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Optoelectronic properties of eutectic-metal-bonded (EMB) GaAs-AlGaAs structures on Si substrates
Solid-Stare Elecfronits Vol. 37. No. I I. pp. 1809-1815,1994 Copyright 0 1994 ElsevierScienceLtd
Pergamon
0038-1101(94)30008-3
Printed in Great Britain. All 0038-I lOI/
rights reserved $7.00+ 0.00
OPTOELECTRONIC PROPERTIES OF EUTECTIC-METAL-BONDED (EMB) GaAs-AlGaAs STRUCTURES ON Si SUBSTRATES R. VENKATASUBRAMANIAN and M. L. TIMMONS Research Triangle Institute, Research Triangle Park, NC 27709, U.S.A. (Received
13 Ocrober
1993; in revised form
29 December
1993)
Abstract-Device-quality GaAs+AIGaAs thin-film hetero-structures have been obtained on Si substrates using a novel approach called eutectic-metal-bonding (EMB). The optoelectronic material properties of the thin-films have been evaluated by a variety of techniques including, Raman spectroscopy, room temperature photoluminescence (PL), and cathodoluminescence (CL) imaging. Transient PL measurement indicates that the minority-carrier lifetime in an EMB GaAs-on-Si thin-film is about 40 times higher than that in state-of-the-art hetero-epitaxial GaAs-on-Si layer. The PL characteristics of the EMB GaAs-on-Si structures have been used to obtain the long-wavelength dispersion values for GaAs thin-film structures. The minority carrier device quality of these thin-films have been evaluated using dark log I-V measurements on n +-p GaAs diodes, spectral-response characterization and solar cell performance data.
INTRODUCITON
of device quality GaAs-AlGaAs materials on Si substrates with low threading dislocation density, still remains unrealized in spite of a significant amount of research. Several approaches that have been explored include, the use of strained-layer superlattices[ 11, thermal cycling and annealing techniques[2], and various hetero-buffers such as Ge[3], Ge/Si,,,Ge,,,,[4], ZnSe[S]. CaF,[6] between the Si substrate and the GaAs-AlGaAs layers. Minority carrier lifetime in hetero-epitaxial GaAs-on-Si, an important parameter indicating the potential usefulness for optoelectronic applications and known to be directly correlated to the dislocation density[7] in the layer, has remained about 2-3 orders below those obtainable in homo-epitaxial GaAs films[8]. At present, the reported highest minority-carrier lifetime in a GaAs hetero-epitaxial film is about 2.5 ns[4]. Alternative approaches to GaAs-on-Si for integrated optoelectronics have been proposed and demonstrated. One approach involves the lift-off of epitaxial GaAs films from GaAs substrates using an AlAs release-layer[9] and then later bonding the films using van der Walls forces onto arbitrary substrates. This technique, besides involving several tedious steps, has not demonstrated high quality materials with the repeated use of the GaAs substrate. Other approaches such as bonding by atomic rearrangement (BAR), developed for InP-on-GaAs[lO], has not been demonstrated for GaAs-on-Si. We have reported11 l] an approach called the eutecticmetal-bonding(EMB) for obtaining device quality GaAs-AlGaAs layers on Si. Recently, this approach has been extended to obtain InP layers on Si subHeteroepitaxy
strates using an EMB GaAs-Si interlayer[l2]. The EMB approach opens up the possibility of integrating several III-V and possibly II-VI compound semiconductor films (such as ZnSe) with Si substrates for optoelectronics, covering a wide range of optical wavelengths (40&l 500 nm). Towards this objective, we review the optoelectronic properties of EMB GaAs-AIGaAs thin-films on Si substrates in this paper. OMVPE GROWTH
AND EUTECTIC
METAL BONDING
Organometallic vapor phase epitaxy (OMVPE) has been used to grow the GaAs-AlGaAs hetero-structures and other device structures, presented in this study, on Ge substrates. The growth is carried out in a conventional atmospheric pressure system. Growth of a GaAs nucleation layer is initiated at 775°C at a growth rate of about 0.03 pm/min. After the growth of -0.2 pm of GaAs layer at this rate, the growth rate is increased to 0.06 pm/min and a layer of 0.3 pm thickness is grown at this rate. This two-step nucleation layer has allowed us to reproducibly obtain high-quality GaAs-AlGaAs epitaxial structures on Ge. An additional 3-pm-thick GaAs layer is grown at 700°C. This total of 3.5~pm-thick buffer has been found to reduce the Ge auto-doping in the subsequent GaAs-AlGaAs active layers. The beneficial effects of using the buffer for improved properties of GaAs-AlGaAs layers are discussed in the next section. This buffer layer is Se-doped with a carrier concentration of about 1 x 10” cmm3. After the epitaxial growth of the structures, a lOO-nm-thick film of gold is evaporated onto the face of the epitaxial structure and on a clean Si substrate.
