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n0-GaInAsSb type II heterojunctions
Infrared Physics & Technology 45 (2004) 169–175 www.elsevier.com/locate/infrared
Electrical and photoelectrical properties of isotype Nþ-GaSb/n0-GaInAsSb type II heterojunctions M.A. Afrailov Department of Physics, Uludag University, G€orukle, Bursa 16059, Turkey Received 3 April 2003
Abstract Dark current–voltage characteristics, spectral response and energy diagrams have been studied for LPE grown isotype heterostructures lattice-matched to GaSb substrates. The dark current mechanisms in the isotype heterostructures were investigated at several temperatures. It is shown that a type II staggered heterojunction can behave as a Schottky diode and the dark current–voltage characteristics of this isotype heterostructures were rectifying over the whole temperature range 90–300 K. The photocurrent sign dependence on photon energy has been studied as a function of forward bias. 2003 Elsevier B.V. All rights reserved. Keywords: Isotype structures; Type II heterojunctions with staggered band alignment; Photo-response; Dark current
1. Introduction Type II heterostructures in the InAs–GaSb system have recently attracted great attention as promising materials for optoelectronic devices (lasers, light-emitting diodes and photodiodes) for the spectral range of 2–5 lm [1–3], which is useful for gas pollutant analysis. The unusual band energy diagram of type II heterojunction results in electron and holes being localized in self-consistent quantum wells on either side of the interface [4]. The importance of this interface results in some new optical, photoelectrical and magnetotransport properties [5]. So an electron channel with high
E-mail address: [email protected] (M.A. Afrailov).
mobility was found at the interface of type II GaInAsSb/InAs broken-gap heterojunctions [6], and intensive interface electroluminescence was observed due to tunneling optical transitions and recombination of spatially separated carriers [7]. Type II heterojunctions on the base of GaSb– Ga1x Inx Asy Sb1y can demonstrate two different kinds of band alignment. In a staggered type II system (x 6 0:24), either the conduction or the valence band of one semiconductor lies outside the band gap of the other material, the band offsets having the same sign. At large band offsets a broken-gap alignment will be formed in type II heterojunctions (x P 0:80), when neither the conduction band nor the valence band of the narrowgap semiconductor is located within the band gap of the wide-gap material [8]. Electrons and holes being mutually transferred through the interface, a
1350-4495/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.infrared.2003.09.001
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M.A. Afrailov / Infrared Physics & Technology 45 (2004) 169–175
high band bending is formed at the heteroboundary. It was shown that a type II broken heterojunction could behave as Schottky diode in which a wide-gap semiconductor (in P–p structure) or a narrow-gap one (in N–n structure) plays the role of metal [4,9]. Photoluminescence of the type II staggered heterostructures was studied in [10]. The present paper deals only with staggered type II heterojunctions based on quaternary Ga1x Inx Asy Sb1y (x ¼ 0:20) solid solution grown liquid phase epitaxy (LPE) by lattice-matched to GaSb substrate. The dark current–voltage characteristics of isotype N–n GaSb–GaInAsSb structures with single heterojunction were investigated and analyzed in the temperature range of 90–300 K, with the intention of identifying various dark current mechanisms dominating in different temperature and bias regions. It is hoped that the obtained results will serve for improving the material and device technology. Moreover, this paper present a new photoelectric phenomenon––the dependence of the photocurrent sign on photon energy as a function of forward bias, which have been first observed in type II heterojunctions based on the isotype GaSb–GaInAsSb structure.
characteristics––by KEITHLEY 590 C–V Analyzer. The photosensitivity spectra were measured with SPM-2 monochromator equipped with a LiF prism and with the use of a globar light source. All measurements were computer controlled.
