Optimizing the wavelength response in one-dimensional p-Si Schottky barrier optical PSDs

Phys. Status Solidi A 208, No. 7, 1718–1725 (2011) / DOI 10.1002/pssa.201026675 p s sa

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applications and materials science

Optimizing the wavelength responsein one-dimensional p-Si Schottkybarrier optical PSDs

Jasmine Henry* and John Livingstone

School of Electrical, Electronic and Computer Engineering, The University of Western Australia, 35 Stirling Highway, Crawley 6009,

Western Australia, Australia

Received 9 November 2010, revised 1 February 2011, accepted 8 February 2011

Published online 9 March 2011

Keywords position sensitive detectors, Schottky barriers, silicon

*Corresponding author: e-mail [email protected], Phone þ61 08 64882537, Fax: þ61 08 64881065

This paper reports on the varying wavelength response of

optical position detectors made of different Schottky barrier

metals, titanium, tantalum and aluminium. The best sensitiv-

ities came from the titanium devices, followed by the tantalum

devices and finally the aluminium devices. The most linear

devices were fabricated from tantalum followed by aluminium

and the worst from titanium. The devices responded well under

low intensity LED illumination indicating that common and

cheap light sources are suitable for producing strong linear

responses in these PSDs. Longwavelength illumination usually

produced the highest sensitivities, though this depended on the

Schottky metal used. Titanium devices averaged an increase of

250% between the responses for short (480 nm) and long

wavelength (873 nm) light. The corresponding increase for

tantalum devices was 125% while it was 116% for aluminium

devices.

� 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Position sensitive detectors (PSDs),produce a linear relation between their electrical output andthe location of a spot of light impinging on a semiconductorjunction by utilizing the lateral photovoltaic effect.Wallmark [1] was the first researcher to fully describe thislateral effect and its potential as an optical position sensor.

PSDs fabricated from a-Si/H have been extensivelyreported with many papers reporting upon novel structuresand applications [2–7]. Some of these innovations are verylarge area devices (5mm� 80mm), flexible substrate PSDsand microcantilever movement detectors. Also reported arecrystalline silicon metal-oxide structures which haveproduced outstanding sensitivities, though over quite shortdistances [8, 9].

Position sensitive detectors are usedmainly for precisionmeasurements such as machine tool alignment, medicalinstrumentation, remote optical alignment and robotic vision[10, 11]. Consequently, PSDs require high resolution andcorrelation between output electrical signal and the positionof the light beam and also must provide continuousinformation. PSDs have no device boundaries or internaldiscontinuities and therefore no ‘blind spots’ in the detectionarea unlike CCDs. Ideally devices should have a highelectrical output, producing for example, millivolts per

linear millimetre movement, but the main feature should bestrong linearity.

1.1 Operating mechanisms PSDs operate in muchthe same way as solar cells, except instead of utilizing atransverse photovoltaic effect, a lateral effect is utilized.When a beam of light is incident on the front surface of atraditional p–n junction, electrons and holes are generated atthe junction. The holes will reinject back into thesemiconductor while electrons will move to the metal sideof the junction. The metal side is more conductive, acting tospread the electrons almost instantaneously, while the lowermobility holes, which are in a less conductive material, tendto remain bunched together. This separation of charge givesrise to the lateral photovoltage measured across the contactseither on the semiconductor side of the junction or the metalside of the junction, though the latter is only useful for certainstructures [8]. Full operating mechanisms can be foundelsewhere [2, 12].

In this work, the figures of merit that have been usedare:

(i) Nonlinearity d ¼ 2s

F¼ 2� RMSdeviation

Full scale


Phys. Status Solidi A 208, No. 7 (2011) 1719

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Nonlinearity – For an ideal device, when the voltageoutput versus distance moved is plotted, a linear relationshipshould be found. An acceptable device has nonlinearities ofless than 15% [3]. This is a measure of the distortion of thesensor output.

(ii) Correlation coefficient, r – this measures the changein one quantity (say distance, x) corresponding to a unitchange in the other (say voltage, y) when the unitsare made comparable. It is defined mathematicallyy�y ¼ s

sysxðx�xÞwhere s is the standard deviation. The

quantity r gives a good indication of device linearity withperfect linearity being indicated when r is j1j.

