CA1332142C - Implantation and electrical activation of dopants into monocrystalline silicon carbide - Google Patents

Abstract

Abstract The invention is a method of ion implantation of dopant ions into a substrate of silicon carbide. In the method, the implantation takes place at elevated temperatures, following which the substrate may be oxidized or annealed.

Description

1332142 - ~
IMPLANTATION AND ELECTRICAL ACTIVATION
OF DOPANTS INTO MONOCRYSTALLINE SILICON CARBIDE ~-~
Field of the Invention The present invention relates to methods ~;
of forming electrically activated semiconductive silicon carbide and to the production of electrical devices from the electrically active silicon carbide. ~
Backaround ~ -Semiconductors are materials generally defined as having electrical conductivities somewhat between the high conductivity characteristics of ~-metals, and the low conductivity characteristics of insulators. Since the invention of the transistor, the development of electrical devices based on semiconductors has revolutionized the electronics industry.
At the present time, silicon remains the most common semiconductor material for doping and ;~
device~manufacture, although in recent years much interest~has been generated in other semiconductor 20~ ;materials including gallium arsenide (GaAs) and indium~phosphide (InPj. Many techniques exist for ;prod w ing~pure crystal~ of these basic elements and `compounds and~for fabricating them into devices and circuits.
~?~ ~ 25 ~ ~ Another material toward which much interest has been directed, but for which limited sucaess in produaing practical crystals and devices has been aahieved, is silicon carbide (SiC). As a semiconductor material, silicon carbide of~ers a number of advantageous characteristics which have -~
`~ long been recognized, one of which is its high -' 1332~2 thermal stability. As an example, although silicon vaporizes at temperatures of approximately 1400DC, silicon carbide remains stable at temperatures approaching 2800C. Fundamentally, this means that silicon carbide can exist as a solid at extremely high temperatures at which silicon--and siiicon based electronic devices- would not only be useless, but completely destroyed.
Second, silicon carbide has a relatively wide band gap, i.e. the energy difference between its valence and conduction bands, of approximately 2.2 electron volts (eV) (beta) or 2.86c (6H ~
alpha). Thi~ is a rather large gap in comparison to that of silicon (l.leV) and means that electrons will have less tendency to move from the valence band to the conduction band solely on the basis of -~
thermal excitation. In practical terms, this allows ~; for silicon carbide-based devices to operate at ; higher temperatures than equivalent silicon-based ;` 20 devices. -~
,," ~ . . .
Additionally, silicon carbide has a high thermal conductivity, a low dielectric constant, a high breakdown electric field, and a high saturated electron drift velocity, meaning that at high electric field levels devices made from it can perform at higher speeds than devices made from any of the other conventional semiconductor materials.
Because of all these inherent characteristics, silicon carbide has been a 30 ! principal and perennial candidate material for application at high temperature, high power, and high speed requirements.
In order to produce a useful electronic device from a semiconductor such as silicon carbide, however, the semiconductor must have some capability ~; for allowing the flow of conducting species from place to place. The two most common species for .'.-:.~' carrying charge are electrons and holes. Electrons are one of the fundamental subatomic particles carrying negative charge, while ~holes~ represent an -~
energy level position within an atom where an electron could be placed, but for some reason is temporarily or permanently absent. Because holes are left behind when electrons move, holes can also -be thought of as moving from place to place and as carrying positive charge.
In silicon carbide, both the silicon atoms and the carbon atoms have identical valence (or "outer shell") electron populations: i.e., four valence electrons. Other than crystal lattice defects and ordiinary thermal population of different energy levels by the electrons, there is no encouragement for electrons or corresponding holes to move from atom to atom, and thereby carry a flow of current. If, however, an appropriate number of slightly different atoms can be added to the crystal, for example aluminum (Al), phosphorous (P), or nitrogen (N), a more conductive material will - -result. The greater conductivity results because atoms such as nitrogen have five valence electrons, while those such as aluminum have only three. Thus, the presence of some nitrogen atoms in a silicon carbide crystal provides a number of extra electrons which would not be present in a purer silicon oarbide crystal. These extra electrons can be ;;
encouraged to move from the nitrogen atoms to empty electron positions in the silicon or carbon atoms, resulting in a flow of current. In a similar but oppositei manner, the presence of atoms such as aluminum which have only three valence electrons ~`~ provides available electron positions into which ~;~ 35 other electrons can move from the silicon or carbon atoms, and thereby carry current.

