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Patterned germanium tunnel junctions for multijunction monolithic cascade solar cells
Solar Cells, 21 (1987) 241 - 252
241
PATTERNED GERMANIUM TUNNEL JUNCTIONS FOR MULTIJUNCTION MONOLITHIC CASCADE SOLAR CELLS* P. K. CHIANG, M. L. TIMMONS and J. A. HUTCHBY Research Triangle Institute, Research Triangle Park, NC 27709 (U.S.A.)
(Received May 1986; accepted July 3, 1986)
Summary The growth and characterization of germanium interconnects is described in this paper. Resistivity measurements indicate that n+-GaAs/p +Ge/p+-GaAs structures may be the most viable interconnect yet reported for monolithic cascade cells. Specific interface resistivities as low as 1 × 10 -4 ~2 cm 2 have been measured. This value surpasses the goal of 1.7 X 10 -4 ~ cm 2 for a cascade cell with a patterned interconnect operating at 1000 Suns under air mass 1.5 conditions. To pattern germanium layers, both wet chemical etching and selective epitaxial growth have been used. Excellent germanium interconnecting patterns have been reproducibly achieved using selective epitaxy. Using the patterned germanium interconnects, cascade action has been unambiguously observed in the A1GaAs/GaAs multijunction cascade solar cells.
1. Introduction Intercell ohmic connections {IOCs) place the multiple junctions of monolithic cascade solar cells in electrical and optical series. Various approaches to developing IOCs have been attempted by several laboratories and include high band gap tunnel junctions [1, 2], laser fusion of top and b o t t o m cells [3], thin germanium layers [4], and multilevel layers of metallization [5]. All have met with only moderate success. High band gap tunnel junctions face a fundamental problem. It is extremely difficult to introduce impurity atoms with concentrations sufficiently high to keep the material degenerate, which is a requirement for low resistivity and tunnelling. Uniformity of the interface between cells is an unsolved problem with laser fusion. Thin germanium layers 75 A thick must be grown to avoid excessive absorption losses. Reproducibility may be a problem. Multilevel metallization, as well as producing three- or four-terminal devices, requires extensive photolithography and masking. *Paper presented at the 7th Photovoltaic Advanced Research and Development Project Review Meeting, Denver, CO, U.S.A., May 13, 1986. 0379-6787/87/$3.50
© Elsevier Sequoia/Printed in The Netherlands
242 In this paper we describe an IOC which combines the advantages of several o f the earlier approaches but has few of the drawbacks. This approach is the patterned germanium IOC. Resistivity measurements show that this IOC is capable of providing the necessary connections for concentrator cells using 1000 Suns under air mass 1.5 conditions. Demonstration of this capability makes the germanium IOC potentially the most viable interco n n ect y e t reported. This paper contains the rationale for the IOC, a description of the germanium growth, results of the resistivity measurements and results showing the operation of a cascade cell using a germanium IOC.
2. Rationale for the germanium intercell ohmic c o n n e c t i o n The initial development of the IOC is intended for GaAs and AIGaAs solar cells. Figure 1 shows the structure of the GaAs/A1GaAs cascade cell with a germanium IOC. Germanium has some very obvious advantages for GaAs-based cells: the lattice mismatch between germanium and GaAs is small, ab o ut 0.08%, and the thermal coefficient of expansion is almost the same for the two materials. These factors favor the epitaxial growth of the mixed Ge/GaAs structures. However, growth of a polar semiconductor (GaAs) on a non-polar one (germanium) is complicated by antiphase domain formation if the growth conditions are not optimized.
[ i! i
[
Metal pn-A+-GaAs lGC°nt aAsact p-AIGaAs
l _l
- .--~,l~-a~ n+-GaAs
"
p+-Ge p+-GaAs p-AIGaAs -
-
-
n-G~s
.
.
~--To Cell ~-BSF + GeIOC
..... ~Bottom Cell .
.
n-AIGaAs n-GaAsSubstrate
J
-
~--BSF ~Substrate
Fig. 1. S t r u c t u r e diagram of an A 1 G a A s / G a A s cascade solar cell using g e r m a n i u m IOC. T h e t o p cell b a n d gap equals 1.9 eV to c u r r e n t m a t c h GaAs using c o n c e n t r a t e d suns, at AM 1.5.
Germanium has another drawback which affects the structure and processing. The band gap is less than that of the GaAs b o t t o m junction, and therefore the tunnel junction is patterned photolithographically to lie directly beneath the surface grid metallization and to contribute no additional shadowing (or absorption) losses with normal solar incidence. In addition, the layers of the germanium IOC are 0.1 pm thick, thus avoiding
243 the need to grow ultrathin layers of germanium reproducibly. Although lithography is involved in the pattern fabrication, process requirements are less critical than are those for multilevel metallization. Incorporating impurities in germanium is easier than incorporating them in GaAs (or in materials with even higher band gaps). Degenerate material is easily fabricated using any of a number of dopants: boron, gallium and aluminum produce p-type behavior whereas arsenic, phosphorus and a n t i m o n y produce n-type behavior. The solid solubility of these impurities is greater than 1 X 1019 cm -3, and all exceed the effective densities of states for both the conduction and the valence bands. Hence the major problem associated with high band gap tunnel junctions (keeping the material degenerately doped) need not be a limiting factor. Device considerations for germanium tunnel junctions are as attractive as the materials properties. Current densities Jp > 1400 A cm -2 have been measured at voltages of 50 mV for good quality discrete germanium tunnel junctions [6]. The specific resistivity for such a device is less than 3.6 × 10 -5 cm 2. Assuming 5% surface coverage by the tunnel junction and concentration to 1000 Suns at AM 1.5, IOCs require specific resistivities of 1.7 × 10 4 cm 2 or less. Thus the germanium device has demonstrated the required performance for a successful IOC. Chemical vapor growth must produce material comparable with that obtained with the recrystallization methods used for the discrete devices if these results are to be duplicated in the cascade structures. None the less, from both materials and device considerations, the germanium IOC is very attractive for the monolithic cascade structure.