1809
R. VENKATASUBKAMANIANand
1810
The Si substrate is rinsed in I: 100 HF solution in water and blown dry before evaporation. The two Au-coated samples are stacked face-to-face in an alloying furnace, placed in intimate physical contact, and bonded to each other at -400 C. The bonding of the GaAs-AIGaAs structure to the Si substrate occurs by the formation of low-temperature eutectics[l I]. This EMB approach involves a lowtemperature bonding process, in contrast to the BAR technique developed for InP-based materials on GaAs. The EMB approach also does not require precise crystallographic alignment of Si and GaAs. unlike between the GaAs and InP substrates in the BAR process. In fact, we have been able to obtain EMB between Si and GaAs for any arbitrary alignment of the two respective crystal orientations. Typical size of the GaAs-AlGaAs structures that have been bonded to Si substrates is about I .5 cm x 2 cm (in this work), and we do not anticipate any immediate difficulty in scaling this process to larger-size wafers. Following the EMB of GaAs-AlGaAs structures on Si, the Ge substrate is selectively removed by a CF,/O: plasma etch. This etch process does not attack GaAs or AlGaAs (upto 90% Al mole-fraction) and leaves excellent surface-quality layers. This can be compared with the epitaxial lift-off process[9] where the HF solution, used to remove the AlAs release layer selectively, precludes the use of high Al-containing (mole fraction of 0.4 and above) layers in thin-film GaAs-AlGaAs structures. The higher Al-containing layers are important for certain device applications such as solar cells. Following the removal of the Ge substrate, the GaAs buffer is removed and the rest of the GaAs-AlGaAs structure is ready for further device processing. The schematic for obtaining thin GaAssAlGaAs films and devices on Si by the EMB technique is indicated in Fig. I. We note that the active devices are processed from the GaAs thin-films, after EMB and Ge substrate-removal. MATERIAL
QUALITY
OF EMB GaA+AlGaAs
THIN-FILMS
Raman spectroscopy was used to evaluate the residual elastic strain in the EMB GaAs-AIGaAs thin films on Si. Figure 2 indicates the Raman spectrum obtained from an EMB GaAs thin-film on Si at 300 K using an Ar+ ion laser excitation at 514.5 nm. We observe the longitudinal optical (LO) phonon at 291.4 cm-‘, the transverse optical (TO) phonon at 268.4 cm-‘, the second-order transverse acoustic (2TA) phonons in the l60cm ’ range and the TO + TA combination at 334cm-‘. The determination of strain indicates that the residual elastic strain in the EMB GaAs-on-Si is lower by an order of magnitude compared to hetero-epitaxial GaAs films deposited directly on Si substrates. Typical strain in hetero-epitaxial GaAs-on-Si films is -2.4 x IO-‘, corresponding to a stress of - 3.0 kbar[l3]. The corresponding strain in the EMB GaAs-on-Si films is
M. L. TIMMONS
rGiiczq
)““L.“““”
*”
EME
pcq (2)
(1)
Ge
Substrate Removal
GaAs
Buffer
(3) Fig.
Selective Buffer Removal And Device Fabrication
GaAs
Devices
(4)
I. Schematic of the steps used to I:dbricatc GaAs -AlGaAs thin films on Si.
EM9
below the detection limit (-0.3 kbar) of Raman spectroscopy. The potential performance of the devices fabricated on the EMB GaAs-on-Si thin-films can be significantly affected by the Gc auto-doping. especially since the GaAs material that is available for device fabrication is the one that was originally closer to the substrate (see Fig. I). This problem can be essentially eliminated by the use of a 3.5-pm-thick GaAs buffer. This buffer can be selectively removed later using AlGaAs stop-etch layers[l4]. Under identical photoluminescence (PL) excitation and detection conditions, the 300K band edge PL intensity of a GaAs acrioe layer has been observed to be over 250 times more intense than that of the GaAs buffer layer, suggesting significantly improved minority carrier lifetime in the active layer compared to that in the buffer. Also, the 300K PL full-width at halfmaximum of this active layer is - 29 meV, comparable to high-quality GaAs homo-epitaxial layers. The high quality of the EMB GaAs-on-Si thinfilms, with the use of a buffer. was evident in the PL decay lifetimes. The PL decay, measured on an EMB AlGaAssGaAs double-hetero (DH) structure. is shown in Fig. 3. The doping level in the GaAs of this DH structure is about active layer 2 x IO” cm ‘. We observe a single exponential transient over two decades with a lifetime of I03 ns. The fast decay near t = 0 is caused by self-absorption ot photoluminescence originating from minority carriers which have diffused away from the front surface of the DH structure[l5]. We also note that the lifetime of 103 ns, for a doping level of - 2 x IO” cm ‘, is a factor of 4 higher than the radiative limit of -25 ns. This radiative limit is estimated from a radiative recombination coefficient (B) of 2 x IO I” cm’ s ‘[8].
Properties
of GaAs-AlGaAs
structures
1811
on Si substrates
9.6 268.4 cm-’ TO mode
8.0
91.4 cm-’ >O mode
2 TA
Combination
1.6
0.0
I
I
I
80
160
240
The minority-carrier
from an EMB GaAs thin film on Si, obtained at 514.5 nm.
lifetime,
exceeding
the radiative
in such GaAs
480
shift (cm-‘)
spectrum
thin-films is related to photonrecycling effects. Such enhancement in lifetime, from photon-recycling effects, has been observed in other GaAs thin-films recently[ 161. The measured lifetime at a laser power of 2.0 mW was -99 ns. The two similar lifetimes indicate that limit,
and TA)
400
320
Wavenumber Fig. 2. Raman
of (TO
using an Ar+ ion laser excitation
traps and deep level defects play a negligible role in minority-carrier recombination. This lifetime of 103 ns is over 40 times larger than the state-of-the-art heteroepitaxial GaAs on Si[4]. The highest lifetime reported previously for a GaAs thin-film, obtained by the cleavage of lateral epitaxial films for transfer (CLEFT) approach, is - 32 ns[15].
Ep = I .408eV P = 0.2mW 5 = 103ns
100 1 0
I 1000
T (ns) Fig. 3. 300K PL decay
from an EMB GaAs/AlGaAs
DH structure
on Si
R. VENKATASUBRAMANIAN and M. L. TIMMONS
1812
virtually absent in the EMB GaAs-on-Si layers. Furthe hetero-epitaxial layer shows a nonther, uniform CL output in contrast to the EMB layer. The dark spots seen in the CL image of the EMB GaAson-Si layer are attributed to possible recombination defects at the EMB GaAs/Au/Si interface or a result of growth on Ge substrates. In any case, the density of such defects is less than 105/cm’. This defect density is about two orders below that obtainable in state-of-the-art hetcroepitaxial GaAs-on-Si, thus explaining the observed near-40-improvement in lifetime. DEVICE
Fig. 4. Cathodoluminescence image from (a) an EMB thin-film GaAs-on-Si compared with that of (b) a heteroepitaxial GaAs layer on Si.
