Dislocation-free Crystals Research Articles

Correlation of Oxygen and Recombination Centers on a Microscale in Asgrown Czochralski Silicon Crystals

ABSTRACTQuantitative microprofiles of the interstitial oxygen concentration and of the excess carrier lifetime, with a spacial resolution of about 20 μm, were obtained in as-grown dislocationfree CZ-Si crystals employing a double laser absorption technique. It was found that maxima (minima) in oxygen concentration along the crystal growth direction coincide with minima (maxima) of the lifetime. It was further found that the relation between changes in oxygen concentration and in lifetime varies in the radial direction indicating that “as-grown” oxygen precipitates are involved in lifetime limiting processes.

Hydrogen Concentration and Distribution in High-Purity Germanium Crystals

High-purity germanium crystals used for making nuclear radiation detectors are usually grown in a hydrogen ambient from a melt contained in a high-purity silica crucible. The benefits and problems encountered in using a hydrogen ambient are reviewed. A hydrogen concentration of about 2×1015cm-3 has been determined by growing crystals in hydrogen spiked with tritium and counting the tritium s-decays in detectors made from these crystals. Annealing studies show that the hydrogen is strongly bound, either to defects or as H2 with a dissociation energy > 3eV. This is lowered to 1.8eV when copper is present. Etching defects in dislocation-free crystals grown in hydrogen have been found by etch stripping to have a density of about 1×107cm-3 and are estimated to contain 108H atoms each.

Electron mobility and minority-carrier lifetime of n-InP single crystals grown by liquid-encapsulated Czochralski method

Correlations between electron mobility, minority-carrier lifetime, impurity concentration, and dislocation density have been studied in LEC (liquid-encapsulated Czochralski)-grown InP single crystals. Mobilities approaching 30 000 cm2/V s at 77 K, which correspond to compensation ratios of 0.2–0.4, have been obtained. Hole lifetimes as high as 3 μs have been obtained in dislocation-free single crystals. The experimental results for lifetimes can be explained by a simple model in which dislocation and impurity are considered as recombination centers.

Growth and crystal quality of inp crystals by the liquid encapsulated czochralski technique

The growth of InP single crystals by the liquid encapsulated Czochralski (LEC) technique has been studied from the standpoint of improving crystal quality. Twin-free crystals have been grown reproducibly in the P direction under the following conditions; (1) using starting material which does not contain fine InP particles, (2) controlling the cone shape of the crystals such that the angle with the growth axis is less than 19.68°, (3) arranging the.hot zone to produce a temperature at the top surface of the B2O3 encapsulant layer below 550°C. It has been confirmed that electrical properties of nominally undoped crystals are dominated by the impurity, Si, and the concentration of Si in an LEC crystal corresponds to that of the starting material. The dislocation densities of undoped LEC InP crystals depend on thermal stresses during the growth process. This knowledge has led to the growth of dislocation-free crystals.

Evidence of residual edge dislocations in czochralski grown Si crystals in 〈110〉 direction

The published work on the type of residual dislocations in Si crystals grown in 〈110〉 direction by Czochralski method indicate that screw dislocations are difficult to be eliminated as they are known to run parallel to the axis of growth. Our work shows on the contrary that it is edge type dislocations which are rather difficult to be eliminated. Extensive X-ray topographic studies have been made to establish the type of dislocations and their movements. Also our work on the growth of dislocation-free Si crystals in 〈110〉direction and their mechanism of dislocation movements are briefly described.

Microdefects in a non-striated distribution in floating-zone silicon crystals

A new type of microdefect has been found in fast-grown dislocation-free floating-zone silicon crystals. The distribution, the concentration and the size distribution have been determined using techniques such as copper and lithium decoration and X-ray topography. The defects in as-grown crystals could not be revealed by means of preferential etching or high voltage electron microscopy. It is concluded that these microdefects are formed by condensation of vacancies which only can become supersaturated when the cooling rate of the crystal is high.

