IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 60, NO. 10, OCTOBER 2013 3149
Impact of Intrinsic Stress in DiamondCapping Layers on the ElectricalBehavior of AlGaN/GaN HEMTs
Ashu Wang, Marko J. Tadjer, Member, IEEE, Travis J. Anderson, Member, IEEE, Roland Baranyai,James W. Pomeroy, Tatyana I. Feygelson, Karl D. Hobart, Member, IEEE, Bradford B. Pate,
Fernando Calle, Member, IEEE, and Martin Kuball
Abstract— A finite-element model coupling 2-D electron gas(2-DEG) density, piezoelectric polarization charge QP, and intrin-sic stress induced by a nanocrystalline diamond capping layer,was developed for AlGaN/GaN high electron mobility transistors.Assuming the surface potential is unchanged by an additionalstress from diamond capping, tensile stress from the diamondcap leads to an additional tensile stress in the heterostruc-ture and, thus an increase in the 2-DEG under the gate.As a result, additional compressive stress near the gate edgeswould develop and lead to decreased 2-DEG in the regionsbetween the source and drain contacts (SDCs). Increased sat-uration drain current will be due to the reduced total resistancebetween SDC. Integration of the 2-DEG density from SDCrevealed a redistribution of sheet density with total sheet chargeconcentration remaining unchanged. The modeling results werecompared with the experimental data from Raman spectroscopyand I–V characterization, and good agreements were obtained.
Index Terms— AlGaN/GaN, electro-thermo-mechanicalcoupling, finite element modeling, nanocrystalline diamond(NCD), stress.
I. INTRODUCTION
GaN and AlN exhibit large piezoelectric and spontaneouspolarization coefficients [1]. A high density of 2-D elec-
tron gas (2-DEG) close to the AlxGa1−xN/GaN heterojunctionis therefore induced to compensate for the polarization charge
Manuscript received March 30, 2013; revised July 19, 2013; acceptedJuly 22, 2013. Date of publication August 7, 2013; date of current versionSeptember 18, 2013. This work was supported in part by the Projects Con-solider RUE under Grant CSD2009-00046, AEGAN under Grant TEC2009-14307-C02-01, CAVE under Grant TEC2012-38247, the Ministerio deEconomía y Competitividad of Spain, the CEI Campus Moncloa, UPM-UCM,Madrid, Spain, and the ASEE. The research at NRL was supported by theOffice of Naval Research. The review of this paper was arranged by EditorK. J. Chen.
A. Wang and F. Calle are with ISOM and Dpto. Ingeniería Electrónica,ETSI Telecomunicación, Universidad Politécnica de Madrid, Madrid 28040,Spain (e-mail: [email protected]; [email protected]).
M. J. Tadjer was with the Universidad Politécnica de Madrid, Madrid 28040,Spain. He is now with the Naval Research Laboratory, Washington, DC 20375USA (e-mail: [email protected]).
R. Baranyai, J. W. Pomeroy, and M. Kuball are with the Center forDevice Thermography and Reliability, University of Bristol, Bristol BS8 1TL,U.K. (e-mail: [email protected]; [email protected];[email protected]).
Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TED.2013.2275031
at this interface, which made the development of advancedAlGaN/GaN high-electron mobility transistors (HEMTs) forradar and communication applications possible.
It is well known that growing AlGaN and GaN layerheteroepitaxially, such as on Si or SiC substrates, as wellas deposition of a passivating dielectric layer, results inintrinsic stresses in the structure. The interaction of thisintrinsic stress on the electrical properties of the device hasbeen investigated previously, including the impact of substratethinning on the 2-DEG density nS [2]. For films deposited athigher temperatures, such as nanocrystalline diamond (NCD),a significant stress will result from the large thermal expansioncoefficient mismatch with the III-nitride heterostructure [3],[4]. Therefore, an NCD film on either the device-side or thesubstrate-side of a GaN HEMT wafer will lead to changes inQ P and nS induced by the additional stress. The objective ofthis paper is to quantify the additional stress a top-side NCDcap will induce on an AlGaN/GaN HEMT.
Studies on how the device performance is influenced bymechanically deforming the heterostructure are well docu-mented [5]–[7]. Experimental studies in mechanical deforma-tion have adopted two experimental paths. On one hand, anexternal force was applied on the whole wafer, with or with-out fabricated HEMTs, and an increased conductivity of the2-DEG with an external tensile was observed [5]. This effectwas attributed to a change of 2-DEG density induced bychange of Q P in both the AlGaN and GaN layers, as wellas a change in the electron effective mass. Consistent withthis observation, an increased dc drain current in AlGaN/GaNHEMTs was also observed with the application of externaluniaxial tensile stress [7]. This change in electrical propertiesof the device has been applied to use AlGaN/GaN HEMTsas pressure sensors [8], [9]. The second approach focusedon the change of the device electrical behavior caused bythe internal stress in the device. This is to illustrate theinteraction of the intrinsic stress in the cap layer, built-in stressin the epitaxial AlGaN/GaN layers, and residual stress in thesubstrate [2], [10].
