Photoconductive Ultraviolet Detectors Research Articles

Epitaxial ZnO on p-Si and its MSM Structure Photoconductive Ultraviolet Detector

In this work, a plasma-assisted molecular-beam epitaxy (PA-MBE) was used to prepare high-quality ZnO thin films on p-type silicon substrates. Be/BeO composite buffer layers were designed to improve the crystal quality of ZnO thin films. Based on the ZnO thin films, we fabricated interdigitated electrode MSM structure photoconductive UV detector by lithography, electron beam evaporation and other traditional semiconductor processes. In addition, dark current, spectral response of the UV detector were measured by responsivity testing system to explore the application of ZnO based UV detectors.

Effect of annealing temperature on the performance of photoconductive ultraviolet detectors based on ZnO thin films

ZnO films prepared by sol–gel method and the ultraviolet detectors based on this material have been investigated in this paper. X-ray diffraction (XRD) patterns showed ZnO films present hexagonal wurtzite structure with a preferential orientation of (002) plane, and the crystallite size of films gradually increased from 44.3 to 54.8 nm as the annealing temperature increased from 400 to 600 °C. Ultraviolet–visible (UV–Vis) spectra indicated that the optical band gap decreases gradually with the increase of annealing temperature, and the minimum band gap is 3.06 eV at 600 °C. Photoluminescence (PL) spectra revealed that increasing annealing temperature can significantly reduce the defects and improve the crystallinity. Finally, gold (Au) coplanar interdigital electrodes were deposited on ZnO film surface and used to fabricate the ultraviolet photodetectors. The response performance of the devices improved as the annealing temperature of ZnO films increased, and the fastest response speed with a rise time of 4.172 s and a fall time of 11.012 s was obtained.

Photoconductive UV detector based on high-resistance ZnO thin film

As a wide bandgap semiconductor material, ZnO has huge potential in applications such as light emitting devices and sensors. Compared with GaN and SiC, ZnO has a bandgap of 3.37 eV and exciton binding energy of 60 meV at room temperature, indicating it is a promising candidate of UV detector. ZnO based metal-semiconductor-metal photoconductive ultraviolet detector has the advantages of high optical gain and strong responsivity. However, due to the photoconductive relaxation and surface effect of the ZnO material, a ZnO-based photoconductive UV detector has a slow response which is defective for practical application. The intrinsic defects typically generated during the synthesis of ZnO, e.g. oxygen vacancy, should be responsible for the slow response. Therefore, we have fabricated the high-resistive ZnO thin film based UV detector and studied its UV response characteristic. High resistance ZnO thin film is fabricated on glass by RF magnetron sputtering and followed by lift-off photolithography to form Al interdigital electrodes. SEM and XRD images show that the as-fabricated ZnO thin film grows with preferential orientation along c-axis. A linear I-V curve under UV illumination indicates the ohmic contact between Al and ZnO. From these results, we can calculate the resistivities to be 3.71×109 Ω · cm and 7.20×106 Ω · cm respectively when in the dark and under 365 nm UV light of 303 μW/cm2. The light-to-dark current ratio is up to 516 with bias of 40 V. Besides, the ZnO thin film detector shows a stable, rapid, repeatible and reproducible response with a rise time of 199 ms and a fall time of 217 ms when exposed to periodically switched UV light illumination at a bias voltage of 40 V. Moreover, the detector has a high selectivity for 365 nm UV light and the responsivity is 0.15 mA/W with the intensity of 303 μW/cm2. Furthermore, the transient response process is analyzed using the theory of surface recombination and bulk recombination of ZnO semiconductor. For a high resistance ZnO thin film based UV detector, the surface recombination process is weakened ascribed to the decrease of intrinsic defects and the bulk recombination process plays a leading role, resulting in the fast response. Results show that high resistivity ZnO thin film based UV detectors have outstanding UV photoresponse characteristics for potential applications in UV/radiation detection.

Optoelectronic characteristics of UV photodetectors based on sol–gel synthesized GZO semiconductor thin films

Ga-doped ZnO (GZO), B–Ga codoped ZnO (GZO:B), and Al–Ga codoped ZnO (GZO:Al) transparent semiconductor thin films and photoconductive ultraviolet (UV) detectors were fabricated on alkali-free glass substrates by the sol–gel route. The doping concentration of Ga was 0.5at.%, and that of Ga plus B or Al was 1at.% in the precursor solutions. The structural, electrical, and optical properties of GZO, GZO:B and GZO:Al thin films were compared and the photoconductivity and photoresponsivity of ZnO-based UV photodetectors are reported. All as-prepared ZnO-based thin films had high transparency in the visible region, and the GZO:Al thin film exhibited the best electrical properties. Current–voltage (I–V) characteristics in dark and illuminated conditions showed that the resistance of the films fell by more than an order of magnitude upon UV light illumination. In this study, we found that the photoconductive UV detector based on GZO:Al film exhibited the highest responsivity, 3.22A/W at 5V bias under UV light illumination.

