DTIC ADA536704: Fermi Level Unpinning of GaSb (100) using Plasma Enhanced Atomic Layer Deposition of Al2O3

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APPLIEDPHYSICSLETTERS97,143502(cid:2)2010(cid:3)
Fermi level unpinning of GaSb „100… using plasma enhanced atomic layer
deposition of Al O
2 3
A. Ali,1,a(cid:2) H. S. Madan,1 A. P. Kirk,2 D. A. Zhao,1 D. A. Mourey,1 M. K. Hudait,3
R. M. Wallace,2 T. N. Jackson,1 B. R. Bennett,4 J. B. Boos,4 and S. Datta1
1PennStateUniversity,UniversityPark,Pennsylvania16802,USA
2TheUniversityofTexasatDallas,Richardson,Texas75080,USA
3VirginiaTech,Blacksburg,Virginia24061,USA
4NavalResearchLaboratory,Washington,DC20375,USA
(cid:2)Received 14 May 2010; accepted 2 September 2010; published online 4 October 2010(cid:3)
N-type and p-type GaSb metal-oxide-semiconductor capacitors (cid:2)MOSCAPs(cid:3) with
atomic-layer-deposited (cid:2)ALD(cid:3) and plasma-enhanced-ALD (cid:2)PEALD(cid:3) Al O dielectrics are studied
2 3
to identify the optimum surface preparation and oxide deposition conditions for a high quality
oxide-semiconductor interface. The ALD Al O /GaSb MOSCAPs exhibit strongly pinned C-V
2 3
characteristics with high interface state density (cid:2)D (cid:3) whereas the PEALD Al O /GaSb MOSCAPs
it 2 3
show unpinned C-V characteristics (cid:2)low D (cid:3). The reduction in Sb O to metallic Sb is suppressed
it 2 3
for the PEALD samples due to lower process temperature, identified by x-ray photoelectron
spectroscopy analysis. Absence of elemental Sb is attributed to unpinning of Fermi level at the
PEALD Al O /GaSb interface. © 2010 American Institute of Physics. (cid:4)doi:10.1063/1.3492847(cid:5)
2 3
Antimonide based compound semiconductors have Both n-type and p-type GaSb (cid:2)100(cid:3) MOS capacitors
gained considerable interest in recent years due to their were fabricated with high-(cid:2) Al O gate dielectric. The
2 3
superior electron and hole transport properties.1,2 Mixed samples were degreased in acetone and ethanol for 5 min
anion InAs Sb quantum-wells (cid:2)QWs(cid:3) with high electron eachandrinsedinisopropylalcohol(cid:2)IPA(cid:3).Thedegreasestep
x 1−x
mobility are candidates for integration with high hole was followed by a dip in concentrated HCl (cid:2)HCl:H O
2
mobility In Ga Sb QW for low power complementary =1:2(cid:3) for 5 min to remove surface oxides followed by an
x 1−x
logic applications.3 Recently, InAs/AlyGa1−ySb heterojunc- IPArinse.GaSbMOScapacitorswerefabricatedwithAl2O3
tion tunnel field effect transistors (cid:2)H-TFETs(cid:3) have been pro- deposited by ALD at 300 °C from trimethylaluminum
posed for low-power high-performance applications.4 Inte- (cid:2)TMA(cid:3) and water or by PEALD at 200 °C from TMA and
grating a high quality dielectric is key to demonstrating a CO .8 Pt/Au gate metallization was done using electron
2
scalable metal-oxide-semiconductor QWFET (cid:2)MOS- beam evaporation after defining gate patterns using optical
QWFET(cid:3)orH-TFETarchitectureforlow-powerlogicappli- lithography. Pd/Au backside Ohmic contacts were deposited
cations.We hypothesize that an ultrathin GaSb surface layer using electron beam evaporation. No post deposition anneal
is more favorable toward high-(cid:2)dielectric integration than was done for any of the samples.
