THE aSiC / cSi ( n ) ISOTYPE HETEROJUNCTION AS A HIGH SENSITIVITY TEMPERATURE SENSOR

The a-SiC/c-Si(n) isotype heterojunction has been studied as a temperature sensor by measuring its reverse current-voltage (IR-V) and reverse voltage-temperature (V-T) characteristics, as well as its reverse current temperature dependence. The IR-V characteristics exhibit an almost constant current, whereas the reverse current Ia depends strongly on T (from 230 K up to 320K). The V-T characteristics, at different reverse currents, reveal a highly temperature sensitive behavior for the a-SiC/c-Si(n) junction. The measured values of temperature sensitivity (AV/AT) was found to be (- -2.5 V/K) in the moderate temperature range, which are much higher than those obtained with bulk semiconductor temperature sensors. The high sensitivity-temperature-range of the a-SiC/c-Si(n) heterojunctions can be controlled electrically within the regim of values from 230K up to 320 K. Finally, the high sensitivity of these devices, in conjunction with the fact that a-SiC films can be used as an add-on to the existing Si technology, emerge new possibilities to the fabrication of high sensitivity temperature microsensors.


INTRODUCTION
The interest in developing high sensitivity semiconductor temperature sensors has been increased during the past few years [1].If integrated   with semiconductor microelectronic circuitry, such sensors could form the basis for a new generation of sensing devices.Although many *Corresponding author.
L. MAGAFAS et al. crystalline microelectronic devices have received considerable atten- tion as temperature sensors, e.g.p-n diodes [2], transistors [3], Bulk Barrier Diodes (BBD) [1] etc., none of them, except from BBD, was found to exhibit a high sensitivity in the range of moderate temperatures.
On the other hand, although a number of other microelectronic devices have been fabricated using a-Si:H or a-SiC:H, e.g.Metal- Amorphous Silicon FETs [4, 5], thin film Transistors [6], Optothy- ristors  [7], and Heterojunction Phototransistors [8] there is no published work in the area of temperature sensors using amorphous semiconductors.Taking, also, into account that localized gap states have a crucial effect on the electrical properties of amorphous semiconductor materials [9,10] and devices [11], it seems to be a great need to study temperature sensor devices based on amorphous semiconductors.
A contribution in this particular direction is the present work, presenting results on the behaviour of a-SiC/c-Si(n) isotype hetero- junctions as highly sensitive temperature sensors, obtained by measuring their reverse current-voltage characteristics at different temperatures, as well as their reverse voltage-temperature character- istics using a current source.The results of these measurements reveal that the a-SiC/c-Si(n) isotype heterojunction is a very attractive device as a high sensitivity temperature sensor, with a sensitivity temperature- range electrically controlled within the region of values from 230 K up to 320 K.
2. FABRICATION OF a-SiC/c-Si(n) HETEROJUNCTIONS Thin films of a-SiC, with a thickness of about tm, were deposited by r.f sputtering on n-type crystalline silicon substrates (5-10 fcm), with ohmic contacts at their back sides.The target used was SiC of constant composition (66 wt% Si 34 wt% C) and 99.8% purity.The RF power was 250W and the target-to-substrate distance was 5.5cm.The substrate temperature was 30C and the sputtering chamber evacuated to a pressure lower than 5 x 10 -7 Torr before the introduction of argon.During the deposition, the flow rate of argon was 20sccm and the pressure in the sputtering chamber 6 rn Torr.Aluminium dots, 5000A thick and 0.785 mm 2 in area, were deposited on the a-SiC layer to form ohmic contacts, as it has been described in our previous works [12,13] The reverse current-voltage (1R-V) and voltage-temperature mea- surements (V-T) of a-SiC/c-Si(n) isotype heterojunctions were carried out within a light-tight cryostat evacuated down to 10 -3 Torr, in the temperature range from 230 K up to 320 K, using a picoameter and a current source controlled by a P.C.

