U.S. patent application number 09/866442 was filed with the patent office on 2002-11-28 for method of manufacturing gan-based p-type compound semiconductors and light emitting diodes.
This patent application is currently assigned to Kopin Corporation. Invention is credited to Chen, Jyh-Chia, Choi, Hong K., Fan, John C.C., Liao, Shirong, Ye, Jinlin.
Application Number | 20020177251 09/866442 |
Document ID | / |
Family ID | 25347631 |
Filed Date | 2002-11-28 |
United States Patent
Application |
20020177251 |
Kind Code |
A1 |
Ye, Jinlin ; et al. |
November 28, 2002 |
METHOD OF MANUFACTURING GAN-BASED P-TYPE COMPOUND SEMICONDUCTORS
AND LIGHT EMITTING DIODES
Abstract
Compound semiconductor material is irradiated with x-ray
radiation to activate a dopant material. Active carrier
concentration efficiency may be improved over known methods,
including conventional thermal annealing. The method may be
employed for III-V group compounds, including GaN-based
semiconductors, doped with p-type material to form low resistivity
p-GaN. The method may be further employed to manufacture GaN-based
LEDs, including blue LEDs, having improved forward bias voltage and
light-emitting efficiency.
Inventors: |
Ye, Jinlin; (South Easton,
MA) ; Chen, Jyh-Chia; (Ellicott City, MD) ;
Liao, Shirong; (South Easton, MA) ; Choi, Hong
K.; (Sharon, MA) ; Fan, John C.C.; (Brookline,
MA) |
Correspondence
Address: |
HAMILTON, BROOK, SMITH & REYNOLDS, P.C.
530 VIRGINIA ROAD
P.O. BOX 9133
CONCORD
MA
01742-9133
US
|
Assignee: |
Kopin Corporation
Taunton
MA
|
Family ID: |
25347631 |
Appl. No.: |
09/866442 |
Filed: |
May 25, 2001 |
Current U.S.
Class: |
438/47 ;
257/E21.326; 257/E21.348 |
Current CPC
Class: |
H01L 33/0095 20130101;
H01L 21/2683 20130101; H01L 21/3245 20130101; H01L 33/325
20130101 |
Class at
Publication: |
438/47 |
International
Class: |
H01L 021/00 |
Claims
What is claimed is:
1. A method of activating a dopant in a compound semiconductor
comprising the step of: irradiating the compound semiconductor with
x-ray radiation.
2. The method of claim 1, wherein the compound semiconductor
comprises a III-V group compound.
3. The method of claim 2, wherein the m-V group compound comprises
a GaN-based compound.
4. The method of claim 3, wherein the GaN-based compound is
represented by the general formula Ga.sub.xAl.sub.1-xN, where
0.ltoreq.x.ltoreq.1.
5. The method of claim 3, wherein the GaN-based compound is
represented by the general formula In.sub.xGa.sub.1-xN, where
0.ltoreq.x.ltoreq.1.
6. The method of claim 1, wherein the compound semiconductor is
irradiated at approximately an ambient temperature.
7. The method of claim 1, wherein the compound semiconductor is
irradiated in an ambient atmosphere.
8. The method of claim 1, further comprising forming a light
emitting device with the compound semiconductor.
9. The method of claim 1, wherein the compound semiconductor is
irradiated for a period in a range of 4-7 minutes.
10. The method of claim 1, wherein the duration of time of the
x-ray irradiation is selected to maximize the carrier concentration
of the compound semiconductor element.
11. The method of claim 1, comprising the additional step of:
during the x-ray irradiation, changing the position of the compound
semiconductor relative to the incident x-ray radiation to alter the
incident angle of the x-ray radiation over a limited angular
range.
12. The method of claim 10, wherein the limited angular range is
approximately 5000 arc sec.
13. The method of claim 1, wherein the p-type dopant is Mg.
14. The method of claim 13, wherein the Mg concentration of a
p-doped layer of the compound semiconductor is approximately
1.times.10.sup.19 cm.sup.-3.
