Influence of Pinhole-Type Defects in AlGaNon rf Performance of AlGaN/GaN HFETs Grownby MOCVD

Jong-Wook Kim1) (a), Jae-Seung Lee (a), Jin-Ho Shin (a), Jae-Hoon Lee (b),

Sung-Ho Hahm (b), Jung-Hee Lee (b), Chang-Seok Kim (c), Jae-Eung Oh (c),and Moo-Whan Shin (d)

(a) LG Electronics Institute of Technology, 16 Woomyeon-Dong, Seocho-Gu,Seoul 137-724, Korea

(b) School of Electronic and Electrical Engineering, Kyungpook National University,1370, Sankyuk-Dong, Buk-Gu, Taegu 702-701, Korea

(c) School of Electrical and Computer Engineering, Hanyang University, Sa-1-Dong,Ansan-Si, Kyuinggi-Do 425-791, Korea

(d) Department of Ceramics Engineering, Myongji University, Nam-Dong, Yongin-Si,Kyuinggi-Do 449-728, Korea

(Received June 21, 2001; accepted July 11, 2001)

Subject classification: 68.35.Dv; 73.20.Hb; 73.61.Ey; 81.15.Gh; 85.30.Tv; S7.14

Growth pressure-dependent generation of trap states in AlGaN/GaN heterostructures was ana-lysed. AlGaN/GaN heterostructures were grown on sapphire substrates at two different pressuresof AlGaN layer growth, 150 and 200 Torr, with other growth parameters unchanged. The mea-sured Al mole fraction in AlGaN layer by XRD was 42 and 25%, respectively. The fabricateddevices using AlGaN/GaN heterostructures exhibited quite different rf performance. The devicewith 25% Al mole fraction showed a lower maximum drain dc current, but more than two timeshigher rf output power density. The results of a pulsed I–V measurement revealed that severetrap-related rf current collapse existed in the devices with 42% Al mole fraction. These trap statesare believed to be generated from pinhole-type defects as shown in the AFM surface image ofAlGaN/GaN heterostructure with Al mole fraction of 42%.

Introduction With the superior material properties, III-nitride based electronic deviceshave been enormously studied since the 1990s [1]. Among them, AlGaN/GaN hetero-structure field effect transistors (HFETs) have already been successfully fabricated andtested for a possible application in high power microwave systems [2]. To date, thestate-of-the-art reported rf power density for AlGaN/GaN HFETs fabricated on sap-phire substrate is 6.6 W/mm (6 GHz) [3]. This high rf power density could be achievedthrough controlling the defects.Currently, the main issue to enhance rf performance of AlGaN/GaN HFETs is to

reduce the trap densities in the bulk and the surface of the AlGaN/GaN heterostruc-ture. Slow-acting trap states cause severe rf current slump and thus decrease outputpower. This current collapse can be explained from the fact that a channel of AlGaN/GaN HFET cannot be fully opened after pinch-off due to electron trapping into slowacting trap states [4], mainly associated with threading dislocations in AlGaN/GaN het-erostructure which were directly observed by scanning capacitance microscopy [5].

1) Corresponding author; Phone: +82 2 526 4892; Fax: +82 2 571 3863;e-mail: jongwkim@lg-elite.com

phys. stat. sol. (a) 188, No. 1, 267–270 (2001)

# WILEY-VCH Verlag Berlin GmbH, 13086 Berlin, 2001 0031-8965/01/18811-0267 $ 17.50þ.50/0

In this study, we compared dc and rf performances of two AlGaN/GaN HFETs fabri-cated with different AlGaN layer grown at 150 and 200 Torr, keeping other growthparameters including low temperature buffer layer and undoped GaN layer unchanged.A remarkable difference on dc and rf performances was observed between these twodevices. A pulsed I–V measurement and atomic force microscopy (AFM) were used toinvestigate the influence of traps on AlGaN/GaN HFETs.

