The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London 47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev. 142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys. 6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. ...

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A comprehensive review of zno materials and devices

In the past several years, research in each of the wide‐band‐gap semiconductors, SiC, GaN, and ZnSe, has led to major advances which now make them viable for device applications. The merits of each contender for high‐temperature electronics and short‐wavelength optical applications are compared. The outstanding thermal and chemical stability of SiC and GaN should enable them to operate at high temperatures and in hostile environments, and also make them attractive for high‐power operation. The present advanced stage of development of SiC substrates and metal‐oxide‐semiconductor technology makes SiC the leading contender for high‐temperature and high‐power applications if ohmic contacts and interface‐state densities can be further improved. GaN, despite fundamentally superior electronic properties and better ohmic contact resistances, must overcome the lack of an ideal substrate material and a relatively advanced SiC infrastructure in order to compete in electronics applications. Prototype transistors have been fabricated from both SiC and GaN, and the microwave characteristics and high‐temperature performance of SiC transistors have been studied. For optical emitters and detectors, ZnSe, SiC, and GaN all have demonstrated operation in the green, blue, or ultraviolet (UV) spectra. Blue SiC light‐emitting diodes (LEDs) have been on the market for several years, joined recently by UV and blue GaN‐based LEDs. These products should find wide use in full color display and other technologies. Promising prototype UV photodetectors have been fabricated from both SiC and GaN. In laser development, ZnSe leads the way with more sophisticated designs having further improved performance being rapidly demonstrated. If the low damage threshold of ZnSe continues to limit practical laser applications, GaN appears poised to become the semiconductor of choice for short‐wavelength lasers in optical memory and other applications. For further development of these materials to be realized, doping densities (especially p type) and ohmic contact technologies have to be improved. Economies of scale need to be realized through the development of larger SiC substrates. Improved substrate materials, ideally GaN itself, need to be aggressively pursued to further develop the GaN‐based material system and enable the fabrication of lasers. ZnSe material quality is already outstanding and now researchers must focus their attention on addressing the short lifetimes of ZnSe‐based lasers to determine whether the material is sufficiently durable for practical laser applications. The problems related to these three wide‐band‐gap semiconductor systems have moved away from materials science toward the device arena, where their technological development can rapidly be brought to maturity.

Large‐band‐gap SiC, III‐V nitride, and II‐VI ZnSe‐based semiconductor device technologies

Gallium nitride (GaN) and its allied binaries InN and AIN as well as their ternary compounds have gained an unprecedented attention due to their wide-ranging applications encompassing green, blue, violet, and ultraviolet (UV) emitters and detectors (in photon ranges inaccessible by other semiconductors) and high-power amplifiers. However, even the best of the three binaries, GaN, contains many structural and point defects caused to a large extent by lattice and stacking mismatch with substrates. These defects notably affect the electrical and optical properties of the host material and can seriously degrade the performance and reliability of devices made based on these nitride semiconductors. Even though GaN broke the long-standing paradigm that high density of dislocations precludes acceptable device performance, point defects have taken the center stage as they exacerbate efforts to increase the efficiency of emitters, increase laser operation lifetime, and lead to anomalies in electronic devices. The p...

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Luminescence properties of defects in GaN

A II-VI compound semiconductor laser diode (10) is formed from overlaying layers of material including an n-type single crystal semiconductor substrate (12), adjacent n-type and p-type guiding lasers (14) and (16) of II-VI semiconductor forming a pn junction, a quantum well active layer (18) of II-VI semiconductor between the guiding layers (14) and (16), first electrode (32) opposite the substrate (12) from the n-type guiding layer (14), and a second electrode (30) opposite the p-type guiding layer (16) from the quantum well layer (18) Electrode layer (30) is characterized by a Fermi energy A p-type ohmic contact layer (26) is doped, with shallow acceptors having a shallow acceptor energy, to a net acceptor concentration of at least 1 x 1017 cm-3, and includes sufficient deep energy states between the shallow acceptor energy and the electrode layer Fermi energy to enable cascade tunneling by charge carriers

Blue-green laser diode

A novel approach to producing p‐type ZnSe epitaxial layers is reported which involves nitrogen atom beam doping during molecular beam epitaxial growth. Net acceptor concentrations as large as 3.4×1017 cm−3 have been measured in nitrogen atom beam doped ZnSe/GaAs heteroepitaxial layers which represents the highest acceptor concentration reported to date for ZnSe:N epitaxial material grown by molecular beam epitaxy. In addition, light‐emitting diodes based on ZnSe:N/ZnSe:Cl, p‐n homojunctions have been found to exhibit dominant electroluminescence in the blue region of the visible spectrum at room temperature.

p-type ZnSe by nitrogen atom beam doping during molecular beam epitaxial growth

We report the growth of zinc blende‐GaN epitaxial films on β‐SiC coated (001) Si substrates using a molecular beam epitaxy approach in which the reactive nitrogen species are generated in a remote 13.56 MHz rf plasma discharge, nitrogen free‐radical source. We postulate, based on optical emission spectroscopy studies of the remote plasma, that, in our study, nitrogen atoms are the species primarily responsible for efficient nitridation. The zinc blende nature of the GaN films was confirmed by in situ reflection high‐energy electron diffraction, ex situ x‐ray diffraction, and ex situ low‐temperature photoluminescence analyses. Our zinc blende‐GaN film growth rates (∼0.3 μm/h) are higher than those reported to date that involve the use of electron cyclotron resonance type reactive nitrogen sources.

