Band Offsets and Interfacial Properties of HfAlO Gate Dielectric Grown on InP by Atomic Layer Deposition
© The Author(s). 2017
Received: 27 December 2016
Accepted: 22 April 2017
Published: 8 May 2017
X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy have been used to determine interfacial properties of HfO2 and HfAlO gate dielectrics grown on InP by atomic layer deposition. An undesirable interfacial InPxOy layer is easily formed at the HfO2/InP interface, which can severely degrade the electrical performance. However, an abrupt interface can be achieved when the growth of the HfAlO dielectric on InP starts with an ultrathin Al2O3 layer. The valence and conduction band offsets for HfAlO/InP heterojunctions have been determined to be 1.87 ± 0.1 and 2.83 ± 0.1 eV, respectively. These advantages make HfAlO a potential dielectric for InP MOSFETs.
KeywordsBand alignments HfAlO dielectric InP Atomic layer deposition
As silicon-based complementary metal-oxide-semiconductor (CMOS) devices approach their fundamental limits when scaled down, the adoption of new technologies is urgently required to meet the demands for higher performance and less power dissipation integrated circuits. The integration of III–V compound semiconductors with high-k gate dielectrics is the leading candidate to address many of these issues [1–6]. III–V compound semiconductors including GaAs, InGaAs, and InP have drawn quite a lot of attention as alternative channel materials due to their high electron mobility and low effective mass over Si. Of these candidates, InP could be promising because it is a material with weak Fermi-level pinning effect and has a high electron saturation velocity (2.5 × 107 cm/s) . However, there are still some bottlenecks impeding the actual implementation of InP channel material. One of the major challenges is finding thermodynamically stable dielectric on InP surface with good interfacial properties like SiO2/Si counterpart . Various chemical treatments have been extensively explored to achieve effective passivation on the InP surface. Such methods include NH4OH, (NH4)2S, H2S, and F treatment [7, 9–11].
Band alignment of high-k/InP interface is of great importance for InP-based MOSFET researches. Recently, Chou et al. measured the band offset between InP (100) and ALD Al2O3 with various passivation methods using internal photoemission (IPE) . The barrier heights from the top of InP valence band (VB) to the bottom of Al2O3 conduction band (CB) and conduction band offset were determined to be 4.05 and 2.7 ± 0.10 eV, respectively. With a high k of ~20–25, HfO2 has been extensively studied as an alternative high-k gate dielectric both in Si-based and III–V technology. The barrier height at HfO2/InP interface is also measured using IPE by Xu et al. to be 3.89 eV for the sample without passivation treatment . To improve the interfacial and thermal properties, gate dielectric engineering has been successfully adopted in high-k/III–V compound semiconductor technology by using a combination of Al2O3 and HfO2 films recently. Kim et al. reported that the In out-diffusion and the subsequent In-related phase generation can be effectively suppressed by introducing Al2O3 to the HfO2 film grown on InP. Moreover, intermixing of Al2O3 with HfO2 to form HfAlO on InP is expected to adjust the band offset. In this study, HfAlO gate dielectrics were grown on InP substrates using alternative cycles atomic layer deposition (ALD) of Al2O3 and HfO2 starting with an ultrathin Al2O layer. ALD has manifested itself as a technique suitable for the semiconductor industry due to its capability of growing uniform and conformal thin films . Accurate controls over chemical stoichiometry and thickness at atomic scale can also be achieved by ALD. The ultrathin Al2O3 layer was introduced to the interface between HfAlO dielectrics and InP substrate to diminish the undesirable interfacial InPxOy layer. The band offset and interfacial properties of the formed HfAlO gate dielectric grown on InP has been investigated using x-ray photoelectron spectroscopy (XPS). The energy band diagrams of the HfAlO/InP heterojunction is then constructed, which can provide vital information on fabricating InP MOSFET.
The heterostructures were prepared on n-type (100) InP wafers with a doping concentration of ~1 × 1017 cm−3. Prior to the ALD process, the InP substrates were degreased using acetone and methanol for 5 min each, followed by a diluted 2% hydrofluoric acid (HF) solution etching for 1 min to remove native oxide. After cleaning, the substrate was immediately transferred to a BENEQ TFS-200 ALD reactor where HfO2 and HfAlO thin films were prepared at 300 °C. Thin HfAlO film was grown using alternative cycles ALD of Al2O3 and HfO2 starting with Al2O3. The precursors used were trimethylaluminum (TMA) and H2O for Al2O3 and tetrakis(ethylmethylamido)hafnium (TEMAH) and H2O for HfO2. Four sets of samples were prepared for XPS measurements: (1) a 4 nm thick HfO2 film grown on InP substrate to detect the interface property of the HfO2/InP heterojunction; (2) a 30 nm thick HfAlO film grown on InP substrate to measure the valence band maximum (VBM) and Al 2p 3/2 of bulk HfAlO; (3) a 4 nm thick HfAlO film grown on InP substrate to determine the energy difference between Al 2p 3/2 and In 3d 5/2 at the HfAlO/InP heterojunction’s interface; and (4) a clean InP substrate (HF-dipped) to measure the VBM and In 3d 5/2 of bulk InP.
