Yasufumi Yabuuchi

Residual impurities in GaN/Al 2 O 3 grown by metalorganic vapor phase epitaxy

Residual impurities in GaN films on sapphire (A12O3) substrates grown by two-step metalorganic vapor phase epitaxy (MOVPE) have been investigated. We have mainly investigated the incorporation of carbon into the GaN films with GaN buffer layers on A12O3 during MOVPE growth, comparing trimethygallium (TMGa) and triethygallium (TEGa) as the typical gallium precursors. The films were characterized by secondary ion mass spectroscopy analysis, photolu-minescence, and Hall measurements. The carbon, hydrogen, and oxygen concentrations increase with decreasing growth temperature in using TMGa. Especially the carbon concentration increases with decreasing a V/III ratio, for both TMGa and TEGa. There is about two times more carbon in the GaN films grown using TEGa than those using TMGa. The carbon from TMGa mainly enhances the D-A pair emission (∼378 nm), which shows the carbon makes an acceptor level at nitrogen sites in GaN. On the other hand, the carbon from TEGa enhances a deep emission (∼550 nm), which shows the carbon makes not only an acceptor level but deep levels at interstitial sites in GaN. The carbon impurities originate from methyl radicals for TMGa, or ethyl radicals for TEGa. It is supposed that, in the case of TEGa, the carbon impurities are not always located at nitrogen sites, but are also located at interstitial sites because of the C-C bonding in ethyl radicals.

Metalorganic Vapor Phase Epitaxy Growth of a High-Quality GaN/InGaN Single Quantum Well Structure Using a Misoriented SiC Substrate

A high-quality GaN/InGaN single quantum well (SQW) structure has been successfully grown using a misoriented 6H-SiC substrate, the face of which is tilted from (0001) toward [1120] by 3.5°, by low pressure metalorganic vapor phase epitaxy (MOVPE). A sharp emission, whose full-width at half maximum (FWHM) was 24.3 meV, from the GaN/InGaN SQW structure was observed at 385nm in the 77K photoluminescence spectrum. From the transmission electron microscopy (TEM) analysis, the dislocations in the GaN film grown on the misoriented substrate were bent from the c-direction, and the threading dislocations to the InGaN film on the GaN film were decreased. For the InGaN film grown on the misoriented substrate, only the sharp band edge emission, whose FWHM was 92.3 meV, was observed at 385 nm in the PL spectrum at 77K. The dislocation density, which was estimated from TEM photographs, in the InGaN film on the GaN film grown on the misoriented substrate was about 5 x 10 8 cm -2 , which was nearly half of that grown on the (0001) substrate.

Anomalous temperature dependence of PL characteristics in ordered InGaAsP strained layer multi-quantum well structure

Clarification of anomalous photoluminescence (PL) characteristics for InGaAsP compressively strained layer multi-quantum well (SL-MQW) structures is studied to realize highly strained SL-MQW lasers with a large number of well layers. PL characteristics such as an increase in PL linewidth and a small temperature dependence of the PL peak wavelength are observed in the SL-MQW structures in which a structure of CuPt-type atomic ordering is observed. Elimination of the ordered structure as well as reduction of the PL linewidth have been confirmed with increasing growth temperature. Although formation of an interfacial undulation (lateral modulation) between SL-MQW layers is also suppressed by the increase in growth temperature, the formation has ultimately limited the maximum strain in SL-MQW. We suggest that the anomalous PL characteristics result from generation of ordering, dominant when the growth temperature is lower, as well as from interfacial undulation, dominant in highly strained SL-MQW when the growth temperature is increased.

Formation of InAsP layer on corrugated InP substrate by MOVPE for buried grating of DFB lasers

Characteristics of the InAsP layer formed on a corrugated InP substrate by a mass-transport mechanism have been investigated for use as the buried grating of distributed feedback (DFB) lasers, The size of the InAsP layer can be controlled by the height of the corrugation, and the arsenic composition in the InAsP layer can be controlled by the AsH/sub 3/ partial pressure. The results of TEM, EDS and PL show that InP is suitable as the buffer layer between the InAsP layer and MQW active layer. Fabricated 1.3 /spl mu/m DFB lasers which have an InAsP layer as an absorptive grating have shown low threshold current and high slope efficiency from -40-+85/spl deg/C, and high reliability has been demonstrated.

Degradation of PL characteristics in strained layer multi-quantum well structure with atomic ordering structure

Clarification of the degradation mechanism of photoluminescence (PL) charac-teristics of InGaAsP compressively strained layer multi-quantum well (SL-MQW) structures on an InP substrate is studied to realize highly strained SL-MQW lasers with a large number of well layers. An unusual temperature-dependence of the PL peak wavelength is observed in the SL-MQW structure: the shift of the PL peak wavelength is reduced with change in measurement temperature. The degradation of PL characteristics is evaluated by the reduc-tion in the PL shift as well as an increase in PL linewidth and a decrease in PL peak intensity. The extent of the PL degradation increases with an increase in the number of well layers and an increase in strain. In the SL-MQW structure with degraded PL characteristics, the structure of CuPt-type atomic ordering is clearly observed for the first time in each barrier layer by high-resolution transmission electron microscope (HR-TEM) measurements. In the SL-MQW structure, neither dislocation nor three-dimensional nucleation is observed by TEM measurements. By increasing the growth temperature, elimination of the ordering structure as well as improvement in the PL characteristics have been confirmed even in the SL-MQW structure with a large amount of strain and a large number of well layers.

Atomic ordering in strained layer multi-quantum well structure

The dependence of photoluminescence (PL) characteristics on both SL-MQW structure and epitaxial growth condition is studied. The degradation of PL characteristics in the SL-MQW is confirmed as a broadening of PL linewidth and an unusual temperature-dependence of PL peak wavelength. By using a high-resolution transmission electron microscope (HRTEM), the structure of atomic ordering is demonstrated over entire barrier layers of the SL-MQW structure with degraded PL characteristics. By increasing growth temperature, elimination of the ordering structure as well as improvement of the PL characteristics have been confirmed even in a SL-MQW structure a with large amount of strain.

Scanning Electron Microscope Observations of As^+-Ion-Implanted Region

Lateral and depth profiles of the ion-implanted region were first observed using cleaved cross-sectional scanning electron microscope. The amorphous region induced by ion bombardment appears dark relative to the crystalline region. It is found that the secondary electrons, as well as the backscattered ones, are influenced by the so-called channeling effect.