The III-V films are grown on large automatically perfect terraces of III-V substrates which have a different lattice constant, with temperature and Group III and V arrival rates chosen to give a Group III element stable surface. The growth is pulsed to inhibit Group III metal accumulation of low temperature, and to permit the film to relax to equilibrium. The method of the invention: (1) minimizes starting step density on sample surface; (2) deposits InAs and GaAs using an interrupted growth mode (0.25 to 2 monolayers at a time); (3) maintains the instantaneous surface stoichiometry during growth (As-stable for GaAs, In-stable for InAs); and (4) uses time-resolved RHEED to achieve aspects (1) through (3)

Grunthaner, Frank J.

Hancock, Bruce R.

Liu, John K.

English

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N O 3- Z " ' _
https://ntrs.nasa.gov/search.jsp?R=19900017369 2020-03-19T21:19:57+00:00Z
MBE GROWTH TECHNOLOGY FOR HIGH QUALITY
STRAINED III-V LAYERS
Inventors: Frank J. Grunthaner
John K. Liu
Bruce R. Hancock
JPL Case No.: 17723
NASA Case No.: NPO-17723-I-CU
Contractor: Jet Propulsion Laboratory Date: December 12, 1989
AWARDS ABSTRACT
The present invention relates to III-V devices employing
strained layers, and, more particularly, to a method for growing
such layers.
In accordance with the present invention, one lattice-mis-
matched III-V film is grown on a substrate of another III-V mate-
rial by use of a discontinuous or interrupted growth mode in mo-
lecular beam epitaxy (MBE). This is done to control the arrival
rate of the various elemental species impinging on the sample sur-
face. These elemental species react to grow the resulting film.
In the case of InAs growth on GaAs, the InAs film is grown under
In-stable conditions (slight excess of In on the surface). Only
1/4 to 2 monolayers of InAs are grown at a time, with periods of
interruption to permit the surface to establish a steady state con-
figuration with the recently grown film. In this manner, multilay-
er films are grown. Fig. 1 depicts apparatus used in the growth
of films on a substrate 18. The apparatus is conventional, and
includes a RHEED (reflective high energy electron diffraction) gun
28. The output from the RHEED is monitored on fluorescent screen
30; when the specular beam intensity is at a maximum, the smoothest
readily accessible surface has been achieved. Fig. 2 depicts the
sequence of opening and closing shutters 20 as a function of time
to deposit a plurality of InAs layers. The surface is first
smoothed with As (As shutter on; t,), then the As, shutter is turned
off for a period of time to permit equilibration of As surface
stoichiometry (t_), then the In shutter is turned on for a period
of time, then the In shutter is turned off for a period of time to
establish InAs stoichiometry (t_), then the As, is turned on. The
In and As, shutters are alternately opened and closed, with a time
delay (t,) between turning off one shutter and opening the other.
The method of the invention is expected to find use in the
growth of low defect density III-V strained layer films on the or-
der of a few tens of monolayers on a III-V substrate of another
material. Such films are useful in quantum well devices, for exam-
ple.
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'' ::t. PATENT APPLICATION
JPL Case No." 17723 _.........=_ _
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NASA Case No.: NPO-17723-I-CU._;I p_.:;>!:;i_:
Attorney Docket: D-89010 L-(Ci_)_).
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PRINT FIGURE_____
MBE GROWTH TECHNOLOGY FOR HIGH QUALITY
STRAINED III-V LAYERS
I0
Oriain of the Invention:
The invention described herein was made in the per-
formance of work under a NASA contract, and is subject to
the provisions of Public Law 96-517 (35 U.S.C. 202) in
which the Contractor has elected not to retain title.
15 Technical Field:
The invention relates to III-V devices employing
strained layers, and, more particularly, to a method for
growing such strained layers.
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Background Art:
Lattice-mismatched materials find use in a variety of
devices employing strained layers, such as optically-ad-
dressed spatial light modulators, high electron mobility
transistors which operate at room temperature, infrared
sensors, and lasers. Lattice-mismatched materials show
different growth modes as compared to such lattice-matched
materials as GaAs/AIGaAs. Thin films of such materials
evidence extensive dislocations and a very rough surface
morphology if grown continuously, as is current convention-
al practice.
In a particular example, it is desired to achieve the