3. Description of germanium growth Pyrolyzing germane (GeH4), a standard chemical vapor technique, provides a convenient method for growing germanium. Growth is described in this section, including reactor operating conditions and characteristics of the selected dopants. The growth takes place in a horizontal reactor operated at atmospheric pressure. The GeH 4 source is a 5 vol.% mixture diluted with pure hydrogen. Hydrogen is also used as the carrier gas, and flow rates have varied from 3 to over 8 1 min -1. Higher flow rates provide optimum growth conditions, minimize a gas phase decomposition which has been reported [7], and are a major factor in reducing growth temperatures to as low as 550 °C. Arsine (ASH3) as a 10 vol.% H2-diluted mixture is the arsenic source used for n-type impurity addition. Trimethylgallium (TMG) and diborane (B2H6) are the sources of the p-type dopants gallium and boron respectively. The doping characteristics of arsenic and boron are shown in Fig. 2. Using ASH3, carrier concentrations reach (0.8 - 2) × 1019 cm 3 at a growth temperature of 625 °C. The insensitivity of the carrier concentration to the AsH 3 flow is explained by the arsenic background which results from deposits
244 1021
r 9 = 570°C
~
B~Tg= ®
'~ 1020
~
~
~
------
625°C GeH 4 = 15 cm3/min
~
g Tg = 625°C GeH4 = 45 cm3/min
As
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1019 .
AS T9 = 625°C GeH 4 = 15 cm3/min
1018
i
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i
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i
i
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100 150 200 250 300 Volumetric Flow of 10% ASH3/90% H 2 or 0.1% B2H6/99.9% H 2 (cm3/rain) Fig. 2. D o p i n g c h a r a c t e r i s t i c s o f a r s e n i c a n d b o r o n as a f u n c t i o n o f A s H 3 a n d B2H 6 f l o w s respectively. 0
50
within the reaction chamber. Increased GeH4 flow produces faster growth and slightly greater arsenic incorporation. Since the effective density of states in the conduction band of germanium is 1.04 × 1019 cm 3 [8], these data show that the material is only marginally degenerate for the lower GeH4 flow. At growth temperatures nearer 750 °C, measured carrier concentrations are greater by as much as a factor of 4. However, the major drawback with arsenic is its diffusivity in germanium, and the higher growth temperature exacerbates this problem. Attempts to use TMG for p-type doping were only partially successful: carrier concentrations reached 7 × 1019 cm 3, but gallium droplet formation was an unsolved problem for practical material growth. Boron, however, has proved to be almost the optimum dopant. A 1000 ppm B2H6 source (diluted with H2) has been used for the doping. As shown in Fig. 2, measured hole concentrations reach almost 4 × 102o cm 3 although surface morphologies degrade for values greater than 2 × 1020 cm 3. The effective density of states in the germanium valence band is 6.0 × 1018 c m 3 [ 8 ] , indicating that all the boron-doped p-type material is deeply degenerate. Also, secondary ion mass spectrometry data suggest that boron does not diffuse appreciably at 625 °C. This establishes the ideal situation for a tunnel junction dopant: degenerate material is easily produced with little tendency for dopant diffusion. The growth temperature has a profound effect on device performance. In the early research, a growth temperature of 750 °C was typically used. No evidence of band-to-band tunnelling was seen in any grown germanium
245
devices, although some reasonably low resistivities were measured. Analyses of resistivities indicate that the junctions are broadened by diffusion of the dopants, principally by that of the arsenic. Therefore, lower temperatures for the germanium deposition are now being employed. Most work reported here has used 625 °C, a temperature compatible with the GaAs b o t t o m cell growth, but temperatures as low as 550 °C have been investigated with good results. The lower temperatures may be significant in obtaining quality GaAs overgrowth on the germanium and in avoiding antiphase domain formation. This is an area currently being investigated.
4. Resistivity measurements for germanium interconnects This program began with the goal of growing germanium homojunctions showing low resistivities. However, subsequent specific resistivity measurements indicate that a completely adequate IOC can be grown using an n+-GaAs/p+-Ge/p+-GaAs sandwich structure. This is a significant result because it simplifies the germanium growth and avoids the problem of arsenic diffusion. The GaAs can be heavily doped with silicon (for n-type material) and magnesium (for p-type material) without significant diffusion. The resulting interfaces are likely to be much less resistive than diffused ones. This has indeed been observed. The measurements leading to this observation are described in this section. The approach to resistivity measurements for the germanium devices is qualitative. Figure 3 shows a typical measured structure. This structure is applicable to either single layers or muttilayer epitaxy and requires chemical etching to evaluate interfaces progressively as they occur deeper in the material. Beginning with contacts deposited onto the material surface, successive levels of contact or interface resistivity are subtracted from the total resistivity measured between two adjacent etched mesas. Contributions from each interface are thereby estimated. We consider this method qualitaa
Contact Ge
-
b
c
d
-
IOC
Subst~te--
I.
Fig. 3. Typical structure used for interface resistivity measurements: Rab , resistivity measured between adjacent mesas; Rcd, resistivity measured between adjacent unetched contacts; R i n t e r f a c e = R a b - - Rcd.
