- Email: [email protected]
Bifacial, large-area silicon sensors for radiative energy signals
Sensors and Actuators,
15 (1988) 243 - 266
BIFACIAL, LARGEAREA ENERGY SIGNALS
243
SILICON SENSORS FOR RADIATIVE
ANDRE1 P. SILARD
Department
of Electronics,
Polytechnic
Institute,
Bucharest,
O.P. 16, 76206 (Romania)
GABRIBL NAN1
IPRS-B&wasa (Enterprise for Radio Components Bucharest, 72996 (Romania) (Received June 30,1987;
and Semiconductors),
in revised form October 15,1987;
accepted January 21,1987)
Abstract A comprehensive design/technological study was conducted with the aim of obtaining highquality, bifacial optical sensors with reproducible parameters on large-area n- and p- silicon wafers. Practical ways of attenuating the severe limitations imposed by different kinds of material (areal) inhomogeneities on the electro-optical performance of large-area singlecrystal silicon sensors for radiative energy signals are described theoretically and tested experimentally. Various procedures leading to a substantial increase of both the emitter and the base contributions to the generated photocurrent are implemented and discussed in detail. The test devices were processed on 2 and 3 inch commercially-available silicon and it was sought to minimize the cost of the cells. The combination of simple design/technological approaches described in this work has ultimately led to the development of low-cost, highquality large-area silicon sensors with good overall electro-optical performance as bifacial devices. The results of this work show clearly that simple, but adequately designed and processed devices fabricated on an industrial scale on large-area silicon wafers could possess parameters similar to those of sophisticated laboratory samples elaborated on small-area silicon chips. With the outlined design/technological approaches, the use of even cheaper, i.e., lower-grade, single-crystal, silicon could still yield large-area sensors for radiative energy signals with fairly acceptable electro-optical performance.
1. Introduction During the past few years substantial advances have been made in the field of large-area silicon sensors for radiative energy signals [l - 41. Both diffused and ion-implanted optical sensors fabricated on 2 inch singlecrystal silicon wafers [l - 41 have equalled the performance of sophisticated Elaevier Sequoia/Printed in The Netherlands
244
laboratory samples elaborated on small-area silicon chips. The reported breakthroughs stemmed from both novel design/technologycal approaches and from a better und~$~d~g of the peculiar physics governing the behaviour of ultra-shallow (0.1 pm to 0.4 pm) silicon junctions fl - 81. The directions of research that could further improve the electro-optical perforrn~~ of both diffused and ion-spited sensors have also been delimited and it was shown that the relative expensiveness of high-quality single-crystal silicon still remains a prohibitive factor in achieving a fairly balanced cost effectiveness in this area of optical sensors [ 1 - 41, A better exploitation of silicon wafer is obtainable in bi&ciaf optical sensors, whose many-fold advantages in several applications (e.g., static and quasistatic concentrating systems, luminescent concentrators, etc.) have been demonstrated in connection with the development of p+-n-n+ devices 5 cm2 in area [9]. The purpose of the present work is to demonstrate the feasibility of high-quality, low-cost bifacial optical sensors with reproducible parameters on large-area (2 and 3 inch) ~ommerc~y~vailable n- and p- silicon wafers, wherein different kinds of areal inhomogeneities play an important limiting role [ 10 - 121. To this end, a comprehensive design/materials/technological study was conducted on lower-grade, #mrnerc~~y~v~ble, large-area, n- and psilicon wafers. Various procedures leading IXIa substantial increase of both the emitter and the base contributions to the generated photocurrent in p+-n-n+ and n+-p-p+ devices are described and tested ~per~e~~~y. Experiments concerning the influence of the anti-reflection (AR) layer, silicon quality, etc. on device performance were also conducted. The emphasis was placed on the min~ization of the sensor cost and the reproducibility of their electro-optical performance in the bifacial mode. The comb~ation of design/technological approaches described here has ultimately led to the development of l&w-cost, high-quality, large-area silicon sensors with good electro~pti~~ pe~orrn~~ as bifacial devices,
The following objectives have been pursued in the design of the developed p+-n-n+ and n+-p-p+ optical sensors: (I) increase of both the base and the emitter ~on~ibutions to the generated photocurrent; (2) decrease of the series resistance R, below the 100 mS2threshold; (3) att~uation of the impact of various ~p~f~tions of the front grid metallization on the device performance; (4) partial elimination of severe limitations imposed by area1 inhomogeneities, including the uniform collection of optically-generated carriers [3,4,10 - 121, on the electro-optical behaviour of the cells; (5) achievement of fairly good electro-optical performance of bifacial devices processed on commercially-available, lower-grade silicon.
245
To increase the base contribution to the generated photocurrent, gettering techniques built into the processing stages (see Section 3) have been used for both types of cells. Lifetime measurements (see Section 4) have shown that the diffusion length of the minority carriers in the corresponds bases exceed, as a rule, the base thickness, thus rendering the backside contacts efficient. Two main design/technological factors determine the features of the emitters in the developed cells. One of the factors is the peculiar impurity profile of the front junctions with a depth xj = 0.25 pm (Fig. 1) obtained by appropriate ~chnolo~cal means. The impurity profile for boron (B), with a shape close to that of the phosphorus (P) diffusion shown in Fig. 1 and described in detail in ref. 3, had the following features: (a) highdopant concentration (- 1021 cme3) in the proximity of the light-incident surface; (b) rapidly falling impurity concentration in the proximity of the aforementioned surface. The combined action of these features decreases the contribution of the front contact region to the series resistance R, of the cells without correspondingly increasing the number of recombinations (especially the Auger processes) in the surface region, a fact that was expected to yield a fairly good optical response of devices in the shorter wavelength portion of the incident light spectrum. To obtain the impurity profile of Fig. 1 for p+-n junctions, i.e., to obtain a high surface concentration without simultaneously evoking high doping effects, a boron (B) diffusion from a boron nitride (BN,) source with hydrogen injection similar to that described elsewhere [ 14, 151 was UfX?d.
The second factor was aimed at a drastic reduction of the impact of different kinds of area1 inhomogeneities, typical of large-area cells, on their electro-optical parameters. To this end, a novel ‘spider’s web’-type (Fig. 2) front grid metalhzation pattern was developed [3, 41 and optimized, according to the prevailing methodologies, for one sun insolation and for a p+- (n+-) emitter sheet resistance of - 200 Q/U. (The actual sheet resistance of the emitter layers was somewhat lower, thus leaving scope for further improvement of the front grid pattern and, hence, of the device performance.)
DISTANCE FFWl
SURFACE
Fig. 1. The impurity profile pattern for the front junctions in developed p+-n-n+ and n+-p-p* large-area silicon sensors for radiative energy signsls.
246
Fig. 2. Photograph of developed ‘spider’s web’-type metallization pattern for 3 inch silicon optical sensors (Scale 1 :l),
The developed contact pattern (Fig. 2) is inherently better suited for a circular silicon configuration in comparison with the classical ‘fishbone’type front grid metallization [l, 21 for the following main reasons: (a) it ensures a better collection of the photogenerated carriers from the periphery of the device; (b) it increases the autonomy in the spacing between the collecting pads by way of reducing both the coverage ratio (shading losses) and power losses on the lateral emitter resistance; (c) it minimizes the impact of local area1 degradation on the performance of the whole device in the illuminated mode [ 3, 41. The total shading losses were kept below 8%. 3. Fabrication 3.1. General considerations The test devices were fabricated on commercially-available 2 inch, (111>,Czochralski, 220 I_cmthick n-wafers, with a starting resistivity of 1.5 2.5 S2 cm and on 180 pm thick p-wafers, with a resistivity in the range 7 - 12 a cm. Both wafer types were lower-grade silicon with more than 5000 dislocations/cm2 (p-silicon) and w 2000 dislocations/cm2 (n-silicon). The 3 inch p+-n-n+ devices were fabricated on (ill), FZ, 230 pm thick n-silicon substrates with a starting resistivity in the range 1.5 to 2.5 LI cm and -2000 dislocations/cm 2. All phosphorus (P) diffusions were performed with a POC13 source. All boron (B) diffusions were performed with a BNs source with hydrogen injection. The combined action of these two processing steps in each single device acted as an excellent gettering, increasing substantially the minority carrier diffusion length in the corresponding bases of the devices.