growth of high quality (low defect density) InAs strained
layer films on GaAs (i00) substrates. Films are required
which are ten to thirty monolayers thick for certain device
applications, but existing technologies give only one mono-
layer, before generation of extensive dislocations.
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The growth of lattice-mismatched materials is extreme-
ly difficult because the system wants to relieve the devel-
oping lattice strain (in the course of film growth) by
spontaneously injecting misfit dislocations. These dislo-
cations are generally accompanied by other point defects
and are electrically active. The resulting material has
very poor electrical properties (low carrier lifetime, Fer-
mi level pinning). This is the general situation obtained
in classical MBE (molecular beam epitaxial) growth of InAs
films on GaAs (i00) surfaces. These films have a lattice
mismatch of more than 7.4% (InAs > GaAs). Classical MBE
growth refers to the continuous deposition of elemental In
on a GaAs surface which is flooded by an excess As4 beam.
The elemental In and As react together on the surface at
elevated temperatures (>200°C) to form the InAs compound.
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Statement of the Invention:
Accordingly, it is an object of the invention to grow
one lattice-mismatched III-V film on a substrate of another
III-V material.
It is an additional object of the invention to grow
such films free of dislocations.
It is yet another object of the invention to extend
the range of film thicknesses achievable with low defect
density beyond the predicted critical thickness of one
monolayer to values between I0 and 30 monolayers.
In accordance with the invention, one lattice-mis-
matched III-V film is grown on a substrate of another III-
V material. As used herein, "substrate" includes not only
a wafer surface, but also another layer or fiim.
The method of the invention involves what is termed a
discontinuous or interrupted growth mode in molecular beam
epitaxy, to control the arrival rate of the various elemen-
tal species impinging on the sample surface. These elemen-
tal species react to grow the resulting film. In the case
of InAs growth on GaAs, the InAs film is grown under In-
stable conditions (slight excess of In on the surface).
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Only 1/4 to 2 monolayers of InAs are grown at a time, with
periods of interruption to permit the surface to establish
a steady state configuration with the recently grown film.
In this manner, multilayer films are grown.
Brief Description of the D_awinus:
Fig. 1 is a schematic view of molecular beam epitaxy
apparatus useful in the practice of the invention.
Fig. 2 is a time plot of shutter opening and closing,
depicting a sequence of depositing a plurality of InAs lay-
ers.
15
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Detailed Description of the Invention:
Referring now to the drawings, wherein like reference
numerals designate like elements throughout, Fig. 1 depicts
molecular beam epitaxy (MBE) apparatus i0 useful in the
practice of the invention. Such apparatus is commercially
available, though customized to particular needs, such as
from Riber (Edison, NJ).
Essentially, the apparatus i0 comprises a housing 12,
which defines a chamber 13. Within the housing 12 is a
cryopanel 14, or liquid nitrogen shroud, surrounding the
entire deposition area 13 to ensure clean, high vacuum.
The cryopanel shrouding also isolates each effusion cell 16
to prevent thermal interference and cross contamination be-
tween cells.
A plurality of effusion cells 16 are used to introduce
specific elements. Typically, heating means (not shown) in
each cell 16 are used to generate a vapor of the element in
that cell, which is directed toward a sample Substrate 18.
Individual shutters 20 control the extent of exposure of
the substrate 18 to a particular element emanating from a
cell 16. The shutters 20 controllably rotate into and out
of position to block or open the aperture from the effusion
cells 16. A main shutter 22 isolates the substrate 18 from
all effusion cells 16.
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4
The relative position of the sample 18 is controlled
by a position control manipulator 24, which the operator
controls. The sample is introduced into the apparatus i0
through a gate valve 26, such as by a track means (not
shown).
A RHEED (reflective high energy electron diffraction)
gun 28 is positioned to provide grazing angle analysis of
the substrate 18. A fluorescent screen 30 is positioned to
intercept scattered electrons from the sample 18 and pro-
vide real-time analysis of the nature of the film being
