Low Frequency Noise in Polycrystalline p- -FeSi2/Ge Heterojunction Solar Cells

A. Bag, C. Mukherjee*, S. Mallik and C. K. Maiti

VLSI Engineering Laboratory, Dept. of Electronics and ECE, Indian Institute of Technology, Kharagpur 721302, India

Phone: +91-3222-281475 Fax: +91-32222-55303 *Email: chhandakkgp@gmail.com

Low-frequency noise characterization of ITO/p- -FeSi2(Al)/Al/n-Ge(100) heterojunction solar cells is re-ported. noise and stress-induced degradation studies are used for the reliability analyses of solar cells. The nature of the burst noise generated from the defects in the p-n junction space-charge region of the solar cell and their bias dependence have been studied in detail. Use of low-frequency noise is shown to be a non-destructive reliability characterization tool for solar cells.

I. INTRODUCTION

For space applications, solar panels with highest reliability are required so that the performance is not degraded or its lifetime is not shortened. Major reliability issues in solar cells are, how-ever, related to defects, dislocations and impurities in the semiconductor used. Noise is one of the major concerns in semiconductor devices that must be understood properly and may be used as a reliability indicator. The fundamental noise sources in semiconductor devices include thermal noise, shot noise and 1/f noise. The excess noise sources include flicker noise, burst noise, generation-recombination (G-R) noise and random telegraph signal [1]. The idea of using low-frequency noise measurements for analyzing device technologies, defect analysis and reliability study has been reported by many re-search groups, including Van der Ziel et al. [2], Vandamme et al. [3], Kleinpenning et al. [4], Sikula et al. [5], Jones [6] and Chobola [7, 8], which led to the advent of reliability physics. Although Dubow and Osterwald [9] was one of earlier re-searchers to propose noise measurements for device reliability estimation, in 1980, Kleinpenning et al. [4] presented the de-tailed theoretical background and experiments of 1/f noise for various types of silicon p-n diodes with predictions on the de-vice lifetime of photovoltaic modules. As the 1/f noise is related to surface states and the burst noise is related to dislocations and metallic impurities, they may provide a good insight into the reliability issues. Low-frequency noise is related to surface states and impurities which with proper characterization may provide a greater insight into the mechanisms and shed light on the reliability issues. Low frequency noise spectroscopy has been used as a powerful technique to study the intrinsic dynamics of carriers and degradation mechanisms of semiconductors.

In this paper, we present the results of our studies on the 1/f

and burst noise as a function of bias voltages in ITO/p- -FeSi2(Al)/Al/n-Ge(100) heterojunction photovoltaic solar cells. Reliability issues are addressed which include the

device degradation, studied under electrical stressing of the solar cells.

II. EXPERIMENTALS

The photovoltaic devices were fabricated on n-type Ge sub-

strates. The substrates were cleaned by dipping in DI water for about half a minute followed by dipping in 10% H2O2 solution. Next, the substrates were immersed into HF solution for 10 min. Finally, the substrates were blown dry with N2, and then im-mediately loaded into the deposition system. A thin Al inter-layer (~10 nm) was deposited prior to deposition of amorphous p-FeSi2 (Al). A layer of ~40 nm thick with Al-containing amorphous p-FeSi2 was deposited by RF co-sputtering of stoichiometric FeSi2 target and pure Al target in Ar ambient at a working pressure of 2.5 mTorr. The devices were then subjected to RTA in N2 ambient at 650°C (Model ULVAC-RIKO, MILA-3000) for 2 min. Heterojunction solar cells were fabri-cated by sputter deposition of ITO (~100 nm thick) at room temperature as top electrode (area ~0.086 cm2) and using Al as Ohmic back contact.

Current density–voltage (J–V) characteristics of the devices

in dark and under AM 1.5 illumination (XES-151S) with a power density of 100 mW/cm2 were measured using Agilent B1500A semiconductor device parameter analyzer. The poly-crystalline -phase formation after RTA at 650°C was con-firmed by x-ray diffraction (results not shown).

Low-frequency noise measurements were carried out at room

temperature using a low-frequency noise measurement setup which includes an Agilent E5263A 2-channel high speed SMU analyzer, a SR570 low noise amplifier (LNA) and an Agilent 35670A dynamic signal analyzer. During measurements, the solar cell was placed in a shielded and light-proof probe box to minimize interference from light and ambient noise. The noise across a 15 load resistance is a measurable quantity from which the noise voltage is measured. The minute fluctuations in the voltage were pre-amplified to a measurable range by the SR570 LNA. Agilent E5263A was used to bias the solar cells. The output of the LNA was then fed to Agilent 35670A dynamic signal analyzer which performs fast Fourier transform on the time domain signal and shows the voltage noise power spectral density (SV) at the output, in the 1 Hz – 10 kHz range. A com-puter interface was connected to the overall system through GPIB connection for noise data acquisition.

