xN Thin Films Research Articles

Structural, optical and electrical properties of undoped and Si-doped Al xGa 1− xN thin films on Si (1 1 1) substrate grown by PA-MBE

We have studied the structural properties of undoped and Si-doped Al x Ga 1− x N/GaN/AlN on Si (1 1 1) substrate prepared by plasma-assisted molecular beam epitaxy (PA-MBE) using high-resolution X-ray diffraction (HR-XRD) and atomic force microscopy (AFM). In comparison with undoped AlGaN, the roughness and dislocation density on the surface of the AlGaN layer decrease with Si doping. Full width half maximum (FWHM) of the undoped and Si-doped samples were equal to 0.69° and 0.52°, respectively. This indicates that the Si doping improves the crystalline quality of the Al x Ga 1− x N layer compared with the undoped one. Raman scattering measurement reveals that the optical phonon modes of A 1(LO) and E 2(H) of the AlGaN show a one-mode and two-modes behavior, respectively. The Fourier-transform infrared reflectance (FTIR) investigation confirms the one-mode (two-mode) behavior of the LO (TO) phonon in our samples. This is in good agreement with Raman measurement. Finally, the barrier height ( Φ B ) of undoped and Si-doped Al x Ga 1− x N samples was found to be 0.86 and 0.74 eV, respectively.

Standard‐free composition measurements of Alx In1–xN by low‐loss electron energy loss spectroscopy

AbstractWe demonstrate a standard‐free method to retrieve compositional information in Alx In1–xN thin films by measuring the bulk plasmon energy (Ep), employing electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM). Two series of samples were grown by magnetron sputter epitaxy (MSE) and metal organic vapor phase epitaxy (MOVPE), which together cover the full com‐ positional range 0 ≤ x ≤ 1. Complementary compositional measurements were obtained using Rutherford backscattering spectroscopy (RBS) and the lattice parameters were obtained by X‐ray diffraction (XRD). It is shown that Ep follows a linear relation with respect to composition and lattice parameter between the alloying elements from AlN to InN allowing for straightforward compositional analysis. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Increased electromechanical coupling in w−ScxAl1−xN

AlN is challenged as the material choice in important thin film electroacoustic devices for modern wireless communication applications. We present the promise of superior electromechanical coupling (kt2), in w−ScxAl1−xN by studying its dielectric properties. w−ScxAl1−xN (0≤x≤0.3) thin films grown by dual reactive magnetron sputtering exhibited low dielectric losses along with minor increased dielectric constant (ε). Ellipsometry measurements of the high frequency ε showed good agreement with density function perturbation calculations. Our data show that kt2 will improve from 7% to 10% by alloying AlN with up to 20 mol % ScN.

The effect of annealing on the surface morphology of strained and unstrained In xAl 1− xN thin films

In x Al 1− x N is a particularly useful group-III nitride alloy because by adjusting its composition it can be lattice matched to GaN. Such lattice-matched layers may find application in distributed Bragg reflectors (DBRs) and high electron mobility transistors (HEMTs). However, compared with other semiconducting nitride alloys, In x Al 1- x N has not been researched extensively. In this study, thin In x Al 1− x N epilayers were grown by metal-organic vapour phase epitaxy (MOVPE) on GaN and Al y Ga 1− y N layers. Samples were subjected to annealing at their growth temperature of 790 °C for varying lengths of time, or alternatively to a temperature ramp to 1000 °C. Their subsequent surface morphologies were analysed by atomic force microscopy (AFM). For both unstrained In x Al 1− x N epilayers grown on GaN and compressively strained epilayers grown on Al y Ga 1− y N, surface features and fissures were seen to develop as a consequence of thermal treatment, resulting in surface roughening. It is possible that these features are caused by the loss of In-rich material formed on spinodal decomposition. Additionally, trends seen in the strained In x Al 1− x N layers may suggest that the presence of biaxial strain stabilises the alloy by suppressing the spinode and shifting it to higher indium compositions.

Preparation of scandium aluminum nitride thin films by using scandium aluminum alloy sputtering target and design of experiments

Scandium aluminum nitride alloy (ScxAl1−xN) thin films were prepared using rf magnetron sputtering with a scandium aluminum alloy (Sc0.42Al0.58) target on n-type (100) silicon substrates. We have investigated the effects of 4 sputtering control factors, which are substrate temperature, sputtering pressure, nitrogen (N2) concentration and cathode power, on the piezoelectric constant d33 of ScxAl1−xN films using design of experiments. Consequently, it is statistically proved that N2 concentration in sputtering gas is the most important control factor. The piezoelectric constant d33 indicates the maximum value of 19.0 pC/N at N2 concentration of 25%. The composition of ScxAl1−xN films prepared under optimized sputtering conditions is Sc0.38Al0.62N, and there is the composition difference between the ScAl alloy target and the thin film. However, the piezoelectric constant of the Sc0.38Al0.62N film is coincident with that of Sc0.38Al0.62N films prepared by dual co-sputtering. Thus, it is possible to prepare high piezoelectric ScxAl1−xN films by using the ScAl alloy sputtering target. ScAl alloy targets are effective for keeping scandium concentration constant in ScxAl1−xN thin films.

