Specification
The invention relates to a bulk nitride mono-crystal especially for use as a substrate for epitaxy. Such a substrate for epitaxy is particularly suitable for preparation of nitride semiconductor layers in a process for manufacturing of various opto-electronic devices.
Known nitride-based opto-electronic devices are manufactured on sapphire or silicon-carbide substrates, differing from the thereafter deposited nitride layers (i.e. heteroepitaxy).
In "the most commonly used Metallo-Orgaruc Chemical Vapor Deposition (MOCVD) method, GaN depositing is performed from ammonia and metallo-organic compounds from the gaseous phase, and the growth rates attained make it impossible to receive a bulk layer. However, MOCVP cannot produce a bulk crystal having a substantial thickness. In order to reduce surface dislocation density a buffer layer is first deposited on sapphire or silicon substrate. However, the reduction of surface dislocation density achieved is not bigger than to about 108/cm".
Another method that has been proposed for the manufacturing of bulk mono-crystalline gallium nitride, involves epitaxial depositing using halogens in the gaseous phase and is called Halide Vapor Phase Epitaxy {HVPE) ["Optical patterning of GaN films" M.K.Kelly, CAmbacher, Appl. Phys. Lett. 69 (12) (1996) and "Fabrication of thin-film InGaN light-emitting diode membranes" W.S.Wrong, T. Sands, Appl. Phys. Lett. 75 (10) (1999)]. The method allows generation of GaN substrates 2 inches in diameter, but the quality is insufficient for application in laser diodes, because the surface density of defects still remains in the 107 to 109/cm2 range. Besides, the HVPE GaN substrates have tilted crystal axes because of distortion caused by epitaxial growth on hetero-substrates, for example on sapphire.
Recently, defect density decrease is attained by using the Epitaxial Lateral Overgrowth (ELOG) method^ In this method, a GaN layer is first grown on the sapphire substrate, and then Si02 is deposited in the form of strips or grids. Next, such a substrate may be used for lateral GaN growing, reducing the defects density to about 107/cm .
Due to significant differences in chemical, physical, crystallographic and electrical properties of substrates such as sapphire or silicon carbide and semiconductor nitride layers deposited thereon by hetero-epitaxy, big technological effort is needed to advance progress in opto-electronics.
On the other hand growth of bulk crystals of gallium nitride and other nitrides of Group XIII elements is also extremely difficult (numbering of the Groups is given according to the IUPAC convention of 1989 throughout this application). Standard methods of crystallization from alloy and sublimation methods are not applicable because of decomposition of the nitrides into metals and N2. In the High Nitrogen Pressure (HNP) method [^Prospects for high-pressure crystal growth of III-V nitrides" S.Porowski et a!., Inst. Phys. Conf. Series, 137, 369 (1998)] decomposition is inhibited by applying a nitrogen atmosphere under high pressure. Growth of crystals is carried out in melted gallium, i.e. in the liquid phase, resulting in production of GaN platelets about 10 mm in size. Sufficient solubility of nitrogen in gallium requires temperatures of about 1500°C and nitrogen pressures of the order of 1500 MPa.
In another known method, supercritical ammonia was proposed to lower the temperature and decrease pressure during the growth process. It was proven in particular that it is possible to obtain crystalline gallium nitride by synthesis from gallium and ammonia, provided that the latter contains alkali-metal amides (KNH2 or LiNH2). The processes were conducted at temperatures of up to 550°C and pressure 500 MPa, yielding crystals of about 5um in size [,,AMMONO method of BN, AIN, and GaN synthesis and crystal growth" R. Dwilihski et aL, Proc. EGW-3, Warsaw, June 22-24, 1998. MRS Internet Journal of Nitride Semiconductor Research, http://nsr.mij.mrs.org/3/25].
Use of supercritical ammonia also allowed recrystallization of gallium nitride within the feedstock comprising finely crystalline GaN [„Crystal Growth of gallium nitride in supercritical ammonia" J.W.Kolis et aL, J. Cryst. Growth 222, 431-434 (2001)]. Recrystallization was made possible by introduction of amide (KNH2) into supercritical ammonia, along with a small quantity of a halogen (KI). Processes conducted at 400°C and 340 MPa gave GaN crystals about 0.5 mm in size. However, no chemical transport processes were observed in the supercritical solution, in particular no growth on seeds.
