U.S. patent application number 11/693763 was filed with the patent office on 2007-07-19 for gallium nitride high electron mobility transistor structure.
Invention is credited to William E. Hoke, John J. Mosca.
Application Number | 20070164313 11/693763 |
Document ID | / |
Family ID | 37431816 |
Filed Date | 2007-07-19 |
United States Patent
Application |
20070164313 |
Kind Code |
A1 |
Hoke; William E. ; et
al. |
July 19, 2007 |
GALLIUM NITRIDE HIGH ELECTRON MOBILITY TRANSISTOR STRUCTURE
Abstract
A semiconductor structure, comprising: a substrate; a first
aluminum nitride (AlN) layer having an aluminum/reactive nitride
(Al/N) flux ratio less than 1 disposed on the substrate; and a
second AlN layer having an Al/reactive N flux ratio greater than 1
disposed on the first AlN layer. The substrate is a compound of
silicon wherein the first AlN layer is substantially free of
silicon.
Inventors: |
Hoke; William E.; (Wayland,
MA) ; Mosca; John J.; (Carlisle, MA) |
Correspondence
Address: |
RAYTHEON COMPANY;c/o DALY, CROWLEY, MOFFORD & DURKEE, LLP
354A TURNPIKE STREET
SUITE 301A
CANTON
MA
02021-2714
US
|
Family ID: |
37431816 |
Appl. No.: |
11/693763 |
Filed: |
March 30, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11132533 |
May 19, 2005 |
7226850 |
|
|
11693763 |
Mar 30, 2007 |
|
|
|
Current U.S.
Class: |
257/192 ;
257/E29.246; 257/E29.25; 257/E29.253 |
Current CPC
Class: |
H01L 29/7785 20130101;
H01L 29/7787 20130101; H01L 29/2003 20130101 |
Class at
Publication: |
257/192 ;
257/E29.246 |
International
Class: |
H01L 31/00 20060101
H01L031/00 |
Claims
1. A semiconductor structure, comprising: a silicon substrate; an
AlN on the silicon substrate; and wherein portions of the AlN layer
in contact with the silicon substrate are substantially free of
silicon; a graded AlGaN layer on the AlN layer; a GaN layer on the
AlGaN layer; and including an AlGaN layer on the GaN layer.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Divisional Application of U.S. patent
application Ser. No. 11/132,533 filed on May 19, 2005, which is
hereby incorporated herein by reference in its entirety.
TECHNICAL FIELD
[0002] This invention relates generally to gallium nitride (GaN)
high electron mobility transistor (HEMT) structures.
BACKGROUND AND SUMMARY
[0003] As is known in the art, GaN HEMT devices require insulating
buffer layers for optimal performance. Unmodulated current flowing
deep in the buffer layer will degrade output power and efficiency.
FIG. 1 illustrates a typical GaN HEMT structure grown on an
insulating SiC substrate. An aluminum nitride (AlN) nucleation
layer is first grown on the substrate since gallium nitride, grown
directly on SiC will exhibit a significant conductivity spike at
the GaN/SiC interface. AlN has a very large bandgap (6.3 eV) which
facilitates high resistivity.
[0004] However, even with an AlN layer we have observed
conductivity spikes in HEMT material. FIG. 2 shows a
doping-thickness plot of a sample which had an AlN nucleation layer
with thickness of 1350 .ANG.. The plot exhibits a conductivity
spike deep in the material at a depth of 10.sup.4 .ANG. or 1 .mu.m.
The reverse bias leakage current was also measured on this sample
using a mercury probe system. As shown in FIG. 3, the leakage
current was significant (20 amperes/cm.sup.2) at -80 volts. The AlN
in the sample shown in FIG. 1 was grown with an Al/N ratio of
approximately 1.57, where the ratio Al/N is the ratio of Al to
reactive nitrogen, it being noted that much more nitrogen flux is
used compared with the aluminum flux, but only a small portion of
the nitrogen flux is reactive. This ratio of Al to reactive
nitrogen indicates that there was approximately 57% excess on the
growth surface of aluminum to reactive nitrogen. It has been
empirically found that Al to reactive nitrogen ratios greater than
1 (aluminum-rich) result in improved films.
[0005] FIG. 4 shows the SIMS (secondary ion mass spectroscopy)
profile of the sample of FIG. 1. The N-type conduction spike is
caused by silicon from the SiC substrate which is migrating through
the AlN layer and piling up at the AlN/GaN interface. GaN is easily
doped by silicon which results in the conductivity. The sample
shown in FIG. 1 contains a 1350 .ANG. AlN layer grown with
Al/N=1.57. The profile shows silicon from the SiC substrate
migrating through the thick AlN layer and piling up at the GaN/AlN
interface.
[0006] We believe that the mechanism for such rapid silicon
diffusion through the thick AlN layer is due to the excess aluminum
on the surface which reacts with the SiC substrate. At our growth
temperature of approximately 750.degree. C., aluminum is a liquid.
