902IEEE ELECTRON DEVICE LETTERS,VOL.36,NO.9,SEPTEMBER2015 Piezoelectricity-Induced Schottky Barrier Height Variations in AlGaN/GaN High Electron
Mobility Transistors
Kaiyuan Yao,Sourabh Khandelwal,Firas Sammoura,Atsushi Kazama,Chenming Hu,and Liwei Lin
Abstract—Drain current of AlGaN/GaN high electron mobility transistors(HEMTs)is measured to decrease/increase with in-plane tensile/compressive external strain.Such a trend is opposite to the conventional theory of direct piezoelectric effect on 2-D electron gas(2DEG).The reason is found to be the dependence of nickel gate barrier height on external strain, which strongly affects HEMTs’threshold voltage and2DEG con-centration.The Ni/AlGaN interface states are proposed to be responsible for strain-induced gate barrier variations,which are important for device performances and sensor applications.
Index Terms—GaN HEMT,Schottky barrier,piezoelectricity.
H IGH electron mobility transistors(HEMTs)based on the
AlGaN/GaN heterostructure featuring two dimensional electron gas(2DEG)have been actively studied for high frequency and high power electronics,as well as chemical and mechanical sensors[1].Applications such as micro strain and pressure sensors have attracted great interests[2]–[5]due to the good piezoelectric properties of AlGaN and GaN along the direction.Externally applied strain can change the net polarization charge density at the AlGaN/GaN interface due to the differences in their piezoelectric coefficients which results in variations in2DEG concentrations[2].This phe-nomenon is defined as the direct piezoelectric effect in this letter.For Ga-faced HEMT devices(this work),an external tensile strain will result in an increased2DEG concentration. However,an opposite trend to the direct piezoelectric effect has been recorded in this work.In order to explain the observed behavior,Schottky gate barrier height changes due to the applied strains for2DEG devices are investigated. Ourfindings show good consistency between analytical and experimental results.As such,piezoelectricity induced Schottky barrier height change could be an important element to be considered for the design of AlGaN/GaN HEMT-based electronic and sensing devices.
Manuscript received June29,2015;accepted July11,2015.Date of publication July14,2015;date of current version August21,2015.This work was supported in part by the Berkeley Sensor and Actuator
老婆饼机Center,a National Science Foundation/Industry/University Cooperative Research Center.The review of this letter was arranged by Editor T.Egawa. K.Yao,F.Sammoura,and L.Lin are with the Department of Mechanical Engineering,University of California at Berkeley,Berkeley,CA94720USA (e-mail:kyyao@berkeley.edu).
S.Khandelwal and C.Hu are with the Department of Electrical Engineer-ing and Computer Science,University of California at Berkeley,Berkeley, CA94720USA.
A.Kazama is with the Research and Development Group,Hitachi,Ltd., Ibaraki319-1414,Japan.
Color versions of one or more of thefigures in this letter are available online at
Digital Object Identifier10.1109/LED.2015.2456178
II.E XPERIMENT
The fabrication process starts with custom-made wafers
with Al0.2Ga0.8N/GaN(30nm/2μm)grown on Si(111)by using the metalorganic chemical vapor deposition(MOCVD)
process[6].Very low wafer bow(−0.5μm and−5.1μm)
were measured by laser profilometry indicating small residual stress.Mesas of40μm in diameter arefirst etched by the
Cl2/BCl3/Ar plasma with a depth of180nm for electrical
isolation.A layer of Ti/Al/Pt/Au(20/80/50/100nm)is evap-
orated for source/drain ohmic contact with a rapid thermal annealing process at850°C for35seconds.The contact
resistance is measured using the transmission line geometry method as1.6×10−5 .cm−2.The Schottky gate contact is constructed by using evaporated Ni/Au(20/200nm).Before the gate metal evaporation process,the exposed AlGaN surface
has beenfirst cleaned by oxygen plasma(1min with RF power
of50W and pressure of180mTorr),and rinsed in diluted NH4OH solution(1:10)for10seconds to remove surface oxide.A200nm thick PECVD SiN x layer is deposited at350°C for the passivation.Without the passivation layer, the drain currentfluctuates and decreases greatly with time and can’t be recorded for meaningful results.Finally,contact pads are opened on the SiN x layer.
电压比较器
电路Two wafers have been fabricated and cut into5cm×2.5cm chips for the strain tests using a standard four-point bending method as shown in Fig. 1.A total of11devices have been measured and reported.The uniaxial stress is applied along the x-direction and the magnitude of the applied force is monitored by a high-precision load cell.The induced strains(εx x,εyy,andεzz)are calculated analytically[7]and simulated numerically using the commercialfinite element software(COMSOL).III.R ESULTS AND D ISCUSSION
Fig.1presents the I D-V G transfer characteristics measured with different magnitudes of strainεx x,under V DS of0.1V. Inset of Fig.1shows microscope photo of a fabricated HEMT device and the schematics of the four-point bending test setup. It is observed that the drain current decreases/increases with respect to tensile/compressive strain under afixed V G,respec-tively.Furthermore,time-resolved variations of I D under fixed V G have been measured with stepwis
e changes ofεx x (not shown here)to confirm that the measured trend of I D with respect toεx x is highly repeatable and hysteresis-free.
To explain the opposite trend(external tensile strain results in decreased2DEG concentration),the J-V characteristics between gate and source are characterized under different magnitudes of strain as shown in Fig.2.It is observed that under afixed voltage,the resulting current density decreases
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YAO et al.:PIEZOELECTRICITY-INDUCED SCHOTTKY BARRIER HEIGHT V ARIATIONS
903
Fig.1.The I D versus V G transfer characteristics of an AlGaN/GaN HEMT device measured with different magnitudes of externally applied strain εxx .Inset shows the schematics of four-point wafer bending setup with simulated εxx profile under a loading force F of 15N,and an optical photo of a fabricated device with detailed
dimensions.