HEMTs in high-power operation condition generate consid-erable heat in the channel. This leads to an elevated lattice tem-perature, furthermore reduces the electron mobility and thusthe output drain current, termed self-heating effect. Diamondcapped on HEMTs was proposed as a heat spreader to reducethe device self-heating effect due to its high-thermal conduc-
3150 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 60, NO. 10, OCTOBER 2013
tivity [11], [12]. The integration with HEMTs can be per-formed by direct deposition on the full processed HEMTs [12],[13], or by the diamond-before-gate approach, in which thediamond deposition is prior to the gate fabrication [11], [14].
The experimental observations have previously beenexplained by an empirical tight-binding model, which assumedthat the AlGaN and GaN layers were subject to identicaldeformation [15]. Therefore, the stress resulting change inthe 2-DEG density was only determined by the differenceof the piezoelectric polarization coefficients of AlGaN andGaN. Mechanical analysis has illustrated the nonuniform stressdistribution in the channel-induced by intrinsic stress in SiNpassivation, without considering its impact on the electricaltransport properties [16]. In fact, the stress in the device doesnot only affect the 2-DEG distribution, but has also structuraldegradations and reliability issues. This also requires thedevelopment of stress-related coupled model for the analysisof inverse piezoelectric and thermal stress [17].
The 2-DEG density can be calculated using a simple analyt-ical expression with an assumption that the strain only exists inthe AlGaN layer [1]. This method leads to uniform distributionof the polarization charge on the AlGaN surface and atthe AlGaN/GaN interface, and also uniform distribution ofthe 2-DEG close to the interface. However, interaction of thestresses in different parts of the device yields to a complexdistribution of the stress in the epitaxial layers, and thusa nonuniform distribution of the 2-DEG. In this case, anumerical method would be necessary for the calculation ofthe 2-DEG density. In this paper, we present a model tocouple the 2-DEG in the channel with the stress in the device.This enabled us to analyze how an intrinsic stress, such asthat induced by passivation and heat spreading layers, affecteddevice electrical behavior. Based on this model, a finite ele-ment method (COMSOL) was used to perform the numericalcalculations [18]. Simulation results are complemented withexperimental device data.
II. EXPERIMENTAL DETAILS
Transistor devices were fabricated on 25-nmAl0.25Ga0.75N/1.3 μm GaN HEMT structures grown bymetal-organic CVD on high-resistivity (1 1 1) Si substrates.Details on transistor device processing, NCD growth andNCD thermal conductivity characterization are providedelsewhere in the literature [11], [14], [19], [20]. The controlHEMT (A-HEMT) was passivated by a 100-nm thick SiNlayer deposited by plasma-enhanced CVD (PECVD). For thediamond-capped HEMT (B-HEMT), a thin 10-nm PECVDSiN layer, followed by an additional 500-nm thick NCD cap,were grown after the mesa isolation and ohmic contact steps,but before Schottky gate metallization. NCD was grown at750 ◦C (∼100 nm/h) and was unintentionally boron dopedfor p− type conductivity. Prior to gate metal liftoff, O2-basedand SF6-based inductively-coupled plasma (ICP) was usedto etch the diamond and SiN layers, respectively. Plasmadamage was minimized by reducing the O2 ICP power asthe AlGaN surface was approached. Process monitoring wasperformed via Hall measurements and scanning electron
Fig. 1. Device structure of a (a) control and (b) diamond capped AlGaN/GaNHEMTs. The gate width is 100 μm, gate length is 1 μm, and distance betweengate and source/drain contact are 2.3 μm/10 μm.
TABLE I
DEVICE PARAMETERS OF THE CONTROL HEMT (A-HEMT) AND THE
DIAMOND-CAPPED HEMT (B-HEMT)
microscope imaging. The devices’ structure and some keyelectrical parameters are shown in Fig. 1 and Table I,respectively. Raman spectroscopic methods were employedfor stress characterization in the GaN heterostructure. AnAr-ion laser with excitation wavelength of 488 nm and aspot size of 0.5 μm, and the measured Raman wavenumberswere averaged over the GaN layer depth. The details ofmicro-Raman spectroscopy analysis are published elsewherein the literature [21]–[23].
III. MODEL DESCRIPTION
A coupled electro-thermo-mechanical model was developedas described in the following using the parameters shown inTable II. In this model, first the stress, Q P , and the nonuniformdistribution of nS in the epitaxial layers were determined bynumerically solving the electromechanical coupling equations.The resulting Q P distribution was thus fed into the Poissonequation, which was solved simultaneously with the continuityequation and the thermal conductivity equation. By solving thethree equations simultaneously, the electrical characteristicsof the device were obtained. In this coupling procedure, theelectrical characteristics of the device under bias did not affectthe Q P distribution, indicating that it was a one-way coupling
WANG et al.: IMPACT OF INTRINSIC STRESS IN DIAMOND CAPPING LAYERS ON THE ELECTRICAL BEHAVIOR OF AlGaN/GaN HEMTs 3151
TABLE II
PARAMETERS USED IN THE SIMULATIONS
method without considering inverse piezoelectric and thermalstress, which have negligible influence on the device electricalbehavior as shown in [24].
