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Optimization of the heteroepitaxy of Ge on GaAs for minority-carrier lifetime
Journal of Crystal Growth 112 (1991) 7—13 North-Holland
7
Optimization of the heteroepitaxy of Ge on GaAs for minority-carrier lifetime R. Venkatasubramanian, M.L. Timmons Research Triangle institute, Research Triangle Park, North Carolina 2 7709, USA
S. Bothra and J.M. Borrego Rensselaer Polytechnic institute, Troy, New York 12180— 3590, USA
Received 9 August 1990; manuscript received in final form 14 December 1990
Growth of Ge on GaAs at reasonably high temperatures, which produces better crystallinity in the Ge, presents serious difficulties due to the dissociation of the GaAs substrate. In this paper, we describe the growth of a low-temperature buffer layer of Ge on GaAs that considerably reduces the effects of decomposition of the GaAs during high-temperature growth of Ge. Using this approach, we present the first report of highly specular, mass-transport-limited, high-temperature growth of Ge on GaAs that is 3) doping level. The factors comparable to the homoepitaxy of Ge, although with a reasonably high residual n-type ( —10w cm affecting the structural, electrical and optical properties of Ge on GaAs, using such an epitaxial growth technique, were studied. Lifetime variations from very low values to about 0.45 j~swere measured by a microwave technique as a function of growth conditions. Significantly, the removal of the surface oxide on the GaAs substrate prior to low-temperature buffer-layer growth, terminating the flow of germane (GeH 4) during the ramp to high growth temperatures, thinner buffer layers, and high-temperature growth of Ge, were found to be important for obtaining long lifetimes.
1. Introduction Germanium has a significant absorption coefficient in the 1.0—1.5 ~tm range for potential solar cell applications, in tandem with GaAs and related materials. Also, the high-quality growth of Ge on GaAs opens up the possibility of integration of photonic devices in the 1.3—1.5 ~tm range in Ge with electronic functions in GaAs. Additionally, the high intrinsic hole mobility in Ge may be of importance in a useful integration of certain Ge devices on a GaAs substrate. It is desirable to grow defect-free Ge on GaAs with high minority-carrier lifetimes for many of these applications, Further, there is very little literature on lifetime measurements in Ge grown by the pyrolysis of GeH4, much less as a function of growth conditions. These measurements will be useful for opti0022-0248/91/$03.50 © 1991
—
mising Ge homoepitaxial structures. Ge does not luminesce, being an indirect-gap material, and unlike GaAs, PL-decay measurements are not useful in obtaining minority-carrier lifetimes. Also, Ge substrates are not available as “semi-insulating”, so that photoconductivity-transient measurements for lifetime are difficult because the parallel substrate/epilayer conductivity swamps out any photoresponse of the epilayer. Thus, to overcome these difficulties and obtain lifetimes in Ge layers, we have used the growth of Ge on semi-insulating GaAs. Using a contactless microwave technique to measure the decay of laser-induced photoconductivity in Ge, we have obtained lifetimes in Ge/ semi-insulating GaAs structures. Although Ge closely lattice-matches GaAs and both materials have similar thermal-expansion coefficients, the growth of Ge on GaAs at reasonably high temperatures, which is thought to
Elsevier Science Publishers B.V. (North-Holland)
8
R. Venkatasubramanian e a!.
/
Optirnizalion of heteroepitaxv of Ge on GaAs
produce better crystallinity in the Ge, is cornplicated by the dissociation of the GaAs substrate. One approach to circumvent this difficulty is to keep an arsine (AsH 3) overpressure on GaAs at higher temperatures until Germane flow is tiated [1]. However, this leads to unwanted reaclions between residual AsH3 and GeH4, even in a high flow-rate, low-pressure system, that can lead to poor Ge layer morphology, In this paper, we present an approach to the growth of Ge on GaAs at temperatures as high as 775 ° C. The use of a buffer layer of Ge grown at low temperatures on GaAs prevents the decomposition of the GaAs substrate during the subsequent high-temperature growth of Ge.
