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Photovoltaic properties and anomalous effects in the ZnSe-GaAs heterojunction
PHOTOVOLTAIC PROPERTIES EFFECTS IN THE ZnSe-GaAs
AND ANOMALOUS HETEROJUNCTION
B. V. ZHUK, I. A. ZHUKOV and A. A. ZLENKO General
Physics Institute.
(Received
Vavilov Street 38, 117942 Moscow.
20 Fehruan, 1985; in reuisedform
U.S.S.R
21 June 1985)
Abstract-The photoelectric properties of the nZnSe-pGaAs heterojunction obtained by deposition of ZnSe on GaAs in the MOCVD process have been investigated. The current-voltage characteristics of the illuminated solar cells are shifted to lower voltages than those expected from the superposition principle. The dependence of quantum efficiency and C-V curves on light intensity and wavelength has been observed. A heterojunction mode1 is proposed for both light and dark conditions. We assumed that near the interface a compensated ZnSe layer exists with negative charged deep centers. As a result electric current in the heterojunction is limited by a barrier in the conduction band. Variation of charge in the compensated layer is connected with electron transfer from deep centers to the conduction band. This process explains the anomalous effects for such a heterojunction. The characteristics calculated b> this model are close to the experimental curves.
INTRODUCIION Lately, investigations in the field of solar cells have led to the development of monocrystalline solar cells mainly based on Si and GaAs and capable of operating under conventional and high-concentration solar radiation, and to the development of thin film polycrystalline solar devices on inexpensive substrates. Generally for polycrystalline samples, heterostructures CdS-Cu, S [1] or Cd 1Zn, _ ,S-CuInSe, [2,3] are used. Meanwhile, attaining high efficiency for these structures has so far been limited by the well-known “crossover effect” of the current-voltage dark and light characteristics (I-V)[l-31 leading to a decrease of open-circuit voltage V,, compared to the theoretical one. An essential spectral dependence on junction capacity and quantum efficiency is observed in such heterostructures. To account for these effects, different models have been suggested in [2,4] on the basis of Anderson’s model and p-i-n diode theory. They assume that the anomalous behaviour of the light characteristics of these heterostructures is due to the dependence of the surface states density on illumination. However, there are no explanations in the framework of these models for significant peculiarities in the behaviour of these heterojunctions. In particular, the experimental dependences of the current-voltage characteristics essentially differ from those available within the models[2]. It should be noted that similar anomalies in the photoelectric properties have been observed for the nZnSe-pGaAs heterostructure[5]. In [6] the characteristics of ZnSe and GaAs are given. The misfit of lattice parameter is equal to about 0.2%. The energies of the electron affinity for ZnSe (4.09 ev) and GaAs (4.07 ev) are close and hence band-offset in conduction band for ZnSe-GaAs heterojunction can be neglected. As
shown in [6] an nZnSe-pGaAs heterostructure is close to that for an optimal solar cell. In this paper we have studied the structures of the nZnSe-pGaAs heterojunction and the anomalies in its photoelectric properties. A model was developed accounting for the anomalous behaviour of these heterostructures. It was shown that the anomalies in photoelectric properties for heterostructures of the CdS-CuInSe type can also be accounted for by means of the suggested heterojunction model. STRUCTURE AND FABRICATION The properties of heterostructures have been studied in a series of publications[5-71. In particular, in [5] the deposition of ZnSe layers on GaAs was considered in a closed system at the substrates temperatures as high as 750-800°C. In [6] the heterojunction was obtained by liquid-phase epitaxy of GaAs on ZnSe substrates. In [7] ZnSe was grown on GaAs from diethylzinc (DEZ) vapours and HzSe in a hydrogen flow at the temperature of 450°C and higher. A transition layer (ZnSe) ~(GaAs), ~, , x = 0.1, where x depends on the condition of growing, appeared to form at the ZnSe-GaAs interface. A 1 pm thick transition layer has been observed in [7], which was interpreted as a Ga,Se, ‘9ZnSe compound. The photovoltaic elements obtained on the basis of ZnSe deposition on GaAs have high-effective series resistance and low quantum efficiency. The ZnSe-GaAs heterojunctions obtained by ZnSe deposition on GaAs in MOCVD process were shown[9] to be good structures for using as solar cells. In this paper, we consider GaA-ZnSe heterojunctions and its influence on photoelectric properties. Epitaxial ZnSe layers have been grown from DEZ and H,Se at temperatures of 350-400°C and pressures in the reactor l-100 Pa. The deposition rate
248
B. V.
ZHIJK
er al.
