The InGaAs-based high electron mobility transistor (HEMT) has been widely studied for the applications in high-speed, low-power and high-frequency circuits due to its low effective electron mass and high electron mobility.^[1,2] Its current flow is controlled by a Schottky metal gate. However, the Schottky gate brings a high leakage current. Recently, the InGaAs-based metal–oxide–semiconductor field-effect transistors (MOSFETs) have received much attention because of their relatively low gate leakage currents and large voltage swings compared with those of the Schottky-gate devices.^[3] Each of these MOSFET and HEMT architectures typically has a higher band gap barrier layer, such as InAlAs, as a surface material, on which high-κ dielectrics will be deposited. Compared with the InP barrier layer, InAlAs provides not only good lattice matching with InGaAs, but also a large conduction band offset with InGaAs, thereby improving the confinement of electrons in the channel. Using in-situ epitaxial growth, a high-quality interface between InGaAs and InAlAs can be obtained. However, the interface between the barrier layer InAlAs and high-k oxide still plays an important role in device operation, affecting the subthreshold swing and the threshold voltage.^[4] So, it is necessary to investigate interfacial and electrical characteristics between the barrier layer InAlAs and high-k oxide.

SiO₂ has been replaced by the high-k dielectric because of its high leakage current when scaling down the device.^[5] Research focuses on high-k dielectrics, including Al₂O₃,^[6] HfO₂,^[7] and so on. Among these high-k dielectrics, the HfO₂ gate dielectric has a high dielectric constant (18–20). However, HfO₂ has a smaller bandgap of 5.45 eV than other high-k dielectrics.^[8] In comparison, Al₂O₃ has a larger band gap of 9 eV, stronger adhesion to many surfaces of material, and serves as a protection layer due to its stabilities against moisture. In recent studies, an HfO₂/InAlAs gate stack has been reported in relation to the surface passivation chemistries of InAlAs.^[9–13] However, interfacial and electrical characteristics of HfO₂/InAlAs MOS have been rarely reported.

In this work, we investigate the interfacial and electrical properties of the ALD-Al₂O₃ and ALD-HfO₂ gate dielectrics on the InAlAs epitaxial layer. Compared with that of the HfO₂/InAlAs capacitor, the capacitance–voltage (C–V) characteristic of the Al₂O₃/InAlAs capacitor is significantly improved, and in particular, the stretch-out of C–V curve for the Al₂O₃/InAlAs capacitor is considerably shrunk. The interface study is done by angle-resolved x-ray photoelectron spectroscopy (AR-XPS) and the electrical characterizations based on the C–V hysteresis, current density–voltage (J–V), and charge-trapping behavior of Al₂O₃ gate stacks on InAlAs are performed to evaluate its potential as an alternative gate dielectric for the InAlAs MOS capacitor.

In this work, a 1.5-μm n-type In_0.5Al_0.5As layer (Si doped to ∼ 1 × 10¹⁷ cm⁻³) was epitaxially grown on a semi-insulating n-type GaAs substrate. Prior to deposition, the samples were first rinsed by using acetone and deionized (DI) water, then soaked in HCl (37%) for 1 min for native oxide removal, and finally dipped in (NH4)₂S (10%) for 15 min to remove elemental As and In, cleaned in deionized water for 3 min and dried in N₂. Two types of ALD dielectrics, i.e., (I) Al₂O₃ (10 nm) and (II) HfO₂ (10 nm) film (sample I: Al₂O₃/InAlAs MOS capacitor, and sample II: HfO₂/InAlAs MOS capacitor), were used for fabricating the high-k/InAlAs MOS capacitors. The ALD-Al₂O₃ film was deposited at 245 °C by using tri(methyl)aluminum (TMA-Al) and H₂O as the precursors. The ALD-HfO₂ film was deposited at 245 °C including an alternating pulse of tetrakis-(ethyl-methyl-amino)-hafnium (TEMA-Hf) and H₂O used as the precursors. Post deposition annealing (PDA) was carried out at 380 °C for 1 min under N₂ ambient by rapid thermal annealing (RTA). Finally, an MOS capacitor structure was produced by sputtering Ti/Pt/Au (20 nm/20 nm/200 nm) to form the gate electrode with an area of 1 × 10⁻⁴ cm².

