Hole concentration test of p-type GaN by analyzing the spectral response of p-n(+) structure GaN ultraviolet photodetector

A new method to test the hole concentration of p-type GaN is proposed, which is carried out by analyzing the spectral response of p-n(+) structure GaN ultraviolet photodetector. It is shown that the spectral response of the photodetector changes considerably with reversed bias. It is found that the difference between photodetector's quantum efficiency at two wavelengths, i.e. 250 and 361 nm, varies remarkably with increasing reversed bias. According to the simulation calculation, the most characteristic change occurs at a reversed voltage under which the p-GaN layer starts to be completely depleted. Based on this effect the carrier concentration of p-GaN can be derived.

A new method to reduce the dark current of GaN based Schottky barrier ultraviolet photodetector

A new method to reduce the dark current of GaN based Schottky barrier ultraviolet photodetector is proposed. In comparision with conventional i-CaN/n(+)-GaN structure, an additional thin p-GaN cap layer is introduced on the i-GaN(n(-)-GaN) in the new structure. The simulation results showed that the additional layer makes the dark current to decrease in the photodetector due to the increase of the Schottky barrier height. The effects of thickness and carrier concentration of p-GaN layer on the dark current of the photodetector were also studied. It is suggested that the dark current of the new structure device could be better reduced by employing p-GaN with higher carrier concentration as the cap layer.

A new p-n structure ultraviolet photodetector with p(-)-GaN active region

A new ultraviolet photodetector of employing p menus type GaN (p(-)-GaN) as the active layer is proposed. It is easy to obtain the p(-)-GaN layer with low carrier concentration. As a result, the depletion region can be increased and the quantum efficiency can be improved. The influence of some structure parameters on the performance of the new device is investigated. Through the simulation calculation, it is found that the quantum efficiency increases with the decrease of the barrier height between the metal electrode and the p(-)-GaN layer, and it is also found that the quantum efficiency can be improved by reducing the thickness of the p(-)-GaN layer. To fabricate the new photodetector with high performance, we should employ thin p(-)-GaN layer as the active layer and reduce the Schottky barrier height.

A new Schottky barrier structure of GaN-based ultraviolet photodetector

A new GaN-based ultraviolet photodetector with Schottky barrior structure is proposed. Comparied with the conventional i-GaN/n(+) -GaN structure, there is an additional thin n-AlGaN cap layer on the i-GaN in the new structure. The simulation result demonstrates that the new structure leads to an increased quantum efficiency in GaN photodetection, since the negative effect of surface states on the photodetector is reduced in the new structure. In addition, it is suggested that the performance of device with the new structure could be further improved by employing an even thinner AlGaN cap layer with higher carrier concentration.

Performance improvement of GaN based Schottky barrier ultraviolet photodetector by adding a thin AlGaN window layer

We propose a new structure of GaN based Schottky barrier ultraviolet photodetector, in which a thin n-type AlGaN window layer is added on the conventional n(-)-GaN/n(+)-GaN device structure. The performance of the Schottky barrier ultraviolet photodetector is found to be improved by the new structure. The simulation result shows that the new structure can reduce the negative effect of surface states on the performance of Schottky barrier GaN photodetectors, improving the quantum efficiency and decreasing the dark current. The investigations suggest that the new photodetector can exhibit a better responsivity by choosing a suitably high carrier concentration and thin thickness for the AlGaN window layer.

Metal-semiconductor-metal ultraviolet photodetector based on GaN

A metal-semiconductor-metal (MSM) ultraviolet photodetector has been fabricated using unintentionally doped n-GaN films grown on sapphire substrates. Its dark current, photocurrent under the illumination with lambda = 360 nm light, responsivity, and the dependence of responsivity on bias voltage were measured at room temperature. The dark current of the photodetector is 1.03 nA under 5 V bias, and is 15.3 nA under 10 V bias. A maximum responsivity of 0.166 A/W has been achieved under the illumination with lambda = 366 nm light and 15 V bias. It exhibits a typical sharp band-edge cutoff at the wavelength of 366 nm, and a high responsivity at the wavelength from 320 nm to 366 nm. Its responsivity under the illumination with lambda = 360 nm light increases when the bias voltage increases.

High responsivity ultraviolet photodetector based on crack-free GaN on Si (111)

A Schottky-based metal-semiconductor-metal photodetector is fabricated on 1 mu m-thick, crack-free GaN on Si (I 11) substrate using an optimized AlxGal-xN/AlN complex buffer layer. It exhibits a high responsivity of 4600A/W at 366nm which may be due to both a crack-free sample and high internal gain. The relationship between responsivity and bias voltage is also investigated. The experiment results indicate that the responsivity increases with the bias voltage and shows a tendency to saturate. (c) 2007 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim.

Role of Ga vacancies in enhancing the leakage current of GaN Schottky barrier ultraviolet photodetectors

The leakage current of GaN Schottky barrier ultraviolet photodetectors is investigated. It is found that the photodetectors adopting undoped GaN instead of lightly Si-doped GaN as an active layer show a much lower leakage current even when they have a higher dislocation density. It is also found that the density of Ga vacancies in undoped GaN is much lower than in Si-doped GaN. The Ga vacancies may enhance tunneling and reduce effective Schottky barrier height, leading to an increase of leakage current. It suggests that when undoped GaN is used as the active layer, it is necessary to reduce the leakage current of GaN Schottky barrier ultraviolet photodetector.

