Fabrication of group III-nitride semiconductor devices
The invention relates to a method to reduce the contact resistance of ohmic contact in group III-nitride high-electron mobility transistor (HEMT). A heavily n-type doped nitride layer with modulation doping is epitaxially grown on selected contact regions for use as ohmic contact layer. The method for producing the n++ ohmic contact layer includes at least the following: deposition of nitride HEMT epitaxial structure on substrates (such as SiC, silicon, sapphire, GaN etc), deposition in-situ or ex-situ mask for selective growth of n-contact, selective etching to create of openings within the mask layer, deposition of modulation doped n++ nitride ohmic contact layer followed by ohmic metal deposition. The modulation doping involves alternating epitaxy of high and low doped nitride layers with common n-type dopant such as Ge, Si etc. The modulation doping significantly increases the range of n-type doping without detrimental effect on the material quality of the contact layer.
1. A method for fabricating a Group III-Nitride semiconductor device, comprising:
forming a buffer layer on a substrate;
forming a barrier layer on the buffer layer;
forming a passivation layer on the barrier layer;
removing partial of the passivation layer and partial of the barrier layer to form two separated trenches; and
performing a regrowth to form two n++ nitride layers with modulation doping on the two trenches;
wherein the modulation doping involves alternating epitaxy of high and low doped layers, the passivation layer prevents the regrowth from forming an n++ nitride layer on the passivation layer, and the two n++ nitride layers are separated; and
the regrowth includes a plurality of repetitive cycles, and each of the repetitive cycles includes two periods of t 1 and t 2 , a Gallium source flow rate is zero during t 1 period and higher than zero during t 2 period, and an additional metalorganic element flow rate during t 1 period is lower than the additional metalorganic element flow rate during t 2 period; and
the modulation doping includes n-type doping, and an in dopant flow rate of the n-type doping during t 1 period is higher than the n dopant flow rate of the n-type doping during t 2 period.
2. The method of claim 1 , wherein the n++ nitride layers are formed by the regrowth using materials selected from the group consisting of GaN, InGaN, AlGaN, AlInGaN, other nitride material(s) and combination thereof.
3. The method of claim 1 , further comprising one of the following:
the high doped layer being formed during the t 1 periods and the low doped layer being formed during the t 2 periods; and
the low doped layer being formed during the t 1 periods and the high doped layer being formed during the t 2 periods.
4. The method of claim 1 , further comprising one or more of the following:
using a Gallium source to provide Gallium required by the regrowth;
using an additional metalorganic source to provide additional metalorganic element required by the regrowth; and
using a dopant source to provide dopant required by the regrowth.
5. The method of claim 4 , further comprising at least one of the following:
the additional metalorganic element being different than Gallium; and
the additional metalorganic element being chosen from a group consisting of In, Al, Group III elements and combination thereof.
6. The method of claim 5 , wherein the n dopant source of the two n++ nitride layers is chosen from a group consisting of Ge with germane gas, Ge with tetramethylgermane vapor and combination thereof.
7. The method of claim 5 , wherein the n dopant source of the two n++ nitride layers is chosen from a group consisting of SiH 4 , Si 2 H 6 , GeH 4 and combination thereof, and wherein the n dopant source of the two n++ nitride layers is balanced in one or more of the following: N 2 , H 2 , Ar or other inert gas.
8. The method of claim 5 , wherein the n dopant source of the two n++ nitride layers is chosen from a group consisting of Si, Ge, Oxygen and combination thereof.
9. The method of claim 1 , wherein the high doped layer has doping level up to 6×10 20 cm −3 to 7×10 20 cm −3 and the low doped layer has doping level at 1×10 19 cm −3 to 1×10 20 cm −3 so as to achieve overall 2×10 20 cm −3 doping level.
10. The method of claim 1 , wherein a thickness of the two n++ nitride layers ranges from 100 nm to 1 um.
11. The method of claim 1 , wherein a thickness of the two n++ nitride layers are larger than a depth of the two separated trenches so that the two n++ nitride layers exposes sidewalk after the regrowth.
12. The method of claim 1 , further comprising removing a top part of the buffer layer during removing the partial of the passivation layer and the partial of the barrier layer.
13. The method of claim 12 , wherein a thickness of the top part of the buffer layer removed ranges from 1 nm to 1 um.
14. The method of claim 1 , wherein the additional metalorganic element flow rate is less than 5 L/min during at least one of periods t 1 and t 2 .
15. The method of claim 1 , further comprising one or more of the following:
the n dopant flow rate being greater than or equal to zero during at least one of periods t 1 and t 2 ; and
the n dopant flow rate being less than 5 L during at least one of periods t 1 and t 2 .
16. The method of claim 1 , further comprising at least one of the following:
a thickness of the layers grown during t 1 and t 2 periods being in the range of 1 nm to 500 nm;
the thickness of the layers grown during t 1 and t 2 periods being adjusted tuning the time duration of each of periods t 1 and t 2 ;
the thickness of the layers grown during t 1 and t 2 periods being adjusted by tuning the Gallium flow rate; and
a growth duration of t 1 or t 2 periods being in the range of 1 second to 15 minutes.
17. The method of claim 1 , wherein the Gallium source flow rate ranges from 0.01 L/min to 5 L/min during t 2 period.