mi-
2. Experimental Growth of Ge was carried out in a horizontalflow, atmospheric-pressure epitaxial reactor with an rf-heated graphite susceptor coated with pyrolytic boron nitride. GaAs substrates, misoriented a few degrees from (100) orientation, were degreased and given a polishing etch prior to epitaxy. This process consisted of a s mm rinse in an etching solution of 1H20: 1H202 : 5H2 504. At the end of this etch, the substrate was rinsed in deionized water, methanol, and then blown dry. Prior to any growth (unless otherwise mdicated), the GaAs substrate was kept under an AsH3 overpressure of 1.0 Torr at a temperature of — 800°C for 10 mm. This step is known to remove the native oxide from the GaAs surface. In some cases, this step was omitted to study the effect of its absence on minority-carrier lifetime in Ge. Following the reducing step, the substrate temperature was lowered to 550°C and maintamed in an H2 flow for about 10 mm to purge the AsH3 from the growth ambient. Following this, the substrate was heated to 590°C, and a flow of GeH4 was initiated. The GeH4 partial pressure was kept constant at 0.0084 Torr, and the H2 carrier flow was about 12 SLM. This GeH4 partial pressure resulted in a growth-rate of — 30 A/mm. Using this growth rate, buffer-layer thicknesses of 0, 90, 900 and 1800 A were used prior to high-temperature growth. Following this buffer-
layer growth at 5900 C, the substrate temperature was raised for a high-temperature growth step at 675. 725 or 775°C. Typical duration of this ramp step was about 3 mm. During the ramp from 590°C to the higher growth temperature, the absence of germane was found to be significant for good Ge layer quality. At the higher growth ternperature, the Ge was grown using a GeH4 partial pressure of 0.063 Torr and an H2 flow rate of 12 SLM. In all cases, the thickness of the grown layer was — 2.5 ~tm. The microwave measurement system, used to obtain the lifetimes in Ge/GaAs structures, has been described in detail elsewhere [2j. A Gunn diode is used as the microwave source and generates 150 mW of microwave power at 36 GHz. A portion of this power is incident on the sample through a parallel plate antenna. The sample is illuminated locally by an AlGaAs laser diode emitting at 850 nm. The area of the sample probed is the region where excess carriers are generated. The induced photoconductivity modulates the microwave power reflected from the sample. This reflected power serves as the probe of the excess-carrier decay. The reflected microwave signal is detected by a fast responding crystal detector, the output of which is a voltage proportional to the incident microwave power at the detector. The laser is driven by a pulser whose pulse width can be varied from 25 to 50 ns. The current delay time of the pulse from the pulser is 3 ns, setting the lower limit for lifetime measurement with this system.
3. Results and discussion A wide variation in surface morphology exists for the Ge epilayers using the growth procedures discussed above. Shown in Figs. la to 1c are the surface features observed in some of the samples with buffer layers. Without the low-temperature buffer, the surface topography is very rough. The absence of GeH4 during the temperature ramp from the buffer-layer growth to the higher growth temperature yields specular surfaces (compare fig. la with figs. lb and Ic). Also, the use of a 900 A thick buffer (fig. ic) results in more complete
R. Venkatasubramanian et a!.
/
9
Optimization of heteroepitaxy of Ge on GaAs
I. Fig. 1. Surface morphology of Ge layers grown on GaAs at 725°C with (a) 90
A
thick buffer at 590°C and GeH
4 flow present during the temperature ramp, (b) 90 A thick buffer and GeH4 flow absent during the ramp and (c) 900 A thick buffer and Gel-I4 flow absent during the ramp. Markers represent 10 Fm.
removal of surface roughness, compared to a 90 A thickdone bufferat(fig. Ib). The high-temperature was 725°C for all three layers in growth fig. 1. The morpholog~iesof the Ge layers grown at 725°C are representative of the morphologies obtained at 675 and 775°C, as long as a buffer layer was included and GeH4 flow was interrupted during the temperature ramp. For all the growth conditions described above, the Ge layers on GaAs substrates are single-crystal. X-ray diffraction, using a conventional singlecrystal diffractometer with Cu K~1and Cu K~2 lines, was used for this purpose. Normal (004) reflections from the epilayer and the substrate are used. In spite of the limited resolution of the diffractometer, the Ge and GaAs diffraction peaks are always resolved when the buffer layer is included. The X-ray diffraction patterns obtained on Ge/GaAs heteroepitaxial structures, grown with a 90 A thick buffer, are shown in fig. 2. In fig. 2h, a Ge layer grown with the GeH4 stopped during the temperature ramp, shows improved resolution between the Ge and GaAs peaks when compared (shown in fig. 2a) to a layer grown without GeH4 flow termination. Spreading-resistance measurements were used to obtain the electrical properties of the Ge layers grown on GaAs. Shown in figs. 3a and 3b are the spreading-resistance profiles obtained on two Ge layers, one grown with GeH4 flowing during the
temperature ramp (n — 1 x 1019 3). crn respectively. 3), and the other without (n — 1 >< l0’~cm Thus, the absence of GeH 4 during the temperature ramp is beneficial in reducing the average
P
_____
~
A ~
(0
~ ~ ~
a
b ~—
. . Fig. 2. Single crystal X-ray diffraction trace of Ge layer grown
on GaAs with a 90 A thick buffer and (a) GeH4 present during the ramp and (h) GeH4 absent during the ramp.