247
The light-incident surface of the devices was passivated with a 100 A thick SiOz layer. As an antireflection (AR) coating, either 800 A SiO or 600 A TiOz layers were used. Standard Ti-Ag-Ni metallization was used for the fronts and backs of the wafers. Contact adherence was ensured by sintering the devices for 10 min at 450 “C in a nitrogen ambient. A 1 pm thick, 1 mm wide oxide layer was preserved on the circular periphery of the wafers in order to prevent shorts caused by edge diffusion. For bifacial cells, the back light-incident surface was processed in a manner identical to the fronts. A cross-section of the fabricated p+-n-n+ devices is presented in Fig. 3.
3 = 1.5 - 2.5 n.cm
Fig. 3. Cross-sectional structure of the developed 2 inch pa-n-n+ silicon optical sensors (see Sections 2 and 3 for the ptiameters of n+-p-p* devices).
3.2. Fabrication of p’-n-n+ sensors At the outset of the fabrication process, an initial total oxidation in wet O2 at 950 “C was performed in order to obtain a SiOz layer 1 pm thick. Following a deoxidation of the rear of the wafers, the backside n-n+ contact was formed by phosphorus diffusion at 850 “C from a POC13 source. The resulting depth of the n-n+ junction was -0.3 pm. To form the p+-n junction, the 1 pm thick oxide on the front surface was etched down, except for the 1 mm wide circular periphery of the wafers. After that, the BN was deposited on the surface. Subsequently, the wafers were introduced into a nitrogen/oxygen ambient at 820 “C to form Bz03. The temperature was raised to 920 “C in about 10 mm. A 1 min Hz/N2 injection at constant temperature followed to realize HB02. Subsequent drive-in diffusion was carried out in a N3 ambient for approximately 25 min. The temperature was then lowered to 820 “C in about 15 min. A wet oxidation in O2 ambient at 750 “C followed. A chemical etch in buffer solution was performed. To passivate the wafers, a 15 min wet N2 ambient oxidation was carried out. The sheet resistance of the p+-layer was in the range 120 - 150 St/Cl. The metallization and AR coating deposition followed.
248
3.3. Fabrication of n’-p-p’ devices The backside p-p+ contact was formed through a boron diffusion from a BN3 source with hydrogen injection according to the procedures oulined in Section 3.2. The fron n+-p junction was formed by phosphorus diffusion from a POCls source according to the procedure outlined in Section 3.2. for the backside contact of p+-n-n+ bifacial devices.
4. Characterization A Zeiss Jena quartz prism monochromator with a xenon arc lamp as light source (simulator of AM 1 spectrum with 100 mW/cm* insolation) coupled to a Tektronix 31/4661 data acquisition system was used to measure the relative spectral response (RSR) of the test devices (Figs. 4 and 5), from which the optical bandwidth B, measured at 50% S,,,, and the peak responsivity wavelength h, (Tables 1 -6) were determined. The values of the peak generated current I, under AM 1 conditions and those of the peak
mg qJL+4 B 80-
-MO
ti
70-
-70
=
60-
-60
3
!io-
-50
-90 -80
-Lo ul 30-
-30
$! 2004.w ,
-20 I 500 I
I
mI
700 I
I 800 L
n 900 I
(4
WAVELENGTH
(nm)
(b)
WAVELENGTH
(nm)
- 10 I1 IO00 4 1100 I <
J?ig. 4. The relative spectral response (RSR) of 2 inch, p+-n-n+ silicon optical sensors under front (a) and back (b) AM 1 illumination (100 mW/cm’ insolation) at 20 ‘C for SiO and TiOa antireflection layers, respectively.
249
do,
I
I
I 5M)
(b)
L
I I I a I I I &IO 700 mm ml WAVELENGTH (nm)
I
I Iwo
I
I llw
Fig. 5. The relative rrpectral response (RSR) of 2 inch, n*-p-p* silicon optical sexwors under front (a) and back (b) AM 1 illumination (100 mW/cm* insolation) at 20 “C for SiO and TiO2 antireflection layers, respectively.
TABLE
1
AM 1, 20 “C electro-optical characteristics, the main parameter6 of the dark currentvoltage diode curves and n-base lifetime 7~ of relevant test 2 inch p*-n-n+ silicon optical sensors under front illumination Device N 4a N 5a N9a N 18b N lgb N 20b N 21b N 23b
(-1
OW
h,
&n
p,
%
rB
n
J,
(pA/cm*)
i$I,cm*)
696 59s 586 59s 610 600 610 600
660 650 660 640 640 630 630 630
720 719 720 722 727 729 730 728
344 344 346 349 366 363 364 360
88 75 66 69 62 83 75 67
63 82 71 ’ 70 71 73 83 66
1.86 1.64 1.81 1.75 1.82 1.64 1.78 1.83
9.1 6.0 7.83 4.8 4.2 3.9 2.8 3.4
6.96 6.95 11.2 5.7 10.3 7.8 8.4 7.4
3
(mA)
a800 A thick SiO AR layer. b600 A thick TiO2 AR layer.
WV
bn)
(~4
260 TABLE 2 AM 1, 20 “C, electro-optical characteristics of relevant test 2 inch p*-n-n+ silicon optical sensors under backside illumination Device
N N N N N N N N
B
4a iTa ga
lBa lga 20b 21b 23b
ha
Inm)
(-1
505 525 520 540 545 545 540 540
900 900 850 850 850 900 850 850
An (mA)
pnl @WI
613 613 615 635 637 637 636 637
270 271 274 284 287 286 289 290
a800 A thick SiO AR layer. b600 A thick TiO2 AR layer. TABLE 3 AM 1, 20 “C electro-optical characteristics, the main parameters of the dark currentvoltage diode curves and p-base lifetime rn of relevant test 2 inch n+-p-p+ silicon optical sensors under front illumination Device P P P P P P P P
11a 17a 2oa 27b 2Bb 29b 30b 31b
B
?B
(nm)
(c(8)
545 530 560 575 570 575 575 570
660 650 670 640 640 630 620 620
685 694 686 700 694 698 699
317 319 319 329 330 332 329 331
57 87 81 77 73 69 71 63
72 55 65 63 74 57 63 71
n
I
dOd
dor
11.8 9.6 9.8 9.6 8.5 8.7 6.1 6.7
11.7 7.81 11.29 6.83 8.75 8.15 6.23 4.65
( PA/cm21 (nA/cm2)
1.91 1.65 1.77 1.76 1.52 1.47 1.92 1.66
a8OO A thick SiO AR layer. b600 A thick TiO2 AR iayer.
delivered power P, into a properly matched load impedance are also shown in Tables 1 - 6 for each of the representative sensors. Several parameters that determine the noise in the photodiode structures (series resistance R,, the dark current, the shunt resistance I&) have also been accurately measured (Fig. 6). The current density of the dark current-voltage curves, analytically expressed by 13,161
J = JOd ew(qv/kT) + JO,
eqWWkT)
(1)
served for the determination of the diode ideality factor II and the diffusion (JOd) and recombination (Jo,) components of the reverse saturation current density Jo according to the method outlined elsewhere [3, 16 1. The values of the series resistance R, were determined from the difference between the dark and AMl-illuminated I-V curves [16]. The base lifetimes rB were
251 TABLE 4 AM 1, 20 “C, electro-optical characteristics of relevant test 2 inch n+-p-p+ silicon optical sensors under backside illumination Device
B
(nm) P I.$& P 338
606
515 505 516 530 520 630 510
P2OB P27b P2Sb P 29b P 30b P 3$b
x,
Ll
Pill
(nm)
(mAI
bW)
900 900 850 850 850 850 850 850
589 596 604 605 620 605 628 621
250 251 263 263 277 271 278 278
a89Q A thick SiO AR layer. b600 A thick TiOz AR layer. TABLE 5 AM 1, 20 “C electrooptical characteristics, the main parameters of the dark currentvoltage diode curves and the n-base lifetime ~8 of relevant test 3 inch p+-n-n+ silicon opt&xl sensors under front illumination
&a 6* $b
686 575 570 576
ZJ b N &Ob N llb
590 580 575
N N N N
la
670 670 670 660 665 660 670 675
1693 1584 1579 1568 1583 1581 1573 1567
756 758 764 783 783 781 775 779
89 97 99 85 99 98 98 99
22.5 24.7 26.8 28.7 26.5 27.2 23.7 26.8
68 72 86 78 69 88 82 79
1.86 1.93 1.94 1.95 1.87 1.93 1.88 1.87
7.45 5.13 7.45 5.35 4.97 5.62 6.12 5.93
7.32 9.32 5.37 8.32 8.33 3.41 3.68 3.54
=BOOA thick SiO AR layer. b660 A thick TiO2 AR layer.
