B. Štěpánek

Sulphur-doped GaSb single crystals

Single crystals of 〈111〉 GaSb doped with sulphur were grown using the Czochralski technique without encapsulant in a hydrogen atmosphere. The homogeneous GaSb single crystals had only a low concentration of sulphur, 1 × 1017 atoms cm−3, due to sulphur evaporation. The most suitable way of alternative sulphur doping was found to be Sb2S3 in a flowing atmosphere of hydrogen and sulphur vapour. Using the distribution coefficient keff calculated from the measured carrier concentration and solidified fraction, it was possible to determine the evaporation rate during crystal growth.

Doping of GaSb single crystals with various elements

Abstract GaSb crystals doped with various elements (Cu, In, Ge, N, S, Se, Te and Mn) were grown using the Czochralski method without encapsulant in a flowing atmosphere of hydrogen from a stoichiometric melt. A suitable dopant was sought, which could cause the decrease of the carrier concentration and/or the increase of the resistivity of GaSb. It has been found that it is very difficult when using this method and the above mentioned dopants. The limit of the lowest carrier concentration is the value of (1–3) × 10 16 cm −3 and of the highest resistivity the value of 0.8 Ω · cm. The arrangement of this growth method is not able to ensure a perfect stirring of the melt during the crystal growth, which is the main problem of the preparation of GaSb wafers having a homogeneous carrier concentration.

GaSb single crystals doped with manganese

Abstract GaSb single crystals doped with manganese were grown using the Czochralski method without encapsulant in a reducing atmosphere flowing hydrogen. Using the Mn concentrations calculated from the Hall measurement, the distribution coefficient ( k eff ) of manganese in GaSb was estimated to be k eff =0.001+-0.0001. The dislocation density, which increases along growth direction, has also been found to be in contrast to other nonvolatile dopants in GaSb.

Copper solubility and distribution in doped GaSb single crystals

For the GaSb single crystals doped with copper (grown using the Czochralski method without encapsulant in flowing atmosphere of hydrogen) the distribution coefficient of copper in GaSb,keff=0.0021±0.0006 was found and the copper solubility in GaSb was discussed. The region of copper solubility in GaSb was analyzed on the thermodynamic basis using chemical phase diagram in the Sb−Ga−Cu system. Due to a rather low solubility of copper, its excessive amount in GaSb caused probably an increase of the dislocation density at the end of the GaSb single crystals.ZusammenfassungFür mit Kupfer versetzte GaSb-Einkristalle (gezüchtet nach der Czochralski Methode ohne Einbettung in dynamischer Wasserstoffatmosphäre) wurde der Verteilungskoeffizient für Kupfer in GaSb mitkeff=0.00210.0006 ermittelt und die Löslichkeit von Kupfer in GaSb diskutiert. Mittels chemischen Phasendiagrammen des Systemes Sb−Ga−Cu wurde auf thermodynamischer Grundlage der Bereich der Kupferlöslichkeit im GaSb System analysiert. Wegen der eher geringen Löslichkeit von Kupfer verursacht seine Überschußmenge in GaSb ein Ansteigen der Versetzungsdichte am Ende von GaSb-Einkristallen.

Electrical properties of Mn-doped GaSb

By analyzing Hall measurements, we have found that Mn is embedded in GaSb grown by the Czochralski method in an electrically active form, with a distribution coefficient of about 0.011. In addition to the Hall measurements, we also studied the d.c. electrical conductivity for several Mn concentrations at temperatures ranging from 4.2 to 300 K. Mn forms an acceptor level lying about 0.016 eV about the valence band. The Mn concentration dependence in the impurity conduction regime (below 30 K) has shown a transition to metallic conduction at a concentration of 8 × 10 23 m −3 , coinciding with the critical concentration in the Mott-Hubbard theory. An alternative to the Mott-Hubbard formula containing only universal constants and quantities directly experimentally observed is derived

Various methods for the growth of GaSb single crystals

Various methods for the growth of GaSb single crystals have been tried. It has been found that the oxidation of the GaSb surface and the low vapor pressure of GaSb material are main problems. For this reason the Czochralski method without encapsulant in a flow of very pure hydrogen seems to be the most suitable technique. Other tested methods have not been acceptable for several reasons (the economic point of view, quality of the crystals, highly complicated equipment, etc.). The Czochralski technique with the use of hydrogen reduces the formation of the oxide layer and makes it possible to grow fully single crystalline, high-quality GaSb with low dislocation density (< 102 cm−2).

Study of low Te-doped GaSb single crystals

Abstract The distribution of tellurium concentration in 〈111〉GaSb single crystals grown using the Czochralski method without encapsulant in an atmosphere of flowing hydrogen was studied. No facets were formed in undoped GaSb, while in the case of Te-doped crystals the facets have appeared at the centre of the GaSb bowl. The formation of facets is dependent on the temperature conditions in the growth surface (its shape) and on the level and the nonhomogeneity of dopant concentration near to the growth interface. It follows that it is difficult to grow single crystals which would have a homogeneous concentration of tellurium in the grown plane of 〈111〉 GaSb.

Decrease in free carrier concentration in GaSb crystals using an ionized hydrogen atmosphere

Abstract GaSb single crystals were grown under a flow of ionized hydrogen using the Czochralski method without encapsulant. The optimal position of a deuterium lamp was studied, and it has been found that the lamp position is not the main factor, in principle. However, better results were achieved in the case of the lamp position being several millimeters above the solid/liquid interface. In addition, it seems to be very likely that donors are passivated more than acceptors, which is confirmed by the growth of Te-doped GaSb. The low Te-doped crystals were p-type in the whole volume, and the free carrier concentration was almost the same and very homogeneous from the top to the bottom of the GaSb crystal, 1.8 − 2.3 × 1016 cm−3.

Copper doping of GaSb single crystals

Abstract For GaSb single crystals doped with copper (grown using the Czochralski method without encapsulant in a flowing atmosphere of hydrogen), the distribution coefficient of copper in GaSb was found to be k eff =0.0021±0.0006 ; the solubility of copper in GaSb is discussed. The region of copper solubility in GaSb was analysed on a thermodynamic basis using chemical phase diagrams for the Sb-Ga-Cu system. Owing to the low solubility of copper, its excessive amount in GaSb probably caused an increase of the dislocation density at the end of the GaSb single crystal.

Thermodynamic aspects of (Te,S)-double-doped GaSb crystal growth

Abstract A series of GaSb single crystals double doped with tellurium and sulphur were grown using the Czochralski method without encapsulant in an atmosphere of flowing hydrogen. The Hall carrier concentration of these crystals was measured and compared with the calculated values. Very good agreement appeared for the sulphur concentration C s for total amounts of dopants (Te and S) smaller than 12 at.%. If C s exceeds a value of 12 at.%, the theoretical and practical values of the Hall concentration differ from each other. It seems that this effect was caused by the creation of a Te-S solid solution and by the subsequent elimination of dopants from the GaSb lattice. This assumption is supported by measurements of dislocation density. In the case of C s ⪆ 12 at. % the dislocations were uniformly distributed on the surface of GaSb〈111〉 sample because the so-called “glide phenomena” could be suppressed by the formation of some precipitates of the Te-S solid solution which might generate dislocations in the whole volume of the GaSb single crystals.