Method for making III-Nitride laser and detection device

A p-i-n structure for use in photoconductors and diodes is disclosed, being formed of an Al.sub.x Ga.sub.1-x N alloy (X=0.fwdarw.1) with In.sub.y Ga.sub.1-Y N (Y=0.fwdarw.1) which as grown by MOCVD procedure with the p-type layer adjacent the substrate. In the method of the subject invention, buffer layers of p-type material are grown on a substrate and then doped. The active, confinement and cap layers of n-type material are next grown and doped. The structure is masked and etched as required to expose a surface which is ion implanted and annealed. A p-type surface contact is formed on this ion-implanted surface which is of sufficiently low resistance as to provide good quality performance for use in a device.

FIELD OF THE INVENTION 
This invention relates to semiconductor III-V alloy compounds, as well as 
to a method of making III-V alloy compounds for use in diode lasers. 
BACKGROUND OF THE INVENTION 
The importance of semiconductor emitters and detectors is rapidly 
increasing along with progress in the opto-electronic field, such as 
optical fiber communication, optical data processing, storage and solid 
state laser pumping. 
GaN-based compounds, especially Al.sub.x Ga.sub.1-x N alloys are the most 
promising material system for high performance and economical ultraviolet 
(UV) emitters photodetectors. With a bandgap energy from 3.4 eV to 6.2 eV, 
UV photodetectors with cut-off wavelengths from 200 nm (AlN) to 365 nm 
(GaN) can be fabricated from this alloy system. The direct bandgap of 
Al.sub.x Ga.sub.1-x N-based detectors are also expected to have better 
intrinsic solar blindness than any other UV photodetectors. This makes 
them ideal for many applications, such as the surveillance and recognition 
of spacecraft, space-to-space communications, the monitoring of welding, 
as well as engines, combustion chambers, and astronomical physics. 
Further, AlN, GaN, InN and their alloys (III-Nitrides) have direct bandgap 
energies from 1.9 eV (659 nm) to 6.2 eV (200 nm, which cover almost the 
whole visible down to mid-ultraviolet wavelength range. Therefore, one of 
the most important applications of these materials is to make visible and 
ultraviolet light-emitting diodes (LED) and laser diodes (LD) with high 
quantum efficiency, which are immediately needed in the current commercial 
markets and can be best achieved by these materials. 
The performance of photoconductors and simple p-n junction photodiodes can 
be very limited in terms of speed, responsivity and noise performance. The 
optimization of GaN-based UV photoconductors requires sophisticated 
structures such as p-i-n layered structures, heterostructures or even 
quantum wells. 
To fabricate these structures and achieve high-performance photodetectors, 
two critical issues need to be addressed. One is the high resistance of 
the p-type layer and its contact, which introduce signal voltage drop and 
excess noise at the contact point. The other problem is introduced by the 
p-type layer annealing procedure. The best way to illustrate these two 
problems is to describe their effect on the performances of current blue 
laser diodes. 
Currently, the demonstrated blue laser diodes are not significantly 
practical since they have to be operated either in pulsed mode or CW at 
low temperature. In addition, their lifetime is short. A typical reported 
blue laser diode structure is a p-i-n structure with a p-type layer on 
top. Because of the high resistance of the p-type layer and its contact, 
excess heating at high current densities is generated, which leads to the 
failure of the device. Other problems exist as a result of the growing 
procedure, which are as follows: First, n-type layers are grown, followed 
by InGaN MQW; Mg-doped layers are then grown. Finally, thermal annealing 
at about 700.degree. C. or low-energy electron beam irradiation (LEEBI) is 
performed to convert the top GaN:Mg or AlGaN:Mg to p-type. Both of these 
procedures will deteriorate the quality of the bottom layers, including 
the promotion of defect and impurity propagation, interfaces deterioration 
and, worse than that, the dissociation of the InGaN active layer and 
interface quality of the InGaN multi-quantum-well, since InGaN begins to 
dissociate at temperatures above 500.degree. C. 
With regard to emitters, III-Nitride based LEDs have been recently 
successfully developed and commercialized, providing coverage from yellow 
to blue. Further, blue laser diodes are known in pulsed mode at room 
temperature and continuous mode at about 40.degree. C. Blue or 
short-wavelength laser diodes are in demand primarily because of their 
immediate need in optical storage and full color flat-panel display. The 
optical storage density is inversely proportional to the square of the 
read-write laser diode. By simply replacing the currently used laser diode 
(780 nm) with blue laser diode (410 nm), the storage density can be 
enhanced by almost four times. 
