1. ARTICLE IN PRESS Journal of Crystal Growth 298 (2007) 835–839 www.elsevier.com/locate/jcrysgro AlGaN/AlN/GaN/SiC HEMT structure with high mobility GaN thin layer as channel grown by MOCVD Xiaoliang WangÃ, Guoxin Hu, Zhiyong Ma, Junxue Ran, Cuimei Wang, Hongling Xiao, Jian Tang, Jianping Li, Junxi Wang, Yiping Zeng, Jinmin Li, Zhanguo Wang Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, PR China Available online 8 December 2006Abstract Enhancement of the electrical properties in an AlGaN/GaN high electron mobility transistor (HEMT) structures was demonstrated byemploying the combination of a high mobility GaN channel layer and an AlN interlayer. The structures were grown on 50 mm semi-insulating (SI) 6H-SiC substrates by metalorganic chemical vapor deposition (MOCVD). The room temperature (RT) two-dimensionalelectron gas (2DEG) mobility was as high as 2215 cm2/V s, with a 2DEG concentration of 1.044 Â 1013 cmÀ2. The 50 mm HEMT waferexhibited a low average sheet resistance of 251.0 O/square, with a resistance uniformity of 2.02%. The 0.35 mm gate length HEMT devicesbased on this material structure, exhibited a maximum drain current density of 1300 mA/mm, a maximum extrinsic transconductance of314 mS/mm, a current gain cut-off frequency of 28 GHz and a maximum oscillation frequency of 60 GHz. The maximum output powerdensity of 4.10 W/mm was achieved at 8 GHz, with a power gain of 6.13 dB and a power added efﬁciency (PAE) of 33.6%.r 2006 Elsevier B.V. All rights reserved.PACS: 72.80.Ey; 81.05.Ea; 81.15.Gh; 85.30.TvKeywords: A1. 2DEG; A3. MOCVD; B1. AlGaN/GaN; B3. HEMT; B3. Power device1. Introduction AlGaN/AlN/GaN heterostructures grown on sapphire substrates and AlN/sapphire templates, respectively [2,3], AlGaN/GaN heterostructures are one of the most which was attributed primarily to the introduction of a thinpromising candidates for high-power and high-frequency AlN interlayer between the GaN and AlGaN layers.microelectronic device fabrication owing to their superior Devices based on this structure grown on SiC demon-material properties [1–6]. They possess a large band gap, strated an output power density of 8.4 W/mm with a powerhigh breakdown ﬁelds, high peak and saturation electron added efﬁciency (PAE) of 28% at 8 GHz , showing thatdrift velocities, and very high sheet charge densities on the AlGaN/AlN/GaN high electron mobility transistorsorder of 1013 cmÀ2 at the interface. In recent years, (HEMTs) structures are very promising for application tosigniﬁcant progress has been made in the improvement of X-band power devices.material quality in AlGaN/GaN heterostructures and in We have previously reported high-quality AlGaN/GaNthe optimization of device structures. AlGaN/GaN hetero- heterostructures on sapphire substrates [8–12]. Recently,structures grown on SiC have already demonstrated a we reported X-band AlGaN/GaN HEMTs with a powerroom temperature (RT) two-dimensional electron gas density of 2.23 W/mm at 8 GHz grown on sapphire ,(2DEG) mobility of 2019 cm2/V s with a sheet electron which used an undoped GaN thin layer with high mobilitydensity of 1.3 Â 1013 cmÀ2 . RT 2DEG mobilities as high as channel. To further enhance the 2DEG transportas 2104 and 2174 cm2/V s have been also obtained in properties, we inserted a thin AlN layer between the high mobility GaN channel layer and the AlGaN barrier layer. ÃCorresponding author. Tel.: +86 10 82304842; fax: +86 10 82304232. In this paper, we present the growth and characterization E-mail address: firstname.lastname@example.org (X. Wang). of this AlGaN/AlN/GaN HEMT structure with high0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.jcrysgro.2006.10.219
