Engineering topics

Gatos, H. C.

Publications and source records attributed to Gatos, H. C..

At least 19 records

NASA NTRS1989

Gettering of donor impurities by V in GaAs and the growth of semi-insulating crystals

Vanadium added to the GaAs melt getters shallow donor impurities (Si and S) and decreases their concentration in the grown crystals. This gettering is driven by chemical reactions in the melt rather than in the solid. Employing V gettering, reproducibly semi-insulating GaAs were grown by horizontal Bridgman and liquid-encapsulated Czochralski techniques, although V did not introduce any midgap energy levels. The compensation mechanism in these crystals was controlled by the balance between the native midgap donor EL2 and residual shallow acceptors. Vanadium gettering contributed to the reduction of the concentration of shallow donors below the concentration of acceptors. The present findings clarify the long-standing controversy on the role of V in achieving semi-insulating GaAs.

Ko, K. Y.↗

NASA NTRS1988

Growth of GaAs from a free surface melt under controlled arsenic pressure in a partially confined configuration

A partially confined configuration for the growth of GaAs from melt in space was developed, consisting of a triangular prism containing the seed crystal and source material in the form of a rod. It is suggested that the configuration overcomes two obstacles in the growth of GaAs in space: total confinement in a quartz crucible and lack of arsenic pressure control. Ground tests of the configuration show that it is capable of crystal growth in space and is useful for studying the growth of GaAs from a free-surface melt on earth. The resulting chemical composition, electrical property variations, and phenomenological models to account for the results are presented.

Gatos, H. C.↗

NASA NTRS1987

Characteristics of GaAs with inverted thermal conversion

GaAs crystals exhibiting inverted thermal conversion (ITC) of resistivity were investigated in conjunction with standard semiinsulating (SI) GaAs regarding characteristics important in device processing. It was established that dislocation density and Si implant activation are unaffected by transformation to the ITC state. However, in ITC GaAs the controlled increase of the EL2 (native midgap donor) concentration during annealing makes it possible to attain resistivities one order of magnitude greater (e.g., about 10 to the 9th ohm cm of 300 K) than those attained in standard SI GaAs (e.g., 10 to the 7th-10 to the 8th ohm cm).

Kang, C. H.↗

NASA NTRS1987

Optical characterization of semi-insulating GaAs - Determination of the Fermi energy, the concentraion of the midgap EL2 level and its occupancy

The key electronic characteristics of semiinsulating GaAs, i.e., the Fermi energy, concentration, and occupancy of the midgap donor EL2, and the net concentration of ionized acceptors can all be determined from high-resolution measurements of the EL2 intracenter absorption. The procedure is based on the measurement of zero-phonon line intensity before and after the complete transfer of EL2 to its metastable state followed by thermal recovery. The procedure is quantitative, involves no fitting parameters, and unlike existing methods, is applicable even when a significant part of the EL2 is ionized.

Lagowski, J.↗

NASA NTRS1987

Arsenic ambient conditions preventing surface degradation of GaAs during capless annealing at high temperatures

Changes in surface morphology and composition caused by capless annealing of GaAs were studied as a function of annealing temperature, T(GaAs), and the ambient arsenic pressure controlled by the temperature, T(As), of an arsenic source in the annealing ampul. It was established that any degradation of the GaAs surface morphology could be completely prevented, providing that T(As) was more than about 0.315T(GaAs) + 227 C. This empirical relationship is valid up to the melting point temperature of GaAs (1238 C), and it may be useful in some device-processing steps.

Kang, C. H.↗

NASA NTRS1986

Inverted thermal conversion - GaAs, a new alternative material for integrated circuits

A new type of GaAs is developed which exhibits inverted thermal conversion (ITC); i.e., it converts from conducting to semiinsulating upon annealing at about 850 C. In device fabrication, its low resistivity prior to high-temperature processing differentiates ITC GaAs from the standard semiinsulating GaAs. The ITC characteristics are obtained through control of the concentration of the midgap donor EL2 based on heat treatment and crystal-growth modification. Thus EL2 does not exist in the conducting state of ITC GaAs. Conversion to the semiinsulating state during 850 C annealing is caused by the formation of EL2.

Lagowski, J.↗

NASA NTRS1986

Effects of macroscopic inhomogeneities on electron mobility in semi-insulating GaAs

It is shown that defect inhomogeneities of sizes larger than the electron mean free path are responsible for the low values and anomalous temperature dependence of the electron mobility in semi-insulating (SI) GaAs. The room-temperature electron mobility values below about 6000 sq cm/V s cannot be uniquely used for the determination of the concentration of ionized defects, since the contribution from inhomogeneities usually exceeds that from scattering by ionized impurities. The effects of the macroscopically inhomogeneous distribution of residual acceptors and the major deep donor EL2 diminish at elevated temperatures between 600 and 900 K, which offers a means for identification of inhomogeneities, and furthermore explains recently reported steplike mobility versus temperature behavior in SI-GaAs.

