Patent Application
20120126239
This invention relates to growth of III-Nitride semiconductor films on silicon substrates, and specifically to methods to manage stress in the films.
As large native substrates for group III-Nitride (III-N) semiconductors are not yet widely available, III-N films, such as GaN and its alloys, are currently grown by heteroepitaxy on suitable non-III-N substrates. Typically, the films are grown on sapphire (Al2O3), silicon carbide (SiC), or silicon substrates. Silicon substrates are emerging as a particularly attractive substrate candidate for III-N layers due to their low cost, wide availability, large wafer sizes, thermal properties, and ease of integration with silicon-based electronics. However, due to the large lattice mismatch and thermal expansion coefficient mismatch between silicon and III-N materials, there is typically a net tensile stress in III-N epitaxial layers deposited directly on silicon substrates. This mismatch can result in cracking of the layers. Therefore thick III-N layers on silicon substrates that are crack-free and that exhibit adequate structural quality can be difficult to achieve. Structures that include additional layers between the III-N layer and the substrate for controlling stress during growth are therefore necessary to allow for growth of thick layers. For example, nucleation and stress management layers may be used.
A typical prior art III-N layer structure for III-N layers grown on silicon, shown in
For the prior art layer structure of
Another prior art layer structure shown in
In many applications in which III-N heteroepitaxial layers are used, it may be necessary that substantially thick III-N epitaxial layers of adequate quality be grown on the foreign substrates. However, with these prior art layer structures, the maximum thickness of the additional epitaxial III-N layer 12 in
In one aspect, a III-N layer structure is described that includes a III-N buffer layer on a foreign substrate, an additional III-N layer, a first III-N structure, and a second III-N layer structure. The first III-N structure atop the III-N buffer layer includes at least two III-N layers, each having an aluminum composition, and the III-N layer of the two III-N layers that is closer to the III-N buffer layer having the larger aluminum composition. The second III-N structure includes a III-N superlattice, the III-N superlattice including at least two III-N well layers interleaved with at least two III-N barrier layers, the barrier layers each having an aluminum composition. The first III-N structure and the second III-N structure are between the additional III-N layer and the foreign substrate.
For layer structures described above, one or more of the following may be applicable. The difference between the aluminum compositions of the at least two III-N well layers and the aluminum compositions of the at least two III-N barrier layers can be less than about 0.5 or less than about 0.2. The thickness of each of the III-N well layers can be between about 20 and 150 nm. The thickness of each of the III-N barrier layers can be less than about 100 Å or less than about 20 Å. The III-N barrier layers can have different thicknesses. The III-N barrier layers can have aluminum compositions between about 1 and 50 percent or between about 1 and 20 percent. The barrier layers can be AlGaN and the well layers can be GaN. The III-N well or barrier layers can be doped with a dopant selected from the group consisting of Fe, Mg, and B. The foreign substrate can be silicon. The foreign substrate can be selected from the group consisting of SiC, sapphire, and zinc oxide. The foreign substrate and the III-N layers each have thermal expansion coefficients, and the thermal expansion coefficient of the foreign substrate is can be smaller than the thermal expansion coefficient of one of the III-N layers. The second III-N structure can be atop the first III-N structure. The III-N buffer layer can be AlN. The additional III-N layer can be GaN or AlGaN. The additional III-N layer can be at least 2 microns thick or at least 5 microns thick. The additional III-N layer can be an epitaxial layer. Further layers atop the additional III-N layer can be included in the structure.
In another aspect, a III-N layer structure is described that includes a III-N buffer layer on a foreign substrate, an additional III-N layer, a first III-N structure, and a second III-N structure. The first III-N structure includes at least two AlxGayN layers where x+y is less than or equal to 1, and the layer of the two layers that is closer to the III-N buffer layer can have the larger aluminum composition. The second III-N structure includes a III-N superlattice, the III-N superlattice including at least two III-N well layers interleaved with at least two III-N barrier layers, the barrier layers each having an aluminum composition. The first III-N structure and the second III-N structure can be between the additional III-N layer and the foreign substrate. For the layer structures described above, one or more of the following may be applicable. Each of the AlxGayN layers can further include an element selected from the group consisting of Indium, Boron, Phosphorus, Arsenic, and Antimony. The difference between the aluminum compositions of the at least two III-N well layers and the aluminum compositions of the at least two III-N barrier layers can be less than about 0.5 or less than about 0.2. The thickness of each of the III-N well layers can be between about 20 and 150 nm. The thickness of each of the III-N barrier layers can be less than about 100 Å or less than about 20 Å. The III-N barrier layers can have different thicknesses. The III-N barrier layers can have aluminum compositions between about 1 and 50 percent or between about 1 and 20 percent. The III-N well or barrier layers can be doped with a dopant selected from the group consisting of Fe, Mg, and B. The barrier layers can be AlGaN and the well layers can be GaN.
The foreign substrate can be silicon or can be selected from the group consisting of SiC, sapphire, and zinc oxide. The foreign substrate and the AlxGayN layers each can have thermal expansion coefficients, and the thermal expansion coefficient of the foreign substrate can be smaller than the thermal expansion coefficient of one of the III-N layers. The second III-N structure can be atop the first III-N structure. The III-N buffer layer can be AlN. The additional layer can be GaN or AlGaN. The additional III-N layer can be at least 2 microns thick or at least 5 microns thick, and can be an epitaxial layer. Further layers atop the additional layer can be included in the structure. The difference in compositions between adjacent III-N layers in III-N layer structures typically needs to be small to minimize the effects of the thermal and lattice mismatches between adjacently grown III-N layers, and also to substantially reduce or eliminate mobile charge in the structure. The layer structures described may allow for sufficiently thick III-N material layers on foreign substrates without inducing undesirable mobile charge in the III-N layers.