Growth of dislocation-free silicon crystals in the 〈110〉 direction for use as neutron monochromators

Silicon crystals are hot-pressed for obtaining increased mosaicity to achieve higher neutron reflectivities. It has been noticed that an even mosaic distribution is obtained by pressing dislocation-free Si crystals grown in the 〈110〉 direction. A new method has been proposed to grow dislocation-free Si crystals in the 〈110〉 direction in a reproducible manner. Several flared up bulges introduced into the stem are mainly responsible for getting rid of the dislocations effectively. The plausible mechanisms responsible for the diversion of axial dislocations and for eliminating them are discussed in the light of earlier studies. Study of the plastic deformation of these crystals using in-situ γ-ray diffractometry revealed that the mosaic distribution is very even, which results in higher neutron reflectivity. Thus, Si crystals grown in the 〈110〉 direction without dislocations are good starting materials for the preparation of efficient neutron monochromators.

Gel and water solution growth, real structure and physical characterization of ferroelectric PbHPO 4

Single crystals of PbHPO 4 have been grown by gel and water solution methods. Both growth techniques have yielded crystals with similar morphology. Their real structure was studied. The gel technique is able to provide dislocation-free and inclusion-free crystals. Water solution crystals are less perfect. The possibility of revealing the ferroelectric domain structure is discussed.

LEC growth and characterization of undoped InP crystals

A necking-in procedure was found to be necessary for suppressing dislocation propagation into the growing crystal from the seed. Having eliminated dislocations in the neck, slip dislocations were generated at the crystal surface which propagated into the crystal interior, the density increasing with increasing diameter of crystal. The critical diameter at which the dislocation generation occurred was found to be dependent on a vertical temperature gradient in the B 2O 3 encapsulant. Dislocation-free undoped InP single crystals have been grown in 〈111〉P direction, up to the diameter of 15 mm when a low temperature gradient in the B 2O 3 layer (∼55°C/cm) was maintained. Carrier concentration profiles along the growth direction undoped InP crystals were found to obey the normal freeze relationship. The carrier concentration in the crystal was found to be dominated by Si in the starting material. The effective distribution coefficient for Si in InP was determined to be 0.55±0.03.

Microdefects in silicon and their relation to point defects

It is generally agreed that microdefects, or swirl defects, in dislocation-free silicon crystals are formed by the agglomeration of point defects. Various models have been proposed, involving Si self-interstitials, vacancies and interstitial-vacancy combinations. Thermal equilibrium as well as non-equilibrium phenomena were held responsible for the origin of these point defects. In this paper all models proposed to explain the formation of swirl defects will be reviewed. It is shown that the assumption of the dominance of self-interstitials in thermal equilibrium can explain most of the experimental observations whereas all other models encounter serious difficulties in trying to describe all aspects of swirl defects.

Preparation of low dislocation density and dislocation-free zinc and copper single crystals

We discuss the effect of growth conditions of Zn and Cu single crystals on their dislocation structure. The perfection of single crystals of pure metals grown from the melt is greatly determined by thermal stresses. A reduction of thermal stresses below a certain critical level produces dislocation-free crystals of copper. In addition, the nonequilibrium vacancy concentration is shown to influence the development of dislocation structures in Zn single crystals. A lower perfection of Zn single crystals than that of Cu crystals grown under similar conditions can be attributed to higher thermal stresses caused by the lattice anisotropy and to a higher efficiency of vacancies for the development of the dislocation structure.

Anomalous x-ray transmission in dislocation-free silicon after electron irradiation

Changes in the anomalous x-ray transmission intensity upon electron irradiation of dislocation-free silicon crystals have been measured. The irradiation at 20 K produced large changes in the transmission of the (220) reflection (i.e., 5%). Some annealing experiments were done. Attempts to understand the large changes suggest that some of the point defects agglomerate forming dislocation loops during irradiation.

