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crystalline silicon heterostructure
Sensors and Actuators A 88 (2001) 139±145
Characterisation and modelling of a two terminal visible/infrared photodetector based on amorphous/crystalline silicon heterostructure D. Caputo, G. de Cesare, M. Tucci* Department of Electronic Engineering, via Eudossiana, 18, 00184 Rome, Italy Received 9 March 2000; received in revised form 12 September 2000; accepted 9 October 2000
Abstract In this work, we report on the detailed characterisation of a two terminal visible/infrared tuneable photodetector both in steady state and transient operation. The device consists of a n-doped amorphous silicon/intrinsic amorphous silicon/p-doped amorphous silicon carbide multilayer structure grown by PECVD on a p-type crystalline silicon wafer doped by a phosphorus diffusion. The steady state behaviour is analysed by an analytical model, which takes into account the absorption coef®cients, the diffusion length, the mobility±lifetime product and the layer thicknesses and allows us to obtain quantitative information on the materials inside the device. The linearity of the detector response is also presented. On the basis of the transient response measured by means of a test circuit simulating the row select TFT and the data line capacitance by discrete components, driving of 2-D array of VIPs is discussed. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Photodetector; Amorphous silicon; Heterostructure; Image scanner; Array
1. Introduction The possibility to realise a two terminal device able to detect light in different spectral ranges upon tuning with the external voltage is a very appealing perspective [1±7]. Heterostructure realised by deposition of a wide band-gap semiconductor on a narrow band-gap one is a way to obtain a stacked biascontrolled photodetector working in a wide spectral range. By using this idea various devices have been presented in literature. In particular, amorphous silicon (a-Si:H)/amorphous silicon carbide (a-SiC:H) ®lms have been used to realise two colour detector tuneable in the blue or red region of the visible spectrum [4] and three colour detector involving three stacked cell of amorphous silicon layers sensitive to the red, green and blue components of the visible spectrum [2,5]. In this work, we characterise in detail the response of a variable spectral response photodetector consisting of n-type a-Si:H/i-type a-Si:H/p-type a-SiC:H multilayer grown by PECVD on a p-type crystalline silicon wafer doped by a phosphorus diffusion [8]. Crystalline silicon (c-Si) used as substrate allows to extend the conversion ef®ciency of amorphous silicon photo-sensor to the near infrared, where the a-Si:H is transparent. * Corresponding author. Present address: ENEA Research Center, LocalitaÁ Granatello, Portici, 80055 Napoli, Italy. Tel.: 39-81-772-3312; fax: 39-81-772-3344. E-mail address: [email protected] (M. Tucci).
In Section 2, device structure and operation are presented. In Section 3, an analytical device model which takes into account the absorption coef®cients and thicknesses of the different materials, the diffusion length of electron in crystalline silicon and mobility±lifetime product of both carriers in undoped amorphous material is described. In Section 4, details of the device fabrication and measurement set-ups for both the steady state and transient characterisation are reported. In Section 5, current±voltage characteristics and quantum ef®ciency curves under different illuminations and bias voltages are reported and reproduced with the presented analytical model. In the same section results of transient measurements are discussed focusing on the array dimensions which can be fabricated with the visible±infrared photodetector (VIP). 2. Device structure and operation A sketch of the device structure is reported in Fig. 1. The device consists of a crystalline silicon p±n junction on which is stacked an amorphous silicon n±i±p structure. The device behaves like a back-to-back diode. The applied voltage forward biases one diode and reverse biases the other one. At V A 2 V the crystalline p±n junction is reversely biased and limits the current in the device. The visible spectrum of the impinging light is absorbed by the amorphous layers and only the long-wavelength region of
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where q is the electron charge, A the device area, Ga(l) the number of electron±hole pair photo-generated per unit volume per second, Vbin the built-in voltage of the p±i±n junction, VA the applied voltage, Vg,c the potential drop across the forward biased crystalline junction and di the thickness of the intrinsic layer. Following (1), the slope of the Iph,a (l,VA) at low bias voltage depends on the mobility± lifetime (mt)a product that is related to the drift diffusion length of the electrons (ln) and holes (lp) in the intrinsic layer p 2 lp ln (2) mta E where E is the electric field assumed constant along the ilayer. The error introduced with this assumption is small [12]. The generation term Ga(l) per unit volume per second in the intrinsic amorphous layer is a function of wavelength, l, by the following relation:
Fig. 1. Section of the photodetector (VIP).
