DESCRIPTION
METHOD AND APPARATUS FOR PRODUCING GROUP-III NITRIDES
The subject invention was made with government support under a research project supported by DARPA Grant No N00014-92-J-1895 and the Office of Air Force Grant No AF97-056 The government has certain rights in this invention
Background of the Invention
The group-Ill nitrides, for example GaN, are promising wide band-gap semiconductors for optical devices in the blue and ultraviolet (S Nakamura, 1997a, 1998b), high-temperature and high-power device applications (H Morkos et al , 1994, T P Chow et al , 1994, J C Zolper et al , 1996) However, the reliability of current state-of the art GaN-based devices such as blue emitter and high-temperature devices is limited Blue diode laser lifetimes are short This is widely attributed to the fact that most GaN devices are grown on lattice mismatched substrates such as sapphire which results in high dislocation densities, typically about 1010 cm 2
Epitaxial GaN films have recently attracted much interest based on their optoelectronic applications as blue-ultraviolet optoelectronic devices and high temperature transistors (S
Nakamura, 1997, 1998) Since bulk GaN substrates are not available currently, the films are generally grown on sapphire, SiC, GaAs, or Si substrates These substrates provides poor lattice and thermal expansion matching to GaN which lead to very high densities of structural defects The identification of an appropriate matched substrate for epitaxial growth might enable the preparation of high quality devices with these semiconductor materials
One growth process for prepanng single crystal films of GaN relies on the vapor phase reaction between GaCl3 and NH3 in a hot-walled reactor, such as a hahde vapor phase epitaxy (HVPE) system Sapphire substrates are often used because they are readily available However, since sapphire is not lattice matched to GaN, GaN and sapphire have very different thermal expansion coefficients Accordingly, the resulting GaN has poor crystalline quality, having high dislocation densities and other lattice imperfections Even so, growth of GaN on sapphire is still common Furthermore, attempts have been made to reduce the occurrence of these dislocations and other lattice imperfections by providing buffer layers, such as A1N or ZnO, between the sapphire and the GaN However, the defects from the substrate mismatch propagate through the buffer layers to the GaN film
Though the first demonstration of the fabrication of single crystal GaN occurred 30 years ago, interest m these matenals for real-world optoelectronic devices has grown only in the last 5-6 years as material quality has improved and controllable p-type doping has finally been achieved A primary difficulty in producing high quality GaN single crystal has been the lack of lattice matching substrates such that high quality GaN single crystal epitaxial films could not be produced. Since high quality bulk GaN substrates have not been available, GaN films are generally grown on sapphire, SiC, or Si substrates. However, III - V nitride compounds having the wurtzite structure which is hexagonal in symmetry, in general, have much smaller lattice constants (a-axis dimension = 3.104 A for A1N, 3.180 A for GaN and 3.533 for InN) as compared to the currently available semiconductor substrates which typically have cubic symmetry Accordingly, sapphire, SiC, and Si provide poor lattice, as well as thermal, matching to GaN which can lead to very high densities of structural defects
The first blue GaN-based light emitting diodes (LEDs) and lasers, are now commercially available. They are fabncated from epitaxial GaN grown on sapphire substrates (S. Nakamura,
1997) The best published lifetime for a GaN-based laser on sapphire is only tens of hours, probably due to the high density of crystal defects. Recently a laser lifetime of 10,000 hours has been reported for devices fabricated on lateral overgrowth epitaxial material on patterned sapphire substrate (S. Nakamura, 1997a, 1998b, 1998c) Apparently the GaN that laterally overgrows on the Sι02 mask (in between the mask openings) has dislocation densities that are orders of magnitude lower than material grown directly on sapphire. Lasers fabricated in this low defect density material have much longer life time.
This epitaxially laterally overgrowth (ELOG) technique involves the growth of a GaN buffer layer on a substrate of, for example, Si, GaAs, Sapphire or SiC. A pattern of SiO,, for example, stripes, is then grown on the GaN buffer layer The Sι02 are about 0.2 μ in thickness and preferably covers about two-thirds of the buffer layer. An example may have 6-8 μm wide Sι02 stripes with 4 μm spacing. As the growth of GaN is continued, the GaN does not grow on the SiO, stripes but, rather, only in the grooves. As the GaN growth m the grooves reaches the height of the SiO, stripes, the GaN continues to grow up, but also begins to grow laterally from the sides of the GaN ridges to eventually form one continuous film. The defect density of the ELOG GaN film can be on the order of 107/cm\ with a reduced number of threading dislocations m GaN layer compared with GaN grown directly on the substrate without the Sι02 stripes. The oπginal substrate mateπal can then be removed, for example via etching, but the Sι02 grooves
are still trapped mside the GaN material. Furthermore, removal of the original substrate can damage the GaN material.
Recently in GaN multi-quantum-well-structure laser diodes (LDs) grown on GaN substrates were demonstrated (S. Nakamura, et al., 1998). The LDs showed a lifetime longer than 780 h despite a large threshold current density. In contrast, the LDs grown on a sapphire substrate exhibited a high thermal resistance and a short lifetime of 200 h under room- temperature continuous-wave operation.
Because of high dissociation pressure of nitrogen over GaN (> 70 kbar at 2300 °C), no one has succeeded m making large bulk GaN single crystal substrates Currently, bulk crystals with dimensions of only a few millimeters can be obtained with high pressure synthesis (20 kbar and 1600°C) (I. Grzegory et al , 1993) and by hydride vapor phase epitaxy (HVPE) on SiC or sapphire substrates with subsequent substrate removal by reactive ion etching, laser pulses, or by polishing. Accordingly, GaN is usually made by heteroepitaxy onto lattice mismatched substrates such as sapphire (S. Nakamura, 1997a) and silicon carbide (Yu V Melnik et al , 1997) with subsequent substrate removal by reactive ion etching or wet chemical etching, by laser pulses or by polishing. Each of these removal procedures can cause residual strain, changes in chemical composition of epitaxial films, etc In addition, not only are the lattice constant of the GaN film and substrate very different, but so are the thermal expansion coefficients, creating additional inducement for the creation of dislocations. These two factors, lattice constant mismatch and very different thermal expansion coefficients, can result m GaN epitaxial films with high densities of dislocations (1010 cm 2), regions of built-in strain, and cracks which often occur due to thermal stress during cooling.
