S. X. Jin

III-nitride blue microdisplays

Prototype blue microdisplays have been fabricated from InGaN/GaN quantum wells. The device has a dimension of 0.5×0.5 mm2 and consists of 10×10 pixels 12 μm in diameter. Emission properties such as electroluminescence spectra, output power versus forward current (L–I) characteristic, viewing angle, and uniformity have been measured. Due to the unique properties of III-nitride wide-band-gap semiconductors, microdisplays fabricated from III nitrides can potentially provide unsurpassed performance, including high-brightness/resolution/contrast, high-temperature/high-power operation, high shock resistance, wide viewing angles, full-color spectrum capability, long life, high speed, and low-power consumption, thus providing an enhancement and benefit to the present capabilities of miniature display systems.

III-nitride ultraviolet light-emitting diodes with delta doping

We present the results on the fabrication and characterization of 340 nm UV light-emitting diodes (LEDs) based on InAlGaN quaternary alloys grown by metalorganic chemical vapor deposition. By employing δ doping in the n- and p-type layers, we have demonstrated enhanced LED structural quality and emission efficiency. Combining with our interconnected microdisk LED architecture, the output power of a 300×300 μm2 bare LED chip measured from the sapphire side reached 50 μW under a standard dc operation condition (20 mA) at 4.6 V and 1.6 mW under a pulsed driving current.

GaN-based waveguide devices for long-wavelength optical communications

Refractive indices of AlxGa1−xN with different Al concentrations have been measured in infrared wavelength regions. Single-mode ridged optical waveguide devices using GaN/AlGaN heterostructures have been designed, fabricated, and characterized for operation in 1550 nm wavelength window. The feasibility of developing photonic integrated circuits based on III-nitride wide-band-gap semiconductors for fiber-optical communications has been discussed.

Size dependence of III-nitride microdisk light-emitting diode characteristics

Individual microdisk blue-light-emitting diodes (μ-LEDs) of varying diameters from 5 to 20 μm have been fabricated from InGaN/GaN quantum wells. Size effects on the μ-LED characteristics, including I–V and L–I characteristics, have been measured. The transient behavior of the μ-LEDs has also been studied. It was found that the turn-on time is on the order of our system response (30 ps) and the turn-off time is on the order of 0.2 ns and shows a strong size dependence. The ability of two-dimensional array integration with advantages of high speed, high resolution, low temperature sensitivity, and applicability under versatile conditions make III-nitride μ-LEDs a potential candidate for light sources in short-distance optical communications.

RECTIFICATION PROPERTIES AND INTERFACE STATES OF HETEROJUNCTIONS BETWEEN SOLID C60 AND N-TYPE GAAS

Solid C60/n‐GaAs heterojunctions have been fabricated by deposition of solid C60 film on the (100)‐oriented epitaxial n‐type GaAs substrates and their electrical characteristics have been measured. The rectification ratio is greater than 106 at a bias of ±1 V. The current for a fixed forward bias is an exponential function of reciprocal temperature, from which the effective potential barrier height of the heterojunction is determined to be 0.58 eV. A trap with an energy level 0.35 eV below the conduction band of GaAs at the C60/GaAs interface has been observed by the deep level transient spectroscopy technique.

HETEROJUNCTIONS OF SOLID C70 AND CRYSTALLINE SILICON : RECTIFYING PROPERTIES AND BARRIER HEIGHTS

Heterojunctions of solid C70 and n‐ or p‐type crystalline Si have been made. Current–voltage measurements show that both C70/n‐Si and C70/p‐Si contacts are rectifying but their directions of rectification are opposite to each other. Thermal activation measurements at a fixed forward bias show an exponential dependence of current on the reciprocal of temperature, from which we determine the effective barrier height as 0.23 eV for C70/n‐Si and 0.27 eV for C70/p‐Si. Relative dielectric constant of solid C70 was determined to be 4.96 through the study of high‐frequency capacitance–voltage characteristics for Ti/C70/p‐Si structures.

NEGATIVE CHARGE STATE OF HYDROGEN SPECIES IN N-TYPE GAAS

Having been exposed to hydrogen plasma, the Te-doped GaAs wafers were deposited with metal Ti, to form the Ti/n-GaAs Schottky barrier diodes (SBDs). The profiles of donor concentration after successive annealing of the hydrogenated SBD at several reverse biases at 100-degrees-C, indicated that the donor concentration in the strong electrical field region of the depletion layer increase monotonically and the donor concentration in the weak electrical field region of the depletion layer decrease monotonically with time. It is confirmed that hydrogen can be present as a negatively charged species in n-type GaAs and thus one can conclude that besides the deep donor level determined previously, hydrogen in GaAs has an acceptor level in the lower half of the energy gap or near the mid gap. There is evidence that the stronger the electrical field in strong field region of depletion layer, the faster the TeH complexes decompose in that region during a reverse bias annealing.

Effects of hydrogen on the Schottky barrier of Ti/n‐GaAs diodes

Having been exposed to hydrogen plasma, Te-doped GaAs wafers were deposited with metal Ti, to form Ti/n-GaAs Schottky barrier diodes (SBD). It was found that due to hydrogenation the Schottky barrier height (SBH) of the SBD decreases from 0.76 to 0.58 eV and the effective Richardson constant A** decreases from 11 to 0.03 A/cm2/K2. Annealing the hydrogenated SBD at different reverse biases at 100-degrees-C produces SBHs that have a one-to-one correlation with the biases of reverse-bias annealing (RBA), i.e., the SBH can be controlled, within the range 0.58-0.74 eV, by the bias of RBA. When the reverse bias is equal to or larger than 3 V, the SBH and the effective Richardson constant of the hydrogenated SBDs after RBA are very near to those of SBDs without hydrogenation. This means that a RBA with a bias higher than a critical value, 3 V in our case, can remove the effects of hydrogenation, but if the SBD is annealed again without a bias at 100-degrees-C for 1 h or more, its SBH and effective Richardson constant recover their hydrogenated values. The main experimental facts can be explained qualitatively by a simple theoretical model.

DEEP LEVELS IN NITROGEN-IMPLANTED N-TYPE GAAS

Ti/n‐GaAs Schottky barrier diodes were prepared on nitrogen‐implanted n GaAs. The Schottky barrier height of the diodes was found to be 0.96 eV, 0.12 eV higher than that of the samples without N implantation. Four distinctive electron traps E1(0.111), E2(0.234), E3(0.415), and E4(0.669) and one hole trap, H(0.545), have been observed with deep level transient spectroscopy. Defect models of these deep levels are proposed and the role of H(0.545) in the Schottky barrier formation is also discussed.

Heterojunctions of solid C60 and crystalline silicon: rectifying properties and energy-band models

Heterojunctions of undoped solid C60 and n- or p-type-doped crystalline Si have been obtained. Current-voltage measurements show that both C60/n-Si and C60/p-Si contacts are rectifying but their directions of rectification are opposite. Thermal activation measurements at a fixed forward bias show an exponential dependence of current on the reciprocal of temperature, from which we determine the effective barrier height as 0.30 eV for C60/n-Si and 0.48 eV for C60/p-Si. Using energy-band models for heterojunctions we assign values to the positions of the conduction and valence bands of the solid C60 relative to those of crystalline Si and derive the electron affinity and band gap of solid C60 film as 3.92 eV and <1.72 eV, respectively.