RESULTS AND DISCUSSION
Figure 1 presents a typical structure of a-SiC/c-Si(n) heterojunctions (Fig. l(a)) and the energy band diagram under thermal equilibrium conditions, as well as under reverse bias conditions (Fig. l(b)).The above energy band diagram was calculated, in our previous study [13], using Anderson's model [14].The conductivity of a-SiC was considered to be n-type, as it was found in our previous works [12,13].The subscripts and 2 refer to a-SiC and to c-Si(n) respectively, so that Eel and Ec2 are the edges of the conduction bands, Evl, Ev2 the edges of the valence bands, 61, 62 the distances between the Fermi level and the corresponding conduction band edges, Egl, gg2 the energy band gaps, Vbil, Vbi2 the partial diffusion potentials, AEc, AEv, the discontinuities of the conduction and valence bands, respectively, and EF the Fermi level under thermal equilibrium conditions.Under reverse bias conditions, the polarity is negative on the a-SiC side of the junction.As it is shown in Figure l(b), under reverse bias conditions electrons from a-SiC have to surpass the potential barrier of the a-SiC/ c-Si(n) junction in order to contribute to the reverse current of the heterojunction.
Figure 2 shows typical reverse log(IR)-V characteristics of a-SiC/c-Si(n) heterojunctions, measured at different temperatures from 230 K up to 320 K.As it clear from this figure, the reverse current, due to the electron transport from a-SiC to c-Si(n), increases with the increase of temperature.On the other hand, for a given temperature, log(I) remains almost constant with the increase of reverse bias voltage, exhibiting a saturation behavior.This behavior of the reverse current is very interesting (for temperature sensing devices), as it will be discussed further below, and it could be attributed to the charge carrier
transport mechanism, in combination with the fact that one side of the heterojunction consists of an amorphous semiconductor (a-SiC).
In an amorphous semiconductor like a-SiC (Fig. l(b)), it is well known that when EF lies below Ec and the temperature is roughly higher than 230K electrons in amorphous semiconductor are transported through extended states of the conduction band [12,15].Thus, as we can see from Figure l(b) under reverse bias conditions, electrons in extended states of the conduction band of a-SiC may be emitted over the barrier into the conduction band of c-Si(n).In this case, for which thermionic emission is the dominant transport mechanism, Anderson's model for isotype heterojunctions predicts that the reverse current obeys the relation [16]: where V1 is applied reverse voltage drop across a-SiC, B is a constant (independent of temperature and applied reverse voltage), and E is the value of the barrier height that electrons in extended states of the conduction band of a-SiC have to surpass in order to reach the conduction band of c-Si(n).However, in the case of a heterojunction like a-SiC/c-Si(n), where one side consists of an amorphous semiconductor, the quantity E should, also, include the a-SiC conductivity activation energy, Ea, which is the energy needed for electrons to make a transition from localized gap states near the conduction band to extended states in the band.V1 is defined by the relation: where n is and V2 is the applied reverse voltage drop across the c-Si(n).Taking into account that the a-SiC/c-Si(n) isotype heterojunction is a one sided junction [13], where the depletion layer extends, mainly, within the c-Si(n), we deduce that V1 should be negligible compared with the total applied reverse bias voltage V.As a result, the value of the barrier height, AEc-Vbil-V1, should remain, almost, constant (_O.18eV), independent of the reverse bias voltage and, therefore, IR should also be independent of the reverse bias voltage.On the other hand, IR should be strongly dependent on T, because the electron concentration in the extended states of the conduction band of a-SiC, n, increases with temperature, according to the relation [10]: Figure 3 shows typical characteristics of log(IRT-1/2) versus IO3/T for two different reverse bias voltages (V-V and 4 I0.It is clear from this figure that a linear relationship is obtained between log(I2T-/2) and 1/T for both values of the reverse voltage, which is consistent with eq. ( 1).It is also clear from this figure that both characteristics have the same slope (E-(1-1/n)qV)/k, where it was found that (E-(1-1/ -3.00 n)q100.36eV).This suggests that compared with E, the quantity (1-1/n)qV can be ignored, in this range of reverse bias voltages.The observed value of E(0.36e 1/) is exactly equal to the sum of the a-SiC conductivity activation energy (= 0.18el0 [17] and the heterojunction barrier height AEe-Vb-VI('O.18eV),suggesting that our previous assumption for E is correct.
The above behavior of a-SiC/c-Si(n) reverse current I as a function of reverse bias voltage, V, and temperature, T, leads to high values for the ratio A V/AT, which is a measure of the temperature sensitivity of a semiconductor sensor [18].In order to evaluate the sensitivity of a-SiC/c-Si(n) isotype heterojunctions as a temperature sensor, V-T measurements were carried out using a current source.
Figure 4 shows typical V-T characteristics at different values of the reverse current, ranging from 25tA up to 2mA.It is clear from this figure that the voltage drop across the junction decreases drastically in all V-T characteristics (with the same slope) as the temperature  increases.From these characteristics, the ratio (AV/AT) was calculated and it was found to be (AV/AT)max-2.5V/K.These values of the temperature sensitivity (AV/AT)max are much higher than those obtained with bulk semiconductor temperature sensors, where this ratio is about -1.2 V/K [1, 18].The above result combined with the fact that the high sensitivity temperature-range of a-SiC/c-Si(n) heterojunctions can be controlled by the reverse current through the device, within the region of values from 230K up to 320K, makes these devices very attractive as temperature sensors.
Differentiating eq. ( 1) and taking into account eq. ( 2), the ratio O V/OT was calculated and it was found to be: ov ov (e-0---OI OT (1 In) T + (1 In) (5) It is clear from eq. ( 5) that, the sensitivity OV/OT, for a given reverse current, decreases as the temperature, T, increases, which is in agreement with experimental results of Figure 4.

CONCLUSIONS
In the present work the a-SiC/c-Si(n) isotype heterojunction has, for the first time, been studied as a temperature sensor by investigating its electrical properties, under reverse bias conditions, and the main results are as follows: i) The reverse current-voltage (IR-V) characteristics at different temperatures (from 230K up to 320K) exhibit a saturation behavior, i.e. the IR for a given temperature is almost independent of V. On the other hand, the current IR increases drastically with the increase of T. The above behavior of IR can be attributed to the form of the energy band levels of the a-SiC/c-Si(n) heterojunction, under reverse bias Conditions, as well as to the temperature dependence of the electron concentration in the extended states of a-SiC.
ii) The reverse bias voltage-temperature (V-T) characteristics at different reverse currents show that V decreases drastically with the increase of T, leading to a high value of the temperature sensitivity ((A V/AT)max'-2.5 V/K) for all characteristics, so that the high sensitivity temperature-range can be controlled electri- cally, within the region of values from 230 K up to 320 K.These observed values of the temperature sensitivity (AV/AT)max in the moderate temperature are much higher than those for bulk semiconductor temperature sensors.Take, also, into account that a-SiC films can be used as an add-on to the existing Si technology, emerge new possibilities to the fabrication of high sensitivity microsensors.