WO1997048499A1

WO1997048499A1 - GaAs SUBSTRATE WITH A PASSIVATING EPITAXIAL GALLIUM SULFIDE FILM AND METHODS FOR FORMING SAME

Info

Publication number: WO1997048499A1
Application number: PCT/US1997/010605
Authority: WO
Inventors: Xiaoyang Zhu
Original assignee: The Board Of Trustees Of Southern Illinois University
Priority date: 1996-06-19
Filing date: 1997-06-19
Publication date: 1997-12-24

Abstract

A passivating epitaxial film of Group III-sulfur material is formed on a III-V compound semiconductor substrate (10) based on the interaction of the substrate with sulfur atoms (40). In a preferred and non-limiting embodiment, en epitaxial Ga2(S2)3 film is formed on a GaAs substrate by exposing a surface (12) of the GaAs substrate to a gaseous sulfur-containing molecule (e.g. H2S), photo-irradiating the sulfur-containing molecule to deposit sulfur atoms on the exposed surface and annealing the sulfur-deposited substrate (10).

Description

GaAs SUBSTRATE WITH A PASSIVATING EPITAXIAL GALLIUM SULFIDE FILM AND METHODS FOR FORMING SAME

This invention was funded, in part, by grants from the U.S. Department of Energy. The U.S. government may have certain rights in this invention.

BACKGROUND OF THE INVENTION The present invention generally relates to the passivation of group III-V type semiconductor materials, and specifically, to the passivation of these materials using sulfur. In a preferred embodiment, the invention relates to the passivation of GaAs by the formation of an epitaxial gallium sulfide film at the surface of a GaAs substrate .

Gallium arsenide (GaAs) and other group III-V compound semiconductor materials have a number of fundamental advantages over silicon. GaAs is a direct band gap material having high electron mobility. As such, GaAs is the material of choice for optoelectronics and has potential applications in high speed digital electronics. GaAs is also used in microwave monolithic integrated circuits for wireless communications and in military electronics where radiation hardness is desired. A prominent obstacle to be overcome before realizing the great potential of GaAs is the lack of a suitable passivation layer. In principle, a passivation layer should (a) eliminate bandgap surface states; (b) prevent chemical reaction between the interface and the ambient; and (c) serve as an energy barrier for electron transport. GaAs and other group III-V compound semiconductors have been passivated using a variety of overlayers, including, for example, large band gap lattice-matched semiconductor materials such as Al_xGa^_xAs and ZnSe, group V elements such as antimony, or group VI elements such as sulfur. (Green and Spicer, "Do We Need a New Methodology for GaAs Passivation?", J. Vac. Sci. Technol . A 11, 1061 [1993] and references cited therein) . While other group VI elements such as selenium and tellurium have also been used to passivate GaAs semiconductors ( see Sandroff and Hegde, "Enhanced Electronic Properties of GaAs Surfaces Chemically Passivated by Selenium Reactions," J. Appl. Phys. 67 (1) [Jan., 1990] ; Chambers and Sundaram, "Structure, chemistry and band bending at Se-passivated GaAs (100) surfaces," Appl. Phys. Lett. 57 (22) , [Nov., 1990] ;

Chambers and Sundaram, "Passivation of GaAs (001) surfaces by incorporation of group VI atoms: a structural investigation," J. Vac. Sci. Technol. B9 (4) [July/Aug., 1991] ) , the predominant group VI passivation efforts have focused on the use of sulfur.

Monolayer sulfur has been deposited onto a III-V compound semiconductor substrate by a variety of methods, including for example, by wet-chemical treatments (Sandroff et al. , "Dramatic Enhancement in the Gain of GaAs/AlGaAs Heterostructure Bipolar Transistor by Surface Chemical Passivation", Appl. Phys. Lett. 51, 33 [1987] ) , and by thermal adsorption or electron-induced decomposition of H₂S (Foord and FitzGerald, "The Adsorption and Thermal Decomposition of Hydrogen Sulphide on GaAs (100) ", Surface Science 106, 29-36 [1994]) .

However, the surface sulfide monolayer is unstable -- tending to dissociate in air.

Several approaches have been taken to address the instability associated with monolayer sulfide. Fowler et al . , in U.S. Patent No. 4,811,077, disclose forming a monolayer of sulfide on the surface of a GaAs substrate and covering the sulfide layer with an overlayer, such as Si0₂- Hou et al . describe an electrochemical treatment of a GaAs surface by which a sulfurized-GaAs layer with a thickness of about 15 A may be formed. (Hou et al. ,

"Electrochemical Sulfur Passivation of GaAs", Appl. Phys. Lett. 60, 2252 [1992]) . Nooney et al. disclose the formation of diffuse, non-epitaxial sulfide layers using photochemical and thermal decomposition of H₂S. (Nooney et al . , "Sulfur Layer Formation of GaAs (100) by Thermal and Photochemical H₂S Dissociation", J. Vac. Sci. Technol., A 13:4, 1837-1848 [1995]) . However, the absence of lattice matching between these proposed passivating layers or overlayers and the GaAs substrate make these methods commercially unattractive. In an alternative approach, U.S. Patent No.

5,300,320 to Barron et al . discloses depositing a cubic phase GaS thin film onto GaAs using chemical vapor deposition (CVD) from a single-source precursor. While the deposited GaS film operates as a stable and effective passivation layer, the single-source CVD approach requires a specially designed precursor, [(tBu)GaS]4, which is complicated and expensive to synthesize and is not completely compatible with existing semiconductor fabrication techniques. In addition to its use for passivating purposes, sulfur has also been used as a dopant to create n-type doped GaAs semiconductors, as disclosed, for example, in U.S. Patent No. 4,725,565 to Oren et al. Sulfur doping has been achieved in group III-V compound semiconductors at concentrations ranging from about 5x10²⁰ atoms/cm³ near the surface to about lxlO¹⁷ atoms/cm³ at depths of several hundred nanometers. (Zhang et al . , "Studies on Eximer Laser Doping of GaAs Using Sulfur Adsorbate as Dopant Source," Appl. Phys. A, 191-195 [1994]) .