10
R. Venkatasubramanian et a!
/ Optimization
1020
1020
1019
~n~~nnnnnnn~
1018
—
n ~
~ 10~~~1016 ~ 10~
1019
—
1018
flflflflfl
io~~-
~
~1o16
-
-
oo
-
1015
~
-
00
.22 1014
o
of heteroepitaxy of Ge on GaAs
-
.~
0
1013
-
~——-—~
Ge GaAs 1012
-
i0~~
10©
-
1012
-
Ge~GaAs ~flnnri
a
~~ii
0
b
iou .51 1.52 2.53 Depth (microns)
0
nri
I
.511.522.53
Depth (microns)
Fig. 3. spreading resistance profile of Ge layer on GaAs grown (a) with GeH
4 present and (b) with Gel-I4 absent during temperature ramp from 590 to 725°C.
hulk doping level by as much as a factor of ten, We also note the large increase in carrier level near Ge/GaAs interface in the sample (fig. 3b), where GeH 4 is turned off during the temperature ramp, presumably due to arsenic doping from the GaAs substrate in the absence of continuous Ge growth. The reason for the lower bulk doping level in fig. 3b is not clear. However, as seen in figs. la and lb. the absence of GeH4 during the temperature ramp leads to better surface morphology and few large-scale defects. This was also evident in the X-ray diffraction data. We note that the main source for As incorporation into Ge grown on GaAs is the constant As evaporation from the back surface of the GaAs substrate and not solidstate diffusion from the GaAs substrate, due to the extremely small self-diffusion coefficients of As in GaAs at high temperatures [3]. Thus, any excess background carrier level in the sample with GeH4 present during the temperature ramp (fig. 3a), but grown at the same temperature (725 C) as the sample without GeH4 during the temperature ramp (fig. 3b), and with the same rate of As °
evaporation from GaAs hack surface, is attrihutable to significantly higher defect densities. 3) in Ge The higher defect density (~1019 cm films, grown with GeH4 present during the temperature ramp, can be understood in a simple manner, as follows. Around 590°C, for the GeH4 input partial pressure employed, the effective rate at which GeH4 decomposes into Ge is probably smaller than the rate at which Ge-absorbed atoms can move on the surface to find a kink-site. At a point somewhere between 590°C and the hightemperature (675—775 C) step, if the GeH4 decomposition rate exceeds the surface mobility-determined rate, growth defects are likely. However, at higher temperatures such as 675 to 775°C. if GeH4 decomposition rate saturates and is once again below the surface-mobility-determined rate (which continues to increase with temperature). we can obtain smooth morphologies at these growth temperatures, as long as a buffer layer is grown at a lower temperature to prevent GaAs decomposition. Using such an approach, we also obtained the °
R. Venkatasubramanian et at.
100
825
775 750 725 700 675 650 I
.~
I
/
Optimization of heteroepitaxy of Ge on GaAs
600
I
growth rate of Ge as a function of growth temperature in the range from 675 to 775°C. The GeH4
I
=
partial pressure was kept constant at 0.063 Torr and H2 flow rate at 12 SLM. In this temperature range, we essentially obtained a temperature-independent growth rate for Ge, as shown by the data of fig. 4. This suggests mass-transport-limited growth of Ge on GaAs without kinetic limitations. For a comparison, growth-rate data of other
-
= GeH~Mole Fraction •
4.4x
11
io~ This work
for a constant GeH4 mole fraction~also shown are data from
authors [1,4] (for Ge on GaAs epitaxy) are also included in fig. 4. These data indicate a considerable thermal activation, probably resulting from kinetic limitations to growth of Ge on GaAs. The optical-absorption characteristics of Ge grown on GaAs near and below the Ge band-edge were also measured. Absorption measurements, especially in the sub-bandgap region, are good indicators of material quality. Shown in fig. 5 are absorption curves of Ge on GaAs, grown at 675, 725 and 775 °C, respectively. These samples had a 900 A thick Ge buffer layer, grown at 590°C.The GaAs substrate underwent a deoxidation step at 800°C,and GeH4 was turned off during temperature ramping. The Ge/GaAs structure grown at
other authors.
725°C exhibits sharper near-band-edge absorp-
A 2.5 x 1O~ Ayers arid Ghandhi • -~10.1
(13
3.0 x iO~ Papazian and Reisman
-
-
102
0.90
I
0.94 0.98
I
1.02
1.06 1.10
I
1.14
1.18
1000/1 Fig. 4. Growth rate of Ge on GaAs between 675 and 7750 C
~
55.000
________________________________________________________
45.000
-
35.000
-
~._
~
//
Ia 0
= ~ 25.000
E 5~
15.000
775°C 675°C
-
5.000
—5.000 6941.4
/
6369.4
I 5797.4 5225.4 Wave Numbers (cm-i)
4653.4
Fig. 5. Optical-absorption characteristics of Ge layers grown on GaAs at 675 to 7750 C using a 90 A thick buffer at 5900 C.