Ga l the spectrum radiation reaches the c-Si junction. Then the measured current is due to the carriers photogenerated in the p-doped crystalline material, where they move by diffusion. On the other side, when the amorphous p±i±n diode is reversely biased (V A ÿ2 V) the measured current is due to the carriers photo-generated in the a-Si:H intrinsic layer where they move by drift. Short-wavelength light is then detected. It is worth to focus on the role of the thicknesses of the aSi:H n-doped and intrinsic layers. The ®rst acts as a ®lter layer reducing the responsivity in the long wavelength region of the crystalline diode, and hence, increasing the separation between the two spectral responses of the device, the second is a trade-off between sensitivity and selectivity, since it allows to have a good response in the blue±green spectral region, and a good transparency to red and near infrared spectral region. 3. Analytical model of the device The steady state behaviour of the device is described by an analytical model taking into account absorption coef®cients and thicknesses of the different materials, diffusion length in the c-Si and the mobility±lifetime product in the amorphous silicon layers. In particular, for the amorphous p±i±n structure we refer to the Crandall analytical relation between photocurrent and applied voltage at ®xed wavelength [12]. This relation relates the reduction in the photocurrent at low ®eld due to electron and hole recombination in the undoped i-layer as given below: mta Iph;a l; VA qAGa l Vbin ÿ VA Vg;c di ÿd 2 = Vbin ÿ VA Vg;c mta 1ÿe i
(1)
N lTTCO l ÿaa-SiC ldp e 1 ÿ eÿaa-Si ldi di
(3)
in which we assume that light is uniformly absorbed and then the diffusion length can be neglected in the i-layer [12]. N(l) is the number of incident photons per square centimetre, TTCO(l) the transmittance of the TCO electrode, aaSiC(l) and aa-Si(l) are the absorption coefficients of the aSiC:H window layer and of the a-Si:H layer, respectively, and dp the thickness of the p-doped layer. The photocurrent in the crystalline silicon can be expressed as a sum of two terms: a constant photocurrent, taking into account the ®ltering of the p±i±n amorphous layers, and a recombination in the depletion region due to defects induced by the diffusion step dc VA ni h ÿ VA ÿVg;a =2VT i e Iph;c l; VA qA Gc lW ÿ 2tc (4) where Gc(l) is the generation per unit volume per second in the crystalline diode, W the thickness of the crystalline diode, ni the intrinsic concentration in the crystalline material, dc(VA) the depletion region in the crystalline diode as a square root function of the applied voltage; tc the lifetime in the depletion region, Vg,a the potential drop across the forward biased amorphous junction and VT the thermal equivalent voltage. The generation term Gc(l) is expressed by the following equation: Gc l
N lTTCO l ÿaa-SiC dp ÿaa-Si di dn ÿac-Si dn0 e W eÿac-Si dc VA 1ÿ 1 ac-Si Lc
(5)
where dn is the thickness of n-doped a-Si:H and dn0 the portion of the n-doped c-Si in which does not extend the depletion region of the crystalline junction. This layer represents a dead layer for the photogeneration process because of its poor hole diffusion length coefficient due
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to the used emitter solar cell diffusion technique for n doped layer and then can be considered as a filter for the incident light. Its thickness has been used as fitting parameter in our model. Lc is the diffusion length in the crystalline bulk material and ac-Si (l) is the absorption coefficient of the crystalline silicon [13]. Then the quantum yield (QY) of the device can be easily evaluated following the equation: QY l; VA
jIph;a l; VA ÿ Iph;c l; VA j qAN l
(6)
4. Experimental details
Fig. 2. Schematic view of the computer controlled experimental set-up utilised to measure the quantum efficiency of the VIP samples.
4.1. Device fabrication Ê A p±n crystalline junction was ®rst realised by a 3000 A diffusion of phosphorus in a p-type silicon h1 0 0i oriented wafer, with r 1 O cm. This diffusion process is the typical used in solar cell production. Particular care was paid in the cleaning procedure of the crystalline surface to avoid contamination and ensure a good ohmic contact at the heterostructure interface between the n-type crystalline silicon and n-type amorphous silicon [9,10]. In particular, the following two steps of cleaning procedure were adopted: 1. Chemical wet etching, in order to remove organic and metallic contamination and oxide layers. 2. High frequency dry etching by exposition to H2 plasma at 4008C just before the deposition of the amorphous layers. Hydrogenated a-Si and a-SiC layers were deposited on the c-Si substrate by Plasma Enhanced Chemical Vapour Deposition (PECVD) in a three chamber system. First, a thick (1 mm) a-Si:H n-doped layer was deposited and then a Ê thick a-Si:H intrinsic layer followed. A 50 A Ê thick 2800 A buffer-layer was deposited between a-Si:H intrinsic and aSiC:H p-type layers, progressively increasing the ¯ux of methane in order to decrease the band-gap mismatch at the interface. This latter buffer prevents the formation of electronic defects due to an abrupt mismatch at the band-edges. Finally, a a-SiC:H p-layer was deposited. The wide band-gap Ê ) of this layer (2 eV) [11] and the very thin thickness (50 A increase light transmittance and then maximise the absorption at shorter wavelength in the intrinsic region. In Table 1,
all the deposition parameters of the p±i±n amorphous layer are reported. A grid-shaped evaporated aluminium front contact with a sputtered transparent conductive oxide (TCO) SnO2 coating is used as window for the incident light. The back contact is realised with a screen printed silver±aluminium paste. 4.2. Steady state characterisations Characterisation of the device has been performed by current±voltage (I±V) and quantum ef®ciency responses in different illumination and bias voltages conditions. Measurements have been performed under a Spectrolab XF10 solar simulator ®ltered with a band-pass ®lter centred at 500 nm (open circles) and 750 nm (triangles) with a FWHM of 10 nm and transmitted intensity of 600 mW/cm2. These two wavelengths, 500 and 750 nm, have been chosen as representative of the detector response in the visible and infrared spectrum, respectively. The total area of the device is are 1 cm2 and the active area 0.2 cm2. In Fig. 2, the set-up utilised to perform the quantum ef®ciency measurement at room temperature is depicted. 4.3. Transient characterisations A detailed analysis of the device transient performance can be obtained with the test circuit shown in Fig. 3. The circuit emulates the driving conditions of a pixel in a matrix of photodetectors. Namely, switch f1 (row select input)
Table 1 Deposition parameters of the amorphous layers Layer
n i buffer p
Gas flow (sccm) SiH4
CH4
PH3
B2H6
40 40 40 40
± ± 0±60 60
10 ± ± ±
± ± ± 5
Pressure (Torr)
RF (mW/cm2)
Temperature (8C)
Ê) Thickness (A
0.3 0.68 0.7 0.7
28 28 28 28
200 200 200 200
10000 2800 50 50
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charge lost during the subsequent integration time. The balance between lost and re-fed charge determines the number of integration/readout cycles necessary to reach the stationary state. Once equilibrium condition is reached both junctions exchange the same amount of charge and thus the charge read out by the sense ampli®er is the integral of the photocurrent (or leakage current if the device is in dark condition) of the reverse biased junction. 5. Results and discussion 5.1. Steady state Fig. 3. Test circuit for the charge readout on VIP. The f switches are on with low logic value at the gate.