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide III-V compound semiconductors such as GaAs having group Ill-sulfur passivation layers. It is also an object of the invention to make such passivated compound semiconductor materials using readily-available source materials and known fabrication equipment. It is an object of the invention to produce semiconductors which are commercially attractive for use in known and developing applications. Briefly, therefore, the present invention is directed to a method for forming a passivating epitaxial film on a III-V compound semiconductor substrate such as GaAs. In the method, sulfur atoms are deposited onto an exposed surface of the III-V semiconductor substrate. The sulfur atoms are preferably deposited onto the substrate surface by exposing the surface to an activated sulfur-containing molecule such as photochemically activated H₂S. The sulfur-deposited substrate is annealed at a temperature and for a period of time sufficient to form an epitaxial group III-sulfide film in the substrate at its surface. That is, the film forms in-si tu in the near-surface region of the substrate. The substrate is preferably annealed at a temperature of at least about 700 K for at least about 1 minute. The deposited sulfur atoms diffuse into the substrate and group V atoms diffuse out of the substrate.

The invention is also directed to a method for forming a gallium sulfide epitaxial film in a GaAs substrate at its surface. In this method, sulfur atoms are deposited onto a surface of the GaAs substrate. The surface of the GaAs substrate onto which sulfur is deposited preferably has a <100> orientation. The sulfur atoms are preferably deposited onto the substrate at a temperature of less than about 300 K. However, the deposition of sulfur atoms can also occur at a temperature of less than about 100 K. The sulfur- deposited substrate is annealed at a temperature of at least about 700 K, and in order of increasing preference, more preferably at least about 750 K, at least about 775 K and about 795 K. The annealing temperature can range from about 700 K to about 830 K, and in order of increasing preference, from about 750 K to about 815 K, from about 775 K to about 815 K, and from about 775 K to about 800 K. The sulfur-depositing and annealing steps are continued -- concurrently, sequentially and/or cyclically -- until an epitaxial gallium sulfide film is formed. The period of time required to form the epitaxial layer will vary with temperature and with the manner in which sulfur is deposited. In a preferred process in which sulfur is cyclically deposited and annealed, the annealing between deposition steps is preferably carried out for at least about 1 minute, preferably at least about 3 minutes and more preferably at least about 5 minutes. The resulting epitaxial gallium sulfide film has a thickness of at least about 0.5 nm (5 A) , and preferably, a thickness ranging from about 0.5 nm (5 A) to about 10 nm (100 A) . In a preferred method, the gallium sulfide film consists essentially of Ga₂(S₂)₃. The gallium sulfide film may also be characterized as comprising a near-surface region having a thickness of at least about 0.5 nm (5 A) with the concentration of sulfur in the near surface region being at least about 2xl0²² atoms/cm³, and preferably at least about 4xl0²² atoms/cm³. The concentration of sulfur in the gallium sulfide film at depths beneath the near surface region is preferably at least about lxlO²² and most preferably at least about 2xl0²² atoms/cm³.

In another method for forming a gallium sulfide epitaxial film on a GaAs substrate, sulfur atoms are deposited onto an exposed surface of the GaAs substrate by exposing the substrate surface to a gaseous sulfur- containing molecule while photo-irradiating the sulfur- containing molecule at the exposed surface. This photo¬ chemical deposition can be effected using ultraviolet radiation having a wavelength shorter than about 500 nm. During the deposition of sulfur atoms, the temperature of the substrate surface can be less than about normal room temperature (about 300 K) . The substrate is annealed at a temperature ranging from about 750 K to about 830 K -- preferably from about 775 K to about 800 K -- for a period of time ranging from about 1 minute to about 2 hours and preferably for a period of time ranging from about 1 minute to about 1 hour. The annealing facilitates sulfur atom diffusion into the substrate and arsenic atom diffusion out of the substrate. The sulfur- depositing and annealing steps are preferably repeated in a cyclic manner until an epitaxial gallium sulfide film having a thickness of at least about 1 nm (10 A) forms in the substrate at its exposed surface.

In another aspect, the invention is directed to methods for the spatially selective formation of an epitaxial gallium sulfide film on a defined portion of a GaAs substrate. One spatially selective method comprises exposing a surface of a GaAs or gallium sulfide-coated substrate to a gaseous sulfur-containing molecule and selectively photo-irradiating the sulfur-containing molecule at a defined portion of the exposed surface. The selective photo-irradiation can be effected by masking the incident light source or by using a focused laser beam. Sulfur atoms deposit onto the defined portion of the substrate surface and diffuse into the substrate. Arsenic atoms diffuse out of the substrate. The localized in-diffusion of sulfur atoms and out- diffusion of arsenic atoms continues until an epitaxial gallium sulfide film forms in the substrate at its surface. An alternative spatially selective method comprises masking a portion of a surface of the GaAs substrate, and depositing sulfur onto the unmasked, exposed surface of the GaAs substrate. Sulfur diffuses into the substrate and arsenic diffuses out of the substrate until an epitaxial gallium sulfide film forms in the substrate at its exposed surface. The invention is directed, as well, to a method for depositing sulfur onto a GaAs or a gallium sulfide (e.g. Ga₂ (S₂) ₃) -coated substrate. In this method, a surface of the substrate is exposed to a gaseous sulfur-containing molecule, while either consecutively or concurrently photo-irradiating the sulfur-containing molecule. The exposed substrate is annealed at a temperature of at least about 700 K for at least about 1 minute.

The invention is directed, moreover, to a sulfur- passivated group III-V compound semiconductor wafer. In the preferred embodiment, the semiconductor wafer is a GaAs wafer comprising a GaAs substrate and an epitaxial gallium sulfide film overlying the substrate. The gallium sulfide film consists essentially of a gallium sulfide compound having an atomic mole ratio of gallium to sulfur of about 1:3 -- such as GaS₃ or Ga₂(S₂)₃. The epitaxial gallium sulfide film overlyies at least a portion of a surface of the substrate and has a thickness of at least about 0.5 nm (5 A) . The Ga₂(S₂)₃ preferably has a thickness of at least about 1 nm (10 A), and additionally or alternatively, comprises a near surface region having a thickness of about 0.5 nm (5 A) with the concentration of sulfur in the near surface region being at least about 2xl0²² atoms/cm³, and preferably at least about 4xl0²² atoms/cm³, and with a GaS_xAS_y transition region wherein the concentration of sulfur is less than about 2x10²².