Table 1 Carrier mobility and specific resistance of ZnSe layers Carrier concentration (cm-“) Mobility (c&/V set) Soecific resistance (Q/cm)
3. lO’(’ 2 10” 2-3.10’” 600 450 200 0.02 0.003 0.1
10” 2-10 0.1-0.5
1
L-_lIsL 2
3
was determined by the pressure of DEZ and H,Se in the reactor and was as high as 0.02-0.2 km/mm. The ZnSe doping with Al was produced by adding vapours of triethylaluminium to the DEZ flow. GaAs with orientation (100) doped with Zn (p = 10” cm ‘) and semiinsulating GaAs were used as substrates. We have studied such layers in [lo]. The mobility and concentration of the carriers in the films with thicknesses about l-10 pm were measured by the Van der Pauw method at room temperature. Table 1 shows the values of carrier mobility and the corresponding values of their concentration. Theoretical mobility in n-ZnSe was calculated for low carrier concentration taking into account only polar optical scattering, as equal to 470 cm*/Vsec at room temperature[ll]. Thus, the values of mobility in our films are close to theoretical and high carrier concentration was obtained without a great decrease of mobility. From high mobility epitaxial layers, we hope for a high quality ZnSe-GaAs heterojunction. We studied photodiodes with carrier concentrations in the ZnSe of 2. 1016 cme3, 2. 10” cm-‘, 3 . 10” cm-’ and with hole concentration lOI cm -’ in the p-GaAs. The thickness of the ZnSe films was about 10 bm. Ohmic contacts were applied to the ZnSe and GaAs layers by depositing drops of a Ga-In alloy followed by annealing in forming gas at 350” c. Dark I- V solar cell characteristics of these heterojunctions are shown in Fig. 1. Under reverse bias voltage the I- V curves may be approximated as: I=I;exp(
where
-A
A = 13.0-14.0,
.(V,-
V)1’2)
V, = 1.3-1.35
V
lo”-
4
05
400
600
800
Ainm:
Fig. 2. Dependence of internal quantum efficiency on light wavelength. Curve (1)--n = 3.10’” cm-s, (2)-r] = 2. 10” A = 470 nm, illumination, (4)--n = 3 10lh cmm3 under with X = 470 nm, (3)--n = 2.10” cmm3 under illumination cme3 without shortwave light.
Such behaviour of the I- V curves is probably due to a barrier in the conduction band. Measurements of the internal quantum efficiency on these diodes was carried out as in [9]. The quantum efficiency decreased with decrease of carrier concentration in the ZnSe. This process becomes sharper when the value of the decreased carrier concentration reached n = 1016 cm13, as shown in Fig. 2. The quantum efficiency in the long wave region of diode sensitivity essentially depends on illumination with a 460-510 nm wavelength. Figure 3 shows the dependence of quantum efficiency for the diode with n = 2.10” cm-’ on the 632 nm wavelength of illumination. The power of illumination at 460-510 nm was about 10 pW/cm2. The quantum efficiency for X = 632 nm versus the power of illumination for X = 472 nm is shown in Fig. 3. The effect of quantum efficiency variation does not depend on power of illumination in the long wavelength region. A similar change of the value of quantum efficiency at illumination in the short wavelength region occurred for diodes with n = lOI cm ‘, however, the quantum efficiency changed from lo- ’ to 3 lo-’ for X = 632 nm at the same power of illumination. Illumination in the short wavelength region led to an essential variation of the I- I/ curves of the diode with n = 5 ‘10” crne3 (Fig. 1).
IbA1 :J@ 4
3
10’
2
1
1
I
I
01
1
03
t
c
U(V)
’
8
07
*
Fig. 1. Dark I- V characteristics of diodes with the area of 0.05 cm2 for various electron concentrations in ZnSe layers. Curve (I)-for n=2.10’7cm-3, T=lOO K; (2)--n=2. lO”cm3, T= 300 K; (3)--n = 3 lo’* cme3, T= 300 K; (4)-under short wavelength illumination in the 460-500 nm region, it = 2 10” cn3&‘;r n = 3 10” cm-s. T=
064 J 500
550
Xtnm)
Fig. 3. Curve (1)-dependence of quantum efficiency with illumination wavelength X = 632 nm on illumination wavelength with intensity equal to 10 pW/cm’, (2)-dependence of quantum efficiency at X = 632 nm on power of illumination with X = 472 nm.
249
Photovoltaic properties in ZnSe-GaAs heterojunction JlA/cm’) 1
2 04 3 0.2.
02
0.4
06
U(V)
Fig. 4. J- V characteristics for various intensities of sunlight (AMO). Curve (1)--P = 3.5 W/cm*, (2)--P = 2.6 W/cm*, (3)--P = 1.6 W/cm*.
