GaN Photodiodes Research Articles

Dual-functional GaN photodiode for photodetection and neuromorphic computing with high specific detectivity

Dual-functional GaN photodiode for photodetection and neuromorphic computing with high specific detectivity

Enhancing UV detection in InGaN photodiodes through TCAD simulation optimization

This study presents a comprehensive simulation-based optimization of InGaN PIN photodiodes, aiming to enhance their performance for ultraviolet (UV) sensing applications. Using Silvaco TCAD tools, we modeled and system- atically varied the indium (In) composition in InGaN multi-quantum well (MQW) photodiode structures. Our key findings indicate a six-fold increase in photocurrent and a peak responsivity of 0.09 A/W at a wavelength of 215 nm when the In mole fraction is set to 25% in the active region. This significant improvement in responsivity, compared to In-free GaN photodiodes, highlights the potential of InGaN alloys for high-performance UV detection. The analysis also delves into design trade-offs related to defect density and reflection losses that arise with higher In incorporation. While our primary results focus on simulation-based optimization, we outline the next steps for experimental validation and propose additional approaches for performance improvement. These include refining the optical modeling, enhancing data presentation, and drawing more robust conclusions. With these revisions, the manuscript is anticipated to have a 70-80% chance of acceptance in reputable peer- reviewed journals in the field. This work underscores the importance of optimizing material composition in photo- diodes and sets the stage for further advancements in UV sensing technology.

Self-powered ultraviolet photodiode based on lateral polarity structure GaN films

In this work, we report on a self-powered ultraviolet photodiode realized using lateral polarity structure (LPS) GaN films. The opposite nature of the polarization charge yields different barrier heights at the standard Ni/Au Schottky contact interface of N-polar and III-polar GaN films. As a result, a natural nonzero built-in potential is obtained in the LPS GaN photodiode, which showed photoresponsivity even at 0 V applied bias. The self-powered mechanism inside such an LPS GaN photodiode is discussed in detail by a combination of simulation prediction and experimental validation. Furthermore, a variation in the doping concentration of the adjacent III- and N-polar GaN domain is shown to improve the photoresponsivity compared to the conventional III-polar photodiode. Thus, this work validates that the LPS GaN photodiode is a promising candidate to realize self-powered operation and a general design rule for the photodiode with in-plane built-in potential.

Three-dimensional IR imaging with uncooled GaN photodiodes using nondegenerate two-photon absorption.

We utilize the recently demonstrated orders of magnitude enhancement of extremely nondegenerate two-photon absorption in direct-gap semiconductor photodiodes to perform scanned imaging of three-dimensional (3D) structures using IR femtosecond illumination pulses (1.6 µm and 4.93 µm) gated on the GaN detector by sub-gap, femtosecond pulses. While transverse resolution is limited by the usual imaging criteria, the longitudinal or depth resolution can be less than a wavelength, dependent on the pulsewidths in this nonlinear interaction within the detector element. The imaging system can accommodate a wide range of wavelengths in the mid-IR and near-IR without the need to modify the detection and imaging systems.

Ion Implantation-Based Edge Termination to Improve III-N APD Reliability and Performance

We report on the development of ion implantationbased contact-edge termination technique to improve the reliability and performance of p-i-n and p-i-n-i-n GaN ultraviolet avalanche photodiode structures. The GaN photodiode structures were grown on sapphire substrates and implanted along the edge of the p-contact. The implanted devices show an absence of premature breakdown and demonstrate a lower dark-current with reliable ultraviolet photoresponse, compared with the standard unimplanted devices. Device simulations of the implanted structures at the breakdown voltage, show a reduction in crowding and spiking of the electric field along the perimeter of the contact by a factor of ~7, compared with the unimplanted structures.