15. The method of claim 14, wherein after the x-ray irradiation,
the p-type carrier concentration of the compound semiconductor is
approximately 1.times.10.sup.18 cm.sup.-3.
16. The method of claim 1, wherein the step of irradiating the
compound semiconductor comprises: generating x-ray radiation with
an x-ray source having an accelerating voltage of approximately 40
kV and an intensity of approximately 0.4 mA; and
17. The method of claim 1, wherein the x-ray irradiates a p-type
impurity doped region of the compound semiconductor to a depth of
approximately 3 .mu.m.
18. A method of fabricating a p-type compound semiconductor
comprising the steps of: growing a compound semiconductor by using
reaction gas containing p-type impurity; and irradiating the
compound semiconductor with x-ray radiation to activate the p-type
impurity.
19. The method of claim 18, wherein the compound semiconductor is
grown by a metalorganic vapor phase deposition process.
20. A method of fabricating compound semiconductor light emitting
diode (LED) comprising the steps of: growing a compound
semiconductor LED structure by an epitaxial growth process; and
irradiating the LED with x-ray radiation to activate a p-type
impurity.
21. The method of claim 20, wherein the LED is a GaN-based LED.
22. The method of claim 20, wherein the LED is a blue LED.
23. The method of claim 20, wherein the LED is a blue LED is a
blue-green LED.
24. The method of claim 20, wherein the LED is a blue LED is a
green LED.
Description
BACKGROUND OF THE INVENTION
[0001] Recently, much attention has been focused on GaN-based
compound semiconductors (e.g., Ga.sub.xAl.sub.1-xN or
In.sub.xGa.sub.1-xN, where 0.ltoreq.x.ltoreq.1) for blue and green
light emitting diode (LED) applications. One important reason is
that GaN-based LEDs have been found to exhibit excellent light
emission at room temperature.
[0002] In general, GaN-based LEDs comprise a multilayer structure
in which n-type and p-type GaN are stacked on a substrate (most
commonly a sapphire substrate), and In.sub.xGa.sub.1-xN/GaN
multiple quantum wells are sandwiched between the p-type and n-type
GaN layers. A number of methods for growing the multilayer
structure are known in the art, including metalorganic chemical
vapor deposition (MOCVD), molecular beam epitaxy (MBE) and hydride
vapor phase epitaxy (HVPE).
[0003] It is also known in the art that these conventional growth
methods for compound semiconductor structures have proven
problematic, particularly with respect to forming a p-type
GaN-based layer suitable for LED applications. In general, GaN
layers formed by known growth methods, such as MOCVD, and doped
with p-type material such as magnesium, behave like a
semi-insulating or high-resistive material. It is generally
understood that this results from hydrogen passivation, or hydrogen
that is present in the reaction chamber bonding with the p-type
dopant and thus preventing the dopant in the GaN from behaving as
an active carrier. Because of this phenomenon, p-type GaN having a
sufficiently low resistivity to form the p-n junction required for
LED applications cannot be produced by conventional techniques.
[0004] Various attempts have been made to overcome the difficulties
in obtaining p-type GaN-based compound semiconductors. In one
technique known as low-energy electron-beam irradiation (LEEBI), a
high-resistive semi-insulating GaN layer, which is doped with a
p-type impurity (Mg), is formed on top of the multilayers of the
GaN compound semiconductor. Then, while maintaining the
semiconductor compound at temperatures up to 600.degree. C., the
compound is irradiated with an electron beam having an acceleration
voltage of 5-15 kV in order to reduce the resistance of the p-doped
region near the sample surface. However, with this method,
reduction in the resistance of the p-doped layer can be achieved
only up to the point that the electron beam penetrates the sample,
i.e. a very thin surface portion less than about 0.5 .mu.m deep.
Furthermore, this method requires heating the substrate to
temperatures up to approximately 600.degree. C. in addition to
high-voltage acceleration of the electron beam.
[0005] Thermal annealing can be used to activate a small fraction
of the dopant as an active carrier. For example, in order to
achieve a carrier concentration of 1.times.10.sup.18 cm.sup.-3, the
concentration of the p-type dopant must be as high as
1.times.10.sup.20 cm.sup.-3.