The thus obtained nitride mono-crystals are of no industrial use as substrates for epitaxy, mainly because of their insufficient size and irregular shape.
Lifetime of optical semiconducting devices depends primarily on crystalline quality of the optically active layers, and especially on surface dislocation density. In case of GaN-based laser diodes, it is beneficial to lower dislocation density in (he GaN substrate layer to less than 106/cm2, and this is extremely difficult in the methods used so far. On the other hand industrial processes for manufacturing such optical semiconducting devices can be performed only on reproducible substrates meeting strict quality specifications.
The present invention aims to provide a bulk nitride mono-crystal, especially for use as a substrate for epitaxy, having quality allowing its use in optc-electronics and electronics. This aim has been achieved by developing a bulk nitride mono-crystal, especially for use as a substrate for epitaxy as defined in the appended claims.
A bulk nitride mono-crystal according to the present invention has parameters as defined in the independent claims 1 and 12, while the preferred features of the same are defined in the respective dependent claims. The present invention relates also to the use of the bulk nitride mono-crystal as a substrate for epitaxy.
The present invention relates to a bulk nitride mono-crystal characterized in that it is a mono-crystal of gallium nitride and its cross-section in a plane perpendicular to c-axis of hexagonal lattice of gallium nitride has a surface area greater than 100 mm2, it is more than 1.0 um thick and its C-plane surface dislocation density is less than 106/cm2, while its volume is sufficient to produce at least one further-processable non-polar A-plane or M-plane plate having a surface area preferably at least 100 mm2.
The present invention relates also to a bulk nitride mono-crystal characterized in that it is a mono-crystal of gallium-containing^ nitride and its cross-section in a plane perpendicular to c-axis of hexagonal lattice of gallium nitride has a surface area greater than 100 mm2, it is more than 1.0 um thick and its C-plane surface dislocation density is less than 106/cm2.
A bulk mono-crystal of gallium-containing nitride according to the present invention is crystallized on the surface of a seed crystal with at least a crystalline layer of gallium-containing nitride, having a dislocation density less than 106 / cm .
A bulk mono-crystal of gallium-containing nitride according to the present invention may be additionally doped with donor and/or acceptor and/or magnetic dopants in concentrations from 1017/ cm3 to 1021 / cm3, depending on the properties required for the intended use of the same, for example as a substrate for epitaxy.
In the particularly preferred embodiment a bulk mono-crystal of gallium-containing nitride according to the present invention has a dislocation density close to 104 /cm2 and at the same time the Full-Width Half-Maximum (FWHM) of the X-ray rocking curve from (0002) plane is close to 60 arcsec.
The bulk nitride mono-crystal according to in the invention -suitable for use as a substrate for epitaxy -is obtained by a method involving dissolution of a respective Group XIII elements feedstock in a supercritical solvent with over-saturation of the supercritical solution with respect to the desired gallium-containing nitride being reached by means of temperature gradient and/or pressure change and crystallization of a desired gallium-containing nitride on a surface of seed crystal, at temperature higher and/or pressure lower than in the dissolution process.
The supercritical solvent contains NH3 and/or its derivatives, and includes ions of elements of Group I -at least potassium or sodium ions, the feedstock consists essentially of gallium-containing nitride and/or its precursors, selected from a group including azides, imides, amido-imides, amides, hydrides, gallium-containing metal compounds and alloys, as well as metallic elements of Group XIII preferably metallic gallium.
Accordingly to the present invention, crystallization of a bulk mono-crystal of gallium-containing nitride takes place in an autoclave, at temperatures from 100°C to 800°C and at pressures from 10 MPa to 1000 MPa and a molar ratio of alkali metal ions to the remaining components of the supercritical solvent ranges from 1:200 tol
:2.
Depositing a bulk mono-crystal of gallium-containing nitride may include lateral growth of gallium-containing nitride on a plurality of surfaces susceptible for such growth, placed on a crystal seed and spaced apart from each other.
The substrate for epitaxy according to the present invention has at least one surface suitable for epitaxial growth of semiconducting nitride layers without any additional pre- treatment.