SUMMARY OF THE INVENTION 
An object, therefore, of the invention is a III-Nitride alloy for use in 
photoconductors and diodes having high quantum efficiency. 
A further object of the subject invention is a GaN-based composition in a 
p-i-n structure of high quality. 
A still further object of the subject invention is an alloy of the 
composition Al.sub.x Ga.sub.1-x N in a p-i-n structure with the p-type 
layers adjacent the substrate. 
Those and other objects are attained by the subject invention wherein an 
Al.sub.x Ga.sub.1-x N alloy (X=0.fwdarw.1) with In.sub.y Ga.sub.1-y N 
(Y=0.fwdarw.1) is grown by MOCVD procedure in a p-i-n structure with the 
p-type layer adjacent the substrate. In the method of the subject 
invention, buffer layers of p-type material are grown on a substrate and 
then doped. These p-type layers are further treated with ion implantation 
of Group I elements. The active layers and cap layers of n-type material 
are next grown and doped. The structure is masked and etched as required 
to expose a surface which is ion implanted and annealed. A p-type surface 
contact is formed on this ion-implanted surface which is of sufficiently 
low resistance as to provide good quality performance for use in a device.

DETAILED DESCRIPTION OF THE INVENTION 
The reactor and associated gas-distribution scheme used herein are 
substantially as described in U.S. Pat. No. 5,384,151. The system 
comprises a cooled quartz reaction tube (diameter 5 cm in the substrate 
area) pumped by a high-capacity roughing pump (120 hr.sup.-1) to a vacuum 
between 10 and 300 Torr. The substrate was mounted on a pyrolytically 
coated graphite susceptor that was heated by rf induction at 1 MHz. The 
pressure inside the reactor was measured by a mechanical gauge and the 
temperature by an infrared pyrometer. A molecular sieve was used to impede 
oil back-diffusion at the input of the pump. The working pressure was 
adjusted by varying the flow rate of the pump by using a control gate 
valve. The gas panel was classical, using 1/4-inch stainless steel tubes 
and Swagelock fittings. Flow rates were controlled by mass flowmeters. 
The reactor was purged with a hydrogen flow of 4 liters min.sup.-1, and the 
working pressure of 78 Torr was established by opening the gate valve that 
separated the pump and the reactor. The evacuation line that was used at 
atmospheric pressure was automatically closed by the opening of the gate 
valve. The gas flow rates were measured under standard conditions, i.e., 1 
atm and 20.degree. C., even when the reactor was at subatmospheric 
pressure. The pressure in the gas panel was regulated by the needle valve 
placed between the gas panel and the reactor. The needle valve was 
adjusted to maintain a constant pressure of 1 atm on the gas panel, 
thereby ensuring reproducibility of flow-rate measurements. 
The gas sources used in this study for the growth of AlGaN by LP-MOCVD are 
listed below. 
______________________________________ 
Group-III Sources 
Group-V Source 
______________________________________ 
In(CH.sub.3).sub.3 
NH.sub.3 
In(C.sub.2 H.sub.5).sub.3 
(CH.sub.3).sub.2 In(C.sub.2 H.sub.5) 
Al(CH.sub.3).sub.3 
Al(C.sub.2 H.sub.5).sub.3 
Ga(CH.sub.3).sub.3 
Ga(C.sub.2 H.sub.3).sub.3 
______________________________________ 
An accurately metered flow of purified H.sub.2 for TMI is passed through 
the appropriate bubbler. To ensure that the source material remains in 
vapor form, the saturated vapor that emerges from the bottle is 
immediately diluted by a flow of hydrogen. The mole fraction, and thus the 
partial pressure, of the source species is lower in the mixture and is 
prevented from condensing in the stainless steel pipe work. 
Pure and diluted ammonia (NH.sub.3) is used as a source of N. The metal 
alkyl or hydride flow can be either injected into the reactor or into the 
waste line by using two-way valves In each case, the source flow is first 
switched into the waste line to establish the flow rate and then switched 
into the reactor. The total gas flow rate is 8 liters min.sup.-1 during 
growth. Stable flows are achieved by the use of mass flow controllers. 