plays the role of a-Si:H TFT that might be integrated on array surface, together with the VIP. The hold capacitor CH emulates the capacitance of the dataline (column) connecting the selected pixel to a charge-sensitive readout ampli®er. The same circuit has been used to evaluate the transient response of an amorphous silicon three colour detector [14]. However, the measurements presented here differ from the previous because the investigated structure is an amorphous±crystalline silicon heterostructure for two colour detection to be used in different applications. The value of capacitor CH has been ®xed at 680 nF, which is much higher than the capacitances of the two junctions of the VIP. In fact, from the deposition parameters we estimated the geometrical values of these two capacitances at 35 and 10 nF for the top and bottom diode, respectively. Switch f2 connects CH to the readout circuit and switch f3 resets the integrated charge. The photosensor is initially precharged by applying a proper value of VA; when f1 switches off, the current ¯owing through the VIP is forced to be zero and the incident light tends to discharge the sensor capacitances. After a given integration time, f1 switches on allowing the recharge of the sensor up to VA and simultaneously sampling the same amount of charge on CH. When f2 switches on, the sampled charge is read out by the ampli®er. When f2 turns off, f3 switches on resetting the output node and the cycle repeats. A qualitative timing diagram is also reported in Fig. 3: we notice that only one switch is kept on at time and that light integration can begin as soon as the sampling operation on CH is terminated. However, these architecture and timing diagram lead to a correct measurement only after some cycles. In fact, at a ®xed value of the external bias VA, when the sensing cycle is repeated the charge stored in the two junctions changes, either in the dark or under illumination. During the integration time the two junctions independently lose charge since the device is under open circuit conditions. During a reading pulse, the two capacitances receive the same amount of charge which, before equilibrium, is different from the
In Fig. 4, experimental photocurrent±voltage characteristics of device are reported. Circles and triangles refer to measured photocurrent measured under 500 and 750 nm monochromatic light, respectively. The measured external quantum yields of the device are reported for two bias conditions (2 V) in Fig. 5. At negative bias voltages the quantum ef®ciency is the same of a single junction amorphous silicon solar cell, and the quantum yield extends from 400 to 700 nm with a peak around 500 nm. At 2 V, the spectral shape is that of a p±n c-Si cell optically ®ltered by a p±i±n a-Si:H, with a peak observed around 800 nm. Fig. 5 clearly shows the excellent spectral separation between the two photoresponses, and peak values over 80% in both cases. Solid lines shown in Figs. 4 and 5 are obtained by the analytical model discussed in Section 3. In order to reduce the number of ®tting parameters in Eqs. (1)±(5), we measured aa-SiC(l) and TTCO(l) on single layers deposited on glass with the same deposition conditions used in the fabrication of the device. Experimental results of absorption
Fig. 4. Experimental (dots) and simulated (continuous line) photocurrent± voltage characteristics of 1 cm2 device under 0.6 mW/cm2 monochromatic light.
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Fig. 7. Transmittance of sputtered TCO. Fig. 5. Experimental (dots) and simulated (continuous line) spectral response of the VIP at V bias 2 V, scaled for an active area of 0.2 cm2.
reported in Fig. 8. The slightly sublinear dependence, observed for both wavelengths can be ascribed to the presence of defects in the intrinsic layer of the amorphous diode and in the depletion region of the crystalline junction due to the diffusion process.
coef®cients and transmittance are shown in Figs. 6 and 7, respectively. Model results reported in Figs. 4 and 5 (solid line) have been achieved using the same set of parameters. In particular, a product mta 1:8 10ÿ9 cm2/V for the intrinsic amorphous layer, dn0 1:4 mm, tc 2 10ÿ9 s and Lc 190 mm for the crystalline material have been utilised. These values are comparable to those reported in literature for both materials [12,13]. The above values of the selected parameters minimise the chi-square merit ®gure. In particular, we found that a variation of 10% of each parameter leads to an increase of 30% of the chi-square. An excellent agreement between simulation and experimental is evident for both type of characterisation. This suggests that the presented model can then be utilised as a tool to optimise the performances of the device by an appropriate design of the thickness of the various layers. As a ®nal steady-state characterisation, the light intensity dependence has been evaluated at 500 and 750 nm and
The charge values read out by the sense ampli®er reported in Fig. 9 are obtained biasing the photodetector with 2 and ÿ2 V, respectively, under a constant illumination. The integration time was varied from 100 ms to long (>1 s) times under 0.6 mW/cm2 of the two illuminations and in dark conditions. For each integration time, the read out charge has been measured after the stationary condition is reached. As it can be seen, correct spectrum sensing can be obtained keeping the integration time under 1 ms. Longer integration times allow a correct light detection in the two bias regime. However, the longer the integration times the higher the dark signal, decreasing the effective S/N ratio of the VIP.
Fig. 6. Absorption coefficient profile of a-SiC:H layer.
Fig. 8. Linearity of the VIP at the two bias and light condition.
5.2. Transient
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The time required to effectively scan a row of pixels is given by the sum of the intervals in which f1 or f2 are set on. We point out that two scans are needed to get the visible (i.e. 500 nm) and infrared (i.e. 750 nm) components of the incident light. Each of these scans requires twice the selection of the whole array, row by row, to allow correct precharging, light integration and recharging/sensing of all the rows. This ®xes the array frame rate as tFRAME 2 2NtSCAN
(7)
where N is the number of the rows in the array. The size and capacitance of the pixel define the tSCAN and the integration time limits the maximum number of rows that can be used in arrays. On the other hand, the duration of the phase in which f1 is off, that is, the integration time tINT is Fig. 9. Experimental response of 1 cm2 VIP biased at 2 V (solid symbols) and at ÿ2 V (open symbols) under radiation at 500 nm (squares), 750 nm (triangles) and in dark conditions (circles).