3. Results and discussions Fig. 1 presents typical current–voltage (I–V ) characteristics of the isotype GaSb/GaInAsSb single heterostructure at several temperatures. Forward bias corresponds to a positive potential on the narrow-gap GaInAsSb side of the heterojunction. It is seen that the dark current–voltage characteristics were rectifying over the whole temperature range 90–300 K. No double saturation branches in the forward or reverse I–V characteristics were observed, which indicates a low density of surface states. The structures studied were grown lattice-matched to an accuracy better than Da=a < 103 and the expected density of surface states did not exceed 6 · 1011 cm2 . In Fig. 2 there are presented energy band diagrams of the N-GaSb/n-GaInAsSb structure at equilibrium (a), forward (b) and reverse (c) applied biases. The band offsets of the GaInx AsSb/GaSb
2. Device fabrication and measurements The structures were fabricated by LPE on n-type GaSb substrates, doped with tellurium to a carrier concentration of (5–7) · 1017 cm3 . The structures were actually single heterostructures in which a narrow-gap n0 -GaInAsSb layer 2–3 lm thick (Eg ¼ 0:52 eV at 300 K) is sandwiched between a Nþ -GaSb layer and a nþ -GaInAsSb layer of the same composition Te-doped to an electron concentration of (3–5) · 1017 cm3 . Mesa-diode samples with a sensitive area 250–300 lm in diameter were fabricated from these structures by standard photolithography. An abrupt junction was found by C–V measurements, with electron concentration 5 · 1015 cm3 in the narrow-gap n0 -GaInAsSb layer. Fabricated devices were mounted into a glass Dewar with a cold shield for detailed electrical measurements at variable temperatures. I–V characteristics of the samples were measured using a KEITHLEY 485 pA meter and capacity–voltage
Fig. 1. Current–voltage characteristics in a linear scale of Nþ GaSb/n0 -GaInAsSb isotype heterostructure at several temperatures. In the inset is the schematic diagram of the structure: (1, 5) Ohmic contacts, (2) nþ -GaInAsSb, (3) narrow-gap n0 GaInAsSb active region, (4) Nþ -GaSb substrate.
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171
Fig. 2. Energy band diagram of a Nþ -GaSb/n0 -GaInAsSb isotype heterostructure in equilibrium (a); under forward (b) and reverse (c) biases.
structures with x ¼ 0:20 were found to be DEC ¼ 0:3 eV and DEV ¼ 0:1 eV [11], which are in reasonable agreement with the data of [12]. From the band diagram one finds that a barrier exists for electrons going from the GaSb to the GaInAsSb as well as for electrons going in the opposite direction. Current transport across a Nþ –n heterojunction is similar to that of a metal–semiconductor junction: diffusion, thermionic emission as well as tunneling of carriers across the barrier can occur. The dominant current transport mechanism at high-temperature region is the thermionic emission of carriers over the barrier, and the current as a function of applied bias V is given by the relation [13] Va qVa I ¼ I0 1 exp 1 ð1Þ Vi kT where I0 ¼
qAR TVi exp k
qVi kT
ð2Þ
Here R is the effective Richardson constant, Va is applied voltage, Vi is the built-in voltage, A is the
junction area and the rest symbols have their usual meanings. Whereas expression (1) is similar to that of metal–semiconductor barrier, it differs in that the temperature dependence is somewhat modified and the reverse bias current increases with voltage. Besides, the small complication is that the applied voltage is distributed between two barriers on either side of the interface. The built-in voltage for Nþ –n heterojunction with doping concentrations Nþ d and Nd is given by the relation [14] þ kT Nd Ncn DEC ln ð3Þ Vi ¼ þ q q Nd Nþ cn where Nþ cn and Ncn are the effective densities of states of the high and low doped region, respectively. 3.1. Forward current–voltage characteristics Fig. 3 shows the forward current–voltage (IF –VF ) characteristics at several temperatures. The junction current as a function of applied bias, can be written in the following empirical form: Va qVa I ¼ I0 1 exp 1 ð4Þ Vi bkT where b ¼ 1 in the case of thermionic current.
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Amperes
1,00E-03
1,00E-05
1,00E-07 T=90 K T=160 K T=230 K
1,00E-09
T=250 K T=300 K
1,00E-11 0
100
200
300
400
500
600
Millivolts
Fig. 3. Forward bias IF –VF characteristics of isotype Nþ -GaSb/ n0 -GaInAsSb structure in semi-logarithmic scale at several temperatures.