Quantum efficiency also increased with reverse voltage bias on the diode. For diodes with n = 2 . 10" cm-- 3 the value of quantum efficiency was close to unity for a reverse voltage bias of 3-5 V. For the diode with n = 1016 cm- 3 the quantum efficiency at the reverse bias voltage increased up to lo-20%. Since the quantum efficiency for the diode with n > 10” cm- 3 is a high value, one can conclude that concentration of recombination centers in the interface of heterostructures is low. Figure 4 shows photo Z-V characteristics for the diode with n = 3.10” cmv3 and area of 5. 10m2 cm2 without antireflection coating, and having an ohmic indium contact in the center of a sample 700 pm in diameter, at different powers of illumination with an AM0 spectrum. Figure 4 shows that the series resistance of the diode together with the contacts is 0.2 a/cm*. This value is close to the theoretical spreading resistance for this diode. The photocurrent density is 26 mA/cm2 and open-circuit voltage V, is 0.62 V at the illumination power of 140 mW with the AM0 spectrum. For understanding of the mechanism of current flow we have investigated the complex differential diode re-
01
sistance variation with bias voltage. The characteristics were observed at frequencies from 200 Hz-80 kHz at room temperature. Figure 5 shows l/C (C is the capacity) as a function of voltage bias at different frequencies of the test signals for a diode with n = 2 10’7cm-3. The thickness of the depletion layer at a zero voltage bias has a value higher than for the abrupt transition case. This indicates the presence of a layer with the carrier concentration different from the volume concentration of the carriers in the heterostructure, The carrier concentration in the layer near the surface of GaAs did not change, as shown by the GaAs photoluminescence spectra after ZnSe deposition.
MODEL Anomalous properties of the nZnSe-pGaAs heterostructures are qualitatively the same as those of n CdS-p CuInSe, heterostructures. They have in common that the energies of the electron affinity are close, therefore there is no barrier for electrons in the conduction band. Wide-forbidden-band materials in these heterostructures tend to selfcompensation. Assume that near the interface there exists a layer of compensated wide-band material with thickness L. Let N, > NJ in this layer, where Nd is the donor concentration in the wide-band material, N, is the deep acceptor concentration with an energy E, < A E (Fig. 6) In this case the Fermi level in the compensated level lies higher than E,. As a result, the levels with energy E, must be filled with electrons. Therefore, a negative space charge with the density q( N, - Nd) may form in an L-thick layer. Depending on the relationship between the values Nd and N, there can be an essential change of the heterojunction band structure, as compared to an abrupt heterojunction, and of its photoelectric properties. We have computed the C- V characteristics at various frequencies and various distributions of accep-
VaP(V)
Fig. 5. Dark C-V characteristics of a diode with area.0.05 cm2 and II = 2. 10” cm-‘. Positive voltage direct bias: Curves (1-2)-theoretical and experimental curves at the frequency of test signal 25 kHz. (3-3)-1000 Hz, (5-6)-500 Hz at room temperature, respectively.
250
B. V. ZHUK er crl.
/
The electric field vanishes at the point where the potential is equal to zero and the electric field and potential are assumed to be continuous at x = 0. Under these conditions we obtain an equation for the potential in the depletion layer with the width L:
GoAs
(5) Fig. 6. Band structure of the ZnSe-GaAs heterojunction.
tars and donors in ZnSe near the heterointerface [12]. It has been shown that the pure differential resistance R of the p-n junction and its capacity C at frequency o can be determined from the equation ~~“(ypn(.r)+iwrt-,,)~‘dx=(R
‘+ioC)-’
(1) where I_Lis the mobility of the current carriers, c,, is the dielectric constant, W is the depletion width of the heterojunction, L,J is the electron charge, and n ( x ) is the current carriers distribution. Analysis of eqn (1) leads to the following dependence of capacitance on frequency:
C( cd)= q,S/d( w)
(4
where S is the junction area, d(w) is the thickness of the depletion layer for which OT > 1, (r is the dielectric relaxation time equal to re,X’, i3 = qpcLn(x), n(_x)=n,,exp(-qcp(x)/kT), where cp(x) is the potential distribution in the junction). For the nZnSe-pGaAs heterostructure, the n(x) distribution corresponds to the electron distribution in the depletion layer. Since there is a large barrier for holes in the valence band, the distribution of holes can be neglected. It follows from eqn (2) that for calculation of the C- V characteristics we must know the potential distribution which can be obtained from Poisson’s equation:
d2gD -q/cco(n(x)-N,+N,). -= dx2
It was assumed that at x < 0 n = Nd and transition layer N, = N, - NJ; therefore, VP:’=
(3)
in the
-q/cc,, . N,{exp(-+(x))+WN~} (4)
where
where L, = (ee,, kT/q2Nj)‘/’ is the Debye length in the n-material. The boundary condition for (5) is q(L) = VI, Vap, where Vn is the diffusion potential and q,!, is the applied voltage bias, Theoretical C-V curves are close to the experimental characteristics at L = 350 nm and N, = 10” cm-j for the barrier concentration in the ZnSe equal to n = 3 lOI6 cm-‘, L = 300 nm. Also for N, = 1.1 10lh for n = 3.10” cmm3, L = 170 nm and N, = 1.5 . 10Lh cm-~’ for n = 3 - 10’s cm- ‘. The C-V curves for samples with n = 3 10”’ cm-’ and n = 3 10" cm-? are not shown, since they are similar to those in Fig 5. Figure 7 shows experimental C-V curves for the same sample as shown in Fig 5, but under illumination with the wavelength about 460-500 nm. Theoretical C-V characteristics are close to the experimental ones provided the concentration of the negative charge centers is decreased to N = 1 ‘10” cm-m? for L = 300 nm. The potential distribution in the junction for different values of the applied voltage bias is shown in Fig 8. Figure 8 shows that there can exist a barrier for electrons in the conduction band. The height of the barrier can essentially change depending on N, As calculations have shown, the barrier height and C-V curves weakly depend on the electron concentration in the ZnSe at n > 3 ‘10” cm ‘. In gen-
G(x) = qq(x)/kT.