All the C–V hysteresis measurements were carried out at a high frequency of 1 MHz at room temperature (295 K). AR-XPS was performed by using a monochromated Al–Kα x-ray source (1486.6 eV). The C 1 s line with a bonding energy of 284.6 eV was used as a reference to eliminate the charge effect during the analysis. AR-XPS is a useful tool for profiling depth in a substrate or a hetero-structure. If necessary, the photoelectron take-off angle, θ (defined as the elevation angle according to the sample surface), was changed by tilting the sample to change the photoelectron escape depth, t, in accordance with

To quantify the density of interfacial trapped charge in high-k oxides on III–V surfaces, the C–V hysteresis measurements are executed at a high frequency (1 MHz) and room temperature (295 K), and the results are shown in Fig. 1.

Fig. 1.

	Figure Option ViewDownloadNew Window
	Fig. 1. C–V characteristics of samples (I) Al2O3/n-InAlAs and (II) HfO2/n-InAlAs, measured at a frequency of 1 MHz.

The upwards line represents the C–V curve measured by the gate voltage swept from −2.0 V to 2.0 V for the Al₂O₃/InAlAs capacitor and from −2.0 V to 2.0 V for HfO₂/InAlAs capacitor respectively, and the downwards line shows the C–V curve contained by the gate voltage swept in the opposite direction. There are obvious accumulation and depletion regions for both MOS capacitors. The hysteresis is likely to be caused by charges trapped at the high-κ/InAlAs interface as well as in the high-κ layer. The C–V curves for sample I show reducing stretch-out compared with those for sample II, which implies that the interfacial trapped charge densities for sample I are lower than those for sample II. In addition, as can be seen from Fig. 1, the difference in the flat-band voltage (ΔV) of C–V hysteresis for sample I is 0.02 V, which is smaller than the ΔV of 0.16 V for sample II. The value of Q_ot (cm⁻²) can be calculated from the following formula through using the C–V measurements:

Table 1.

Table 1.

Table 1. Electrical properties of MOS capacitors with an area of 100 μm×100 μm for the Al2O3 and HfO2 layers. . Dielectric material Vhys/V Cox/(μF/cm2) Qot/cm−2 Al2O3 layer 0.02 0.3599 7.1980×1010 HfO2 layer 0.16 0.5478 8.7648×1011

Table 1. Electrical properties of MOS capacitors with an area of 100 μm×100 μm for the Al2O3 and HfO2 layers. .

In order to further study the interface quality of the two samples, the interface state density D_it is analyzed. D_it can be extracted from the C–V curves by the Terman method ^[14] as follows:

Fig. 2.

	Figure Option ViewDownloadNew Window
	Fig. 2. Curves of Dit versus Et–Ei for sample I: Al2O3/InAlAs and sample II: HfO2/InAlAs.

The midgap D_it for sample I is 2.5×10¹¹ cm⁻²·eV⁻¹, while the value of D_it for sample II is 3×10¹² cm⁻²·eV⁻¹. It is found that the interface trap density for sample I is lower than that for sample II, which means that the Al₂O₃ layer can effectively reduce the interface trap density, and thus improve the interfacial characteristics on the InAlAs layer. Moreover, Al₂O₃ has better lattice matching with the InAlAs epitaxial layer, which may contribute to the low dangling bonds, such as In–, As–, O–. The Al₂O₃ is known for its high-temperature stability against recrystallization. But HfO₂ forms polycrystalline after annealing 600 °C from the amorphous film. Thus HfO₂ is much weaker especially in the temperature stability. This indicates that a thermally stable and reliable Al₂O₃/InAlAs capacitor has been obtained.

To investigate the interfacial properties of the HfO₂ and Al₂O₃ stack films on InAlAs, the chemical states are analyzed by AR-XPS, and the spectra are fitted to Gaussian–Lorentzian functions. Figure 3(a) shows the Al 2p XPS photoelectron spectra obtained from 380 °C annealing for sample I at different take-off angle θ. For sample I, the Al 2p spectrum shows the peak at 74.2 eV, which indicates the formation of Al₂O₃ at θ = 90°. It is shown that there is no obvious change in the peak of Al 2p at θ = 10° and 30°, which suggests that there is no detectable chemical reaction in the Al₂O₃ layer. This is also confirmed by the O1s XPS photoelectron spectra shown in Fig. 5, which shows that no other aluminum compounds are formed in the Al₂O₃ layer. The XPS probing on the Hf 4f of sample II is shown in Fig. 3(b). For sample II, the Hf 4f spectrum shows two components 4f_5/2 at 18.18 eV and 4f_7/2 at 16.48 eV with a spin–orbital splitting of 1.86 eV at θ = 10°, indicating the formation of HfO₂.^[15] Figure 3(b) shows that the peaks shift to higher binding energies of 18.41 eV and 16.76 eV in the case of θ=30° and 90° uniformly. The binding energies of Hf 4f_7/2 and 4f_5/2 in the HfAlO film are 16.76 eV and 18.41 eV respectively,^[16] therefore it is concluded that a thin Hf–O–Al mixture layer is formed at the interface of samples II. This is nicely consistent with the changes in the O 1s spectra in Fig. 5.

