RU2168237C2 - Method for producing nitride film on surfaces of semiconductor compounds - Google Patents

Abstract

FIELD: semiconductor engineering. SUBSTANCE: method used for passivating surfaces of semiconductor compounds A³B⁵ and devices built around them and for treating surfaces to be subjected to epitaxial growth of GaN involves removal of oxides from semiconductor surface followed by its neutralization in environment containing hydrazine N₂H₄. Novelty is that oxide removal and neutralization are effected by dipping semiconductor surface in aqueous alkali solution of hydrazine N₂H₄ and water-soluble monosulfide. Concentration of hydrazine in mentioned solution is 2-10 mole/l and that of sulfide salt of group 1 metal is 0.05-015 mole/l. Sulfide salt of group 1 metal (such as Na₂S) or (NH₄)₂S may be introduced as soluble monosulfide. Single-layer films coherent to original semiconductor surface to minimize density of surface states in forbidden gap may be produced by using this method. EFFECT: provision for obtaining crystalline surface. 4 cl, 1 tbl

Description

The invention relates to the field of semiconductor technology and can be used to electronically and chemically passivate the surface of A ³ B ⁵ semiconductor compounds and devices based on them, as well as to prepare the surface of these semiconductor compounds for subsequent epitaxial growth of gallium nitride GaN on it.

Currently, nitride compounds, such as boron nitride, silicon nitride, are widely used as passivating coatings on the surface of A ³ B ⁵ semiconductors and devices based on them. Such films are usually obtained from the gas phase by chemical and plasma chemical deposition methods. However, the amorphous passivating layer – semiconductor interface is defective, which does not allow one to significantly reduce the density of the intrinsic surface states of the semiconductor and, accordingly, provide the necessary decrease in leakage currents and an increase in the breakdown voltage of device structures.

Thus, a known method of producing layers of boron nitride on the substrates of their semiconductors A ³ B ⁵ , including placing the substrate in the heated zone of the reactor, remote from the zone of plasma created by RF discharge with a frequency of 13.56 MHz, and the introduction of borazole and helium into the reactor. The temperature of the substrate is maintained in the range of 160 - 200 ^o C, the distance between the substrate and the edge of the inductor is set in the range of 10 - 14 cm; the power supplied to the inductor is 6 to 10 watts. The process is carried out at a partial pressure of helium (3 - 8) • 10 ^-2 Torr and a partial pressure of borazole (2 - 3) • 10 ^-2 Torr. Helium is introduced through the RF discharge zone, and borazole is introduced behind the inductor zone at a distance of no more than 1 cm from its edge (see RF patent N 2012092, class H 01 L 21/318, published on April 30, 94).

 As mentioned above, when using the known method, the interface: the amorphous passivating layer — the semiconductor turns out to be defective, which does not provide a significant decrease in the density of the intrinsic surface states of the semiconductor and, accordingly, does not provide the necessary decrease in leakage currents and an increase in the breakdown voltage of instrument structures.

 A known method of producing a nitrated layer on a substrate from a semiconductor material, including the formation of an oxide layer on a substrate and its subsequent thermal nitration in a nitrogen-containing gas medium in the field of electromagnetic radiation. The wavelength range of electromagnetic radiation is selected from the condition of ionization of a nitrogen-containing gas medium (see RF patent N 2008745, class H 01 L 21/318, published 02/28/94).

 The known method allows to improve the electrophysical parameters and radiation resistance of the nitrated oxide layer at a thickness of more than 100 A, however, such a coating is amorphous and has an inhomogeneous composition over the thickness of the layer. In addition, the known method is multi-stage and requires the use of sophisticated technological equipment, including vacuum, gas distribution and electrical circuits.

The closest set of essential features to the claimed technical solution is a method of producing a nitride film on the surface of semiconductor compounds A ³ B ⁵ , adopted as a prototype. The prototype method includes the preliminary desorption of oxides from the surface of the semiconductor compound at high temperatures (600 ^o C) in the stream of arsenic vapor and nitridation of the surface of the semiconductor using vapor of hydrazine N ₂ H ₄ at 480 ^o C (see Antipov V.G., Zubrilov A .S., Merkulov A.V. et al., Molecular Beam Epitaxy of Cubic GaN on GaAs (001) Substrates Using Hydrazine, FTP, vol. 29, issue 10, pp. 1812 - 1821).

