- Email: [email protected]
OBIC and EBIC investigations on GaAs shallow homojunction solar cells
Solar Energy Materials 17 (1988) 457-469 North-Holland, Amsterdam
457
OBIC AND EBIC INVESTIGATIONS ON GaAs SHALLOW HOMOJUNCTION SOLAR CELLS A. CAMANZI, A. PARRETTA Eniricerche SpA, Via E. Ramarini 32, 00015 Monterotondo, Rome, Ita~
M. GAROZZO and M° VITTORI ENEA, C~vaccia, Rome, Ita~ Received 10 December 1987; in revised form 22 February 1988 Very shallow p + - n GaPs homojunction solar cells are prepared on both bulk and deposited thin film materials by utilizing a solid state diffusion process, and investigated by optical and electron beam induced current techniques (OBIC and EBIC respectively). Some specimens are also analyzed by the scanning transmission technique (STEBIC). A spatially resolved spectral scanning apparatus is used in the OBtC experiments. Local (50-80/xm) spectral responses are worked out in order to achieve a quantitative estimate of the relevant cell parameters, such as junction depth and minority carrier diffusion length. Data are reported for two shallow homojunctions quite homogeneous in depth. A large spreading of the minority carrier diffusion length values (Lp--0.02-0.8/tin) results from the analysis of bulk devices; higher and more homogeneous values are obtained on thin films (Lp=1.2-2.3 /~m). Very small regions ( < 1 /tm) of electrical recombination are detected by EBIC and STEM-STFB!C and compared with their morphologies. The electrically active defects observed in epitaxial films are not generally correlated to dislocations or stacking faults; they seem rather due to local off-stoichiometry or not-resolved impurity clusters. Moreover, the density of these defects in epitaxial fdms is lower than in bulk specimens. This fact can explain the superior quality of the shallow junctions in epitaxially grown layers in comparison with the commercial single crystal ones. Homogeneity and high doping level of diffused layers prove that this sofid state diffusion process is suitable for realizing high efficiency solar ceils.
1. Introduction GaAs is a widely employed material for high-efficiency solar cells and more generally for electronic devices fsee, e.g., ref. [l]). Shallow homojunctions singlecrystal solar cells show conversior, efficiencies higher than 22~ [2]; moreover, with new developments, it is expected to reach efficiencies very close to the theoretical value (27~). The need of low cost and high quality devices has led to the investigation of thin film active structures grown on low cost silicon substrates (see~ e.g., refs. [3-5]) and on reusable GaAs substrates (CLEFT process) [6]. Diffused GaAs shallow homojunction p+-n structures have been obtained by Borrego et al. [7]. We have recently prepared thin film devices by employing a simple open-tube Zn solid state diffusion method [8]~ Conversion effici~-~;e~ ~dgher than 14~ were obtained on 1 cm2 epitaxially grown films. High carrier collection 0165-1633/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
458
A. Camanzi et oL / OBIC and EBIC study of OaAs homojunction solar cells
efficiencies resulted for very thin (<0.1 ~tm), highly doped (NA = 1 0 2o cm -3) emitter layers. In this work we show that the diffused layer thickness is uniform enough on the whole sanlple surface, even for very thin p + layers ( < 0.1/tm). The relationship between ~he photovoltaic response and the structural properties was investigated. Optical and electron beam induced current techniques (OBIC and EBIC) allowed us to locally estimate both junction depth (Xj) and minority carrier diffusion length (Lp). The quality of p+-n thin epitaxial films is compared with that of bulk ones. Moreover the recombination center density in both kinds of materials was determined. Electrical recombination maps were obtained by EBIC on Schottky diode~ in order to evaluate the intrinsic material features. Thinned specimens were examined in a scanning transmission electron microscope by STEM-STEBIC, directly comparing the electrical activity in the induced current mode and the crystal defects in the diffraction contrast mode at high resolution [9].
2. Experimental In the present work two kinds of specimens were investigated: (100) oriented, n-doped (Te; N D ffi 6 × 1016cm-3) GaAs single-crystal from MCP and thin n-doped (S) GaAs epitaxial films. The latter were grown by an MO-CVD process ou n "~(Te; N D - - 5 × 10 ls cm-3), (100) oriented 2 ° off towards the (110) direction, GaAs single-crystal substrates from MCP. Both kinds of specimens were processed by sofid state diffusion in order to obtain p + (Zn) doped shallow junctions. Details on preparation of the epitaxial films, doping process and electrical contact deposition have been previously reported [8]. The top contact grid on very thin emitters ( < 20 nm) used Pd instead of electroplated gold to avoid any shunt effect. Palladium was electro-less plated at room temperature by using the following solution: 0.5 g of PdCl2 with 50 ml HCI and 950 ml H20 [10]. OBIC measurements were performed by the fight scanning spot apparatus already described [11]. The light beam was generated by a 400 W quartz iodine lamp and focused on the cell at a size of 50-80/zm in diameter. Local spectral responses and photocurrent profiles were recorded on a plotter. EBIC was carried out on Schottky diodes manufactured by evaporating 0.6 mm wide gold dots through a mica mask. The electrodes were thin, generally less than 50 rim. The specimens were examined with the surface perpendicular to the electron beam. The thinning procedure and Schottky barrier preparation, suitable for STEBIC observations, have been previously described [12]. Before gold evaporation, every specimen was submitted to the same preliminary cleaning techniques in HCI: H20 -- 1 : 10 and H2SO4 : H 2 0 : H 2 0 2 -- 1 : 5 : 1. A 150MK-2 Cambridge SEM operating up to 40 kV and a JEOL 200 B TEM-STEM were used to obtain EBIC and STEBIC images respectively.
