SiGeC semiconductor crystals and the method producing the same

Details SiGeC semiconductor crystals and the method producing the same SiGeC semiconductor crystals and the method producing the same

2003-03-10

2003-12-09

Jones, Deborah (Department: 1775)

Stock material or miscellaneous articles

Composite

Of silicon containing

C428S448000, C428S698000, C428S336000, C117S939000, C117S951000

Reexamination Certificate

active

06660393

ABSTRACT:

TECHNICAL FIELD
The present invention relates to an SiGeC semiconductor crystal applicable to a bipolar transistor or a field-effect transistor and a method for producing the same.
BACKGROUND ART
The present invention relates to an SiGeC semiconductor crystal, which is a Group IV mixed crystal semiconductor, and a method for producing the same.
Conventionally, attempts have been made at fabricating a semiconductor device which operates faster than known Si semiconductor devices by stacking a Si layer and a semiconductor layer containing Si as a main ingredient thereof so as to form a heterojunction. Si
1−x
Ge
x
and Si
1−x−y
Ge
x
C
y
, which are mixed crystal semiconductors each formed using a Group IV element that is in the same group as Si, are expected as candidates for a material for forming a heterojunction with the Si layer. Particularly, as for an Si
1−x−y
Ge
x
C
y
mixed crystal semiconductor that is formed from three different elements, its band gap and lattice constant can be independently controlled by adjusting its composition, resulting in greater flexibility in semiconductor device design. Therefore, the Si
1−x−y
Ge
x
C
y
mixed crystal semiconductor has attracted much attention. For example, a lattice matching between Si
1−x−y
Ge
x
C
y
and Si crystals can be made by properly adjusting the composition of Si
1−x−y
Ge
x
C
y
. A heterobarrier (band offset) can be also formed on both a conduction band edge and a valence band edge around the interface of the heterojunction between the Si and Si
1−x−y
Ge
x
C
y
layers by properly adjusting the composition of Si
1−x−y
Ge
x
C
y
. Japanese Unexamined Patent Publication No. 10-116919, for example, discloses a field-effect transistor in which a two-dimensional electron gas serves as a carrier and which can operate at a high speed by utilizing a heterobarrier formed on the conduction band edge near the interface of Si/SiGeC layers.
Meanwhile, for producing Si
1−x−y
Ge
x
C
y
mixed crystals, use is now made of, for example, a chemical vapor deposition (CVD) process in which respective source gases of elements Si, Ge and C are dissolved so as to induce epitaxial growth of those elements on the Si or SiGe layers, or a molecular beam epitaxy (MBE) process in which respective source solids of the elements are heated and vaporized so as to induce crystal growth of the elements. In order to use an Si
1−x−y
Ge
x
C
y
layer as a part of a semiconductor device, the Si
1−x−y
Ge
x
C
y
layer is required to be doped with an impurity for generating a carrier, which will be a dopant so as to control the conductivity and specific resistance of the Si
1−x−y
Ge
x
C
y
layer. In the Si
1−x−y
Ge
x
C
y
layer, boron (B) and phosphorus (P) are used as a p-type dopant and an n-type dopant, respectively, in many cases. It is well known that the conductive type and specific resistance of a growth layer can be adjusted by doping the layer with a dopant during crystal growth.
Problems to be solved
FIG. 4
is a graph indicating the result of an experiment conducted by the inventors for the purpose of consideration as to doping of an Si
1−x−y
Ge
x
C
y
layer and shows how the specific resistance of the Si
1−x−y
Ge
x
C
y
layer changed depending on the C content thereof. The Si
1−x−y
Ge
x
C
y
layer as a sample from which the data was collected is as-grown one obtained by being epitaxially grown by a CVD process with the use of Si
2
H
6
, GeH
4
and SiH
3
CH
3
as respective source gases of elements of Si, Ge, and C and B
2
H
6
as a source gas of boron (B) which is a p-type impurity (dopant) (i.e., through in-situ doping). In this experiment, the flow rates of Si
