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Improved electrical and interfacial properties of RF-sputtered HfAlOx on n-GaAs with effective Si passivation
Applied Surface Science 256 (2010) 6618–6625
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Improved electrical and interfacial properties of RF-sputtered HfAlOx on n-GaAs with effective Si passivation P.S. Das a,b,∗ , A. Biswas b a b
Dept. of Electronics and ECE, Indian Institute of Technology, Kharagpur 721302, India Institute of Radio Physics and Electronics, University of Calcutta, 92 A P C Road, Kolkata 700009, India
a r t i c l e
i n f o
Article history: Received 6 December 2009 Received in revised form 18 April 2010 Accepted 18 April 2010 Available online 24 April 2010 Keywords: GaAs High-k XPS Interfacial characteristic Current conduction mechanism
a b s t r a c t In this paper, we present the effects of ultrathin Si interfacial layer on the physical and electrical properties of GaAs MOS capacitors fabricated using RF-sputtered HfAlOx gate dielectric. It is found that HfAlOx /Si/nGaAs stack exhibits excellent electrical properties with low frequency dispersion (∼4.8%), hysteresis voltage (0.27 V) and interface trap density (1.3 × 1012 eV−1 cm−2 ). The current density of 3.7 × 10−5 A/cm2 is achieved with an equivalent-oxide-thickness of 1.8 nm at VFB + 1 V for Si-passivated HfAlOx films on n-GaAs. X-ray photoelectron spectroscopy (XPS) analysis shows that the suppression of low-k interfacial layer formation is accomplished with the introduction of ultrathin Si interface control layer (ICL). Thus the introduction of thin layer of Si between HfAlOx dielectrics and GaAs substrate is an effective way to improve the interface quality such as low frequency dispersion, hysteresis voltage and leakage current. Additionally, current conduction mechanism has been studied and the dominant conduction mechanisms are found to be Schottky emission at low to medium electric fields and Poole–Frenkel at high fields and high temperatures under substrate injection. In case of gate injection, the main current conduction at low field is found to be the Schottky emission at high temperatures. © 2010 Elsevier B.V. All rights reserved.
1. Introduction During the last decade, substrate engineering, e.g. pseudomorphically strained-SiGe channels grown on Si substrates for p-MOSFETs and strained-Si channels on relaxed SiGe buffer layers with the graded Ge for n-MOSFETs, has been pursued to improve the carrier mobility in the channel [1]. As the complementary metal-oxide-semiconductor (CMOS) technology develops beyond 22 nm nodes, high priority has been assigned to III–V semiconductors as channel materials motivated by the need to increase the channel mobility as well as to reduce power consumption. III–V compound semiconductors offer the advantages of high electron mobilities, rich band gap engineering, and high breakdown fields and thus are likely to outperform silicon in certain metal-oxidesemiconductor (MOS) applications such as high-speed and high power devices [2,3]. However, surface oxidation occurring on the surface of III–V semiconductors can introduce a high defect density into the devices, which detrimentally affects the electrical characteristics of devices causing the Fermi level pinning [4,5]. One key challenge in the III–V technology is to identify thermodynamically stable insulators on the III–V’s that give a low interfacial density
of states (Dit ) and a low leakage current. Among the III–V compound semiconductors, current ly, GaAs is being widely studied as a potential channel material replacement of Si. During the last decade, high-k dielectric films such as HfO2 , Al2 O3 , ZrO2 and TiO2 are extensively researched for their potential as substitution for SiO2 . Recently, Hf-based dielectrics have been considered as very promising high permittivity material for future gate dielectric applications. But, direct deposition of HfO2 on GaAs exhibited anomalous characteristics with large frequency dispersion, hysteresis, low effective mobility and also high leakage current [6,7]. The interfacial layer growth may be overcome by introducing interface control layers (ICLs) such as Si, Ge, and Gd2 O3 [9–11]. In addition, there have been some attempts to adopt composite films of Al2 O3 and HfO2 on GaAs substrates [8,12]. In this paper, we studied the electrical and structural properties of RF-sputter deposited HfAlOx on n-GaAs substrates in the presence of the ultrathin silicon interface control layer. X-ray photoelectron spectroscopy (XPS) analysis was performed to investigate the interfacial properties of GaAs MOS capacitors.
2. Experiments ∗ Corresponding author at: Dept. of Electronics and ECE, Indian Institute of Technology, Kharagpur 721302, India. Tel.: +91 3222 281475; fax: +91 3222 255303. E-mail address: [email protected] (P.S. Das). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.04.058
MOS capacitors were fabricated on Te-doped n-GaAs (1 0 0) wafers with a carrier density ∼1 × 1016 cm−3 . The samples were degreased in methanol, acetone and 2-propanol successively for
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Fig. 1. High frequency C–V characteristics of MOS capacitors on n-GaAs with Sand Si-passivation for voltage sweep from inversion to accumulation and back to inversion at a ramp rate of 0.1 V/s.
Fig. 3. (a) Current density versus applied bias plot (J–V characteristics) of HfAlOx thin film on n-GaAs with S- and Si-passivation and (b) gate leakage current density at VFB ± 1 V versus EOT. For comparison, reported data of high-k on Si, SiO2 on Si, HfAlOx thin films on GaAs are also plotted together.
Fig. 2. (a) and (b) Frequency dispersion characteristics of GaAs MOS capacitors with S- and Si-passivation.
2 min each. Then the substrates were treated with 50% HCl for 5 min and heated in NH4 OH at 60 ◦ C for 10 min to remove native oxide. After wet chemical cleaning, the samples were heated into 40% ammonium sulfide solution for 10 min for S-passivation of GaAs surface. The HfAlOx films were deposited at 50 ◦ C by RF co-sputtering of Al2 O3 and HfO2 targets in 100 W powers in Ar atmosphere at 10−3 mbar working pressure. Prior to the dielectric deposition, ultrathin silicon layer of 1.5 nm was deposited as interface control layer (ICL) on some of the samples by RF sputtering in the same ambient. Thickness of the dielectric layer was measured to be ∼7 nm using an ellipsometer. The post deposition annealing (PDA) was carried out in an N2 ambient at 600 ◦ C for 1 min by rapid thermal annealing. Finally, Al electrode was formed by thermal evaporation with the gate area of 1.96 × 10−3 cm2 at 10−6 mbar using the shadow masking technique. In order to make the backside ohmic contact Au/Ge/Au was deposited at a pressure of 10−6 mbar. For chemical analysis, high-resolution XPS was performed using VG ESCALAB 220i-XL system. Capacitance–voltage (C–V) measurements were made using Agilent E4980A LCR meter and current–voltage (I–V) measurements were performed with HP4156C semiconductor parameter analyzer, respectively.
