I. Yamakawa

Atomic-scale observation of interfacial roughness and As–P exchange in InGaAs/InP multiple quantum wells

Cross-sectional scanning tunneling microscopy (XSTM) has been used to study interfacial properties of InP-on-InGaAs interfaces in InGaAs/InP multiple quantum wells grown by metalorganic vapor phase epitaxy with a growth interruption. XSTM has enabled us to separately identify step-like roughness and distributions of As atoms incorporated in the InP layer near the interface. The As composition profile along the growth direction analyzed from distributions of As atoms in XSTM images shows an exponential variation with distance from the InP-on-InGaAs interface. It is found that the growth interruption of 30 s reduces considerably the roughness amplitude to 0.45 nm from 1.1 nm and increases the coherent length from 22 to 27 nm.

Scanning tunneling microscopy study of interfacial structure of InAs quantum dots on InP(001) grown by a double-cap method

The interfacial properties of InAs self-assembled quantum dots (QDs) on InP(001) grown by the double-capped method by metal-organic chemical-vapor deposition have been investigated by means of cross-sectional scanning tunneling microscopy (STM). Truncated pyramidal QDs with a monolayer-step height in the range of 6–14 ML are observed in the STM images, and their top and bottom interfaces are extremely sharp. On the side of the QDs, however, segregation of As atoms is observed, which suggests that the migration of As atoms from the QDs takes place by As∕P exchange during the cap and etching processes in the double-cap procedure.

Cross-Sectional Scanning Tunneling Microscopy Study of Interfacial Roughness in an InGaAs/InP Multiple Quantum Well Structure Grown by Metalorganic Vapor Phase Epitaxy

Interfacial roughness of InxGa1-xAs/InP (x=0.53) multiple quantum wells (MQW) grown by metalorganic vapor phase epitaxy has been investigated by cross-sectional scanning tunneling microscopy (STM). The MQW structure is composed of 125 periods of 11-nm-wide well layers and 44-nm-wide barrier layers on an InP (001) substrate. The observed STM images have revealed that the InGaAs-on-InP interface is extremely sharp compared to the InP-on-InGaAs interface; the roughness and terrace size on the InGaAs-on-InP interface are 1–2 monolayers (ML) and 31 nm, respectively, while they are 3–4 ML and 9 nm on the InP-on-InGaAs interface. The large roughness observed for the InP-on-InGaAs interface is interpreted in terms of the incorporation of residual As atoms on the InGaAs surface.

Scanning-tunneling-microscopy observation of heterojunctions with a type-II band alignment in ZnSe∕BeTe multiple quantum wells

Heterojunctions of ZnSe∕BeTe multiple quantum wells (MQW) with a type-II band alignment have been investigated by cross-sectional scanning tunneling microscopy (STM). The brightness of the ZnSe and BeTe layers in the cross-sectional STM image is inverted between filled- and empty-state images, taken by switching the bias polarity of the sample bias voltage in constant current mode. Such inversion of the brightness indicates changes in the band offsets of the conduction and valence bands between the ZnSe and BeTe layers of the type-II MQW. The roughness of interfaces in the filled state images has also been investigated on an atomic scale. It is found that the roughness amplitude Δ, and the correlation length Λ, which characterize the observed interfacial roughness, are comparable to the values observed for III-V heterostructures.

Composition Profile of ZnSe/BeTe Multiple Quantum Well Structures Studied by Cross-Sectional Scanning Tunneling Microscopy

The compositional distribution of ZnSe/BeTe interfaces in ZnSe/BeTe multiple quantum wells with a type-II band alignment has been investigated using cross-sectional scanning tunneling microscopy and X-ray diffraction measurements. The filled- and empty-state images revealed that Zn–Te and Be–Se bonds exist at the ZnSe/BeTe interface within the range of 2–4 monolayers along the growth direction. The transition layer between the ZnSe layer and the BeTe layer is composed of the BeZnSeTe quaternary alloy. X-ray diffraction analysis confirmed the existence of a (ZnTe)0.58(BeSe)0.42 transition layer with a width of 0.40 nm at the ZnSe/BeTe interface.

Cathodoluminescence Study of Quantum-Size and Alloying Effects in Single Fractional Monolayer CdSe/ZnSe Structures

Luminescence properties of single fractional monolayer CdSe/ZnSe structures with nominal thickness of 1.25 to 3.6 monolayers have been investigated by means of cathodoluminescence (CL) spectroscopy. The CL spectra exhibit sharp peaks superimposed on a broad luminescence band due to individual CdSe-enriched dots spontaneously formed in the CdSe layer. By fitting the CL spectra to the transition energies calculated by a quantum disk model taking into account compositional alloying and dot sizes, we have determined Cd concentrations in CdSe-enriched dots. The results are in good agreement with structural data, and indicate that the blue shift of the dot luminescence with decreasing nominal thickness is due to the alloying effect.

Composition Profile of MOVPE Grown InP/InGaAs/InP Quantum Well Structures Studied by Cross‐Sectional Scanning Tunneling Microscopy

Interfacial properties of InP/In0.53Ga0.47As/InP quantum well structures grown by metalorganic vapor phase epitaxy have been investigated by means of cross‐sectional scanning tunneling microscopy (XSTM). Composition profiles of Ga and As atoms derived from XSTM images reveal the formation of the compositional transition layer at the InP‐on‐InGaAs interface. The transition layer width of the Ga fraction is independent of both the growth interruption time and the InGaAs layer width. This result suggests that the transition layer of the Ga composition is formed by the Ga/In exchange during the overgrowth of the InP layer on the InGaAs layer.

Sharp Interfacial Structure of InAs/InP Quantum Dots Grown by a Double‐Cap Method: A Cross‐Sectional Scanning Tunneling Microscopy Study

Interfacial properties of InAs quantum dots (QDs) grown by a double‐cap method in metalorganic chemical vapor deposition have been investigated by cross‐sectional scanning tunneling microscopy (XSTM). XSTM images reveal that top and bottom interfaces of the InAs QD are extremely sharp. QDs with a monolayer‐stepped height in the range 6–14 ML are observed, which indicates that the double‐cap method can produce QDs with a well‐defined height.