Due to the large gate width/length aspect ratio, the simula-tions are restricted in two dimensions (see the simulated devicestructures shown in Fig. 1, with dimensions consistent withthe fabricated ones). Therefore, a plane-strain approximationwas used for the mechanical simulation. For A-HEMT, the2-DEG is simulated by assuming uniform built-in tensile strainin AlGaN layer but no strain in GaN [1]. Strain in AlGaN layercan be expressed as Sx x = Syy = (αGaN −αAlGaN)/αAlGaN. Inthis case, Q P only appears on the surface of AlGaN and atthe AlGaN/GaN interface, but not in the GaN bulk. In idealconditions without considering traps or defects, the followingpolarization charge is presented: surface polarization chargeσS = PPE(AlGaN) + PSP(AlGaN) on the surface of AlGaNlayer, and interfacial polarization charge σS = PPE(AlGaN) +PSP(AlGaN) − PSP(GaN) at the AlGaN/GaN interface, wherePPE and PSP denote piezoelectric and spontaneous polarizationvector, respectively. In the simulation, only the in-plane initialconstant strain for AlGaN is provided; accordingly the out-of-plane strain will be numerically determined. For B-HEMT, Q P
for AlGaN and GaN is affected by the intrinsic stress in thecap layer since they are fully mechanically-coupled system,leading to the nonuniform distribution of Q P on the AlGaNsurface and at the AlGaN/GaN interface, as well as in theAlGaN and GaN bulk. This intrinsic stress is assumed as in-plane named σx x along the gate length (x-direction) and σyy
along the gate width (y-direction). With the initial strain inAlGaN and the intrinsic stress in diamond layer, we solvethe coupled mechanical and electrostatic equations with theconstitutive equations
σi j = ci jkl skl − eki j Ek
Di = ei jk s jk − εi j E j + Psi (1)
to obtain the electric displacement vector Di as well asthe 2-DEG density ns for A- and B-HEMT, respectively.In (1), σi j is the stress tensor, ei jk is the elastic stiffness tensor,
Skl is the strain tensor, ei jk is the piezoelectric coefficienttensor, εi j is the permittivity tensor, Ek is the electric fieldvector, and Ps
i is the spontaneous polarization vector. In themechanical simulations, the boundary condition was free ofstress on top of and at the sidewall of the simulated structure,whereas it was fixed constraint at the bottom of the simulatedstructure (Fig. 1).
In order to obtain the electrical characteristics of thedevice, we have to solve the following relevant semiconductorequations.
1) Poisson Equation:
∇ · D = qn (2)
where q is the basic charge unit and n is the electron con-centration. The electric displacement vector D is propagatedfrom Di in (1). The polarization vector Pi = ei jk S jk + Ps
i inAlGaN/GaN obtained from (1) is assumed to be unchangedwith the drain and gate voltage. This means that the inversepiezoelectric polarization effect is not considered. Only theelectron charge is included in the right side of (2). The holes,unintentional ionized impurities, and other nonideal charges,i.e., traps or defects are also omitted.
2) Current Continuity Equation:
1
q∇ · jn = 0, jn = −qnμn∇φn . (3)
Here, electron concentration n is dependent on electricalpotential and quasi-Fermi level φn . It is calculated by Fermi–Dirac statistics using the Aymerich–Humet approximation[25]. We choose φn instead of n as the dependent variableto avoid the nonphysical phenomenon of negative carrierconcentration. jn is the drift-diffusion electron current solvedin the GaN layer as electrons mainly concentrate on the GaNside of the AlGaN/GaN interface. Hole current was neglected.
3) Thermal Conduction Equation:
∇ · k∇T + jn · E = 0 (4)
where T is the temperature, k is the thermal conductivity, Eis the electric field, and jn · E is the joule heat generatedby the electron current. AlGaN/GaN HEMTs always workin high-power condition, which would present a significantself-heating effect. Therefore, incorporating the thermal effectis necessary for the improved accuracy of the simulation.Isothermal boundary condition is applied at the bottom of thesubstrate representing a perfect heat sink to room temperature.Because the generated heat is highly concentrated at the drainedge of the gate, the area for heat transfer to air is verysmall, leading to negligible heat dissipation from the top ofthe device.
Indeed, the numerical experiments show that the temper-ature distribution in the device is unchanged when tryingdifferent thermal exchange coefficients ranging from 10 to100 W/(m2K). Thus, an adiabatic boundary condition isapplied on the surface, as well as at the sidewall of thedevice.
3152 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 60, NO. 10, OCTOBER 2013
IV. RESULTS AND DISCUSSIONS
A. 2-DEG Response to Interfacial Q P
We follow with an analysis on how the 2-DEG changeswith Q P -induced by the intrinsic stress in the diamond layer.From the simulation point of view, Q P can be determinedcompletely by the stress, but to calculate 2-DEG density thesurface potential is needed. For A-HEMT, we do not carethe negative surface charge of the AlGaN layer (actually, nodefinite theory to describe it because of the surface states), butinstead fixing the surface potential to make the 2-DEG densitysimilar to Hall measurement results.