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Fig. 4. (a) High-resolution XPS of Ga 3d spectra of thin HfAlOx overlayer on S-passivated n-GaAs, (b) high-resolution XPS of Ga 3d spectra of thin HfAlOx overlayer on Si-passivated n-GaAs, (c) high-resolution XPS of As 3d spectra of thin HfAlOx overlayer on S-passivated n-GaAs and (d) high-resolution XPS of As 3d spectra of thin HfAlOx overlayer on Si-passivated n-GaAs and the inset shows the Si 2p spectrum for HfAlOx /Si/n-GaAs.
3. Results and discussions 3.1. Electrical characterization Fig. 1 shows high frequency (HF) C–V characteristics of GaAs MOS capacitors of 7 nm sputtered HfAlOx dielectrics. One can easily observe that the accumulation capacitance (Cacc ) increases by 24% with the introduction of Si interface control layer for the same thickness of HfAlOx . The average value of dielectric constant (ε = Cacc d/ε0 A) calculated from accumulation capacitance are 16.1 and 13.1 for Si- and S-passivated samples, respectively. The quantities ε0 , Cacc /A and d denote, respectively, the permittivity in vacuum, the accumulation capacitance per unit area and the thickness of the HfAlOx film. The equivalent oxide thickness (EOT) is found to be 1.8 nm and 2.1 nm for MOS capacitors with Siand S-passivation, respectively. The C–V characteristics are directly related to the charged defects in the insulator and at the insulator/semiconductor interface. In fact, the unbalanced charges arising from defects will cause a C–V flat-band voltage VFB to shift from the ideal condition. The shift value (Vf ) can be used to quantify the number of fixed charges [Nf (cm−2 )] in the dielectric films using the equation: Nf =
Cox (MS − VFB ) qA
ϕMS being the metal-semiconductor work function difference. The flat-band voltages (VFB ) for HfAlOx and HfAlOx /Si are 0.6 and 0.2 V, respectively. The positive VFB for both the cases indicates a neg-
ative oxide charge trapping in the films. The fixed oxide charge density (Nf ) calculated from flat-band voltages are 6.5 × 1011 cm−2 and 3.5 × 1012 cm−2 for samples with and without Si ICL, respectively. Moreover, the number of trapped (negative) charges [Not (cm−2 )] has been evaluated from the hysteresis observed in the C–V curve. Dalapati et al. [8] have reported a hysteresis voltage (VFB ) of 0.9 V for directly deposited HfAlOx on n-GaAs substrate whereas VFB is 0.55 V in our case. By using the HfAlOx /Si/n-GaAs stack the hysteresis voltage can further be improved to 0.11 V. From the hysteresis characteristics, the calculated trapped charge density (Nbt = Cox VFB /Aq) were found to be 2.3 × 1012 cm−2 and 4.1 × 1012 cm−2 with and without Si ICL, respectively. From Fig. 1, it is also observed that C–V curves stretch out along the voltage axis, which indicates high density of interface states for directly deposited HfAlOx on nGaAs. However, the stretch out effect decreases in presence of Si ICL. Evaluation of the fast interface state density (Dit ) by Terman [13] method has also shown the improved interface characteristics for Si ICL samples. The minimum value of Dit for HfAlOx /n-GaAs is 6.2 × 1012 eV−1 cm−2 , which is nearly the same as reported by Dalapati et al. [8]. With the incorporation of a Si ICL in between HfAlOx and n-GaAs, the value of Dit as low as 1.3 × 1012 eV−1 cm−2 is obtained and thus a significant improvement is achieved. Fig. 2 shows typical C–V curves of MOS capacitors at different frequencies ranging from 1 kHz to 1 MHz for frequency dispersion analyses. In case of a HfAlOx /n-GaAs stack, the percentage frequency dispersion (C) and flat-band voltage change (V) are found to be 13.1% and 250 mV. Suri et al. [12] reported improved frequency dispersion characteristics with V and C decreasing to 40 mV and 6%
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Fig. 6. Current density versus applied voltage (J–V) plot for GaAs MOS capacitor at various temperatures for samples with and without Si ICL. Fig. 5. (a) O 1s spectra of bulk HfAlOx overlayer on S-passivated n-GaAs and (b) O 1s spectra of bulk HfAlOx overlayer on Si-passivated n-GaAs.
with HfAlOx gate dielectrics on p-GaAs. But, it is also learnt form the literature that for any high-k/n-GaAs gate stack, the frequency dispersion is higher compared with high-k/p-GaAs gate stack, which is due to the intrinsic surface nature of n-GaAs [8]. Dalapati et al. [8] reported 10% dispersion in accumulation capacitance for HfAlOx /nGaAs stack. However, with the incorporation of the Si ICL, V and C decrease to 4.8% and 60 mV in our device. Thus, it is obvious that the Si ICL reduces Fermi level pinning in GaAs MOS capacitors and improves the overall dielectric properties of the film efficiently which may be due to the decrease in the formation of the low-k interfacial layer re-growth. Fig. 3(a) depicts the leakage current density characteristics of HfAlOx /n-GaAs and HfAlOx /Si/n-GaAs stacks. The leakage current density for HfAlOx film measured at VFB + 1 V is 1.95 × 10−4 A/cm2 . This rather leaky characteristic is attributed to the relatively high density of interface states and/or interfacial oxides induced lowering of conduction band offset between HfAlOx and GaAs. In the presence of a Si ICL, the current density reduces to 3.7 × 10−5 A/cm2 at VFB + 1 V. It is worth to point out that the Si ICL lowers the leakage current density by nearly one order of magnitude even for a lower value of EOT [Fig. 3(b)]. Fig. 3(b) shows the gate leakage current density at VFB ± 1 V versus EOT and compared with some
recently published results. The improvement of leakage current for Si-passivated sample is supposed to be due to reduction of GaAs chemical states and thus, hindering the conduction of leakage currents. The result shows that leakage currents for HfAlOx /n-GaAs and HfAlOx /Si/n-GaAs stack are lower as compared with that in SiO2 on Si, but larger than that in a high-k/Si structure. Interestingly, for the corresponding value of EOTs, the ratio of current densities for HfAlOx /Si/n-GaAs and high-k/Si in our case is around 7.6 ± 0.5, which is lower compared to the result of Suri et al. [12] for a HfAlOx /p-GaAs stack (20 ± 0.05). 3.2. X-ray photoelectron spectroscopy analysis In order to investigate the interface quality, another set of 4 nm thick samples of thin films of HfAlOx on silicon and sulfur passivated n-GaAs were prepared using same process steps and analyzed by high-resolution X-ray photoelectron spectroscopy using the VG ESCALAB 220i-XL XPS system. The Ga 3d XPS spectrum can be fitted into two peaks at 19.2 and 20.4 eV, which correspond to Ga–As and Ga–O from Ga2 O3 [5], as shown in Figs. 4(a) and (b) for S and Si-passivated samples, respectively. For S-passivated sample, the percentage of Ga–O peak area compared to total Ga 3d area is about 30.5% and reduces to 9.5% for Si-passivated n-GaAs. Fig. 4(c) and (d) depicts the As 3d spectra of thin HfAlOx layer on sulfur and silicon passivated n-GaAs substrate. The As 3d spectra show As 3d5/2 and
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Fig. 8. Arrhenius plots for Schottky emission at various electric fields under substrate injection with and without Si-passivation. Fig. 7. Characteristics of Schottky emission (a) GaAs/HfAlOx and (b) GaAs/Si/HfAlOx stacks under substrate injection, respectively.
