High temperature, high voltage operation of diamond Schottky diode

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Diamond and Related Materials 7 ( 19981581 584

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High temperature, high voltage operation of diamond Schottky diode A. Vescan *, I. Daumiller, P. Gluche, W. Ebert, E. Kohn Department of Electron Devices and Circuits. University ~" UIm. 89081 Uhn. Germany

Received 23 June 1997; accepted 24 September 1997

Abstract

Very high temperature operation of homoepitaxial diamond Schottky diodes is demonstrated with rectifying behaviour up to 800 C. The Schottky material is p *-Si with a chemically stabilized Si-diamond interface, leading to significantly reduced thermal activation of the reverse currents. On diamond films with low surface doping concentration 150 V breakdown voltage at room temperature is observed. © 1998 Elsevier Science S.A. Kevwords." Diamond Schottky ~.tiode: High temperature: High voltage

I. introduction

High temperature applications of diamond devices are extremely promising due to the high band-gap, which limits the intrinsic carrier concentration to < 10 ~° cm-3 up to 1000 'C. In addition, the high breakdown field, which is expected to be above 107V cm[I], indicates the potential for high w)ltage operation. However, the high voltage/high temperature performance of diamond Schottk}-diodes had been limited by material defects and the stability of the metal d i a mond interface at high temperatures. State-of the-art Schottky-diodes, t'abricated using Au as contact material, due to its chemical inertness, exhibit poor adhesion and mechanical softness and are therefore critical for high temperature operation. Therefore, a silicon-based contact system was developed, which shows excellent adhesion and is stable up to 600 'C, [2]. In this paper optimization of the Si-based contact system is reported, which enables operation at temperatures up to 800 C.

2. Experimental

The diode structures wcre fabricated on highly boron doped ( llb with NA > 10:° c m 3) single crystal substrates with 3 × 3 mm 2 area. The substrate material was superpolished [3] and wet-chemically cleaned prior to the * Corresponding author. Tel: +49 731 51126181' I-ax: + 49 731 502615I" e-mail: vescaq(aebs.e-tec!mik,uni-uim.de 0925-9635/98.'$19.00 ,~t::'1998 Elsevier Science B.V. All rights reserved. PIi S0925-9635(97)(t0200-8

deposition of the active epitaxial layer. Epitaxial growth was performed in a standard ASTEX microwave chemical vapour deposition chamber, modified to enable boron doping using a thin boron rod. To obtain a steep transition from the highly doped back contact to the low doped active layer, the boron rod is inserted into 1he plasma for a short period of time at the beginning of the growth. Subsequently the film is grown out wilh the residual boron in the chamber. With this doping method a steep doping prolile can be achieved. Thc surface boron concentration is dependent on the gro~, 111 time and can be as low as ca 2 × 10~cm ~, as shown in Fig. 1, at an actwe layer thickness of 0.5 ! t.tm. After epitaxy the diamond film was treated m an oxygen plasma in order to obtain an oxygen terminated surface. This leads to suppression of the surface conductivity of the as grown, hydrogen terminated diamond surface [4], serving for surface passivation. Also. the barrier height on the oxygen terminated surlace becomes metal work function independent. On the low doped diamond surface a Si-based conlact metailization system was deposited by sputtering and patterned by lift-off. The amorphous silicon contact layer is degeneratly boron-doped, showing quasi-metallic behaviour. Using a special passivation method, the Si-diamond interface was stabilized to increase the recrystallization temperature and inhibit solid state surface reactions, leading to enhanced thermal stability. For a technical diode structure the Schottky contact material has to be complemented with a cap-layer ~i.e. Au). Intermixing of the Si and the Au cap-layer lwhich

,-I. I'es('an et al. Diamoml aml Rehtted MateriaL~ 7 (1998) 581 584

582

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bias voltage (V) Fig. 3. /V-characteristics of Si-diamond diode at 50 C with 150,V breakdown voltage. •

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,

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Fig. !. Doping protile from capacitance- voltage measurement of pulse doped epitaxial lilra on p'-substrate.

takes place at ca 370 C ) is supressed by using an amorphous Si:W alloy stuffed with nitrogen as a difussion barrier. The (Si:W):N layer was deposited by sputtering in an 95%Ar.'5%N,, gas mixture. This contact system is stable up to ca 800 C , where the diffusion barrier failed, but. may be further optimized using higher nitrogen content in the diff~Jsion barrier. The thermal stability of the contact syste,.'a is demonstrated in Fig. 2, where the IV characteristics (at R'[) after annealing in vacut,m at different tenmeratures is shown. Up to 700 C the reverse current density is basically unchanged. Only after prolonged annealing at 800 C (Ibr >30rain) degradation is observed, however, still showing rectifying behaviour.

3. Characteristics at elevated temperatures ( T_<300 C ) The current voltage characteristics of a diode at 50 C (tabricated on the sample corresponding to the doping 10 4-_

profile in Fig. 1) is shown in Fig. 3. The forward current density is limited > 2 V by the series resistance. Nevertheless, 1 kA cm---' are obtained. Up to ca in the measurement setup. Above 75 V the diode current becomes dominant, showing irreversible breakdown at V,~r= 150 V. This is the highest breakdown voltage reported so far for diamond Schottky diodes. To determine the breakdown field, it is assumed that the breakdown occurs across the low doped part of the space charge layer (SCL). Extrapolating the doping profile (dotted line in Fig. i } indicates that the doping concentration increases steeply to 1020cm '-3 at w=500nm. Therefore the SCL width w will be pinned at this depth. Neglecting the influence of the doping protile on the electric field, the breakdown field results to E n R = VUR/W=3 x I(PV cm t . Tiffs value represents a lower limit for E,r, which is a l~lctor of two higher than previou.,,ly reported for Au-Schottky contacts on low doped diamond [51. The diode characteristics up to 30() C is shown in Fig. 4. In contrast to previously published results [6], only negligible activation of the reverse current is observed. This indicates, that either the density of electrically active surface defects could be ;educed, or more probably that injection across defects is suppressed, due to the chemically passivated interface. The diode temperature behaviour in this range can be well explained by

annealed at 800"C

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~ite.._=,~ . , ,:_~____( 0 20 40 60 80 1O0 bias voltage (V) Fig. 2. IV-characteristics of Si-based contacts as deposited and after annealing.

