Chyuan‐Wei Chen

Liquid-crystal alignment on a-C:H films by nitrogen plasma beam scanning

A plasma beam scanning treatment has been developed to modify the surface of the hydrogenated amorphous carbon a-C:H film on the indium tin oxide glass. The plasma beam scanning treatment makes the a-C:H film an excellent layer for liquid-crystal alignment. The qualities of a-C:H films were characterized by using atomic force microscope, micro-Raman spectroscopy, and field-emission scanning electron microscope. The ultrathin a-C:H films were deposited at 50% CH4/H2+CH4 gas ratio, 100 W radio-frequency power, and a gas pressure of 10 mtorr for 15 min by capacitive-coupled plasma chemical-vapor deposition method. The twist nematic cells were filled with liquid crystal ZLI-2293 on the a-C:H film treated with different nitrogen plasma beam scanning time. The grooving mechanism is considered not responsible for the liquid-crystal LC alignment. Raman spectra suggest that a bond-breaking process of aromatic rings occurs in the aC:H film. The O1s ,C 1s, and N1s core-level spectra support that the nitrogen plasma beam scanning treatment induces a bond-breaking process of aromatic rings to create available carbon dangling bonds for the formation of C‐O bonds. The newly formed C‐O bonds are “directional,” which favor the LC alignment on the a-C:H film.

Growth-temperature dependence of electrical and luminescent properties of high-quality GaSb grown by liquid-phase epitaxy

The growth‐temperature dependence of electrical and photoluminescent properties from high‐quality GaSb layers grown by liquid‐phase epitaxy has been studied. At the growth temperature of 600 °C, a hole concentration of ∼ 1 × 1017 cm−3 is obtained and the 16 K photoluminescence spectrum is dominated by the line BE2 at 802.9 meV associated with excitons bound to acceptors, and a stronger band‐acceptor emission band at 777.8 meV. With reducing the growth temperature, the hole concentration gradually decreases, as does the line BE2 in the photoluminescence spectrum. The GaSb layer conduction converts from p to n with a minimum hole concentration of 2–6 × 1015 cm−3 when the growth temperature is below 450 °C. The line D located at 808.2 meV, due to a donor‐bound exciton transition, becomes dominant and the band‐acceptor emission becomes very weak at lower growth temperatures. The is the first report on the growth‐temperature dependence of the excitonic transitions from high‐quality GaSb layers.

On the mechanism of enhancement on electron field emission properties for ultrananocrystalline diamond films due to ion implantation

The effects of N and C ion implantations on modifying the structural and field emission properties of ultrananocrystalline diamond (UNCD) films were investigated. Low dose ion implantations possibly introduced point defects, which were easily removed by the annealing process. The nature of the doping species, N or C, was immaterial. In contrast, high dose N ion implantation induced the formation of the amorphous phase, which was converted into the graphitic phase after annealing, and improved the field emission properties (Je was increased to ~6.3 mA cm−2 at 20 V µm−1). However, the high dose C ion implantation induced the graphitic phase directly, which degraded the field emission characteristics, i.e. Je was lowered to ~0.6 mA cm−2 at 20 V µm−1. The variations in the electron field emission properties for ion-implanted UNCD films are accounted for by the nature of the induced defects and the electron transfer doping mechanism.

Growth and characterization of high‐quality In0.32Ga0.68P layers on GaAs0.61P0.39 substrates by liquid‐phase epitaxy

High‐quality In1−xGaxP epitaxial layers were grown on GaAs0.61P0.39 substrates by liquid‐phase epitaxy using a supercooling technique. The growth conditions and properties of the undoped In1−xGaxP layers are described in detail. The lattice mismatch normal to the wafer surface between the In1−xGaxP layer and GaAs0.61P0.39 substrate varies linearly with the supercooled temperature of the growth solution. Low‐carrier‐concentration undoped epitaxial layers can be grown from an In solution baked at temperature higher than 900 °C for 10 h and with a suitable supersaturation temperature. The lowest carrier concentrations of 8–20×1015 cm−3 measured by the capacitance‐voltage method have been achieved in the layers grown with a 9–12 °C supercooled temperature. These samples with a lattice mismatch of ∼+0.15% also show the narrowest full widths at half maximum of photoluminescence peaks of 36 meV at 300 K, 11.5 meV at 16 K, and 7.5 meV at 4.5 K. The electrical and optical properties of the In1−xGaxP epitaxial laye...

Photoluminescence study of rapid thermal annealing from nitrogen‐implanted In0.32Ga0.68P

Photoluminescence studies were performed to evaluate the results of rapid thermal annealing of nitrogen‐implanted In0.32Ga0.68P layers, which were grown on GaAs0.61P0.39 substrates by a supercooling liquid‐phase‐epitaxial method. When the annealing temperature used is between 600 and 840 °C with 30 s duration, the N isoelectronic trap can be activated with an activation energy of 0.48 eV which is necessary to place N atoms into P sites. The 9 K photoluminescence spectrum is dominated by the sharp near‐band‐gap peak EgΓ and the broad N‐related band Nx. The N level is located ∼110 meV below the Γ‐band minimum for the In0.32Ga0.68P alloy. By selecting different annealing temperatures and times, the optimum annealing condition to obtain the strongest emission intensity of the band Nx is at T=800 °C and 30 s duration.