12
R. Venkatasubramanian et a!.
/
Optimization of heteroepitaxy of Ge on GaAs
Table I Minority-carrier lifetime of Ge on GaAs as a function of various growth schemes Sample
Deoxidation
Buffer-layer
GeH
No.
at 800°C
thickness
4 flow during ramp
Growth temperature
(A)
Lifetime (ns)
(absent)
(° C)
2
No No
900 900
No Yes
675 675
3 4 5 6 7
No No No No Yes
1800 900 900 900 900
Yes No Yes Yes Yes
675 725 725 675 675
<3 65 100
8 9
Yes Yes
900 900
Yes Yes
725 775
90 150
10 11 12
Yes Yes Yes
90 90 90
No Yes Yes
725 725 725
<3 440 420
3
tion and a higher peak transmission in the subbandgap region. These are attributed to reduced defects/impurities in the Ge layers grown at 725°C compared to those grown at 675 and
thinner buffer layers (90 A buffers seem better than 900 A thick buffers), and higher growth temperature all appear to improve the lifetime in Ge layers. The longest lifetime that has been mea-
775°C.
sured is 440 ns and it is likely that similar values are obtainable in Ge homoepitaxial layers grown at — 725°C, for similar doping levels of n — I X 3. 1018 cm
Based
on
absorption
characteristics,
725 ° C appears as an optimum growth temperature for Ge/GaAs epitaxy. A similar observation has been made the homoepitaxy analysis, of Ge [Sj. where based on in spreading-resistance it was argued that defects in Ge layers were probably minimized. The GaAs/Ge heteroepitaxial data supports this conclusion, although lifetime values observed in Ge layers as a function of growth temperature which are discussed below, suggest 775°C may yield better lifetimes in Ge. The minority-carrier hole lifetimes in Ge layers, as a function of various growth procedures described above, were measured by the microwave technique. The lifetime values are summarized in table 1. When lifetimes are near the resolution limit (— 3 ns) of the equipment, the photoconductivity is found to be weak and it is difficult to measure accurate lifetimes. The earlier conclusions about the structural, electrical, and optical properties of the Ge-on-GaAs layers as a function of growth conditions apply equally well to minoritycarrier lifetime. Significantly, the inclusion of a 800°C deoxidation step for GaAs substrate prior to low-temperature buffer-layer growth, the absence of GeH 4 during the temperature ramp,
4. Summary We have described a new approach to the beteroepitaxy of Ge on GaAs at growth temperatures where the dissociation of the GaAs substrate becomes a problem. Using Ge buffers grown at low temperatures, we have obtained specular, masstransport-limited growth of Ge on GaAs that is comparable to the homoepitaxy of Ge. However, the best films grown using this approach have background carrier concentration in the range of — 3. Surface-morphology features, X-ray 1fl~~ cm electrical characteristics, and opticaldiffraction, absorption data are presented as a function of processing conditions. Minority-carrier lifetimes in Ge layers on semi-insulating GaAs, measured by a microwave technique, are also presented as a function of the various growth schemes. Lifetimes as long as 440 ns have been obtained for Ge in Ge/GaAs heterostructures.
R. Venkatasubramanian et a!.
/
Optimization of heteroepitaxy of Ge on GaAs
Acknowledgements
13
References [11 J.E. Ayers and S.K. Ghandhi, J. Crystal Growth 89 (1988)
The authors are pleased to acknowledge the
371.
support of the Department of the Air Force,
[2] R. Venkatasubramanian, S. Bothra, S.K. Ghandhi and J.M.
Wright Research and Development Center (AFSC), Wright-Patterson Air Force Base, Ohio 45433-6523, under Contract No. F33615-87-C2804 for this work. Air Force Project Engineer is Mr. Kitt Reinhardt. Also, we wish to thank Ms. Vicki Michael Hewitt for assistance with manuscript preparation.
Borrego, in: Proc. IEEE Photovoltaic Specialists Conf., 1988, p. 689. [3] D.L. Kendall, in: Semiconductors and Semimetals, Vol. 4, Eds. R.K. Willardson and A.C. Beer (Academic Press, New York, 1968) p. 163. [4] S.A. Papazian and A. Reisman, J. Electrochem. Soc. 115 (1968) 961. [5] R. Venkatasubramanian, R.T. Pickett and M.L. Timmons, J. AppI. Phys. 66 (1989) 5662.