(iii) Spatial resolution – this indicates the minimumdistance that can be measured when the light spot is movedfrom one position to another. The spatial resolution ofdevices is measured by calculating the nonlinearity ofdevices for data produced using different increments.Reported here are the results for 500mm increments. Lossof linearity occurs close to contacts, caused by the width ofthe spot, so that the wider the spot the earlier nonlinearityoccurs. This tailing off is due to device edge effects related toelectric field distribution [3].

2 Experimental2.1 Device fabrication Devices were fabricated from

p-type (100) silicon obtained from a variety of sourcesdepending on the resistivity of the substrates. This issummarized in Table 1. Four devices types were chosen toillustrate the behaviour of the devices for each metal. DeviceType A is a 20V cm substrate with a thin metal film(transmittance 70–80%), Type B is a 20V cm substrate witha thick metal film (transmittance 20–40%, Type C has a150V cm substrate (metal film transmittance 10–20%) andType D has a 1600V cm substrate (metal film transmittance10–20%). Itwas found that themetal film thickness hadmoreofan effect on sensitivity for lower resistivity substrate devicesand little effect on the higher resistivity substrate devices.

Wafers were cut to 20mm� 10 mm samples and thencleaned and etched using standard procedures. The Schottkymetals were Ti (99.99%), Al (99.95%) and Ta (99.95%) andall were deposited using electron-beam evaporation. Thethickness of the films was varied as detailed in the Resultstables and monitored by a Telemark film thickness monitor

Table 1 Substrate details.

substrateresistivity(V cm)

filmtransmission(%)

device type source

20 70–80 Type A thin film Waferworld Inc.20–40 Type B thick film Uniwafer

150 10–20 Type C Evergreensemiconductormaterials

1600 10–20 Type D Evergreensemiconductormaterials

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and calibrated using a surfometer. Film thicknesses arewithin 10% of stated values. The background pressure wasaround 10�6 Torr, the deposition rates were about 5–10 A/sfor Ti, 5–10 A/s for Al and 1–2 A/s for Ta. Wire leads werethen attached to the front surface and back contacts. The finalactive region of devices, once contacts were in place wasapproximately 8.5–11.5mm along the long axis (substratelength 20mm), depending on the light source. Devicesmeasured here have completely stabilized over time, withsensitivities being similar to those measured some time ago,implying that any material changes relating to oxides andinterfaces have reached a stable state.

2.2 Device measurement procedures Deviceswere tested under a variety of optical sources as listed inTable 2, with no background illumination. The laser beamand LEDs were focused to a spot of diameter 0.5–0.75mm.The spectral output of each light source was measured tocheck spectral spread and peak wavelength. The spectralhalf-widths were obtained from manufacturer data sheets.

The light intensity of 1mW was chosen as it was thehighest wattage that ALL the light sources could deliver.

For position measurements, the devices were configuredin photovoltaic mode, utilizing the semiconductor contacts,and voltage was measured as a function of light beamposition. These were then plotted, with the slope of the linegiving mV/increment (sensitivity) and the correlation co-efficient giving a measure of linearity. Additionally thenonlinearity, as described earlier, was also calculated as thisgives amore exact measurement of output distortion than thecorrelation coefficient.

Devices operated in the photovoltaic mode, the simplestconfiguration, with the measured voltage being measuredbetween two coplanar contacts, with the other two contactsshort-circuited as in Fig. 1. Note that devices can bemeasured across either the semiconductor or the metal sideof the junction. The best result can be obtained from eitherside depending on the relative resistivities of each side. Thespatial resolutions and nonlinearities were measured andfound to be better than 10mmand less than 6%, respectively.

3 Results Results of PSDs under each light sourcewhere s is sensitivity (mV/mm) and r is the correlation co-efficient of position versus voltage output.

Tables 3–5 show the results for Al, Ta and Ti PSDs undereach light source where s is sensitivity (mV/mm) and r is the

Table 2 Light sources.

light source peakwavelength(nm)

opticalpower(mW)

spectralhalf-width(nm)

blue LED 480 1 20green LED 521 1 35red LED 663 1 30IR LED 873 1 80laser diode 635 1


1720 J. Henry and J. Livingstone: Optimizing the wavelength response in 1D p-Si Schottky barrier optical PSDsp

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Figure 1 PSDconfiguredinphotovoltaicmode.Anarrowindicatesthe impinging light beam.

correlation coefficient of position versus voltage output.Nonlinearity, d, as calculated from the formula stated earlierand the length over which the output was taken is also given.

