Aluminum nitride, Scandium nitride, and Aluminum-Scandium-Nitride ternary alloys : Structural, optical, and electrical properties.

摘要

Al and Sc are iso-electric, both of which have three valence electrons. Their nitrides AlN and ScN both have high melting points, high hardness, and good chemical inertness. And their distinct properties find applications in different areas: AlN in piezoelectric acoustic-wave devices, and ScN as candidate for high-temperature thermoelectricity. While there are unsettled problems to solve for AlN and ScN alone, which are to obtain tilted c-axis texture in AlN for shear mode acoustic-wave devices to maximize performance, and to determine electronic band structure of ScN that has been long debated due to free carrier effect, the alloying between AlN and ScN is also intriguing in that the ternary alloy Al-Sc-N connects their similarity and opens even wider possibility and greater potential. The significantly enhanced piezoelectric coefficient in the alloy compared to pure AlN is one of the best examples that is little understood, and alternate bandgap engineering in LED fabrication would probably be another contribution from the alloy. Structural, optical, and electrical properties of AlN, ScN, and Al-Sc-N ternary alloys are thus studied in order to answer these questions, and to explore more fundamental physics characteristics within these nitride materials. For the purpose of achieving tilted c-axis texture in AlN, off-axis deposition is conducted with a variable deposition angle &agr; = 0-84° in 5 mTorr pure N2 at room temperature. XRD pole figure analysis show that layers deposited from a normal angle (&agr; = 0°) exhibit fiber texture, with the c-axis tilted by 42+/-2° off the substrate normal. However, as &agr; is increased to 45°, two preferred in-plane grain orientations emerge, with populations I and II having the c-axis tilted towards and away from the deposition flux, by 53+/-2° and 47+/-1° off the substrate normal, respectively. Increasing alpha further to 65 and 84°, results in the development of a single population II with a 43+/-1° tilt. The observed tilt is ideal for shear mode electromechanical coupling, which is maximized at 48°. And this developing bi-axial texture is attributed to evolutionary competitive growth mode which selects out-of-plane and in-plane orientation by nuclei growth rates. In order to determine electronic band structure of ScN, simulation and experimental results are combined. First-principle simulation with HSE exchange function suggests rock-salt ScN to be indirect semiconductor with indirect gap of 0.92 eV and direct gap of 2.02 eV at X point, as well as electron transport effective mass of 0.33+/-0.05 m0 at conduction band bottom. In experiment, epitaxial ScN thin films deposited on single-crystal MgO 001 substrates by reactive sputtering are found unintentionally doped to be degenerate n-type semiconductor with electron density between 1.12x10 20 and 12.8x1020 cm-3. Direct bandgap determined by optic absorption method is observed decreasing with carrier concentration due to Bursten-Moss effect in a roughly linear trend, yielding an extrapolated intrinsic gap value of 2.1 eV at zero carrier density. Electron transport effective mass is also calculated from fitted plasma frequency, which is 0.40+/-0.02 m0. The overall great agreement between simulation and experiment can be concluded in regard of bandgap and effective mass, as well as optic reflectance level. Al-Sc-N ternary alloys can be categorized into two regions: Al-rich wurtizte Al1-xScxN and Sc-rich rock-salt Sc1-xAlxN. For Al1-xScxN, multiple phenomena are observed in experiment as Sc concentration x increases, from epitaxial samples deposited on sapphire 0001 substrates at 850°C: (1) Anisotropic lattice expansion with a = 3.111+0.744x A while c remains at 4.989+/-0.005 A, implying elongation of bond lengths as well as decrease in bond angle from 108.2° towards ~90° corresponding to meta-stable h-ScN. (2) Dielectric constant increases with Sc concentration as epsiloninfinity = 4.15 + 3.2x, correlated to the almost linear reduction in direct bandgap Eg = 6.15-9.32x (eV) which itself has implication in III-V compound bandgap engineering. (3) All optical phonons mode frequencies measured from IR specular reflectance and Raman scattering red-shift as more Sc atoms are incorporated: &ohgr;[E2(H)] = 658 - 233x, &ohgr;[A1(TO)] = 612 - 159x, &ohgr;[E1(TO)] = 681 - 209x, &ohgr;[A1(LO)] = 868 - 306x (all units in cm-1). The phonon softening effect is understood as the increase in ionicity and weakening in covalent bond strength, which are believed to be the major factors leading to piezoelectric enhancement. Similarly, Sc1-xAlxN ternary alloy also exhibit changes in lattice structure and bandgaps as seen in samples expitaxially deposited on MgO(001) substrate at 950°C. Measured relaxed lattice constant assuming a Poisson ratio of 0.2 is shrinking nonlinearly with Al concentration x because rock-salt AlN has a smaller lattice constant than ScN. X point bandgap value and near-Gamma-point interband transition energy are greater with more Al incorporated: Eg(X) = 2.50+2.51 x (eV), and Eg(Gamma) = 3.80 + 1.45 x (eV), implying that band structure of this ternary alloy is possibly under the effect of rock-salt AlN indirect bandgap that is around 5 eV from simulation. Carrier weak localization is also observed at low temperature and becomes dominant as Al concentration increases, in which normal phonon scattering in ScN is over-taken by electron coherent back-scattering by Al atoms. Overall, structural, optical, and electrical characterization and analysis have been conducted on AlN, ScN, and Al-Sc-N ternary alloy materials, the results of which could help to improve existing processes and also to understand more fundamental properties.