Shin-ichi Ohta

Cathodoluminescence image of defects and luminescence centers in ZnS/GaAs(100)

The spatial distributions of the cathodoluminescence (CL) emissions from thin ZnS films on GaAs(100) have been examined by the low‐temperature CL measurement system combined with a transmission electron microscope (TEM). The correlation between these CL emissions and structural defects were studied by comparing the monochromatic CL images with the TEM images for both plan‐view and cross‐sectional observations. It is found that the neutral acceptor–bound exciton associated emission (A0,X) and the free‐electron‐to‐ionized acceptor transition emission (e,A) are affected by the stacking fault distribution. The localization of the emission due to the deep‐level emission transition near the interface suggest the diffusion of Ga atoms from the GaAs substrate. The characteristic distributions of the CL emission regions can be explained by considering the competitions among the recombination channels of those radiative processes for each type of an excess carrier, an electron, or a hole.

Molecular beam epitaxial growth of ZnS on a (100)-oriented Si substrate

Abstract ZnS films grown on (100) Si substrates were successfully obtained by molecular beam epitaxy. The dependence of the crystal quality on growth conditions was studied. The substrate temperature ( T s ) and the molecular beam flux ration of Zn to S ( J Zn / J S affect the crystal quality and the growth rate of the ZnS films profoundly. Crystallographic quality and surface morphology were significantly improved by growing at temperatures higher than 300°C. Below 300°C, growth orientation and growing planes of ZnS are different from others, namely ZnS(111) ‖ Si(511). The films grown at higher T s ( > 300°C) have the relation of ZnS(100) ‖ Si(100). By increasing J Zn / J S , the surface morphology became worse. It is found that the optimum growth conditions are T s = 340° C and J Zn / J S =1 or 2. The growth rate of ZnS decreases with incresing T s , and increases with increasing J Zn / J S and t hen saturates above J Zn / J S = 3. From these results, it is presumable that ZnS growth is controlled by the sticking coefficient of the minority molecular species.

Growth and characterization of CdS epilayers on (100)GaAs by atomic layer epitaxy

Abstract Monocrystalline CdS films are successfully grown on (100)GaAs at the low temperature of 340°C by atomic layer epitaxy (ALE) using a molecular beam epitaxy (MBE) system. The growth orientation of the epilayer is 〈100〉 of a zincblende structure with no twinning. The film thickness is in good agreement with the predesigned atomic layer numbers. The variation of the film thickness is uniform in the whole area of the epilayer and the surface morphology is flat and smooth. The photoluminescence spectra show dominant near-band-edge emission, suggesting a high crystallographic quality. It is indicated that the ALECdS epilayer is advantageous, especially for the ultrathin multilayer structure preparation.

High quality zinc sulfide epitaxial layers grown on (100) silicon by molecular beam epitaxy

ZnS films were successfully grown by molecular beam epitaxy on (100) Si substrates. These high quality films, grown at a substrate temperature of 340 °C with a molecular beam flux ratio of Zn to S of unity, exhibited high crystallographic quality and revealed a flat surface. Secondary ion mass spectroscopic analysis showed that the film contained little impurity. In photoluminescence, a broad peak was observed at 2.6 eV at room temperature while an additional sharp peak at 3.6 eV appeared at 77 K. The latter is considered to be band‐to‐band emission and the former may be due to the Cu‐blue emission.

Growth of CdS/ZnS superlattices at low temperature by atomic layer epitaxy

The (CdS)n/(ZnS)m superlattices (SLs) have been prepared on (100)GaAs at low growth temperature of 185 or 190°C by atomic layer epitaxy using a molecular beam epitaxy system. The SL growth progresses with the surface reconstruction change from c(2 × 2)-Zn to (2 × 1)-S for ZnS and from (1 × 1)-Cd or c(2 × 2)-Cd to (2 × 1)-S for CdS. The SL structures are confirmed by the satellite peaks observed in X-ray diffraction spectra. The SLs exhibit flat and smooth surfaces. The photoluminescence spectrum of the ((CdS)8/(ZnS)20)55 SL shows a sharp peak at 440 nm and suggests that the light emission originated from the quantum level in the SL.