246 tive in n a t u r e because it does n o t take into a c c o u n t c u r r e n t spreading, surface leakage and bulk losses. These factors appear to be m i n o r , however, c o m p a r e d with c o n t a c t and interface resistances. T h e specific resistivity goal for the IOC is 1.7 × 10 ..4 ~2 cm z. This value results f r o m p a t t e r n i n g which increases the c u r r e n t density in the IOC to 250 - 300 A cm z for solar conc e n t r a t i o n s o f 1 0 0 0 Suns of AM 1.5; the voltage d r o p across the IOC m u s t be limited to 50 m V or less. Few devices grown at high t e m p e r a t u r e s (greater than 700 °C) s h o w e d t u n n e l j u n c t i o n behavior. In c o n t r a s t , in devices grown at t e m p e r a t u r e s as low as 625 °C, the back-to-back tunnelling characteristic is present. As shown in Fig. 4, curve 1 was measured b e t w e e n adjacent c o n t a c t s on the u n e t c h e d surface, and curve 2 was m e a s u r e d b e t w e e n adjacent e t c h e d mesas. The specific resistivity is a p p r o x i m a t e l y I × 10 2 ~ cm 2, which is still s o m e w h a t high b u t includes a measurable c o n t r i b u t i o n f r o m the contacts. J u n c t i o n b r o a d e n i n g a p p a r e n t l y also plays a role in this device. T h e device which p r o d u c e d the c u r r e n t density v s . voltage curve o f Fig. 4 contains a p+-Ge layer and an n+-Ge layer grown on a z i n c - d o p e d p+-GaAs substrate. No cont r i b u t i o n f r o m the n+-Ge-p+-GaAs i n t e r f a c e is a p p a r e n t in the c u r r e n t density v s . voltage characteristic which leads to the assumption t h a t this interface resistivity is c o n s i d e r a b l y less t h a n t h a t o f the m e a s u r e d interface. Low specific resistivities had previously been m e a s u r e d for p+-Ge p+-GaAs interfaces (less than or equal to 1.0 × 10 4 ~ cm 2) and p+-Ge-n+-GaAs interfaces (less than or equal to 1.0 × 10 4 ~ cm2), as can be seen in Figs. 5 and 6 respectively. These data suggest t h a t a single, d e e p l y degenerate, p - t y p e layer sandwiched b e t w e e n n +- and p+-GaAs layers can substitute for the g e r m a n i u m tunnel j u n c t i o n . This is similar to the original suggestion of Fraas [4] e x c e p t t h a t the p a t t e r n i n g eliminates the need to grow ultrathin layers. The resulting s t r u c t u r e also relies on a h e t e r o t u n n e l l i n g interface b e t w e e n the g e r m a n i u m and the GaAs (or A1GaAs) and, in t h a t respect, is similar t o the A1GaAs/GaAs tunnelling j u n c t i o n r e p o r t e d by Miller e t al. [9]. 1
r
/
2
/./ I
/
/
IA Ili
I:: /
Vertical scale = 5 mA/div Horizontal scale = 0 , 1 V/div +
Fig. 4. Current density vs. voltage curve of a p+-Ge/n+-Ge structure grown on a P2 -GaAs substrate at 625 °C. The specific resistivity in the region of origin is 1 × 10 2 ~ cm .
247 1
f
J/ ....
////
/
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p+- Ge/p +-GaAs Ver: 50 mA/div Hor: 0.05 V/div
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f
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p+- Ge/n+- GaAs Ver: 50 mA/div Hor: 0.1 V/div
Fig. 5. Current density v s . voltage characteristics for a p+-Ge/p+-GaAs structure (growth temperature, 625 °C): curve 1, measured between adjacent contacts on unetched surface; curve 2, measured from etched mesa to etched mesa. Fig. 6. Current density v s . voltage characteristics for a p+-Ge/n÷-GaAs structure (growth temperature, 625 °C): curve 1, measured between adjacent contacts on unetched surface; curve 2, measured from etched mesa to etched mesa. 312
12
/
/
I
J
I I
Ver: 20 mA/div Hor: 0.1 V/div
(a)
Ver: 20 mA/div Hor: 0.1 V/div
(b)
Fig. 7. Current density v s . voltage curves of an n+-GaAs/p+-Ge/p+-GaAs structure: (a) curves 1 and 2 measured between adjacent pairs of contact dots on unetched surface (contact resistivity approximately 1 x 10 -3 ~ cm2); (b) curves 1, 2 and 3 measured between adjacent etched mesas, curves 1 and 2 use the same contact pairs as curves 1 and 2 in (a). The Ge-GaAs interfaces contribute no measurable resistance increases.
Structures consisting evaluated. Figure 7 shows one such structure. Figure before the etching process.
of n+-GaAs/p+-Ge/p+-GaAs have been grown and the c u r r e n t d e n s i t y vs. v o l t a g e c h a r a c t e r i s t i c of 7(a) s h o w s m e a s u r e m e n t s m a d e b e t w e e n c o n t a c t s Figure 7(b) shows the measurements made after
248 etching through the germanium into the p+-GaAs substrate, and curves 1 and 2 were measured between the same pairs of contacts as curves 1 and 2 in Fig. 7(a). There is no measurable change in the current density vs. voltage curves before and after etching. Contact resistivity dominates the measurement and may be as much as several orders of magnitude greater than the indicating that the interface specific resistivities total less than 1 × 10 4 ~2 cm 2. This value surpasses the requirement of the cascade structure and renders the germanium sandwich structure potentially the most viable IOC yet described for two-terminal monolithic devices.
5. Description of selective germanium epitaxy To pattern germanium layers, wet chemical etching has been used in earlier research. The p+-Ge etches extremely slowly which enhances undercutting during the pattern formation. On occasion, the germanium fingers have been lost. To improve the pattern formation, a selective germanium epitaxial growth technique has been developed. In this experiment, SiO: masks 3000 A thick are grown at 240 °C using plasma-assisted chemical vapor deposition onto polished GaAs surfaces. Windows are defined by standard photolithographic techniques. After the germanium growth, samples are dipped in the buffered HF to lift off unwanted material, leaving the patterned germanium as shown in Fig. 8. A magnified scanning electron micrograph of the surface can be seen in Fig. 9. The selectivity of germanium growth on the exposed substrate compared with that on the SiO: mask depends on the growth temperature. Good selectivity is obtained at growth temperatures below 570 °C. Above 570 °C, non-reproducibility is observed. This may be due to the oxide contamination on the substrates as reported by Ishii e t al. [10]. Further study is needed to identify the major problems affecting the selectivity.
Mag = 280x Fig. 8. Micrograph showing the surface of a patterned germanium layer selectively grown on a GaAs substrate. (Magnification, 280x .)