Al (liquid)+SiCAl.sub.4C.sub.3+Al (liquid+Si) (1) Since silicon is
now in a liquid state, it can rapidly move through the AlN film
which is grown with excess aluminum.
[0007] Now that we have discovered the source of the doping
(silicon) and mechanism for the rapid dopant migration, a structure
is provided having a two-step AlN nucleation layer. In the old
process the silicon concentration in the AlN peaked at greater than
1.times.10.sup.20 cm.sup.-3. In the process according to the
invention, the silicon concentration peaked at less than
3.times.10.sup.18 cm.sup.-3. Thus, in accordance with the invention
the AlN is substantially free of silicon.
[0008] In accordance with the invention, it has been demonstrated
that good AlN material quality may be obtained with the entire AlN
layer being conventionally grown by molecular beam epitaxy (MBE)
with an Al to reactive nitrogen ratio greater than 1
(aluminum-rich).
[0009] In one embodiment, an initial AlN layer is grown with an Al
to reactive nitrogen ratio less than 1 (nitrogen-rich) so that
there is no free aluminum to react with the SiC surface. Once the
SiC surface has been completely covered by AlN, the Al to reactive
nitrogen ratio is increased to greater than 1 (aluminum-rich) for
the rest of the layer to improve the material quality. The initial
layer can be thin (30-200 .ANG.) since its function is to cover the
SiC surface. By making this layer thin, the roughness associated
with growing AlN nitrogen-rich is minimized. The second AlN layer
grown Al-rich improves the material quality.
[0010] In accordance with the invention, a semiconductor structure
is provided having a substrate, a first AlN layer having an Al to
reactive nitrogen ratio less than 1 disposed on the substrate, and
a second AlN layer having an Al to reactive nitrogen ratio greater
than 1 disposed on the first AlN layer.
[0011] In one embodiment, the substrate is a compound of silicon
and wherein the first AlN layer is substantially free of
silicon.
[0012] In accordance with the invention, a method is provided for
forming a semiconductor structure comprises: growing a layer of AlN
on a substrate comprising a compound of silicon with the reactive
nitrogen flux greater than the aluminum flux; and changing the
aluminum and reactive nitrogen fluxes such that the aluminum flux
is greater than the reactive nitrogen flux.
[0013] The details of one or more embodiments of the invention are
set forth in the accompanying drawings and the description below.
Other features, objects, and advantages of the invention will be
apparent from the description and drawings, and from the
claims.
DESCRIPTION OF DRAWINGS
[0014] FIG. 1 is a GaN HEMT structure according to the PRIOR
ART;
[0015] FIG. 2 is a doping-depth plot for the PRIOR ART GaN HEMT of
FIG. 1;
[0016] FIG. 3 is a Mercury probe I-V leakage measurement of the
PRIOR ART GaN HEMT of FIG. 1;
[0017] FIG. 4 is a SIMS depth profile of PRIOR ART GaN HEMT of FIG.
1;
[0018] FIG. 5 is a GaN structure according to the invention;
[0019] FIG. 6 is a SIMS depth profile of the GaN HEMT structure of
FIG. 5;
[0020] FIG. 7A is a doping-depth plot for the GaN HEMT structure of
FIG. 5;
[0021] FIG. 7B is a Mercury probe I-V leakage measurement of the
GaN HEMT structure of FIG. 5; and
[0022] FIG. 8 is a process flow chart of the method used to form
the GaN HEMT structure of FIG. 5.
[0023] Like reference symbols in the various drawings indicate like
elements.
DETAILED DESCRIPTION
[0024] Referring now to FIG. 5, a GaN HEMT structure 10 is shown to
prevent buffer conductivity due to silicon migration from a
substrate 12. The substrate 12 is a compound of silicon. Here, the
compound is silicon carbide, SiC. The GaN HEMT structure 10 is
formed with a two-step AlN layer 14 grown with a significant
reduction in silicon migration. The first AlN layer 14a was 115
.ANG.-thick with an Al to reactive nitrogen ratio of 0.97 on the
surface of the substrate 12. The second layer 14b was 305
.ANG.-thick with an Al to reactive nitrogen ratio of 1.21 on the
first layer 14a giving a total AlN layer 14 thickness of 420 .ANG..
The SIMS profile for the structure 10 is shown in FIG. 6. A
comparison with FIG. 4 shows that the maximum silicon (Si) peak at
the GaN/AlN interface has been reduced by a factor of 100. Thus,
the AlN layer 14 is substantially free of silicon. These
improvements were obtained even though the total AlN thickness in
FIG. 5 is one-third that in FIG. 1, discussed above.
[0025] Completing the structure 10, a GaN layer 16 is grown on the
second layer 14b AlN. Optionally, an Al.sub.xGa.sub.1-xN layer 15
may be grown under the GaN layer 16.
