ARTICLE IN PRESS Microelectronics Journal 40 (2009) 435– 438
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Novel silicon high sensitive photonic sensor A. Axelevitch , G. Golan Holon Institute of Technology, EEE Department, P.O. Box 305, Holon 58102, Israel
a r t i c l e in fo
abstract
Available online 17 July 2008
Light depended resistors (LDR) or photoresistors are semiconductor devices that are changing resistance under illumination. These devices have many applications in industrial controls: item counters, presence and proximity sensors, flame detectors, photometric devices, etc. If the light falling on the device has energy which is greater than the bandgap of the semiconductor, photons absorbed by the semiconductor excite electron–hole pairs which result in lowering the resistance of the semiconductor. Generally, these devices are made of semiconductors such as CdS or CdSe using a thin film technology, since they have traps and misfits in their atomic structure, leading to high dark current and noise. In this work, we describe a novel approach for a novel family of high sensitive light detectors made of single-crystalline silicon. Basic sensor was built in a flat shape providing lateral electrical transport of excited charged carriers. Simple laboratory methods were used to diffuse impurities on both sides of the sensor. The sample shows high sensitivity due to light intensity variation from dark to strong light (96,000 lx). A 30 times variation in the sample resistance was obtained. & 2008 Elsevier Ltd. All rights reserved.
Keywords: Silicon photonic sensor Lateral electrical transport
1. Introduction Semiconductor light detectors can be divided into two major categories: junction and bulk effect devices. Junction devices, when operated in the photoconductive (PC) mode, utilize the reverse characteristic of a PN junction. Under reverse bias, the PN junction acts as a light controlled current source. Output is proportional to incident illumination and is relatively independent of implied voltage. Silicon junction photodiodes are examples of this type of detectors. In contrast, bulk effect photoconductors have no junction. In these devices, the bulk resistivity decreases with increasing illumination, allowing for more photocurrent to flow. This resistive characteristic gives the bulk effect within photoconductors a unique quality: The signal current from the detector can be varied over a wide range by adjusting the applied voltage [1]. Another way to classify photosensors is by distinguishing between two types of incident light: the PC and the photovoltaic (PV) [2]. The difference between these types is defined by the construction method of the above-mentioned sensors. The PC type, which is known also as the coplanar type, has two metal Ohmic contact electrodes disposed one against the other on a slab of semiconductor (in the form of bulk or thin film). The PV type, which is known also as the diode type, has a semiconductor slab or thin film sandwiched between a top and a bottom electrodes in the vertical direction. The PV-type sensors are commonly made as Corresponding author.
E-mail address:
[email protected] (A. Axelevitch). 0026-2692/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2008.06.012
Schottky diodes or p-type/intrinsic/n-type (PIN) diodes. The pin diodes are becoming predominant compared to the Schottky diodes in signal-to-noise ratio and other image properties. The PC sensors have a lateral flow of photocurrent due to its structure, so that the carriers move in the space between the metal electrodes. In the PV type, the carriers move between the top and bottom electrodes in the vertical direction. If we compare the two types of sensors, the PC type is simpler in construction. Its manufacturing process has smaller number of operations and it is lower in cost. However, the sensitivity of these sensors may be improved using integration of both types of sensors. The main goal of this work was investigation of the possibility to create photoconductive sensor using both types of the light response.
