Method of feeding gases into a reactor to grow epitaxial structures based on group III nitride metals and a device for carrying out said method

Abstract

The invention relates to methods for the chemical application of coatings by the decay of gaseous compounds, in particular to methods for injecting gases into a reaction chamber. The invention also relates to means for feeding gases into a reaction chamber, said means providing for the regulation of streams of reactive gases, and ensures the possibility of obtaining multi-layer epitaxial structures having set parameters and based on nitrides of group III metals while simultaneously increasing the productivity and cost-effectiveness of the process of the epitaxial growth thereof. Before being fed into a reactor, all of the gas streams are sent to a mixing chamber connected to the reactor, and are then fed into the reactor via a flux former under laminar flow conditions. The mixing chamber and the flux former are equipped with means for maintaining a set temperature. As a result of these solutions, a gaseous mixture with set parameters is fed into the reactor, and the formation of vortices is simultaneously prevented. The maximum allowable volume of the mixing chamber is chosen to take into account the process parameters and the required rarity of heterojunctions.

Claims

1. A method of delivering gas into a rector for epitaxial growth of group III nitrides, comprising delivering into a reactor of at least two reactive gas flows, at least one of which is mixed with a carrier gas, using trimethylaluminum, trimethylindium, trimethylgallium, triethylgallium, and their mixtures as a reactive gas—a source of group III metals, and using ammonia as a reactive gas—a source of nitrogen, wherein the gas flows before injection into a reactor are directed to at least one connected with a reactor mixing chamber for formation of gas mixture, and then gas mixture is delivered into a reactor via flow former shaped for laminar flow conditions promotion, and the walls of mixing chamber and flow former are heated and kept at temperature of 40° C. to 400° C., and the internal volume of the mixing chamber fulfills the conditions:
V<Q.Math.(P.sub.st/P).Math.(T/T.sub.st).Math.1 sec, where V is the internal volume of a mixing chamber, cm.sup.3; Q is the total summed flow rate through a chamber, cm.sup.3/s under standard conditions; P.sub.st, T.sub.st are standard values of pressure and temperature (P=10.sup.5 Pa, T=273.15 K); P is a pressure in the mixing chamber; T is a minimal temperature in the mixing chamber.

2. The method of claim 1, wherein hydrogen or hydrogen-nitrogen mixture with nitrogen content no more than 5% is used as a carrier gas.

3. The method of claim 1, wherein nitrogen is used as a carrier gas.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The method and device for supplying gases into a reactor to grow group III nitride epitaxial structures are illustrated by the drawings and graphs:

(2) FIG. 1. System of gas injection into a reactor, side view;

(3) FIG. 2. System of gas injection into a reactor, top view;

(4) FIG. 3. Surface distribution of growth rate of AlGaN alloy across the rotating substrate in the horizontal reactor;

(5) FIG. 4. Surface distribution of growth rate of AlGaN alloy across non-rotating substrate in the horizontal reactor;

(6) FIG. 5. Surface distribution of composition of AlGaN alloy across the rotating substrate in the horizontal reactor;

(7) FIG. 6. Surface distribution of composition of AlGaN s alloy across over the non-rotating substrate in the horizontal reactor;

(8) FIG. 7. X-ray diffraction curve of III nitride epitaxial structure on the silicon carbide substrate;

(9) FIG. 8. Distribution map of resistance of a conducting channel of III nitride epitaxial structure over the substrate with a diameter of 100 mm.

EMBODIMENT OF THE INVENTION

(10) The suggested device for supplying gases into a reactor to grow III nitride epitaxial structures contains at least two inlets 1 and 2 to supply reactive gases in the system of gas injection in a reactor. The system of gas injection in a reactor contains at least one mixing chamber 3 to prepare a gas mixture and flow former 4 connecting the mixing chamber with inlet in a reactor 5, where there is substrate holder 6 with no less than one substrate placed on it and substrate heater 7. The mixing chamber and flow former are equipped with the tools to keep given temperature 8. In the particular case, the system can comprise several chambers, e.g., contain five chambers 9, 10, 11, 12, and 13, each of which is connected with one of flow formers 14, 15, 16, 17, and 18 that connect the chambers with inlet in a reactor.

(11) The suggested device can be used to grow epitaxial structures based on group III nitrides by MOVPE (MOCVD) methods in combination with reactors with different geometry, such as horizontal, planetary, and vertical.

