a-Si:H Schottky diode for hydrogen detection

Sensors

and Actuators,

4 (1983)

349

349 - 356

TRANSPORT PROPERTIES OF A Pd/INSULATOR/a-Si:H DIODE FOR HYDROGEN DETECTION*

A D’AMICO,

G FORTUNATO

Istrtuto dr Elettromca Rome (Italy)

dello

SCHOTI’KY

and G PETROCCO

Stato

Solrdo

de1 C N R

Vta Cmeto

Romano,

42-00156

C COLUZZA Istttuto

dl Aszca

“G Marconi”

Unwersita

degll Stud1 dz Roma,

Rome

(Italy)

Abstract A new metal-msulator-semiconductor (MIS) Schottky diode sensitive to hydrogen has been fabncated and tested Its mam feature 1s the utlllzatlon of low-cost hydrogenated amorphous sllrcon (a-S1 H) obtamed by the gIow discharge technique The catalytic metal IS palladium and the insulator IS a thm film (20 a) s&on dioxide layer Experunental results mdlcate that it 1s possible to achieve mverse current variations of more than 3 orders of magnitude during the adsorption or desorptlon of hydrogen Results obtained on C/V measurements at room temperature and on I/V characterlstlcs m the range 22 - 120 “C are given and discussed Prehmmary conslderatlons of the stability problems are also given

1. Introduction In the last few years many kinds of chemically-sensltlve semiconductor devices have been studied for hydrogen detectlon Among these are the MIS and MOS capacitors and transistors 11 - 4] and MIS [5,8] and MS [9, lo] Schottky beer diodes In most of these devices single crystal ahcon has been used as the semiconductor material The reason for the hydrogen senwtlvlty 1s the well-known catalytic behavlour of palladium or platinum m combmatlon with the intrinsic transport properties of the structure used [2,7]. In this context, two mam hypotheses are mentioned m the references one deals with the variation of the contact potential between the Pd and the semiconductor [2,4], the other refers to the current control due to the varlatlon m the number of interface states mduced durmg hydrogen adsorptlon or desorptlon [8,11] *Based on a Paper presented at Solid-State May 31 - June 3,1983 0250-6874/83/$3

00

Transducers 83, Delft, The Netherlands,

0 Elsevler Sequola/Prmted

m The

Netherlands

350

In this work we will consider a new MIS structure, where the metal 1s still a thm Pd film, but the semlconductor 1s a thm f&n of hydrogenated amorphous sillcon (a-S1 H) The msulator IS a thin f&n of naturally grown oxide whose thickness of about 20 ii has been estimated according to the oxldatlon tune law deduced by Ponpon and Bourdon [ 121 The nnportance of such an msulatmg layer IS relevant because It must prevent the dlffuslon of Pd atoms mto the a-& H to form slhcldes which strongly reduce the sensltlvlty to H, [6] On the other hand, this oxide layer should not be too thick m order to allow the flow of the current The use of a-S1 H 1s promlsmg for producmg hydrogen sensors with low-cost thin film technoIogy, as a recent paper [ 151 gives prehmmary data concerning the sensltlvlty to Hz at room temperature of a similar MIS a-S1 Hbased diode Here we will concentrate on results obtained on the temperature behavlour of I/V curves m the range 22 - 120 K for a given Hz concentration m an N2 + H, mixture and on C/V measurements Response time curves taken at 47 “C are also shown and discussed

2 Preparation of the MIS structure Figure 1 shows the MIS diode considered m this work, thicknesses of the films as well as the dunenslons are also indicated The technological steps used to fabricate such a structure are the following (1) Etchmg and cleanmg of a surface glass (substrate) (2) Deposltlon of - 3000 ii of chrommm by the r f sputtermg techmque

