1. Solid-State Electronics 49 (2005) 802–807
www.elsevier.com/locate/sse
AlGaN/GaN field effect transistors with C-doped GaN buffer layer
as 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. Hunt
Abstract
The effectiveness of Ammonia Molecular Beam Epitaxy (MBE) grown carbon-doped GaN buffer layer as an electrical isolation
template 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 layer
over sapphire substrates. The measurement of the gate-to-source voltage (VGS) dependent drain current (ID) demonstrated excellent
dc 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 leakage
current was less than 1 lA/mm at the subthreshold voltage (Vth = À5.2 V). Inter-devices isolation current (IISO) measurements
demonstrated 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.Ey
Keywords: GaN; FET; MBE; Carbon doping; Heterostructure
1. 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 addressed
tion technologies, AlGaN/GaN heterostructure field the electron mobility (l) dependence carrier densities
effect transistors (HFET) have reached nowadays a very (ns) [7–15]. However, there have been only few data on
advanced position and have clearly demonstrated their the growth of 2DEG structures with high nsl values
capability for high-power and high frequency applica- (>1016 VÀ1 sÀ1). Achievement of these latter values
tions [1–6]. Molecular Beam Epitaxy (MBE) and requires growth of AlGaN/GaN heterostructures with
Metal-Organic Chemical Vapor Deposition (MOCVD) high 2DEG mobility (P103 cm2/V s) at ns values in the
techniques have been successfully used for growth of few 1013 cmÀ2 ranges. Strong decrease of the 2DEG
AlGaN/GaN two-dimensional electron gas (2DEG) mobility with increasing the sheet carrier density in the
structures 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 [13].
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 buffer
0038-1101/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.sse.2005.01.012

2. S. Haffouz et al. / Solid-State Electronics 49 (2005) 802–807 803
layer will not only lead to high leakage currents and The growth of nitride material in our MBE/MSE dual
therefore 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 by
have been proposed. Increasing the N flux during MSE [23]) followed by high-temperature nitride epilayers.
MBE grown GaN layers changes the profile of the For the present study, 20-nm-thick AlN nucleation layer
structural defects and residual impurities in this layer was deposited at 885 °C by dc reactive sputter mode using
and leads to highly resistive buffer layers [16]. By high purity Al target, 25 sccm NH3 flow, 100 sccm Ar flow
adjusting the recrystallisation time of the nucleation and 60 W power. The growth rate was about 1 nm/min.
layer, Bougrioua et al. [17] have demonstrated the Achievement of high quality overgrown layer, as well as
growth of highly resistive GaN layers in a MOCVD insulating template was accomplished by depositing
reactor. Using Fe [18], Be [19] and Zn [20] as accep- 2-lm-thick C-doped GaN layer at 930 °C using 1 sccm
tor-like point defects, semi-insulating GaN films were methane (CH4) flow and low-energy saddle field ion
also successfully grown by respectively MOCVD, source for cracking the CH4 [21]. The growth rate of this
MBE and Hydride Vapor Phase Epitaxy (HVPE) tech- layer was about 0.80 lm/h with Ga cell temperature of
nique. Within our group, we have previously demon- 1000 °C and NH3 flow of 100 sccm. X-ray measurements
strated the growth of semi-insulating C-doped GaN showed that the full width at half maximum (FWHM) of
buffer layer with good structural properties and excel- the (0 0 0 2) peak in x-scan was about 57000 . Resistivity of a
lent reproducibility and reliability [21,22]. This layer few MX cm was reproducibly achieved. Secondary ion
was systematically used as a template prior to growth mass spectroscopy analysis of the C-doped GaN template
of 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 [19] has so far inves- Achieving higher carbon concentration was found to be
tigated 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 a
nsl product values. Excellent electronic properties have two-dimensional electron gas structure that consists of
been achieved as revealed by an nsl product linearly 200-nm-thick undoped GaN channel layer followed by
increasing 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 at
gate 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 using
of the pinch-off characteristic is demonstrated by 0.75 lm optical-gate-length. The mesa isolation was
measuring the dependence of the logarithm of the drain accomplished using chemically assisted ion beam etching
current on the gate-to-source voltage for various drain– (CAIBE) technique [24]. The Ohmic contacts were
source 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 [25]. Low contact
ments 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 was
the 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 a
transistors 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, is
sputter epitaxy (MSE) facility. Prior to growth, 2 0 0 basal lower than would be expected from the values expected
plane sapphire substrates were first back-coated with based on the sheet carrier density. Finally, the gate
molybdenum 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 by
HF for 1 min, rinsed in deionized water and dried with e-beam evaporated Pt/Au layers (100/200 nm). The
nitrogen flow. devices have not been passivated.