[0006] Also, the high level of doping required for the thermal
annealing method degrades Hall mobility of the p-type GaN, with a
typical value of Mg-doped GaN of only 20 cm.sup.2/v-s. Moreover,
because of the heavy doping and the degraded top layer
crystallinity, the forward bias voltage of GaN-based LEDs cannot be
made as low as desired, and the light-emitting efficiency is
decreased.
[0007] Furthermore, the annealing temperature is typically more
than 800.degree. C., which is higher than the temperature used for
forming the light emitting layers. These high temperatures may
additionally degrade the light-emitting efficiency of the
device.
SUMMARY OF THE INVENTION
[0008] A method for activating a dopant in a semiconductor
comprises irradiating the semiconductor with x-ray radiation. This
process can be used to fabricate low-resistance p-type compound
semiconductors for example, including III-V Group compound
semiconductors, such as GaN-based semiconductors. The x-ray
radiation may be generated by an x-ray source and directed to the
surface of the doped compound semiconductor wafer. In certain
embodiments, the x-ray source has an accelerating voltage of
approximately 40 kV and an intensity of approximately 0.4 mA, for
example. Additionally, the position of the sample may be changed
with respect to the incident x-ray radiation so that the incident
angle of the x-ray radiation sweeps through a limited angular range
during the irradiation process. According to at least one
embodiment, the limited angular range is approximately 100-5000 arc
sec.
[0009] In contrast to known methods for activating dopants in
semiconductor materials, this method can be performed at room
temperature in an atmospheric, clean room, or processing chamber
environment. The time duration of the x-ray irradiation may also be
selectively controlled to maximize the carrier concentration of the
irradiated samples. Typically, when using an x-ray system such as
Model QC1A available from Bede Scientific, Inc., an irradiation
time between approximately 4 and 7 minutes is used to achieve the
desired dosage.
[0010] The present invention further relates to a method for
improving the doping efficiency of p-type compound semiconductors.
According to certain embodiments, a compound semiconductor is doped
with a p-type impurity, such as magnesium, and having a dopant
concentration of approximately 1.times.10.sup.19 cm.sup.-3, is
irradiated with x-ray radiation to produce a p-type compound
semiconductor with a carrier concentration of 1.times.10.sup.18
cm.sup.-3.
[0011] A method of fabricating a p-type compound semiconductor is
additionally disclosed, the method comprising the steps of growing
a compound semiconductor by using reaction gas containing p-type
impurities, and then irradiating the compound semiconductor with
x-ray radiation to activate the p-type impurity.
[0012] A method of fabricating a compound semiconductor light
emitting diode (LED) is further disclosed, the method comprising
the steps of growing a compound semiconductor LED structure using a
known epitaxial growth process, and then irradiating the LED with
x-ray radiation to activate a p-type impurity. The LED may be a
GaN-based LED, and in particular embodiments, a GaN-based blue LED.
In general, the LEDs produced with the x-ray irradiation method of
the present invention demonstrate improved forward bias voltage and
light-emitting efficiency over previously known methods.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1a is a schematic view of a III-V Group layered
structure to be irradiated with x-ray radiation according to the
principles of the present invention;
[0014] FIG. 1b is a schematic view of an LED device manufactured in
accordance with the principles of the present invention;
[0015] FIG. 2 is a schematic diagram illustrating an x-ray source
irradiating a compound semiconductor according to the principles of
the present invention;
[0016] FIG. 3 is a graph illustrating the carrier concentration of
Mg-doped p-GaN irradiated by x-ray radiation compared to the
carrier concentration of Mg-doped p-GaN treated by conventional
post-growth thermal annealing;
[0017] FIG. 4 is a graph illustrating the relationship between
carrier concentration and x-ray irradiation time according to the
principles of the present invention;
[0018] FIG. 5 is a graph illustrating the Hall mobility of Mg-doped
p-GaN irradiated by x-ray radiation compared to the Hall mobility
of Mg-doped p-GaN treated by conventional post-growth thermal
annealing;
[0019] FIG. 6 is a graph comparing the photoluminescence intensity
of an Mg-doped GaN-based blue light emitting diode (LED) irradiated
by x-ray radiation, an Mg-doped GaN-based blue LED treated by
conventional post-growth thermal annealing, and an Mg-doped
GaN-based blue LED with no post-growth treatment;
[0020] FIG. 7 is a graph illustrating the relationship between the
current and forward bias voltage (I-V curve) of a GaN-based blue
LED irradiated by x-ray radiation, and a GaN-based blue LED treated
by conventional post-growth thermal annealing; and
[0021] FIG. 8 is a graph comparing the electroluminescence (EL)
spectra of a GaN-based blue LED irradiated by x-ray radiation and a
GaN-based blue LED treated by conventional post-growth thermal
annealing.
