T. S. Wu

High luminous efficiency thin‐film electroluminescent devices with low resistivity insulating materials

In order to fabricate high brightness and high efficiency thin‐film electroluminescent (EL) devices, the emission characteristics of devices employing low resistivity and high dielectric constant materials, such as radio‐frequency‐sputtered HfO2 films, have been studied. It was found that the EL device with a glass/indium tin oxide/BaTiO3/ZnS:TbF3/HfO2/Ta2O5/HfO2/Al structure exhibited higher brightness and higher efficiency than the other devices. The highest luminous efficiency (η) and brightness of 0.9 lm/W and 1000 cd/m2, respectively, were obtained at 1‐kHz sinusoidal wave voltage excitation. This was mainly due to the insulating layers adjacent to the active layer, which have low resistivity and high dielectric constant. So, it has higher density of interface states and deeper interface states at HfO2–ZnS and BaTiO3–ZnS interfaces.

Temperature dependent characteristics of a PIN avalanche photodiode (APD) in Ge, Si and GaAs

A theoretical assessment is presented based on a modification of Baraffs theory in order to compare the temperature dependence of several characteristics, including breakdown voltage, excess noise factor, effective ionization rate ratio and efficiency in Ge, Si and GaAs PIN avalanche photodiodes. The temperature coefficient of avalanche breakdown voltage in a high field region is studied. Finally the response time of a PIN APD in these materials is discussed.

Heteroepitaxial growth of gallium antimonide on GaAs by low pressure MOVPE

Abstract Undoped GaSb epilayers have been grown on (100) GaAs semi-insulating substrates by low pressure metalorganic vapor phase epitaxy (MOVPE). The effects of growth temperatures and TMSb/TEGa mole fraction ratios on the epitaxial properties of surface morphology, growth rate, carrier concentration and hole mobility (measured at 300 and 77 K) have been studied. For TMSb/TEGa mole fraction ratio of 6.84, a smooth surface morphology can be obtained at growth temperature of 550°C. At 550°C, the epitaxial surface grown for V / III = 6.64 was more smooth and mirror-like than that grown for V / III = 6.84. The growth rate increases with growth temperature. The hole concentration increases and mobility decreases with growth temperature between 520 and 635°C for V / III = 6.84. For 550°C grown epilayers: as the V/III ratio increased above 6.64 or decreased below 6.64, the hole concentration increased and the hole mobility decreased. The lowest concentration 1.8 × 10 16 cm −3 (77 K ) and the highest mobility 1447 cm 2 /V s (77 K) can be obtained for a V/III ratio of 6.64 at 550°C.

Influence of growth temperature upon the in solid composition in InxGa1-xSb epilayers grown by metalorganic chemical vapor deposition

InxGa1−xSb epitaxial layers have been grown on (100) GaSb substrates under TMSb/(TMIn+TEGa) ratio of 5.8 and system pressure of 170 Torr at different substrate temperatures (550–630 °C). From analyses of both the x‐ray diffraction and photoluminescence (PL), the indium solid mole fraction was found to decrease with increasing growth temperature. The x‐ray diffraction patterns degrade with increasing lattice mismatch (or with increasing In mole fraction in the solid). The photoluminescence peak wavelength decreases with increasing growth temperature. The optimum growth temperature is about 600 °C at which the good quality InxGa1−xSb epilayers with the most mirrorlike surface, narrowest PL full width at half‐maximum (9 meV) and strongest PL intensity can be obtained. The narrowest full width at half‐maximum value is only 9 meV. The full width at half‐maximum of 10‐K photoluminescence peaks was found to increase with increasing lattice mismatch due to the increasing misfit dislocations and strains existing a...

Antireflective and Radiation Resistant ZnO Thin Films for the Efficiency Enhancement of GaAs Photovoltaics

In this paper, zinc oxide (ZnO) thin films as an antireflective (AR) coating layer have been successfully fabricated on GaAs solar cells by the sol―gel method. ZnO films were prepared chemically by spin coating the gel with an aqueous solution of zinc acetate and ethanolamine. The current―voltage measurements of the solar cells confirmed the increase of the short-circuit current induced by the AR effect. The open-circuit voltage and fill factor were also improved by the surface passivation. As a result, the conversion efficiency of the cells without an AR coating (8.2%) was significantly enhanced to 13.6%. The results indicate that the chemical deposition of ZnO was effective for the AR coating of GaAs solar cells. Additionally, we demonstrate that the cells coated with radiation resistant ZnO films exhibit less efficiency decay than the devices without such treatment. Under the maximum proton fluence of 10 13 cm ―2 , the conversion efficiency decay was reduced to 69.8%, while the solar cells without ZnO films showed an efficiency decay of 83.1%.