2. ARTICLE IN PRESS836 X. Wang et al. / Journal of Crystal Growth 298 (2007) 835–839mobility GaN channel. The structures were grown on semi- recessed gate etching was performed by reactive ion etchinginsulating (SI) 6H-SiC substrates by metalorganic chemical (ICP) technology, and then the Schottky gates withvapor deposition (MOCVD). We achieved a high 2DEG 0.35 mm gate length were formed by electron beammobility of 2215 cm2/V s with a 2DEG concentration of lithography. The gate metallization was realized by using1.044 Â 1013 cmÀ2 in this structure at room temperature. electron-beam evaporated Ni/Au. Finally, the device sur-The 0.35 mm gate length HEMT devices fabricated on this face was passivated using silicon nitride.material structure, exhibited a maximum drain currentdensity of 1300 mA/mm, a maximum extrinsic transcon- 3. Results and discussionductance of 314 mS/mm, a current gain cut-off frequencyof 28 GHz and a maximum oscillation frequency of Double crystal X-ray diffraction (DCXRD) measure-60 GHz. The maximum output power density was ments were performed to characterize the structural4.10 W/mm at 8 GHz, with a power gain of 6.13 dB and aPAE of 33.6%. properties and crystalline qualities of the AlGaN/AlN/ GaN HEMT structure with a high mobility GaN thin layer as channel. The measured DCXRD spectrum of the2. Device structure and fabrication structure is shown in Fig. 2, and the GaN(0 0 0 2), AlGaN(0 0 0 2), SiC(0 0 0 6), and AlN(0 0 0 2) peaks are The AlGaN/AlN/GaN HEMT structures of this work perfectly aligned in sequence from the low-angle side. Thewere grown on 50 mm SI 6H-SiC substrates by MOCVD. inset in Fig. 2 shows that the intense GaN(0 0 0 2) peak hasAll layers were grown with unintentional doping. Fig. 1 a small FWHM of 3.18 arcmin in a rocking curve scan forgives the schematic diagram of the present structure. the same sample, which indicates the high epitaxial qualityTrimethylgallium (TMG), trimethylaluminum (TMA), of the GaN layer.and ammonia (NH3) were used as sources of Ga, Al, and Sheet resistance and resistance uniformity of the AlGaN/N, respectively. Nitrogen (N2) and hydrogen (H2) were AlN/GaN HEMT structures were measured with Lehight-used as carrier gases. Before growth, the 6H-SiC substrate on contactless sheet resistivity mapping across the 50 mmwas baked in H2 ambient at 1000 1C to remove surface wafer. The typical measurement result is shown in Fig. 3.contaminations. The growth of the HEMT structure began The wafer displayed a maximum resistance of 260.2 O/with a thin AlN nucleation layer, which deposited on the square and a minimum value of 241.2 O/square. The6H-SiC substrate. Next, a 1.5 mm undoped high resistivity average sheet resistance was 251.0 O/square, with theGaN buffer layer was deposited at 1000 1C. Then a 100 nm resistance uniformity of 2.02%, showing the high qualityhigh mobility GaN channel layer was deposited at 1050 1C, of the GaN channel layer and good resistance uniformityfollowed by deposition of a 1 nm AlN interlayer and a across the whole wafer. So, the grown HEMT wafer is very20 nm undoped AlGaN barrier layer at 1000 1C. The high suitable for the fabrication of large periphery powermobility GaN channel layer and AlN interlayer were devices.simultaneously employed, which had been proved to Hall effect measurements of the AlGaN/AlN/GaNfurther improve the 2DEG transport properties by our HEMT structures were carried out using the Van derearlier reported results [10–13]. Pauw technique in the temperature range from 293 to For the device fabrication, the drain and source ohmic 650 K. Fig. 4 shows the temperature dependence of thecontacts were formed by rapid thermal annealing ofevaporated Ti/Al/Ni/Au, and the measured ohmic contact GaN(0002)resistance was 0.4 O cm. Then, the device isolation wasrealized by using multiple-energy boron (B) ion implanta- 105 SiC(0006)tion. A silicon nitride ﬁlm was then deposited usingplasma-enhanced chemical vapor deposition (PECVD).The Intensity (a.u.) 104 AlGaN(0002) 103 AlN(0002) 102 101 32 33 34 35 36 37 38 ω/2θ (degree) Fig. 2. DCXRD spectrum of the AlGaN/AlN/GaN HEMT structure onFig. 1. The schematic diagram of the present AlGaN/AlN/GaN HEMT SiC substrates. The inset shows the rocking curve of the GaN (0 0 0 2)structure with high-mobility GaN channel on SiC substrates. peak.