Dopants usable in the method of the subject invention are as follows: 
______________________________________ 
n dopant 
p dopant 
______________________________________ 
H.sub.2 Se 
(CH.sub.3).sub.2 Zn 
H.sub.2 S 
(C.sub.2 H.sub.5).sub.2 Zn 
(CH.sub.3).sub.3 Sn 
(C.sub.2 H.sub.5).sub.2 Be 
(C.sub.2 H.sub.5).sub.3 Sn 
(CH.sub.3).sub.2 Cd 
SiH.sub.4 
(.eta.C.sub.2 H.sub.5).sub.2 Mg 
Si.sub.2 H.sub.6 
______________________________________ 
The substrate can be GaAs, Si, Al.sub.2 O.sub.3, MgO, SiC, ZnO, 
LiGaO.sub.2, LiAlO.sub.2, MgAl.sub.2 O.sub.4 or GaN. Preferably, sapphire 
(Al.sub.2 O.sub.3) is used as the substrate. The epitaxial layer quality 
is sensitive to the pretreatment of the substrate and the alloy 
composition. Pretreatment of the substrates prior to epitaxial growth was 
thus found to be beneficial. One such pretreatment procedure is as 
follows: 
1. Dipping in H.sub.2 SO.sub.4 for 3 minutes with ultrasonic agitation; 
2. Rinsing in Deionized H.sub.2 O; 
3. Rinsing in hot methanol; 
4. Dipping in 3% Br in methanol at room temperature for 3 minutes 
(ultrasonic bath); 
5. Rinsing in hot methanol; 
6. Dipping in H.sub.2 SO.sub.4 for 3 minutes; 
7. Rinsing in deionized H.sub.2 O, and 
8. Rinsing in hot methanol. 
After this treatment, it is possible to preserve the substrate for one or 
two weeks without repeating this treatment prior to growth. 
The invention is described in accordance with the drawings and, in 
particular, with respect to FIGS. 1 and 2. FIG. 1 is a cross-section of a 
III-Nitride based laser diode structure formed in accordance with the 
subject invention for use on a photo emitter. FIG. 2 is a cross-section of 
an embodiment of III-Nitride based laser diode structure for use in a 
photo detector. 
Growth takes place by introducing metered amounts of the group-III alkyls 
and the group-V hydrides into a quartz reaction tube containing a 
substrate placed on an rf-heated susceptor surface. The hot susceptor has 
a catalytic effect on the decomposition of the gaseous products; the 
growth rate is proportional to the flow rate of the group-III species, but 
is relatively independent of temperature between 500.degree. and 
600.degree. C. and of the partial pressure of group-V species as well. The 
gas molecules diffuse across the boundary layer to the substrate surface, 
where the metal alkyls and hydrides decompose to produce the group-III and 
group-V elemental species. The elemental species move on the hot surface 
until they find an available lattice site, where growth then occurs. 
For best results, all surfaces of the growth reaction chamber are coated 
with a barrier coating capable of withstanding high temperatures and not 
reacting with the reactants and dopants utilized therein at such high 
temperatures. Preferably, a coating of AlN or of SiC is grown in situ in 
the reaction chamber to cover all surfaces therein. There is thus formed a 
stable layer that prevents oxygen and other impurities originating within 
the reaction chamber from reacting with the semiconducting layer to be 
grown. 
High quality AlGaN/GaN may be grown in the method of the subject invention 
by low pressure metallorganic chemical vapor deposition (LP-MOCVD). The 
layers of the heterostructure are grown by an induction-heated horizontal 
cool wall reactor. Trimethylindium (TMI), Trimethylaluminum (TmAl) and 
Triethylgallium (TEG) are used as the sources of Indium and Gallium. Pure 
and diluted ammonium gas (NH.sub.3) is used as the N source. Sample is 
grown on a sapphire substrate. A buffer layer of AlN and thin contact and 
confinement layers of GaN, Al.sub.x Ga.sub.1-x N (X=0.fwdarw.1) and 
In.sub.x Ga.sub.x-1 N are individually laid on the substrate at 
thicknesses from 50.ANG.to 1 .mu.m. The undoped active layer may be 
In.sub.x Ga.sub.1-x N (0.ltoreq.X .ltoreq.1) , Al.sub.y Ga.sub.1-y N 
(0.ltoreq.Y.ltoreq.1) or the superlattice structure of GaN/In.sub.x 
Ga.sub.1-x N (0.ltoreq.X.ltoreq.1). The optimum growth conditions for the 
respective layers are listed in Table 1. The confinement of the active 
layer for the subject invention may be as a heterostructure, separate 
confinement heterostructures or with a quantum well. 