In the preferred embodiment, the present invention relates to a bulk nitride mono-crystal grown in a direction parallel to c-axis of hexagonal lattice of gallium nitride seed in a supercritical NH3 containing gallium-complex compounds at Ga:NH3 molar ratio of more than 1 :50, in order to have a thickness high enough to obtain at least one further- processable A-plane or M-plane gallium-nitride substrate.
In still further preferred embodiment, the present invention relates to a bulk nitride mono-crystal grown on a seed having no substantial tilted crystal axis by means of a supercritical NH3 containing gallium-complex compounds, having not so much surface roughness as to decrease lifetime of a nitride semiconductor device formed thereon.
The bulk nitride mono-crystal wherein the seed and the bulk nitride mono-crystal consist essentially of gallium nitride and the seed is in form of a flat plate with two parallel faces perpendicular to c-axis of hexagonal lattice of gallium-containing nitride, (0001) and (000-1), while two bulk mono-crystals of gallium nitride are crystallized on both such faces of the seed crystal.
The bulk nitride mono-crystal wherein it is crystallized on a seed with one of the faces perpendicular to c-axis of hexagonal lattice of gallium-containing nitride, (0001) or (000-1), covered by metallic plate made preferably silver.
The bulk nitride mono-crystal wherein it is crystallized on a seed with one of the faces perpendicular to c-axis of hexagonal lattice of gallium-containing nitride, (0001) or (000-1), coated by metallic layer, preferably made silver.
The bulk nitride mono-crystal wherein it is crystallized on a seed with one of the faces perpendicular to c-axis of hexagonal lattice of gallium-containing nitride, (0001) or (000-1), blocked by arranging on that plane of the seed a second seed crystal of the same size with the same face: (0001) or (000-1) facing the corresponding face to be blocked of the first seed.
The bulk nitride mono-crystal wherein it is crystallized on nitrogen-terminated (000-1) face of the seed only.
The bulk nitride mono-crystal wherein it has a better surface quality than a bulk mono-crystal that can be crystallized on gallium- terminated (0001) face of the seed crystal.
The bulk nitride mono-crystal wherein it has lower surface dislocation density than a bulk mono-crystal that can be crystallized on gallium-terminated (0001) face of the seed crystal.
The bulk nitride mono-crystal wherein it has a better electrical resistivity than a bulk mono-crystal that can be crystallized on gallium- terminated (0001) face of the seed crystal,
The bulk nitride mono-crystal wherein it has lower values of full width at half maximum (FWHM) of the X-ray rocking curve than a bulk mono-crystal that can be crystallized on gallium-terminated (0001) face of the seed crystal.
The accompanying drawings illustrate the present invention and Fig. 1 shows a dependence of GaN solubility in supercritical ammonia containing potassium amide (at molar ratio of KNH2:NH3=0,07) on pressure at T=400°C and T=500°C, Fig. 2 shows florescence microscope view of a fracture of a substrate for epitaxy according to the invention, Fig. 3a presents SIMS (Secondary Ion Mass Spectroscopy) profiles of a sample of a bulk nitride mono-crystal, especially for use as a substrate for epitaxy according to the invention with a high Group I metals content, while Fig. 3b shows for comparison SIMS profiles of a sample of gallium nitride obtained by HVPE method and having a very low content of Group I metals, Fig. 4 presents an X -ray rocking curve from (0002) plane of the bulk GaN mono-crystal according to the invention, Fig. 5 shows change of temperature inside the autoclave in time at a constant pressure in Example 1, Fig. 6 presents change of pressure in time inside the autoclave at constant temperature in Example 2, Fig. 7 presents change of temperature in time in the autoclave at constant volume in Example 3, Fig. 8 presents change of temperature in time for Example 4, Fig. 9 presents change of temperature in time for Example 5, Fig. 10 presents change of temperature in time for Example 6, Fig. 11 to 15 present change of temperature in time for Example 7 to 11, Fig. 16-18 illustrate subsequent phases of manufacturing three exemplary bulk nitride mono- crystals according to the present invention -formed by lateral growth method. Additionally, Fig. 19 and 20 show sectional views of opto-electronic devices -a ridge
type laser diode, based on a substrate for epitaxy in form of a bulk nitride mono-crystal according to the invention, and nitride semiconductor laser device as described herein, respectively. In the present invention the following definitions apply.
