Cap layers and/or passivation layers for nitride-based transistors, transistor structures and methods of fabricating same

FIELD OF THE INVENTION

0001The present invention relates to semiconductor devices and, more particularly, to transistors that incorporate nitride-based active layers.

BACKGROUND

0002Materials such as silicon (Si) and gallium arsenide (GaAs) have found wide application in semiconductor devices for lower power and (in the case of Si) lower frequency applications. These, more familiar, semiconductor materials may not be well suited for higher power and/or high frequency applications, however, because of their relatively small bandgaps (<i>e.g</i>., 1.12 eV for Si and 1.42 for GaAs at room temperature) and/or relatively small breakdown voltages.

0003In light of the difficulties presented by Si and GaAs, interest in high power, high temperature and/or high frequency applications and devices has turned to wide bandgap semiconductor materials such as silicon carbide (2.996 eV for alpha SiC at room temperature) and the Group III nitrides (<i>e.g.</i>, 3.36 eV for GaN at room temperature). These materials, typically, have higher electric field breakdown strengths and higher electron saturation velocities as compared to gallium arsenide and silicon.

0004A device of particular interest for high power and/or high frequency applications is the High Electron Mobility Transistor (HEMT), which, in certain cases, is also known as a modulation doped field effect transistor (MODFET). These devices may offer operational advantages under a number of circumstances because a two-dimensional electron gas (2DEG) is formed at the heterojunction of two semiconductor materials with different bandgap energies, and where the smaller bandgap material has a higher electron affinity. The 2DEG is an accumulation layer in the undoped ("unintentionally doped"), smaller bandgap material and can contain a very high sheet electron concentration in excess of, for example, 10¹³ carriers/cm². Additionally, electrons that originate in the wider-bandgap semiconductor transfer to the 2DEG, allowing a high electron mobility due to reduced ionized impurity scattering.

0005This combination of high carrier concentration and high carrier mobility can give the HEMT a very large transconductance and may provide a strong performance advantage over metal-semiconductor field effect transistors (MESFETs) for highfrequency applications.

0006High electron mobility transistors fabricated in the gallium nitride/aluminum gallium nitride (GaN/AlGaN) material system have the potential to generate large amounts of RF power because of the combination of material characteristics that includes the aforementioned high breakdown fields, their wide bandgaps, large conduction band offset, and/or high saturated electron drift velocity. A major portion of the electrons in the 2DEG is attributed to polarization in the AlGaN. HEMTs in the GaN/AlGaN system have already been demonstrated. <patcit id="pcit0001" dnum="US5192987A"><text>U.S. Patents 5,192,987</text></patcit> and <patcit id="pcit0002" dnum="US5296395A"><text>5,296,395</text></patcit> describe AlGaN/GaN HEMT structures and methods of manufacture. <patcit id="pcit0003" dnum="US6316793B"><text>U.S. Patent No. 6,316,793, to Sheppard et al.</text></patcit>, which is commonly assigned and is incorporated herein by reference, describes a HEMT device having a semi-insulating silicon carbide substrate, an aluminum nitride buffer layer on the substrate, an insulating gallium nitride layer on the buffer layer, an aluminum gallium nitride barrier layer on the gallium nitride layer, and a passivation layer on the aluminum gallium nitride active structure. <patcit id="pcit0004" dnum="WO2004008495A"><text>WO 2004/008495</text></patcit> discloses a group III Nitride HEMP having a substrate layer, a channel layer on the substrate layer, a first cap layer on the channel layer, and a second cap layer on the first cap layer. In one example, the first cap layer is formed of AlN and the second cap layer is formed of AlGaN.

0007<patcit id="pcit0005" dnum="US6498111B"><text>US-6,498,111</text></patcit> describes a method for protecting the surface of a semiconductor material from damage and dopant passivation. A barrier layer of dense or reactive material is deposited on the semiconductor material shortly after growth. The semiconductor material can then be removed from the reactor with little or no passivation of the dopant species. The barrier layer can be removed using a variety of etching processes.

SUMMARY OF THE INVENTION

0008The present invention provides a Group III-nitride high electron mobility transistor according to claim 1 and a method of fabricating a Group III-nitride high electron mobility transistor in line with claim 5. In particular embodiments of the present invention, the protective layer has a thickness of from about 0.1 nm to about 1 nm. In certain embodiments, the protective layer has a thickness of about one mono layer.