The process of interaction of the surface of semiconductor compounds A ³ B ⁵ with hydrazine vapors in the known method leads to a chemical reaction of the replacement of element 5 of the semiconductor group by nitrogen with the formation of the corresponding nitride. To ensure the crystallinity of such a layer, the process is carried out at temperatures not lower than 450 ^o C. However, the use of high temperatures leads to the formation of relatively thick (more than 10 monomolecular layers) films in which relaxation defect formation processes occur (microcracks, packaging defects, misfit dislocations occur). Thus, the nitride films obtained during the substitution process are not coherent to the initial surface of the semiconductor and, in the case of subsequent growth of epitaxial GaN layers on them, become a source of germinating crystalline defects. The prototype method does not ensure the planarity of the interface between the substrate and the nitride layer and the uniformity of the thickness of this layer. In addition, the known method for producing a nitride film is not applicable for passivation of the surface of instrument structures with pn and heterojunctions and contact metal layers, which is due to parasitic chemical and diffuser processes that develop on the surface and in the bulk of multicomponent structures under processing conditions. The known method is not applicable to aluminum-containing compounds A ³ B ⁵ due to the impossibility of removing natural oxide during thermal annealing.

The objective of the invention was to develop such a method for producing a nitride film on the surface of semiconductor compounds A ³ B ⁵ , which would provide a monolayer nitride film coherent to the initial surface of the semiconductor crystal, creating a planar, defect-free semiconductor – nitride layer interface and, thus, would allow minimize the density of surface states lying in the band gap of the semiconductor.

The problem is solved in that in the method for producing a nitride film on the surface of semiconductor compounds A ³ B ⁵ , including the removal of oxides from said surface and its subsequent nitridation in a medium containing hydrazine N ₂ H ₄ , the removal of oxides and nitridation is carried out by immersion of the aforementioned surface in alkaline an aqueous solution of hydrazine and water-soluble monosulfide. The concentration of hydrazine in the solution can be 2-10 mol / L, and the concentration of water-soluble monosulfide can be 0.05 - 0.15 mol / L. As the water-soluble monosulfide, a sulfide salt of a group 1 metal (e.g., Na ₂ S or (NH ₄ ) ₂ S) can be added.

The essence of the proposed method for producing a nitride film is to create a coherent adsorption coating of nitrogen atoms bonded to elements of group 3 of the semiconductor compound during treatment with an alkaline hydrazine-containing aqueous solution. The function of the SH ^- anions formed in the solution is to remove group 5 elements from the treated surface in the form of soluble thio compounds. The process of formation of a nitride film is based on dissociative adsorption of hydrazine molecules on electrophilic centers formed by atoms of the elements of group 3 of the surface of the semiconductor compound A ³ B ⁵ . When the initial surface of the semiconductor compound interacts with an alkaline nitridizing solution, the natural oxide initially dissolves on the surface of the semiconductor compound A ³ B ⁵ . Then, the redox process occurs at the heterophase semiconductor – electrolyte interface, which consists in the transport of conduction electrons from the semiconductor to hydrogen cations in solution and the formation of electrophilic adsorption centers on positively charged semiconductor atoms. Next, selective adsorption of SH- anions on the atoms of group 5 elements and nucleophilic hydrazine molecules on the atoms of group 3 elements occurs, followed by desorption of soluble thio compounds of group 5 elements and dissociation products of adsorbed hydrazine molecules. The end result of these processes is the formation of a film of nitrogen atoms chemisorbed on the atoms of the elements of group 3 in the nodal positions of the crystal lattice of the semiconductor compound A ³ B ⁵ . The optimal concentration of hydrazine in the solution is 2 - 10 mol / L, and the concentration of soluble monosulfide is 0.05 - 0.15 mol / L. At a hydrazine concentration of less than 2 mol / L, the continuity of the nitride film may not be achieved. The introduction of monosulfide in a concentration exceeding 0.15 mol / L can lead to the appearance of sulfur in the resulting adsorption coating, which reduces the degree of filling of the surface of the semiconductor compound with nitrogen. At a sulfide salt concentration of less than 0.05 mol / L, complete removal of group 5 elements from the surface layer of the semiconductor compound may not occur.

The authors did not find in the patent and other scientific and technical literature descriptions of a similar method for producing a nitride film on the surface of the semiconductor compound A ³ B ⁵ , which would contain the claimed combination of essential features, which indicates, according to the authors, that the proposed method meets the criterion of "novelty."

