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GaAs shallow homojunction solar cells fabricated on thin epitaxial films by a simple Zn solid state diffusion method
Solar Energy Materials 14 (1986) 29-49 North-Holland, A m s t e r d a m
29
GaAs S H A L L O W H O M O J U N C T I O N S O L A R CELLS FABRICATED O N T H I N EPITAXIAL FILMS BY A S I M P L E Zn S O L I D STATE DIFFUSION METHOD
M. GAROZZO *, A. PARRETTA, G. MALETTA, V. ADONCECCHI and M. GENTILI ** Eniricerche, S.p.A., Via Ramarini 32, 00015 Monterotondo, Rome, Italy Received 28 June 1985; in revised form 3 April 1986 Computer simulation shows that high photocurrent densities can be achieved on p ÷ - n shallow G a A s solar cells by using high quality thin epitaxial films. The simple addition of a back surface field also improves open circuit voltages and conversion efficiencies. Unintentionally n-doped GaAs epitaxial films ( = 2 t~m thick) were grown by metal organic chemical vapor deposition. A simple 0.5 rtm thick SiO 2 layer was used as a cap in the open-tube Zn solid-state diffusion technique. Diffusion processes were carried out on both commercial bulk material and epitaxial films at low temperature ( < 600°C) by using the Z n O / S i O 2 mixture as a doping source. The p+ layers were well controlled in thickness in the range 250-14000 A, heavily doped ( --1020 cm - 3 ) and very homogeneous. No damage was observed on the G a A s surface after the diffusion process. A best fit of the experimental photoresponse spectra yielded reliable values of the diffused layers thicknesses and of the minority carrier diffusion lengths. These were in the range 0.2-1.3 I~m for commercial material and 3.5-5.5 ~tm for epitaxial films. Good agreement was found between the experimental photocurrent values and the calculated ones in both bulk and thin film devices. The thin film photocurrent density reached values as high as 24.2 m A / c m 2. A conversion efficiency of 14.1% at AM1 was obtained on a thin film device with an area of 1 cm 2 and with a p+ layer thickness of 700 A.
1. Introduction
It is well known that gallium arsenide solar cells show the highest conversion efficiency. This material presents some difficulties in terrestrial large scale application because of the high cost of gallium and the sophisticated technologies involved. Owing to the high value of the gallium arsenide absorption coefficient, it is, nevertheless, possible to fabricate high performance thin film devices, taking advantage of the high quality of the material grown by chemical vapor deposition. Besides that, GaAs shows other interesting properties: it is radiation resistant and it is suitable for monolithic tandem cells. From this point of view it could be much more promising than silicon for very high efficiency solar cells. In order to overcome the problems related to the cost, germanium-coated silicon [1-3] and CLEFT [4] processes seem to be very interesting. The metal organic chemical vapor deposition (MO-CVD) seems moreover to be the most promising solution for large scale production [1]. * Present address: ENEA, Casaccia, Rome, Italy. ** Present address: lESS, Via Cineto R o m a n o 42, Rome, Italy.
M. Garozzo et al. / GaAs shallow homojunction solar cells
30
In GaAs concentration solar cells a GaA1As top layer is generally used in order to strongly reduce the surface recombination velocity. Unfortunately the use of this ternary compound entails some technological problems owing to the high aluminum reactivity, the need of a very accurate control of the alloy composition and the difficulty to obtain satisfactory Ohmic contacts. On the other hand in the case of flat panels high conversion efficiencies are obtained by preparing GaAs p - n homojunctions [5]. In this paper we show, by a computer simulation, that, in order to reduce surface recombination as much as possible, the top layer must be very thin (0.05-0.1 ~m) and then heavily doped to avoid sheet resistance. Our calculations also show that the use of high quality thin epitaxial films as an active material results in a remarkable enhancement of spectral photoresponse owing to the large values of the minority carrier diffusion lengths. This fact, in combination with an easily added back surface field (BSF), allows high conversion efficiencies ( > 19%) to be reached. GaAs p + - n junction solar cells fabricated by Borrego et al. [6] by open-tube solid-state diffusion are very interesting for both the accurate control of the junction depth and the possibility of a large scale production. In this paper we present a method for fabricating p + - n junctions by zinc diffusion from Zn-doped silica as a source, covered by a SiO 2 layer only. The open-tube diffusion was performed at a relatively low temperature, thus avoiding the use of phosphosilicate glass (PSG) as a cap. The results on the Zn diffusion coefficient are quite similar to those obtained with a PSG cap [7] and to those obtained by the box-diffusion technique [8]. The diffused layers are heavily doped ( = 1020 cm-3), homogeneous and well controlled in thickness. p ÷ - n devices were fabricated in this way on both bulk material and MO-CVD grown thin films. The minority carrier diffusion length Lp and the junction depth Xj were derived, in both cases, by the best fit of the experimental internal spectral response. The values of Lp obtained for the epitaxial layers are almost one order of magnitude larger than those in the bulk material. By using these values of Lp and Xj, the expected values of the photocurrent density were calculated. A good agreement was found between these theoretical values and the experimental ones. The photocurrent density of the thin film devices is about 20% greater than that measured on bulk material, as expected from the computer simulation. The best photovoltaic performance consists in a conversion efficiency of 14.1% at AM1 on an AR coated epitaxial thin film device with an area of 1 cm2. 2. GaAs p+-n homojunction solar cells: a computer simulation
2.1. Charge collection The photocurrent density JL is expressed by the following equation:
JL = q f Q(X) F(X) (1 - R ( X ) ) d X , J
(1)
31
M. Garozzo et aL / GaAs shallow homojunction solar cells
where q is the magnitude of the electron charge, F(~,) is the density of the incident solar photon flux per unit bandwidth, R()~) is the device reflectance and Q()~) is the internal spectral response: Q(2t) = Q,(2t) + Qp(X) + Qdr(X), (2) where Qn(~) is the contribution of the electrons from the p-side, Qp(~) is that of the holes from the n-side and Qdr(X) from the depletion region. Adopting the simple model of an abrupt p-n homojunction, the three terms of the internal spectral response are given by the following equations [9]: Qo(X)
aLn
-an' - ln" + aL n Ln
a2LZ"-I ~Sn~'nsinh(L~)+cosh(~) exp( - aXJ) { ~ c ° s h ( L ~ ) + sinh(~) }
Sn' Lnrnsinh( L~ ) + c°sh( L~ ) - a L n exp(- aXj)),
Qp(X)
(3)
aLp exp(-a(Xj+ W)} z~'Sa Lp-1 [ x
Sp'rP{cosh(~p ) - exp(- all)) + sinh(~p ) ~L~ -
S~--;~C--- ~ - . . . .
-
G
smh(--1 + cosh(--1
\G ]
aLp
~G ]
exp(- all)
]
+ Sp~'p [ H ) (~_~p) ' sin (K + cos
(4)
0ar(~k) = exp(-aXj) [1 - exp(-aW)], (5) where Lp, 'rp and Sp are the minority carrier diffusion length, lifetime and surface recombination velocity in the n-side and L., .r., S. are the same quantities in the p-side, a = a(k) is the GaAs absorption coefficient, Xj is the junction depth, W is the depletion layer thickness and H is the base region thickness minus W.
M. Garozzo et al. / GaAs shallow homojunction solar cells
32 1.0
a
o
0.8
Z 0 0.6 o. oO I.U
j
-J 0 . 4 I-w
n 0.2 O0
o.o 400
5~)0
i 6 0I 0 700 WAVELENGTH
8 0I 0 (nm)
9()0
1000
Fig. 1. Calculated internal spectral response for bulk devices with Lp = 0.5 ~m. (a) Xj = 500 A; (b)
xj = 1000 A; (c) x~ = 2000 A.
The junction depth is effective on all the three terms of the spectral response; it affects therefore the photoresponse in the whole spectral range. The behaviour of Q ( ~ ) at different values of ~ may be evaluated assuming S n = 10 7 c m / s , 7n = 0.6 ns, L, = 1.6 ~ m , L p = 0.5 p~m and the donor concentration N o = 6 × 1016 cm 3, typical values for devices realized on bulk material [10]. If an Ohmic back contact is assumed (Sp = oo), Q(~,) is, moreover, independent of 1-p. Absorption coefficient data were derived by Aspnes and Studna [11]. The few data below 1.5 eV were calculated from the relation [12] a - ( h i , - E g ) 1/2 and normalized to match the experimental ones. Doping effects were not accounted for. Fig. 1 shows calculated internal spectral responses for bulk devices with Xj = 500 A, 1000 ,~ and 2000 ,~ respectively and with the above reported values for the other parameters. The spectral response is found to increase with decreasing junction depth, especially at short-medium wavelengths, owing to the high surface recombination velocity and to the very large absorption coefficient. At long wavelengths the carrier collection is strongly dependent on the transport properties of the active material, particularly on the minority carrier diffusion length L p . Devices fabricated on epitaxially grown thin films show values of Lp as high as 5 ~tm at an appropriate donor concentration (1016-1017cm 3) [6]. Moreover the epitaxial deposition of the active material makes it easier to add a back surface field, which increases the carrier collection efficiency even more ( s p = 0). Fig. 2 shows the internal spectral response calculated for a 2 ~m thick epitaxial film with Lp = 5 ~m and Sp = 0 at different values of Xj. All other parameters are
33
M. Garozzo et al. / Gads shallow homojunction solar cells 1.0
I
I
I
I
I 500
I 600
I 700
I 800
O 0.8 U,,I z O n
O.6
uJ n,. ..I
0.4
I"' 0.2 a. u~
0,0 400
WAVELENGTH
I, 900
1000
(nm)
Fig. 2. Calculated internal spectral response for thin film devices with Lp = 5 ~m. (a) Xj = 500 ,~; (b)
xj = ]0oo A; (c) xj = 2000 A. the same as for fig. 1. Also in this c a s e Q p ( ~ ) does not depend on rp. A c o m p a r i s o n between the curves of fig. 2 and fig. 1 clearly shows the large e n h a n c e m e n t of the internal spectral response at medium-long wavelengths. 1,0
w
i
I
I
I
800
900
A
O.8 iJJ (,.) z
<~ 0.6 u,l .J ~: 0 . 4
°1% 400
500
I 600 700 WAVELENGTH
1000
(nm)
Fig. 3. Typical reflectance spectrum of the 880 ,~ thick anodic oxide covered GaAs samples.