Fig. 6. The lumped equivalent circuit of silicon photodiode structures.
measured according to the OCVD method and Kingston’s method, respectiv’ely. The shunt resistake RSh is shown in Table 6 for 3 inch, p+-n-n+ devices by way of illustration. The measurements of electro-optical parameters were independently pelCformed at three different locations*, where a cross-calibration with reference devices was made.
252 TABLE 6 AM 1, 20 “C! electro-opticd characteristics of relevant test 3 inch p+-n-n+ silicon optical sensors under backside illumination Device
h
pill
(nm)
N N N N N N N N
la 3” Sa 7b 8b Sb lob lib
510 505 515 510 515 520 525 525
900
900 900 850 850 850 850 850
1358 1354 1359 1362 1363 1365 1368 1369
WV
Rs WQ
607 604 608 612 613 615 615 614
87 76 73 83 73 72 88 99
n
23.7 22.9 24.7 25.7 28.3 29.7 28.7 28.8
1.83 1.81 1.91 1.77 1.83 1.87 1.89 1.78
JOd
JOr
(pA/cm2)
(nA/cm’)
6.27 6.47 6.38 6.42 5.72 5.92 6.17 6.35
4.37 4.17 4.56 4.17 3.92 4.72 4.22 4.76
WOO A thick SiO AR layer. b600 A thick TiO2 AR layer.
5. Discussion The subsequent discussion is restricted to the most representative p+-n-n+ and n+-p-p+ silicon optical sensors. By eliminating both the several test units with the highest values of Im and I3 and those that failed to have both I, and B above the bottom values of Tables 1 -6, the selected devices reflect the dominunt trends observed amo~lg the fabricated 2 inch and 3 inch silicon optical sensors, The following peculiar features of the devices are noteworthy: (1) The devices possess a wide-band RSR in the bifacial mode: the optical bandwidth B, measured at 50% S,,, (Figs. 4 and 5) currently has values in the range 550 nm - 600 nm under AM 1 front illumination (Tables 1, 3 and 5), while values in the range 500 nm - 540 nm are commonplace for AM 1 backside illumination (Tables 2,4 and 6); (2) The peak responsivity wavelength A, (Figs. 4, 5, Tables 1,3,5) for all cells under front illumination is shifted toward the visible portion of the solar s-&rum. The wavelength h, is located around 600 - 669 nm, instead of 750 900 nm as in usual cells [16, 173 or in the backside-illuminated devices reported in this work. The aforementioned shift of X, enabled the fabricated devices to take greater advantage of the incident light power. (3) All samples possess a good blue response (X = 0.4 pm) accompanied by a relatively substantial quantum efficiency (QE) at longer wavelength (h > 1.0 pm) of the incident light under both front and backside AM 1 illumination (Figs. 4 and 5). An excellent QE is achieved for incident light in the wavelength range X = 0.6 firn to 0.9 pm. *IPRS-Bgneaaa (location of one of the authom), ICEFIZ (Central Imatitute its, Bucharest), ICPE (R and D Institute for Electrical Engineering, Bucharest).
of Phye-
263
(4) The use of a TiOz AR layer (instead of its SiO counterpart) increases the optical bandwidth of front-illuminated devices, primarily through the increase of quantum efficiency at longer wavelengths of the incident light. A shift of wavelength X, by 10 - 30 nm toward the visible portion of the light spectrum in the frontcilluminated cells using a Ti02 AR layer was recorded (Figs. 4, 5, Tables 1, 3, 5). The same shift of X, in backside-illuminated cells was roughly 50 nm (Figs. 4, 5, Tables 2, 4,6). (5) The optical features outlined in (4) enabled the peak generated current I, in both front- and back-illuminated cells using a TiOz AR layer to increase (Tables 1 - 6). (6) The test sensors possess a relatively large value of the minority carrier lifetime TB in the bases (Tables 1, 3 and 5), thanks to the boronand phosphorus-based gettering process built into the processing steps (Section 3). As a result, the devices with TB > 40 ps inherently had a diffusion length of the minority carriers in the bases (holes in the n-base of p+-n-n+ devices, electrons in the p-base of n+-p-p+ samples) equal to or larger than the silicon wafer thickness. This feature had a positive impact on the RSR parameters and on the values of the generated photocurrent. (7) The series resistance R, has been reduced to values below the 100 mS1 threshold (Tables 1 -6), which is a considerable achievement, especially in large-area optical sensors. This feature was mirrored by the enhanced value of the peak power P, delivered into a properly matched load. (8) The value of P, was somewhat limited by the quality of the starting silicon. This fact was also reflected in the somewhat higher than usual [3, 16.1 values of the diffusion component Jod (Tables 1,3 and 5) of the reverse saturation current density Jo, especially for n+-p-p+ devices. (9) The extremely high values of the peak generated current Im under AM 1 conditions (Tables 1 - 6) express the excellent responsivity R [ 173. The following factors endowed the fabricated devices with high values of 1, and R : (a) the relatively large optical bandwidth B; (b) the high quantu.W efficiency, both at short (0.4 I.crn- 0.5 pm) and longer (0.8 pm - 1.1 pm) wavelengths of the incident light; (c) the relatively large base lifetime rB, which has the result that a large amount of carriers, optically generated in the bulk of samples with high quantum efficiency, do actually survive’ ‘ and contribute to the photocurrent generated; (d) the implementation of the novel metallization grid (Fig. 2), a step that both attenuated the impact of area1 inhomogeneities on the performance of the cells and reduced the shading losses; (e) the increased contribution of the p+-emitter to the generated photocurrent, thanks to both the surface passivation and the features of the impurity profile (Fig. 1). It should be noted that test cells exhibited a higher than usual quantum efficiency for photons with energies E Q 1.2 eV and that any eventual losses in the blue response were more than compensated by an increase in the infrared response, especially at wavelengths X > 1 .O Mm (Figs. 4 and 5). These features of the developed devices behaviour as photovoltaic converters. Thus:
are correlated
with
then
254
(1) The p+-n-n+ cells have a reproducible conversion efficiency q = 1’7 18% under front AM 1 illumination and Q= 13 - 14% under back AM 1 illumination conditions. (2) The n+-p-p+ cells fabricated on lower-quality p-type silicon have a conversion efficiency q = 15.5 - 16.5% and 77= 12 - 13.2% under front and back AM 1 illumination conditions, respectively. Taking into account the cell size and the quality of the starting silicon, the obtained values of conversion efficiency q and the resulting values for the short-circuit density J,, (ratio of Im to cell area) compare well with the best data reported to date [l - 4,9,13,16,17]. 6. Concluding remarks All the reported results were obtained in an industrial environment, not under laboratory conditions. Thus, this work shows clearly that properly designed/processed silicon optical sensors elaborated on large-area, lowergrade n- and p-wafers can possess electro-optical parameters in the bifacial mode similar to those of sophisticated laboratory samples using small-area silicon chips. The performance improvements of large-area silicon sensors for radiative energy signals reported in this work could undoubtedly have a major beneficial impact on the further development of photovoltaic converters, light-sensitive devices, different types of photodetectors, etc. The outlined design/technological improvements are all the more important as they were tested on an industrial scale on large-area wafers and it is usually expected that ‘cell efficiency will generally decrease with increasing the cell size’ [ 181. To conclude, this work has demonstrated the feasibility of high-quality, low-cost bifuciul sensors for radiative energy signals with reproducible electro-optical parameters on 2 inch and 3 inch commercially-available silicon wafers. The implementation of usual refinements (textured surface, double AR layer, etc.) in conjunction with the design/technological approaches of this work could further improve the parameters of these sensors. Alternatively, the use of even cheaper silicon could still yield large-area sensors for radiative energy signals with fairly acceptable electroaptical performance under both front and backside illumination. Reference 1 A. P. Siiard, Silicon optical ~ensom fabricated through masked ion implantation, Sensors and Actuotons, 7 (1986) 177 - 187. 