Shigeya Naritsuka

Electron traps in AlGaAs grown by molecular‐beam epitaxy

Using deep‐level transient capacitance spectroscopy we have investigated deep electron traps in n‐AlGaAs grown by molecular‐beam epitaxy (MBE). The thermal activation energies of seven traps, labeled ME1–ME7, observed in this study increase with increasing Al content(x) up to the direct‐indirect crossover point (x∼0.42), but show only a small change with further increases in Al content. Traps ME4–ME7 are dominant in samples with x≤0.2. Traps ME4–ME6 strongly depend on the growth ambient. The concentration of ME7 is almost independent of the ambient in the growth chamber but decreases rapidly with increasing growth temperature. ME7 is a native defect and can almost certainly be identified with the trap EL2 observed in bulk and vapor‐phase epitaxially grown GaAs. Traps ME4–ME6 are probably formed by impurities involving oxygen such as CO, H2O, and AsO in the growth ambient. All of the traps, ME5–ME7 are clearly responsible for a decrease in the photoluminescence intensity of MBE grown AlGaAs.

InP Layer Grown on (001) Silicon Substrate by Epitaxial Lateral Overgrowth

An InP epitaxial layer with a dislocation-free area was obtained for the first time on (001) Si substrate by using the epitaxial lateral overgrowth (ELO) technique. Most etch pits appeared in the region over the seed area on the ELO stripe. This indicates that the SiO2 film between the lateral overgrowth layer and the substrate prevented the propagation of the dislocations from the substrate to the lateral overgrowth layer. Spatially resolved photoluminescence showed that the optical quality of an InP ELO layer grown on Si was almost the same as that of a homoepitaxially grown InP layer and that the ELO technique is also useful to relieve stress caused by both the lattice mismatch and the difference in thermal expansion coefficient.

Effects of well number, cavity length, and facet reflectivity on the reduction of threshold current of GaAs/AlGaAs multiquantum well lasers

The optimum design for reducing the threshold current of GaAs/AlGaAs multiquantum well lasers is determined experimentally. The lowest threshold current density is realized by using single and multiquantum wells at long and short cavity lengths, respectively. The threshold current has a minimum at the optimum cavity length: the minimum threshold current is smaller for a larger number of quantum wells, and the optimum cavity length is inversely proportional to the number of wells. Experiments are compared to the theory developed by P.W.A. McIlroy, et al. (ibid., vol.21, no.12, p.1958-63, 1985) and limiting performances of quantum well lasers with various numbers of wells are presented. The reduction of the threshold current by high reflectivity coatings is also demonstrated, and a threshold current as low as 1.86 mA at 15 degrees C is reported. >

Effect of growth temperature on epitaxial lateral overgrowth of GaAs on Si substrate

The dependence of epitaxial lateral overgrowth (ELO) of GaAs on Si on the growth temperature of liquid phase epitaxy (LPE) has been studied. It is shown that the ratio of the ELO width to the thickness (ELO ratio) depends strongly on LPE growth temperature. When the growth temperature is chosen as high as 620°C, the ELO ratio equals 5.8. However, when the growth temperature is decreased to 500°C, the ELO ratio is increased to as large as 14. Molten KOH etching showed that only a few pits are present on the ELO layers. It is concluded that by employing low growth temperature, GaAs epitaxial layers with large ELO ratio and with only a few dislocations can be obtained.

Epitaxial lateral overgrowth of InP by liquid phase epitaxy

Abstract Epitaxial lateral overgrowth (ELO) of InP has been conducted for the first time by liquid phase epitaxy (LPE). The overgrowth has been studied for seeds in SiO 2 of 2.5 to 7 μm width using starlike and parallel patterns. A wide and flat ELO layer was grown on (001) and (111)B oriented substrates at growth temperature between 773 and 873 K. No etch pits were found in the overgrowth regions. For (001) oriented substrates, the seed aligned away from 〈100〉 and 〈110〉 and their equivalent directions brought the maximum overgrowth up to 100 μm whereas the seed aligned adjoining these directions brought almost no overgrowth. On the other hand, for (111)B oriented substrates, the seed aligned away from 〈110〉 and its equivalent directions brought the maximum overgrowth up to 140 μm whereas the seed aligned adjoining these directions brought almost no overgrowth. The maximum ratio of width to thickness of 47 for (111)B ELO layer was obtained. This value is much larger than that of the (001) ELO layer of 17.