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In terms familiar to the semiconductor industry, such added atoms are referred to as ndopants, n and the process of introducing them into semiconductor materials is known as ~doping. n By doping a semiconductor material such as silicon carbide with either atoms with more valence electrons (n-type doping) or fewer valence electrons, (p-type doping): a semiconductor material can result which has certain specific electrical characteristics, and through which current can be made to flow under particular controlled conditions. -~
Such materials can be fabricated into devices of ~
many types, common examples of which are diodes, ;
capacitors, junction transistors, and field effect transistors, all of which in turn can be built up -~
into circuits and more complicated devices. ~-; Accordingly, in order to produce useful `~ semiconductor electronic devices, at least three basic requirements must be met: first, an appropriate semiconductor material must be j'~ available, often in the form of a single crystal or ;~
~i~ a monocrystalline thin film; second, the ability ;~
;~ must exist to dope the semiconductor material in the desired manner; and third, proper techniques must be ;
developed for forming devices from the doped materials. ;
Accordingly, much interest, research, publication activity, and indeed patent literature, ;~ has been directed at producing silicon carbide, ` 1-'; 3o ! doping it, and manufacturing devices from it. In ~;
~ spite of this high level of attention, commercial ;~
-~ devices formed from silicon carbide have to date ;
failed to move beyond the literature or the research lab into the commercial marketplace.
Two main categories of failure exist: a lack of any reproducible and precise methods for .
forming the necessary single crystals of silicon :~ 1332~42 carbide essential for device manufacture; and the lack of successful doping techniques which to date have failed to result in doped monocrystalline silicon carbide having high enough purities, low enough defect levels, and sufficient electrical activation of the dopant species to form any commercially useful devices.
Recently, however, techniques have been developed for successfully gxowing monocrystalline silicon carbide of high purity and low defect level in each of the two most common forms of silicon carbide, the cubic or beta structure and the hexagonal 6H alpha structure. These developments are the subject of co-pending Canadian patent applications assigned to the asisignee of the present invention, ~;
specifically nGrowth of Beta-SiC Thin Films and Semiconductor Devices Fabricated Thereon, n Serial No. 581,147 filed October 25, 1988, and Homoepitaxial Growth of Alpha-SiC Thin Films and Semiconductor Devices Fabricated Thereon,~ Serial No. 581,144 filed october 25, 1988.
~ From the doping standpoint, a number of ;~ methods exist for introducing an appropriate dopant into a semiconductor substrate. These include diffusion, chemical vapor deposition, and ion implantation. In ion implantation, ions of the `~ dopant atoms are formed by any appropriate method, for example by application of a strong enough field to strip one or more electrons from each dopant -~
30 ~ atom. The ions are then accelerated, typically through a mass spectrometer to further separate and ~;~
accelerate the desired dopant atoms, and finally directed into a target material (usually a single crystal or monocrystalline thin film) at energies typically between about 50 and about 300 kilo electron volts (KeV). On an atomic level, this results in severe collisions between the accelerated '':'~ `' .