QUALITY
OF THE
EMB
CaAs THIN
FILMS
The PL spectrum from a 6-/(m-thick EMB thinfilm GaAs-on-Si is shown in Fig. 5. The pronounced oscillations for the sub-bandgap (; > 8700 A) PL due to FabryyPerot action (multiple-beam interference in a cavity-like structure) from the reflection of photons. from the Au-layer used for EMB, can be noticed. The clear interference fringes also attest to the smoothness (optical-quality) of the EMB GaAs-Si interface for photonic device applications. The mode spacing can be used to obtain the long wavelength dispersion values of GaAs. Assuming that the entire structure acts like a single vertical FabryyPerot cavity of length I with an average
The EMB GaAs thin films on Si were studied by cathodoluminescence (CL) in a scanning electron microscope. Figure 4, compares the CL image of an EMB GaAs-on-Si layer with that of a heteroepitaxial GaAs-on-Si layer. The heteroepitaxial GaAs layers were grown on nominal (100) substrates. Therefore, we observe the anti-phase domain boundaries (APDs), seen as closed loops in the CL image, in the hetero-epitaxial GaAs-on-Si layer. Such APDs are
0.024 eV I-1
L
8000
8500
9000
9500
10,000
Wavelength Fig. 5. 300K PL spectrum
I 1,000
10,500
I 1,500
12.000
(.&)
from a 6-pm-thick
EMB GaAs
thin-film
on Si
I2.SOO
I3.000
Properties of GaAs-AlGaAs cavity refractive obtain[ 171:
index of n at the wavelength
structures on Si substrates
1, we
1813
Front Contact 1
Au&N/Au
1
S1 = 1*/21(n - 1 dnldl), where 61 is the observed mode spacing at 1. Rewriting the relation to obtain the dispersion as a function of the observed mode spacing, we obtain:
Emitter
n+GaAs
Base
p-GaAs
(2)
Back Surface Field
Using the mode spacing at the various wavelengths seen in Fig. 5 and a value of n of 3.6 at the GaAs band edge[l8], we obtain the dispersion values for the subbandgap region in GaAs. The calculated values are plotted in Fig. 6. These values are useful if a strainedlayer-type Ga,In, _,As (with a band gap of _ 1.0 eV) lasing region is incorporated between GaAs cladding regions and the structure can be eutectic-metalbonded to a Si substrate. Such a vertical-cavity, surface-emitting device has been proposed for integrated optoelectronics in the 1.3 pm wavelength range[ 191. The device quality of the EMB GaAs-AlGaAs thin-films were evaluated by GaAs solar cell test structures. The schematic of the GaAs cell structure fabricated on an EMB GaAs-on-Si thin-film, is shown in Fig. 7. The Au-coating (for EMB) on the backside of the thin-film cell is believed to be a good reflector for utilizing possible photonrecycling effects. This was evident in the excellent measured Jsc (27.8 mA/cm*) values of the solar cells, consistent with the spectral-response data. The I-V data measured on a solar cell fabricated with the EMB thin-film GaAs-on&, is shown in Fig. 8. The V, of the cell is 0.9455 V. However, a fill-factor of only 0.7825, apparently limits the cell efficiency to 17.9%. The fill-factor is limited by the series resistance probably from current transport across the EMB GaAs-Si interface, as discussed further below. The spectral-response data for the same cell structure is shown in Fig. 9. The near-flat spectral-response (> 90%) in the spectral-range of 700-870 nm
Back Contact Layer
(I/n)(dn/dl)
= (l/n) - (n/21&).
4.0 pm
p-A’ o.$j”o.Ps
0.2 pm
p+ GaAs
2.0 pm
AU EMB
Back Contact
c
AU
1
TilAu
1
Fig. 7. Schematic of a solar cell structure on an EMB thin-film GaAs-on&
indicates excellent diffusion length in the base region of the EMB GaAs thin-film solar cell. The spectralresponse of the n+-p GaAs solar cell, which has a thin emitter of only 0.1 pm depth, has been used to obtain an estimate of minority-carrier diffusion length in the base. The photogenerated current, J,(1) at a wavelength 1, in an n +-p solar cell can be approximated under monochromatic illumination (especially for wavelengths very close to the band edge and for a thin emitter cell) as[20]:
J,(l) = qN(l)[l
- R(A)] 1 - exp(aW) [
+ $$+i
{(a& - l)exp(-EW)
n
+ PI],
(3)
AM1.5 D, IkW/m* 3.5 r3.0 2.5 -
4 q g ;: e ”;
2.0 1.5l.O0.5 0.0 -0.5
0
850
870
890
910
Wavelength
930
950
(nm)
Fig. 6. Dispersion curve for a GaAs thin-film in the subbandgap
region.
’ -0.2
I
I
I
I
I
I
0
0.2
0.4
0.6
0.8
1.0
Voltage (V) Fig. 8. I-V data measured under AM I .5 for the EMB thinfilm GaAs-on-Si solar cell. V, = 0.9455, I, = 3.287 mA, FF = 0.7825, efficiency = 17.9% (active-area), cell active area = 0.14 I cm2.
