K. K. Shih

Some optical properties of the AlxGa1−xAs alloys system

Optical transmission data covering the Γ15V–Γ1C absorption edge are presented, together with photoluminescence (PL) results, for AlxGa1−xAs crystals of high purity [n (293 K) <1017 cm−3] for 0<x<0.9. The results indicate that the fundamental absorption edge is domnnated by electron‐hole interaction even in an an alloy. In the direct‐gap region the bound‐exciton ground‐state peak is clearly resolved. In the indirect‐gap region the relaxation of crystal momentum conservation in optical transitions for an alloy apparently has significant effects on the Γ15V–Γ1C absorption edge, and the discrete peak in absorption spectra disappears. From the variation of the absorption coefficient αth at the Γ15V–Γ1C edge with x, we can deduce the corresponding variation of the excitonic binding energy Eex, excitonic effective mass m*, and electron mass mc for the Γ1C minimum. Extrapolated values for these quantities in AlAs are Eex=5 meV, m*=0.060 m0, and mc=0.11 m0 for a value αth = 2.3×104 cm−1. Further, there is a shift ...

Ti/Ti‐N Hf/Hf‐N and W/W‐N multilayer films with high mechanical hardness

Ti/Ti‐N, Hf/Hf‐N, and W/W‐N multilayer films with very thin individual metal and metal‐nitride layers were developed, with hardness of the structure much higher than that of single‐layer nitride films. Hardnesses with value between 3500 and 5000 kg/mm2 or higher were observed in the multilayer films. This is an improvement over that of single‐layer metal‐nitride films where hardnesses between 2200 and 2800 kg/mm2 are typically achieved. The improvement is thought to be due to the fact that the grains are restricted to a very small size in the thin individual layers of the sandwich structure. The multilayer films, in general, have better adhesion and less defects than the single‐layer films. These films were prepared on room‐temperature substrates by a simple sputtering process and have potential applications, as in hard coatings.

Growth of Smooth Uniform Epitaxial Layers by Liquid‐Phase‐Epitaxial Method

The liquid‐phase‐epitaxial (LPE) process has proven to be a very effective technique for the formation of high‐quality III‐V semiconductor layers. It has found widespread use because devices made by this method produce high‐efficiency LEDs and low‐threshold lasers. A limitation of this method has been the difficulty of making smooth high‐quality surfaces. It will be shown that the use of a specially designed LPE growth apparatus makes possible the growth of reproducible uniform economical and, above all, smooth photolithographic processable layers. This process has made possible the formation of planar monolithic AlxGa1−xAs LEDs with luminance characteristics as high as 104 ft L at a current density of ∼40 A/cm2.

Properties of Co‐N, Co‐Fe‐N, and Co‐Zr‐N films prepared by rf sputtering in nitrogen‐argon gas mixtures

The structure and properties of Co‐N, Co‐Fe‐N, and Co‐Zr‐N films, prepared by rf reactive sputtering using nitrogen and argon gases, have been studied. The resistivity and coercivity of Co‐N, Co‐Fe‐N, and Co‐Zr‐N films were determined as a function of nitrogen partial pressure. It was found that the properties of Co‐N and Co‐Fe‐N films were very similar where the properties were determined mainly by the nitride phases in these films. The resistivity of both Co‐N and Co‐Fe‐N films increased with the increase of nitrogen pressure. The coercivity of both films decreased with an initial increase of nitrogen pressure, then increased with a further increase of pressure so that there is a region of nitrogen pressures where the coercivity is at its lowest value. The low coercivity is attributed to the formation of the Co4N phase in Co‐N films and both Co4N and Fe4N phases in Co‐Fe‐N films. For Co‐Zr‐N films, resistivity first decreased with an increase of nitrogen pressure, then increased with a further increase ...