As 3d3/2 doublets for GaAs substrate at 41.2 and 41.8 eV, respectively [14]. Yet another peak is observed at 44.3 eV, which is due to As–O from As2 O3 [14]. However, the peak intensity is quite low for Si-passivated samples. In addition the percentage of the As–O area compared to the total area of As 3d are 25.4% and 11.3% for S and Si-passivated GaAs, respectively. The As–O and Ga–O bonds were reported to act as defect states in the band gap and hence play a critical role in degrading device electrical property [15]. This is why the formation of As–O and Ga–O bonds was effectively suppressed using Si interface control layer as also predicted from frequency dispersion analyses. The Si 2p spectrum for HfAlOx /Si/nGaAs is shown in the inset of Fig. 4(d). The peak at 103.2 eV is due to the Si–O bonds from SiO2 . Further no peak is found at 99.9 eV for the Si–Si bonds, which indicates that the Si ICL was fully oxidized. Fig. 5(a) and (b) shows the O 1s spectra for bulk HfAlOx on S and Sipassivated n-GaAs, respectively. The spectra can be deconvoluted into two Gaussian peaks at 530 eV and 532.5 eV for HfAlOx /n-GaAs. An energy shift of 0.2 eV for silicon passivated samples is noticed, which may arise due to different interface dipoles between high-k and GaAs. The atomic percentage of Hf:Al was estimated to be 21:79 which is equivalent to the mole fraction of (HfO2 )0.21 (Al2 O3 )0.79 .
3.3. Conduction mechanism analysis The temperature dependency of the gate leakage current of 7 nm HfAlOx thin films on n-GaAs substrates with and without silicon interface control layer (ICL) was studied to identify the electrical conduction mechanism under substrate and gate injections. Fig. 6 shows the current density versus applied voltage (J–V) plot in the temperature range 25–150 ◦ C with and without Si ICL, respectively. In-depth analysis of Fig. 6 reveals that dominant conduction mechanism at low and medium electric fields is Schottky emission whereas Poole–Frenkel conduction takes over at higher field under substrate injection and in case of gate injection, the Schottky emission dominates the current conduction at low field for both the samples. The current density due to standard Schottky emission [16] can be expressed as
∗ 2
JSE = A T
exp
−q(˚B −
qE/4ε0 εr )
kT
(1)
where A∗ = (4qm∗hk k2 /h3 ) = 120(m∗hk /m0 ) (A/cm2 K2 ), JSE is the current density, A* is the effective Richardson constant, T is the absolute temperature, q is the electronic charge, q˚B is the Schottky barrier height, E is the applied electric field, k is the Boltzmann’s constant, h is the Planck’s constant, ε0 is the free space permittivity, εr is the dynamic dielectric constant of the gate dielectric, m0
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Fig. 9. Characteristics of Poole–Frenkel conduction (a) GaAs/HfAlOx and (b) GaAs/Si/HfAlOx stacks under substrate injection, respectively. Fig. 10. Arrhenius plots for Poole–Frenkel conduction at various electric fields under substrate injection with and without Si ICL.
is the free electron mass and m* is the effective mass electrons in HfAlOx . The dynamic dielectric constant (εr ) should be nearly equal to square of the measured the refractive index (εr ∼ n2 ). For standard Schottky emission, a plot of ln(J/T2 ) versus E1/2 should be a straight line. The experimental data in the region of high temperatures (75–150 ◦ C) together with low to medium electric fields (0.50–2.86 MV/cm) fit the Schottky emission theory very well under the substrate injection for GaAs/HfAlOx stack as shown in Fig. 7(a). For GaAs/Si/HfAlOx stack, Schottky emission occurs within the electric field of 0.47–1.17 MV/cm and in the temperature ranging between 25 ◦ C and 150 ◦ C as demonstrated in Fig. 7(b). The average values of the fitted dynamic dielectric constant εr are 3.25 and 3.3 for samples with and without Si ICL, respectively. Fig. 8 shows the Arrhenius plot where the ratio of gate leakage current density to square of the temperature is plotted on a semi-log plot as a function of inverse of temperature for different applied fields. It is observed that data approximately follow a straight line, indicating that leakage current varies exponentially with 1/T. Moreover, barrier height (˚B ) at HfAlOx and n-GaAs interface are about 1.45 ± 0.04 eV and 1.41 ± 0.05 eV for samples with and without Si ICL, respectively, extracted from the slopes of Fig. 8. These values take into account the interfacial effects. However, in the high temperature range 75–150 ◦ C and high electric field, Poole–Frenkel conduction takes over. The corre-
sponding leakage current density in dielectrics can be expressed by employing the following equation [16]:
JP–F = qNC E exp
−q(˚t − q
qE/4ε0 εr )
kT
,
(2)
where is the mobility of electrons, NC = 2(2m*kT/h2 )3/2 is the effective density of states function in the conduction band, ˚t is the trap energy level and other quantities are the same as defined earlier. Additionally, the plot of ln(J/E) versus E1/2 should be a straight line with a slope εr . The experimental data in the domains of high temperatures (75–150 ◦ C) and high electric fields (3.57–6.28 MV/cm) are best described by P–F conduction as shown in Fig. 9(a) for samples without Si-passivation. Fig. 9(b) shows the P–F plot for samples with Si-passivation with high electric fields (3.64–5.88 MV/cm) and high temperatures (75–150 ◦ C). The average value of fitted dynamic dielectric constants εr are 3.26 and 3.27 for samples with and without Si ICL, respectively. From the Arrhenius plot as shown in Fig. 10 trap barrier heights (˚t ) are extracted to be about 0.7 ± 0.03 and 0.95 ± 0.04 eV for samples with and without Si ICl, respectively. Moreover, the intercept of the P–F plot is a function of the mobility of electrons, and density of states
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Fig. 11. Characteristics of Schottky emission under gate injection for samples with and without Si ICL.
