M. Aven

Some Electrical and Optical Properties of ZnSe

Single crystals of ZnSe have been prepared by the vapor growth technique and optical and electrical measurements on these crystals are reported. Analysis of the reststrahlen reflection peak gives 0.026 ev for the transverse optical phonon energy. The longitudinal optical phonon energy is 0.031 ev as calculated from the transverse phonon energy, the static dielectric constant, e0=8.1±0.3, and the high‐frequency dielectric constant, e∞=5.75±0.1. The effective ionic charge calculated from the Szigetti formula is 0.7±0.1. Exciton absorption peaks associated with the valence and conduction bands in the vicinity of Γ were observed at liquid hydrogen temperature with the principal peak at 2.81±0.01 ev. The exciton reduced mass 0.1 m0 combined with the room temperature electron‐to‐hole mobility ratio of 12, obtained by preliminary transport measurements on n‐ and p‐type ZnSe gives tentative values of 0.1 m0 and 0.6 m0 for the electron and hole masses, respectively.Reflectance was determined by various methods in ...

Some Properties of Zinc Sulfide Crystals Grown from the Melt

Hexagonal zinc sulfide crystals were obtained by controlled cooling of melted zinc sulfide. Both pure and activated zinc sulfide powders were used. The density of pure melt grown crystals was found to be higher than that of natural zinc blende crystals or crystals grown by evaporation. The stability of the pure hexagonal crystals towards transformation to the cubic zinc‐blende structure in the interval of temperature 700–1150°C was investigated. A full conversion to the cubic phase was never observed. The experimental evidence indicates the transition point to be above 1150°C.

ELECTRICAL TRANSPORT AND CONTACT PROPERTIES OF LOW RESISTIVITY n‐TYPE ZINC SULFIDE CRYSTALS

This Letter describes some electrical contact and transport properties of ZnS single crystals having room-temperature resistivities in the range of 1 to 10 ohm-cm. Previous electrical transport measurements on ZnS have been done mainly at high temperatures or under photoexcitation. Electrical contacts to ZnS which display ohmic characteristics at room temperature have been described by Alfrey and Cooke. A serious limitation to a more extensive investigation of the electrical properties of ZnS has been the difficulty in providing ZnS crystals with contacts which would stay ohmic at low temperatures. It has also been difficult to dope ZnS n-type without simultaneously introducing large concentrations of native acceptor defects.

High Electron Mobility in Zinc Selenide Through Low‐Temperature Annealing

Electron mobility in ZnSe has been measured between 40° and 400°K. It is shown that through repeated annealing in liquid Zn the mobility maximum can be increased to 12 000 cm2/V sec. This is one of the highest mobilities measured for semiconductors with band gaps as wide as that of ZnSe (2.7 eV). The increase in mobility is mainly due to elimination of doubly charged acceptor states. The residual scattering is believed to be due, in part, to charged isolated impurities and, in part, to paired impurity dipoles.

Injection Electroluminescence in ZnS and ZnSe

Injection electroluminescence has been observed in Cu2S‐ZnS and Cu2Se‐ZnSe heterojunctions. The light emission occurs through hole injection from the p‐type Cu chalcogenide into n‐type ZnS or ZnSe. At room temperature the light emission from the Cu2S‐ZnS and the Cu2Se‐ZnSe junctions originates at the Cu or self‐activated luminescence centers. At 77°K the Cu2Se‐ZnSe diodes exhibit emission bands peaking at 1.96, 2.07, 2.36, and 2.68 eV. The dominance of any one of these bands over the others can be achieved by appropriate preparative techniques. In the diodes emitting predominantly in the 2.36‐eV band the light intensity varies approximately as the 32 power of current. The lowest voltage threshold observed for visible light emission is 1.5 V.Both the luminescence and the electrical characteristics of the prepared diodes are in accord with a tentative model for the band structure of Cu chalcogenide‐II‐VI compound heterojunctions. The particular model for the Cu2Se‐ZnSe diodes is presented and discussed in d...

Mobility of Holes and Interaction between Acceptor Defects in ZnTe

Electrical transport studies in p‐type ZnTe have confirmed that in the temperature range of about 80° to 500°K the hole mobility is principally limited by LO phonon scattering. An effective mass ratio of 0.6 gives the best fit to the experimental data. The highest mobility, 6500 cm2/V·sec at 35°K, was observed in a crystal with an active defect center concentration of 2×1015 cm−3. The ionization energy of the first charge state of a native acceptor, believed to be a Zn vacancy, was found to be ≥0.057±0.002 eV. Besides acting as shallow acceptor defects in ZnTe, the elements Li, Na, and P were found to promote the incorporation of substantial concentrations of native defects. High concentrations of Li, under excess Zn firing conditions, were observed to lead to highly compensated, low‐mobility material, suggestive of self‐compensation of the Li dopant.

Barrier Heights and Contact Properties of n‐Type ZnSe Crystals

Barrier‐height measurements performed on ultrahigh‐vacuum‐cleaved and on chemically etched ZnSe crystals have shown that for most metals the barrier height increases linearly with its electronegativity. The highly reactive metals Mg, Ca, and Ba were found to be an exception, showing a decrease in barrier height with increasing electronegativity. Aluminum, when diffused into ZnSe from a film deposited onto a vacuum‐cleaved surface, was found to produce a low‐resistivity surface layer, followed by a highly compensated region in the crystal. This effect is suggested to arise from copious generation of Zn vacancies at the Al‐doped crystal surface, and their influx, ahead of the Al, into the crystal.

Mechanism of Charge Transport and Light Emission in ZnSexTe1−xp‐n Junctions

Electrical transport, photoconductivity, and luminescence measurements have been performed on high‐resistivity p‐ and n‐type ZnSexTe1−x crystals. p‐n junction diodes prepared from such material, when cooled from 300° to 80°K in the dark, pass very little current and will not emit light. A momentary illumination with radiation of hν>1.9 eV will initiate a steady, efficient light emission, and produce an increase in current by a factor of about 1010. The experimental data obtained will be used to propose a mechanism for the charge transport and light emission in ZnSexTe1−x p‐n junctions which involves a self‐induced photoconductivity process in both the p‐ and the n‐type bulk of the diodes.

Gunn Effect in ZnSe

We have observed the Gunn effect in n‐type ZnSe crystals subjected to pulsed electric fields. At room temperature the threshold field is about 38 kV/cm, in rough agreement with the dielectric breakdown field as calculated from Strattons formulas. In samples showing a spiking mode of current instability, the field outside the domain E1≈22 kV/cm, the domain field E2≥100 kV/cm, the domain drift velocity vdd≈1.0×107 cm/sec, and the peak‐to‐valley current ratio is about 3:2. Thus, the high‐field behavior of ZnSe is similar to that of GaAs, except for the higher characteristic fields. However, current runaway limits operation of ZnSe units to very short pulses and fields near threshold.

Scanning electron microscopy and cathodoluminescence of ZnSex Te1 − xp‐n junctions

The structure of ZnSex Te1−xp‐n junction diodes has been examined under cathode‐ray excitation. The cathodoluminescence efficiency of p‐type ZnSex Te1−x drops by a factor of 102−103 between 77 and 300 °K, whereas that of n‐type material stays approximately constant in the same temperature range. Junction profiles obtained by cathodoluminescence, cathodoconductivity, and secondary electron emission indicate that the radiative recombinations occur on the p‐type side of the diodes. The large drop in the diode efficiency between 77 and 300 °K is thus attributable to the decrease of the radiative recombination efficiency of p‐type ZnSex Te1−x.