Taking, S., MacFarlane, D. and Wasige, E. (2011) AlN/GaN-based MOS-HEMT technology: processing and device results. Active and Passive Electronic Components, 2011 (821305). ISSN 0882-7516.

http://eprints.gla.ac.uk/54418 Deposited on: 02 August 2011

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Hindawi Publishing CorporationActive and Passive Electronic ComponentsVolume 2011, Article ID 821305, 7 pagesdoi:10.1155/2011/821305

Research Article

AlN/GaN-Based MOS-HEMT Technology:Processing and Device Results

S. Taking, D. MacFarlane, and E. Wasige

High Frequency Electronics Group, School of Engineering, University of Glasgow, Glasgow G12 8LT, UK

Correspondence should be addressed to S. Taking, sanna@elec.gla.ac.uk

Received 1 October 2010; Accepted 6 December 2010

Academic Editor: David Moran

Copyright © 2011 S. Taking et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Process development of AlN/GaN MOS-HEMTs is presented, along with issues and problems concerning the fabrication processes.The developed technology uses thermally grown Al2O3 as a gate dielectric and surface passivation for devices. Significantimprovement in device performance was observed using the following techniques: (1) Ohmic contact optimisation using Al wetetch prior to Ohmic metal deposition and (2) mesa sidewall passivation. DC and RF performance of the fabricated devices will bepresented and discussed in this paper.

1. Introduction

The search for improved high power and high frequencyperformance has called attention to the most recent devel-opment in aluminium nitride/gallium nitride- (AlN/GaN-)based high electron mobility transistors (HEMTs) whichtarget future microwave power devices. Key properties ofthis material system are high 2DEG sheet carrier concen-tration at the heterojunction interface, high carrier electronvelocity, and large electric breakdown field, and so superiorperformance compared to conventional AlGaN/GaN devicescould be achieved. With improvements in material growthand processing techniques, record performances made in thismaterial system include 2DEG sheet carrier concentrationover 3 × 1013 cm−2 with very low sheet resistance, Rsh,

2 Active and Passive Electronic Components

Ohmic Ohmic

Contactto 2 DEG

1 nm GaN3 nm AIN

3.8 μm GaN 2 DEG

Figure 1: Optimised TLM processing summary for unprotectedand unpassivated AlN/GaN HEMT samples. Processing includes (a)sample cleaning with acetone, isopropanol, and deionised water, (b)deoxidation, (c) Ohmic metallisation, (d) Ohmic annealing, and (e)TLM measurements.

OhmicAl2O3 Al2O3 Al2O3

Ohmic

Contactto 2 DEG

1 nm GaN3 nm AIN

3.8 μm GaN 2 DEG

Figure 2: Optimised TLM processing summary for protected andpassivated AlN/GaN MOS-HEMT samples. Processing includes(a) sample cleaning with only deionised water and deoxidation,(b) 2 nm Al deposition, (c) etching Al from Ohmic contact regions,(d) thermal oxidation of Al, (e) Ohmic metallisation, (f) Ohmicannealing, and (g) TLM measurements.

transconductance, GMAX, which results in the enhancementof the current gain cutoff frequency, fT , as well as maximumfrequency of oscillation, fMAX, of the devices. For thesereasons, Ohmic contact optimisation processing for HEMTand MOS-HEMT in the AlN/GaN material systems is crucialto achieving good device performance. Details of the Ohmiccontact process optimisation were reported in [9] and aresummarised here.

Ohmic contacts on both protected (with 2 nm evapo-rated Al which is later oxidised to form Al2O3) and unpro-tected (as grown) AlN/GaN samples were fabricated andcharacterised. Figure 1 shows the optimised transmissionline method (TLM) processing summary for unprotectedand unpassivated AlN/GaN HEMT samples while Figure 2shows the optimised TLM processing summary for protectedand passivated AlN/GaN MOS-HEMT samples. A summaryof the optimised RC and Rsh values on HEMT and MOS-HEMT is shown in Table 1. The sheet resistance of theprotected sample (159Ω/�) is about one third that ofthe unprotected one (450Ω/�). Clearly, protection of thesamples during processing is the key to good performance.On the other hand, the TLM results of unprotected andunpassivated samples exhibited very low contact resistancesfor this material system with an average value of 0.31Ω·mm.This result provides an indication of how Ohmic contactsmay be processed for a protected sample.