Fig. 12. Arrhenius plots for Schottky emission at different electric fields under gate injection with and without Si-passivation.
function, NC and is expressed as Intercept = ln(qNC ) −
q˚t kT
(3)
Using Eq. (3) and assuming the effective mass of electrons in HfAlOx to be 0.4m0 , the electron mobility is determined as 6.8 × 10−4 and 1.4 × 10−4 cm2 /V-s employing samples with and without Si ICL, respectively. Under negative bias conditions, the dominant current conduction mechanism is found to be Schottky emission at low fields ranging 0.11–1.64 MV/cm for Si-passivated samples and 0.1–1.8 MV/cm for non-passivated samples and in the high temperatures between 75 ◦ C and 150 ◦ C. Fig. 11 shows the corresponding Schottky plot of ln(J/T2 ) versus E1/2 under the negative bias, also referred to the gate injection. Further the average values of the fitted dynamic dielectric constant εr are 3.24 and 3.22 for samples with and without Si ICL, respectively. So, the refractive index of HfAlOx is around 1.80 (εr ∼ n2 ). The barrier heights (˚B ) at the interface of metal gate and high-k have been evaluated from the Arrhenius plot as shown in Fig. 12 and are determined to be 1.5 ± 0.05 and 1.46 ± 0.07 eV for Si-passivated and non-passivated samples, respectively. All the parameters extracted from conduction mechanism are tabulated in Table 1. Using the two Schottky barrier heights obtained in this work and taking the electron affin-
ity of GaAs as 4.07 eV [17] and the work function of Al as 4.28 eV [18,19] and the energy band diagram of the Al–HfAlOx –n-GaAs and Al–HfAlOx –Si–n-GaAs systems can be constructed as shown in Fig. 13.
Table 1 A comparison of electrical parameters for HfAlOx gate dielectrics on n-GaAs substrate with and with silicon inter-layer. Properties
n-GaAs/HfAlOx
n-GaAs/Si/HfAlOx
Dielectric constant EOT (nm) Flat-band voltage (V) Fixed oxide charge density (cm−2 ) Hysteresis voltage (V) Border trap density (cm−2 ) Interface state density (eV−1 cm−2 ) Frequency dispersion (%) Current at VFB + 1 V (A/cm2 ) Barrier height (high-k & GaAs) in eV Trap energy in high-k (eV) Electronic mobility (cm2 /V-s) Barrier height (Al & high-k) in eV Electron affinity of high-k (eV)
13.1 2.1 0.6 3.5 × 1012 0.55 4.1 × 1012 6.2 × 1012 13.1 1.95 × 10−4 1.41 ± 0.05 0.95 ± 0.04 1.4 × 10−4 1.46 ± 0.07 2.82
16.1 1.8 0.2 6.5 × 1011 0.11 2.3 × 1012 1.3 × 1012 6 3.7 × 10−5 1.45 ± 0.04 0.7 ± 0.03 6.8 × 10−4 1.5 ± 0.05 2.75
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been studied and the dominant conduction mechanisms are found to be Schottky emission at low to medium electric fields and Poole–Frenkel at high fields and high temperatures under substrate injection. In case of gate injection, the main current conduction at low field is found to be the Schottky emission at high temperatures. In particular, we have determined the some material and transport parameters for GaAs MOS capacitors with Al/HfAlOx /Si gate stacks: (i) HfAlOx /n-GaAs barrier height: 1.45 ± 0.04 eV, (ii) Al/HfAlOx barrier height: 1.5 ± 0.05 eV, (iii) a trap level at 0.7 ± 0.03 eV below the conduction band of HfAlOx contributing to Poole–Frenkel conduction, (iv) electron mobility () in HfAlOx : 6.8 × 10−4 cm2 /V-s and (v) electron affinity of HfAlOx with Si ICL: 2.75 eV. These parameters will be very useful for carrying out modeling and simulation of GaAs-based MOSFETs with Al/HfAlOx /Si gate stacks in the future. The energy band diagram of the Al–HfAlOx –n-GaAs and Al–HfAlOx –Si–n-GaAs systems are also constructed from current conduction analysis. References
Fig. 13. The energy band diagram of the GaAs MOS system before contact.