Results reported in [14] show instead that longer integration times (at least >10 ms) are needed for a correct detection of the three different spectrum components. This is due to the different geometrical capacitance values of the junctions in the two structures. Of course, since the detector presented here allows a shorter integration time, a larger number of rows can be driven in a matrix application (see below). Following the above discussion, the different saturation levels of the readout charge for the 500 and 750 nm wavelengths are due to the different values of photocurrent induced in the reverse biased junction. It is worth noticing that in Fig. 9 there is a difference between dark readout charge and the visible one (500 nm) when the front diode is forward biased and back diode is reverse biased (V bias 2 V). The saturated read out charge is higher in dark condition than under 500 nm and a crossing of the two curves occurs around 10 ms. Furthermore, the read-out charge in dark condition and under 750 nm does not show a clear saturation as instead it occurs for the measurement under 500 nm. This lack of saturation is an anomalous behaviour since the readout charge has to saturate when the currents discharging the two stacked capacitances of the VIP (equivalent to the p±i±n and n±p structures) reach the value of the switch off leakage current. In particular, the integration time needed to reach saturation should decrease with increasing photocurrent. Instead, the curve obtained in dark condition and under 750 nm illumination seem to show the same increase in the read-out charge as due to some capacitor that is still charging almost linearly. At the moment we are not able to explain this behaviour, that has been observed also in different photodetectors driven in the same way [14], however, as discussed above, the correct operation of the device is ensured if 1 ms integration time is used.
tINT N ÿ 1tSCAN
tFRAME 4
(8)
The value of tFRAME is typically ®xed by the application and determines the useful integration times. In order to investigate these integration times, referring to the circuit of Fig. 3, the duration of the sampling phase with f1 on was ®xed at 8 ms. The readout (f2 on, f3 off) and reset (f2 off, f3 on) phases were kept long enough to fully expire the corresponding transients; the sum of these two phases gives the integration time (f1 off). Although in this work the crystalline substrate has been used only for the detection in the infrared radiation, it can also allow the integration of the driving circuitry of a 2-D photodetector array as reported in,1 where photodetectors are integrated in stacked con®guration with the crystalline substrate. From Fig. 9, taking into account the linearity behaviour reported in Fig. 8, we can note that ®xing the integration time at 1 ms, it is possible to reduce to 300 mW/cm2 the intensity of incident light continuing to obtain good spectral detection in both bias conditions. This intensity can be considered as the lowest intensity which allows to achieve an acceptable signal-to-noise ratio. 6. Conclusions An a-Si:H/c-Si heterostructure photosensor for the detection of visible and infrared light is realised, modelled and characterised both in steady state and transient operation. The device exhibits an excellent spectral separation controlled by a bias of a few volts, with QY 0:86 at l 480 nm and QY 0:8 at l 780 nm. The very good agreement between experimental and modelled results indicates that the reported analytical model can be used a tool to give both quantitative information's on the material quality inside the device and predictions for the optimisation of the device structure. In particular, a product 1
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D. Caputo et al. / Sensors and Actuators A 88 (2001) 139±145
mta 1:8 10ÿ9 cm2/V for the intrinsic amorphous layer, tc 2 10ÿ9 s and Lc 190 mm for the crystalline region have been extracted. A linear behaviour over two orders of magnitude of incident monochromatic radiation has been achieved. The transient behaviour of the VIP under 500 and 750 nm monochromatic varying the light integration time has been investigated. We found that for this device a correct detection is obtained using an integration time lower than 1 ms. On the basis on this results and on the linear response of the device, we found that is possible to reduce to 300 mW/cm2 the intensity of incident light continuing to obtain good spectral detection in both bias conditions.
References [1] H.K. Tsai, S.C. Lee, Amorphous SiC/Si three color detector, Appl. Phys. Lett. 52 (1988) 275. [2] H. Stiebig, C. Ulrichs, T. Kulessa, J. Folsch, F. Finger, H. Wagner, Transient photocurrent response of a-Si:H based three color nipin detector, J. Non-Cryst. Solids 198±200 (1996) 1185. [3] K. Eberhardt, T. Neidlinger, M. Schubert, Three color sensor based on amorphous n±i±p±i±n layer sequence, IEEE Trans. Electron Devices 42 (1995) 1763. [4] G. de Cesare, F. Irrera, F. Lemmi, F. Palma, Tunable photodetectors based on amorphous Si/SiC heterostructures, IEEE Trans. Electron Devices 42 (1995) 835.