Walukiewicz, W.↗

NASA NTRS1986

Optical and transient capacitance study of EL2 in the absence and presence of other midgap levels

A high-resolution optical study was carried out on GaAs crystals grown by horizontal Bridgman and liquid-encapsulated-Czochralski methods. An excellent correlation was found between the intensity of the 1.039-eV no-phonon line and the characteristic absorption of EL2, the major deep donor level in GaAs. A correlation was also found between the characteristic optical absorption of EL2 and its concentration as determined by junction capacitance measurements. The presence of EL0, another midgap level contained in heavily oxygen-doped crystals at concentration always less than those of EL2, had no effect on the optical spectra, but altered the capacitance measurements. Accordingly, an accurate calibration for the determination of EL2 by optical absorption was obtained from capacitance measurements on crystals containing only EL2; in this way the uncertainties introduced by other midgap levels were eliminated.

Skowronski, M.↗

NASA NTRS1986

Crystal growth of device quality GaAs in space

It was established that the findings on elemental semiconductors Ge and Si regarding crystal growth, segregation, chemical composition, defect interactions, and materials properties-electronic properties relationships are not necessarily applicable to GaAs (and to other semiconductor compounds). In many instances totally unexpected relationships were found to prevail. It was further established that in compound semiconductors with a volatile constituent, control of stoichiometry is far more critical than any other crystal growth parameter. It was also shown that, due to suppression of nonstoichiometric fluctuations, the advantages of space for growth of semiconductor compounds extend far beyond those observed in elemental semiconductors. A novel configuration was discovered for partial confinement of GaAs melt in space which overcomes the two major problems associated with growth of semiconductors in total confinement. They are volume expansion during solidification and control of pressure of the volatile constituent. These problems are discussed in detail.

Gatos, H. C.↗

NASA NTRS1985

Native hole trap in bulk GaAs and its association with the double-charge state of the arsenic antisite defect

A dominant hole trap has been identified in p-type bulk GaAs employing deep level transient and photocapacitance spectroscopies. The trap is present at a concentration up to about 4 x 10 to the 16th per cu cm, and it has two charge states with energies 0.54 + or - 0.02 and 0.77 + or - 0.02 eV above the top of the valence band (at 77 K). From the upper level the trap can be photoexcited to a persistent metastable state just as the dominant midgap level, EL2. Impurity analysis and the photoionization characteristics rule out association of the trap with impurities Fe, Cu, or Mn. Taking into consideration theoretical results, it appears most likely that the two charge states of the trap are the single and double donor levels of the arsenic antisite As(Ga) defect.

Lagowski, J.↗

NASA NTRS1985

Metastability of the midgap level EL 2 in GaAs - Relationship with the As antisite defect

It is found that the rate of the photoinduced transition of the GaAs midgap level EL 2 to its metastable state increases as its occupation increases. High-resolution optical spectra of this transition exhibit a sharp peak very similar to the no-phonon line of the intracenter absorption of the As antisite defect. These findings show that the transition to the metastable state is initiated from the ground state 1A1, and it is finalized via the excited state 1T2 of the neutral As antisite defect. They thus provide a new basis for the critical assessment of the EL 2 metastability models and further confirmation of the association of EL 2 with the isolated As antisite defect.

Skowronski, M.↗

NASA NTRS1985

Partially confined configuration for the growth of semiconductor crystals from the melt in zero-gravity environment

A novel partially confined configuration is proposed for the crystal growth of semiconductors from the melt, including those with volatile constituents. A triangular prism is employed to contain the growth melt. Due to surface tension, the melt will acquire a cylindrical-like shape and thus contact the prism along three parallel lines. The three empty spaces between the cylindrical melt and the edges of the prism will accommodate the expansion of the solidifying semiconductor, and in the case of semiconductor compounds with a volatile constituent, will permit the presence of the desired vapor phase in contact with the melt for controlling the melt stoichiometry. Theoretical and experimental evidence in support of this new type of confinement is presented.

Lagowski, J.↗

NASA NTRS1985

Crystal growth of device quality GaAs in space

The present program has been aimed at solving the fundamental and technological problems associated with Crystal Growth of Device Quality in Space. The initial stage of the program was devoted strictly to ground-based research. The unsolved problems associated with the growth of bulk GaAs in the presence of gravitational forces were explored. Reliable chemical, structural and electronic characterization methods were developed which would permit the direct relation of the salient materials parameters (particularly those affected by zero gravity conditions) to the electronic characteristics of single crystal GaAs, in turn to device performance. These relationships are essential for the development of optimum approaches and techniques. It was concluded that the findings on elemental semiconductors Ge and Si regarding crystal growth, segregation, chemical composition, defect interactions, and materials properties-electronic properties relationships are not necessarily applicable to GaAs (and to other semiconductor compounds). In many instances totally unexpected relationships were found to prevail.