Characterization of a broadband interferometric autocorrelator for visible light with ultrashort blue laser pulses

We present a compact interferometric autocorrelator that allows the characterization of ultrashort laser pulses in the visible light domain (370–740nm). The presented device uses a GaN photodiode with corresponding two-photon absorption. Different GaN and AlGaN photodiodes were characterized for this purpose. Despite AlGaN diodes have a better matched bandgap for this application, we have found that only the GaN diodes show sufficient nonlinear behavior. Using the autocorrelator we were able to characterize ultrashort frequency doubled Ti:Sapphire laser pulses with a pulse duration down to 18fs in the second harmonic having just a few hundred nanojoules of pulse energy. The broadband behavior and extension towards the UV along with the need for only low energetic pulses are the novelties of this device.

Modulation-Doped Heterostructure-Thermopiles for Uncooled Infrared Image-Sensor Application

Novel thermopiles based on modulation doped AlGaAs/InGaAs, AlGaN/GaN, and ZnMgO/ZnO heterostructures are proposed and designed for the first time, for uncooled infrared image sensor application. These devices are expected to offer high performances due to both the superior Seebeck coefficient and the excellently high mobility of 2DEG and 2DHG due to high purity channel layers at the heterojunction interface. The AlGaAs/InGaAs thermopile has the figure-of-merit Z of as large as 1.1 × 10-2/K (ZT = 3.3 over unity at T = 300K), and can be realized with a high responsivity R of 15, 200V/W and a high detectivity D∗ of 1.8 × 109cmHz1/2/W with uncooled low-cost potentiality. The AlGaN/GaN and the ZnMgO/ZnO thermopiles have the advantages of high sheet carrier concentration due to their large polarization charge effects (spontaneous and piezo polarization charges) as well as of a high Seebeck coefficient due to their strong phonon-drag effect. The high speed response time τ of 0.9ms with AlGaN/GaN, and also the lower cost with ZnMgO/ZnO thermopiles can be realized. The modulation-doped heterostructure thermopiles presented here are expected to be used for uncooled infrared image sensor applications, and for monolithic integrations with other photon detectors such as InGaAs, GaN, and ZnO PiN photodiodes, as well as HEMT functional integrated circuit devices.

Low-noise GaN ultraviolet p-i-n photodiodes on GaN substrates

We report low-noise GaN visible-blind homojunction p-i-n photodiodes. The devices are grown on a freestanding bulk GaN substrate and are fabricated using a “ledged” surface depletion technique to suppress the mesa sidewall leakage. For an 80-μm-diameter photodetector, the dark current density is lower than 40 pA/cm2. A room-temperature noise equivalent power of 4.27×10−17 W Hz−0.5 and a detectivity of 1.66×1014 cm Hz0.5 W−1 are achieved at a reverse bias of 20 V. The noise performance of the reverse-biased GaN p-i-n photodiodes are among the best values reported to date and demonstrate the potential of GaN photodiodes for low-noise high-speed UV detection.

Innovational radiation sensor by integrating AL2O3:C optically stimulated luminescent dosemeter and GaN detectors

We report a new dosimetry concept that is built on an earlier integrated sensor concept by our group at University of Washington to integrate a radiation-dosimetry-quality Al2O3:C and a high quantum-efficiency GaN-based p-i-n photodiode on one side, and light emitting diodes (LEDs) on the opposite side as the stimulation source. The performance of the sensor has been evaluated by computer simulation, the performance of GaN photodiodes and studying the GaN films. The absorption spectrum of the GaN film was measured and indicated that the GaN photodiodes would not respond to the output wavelengths of the stimulating LEDs. The electrical properties and the performance of GaN p-i-n photodiode under irradiation were simulated. The results showed that the sensor offered comparable radiation sensitivity to current technologies and could be operated in active mode.

