Sb-mediated growth of n- and p-type AlGaAs by molecular beam epitaxy
S. R. Johnson, Yu. G. Sadofyev, D. Ding, Y. Cao, S. A. Chaparro, K. Franzreb, and Y.-H. Zhang
Citation: Journal of Vacuum Science & Technology B 22, 1436 (2004); doi: 10.1116/1.1705579
View online: http://dx.doi.org/10.1116/1.1705579
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Published by the AVS: Science & Technology of Materials, Interfaces, and Processing
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Sb-mediated growth of n- and p-type AlGaAs by molecular beam epitaxy
S. R. Johnson,a) Yu. G. Sadofyev, D. Ding, Y. Cao, S. A. Chaparro, K. Franzreb,
and Y.-H. Zhang
Center for Solid State Electronics Research and Department of Electrical Engineering,
Arizona State University, Tempe, Arizona 85287-6206

共Received 27 October 2003; accepted 24 February 2004; published 7 June 2004兲
The Sb-mediated growth of Al0.65Ga0.35As is studied for Sb/III flux ratios from 0% to 2% and
growth temperatures from 580 to 720 °C. The surface morphology and electrical properties are
found to strongly depend on both the growth temperature and Sb flux. As an isoelectronic dopant,
Sb improves the conductivity of n-Al0.65Ga0.35As and reduces the conductivity of p-Al0.65Ga0.35As.
As a surfactant, Sb improves the surface morphology of Al0.65Ga0.35As at all growth temperatures
with the most dramatic improvement occurring at 670 °C. The smoothest surface 共0.2 nm
root-mean-square roughness height兲 was obtained at 700 °C using a Sb/III flux ratio of 0.02. This
work demonstrates that Sb-mediated molecular beam epitaxy growth of n-AlGaAs effectively
eliminates the ‘‘forbidden temperature gap’’ for device quality n-AlGaAs. © 2004 American
Vacuum Society. 关DOI: 10.1116/1.1705579兴

I. INTRODUCTION
Today, molecular beam epitaxy 共MBE兲 is widely used to
grow optoelectronic devices, such as, in-plane lasers and vertical cavity surface emitting lasers 共VCSELs兲 that cover
wavelength ranges from visible to near infrared. These devices typically require a significant amount of Al containing
alloys to achieve low index optical confinement layers and
high band gap carrier confinement layers. AlGaAs as a nearly
lattice-matched alloy, in particular, is widely used on the
GaAs substrate, making it the material of choice for GaAs
based lasers. Due to the wide range of temperatures required
for active materials, such as GaAs, InGaAs, and GaAsSb, it
is important that high quality AlGaAs growth is accessible
over a wide range of substrate temperatures. Furthermore,
good control of surface morphology and deposition rate is
important for VCSELs, since accurate layer thickness and
smooth interfaces are critical to good device performance.
The existence of a forbidden substrate temperature gap for
the growth of AlGaAs layers by MBE has been widely
reported.1– 4 The surface morphology of Alx Ga1⫺x As (x
⬎0.2) is smooth for lower growth temperatures 共550–
600 °C兲, rough for intermediate temperatures 共620– 680 °C兲,
and smooth for high temperatures 共700–750 °C兲. These temperature ranges may vary slightly with the Al composition.
The lower growth temperatures result in epitaxial layers with
smooth surface morphology that have less than optimal electrical properties compared to layers grown at higher temperatures. At the highest growth temperatures the sticking coefficient of Ga is less than unity and strongly dependent on the
growth temperature, resulting in poor layer composition and
thickness control. Therefore, compromises are typical when
choosing adequate growth temperatures for GaAs/AlGaAs
distributed Bragg reflectors in VCSEL devices.
Sb as a surfactant has been proposed5 for the growth of
a兲

Author to whom correspondence should be addressed; electronic mail:
shane.johnson@asu.edu