Growth of zinc blende‐GaN on β‐SiC coated (001) Si by molecular beam epitaxy using a radio frequency plasma discharge, nitrogen free‐radical source

We report the electrical characteristics of heavily Si‐doped zinc blende GaN epilayers deposited on β‐SiC coated (001) Si substrates. The β‐GaN films were grown by molecular beam epitaxy using a rf plasma discharge, nitrogen free‐radical source, and the doping concentration in the films was controlled over the range 1.5×1018–3.0×1020 cm−3 by suitably adjusting the temperature of a Si effusion cell. We have found that Si incorporation in β‐GaN results in a relatively deep donor level (∼62 meV below the conduction band edge at a carrier concentration at room temperature of 1018 cm−3). Also, we present evidence of simultaneous high mobility conduction band conduction (dominant at high temperatures) and low mobility impurity band conduction (dominant at temperatures <70 K) in heavily doped (nRT≳1019 cm−3) material.

Growth by molecular beam epitaxy and electrical characterization of Si‐doped zinc blende GaN films deposited on β‐SiC coated (001) Si substrates

Multilayerepitaxial structures consisting of In x Ga1−x As layers of various compositions were grown on GaAs substrates by the molecular beam epitaxy technique. Dislocation evolution and residual strain in these heterostructures were studied using cross‐sectional transmission electron microscopy (XTEM) and high‐resolution x‐ray diffraction analyses, respectively. The multilayerheterostructures were designed such that the compositional difference between two adjacent In x Ga1−x As layers in the stack was less than a critical compositional difference of Δx=0.18, taking partial lattice‐relaxation into account. XTEM studies of the stacked structures indicated dislocation evolution to be confined to the GaAs substrate and the In x Ga1−x As layers underlying the top In x Ga1−x As layer in the stack, the top In x Ga1−x As layer being essentially dislocation‐free. This phenomenon is attributed to a monotonic increase in the yield strength of In x Ga1−x As at the appropriate growth temperatures with increasing values of x. Such behavior appears to persist up to an In x Ga1−x As composition of approximately x=0.5, whereupon a further increase in composition results in dislocation evolution in the top layer of the stack. It is postulated that the yield strength of In x Ga1−x As decreases with increasing values of x beyond x=0.5. Extremely low dislocation density In x Ga1−x As material was grown on GaAs using the stacked structure approach as evidenced by etch pit analysis. For example, dislocation densities of 1–2×103/cm2 and 5–6×103/cm2 were recorded from In0.35Ga0.65As and In0.48Ga0.52As top layers, respectively. Such In x Ga1−x As alloys would be potentially suitable for the fabrication of photonic devices operating at 1.3 μm (x=0.35) and 1.55 μm (x=0.48).

Application of ‘‘critical compositional difference’’ concept to the growth of low dislocation density (<104/cm2) InxGa1−xAs (x≤0.5) on GaAs

Nitrogen‐doped ZnSe layers (∼0.5 μm thick) have been grown by molecular beam epitaxy (MBE) onto (100)‐oriented GaAs substates using both N2 and NH3 gas sources as alternative sources of nitrogen. The low‐temperature photoluminescence spectra obtained from N‐doped ZnSe layers were dominated by strong donor‐acceptor (D‐A) pair recombination emission. A no‐phonon emission peak (QN0 ) at 2.699 eV together with four distinct longitudinal optical (LO) phonon replicas of QN0 were readily resolved. The amplitudes of this series of D‐A pair recombination peaks were found to increase in a monotonic fashion with increasing overpressure of N2 and NH3 in the MBE growth chamber. In addition an acceptor‐bound exciton peak (IN1 ) was detected at 2.7917 eV from layers grown using large N2 or NH3 overpressures (∼10−4 mbar.).

R. M. Park

Papers

p-type ZnSe by nitrogen atom beam doping during molecular beam epitaxial growth

Growth of zinc blende‐GaN on β‐SiC coated (001) Si by molecular beam epitaxy using a radio frequency plasma discharge, nitrogen free‐radical source

Growth by molecular beam epitaxy and electrical characterization of Si‐doped zinc blende GaN films deposited on β‐SiC coated (001) Si substrates

Application of ‘‘critical compositional difference’’ concept to the growth of low dislocation density (<104/cm2) InxGa1−xAs (x≤0.5) on GaAs

Photoluminescence properties of nitrogen‐doped ZnSe grown by molecular beam epitaxy