34
M. Garozzo et al. / GaAs shallow homojunction solar cells
Table 1 Calculated photocurrent densities for commercial bulk material and epitaxial thin film at three values of
xj
Xj (]k)
JL (mA/cm2) for commercial bulk material
JL (mA/cm2) for epitaxial thin film
500 1000 2000
20.5 18.6 15.9
25.3 22.9 19.2
Starting from the spectral responses shown so far, an evaluation of photocurrent densities can be made by using eq. (1). The incident solar photon flux F(X) at AM1 was obtained by a logarithmic interpolation of standard tables data at AM0 and AM1.5 [13]. Photocurrent densities were then normalized for a total irradiance of 100 m W / c m 2. The device reflectance R(~,) was measured on GaAs samples covered by an 880 ,~ thick anodic oxide and is shown in fig. 3. The values found for the photocurrent density are shown in table 1. High values of the photocurrent density can be obtained by means of very thin top layers and by employing high quality thin films. This latter improvement is around 20%.
2.2. Open circuit voltage, fill factor and conversion efficiency In GaAs p - n diodes the dark current is the sum of the injection current and the space charge recombination current. The first contribution however becomes predominant at higher voltages; if we assume that at the open circuit voltage the transport mechanism results essentially from carrier diffusion, the open circuit voltage is given by the following equation: Vo~= k__T_TIn ( J L / J g i f f + 1). (6) q If NA >> Nd (p+-n solar cell), the reverse dark current density jdirf is given by the following equation [14]: jgiff =
q n2Lp [ ( Sprp/Lp) ND---~p[ (Sprp/Lp)
cosh(H/Lp) sinh(H/Lp)
+ sinh(H/Lp) ] + cosh(H/Lp)
(7)
where n i is the intrinsic carrier concentration (for GaAs n~ = 1.1 × 107 cm -3 at T = 300 K). In bulk solar cells ( H >> Lp, Sp = ~ ) eq. (7) becomes: jgi+f =
n~Lp q ND~.p,
(7')
while in thin film solar cells with BSF (Sp = 0) eq. (7) becomes: j0diff=
n2Lp
{H
q~--~-Dzptgh~ ~p ).
(7")
M. Garozzo et al. / GaAs shallow homojunction solar cells
~
1.1
I
1'
1
35
I"
O 0
........ 0.9 0.0
.... l 0.2
0A
i ..................
0.6
0.8 H/Lp
1.0
Fig. 4. Open circuit voltage of (a) bulk devices and (b) thin film devices as a function of H / L p . N D = 6 × 1016 cm -3 in both cases.
Since the ratio Lp//'rp shows lower values in epitaxially grown matorial [15] than in the bulk one [10], we expect in the first case an improvement in the open circuit voltage. This improvement is further enhanced by the last term in eq. (7"). Since H >_ 2 ~m for a reasonable solar energy absorption, the term tgh(H/Lp)is effective only if the value of the minority carrier diffusion length is high enough. A comparison between the values of Vo~ in a thin film solar cell as a function of H/Lp and those obtained in a bulk solar cell can be made by using Lp = 5 ~m, ~-p= 200 ns and L p = 0.5 ~m, 'rp = 3 ns respectively. In both cases it is assumed that JL = 20 m A / c m 2 and N D = 6 × 1016 cm -3. Fig. 4 shows the two different contributions to the enhancement of Vow.The first one (AV1) is due to the higher material quality and does not depend on H; the second one (AVE) is related to the presence of a BSF and can be optimized by putting H = 2 ~tm.
Table 2 Photovoltaic performances expected for devices on commercial bulk and high quality epitaxial thin film material Active material Commercial bulk Epitaxial thin film ( H = 2 ~m)
jdiff
JL
Voc
( m A / c m 2)
( m A / c m 2)
(mV)
5.4 × 10-15 2.8 × 10 -16
20.5 25.3
929 1012
FF
Efficiency (%)
0.75 0,75
14.3 19.2
36
M. Garozzo et al. / GaAs shallow homojunction solar cells
Following the hypothesis of a diffusion-controlled transport mechanism, the use of BSF thin film devices can then improve the open circuit voltage by more than 8%. While the above assumption is reasonable to calculate Voc, it is not very realistic to evaluate the fill factor (FF), since, at low voltage, the recombination current density is generally not negligible. In order to estimate the conversion efficiency we assume the fill factor of both devices to be 0.75, which was experimentally obtained in several devices. It is thus possible to compare the simulated conversion efficiencies of bulk and BSF thin film devices. The photovoltaic performances of both devices, in table 2, were calculated by using the above reported values for the electrical parameters and by assuming a top layer thickness of 500 A. This comparison shows that the use of thin films can improve the conversion efficiency of GaAs p * - n solar cells about 30%.
3. Experimental 3.1. S a m p l e s
Two different kinds of samples were used in the present work. The commercially-obtained bulk materials by MCP Electronic Materials were (100) oriented n-doped (Te) single crystals with N D = 6 × 1016 c m - 3 and /~H = 3300 c m 2 V l s-1. These samples were degreased in acetone and then etched by a H 2 0 : H202 : H 2 S O 4 = 1 : 1:5 mixture at 50°C for 1 min. The other samples were epitaxial films grown by MO-CVD. The epitaxial growths were performed in a horizontal reactor operating in hydrogen at atmospheric pressure. The SiC-coated graphite susceptor was heated by rf. Trimethylgallium (TMG) and arsine (ASH3) were the reagents; their molar fractions were 1.9 × 10 -4 and 5 × 10 -3 respectively. The growth temperature was 670°C. The substrates were n + (ND = 5 × 1018 cm -3) commercial GaAs (100) oriented 2 ° off towards the (110) direction. The thickness of the epitaxial films, derived from the weight increment after the deposition process, was in the range 2-2.8 ~m. The films were unintentionally n-doped with N D = 7 × 1015-1 × 1016 c m 3. A typical value for the room temperature Hall mobility was 4000 cm2V l s - 1 Both kinds of samples were cleaned in the H C 1 / H 2 0 = 1 : 1 mixture just before the oxide deposition. 3.2. Solid-state diffusion
The Z n O - S i O 2 dopant source and the silica layer were single-run deposited by chemical vapor deposition in a resistance-heated, cold-wall vertical reactor, following the deposition method proposed by Shealy et al. [16]: the layers were obtained by diethylzinc (DEZ) and silane (Sill4) oxidation at 360°C using nitrogen as a carrier gas. The D E Z : Sill 4 ratio waS'4:5. Typical growth rates were in the range 200-300 A / m i n for both deposition processes, as resulted from interferometric measurements. Optimized values were 0.4 9 m for the Z n O - S i O 2 film thickness and
M. Garozzo et al. / GaAs shallow homojunction solar cells
37
0.5 ~m for the silica film. Before diffusion, the edges of the glassy layers were removed by H F in order to prevent any shunt effect. Diffusions were carried out in an open-tube furnace with a 85:15 nitrogen:hydrogen gas mixture flowing at 15 c m / m i n . Diffusion temperatures from 530 to 580°C and diffusion times from 45 min to 3 h were used. Junction depths and carrier concentration profiles were derived from a set ot two-points sheet resistance measurements carried out after successively removin~ well controlled G a A s layers. Two Ohmic contacts on the p ÷ layers were realized and protected by an insulating paint. GaAs anodic oxidation and subsequent oxide stripping by HC1 were used to peel the p+ layers [17,18]. A 3% aqueous solution ot tartaric acid in two parts of propylene glycol was the anodization electrolyte; the thickness of the removed layers was 13.3 A per volt of anodization potential as confirmed by interferometric measurements on the oxide layers. The oxidation process could be carried out up to the p+ layer exhaustion. The presence of a reverse biased barrier at the n-GaAs/electrolyte interface prevented any further anodic oxidation.
3.3. Device fabrication Ohmic back contacts were realized by vacuum evaporation of the A u - S n - A u structure [19]. Samples were then annealed at 415°C for 10 min. N o appreciable alteration of the previously diffused surface was observed. The antireflection coatin~ (ARC) was realized by anodic oxidation with a voltage drop of 44 V, correspondin~ to an oxide thickness of about 880 ,~. The grid structure of the solar cell was the~ photolitographically defined. G a A s was gold electroplated in the cyanide-less ECF6£ Engelhard solution at 55°C for 90 s. The typical deposition rate was about 0.1 ~t/min.
4. Results and discussion
4.1. Solid-state zinc diffusion As previously discussed very thin window layers are needed to reach higl7 photocurrent density values. From this point of view junctions realized by solid state diffusion are more suitable than those obtained by CVD. Solid state diffusion assures a more accurate control of the window laye~ thickness and of its homogeneity. The latter requirement is very important in the case of very thin p ÷ layers in order to avoid any device short circuit. Furthermore by solid state diffusion it is possible to obtain a heavily doped diffused layer and consequently, a very low sheet resistance value. Since SiO 2 has been widely employed as an encapsulant for silicon, m a n y author, [20,21] have attempted to use it as an encapsulant also for GaAs. Unfortunatel 3 silicon dioxide films tend to crack during high temoperature (800-900°C) diffusior processes if their thickness exceeds about 6000 A, the expansion coefficient ol
38
M. Garozzo et al. / GaAs shallow homojunction solar cells
silicon dioxide being one order of magnitude smaller than that of gallium arsenide [22]. Furthermore, at 750°C gallium out-diffusion occurs through SiO 2 caps [23]. A reproducible solid state diffusion of zinc in gallium arsenide was reported for the first time by Shealy et al. [24] using a cap layer of phosphosilicate glass with 20 wt.% of P205 in SiO 2, in order to match thermal expansion coefficients. This technique was successfully employed to prepare shallow p + - n homojunctions on bulk GaAs [25]. Borrego et al. [6] have fabricated a photovoltaic cell with a conversion efficiency of 12.6% at AM1 by using the same technique. Nevertheless zinc is a very fast diffuser in gallium arsenide [26], and it is thus possible to obtain shallow p + - n homojunctions by using SiO 2 instead of PSG as a cap. We have performed these diffusion processes at relatively low diffusion temperatures ( < 600°C) and with a well controlled value of the SiO 2 layer thickness (0.5 ~tm). In this case the SiO 2 film is thin enough to avoid any crack and thick enough to prevent any out-diffusion from the gallium arsenide surface. For temperatures lower than 600°C, furthermore, the arsenic vapour pressure is negligible [27], and the diffusion coefficients of arsenic, gallium [28] and zinc [24] into silicon dioxide are very small. Fig. 5 shows the doping profiles obtained on two samples diffused at 545°C for 1 h and at 570°C for 1.5 h respectively. The values of the carrier concentration were derived from the resistivity measurements, which are described in section 3.2, assuming a hole drift mobility of 40 cm 2 V - ] s - ] [25]. The corresponding diffusion depths d o are 1900 and 6800 .~, where d o is defined as the maximum thickness value at which it is still possible to carry out the anodic oxidation. 21 10
!
