Study on the Physical and Leakage Current Characteristics of an Optimized High-k/InAlAs MOS Capacitor with a HfO₂–Al₂O₃ Laminated Dielectric

Abstract

High-k/n-InAlAs MOS capacitors are popular for the isolated gate of InAs/AlSb and InAlAs/InGaAs high-electron mobility transistors. In this study, a new kind of high-k/n-InAlAs MOS-capacitor with a HfO₂–Al₂O₃ laminated dielectric was successfully fabricated using an optimized process. Compared with the traditional HfO₂/n-InAlAs MOS capacitor, the new device has a larger equivalent oxide thickness. Two devices, with a HfO₂ (8 nm)–Al₂O₃ (4 nm) laminated dielectric and a HfO₂ (4 nm)–Al₂O₃ (8 nm) laminated dielectric, respectively, were studied in comparison to analyze the effect of the thickness ratios of HfO₂ and Al₂O₃ on the performance of the devices. It was found that the device with a HfO₂ (4 nm)–Al₂O₃ (8 nm) laminated dielectric showed a lower effective density of oxide charges, and an evidently higher conduction band offset, making its leakage current achieve a significantly low value below 10⁻⁷ A/cm² under a bias voltage from −3 to 2 V. It was demonstrated that the HfO₂–Al₂O₃ laminated dielectric with a HfO₂ thickness of 4 nm and an Al₂O₃ thickness of 8 nm improves the performance of the high-k dielectric on InAlAs, which is advantageous for further applications.

1. Introduction

Owing to the requirements of high speed, low dissipation, and low noise for modern integrated circuits, InAs/AlSb and InAlAs/InGaAs high-electron mobility transistors (HEMTs) are receiving great attention as new III–V compound devices because of their high electron mobility and electron saturation drift speed [1,2,3,4]. Because of its good compatibility with AlSb, InAs, and InGaAs, InAlAs is one of the most promising materials for the protection layer of InAs/AlSb and InAlAs/InGaAs HEMTs to enhance the carrier density in the channel [1,3]. Depositing a high-k dielectric film on InAlAs as a MOS capacitor can effectively suppress the leakage current [5,6,7,8,9]. However, the lack of reasonable high-k dielectrics is still a major problem that limits the performance of the isolated gate. HfO₂ is the main candidate for the high-k dielectric because of its high dielectric constant [5,7], but its direct deposition on InAlAs limits its performance owing to the poor lattice match with InAlAs [8]. Therefore, a thin Al₂O₃ film inserted between InAlAs and HfO₂ as a buffer layer has been proposed [8]. This structure can increase the quality of the MOS capacitor by providing a better match with InAlAs. Reference [8] initially reported the electrical and interfacial characteristics of a HfO₂–Al₂O₃/n-InAlAs MOS-capacitor. However, in [8], the impact of the thickness of HfO₂ and Al₂O₃ on the performance of the device was not investigated, and the leakage current performance, which is considered as the most important electrical characteristic of the MOS-capacitor was not mentioned either. In order to optimize the fabrication process and improve the performance of the high-k/InAlAs MOS-capacitor with an HfO₂–Al₂O₃ laminated dielectric, a new kind of HfO₂–Al₂O₃/n-InAlAs MOS-capacitor was manufactured in this study, and its optimized fabrication process was discussed in detail. Two samples with HfO₂ (8 nm)/Al₂O₃ (4 nm) and HfO₂ (4 nm)/Al₂O₃ (8 nm) were designed and prepared to investigate the effect of the different thickness ratios of HfO₂ and Al₂O₃. Devices were tested by atomic force microscope (AFM), Focused Ion beam (FIB), scanning electron microscope (SEM), and X-ray photoelectron spectroscopy (XPS) to describe the physical characteristics. Based on the capacitance (C)–voltage (V) and current (I)–V test results, their physical characteristics were analyzed and their electrical characteristics, including the leakage current, were investigated in detail.