2 A. P. Silard, Ma&ed ion-implanted silicon solar cellaloptical se~oru, in Phyrice of Semiconductor Devices, World Scientific Publishing Co. : Philadelphia/Siiapore, 1986, pp. 184 - 193; A. P. Silard, Using silicon dioxide a~ device engineering parameter in the fabrication of photovoItaic cells through tided ion implantation, Sol. Energy Mater,. 14 (1986) 95 - 105. 3 A. P. Siiard, Electra-optical performance of large-area eilicon senoom for radiative energy signals, Sensors und Actuators, 12 (1987) 23 - 34. 4 A. P. Silard, F. Pera and G. Nani, Development of preferentiiy current-generating silicon solar celle, Appl Phys. Lett., 48 (10) (1986) 673 - 675; A. P. Silard, F. Pera
5
6 7 8 9 10 11
12 13 14 15 16 17 18
and G. Nani, High-efficiency, large-area p+-n-n* silicon solar cells, SolidStcrte Electron., 30 (1987) 397 - 401. A. P. Silard and M. J. Dut& Analyti of electric field in very shallow silicon junctions, Int. J. Electron., 61 (1986) 627 - 638. A. P. Shard, Analysis of electric field diitributions in ultra-shallow silicon junctions un&r thermodynamic eq&librium conditiona, Int. J. EZectron., 62 (1987), in press. A. P. Shard and M. J. Du$& Inveotigation of the majority carriers diffusion coefficients behavioural pattern in degenerately-doped dlicon, Int. b. Electron., 62 (19871, in press. A. P. Silard and M. J. Duw, The effective htrinsic concentration nie in degenerately doped silicon, J. AppZ. Phye., (Oct. 15,1987), in press. A. Cuevas, A. Luque, J. Eguren and J. de1 Alamo, High efficiency bifacial back surface field solar cells, SOL Cella, 3 (1981) 337 - 340. F. A. Lindhohn and C. T. Sah, Fundamental electronic mechanisms limiting the performance of solar cells, IEEE Trans. Electron Devices, ED-24 (1977) 299 - 304. F. A. Lindholm, J. A. Mazer, J. R. Davis and J. J. Arreola, Degradation of solar cell performance by areal inhomogeneities, Solid-State Electron., 23 (1980) 967 - 971. A. W. Penn, Large-area p/n junctions for the detection of nuclear radiation, in S. C, Jain and S. Radhakrishna (eds.), PhySiC8 of Semiconductor Devices, Wiley Eastern, New Delhi, 1982, pp. 84 - 95. M. A. Green, A. W. Blakers, J. Shi, E. M. Keller and S. R. Wenham, Hiihefficiency silicon solar cells, IEEE Tmns. Electron Devices, ED-31 (1984) 679 - 683. G. Pignatel and G. Quierolo, AES study of boron diffusion in silicon from a boron nitride source with hydrogen injection, J. Electrochem. Sot., 126 (1979) 1805 - 1810. E. Dominguez and M. Jaraiz, A non-Fickian model of the boron in silicon diffusion from BN source, J. Electrochem. Sot.. 133 (1986) 1895 - 1900. R. N. Hall, Silicon photovoltaic cells (Review paper), Solid-State Electron., 24 (1981) 595 - 616. S. M. Sze, Physics of Semiconductors Devices, Wiley, New York, 1981, pp. 681 - 816. M. A. Green, Zhao Jianhua, A. W. Biaksrs, M. Taouk and S. Narayanan, 25 - percent efficient low-resistivity silicon concentrator solar cells, IEEE Electron Device Lett., EDL-7 (1986) 583 - 585.
Biographies Gabriel Nuni was born in Bucharest, Romania, on March 22,1954. He graduated from the Department of Electronics, Bucharest Polytechnic Institute in 1979, with a thesis on power transistor thermal modelling. The same year he joined IPRS-Bgneasa (enterprise for Radio Components and Semiconductors) Bucharest, and has worked there ever since. His main areas of interest are power transistor design and silicon technology. In 1981 he went through a power transistors production management training program in the U.S.A. Since 1984 he has been involved in solar energy conversion research programs and has contributed to the development of large-area, all-diffused silicon solar cells/optical sensors. In 1986 he joined on a part-time basis the project concerning the development of doubleinterdigitated (TIL) power transistors. G. Nani hascoauthored over fifteen papers in refereed scientific journals and technical digests of professional conferences on silicon solar cells/ optical sensors and novel power transistors. He holds five patents in the area of power transistor design and silicon technology.
Andrei P. Silard was born in Timigoara, Romania, on April 30, 1944. He received the B.S. degree in electronic engineering in 1967 with the highest distinction following work on the measurement of thermal par&neters of thyristors. In 1976 he received the Ph.D. in electrical engineering from the Bucharest Polytechnic Institute (BPI) with a thesis consisting of a comprehensive theoretical/experimental investigation of the physical mechanisms in power amplifying gate thyristors. The same year he received the B.S. degree in history from Bucharest University. At the Electronics Research Institute in Bucharest (1967 - 73), he worked in the field of power circuit applications of thyristors, especially for induction heating, and was involved in the development and characterization of h.f. low-loss ferrites. In 1974 he joined the teaching staff of the Department of Electronics, BPI. Up to 1982 he initiated and completed several research projects, whose results led to a better understanding of electrothermal failure mechanisms in power rectifiers under surge conditions, of basic thyristor physics in various modes of operation and contributed to an optimized design of original light-activated thyristor structures. He is now teaching courses on electronic measurement and control apparatus and semiconductor power devices in the Department. Since 1978, he has pioneered research work on novel power semiconductor and photonic devices. He contributed to the inception and development of the double-interdigitated (TIL) concept, which resulted in the fabrication of a new class of electrothermally fail-safe power devices: bipolar transistors, gate turn-off (GTO) and gate-assisted turn-off (GAT) thyristors. Subsequently, he contributed to the development of high-voltage, ultra-fast turning-on TIL-type transistors with base wells (BW). His research contributed to the development of highquality solar cells/optical sensors on large-area (2 and 3 inch) silicon wafers, both through diffusion and ion implantation. His investigations have also led to the clarification and reformulation of several fundamental issues in the physics of ultra-shallow, heavily-doped silicon junctions. Dr. Silard has published over 100 papers in refereed scientific journals and technical digests of professional conferences (such as IEDM, IAS, ESSDERC, CSSDM, IECON, etc.) and has given invited lectures at international meetings on microelectronics in India and Brazil. He co-holds seven patents on power devices and ICs and has contributed to two books on power rectifiers. In 1983 he received the 1981 Technical Sciences Award of the Romanian Academy for contributions to the electrothermal investigation of power devicesand ICs. He has been a Senior Member of the IEEE since 1982. He has published extensively on the philosophy of -history, both in Romania and in the United States. He has authored over 20 studies and papers on the impact of science, particularly of electronics, on contemporary societies.
