Nb2O5 thin film deposited by direct liquid injection-chemical vapor deposition method using Nb(OC2H5)5 precursor
- Nb2O5 thin film deposited by direct liquid injection-chemical vapor deposition method using Nb(OC2H5)5 precursor
- Bharti, Dinesh Chand
- Date Issued
- The niobium (V) oxide (Nb2O5) is emerging as an advance material for wide range of applications due to its excellent chemical and physical properties. It has attracted much attention as a promising candidate for the future dynamic random access memory and complementary metal oxide semiconductor devices, owing to its high‐κ value of up to 200. The literature studies show that the electrical properties of Nb2O5 are very sensitive to the synthetic routes and deposition conditions. There are several factors that affect the quality of niobium oxide film such as composition (a mixed phase), interface (lattice mismatch, native oxide and transition layer), defect states (oxygen vacancy, interstitial oxygen or metal) and crystal structure (polymorph, texture and grain size) etc. A correlation study between bulk composition and electrical properties has never been reported. In this work, we investigated the effect of growth conditions on film properties. In more details, the influence of reactant’s molar ratios [oxygen: precursor] and deposition temperatures on growth rate, stoichiometry, crystal structure, morphology, dielectric constant and leakage current were studied. The niobium oxide thin films were deposited onto Si (100) substrate by direct liquid injection-chemical vapor deposition method using Nb(OC2H5)5 precursor. Two growth regimes were observed as a function of deposition temperature. The maximum growth rate was observed at a lower oxygen/precursor molar ratio of 150 and deposition temperature of 360 °C. The X-ray photoelectron spectroscopic studies revealed that a mixed phase of niobium oxides (NbO, NbO2 and Nb2O5) in the bulk. The amount of each phase was quite substantial. The higher deposition temperature and oxygen molar ratio results into interface reaction. The films deposited at 340 and 360 °C were amorphous. Therefore, the X-ray diffraction pattern showed the broad peaks. Increasing of oxygen/precursor molar ratios did not help in crystallinity, whereas the grains were formed. Therefore, the grains were weakly crystalline. The crystalline film was obtained at 400 °C and film’s crystallinity was decreased with increase in oxygen molar ratio. The atomic force microscopic image showed a smooth surface for an amorphous film and grains for a weakly crystalline film. The surface roughness of weakly crystalline and crystalline films was very sensitive to the deposition temperatures and reactant’s molar ratios. The capacitance-voltage curve showed a positive shift in flat band voltage was due to excess oxygen and defect state. The capacitance-voltage hysteresis was attributed to the mixed phases of niobium oxide and remained similar as function of deposition condition. The current-voltage curve showed the higher leakage current density for higher surface roughness. It was found that the decrease in surface roughness improved the leakage current. The films dielectric constant was improved by increasing the oxygen ratios. At molar ratio of 150, a high dielectric constant value of 47 at 340 °C was obtained for an amorphous film and corresponding leakage current density was 2.0×10-5 A/cm2 at 1 MV/cm. No break down voltage was observed for any of sample.
Further, we investigated the effect of post annealing temperature on composition, crystal structure and electrical property of Nb2O5 in order to obtain a pure crystalline phase. It was monitored that initial growth condition affects the overall film’s properties. The film was deposited at 340 °C under different oxygen ratio R0, R75 and R600 and subsequently post annealed at 500, 600 and 700 °C for 1 hour in Ar gas atmosphere. The films were also deposited at 360 and 400 °C and annealed under same condition for comparison. The film was deposited at 340 °C under R75 and annealed at 500 °C exhibited the mixed states of niobium (Nb2+, Nb4+ and Nb5+)
this is due to presence of amorphous layer. Therefore, film was a mixture of amorphous and crystalline phases. While annealing at 600 °C revealed a pure state of niobium (Nb5+) and pattern did not change as a function of sputtering time. Similarly a single state of niobium (Nb5+) was obtained at 700 °C. In contrast, the film was deposited under higher oxygen/precursor molar ratio of 600 (R600) showed a complex multiplet structure of mixed states of niobium (Nb2+, Nb4+ and Nb5+) at annealing temperatures of 500, 600 and 700 °C. The X-ray photoelectron spectroscopic studies revealed that the higher oxygen ratio during deposition includes excess interstitial oxygen at interface and these intend to form an amorphous transition layer and also affected the overall morphology of film. All the films were crystallized into orthorhombic Nb2O5 (T-phase) as a function of annealing temperature. However, crystalline films showed the textured growth depending on oxygen molar ratios during the deposition. The film was deposited at 340 °C under R75 and annealed at 600 °C exhibited a predominant a‐axis (200) texture of Nb2O5 orthorhombic structure. While, the film was deposited at 340 °C under R600 and annealed at same condition showed a predominant c‐axis (001) texture. The annealing temperature of 700 °C yielded always randomly oriented films. The surface roughness was increased with an increase in grain size of a crystalline film. The film was exhibited a completely different morphology due to high temperature and random orientation at post annealing temperature of 700 °C. The film was grown at 340 °C under R75 and post annealed at 600 °C completely improved the capacitance-voltage hysteresis, which was observed in as deposited film. As evident from X-ray photoelectron spectroscopic analysis, a mixed state of Nb (Nb2+, Nb4+ and Nb5+) was obtained for an amorphous film and a pure crystalline Nb2O5 film (Nb5+) at post annealing temperature of 600 °C. The current‐voltage curve showed a low leakage current for an amorphous film and increased one order for a crystalline film at 600 °C, which was due to crystallinity and the increase in surface roughness. The high dielectric constant value of 51 was obtained for a pure crystalline film and corresponding leakage current density was 2.7×10-5 A/cm2 at 1 MV/cm respectively.
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