Bun Lee

Naturally formed InxAl1-xAs/InyAl1-yAs vertical superlattices

InxAl1−xAs/InyAl1−yAs vertical superlattices (VS) were naturally formed by phase separation during the growth of InAlAs/InP layers at temperatures in the range 565–615 °C by metalorganic chemical vapor deposition. The VS lies perpendicular to the (001) growth plane. As the growth temperature increased from 565 to 615 °C, the VS decreased in thickness from ∼15 to ∼6 nm, and became less planar and uniform. Transmission electron diffraction results showed the occurrence of CuPt‐type ordering in some of the layers. Band‐gap reduction of ∼300 meV was observed in the InAlAs layers grown at temperatures ranging from 565 to 615 °C. Such a large reduction in band‐gap energy was attributed to combined effects of the VS and CuPt‐type ordering.

Optical properties of InGaAs linear graded buffer layers on GaAs grown by metalorganic chemical vapor deposition

We report optical characteristics of linear graded InxGa1−xAs (XIn=0–0.58) buffer layers grown on GaAs by low‐pressure metalorganic chemical vapor deposition. Two types of wirelike surface structures were observed from the layers grown at two different temperatures. Low‐temperature photoluminescence (PL) and double‐crystal x‐ray diffractometric measurements indicate that the PL energy and the relaxation of the graded layers were strongly dependent on the top surface structure. InGaAs cap layers were grown on top of the graded buffer layers with a variation of indium composition. A strong PL signal was observed from the top region of the graded layer grown with a lattice‐matched cap layer. It suggests that the top region of the grade, similar to a graded well structure, is compressively strained but is of high structural quality without dislocations.

EVOLUTION OF THE SURFACE CROSS-HATCH PATTERN IN INXGA1-XAS/GAAS LAYERS GROWN BY METAL-ORGANIC CHEMICAL VAPOR DEPOSITION

The evolution of the cross‐hatch pattern (CHP) in InxGa1−xAs/GaAs heterostructures has been studied. It is found that stress is concentrated at the valleys of the CHP from the results of crack formation at the CHP valleys in the thick GaAs cap layer grown on an InGaAs layer. Residual strain in the InGaAs/GaAs epitaxial layer showing a CHP is confined along the valleys of the CHP with a nonuniform distribution throughout the epitaxial layer. The skeleton of the CHP is formed at the beginning of the rapid strain relaxation period and the depth of the CHP valleys increases after most of the strain has been released. We propose that the development of the CHP in the later stage of the growth takes place by the growth suppression at the CHP valleys due to the high level of stress concentration.

Observation of phase separation and ordering in the InAlAs epilayer grown on InP at the low temperature

Abstract We have studied the phase separation and ordering phenomenon of InAlAs epilayers grown on InP substrate by metal–organic chemical vapor deposition (MOCVD). From the intensity and full width half maximum FWHM of double-crystal X-ray diffraction (DCXRD) and photoluminescence (PL), we observed that the structural and optical quality of InAlAs epilayers were improved as growth temperature increased. The band-gap reduction due to phase separation and ordering is 291, 246 and 28 meV in the InAlAs epilayers grown at 565, 615 and 700°C, respectively. The maximum degree of phase separation was obtained from the InAlAs epilayer grown at 565°C. However, the maximum degree of ordering was obtained at the medium growth temperature. A rapid thermal annealing experiment showed a maximum band-gap shift of 78 meV at 880°C for 3 min. Transmission electron microscopy (TEM) showed that the origin of the blue shift of the band-gap was the complete disappearance of ordering and most of the total band-gap reduction (≈3/4) occurred by phase separation.

Precise control of 1.55 μm vertical-cavity surface-emitting laser structure with InAlGaAs/InAlAs Bragg reflectors by in situ growth monitoring

The vertical-cavity surface-emitting laser (VCSEL) structure designed at 1.55 μm was grown by a low-pressure metalorganic chemical vapor deposition method. In situ laser reflectometry, using both 0.633 and 1.53 μm wavelengths simultaneously, was employed to control the exact optical thickness over the whole growth time. The distributed Bragg reflectors (DBRs) were grown with alternate In0.53Al0.13Ga0.34As and In0.52Al0.48As λ/4 wavelength layers. The oscillatory reflection signals obtained by the monitoring laser at 1.53 μm gave information for designing the center wavelength of the DBR. The reflectance spectrum of the VCSEL structure showed an excellent square shaped wide flatband (greater than 90 nm) where the reflectivity reached a plateau as expected by the in situ monitoring data.