Structure and orientation of films prepared by reactive sputtering of Ni, Fe, and NiFe in Ar/N2 mixtures

Using controlled N2/Ar sputtering gas mixtures, rf‐sputtered films were prepared from Fe, Ni, and Ni81Fe19 targets, and their structure and orientation were studied by x‐ray diffraction. When no N2 was introduced during sputtering, all the films were oriented in the highest density planes; i.e., in (110), (111), and (111) for α‐Fe, Ni, and γ‐Ni81Fe19, respectively. With increasing N2 introduction, however, the orientation shifted to lower density planes, and eventually nitrides were formed. Multilayer films of metal and nitride were prepared with the Ni81Fe19 target by repetitive supply of N2 for short periods during sputtering. Strong orientation effects were observed depending on the number of layers grown. In addition, the x‐ray diffraction of these films presented evidence of epitaxial growth of the nitrides (Ni,Fe)4N on the alloy layers.

Fe/Fe‐N multilayer films with low coercivity zero magnetostriction and high saturation magnetization

A method of fabricating of Fe/Fe‐N multilayer films in the form of alternating thin films of Fe separated by very thin interlayers of Fe‐N sputtered in the presence of nitrogen is described. It was found that the properties of these films depended on the thickness of Fe and Fe‐N layers and the bias voltage. There is a region where the composite films have coercivity less than 1 Oe, with zero or near‐zero magnetostriction. The normally high saturation magnetization of Fe with value of 20 kG was preserved.

Preparation of Efficient Electroluminescent Diodes from p‐on‐n Liquid‐Phase Epitaxial Layers of GaP

Efficient red‐emitting GaP diodes have been fabricated from p‐on‐n layers formed by liquid epitaxy in a closed‐tube system. The external quantum efficiency may increase by a factor of two to three after the diodes are heat‐treated.

Properties of GaP Green‐Light‐Emitting Diodes Grown by Liquid‐Phase Epitaxy

Efficient and reproducible green‐light‐emitting diodes have been made by a vertical liquid‐phase epitaxial method. External quantum efficiencies of uncoated diodes as high as 3×10−4 at 300°K were obtained. The I‐V characteristics of these diodes show an exponential behavior of the form I = I0 exp eV/βKT; values of β were generally about 2 at 300° and 77°K. The emission intensity varies with I2 at low currents, and changes to an approximately linear dependence at high currents. Emission spectra have been studied as a function of current and temperature from 27° to 670°K. Seven emission lines in the green region of the spectrum have been observed, and the origins of these radiative recombinations have been studied. At low temperatures, four lines are identified as pair emission. One line at intermediate temperatures is believed to be due to free‐hole‐to‐donor emission, and two lines at high temperatures may be attributed to free‐exciton recombination.

Detection and origins of crystal defects in GaAs/GaAlAs LPE layers

We report the discovery that shallow growth pits develop on the surfaces of GaAs/GaAlAs double‐heterostructure wafers prepared by liquid phase epitaxy where the surfaces are intersected by crystal defects. The growth pits enable nondestructive defect characterization of such wafers to be made by optical microscopy. It is found that pits due to stacking faults are more readily detected than pits due to undissociated dislocations. The results of a search for the origins of the defects in the material are also presented. Characteristic ’’star’’ patterns develop when the material is etched. TEM observations show that these are due to clusters of crystal defects which originate at inclusions buried in the grown material.

Electroluminescence from Ge‐Doped GaP p—n Junctions

The electroluminescence from GaP containing Ge has been studied. p—n junctions were formed by the liquid epitaxy method. The peak of the dominant emission from these diodes is near 1.96 eV at 77°K. The maximum external quantum efficiency at 77°K was 8%. The emission appears to involve Zn and Ge pairs even though these impurities were doped on opposite sides of the junction. It was not possible to ascribe any emission lines to recombination involving Te and Ge. Current, voltage, and intensity dependences of the diodes have been measured. The results indicate that radiative recombination between Zn and Ge at 77°K is predominantly in the space‐charge region. At low currents, photon‐assisting tunneling becomes important.