Fig. 7. C-V characteristics of diode under illumination with X = 470-500 nm: Curves (l-2) are theoretical and experimental curves at w = 25 kHz, (3-4)-10(x) HZ. (5-6) -500 Hz at room temperature, respectively.
Photovoltaic
properties
in ZnSe-GaAs
Fig. 8. Potential distribution in conduction band of the heterostructure for n = 10” cmm3, L = 300 nm. Curves (1,3,5)-for NA = 1.2’ 10n’ cme3 and Vd- VUp= 1.1 V, 0.55 V, respectively; (2,4,6)-for N, = 1.1016 cmm3.
these characteristics depend on concentration of the charge centers in the transition layer and on its thickness (Fig 8). Analysis of the solution of eq (1) has shown that at direct voltage bias one can neglect the influence of the current on C-V characteristics, if, the current densities are similar to these for Fig 1. It should be noted that the experimental C-V characteristics are close to the theoretical value at Vn = 1 V. This discrepancy with theory can be explained by the surface states at the ZnSe/GaAs interface which lead to the band bending in the GaAs by the value of 0.2-0.3 eV. It should be noted that the calculations were obtained under the boundary condition cp( L) = V, tp which is valid only at high hole concentration. It follows from eq (1) that this condition is fulfilled doping with an accuracy of - kT for an arbitrary level in p-GaAs if there is a barrier for electrons in the conduction band. It is known that when doping ZnSe with Ga and In up to the level of 10’9-1020 crnm3, a compensated material with high resistance is formed [13]. In [13] ZnSe doping with In and Ga to the values of 10’x-1020 cme3 is investigated and deep compensated acceptor centers are shown to have ionization energies of 0.59 eV and 0.41 eV for In, and 0.55 eV for Ga. So, the presence of the compensated layer in the ZnSe of the diodes is associated with the formation of a Ga-doped 0.1-3 pm thick layer near the heterojunction boundary. This may be due to both the Ga diffusion from the substrate during growing and the Ga transfer by alkyl radicals formed by decomposition of DEZ at the initial stages of growing.
eral
DISCUSSION
AND CONCLUSION
On the basis of the model considered we can draw the following conclusions to account for the behaviour of the ZnSe/GaAs heterostructure solar cells.
251
heteroJunction
1. An increasing quantum efficiency under reverse voltage bias is explained by an increasing electric field in the space charge region. As a result the losses from surface and volume recombination decrease. 2. The “crossover” effects of the dark and light I-V curves, the change of capacitance of the p-n junctions and the increase of quantum efficiency of diodes in the long-wave length region under illumination can be explained by the fact that dark and light current is limited by a barrier in the conduction band. The probability of tunneling is determined by the height of the barrier. Under short-wave length illumination of a diode, electron transfer from deep acceptor centers to the conduction band occurs. As a result, the concentration of the negatively charged acceptor centers decreases and the barrier for electron is reduced. From the spectral curves in Fig 3 we can estimate the ionization energy of deep acceptor centers to be equal to 0.5-0.6 eV for the compensating centers in the ZnSe layers, which is close to the value of [13]. Since the band structure of the CdS-CuInSe, heterojunctions is analogous to that of the ZnSeeGaAs heterojunctions, the anomaly in this structure can be explained by the same model. Using the data of [3], the energy of deep acceptor levels in CdS, which is equal to about 0.8 eV can be estimated. This fact is apparently accounted for by the diffusion of Cu into CdS, which leads to the formation of a compensated layer in the CdS. 3. The decrease of quantum efficiency with decreasing carrier concentration in wide-band material is connected with increasing thickness of the compensated layer and rise of the barrier for electrons in the conduction band.
