structural and electrical properties of thin ho2o3 gate dielectrics
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Thin Solid Films 519 (2010) 923–927
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Structural and electrical properties of thin Ho2O3 gate dielectrics
Tung-Ming Pan a,⁎, Wei-Tsung Chang a, Fu-Chien Chiu b
a Department of Electronics Engineering, Chang Gung University, Taoyuan 333, Taiwan, ROCb Department of Electronic Engineering, Ming-Chuan University, Taoyuan 333, Taiwan, ROC
⁎ Corresponding author. Tel.: +886 3 211 8800x3349E-mail address: [email protected] (T.-M. Pan)
0040-6090/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.tsf.2010.09.002
a b s t r a c t
a r t i c l e i n f oArticle history:Received 29 November 2009Received in revised form 1 September 2010Accepted 2 September 2010Available online 7 September 2010
Keywords:Holmium oxideGate dielectricsX-ray diffractionX-ray photoelectron spectroscopySputtering
This paper describes the structural properties and electrical characteristics of thin Ho2O3 gate dielectricsdeposited on silicon substrates by means of reactive sputtering. The structural and morphological features ofthese films after postdeposition annealing were studied by X-ray diffraction, atomic force microscopy, and X-ray photoelectron spectroscopy. It is found that Ho2O3 dielectrics annealed at 700 °C exhibit a thinnercapacitance equivalent thickness and excellent electrical properties, including the interface trap density andthe hysteresis in the capacitance–voltage curves. Under constant current stress, the Weibull slope of thecharge-to-breakdown of the 700 °C-annealed films is about 1.7. These results are attributed to the formationof well-crystallized Ho2O3 structure and the reduction of the interfacial SiO2 layer.
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1. Introduction
The continuous scaling of metal-oxide-semiconductor (MOS)technology is a major driving force in the development of thesemiconductor industry. The performance of integrated circuits can beimproved by reducing the key dimensions in the circuits. The scalingrule of metal-oxide-semiconductor field-effect transistors (MOSFETs)includes the decreasing thickness of the gate oxide (SiO2). However, ahigh tunneling current is produced when the gate oxide thickness issmaller than 2 nm [1]. Since the direct tunneling current is principallydetermined by the SiO2 thickness, it is possible to inhibit the directtunneling current through the SiO2 of the MOSFET using a high-permittivity (k) gate dielectric with a thicker film thickness tomaintain the same capacitance and driving current. Due to therequirements of large capacitance density and low leakage current inadvanced high-k capacitors, various dielectric materials are investi-gated. Among the candidates being considered, HfO2 is a promisinghigh-k dielectric because of its high k-value, wide band gap, and goodprocess compatibility [2]. However, HfO2 is prone to crystallization atrelatively low temperature (~500 °C), whichmay lead to high leakagecurrents through the grain boundaries and low channel mobility dueto fixed charges and/or dipoles at the HfO2/Si interface in theconventional gate-first fabrication process [3,4].
Recently, rare-earth (RE) metal oxides, such as La2O3 [5], Pr2O3 [6],Nd2O3 [7], Gd2O3 [8], Er2O3 [9], and Yb2O3 [10], were extensively
investigated for the applications of optoelectronic, logic, and memorydevices due to those high resistivities, high dielectric constants, andlarge band gaps. Ho2O3 is a typical RE oxide, which is attractive tosubstitute SiO2 in the CMOS devices due to its dielectric constant of13.1, large energy band of 5.3 eV, and high chemical and thermalstability in contact with Si [11–13]. Furthermore, Ho2O3 has thehighest lattice energies among the RE oxides and is easy to becrystallized prior to an annealing process. Paivasaari et al. [13]reported the electrical characteristics and physical properties ofHo2O3 thin film grown onto silicon (100) substrate using atomic layerdeposition. The electrical properties seem to be related to thepostdeposition annealing (PDA) processes [14]. To well clarify theelectrical properties, the structures of bulk and interfacial layer needto be addressed in detail [15]. In this paper, the PDA effects on thephysical and electrical properties of Ho2O3 gate dielectrics depositedon Si (100) substrate were studied. X-ray diffraction (XRD) was usedto identify the growth directions and crystallinity of the Ho2O3 films.Atomic force microscopy (AFM) was used to analyze the surfaceroughness of the dielectric films after annealing at differenttemperatures. X-ray photoelectron spectroscopy (XPS) was used tostudy the chemical structure of the Ho2O3 films. It is found that thetemperature plays an important role in the formation of the silicateand interfacial layer. Accordingly, the PDA effects on the electricalcharacteristics of the Ho2O3 films were realized.