This suggests that if low dislocation density bulk GaN substrates were available, device life times approaching the 50,000 target for reliable CD-ROM storage devices could readily be achieved. Similar improvements can be expected with respect to the reliability of other GaN- based devices such as heteroj unction bipolar transistors and modulation-doped field effect transistors for high-temperature electronics and uncooled avionics
Brief Summary of the Invention The subject invention pertains to a method and device for producing large area single crystalline III-V nitride compound semiconductor substrates with a composition AlxInyGa,. x y N (where 0 < x < 1 , 0 < y < 1, and 0 < x + y < l). In a specific embodiment, a crystal GaN substrates with low dislocation densities (~ 107 cm"2) can be produced. These substrates, for example, can be used to fabricate lasers and transistors. Large bulk single crystals of III-V compounds can be produced in accordance with the subject invention by, for example, utilizing
the rapid growth rates afforded by hydride vapor phase epitaxy (HVPE) and growing on oxide substrates such as lithium gallate LiGaO, substrates Lithium gallate has a close lattice mismatch (-1%) to GaN. LiGaO, has an orthorhombic structure with lattice parameters of a = 5.402 A , b = 6.372 A and c = 5.007 A. Bulk single crystals of LiGaO, can be grown from a melt by the Czochralski technique. Lithium gallate crystals were obtained from Crystal
Photonics, Inc. and subsequently sliced and polished on both sides. A thin MOVPE (metal organic vapor phase epitaxy) GaN film can be grown on the lithium gallate substrates to protect the oxide substrate from attack by HCl duπng HVPE. The oxide substrate can be self-separated from the GaN film after special substrate treatment procedure and cooling process. Examples of oxide substrates include LiGaO,, LιA102, MgAlSc04, Al2Mg04, and LιNd02. In a specific embodiment, ALN can be grown on LιA102, preferably after surface mtπdation.
The subject invention also relates to an apparatus which can alternately perform MOVPE and HVPE, without removing the substrate This eliminates the need to cool the substrate between the performance of the different growth techniques The subject invention can utilize a technique for the deposition of GaN which can alternate between MOVPE and HVPE, combining the advantages of both. In this process, during HVPE, tπmethylgallium (TMG) can first be reacted with HCl in the source zone of the hot wall reactor (see Figure 1A) to form chlorinated gallium species. For example, TMG and HCl can be reacted according to the following reaction: Ga(CH3), + HCl - GaCl + 3CH4
Preferably, the methyl radicals can be converted to methane gas such that negligible carbon is incorporated in the GaN films. The stream can then be combined with NH3 in the downstream mixing zone and directed toward a substrate where deposition of GaN occurs. For example, the stream can be combined with NH3 resulting in GaN deposition m accordance with the following reaction:
GaCl + NH3 - GaN + HCl + H2 The advantages of this technique include: the ability to deposit GaN by either MOVPE or HVPE m the same reactor, high growth rates, rapid reactant switching, lower background impurities with HCl (the Cl retains metal impurities in the vapor phase), in-situ etching, elimination of HVPE source problems and finally improvement of NH-, cracking.
Preferably, LiGaO, substrate nitπdation is utilized for GaN film/LiGaO, substrate self- separation which can cause the GaN film to "lift off the substrate, such that substrate removal in HCl by wet chemical etching is not needed.
Changes in the surface morphology, chemical composition and crystal structure of the (001) LiGaO, substrate as a function of nitπdation agent, temperature and time, and showed the
influence of surface morphology of the mtnded layer on the subsequent growth of GaN films and film/substrate self-separating.
The subject invention relates to a method for producing III-V nitride compound semiconductor substrates, comprising the steps of. growing a first III-V nitride compound semiconductor onto an oxide substrate by MOVPE; and growing an additional III-V nitride compound semiconductor by HVPE onto the first III-V nitride compound semiconductor grown by MOVPE. This method can be utilized to grow a first and additional III-V nitride compound semiconductors each having a composition given by Al In%Ga, v N (where 0 < x < 1, 0 < y < 1, and 0 < x + y < 1). The first and the additional III-V nitride compound semiconductors can have different compositions or each have the same composition The oxide substrate can have an orthorhombic structure with a good lattice match to the first III-V nitride compound semiconductor. For example, the oxide substrate can be selected from the group consisting of LiGaO,, LιA102, MgAlSc04, Al2Mg04, and LiNdO, Preferably, the oxide substrate has a surface area for III-V nitride compound semiconductor growth of at least 10 10 mm2. In a preferred embodiment, the first and the additional III-V nitride compound semiconductors are both GaN. At least 0.2 μm of GaN can be grown during the MOVPE growth step. The step of growing GaN onto the LiGaO, substrate by MOVPE can be conducted in a low pressure honzontal cold-wall MOCVD reactor with tπethylgallium (TEGa) and ammonia (NH3) as precursors and N2 as a carrier gas. Advantageously, the oxide substrate, for example the LιGa02 substrate, can be maintained at an elevated temperature between the step of growing GaN by MOVPE and the step of growing GaN by HVPE. Also, the step of growing GaN by MOVPE and the step of growing GaN by HVPE can each take place in the same reactor
The MOVPE grown GaN can serve to protect the LiGaO, substrate from attack by HCl duπng the HVPE growth of GaN. If desired, additional GaN can be grown by MOVPE onto the
GaN grown by HVPE, producing a high quality surface After the final layer is grown, the GaN can be cooled in for example nitrogen flow, to room temperature. The step of growing additional GaN by HVPE can involve the step of first reacting tπmethylgallium (TMG) with HCl in a source zone of a hot wall reactor to form a stream comprising a chlorinated gallium species. For example, the TMG can be reacted with HCl according to the following reaction-
