Doctoral Dissertations

Date of Award

12-1993

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Metallurgical Engineering

Major Professor

Douglas H. Lowndes

Committee Members

James R. Thompson, Dave Geohegan, Jay Jellison

Abstract

Pulsed KrF (248 nm) laser ablation of polycrystalline ZnS and ZnSe targets was used to grow high quality, fully epitaxial ZnS and ZnSe thin films on GaAs (001), GaAs (111). Lower quality single crystal films resulted when ZnS was grown on GaP(001), while polycrystalline films resulted when ZnS was grown on Si(001) and Ge(001). The optimal growth temperature was determined from the rocking curve FWHM of the (004) x-ray diffraction (XRD) peak. For ZnS the optimal growth temperature was found to be Tg = 325°ree;C; while the optimal temperature for PLA-growth of ZnSe it was Tg = 300°ree;C.

For the films grown on GaAs, XRD showed that the in-plane film and GaAs [100] and [010] axes were aligned (i.e., true three-dimensional epitaxy). For ZnS grown on GaAs(111), XRD showed that the in-plane directions of the ZnS were aligned in two types of domains with respect to the underlying GaAs substrate. This is consistent with a zinc blende structure with a large degree of hexagonal stacking. The XRD pattern from the ZnSe film grown on GaAs(111) also indicated that the in-plane directions of the ZnSe might have been aligned in two types of domains with respect to the GaAs substrate.

Stacking faults were found to be the dominant defects present in the ZnS films. The stacking fault density was determined to have an upper bound of ρsf ~ 2.5 x 1010 cm-cm-3. This stacking fault density is less than that in ZnS films grown by conventional techniques (e.g., MOCVD). Furthermore, the widths of the rocking curves of the PLA-grown ZnS films were less than for MBE-grown ZnS films of comparable thickness. Misfit dislocations were found to be the dominant defects present in the ZnSe films. The rocking curve width from the ZnSe(004) reflection, for a 225 nm thick film grown at the optimal Tg, implied a misfit dislocation density within the film of ρd = 1.6 x 1014 cm-3.

The band structures of the ZnS and ZnSe films grown on GaAs(001) were investigated by inspecting the dielectric function (ε1, ε2). For ZnS and ZnSe, ε2 approached zero below the bandedge indicating that both the PLA-grown ZnS and ZnSe films were of high optical quality. The E0 and Ε0+Δ0 transitions (Γ-point) also were clearly seen in the dielectric function spectrum of ZnS, while the Ε1 and Ε1+Δ1 transitions (L-point) were not seen because they occur above the upper limit of investigation. The Ε0, Ε0+Δ0, Ε1, and Ε1+Δ1 transitions all were seen clearly in the dielectric function spectrum of the ZnSe films.

Photoluminescence measurements of the ZnS and ZnSe films on GaAs(001) showed donor and acceptor-bound excitonic emissions. Both films also showed free excitonic emissions. In the ZnSe film this free exciton peak was split into the heavy- and light-hole components because of the strain present in the film.

A multitarget holder was installed that allowed in situ selection of either a ZnS or a ZnSe target. By alternately ablating each target, strained layer superlattices of the form (ZnSe)m-(ZnS)n were grown. The θ/2θ XRD pattern from a sample having 65 periods of compositional modulation showed several orders of superlattice satellite peaks. The SLS compositional modulation period, determined from the separation of adjacent superlattice peaks, was L4.8 nm. This period compared well (only 4% difference) with the desired value.

A new technique that utilized ablation of ZnSe into a low-pressure H2S gas ambient was developed to permit fabrication of ZnSe1-xSx alloy films. The variation of sulfur content (x) with H2S partial pressure was found to follow the relation x = A[P(H2S)]B, with B~1/2. The ability to grow films having predetermined compositions was evaluated by using this relation to determine the H2S partial pressure needed to grow an (001)-oriented alloy film having x = 0.075; the resulting sulfur content for this film was x = 0.06. Room temperature photoluminescence of the (001) ZnSe1-xSx films showed that the bandgap increased with increasing sulfur content; this behavior is consistent with sulfur entering the lattice on the selenium sites. To evaluate this spatial control over film composition, a ZnSe1-xSx multilayered epitaxial structure was fabricated that simultaneously incorporated both continuously graded and successive abrupt, periodic compositional changes.

This same technique also was applied to incorporate doping elements into semiconductor films by growing ZnS films in low pressure HCl ambients. Room temperature photoluminescence of a ZnS:Cl film grown in a 0.1% (-2 µTorr) HCl ambient showed a broad emission between 2 and 3 eV. This emission is due to transitions through deep levels associated with Cl complexes, and demonstrates that Cl was incorporated into the ZnS.

For both ZnS and ZnSe, a growth rate anisotrophy was observed for growth on the (001) and (111) surfaces. The difference in growth rates is due to differences in desorption rates on these surfaces. This difference in the rates is due to the difference in: (i) the site densities of the two surfaces, and (ii) the activation energies of the desorption rate constant.

Growth of the ZnSe1-xSx alloys in ambients with and without He buffer gas showed that the alloy-formation mechanism depends only on the H2S partial pressure and not the total ambient pressure. It also was found that the alloy composition is controlled by the dissociation of H2S.

To assist in identifying the rate-limiting step, ZnSe1-xSx films were grown in a P(H2S) = 22 mTorr ambient at various Tg. For the (001)-oriented alloys, the sulfur content was found to increase with increasing temperature, while the (111)-oriented alloys showed a constant sulfur content at all Tg. The slope of the line for the (001)-oriented alloy data gave an activation energy of 0.23 eV. It is shown that these data are consistent with the rate-limiting step being the replacement of the selenium with sulfur by adsorption.

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