By employing the structure in Figure 2 for TLMprocessing, optimisation of wet etching using 16H3PO4 :HNO3 : 2H2O Al etch solution prior to Ohmic metallisationproduced very low contact resistance as well as very low sheetresistance as reported in [9]. Figure 3 shows the measuredI-V characteristics on 5 μm TLM gap spacing of annealedOhmic contacts under different Al etch times prior to Ohmic

0

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Cu

rren

t(m

A)

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B4: etch Al 1 minB1: etch Al 10 sB2: etch Al 20 s

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Voltage (V)

2 3

Figure 3: Current-voltage (I-V) characteristics on 5 μm TLM gapspacing of annealed Ohmic contacts under different Al etch timesprior to Ohmic metal deposition.

Table 1: Summary of results for the optimised RC and Rsh values onHEMT and MOS-HEMT samples in the AlN/GaN material system.

Sample Description RC, Ω·mm Rsh, Ω/�

AUnprotected and unpassivated(HEMT)

0.31 480

B2Protected and passivated(MOS-HEMT)

0.49 159

metal deposition. The processing methods for sample B2,on which Al was etched for 20 secs gave the best I-V plotas compared to other etching times. The average values ofRC and Rsh for this sample were 0.49Ω·mm and 159Ω/�,respectively. By using the correct Al etch time, it seems thatthe top surface of the semiconductor is etched leaving agood clean surface for metallisation. However, if the samplewas left longer in the etchant the contact resistance risesindicating that further undesirable reactions may be takingplace. Figure 4 shows the measured contact resistance RC incomparison with other published work for AlN/GaN-baseddevices. It is clear that the adopted approach here results inone of the lowest contact resistance values for this materialsystem.

2.2. Gate Wrap-Around MOS-HEMT Optimisation. A gatewrap-around layout technique [10], where the gate electrodeencircles the drain as shown in Figure 5, was employed forprocess development and optimisation on AlN/GaN HEMTstructures. This technique consists only of Ohmic and gatemetallisation, eliminating the mesa isolation step. Duringprocess development, 10 mm× 10 mm samples cleaved froma 2-inch wafer were used. Device fabrication starts withstandard sample cleaning using acetone, isopropanol, anddeionised water. The optimised Ohmic contact processing

Active and Passive Electronic Components 3

0.4

0.8

Rc′

(Ω.m

m)

1.2

1.6

2

600 675 750 825 900

[7]

[3]

[15]

[16]

[13]

[this paper], [14][12]

Annealing temperature (◦C)

Figure 4: Comparison of Ohmic contact resistance, RC, on AlN/GaN-based devices as a function of annealing temperatures fromvarious publications.

300 μm

50 μmS G D

Figure 5: SEM micrograph of completed gate wrap-around MOS-HEMT layout. Inset: Device with LSD = 6μm and LG = 3μm.

in Figure 1 was employed for fabrication of unprotectedand unpassivated AlN/GaN HEMT devices. Deoxidation wasdone on the Ohmic contact regions by HCl : 4H2O solutionprior to Ohmic metal deposition. Ohmic metal contacts wereformed by evaporation of Ti/Al/Ni/Au, followed by a lift-offprocess, and then annealing at 800◦C for 30 secs. Thereafter,gate metal contacts were formed by evaporation of Ni/Au andfollowed by lift-off process.

AlN/GaN structures are known to be very sensitive toprocessing liquids, and so unprotected and unpassivatedAlGaN/GaN HEMTs (from same/similar growth conditions)were also processed and fabricated to provide comparativedata. DC measurements were done by contacting the probeneedles directly on top of the source (S), drain (D), andgate (G) structures. All measurements were made at roomtemperature using Agilent’s B1500A Semiconductor Param-eter Analyzer. Figure 6(a) shows the IDS-VDS characteristicsof fabricated unprotected and unpassivated 3 μm × 100 μmdevices on AlGaN/GaN HEMT structure. Devices made onthis material system exhibited good gate control of draincurrents up to a gate bias of 1 V and achieved a maximumdrain current of ∼800 mA/mm. The devices also showedboth good pinch-off and good saturation characteristics. Onthe other hand, devices made on AlN/GaN HEMT structure

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Excessive gate current

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(b)

Figure 6: IDS-VDS characteristics of fabricated unprotected andunpassivated with 3 μm × 100 μm device (a) AlGaN/GaN HEMTand (b) AlN/GaN HEMT.

exhibited very high leakage currents, did not pinchoff, andthe drain current was very low as shown in Figure 6(b).

