Group II-VI Materials

Group II-VI Materials are presently important for optoelectronics in two device applications at opposite spectral regions, as well as in photovoltaics. The two device applications are the blue and near UV emitters (ZnS, ZnSe) and the near to far IR detectors (HgCdTe). 

CdTe is a II-VI material actively under development as a buffer layer on GaAs for subsequent HgCdTe growth and as a solar eell. CuInSea is also under development for solar cells. Both have bandgaps near 1.5 eV (the peak in the transmitted solar spee-trum). have surpassed the 10% efficiency range, and are relatively cost competitive. However, each of these compounds presents long term environmental pollution concerns. 

The III-V group alloys are by far the most studied compound semiconductors and consist of a wide variety of elements In, Ga, As, P, Sb, and N. The Ill-nitrides are split out as in a separate section (4.5.3) since the nitrides also include BN. 

The mixed alloy arsenides are important to a wide range of optoelectronic applications. The addition of A1 to GaAs allows for compositional control of the bandgap and formation of heterojunctions from various alloys written as Al Gai- As. In turn, a great variety of devices may be fabricated. A difficulty in fabricating these alloys is the reactivity of Al. A1 and its primary sources TMAl and TEAl are very reactive to oxygen. TMAl is the most popular reactant. Oxygen incorporation readily occurs because AlyO precipitates are involatile (GaO which readily evaporates from the deposition 

InP is another popular optoelectronic III-V alloy demonstrating high mobilities ( 200000 crn /Vs at 77 K, 5 000 at room temperature). InP substrates are available in 50 and 75 mm diameters, and larger diameters are being pursued. Typical source reactants are PHj and TMIn. The problems have been in stable transport of reactant materials and film purity. Both problems have been addressed purity has been greatly improved over the past several years, the bubbler internal geometry has been improved, and In sources have been configured to operate both in parallel or in series. 

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Group II-VI materials (e.g., CdTe, HgCdTe) Detectors Vertical Bridgman, horizontal Bridgman... 

The film thickness uniformity and film composition depend, in part, on the molecular or atomic flux variation across a substrate. This flux variation is a function of the directionality of the evaporation source (i.e., the molecular-beam distribution) and the orientation of the substrate relative to the source (7). In this section, the fundamental equations that describe the distribution of the incident flux will be introduced, along with the solution of these equations for the evaporation of group II-VI compound-semiconductor materials. 

The behavior noted for ZnS and (Cd ZnjS has been reported by others for group II-VI (21) and III-V (23) compound semiconductors. Because the analysis of the surface reaction zone is based on conservation of mass without regard for the mechanism of transporting reactants to the substrate, the framework should be applicable for the engineering analysis of the deposition of a broad group of compound-semiconductor electronic materials by both... 

Synthesis of Core-Shell Nanocrystals with InAs Cores Preparation of the InAs core-shell nanocrystals is carried out in a two-step process. In the first step, the InAs cores are prepared using the injection method with TOP as solvent. This allowed hundreds of milligrams of nanocrystals per synthesis to be obtained (as detailed above), with a size-selective precipitation being used to improve the size distribution of cores to a 10%. In the second step, the shells of the various materials were grown on the prepared cores. This two-step approach followed methods developed for the synthesis of group II-VI core-shell nanocrystals. 

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