Substrate temperature effect on crystallographic quality and surface morphology of zinc sulfide films on (100)‐oriented silicon substrates by molecular‐beam epitaxy

The crystallographic quality and the surface morphology of ZnS films on (100) Si substrates by molecular‐beam epitaxy are profoundly dependent on the substrate temperature Ts. When the Ts was below 300 °C, single crystalline films with twins grew along 〈100〉 directions in the initial stage of growth. However, when the film thickness was increased, the growth orientation changed from 〈100〉 to two 〈511〉 directions of the Si substrate. Namely, the relation of growth orientations between overgrowth and substrate is ZnS(111)∥Si(511). When the Ts was set between 340 and 370 °C, the grown films were single crystal with twins appearing initially and grew with a 〈100〉 orientation, but decreased with increasing film thickness. Especially in the case of Ts=340 °C, the ZnS film became best crystallized and exhibited the twin‐free streaky pattern in the reflection high‐energy electron diffraction pattern with a smooth surface morphology.

Orientation of CuGaS2 thin films on (1 0 0) GaAs and GaP substrates

Abstract CuGaS 2 films have been grown on (1 0 0) GaAs and GaP wafers by the vacuum evaporation technique using constituent elementary substances as sources. RHEED patterns of the films that use 〈0 1 1〉 azimuths in a substrate show two different patterns in mutually orthogonal directions. In one direction, the observed pattern is a single pattern which illustrates that CuGaS 2 grows in the [0 0 1] orientation direction (the c -axis growth). In another direction, however, additional patterns appeared. This phenomenon indicates that one pair of the 180 rotated {1 1 2} twins exists in addition to c -axis growth in a film, and it also indicates that additional orientations except for c -axis growth are reduced in films grown on compound substrates as compared with ones on (1 0 0) Si.

Preparation and characterization of AgGaS2 thin films by vacuum evaporation

Abstract AgGaS 2 thin films have been prepared by the vacuum evaporation method on GaAs(100). Elemental silver, gallium and sulfur have been used as the source materials. The X-ray diffraction and the RHEED analysis show that the good epitaxial films with the c -axis normal to the GaAs(100) substrate are grown for the stoichiometric composition. The non-stoichiometric films show that the two different orientational types coexist in a film. One type is that the c -axis is normal to the substrate. The other type is that the c -axis is parallel to the two a -axes of GaAs.

Correlation between cathodoluminescence and structural defects inZnS/GaAs(100) andZnS/eGaAs(100) studied by transmission electron microscopy

The spatial distributions of the cathodoluminescence (CL) emissions from thin ZnS and ZnSe films on GaAs(100) have been examined by the low-temperature CL imaging system combined with a transmission electron microscope (TEM). The correlation between these CL emissions and structural defects were studied by comparing the monochromatic CL images with TEM images for both plan-view and cross-sectional observations. It is found that the bound exciton associated emission (A0, X) and the (e, A) emission are affected by the stacking fault distribution. The localization of the emission due to the DLE transition near the interface suggest the diffusion of Ga or As atoms from the GaAs substrate.

Growth and characterization of (CdS)4(ZnS)16 superlattices

Abstract CdS ZnS superlattices (SLs), which contain 4 molecular layers of CdS and 16 molecular layers of ZnS, were successfully obtained on (100)GaAs substrates by means of atomic layer epitaxy (ALE) using a molecular beam epitaxy (MBE) system. It is suggested that each constituent layer has compressive and tensile strain. The layer configuration of the SL is in good agreement with the predesigned scheme, as shown by the results of X-ray diffraction analysis and transmission electron microscope pictures. This emphasizes that the ALE growth is effective in forming fine multilayer structures. The photoluminescence spectrum shows a dominant sharp peak at around 450 nm. The cathodoluminescence spectrum measured at room temperature also indicates a sharp peak at 453 nm. These emissions have originated from quantum levels formed in CdS well layers in the SL.