Patent Application
20140151712
Enhancement-mode (E-mode) high electron-mobility transistors (HEMTs), commonly employ a gate recess in the barrier layer of the transistor in order to obtain the desired threshold, or “turn-on” voltage, VT, at which the HEMT begins to conduct on-state current. Dry etching, such as reactive ion etching (RIE) or inductively coupled plasma (ICP) etching, is typically employed to fabricate the gate recess in HEMTs. However, VT depends, in part, upon the thickness of the barrier layer between the gate electrode and channel layer of the HEMT. Also, RIE and ICP processes can degrade electron mobility in the channel layer. Furthermore, it is difficult to control vertical scaling by use of RIE or ICP because of the lack of an etch stop in the nitride materials employed to form the barrier layer of HEMTs.
Therefore, a need exists for a HEMT and a method of fabricating a HEMT that overcomes or minimizes the above-referenced problems.
The invention is related to an epitaxial structure, such as is employed in a high electron mobility transistor (HEMT) and a method of forming the epitaxial structure, wherein the depth of the gate recess formed in the barrier layer can be controlled during fabrication of the epitaxial structure.
In one embodiment, the invention is an epitaxial structure, such as is employed in a HEMT, that includes a substrate, a buffer layer on the substrate wherein the buffer layer includes gallium nitride, a channel layer over the buffer layer consisting essentially of Inx Ga1-xN, where 0≦x≦1, and wherein the channel layer includes a 2-dimensional electron gas region distal to the buffer layer. A first barrier layer over the channel layer is formed at a first temperature, and a second barrier layer over the first barrier layer is formed at a second temperature that is lower than that of the first temperature, at which the first barrier layer is formed.
In one embodiment, the invention is a method of forming an epitaxial structure, such as is employed in a HEMT, and includes the steps of forming a substrate, forming a buffer layer on the substrate, forming a channel layer over the buffer layer, wherein the channel layer consists essentially of Inx Ga1-x N, where 0>x>1. A first barrier layer is formed over the channel at a first temperature, and a second barrier layer is formed over the first barrier layer at a temperature lower than that of the first temperature, whereby a 2-dimensional electron gas region is formed in the channel layer distal to the buffer layer as a result of forming at least one of the first and second barrier layers. A recess is then formed in the second barrier layer.
This invention has several advantages. For example, the formation of a barrier layer formed at a relatively high temperature between the channel layer and a barrier layer formed at a relatively low temperature serves as an etch stop during wet etching of the low temperature barrier layer to form a gate recess in the epitaxial structure. Further, the relative thickness of the high temperature and low temperature barrier layers can be controlled to thereby ensure that the total thickness of the low-temperature and high-temperature barrier layer components is more than the critical thickness needed to introduce a 2-dimensional electron gas into the channel layer of the as-grown structure. In general, independent control of barrier thickness between the gate and channel layer can be employed to determine the threshold voltage of a resulting HEMT employing the epitaxial structure of the invention.
The invention is directed to an epitaxial structure, such as an epitaxial structure employed in a high electron mobility transistor (HEMT) structure, that includes a first barrier layer that is formed at a first temperature, and a second barrier layer over the first barrel barrier layer formed a second temperature, lower than that of the first temperature, and to a method of forming such an epitaxial structure.
In one embodiment, the invention is an epitaxial structure, such as an enhancement mode (E-mode) high electron-mobility transistor (HEMT) structure. As understood by those of skill in the art, an epitaxial structure can contain many distinct layers that are, either collectively or individually, designed to achieve desired device characteristics. An epitaxial structure is typically formed over a substrate. Examples of substrate materials for GaN-based epitaxial structures include sapphire (Al2O3), silicon carbide (SiC), silicon (Si), gallium nitride (GaN), or aluminum nitride (AlN). For a GaN-based FET, such as a HEMT, a buffer layer with high electrical resistivity is typically formed over the substrate. For HEMT epitaxial structures, a channel layer is typically formed over the buffer layer and a barrier layer is typically formed over the channel layer. The barrier layer should be formed from a material with larger bandgap than the material used to form the channel layer. When the barrier layer and channel layer are formed using appropriate materials and methods, a large electron concentration can be developed in the channel layer adjacent to the barrier layer. The electrons in this region exhibit high mobility and this collective group of electrons is referred to as a 2-dimensional electron gas (2DEG). It should be noted that certain optional layers may be present or absent in a HEMT epitaxial structure depending on its design. Of particular note, a channel layer may not be employed and, if a large bandgap barrier layer is formed over a buffer layer with smaller bandgap, a 2DEG can be formed in the region of the buffer layer adjacent to the barrier layer. There may also be intervening layers between the channel layer and the barrier layer. A common example is a spacer layer formed directly on the channel layer and between the channel layer and barrier layer. Such a spacer layer can be used to enhance properties of the 2DEG such as electron mobility and electron concentration. For GaN-based HEMTs, a common structure employs a GaN buffer layer, GaN channel layer, AN spacer layer, and AlGaN barrier layer, although many permutations are possible. Suitable material properties (e.g., bandgap) for the respective layers of GaN-based epitaxial structures are known in the art.