Fig. 3.

	Figure Option ViewDownloadNew Window
	Fig. 3. XPS spectra of (a) Al 2p in sample I and (b) Hf 4f in sample II at different take-off angle θ (10°, 30°, and 90°).

To further investigate the interfaces of sample I and sample II, XPS analyses are performed on elements O and As in the gate dielectric structures. Figure 4 gives the information about the As 3d core level spectra for HfO₂ and Al₂O₃ on the n-InAlAs at the different values of take-off angle θ.

Fig. 4.

	Figure Option ViewDownloadNew Window
	Fig. 4. As 3d photoelectron spectra of sample I and sample II at the different values of take-off angle θ (10°, 30°, and 90°).

Figure 5 shows the O 1s core level spectra for sample I and sample II at θ = 90°. In Fig. 4, neither As– nor As–O is detected at θ = 10°, and only a little As–O is detected at θ = 30° for sample I, which reveals that Al₂O₃ can be effective in lowering the diffusion of As for sample I. Meanwhile for sample II, a great amount of As₂O₃ is seen at θ = 10° and 30°, which suggests that As has great ability to diffuse in the HfO₂ gate dielectric. Figure 5 reveals that a certain number of O dangling bonds are shown in the HfO₂ sample, and it is the reason for forming interface trap density.^[17,18]

Fig. 5.

	Figure Option ViewDownloadNew Window
	Fig. 5. O1s photoelectron spectra at the take-off angle θ = 90° for sample (I) Al2O3/InAlAs capacitor, and sample (II) HfO2/InAlAs capacitor.

The gate leakage currents of sample I and sample II can be seen in Fig. 6. At a gate voltage of +1 V, the gate leakages of the Al₂O₃ and HfO₂ samples are 1.37×10⁻⁶ A/cm² and 3.22 × 10⁻⁶ A/cm², respectively. The result indicates that sample I shows an appreciably lower gate leakage current than sample II. The gate leakage line is well fitted by the Fowler–Nordheim (FN) tunneling model (data not shown), thus the FN tunneling is the dominant current-conduction mechanism for sample I. The FN tunneling conduction demands a low density of oxide traps and a big band offset, which are required by the large bandgap and the high-quality oxide film of Al₂O₃. The decrease of leakage current density of sample I may be attributed to high-thermodynamic stability, high quality, robustness and large conduction band offset between Al₂O₃ and InAlAs. The Al₂O₃ is known for its high temperature stability against recrystallization and its large measured conduction band offset with many semiconductor materials; while the higher gate leakage current of sample II may be explained by the smaller band-gap of HfO₂ and the change from amorphous HfO₂ films to polycrystalline after 600 °C annealing. HfO₂ is much weaker especially in the temperature stability. Moreover, the poor lattice-matching and interfacial quality between HfO₂ and InAlAs assist the carriers in tunneling the gate dielectric to form the high leakage current.

Fig. 6.

	Figure Option ViewDownloadNew Window
	Fig. 6. J–V characteristics for sample I: Al2O3/InAlAs and sample II: HfO2/InAlAs.

In this work, the characteristics of the ALD-Al₂O₃/InAlAs and ALD-HfO₂/InAlAs MOS capacitors are investigated by AR-XPS. It is found that the Al₂O₃ layer can prevent the As 3d from diffusing, and the interfacial oxidation and trap charge from forming. Moreover, the C–V hysteresis measurements are carried out at a high frequency (1 MHz) and room temperature (295 K). Compared with the HfO₂/InAlAs MOS capacitor, the Al₂O₃/InAlAs MOS capacitor exhibits good electrical properties in narrowing down the hysteresis loop, shrinking stretch-out of C–V characteristics, and significantly reducing the oxide trapped charge density (Q_ot) and the interface state density (D_it). Additionally, the gate leakage current densities of 1.37 × 10⁻⁶ A/cm² and 3.22 × 10⁻⁶ A/cm² at +1 V for the Al₂O₃/InAlAs and HfO₂/InAlAs capacitors respectively also prove that there are better electrical properties for the Al₂O₃/InAlAs capacitor. Therefore, the Al₂O₃/InAlAs MOS capacitor can make the device achieve good performances.