0009In still further embodiments of the present invention, the gate contact is on the protective layer. The protective layer may be formed in-situ with forming the barrier layer.

0010Various combinations and/or sub-combinations of cap layers, passivation layers, protective layers and/or anneals of passivation layers may also be provided according to some embodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

0011<figref idref="f0001"><b>Figure 1</b></figref> is a cross-sectional schematic drawing illustrating transistors having ohmic contacts on a protective layer according to some embodiments of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

0012The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. However, this invention should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the thickness of layers and regions are exaggerated for clarity. Like numbers refer to like elements throughout. As used herein the term "and/or" includes any and all combinations of one or more of the associated listed items.

0013The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and/or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

0014It will be understood that when an element such as a layer, region or substrate is referred to as being "on" or extending "onto" another element, it can be directly on or extend directly onto the other element or intervening elements may also be present. In contrast, when an element is referred to as being "directly on" or extending "directly onto" another element, there are no intervening elements present. It will also be understood that when an element is referred to as being "connected" or "coupled" to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being "directly connected" or "directly coupled" to another element, there are no intervening elements present. Like numbers refer to like elements throughout the specification.

0015It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the present invention.

0016Furthermore, relative terms, such as "lower" or "bottom" and "upper" or "top," may be used herein to describe one element's relationship to another elements as illustrated in the Figures. It will be understood that relative terms are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures. For example, if the device in the Figures is turned over, elements described as being on the "lower" side of other elements would then be oriented on "upper" sides of the other elements. The exemplary term "lower", can therefore, encompass both an orientation of "lower" and "upper," depending of the particular orientation of the figure. Similarly, if the device in one of the figures is turned over, elements described as "below" or "beneath" other elements would then be oriented "above" the other elements. The exemplary terms "below" or "beneath" can, therefore, encompass both an orientation of above and below. Furthermore, the term "outer" may be used to refer to a surface and/or layer that is farthest away from a substrate.

0017Embodiments of the present invention are described herein with reference to cross-section illustrations that are schematic illustrations of idealized embodiments of the present invention. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, embodiments of the present invention should not be construed as limited to the particular shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. For example, an etched region illustrated as a rectangle will, typically, have tapered, rounded or curved features. Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the precise shape of a region of a device and are not intended to limit the scope of the present invention.

0018Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

0019It will also be appreciated by those of skill in the art that references to a structure or feature that is disposed "adjacent" another feature may have portions that overlap or underlie the adjacent feature.

0020Embodiments of the present invention may be particularly well suited for use in nitride-based devices such as Group III-nitride based HEMTs. As used herein, the term "Group III nitride" refers to those semiconducting compounds formed between nitrogen and the elements in Group III of the periodic table, usually aluminum (Al), gallium (Ga), and/or indium (In). The term also refers to ternary and quaternary compounds such as AlGaN and AlInGaN. As is well understood by those in this art, the Group III elements can combine with nitrogen to form binary (<i>e.g.</i>, GaN), ternary (<i>e.g</i>., AlGaN, AlInN), and quaternary (<i>e.g.</i>, AlInGaN) compounds. These compounds all have empirical formulas in which one mole of nitrogen is combined with a total of one mole of the Group III elements. Accordingly, formulas such as Al_xGa_1-xN where 0 ≤ x ≤ 1 are often used to describe them.