377978-1-4799-0480-8/13/$31.00 c©2013 IEEE

III. RESULTS AND DISCUSSIONS PV properties of the ITO/p- -FeSi2(Al)/Al/n-Ge(100) het-

erojunctions were measured using a solar simulator (XES-151S) with a power density of 100 mW/cm2. The J–V characteristics of the ITO/p- -FeSi2(Al)/Al/n-Ge(100) heterojunction solar cells both under dark and AM 1.5 (100 mW/cm2) simulated solar irradiation condition is shown in Fig. 1. The device exhibits an open-circuit voltage (VOC) of 0.055 volt, a short-circuit current density (JSC) of 0.58 mA/cm2 under illuminated condition. The fill factor (FF) and the power conversion e ciency ( ) were estimated to be 24.21% and 0.008%, respectively. The fill factor (FF) and the power conversion efficiency ( ) of the device was measured using the relations:

m m

sc oc

I VFFI V

(1)

and oc sc

in

V I FFP

(2)

where VOC is the open-circuit voltage, ISC is the short circuit current, FF is the �ll factor, is the power conversion efficiency and Pin is the incident light power density, Im and Vm are the current and voltage at the maximum power point.

-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3

-3

-2

-1

0

1

2

3

Dark Current Photo Current

Cur

rent

Den

sity

(mA

/cm

2 )

VOC~ 55 mV

JSC~0.582 mA/cm2

Applied Bias (V)

ITO/ -FeSi2(Al)/Al/n-Ge

Fig. 1. Current density-voltage (J–V) characteristics of ITO/p- -FeSi2(Al)/Al/n-Ge(100) heterojunction solar cells in dark (open sym-bol) and under AM 1.5 illumination (100 mW/cm2) simulated solar irradiation condition (solid symbol).

Excess noise has a 1/f spectral density. Noise spectral density increases with damage and stress. The 1/f excess noise is cha-racterized by generalized Hooge’s formula [10] given as:

2

2,Q II Q

F I S fS Ff I

(3)

where SI is the current noise spectral density, f is the frequency, I is the device current and FQ is the quality factor. FQ is the factor

used to judge device quality which is given generally by FQ = /N with being the Hooge’s constant and N being the total

number of charged carriers (which contributes to the noise generation) in the sample. Fig. 2 shows the voltage noise power spectral density, SV, of the ITO/p- -FeSi2(Al)/Al/n-Ge(100) solar cells. The noise is of 1/f m type with m~1.6.

101 102 103 10410-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

S V (V

2 /Hz)

Frequency (Hz)

VF= 0.2 V VF= 0.6 V VF= 1.0 V VF= 1.4 V

~1/ 1.6

Fig. 2. 1/f noise spectral density versus frequency for the ITO/p- -FeSi2(Al)/ Al /n-Ge(100) heterojunction solar cell depicting bias dependency with forward bias.

0.0 0.5 1.0 1.5 2.0

10-13

10-12

10-11

S V (V

2 /Hz)

VF (Volts)

=1 kHz

SVM

Fig. 3. Voltage noise spectral densities of the samples as a function of forward bias voltage at 1 kHz showing a maximum (SVM) at a forward bias of 1.38 V due to power matching.

Fig. 3 illustrates the explicit voltage dependency of SV with forward bias at 1 kHz. The noise spectra attain the maximum (SVM) at about 1.38 V with SVM being 2.2×10-11 V2/Hz. The noise spectral density measured across the load resistance versus the

378 2013 20th IEEE International Symposium on the Physical and Failure Analysis of Integrated Circuits (IPFA)

forward bias has its maximum value when the dynamic resis-tance of the p-n junction equals the load resistance (power match condition). Fig. 4 shows the current noise spectral den-sity, SI, for both forward and reverse currents. It is seen that the SI follows the square law relation with the forward current, IF, in the generation-recombination current region up to a current of 10-4 A. The linear region signifies the diffusion current com-ponent. The quality factor, FQ = fSI /I2 [10], in forward direction is equal to 1.7×10-7 for a current of 8.7×10-5 A.

10-5 10-4 10-3

10-19

10-18

10-17

~I0.45

~I

~I1.7

S I (A2 /H

z)

IF, IR (A)

Forward Bias Reverse Bias

~I2

Fig. 4. Excess current noise spectral densities of the samples are shown as a function of forward and reverse bias currents at a frequency of 1 kHz. The excess noise becomes more than the shot noise (2qI) and demonstrates a gen-eration-recombination current dependence up to 10-4 A. Afterwards, the diffu-sion current becomes dominant. The current noise spectral density SI is shown as a function of forward and reverse currents. It is seen that the SI follows the square law relation with the forward current IF in the genera-tion-recombination current region up to a current of 10-4 A. After that the noise current PSD becomes linear which signifies the diffusion current component. For reverse bias the PSD is proportional to I 0.45

after the square law region. It is observed that the solar cells display a considerable amount of excess noise, which exceeds the thermal noise or shot noise power spectral densities.