2. Experimental details
The MOS devices were fabricated on 4-in. p-type (100) Si waferswith a resistivity of 5–10 Ωcm. After standard RCA cleaning, the Sisubstrate was dipped in 1% HF solution for 10 s to remove the native
20 30 40 50 60
Ho2O
3 (222)
Ho2O
3 (440)
Ho2O
3 (622)
Ho2O
3 (026)
Ho2O
3 (400)
800oC
700oC
600oC
W/O
Inte
nsi
ty (
arb
.un
its)
2θ (degree)
Fig. 1. XRD patterns of Ho2O3 dielectric films annealed at different temperatures.
924 T.-M. Pan et al. / Thin Solid Films 519 (2010) 923–927
oxide. An 11 nm Ho2O3 thin film was deposited on the Si substratethrough reactive sputtering of a holmium target with a base pressureof about 1.33×10−4 Pa. Then, samples were subjected to rapidthermal annealing (RTA) in an O2 ambient (6.67×104Pa) for 30 s attemperatures ranging from 600 to 800 °C. After the deposition of a4000 Å Al film, the gate areas of MOS devices were defined byphotolithography and wet etching. Finally, the backside contact of Siwafer was cleaned with a buffer HF solution and subsequentlydeposited a 4000 Å Al film by a thermal coater.
The film structure of the Ho2O3 films annealed at varioustemperatures was investigated using a Bruker-AXS D5005 diffrac-tometer with a Cu Kα (λ=1.542 Å) radiation, operated at 40 kV and30 mA. The composition and chemical bonding in each Ho2O3 filmwere analyzed using a Thermo VG ScientificMicrolab 350 systemwitha monochromatic Al Kα (1486.7 eV) source, operated at 300 W. Thechemical shift in the spectra was corrected using the C ls peak fromadventitious carbon at a binding energy of 285 eV. The curve-fitanalyses were performed for O 1s and Si 2p photopeaks usingLorentzian–Gaussian functions in the effort to identify the function-alities associated with each element. The surface morphology androughness of the films were analyzed using an NT-MDT Solver P47(AFM). The AFMwas operated in the tapping mode with a scan rate of
166 164 162 160 1581.2x104
1.8x104
2.4x104
3.0x104
800oC
700oC
600oC
W/O
Inte
nsi
ty (
cou
nts
/s)
Binding energy (eV)
Ho2O
3
536 534 5322.0x104
4.0x104
6.0x104
8.0x104
(b) O 1s
Inte
nsi
ty (
cou
nts
/s)
Binding e
SiO2
(a) Ho 4d5/2
Fig. 2. XPS spectra of (a) Ho 4d, (b) O 1s, and (c) Si 2p
1 Hz for imaging; the scan size for measurement of the roughness was1 μm. The root-mean-square (rms) roughness was measured from theAFM height images. All measurements were carried out in air usingsilicon cantilevers with a resonance frequency of 100–200 kHz. Highfrequency capacitance–voltage (C–V) curves were measured usingHewlett-Packard (HP) 4285A LCR meter. The capacitance equivalentthickness (CET) was extracted in the accumulation region (noquantum mechanical correction was applied). The electrical proper-ties of MOS capacitors were measured using the HP 4156Csemiconductor parameter analyzer.
3. Results and discussion
3.1. Physical properties
To investigate the crystalline structures of the Ho2O3 films afterRTA treatments, XRD measurements were made. The temperature-induced crystalline structure of Ho2O3 was known by the XRDanalyses. For as-deposited films, there were five weak reflection peaksof (222), (400), (440), (026), and (622) found in the XRD patterns, asshown in Fig. 1. A stronger (400) peak was observed for the sampleannealed at 600 °C and 700 °C. The (400) peak is more intensive thanthe other reflection peaks (222), (440), (026), and (622). The filmannealed at 700 °C exhibits a stronger (400) peak than that annealedat 600 °C. However, the (400) peak becomes very small after 800 °Cannealing. It implies that an amorphous silica and Ho-silicate at theHo2O3/Si interface were formed due to oxygen diffusion through theHo2O3 film towards the Si substrate [15].