GaAs and other III-V compound semiconductors passivated according to the methods of the present invention will find application in optoelectronics, high speed digital electronics, microwave monolithic integrated circuits for wireless communications, and other uses. Advantageously, the methods presented herein are compatible with existing semiconductor fabrication methodologies. These methods can be carried out on a commercial scale using commercially available reactant sources and using equipment known and available in the art . The methods of the invention can be employed to efficiently and relatively inexpensively form thick, stable passivating layers which have a crystal lattice structure that matches the lattice of the underlying substrate semiconductor material . The epitaxial nature of the resulting group III-sulfur passivated semiconductors confers advantages over non-epitaxial passivation layers of the prior art, particularly with respect to structural integrity and possibly lower defect entities. Other features and objects of the present invention will be in part apparent to those skilled in the art and in part pointed out hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS Figures 1(a) through 1(f) are schematic illustrations of one of the approaches for forming a gallium sulfide thin film on GaAs by the method of the invention.

Figure 2 is a set of plots of X-ray photoelectron spectra (XPS) for (a) clean GaAs (100) and (b-f) the GaAs surface following 1, 3, 8, 11, and 15 cycles, respectively, of S deposition (photo-assisted deposition from H₂S at 115 K for 3 minutes) and annealing (775 K for 5 minutes) . Figure 3 shows a set of X-ray photoelectron spectra (XPS) of (a) a Ga₂S₃ film used for calibration, (b) a gallium sulfide film formed on GaAs by 15 cycles of photo-assisted deposition of sulfur using H₂S and annealing as in Fig. 2, and (c) a gallium sulfide film formed on GaAs by hot-filament assisted deposition of sulfur (750 K) using H₂S as the S source gas.

Figures 4 (a) through 4 (d) show low energy electron diffraction (LEED) results. Fig. 4(a) shows a gallium sulfide thin film formed on GaAs (100) using a cyclic sulfur-depositing (photochemical deposition using H₂S) and annealing (700 K - 750 K) technique. Fig. 4(b) shows the same gallium sulfide film as in Fig 4 (a) after further annealing at 785 K for 20 minutes. Fig. 4(c) shows the same gallium sulfide film as in Fig. 4(b) after still further annealing at 795 K for 20 minutes. Fig. 4(d) shows the GaAs (100) - (2x1) S surface for comparison. An electron energy of 75 eV was used for Figure 4 (a) and 117 ± 2 eV for Figures 4 (b) through 4 (d) .

Figure 5 shows a set of thermal desorption spectra of AsS+ recorded after 1-8 cycles of sulfur deposition on GaAs at 115 K. Note the shift in the y-axis scale for spectra 7 and 8. The temperature ramp was 3 K/s .

Figures 6 (a) and 6 (b) show the coverage of sulfur on the GaAs (100) surface as a function of deposition cycle (both with and without photon irradiation) (Fig. 6(a)), and as a function of H₂S exposure (Fig. 6(b)) . In both figures, sulfur coverage is reported as the number of monolayers (ML) . In Fig. 6(b), H₂S exposure is in Langmuires (L) (10^"6 torr s) . Figure 7 shows a depth profile of a gallium sulfide film formed on the GaAs substrate by hot-filament assisted (1800 K) deposition of sulfur on a GaAs substrate (750 K) using H₂S. The S/Ga atomic ratios (solid circles) and the As/Ga atomic ratios (open circles) were obtained from XPS measurements after Ar⁺ sputtering (8 μA/cm²) for various times. The estimated depth is shown on top and assumes a sputtering yield of unity and lattice constants of those for GaAs.

The invention is described in further detail below with reference to the figures, in which like items are numbered the same throughout.

DETAILED DESCRIPTION OF THE INVENTION In the present invention, a passivating film of group Ill-sulfur material is formed on a III-V compound semiconductor substrate based on the interaction of the substrate with sulfur atoms. The sulfur atom replaces the group V atom from the substrate lattice to form a group III-sulfide film which is epitaxial or lattice-matched to the III-V substrate. Advantageously, this substitution reaction can be driven to occur beyond the monolayer region, such that the resulting group III-S epitaxial film is an effective and stable passivating layer for the III-V compound semiconductor. Although the present invention is described herein with respect to its preferred embodiment -- the formation of a gallium sulfide passivating film on a GaAs substrate -- such description is to be considered exemplary of the principles of the invention, and non-limiting with respect to the scope of III-V compound semiconductor materials and III-S passivating films which the invention encompasses . The principles described herein may be applied to form gallium sulfide or other group III- sulfide passivating films on GaAs or on other III-V compound semiconductors or alloys thereof, such as InP, InAs, InGaAs, GaAlAs, InGaAsP, etc.

Briefly, sulfur atoms and/or activated sulfur species are deposited onto an exposed surface of the GaAs substrate. Where activated sulfur species are deposited, these species serve as a ready source of sulfur atoms at the substrate surface. The sulfur-deposited substrate is annealed at conditions which cause sulfur atoms to diffuse into the substrate and replace arsenic atoms in the GaAs crystal lattice, and to cause arsenic atoms to diffuse out of the substrate. When a sufficient amount of sulfur atoms are deposited and diffusion of sulfur atoms into the GaAs substrate is continued for a sufficient amount of time at a temperature greater than about 700 K, the result is a cubic phase, epitaxial, crystalline gallium sulfide thin-film on the GaAs substrate. Without being bound by theory, it appears that annealing temperatures less than about 700 K are not sufficient to result in the formation of an epitaxial layer within commercially reasonable times -- most likely due to inadequate in-diffusion of sulfur atoms, inadequate driving force for the S/As exchange reaction, and/or inadequate out-diffusion of arsenic atoms. Advantageously, such in si tu formation of epitaxial gallium sulfide can be accomplished using conventional precursors. In a preferred embodiment, the in-diffusion of sulfur, the sulfur-arsenic replacement reaction and the out-diffusion of arsenic is facilitated by photochemically depositing sulfur atoms onto the substrate, annealing, and then repeatedly photo- depositing sulfur and annealing in a cyclic manner. Without being bound by theory, the activation of a sulfur-containing molecule, by photo-chemical or other means, is believed to generate an activated sulfur species and/or sulfur atom present on the surface of the substrate. The activated sulfur species can be a sulfur- containing molecule in an excited energy state, a sulfur- containing free radical or a sulfur-containing ion.