Fig.2.The gate-to-source J -V measurements under forward bias with different magnitudes of externally applied strain,εxx .Inset:extracted values of Schottky barrier height versus εxx ,and a cross-sectional view of the measurement setup.
under the tensile strain and increases under compressive strain.Previously,researchers have found that under a small for-ward bias,gate-to-source J-V characteristics follow the law of thermionic emission [8]with Schottky barrier height,φb .Therefore variations of current in Fig.2indicate changes of φb ,as shown by the inset.The slope between the barrier variation φb and εx x is ∼55μV per microstrain in this work.
The gate Schottky barrier height φb has a strong effect on the transfer characteristics of AlGaN/GaN HEMT.Threshold voltage V th and 2DEG electron concentration n s are extracted from measured I D −V G curves under different strain using the published AlGaN/GaN HEMT device model [9],[10].The Hall measurement hasn’t been employed here as the four point wafer bending setup is made of stainless steel.Variations of V th and n s (denoted V th and n s )in presence of εx x
are
Fig.3.Comparison of the Schottky barrier effect,direct piezoelectric effect,and their combined effect w
ith measured results for (a) V th and (b) n s with respect to εxx .The yellow and blue shaded areas are calculation uncertainties based on previously reported piezoelectric and stiffness coefficients [11]–[15].
plotted in Fig.3.The analytical expression of V th is given by [10]:
V th =φb −
E C q −σtot d AlGaN εAlGaN
quartz插件(1)
where q is electron charge,σtot is the total polarization charge
density at the AlGaN/GaN interface;d AlGaN and εAlGaN are the thickness and permittivity of the AlGaN barrier layer,respectively;and E C is the AlGaN/GaN heterojunction band offset.In Fig.3a,the Schottky barrier effect calculation is simply the slope of measured φb −εx x ,while the direct piezoelectric effect is calculated by:
σtot =(p AlGaN 31−p GaN 31)(εx x +εyy )+(p AlGaN 33−p GaN
33)εzz
(2)where σtot is the strain-induced change of σtot ;p AlGaN
31
,p GaN 31,p AlGaN 33and p GaN 33are material piezoelectric coefficients.In Fig.3b,the values of n s are calculated based on [16]:
n s =
σtot
q −(εAlGaN d AlGaN q 2
)e φb (3)
The yellow and blue shaded areas indicate the calculation
uncertainties based on previously reported piezoelectric and elastic stiffness coefficients [11]–[15].For both V th and n s ,the Schottky barrier effect and direct piezoelectric effect give opposite co
ntributions as shown.The experimentally measured results in this work matches well with the combined effect.The above analyses and results support the proposed effect of piezoelectricity induced Schottky barrier height changes.The external tensile stress results in tensile strain εxx and compressive strains εyy and εzz while p 33(p zz )is positive and p 31(p zx )is negative.The overall net piezoelectric effect is an increase of the negative polarization charges at the top surface
904IEEE ELECTRON DEVICE LETTERS,VOL.36,NO.9,SEPTEMBER
棉絮加工
2015
Fig.4.(a)Capacitance-voltage characteristics of Ni/Au contacts on the
AlGaN/GaN devices.Inset-dependence of conductance on angular fre-
quency.(b)Comparing experimental and modeled φb−εxx relation.The red and yellow shaded areas are uncertainties based on reported material
coefficients[11]–[15].
of AlGaN barrier.In response,more interface donor states may be ionized to hold positive charges,and the conduction band edge energy of AlGaN at the Ni/AlGaN interface moves upward leading to an increase in gate barrier height.Another factor is the charge changes at the gate due to the applied strain.The gate capacitance,C G,is the parallel plate capac-itance between gate metal and2DEG before depletion.For example,under a strainεx x of1.5×10−4, Q G is calculated as 1.5×10−7V G C/m2(using dielectric constant and poison ratio of AlGaN from[16]).The charge changes due to the piezoelec-tric effect is calculated(right side of Eq.4)as7.2×10−5C/m2, which is2∼3orders of magnitude higher than that of Q G (for typical V G in our experiments).Therefore,the gate charge changes due to strain are neglected and net charge changes are expressed as:
q2 φb D it≈p AlGaN
31(εx x+εzz)+p AlGaN
33
εyy(4)
where D it is the interface state density.Under the same applied strain,the right hand side of Eq.4is the same and smaller D it results in a larger Schottky barrier varia-tion.Here,capacitance and conductance of a circular-shape (70μm radius)Ni/Au Schottky contact have been measured with respect to frequency in Fig.4a and D it is extracted as 4.5±0.4×1012cm−2eV−1[17]-a low number as compared with prior reports[18]–[20].Figure4b shows the values of φb extracted from experimental data and compared with the model(Eq.4)with very good agreements over the
applied strain.If D it is1.8×1013cm−2eV−1or larger,the strain-induced Schottky barrier variation becomes negligible as shown by the red shaded area in Fig.4b.
IV.S UMMARY
The nickel-based Schottky gate structure for the AlGaN/GaN HEMT devices has been studied and piezoelectricity induced Schottky barrier height variations have been characterized.The observed barrier height variations with respect to the applied strain are attributed to the Ni/AlGaN interface state charges.This effect is shown to have a strong influence on the HEMT threshold voltage and 2DEG electron concentrations.
A CKNOWLEDGMENT
The authors would like to thank Drs. A. D.Koehler, E.D.Le Boulbar and J.Derluyn for valuable discussions,and thank Dr.C.Yang and R.Rivers for help on device fabrication. The devices are fabricated at the Berkeley Marvell Nanolab.
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