R. VENKATASUBRAMANIAN and M. L. TIMM~NS
200
300 400
500 600 700 Wavelength
800
900 1000
(nm)
Fig. 9. Spectral response measurement from the 17.9%efficient EMB thin-film GaAs-on-Si solar cell. Light bias = I .OOmA, zero voltage bias.
where p is given by: 2[exp(-cd) R=
-exp(-aW)expjW)l
Substituting the measured value of internal quantum efficiency at the wavelength of 801 nm, WCcan iteratively solve for the electron diffusion length in the p-base. Similarly, we can evaluate the diffusion length using the measured internal quantum efficiency for other wavelengths close to the band edge of GaAs. We estimate an average diffusion length in the p-base of about 7 k 2 pm. This value is consistent with a lifetime of - 100 ns, discussed earlier in the EMB GaAs thin-film at similar (2 x lO”cm ‘) doping levels, and the expected minority-carrier (electron) mobility in p-GaAs. This diffusion length also attests to the device-quality EMB GaAs thin-films for minority-carrier device applications. Dark I-V measurements were also used to compare the quality of n +-p GaAs diodes fabricated on EMB GaAs-on-Si thin-films with those fabricated on GaAs substrates. The two measurements are indicated in Fig. IO. The saturation current density of the diodes (2 x 10m9A/cm*) fabricated on the EMB material is about an order of magnitude higher than typically observed in the homoepitaxial material. The higher dark saturation current density is attributable to possible defects as a result of growth on Ge rather than the EMB process. This is because the ideality factor, upto a voltage bias of 0.6 V, of the diode indicates that the dark-current is probably from depletion-region generation. The higher dark current also explains the lower P’,, values obtained in solar cells fabricated on EMB GaAs-on-S;, by about SOmV, compared to typical GaAs solar cells fabricated on homo-epitaxial materials. EMB GaAs-on-Si diodes also indicate series resistance effects at the higher current densities. This is consistent with the lower fill-factor of the GaAs solar cells fabricated on the EMB layers. The series resistance is attributed to the poor electrical conductivity of the p +SiI’Au/p ’ -GaAs EMB interface (see Fig. 7). This interface has been observed to have a specific electrical rcsistivity of about 1 R-cm’. This high resistivity is related to the non-ohmicity of the Au/p ’ -GaAs and/or Au/p + Si components of the interface. even after the sintering
Lexp(“-? -exp&~ “)(4)
and N(1) is the incident photon flux, R(1) is the reflectance of the surface, CC(I) is the absorption coefficient in the semiconductor, Wis the width of the depletion layer in the p-base, L, is the electron diffusion length in the p-base, q is the electronic charge, and L is the thickness of the base. The internal quantum efficiency at a wavelength I, I(I), can be written as: I(i)=
I -exp(*W)+& n
L
x {(CCL,- I)exp(-aW)+fi}
1
(5)
For a nominal base doping level of -4 x 10’7cm-3, under short-circuit spectral-response measurement conditions, we obtain a junction depletion layer width of 0.03 pm. The thickness of the base of the cell is -4.0 pm. At a wavelength of 801 nm, we assume an absorption coefficient of 2.0 x 104cm~‘[21] in GaAs. Under these conditions, the expressions for p reduces to:
and the internal quantum
;
efficiency can be written as:
rom7 - EMBGaAson
0
-0.2
Si,/'
-0.4
-0.6
-0.x
I -1.0
Voltage (V) “ii=[l
+&[‘“p$$-
‘II.
(7)
Fig. IO. Dark log I-V data on two n + p GaAs diodes. one grown
on a single-crystal GaAs substrate and the other fabricated on an EMB thin-film GaAs-on-Si.
Properties of GaAssAlGaAs step at -400°C (carried out during the EMB process). The ohmicity of this interface is a concern in device applications such as solar cells and also in other possible high current devices like LEDs and lasers. We have recently developed a variation of this p+-%/Au/p+-GaAs EMB scheme by utilizing a n +-Si/AulSnlAuln +-GaAs EMB structure. The presence of Sn, an n-type dopant in GaAs, is believed to lead to a better ohmic contact at the Au/n +-GaAs interface. The n +-SilAu/Sn/Auln +-GaAs EMB interface has indicated a specific resistivity of 4.2 x 10m2R-cm’, about 25 times less than that for the p +-SilAujp +-GaAs EMB interface[22]. Thus p+-n GaAs device structures on n +-Si substrates, when utilizing EMB approach, may be appropriate for reducing ohmic losses.
CONCLUSIONS
In conclusion we have demonstrated a novel approach to obtain device-quality GaAs-AlGaAs heterostructures on Si substrates using eutectic metal bonding (EMB). The EMB approach has led to a significant improvement in minority-carrier lifetimes (- 100 ns) compared to direct hetero-epitaxial growth of GaAs-AlGaAs structures on Si substrates. The material quality of the EMB GaAs layers have been studied using PL and CL imaging techniques. Raman spectroscopy indicates negligible strain in the EMB GaAs-on-Si thin-films. The room temperature PL data on the EMB GaAs thin-films have been used to obtain the long-wavelength dispersion values for GaAs. The device quality of the EMB GaAs-AlGaAs structures have been evaluated by dark Z-V measurements. Solar cells fabricated on the EMB GaAs thin-films indicate efficiencies of - 17.9% under AM I .5 simulation. The spectral response of the solar cells indicate that the minority carrier diffusion lengths are in the range of 7 pm in the EMB GaAs thin-films, in agreement with the measured minoritycarrier lifetimes (100 ns). Therefore, we believe the EMB approach promises to be very useful for integration of GaAs optical devices with Si electronic devices. Acknowledgements-We like to acknowledge the technical assistance of Mr T. S. Colpitts in the OMVPE growth of GaAs-AlGaAs structures and Mr D. Malta with the CL imaging data. We also like to acknowledge the assistance of Dr Keith Emery and Dr Richard Ahrenkiel of the National Renewable Energy Laboratory (NREL), Golden, Colorado, for their assistance with the solar cell measurements and lifetime data. Portions of this work were conducted as a
structures on Si substrates result of a subcontact from the NREL as the technical monitor.