Devices formed by layer structures that include or are formed of III-N semiconductor layers, such as GaN and its alloys, grown atop foreign substrates, (i.e., substrates that differ substantially in composition and/or lattice structure from that of the deposited layers), such as silicon (Si), silicon carbide (SiC), or sapphire (Al2O3), are described herein. As used herein, the terms III-Nitride or III-N materials, layers or devices refer to a material or device comprised of a compound semiconductor material according to the stoichiometric formula AlxInyGazN, where x+y+z is about 1. Here, x, y, and z are compositions of Al, In and Ga, respectively.
Each layer of the first III-N structure 40 can have a substantially uniform Al composition within the layer, the layer closest to the substrate 10 having the largest Al composition, and each subsequent layer having an Al composition which is smaller than that of the layer directly beneath it, such that the layer farthest from the substrate has the smallest Al composition.
The second III-N structure 50, also described in detail below, is a III-N superlattice, or a III-N superlattice with a modulated composition. As used herein, a superlattice is a series of semiconductor layers stacked in a single direction for which, with the possible exception of the outermost layers, each intermediate layer directly contacts two other superlattice layers, both of which have either a larger or a smaller energy bandgap than that of the intermediate layer directly contacting it. The two superlattice layers are on opposite sides of the intermediate layer. The layers with energy bandgaps larger than those of the adjacent superlattice layers are referred to as barrier layers. The layers with energy bandgaps smaller than those of adjacent superlattice layers are referred to as well layers. As used herein, a superlattice with modulated composition is a superlattice in which the compositional makeup of different barrier layers or different well layers varies. For example, a GaN/AlGaN superlattice with modulated composition is a superlattice with AlGaN barrier layers that vary in aluminum composition from one barrier layer to the other. A GaN/AlGaN superlattice with modulated composition can include the following sequence of layers: GaN, AlxGa1-xN, GaN, AlyGa1-yN, GaN, AlzGa1-zN, where x, y, and z are not all equal. The well layers are GaN and the barrier layers are AlGaN with dissimilar aluminum compositions. A superlattice of modulated composition, in addition to having a variation in composition of the layers, can also have a variation in layer thicknesses, such that the thickness of well and barrier layers can change from one layer to another. The second III-N structure 50 includes periodically alternating layers of III-N well layers and barrier layers, and can include at least two sets of III-N well and barrier layers, but typically may include more.
The inclusion of the first III-N structure 40 in the III-N multilayer structure of
Consequently, the III-N layer structure of
Abrupt changes in Al composition between adjacent III-N well and barrier layers in the second III-N structure 50 can induce undesirable excess electrons or two-dimensional electron gasses (2DEGs) in the III-N well layers if there is too high of an Al compositional difference between the two layers. Layer structures with small compositional differences between adjacent III-N layers, as in the III-N layer structure of
The first III-N structure 40, an example of which is illustrated schematically in
For example, III-N layer 41 can be AlxGa1-xN and have an aluminum composition x of about 0.6. III-N layer 42, atop III-N layer 41, can be AlaGa1-aN and have an aluminum composition, a, which is less than that of III-N layer 41, such as about 0.5. Likewise, each subsequent III-N layer, which in
III-N barrier layers 51, 53, and 55 can have low aluminum composition, such as between about 1 and 50 percent, between about 2 and 20 percent, or between about 2 and 10 percent. Each III-N barrier layer in the second III-N structure 50 can have dissimilar aluminum composition to that of other III-N barrier layers, and all III-N barrier layers 51, 53, and 55 can have aluminum composition less than or equal to about 0.5, 0.2, or 0.1. For example, III-N barrier layer 51 can be AlxGa1-xN with aluminum composition x of about 0.1. III-N barrier layer 53 can be AlyGa1-yN with aluminum composition y of about 0.05. III-N barrier layer 55 can be AlzGa1-zN with aluminum composition z of about 0.01. The second III-N layer structure 50 can include additional or fewer well/barrier layers than those shown in the example of
In some implementations, at least one of the well and/or barrier layers in the second III-N structure 50 is doped, such as with Fe, Mg, or B, in order to compensate or eliminate any mobile charge that may have been induced in these layers. Inclusion of these dopants (particularly in large concentrations) in III-N devices such as transistors or HEMTs has been known to cause adverse effects, such as DC-to-RF dispersion.
However, because of the relatively small differences in composition between the well and barrier layers in the second III-N structure 50, the compensating dopant concentration can be made small while still substantially eliminating or compensating the mobile charge in the structure. For example, the compensating dopant concentration can be made smaller than in a similar layer structure that supports approximately the same amount of strain energy in III-N epitaxial layer 60, but does not include a first III-N structure 40, since a layer structure without a first III-N structure 40 would require larger compositional differences between adjacent well and barrier layers in the superlattice structure.
Other possible additions or modifications to the layer structure of
A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the techniques and structures described herein. Accordingly, other implementations are within the scope of the following claims.