Ga (CH3)3 + HCl → GaCl + 3 CH4. Preferably, the methyl radicals are converted to methane gas such that neghble carbon is incorporated in the GaN. The step of growing additional GaN by HVPE can further involve the step of combining the stream with NH3 in a downstream mixing zone and directing the stream toward the GaN grown by MOVPE on the substrate where growth of additional GaN can occurs
Upon combining the stream with NH3 the deposition of GaN can occur. For example, the stream can be combined with NH3 resulting in GaN deposition in accordance with the following reaction:
GaCl + NH3 - GaN + HCl + H2 After the step of growing additional GaN by HVPE the LiGaO, can be removed from the GaN by, for example, wet chemical etching. Preferably, the GaN can be lifted off the LιGa02 substrate. In a preferred embodiment, prior to the growth of GaN onto a LiGaO, substrate by MOVPE, nitπdation of the LiGaO, substrate can be performed This substrate nitπdation can cause a reconstruction of the substrate surface and the formation of a thin layer of mtnded mateπal having the same orientation as the substrate This substrate tπdation can involve the steps of: heating the substrate m the presence of nitrogen; and exposing a surface of the substrate to NH3. Preferably the substrate is heated for a period of time ranging from about 10 minutes to 15 minutes in a temperature range of about 800 °C to about 850°C, and the substrate surface is exposed to NH3 for a peπod of time ranging from about 30 seconds to about 10 minutes m a temperature range of about 800 °C to about 900 °C
After growing additional GaN by HVPE, the LιGa02 substrate and the GaN can be separated. This separation can be accomplished by the application of mechanical force such that the GaN lifts off of the LiGaO, substrate. After the GaN is separated from the LiGaO, substrate, the LiGaO, can then be reused to grow additional GaN The subject method can be used to produce a large area free standing GaN crystal, having a dislocation density less than 108 cm 2. The surface area of these crystals can be at least 10"4 m2, and have been as large as a 2 inch diameter circular wafer In a specific embodiment, a GaN crystal has been produced with a dislocation density less than 107 cm 2 and a useable substrate area greater than 10"2 m2. The subject invention also relates to a method of preparing the surface of an oxide substrate, comprising the steps of: heating an oxide substrate in the presence of nitrogen, exposing a surface of the oxide substrate to NH3. This method is applicable to oxide substrates such as LιGa02, LιA102, MgAlSc04, Al2Mg04, and LiNdO, The oxide substrate, for example LiGaO,, can be heated for a penod of time ranging from about 10 minutes to about 12 minutes in a temperature range of about 800°C to about 850°C. The surface of the oxide substrate can be exposed to NH3 for a period of time ranging from about 30 seconds to about 10 minutes in a temperature range of about 800 °C to about 900°C. The substrate can be heated in the presence of nitrogen, for example flowing N2 over the oxide surface at a flow rate in the range from about 2 L/min to about 5 L/min. This method can improve the smoothness of the surface of the oxide substrate.
The subject invention also pertains to a device for producing GaN crystals having a means for performing metal organic vapor phase epitaxy (MOVPE) on the surface of the substrate and a means for performing hydride vapor phase epitaxy (HVPE) on a surface of a substrate. The device can transition from MOVPE to HVPE in situ.
Advantageously, the substrate does not have to be removed from the device between MOVPE and HVPE and, therefore, the substrate can be maintained at elevated temperatures during transition from MOVPE to HVPE.
Brief Description of the Drawings
Figure 1 A illustrates schematically an H-MOVPE reactor in accordance with the subject invention.
Figure IB shows a schematic structure of a sequence of layers which can be used to produce thick GaN films. Figure 2 shows the results of an XRD analysis which was performed on a free-standing
GaN sample produced in accordance with the subject invention
Figure 3A and 3B show auger electron spectroscopy (AES) spectra of a bulk single crystal GaN substrate from the top and the GaN/LιGaO interface after separation, respectively Figures 3C and 3D show AES spectra of a LιGa02 substrate before (after nitπdation step) and after growth, respectively.
Figures 3E and 3F show micro Raman scattering spectra of a bulk single crystal GaN substrate from the top and the GaN/LιGa02 interface after separation, respectively.
Figure 4 illustrates the growth rates of GaN on LιGa02 by HVPE as a function of the HCl/Ga molar ratio, resulting from experiments in accordance with the subject invention. Figure 5 illustrates schematically a reactor design in accordance with the subject invention.
Figure 6A illustrates schematically an mlet section side view m accordance with the subject invention.
Figure 6B illustrates schematically an extended view of an mlet section in accordance with the subject invention.
Detailed Disclosure of the Invention The subject invention pertains to a method and device for producing large area single crystalline III-V nitride compound semiconductor substrates with a composition AlxInyGa, .v N (where 0 < x < 1, 0 < y < 1, and 0 < x + y < 1). In a specific embodiment, GaN substrates, with
low dislocation densities (~ 107 cm2) can be produced. These crystalline III-V substrates can be used to fabricate lasers and transistors. Large area free standing single crystals of III-V compounds, for example GaN, can be produced in accordance with the subject invention. By utilizing the rapid growth rates afforded by hydride vapor phase epitaxy (HVPE) and growing on lattice matching orthorhombic structure oxide substrates, good quality III-V crystals can be grown. Examples of oxide substrates include LiGaO,, LiAlO 3 MgAlScO 4 Al MgO , ,and LιNd02.