AlyIn1−ySb barrier layer for the QWFET or AlyGa1−ySb Capacitance voltage (cid:2)C-V(cid:3) and conductance voltage
channellayerfortheH-TFET,asitavoidsAlattheinterface (cid:2)G-V(cid:3) measurements were obtained with HP 4285A preci-
and the associated surface oxidation. Hence, it is imperative sion LCR meter. Figure 1(cid:2)a(cid:3) shows the C-V measurements
to carefully examine the electrical characteristics of on ALD Al O /n-GaSb and PEALD Al O /n-GaSb MOS
high-(cid:2)/GaSb semiconductor interface. capacitors.T2he3ALDsampleshowsweakly2p3innedC-Vchar-
GaSb has a highly reactive surface and on exposure to acteristicswithveryfastinterfacetrapdensity(cid:2)D (cid:3) response
it
air it will form a native oxide layer composed of Ga O and
2 3 whereas PEALD sample demonstrates good Fermi level
Sb O (cid:2)2GaSb+3O →Ga O +Sb O (cid:3). The Sb O can fur-
2 3 2 2 3 2 3 2 3 modulation.TheaccumulationsideofC-Vcharacteristicsfor
ther react with the GaSb surface forming elemental Sb and
the PEALD sample shows the effect of high D near the
Ga2O3 (cid:2)Sb2O3+2GaSb→Ga2O3+4Sb(cid:3).5,6 Chemical treat- conduction band (cid:2)CB(cid:3) and the negative bias regiimt e shows
ments based on HCl are effective in removing native oxides the effect of inversion response along with a low D re-
on GaSb.6,7 Here, we study the effects of HCl treatment on sponse.Theaccumulationsideoftheadmittancedataisitana-
thecapacitance-voltagecharacteristics(cid:2)C-V(cid:3)andthesurface lyzed using the standard depletion/accumulation model9 and
chemistry of n-type and p-type GaSb(cid:2)100(cid:3) MOS capacitors theinversionsideusinganalternative.10Thecircuitmodelin
fabricated with both atomic-layer-deposited (cid:2)ALD(cid:3) and
inversion accounts for the supply of minority carriers
plasma enhancedALD (cid:2)PEALD(cid:3) Al O dielectrics. PEALD
2 3 through bulk thermal generation and diffusion across the
was employed to reduce the thermal budget of dielectric
space charge layer along with interface state contribution,
deposition,particularlyimportantforantimonidebasedsemi-
which enables accurate modeling of the admittance data in
conductors. In this Letter, we demonstrate an unpinned inversion.Figure1(cid:2)b(cid:3)showsC-VcharacteristicsoftheALD
Fermi level in GaSb MOS system with a high-(cid:2)PEALD Al O /p-GaSb and PEALD Al O /p-GaSb samples with
2 3 2 3
Al O dielectric, using temperature resolved admittance
2 3 HCl treatment for different temperatures. The ALD sample
spectroscopy and monochromatic x-ray photoelectron spec-
shows strongly pinned C-V characteristics with both accu-
troscopy (cid:2)XPS(cid:3) analysis.
mulationandinversionregimescompletelydominatedbyin-
terface states. For the PEALD sample, there is minimal dis-
a(cid:3)Electronicmail:[email protected]. persion of capacitance in accumulation due to less Dit near
0003-6951/2010/97(cid:3)14(cid:2)/143502/3/$30.00 97,143502-1 ©2010AmericanInstituteofPhysics
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4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER
Fermi level unpinning of GaSb (100) using plasma enhanced atomic layer
5b. GRANT NUMBER
deposition of Al2O3
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143502-2 Alietal. Appl.Phys.Lett.97,143502(cid:3)2010(cid:2)
FIG. 2. (cid:2)Color online(cid:3) Conductance (cid:2)G/(cid:3)(cid:3) contours of n-type and p-type
GaSbMOSCAPswithPEALDAlO showinggoodFermilevelmodulation
2 3
towardvalenceandconductionbandsides.
holes whereas it forms a Schottky barrier for electrons
(cid:2)(cid:6)0.6 eV Schottky barrier height for electrons due to the
Fermi stabilization energy in GaSb being (cid:6)0.1 eV from the
VB.11 This Schottky barrier gives rise to higher contact re-
sistance for electrons at lower temperatures, which causes
frequency dispersion in the accumulation capacitance of the
n-type metal-oxide-semiconductor capacitors (cid:2)MOSCAPs(cid:3).
For the p-type devices this is not a problem as the contacts
are Ohmic for holes, and the measurement can be done at
very low temperature. Hence, the above measurement tem-
peratures were employed.
Figure 2 shows the conductance contour map (cid:2)G/(cid:3)(cid:3) of
n-typeandp-typePEALDsamplesasafunctionofgatevolt-
age and small-signal frequency. The V-shaped trajectory of
the peak value of conductance, (cid:2)G/(cid:3)(cid:3) , along the fre-
peak
quency axis shows that the Fermi level moves freely on ei-
ther side of the midgap of GaSb. PEALD Al O /GaSb
2 3
samples demonstrate an unpinned Fermi level at the inter-
face, even though the Fermi stabilization energy for GaSb is
0.1 eV from the VB.11 The extracted capture cross section
FIG.1. (cid:2)Coloronline(cid:3)C-Vcharacteristicsasafunctionoffrequencyof(cid:2)a(cid:3) values are 9(cid:4)10−15 cm2 for traps near the VB and 8
n-typeALDandPEALDsampleswithHCltreatmentand(cid:2)b(cid:3)p-typeALD (cid:4)10−19 cm2 for traps near the CB, indicating that the traps
and PEALD samples with HCl treatment. Measurement temperature is in-
near the VB are faster than those near the CB. Hence, the
dicatedinthefigure.
