Fabrication of defect free III-nitride materials

Details Fabrication of defect free III-nitride materials Fabrication of defect free III-nitride materials

1999-08-06

2001-08-07

Christianson, Keith (Department: 2813)

Semiconductor device manufacturing: process

Formation of semiconductive active region on any substrate

On insulating substrate or layer

Reexamination Certificate

active

06271104

ABSTRACT:

BACKGROUND OF THE INVENTION
This application relates to a method for preparing semiconductor materials and devices, and more particularly to a method of preparing defect free layers of semiconductor materials for use as a substrate or a buffer layer in a semiconductor device.
The growth of high quality, defect free semiconductor materials for use as a substrate or a buffer layer in a semiconductor device is a prerequisite for the subsequent growth of high quality semiconductor layers in the device.
The preparation of defect free semiconductor materials is easiest when a substrate of the same material is used. However, often times this is not possible and defects are created during the preparation of the semiconductor at the interface between the semiconductor and the substrate, as a result of the lattice and thermal mismatch between the semiconductor and the substrate materials. Buffer layers and other multi-layer schemes are often used to form a transition between the substrate and the semiconductor layer, but these have proven insufficient in many cases.
For example, in the case of III-Nitride semiconductor materials, many of the defects propagate nearly along the growth direction, that is perpendicular to the substrate/layer interface, without stopping as the film is grown thicker. It is therefore necessary to effectively stop the propagation of these defects.
SUMMARY OF THE INVENTION
An object then of the subject invention is the growth of a high quality layer of semiconductor material.
A further object is the growth of defect-free III-Nitride materials.
These and other objects are attained by the subject invention which comprises the method of preparing defect-free semiconductor material layers by growing the semiconductor material buffer layer under optimum conditions on the substrate; the buffer layer was grown thin (10 Å-10000 Å); the buffer layer is next masked with a dielectric film which is then etched to open periodic seed windows having a width of 0.1-50 &mgr;m and periods of 0.2-500 &mgr;m. The length of the seed window can be as long as needed and may reach the edges of the substrate wafer. Another layer of a III-V or II-VI material is then grown in the longitudinal direction from the seed window, followed by lateral growth of the same material to form an epitaxial film and a structure which provides a defect free surface for further epitaxial layers. The substrate may also be removed by an etching solution to yield a defect free semiconductor material layer for use as a substrate.
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 (120hr
−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 ¼-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
−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° 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 GaN, AlGaN, and GaInN by LP-MOCVD are listed below.
Group-III Sources
Group-V Source
Al(CH
3
)
3
t-butylamine
Al(C
2
H
5
)
3
NH
3
In(CH
3
)
3
In(C
2
H
5
)
3
(CH
3
)
2
In(C
2
H
5
)
Ga(CH
3
)
3
Ga(C
2
H
3
)
3
An accurately metered flow of purified H
2
or N
2
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 or N
2
. 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.
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 about 8 liters min
−1
during growth. Stable flows are achieved by the use of mass flow controllers.
The dielectric film etching solution used in the subject invention may be any of those commonly used, such as a solution of sulfuric acid and hydrogen peroxide. Alternatively, a solution of HF and ammonium fluoride may be used.
The substrate can be GaAs, Si, Al
2
O
3
, MgO, SiC, ZnO, LiGaO
2
, LiAlO
2
, MgAl
2
O
4
or GaN. Preferably, Si 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 the procedure of the subject invention was thus found to be beneficial. One such pretreatment procedure is as follows:
1. Dipping in H
2
SO
4
for 3 minutes with ultrasonic agitation;
2. Rinsing in Deionized H
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
2
SO
4
for 3 minutes;
7. Rinsing in deionized H
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 epitaxial layers of the subject invention may be grown by low-pressure metalorganic chemical vapor deposition (LP-MOCVD), although MBE, LPE and VPE procedures may be used with equal effect.
Growth takes place in MOCVD procedures 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 and 600° 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 III-V layers may be grown in the method of the subject invention by low pressure metalorganic chemical vapor deposition (LP-MOCVD) although MBE, LPE and VPE may be used as stated above. The layers are grown by an induction-heated horizontal cool wall reactor. The sources set forth above are

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