Al gan gan field effect transistors with c-doped gan buffer layer as an electrical isolation template grown by molecular beam epitaxy
Solid-State Electronics 49 (2005) 802–807 www.elsevier.com/locate/sseAlGaN/GaN ﬁeld eﬀect transistors with C-doped GaN buﬀer layeras an electrical isolation template grown by molecular beam epitaxy S. Haﬀouz *, H. Tang, J.A. Bardwell, E.M. Hsu, J.B. Webb, S. Rolfe Institute for Microstructural Sciences, National Research Council Canada, Montreal Rd. M-50, Ottawa, Canada K1A 0R6 Received 5 March 2004; received in revised form 23 November 2004 The review of this paper was arranged by Prof. C. HuntAbstract The eﬀectiveness of Ammonia Molecular Beam Epitaxy (MBE) grown carbon-doped GaN buﬀer layer as an electrical isolationtemplate was investigated. AlGaN/GaN ﬁeld eﬀect transistor structures with a product of sheet electron density and mobility (nsl),linearly increasing from 1.5 · 1016 VÀ1 sÀ1 to 2 · 1016 VÀ1 sÀ1 with ns, were grown on 2-lm-thick carbon-doped GaN buﬀer layerover sapphire substrates. The measurement of the gate-to-source voltage (VGS) dependent drain current (ID) demonstrated excellentdc pinch-oﬀ characteristics as revealed by an on-to-oﬀ ratio of 107 for a drain–source voltage (VDS) up to 15 V. The gate leakagecurrent was less than 1 lA/mm at the subthreshold voltage (Vth = À5.2 V). Inter-devices isolation current (IISO) measurementsdemonstrated IISO values in the low pico-amperes ranges indicating a complete suppression of the parallel conduction paths.Small-signal rf measurements demonstrated a fmax/ft ratio as high as 2.9 attesting the absence of charge coupling eﬀects.Ó 2005 Elsevier Ltd. All rights reserved.PACS: 85.30.Tv; 81.15.Hi; 73.61.EyKeywords: GaN; FET; MBE; Carbon doping; Heterostructure1. Introduction sity and electron mobility (nsl) is of great importance for fabrication of high performance ﬁeld eﬀect transis- With improved growth material quality and fabrica- tors. In literature, considerable studies have addressedtion technologies, AlGaN/GaN heterostructure ﬁeld the electron mobility (l) dependence carrier densitieseﬀect transistors (HFET) have reached nowadays a very (ns) [7–15]. However, there have been only few data onadvanced position and have clearly demonstrated their the growth of 2DEG structures with high nsl valuescapability for high-power and high frequency applica- (>1016 VÀ1 sÀ1). Achievement of these latter valuestions [1–6]. Molecular Beam Epitaxy (MBE) and requires growth of AlGaN/GaN heterostructures withMetal-Organic Chemical Vapor Deposition (MOCVD) high 2DEG mobility (P103 cm2/V s) at ns values in thetechniques have been successfully used for growth of few 1013 cmÀ2 ranges. Strong decrease of the 2DEGAlGaN/GaN two-dimensional electron gas (2DEG) mobility with increasing the sheet carrier density in thestructures on various types of substrates. Growth of 1–2 · 1013 cmÀ2 ranges has been observed in AlGaN/2DEG structures with high product of sheet carrier den- GaN structures grown by MOCVD technique . On another hand, achievement of highly insulating * Corresponding author. Tel.: +1 613 991 0761; fax: +1 613 990 0202. GaN buﬀer prior to deposition of AlGaN/GaN struc- E-mail address: souﬁen.haﬀouz@nrc-cnrc.gc.ca (S. Haﬀouz). tures has not been an easy task. A conductive buﬀer0038-1101/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.doi:10.1016/j.sse.2005.01.012
S. Haﬀouz et al. / Solid-State Electronics 49 (2005) 802–807 803layer will not only lead to high leakage currents and The growth of nitride material in our MBE/MSE dualtherefore a poor pinch-oﬀ characteristics but also will mode system was performed following the established 2-degrade the rf performances of the HFETs at high fre- step deposition procedure that consists of a low-tempera-quencies. To resolve this problem, a few approaches ture nucleation layer (in our case is AlN deposited byhave been proposed. Increasing the N ﬂux during MSE ) followed by high-temperature nitride epilayers.MBE grown GaN layers changes the proﬁle of the For the present study, 20-nm-thick AlN nucleation layerstructural defects and residual impurities in this layer was deposited at 885 °C by dc reactive sputter mode usingand leads to highly resistive buﬀer layers . By high purity Al target, 25 sccm NH3 ﬂow, 100 sccm Ar ﬂowadjusting the recrystallisation time of the nucleation and 60 W power. The growth rate was about 1 nm/min.layer, Bougrioua et al.  have demonstrated the Achievement of high quality overgrown