REFERENCES 1. A. Rothwarf, SO/UT Cells 2, 115 (1980. 2. R. R. Potter and .I. R. Sites, IEEE Truns. Elecrron De?. ED-31, 571 (1984). 3. S. S. Hegedus, IEEE Trans. Electron De!). ED-31, 629 (1984). 4. A. Rothwarf, IEEE Trans. Electron Dw. ED-31, 1513 (1984). 5. Y. Shirakawa and H. Kukimoto. J. Appl. Ph,s. 51. 5855 (1980). 6. A. G. Mimes and D. L. Feucht, Hetwojunctrons und Merui-Semiconducror Juncrlons, Academic, New York (1972). 7. P. Blanconmer, M. Cerelet, P. Henos and A. M. JeanLouis, Thin Sohd Films 55, 375 (1978). 8. P. V. Gaugash, V. A. Kasyan, V. I. Korolkov and N. R. Rakhimov, Fizrku i tekhniku poluprovodmkor> 9. 1879 (1975). 9. G. G. Devyatikh, B. V. Zhuk, A. A. Zlenko, A. M. Prokhorov, V. K. Khamilov and G. P. Shipulo, Psma ~1 Zhurnal Teoreticheskoi Fizikl 10, 118 (1984). 10. G. G. Devyatikh. B. V. Zhuk. A. A. Zlenko. E. U. Korovina, E. N. Razov, V. K. Khamilov and M. F. Churbanov, Izu. AN SSSR, Neorgunicheskie maieriab 19. 2048 (1983). 11. T. Yao. M. Ogura, S. Matsuoka and T. Morishita. Appl. Phys. Letr. 43, 499 (1983). 12. W. W. Anderson, Solid-Sr. Electron. 25. 1033 (1982). 13. A. A. Qidwai and J. Woods, J. Crystal C;rouath 59. 217 (1982).
AND ANOMALOUS HETEROJUNCTION
B. V. ZHUK, I. A. ZHUKOV and A. A. ZLENKO General
Physics Institute.
(Received
Vavilov Street 38, 117942 Moscow.
20 Fehruan, 1985; in reuisedform
U.S.S.R
21 June 1985)
Abstract-The photoelectric properties of the nZnSe-pGaAs heterojunction obtained by deposition of ZnSe on GaAs in the MOCVD process have been investigated. The current-voltage characteristics of the illuminated solar cells are shifted to lower voltages than those expected from the superposition principle. The dependence of quantum efficiency and C-V curves on light intensity and wavelength has been observed. A heterojunction mode1 is proposed for both light and dark conditions. We assumed that near the interface a compensated ZnSe layer exists with negative charged deep centers. As a result electric current in the heterojunction is limited by a barrier in the conduction band. Variation of charge in the compensated layer is connected with electron transfer from deep centers to the conduction band. This process explains the anomalous effects for such a heterojunction. The characteristics calculated b> this model are close to the experimental curves.
INTRODUCIION Lately, investigations in the field of solar cells have led to the development of monocrystalline solar cells mainly based on Si and GaAs and capable of operating under conventional and high-concentration solar radiation, and to the development of thin film polycrystalline solar devices on inexpensive substrates. Generally for polycrystalline samples, heterostructures CdS-Cu, S [1] or Cd 1Zn, _ ,S-CuInSe, [2,3] are used. Meanwhile, attaining high efficiency for these structures has so far been limited by the well-known “crossover effect” of the current-voltage dark and light characteristics (I-V)[l-31 leading to a decrease of open-circuit voltage V,, compared to the theoretical one. An essential spectral dependence on junction capacity and quantum efficiency is observed in such heterostructures. To account for these effects, different models have been suggested in [2,4] on the basis of Anderson’s model and p-i-n diode theory. They assume that the anomalous behaviour of the light characteristics of these heterostructures is due to the dependence of the surface states density on illumination. However, there are no explanations in the framework of these models for significant peculiarities in the behaviour of these heterojunctions. In particular, the experimental dependences of the current-voltage characteristics essentially differ from those available within the models[2]. It should be noted that similar anomalies in the photoelectric properties have been observed for the nZnSe-pGaAs heterostructure[5]. In [6] the characteristics of ZnSe and GaAs are given. The misfit of lattice parameter is equal to about 0.2%. The energies of the electron affinity for ZnSe (4.09 ev) and GaAs (4.07 ev) are close and hence band-offset in conduction band for ZnSe-GaAs heterojunction can be neglected. As
shown in [6] an nZnSe-pGaAs heterostructure is close to that for an optimal solar cell. In this paper we have studied the structures of the nZnSe-pGaAs heterojunction and the anomalies in its photoelectric properties. A model was developed accounting for the anomalous behaviour of these heterostructures. It was shown that the anomalies in photoelectric properties for heterostructures of the CdS-CuInSe type can also be accounted for by means of the suggested heterojunction model. STRUCTURE AND FABRICATION The properties of heterostructures have been studied in a series of publications[5-71. In particular, in [5] the deposition of ZnSe layers on GaAs was considered in a closed system at the substrates temperatures as high as 750-800°C. In [6] the heterojunction was obtained by liquid-phase epitaxy of GaAs on ZnSe substrates. In [7] ZnSe was grown on GaAs from diethylzinc (DEZ) vapours and HzSe in a hydrogen flow at the temperature of 450°C and higher. A transition layer (ZnSe) ~(GaAs), ~, , x = 0.1, where x depends on the condition of growing, appeared to form at the ZnSe-GaAs interface. A 1 pm thick transition layer has been observed in [7], which was interpreted as a Ga,Se, ‘9ZnSe compound. The photovoltaic elements obtained on the basis of ZnSe deposition on GaAs have high-effective series resistance and low quantum efficiency. The ZnSe-GaAs heterojunctions obtained by ZnSe deposition on GaAs in MOCVD process were shown[9] to be good structures for using as solar cells. In this paper, we consider GaA-ZnSe heterojunctions and its influence on photoelectric properties. Epitaxial ZnSe layers have been grown from DEZ and H,Se at temperatures of 350-400°C and pressures in the reactor l-100 Pa. The deposition rate
248
B. V.
ZHIJK
er al.