As the device structure shows (Fig. 1), the diamond layer isdeposited on SiN but not directly on the surface of the AlGaNlayer, so both A- and B-HEMT should have same contact prop-erties at the AlGaN/SiN interface from the chemical point ofview. However, the surface Q P of AlGaN should be differentbecause of the additional Q P -induced by intrinsic stress in thediamond layer for B-HEMT (spontaneous polarization chargeis not influenced by stress, so it was kept unchanged). Mindingthis difference, we still assume the surface potential of aB-HEMT is the same as that of a A-HEMT. Generally speak-ing, this assumption should be somewhat reasonable due to thevery high density surface states always existing on the surfaceof AlGaN, leading to Fermi level pinning. This was used toexplain the 2-DEG density dependence on AlGaN thickness[26]. With this assumption in mind, the numerical calculationsshow that the additional positive Q P at the AlGaN/GaNinterface-induced by external stress leads to additional 10%negative charge on the AlGaN surface and 90% 2-DEG, whichmeans that change of the interfacial Q P mainly causes the2-DEG other than the AlGaN surface charge to compensate it.This presents the potential to improve the device performanceby stress engineering.
B. Experimental Validation of Simulated Stress
Raman spectroscopy was used to determine the stress-induced by the diamond capping layer. The Raman spectrawere obtained at a distance of about 4 μm from the NCDedge near the drain side of the gate. The phonon frequencyshift of the GaN layer in the device structure as a function oftemperature is shown in Fig. 2(a). From the phonon frequencydifference, the additional stress-induced by the diamond layer(assumed to be biaxial) was determined using a −1.25 (GPa ·cm)−1 deformation potential. The result is shown in Fig. 2(b).
To validate the model, thermo-mechanical simulations wereperformed to obtain the stress as a function of temperature.In the simulations, the heat dissipation in the channel wascalculated as jn · E, where jn is the current density and Eis the electric field. The temperature and thermal stress thencan be determined by this heat dissipation with the thermalconductivity and mechanical parameters shown in Table II.Fig. 2(b) compares the measured additional stress in theGaN layer with simulated additional stress at the drain edgeof the gate. The measured additional stress had indicatedtensile stress, whereas the simulated one was compressive[σx x < 0 for x < 1.0 μm and x > 2.0 μm in Fig. 4(a)].The reason for the discrepancy lies in the exposure of the SiN
Fig. 2. Comparison of Raman shift for (a) A- and B-HEMT and (b) additionalstress induced by the diamond cap layer determined by the Raman shift andthermo-mechanical simulations as a function of temperature.
nucleation layer to the high temperature of NCD growth, thuschanging its stoichiometry and increasing its contribution tothe tensile profile in the access region. Future experiments willbe performed to improve dielectric thermal stability.
Interestingly, the temperature dependence of both simulatedand measured additional stress was consistent [Fig. 2(b)].The temperature dependence of the additional stress wasattributed to the difference of the thermal stress caused by thedifferent thickness and thermal expansion coefficient of theSiN (A-HEMT) and diamond (B-HEMT) cap layer (Table II).
Fig. 3 shows the 2-D distribution of stress σx x in A- andB-HEMT. We can see that the discontinuity of the diamondlayer at the gate edge leads to nonuniform and relative largeσx x there for B-HEMT.
Fig. 4 exhibits the 1-D distribution of the additional stressσx x (a), piezoelectric polarization PPZ (b), 2-DEG density ns
(c), and electric field magnitude |E | (d) along the channelfor B-HEMT with varied thickness of diamond layer. As thegraphs show, the A-HEMT has no stress and PPZ becauseof the assumption of zero strain in the GaN buffer [blackline in (a) and (b)]. As a result, the 2-DEG density wasuniformly distributed [black line in (c)]. For the B-HEMT,the intrinsic tensile stress in the diamond layer-induced addi-tional nonuniform tensile stress, and thus the increase in2-DEG density under the gate up to the gate edge. For thesource/drain region, additional compressive stress, and thusdecreased 2-DEG density are induced. The simulation resultsshow that integration of the 2-DEG between the source anddrain contacts (SDCs) (1.41 × 1010 cm−1) is the same forboth the A- and B-HEMT. This means that whereas the2-DEG was redistributed across the SDC-induced by thestress in the diamond layer, the total concentration remainsunchanged, since the mechanical coupling between the dia-mond layer and the heterostructure is internal. Note that, this isdifferent from the case where a change in the 2-DEG density is
WANG et al.: IMPACT OF INTRINSIC STRESS IN DIAMOND CAPPING LAYERS ON THE ELECTRICAL BEHAVIOR OF AlGaN/GaN HEMTs 3153
Fig. 3. 2-D distribution of stress in the A- and B-HEMT.
induced by external bending on the whole wafer [15], in whichthe 2-DEG would change in the same way across the wholeregion between SDC, and its integration from SDCs would bechanged with the external stress. The asymmetrical distributionof 2-DEG under the gate was attributed to the differentgeometrical spacing between the gate and the source/draincontacts (GSDC). A large change of the 2-DEG happens inthe area near the gate edge, compared with a relatively smallchange for the S/D regions. This can be explained that inaddition to the large normal stress, a noticeable shear stress isalso induced at the gate edge due to the discontinuity of thediamond layer. By reducing the gate length, the two gate edgesbecome closer, leading to higher additional 2-DEG under thegate.
