Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~.271393
.
P~ 84.574 1 21-08-1985
Metho~ of manufact ~ing a semi-insulating single crystal of gallium
indium arsenide.
The invention relates to a method of manufacturing a single
crystal of gallium arsenide (Ga~s) of semi-insulating type including
indium (In) for reducing dislocations.
The invention can ke used in the manufacture of semi-insula-
ting substrates for semiconductor devices in those cases in which a
high integration density and high performances are required.
A method of manufacturing such a single crystal is knownfrom British Patent GB~PS 1242410 in which method mono-crystalline lay-
ers of galium arsenide are grown from a solution, into which indium is
incorporated in a cluantity less than 5% so that, due to the lcw segre-
gation coefficient of indium, the quantity of indium in the finished
single crystal is negligible (less than 0.5%). As a result of the addi-
tion of indium to the solution the crystalline qualities of the gallium
arsenide layers are materially impro~ed.
The known method, how~ver, which is the method of epitaxy from
the liquid phase~ is not suitable for obtaining very bulky single crys-
tals. Besides, impurities, such as ehromium, which have to ke included
in the bath in order to obtain a crystal having, for example, semi-in-
sulating properties, have very low se~regation c oefficients. Consecluent-
ly, they are not uniformly incorporated into the cry~tal and have a
tendency to diffuse unduly in the epitaxial layers which can be formed
afterwards on this crystal when used as a substrate.
Object of the invention is obtaining a bulky single crystal
without dislocations ~nd of the semi-insulating type without addition
of disturbing impurities. Therefore, according to the invention, the
method mentioned before is characterized in that the single erystal is
formed from a m~lten bath, in which the mass of arsenic (As) withrespect
to the total mass of arsenic plus gallium (Ga + A~s) is higher than 45
and lower than 51~, in that indium (In) is introducecl in the form of
indium arsenidP (InAs), the mass of indium arsenide (In~s) with re~spect
to the mass of arsenic plus gallium (Ga ~ As) is higher than 5% and
lower than 10~ and in that this single crystal is then subjected to a
sinter mg treatment.
39;3
P~ 84.574 2 21-08~1985
The crystal obtained Erom the b~ath contains a~out 1~ indium
and has a p-type conc~lctivity. D~le to the sintering treatment the level
of deep donors ~L2 is increased and the crystal becomes semi-insulating.
The advantage of the method in accordance wlth the invention
is therefore that it provides a single crystal which is semi-insulating
and free frc~m disturbing dopants.
In order that the invention may be readily carried out, it
will now be described more fully with reference to the accompanying
drawing, in which:
Fig. 1 is a sectional view of the distribution of the defect
EL2 in a crysta] of gallium arsenide doped with indium (Ga1_xInx As),
where Fig. 1a shows this distribution before sintering in a sectional
view perpendicular to the pulling axis, Fig. 1b shows this distribution
before sintering in a sectional view parallel to the pulling axis, Fig.
1S 1c shows this distribution after sintering in a sectional view perpen-
dicular to the pulling axis and Fig. 1d shows this distribution after
sintering in a sectional view parallel to the pulling axis.
According to the invention, a bulk~7 single crystal devoid of
dislocations is obtained ~y pulling, by means of the Czochralski method,
of a single crystal of gallium arsenide doped by means of an iso-elec-
tronic element, such as indium, in quantities of 1019 to 2.102
atoms/cm3. Such a single crystal corresponds to the general formula:
Ga1 xIn As,
in which the concentration x is of the order of 1%.
Pulliny by means of the Czochralski method certainly permits
of obtaining single erystals of large cross-section, but gives rise to
difficulties, such as, for example, tensi~ns in the crystal. These
difficulties can be solved only ky defining in a specific manner the
different parameters of the pulling process.
By a suitab]e pulling method and by a suitable doping with
indium, crystals having a high crystalline quality required for suk-
strates of integrated circuits can ke obtained. However, a further
problem has still to be solved, which consists in forming such a crys-
tal having the properties of a semi-insulator.
The electrical properties of gallium arsenide are given by
the combination of the active centres present in the material and more
particularly by their respective concentrations and their energy level
in the forbidden band. This combination permits of determining the
39~3
P~E` 84.574 3 21-08-1985
~ermi level. If this level is in the proximity of the conduction band,
the crystal is of the n-conductivity type, ancl, if this level is in
the proximity of the valency bandl the crystal is of the E~conductivity
type. If the level is blocked in the midclle of the forbidden band, the
crystal is semi insulating and has a high resistivity. The active
centres can be of two types:
- impurities (residual or introduced at will)
- natural defects, among which EL2, which have energy levels
lying approximately in the middle of the forbidden band for
gallium arsenide.
The electrical type of the undoped crystals of gallium arseni-
de consequently first depends upon the stoichiometry of the pulling
bath during their processing. If the bath is rich of gallium, the crys-
tal has a E~conductivity ty~e and has an excess of acceptors. If on the
contrary the bath is rich of arsenic, the crystal has an n-conductivity
type and has an excess of donors.
In these conditions, in order to obtain a semi-insulating
crystal according to the invention, the starting material is a crystal
of ~-conductivity type, in which a suitable quantity of deep donors is
caused to appear.
Ps shown in Figs. 1a and 1b, it can e ascertained that a
crystal of gallium indium arsenide obtained according to the invention
has a level of concentration of the defects EL2 which is simultaneously:
- very hcm~geneous and
~ very lcwl of the order of 3 to 5 . 1015 cm 3.
