J. J. Tietjen

THE PREPARATION AND PROPERTIES OF VAPOR‐DEPOSITED SINGLE‐CRYSTAL‐LINE GaN

Single‐crystalline, colorless, GaN has been prepared by a vapor‐phase growth technique previously used to prepare GaAs, GaP, and GaSb. These crystals are the first reported speciments of GaN suitable for good electrical and optical evaluation of this compound. It has been determined that GaN has a direct energy bandgap of 3.39 eV, and that undoped crystals prepared by this method have a very high inherent electron concentration, typically above 1019/cm3, which is probably related to a high density of nitrogen vacancies. Conducting p‐type specimens have been prepared using Ge as the dopant; but this result has been difficult to reproduce, and the samples have been electrically inhomogeneous.

Detection of Selenium Clustering in GaAs by Transmission Electron Microscopy

Direct evidence has been found for the existence of coherent particles and small precipitates in bulk Se‐doped GaAs (n=2×1018 cm−3) and in Se‐doped, vapor‐grown epitaxial GaAs (n=4×1018; 1×1019 cm−3). Data obtained from moire fringe contrast indicate a probable particle composition of Ga2Se3. In samples with more than 2×1018 carriers cm−3, the major part of the total Se content is in the form of particles, and this accounts for the difference between the total Se concentration and the electrically active Se concentration. Diffusion in the solid can account for particle growth in the Bridgman‐grown crystal, but not in the vapor‐grown epitaxial material.

Current status of negative electron affinity devices

The introduction of electron emitters utilizing negative electron affinity has greatly improved the performance of many conventional light-sensing devices. The unique properties of such emitters have also made possible devices which were heretofore not feasible. Since their arrival as laboratory curiosities about five years ago, these emitters have had a large impact in the area of low-light-level detection, particularly scintillation counting. Recent advances in materials technology and surface activation processes have brought negative electron affinity photocathodes to the market place for use as detectors for both the Nd and GaAs lasers. In both cases, the detectors are more than an order of magntiude more sensitive to the laser light than previous photocathodes, and the dark currents (thermionic emission from the cathodes which can be limiting in low-light-level use) are several orders of magnitude less. Several other applications of negative electron affinity are presently under development which may further affect photocathodes, photomultipliers, imaging devices, and even the time-honored thermionic cathode. The operating principles of this type of electron emitter, the present state of the art and its effect on device performance, and the possible developments in the near future are discussed.

Stresses in Heteroepitaxial Layers: GaAs1−xPx on GaAs

A model is presented showing that there is a previously unrecognized source of stress in epitaxially grown heterojunction structures, specifically caused by a set of inclined dislocations formed by misfit dislocations which turn upwards at the heterojunction. This stress is indirectly related to the lattice mismatch at the heterojunction. For small lattice mismatch, the inclined dislocations are in an ordered array and cause the layer to bend upon removal from the substrate. For large lattice mismatch, the inclined dislocations are random so that there are only localized stresses and no net bending stress. A series of heterojunctions of GaAs1−xPx vapor grown onto GaAs were prepared, and the GaAs1−xPx constant‐composition layers were removed from the substrate. The bending of the layers observed and the dislocation morphologies revealed in the layers by transmission electron microscopy, demonstrate the validity of the above model. In GaAs0.8P0.2 grown on GaAs, the stress due to lattice mismatch exceeds tha...

AN OPTOELECTRONIC COLD CATHODE USING AN AlxGa1−xAs HETEROJUNCTION STRUCTURE

An efficient optoelectronic cold cathode has been made which includes a Si‐compensated AlxGa1−xAs electroluminescent diode covered with an absorbing p‐type GaAs layer having a negative electron affinity surface. This structure is designed to minimize current crowding in the vicinity of the Ohmic contact. An over‐all efficiency of 1.1×10−3 (current emitted into vacuum/diode current) has been achieved. This represents a factor of 102–103 improvement over previous p‐n junction or optically coupled cold cathode structures.

NEW HIGH‐GAIN DYNODE FOR PHOTOMULTIPLIERS

A new photomultiplier is described in which the recently reported GaP(Cs) material is used for the first dynode to obtain high secondary emission gain. Gain factors of 20 to 40 have been measured resulting in greatly improved performance of the new photomultiplier at very low light levels.

Influence of Deposition Temperature on Composition and Growth Rate of GaAsx P1−x Layers

Epitaxial layers of GaAsxP1−x alloys were deposited in an open‐tube vapor‐transport apparatus. It was found that the growth rate and the composition of the deposited alloy strongly depend on the deposition temperature. Higher deposition temperatures lead to an increase in the P content of the alloy. The growth rate reached a maximum at 830°C and then decreased with further decrease in the deposition temperature, contrary to expectations based on the thermodynamics of the growth system. A time‐of‐flight mass spectrometer was coupled to the growth system, and the composition and the chemical reactivity of the vapor species was studied. The results of these studies offer an explanation for the observed dependence of the composition of the deposited alloy on the deposition temperature.

GaAs1−xPx AS A NEW HIGH QUANTUM YIELD PHOTOEMISSIVE MATERIAL FOR THE VISIBLE SPECTRUM

A new photocathode having high quantum yield for visible and ultraviolet radiation is reported. The cathode consists of a cesium‐activated strongly p‐type doped GaAs1−xPx alloy.

Vapor-Phase Growth of Several III–V Compound Semiconductors

A review is presented of a vapor-phase growth method that has been developed for the synthesis of a broad spectrum of III–V compounds. The predominant feature of this technique is the use of gases as the source chemicals, thereby providing improved control of the chemical composition, homogeneity, crystalline perfection, and impurity concentrations and distributions of the epitaxial layers. As a result, a number of notable advances have been made with respect to the material properties and device utilization of several III–V compounds. The chemistry of the deposition processes was studied by means of a mass spectrometer coupled to the vapor-phase growth system. Results of these studies are presented and discussed.

Vapor phase growth of gallium arsenide microwave diodes

Abstract Gallium arsenide microwave diodes have been prepared by a novel vapor phase growth technique with the highest combination of reverse breakdown voltages and cut-off frequencies reported to date. This result is attributed to three inherent properties of this technique. First, structures of gallium arsenide can be prepared with a high purity layer directly next to layers doped either n - or p -type over a wide range of selected resistivities. Second, high quality, abrupt p - n junctions can be incorporated in these crystals without sacrifice of desired bulk properties. Third, the devices do not have to be heated to elevated temperatures after vapor growth, so that contamination of the high purity region is virtually eliminated.