Method and apparatus for growing indium oxide (In2O3) single crystals and indium oxide (In2O3) single crystal

Patent metadata

Field	Value
Publication number	US-10208399-B2
Application number	US-201214394858-A
Country	US
Kind code	B2
Filing date	Apr 24, 2012
Priority date	Apr 24, 2012
Publication date	Feb 19, 2019
Grant date	Feb 19, 2019

Abstract

A method and apparatus for growing truly bulk In 2 O 3 single crystals from the melt, as well as melt-grown bulk In 2 O 3 single crystals are disclosed. The growth method comprises a controlled decomposition of initially non-conducting In 2 O 3 starting material ( 23 ) during heating-up of a noble metal crucible ( 4 ) containing the In 2 O 3 starting material ( 23 ) and thus increasing electrical conductivity of the In 2 O 3 starting material with rising temperature, which is sufficient to couple with an electromagnetic field of an induction coil ( 6 ) through the crucible wall ( 24 ) around melting point of In 2 O 3 . Such coupling leads to an electromagnetic levitation of at least a portion ( 23.1 ) of the liquid In 2 O 3 starting material with a neck ( 26 ) formation acting as crystallization seed. During cooling down of the noble metal crucible ( 4 ) with the liquid In 2 O 3 starting material at least one bulk In 2 O 3 single crystal ( 28.1, 28.2 ) is formed. We named this novel crystal growth method the “Levitation-Assisted Self-Seeding Crystal Growth Method”. The apparatus for growing bulk In 2 O 3 single crystals from the melt comprises an inductively heated thermal system with a noble metal crucible ( 4 ) and evacuation passages ( 22, 22.1 ) for gaseous decomposition products of In 2 O 3 , while keeping very low temperature gradients. Various configurations of the induction coil ( 6 ), the noble metal crucible ( 4 ) and a lid ( 12 ) covering the crucible can be utilized to obtain very low temperature gradients, sufficient evacuation passages and a high levitation force. The electrical properties of the melt grown In 2 O 3 single crystals can be modified in a wide range by at least one heat treatment in suitable atmospheres and appropriate temperatures.

First claim

The invention claimed is:

1. A method for growing bulk In 2 O 3 single crystals, comprising the steps of:
i) providing into a growth chamber a thermal system comprising a noble metal crucible containing an initially non-conducting In 2 O 3 starting material, a crucible thermal insulation surrounding the noble metal crucible at a bottom and a side, and an induction coil disposed around the noble metal crucible and coupled to a radio frequency generator, wherein a crucible wall thickness (th) is not greater than a penetration depth of an eddy current induced in the noble metal crucible wall by the induction coil:
th≤δ =√{square root over (1/(π f μσ))}

where δ is the penetration depth (or a skin depth), f is a generator frequency, μ is a permeability, and σ is an electrical conductivity of the noble metal crucible, the generator frequency being in a range between 1 kHz and 2 MHz;
ii) closing the noble metal crucible with a cover of the thermal system comprising at least a lid and having at least one evacuation passage for removing gaseous decomposition products of In 2 O 3 from an interior of the noble metal crucible and keeping the interior of the noble metal crucible in communication with a growth atmosphere, wherein a cross-section area of the at least one evacuation passage is 30% or less of a cross-section area of the crucible;
iii) introducing at least into the thermal system the growth atmosphere, which provides oxygen partial pressure between 0.005 bar and 0.2 bar around a melting point of In 2 O 3 ;
iv) heating the In 2 O 3 starting material by conduction and radiation from the noble metal crucible which in turn is inductively heated by the induction coil;
v) keeping at least inside the thermal system an oxygen partial pressure which is insufficient to avoid In 2 O 3 decomposition, thus decomposing in a controlled way the In 2 O 3 starting material during heating of the noble metal crucible, thereby increasing electrical conductivity of the In 2 O 3 starting material with rising temperature;
vi) inducing, through the wall of the noble metal crucible, an eddy current in the In 2 O 3 starting material, which becomes conducting around its melting point;
vii) melting the In 2 O 3 starting material to form a melt comprising liquid In 2 O 3 starting material by a combination of conductive, radiative, and electrical heating, the electrical heating being caused by the eddy current induced directly in the In 2 O 3 starting material;
viii) electromagnetically levitating at least a portion of the liquid In 2 O 3 starting material and forming at the same time a neck portion of the liquid In 2 O 3 extending from the levitating top portion of the melt towards a bottom portion of the liquid In 2 O 3 starting material at the crucible bottom, or towards the crucible bottom if all the liquid In 2 O 3 starting material is levitating;
ix) monitoring and detecting the melting point of the In 2 O 3 starting material by at least one melting detection means;
x) cooling the noble metal crucible containing the liquid In 2 O 3 starting material to room temperature; and,
xi) solidifying during cooling down the liquid In 2 O 3 starting material and forming at the same time at least one bulk In 2 O 3 single crystal from the levitating and non-levitating portions of the liquid In 2 O 3 starting material, which solidifies on top and bottom sides of the liquid neck portion acting as a seed, or on a top side of the liquid neck portion if all liquid In 2 O 3 starting material is levitating.

