H. Ishikura

Stable avalanche-photodiode operation of ZnSe-based p+–n structure blue-ultraviolet photodetectors

A high-field operation of p+–n structure blue-ultraviolet photodetectors has been studied using ZnSe-based II–VI wide-band-gap compound semiconductors grown by molecular-beam epitaxy (MBE). With an improved crystal quality on macrodefects (dislocations and stacking faults) during an initial MBE growth as well as an optimized device structure including a complete superlattice ohmic-contact layer, a stable high electric-field operation up to 8×105 V/cm is established. It is demonstrated that the p+–n ZnSe photodiodes have shown a stable avalanche-photodiode operation with a large avalanche gain of G=60 in the blue-ultraviolet region at room temperature.

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.

Persistent Photoconductivity (PPC) and Related Deep Metastable Center in MBE Grown p-Type ZnMgSSe

A persistent-photoconductivity (PPC) effect and the related deep metastable centers have been investigated for a p-type ZnMgSSe quaternary system grown by molecular beam epitaxy. A low temperature photo-excitation revealed a marked PPC effect in nitrogen doped p-type Zn1-xMgxSySe1-y (0.05<x<0.18, 0.08<y<0.2) with an exponential temperature dependence of the carrier decay time constant: τ(T) = τ0 exp (-490±50 meV/kT). It is also confirmed that the low-temperature photo-excitation has caused a new deep metastable hole trap center with thermal hole activation energy of 650 meV. This metastable deep center is evidenced to be responsible for the observed PPC effect.

Optimization of ZnSe/ZnTe superlattice structured p-contact for ZnSe-based optical devices

We have optimized the structure of ZnSe/ZnTe superlattice electrode by comparison between experimental characteristics and theoretical calculations. Quantum levels in ZnSe/ZnTe superlattice and tunneling carrier densities are evaluated by using finite element method and transfer matrix method, respectively. It is found that important parameters for the design of the superlattice electrode are (a) high ground quantum level of hole, and (b) large tunneling carrier density. A new optimum structure with very high tunneling current density is proposed.

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).