Liquid Encapsulated Czochralski GaAs Research Articles

The investigation of lineage propagation in liquid-encapsulated Czochralski GaAs using cathodoluminescence imaging of cleaved wafers

A novel technique, using a combination of conventional KOH dislocation etching and cathodoluminescence imaging of cleaved wafers, has been used to investigate the propagation of lineage features in liquid-encapsulated Czochralski GaAs. Using this technique, three-dimensional pictures of these dislocation structures have been obtained.

Influence of boron related defects on activation of silicon implanted into undoped semi-insulating GaAs

The dependence of sheet carrier concentration of a Si-implanted layer on lattice parameters of undoped liquid encapsulated Czochralski GaAs was investigated. Crystals were grown from near-stoichiometric melts with varying boron concentrations. Sheet carrier concentration increases with an increase in lattice parameters due to the reduced boron concentration. An anomalous reduction in the lattice parameter suggests that native defects such as BGa VAs are responsible for the increase in the occupancy of implanted Si atoms on As sites.

Microdefects in dislocation-free liquid-encapsulated Czochralski GaAs

The effects of different growth conditions on microdefect generation in dislocation-free liquid-encapsulated Czochralski GaAs crystals are investigated using KOH etching technique and infrared light scattering tomography. It is found that microdefects are detected as micropits by KOH etching and as scatterers by infrared light scattering tomography, but are not detected by transmission electron microscopy. It is also found that the formation of these microdefects begins at below 1000 °C during the cooling process of the growth and strongly depends on the cooling rate. Other findings—melt composition and impurities (In,B), and the existence of denuded zones around dislocations—have led the authors to conclude that the microdefects are microprecipitates of excess arsenic atoms.

GaAs MESFET device dependences on ion-implant tilt and rotation angles

Lightly Cr-doped liquid-encapsulated Czochralski (LEC) GaAs wafers were implanted with 5*10/sup 12/ 100-keV Si/sup 29/ ions/cm/sup 2/ at tilt angles between 0 and 13 degrees and at rotation angles between 0 and 45 degrees C. Capacitance-voltage measurements were then made to determine electron profiles. It was found that cross-wafer device uniformity can be improved using implant tilt angles greater than 9 degrees . For microwave MESFET devices, the maximum transconductances at low I/sub DS/ are achieved using tilt angles greater than 6 degrees and rotation angles greater than 30 degrees . >

Characteristics of electron traps in Si-implanted and rapidly thermal-annealed GaAs

Rapid thermal annealing (RTA) using halogen lamps for Si-implanted (250-keV, 2×1013 cm−2 dose) semi-insulating liquid-encapsulated Czochralski GaAs has been investigated by deep level transient spectroscopy. RTA has been performed at 700, 800 and 900 °C for 6 s with SiO2 encapsulation. For comparison, conventional furnace annealing (FA) has also been performed at 800 °C for 20 min. In the RTA samples at 700 °C, three electron traps NI1 (Ec−0.53 eV), NI4 (Ec−0.63 eV), and EL2 (Ec−0.78 eV) are observed on the Cr-doped substrate, while the traps NI1, EL2, and NI3 (Ec−0.47 eV) are observed on the undoped substrate. In RTA samples above 800 °C, the electron trap NI2 (Ec−0.55 eV) or EL2 is dominant and traps NI1, NI3, and NI4 are not detected. It is possible that three traps NI1, NI3, and NI4 are residual defects produced by Si implantation. The low concentration of the trap EL2 in FA samples is due to the out-diffusion of it. This phenomenon can be related with the change of the stoichiometry in the implanted layer by the formation of the transition region at the SiO2 /GaAs interface during FA. The notable spatial variation of the trap NI2 is observed across the wafer. On the other hand, the peculiar spatial distribution of the trap EL2 is not observed across the RTA sample. Furthermore, the trap EL2 concentration does not depend on RTA temperatures. It is possible that the spatial distribution of the trap EL2 across the RTA sample is changed by Si implantation and RTA.