A. Camanzi et el. / OBIC and EBIC study of GaAs homojunction solar cells
459
3. Results and discussion
3.1. OBIC measurements The analysis of charge collection in GaAs p+-n shallow homojunction solar cells was extensively carried out in our previous work [8]. The internal spectral response in shallow homojunctions depended only on the parameters Xj, Lp and Sn/'Itn, where Sn and Pn are the surface recombination velocity and mobility in the p-side, respectively. The spectral response is differently affected by Xj, Lp and Sn//tn hi various spectral regions. These parameters can therefore be determined by fitting the data in the wavelength range 350-900 nm [8]. In this work we apply this method to perform measurements of Lp and Xj on selected specimen points. The photoresponse is ~es2e~ntially affected by Xj in the blue region and by Lp in the red. As a result it is possible to select two proper wavelengths in o~der to quicHy check the variability of these parameters througD~out the cell. The internal quantum efficiency at 450 nm versus Xj (fig. 1) does not depend practically on Lp, when values between 0.82 and 3 pm are used. The same quantity calculated at 850 nm as a function of Lp (fig. 2) does not change much by varying Xj. Following this approach Xj and Lp c a n be separately evaluated. Fig. 3 shows a typical photocurrent lateral profile recorded at 450 and 850 nm on a bulk sample. Some strong current drops are observed, affecting both curves in a
1,0
5
I
I
0.8
B
i
'
)" = 450 nm
0.6
tu
0.4
O
0.2
0.0
0
50
100 JUNCTION DEPTH
150
200
250
X I(nm)
Fig. 1. Calculated internal quantum efficiency at ~ = 450 nm versus X~ for a bulk device with ND=6X10 l~ cm -3 and $ . / ~ . = 1 . 5 X 1 0 4 V cm -1. The curve is practically independent of Lp for values between 0.2 and 3 Fro.
A. Camanzi et aL / OBIC and £BIC study of GaAs homojunction solar cells
460
1.0
,
,
,
L= 8 5 0 n m 0.8 (3
Z i
0.6
u. u. I&J
:E 0.4 I-Z 9C
0
0.2
0.0
~0
.25
.5 DIFFUSION
.75
LENGTH
1.0
Lp(/m)
Fig. 2. Calculated internal quantum efficiency at A = 850 nm versus Lp for a bulk device with N D ----6 × 1016 ,~tn- 3 a n d S a / / t n : 1.5 × 10 4 V c m - t. A little dependence on Xj is shown.
different way. For a better understanding of such a behaviour the full spectral response has been performed on several selected points along the photocurrent profiles, for a more careful measurement of Xj and Lp. The values of Xj resulted to be very low and practically constant (10 nm) on the whole sample. Lp values (see table 1) are high enough for points belonging to flat zones of the photocurrent profiles and quite similar ( - 0.5/tm) to that obtained by illuminating the whole cell. On the other hand, very low Lp values were recorded for
=J ---
w
IZ u4
~. = 4 5 O n t o ). = 8 5 0 n m
o,2
_~.,
]
R,
'N .
W U
0 Z
a_
~j I
|
'
I
I
i
i
X
Fig, 3. Photocurrent lateral profdes of the bulk device recorded at two different wavelengths (450 and 850 nm). The x axis indicates an in-plane spatial coordinate normal to the central bar of the grid, The vertical dotted lines indicate the points at which the spectral response was made~
A. Camanzi et al. / OBIC and EBIC study of 6aAs homojunction solar cells
461
Table 1 Photocurrent at k = 850 nm for some points of the same bulk device as in fig. 3 Point
Photocurrent (a.u.)
Lp (~m)
A1 A2 B1 B2 B3 C1 C2 C3 D1 D3
102 44 97 39 37 37 90 116 20 96
0.43 0.05 0.632 0.02 0.03 0.02 0.392 0.808 0.02 0.376
The same letter is used for points on the same segment. The Lp values are reported as obtained from the best fit of the internal spectral responses. Xj resulted to be constant and equal to 10 nm.
points where the photocurrent profiles collapse. For such points, the internal spectral responses are not well fitted using the nominal donor concentration ND ( N D ---6 X 1016 c m - 3 ) , while a better result is obtained using higher ND values ( N D = 6 X 1017 c m - 3 ) . This assumption agrees with the strong reduction of the photocurrent profile occurring also at 450 nm in spite of the fact that a constant value of Xj is always obtained. This behaviour can be explained by a local junction shunt arising from the tunneling mechanism, allowed by the high value of the donor concentration. Fig. 4 shows the experimental quantum efficiency at 850 nm versus
1D ~. : 8 5 0 nm 0,8
0.4
O
0.2
0.0
0.0
I .25
! .5
! .75
1,0
DIFFUSION LENGTH LO(~ m) Fig. 4. Experimental quantum efficiency data obtained at ~. = 850 nm from the internal spectral responses made at the points of table 1 (bulk device). The theoretical curve of fig. 2 is shown for comparison.