2. Materials and Methods

The structure diagram of the MOS capacitor is shown in Figure 1. To match with InAlAs and achieve better performance, InP was selected as the substrate rather than GaAs, which were frequently used in other studies [7,8,9]. InP is semi-insulating and a 350-μm-thick substrate was used. To decrease the lattice mismatch with InAlAs, a 200-nm-thick InP buffer layer was grown on the InP substrate by MBE at 470 °C. A 500-nm-thick Si-doped In_0.5Al_0.5As layer with a doping concentration of 1 × 10¹⁷ cm⁻³ was deposited on the InP buffer layer. The high-k dielectric was then deposited by atomic layer deposition (ALD) [10,11,12,13]. The details of the ALD process to produce the HfO₂–Al₂O₃ laminated dielectric are given in

Table 1. ALD process to prepare the HfO₂–Al₂O₃ dielectric.

Dielectric	Precursor	Pulse Time (s)	Deposition Temperature (°C)	Pressure (mbar)	Deposition Speed (nm/s)
Al2O3	TMT + N2 + H2O + N2	0.5 + 2 + 0.5 + 1	245	2.3	0.1
HfO2	TEMAH + N2 + H2O + N2	1 + 2 + 1 + 2	245	2.3	0.1

. We controlled the ALD process circle to manufacture different dielectric thicknesses. A post-deposition annealing (PDA) process was applied to increase the quality of the oxide-semiconductor interface [14,15,16,17]. The process involved heating the film from ambient temperature to 380 °C in N₂ over 15 s, annealing for 60 s_, and then cooling to ambient temperature over 300 s [6,7]. Finally, a Ti (20 nm)/Pt (20 nm)/Au (200 nm) metal structure was grown as the electrode by the magnetron sputtering technique. We grew two electrodes with different areas. The area of the small one is 150 μm × 150 μm (marked as C₁ in Figure 1), and the area of the large one is 1500 × 1500 μm² (marked as C₂ in Figure 1). When a voltage is applied between the two electrodes, C₁ is connected in series with C₂. Because the area of C₂ is 100 times larger than the area of C₁, the influence of C₂ on the total capacitance can be neglected, and the measured capacitance can be approximately equal to the value of C₁. The electrodes that grow on top of the oxide layer can avoid the pollution caused by the back contact process used in [8] (e.g., the metal directly deposited on the surface of InAlAs).

A simple HfO₂/n-InAlAs MOS capacitor with an oxide thickness of 12 nm and two HfO₂–Al₂O₃/n-InAlAs MOS capacitors with HfO₂ (8 nm)/Al₂O₃ (4 nm) and HfO₂ (4 nm)/Al₂O₃ (8 nm) laminated dielectrics were fabricated. These three samples were designed to have the same physical oxide thickness of 12 nm for convenient comparison. The surface roughness RMS (root-mean-square) values of the three samples which are taken from the AFM test are observed as small, around 0.5 nm. The 5 μm × 5 μm AFM graph of the HfO₂ (4 nm)/Al₂O₃ (8 nm) MOS capacitor is show in Figure 2a. The FIB-SEM image of the across-section of the HfO₂ (4 nm)/Al₂O₃ (8 nm) MOS capacitor is shown in Figure 2b. These indicate a compactable and homogeneous device structure.

3. Results and Discussions

The accumulated capacitances of the MOS-capacitors C_OX were measured by the high-frequency capacitance method at 1 MHz, and the results are shown in Figure 3. The HfO₂/InAlAs MOS capacitor has a highest C_OX of 0.68 μF/cm², while the C_OX is lower with an inserted Al₂O₃ thin film. The C_OX value decreases with increasing thickness of the inserted Al₂O₃ layer. The highest C_OX values of the HfO₂ (8 nm)/Al₂O₃ (4 nm)/InAlAs and HfO₂ (4 nm)/Al₂O₃ (8 nm)/InAlAs MOS capacitors are 0.517 and 0.355 μF/cm², respectively. It is noted that C_OX shows a decrease as the MOS capacitor is driven further into accumulation. This is induced by the obvious leakage current under the high voltage bias condition. In addition, a low C–V hysteresis is observed for the sample with HfO₂ (4 nm)/Al₂O₃ (8 nm) laminated dielectrics. The reason can be explained by its lowest density of oxide charges that we discuss herein.