Sulfur atoms can be in the ground state, in an excited energy state or in an ionic state. Sulfur atoms deposited directly or derived from the activated sulfur moiety diffuse into the substrate and induce an extensive replacement reaction beyond the top-most surface of the substrate. The resulting gallium sulfide film is epitaxial and forms in thicknesses ranging from about 0.5 nm (5 A) to about 6 nm (60 A) . The photochemical deposition method can be extended to prepare epitaxial gallium sulfide films having a thickness of about 10 nm (100 A) . Alternative sulfur deposition methods, such as thermal activation methods, can also be used to prepare epitaxial gallium sulfide films having a thickness of about 10 nm (100 A) over a GaAs substrate. The passivating effect of such epitaxial gallium sulfide films is useful for a variety of device fabrication and integrated circuit technologies.

Referring to Figures 1(a) through 1(e) , the GaAs substrate 10 is preferably crystalline GaAs which, in the (100) direction, comprises alternating layers of gallium atoms 20 and arsenic atoms 30 arranged in a cubic lattice. (Fig. 1(a)) . The GaAs substrate 10 has a surface 12 which can be Ga-terminated or As-terminated. The substrate surface 12 is preferably an exposed Ga-rich GaAs surface. While a <100> orientation is preferred, the GaAs substrate can also have a <110>, <lll> or other orientations.

Sulfur atoms 40 are deposited onto the surface 12 of the GaAs substrate 10. (Fig. 1(b)) . The sulfur atoms are preferably deposited onto the substrate surface by exposing the surface to an activated sulfur-containing molecule. In a preferred process, sulfur is photochemically deposited by exposing the GaAs substrate surface to a sulfur-containing molecule, photo- irradiating the sulfur-containing molecule and/or the surface, and causing sulfur atoms to deposit onto the substrate surface. Although the substrate surface is preferably photo-irradiated at the same time that the surface is being dosed with a sulfur-containing molecule (Example 1) , it is also effective to dose the surface with a sulfur-containing molecule first and then photo- irradiate the surface (Example 6) . Moreover, other methods for depositing activated sulfur species and/or sulfur atoms onto the surface of the GaAs substrate can be employed. Dosing concurrently with thermal activation using a high temperature source (e.g. hot filament) in close proximity to the dosed substrate is a preferred alternative method. (See Example 7) . Less preferred methods for depositing activated sulfur species and/or sulfur atoms onto the surface of the GaAs substrate include exposing the surface to a sulfur-containing molecule and activating the sulfur-containing molecule using plasma excitation, electron irradiation, etc.

The preferred photochemical method described herein offers advantages over conventional sulfur deposition methods. Both the rate of deposition and the extent of sulfur coverage on the substrate surface can be greatly enhanced by photo-chemically depositing the sulfur. Additionally, lower substrate temperatures can be used for depositing activated sulfur-containing species or sulfur atoms. For example, when gaseous H₂S is photo- irradiated, a reactive sulfur species such as HS and/or sulfur atoms are generated even at temperatures less than about 30OK. Example 6 compares photochemical and conventional thermal methods for depositing sulfur onto a GaAs substrate.

In the photo-deposition process, the sulfur- containing molecule is preferably a commercially available gaseous molecule such as H₂S. Other sulfur- containing source gasses include carbonyl sulfide (SCO) , carbon disulfide (CS₂) , dimethyl sulfide ((CH₃)₂S) , sulfur tetrafluoride (SF₄) , sulfur hexafluoride (SF₆) , etc. The substrate can be exposed to the gaseous molecules by admitting the gaseous molecule into a chamber suitable for such deposition and housing the GaAs substrate. The sulfur-containing molecule may be irradiated in the chamber using ultraviolet light (e.g. from an eximer laser or an Hg arc lamp) . The photon energy is preferably about or greater than about 2 eV with a preferred average power of about or greater than about 1 W. Without being bound by theory, irradiation of the sulfur-containing molecule may occur while the molecule is at the substrate surface 12 or in the atmosphere above the surface 12. The photo-energy of the incident light activates the sulfur-containing molecule, and facilitates adsorption of sulfur atoms onto the substrate surface. The substrate temperature during irradiation is not narrowly critical, and can range from about as low as cryogenically achievable (e.g. about 77 K using liquid nitrogen) to about 830 K. When a cyclic deposition- annealing process (described below) is used for forming the gallium sulfide film, the temperature of the substrate during irradiation preferably ranges from about 77 K to about 500 K, and more preferably, from about 77 K to about room temperature. When the gallium sulfide film is formed by a non-cyclic, simultaneous deposition and annealing process (described below) , the temperature of the substrate during irradiation preferably ranges from about 700 K to about 830K, and may vary within this range throughout the process. The pressure during irradiation is also not narrowly critical. While ultra-high-vacuum pressures (e.g. about lxlO^"10 torr or 1.33322xl0^"5 Pa) are preferred for analytic evaluation of the resulting gallium sulfide film, higher pressures are preferred for commercial applications of the present invention. Additionally, relatively higher pressures are preferred at higher temperatures than at lower temperatures. A preferred pressure range is from about lxlO^"6 torr to about 1 torr. After irradiation, sulfur atoms deposit onto the surface 12 of the GaAs substrate 10. Without being bound by theory, H₂S adsorbs in both molecular and dissociative forms onto the surface of a GaAs (100) substrate at a substrate temperature of about 90 K. Photo-irradiation electronically excites adsorbed H₂S and leads to hydrogen desorption from the surface, at relatively lower temperatures than desorption would occur without such irradiative excitation. In contrast, where H₂S is thermally deposited without photo-irradiation: molecular H₂S dissociates on the surface at about 200 K; some of the surface HS species (formed from dissociated H₂S and bonded to both Ga and As atoms) recombine and desorb from the surface at about 300 K; and the remaining surface HS species irreversibly dissociate to surface S at substrate temperatures of about 550 K following H₂ desorption at about 500 K. As discussed below, the surface deposition of sulfur atoms after photo- irradiation may be facilitated by the subsequent or simultaneous annealing of the substrate used to effect diffusion.

The spatially-selective, localized formation of gallium sulfide films on the surface of a GaAs substrate is advantageously achievable based on the methods presented herein. For example, sulfur-containing molecules can be spatially-selectively irradiated using a masked photon-source projection or a focused laser beam (e.g. laser writing) as the excitation energy. Alternatively, a portion of the GaAs surface may be masked (for example, with silicon oxide) with the unmasked exposed portion being open for formation of a gallium sulfide film. The mask may be removed after the selective growth of gallium sulfide. Advantageously, the masking of the photon-source eliminates the need to remove a surface maskant by subsequent etching.