Experiments and Discussion The layer structures used in this work consisted of a lowtemperature grown thin GaN buffer layer, a 2.4 mm thick undoped GaN layer, and anundoped AlxGa1––xN layer, successively grown by metal organic chemical vapor deposi-tion (MOCVD) on a 2-inch sapphire substrate. Growth parameters were unchangedexcept that the respective pressures during the growth of AlGaN layer were 150 Torrfor one (sample A) and 200 Torr for the other (sample B). Al mole fractions of un-doped AlGaN, grown at 150 and 200 Torr, were measured by XRD and turned out tobe 42 and 25%, respectively.The as-grown AlGaN/GaN heterostructures were first mesa-etched by ECR-RIE for

electrical isolation, and ohmic contacts were formed by the deposition of Ti/Al/Pt/Au.The ohmic contact resistivity of both devices was lower than mid 10––7 W cm2. And then,the gate metals of Ni/Au were deposited, and Si3N4 was used for surface passivation.The device fabrication was completed by Au plating for an air bridge inter-connection.The gate length of the devices was 0.6 mm. The maximum drain current (ID max) ofsample A and B at gate bias of +1 V was 820 mA/mm and 625 mA/mm, respectively.Larger output current of sample A was as expected due to an increased density of two-dimensional electron gas (2DEG), attributed to higher Al mole fractions. In contrast, rfoutput power measurement (un-cooled, VDS = 15 V, class A operation) showed some-what unexpected results. Figure 1 exhibits the results of the load-pull measurement for2 � 94 mm wide gate AlGaN/GaN HFETs. The saturated output power density at2.1 GHz of sample A and B was 0.47 W/mm and 1.19 W/mm, respectively, even thoughthe dc current level for sample B is lower than that of sample A. In addition, thepower added efficiency of sample B was almost three times higher than that of sampleA. During the load-pull measurement (class A operation) of sample A, the level ofdrain dc current was decreased down to 50% of the original current level as an input rfpower was increased.

268 Jong-Wook Kim et al.: Influence of Pinhole-Type Defects in AlGaN/GaN HFETs

Fig. 1. Results of load-pull measurement of 2 � 94 mm wide AlGaN/GaN HFETs. a) 150 Torr andb) 200 Torr grown AlGaN layer

The discrepancy between dc and rf performances for sample A and B could be ex-plained by the results obtained from pulsed I–V measurement, carried out with a normaland a pinch-off mode. The initial bias conditions for a normal mode were VDS = 0 V andVGS = 0 V, and those for a pinch-off mode were VDS = ––6 Vand VGS = 15 V. Pulse signals(1 ms) of each drain and gate bias point were applied, and drain currents were measuredby digital oscilloscope. The results for 2 � 94 mm wide gate AlGaN/GaN HFETs areshown in Fig. 2. The normal mode results of both devices are similar to conventional draindc I–V curves because there is no electrical stress during the initial bias conditions. How-ever, the pinch-off mode measurement shows a remarkable difference between two de-vices. As shown in Fig. 2a, sample A showed a severe current collapse at a pinch-offmode. At initial bias points of VDS < ––6 V and VGS > 15 V, much severe current collapsewas observed in sample A. This is because a fully depleted channel at the initial biaspoints of gate and drain was not completely opened. It is reported that current collapse ismainly caused by trap states related with threading dislocations [5].

phys. stat. sol. (a) 188, No. 1 (2001) 269

Fig. 2. Results of pulsed I–V measurement of 2 � 94 mm wide AlGaN/GaN HFETs. a) 150 Torrand b) 200 Torr grown AlGaN layer

a) b)

Fig. 3 (colour). AFM surface images AlGaN/GaN heterostructures. a) 150 Torr and b) 200 Torrgrown AlGaN layer

AFM measurement was used for the characterization of two AlGaN/GaN hetero-structures. Figure 32) shows AFM surface images of AlGaN/GaN heterostructures. Pin-hole-type defects were observed on the surface and are believed to be strongly relatedwith threading dislocations. The only difference of a growth condition between thesetwo samples was a total pressure during the AlGaN layer growth. Thus, it was expectedthat higher strain in Al0.42Ga0.58N layer by larger lattice mismatch influenced the propa-gation of threading dislocations along c-axis from undoped GaN to AlGaN layer andthe generation of pinhole-type defects on its surface.

Conclusions Pressure-dependent trap state generation in AlGaN/GaN heterostruc-tures was analysed. AlGaN/GaN HFETs with an AlGaN layer grown at 150 and200 Torr showed remarkable differences in rf performances. The pressure during thegrowth of AlGaN might influence the propagation of threading dislocations along c-axisfrom undoped GaN, which seems to be responsible for the generation of pinhole-typedefects on the surface. More than two times higher rf output power density for sampleB was measured at 2.1 GHz. Further study on the propagation mechanism of threadingdislocations will be continued.

Acknowledgements This work is financially supported by the Ministry of Commerce,Industry, and Energy (#990-02-03) in Korea.

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270 Jong-Wook Kim et al.: Influence of Pinhole-Type Defects in AlGaN/GaN HFETs

2) Colour figure is published online (www.physica-status-solidi.com).