The Table gives the concentration of defects EL2 f the
different zones of this erystal for a ~etter understanding of the
figures.
TABLE
__ . . .................... _ _ _ _
Reference numerals of the zones Concentration of defects EL2
, , _ _
1 3 to 4.9 . 1015 cm 3
35 2 5 to 6.9 . 1015 . cm 3
3 7 to 9.9 . 1015 c~-3
4 10 to12.4 . 1015 . cm 3
_
~11.;~'7~3~
,
PHF 84.574 4 21-08-1985
The ~ehaviour of the defect EL2 in gallium arsenide is
known Erom the publication of D~ RUMSBY et al, in GaAs I.C. Symposium
1983, I.E.E,E., entitled "Improved uniformity of L.E.C. undoped Gallium
Arsenide produced by high temperatllre annealing". This publication in-
dicates that there is a solution for rendering the electrical propertiesof an undoped gallium arsenide crystal homogeneous. This solution con-
sists in sintering this crystal at a temperature of 950 for at least
5 hours. Due to this treatment, the values of the concentrations of the
defects EL2 are levelled. Although the publication indicates the means
for obtaining homDgeneous electrical properties and more particularly
a constant concentration level of the defects in an undoped crystal of
gallium arsenide, it does not state anything about the behaviour of the
level of defects EL2 in a crystal of gallium indium arsenide and about
a method of obtaining a semi-insulating crystal without disturbing
lS dopants.
According to the invention, the last-mentioned problem is
solved by first forming a single crystal of gallium indium arsenide by
means of the method described above from a bath rich in gallium so that
the single crystal obtained is of p-conductivity type, and by then sub-
jecting this single crystal to a single sintering treatment.
In fact is has been found that due to this operation, thelevel of defects EL2 of the crystal of indium-doped gallium arsenide,
which had a low level of these defects, as shown in Figures 1a and 1b,
rises in a surprising manner, as shown in Figures 1c and 1d, whilst
remaining uniform.
Thi~ result is the more surprising as if the prior art indi-
cated that uniformuty of the level of the defects EL2 was obtained in
a crystal of substantially similar ccmposition, it did not indicate
that the value thereof was considerab]y changed. Due to the fact that
in a crystal according to the invention the uniformity of the defects
EL2 was obtained from the origin, it was not obvious to carry out a
sintering treatment for finding a solution for the problem.
However, after the sintering treatment, the concentration of
deep donors in the single crystal of gallium indium arsenide has be-
35 come sufficient for this single crystal to be semi-insulating. The
addition of disturbing impurities, such as chromium, is thus avoided.
It can be readily imagined that the level of defects EL2 is
connected with an intrinsic defect of the crystal which could be the
~;~73L3~ 33
PHF 84.574 5 21-08-198S
positioning of arsenic atoms at the gallium sites (anti-sites). The
results obtained by the use of the invention suggest that, in contrast
with what was assumed hitherto, the homogenization of the defects EI,2
duriny a sintering treatment of undoped gallium arsenide is not due to
the diffusion of defects existing in the material, but on the contrary
to the evolution towards a thermodynamic equilibrium. The increase of
the level of defects EL2 in indium-doped gallium arsenide during the
sintering treatment would then be due to the evolution towards such an
equilibrium.
Thus a eriori unexFected result conseq~lently pe~mits of obtai-
ning, from indiumrdoped gallium arsenide crystals formed in accordance
with the invention, semi-insulating substrates which have great advan-
tages due to the fact that they are devoid of dislocations. This per-
mits of for~ing at their surfaces epitaxial layers which are as perfect
as possible and miniaturized circuit elements whose properties are
repetitive.
Such a method may be carried out in the ollowing manner. The
indium-doped gallium arsenide crystal is formed by the pulling method of
Czochralski with liquid encapsulation in a crucible of quartz or of
20 boropolynitride (BPN). The bath is rich in gàllium, which ~ecomes mani
fest by the fact that the concentration of the mass of gallium with
respect to the mass of arsenic plus gallium in the molten bath is higher
than 49% or by the fact that the concentration of the mass of arsenic
with respect to the mass of arsenic plus gallium in the molten bath lies
25 between 45 and 51%. The doping with indium can be effected by introducing
into the bath a quantity of indium arsenide defined by the ratio between
the mass ~ nAs f indium arsenide and the mass mGaAS of gallium arsenide
' ~ InAs/ Ga~s ~ 0,1
which is obtained, for example, by masses of
550 g of arsenic (As) associated with
510 g of gallium ~Ga) and with
70 g of indium arsenide (In~s) in the molten
bath.
The pulling speed is low, for example, lower than 15 mm per
35 hour. The general formula of the crystal obtained is:
Ga1 xInxAs,
in which the concentration ~ ~0,01.
A suitable sintering temperature lies between 800C and 1000C,
.
~71~Y3;.~
PHF 84.574 6 21-08-1985
for example, is of the order of 900C. This sintering treatment has
to be effected Eor at least 1 hcur~
Before the sintering treatment, the level of deects EL2
lies b~tween 3 and 5.1015 cm 3 (Figs. 1a and 1b). Afte~ the sintering
treatm~nt, this level is > 12.1015 cm 3, as is illustrated in Figs. 1b
and 1c.
It is clear that certain variations are possible in the comr
position of the starting III-V compound and in the kind of sintering
treatment to whlch it is subjected without departing from the scope of
the invention as defined by the appended claims.