The structural and compositional changes of the Ho2O3 film afterRTA treatments were examined by XPS. Fig. 2(a) depicts the Ho 4dXPS spectra of the Ho2O3 films annealed at various temperatures. TheHo 4d reference peak of Ho2O3 is located at 161.8 eV [16]. Withreference to the peak position of Ho 4d, the Ho2O3 films annealed atmoderate temperatures (≤700 °C) contain less Si than did at hightemperature (800 °C). In addition, the Ho 4d peak positionwas shiftedabout 1.2 eV for the films after 800 °C annealing. This finding indicatesthe formation of low-k interfacial layer because of the silicon atomsfrom the substrate out diffusing into the Ho2O3/Si interface [15]. Fig. 2(b) depicts the O 1s XPS spectra of the Ho2O3 films before and afterRTA treatments. The appropriate curve-fittings for the peak positionswere also shown in Fig. 2(b). Every fitted peak followed the shape ofthe Lorentzian–Gaussian function. The O 1s peaks at 533.0, 531.2, and
800oC 800oC
700oC 700oC
600oC 600oC
Ho2O
3
530 528 526
W/O W/O
nergy (eV)
Ho silicate
SiO2
106 104 102 100 98 96
4.0x103
8.0x103
1.2x104
Silicate
Si substrate
(c) Si 2p
Inte
nsi
ty (
cou
nts
/s)
Binding energy (eV)
for Ho2O3 films annealed at various temperatures.
Rrms=0.60 nm
(b) 600°C (a) W/O
Rrms=0.75 nm
Rrms=0.86 nm
(c) 700 °C (d) 800°C
Rrms=0.82 nm
10 nm
5 nm
0 nm
10 nm
5 nm
0 nm
15 nm
7.5 nm
0 nm0 nm
7.5 nm
15 nm
Fig. 3. AFM surface image of Ho2O3 films annealed at different temperatures.
925T.-M. Pan et al. / Thin Solid Films 519 (2010) 923–927
529.8 eV correspond to the bonds of Si–O [17], Ho–O–Si, and Ho–O[16], respectively. The peak feature at 531.2 eV (Ho-silicate) is clearlydifferent from those in SiO2 (high-energy feature at 533 eV) andHo2O3 (low-energy feature at 529.8 eV). It implies that the chemicalstate is attributed to the mixture of Ho2O3 and SiO2, i.e., thenonstoichiometric HoSixOy. In this work, the as-deposited film ismainly composed of Ho2O3 and Ho-silicate. The O 1s peak intensitycorresponding to Ho-silicate is approximately constant for the sampleannealed at 600 and 700 °C, however, it decreased dramatically forthat annealed at 800 °C. The O 1s peak intensity corresponding to SiO2
is increased with increasing the anneal temperature. The O 1s peakintensity corresponding to Ho2O3 is also increased with increasing theanneal temperature, but the peak vanishes for the 800 °C-annealedfilms. It implies that most of holmium and oxygen moving from theHo2O3 film may be reacted to form a thick low-k interfacial layerduring high temperature annealing. In Fig. 2(c), the Si 2p XPS spectrafor the Ho2O3 films after RTA treatments were composed of threedifferent component peaks at binding energies of 99.3, 102.4, and103.4 eV, respectively. The Si substrate peak was fixed at 99.3 eV,whereas the peak position of SiO2 was assigned at 103.4 eV [17]. TheSi 2p peak at 102.4 eV can be attributed to Si in Ho-silicate. The Si 2ppeak intensity corresponding to SiO2 is rather constant up to 700 °Cbut suddenly increases at 800 °C, while the O 1s peak intensitycorresponding to silicate is rather constant. This indicates that theSiO2 layer grew monotonically at the film/substrate interface. Incontrast, the Si 2p peak position (corresponding to SiO2) of the filmafter RTA at 800 °C has shifted by about 0.4 eV relative to SiO2 peak.
Moreover, there is a certain increase in the SiO2 peak intensity. Thissuggests that oxygen, which is responsible for the interfacial SiO2
formation, was supplied from the Ho2O3 film itself grown with highoxygen concentration. The reason for the enhanced oxidation of the Sisubstrate during RTA can be understood. This is not surprisingbecause the oxygen diffusion in rare-earth oxides is very high [18].
The AFM surface images of the Ho2O3 sensing films after RTA atvarious temperatures are shown in Fig. 3. The surface roughness of theHo2O3 films clearly increased upon increasing the RTA temperaturebut suddenly decreased at 800 °C. The increase in surface roughnessafter RTA treatment is plausibly due to grain growth and/orcrystallization during annealing. During high temperature annealing,the oxygen atom removed from the film mostly migrated to theinterface and increased the formation of an amorphous silica layer,leading to a smaller surface roughness.