"Electrical microscopy" of test parameter inhomogeneities resulting from microdefects in processed silicon wafers

Experimental results are presented on reverse current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</sub> ) inhomogeneities in n+-p diode arrays fabricated on wafers selected from dislocation-free Czochralski-grown silicon crystals. The crystals are boron doped with a resistivity of 10 Ω· cm and a (100) growth orientation. I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</sub> test results are presented as a test map using a computerized test system with a data graphics capability. The I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</sub> test "micrographs" reveal a direct correlation with the microdefect distribution in the original unprocessed wafers as observed with optical microscopy and chemical decoration etch methods. The microdefect distributions in the crystal result from growth interface instabilities formed during the crystal-growth process. It is observed that the microdefect distribution is dominant in all I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</sub> test maps and thus provides a graphic link between charge carrier transp01rt in the depletion region of p-n diodes and the microdefects formed during crystal growth which generate the excess I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</sub> current. This experimental method can be used to establish correlations between electrical test parameter nonuniformities in semiconductor devices which result from inhomogeneities in charge carrier transport and diffusion in the material, and structural inhomogeneities which are formed during crystal growth and subsequent wafer processing.

Deformation studies of initially dislocation-free copper crystals 111.In situX-ray topography

Abstract X-ray topographs have been taken in situ during tensile plastic deformation of initially dislocation-free copper single crystals. In as-produced crystals no dislocations were generated out to the abrupt yield point, which was comparable with the Instron measurements reported in Part I (Kamada and Tanner 1974). Yielding proceeded by rapid propagation of the deformation front down the crystal and no anomalous transmission topographs were obtained in the easy-glide region. Evidence is produced to suggest that the initial sources are associated with dislocations introduced near the grips during mounting. Synchrotron X-ray topographs, taken sequentially on nuclear plates, of a crystal electroplated with 1 μm Cr showed markedly different features. At a stress level of 91 g/mm2 yielding occurred as coarse, inhomogeneous, single slip; this stress level being in very good agreement with that measured in an Instron on similar crystals and reported in Part II (Buckley-Golder and Tanner 1979). At a stress of...

The effect of doping on the formation of swirl defects in dislocation-free czochralski-grown silicon crystals

During the growth of macroscopically dislocation-free Czochralski silicon crystals three types of swirl defects (types A, B and C) can form. The effect of doping with electrically active impurities (B, Ga, Sb, P and As) and isoelectronic impurities (Sn) on the formation of these microdefects has been investigated by means of preferential etching, copper and lithium decoration, X-ray transmission topography and EBIC-mode SEM. Doping with donors (concentration > 10 17 cm -3) suppresses the formation of A swirl defects (dislocation loops). Doping with acceptors (concentration > 10 17 cm -3) eliminates the formation of B and C swirl defects, while doping with tin has no effect. The new experimental data strongly suggest that the formation of the different types of swirl defects is due to parallel condensation processes involving silicon interstitials and vacancies. The observed doping effects are explained in terms of complex formation as a result of coulomb attraction between dopants and charged thermal point defects.

Quantitative determination of microdefect density in dislocation-free silicon by preferential chemical etching

A phenomenological equation is derived which relates the observed surface density of hillock defects in dislocation-free Czochralski silicon crystals to the bulk density, the etch rate, the etch time and a parameter which we call the defect survival time which is defined as the time interval between the defect being revealed by the etch and the feature being totally etched away. Application of a practical method, designated etching analysis with time (EAT), to the quantitative analysis of two silicon samples from the same dislocation-free crystal is described. Excellent agreement between theory and experiment is obtained.