Spatially selective formation of a gallium sulfide passivating film finds application in device manufacturing processes, including applications where other materials are deposited or formed over the localized passivating layer.

The sulfur atoms 40 on the surface 12 of the GaAs substrate 10 are allowed to diffuse into the substrate 10 while arsenic atoms 30 diffuse out of the substrate 10. The sulfur in-diffusion, S/As exchange reaction, and arsenic out-diffusion can occur simultaneously with the deposition of sulfur atoms onto the substrate or sequentially to such deposition. In either case, the S/As diffusion exchange is preferably effected by annealing the sulfur-deposited substrate (Fig. 1(b)) at a temperature ranging from about 700 K to about 830 K, more preferably from about 700 K to about 800 K, even more preferably from about 750 K to about 815 K, and still more preferably from about 775 K to about 800 K. An annealing temperature of about 800 K is a most preferred temperature. The annealing time is not narrowly critical, and can range from about 1 minute to several hours or longer, depending on the annealing temperature. At temperatures ranging from about 700 K to about 830 K, the annealing time preferably ranges from about 1 minute to about four hours, more preferably from about 5 minutes to about 1 hour and most preferably from about 5 minutes to about 30 minutes. Without being bound by theory, heating to temperatures in this range causes some of the sulfur atoms 40 present on the surface 12 of the substrate 10 to diffuse into the GaAs lattice to replace arsenic atoms in the GaAs lattice. (Fig. 1(c)) . The arsenic atoms 30 diffuse out of the substrate lattice, and may combine with sulfur atoms 40 on the surface 12 of the substrate as arsenic-sulfide. The AsS species 50, along with S₂ 42 from extra sulfur atoms 40, desorb into the gas phase during annealing, therby forming an epitaxial gallium sulfide passivating layer over the GaAs substrate.

In a preferred process for preparing a gallium sulfide film on a GaAs substrate, the sulfur in- diffusion, S/As exchange reaction and arsenic out- diffusion is effected by sequentially photochemically depositing sulfur atoms onto the surface of the substrate as described above, annealing as described above and then repeating the steps of depositing sulfur onto the substrate and annealing at least once. Multiple deposition-annealing cycles result in an extensive substitution of sulfur atoms for arsenic atoms in the GaAs crystalline lattice (Fig. 1(d)) , and ultimately in the formation of an epitaxial gallium sulfide layer. (Fig. 1(e)) . In an alternative process, the deposition of multiple layers of sulfur atoms onto the GaAs substrate, the in-diffusion of sulfur atoms, replacement of arsenic atoms, and out-diffusion of arsenic atoms can occur simultaneously. In this alternative process, while the particular diffusion-affecting parameters are generally not narrowly critical, the simultaneous deposition and annealing may be carried out at temperatures greater than about 700 K to ensure out- diffusion of arsenic-sulfide. Relatively higher pressures of sulfur-containing source gasses are required at such higher temperatures. Regardless of whether the deposition and diffusion steps are carried out sequentially or simultaneously, the surface- photodissociation of H₂S leads to the deposition of sulfur atoms, either directly or through an activated H₂S molecule, which diffuses into the GaAs surface.

The resulting gallium sulfide film 15 comprises a relatively pure gallium sulfide lattice structure which, based on LEED pictures, is crystalline and epitaxial to the GaAs substrate. A comparison of the XPS spectra of the gallium sulfide film 15 to that of a calibration Ga₂S₃ spectra, shown in Fig. 3, demonstrates that the near surface region of the formed gallium sulfide film has a stoichiometry of GaS₃ with an atomic Ga:S mole ratio of 1:3. Based on electron counting rules, the stable solid compound is likely in the form of a polysulfide (e.g.

Ga₂(S₂)₃) . The gallium sulfide film 15 forms in si tu in the GaAs substrate at its exposed surface by replacing As atoms with S atoms. The thickness of the passivating gallium sulfide layer can range from about 0.5 nm (5 A) to about 10 nm (100 A) , and preferably is at least about 1 nm (10 A) thick. As shown in Fig. 1(f) , the gallium sulfide film 15 includes a near-surface region 16, defined herein as being the region of the gallium sulfide film 15 which is most near the surface of the gallium sulfide film and having a thickness which is the same as the thickness of a unit cell of crystalline gallium sulfide: about 0.5 nm (5 A) to about 0.6 nm (6 A) . The concentration of sulfur atoms in the near surface region 16 of the gallium sulfide film 15 is preferably at least about 2xl0²² atoms/cm³ and more preferably at least about 4xl0²² atoms/cm³. The concentration of sulfur in the gallium sulfide film at depths beneath the near surface region is preferably at least about lxlO²² and most preferably at least about 2xl0²² atoms/cm³. However, the concentration of sulfur in the gallium sulfide film may vary continuously with increasing depth, as illustrated by the depth profile of a thick film formed via hot- filament assisted deposition and annealing (Fig. 7) . As a non-limiting example, in a gallium sulfide passivated GaAs semiconductor wafer which comprises a 2 nm (20 A) thick gallium sulfide film, the surface-most 1.5 nm (15 A) -- including the near surface region -- may have a sulfur concentration of at least about 2-4xl0²² atoms/cm³ and the 0.5 nm (5 A) of the gallium sulfide film immediately underlying this top-most 1.5 nm (15 A) may have a sulfur concentration of at least about l-2xl0²² atoms/cm³. The depth of the gallium sulfide film having a higher sulfur concentration (2xl0²² atoms/cm³) may be any depth which is below the near surface region. A transition region 17 lies between the gallium sulfide film 15 and the bulk region 19 of the GaAs substrate 10. The transition region 17 is defined by the extent to which arsenic atoms are replaced by sulfur atoms, and is represented by the formula GaSχAs_y. For purposes of the present invention, the transition region 17 includes GaS_χAs_y wherein the concentration of sulfur is less than about 2xl0²².

