Wurtzite structure type semiconductors

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.

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 . 

Zinc oxide may crystallize in either the wurtzite or, more rarely, the zinc blende structure, the former being more stable at lower temperatures. In both structures the zinc and oxygen ions are tetrahedrally co-ordinated to each other. The bonding is intermediate between the completely ionic and the completely covalent, both ions being more polarizable than in a perfect ionic crystal and carrying an effective charge of only 0.5e. As is well known, non-stoicheio-metric ZnO, with excess of Zn, is an n-type semiconductor with a band gap of 3.2 eV. 

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). 

Although the eommon semieonduetor materials share this basie diamond/zinc blende lattiee strueture, some semieonduetor erystals are based on a hexagonal elose-paeked (hep) lattice. Examples are CdS and CdSe. In this example, all of the Cd atoms are located on one hep lattice whereas the other atom (S or Se) is located on a second hep lattice. In the spirit of the diamond and zinc blende lattices, the complete lattice is constructed by interpenetrating these two hep lattices. The overall crystal structure is called a wurtzite lattice. Type IV-VI semiconductors (PbS, PbSe, PbTe, and SnTe) exhibit a narrow bandgap and have been used for infrared detectors. The lattice structure of these example IV-VI semiconductors is the simple cubic lattice (also called an NaCl lattice). 

Intrinsic, wide-bandgap senuccHiductor. The cubic foon, (zinc blende structure) has a band gap of 3.54 eV at 300 K whereas the hexagonal fenm wurtzite) has a band g of 3.91 eV. Can be doped as both an n-type orp-type semiconductor. It may exhibit phcejdiCHescence, due to impurities, on illuminati[Pg.201]

Because of the importance of semiconductors in our everyday lives, it is important to draw attention to the structural similarities that exist within this group of materials. The majority of semiconductors exhibit either a diamond (Fig. 6.20) or zinc blende (Fig. 6.19) structure type, with the wurtzite structure (Fig. 6.21) being less common. In each of these prototype structures, atoms are in tetrahedral environments. Diamond-type stmetures are adopted by Si and Ge, and the addition of dopants (see Section 6.9)... 

Doped ZnO ZnO is a simple oxide crystallising in the hexagonal wurtzite structure. This oxide shows a broad spectrum of properties and has been the subject of intensive research for applications as a photocalyst, a magnetic semiconductor and also very recently as a piezoelectric, For electronic properties, the doped ZnO oxides exhibit a n-type character. It is also a TCO with a bandgap of 3.5 eV which is used in applications such as photovoltaic cells or flat... 

ZnO with a wurtzite structure is naturally an n-type semiconductor because of deviation from stoichiometry due to the presence of intrinsic defects such as O vacancies (Vo) and Zn interstitials (Zn ). Undoped ZnO shows n-type conductivity with electron densities as high as 10 cm [5], a value that fortunately has been reduced by molecular beam epitaxy (MBE) to about lO cm [9, 10] and by hydrothermal growth to below 10 " cm [11]. 

A smaller class of type II alloys of II-VI binaries also exists, including the (CdS) ,(ZnSe)i (CdS) ,(ZnTe)i (CdSe) ,(ZnSe)i (CdS) ,(CdTe)i-. (CdSe)x(CdTe)i i , and (CdS) c(ZnS)i i systems, which transform at some critical composition from the W to the ZB structure. Importantly, the transition temperatures are usually well below those required to attain a thermodynamically stable wurtzite form for the binary constituents (e.g., 700-800 °C for pure CdS and > 1,020 "C for pure ZnS). The type 11 pseudobinary CdxZni jcSe is of considerable interest in thin film form for the development of tandem solar cells as well as for the fabrication of superlattices and phosphor materials for monitors. The CdSe Tei-x alloy is one of the most investigated semiconductors in photoelectrochemical applications. 

This topic will be discussed in two steps, firstly the crystals with 3-dimensional structures (metal nanoparticles and 4-coordinate semiconductors) and then those with a layered stmcture (Bi, Se, Te, etc). A decrease in the crystal sizes can result in a change of the stmcture type if the surface energy gain exceeds the enthalpy of the corresponding phase transition. Thus, Co has the structure of the hep type in the bulk, the/cc type in 10-20 nm particles and the bcc type in 2-5 nm particles [33], Particles of In with the diameter of <5 nm have the/cc structure, and those from 5 nm upward to the bulk have the fecf-lattice [34]. Agl adopts the cubic stmcture in the particles larger than 50 nm and the hexagonal one in smaller crystals [35]. In As has the wurtzite (w) stmcture up to 40 nm and the sphalerite (zb) structure in grains of >80 nm [36]. Nano-CdS has the the zb stmcture for D = 4 nm, while the w-phase is stable for the bulk material [37, 38]. On the contrary, MnSe was obtained in the w-form in nanoparticles, whereas the zb phase is stable for bulk crystals [37, 39]. 

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