15 (1988) 243 - 266
BIFACIAL, LARGEAREA ENERGY SIGNALS
243
SILICON SENSORS FOR RADIATIVE
ANDRE1 P. SILARD
Department
of Electronics,
Polytechnic
Institute,
Bucharest,
O.P. 16, 76206 (Romania)
GABRIBL NAN1
IPRS-B&wasa (Enterprise for Radio Components Bucharest, 72996 (Romania) (Received June 30,1987;
and Semiconductors),
in revised form October 15,1987;
accepted January 21,1987)
Abstract A comprehensive design/technological study was conducted with the aim of obtaining highquality, bifacial optical sensors with reproducible parameters on large-area n- and p- silicon wafers. Practical ways of attenuating the severe limitations imposed by different kinds of material (areal) inhomogeneities on the electro-optical performance of large-area singlecrystal silicon sensors for radiative energy signals are described theoretically and tested experimentally. Various procedures leading to a substantial increase of both the emitter and the base contributions to the generated photocurrent are implemented and discussed in detail. The test devices were processed on 2 and 3 inch commercially-available silicon and it was sought to minimize the cost of the cells. The combination of simple design/technological approaches described in this work has ultimately led to the development of low-cost, highquality large-area silicon sensors with good overall electro-optical performance as bifacial devices. The results of this work show clearly that simple, but adequately designed and processed devices fabricated on an industrial scale on large-area silicon wafers could possess parameters similar to those of sophisticated laboratory samples elaborated on small-area silicon chips. With the outlined design/technological approaches, the use of even cheaper, i.e., lower-grade, single-crystal, silicon could still yield large-area sensors for radiative energy signals with fairly acceptable electro-optical performance.
1. Introduction During the past few years substantial advances have been made in the field of large-area silicon sensors for radiative energy signals [l - 41. Both diffused and ion-implanted optical sensors fabricated on 2 inch singlecrystal silicon wafers [l - 41 have equalled the performance of sophisticated Elaevier Sequoia/Printed in The Netherlands
244
laboratory samples elaborated on small-area silicon chips. The reported breakthroughs stemmed from both novel design/technologycal approaches and from a better und~$~d~g of the peculiar physics governing the behaviour of ultra-shallow (0.1 pm to 0.4 pm) silicon junctions fl - 81. The directions of research that could further improve the electro-optical perforrn~~ of both diffused and ion-spited sensors have also been delimited and it was shown that the relative expensiveness of high-quality single-crystal silicon still remains a prohibitive factor in achieving a fairly balanced cost effectiveness in this area of optical sensors [ 1 - 41, A better exploitation of silicon wafer is obtainable in bi&ciaf optical sensors, whose many-fold advantages in several applications (e.g., static and quasistatic concentrating systems, luminescent concentrators, etc.) have been demonstrated in connection with the development of p+-n-n+ devices 5 cm2 in area [9]. The purpose of the present work is to demonstrate the feasibility of high-quality, low-cost bifacial optical sensors with reproducible parameters on large-area (2 and 3 inch) ~ommerc~y~vailable n- and p- silicon wafers, wherein different kinds of areal inhomogeneities play an important limiting role [ 10 - 121. To this end, a comprehensive design/materials/technological study was conducted on lower-grade, #mrnerc~~y~v~ble, large-area, n- and psilicon wafers. Various procedures leading IXIa substantial increase of both the emitter and the base contributions to the generated photocurrent in p+-n-n+ and n+-p-p+ devices are described and tested ~per~e~~~y. Experiments concerning the influence of the anti-reflection (AR) layer, silicon quality, etc. on device performance were also conducted. The emphasis was placed on the min~ization of the sensor cost and the reproducibility of their electro-optical performance in the bifacial mode. The comb~ation of design/technological approaches described here has ultimately led to the development of l&w-cost, high-quality, large-area silicon sensors with good electro~pti~~ pe~orrn~~ as bifacial devices,
The following objectives have been pursued in the design of the developed p+-n-n+ and n+-p-p+ optical sensors: (I) increase of both the base and the emitter ~on~ibutions to the generated photocurrent; (2) decrease of the series resistance R, below the 100 mS2threshold; (3) att~uation of the impact of various ~p~f~tions of the front grid metallization on the device performance; (4) partial elimination of severe limitations imposed by area1 inhomogeneities, including the uniform collection of optically-generated carriers [3,4,10 - 121, on the electro-optical behaviour of the cells; (5) achievement of fairly good electro-optical performance of bifacial devices processed on commercially-available, lower-grade silicon.
245
To increase the base contribution to the generated photocurrent, gettering techniques built into the processing stages (see Section 3) have been used for both types of cells. Lifetime measurements (see Section 4) have shown that the diffusion length of the minority carriers in the corresponds bases exceed, as a rule, the base thickness, thus rendering the backside contacts efficient. Two main design/technological factors determine the features of the emitters in the developed cells. One of the factors is the peculiar impurity profile of the front junctions with a depth xj = 0.25 pm (Fig. 1) obtained by appropriate ~chnolo~cal means. The impurity profile for boron (B), with a shape close to that of the phosphorus (P) diffusion shown in Fig. 1 and described in detail in ref. 3, had the following features: (a) highdopant concentration (- 1021 cme3) in the proximity of the light-incident surface; (b) rapidly falling impurity concentration in the proximity of the aforementioned surface. The combined action of these features decreases the contribution of the front contact region to the series resistance R, of the cells without correspondingly increasing the number of recombinations (especially the Auger processes) in the surface region, a fact that was expected to yield a fairly good optical response of devices in the shorter wavelength portion of the incident light spectrum. To obtain the impurity profile of Fig. 1 for p+-n junctions, i.e., to obtain a high surface concentration without simultaneously evoking high doping effects, a boron (B) diffusion from a boron nitride (BN,) source with hydrogen injection similar to that described elsewhere [ 14, 151 was UfX?d.
The second factor was aimed at a drastic reduction of the impact of different kinds of area1 inhomogeneities, typical of large-area cells, on their electro-optical parameters. To this end, a novel ‘spider’s web’-type (Fig. 2) front grid metalhzation pattern was developed [3, 41 and optimized, according to the prevailing methodologies, for one sun insolation and for a p+- (n+-) emitter sheet resistance of - 200 Q/U. (The actual sheet resistance of the emitter layers was somewhat lower, thus leaving scope for further improvement of the front grid pattern and, hence, of the device performance.)
DISTANCE FFWl
SURFACE
Fig. 1. The impurity profile pattern for the front junctions in developed p+-n-n+ and n+-p-p* large-area silicon sensors for radiative energy signsls.
246
Fig. 2. Photograph of developed ‘spider’s web’-type metallization pattern for 3 inch silicon optical sensors (Scale 1 :l),
The developed contact pattern (Fig. 2) is inherently better suited for a circular silicon configuration in comparison with the classical ‘fishbone’type front grid metallization [l, 21 for the following main reasons: (a) it ensures a better collection of the photogenerated carriers from the periphery of the device; (b) it increases the autonomy in the spacing between the collecting pads by way of reducing both the coverage ratio (shading losses) and power losses on the lateral emitter resistance; (c) it minimizes the impact of local area1 degradation on the performance of the whole device in the illuminated mode [ 3, 41. The total shading losses were kept below 8%. 3. Fabrication 3.1. General considerations The test devices were fabricated on commercially-available 2 inch, (111>,Czochralski, 220 I_cmthick n-wafers, with a starting resistivity of 1.5 2.5 S2 cm and on 180 pm thick p-wafers, with a resistivity in the range 7 - 12 a cm. Both wafer types were lower-grade silicon with more than 5000 dislocations/cm2 (p-silicon) and w 2000 dislocations/cm2 (n-silicon). The 3 inch p+-n-n+ devices were fabricated on (ill), FZ, 230 pm thick n-silicon substrates with a starting resistivity in the range 1.5 to 2.5 LI cm and -2000 dislocations/cm 2. All phosphorus (P) diffusions were performed with a POC13 source. All boron (B) diffusions were performed with a BNs source with hydrogen injection. The combined action of these two processing steps in each single device acted as an excellent gettering, increasing substantially the minority carrier diffusion length in the corresponding bases of the devices.