Ordering effect on band-gap lowering in lattice-matched InAlAs epilayers grown on InP by metal–organic chemical-vapor deposition

Band-gap lowering due only to the cation ordering effect is investigated in InAlAs layers grown on InP by using photoluminescence measurement. Double-crystal x-ray diffraction and Rutherford backscattering measurements confirm that both of the InAlAs epilayers studied, grown at 700 and 750 °C, are lattice matched with InP substrates. Through transmission electron diffraction measurements, it is observed that a CuPt-type ordering structure is formed in the InAlAs layers grown at 700 °C but not in the layers at 750 °C. Photoluminescence measurements at 1.7 K reveal that the band-gap energy of the ordered InAlAs is smaller by 60 meV than that of the unordered InAlAs.

Measurement of index of refraction of InxAl1−xAs epitaxial layer using in situ laser reflectometry

We report the application of in situ laser reflectometry in measuring index of refraction and other optical properties of InxAl1−xAs (0≤x≤1) epitaxial layers grown on GaAs substrates by low‐pressure metal‐organic chemical vapor deposition method. Two different lasers were used: One was a He–Ne laser operating at 0.6328 μm and the other was a single mode distributed‐feedback (DFB) laser diode operating at 1.530 μm. The two laser beams were simultaneously directed on the growing layer at an angle of 71° from the surface normal. The oscillatory signals showed a variety of patterns depending on the index of refraction of the epitaxial layer of InxAl1−xAs (0≤x≤1). The initial oscillatory signal pattern was found to be reversed at the composition of x=0.75, where the index of refraction of the InxAl1−xAs layer is the same as that of the GaAs substrate. The absorption coefficients and indices of refraction of the InxAl1−xAs epitaxial layers were obtained for the entire range of composition at the growth temperat...

Large non‐biased all‐optical bistability in an electroabsorption modulator using p‐i‐n‐i‐p diode and asymmetric Fabry–Perot cavity structure

We have constructed a non‐biased all‐optical bistable device using electroabsorption in multiple quantum wells (MQWs). The device is made of a serial connection of two identical double p‐i‐n (p‐i‐n‐i‐p) diode structures within asymmetric Fabry–Perot (AFP) cavity. This scheme ensures both large internal electric field swing and full exploitation of the light intensity. From the measurement of the diode properties, the non‐biased bistability parameters—reflection change of 19%, contrast ratio of 17:1, and bistable loop width of 70%—were obtained.

In-Situ Monitoring of In0.53Al0.13Ga0.34As/In0.52Al0.48As 1.55 µm Vertical Cavity Surface Emitting Laser Structure Grown by Metal Organic Chemical Vapor Deposition

A 1.55 µm vertical cavity surface emitting laser (VCSEL) structure was grown by low pressure metalorganic chemical vapor deposition method. In-situ laser reflectometry with a wavelength near resonant position of VCSEL structure was employed to monitor the optical thickness over the whole growth time. The distributed Bragg reflectors (DBRs) were grown with alternate In0.53Al0.13Ga0.34As and In0.52Al0.48As quarter lambda wavelength layers. The growth time of each layer in the DBR was determined through real time feedback of growth monitoring in the growth of In0.52Al0.48As and In0.53Al0.13Ga0.34As buffer layers prior to beginning the subsequently grown DBR structure. The oscillatory reflection signals by the laser operating at 1.53 µm gave information for the position of stop band in the reflectivity spectrum of the VCSEL structure. The reflectivity spectrum of VCSEL structure showed excellent square-shaped flat band wider than 50 nm where the reflectivity reached a plateau as expected by the in-situ monitoring data.

Lattice relaxation in InxAl1−xAs/GaAs heterostructure grown on exact oriented and misoriented GaAs substrates

We have examined lattice relaxation properties of 2 μm thick InxAl1−xAs epitaxial layers grown simultaneously on a (100) exact oriented GaAs substrate and on a 2° misoriented GaAs substrate by low pressure metalorganic chemical vapor deposition method. Double crystal x‐ray diffractometry has been used to determine the alloy compositions at various azimuthal angles. As a result of (400) x‐ray rocking curve analysis, it was found that the full width at half‐maximum (FWHM) of x‐ray rocking curves for both samples were strongly dependent on the azimuthal angle, indicating that the lattice relaxation depended greatly on the substrate orientation. From the data of {511} asymmetric measurements for four different directions, the In contents of the epitaxial layers found were 24%. The In0.24Al0.76As epitaxial layers grown on an exact oriented substrate and on a misoriented substrate both showed 82% relaxation, which was a larger value in comparison with the similarly mismatched InGaAs/GaAs heterostructure. The pe...