249
PH-_ Ge
P~--GaAs
Fig. 9. Scanning electron micrograph of a germanium sample selectively grown on a GaAs substrate.
To test the germanium interconnects, GaAs p - n b o t t o m cells were grown on p+-GaAs substrates with patterned germanium interconnecting layers. The cells show photovoltaic action and current flow through the germanium interconnects because the p-type substrates and n+-GaAs buffer layers produce a potential barrier. The current density v s . voltage characteristic for such a cell is given in Fig. 10 for dark and illuminated conditions. The junction quality, however, is poor because the overgrowth conditions of the GaAs b o t t o m cell have not been optimized.
'1 Curve 1. 1 sun Ver: 0,5 mA/div Hor: 0,5 V/div Curve 2. 7 suns Ver: 2.0 mNdiv Hor: 0,5 V/div Fig. 10. Current density v s . voltage curve for a GaAs p - n b o t t o m cell grown on a p+-GaAs substrate with a patterned germanium IOC: Voc=0.48 V (1 Sun) and 0.61 V (7 Suns); J s c = 1 4 m A cm -2 (1 Sun); cell area, 0.029 cm 2.
250 To summarize, good germanium selective epitaxial growth has been achieved at growth temperatures lower than 570 °C. The current conduction through a patterned structure has been clearly demonstrated. The overgrowth of the GaAs on germanium needs to be improved.
6. Development of cascade cells using germanium interconnects The next stage in germanium IOC development is clearly the demonstration of high efficiency cascade solar cells. The first steps towards this end have been taken. In this section an unambiguous demonstration of voltage addition in a cascade cell is presented. The perceived problem areas which must be addressed if projected efficiencies are to be realized are also identified. In earlier work, poor understanding and control of the magnesium source used for p-type doping in GaAs and A1GaAs prevented growth of good quality junctions and hampered attempts to provide a clear-cut cascade demonstration using two-layer germanium IOCs. A cell for which spectral response measurements suggested collection from the two junctions failed to show the characteristic double breakdown of the cascade structure. The low Voc (about 1 V) kept a single junction as a possible voltage source rather than two poor junctions showing voltage addition. Solving the magnesium-related problems led to improved junction growth for top and b o t t o m cells, and cascade structures have been attempted which employ the Ge/GaAs sandwich IOC. Unambiguous cascade action has been observed. The dark and illuminated current density v s . voltage curve for the best of these cells is shown in Fig. 11. The cell area is 0.19 cm 2 and the illumination (from a filtered solar simulator) is about 7 Suns at AM 0. Voc = 1.44 V, a value greater than any single-junction measurement made in our laboratory and one which could only result from voltage addition of the two junctions. The reverse characteristic confirms the observation that double breakdown is present, with the A1GaAs and GaAs junctions breaking down at 2 V and 5 V respectively. Major performance limitations arise from the A1GaAs junctions. No significant optimization has yet been attempted. The major problem is thought to be moisture in the AsH 3. The growth temperature for the A1GaAs in the best cascade cell was 700 °C without moisture-removal apparatus on the AsH3 gas line. Growth above 750°C, where the moisture-related problems ameliorate, results in very poor overgrowth on the germanium. The cascade cells using 750 °C for the A1GaAs growth show double breakdown, but Voc values are lower. Poorer quality A1GaAs junctions grown on the germanium are believed to be responsible. This indicates the need to achieve quality A1GaAs growth at temperatures below 700 °C and is one of the major focal points of our present research efforts. To summarize the main points, cells showing unambiguous cascade action have been grown and A1GaAs junctions, grown at low temperature
251
/
Forward: 0.5 V/div 0.1 mA/div Reverse: 1.0 V/div 1.0 mA/div
Fig. 11. Current density v s . voltage curve of cascade solar cell using n+-OaAs-p+-Ge-p+GaAs IOC: Voc=1.44 V (7 Suns);Jsc=l.5 mA cm 2;cell area, 0.19 cm 2.
without moisture scrubbing on gas lines, limit device performance at the present time.
7. Conclusions In this paper three major results have been described. Firstly, resistivity measurements indicate that the n+-GaAs/p+-Ge/p+-GaAs structures are viable IOCs, potentially to concentrations as high as 1000 Suns. Secondly, germanium interconnecting patterns have been reproducibly achieved using selective epitaxy. Thirdly, cascade cells using the germanium IOC unambiguously show voltage addition. The factors limiting the germanium growth and processing and the A1GaAs growth have been identified. For germanium, the main problem is optimizing GaAs overgrowth. A1GaAs problems are (i) moisture removal from reactant gas lines, and (ii) developing low growth temperatures without sacrificing material quality. As the technology improves and these problems are solved, the cascade structure using germanium IOCs may well fulfill the modeled predictions for high efficiencies.
Acknowledgments This research is funded by the Solar Energy Research Institute under Subcontract XL-4-03032-6. This support is gratefully acknowledged.
252 T h e a u t h o r s are g r a t e f u l f o r t h e a s s i s t a n c e o f G. F o u n t a i n , R. P i c k e t t , T. S. C o l p i t t s a n d G. S o l o m o n f o r m u c h o f t h e l a b o r a t o r y w o r k a n d t o S. R a y f o r m a n u s c r i p t p r e p a r a t i o n .