3.1 Aluminium devices From the lowest averageLED output (5.28mV/mm), for the 480 nm LED to the bestaverage LED output (11.42mV/mm), which was for the873 nmLED, there was an increase of in sensitivity of 116%.The laser results were not included in this calculation sincethe laser spot is SMALLER than the LED light spots.

Al devices had the least clear-cut trend betweensensitivity and wavelength – as can be seen in Fig. 2.Generally speaking, (i) longer wavelength light produced

Table 3 Results for aluminium devices.

device type A B

aluminium film thickness (A) 100 300approx. transmittance (%) 70 40blue 480 nmS (mV/mm) 0.91 1.06r 0.9993 0.99linear distance (mm) 9.5 10.0nonlinearity (%) 2.41 4.20

green 521 nmS (mV/mm) 1.06 1.19r 0.9981 0.99linear distance (mm) 10.5 10.5nonlinearity (%) 2.57 2.31

red 663 nmS (mV/mm) 2.41 2.56r 0.9989 0.99linear distance (mm) 8.5 10.0nonlinearity (%) 3.01 4.86

IR 873 nmS (mV/mm) 6.24 7.30r 0.999 0.99linear distance (mm) 8.5 10.0nonlinearity (%) 2.83 2.47

laser 635 nmS (mV/mm) 0.952 1.84r 0.9961 0.99linear distance (mm) 11.5 11.0nonlinearity (%) 5.45 2.29


higher sensitivities while the shorter wavelength lightproduced poorer sensitivities, (ii) lengths of linear regionsare shorter for the red and infrared LED illumination whilealso having higher nonlinearities (Fig. 2).

3.2 Tantalum devices In Table 4 we can see thatsensitivity of the tantalum PSDs increases with increas-ing wavelength. The linear regions are greater for theshorter wavelength light. In comparison to the Al devices inTable 3, sensitivities are lower but linearity is higher(nonlinearity is lower) and linear distances are greater (Fig. 3).

From the lowest average output (4.96mV/mm), for the521 nm LED to the best average output (11.15mV/mm) forTa devices, which was for 873 nm LED to, there was anincrease of in sensitivity of 125%.

3.3 Titanium devices From Table 5 it can be seenthat for the titanium PSDs, sensitivity is proportional towavelength of light and that there is a distinctly highernonlinearity for these devices compared with the shorterwavelength light (Fig. 4). This is similar to the results foundfor Al and Ta PSDs, although the sensitivities here arehigher, while linearity is lower than that found for tantalumand about the same as that found for aluminium.

There was a 250% improvement in titanium devicesensitivities between the lowest average output (which wasfor 480 nm LED) and the highest average output (which wasfor the 873 nm LED).

C D average

600 42010 15

3.76 15.40 5.2890 0.9995 0.9990 0.9992

9.5 10.0 9.81.98 2.73 2.89

3.94 15.28 5.35093 0.9992 0.9990 0.9993

9.5 11.0 10.382.53 1.95 2.34

6.50 23.12 8.6672 0.9981 0.9960 0.9975

9.0 9.0 9.133.91 5.76 4.39

13.46 18.68 11.4293 0.9975 0.9965 0.9981

9.5 9.0 9.254.73 5.64 3.920

4.26 12.78 4.96088 0.9996 0.9993 0.9985

10.5 9.0 10.501.82 2.61 3.040

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Table 4 Results for tantalum devices.

device type A B C D average

tantalum film thickness (A) 100 280 400 400approx. transmittance 70% 26% 20% 20%blue 480 nmS (mV/mm) 2.75 0.56 6.23 10.3 4.96r 0.9996 0.9990 0.9993 0.9999 0.9995linear distance (mm) 11.5 10.5 11.0 12.0 11.3nonlinearity (%) 1.82 2.73 2.38 0.74 1.92

green 521 nmS (mV/mm) 2.86 0.68 0.68 10.64 5.22r 0.9991 0.9997 0.9988 0.9990 0.9992linear distance (mm) 12.5 10.0 12.0 12.0 11.63nonlinearity (%) 2.70 1.51 2.97 0.91 2.03

red 663 nmS (mV/mm) 4.18 1.18 15.88 13.40 8.66r 0.9999 0.9980 0.9989 0.9967 0.9984linear distance (mm) 12.5 10.0 7.0 10.0 9.88nonlinearity (%) 0.97 3.98 2.99 5.08 3.26