247
The light-incident surface of the devices was passivated with a 100 A thick SiOz layer. As an antireflection (AR) coating, either 800 A SiO or 600 A TiOz layers were used. Standard Ti-Ag-Ni metallization was used for the fronts and backs of the wafers. Contact adherence was ensured by sintering the devices for 10 min at 450 “C in a nitrogen ambient. A 1 pm thick, 1 mm wide oxide layer was preserved on the circular periphery of the wafers in order to prevent shorts caused by edge diffusion. For bifacial cells, the back light-incident surface was processed in a manner identical to the fronts. A cross-section of the fabricated p+-n-n+ devices is presented in Fig. 3.
3 = 1.5 - 2.5 n.cm
Fig. 3. Cross-sectional structure of the developed 2 inch pa-n-n+ silicon optical sensors (see Sections 2 and 3 for the ptiameters of n+-p-p* devices).
3.2. Fabrication of p’-n-n+ sensors At the outset of the fabrication process, an initial total oxidation in wet O2 at 950 “C was performed in order to obtain a SiOz layer 1 pm thick. Following a deoxidation of the rear of the wafers, the backside n-n+ contact was formed by phosphorus diffusion at 850 “C from a POC13 source. The resulting depth of the n-n+ junction was -0.3 pm. To form the p+-n junction, the 1 pm thick oxide on the front surface was etched down, except for the 1 mm wide circular periphery of the wafers. After that, the BN was deposited on the surface. Subsequently, the wafers were introduced into a nitrogen/oxygen ambient at 820 “C to form Bz03. The temperature was raised to 920 “C in about 10 mm. A 1 min Hz/N2 injection at constant temperature followed to realize HB02. Subsequent drive-in diffusion was carried out in a N3 ambient for approximately 25 min. The temperature was then lowered to 820 “C in about 15 min. A wet oxidation in O2 ambient at 750 “C followed. A chemical etch in buffer solution was performed. To passivate the wafers, a 15 min wet N2 ambient oxidation was carried out. The sheet resistance of the p+-layer was in the range 120 - 150 St/Cl. The metallization and AR coating deposition followed.
248
3.3. Fabrication of n’-p-p’ devices The backside p-p+ contact was formed through a boron diffusion from a BN3 source with hydrogen injection according to the procedures oulined in Section 3.2. The fron n+-p junction was formed by phosphorus diffusion from a POCls source according to the procedure outlined in Section 3.2. for the backside contact of p+-n-n+ bifacial devices.
4. Characterization A Zeiss Jena quartz prism monochromator with a xenon arc lamp as light source (simulator of AM 1 spectrum with 100 mW/cm* insolation) coupled to a Tektronix 31/4661 data acquisition system was used to measure the relative spectral response (RSR) of the test devices (Figs. 4 and 5), from which the optical bandwidth B, measured at 50% S,,,, and the peak responsivity wavelength h, (Tables 1 -6) were determined. The values of the peak generated current I, under AM 1 conditions and those of the peak
mg qJL+4 B 80-
-MO
ti
70-
-70
=
60-
-60
3
!io-
-50
-90 -80
-Lo ul 30-
-30
$! 2004.w ,
-20 I 500 I
I
mI
700 I
I 800 L
n 900 I
(4
WAVELENGTH
(nm)
(b)
WAVELENGTH
(nm)
- 10 I1 IO00 4 1100 I <
J?ig. 4. The relative spectral response (RSR) of 2 inch, p+-n-n+ silicon optical sensors under front (a) and back (b) AM 1 illumination (100 mW/cm’ insolation) at 20 ‘C for SiO and TiOa antireflection layers, respectively.
249
do,
I
I
I 5M)
(b)
L
I I I a I I I &IO 700 mm ml WAVELENGTH (nm)
I
I Iwo
I
I llw
Fig. 5. The relative rrpectral response (RSR) of 2 inch, n*-p-p* silicon optical sexwors under front (a) and back (b) AM 1 illumination (100 mW/cm* insolation) at 20 “C for SiO and TiO2 antireflection layers, respectively.
TABLE
1
AM 1, 20 “C electro-optical characteristics, the main parameter6 of the dark currentvoltage diode curves and n-base lifetime 7~ of relevant test 2 inch p*-n-n+ silicon optical sensors under front illumination Device N 4a N 5a N9a N 18b N lgb N 20b N 21b N 23b
(-1
OW
h,
&n
p,
%
rB
n
J,
(pA/cm*)
i$I,cm*)
696 59s 586 59s 610 600 610 600
660 650 660 640 640 630 630 630
720 719 720 722 727 729 730 728
344 344 346 349 366 363 364 360
88 75 66 69 62 83 75 67
63 82 71 ’ 70 71 73 83 66
1.86 1.64 1.81 1.75 1.82 1.64 1.78 1.83
9.1 6.0 7.83 4.8 4.2 3.9 2.8 3.4
6.96 6.95 11.2 5.7 10.3 7.8 8.4 7.4
3
(mA)
a800 A thick SiO AR layer. b600 A thick TiO2 AR layer.
WV
bn)
(~4
260 TABLE 2 AM 1, 20 “C, electro-optical characteristics of relevant test 2 inch p*-n-n+ silicon optical sensors under backside illumination Device
N N N N N N N N
B
4a iTa ga
lBa lga 20b 21b 23b
ha
Inm)
(-1
505 525 520 540 545 545 540 540
900 900 850 850 850 900 850 850
An (mA)
pnl @WI
613 613 615 635 637 637 636 637
270 271 274 284 287 286 289 290
a800 A thick SiO AR layer. b600 A thick TiO2 AR layer. TABLE 3 AM 1, 20 “C electro-optical characteristics, the main parameters of the dark currentvoltage diode curves and p-base lifetime rn of relevant test 2 inch n+-p-p+ silicon optical sensors under front illumination Device P P P P P P P P
11a 17a 2oa 27b 2Bb 29b 30b 31b
B
?B
(nm)
(c(8)
545 530 560 575 570 575 575 570
660 650 670 640 640 630 620 620
685 694 686 700 694 698 699
317 319 319 329 330 332 329 331
57 87 81 77 73 69 71 63
72 55 65 63 74 57 63 71
n
I
dOd
dor
11.8 9.6 9.8 9.6 8.5 8.7 6.1 6.7
11.7 7.81 11.29 6.83 8.75 8.15 6.23 4.65
( PA/cm21 (nA/cm2)
1.91 1.65 1.77 1.76 1.52 1.47 1.92 1.66
a8OO A thick SiO AR layer. b600 A thick TiO2 AR iayer.
delivered power P, into a properly matched load impedance are also shown in Tables 1 - 6 for each of the representative sensors. Several parameters that determine the noise in the photodiode structures (series resistance R,, the dark current, the shunt resistance I&) have also been accurately measured (Fig. 6). The current density of the dark current-voltage curves, analytically expressed by 13,161
J = JOd ew(qv/kT) + JO,
eqWWkT)
(1)
served for the determination of the diode ideality factor II and the diffusion (JOd) and recombination (Jo,) components of the reverse saturation current density Jo according to the method outlined elsewhere [3, 16 1. The values of the series resistance R, were determined from the difference between the dark and AMl-illuminated I-V curves [16]. The base lifetimes rB were
251 TABLE 4 AM 1, 20 “C, electro-optical characteristics of relevant test 2 inch n+-p-p+ silicon optical sensors under backside illumination Device
B
(nm) P I.$& P 338
606
515 505 516 530 520 630 510
P2OB P27b P2Sb P 29b P 30b P 3$b
x,
Ll
Pill
(nm)
(mAI
bW)
900 900 850 850 850 850 850 850
589 596 604 605 620 605 628 621
250 251 263 263 277 271 278 278
a89Q A thick SiO AR layer. b600 A thick TiOz AR layer. TABLE 5 AM 1, 20 “C electrooptical characteristics, the main parameters of the dark currentvoltage diode curves and the n-base lifetime ~8 of relevant test 3 inch p+-n-n+ silicon opt&xl sensors under front illumination
&a 6* $b
686 575 570 576
ZJ b N &Ob N llb
590 580 575
N N N N
la
670 670 670 660 665 660 670 675
1693 1584 1579 1568 1583 1581 1573 1567
756 758 764 783 783 781 775 779
89 97 99 85 99 98 98 99
22.5 24.7 26.8 28.7 26.5 27.2 23.7 26.8
68 72 86 78 69 88 82 79
1.86 1.93 1.94 1.95 1.87 1.93 1.88 1.87
7.45 5.13 7.45 5.35 4.97 5.62 6.12 5.93
7.32 9.32 5.37 8.32 8.33 3.41 3.68 3.54
=BOOA thick SiO AR layer. b660 A thick TiO2 AR layer.