The following examples illustrate the principles and advantages of the invention. EXAMPLES The experiments set forth in Examples 1-5 were performed in an ultra-high vacuum (UHV) chamber. The system was equipped with a low energy electron diffraction (LEED) apparatus, an x-ray photoelectron spectrometer (XPS) , and a quadrupole mass spectrometer (QMS) . The GaAs sample was a slice (15x10x0.4 mm) of an n-type GaAs (100) wafer with Te-doping level of 10¹⁷/cm³. The sample was cooled with liquid nitrogen and resistively heated through a Ta film deposited to the back of the sample. Cleaning was achieved by Ar⁺ sputtering and annealing to yield a "4x6" LEED pattern, corresponding to a Ga-rich GaAs surface. H₂S was dosed through a collimated doser with a calibrated leak. 193 nm laser irradiation was provided from an ArF excimer laser. In XPS measurement, x-ray was from an Al anode (1486.6 eV) incident at surface normal. Photoelectrons were detected at 60° with respect to surface normal. All binding energies were referenced to that of As_3d(5/2, (not resolved from As_3d(3/2)) from GaAs, which was taken to be 40.7 eV.

The experimental conditions for Example 6 are presented therein.

Example 1: Cyclic Photon-Enhanced Deposition of H_?S / Annealing to Form Epitaxial Gallium Sulfide Film on GaAs Substrate

The GaAs substrate was exposed to H₂S and simultaneously photo-irradiated with 193 nm laser radiation at a temperature of 115 K to deposit reactive sulfur species or sulfur atoms on the surface of the substrate. The sulfur-deposited substrate was subsequently annealed at 700-800 K to cause the sulfur atoms to diffuse into the GaAs substrate. This photochemical deposition and annealing cycle was repeated, with each cycle consisting of exposing the GaAs surface to H₂S (5xl0¹³ molecules/cm²sec) and 193 nm laser irradiation (50 Hz, 5 mJ/cm²pulse) for 3 minutes at 115 K, followed by annealing at 775 K for 5 minutes.

Example 2 : XPS Analysis and Confirmation of S-As Replacement Reaction Mechanism

Analysis of the gallium sulfide film prepared in Example 1 by XPS firmly established the As-S replacement reaction mechanism wherein sulfur atoms replace arsenic atoms from the GaAs lattice. Figure 2 shows a set of X-ray photoelectron spectra (XPS) of: (a) clean

GaAs(lOO) ; (b-f) the GaAs surface after 1, 3, 8, 11 and 15 cycles, respectfully, of laser-assisted sulfur deposition and annealing. Each deposition cycle consisted of: (1) simultaneously exposing the GaAs surface to H₂S (5xl0¹³ molecules / cm² sec) and to 193 nm laser irradiation (50 Hz, 5mJ / cm² pulse) for three minutes at 115 K; and then annealing at 775 K for five minutes. With increased sulfur deposition and annealing cycles, the Ga intensity remained constant, while the sulfur intensity grew at the expense of As. After about 15 cycles of deposition (spectrum (f) in Fig. 2) the As3p intensity becomes negligible and the spectrum is dominated by Ga and S. The binding energies of Ga3p(3/2) and Ga3p(l/2) in gallium sulfide, spectrum (f) , were 105.2 ± 0.2 eV and 108.6 ± 0.2 eV, respectively, which were 0.8 eV higher than the corresponding values for GaAs. Within experimental uncertainty, these values were similar to those of cubic phase CVD GaS (Mclnnes, et al . , "Enhancement of photoluminescence intensity of GaAs with cubic phase GaS chemical vapor deposited using a structurally designed single-source precursor," Appl. Phys. Lett. 62, 711 [1993]) and to hexagonal GaS or Ga₂S₃ (McGuire et al . , "Study of core electron binding energies in some group Ilia, Vb, and VIb compounds," Inorganic Chemistry, 12, 2450 [1973]) . The binding energy of S2p in the gallium sulfide film was 162.1 ± 0.2 eV.

Figure 3 compares the XPS spectra of the gallium sulfide film 15 to that of a calibration Ga₂(S₂)₃ spectra. Based on these spectra, the near surface region of the formed gallium sulfide film has a stoichiometry of GaS₃, and is likely in the form of a polysulfide such as Ga₂(S₂)₃) .

Example 3 : Thickness of Gallium Sulfide Film Based on XPS Analysis Data

The thickness of the gallium sulfide film prepared via photo-assisted decomposition of H₂S and annealing in Example 1 was estimated from XPS data. The lower limit was obtained from the attenuation of the As3p intensity, assuming an abrupt interface between gallium sulfide and GaAs. After 15 cycles of deposition, the As3p intensity decreased to ≤ 10% that of the clean GaAs surface. Using a universal mean free path (L) of 19 A at a kinetic energy of 1340 eV and a detection angle of 60°, the gallium sulfide film thickness was ≥ 22 A. The actual film was thicker, involving a slowly varying transition region with a composition of GaSχAs_y.

The thickness of the gallium sulfide film prepared via thermal-activated decomposition of H₂S and annealing was determined to be at least about 10 nm (100 A) . (See Ex. 7) .

Example 4 : Crystalline Quality of Gallium Sulfide Film and Variation Thereof Based on Annealing Temperature A gallium sulfide film was formed as per Example 1, and the crystalline quality of the films were determined by low energy electron diffraction (LEED) . Figure 4 (a) shows a typical LEED picture for an as-deposited gallium sulfide film on GaAs (100) (-15 cycles of deposition) prepared using cyclic depositing / annealing steps with the annealing temperature ranging from about 700 K to about 750 K during each cycle. The four fold symmetry clearly established the epitaxial nature, but the diffusive spots revealed disordering and small domain size within the film. The crystalline quality of this as-deposited film was significantly improved by further annealing at 785 K (Fig. 4(b)) , which shows a (2x1) reconstructed surface. Still further annealing at 795 K yielded a very sharp (2x1) diffraction pattern, (Fig. 4(c)), indicating excellent crystalline quality. This diffraction pattern was similar to the (2x1) pattern for one monolayer sulfur covered GaAs (100) , (Fig. 4(d)) , which was obtained from repeated H₂S adsorption and annealing without laser irradiation. When the gallium sulfide filmed formed by the aforementioned cyclic depositing / annealing steps was supplmented by further annealing at temperatures up to about 800 K, XPS measurement showed no significant change in the surface was observed, thereby indicating the stability of the film. That is, further S-As exchange between this gallium sulfide single crystalline film and the GaAs substrate appeared to be negligible in this temperature region.

