Group IIVI Semiconductor Growth Parameters

The Group II-VI semiconductors have been used for electroluminescent films (zinc sulfide and zinc selenide), infrared and far-infrared photodetectors (mercury cadmium telluride), thin-film photovoltaic devices (cadmium telluride), and, most recently, blue semiconductor lasers (zinc selenide).

The wide-bandgap materials described below include zinc selenide (ZnSe), zinc sulfide (ZnS), and zinc sulfide selenide (ZnSxSe1-x).

Both ZnSe and ZnS can be grown on GaAs or GaP substrates by using diethylzinc (DEZn) or dimethylzinc (DMZn) as the source of zinc, and either hydrogen sulfide (H2S) or hydrogen selenide (H2Se) respectively. However, the MOCVD growth of wide-bandgap selenides and sulfides has two major problems that are not encountered in the growth of Group III-V materials. The first is a premature reaction between DMZn and DEZn and H2Se or H2S, and the second is the formation of native defects, such as zinc, selenium, and sulfur vacancies at high growth temperatures. The first problem leads to poor surface morphology, whereas the second problem seems to be inherent to wide-bandgap semiconductors, especially materials that contain relatively high vapor pressure elements, such as ZnSe and ZnS.

The first problem can be solved by using both Group II and Group VI dialkyl sources, such as DMZn or DEZn and dimethylselenium (DMSE) or diethylselenium (DESe) because of the high decomposition temperature of these sources. The second problem can be reduced by decreasing the growth temperature. By using laser- or plasma-assisted MOCVD growth techniques, the growth temperature can be reduced to the range of 350 to 400 °C (660 to 750 °F) (Ref 155). Alternate precursors, such as Lewis acid-base adducts of dialkylzincs with dialkyl selenides or dialkyl sulfides and either H2Se or H2S, also reduce the growth temperature, although the premature reaction is not completely eliminated with these source combinations (Ref 156).

ZnSxSe1-x layers are lattice matched to GaAs and GaP substrates at x = 0.06 and x = 0.83, respectively. They can be grown at 500 °C (930 °F) using the precursors diethylsulfur (DES), DEZn, and DESe. The epitaxial layer shows excellent morphology, as evidenced by a narrow x-ray rocking curve (Ref 157). The growth temperature can be lowered by using an alternative source combination of the adduct, such as DMZn-DMSe, H2S, and H2Se, but it then becomes more difficult to control the alloy composition and attain lattice-matched layers. The growth mechanism that impedes control of the composition in this case is not clear. Photo-assisted CVD using alkyls of zinc, sulfur, and selenium as source materials is an effective low-temperature technique for the deposition of lattice-matched ZnSSe layers.

The narrow-bandgap materials described below include cadmium telluride (CdTe), mercury telluride (HgTe), and mercury cadmium telluride (HgCdTe).

Cadmium telluride can be grown by directly combining the vapors of the two elements, which are carried by hydrogen or nitrogen. Cadmium vaporizes at 756 °C (1390 °F), and tellurium, at 990 °C (1815 °F) (Ref 158). This material also can be grown by MOCVD through this reaction:

which takes place at substrate temperatures between 150 and 250 °C (300 and 480 °F) under the activation of an excimer laser (Ref 159).

Mercury telluride can be grown by using plasma-enhanced CVD, with dimethylmercury (DMHg) and dimethyltellurium (DMTe) as the mercury and tellurium sources, respectively. Williams et al. demonstrated a growth rate of 4 pm/h (160 pin./h) at a deposition temperature of 85 °C (185 °F), a pressure of 65 Pa (0.5 torr) and an RF power of 2 W at 15 MHz (Ref 160).

which takes place in the temperature range of 325 to 350 °C (620 to 660 °F), or through the reaction:

(C2H5)2 Te + (CH3)2Cd + nU2 -> CdTe + hydrocarbons

For HgCdTe, the selection of MOCVD growth temperature is a problem. The most commonly used tellurium precursor, diethyltellurium (DETe), pyrolyzes effectively only at a temperature of approximately 410 °C (770 °F). This is higher than the pyrolysis temperature of dimethylcadmium (DMCd), the Group II alkyl commonly used. The high temperatures require a very high mercury pressure, which is supplied by heating elemental mercury. The combination of a high mercury concentration and a high molecular weight leads to convection cells in the vapor. It also results in the creation of large concentrations of mercury vacancies, which act as acceptors. Finally, the self-diffusion coefficients at 400 °C (750 °F) are large, precluding the growth of superlattice structures with abrupt interfaces. Lowering the growth temperature becomes the major consideration of the growth of HgCdTe. This material has been grown at 440 °C (825 °F) on GaAs and InSb substrates by using DETe and DMCd (Ref 161).

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