In a preferred embodiment, lithium gallate (LιGa02) substrates can be utilized to grow good quality GaN crystals. In a specific embodiment, lithium gallate has a close lattice mismatch (-1%) to GaN. LiGaO, has an orthorhombic structure with lattice parameters of a =
5.402 A , b = 6.372 A and c = 5.007 A. Bulk single crystals of LiGaO, can be grown from a melt by the Czochralski technique In a specific embodiment, lithium gallate crystals were obtained from Crystal Photonics, Inc. and subsequently sliced and polished on both sides. A thin GaN film can be grown, for example by MOVPE (metal organic vapor phase epitaxy), on the lithium gallate substrates. This GaN film can act to protect the oxide substrate from attack by HCl during growth of a thicker GaN film by, for example, HVPE The oxide substrate can be subsequently self-separated from the GaN film after special substrate treatment procedure and cooling process Alternatively, the oxide substrate can be removed by other well known techniques such as wet chemical etching. Additional III-V nitride materials which can be grown via the subject method include, for example, A1N and InN
If desired, the substrate can be moved back and forth between MOVPE and HVPE growing apparatus, while maintaining the substrate is an appropriate environment. The subject invention also relates to a method and apparatus, for the deposition of III-V compounds, which can alternate between MOVPE and HVPE, combining the advantages of both. In a preferred embodiment, a hybrid reactor in accordance with the subject invention can go back and forth between MOVPE and HVPE in situ so that the substrate does not have to be transported between reactor apparatus. Preferably, this hybrid reactor allows both the MOVPE and HVPE growth to occur in a hot-wall reactor
In a specific embodiment, the subject method and apparatus can be utilized for the deposition of GaN. Preferably, a thin layer of GaN is first grown on the substrate via MOVPE.
Although, the entire thick film of GaN can be grown by MOVPE, MOVPE is much slower than HVPE and, therefore, more time consuming for growing a thick film. Accordingly, HVPE can be used to grow GaN on top of the thin layer of GaN grown by MOVPE. Alternatively, the entire GaN layer can be grown by HVPE. In this process, during HVPE, tπmethylgallium (TMG) can first be reacted with HCl in the source zone of the hot wall reactor (see Figure 1 A)
to form chlorinated gallium species. For example, TMG and HCl can be reacted according to the following reaction:
Ga(CH3)3 + HC1 - GaCl + 3CH4 Preferably, essentially all methyl radicals are immediately converted to methane gas, such that essentially no carbon is incorporated in the GaN films. The stream can then be combined with
NH3 m the downstream mixing zone and passed over a substrate where deposition of GaN occurs For example, the stream can be combined with NH3 in accordance with the following reaction:
GaCl + NH3 - GaN + HCl + H, The advantages of this technique can include one or more of the following: the ability to perform MOVPE or HVPE growth in the same reactor, high growth rates, rapid reactant switching, lower background impurities with HCl (the Cl retains metal impurities in the vapor phase), in-situ etching, elimination of HVPE source problems, and improvement of NH3 cracking. Prior to the initial step of growing the III-V compound on the oxide substrate, the substrate can be treated to prepare the oxide surface tor growth of the group-Ill nitride film. This treatment can enhance the ability to mechanically separate the oxide substrate from the III- V film after growth. Preferably, LiGaO, substrate nitπdation is utilized prior to GaN growth, in order to enhance for GaN film/LiGa02 substrate self-separation Such nitπdation can cause the GaN film to "peel off the substrate, such that substrate removal in HCl by wet chemical etching is not needed. The use of a nitrogen carrier gas can be utilized to achieve high structural quality GaN on LιGaOz. Furthermore, the nitπdation of LiGaO,, for example using NH3 prior to growth, can improve the film quality. For the growth of thick GaN films it is a preferred step, and can enhance film-substrate self-separation. Nitπdation of the LiGaO, substrate can lead to the reconstruction of the substrate surface and to the formation of a thin layer of tnded material having the same orientation as the substrate The nitπdation is thought to supply nucleation centers to promote the growth of GaN through a decrease in the interfacial free energy between the film and substrate. The mtnded layer can also reduce the diffusion of Li into the GaN film. Nitndation causes changes in the surface morphology, chemical composition and crystal structure of the oxide, for example (001) LιGa02, substrate as a function of nitndation agent, temperature and time. These changes m the surface morphology of the mtnded layer influence the subsequent growth of GaN films and film/substrate self-separating.
Referring to Figure IB, a schematic structure of a sequence of layers which can be used to produce thick GaN films is shown. Substrates can first be pre-heated in nitrogen, followed
by a nitndation step using NH3. Seed GaN crystals can be grown by MOVPE on (OOl)LιGaO, substrates to protect substrate from HCl attack GaN layers can be grown on the mtnded surfaces at, for example, 850°C and atmospheric pressure MOVPE GaN film thickness of 0.2 - 0.3 μm can be grown first, followed by a thick GaN layer grown by HVPE. In a specific example, the HVPE grown GaN can be grown at 850-950°C at HCl/Ga ratio of 2.0 and NH3 flow 250 seem. The estimated growth rate for HVPE is 50 - 70 μm/hr. Typical HVPE GaN thickness can range from 100 to 300 μm. If desired, as a last step, growth of a thin (0.1 - 0.2 μm) MOVPE GaN layer can be performed to improve the surface morphology of the growing film Samples can be slowly cooled to room temperature in, for example, nitrogen flow. The LiGaO, substrate can be removed, for example by wet chemical etching. Alternatively, the
LiGaO, substrate can be mechanically separated from the GaN film. The LiGaO, substrate nitndation and cooling processes are preferred to encourage film-substrate self-separation and cause the GaN film to "lift off the LiGaO, substrate The LiGaO, can then be reused to grow additional GaN crystals. XRD was used to assess the crystallmity of thick GaN films grown m accordance with the subject invention. Referring to Figure 2, only two peaks occurring at 2Θ = 34.67° and 2Θ = 73.01 °may be indexed as the (0002) and (0004) diffraction peaks of GaN. Thus, single crystal (0001) - oriented hexagonal GaN growth on LiGaO, by H-MOVPE was observed.