These results, together with the TLM results describedin the previous subsection, showed that there were someissues with processing of AlN/GaN HEMT structure whichare not seen in AlGaN/GaN HEMTs. Exposure to differentprocessing chemicals such as resist developer and solventssolutions could help reduce the Ohmic contact resistancebut at the same time this may have led to the degradationof the quality of the AlN/GaN epilayer structures. Similarobservations were made by Fan et al. [11] on the formationof low Ohmic contact on n-GaN materials, where reducedOhmic contact resistance was caused by the damage of theRIE process employed prior to deposition of the Ohmiccontact metallisation. The devices however suffered fromsurface sensitivity and high leakage currents. It is thereforenecessary to protect the AlN/GaN epitaxial layers duringdevice processing.

4 Active and Passive Electronic Components

1 nm GaN3 nm AlN

3.8 μm GaN 2 DEG

(a)

2 nm Al1 nm GaN3 nm AlN

3.8 μm GaN 2 DEG

(b)

S D

1 nm GaNAl2O3Al2O3 Al2O3

3 nm AlN3.8 μm GaN 2 DEG

(c)

1 nm GaNAl2O3Al2O3 Al2O3

3 nm AlN

3.8 μm GaN 2 DEG

(d)

S DG

1 nm GaNAl2O3Al2O3 Al2O3

3 nm AlN3.8 μm GaN 2 DEG

(e)

Figure 7: Process flow for fabrication of protected and passivatedAlN/GaN MOS-HEMTs using the gate wrap-around technique.Processing includes (a) sample cleaning and deoxidation, (b) 2 nmAl deposition, (c) etching Ohmic regions and thermal oxidationof Al, (d) Ohmic metallisation and annealing, and (e) gatemetallisation and device measurements.

A new process for the fabrication AlN/GaN-based deviceswas therefore developed. It involved employing thermallygrown Al2O3 for protection of the very sensitive AlN epilayerfrom exposure to liquid chemicals during processing [6]as earlier described for TLM experiments (Figure 2). ThisAl2O3, which is formed by thermal oxidation of evaporatedAl, acts as a surface passivate and as a gate dielectric for thetransistors. Figure 7 shows the process flow for fabrication ofprotected and passivated AlN/GaN MOS-HEMT using thegate wrap-around technique.

To further directly explore the impact of Ohmic contactsoptimisation on device performance, devices were fabricatedin which the etching time of the Al in Ohmic contact regionwas varied. Figure 8 shows the typical IDS-VDS characteristicsof fabricated 3 μm × 100 μm gate AlN/GaN MOS-HEMTdevices with different etching times, 10 secs and 20 secs. Itis clear that a 20-sec Al etch has a significant impact onthe device performance with the drain current at zero gatevoltage (IDSS) more than double that of a device in whichthe etching time was 10 secs. Compared to similar resultsfor the AlN/GaN HEMT (unprotected and unpassivateddevice in Figure 6(b)) on the same epilayer structure, theseresults show that protecting and passivating the AlN/GaNlayers during processing yield AlN/GaN MOS-HEMT withfar superior and excellent transistor characteristics [6].

2.3. Mesa AlN/GaN MOS-HEMT Optimisation. The devel-oped process technology was extended to realise AlN/GaNMOS-HEMTs using the conventional mesa isolation tech-nique for devices. The process flow is similar to that forthe gate wrap-around devices (Figure 7) but with additionalmesa isolation and the bond pad steps. Figure 9 shows

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(mA

/mm

)

800

1200

0 2 4 6 8 10

20 s Al etch10 s Al etch

VDS (V)

Figure 8: IDS against VDS characteristics of fabricated 3 μm ×100 μm gate AlN/GaN MOS-HEMT devices with different etchingtimes using the simplified gate wrap-around method. The devicesare biased from VGS = +3 V to −4 V with step size of 1 V.

GateAl2O3

1 nm GaN3 nm AlN

Unpassivatedmesa sidewall

edge

3.8 μm GaN2 DEG

(a)

GateAl2O3

1 nm GaN3 nm AlN

Passivated mesasidewall edge

3.8 μm GaN2 DEG

(b)

Gate

Source

5 μmMesa sidewall edge

Drain

(c)

Figure 9: Schematics cross-section of fabricated MOS-HEMT (a)without mesa sidewalls edge passivation, (b) with mesa sidewallsedge passivation, and (c) top-view SEM micrograph of completedtwo-finger 2.5 μm gate length device.

the schematic cross-section of fabricated MOS-HEMT with(a) unpassivated and (b) passivated mesa sidewalls, respec-tively. Figure 9(c) shows the topview SEM micrograph ofcompleted two-finger 2.5 μm gate length device.