2. Experimental details Fig. 1(a) represents a 3-D sketch of the sensor. It was designed with a goal to utilize maximum of the surface area as an active region. A photosensor consists of a slab of high-resistive singlecrystalline silicon with highly doped opposite sides of a slab. Ohmic contacts attached to the doped sides complete the design of the sensor. When incident light falls on the surfaces of the sensor, pairs of charged carriers are generated in the semiconductor material and enable the response current. Charged carriers generated in the body of silicon move horizontally through singlecrystalline silicon under influence of electrical fields created on the sides of the slab and under the bias. Silicon sensors were made from high-resistive n-type single-crystalline silicon with the
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A. Axelevitch, G. Golan / Microelectronics Journal 40 (2009) 435–438
following parameters: resistivity r ¼ 1.1 104 O cm, thickness of 0.44 mm, width of 1 cm, and length of 2 cm. The silicon was cut in the (1 0 0) plane and both large surfaces were polished. External view of the typical sample is presented in Fig. 1(b). To prepare an n-type doping at both sides of the silicon slab, this side was covered by an orthophosphoric acid solution, H3PO4 in ethyl alcohol (1:1). The sample was placed in a vacuum chamber at 3 mm distance from a heating source (a tungsten wire in vacuum). Heating was done for 5–10 min in vacuum of 3 105 Torr at a temperature of 500 1C. Temperature of the sample was measured using a thermocouple of K-type. Fig. 2
presents our sample coated with orthophosphoric acid in a vacuum chamber before the heat treatment. The second side of the sample was coated by an In–Ga eutectic alloy and treated in the same conditions as a first side. Electrical characterization of the samples was done using a standard four-point probe measurement technique. Charged carriers and their behavior under heating were studied using the hot-probe evaluation technique [3]. The photocurrent behavior under various intensity illuminations was measured with bias and without it. The illumination level was measured using a digital light meter TES 1332A. Charge carriers concentrations in various parts of the samples were estimated using the common formulation of the well-known dynamical parameters of silicon: mn ¼ 1350 cm2/V s and mp ¼ 450 cm2/V s [4]:
s ¼ qðmn N þ mp PÞ.
(1)
Following the measurement phase, one of the sensors was etched in a buffer solution of a hydrofluoric acid for 2 min. Then, all of the measurements were repeated.
3. Experimental results
Fig. 1. A 3-D view of a typical experimental sensor: (a) a theoretical drawing of the photoconductive sensor and (b) an experimental view.
Fig. 3 presents experimental characteristics of the sheet resistance measured in various parts of the sample: in the doped sides and in the central zone (undoped region). All of the measured characteristics are straight lines, showing Ohmic type of the measured contacts. This behavior enables to calculate the resistivity of all measured regions and estimate the relating charged carrier concentration. Resistivity of the undoped part of the sample was obtained as 1.1 104 O cm. After etching, this resistivity remains the same even in dark. However, in light of 750 lx (day light), the resistivity was decreased up to 0.95 104 O cm due to generation pairs of charged carriers. Fig. 4(a) demonstrates that the active (undoped) part of the slab is of n-type nature. This follows from the positive influence of the measured voltage, generated by the hot-probe experiment, shown in the figure. In this experiment, the thermally excited majority
Fig. 2. A thermal annealing vacuum setup.
ARTICLE IN PRESS A. Axelevitch, G. Golan / Microelectronics Journal 40 (2009) 435–438
1.6
P doped edge, ρ = 2.6∗104 Ω∗cm Undoped part, ρ = 1.1∗104 Ω∗cm In-Ga doped edge, ρ = 1.4∗104 Ω∗cm
10
1.2
Measured voltage, mV
Measured voltage, V
1.4
437
1.0 0.8 0.6 0.4
8 6
40° C 80° C
4 2 0
0.2 15
20
25 30 Applied current, μA
-2
35
0
10
20
30 40 50 60 70 Processing time, sec
80
90 100 110
Fig. 3. Sheet resistance characterization to various parts of the sample.
The proposed construction method of the new sensor assumes a lateral transport mechanism of charged carriers through the active part of the sensor. This electrical transport must be driven by two different forces: external electrical field (a bias voltage) and internal electrical field built by two diode structures connected in series. External electrical field may be estimated simply using the formula of a flat capacitor. Internal field may be estimated using the solution of the Poisson’s equation: qNA xp
0 r
¼
20 40° C 80° C
15 10 5 0 0
qN D xn
,
(2)
19
19
0 r
C is the elementary charge, NA ¼ ND ¼ 10 where q ¼ 1.6 10 cm3 is the required impurity density at both sides of the sensor element, xn ¼ xp is the depletion region in the heavy-doped sides of the sample, e0 ¼ 8.85 1012 F/cm is the permittivity constant of vacuum, and er ¼ 11.8 is the relative dielectric constant of silicon.