References 1 S. M. Bedair, M. F. Lamorte, J. R. Hauser and K. W. Mitchell, Tech. Dig. IEEE Int. Electron Device Meet. IEEE, New York, 1978, p. 250. 2 M. L. Timmons and S. M. Bedair, Proc. 15th IEEE Photovoltaic Specialists' Conf., Orlando, FL, 1981, IEEE, New York, 1981, p. 1289. 3 H. T. Yang and S. W. Zehr, Proc. 15th IEEE Photovoltaic Specialists' Conf., Orlando, FL, 1981, IEEE, New York, 1981, p. 1357. 4 L. M. Fraas, Proc. 15th IEEE Photovoltaic Specialists' Conf., Orlando, FL, 1981, IEEE, New York, 1981, p. 1353. 5 R. A. LaRue, P. G. Borden, M. J. Ludowise, P. E. Gregory and W. T. Dietze, Proc. 16th IEEE Photovoltaic Specialists' Conf., San Diego, CA, 1982, IEEE, New York, 1982, p. 228. 6 R. M. Minton and R. Glicksman, Solid-State Electron., 7 (1964) 491. 7 E. A. Roth, H. Gassenberger and A. J. Amick, RCA Rev., 24 (1963) 499. 8 S. M. Sze, Physics o f Semiconductor Devices, Wiley, New York, 1969, p. 57. 9 D. L. Miller, S. W. Zehr and J. S. Harris, Jr., J. Appl. Phys., 53 (1982) 774. 10 H. Ishii, Y. Takahashi and J. Murota, Appl. Phys. Lett., 47 (1985) 863.
241
PATTERNED GERMANIUM TUNNEL JUNCTIONS FOR MULTIJUNCTION MONOLITHIC CASCADE SOLAR CELLS* P. K. CHIANG, M. L. TIMMONS and J. A. HUTCHBY Research Triangle Institute, Research Triangle Park, NC 27709 (U.S.A.)
(Received May 1986; accepted July 3, 1986)
Summary The growth and characterization of germanium interconnects is described in this paper. Resistivity measurements indicate that n+-GaAs/p +Ge/p+-GaAs structures may be the most viable interconnect yet reported for monolithic cascade cells. Specific interface resistivities as low as 1 × 10 -4 ~2 cm 2 have been measured. This value surpasses the goal of 1.7 X 10 -4 ~ cm 2 for a cascade cell with a patterned interconnect operating at 1000 Suns under air mass 1.5 conditions. To pattern germanium layers, both wet chemical etching and selective epitaxial growth have been used. Excellent germanium interconnecting patterns have been reproducibly achieved using selective epitaxy. Using the patterned germanium interconnects, cascade action has been unambiguously observed in the A1GaAs/GaAs multijunction cascade solar cells.
1. Introduction Intercell ohmic connections {IOCs) place the multiple junctions of monolithic cascade solar cells in electrical and optical series. Various approaches to developing IOCs have been attempted by several laboratories and include high band gap tunnel junctions [1, 2], laser fusion of top and b o t t o m cells [3], thin germanium layers [4], and multilevel layers of metallization [5]. All have met with only moderate success. High band gap tunnel junctions face a fundamental problem. It is extremely difficult to introduce impurity atoms with concentrations sufficiently high to keep the material degenerate, which is a requirement for low resistivity and tunnelling. Uniformity of the interface between cells is an unsolved problem with laser fusion. Thin germanium layers 75 A thick must be grown to avoid excessive absorption losses. Reproducibility may be a problem. Multilevel metallization, as well as producing three- or four-terminal devices, requires extensive photolithography and masking. *Paper presented at the 7th Photovoltaic Advanced Research and Development Project Review Meeting, Denver, CO, U.S.A., May 13, 1986. 0379-6787/87/$3.50
© Elsevier Sequoia/Printed in The Netherlands
242 In this paper we describe an IOC which combines the advantages of several o f the earlier approaches but has few of the drawbacks. This approach is the patterned germanium IOC. Resistivity measurements show that this IOC is capable of providing the necessary connections for concentrator cells using 1000 Suns under air mass 1.5 conditions. Demonstration of this capability makes the germanium IOC potentially the most viable interco n n ect y e t reported. This paper contains the rationale for the IOC, a description of the germanium growth, results of the resistivity measurements and results showing the operation of a cascade cell using a germanium IOC.
2. Rationale for the germanium intercell ohmic c o n n e c t i o n The initial development of the IOC is intended for GaAs and AIGaAs solar cells. Figure 1 shows the structure of the GaAs/A1GaAs cascade cell with a germanium IOC. Germanium has some very obvious advantages for GaAs-based cells: the lattice mismatch between germanium and GaAs is small, ab o ut 0.08%, and the thermal coefficient of expansion is almost the same for the two materials. These factors favor the epitaxial growth of the mixed Ge/GaAs structures. However, growth of a polar semiconductor (GaAs) on a non-polar one (germanium) is complicated by antiphase domain formation if the growth conditions are not optimized.
[ i! i
[
Metal pn-A+-GaAs lGC°nt aAsact p-AIGaAs
l _l
- .--~,l~-a~ n+-GaAs
"
p+-Ge p+-GaAs p-AIGaAs -
-
-
n-G~s
.
.
~--To Cell ~-BSF + GeIOC
..... ~Bottom Cell .
.
n-AIGaAs n-GaAsSubstrate
J
-
~--BSF ~Substrate
Fig. 1. S t r u c t u r e diagram of an A 1 G a A s / G a A s cascade solar cell using g e r m a n i u m IOC. T h e t o p cell b a n d gap equals 1.9 eV to c u r r e n t m a t c h GaAs using c o n c e n t r a t e d suns, at AM 1.5.
Germanium has another drawback which affects the structure and processing. The band gap is less than that of the GaAs b o t t o m junction, and therefore the tunnel junction is patterned photolithographically to lie directly beneath the surface grid metallization and to contribute no additional shadowing (or absorption) losses with normal solar incidence. In addition, the layers of the germanium IOC are 0.1 pm thick, thus avoiding
243 the need to grow ultrathin layers of germanium reproducibly. Although lithography is involved in the pattern fabrication, process requirements are less critical than are those for multilevel metallization. Incorporating impurities in germanium is easier than incorporating them in GaAs (or in materials with even higher band gaps). Degenerate material is easily fabricated using any of a number of dopants: boron, gallium and aluminum produce p-type behavior whereas arsenic, phosphorus and a n t i m o n y produce n-type behavior. The solid solubility of these impurities is greater than 1 X 1019 cm -3, and all exceed the effective densities of states for both the conduction and the valence bands. Hence the major problem associated with high band gap tunnel junctions (keeping the material degenerately doped) need not be a limiting factor. Device considerations for germanium tunnel junctions are as attractive as the materials properties. Current densities Jp > 1400 A cm -2 have been measured at voltages of 50 mV for good quality discrete germanium tunnel junctions [6]. The specific resistivity for such a device is less than 3.6 × 10 -5 cm 2. Assuming 5% surface coverage by the tunnel junction and concentration to 1000 Suns at AM 1.5, IOCs require specific resistivities of 1.7 × 10 4 cm 2 or less. Thus the germanium device has demonstrated the required performance for a successful IOC. Chemical vapor growth must produce material comparable with that obtained with the recrystallization methods used for the discrete devices if these results are to be duplicated in the cascade structures. None the less, from both materials and device considerations, the germanium IOC is very attractive for the monolithic cascade structure.