`~ 13321~2 ions and the atoms of the target crystal. This initially can result in two problems. First, the dopant ions may not be in positions at which -~
electrons and holes can be transferred, i.e. they are not yet ~electrically activated.~ Secondly, a great deal of damage to the target crystal results, -~
and in particular, atoms in the crystal lattice are -~
dislodged from their proper positions to a greater or lesser extent. As is known to those familiar with semiconductor crystallography, such damaged crystals often do not have the electrical properties required for useful semiconductor devices.
Accordingly, various attempts and techniques have ~-been developed for dealing with the damage done ~;
during implantation.
A first technique is to heat or anneal the doped substrate material following implantation.
This heating step, when followed by an appropriate rate of cooling, should theoretically encourage the atoms in the crystal lattice, most of which are atoms of the semiconductor, to recrystallize in an' ~.'.'7'' orderly fashion, thus repairing the damage to the sem~conductor crystal lattice and allowing the ~-~
dopant atoms to position themselves for electrical activation. Nevertheless, crystal defects such as;~
dislocations,~stacking faults, twins, other defects -~
or combinations of the same, tend to remain following such annealing.
Other researchers have attempted to raise ~ ~i the temperature of the target during implantation, ' an example of which is U.S. Patent No. 3,293,084 to `
;:`$~ McCaldin (December 20, 1966), which discusses ion implantation of silicon, germanium or silicon and germanium alloys with sodium, potassium, rubidium,`~-or cesium as the doping atom. In 1966, however, -~ analytical tools such as transmission electron microscopy (TEM) and deep level transient ; 1332142 spectroscopy (DLTS) were unavailable and the residual defects formed under these conditions remained undiscovered.
Accordingly, the more recent development of these analytical tools has demonstrated that ion implantation in silicon conducted at high temperatures results in lattice defects. As ~;
presently understood, when silicon is ion implanted at high temperatures, sufficient energy is imparted to the lattice by the incoming ions to cause ~ ' individual point defects to arrange themselves in a ' lower energy configuration. These configurations include planar (stacking faults) and line -~
(dislocations or loops) defects, with line defects forming somewhat more often. These defects are, of course, detrimental to the operation of any ' resulting device formed from that material.
Accordingly, in recent years, emphasis has shifted to ion implantation which is conducted when the target i6 at a rather low temperature, specifically temperatures on the order of the boiling point of liquid nitrogen (77~K, -196C).
Under such circumstances, the implantation bombardment of ions creates a totally amorphous region in the target crystal, i.e. one in which no `;~
specific crystal structure is present. Performing anneàling following the low temperature implantation `
encourages the implanted region--i.e. the layer -~
represented by the depth to which the bombarding 30l ions have penetrated--to recrystallize into a l~ayer `~ which resembles an epitaxial growth portion, giving this techn'ique the name "solid-phase-epitaxy.
~m~ Under these conditions, the majority of the defects formed by the initial bombardment remain at the ' boundary between'the recrystallized layer and the layer which was too deep in the crystal to be affected by the bombardment.

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~ 1332142 Such low temperature implantation followed by annealing represents today~s best technology for producing doped silicon materials for electrical -~;
devices. Indeed, the quality and performance of electrical devices formed from any given semiconductor material is one of the best indications of the quality of the original material ~ -and of the doping technique used to give it its desired properties.
Accordingly, in attempting to find a suitable ion implantation technique for adding dopants to silicon carbide, researchers have attempted to reproduce those techniques found successful with silicon alone. For example, Tohi et al., U.S. Patent Nos. 3,629,011 and 3,829,333, '`
discuss techniques for implanting ions in silicon ;-~;
carbide at room or /'ordinary~ temperatures or at ~ -relatively low temperatures following which the bombarded silicon carbide is annealed at high ~-temperatures (up to 1600C~. To date, however, this has proved unsuccessful in producing any device quality doped silicon carbide crystals. In an ;~
attempt similar to those described by Tohi, Edmond ~ ~
~;; et al. found that implanting dopants into silicon ;
carbide at liquid nitrogen temperatures indeed produced amorphous layers, but annealing resulted in polycrystalline forms of silicon carbide or defective single crystals of silicon carbide, neither of which were suitable for manufacturing ;~
30 , electrical devices, J.A. Edmond, S.P. Withrow, H.S. ;~
Kong and R.F. Davis, Beam Solid InteraDctions and Phase Transformations, edited by H. Kurz, G.L. Olson and J.M. Poate (Materials Research Society, Pittsburgh, 1986), p. 395. ~;~
Other attempts have likewise been made to implant silicon carbide at ordinary or ~room~
temperatures. At room temperatures (approximately '~
, '' ~.