3. ARTICLE IN PRESS X. Wang et al. / Journal of Crystal Growth 298 (2007) 835–839 837 product obtained in our grown structures make these AlGaN/AlN/GaN structures very suitable for high-tem- perature and high-power applications. On-wafer DC characteristics were measured using HP4142 Semiconductor parameter analyzer with the gate bias ranging from 2 to À4 V in steps of À1 V. Fig. 5 shows the typical current–voltage (Ids–Vds) characteristics for the device with 0.35 mm Â 1.0 mm gate periphery and 4 mm source–drain spacing, and the device demonstrates high current density. The maximum drain current density was 1300 mA/mm at a gate bias of 2 V. The ohmic contact resistance of the device was only 0.4 O cm, resulting in a low knee voltage of about 4 V even at a higher gate bias of 2 V. Fig. 6 shows the DC transfer characteristics of the same device, and a maximum extrinsic transconductance ofFig. 3. Sheet resistance mapping of the 50 mm AlGaN/AlN/GaN HEMT 314 mS/mm was obtained at a gate bias of À2.6 V. Thewafer on SiC substrates. device pinched off completely at a gate bias of À3.6 V. From Fig. 4, we also can see that the extrinsic transcon- 2500 2.5 ductance maintains a high value of above 260 mS/mm over a broad gate bias range of À3 to À1 V, showing the superior current handling of the device. Finally, the Concentration(1013cm-2) 2000 2.0 measured gate to drain breakdown voltage of the device Mobility(cm2/V.s) 1500 1.5 was about between 80 and 100 V. Small-signal radio frequency (RF) performance was 1000 1.0 characterized on-wafer by scattering (S) parameter mea- surements. Fig. 7 shows the short circuit current gain (h21) and maximum stable gain (MSG) against frequency for the 500 0.5 device with 0.35 mm Â 1.0 mm gate periphery. At a drain bias of 10 V and a gate bias of À2 V, the current gain cutoff 0 0.0 frequency was measured to be 28 GHz. The maximum 300 400 500 600 700 oscillation frequency of 60 GHz was also extrapolated at Temperature(K) À20 dB/decade from MSG data.Fig. 4. Temperature dependence of the 2DEG mobility and concentration Large signal measurements were performed using a load-in the AlGaN/AlN/GaN HEMT structure on SiC substrates. pull system at 8 GHz. Fig. 8 shows the power performance of the device with a 0.35 mm Â 1.0 mm gate periphery. The2DEG mobility and 2DEG concentration in the AlGaN/ device was biased at a drain bias of 35 V. At an input powerAlN/GaN HEMT structure with a high mobility GaN of 30 dBm, the device displayed a maximum output powerlayer as channel. From the ﬁgure, the high RT 2DEG density of 4.10 W/mm with a PAE of 33.6%. The measuredmobility of 2215 cm2/V s was achieved, with a 2DEGconcentration of 1.044 Â 1013 cmÀ2. When the temperaturewas increased to 400 and 500 K, the mobility still remained 1.4 VGS:at 1159 and 750 cm2/V s, respectively. To our knowledge, Start=+2Vthe achieved RT 2DEG mobility in this work is one of the 1.2 Step=-1Vhighest results ever reported for AlGaN/GaN HEMTstructures grown on SiC substrates, which originated both 1.0 IDS(A/mm)from the optimized HEMT structure and high epitaxial 0.8quality. The combined employment of the high mobilityGaN channel layer and AlN interlayer, can signiﬁcantly 0.6improve the 2DEG transport properties and crystalquality, which is proved by the Hall measurements in this 0.4research and our earlier reported results [10–13]. From Fig.4, we also can conclude that the RT 2DEG mobility— 0.2concentration (n Â m) product is as high as 2.31 Â 1016/V s, 0.0and the results are in good agreement with the results 0 2 4 6 8 10calculated from sheet resistance mapping. As we know, the VDS(V)electrical performance of HEMT devices is closely relatedto the product of n Â m. Thus the high mobility and n Â m Fig. 5. DC Ids–Vds characteristics of a typical AlGaN/AlN/GaN HEMT.
