High Perfomance Integrated Temperature Sensor based on

Amorphous Silicon Diode for Photonics on CMOS

Sandro Rao, Giovanni Pangallo and Francesco Della Corte

Università degli Studi “Mediterranea” - DIIES - Via Graziella Feo di Vito 89122, Reggio Calabria, Italy

Keywords: p-i-n diode, Temperature Sensor, Amorphous Silicon, Photonic Integrated Circuit.

Abstract: A temperature sensor based on a photonic layer-integrated hydrogenated amorphous silicon p-i-n diode is

presented. The linear dependence of the voltage drop across the forward-biased diode on temperature, in a

range from 30 °C up to 170 °C, has been used for thermal sensing. A high sensitivity of 11.9 mV/°C in a

biasing current range ≈34-40 nA has been measured.

1 INTRODUCTION

Hydrogenated amorphous silicon (a-Si:H) is a

promising platform enabling the desired matching

between electronics and on-chip photonics (Della

Corte, 2013). Thin layers of a-Si:H can be in fact

deposited using the CMOS-compatible low-

temperature plasma-enhanced chemical vapor

deposition technique (LT-PECVD), with no impact at

all on the microelectronic layers. Moreover, a-Si:H

could be deposited on different substrates where

crystalline silicon (c-Si) could not, be it a glass, a

metal, an already processed silicon wafer, or even

plastic.

Generally, on-chip temperature measurements are

explored for thermal variation compensation in many

sensing devices such as humidity, pressure, flow,

stress and gas concentration sensors (Mansoor, 2015).

Moreover, many Si-photonic active devices are

temperature-dependent, namely they are sensible to

the environment temperature variations due to the

large thermo-optic coefficient of Si. The thermal

sensitivity of the resonant wavelength for silicon ring

resonators is, i.e., of 100 pm/°C about (Yamada,

2011) or, to mention just another example, in a Mach

Zehnder (MZ) interferometer the TO effect is

responsible of a wavelength shift of 90 pm/°C

(Selvaraja, 2010). Consequently, such devices are not

practical without thermal compensation. Thermal

challenges need to be resolved in order to advance the

Silicon Photonics for future network-on-chip

interconnection systems.

In this work, a temperature sensor based on a

waveguide-integrated a-Si:H p-i-n diode is presented.

The linear dependence of the voltage drop across the

forward-biased diode on temperature variations from

T=30°C up to 170 °C has been accurately measured.

Similar sensors based on a-Si:H diodes were

already reported in literature showing however

sensitivities lower than 3.3 mV/°C in a temperature

range from T=30 up to 80 °C (De Cesare, 2015).

2 AMORPHOUS SILICON P-I-N

DIODE TEMPERATURE

SENSOR

The diode temperature sensor was integrated in

proximity to a Mach Zehnder interferometer (MZI),

Figure 1(a), and in particular close to the MZI arm

where the propagating optical signal phase shift is

achieved by electric-field induced p-i-n diode carrier

depletion. The schematic layout of the realized device

is shown in Figure 1(d) together with its geometrical

dimensions. More details about the MZI fabrication

are provided in (Rao, 2012).

The schematic cross section of the fabricated a-

Si:H waveguide vertical-integrated p-i-n diode is

shown Figure 1 (c). It consists of an intrinsic a-Si:H

layer, 2-μm-thick, between a p-doped a-SiC:H, 2-μm-

thick, and an n-doped a-SiC:H, 300-nm-thick. The p-

i-n cathode top contact is a 200-nm-thick Al layer.

The active area of device is 2.25·10

-4

Rao, S., Pangallo, G. and Corte, F.

High Perfomance Integrated Temperature Sensor based on Amorphous Silicon Diode for Photonics on CMOS.

DOI: 10.5220/0005744903690372

In Proceedings of the 4th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2016), pages 371-374

ISBN: 978-989-758-174-8

371

Figure 1: (a) A Optical microscope (top view) of the MZI a-Si:H based modulators and temperature sensors. (b) Top view

and schematic cross section (c) of integrated a-Si:H p-i-n diode temperature sensor. (d) Schematic MZI, plot not in scale.

As well-known, the I

current flowing in a p-i-n

diode at a given applied voltage V

can be analytically

described using the following formula:













 1

eII



(1)

where η is the ideality factor, I

is the saturation

current, q is the electric charge and k is the Boltzmann

constant.

The characterization of the sensor output has been

performed under forward bias condition where, at

constant DC current, the voltage across the diode is

linearly dependent on the temperature.

In fact, for qV

>>ηkT the voltage dependence on

temperature can be obtained from (1), yielding:















V ln



(2)

Equation (2) makes explicit the linear dependence

-T as long as the non-linear contribution of Is can

be considered negligible with respect to I

3 EXPERIMENTAL RESULTS

In our setup, the p-i-n diodes have been biased with a

current I

kept constant in the whole temperature

range. The devices were tested in a climatic chamber

(Galli Genviro-030-C) setting the reference

temperature through its internal PID digital

microcontroller. A calibrated PT100 sensor, with an

accuracy of ±0.3 °C, was placed in contact with the

device under test in order to monitor, during the

measurements, the exact temperature set points

gradually varied from (to) 30 to (from) 170 °C.