I
I
I
I
I
I
? E
1020 59. °-o~°A
A &&
•
AAA
AA
& A
O0 0
0
Z
0
0
0
10'" pz uJ ~J z
1018 O O nLU n--- 1017 ~1
QC
<[ O
lO
16
0,0
I
I
0.1
0.2
I---4
I
i
i
I
0.3 0.4 0.5 0.6 p+- LAYER DEPTH ()Jm)
I 02
0.8
Fig. 5. Doping profiles of two samples diffused at different conditions. Circles: 545°C for 1 h; triangles: 570°C for 1.5 h. The bars indicate the diffusion depths.
M. Garozzo et al. / GaAs shallow hornojunction solar cells
39
The obtained doping levels are very high ( = 10 20 c m - 3 ) and equal to those obtained by employing PSG as a cap [25]; in this way we can confirm that the SiO 2 layer is an effective cap. On the other hand the flatness of the curve in fig. 5 corresponding to the deeper diffusion and its abrupt fall near d o shows that the mixture Z n O - S i O 2 behaves as an infinite doping source, and then the zinc diffusion process is well described by a complementary error function. In this case the diffusion depth is given by:
(8)
do -- 1.1(Dr) 1/2, where t is the diffusion time and D is the diffusion coefficient: D = D o exp( - Ea/kT),
(9)
where E a is the activation energy. A set of diffusions at different temperatures (530-580°C) and times (45 m i n - 3 h) was carried out in order to obtain D as a function of the reciprocal temperature. The diffusion depths were in the range 700-11 000 ,~. The experimental data of D
-11
10
W
E
I-. z iii
m U.
16'3
{ la"l
16'" 1.1 RECIPROCAL
1.2 TEMPERATURE
1.3
1.4
1 0 ~ T ( K 1)
Fig. 6. Zn diffusion coefficient as a function of the reciprocal temperature.
40
M. Garozzo et al. / GaAs shallow homojunction solar cells
are shown in fig. 6. The best fit of these points, obtained using eqs. (8) and (9), is indicated as a continuous line. It corresponds to D o = 2.46 × 1013 cm2/s and E a = 4.6 eV. In all diffused samples we have obtained the same doping level with no evidence of damage to the GaAs surface. Some measurements in progress [29], performed by the scanning light spot technique [30], show that the diffused layers thickness is very uniform on the whole sample surface, even for very thin p+ layers (200-300 A). This technique is as practical and reproducible as the open-tube zinc diffusion employing PSG as a cap; furthermore our process is more convenient from the point of view of simplicity and safety. 4. 2. Spectral response and photocurrent
Using the above described results, we have prepared several devices on both commercial bulk materials and MO-CVD grown epitaxial thin films. These films were grown on heavily n-doped substrates to have a BSF. The external spectral response Qe(~) of such devices was measured in the 400-1000 nm spectral range, using the same computerized apparatus described in refs. [30,31]. Figs. 7 and 8 show the experimental external spectral responses of a bulk (sample m61) and of a thin film (sample M213C) device. Both devices have a 880 A thick ARC. The epitaxial layer of the sample M213C was 2.1 ~tm thick and its intrinsic donor concentration was 7 × 10 ~5 cm -3, as resulted from C - 2 - V measurements. A quantitative estimate of the most interesting device parameters, such as the junction depth ~ and the minority carrier diffusion length Lp, can be obtained by
1.0
I
I
I
I
I
A
0.8
m 61
UJ (/)
z
o
0.6
a. or) uJ
0,4 ,-I
n.I-. 0,2 a.
ffJ 0.0 400
500
600 700 WAVELENGTH
800 (nm)
900
Fig. 7. Experimental external spectral response of the m61 bulk device.
1000
M. Garozzo et al. / GaAs shallow homojunction solar cells
1.0
v 0
|
I
I
I
41
I
0.8 M 213 C
kg
0.6 0 tl) tu iv.
0A
-I
,¢ tr IO
0.2
O. t/)
0~
I 500
400
I I 600 700 WAVELENGTH
800 (nm)
900
1000
Fig. 8. Experimental external spectral response of the 10213C thin film device.
carrying out the best fit of the experimental internal spectral response. We prove that it is possible to have a meaningful fit of Q(?~) by using eqs. (2)-(5), in spite of the presence of several parameters. The number of these parameters can in fact be remarkably reduced if we analyse each single term on the right hand side of eq. (2). The electron contribution Q,(X) is simplified by two different approximations. Since we analyse just shallow homojunctions, it is possible to write X / L n << 1. Furthermore, owing to the high value of the GaAs absorption coefficient, we have 2 2 a L n >> l. Since L n = (Dn-rn) 1/2 and the minority carrier diffusion coefficient D, is related to the electron mobility by the Einstein relationship D n = VT#,, with V T = k T / q = 25.85 mV at room temperature, eq. (3) becomes: a--V-~v~(1 - e-~X~) + 1 Qn()~) =
XjSn
- e-'~x~ '
(3')
+ ]
and in Qn(X) only two parameters are present: Xj and the S . / I ~ . ratio. As regards Qp(X), this term must be written in two different ways corresponding to bulk and BSF thin film devices respectively. For bulk devices (Sp = o0, H / L p >> 1) eq. (4) becomes: Qp(X) =
aLp
e x p [ - a ( X j + W)] aLp + 1
(4')
M. Garozzoet al. / GoAsshallowhomojunction solar cells
42
while for BSF thin film devices (Sp = 0) eq. (4) becomes:
Qp(X ) = aLo e x p [ - a ( Xj + W)]
sinh( H / L p ) + HaLp / L pe) x p ( - a H ) (4")
In eq. (4") H is not a parameter because H = H 0 - Xj - W = H(Xj), where H 0 is the known epitaxial layer thickness. Qp(;k) depends therefore, in both cases, on Xj and Lp only. Q ( ~ ) given in eq. (2) is then a function of only three parameters: Xj, Lp and Sn//~ ". The first one is effective in the whole spectral range, especially in the s h o r t - m e d i u m wavelength region. The second one is effective in the long wavelength region. The ratio Sn//~, is effective only in the blue region, where it strongly affects the internal quantum efficiency shape. In fig. 9 dots are the experimental values of the internal spectral response of the sample m61 obtained from the data of Qe(X) and from the experimental reflectance spectrum. The continuous line shows the best fit obtained from eqs. (3'), (4') and (5). The fit gives Lp = 0.63 ~m, Xj = 2000 ,~ and Sn/~n 1.5 X 104 V / c m . The best fit of Q(?,) for the sample M213C (fig. 10), obtained in the same way as for the sample m61 apart from replacing eq. (4') by eq. (4"), gives Lp = 5.4 ~m, Xj = 660 ,~ and the same value of Sn/l-t n" AS expected the improvement at short wavelengths is due to the thinner p ÷ layer, while the enhanced response at medium-long wavelengths arises from the high value of the minority carrier diffusion length. =
1.0
m 61
o 0.8 LU U:I
z 0.6 o
(3.. (/) I,I n"
0.4 .J E: I.-
0.2 O. (/)
0.0
400
I 500
I I 600 700 WAVELENGTH
I 800 (nm)
I 900
1000
Fig. 9. Experimental internal spectral response of the sample m61: the dots are the experimental values and the continuous line is the best fit curve.
43
M. Garozzo et al. / GaAs shallow homojunction solar cells 1.0
I
I
I
I
I
I 500
I 600
I 700
I 800
I 900
0 0.8 UJ
0.6
o
IX
GO UJ IZ
0.4
.-I
n,, I-
0.2
U.I IX
0.0 400
WAVELENGTH
1000
(nm)
Fig. 10. Experimental internal spectral response of the sample M213C: the dots are the experimental values and the continuous line is the best fit curve.
I n table 3 we report the j u n c t i o n depths a n d the m i n o r i t y carrier diffusion lengths as o b t a i n e d from the best fit of the i n t e r n a l spectral response of several bulk devices. It can be seen that the values of Lp are in the range 0.2-1.3 ~tm a n d that the evaluation of Lp is i n d e p e n d e n t o n the value f o u n d for the j u n c t i o n depth. T h e same quantities for thin film devices are shown in table 4 together with their epitaxial layer thickness. I n this case the values of Lp are almost one order ot m a g n i t u d e larger (3.5-5.5 ~m). As regards to the ratio Sn//~tn, the same value (1.5 × 10 a V / c m ) resulted from al] best fits performed o n b o t h bulk a n d thin film devices. This is a resonable result, as
Table 3 Xj and Lp as obtained from the best fit of the internal quantum efficiency in commercial bulk device~, Sample
Xj (m)
Lp (Ixm)
m48 m49 m51 m53 m55 m61 m63A m63B m63C m63D m64
900 14000 1400 1800 350 2000 1600 2600 6600 750 250
0.88 0.82 1.25 0.47 0.86 0.63 0.78 1.26 0.22 1.26 0.50
44
M. Garozzo et al. / GaAs shallow homojunction solar cells
Table 4 Xj and Lp as obtained from the best fit of the internal q u a n t u m efficiency in epitaxial thin film devices. H 0 is the epitaxial layer thickness Sample
H 0 (jzm)
Xj (A)
Lp (~tm)
M182 M192 M193 M212 M213A M213B M213C
2.7 2.6 2.5 2.2 2.1 2.1 2.1
2500 2500 1150 1450 1500 700 660
5.0 3.5 5.5 3.9 4.9 4.9 5.4
the electrical properties of the heavily doped diffused layers do not depend on the quality of the employed material, but they are strictly correlated to the solid state diffusion conditions. In order to check the reliability of our best fit method, some devices were peeled using anodic oxidation and oxide removal; the diffusion depth d o was obtained peeling the film until the anodic oxidation could be carried out as explained in section 3.2. Fig. 11 shows the comparison between the values of d o and the values of Xj obtained from the photoresponse best fit, previously performed on the same samples. The straight line represents the expected correlation between the two quantities. The good agreement shows that the used best fit method is very reliable. 10
,
,
,
,
,
,
,
,
,
v
,
,
,
,
,
,
,
,
,
=k
x 1
"r" I'e~ a Z
O t-
0.1
Z
0.01
0.01
I
,
0.1
DIFFUSION
I
1
DEPTH
10
do(pm)
Fig. 11. Comparison between the junction depth Xj, as obtained from the photoresponse best fit, and the diffusion depth d o , as obtained from the peeling. The continuous line represents the expected correlation.