Free-standing bulk GaN crystals with a size of 10 x 10 x 0 3 mm3 were obtained without any mechanical or chemical treatment. In addition, 2 inch GaN substrates have also been grown in accordance with the subject invention.
GaN bulk crystals grown in accordance with the subject invention have shown had a full width at half maximum (FWHM) ω - scan of 46.7 arc sec Average values of FWHM for bulk GaN ranged from 100 to 300 arc sec. We investigated both sides (top and bottom) of a GaN crystal grown by the subject method. Figures 3 A and 3B show auger electron spectroscopy (AES) spectra of a bulk single crystal GaN substrate from the top and from the GaN/LιGa02 interface after separation, respectively. The AES spectra appear to show essentially identical chemical composition on top of the film and at the film-substrate interface. No residual contamination was detected. Figures 3C and 3D show the AES spectra of the LiGaO, substrate before growth (after nitndation step) and after growth, respectively. These spectra are essentially identical as well. No trace of substrate decomposition was detected. It appears likely that separation occurs at the interface of the MOVPE GaN film and the minded layer of the LιGa02 substrate. Figure 3E and 3F show micro Raman scattenng spectra of a bulk single crystal GaN substrate from the top and from the GaN/LιGa02 interface after separation, respectively Based on micro Raman scattering
measurements (Ar-ion laser 100 mw, 514 nm; 200 μm slit width) no shifting on the position of the most strain-sensitive phonon E2 (570 cm-1) was detected. This corresponds to a residual- deformation free GaN crystal.
Surface morphology was examined by Atomic Force Microscopy (AFM), and the RMS roughness determined from the measurements was Rg = 0.03 nm for the HVPE surface. Adding the top MOVPE layer reduced the surface roughness by an order of magnitude (Rg = 0.03 nm). This illustrates the advantage of the last MOVPE GaN growth step.
We have demonstrated the H-MOVPE (hydπde metal organic phase epitaxy) growth of thick (100 - 300 μm) GaN films on 10 x 10 mm2 LιGa02 and removal of the film from the substrate. The GaN substrates grown have had a flat monocrystalline surface without any mechanical or chemical treatment In addition, no cracks or residual stam were observed.
In a specific embodiment, the GaN films can be deposited in a low pressure horizontal cold-wall MOCVD reactor with tπethylgallium (TEGa) and ammonia (NH3) as precursors and N2 as a earner gas at substrate temperature T = 650 - 900 °C, V/III ratio = 3000, reactor pressure = 100 Torr.
A substrate nitndation procedure can include pre -heating the substrates in nitrogen (N2) for 10 mm. at 850°C in an MOCVD reactor, followed by a nitπdation step using NH3. The exposure to NH3 can vary, for example, from 30 sec to 10 mm., at a temperature from 650 to 900°C. GaN layers can subsequently be grown on the mtnded substrate surface. Surface morphology of substrates and films grown in accordance with the subject method were determined by Atomic Force Microscopy (AFM) Chemical composition of the substrates and films was analyzed by Auger Electron Spectroscopy (AES) X-Ray Photoelectron Spectroscopy (ESCA), Secondary Ion Mass Spectroscopy (SIMS) and by Secondary Neutral Mass Spectroscopy (SNMS), while the structural quality was analyzed by Transmission Electron Spectroscopy (TEM). The surface morphology of (001) LiGaO, substrates as determined by
Atomic Force Microscopy (AFM) showed a dramatic improvement after nitndation, and the RMS roughness determined from the measurements is summarized in Table 1.
Table 1. RMS surface roughness as a function of NH3 and N2 expose temperature
For samples treated with ammonia, the surface roughness was observed to significantly decrease while the surface nitrogen concentration increased, as compared with as-received substrates. A distinct N
KLL peak was observed on NH
3 pretreated surfaces, indicating that nitrogen was incorporated into the LiGaO, surface layer.
Before nitndation a damaged disordered region about 10-15 nm deep appears to be formed at the LiGaO, surface. After NH3 treatment the disordered region observed for the as- received LiGaO, has disappeared. The NH3 treated surface exhibits a high degree of crystalline quality, with an atomically flat surface having steps less than 5 A observed. Accordingly, nitndation of the LiGaO, improves surface structure.
NH-, Pretreatment in NH3 at 650 °C produced a very rough surface, with apparent three dimensional growth mode. The surface morphology of GaN films grown on (001) LiGaO, pretreated at 800 and 900°C are very smooth, consistent with a two dimensional growth mode.
Accordingly, the mtnded layer improves the surface morphology of the resulting GaN films.
It appears that nitndation of the LiGaO, substrate leads to the reconstruction of the substrate surface and to the formation of a thm layer of mtnded material having the same orientation as the substrate. The essential role of nitndation is thought to be the supply of nucleation centers for GaN which have the same orientation as the substrate and the promotion of the growth of high quality GaN films due to the decrease m mterfacial free energy between the film and substrate. It also may play a role in preventing Li diffusion from the substrate into the GaN film.
Example 1 — GaN Growth
Preferably, the surface of a LιGa02 substrate is treated before the growth of any GaN. This surface preparation step can involve flowing N, over the surface of the LiGaO, substrate. For example, a N2 flow of 1.7 L/min can be allowed to flow over the LiGaO, surface for about 10 minutes, at a temperature of approximately 850°C. Next, a nitπdation step can be performed where NH3 and N2 can be allowed to flow over the surface of the LiGaO, substrate. For example, NH3 of 500 seem with N2 of 1.35 L/mm can be allowed to flow over the surface, at a temperature of approximately 850°C. Improvements to the surface can be achieved over a wide range of temperatures. For example, experiments conducted in the range of 800 - 900°C have shown surface improvement. This nitπdation of the LιGa02 surface can smooth the surface,
which can promote two dimensional growth of GaN layers subsequently grown on the nitπdated LiGaO, surface. Without the nitndation step the LiGaO, can tend to promote three dimensional growth of GaN, which can lead to a very rough GaN surface as well as defects in the GaN which is grown. One benefit of the nitπdation may be to supply nucleation centers of GaN having the same orientation as the substrate. These nucleation centers can contribute to the promotion of two-dimensional GaN film growth through a decrease in mterfacial free energy between the film and substrate. Surface flatness is further improved with increasing exposure to NH3.