Fig. 4(d) shows that the large change of electric fieldhappens near the gate edge, which may affect the devicebreakdown voltage. Fig. 4(e) shows that the average additionalstress and the average 2-DEG density in the channel under thegate increase as the diamond thickness increases. This effectgoes away for diamond layers thicker than 2 μm. This casehas been illustrated experimentally for the device with SiNpassivation [10].
C. Influence of Additional Stress on Electrical Behavior
Fig. 5 compares the electrical measurements and simulationsof the drain current IDS as a function of the drain voltageVDS (a) and gate voltage VGS (b), for both the A- andB-HEMT. In the case of the B-HEMT, the simulations includethe coupling with the intrinsic stress in the diamond layer.Fig. 5(a) shows the simulation results were in very goodagreement with the IDS–VDS experimental data for the operat-ing conditions considered. Fig. 5(b) shows that the simulatedIDS of B-HEMT was higher than that of A-HEMT, whichis contrary to the experimental result at around VGS = 1 V(VDS = 0.1 V). The reason is that, we did not consider gateleakage current in the simulation.
The inset graph in Fig. 5(b) shows how the average2-DEG density under the gate depends on VGS. Comparedwith a A-HEMT, the increment in nS in a B-HEMT does notchange with VGS for VGS > Vt as shown in the graph such thatnS has a relatively large increase at low VGS, which further
Fig. 4. Distribution of (a) nonuniform additional stress σxx , (b) piezoelectricpolarization PPZ PPZ, (c) 2-DEG density ns , (d) electric field magnitude alongthe channel, and (e) average 2-DEG density (left axis) and average additionalstress (right axis) in the channel under the gate for B-HEMT with variedthickness of diamond layer (0.1–2 μm). σxx , PPZ, and |E| are extracted1 nm below the AlGaN/GaN interface.
yields to a relative large IDS increase at low VGS as shownin Fig. 5(a). The reason is that change of the 2-DEG onlydepends on the additional polarization charge-induced by thediamond layer, and at the same time VGS has no influence onthe polarization charge (not considering inverse piezoelectricpolarization effect). This phenomenon has been equivalently
3154 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 60, NO. 10, OCTOBER 2013
Fig. 5. Comparision of experimental and simulation results for IDS as affunction of (a) VDS and (b) VGS. Inset in (b) shows the average 2-DEGdensity ns under the gate as a function of VGS for A- and B-HEMT.
illustrated in AlGaN/GaN HEMTs pressure sensors, as IDSis more sensitive to pressure at lower gate fields [8], [9].In addition, this graph also shows that the threshold voltagewas negatively shifted due to the increased 2-DEG densityunder the gate (Table I).
In accordance with our experimental work, we only dis-cussed the intrinsic stress from the diamond layer impact onthe device electrical behavior. However, this model should besuitable to generalize the effect of any capping layer, such asa SiN passivation layer traditionally used to mitigate currentcollapse effects during RF operation, on the stress profile inthe device [27], [28]. The intrinsic stress in such passivationlayers has been shown to be as high as 3 GPa [29], which couldpresent significant reliability concerns in a scaled device.
To analyze how intrinsic stress in the capping layers affectsdevice scaling, we extended our simulations with a 3-GPaintrinsic stress using the verified model for a long-channelHEMT. Fig. 6(a) shows the maximum σx x in the channel as afunction of the gate length LG and the distance between the
Fig. 6. (a) Simulation results for the maximum additional stress σxx inthe channel (1 nm below the AlGaN/GaN interface) induced by the caplayer with intrinsic stress σxx = σyy = 3 GPa as a function of thegate length LG and the distance between the gate and drain contact LGD.(b) and (c) GSDC resistance increase |�RS/D|, gate resistance decrease|�RCH|, total resistance decrease |�RT |, and drain current increase |�IDS|compared to A-HEMT as a function of LG and LGD, respectively. In all thegraphs, LGD = LGS = 4 μm when LG is varied, and LGS = 4 μm andLG = 1 μm when LGD is varied.
gate and drain contact LGD. As the graph shows, σx x increasesas LGD increases or LG decreases. As severe reliability effectsdue to structural stress near the gate have been reported [30],[31], it has become clear that a multilayer device cappingstrategy could be engineered to minimize the total stress inthe heterostructure. The stress contribution of a novel materialsuch as NCD can be calibrated on a HEMT structure of knownintrinsic stress. Consequently, stress-induced by the NCD capcould be mitigated when a cap layer stack such as NCD/SiNis employed.
To analyze the effect of sheet charge redistribution underthe additional stress of the cap layer, we can considerIDS as a function of the total resistance RT betweenSDC. RT would be divided into two constituent resistances:RT = RS/D + RCH where RS/D and RCH were the resis-tances between GSDC (extrinsic resistance) and under thegate (channel resistance), respectively. RCH is modulated byVGS and RS/D is independent on the operating conditions.