Temperature dependence of the photoluminescence of Zn‐doped In0.32Ga0.68P grown on GaAs0.61P0.39 substrates

The photoluminescence (PL) spectra of Zn‐doped In0.32Ga0.68P epitaxial layers grown on GaAs0.61P0.39 substrates by liquid‐phase epitaxy has been investigated in the temperature range of 8–300 K. The radiative recombination processes of the direct In0.32Ga0.68P alloys for which the composition is near the direct–indirect band gap crossover point have been studied at various temperatures. At higher temperatures (≳150 K) only one emission band corresponding to free‐electron‐to‐free‐hole transition dominates. Two peaks and one broad band are observed in the PL spectrum when the temperature is below 100 K. The peak denoted by A is due to direct interband radiative recombination. The temperature dependence of the band gap in In0.32Ga0.68P layers can be expressed as 2.25 − [1.79 × 10−3T2/(T + 1236)] eV. The peak denoted by B, exhibited by undoped or moderately Zn‐doped (p ≤ 3 × 1018 cm−3) InGaP samples, is attributed to the conduction‐band‐to‐acceptor transition. A third broad band (C) dominates at low temperatu...

Stress relaxation in the GaN∕AlN multilayers grown on a mesh-patterned Si(111) substrate

300×300μm2 crack-free GaN∕AlN multilayers of 2μm in thickness have been successfully grown on the Si(111) substrate patterned with the SixNy mesh by metal-organic chemical-vapor deposition. The in-plane stress exhibits a U-shape distribution across the “window” region, supported by the Raman shift of the GaN E2(TO) mode. This indicates a stress relaxation abruptly occurring near the edge of the window region due to the freestanding surface (11¯01) or (112¯2). The in-plane stress is almost relaxed at the corner of the window region due to three freestanding surfaces (11¯01), (112¯2), and (101¯1). The maximum in-plane stress is located near the surface of the multilayers at the center of the window region, supported by the Raman measurements and the failure observations. The role of the SixNy mesh in the stress relaxation is discussed.

Liquid‐phase epitaxial growth and characterization of InGaAsP layers grown on GaAsP substrates for application to orange light‐emitting diodes

The growth conditions of undoped InGaAsP layers grown on GaAs0.61P0.39 substrates and effects of Te and Zn‐doping on electrical and optical properties have been examined in detail. The narrowest full widths at half‐maximum (FWHM) of photoluminescence (PL) spectra were measured to be 40 meV at 300 K and 12 meV at 8 K for an undoped InGaAsP sample with a background electron concentration of 2×1016 cm−3. Room‐temperature carrier concentrations in the range of 1.8×1017–3.4×1018 cm−3 for n‐type dopant and 1.6×1017–2.8×1018 cm−3 for p‐type dopant are obtained reproducibly. The 50 K PL spectra of Zn‐doped layers show three distinctive peaks and their relative intensities change with various hole concentrations. Visible light‐emitting diodes (LEDs) emitting at 619 nm and employing the InGaP/InGaAsP/InGaP double‐heterostructure (DH) grown on a lattice‐matched GaAs0.61P0.39 substrate have been fabricated. The DH LEDs are characterized by current‐voltage (I‐V) measurement, electroluminescence (EL), light output powe...

Photoluminescence Study of Heavily Te-doped GaAs Grown by Liquid-Phase Epitaxy

Systematic studies of photoluminescence (PL) are used to characterize the heavily Te-doped GaAs layers with electron concentrations of 9.4×1018-2.3×1019 cm-3. For the undoped layer with an electron concentration of 1×1015 cm-3, the near-band-to-band transition is found to dominate the low-temperature PL spectra. While at concentrations above 1018 cm-3, both the band filling as well as band tailing due to the carrier scattering with the ionized donor impurities and band shrinkage due to the exchange interaction between free carriers are considered to account for the observed luminescence behavior. The dependence of spectral shape and broadening on the doping level, excitation power and temperature has been investigated in detail. At concentrations above 1×1019 cm-3, the low-temperature PL spectra is mainly dominated by the low energy, band-edge artifact peak B at 1.488 eV which passes through the substrate, reflects off the back surface, and is emitted from the epitaxial surface. The low-energy transition becomes stronger with doping due to the increase in the band shrinkage.

Tellurium and zinc doping in In0.32Ga0.68P layers grown by liquid‐phase epitaxy

In0.32Ga0.68P epitaxial layers doped with Te and Zn were grown on (100) GaAs0.61P0.39 epitaxial substrates by liquid‐phase epitaxy using a supercooling method. The electrical properties of doped layers were determined by C‐V measurements at 300 K. Room‐temperature carrier concentrations ranging from 9×1016 to 2×1018 cm−3 for n‐type and from 3×1016 to 6×1018 cm−3 for p‐type dopants are obtained reproducibly. The full width at half maximum value of the 300 K photoluminescent spectrum increases with carrier concentration for Te‐ and Zn‐doped layers. The relative intensity of 300‐K photoluminescent peak presents the maximum values at 1×1018 and 6×1017 cm−3 for electron and hole concentrations, respectively. The 100‐K photoluminescent spectra show three distinctive peaks and their relative intensities change with hole concentrations. Finally, the relationship between the acceptor ionization energy and hole concentration is described.