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2.1.2 Physical Properties

Directly measured density of wurtzite InN is 6.89×103 kg m-3 at 25 °C [Hah40]. A

comparable value of 6.81×103 kg m-3 has been estimated from X-ray data [Pea67]. The cell volume, taken in conjunction with a molar mass of 128.827 g mol-1, yields densities of (6.81±0.05)×103 kg m-3 and 6.97×10 kg m-3 for the wurtzite and zinc blende polytypes, respectively. Bulk modulus has been calculated from first principles by a local-density approximation [Cam90] and by a linear muffin-tin orbital method [Kub89], suggesting a value of B = 165 GPa.

The five distinguishable second-order elastic moduli in a hexagonal crystal are c11, c12, c13, c33 and c44. Other researchers have utilized empirical and theoretical approaches to calculate the thermoelastic properties of the wurtzite structure InN [She91, Kim96a, Wri97, Mar98, Chi99]. Table 2-3 summarizes the room-temperature elastic constants from both experimental and theoretical results. Estimates of the principal transverse and

longitudinal elastic constants ct and cl are given in Table 2-4. Table 2-3. Elastic constants of wurtzite InN at room temperature.

[Wan01]

Table 2-4. Physical properties of InN.

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[Edg94], (reprinted from the Institute of Electrical Engineers with the permission of INSPEC)

The piezoelectric constant has not been reported, but its dependence on the dielectric

constants εr and e14 [Wol89] allows values of about 50 % of those found in AIN to be

inferred [Chi94].

Indium nitride has twelve phonon modes at the zone centre (symmetry group C6v),

three acoustic and nine optical with the acoustic branches essentially zero at k = 0. The

IR active modes are E1 (LO), E1(TO), A1(LO) and A1(TO). A transverse optical mode has

been identified at 478 cm-1(59.3 meV) by reflectance and 460 cm-1 (57.1 meV) by

transmission [Tan88]. In both reports the location of a longitudinal optical mode is

inferred from the Brout sum rule, giving respective values of 694cm-1 (86.1 meV) and

719cm-1 (89.2 meV).

In summary, the physical properties of InN films were briefly discussed, especially

the elastic constants used to calculate the strain energy and thus estimate the critical

thickness of InN film.