462
A. Camanzi et al. / OBIC and EBIC study of Gads homojunction solar cells
E4 I ,,1~
¢i w I-
zt u n,
EI2
• 't ! ~.* 1%#41.
. . ~
E3 ¢
'
'
~4rt
i
i
..~ll
I
I
t t
/
~
J .-=,,
I.*-.
II, I
!
)1
I:
~t
=6SOma
I
ee
0 0 I-
0 z ~L
X
Fig. 5. Photocurrent lateral profiles of the thin film device recorded at two different wavelengths (450 and
850 nm).
the Lp values. A good agreement between the experimental points and the theoretical curve is obtained. The photocurrent profiles for epitaxial film devices appear more uniform than for bulk ones. Fig. 5 shows a typical result. In this case, it was possible to achieve a good fit of the experimental spectral responses just using the same N v value, (1-2) × 1017cm-3 (see table 2). The Xj values are typically in the range 70.0-100.0 nm. The smaller values at the sample edges (points E4, V3, G2) are due to the edge effect of the doping source. The L p values (1.2-2.3/zm), are higher than in the bulk device~ in spite of higher donor concentration. Epitaxial material is more uniform from the point of view of transport properties.
Table 2 Photocurrent at k -- 450 and 850 nm for some points of the same thin film device as in fig. 5 Point
Photocurrent (a.u.)
Xj (nm)
k == 450 n m
E1 E2 E3 E4 F1 F2 F3 G1 G2
114 103 11~ 162 116 124 170 115 157
Photocurrent (a.n.)
Lp (/tm)
k = 850 n m
100 90 70 10 90 70 12 70 24
160 148 133 123 162 132 127 133 130
1.85 2.3 1.39 1.19 2.09 1.38 1.24 1.50 1.37
The same letter is ,Jsed for points on the same segment. The Xa and Lp values are reported as obtained from the best fit of the internal spectral responses. The points E4, F3 and G2 are near the sample edge.
A. Canmnzi et al. / OBIC and EBIC study of 6oAs homojunction solar cells
463
Fig. 6. A typicalthermalpit observedon the GaAs surface,after doping. 3.2. S E M - E B I C
observations
SEM channeling patterns on the epitaxial layers show, besides the same orientation as the substrate, their high crystallographic quality. Before doping, but after light chemical etching for a short time, the surface of both kinds of specimens was h i l l y specular and smooth. After Zn doping, the most frequently observed defects are thermal pits due to a thermal etching (see fig. 6); their concentration in epitaxial layer devices is three orders of magnitude lower than in MCP bulk ones, where they frequently appear accumulated (table 3). All morphological defects behave as charge carrier recombination centers, when observed in the EBIC mode; inhomogeneity regions appear as brighter areas. Very deep defects are found in Schottky diodes prepared otx MCP material. The recombination centers seem to be associated to crystallographic defects, such as stacking faults and dislocations [13]. The EBIC contrast increases with the accelerating voltage and consequently with the beam penetration, showing their bulk nature (figs. 7a, 7b, 8a and 8b). On the other hand, EBIC images on Schottky diodes
Table 3 Defects observedin the variousGaAs structures (the averagesize and density ~e reported as obtained by an image analysis) Defect
Averagesize (~m)
Density (cm-2)
Kind of structure
Thermal pit Thermal pit Dot-like Dot-like
2 2 2 20
5 m 10 2 5 x 10s 103
Shallowjunction on epitaxial film Shallowjunction on bulk crystal Schottkydiode on epitaxial film Schottkydiode on bulk crystal
10 6
464
A. Camanzi et aL / OBIC and EBIC study of Go.As homojunction solar cells
Fig. 7. Increase of the EBIC contrast with increasing beam energy: (a) 20 kV, (b) 38 kV.
manufactured on epitaxial material show dark "loops" (fig. 9), and their contrast is not typical of linear or extended defects present in bulk material [13,14]. Their density and average size are reported in table 3.
A. Camanzi et al. / OBIC and EBIC study of GaAs homojunction solar ceils
465
Fig. 8. (a) SEM image observed on MCP Schottky diode in secondary electron mode. (b) The same area observed in EBIC mode, at an energy of 40 kV. The a.rrows indicate powder particles on the specimen surface. Black dots are very deep recombination defects.
The higher concentration of thermal pits on MCP bulk specimens after doping is due to a higher density of crystalline defects; this is confirmed by the large number of "dot-like" recombination centers observed in Schottky structures.
466
A. Camanzi et ag / OBIC and EBIC study of Gads homojunction solar cells
4,ttm Fig, 9. Loop-shape recombination centers.
Some particular defects are observed in epitaxial structures which present a charge collection efficiency higher than background. Since their contrast is not inverted by increasing the beam energy, they cannot be interpreted as thickness variations of the emitter [151. They can be interpreted as doping defects and further investigation of this hypothesis is in progress. On the other hand, EBIC contrast inversion with increasing beam voltage is observed where the junction thickness is reduced on the thermal pits [161. In order to investigate the nature of the loop-like defects observed in epitaxial Schottky diodes, a deeper analysis has been carried out by STEM-STEBIC. Any STEM diffraction contrast was observed corresponding to the dark dots in STEBIC (figs. 10a and 10h). Consequently these recombhlation centers cannot be associated to extended or linear crystal defects. Their electrical contrast may arise from point defect dusters, with no detectable diffraction contrast. From the comparison between Schottky and homojuncfion devices it is possible to support the idea that the pits observed in the two kinds of specimens have a different origin. A detailed discussion about the STEBIC-STEM contrast on defects in GaAs, particularly in epitaxial layers, will be presented separately [17]. Here, we can only report that this technique allows a better spatial resolution, which results in a size of dark spots of 0.1/~m, an order of magnitude smaller than those in EBIC-SEM mode [121.