Fig. 6. The lumped equivalent circuit of silicon photodiode structures.
measured according to the OCVD method and Kingston’s method, respectiv’ely. The shunt resistake RSh is shown in Table 6 for 3 inch, p+-n-n+ devices by way of illustration. The measurements of electro-optical parameters were independently pelCformed at three different locations*, where a cross-calibration with reference devices was made.
252 TABLE 6 AM 1, 20 “C! electro-opticd characteristics of relevant test 3 inch p+-n-n+ silicon optical sensors under backside illumination Device
h
pill
(nm)
N N N N N N N N
la 3” Sa 7b 8b Sb lob lib
510 505 515 510 515 520 525 525
900
900 900 850 850 850 850 850
1358 1354 1359 1362 1363 1365 1368 1369
WV
Rs WQ
607 604 608 612 613 615 615 614
87 76 73 83 73 72 88 99
n
23.7 22.9 24.7 25.7 28.3 29.7 28.7 28.8
1.83 1.81 1.91 1.77 1.83 1.87 1.89 1.78
JOd
JOr
(pA/cm2)
(nA/cm’)
6.27 6.47 6.38 6.42 5.72 5.92 6.17 6.35
4.37 4.17 4.56 4.17 3.92 4.72 4.22 4.76
WOO A thick SiO AR layer. b600 A thick TiO2 AR layer.
5. Discussion The subsequent discussion is restricted to the most representative p+-n-n+ and n+-p-p+ silicon optical sensors. By eliminating both the several test units with the highest values of Im and I3 and those that failed to have both I, and B above the bottom values of Tables 1 -6, the selected devices reflect the dominunt trends observed amo~lg the fabricated 2 inch and 3 inch silicon optical sensors, The following peculiar features of the devices are noteworthy: (1) The devices possess a wide-band RSR in the bifacial mode: the optical bandwidth B, measured at 50% S,,, (Figs. 4 and 5) currently has values in the range 550 nm - 600 nm under AM 1 front illumination (Tables 1, 3 and 5), while values in the range 500 nm - 540 nm are commonplace for AM 1 backside illumination (Tables 2,4 and 6); (2) The peak responsivity wavelength A, (Figs. 4, 5, Tables 1,3,5) for all cells under front illumination is shifted toward the visible portion of the solar s-&rum. The wavelength h, is located around 600 - 669 nm, instead of 750 900 nm as in usual cells [16, 173 or in the backside-illuminated devices reported in this work. The aforementioned shift of X, enabled the fabricated devices to take greater advantage of the incident light power. (3) All samples possess a good blue response (X = 0.4 pm) accompanied by a relatively substantial quantum efficiency (QE) at longer wavelength (h > 1.0 pm) of the incident light under both front and backside AM 1 illumination (Figs. 4 and 5). An excellent QE is achieved for incident light in the wavelength range X = 0.6 firn to 0.9 pm. *IPRS-Bgneaaa (location of one of the authom), ICEFIZ (Central Imatitute its, Bucharest), ICPE (R and D Institute for Electrical Engineering, Bucharest).
of Phye-
263
(4) The use of a TiOz AR layer (instead of its SiO counterpart) increases the optical bandwidth of front-illuminated devices, primarily through the increase of quantum efficiency at longer wavelengths of the incident light. A shift of wavelength X, by 10 - 30 nm toward the visible portion of the light spectrum in the frontcilluminated cells using a Ti02 AR layer was recorded (Figs. 4, 5, Tables 1, 3, 5). The same shift of X, in backside-illuminated cells was roughly 50 nm (Figs. 4, 5, Tables 2, 4,6). (5) The optical features outlined in (4) enabled the peak generated current I, in both front- and back-illuminated cells using a TiOz AR layer to increase (Tables 1 - 6). (6) The test sensors possess a relatively large value of the minority carrier lifetime TB in the bases (Tables 1, 3 and 5), thanks to the boronand phosphorus-based gettering process built into the processing steps (Section 3). As a result, the devices with TB > 40 ps inherently had a diffusion length of the minority carriers in the bases (holes in the n-base of p+-n-n+ devices, electrons in the p-base of n+-p-p+ samples) equal to or larger than the silicon wafer thickness. This feature had a positive impact on the RSR parameters and on the values of the generated photocurrent. (7) The series resistance R, has been reduced to values below the 100 mS1 threshold (Tables 1 -6), which is a considerable achievement, especially in large-area optical sensors. This feature was mirrored by the enhanced value of the peak power P, delivered into a properly matched load. (8) The value of P, was somewhat limited by the quality of the starting silicon. This fact was also reflected in the somewhat higher than usual [3, 16.1 values of the diffusion component Jod (Tables 1,3 and 5) of the reverse saturation current density Jo, especially for n+-p-p+ devices. (9) The extremely high values of the peak generated current Im under AM 1 conditions (Tables 1 - 6) express the excellent responsivity R [ 173. The following factors endowed the fabricated devices with high values of 1, and R : (a) the relatively large optical bandwidth B; (b) the high quantu.W efficiency, both at short (0.4 I.crn- 0.5 pm) and longer (0.8 pm - 1.1 pm) wavelengths of the incident light; (c) the relatively large base lifetime rB, which has the result that a large amount of carriers, optically generated in the bulk of samples with high quantum efficiency, do actually survive’ ‘ and contribute to the photocurrent generated; (d) the implementation of the novel metallization grid (Fig. 2), a step that both attenuated the impact of area1 inhomogeneities on the performance of the cells and reduced the shading losses; (e) the increased contribution of the p+-emitter to the generated photocurrent, thanks to both the surface passivation and the features of the impurity profile (Fig. 1). It should be noted that test cells exhibited a higher than usual quantum efficiency for photons with energies E Q 1.2 eV and that any eventual losses in the blue response were more than compensated by an increase in the infrared response, especially at wavelengths X > 1 .O Mm (Figs. 4 and 5). These features of the developed devices behaviour as photovoltaic converters. Thus:
are correlated
with
then
254
(1) The p+-n-n+ cells have a reproducible conversion efficiency q = 1’7 18% under front AM 1 illumination and Q= 13 - 14% under back AM 1 illumination conditions. (2) The n+-p-p+ cells fabricated on lower-quality p-type silicon have a conversion efficiency q = 15.5 - 16.5% and 77= 12 - 13.2% under front and back AM 1 illumination conditions, respectively. Taking into account the cell size and the quality of the starting silicon, the obtained values of conversion efficiency q and the resulting values for the short-circuit density J,, (ratio of Im to cell area) compare well with the best data reported to date [l - 4,9,13,16,17]. 6. Concluding remarks All the reported results were obtained in an industrial environment, not under laboratory conditions. Thus, this work shows clearly that properly designed/processed silicon optical sensors elaborated on large-area, lowergrade n- and p-wafers can possess electro-optical parameters in the bifacial mode similar to those of sophisticated laboratory samples using small-area silicon chips. The performance improvements of large-area silicon sensors for radiative energy signals reported in this work could undoubtedly have a major beneficial impact on the further development of photovoltaic converters, light-sensitive devices, different types of photodetectors, etc. The outlined design/technological improvements are all the more important as they were tested on an industrial scale on large-area wafers and it is usually expected that ‘cell efficiency will generally decrease with increasing the cell size’ [ 181. To conclude, this work has demonstrated the feasibility of high-quality, low-cost bifuciul sensors for radiative energy signals with reproducible electro-optical parameters on 2 inch and 3 inch commercially-available silicon wafers. The implementation of usual refinements (textured surface, double AR layer, etc.) in conjunction with the design/technological approaches of this work could further improve the parameters of these sensors. Alternatively, the use of even cheaper silicon could still yield large-area sensors for radiative energy signals with fairly acceptable electroaptical performance under both front and backside illumination. Reference 1 A. P. Siiard, Silicon optical ~ensom fabricated through masked ion implantation, Sensors and Actuotons, 7 (1986) 177 - 187. 