In a specific embodiment, an initial GaN film can be grown on the LιGa02 surface at a temperature of about 850°C, utilizing a mam N2 flow rate of 1 35 L/mm (total N2 flow rate of 1.7 L/mm). Dunng MOVPE, TMG can be combined with NH3 at a preferred temperature range of 150°C - 250°C During HVPE, HCl can be mixed with tπmethylgallium (TMG), or Ga(CH3)3, at a temperature of about 250°C - 500°C, and preferably 300°C - 400°C, to form GaCl and CH4 which are then introduced to the mam growth chambers Growth of the GaN can then be performed, for example, in the temperature range 600 °C - 1 100°C, preferably in the range 700°C - 1000°C, and more preferably in the range 800°C - 950°C
Example 2 — Oxide Substrate Surface Pretreatment
A series of expenments were conducted involving the exposure of the oxide substrates to N2 at various temperatures. It was observed that no apparent surface degradation occurred. Subsequent GaN films grown in N2 on LiGaO, on 850 °C exhibited excellent surface and crystalline quality The FWHM of films grown in N2 were more than an order of magnitude lower (< 160 arc sec) than those grown m H2
Pretreatment with NH3 had a significant effect on the quality of the subsequently grown
GaN, which were deposited m a low pressure horizontal cold-wall MOCVD reactor with tnethylgalhum (TEGa) and ammonia (NH3) as precursors and N2 as the carrier gas, at substrate temperature between 650 and 900 °C, V/III ratio = 3000, and a reactor pressure of 100 Torr.
Substrates were preheated in nitrogen (N2) for 10 mm at 850°C before nitndation m the reactor.
This was followed by a nitndation step using NH3 (1500 seem) The exposure to NH3 varied from 30 s to 10 mm. Thin GaN layers were grown on the mtnded substrate surface using a growth time of 1 mm (estimated thickness 85 to 100 A). The surface morphology of the pretreated substrate prior to growth was determined by Atomic Force Microscopy (AFM) and the chemical composition analyzed by Auger Electron Spectroscopy (AES). The results are shown in the table below.
RMS surface roughness and N content of treated LiGaO, surfaces
X-Ray Photoelectron Spectroscopy (ESCA) spectra for the NH3 treated substrate surfaces showed chemical shifts for the Ga3d peak, suggesting, for example, that Ga-N bonds formed m the near surface regions.
There was also concern that Li diffusion into the GaN film would deteriorate the electrical properties of the matenal. Using Secondary Neutral Mass Spectrometry (SNMS), Li profiles were measured in the GaN films. The resulting data indicated a significant difference in the value of the lithium diffusion coefficient, depending on the substrate pretreatment. In particular, when the substrate is pretreated with NH, the lithium diffusion coefficient was very small and, in a specific embodiment, decreased from (5±3) 10 π to (6±2) 10 18 cm2/sec as the substrate pretreatment time decreased from 10 minutes to 30 seconds
In order to assess what phases are expected to be formed on the substrate surface, a thermodynamic simulation of the nitndation experiments has been performed using the Thermo- Calc databank (along with the thermochemical data for LιGa02 evaluated by Dr. A. Davydov). The complex chemical equilibrium involving gaseous and condensed phases at different temperatures, pressures and NH3/N2 ratios has been computed by minimizing the total Gibbs free energy in the Li-Ga-O-N-H system. Results of this modeling suggest that nitndation of LiGaO, at temperatures below about 800°C can lead to the formation of stable GaN, Lι20 and liquid Ga droplets, as well as metastable LiH and LiOH compounds. At higher temperatures formation of the above phases are not thermodynamically favorable and only GaN and LiGaO, compounds can co-exist with the gas phase.
These predictions support the results of the AES, SIMS analyses of substrates which have undergone nitndation in accordance with the subject invention, which suggests that GaN and other mixed oxides, hydndes and nitrides could be formed m the 50-100 A surface layer on the (001)LιGaO2 substrate duπng the nitπdation
There is no evidence of nitrogen incorporation in case of nitrogen pretreatment at the same temperature.
The microstructures of the near surface region of an as-received LiGaO, substrate and a mtnded LiGaO, substrate was analyzed by HRTEM. Before nitπdation an approximately 10- 15 nm deep disordered region was formed at the LiGaO, surface. High contrast in that region indicated that some degree of stress still remained m the disordered region. After treatment with NH3 the discordered region observed for the as-received LiGaO, substrate disappeared and the NH3-treated substrate surface exhibited a high degree of crystalline quality. Atomically flat surfaces were observed, with steps of less than 5 A measured. The lattice images at surface were bent with a higher contrast compared to the LiGaO, indicating either a change in lattice parameters of the LiGaO, or formation of another phase Nitndation of the LiGaO, can improve surface structure and promote surface reconstruction, and, in particular, the formation of a thin layer of mtnded material having the same orientation as the substrate. In fact, SIMS analysis indicates that GaN forms on the mtnded LiGaO, Accordingly, the nitndation of LiGaO, substrates prior to GaN growth appears to have a positive effect. It is believed that a surface reaction product may be formed that promotes recrystallization of the underlying LιGa02 and shows a lattice parameter very close to that of GaN. Furthermore, this reaction product may serve as an efficient barrier for Li transport into the GaN. The quality of the GaN grown on the pretreated LiGaO, substrates was remarkably high. The surfaces were atomically flat, and the bulk microstructure was excellent as judged by
Transmission Electron Spectroscopy (TEM) micrographs and HRXRD analysis. Indeed, FWHM's on the order of 25 sec ' were observed
Example 3 — GaN deposition This example descnbes a technique for the deposition of gallium nitπde which combines the advantages of MOCVD and hydride VPE. In this process, tπmethylgallium (TMG) is first reacted with HCl in the source zone of a hot wall reactor to form chlorinated gallium species according to the following reaction:
Ga(CH3)3 + xHCl + - GaCl + 3CH4 This stream is then combined with NH3 m the downstream mixing zone and passed over a substrate where deposition of GaN occurs by the following reaction:
GaClx + NH3 - GaN(s) + xHCl + 14(3 -x)H2
The advantages of this technique include a high growth rate (~ 100 times rates observed in
MOCVD), high puπty (the Cl retains metal impurities m the vapor phase), and the easy source delivery associated with MOCVD.