A. Camanzi et aL / OBIC and EBIC study of GaAs homojunct~.on solar cells
46/
Fig. 10. (a) Scanning transmission electron microscopy image of a thinned epitaxial structure. Only a scratch is visible. (b) Scanning transmission electron beam induced current image of title azea (s) showing loops, black dots and the scratch as electrical recombination centers.
468
A. Camanzi et al. / OBIC and EBIC study of 6oAs homojunction solar cells
4. Conclusions OBIC and spectral response measurements have been performed on GaAs shallow homojunction solar cells realized by a solid state diffusion process both on bulk and epitaxial material. Bulk commercial material seems to contain regions with a doping level much higher than the nominal value, giving rise to localized shunts. Epitaxial material shows much larger values of the minority diffusion length and is more homogeneous from the electrical point of view. OBIC photocurrent profdes match very well with the presence of electrical recombination centers as revealed by EBIC on Schottky diodes; their density corresponds to that of the thermal pits resulting from the diffusion process. Lower photocurrent values and higher inhomogeneity observed on bulk samples arise from their higher defect concentration. This also explains the smaller Lp values, probably limited by the impurity precipitation asscgiated with lattice defects. In bulk material the recombination centers can be easily correlated with linear and extended defects. The nature of electrical defects in epitaxial films has not been identified owing to the absence of TEM diffraction contrast and further investigations are required. The OBIC technique appears to be a really powerful tool for a fast characterization of the local device performance. The results are in good agreement with EBIC observations; however EBIC allows a better spatial resolution and depth locali7ation of the defects. A better comparison between the results from the two techniques can be achieved by improving the OBIC spatial resolution through the use of lasers as light sources. This direct correlation could resul~ in a deeper understanding of the recombination center nature. With this purpose, an important contribution can be obtained by STEM-STEBIC methods, allowing identification of the defect featu; ~ by diffraction contrast, even if the required small specimen thicknesses could introduce some experimental difficulties [17].
Acknowledgements The assistance of P. Alessandrini, V. Adoncecchi and G. Maletta in specimen preparing and characterizing was greatly appreciated.
References [1] See,for instance,J.C.C. Fan, SolarCells 12 (1984)51; A.Y. Cho, in: Proc. 17thIntern.Conf. on the Physicsof Semiconductors,San Francisco,CA, 1984, Ed. J.D. Chadiand W.A. Harrison(Springer,New York, 1985)p. 1515.
A. Camanzi et ai. / OBIC and EBIC study of Gads homojunction solar cells
469
[2] R.P. Gale, J.C.C. Fan, G.W. Turner and IlL. Chapman, in: Proc. 17th IEEE Photovoltaic Specialists Conf., Orlando (1984) p. 1422. [3] See, for instance, M. Yamaguchi, A. Yamamoto, Y. Itoh and T. Nishioka, in: Proc. 19th IEEE Photovoltaic Specialists Conf. (1986); I l Fischer, H. Morkos, D.A. Neumann, H. Zabel, C. Choi, O. Otsuka, M. Longerbone and L.P. Erickson, J. Appl. Phys. 60 (1986) 1640. [4] M. Garozzo, G. Conte, F. Evangelisti and G. Vitali, Appl. Phys. Letters 41 (1982) 1070. [5] M. Garozzo, D. MarBadonna and A. Parretta, Mater. Chem. Phys. 9 (1983) 157. [6] J.C.C. Fan, R.W. McClelland and B.D. King, in: Proc. 1711, IEEE Photovoltaic Specialists Conf., Orlando (1984) p. 31. [7] J.M. Borrego, IlP. Kecney, I.B. Bhat, K.N. Bhat, L.G. Sundaram and S.K. Ghandhi, in: Proc. 16th IEEE Photovoltaic Specialists Conf., San Diego {~982) p. 1157. [8] M. Garozzo, A. Parretta, G. Malet~% V. Adoncecchi and M. Gentili, Solar Energy Mater. 14 (1986) 29. [9] P.M. Petroff, D.V. Lang, J.L. Strudel and R.A. Logan, Scanning Electron Microsc. 1 (1978) 325. [10] C. Flores, Solar Cells 9 (1983) 169. ~11] L. De Angelis, E. Scaf~, F. Galluzzi, L. Fomarini and B. Scrosati, J. Electrochem. SOc. 129 (1982) 1237. [12] A. Camanzi, M. Vittori, A. Parretta and P. Aiessandrini, in: Proc. llth Intern. Congr. on Electron Microscopy, Kyoto (1986) p. 395. [13] D. Laister and G.M. Je~kins, J. Mater. Sci. 3 (1968) 584. [14] S.M. Davidson, J. Microsc. 110 (1977) 177. [15] D. Fathy and U. Valdr~, J. Microsc. Spettrosc. Elettron. 5 (1980) 175. [16] C. Donolato, Phys. Status Solidi (a) 65 (1981) 445. [17] A. Camanzi, M. Garozzo, A. Parretta and M. Vittori, to be published.