~ 1332~42 g 293K to 298K), no technique which produces consistent results has been developed. Some crystals tend to remain crystalline, while others become amorphous in a manner similar to that which occurs that upon low temperature implantation. The layers which remain crystalline during bombardment tend to recover properly upon annealing, but certain problems in the technique remain. At certain `
dosages of the dopants (dosage control being a requirement for imparting desired electrical properties to the target material), the crystal structure became amorphous. Furthermore, successful ; annealing had to be conducted at temperatures of approximately 1800C. These temperatures are well ~:
above the vaporization point of the silicon substrates upon which silicon carbide was always deposited prior to the concurrent inventions discussed earlier which produce silicon carbide upon silicon carbide.
Accordingly, there are no known successful --~ techniques in the art for producing device quality, single crystal, electrically activated doped silicon carbide semiconductive materials using conventional ion implantation techniques.
It is therefore an object of the present invention to provide a method of producing either n or p-dsped regions in silicon carbide suitable for semiconductor electrical devices.
It is another object of the invention to produce appropriately doped monocrystalline silicon carbide in a manner which prevents amorphization of the silicon carbide crystal during the doping ~i~` technique while appropriately electrically activating the dopant introduced.
It is a further object of the invention to provide a method for producing doped monocrystalline silicon carbide by ion beam implantation.
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-10- ~ , It is another object of the invention to provide a method for producing a doped silicon carbide by ion beam implantation conducted at temperatures high enough to prevent amorphization of the silicon carbide followed by removal of the ~
highly conductive near surface layer typically ;~` -formed from ion implantation, and by electrical ~ ~
activation of the dopant atoms introduced by the ion ~ -beam.
It is a further object of the invention to ~i-produce electrical devices of useful commercial quality using doped portions of silicon carbide formed according to the ion implantation techniques of the present invention.
Summary of the Invention The present invention comprises a method -~
of producing a doped region of silicon carbide suitable for semiconductor electrical devices operable at high temperatures. The method comprises directing an ion beam of dopant ions onto a substrate of silicon carbide in which the silicon carbide is maintained at a temperature high enough to prevent amorphization of, and minimize damage to, ~; the silicon carbide crystal lattice during the ion bombardment. The highly conductive near surface ~ ~ layer which results from the k~ombardment is removed t~; from the silicon carbide and the silicon carbide is annealed to electrically activate the dopant ~
introduced by the ion beam. ~`
30l Description of the Drawings ~ ,~
The foregoing and other objects, advantages and features of the invention, and the ;~ manner in which the same are accomplished will become more readily apparent upon consideration of ;~
the following detailed description of the invention taken in conjunction with the accompanying drawings, ~ . .

--` 13321~L2 which illustrate preferred and exemplary embodiments, and wherein~
Figure 1 is the Rutherford Backscattering spectrum for gallium implanted into silicon carbide `~
according to the present invention;
Figure 2 is the corresponding spéctrum for aluminum; and Figure 3 is the corresponding spectrum for nitrogen.
Detailed Description In an initial embodiment of the present invention, ions of aluminum, gallium and nitrogen were implanted into beta silicon carbide thin films while those films were maintained at temperatures of 623K, 823K and 1023K (350~C, 550C and 750C, respectively). These thin films can be produced as described in the aforementioned co-pending Canadian ;~
applications Serial Nos. 581,147 and 581,144, ~ "Growth of Beta-SiC Thin Fiims and Semiconductor -'J`.'~ ~ 20 Devices Fabricated Thereon, n and ~Homoepitaxial Growth of Alpha-SiC Thin Films and Semiconductor Devices Fabricated Thereon,n respectively. The resulting doped crystals were analyzed using Rutherford Backscattering ~RBS)/ion channeling ~ ;
techniques. These demonstrated that implantation of any of the above species at 623K resulted in only slight crystal lattice damage. In comparison, ion ~ implantation conducted at the same dopant dosage and -~ energy levels at room temperature resulted in either extensive lattice damage or, in the case of gallium, amorphization. When samples implanted at 1023K ~`;
`~ were analyzed, lattice damage was almost ~ non-existent. ~; -,;:
These beneficial effects of higher ~` 35 temperature implantation do not, however, by ;
themselves necessarily result in the complete `~ electrical activation of the implanted dopant.

