaN NR HEMT structures have a gain in the range of the 105–107, a sublinear behaviour with the excitation power and a UV/visible contrast of more than three orders of magnitude. In spite of the larger surface-to-volume ratio, sublinearities and persistent photocurrent effects are less intense in NR structures compared to planar devices, and they decrease at higher excitation power. The asymmetric NR morphology does not induce a preferential coupling of polarized light in the NR structure; since light is mostly absorbed in the GaN buffer layer and photogenerated charges are efficiently collected by the HEMT channel.

Novel III‐nitride based transparent photodetectors for standing wave interferometry

AbstractThe concept of a standing wave (SW) interferometer has been extensively investigated for the last decade. A key problem within these efforts is the development of a transparent ultra‐thin photodetector for sampling the intensity profile of the generated SW. We report on the simulation and fabrication as well as on the structural and optoelectronical characterization of highly transparent AlGaN/GaN double‐heterostructure and InGaN metal–semiconductor–metal photodetectors based on intraband and band‐to‐band transitions, respectively. Both detectors have a 20 nm thick optically active layer designed for the light absorption at the wavelength of ∼633 nm. Besides good structural properties of both Al0.35Ga0.65N/GaN and In0.56Ga0.44N structures, the absorbance of the InGaN‐based detector was found ∼20 times higher compared to the AlGaN/GaN photodetectors. The temporal characteristics of the detectors are limited by both the RC‐constant and by the persistent photocurrent effect. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Space charge limited electron transport in AlGaN photoconductors

Electrical properties and photoresponse of AlGaN based photodetectors were studied demonstrating an adverse effect of the broad-band trap distribution on the spectral, electrical, and time-response characteristics. It was found that n-type conduction mechanism is space charge limited indicating a strong carrier trapping effect. In particular, we show that dark current, photoresponsivity, and response time are determined by the slow-varying energy distribution of traps located above the equilibrium Fermi level. We demonstrate also that both the deep recombination centers and the localized shallow states do not impact the response time of the photodetector, and the persistent photocurrent effect is directly connected to the voltage-induced injection and trapping of the excess carriers.

ITON Schottky contacts for GaN based UV photodetectors

Lateral Schottky ultraviolet detectors were fabricated in GaN using indium-tin-oxynitride (ITON) as a contact metal. The GaN semiconductor material was grown on 2 in. sapphire substrate by metal–organic chemical vapor deposition (MOCVD). The Schottky contact has been realized using ITON that has been deposited using sputter techniques. I– V characteristics have been measured with and without UV illumination. The device shows photo-to-dark current ratios of 10 3 at −1 V bias. The spectral responsivity of the UV detectors has been determined. The high spectral responsivity of more than 30 A/W at 240 nm is explained by a high internal gain caused by generation–recombination centers at the ITON/GaN interface. Persistent photocurrent effect has been observed in UV light (on–off) switching operation, time constant and electron capture coefficient of the transition has been determined.

Radiation-induced effects in GaN by photoconductivity analysis

Spectral photoconductivity is a very efficient tool to explore the forbidden gap of wide gap semiconductors, detecting band-to-band or deep level-to-band transitions of the charge carriers. In this paper we present some results obtained by spectral photoconductivity concerning HVPE (hydride vapour phase epitaxy) grown gallium nitride submitted to high energy proton irradiation. The behaviour of the material before and after irradiation may give information on the characteristics of the deep bands and their association to surface or bulk defects. In particular we observed how irradiation quenches the yellow band and correspondingly inhibits persistent photocurrent effects. Persistent photocurrent measurements carried out at different temperatures give further insights on the radiation-induced effects. The possible correlation among the deep-related transitions and the persistent photo-effects is here proposed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Persistent photocurrent spectroscopy of GaN metal–semiconductor–metal photodetectors on long time scale

Some of the trapping mechanisms responsible for the persistent photocurrent effects in GaN metal–semiconductor–metal photodetectors have been studied on a time scale of several hours by analyzing the photocurrent decay kinetic as a function of temperature. The analysis of the Arrhenius plot of the decay kinetic on a long time scale shows two activation energies of about 140meV and 1eV. Such values are in good agreement with two slope changes observed in the room temperature photocurrent spectrum around 2.40 and 3.25eV, in the GaN energy band gap. According to the Lucovsky theory the 140meV activation energy was interpreted as due to the transition from a deep localized state to the conduction band edge, whereas the 1eV activation energy was interpreted as due to the transition from the valence band edge to a deep localized state. Therefore, the persistent photocurrent on long time scale is primarily due to the presence of donor deep and acceptor deep states generated by gallium vacancies, gallium antisites, and carbon impurities.

Interface roughness scattering in thin, undoped GaInP/GaAs quantum wells

Electronic transport properties of very thin undoped GaInP/GaAs quantum wells have been measured by temperature dependent low field Hall effect and by Shubnikov–de Haas effect. Strong Shubnikov–de Haas oscillations were observed after increasing the electron concentration via the persistent photocurrent effect. Low temperature mobilities of up to 70 000 cm2/V s at carrier concentrations of 6.5×1011 cm−2 were observed in a 20 Å quantum well. The results are compared with the theory of interface roughness scattering which indicates extremely smooth interfaces; however, discrepancies between experiment and theory are observed.

Photocurrent transients in semi-insulating GaAs, effects of EL2 and other defects

Photocurrent transient effects in undoped semi-insulating gallium arsenide have elicited conflicting explanations. The photocurrent quenching, enhanced photocurrent, and persistent photocurrent effects are reviewed and new results on these effects are reported. A comparison of the photocurrent transients with optical absorption quenching along with other experiments shows that photocurrent quenching is due to the metastable transformation of the deep donor EL2 and that the enhanced and persistent photocurrent effects are due to a seperate effect that is most likely a metastable transformation of another defect. Thermal recovery experiments show that the activated state responsible for the enhanced photocurrent recovers prior to the recovery of the metastable state of EL2. Furnace annealing experiments are presented showing that the enhanced and persistent photocurrent effects are significantly reduced after anneals at 500 °C, well below the annealing temperature of quenchable EL2. After reviewing existing models we suggest that the absence of a persistent photocurrent immediately after quenching is due to compensation of residual impurities by the second level of EL2. A complex defect, perhaps including EL6 as an intermediary, is proposed for the defect responsible for the charge transfer resulting in the enhanced and persistent photocurrent effects.

Temperature dependence of the persistent photocurrent in Czochralski gallium arsenide.

The persistent-photocurrent effect in bulk, semi-insulating GaAs has been studied as a function of temperature. The persistent photocurrent is activated by illumination with 1.13-eV light and is preceded by an enhanced photocurrent during long-term illumination after the initial photocurrent has decayed in a quenching process similar to that of the deep donor EL2. Both the magnitude of the persistent photocurrent, and its decay rate are strongly temperature dependent and show significant reduction above 40 K. These results indicate that while the metastable transformation of EL2 is required for the activation of the persistent photocurrent, another unidentified metastable process after this is also required.