 It is known to obtain nitride films using hydrazine (see prototype method). However, in the known method, it is necessary to first remove the oxide layer, and to carry out nitridation in hydrazine vapors at high temperature. In the claimed method, the process of removing oxides and the formation of a nitride film occurs in an aqueous solution of hydrazine and monosulfide, which provides a new technical result - the formation of a crystalline monolayer of nitride, a coherent initial surface of a semiconductor crystal and suitable for use as a substrate during epitaxial crystallization.

The inventive method for producing a nitride film on the surface of semiconductor compounds A ³ B ⁵ is as follows.

 The semiconductor wafer is degreased in carbon tetrachloride and then washed successively with acetone and deionized water. An alkaline aqueous solution of hydrazine and a water-soluble monosulfide is prepared, preferably with a concentration of hydrazine of 2-10 mol / L and monosulfide of 0.05-0.15 mol / L. The defatted and washed plate of the semiconductor compound is immersed in the prepared solution at room temperature and kept there for several minutes. Then the plate is removed from the solution, repeatedly washed with water and dried in air.

Examples of specific performance of the method. In the above procedure, alkaline aqueous solutions of hydrazine and sodium sulfate Na ₂ S of various concentrations were prepared. The processed GaAs crystals are immersed in the prepared solutions and kept at room temperature for 10 minutes. Three methods were used to evaluate the quality of the obtained nitride films: diffraction of slow electrons at a temperature of 400 ^o C; Auger electron spectrometry; photoluminescence.

 Comparing the data obtained by these methods, they made a conclusion about the structural perfection and composition of the adsorption film. The initial concentration of the components in the solutions, as well as the quality characteristics of the obtained nitride films are shown in the table.

 As can be seen from the data given in the table, adsorption films formed in hydrazine-sulfide alkaline solutions most fully satisfy the conditions of continuity of the nitride coating and its crystallinity in the above ranges of concentrations of the solution components.

GaAs (100) substrates nitrided by the claimed method were used to grow molecular-beam epitaxy by means of molecular beam epitaxy for growing ZnSe epitaxial layers and AlGaAs solid solutions. The epitaxy temperatures were 290 and 570 ^o C, respectively. Through the use of the proposed method for processing the surface of the substrate, it became possible to exclude the stage of high-temperature pre-epitaxial annealing of the substrates. The crystalline state of the growth surface of the substrates was controlled by fast electron diffraction. Both in the growth of ZnSe layers and in the growth of layers of AlGaAs solid solutions, a (1 x 1) diffraction pattern was observed, which indicates the absence of surface reconstruction usual for GaAs (100). At the initial stage of ZnSe epitaxy (3-4 monolayers), multiple three-dimensional nuclei were formed, which then coalesced into a continuous epitaxial film growing according to the layer mechanism. The layers grown on substrates treated by the present method have two orders of magnitude lower density of stacking faults and more than an order of magnitude lower density of inclined dislocations than ZnSe layers grown on non-processed substrates. In the case of growing AlGaAs on the nitridated surface of the substrates treated by the claimed method, epitaxial growth already at the initial stage occurred by the layered mechanism.

The results obtained indicate that the GaAs (100) surfaces treated by the claimed method after heating in vacuum to 290 ^° C do not have an amorphous coating and end with a layer of atoms coherently connected with the crystal, other than Ga or As atoms. Coatings created by the inventive method provide the possibility of epitaxial growth of films of compounds A ² B ⁶ and A ³ B ⁵ on the studied surfaces.

Claims (4)

1. A method of producing a nitride film on the surface of semiconductor compounds A ³ B ⁵ , including the removal of oxides from said surface and its subsequent nitridation in a medium containing hydrazine N ₂ H ₄ , characterized in that the removal of oxides and nitridation is carried out by immersion of said surface in alkaline aqueous a solution of hydrazine N ₂ H _{4 with a} concentration of 2 to 10 mol / L and a water-soluble monosulfide with a concentration of 0.05 to 0.15 mol / L.  2. The method according to claim 1, characterized in that the sulfide salt of a metal of group 1 is introduced as a water-soluble monosulfide. 3. The method according to claim 2, characterized in that Na ₂ S is introduced as a sulfide salt of a metal of group 1 4. The method according to claim 1, characterized in that as the water-soluble monosulfide enter (NH ₄ ) ₂ S.

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