1815 with Mr T. S. Basso
REFERENCES
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OPTOELECTRONIC PROPERTIES OF EUTECTIC-METAL-BONDED (EMB) GaAs-AlGaAs STRUCTURES ON Si SUBSTRATES R. VENKATASUBRAMANIAN and M. L. TIMMONS Research Triangle Institute, Research Triangle Park, NC 27709, U.S.A. (Received
13 Ocrober
1993; in revised form
29 December
1993)
Abstract-Device-quality GaAs+AIGaAs thin-film hetero-structures have been obtained on Si substrates using a novel approach called eutectic-metal-bonding (EMB). The optoelectronic material properties of the thin-films have been evaluated by a variety of techniques including, Raman spectroscopy, room temperature photoluminescence (PL), and cathodoluminescence (CL) imaging. Transient PL measurement indicates that the minority-carrier lifetime in an EMB GaAs-on-Si thin-film is about 40 times higher than that in state-of-the-art hetero-epitaxial GaAs-on-Si layer. The PL characteristics of the EMB GaAs-on-Si structures have been used to obtain the long-wavelength dispersion values for GaAs thin-film structures. The minority carrier device quality of these thin-films have been evaluated using dark log I-V measurements on n +-p GaAs diodes, spectral-response characterization and solar cell performance data.
INTRODUCITON
of device quality GaAs-AlGaAs materials on Si substrates with low threading dislocation density, still remains unrealized in spite of a significant amount of research. Several approaches that have been explored include, the use of strained-layer superlattices[ 11, thermal cycling and annealing techniques[2], and various hetero-buffers such as Ge[3], Ge/Si,,,Ge,,,,[4], ZnSe[S]. CaF,[6] between the Si substrate and the GaAs-AlGaAs layers. Minority carrier lifetime in hetero-epitaxial GaAs-on-Si, an important parameter indicating the potential usefulness for optoelectronic applications and known to be directly correlated to the dislocation density[7] in the layer, has remained about 2-3 orders below those obtainable in homo-epitaxial GaAs films[8]. At present, the reported highest minority-carrier lifetime in a GaAs hetero-epitaxial film is about 2.5 ns[4]. Alternative approaches to GaAs-on-Si for integrated optoelectronics have been proposed and demonstrated. One approach involves the lift-off of epitaxial GaAs films from GaAs substrates using an AlAs release-layer[9] and then later bonding the films using van der Walls forces onto arbitrary substrates. This technique, besides involving several tedious steps, has not demonstrated high quality materials with the repeated use of the GaAs substrate. Other approaches such as bonding by atomic rearrangement (BAR), developed for InP-on-GaAs[lO], has not been demonstrated for GaAs-on-Si. We have reported11 l] an approach called the eutecticmetal-bonding(EMB) for obtaining device quality GaAs-AlGaAs layers on Si. Recently, this approach has been extended to obtain InP layers on Si subHeteroepitaxy
strates using an EMB GaAs-Si interlayer[l2]. The EMB approach opens up the possibility of integrating several III-V and possibly II-VI compound semiconductor films (such as ZnSe) with Si substrates for optoelectronics, covering a wide range of optical wavelengths (40&l 500 nm). Towards this objective, we review the optoelectronic properties of EMB GaAs-AIGaAs thin-films on Si substrates in this paper. OMVPE GROWTH
AND EUTECTIC
METAL BONDING
Organometallic vapor phase epitaxy (OMVPE) has been used to grow the GaAs-AlGaAs hetero-structures and other device structures, presented in this study, on Ge substrates. The growth is carried out in a conventional atmospheric pressure system. Growth of a GaAs nucleation layer is initiated at 775°C at a growth rate of about 0.03 pm/min. After the growth of -0.2 pm of GaAs layer at this rate, the growth rate is increased to 0.06 pm/min and a layer of 0.3 pm thickness is grown at this rate. This two-step nucleation layer has allowed us to reproducibly obtain high-quality GaAs-AlGaAs epitaxial structures on Ge. An additional 3-pm-thick GaAs layer is grown at 700°C. This total of 3.5~pm-thick buffer has been found to reduce the Ge auto-doping in the subsequent GaAs-AlGaAs active layers. The beneficial effects of using the buffer for improved properties of GaAs-AlGaAs layers are discussed in the next section. This buffer layer is Se-doped with a carrier concentration of about 1 x 10” cmm3. After the epitaxial growth of the structures, a lOO-nm-thick film of gold is evaporated onto the face of the epitaxial structure and on a clean Si substrate.