7
Optimization of the heteroepitaxy of Ge on GaAs for minority-carrier lifetime R. Venkatasubramanian, M.L. Timmons Research Triangle institute, Research Triangle Park, North Carolina 2 7709, USA
S. Bothra and J.M. Borrego Rensselaer Polytechnic institute, Troy, New York 12180— 3590, USA
Received 9 August 1990; manuscript received in final form 14 December 1990
Growth of Ge on GaAs at reasonably high temperatures, which produces better crystallinity in the Ge, presents serious difficulties due to the dissociation of the GaAs substrate. In this paper, we describe the growth of a low-temperature buffer layer of Ge on GaAs that considerably reduces the effects of decomposition of the GaAs during high-temperature growth of Ge. Using this approach, we present the first report of highly specular, mass-transport-limited, high-temperature growth of Ge on GaAs that is 3) doping level. The factors comparable to the homoepitaxy of Ge, although with a reasonably high residual n-type ( —10w cm affecting the structural, electrical and optical properties of Ge on GaAs, using such an epitaxial growth technique, were studied. Lifetime variations from very low values to about 0.45 j~swere measured by a microwave technique as a function of growth conditions. Significantly, the removal of the surface oxide on the GaAs substrate prior to low-temperature buffer-layer growth, terminating the flow of germane (GeH 4) during the ramp to high growth temperatures, thinner buffer layers, and high-temperature growth of Ge, were found to be important for obtaining long lifetimes.
1. Introduction Germanium has a significant absorption coefficient in the 1.0—1.5 ~tm range for potential solar cell applications, in tandem with GaAs and related materials. Also, the high-quality growth of Ge on GaAs opens up the possibility of integration of photonic devices in the 1.3—1.5 ~tm range in Ge with electronic functions in GaAs. Additionally, the high intrinsic hole mobility in Ge may be of importance in a useful integration of certain Ge devices on a GaAs substrate. It is desirable to grow defect-free Ge on GaAs with high minority-carrier lifetimes for many of these applications, Further, there is very little literature on lifetime measurements in Ge grown by the pyrolysis of GeH4, much less as a function of growth conditions. These measurements will be useful for opti0022-0248/91/$03.50 © 1991
—
mising Ge homoepitaxial structures. Ge does not luminesce, being an indirect-gap material, and unlike GaAs, PL-decay measurements are not useful in obtaining minority-carrier lifetimes. Also, Ge substrates are not available as “semi-insulating”, so that photoconductivity-transient measurements for lifetime are difficult because the parallel substrate/epilayer conductivity swamps out any photoresponse of the epilayer. Thus, to overcome these difficulties and obtain lifetimes in Ge layers, we have used the growth of Ge on semi-insulating GaAs. Using a contactless microwave technique to measure the decay of laser-induced photoconductivity in Ge, we have obtained lifetimes in Ge/ semi-insulating GaAs structures. Although Ge closely lattice-matches GaAs and both materials have similar thermal-expansion coefficients, the growth of Ge on GaAs at reasonably high temperatures, which is thought to
Elsevier Science Publishers B.V. (North-Holland)
8
R. Venkatasubramanian e a!.
/
Optirnizalion of heteroepitaxv of Ge on GaAs
produce better crystallinity in the Ge, is cornplicated by the dissociation of the GaAs substrate. One approach to circumvent this difficulty is to keep an arsine (AsH 3) overpressure on GaAs at higher temperatures until Germane flow is tiated [1]. However, this leads to unwanted reaclions between residual AsH3 and GeH4, even in a high flow-rate, low-pressure system, that can lead to poor Ge layer morphology, In this paper, we present an approach to the growth of Ge on GaAs at temperatures as high as 775 ° C. The use of a buffer layer of Ge grown at low temperatures on GaAs prevents the decomposition of the GaAs substrate during the subsequent high-temperature growth of Ge.
