SEMICONDUCTOR ALLOYS

A useftil applicadon of time-dependent PL is the assessment of the quality of thin III-V semiconductor alloy layers and interfaces, such as those used in the fabri-cadon of diode lasers. For example, at room temperature, a diode laser made with high-quality materials may show a slow decay of the acdve region PL over several ns, whereas in low-quality materials nonradiative centers (e.g., oxygen) at die cladding interface can rapidly deplete the free-carder population, resulting in much shorter decay times. Measurements of lifetime are significandy less dependent on external condidons than is the PL intensity. 

Wei S-H, Zunger A (1987) Total-energy and band-structure calculations for the semi-magnetic Cdi-xMnxTe semiconductor alloy and its binary constituents. Phys Rev B 35 2340-2365... 

Rajeshwar K (1992) Electrosynthesized thin films of group II-VI compound semiconductors, alloys and superstractures. Adv Mater 4 23-29... 

Figure 9.56 The structure of a typical QD nanocrystal includes a semiconductor alloy core surrounded by a shell consisting of a different alloy structure. Early QD compositions involved the use of CdSe core with a ZnS shell, but many different alloy compositions have been used and are possible.

As a final comment on terminology, we note that elemental semiconductors are formed from a single element, e.g., Si or Ge, whereas compound semiconductors are formed from two binary), three ternary), four quaternary), or, rarely, more elements. Semiconductor alloys refer to solid solutions where either one anion or one cation can substitute for another, or possibly two or more such substitutions can occur for a binary semiconductor AB a simple alloy with C would be represented as Ai CjcB. Semiconductors are often classified by the group numbers in the periodic table. Thus, for example, I-VII semiconductors include Cul and AgBr, II-VI semiconductors include ZnS, CdTe, and HgTe, III-V semiconductors include GaAs, GaN, InP, and InSb, and IVx-VIv semiconductors include PbSe and Sn02. Fundamental physical properties are compiled in a recent handbook [22]. 

Some of the major questions that semiconductor characterization techniques aim to address are the concentration and mobility of carriers and their level of compensation, the chemical nature and local structure of electrically-active dopants and their energy separations from the VB or CB, the existence of polytypes, the overall crystalline quality or perfection, the existence of stacking faults or dislocations, and the effects of annealing upon activation of electrically-active dopants. For semiconductor alloys, that are extensively used to tailor optoelectronic properties such as the wavelength of light emission, the question of whether the solid-solutions are ideal or exhibit preferential clustering of component atoms is important. The next... 

The isotropic chemical shift, the trace of the chemical shift tensor, is one of the basic NMR parameters often measured for both spin-1/2 and quadrupolar nuclei. The CSA can also be measured in non-cubic environments, such as the n3Cd nuclei experience in the chalcopyrite structure of crystalline CdGeAs2 [141] or CdGeP2 [142], and the 31P nuclei in the latter compound [142], Although the isotropic chemical shift can be measured from the NMR spectrum of a static powder because the CSA is zero in many cases because of cubic symmetry of the lattice, improved resolution is obtained by using MAS to remove dipolar couplings. Two particular areas where the isotropic chemical shifts have proven very informative will now be discussed, semiconductor alloys and semiconductor polytypes. 

These chalcopyrite semiconductors are of interest for possible optoelectronic and photovoltaic applications. Several such II-IV-V2 semiconductor alloys have been studied two where the anions are substituted, CdGeAs2 XPX and ZnGeAs2 YPY [ 162], and one where the cations are substituted, ZnxCd xGeP2 [163]. 

In summary, NMR techniques based upon chemical shifts and dipolar or scalar couplings of spin-1/2 nuclei can provide structural information about bonding environments in semiconductor alloys, and more specifically the extent to which substitutions are completely random, partially or fully-ordered, or even bimodal. Semiconductor alloys containing magnetic ions, typically transition metal ions, have also been studied by spin-1/2 NMR here the often-large frequency shifts are due to the electron hyperfine interaction, and so examples of such studies will be discussed in Sect. 3.5. For alloys containing only quadrupolar nuclei as NMR probes, such as many of the III-V compounds, the nuclear quadrupole interaction will play an important and often dominant role, and can be used to investigate alloy disorder (Sect. 3.8). 

The study of local or long-range ordering in semiconductor alloys based upon the effects on NQCCs has been carried out in a number of cases. These studies are analogous to the study of ordering in In Ga P by means of the chemical shift interaction, as described in Sect. 3.3.1. 

Oranges, citric acid in, 6 632t ORBIT PRINT SELECT software, 18 243 Orbitrap, 15 662-663 Orb web, structure of, 22 630 Ordered intermetallic alloys, 13 530 Order, in amorphous semiconductor structure, 22 128-129 Ordering, in ternary semiconductor alloy preparation, 22 158-159 Order of addition, in large-scale... 

Ternary heterogeneous azeotrope, 8 823 Ternary plutonium oxides, 19 689 Ternary semiconductor alloys,... 

Various materials have been synthetized in reverse micelles (3). Cadmium sulfide and cadmium selenide semiconductors (38-47) were the first materials prepared by this method. This has been extended to semiconductor alloys such as... 

Source Considerations. Many CVD sources, especially sources for or-ganometallic CVD, such as Ga(CH3)3 and Ga(C2H5)3, are liquids at near room temperatures, and they can be introduced readily into the reactor by bubbling a carrier gas through the liquid. In the absence of mass-transfer limitations, the partial pressure of the reactant in the gas stream leaving the bubbler is equal to the vapor pressure of the liquid source. Thus, liquid-vapor equilibrium calculations become necessary in estimating the inlet concentrations. For the MOCVD of compound-semiconductor alloys, the computations have also been used to establish limits on the control of bubbler temperature to maintain a constant inlet composition and, implicitly, a constant film composition (79). Similar gas-solid equilibrium considerations govern the use of solid sources such as In(CH3)3. 

Cl. 1 Ohmic contacts to GaN and the III-V nitride semiconductor alloys C1.2 Schottky barrier contacts to GaN Cl.3 Band offsets at interfaces between AIN, GaN and InN... 

The technique has the flexibility to grow a multitude of compound semiconductor alloys by varying the composition of the source gas. 

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