3.2. Electrical characterization
C–Vmeasurements weremade to investigate the RTA temperatureeffects on the Ho2O3 films. Fig. 4 displays the high frequency(100 kHz) C–V curves of the Ho2O3 MOS capacitors annealed atvarious temperatures. The dielectric constant (i.e. k-value) of gatedielectrics was extracted from the C–V data in the accumulationmode[2]. The k-values of the as-deposited film and films annealed at 600,700, and 800 °C are about 6.5±0.6, 8.7±0.7, 10.1±0.8, and 7.7±0.6,respectively. Paivasaari et al. demonstrated that the k-value of Ho2O3
thin film deposited on the native-SiO2/n-Si is 9.9 [13]. Shannon
-4 -3 -2 -1 00.0
0.2
0.4
0.6
0.8
W/O
600 oC
700 oC
800 oC
Gate voltage (V)
Cap
acit
ance
(μF
/cm
2 )
Fig. 4. C–V curves of MOS devices incorporating Ho2O3 dielectrics annealed at varioustemperatures.
-8 -6 -4 -2 010-9
10-7
10-5
10-3
10-1
W/O
600oC
700oC
800oC
Cu
rren
t d
ensi
ty (
A/c
m2 )
Gate voltage (V)
Fig. 6. J–V curves of Ho2O3 gate dielectrics annealed at various temperatures.
926 T.-M. Pan et al. / Thin Solid Films 519 (2010) 923–927
reported that the k-value of Ho2O3 film is 13.1 [19]. The as-depositedsample has a lower k-value, which results in the reduction of thecapacitance at accumulation regime. This result may come from theincompletely crystallized Ho2O3 induced by the large amount ofdefects. Based on the high frequency C–V curves, the negative flatbandvoltage (VFB) results from the positive fixed charges located at theHo2O3/interfacial layer. The origin of the positive charges in the Ho2O3
film may be attributed to the high density of defects induced from theoxygen vacancies and broken bonds [15]. In this work, the Ho2O3 MOScapacitor annealed at 700 °C exhibits the highest capacitance and thelowest VFB shift. It implies that the oxygen-related defects are reducedand the compositional homogeneity of the Ho2O3 dielectric is restoredduring 700 °C annealing. Accordingly, the effective dielectric constantis increased and the positive charge density is reduced. However, theHo2O3 film annealed at 800 °C shows a smaller capacitance and alarger VFB than that annealed at 700 °C. This results from theformation of a thicker interfacial layer at the Ho2O3/Si interface,which is consistent with the analyses in Fig. 2.
Fig. 5 shows the interface state densities and hysteresis voltages ofthe Ho2O3 MOS capacitors after different RTA temperatures. Theinterface state densities (Dit) were calculated by the Hill's method[20]. Experimental results showed that the minimum Dit wasobtained for the films annealed at 700 °C. The minimum Dit is about3×1011 eV−1 cm−2. The Dit value decreases with increasing the RTAtemperature. The decreased Dit may come from the films tended toform the pure Ho2O3 and interfacial layer structure after RTA, which
100
101
102
RTA temperature
Dit
(x10
11 1
/cm
2 -eV
)
W/O 700oC 800oC600oC101
102
103
Δ V
(m
V)
Fig. 5. Interfacial densities of state and hysteresis voltages of Ho2O3 dielectric filmsplotted as a function of the RTA temperature.
causes the reduced Dit. In this work, we investigated the C–Vhysteresis which may relate to the chemical contaminations, thestress induced defects, and/or themobile ions [21]. The C–V hysteresismeasurement was made by sweeping the voltage from +1 to −4 Vthen back again to extract the difference of flatband voltages. Thehysteresis results were mostly related to the defects existed in theHo2O3 and/or at the Ho2O3/SiO2 interface. Experimental resultsshowed that the hysteresis voltage decreases with increasing theRTA temperature, but the hysteresis voltage increases after the 800 °Cannealing. The increased hysteresis voltage may come from thecompensating effect of defects and unsaturated bonds during 800 °Cannealing. In general, the as-deposited dielectric film is deficient in Oatoms. Therefore, the oxygen vacancies exhibit in the oxide film and/or at the oxide/Si interface. After high temperature annealing, theoxygen vacancies are filled out, which reduces the amounts of fixedoxide charges within the film and interface traps at the interface.Nevertheless, the hysteresis voltage of the film annealed at 800 °C ishigher than that annealed at 700 °C, which is attributed to a thickerinterface layer formed at the oxide/Si interface. This may result in theincreasing of defects in the film. Furthermore, the high defect densitywithin the oxide may also lead to the enhancement of C–V hysteresis.