Gatos, H. C.↗

NASA NTRS1985

Crystal Growth of Device Quality Gaas in Space

The GaAs research evolves about these key thrust areas. The overall program combines: (1) studies of crystal growth on novel approaches to engineering of semiconductor material (i.e., GaAs and related compounds); (2) investigation and correlation of materials properties and electronic characteristics on a macro- and microscale; and (3) investigation of electronic properties and phenomena controlling device applications and device performance. This effort is aimed at the essential ground-based program which would insure successful experimentation with and eventually processing of GaAs in near zero gravity environment. It is believed that this program addresses in a unique way materials engineering aspects which bear directly on the future exploitation of the potential of GaAs and related materials in device and systems applications.

Gatos, H. C.↗

NASA NTRS1985

EBIC spectroscopy - A new approach to microscale characterization of deep levels in semi-insulating GaAs

The microscale characterization of electronic defects in (SI) GaAs has been a challenging issue in connection with materials problems encountered in GaAs IC technology. The main obstacle which limits the applicability of high resolution electron beam methods such as Electron Beam-Induced Current (EBIC) and cathodoluminescence (CL) is the low concentration of free carriers in semiinsulating (SI) GaAs. The present paper provides a new photo-EBIC characterization approach which combines the spectroscopic advantages of optical methods with the high spatial resolution and scanning capability of EBIC. A scanning electron microscope modified for electronic characterization studies is shown schematically. The instrument can operate in the standard SEM mode, in the EBIC modes (including photo-EBIC and thermally stimulated EBIC /TS-EBIC/), and in the cathodo-luminescence (CL) and scanning modes. Attention is given to the use of CL, Photo-EBIC, and TS-EBIC techniques.

Li, C.-J.↗

NASA NTRS1985

EL2 and related defects in GaAs - Challenges and pitfalls

The incorporation process of nonequilibrium vacancies in melt-grown GaAs is strongly complicated by deviations from stoichiometry and the presence of two sublattices. Many of the microdefects originating in these vacancies and their interactions introduce energy levels (shallow and deep) within the energy gap. The direct identification of the chemical or structural signature of these defects and its direct correlation to their electronic behavior is not generally possible. It is necessary, therefore, to rely on indirect methods and phenomenological models and deal with the associated pitfalls. EL2, a microdefect introducing a deep donor level, has been in the limelight in recent years because it is believed to be responsible for the semi-insulating behavior of undoped GaAs. Although much progress has been made towards understanding its origin and nature, some relevant questions remain unanswered. An attempt is made to assess the present status of understanding of EL2 in the light of most recent results.

Gatos, H. C.↗

NASA NTRS1985

EL2 and related defects in GaAs - Challenges and pitfalls

The incorporation process of nonequilibrium vacancies in melt-grown GaAs is strongly complicated by deviations from stoichiometry, and the presence of two sublattices. Many of the microdefects originating in these vacancies and their interactions introduce energy levels (shallow and deep) within the energy gap. The direct identification of the chemical or structural signature of these defects and its direct correlation to their electronic behavior is not generally possible. It is therefore necessary to rely on indirect methods and phenomenological models and be confronted with the associated pitfalls. EL2, a microdefect introducing a deep donor level, has been in the limelight in recent years because it is believed to be responsible for the semi-insulating behavior of undoped GaAs. Although much progress has been made towards understanding its origin and nature, some relevant questions remain unanswered. An attempt is made to assess the present status of understanding of EL2 in the light of the most recent results.

Gatos, H. C.↗

NASA NTRS1985

Nonstoichiometric defects in GaAs and the EL2 bandwagon

In the present paper, an attempt is made to formulate a common framework for a discussion of nonstoichiometric defects, especially EL2 and dislocations. An outline is provided of the most important settled and unsettled issues, taking into account not only fundamental interests, but also urgent needs in advancing IC technology. Attention is given to stoichiometry-controlled compensation, the expected role of melt stoichiometry in electrical conductivity for the basic atomic disorders, defect equilibria-dislocations and EL2, and current issues pertaining to the identification of EL2. It is concluded that nonstoichiometric defects play a critical role in the electronic properties of GaAs and its electronic applications. Very significant progress has been recently made in learning how to adjust melt stoichiometry in order to maximize its beneficial effects and minimize its detrimental ones.

Lagowski, J.↗