1809
R. VENKATASUBKAMANIANand
1810
The Si substrate is rinsed in I: 100 HF solution in water and blown dry before evaporation. The two Au-coated samples are stacked face-to-face in an alloying furnace, placed in intimate physical contact, and bonded to each other at -400 C. The bonding of the GaAs-AIGaAs structure to the Si substrate occurs by the formation of low-temperature eutectics[l I]. This EMB approach involves a lowtemperature bonding process, in contrast to the BAR technique developed for InP-based materials on GaAs. The EMB approach also does not require precise crystallographic alignment of Si and GaAs. unlike between the GaAs and InP substrates in the BAR process. In fact, we have been able to obtain EMB between Si and GaAs for any arbitrary alignment of the two respective crystal orientations. Typical size of the GaAs-AlGaAs structures that have been bonded to Si substrates is about I .5 cm x 2 cm (in this work), and we do not anticipate any immediate difficulty in scaling this process to larger-size wafers. Following the EMB of GaAs-AlGaAs structures on Si, the Ge substrate is selectively removed by a CF,/O: plasma etch. This etch process does not attack GaAs or AlGaAs (upto 90% Al mole-fraction) and leaves excellent surface-quality layers. This can be compared with the epitaxial lift-off process[9] where the HF solution, used to remove the AlAs release layer selectively, precludes the use of high Al-containing (mole fraction of 0.4 and above) layers in thin-film GaAs-AlGaAs structures. The higher Al-containing layers are important for certain device applications such as solar cells. Following the removal of the Ge substrate, the GaAs buffer is removed and the rest of the GaAs-AlGaAs structure is ready for further device processing. The schematic for obtaining thin GaAssAlGaAs films and devices on Si by the EMB technique is indicated in Fig. I. We note that the active devices are processed from the GaAs thin-films, after EMB and Ge substrate-removal. MATERIAL
QUALITY
OF EMB GaA+AlGaAs
THIN-FILMS
Raman spectroscopy was used to evaluate the residual elastic strain in the EMB GaAs-AIGaAs thin films on Si. Figure 2 indicates the Raman spectrum obtained from an EMB GaAs thin-film on Si at 300 K using an Ar+ ion laser excitation at 514.5 nm. We observe the longitudinal optical (LO) phonon at 291.4 cm-‘, the transverse optical (TO) phonon at 268.4 cm-‘, the second-order transverse acoustic (2TA) phonons in the l60cm ’ range and the TO + TA combination at 334cm-‘. The determination of strain indicates that the residual elastic strain in the EMB GaAs-on-Si is lower by an order of magnitude compared to hetero-epitaxial GaAs films deposited directly on Si substrates. Typical strain in hetero-epitaxial GaAs-on-Si films is -2.4 x IO-‘, corresponding to a stress of - 3.0 kbar[l3]. The corresponding strain in the EMB GaAs-on-Si films is
M. L. TIMMONS
rGiiczq
)““L.“““”
*”
EME
pcq (2)
(1)
Ge
Substrate Removal
GaAs
Buffer
(3) Fig.
Selective Buffer Removal And Device Fabrication
GaAs
Devices
(4)
I. Schematic of the steps used to I:dbricatc GaAs -AlGaAs thin films on Si.
EM9
below the detection limit (-0.3 kbar) of Raman spectroscopy. The potential performance of the devices fabricated on the EMB GaAs-on-Si thin-films can be significantly affected by the Gc auto-doping. especially since the GaAs material that is available for device fabrication is the one that was originally closer to the substrate (see Fig. I). This problem can be essentially eliminated by the use of a 3.5-pm-thick GaAs buffer. This buffer can be selectively removed later using AlGaAs stop-etch layers[l4]. Under identical photoluminescence (PL) excitation and detection conditions, the 300K band edge PL intensity of a GaAs acrioe layer has been observed to be over 250 times more intense than that of the GaAs buffer layer, suggesting significantly improved minority carrier lifetime in the active layer compared to that in the buffer. Also, the 300K PL full-width at halfmaximum of this active layer is - 29 meV, comparable to high-quality GaAs homo-epitaxial layers. The high quality of the EMB GaAs-on-Si thinfilms, with the use of a buffer. was evident in the PL decay lifetimes. The PL decay, measured on an EMB AlGaAssGaAs double-hetero (DH) structure. is shown in Fig. 3. The doping level in the GaAs of this DH structure is about active layer 2 x IO” cm ‘. We observe a single exponential transient over two decades with a lifetime of I03 ns. The fast decay near t = 0 is caused by self-absorption ot photoluminescence originating from minority carriers which have diffused away from the front surface of the DH structure[l5]. We also note that the lifetime of 103 ns, for a doping level of - 2 x IO” cm ‘, is a factor of 4 higher than the radiative limit of -25 ns. This radiative limit is estimated from a radiative recombination coefficient (B) of 2 x IO I” cm’ s ‘[8].
Properties
of GaAs-AlGaAs
structures
1811
on Si substrates
9.6 268.4 cm-’ TO mode
8.0
91.4 cm-’ >O mode
2 TA
Combination
1.6
0.0
I
I
I
80
160
240
The minority-carrier
from an EMB GaAs thin film on Si, obtained at 514.5 nm.
lifetime,
exceeding
the radiative
in such GaAs
480
shift (cm-‘)
spectrum
thin-films is related to photonrecycling effects. Such enhancement in lifetime, from photon-recycling effects, has been observed in other GaAs thin-films recently[ 161. The measured lifetime at a laser power of 2.0 mW was -99 ns. The two similar lifetimes indicate that limit,
and TA)
400
320
Wavenumber Fig. 2. Raman
of (TO
using an Ar+ ion laser excitation
traps and deep level defects play a negligible role in minority-carrier recombination. This lifetime of 103 ns is over 40 times larger than the state-of-the-art heteroepitaxial GaAs on Si[4]. The highest lifetime reported previously for a GaAs thin-film, obtained by the cleavage of lateral epitaxial films for transfer (CLEFT) approach, is - 32 ns[15].
Ep = I .408eV P = 0.2mW 5 = 103ns
100 1 0
I 1000
T (ns) Fig. 3. 300K PL decay
from an EMB GaAs/AlGaAs
DH structure
on Si
R. VENKATASUBRAMANIAN and M. L. TIMMONS
1812
virtually absent in the EMB GaAs-on-Si layers. Furthe hetero-epitaxial layer shows a nonther, uniform CL output in contrast to the EMB layer. The dark spots seen in the CL image of the EMB GaAson-Si layer are attributed to possible recombination defects at the EMB GaAs/Au/Si interface or a result of growth on Ge substrates. In any case, the density of such defects is less than 105/cm’. This defect density is about two orders below that obtainable in state-of-the-art hetcroepitaxial GaAs-on-Si, thus explaining the observed near-40-improvement in lifetime. DEVICE
Fig. 4. Cathodoluminescence image from (a) an EMB thin-film GaAs-on-Si compared with that of (b) a heteroepitaxial GaAs layer on Si.
