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. ...

/pdf/a-comprehensive-review-of-zno-materials-and-devices-21a3zjadem.pdf

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

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

Positron annihilation spectroscopy is particularly suitable for studying vacancy-type defects in semiconductors. Combining state-of-the-art experimental and theoretical methods allows for detailed identification of the defects and their chemical surroundings. Also charge states and defect levels in the band gap are accessible. In this review the main experimental and theoretical analysis techniques are described. The usage of these methods is illustrated through examples in technologically important elemental and compound semiconductors. Future challenges include the analysis of noncrystalline materials and of transient defect-related phenomena.

/pdf/defect-identification-in-semiconductors-with-positron-kn4qxl00fh.pdf

Defect identification in semiconductors with positron annihilation: Experiment and theory

ZnSe layers have been grown by molecular beam epitaxy on high-purity, high-quality ZnSe wafers [(100) oriented] cut from ingots grown by the iodine vapor transport method. Photoluminescence (PL) analysis indicates the homoepitaxial ZnSe material to be of ultra-high purity as well as being strain-free relative to ZnSe/(100) GaAs layers which exhibit in-plane biaxial tension. The 10 K PL spectra recorded from homoepitaxial layers exhibit unsplit free- and donor-bound exciton transitions of comparable intensity together with a strong peak at 2.7768 eV believed to be the so-called Iv transition in relaxed ZnSc. The ultra-high purity nature of the homoepitaxial layers is attributed to the high purity of the substrate material in addition to the use of high purity Zn and Se source materials in this work.

M. B. Troffer

Papers

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

Strain-free, ultra-high purity ZnSe layers grown by molecular beam epitaxy