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[5] G. de Cesare, F. Irrera, F. Lemmi, F. Palma, Amorphous Si/SiC threecolor detector with adjustable threshold, Appl. Phys. Lett. 66 (1995) 1178. [6] Y.K. Fang, J.B. Hwang, K.H. Cheng, C.R. Liu, M.J. Tsai, L.C. Kuo, An amorphous Sic/Si heterojunction p±i±n diode for low noise and high sensitivity UV detector, IEEE Trans. Electron Devices 39 (1992) 292. [7] D. Caputo, G. de Cesare, F. Irrera, F. Palma, Solar blind UV photodetectors for large area applications, IEEE Trans. Electron Device 43 (1996) 1351. [8] G. de Cesare, F. Galluzzi, F. Irrera, D. Lauta, F. Ferrazza, M. Tucci, Variable spectral response photodetector based on crystalline/ amorphous silicon heterostructure, J. Non-Cryst. Solids 198±200 (1996) 1189. [9] F. Rubinelli, S. Albornoz, R. Buitrago, Amorphous crystalline silicon isotype heterojunction: electrostatic potential distribution and C(V) curves, Solids-St. Electron. 28 (1985) 741. [10] M. Tanaka, M. Taguchi, T. Matsuyama, T. Sawada, S. Tsuda, S. Nakano, H. Hanafusa, Y. Kuwano, Development of new a-Si/c-Si heterojunction solar cells: ACJ-HIT (Artificial Constructed Junction Heterojunction with Intrinsic Thin Layer), Jpn. J. Appl. Phys. 31 (1992) 3518. [11] G. de Cesare, F. Galluzzi, G. Guattari, G. Leo, R. Vincenzoni, E. Bemporad, Structural, optical and electronic properties of wide band gap amorphous carbon±silicon alloys, Diam. Rel. Mat. 2 (1993) 773. [12] R.S. Crandall, Transport in hydrogenated amorphous silicon p±i±n solar cells, J. Appl. Phys. 53 (1982) 3350. [13] M.A. Green, M.J. Keevers, Optical properties of intrinsic silicon at 300 K, Prog. Photovoltaic: Res. Appl. 3 (1995) 189. [14] F. Irrera, F. Lemmi, F. Palma, Transient behaviour of adjustable threshold a-Si:H/a-SiC:H three color detector, IEEE Trans. Electron Devices 44 (1997) 1410.
Characterisation and modelling of a two terminal visible/infrared photodetector based on amorphous/crystalline silicon heterostructure D. Caputo, G. de Cesare, M. Tucci* Department of Electronic Engineering, via Eudossiana, 18, 00184 Rome, Italy Received 9 March 2000; received in revised form 12 September 2000; accepted 9 October 2000
Abstract In this work, we report on the detailed characterisation of a two terminal visible/infrared tuneable photodetector both in steady state and transient operation. The device consists of a n-doped amorphous silicon/intrinsic amorphous silicon/p-doped amorphous silicon carbide multilayer structure grown by PECVD on a p-type crystalline silicon wafer doped by a phosphorus diffusion. The steady state behaviour is analysed by an analytical model, which takes into account the absorption coef®cients, the diffusion length, the mobility±lifetime product and the layer thicknesses and allows us to obtain quantitative information on the materials inside the device. The linearity of the detector response is also presented. On the basis of the transient response measured by means of a test circuit simulating the row select TFT and the data line capacitance by discrete components, driving of 2-D array of VIPs is discussed. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Photodetector; Amorphous silicon; Heterostructure; Image scanner; Array
1. Introduction The possibility to realise a two terminal device able to detect light in different spectral ranges upon tuning with the external voltage is a very appealing perspective [1±7]. Heterostructure realised by deposition of a wide band-gap semiconductor on a narrow band-gap one is a way to obtain a stacked biascontrolled photodetector working in a wide spectral range. By using this idea various devices have been presented in literature. In particular, amorphous silicon (a-Si:H)/amorphous silicon carbide (a-SiC:H) ®lms have been used to realise two colour detector tuneable in the blue or red region of the visible spectrum [4] and three colour detector involving three stacked cell of amorphous silicon layers sensitive to the red, green and blue components of the visible spectrum [2,5]. In this work, we characterise in detail the response of a variable spectral response photodetector consisting of n-type a-Si:H/i-type a-Si:H/p-type a-SiC:H multilayer grown by PECVD on a p-type crystalline silicon wafer doped by a phosphorus diffusion [8]. Crystalline silicon (c-Si) used as substrate allows to extend the conversion ef®ciency of amorphous silicon photo-sensor to the near infrared, where the a-Si:H is transparent. * Corresponding author. Present address: ENEA Research Center, LocalitaÁ Granatello, Portici, 80055 Napoli, Italy. Tel.: 39-81-772-3312; fax: 39-81-772-3344. E-mail address: [email protected] (M. Tucci).
In Section 2, device structure and operation are presented. In Section 3, an analytical device model which takes into account the absorption coef®cients and thicknesses of the different materials, the diffusion length of electron in crystalline silicon and mobility±lifetime product of both carriers in undoped amorphous material is described. In Section 4, details of the device fabrication and measurement set-ups for both the steady state and transient characterisation are reported. In Section 5, current±voltage characteristics and quantum ef®ciency curves under different illuminations and bias voltages are reported and reproduced with the presented analytical model. In the same section results of transient measurements are discussed focusing on the array dimensions which can be fabricated with the visible±infrared photodetector (VIP). 2. Device structure and operation A sketch of the device structure is reported in Fig. 1. The device consists of a crystalline silicon p±n junction on which is stacked an amorphous silicon n±i±p structure. The device behaves like a back-to-back diode. The applied voltage forward biases one diode and reverse biases the other one. At V A 2 V the crystalline p±n junction is reversely biased and limits the current in the device. The visible spectrum of the impinging light is absorbed by the amorphous layers and only the long-wavelength region of
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where q is the electron charge, A the device area, Ga(l) the number of electron±hole pair photo-generated per unit volume per second, Vbin the built-in voltage of the p±i±n junction, VA the applied voltage, Vg,c the potential drop across the forward biased crystalline junction and di the thickness of the intrinsic layer. Following (1), the slope of the Iph,a (l,VA) at low bias voltage depends on the mobility± lifetime (mt)a product that is related to the drift diffusion length of the electrons (ln) and holes (lp) in the intrinsic layer p 2 lp ln (2) mta E where E is the electric field assumed constant along the ilayer. The error introduced with this assumption is small [12]. The generation term Ga(l) per unit volume per second in the intrinsic amorphous layer is a function of wavelength, l, by the following relation:
Fig. 1. Section of the photodetector (VIP).