Thermal stability, microstructure and mechanical properties of Ti 1 − xZr xN thin films

Single-phase [NaCl]-structure Ti 1 − x Zr x N thin films (0 < x < 1) have been deposited using cathodic arc plasma deposition. The films were investigated using X-ray diffraction (XRD), transmission electron microscopy, differential scanning calorimetry (DSC), and nanoindentation. Density functional theory calculations on phase stabilities show that the pseudo-binary TiN–ZrN system exhibits a miscibility gap, extending over 0 ≤ x ≤ 0.99 at 1000 °C, with respect to phase transformation from a solid solution into a two-phase mixture of [NaCl]-structure TiN and ZrN components. The films were found to retain their as-deposited single-phase structure during post-deposition annealing at 600 °C (18 h), 700 °C (12 h), 1100 and 1200 °C (2 h), and for as long as 195 h at 600 °C. DSC revealed no heat flow during annealing, similar to TiN, and only the x = 0.53 film exhibited a slight increase in XRD peak broadening after annealing at 1200 °C, consistent with spinodal decomposition. This effective thermal stability of the alloys is explained by the combination of a limited driving force for phase transformation and an insufficient atom diffusivity. In terms of mechanical properties, films with composition deepest within the miscibility gap showed a hardness of ∼ 30 GPa after annealing at 1100–1200 °C; a value only moderately lower than in the as-deposited condition. The principal hardening mechanism for the Ti 1 − x Zr x N films is proposed to be solid-solution hardening through local lattice strain fields originating from difference in atomic radius of Ti and Zr. The material system is thus promising for cutting tool applications.

Femtosecond carrier dynamics of InxGa1−xN thin films grown on GaN (0001): Effect of carrier-defect scattering

Ultrafast carrier dynamics in ultrathin InxGa1−xN ternary alloys were investigated in detail, using femtosecond transient differential optical absorption measurements. Measurements were performed with probing wavelengths above and below the band edge of the materials. Furthermore, we performed a structural characterization by x-ray diffraction reciprocal space maps and we found that the alloys with the higher InN mole fraction (x=0.89 and x=0.43) present large lattice mismatch-strain relaxation whereas the lower InN content alloy was fully coherent with the underlying GaN (0001) layer. Our analysis showed that the observed nonradiative carrier dynamics was strongly related with the carrier-defect scattering of the materials. Our measurements in conjunction with the numerical analysis showed that when we excite these alloys with ultrashort laser pulses the background carrier concentration for In-rich InGaN samples participates to the photoexcited carrier relaxation process via carrier momentum scattering. For the higher InN content alloys (x=0.89 and 0.43) the relaxation times were short (0.4–1.4ps) whereas for the full strained alloy (x=0.07) a slower nonradiative relaxation time (∼25ps) was observed. Finally, the energy loss rate of this material (2.05±0.10meV∕fs) as well as the optical phonon lifetime (44±2fs) were extracted.

Magnetic properties of MnxTi1−xN thin films grown by plasma-assisted molecular beam epitaxy

High-quality MnxTi1−xN thin films were grown on MgO(001) substrates using plasma-assisted molecular beam epitaxy. Magnetic measurements evidence the presence of ferromagnetism with Curie temperature exceeding 380K. X-ray photoelectron spectroscopy indicates that the Mn ions are in a divalent state and uniformly substitute on Ti cation sites, consistent with the ferromagnetism that correlates with Mn substitution on Ti sites. The origin of the ferromagnetism might be attributed to itinerant-carrier mediated Rudermann-Kittel-Kasuya-Yosida (RKKY)-type long-range coupling which allows for arbitrary itinerant-carrier spin polarization and dynamic correlations.

Off-axis texture in nanostructured Ti 1 − xAl xN thin films

Standard θ-2 θ and pole figure X-ray diffraction techniques were used to study structural properties and preferred orientation of nanostructured TiAlN thin films prepared using cathodic arc deposition. Systematic collection of reflections from lattice planes {111}, {200}, {220}, and {311} showed that the in-plane orientation of crystallites exhibits cylindrical symmetry with random distribution of crystallites, while the out-of-plane growth presents strong texture which is inclined with respect to the surface normal. This brings the crystallographic orientation of fibre texture towards high indices lattice planes (113) and (115) contrary to traditionally grown films that develop a preferred orientation following to low indices close-packed planes (111) and (200). The origin of inclined growth is discussed taking into account the role of crystallographic defects in particular twin faults that develop in the lattice of growing crystal and change the stacking sequences of atom layers.