13321~2 Therefore, in order to optimize electrical activation, samples according to the present invention were annealed at about 1473K (1200C) for up to 30 minutes following implantation.
Differential capacitance-voltage, spreading resistance, and sheet resistance technique~
demonstrated that activation of both p-type and n-type species (aluminum and nitrogen) had all been successfully accomplished. The maximum percentage of activated and room temperature ionized dopants ~
were 0.5 percent and 60 percent for aluminum and ~-nitrogen, respectively. This corresponds to activation levels of approximately 40 percent and 95 percent, respectively, based upon normal carrier;;,~
concentration calculations usinq the respective `
ionization energies at room temperature.
In one embodiment of the invention, each`;~
: sample of beta silicon carbide was mechanically polished, chemically oxidized, and then etched in -~
- 20 hydrofluoric acid (HF) to produce a clean, undamaged ;`-~
and smooth surface prior to bombardment. Initial -~
electrical measurements using differential ~;-capacitance-voltage measurements were also made before implanting for base line comparative reference purposes. The ion implantation was again conducted primarily at the three temperatures of ~
623~K, 823K, and 1023K. Some generally --~-insignificant deviation from these temperatures occurred as the result of temperature variations in the sample holder.
In practice, the ion beam was generated using a hot filament source and was directed towards , ,~ . .
the beta silicon carbide thin film using an offset angle of 7~ from the sample surface normal in order 35 to reduce channeling effects. Table I summarizes ~ ;
;~ the implantation conditions utilized for this work, in which N equals the electron density, p equals the ~-~

~ :-hole density, both expressed in exponential notation (e.g., 3E16 represents 3 X l016 carriers per cubic centimeter) as carriers per cubic centimeter ([n or p]/cm3) and the implant dosage is expressed as atoms per square centimeter (atoms/cm2), also in exponential notation.
Table I. Conditions and results of high temperature implantation of Al and N in Beta-SiC
Implant Sample Starting Implant Energy Implant Implant Final No. n or p Species (keV) Dose Temp.(K! n or p ;~ 1. n=3E16 Al+ 130 3.9E14 1023 p=lE16 ~;
2. n=2E16 Al ~ 185 1.8E14 1023 p=4E16 3. n=3E16 Al+ 160 2.3E14 1023 p=lE16 4. n=4E16 Al+ 100 4.8E14 1023 p=4E16 5. n=2E16 Al+ 185 6.3E14 623 p=SE15 ! . "