Fig. 6 displays the gate leakage current as a function of gate voltageacross the Ho2O3 dielectric annealed at different temperatures. At VFB-1 V, the leakage current of Ho2O3 dielectric without anneal is higherthan that with RTA treatments. This may come from the damages ofthe composition and structure in Ho2O3 film during the sputtering
-3
-2
-1
0
1
2
10-3 10-2 10-1 100
Area=3.14x10-4
T=300 k J=0.4 mA/cm2J=3.2 mA/cm2
β β =1.7β =1.7
J=1.6 mA/cm2
Qbd (C/cm2)
ln(-
ln(1
-F))
β β =1.2
Fig. 7. Weibull distribution of charge-to-breakdown of Ho2O3 dielectrics annealed at700 °C for various constant current stresses.
-3
-2
-1
0
1
2
10-4 10-3 10-2 10-1 100 101
J=0.4 mA/cm2
Area=3.14x10-4
(b)
T=400 k T=350 k T=300 k
-3
-2
-1
0
1
2
10-7 10-5 10-3 10-1 101
(a)
J=0.4 mA/cm2
T=300 k
β=1.7
β=1.7
β=1.4
β=1.4
β=1.3
β=1.5
Area=1.26x10-3
Area=5.02x10-3 Area=3.14x10-4
Qbd (C/cm2)
Qbd (C/cm2)
ln(-
ln(1
-F))
ln(-
ln(1
-F))
Fig. 8. Weibull distribution of charge-to-breakdown for Ho2O3 dielectrics annealed at700 °C for (a) various capacitor areas and (b) various stressing temperatures.
927T.-M. Pan et al. / Thin Solid Films 519 (2010) 923–927
process. For Ho2O3 film annealed at 700 °C, the leakage currentdensity is around 83 nA/cm2. The optimal annealing at 700 °C causesthe reduced crystal defects in Ho2O3 and the formation of aninterfacial layer at the Ho2O3/Si interface and the improved electricalcharacteristics.
To study the breakdown mechanism of the Ho2O3 dielectric after700 °C annealing, constant current stress (CCS) was applied toevaluate the charge-to-breakdown (QBD). Fig. 7 shows the QBD
Weibull distribution of Ho2O3 thin film under different currentdensities. The Weibull slope is about 1.7 for both 0.4 and 1.6 mA/cm2. It indicates that the trap generation during stresses isindependent of the current density ranging from 0.4 to 1.6 mA/cm2.Nevertheless, the Weibull slope becomes smaller under CCS of3.2 mA/cm2. It implies that the critical trap densitywas soon producedduring the high current stress, which leads to the dielectricbreakdown of thin Ho2O3 films. Namely, the breakdown process canbe accelerated by the high current stress. Fig. 8(a) depicts the QBD
Weibull plot of Ho2O3 films annealed at 700 °C for different capacitorareas. The Weibull slope of QBD distribution is related to the capacitorarea [22]. The Weibull slope decreased with increasing the capacitorarea, suggesting the high number of extrinsic defects in the large area.Fig. 8(b) displays the QBD Weibull plot of Ho2O3 dielectric filmannealed at 700 °C under different temperature stresses. The Weibullslope of QBD distribution decreased with increasing the stressedtemperature. The smallerWeibull slope at elevated temperature couldbe due to temperature insensitive defects and possible redistributionin Ho2O3 as well as the increase in the number of generated traps.Eventually, the dielectric breakdown is triggered by the formation oftraps in a conductive path between the anode and cathode.
4. Conclusion
In this paper, we reported a high-k Ho2O3 gate dielectric grown(100) Si substrate through reactive sputtering. The optimal annealingtemperature was 700 °C. The film structure and interface propertieswere identified by XRD and XPS, respectively. In addition, the effectsof stressed current density, MOS capacitor area, and temperature onthe charge-to-breakdown characteristics for the 700 °C-annealedHo2O3 film were studied.
Acknowledgment
This work was supported by the National Science Council, Taiwan,Republic of China, under contract no. NSC-97-2221-E-182-050-MY3.
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