45
M. Garozzo et al. / GaAs shallow homojunction solar cells
Table 5 Calculated and experimental photocurrent densities at AM1 for bulk and thin film devices Sample
Xj (m)
Lp (l~m)
JL (mA//cm2) Calculated
Experimental
Bulk m61 m51 m48
2000 1400 900
0.63 1.25 0.88
16.3 19.3 20.3
16.1 18.2 18.4
Thin films M213A M213B M213C
1500 700 660
4.9 4.9 5.4
21.0 24.5 24.7
19.0 24.1 24.2
As a further demonstration, we have used the values of Xj and Zp from the photoresponse best fit to calculate the photocurrent densities at AM1 for some devices. Such calculated values are compared in table 5 with those experimentally obtained on the same samples at simulated AM1, by considering only the active area of the devices. Also in this case a good agreement is obtained. The experimental values reported in table 5 demonstrate that the photocurrent density is, as expected, strongly affected by the junction depth. It is also apparent that the use of high quality thin films increases the photocurrent density by about 20%. We can therefore conclude that, as regards to the charge collection, our experimental results confirm the reliability of the computer simulation. 4. 3. Solar cell efficiency The solar energy conversion efficiency was in the 10-12% range for bulk devices and in the 12-14% range for thin film devices. The best result was achieved on the 1 cm 2 active area thin film device M213B. We obtained, under simulated AM1 conditions (fig. 12), a short circuit current density of 24.1 m A / c m 2, an open circuit voltage of 770 mV, a fill factor of 0.76 and a conversion efficiency of 14.1%. In fig. 13 the external spectral response, measured by avoiding any light reflection from the grid, is reported. Fig. 14 gives the dark current density-voltage characteristics at direct polarization. The curve shows a depletion layer recombination mechanism, dominating at low voltages ( < 400 mV). This fact is typical for diodes realized on GaAs as well as on any other high energy gap semiconductor. For this reason it is quite difficult tc obtain fill factor values much greater than 0.76. As regards to the open circuit voltage, it is limited by the transport mechanism, which, even at a relatively high voltage, cannot be described as a pure diffusion-controlled mechanism. The slope of the log J - V curve at V > 500 mV corresponds tc n = 1.27 and Jo = 10-a2 A/cm2 (fig. 14). This value of J0 is much higher than expected in the case of a diffusion-controlled transport mechanism. At a dopin~
M. Garozzo et al. / GaAs shallow homojunction solar cells
46 40
I
I M 213
I
I
600
800
B
30 E I- 20 z LU OC ~C .-~ 10-
0 0
I
I
200
400
I
VOLTAGE Fig. 12. Photo l - V p o w e r point.
1000
(mV)
characteristics of the M213B device at s i m u l a t e d AM1. The dot is the m a x i m u m
level of 7 x 1015 cm 3 and using the reported values for the other parameters of the M213B device, we should have, in fact, J0 = 2.7 × 10 t8 A / c m 2. The value of Vo~(770 mV) is then lower than that expected (950 mV). A higher doping of the active material, for example N D -- 6 x 1016 c m - 3 , should
1.0
I
I
i
I
@
O
0,8
i,i Z
O ilL. t/) er
0.6
0.4
.J ne I-O ILl a.
0.2
0.0 400
I
500
I
I
600 700 WAVELENGTH
I
800 (nm)
IN 900
Fig. 13. E x p e r i m e n t a l external spectral response of the M213B device.
1000
47
M. Garozzo et al. / GaAs shallow homojunction solar cells -1
lO
|
I
M 213
-2
I
l
B
lO E
-3 lO
>. I--4
u> 10 z 14.1
16 s UJ
u
/I
-6
lO
I
n = 1.27 J ° = 1()12 A / c m 2
-7
lO
I
I I I
-8
10
0.0
I
I
I
I
0.2
0.4
0.6
0,8
VOLTAGE
(VI
1.0
Fig. 14. Dark log J - V characteristics of the M213B device.
increase the importance of the diffusion transport mechanism enhancing the open circuit voltage and, consequently, the conversion efficiency.
5. Conclusion We have demonstrated, on the basis of a computer simulation, that an improvement of about 30% can be obtained in the solar energy conversion efficiency of shallow p + - n GaAs solar cells by using high quality epitaxial thin film as an active region. GaAs epitaxial thin films were grown by metal organic chemical vapor deposition. Shallow homojunctions were prepared by open-tube Zn solid state diffusion at low temperature (< 600°C), by using the mixture ZnO/SiO 2 as a doping source and a simple SiO2 layer as a cap; this technique is quite simple and does not imply any safety problem. The p+ layers were well controlled in thickness, heavily doped
48
M. Garozzo et al. / GaAs shallow homojunction solar cells
a n d v e r y h o m o g e n e o u s . N o d a m a g e was o b s e r v e d on the G a A s s u r f a c e a f t e r the d i f f u s i o n process. A best fit m e t h o d was a p p l i e d to the e x p e r i m e n t a l p h o t o r e s p o n s e s p e c t r a o f d e v i c e s p r e p a r e d on b o t h bulk m a t e r i a l s a n d thin e p i t a x i a l films. A c o m p a r i s o n b e t w e e n the two d i f f e r e n t k i n d s of d e v i c e s s h o w s that in the e p i t a x i a l films the m i n o r i t y c a r r i e r d i f f u s i o n l e n g t h is a l m o s t o n e o r d e r o f m a g n i t u d e l a r g e r t h a n in the b u l k m a t e r i a l a n d the r e s p o n s e at l o n g w a v e l e n g t h s is r e m a r k a b l y i n c r e a s e d . E x p e r i m e n t a l p h o t o c u r r e n t d e n s i t i e s w e r e in g o o d a g r e e m e n t w i t h c a l c u l a t e d v a l u e s a n d i n c r e a s e d by a b o u t 2070 on p a s s i n g f r o m b u l k to thin film devices. A c o n v e r s i o n e f f i c i e n c y of 14.1% at A M 1 o n a 1 c m 2 a r e a thin film d e v i c e was o b t a i n e d . T h i s e f f i c i e n c y v a l u e can be f u r t h e r i m p r o v e d by i n t e n t i o n a l l y d o p i n g the a c t i v e layer in o r d e r to r e d u c e J0 i.e. to i n c r e a s e V,~.
Acknowledgements T h e a u t h o r s are v e r y g r a t e f u l to Dr. F. G a l l u z z i for h e l p f u l discussions. T h e y w o u l d also like to a c k n o w l e d g e Dr. G. G r i l l o for h a v i n g w o r k e d o u t the A M 1 solar s p e c t r u m data.
References [1] [2] [3] [4] [5] [6] [71 [8] [9] [10] [11] [12l [13]
[14] [15] [16] [/7] [18] [19] [20] [21] [22]
R.J. Stirn, K.L. Wang and Y.C.M. Yeh, 15th Rec. IEEE Photo. Spec. Conf., Orlando (1981) p. 1045. M. Garozzo, G. Conte, F. Evangelisti and G. Vitali, Appl. Phys. Lett. 41 (1982) 1070. M. Garozzo, D. Margadonna and A. Parretta, Mat. Chem. Phys. 9 (1983) 157. C.O. Bozler, R.W. McClelland, J.P. Salerno and J.C.C. Fan, J. Vac. Sci. Technol. 20 (1982) 720. J.C.C. Fan, C.O. Bozler and R.L. Chapman, Appl. Phys. Lett. 32 (1978) 390. J.M. Borrego, R.P. Keeney, I.B. Bhat, K.N. Bhat, L.G. Sundaram and S.K. Ghandhi, 16th Rec. IEEE Photo. Spec. Conf., San Diego (1982) p. 1157. R.J. Field and S.K. Ghandhi, J. Electrochem. Soc. 129 (1982) 1567. A.J. Spring Thorpe and M.N. Svilans, Gallium Arsenide and Related Compounds 1982 (The Institute of Physics, Bristol and London, 1983) p. 589. H.J. Hovel, Semiconductors and Semimetals, vol. 11 (Academic Press, New York, 1975) p. 17. H.J. Hovel, Semiconductors and Semimetals, vol. 11 (Academic Press, New York, 1975) p. 25. D.E. Aspnes and A.A. Studna, Phys. Rev. B27 (1983) 985. S.M. Sze, Physics of Semiconductor Devices (Wiley, New York, 1981) p. 40. Standard Procedures for Terrestrial Photovoltaic Performance Measurements, Specification No. 101, Issue 2, Commission of the European Communities, Joint Research Centre - Ispra Establishment, Italy, EUR 7078 EN (1980). H.J. Hovel, Semiconductors and Semimetals, vol. 11 (Academic Press, New York, 1975) p. 51. G.W. 't Hooft, C. Van Opdorp, H. Veenvliet and A.T. Vink, J. Crystal Growth 55 (1981) 173. J.R. Shealy, B.J. Baliga, R.J. Field and S.K. Ghandhi, J. Electrochem. Soc. 128 (1981) 558. H. Hasegawa and H.L. Hartnagel, J. Electrochem. Soc. 123 (1976) 713. T. Ishii and B. Jeppsson, J. Electrochem. Soc. 124 (1977) 1784. R. Gulati, R.K. Purohit and I. Chandra, J. Inst. Telecomm. Engrs. 15 (1969) 815. A.G. Foyt, J.P. Donnelly and W.T. Lindley, Appl. Phys. Lett. 14 (1969) 73. W. von Munch, GaAs Planar Technology, IBM J. Res. Develop, 10 (1966) 438-445. B.J. Baliga and S.K. Ghandhi, IEEE Trans. Electron Devices ED-19 (1972) 761.