457
OBIC AND EBIC INVESTIGATIONS ON GaAs SHALLOW HOMOJUNCTION SOLAR CELLS A. CAMANZI, A. PARRETTA Eniricerche SpA, Via E. Ramarini 32, 00015 Monterotondo, Rome, Ita~
M. GAROZZO and M° VITTORI ENEA, C~vaccia, Rome, Ita~ Received 10 December 1987; in revised form 22 February 1988 Very shallow p + - n GaPs homojunction solar cells are prepared on both bulk and deposited thin film materials by utilizing a solid state diffusion process, and investigated by optical and electron beam induced current techniques (OBIC and EBIC respectively). Some specimens are also analyzed by the scanning transmission technique (STEBIC). A spatially resolved spectral scanning apparatus is used in the OBtC experiments. Local (50-80/xm) spectral responses are worked out in order to achieve a quantitative estimate of the relevant cell parameters, such as junction depth and minority carrier diffusion length. Data are reported for two shallow homojunctions quite homogeneous in depth. A large spreading of the minority carrier diffusion length values (Lp--0.02-0.8/tin) results from the analysis of bulk devices; higher and more homogeneous values are obtained on thin films (Lp=1.2-2.3 /~m). Very small regions ( < 1 /tm) of electrical recombination are detected by EBIC and STEM-STFB!C and compared with their morphologies. The electrically active defects observed in epitaxial films are not generally correlated to dislocations or stacking faults; they seem rather due to local off-stoichiometry or not-resolved impurity clusters. Moreover, the density of these defects in epitaxial fdms is lower than in bulk specimens. This fact can explain the superior quality of the shallow junctions in epitaxially grown layers in comparison with the commercial single crystal ones. Homogeneity and high doping level of diffused layers prove that this sofid state diffusion process is suitable for realizing high efficiency solar ceils.
1. Introduction GaAs is a widely employed material for high-efficiency solar cells and more generally for electronic devices fsee, e.g., ref. [l]). Shallow homojunctions singlecrystal solar cells show conversior, efficiencies higher than 22~ [2]; moreover, with new developments, it is expected to reach efficiencies very close to the theoretical value (27~). The need of low cost and high quality devices has led to the investigation of thin film active structures grown on low cost silicon substrates (see~ e.g., refs. [3-5]) and on reusable GaAs substrates (CLEFT process) [6]. Diffused GaAs shallow homojunction p+-n structures have been obtained by Borrego et al. [7]. We have recently prepared thin film devices by employing a simple open-tube Zn solid state diffusion method [8]~ Conversion effici~-~;e~ ~dgher than 14~ were obtained on 1 cm2 epitaxially grown films. High carrier collection 0165-1633/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
458
A. Camanzi et oL / OBIC and EBIC study of OaAs homojunction solar cells
efficiencies resulted for very thin (<0.1 ~tm), highly doped (NA = 1 0 2o cm -3) emitter layers. In this work we show that the diffused layer thickness is uniform enough on the whole sanlple surface, even for very thin p + layers ( < 0.1/tm). The relationship between ~he photovoltaic response and the structural properties was investigated. Optical and electron beam induced current techniques (OBIC and EBIC) allowed us to locally estimate both junction depth (Xj) and minority carrier diffusion length (Lp). The quality of p+-n thin epitaxial films is compared with that of bulk ones. Moreover the recombination center density in both kinds of materials was determined. Electrical recombination maps were obtained by EBIC on Schottky diode~ in order to evaluate the intrinsic material features. Thinned specimens were examined in a scanning transmission electron microscope by STEM-STEBIC, directly comparing the electrical activity in the induced current mode and the crystal defects in the diffraction contrast mode at high resolution [9].
2. Experimental In the present work two kinds of specimens were investigated: (100) oriented, n-doped (Te; N D ffi 6 × 1016cm-3) GaAs single-crystal from MCP and thin n-doped (S) GaAs epitaxial films. The latter were grown by an MO-CVD process ou n "~(Te; N D - - 5 × 10 ls cm-3), (100) oriented 2 ° off towards the (110) direction, GaAs single-crystal substrates from MCP. Both kinds of specimens were processed by sofid state diffusion in order to obtain p + (Zn) doped shallow junctions. Details on preparation of the epitaxial films, doping process and electrical contact deposition have been previously reported [8]. The top contact grid on very thin emitters ( < 20 nm) used Pd instead of electroplated gold to avoid any shunt effect. Palladium was electro-less plated at room temperature by using the following solution: 0.5 g of PdCl2 with 50 ml HCI and 950 ml H20 [10]. OBIC measurements were performed by the fight scanning spot apparatus already described [11]. The light beam was generated by a 400 W quartz iodine lamp and focused on the cell at a size of 50-80/zm in diameter. Local spectral responses and photocurrent profiles were recorded on a plotter. EBIC was carried out on Schottky diodes manufactured by evaporating 0.6 mm wide gold dots through a mica mask. The electrodes were thin, generally less than 50 rim. The specimens were examined with the surface perpendicular to the electron beam. The thinning procedure and Schottky barrier preparation, suitable for STEBIC observations, have been previously described [12]. Before gold evaporation, every specimen was submitted to the same preliminary cleaning techniques in HCI: H20 -- 1 : 10 and H2SO4 : H 2 0 : H 2 0 2 -- 1 : 5 : 1. A 150MK-2 Cambridge SEM operating up to 40 kV and a JEOL 200 B TEM-STEM were used to obtain EBIC and STEBIC images respectively.