The conditions necessary for formation of GaCl from TMGa and HCl were explored. The reaction of TMGa with HCl to produce GaCl was expected to occur within a narrow temperature range, with the upper bound set by the decomposition temperature of TMGa since decomposition before the reaction with HCl might yield liquid gallium wall deposition. Such liquid gallium droplets can form upstream in the growth zone and can restrict the flow of gallium to the water. Also, the flux of gallium might then continue even when the TMG flow is halted, until all of the droplets have evaporated. If the temperature is too low, an adduct compound likely will form between the HCl and TMGa. Furthermore, the formation of gallium trichloride (GaCl3) is fhermodynamically favored over the monochloπde (GaCl). GaCl3 is a liquid and gives inefficient growth. An important consideration is the location and temperature at which the HCl is mixed with the TMGa stream. It was found that in a temperature range of 250 to 350°C, GaN could be grown at deposition rates on the order of ~ 40 μm/hr.
Films were grown at temperatures ranging from 450°C to 975 °C, at HCl/Ga ratios from 0 to 12, and at NH3 flows from 100 to 500 seem. The surface features of films grown at 900°C included hexagonal pyramids and mesas, as observed on conventional hydride films. The smoothest films had feature sizes of approximately 50 nm, as measured by a stylus profilometer In contrast, conventional hydride films have reported surface roughnesses on the order of microns. Lateral and longitudinal uniformity were also improved with this deposition technique The most uniform merged hydπde films were uniform to ±3 μm across a 1 cm x 1 cm substrate, which is significantly better than the 20% results for conventional hydride films.
The growth rate was measured in the range 500 ° C to 950 ° C Growth appeared reaction- limited below 750 °C, and diffusion-limited above this temperature
The variation of the growth rate with the HCl/Ga ratio was also studied at 750°C. At an HCl flow rate of zero, the growth rate was low, typical of MOCVD growth. As HCl was added the growth rate increased and proceeded through a maximum near equimolaπty, after which it tailed off to zero with high excesses of HCl. This is consistent with fhermodynamic predictions and conventional hydride results which demonstrate lower growth rates with increased HCl concentration.
The growth rate was found to be independent of the NH3 flow rate for V/III ratios greater than 100. Qualitatively, films grown with higher NH3 flow rates appeared smoother, and films seemed less prone to thermal cracking with higher NH, concentrations.
Average values of full width at half maximum (FWHM) for bulk GaN grown in accordance with the subject invention ranged from 100 to 300 arc seconds, with a GaN bulk crystal having a FWHM ω-scan of 46.7 arc seconds.
The best hydride films showed an lnstrmsic n-type carrier concentration of 1.5 x lO'Vcm3 (FTIR), which is consistent with literature results for hydride VPE. Secondary Ion Mass Spectroscopy (SIMS) analysis of both hydride and MOCVD films grown in the merged- hydπde reactor showed the presence of Cl, C, and O. Comparative analyses showed that the film grown with the merged hydride technique incorporated more chloride than the standard
MOCVD technique, but, more importantly, incorporated less carbon and oxygen.
The merged hydride technique for the chemical vapor deposition of gallium nitride can grow high quality gallium nitride film, and, in particular high quality thick films Growth rates comparable to conventional hydride VPE have been achieved for single-crystalline film using metal organic reactants. Crystalline quality for as-grown films proved superior to conventional
MOCVD for films grown in the same reactor, and comparable to FWHM values found in the literature
Example — 4 A HVPE system which was onginally designed for growth of GaN films can use gallium chloride (GaCl) and ammonia as the reactants for GaN growth A schematic diagram of such a reactor is shown in Figure 1A. Present MOCVD reactors using trimethylgallium (TMG) and NH, for growing GaN have growth rates of 1 μm/hr or less such that there is no practical way to grow free standing GaN wafers in such slow reactors In contrast, growth rates in a HVPE machine as shown in Figure 1 A are higher, ranging as high as 200 microns per hour (3-4 microns per minute) Recent results confirm that GaN samples exceeding a millimeter thickness can be grown in several hours using HVPE In a specific embodiment, TMG rather than liquid gallium as the Ga source can be utilized Before entenng the mam reaction zone, the TMG can be mixed with HC 1 , sustaining the reaction, HCl + (CH3)3Ga + H2 - GaCl + 3CH4 (1)
Preferably, essentially all methyl radicals are immediately converted to methane gas, such that essentially no carbon is incorporated in the GaN film The subsequent reaction in the mam tube can be represented as
GaCl + NH3 + H2 → GaN + HC1+ 2H2 (2) A HVPE system similar to the one shown in Figure 1 A can be operated the traditional way using hydrogen chloride to transport gallium for reaction with ammonia to form free standing GaN wafers.
Preferably, lithium gallate (LιGa02) is utilized as the substrate for starting the GaN layer. LiGaO, is almost perfectly lattice matched to GaN, having lattice parameters a = 3.186A and c = 5.007A, compared to a = 3.189A and c = 5.185 A for GaN Although excellent quality
GaN films can be deposited on lattice-matched LiGaO, substrates at least three potential problems may exist: (1) LιGa02 may be corroded by hydrogen and HCl; (2) LiGaO, is not conducting; and (3) Li ions may diffuse from the LiGaO, substrate into the GaN film, making it highly conducting. In order to overcome these deficiencies, a LiGaO, substrate can have a thm GaN buffer layer grown on it by MOCVD. Preferably, the LiGaO, first undergoes nitndation.