WANG et al.: IMPACT OF INTRINSIC STRESS IN DIAMOND CAPPING LAYERS ON THE ELECTRICAL BEHAVIOR OF AlGaN/GaN HEMTs 3155
As we discussed before, for B-HEMT the lateral redistributionof sheet charge between SDC would lead to the increased RS/D(decreased 2-DEG density) and decreased RCH (increased2-DEG density). However, the decreased RCH is higher thanthe increased RS/D which leads to an overall decreased RT ,and thus a increased IDS. Specific simulation results areshown in Fig. 6(b) and (c), which show that IDS would havesignificant improvement by around 20% for the device withrelative short gate length (LG = 0.5 μm) or long distancebetween gate and drain contact (LGD = 15 μm).
D. Discussion
Due to the lack of systematic studies on the dependence ofsurface potential on additional stress applied on the device,we assumed Fermi-level pinning on the surface of AlGaN andobtained reasonable agreement of simulation and experimentalresults. We attributed the intrinsic tensile stress in the diamondlayer to the increase in IDS. Based on this model, if thetensile stress is replaced with compressive stress, the decreased2-DEG density under the gate region (increased RG ),increased 2-DEG density between GSDC (decreased RS/D),and therefore the decreased saturation IDS would be obtained.This shows a similar behavior to that observed experimentally,in which AlGaN/GaN HEMTs were capped with diamond-likecarbon layer with the ∼6-GPa high compressive stress [32].
In general, mechanical degradation such as crack/pit alwayshappens at the drain side of the gate edge, where highlylocalized heat dissipation, high thermal stress, and high inversepiezoelectric stress are present, and these aspects were pro-posed to explain the mechanical degradation [17], [30], [33],[34]. Our work shows that the intrinsic stress in the cap layergives also highly localized additional stress at the gate edge[Fig. 6(a)], which therefore presents a probability to aggra-vate or mitigate the mechanical degradation. This aspect hasbeen illustrated by the simulation, showing that the compres-sive/tensile stress in the passivation layer can decrease/increasethe total stress at the gate edge [17]. Combined with ourconclusions, this means that tensile stress in the cap layerwould improve the device electrical behavior, but aggravatethe mechanical degradation; conversely, compressive stress inthe cap layer would mitigate the mechanical degradation, butreduce the electrical behavior.
V. CONCLUSION
We presented an electro-thermo-mechanical coupled modelto analyze how the intrinsic stress in the capped layer affectsthe electrical behavior of AlGaN/GaN HEMTs. The simulationresults were verified by the experimental data from Ramanspectroscopy and I–V characterization for the diamond cappeddevice. The stress in the capped layer induces nonuniform2-DEG distribution in the channel. Change of the AlGaN/GaNinterfacial Q P mainly causes the 2-DEG other than the AlGaNsurface charge to compensate it, which presents a potential toimprove the device performance by stress engineering. Due tothe discontinuity of the capped layer at the gate edge, unlikethe whole wafer bending by external stress, intrinsic stressin the capped layer would induce the 2-DEG under the gate
showing opposite variation to that in other regions betweenSDC. Since mechanical coupling between the device andcapped layers is internal, the 2-DEG would be redistributedby the additional stress from the cap layer while remaining itstotal concentration between SDC unchanged. For the deviceoperating in saturation condition, change of RCH is the domi-nating factor of change of RT , so the decreased RCH-inducedby tensile stress in the capped layer leads to decreased RT ,and therefore increased drain current. Extended simulationswith this validated model showed significant improvement ofthe device electrical behavior would be achieved.
REFERENCES
[1] O. Ambacher, J. Smart, J. R. Shealy, N. G. Weimann, K. Chu, M. Mur-phy, W. J. Schaff, L. F. Eastman, R. Dimitrov, L. Wittmer, M. Stutzmann,W. Rieger, and J. Hilsenbeck, “Two-dimensional electron gases inducedby spontaneous and piezoelectric polarization charges in N- and Ga-face AlGaN/GaN heterostructures,” J. Appl. Phys., vol. 85, no. 6,pp. 3222–3233, Mar. 1999.
[2] M. Azize and T. Palacios, “Effect of substrate-induced strain in thetransport properties of AlGaN/GaN heterostructures,” J. Appl. Phys.,vol. 108, no. 2, pp. 023707-1–023707-4, Jul. 2010.
[3] X. Xiao, B. W. Sheldon, Y. Qi, and A. K. Kothari, “Intrinsic stressevolution in nanocrystalline diamond thin films with deposition tem-perature,” Appl. Phys. Lett., vol. 92, no. 13, pp. 131908-1–131908-3,Apr. 2008.
[4] H. Li, B. W. Sheldon, A. Kothari, Z. Ban, and B. L. Walden, “Stressevolution in nanocrystalline diamond films produced by chemical vapordeposition,” J. Appl. Phys., vol. 100, no. 9, pp. 094309-1–094309-9,Nov. 2006.