mi-
2. Experimental Growth of Ge was carried out in a horizontalflow, atmospheric-pressure epitaxial reactor with an rf-heated graphite susceptor coated with pyrolytic boron nitride. GaAs substrates, misoriented a few degrees from (100) orientation, were degreased and given a polishing etch prior to epitaxy. This process consisted of a s mm rinse in an etching solution of 1H20: 1H202 : 5H2 504. At the end of this etch, the substrate was rinsed in deionized water, methanol, and then blown dry. Prior to any growth (unless otherwise mdicated), the GaAs substrate was kept under an AsH3 overpressure of 1.0 Torr at a temperature of — 800°C for 10 mm. This step is known to remove the native oxide from the GaAs surface. In some cases, this step was omitted to study the effect of its absence on minority-carrier lifetime in Ge. Following the reducing step, the substrate temperature was lowered to 550°C and maintamed in an H2 flow for about 10 mm to purge the AsH3 from the growth ambient. Following this, the substrate was heated to 590°C, and a flow of GeH4 was initiated. The GeH4 partial pressure was kept constant at 0.0084 Torr, and the H2 carrier flow was about 12 SLM. This GeH4 partial pressure resulted in a growth-rate of — 30 A/mm. Using this growth rate, buffer-layer thicknesses of 0, 90, 900 and 1800 A were used prior to high-temperature growth. Following this buffer-
layer growth at 5900 C, the substrate temperature was raised for a high-temperature growth step at 675. 725 or 775°C. Typical duration of this ramp step was about 3 mm. During the ramp from 590°C to the higher growth temperature, the absence of germane was found to be significant for good Ge layer quality. At the higher growth ternperature, the Ge was grown using a GeH4 partial pressure of 0.063 Torr and an H2 flow rate of 12 SLM. In all cases, the thickness of the grown layer was — 2.5 ~tm. The microwave measurement system, used to obtain the lifetimes in Ge/GaAs structures, has been described in detail elsewhere [2j. A Gunn diode is used as the microwave source and generates 150 mW of microwave power at 36 GHz. A portion of this power is incident on the sample through a parallel plate antenna. The sample is illuminated locally by an AlGaAs laser diode emitting at 850 nm. The area of the sample probed is the region where excess carriers are generated. The induced photoconductivity modulates the microwave power reflected from the sample. This reflected power serves as the probe of the excess-carrier decay. The reflected microwave signal is detected by a fast responding crystal detector, the output of which is a voltage proportional to the incident microwave power at the detector. The laser is driven by a pulser whose pulse width can be varied from 25 to 50 ns. The current delay time of the pulse from the pulser is 3 ns, setting the lower limit for lifetime measurement with this system.
3. Results and discussion A wide variation in surface morphology exists for the Ge epilayers using the growth procedures discussed above. Shown in Figs. la to 1c are the surface features observed in some of the samples with buffer layers. Without the low-temperature buffer, the surface topography is very rough. The absence of GeH4 during the temperature ramp from the buffer-layer growth to the higher growth temperature yields specular surfaces (compare fig. la with figs. lb and Ic). Also, the use of a 900 A thick buffer (fig. ic) results in more complete
R. Venkatasubramanian et a!.
/
9
Optimization of heteroepitaxy of Ge on GaAs
I. Fig. 1. Surface morphology of Ge layers grown on GaAs at 725°C with (a) 90
A
thick buffer at 590°C and GeH
4 flow present during the temperature ramp, (b) 90 A thick buffer and GeH4 flow absent during the ramp and (c) 900 A thick buffer and Gel-I4 flow absent during the ramp. Markers represent 10 Fm.
removal of surface roughness, compared to a 90 A thickdone bufferat(fig. Ib). The high-temperature was 725°C for all three layers in growth fig. 1. The morpholog~iesof the Ge layers grown at 725°C are representative of the morphologies obtained at 675 and 775°C, as long as a buffer layer was included and GeH4 flow was interrupted during the temperature ramp. For all the growth conditions described above, the Ge layers on GaAs substrates are single-crystal. X-ray diffraction, using a conventional singlecrystal diffractometer with Cu K~1and Cu K~2 lines, was used for this purpose. Normal (004) reflections from the epilayer and the substrate are used. In spite of the limited resolution of the diffractometer, the Ge and GaAs diffraction peaks are always resolved when the buffer layer is included. The X-ray diffraction patterns obtained on Ge/GaAs heteroepitaxial structures, grown with a 90 A thick buffer, are shown in fig. 2. In fig. 2h, a Ge layer grown with the GeH4 stopped during the temperature ramp, shows improved resolution between the Ge and GaAs peaks when compared (shown in fig. 2a) to a layer grown without GeH4 flow termination. Spreading-resistance measurements were used to obtain the electrical properties of the Ge layers grown on GaAs. Shown in figs. 3a and 3b are the spreading-resistance profiles obtained on two Ge layers, one grown with GeH4 flowing during the
temperature ramp (n — 1 x 1019 3). crn respectively. 3), and the other without (n — 1 >< l0’~cm Thus, the absence of GeH 4 during the temperature ramp is beneficial in reducing the average
P
_____
~
A ~
(0
~ ~ ~
a
b ~—
. . Fig. 2. Single crystal X-ray diffraction trace of Ge layer grown
on GaAs with a 90 A thick buffer and (a) GeH4 present during the ramp and (h) GeH4 absent during the ramp.