IR 873 nmS (mV/mm) 5.48 4.20 20.16 14.74 11.15r 0.9993 0.9994 0.9995 0.9991 0.9993linear distance (mm) 11.5 9.0 10.0 11.0 10.4nonlinearity (%) 2.42 2.15 1.97 2.42 2.24

laser 635 nmS (mV/mm) 3.42 1.42 8.76 10.68 6.07r 0.9990 0.9995 0.9984 0.9995 0.9991linear distance (mm) 12.5 11.5 9.0 10.0 10.8nonlinearity (%) 2.24 2.05 3.67 1.89 2.46

Table 5 Results for titanium devices.

device type A B C D average

titanium film thickness (A) 200 400 500 350approx. transmittance 80 30 15 10blue 480 nmS (mV/mm) 1.26 0.64 3.78 12.41 4.46r 0.9992 0.9992 0.9992 0.9991 0.99917linear distance (mm) 12.0 8.5 9.0 12.0 10.4nonlinearity (%) 2.45 1.99 2.41 2.45 2.33

green 521 nmS(mV/mm) 1.14 0.74 5.70 12.74 5.08r 0.9978 0.9987 0.9986 0.9981 0.9983linear distance (mm) 12 11.0 11.0 11.5 11.4nonlinearity (%) 4.45 3.26 2.75 3.53 3.50

red 663 nmS (mV/mm) 2.86 2.18 12.6 16.0 8.40r 0.9994 0.9990 0.9986 0.9977 0.9987linear distance (mm) 11.0 9.0 9.0 9.0 9.5nonlinearity (%) 2.27 2.79 3.48 4.04 3.15

IR 873 nmS (mV/mm) 5.64 5.84 26.14 24.88 15.62r 0.9985 0.9944 0.9992 0.9973 0.9974linear distance (mm) 11.5 9.0 9.0 9.5 9.8nonlinearity (%) 3.61 7.31 2.56 4.47 4.49

laser 635 nmS (mV/mm) 1.20 poor 5.92 6.80 4.64r 0.9993 0.9995 0.9925 0.9971linear distance (mm) 10 - 12.5 9 10.5nonlinearity (%) 2.22 - 1.88 4.32 2.81

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Figure 4 (online colour at: www.pss-a.com) The relationshipbetween device sensitivity and incident light wavelength for tita-nium devices. Devices have higher sensitivities at longer wave-lengths.

Figure 2 (online colour at: www.pss-a.com) The relationshipbetween device sensitivity and incident light wavelength foraluminium devices. Devices have higher sensitivities at longerwavelengths.

Figure 3 (online colour at: www.pss-a.com) The relationshipbetween device sensitivity and incident light wavelength for tanta-lum devices. Devices have higher sensitivities at longer wave-lengths.

In general it can be said that for all the PSDs, lowersensitivities gave better linearities as illustrated by lowernonlinearity measurements. The mechanism for this isoutlined in the Section 4.

4 Discussion Table 6 sums up the best individualperformances of all the devices reported in Tables 3–5for each of the metals. Also included are the ranges ofaverage values of sensitivities, nonlinearity and linear

Table 6 Summary of best results from graphs and tables above.

Al

best averages for each metalsensitivity (mV/mm) 11.42r 0.9981light source (nm) 873

individual bestsensitivity (mV/mm) 23.12r 0.9960light source (nm) 663

range of average sensitivity of (mV/mm) 5.28–1range of average nonlinearity (%) 2.885–range of average linear region lengths (mm) 9.13–1


distances for each metal used. In practical terms, it ispossible for certain structures to be chosen to optimizethe response required for specific applications/lightwavelengths.

From Table 6 it can be seen that aluminium devices hadthe poorest linearities while still not producing the bestsensitivities. Tantalum had the best linearities and longestlinear regions but lowest sensitivities. Titanium deviceswere, overall, the best devices in terms of sensitivities whilestill producing long linear regions. The differences inperformance of each metal are attributed to the opticalproperties of each metal.