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ill

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Fig. 4. Diode characteristics up to 3()0 C.

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A. I;esran et al. , Diamond amt Rehtted Materuds 7 (1998) 581-584

an interfacial layer according to Ref. [7], increasing the ideality factor to n > 1.5 (see Fig. 6a). The exponential voltage dependence of the reverse current, as seen at 300 :C, can also be related to this effect.

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4. High temperature operation (T> 500 C) The high temperature characteristics of a S]-diamond Schottky diode is shown in Fig. 5. To avoid etching of the diaomnd surface in air at high temperatures, the measurement was performed on a heated stage mounted in a vacuum chamber. The sample was stressed at each temperature for 15 min. Up to 500 ':'C the same weak temperature activation of the reverse current is observed as already mentioned. However, increasing the temperature beyond 500 C a significant change in the reverse current is seen. The thermal activation of the reverse current is considerably increased. The temperature is now in the range, where the recrystallization of the amorphous Si-contact layer.is known to take place [2]. Also, the formation of an interfacial SiC layer cannot be excluded [8]. Nevertheless, due to the high barrier height, the diode structure remains rectifying up to 800 'C. The barrier height was extracted from a Richardson plot (taking the points with low ideality factors) to ca ~/~b= 1.9 eV. In the high temperature limit the ideality factors approach unity (Fig. 6a) at high current levels. The forward current is limited by the series resistance, which is temperature actiwtted. The activation energy is iI~ the low temperature range ca 0.25 eV (Fig. 6b), dominated by the resistance of the low doped epitaxial layer. Increasing the temperature the slight deviation from the exponential behaviour indicales increasing contribution of the substrate rcsistancc (which show typical activation energies 0. ! eV ). Above 600 C a steep decrease in series resistance, originating from a parallel current path, is observed. The activation energy of this path is 1.5 eV, close to the value of the surface potential of oxygen terminated diamond surfaces. Therefore it is speculated that surface leakage currents may be the cause, however, a more detailed investigation is needed. 103 !

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1000IT (11K) Fig. 6. (a) Richardson plot and t b) series rcsi:'ance of the diode slmv, n in Fig. 5.

Fo~" example, a guard ring configuration may give more insight into this effect.

5. Conclusion The operation of diamond Schottky-diodcs tip to 81)t) C have been demonstrated. Using a Si-based mctailization the influence of defects at the diamond surface can be almost completely eliminated at elevated temperatures, leading to a weak temperature activation of the reverse currents. At higher temperatures some thermal activation is observed. The results indicate that further optimization of the contact system and the Si diamond interlace may suppress these effects. "lhe low doping concentration of the diamond films leads to a breakdown voltage of 150 V, the highest value reported to date. Estimation of the breakdown lieid across an cpitaxhd layer thickness of 450 nm results in 3 × 10" V cm ~. [-or high temperature applications a suitable passivation for the diamond surface has still to be dcve,oped to avoid etching of the diamond in air and suppress surface leakage.

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Acknowledgement :

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Fig. 5. Oper:3tion of Si diamond diode up to ~()(1 C in v;,iCLlqnl.

This work was supported by the Deutsche Forschungsgemein:.;chaft in the frame of the Tri-national "D-,% ~H"-consorfium.

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A. Vescan et al. / Diamomt am/Rehtted Matt'rials 7 ( ! 998) 581--584

References [!] J.E. Field, The Properties of Diamond. Academic Press, New York, 1979. [2] P. Gluche, W. Ebert, A. Vescan, E. Kohn, Silicon based contacts on di~mo!ld with high barrier height and thermal stability, in: 3rd High Temperatur,' Electronics Conference. Albuquerque. NM, USA, 9-14 June 1996. [31 P. Mnzinger, O. Weis, Growth of homoepitaxial diamond films on superpolished substrates in a pulsed microwave plasma, Diamond Relat. Mater. 4 (1995) 458. [4] H. Kawarada, Hydrogen-terminated diamond surfaces and interfaces, Surf. Sci. Rep. 26 (1996) 205.

[5] W. Ebert, A. Vescan, P. Gluche, E. Kohn, High-volte, ge Schottky diode on epitaxial diamond layer, Diamond Relat. Mater. 6 (1997) 329. [6] A. Vescan, W. Ebert, T.H. Borst, l / V Characteristics of epitaxial Schottky Au barrier diode on p + diamond substrate, Diamond Relat. Mater. 4 (1995) 661. [7] H. Rhoderick, R.H. Williams, Metal-Semiconductor Contacts. Clarendon Press, Oxlbrd, 1988. [8] A. Vescan, W. Ebert, T.H, Borst, M. Pitter, M. Hugenschmidt, R.J. Behm, J. Gerster, R. Sauer, E. Kohn. Si-based contacts on epitaxial diamond films. Electrochemical Soc. Proc. 94-4(1995).