Example 5 : Gallium Sulfide Growth Rate versus Film Thickness: Confirmation of Diffusion Mechanism

Thermal desorption measurement studies were performed to confirm the out-diffusion of arsenic. After laser-assisted photochemical sulfur deposition on GaAs (100) at low temperatures (100 - 300 K) , thermal desorption spectroscopy (TDS) measurement shows the desorption of S₂ at -400 K (not shown) , arsenic sulfide species at 400 - 800 K, and Ga-containing species above about 830 K (not shown) . The desorption of Ga containing species above 830 K corresponded to the decomposition/desorption of the gallium sulfide film. Figure 5 shows a set of thermal desorption spectra of AsS+ taken after various cycles of S deposition. The exact stoichiometry of the arsenic sulfide species was not determined. However, as the major quadrupole mass spectrometer signal is AsS+, with a cracking signal of

As+, the assigned stoichiometry was AsS. With increasing deposition/annealing cycles, the amount of AsS desorption decreased while the peak temperature increased (from about 500 K for the 1st cycle to about 650 K for the 8th) , in excellent agreement with a diffusion model. Moreover, this data demonstrates that annealing temperatures of at least about 700 K are advantageous to cause adequate in-diffusion of sulfur atoms, to drive the sulfur-arsenic exchange reation and/or to cause adequate out-diffusion of arsenic atoms, and ultimately, to create a relatively thick epitaxial passivating layer on a GaAs substrate.

Example 6: Comparison of Photo-chemical and Conventional Thermal Methods for Depositing Sulfur onto a GaAs Substrate

Experiments were performed in an ultrahigh vacuum (UHV) chambers (base pressure lxlO^"10 torr) . The main feature of the chamber was a VSW-EA125 hemispherical electron energy analyzer, equipped with a five channel detector, for soft X-ray photoelectron spectroscopy

(SXPS) . Excitation radiation (hn = 260 eV) for the core level spectroscopy was obtained from the spherical grating monochromator on the U13UA beam line at the National Synchrotron Light Source. The angles of light incidence and electron detection were 35° and 30°, respectively, from surface normal. GaAs samples (15x10x0.5 mm) were obtained as slices of a semi-insulating GaAs (100) wafer. Each sample was held by the edges with two Ta clips and attached to a liquid- nitrogen cooled sample holder. A 3000 A Ta film was deposited on the back of the sample for resistive heating. Temperature was measured by a chrome1-alumel thermocouple spot-welded to a small Ta clip, which was attached to the bottom edge of the sample. The GaAs samples were cleaned by Ar+ sputtering (500-600 eV) at 400 K and annealing at 750 K - 800 K, corresponding to a Ga-rich GaAs (100) surface. Absolute sulfur coverage was determined from the decrease in the Ga3d and As3d soft x- ray photoelectron spectra (SXPS) peak intensity from substrate GaAs following saturation H₂S adsorption at 90 K. All other sulfur coverages were referenced to this surface based on integrated S2p SXPS peak areas.

Based on a comparison of three approaches for depositing sulfur, both the rate and extent of sulfur deposition was greatly enhanced by photo-irradiation. In a first thermal deposition process -- without photo- irradiation -- the Ga-rich GaAs surface was exposed to H₂S (about 10^~7 torr) at a constant substrate temperature of 550 K. This temperature had previously been determined to be the optimal temperature for efficient thermal deposition of sulfur. (Foord and FitzGerald, "The Adsorption and Thermal Decomposition of Hydrogen Sulphide on GaAs (100) ", Surface Science 106, 29-36 [1994] ) . The surface was exposed to H₂S by admitting the gas through an aperture-controlled doser with the doser tube (ID -1 cm) terminated at -1mm from the sample surface. Reproducible dosing was obtained by controlling the gas pressure behind the aperture. As shown in Figure 6(b), a saturation sulfur coverage of about 0.8 monolayer (ML) was reached at exposures greater than about 500 Langmuires .

In a second thermal deposition approach, -- again without photo-irradiation -- the Ga-rich GaAs surface was exposed to 20 Langmuires H₂S at 90 K followed by annealing at 750 K for three minutes. This dosing-annealing cycle was repeated. Exposure to H₂S was as described above. As shown in Figure 6(a) (triangles) , this thermal deposition process resulted in less than about 0.6 ML of sulfur being deposited. Comparison of this data to the data in Fig. 6(b) demonstrates that this thermal deposition process was less efficient than thermal deposition at a constant substrate temperature of 550 K.

In a photo-deposition approach, the Ga-rich GaAs surface was exposed to 20 Langmuires H₂S at a substrate temperature of 90 K for H₂S adsorption, as described above. The sample was then irradiated at 90 K with white light synchrotron radiation (0 - 500 eV) with an integrated photon flux of about 10¹⁵-10¹⁶ photons/cm²s. The irradiation was carried out until negligible molecular H₂S remained on the surface (about -100 seconds) . After irradiation, the substrate was annealed at 750 K for three minutes. The dose-irradiation- annealing cycle was then repeated. As shown in Figure 6 (a) (circles) , sulfur coverage well in excess of 1 ML was obtained. Comparison to the data from the thermal deposition approaches demonstrates that the rate and extent of photo-assisted sulfur deposition was significantly enhanced relative to the processes which did not photo-activate the sulfur-containing molecules.

Example 7: Hot-Filament Assisted Deposition of Sulfur onto a GaAs Substrate

The efficient deposition of sulfur on GaAs and the growth of an epitaxial gallium sulfide film on the substrate surface was achieved using thermally activated H₂S. Briefly, a GaAs substrate surface was exposed to H₂S at a pressure of about 0.02 torr with a hot W filament (about 1800 K) placed at approximately 8 mm above the GaAs substrate during the exposure to H₂S. The thermal- activated deposition was continued for a period of about 10 minutes. The filament activated the H₂S and allowed for deposiiton of sulfur onto the surface of the substrate. The surface temperature was about 750 K during deposition of sulfur, thereby allowing for simultaneous annealing during deposition.

The XPS spectra of the resulting gallium sulfide epitaxial layer is shown in Fig. 3 as spectrum (c) .

