Wurtzite structure properties

The unique properties of the stable d5 Mn2+ ion is reflected by the fact that MnS and MnSe crystallize in three modifications, in the rocksalt, the cubic sphalerite and the hexagonal wurtzite structure. While in the NiAs structure of MnTe the cations occupy the octahedral holes of a hexagonal close-packing of anions they occupy half of the tetrahedral holes of this packing in the ZnO type modification of MnS and MnSe. The non-metallic character is evident already from the fact that the structure is undistorted (c/a = 1.61 for MnS and 1.63 for MnSe) and that the cations really are at the centres of one set of tetrahedral holes and not at the centre of the bipyramidal holes composed of two tetrahedra of the two different sets. 

Again, many calculations were undertaken to understand the electronic and structural properties of GaN, both in the wurtzite and the cubic zincblende phase. The results concerning the mechanical properties are summarised in TABLE 6. 

Moreover, since the wurtzite structure already has a C2v symmetry, there is not much scope for improvement of the hole density-of-states by utilising strained quantum wells [11]. Only by using structures grown on special surfaces (r-face), can an improvement of the lasing properties by using strained quantum wells be expected [12]. 

One of the most basic properties of the optical gain in nitride-based structures is its polarisation dependence. In contrast to other bulk III-V materials, where the optical gain is isotropic, nitride structures exhibit almost exclusively TE-mode gain [14]. This is due to the symmetry-induced splitting of the valence bands in the wurtzite structure. 

Zinc oxide (ZnO, wurtzite structure) eliminates oxygen on heating to form nonstoichio-metric colored phases, Zni+xO with x < 70 ppm. ZnO is almost transparent and is used as white pigment, polymer stabilizer, emollient in zinc ointments, creams and lotions, as well as in the production of Zu2Si04 for TV screens. A major application is in the rubber industry to lower the temperatures and to raise the rate of vulcanization. Furthermore, it is an n-type semiconductor (band gap 3.37 eV) and shows piezoelectric properties, making zinc oxide useful for microsensor devices and micromachined actuators. Other applications include gas sensors , solar cell windows and surface acoustic devices. ZnO has also been considered for spintronic application because of theoretical predictions of room-temperature ferromagnetism . 

Aluminum nitride (AIN) differs from other A B compounds in its hexagonal wurtzite structure. The lattice parameters of this compound are a = 3.111 k, c = 4.978 A, and c/a = 1.600 [1]. Thus, the structure of aluminum nitride differs from the perfect wurtzite structure, which consists of regular tetrahedra and is characterized by the axial ratios c/a = 1.633. Moreover, aluminum nitride has distinctive physical and physicochemical properties. 

The results of studies conducted to date have indicated that O-substitution in wurtzite-structured GaN, and N-substitution in Ga203 phases, can be achieved effectively by using a variety of chemical and physical approaches, and that this has resulted in an important tuning of the optical and electronic properties. As yet, the oxygen and nitrogen substitution limits have not been determined, and the effects of... 

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