0021Suitable structures and techniques for fabricating GaN-based HEMTs that may utilize embodiments of the present invention are described, for example, in commonly assigned <patcit id="pcit0006" dnum="US6316793B"><text>U.S. Patent 6,316,793</text></patcit> and <patcit id="pcit0007" dnum="US20020066908A1A"><text>U.S. Patent Publication No. 2002/0066908A1 filed July 12, 2001</text></patcit> and published June 6, 2002, for "ALUMINUM GALLIUM NITRIDE/GALLIUM NITRIDE HIGH ELECTRON MOBILITY TRANSISTORS HAVING A GATE CONTACT ON A GALLIUM NITRIDE BASED CAP SEGMENT AND METHODS OF FABRICATING SAME," <patcit id="pcit0008" dnum="US20020167023A1A"><text>U. S. Patent Publication No. 2002/0167023A1 to Smorchkova et al., published November 14, 2002</text></patcit>, entitled "GROUP-III NITRIDE BASED HIGH ELECTRON MOBILITY TRANSISTOR (HEMT) WITH BARRIER/SPACER LAYER", <patcit id="pcit0009" dnum="US61784303" dnum-type="L"><text>U.S. Patent Application Serial No. 10/617,843</text></patcit> filed July 11, 2003 for "NITRIDE-BASED TRANSISTORS AND METHODS OF FABRICATION THEREOF USING NON-ETCHED CONTACT RECESSES," <patcit id="pcit0010" dnum="US77288204" dnum-type="L"><text>U.S. Patent Application Serial No. 10/772,882 filed February 5, 2004</text></patcit> for "NITRIDE HETEROJUNCTION TRANSISTORS HAVING CHARGE-TRANSFER INDUCED ENERGY BARRIERS AND METHODS OF FABRICATING THE SAME," <patcit id="pcit0011" dnum="US89772604" dnum-type="L"><text>U.S. Patent Application Serial No. Serial No.: 10/897,726, filed July 23, 2004</text></patcit> entitled "METHODS OF FABRICATING NITRIDE-BASED TRANSISTORS WITH A CAP LAYER AND A RECESSED GATE," <patcit id="pcit0012" dnum="US84961704" dnum-type="L"><text>U.S. Patent Application Serial No. 10/849,617, filed May 20, 2004</text></patcit> entitled "METHODS OF FABRICATING NITRIDE-BASED TRANSISTORS HAVING REGROWN OHMIC CONTACT REGIONS AND NITRIDE-BASED TRANSISTORS HAVING REGROWN OHMIC CONTACT REGIONS," <patcit id="pcit0013" dnum="US84958904" dnum-type="L"><text>U.S. Patent Application Serial No. 10/849,589, filed May 20, 2004</text></patcit> and entitled "SEMICONDUCTOR DEVICES HAVING A HYBRID CHANNEL LAYER, CURRENT APERTURE TRANSISTORS AND METHODS OF FABRICATING SAME" and <patcit id="pcit0014" dnum="US20030020092A"><text>U.S. Patent Publication No. 2003/0020092 filed July 23, 2002</text></patcit> and published January 30, 2003 for "INSULATING GATE ALGAN/GAN HEMT"

0022Exemplary devices according to some embodiments of the present invention are now described with reference to <figref idref="f0001"><b>Figure 1</b></figref><b>.</b> A substrate <b>10</b> is provided on which nitride based devices may be formed. In particular embodiments of the present invention, the substrate <b>10</b> may be a semi-insulating silicon carbide (SiC) substrate that may be, for example, 4H polytype of silicon carbide. Other silicon carbide candidate polytypes include the 3C, 6H, and 15R polytypes. The term "semi-insulating" is used descriptively rather than in an absolute sense. In particular embodiments of the present invention, the silicon carbide bulk crystal has a resistivity equal to or higher than about 1x10⁵ Ω-cm at room temperature.

0023Optional buffer, nucleation and/or transition layers (not shown) may be provided on the substrate <b>10.</b> For example, an AlN buffer layer may be provided to provide an appropriate crystal structure transition between the silicon carbide substrate and the remainder of the device. Additionally, strain balancing transition layer(s) may also be provided as described, for example, in commonly assigned <patcit id="pcit0015" dnum="US20030102482A1"><text>U.S. Patent Publication No. 2003/0102482A1, filed July 19, 2002</text></patcit> and published June 5, 2003, and entitled "STRAIN BALANCED NITRIDE HETROJUNCTION TRANSISTORS AND METHODS OF FABRICATING STRAIN BALANCED NITRIDE HETEROJUNCTION TRANSISTORS," or United States Patent Publication No.<patcit id="pcit0016" dnum="US20040012015A1"><text> 2004/0012015 A1, filed July 19, 2002</text></patcit> and published January 22, 2004, and entitled "STRAIN COMPENSATED SEMICONDUCTOR STRUCTURES AND METHODS OF FABRICATING STRAIN COMPENSATED SEMICONDUCTOR STRUCTURES".