10
20
30 40 50 60 70 Processing time, sec
80
90 100 110
Measured voltage, V
20 40° C 15
80° C
10 5 0 0
4. Discussion
Em ¼
Measured voltage, mV
25 carriers are translated within the semiconductor from the hot probe to the cold probe. Therefore, the positive measured voltage characterises the negative type of majority carriers. Additional heating of the ‘‘hot-probe’’ results in a significant growth of the charged carriers. Resistivity of the doped sides is also decreased after etching and under illumination. Growth of resistivity in the doped sides, relating to the undoped part of the sample, may be explained by creation of interstitial impurities within the silicon crystal during diffusion. Thus, we need an additional annealing to the sample in order to activate the inserted impurities. Fig. 4(a–c) presents the dynamical characteristics of the undoped part and doped sides of the sample during the ‘‘hotprobe’’ experiment. As shown, all parts of the sample are having n-type conductivity. Various resistivity or charged carriers concentration in these parts results in various values of the measured voltage at the same temperatures. Fig. 5 presents experimental relationship between the illumination and the resistance of the sensor. As shown, resistance of the sample is dramatically decreased under illumination. This is due to a raise in the electron–hole pair concentration as a result of the optical absorption. A large surface area of the sensor enables the absorbance of many photons and thus increases its sensitivity. Fig. 6 illustrates the electrical behavior of the sensor under irradiation with various biases in opposite sides. The sensor behaves in a similar manner with various bias voltages.
10
20
30 40 50 60 70 Processing time, sec
80
90 100 110
Fig. 4. Hot-probe characteristics of the sensor: (a) undoped (active) part of the sample, (b) In–Ga-doped side of the sample, and (c) P-doped side of the sample.
A built-in potential, grown due to the impurity concentration in the edges of the ideal sample (pure intrinsic single-crystalline silicon) at room temperature, is equal to V b ¼ V T ln
N D ni NA n 1019 ¼ V T ln 2 i ¼ 0:026 ln 0:53V 2 ni ni 1:5 1010
(3)
where VT ¼ 0.026 V is the environmental potential and ni ¼ 1.5 1010 cm3 is the intrinsic concentration of the charged carriers in silicon at room temperature. The length of the depletion regions xn and xp may be estimated using the following
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800
and xpi, situated within the intrinsic part is estimated using the same equation. It gives 2.6 mm from each side. Obtained sensor elements that were built in laboratory conditions behaved differently from the described ideal case. As shown, the difference in the charged carriers concentration in various parts of the samples was not large. Therefore, lower values of the response current were obtained and lower electrical fields were built within the sensor.
Dark
700 Resistance, kΩ
600 500 400 300 200 Strong light
Day light
100
A novel family of Si photoconductive sensors was created. Due to some lab difficulties in diffusion from both sides, we may consider this work as feasibility proof for application of both photoresponse mechanisms in the same silicon sensor. The obtained results show the following points:
0 1
10
100 1000 Illuminance, lx
10000
Fig. 5. Characterization of the novel experimental silicon sensor
Measured current, mA
1. a novel silicon sensor was created in laboratory conditions; 2. sample laboratory methods for diffusion of impurities in silicon were tested and studied; 3. the obtained results show high sensitivity at the sensor; and 4. this novel sensor may be considered as a promising device with various applications.
Bias voltage:
3.5
5V
3.0
10 V
2.5
15 V
5. Conclusions
20 V 2.0 References
1.5 1.0 0.5 0.0 0
20000
40000
60000
80000
100000
Illuminance, lx Fig. 6. Electrical behavior of the novel sensor within an electrical circuit.
equation: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 20 r V b 20 r V b xn ¼ ¼ xp ¼ ¼ 83 nm: qN D qN A
(4)
Therefore, a maximal electrical field built-in within the intrinsic part, is estimated, as 1.3 105 V/cm. The depletion regions xni
[1] PerkinElmer, Inc., 2004, /http://optoelectronics.perkinelmer.com/content/ ApplicationNotes/APP_PhotocellIntroduction.pdfS. [2] J. Ryu Il, TFT-LCD Research Center, Kyung Hee University, 2001, /http:// tftlcd.khu.ac.kr/research/sensor_p/chap1.pdfS. [3] G. Golan, A. Axelevitch, B. Gorenstein, V. Manevych, Hot-probe method for evaluation of impurities concentration in semiconductors, Microelectron. J. 37 (9) (2006) 910–915. [4] S.M. Sze, Physics of Semiconductor Devices, Wiley, New York, 1981.