2. The method according to claim 1 , further comprising, between step vii) melting and step x) cooling, the step of:
xii) holding the In 2 O 3 starting material in a liquid phase for a predetermined time, to allow In 2 O 3 to decompose and thus increase its electrical conductivity.

3. The method according to claim 2 , further comprising the step of:
xiii) overheating the In 2 O 3 starting material in the liquid phase by a maximum of 5% in relation to a detected melting point, to allow In 2 O 3 to decompose and thus increase its electrical conductivity.

4. The method according to claim 1 , wherein the growth atmosphere provides oxygen, which changes in partial pressure from about 0 bar to about 0.1 bar with rising temperature from room temperature to a temperature in the range of 1950° C. to 2100° C.

5. The method according to claim 1 , wherein a cooling rate of the noble metal crucible from the In 2 O 3 liquid phase to the solidification point of the In 2 O 3 liquid phase is between 50 K/h and 2000 K/h.

6. The method according to claim 1 , wherein the at least one evacuation passage is in the form of at least one evacuation opening, wherein a cross-section area of the at least one evacuation opening is between 0.25% and 30% of the crucible cross-section area.

7. The method according to claim 6 , wherein the cross-section area of the at least one evacuation opening is between 0.25% and 10% of the crucible cross-section area.

8. The method according to claim 1 , wherein the induction coil is cylindrical, and the noble metal crucible is disposed within the induction coil in such a way that at least part of the liquid In 2 O 3 starting material is located above a middle plane of the induction coil.

9. The method according to claim 1 , wherein the crucible wall thickness (th) is between 0.5 and 3 mm, and the RF generator frequency is between 5 kHz and 100 kHz.

10. The method according to claim 1 , wherein the melting detection means comprises at least one selected from the group consisting of: a pyrometer, a thermocouple, both the pyrometer and the thermocouple being adapted to detect a temperature of any part of the thermal system, a weighing unit adapted to detect mass losses of the In 2 O 3 starting material, and a mass spectrometer to detect decomposition products of the In 2 O 3 starting material.

11. An apparatus for growing bulk In 2 O 3 single crystals from a melt, comprising:
a growth chamber;
a radio frequency (RF) generator;
an induction coil coupled to the RF generator and disposed inside the growth chamber;
a thermal system disposed within the induction coil in the growth chamber and being in communication with a growth atmosphere, which growth atmosphere provides oxygen partial pressure between 0.005 bar and 0.2 bar around a melting point of In 2 O 3 , the thermal system comprising:
i) a noble metal crucible for containing an initially non-conducting In 2 O 3 starting material and subsequently grown crystals, the noble metal crucible being disposed within the induction coil and having a wall thickness (th), which wall thickness is not greater than a penetration depth of an eddy current induced in the noble metal crucible wall by the induction coil:
th≤δ =√{square root over (1/(π f μσ))}

where δ is the penetration depth (or a skin depth), f is a generator frequency, p is a permeability, and σ is an electrical conductivity of the noble metal crucible, the generator frequency being in a range between 1 kHz and 2 MHz, wherein crucible and coil shapes are adapted to electromagnetically levitate a portion of the liquid In 2 O 3 starting material and to form a neck portion of the liquid In 2 O 3 extending from a levitating top portion of the melt toward a bottom portion of the liquid In 2 O 3 starting material at the crucible bottom or toward a crucible bottom if all the liquid In 2 O 3 starting material is levitating;
ii) crucible thermal insulation surrounding a bottom and a side wall of the noble metal crucible;
iii) a cover enclosing the noble metal crucible from a top of the crucible, the cover comprising at least a lid and having at least one evacuation passage adapte

Assignees

• Galazka Zbigniew
• Fornari Roberto
• Uecker Reinhard
• Forschungsverbund Berlin Ev

Inventors

• Galazka Zbigniew
• Fornari Roberto
• Uecker Reinhard

Classifications

• H01F1/01

  of inorganic materials (H01F1/44 takes precedence) · CPC title

• C30B11/007

  Mechanisms for moving either the charge or the heater · CPC title

• C30B30/04 Primary

  using magnetic fields · CPC title

• C30B11/14

  characterised by the seed, e.g. its crystallographic orientation · CPC title

• Y10T117/1004

  with means for measuring, testing, or sensing · CPC title