Ga l the spectrum radiation reaches the c-Si junction. Then the measured current is due to the carriers photogenerated in the p-doped crystalline material, where they move by diffusion. On the other side, when the amorphous p±i±n diode is reversely biased (V A ÿ2 V) the measured current is due to the carriers photo-generated in the a-Si:H intrinsic layer where they move by drift. Short-wavelength light is then detected. It is worth to focus on the role of the thicknesses of the aSi:H n-doped and intrinsic layers. The ®rst acts as a ®lter layer reducing the responsivity in the long wavelength region of the crystalline diode, and hence, increasing the separation between the two spectral responses of the device, the second is a trade-off between sensitivity and selectivity, since it allows to have a good response in the blue±green spectral region, and a good transparency to red and near infrared spectral region. 3. Analytical model of the device The steady state behaviour of the device is described by an analytical model taking into account absorption coef®cients and thicknesses of the different materials, diffusion length in the c-Si and the mobility±lifetime product in the amorphous silicon layers. In particular, for the amorphous p±i±n structure we refer to the Crandall analytical relation between photocurrent and applied voltage at ®xed wavelength [12]. This relation relates the reduction in the photocurrent at low ®eld due to electron and hole recombination in the undoped i-layer as given below: mta Iph;a l; VA qAGa l Vbin ÿ VA Vg;c di ÿd 2 = Vbin ÿ VA Vg;c mta 1ÿe i
(1)
N lTTCO l ÿaa-SiC ldp e 1 ÿ eÿaa-Si ldi di
(3)
in which we assume that light is uniformly absorbed and then the diffusion length can be neglected in the i-layer [12]. N(l) is the number of incident photons per square centimetre, TTCO(l) the transmittance of the TCO electrode, aaSiC(l) and aa-Si(l) are the absorption coefficients of the aSiC:H window layer and of the a-Si:H layer, respectively, and dp the thickness of the p-doped layer. The photocurrent in the crystalline silicon can be expressed as a sum of two terms: a constant photocurrent, taking into account the ®ltering of the p±i±n amorphous layers, and a recombination in the depletion region due to defects induced by the diffusion step dc VA ni h ÿ VA ÿVg;a =2VT i e Iph;c l; VA qA Gc lW ÿ 2tc (4) where Gc(l) is the generation per unit volume per second in the crystalline diode, W the thickness of the crystalline diode, ni the intrinsic concentration in the crystalline material, dc(VA) the depletion region in the crystalline diode as a square root function of the applied voltage; tc the lifetime in the depletion region, Vg,a the potential drop across the forward biased amorphous junction and VT the thermal equivalent voltage. The generation term Gc(l) is expressed by the following equation: Gc l
N lTTCO l ÿaa-SiC dp ÿaa-Si di dn ÿac-Si dn0 e W eÿac-Si dc VA 1ÿ 1 ac-Si Lc
(5)
where dn is the thickness of n-doped a-Si:H and dn0 the portion of the n-doped c-Si in which does not extend the depletion region of the crystalline junction. This layer represents a dead layer for the photogeneration process because of its poor hole diffusion length coefficient due
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to the used emitter solar cell diffusion technique for n doped layer and then can be considered as a filter for the incident light. Its thickness has been used as fitting parameter in our model. Lc is the diffusion length in the crystalline bulk material and ac-Si (l) is the absorption coefficient of the crystalline silicon [13]. Then the quantum yield (QY) of the device can be easily evaluated following the equation: QY l; VA
jIph;a l; VA ÿ Iph;c l; VA j qAN l
(6)
4. Experimental details
Fig. 2. Schematic view of the computer controlled experimental set-up utilised to measure the quantum efficiency of the VIP samples.
4.1. Device fabrication Ê A p±n crystalline junction was ®rst realised by a 3000 A diffusion of phosphorus in a p-type silicon h1 0 0i oriented wafer, with r 1 O cm. This diffusion process is the typical used in solar cell production. Particular care was paid in the cleaning procedure of the crystalline surface to avoid contamination and ensure a good ohmic contact at the heterostructure interface between the n-type crystalline silicon and n-type amorphous silicon [9,10]. In particular, the following two steps of cleaning procedure were adopted: 1. Chemical wet etching, in order to remove organic and metallic contamination and oxide layers. 2. High frequency dry etching by exposition to H2 plasma at 4008C just before the deposition of the amorphous layers. Hydrogenated a-Si and a-SiC layers were deposited on the c-Si substrate by Plasma Enhanced Chemical Vapour Deposition (PECVD) in a three chamber system. First, a thick (1 mm) a-Si:H n-doped layer was deposited and then a Ê thick a-Si:H intrinsic layer followed. A 50 A Ê thick 2800 A buffer-layer was deposited between a-Si:H intrinsic and aSiC:H p-type layers, progressively increasing the ¯ux of methane in order to decrease the band-gap mismatch at the interface. This latter buffer prevents the formation of electronic defects due to an abrupt mismatch at the band-edges. Finally, a a-SiC:H p-layer was deposited. The wide band-gap Ê ) of this layer (2 eV) [11] and the very thin thickness (50 A increase light transmittance and then maximise the absorption at shorter wavelength in the intrinsic region. In Table 1,