Table 1 Carrier mobility and specific resistance of ZnSe layers Carrier concentration (cm-“) Mobility (c&/V set) Soecific resistance (Q/cm)
3. lO’(’ 2 10” 2-3.10’” 600 450 200 0.02 0.003 0.1
10” 2-10 0.1-0.5
1
L-_lIsL 2
3
was determined by the pressure of DEZ and H,Se in the reactor and was as high as 0.02-0.2 km/mm. The ZnSe doping with Al was produced by adding vapours of triethylaluminium to the DEZ flow. GaAs with orientation (100) doped with Zn (p = 10” cm ‘) and semiinsulating GaAs were used as substrates. We have studied such layers in [lo]. The mobility and concentration of the carriers in the films with thicknesses about l-10 pm were measured by the Van der Pauw method at room temperature. Table 1 shows the values of carrier mobility and the corresponding values of their concentration. Theoretical mobility in n-ZnSe was calculated for low carrier concentration taking into account only polar optical scattering, as equal to 470 cm*/Vsec at room temperature[ll]. Thus, the values of mobility in our films are close to theoretical and high carrier concentration was obtained without a great decrease of mobility. From high mobility epitaxial layers, we hope for a high quality ZnSe-GaAs heterojunction. We studied photodiodes with carrier concentrations in the ZnSe of 2. 1016 cme3, 2. 10” cm-‘, 3 . 10” cm-’ and with hole concentration lOI cm -’ in the p-GaAs. The thickness of the ZnSe films was about 10 bm. Ohmic contacts were applied to the ZnSe and GaAs layers by depositing drops of a Ga-In alloy followed by annealing in forming gas at 350” c. Dark I- V solar cell characteristics of these heterojunctions are shown in Fig. 1. Under reverse bias voltage the I- V curves may be approximated as: I=I;exp(
where
-A
A = 13.0-14.0,
.(V,-
V)1’2)
V, = 1.3-1.35
V
lo”-
4
05
400
600
800
Ainm:
Fig. 2. Dependence of internal quantum efficiency on light wavelength. Curve (1)--n = 3.10’” cm-s, (2)-r] = 2. 10” A = 470 nm, illumination, (4)--n = 3 10lh cmm3 under with X = 470 nm, (3)--n = 2.10” cmm3 under illumination cme3 without shortwave light.
Such behaviour of the I- V curves is probably due to a barrier in the conduction band. Measurements of the internal quantum efficiency on these diodes was carried out as in [9]. The quantum efficiency decreased with decrease of carrier concentration in the ZnSe. This process becomes sharper when the value of the decreased carrier concentration reached n = 1016 cm13, as shown in Fig. 2. The quantum efficiency in the long wave region of diode sensitivity essentially depends on illumination with a 460-510 nm wavelength. Figure 3 shows the dependence of quantum efficiency for the diode with n = 2.10” cm-’ on the 632 nm wavelength of illumination. The power of illumination at 460-510 nm was about 10 pW/cm2. The quantum efficiency for X = 632 nm versus the power of illumination for X = 472 nm is shown in Fig. 3. The effect of quantum efficiency variation does not depend on power of illumination in the long wavelength region. A similar change of the value of quantum efficiency at illumination in the short wavelength region occurred for diodes with n = lOI cm ‘, however, the quantum efficiency changed from lo- ’ to 3 lo-’ for X = 632 nm at the same power of illumination. Illumination in the short wavelength region led to an essential variation of the I- I/ curves of the diode with n = 5 ‘10” crne3 (Fig. 1).
IbA1 :J@ 4
3
10’
2
1
1
I
I
01
1
03
t
c
U(V)
’
8
07
*
Fig. 1. Dark I- V characteristics of diodes with the area of 0.05 cm2 for various electron concentrations in ZnSe layers. Curve (I)-for n=2.10’7cm-3, T=lOO K; (2)--n=2. lO”cm3, T= 300 K; (3)--n = 3 lo’* cme3, T= 300 K; (4)-under short wavelength illumination in the 460-500 nm region, it = 2 10” cn3&‘;r n = 3 10” cm-s. T=
064 J 500
550
Xtnm)
Fig. 3. Curve (1)-dependence of quantum efficiency with illumination wavelength X = 632 nm on illumination wavelength with intensity equal to 10 pW/cm’, (2)-dependence of quantum efficiency at X = 632 nm on power of illumination with X = 472 nm.
249
Photovoltaic properties in ZnSe-GaAs heterojunction JlA/cm’) 1
2 04 3 0.2.