2
H
6
and GeH
4
and the temperature of the Si
1−x−y
Ge
x
C
y
layer during the epitaxial growth thereof were kept constant and only the flow rate of SiH
3
CH
3
was changed. As shown in
FIG. 4
, as for the sample having a C content of 0.45% or less, even when the C content was changed, the specific resistance of the sample stayed almost constant and relatively low. In contrast, as for the Si
1−x−y
Ge
x
C
y
layer having a C content of 1.6%, the specific resistance thereof remarkably increased. That is to say, it was clearly shown that clearly shown that the specific resistance of the Si
1−x−y
Ge
x
C
y
layer which had been epitaxially grown by this method increased to the level at which the layer would be no longer suitable for use as an active region of a semiconductor device (e.g., a channel region of FET, a base layer of a bipolar transistor).
FIG. 5
is a graph indicating the result of the secondary ion mass spectroscopy on a sample formed basically in the same method as the sample from which the data shown in
FIG. 4
was collected and shows how the boron concentration of the Si
1−x−y
Ge
x
C
y
layer changed depending on the C content thereof. This is an experiment that was conducted to examine whether the specific resistance shown in
FIG. 4
was affected by the boron concentration, because the doping efficiency of boron slightly changes, depending upon the C content of the Si
1−x−y
Ge
x
C
y
layer, when boron is introduced into the Si
1−x−y
Ge
x
C
y
layer by an in-situ doping process. Note that the sample from which the data of
FIG. 5
was collected is not identical to the sample from which the date of
FIG. 4
was collected. As shown in
FIG. 5
, the B concentration of the Si
1−x−y
Ge
x
C
y
layer did not largely depend on the C content thereof. In addition, as also shown in
FIG. 5
, the B concentration of the Si
1−x−y
Ge
x
C
y
layer tended to increase as the C content of the Si
1−x−y
Ge
x
C
y
layer increased. That is to say, it was confirmed that the increase in the specific resistance of the sample having a B concentration of 1.6% shown in
FIG. 4
was not caused due to the shortage of B concentration.
The inventors then assumed that an increase in specific resistance of regions having a relatively high C content in the Si
1−x−y
Ge
x
C
y
layer would be caused by B having not sufficiently been activated during an epitaxial growth associated with an in-situ doping. Conventionally, with an in-situ doping of a dopant during an epitaxial growth of a semiconductor layer (e.g., a Si layer or an Si
1−x−y
Ge
x
C
y
layer) using a CVD process, an annealing process for activating the dopant is considered as unnecessary because the dopant is activated concurrently with the epitaxial growth of the semiconductor layer, unlike an impurity doping by an ion implanting process. As shown in
FIG. 4
, the Si
1−x−y
Ge
x
C
y
layer, with its C content of 0.45% or less, has a relatively low specific resistance in the as-grown state. In such a case the Si
1−x−y
Ge
x
C
y
layer as-grown can be therefore used for an active region of a semiconductor device. However, it is likely that when the C content of the Si
1−x−y
Ge
x
C
y
layer increases, some phenomenon causing problems that cannot be solved by the conventional technology will appear. Particularly, it is empirically known that various properties of the layer are largely changed when the C content of the Si
1−x−y
Ge
x
C
y
layer increases to over 1%. Therefore, around 1% of C content can be considered to be the critical value where the specific resistance of the Si
1−x−y
Ge
x
C
y
layer starts increasing, although the data of
FIG. 4
is not enough to confirm that.
DISCLOSURE OF INVENTION
It is an object of the present invention to provide an Si
1−x−y
Ge
x
C
y
semiconductor crystal applicable as an active region of a semiconductor device and a method for producing the same by taking measures to activate boron (B), particularly for an Si
1−x−y
Ge
x
C
y
layer having a relatively high degree of carbon (C) content which goes just over 1%.
A method for

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