amorphous silicon carbide UV sensor

Journal of Non-Crystalline Solids 352 (2006) 1818–1821 www.elsevier.com/locate/jnoncrysol

Innovative window layer for amorphous silicon/amorphous silicon carbide UV sensor D. Caputo a

a,*

, G. de Cesare a, A. Nascetti a, M. Tucci

a,b

Department of Electronic Engineering, University of Rome, ‘La Sapienza’ via Eudossiana, 18 00184 Rome, Italy b Enea Res. Center, Casaccia via Anguillarese, 301 00060 S. Maria di Galeria, Rome, Italy Available online 29 March 2006

Abstract In this work, we utilize an ultra-thin (below 5 nm) chromium silicide film as window layer on the top of an UV photodetector. The sensor is based on hydrogenated amorphous silicon (a-Si:H) and silicon carbide (a-SiC:H) n–i–p structure deposited on glass substrate with a grid-shaped top-electrode. The innovative layer is formed on the top of the amorphous films and acts as a shunt of the p-layer. The chromium silicide film leads to two advantages: first, it avoids the forward self-biasing polarization of the region between finger electrodes and second, it eliminates the effect of the boron activation in the p-doped layer under UV radiation. Ó 2006 Elsevier B.V. All rights reserved. PACS: 73.64.Jc; 73.40. c; 42.79.Pw.Qx Keywords: Silicon; Heterojunctions; Sensors

1. Introduction Detection of ultraviolet (UV) and, in particular, of the vacuum ultraviolet (VUV) radiation is currently a scientific sector under development. Along with the traditional fields of applications (astrophysics observations from space [1], laser produced plasmas spectroscopy [2], monitoring of the pulsed laser used for photolitography [3], particle detection [4], dosimetry [5]), new fields related to biomedical applications are becoming relevant [6]. UV detectors are usually based on crystalline silicon device covered by UV antireflection coating for enhanced response [7], on diamond [4] based device for their solarblindness property or on micro-channel plates [8] for their excellent sensitivity. Amorphous silicon UV detectors have been presented in the last few years [9,10] in order to obtain a compact and low cost detector. These devices are based on p–i–n amorphous silicon (a-Si:H)/amorphous silicon

*

Corresponding author. Tel.: +39 06 4458 5832; fax: +39 06 4742647. E-mail address: [email protected] (D. Caputo).

0022-3093/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.09.052

carbide (a-SiC:H) heterojunction, where the radiation is impinging through a grid-shaped front-electrode. In this paper, we present an improvement of our previous device, which enhances the charge collection efficiency of the device and at the same time solves the metastability problems related to the UV induced dopant activation in the p-layer which was observed for the a-Si:H/a-SiC:H device [11]. The new device uses an innovative window layer, a very thin chromium silicide (CrSi) film obtained with technological processes at room temperature. 2. Device metastability In Fig. 1(a), the basic structure of the device, grown by plasma enhanced chemical vapor deposition (PECVD), is reported. In particular, we have deposited a stack of 50 nm thick a-Si:H n-type, 300 nm thick a-Si:H i-type and 5 nm thick a-SiC:H p-type. Radiation penetrates through an aluminum metal grid evaporated on the top of the structure. Due to the high absorption coefficient of a-SiC:H in the UV range, photogeneration occurs in the p-layer, which acts as active layer.

D. Caputo et al. / Journal of Non-Crystalline Solids 352 (2006) 1818–1821

UV light ΔV1

ΔV2

ΔV3

ΔV4

ΔV5

Iph

Iph

Iph

Iph

Iph

p-type i-type n-type

a) Vpin ΔV5 ΔV4 ΔV3

Voc

ΔV2 ΔV1

b)

x

Fig. 1. (a) Cross section of UV the photodiode, in proximity of a finger electrode. DV represents the potential drop along the p-type layer caused by the photocurrent flowing through it. (b) Qualitative behavior of the voltage across the n–i–p structure (Vpin) due to the forward self-biasing of the junction as a function of the distance from the grid electrode.