conductance plots were done at lower temperature for the
p-type samples where the conductance peaks were visible.
thevalenceband(cid:2)VB(cid:3)whereastheinversionresponseshows The capture cross section values are consistent with Ref. 12,
contributions from minority carriers as well as high D near where irradiated p-type GaSb showed acceptor type traps
it
theCB.The200KC-VshowsaclearFermilevelmovement near the VB with higher capture cross-section.
from accumulation to depletion for the PEALD sample Figure 3 shows the monochromatic XPS analysis of the
whereas ALD sample exhibits pinned C-V characteristics. Sb 4d region for the ALD and PEALD samples with HCl
C-V measurements for the n-type (cid:2)p-type(cid:3) MOS capacitors treatment.13AllPEALDandALDsamplesshowpresenceof
were done at 300 and 250 K (cid:2)300 and 200 K(cid:3).The reason is Ga-oxides. TheALD samples show no detectable Sb-oxides
that the backside contact (cid:2)Pd/Au(cid:3) on GaSb is Ohmic for whereasthePEALDsampleshavesignificantSb-oxides.As
FIG.3. (cid:2)Coloronline(cid:3)XPSdatacomparingtheconcen-
trationofSbO inALDandPEALDsamples.Reduc-
2 3
tioninSbO inthe(cid:2)highertemperature(cid:3)ALDsamples
2 3
isevident.
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143502-3 Alietal. Appl.Phys.Lett.97,143502(cid:3)2010(cid:2)
GaSb,andisinferiortothe200 °CPEALDsample.Wealso
speculate that the oxide anions in the plasma during the
PEALD process may more efficiently oxidize the elemental
Sb back to Sb O .
2 3
Figure 4(cid:2)a(cid:3) shows the D extracted from the n-type and
it
p-type samples.The D extracted from the n-type MOSCAP
it
data using the inversion mode model (cid:2)Ref. 9(cid:3) is consistent
withtheanalysisonthep-typesample.Figure4(cid:2)b(cid:3)showsthe
Fermi level movement efficiency of the ALD and PEALD
samples. Fermi level movement efficiency is defined as the
ratio of the change in the Fermi level at the oxide-
semiconductor interface in the presence of D to the change
it
in Fermi level without D , for a unit applied gate voltage.
it
Mathematically, it can be represented as follows:
1+C /C
E movement efficiency= dep ox %. (cid:2)1(cid:3)
F 1+(cid:2)C +C (cid:3)/C
dep it ox
ThePEALDsamplewithHClpretreatmentshowsa90%E
F
movement efficiency near the VB making it suitable for
MOS-QWFET or H-TFET device operations, where the
Fermi level sweeps near the VB of GaSb.
In summary, we have demonstrated GaSb MOS capaci-
tors (cid:2)p-type and n-type(cid:3) with unpinned Fermi level using
PEALD Al O by minimizing elemental Sb at the
2 3
GaSb/Al O interface. The reduction of Sb O to metallic
2 3 2 3
Sb is suppressed with PEALD due to lower deposition tem-
perature,whichisconfirmedbyXPSanalysis.TheD islow
it
near the valence band which makes GaSb-PEALD Al O a
2 3
good interface for composite barrier design with Al In Sb
y 1−y
and Al Ga Sb barrier layers in mixed-anion MOS-
y 1−y
QWFETs or H-TFETs for ultralow power logic applications.
FIG. 4. (cid:2)Color online(cid:3) (cid:2)a(cid:3) Extracted D from n-type and p-type samples
it
showinglowD nearVBforPEALDMOSCAPs.Theinversionsideforthe Someoftheauthorsgratefullyacknowledgethefinancial
it
n-typeMOSCAPswasmodeledtoevaluateDit(cid:2)Ref.9(cid:3)whichisconsistent support from the FCRP for Materials, Structures, and De-
with the data obtained from p-type samples. (cid:2)b(cid:3) Fermi level movement vices (cid:2)MSD(cid:3) sponsored by the SRC and DARPA.
efficiencyofGaSbMOSCAPswithALDandPEALDAlO .
2 3
1S.Datta,T.Ashley,J.Brask,L.Buckle,M.Doczy,M.Emeny,D.Hayes,
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T.Emeny,M.Fearn,D.G.Hayes,K.P.Hilton,M.K.Hudait,R.Jefferies,
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temperatures.15TheALD, which was done at 300 °C, could
Uren,D.J.Wallis,P.J.Wilding,andR.Chau,Tech.Dig.-Int.Electron
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2 3
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