For the heterojunctions under consideration, the barrier height is higher than kT =q and the depletion region in the substrate is about 106 cm wide, mechanism of current flowing in this case is mainly governed by the emission theory [15]. The comparison was theoretically calculated (Eq. (4)) and experimental current–voltage characteristics in the forward bias region for several temperatures are shown in Fig. 4. It is seen that the b-factor determined from the slope of the (IF –VF ) characteristics at high-temperatures (T P 250 K) for small forward bias region, is equal to b ¼ 1:1–1:3 and indicates the predominantly thermionic nature of the current. Indeed, small forward - bias applied to the heterojunction lowers its barriers. That is, in
the heterostructure under study the current is due to thermionic emission of electrons over the barrier and flows from GaSb to the narrow-gap semiconductor. The discrepancy from the theory for large forward bias voltages can be explained as due to the effect of series resistance. At low temperatures the tunnelling processes become dominant, because of improbability of passing over barriers. This is confirmed by large bfactor values obtained (b ¼ 2:7 and 7.7) for two linear portions of the IF –VF characteristic at T ¼ 160 K (Fig. 4) and for two linear portions of the IF –VF characteristic (b ¼ 3.6 and 10.2) at T ¼ 90 K (Fig. 3). Thus the variation of b-factor as a function of temperature can be explained by tunnelling and by distribution of the applied voltage between the two potential barriers. 3.2. Reverse current–voltage characteristics Fig. 5 shows the reverse current–voltage (IR –VR ) characteristics as a function of temperature measured for the same structures, as shown above in logarithmic scale. Reverse voltage corresponds to negative potential on the narrow-gap GaInAsSb side of the heterojunction. It is seen that at hightemperatures (T > 230 K) a lightly saturation of the reverse current was observed for small biases (V < 500 mV), indicating that in the case a thermionic emission current mechanism prevails. This current is due to thermal activation of electrons from GaInAsSb into GaSb over the barrier
1,00E+00 1 − β = 1.1 (300 K) 2 − β = 1.3 (250 K) 3 − β = 2.7 (160 K)
1,00E-01
1
1,00E-02
2
1,00E-03
3
1,00E-03 1,00E-04
1,00E-04
Amperes
Amperes
1,00E-02
1,00E-05 1,00E-06
I (theor., 300 K) I (exper., 300 K) I (theor., 250 K) I (exper., 250 K) I (theor., 160 K) I (exper., 160 K)
1,00E-07 1,00E-08 1,00E-09
1,00E-05 1,00E-06 T=300 K
1,00E-07
T=250 K T=230 K
0
50
100
150
200
250
300
350
400
450
500
1,00E-08
T=203 K T=160 K
Millivolts
Fig. 4. Comparison of theoretically calculated (continuous line curve) and experimental (discrete points) current–voltage characteristics in the forward bias region at several temperatures.
1,00E-09 10
100
1000
10000
Millivolts
Fig. 5. Reverse bias IR –VR characteristics in logarithmic scale at several temperatures.
M.A. Afrailov / Infrared Physics & Technology 45 (2004) 169–175
at the heterointerface. Since, the barrier height seen from the GaInAsSb remains unchanged, the reverse current increases with bias due to electrons partly tunnelling from the GaInAsSb into GaSb. Fig. 6 shows IR versus 103 /T as a function of the reverse bias. The activation energy estimated from this figure in the high-temperature region is equal to Ea ¼ 0:288 eV. This value is very close to the barrier height DEC at the interface for electrons in GaInAsSb, which is in reasonable agreement with the data obtained in [11,12]. This is also evidence of the predominant contribution of the thermionic emission mechanism to dark current flow at high-temperature region. For large values of reverse bias the current determined by tunnelling over the whole temperature range of 90–300 K. Indeed, a large reverse bias can result in the barrier to become thin enough for significant tunnelling of electrons from GaInAsSb to GaSb. Estimations show that at low temperatures the tunnelling mechanism of the current flowing dominates, since carrier passing over the barrier becomes improbable. This is confirmed by the weak temperature dependence of the reverse current in the low temperature region. 3.3. Photoelectric properties As stated above, spatial separation of electrons and holes at the type II heterointerface leads to new quantum effects that manifest themselves in phenomenon of photocurrent amplification on N–n heteroboundary [4]. A novel photoelectric 1,00E-01 V = -0,2 V
1,00E-02
V = -0,4 V V = -0,6 V
Amperes
1,00E-03
V = -3,0 V
1,00E-04 1,00E-05 1,00E-06 1,00E-07 1,00E-08 1,00E-09 2
4
6
8
10
12
1000/T, K-1
Fig. 6. The reverse current as a function of reciprocal temperature at different reverse applied bias.