QUALITY
OF THE
EMB
CaAs THIN
FILMS
The PL spectrum from a 6-/(m-thick EMB thinfilm GaAs-on-Si is shown in Fig. 5. The pronounced oscillations for the sub-bandgap (; > 8700 A) PL due to FabryyPerot action (multiple-beam interference in a cavity-like structure) from the reflection of photons. from the Au-layer used for EMB, can be noticed. The clear interference fringes also attest to the smoothness (optical-quality) of the EMB GaAs-Si interface for photonic device applications. The mode spacing can be used to obtain the long wavelength dispersion values of GaAs. Assuming that the entire structure acts like a single vertical FabryyPerot cavity of length I with an average
The EMB GaAs thin films on Si were studied by cathodoluminescence (CL) in a scanning electron microscope. Figure 4, compares the CL image of an EMB GaAs-on-Si layer with that of a heteroepitaxial GaAs-on-Si layer. The heteroepitaxial GaAs layers were grown on nominal (100) substrates. Therefore, we observe the anti-phase domain boundaries (APDs), seen as closed loops in the CL image, in the hetero-epitaxial GaAs-on-Si layer. Such APDs are
0.024 eV I-1
L
8000
8500
9000
9500
10,000
Wavelength Fig. 5. 300K PL spectrum
I 1,000
10,500
I 1,500
12.000
(.&)
from a 6-pm-thick
EMB GaAs
thin-film
on Si
I2.SOO
I3.000
Properties of GaAs-AlGaAs cavity refractive obtain[ 171:
index of n at the wavelength
structures on Si substrates
1, we
1813
Front Contact 1
Au&N/Au
1
S1 = 1*/21(n - 1 dnldl), where 61 is the observed mode spacing at 1. Rewriting the relation to obtain the dispersion as a function of the observed mode spacing, we obtain:
Emitter
n+GaAs
Base
p-GaAs
(2)
Back Surface Field
Using the mode spacing at the various wavelengths seen in Fig. 5 and a value of n of 3.6 at the GaAs band edge[l8], we obtain the dispersion values for the subbandgap region in GaAs. The calculated values are plotted in Fig. 6. These values are useful if a strainedlayer-type Ga,In, _,As (with a band gap of _ 1.0 eV) lasing region is incorporated between GaAs cladding regions and the structure can be eutectic-metalbonded to a Si substrate. Such a vertical-cavity, surface-emitting device has been proposed for integrated optoelectronics in the 1.3 pm wavelength range[ 191. The device quality of the EMB GaAs-AlGaAs thin-films were evaluated by GaAs solar cell test structures. The schematic of the GaAs cell structure fabricated on an EMB GaAs-on-Si thin-film, is shown in Fig. 7. The Au-coating (for EMB) on the backside of the thin-film cell is believed to be a good reflector for utilizing possible photonrecycling effects. This was evident in the excellent measured Jsc (27.8 mA/cm*) values of the solar cells, consistent with the spectral-response data. The I-V data measured on a solar cell fabricated with the EMB thin-film GaAs-on&, is shown in Fig. 8. The V, of the cell is 0.9455 V. However, a fill-factor of only 0.7825, apparently limits the cell efficiency to 17.9%. The fill-factor is limited by the series resistance probably from current transport across the EMB GaAs-Si interface, as discussed further below. The spectral-response data for the same cell structure is shown in Fig. 9. The near-flat spectral-response (> 90%) in the spectral-range of 700-870 nm
Back Contact Layer
(I/n)(dn/dl)
= (l/n) - (n/21&).
4.0 pm
p-A’ o.$j”o.Ps
0.2 pm
p+ GaAs
2.0 pm
AU EMB
Back Contact
c
AU
1
TilAu
1
Fig. 7. Schematic of a solar cell structure on an EMB thin-film GaAs-on&
indicates excellent diffusion length in the base region of the EMB GaAs thin-film solar cell. The spectralresponse of the n+-p GaAs solar cell, which has a thin emitter of only 0.1 pm depth, has been used to obtain an estimate of minority-carrier diffusion length in the base. The photogenerated current, J,(1) at a wavelength 1, in an n +-p solar cell can be approximated under monochromatic illumination (especially for wavelengths very close to the band edge and for a thin emitter cell) as[20]:
J,(l) = qN(l)[l
- R(A)] 1 - exp(aW) [
+ $$+i
{(a& - l)exp(-EW)
n
+ PI],
(3)
AM1.5 D, IkW/m* 3.5 r3.0 2.5 -
4 q g ;: e ”;
2.0 1.5l.O0.5 0.0 -0.5
0
850
870
890
910
Wavelength
930
950
(nm)
Fig. 6. Dispersion curve for a GaAs thin-film in the subbandgap
region.
’ -0.2
I
I
I
I
I
I
0
0.2
0.4
0.6
0.8
1.0
Voltage (V) Fig. 8. I-V data measured under AM I .5 for the EMB thinfilm GaAs-on-Si solar cell. V, = 0.9455, I, = 3.287 mA, FF = 0.7825, efficiency = 17.9% (active-area), cell active area = 0.14 I cm2.