High-resolution transmission electron microscopy (HR-TEM) was used to obtain the images of high-k/InP interface at atomic scale. Both TEM with low and high magnification were performed to investigate the interfacial profile and fringe atom arrangement. Accurate film thicknesses can also be measured directly. The XPS spectra were recorded using a Thermo Scientific ESCALAB 250 XPS system equipped with a monochromatic Al Kα source (hv = 1486.6 eV). The source power is 150 W (15 kV × 10 mA) at a takeoff angle of 90°. Scans with a step of 0.05 eV and pass energy of 20 eV were performed for 20 times for binding energy of specific elements. The chemical compositions for the prepared HfAlO thin film were detected to be 14.23% for Hf, 22.36% for Al, and 62.55% for O, respectively. Valence band scans with a step of 0.01 eV and pass energy of 20 eV were performed for 30 times for valence band spectra. Charge correction was performed using the known position of C 1s spectra at 284.8 eV. The XPS spectrometer energy scale was also calibrated using Cu 2p 3/2, Ag 3d 5/2, and Au 4f 7/2 photoelectron lines located at 932.67, 368.26, and 83.98 eV, respectively.
Results and Discussion
Binding energies (in eV) of core level spectra for all the samples and the valence band maximum (VBM) values (in eV) for the bulk InP and HfAlO (30 nm)/InP samples
HfAlO (30 nm)/InP
HfAlO (4 nm)/InP
In 3d 5/2
Hf 4f 7/2
Al 2p 3/2
0.79 ± 0.05
2.89 ± 0.05
The energy band gaps (E g) measured by spectroscopic ellipsometry are 5.70 and 6.04 eV for as-deposited HfO2 and HfAlO films, respectively. Using 1.34 eV energy gap for InP , the ΔE c at the HfAlO /InP interface is thus calculated to be 2.83 ± 0.1 eV. According to the investigation above, both of the ΔE v and ΔE c values are larger than 1 eV, which means the HfAlO film supplies enough barrier heights for both electrons and holes. Compared with the ΔE c extracted to be 2.55 eV at the interface of HfO2/InP by Xu et al. , the ΔE c for HfAlO/InP is a little larger. Furthermore, the ΔE v at the interface of HfO2/InP can also be calculated to be 1.81 eV using equation (2), which is a little smaller than that of HfAlO/InP heterojunction. As a result, the dielectric HfAlO film can suppress both the electrons induced and holes induced leakage current more effectively than the HfO2 film when InP is chosen as channel material. Considering the better interface condition and higher potential barrier heights, HfAlO can be relatively preferable than HfO2 as the dielectric film for InP.
In summary, interfacial properties for HfO2/InP and HfAlO/InP heterojunctions have been studied by XPS and HR-TEM. When HfO2 is deposited on InP substrate by ALD, an undesirable interfacial InPxOy layer is easily formed at the HfO2/InP interface, which will degrade the electrical properties. Fortunately, a thin Al2O3 layer can act as a barrier layer to form an abrupt interface between HfAlO and InP channel layer. The band alignment at the interface of HfAlO/InP heterojunction was studied, and the VBM for HfAlO/InP is 1.87 ± 0.1 eV at interface, leading to a ΔE c of 2.83 ± 0.1 eV for HfAlO/InP based on the analysis. This result indicates that HfAlO dielectric film can supply sufficient barrier heights for holes and electrons when InP is chosen as channel material. More researches are needed to make further efforts to optimize the heterojunction’s interface condition.
This work was supported by the National Natural Science Foundation of China (No. U1632121, 51102048, and 61376008), SRFDP (No. 20110071120017), and the Innovation Program of Shanghai Municipal Education Commission (14ZZ004).
LFY and TW carried out the deposition along with its characterization (XPS, TEM). YZ participated in the characterization. LFY, TW, YZ, and HLL participated in the analysis and discussion of the results obtained from the experiments. HLL supervised this study. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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