300ym

I

Fig

I,

_

I,.-_

1 SchematIc of the MIS diode

351

(3) Depoatlon of - 300 ii of n+doped a-S1 H using a glow discharge m an SlH4 + 1% PH3 gas mixture to provide the back ohmic contact (4) Deposltlon by the glow discharge technique of about 6000 a of an undoped layer of a-S1 H (5) Growth of an oxide layer (20 ii) by oxldatlon m air of the a-S1 H surface (6) Thermal evaporation of Pd (350 A) and lift-off technique (L 0 T ) for defmltlon of the catalytic area (7) Gold evaporation (2500 a) and L 0 T for formation of contacts

3. Results and dlscusslon The transport measurements were performed by placing the sample m the dark, m a~ or m the presence of Nz + Hz fluxes with different concentrations of Hz, almost at atmospheric pressure The I/V characterlstlcs of the MIS diode based on a-S1 H can be described by both the dlffuslon mechanism 1131 and by tunnellmg through the thm oxide layer [14] In this case, the followmg relatlonshlp for the current density holds

J = wGJcE, ew(-WKT)T

[em (-g

-I)] =Jo[exP(-g -I)]

where pC IS the electron mobility m the conduction band (c b ), NC IS the effective density of states m the c b , E, 1s the electric field at the surface of the semiconductor, GB 1s the barrier height, 7 1s the transmlsslon coefficient across the oxide layer and n IS the quality factor which, m this structure, takes mto account that part of the applied voltage which 1s developed across the msulatmg layer [ 141 In Fig 2 forward and reverse I/V characterlstlcs for a typical MIS drode are shown They refer to data taken when the device IS m air or m an Nz + H, mixture with 280 ppm of H, at room temperature It 1s worth pomtmg out the large variation of the reverse I/V curve between the two stationary condltlons relative to the adsorptIon and desorptlon processes More than 3 orders of magnitude difference 1s obtamed, comparable with the best sensltlvlty observed m single crystal s&con-based diodes [ 51 From the slope and the mtercept to zero bias of the forward characterlstlc m ar, rt LSpossible to estunate the n and Jo values, which are equal to 1 .ll and 3 X lo- lo A/cm* respectively These values agree with a good quality structure In hydrogen the forward characterlstlc becomes hmlted by the series resistance and nH, JOH cannot be evaluated with the above procedure In order to investigate the transport property vaatlons induced by the hydrogen absorption, the barrier height values have been determmed by the temperature dependence of the saturation current, and the built-m potential has been evaluated by the C/V relatlonshlp

352

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Fig 2 I/V curves of the MIS structure exposed to air (0) and to 280 ppm Hz m N2 + Hz mixture (e) at room temperature Fig 3 Saturation current m air (0) and in 0 5% H2 m Nz f H2 mixture (+) as a function of 103/T From the slope It 1s possible to determine the barrier height 1 04 eV and 0 56 eV respectively for the two condkons In the first case we determined, at different temperatures up to 120 “C m ax, the saturation current values Jo,while from the I/V reverse curves, m the presence of a flux of 0 5% H, m an N, + Hz mixture, the Jaw values have been estnnated Figure 3 shows both the Jo and JOH behavlours uersus 103/T These two curves allow the determrnatlon of the barrier height @a m air (1 04 V) and $BH m H, (0 56 eV) The correspondmg lowering m the barrier height m this case 1s then equal to 0 48 eV The C/V measurements were obtained at a signal frequency of ‘7 Hz usmg both a Prmceton Applied Research CV system Model CV 410 and a lock-m amghfier, Model 124 A Plotting l/C* uersus the applied voltage, it has been possible to determine the built-m potentials V,, V,, as shown m Fig 4, where the data refer to ax and to 0 5% H, at room temperature The values of V, and V,, for the

353

4r------

-0s

04

02

0

-02

-04

-06

-08

V (VOLT)