(12) The suggested device can be also used in combination with reactors that have several injection zones for reactive gases, through which reactive gas flows different in value are injected to compensate typical for these reactors nonuniformity of a growth rate over the substrate holder area. In this case, the suggested device should contain several mixing chambers connecting with a reactor through separate flow formers or a common flow former. During the growth of separate epitaxial layers that form a multilayered structure, exact values of reactive gas concentrations in these chambers can differ to provide the uniform growth rate over the substrate holder.

(13) The suggested device can also be used in combination with reactors that contain additional zones for injection of purge gases (hydrogen, nitrogen, ammonia or their mixtures), the purpose of which is to suppress unwanted inhomogeneities inherent to these reactors or parasitic deposition of materials on the structural elements of a reactor.

(14) The method and device for supplying gases into a reactor to grow III nitride epitaxial structure were tested during the epitaxial growth of Al.sub.xGa.sub.1-xN alloy in the horizontal reactor with a width of 145 mm, height of 20 mm, distance from zone of gas injection in a reactor to substrate holder center of 150 mm, and with one substrate of 100 mm in a diameter. The system of gas injection into a reactor consisted of mixing chamber 3 connecting with inlet in a reactor 5 by means of flow former 4. The characteristics of reactive gas flows were chosen based on the condition for providing x≠0.14.

(15) The characteristics of growth processes of an epitaxial layer using the mixing chamber as well as the separate inlets for injection of reactive gases into a reactor are listed in Table 1.

(16) TABLE-US-00001 TABLE 1 Comparative characteristics of the growth process of Al.sub.0.14Ga.sub.0.86N epitaxial layer when injecting reactive gases through separate inlets and using the mixing chamber to prepare a gas mixture Characteristics of Characteristics of technological process using technological Parameters of injection of reactive gases process with a use technological process through separate inlets of mixing chamber Temperature 1100° C. 1100° C. of substrate holder Pressure in reactor  200 mbar  200 mbar Hydrogen flow   30 SLM   30 SLM Ammonia flow   10 SLM   10 SLM TMA flow   25 μmol/min   25 μmol/min TMG flow  225 μmol/min  166 μmol/min Averaged growth rate 0.73 μm/h 1.05 μm/h AlN molar fraction in 0.13 0.14 AlGaN alloy

(17) Based on the analysis of results given in table 1 and in FIG. 3, FIG. 4, FIG. 5, and FIG. 6, it can be concluded that a use of the suggested method and device to grow epitaxial structures makes it possible to increase the growth rate by 40% with simultaneous decrease in the trimethylgallium flow rate by 25%, which is evidence that the profitability and efficiency of the process increase. As follows from FIG. 4, when using the suggested method and device the epitaxial layer growth rate in the direction along reactor axis changes slightly but is decreases with increasing the distance from inlet in a reactor, which testifies a high level of trimethylgallium utilization. At the same time, when using the separate inlets, the trimethylgallium concentration near the substrate decreases, which leads to the substantially nonuniform growth rate along the reactor axis and the growth rate is minimum near the injector and increases with increasing the distance from it. The latter means that considerable amount of trimethylgallium is not used during the synthesis of an epitaxial layer and leaves a reactor being unutilized, which results in the low profitability of the process.

(18) As follows from FIG. 5 and FIG. 6, when using the suggested method and device high uniformity of AlN molar fraction in the AlGaN epitaxial layer over the substrate area is reached.

(19) Using the same reactor, the suggested method and device were tested during the growth of the III nitride multilayered epitaxial structure of high electron mobility transistor on the silicon carbide substrate with a diameter of 100 mm

(20) The composition of the transistor structure in sequence starting from the substrate:

(21) 1. Al.sub.0.35Ga.sub.0.65N buffer layer with a thickness of 0.2 μm;

(22) 2. GaN buffer layer with a thickness of 2.4 μm;

(23) 3. GaN channel layer with a thickness of 0.4 μm;

(24) 4. AlN barrier layer with a thickness of 0.75 nm;

(25) 5. Al.sub.0.25Ga.sub.0.75N barrier layer with a thickness of 21 nm.

(26) To supply gases into a reactor, the gas injection system that comprises mixing chamber 3 to prepare a gas mixture and flow former 4 connecting the mixing chamber with inlet and providing gas injection into a reactor as a laminar flow was used. The mixing chamber and flow former were heated up to a temperature of 150° C. using heater 8.