1436

J. Vac. Sci. Technol. B 22„3…, MayÕJun 2004

smoother AlGaAs in the forbidden temperature gap where
the Ga sticking coefficient is close to unity and the Sb incorporation rate is close to zero. Since most device applications
require a substantial Al mole fraction, we chose to study the
Sb-mediated growth of Si and Be doped Al0.65Ga0.35As layers with emphasis on optimizing the electrical properties in
addition to the more widely studied surface properties.
II. EXPERIMENTAL RESULTS
A set of Si doped (N Si⫽2.0⫻1018 cm⫺3 ) Al0.65Ga0.35As
samples were grown at various temperatures 共580, 610, 640,
670, 700, and 720 °C兲 and various Sb/III flux ratios
共0.00, 0.005, 0.01, and 0.02兲. In addition to the extensive
set of n-type samples, several Be doped (N Be⫽2.0
⫻1018 cm⫺3 ) Al0.65Ga0.35As samples were grown at 580 and
640 °C with Sb/III flux ratios of 0.00 and 0.02. The growth
was done using a V80H VG solid source MBE system
equipped with Ga, Al, Si, and Be effusion cells and As and
Sb valved crackers. The As and Sb cracking zones were operated at 1000 °C to ensure that the group-V species were
cracked. The samples were grown on semi-insulating GaAs
共001兲 substrates starting with a 30 nm undoped GaAs buffer
layer, followed by a 200 nm thick undoped Al0.65Ga0.35As
spacer layer and a 1000 nm thick Si 共or Be兲 doped
Al0.65Ga0.35As layer. The undoped spacer provides a barrier
to free carrier penetration, thus eliminating the possible formation of a two-dimensional electron gas in the GaAs buffer
layer that could have an adverse effect on electrical measurements of the samples. Finally, the doped Al0.65Ga0.35As layer
was capped with a thin 10 nm Si 共or Be兲 doped GaAs contact
layer, whose thickness was chosen to be thick enough to
provide electrical contact and yet thin enough not to obscure
the surface morphology or conductivity of the underlying
AlGaAs layer.
The group-V flux values are given in absolute terms, with
Sb/III⫽Sb/共Ga⫹Al兲, where, for example, the flux of each
element is in units of incident atoms per unit area per unit
time. The Al0.65Ga0.35As layers are random alloy and were

1071-1023Õ2004Õ22„3…Õ1436Õ5Õ$19.00

©2004 American Vacuum Society

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1437

Johnson et al.: Sb-mediated growth of n- and p-type AlGaAs

FIG. 1. Surface morphology of 1.2 ␮m thick Si doped Al0.65Ga0.35As layers
grown at various Sb/III flux ratios and substrate temperatures. 共a兲 Length
scale of the surface roughness in the 关110兴 direction. 共b兲 rms height of the
surface roughness.

grown under a group-V stabilized 2⫻4 surface reconstructed
for both Sb-mediated and Sb-free growth. The As2 overpressure was varied depending on the growth temperature, with
As/III⫽1.5 at 580, 610, and 640 °C, As/III⫽1.8 at 670 °C,
and As/III⫽2.0 at 700 and 720 °C. The absolute V/III flux
ratios were established using the onset of the Ga rich surface
reconstruction 共or Ga droplet formation兲 to find the one-toone flux ratio 共Sb/Ga⫽1.0 or As/Ga⫽1.0兲, from which each
group-V flux curve was normalized in absolute terms. The
growth 共substrate兲 temperatures were accurately measured by
means of the substrate band edge using diffuse reflection
spectroscopy6 and were controlled via a substrate thermocouple which was typically about 100 °C higher than the
substrate temperature. The samples were investigated using
atomic force microscopy 共AFM兲, x-ray diffraction 共XRD兲,
secondary ion mass spectroscopy 共SIMS兲, and Hall effect
measurements in the van der Pauw contact geometry.
The surface morphology of each sample was examined by
AFM. The length scale of the roughness in the 关110兴 direction, L 关 110兴 , and the root mean square 共rms兲 height of the
surface roughness obtained from these measurements is reported in Fig. 1 for the various Sb fluxes and growth temperatures. Note that both the sample grown at 580 °C with
Sb/III⫽0.005 and the sample grown at 700 °C with Sb/III
⫽0.00 exhibit two length scales, as shown in Fig. 1共a兲. The
length scales were ascertained through two-dimensional 共2D兲
fast Fourier transform 共FFT兲 analysis of the AFM images and
only observed in the 关110兴 direction. The L 关 110兴 length scales