6. n=2E16 Al+ 185 6.3E14 823 p=lE16 --~; 20 7. n=2E16 Al~ 185 6.3E14 1023 p=lE16 8. n=2E16 Al+ 185 1.3E15 1023 p-6E16 ` ~ 9. n=2E16Al+,AI++150,300 4.3,6.8E14 1053 p=SE15 ~, 10. n=2E16 Al+ 300 6.8E14 1023 p=SE16 -`;
11. p=9E17 N+ 907180 l.l,l.~E14 973 n=2E18 12. p=4E17 N+,N++ 180,360 1.7,2.2E14 993 n=4E18 13. p=2E17 N+ 90,180 0.9,1.3E14 623 n=lE18 14. p=lE17 N+. 90,180 0.9,1.3E14 823 n=3E18 15. p=2E16 N+ 90,180 0.9,1.3E14 1023 n=3E18 i As set forth earlier herein, in order to structurally characterize the residual lattice d~mage caused by the implantation at the higher temperatures, Rutherford Backscattering (RBS)/ion ~
c~hanneling tochniques were used. The backscattering ~ -analyses were producing using 2.0 MeV helium ions ;~
incidënt along the [110] axial direction. These tests werefperformed for samples implanted with each ~-species at room temperature (298K, 25C) as well as the three elevated temperatures. Differential ~
capacitance-voltage, spreading resistance and sheet -;-resistance measurements were made in order to x~ electrically characterize the layers. Initial meaBurements indicated that highly conductive ~ -surface layers existed in the samples, leading to an undesirable excess leakage current. Although applicants do not wish to be bound by any particular theory, it appears that the conductive surface layer may represent a silicon-rich surface layer which results as the kombardment forces carbon away from the surface and into the crystal.
Accoxdingly, the invention further comprises removing this highly conductive near surface layer from the silicon carbide which results - -from this effect of the directed ion beam. In a ~ -preferred embodiment of the invention, the step of ;~-removing the highly conductive near surface layer comprises oxidizing the highly conductive near surface layer and then removing the thus oxidized near surface layer. Further to the preferred embodiment, the step of oxidizing the near surface layer comprises heating the silicon carbide in the ~ -presence of oxygen at a temperature and for a time ~`~
period sufficient to substantially oxidize all of the highly conductive near surface layer, with temperature ranges of 1000 to 1500C being desirable and a temperature of about 1200C most preferred. A
typical time period was 30 minutes. This removal step generally removed no more than approximately 500 angstroms of material from the surface. ~
`~ In another embodiment, the highly - ~-conductive near surface layer can be removed using various dry or wet etching techniques. Typical dry -~ etching techniques include reactive ion etching,reactive ion beam etching, and plasma etching.
Typical wet etching techniques include molten salt etching techniques using a variety of salt ~-combinations.
Finally, in order to successfully complete the electrical activation of the dopant species, the silicon carbide is further annealed. In preferred embodiments of the invention, the step of annealing the silicon carbide comprises heating the silicon carbide at a temperature of between about 1000 Centigrade and about 1500 Centigrade, with a temperature of approximately 1200 Centigrade most preferred. In the most preferred embodiment of the invention, the oxidizing removal of the highly conductive near surface layer is conducted by heating the sample in the presence of dry oxygen, following which the oxygen is replaced by argon while heating at the same temperature and for the same time period continues. In yet another embodiment, the removal of the highly conductive near surface layer and the annealing to electrically activate the dopant can be combined into a single step in which the bombarded silicon carbide is heated in the presence of oxygen for a time and temperature sufficient to both remove the highly ~ -conductive near surface layer and to electrically activate the dopant introduced by the ion beam.
Figures 1, 2 and 3 represent the RBS
spectra for gallium, aluminum, and nitrogen implanted silicon carbide respectively. The spectra ~;~ illustrate the damage accumulation as a function of implant temperature. It will be seen in each case that damage decreased with increasing implant temperature, with the spectra obtained from the 1023~K implants being nearly coincidental with that of the virgin aligned undoped beta silicon carbide.
This extremely low damage level was also demonstrated when the silicon carbide was implanted with aluminum or nitrogen at 823K; however, ; ~-gallium, with a much larger atomic weight, tended to ~
require the 1023~K implantation temperature in order ~ `
~ to minimize induced residual damage. The electrical -~
;~ 35 properties for each implanted sample are also listed in Table I. These demonstrate that increasing the implant temperatures from 623K to 823~K increases . ~

13321~2 . ~

dopant activation, but implanting at 1023K does not `~
cause a corresponding additional increase in this parameter.
In order to further study the implantation of nitrogen (N) in beta silicon carbide at elevated temperatures, p-type substrates were initially utili2ed. In every case, an n-type conducting layer resulted from the introduction of nitrogen. The percentage of ionized activated dopant ranged from about 20 percent for the 623K implant to about 60 percent for the 823K or greater implant. As was the case in the study of aluminum, maximum activation was achieved at the 823K implant.
Accordingly, the invention demonstrates three major advantages: residual damage to the ~ ~
crystal lattice is greatly reduced; improved ! ~ ~ ;
activation of the dopants is achieved at much lower annealing temperatures (1200~C versus 1800~C); and -~
the above processes can take place at temperatures 2Q at which silicon remains stable such that silicon substrates can be used in all of these processes where desirable or necessary.
~i~ The success provided by the present invention in producing doped regions with very few Z5 residual defects in single crystal silicon carbide ~
ilms has been demonstrated by a corresponding ;~ -success in producing electronic devices of ~-commercial-level performance and quality using ~
c ` ~ silicon carbide implanted according to the present ;~j invention.
Example 1: MOSFET
~;~ A metal oxide semiconductor field effect `~ transistor (MOSFET) was fabricated in 1.2 micron thick n-type layers on approximately 0.5 micron thick p-type beta silicon thin film crystals grown by chemical vapor deposition on 6H alpha silicon carbide crystals. The thin film was grown as 1332~42 described in the concurrently pending applications set forth earlier. The dopant for the p-type portion was aluminum which diffused from a heavily aluminum doped 6H alpha silicon carbide substrates.
The thin films were polished, oxidized to remove the polishing damage, and etched in hydrofluoric acid (HF) to remove the remaining oxide film. The gate oxide was then grown and cleaned. A
500 micron thick film of polysilicon was applied to the oxide via low pressure CVD, doped, and then patterned to form the gate contacts. The N type source and drain areas were then formed by the high temperature ion implantation of nitrogen through the --oxide, according to the present invention. Windows for the source and drain were formed and tantalum silicide (TaSi2) was then applied and patterned and annealed.
~ MOSFETs formed in this manner showed very -~ stable drain current saturation out to a drain source voltage of 25 volts. The drain characteristics of this device were also determined r'~ at 673K and at every 50~K interval to a maximum of 923K. Satisfactory operation of all of these transistors was observed up to 923K. Stable saturation and channel cut-off were achieved throughout this temperature range at drain-source ` voltages exceeding 25 volts. Transconductance increased with temperature to 573K, and decreased with temperature after 673K.
~;~ 30 Example 2: P-N Junction DiodeStandard semiconductor diodes were fabricated in single crystal silicon carbide using the high temperature ion implantation of the present invention. Two device configurations were ;;~;
``~ 35 developed, planar and mesa. The diodes had the following characteristics: rectification was observed with leakage currents ranging from 5 --~