Fig 4 l/C2 uersus V for the MIS diode m au (e) and m 0 5% Hz m N2 + H2 mixture (0) The mtercepts and the slope are a measure of the budt-m potential and of the space charge den&y respectively The values obtained for the two condltlons are 0 56 eV for V, and 0 07 eV for VBH, while the slopes give for the space charge density a value of 6 0 x lOI cm-3 in an (IV) and 5 3 x 1015cme3 m Hz (NH)

two condltlons are given by 0 56 eV and 0 07 eV respectnrely, with a lowering of 0 49 eV after the admlsslon of H, By the slope of the curves m Frg 4, the calculated space charge den&y N 1s equal to 6 X 1015 cm-’ (m air) and seems to change only a httle under Hz exposure Furthermore, no features appear when the device 1s exposed to hydrogen Consequently the mechanism for the hydrogen senetlvlty seems to be connected with a change m the contact potential V, which 1s reflected m a reduction of both the barrier height &, and the built-m potential V, [2) This idea also agrees with the conslderatlon that m a-S1 H the density of states m the gap 1s so high that a slgrnflcant further Increase due to the Hzinduced trapping states seems improbable It 1s mterestmg to note that the values obtamed for the reduction of V,,resulting from both the temperature dependence of the saturation current and by the C/V measurements, are m good agreement with each other and comparable to those obtamed by other authors m angle-crystal based MIS structures [2] Finally, consldermg the values of 4B and V, or $BH and VBH,we can approxnnately evaluate the posltlon of the Fermi level m the a& H semiconductor matenal used, this 1s about 0 5 eV below the conduction band, in agreement with the actlvatlon energy value AE - 0 53 eV obtained by the senes conductance analysis of the diode

354 02 i

T-47

C

T=47”C 0

01

% H2 I

’

Fig

012345

10 f(MIN)

5 Output current response uersus

Nz, (b) for 100 ppm Hz m Hz + NZ

time of the MIS structure (a) for 0 5% Hz m Hz +

Figure 5(a) shows a typical output current response monitored as a function of tune with the mjectlon and removal of Hz, and subsequent mjectlon of O2 m the test chamber, of the MIS diode when a voltage polarization of -0 5 V and an H, concentration of 0 5% are used The time requu-ed to reach 90% of the saturation value I,,, 1s less than 1 minute at 47 “C This time 1s normally controlled by the followmg parameters hydrogen concentration, device temperature, volume of the test chamber and gas flux mtensity During the desorptlon process m the presence of a flux of oxygen, the then an almost exponentml decay fall tune 1s a few seconds until 0 3 I,,,, whose tune constant 1s about 1 5 mm, with the condltlon shown m Fig 5(a), puts the device m the orlgmal stationary condltlon These results are m agreement with the transient behavlour of analogous devices discussed in ref 16 Figure 5(b) shows another typical response when an H2 concentration of 100 ppm 1s used This time can be lowered, for instance, by reducmg the chamber test volume which m our experiment IS about lo3 cm3 The output current behavlour observed by analysmg many absorption and desorptlon cycles shows that on the same day both the mltlal and the saturation levels are restored wlthm an accuracy of less than 2% (short time stability) A comparison of measurements made on different days a week apart (long tune stablhty) shows that the accuracy decreases to about 8 12% when 0 5% Hz 1s used and to about 5 - 9% m the case of 100 ppm H, There may be several reasons for such mstabllltles a first posslblhty can be the aging of the diode, m fact accelerating procedures such as annealmg at about 130 “C seem to increase the stablhty of the response of the structure

355

A second posslblhty arises from contammatlon of the Pd which can decrease its catalytic efflclency Finally another reason may be the dlffuslon of atomic hydrogen through the oxide and the first few monolayers of the a-S1 H This process, which can to a certam extent influence the transport current, needs to be investigated further