Diffusion of Te or Zn into GaAs from Doped SiO2 Films by Rapid Thermal Processing

ABSTRACTThe n+ layers on semi-insulating liquid encapsulated Czochralski GaAs and p+ layers on Si-doped n-type GaAs were formed by rapid thermal diffusion (RTD) from Te- and Zn-doped oxide films, respectively. The Zn diffusion coefficient of the RTD sample at 850°C for 6s with the heating rate of 50°C/s is about two orders of magnitude higher than that of a similar furnacediffused sample at the same temperature. In addition, Zn and Te diffusion are strongly enhanced by the high heating rate of RTD. The shallow and abrupt p+n junction in GaAs is formed by RTD of Zn with the low heating rate. This shallow p+n junction is suitable for the construction of a photodiode. It is observed that the short wavelength spectral response (<800 nm) of the photodiode fabricated by RTD from Zn-doped oxide film decreases as the heating rate of RTD increases. Deep levels in these photodiodes were characterized by deep level transient spectroscopy. A electron trap EZ (Ec -0.57eV) is produced by RTD of Zn in the n-type substrate under p+n junction. The concentration of this trap is independent of the heating rate of RTD.

Analysis of scanning deep level transient spectroscopy

Fundamental processes such as charge generation, trapping and emission, and collection in scanning deep level transient spectroscopy (SDLTS) are analyzed. It is concluded that SDLTS is more sensitive to minority-carrier traps than to majority-carrier traps and that a two-dimensional map of SDLTS peak signal magnitude, i.e., SDLTS image, of minority-carrier traps does not necessarily show the trap concentration distribution. A practical equation for trap concentration is derived by utilizing an electron beam induced current. Concentration of a hole trap with an activation energy 0.45 eV in an n-type as-grown liquid encapsulated Czochralski GaAs is calculated to be 1×1016 cm−3.

Infrared laser scanning microscopy in transmission: A new high-resolution technique for the study of inhomogeneities in bulk GaAs

This letter describes the use of the infrared laser scanning microscope in transmission to observe inhomogeneities in bulk GaAs. The results show that the technique achieves a high lateral resolution of ≂2 μm, and is able to detect low contrast intensity fluctuations down to 0.2%. The images obtained show contrast due to both particle scattering and EL2 absorption. The method is nondestructive, is suitable for bulk specimens, and enables three-dimensional defect distributions to be determined. As examples we show results from both In-doped and undoped liquid encapsulated Czochralski GaAs specimens.

Photoluminescence in electrically reversible (semiconducting to semi-insulating) bulk GaAs

A photoluminescence study has been made of electrically reversible, bulk, liquid-encapsulated Czochralski GaAs at temperatures 2–300 K. The reversibility from the semiconducting to the semi-insulating state is made by slow or fast cooling, respectively, following a 5-h, 950 °C heat treatment in an evacuated quartz ampoule. A donor level at Ec−0.13 eV and two acceptor levels at Ev+0.069 eV and Ev+0.174 eV are produced after the heat treatment. Only the acceptor levels were detected by photoluminescence. A tentative model assigning the acceptor to the intrinsic defect pair VGa-GaAs is discussed.

A study of dislocations in In-doped LEC GaAs crystals

Grown-in dislocations in In-doped GaAs grown by the liquid-encapsulated Czochralski (LEC) technique have been studied by X-ray transmission topography, AB etching and transmission electron microscopy. The observed dislocations have been classified into three types by their configurations. They include edge-type dislocations and helices starting from the interface between the seed and the grown crystal. The elimination procedures for these dislocations have been suggested from the observed dislocation configurations.

Slip dislocation propagation in In-doped liquid encapsulated Czochralski GaAs during crystal growth

A propagation rule was studied for two types of slip dislocations arising during crystal growth. In order to specify the dislocation distribution revealed by x-ray topography, we considered the relationship among the slip system, thermal gradient, and stress field. One type of slip dislocation was found to propagate on slip systems having a maximum Schmid factor in a uniform radial stress field, while another well known type occurred in a tangential stress field. It was concluded that different slip systems can be activated by specific stress fields caused by different thermal gradients under various growth conditions.

