Lingyan Shen

Improvement of Al2O3 Films on Graphene Grown by Atomic Layer Deposition with Pre-H2O Treatment

We improve the surface of graphene by atomic layer deposition (ALD) without the assistance of a transition layer or surface functionalization. By controlling gas-solid physical adsorption between water molecules and graphene through the optimization of pre-H2O treatment and two-step temperature growth, we directly grew uniform and compact Al2O3 films onto graphene by ALD. Al2O3 films, deposited with 4-cycle pre-H2O treatment and 100-200 °C two-step growing process, presented a relative permittivity of 7.2 and a breakdown critical electrical field of 9 MV/cm. Moreover, the deposition of Al2O3 did not introduce any detective defects or disorders in graphene.

HfO2 dielectric film growth directly on graphene by H2O-based atomic layer deposition

Due to its exceptionally high carrier mobility, International Technology Roadmap for Semiconductors considers graphene to be among the candidate materials for postsilicon electronics. In order to realize graphene-based devices, thin and uniform-coverage high-κ dielectrics without any pinholes on top of graphene is required. There are no dangling bonds on defect-free graphene surface; it is difficult to grow uniform-coverage high-κ dielectrics on graphene directly by atom layer deposition. Meanwhile, degradation of defects in graphene/high-κ structure is necessary for the optimization of high-κ dielectrics fabrication technology. Here the authors report on a H2O-based atom layer deposition method used for HfO2 growth, where physically adsorbed H2O molecules on graphene surface act as oxidant, and self-limit react with metal precursors to form HfO2 film onto graphene directly. Raman spectra reveal H2O-based atom layer deposition method will not introduce defects into graphene. The surface root mean square o...

Direct growth of Sb2Te3 on graphene by atomic layer deposition

The direct growth of Sb2Te3 on graphene is achieved by atomic layer deposition (ALD) with pre-(Me3Si)2Te treatment. The results of atomic force microscopy (AFM) indicate Volmer–Weber island growth is the dominant growth mode for ALD Sb2Te3 growth on graphene. High resolution transmission electron microscopy (HRTEM) analysis reveals perfect crystal structures of Sb2Te3 on graphene and no interface layer generation. The characterization of X-ray photoelectron spectroscopy (XPS) implies the impermeability of graphene can maintain Sb2Te3 intact and isolate the adverse effects of substrates. Our study provides a step forward to grow high quality Sb2Te3 at low temperature and expand the potential applications of graphene in ALD techniques.

Low-temperature plasma-enhanced atomic layer deposition of HfO2/Al2O3 nanolaminate structure on Si

HfO2/Al2O3 nanolaminate was deposited on a Si substrate by plasma-enhanced atomic layer deposition at 150 °C with in situ plasma treatment. Unilayer HfO2 and Al2O3 films were prepared for comparison. Films were treated by rapid thermal annealing at 870 °C in a nitrogen atmosphere for 30 s. Al atoms in the HfO2/Al2O3 nanolaminate diffuse into HfO2 layers during rapid thermal annealing, facilitating the formation of tetragonal HfO2. The HfO2/Al2O3 nanolaminate has an effective dielectric constant of 20.7, a breakdown electric field of 7.4 MV/cm, and leakage current of 2.3 × 10−5 mA/cm2 at a gate bias of Vg = −1 V. The valence band offset, conduction band offset, and the band gap of the film are 2.75, 1.96, and 5.83 eV, respectively.

Al2O3–Gd2O3 double-films grown on graphene directly by H2O-assisted atomic layer deposition

We demonstrate the direct Al2O3–Gd2O3 double-films growth on graphene by H2O-assisted atomic layer deposition (ALD) using a hexamethyl disilazane precursor {Gd[N(SiMe3)2]3}. No defects are brought into graphene as shown by Raman spectra; the surface root-mean-square (RMS) roughness of the Al2O3–Gd2O3 double-films is down to 0.8 nm, comparable with the morphology of pristine graphene; the films are compact and continuous, and the relative permittivity is around 11, which indicate that H2O-assisted ALD can prepare high quality dielectric films on graphene.