Also observed was that the sensitivity of the PSDs,regardless of metal used, increased with increasing wave-length in most devices and this could be due to;

(i) t

1.424.3850.38

he fact that, for the sameradiantflux,numberofphotonsincreases with decreasing wavelength and/or

(ii) o
ptical work functions of the metals and/or (iii) t he optical properties of the metal films.
Thefirst factor is largely self-explanatory– if twoelectro-
magnetic waves have the same radiant energy (total opticalpower of electromagnetic radiation) but different wave-length, the longer wavelength photons have less energy,but there are more of them. Calculating the photon density
Ti Ta

15.62 11.150.9974 0.9993873 873

26.14 20.160.9992 1.0000873 8734.46–15.62 4.96–11.152.325–4.490 1.920–3.2559.50–11.38 9.88–11.63

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for theLEDsources, the IR(890 nm)LEDhas twice thephotondensity as the blue (480 nm) LED. The respective responsesshow that the IR LED produces more than proportionatelyhigherresponses than theblueLEDformostdevices indicatingthat other mechanisms must also be at work.

As for the second factor, from our results we surmise thatthe metal work function does not have a large impact ondevice sensitivity, i.e. the size of the barrier is not a majordetermining factor in device performance, as long as a barrieris formed so that carriers can be separated. The workfunctions of Al, Ti and Ta, respectively, are: �4.1, 4.33 and4.0–4.8 eV. Theoretically Al should produce the highest SBheight while Ta should produce the lowest.

Possibly the third factor is most influential, i.e. lightpenetration depth. This is important as this determines wherecarriers are generated and whether it is close or far from thecollecting contacts and hence whether recombination plays amajor role in device performance. The optical constants, nand k are important. It is known that n and k are dependentupon film deposition, morphology and incident lightwavelength and this will also be discussed here.

4.1 Factors affecting optical properties of metalfilms In general electron-beam deposition of metalsproduces films closest in physical properties to the bulkmaterial compared with slower deposition techniques suchas sputtering. Faster, colder deposition conditions producethe best films in reactive and easily oxidized metals (such asAl), although Ta is a partial exception to this [13–15]. Taneeds to be deposited in UHV and at high rates in order toavoid the formation of TaxOy. In this work Ta films weredeposited at around 10�6 Torr and so it is likely that at theslow deposition rates here, the Ta films will be at leastpartially oxidized [16]. The key point of this argument is thatTa evaporated on to room temperature substrates has a ß-Tacrystal structure with a free-electron lifetime and mean freepath about 10 times less than that in a-Ta, i.e. with anelectron lifetime about 3.8� 10�15 s and so recombination islikely to be an issue in the devices here [17].

Secondary ion mass spectroscopy (SIMS) work wasundertaken previously in other work to study the effects ofaging on the Schottky barrier junctions. The results showedthat titanium junctions had little change with aging and thatfilmswere oxidizedwhen newly deposited. Aluminiumfilmsshowed an increased presence of oxygen with aging andcorresponded to improved sensitivities perhaps due to filmsbecoming more transparent. Tantalum films showed aninitially oxidized film and an increased oxygen content at thesilicon surface with aging and this was also accompanied byan improved sensitivity. The devices here are 12 months oldand therefore expected to have fully oxidized [18].

Film thickness has no direct bearing on n and k of acontinuous metal film – instead n and k vary with incidentlight wavelength. However, very thinmetal filmsmay in factnot be continuous (these may form islands or a semicontin-uous film) and this will affect both the optical and electricalproperties of a material greatly. Discontinuous metal films

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result in anomalous optical properties quite different to bulkmaterials. Typical is the transmittance of a material – it isminimum in the visible rangewhile highly transmitting in theinfrared range and this is entirely related to filmmorphology[19]. Thin films have shorter diffusion lengths where thickerfilms have an almost infinite diffusion length but the films areless permeable to light.

Very thin films have large physical stresses caused bymany factors relating to the way the film deposits and formsand most critically, are the film thickness and depositionrate. For example, Al films between 200 and 260 A have largeholes regardless of the deposition rate and truly continuousfilms occur at 600 A [20, 21]. At high deposition rates a largenumber of atoms arrive at the surface per unit time and thedensity of stable nuclei is large – these clusters grow with theaddition of more atoms. The critical evaporation rate toproduce maximum grain size was found to be around 17 A/s(higher than any rates used in this work) – fast rates alsoproduce a rougher film [20]. This implies that the Al films inthis work are semi-continuous and partially oxidized.