,.',` ~.

~332~4'~ :

microamps (uA) to 50 uA at 300K and 673K, respectively, at a reverse bias of 5V. Forward bias current-voltage characteristics revealed the presence of space-charge-limited current flow, consistent with the characteristics of wide bandgap : -semiconductors such as silicon carbide. ~ .
In the drawings and specification, particular examples and embodiments have been described. It will be understood, however, that : 10 these have been set forth as examples, rather than limitations, the scope of the invention being :~
defined by the claims. .

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Claims (24)

1. A method of producing an n or p-doped region of silicon carbide suitable for semiconductor electrical devices, the method comprising:
directing an ion beam of dopant ions onto a monocrystalline silicon carbide substrate in which the silicon carbide is maintained at a temperature high enough to substantially prevent damage to the silicon carbide crystal lattice and to position implanted dopant ions from the ion beam in the near vicinity of substitutional lattice sites in the monocrystalline silicon carbide substrate;
removing the highly conductive near surface layer from the monocrystalline silicon carbide substrate which layer results from the directed ion beams independently of the effects of maintaining the substrate at the high temperature of implantation or of the temperature of any subsequent heating; and thereafter heating the monocrystalline silicon carbide substrate to a temperature higher than the temperature at which the substrate is maintained during implantation while low enough to prevent silicon atoms from evaporating from the substrate in any substantial amounts, and which heating supplies sufficient kinetic energy to the implanted dopant ions to encourage the implanted dopant ions to move to and occupy substitutional lattice sites thereby electrically activating the implanted dopant ions.

2. A method according to Claim 1 further comprising annealing the silicon carbide to electrically activate the dopant introduced by the ion beam.

3. A method according to Claim 2 wherein the step of annealing the silicon carbide follows the step of removing the highly conductive near surface layer.

4. A method according to Claim 1 wherein the step of maintaining the silicon carbide at a temperature high enough to substantially prevent damage to the silicon carbide crystal lattice comprises maintaining the silicon carbide at a temperature of 823K or greater.

5. A method according to Claim 1 wherein the step of maintaining the silicon carbide at a temperature high enough to substantially prevent damage to the silicon carbide crystal lattice comprises maintaining the silicon carbide at a temperature of 1023K or greater.

6. A method according to Claim 1 wherein the step of removing the highly conductive near surface layer comprises dry etching.

7. A method according to Claim 1 wherein the step of removing the highly conductive near surface layer comprises wet etching.

8. A method according to Claim 1 wherein the step of removing the highly conductive near surface layer comprises removing not more than approximately 500 angstroms of material from the silicon carbide substrate.