M. Garozzo et al. / GaAs shallow homojunction solar cells
[23] [24] [25] [26] [27] [28] [29] [30] [31]
49
J. Gyulai, J.W. Mayer, I.V. Mitchell and V. Rodriguez, Appl. Phys. Lett. 17 (1970) 332. J.R. Shealy, B.J. Baliga and S.K. Ghandhi, IEEE Electron Device Lett. EDL-I (1980) 119. S.K. Ghandhi and R.J. Field, Appl. Phys. Lett. 38 (1981) 267. L.R. Weisberg and J. Blanc, Phys. Rev. 131 (1963) 1548. L. Hollan, J.P. Hallais and J.C. Brice, Current Topics in Materials Science, vol. 5 (North-Holland, Amsterdam, 1980) p. 179. H. Hayashi, K. Kikuchi, T. Yamaguchi and T. Nakahara, Gallium Arsenide and Related Compounds 1980 (The Institute of Physics, Bristol and London, 1981) p. 275. G. Garozzo, A. Parretta and G. Maletta, unpublished data. L. De Angelis, E. Scaf~, F. Galluzzi, L. Fornarini and B. Scrosati, J. Electrochem. Soc. 129 (1982) 1237. E. Scaf+, G. Maletta, R. Tomaciello, P. Alessandrini, A. Camanzi, L. De Angelis and F. Galluzzi, Solar Cells 10 (1983) 17.
29
GaAs S H A L L O W H O M O J U N C T I O N S O L A R CELLS FABRICATED O N T H I N EPITAXIAL FILMS BY A S I M P L E Zn S O L I D STATE DIFFUSION METHOD
M. GAROZZO *, A. PARRETTA, G. MALETTA, V. ADONCECCHI and M. GENTILI ** Eniricerche, S.p.A., Via Ramarini 32, 00015 Monterotondo, Rome, Italy Received 28 June 1985; in revised form 3 April 1986 Computer simulation shows that high photocurrent densities can be achieved on p ÷ - n shallow G a A s solar cells by using high quality thin epitaxial films. The simple addition of a back surface field also improves open circuit voltages and conversion efficiencies. Unintentionally n-doped GaAs epitaxial films ( = 2 t~m thick) were grown by metal organic chemical vapor deposition. A simple 0.5 rtm thick SiO 2 layer was used as a cap in the open-tube Zn solid-state diffusion technique. Diffusion processes were carried out on both commercial bulk material and epitaxial films at low temperature ( < 600°C) by using the Z n O / S i O 2 mixture as a doping source. The p+ layers were well controlled in thickness in the range 250-14000 A, heavily doped ( --1020 cm - 3 ) and very homogeneous. No damage was observed on the G a A s surface after the diffusion process. A best fit of the experimental photoresponse spectra yielded reliable values of the diffused layers thicknesses and of the minority carrier diffusion lengths. These were in the range 0.2-1.3 I~m for commercial material and 3.5-5.5 ~tm for epitaxial films. Good agreement was found between the experimental photocurrent values and the calculated ones in both bulk and thin film devices. The thin film photocurrent density reached values as high as 24.2 m A / c m 2. A conversion efficiency of 14.1% at AM1 was obtained on a thin film device with an area of 1 cm 2 and with a p+ layer thickness of 700 A.
1. Introduction
It is well known that gallium arsenide solar cells show the highest conversion efficiency. This material presents some difficulties in terrestrial large scale application because of the high cost of gallium and the sophisticated technologies involved. Owing to the high value of the gallium arsenide absorption coefficient, it is, nevertheless, possible to fabricate high performance thin film devices, taking advantage of the high quality of the material grown by chemical vapor deposition. Besides that, GaAs shows other interesting properties: it is radiation resistant and it is suitable for monolithic tandem cells. From this point of view it could be much more promising than silicon for very high efficiency solar cells. In order to overcome the problems related to the cost, germanium-coated silicon [1-3] and CLEFT [4] processes seem to be very interesting. The metal organic chemical vapor deposition (MO-CVD) seems moreover to be the most promising solution for large scale production [1]. * Present address: ENEA, Casaccia, Rome, Italy. ** Present address: lESS, Via Cineto R o m a n o 42, Rome, Italy.
M. Garozzo et al. / GaAs shallow homojunction solar cells
30
In GaAs concentration solar cells a GaA1As top layer is generally used in order to strongly reduce the surface recombination velocity. Unfortunately the use of this ternary compound entails some technological problems owing to the high aluminum reactivity, the need of a very accurate control of the alloy composition and the difficulty to obtain satisfactory Ohmic contacts. On the other hand in the case of flat panels high conversion efficiencies are obtained by preparing GaAs p - n homojunctions [5]. In this paper we show, by a computer simulation, that, in order to reduce surface recombination as much as possible, the top layer must be very thin (0.05-0.1 ~m) and then heavily doped to avoid sheet resistance. Our calculations also show that the use of high quality thin epitaxial films as an active material results in a remarkable enhancement of spectral photoresponse owing to the large values of the minority carrier diffusion lengths. This fact, in combination with an easily added back surface field (BSF), allows high conversion efficiencies ( > 19%) to be reached. GaAs p + - n junction solar cells fabricated by Borrego et al. [6] by open-tube solid-state diffusion are very interesting for both the accurate control of the junction depth and the possibility of a large scale production. In this paper we present a method for fabricating p + - n junctions by zinc diffusion from Zn-doped silica as a source, covered by a SiO 2 layer only. The open-tube diffusion was performed at a relatively low temperature, thus avoiding the use of phosphosilicate glass (PSG) as a cap. The results on the Zn diffusion coefficient are quite similar to those obtained with a PSG cap [7] and to those obtained by the box-diffusion technique [8]. The diffused layers are heavily doped ( = 1020 cm-3), homogeneous and well controlled in thickness. p ÷ - n devices were fabricated in this way on both bulk material and MO-CVD grown thin films. The minority carrier diffusion length Lp and the junction depth Xj were derived, in both cases, by the best fit of the experimental internal spectral response. The values of Lp obtained for the epitaxial layers are almost one order of magnitude larger than those in the bulk material. By using these values of Lp and Xj, the expected values of the photocurrent density were calculated. A good agreement was found between these theoretical values and the experimental ones. The photocurrent density of the thin film devices is about 20% greater than that measured on bulk material, as expected from the computer simulation. The best photovoltaic performance consists in a conversion efficiency of 14.1% at AM1 on an AR coated epitaxial thin film device with an area of 1 cm2. 2. GaAs p+-n homojunction solar cells: a computer simulation
2.1. Charge collection The photocurrent density JL is expressed by the following equation:
JL = q f Q(X) F(X) (1 - R ( X ) ) d X , J
(1)
31
M. Garozzo et aL / GaAs shallow homojunction solar cells
where q is the magnitude of the electron charge, F(~,) is the density of the incident solar photon flux per unit bandwidth, R()~) is the device reflectance and Q()~) is the internal spectral response: Q(2t) = Q,(2t) + Qp(X) + Qdr(X), (2) where Qn(~) is the contribution of the electrons from the p-side, Qp(~) is that of the holes from the n-side and Qdr(X) from the depletion region. Adopting the simple model of an abrupt p-n homojunction, the three terms of the internal spectral response are given by the following equations [9]: Qo(X)
aLn
-an' - ln" + aL n Ln
a2LZ"-I ~Sn~'nsinh(L~)+cosh(~) exp( - aXJ) { ~ c ° s h ( L ~ ) + sinh(~) }
Sn' Lnrnsinh( L~ ) + c°sh( L~ ) - a L n exp(- aXj)),
Qp(X)
(3)
aLp exp(-a(Xj+ W)} z~'Sa Lp-1 [ x
Sp'rP{cosh(~p ) - exp(- all)) + sinh(~p ) ~L~ -
S~--;~C--- ~ - . . . .
-
G
smh(--1 + cosh(--1
\G ]
aLp
~G ]
exp(- all)
]
+ Sp~'p [ H ) (~_~p) ' sin (K + cos
(4)
0ar(~k) = exp(-aXj) [1 - exp(-aW)], (5) where Lp, 'rp and Sp are the minority carrier diffusion length, lifetime and surface recombination velocity in the n-side and L., .r., S. are the same quantities in the p-side, a = a(k) is the GaAs absorption coefficient, Xj is the junction depth, W is the depletion layer thickness and H is the base region thickness minus W.
M. Garozzo et al. / GaAs shallow homojunction solar cells
32 1.0
a
o
0.8
Z 0 0.6 o. oO I.U
j
-J 0 . 4 I-w
n 0.2 O0
o.o 400
5~)0
i 6 0I 0 700 WAVELENGTH
8 0I 0 (nm)
9()0
1000
Fig. 1. Calculated internal spectral response for bulk devices with Lp = 0.5 ~m. (a) Xj = 500 A; (b)
xj = 1000 A; (c) x~ = 2000 A.
The junction depth is effective on all the three terms of the spectral response; it affects therefore the photoresponse in the whole spectral range. The behaviour of Q ( ~ ) at different values of ~ may be evaluated assuming S n = 10 7 c m / s , 7n = 0.6 ns, L, = 1.6 ~ m , L p = 0.5 p~m and the donor concentration N o = 6 × 1016 cm 3, typical values for devices realized on bulk material [10]. If an Ohmic back contact is assumed (Sp = oo), Q(~,) is, moreover, independent of 1-p. Absorption coefficient data were derived by Aspnes and Studna [11]. The few data below 1.5 eV were calculated from the relation [12] a - ( h i , - E g ) 1/2 and normalized to match the experimental ones. Doping effects were not accounted for. Fig. 1 shows calculated internal spectral responses for bulk devices with Xj = 500 A, 1000 ,~ and 2000 ,~ respectively and with the above reported values for the other parameters. The spectral response is found to increase with decreasing junction depth, especially at short-medium wavelengths, owing to the high surface recombination velocity and to the very large absorption coefficient. At long wavelengths the carrier collection is strongly dependent on the transport properties of the active material, particularly on the minority carrier diffusion length L p . Devices fabricated on epitaxially grown thin films show values of Lp as high as 5 ~tm at an appropriate donor concentration (1016-1017cm 3) [6]. Moreover the epitaxial deposition of the active material makes it easier to add a back surface field, which increases the carrier collection efficiency even more ( s p = 0). Fig. 2 shows the internal spectral response calculated for a 2 ~m thick epitaxial film with Lp = 5 ~m and Sp = 0 at different values of Xj. All other parameters are
33
M. Garozzo et al. / Gads shallow homojunction solar cells 1.0
I
I
I
I
I 500
I 600
I 700
I 800
O 0.8 U,,I z O n
O.6
uJ n,. ..I
0.4
I"' 0.2 a. u~
0,0 400
WAVELENGTH
I, 900
1000
(nm)
Fig. 2. Calculated internal spectral response for thin film devices with Lp = 5 ~m. (a) Xj = 500 ,~; (b)
xj = ]0oo A; (c) xj = 2000 A. the same as for fig. 1. Also in this c a s e Q p ( ~ ) does not depend on rp. A c o m p a r i s o n between the curves of fig. 2 and fig. 1 clearly shows the large e n h a n c e m e n t of the internal spectral response at medium-long wavelengths. 1,0
w
i
I
I
I
800
900
A
O.8 iJJ (,.) z
<~ 0.6 u,l .J ~: 0 . 4
°1% 400
500
I 600 700 WAVELENGTH
1000
(nm)
Fig. 3. Typical reflectance spectrum of the 880 ,~ thick anodic oxide covered GaAs samples.