Solution-Processed Photoconductive UV Detectors Based on ZnO Nanosheets

A low-cost and complexity-free demonstration of two relatively fast ultraviolet (UV) detectors based on ZnO nanosheets are reported in this letter for the first time. Highly crystalline ZnO nanosheets have been synthesized by a sonochemical method. The proposed UV detectors were fabricated through coating the synthesized materials on Cu electrodes. Experimentally obtained responsivity, detectivity, absorption, and I-V characteristics along with the quantum efficiency are reported. Comparison of the conventional ZnO photo-sensors with nanoparticle-based ones reveals that the proposed nanosheets photosensors have faster response time in the UV region, which might be attributed to low density of defect states (created during synthesis process) on nanosheets surfaces.

Photoconductive UV Detectors Based on ZnO Films Prepared by R.F. Magnetron Sputtering Method

Highly c-axis oriented zinc oxide (ZnO) thin films were deposited on glass substrates by radio frequency (r.f.) sputtering. The photoconductor UV detector based on ZnO films, having a metal-semiconductor-metal (MSM) structure with interdigitation configuration, were fabricated by using aluminium (Al) as a contact metal. The characteristics of dark and photocurrent of the ultraviolet (UV) detector and the UV photo-response of the detector were investigated. The linear current-voltage (I-V) characteristics under both forward and reverse bias exhibit ohmic metal-semiconductor contacts. Under illumination by monochromatic light at a wavelength of 365~nm, the photo-generated current was measured to be 0.56 $\mu$A at a bias of 6 V. The photo-response decay in these devices is slow.

Fabrication and performance of N-doped ZnO UV photoconductive detector

We report study on the fabrication and characterization of an ultraviolet (UV) photodetectors based on N-doped ZnO films. The N-doped ZnO films with 10at% N doping are spray deposited on to alumina substrates. The photoconductive UV detector based on N-doped ZnO thin films, having a metal–semiconductor–metal (MSM) configuration are fabricated using Al as a contact metal. The dependence of I–V characteristic under dark and illumination, spectral and transient photoresponse of the detector are investigated. The linear current–voltage (I–V) characteristics under forward bias exhibit ohmic metal–semiconductor contact. The UV photoconductive effect is observed showing fast response with switching on/off UV light illumination. The neutralization of photogenerated holes by negatively charged oxygen ion plays a key role in the photoconductive characteristics of N-doped ZnO polycrystalline films.

Large area Ag–TiO 2 UV radiation sensor fabricated on a thermally oxidized titanium chip

A Schottky-type ultraviolet (UV) light sensor is fabricated on a thermally oxidized titanium chip. The device is of Ag–TiO 2–Ti structure, and the Schottky junction between silver and rutile is formed subsequent to the vacuum deposition of silver on the thermally grown rutile layer by a controlled thermal annealing in air. The device operates at −300 mV biasing established between silver and titanium electrodes. The dark reverse current of this diode increases four orders of magnitude when illuminated with UV light (355 nm) of 10 μW/mm 2 intensity. The device is almost insensitive to visible light and requires no filtering when used for ambient UV level detection. The operation mechanism of the device is described by photonic electron–hole pair generation in the carrier depleted titanium dioxide layer adjacent to the silver electrode. The electrode-to-electrode distance is 1 μm only affording much faster performance compared to photoconductive UV detectors fabricated based on titanium dioxide; the response and recovery times of the device are 10 ms and 17 ms, respectively. At its standby mode, a 1 mm 2 active area device consumes less than 10 pW of electric power. Sensors with sensitive areas as large as ∼10 mm 2 are easy to fabricate. The fabricated devices are rugged, resistant to UV degradation, and cost effective.

Response time improvement of AlGaN photoconductive detectors by adjusting crystal-nuclei coalescence process in metal organic vapor phase epitaxy

AlGaN photoconductive ultraviolet detectors are fabricated to study their time response characteristics. Persistent photoconductivity, a deterring factor for the detector response time, is found to be strongly related to the grain boundary density in AlGaN epilayers. By improving the crystal-nuclei coalescence process in metal organic vapor phase epitaxy, the grain-boundary density can be reduced, resulting in an-order-of-magnitude decrease in response time.

Monolithically integrated UV/IR‐photodetectors based on an AlN/GaN‐based superlattice grown on an AlGaN buffer layer

AbstractIn this article, we demonstrate closely spaced, monolithically integrated photodetectors in two largely different wavelength ranges. The epitaxial structure of the devices was grown by plasma‐assisted molecular‐beam epitaxy on an AlN‐on‐sapphire template; it consists of a Si‐doped AlGaN thin film, and a nearly strain compensated 40 period AlN/GaN superlattice with 1.0 nm thick GaN quantum wells and 2.0 nm thick AlN barriers. The entire structure is covered with an AlGaN cap layer. The superlattice acts as active region for the infrared detector, while the AlGaN buffer layer serves as active area for the ultraviolet detector. While the photovoltaic near‐infrared detector has a center wavelength of 1.37 μm, the photoconductive ultraviolet detector has a long wavelength cutoff at 250 nm. The two detectors could be operated up to room temperature with reasonable sensitivities. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Monolithically integrated AlGaN/GaN/AlN-based solar-blind ultraviolet and near-infrared detectors