[0022] The foregoing and other objects, features and advantages of
the invention will be apparent from the following more particular
description of preferred embodiments of the invention, as
illustrated in the accompanying drawings in which like reference
characters refer to the same parts throughout the different views.
The drawings are not necessarily to scale, emphasis instead being
placed upon illustrating the principles of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0023] Turning now to FIG. 1a, a layered III-V Group compound
semiconductor structure is illustrated schematically. As shown, the
structure comprises a substrate (in this case, a sapphire
substrate), a first layer of an n-type doped compound semiconductor
(e.g. GaN), multiple quantum wells (e.g. In.sub.xGa.sub.1-xN/GaN),
and a top layer of p-type doped compound semiconductor (e.g. GaN).
According to one aspect, the present invention relates to a method
for irradiating a compound semiconductor, including a compound
semiconductor as illustrated in FIG. 1a, with x-ray radiation. The
present invention also relates to a method for fabricating a
compound semiconductor light emitting diode (LED), comprising the
steps of growing a compound semiconductor LED structure by an
epitaxial growth process and irradiating the LED with x-ray
radiation to activate a p-type impurity. FIG. 1b schematically
illustrates the structure of a finished LED device, the LED device
fabricated from a layered III-V Group compound semiconductor, such
as the structure shown in FIG. 1a.
[0024] Turning now to FIG. 2, an x-ray source 1 comprises a device
for generating x-ray radiation and directing one or more beams of
x-ray radiation 3 at a target area. A conventional double-crystal
x-ray diffractometer may be employed as a suitable source for the
x-ray radiation. According to some embodiments, the x-ray source
tube has an accelerating voltage of 40 kV and an intensity of 0.4
mA for generating the x-ray radiation.
[0025] Also illustrated in FIG. 2 is a sample 2 comprising a
compound semiconductor, such as a III-V group compound
semiconductor wafer, which is positioned for receiving incident
x-ray radiation from the x-ray source. As illustrated here, the
sample may be located directly below the x-ray source. In certain
embodiments, the position of the sample relative to the incident
x-ray radiation may be changed during the irradiation process for
reasons which will become apparent below. As shown in FIG. 2, the
sample is positioned such that the x-ray radiation is incident upon
the surface of the sample at an angle, .theta.. According to one
aspect, the sample may be tilted or rocked with respect to at least
one axis to change the incident angle .theta. through a limited
angular range. One method for achieving this tilting motion is to
position the sample on a stage 4 that is capable of a rocking
motion with respect to at least one axis.
[0026] Alternatively, the sample may maintain stationary while the
x-ray source alters the incident angle of the x-ray radiation
through the limited angular range.
[0027] The present invention generally relates to a process of
irradiating a compound semiconductor containing a p-type impurity
with x-ray radiation. According to certain embodiments, the present
invention additionally comprises fabricating the compound
semiconductor containing the p-type impurity through an epitaxial
growth process, followed by a further step of irradiating the
compound with x-ray radiation. The irradiation process may be
employed in a clean room environment, and may even be performed
within the growth chamber itself. Moreover, the step of x-ray
irradiation can be performed at room temperature, and in an ambient
atmospheric environment.