4. Conclusions In this brief, we present the impacts of an ultrathin Si interfacial layer on the electrical properties of GaAs MOS capacitors fabricated using RF-sputtered HfAlOx as the dielectric. It is found that the Si-passivated GaAs MOS capacitor exhibits excellent electrical properties compared with the non-passivated one’s. The HfAlOx /Si/n-GaAs stack has low frequency dispersion (∼4.8%), which is attributed to the reduction of interfacial oxides (GaAs:O) at the interface. An equivalent-oxide-thickness of 1.8 nm and leakage current density of 3.7 × 10−5 A/cm2 has been achieved at VFB + 1 V with low interface states density, Dit of 1.3 × 1012 eV−1 cm−2 and border trap density, Nbt of 2.3 × 1012 cm−2 for Si-passivated HfAlOx films on n-GaAs. X-ray photoelectron spectroscopy (XPS) analysis shows that the suppression of low-k interfacial layer formation is accomplished with the introduction of ultrathin Si interface control layer (ICL). In conclusion, the incorporation of a thin layer of Si between the HfAlOx dielectric and GaAs substrate is an effective way to improve the interface quality such as frequency dispersion, hysteresis voltage and leakage current. Therefore, HfAlOx /Si stack can be used as the potential gate insulator for GaAs-based MOS devices. Additionally, current conduction mechanism has
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Improved electrical and interfacial properties of RF-sputtered HfAlOx on n-GaAs with effective Si passivation P.S. Das a,b,∗ , A. Biswas b a b
Dept. of Electronics and ECE, Indian Institute of Technology, Kharagpur 721302, India Institute of Radio Physics and Electronics, University of Calcutta, 92 A P C Road, Kolkata 700009, India
a r t i c l e
i n f o
Article history: Received 6 December 2009 Received in revised form 18 April 2010 Accepted 18 April 2010 Available online 24 April 2010 Keywords: GaAs High-k XPS Interfacial characteristic Current conduction mechanism
a b s t r a c t In this paper, we present the effects of ultrathin Si interfacial layer on the physical and electrical properties of GaAs MOS capacitors fabricated using RF-sputtered HfAlOx gate dielectric. It is found that HfAlOx /Si/nGaAs stack exhibits excellent electrical properties with low frequency dispersion (∼4.8%), hysteresis voltage (0.27 V) and interface trap density (1.3 × 1012 eV−1 cm−2 ). The current density of 3.7 × 10−5 A/cm2 is achieved with an equivalent-oxide-thickness of 1.8 nm at VFB + 1 V for Si-passivated HfAlOx films on n-GaAs. X-ray photoelectron spectroscopy (XPS) analysis shows that the suppression of low-k interfacial layer formation is accomplished with the introduction of ultrathin Si interface control layer (ICL). Thus the introduction of thin layer of Si between HfAlOx dielectrics and GaAs substrate is an effective way to improve the interface quality such as low frequency dispersion, hysteresis voltage and leakage current. Additionally, current conduction mechanism has been studied and the dominant conduction mechanisms are found to be Schottky emission at low to medium electric fields and Poole–Frenkel at high fields and high temperatures under substrate injection. In case of gate injection, the main current conduction at low field is found to be the Schottky emission at high temperatures. © 2010 Elsevier B.V. All rights reserved.
1. Introduction During the last decade, substrate engineering, e.g. pseudomorphically strained-SiGe channels grown on Si substrates for p-MOSFETs and strained-Si channels on relaxed SiGe buffer layers with the graded Ge for n-MOSFETs, has been pursued to improve the carrier mobility in the channel [1]. As the complementary metal-oxide-semiconductor (CMOS) technology develops beyond 22 nm nodes, high priority has been assigned to III–V semiconductors as channel materials motivated by the need to increase the channel mobility as well as to reduce power consumption. III–V compound semiconductors offer the advantages of high electron mobilities, rich band gap engineering, and high breakdown fields and thus are likely to outperform silicon in certain metal-oxidesemiconductor (MOS) applications such as high-speed and high power devices [2,3]. However, surface oxidation occurring on the surface of III–V semiconductors can introduce a high defect density into the devices, which detrimentally affects the electrical characteristics of devices causing the Fermi level pinning [4,5]. One key challenge in the III–V technology is to identify thermodynamically stable insulators on the III–V’s that give a low interfacial density
of states (Dit ) and a low leakage current. Among the III–V compound semiconductors, current ly, GaAs is being widely studied as a potential channel material replacement of Si. During the last decade, high-k dielectric films such as HfO2 , Al2 O3 , ZrO2 and TiO2 are extensively researched for their potential as substitution for SiO2 . Recently, Hf-based dielectrics have been considered as very promising high permittivity material for future gate dielectric applications. But, direct deposition of HfO2 on GaAs exhibited anomalous characteristics with large frequency dispersion, hysteresis, low effective mobility and also high leakage current [6,7]. The interfacial layer growth may be overcome by introducing interface control layers (ICLs) such as Si, Ge, and Gd2 O3 [9–11]. In addition, there have been some attempts to adopt composite films of Al2 O3 and HfO2 on GaAs substrates [8,12]. In this paper, we studied the electrical and structural properties of RF-sputter deposited HfAlOx on n-GaAs substrates in the presence of the ultrathin silicon interface control layer. X-ray photoelectron spectroscopy (XPS) analysis was performed to investigate the interfacial properties of GaAs MOS capacitors.
2. Experiments ∗ Corresponding author at: Dept. of Electronics and ECE, Indian Institute of Technology, Kharagpur 721302, India. Tel.: +91 3222 281475; fax: +91 3222 255303. E-mail address: [email protected] (P.S. Das). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.04.058
MOS capacitors were fabricated on Te-doped n-GaAs (1 0 0) wafers with a carrier density ∼1 × 1016 cm−3 . The samples were degreased in methanol, acetone and 2-propanol successively for
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Fig. 1. High frequency C–V characteristics of MOS capacitors on n-GaAs with Sand Si-passivation for voltage sweep from inversion to accumulation and back to inversion at a ramp rate of 0.1 V/s.
Fig. 3. (a) Current density versus applied bias plot (J–V characteristics) of HfAlOx thin film on n-GaAs with S- and Si-passivation and (b) gate leakage current density at VFB ± 1 V versus EOT. For comparison, reported data of high-k on Si, SiO2 on Si, HfAlOx thin films on GaAs are also plotted together.
Fig. 2. (a) and (b) Frequency dispersion characteristics of GaAs MOS capacitors with S- and Si-passivation.
2 min each. Then the substrates were treated with 50% HCl for 5 min and heated in NH4 OH at 60 ◦ C for 10 min to remove native oxide. After wet chemical cleaning, the samples were heated into 40% ammonium sulfide solution for 10 min for S-passivation of GaAs surface. The HfAlOx films were deposited at 50 ◦ C by RF co-sputtering of Al2 O3 and HfO2 targets in 100 W powers in Ar atmosphere at 10−3 mbar working pressure. Prior to the dielectric deposition, ultrathin silicon layer of 1.5 nm was deposited as interface control layer (ICL) on some of the samples by RF sputtering in the same ambient. Thickness of the dielectric layer was measured to be ∼7 nm using an ellipsometer. The post deposition annealing (PDA) was carried out in an N2 ambient at 600 ◦ C for 1 min by rapid thermal annealing. Finally, Al electrode was formed by thermal evaporation with the gate area of 1.96 × 10−3 cm2 at 10−6 mbar using the shadow masking technique. In order to make the backside ohmic contact Au/Ge/Au was deposited at a pressure of 10−6 mbar. For chemical analysis, high-resolution XPS was performed using VG ESCALAB 220i-XL system. Capacitance–voltage (C–V) measurements were made using Agilent E4980A LCR meter and current–voltage (I–V) measurements were performed with HP4156C semiconductor parameter analyzer, respectively.