Relationship between solid and vapor phase compositions for InxGa1−xSb epilayers grown by MOCVD

Abstract In x Ga 1− x Sb epitaxial layers have been grown on (100) GaSb substrates at 600°C and 170 torr under different group V/group III ratios (5.3–6.2). From both analyses of X-ray diffraction and photoluminescence (PL), the variation of In solid composition ( x s ) with vapor phase composition ( x v ) was studied. The distribution coefficient (the ratio x s to x v ) is found to be smaller than unity. The result is similar to but slightly higher than those reported by Bougnot et al. The energy gaps of In x Ga 1− x Sb epilayers were deduced from photoluminescent peaks (10 K). The relationship between energy gap (10 K) and in solid composition was studied and compared with the numerical results reported by Auvergne et al. The effects of group V/group III ratios on epitaxial surface morphologies and PL were also studied. The optimum V/III ratio is about 5.8 under which good-quality In x Ga 1− x Sb epilayers with a mirror-like surface, a narrow full width at half maximum (9 meV) and a strong intensity of PL can be obtained for epitaxial growth at 600°C, 170 torr.

Characterization of p-GaAs by low pressure MOCVD using DEZ as dopant

Abstract Zinc-doped GaAs epilayers grown by low pressure metalorganic chemical vapor deposition (LP-MOCVD) are studied. Triethylgallium (TEG) and arsine (AsH 3 ) are used as Ga and As source, respectively. Diethylzinc (DEZ) is used as p-type dopant. Layers of high crystalline quality can be obtained. The influence of growth parameters such as DEZ mole fraction, growth temperatures and AsH 3 mole fraction on hole concentration are measured and discussed. These results can be explained well by a simple qualitative model. The hole concentration is proportional to the concentration of gallium vacancies. The I – V characteristics of Schottky diodes and p-n junctions are discussed. The ideality factor is about 1.3.

Metalorganic chemical vapor deposition of ZnSe thin films on ITO/ glass substrates

Abstract High quality ZnSe/ITO/glass thin films have been investigated by low-pressure metalorganic chemical vapor deposition. The full-width at half maximum (FWHM) of the cubic (111) plane depends on the [H 2 Se]/[DMZn] ratio and substrate temperature and the value of FWHM was less than 0.4°. The lattice constant and energy gap obtained were 5.6863 A and 2.6957 eV, respectively. Specular ZnSe/ITO/glass can be grown in our system. A uniform thickness within 3% over the whole substrate (250 x 180 mm 2 ) was obtained.

Growth, Fabrication, and Characterization of InGaAsN Double Heterojunction Solar Cells

In this paper, we have demonstrated fabrication and characterization of p-GaAs/i-InGaAsN/n-GaAs double heterojunction solar cells (DHJSCs). The intrinsic InGaAsN absorption layers which were lattice-matched to GaAs substrate with photoresponse to 1 eV were grown by the metal organic chemical vapor deposition method. The samples were studied experimentally by varying the growth temperature of intrinsic InGaAsN absorption layers. By adjusting the indium and nitrogen content, we have grown InGaAsN epilayers which were lattice-matched to GaAs substrate at various growth temperatures. Among three intrinsic layer growth temperatures, it was found that the InGaAsN DHJSC with the intrinsic layer growth temperature of 550 °C can get the largest absorption region. Under the AM 1.5 direct spectrum, the DHJSCs with absorption layer which were grown at 550 °C have open-circuit voltages ranging from 0.295 V, short-circuit currents of 14.4 mA/cm2, and fill factor of 51.2%. The conversion efficiency of InGaAsN double heterojunction solar cells achieves 2.38%, and the absorption wavelength region would extend to 1200 nm.

A new δ-doped InGaAs/GaAs pseudomorphic high electron mobility transistor utilizing a strained superlattice spacer

Abstract A new δ-doped In 0.18 Ga 0.82 As/GaAs pseudomorphic high electron mobility transistor (HEMT) utilizing an In 0.18 Ga 0.82 As/GaAs strained superlattice spacer grown by low-pressure metal organic chemical vapor deposition (LP-MOCVD) has been prepared. For a gate length of 5 μm, the present structure reveals superior saturation current density (180 mA/mm at 300 K and 230 mA/mm at 77 K) and extrinsic transconductance (103 mS/mm at 300 K and 142 mS/mm at 77 K). The current-voltage characteristics are superior to those of reported similar InGaAs/GaAs structure with gate length of 2 μm grown by molecular beam epitaxy (MBE). Also, because of the undoped cap layer grown on the top of the δ-doped GaAs, a breakdown voltage of 10 V has been achieved.