[0028] In general, the present method may be employed to
manufacture a p-type semiconductor element from, for instance, a
III-V group compound, such as GaN. There are at least three
commonly utilized growth methods for GaN-based semiconductors,
including metalorganic chemical vapor deposition (MOCVD), molecular
beam epitaxy (MBE) and hydride vapor phase epitaxy (HVPE). Of
these, MOCVD is the most common method. In this process,
metalorganic compound gases such as trimethylgallium (TMG),
trimethylindium (TMI), and trimethylaluminum (TMA), along with
ammonium, are introduced into a reactor where a substrate,
typically sapphire, is located. The epitaxial film of GaN-based
semiconductors is grown on the substrate at high temperatures,
typically between 700.degree. C. and 1000.degree. C. To improve
crystallinity, either an AlN buffer layer or a GaN buffer layer is
first grown on the substrate at a lower temperature (e.g.
600.degree. C.) and the GaN compound is grown on the buffer layer
at the higher temperatures.
[0029] By supplying one or more impurity gases during the growth of
the film, a multilayer structure made of n-type and/or p-type GaN
compound semiconductors can be manufactured. There are a number of
known n-type impurities, such as Si, that are readily used, thus
rendering it relatively straightforward to grow characteristic
n-type semiconductor layers.
[0030] However, employing common p-type impurities, such as Mg or
Zn, will not result in an equivalently repeatable, well-defined
p-type layer. Generally, p-doped layers formed with conventional
techniques behave as a semi-insulating or highly-resistive layers.
Because many of the most useful applications for GaN-based
semiconductor structures, including LED applications, depend upon
the characteristics of a p-n junction, without a characteristic
p-type layer with sufficiently low resistance, these practical
applications become difficult, if not impossible, to achieve, and
any advantages inherent in the GaN-based compound are lost.
[0031] According to one aspect, the present invention provides a
method for overcoming these and other difficulties by irradiating
the p-doped compound semiconductor with x-ray radiation to activate
the p-type impurities in the doped layer. It is generally
understood in the art that p-type layers formed by conventional
methods exhibit high resistance due to gasses in the reaction
chamber bonding with p-type impurities to prevent them from acting
as acceptors. More specifically, for a GaN-based compound
semiconductor grown by the MOVCD method, NH.sub.3 is typically used
as the source of the N atoms. During the growth process, NH.sub.3
is decomposed to generate hydrogen atoms. These hydrogen atoms
react with the p-type dopant, such as Mg, and form Mg--H bonds or
Mg--H complexes. Thus, GaN into which p-type impurity is doped is
highly resistive or semi-insulating. Irradiating the p-doped
semiconductor compound with high-energy x-ray photons, however,
breaks the Mg--H bonds and releases the hydrogen from the Mg--H
bond to leave a characteristic p-type GaN layer. The p-type
impurity, Mg, then behaves as an active acceptor.
[0032] According to another aspect, the present invention relates
to an improved method for activating a p-type impurity in a
compound semiconductor material. As discussed above, the LEEBI
method may be employed to convert high-resistive GaN material into
a p-type conductive material. However, the effectiveness of this
method is limited by the penetration of the electron beam: a very
thin surface portion less than a depth of about 0.5 .mu.m. By
contrast, the x-ray radiation of the present invention is capable
of activating the entire p-doped region at depths greater than 0.5
.mu.m and up to 3 .mu.m. Moreover, the present method can be
advantageously employed over a wide range of temperatures,
including room temperature. The LEEBI method entails heating and
maintaining the substrate at a temperature of approximately
600.degree. C.
[0033] The present invention further relates to a method for
improving the activation efficiency for a p-type compound
semiconductor. FIG. 3 is a graph illustrating the carrier
concentration of Mg-doped p-GaN irradiated with x-ray radiation
compared to Mg-doped p-GaN treated by conventional thermal
annealing. All of the samples were grown under the same conditions:
first by growing a GaN buffer layer on a sapphire substrate by the
MOVCD method, then forming a 1.5 .mu.m thick semi-insulating
unintentionally doped GaN layer on the buffer layer, and finally
growing a 2 .mu.m thick GaN layer doped with Mg as a p-type
impurity on top of the semi-insulating layer. The doping
concentration for all samples was approximately 1.times.10.sup.20
cm.sup.-3. The carrier concentration was measured by the
conventional capacitance-voltage (C-V) method. The data plotted in
FIG. 3 was extracted from the same irradiation conditions and
thermal annealing conditions for each group. The conventional
thermal annealing was carried out in an MOCVD reactor at a constant
temperature of 820.degree. C. for 20 minutes in an N.sub.2
atmosphere. The exposure time for the x-ray irradiated samples was
4 minutes.