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Fig. 4. (a) High-resolution XPS of Ga 3d spectra of thin HfAlOx overlayer on S-passivated n-GaAs, (b) high-resolution XPS of Ga 3d spectra of thin HfAlOx overlayer on Si-passivated n-GaAs, (c) high-resolution XPS of As 3d spectra of thin HfAlOx overlayer on S-passivated n-GaAs and (d) high-resolution XPS of As 3d spectra of thin HfAlOx overlayer on Si-passivated n-GaAs and the inset shows the Si 2p spectrum for HfAlOx /Si/n-GaAs.
3. Results and discussions 3.1. Electrical characterization Fig. 1 shows high frequency (HF) C–V characteristics of GaAs MOS capacitors of 7 nm sputtered HfAlOx dielectrics. One can easily observe that the accumulation capacitance (Cacc ) increases by 24% with the introduction of Si interface control layer for the same thickness of HfAlOx . The average value of dielectric constant (ε = Cacc d/ε0 A) calculated from accumulation capacitance are 16.1 and 13.1 for Si- and S-passivated samples, respectively. The quantities ε0 , Cacc /A and d denote, respectively, the permittivity in vacuum, the accumulation capacitance per unit area and the thickness of the HfAlOx film. The equivalent oxide thickness (EOT) is found to be 1.8 nm and 2.1 nm for MOS capacitors with Siand S-passivation, respectively. The C–V characteristics are directly related to the charged defects in the insulator and at the insulator/semiconductor interface. In fact, the unbalanced charges arising from defects will cause a C–V flat-band voltage VFB to shift from the ideal condition. The shift value (Vf ) can be used to quantify the number of fixed charges [Nf (cm−2 )] in the dielectric films using the equation: Nf =
Cox (MS − VFB ) qA
ϕMS being the metal-semiconductor work function difference. The flat-band voltages (VFB ) for HfAlOx and HfAlOx /Si are 0.6 and 0.2 V, respectively. The positive VFB for both the cases indicates a neg-
ative oxide charge trapping in the films. The fixed oxide charge density (Nf ) calculated from flat-band voltages are 6.5 × 1011 cm−2 and 3.5 × 1012 cm−2 for samples with and without Si ICL, respectively. Moreover, the number of trapped (negative) charges [Not (cm−2 )] has been evaluated from the hysteresis observed in the C–V curve. Dalapati et al. [8] have reported a hysteresis voltage (VFB ) of 0.9 V for directly deposited HfAlOx on n-GaAs substrate whereas VFB is 0.55 V in our case. By using the HfAlOx /Si/n-GaAs stack the hysteresis voltage can further be improved to 0.11 V. From the hysteresis characteristics, the calculated trapped charge density (Nbt = Cox VFB /Aq) were found to be 2.3 × 1012 cm−2 and 4.1 × 1012 cm−2 with and without Si ICL, respectively. From Fig. 1, it is also observed that C–V curves stretch out along the voltage axis, which indicates high density of interface states for directly deposited HfAlOx on nGaAs. However, the stretch out effect decreases in presence of Si ICL. Evaluation of the fast interface state density (Dit ) by Terman [13] method has also shown the improved interface characteristics for Si ICL samples. The minimum value of Dit for HfAlOx /n-GaAs is 6.2 × 1012 eV−1 cm−2 , which is nearly the same as reported by Dalapati et al. [8]. With the incorporation of a Si ICL in between HfAlOx and n-GaAs, the value of Dit as low as 1.3 × 1012 eV−1 cm−2 is obtained and thus a significant improvement is achieved. Fig. 2 shows typical C–V curves of MOS capacitors at different frequencies ranging from 1 kHz to 1 MHz for frequency dispersion analyses. In case of a HfAlOx /n-GaAs stack, the percentage frequency dispersion (C) and flat-band voltage change (V) are found to be 13.1% and 250 mV. Suri et al. [12] reported improved frequency dispersion characteristics with V and C decreasing to 40 mV and 6%
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Fig. 6. Current density versus applied voltage (J–V) plot for GaAs MOS capacitor at various temperatures for samples with and without Si ICL. Fig. 5. (a) O 1s spectra of bulk HfAlOx overlayer on S-passivated n-GaAs and (b) O 1s spectra of bulk HfAlOx overlayer on Si-passivated n-GaAs.
with HfAlOx gate dielectrics on p-GaAs. But, it is also learnt form the literature that for any high-k/n-GaAs gate stack, the frequency dispersion is higher compared with high-k/p-GaAs gate stack, which is due to the intrinsic surface nature of n-GaAs [8]. Dalapati et al. [8] reported 10% dispersion in accumulation capacitance for HfAlOx /nGaAs stack. However, with the incorporation of the Si ICL, V and C decrease to 4.8% and 60 mV in our device. Thus, it is obvious that the Si ICL reduces Fermi level pinning in GaAs MOS capacitors and improves the overall dielectric properties of the film efficiently which may be due to the decrease in the formation of the low-k interfacial layer re-growth. Fig. 3(a) depicts the leakage current density characteristics of HfAlOx /n-GaAs and HfAlOx /Si/n-GaAs stacks. The leakage current density for HfAlOx film measured at VFB + 1 V is 1.95 × 10−4 A/cm2 . This rather leaky characteristic is attributed to the relatively high density of interface states and/or interfacial oxides induced lowering of conduction band offset between HfAlOx and GaAs. In the presence of a Si ICL, the current density reduces to 3.7 × 10−5 A/cm2 at VFB + 1 V. It is worth to point out that the Si ICL lowers the leakage current density by nearly one order of magnitude even for a lower value of EOT [Fig. 3(b)]. Fig. 3(b) shows the gate leakage current density at VFB ± 1 V versus EOT and compared with some
recently published results. The improvement of leakage current for Si-passivated sample is supposed to be due to reduction of GaAs chemical states and thus, hindering the conduction of leakage currents. The result shows that leakage currents for HfAlOx /n-GaAs and HfAlOx /Si/n-GaAs stack are lower as compared with that in SiO2 on Si, but larger than that in a high-k/Si structure. Interestingly, for the corresponding value of EOTs, the ratio of current densities for HfAlOx /Si/n-GaAs and high-k/Si in our case is around 7.6 ± 0.5, which is lower compared to the result of Suri et al. [12] for a HfAlOx /p-GaAs stack (20 ± 0.05). 3.2. X-ray photoelectron spectroscopy analysis In order to investigate the interface quality, another set of 4 nm thick samples of thin films of HfAlOx on silicon and sulfur passivated n-GaAs were prepared using same process steps and analyzed by high-resolution X-ray photoelectron spectroscopy using the VG ESCALAB 220i-XL XPS system. The Ga 3d XPS spectrum can be fitted into two peaks at 19.2 and 20.4 eV, which correspond to Ga–As and Ga–O from Ga2 O3 [5], as shown in Figs. 4(a) and (b) for S and Si-passivated samples, respectively. For S-passivated sample, the percentage of Ga–O peak area compared to total Ga 3d area is about 30.5% and reduces to 9.5% for Si-passivated n-GaAs. Fig. 4(c) and (d) depicts the As 3d spectra of thin HfAlOx layer on sulfur and silicon passivated n-GaAs substrate. The As 3d spectra show As 3d5/2 and
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Fig. 8. Arrhenius plots for Schottky emission at various electric fields under substrate injection with and without Si-passivation. Fig. 7. Characteristics of Schottky emission (a) GaAs/HfAlOx and (b) GaAs/Si/HfAlOx stacks under substrate injection, respectively.
