Wurtzite-structured GaN

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... 

A product presentation for harmonic oscillator lineshapes with Lorentzian broadening is employed for the calculation of the lattice contribution to the dielectric function of uniaxial crystals, such as wurtzite-structure GaN [37] ... 

MnTe, AIN, GaN, InN, SiC and NH4F. The wurtzite structure is, in a sense, intermediate between NaCl and ZnS structures. 

AIN, GaN and InN crystallise in the wurtzite structure which is characterised by lattice parameters a and c, as well as by u-value (u = b/c, where b is a bond-length in the c-direction). For the ideal wurtzite structure, c/a = 1.633 and u = 0.375. In contrast to the cubic sphalerite structure, the wurtzite structure offers two possibilities to deviate from the ideal arrangement, by changing the c/a ratio and by changing the u value. Such deviations are often observed in wurtzite-type structures [1] but there exists a strong correlation between the c/a ratio and the u parameter if c/a decreases, then u increases in such a way that the four tetrahedral distances remain nearly constant and the tetrahedral angles are distorted [2]. The bond lengths would be equal if ... 

Application of high pressure changes the c/a ratio to lower values for AIN [5] and InN [6] (stabilising the wurtzite structure), whereas for GaN this ratio remains unchanged [6],... 

The GaN platelet crystals had the shape of elongated hexagons (FIGURE 1) and crystallised with the wurtzite structure. 

In this Datareview, the crystalline quality of GaN and also of ternary alloys on GaN, which have wurtzite structure, characterised by a triple-axis high resolution X-ray diffraction system, is reviewed. 

The GaN crystals grown by this method have wurtzite structure and they are mainly in the form of hexagonal platelets or hexagonal needles. High supersaturation favours growth in the c-direction which leads to needle-like forms. 

The most common way of fabricating GalnN/GaN heterostructures is by means of heteroepitaxy of GaN and GalnN on sapphire or SiC substrates. In fact, this leads to wurtzite GaN, which is now the commonly used modification. Even though there is considerable effort being devoted to producing cubic GaN structures, we will restrict our discussion to the hexagonal phase, since practically all the work on heterostructures concentrates on the wurtzite structure. 

The wurtzite structure is adopted by most of the remaining compounds comprised of elements from the same groups as zinc blende, but which do not take the zinc blende strucmre, for example, AIN, InN, CdSe. The strucmre seems to be able to accommodate larger electronegativity differences between the constiment atoms, as in BeO, GaN, and ZnO. For these more ionic compounds, the wurtzite unit cell must be more stable than that of zinc blende, to an extent governed by the specific bonding forces in each case. 

Most wurtzite-type crystals are direct band-gap materials (2fP-SiC is an exception) and interband transitions can take place between these three Fils and the T7 CB minimum. These materials are anisotropic and this anisotropy reflects on the selection rules for the optical transitions and on the effective masses. The Tg (A) —> T7 (CB) transitions are only allowed for ETc while the two T7 (B. C) —> T7 (CB) transitions are allowed for both polarizations. However, the relative values of the transition matrix elements for the T7 (B, C) —> T7 (CB) transitions can vary with the material. For instance, in w-GaN, the T7 (B) —> T7 (CB) transition is predominantly allowed for ETc while the T7 (C) — I 7 (CB) transition is predominantly allowed for E//c [22]. Table 3.7 gives band structure parameters of representative materials with the wurtzite structure. 

Table 3.T. Selected band structure parameters of four compounds with the wurtzite structure. The energies for ZnO and GaN are given at LHeT and at 80 K for CdSe and CdS (the effective masses are expressed in units of me)...

Light-brown AIN, light-gray GaN, and dark-brown InN form with the wurtzite structure [182, 183], The electrical conductivity increases from AIN to GaN to InN [184], in addition GaN and InN both show diamagnetic susceptibility (yM = —2.8 and — 4.1 x 10 5, respectively) [185], In,ON17F4, is known with an ordered fluorite superstructure as determined by neutron diffraction [186]. 

We first discuss the results based on a GaN (2x2x2) supercell having wurtzite structure. Two Ga atoms in this supercell were replaced with Mn atoms, corresponding to a GanMniNig supercell and a 12.5 % Mn doping concentration. Note that in recent experiments Mn concentration... 

The nitrides AIN, GaN and InN are known. Only aluminum reacts directly with nitrogen. GaN is obtained on reaction of Ga or Ga203 at 600-1000° with NH3 and InN by pyrolysis of (NH4)3InF6. All have a wurtzite structure (Fig. 2-3). They are fairly hard and stable, as might be expected from their close structural relationship to diamond and the diamond-like BN. 

There is a great number of mostly covalent and tetrahedral binary IV-IV, III-V, II-VI and I-VII semiconductors. Most crystallize in the zincblende structure, but some prefer the wurtzite structure, notably GaN [11,12]. While the bonding in all of these compounds (and their alloys) is mostly covalent, some ionic character is always present because of the difference in electron affinity of the constituent atoms. 

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