L. A. Kolodziejski

High resolution electron microscope study of epitaxial CdTe‐GaAs interfaces

CdTe films have been grown on (100) GaAs substrates with two different epitaxial relations: (111)CdTe∥(100)GaAs and (100)CdTe∥(100)GaAs. High resolution electron microscope observation of these two types of interfaces was carried out in order to investigate the role of the substrate surface microstructure in determining which type of epitaxy occurs. The interface of the former type shows a direct contact between the CdTe and GaAs crystals, while the interface of the latter type has a very thin layer (∼10 A in thickness), which is most likely an oxide, between the two crystals. These observations suggest that the GaAs substrate preheating cycle prior to CdTe film growth is crucial in determining which type of epitaxy occurs in this system.

Molecular beam epitaxy of diluted magnetic semiconductor (Cd1−xMnxTe) superlattices

In this letter we report the first growth of diluted magnetic semiconductor superlattices. Bright and dark field transmission electron microscopy images confirm that the superlattices consist of distinct periodic layers of uniform composition and thickness. The photoluminescence spectra of the superlattices are found to be over three orders of magnitude more intense when compared to uniform thin films (and bulk samples) grown under similar conditions. The reason for this large increase is not clearly understood but this effect may have important consequences for future applications of these novel materials in optoelectronic devices.In this letter we report the first growth of diluted magnetic semiconductor superlattices. Bright and dark field transmission electron microscopy images confirm that the superlattices consist of distinct periodic layers of uniform composition and thickness. The photoluminescence spectra of the superlattices are found to be over three orders of magnitude more intense when compared to uniform thin films (and bulk samples) grown under similar conditions. The reason for this large increase is not clearly understood but this effect may have important consequences for future applications of these novel materials in optoelectronic devices.

Nucleation and characterization of pseudomorphic ZnSe grown on molecular beam epitaxially grown GaAs epilayers

The heteroepitaxial growth of ZnSe on GaAs epilayers grown by molecular beam epitaxy is found to occur via a two‐dimensional growth mechanism. Alternatively, nucleation on a GaAs substrate exhibits three‐dimensional growth characteristics. The differentiation of the type of nucleation is evidenced by reflection high‐energy electron diffraction intensity oscillations, as well as the dynamic behavior of the diffraction patterns. Photoluminescence measurements of pseudomorphic ZnSe epilayers grown on GaAs epilayers provide a direct measurement of ZnSe deformation potentials.

Wide gap II‐VI superlattices of ZnSe‐Zn1−xMnxSe

In this letter we report the first growth of wide gap II‐VI semiconductor superlattices of Zn1−xMnxSe (0<x<0.51). The superlattices are grown by molecular beam epitaxy. Bulk crystals of Zn1−xMnxSe (0<x<0.57) grown in the past have shown a predominance of the zincblende phase only up to x=0.3; above this mole fraction a predominance of the hexagonal phase is observed. For the superlattices and epilayers reported here, only the zincblende phase (100) is present over the entire composition range investigated. Transmission electron microscopy shows clear evidence of the superlattice structure. Photoluminescence measurements of ZnSe epilayers show a dominant free‐exciton feature while the Zn1−xMnxSe epilayers exhibit two distinct photoluminescence peaks. The relative intensities of band‐to‐band transitions and Mn‐related transitions are somewhat comparable for the epilayers. However, the superlattices having ZnSe in the wells show virtually no Mn‐related emission regardless of the mole fraction of Mn in the ba...

Molecular beam epitaxy of Cd1−xMnxTe

In this letter we report the first growth of high‐quality, single crystal thin films of Cd1−xMnxTe; the films are deposited by molecular beam epitaxy. Cd1−xMnxTe is one of a newly developed class of magneto‐optic materials called diluted magnetic semiconductors. Films with mole fractions of up to 50% Mn have been grown with no indication of any MnTe phases. A sputtered Auger electron spectroscopy depth profile shows that the Mn is uniformly incorporated throughout the film.

Two‐dimensional metastable magnetic semiconductor structures

Metastable zinc‐blende MnSe has been grown by molecular beam epitaxy. The magnetic semiconductor has been incorporated into three novel superlattice structures which include a binary ZnSe/MnSe superlattice, a comb superlattice, and a ZnSe/(Zn,Mn)Se superlattice structure consisting of ZnSe wells perturbed by the insertion of ultrathin layers (two to three monolayers) of MnSe. Reflection high‐energy electron diffraction and transmission electron microscopy reveal the zinc‐blende crystal structure of the MnSe layers. Preliminary optical measurements show agreement with anticipated exciton emission energies; such spectra show large magnetic field induced shifts indicating that, although MnSe is antiferromagnetic, these ‘‘two‐dimensional’’ magnetic sheets exhibit paramagnetic behavior.