[0034] As shown in FIG. 3, the high-resistivity GaN-based compound
semiconductors were converted to p-type semiconductors under both
methods. However, the carrier concentration produced by the x-ray
irradiation of the present invention is 4-10 times higher than that
produced by thermal annealing. Assuming that the percentage of
activation does not change with doping concentration, the doped Mg
concentration should be as high as 1.times.10.sup.20 cm.sup.-3 for
the post-growth thermal annealing in order to achieve a p-type
carrier concentration of 1.times.10.sup.18 cm.sup.-3. However, if
the x-ray irradiation method is employed instead, the Mg
concentration need only be 1.times.10.sup.19 cm.sup.-3 to obtain
the equivalent carrier concentration.
[0035] FIG. 4 illustrates the relationship between the x-ray
irradiation time and the carrier concentration of a GaN-based
compound semiconductor. In this example, the GaN-based compound
semiconductor was again formed by growing a GaN buffer layer on the
sapphire substrate by the MOCVD method, growing a 1.5 .mu.m thick
semi-insulating unintentionally doped GaN layer on the buffer
layer, and growing a 2 .mu.m thick GaN layer doped with Mg as a
p-type impurity on the semi-insulating layer. In this example,
however, the samples were x-ray irradiated for different durations.
As apparent from FIG. 4, as the irradiation time increases the
carrier concentration initially increases sharply, and then reaches
a level of approximately 1.times.10.sup.19 cm.sup.-3 when the
irradiation time is about 4 minutes. For irradiation times between
4 and 7 minutes, the carrier concentration remains about the same.
For longer irradiation times, however, the concentration gradually
decreases, most likely due to recombination of the Mg--H bonds and
complexes. Preferably, the duration of the x-ray irradiation is
selected to maximize the resultant carrier concentration of the
p-type semiconductor material.
[0036] According to another aspect of the present invention, the
high carrier efficiency of the x-ray irradiated p-type compound
semiconductor may be further improved by changing the incident
angle of the x-ray radiation during the irradiation process. For
instance, the targeted sample may be tilted over a angular small
range so that the x-ray incident angle likewise changes in that
small range. Alternatively, the angle of the incident radiation may
be changed directly by, for instance, moving the x-ray source to
alter the angle at which the x-ray beams are emitted. In one
embodiment, the incident angle is altered through a limited angular
range of 5000 arc sec.
[0037] FIG. 5 is a graph comparing the Hall mobility between
thermally annealed GaN compound semiconductor layers and x-ray
irradiated compound semiconductor layers. As before, each GaN-based
compound semiconductor was prepared by growing a GaN buffer layer
on the sapphire substrate by the MOCVD method, growing a 1.5 .mu.m
thick semi-insulating unintentionally doped GaN layer on the buffer
layer, and growing a 2 .mu.m thick GaN layer doped with Mg as a
p-type impurity on the semi-insulating layer. A first group of
samples underwent conventional thermal annealing at 820.degree. C.
for 20 minutes, while the second group were irradiated by x-ray
radiation for 4 minutes according to the principles of the present
invention. The doping concentrations for the thermally annealed and
x-ray irradiated samples were 1.times.10.sup.20 cm.sup.-3 and
1.times.10.sup.19 cm.sup.-3, respectively, and the post-treatment
carrier concentration for both groups was approximately
1.times.10.sup.18 cm.sup.-3. FIG. 5 illustrates the Hall mobility
obtained by van der Paw measurement plotted vs. sample series
number.