10
R. Venkatasubramanian et a!
/ Optimization
1020
1020
1019
~n~~nnnnnnn~
1018
—
n ~
~ 10~~~1016 ~ 10~
1019
—
1018
flflflflfl
io~~-
~
~1o16
-
-
oo
-
1015
~
-
00
.22 1014
o
of heteroepitaxy of Ge on GaAs
-
.~
0
1013
-
~——-—~
Ge GaAs 1012
-
i0~~
10©
-
1012
-
Ge~GaAs ~flnnri
a
~~ii
0
b
iou .51 1.52 2.53 Depth (microns)
0
nri
I
.511.522.53
Depth (microns)
Fig. 3. spreading resistance profile of Ge layer on GaAs grown (a) with GeH
4 present and (b) with Gel-I4 absent during temperature ramp from 590 to 725°C.
hulk doping level by as much as a factor of ten, We also note the large increase in carrier level near Ge/GaAs interface in the sample (fig. 3b), where GeH 4 is turned off during the temperature ramp, presumably due to arsenic doping from the GaAs substrate in the absence of continuous Ge growth. The reason for the lower bulk doping level in fig. 3b is not clear. However, as seen in figs. la and lb. the absence of GeH4 during the temperature ramp leads to better surface morphology and few large-scale defects. This was also evident in the X-ray diffraction data. We note that the main source for As incorporation into Ge grown on GaAs is the constant As evaporation from the back surface of the GaAs substrate and not solidstate diffusion from the GaAs substrate, due to the extremely small self-diffusion coefficients of As in GaAs at high temperatures [3]. Thus, any excess background carrier level in the sample with GeH4 present during the temperature ramp (fig. 3a), but grown at the same temperature (725 C) as the sample without GeH4 during the temperature ramp (fig. 3b), and with the same rate of As °
evaporation from GaAs hack surface, is attrihutable to significantly higher defect densities. 3) in Ge The higher defect density (~1019 cm films, grown with GeH4 present during the temperature ramp, can be understood in a simple manner, as follows. Around 590°C, for the GeH4 input partial pressure employed, the effective rate at which GeH4 decomposes into Ge is probably smaller than the rate at which Ge-absorbed atoms can move on the surface to find a kink-site. At a point somewhere between 590°C and the hightemperature (675—775 C) step, if the GeH4 decomposition rate exceeds the surface mobility-determined rate, growth defects are likely. However, at higher temperatures such as 675 to 775°C. if GeH4 decomposition rate saturates and is once again below the surface-mobility-determined rate (which continues to increase with temperature). we can obtain smooth morphologies at these growth temperatures, as long as a buffer layer is grown at a lower temperature to prevent GaAs decomposition. Using such an approach, we also obtained the °
R. Venkatasubramanian et at.
100
825
775 750 725 700 675 650 I
.~
I
/
Optimization of heteroepitaxy of Ge on GaAs
600
I
growth rate of Ge as a function of growth temperature in the range from 675 to 775°C. The GeH4
I
=
partial pressure was kept constant at 0.063 Torr and H2 flow rate at 12 SLM. In this temperature range, we essentially obtained a temperature-independent growth rate for Ge, as shown by the data of fig. 4. This suggests mass-transport-limited growth of Ge on GaAs without kinetic limitations. For a comparison, growth-rate data of other
-
= GeH~Mole Fraction •
4.4x
11
io~ This work
for a constant GeH4 mole fraction~also shown are data from
authors [1,4] (for Ge on GaAs epitaxy) are also included in fig. 4. These data indicate a considerable thermal activation, probably resulting from kinetic limitations to growth of Ge on GaAs. The optical-absorption characteristics of Ge grown on GaAs near and below the Ge band-edge were also measured. Absorption measurements, especially in the sub-bandgap region, are good indicators of material quality. Shown in fig. 5 are absorption curves of Ge on GaAs, grown at 675, 725 and 775 °C, respectively. These samples had a 900 A thick Ge buffer layer, grown at 590°C.The GaAs substrate underwent a deoxidation step at 800°C,and GeH4 was turned off during temperature ramping. The Ge/GaAs structure grown at
other authors.
725°C exhibits sharper near-band-edge absorp-
A 2.5 x 1O~ Ayers arid Ghandhi • -~10.1
(13
3.0 x iO~ Papazian and Reisman
-
-
102
0.90
I
0.94 0.98
I
1.02
1.06 1.10
I
1.14
1.18
1000/1 Fig. 4. Growth rate of Ge on GaAs between 675 and 7750 C
~
55.000
________________________________________________________
45.000
-
35.000
-
~._
~
//
Ia 0
= ~ 25.000
E 5~
15.000
775°C 675°C
-
5.000
—5.000 6941.4
/
6369.4
I 5797.4 5225.4 Wave Numbers (cm-i)
4653.4
Fig. 5. Optical-absorption characteristics of Ge layers grown on GaAs at 675 to 7750 C using a 90 A thick buffer at 5900 C.