4.2 Penetration depths The refractive index of themetal films at each wavelength of light should be consideredto determine if the metal films simply become moretransparent at higher wavelengths of light.

From n and k values, representing the real and quadraturecomponents of the refractive index of a material, theabsorption coefficient, a, can be calculated froma ¼ 2vk=c. The value of 1/a gives the skin, or penetrationdepth of light through a material. For a material to betransparent the penetration depth should be greater than thematerial’s thickness [22]. As stated previously, n and kvalues ‘do not vary with film thickness’, but do vary withincident light wavelength [23]. This would explain whydevice output tends not to vary much with film thickness(except for high resistivity substrates, where aspects ofresistivity are a major mechanism in determining devicesensitivity). FromTable 7, values of n and k for Ti, Al and Taare given for wavelengths close to those of the light sourcesused in this work, and the corresponding calculatedpenetration depths. There is a large range of data for therefractive indices of Ta reported, but this does not make a lotof difference to the penetration depths calculated, as k is themain factor for each metal.

From Table 7 the calculations show that the penetrationdepths increase with increasing light wavelength and are asfollows:

(i) A
l penetration depth: 72–95 A (ii) T i penetration depth: 118–168 A (iii) T a penetration depth: 400–630 A.
These figures are indicative of the relative penetrationdepths, rather than actual depths. Previous discussionindicates the strong likelihood of film oxidation which willincrease the penetration depth of the light sources. It doeshowever give an indication of the relative depths of light



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Table 7 Calculated penetration depths for metals at varying wavelengths.

wavelength (nm) n k v a (m�1) 1/a (m) metal reference

488 2.44 3.3 3.8626� 1015 84977434.43 1.17678� 10�8 Ti [24]514 2.54 3.43 3.66723� 1015 83857227.24 1.1925� 10�8 Ti [24]618 2.76 3.84 3.05009� 1015 78082236.89 1.2807� 10�8 Ti [24]660 2.86 3.96 2.85599� 1015 75398160 1.32629� 10�8 Ti [24]836 3.29 3.96 2.25473� 1015 59524863.16 1.67997� 10�8 Ti [24]492 0.64 5.5 3.83121� 1015 140477601.6 7.11857� 10�9 Al [25]546 0.82 5.99 3.4523� 1015 137861715 7.25365� 10�9 Al [25]650 1.3 7.11 2.89993� 1015 137456645.5 7.27502� 10�9 Al [25]850 2.08 7.15 2.21759� 1015 105705263.5 9.46027� 10�9 Al [25]450 2.84 0.69 4.18879� 1015 19268418.67 5.18984� 10�8 Ta [26]546.1 2.43 0.7 3.45166� 1015 16107767.81 6.20818� 10�8 Ta [26]546.1 3.5 0.69 3.45166� 1015 15877656.84 6.29816� 10�8 Ta [27]546.1 2.3 0.7 3.45166� 1015 16107767.81 6.20818� 10�8 Ta [28]632 1.59 1.24 2.98252� 1015 24655516.46 4.05589� 10�8 Ta [26]632 2.46 1.05 2.98252� 1015 20877655.06 4.78981� 10�8 Ta [29]472 2.52 2.96 3.99355� 1015 78805986.44 1.26894� 10�8 bulk Ta [25]622 2.13 2.89 3.03047� 1015 58387106.75 1.71271� 10�8 bulk Ta [25]

penetration for each of the Schottky metal films, i.e. Al hasthe shallowest penetration depth while Ta has the deepestpenetration. By calculating the penetration depths theapproximate region of carrier generation can be ascertained.If carriers are too far from the effective p–n junctionregion (either too close to the surface or too deep into thesemiconductor), then they are more likely to suffer fromrecombination before collection by contacts.

4.2.1 Aluminium films Aluminium has very shallowpenetration depth and this indicates that light does notpenetrate deeply into the junction and therefore fewercarriers will be generated in the region of the junction andresults in poorer responses. Also the oxidation of the films isunlikely to be uniform for each device – thinner films willhave a higher proportion of oxidation than thicker films andso results will be affected by film thickness variation sincegood PSD operation relies on uniform layers. Very thin filmsare also unlikely to produce a good, uniform Schottkybarrier. It has been documented that thermally evaporated Alfilms comprises of islands which are discontinuous untilabout 500–600 A in thickness where the islands begin tocoalesce [21, 30]. Techniques such as electron-beamevaporation, where the particles have higher energy,continuous films are formed at lower thicknesses [19]. TheAl films reported here are around 200–300 A, and it wasfound that devices with thicknesses lower than this did notperform well. Al has the lowest average of the responses.