A GaN layer, for example on the order of 300-400 μm thick, can then be grown by HVPE. The LιGa02 layer can then be removed by, for example, an HCl etch, resulting in a free standing GaN wafer. Preferably, the GaN wafer can be lifted off of the LiGaO, layer without the need for etching. High growth rates can be obtained when approximately equal molar ratios of HCl and
TMG are admitted to the reaction tube. As shown in Figure 4, growth rates as high as 85 μm/hr were achieved. Excess HCl tends to lower the growth rate since it interferes with reaction (2) Layers of GaN are typically grown at temperatures of 850-900°C, and carrier concentrations in the films are at the level of 1 x 1018 cm 3. The GaN films are excellent single crystals as shown by the x-ray diffraction pattern m Figure 2.
LiGaO, was originally explored in 1965, when the crystal structure was elucidated LιGa02 is formed by the substitution of equal concentrations of Li and Ga for Zn in ZnO. The crystal structure is nearly wurtzite (like ZnO), but due to the very small size of the Li ion, the structure is slightly distorted. In order to account for this distortion, LiGaO, is classified as orthorhombic. LiGaO, melts around 1600 , unlike ZnO which decomposes. Crystal Photoms
Inc., Sanford, FL, prepares large crystalline boules of these materials by the Czochralski melt pulling technique. It was found that these melts have a problem with loss of lithium through the evaporation of Lι,0. Thus excess Lι20 has to be added to the melts. Boules up to 6 inches long can be grown, featuπng 2 inch diameters. Growth rates of at least 2mm/hour can be achieved, much faster than either SiC or ZnO. These boules can then be cut into wafers, and polished.
The LiGaO, substrates received from Crystal Photonics were polished on both sides, and had the (001) oπentation. The 38 mm diameter wafers were sawed into four equal quarters, or 10 x 10 mm samples, with a diamond wheel, and cleaned with standard organic solvents pπor to being loaded in the growth chamber. We found that attempts to etch the surface of LiGaO, with common inorganic acids such as HCl quickly ruin the appearance of the surface.
Therefore, the wafers were simply loaded into the reaction chamber without further processing.
The samples were heated m flowing nitrogen. Typically, the nitrogen flowrate was maintained at 5 l/min. The gallium source was tπethylgalhum (TEG), held at 15°C; the bubbler pressure was held at 740 Torr, with a earner gas flowrate of 50 seem. This corresponds to 11.5 μmole/mm of TEG. 3 l/min of NH3 were admitted to the chamber. The sample was heated
slowly m an inert nitrogen ambient. The growths were performed at 850-870°C for one hour. After one hour of growth, the reactant gas flows were stopped, and the sample was slowly cooled to room temperature.
GaN films on LiGaO, substrates grown in accordance with the subject invention were investigated using cross-sectional Transmission Electron Spectroscopy (TEM). One sample had a density of threading dislocations of only 107 cm"2 at a distance greater than 0.3 microns from the interface. Densities were about 109 cm 2 at the interface. Accordingly, it may be possible to make GaN LEDs by standard GaAs processing techniques because the substrate will be conducting. The subject invention relates to a method and apparatus, for the deposition of III-V compounds, which can alternate between MOVPE and HVPE, combining the advantages of both. In particular, the subject hybnd reactor can go back and forth between MOVPE and HVPE in situ so that the substrate does not have to be transported between reactor apparatus and, therefore, cooled between the performance of different growth techniques.
Example — 5 Reactor and Inlet Section Design
Figures 5, 6A, and 6B show schematically portions of a reactor design in accordance with the subject invention. This design can allow planar composition changes throughout the film in two ways. First, it can incorporate dopants by standard MOCVD techniques. Second, it can grow planar heterostructures at high growth rates by adding another metalorgamc (such as tπmethylalummum (TMA) or tπmethylindium (TMI)) to the reactant stream. Accordingly, two different compositions of film may be alternated with this method.
A reactor design is shown in Figure 5 and an mlet section is shown m Figures 6A and 6B. The reactor is a hot-wall design, housed in a clamshell furnace with six independently controlled temperature zones. Each of the reactor tubes is fabricated from quartz, and all are concentnc, with the center tube of the mlet having an adjustable length so that the TMG reaction temperature may be controlled by adjusting either the tube length or the furnace zone temperature. The mlet section is attached to the main reactor tube by a 2-inch Ultratorr fitting.
The substrate can be loaded upon a quartz, sled-like wafer holder, which can then be manually loaded into a Pyrex load lock by means of a forked quartz rod. The load lock can be pumped to rough vacuum and then refilled with nitrogen. The gate valve is opened, and the sled is loaded into the reactor by translating the rod. The rod is drawn back into the load lock and secured. Then the reactor can then be pumped down and purged, for example, three times to desorb any contaminants from the reactor walls. The gate valve is closed, and the wafer is permitted to come to thermal equihbπum under nitrogen. Ammonia flow can be started to the
run side of the reactor, and HCl and TMG flows can be started t the vent side. After about 60 seconds of nitndation, the HCl and TMG can be shunted to the run side.
At the end of the growth run, the HCl and TMG can be reshunted to the vent, and then all reactant flows stopped. An exception to this case is made when lithium gallium oxide (LGO) is the substrate being used, in which case NH3 is flowed until the furnace temperature drops below a given value, typically 600 °C. The reactor can again be purged with nitrogen for ten minutes, and unless the growth is on LGO, the substrate-bearing wafer holder is removed from the reactor. If the growth is on LGO, the reactor is allowed to cool to room temperature, at which time the film is removed.
Specific flows for single-crystal growth can be as follows:
It should be understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled m the art and are to be included within the spiπt and purview of this application and the scope of the appended claims.