Thermal annealing of semi-insulating GaAs under controlled arsenic pressure

Bulk, undoped, semi-insulating, liquid encapsulated Czochralski GaAs crystals have been annealed under controlled arsenic pressures, and the uniformities of EL2 concentration and resistivity were investigated. Crystals were grown from different melt compositions from Ga rich to As rich. Arsenic pressure was changed from 0 (vacuum) to 3.4×103 Torr. As a result, it was found that both EL2 concentration and resistivity in the crystal grown from As-rich melt have the lowest fluctuations around 530 Torr arsenic pressure.

Characterization of GaAs self-aligned refractory-gate metal-semiconductor field-effect transistor (MESFET) integrated circuits

GaAs metal-semiconductor field-effect transistors (MESFETs) and other integrated-circuit elements were characterized by including extensive process test sites on wafers with digital logic and memory circuits. A self-aligned, refractory-gate enhancement/depletion (E/D) process was employed which included 47SiF+ channel and source/drain implants, capless arsenic overpressure furnace annealing, WSi0.11 gate metal with in situ sputter cleaning, Ni-Au-Ge ohmic contacts, Si3N4 or SiO2 insulation, and Ni-Au wiring. On-water threshold voltage standard deviations as low as 31 mV for 1-μm E-FETs and 49 mV for 1-μm D-FETs were measured using 51-mm standard semi-insulating liquid-encapsulated Czochralski GaAs substrates. Threshold voltage control from wafer to wafer was of order 100 mV. Schottky diode barrier height was about 0.73 eV with an ideality of 1.2, although small self-aligned Schottky gates often showed excess conduction believed to occur at the gate edges. FET square-law coefficient, subthreshold leakage, gate capacitance, backgating, contact resistance, and wiring and insulation characteristics were also measured and found satisfactory. Fully functional 1-μm gate E/D MESFET circuits including a 4×4 bit multiplier, a 4×4 crosspoint switch, a 448-bit static RAM, and an integrated photodiode amplifier were demonstrated.

Slow-relaxation phenomena in photoconductivity for semi-insulating GaAs. II. Distribution among samples

Photoelectric fatigue and spike photoconductivity (SPC) are systematically studied in twenty-three pieces of undoped semi-insulating liquid-encapsulated Czochralski GaAs crystal (six ingots, four suppliers). Distributions among samples for photosensitivity, SPC-decay time constant, SPC spectrum, and oscillatory behavior are discussed. Through these studies, the metastable state X* (some kind of excited state of the main deep center X present in GaAs) is found to be classified into two states X*a and X*f , according to whether the electronic transition X*→X due to irradiation with the photon energy near 0.80 eV is allowed or forbidden, respectively. State X*a is responsible for the SPC phenomenon, while both states X*a and X*f are responsible for photoelectric fatigue. Moreover, the concentration of state X*a is estimated to be about 10−3 times that of state X*. These results suggest that there exist various configurations of metastable states.

Reversible electrical properties of LEC GaAs

Undoped, low-pressure, liquid-encapsulated Czochralski GaAs can be reversibly changed from conducting (ρ ∼ 1Ω-cm) to semi-insulating (ρ ∼ 107Ω-cm) by either slow or fast cooling, respectively, after a 5 hr, 950° C soak in an evacuated quartz ampoule. The semi-insulating wafers are very uniform and lead to tight threshold-voltage control in direct-implant MESFET’s. We have studied crystals in both states by temperature-dependent Hall effect, photoluminescence, IR absorption, mass spectroscopy, and DLTS. It is shown that donor and acceptor concentrations are typically more than an order of magnitude greater than the C and Si concentrations, which are both less than 3 × 1014 cm−3. The EL2 concentration remains relatively constant at about 1.0 × 1016 cm−3. Thus, the normal EL2-Si-C compensation model does not apply. The most likely explanation for the reversibility involves a delicate balance between native-defect donors and acceptors in equilibrium at 950° C, but with the donors dominating after a slow cool, and the acceptors after a fast cool. A consistent model includes a dominant donor at Ec 0.13eV, probably VAs – AsGa, and a dominant acceptor at Ev + 0.07eV, probably VGa GaAs. In this model, vacancy motion is very important during the slow cool. Such processes must be strongly considered in the growth of bulk, high-purity GaAs.