Doping is preferably conducted with bis-cyclopentadienyl magnesium 
(BCP.sub.2 Mg) for p-type doping and silane (SiH.sub.4) for n-type doping. 
Doping is performed through a BCP.sub.2 Mg bubbler with H.sub.2 as carrier 
gas and at temperatures from -15.degree. C. to ambient temperatures at 
20-90 cm.sup.3 min..sup.- 1 and onto either a hot or cooled substrate 
(535.degree. C.). SiH.sub.4 may be simply directed at ambient temperatures 
onto the hot substrate at 20-90 cm.sup.3 min..sup.-1. 
In a preferred doping method for incorporating the maximum amount of p-type 
dopant on the layer, once the p-type layer to be doped is fully grown, the 
heat source is terminated and the substrate allowed to cool; the metal and 
hydride sources are terminated; the dopant flow, for instance DEMg, is 
initiated at the temperatures indicated for diffusion onto the cooled 
substrate/epilayer which has been previously grown. After about 2-3 
minutes, the dopant flow is terminated and the next epilayer grown. By 
this method, it is found that 10.sup.20 atoms/cm.sup.3 of Mg may be placed 
on the top surface of the epilayer. 
When ion implanting, the principle of coulomb pairing is observed. In other 
words, copper, with two available electrons, is paired with the four 
available electron orbitals of Germanium. In like fashion, the one 
available electron of Silver can also be paired with the available 
electron orbitals of Silicon or Germanium. Copper, Mercury or Silver ions 
may be implanted by the following procedure: 
The ions are implanted using a Penning source. An energy of 700 KeV is used 
with a maximum beam intensity of 100 nA.sup.-2. The implantation is tilted 
70.degree. from the &lt;100&gt; axis. After implantation, the objects were 
annealed capless in a flowing high-purity 0.4% PH.sub.3 /H.sub.2 mixture 
at 700.degree. C. for ten minutes. 
TABLE 1 
______________________________________ 
Optimum growth conditions of a Al.sub.x Ga.sub.1-x N/In.sub.x Ga.sub.1-x 
structure. 
AlGaN InGaN GaN 
______________________________________ 
Growth Pressure 
76 76 76 
Growth Temperature 
535 535 535 
(.degree.C.) 
Total H.sub.2 3low 3 3 
(liter/min) 
Al(C.sub.2 H.sub.5) 
30 -- -- 
TMI (cc/min) -- 200 -- 
TEG (cc/min) 120 120 120 
NH.sub.3 (cc/min) 
300 300 300 
Growth Rate 150 300 250 
(.ANG./min) 
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EXAMPLES 1-3 
A III-Nitride based laser diode structure as set forth in FIG. 1 is 
prepared. After coating the reaction chamber with AlN at a temperature of 
1000.degree. C., a thin (350.ANG.) AlN buffer layer is first grown on a 
sapphire substrate prepared as set forth above. A contact layer of GaN 
(2000.ANG.) is next grown and doped with magnesium to a concentration of 
5.times.10.sup.18 -10.sup.20 atoms/cm.sup.-3. A p-type confinement layer 
of In.sub.y Ga.sub.1-y N (300.ANG.) is next grown and doped with 
magnesium, following which a layer of Al.sub.x Ga.sub.1-x N (1500.ANG.) is 
grown and doped with magnesium. Finally, a layer of GaN is grown and doped 
with magnesium. The magnesium doped layers are next annealed at a 
temperature of about 700.degree. C. After annealing, the hole 
concentration is 5.times.10.sup.19 -10.sup.18 cm.sup.-3. Active layers of 
InGaN MQW (50.ANG.) are next grown followed by confinement layers of GaN 
(500.ANG.), Al.sub.1 Ga.sub.1-x N(500.ANG.) and GaN (500.ANG.), all doped 
with silicon followed by a buffer layer of GaN (500.ANG.) doped with 
silicon for an electron concentration of 5.times.10.sup.17 -10.sup.18 
cm.sup.-3 and a cap or contact layer of GaN (200.ANG.) highly doped with 
silicon as set forth above, i.e. to a concentration of 10.sup.20 
atoms/cm.sup.3. The structure is then masked and etched with reactive ion 
etching to expose the lower contact GaN:Mg surface, which surface is then 
ion-implanted with copper ions to provide a conductive surface thereon. 