Effect of p-GaN layer thickness on the performance of p-i-n structure GaN ultraviolet photodetectors

We investigated the influence of thickness of p-GaN layer on the performance of p-i-n structure GaN ultraviolet photodetector. Through the simulation calculation, it was found that both the quantum efficiency and dark current of device decrease when employing thicker p-GaN layer, while both the quantum efficiency and dark current increase with decreasing thickness of p-GaN layer. It is suggested that the Schottky contact junction between the metal and p-GaN may be responsible for the incompatible effect. We has to make a suitable choice of the thickness of p-GaN in the device design according to the application requirement.

Homoepitaxial growth of 4H-SiC multi-epilayers and its application to UV detection

Homoepitaxial growth of 4H-SiC p(+)/pi/n(-) multi-epilayer on n(+) substrate and in-situ doping of p(+) and pi-epilayer have been achieved in the LPCVD system with SiH4+C2H4+H-2. The surface morphologies, homogeneities and doping concentrations of the n(-)-single-epilayers and the p(+)/pi/n(-) multi-epilayers were investigated by Nomarski, AFM, Raman and SIMS, respectively. AFM and Raman investigation showed that both single- and,multi-epilayers have good surface morphologies and homogeneities, and the SIMS analyses indicated the boron concentration in p+ layer was at least 100 times higher than that in pi layer. The UV photodetectors fabricated on 4H-SiC p(+)/pi/n(-) multi-epilayers showed low dark current and high detectivity in the UV range.

A GaN photodetector integrated structure for wavelength characterization of ultraviolet light

An interesting GaN photodetector structure, which can be used for characterizing the wavelength of incident ultraviolet light, is proposed. It is composed of two back-to-back integrated diodes, i.e. p-n and p-i-n GaN ultraviolet photodiodes with different spectral response. The wavelength of monochromatic ultraviolet light could be identified by measuring the photocurrent ratio value through a simple electronic circuit.

Investigation of responsivity decreasing with rising bias voltage in a GaN Schottky barrier photodetector

The unexpected decrease in measured responsivity observed in a specific GaN Schottky barrier photodetector (PD) at high reverse bias voltage was investigated and explained. Device equivalent transforms and small signal analysis were performed to analyse the test circuit. On this basis, a model was built which explained the responsivity decrease quantitatively. After being revised by this model, responsivity curves varying with bias voltage turned out to be reasonable. It is proved that the decrease is related to the dynamic parallel resistance of the photodiode. The results indicate that with a GaN Schottky PD, the choice of load resistance is restricted according to the dynamic parallel resistance of the device to avoid responsivity decay at high bias voltage.

Influence of Mg content on molecular-beam-epitaxy-grown ZnMgS ultraviolet photodetectors

Zn1-xMgxS-based Schottky barrier ultraviolet (UV) photodetectors were fabricated using the molecular-beam-epitaxy (MBE) technique. The influence of Mg content on MBE-grown Zn1-xMgxS-based UV photodetectors has been investigated in details with a variety of experimental techniques, including photoresponse (PR), capacitance-voltage, deep level transient Fourier spectroscopy (DLTFS) and photoluminescence (PL). The room-temperature PR results show that the abrupt long-wavelength cutoffs covering 325, 305 295. and 270 nm with Mg contents of 16%, 44%, 57%, and 75% in the Zn1-xMgxS active layers, respectively, were achieved. But the responsivity and the external quantum efficiency exhibited a slight decrease with the Mg content increasing. In good agreement with the PR results, both of the integrated intensity of the PL spectra obtained from Zn1-xMgxS thin films with different Mg compositions (x = 31% and 52%, respectively) and the DLTFS spectra obtained from Zn1-xMgxS-based (x = 5% and 45%, respectively) UV photodetector samples clearly revealed a significant concentration increase of the non-radiative deep traps with increasing Mg containing in the ZnMgS active layers. Our experimental results also indicate that the MBE-grown ZnMgS-based photodetectors can offer the promising characteristics for the detection of short-wavelength UV radiation. (C) 2004 Elsevier B.V. All rights reserved.

Fabrication and characterization of metal-semiconductor-metal (MSM) ultraviolet photodetectors on undoped GaN/sapphire grown by MBE

Interdigital metal-semiconductor-metal (MSM) ultraviolet photoconductive detectors have been fabricated on undoped GaN films grown by molecular beam epitaxy (MBE), Response dependence on wavelength, applied current, excitation powers and chopper frequency has been extensively investigated. It is shown that the photodetector's spectral response remained nearly constant for wavelengths above the band gap and dropped sharply by almost three orders of magnitude for wavelengths longer than the band gap. It increases linearly with the applied constant current, but very nonlinearly with illuminating power. The photodetectors showed high photoconductor gains resulting from trapping of minority carriers (holes) at acceptor impurities or defects. The results demonstrated the high quality of the GaN crystal used to fabricate these devices. (C) 2000 Elsevier Science B.V. All rights reserved.

Vertical PIN ultraviolet photodetectors based on 4H-SiC homoepilayers

Vertical PIN ultraviolet photodetectors based on 4H-SiC homoepilayers are presented. The growth of the 4H-SiC homoepilayers was carried out in a LPCVD system. The size of the active area of the photodetector was 300 x 300 mu m(2). The dark and illuminated I-V characteristics were measured at reverse biases from 0 V to 30 V at room temperature. The illuminated current was at least two orders of magnitude higher than the dark current at a bias of below 12 V. The photoresponse was measured from 200 nm to 400 nm at different reverse biases and the peak values of the photo response were located at 3 10 nm. The calculated spectral detectivity D* was shown to be higher than 10(13) cmHz(1/2)/W from 260 to 360 nm with a peak value of 5.9 x 10(13) cmHz(1/2) /W at 310 nm. The peak value of the photoresponse was hundreds of times higher than the response at 400 nm, which showed the device had good visible blind performance. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.