1437

are a result of surface trenches and ridges orientated perpendicular to, and periodic in, the 关110兴 direction.
The smoothing effect of the Sb surfactant in the forbidden
temperature gap is evident and the amount of Sb flux required for smoothing tends to increase with the substrate
temperature. A Sb/III flux ratio of 0.005 is sufficient for substrate temperatures below 620 °C, whereas a Sb/III flux ratio
of 0.01 to 0.02 is necessary in the forbidden temperature gap.
The length scale of the roughness in the forbidden temperature gap is insensitive to the Sb/III flux ratio, holding constant at about 200 nm, albeit the rms roughness height is
reduced from over 3 to less than 1 nm. On the other hand, the
roughness length scale abruptly doubles at 700 °C where
higher growth temperatures result in smother surfaces with
rms roughness heights below 1 nm.
To correlate the quantitative roughness shown in Fig. 1
with the qualitative optical characterization that is commonly
used to describe the roughness of epitaxial films, the following observations are offered. The smoothest AlGaAs layers
共⬃0.2 nm rms roughness兲 were grown at 700 and 720 °C
with a Sb/III flux ratio of 0.02. Remarkably, these surfaces
have the same low levels of roughness as high quality MBE
grown GaAs and have a flat mirror like appearance under a
high-magnification optical microscope 共with or without
phase contrast兲 and exhibit no optical scattering under a
bright white-light source. For higher rms roughness levels
around 0.5 nm the samples show some texture under an optical microscope and appear hazy under a bright light. For
surfaces with poor morphology, the rms roughness exceeds 1
nm and the surface texture is clearly visible under an optical
microscope and the samples appear hazy under ambient
lighting. These optical scattering observations are valid when
the roughness length scales are on the order of the wavelengths of visible light, which is typically the case for MBE
grown material and the reason why optical scattering is such
a common qualitative technique used by MBE growers.
The XRD measurements reveal that about 40% of the
incident Sb flux is incorporated at the lowest substrate temperature 共580 °C兲. As the substrate temperature increases the
Sb incorporation decreases exponentially, quickly dropping
below concentrations measurable by XRD. Therefore to determine the temperature dependence of the Sb sticking coefficient, S Sb 共or incorporation兲, SIMS measurements were
done on the samples grown under the highest 共0.02兲 Sb/III
flux. The XRD measurements were used to assign absolute
numbers to the SIMS data shown in Fig. 2. The Sb sticking
coefficient 共left-hand axis兲 and the Sb concentration 共righthand axis兲 decrease by two orders of magnitude over the
temperature range shown, resulting in a low 共but nonzero兲 Sb
concentration of about 2⫻1018 cm⫺3 in samples grown at
700 °C. Even at the highest growth temperatures Sb is incorporated as an isoelectronic dopant. The Sb sticking coefficient exhibits an Arrhenius behavior 共solid line fit to the
data兲 with characteristic energy 2.76⫾0.15 eV. The equation
with the best-fit parameters is shown in Fig. 2 where kT is in
units of absolute temperature. The constant in front of the

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1438

Johnson et al.: Sb-mediated growth of n- and p-type AlGaAs

FIG. 2. Sb sticking coefficient and codoping concentration in MBE grown
Al0.65Ga0.35As:Sb as a function of the substrate temperature for a Sb/III flux
ratio of 0.02.

exponential is 4.0⫻1018 cm⫺3 for the Sb concentration and
1% for the Sb sticking coefficient.
Figure 3 shows the room temperature Hall mobility
and
resistivity
of
the
Si
doped
(N Si⫽2
⫻1018 cm⫺3 ) Al0.65Ga0.35As layers discussed above and
shown in Fig. 1. The electron mobility increases with the
growth temperature and is enhanced by Sb codoping. Remarkably, at the lower growth temperatures where the Sb
codoping levels are large (⬃1020 cm⫺3 ), Sb as an isoelec-

FIG. 3. Electrical properties of 1.0 ␮m thick Si doped Al0.65Ga0.35As layers
grown at various Sb/III flux ratios and substrate temperatures. 共a兲 Electron
mobility, with the electron concentration and doping efficiency shown in the
inset. 共b兲 Sheet resistance and resistivity; the solid lines are fits to the data
for samples grown without Sb 共upper curve兲 and with Sb 共lower curve兲.

