Wurtzite structure semiconductors

Surfaces of real crystals never adopt the bulk-truncated structures shown in Fig. 4.5. They reconstruct or relax (inwards or outward movement of the atoms) to minimize their surface energy [42]. Known surface structures of zincblende and wurtzite structure semiconductors are summarized in [43]. Nonpolar surfaces of wurtzite (1120) and (1010) surfaces show no lateral surface reconstructions and are supposed to have a structure similar to the well-known zincblende (110) surface, which is characterized by an inward relaxation of the surface cations and partial electron transfer from the surface cation dangling bond to the surface anion dangling bond [42,43]. 

In the linear elasticity theory, the relation between the stress and strain tensors and e,j for wurtzite structure semiconductors, such as GaN is given by ... 

II-VI semiconductors, such as CdSe and CdS, normally have the wurtzite structure (see Chapter 1) where each element is tetrahedrally coordinated. Under high pressures (2 GPa), these transform to the six-coordinate NaCl (rock salt) structure. However, if pressure is applied to a CdSe nanocrystal of about 4 nm in diameter, it now takes much more pressure, about 6 GPa, to transform it to the rock salt structure. It is thought that this may be a resistance to the exposure of high-index crystal planes... 

The parameter is obtained by relating the static dielectric constant to Eg and taking in such crystals to be proportional to a - where a is the lattice constant. Phillips parameters for a few crystals are listed in Table 1.4. Phillips has shown that all crystals with a/ below the critical value of0.785 possess the tetrahedral diamond (or wurtzite) structure when f > 0.785, six-fold coordination (rocksalt structure) is favoured. Pauling s ionicity scale also makes such structural predictions, but Phillips scale is more universal. Accordingly, MgS (f = 0.786) shows a borderline behaviour. Cohesive energies of tetrahedrally coordinated semiconductors have been calculated making use... 

By the use of mainly LEED and lately ion scattering techniques the location of many atomic adsorbates, their bond distances and bond angles from their nearest neighbor atoms have been determined. The substrates utilized in these investigations were low Miller Index surfaces of fee, hep and bcc metals in most cases, and low Miller Index surfaces of semiconductors that crystallize in the diamond, zincblende and wurtzite structures in some cases that could be cleaned and ordered with good reproducibility. 

A major portion of the effort on semiconductors has been expended on the binary compounds having the zinc blende or wurtzite structure. These are commonly classified by the A group numbers such as III-V (InAs) and II-VI (CdTe) and have what may loosely be described as a 1 to 1 cation-anion ratio. However, another series of compounds that has become of increased interest can be generally classified as the IV-VI compounds. Specifically, these are the chal-cogenides of germanium, tin, and lead. In this discussion, we present some experimental observations on the tellurides of these group IV A elements. 

One of the most widely studied types of semiconductor NCs is CdSe. The hexagonal wurtzite structure that pertains for large CdSe crystals is shown in Fig. la, and a model of a nanocrystal is pictured in Fig. b. The size of such NCs can be controlled by reaction... 

Figure 1. Energy band structure and the symmetry of the free exciton ground state in wurtzite-type semiconductors for (a) ZnO and (b) GaN, CdSe, CdS.

In certain cases, when the semiconductor is composed of elements with very different radii and electronegativities, the lattice site in the tetrahedral position of a face-centered cubic structure becomes too small to accommodate one of the ions in the semiconductor. To maintain a lattice with a 1 1 cation anion stoichiometry, the solid adopts a structure in which the atoms are instead hexagonally close-packed. As in the zinc blende structure, only half of the possible tetrahedral sites in this hexagonal structure are filled with atoms. This arrangement, which is adopted by semiconductors such as ZnO and CdSe (Figure 1(d)), is called 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 . 

As mentioned, elemental silicon has the diamond structure. Silicon carbide, SiC, occurs in many crystalline forms, some based on the diamond structure and some on the wurtzite structure (see Figures 7-6 and 7-8(b)). It can be made from the elements at high temperature. Carborundum, one form of silicon carbide, is widely used as an abrasive, with a hardness nearly as great as diamond and a low chemical reactivity. SiC has now garnered interest as a high-temperature semiconductor. 

HgSe is a semiconductor which can crystallize in both the zinc blende and wurtzite structures. At high temperatures the hexagonal close-packed wurtzite structure is preferred, but at lower temperatures the cubic close-packed form dominates. 

The (110) surface of the zincblende structure compound semiconductors and the ( lOllO) surface of the wurtzite structure compound semiconductors... 

Cadmium(II) oxide (formed by heating Cd in O2, and varying in colour from green to black) adopts an NaCl structure. It is insoluble in H2O and alkahs, but dissolves in acids, i.e. CdO is more basic than ZnO. Addition of dilute alkali to aqueous solutions of Cd precipitates white Cd(OH)2, and this dissolves only in concentrated alkali to give [Cd(OH)4] (contrast [Zn(OH)4] , 21.68). Equation 21.5 showed the role of Cd(OH)2 in NiCd cells. Yellow CdS (the stable a-form has a wurtzite structure. Figure 5.20) is commercially important as a pigment and phosphor CdSe and CdTe are semiconductors (see Section 22.2). 

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. 

The same analysis that has been made here for the zincblende semiconductors can of course be carried out for the wurtzite structure. For comparison with experiment, another approach is simpler that is to compare the formulae obtained here with effective cubic elastic constants, which were obtained by Martin (1972b). These were estimates of what the constants of wurtzite compounds would... 

Of the group 13 metals, only Al reacts directly with N2 (at 1020 K) to form a nitride AIN has a wurtzite-type structure and is hydrolysed to NH3 by hot dilute alkali. Gallium and indium nitrides also crystallize with the wurtzite structure, and are more reactive than their B or Al counterparts. The importance of the group 13 metal nitrides, and of the related MP, MAs and MSb (M = Al, Ga, In) compounds, lies in their applications in the semiconductor industry (see also Section 19.4). 

The oxygen lattice of ordinary ice is similar to that of the wurtzite structure, observed with semiconductors, in which both A and B... 

Band Structures of Aluminium Compounds. Aluminium Nitride (AIN). Aluminium nitride is a direct-gap semiconductor. Since it crystallizes in the wurtzite structure, the band structure (Fig. 4.1-66) differs from that of most of the other III-V compounds. 

Besides group IV, the compounds by the atoms in groups III-V are also semiconductors, such as BN, BP, Bas, AIN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, and InSb. Except for the nitrides, all of these compounds crystallize into the zincblende structure. The nitrides are stable in the wurtzite structure. Meanwhile, the mixrnre crystals made of binary III-V compounds also have semiconducting properties, such as (Ga,Al)As, Ga(As,P),(In,Ga)As, and (In,Ga)(As,P). 

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