all the deposition parameters of the p±i±n amorphous layer are reported. A grid-shaped evaporated aluminium front contact with a sputtered transparent conductive oxide (TCO) SnO2 coating is used as window for the incident light. The back contact is realised with a screen printed silver±aluminium paste. 4.2. Steady state characterisations Characterisation of the device has been performed by current±voltage (I±V) and quantum ef®ciency responses in different illumination and bias voltages conditions. Measurements have been performed under a Spectrolab XF10 solar simulator ®ltered with a band-pass ®lter centred at 500 nm (open circles) and 750 nm (triangles) with a FWHM of 10 nm and transmitted intensity of 600 mW/cm2. These two wavelengths, 500 and 750 nm, have been chosen as representative of the detector response in the visible and infrared spectrum, respectively. The total area of the device is are 1 cm2 and the active area 0.2 cm2. In Fig. 2, the set-up utilised to perform the quantum ef®ciency measurement at room temperature is depicted. 4.3. Transient characterisations A detailed analysis of the device transient performance can be obtained with the test circuit shown in Fig. 3. The circuit emulates the driving conditions of a pixel in a matrix of photodetectors. Namely, switch f1 (row select input)
Table 1 Deposition parameters of the amorphous layers Layer
n i buffer p
Gas flow (sccm) SiH4
CH4
PH3
B2H6
40 40 40 40
± ± 0±60 60
10 ± ± ±
± ± ± 5
Pressure (Torr)
RF (mW/cm2)
Temperature (8C)
Ê) Thickness (A
0.3 0.68 0.7 0.7
28 28 28 28
200 200 200 200
10000 2800 50 50
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charge lost during the subsequent integration time. The balance between lost and re-fed charge determines the number of integration/readout cycles necessary to reach the stationary state. Once equilibrium condition is reached both junctions exchange the same amount of charge and thus the charge read out by the sense ampli®er is the integral of the photocurrent (or leakage current if the device is in dark condition) of the reverse biased junction. 5. Results and discussion 5.1. Steady state Fig. 3. Test circuit for the charge readout on VIP. The f switches are on with low logic value at the gate.
plays the role of a-Si:H TFT that might be integrated on array surface, together with the VIP. The hold capacitor CH emulates the capacitance of the dataline (column) connecting the selected pixel to a charge-sensitive readout ampli®er. The same circuit has been used to evaluate the transient response of an amorphous silicon three colour detector [14]. However, the measurements presented here differ from the previous because the investigated structure is an amorphous±crystalline silicon heterostructure for two colour detection to be used in different applications. The value of capacitor CH has been ®xed at 680 nF, which is much higher than the capacitances of the two junctions of the VIP. In fact, from the deposition parameters we estimated the geometrical values of these two capacitances at 35 and 10 nF for the top and bottom diode, respectively. Switch f2 connects CH to the readout circuit and switch f3 resets the integrated charge. The photosensor is initially precharged by applying a proper value of VA; when f1 switches off, the current ¯owing through the VIP is forced to be zero and the incident light tends to discharge the sensor capacitances. After a given integration time, f1 switches on allowing the recharge of the sensor up to VA and simultaneously sampling the same amount of charge on CH. When f2 switches on, the sampled charge is read out by the ampli®er. When f2 turns off, f3 switches on resetting the output node and the cycle repeats. A qualitative timing diagram is also reported in Fig. 3: we notice that only one switch is kept on at time and that light integration can begin as soon as the sampling operation on CH is terminated. However, these architecture and timing diagram lead to a correct measurement only after some cycles. In fact, at a ®xed value of the external bias VA, when the sensing cycle is repeated the charge stored in the two junctions changes, either in the dark or under illumination. During the integration time the two junctions independently lose charge since the device is under open circuit conditions. During a reading pulse, the two capacitances receive the same amount of charge which, before equilibrium, is different from the
In Fig. 4, experimental photocurrent±voltage characteristics of device are reported. Circles and triangles refer to measured photocurrent measured under 500 and 750 nm monochromatic light, respectively. The measured external quantum yields of the device are reported for two bias conditions (2 V) in Fig. 5. At negative bias voltages the quantum ef®ciency is the same of a single junction amorphous silicon solar cell, and the quantum yield extends from 400 to 700 nm with a peak around 500 nm. At 2 V, the spectral shape is that of a p±n c-Si cell optically ®ltered by a p±i±n a-Si:H, with a peak observed around 800 nm. Fig. 5 clearly shows the excellent spectral separation between the two photoresponses, and peak values over 80% in both cases. Solid lines shown in Figs. 4 and 5 are obtained by the analytical model discussed in Section 3. In order to reduce the number of ®tting parameters in Eqs. (1)±(5), we measured aa-SiC(l) and TTCO(l) on single layers deposited on glass with the same deposition conditions used in the fabrication of the device. Experimental results of absorption
Fig. 4. Experimental (dots) and simulated (continuous line) photocurrent± voltage characteristics of 1 cm2 device under 0.6 mW/cm2 monochromatic light.
D. Caputo et al. / Sensors and Actuators A 88 (2001) 139±145
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Fig. 7. Transmittance of sputtered TCO. Fig. 5. Experimental (dots) and simulated (continuous line) spectral response of the VIP at V bias 2 V, scaled for an active area of 0.2 cm2.
reported in Fig. 8. The slightly sublinear dependence, observed for both wavelengths can be ascribed to the presence of defects in the intrinsic layer of the amorphous diode and in the depletion region of the crystalline junction due to the diffusion process.