The device, as described above, suffers of two main problems. The first is due to the high resistivity of the p-layer, which causes a potential drop between the fingers of the grid electrode. As reported in Fig. 1(b), for a device biased in short circuit condition, the voltage across the p–i–n structure (Vpin) increases with the distance from the finger of the grid electrode. This potential drop leads to forward self-biasing of the device, negatively affecting the collection efficiency and leads to a limit of the ratio between the active area and the device area (fill factor of the grid). It should be noted here that due to this phenomenon, the extent of the area which actually contributes to the output signal depends on the intensity of the impinging radiation and therefore, this effect introduces a non-linearity in the sensor response. The second issue is related to the boron activation induced by the UV radiation, which causes a decrease of the a-SiC:H p-layer resistivity. This variation depends on the light intensity and leads to an increase of collection efficiency during the device operation. This effect is metastable and recovers, when the light is turned off, in a few tens of minutes. It is evident that such metastability significantly hinders any practical application of this device. 3. Device structure In order to overcome both issues mentioned before, we introduced a high conductivity chromium silicide film on the top of the silicon carbide p-layer. The idea behind this innovative device structure is to reduce the resistivity by introducing a surface shunt on the p-layer. Formation of CrSi film occurs at room temperature at the interface between silicon and chromium. In particular, the chromium wet etching performed with a solution of 30 g ceric ammonium nitrate, 9 ml glacial acetic acid and 200 ml deionized water, is not able to remove the alloy film.

The thickness of the silicide layer is below 5 nm as calculated from the reactive ion etching time and the etch rate data of our system. Though very thin, the CrSi layer obtained on a single n-doped layer grown on a 7059 corning glass substrate shows a resistance between two coplanar electrodes six orders of magnitude lower than the one of the p-layer. After several tests on amorphous silicon materials, we found that the chromium silicide is present when the chromium is evaporated on a n-type film, while its formation is inhibited on p-type or i-type material. In order to have a CrSi layer on top of the UV sensor the original device structure was modified adding two technological steps. The first is the deposition of a ultra-thin (close to 1 nm) n-doped amorphous silicon film (dn) on the top p-type layer. This was achieved with a 5s PECVD process at 180 °C with the same recipe reported in [10]. The second step is the thermal evaporation of a three-layer chromium–aluminum–chromium (15/300/15 nm) stack as top metal electrode. This stack allows to reduce the sheet resistance of the metal contact while preventing aluminum diffusion into amorphous silicon and reducing the mechanical stress associated with thick chromium films. From a technological point of view, the new structure does not introduce further photolithographic steps and the whole device fabrication requires the same mask set utilized for the previous device. In our process the patterning of the grid electrode is done before the mesa process that defines the pixel area in order to reduce the contaminations on the sensor side-walls. In Fig. 2 the current–voltage characteristics of a 1 mm2 device are reported. It is worth pointing out here that the presence of the p–n junction on top of the structure (glass/electrode/n–i–p–dn/electrode) does not affect the electrical characteristics of the n–i–p diode. In fact, when positive voltage is applied to the grid electrode the current

10 -6 10 -7 10 -8 10 -9 Current (A)

Grid finger

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10-10 10-11 10-12 10-13 10-14 -0.8

-0.6

-0.4

-0.2 0 0.2 Voltage (V)

0.4

0.6

0.8

Fig. 2. Current–voltage curve of the n–i–p–dn–CrSi structure under dark condition. The device area is 1 mm2.

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increases as expected in a forward biased junction without any evidence of double-diode characteristic. The high defect density of the doped materials ensures the formation of an ohmic contact between the two top doped layers [12].

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4. Results

40

Photocurrent (nA)

From our experiments we realized several devices with different grid electrode geometries. In particular, for each sensor the finger width has been set to 50 lm, while the distance between fingers was varied. In Fig. 3 we report the quantum efficiency (QE) of three 4 mm2 devices on which the CrSi layer is not present. The measurement has been performed in the range 200–300 nm under short circuit condition for values of finger distance equal to 50 lm (diamonds), 100 lm (circles), 200 lm (squares). The percentage increase of QE is equal to percentage increase of the illuminated area for the two devices with 50 and 100 lm finger distance indicating that the collection efficiency is equal for these grid geometries and radiation intensity. A tendency to saturation of QE values is evident for devices with finger distance greater than 100 lm, indicating that the increase of the photocurrent with the illuminated area is balanced by a reduction of the collection efficiency. These results demonstrate that for the given radiation intensity, the extension of the charge collection distance around the grid fingers is less than 100 lm. The behavior described above is not present when the CrSi window layer is added over the device structure. In Fig. 4 we report the photocurrent of a set of 4 mm2 area devices in short circuit condition as a function of the grid fill factor (1 means no grid at all). The radiation intensity was about 0.5 lW centered at 253.4 nm. The observed lin-

