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The authors report the growth of ZnO/ZnMgO multiple quantum well (MQW) structures by plasma-assisted molecular beam epitaxy. A set of three MQW samples with different well thicknesses were grown on c -plane sapphire substrates. Structural and optical properties of the samples were characterized by reflection high-energy electron diffraction, high-resolution x-ray diffraction (XRD) and photoluminescence measurements. Clear superlattice satellite peaks and thickness fringes observed in XRD measurements indicate the formation of periodic structure with good interfacial quality and high crystalline quality. Mid infrared absorptions around 3 μm are observed from Fourier transform infrared spectroscopy measurement. The polarization-dependent absorption proves that the absorptions are originated from intersubband transitions.

In order to increase the conduction band offset of the ZnCdMgSe-based material system we studied the incorporation of strained CdSe layers to obtain deeper quantum wells for shorter wavelength intersubband transitions than those obtained in lattice-matched structures. Five CdSe/ZnSe/ZnCdMgSe multi-quantum wells (QW) samples grown by molecular beam epitaxy are studied in detail by transmission electron microscopy (TEM), X-ray diffraction (XRD), cw-photoluminescence (PL), and Fourier Transform Infrared (FTIR) absorption experiments. TEM and XRD results confirmed good structural quality of the samples. All the multi-QW PL energies were below the ZnCdSe lattice-matched to InP alloy bandgap (2.1 eV), which serves as first evidence of having achieved deeper quantum wells. FTIR absorptions from 3.83 to 2.56 μm were measured, shorter than those achieved by the lattice matched system. Simulations based on these results predict that absorptions as low as 2.18 μm can be obtained with these materials.

CdTe layers were grown on (0 0 1) GaAs substrates with a ZnSe buffer layer by molecular beam epitaxy. The high-quality ZnSe (0 0 1) epitaxy on GaAs allows us to control the II–VI/III–V heterovalent interface prior to the CdTe deposition. By subsequently adjusting the interface between ZnSe and CdTe, it was possible to selectively obtain the growth of either (0 0 1) or (1 1 1) CdTe epitaxial layers on the (0 0 1) ZnSe/GaAs substrate. Reflection high energy electron diffraction indicates that the nucleation of the CdTe epitaxial layers turns two-dimensional within a few seconds of initiating the growth. X-ray diffraction and photoluminescence measurements indicate that both the (1 1 1) and the (0 0 1) CdTe films are of high structural quality despite the large lattice constant mismatch of 14.6% between CdTe and the (0 0 1) ZnSe/GaAs substrate.

In this paper, we present our recent progress towards the realization of short wavelength intersubband devices with wide band gap II–VI selenide semiconductors. Intersubband electroluminescence at 4.8 μm was observed in a ZnCdSe/ZnCdMgSe quantum cascade structure at temperatures up to room temperature. We also developed a new ZnCdSe/MgSe quantum structure with metastable MgSe barriers. Intersubband absorption in the 3–5 μm range was observed. Calculations suggest that with this structure intersubband transitions as short as 1.55 μm may be achieved.

The authors report the effect of a low-temperature buffer layer of Cu 2 O (LTB-Cu 2 O) on the growth of Cu 2 O thin films. The samples were prepared by radio frequency-magnetron sputtering. It was found that with the introducing of LTB-Cu 2 O, the linewidth of the X-ray diffraction peak decreases and the Cu 2 O thin films have preferred (1 1 1) orientation. By introducing LTB-Cu 2 O, the grain size sufficiently increases and the surface flatness improves, as indicated from the atomic force microscopy measurement. Under the optimum growth conditions, hole mobility of 256 cm 2 V −1 s −1 , which is the highest value reported so far, with the hole concentration of 1×10 14 cm −3 was achieved.

We report growth by molecular beam epitaxy of a Zn x Cd (1− x ) Se/Zn x ′ Cd y ′ Mg (1− x ′− y ′) Se quantum cascade (QC) structure that was designed for electroluminescence (EL) at ∼4.5 μm. The QC active region is comprised of a two-well asymmetric coupled quantum well (ACQW) structure, which was thoroughly investigated to establish the growth conditions for the QC structure. After growth of several ACQW test samples, the conditions for growth were optimized for the QC structure. In-situ RHEED, photoluminescence (PL), high-resolution X-ray diffraction (HR-XRD) and Fourier transform infrared (FTIR) spectroscopy results were used for characterization. The QC structure grown under these conditions exhibited EL at 4.8 μm, in excellent agreement with the design value.