Etch structure on microdefects in floating-zone silicon wafers as affected by dopants and surface treatment

The microdefect distribution in dislocation-free floating-zone silicon crystals has been investigated by means of preferential etching. The etch structures developed on the microdefects are found to depend upon the presence of dopants and the surface treatment of the crystal sections. Strong indications were obtained that microdefects in chemically–mechanically polished slices are saturated with atomic hydrogen, developed during the polishing treatment, and which affects the etching behavior of the microdefects in a different way than hydrogen that dissolved in the crystal during the growth process. Low concentrations of dopants appeared to suppress the effect of hydrogen introduced during polishing.

Study of point-defect aggregates in nearly perfect silicion single crystals using a high-resolution diffuse X-ray scattering technique

High-resolution diffuse X-ray scattering (DXS) measurements have been made on dislocation-free silicon single crystals of the following types: (1) as-grown specimen with no heat treatment (labelled as NHT); (2) specimen heated at 1273 K under oxygen for 10 h [labelled as HT(1)] and (3) specimen heated at 1273 K under oxygen for 50 h [labelled as HT(2)]. Reciprocal space around the 111 reciprocal-lattice point in (1,-1,1) geometry has been explored using a Cu Kα exploring beam. From the distribution of DXS intensity it was found that for sample NHT, the observed DXS is predominantly due to aggregates of vacancies. Heating the specimen under oxygen produces defect aggregates of interstitial type. Infrared absorption measurements showed that the area under the 9 μm absorption band decreases, suggesting that oxygen ions aggregate into clusters of interstitial type. Transmission X-ray topographs also support these results.

Growth and characterization of Indium-doped silicon for extrinsic IR detectors

Experimental growth and characterization studies of extrinsic indium-doped silicon for 3- to 5-µm focal-plane array applications have been carried out. Large, 2- and 3-in-diameter, -oriented indium-doped silicon crystals were prepared by Czochralski crystal pulling. The growth conditions affecting crystalline perfection, maximum dopant concentration and uniformity, and the residual shallow acceptor impurity content in grown crystals were investigated. In addition, effects of carbon content on the concentration of the 0.11-eV defect level in indium-doped silicon have been studied. The results demonstrate that near dislocation-free crystals containing indium concentrations up to 5 × 1017cm-3can be achieved at low growth rates to delay the onset of constitutional supercooling, while unwanted boron and aluminum impurities can be maintained at levels approaching 1 × 1013cm-3, when high purity synthetic-quartz crucibles are utilized to minimize melt contamination. Phosphorus-compensated indium-doped infrared (IR) detector performance and test photodetectors show peak responsivities up to 10 A/W (5.9 µm, 1000 V/cm, 50 K). Monolithic CCD test structures fabricated on Czochralski indium-doped silicon substrates show lower responsivities, however, due to detrimental effects of high-temperature CCD processing.

Real-Time X-Ray Topographic Observation of Melting and Growth of Silicon Crystals

ABSTRACTA technique for the direct viewing of topographic images was developed based on a television system using an x-ray sensing PbO-vidicon camera tube (resolution 25 μm). Melting and growth processes of plate-shaped crystals were observed in an argon flow by this technique. The observation showed that dislocations have very high mobilities by superheating and the equilibrium dislocation density of zero is achieved just before melting. In melting of dislocation-free crystals, liquid drops (locally molten regions) were observed inside the crystals. While, crystallites appeared in the supercooled melt near the growth interface. It was found that both the drops and crystallites are not formed in 5% hydrogen/95% argon environment. Melting behavior of crystals covered with oxide films indicates that the solid-liquid interfacial free energy is greatly lowered by oxygen impurity. This oxygen effect may be responsible for both the drop and crystallite formation. Origin of swirl defects in dislocation-free crystals is discussed in the basis of these observations. Both composite and common a/2&lt;110&gt; dislocations were observed near the growth interface: The former are composed of three a/2&lt;110&gt; dislocations and are present at the interface so that, as the crystal grows, they propagate into the new crystal. They influence the interface morphology, when their Burgers vectors have the component perpendicular to the interface. The common dislocations cannot exist stably at the interface and are driven into the newly grown dislocation-free region by thermal stresses.