Closely spaced, monolithically integrated photodetectors in two largely different wavelengths ranges are demonstrated. The device structure was grown by plasma-assisted molecular-beam epitaxy on an AlN-on-sapphire template, and it consists of a Si-doped AlGaN thin film, and a nearly strain compensated 40 period AlN/GaN superlattice with 1.0 nm-thick GaN quantum wells and 2.0 nm-thick AlN barriers. The entire structure is covered with an AlGaN cap. The superlattice constitutes the active region for the infrared detector, while the AlGaN buffer layer serves as active area for the ultraviolet detector. The photoconductive ultraviolet detector has a long wavelength cutoff at 250 nm, whereas the photovoltaic near-infrared detector has a centre wavelength of 1.37 µm.

Photoconductive ultraviolet detectors based on ZnO films

Properties of photoconductive ultraviolet detectors fabricated on ZnO films were presented. Highly c-axis oriented ZnO films were grown on glass substrates by pulsed laser deposition. Ultraviolet photodetectors were fabricated based on metal–semiconductor–metal planar structures. The photoresponsivity and the quantum efficiency are much higher in the ultraviolet range than in the visible range, and the peak values are around 360nm. Photocurrent transients show that the detector has a large photocurrent with the peak value of 2.8mA, and a slow photoresponse with a rise time of 5min and a decay time of 7min. The response curve of the detector is fitted well with exponential curve. The large photocurrent should result from the both effects of the accumulation of conduction electrons and the decrease of the barrier height between crystallites. The relaxation time constant τ obtained from the curve fitting represents the time accumulation during the process. The neutralization of photogenerated holes by negatively charged oxygen ions plays a key role in the photoconductive characteristics of ZnO polycrystalline films.

ZnO thin film photoconductive ultraviolet detector with fast photoresponse

C-axis preferred oriented ZnO thin films were prepared on quartz substrates by RF sputtering. Photoconductive ultraviolet detector with planar interdigital electrodes was fabricated on ZnO thin film by the lift off technique. Linear I– V characteristic was observed under dark or 365 nm UV light illumination and has obvious difference. The photoresponsivity of 365 nm at 5 V bias is 18 A/W. The response time measure set mainly contains KrF excimer laser with the pulse width of 30 ns and the oscillograph with the bandwidth of 200 MHz. The result shows fast photoresponse with a rise time of 100 ns and fall time of 1.5 μs.

Study of Photocurrent Properties of GaN Ultraviolet Photoconductor Grown on 6H-SiC Substrate

The properties of a photoconductive ultraviolet detector based on a GaN epilayer grown on a 6H-SiC substrate using metal-organic chemical vapor deposition were investigated. We obtained the detectable energy span of the device up to the ultraviolet region by photocurrent measurement. The spectral responsivity remained nearly constant for wavelengths ranging from 250 to 365 nm and dropped by three orders of magnitude within 15 nm of the band edge from 365 nm to 380 nm.The detector was measured to have a responsivity of 133 A/W at a wavelength of 360 nm under a 5 V bias, and the voltage-dependent responsivity was evatuated. Furthermore, a convenient method to determine the response time was developed. The relationship between response time and bias was obtained.

High-Quality Photoconductive Ultraviolet GaN/6H-SiC Detector and Its Properties

ABSTRACTThe properties of photoconductive ultraviolet detector based on GaN epilayer grown on 6H-SiC substrate by metalorganic chemical vapor deposition were investigated in this paper. We obtained the detectable energy span of the device up to ultraviolet by photocurrent measurement. The spectral responsivity remained nearly constant for wavelengths from 250 to 365 nm and dropped by three orders of magnitude within 10 nm of the band edge (by 380 nm). The detector was measured to have a responsivity of 133 A/W at a wavelength of 360 nm under a 5-V bias, and the voltage-dependent responsivity was performed. Furthermore, an easy method was developed to determine the response time, and the relationship between response time and bias was obtained.

High-responsivity photoconductive ultraviolet sensors based on insulating single-crystal GaN epilayers

We report on the fabrication and characterization of photoconductive ultraviolet detectors based on insulating single-crystal GaN. The active layer (GaN) was deposited over basal-plane sapphire substrates using a unique switched atomic-layer-epitaxy process. The sensors were measured to have a responsivity of 2000 A/W at a wavelength of 365 nm under a 5-V bias. The responsivity remained nearly constant for wavelengths from 200 to 365 nm and dropped by three orders of magnitude within 10 nm of the band edge (by 375 nm). We estimate our sensors to have a gain of 6×103 (for wavelength 365 nm) and a bandwidth in excess of 2 kHz. The photosignal exhibited a linear behavior over five orders of incident optical power, thereby implying a very large dynamic range for these GaN-based ultraviolet sensors.