173
phenomenon––the dependence of the photocurrent sign on photon energy as a function of forward bias in Nþ -GaSb/n0 -GaInAsSb structures have been studied in this work. This effect is due to modulation of the barrier transparency at the interface limiting the tunnel transitions of the conduction electrons and to the localization of photoholes in the potential well at the type II interface. In this type of N–n junction a photovoltage will be produced, which may depend in sign on the photon energy. It is important to note that in our case the sign reversal of the photocurrent should not be attributed to the surface states at the interface as in [16] because the structures studied were well lattice-matched and observes were no double saturation branches on the forward or reverse I–V characteristics as opposed to results of [16]. A fairly deep (up to 100 meV) quantum well for electrons is formed at the interface in the GaInAsSb/GaSb structure on the side of narrow-gap quaternary layer (see Fig. 2). Simultaneously, a barrier for electrons arises in the conduction band at the interface. The barrier depth and width vary with the composition of solid solution and the layer doping level. If we excite the structure with light whose wavelength falls in the intrinsic absorption band of the narrow-gap semiconductor, non-equilibrium holes are generated and eventually tunnel through the barrier in the valence band. As a result they are captured by the native acceptors in the band-bending region on the GaSb side. The barrier transmission for electrons in the conduction band is extremely sensitive to the presence of non-equilibrium holes at the interface. The accumulation of these holes results in narrowing of the barrier, which augments the tunneling of conduction electrons. The spectral response of the isotype GaSb– GaInAsSb heterostructure as a function of forward biases at room temperature is presented in Fig. 7. At zero or small forward bias, the photocurrent is positive, which corresponds to photoelectrons crossing the barrier towards GaSb and to photoholes drifting and diffusing in the opposite direction. The forward bias of several millivolts applied to the structure lowers the energy barriers at the
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M.A. Afrailov / Infrared Physics & Technology 45 (2004) 169–175 120 100
T=300 K
Photocurrent (arb.units)
80 60 40 20 0 -20 -40
0V 3 mV 4,1mV 16 mV -16 mV
-60 -80 -100 -120 0.6
1.1
1.6
2.1
2.6
Wavelength (µm)
Fig. 7. Spectral response of a Nþ -GaSb/n0 -GaInAsSb isotype heterostructure as a function of the forward bias at room temperature.
heterojunction, and the energy bands become partly unbent (see Fig. 2b). It becomes possible for non-equilibrium electrons photogenerated in GaSb to pass to GaInAsSb layer and move further toward the positive contact. As a result the negative photocurrent appears firstly in this spectral range beginning from k 1:8 lm. At larger forward bias, electrons photogenerated in the GaInAsSb layer are no more able to cross the barrier and holes accumulate at the interface. The resulting narrowing of the barrier could enhance the dark current due to electrons coming from GaSb, which thus appears as a negative photocurrent in the spectral range k > 1:8 lm (see Fig. 7). If we apply a reverse bias of several millivolts to the structure (see Fig. 2c), photogenerated electrons in GaInAsSb and photoholes in GaSb begin to tunnel through the interface and the photocurrent is determined by flowing photoelectrons from GaInAsSb to GaSb and by photoholes flowing in opposite direction. As a result, the photocurrent becomes positive. 4. Conclusion In this letter we have considered the electrical and photoelectric properties of a single type II
heterojunction in the GaSb/GaInAsSb system with the staggered band alignment. It is shown that a type II staggered heterojunction can behave as Schottky diode and the dark current–voltage characteristics of this heterostructure are rectifying over the whole temperature range of 90–300 K. No double saturation branches in the forward or reverse I–V characteristics were observed, which indicates a low density of surface states in this heterostructure. The qualitative comparison of experimental results with theory shows that various components of current flowing (diffusion, thermal emission tunnelling) through the heterojunction make contribution to dark current flow, dominating at different temperatures. The sign reversal of the photocurrent on photon energy on forward bias has been studied. This effect is due to modulation of the barrier transparency at the interface limiting the tunnel transitions of the conduction electrons and to the localization of photoholes in the potential well at the type II interface. The sign reversal of the photocurrent on photon energy as a function of applied voltage takes place only on the forward bias. Studies of these heterostructures will provide the physical basis for the fabrication of photodetectors operating in the wavelength range of 2–5 lm, important for gas pollutant analysis (CO2 , CO), combustion diagnostics and a null detecting frequency meter operating in the near IR region at room temperature.
Acknowledgements I thank Prof. M. P. Mikhailova and Dr. Yu.P. Yakovlev who are both with the Ioffe PhysicoTechnical Institute of RAS (St. Petersburg, Russia) for discussion, encouragement.