R. VENKATASUBRAMANIAN and M. L. TIMM~NS
200
300 400
500 600 700 Wavelength
800
900 1000
(nm)
Fig. 9. Spectral response measurement from the 17.9%efficient EMB thin-film GaAs-on-Si solar cell. Light bias = I .OOmA, zero voltage bias.
where p is given by: 2[exp(-cd) R=
-exp(-aW)expjW)l
Substituting the measured value of internal quantum efficiency at the wavelength of 801 nm, WCcan iteratively solve for the electron diffusion length in the p-base. Similarly, we can evaluate the diffusion length using the measured internal quantum efficiency for other wavelengths close to the band edge of GaAs. We estimate an average diffusion length in the p-base of about 7 k 2 pm. This value is consistent with a lifetime of - 100 ns, discussed earlier in the EMB GaAs thin-film at similar (2 x lO”cm ‘) doping levels, and the expected minority-carrier (electron) mobility in p-GaAs. This diffusion length also attests to the device-quality EMB GaAs thin-films for minority-carrier device applications. Dark I-V measurements were also used to compare the quality of n +-p GaAs diodes fabricated on EMB GaAs-on-Si thin-films with those fabricated on GaAs substrates. The two measurements are indicated in Fig. IO. The saturation current density of the diodes (2 x 10m9A/cm*) fabricated on the EMB material is about an order of magnitude higher than typically observed in the homoepitaxial material. The higher dark saturation current density is attributable to possible defects as a result of growth on Ge rather than the EMB process. This is because the ideality factor, upto a voltage bias of 0.6 V, of the diode indicates that the dark-current is probably from depletion-region generation. The higher dark current also explains the lower P’,, values obtained in solar cells fabricated on EMB GaAs-on-S;, by about SOmV, compared to typical GaAs solar cells fabricated on homo-epitaxial materials. EMB GaAs-on-Si diodes also indicate series resistance effects at the higher current densities. This is consistent with the lower fill-factor of the GaAs solar cells fabricated on the EMB layers. The series resistance is attributed to the poor electrical conductivity of the p +SiI’Au/p ’ -GaAs EMB interface (see Fig. 7). This interface has been observed to have a specific electrical rcsistivity of about 1 R-cm’. This high resistivity is related to the non-ohmicity of the Au/p ’ -GaAs and/or Au/p + Si components of the interface. even after the sintering
Lexp(“-? -exp&~ “)(4)
and N(1) is the incident photon flux, R(1) is the reflectance of the surface, CC(I) is the absorption coefficient in the semiconductor, Wis the width of the depletion layer in the p-base, L, is the electron diffusion length in the p-base, q is the electronic charge, and L is the thickness of the base. The internal quantum efficiency at a wavelength I, I(I), can be written as: I(i)=
I -exp(*W)+& n
L
x {(CCL,- I)exp(-aW)+fi}
1
(5)
For a nominal base doping level of -4 x 10’7cm-3, under short-circuit spectral-response measurement conditions, we obtain a junction depletion layer width of 0.03 pm. The thickness of the base of the cell is -4.0 pm. At a wavelength of 801 nm, we assume an absorption coefficient of 2.0 x 104cm~‘[21] in GaAs. Under these conditions, the expressions for p reduces to:
and the internal quantum
;
efficiency can be written as:
rom7 - EMBGaAson
0
-0.2
Si,/'
-0.4
-0.6
-0.x
I -1.0
Voltage (V) “ii=[l
+&[‘“p$$-
‘II.
(7)
Fig. IO. Dark log I-V data on two n + p GaAs diodes. one grown
on a single-crystal GaAs substrate and the other fabricated on an EMB thin-film GaAs-on-Si.
Properties of GaAssAlGaAs step at -400°C (carried out during the EMB process). The ohmicity of this interface is a concern in device applications such as solar cells and also in other possible high current devices like LEDs and lasers. We have recently developed a variation of this p+-%/Au/p+-GaAs EMB scheme by utilizing a n +-Si/AulSnlAuln +-GaAs EMB structure. The presence of Sn, an n-type dopant in GaAs, is believed to lead to a better ohmic contact at the Au/n +-GaAs interface. The n +-SilAu/Sn/Auln +-GaAs EMB interface has indicated a specific resistivity of 4.2 x 10m2R-cm’, about 25 times less than that for the p +-SilAujp +-GaAs EMB interface[22]. Thus p+-n GaAs device structures on n +-Si substrates, when utilizing EMB approach, may be appropriate for reducing ohmic losses.
CONCLUSIONS
In conclusion we have demonstrated a novel approach to obtain device-quality GaAs-AlGaAs heterostructures on Si substrates using eutectic metal bonding (EMB). The EMB approach has led to a significant improvement in minority-carrier lifetimes (- 100 ns) compared to direct hetero-epitaxial growth of GaAs-AlGaAs structures on Si substrates. The material quality of the EMB GaAs layers have been studied using PL and CL imaging techniques. Raman spectroscopy indicates negligible strain in the EMB GaAs-on-Si thin-films. The room temperature PL data on the EMB GaAs thin-films have been used to obtain the long-wavelength dispersion values for GaAs. The device quality of the EMB GaAs-AlGaAs structures have been evaluated by dark Z-V measurements. Solar cells fabricated on the EMB GaAs thin-films indicate efficiencies of - 17.9% under AM I .5 simulation. The spectral response of the solar cells indicate that the minority carrier diffusion lengths are in the range of 7 pm in the EMB GaAs thin-films, in agreement with the measured minoritycarrier lifetimes (100 ns). Therefore, we believe the EMB approach promises to be very useful for integration of GaAs optical devices with Si electronic devices. Acknowledgements-We like to acknowledge the technical assistance of Mr T. S. Colpitts in the OMVPE growth of GaAs-AlGaAs structures and Mr D. Malta with the CL imaging data. We also like to acknowledge the assistance of Dr Keith Emery and Dr Richard Ahrenkiel of the National Renewable Energy Laboratory (NREL), Golden, Colorado, for their assistance with the solar cell measurements and lifetime data. Portions of this work were conducted as a
structures on Si substrates result of a subcontact from the NREL as the technical monitor.
1815 with Mr T. S. Basso
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