02
0.4
06
U(V)
Fig. 4. J- V characteristics for various intensities of sunlight (AMO). Curve (1)--P = 3.5 W/cm*, (2)--P = 2.6 W/cm*, (3)--P = 1.6 W/cm*.
Quantum efficiency also increased with reverse voltage bias on the diode. For diodes with n = 2 . 10" cm-- 3 the value of quantum efficiency was close to unity for a reverse voltage bias of 3-5 V. For the diode with n = 1016 cm- 3 the quantum efficiency at the reverse bias voltage increased up to lo-20%. Since the quantum efficiency for the diode with n > 10” cm- 3 is a high value, one can conclude that concentration of recombination centers in the interface of heterostructures is low. Figure 4 shows photo Z-V characteristics for the diode with n = 3.10” cmv3 and area of 5. 10m2 cm2 without antireflection coating, and having an ohmic indium contact in the center of a sample 700 pm in diameter, at different powers of illumination with an AM0 spectrum. Figure 4 shows that the series resistance of the diode together with the contacts is 0.2 a/cm*. This value is close to the theoretical spreading resistance for this diode. The photocurrent density is 26 mA/cm2 and open-circuit voltage V, is 0.62 V at the illumination power of 140 mW with the AM0 spectrum. For understanding of the mechanism of current flow we have investigated the complex differential diode re-
01
sistance variation with bias voltage. The characteristics were observed at frequencies from 200 Hz-80 kHz at room temperature. Figure 5 shows l/C (C is the capacity) as a function of voltage bias at different frequencies of the test signals for a diode with n = 2 10’7cm-3. The thickness of the depletion layer at a zero voltage bias has a value higher than for the abrupt transition case. This indicates the presence of a layer with the carrier concentration different from the volume concentration of the carriers in the heterostructure, The carrier concentration in the layer near the surface of GaAs did not change, as shown by the GaAs photoluminescence spectra after ZnSe deposition.
MODEL Anomalous properties of the nZnSe-pGaAs heterostructures are qualitatively the same as those of n CdS-p CuInSe, heterostructures. They have in common that the energies of the electron affinity are close, therefore there is no barrier for electrons in the conduction band. Wide-forbidden-band materials in these heterostructures tend to selfcompensation. Assume that near the interface there exists a layer of compensated wide-band material with thickness L. Let N, > NJ in this layer, where Nd is the donor concentration in the wide-band material, N, is the deep acceptor concentration with an energy E, < A E (Fig. 6) In this case the Fermi level in the compensated level lies higher than E,. As a result, the levels with energy E, must be filled with electrons. Therefore, a negative space charge with the density q( N, - Nd) may form in an L-thick layer. Depending on the relationship between the values Nd and N, there can be an essential change of the heterojunction band structure, as compared to an abrupt heterojunction, and of its photoelectric properties. We have computed the C- V characteristics at various frequencies and various distributions of accep-
VaP(V)
Fig. 5. Dark C-V characteristics of a diode with area.0.05 cm2 and II = 2. 10” cm-‘. Positive voltage direct bias: Curves (1-2)-theoretical and experimental curves at the frequency of test signal 25 kHz. (3-3)-1000 Hz, (5-6)-500 Hz at room temperature, respectively.
250
B. V. ZHUK er crl.
/
The electric field vanishes at the point where the potential is equal to zero and the electric field and potential are assumed to be continuous at x = 0. Under these conditions we obtain an equation for the potential in the depletion layer with the width L:
GoAs
(5) Fig. 6. Band structure of the ZnSe-GaAs heterojunction.
tars and donors in ZnSe near the heterointerface [12]. It has been shown that the pure differential resistance R of the p-n junction and its capacity C at frequency o can be determined from the equation ~~“(ypn(.r)+iwrt-,,)~‘dx=(R
‘+ioC)-’
(1) where I_Lis the mobility of the current carriers, c,, is the dielectric constant, W is the depletion width of the heterojunction, L,J is the electron charge, and n ( x ) is the current carriers distribution. Analysis of eqn (1) leads to the following dependence of capacitance on frequency:
C( cd)= q,S/d( w)
(4
where S is the junction area, d(w) is the thickness of the depletion layer for which OT > 1, (r is the dielectric relaxation time equal to re,X’, i3 = qpcLn(x), n(_x)=n,,exp(-qcp(x)/kT), where cp(x) is the potential distribution in the junction). For the nZnSe-pGaAs heterostructure, the n(x) distribution corresponds to the electron distribution in the depletion layer. Since there is a large barrier for holes in the valence band, the distribution of holes can be neglected. It follows from eqn (2) that for calculation of the C- V characteristics we must know the potential distribution which can be obtained from Poisson’s equation:
d2gD -q/cco(n(x)-N,+N,). -= dx2
It was assumed that at x < 0 n = Nd and transition layer N, = N, - NJ; therefore, VP:’=
(3)
in the
-q/cc,, . N,{exp(-+(x))+WN~} (4)
where
where L, = (ee,, kT/q2Nj)‘/’ is the Debye length in the n-material. The boundary condition for (5) is q(L) = VI, Vap, where Vn is the diffusion potential and q,!, is the applied voltage bias, Theoretical C-V curves are close to the experimental characteristics at L = 350 nm and N, = 10” cm-j for the barrier concentration in the ZnSe equal to n = 3 lOI6 cm-‘, L = 300 nm. Also for N, = 1.1 10lh for n = 3.10” cmm3, L = 170 nm and N, = 1.5 . 10Lh cm-~’ for n = 3 - 10’s cm- ‘. The C-V curves for samples with n = 3 10”’ cm-’ and n = 3 10" cm-? are not shown, since they are similar to those in Fig 5. Figure 7 shows experimental C-V curves for the same sample as shown in Fig 5, but under illumination with the wavelength about 460-500 nm. Theoretical C-V characteristics are close to the experimental ones provided the concentration of the negative charge centers is decreased to N = 1 ‘10” cm-m? for L = 300 nm. The potential distribution in the junction for different values of the applied voltage bias is shown in Fig 8. Figure 8 shows that there can exist a barrier for electrons in the conduction band. The height of the barrier can essentially change depending on N, As calculations have shown, the barrier height and C-V curves weakly depend on the electron concentration in the ZnSe at n > 3 ‘10” cm ‘. In gen-
G(x) = qq(x)/kT.
