Structural and Optical Properties of Strained-Layer Superlattices

Citation

Miles, Richard Henry (1989) Structural and Optical Properties of Strained-Layer Superlattices. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/042y-d234. https://resolver.caltech.edu/CaltechETD:etd-02082007-093744

Abstract

This thesis describes investigations into the optical and structural properties of strained-layer superlattices. The purpose of the work was twofold: to establish the merits of strained-layer structures in applications, particularly to optoelectronics; and to examine structural characteristics of superlattices in which the lattice-mismatch between adjacent layers is large. Optical properties of CdTe/ZnTe superlattices have been examined through photoluminescence experiments. Observed band gaps have been compared with those expected from calculations of electronic band structure, including effects that are due to strain. Band gaps of a variety of II-VI superlattices have been calculated based on the agreement between theory and experiment in the CdTe/ZnTe system. The accommodation of lattice mismatch has been investigated for CdTe/ZnTe and Ge_0.5Si_0.5/Si superlattices. The assumptions behind traditional single-film critical thicknesses and their extensions to multilayer structures were of particular interest in these studies.

In Chapter 2 we use photoluminescence experiments to examine the optical properties of CdTe/ZnTe superlattices grown on a variety of Cd_xZn_1-xTe buffer layers. The work was motivated by interest in wide-band-gap II-VI's as possible visible light emitters and detectors and, more generally, by interest in the effects of strain and dislocations on the optical properties of strained-layer superlattices. Photoluminescence from the superlattices is observed to be several orders of magnitude more intense than from a Cd_0.37Zn_0.63Te alloy. Spectra are dominated by Gaussian distributions of excitonic lines. The 20-30meV widths of these distributions show that superlattice layer thicknesses were controlled to approximately one monolayer. Identifying the superlattice band gaps as the high-energy edges of the observed excitonic luminescence yields sample energy gaps substantially lower than expected for alloys. Observed gaps are in excellent agreement with those calculated from a ^→k • ^→p model, assuming strain appropriate to a free-standing structure. This configuration is one in which dislocations at the superlattice/buffer-layer interface have redistributed strain within an otherwise dislocation-free superlattice in manner that minimizes the elastic strain energy within the structure. The free-standing configuration is argued to be plausible in view of calculated critical thicknesses and strain relaxation rates. Calculations of the effects of a free-standing strain on the electronic band structure of CdTe/ZnTe superlattices show that strain can substantially reduce band gaps (on the order of 100meV for a 6% mismatch), and causes transitions from type-I to type-II band alignments. Attempts to observe laser oscillation in these CdTe/ZnTe superlattice structures have proven unsuccessful to date, although Cd_0.25Zn_0.75Te/ZnTe structures have recently been reported to lase.

Chapter 3 describes a structural study of the CdTe/ZnTe superlattices examined in Chapter 2. Strain fields and dislocation densities are inferred from x-ray diffraction, in situ reflection high-energy electron diffraction (RHEED), and transmission electron microscopy (TEM). All of our samples are observed to exceed the critical thickness for the nucleation of misfit-accommodating dislocations. Although each of the structures appears to be highly defective, the free-standing limit appears to be plausible, as defect densities drop substantially within a micron of the superlattice/buffer-layer interface, regardless of the buffer layer used. Although several samples substantially exceed predicted critical thicknesses, the sample that shows the smallest degree of residual strain lies below limits derived from a previous empirical study. This result demonstrates that dislocation formation in superlattices is not appropriately characterized by applying traditional critical thickness models to an alloy of equivalent total thickness and average composition. Variations in strain fields appear to be correlated with sample growth conditions. As growth parameters are neglected in traditional energy-balancing models of critical thickness, it is argued that activation barriers associated with the nucleation or glide of dislocations can substantially inhibit the relaxation of strain beyond the equilibrium limits.

In Chapter 4 we demonstrate that the accommodation of lattice mismatch in Ge_0.5Si_0.5/Si superlattices is highly dependent on the conditions under which a sample is grown. Dislocation densities of 1.5 x 10⁵cm^-1 drop to levels undetectable by TEM (< 10⁵cm^-2) as the growth temperature of compositionally identical superlattices is lowered from 530°C to 365°C. Thus, by lowering growth temperatures, it is possible to freeze a structure in a highly strained metastable state well beyond the critical thickness limits calculated by equilibrium theories. There appears to be a large kinetic barrier blocking dislocation nucleation or glide; the effect we observe cannot be explained by mismatched thermal expansion coefficients alone. These results are contrary to initial studies of Ge_xSi_1-x alloys, which appear to display critical thicknesses relatively independent of temperature over the ranges described here. Recognizing that defect creation can be inhibited in severely mismatched superlattices should be important in growing heavily strained films of high quality.

Finally, the Appendix contains maps of band gap as a function of layer thicknesses for a variety of II-VI superlattice systems, calculated using the Bastard model described in Chapter 2. Agreement with experiment is good for the CdTe/ZnTe superlattices examined here. As mentioned in Chapter 1, comparison of these calculated gaps with those measured experimentally leads to a prediction of ΔE_v = 1.0 ± 0.1eV for the ZnSe/ZnTe valence band offset.

Thesis Files