12
R. Venkatasubramanian et a!.
/
Optimization of heteroepitaxy of Ge on GaAs
Table I Minority-carrier lifetime of Ge on GaAs as a function of various growth schemes Sample
Deoxidation
Buffer-layer
GeH
No.
at 800°C
thickness
4 flow during ramp
Growth temperature
(A)
Lifetime (ns)
(absent)
(° C)
2
No No
900 900
No Yes
675 675
3 4 5 6 7
No No No No Yes
1800 900 900 900 900
Yes No Yes Yes Yes
675 725 725 675 675
<3 65 100
8 9
Yes Yes
900 900
Yes Yes
725 775
90 150
10 11 12
Yes Yes Yes
90 90 90
No Yes Yes
725 725 725
<3 440 420
3
tion and a higher peak transmission in the subbandgap region. These are attributed to reduced defects/impurities in the Ge layers grown at 725°C compared to those grown at 675 and
thinner buffer layers (90 A buffers seem better than 900 A thick buffers), and higher growth temperature all appear to improve the lifetime in Ge layers. The longest lifetime that has been mea-
775°C.
sured is 440 ns and it is likely that similar values are obtainable in Ge homoepitaxial layers grown at — 725°C, for similar doping levels of n — I X 3. 1018 cm
Based
on
absorption
characteristics,
725 ° C appears as an optimum growth temperature for Ge/GaAs epitaxy. A similar observation has been made the homoepitaxy analysis, of Ge [Sj. where based on in spreading-resistance it was argued that defects in Ge layers were probably minimized. The GaAs/Ge heteroepitaxial data supports this conclusion, although lifetime values observed in Ge layers as a function of growth temperature which are discussed below, suggest 775°C may yield better lifetimes in Ge. The minority-carrier hole lifetimes in Ge layers, as a function of various growth procedures described above, were measured by the microwave technique. The lifetime values are summarized in table 1. When lifetimes are near the resolution limit (— 3 ns) of the equipment, the photoconductivity is found to be weak and it is difficult to measure accurate lifetimes. The earlier conclusions about the structural, electrical, and optical properties of the Ge-on-GaAs layers as a function of growth conditions apply equally well to minoritycarrier lifetime. Significantly, the inclusion of a 800°C deoxidation step for GaAs substrate prior to low-temperature buffer-layer growth, the absence of GeH 4 during the temperature ramp,
4. Summary We have described a new approach to the beteroepitaxy of Ge on GaAs at growth temperatures where the dissociation of the GaAs substrate becomes a problem. Using Ge buffers grown at low temperatures, we have obtained specular, masstransport-limited growth of Ge on GaAs that is comparable to the homoepitaxy of Ge. However, the best films grown using this approach have background carrier concentration in the range of — 3. Surface-morphology features, X-ray 1fl~~ cm electrical characteristics, and opticaldiffraction, absorption data are presented as a function of processing conditions. Minority-carrier lifetimes in Ge layers on semi-insulating GaAs, measured by a microwave technique, are also presented as a function of the various growth schemes. Lifetimes as long as 440 ns have been obtained for Ge in Ge/GaAs heterostructures.
R. Venkatasubramanian et a!.
/
Optimization of heteroepitaxy of Ge on GaAs
Acknowledgements
13
References [11 J.E. Ayers and S.K. Ghandhi, J. Crystal Growth 89 (1988)
The authors are pleased to acknowledge the
371.
support of the Department of the Air Force,
[2] R. Venkatasubramanian, S. Bothra, S.K. Ghandhi and J.M.
Wright Research and Development Center (AFSC), Wright-Patterson Air Force Base, Ohio 45433-6523, under Contract No. F33615-87-C2804 for this work. Air Force Project Engineer is Mr. Kitt Reinhardt. Also, we wish to thank Ms. Vicki Michael Hewitt for assistance with manuscript preparation.
Borrego, in: Proc. IEEE Photovoltaic Specialists Conf., 1988, p. 689. [3] D.L. Kendall, in: Semiconductors and Semimetals, Vol. 4, Eds. R.K. Willardson and A.C. Beer (Academic Press, New York, 1968) p. 163. [4] S.A. Papazian and A. Reisman, J. Electrochem. Soc. 115 (1968) 961. [5] R. Venkatasubramanian, R.T. Pickett and M.L. Timmons, J. AppI. Phys. 66 (1989) 5662.