[5] B. S. Kang, S. Kim, J. Kim, F. Ren, K. Baik, S. J. Pearton, B.P. Gila, C. R. Abernathy, C.-C. Pan, G.-T. Chen, J.-I. Chyi, V.Chandrasekaran, M. Sheplak, T. Nishida, and S. N. G. Chu, “Effectof external strain on the conductivity of AlGaN/GaN high-electron-mobility transistors,” Appl. Phys. Lett., vol. 83, no. 23, pp. 4845–4847,Dec. 2003.
[6] A. D. Koehler, A. Gupta, M. Chu, S. Parthasarathy, K. J. Linthicum,J. W. Johnson, T. Nishida, and S. E. Thompson, “Extraction ofAlGaN/GaN HEMT gauge factor in the presence of traps,” IEEEElectron Device Lett., vol. 31, no. 7, pp. 665–667, Jul. 2010.
[7] C.-T. Chang, S.-K. Hsiao, E. Y. Chang, C.-Y. Lu, J.-C. Huang, andC.-T. Lee, “Changes of electrical characteristics for AlGaN/GaN HEMTsunder uniaxial tensile strain,” IEEE Electron Device Lett., vol. 3, no. 3,pp. 213–215, Mar. 2009.
[8] G. Vanko, M. Drzik, M. Vallo, T. Lalinsky, V. Kutis, S. Stancik, I. Ryger,and A. Bencurova, “AlGaN/GaN C-HEMT structures dynamic stressdetection,” Sens. Actuators A, Phys., vol. 172, no. 1, pp. 98–102,Dec. 2011.
[9] Y. Liu, P. P. Ruden, J. Xie, H. Morkoc, and K.-A. Son, “Effect ofhydrostatic pressure on the dc characteristics of AlGaN/GaN hetero-junction field effect transistors,” Appl. Phys. Lett., vol. 88, no. 1,pp. 013505-1–013505-3, Jan. 2006.
[10] C. M. Jeon and J.-L. Lee, “Effects of tensile stress induced bysilicon nitride passivation on electrical characteristics of AlGaN/GaNheterostructure field-effect transistors,” Appl. Phys. Lett., vol. 86, no. 17,pp. 172101-1–172101-3, Apr. 2005.
[11] M. J. Tadjer, T. J. Anderson, K. D. Hobart, T. I. Feygelson, J. D. Cald-well, C. R. Eddy, F. J. Kub, J. E. Butler, B. Pate, and J. Melngailis,“Reduced self-heating in AlGaN/GaN HEMTs using nanocrystallinediamond heat-spreading films,” IEEE Electron Device Lett., vol. 33,no. 1, pp. 23–25, Jan. 2012.
[12] M. S. Eggebert, P. Meisen, F. Schaudel, P. Koidl, A. Vescan,and H. Leier, “Heat-spreading diamond films for GaN-based high-power transistor devices,” Diamond Relat. Mater., vol. 10, nos. 3–7,pp. 744–749, Mar./Jul. 2001.
[13] M. Alomari, M. Dipalo, S. Rossi, M.-A. Diforte-Poisson, S. Delage,J.-F. Carlin, N. Grandjean, C. Gaquiere, L. Toth, B. Pecz, and E. Kohn,“Diamond overgrown InAlN/GaN HEMT,” Diamond Relat. Mater.,vol. 20, no. 4, pp. 604–608, Apr. 2001.
3156 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 60, NO. 10, OCTOBER 2013
[14] T. J. Anderson, M. J. Tadjer, K. D. Hobart, T. I. Feygelson, J. D. Cald-well, M. A. Mastro, J. K. Hite, C. R. Eddy, Jr., F. J. Kub, and B.B. Pate, “Profiling the temperature distribution in AlGaN/GaN HEMTswith nanocrystalline diamond heat spreading layers,” in Proc. DeviceRes. Conf., Apr. 2012, pp. 1–3.
[15] M. Chu, A. D. Koehler, A. Gupta, T. Nishida, and S. E. Thomp-son, “Simulation of AlGaN/GaN high-electron-mobility transistor gaugefactor based on two-dimensional electron gas density and electronmobility,” J. Appl. Phys., vol. 108, no. 10, pp. 104502-1–104502-6,Nov. 2010.
[16] M. A. Mastro, J. R. LaRoche, N. D. Bassim, and C. R. Eddy, Jr.,“Simulation on the effect of non-uniform strain from the passiva-tion layer on AlGaN/GaN HEMT,” Microelectron. J., vol. 36, no. 8,pp. 705–711, Aug. 2005.
[17] M. G. Ancona, S. C. Binari, and D. J. Meyer, “Fully coupled ther-moelectromechanical analysis of GaN high electron mobility transistordegradation,” J. Appl. Phys., vol. 111, no. 7, pp. 074504-1–074504-16,Apr. 2012.