9. A method according to Claim 1 wherein the step of removing the highly conductive near surface layer comprises heating the silicon carbide in the presence of oxygen at a temperature and for a time period sufficient to substantially oxidize all of the highly conductive near surface layer.

10. A method according to Claim 9 wherein the step of heating the silicon carbide in the presence of oxygen comprises heating the silicon carbide at a temperature of between about 1000° and 1500°
centigrade.

11. A method according to Claim 9 wherein the step of heating the silicon carbide in the presence of oxygen comprises heating the silicon carbide to a temperature of about 1200° centigrade.

12. A method according to Claim 2 wherein the step of annealing the silicon carbide comprises heating the silicon carbide at a temperature and for a time period sufficient to substantially oxidize all of the highly conductive near surface layer.

13. A method according to Claim 12 wherein the step of heating the silicon carbide comprises heating the silicon carbide to a temperature of between about 1000° and 1500° centigrade.

14. A method according to Claim 12 wherein the step of heating the silicon carbide comprises heating the silicon carbide to a temperature of about 1200° centigrade.

15. A method according to Claim 1 wherein the step of directing an ion beam of dopant ions comprises directing an ion beam of aluminum ions.

16. A method according to Claim 1 wherein the step of directing an ion beam of dopant ions comprises directing an ion beam of nitrogen ions.

17. A method according to Claim 1 wherein the step of directing an ion beam of dopant ions comprises directing an ion beam of gallium ions.

18. A method of producing an n or p-doped region of silicon carbide suitable for semiconductor electrical devices, the method comprising:
directing an ion beam of dopant ions onto a substrate of silicon carbide in which the silicon carbide is maintained at a temperature high enough to substantially prevent damage to the silicon carbide crystal lattice and to position implanted dopant ions from the ion beam in the near vicinity of substitutional lattice sites in the monocrystalline silicon carbide substrate;
heating the silicon carbide in the presence of oxygen at a temperature of between about 1000° and about 1500° centigrade for a time period sufficient to substantially oxidize all of the highly conductive near surface layer which results from the directed ion beam; and thereafter heating the silicon carbide in the presence of a noble gas at a temperature of between about 1000° and about 1500° centigrade which supplies sufficient kinetic energy to the implanted dopant ions to encourage the implanted dopant ions to move to and occupy substitutional lattice sites thereby electrically activating the implanted dopant ions.

19. A method according to Claim 18 wherein the step of heating the silicon carbide in the presence of oxygen comprises heating the silicon carbide at a temperature of about 1200° centigrade.

20. A method according to Claim 18 wherein the step of heating the silicon carbide in the presence of a noble gas comprises heating the silicon carbide in the presence of argon at a temperature of about 1200° centigrade.

21. A method of producing a n or p-doped region of silicon carbide suitable for semiconductor electrical devices, the method comprising:
directing an ion beam of dopant ions onto a substrate of silicon carbide in which the silicon carbide is maintained at a temperature high enough to substantially prevent damage to the silicon carbide crystal lattice and to position implanted dopant ions from the ion beam in the near vicinity of substitutional lattice sites in the monocrystalline silicon carbide substrate;
removing the highly conductive near surface layer which results from the directed ion beam from the silicon carbide by heating the silicon carbide in the presence of oxygen at a temperature of between about 1000° and about 1500° centigrade for a time period sufficient to substantially oxidize all of the highly conductive near surface layer; and introducing a noble gas to substantially displace all of the oxygen in the closed system while continuing to heat the silicon carbide at a temperature of between about 1000° and about 1500°
centigrade which supplies sufficient kinetic energy to the implanted dopant ions to move to and occupy substitutional lattice sites thereby electrically activating the implanted dopant ions following the displacement of oxygen by the introduced noble gas.

22. A method according to Claim 21 wherein the step of heating the silicon carbide in the presence of oxygen comprises heating the silicon carbide at a temperature of about 1200° centigrade.

23. A method according to Claim 21 wherein the step of introducing a noble gas while continuing to heat the silicon carbide comprises heating the silicon carbide at a temperature of about 1200°C.

24. A method according to Claim 21 wherein the step of directing an ion beam of dopant ions comprises directing an ion beam consisting of ions selected from the group aluminum, nitrogen or gallium.