Wide-gap II-VI superlattices

At this time, there is evident a sharp increase of interest in the II-VI class of semiconducting compounds largely due to recent success in the growth of these materials by molecular beam epitaxy (MBE). One of the more important areas for application of high-quality II-VI films is infrared imaging where CdTe deposited by MBE onto GaAs substrates is proposed as the substrate for subsequent HgCdTe and HgTe/CdTe superlattice deposition. Moreover, interest in the wide-gap II-VI compounds is stimulated by the need for electronically addressable flat panel display devices, and for the development of wide-gap (blue) LED and injection laser devices. For applications in the blue portion of the visible spectrum, ZnSe and ZnS have long been favored candidates. Very high quality ZnSe has recently been grown by MBE. The photoluminescence spectra of the MBE-grown ZnSe samples grown at Purdue University and elsewhere strongly suggests that, in many cases, the film quality exceeds that obtainable in bulk form. In addition to the Conventional II-VI materials, a new class of materials called diluted magnetic semiconductors (DMS) are currently receiving considerable attention. DMS are II-VI semiconductors such as CdTe or ZnSe with a fraction of the group II element substituted by a magnetic transition element such as Mn. The incorporation of Mn leads to very large magnetooptic effects, on the Order of several hundred times that exhibited by conventional semiconductors of a comparable bandgap. An especially significant feature of II-VI DMS materials is the increase in bandgap resulting from Mn incorporation. The band-gap increases with Mn mole fraction in a manner similar to the effect of Al in the (Ga, Al)As system, and with similar implications to the creation of quantum well structures and superlattices. The first part of this paper reviews our recent work involving quantum well structures in the (Zn, Mn)Se system, with the second part emphasizing (Cd, Mn)Te. Included in the discussion of superlattice structures in (Cd, Mn)Te is a description of recent results concerning the heteroepitaxial growth of CdTe on GaAs-materials having a lattice constant mismatch of 14.6 percent.

Stimulated emission and laser oscillations in ZnSe‐Zn1−xMnxSe multiple quantum wells at ∼453 nm

In this letter we report the first observation of stimulated emission and laser oscillations occurring in ZnSe‐Zn1−xMnxSe quantum well structures. The results were obtained in superlattices consisting of alternating layers of ZnSe and Zn1−xMnxSe on a ZnSe buffer layer grown by molecular beam epitaxy on GaAs. Gain spectra were measured on these samples and thresholds for stimulated emission determined for various emission wavelengths. Optically pumped lasers were fabricated from one of these quantum well structures and found to operate in the blue portion of the visible spectrum from 451.5 to 455 nm. Lasing was observed up to 80 K.

Pseudomorphic ZnSe/n‐GaAs doped‐channel field‐effect transistors by interrupted molecular beam epitaxy

The fabrication and current‐voltage characteristics of the first depletion‐mode field‐effect transistors based on a pseudomorphic ZnSe/n‐GaAs heterointerface are described. The devices are doped‐channel field‐effect transistors produced by means of interrupted growth with the use of two separate molecular beam epitaxy systems. Very strong (visible to the naked eye) reflection high‐energy electron diffraction intensity oscillations persist for 120 periods when ZnSe is nucleated on the GaAs epilayer. The current‐voltage characteristics of the transistors are close to ideal; channel modulation indicates that the Fermi level is not pinned at the ZnSe/GaAs interface.

ZnSe-ZnMnSe and CdTe-CdMnTe superlattices

We report the growth and characterization of superiattices of a new wide-gap, zincblende material system, Zn1 − xMnxSe. ZnSe exhibiting dominant free excitonic emission in photoluminescence (PL) is the well material, while wider-band-gap Zn1 − xMnxSe (0.23 < x < 0.66) forms the barrier material. PL measurements show greatly enhanced quantum efficiency compared to films of the ZnSe well material, while transmission electron microscopy shows extremely abrupt interfaces by the presence of seventh-order satellite spots. Previously reported superiattices in the CdTe-CdMnTe material system were grown with the (111) orientation, and exhibit unique excitonic properties believed related to the (111) interfaces. Using various techniques to select (111) or (100) heteroepitaxy of CdTe on (100) GaAs, we report the first (100) superlattices of this material, and compare the optical properties to the previous (111) structures.