0024Appropriate SiC substrates are manufactured by, for example, Cree, Inc., of Durham, N.C., the assignee of the present invention, and methods for producing are described, for example, in <patcit id="pcit0017" dnum="USRE34861E"><text>U. S. Patent Nos. Re. 34,861</text></patcit>; <patcit id="pcit0018" dnum="US4946547A"><text>4,946,547</text></patcit>; <patcit id="pcit0019" dnum="US5200022A"><text>5,200,022</text></patcit>; and <patcit id="pcit0020" dnum="US6218680B"><text>6,218,680</text></patcit>, the contents of which are incorporated herein by reference in their entirety. Similarly, techniques for epitaxial growth of Group III nitrides have been described in, for example, <patcit id="pcit0021" dnum="US5210051A"><text>U. S. Patent Nos. 5,210,051</text></patcit>; <patcit id="pcit0022" dnum="US5393993A"><text>5,393,993</text></patcit>; <patcit id="pcit0023" dnum="US5523589A"><text>5,523,589</text></patcit>; and <patcit id="pcit0024" dnum="US5592501A"><text>5,592,501</text></patcit>

0025Although silicon carbide may be used as a substrate material, embodiments of the present invention may utilize any suitable substrate, such as sapphire, aluminum nitride, aluminum gallium nitride, gallium nitride, silicon, GaAs, LGO, ZnO, LAO, InP and the like. In some embodiments, an appropriate buffer layer also may be formed.

0026A channel layer <b>20</b> is provided on the substrate <b>10.</b> The channel layer <b>20</b> may be deposited on the substrate <b>10</b> using buffer layers, transition layers, and/or nucleation layers as described above. The channel layer <b>20</b> may be under compressive strain. Furthermore, the channel layer and/or buffer nucleation and/or transition layers may be deposited by MOCVD or by other techniques known to those of skill in the art, such as MBE or HVPE .

0027In some embodiments of the present invention, the channel layer <b>20</b> is a Group III-nitride, such as Al_xGa_1-xN where 0 ≤ x &lt; 1, provided that the energy of the conduction band edge of the channel layer <b>20</b> is less than the energy of the conduction band edge of the barrier layer <b>22</b> at the interface between the channel and barrier layers. In certain embodiments of the present invention, x = 0, indicating that the channel layer <b>20</b> is GaN. The channel layer <b>20</b> may also be other Group III-nitrides such as InGaN, AlInGaN or the like. The channel layer <b>20</b> may be undoped ("unintentionally doped") and may be grown to a thickness of greater than about 2 nm. The channel layer <b>20</b> may also be a multi-layer structure, such as a superlattice or combinations of GaN, AlGaN or the like.

0028A barrier layer <b>22</b> is provided on the channel layer <b>20.</b> The channel layer <b>20</b> may have a bandgap that is less than the bandgap of the barrier layer <b>22</b> and the channel layer <b>20</b> may also have a larger electron affinity than the barrier layer <b>22.</b> The barrier layer <b>22</b> may be deposited on the channel layer <b>20.</b> In certain embodiments of the present invention, the barrier layer <b>22</b> is AlN, AlInN, AlGaN or AlInGaN with a thickness of between about 0.1 nm and about 40 nm. Examples of layers according to certain embodiments of the present invention are described in <patcit id="pcit0025" dnum="US20020167023A1"><text>U.S. Patent Publication No. 2002/0167023A1, to Smorchkova et al.</text></patcit><i>,</i> entitled "GROUP-III NITRIDE BASED HIGH ELECTRON MOBILITY TRANSISTOR (HEMT) WITH BARRIER/SPACER LAYER". In particular embodiments of the present invention, the barrier layer <b>22</b> is thick enough and has a high enough Al composition and doping to induce a significant carrier concentration at the interface between the channel layer <b>20</b> and the barrier layer <b>22</b> through polarization effects. The barrier layer <b>22</b> may be a Group III-nitride and has a bandgap larger than that of the channel layer <b>20</b> and a smaller electron affinity than the channel layer <b>20.</b> Accordingly, in certain embodiments of the present invention, the barrier layer <b>22</b> is AlGaN, AlInGaN and/or AlN or combinations of layers thereof. The barrier layer <b>22</b> may, for example, be from about 0.1 nm to about 40 nm thick, but is not so thick as to cause cracking or substantial defect formation therein. In certain embodiments of the present invention, the barrier layer <b>22</b> is undoped or doped with an n-type dopant to a concentration less than about 10¹⁹ cm^-3. In some embodiments of the present invention, the barrier layer <b>22</b> is Al_xGa_1-xN where 0&lt;x≤1. In particular embodiments, the aluminum concentration is about 25%. However, in other embodiments of the present invention, the barrier layer <b>22</b> comprises AlGaN with an aluminum concentration of between about 5% and about 100%. In specific embodiments of the present invention, the aluminum concentration is greater than about 10%.