2 A. P. Silard, Ma&ed ion-implanted silicon solar cellaloptical se~oru, in Phyrice of Semiconductor Devices, World Scientific Publishing Co. : Philadelphia/Siiapore, 1986, pp. 184 - 193; A. P. Silard, Using silicon dioxide a~ device engineering parameter in the fabrication of photovoItaic cells through tided ion implantation, Sol. Energy Mater,. 14 (1986) 95 - 105. 3 A. P. Siiard, Electra-optical performance of large-area eilicon senoom for radiative energy signals, Sensors und Actuators, 12 (1987) 23 - 34. 4 A. P. Silard, F. Pera and G. Nani, Development of preferentiiy current-generating silicon solar celle, Appl Phys. Lett., 48 (10) (1986) 673 - 675; A. P. Silard, F. Pera
5
6 7 8 9 10 11
12 13 14 15 16 17 18
and G. Nani, High-efficiency, large-area p+-n-n* silicon solar cells, SolidStcrte Electron., 30 (1987) 397 - 401. A. P. Silard and M. J. Dut& Analyti of electric field in very shallow silicon junctions, Int. J. Electron., 61 (1986) 627 - 638. A. P. Shard, Analysis of electric field diitributions in ultra-shallow silicon junctions un&r thermodynamic eq&librium conditiona, Int. J. EZectron., 62 (1987), in press. A. P. Shard and M. J. Du$& Inveotigation of the majority carriers diffusion coefficients behavioural pattern in degenerately-doped dlicon, Int. b. Electron., 62 (19871, in press. A. P. Silard and M. J. Duw, The effective htrinsic concentration nie in degenerately doped silicon, J. AppZ. Phye., (Oct. 15,1987), in press. A. Cuevas, A. Luque, J. Eguren and J. de1 Alamo, High efficiency bifacial back surface field solar cells, SOL Cella, 3 (1981) 337 - 340. F. A. Lindhohn and C. T. Sah, Fundamental electronic mechanisms limiting the performance of solar cells, IEEE Trans. Electron Devices, ED-24 (1977) 299 - 304. F. A. Lindholm, J. A. Mazer, J. R. Davis and J. J. Arreola, Degradation of solar cell performance by areal inhomogeneities, Solid-State Electron., 23 (1980) 967 - 971. A. W. Penn, Large-area p/n junctions for the detection of nuclear radiation, in S. C, Jain and S. Radhakrishna (eds.), PhySiC8 of Semiconductor Devices, Wiley Eastern, New Delhi, 1982, pp. 84 - 95. M. A. Green, A. W. Blakers, J. Shi, E. M. Keller and S. R. Wenham, Hiihefficiency silicon solar cells, IEEE Tmns. Electron Devices, ED-31 (1984) 679 - 683. G. Pignatel and G. Quierolo, AES study of boron diffusion in silicon from a boron nitride source with hydrogen injection, J. Electrochem. Sot., 126 (1979) 1805 - 1810. E. Dominguez and M. Jaraiz, A non-Fickian model of the boron in silicon diffusion from BN source, J. Electrochem. Sot.. 133 (1986) 1895 - 1900. R. N. Hall, Silicon photovoltaic cells (Review paper), Solid-State Electron., 24 (1981) 595 - 616. S. M. Sze, Physics of Semiconductors Devices, Wiley, New York, 1981, pp. 681 - 816. M. A. Green, Zhao Jianhua, A. W. Biaksrs, M. Taouk and S. Narayanan, 25 - percent efficient low-resistivity silicon concentrator solar cells, IEEE Electron Device Lett., EDL-7 (1986) 583 - 585.
Biographies Gabriel Nuni was born in Bucharest, Romania, on March 22,1954. He graduated from the Department of Electronics, Bucharest Polytechnic Institute in 1979, with a thesis on power transistor thermal modelling. The same year he joined IPRS-Bgneasa (enterprise for Radio Components and Semiconductors) Bucharest, and has worked there ever since. His main areas of interest are power transistor design and silicon technology. In 1981 he went through a power transistors production management training program in the U.S.A. Since 1984 he has been involved in solar energy conversion research programs and has contributed to the development of large-area, all-diffused silicon solar cells/optical sensors. In 1986 he joined on a part-time basis the project concerning the development of doubleinterdigitated (TIL) power transistors. G. Nani hascoauthored over fifteen papers in refereed scientific journals and technical digests of professional conferences on silicon solar cells/ optical sensors and novel power transistors. He holds five patents in the area of power transistor design and silicon technology.
Andrei P. Silard was born in Timigoara, Romania, on April 30, 1944. He received the B.S. degree in electronic engineering in 1967 with the highest distinction following work on the measurement of thermal par&neters of thyristors. In 1976 he received the Ph.D. in electrical engineering from the Bucharest Polytechnic Institute (BPI) with a thesis consisting of a comprehensive theoretical/experimental investigation of the physical mechanisms in power amplifying gate thyristors. The same year he received the B.S. degree in history from Bucharest University. At the Electronics Research Institute in Bucharest (1967 - 73), he worked in the field of power circuit applications of thyristors, especially for induction heating, and was involved in the development and characterization of h.f. low-loss ferrites. In 1974 he joined the teaching staff of the Department of Electronics, BPI. Up to 1982 he initiated and completed several research projects, whose results led to a better understanding of electrothermal failure mechanisms in power rectifiers under surge conditions, of basic thyristor physics in various modes of operation and contributed to an optimized design of original light-activated thyristor structures. He is now teaching courses on electronic measurement and control apparatus and semiconductor power devices in the Department. Since 1978, he has pioneered research work on novel power semiconductor and photonic devices. He contributed to the inception and development of the double-interdigitated (TIL) concept, which resulted in the fabrication of a new class of electrothermally fail-safe power devices: bipolar transistors, gate turn-off (GTO) and gate-assisted turn-off (GAT) thyristors. Subsequently, he contributed to the development of high-voltage, ultra-fast turning-on TIL-type transistors with base wells (BW). His research contributed to the development of highquality solar cells/optical sensors on large-area (2 and 3 inch) silicon wafers, both through diffusion and ion implantation. His investigations have also led to the clarification and reformulation of several fundamental issues in the physics of ultra-shallow, heavily-doped silicon junctions. Dr. Silard has published over 100 papers in refereed scientific journals and technical digests of professional conferences (such as IEDM, IAS, ESSDERC, CSSDM, IECON, etc.) and has given invited lectures at international meetings on microelectronics in India and Brazil. He co-holds seven patents on power devices and ICs and has contributed to two books on power rectifiers. In 1983 he received the 1981 Technical Sciences Award of the Romanian Academy for contributions to the electrothermal investigation of power devicesand ICs. He has been a Senior Member of the IEEE since 1982. He has published extensively on the philosophy of -history, both in Romania and in the United States. He has authored over 20 studies and papers on the impact of science, particularly of electronics, on contemporary societies.