The n-contact is formed by deposition of Au, while the p-contact is formed 
by deposition of Pt/Au and annealing at 450.degree. C. for 30 s. The laser 
diode structure is then functional. 
Three separate structures are grown, with the different values for X and Y 
as set forth in Table 2. 
TABLE 2 
__________________________________________________________________________ 
Example 
X or Y 
In.sub.y GA.sub.1-y N 
Al.sub.x Ga.sub.1-x N 
In.sub.y Ga.sub.1-y N 
Al.sub.x Ga.sub.1-x N 
__________________________________________________________________________ 
1 0.0 GaN:Mg GaN:Mg GaN GaN:Si 
2 .75 
In.sub.0.75 Ga.sub.0.25 N:Mg 
Al.sub.0.75 Ga.sub.0.25 N:Mg 
In.sub.0.75 Ga.sub.0.25 N 
Al.sub.0.75 Ga.sub.0.25 N:Si 
3 0.0 InN:Mg AlN:Mg GaN AlN:Si 
__________________________________________________________________________ 
EXAMPLES 4-6 
A III-Nitride based laser diode structure as set forth in FIG. 2 is 
prepared. The reaction chamber is coated with AlN as in Example 1. A thin 
(350.ANG.) AlN buffer layer is then grown on a sapphire substrate prepared 
as set forth above. A lower contact layer of GaN (2000.ANG.) is grown and 
doped with magnesium. Al.sub.x Ga.sub.1-x N is next grown (1 .mu.m) and 
doped with magnesium to a concentration of 10.sup.20 atoms/cm.sup.-3. The 
magnesium doped layer is next annealed at a temperature of about 
700.degree. C. resulting in a hole concentration of 5.times.10.sup.19 
atoms/cm.sup.-3. Two active layers of InGaN MQW (50.ANG.) are next grown: 
Two layers of Al.sub.x Ga.sub.1-x N are next grown (150-300.ANG.) both 
doped with silicon, the upper layer being doped to a concentration of 
10.sup.20 atoms/cm.sup.3. The structure is then masked and etched by 
reactive ion etching to expose the lower contact AlGaN:Mg surface, which 
surface is then ion-implanted with copper ions to provide a conductive 
surface thereon. An n-contact is then formed by deposition of Au, while 
the p-contact is formed on the p-type surface by deposition of Pt/Au and 
annealing at 450.degree. C. for 30 s. The laser diode structure is then 
functional. 
Three separate structures are grown, with the different values for X and Y, 
as set forth in Table 3. 
TABLE 3 
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Example 
X or Y Al.sub.x GA.sub.1-x N:Mg 
In.sub.y Ga.sub.1-y N 
Al.sub.y Ga.sub.1-y N:Si 
______________________________________ 
4 0 GaN GaN GaN 
5 0.75 Al.sub.0.75 Ga.sub.0.25 N 
In.sub.0.75 Ga.sub.0.25 N 
Al.sub.0.75 Ga.sub.0.25 N 
6 1.0 AlN InN AlN 
______________________________________ 
FIG. 3 shows the detectivity of Al.sub.x Ga.sub.1-x N photodetectors, such 
as in Examples 1-6, (X=0.fwdarw.1.0) . The detectivity increases with 
wavelength, showing high quantum efficiency. Near the bandgap, it exhibits 
a sharply decreasing detectivity. The cutoff wavelength is tuned by the Al 
composition, which varies from about 360 nm for GaN to 200 nm for AlGaN. 
The structures of Examples 1-6 preserve the efficiencies of the p-i-n 
structures, while maintaining low defects, good interface quality, and 
good conduction at both the n and p contact points. The high doping level 
and the ion-implantation step provides a conductive p-layer surface while 
the necessary destructive annealing steps are performed prior to growth of 
the active layers, thereby retaining the properties necessary for a 
high-quality device. 
While the invention has been described with reference to a preferred 
embodiment, it will be understood by those skilled in the art that various 
changes may be made and equivalents may be substituted for elements 
thereof without departing from the scope of the invention. In addition, 
many modifications may be made to adapt a particular situation or material 
to the teachings of the invention without departing from the essential 
scope thereof. Therefore, it is intended that the invention not be limited 
to the particular embodiment disclosed as the best mode contemplated for 
carrying out this invention, but that the invention will include all 
embodiments and equivalents falling within the scope of the appended 
claims. 
Various features of the invention are set forth in the following claims.