1438

tronic dopant still improves the electron mobility, indicating
that Sb codoping does not add to electron scattering. In the
640 to 670 °C growth temperature range, the optimal Sb
codoping level is about 3⫻1018 cm⫺3 共Sb/III⫽0.005–0.01兲.
For the highest growth temperature 共720 °C兲, the mobility
decreases because the Al mole fraction erroneously increases
from 0.65 to 0.70 when the Ga sticking coefficient decreases
from close to unity at 700 °C to 0.8 at 720 °C. The Al mole
fraction 共and hence Ga sticking兲 is determined from strain
measurements of the AlGaAs layer using XRD. At high
growth temperatures the contribution of incorporated Sb to
strain is roughly equivalent to ⫹1% in Al mole fraction and
is disregarded in the Ga sticking coefficient analysis.
Despite the fact that growth temperature and Sb codoping
have a dramatic effect on the electron mobility, the overall
contribution to the conductivity is not as spectacular because
of a strong dependence of the electron concentration 共doping
efficiency兲 on the growth temperature, as shown in the inset
of Fig. 3共a兲. The electron concentration is given on the lefthand axis and the Si doping efficiency is given on the righthand axis. Here the Si doping efficiency is defined as
n AlGaAs /n GaAs 共Hall measurements兲 for a given Si doping
flux; note that we are assuming that n GaAs⬃N Si , neglecting
the fact that a small fraction of Si incorporates as an acceptor
in GaAs. Although not as absolutely accurate as Hall measurements, SIMS measurements tend to validate our assumption, where we found that N SIMS⬃7⫻1017 cm⫺3 for Sidoped GaAs with n GaAs⫽1⫻1018 cm⫺3 . The accuracy of the
SIMS data is limited by the ⫾40% absolute accuracy of the
Si implant samples used as SIMS reference standards.
The trend in doping efficiency, shown by the solid line,
Fig. 3共a兲 inset, is independent of the Sb codoping levels and
has a minimum of 10% at 660 °C, right in the heart of the
forbidden temperature gap. The best doping efficiency 共25%兲
occurs at the lowest growth temperature 共580 °C兲. Notwithstanding the decline in electron concentration, the conductivity of the AlGaAs layer increases with the growth temperature, as shown by the plot in Fig. 3共b兲 with resistivity on the
right-hand axis and sheet resistance on the left-hand axis.
The solid lines are fits to the data from samples grown without Sb 共upper curve兲 and with Sb 共lower curve兲. On average,
the sheet resistance decreases at a rate of ⫺5 ⍀/°C and is
further reduced by 120 ⍀/䊐 when Sb codoping is present.
The impact of Sb as an isoelectronic dopant in
Al0.65Ga0.35As is presented in Fig. 4 for the various growth
temperatures. The Sb levels shown here are calculated using
the Sb sticking coefficient given in Fig. 2 and the incident
Sb/III flux rates. Note that the smallest Sb concentrations
共left-hand side of each curve兲 are for the samples grown with
the Sb source off 共Sb/III⫽0.00兲, where the unintentional Sb
comes from the Sb background in the growth chamber. In
this case, the Sb codoping level calculation is based on the
typical 1018 cm⫺3 Sb background levels observed by SIMS
measurements on samples grown at 580 °C. The increase in
electron mobility with higher Sb codoping levels is clearly
seen in Fig. 4共a兲. On the other hand, in Fig. 4共b兲, the electron

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1439

Johnson et al.: Sb-mediated growth of n- and p-type AlGaAs

FIG. 4. Electrical properties as a function of the Sb codoping concentration
for 1.0 ␮m thick Si doped Al0.65Ga0.35As layers grown at various substrate
temperatures. 共a兲 Electron mobility. 共b兲 Electron concentration.

concentration shows no observable dependence on the Sb
codoping levels.
The results of the Be doped Al0.65Ga0.35As samples are
summarized in Fig. 5. The rms height of the surface roughness is shown in Fig. 5共a兲 and is less than 1 nm for all four
samples. Of the samples grown at 640 °C, the Be doped
samples are about three times smoother than the Si doped
samples. It is possible that surface segregation of the Be
dopant serves as surfactant, resulting in a smoother growth
surface at 640 °C. The Be doped samples grown at 580 °C do
not exhibit a roughness length scale and the samples grown
at 640 °C display a length scale around 200 nm in the 关110兴
direction, similar to that observed in the Si doped samples
discussed above. The hole mobility and material resistivity
of Be doped Al0.65Ga0.35As are shown in Fig. 5共b兲. Unlike
the Si doped material, the Be doped material does not exhibit
a change in doping efficiency with the growth temperature;
the doping efficiency is 75% for all four samples. The Be
impurity and hole concentrations are listed in Fig. 5共b兲. Similar to the Si doped samples, the carrier mobility and material
conductivity of the Be doped samples increase with the
growth temperature, however, unlike the Si doped samples,
Sb as a codopant substantially reduces the carrier mobility
and material conductivity.
III. DISCUSSION
Sb acts as both a surfactant and an isoelectronic dopant in
AlGaAs. As a surfactant, Sb provides improved surface mor-

