Zoltan K. Kun

Growth and characterization of ZnSe and homogeneous ZnSxSe1−x crystals

Abstract ZnSe and homogeneous ZnSxSe1−x crystals (x varied from 0.8 to 0.2) were grown by the iodine vapor transport technique. We found that crystal growth is aided by a slight excess of S or Se in the growth capsule. The homogeneity of ZnSxSe1−x boules was assured by the large diameter and short length of the growth ampoule. These dimensions also favored higher growth speeds: 0.6 to 0.7 g per day. Boules, on the average, 30 mm long and 15 mm in diameter could be grown. Away from the seed end they contained one or two large grains. They were characterized by X-ray, residual impurity analysis and etch pit counts. The transporting halide (iodine) incorporated in the material in the 200 to 400 ppm concentration range. In ZnSe crystals where precision measurements could be made, iodine was found to increase the lattice constant by 10-3 A per 50 ppm iodine.

The variation of residual impurities in ZnSe crystals used in light‐emitting diode fabrications

Recent reports in the literature suggest that residual impurities, probably interstitial lithium, are responsible for the strong compensation of p‐type conductivity in ZnSe. Data presented here support these findings. ZnSe boules that had high Li content, as determined from the pair band of the emission spectra, produced poor light‐emitting diodes (LED’s). At the same time LED’s were consistently good when boules that contained less residual Li were used. Ion microprobe measurements showed that one of the processes that takes place during LED fabrication is the reduction of the residual Li content. These two findings together show that low Li content is a prerequisite for good LED formation.

Growth and characterization of thallium arsenic selenide crystals for nonlinear optical applications

Abstract The synthesis, growth, and characterization of large single crystals of the efficient nonlinear optical sulfosalt material thallium arsenic selenide (TAS) are reviewed. Recent significant improvements in crystal growth and purification technology have enabled us to grow high optical quality crystals as measured by etchpit, x-ray topographic, and birefringent interferometric characterization methods. The excellent crystal quality is reflected in the high CO 2 frequency conversion efficiencies obtained with TAS. In addition to quality improvement in 2.5 cm diameter crystals, larger crystals, up to 5 cm in diameter, have now been grown by a modified Bridgman technique. Etching of crystals grown under a high axial thermal gradient indicated a higher defect density near the crystal surface than at the corresponding central region. In addition, high defect densities were found at the seed-crystal joint. This suggests that the etchpits are related to thermal stresses induced by the temperature distribution in a growing crystal. X-ray topographic analyses confirmed the etching studies, and also revealed a subgrain structure in some crystals which is related to the crystal stress distribution and to seeding defects. Birefringent interference patterns, a measure of optical uniformity, show a direct correlation between subgrain structure and optical homogeneity of the crystals. Improved growth control has eliminated these defects. The nonlinear optical susceptibilities of TAS are large, nearly three times those of proustite. Crystals grown in our laboratory have shown high values of second, third, and fourth harmonic conversion efficiency of the output from a CO 2 laser. Fifth harmonic generation has also been demonstrated. A 4.8 cm long crystal, 25 mm in diameter, was used to double 4.8 μm to 2.4 μm with an efficiency of 6.4% for the relatively low fluence of 8 mJ/cm 2 in an 80 nsec pulse.

The influence of chlorine on the crystal structure and electroluminescent behavior of ZnS:Mn Films in thin film electroluminescent devices

Comparative experiments showed that the addition of chlorine at the preparation of the ZnS:Mn source materials for TFEL device fabrication can enhance grain growth in the ZnS:Mn films during post-deposition anneal. Correspondingly the luminescent properties of these films improved as compared to those without chlorine. Depending upon the Mn concen-tration and annealing temperature, both the saturation brightness and the peak luminous efficiency have increased by up to a factor of three. Optimized experimental para-meters resulted 810fL saturation brightness and 0.6l/W peak luminous efficiency. A practical significance is that useful TFEL devices can be fabricated using source materials with Cl, addition when post deposition annealing temperatures have to be kept low.

Temporal response of high-resolution acoustooptic tunable filters

In this paper we report acoustooptic tunable filters (AOTF) for the infrared made using thalium arsenic selenide in a collinear configuration. Resolution near 1 wave number can be achieved for interaction lengths of 5-cm, provided the optical uniformity is sufficiently high. Resolution measurements in collinear AOTFs is complicated by acoustic resonances. We have analyzed the temporal response of an AOTF to pulsed rf excitation, in which there are no resonances and from which the resolution can be inferred. We have applied this analysis to a 4-cm interaction length AOTF and determined a uniformity in the optical birefringence of a few parts in 10,000. This was in good agreement with an interpretation of linear sweep frequency response measurements.

Thallium arsenic sulfide acoustooptic Bragg cells

Thallium arsenic sulfide acoustooptic Bragg cells are introduced. Theoretical discussions include isotropic and anisotropic configurations. Experimental results are reported for high efficiency-bandwidth cells. Crystal growth conditions and fabrication techniques are also discussed.

Crystal growth aspects of the fabrication of high output thin film electroluminescent edge emitterTM arrays

Abstract Edge emitter TFEL samples were prepared by vacuum deposition. The composition of the source materials for the ZnS:Mn films was varied, and so was the substrate temperature during deposition. In particular, the source materials had an addition of Cl, and the substrate temperature was kept low during deposition. The edge emitter with the lowest attenuation was deposited from the highest Cl containing source material and at a substrate temperature that was initially high but was decreasing quickly during deposition. We believe that the likely reason for low attenuation is grain growth in the ZnS film which is not accompanied by the formation of intergranular cavities.

Test Station For Thin Film Electroluminescent (TFEL) Edge Emitter Array, A Potential Low-Cost, All Solid State Imaging Device

A test station was developed to evaluate the TFEL edge emitter array in various imaging applications. It consists of 400-dpi and 20-dpi resolution edge emitter array test samples, a sample mount that accepts both kinds of edge emitters, and an electronic drive system. The drive system provides variable drive frequency and excitation voltage, internal test patterns, and an option for external data input. As an example of its capabilities, electrophotographic printing samples are shown.

The growth of homogeneous NbO2 single crystals by the float zone method

NbO2 is a potentially useful solid state switching material which is difficult to grow as large homogeneous single crystals. Due to its reactivity with most container materials, NbO2 must be grown by a containerless technique. One of the containerless methods, the focussed light float zone technique was employed to grow large (0.5 by 3.3 cm) oriented single crystals of NbO2. We found that the NbO2 powder starting material, a product of hydrogen reduction, contains small amounts of lower Nb oxides. These, in the form of high electrical conductivity inclusions, usually render the NbO2 useless as a switch. We have successfully suppressed inclusion formation during crystal growth by raising the temperature of the molten zone.

p‐type conductive alloys in the Zn‐Ga‐P‐S quaternary system

It was reported in the literature that high electrical conductivities, either n or p type, cannot be obtained in the GaP‐ZnS system at higher than 5 mol% ZnS. This behavior is similar to that of the other (II‐VI)x(III‐V)1−x pseudobinary alloys. It is reported in this paper that degenerate p‐type conductivity can be achieved in those Zn‐Ga‐P‐S quaternary alloys which slightly deviate from the pseudobinary composition. The required conversion treatment is high‐temperature annealing in zinc vapor. A comparison of the crystal growth technique with those of other researchers suggests that the deviation from the pseudobinary composition originated from the large excess of Ga in the closed system. This effect was enhanced by precipitating the crystals from the Ga solvent by cooling over a wide temperature interval. The significance of finding the high p‐type conductivity was that it led to our present technique of p‐type doping of ZnSe and ZnSxSe1−x.