layer, as well asgrowth of highly resistive GaN layers in a MOCVD insulating template was accomplished by depositingreactor. Using Fe , Be  and Zn  as accep- 2-lm-thick C-doped GaN layer at 930 °C using 1 sccmtor-like point defects, semi-insulating GaN ﬁlms were methane (CH4) ﬂow and low-energy saddle ﬁeld ionalso successfully grown by respectively MOCVD, source for cracking the CH4 . The growth rate of thisMBE and Hydride Vapor Phase Epitaxy (HVPE) tech- layer was about 0.80 lm/h with Ga cell temperature ofnique. Within our group, we have previously demon- 1000 °C and NH3 ﬂow of 100 sccm. X-ray measurementsstrated the growth of semi-insulating C-doped GaN showed that the full width at half maximum (FWHM) ofbuﬀer layer with good structural properties and excel- the (0 0 0 2) peak in x-scan was about 57000 . Resistivity of alent reproducibility and reliability [21,22]. This layer few MX cm was reproducibly achieved. Secondary ionwas systematically used as a template prior to growth mass spectroscopy analysis of the C-doped GaN templateof AlGaN/GaN structures. Though up to now, a few using a methane ﬂow rate as low as 1 sccm revealed car-reports have demonstrated the growth of insulating bon concentration in the range of 2–8 · 1018 cmÀ3.GaN buﬀer layer, only one study  has so far inves- Achieving higher carbon concentration was found to betigated the eﬀectiveness of an insulating GaN buﬀer relatively straightforward by simply increasing the meth-layer as an electrical isolation template in ﬁeld eﬀect ane ﬂux, however, the crystal quality gets worse and there-transistors. fore the quality of the overgrown layers (2DEG structure) In this article, we ﬁrst report on the growth of will be aﬀected as well.AlGaN/GaN ﬁeld eﬀect transistor structures with high The growth procedure is completed by depositing ansl product values. Excellent electronic properties have two-dimensional electron gas structure that consists ofbeen achieved as revealed by an nsl product linearly 200-nm-thick undoped GaN channel layer followed byincreasing from 1.5 · 1016 VÀ1 sÀ1 to 2 · 1016 VÀ1 sÀ1 undoped AlGaN barrier. During AlGaN/GaN deposi-with ns from 1.2 to 2 · 1013 cmÀ2. Further, we investi- tion, the substrate temperature was kept unchanged atgate the eﬀectiveness of the C-doped GaN buﬀer layer 930 °C.as an electrical isolation template. A detailed picture Field eﬀect transistors have been fabricated usingof the pinch-oﬀ characteristic is demonstrated by 0.75 lm optical-gate-length. The mesa isolation wasmeasuring the dependence of the logarithm of the drain accomplished using chemically assisted ion beam etchingcurrent on the gate-to-source voltage for various drain– (CAIBE) technique . The Ohmic contacts weresource voltages. The absence of any parallel conduction achieved by evaporating a thin Ti/Al/Ti/Au layers (20/path is also evidenced by inter-devices isolation current 100/45/55 nm) followed by rapid thermal annealing at(IISO) measurements. Finally, small-signal rf measure- 800 °C for 120 s in N2 atmosphere . Low contactments shows an fmax/ft ratio as high as 2.9 attesting to resistance with value in the range of 0.5–0.7 X mm wasthe absence of charge coupling eﬀects. obtained, based on circular transmission line measure- ments. The sheet resistance, which was also measured, was consistent with the results on the unpatterned wafer.2. Experimental details It should be noted that the ohmic metal probe pads were located on the mesa ﬂoor, with ohmic metal wrapping The growth of AlGaN/GaN structures for ﬁeld eﬀect up the sloping sidewalls of the mesa. This results in atransistors fabrication has been carried out using thinner metal layer on the mesa sidewalls and an addi-Ammonia Molecular Beam Epitaxy (MBE) technique tional series resistance in the device. Thus, the dc perfor-(SVT Associates) that is also equipped by a magnetron mance, and speciﬁcally the drain current density, issputter epitaxy (MSE) facility. Prior to growth, 2 0 0 basal lower than would be expected from the values expectedplane sapphire substrates were ﬁrst back-coated with based on the sheet carrier density. Finally, the gatemolybdenum to facilitate radiation heating. Further, Schottky contacts were achieved by sputtering 30-nm-they were vapor-cleaned in chloroform, dipped in 10% thick Pt ﬁlm (to improve the adhesion) capped byHF for 1 min, rinsed in deionized water and dried with e-beam evaporated Pt/Au layers (100/200 nm). Thenitrogen ﬂow. devices have not been passivated.