Interface engineering of an AlNO/AlGaN/GaN MIS diode induced by PEALD alternate insertion of AlN in Al2O3

In this paper, AlNO nano-films have been deposited on an AlGaN/GaN heterojunction by alternating growth of AlN and Al2O3 using plasma enhanced atomic layer deposition (PEALD). With optimized AlN layer insertion in Al2O3, the oxygen is effectively blocked from diffusing to the AlGaN surface and the formation of detrimental Ga–O bonds is significantly suppressed. Owing to the negative fixed charges in Al2O3, provided by the incorporated nitrogen, the flat band voltage (Vfb) of the AlNO/AlGaN/GaN metal–insulator–semiconductor (MIS) diode exhibits a positive shift of 1.50 V, compared with the Al2O3/AlGaN/GaN MIS diode. Markedly reduced hysteresis and frequency-dispersion in the C–V characteristics have also been observed at the AlNO/AlGaN interface. Furthermore, the interface states density (Nit) at the AlNO/AlGaN interface has been reduced by one order of magnitude compared with the Nit at the Al2O3/AlGaN interface, and the border traps density (Nbt) near the AlNO/AlGaN interface is also identified to be reduced by the insertion of AlN layers into Al2O3. The PEALD induced optimization of AlNO deposition on the AlGaN/GaN heterojunction provides a pathway to the fabrication of AlGaN/GaN high electron mobility transistors (HEMTs) with low interface trap density.

Properties of HfO2/La2O3 nanolaminate films grown on an AlGaN/GaN heterostructure by plasma enhanced atomic layer deposition

HfO2/La2O3 nanolaminate (HLOL) films were grown on AlGaN/GaN heterostructure by plasma enhanced atomic layer deposition (PEALD). After high-temperature rapid thermal annealing (RTA), the properties of the films were characterized. Si (coming from the La2O3 metal precursor) and La atoms diffuse into the HfO2 layer, cubic and tetragonal phase HfO2 exist in the HLOL film. The HLOL film has an effective dielectric constant of 35.7 and a breakdown electric field of 5.5 MV cm−1. Moreover, the HLOL film is an effective barrier to suppress the gate leakage current, which is 5 orders of magnitude lower compared to a conventional Schottky diode. The HLOL dielectric film on AlGaN/GaN could also improve the gate voltage swing range.

Negative differential resistance in the I–V curves of Al2O3/AlGaN/GaN MIS structures

Negative differential resistance (NDR) induced by inter-valley electron transfer is often observed in Gunn diodes. In this paper, we report the observation of this novel NDR phenomenon in Al2O3/AlGaN/GaN metal–insulator–semiconductor (MIS) structures. This study offers new understanding on the gate characteristics of GaN HEMTs. The NDR is found to be more prominent at low temperature. The voltage (VNDR) at the onset of the NDR is temperature dependent and decreases with temperature. Measurement results support the possibility that the NDR originates from the inter-valley electron transfer in conjunction with tunneling. The fitting results of the measured I–V with tunneling models reveal that TAT (trap assisted tunneling) is the dominating mechanism at low gate bias and FNT (Fowler–Nordheim tunneling) is dominant at relatively high gate voltage. Moreover, the energy difference between valley Γ and valley M–L in the GaN conduction band can be estimated by the voltage (VNDR), which is linearly related to the crystal temperature.

Effects of rapid thermal annealing on the properties of HfO2/La2O3 nanolaminate films deposited by plasma enhanced atomic layer deposition

In this work, HfO2/La2O3 nanolaminate films were deposited on Si substrates by plasma enhanced atomic layer deposition with in situ plasma treatment. Different annealing treatments were adopted to change films structure and performance. The upper HfO2 layers in HfO2/La2O3 nanolaminates were easily crystallized after annealing at 800 °C, while all the La2O3 layers kept amorphous. X-ray photoelectron spectroscopy results indicated that LaO(OH) and La(OH)3 peaks became weak, H2O molecules in laminates evaporated during high-temperature annealing. Band diagram analysis showed that valence band offset and band gap widened after 800 °C annealing. Annealing, especially 800 °C annealing, had gentle effect on leakage current, but could obviously change capacitance and permittivity due to tetragonal and cubic phase formed in the HfO2 film.

Performance Improvement and Current Collapse Suppression of Al 2 O 3 /AlGaN/GaN HEMTs Achieved by Fluorinated Graphene Passivation

In this letter, fluorinated graphene (FG) is utilized to passivate GaN surface for a metal–insulator–semiconductor high electron mobility transistor (MIS HEMT). The FG-MIS HEMT achieves better DC characteristics than a traditional MIS HEMT, including larger saturation drain current density (34.3%), higher peak trans-conductance (14.4%), lower ON-resistance (21.6%), and lower off-state leakage. Moreover, current collapse measurement reveals that not only can FG suppress the drain saturation current reduction of MIS HEMT from 41.8% to 8.1% at off-state drain bias of 50 V, but also it prevents dynamic ON-resistance increasing with off-state stress. The coverage of FG on GaN surface can prevent GaN being oxidized and N diffusion from GaN during gate dielectric deposition, thus suppressing the formation of Ga-O bonds and Ga dangling bonds, leading to an excellent interface condition for Al2O3/GaN with reduced fixed interface charges. Therefore, significant passivation effect is achieved.