Kouji Ohtsuka

Improved Characteristics of Blue and Green InGaN-Based Light-Emitting Diodes on Si Grown by Metalorganic Chemical Vapor Deposition

We report significantly improved characteristics of InGaN multiple-quantum well blue and green light-emitting diodes (LEDs) on Si (111) substrates using metalorganic chemical vapor deposition. A high-temperature-grown thin AlN layer and AlN/GaN multilayers have been used for the growth of high-quality active layer on Si substrate. The blue LED on Si exhibited an operating voltage of 4.1 V, a series resistance of 30 Ω, an optical output power of 18 µW and a peak emission wavelength of 478 nm with a full width at half maximum of 22 nm at 20 mA drive current. These characteristics are comparable to those of LED on sapphire substrate. The green LED was also fabricated on Si substrate successfully.

Optical Properties of (AlxGa1-x)0.5In0.5P Quaternary Alloys

The dielectric function spectra e(E) of (Alx Ga1-x )0.5In0.5P quaternary alloys lattice-matched to GaAs have been measured at room temperature in the 1.2–5.5-eV photon-energy range using spectroscopic ellipsometry (SE). The measured SE spectra show distinct structures at energies of the E0, E1 and E2 critical points. These spectra are analyzed on the basis of a simplified model of interband transitions. This model enables us to obtain the, optical response in (Alx Ga1-x )0.5In0.5P alloys of arbitrary composition x and photon energy E=\hbarω. Results are presented for the complex refractive index (n*=n+ ik), absorption coefficient (α) and normal-incidence reflectivity (R) of these alloys.

Refractive index of (AlxGa1−x)0.5In0.5P quaternary alloys

Refractive‐index dispersion in (AlxGa1−x)0.5In0.5P quaternary alloys has been measured at room temperature in the 1.2–2.9 eV photon‐energy range by using spectroscopic ellipsometry. The measured data are fitted to the first‐order Sellmeier equation. The resulting expression can be used in the design of various waveguiding devices.

Ellipsometric and thermoreflectance spectra of (AlxGa1−x)0.5In0.5P alloys

Spectroscopic‐ellipsometry (SE) and thermoreflectance (TR) spectra of (AlxGa1−x)0.5In0.5P quaternary alloys are presented. Both measurements are carried out on the same samples in the 1–6 eV photon‐energy range at room temperature. These spectra are analyzed based on a simple model of the interband transitions. The results are in satisfactory agreement with the experimental SE and TR data over the entire range of photon energies. The composition dependence of the interband critical‐point and indirect‐band‐gap energies is also discussed.

Hydrogen plasma passivation of GaAs on Si substrates for solar cell fabrication

The effects of hydrogen plasma treatment and postannealing on GaAs solar cells on Si substrates have been investigated. It is found that postannealing temperature is an important parameter to obtain GaAs on Si with a long minority carrier lifetime. The minority carrier lifetime is increased and the deep level concentration is decreased by the hydrogen plasma treatment. Even after 450 °C postannealing with the complete recovery of the shallow impurity level, the minority carrier lifetime is still longer and the deep level concentration is lower than those of the as-grown sample. It means that the defects in GaAs on Si are passivated by hydrogen. The efficiency of GaAs-on-Si solar cell (air mass 0, 1 sun) is improved from 16.6% (as-grown) to 18.3% by the hydrogen plasma passivation and to 17.2% by the hydrogen plasma passivation and postannealing at 450 °C.

Hydrogen Plasma Passivation and Improvement of the Photovoltaic Properties of a GaAs Solar Cell Grown on Si Substrate

The effects of hydrogen (H) plasma passivation of metalorganic chemical vapor deposition (MOCVD) grown GaAs solar cells on a Si substrate have been studied. After H plasma passivation and postannealing in AsH3 ambient, the conversion efficiency of the GaAs solar cell grown on a Si substrate is increased from 16.3 to 17.2% at AM0. This improvement is thought to be due to the passivation of the defect-associated recombination centers by H plasma exposure and the recovery of the plasma-induced damages by postannealing the passivated solar cells in AsH3 ambient at 450°C.

Electroreflectance study of (AlxGa1−x)0.5In0.5P alloys

Electroreflectance spectroscopy has been applied to the study of (Alx Ga1-x )0.5In0.5P quaternary systems lattice-matched to GaAs. The measurements are made at room temperature in the photon-energy range of 1.7–5.8 eV. The composition dependence of the E0, E0+Δ0, E1, E2 and E2+δ gaps has been determined. The lowest direct band gap is found to be given by E0(x)=1.90+0.57x+0.11×2 eV. The E0+Δ0, and E1 gaps are written as similar quadratic equations. On the other hand, the variation of the E2 and E2+δ gaps is found to be approximately linear. The lowest direct-to-indirect gap crossover is also estimated to occur at x~0.63.

Temperature dependence of electroreflectance spectroscopy in Ga0.5In0.5P alloy

Electroreflectance (ER) measurements have been carried out to investigate critical point (CP) structures in weakly ordered Ga0.5In0.5P alloy at temperatures between 11 and 300 K. The measured ER spectra reveal distinct structures at ~1.9–2.0 eV [E0/(E0 + Δ0)], ~3.2–3.6 eV [E1/(E1 + Δ1)] and ~4.7–5.1 eV [E2/(E2 + δ)]. These structures are well explained by a three-dimensional [E0/(E0 + Δ0)] and a two-dimensional one-electron CP line shape [E1/(E1 + Δ1) and E2/(E2 + δ)]. The temperature dependence of the CP parameters (energy, amplitude and broadening parameter) has been determined and analysed using the Varshni equation and an analytical four-parameter expression developed for the explanation of the band-gap shrinkage effect in semiconductors. The spin–orbit split off energies have also been determined to be Δ0 ~ 80 meV and Δ1 ~ 256 meV, which are almost temperature independent.