804 S. Haﬀouz et al. / Solid-State Electronics 49 (2005) 802–8073. AlGaN/GaN ﬁeld eﬀect transistor structures 2000 2.4 T=300K 1750 2.2 2DEG mobility, µ(cm2/Vs) It is well established that the electrical properties 2.0of wurtzite structure of III-Nitrides in [0 0 0 1] direc- 1500 1.8 nsµ (1016V-1S-1)tion results from a combination eﬀect of spontaneous 1250 1.6and piezoelectric polarization ﬁelds. The polarization- 1.4 1000induced electrostatic charge densities were reported to 1.2be as high as few 1013e/cm2 at the heterojunction inter- 750 1.0 0.8face [26,27]. Particularly, due to the large band oﬀset 500 0.6and strong piezoelectric eﬀect, AlGaN/GaN hetero- 0.4structure forms two-dimensional electron gas (2DEG) 250 0.2with very high electron densities (few 1013 cmÀ2) even 0 0.0without intentional doping . The sheet carrier con- 1.0 1.2 1.4 1.6 1.8 2.0 2.2centration of the 2DEG located at the AlGaN/GaN Sheet electron density, ns(1013cm-2)interface of nominally undoped structures can be writ-ten as  Fig. 1. Room-temperature two-dimensional electron mobility vs sheet carrier density. The resulting product of the sheet carrier density and mobility (nsl) is also plotted. rðxÞ e0 eðxÞns ðxÞ ¼ À ½e/B ðxÞ þ EF ðxÞ À DEc ðxÞ; e de2where r(x) is the total (spontaneous and piezoelectric) tures and/or SiC substrate, have been used to achievepolarization-induced charge density at the AlGaN/ such a high value. Because the sheet resistivity (Rsh)GaN interface, e(x) is the dielectric constant, d is the is inversely proportional to the ns product, the mea-AlGaN barrier thickness, e/B(x) is the surface barrier sured Rsh (not shown here) has continuously decreasedheight, EF is the Fermi-level position with respect to from 401 X/sq down to 323 X/sq when the nsl productthe conduction band edge and DEc is the conduction increased from 1.5 · 1016 VÀ1 sÀ1 to $2 · 1016 VÀ1 sÀ1.band oﬀset between AlGaN and GaN. The small scattering of the data within the eye-guiding Above a critical value , the increase of the AlGaN line (solid line shown in Fig. 1) is indicative of thethickness (d) would lead to an enhancement of the elec- excellent reproducibility and yield of growing such het-tron transfer from the surface or bulk states to the het- erostructures by our MBE system. The use of higherointerface states and therefore increases the 2DEG growth temperature (930 °C) and the good control ofcarrier density. Meanwhile, a larger band discontinuity the Al ﬂux that is improved by the specially designedintroduced by higher Al composition of the barrier layer cell with water cooled cold lip to avoid creeping ofleads to a better carrier conﬁnement, stronger spontane- Al from the crucible, had provided better uniformityous and piezoelectric ﬁelds and therefore higher carrier over the 2 0 0 wafers and excellent reproducibility anddensity. Within this methodology, and in order to yield. Detailed study on the uniformity over hall waferachieve high ns values, we grow pseudomorphic Alx- will be reported elsewhere .Ga1ÀxN/GaN structures by increasing the AlGaN bar- The measured electron mobility of 103 cm2/V s at sheetrier thickness and Al content in the range of 18–24 nm carrier density of $2 · 1013 cmÀ2 in our 2DEG structureand 29–43%, respectively. is consistent with the theoretically predicted value Fig. 1 depicts the room temperature evolution of the (l $ 1.1 · 103 cm2/V s at ns $ 2 · 1013 cmÀ2) calculated2DEG mobility as the function of the carrier density by Farvacque and Bougrioua  by taking into accountfor the complete set of experiments. The Hall