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The compound nitride semiconductors belong to the III-V semiconductor material family. They form partially ionic crystals [19] whose band gap has wide variety of energies. In fact, the three main binary materials of this family, namely AlN , GaN and InN , have band gaps ranging from 6.13 eV for AlN , 3.44 eV for GaN and even below 0.7 eV for InN [20–23].

Some of these materials where first obtained during the mid 60s. The first metal- insulator-semiconductor (MIS) light emitting diodes (LEDs) were developed at mid 70s, indeed. However, few attention was paid to the III-N material family until S. Nakamura achieved to produce blue LEDs at his lab in Nichia Corporation, a work that has been awarded the 2014 Nobel Prize. A review on the history of the develop- ment of GaN based LEDs may be found at [24].

The growth of these materials possess a serious practical problem. The bulk growth of GaN crystals has to be carried out at high pressure and high tempera- ture conditions [25, 26]. The technique is based in the dilution of nitrogen in liquid gallium. Although the crystal quality is improving, this method provides small wafer sizes and with high defect density. Therefore, the most spread technique for III-N material growth is the heteroepitaxy on various substrates.

There are many materials which may serve as substrates but only a few unfold to be suitable from different points of view. The main issue is the lattice and thermal expansion coefficient mismatch between the substrate and the III-N overgrown mate- rial. This produces the appearance of a large defect density in the epilayers, specially dislocations appear in order to relax the accumulated stain energy. For instance, the lattice mismatch between GaN and sapphire (Al2O3), a commonly used substrate

for nitride wafers, is as high as 13.8%. There are several way of reducing the defect density, being the use of an accomodation buffer layer between the substrate and the epilayer the most usual [27–29]. Indeed, the early efforts to reduce the defect density and obtain a functional material have been overcomed, making easy to find nowadays wafers which provide a very good device quality. Nevertheless, the strain issue is of outermost importance for MEMS device, since they rely on the mechanical properties of the functional material as well as on the electrical ones. The influence of residual stress in the material and the mechanical properties of III-N material will be analysed and discussed in this dissertation in the corresponding chapter.

Figure 1.1: Main crystal structures which form the III-N materials.

Crystal structure

There are three crystal forms which the III-N materials may adopt [30]: wurtzite, zincblende and rock-salt. However, the wurtzite structure is the most thermodynam- ically stable, whereas the zincblende phase has been stabilized by means of epitaxial growth on the (001) planes of cubic crystals. The third phase may only be obtained at high pressures. In figure 1.1 the unit cells corresponding to the two first structures have been depicted.

The wurtzite structure presents an hexagonal unit cell. Consequently, two lat- tice parameters are defined, namely a and c. Every cell contains six atoms of each element. It can be thought of as two interpenetrating hexagonal sublattices, each formed by atoms corresponding to one element. This sublattices are displaced one with respect to the other 5/8 of the cell height along the c-axis. The symmetry of this cell corresponds to the P 63mc(C6v4 )point group.

The two aforementioned forms have certain similarities. In both cases each atom of the group III is coordinated with four nitrogen atoms and vice-versa, each nitrogen atom is coordinated with four group III atoms. The main difference between both structures is the stacking sequence. For the wurtzite phase the stacking sequence is ABABAB along the <0001> direction. Whereas for the zincblende phase the se- quence is ABCABC along the <111> direction. Both cases have been sketched in figure 1.2, where the differences between them are clearly observable. The bonding energy are very high, which translates in a high stability. The bonding energy of AlN is 2.88 eV , for GaN is 2.2 eV and for InN is 1.93 eV .

Figure 1.2: Plane stack sequence of the two most common III-N crystal structures.

Electrical properties

GaN grown by epitaxial techniques often presents a residual n type doping with electron concentrations ranging from 1018_cm−3 _{to 10}16_cm−3_{, depending on the qual-}

ity of the epilayer. Various origins to this high electron concentration have been suggested [31]:

• Nitrogen vacancy

• Ga interstitial as native defect • Contaminant atoms such as Si or O

Nevertheless, the growth techniques have been improved, making it easy to grow p type GaN layers. Therefore, GaN can be grown to form p-n junctions although achieving very heavy p type doping is still challenging.

Additionaly, the III-N materials exhibit piezoelectric and pyroelectric effects which can be played with in order to create thin layers of high mobility and high concentra- tion carriers. Consequently, the device engineer has more parameters to play with, adding the stress to the band engineering and doping levels.

The electrical properties of nitride semiconductors are excellent. The electron mobility is very high specially if a 2DEG is formed at a properly tailored heterointer- face, allowing the application of GaN based devices to high frequency applications. The hole mobility is lower than the electron but sufficiently high for the considera- tion of bipolar devices such as pn diodes. The fabrication of ohmic contacts to GaN

has been widely researched and many different metalization and annealing schemes have been published; for instance, a common achievable value of the specific con- tact resistance is 10−8 _Ωcm−2 _{[32] and saturation velocities of 3 · 10}7 _cms−1_{. Due}

to the extremely high stability and large band gap energy of the GaN and Al-rich AlInGaN alloys the properties that have been described remain stable even at high temperatures. Consequently the III-N material are suited for high temperature or high dissipated power applications.

Piezoelectric properties

Nitrides do not possess inversion symmetry, therefore they present piezoelectric effect along the [0001] direction. This effect is larger than for other III-V mate- rials [33], being the piezoelectric constants for III-N up to 10 times higher than for the other III-V materials, see table 1.1. Therefore, when stress is applied along certain directions, a distortion in the valence charge of the unit cell is produced, arising a large internal electric field [34]. Spontaneous polarizations also appear at the interfaces between materials along the c-axis of the wurtzite crystal structure. These polarizations may increase or decrease the charge density at the interface. This principle applies to the AlGaN/GaN heterojunction without external stresses, allowing the formation of the aforementioned two dimensional electron gas (2DEG). These properties are exploited in practical devices such as high electron mobility transistors (HEMTs) [33], without the need of doping at the barrier as it is with the AlGaAs/GaAsheterojunction.

Other devices use the piezoelectric effect such as the surface acoustic wave de- vices (SAWs), the thin film bulk acoustic resonators (FBARs) or the piezoelectric MEMS resonators. All these devices use the high piezoelectric response of the III-N materials as transduction principle between the mechanical and electrical domain. The high sensitivity of these devices to surface modifications or external forces allow the fabrication of very sensitive sensors and actuators. These properties remain even at high temperatures as mentioned before, therefore the III-N materials are specially suited for harsh environment sensors.