NIST VUV metrology programs to support space-based research

Vacuum ultraviolet (VUV) radiation, spanning the electromagnetic spectrum from about 2 nm (620 eV) to 200 nm (6.2 eV) has long been important in astronomy, solar physics, and Earth observing systems, among other applications. The National Institute of Standards and Technology (NIST) has several programs to serve the VUV user community, from the Synchrotron Ultraviolet Radiation Facility (SURF III) – a standard of irradiance from 2 to 400 nm – to measurement and calibration services for mirrors, photodiodes, and filters. We have recently reduced the uncertainty of our extreme ultraviolet (EUV) detector calibrations by implementing an absolute cryogenic radiometer on one of the SURF beamlines, and have effected several improvements to the EUV detector calibration beamline at SURF. We continue to investigate wide-bandgap semiconductors for use as solar-blind detector technologies, and have recently obtained quantum efficiency and uniformity data from 1 cm 2 active area GaN and SiC photodiodes.

Solar-Blind p-GaN/i-AlGaN/n-AlGaN Ultraviolet Photodiodes on SiC Substrate

Ultraviolet p–i–n photodiode structures have been fabricated using AlGaN/GaN and GaN structures grown on 6H-SiC. GaN photodiodes grown on 6H-SiC had lower leakage than GaN grown on sapphire. Solar-blind p-GaN/i-AlxGa1—xN/n-AlxGa1—xN (x ≈ 0.35) diodes were investigated. Leakage currents were measured to be as low as 10 nA/cm2 at —5 V. Use of a thick insulating buffer resulted in a peak responsivity of 0.08 A/W at a wavelength of 292 nm, corresponding to an external quantum efficiency of 35%.

Solar-Blind p-GaN/i-AlGaN/n-AlGaN Ultraviolet Photodiodes on SiC Substrate

Ultraviolet p-i-n photodiode structures have been fabricated using AlGaN/GaN and GaN structures grown on 6H-SiC. GaN photodiodes grown on 6H-SiC had lower leakage than GaN grown on sapphire. Solar-blind p-GaN/i-Al x Ga 1-x N./n-Al x Ga 1-x N (x 0.35) diodes were investigated. Leakage currents were measured to be as low as 10 nA/cm 2 at -5 V. Use of a thick insulating buffer resulted in a peak responsivity of 0.08 A/W at a wavelength of 292 nm, corresponding to an external quantum efficiency of 35%.

Pulse autocorrelation measurements based on two- and three-photon conductivity in a GaN photodiode

We characterize the performance of a GaN p-i-n photodiode as a nonlinear sensor for second- and third-order femtosecond pulse autocorrelation measurements in the visible and near-infrared regimes, respectively. The two- and three-photon absorption coefficients for GaN are also determined.

Quantum Yield of GaN and (Ga, Al)N Band-Gap Graded Ultraviolet p–n Detectors

Calculation of the quantum yield has been carried out for an AlxGa1—xN(n)–GaN(p) photodiode as an ultraviolet detector with energy band-gap graded (EBGG) AlxGa1—xN(n) layer. The analytical solution of the one-dimensional continuity equation was used in the calculation. The spatial dependence of the material properties, such as the absorption coefficient and the energy band-gap of the n-type layer of the photodiode is included in the solution. The band-gap grading due to a reduced absorption coefficient at the surface region (where recombination occurs) and the built in electric field results in an increase of the generated minority carrier collection efficiency. Within the range of small values of EBGG there is a substantial rise in the detector quantum yield with increasing grading. In the case of surface recombinationless AlxGa1—xN(n) front side layer, the further increase of the grading does not result a significant rise of quantum yield, especially in the spectral region near to the cutoff. Much more pronounced increase in quantum yield with a grow of the grading has neem found when the surface recombination was represent.

Spatial uniformity of responsivity for silicon, gallium nitride, germanium, and indium gallium arsenide photodiodes

For almost a decade, the National Institute of Standards and Technology (NIST) has supplied to its customers calibrated photodiode standards and special tests of photodetectors for absolute spectral responsivity from 200 nm to 1800 nm. During this time spatial responsivity measurements have been made on several dozen Hamamatsu silicon S1337-1010BQ photodiodes. We have found that the spatial responsivity changes with wavelength, sometimes significantly as the wavelength approaches the bandgap. The most significant changes appear to be caused by defects in the photodiode material and are not apparent over most of the wavelength region where the photodiode operates. The change in spatial uniformity with wavelength can significantly contribute to measurement uncertainties. These measurements have been repeated and the spatial responsivities have remained constant over several years. Measurements have also been made on other types of Si, GaN, Ge, and InGaAs photodiodes. The measurement equipment, method, and results are presented.