measure- the scattering mechanisms associated with phonons,ment results clearly showed that the carrier density had carrier–carrier interactions, dislocations and ionizedcovered the $1.2–2 · 1013 cmÀ2 range. Their corre- impurities. However, in LP-MOCVD grown AlGaN/sponding room temperature electron mobility was at GaN structures , strong decrease of the 2DEGleast 1000 cm2/V s and reached a maximum of mobility from about 1250 to 200 cm2/V s was observed1250 cm2/V s for ns value of 1.25 · 1013 cmÀ2. The nsl when the sheet carrier density increases from 1.2 toproduct value, which is an important parameter for 2 · 1013 cmÀ2. This pronounced decrease of the mobilityachievement of high performance HFET, has linearly with carrier density was mainly attributed to the scatter-increased from 1.5 to $2 · 1016 VÀ1 sÀ1 when the car- ing mechanisms associated with strain-relaxation inducedrier density increased from 1.2 to $2 · 1013 cmÀ2 (see defects . Using our MBE system, we have been able toFig. 1). According to our knowledge, only a few grow pseudomorphically AlGaN barrier layers on GaNreports [6–8,30] have so far demonstrated nsl product epilayer with high aluminum content and thereforevalue higher than 2 · 1016 VÀ1 sÀ1. In all these reports, keeping the mobility remarkably high (P103 cm2/V s)doped AlGaN barrier, AlGaN/AlN/GaN heterostruc- for sheet carrier density up $2 · 1013 cmÀ2.
S. Haﬀouz et al. / Solid-State Electronics 49 (2005) 802–807 8054. C-doped GaN buﬀer layer as an electrical isolation 10 3template T=300K Drain Current, ID (mA/mm) 2 10 Devices were fabricated on a wafer with a 2DEG 1 10structure with ns and l of 1.7 · 1013 cmÀ2 and V DS=15V1120 cm2/V s, respectively. The measured sheet resistiv- 10 0 VDS=10V On-to-Off ratioity and aluminum content in the barrier layer were VDS=5V ~10 7 -1328 X/sq and 36%, respectively. Fig. 2 displays the typ- 10ical room temperature drain current–voltage (I–V) char- -2 10acteristics. The fabricated HFET exhibited maximumcurrent densities as high as 900 mA/mm and transcon- -3 10 Pinch-off voltageductance peak value of about 180 mS/mm. By sweeping -4the gate–source voltage from 3 V down to around À5 V, 10 -10 -8 -6 -4 -2 0 2 4 6we have been able to turn oﬀ the devices withoutany problem. However, using undoped GaN template, Gate-to-Source Voltage, VGS (V)which usually relatively highly conductive (electron con- Fig. 3. VGS-dependent drain current (ID) at diﬀerent source-to-draincentration in the range of 1017 cmÀ3), we found that is voltages (VDS) in the AlGaN/GaN ﬁeld eﬀect transistor with nslnot possible to pinch-oﬀ the device completely and we product value of 1.9 · 1016 VÀ1 sÀ1.have not able to obtain a properly working devices. In order to check carefully the pinching-oﬀ character-istics of the devices grown on highly insulating C-doped 10-2template and to obtain more information on the leakage Gate Leakage Current, IG (mA/mm) T=300Kcurrents, we have carried out measurements of the gate- 10-3to-source voltage (VGS) dependent drain current (ID) Gate Leakage Current,IG (mA/mm) 10-1and gate leakage current (IG). The results are depicted 10-4 VGS= Vth= -5.2Vin Figs. 3 and 4, respectively. Two important pieces ofinformation can be deduced from the ID–VGS curves. 