Mechanical properties of AlxGa1−xN films with high Al composition grown on AlN/sapphire templates

Mechanical properties of AlxGa1−xN thin films with high Al composition (0.33⩽x⩽1) grown on AlN/sapphire templates have been investigated by means of the nanoindentation technique. It is found that Young’s modulus E of the films increases with increasing Al composition. In addition, it is also found that the occurrence of the clear and sudden displacement discontinuity (“pop-in”) in the plastic deformation (PD) process is dependent on Al composition in AlxGa1−xN films. The higher Al composition results in less occurrence of the pop-in in the PD process of the films. With increasing Al composition, it is believed that the increase of the bond strength and the decrease of the lattice mismatch between AlxGa1−xN films and AlN/sapphire templates result in greater resistance to the formation of dislocations, which is responsible for the pop-in behavior in AlxGa1−xN films.

Surface phonon polariton mode of wurtzite structure AlxGa1−xN (⩽x⩽1) thin films

Surface phonon polariton (SPP) mode of wurtzite structure AlxGa1−xN (0⩽x⩽1) thin films is investigated experimentally and theoretically. The influences of the alloy composition x on the position and the line width of SPP mode are reported. The results showed that the SPP absorption peak exhibits one-mode behavior and is subjected to broadening as x is increased. Reasonable agreement between the experimental and the theoretical results is also obtained. Finally, the composition dependence of the SPP mode with bowing parameter of −72.5cm−1 is determined.

Yttrium-induced structural changes in sputtered Ti 1− xAl xN thin films

Metastable Ti1−xAlxN thin films synthesized by plasma-assisted vapour deposition crystallize in the industrially preferred cubic NaCl (c) structure with AlN mole fractions (x) ⩽ 0.7. We reveal, via X-ray diffraction and transmission electron microscopy, that the addition of yttrium to c-Ti1−xAlxN shifts the solubility limit to lower AlN contents, resulting in mixed c- and hexagonal ZnS–wurtzite (w) phase (Ti1−xAlx)1−yYyN coatings. Ab initio calculations yield a ∼23% decrease in the solubility limit in c-Ti1−xAlxN, from x ∼ 0.69 to 0.56 with the addition of y = 12.5 at.% Y.

Influence of silicon doping on vacancies and optical properties of AlxGa1−xN thin films

The authors have used positron annihilation spectroscopy and photoluminescence measurements to study the influence of silicon doping on vacancy formation in AlGaN:Si structures. The results show a correlation between the Doppler broadening measurements and the intensity from 510nm photoluminescence transition. The reduction in the W parameter when the [Si]∕[Al+Ga] fraction in the gas phase is above 3×10−4 indicates that the positrons annihilate in an environment where less Ga 3d electrons are present, i.e., they are trapped in group-III vacancies. The observation of vacancies at these silicon concentrations coincides with the onset of the photoluminescence transition at 510nm.

Systematic theoretical investigations of compositional inhomogeneity in In xGa 1−xN thin films on GaN(0 0 0 1)

We systematically investigate the compositional inhomogeneity near surface, interface, and dislocation in In x Ga 1− x N thin films on GaN(0 0 0 1) by using our empirical interatomic potential and the Monte Carlo (MC) method. The compositional inhomogeneity is discussed by evaluating individual contribution such as strain relief at the surface and the interface between In x Ga 1− x N and GaN with/without misfit dislocations. The empirical potential calculations reveal that the dislocation core energy for bulk InN (1.51 eV) is smaller than that of GaN (1.81 eV). This suggests that In atoms preferentially reside in the lattice sites near the dislocation core in In x Ga 1− x N. The MC simulation clarifies that In surface segregation is found in In x Ga 1− x N thin films pseudomorphically grown on GaN(0 0 0 1), where the surface composition of In is greater than that of bulk In composition because of strain relief and bond energy profit of In atoms at the surface. Further MC simulation for the system including both surface and misfit dislocations implies that the In atoms segregate at the surface strongly while In atoms segregate near the misfit dislocations weakly.

Electrical transport properties of ferromagnetic GaxCr1−xN thin films

The authors report the transport properties of ferromagnetic Ga0.97Cr0.03N films with a Curie temperature of over 900K. Samples synthesized under conditions that produce the maximum occupancy of Cr atoms on the Ga site (∼90%) and the maximum ferromagnetic moment of ∼0.6μB∕Cr (775°C growth temperature) exhibit a T−1∕4 dependence of resistivity, indicative of Mott [J. Non-Cryst. Solids 1, 1 (1968)] phonon-assisted variable range hopping between deep level defects. The observation of variable range hopping in ferromagnetic films indicates that ferromagnetism results from a double-exchange-type mechanism. Films grown at higher temperatures (&amp;gt;800°C) show enhanced Cr segregation, properties associated with percolative transport, and very small, if any, ferromagnetic moments.