Fig. 7. C-V characteristics of diode under illumination with X = 470-500 nm: Curves (l-2) are theoretical and experimental curves at w = 25 kHz, (3-4)-10(x) HZ. (5-6) -500 Hz at room temperature, respectively.
Photovoltaic
properties
in ZnSe-GaAs
Fig. 8. Potential distribution in conduction band of the heterostructure for n = 10” cmm3, L = 300 nm. Curves (1,3,5)-for NA = 1.2’ 10n’ cme3 and Vd- VUp= 1.1 V, 0.55 V, respectively; (2,4,6)-for N, = 1.1016 cmm3.
these characteristics depend on concentration of the charge centers in the transition layer and on its thickness (Fig 8). Analysis of the solution of eq (1) has shown that at direct voltage bias one can neglect the influence of the current on C-V characteristics, if, the current densities are similar to these for Fig 1. It should be noted that the experimental C-V characteristics are close to the theoretical value at Vn = 1 V. This discrepancy with theory can be explained by the surface states at the ZnSe/GaAs interface which lead to the band bending in the GaAs by the value of 0.2-0.3 eV. It should be noted that the calculations were obtained under the boundary condition cp( L) = V, tp which is valid only at high hole concentration. It follows from eq (1) that this condition is fulfilled doping with an accuracy of - kT for an arbitrary level in p-GaAs if there is a barrier for electrons in the conduction band. It is known that when doping ZnSe with Ga and In up to the level of 10’9-1020 crnm3, a compensated material with high resistance is formed [13]. In [13] ZnSe doping with In and Ga to the values of 10’x-1020 cme3 is investigated and deep compensated acceptor centers are shown to have ionization energies of 0.59 eV and 0.41 eV for In, and 0.55 eV for Ga. So, the presence of the compensated layer in the ZnSe of the diodes is associated with the formation of a Ga-doped 0.1-3 pm thick layer near the heterojunction boundary. This may be due to both the Ga diffusion from the substrate during growing and the Ga transfer by alkyl radicals formed by decomposition of DEZ at the initial stages of growing.
eral
DISCUSSION
AND CONCLUSION
On the basis of the model considered we can draw the following conclusions to account for the behaviour of the ZnSe/GaAs heterostructure solar cells.
251
heteroJunction
1. An increasing quantum efficiency under reverse voltage bias is explained by an increasing electric field in the space charge region. As a result the losses from surface and volume recombination decrease. 2. The “crossover” effects of the dark and light I-V curves, the change of capacitance of the p-n junctions and the increase of quantum efficiency of diodes in the long-wave length region under illumination can be explained by the fact that dark and light current is limited by a barrier in the conduction band. The probability of tunneling is determined by the height of the barrier. Under short-wave length illumination of a diode, electron transfer from deep acceptor centers to the conduction band occurs. As a result, the concentration of the negatively charged acceptor centers decreases and the barrier for electron is reduced. From the spectral curves in Fig 3 we can estimate the ionization energy of deep acceptor centers to be equal to 0.5-0.6 eV for the compensating centers in the ZnSe layers, which is close to the value of [13]. Since the band structure of the CdS-CuInSe, heterojunctions is analogous to that of the ZnSeeGaAs heterojunctions, the anomaly in this structure can be explained by the same model. Using the data of [3], the energy of deep acceptor levels in CdS, which is equal to about 0.8 eV can be estimated. This fact is apparently accounted for by the diffusion of Cu into CdS, which leads to the formation of a compensated layer in the CdS. 3. The decrease of quantum efficiency with decreasing carrier concentration in wide-band material is connected with increasing thickness of the compensated layer and rise of the barrier for electrons in the conduction band.
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