The equivalent oxide thickness (EOT) values of the three samples were determined from the C–V curves [7,9]. The results are given in

Table 2. Physical and electrical parameters of the HfO₂/n-InAlAs and HfO₂–Al₂O₃/n-InAlAs MOS capacitors.

Parameter	HfO2/n-InAlAs	HfO2 (8nm)/Al2O3 (4nm)/n-InAlAs	HfO2 (4nm)/Al2O3 (8nm)/n-InAlAs
COX (μF/cm2)	0.680	0.517	0.355
EOT (nm)	5.08	6.68	9.73
εOX	9.22	7.01	4.81
CFB (μF/cm2)	0.374	0.319	0.249
VFB (V)	−0.31	−0.44	−0.23
Neff (cm−2)	1.83 × 1012	1.81 × 1012	0.78 × 1012
ΔECB (eV)	1.120	1.179	1.563

. Inserting Al₂O₃ increases the EOT. When the thickness of Al₂O₃ is 4 and 8 nm, the EOT values are 6.68 and 9.73 nm, respectively. Because the electric field intensity E_i is inversely proportional to the EOT for fixed bias voltage V_g, the higher EOT indicates that inserting an Al₂O₃ film between InAlAs and HfO₂ decreases E_i under the same V_g, which will help to decrease the leakage current. It is noted that the increased EOT would degrade the gate control ability of the device. The equivalent dielectric constant ε_OX was calculated according to the C–V test data [18]. The two samples with a HfO₂/Al₂O₃ laminated dielectric have lower ε_OX values than the sample with only a HfO₂ dielectric, and the ε_OX value decreases as the thickness of the Al₂O₃ film increases. This can be explained by the lower dielectric constant of Al₂O₃ than HfO₂.

The effective density of oxide charges N_eff, which indicates the quality of the MOS capacitor, and can be estimated from the flat-band capacitance C_FB and flat-band voltage V_FB [7,19]. The estimated values for the samples are listed in Table 2. First, we will compare the N_eff values of the samples with a single HfO₂ dielectric and a HfO₂ (8 nm)/Al₂O₃ (4 nm) laminated dielectric. Because Al₂O₃ possesses better matching with InAlAs than HfO₂, inserting an Al₂O₃ film improves the interface quality [8,9], which helps to suppress spreading of the impurities in the dielectric layer. However, another interface is generated between Al₂O₃ and HfO₂ because of insertion of the Al₂O₃ film, leading to generation of additional interface states, which will induce an increase in traps. These competing effects mean that the N_eff values of the two samples are similar. The N_eff values are 0.78 × 10¹² cm⁻² for the sample with a HfO₂ (4 nm)/Al₂O₃ (8 nm) laminated dielectric and 1.81 × 10¹² for the sample with a HfO₂ (8 nm)/Al₂O₃ (4 nm) laminated dielectric. Thus, N_eff decreases as the thickness of Al₂O₃ increases. This reduction can be explained by the better interfacial quality of the sample with a thicker Al₂O₃ layer. In addition, the stability of Al₂O₃ is higher than HfO₂ and the oxygen atoms in Al₂O₃ are more difficult to remove by other impurity bonds (e.g., In– and As–) to form In–O and As–O traps, so the sample with a thicker Al₂O₃ layer has lower N_eff. For verification, XPS was performed to check the diffusion states of the As and In elements in the oxide layer. The XPS spectra before and after 30 s etching on the oxide layer are shown in Figure 4 (30 s etching is estimated to reach 6nm depth of oxide layer). The XPS spectra is shown in Figure 4. In general, the As 3d and In 3d peaks before etching are lower than those after 30 s etching because of the blocking action of the high-k dielectric on impurities. Compared with the Hf–O bond, the Al–O bond shows a stronger blocking effect on diffusion of impurity particles, so the sample with HfO₂ (4 nm)/Al₂O₃ (8 nm) has a lower concentration of As than the sample with HfO₂ (8 nm)/Al₂O₃ (4 nm). Although Al₂O₃ can block the diffusion of As, the 4 nm thickness does not perform a good blocking role, indicating that Al₂O₃ must have a reasonable thickness for a good blocking effect on impurities. Diffusion of As in the oxide layer is suppressed when the thickness of Al₂O₃ is increased to 8 nm, which suppresses the formation of charge traps in the oxide layer and reduces the N_eff value. In addition, the In content before etching is negligible because of the weak diffusion of In in the oxide layer. The lower N_eff is helpful to suppress the hysteresis phenomenon in the C–V curve. This explains the lowest C–V hysteresis for the sample with the HfO₂ (4 nm)/Al₂O₃ (8 nm) laminated dielectric (Figure 3).