4. ARTICLE IN PRESS838 X. Wang et al. / Journal of Crystal Growth 298 (2007) 835–839 360 1.6 linear gain and power gain at peak PAE were 13.0 and gm 6.13 dB, respectively. Compared with our previously 300 fabricated AlGaN/GaN HEMTs on sapphire substrates IDS 1.2 , the power density was dramatically improved from 240 2.23 to 4.10 W/mm at 8 GHz. The enhancement in power gm(mS/mm) IDS(A/mm) performance was mainly attributed to the combination of 180 0.8 high thermal properties of SiC and high quality of the 120 epitaxial structure. The high thermal conductivity of SiC 0.4 substrate can make heat in AlGaN/GaN HEMT sink 60 through the substrate promptly, and then the high output power density of the device was realized. In contrast, the 0 0.0 output power of the devices on sapphire was seriously -6 -4 -2 0 2 dissipated by self-heating due to the poor thermal VGS(V) conductivity of sapphire, limiting the possible achieved maximum output power level .Fig. 6. DC transfer characteristics of a typical AlGaN/AlN/GaN HEMTwith 0.35 mm Â 1.0 mm gate periphery on SiC substrates with0.35 mm Â 1.0 mm gate periphery on SiC substrates. 4. Conclusion 50 A high RT 2DEG mobility of 2215 cm2/V s with a 2DEG concentration of 1.044 Â 1013/cmÀ2 was realized in AlGaN/ MSG AlN/GaN structures with high mobility GaN channels on 40 SI SiC substrates grown by MOCVD. The 50 mm HEMT |h21| wafer exhibited a low average sheet resistance of 251.0 O/|h21| / dB, MSG / dB |h21| square, with a resistance uniformity of 2.02%. The 30 enhancement of electrical properties in the structures was attributed to the combined use of a high-mobility GaN 20 channel layer, an AlN interlayer, and SiC substrates. MSG HEMT devices with 0.35 mm Â 1.0 mm gate periphery were fabricated on this material structure, and the devices 60GHz 10 exhibited a maximum drain current density of 1300 mA/ 28GHz mm, a maximum extrinsic transconductance of 314 mS/ mm, a current gain cutoff frequency of 28 GHz and a 0 maximum oscillation frequency of 60 GHz. The maximum 1E8 1E9 1E10 output power density of 4.10 W/mm was achieved at Frequency / Hz 8 GHz, with an associated gain of 6.13 dB and a powerFig. 7. Short circuit current gain (h21) and maximum stable gain (MSG) of added efﬁciency of 33.6%. The results demonstrate thea typical AlGaN/AlN/GaN HEMT with 0.35 mm Â 1.0 mm gate periphery potential of GaN-based HEMTs for high-power applica-on SiC substrates. tions at X-band frequencies. 38 40 Acknowledgements 36 Pout Gain This work was supported by the Knowledge Innovation 34 30 Program of Chinese Academy of Sciences (no. KGCX2- PAE Pout(dBm), PAE(%) 32 SW-107-1), the National Natural Science Key Foundation Gain(dB) 30 of China (no. 60136020), the State Key Development 20 Program for Basic Research of China (nos. 513270505, 28 G20000683, 2002CB311903), and National High Technol- 26 ogy R&D Program of China (no. 2002AA305304). The 24 10 authors acknowledge Prof. Tangsheng Chen in Nanjing Electronic Devices Institute for the fabrication of the 22 AlGaN/AlN/GaN HEMT devices. 20 0 5 10 15 20 25 30 Pin(dBm) ReferencesFig. 8. Power performance of a typical AlGaN/AlN/GaN HEMT with  R. Gaska, J.W. Yang, A. Osinsky, Q. Chen, M. Asif Khan, A.O.0.35 mm Â 1.0 mm gate periphery on SiC substrates. Orlov, G.L. Snider, M.S. Shur, Appl. Phys. Lett. 72 (1998) 707.
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