Thermal decomposition of Zr 1 − x Al xN thin films deposited by magnetron sputtering

The thermal stability of the metastable fcc Zr 1 − x Al xN thin films has been investigated by heating the samples in ultra high vacuum ( P < 10 − 7 Pa). For this purpose, two films with chemical composition Zr 0.65Al 0.35N and Zr 0.57Al 0.43N were grown by reactive magnetron sputtering. The films were heated for duration of 60 min between 400–850 °C by steps of 50 °C. The structural evolution of the annealed films has been investigated by using X-ray diffraction (XRD), transmission electron spectroscopy (TEM) and by atomic force microscopy (AFM). Results show that up to annealing temperature of 600 °C the pristine structural and mechanical properties of all the films are retained. At annealing temperatures above 600 °C, important structural modification result as deduced from the shift of the XRD peaks towards the low 2θ values indicating changes in the unit cell dimension. Structural modifications are accompanied by hardness enhancement. In Zr 0.57Al 0.43N films annealed at 850 °C, chemical analysis by X-ray energy dispersive spectrometry evidenced Al-rich regions inhomogeneously distributed in the plane of the film. The results are discussed in terms of the transformation of the fcc Zr 1 − x Al xN structure in a two-phase system composed of Al-poor Zr 1 − x Al xN and low crystallized h-AlN via spinodal decomposition.

Application of x-ray metrology in the characterization of metal gate thin films

Physical characterization of as-deposited and 1000°C annealed TaxAl1−xN thin films was performed using several characterization techniques. X-ray diffraction metrology shows that the crystallographic phase and crystallinity of TaN and AlN films vary with composition. The unannealed TaN film was cubic TaN, while the annealed TaN was mixed hexagonal Ta2N and hexagonal TaN. Both AlN films were hexagonal, with the annealed film having a higher crystalline fraction. However, TaxAl1−xN films displayed a relatively small change in crystalline fraction and crystallography. Well behaved capacitors were fabricated, and a work function of ∼5.0eV was obtained.

Spontaneous compositional superlattice and band-gap reduction in Si-doped AlxGa1−xN epilayers

Combined transmission electron microscopy (TEM), x-ray diffraction, and cathodoluminescence spectroscopy measurements of AlxGa1−xN thin films grown by molecular-beam epitaxy reveal spontaneous modulation, phase separation, and band-gap reductions that vary systematically with AlN mole fraction across the full alloy series. At low AlN mole fraction (x⩽0.5), AlGaN epilayers display pronounced phase separation. With increasing AlN mole fraction, phase separation is strongly suppressed by the formation of spontaneous modulation, which high spatial resolution TEM techniques unambiguously determine to be an atomic-scale compositional superlattice. Superlattice-induced reductions from band gaps expected for compositionally disordered epilayers exceed several hundred meV for the Al-rich average alloy composition.

Spectroscopic studies of the infrared emission from Tm doped Al x Ga 1– x N thin films

The infrared (IR) emission properties of in-situ Tm doped AlxGa1–xN thin films (0 ≤ x ≤ 1) prepared by solid-source molecular-beam epitaxy were investigated. All samples exhibited 1.48 µm photoluminescence (PL) from the 3H4→3F4 transition of Tm3+. The absolute intensity of the 1.48 µm PL varied strongly with Al content and reached a maximum for Al0.81Ga0.19N:Tm. The integrated 1.48 (m PL from Al0.81Ga0.19N:Tm was weakly temperature dependent and decreased by less than a factor of two between 15 K and 300 K. The PL lifetime of the 3H4 state decreased only slightly from ∼102 (s to ∼86 (s for the same temperature range. The results suggest that non-radiative decay processes in the 3H4 level are small in Al0.81Ga0.19N:Tm, which is supported by the well-known energy-gap law for multiphonon relaxations. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Thermal conduction in AlxGa1−xN alloys and thin films

We report on experimental and theoretical investigation of thermal conduction in AlxGa1−xN alloys. A focus of this study is on understanding the effect of the Al mass fraction x and temperature on thermal conductivity in AlxGa1−xN thin films. The thermal conductivity of a set of AlxGa1−xN thin films as well as a pure GaN sample was measured using the differential 3ω technique in the temperature range from 80 to 400 K. Application of the virtual-crystal model allowed us to elucidate the strength of the mass-difference and strain-field-difference phonon scattering in AlxGa1−xN alloy system. The obtained thermal-conductivity temperature dependence indicates the high degree of disorder in the system. The measured variation of the thermal conductivity with the Al fraction x is in good agreement with the theory predictions. The measured data and calculation procedure are useful for evaluating the self-heating effect in AlxGa1−xN/GaN heterostructure field-effect transistors and for the device structure optimization.