H. Kasada

Highly Efficient Blue–Ultraviolet Photodetectors Based on II–VI Wide-Bandgap Compound Semiconductors

II-VI ZnSe (binary) and ZnSSe (ternary) widegap compound semiconductor based photovoltaic devices (p-i-n and avalanche photodiodes (APDs)) are developed for the blue-violet (460-400 nm) optical wavelength region by molecular beam epitaxial (MBE) growth. The ternary compound ZnSSe p-i-n structure photodiodes, grown with complete lattice matched condition on GaAs substrates, exhibit high external quantum efficiencies of 80-70% in the blue-violet optical region with extremely low dark currents (∼pA/mm 2 ), These diodes reveal stable and long-lived operation at 300 K. Also presented is ZnSe and ZnSSe based blue-violet APD devices, fabricated by precise defect control and process techniques. This new device has revealed large signal gain (G) of G = 50 for ZnSe, and G = 60 for the ZnSSe APD device under high electric field strength of (0.8-1.1) × 10 6 V/cm. Important device parameters, ionization coefficients of photo-injected electrons and holes, and device stability are also discussed.

Efficient blue-green light-emitting diodes of ZnSSe:Te/ZnMgSSe DH structure grown by molecular-beam epitaxy

We have investigated the optical properties of Te-doped ternary ZnSSe epilayers grown on (100) GaAs substrates by molecular-beam epitaxy for the first time. The case of ZnS x Se 1-x-y : Te y (x = 0.11, y = 0.042) specimen is represented by a good crystal quality; a good lattice matching to GaAs (Δa/a 1000 h) operated at 3 A/cm 2 (300 K).

Widegap II–VI Compound Optical Modulators of ZnSe/ZnMgSSe Single and Asymmetric-Coupled Quantum Wells

We report on widegap II-VI compound optical modulators of ZnSe/ZnMgSSe single and asymmetric coupled quantum wells (ACQWs) for blue-violet short wavelength region. Effective mass approximated calculations have shown that a ZnSe(10 ML)-ZnMgSSe(9 ML)-ZnSe(7 ML) ACQW exhibits a weak Stark effect region (E 40 kV/cm). The optical modulators with p-i-n structure, fabricated on GaAs substrate by molecular beam epitaxy(MBE), have shown light modulation at a wavelength of 443 nm under a reverse bias of 36 V at room temperature. A modulation depth of 29% was observed under a field strength of 270 kV/cm with a transmission configuration. The ACQW modulator exhibits an enhanced Stark shift of about 20 meV, which is twice as large as the SQW value, in the low field region of 30-100 kV/cm.

High Efficiency and Long‐Lived Green and Blue Light Emitting Diodes Based on ZnSSe:Te Active Layer Grown by Molecular Beam Epitaxy

Bright green and blue light emitting diodes have been developed based on ZnSSe:Te active layer, with high crystal quality and a close lattice-match to GaAs substrate. The green LEDs exhibit a fairly long lifetime (>2000 h) at room temperature, which is attributable to the crystal-hardening effect by Te-doping. The devices show slow-mode degradation due to the gradual enhancement of microscopic defects in p-ZnMgSSe cladding layer. No detectable changes in dark-spot and dark-line formation due to macroscopic defects are observed during device operation.

Blue‐Violet Avalanche‐Photodiode (APD) and its Ionization Coefficients in II–VI Wide Bandgap Compound Grown by Molecular Beam Epitaxy

We present II-VI wide bandgap semiconductor ZnSe and ZnSSe based blue-violet avalanche photo-diode (APD) operation grown on GaAs substrate by molecular beam epitaxy (MBE). Because of highly improved crystal quality of the active layer, the ZnSSe APD reveals stable and high avalanche gain (G ≥ 60) in the blue-violet optical region under high field operation condition (E = 1.15 x 10 6 V/cm) at room temperature (RT). High electric field induced ionization coefficients α (for electrons) and β (for holes) are determined as a function of electric field strength E(V/cm); α(E) = 1.7 × 10 7 exp (-4.9 × 10 6 /E) and β(E) = 3.7 × 10 6 exp (-4.0 × 10 6 /E).