Diamond and Related Materials 11(2002) 392–395

0925-9635/02/$ - see front matter� 2002 Elsevier Science B.V. All rights reserved.PII: S0925-9635Ž01.00610-0

Nitrogen and phosphorus implanted MESFETs in semi-insulating 4H-SiC

J.B. Tucker , S. Mitra , N. Papanicolaou , A. Siripuram , M.V. Rao *, O.W. Hollanda a b a a, c

Department of Electrical and Computer Engineering, George Mason University, Fairfax, VA 22030-4444, USAa

Naval Research Laboratory, Washington, DC 30275, USAb

Oak Ridge National Laboratory, Oak Ridge, TN 37831-6048, USAc

Abstract

Nitrogen and phosphorus ion implantation were used to fabricate 2mm gate length, n-channel Metal-Semiconductor Field-Effect-Transistors(MESFETs) in semi-insulating bulk 4H-SiC. In order to create the channel region, either nitrogen or phosphorusion-implantations was performed to a depth of 300 nm at room temperature to a volumetric concentration of 5=10 cm . The17 y3

sourceydrain regions were created by nitrogen implantation to a volumetric concentration of 2=10 cm , regardless of the19 y3

species used for the channel implantation. Annealing for a duration of 15 min at 14508C (for nitrogen-implanted channels) or1500 8C (for phosphorus-implanted channels) activated the implants. This study utilized aluminum Schottky gates for the FETs.Both the nitrogen and phosphorus-implanted channel MESFETS exhibited pinch-off voltages at approximately 18 V and the drainsaturation currents between 30 and 40 mA.� 2002 Elsevier Science B.V. All rights reserved.

Keywords: SiC; MESFET; Ion-implantation; Annealing

1. Introduction

Semi-insulating(SI) 4H-SiC stands as an attractivematerial to fabricate high-frequency, high-power devicesw1,2x. Currently, semi-insulating 4H-SiC is available inthe form of bulk substrates from the vendors. Nitrogenand aluminum ion-implantations into bulk semi-insulat-ing material have yielded nearly identical implant acti-vations when compared to epitaxial layersw3x. Selectivearea ion-implantation is regarded as an attractive methodto fabricate MESFETs in SI 4H-SiC due to the ease ofinter-device isolation without loss of planarity. On theother hand, epitaxial layer technology requires mesaetching for inter-device isolation, which compromisesthe planarity of the device and consequently the deviceyield.

In this work, fully ion-implanted n-channel MESFETswere fabricated by using donor ion implantations forboth channel and sourceydrain regions. For comparison,channel regions were created by using both nitrogen andphosphorus ion implantations. One of the primary attrib-utes of comparison was channel carrier mobility. Recent-ly, it has been reported that the phosphorusion-implantation yields a higher carrier mobility than

*Corresponding author. Tel:q703-993-1612; fax:q703-993-1601.E-mail address: rmulpuri@osf1.gmu.edu(M.V. Rao).

nitrogen implantation for similar doping concentrationsw4x. In this study, the sourceydrain implantations wereperformed using nitrogen as the implant species regard-less of the donor specie used for the channel implanta-tion. Adequate phosphorus activation for the relativelyhigh sourceydrain doping concentrations requires ele-vated ion-implantation temperaturesw4–7x. Therefore,sourceydrain regions, in this study, were created usingnitrogen implantations. We attempted to make thedevices using a simple photo-resist implant mask tech-nology, which requires the implants to be performed atroom temperature.

2. Experiment

Bulk, Si-face, 88 off-axis, vanadium doped, SI 4H-SiC substrates were used in this study. The nitrogen andphosphorus implant schedules used for the channel andsourceydrain regions are given in Table 1. All implantswere performed at room temperature using a 1.5-mmlayer of patterned photoresist as an implant mask. A200–300 nm AIN encapsulation layer was used toprotect the SiC wafer surface during annealing. Postimplant annealing was performed for 15 min at 14508C for samples with nitrogen-implanted channels and at1500 8C for samples with phosphorus-implanted chan-

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Table 1Implant schedules used for channel and sourceydrain doping in thisstudy

Implant Energy(keV) Dose(cm )-2

Nitrogen(channel) 20 1.7=10112

45 3.0=1012

80 4.0=1012

130 5.0=1012

200 8.0=1012

Phosphorus(channel) 30 1.10=1012

65 1.62=1012

110 2.29=1012

170 3.32=1012

275 6.80=1012

Nitrogen(sourceydrain) 20 5.7=1013

45 1.0=1014

80 1.33=1014

130 1.67=1014

200 2.67=1014

Fig. 1. (a) Schematic of the cross-sectional view of the phosphorus-implanted channel MESFET and(b) forward current–voltage char-acteristics of the Al Schottky gate on the phosphorus-implantedchannel.