1439

FIG. 5. Surface morphology 共of 1.2 ␮m thick兲 and the electrical properties
共of 1.0 ␮m thick兲 Be doped Al0.65Ga0.35As layers grown at various Sb/III
flux ratios and substrate temperatures. 共a兲 rms height and 关110兴 length scale
of the surface roughness. 共b兲 Hole mobility and resistivity.

phology with the most dramatic improvement occurring in
the forbidden temperature gap. During Sb-mediated growth a
steady-state surface population of Sb is maintained, enhancing the surface diffusion kinetics of the Ga and Al adatoms.
The three-dimensional morphology of AlGaAs layers grown
in the forbidden temperature gap is thought to originate from
less than ideal Ga and Al transport caused by oxygen gettering on the surface,7 strong anisotropy of surface diffusion,8
the strong Al–As bond,9 and diffusion barriers at island
edges.10
The anisotropic diffusion at the atomic scale presumably
drives the 200 nm length scale roughness observed in the
关110兴 direction. In which case, the group-V terminated dimer
rows perpendicular to the 关110兴 direction oppose group-III
adatom diffusion in that direction, while the group-III terminated trenches between the dimer rows assist group-III adatom diffusion in the perpendicular 关 1̄10兴 direction. At higher
temperatures above the forbidden temperature gap, Ga and
Al diffusion is enhanced, resulting in smoother films. Below
the forbidden temperature gap, surface impurities are thought
to be more readily incorporated, interfering less with surface
mobility, and the Al adatoms tend to stay put thereby avoiding roughening due to anisotropic diffusion.
Isoelectronic doping reduces deep levels11 which improves the material properties of Si doped AlGaAs where
deep-level traps are not simply arsenic or gallium vacancies, but possibly impurities or gallium/arsenic related complexes.12 Furthermore, isoelectronic doping can reduce unin-

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1440

Johnson et al.: Sb-mediated growth of n- and p-type AlGaAs

tentional impurities and dislocation density in III–V
materials.13 As an isoelectronic dopant, Sb yields higher
electron mobility and lower sheet resistance in n-AlGaAs:Si
and lower hole mobility and higher sheet resistance in
p-AlGaAs:Be. An interesting result given that Sb occupies
group-V sites and as n and p dopants, both Si and Be impurities occupy group-III sites.
The overall doping efficiency of Si in Al0.65Ga0.35As is
low because the dominant Si donor level sits well below the
conduction band14 and some of the dopant atoms are likely
incorporated as acceptors due to the amphoteric nature of Si.
The notable temperature dependence of the doping efficiency
may be due to an amphoteric behavior of Si atoms in AlGaAs that depends on the growth temperature or that the Si
donor activation depends on the growth temperature. Compensation by defects is another possibility, in which case the
defect formation would have to depend only on the temperature and not on the Sb codoping or surface morphology. On
the other hand, the doping efficiency of Be in Al0.65Ga0.35As
is independent of the growth temperature and Sb codoping.
IV. CONCLUSIONS
Sb-mediated growth improves both the electrical properties and the surface morphology of Si doped Al0.65Ga0.35As;
as an isoelectronic dopant Sb increases the electron mobility
and as a surfactant Sb reduces surface roughness. The most
impressive results occur in the 620– 680 °C temperature

1440

range, essentially eliminating the forbidden temperature gap
for n-AlGaAs:Si growth in devices. Sb-mediated growth improves the surface morphology and degrades the electrical
properties of Be doped Al0.65Ga0.35As; as a surfactant Sb
reduces surface roughness and as an isoelectronic dopant Sb
decreases hole mobility.
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