34
M. Garozzo et al. / GaAs shallow homojunction solar cells
Table 1 Calculated photocurrent densities for commercial bulk material and epitaxial thin film at three values of
xj
Xj (]k)
JL (mA/cm2) for commercial bulk material
JL (mA/cm2) for epitaxial thin film
500 1000 2000
20.5 18.6 15.9
25.3 22.9 19.2
Starting from the spectral responses shown so far, an evaluation of photocurrent densities can be made by using eq. (1). The incident solar photon flux F(X) at AM1 was obtained by a logarithmic interpolation of standard tables data at AM0 and AM1.5 [13]. Photocurrent densities were then normalized for a total irradiance of 100 m W / c m 2. The device reflectance R(~,) was measured on GaAs samples covered by an 880 ,~ thick anodic oxide and is shown in fig. 3. The values found for the photocurrent density are shown in table 1. High values of the photocurrent density can be obtained by means of very thin top layers and by employing high quality thin films. This latter improvement is around 20%.
2.2. Open circuit voltage, fill factor and conversion efficiency In GaAs p - n diodes the dark current is the sum of the injection current and the space charge recombination current. The first contribution however becomes predominant at higher voltages; if we assume that at the open circuit voltage the transport mechanism results essentially from carrier diffusion, the open circuit voltage is given by the following equation: Vo~= k__T_TIn ( J L / J g i f f + 1). (6) q If NA >> Nd (p+-n solar cell), the reverse dark current density jdirf is given by the following equation [14]: jgiff =
q n2Lp [ ( Sprp/Lp) ND---~p[ (Sprp/Lp)
cosh(H/Lp) sinh(H/Lp)
+ sinh(H/Lp) ] + cosh(H/Lp)
(7)
where n i is the intrinsic carrier concentration (for GaAs n~ = 1.1 × 107 cm -3 at T = 300 K). In bulk solar cells ( H >> Lp, Sp = ~ ) eq. (7) becomes: jgi+f =
n~Lp q ND~.p,
(7')
while in thin film solar cells with BSF (Sp = 0) eq. (7) becomes: j0diff=
n2Lp
{H
q~--~-Dzptgh~ ~p ).
(7")
M. Garozzo et al. / GaAs shallow homojunction solar cells
~
1.1
I
1'
1
35
I"
O 0
........ 0.9 0.0
.... l 0.2
0A
i ..................
0.6
0.8 H/Lp
1.0
Fig. 4. Open circuit voltage of (a) bulk devices and (b) thin film devices as a function of H / L p . N D = 6 × 1016 cm -3 in both cases.
Since the ratio Lp//'rp shows lower values in epitaxially grown matorial [15] than in the bulk one [10], we expect in the first case an improvement in the open circuit voltage. This improvement is further enhanced by the last term in eq. (7"). Since H >_ 2 ~m for a reasonable solar energy absorption, the term tgh(H/Lp)is effective only if the value of the minority carrier diffusion length is high enough. A comparison between the values of Vo~ in a thin film solar cell as a function of H/Lp and those obtained in a bulk solar cell can be made by using Lp = 5 ~m, ~-p= 200 ns and L p = 0.5 ~m, 'rp = 3 ns respectively. In both cases it is assumed that JL = 20 m A / c m 2 and N D = 6 × 1016 cm -3. Fig. 4 shows the two different contributions to the enhancement of Vow.The first one (AV1) is due to the higher material quality and does not depend on H; the second one (AVE) is related to the presence of a BSF and can be optimized by putting H = 2 ~tm.
Table 2 Photovoltaic performances expected for devices on commercial bulk and high quality epitaxial thin film material Active material Commercial bulk Epitaxial thin film ( H = 2 ~m)
jdiff
JL
Voc
( m A / c m 2)
( m A / c m 2)
(mV)
5.4 × 10-15 2.8 × 10 -16
20.5 25.3
929 1012
FF
Efficiency (%)
0.75 0,75
14.3 19.2
36
M. Garozzo et al. / GaAs shallow homojunction solar cells
Following the hypothesis of a diffusion-controlled transport mechanism, the use of BSF thin film devices can then improve the open circuit voltage by more than 8%. While the above assumption is reasonable to calculate Voc, it is not very realistic to evaluate the fill factor (FF), since, at low voltage, the recombination current density is generally not negligible. In order to estimate the conversion efficiency we assume the fill factor of both devices to be 0.75, which was experimentally obtained in several devices. It is thus possible to compare the simulated conversion efficiencies of bulk and BSF thin film devices. The photovoltaic performances of both devices, in table 2, were calculated by using the above reported values for the electrical parameters and by assuming a top layer thickness of 500 A. This comparison shows that the use of thin films can improve the conversion efficiency of GaAs p * - n solar cells about 30%.
3. Experimental 3.1. S a m p l e s
Two different kinds of samples were used in the present work. The commercially-obtained bulk materials by MCP Electronic Materials were (100) oriented n-doped (Te) single crystals with N D = 6 × 1016 c m - 3 and /~H = 3300 c m 2 V l s-1. These samples were degreased in acetone and then etched by a H 2 0 : H202 : H 2 S O 4 = 1 : 1:5 mixture at 50°C for 1 min. The other samples were epitaxial films grown by MO-CVD. The epitaxial growths were performed in a horizontal reactor operating in hydrogen at atmospheric pressure. The SiC-coated graphite susceptor was heated by rf. Trimethylgallium (TMG) and arsine (ASH3) were the reagents; their molar fractions were 1.9 × 10 -4 and 5 × 10 -3 respectively. The growth temperature was 670°C. The substrates were n + (ND = 5 × 1018 cm -3) commercial GaAs (100) oriented 2 ° off towards the (110) direction. The thickness of the epitaxial films, derived from the weight increment after the deposition process, was in the range 2-2.8 ~m. The films were unintentionally n-doped with N D = 7 × 1015-1 × 1016 c m 3. A typical value for the room temperature Hall mobility was 4000 cm2V l s - 1 Both kinds of samples were cleaned in the H C 1 / H 2 0 = 1 : 1 mixture just before the oxide deposition. 3.2. Solid-state diffusion
The Z n O - S i O 2 dopant source and the silica layer were single-run deposited by chemical vapor deposition in a resistance-heated, cold-wall vertical reactor, following the deposition method proposed by Shealy et al. [16]: the layers were obtained by diethylzinc (DEZ) and silane (Sill4) oxidation at 360°C using nitrogen as a carrier gas. The D E Z : Sill 4 ratio waS'4:5. Typical growth rates were in the range 200-300 A / m i n for both deposition processes, as resulted from interferometric measurements. Optimized values were 0.4 9 m for the Z n O - S i O 2 film thickness and
M. Garozzo et al. / GaAs shallow homojunction solar cells
37
0.5 ~m for the silica film. Before diffusion, the edges of the glassy layers were removed by H F in order to prevent any shunt effect. Diffusions were carried out in an open-tube furnace with a 85:15 nitrogen:hydrogen gas mixture flowing at 15 c m / m i n . Diffusion temperatures from 530 to 580°C and diffusion times from 45 min to 3 h were used. Junction depths and carrier concentration profiles were derived from a set ot two-points sheet resistance measurements carried out after successively removin~ well controlled G a A s layers. Two Ohmic contacts on the p ÷ layers were realized and protected by an insulating paint. GaAs anodic oxidation and subsequent oxide stripping by HC1 were used to peel the p+ layers [17,18]. A 3% aqueous solution ot tartaric acid in two parts of propylene glycol was the anodization electrolyte; the thickness of the removed layers was 13.3 A per volt of anodization potential as confirmed by interferometric measurements on the oxide layers. The oxidation process could be carried out up to the p+ layer exhaustion. The presence of a reverse biased barrier at the n-GaAs/electrolyte interface prevented any further anodic oxidation.
3.3. Device fabrication Ohmic back contacts were realized by vacuum evaporation of the A u - S n - A u structure [19]. Samples were then annealed at 415°C for 10 min. N o appreciable alteration of the previously diffused surface was observed. The antireflection coatin~ (ARC) was realized by anodic oxidation with a voltage drop of 44 V, correspondin~ to an oxide thickness of about 880 ,~. The grid structure of the solar cell was the~ photolitographically defined. G a A s was gold electroplated in the cyanide-less ECF6£ Engelhard solution at 55°C for 90 s. The typical deposition rate was about 0.1 ~t/min.
4. Results and discussion
4.1. Solid-state zinc diffusion As previously discussed very thin window layers are needed to reach higl7 photocurrent density values. From this point of view junctions realized by solid state diffusion are more suitable than those obtained by CVD. Solid state diffusion assures a more accurate control of the window laye~ thickness and of its homogeneity. The latter requirement is very important in the case of very thin p ÷ layers in order to avoid any device short circuit. Furthermore by solid state diffusion it is possible to obtain a heavily doped diffused layer and consequently, a very low sheet resistance value. Since SiO 2 has been widely employed as an encapsulant for silicon, m a n y author, [20,21] have attempted to use it as an encapsulant also for GaAs. Unfortunatel 3 silicon dioxide films tend to crack during high temoperature (800-900°C) diffusior processes if their thickness exceeds about 6000 A, the expansion coefficient ol
38
M. Garozzo et al. / GaAs shallow homojunction solar cells
silicon dioxide being one order of magnitude smaller than that of gallium arsenide [22]. Furthermore, at 750°C gallium out-diffusion occurs through SiO 2 caps [23]. A reproducible solid state diffusion of zinc in gallium arsenide was reported for the first time by Shealy et al. [24] using a cap layer of phosphosilicate glass with 20 wt.% of P205 in SiO 2, in order to match thermal expansion coefficients. This technique was successfully employed to prepare shallow p + - n homojunctions on bulk GaAs [25]. Borrego et al. [6] have fabricated a photovoltaic cell with a conversion efficiency of 12.6% at AM1 by using the same technique. Nevertheless zinc is a very fast diffuser in gallium arsenide [26], and it is thus possible to obtain shallow p + - n homojunctions by using SiO 2 instead of PSG as a cap. We have performed these diffusion processes at relatively low diffusion temperatures ( < 600°C) and with a well controlled value of the SiO 2 layer thickness (0.5 ~tm). In this case the SiO 2 film is thin enough to avoid any crack and thick enough to prevent any out-diffusion from the gallium arsenide surface. For temperatures lower than 600°C, furthermore, the arsenic vapour pressure is negligible [27], and the diffusion coefficients of arsenic, gallium [28] and zinc [24] into silicon dioxide are very small. Fig. 5 shows the doping profiles obtained on two samples diffused at 545°C for 1 h and at 570°C for 1.5 h respectively. The values of the carrier concentration were derived from the resistivity measurements, which are described in section 3.2, assuming a hole drift mobility of 40 cm 2 V - ] s - ] [25]. The corresponding diffusion depths d o are 1900 and 6800 .~, where d o is defined as the maximum thickness value at which it is still possible to carry out the anodic oxidation. 21 10
!