coef®cients and transmittance are shown in Figs. 6 and 7, respectively. Model results reported in Figs. 4 and 5 (solid line) have been achieved using the same set of parameters. In particular, a product mta 1:8 10ÿ9 cm2/V for the intrinsic amorphous layer, dn0 1:4 mm, tc 2 10ÿ9 s and Lc 190 mm for the crystalline material have been utilised. These values are comparable to those reported in literature for both materials [12,13]. The above values of the selected parameters minimise the chi-square merit ®gure. In particular, we found that a variation of 10% of each parameter leads to an increase of 30% of the chi-square. An excellent agreement between simulation and experimental is evident for both type of characterisation. This suggests that the presented model can then be utilised as a tool to optimise the performances of the device by an appropriate design of the thickness of the various layers. As a ®nal steady-state characterisation, the light intensity dependence has been evaluated at 500 and 750 nm and
The charge values read out by the sense ampli®er reported in Fig. 9 are obtained biasing the photodetector with 2 and ÿ2 V, respectively, under a constant illumination. The integration time was varied from 100 ms to long (>1 s) times under 0.6 mW/cm2 of the two illuminations and in dark conditions. For each integration time, the read out charge has been measured after the stationary condition is reached. As it can be seen, correct spectrum sensing can be obtained keeping the integration time under 1 ms. Longer integration times allow a correct light detection in the two bias regime. However, the longer the integration times the higher the dark signal, decreasing the effective S/N ratio of the VIP.
Fig. 6. Absorption coefficient profile of a-SiC:H layer.
Fig. 8. Linearity of the VIP at the two bias and light condition.
5.2. Transient
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D. Caputo et al. / Sensors and Actuators A 88 (2001) 139±145
The time required to effectively scan a row of pixels is given by the sum of the intervals in which f1 or f2 are set on. We point out that two scans are needed to get the visible (i.e. 500 nm) and infrared (i.e. 750 nm) components of the incident light. Each of these scans requires twice the selection of the whole array, row by row, to allow correct precharging, light integration and recharging/sensing of all the rows. This ®xes the array frame rate as tFRAME 2 2NtSCAN
(7)
where N is the number of the rows in the array. The size and capacitance of the pixel define the tSCAN and the integration time limits the maximum number of rows that can be used in arrays. On the other hand, the duration of the phase in which f1 is off, that is, the integration time tINT is Fig. 9. Experimental response of 1 cm2 VIP biased at 2 V (solid symbols) and at ÿ2 V (open symbols) under radiation at 500 nm (squares), 750 nm (triangles) and in dark conditions (circles).
Results reported in [14] show instead that longer integration times (at least >10 ms) are needed for a correct detection of the three different spectrum components. This is due to the different geometrical capacitance values of the junctions in the two structures. Of course, since the detector presented here allows a shorter integration time, a larger number of rows can be driven in a matrix application (see below). Following the above discussion, the different saturation levels of the readout charge for the 500 and 750 nm wavelengths are due to the different values of photocurrent induced in the reverse biased junction. It is worth noticing that in Fig. 9 there is a difference between dark readout charge and the visible one (500 nm) when the front diode is forward biased and back diode is reverse biased (V bias 2 V). The saturated read out charge is higher in dark condition than under 500 nm and a crossing of the two curves occurs around 10 ms. Furthermore, the read-out charge in dark condition and under 750 nm does not show a clear saturation as instead it occurs for the measurement under 500 nm. This lack of saturation is an anomalous behaviour since the readout charge has to saturate when the currents discharging the two stacked capacitances of the VIP (equivalent to the p±i±n and n±p structures) reach the value of the switch off leakage current. In particular, the integration time needed to reach saturation should decrease with increasing photocurrent. Instead, the curve obtained in dark condition and under 750 nm illumination seem to show the same increase in the read-out charge as due to some capacitor that is still charging almost linearly. At the moment we are not able to explain this behaviour, that has been observed also in different photodetectors driven in the same way [14], however, as discussed above, the correct operation of the device is ensured if 1 ms integration time is used.
tINT N ÿ 1tSCAN
tFRAME 4
(8)
The value of tFRAME is typically ®xed by the application and determines the useful integration times. In order to investigate these integration times, referring to the circuit of Fig. 3, the duration of the sampling phase with f1 on was ®xed at 8 ms. The readout (f2 on, f3 off) and reset (f2 off, f3 on) phases were kept long enough to fully expire the corresponding transients; the sum of these two phases gives the integration time (f1 off). Although in this work the crystalline substrate has been used only for the detection in the infrared radiation, it can also allow the integration of the driving circuitry of a 2-D photodetector array as reported in,1 where photodetectors are integrated in stacked con®guration with the crystalline substrate. From Fig. 9, taking into account the linearity behaviour reported in Fig. 8, we can note that ®xing the integration time at 1 ms, it is possible to reduce to 300 mW/cm2 the intensity of incident light continuing to obtain good spectral detection in both bias conditions. This intensity can be considered as the lowest intensity which allows to achieve an acceptable signal-to-noise ratio. 6. Conclusions An a-Si:H/c-Si heterostructure photosensor for the detection of visible and infrared light is realised, modelled and characterised both in steady state and transient operation. The device exhibits an excellent spectral separation controlled by a bias of a few volts, with QY 0:86 at l 480 nm and QY 0:8 at l 780 nm. The very good agreement between experimental and modelled results indicates that the reported analytical model can be used a tool to give both quantitative information's on the material quality inside the device and predictions for the optimisation of the device structure. In particular, a product 1
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D. Caputo et al. / Sensors and Actuators A 88 (2001) 139±145
mta 1:8 10ÿ9 cm2/V for the intrinsic amorphous layer, tc 2 10ÿ9 s and Lc 190 mm for the crystalline region have been extracted. A linear behaviour over two orders of magnitude of incident monochromatic radiation has been achieved. The transient behaviour of the VIP under 500 and 750 nm monochromatic varying the light integration time has been investigated. We found that for this device a correct detection is obtained using an integration time lower than 1 ms. On the basis on this results and on the linear response of the device, we found that is possible to reduce to 300 mW/cm2 the intensity of incident light continuing to obtain good spectral detection in both bias conditions.
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