Optimization of growth condition for wide dislocation-free GaAs on Si substrate by microchannel epitaxy

The growth condition dependence of the microchannel epitaxy (MCE) width-to-thickness ratio (W/T ratio) was studied systematically. The result shows that by optimizing the line seed separation and growth temperature, the GaAs layer with a high W/T ratio has been grown on GaAs-coated Si substrate, respectively, at 500 μm and 530°C. In this work, GaAs MCE layers with a high W/T ratio of 17.5 were obtained. Furthermore, by employing the optimized growth condition, GaAs layers with a wide dislocation-free region on a Si substrate were obtained after the growth of 7 h. Molten KOH etching showed that the width of the dislocation-free region is about 43 μm for the sample grown for 3 h. The width is enough to fabricate devices such as vertical cavity surface emitting lasers (VCSELs).

Composition dependence of photoluminescence of AlxGa1−xAs grown by molecular beam epitaxy

Low‐temperature (∼4 K) photoluminescence of lightly Si‐doped AlxGa1−xAs grown by molecular beam epitaxy has been studied. The defect‐related emissions, due to the defect exciton (d, X) and the defect complex (d), have been identified. The peak energies of these emissions, which are 1.505 eV (d, X) and 1.474 eV (d) for GaAs, have been determined as a function of the mole fraction x ( x<0.45). The energy difference between the donor–bound‐exciton (BE) peak and the defect‐exciton peak is almost constant (∼9 meV). In contrast, the energy difference of the BE peak and the defect‐complex peak increases from 40 meV at x=0 to about 70 meV at x∼0.4. In addition, the ionization energies of carbon and silicon acceptors in AlxGa1−xAs have been determined as a function of x and compared with theoretical calculations.

Spatially resolved photoluminescence of laterally overgrown InP on InP-coated Si substrates

Optical properties of InP grown by ELO (epitaxial lateral overgrowth) on InP-coated Si substrates were characterized by spatially resolved photoluminescence. Intensity of the photoluminescence spectra of these layers was found to be as strong as that of InP homoepitaxially grown on InP substrates. The luminescence spectra showed a small FWHM of 23 meV and no shift in wavelength was seen across the ELO layers. This means almost no stress exists in InP ELO layers on Si substrates. It is concluded that stress free InP has been obtained successfully on Si substrates. Simulation to evaluate stress in ELO layers was conducted and a good agreement with the experiments was obtained.

Interface supersaturation in microchannel epitaxy of InP

By employing microchannel epitaxy (MCE), it became possible to reduce the dislocation density in liquid phase epitaxy (LPE) of InP so that steps supplied from only one screw dislocation can cover the whole surface of MCE island. This enabled us to measure interstep distance by AFM and to calculate interface supersaturation even in metallic solution with the help of Cabrera and Levine formula by assuming appropriate value of interface free energy. The interface supersaturation was found being ranged from 0.02 to 0.05 upon various experimental conditions. A minimum interface supersaturation was realized when the growth temperature and cooling rate were chosen as 500°C and 0.05°C/min, respectively. The ratio of width to thickness of the grown MCE layer, which is defined as W/T ratio, was found to increase rapidly with the decrease of the interface supersaturation. A W/T ratio as high as 20 was accomplished under the growth condition where minimum interface supersaturation can be realized.

Characteristics of molecular‐beam epitaxially grown pair‐groove‐substrate GaAs/AlGaAs multiquantum‐well lasers

Growth and device characteristics of an index‐guided GaAs/AlGaAs multiquantum‐well (MQW) laser, called a pair‐groove‐substrate (PGS) MQW laser, are described in detail. The laser structure is fabricated by using single‐step molecular‐beam epitaxy on a (001) GaAs substrate with a pair of etched grooves along the 〈110〉 direction. A mesa between a pair of grooves, where the lasing action occurs, becomes narrow during growth, and the narrow mesa offers lateral waveguiding that stabilizes a fundamental transverse mode. The superior crystalline quality of the mesa top, which is examined by a microprobe photoluminescence technique, serves to lower the lasing threshold currents. The lasers with mesa widths below 2 μm show stable transverse mode operation with a low threshold current of 20 mA, as well as a high external differential quantum efficiency of 68%. The low threshold and high characteristic temperature accomplish a high‐temperature continuous‐wave operation at 153 °C for the lasers mounted on silicon he...