Based on this data, the surface and near-surface region is exclusive gallium sulfide -- Ga₂(S₂)₃. A depth profile of the resulting gallium sulfide film (Fig. 7) shows that As is detectable only at a depth beyond 100 A (10 nm) -- indicating that the gallium sulfide film is at least about 100 A (10 nm) thick. At depths greater than about 100 A (10 nm) , and extending to a thickness of about 600 A (60 nm) , the sulfide film is shown to be properly characterized by a formula of GaSχAs_y.

In light of the detailed description of the invention and the examples presented above, it can be appreciated that the several objects of the invention are achieved. The explanations and illustrations presented herein are intended to acquaint others skilled in the art with the invention, its principles, and its practical application. Those skilled in the art may adapt and apply the invention in its numerous forms, as may be best suited to the requirements of a particular use. Accordingly, the specific embodiments of the present invention as set forth are not intended as being exhaustive or limiting of the invention.

Claims

WE CLAIM:

1. A method for forming a gallium sulfide epitaxial film in si tu in a GaAs substrate, the method comprising depositing sulfur atoms onto a surface of the GaAs substrate, and annealing the sulfur-deposited substrate at a temperature of at least about 700 K, the sulfur-depositing and annealing steps being continued at least until an epitaxial gallium sulfide film having a thickness of at least about 0.5 nm (5 A) is formed in the GaAs substrate at its surface.

2. The method of claim 1 wherein the sulfur- depositing and annealing steps are continued at least until an epitaxial gallium sulfide film having a thickness of at least about 1 nm (10 A) is formed.

3. The method of claim 1 wherein the sulfur- depositing and annealing steps are continued at least until an epitaxial gallium sulfide film having a thickness of at least about 6 nm (60 A) is formed.

4. The method of claim 1 wherein the surface of the GaAs substrate onto which sulfur is deposited has a <100> orientation.

5. The method of claim 1 wherein the gallium sulfide film consists essentially of Ga₂(S₂)₃.

6. The method of claim 1 wherein the sulfur- depositing and annealing steps are continued at least until an epitaxial gallium sulfide film having a thickness of at least about 1 nm (10 A) is formed, the gallium sulfide film comprising a near-surface region having a thickness of at least about 0.5 nm (5 A) , the concentration of sulfur in the near-surface region being at least about 4xl0²² atoms/cm³.

7. The method of claim 1 wherein the sulfur atoms are deposited onto the substrate surface by exposing the surface to an activated sulfur-containing molecule.

8. The method of claim 1 wherein the sulfur atoms are photochemically deposited onto the substrate surface.

9. The method of claim 1 wherein the sulfur is deposited onto the substrate surface by exposing the surface to a gaseous sulfur-containing molecule and photo-irradiating the sulfur-containing molecule.

10. The method of claim 9 wherein the gaseous sulfur-containing molecule is spatially-selectively irradiated.

11. The method of claim 1 further comprising masking a portion of a surface of the gallium arsenide substrate, wherein the sulfur atoms are deposited onto the exposed, unmasked portion of the substrate surface.

12. The method of claim 1 wherein the sulfur- deposited substrate is annealed at a temperature of at least about 750 K.

13. The method of claim 1 wherein the sulfur- deposited substrate is annealed at a temperature of at least about 775 K.

14. The method of claim 1 wherein the sulfur- deposited substrate is annealed at a temperature ranging from about 750 K to about 830 K.

15. The method of claim 1 wherein the sulfur- deposited substrate is annealed at a temperature ranging from about 775 K to about 800 K.

16. The method of claim 1 wherein the sulfur atoms are deposited onto the substrate at a temperature of greater than about 700 K and the sulfur-deposited substrate is annealed at a temperature ranging from about 700 K to about 830 K.

17. The method of claim 1 wherein the sulfur atoms are deposited onto the substrate at a temperature of less than about 300 K and the sulfur-deposited substrate is subsequently annealed at a temperature ranging from about 700 K to about 830 K.

18. The method of claim 17 wherein the sulfur- deposited substrate is annealed at a temperature ranging from about 750 K to about 815 K for at least about 1 minute and the sulfur-depositing and annealing steps are repeated at least five times.

19. The method of claim 17 wherein the sulfur- deposited substrate is annealed at a temperature ranging from about 775 K to about 800 K for at least about 3 minutes and the sulfur-depositing and annealing steps are repeated at least 10 times.

20. A method for forming a gallium sulfide epitaxial film on a GaAs substrate, the method comprising depositing sulfur atoms onto an exposed surface of the GaAs substrate by exposing the substrate surface to a gaseous sulfur-containing molecule and photo-irradiating the sulfur-containing molecule at the exposed surface using ultraviolet radiation having a wavelength shorter than about 500 nm at a temperature of less than about 300

K, annealing the sulfur-deposited substrate in a cyclic-annealing step at a temperature ranging from about

750 K to about 800 K for a period of time ranging from about 3 minutes to about 1 hour, and repeating the sulfur-depositing and cyclic-annealing steps at least 10 times.

21. The method of claim 20 further comprising annealing the sulfur-deposited substrate after the sulfur-depositing and cyclic-annealing steps have been repeated at least 10 times, the further annealing being at a temperature ranging from about 775 K to about 800 K for at least about 15 minutes.

22. A method for depositing sulfur onto a GaAs substrate, the method comprising exposing a surface of the GaAs substrate to a gaseous sulfur-containing molecule, photo-irradiating the sulfur-containing molecule, and annealing the exposed substrate at a temperature ranging from about 700 K to about 830 K for at least about 1 minute.

23. The method of claim 22 wherein the sulfur- containing molecule is photo-irradiated at the substrate surface.

24. The method of claim 22 wherein the steps of exposing the substrate surface to a sulfur-containing molecule and photo-irradiating the sulfur-containing molecule are carried out concurrently.

25. The method of claim 22 wherein the sulfur- containing molecule is photo-irradiated using ultraviolet radiation having a wavelength shorter than about 500 nm.

26. A passivated GaAs semiconductor wafer comprising a GaAs substrate, an epitaxial gallium sulfide film overlying at least a portion of a surface of the substrate, the gallium sulfide film consisting essentially of gallium and sulfur atoms in an atomic ratio of Ga:S of about 1:3 and having a thickness of at least about 0.5 nm (5 A) .

27. The passivated GaAs semiconductor of claim 26 wherein the gallium sulfide film consists essentially of Ga₂ (S₂) ₃.