Noise spectral density as a device reliability estimator

Solar Cells, 1 (1979/80) 315 - 319 © Elsevier Sequoia S.A., Lausanne -- Printed in the Netherlands

NOISE SPECTRAL DENSITY ESTIMATOR*

AS A DEVICE

315

RELIABILITY

JOEL DUBOW and CARL OSTERWALD

Department of Electrical Engineering, Colorado State University, Fort Collins, Colo. 80523 (U.S.A.) (Received August 3, 1979; accepted October 19, 1979)

Summary The measurement of noise spectral density is proposed as an estimator of solar cell device reliability. The use of associated temperature and bias stresses is investigated as a non-destructive rapid test method. A novel technique which employs the cooling rather than the heating of the device was utilized. In certain cases the predominant failure mechanisms may be derived. Data are presented on indium-tin oxide/silicon as well as p - n junction devices.

1. Introduction The viability of terrestrial photovoltaic technology depends in a large measure on the operational lifetime of the individual solar cells. This lifetime is largely limited by the degradation mechanisms present in the device and its interconnections. The dominant degradation mechanisms are usually extrinsic and n o t intrinsic to the device. For example, adsorbed water, moisture-induced corrosion and contamination [1] as well as ion migration [2] are key failure mechanisms in devices and integrated circuits. Most failure mechanisms follow the Arrhenius relation A exp (--Ea/kT). The rate of degradation is also a function of the stress level under which the device operates. Stressors are usually classified as applications of thermal, electrical, optical and chemical excitations. Degradation mechanisms may be classified as induced by one of the stressors. In addition, a number of devices fail prematurely because of flaws in fabrication. Table 1 lists typical degradation mechanisms and their dependence on temperature, bias and time [ 2]. Indium-tin oxide (ITO) semiconductor-insulator-silicon (SIS) cells exhibit k n o w n thermal degradation caused b y growth of the interfacial layer. In addition, the ionic diffusion mechanisms are magnified in large-area devices such as solar cells. This ion diffusion can cause shorting, soft breakdown, *Paper presented at the Photovoltaic Material and Device Measurement Workshop, "Focus on Polycrystalline Thin Film Cells", Arlington, Virginia, U.S.A., June 11 - 13, 1979.

316 TABLE 1 Device degradation and failure mechanism models

Indicator

Mechanism

t

T

V

Capacitance C

Ionic diffusion ambient to surface

C = C O + A t 1/2

exp(--A/kT)

exp(--B/V)

Resistance R

Intermetallic cornpound formation

R = e x p (at)

exp ~exp(--A/kT)}

IR

Field-induced motion of mobile ions

I R = K (1 - - e x p

exp (--exp (--A/kT)}

KV

IR

Motion of ions on surface oxide

I R = f ( l n t) o r f ( t n)

exp (--K/kT)

Infinite

IR

P r e c i p i t a t i o n in bulk

I R = At or

e x p [{--(F n + Fd)/kT} n ]

e x p (--B/V)

(--t/to)}

x / ( D t ) 1/2

enhanced leakage current, enhanced recombination current and contact failure. The noise spectral density test presented here includes temperature and bias stresses as well as, potentially, optical and chemical stresses and is nondestructive and relatively rapid. It is novel in that it involves cooling the device rather than heating it. It will function primarily as a sorting tool. However, in certain cases it may also indicate the predominant failure mechanism. The tests described here were made on ITO/Si solar cells as well as p - n junction diodes.

2. Reverse current-voltage test Walsh [ 3 ] and Cocca ~ 4 ] have developed tests utilizing the temperatureand bias-dependent reverse current-voltage characteristics. These tests look for hysteresis in the reverse current-voltage curve as well as doubly or more activated reverse current-temperature curves. The hysteresis is considered "positive" if, after reverse biasing the device to the knee of the breakdown characteristic, the reverse current at a given voltage is lower on the reduction of bias than on the increase. The hysteresis is "negative" if the current increases (i.e. counterclockwise curve). The tests have been shown to correlate reasonably well with extended life tests [ 2]. The problems with the Walsh and Cocca tests are that they are hard to interpret and are sometimes destructive. The noise spectral density test alleviates these problems by using breakdown voltages below the avalanche breakdown voltage of the device and low temperatures. The test is based u p o n deviations from ideal diode shot (and thermal) noise. Noise in metal-insulator-silicon and SIS devices as well as in PIN devices has been studied previously [5 - 7]. Ideal SIS diodes in reverse bias exhibit shot noise plus a much smaller thermal noise component. A non-ideal device exhibits shot noise, thermal noise, generation-recombina-

