Sadao Adachi

Optical properties of PbS

Spectroscopic ellipsometry has been used to measure the complex dielectric function e(E)=e1(E)+ie2(E) of PbS single crystals. The measurements are made on cleaved PbS surfaces in the 1.5–5.3 eV photon-energy range at room temperature. It is found that the optical properties of PbS are very sensitive to air exposure. The observed e(E) spectra reveal distinct structures at energies of the E1, E2, and E3 critical points (CPs). These data and the literature values are used for the modeling of the optical constants of PbS over the 0.3–5.3 eV photon-energy range, including the band-gap (E0∼0.4 eV) region. It is shown that the three-dimensional M0 CP (E0) and damped harmonic-oscillator model (E1, E2, and E3) provide satisfactory fits to the experimental optical-constant data.

Zinc Oxide (ZnO)

Zinc oxide (ZnO) is a compound crystallizing in the wurtzite structure, and has a wide band gap ∼3.4 eV at 300 K. It has been used extensively as a photoconducting and fluorescent material, being effective in the visible-to-UV spectral region. The excellent optical, piezoelectric, and acoustooptic properties of a thin ZnO film also make it possible to fabricate solar cell windows [1], gas sensors [2], surface acoustic wave devices [3], and integrated acoustooptic devices [4]. Optically pumped lasing from ZnO platelets have also been observed at very low pump power (∼4 W/cm2) [5].

Zinc Selenide (ZnSe)

Zinc selenide (ZnSe) crystallizes in the cubic, zinc-blende structure, although it is also possible to grow hexagonal wurtzite form. ZnSe is the prototype of wide-band-gap II–VI semiconductors and plays an important role in the development of blue-green or blue injection lasers (see, e.g., Ref. [1]). Many attempts have, thus, been made to achieve epitaxial growth of ZnSe layers on GaAs substrates [2] because of the relatively small lattice mismatch between GaAs and ZnSe (∼0.27%). Its large band gap and closely lattice-matched nature make it also attractive for use as a passivation layer for GaAs and as an insulating layer of GaAs field-effect transistors [3]. ZnSe is also promising material for nonlinear optical devices (see, e.g., Ref. [4]).

Cadmium Telluride (CdTe)

Cadmium telluride (CdTe) is a direct-band-gap semiconductor (E g∼1.5 eV at 300 K) crystallizing in the cubic, zinc-blende structure. CdTe has been devoted to the development of semiinsulating crystals for applications in RT γ-ray detectors and as electro-optic and acoustooptic devices (see, e.g., [1]). As thin films, this material has also been used in solar cells, liquid-crystal imaging devices, and IR optics [2]. CdTe(111)B films with a 5 in. diameter have been grown on Si(100) substrates by MBE [3].

Zinc Telluride (ZnTe)

Zinc telluride (ZnTe) is a wide-band-gap II–VI semiconductor (E g=2.25 eV at 300 K) crystallizing in the cubic, zinc-blende structure. This material is promising for application as a purely green light-emitting diode. Most wide-band-gap II–VI semiconductors can be easily doped n type, but resist p-type doping. The most notable exception is ZnTe which is easily doped p type, but not n type. It is, thus, quite a challenge to achieve n-type doping of ZnTe. Recently, successful growth of low-resistivity (as low as 0.1–0.3 Ω-cm) n-type ZnTe epitaxial layers has been achieved by MOCVD on Bridgman-grown (100)ZnTe substrates with the use of triethylaluminium as the dopant [1]. This result may be encouraging in the search for various optoelectronic devices based on ZnTe.

Gallium Arsenide (GaAs)

Regarding device applications, GaAs is currently one of the most versatile semiconductors in use [1-3]. From an aspect of the solid-state phySiCs, GaAs is extremely interesting as prototypal direct-band-gap semiconductor. A review of many phySiCal and semiconducting properties of GaAs has been given in Refs. [4–8].

Indium Arsenide (InAs)

Indium arsenide (InAs) is a direct-band-gap semiconductor crystallizing in the zinc-blende structure. The relatively small direct band gap (∼0.36 eV at 300 K, Ref. [1]) and resulting high electron mobility (μn≥1×104 cm2/Vs at 300 K, Ref. [1]) have made this material interesting for use in Hall-effect device applications (see Ref. [2]). As a substrate, or active layer, InAs can be employed in conjunction with many semiconductors such as AlGaSb or InAsSb and has interesting heterojunction potential for modulation-doped field effect transistors, lasers and detectors in the wavelength range up to 10 μm, and quantum-well structures [2],

Lead Selenide (PbSe)

There has been considerable interest in lead-salt-type crystals (PbS, PbSe, and PbTe) because they have a number of interesting phySiCal properties as well as numerous potential applications. The highest pulsed operation temperature of 282 K was reported for a wavelength of 4.2 μm from a PbSe-based injection laser [1]. A comprehensive review of the energy-band structures and some optical properties of the lead-salt-type crystals has been carried out [2].

Tin Telluride (SnTe)

The lead chalcogenides, PbS, PbSe, and PbTe, crystallize in the rocksalt structure, while in tin telluride (SnTe) a ferroelectric phase transition from a rocksalt structure (s-SnTe) to a rhombohedral structure really occurs. The rocksalt structure is stable above -100 K. SnTe always behaves as a heavily doped p-type semiconductor. Because of its narrow band gap, SnTe is useful for IR optical devices.

Mercury Telluride (HgTe)

Mercury telluride (HgTe) is a IIb–VIb semiconductor crystallizing in the zinc-blende structure. HgTe forms alloys with many other IIb–VIb compounds. The Cd x Hg1-x Te alloy system is till the major material used for infrared detectors [1,2], By varying the x composition from zero (HgTe) to unity (CdTe) the energy gap varies from ∼ -0.3 to ∼1.6 eV. This promises that detectors with peak responses over a wide wavelength range can be fabricated from this alloy system. Because of the unique band structure (i.e., “negative” energy gap, “inverted” band structure), this material is also interesting from many phySiCal points of view (see, e.g., Ref. [3]).