Y. Okuno

Structural and optoelectronic properties and their relationship with strain relaxation in heteroepitaxial InP layers grown on GaAs substrates

Heteroepitaxial layers of InP with thickness D ranging from 0.1 to 6.0 μm were grown by low‐pressure metalorganic chemical vapor deposition on (001) surfaces of GaAs substrates. Their dislocation structure, induced strains, and nature of the radiative recombinations were investigated as a function of D with transmission electron microscopy, x‐ray diffraction, and photoluminescence spectroscopy. For D<2 μm, the films are highly dislocated with a tangle of interfacial and threading dislocations above the heterointerface. The spatial extent of the interfacial dislocations and the density of threading dislocations increase with increasing D. For D≳2 μm the portion of the layers away from the heterointerface by more than 1.5 μm shows a decrease in the density of threading dislocations and a dramatic improvement in the crystalline quality with increasing D. Typical dislocation densities in the neighborhood of the top surface are in the mid 107 cm−2 range when D surpasses 4.0 μm. Concomitant with the improved crystalline quality, the following observations are made. Firstly, the full width at half maximum of the x‐ray rocking curves diminishes from values larger than 500 arcsec for D<1.0 μm to about 200 arcsec for D≳4.0 μm. Secondly, the near‐band‐edge photoluminescence transitions, which for D<2.0 μm are predominantly determined by defect‐induced band tailing, display excitonic character. Thirdly, below‐band‐gap transitions due to interfacial defects decrease in intensity. Biaxial compressive strain is present in the layers because of lattice mismatch and differences in linear thermal expansion with the substrate. The strain removes the degeneracy between the light‐ and heavy‐hole states at the top of the valence band, and consequently with increasing temperature above 12 K recombinations from the conduction to the split valence bands are observed in the photoluminescence spectra for all D. The identification of such transitions follows from their temperature dependence and the activation energy yield for the thermalization of the holes. The measured valence‐band splitting decreases from 12.5 meV for D=0.3 μm to saturation values of 5.6 meV for D≳3.0 μm, indicating strain relaxation with D in qualitative agreement with x‐ray determinations. Quantitative differences between both methods are realized and are attributed to a temperature dependence of the differential linear thermal expansion. The contribution to the strain from the lattice mismatch is much larger than expected from equilibrium models. The dislocation generation at different stages during the growth is inferred from the strain relaxation against D and the observed location of the dislocations throughout the layers.

Anti‐phase direct bonding and its application to the fabrication of InP‐based 1.55 μm wavelength lasers on GaAs substrates

We propose anti‐phase direct bonding and report on the first demonstration of its application to device fabrication. Cross‐sectional observation by high‐resolution transmission electron microscope showed that InP and GaAs wafers bonded at the atomic level and the misfit dislocations were localized at the bonding interface. Then InP‐based 1.55 μm wavelength lasers were fabricated on GaAs. The performance of the lasers was approximately equal to that of the lasers formed by in‐phase direct bonding. Moreover, stable operation was possible for more than 1000 h at 50 °C.

Threading dislocation reduction in InP on GaAs by thin strained interlayer and its application to the fabrication of 1.3-μm-wavelength laser on GaAs

This paper examines the reduction of threading dislocation by a thin strained interlayer (SIL), and a long-wavelength laser fabricated on a GaAs substrate with SILs. A cross-sectional transmission electron microscope is used to observe the dislocation blocking ability of an SIL made of InGaP, which is inserted in an InP layer grown on a GaAs substrate. The characteristics of the BH laser emitting at 1.3 µm on GaAs are investigated. Most of them are improved by SIL insertion. In particular, threshold current is reduced to 70% on average and is also distributed more uniformly.

Heteroepitaxial GaAs layers on InP substrates: Radiative recombinations, strain relaxation, structural properties, and comparison with InP layers on GaAs

GaAs layers grown on (001) InP surfaces by low‐pressure metalorganic chemical vapor deposition were investigated with photoluminescence spectroscopy, x‐ray diffraction, Raman scattering, and transmission electron microscopy. Correlations between the optoelectronic properties, the strain relaxation, and the structural defects were established for layer thickness D ranging between 0.1 and 3.0 μm. A comparison with the case of InP layers grown on GaAs substrates is presented. Radiative recombinations to split light‐ and heavy‐hole valence bands near the zone center are seen at 12 K in the photoluminescence spectra. The splitting is due to a biaxial tensile strain. With increasing temperature, the heavy‐hole transitions gain intensity and at around 140 K they are the only features in the spectra. In the 12–50 K temperature range the intensity ratio between the heavy‐ and light‐hole transitions also depends on laser power. The hole activation energy determined from the temperature dependence of the intensity r...