317 TABLE 2 Diode current and noise mechanisms Is

= T3/2exp

Si(s)

= 2gl s

(--Eg/kT) s h o t noise

T Si(w) -

w2T2

1+

generation-recombination

noise

kTNT(e) Si(w) = K

-

-

f l i c k e r noise

af Si(w) =

4KTG

thermal noise

tion noise and flicker noise. Table 2 summarizes diode noise mechanisms and their spectral dependences. Ideal shot noise is only weakly temperature dependent. At a single temperature it is difficult to interpret or to separate out individual noise mechanisms. However, since the various noise mechanisms exhibit different temperature dependences, the existence of substantial amounts of leakage, recombination and contact problems tends to show up in the temperature dependence of the noise spectral density. The biases involved create relatively high fields which enhance ionic diffusion. The temperature variation sweeps the Fermi level so that certain traps become recombination centers and vice versa. Measurements were made using a cold finger with a thermoelectric cooling element for cooling, temperature control and heating. The device was attached to the thermoelectric element and connected through a batteryoperated Quantec 205C pre-amplifier. The noise spectral density was recorded in a Hewlett Packard 3580A spectrum analyzer. The noise spectral density and the reverse current-voltage characteristics of typical devices are shown in Fig. 1. It is seen that the device with the most non-ideal reverse current characteristic also exhibits the greatest variation and anomalous temperature dependence in noise spectral density. The hysteresis in the reverse current-voltage characteristics is indicative of ion motion on the surface of the devices. The relatively soft breakdown characteristics exhibited in the reverse current-voltage curve of these devices are also indicative of edge effects and imperfections in the device. Low conversion efficiency does n o t correlate with reverse bias hysteresis and variation in noise spectral density. These devices also failed an extended high temperature test at variable rates. Recent data, including detailed I - V and automatic admittance analyses, confirm these trends [ 10]. While these results do not conclusively establish the validity of the technique, the trends indicate its potential utility and consistency with the hypothesis. The trends need to be statistically validated in future work. However, experience with solar cells and with conventional small signal p - n junction diodes [3] points to the potential utility of the temperaturedependent noise spectral density as a device reliability screen and, in some

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cases, diagnostic tool. Previous work on 250 p - n junction diodes has shown that three frequencies and two temperatures are enough to establish a reasonable correlation with device reliability estimates [ 2]. Since water tends to adsorb relatively freely on oxide surfaces [ 8, 9] the potential for ionic contamination is great and extrinsic to the basic failure mechanism in the junction itself. The data obtained so far indicate that fabrication efforts should be directed to passivating the edges of the cell to minimize the effects of ion migration and possible leakage around the cell periphery. Intuitively, effects which have been seen in p - n junction diodes and ITO solar cells will be common to all types of solar cells and not just to ITO. Also, these effects will be accentuated in large-area devices. For example, results from the Jet Propulsion Laboratory large-scale array program indicate that premature failure in field-operated solar cells due to corrosion and ionic contamination is common in commercial cells. References

1 G. Cerofolini and C. Rovere, Ion migration failure in integrated circuits, Thin Solid Films, 4 7 (1972) 83. 2 E. Yon and J. DuBow, Silicon surface passivation for devices, NASA N G R 36-003-067, 1971.

319 3 T. Walsh, A technique for determining life capability of individual semiconductors, ASTM Reliability Conf., Chicago, 1969. 4 F. Cocca, Effects of low temperature on reverse I - V characteristics of silicon diodes, NASA ERC Publications, May 1969. 5 A. Vanderziel and R. Perala, Study of noise in semiconductor devices to noise in PIN diodes, NTIS AD 821 498, September 1967. 6 V. Kumar and W. Dahlke, Low frequency noise in Cr-SiO2-n-Si tunnel diodes, IEEE Trans. Electron Devices, 24 (1977) 146. 7 T. Kleinpenning, Low frequency noise in Schottky barrier diodes, Solid-State Electron, 22 (1979) 121. 8 E. Schlegel, Behavior of surface ions on semiconductor devices, IEEE Trans. Electron. Devices, 15 (1968) 973. 9 M. Atalla, Stability of thermally oxidized silicon junctions in wet atmospheres, Proc. IEEE, 106 (1959) 1130. 10 J. DuBow and P. Smith, Cell and panel reliability estimators, Proc. 15th Large-scale Solar Array Project Integration Meeting, Jet Propulsion Laboratory, January 1980.