A. Camanzi et el. / OBIC and EBIC study of GaAs homojunction solar cells
459
3. Results and discussion
3.1. OBIC measurements The analysis of charge collection in GaAs p+-n shallow homojunction solar cells was extensively carried out in our previous work [8]. The internal spectral response in shallow homojunctions depended only on the parameters Xj, Lp and Sn/'Itn, where Sn and Pn are the surface recombination velocity and mobility in the p-side, respectively. The spectral response is differently affected by Xj, Lp and Sn//tn hi various spectral regions. These parameters can therefore be determined by fitting the data in the wavelength range 350-900 nm [8]. In this work we apply this method to perform measurements of Lp and Xj on selected specimen points. The photoresponse is ~es2e~ntially affected by Xj in the blue region and by Lp in the red. As a result it is possible to select two proper wavelengths in o~der to quicHy check the variability of these parameters througD~out the cell. The internal quantum efficiency at 450 nm versus Xj (fig. 1) does not depend practically on Lp, when values between 0.82 and 3 pm are used. The same quantity calculated at 850 nm as a function of Lp (fig. 2) does not change much by varying Xj. Following this approach Xj and Lp c a n be separately evaluated. Fig. 3 shows a typical photocurrent lateral profile recorded at 450 and 850 nm on a bulk sample. Some strong current drops are observed, affecting both curves in a
1,0
5
I
I
0.8
B
i
'
)" = 450 nm
0.6
tu
0.4
O
0.2
0.0
0
50
100 JUNCTION DEPTH
150
200
250
X I(nm)
Fig. 1. Calculated internal quantum efficiency at ~ = 450 nm versus X~ for a bulk device with ND=6X10 l~ cm -3 and $ . / ~ . = 1 . 5 X 1 0 4 V cm -1. The curve is practically independent of Lp for values between 0.2 and 3 Fro.
A. Camanzi et aL / OBIC and £BIC study of GaAs homojunction solar cells
460
1.0
,
,
,
L= 8 5 0 n m 0.8 (3
Z i
0.6
u. u. I&J
:E 0.4 I-Z 9C
0
0.2
0.0
~0
.25
.5 DIFFUSION
.75
LENGTH
1.0
Lp(/m)
Fig. 2. Calculated internal quantum efficiency at A = 850 nm versus Lp for a bulk device with N D ----6 × 1016 ,~tn- 3 a n d S a / / t n : 1.5 × 10 4 V c m - t. A little dependence on Xj is shown.
different way. For a better understanding of such a behaviour the full spectral response has been performed on several selected points along the photocurrent profiles, for a more careful measurement of Xj and Lp. The values of Xj resulted to be very low and practically constant (10 nm) on the whole sample. Lp values (see table 1) are high enough for points belonging to flat zones of the photocurrent profiles and quite similar ( - 0.5/tm) to that obtained by illuminating the whole cell. On the other hand, very low Lp values were recorded for
=J ---
w
IZ u4
~. = 4 5 O n t o ). = 8 5 0 n m
o,2
_~.,
]
R,
'N .
W U
0 Z
a_
~j I
|
'
I
I
i
i
X
Fig, 3. Photocurrent lateral profdes of the bulk device recorded at two different wavelengths (450 and 850 nm). The x axis indicates an in-plane spatial coordinate normal to the central bar of the grid, The vertical dotted lines indicate the points at which the spectral response was made~
A. Camanzi et al. / OBIC and EBIC study of 6aAs homojunction solar cells
461
Table 1 Photocurrent at k = 850 nm for some points of the same bulk device as in fig. 3 Point
Photocurrent (a.u.)
Lp (~m)
A1 A2 B1 B2 B3 C1 C2 C3 D1 D3
102 44 97 39 37 37 90 116 20 96
0.43 0.05 0.632 0.02 0.03 0.02 0.392 0.808 0.02 0.376
The same letter is used for points on the same segment. The Lp values are reported as obtained from the best fit of the internal spectral responses. Xj resulted to be constant and equal to 10 nm.
points where the photocurrent profiles collapse. For such points, the internal spectral responses are not well fitted using the nominal donor concentration ND ( N D ---6 X 1016 c m - 3 ) , while a better result is obtained using higher ND values ( N D = 6 X 1017 c m - 3 ) . This assumption agrees with the strong reduction of the photocurrent profile occurring also at 450 nm in spite of the fact that a constant value of Xj is always obtained. This behaviour can be explained by a local junction shunt arising from the tunneling mechanism, allowed by the high value of the donor concentration. Fig. 4 shows the experimental quantum efficiency at 850 nm versus
1D ~. : 8 5 0 nm 0,8
0.4
O
0.2
0.0
0.0
I .25
! .5
! .75
1,0
DIFFUSION LENGTH LO(~ m) Fig. 4. Experimental quantum efficiency data obtained at ~. = 850 nm from the internal spectral responses made at the points of table 1 (bulk device). The theoretical curve of fig. 2 is shown for comparison.
462
A. Camanzi et al. / OBIC and EBIC study of Gads homojunction solar cells
E4 I ,,1~
¢i w I-
zt u n,
EI2
• 't ! ~.* 1%#41.