Investigation on direct bonding of III–V semiconductor wafers with lattice mismatch and orientation mismatch

The direct bonding of various III–V wafers with mismatches in terms of lattice constants and surface orientations was systematically investigated for an In–Ga–As–P system. Many wafer combinations were bonded with sufficient mechanical strength, despite those mismatches. The bonding interface of (001) GaP and (110) InP was observed by transmission electron microscopy and found to be bonded at the atomic level without any defects occurring. The electrical property of the bonding interface was examined for several bonded structures of GaAs and InP. The results support a novel concept ‘‘free‐orientation integration,’’ which should be achieved by direct bonding.

Fabrication of (001) InP‐based 1.55‐μm wavelength lasers on a (110) GaAs substrate by direct bonding (A prospect for free‐orientation integration)

We examine the direct bonding of (001) InP and (110) GaAs and demonstrate its application to device fabrication. Cross‐sectional observation shows that these wafers can be united without generating dislocation. (001) InP‐based 1.55‐μm wavelength lasers are fabricated on (110) GaAs. The light–current characteristics of the lasers are almost identical to those of lasers fabricated on (001) GaAs, while the turn‐on voltage is higher by about 0.4 V due to the large band discontinuity. The results show that the direct bonding technique is promising for allowing new concept ‘‘free‐orientation integration.’’

Study of threading dislocation reduction by strained interlayer in InP layers grown on GaAs substrates

Abstract We report the optimization of the properties of a strained interlayer (SIL) which is applied to reduce density of threading dislocations generated in an InP layer grown on GaAs. The effect of each SIL as a defect filtering layer was measured by X-ray diffraction on an InP layer which comprises the SIL. All the layers were grown by low-pressure metalorganic vapor phase epitaxy. Ternary compounds such as InGaP and InGaAs were investigated as materials for SILs. First In 1- x Ga x P was examined by varying its thickness and composition, and x = 0.2 with 400 A thickness was found to be the most effective. The ways in which threading dislocations were reduced by SILs with different composition x were compared, in order to discuss the strain relaxation behavior in corresponding compounds. Examination of InGaAs revealed that not only the lattice parameter, but also other material properties affect the way in which the SIL reduce dislocations. We also found that an SIL which has a larger lattice constant that InP did not reduce dislocation density. These results are explained in terms of misfit dislocation networks, which are believed to be formed at SIL interfaces and to cause the dislocation reduction effect.

Crystal growth and fabrication of a 1.3-μm-wavelength multiple-quantum-well laser on a (211)A InP substrate

We demonstrate the fabrication of a long-wavelength laser on a (211) InP substrate, with the expectation of reducing threshold current density. We found that InGaAsP single quantum wells (SQWs) could be fabricated with good optical properties provided the SQW layers were not made too thin. A laser that had an unstrained multiple-quantum-well active layer emitting at 1.3 μm was fabricated on a (211)A InP substrate. Its threshold current density was 900 A/cm2, which is comparable to the value for the same type of laser on a (100) substrate. These results suggest that long-wavelength lasers with satisfactory quality can be fabricated on a (211)A substrate.

Direct Wafer Bonding of a (001) InP-Based Strained Multiple Quantum Well on a (110) Si Substrate with a GaAs Buffer Layer, Aligning Cleavage Planes of InP and Si

We demonstrate direct wafer bonding of a (001) InP-based structure and a (110) Si substrate with a GaAs buffer layer. Cleavage planes of the InP and Si are aligned to obtain a smooth cleaved facet of the InP. Cross-sectional transmission electron microscope observation shows that the InP region is dislocation-free, and that an unexpected In region is formed at the bonded interface. Strong intensity is achieved in the photoluminescence spectrum from the strained multiple quantum well in the InP region, although a peculiar feature is observed in the spectrum measured at room-temperature. The results confirm that our method of direct bonding makes it possible to achieve smooth cleaved facets and high crystalline quality in a III?V layer fabricated on a Si substrate.

Dislocation reduction in InP layers grown on sawtooth-patterned GaAs substrates

Abstract We report on the crystalline improvement of InP layers grown on GaAs substrates by using sawtooth-patterned substrates. The period of sawtooth pattern was 1.2 μm and the layers were grown by low-pressure metalorganic chemical vapor deposition. Cross-sectional transmission electron microscope observation is applied to examine the effectiveness of the sawtooth-shaped hetero-interface for the reduction of threading dislocations in InP layers which accommodate lattice mismatch. A remarkable decrease in the full width at half-maximum values of the X-ray diffraction peaks was obtained, signifying a dramatic crystalline improvement due to the reduction of dislocations by the sawtooth-shaped interface. The unevenness of the layer surface which originates from the sawtooth shape of the substrate can be readily smoothed by thermal cycle annealing, which is accompanied by additional crystalline improvement. Moreover, an attempt to put a thin strained interlayer in the epitaxial layer as a defect filtering tool is also presented.