As 3d3/2 doublets for GaAs substrate at 41.2 and 41.8 eV, respectively [14]. Yet another peak is observed at 44.3 eV, which is due to As–O from As2 O3 [14]. However, the peak intensity is quite low for Si-passivated samples. In addition the percentage of the As–O area compared to the total area of As 3d are 25.4% and 11.3% for S and Si-passivated GaAs, respectively. The As–O and Ga–O bonds were reported to act as defect states in the band gap and hence play a critical role in degrading device electrical property [15]. This is why the formation of As–O and Ga–O bonds was effectively suppressed using Si interface control layer as also predicted from frequency dispersion analyses. The Si 2p spectrum for HfAlOx /Si/nGaAs is shown in the inset of Fig. 4(d). The peak at 103.2 eV is due to the Si–O bonds from SiO2 . Further no peak is found at 99.9 eV for the Si–Si bonds, which indicates that the Si ICL was fully oxidized. Fig. 5(a) and (b) shows the O 1s spectra for bulk HfAlOx on S and Sipassivated n-GaAs, respectively. The spectra can be deconvoluted into two Gaussian peaks at 530 eV and 532.5 eV for HfAlOx /n-GaAs. An energy shift of 0.2 eV for silicon passivated samples is noticed, which may arise due to different interface dipoles between high-k and GaAs. The atomic percentage of Hf:Al was estimated to be 21:79 which is equivalent to the mole fraction of (HfO2 )0.21 (Al2 O3 )0.79 .
3.3. Conduction mechanism analysis The temperature dependency of the gate leakage current of 7 nm HfAlOx thin films on n-GaAs substrates with and without silicon interface control layer (ICL) was studied to identify the electrical conduction mechanism under substrate and gate injections. Fig. 6 shows the current density versus applied voltage (J–V) plot in the temperature range 25–150 ◦ C with and without Si ICL, respectively. In-depth analysis of Fig. 6 reveals that dominant conduction mechanism at low and medium electric fields is Schottky emission whereas Poole–Frenkel conduction takes over at higher field under substrate injection and in case of gate injection, the Schottky emission dominates the current conduction at low field for both the samples. The current density due to standard Schottky emission [16] can be expressed as
∗ 2
JSE = A T
exp
−q(˚B −
qE/4ε0 εr )
kT
(1)
where A∗ = (4qm∗hk k2 /h3 ) = 120(m∗hk /m0 ) (A/cm2 K2 ), JSE is the current density, A* is the effective Richardson constant, T is the absolute temperature, q is the electronic charge, q˚B is the Schottky barrier height, E is the applied electric field, k is the Boltzmann’s constant, h is the Planck’s constant, ε0 is the free space permittivity, εr is the dynamic dielectric constant of the gate dielectric, m0
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Fig. 9. Characteristics of Poole–Frenkel conduction (a) GaAs/HfAlOx and (b) GaAs/Si/HfAlOx stacks under substrate injection, respectively. Fig. 10. Arrhenius plots for Poole–Frenkel conduction at various electric fields under substrate injection with and without Si ICL.
is the free electron mass and m* is the effective mass electrons in HfAlOx . The dynamic dielectric constant (εr ) should be nearly equal to square of the measured the refractive index (εr ∼ n2 ). For standard Schottky emission, a plot of ln(J/T2 ) versus E1/2 should be a straight line. The experimental data in the region of high temperatures (75–150 ◦ C) together with low to medium electric fields (0.50–2.86 MV/cm) fit the Schottky emission theory very well under the substrate injection for GaAs/HfAlOx stack as shown in Fig. 7(a). For GaAs/Si/HfAlOx stack, Schottky emission occurs within the electric field of 0.47–1.17 MV/cm and in the temperature ranging between 25 ◦ C and 150 ◦ C as demonstrated in Fig. 7(b). The average values of the fitted dynamic dielectric constant εr are 3.25 and 3.3 for samples with and without Si ICL, respectively. Fig. 8 shows the Arrhenius plot where the ratio of gate leakage current density to square of the temperature is plotted on a semi-log plot as a function of inverse of temperature for different applied fields. It is observed that data approximately follow a straight line, indicating that leakage current varies exponentially with 1/T. Moreover, barrier height (˚B ) at HfAlOx and n-GaAs interface are about 1.45 ± 0.04 eV and 1.41 ± 0.05 eV for samples with and without Si ICL, respectively, extracted from the slopes of Fig. 8. These values take into account the interfacial effects. However, in the high temperature range 75–150 ◦ C and high electric field, Poole–Frenkel conduction takes over. The corre-
sponding leakage current density in dielectrics can be expressed by employing the following equation [16]:
JP–F = qNC E exp
−q(˚t − q
qE/4ε0 εr )
kT
,
(2)
where is the mobility of electrons, NC = 2(2m*kT/h2 )3/2 is the effective density of states function in the conduction band, ˚t is the trap energy level and other quantities are the same as defined earlier. Additionally, the plot of ln(J/E) versus E1/2 should be a straight line with a slope εr . The experimental data in the domains of high temperatures (75–150 ◦ C) and high electric fields (3.57–6.28 MV/cm) are best described by P–F conduction as shown in Fig. 9(a) for samples without Si-passivation. Fig. 9(b) shows the P–F plot for samples with Si-passivation with high electric fields (3.64–5.88 MV/cm) and high temperatures (75–150 ◦ C). The average value of fitted dynamic dielectric constants εr are 3.26 and 3.27 for samples with and without Si ICL, respectively. From the Arrhenius plot as shown in Fig. 10 trap barrier heights (˚t ) are extracted to be about 0.7 ± 0.03 and 0.95 ± 0.04 eV for samples with and without Si ICl, respectively. Moreover, the intercept of the P–F plot is a function of the mobility of electrons, and density of states
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Fig. 11. Characteristics of Schottky emission under gate injection for samples with and without Si ICL.