The bias current I

was varied in a range from 1

nA to 100 nA and the corresponding voltage drop V

across the a-Si:H p-i-n diode was measured by using

the Agilent 4155C Semiconductor Parameter

Analyser. In Figure 2 we report the I

-V

characteristics, for different temperatures in a range

from 30 up to 170°C.

Figure 2: Forward Current-Voltage Characteristics for

Temperatures Ranging from 30 up to 170°C. the Inset

Shows a Detail of the I

-V

-T Characteristics in Linear

Scale.

From I

-V

-T measurements, the highly linear

dependence of the drop voltage across the p-i-n diode

on different temperatures have been extracted as

shown in Figure 3.

In our analysis, the coefficient of determination

) has been calculated to evaluate the agreement

between the experimental measurements and their

linear best-fit, f

(T). In particular, R

allowed us to

quantify the sensor linearity goodness by fitting the

experimental data with a linear model.

AOMat 2016 - Special Session on Advanced Optical Materials

372

In the same figure, the measured data are fitted

with the best-calculated linear model showing a good

degree of linearity (R

>0.99) for the whole considered

range of I

, 3.7 nA to 100 nA.

Figure 3: Measured (points) forward voltages versus

temperature at different polarization currents. Experimental

data are fitted with the best-calculated linear model f

(T).

The sensor sensitivity, S, is defined as the

temperature derivative of equation (2) and, therefore,

it can be calculated from the slope of the V

-T

characteristics. The calculated values of S for the five

cases shown in the figure are reported in Table I.

For I

=3.7 nA the sensitivity is 10.39 mV/°C and

increases up to 12.31 mV/°C for I

=100 nA.

Table 1: Sensor sensitivity, S, calculated from V

-T

characteristics.

lines of Fig. 5 I

[nA] S [mV/°C]

a 3.7 10.39

b 38 11.9

c 52 11.99

d 76 12.12

e 100 12.31

A more detailed analysis of R

and S is shown in

Figure 4 for all values of I

in steps of 100 pA.

It is worth noting that the coefficient of

determination varies by only 0.25% from an average

of R

=0.9972 over the considered temperature range

leading to a temperature sensor with a highly linear

behavior in a wide range of biasing currents. The

maximum of R

~0.9996 has been calculated in the

current range ≈34-40 nA corresponding to a

sensitivity S=11.9 mV/°C.

To evaluate the mismatch between the calculated

linear best-fit, f

(T), and the experimental

measurements, the corresponding root mean square

error (rmse) was first calculated and subsequently

converted into a temperature error value using the

following formula:

TfTV

iLiD









))()((

(3)

where n is the number of the temperature set

points.

The calculated plot, e

versus I

, for the

considered temperature range is reported in Figure 5.

is always lower than 5 °C while the minimum

=0.87 °C is obtained for I

=37.3±3.3 nA.

Figure 4: Coefficient of determination and sensitivity

calculated for bias currents between I

=3.7 nA-100 nA.

Figure 5: r.m.s. error versus bias current.

4 CONCLUSIONS

A temperature sensor based on a waveguide

integrated a-Si:H p-i-n diode has been designed and

characterized. The linear dependence of the voltage

drop across the forward-biased diode on temperature,

in a range from 30° up to 170°C was demonstrated.

Measurements showed both a high degree of

linearity (R

=0.9996) and a high sensitivity (S=11.9

mV/°C) in the biasing current range ≈34-40 nA.

High Perfomance Integrated Temperature Sensor based on Amorphous Silicon Diode for Photonics on CMOS

373

Such devices can be integrated into photonic

integrated circuits (PICs) for sensing applications and

in CMOS compatible photonic active devices for

which the temperature variation is an issue.

REFERENCES

Della Corte, F.G. & Rao, S., 2013. Use of amorphous

silicon for active photonic devices. IEEE Transactions

on Electron Devices, 60(5), pp.1495–1505.

Mansoor, M. et al., 2015. Silicon diode temperature

sensors—A review of applications. Sensors and

Actuators A: Physical, 232, pp.63–74.

Yamada, S. et al., 2011. Experimental investigation of

thermo-optic effects in SiC and Si photonic crystal

nanocavities. Optics Letters, 36(20), pp. 3981.

Selvaraja, S. K. et al., 2010. Thermal trimming and tuning

of hydrogenated amorphous silicon nanophotonic

devices. Appl. Phys. Lett. 97, pp 071120

Be Cesare, G., Nascetti, A. & Caputo, D., 2015. Amorphous

Silicon p-i-n Structure Acting as Light and Temperature

Sensor. Sensors, 15(6), pp.12260–12272.

Rao, S. et al., 2012. A 25 ns switching time MachZehnder

modulator in as-deposited a-Si:H. Optics Express,

20(9), pp. 9351.

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