[0038] As is apparent from FIG. 5, the Hall mobility of the x-ray
irradiated p-type GaN compound semiconductor is 25-35 cm.sup.2/v-s,
while the Hall mobility of the thermally annealed p-type GaN
compound semiconductor is only 10-20 cm.sup.2/v-s for the same
level of carrier concentration. The higher Hall mobility for the
x-ray irradiated samples implies a better crystallinity, which
would result in a lower resistance. Thus, an LED employing an x-ray
irradiated Mg-doped GaN layer as the p-layer exhibits a lower
forward bias voltage than presently allowed using conventional
manufacturing techniques.
[0039] According to yet another aspect of the present invention, a
method of fabricating a compound semiconductor light emitting diode
(LED) comprises the steps of growing a compound semiconductor LED
by an epitaxial growth method and irradiating the LED with x-ray
radiation to activate a p-type impurity. In one embodiment, a
GaN-based LED structure is prepared by forming, in the following
order, a GaN buffer layer on a sapphire substrate, a 1 .mu.m thick
undoped GaN layer, a 2 .mu.m thick Si-doped GaN layer, 5 periods of
unintentionally doped In.sub.xGa.sub.1-xN/GaN multiple quantum
wells, and a 0.3 .mu.m thick Mg-doped GaN layer. The thickness of
the wells and barriers are 30 and 100 Angstroms, respectively. The
Mg-doped GaN layer may then be irradiated by x-ray radiation
according to the principles described above to obtain a low
resistivity p-type layer.
[0040] Employing the x-ray irradiation method of the present
invention, a GaN-based compound semiconductor LED may be
manufactured with lower forward bias voltage and higher efficiency
than is attainable using current fabrication techniques. FIG. 6 is
a graph comparing the photoluminescence (PL) intensity of GaN-based
LED structures as just described where a first group of LEDs has
been x-ray irradiated, a second group has undergone conventional
thermal annealing, and a third group has not undergone any
post-growth treatment. The LED layers were illuminated by a He--Cd
laser source to measure the intensity of the photoluminescence as
an evaluation of the crystallinity. The higher PL measurements
reflect better sample crystallinity.
[0041] The PL peak at .about.480 nm, which is associated with the
quantum wells, is much lower for the thermally annealed sample than
that of the untreated sample. The x-ray irradiated sample, however,
compared favorably to the untreated sample, thus indicating that
the thermal annealing process degrades the quality of GaN-based
structure in a way that x-ray irradiation does not. Moreover, for
the x-ray irradiated samples, there is a strong peak at .about.440
nm, which is associated with the activated Mg, thus indicating that
x-ray irradiation yields much stronger Mg activation than no
post-growth treatment and conventional thermal annealing.
[0042] FIG. 7 is a graph showing the relationship between the
current and forward bias voltage (I-V curve) for GaN-based blue
LEDs treated by x-ray irradiation and conventional thermal
annealing. As is apparent from the graph, the forward bias voltage
(defined at 20 mA current) for the x-ray irradiated LEDs is 0.35 V,
which is lower than the 0.37 V for the thermal-annealed LEDs. This
is not surprising, considering that, as has been demonstrated
above, the thermally annealed Mg-doped GaN layer has a lower
carrier concentration, lower Hall mobility and degraded
crystallinity relative to the comparable x-ray irradiated layer. As
a result of these factors, the thermal annealing process produces a
more resistive p-GaN layer, and accordingly, the forward bias
voltage for the LED is higher.
[0043] FIG. 8 is a graph comparing the electroluminescence (EL)
spectra of GaN-based compound semiconductor blue LEDs fabricated by
thermal annealing and x-ray irradiation. These LEDs were prepared
in the same way as described above, with the same carrier
concentration in the top p-GaN layer. The EL spectrum was measured
by applying 20 mA current to GaN-based blue LEDs, and detecting the
intensity of the illuminated light with a Si photodetector. As is
apparent from FIG. 8, the EL intensity of the x-ray irradiated
GaN-based blue LED is 50% higher than that of the thermally
annealed LED.
[0044] While this invention has been particularly shown and
described with references to preferred 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.
* * * * *