It should be noted that when a layer is referred to as being “on” or “over” another layer or substrate, it can be directly on the layer or substrate, or an intervening layer also may be present. A layer that is “directly on” another layer or substrate means that no intervening layer is present. It should also be understood that when a layer is referred to as being “on” or “over” another layer or substrate, it may cover the entire layer or substrate, or a portion of the layer or substrate.
In one embodiment of the invention, shown in
Buffer layer 14 is deposited over substrate 12. Suitable materials of buffer layer 14 include those that are known in the art, such as GaN, aluminum nitride (AlGaN) and indium gallium nitride (InGaN). Buffer layer 14 is deposited by a suitable method, such as is known in the art. Examples of suitable methods of deposit of buffer layer include metal-organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). Back barrier layer 16 is an optional component of the epitaxial structure of the invention. Back barrier layer 16 operates to help improve the confinement of electrons in the channel. Back barrier layer 16 can be fabricated by a suitable method, such as MOCVD or MBE, as is known in the art. Typically, suitable thicknesses of substrate 12, buffer layer 14 and back barrier 16 are typical of those known in the art. The average thickness of buffer layer 14 typically is in a range of between about 500 nm and about 10 μm, and that of back barrier layer 16 typically is in a range of between about 10 nm and about 1000 nm.
Channel layer 18 is formed over back barrier layer 16, if present. Otherwise, channel layer 18 is formed over buffer layer 14. Channel layer 18 is formed of InxGa1-xN, where 0≦x≦1. Channel layer 18 is nominally-undoped. Typically, channel layer 18 is formed by a method known to those of skill in the art, such as metal-organic chemical vapor deposition (MOCVD) and beam epitaxy (MBE).
First aluminum nitride (AlN) barrier layer 20 is formed over the channel layer 18. As defined herein, a “barrier layer” is a layer in an epitaxial structure that has a wider band gap than the channel layer it overlays. First AlN barrier layer 20 is a crystalline material. Examples of suitable methods of forming first barrier layer 20 are known to those of skill in the art and include, for example, MOCVD or MBE. In one embodiment, first AlN barrier layer 20 is formed at a temperature in a range of between about 900° C. and about at 1300° C. Preferably, first barrier layer 20 is formed at a temperature in a range of between about 900° C. and about 1200° C. In one embodiment, first barrier layer 20 has a thickness in a range between about 0.5 nm and about 3.0 nm. In a preferred embodiment, the first barrier layer 20 has a thickness of the range between about 0.5 nm and about 2.0 nm.
Second barrier layer 22 is formed over first barrier layer 20. The second barrier layer 22 is a polycrystalline or an amorphous material and contains more material defects than first barrier layer 20. Preferably, first barrier layer 20 and second barrier layer 22 are formed of the same material, albeit at different temperatures. Second barrier layer 22 is formed at a temperature lower than that at which first barrier layer 20 was formed. In one embodiment, second barrier layer 22 is formed at a temperature in a range of between about 300° C. and about 800° C. Typically, the average thickness of second barrier layer 22 is in a range of between about 1.0 nm and about 10 nm. Preferably, the average thickness of second barrier layer 22 is in a range between about 1 nm and about 100 nm. 2-dimensional gas 26 is formed in channel layer 18 consequent to forming the barrier layers.
Preferably, the cumulative average thickness of first and second barrier layers is in a range of between about 5 nm and about 20 nm. Alternatively, the average cumulative thickness of first and second barrier layers is at least about 2 nm.
In a second embodiment of the invention shown in
As is known to those skilled in the art, source 38 and drain 40 terminals in electrical communication with second barrier layer are component parts of a HEMT, and are shown as components of the embodiment of the invention represented in
The following example represents a non-limiting embodiment of the invention.
A wet etching test using room temperature potassium hydroxide (KOH) solution as the etchant was performed with the LT AlN and HT AlN separately. HT AlN is very resistive to KOH etching. After soaking the sample in KOH for 60 seconds, no obvious etching effect was observed. LT AlN reacted very actively with KOH. Within 15 seconds, 955 nm LT AlN was removed, which gave an etch rate of 63.7 nm/s.
While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.
The teachings of all patents, published applications and references cited herein are incorporated by reference in their entirety.
This application claims the benefit of U.S. Provisional Application No. 61/656,882, filed on Jun. 7, 2012, the teachings of which are incorporated herein by reference in their entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61656882 | Jun 2012 | US |