0029Ohmic metal is patterned to provide ohmic contact material patterns that when annealed provide the ohmic contacts <b>30.</b>

0030Transistors according to embodiments of the present invention may be fabricated utilizing techniques such as those discussed, for example, as described in <patcit id="pcit0026" dnum="US84961704" dnum-type="L"><text>U. S. Patent Application Serial No. 10/849,617, filed May 20, 2004</text></patcit> and entitled "METHODS OF FABRICATING NITRIDE-BASED TRANSISTORS HAVING REGROWN OHMIC CONTACT REGIONS AND NITRIDE-BASED TRANSISTORS HAVING REGROWN OHMIC CONTACT REGIONS" and <patcit id="pcit0027" dnum="US89772604" dnum-type="L"><text>U.S. Patent Application Serial No. 10/897,726, filed July 23, 2004</text></patcit> and entitled "METHODS OF FABRICATING NITRIDE-BASED TRANSISTORS WITH A CAP LAYER AND A RECESSED GATE".

0031A protective layer <b>64</b> is provided on the barrier layer <b>22.</b> As is illustrated in <figref idref="f0001"><b>Figure 1</b></figref><b>,</b> the ohmic contacts are provided on the protective layer <b>64.</b> The gate contact 32 may also be provided on the protective layer <b>64.</b> In the present invention, the ohmic contacts <b>30</b> are provided directly on the protective layer <b>64</b> and the gate contact <b>32</b> may also be provided directly on the protective layer <b>64.</b>

0032In an embodiment where the protective layer 64 is formed to a thickness of from about 0.1 nm to about 1 nm: the protective layer 64 may be a SiN layer deposited prior to formation of the ohmic contacts 30 and the gate contact 32; alternatively, the protective layer 64 may be a BN or MgN layer; and the protective layer 64 may be a single layer, or, protective layer 64 may be provided as multiple layers, such as a layer of SiN and a layer of AlN. MgN may be especially suitable for use with p-type devices as additional doping may be provided upon anneal of the ohmic contact material.

0033In an embodiment where in the protective layer 64 comprises MgN, the protective layer 64 may have a thickness of from about 0.1 nm to about 1 nm and, in some embodiments, may have a thickness of about one monolayer. Because the protective layer 64 is thin, there is no need to recess the ohmic contacts through the protective layer 64. Reliability may be improved through better surface state control and lower gate leakage current in comparison to devices without such a protective layer.

0034The protective layer 64 may be formed in-situ with the formation of the barrier layer. In an embodiment where the protective layer 64 is formed to a thickness of from about 0.1 nm to about 1 nm, because the protective layer 64 is very thin, there may be very little additional fabrication cost other than providing a Si source, B source or Mg source and only a small additional growth time to deposit the thin protective layer 64. Furthermore, because the protective layer 64 is thin, no additional steps may be needed to form recesses for the gate and no additional steps are needed to form recesses for the ohmic contacts.

0035While embodiments of the present invention have been described herein with reference to particular HEMT structures, the present invention should not be construed as limited to such structures. For example, additional layers may be included in the HEMT device while still benefiting from the teachings of the present invention. The additional layers may include a compositionally graded transition layer or layers.

0036Furthermore, the barrier layer 22 may also be provided with multiple layers as described in <patcit id="pcit0028" dnum="US20020167023A1"><text>U.S. Patent Publication No. 2002/0167023A1</text></patcit> cited above. Thus, embodiments of the present invention should not be construed as limiting the barrier layer to a single layer but may include, for example, barrier layers having combinations of GaN, AlGaN and/or AlN layers. For example, a GaN, AlN structure may be utilized to reduce or prevent alloy scattering. Thus, embodiments of the present invention includes nitride based barrier layers, such nitride based barrier layers may include AlGaN based barrier layers, AlN based barrier layers and combinations thereof.