Method of fabrication of semiconducting compounds of...

Details Method of fabrication of semiconducting compounds of... Method of fabrication of semiconducting compounds of...

1999-12-06

2001-12-11

Christianson, Keith (Department: 2813)

Semiconductor device manufacturing: process

Making device or circuit emissive of nonelectrical signal

Compound semiconductor

Reexamination Certificate

active

06329215

ABSTRACT:

FIELD OF THE INVENTION
This Invention relates to the method of fabrication nitride semiconductors A
3
B
5
of p- and n-type electric conductivity for manufacturing of optoelectronic devices.
OBJECT AND SUMMARY OF THE INVENTION
The subject of the Invention relates to the method of fabrication of nitride semiconductors A
3
B
5
such as GaN, AIN, InN and their solid solutions, characterized by p- and n-type electric conductivity, low electric resistivity, high intensity of emitted light and high structural quality.
The semiconductors obtained by this method are applied in the construction of optoelectronic light emitting devices, light detectors and electric current amplifiers such as e.g.: highly efficient blue and green diodes, laser diodes, high power lasers, ultraviolet detectors and high temperature field effect transistors.
The layers of nitrides A
3
B
5
are obtained most frequently by metaloorganic chemical vapor deposition (MOCVD) on sapphire substrates. In this method the metaloorganic compound fulfills the role of the gas assuring the existence of chemical reaction leading to creation of the nitride A
3
B
5
for example trimethylgallium and/or trimethylaluminium and ammonia are injected together with the hydrogen carrier gas to the reactor chamber where the sapphire substrate is located. Next, keeping the epitaxial growth temperature in the range 800-1000° C., the heteroepitaxial layers of nitrides A
3
B
5
are obtained. The proper choice of the gas containing the dopant leads to the fabrication of the nitride of the prescribed type of conductivity. Silicon, substituting Ga or Al in the nitride represents the most frequently used n-type impurity i.e. donor. High concentrations of Si donors, higher than 10
19
cm
−3
can be introduced without much trouble to the nitride layers. This corresponds to the similar concentration of free electrons, responsible for the electron type electric conductivity in the n-type layers.
Magnesium is commonly used as type p-type impurity i.e. acceptor. The growth of nitride Mg-doped structures is conducted in the method where hydrogen is used as carrier gas, which leads to the growth of highly resistive layers. This is consequence of the creation of electrically neutral complex defects containing Mg
−
acceptors and H
+
donors. In on order to obtain p-type conductive nitride layers, the structure containing Mg
+
acceptors and H
+
donors is annealed in the nitrogen atmosphere in the temperatures 400° C. or higher. It is known, however, that the increase of the annealing temperature increases effectiveness of the conversion of the insulating Mg-doped nonconductive layers into the p-type layers, but at the same time, the natural limitation of this method is requirement to keep this temperature below the nitride chemical decomposition temperature. For example, for GaN and the nitrogen pressures not exceeding 200 bars, T{tilde over (=)}1000-1100° C. is the maximal annealing temperature. In order to prevent the nitride decomposition during annealing, aside from the increase to the nitrogen pressure, the “cap” of Si
3
N
4
or SiO
2
was used. This cap hindered the escape of nitrogen from the surface of the nitride layer, allowing the application of the annealing temperature close to 1100° C. Despite this, the problem of the obtaining of p-type layers of low resistivities has not been solved which translates into high concentration of the hole type carriers. It is difficult the overcome the limiting hole concentration of 5×10
17
cm
−3
, despite that the concentration of Mg substituting Ga can be higher than 10
19
cm
−3
.
There exists equally known other method of fabrication of doped nitride structures A
3
B
5
. The method relies on the introduction of p- and n-type impurities into the thin subsurface nitride layer by high energy ion implantation: Mg or Ca in order to obtain p-type and Si or O ions for n-type. Sometimes, for the creation of the prescribed layer it is necessary to use the simultaneous implantation by P ions.
The implantation process leads to nonconductive, optically inactive semiconductor layers of seriously destroyed crystalline structure. In order to obtain the layers with electrically and optically active impurities it is necessary to use annealing of the implanted structures in high temperatures. It leads to removal of the radiative defects, preventing the active action of the implanted ions as an active impurities, responsible for light emission by semiconductor structure.
As stems from the presented description, the processes of the annealing of the nitride structures are used in both above mentioned methods of the creation of p-type material and in case of implantation, also for the n-type layers. In both cases, the annealing temperature cannot exceed 1000-1100° C., due to chemical instability of these materials in high temperature and low pressure. The time limitation of the annealing processes are related to the same problem of stability.
The goal of the Invention is to introduce the new method of fabrication of the structures semiconductor nitrides GaN, AIN, InN and their solid solutions, containing layers of p- and/or n-type of the improved structural quality and increased effectiveness of light emission.
The processes leading to the creation of structures of prescribed electrical, optical and structural properties follow deposition of the semiconductor layers of nitrides A
3
B
5
on the conductive or isolating substrate of the bulk crystal (Ga,Al.,In)N or sapphire. In order to grow the layered structure, the well known MOCVD or MBE methods are used. So prepared structure is subjected to the high pressure and high temperature influence in high pressure diffusional chamber.
In connection with the above, the paramount element of the presented Invention is the execution of the controlled modification of physical properties of homoepitaxial and heteroepitaxial structures of semiconductor nitrides A
3
B
5
deposited on the conductive substrate subjected the introductory processing or isolating substrate, in high pressure diffusional chamber filled with one or multi-component gas, compressed to pressure between 1000 and 20000 bar. This allows the execution of the doping processes to p- and n-type and modification of the structure properties in order to obtain the parts of the optoelectronic devices of increased efficiency of light emission or the low resistivity parts of electronic devices. These processes are effected in the temperature equal to 1000-1800° C.
The structures obtained in the framework of the Invention are characterized by the quality of the crystallographic structure higher than the heteroepitaxial structures obtained by annealing without pressure or under low gas pressure.
The method according to the Invention uses as the conductive substrate, the bulk crystal of semiconductive compound A
3
B
5
, annealed in high pressure, high temperature diffusional chamber in the temperature 1000-1800° C., under high pressure of single or multi-element gas in the range 1000-20000 bar in the presence of the dopant source..
The high pressure annealing, according to the Invention is effected in one component gas nitrogen or argon or in these gas mixture described by the formula z N+(1−z)Ar, where 0≦z≦1.
As isolating substrate, the bulk semiconductor compound A
3
B
5
crystal is used, doped with magnesium or sapphire Al
2
O
3
.
The used homoepitaxial or heteroepitaxial layers are represented by the compounds of the following chemical formula Ga
x
Al
1−x
N where 0≦x ≦1 and Ga
y
In
1−N
where 0≦y≦1.
The p- and n-type dopants are applied in various stages of the described processes. They are introduced during both growth of the isolating substrate and doping of homoepitaxial and heteroepitaxial layers. In case of the layers of p- and n-type the dopants are provided by the external and/or internal source.
The external source of the p- and n-type dopants is the piece of material containing one of the following elements: Mg, Ca, Be, Zn,

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