[18] [Online]. Available: http://www.comsol.com[19] J. E. Butler and A. V. Sumant, “The CVD of nanodiamond matertials,”
Chem. Vapor Deposit., vol. 14, nos.7–8, pp. 145–160, 2008.[20] E. Bozorg-Grayeli, A. Sood, M. Asheghi, V. Gambin, R. Sandhu,
T. I. Feygelson, B. B. Pate, K. D. Hobart, and K. E. Goodson,“Thermal conduction inhomogeneity of nanocrystalline diamond filmsby dual-side thermoreflectance,” Appl. Phys. Lett., vol. 102, no. 11,pp. 111907-1–111907-4, 2013.
[21] T. Batten, J. W. Pomeroy, M. J. Uren, T. Martin, and M. Kuball, “Simul-taneous measurement of temperature and thermal stress in AlGaN/GaNhigh electron mobility transistors using Raman scattering spectroscopy,”J. Appl. Phys., vol. 106, no. 9, pp. 094509-1–094509-4, Nov. 2009.
[22] A. Sarua, M. Kuball, and J. E. Van Nostrand, “Phonon deformationpotentials of the E2(high) phonon mode of Alx Ga1−x N,” Appl. Phys.Lett., vol. 85, no. 12, pp. 2217–2219, 2004.
[23] A. Sarua, H. Ji, M. Kuball, M. J. Uren, T. Martin, K. J. Nash,K. P. Hilton, and R. S. Balmer, “Piezoelectric strain in AlGaN/GaNheterostructure field-effect transistors under bias,” Appl. Phys. Lett.,vol. 88, no. 10, pp. 103502-1–103502-3, 2006.
[24] M. Auf der Maur, G. Romano, and A. Di Carlo, “Electro-thermo-mechanical simulation of AlGaN/GaN HEMTs,” in Proc. 15th IWCE,May 2012, pp. 1–4.
[25] X. Aymerich-Humet, F. Serra-Mestres, and J. Millan, “An analyticalapproximation for the Fermi-Drac integral F3
2 (η),” Solid-State Electron.,
vol. 24, no. 10, pp. 981–982, Oct. 1981.
[26] J. P. Ibbetson, P. T. Fini, K. D. Ness, S. P. DenBaars, J. S. Speck, andU. K. Mishra, “Polarization effects, surface states, and the source ofelectrons in AlGaN/GaN heterostructure field effect transistors,” Appl.Phys. Lett., vol. 77, no. 2, pp. 250–252, Jul. 2000.
[27] G. Koley, V. Tilak, L. F. Eastman, and M. G. Spencer, “Slow transientsobserved in AlGaN HFETs: Effects of SiNx passivation and UVillumination,” IEEE Trans. Electron Devices, vol. 50, no. 4, pp. 886–893,Apr. 2003.
[28] X. Hu, A. Koudymov, G. Simin, J. Yang, M. A. Khan, A. Tarakji,M. S. Shur, and R. Gaska, “Si3N4/AlGaN/GaN metal-insulator-semiconductor heterostructure field-effect transistors,” Appl. Phys. Lett.,vol. 79, no. 17, pp. 2832–2834, Oct. 2001.
[29] S. E. Thompson, G. Sun, Y. S. Choi, and T. Nishida, “Uniaxial-process-induced strained-Si: Extending the CMOS roadmap,” IEEETrans. Electron Devices, vol. 53, no. 5, pp. 1010–1020, May 2006.
[30] P. Makaram, J. Joh, J. A. del Alamo, T. Palacios, and C. V. Thompson,“Evolution of structural defects associated with electrical degradationin AlGaN/GaN high electron mobility transistors,” Appl. Phys. Lett.,vol. 96, no. 23, pp. 233509-1–233509-3, Jun. 2010.
[31] J. A. del Alamo and J. Joh, “GaN HEMT reliability,” Microelectron.Rel., vol. 49, pp. 1200–1206, Jul. 2009.
[32] X. Liu, B. Liu, E. K. F. Low, W. Liu, M. Yang, L. S. Tan, K. L. Teo,and Y. C. Yeo, “Local stress induced by diamond-like carbon liner inAlGaN/GaN metal-oxide-semiconductor high-electron mobility transis-tors and impact on electrical characteristics,” Appl. Phys. Lett., vol. 98,no. 18, pp. 183502-1–183502-3, May 2011.
[33] J. Joh and J. A. del Alamo, “Critical voltage for electrical degradationof GaN high-electron mobility transistors,” IEEE Electron Device Lett.,vol. 29, no. 4, pp. 287–289, Apr. 2008.
[34] U. Chowdhury, J. L. Jimenez, C. Lee, E. Beam, P. Saunier, T. Balistreri,S.-Y. Park, T. Lee, J. Wang, M. J. Kim, J. Joh, and J. A. del Alamo,“TEM observation of crack- and pit-shaped defects in electricallydegraded GaN HEMTs,” IEEE Electron Device Lett., vol. 29, no. 10,pp. 1098–1100, Oct. 2008.
[35] J. Das, H. Oprins, H. Ji, A. Sarua, W. Ruythooren, J. Derluyn,M. Kuball, M. Germain, and G. Borghs, “Improved thermal performanceof AlGaN/GaN HEMTs by an optimized flip-chip design,” IEEE Trans.Electron Devices, vol. 53, no. 11, pp. 2696–2701, Nov. 2006.
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