3. Description of germanium growth Pyrolyzing germane (GeH4), a standard chemical vapor technique, provides a convenient method for growing germanium. Growth is described in this section, including reactor operating conditions and characteristics of the selected dopants. The growth takes place in a horizontal reactor operated at atmospheric pressure. The GeH 4 source is a 5 vol.% mixture diluted with pure hydrogen. Hydrogen is also used as the carrier gas, and flow rates have varied from 3 to over 8 1 min -1. Higher flow rates provide optimum growth conditions, minimize a gas phase decomposition which has been reported [7], and are a major factor in reducing growth temperatures to as low as 550 °C. Arsine (ASH3) as a 10 vol.% H2-diluted mixture is the arsenic source used for n-type impurity addition. Trimethylgallium (TMG) and diborane (B2H6) are the sources of the p-type dopants gallium and boron respectively. The doping characteristics of arsenic and boron are shown in Fig. 2. Using ASH3, carrier concentrations reach (0.8 - 2) × 1019 cm 3 at a growth temperature of 625 °C. The insensitivity of the carrier concentration to the AsH 3 flow is explained by the arsenic background which results from deposits
244 1021
r 9 = 570°C
~
B~Tg= ®
'~ 1020
~
~
~
------
625°C GeH 4 = 15 cm3/min
~
g Tg = 625°C GeH4 = 45 cm3/min
As
g "v"
tj
1019 .
AS T9 = 625°C GeH 4 = 15 cm3/min
1018
i
i
i
i
i
i
i
i
i
i
I
100 150 200 250 300 Volumetric Flow of 10% ASH3/90% H 2 or 0.1% B2H6/99.9% H 2 (cm3/rain) Fig. 2. D o p i n g c h a r a c t e r i s t i c s o f a r s e n i c a n d b o r o n as a f u n c t i o n o f A s H 3 a n d B2H 6 f l o w s respectively. 0
50
within the reaction chamber. Increased GeH4 flow produces faster growth and slightly greater arsenic incorporation. Since the effective density of states in the conduction band of germanium is 1.04 × 1019 cm 3 [8], these data show that the material is only marginally degenerate for the lower GeH4 flow. At growth temperatures nearer 750 °C, measured carrier concentrations are greater by as much as a factor of 4. However, the major drawback with arsenic is its diffusivity in germanium, and the higher growth temperature exacerbates this problem. Attempts to use TMG for p-type doping were only partially successful: carrier concentrations reached 7 × 1019 cm 3, but gallium droplet formation was an unsolved problem for practical material growth. Boron, however, has proved to be almost the optimum dopant. A 1000 ppm B2H6 source (diluted with H2) has been used for the doping. As shown in Fig. 2, measured hole concentrations reach almost 4 × 102o cm 3 although surface morphologies degrade for values greater than 2 × 1020 cm 3. The effective density of states in the germanium valence band is 6.0 × 1018 c m 3 [ 8 ] , indicating that all the boron-doped p-type material is deeply degenerate. Also, secondary ion mass spectrometry data suggest that boron does not diffuse appreciably at 625 °C. This establishes the ideal situation for a tunnel junction dopant: degenerate material is easily produced with little tendency for dopant diffusion. The growth temperature has a profound effect on device performance. In the early research, a growth temperature of 750 °C was typically used. No evidence of band-to-band tunnelling was seen in any grown germanium
245
devices, although some reasonably low resistivities were measured. Analyses of resistivities indicate that the junctions are broadened by diffusion of the dopants, principally by that of the arsenic. Therefore, lower temperatures for the germanium deposition are now being employed. Most work reported here has used 625 °C, a temperature compatible with the GaAs b o t t o m cell growth, but temperatures as low as 550 °C have been investigated with good results. The lower temperatures may be significant in obtaining quality GaAs overgrowth on the germanium and in avoiding antiphase domain formation. This is an area currently being investigated.
4. Resistivity measurements for germanium interconnects This program began with the goal of growing germanium homojunctions showing low resistivities. However, subsequent specific resistivity measurements indicate that a completely adequate IOC can be grown using an n+-GaAs/p+-Ge/p+-GaAs sandwich structure. This is a significant result because it simplifies the germanium growth and avoids the problem of arsenic diffusion. The GaAs can be heavily doped with silicon (for n-type material) and magnesium (for p-type material) without significant diffusion. The resulting interfaces are likely to be much less resistive than diffused ones. This has indeed been observed. The measurements leading to this observation are described in this section. The approach to resistivity measurements for the germanium devices is qualitative. Figure 3 shows a typical measured structure. This structure is applicable to either single layers or muttilayer epitaxy and requires chemical etching to evaluate interfaces progressively as they occur deeper in the material. Beginning with contacts deposited onto the material surface, successive levels of contact or interface resistivity are subtracted from the total resistivity measured between two adjacent etched mesas. Contributions from each interface are thereby estimated. We consider this method qualitaa
Contact Ge
-
b
c
d
-
IOC
Subst~te--
I.
Fig. 3. Typical structure used for interface resistivity measurements: Rab , resistivity measured between adjacent mesas; Rcd, resistivity measured between adjacent unetched contacts; R i n t e r f a c e = R a b - - Rcd.