The leakage currents of the MOS capacitors are shown in Figure 5. For negative bias voltage, compared with the reported results in [7,9], the leakage current density of the new HfO₂ (4 nm)/Al₂O₃ (8 nm)/n-InAlAs MOS capacitor is significantly lower in the V_g range from −3 to 2 V (below 10⁻⁷ A/cm²), and it is one order of magnitude lower than that of the HfO₂/n-InAlAs MOS capacitor. One reason is that inserting an Al₂O₃ film enhances the match between HfO₂ and InAlAs by suppressing the formation of a low-k interfacial layer. In addition, Al₂O₃ has a higher barrier height than HfO₂, which makes the HfO₂–Al₂O₃/n-InAlAs MOS capacitor possess less accessibility for carriers to overcome the barrier and form leakage current. Under positive bias voltage, electrons cross the barrier at the high-k/InAlAs interface (φ_B) and contribute to the leakage current [20,21]. In general, the conduction band offset (ΔE_CB) between oxide and the semiconductor layer is used to determine φ_B [6]. To further investigate the reason for the barrier effect on the leakage current, ΔE_CB was calculated by the Krant method [6]. The results are listed in Table 2. It should be noted that the sample with HfO₂ (4 nm)/Al₂O₃ (8 nm) laminated dielectric shows the highest band offset ΔE_CB (1.563 eV), which can be explained by its higher barrier height of Al₂O₃, so the sample with an 8-nm-thick Al₂O₃ film shows significantly lower leakage current. The sample with HfO₂ (8 nm)/Al₂O₃ (4 nm) laminated dielectric shows a competitive ΔE_CB value single (1.179 eV) with the sample with HfO₂ dielectric (1.120 eV). However, the sample with HfO₂ (8 nm)/Al₂O₃ (4 nm) laminated dielectric shows a higher leakage current than the sample with single HfO₂ dielectric when a positive bias voltage is applied. It can be explained as following: A 4nm Al₂O₃ layer is too thin to suppress the leakage current effectively; meanwhile, the additional states on the Al₂O₃-HfO₂ interface degrade the leakage current. Therefore, the sample with HfO₂ (8 nm)/Al₂O₃ (4 nm) dielectric shows the lowest leakage. The leakage current mechanism of devices is complicated, and will be investigated in detail in our next work.

The physical and electrical parameters of the HfO₂/n-InAlAs and HfO₂–Al₂O₃/n-InAlAs MOS capacitors are compared in Table 2.

4. Conclusions

In conclusion, compared with the HfO₂/n-InAlAs MOS capacitor, the HfO₂–Al₂O₃/n-InAlAs MOS capacitor has the higher EOT and lower N_eff, which help to suppress the leakage current. The HfO₂–Al₂O₃/n-InAlAs MOS capacitor has a high conduction band offset, making its leakage current below 10⁻⁷ A/cm² under bias voltage from −3 to 2 V. Therefore, the Al₂O₃–HfO₂ laminated dielectric improves the high-k gate dielectric on InAlAs and suppresses the leakage current. The HfO₂–Al₂O₃/n-InAlAs MOS capacitor with HfO₂ thickness of 4 nm and Al₂O₃ thickness of 8 nm is a good candidate for the isolated gate of InAs/AlSb and InAlAs/InGaAs HEMTs.