Stoichiometry dependence of electrical activation efficiency in Si implanted layers of undoped, semi-insulating GaAs

Stoichiometry dependences of the electrical activation efficiency in Si implanted layers have been investigated for undoped, semi-insulating (SI) liquid encapsulated Czochralski (LEC) GaAs grown from melts of different compositions. The absolute value of lattice parameter was used as a measure for deviation from stoichiometry in GaAs. Irrespective of the annealing procedures, the electrical activation efficiency tends to increase and saturate to a maximum value as the crystals become As-rich, i.e., the lattice parameter increases from 0.565358 to 0.565364 nm. These results demonstrate that the precise control of stoichiometry greatly improves the reproducibility of LEC growth of undoped SI GaAs suitable for GaAs integrated circuits.

On the Interpretation of Defect Etching Behavior of Undoped, Semi‐Insulating, Liquid Encapsulated Czochralski GaAs Substrates

When (100) wafers of undoped semi‐insulating LEC are etched in the AB etch, both grooved etch features and ridges are produced at dislocations. Both these features have been studied by a correlation of etching, scanning electron microscopy in the wavelength dispersive cathodoluminescence mode (SEM‐WDCL), and transmission electron micros‐copy (TEM). It has been shown that a groove corresponds to a dislocation surrounded by matrix with a relatively slower etch rate whereas a ridge corresponds to a dislocation surrounded by matrix with a higher etch rate. These areas of different etch rate are shown by SEM‐WDCL to correspond to areas with slighly different Fermi levels . The AB etch rate dependence on is confirmed by determining the etch rate for samples with widely separated Fermi levels. An alternative explanation which ascribes grooved features to heavy decoration with As precipitates is shown to be inconsistent with TEM observations.

Influence of melt composition on the longitudinal distribution of midgap native donor concentration in semi-insulating liquid-encapsulated Czochralski GaAs crystal

The longitudinal distribution of midgap native donor concentration was investigated in detail using semi-insulating liquid-encapsulated Czochralski GaAs crystals grown from various melt compositions. The midgap native donor concentration was found to decrease with fractional solidification under Ga-rich conditions, to increase under As-rich conditions, and not to change under stoichiometric conditions. Moreover, it varied with the magnetic field intensity and the crystal-pulling speed depending on the melt composition. These phenomena were consistently explained by assuming that the midgap native donor acts like a dopant with a certain segregation coefficient k, where k<1 under As-rich conditions, k=1 under stoichiometric conditions, and k>1 under Ga-rich conditions. This is in good agreement with the expected segregation of As atoms determined from a phase diagram with a narrow solid-solution region.

Semiconducting/semi-insulating reversibility in bulk GaAs

Bulk, liquid-encapsulated Czochralski GaAs may be reversibly changed from semiconducting (ρ∼1 Ω cm) to semi-insulating (ρ∼107 Ω cm) by slow or fast cooling, respectively, following a 5 h, 950 °C soak in an evacuated quartz ampoule. This effect has been studied by temperature-dependent Hall-effect, photoluminescence, infrared absorption, mass spectroscopy, and deep level transient spectroscopy measurements. Except for boron, the samples are very pure, with carbon and silicon concentrations less than 3×1014 cm−3. Donor and acceptor concentrations, on the other hand, are in the mid 1015 cm−3 range, which means that the compensation is primarily determined by native defects, not impurities. A tentative model includes a donor at EC−0.13 eV, attributed to VAs−AsGa, and an acceptor at EV+0.07 eV, attributed to VGa−GaAs.

Elimination of grown-in dislocations in In-doped liquid encapsulated Czochralski GaAs

Abstract Dislocation behavior is investigated in the seed and grown crystal at the initial growth stage of In-doped liquid encapsulated Czochralski (LEC) GaAs, and a novel method is proposed for eliminating grown-in dislocations propagating in the growth direction from the seed near crystal center. The grown-in dislocations can be explained by the reaction of two or more misfit dislocations generated at the interface between the seed and grown crystal. A dislocation-free seed doped with indium of the same concentration as the crystal to be grown is essential for eliminating these dislocations. This technique is very effective for the growth of large-sized, completely dislocation-free crystals.