Fig. 2. Reverse current–voltage characteristics of the Al Schottky gateon the phosphorus-implanted channel in the temperature range of RTto 300 8C.

nels, using an inductively heated graphite susceptor inargon ambient. Alloyed Ni(100 nm) formed the Ohmiccontacts for the sourceydrain regions. Alloying wasdone in a vacuum furnace at 12008C for 3 min. TheAl Schottky gate was formed by a lift-off lithographyprocess. The gate is 2mm in length and 280mm wide.Devices were characterized for their current–voltage,capacitance–voltage and ohmic contact resistancebehavior at various temperatures between 30 and 3508C. Van der Pauw Hall measurements were also per-formed on the control structures to obtain the bulkchannel carrier mobility.

3. Results and discussion

Results on phosphorus-implanted channel MESFETdevices are presented below and compared with thecorresponding values on nitrogen-implanted channeldevices. Detailed results on nitrogen-implanted channeldevices have been presented elsewherew8x. A schematicof the cross-sectional view of the phosphorus-implantedchannel device structure is given in Fig. 1a. Van derPauw Hall measurements of on-wafer test patterns atroom temperature showed bulk electron mobilities of;240 and;200 cm yVØs in samples with nitrogen-2

and phosphorus-implanted channels, respectively. Thecorresponding substitutional doping concentrationsobtained byC–V measurements on the Schottky gatewere;3=10 cm for both samples. These measure-17 y3

ments suggest that phosphorus implantation does notyield higher carrier mobility than nitrogen implantationfor the low doping concentrations. This result differsfrom previous reportsw4,7x of higher carrier mobility inphosphorus-implanted material when compared to nitro-gen-implanted material. Previous reports involved higherP doping concentrations than those used in this study.

Fig. 1b shows the Al Schottky gate forward current–voltage (I–V) characteristics at room temperature forthe MESFETs with phosphorus-implanted channels. TheAl metal gate Schottky diode exhibited an ideality factorbetween 1.8 and 2 at forward bias currents in the rangeof pA to nA. The forward logI–V curve is linear overthis current range. An Al Schottky junction built-inpotential of;2.0 V was obtained from theI–V char-acteristics. Fig. 2 shows reverse current–voltage char-acteristics of the same Al Schottky gate diode in thetemperature range of 30–3008C. For samples withphosphorus-implanted channels, a significantly low gateleakage current of;1 nA was recorded at a gate voltage

394 J.B. Tucker et al. / Diamond and Related Materials 11 (2002) 392–395

Fig. 3. DC transfer characteristics of the phosphorus-implanted chan-nel MESFET at room temperature.

Fig. 4. DC drain output characteristics of the phosphorus-implantedchannel MESFET at(a) RT and(b) 300 8C.

of y20 V at room temperature, regardless of the factthat no dielectric passivation was used. At a highertemperature(300 8C) the gate showed a relatively non-leaky behavior with a leakage current of;1 mA aty20 V. The low reverse current observed here isbelieved to be due to the absence of reactive ion etchingdamage under the gate. No etching damage exists in thepresent device since we did not use a recess under thegate for phosphorus-implanted devices.

The Ni sourceydrain ohmic contact resistance for thephosphorus-implanted channel MESFETs was measuredthrough the use of Transmission Line Model(TLM)patterns. The contacts were found to have a specificcontact resistance(r ) of ;10 V-cm , which isy2 2

c

comparable to the value measured earlierw8x in sampleswith nitrogen-implanted channels. This value is largerthan desired for MESFETs and may limit device per-formance. The ohmic contact resistance can probably beminimized by post implant annealing the devices at ahigher temperature or by using elevated temperaturesourceydrain implantations. Elevated temperature phos-phorus implantations yielded sheet resistances as low as100 Vyh in 4H-SiC w4–7x. The measured inter-deviceisolation resistivity was more than 10V-cm for an10

inter-device separation of 450mm. This fact confirms amore than sufficient isolation behavior.