[0026] An Al.sub.xGa.sub.1-xN layer 18 is grown on the GaN layer
16. A cap layer 20 of GaN may be grown on the Al.sub.XGa.sub.1-xN
layer. Source (S) and drain (D) electrodes are formed in ohmic
contact with the layer 20 if there is a cap and layer 18 if there
is no cap and a gate electrode (G) is formed in Schottky contact
with layer 20 if there is a cap and layer 18 if there is no cap in
any conventional manner after removal from the MBE machine.
[0027] Corresponding improvements are observed in the doping-depth
profile, FIG. 7A, and Current (I)-voltage (V) i.e., I-V, leakage
results shown in FIG. 7B. In comparing FIGS. 2 and 7A, the doping
spike has been significantly reduced, if not eliminated. In
comparing FIG. 3 and FIG. 7B, it is noted that the leakage current
at -80 volts (FIG. 3) has dropped almost 4 orders of magnitude from
20 amperes/cm.sup.2 to 2.8.times.10.sup.-3 amperes/cm.sup.2 (FIG.
7B), where FIG. 7A is a doping-depth plot for the GaN HEMT 10 using
the two-step AlN buffer layer 14 and FIG. 7B is the mercury probe
I-V leakage measurement of the structure of FIG. 5 that shows at
-80 volts the leakage current is 2.8 mA/cm.sup.2.
[0028] It is noted that the nitrogen molecule, N.sub.2, is too
stable to react with aluminum to form AlN. The process we use is to
flow the nitrogen gas through a radio frequency (RF) plasma which
excites some of the nitrogen molecules to form reactive nitrogen (a
combination of nitrogen atoms and excited nitrogen molecules,
N.sub.2). The amount of reactive nitrogen that reacts with aluminum
is only approximately 1% of the total nitrogen flow. That is why we
have to calibrate the machine. Consequently, there is much more
nitrogen flux hitting the substrate than aluminum flux. The flux
ratio of interest is the Al to reactive nitrogen ratio, not the Al
to nitrogen ratio. Nitrogen-rich growth conditions means that the
rate of reactive nitrogen atoms (which can react with aluminum)
impinging on the substrate surface is higher than the rate of
aluminum atoms hitting the substrate surface. The opposite
condition is aluminum-rich.
[0029] More particularly, we normally change from nitrogen-rich to
aluminum-rich by adjusting the rate of aluminum atoms (also called
the aluminum flux) impinging on the substrate surface. Aluminum is
evaporated and the vapor pressure which determines the aluminum
flux increases exponentially with the aluminum furnace temperature.
We determine when the Aluminum flux=Reactive nitrogen flux by the
following means: First Al.sub.xGa.sub.1-xN is grown. (The same
nitrogen plasma conditions will be used for AlN as was used for the
AlGaN. If the same nitrogen plasma conditions are not used, we
estimate the effect of the different conditions on the nitrogen
flux.) From x-ray measurements the aluminum concentration is
measured. For example, assume it was Al.sub.0.25Ga.sub.0.75N. To
grow with Al flux=N reactive flux for AlN then means we need 4
times the aluminum flux so the furnace temperature must be
increased to achieve 4 times the vapor pressure. If we use a higher
aluminum furnace temperature (and consequently higher vapor
pressure) the growth conditions are aluminum-rich.
[0030] In the process according to the invention, the silicon
concentration peaked at less than 3.times.10.sup.18 cm.sup.-3.
Thus, in accordance with the invention the AlN is substantially
free of silicon
[0031] The process for forming the structure of FIG. 5 is,
referring to FIG. 8, as follows:
[0032] Calibrate a molecular beam epitaxy (MBE) machine by
determining when the Al flux equals the reactive N flux at the
substrate surface, Step 800. Place substrate having a compound of
silicon into the calibrated MBE machine, Step 802. Direct onto the
substrate 12 surface a flux of aluminum atoms and a flux of
reactive nitrogen atoms, Step 804. Adjust aluminum and reactive
nitrogen fluxes such that the reactive nitrogen flux is greater
than the aluminum flux, Step 804. Grow a layer 14a of AlN until a
thickness of 30-300 angstroms, Step 806. Change the aluminum and
reactive nitrogen fluxes such that the aluminum flux is greater
than the reactive nitrogen flux, Step 808. Grow a layer 14b of AlN
until a thickness of 50-1000 angstroms, Step 810. Grow a graded
Al.sub.XGa.sub.1-xN layer 15, with aluminum concentration
ramped-down from x=1 to x=0 and having a thickness of 0 angstroms
(i.e., no grade) to 2000 angstroms, Step 812. Grow a GaN layer 16
having a thickness of 0.3-3.0 microns, Step 814. Grow
Al.sub.xGa.sub.1-xN layer 18 with 0.05<x<0.40 and thickness
of 100-500 angstroms, Step 816. Grow GaN cap layer 20 with
thickness of 0 angstroms (no cap) to 300 angstroms, Step 818.
[0033] A number of embodiments of the invention have been
described. Nevertheless, it will be understood that various
modifications may be made without departing from the spirit and
scope of the invention. Accordingly, other embodiments are within
the scope of the following claims.
* * * * *