4.2.2 Tantalum films The penetration depth of Ta isfar larger than that calculated for Al and Ti, indicating thatcarriers will be generated quite far from the junction anddeeper into the semiconductor leaving carriers further totravel before collection by contacts and therefore probablymore prone to recombination than the other two metals sincecarriers will be generated furthest away from the collectioncontacts.


4.2.3 Titanium films The Ti devices had the bestaverage sensitivities and overall performed the best of thethree Schottky metals. The calculated penetration depthsare less than most of the film thicknesses and if films werepure titanium then light would not reach the junction.Evidence from other researchers and our SIMS results [18]suggests that the Ti films are partially oxidized allowing for adeeper penetration of light. What can be surmised is that thelonger wavelengths of light are able to penetrate deeper intothe device and closer to the junction leading tomore efficientcarrier collection resulting in higher sensitivities. Ti mayhave the best compromise of Al with low penetration and Tapenetration too deep with regard to film thickness (and pathlength through the film), position of the junction and filmconductivity/carrier diffusion length. Titanium films havebeen found to be electrically, and therefore probablyphysically, continuous even at 25–30 A, the films used inthis work are around 200–400 A [31].

A factor pertinent to all devices, if one considers theabsorption co-efficient of silicon [32, 33] which decreasessteadily with wavelength, while reflectance [34] remainslargely constant (i.e. more is transmitted to the junction),then this implies that the penetration depth increases withincreasing wavelength and adds to the increased response athigher wavelengths. This is true for all the metals and sodifferences in PSD responses for eachmetal are related to theoptical properties of each metal.

Another possible factor influencing PSD behaviour isfrom a ‘lateral optical effect’ described by Martins andFortunato [35]. The paper refers to a model explaining whyPSD performance can be adversely affected by low lightlevels incident upon low conductivity layers in a PSD. Theremay be influence from this factor in the devices here but it isconsidered that the effect would be very small in comparisonwith other factors. The light levels referred to are 1–100mW,while in this work the light levels are at least ten times this:1000mW(1mW). Furthermore the conductivity of themetal

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layers in this work are thought to be substantially higher thanthe doped amorphous silicon layers used in the Martins’work.

4.3 Effect of wavelength on device non-linearity In terms of the effect of wavelength on devicenonlinearity, it can be very generally said that all devices hadthe worst average nonlinearity under the longer wavelengthlight (red or IR LED) while the best average nonlinearitiesoccurred for the shorter wavelengths (blue or green LED).This is related to the fact that the highest sensitivities aremeasured at longwavelengths and the lowest sensitivities aremeasured at short wavelengths. This can be explained interms of the ‘lateral energy’ of carriers as Yu and Wangexplain [9]. Carriers have a lateral energy EL/hn�Eg. Thelarger the lateral energy then the better the diffusion lengthand so at the longest wavelengths,EL is lowest and so carriersare less likely to reach the contacts and hence a poorerperformance results.

5 Conclusions Good PSD operation cannot be pre-dicted by considering metal and semiconductor workfunctions and Schottky barrier formation alone.

It was shown that most of the PSDs produce highersensitivities with increasing wavelengths. This can beattributed to the following:

(i) f

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or the same radiant energy, longerwavelength light hasa larger number of photons and

(ii) th
e optical properties of the device structures. Therefractive indices of metals play a major role in PSDsensitivities.
(iii) s
iliconabsorptioncharacteristicswhere there increasinglight absorption for increasing light wavelength.
As for the differences in PSD behaviour for each metal:the best sensitivities were from Ti, then Al and finally TaPSDs. This is due to the optical properties of each metal,where Ti films allow for the best light penetration depths ofthe three metals. Al and Ta films produce light penetrationwhich are either too shallow or too deep, resulting in carriersrecombining before collection and hence lower sensitivities.Carriers furthest away from the contacts have further totravel to reach contacts and suffer from recombinationespecially since thinmetal films have lower diffusion lengthsthan thicker films and bulk materials. Work function is notconsidered to play a role in the differences in results found.

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Optimizing the wavelength response in one-dimensional p-Si Schottky barrier optical PSDs

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