The transfer characteristics of the working MESFETs(WyLs280y2) are shown in Fig. 3. The mutual tran-sconductance(g ) was found to be 2.33 mS at aV ofm gs

y5 V and V of 25 V. The resulting channel carrierds

mobility was calculated to be;32 cm yVØs. The same2

range of mobility values was also obtained for MESFETswith a nitrogen-implanted channel region. The drainoutput characteristics of the same device are shown inFig. 4. The drain conductance measured from the slopeof the curve in the linear region forV s0 V was foundgs

to be 1.80 mS and the resulting channel carrier mobilitywas calculated to be 40 cmyVØs. The difference2

between the carrier mobility values calculated using the

drain transconductance and the mutual transconductancecould be attributed to the difference in gate biasingvoltage w8x. The maximum saturation drain currentdensity, I , was found to be;30 mA for MESFETsdss

with phosphorus-implanted channels. This value is com-parable to the saturation currents measured in the MES-FETs with nitrogen-implanted channels.

To study the temperature stability of the devices, theMESFETs were characterized at various temperaturesbetween room temperature and 3008C. As shown inFig. 4b, even at 3008C the devices showed a stablebehavior with small changes in device parameters. Boththe drain transconductance and the mutual transcon-ductance increased initially with temperature and thenbegan to decrease with further increases in temperature(in the range of 2008C and higher). The temperaturedependency of the effective channel carrier mobilityfollowed the same trend as that of the transconductancesdue to their direct interdependency. In this regard,MESFETs with phosphorus-implanted channels exhibit-ed characteristics similar to those with nitrogen-implant-ed channels. The saturation drain current,I , increaseddss

initially with temperature due to the ionization of donorsin the channel region, but decreased afterwards due to a

395J.B. Tucker et al. / Diamond and Related Materials 11 (2002) 392–395

lower mobility value at a higher temperature(tempera-ture beyond 2008C).

4. Conclusions

The phosphorus-implanted channel MESFETs exhib-ited similar current–voltage characteristics when com-pared to the nitrogen-implanted channel MESFETs.However, the effective channel carrier mobility calcu-lated from the transconductance in both the devices is4–5 times smaller than the bulk Hall carrier mobility.The poor transconductance of these devices is believedto be due to a high sourceydrain ohmic contact resistancemeasured in the devices. This contact resistance needsto be reduced in order to realize the maximum potentialof these simple fully implanted MESFET structures inthe SI 4H-SiC substrate. This could be achieved eitherby using elevated temperature sourceydrain implantationor by using higher annealing temperatures.

Acknowledgments

The authors thank R.D. Vispute of University ofMaryland and M. Derenge of Army Research Laboratory

for their help with the capping and annealing. This workwas supported by the National Science Foundation undergrant � ECS-9711128 and by the Division of MaterialSciences, US Department of Energy, under contract DE-AC05OOOR22725 with Oak Ridge National Laboratory,managed by UT-Battelle, LLC.

References

w1x J. Baliga, Inst. Phys. Conf. Ser. No. 142(1996) 1.w2x W. Moore, EPRI J. 3(1997) 1, Nov.yDec.w3x M.V. Rao, J.B. Tucker, M.C. Ridgeway, O.W. Holland, N.

Papanicolaou, J. Mittereder, J. Appl. Phys. 86(1999) 752.w4x M.A. Capano, R. Santhakumar, R. Venugopal, M.R. Melloch,

J.A. Cooper Jr, J. Electron. Mater. 29(2000) 210.w5x J.A. Gardner, A. Edwards, M.V. Rao, N. Papanicolaou, G.

Kelner, O.W. Holland, M.A. Capano, M. Ghezzo, J. Kretchmer,J. Appl. Phys. 83(1998) 5118.

w6x V. Khemka, R. Patel, N. Ramungal, T.P. Chow, M. Ghezzo, J.Kretchmer, J. Electron. Mater. 28(1999) 167.

w7x E.M. Handy, M.V. Rao, O.W. Holland, K.A. Jones, M.A.Derenge, N. Papanicolaou, J. Appl. Phys. 88(2000) 5630.

w8x J.B. Tucker, N. Papanicofaou, M.V. Kao, O.W. Holiand, Dia-mond Rel. Mater.(in review).