Optical and morphological studies are used to investigate the effects of chemical composition and, in particular, the magnesium content of the Zn x Cd y Mg 1− x − y Se barrier layers on the size, density and uniformity of CdSe self-assembled quantum dots (QDs). A reduction of the uncapped QD size, as well as a blue shift of the capped QD photoluminescence peak position by increasing Mg concentration in the Zn x Cd y Mg 1− x − y Se barrier has been demonstrated by changing the Mg cell temperature during growth. In addition, a more uniform and more densely packed QD layer has been observed with an increase of the MgSe fraction in the Zn x Cd y Mg 1− x − y Se barrier layer using three-dimensional topographic atomic force microscopy images of the surface of uncapped QDs. Results point to Mg as a chemical factor that induces QD formation, either by increasing the density of atomic steps or/and by changing the energy of the Zn x Cd y Mg 1− x − y Se surface.

The effect of Mn as a surfactant on the formation of InAs nanostructures was studied. The shape of the nanostructures changed drastically when a layer of Mn was added on the growth front before InAs growth. When 6 ML 2 1 1 (6 monolayers in (2 1 1) plane) of InAs was grown on GaAs (2 1 1)B with an initial Mn coverage of 0.5 ML, quantum wire-like structures (QWRs) were formed at growth temperatures ranging from 450 to 510°C. With the same initial coverage of Mn, two kinds of nanostructures, quantum dashes (QDHs) and quantum dots (QDs) were formed at 400 or 425°C, whereas only QDs resulted at 380°C. Nanostructures on (1 0 0) and (3 1 1)B surfaces were also investigated and compared.

The authors present the design and growth of zincblende MgSe/Zn x Cd 1 −x Se multiple quantum wells (MQWs), which have a large conduction band offset of 1.2 eV, useful for intersubband (ISB) device applications. The samples were grown on (0 0 1)-InP substrates by molecular beam epitaxy. In situ reflection high energy electron diffraction (RHEED) shows that thin MgSe layers with zincblende structure can be epitaxially grown on InP substrates. Structural degradation of MQW structures due to phase transition of MgSe from zincblende to rocksalt is suppressed and structural quality improved with the introduction of a thick Zn x Cd 1 −x Se spacer layer. The ISB absorptions in 3.35–4.9 μm have been observed in MgSe/Zn x Cd 1 −x Se structure. The dependence of ISB absorption on the MgSe barrier thickness is also studied in this structure.

InAs and (In,Mn)As nanostructures grown on GaAs(1 0 0), (2 1 1)B and (3 1 1)B substrates have been studied. Quantum dots (QDs) were observed when InAs was grown on GaAs(1 0 0) (or (3 1 1)B). QDs with bimodal size distribution were formed when InAs was deposited on GaAs(2 1 1)B at lower growth temperatures (" 4 ) whereas quantum dashes (QDHs) were observed at higher " 4 . (In,Mn)As QDs grown on GaAs(1 0 0) showed a broad range of dot sizes with irregular shape. (In,Mn)As QDs with bimodal size distribution were observed for the structure grown on GaAs(3 1 1)B. (In,Mn)As QDs grown on GaAs(2 1 1)B showed improved size uniformity compared to those grown on GaAs (1 0 0) and (3 1 1)B. The e!ects of Mn as a surfactant on InAs nanostructures were also studied. ( 1999 Elsevier Science B.V. All rights reserved.

High-resistivity and p-type ZnSe-based II–VI semiconductor superlattices were prepared by molecular beam epitaxy on (100) GaAs or (100) InP substrates. The modulation-doped ZnSe-ZnTe superlattice by using antimony as a dopant has a hole concentration of 5×10 14 cm -3 . The absorption spectra of undoped ZnSe-ZnS superlattices were measured at temperatures ranging from 16 K to room temperature and three absorption peaks corresponding to 1E-1HH, 1E-1LH and 1E-3HH transitions were observed for the first time. Two absorption peaks corresponding to the transitions of the first heavy-hole and light-hole to the conduction band were also observed in the room temperature absorption spectra of ZnSe-ZnTe superlattices.

The refractive indicies of ZnSe-ZnTe and ZnSe-ZnS strained-layer superlattices (SLSs) were determined by double-beam reflectance measurements at 300 K. A discontinuity was found in the index spectra at about the energy equal to the transition energy E 1 (e−h) between n =1 confined electron and hole levels. Large difference was found between the indices of the SLSs and those of the compositional bulk materials. Using the effective infinite-well model within effective mass approximation and taking into account the large strain effect, we also calculated the shifts of the exciton peaks with applied electric fields for ZnSe-ZnS multiple quantum wells (MQWs).