Reduction of Dark Current Under Reverse Bias in a-Si:H p-i-n Photodetectors

Abstract

This paper presents the development of low dark current amorphous silicon (a-Si:H) based heterojunction photodiodes. A series of p-i-n and n-i-p structures have been deposited by plasma-enhanced chemical vapor deposition (PECVD). Junction properties and carrier transport are investigated in terms of dark and light current-voltage characteristics, time dependence of the dark current, and spectral photoresponse measurements. It is demonstrated that a thin (∼4 nm) undoped a-SiC:H buffer layer introduced between the p and i layers reduces the leakage current and improves the diode ideality factor. A dark current density of ∼10 pA/cm2 at reverse bias of 1 V was achieved for the n-i-p structure. Optimization of device design for further improvement of dark current and photoresponse is discussed.

Large Area Digital X‐ray Imaging

“…Figure 1.1.22 shows the internal quantum efficiency of the n-i-d i -p diode as a function of wavelength. As can be seen, the recombination at the p/i interface limits the quantum efficiency in the 400-460 nm region [44]. At a wavelength of 550 nm, the quantum efficiency reaches the maximum value of 80% and subsequently degrades at higher wavelengths owing to decreasing absorption.…”

“…The dark current of the p-i-n photodiode is limited by a thermal generation current caused by the thermal excitation of electrons and holes from bulk gap states to the band edges [40,41]. Beside bulk thermal generation, the additional sources of the dark current are identified to be contact injection, edge leak-A1.1.3 Detectors 23 age, macrostructural shunt paths, and emission of the carriers from defect states at the p-i and i-n interfaces [42][43][44]. The a-SiC : H layer is deposited by the dissociation of a gas mixture of SiH 4 and CH 4 in a PECVD chamber, where the ratio of CH 4 /SiH 4 flow rates determines the band gap of the material, as shown in Figure 1.1.18.…”

“…The a-SiC : H layer is deposited by the dissociation of a gas mixture of SiH 4 and CH 4 in a PECVD chamber, where the ratio of CH 4 /SiH 4 flow rates determines the band gap of the material, as shown in Figure 1.1.18. For the p-type aSiC : H layers, the material is doped by addition of (*1%) trimethylborane (TMB), which has a lower defect density and higher photoresponse in comparison with samples deposited using diborane (B 2 H 6 ) [44]. In addition, band gap narrowing, which occurs with B 2 H 6 , is not observed in the TMB case as a function of the CH 4 /SiH 4 ratio [45].…”

“…The dark current-voltage characteristics at room temperature for the fabricated structures are shown in Figure 1.1.20 [44]. Although all the samples have identical intrinsic a-Si : H layer thickness, the dark currents are distinctly different.…”

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“…As the electric field increases, injection from the doped layer and the Poole-Frenkel effect in the high-field region (close to the p-i interface) become dominant. In order to minimize this sharp increase of leakage current with growing electric field, several solutions have been proposed in the literature: (a) An increase of the p-layer thickness [34,35], (b) the introduction of a double p-layer [36] or (c) a buried p-layer [35], or (d) the introduction of a buffer layer at the p-i interface [37].All these solutions can be easily implemented and require minimal modifications of the fabrication process. However, it is not yet clear which solution offers the lowest leakage current as interface properties (rather than the intrinsic or doped layer properties) usually play the major role in controlling leakage.…”