Process for obtaining a layer of single-crystal germanium on...

Details Process for obtaining a layer of single-crystal germanium on... Process for obtaining a layer of single-crystal germanium on...

2001-03-09

2003-03-25

Hiteshew, Felisa (Department: 1765)

Single-crystal, oriented-crystal, and epitaxy growth processes;

Forming from vapor or gaseous state

With decomposition of a precursor

C117S089000, C117S097000, C117S102000, C117S105000

Reexamination Certificate

active

06537370

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in a general way to a process for obtaining a layer of single-crystal germanium on a substrate of single-crystal silicon.
2. Description of the Relevant Art
Silicon (Si) is the basic compound of microelectronics. It is currently available on the market in the form of wafers 200 mm in diameter. The performance limits of integrated circuits are in fact therefore those associated with the intrinsic properties of silicon. Among these properties, mention may be made of the electron mobility.
Germanium (Ge), which belongs to column IV of the Periodic Table of Elements, is a semiconductor. It is potentially more beneficial than Si since (i) it has a higher electron mobility, (ii) it absorbs well in the infrared range and (iii) its lattice parameter is greater than that of Si, thereby allowing heteroepitaxial structures using the semiconductor materials of columns III-V of the Periodic Table.
Unfortunately, germanium does not have a stable oxide and there are no large-diameter germanium wafers on the market, except at prohibitive prices.
Si
1−x
Ge
x
alloys have already been grown on substrates of single-crystal Si. The alloys obtained only rarely have germanium contents exceeding 50% in the alloy.
Moreover, when SiGe alloys are grown on single-crystal Si, the growth of the SiGe alloy is initially single-crystal growth. The greater the thickness of the layer and the higher its germanium content, the more the layer becomes “strained”. Above a certain thickness, the “strain” becomes too high and the layer relaxes, emitting dislocations. These dislocations have a deleterious effect on the future circuits which will be constructed on this layer and the relaxation of the layers causes certain advantages of the strained band structure (offsetting of the conduction and valence bands depending on the strain states; Si/SiGe or SiGe/Si) to be lost. Corresponding to each composition and to each production temperature there is therefore a maximum thickness of strained layer.
In some applications, the concept of “relaxed substrates” has been developed, that is to say Si
1−x
Ge
x
layers are grown on silicon so as to exceed the critical thickness for a given composition, but by adjusting the deposition parameters for the layers so that the dislocations emitted do not propagate vertically but are bent over as to propagate in the plane of the layer in order subsequently to evaporate at the edges of the wafer. Growth therefore takes place from layers increasingly rich in germanium, it being possible for the germanium gradient to change stepwise or in a continuous fashion.
However, the films obtained by this “relaxed substrate” process either have a relatively low (<50%) degree of germanium enrichment or have an unacceptable emergent-dislocation density for applications in microelectronics.
Thus, the article entitled “Stepwise equilibrated graded Ge
x
Si
1−x
buffer with very low threading dislocation density on Si (001)”, by G. Kissinger, T. Morgenstern, G. Morgenstern and H. Richter, Appl. Phy. Lett. 66(16), Apr. 13, 1995, describes a process in which the sequence of the following layers is deposited on a substrate:
250 nm Ge
0.05
Si
0.95
+100 nm Ge
0.1
Si
0.9
+100 nm Ge
0.15
Si
0.85
+150 nm Ge
0.2
Si
0.8
.
After deposition, each layer undergoes in situ annealing in hydrogen at 1095° C. or 1050° C. By way of comparison, similar sequences of layers have been deposited, but without annealing.
A 300 nm layer of Ge
x
Si
1−x
of the same composition as the upper buffer layer is also deposited on the latter.
The specimens that did not undergo intermediate annealing have an emergent-dislocation density of 10
6
cm
−2
, whereas the specimen that did undergo annealing has an emergent-dislocation density of 10
3
-10
4
cm−
2
.
The article entitled “Line, point and surface defect morphology of graded, relaxed GeSi alloys on Si substrates”, by E. A. Fitzgerald and S. B. Samavedam, Thin Solid Films, 294, 1997, 3-10, describes the fabrication of relaxed substrates comprising up to 100% germanium. However, the process employed takes a long time (more than about 4 hours per wafer) and is consequently unattractive from an industrial standpoint. Moreover, this process is not reversible, that is to say it does not allow pure silicon to be deposited on a germanium substrate.
Furthermore, during the fabrication of such relaxed substrates, a surface roughness is observed which increases depending on the deposition conditions and which may have negative effects—since they are cumulative—that is to say the onset of roughness can only become greater as the deposition proceeds.
A deposition process has also been proposed which makes it possible to form, on a silicon substrate, Si
1−x
Ge
x
layers (x varying from 0 to 1), possibly going as far as a layer of pure Ge, and having a low emergent-dislocation density.
The essential characteristic of this process includes, during the chemical vapor deposition, continuously modifying the stream of active gases (SiH
4
and GeH
4
, for example) at the same time as the deposition temperature is varied. Thus, the emitted dislocations are rapidly rejected and removed in order to gradually relax the growing layer. Thus, it is possible to obtain these relaxed substrates going from a Ge concentration of zero (Si substrate) to a Ge concentration of 100% with a 4 to 5 &mgr;m film whereas the prior techniques require interlayers of more than 10 &mgr;m (typically about 25 &mgr;m).
The advantages of the latter technique are therefore a smaller thickness of interlayer, so as to obtain a relaxed-substrate surface layer, and a low defect (emergent dislocation) density of about 10
5
defects/cm
2
(compared with 10
6
for the prior processes).
However, this process still requires the deposition of an interlayer having a Ge concentration gradient, which requires film thicknesses of about 4 to 5 &mgr;m.
Moreover, this technique also requires long deposition times, of more than one hour per wafer treated in certain cases, thereby reducing the hourly output of wafers and increasing the fabrication cost of the wafers.
SUMMARY OF THE INVENTION
Disclosed herein is a novel process for depositing a layer of pure single-crystal germanium on a substrate of single-crystal silicon, which does not require the deposition of an interlayer with a concentration gradient.
Disclosed herein is also such a deposition process giving low densities of residual emergent dislocations, of less than 10
3
defects/cm
2
of surface.
Additionally such a process allows a layer to be obtained in a very short time and with small thickness (about 10 minutes for a 1 &mgr;m layer of pure Ge).
According to a first method, the process for forming a layer of pure single-crystal germanium on a substrate of single-crystal silicon includes:
(a) temperature stabilization of the single-crystal silicon substrate at a first predetermined stabilized temperature (T
1
) of 400° C. to 500° C., preferably of 430° C. to 460° C.;
(b) chemical vapor deposition (CVD) of germanium at said first predetermined temperature (T
1
) until a germanium base layer with a predetermined thickness of less than a desired final thickness obtained on the substrate;
(c) increase in the germanium chemical vapor deposition temperature from the first predetermined temperature (T
1
) to a second predetermined temperature (T
2
) ranging from 750° C. to 850° C., preferably from 800° C. to 850° C.; and
(d) continuation of the germanium chemical vapor deposition at said second predetermined temperature (T
2
) until the desired final thickness of the single-crystal germanium layer is obtained.
According to a second method, the process for forming a layer of pure single-crystal germanium on a substrate of single-crystal silicon comprise, after step (c) and before step (d):
(c
1
) a step in which the germanium CVD deposition is stopped and the temperature is lowered from the second predetermined temperature (T
2
) down to a third predetermine

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