grown.
An ion gauge 32 is used to measure the relative inten-
sity of the molecular beams when the gauge is rotated into
the growth position (done with the position control manipu-
lator 24).
A quadrupole mass spectrometer 34 is used to measure
the background chamber gas impurities.
An ultra-high vacuum (P < ixl0 -I° Torr base pressure) is
pulled on the chamber 13, as is conventional.
In the novel approach of the invention, five different
aspects of the problem are addressed to achieve the growth
of electrical quality InAs/GaAs structures. These reduce
to real-time control of the surface morphology during
growth using RHEED intensity analysis and deposition of the
InAs in a discontinuous or interrupted procedure. The con-
trol of surface morphology is predicated on the inference
that steps and kinks at the edges of extended terraces act
as nucleation sites for the generation of dislocations, or
of InAs faceted islands. The smoothest starting surface
therefore has the fewest steps or kinks and shows the long-
est terrace lengths. This condition is established by an-
nealing the sample starting surface in As, flux while moni-
toring the RHEED specular beam intensity. When the inten-
sity is a maximum, the smoothest readily accessible surface
has been achieved.
The discontinuous or interrupted growth mode is used
to take advantage of the capability of MBE to precisely
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5
control the arrival rate of the elemental species impinging
on the sample surface which react to grow the resulting
film. Typical MBE growth conditions call for a high excess
of As, (two to ten times greater than required by stoichiom-
etry) to be continuously delivered to the sample surface.
In this case, the growth rate is controlled by the arrival
of the Group III element (Ga or In). This approach rapidly
roughens the surface in time and gives a three-dimensional
morphology with extreme electrical and structural defects.
This is related to the low surface mobility of In on GaAs
under strongly As, stabilized conditions, and to the ten-
dency for InAs to grow preferentially on InAs as well as
InAs facets.
In the approach of the invention, the InAs film is
grown under In-stable conditions (slight excess of In on
surface). By this is meant up to about 10% excess of In
over stoichiometric In. Preferably, however, the excess In
is maintained as close to stoichiometry as possible, to
minimize defects. Too much In balls up, causing cluster
defects, which adversely affect electrical properties.
More than stoichiometric As again quenches the In surface
mobility, giving clustered growth with attendant disloca-
tions and electrical defects.
The process of the invention gives very high surface
migration rates, and a very uniform, smooth over-layer.
Unfortunately, if the films are grown continuously under
these conditions, an In excess in the film would be
achieved which would also destroy the structural and elec-
trical quality of the film. Therefore, 1/4 to 2 monolayers
of InAs are variously deposited for time periods of a few
seconds. The In and As shutters are then closed to inter-
rupt the In growth and to allow the surface to equilibrate
with the recently deposited film.
Fig. 2 depicts the process of the invention, first
smoothing the surface with As (As shutter on; tl), then
turning the As, shutter off for a period of time to permit
equilibration of As surface stoichiometry (t2), then turning
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6
the In shutter on for a period of time, then turning the In
shutter off, then delaying for a period of time to estab-
lish InAs stoichiometry (t_), then turning on the As, shut-
ter. The In and As, shutters are alternately opened and
closed, with a time delay (t,) between turning off one shut-
ter and opening the other.
The control of the In and As shutters is mechanically
limited. Accordingly, precise control of In and As is
achieved by opening the shutters for the minimum time pos-
sible, while controlling the flux of the elemental gun be-
hind the shutter by temperature control. An increase in
cell temperature of 5°C doubles the flux.
In the typical Riber apparatus, the minimum shutter
opening and closing times are 120 msec each. Other commer-
cial apparatus may vary.
The low pressure of background arsenic consumes the
excess indium, returning the film to stoichiometry. While
the growth has been interrupted, the atoms of the surface
translate rapidly, giving rise to a smoother film. This
method minimizes the evolving step density if the tempera-
tures are appropriately chosen, and consequently, the film
approaches a non-equilibrium or metastable configuration.