. . ~
E3 ¢
'
'
~4rt
i
i
..~ll
I
I
t t
/
~
J .-=,,
I.*-.
II, I
!
)1
I:
~t
=6SOma
I
ee
0 0 I-
0 z ~L
X
Fig. 5. Photocurrent lateral profiles of the thin film device recorded at two different wavelengths (450 and
850 nm).
the Lp values. A good agreement between the experimental points and the theoretical curve is obtained. The photocurrent profiles for epitaxial film devices appear more uniform than for bulk ones. Fig. 5 shows a typical result. In this case, it was possible to achieve a good fit of the experimental spectral responses just using the same N v value, (1-2) × 1017cm-3 (see table 2). The Xj values are typically in the range 70.0-100.0 nm. The smaller values at the sample edges (points E4, V3, G2) are due to the edge effect of the doping source. The L p values (1.2-2.3/zm), are higher than in the bulk device~ in spite of higher donor concentration. Epitaxial material is more uniform from the point of view of transport properties.
Table 2 Photocurrent at k -- 450 and 850 nm for some points of the same thin film device as in fig. 5 Point
Photocurrent (a.u.)
Xj (nm)
k == 450 n m
E1 E2 E3 E4 F1 F2 F3 G1 G2
114 103 11~ 162 116 124 170 115 157
Photocurrent (a.n.)
Lp (/tm)
k = 850 n m
100 90 70 10 90 70 12 70 24
160 148 133 123 162 132 127 133 130
1.85 2.3 1.39 1.19 2.09 1.38 1.24 1.50 1.37
The same letter is ,Jsed for points on the same segment. The Xa and Lp values are reported as obtained from the best fit of the internal spectral responses. The points E4, F3 and G2 are near the sample edge.
A. Canmnzi et al. / OBIC and EBIC study of 6oAs homojunction solar cells
463
Fig. 6. A typicalthermalpit observedon the GaAs surface,after doping. 3.2. S E M - E B I C
observations
SEM channeling patterns on the epitaxial layers show, besides the same orientation as the substrate, their high crystallographic quality. Before doping, but after light chemical etching for a short time, the surface of both kinds of specimens was h i l l y specular and smooth. After Zn doping, the most frequently observed defects are thermal pits due to a thermal etching (see fig. 6); their concentration in epitaxial layer devices is three orders of magnitude lower than in MCP bulk ones, where they frequently appear accumulated (table 3). All morphological defects behave as charge carrier recombination centers, when observed in the EBIC mode; inhomogeneity regions appear as brighter areas. Very deep defects are found in Schottky diodes prepared otx MCP material. The recombination centers seem to be associated to crystallographic defects, such as stacking faults and dislocations [13]. The EBIC contrast increases with the accelerating voltage and consequently with the beam penetration, showing their bulk nature (figs. 7a, 7b, 8a and 8b). On the other hand, EBIC images on Schottky diodes
Table 3 Defects observedin the variousGaAs structures (the averagesize and density ~e reported as obtained by an image analysis) Defect
Averagesize (~m)
Density (cm-2)
Kind of structure
Thermal pit Thermal pit Dot-like Dot-like
2 2 2 20
5 m 10 2 5 x 10s 103
Shallowjunction on epitaxial film Shallowjunction on bulk crystal Schottkydiode on epitaxial film Schottkydiode on bulk crystal
10 6
464
A. Camanzi et aL / OBIC and EBIC study of Go.As homojunction solar cells
Fig. 7. Increase of the EBIC contrast with increasing beam energy: (a) 20 kV, (b) 38 kV.
manufactured on epitaxial material show dark "loops" (fig. 9), and their contrast is not typical of linear or extended defects present in bulk material [13,14]. Their density and average size are reported in table 3.
A. Camanzi et al. / OBIC and EBIC study of GaAs homojunction solar ceils
465
Fig. 8. (a) SEM image observed on MCP Schottky diode in secondary electron mode. (b) The same area observed in EBIC mode, at an energy of 40 kV. The a.rrows indicate powder particles on the specimen surface. Black dots are very deep recombination defects.
The higher concentration of thermal pits on MCP bulk specimens after doping is due to a higher density of crystalline defects; this is confirmed by the large number of "dot-like" recombination centers observed in Schottky structures.
466
A. Camanzi et ag / OBIC and EBIC study of Gads homojunction solar cells
4,ttm Fig, 9. Loop-shape recombination centers.
Some particular defects are observed in epitaxial structures which present a charge collection efficiency higher than background. Since their contrast is not inverted by increasing the beam energy, they cannot be interpreted as thickness variations of the emitter [151. They can be interpreted as doping defects and further investigation of this hypothesis is in progress. On the other hand, EBIC contrast inversion with increasing beam voltage is observed where the junction thickness is reduced on the thermal pits [161. In order to investigate the nature of the loop-like defects observed in epitaxial Schottky diodes, a deeper analysis has been carried out by STEM-STEBIC. Any STEM diffraction contrast was observed corresponding to the dark dots in STEBIC (figs. 10a and 10h). Consequently these recombhlation centers cannot be associated to extended or linear crystal defects. Their electrical contrast may arise from point defect dusters, with no detectable diffraction contrast. From the comparison between Schottky and homojuncfion devices it is possible to support the idea that the pits observed in the two kinds of specimens have a different origin. A detailed discussion about the STEBIC-STEM contrast on defects in GaAs, particularly in epitaxial layers, will be presented separately [17]. Here, we can only report that this technique allows a better spatial resolution, which results in a size of dark spots of 0.1/~m, an order of magnitude smaller than those in EBIC-SEM mode [121.