(27) During the growth of multilayered epitaxial structures, it is necessary to change abruptly the chemical composition of an injected mixture of carrier gas and reactive gases with the aim to provide the sharpness of interfaces between separate layers that form a multilayered structure at the scale of an atomic layer (˜0.25 nm). At actual growth rates this means that for all the growth conditions used in the technological process of epitaxial structure growth the following condition should be satisfied
V<Q.Math.(Pst/P).Math.(T/Tst).Math.1 sec,
where V is the internal volume of a mixing chamber, cm.sup.3;

(28) Q is the total summed flow rate through a chamber cm.sup.3/s, under standard conditions;

(29) P.sub.st/P is the ratio of pressure under standard conditions to maximum pressure in a mixing chamber;

(30) T/T.sub.st is the ratio of minimum temperature in a mixing chamber to temperature under standard conditions.

(31) When this condition is satisfied, in 5 s after changing the chemical composition of gases injected in the mixing chamber the composition of a gas mixture at output of the mixing chamber will change by more than 99%, which can be regarded as a total composition change.

(32) The volume of mixing chamber 3 was 60 cm.sup.3. The values of Q.Math.(Pst/P).Math.(T/Tst).Math.1 sec when growing all 5 layers forming the transistor structure are listed in Table 2. Based on the data given in Table 2 it can be concluded that the condition is satisfied for all the layers.

(33) TABLE-US-00002 TABLE 2 Process parameters vs. volume of the mixing chamber. Q, cm3/s under standard Layer conditions Pst/P T/Tst Q•(Pst/P)•(T/Tst)•1 sec 1 583 10 1.55 9035 2 218 2.5 1.55 845 3 358 1.25 1.55 693 4 583 5 1.55 4518 5 583 5 1.55 4518

(34) The conditions and results of the carried out experiment are given in Table 3.

(35) TABLE-US-00003 TABLE 3 Characteristics of the growth process of layers that form a III nitride epitaxial structure of a high electron mobility transistor on the silicon carbide substrate of 100 mm in a diameter. TMGa, TMAl, Growth Duration T, P, H.sub.2, N.sub.2, NH.sub.3, μmol/ μmol/ rate, of growth, Thickness, Layer ° C. mbar slm slm slm min min μm/h min μm 1 1080 100 30 0 5 90 50 1.5 8 0.2 2 1060 400 8.4 0 4.66 500 0 8 18 2.4 3 1060 800 13.8 0 7.66 600 0 6 4 0.4 4 1010 200 6 24 5 0 16 0.065 0.66 7.5 .Math. 10.sup.−4 5 1010 200 6 24 5 36 16 0.26 4.87 0.021 Total 35.5 ~3

(36) The duration of the technological process (except of reactor heating and cooling, the duration of which is unaffected by the method of supplying gases into a reactor) was 35.5 min. This is several times less than when using the known methods and equipment, which confirms the high production efficiency of the suggested method and device for carrying out the method.

(37) Based on the analysis of data given in Table 3, one can conclude that the epitaxial structure growth process can successfully proceed over a wide range of pressures (100-800 mbar) at substantially different total flow rates of gases passing through a reactor (13-35 slm). It can be also noted that the process proceeds effectively both when using only hydrogen as a carrier gas (layers 1-3) and when using nitrogen and hydrogen mixture with prevalence of nitrogen (layers 4 and 5, for which the presence of nitrogen in a reactor is necessary because of the peculiarities of surface chemical reactions).

(38) The thickness and composition of epitaxial layers of the structure were determined based on the results of high-resolution X-ray diffractometry. The X-ray diffraction curve for the obtained epitaxial structure is shown in FIG. 7. A good agreement of maximum positions and shape of the experimental curve (solid line) with maximum positions and shape of the calculated curve (dashed line, for visual clarity shifted up along vertical axis) has shown that the obtained layer parameters of the grown structure, in particular, the thickness and chemical composition of separate epitaxial layers, almost coincide with the calculated results.

(39) The high uniformity of obtained structure over the substrate was confirmed by means of resistivity mapping. The resistivity map of the epitaxial structure over the substrate with a diameter of 100 mm is shown in FIG. 8. The extremely low value of nonuniformity (2 Ω/square) in comparison with a mean value (323 Ω/square) corresponds to the magnitude of relative nonuniformity of 0.6%, which corroborates that the suggested method and device can provide a growth of area-uniform multilayered epitaxial structures.