Using the RHEED to monitor the evolving crystallography of
the deposited film, the next layer deposition sequence is
reinitiated as soon as the R HEED specular beam intensity is
a maximum and the surface lattice constant becomes undis-
tinguishable from bulk GaAs. The time constant for this
film morphology recovery or annealing sequence is in the
range of a few seconds to one to two minutes.
The interrupted sequence therefore makes it possible
to grow In-stable InAs films (or other III-stable III-V
films), recover the film stoichiometry, let the film strain
relax in the direction of smoother films (lower entropy),
and maximize the average terrace length for the next film
deposition step. The use of RHEED makes it possible for
the experimenter to control stoichiometry without compro-
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7
mising the layer-by-layer growth mode needed for high qual-
ity interfaces.
This approach reduces the variation in thickness of
strained quantum wells, giving better interface quality,
and strongly impacts the defect density in the film and/or
the interfaces. This should give rise to lower concentra-
tions of dislocations and other near-interfacial electri-
cally active defects.
Reasonable electrical quality in these InAs/GaAs mul-
tilayers will make possible a series of new electronic
structures by breaking the conventional perception about
the lattice-mismatched problem. Lattice mismatched materi-
als give new experimental opportunities for design of elec-
tronic properties through band gap engineering as needed.
This approach supports a narrow band gap system buried
in GaAs which is ideal for optically addressed spatial
light modulator structures. The large band offset in the
conduction band between InAs and GaAs should make possible
HEMT structures with strong z-axis confinement. Since the
starting InAs has a high electron mobility intrinsically,
and since strain should effect the inter-band alignment
which gives rise to ballistic electron scattering, this
material should give rise to two-dimensional electron con-
finements and very high mobilities in thin quantum well
structures.
The experimental method of independently assessing the
stoichiometry, reconstruction and morphology of the evolv-
ing film using RHEED as a primary in situ characterization
probe is applicable to many other material systems showing
lattice mismatch, such as GaAs/InAs, GaAs/Si, InAs/Si,
InS_As:_./GaS_AS1_y, and AiAs/InAs.
As indicated above, the number of monolayers grown
depends on substrate temperature and flux of elements.
This is easily determined by monitoring RHEED specular
intensity, which is maximized for a given monolayer deposi-
tion.
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The period of time that all shutters are closed is
also determined by RHEED analysis. When the specular
intensity reaches a maximum value, the shutters are kept
closed for about three times the time it took to reach
maximum intensity, keeping the substrate temperature con-
stant.
It is preferred that the substrate temperature be
maintained at as low a value as possible. In the case of
InAs formation, the temperature may range from about 250 °
to 575°C, with times of shutter cessation ranging from
about 5 to 90 sec, the shorter times being associated with
the higher temperatures. At a temperature of 450°C, a time
of shutter cessation of about i0 to 15 sec is employed; 30
monolayers of InAs on GaAs have been grown under such con-
ditions.
MBE GROWTHTECHNOLOGYFOR HIGH QUALITY
STRAINED III-V LAYERS
5
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ABSTRACT OF THE DISCLOSURE
III-V films are grown on large automatically perfect
terraces of III-V substrates which have a different lattice
constant, with temperature and Group III and V arrival
rates chosen to give a Group III element stable surface.
The growth is pulsed to inhibit Group III metal accumula-
tion of low temperature, and to permit the film to relax to
equilibrium. The method of the invention i) minimizes
starting step density on sample surface; 2) deposits InAs
and GaAs using an interrupted growth mode (0.25 to 2 mono-
layers at a time); 3) maintains the instantaneous surface
stoichiometry during growth (As-stable for GaAs, In-stable
for InAs); and 4) uses time-resolved RHEED to achieve as-
pects (i) - (3).
28
I0
54/
12
14
14
24
Fig. 1.
In
I L J I I I
As [
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I _2
I
I
II
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II
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Fig. 2.


MBE growth technology for high quality strained III-V layers

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