13.2.2 III−Nitride

The group III-nitride compounds belong to group III–V compound semiconductors, and

the more common materials include AlN, GaN, InN, and their alloys. The high structural

quality, flexibility, spontaneous and piezoelectric polarization, direct bandgap, and good

biocompatibility of III-nitride compounds increased the interest for applications in wear­

able UV detectors and biosensing. Additionally, the III-nitride compounds exhibit con­

siderable bond energies of 2.28 eV for AlN, 2.2 eV for GaN, 1.93 eV for InN [13], resulting in

a high melting point, greater chemical stability, and good mechanical strength.

The III-nitride semiconductors can crystallize in the thermodynamically stable WZ, and

the ZB structure is metastable [13]. For all the three compounds, although the WZ phase

is energetically favorable, the calculated energy difference between the WZ and ZB is

small [14]. At high pressures, the crystal RS structure can be formed from the WZ phase,

and in this process, the covalent bond character changes to ionic [14].

The polarizations present in a structure will have direct effects on the optical and

electrical properties of the materials. In the III-nitrides, besides their piezoelectric prop­

erties (where a strain is required), the central asymmetry and the significant ionicity of III-

N bonds result in spontaneous polarization (polarization at zero strain) along the c-axis

([0001] – the axis that is perpendicular to the hexagonal layers) [14]. Additionally, the

crystal structure of these materials also exhibits crystallographic polarity, and the choice

of the correct plane polarity is essential depending on the desired property of the ma­

terial. A [0001] plane when terminated by group III atoms is denominated by c-plane,

while a [0001^¯] plane terminated by nitrogen atoms is denominated by N-polar, and both

of them are polar planes. The c-plane structure results in high electron mobility (see

Section 13.3 – Table 13.1), and it is commonly applied in electronic devices, such as

heterojunction field-effect transistors (HFETs) [2]. Although the c-plane in WZ of nitrides

is more common, the literature showed greater optical efficiency in structures with

nonpolar or semipolar planes, enabling LED applications [15]. The m-plane (or [101^¯0]

plane) and a-plane (or [112^¯0] plane) are nonpolar, while the other planes are semipolar

(Figure 13.2), and in these cases, the internal electric field under the planes is induced by

the reduction of the polarization.

13.2.3 Silicon Carbide – SiC

Formed by earth-abundant elements, silicon carbide compounds are a very well-known

class of wide-band-gap semiconductors due to their unique physical and chemical

TABLE 13.1

Typical Properties of Common WBG Semiconductors from Each Family [ 1, 2, 6– 12]

II–VI family

III-nitride family

SiC family

ZnO

CdS

AlN

GaN

InN

3C-SiC 4H-SiC 6H-SiC

Bandgap (eV)

3.37

2.50

6.20

3.40

0.70

2.36

3.26

3.02

Melting point (°C)

1,975

1,750 (100 atm)

3,000

2,500

1,100

2,830

2,830

2,830

Breakdown electric field

(MV/cm)

4.0

1.0–2.5

1.8

5.0

–

1.2

2.0

2.4

Electron mobility (cm² V⁻¹ s⁻¹)

210

210

300

1,000

3,200

1,000

1,000

450

Thermal conductivity

(W cm⁻¹K⁻¹)

0.6–1.16

0.2

2.5

2.27

1.2

4.5

4.5

4.5

206

Bioelectronics