10-5 10-2The ﬁrst one is the steepness of the slope in the ON– 10-6OFF transition region (À5.3 V < VGS < À3 V). In fact, 10-3the rapid decrease of the drain current with decreasing 10-7the gate-to-source voltage indicates the sharp pinching-oﬀ of our devices. By extrapolation, we deduced a 10-8 10-4 0 5 10 15 20precise value of pinch-oﬀ voltage (also known as sub- Drain-to-Source Voltage,VDS (V)threshold voltage) that is equal to À5.2 V for a VDS of 10-9 -20 -18 -16 -14 -12 -10 -8 -6 -4 -2 05 V and 10 V. We note also a small increase of thepinch-oﬀ voltage (Vth = À5.3 V) by increasing the Gate-to-Source Voltage, VGS (V) Fig. 4. VGS-dependent gate leakage current (IG). The insert depicts the IG vs VDS at the subthreshold voltage (VGS = Vth = À5.2 V). 1000 T=300K VGS =3V Step= -1V drain–source voltage (VDS = 15 V). The second impor- Drain Current, ID (mA/mm) 800 tant information is the amount of the drain current that is still ﬂowing in the OFF state, which is possibly origi- 600 nating from the GaN buﬀer layer and/or from the GaN/ AlN/sapphire interfaces. The measured ON-to-OFF cur- rent ratio was as high as $107 attesting the very low 400 leakage current. This clearly indicates that there is no parallel conduction paths through the C-doped GaN 200 layer and it is also isolating properly the channel layer from the underneath structure. The gate leakage current has been also measured for a VGS up to À20 V as shown 0 in Fig. 4. The value of IG was only 0.4 lA/mm at the 0 5 10 15 Drain-to-Source Voltage, VDS (V) subthreshold voltage and increases only to 1 lA/mm at VGS of À20 V. The insert of Fig. 4 shows the dependenceFig. 2. Typical IDS–VDS characteristics of the AlGaN/GaN ﬁeld eﬀect of gate leakage current on drain–source voltage at thetransistor structure with nsl product value of 1.9 · 1016 VÀ1 sÀ1. pinch-oﬀ voltage (VGS = À5.2 V). An increase of the
806 S. Haﬀouz et al. / Solid-State Electronics 49 (2005) 802–807gate leakage with increasing drain–source voltage is ob- plate. Excellent dc pinch-oﬀ characteristics, very lowserved and is in agreement with the results obtained by leakage currents and good rf performances wereArulkumaran et al. . It should be pointed out that demonstrated.the value of the gate leakage current measured on ourHFET devices is reasonably low compared to the valuesreported in the literature [15,34–36]. However, a closer Acknowledgementlook reveals not only that the leakage current is stilllarger than the expected reverse saturation current value We gratefully acknowledge the assistance of C.given by the thermionic emission (TE) transport model, Storey and D. Kuan with the rf measurements, thebut also, reveals strong dependence of gate leakage assistance of R. Wang with X-ray diﬀraction measure-current on the reverse voltage (at least up to À10 V). ments and the helpful discussions with S. McAlister.Recently, some eﬀort has been made in order to under-stand the mechanism of gate leakage current in AlGaN/GaN HFETs. A possible mechanism, using thin surface Referencesbarrier (TSB) model, was recently proposed by Hase-gawa et al. . This model assumes the presence of high  Shealy JR, Kaper V, Tilak V, Prunty T, Smart JA, Green B, et al.density of deep donor defects (at EC-0.37 eV) near J Phys Condens Matter 2002;14:3499.AlGaN surface, causing a narrowing of the Schottky  Chini A, Coﬃe R, Meneghesso G, Zanoni E, Buttari B, Heikman S, et al. Electron Lett 2003;39:625.barrier in such a way that electrons can tunnel through  Bardwell JA, Liu Y, Tang H, Webb JB, Rolfe SJ, Lapointe L.this barrier in both forward and reverse direction by Electron Lett 2003;39:654.means of the thermionic ﬁeld-emission (TFE) or the  Kumar V, Lu W, Schwindt R, Kuliev A, Simin G, Yang J, et al.ﬁeld-emission (FE) mechanism, depending on the tem- IEEE Electron Dev Lett 2002;23:455.perature. 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