A. Camanzi et aL / OBIC and EBIC study of GaAs homojunct~.on solar cells
46/
Fig. 10. (a) Scanning transmission electron microscopy image of a thinned epitaxial structure. Only a scratch is visible. (b) Scanning transmission electron beam induced current image of title azea (s) showing loops, black dots and the scratch as electrical recombination centers.
468
A. Camanzi et al. / OBIC and EBIC study of 6oAs homojunction solar cells
4. Conclusions OBIC and spectral response measurements have been performed on GaAs shallow homojunction solar cells realized by a solid state diffusion process both on bulk and epitaxial material. Bulk commercial material seems to contain regions with a doping level much higher than the nominal value, giving rise to localized shunts. Epitaxial material shows much larger values of the minority diffusion length and is more homogeneous from the electrical point of view. OBIC photocurrent profdes match very well with the presence of electrical recombination centers as revealed by EBIC on Schottky diodes; their density corresponds to that of the thermal pits resulting from the diffusion process. Lower photocurrent values and higher inhomogeneity observed on bulk samples arise from their higher defect concentration. This also explains the smaller Lp values, probably limited by the impurity precipitation asscgiated with lattice defects. In bulk material the recombination centers can be easily correlated with linear and extended defects. The nature of electrical defects in epitaxial films has not been identified owing to the absence of TEM diffraction contrast and further investigations are required. The OBIC technique appears to be a really powerful tool for a fast characterization of the local device performance. The results are in good agreement with EBIC observations; however EBIC allows a better spatial resolution and depth locali7ation of the defects. A better comparison between the results from the two techniques can be achieved by improving the OBIC spatial resolution through the use of lasers as light sources. This direct correlation could resul~ in a deeper understanding of the recombination center nature. With this purpose, an important contribution can be obtained by STEM-STEBIC methods, allowing identification of the defect featu; ~ by diffraction contrast, even if the required small specimen thicknesses could introduce some experimental difficulties [17].
Acknowledgements The assistance of P. Alessandrini, V. Adoncecchi and G. Maletta in specimen preparing and characterizing was greatly appreciated.
References [1] See,for instance,J.C.C. Fan, SolarCells 12 (1984)51; A.Y. Cho, in: Proc. 17thIntern.Conf. on the Physicsof Semiconductors,San Francisco,CA, 1984, Ed. J.D. Chadiand W.A. Harrison(Springer,New York, 1985)p. 1515.
A. Camanzi et ai. / OBIC and EBIC study of Gads homojunction solar cells
469
[2] R.P. Gale, J.C.C. Fan, G.W. Turner and IlL. Chapman, in: Proc. 17th IEEE Photovoltaic Specialists Conf., Orlando (1984) p. 1422. [3] See, for instance, M. Yamaguchi, A. Yamamoto, Y. Itoh and T. Nishioka, in: Proc. 19th IEEE Photovoltaic Specialists Conf. (1986); I l Fischer, H. Morkos, D.A. Neumann, H. Zabel, C. Choi, O. Otsuka, M. Longerbone and L.P. Erickson, J. Appl. Phys. 60 (1986) 1640. [4] M. Garozzo, G. Conte, F. Evangelisti and G. Vitali, Appl. Phys. Letters 41 (1982) 1070. [5] M. Garozzo, D. MarBadonna and A. Parretta, Mater. Chem. Phys. 9 (1983) 157. [6] J.C.C. Fan, R.W. McClelland and B.D. King, in: Proc. 1711, IEEE Photovoltaic Specialists Conf., Orlando (1984) p. 31. [7] J.M. Borrego, IlP. Kecney, I.B. Bhat, K.N. Bhat, L.G. Sundaram and S.K. Ghandhi, in: Proc. 16th IEEE Photovoltaic Specialists Conf., San Diego {~982) p. 1157. [8] M. Garozzo, A. Parretta, G. Malet~% V. Adoncecchi and M. Gentili, Solar Energy Mater. 14 (1986) 29. [9] P.M. Petroff, D.V. Lang, J.L. Strudel and R.A. Logan, Scanning Electron Microsc. 1 (1978) 325. [10] C. Flores, Solar Cells 9 (1983) 169. ~11] L. De Angelis, E. Scaf~, F. Galluzzi, L. Fomarini and B. Scrosati, J. Electrochem. SOc. 129 (1982) 1237. [12] A. Camanzi, M. Vittori, A. Parretta and P. Aiessandrini, in: Proc. llth Intern. Congr. on Electron Microscopy, Kyoto (1986) p. 395. [13] D. Laister and G.M. Je~kins, J. Mater. Sci. 3 (1968) 584. [14] S.M. Davidson, J. Microsc. 110 (1977) 177. [15] D. Fathy and U. Valdr~, J. Microsc. Spettrosc. Elettron. 5 (1980) 175. [16] C. Donolato, Phys. Status Solidi (a) 65 (1981) 445. [17] A. Camanzi, M. Garozzo, A. Parretta and M. Vittori, to be published.