Fig. 12. Arrhenius plots for Schottky emission at different electric fields under gate injection with and without Si-passivation.
function, NC and is expressed as Intercept = ln(qNC ) −
q˚t kT
(3)
Using Eq. (3) and assuming the effective mass of electrons in HfAlOx to be 0.4m0 , the electron mobility is determined as 6.8 × 10−4 and 1.4 × 10−4 cm2 /V-s employing samples with and without Si ICL, respectively. Under negative bias conditions, the dominant current conduction mechanism is found to be Schottky emission at low fields ranging 0.11–1.64 MV/cm for Si-passivated samples and 0.1–1.8 MV/cm for non-passivated samples and in the high temperatures between 75 ◦ C and 150 ◦ C. Fig. 11 shows the corresponding Schottky plot of ln(J/T2 ) versus E1/2 under the negative bias, also referred to the gate injection. Further the average values of the fitted dynamic dielectric constant εr are 3.24 and 3.22 for samples with and without Si ICL, respectively. So, the refractive index of HfAlOx is around 1.80 (εr ∼ n2 ). The barrier heights (˚B ) at the interface of metal gate and high-k have been evaluated from the Arrhenius plot as shown in Fig. 12 and are determined to be 1.5 ± 0.05 and 1.46 ± 0.07 eV for Si-passivated and non-passivated samples, respectively. All the parameters extracted from conduction mechanism are tabulated in Table 1. Using the two Schottky barrier heights obtained in this work and taking the electron affin-
ity of GaAs as 4.07 eV [17] and the work function of Al as 4.28 eV [18,19] and the energy band diagram of the Al–HfAlOx –n-GaAs and Al–HfAlOx –Si–n-GaAs systems can be constructed as shown in Fig. 13.
Table 1 A comparison of electrical parameters for HfAlOx gate dielectrics on n-GaAs substrate with and with silicon inter-layer. Properties
n-GaAs/HfAlOx
n-GaAs/Si/HfAlOx
Dielectric constant EOT (nm) Flat-band voltage (V) Fixed oxide charge density (cm−2 ) Hysteresis voltage (V) Border trap density (cm−2 ) Interface state density (eV−1 cm−2 ) Frequency dispersion (%) Current at VFB + 1 V (A/cm2 ) Barrier height (high-k & GaAs) in eV Trap energy in high-k (eV) Electronic mobility (cm2 /V-s) Barrier height (Al & high-k) in eV Electron affinity of high-k (eV)
13.1 2.1 0.6 3.5 × 1012 0.55 4.1 × 1012 6.2 × 1012 13.1 1.95 × 10−4 1.41 ± 0.05 0.95 ± 0.04 1.4 × 10−4 1.46 ± 0.07 2.82
16.1 1.8 0.2 6.5 × 1011 0.11 2.3 × 1012 1.3 × 1012 6 3.7 × 10−5 1.45 ± 0.04 0.7 ± 0.03 6.8 × 10−4 1.5 ± 0.05 2.75
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been studied and the dominant conduction mechanisms are found to be Schottky emission at low to medium electric fields and Poole–Frenkel at high fields and high temperatures under substrate injection. In case of gate injection, the main current conduction at low field is found to be the Schottky emission at high temperatures. In particular, we have determined the some material and transport parameters for GaAs MOS capacitors with Al/HfAlOx /Si gate stacks: (i) HfAlOx /n-GaAs barrier height: 1.45 ± 0.04 eV, (ii) Al/HfAlOx barrier height: 1.5 ± 0.05 eV, (iii) a trap level at 0.7 ± 0.03 eV below the conduction band of HfAlOx contributing to Poole–Frenkel conduction, (iv) electron mobility () in HfAlOx : 6.8 × 10−4 cm2 /V-s and (v) electron affinity of HfAlOx with Si ICL: 2.75 eV. These parameters will be very useful for carrying out modeling and simulation of GaAs-based MOSFETs with Al/HfAlOx /Si gate stacks in the future. The energy band diagram of the Al–HfAlOx –n-GaAs and Al–HfAlOx –Si–n-GaAs systems are also constructed from current conduction analysis. References
Fig. 13. The energy band diagram of the GaAs MOS system before contact.
4. Conclusions In this brief, we present the impacts of an ultrathin Si interfacial layer on the electrical properties of GaAs MOS capacitors fabricated using RF-sputtered HfAlOx as the dielectric. It is found that the Si-passivated GaAs MOS capacitor exhibits excellent electrical properties compared with the non-passivated one’s. The HfAlOx /Si/n-GaAs stack has low frequency dispersion (∼4.8%), which is attributed to the reduction of interfacial oxides (GaAs:O) at the interface. An equivalent-oxide-thickness of 1.8 nm and leakage current density of 3.7 × 10−5 A/cm2 has been achieved at VFB + 1 V with low interface states density, Dit of 1.3 × 1012 eV−1 cm−2 and border trap density, Nbt of 2.3 × 1012 cm−2 for Si-passivated HfAlOx films on n-GaAs. X-ray photoelectron spectroscopy (XPS) analysis shows that the suppression of low-k interfacial layer formation is accomplished with the introduction of ultrathin Si interface control layer (ICL). In conclusion, the incorporation of a thin layer of Si between the HfAlOx dielectric and GaAs substrate is an effective way to improve the interface quality such as frequency dispersion, hysteresis voltage and leakage current. Therefore, HfAlOx /Si stack can be used as the potential gate insulator for GaAs-based MOS devices. Additionally, current conduction mechanism has
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