The noise studies for reliability analysis are focused on the

identification and analysis of failure mechanisms, defects and traps, device quality assessment and reliability prediction. The sensitivity of the noise measurement is the main reason for using it as a non-destructive prediction tool in reliability analysis and device lifetime assessment. The reliability issues are indicated by transport mechanisms of charged carrier in the forward and reverse current directions [11]. Basic transport mechanisms include generation-recombination, diffusion, breakdown (zener and avalanche), and high injection. Apart from these, additional charge carrier transport may take place in p-n junctions, which may result in excess current.

Fig. 5 shows the voltage noise power spectral density of the solar cells as a function of electrical stress time at three different frequencies. Constant voltage stressing has been done for 55 minutes at a reverse voltage of -1.6 V and SV is recorded during the stress time. It is observed that the noise spectral density increases with stress-time which clearly indicates gradual dete-rioration in device quality which is a quality indicator of device life-time, i.e., it improves the understanding of the device deg-radation mechanisms.

0 10 20 30 40 50 6010-11

10-10

10-9

10-8

10-7

10-6

10-5

S V (V

2 /Hz)

Stressing Time (min)

CVS @ 1.6 V

@ 96 Hz @ 512 Hz @ 1024 Hz

Fig. 5. Voltage noise spectral densities of the samples are shown as a function of electrical stress time at three different frequencies for a constant voltage stress of duration 55 minutes at -1.6 V. Constant voltage stressing indicates an increase in the noise level and therefore reliability degradation.

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

4.470

4.485

4.500

5.0100

5.0125

20.85

20.90

51.84

51.86

I (m

A)

Time (s)

ITO/ -Fesi2 (Al)/ Al/n-Ge

IBN

=0.03 mA

@ 0.8 V

@ 0.6 V

@ 0.4 V

@ 0.2 V

Fig. 6. Time behavior of the noise current showing burst noise for different forward bias voltages with an amplitude IBN.

2013 20th IEEE International Symposium on the Physical and Failure Analysis of Integrated Circuits (IPFA) 379

Fig. 6 shows the burst noise (random telegraph noise) at four

different bias voltages observed as fluctuation between two current levels for a time window of 2 seconds in the solar cells indicating presence of defects and dislocations in the space-charge region of the p-n junction. So, the burst noise response indicates device quality. Hsu et al. suggested that burst noise is due to bistable fluctuation of the potential barrier af-fected by a defect in the space-charge region involving a single charge [12]. The noise fluctuation basically reveals current or voltage impulses featuring two or more amplitude levels, which may be described as non-stationary and random fluctuations, over which `pure noise' is superimposed. Potential barrier in the region with defects is lower compared to the barrier in the defect free region of the diode, which makes the current density in the defect-regions higher. Under low-bias conditions, the burst noise amplitude IBN may be expressed as [12]:

,BNq AI V

nkT A (4)

where I is the DC current flowing through the p-n junction, q V is the magnitude of the effective potential barrier fluctuation,

A/A is the relative change of the effective area and n is the factor of ideality of I-V characteristics. In the time-domain signal, two time constants can be defined: time at high current level, + during which the generation-recombination centre is empty (capture time) and time at low current level, - during which the electron is in the trap (emission time). The burst noise amplitude, IBN, of the noise current is calculated from Fig. 6 as 3×10-5 A for the bias voltage of 0.6 V. 1/f noise and burst noise are closely related to each other as well as to device structure imperfections such as surface defects, impurities and disloca-tions. In particular, 1/f noise is mainly related to surface states whereas the burst noise is related to lattice dislocations and heavy metal impurities.

IV. CONCLUSIONS Low frequency noise characteristics have been studied in

detail for germanium iron disilicide heterojunction solar cells. Bias dependency of 1/f and burst noise is studied. Stress-induced build-up of defects, degradations and disloca-tions are observed from the 1/f noise and burst noise spectra. Low frequency noise spectroscopy is shown to be a powerful characterization tool for the reliability analyses in heterojunc-tion solar cells.

ACKNOWLEDGMENT

The authors acknowledge DST, New Delhi (project sanction number: DST/TM/SERI/2k11/100/(G)) for supporting the solar cell work reported here.

REFERENCES

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[8] Z. Chobola, "Impulse noise in silicon solar cells," Microelectron. J., vol. 32, no. 9, pp. 707-711, Sep. 2001.

[9] J. Dubow, and C. Osterwald, "Noise Spectral Density as a Device Reliability Estimator," Sol. Cells, vol. 1, no. 3, pp. 315-319, May. 1980.

[10] F. N. Hooge, “1/f noise,” Physica B, vol. 83, no. 1, pp. 14-23, May. 1976. [11] J. Xu, D. Abbot, and Y. Dai, "1/f, g–r and burst noise used a screening

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[12] S. T. Hsu, R. J. Whittier, and C.A. Mead, “Physical model for burst noise in semiconductor devices”, Solid-State Electron., vol. 13 (7), pp. 1055-1056, 1970.

380 2013 20th IEEE International Symposium on the Physical and Failure Analysis of Integrated Circuits (IPFA)