Active and Passive Electronic Components 5

0

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(mA

/mm

)

300

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500

0 2 4 6 8 10

Devices do not fully pinch-off

WG = 2× 300 μmWG = 2× 400 μm

VDS (V)

Figure 10: IDS against VDS characteristics of fabricated two-finger3 μm gate length AlN/GaN MOS-HEMT mesa devices with unopti-mised etching time (Al etch for 10 secs). The devices had no mesasidewall passivation and were biased from VGS = +3 V to −4 V witha step size of 1 V.

3. Characterisation, Results, and Discussion

Initially, mesa devices were fabricated without mesa sidewallpassivation and with an unoptimised Ohmic contact process.Figure 10 shows typical IDS-VDS characteristics of a 3 μm gatelength AlN/GaN MOS-HEMT device made this way. Thedevices exhibited high knee voltages (high Ohmic contactresistance) and very high leakage currents. The reason forthe high leakage currents seemed to be the contact betweenthe gate metal and the exposed mesa sidewalls edge asillustrated in Figure 9(a). To solve this problem, the deviceswere passivated with an additional layer of thermally grownAl2O3 on the mesa sidewalls edge as shown in Figure 9(b).Significant improvement in the DC characteristics of thefabricated devices using this new process was observed. Themeasured IDS-VDS characteristics and the device transcon-ductance versus gate voltage are shown in Figure 11. Thedrain current and transconductance are observed to decreasewith gate width. This is attributed to selfheating effects.

The small signal RF performance of this device was alsomeasured (not shown here). A unity current gain cutofffrequency, fT , and power gain cutoff frequency, fMAX, of2.8 and 7.9 GHz were obtained for a two-finger 2.5 μm ×100 μm device, respectively, for a device biased at VDS = 4 Vand VGS = −1 V. Devices with gate length of 0.2 μm and0.5 μm were also fabricated using the processing with mesasidewalls passivation [17]. Excellent DC and RF performancewas observed from the fabricated device as shown inFigure 12. fT and fMAX of 50 GHz and 40 GHz, respectively,were achieved for the 0.2 μm devices, and of 20 GHz and30 GHz, respectively, for the 0.5 μm devices. The DC andRF measurements were made at room temperature usingthe Agilent’s B1500A Semiconductor Parameter Analyzerand E8361A PNA Network Analyzer, respectively. Each

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mm

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−6 −4 −2 0

GmIDS

VGS (V)

WG = 2× 100 μm

WG = 2× 200 μm 0

300

I DS

(mA

/mm

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600

(b)

Figure 11: (a) IDS against VDS and (b) Gm against VGS character-istics of fabricated two-finger 2.5 μm gate length AlN/GaN MOS-HEMT devices with optimised 20 s of etching time with passivatedmesa sidewalls. The devices are biased from VGS = +3 V to −4 Vwith step size of 1 V.

10 mm × 10 mm sample had approximately 70 devices, andthe variation in device performance on a sample was under5%. Two different samples from neighbouring parts of awafer had comparable device characteristics indicating goodwafer uniformity and reproducibility of the process.

4. Conclusion

The processing of AlN/GaN-based HEMTs has beendescribed and discussed. The sensitivity of the AlN/GaNepitaxial layer structure necessitated the introduction ofspecial processing requirements and the use of thermally

6 Active and Passive Electronic Components

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(mA

/mm

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VGS = 3 V to −6 VStep 1 V

LG = 0.2 μmLG = 0.5 μm

VDS (V)

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n(d

B)

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1 10 100

LG =0.5 μmfT = 20 GHzfmax = 30 GHz

LG =0.2 μmfT = 50 GHzfmax = 40 GHz

fTfmax

Frequency (GHz)

(b)

Figure 12: (a) IDS against VDS characteristics of fabricated two-finger 100 μm gate width AlN/GaN MOS-HEMT with gate lengthsof 0.2 μm and 0.5 μm, (b) the small-signal RF performances. Thedevices are biased at VGS = −2.5 V and VDS = 10 V.

grown Al2O3 as a gate dielectric and device passivation.Excellent DC and RF characteristics on AlN/GaN MOS-HEMTs were achieved but further reduction in the Ohmiccontact resistance is still required before the full potentialof this material system can be realised. The achieved resultsindicate the potential of AlN/GaN MOS-HEMT technologyfor high frequency and high power applications.

Acknowledgment

The authors would like to thank staff at the James WattNanofabrication Centre (JWNC), University of Glasgow, forsupporting and assisting this work.

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