246 tive in n a t u r e because it does n o t take into a c c o u n t c u r r e n t spreading, surface leakage and bulk losses. These factors appear to be m i n o r , however, c o m p a r e d with c o n t a c t and interface resistances. T h e specific resistivity goal for the IOC is 1.7 × 10 ..4 ~2 cm z. This value results f r o m p a t t e r n i n g which increases the c u r r e n t density in the IOC to 250 - 300 A cm z for solar conc e n t r a t i o n s o f 1 0 0 0 Suns of AM 1.5; the voltage d r o p across the IOC m u s t be limited to 50 m V or less. Few devices grown at high t e m p e r a t u r e s (greater than 700 °C) s h o w e d t u n n e l j u n c t i o n behavior. In c o n t r a s t , in devices grown at t e m p e r a t u r e s as low as 625 °C, the back-to-back tunnelling characteristic is present. As shown in Fig. 4, curve 1 was measured b e t w e e n adjacent c o n t a c t s on the u n e t c h e d surface, and curve 2 was m e a s u r e d b e t w e e n adjacent e t c h e d mesas. The specific resistivity is a p p r o x i m a t e l y I × 10 2 ~ cm 2, which is still s o m e w h a t high b u t includes a measurable c o n t r i b u t i o n f r o m the contacts. J u n c t i o n b r o a d e n i n g a p p a r e n t l y also plays a role in this device. T h e device which p r o d u c e d the c u r r e n t density v s . voltage curve o f Fig. 4 contains a p+-Ge layer and an n+-Ge layer grown on a z i n c - d o p e d p+-GaAs substrate. No cont r i b u t i o n f r o m the n+-Ge-p+-GaAs i n t e r f a c e is a p p a r e n t in the c u r r e n t density v s . voltage characteristic which leads to the assumption t h a t this interface resistivity is c o n s i d e r a b l y less t h a n t h a t o f the m e a s u r e d interface. Low specific resistivities had previously been m e a s u r e d for p+-Ge p+-GaAs interfaces (less than or equal to 1.0 × 10 4 ~ cm 2) and p+-Ge-n+-GaAs interfaces (less than or equal to 1.0 × 10 4 ~ cm2), as can be seen in Figs. 5 and 6 respectively. These data suggest t h a t a single, d e e p l y degenerate, p - t y p e layer sandwiched b e t w e e n n +- and p+-GaAs layers can substitute for the g e r m a n i u m tunnel j u n c t i o n . This is similar to the original suggestion of Fraas [4] e x c e p t t h a t the p a t t e r n i n g eliminates the need to grow ultrathin layers. The resulting s t r u c t u r e also relies on a h e t e r o t u n n e l l i n g interface b e t w e e n the g e r m a n i u m and the GaAs (or A1GaAs) and, in t h a t respect, is similar t o the A1GaAs/GaAs tunnelling j u n c t i o n r e p o r t e d by Miller e t al. [9]. 1
r
/
2
/./ I
/
/
IA Ili
I:: /
Vertical scale = 5 mA/div Horizontal scale = 0 , 1 V/div +
Fig. 4. Current density vs. voltage curve of a p+-Ge/n+-Ge structure grown on a P2 -GaAs substrate at 625 °C. The specific resistivity in the region of origin is 1 × 10 2 ~ cm .
247 1
f
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////
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p+- Ge/p +-GaAs Ver: 50 mA/div Hor: 0.05 V/div
7
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f
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Fig. 5. Current density v s . voltage characteristics for a p+-Ge/p+-GaAs structure (growth temperature, 625 °C): curve 1, measured between adjacent contacts on unetched surface; curve 2, measured from etched mesa to etched mesa. Fig. 6. Current density v s . voltage characteristics for a p+-Ge/n÷-GaAs structure (growth temperature, 625 °C): curve 1, measured between adjacent contacts on unetched surface; curve 2, measured from etched mesa to etched mesa. 312
12
/
/
I
J
I I
Ver: 20 mA/div Hor: 0.1 V/div
(a)
Ver: 20 mA/div Hor: 0.1 V/div
(b)
Fig. 7. Current density v s . voltage curves of an n+-GaAs/p+-Ge/p+-GaAs structure: (a) curves 1 and 2 measured between adjacent pairs of contact dots on unetched surface (contact resistivity approximately 1 x 10 -3 ~ cm2); (b) curves 1, 2 and 3 measured between adjacent etched mesas, curves 1 and 2 use the same contact pairs as curves 1 and 2 in (a). The Ge-GaAs interfaces contribute no measurable resistance increases.
Structures consisting evaluated. Figure 7 shows one such structure. Figure before the etching process.
of n+-GaAs/p+-Ge/p+-GaAs have been grown and the c u r r e n t d e n s i t y vs. v o l t a g e c h a r a c t e r i s t i c of 7(a) s h o w s m e a s u r e m e n t s m a d e b e t w e e n c o n t a c t s Figure 7(b) shows the measurements made after
248 etching through the germanium into the p+-GaAs substrate, and curves 1 and 2 were measured between the same pairs of contacts as curves 1 and 2 in Fig. 7(a). There is no measurable change in the current density vs. voltage curves before and after etching. Contact resistivity dominates the measurement and may be as much as several orders of magnitude greater than the indicating that the interface specific resistivities total less than 1 × 10 4 ~2 cm 2. This value surpasses the requirement of the cascade structure and renders the germanium sandwich structure potentially the most viable IOC yet described for two-terminal monolithic devices.