I
I
I
I
I
I
? E
1020 59. °-o~°A
A &&
•
AAA
AA
& A
O0 0
0
Z
0
0
0
10'" pz uJ ~J z
1018 O O nLU n--- 1017 ~1
QC
<[ O
lO
16
0,0
I
I
0.1
0.2
I---4
I
i
i
I
0.3 0.4 0.5 0.6 p+- LAYER DEPTH ()Jm)
I 02
0.8
Fig. 5. Doping profiles of two samples diffused at different conditions. Circles: 545°C for 1 h; triangles: 570°C for 1.5 h. The bars indicate the diffusion depths.
M. Garozzo et al. / GaAs shallow hornojunction solar cells
39
The obtained doping levels are very high ( = 10 20 c m - 3 ) and equal to those obtained by employing PSG as a cap [25]; in this way we can confirm that the SiO 2 layer is an effective cap. On the other hand the flatness of the curve in fig. 5 corresponding to the deeper diffusion and its abrupt fall near d o shows that the mixture Z n O - S i O 2 behaves as an infinite doping source, and then the zinc diffusion process is well described by a complementary error function. In this case the diffusion depth is given by:
(8)
do -- 1.1(Dr) 1/2, where t is the diffusion time and D is the diffusion coefficient: D = D o exp( - Ea/kT),
(9)
where E a is the activation energy. A set of diffusions at different temperatures (530-580°C) and times (45 m i n - 3 h) was carried out in order to obtain D as a function of the reciprocal temperature. The diffusion depths were in the range 700-11 000 ,~. The experimental data of D
-11
10
W
E
I-. z iii
m U.
16'3
{ la"l
16'" 1.1 RECIPROCAL
1.2 TEMPERATURE
1.3
1.4
1 0 ~ T ( K 1)
Fig. 6. Zn diffusion coefficient as a function of the reciprocal temperature.
40
M. Garozzo et al. / GaAs shallow homojunction solar cells
are shown in fig. 6. The best fit of these points, obtained using eqs. (8) and (9), is indicated as a continuous line. It corresponds to D o = 2.46 × 1013 cm2/s and E a = 4.6 eV. In all diffused samples we have obtained the same doping level with no evidence of damage to the GaAs surface. Some measurements in progress [29], performed by the scanning light spot technique [30], show that the diffused layers thickness is very uniform on the whole sample surface, even for very thin p+ layers (200-300 A). This technique is as practical and reproducible as the open-tube zinc diffusion employing PSG as a cap; furthermore our process is more convenient from the point of view of simplicity and safety. 4. 2. Spectral response and photocurrent
Using the above described results, we have prepared several devices on both commercial bulk materials and MO-CVD grown epitaxial thin films. These films were grown on heavily n-doped substrates to have a BSF. The external spectral response Qe(~) of such devices was measured in the 400-1000 nm spectral range, using the same computerized apparatus described in refs. [30,31]. Figs. 7 and 8 show the experimental external spectral responses of a bulk (sample m61) and of a thin film (sample M213C) device. Both devices have a 880 A thick ARC. The epitaxial layer of the sample M213C was 2.1 ~tm thick and its intrinsic donor concentration was 7 × 10 ~5 cm -3, as resulted from C - 2 - V measurements. A quantitative estimate of the most interesting device parameters, such as the junction depth ~ and the minority carrier diffusion length Lp, can be obtained by
1.0
I
I
I
I
I
A
0.8
m 61
UJ (/)
z
o
0.6
a. or) uJ
0,4 ,-I
n.I-. 0,2 a.
ffJ 0.0 400
500
600 700 WAVELENGTH
800 (nm)
900
Fig. 7. Experimental external spectral response of the m61 bulk device.
1000
M. Garozzo et al. / GaAs shallow homojunction solar cells
1.0
v 0
|
I
I
I
41
I
0.8 M 213 C
kg
0.6 0 tl) tu iv.
0A
-I
,¢ tr IO
0.2
O. t/)
0~
I 500
400
I I 600 700 WAVELENGTH
800 (nm)
900
1000
Fig. 8. Experimental external spectral response of the 10213C thin film device.
carrying out the best fit of the experimental internal spectral response. We prove that it is possible to have a meaningful fit of Q(?~) by using eqs. (2)-(5), in spite of the presence of several parameters. The number of these parameters can in fact be remarkably reduced if we analyse each single term on the right hand side of eq. (2). The electron contribution Q,(X) is simplified by two different approximations. Since we analyse just shallow homojunctions, it is possible to write X / L n << 1. Furthermore, owing to the high value of the GaAs absorption coefficient, we have 2 2 a L n >> l. Since L n = (Dn-rn) 1/2 and the minority carrier diffusion coefficient D, is related to the electron mobility by the Einstein relationship D n = VT#,, with V T = k T / q = 25.85 mV at room temperature, eq. (3) becomes: a--V-~v~(1 - e-~X~) + 1 Qn()~) =
XjSn
- e-'~x~ '
(3')
+ ]
and in Qn(X) only two parameters are present: Xj and the S . / I ~ . ratio. As regards Qp(X), this term must be written in two different ways corresponding to bulk and BSF thin film devices respectively. For bulk devices (Sp = o0, H / L p >> 1) eq. (4) becomes: Qp(X) =
aLp
e x p [ - a ( X j + W)] aLp + 1
(4')
M. Garozzoet al. / GoAsshallowhomojunction solar cells
42
while for BSF thin film devices (Sp = 0) eq. (4) becomes:
Qp(X ) = aLo e x p [ - a ( Xj + W)]
sinh( H / L p ) + HaLp / L pe) x p ( - a H ) (4")
In eq. (4") H is not a parameter because H = H 0 - Xj - W = H(Xj), where H 0 is the known epitaxial layer thickness. Qp(;k) depends therefore, in both cases, on Xj and Lp only. Q ( ~ ) given in eq. (2) is then a function of only three parameters: Xj, Lp and Sn//~ ". The first one is effective in the whole spectral range, especially in the s h o r t - m e d i u m wavelength region. The second one is effective in the long wavelength region. The ratio Sn//~, is effective only in the blue region, where it strongly affects the internal quantum efficiency shape. In fig. 9 dots are the experimental values of the internal spectral response of the sample m61 obtained from the data of Qe(X) and from the experimental reflectance spectrum. The continuous line shows the best fit obtained from eqs. (3'), (4') and (5). The fit gives Lp = 0.63 ~m, Xj = 2000 ,~ and Sn/~n 1.5 X 104 V / c m . The best fit of Q(?,) for the sample M213C (fig. 10), obtained in the same way as for the sample m61 apart from replacing eq. (4') by eq. (4"), gives Lp = 5.4 ~m, Xj = 660 ,~ and the same value of Sn/l-t n" AS expected the improvement at short wavelengths is due to the thinner p ÷ layer, while the enhanced response at medium-long wavelengths arises from the high value of the minority carrier diffusion length. =
1.0
m 61
o 0.8 LU U:I
z 0.6 o
(3.. (/) I,I n"
0.4 .J E: I.-
0.2 O. (/)
0.0
400
I 500
I I 600 700 WAVELENGTH
I 800 (nm)
I 900
1000
Fig. 9. Experimental internal spectral response of the sample m61: the dots are the experimental values and the continuous line is the best fit curve.
43
M. Garozzo et al. / GaAs shallow homojunction solar cells 1.0
I
I
I
I
I
I 500
I 600
I 700
I 800
I 900
0 0.8 UJ
0.6
o
IX
GO UJ IZ
0.4
.-I
n,, I-
0.2
U.I IX
0.0 400
WAVELENGTH
1000
(nm)
Fig. 10. Experimental internal spectral response of the sample M213C: the dots are the experimental values and the continuous line is the best fit curve.
I n table 3 we report the j u n c t i o n depths a n d the m i n o r i t y carrier diffusion lengths as o b t a i n e d from the best fit of the i n t e r n a l spectral response of several bulk devices. It can be seen that the values of Lp are in the range 0.2-1.3 ~tm a n d that the evaluation of Lp is i n d e p e n d e n t o n the value f o u n d for the j u n c t i o n depth. T h e same quantities for thin film devices are shown in table 4 together with their epitaxial layer thickness. I n this case the values of Lp are almost one order ot m a g n i t u d e larger (3.5-5.5 ~m). As regards to the ratio Sn//~tn, the same value (1.5 × 10 a V / c m ) resulted from al] best fits performed o n b o t h bulk a n d thin film devices. This is a resonable result, as
Table 3 Xj and Lp as obtained from the best fit of the internal quantum efficiency in commercial bulk device~, Sample
Xj (m)
Lp (Ixm)
m48 m49 m51 m53 m55 m61 m63A m63B m63C m63D m64
900 14000 1400 1800 350 2000 1600 2600 6600 750 250
0.88 0.82 1.25 0.47 0.86 0.63 0.78 1.26 0.22 1.26 0.50
44
M. Garozzo et al. / GaAs shallow homojunction solar cells
Table 4 Xj and Lp as obtained from the best fit of the internal q u a n t u m efficiency in epitaxial thin film devices. H 0 is the epitaxial layer thickness Sample
H 0 (jzm)
Xj (A)
Lp (~tm)
M182 M192 M193 M212 M213A M213B M213C
2.7 2.6 2.5 2.2 2.1 2.1 2.1
2500 2500 1150 1450 1500 700 660
5.0 3.5 5.5 3.9 4.9 4.9 5.4
the electrical properties of the heavily doped diffused layers do not depend on the quality of the employed material, but they are strictly correlated to the solid state diffusion conditions. In order to check the reliability of our best fit method, some devices were peeled using anodic oxidation and oxide removal; the diffusion depth d o was obtained peeling the film until the anodic oxidation could be carried out as explained in section 3.2. Fig. 11 shows the comparison between the values of d o and the values of Xj obtained from the photoresponse best fit, previously performed on the same samples. The straight line represents the expected correlation between the two quantities. The good agreement shows that the used best fit method is very reliable. 10
,
,
,
,
,
,
,
,
,
v
,
,
,
,
,
,
,
,
,
=k
x 1
"r" I'e~ a Z
O t-
0.1
Z
0.01
0.01
I
,
0.1
DIFFUSION
I
1
DEPTH
10
do(pm)
Fig. 11. Comparison between the junction depth Xj, as obtained from the photoresponse best fit, and the diffusion depth d o , as obtained from the peeling. The continuous line represents the expected correlation.