45

20 0.5

0.6

0.7

0.8 Fill Factor

0.9

1

1.1

Fig. 4. Short circuit photocurrent of 4 mm2 devices including the chromium silicide film reported as a function of the fill factor of the grid electrode. Fill factor equal to 1 means absence of the grid. The radiation intensity was about 0.5 lW centered at 253.4 nm. The solid line is a linear interpolation.

ear increase states that for all the devices the whole illuminated area contributes in the same way to the photocurrent: the collection efficiency is no longer a function of the distance from the grid fingers. This proves that the chromium silicide acts effectively as an UV transparent shunt layer, ensuring that even for the device without grid electrode (i.e., with a contact ring along the device perimeter) the potential drop between the border and the center of the device, equal to 1 mm, is negligible.

4

200 μm 100 μm 50 μm

3 Normalized Photocurrent

0.18 Quantum Efficiency

30

25

0.2

0.16 0.14

0.12

0.1 0.08 200

35

n-i-p-δn-CrSi n-i-p

3

2

2

1

220

240 260 Wavelength (nm)

280

300

Fig. 3. Quantum efficiencies in the UV range of 4 mm2 devices fabricated without the chromium silicide layer. The different curves refer to different finger grid distances. The width of the metal finger is 50 lm for all the sensors.

1 100

101

102

103

104

Time (s) Fig. 5. Time evolution of the photocurrent under 253.4 nm radiation for devices with (circles) and without (diamonds) the chromium silicide window layer. Each photocurrent has been normalized to its initial value.

D. Caputo et al. / Journal of Non-Crystalline Solids 352 (2006) 1818–1821

From the design point of view, this innovative window layer allows to use a larger distance between the finger electrodes, leading to a more favorable ratio between active area and the sensor area and therefore, to an increase of sensor efficiency. The presence of the CrSi layer allows to solve also the second issue of the previous device, related to the metastability of the boron activation under UV radiation. In Fig. 5 the time evolution of the photocurrent under UV radiation for two devices, with (filled circles) and without (open circles) the chromium silicide is reported. Both curves are normalized to their initial values. The photocurrent of the old device increases due to the boron activation of the p-doped layer, while in the new one the photocurrent does not change with time. The stability of the new device is due to the presence of the CrSi film, which hides the resistivity variation of the p-doped layer under UV light. 5. Conclusion A very thin (below 5 nm) chromium silicide layer has been utilized as window layer on the top of a n–i–p aSi:H/a-SiC:H UV photodiode in order to increase sensor performances. The photodiode is a glass/TCO/n–i–p–dn/ metal a-Si:H structure where the radiation is impinging on the grid electrode. The chromium silicide film allows to overcome problems related to the high resistivity of

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the p-layer and to boron activation under UV radiation. In fact, the chromium silicide acts as a surface shunt for the high resistivity p-layer avoiding forward self-biasing polarization of the region between the finger electrodes and hiding the resistivity variation of the p-doped layer under UV light. References [1] M.P. Ulmer, M. Razeghi, E. Bigan, Proc. SPIE 2397 (1995) 210. [2] J.B. Morris, B.E. Forch, A.W. Miziolek, Appl. Spectrosc. 44 (1990) 1040. [3] Webpage. [4] A. Mainwood, Semicond. Sci. Techol. 15 (2000) 55. [5] L.E. Quintern, Y. Furusawa, F. Fukutsu, H. Holtschmidt, J. Photochem. Photobiol. B 37 (1997) 158. [6] X. Zhongqi, N. Tsutomu, A. Akihiro, H. Takeshi Hirokawa, Electrophoresis 25 (2004) 3875. [7] A. Ghazi, H. Zimmermann, P. Seegebrecht, IEEE Trans. Electron Dev. 49 (2002) 1124. [8] J.L. Wiza, Nucl. Instrum. Methods 162 (1979) 587. [9] F. Mutze, K. Seibel, B. Schneider, M. Hillebrand, F. Blecher, T. Lule, H. Keller, P. Rieve, M. Wagner, M. Bohm, Mater. Res. Soc. Symp. Proc. 557 (1999) 815. [10] G. de Cesare, F. Irrera, F. Palma, M. Tucci, E. Jannitti, G. Naletto, P. Nicolosi, Appl. Phys. Lett. 67 (1995) 335. [11] D. Caputo, G. de Cesare, F. Irrera, M. Tucci, J. Non-Cryst. Solids 227–230 (1998) 1316. [12] R.A. Street, in: Hydrogenated Amorphous Silicon, Cambridge University Press, 1991.