ZnS/Si/ZnS Quantum Well Structures for Visible Light Emission
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INTRODUCTION There is great interest in the development of new materials and structures which can efficiently allow the transfer of information at ultra-high speed throughout various advanced data processor or telecommunications systems, whether from chip to chip, board to board, machine to machine, or machine to human operator (displays). Unfortunately, although silicon is the basic building block for most microelectronic circuits, it has never proven possible to develop silicon light-emitting diodes (LEDs) which could provide optical transfer of information. Hybrid
approaches such as growth of light-emitting GaAs on Si have given disappointing results.' True wafer scale integration will only become a reality if all of the light-emission, light detection, and electronic amplification functions can be implemented directly in a single silicon wafer. Quantum confined silicon using ZnS may provide the missing functionality. 2 To investigate the properties of quantum confined silicon with a controlled, planar device structure, we have undertaken a study to grow silicon quantum wells (QW) using chemical vapor deposition.3
THEORY AND EXPERIMENT
Experiment The basic structure chosen for this study consists of silicon confined by lattice-matched zinc sulfide cladding layers. ZnS can provide effective barriers to Si because it has a wide band gap (3.6 eV) while being closely lattice matched to Si and ZnS (a = 5.4307 A and 5.4093 A, respectively), favorable for the growth of films with low dislocation densities. ZnS can be doped n-type, while the Si substrate can be made p-type, which should allow injection of charge carriers into the quantum structures followed by radiative recombination; since the refractive 295
Mat. Res. Soc. Symp. Proc. Vol. 405 ©1996 Materials Research Society
index of ZnS is significantly lower than that of silicon, an optical waveguide should be possible. General deposition conditions were described earlier. 3 Films were grown in a commercial MOCVD reactor featuring a horizontal reaction chamber and an RF heated susceptor tilted from horizontal. Reactions were carried out at a pressure of 50 torr, with a hydrogen carrier gas flow rate of 4 slpm. First, a film of ZnS was grown on the silicon to act as a lower cladding layer. ZnS was grown using diethyl zinc (DEZ) and hydrogen sulfide as reactant gases; single crystal films were grown at a substrate temperatures of 600'C. Silicon quantum wells were deposited using disilane onto the ZnS cladding layer, typically at 600'C. Most samples featured multiple quantum wells (MQW), with each silicon deposition followed by a ZnS film. Upon completion of the final silicon quantum well deposition step, the upper ZnS cladding region was grown. In one run, the DEZ and H2S were allowed to continue flowing during the silicon deposition. Since we have found no evidence of any solubility between Si and ZnS, it is possible that in this sample, Si quantum dots may have precipitated in the ZnS host.
Theory
It is possible to estimate the effects of quantum confinement
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