45
M. Garozzo et al. / GaAs shallow homojunction solar cells
Table 5 Calculated and experimental photocurrent densities at AM1 for bulk and thin film devices Sample
Xj (m)
Lp (l~m)
JL (mA//cm2) Calculated
Experimental
Bulk m61 m51 m48
2000 1400 900
0.63 1.25 0.88
16.3 19.3 20.3
16.1 18.2 18.4
Thin films M213A M213B M213C
1500 700 660
4.9 4.9 5.4
21.0 24.5 24.7
19.0 24.1 24.2
As a further demonstration, we have used the values of Xj and Zp from the photoresponse best fit to calculate the photocurrent densities at AM1 for some devices. Such calculated values are compared in table 5 with those experimentally obtained on the same samples at simulated AM1, by considering only the active area of the devices. Also in this case a good agreement is obtained. The experimental values reported in table 5 demonstrate that the photocurrent density is, as expected, strongly affected by the junction depth. It is also apparent that the use of high quality thin films increases the photocurrent density by about 20%. We can therefore conclude that, as regards to the charge collection, our experimental results confirm the reliability of the computer simulation. 4. 3. Solar cell efficiency The solar energy conversion efficiency was in the 10-12% range for bulk devices and in the 12-14% range for thin film devices. The best result was achieved on the 1 cm 2 active area thin film device M213B. We obtained, under simulated AM1 conditions (fig. 12), a short circuit current density of 24.1 m A / c m 2, an open circuit voltage of 770 mV, a fill factor of 0.76 and a conversion efficiency of 14.1%. In fig. 13 the external spectral response, measured by avoiding any light reflection from the grid, is reported. Fig. 14 gives the dark current density-voltage characteristics at direct polarization. The curve shows a depletion layer recombination mechanism, dominating at low voltages ( < 400 mV). This fact is typical for diodes realized on GaAs as well as on any other high energy gap semiconductor. For this reason it is quite difficult tc obtain fill factor values much greater than 0.76. As regards to the open circuit voltage, it is limited by the transport mechanism, which, even at a relatively high voltage, cannot be described as a pure diffusion-controlled mechanism. The slope of the log J - V curve at V > 500 mV corresponds tc n = 1.27 and Jo = 10-a2 A/cm2 (fig. 14). This value of J0 is much higher than expected in the case of a diffusion-controlled transport mechanism. At a dopin~
M. Garozzo et al. / GaAs shallow homojunction solar cells
46 40
I
I M 213
I
I
600
800
B
30 E I- 20 z LU OC ~C .-~ 10-
0 0
I
I
200
400
I
VOLTAGE Fig. 12. Photo l - V p o w e r point.
1000
(mV)
characteristics of the M213B device at s i m u l a t e d AM1. The dot is the m a x i m u m
level of 7 x 1015 cm 3 and using the reported values for the other parameters of the M213B device, we should have, in fact, J0 = 2.7 × 10 t8 A / c m 2. The value of Vo~(770 mV) is then lower than that expected (950 mV). A higher doping of the active material, for example N D -- 6 x 1016 c m - 3 , should
1.0
I
I
i
I
@
O
0,8
i,i Z
O ilL. t/) er
0.6
0.4
.J ne I-O ILl a.
0.2
0.0 400
I
500
I
I
600 700 WAVELENGTH
I
800 (nm)
IN 900
Fig. 13. E x p e r i m e n t a l external spectral response of the M213B device.
1000
47
M. Garozzo et al. / GaAs shallow homojunction solar cells -1
lO
|
I
M 213
-2
I
l
B
lO E
-3 lO
>. I--4
u> 10 z 14.1
16 s UJ
u
/I
-6
lO
I
n = 1.27 J ° = 1()12 A / c m 2
-7
lO
I
I I I
-8
10
0.0
I
I
I
I
0.2
0.4
0.6
0,8
VOLTAGE
(VI
1.0
Fig. 14. Dark log J - V characteristics of the M213B device.
increase the importance of the diffusion transport mechanism enhancing the open circuit voltage and, consequently, the conversion efficiency.
5. Conclusion We have demonstrated, on the basis of a computer simulation, that an improvement of about 30% can be obtained in the solar energy conversion efficiency of shallow p + - n GaAs solar cells by using high quality epitaxial thin film as an active region. GaAs epitaxial thin films were grown by metal organic chemical vapor deposition. Shallow homojunctions were prepared by open-tube Zn solid state diffusion at low temperature (< 600°C), by using the mixture ZnO/SiO 2 as a doping source and a simple SiO2 layer as a cap; this technique is quite simple and does not imply any safety problem. The p+ layers were well controlled in thickness, heavily doped
48
M. Garozzo et al. / GaAs shallow homojunction solar cells
a n d v e r y h o m o g e n e o u s . N o d a m a g e was o b s e r v e d on the G a A s s u r f a c e a f t e r the d i f f u s i o n process. A best fit m e t h o d was a p p l i e d to the e x p e r i m e n t a l p h o t o r e s p o n s e s p e c t r a o f d e v i c e s p r e p a r e d on b o t h bulk m a t e r i a l s a n d thin e p i t a x i a l films. A c o m p a r i s o n b e t w e e n the two d i f f e r e n t k i n d s of d e v i c e s s h o w s that in the e p i t a x i a l films the m i n o r i t y c a r r i e r d i f f u s i o n l e n g t h is a l m o s t o n e o r d e r o f m a g n i t u d e l a r g e r t h a n in the b u l k m a t e r i a l a n d the r e s p o n s e at l o n g w a v e l e n g t h s is r e m a r k a b l y i n c r e a s e d . E x p e r i m e n t a l p h o t o c u r r e n t d e n s i t i e s w e r e in g o o d a g r e e m e n t w i t h c a l c u l a t e d v a l u e s a n d i n c r e a s e d by a b o u t 2070 on p a s s i n g f r o m b u l k to thin film devices. A c o n v e r s i o n e f f i c i e n c y of 14.1% at A M 1 o n a 1 c m 2 a r e a thin film d e v i c e was o b t a i n e d . T h i s e f f i c i e n c y v a l u e can be f u r t h e r i m p r o v e d by i n t e n t i o n a l l y d o p i n g the a c t i v e layer in o r d e r to r e d u c e J0 i.e. to i n c r e a s e V,~.
Acknowledgements T h e a u t h o r s are v e r y g r a t e f u l to Dr. F. G a l l u z z i for h e l p f u l discussions. T h e y w o u l d also like to a c k n o w l e d g e Dr. G. G r i l l o for h a v i n g w o r k e d o u t the A M 1 solar s p e c t r u m data.
References [1] [2] [3] [4] [5] [6] [71 [8] [9] [10] [11] [12l [13]
[14] [15] [16] [/7] [18] [19] [20] [21] [22]
R.J. Stirn, K.L. Wang and Y.C.M. Yeh, 15th Rec. IEEE Photo. Spec. Conf., Orlando (1981) p. 1045. M. Garozzo, G. Conte, F. Evangelisti and G. Vitali, Appl. Phys. Lett. 41 (1982) 1070. M. Garozzo, D. Margadonna and A. Parretta, Mat. Chem. Phys. 9 (1983) 157. C.O. Bozler, R.W. McClelland, J.P. Salerno and J.C.C. Fan, J. Vac. Sci. Technol. 20 (1982) 720. J.C.C. Fan, C.O. Bozler and R.L. Chapman, Appl. Phys. Lett. 32 (1978) 390. J.M. Borrego, R.P. Keeney, I.B. Bhat, K.N. Bhat, L.G. Sundaram and S.K. Ghandhi, 16th Rec. IEEE Photo. Spec. Conf., San Diego (1982) p. 1157. R.J. Field and S.K. Ghandhi, J. Electrochem. Soc. 129 (1982) 1567. A.J. Spring Thorpe and M.N. Svilans, Gallium Arsenide and Related Compounds 1982 (The Institute of Physics, Bristol and London, 1983) p. 589. H.J. Hovel, Semiconductors and Semimetals, vol. 11 (Academic Press, New York, 1975) p. 17. H.J. Hovel, Semiconductors and Semimetals, vol. 11 (Academic Press, New York, 1975) p. 25. D.E. Aspnes and A.A. Studna, Phys. Rev. B27 (1983) 985. S.M. Sze, Physics of Semiconductor Devices (Wiley, New York, 1981) p. 40. Standard Procedures for Terrestrial Photovoltaic Performance Measurements, Specification No. 101, Issue 2, Commission of the European Communities, Joint Research Centre - Ispra Establishment, Italy, EUR 7078 EN (1980). H.J. Hovel, Semiconductors and Semimetals, vol. 11 (Academic Press, New York, 1975) p. 51. G.W. 't Hooft, C. Van Opdorp, H. Veenvliet and A.T. Vink, J. Crystal Growth 55 (1981) 173. J.R. Shealy, B.J. Baliga, R.J. Field and S.K. Ghandhi, J. Electrochem. Soc. 128 (1981) 558. H. Hasegawa and H.L. Hartnagel, J. Electrochem. Soc. 123 (1976) 713. T. Ishii and B. Jeppsson, J. Electrochem. Soc. 124 (1977) 1784. R. Gulati, R.K. Purohit and I. Chandra, J. Inst. Telecomm. Engrs. 15 (1969) 815. A.G. Foyt, J.P. Donnelly and W.T. Lindley, Appl. Phys. Lett. 14 (1969) 73. W. von Munch, GaAs Planar Technology, IBM J. Res. Develop, 10 (1966) 438-445. B.J. Baliga and S.K. Ghandhi, IEEE Trans. Electron Devices ED-19 (1972) 761.
M. Garozzo et al. / GaAs shallow homojunction solar cells
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49
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