5. Description of selective germanium epitaxy To pattern germanium layers, wet chemical etching has been used in earlier research. The p+-Ge etches extremely slowly which enhances undercutting during the pattern formation. On occasion, the germanium fingers have been lost. To improve the pattern formation, a selective germanium epitaxial growth technique has been developed. In this experiment, SiO: masks 3000 A thick are grown at 240 °C using plasma-assisted chemical vapor deposition onto polished GaAs surfaces. Windows are defined by standard photolithographic techniques. After the germanium growth, samples are dipped in the buffered HF to lift off unwanted material, leaving the patterned germanium as shown in Fig. 8. A magnified scanning electron micrograph of the surface can be seen in Fig. 9. The selectivity of germanium growth on the exposed substrate compared with that on the SiO: mask depends on the growth temperature. Good selectivity is obtained at growth temperatures below 570 °C. Above 570 °C, non-reproducibility is observed. This may be due to the oxide contamination on the substrates as reported by Ishii e t al. [10]. Further study is needed to identify the major problems affecting the selectivity.
Mag = 280x Fig. 8. Micrograph showing the surface of a patterned germanium layer selectively grown on a GaAs substrate. (Magnification, 280x .)
249
PH-_ Ge
P~--GaAs
Fig. 9. Scanning electron micrograph of a germanium sample selectively grown on a GaAs substrate.
To test the germanium interconnects, GaAs p - n b o t t o m cells were grown on p+-GaAs substrates with patterned germanium interconnecting layers. The cells show photovoltaic action and current flow through the germanium interconnects because the p-type substrates and n+-GaAs buffer layers produce a potential barrier. The current density v s . voltage characteristic for such a cell is given in Fig. 10 for dark and illuminated conditions. The junction quality, however, is poor because the overgrowth conditions of the GaAs b o t t o m cell have not been optimized.
'1 Curve 1. 1 sun Ver: 0,5 mA/div Hor: 0,5 V/div Curve 2. 7 suns Ver: 2.0 mNdiv Hor: 0,5 V/div Fig. 10. Current density v s . voltage curve for a GaAs p - n b o t t o m cell grown on a p+-GaAs substrate with a patterned germanium IOC: Voc=0.48 V (1 Sun) and 0.61 V (7 Suns); J s c = 1 4 m A cm -2 (1 Sun); cell area, 0.029 cm 2.
250 To summarize, good germanium selective epitaxial growth has been achieved at growth temperatures lower than 570 °C. The current conduction through a patterned structure has been clearly demonstrated. The overgrowth of the GaAs on germanium needs to be improved.
6. Development of cascade cells using germanium interconnects The next stage in germanium IOC development is clearly the demonstration of high efficiency cascade solar cells. The first steps towards this end have been taken. In this section an unambiguous demonstration of voltage addition in a cascade cell is presented. The perceived problem areas which must be addressed if projected efficiencies are to be realized are also identified. In earlier work, poor understanding and control of the magnesium source used for p-type doping in GaAs and A1GaAs prevented growth of good quality junctions and hampered attempts to provide a clear-cut cascade demonstration using two-layer germanium IOCs. A cell for which spectral response measurements suggested collection from the two junctions failed to show the characteristic double breakdown of the cascade structure. The low Voc (about 1 V) kept a single junction as a possible voltage source rather than two poor junctions showing voltage addition. Solving the magnesium-related problems led to improved junction growth for top and b o t t o m cells, and cascade structures have been attempted which employ the Ge/GaAs sandwich IOC. Unambiguous cascade action has been observed. The dark and illuminated current density v s . voltage curve for the best of these cells is shown in Fig. 11. The cell area is 0.19 cm 2 and the illumination (from a filtered solar simulator) is about 7 Suns at AM 0. Voc = 1.44 V, a value greater than any single-junction measurement made in our laboratory and one which could only result from voltage addition of the two junctions. The reverse characteristic confirms the observation that double breakdown is present, with the A1GaAs and GaAs junctions breaking down at 2 V and 5 V respectively. Major performance limitations arise from the A1GaAs junctions. No significant optimization has yet been attempted. The major problem is thought to be moisture in the AsH 3. The growth temperature for the A1GaAs in the best cascade cell was 700 °C without moisture-removal apparatus on the AsH3 gas line. Growth above 750°C, where the moisture-related problems ameliorate, results in very poor overgrowth on the germanium. The cascade cells using 750 °C for the A1GaAs growth show double breakdown, but Voc values are lower. Poorer quality A1GaAs junctions grown on the germanium are believed to be responsible. This indicates the need to achieve quality A1GaAs growth at temperatures below 700 °C and is one of the major focal points of our present research efforts. To summarize the main points, cells showing unambiguous cascade action have been grown and A1GaAs junctions, grown at low temperature
251
/
Forward: 0.5 V/div 0.1 mA/div Reverse: 1.0 V/div 1.0 mA/div
Fig. 11. Current density v s . voltage curve of cascade solar cell using n+-OaAs-p+-Ge-p+GaAs IOC: Voc=1.44 V (7 Suns);Jsc=l.5 mA cm 2;cell area, 0.19 cm 2.
without moisture scrubbing on gas lines, limit device performance at the present time.
7. Conclusions In this paper three major results have been described. Firstly, resistivity measurements indicate that the n+-GaAs/p+-Ge/p+-GaAs structures are viable IOCs, potentially to concentrations as high as 1000 Suns. Secondly, germanium interconnecting patterns have been reproducibly achieved using selective epitaxy. Thirdly, cascade cells using the germanium IOC unambiguously show voltage addition. The factors limiting the germanium growth and processing and the A1GaAs growth have been identified. For germanium, the main problem is optimizing GaAs overgrowth. A1GaAs problems are (i) moisture removal from reactant gas lines, and (ii) developing low growth temperatures without sacrificing material quality. As the technology improves and these problems are solved, the cascade structure using germanium IOCs may well fulfill the modeled predictions for high efficiencies.
Acknowledgments This research is funded by the Solar Energy Research Institute under Subcontract XL-4-03032-6. This support is gratefully acknowledged.
252 T h e a u t h o r s are g r a t e f u l f o r t h e a s s i s t a n c e o f G. F o u n t a i n , R. P i c k e t t , T. S. C o l p i t t s a n d G. S o l o m o n f o r m u c h o f t h e l a b o r a t o r y w o r k a n d t o S. R a y f o r m a n u s c r i p t p r e p a r a t i o n .
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