3. 804 S. Haffouz et al. / Solid-State Electronics 49 (2005) 802–807
3. 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 2.0
of 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 1.6
and piezoelectric polarization fields. The polarization- 1.4
1000
induced electrostatic charge densities were reported to 1.2
be as high as few 1013e/cm2 at the heterojunction inter- 750 1.0
0.8
face [26,27]. Particularly, due to the large band offset
500 0.6
and strong piezoelectric effect, AlGaN/GaN hetero-
0.4
structure forms two-dimensional electron gas (2DEG) 250
0.2
with very high electron densities (few 1013 cmÀ2) even 0 0.0
without intentional doping [28]. The sheet carrier con- 1.0 1.2 1.4 1.6 1.8 2.0 2.2
centration of the 2DEG located at the AlGaN/GaN Sheet electron density, ns(1013cm-2)
interface of nominally undoped structures can be writ-
ten as [29] 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 eðxÞ
ns ðxÞ ¼ À ½e/B ðxÞ þ EF ðxÞ À DEc ðxÞŠ;
e de2
where r(x) is the total (spontaneous and piezoelectric) tures and/or SiC substrate, have been used to achieve
polarization-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 decreased
height, EF is the Fermi-level position with respect to from 401 X/sq down to 323 X/sq when the nsl product
the 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 [27], the increase of the AlGaN line (solid line shown in Fig. 1) is indicative of the
thickness (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 high
erointerface states and therefore increases the 2DEG growth temperature (930 °C) and the good control of
carrier density. Meanwhile, a larger band discontinuity the Al flux that is improved by the specially designed
introduced by higher Al composition of the barrier layer cell with water cooled cold lip to avoid creeping of
leads to a better carrier confinement, stronger spontane- Al from the crucible, had provided better uniformity
ous and piezoelectric fields and therefore higher carrier over the 2 0 0 wafers and excellent reproducibility and
density. Within this methodology, and in order to yield. Detailed study on the uniformity over hall wafer
achieve high ns values, we grow pseudomorphic Alx- will be reported elsewhere [31].
Ga1ÀxN/GaN structures by increasing the AlGaN bar- The measured electron mobility of 103 cm2/V s at sheet
rier thickness and Al content in the range of 18–24 nm carrier density of $2 · 1013 cmÀ2 in our 2DEG structure
and 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) calculated
2DEG mobility as the function of the carrier density by Farvacque and Bougrioua [32] by taking into account
for 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 ionized
covered 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 [13], strong decrease of the 2DEG
least 1000 cm2/V s and reached a maximum of mobility from about 1250 to 200 cm2/V s was observed
1250 cm2/V s for ns value of 1.25 · 1013 cmÀ2. The nsl when the sheet carrier density increases from 1.2 to
product value, which is an important parameter for 2 · 1013 cmÀ2. This pronounced decrease of the mobility
achievement 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 induced
rier density increased from 1.2 to $2 · 1013 cmÀ2 (see defects [32]. Using our MBE system, we have been able to
Fig. 1). According to our knowledge, only a few grow pseudomorphically AlGaN barrier layers on GaN
reports [6–8,30] have so far demonstrated nsl product epilayer with high aluminum content and therefore
value 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.

4. S. Haffouz et al. / Solid-State Electronics 49 (2005) 802–807 805
4. C-doped GaN buffer layer as an electrical isolation 10
3
template T=300K
Drain Current, ID (mA/mm)
2
10
Devices were fabricated on a wafer with a 2DEG 1
10
structure with ns and l of 1.7 · 1013 cmÀ2 and
V DS=15V
1120 cm2/V s, respectively. The measured sheet resistiv- 10
0
VDS=10V On-to-Off ratio
ity and aluminum content in the barrier layer were VDS=5V ~10
7
-1
328 X/sq and 36%, respectively. Fig. 2 displays the typ- 10
ical room temperature drain current–voltage (I–V) char- -2
10
acteristics. The fabricated HFET exhibited maximum
current densities as high as 900 mA/mm and transcon- -3
10 Pinch-off voltage
ductance peak value of about 180 mS/mm. By sweeping
-4
the gate–source voltage from 3 V down to around À5 V, 10
-10 -8 -6 -4 -2 0 2 4 6
we have been able to turn off the devices without
any 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-drain
centration in the range of 1017 cmÀ3), we found that is voltages (VDS) in the AlGaN/GaN field effect transistor with nsl
not 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
10-2
template and to obtain more information on the leakage Gate Leakage Current, IG (mA/mm)
T=300K
currents, we have carried out measurements of the gate- 10-3
to-source voltage (VGS) dependent drain current (ID)
Gate Leakage Current,IG (mA/mm) 10-1
and gate leakage current (IG). The results are depicted 10-4 VGS= Vth= -5.2V
in Figs. 3 and 4, respectively. Two important pieces of
information can be deduced from the ID–VGS curves. 10-5 10-2
The first one is the steepness of the slope in the ON–
10-6
OFF transition region (À5.3 V < VGS < À3 V). In fact,
10-3
the rapid decrease of the drain current with decreasing 10-7
the 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 20
precise 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 0
5 V and 10 V. We note also a small increase of the
pinch-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 dependence
Fig. 2. Typical IDS–VDS characteristics of the AlGaN/GaN field effect of gate leakage current on drain–source voltage at the
transistor structure with nsl product value of 1.9 · 1016 VÀ1 sÀ1. pinch-off voltage (VGS = À5.2 V). An increase of the

5. 806 S. Haffouz et al. / Solid-State Electronics 49 (2005) 802–807
gate leakage with increasing drain–source voltage is ob- plate. Excellent dc pinch-off characteristics, very low
served and is in agreement with the results obtained by leakage currents and good rf performances were
Arulkumaran et al. [33]. It should be pointed out that demonstrated.
the value of the gate leakage current measured on our
HFET devices is reasonably low compared to the values
reported in the literature [15,34–36]. However, a closer Acknowledgement
look reveals not only that the leakage current is still
larger 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, the
but 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/
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