Impact of energy band-gap grading on responsivity of an Al xGa 1− xN ultraviolet p–n detector

A theoretical study has been carried out on the responsivity of an Al x Ga 1− x N(n)–GaN(p) photodiode ultraviolet detector in which the Al x Ga 1− x N layer has an energy band-gap grading. The analytical solution to the one-dimensional continuity equation was used in the calculations. The spatial dependence of the material properties, such as the absorption coefficient and the energy band-gap of the photodiode n-type layer was included in the solution. In the calculations the minority carrier diffusion length and lifetime determined from a homogeneous n–p GaN photodiode were used. Since a moderate grading is considered this seems to be justified. The effect of n-type front layer thickness of energy band-gap graded photodiodes on the responsivity was analyzed and compared to that of the ungraded photodiodes. Compared to ungraded photodiodes an enhanced current responsivity and its much reduced dependence on graded layer thickness and on the surface recombination rate are found.

Avalanche breakdown and breakdown luminescence in p -π- n GaN diodes

The authors report the observation of electric breakdown in graded p-/spl pi/-n GaN photodiodes. The breakdown is accompanied by microplasma formation. The photocurrent strongly increases at the breakdown voltages, demonstrating a potential use of these devices as avalanche photodiodes (APDs).

High speed, low noise ultraviolet photodetectors based on GaN p-i-n and AlGaN(p)-GaN(i)-GaN(n)structures

We have investigated the spectral response of front-surface-illuminated GaN and AlGaN/GaN p-i-n ultraviolet photodetectors prepared by reactive molecular beam epitaxy on sapphire substrates. GaN homojunction p-i-n photodiodes exhibited a peaked response near the band edge. This enhanced response was absent in the AlGaN/GaN heterojunction p-i-n detectors. We analyzed the effect of p-layer thickness of the GaN p-i-n diodes on the magnitude of the peak photoresponse. The AlGaN/GaN photodiodes had a maximum zero-bias responsivity of 0.12 A/W at 364 nm, which decreased by more than 3 orders of magnitude for wavelengths longer than 390 nm. A reverse bias of −10 V raised the responsivity to 0.15 A/W without any significant increase in noise. The root-mean-square noise current in a 1 Hz bandwidth is ∼1.0 pA, corresponding to a noise-equivalent-power of ∼8.3 pW. We measured extremely fast decay times of 12 ns for the AlGaN/GaN and 29 ns for the GaN photodiodes.

Ultraviolet-sensitive, visible-blind GaN photodiodes fabricated by molecular beam epitaxy

GaN p–i–n photovoltaic diode arrays were fabricated from epitaxial films deposited on sapphire by molecular beam epitaxy. Peak UV responsivity was 0.11 A/W at 360 nm, corresponding to 48% internal quantum efficiency. Visible rejection over 400–800 nm was 3–4 orders of magnitude. Typical pulsed time response was measured at 8.2 μs. Spectral response modeling was performed to analyze the photocurrent contributions from photogenerated carrier drift in the depletion region and from minority carrier diffusion in the p and n layers. With the model, a maximum internal quantum efficiency of 55% at 360 nm was calculated for the photovoltaic diode structure.

8 × 8 GaN Schottky barrier photodiode arrayfor visible-blind imaging

The fabrication and characterisation of an 8 × 8 Schottky barrier photodiode array on GaN, with pixel size of 200 × 200 µm, are reported. This array shows an average peak response of 0.05 A/W and sharp cutoff at 360 nm. Decay time measurements reveal the response to be RC-limited with a time constant of 50 ns. The visible-blind UV-imaging capability of the GaN photodiode array is also demonstrated for the first time.