4. Conclusions A new MIS diode sensitive to hydrogen using hydrogenated amorphous s&on as the semiconductor materml has been fabricated and tested Its mam characterlstlc IS the large reverable varlatron (more than 3 orders of magnitude with the condltlons of Fig 2) of the reverse current after the adsorption and desorptlon processes have taken place In these devices the mechanism that seems to be responsible for the sensltlvlty 1s the variation of the contact potential, as ISpointed out by C/V measurements As m other slmllar structures mentioned m the literature which utilize Pd as a metal and single crystal slllcon as semiconductor, the stability of the structures 1s still a problem to be more deeply investigated Pd aging and polsonmg, as well as the modlflcatlon of both the Pd/S102 and S102-a-S1 H interfaces due to a possible diffusion of H atoms occurrmg during the absorption process, can alter the over-all performance of the device m terms of sensltmlty and stability, although m some appllcatlons the results obtamed with the MIS diode dlscussed here can already be considered satisfactory The slmphclty of the fablncatlon steps and the advantages of thm f&n technology remam as the mam posltlve features of this structure

References 1 I Lundstrom,M S Shlvaraman and C Svensson, A hydrogen sensltlve Pd-gate MOS transistor, J Appl Phys , 46 (1975) 3876 2 I Lundstrom, Hydrogen sensltlve MOS-structure Part 1 Prmclple and apphcatlons, Sensor and Actuators, 1 (1981) 403 - 426 3 I Lundstrom, M S Shlvaraman and C Svensson, Chermcal reactlons on palladmm surfaces studied with Pd-MOS structures, Surface Sczence, 64 (1977) 497 4 M C Steele, J W Hlle and B A MacIver, Hydrogen sensltlve palladium gate MOS capacitor, J Appl Phys, 47 (1976) 2537 5 F Rouths, S Ashok, J Fonash and J M Rouths, A study of Pd/Sl MIS Schottky barrierdiode hydrogen detector, IEEE Trans Elect Deu , ED-28 (9) (Sept ) (1981) 6 S J Fonash, H Huston and S Ashok, Conductmg MIS diode gas detectors the Pd/ SlO,/Sl hydrogen sensor, Sensors and Actuators, 2 (1982) 363 - 369 7 M S Shlvaraman, I Lundstrom, C Svensson and H Ammarsten, Hydrogen sensltivlty of palladium thm oxide slhcon Schottky barriers,Elec Lett , 12 (1976) 483 8 J N Zemel, B Keramatl, C W Spwak and A D’Amlco, NATO-Advanced Study Instztute on Chemzcally Sensztzve Electronac Devzces. 1980, Sensors and Actuators, 1 (1981) 427

Hzghtstown

NJ, June 9 - 21,

356 9 M C Steele and B A MacIver, Palladmm cadmium-sulfide Schottky diodes for hydrogen detectron, Appl Phys Lett , 29 (1976) 687 10 K Ito, Hydrogen sensltlve Schottky barrier diodes, Surface Scrence, 86 (1979) 345 11 B Keramatl and N J Zemel, Pd-thm SlC&-51 diode I Isothermal varlatlon of H2 mduced mterfacral trappmg states, J Appl Phys , 53 (1982) 1091 12 J P Ponpon and B Bourdon, Oxldatlon of glow-discharge a-S1 H, Sol State Electr , 25 (1982) 875 13 C R Wronsky, D E Carlson and R E Damel, Schottky-barrier characterlstlcs of Phys Left, 29 (1976) 602 metal amorphous sillcon diodes, Appl 14 H C Card and E H Rhoderlck, Studies of tunnel MOS diodes I Interface effects in &con Schottky diodes, J Phys D Appl Phys , 4 (1971) 1589 15 A D’Amlco, G Fortunato, G Petrocco, C Coluzza, Pd/a-S1 H metal-msulatorsemiconductor Schottky barrier diode for hydrogen detection, Appl Phys Left, 4.2

(1983) 16

964

I Lundstrom and D Soderberg, terlzatlon, Sensors and Actuators,

Hydrogen sensltlve MOS 2 (1981 - 82) 105 - 138

structures

Part 2

Charac-