Gallium-containing nitride means a nitride of gallium and optionally other element(s) of group XIII. It includes, but is not restricted to, the binary compound GaN, ternary compounds such as AlGaN, InGaN and also AlInGaN, where the ratio of the other-elements of group XIII to Ga can vary in a wide range.
Bulk mono-crystal of gallium-containing nitride means a mono-crystal especially for use as a substrate for epitaxy made of gallium-containing nitride from which opto-electronic devices such as LED or LD can be formed by epitaxial methods such as MOCVD and HVPE.
A fiirther-processable non-polar A- or M-plane plate means a plate possessing A- or M-plane surfaces which are suitablefor epitaxial deposition of nitride layers, and for manufacturing thereon of at least one nitride opto-electronic device, preferably a nitride semiconductor laser structure. Such a plate should be of a size allowing further processing by MOCVD , MBE or other methods of epitaxial deposition thereon of nitride layers, the surface area being preferably higher thanlO mm , most preferably higher than 100 mm".
Supercritical solvent means a fluid in a supercritical state. Essentially, the supercritical solvent contains a nilrogen-containing solvent and ions of alkali metals. It can also contain other components in addition to the solvent itself as long as these components do not substantially influence or disturb function of supercritical solvent.
Supercritical solution is used when referring to the supercritical solvent when it contains Group XIII element(s), in particular gallium - in a soluble form originating from the dissolution of feedstock containing Group XIII elements), in particular gallium.
Dissolution of feedstock means a process (either reversible or irreversible) in which said feedstock is taken up to the supercritical solvent as Group XIII elements), in particular gallium in a soluble form, possibly Group XIII element(s)-complex compounds, in particular gallium-complex compounds.
Group XIII element(s)-complex compounds, in particular gallium-complex compounds are complex compounds, in which a Group XIII element(s), in particular
- r
gallium atom is a coordination center surrounded by ligands, such as NH3 molecules or itsU derivatives, like NH2", NH2", etc.
Over-saturation of supercritical solution with respect to gallium-containing nitride means that the concentration of gallium in a soluble form in said solution is higher than that in equilibrium (i.e. it is higher than solubility). In the case of dissolution of galHum-containing nitride in a closed system, such an over-saturation can be achieved by either increasing the temperature and/or decreasing the pressure.
Autoclave means a closed container which has a reaction chamber where the ammonobasic process according to the present invention is carried out.
When evaluating the properties of a bulk nitride mono-crystal of the present invention various parameters can be measured and various evaluation methods can be employed, all well known in the art.
One of the important parameters is (Surface) Dislocation Density. In some publications the term "Etch Pit Density" (or EPD) is used when quality of a nitride mono-crystal is discussed. It has been already proved by microscopic observations that crystals can be etched more efficiently in near-dislocation region. So, if the amount of dislocations is not too high, counting of etch pits is the easiest way of determining of dislocation density. However, used etching procedure should be confirmed by TEM measurements. When evaluating the surface dislocation density of bulk nitride mono-crystals of the present invention the values of the parameter were obtained from the cathodoluminescence maps by microscopic observation of dark points on the surface of electron-beam-excited crystal. Such dark points could be near-dislocation regions, due to creation of irradiative recombination centers. This is another technique of determining of dislocation density and the results obtained have been confirmed by TEM measurements. Throughout the present specification Dislocation Density and EPD are used as equivalent terms.
Measurements of FWHM of the X-ray rocking curve and SIMS (Secondary Ion Mass Spectroscopy) profiles of the samples of bulk nitride mono-crystals according to the invention were employed in a course of evaluation of the quality of the samples obtained.
In accordance with the present invention a bulk mono-crystal of gallium-containing nitride have both large size and high_ quality. Such a bulk mono-crystal of gallium-containing nitride can have a surface area of more than 2 cm*" and a surface dislocation
density of less than 106 / cm2, having a thickness of at least 200 um (preferably at least 500 |im) and FWHM of X-ray rocking curve from (0002) plane of 50 arcsec or less,
Such mono-crystals can be grown on gallium-containing nitride crystal seeds and in turn they may subsequently serve as seeds for next mono-crystal growth processes.
As it was explained above, a bulk mono-crystal of gallium-containing nitride is a crystal of gallium nitride and optionally other element(s) of Group XIII. These compounds can be represented by the formula AlxGa].x.yInyN, wherein 0