References [1] A.N. Baranov, N. Bertu, Y. Cumminal, J. Cumminay, G. Boissier, C. Alibert, A. Joullie, Appl. Phys. Lett. 71 (1997) 735–737. [2] C.N. Lin, R.Q. Yang, S.J. Murry, S.S. Pey, C. Yan, J. McDaniel, M. Falkon, IEEE Photon. Technol. Lett. 9 (1997) 1573–1575.
M.A. Afrailov / Infrared Physics & Technology 45 (2004) 169–175 [3] M.P. Mikhailova, N.D. Stoyanov, O.V. Andreychuk, K.D. Moiseev, I.A. Andreev, Yu.P. Yakovlev, M.A. Afrailov, IEE Proc. Optoelectron. 149 (2002) 41–44. [4] M.P. Mikhailova, A.N. Titkov, Semicond. Sci. Technol. 9 (1994) 1279–1295. [5] M.P. Mikhailova, K.D. Moiseev, V.A. Berezovets, R.V. Parfeniev, N.I. Bazhenov, V.A. Smirnov, Yu.P. Yakovlev, IEE Proc. Optoelectron. 145 (1998) 268–274. [6] T.I. Voronina, B.E. Zhurtanov, T.S. Lagunova, M.P. Mikhailova, K.D. Moiseev, A.E. Rozov, Yu.P. Yakovlev, Semiconductors 35 (2001) 345–351. [7] N.I. Bazhenov, G.G. Zerga, M.P. Mikhailova, K.D. Moiseev, V.A. Smirnov, O.Yu. Soloveva, Yu.P. Yakovlev, Semiconductors 31 (1997) 658–661. [8] M.A. Afrailov, A.N. Baranov, A.P. Dimitriev, M.P. Mikhailova, Yu.P. Smorchkova, I.N. Timchenko, V.V. Shertnev, Yu.P. Yakovlev, I.N. Yassievich, Sov. Phys. Semicond. 24 (1990) 876–882. [9] I.A. Andreev, A.N. Baranov, M.P. Mikhailova, K.D. Moiseev, A.V. Pentsov, Yu.P. Smorchkova, V.V. Shertnev,
[10]
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[16]
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Yu.P. Yakovlev, Sov. Tech. Phys. Lett. 18 (1992) 135– 137. E. Hulicius, J. Oswald, J. Pangrac, T. Simecek, A.N. Titkov, O.B. Gusev, Abstracts of 12th General Conf. of the EPS Condenced Matter Division, vol. 16A, Praque, 1992. M.A. Afrailov, M.P. Mikhailova, N.R. Rahimov, Tr. J. Phys. 21 (1997) 1229–1232. M.S. Bresler, O.B. Gusev, A.N. Titkov, V.N. Cheban, Yu.P. Yakovlev, E. Hulicius, I. Oswald, I. Pangrac, T. Simecek, Semiconductors 27 (1993) 341–346. M.S. Tyagi, Introduction to Semiconductor Materials and Devices, Wiley, New York, 1991. J. Bart Van Zeghbroeck, Principles of Electron Devices, University of Colorado, Boulder, 1996. A.G. Milnes, D.L. Feucht, Heterojunction and Metal– Semiconductor Junctions, Academic Press, New York, 1972, Mir, Moscow, 1975. M.J. Hampshire, T.I. Pritchard, R.D. Tomlinson, Solid State Electron. 13 (1970) 1073–1076.
Electrical and photoelectrical properties of isotype Nþ-GaSb/n0-GaInAsSb type II heterojunctions M.A. Afrailov Department of Physics, Uludag University, G€orukle, Bursa 16059, Turkey Received 3 April 2003
Abstract Dark current–voltage characteristics, spectral response and energy diagrams have been studied for LPE grown isotype heterostructures lattice-matched to GaSb substrates. The dark current mechanisms in the isotype heterostructures were investigated at several temperatures. It is shown that a type II staggered heterojunction can behave as a Schottky diode and the dark current–voltage characteristics of this isotype heterostructures were rectifying over the whole temperature range 90–300 K. The photocurrent sign dependence on photon energy has been studied as a function of forward bias. 2003 Elsevier B.V. All rights reserved. Keywords: Isotype structures; Type II heterojunctions with staggered band alignment; Photo-response; Dark current
1. Introduction Type II heterostructures in the InAs–GaSb system have recently attracted great attention as promising materials for optoelectronic devices (lasers, light-emitting diodes and photodiodes) for the spectral range of 2–5 lm [1–3], which is useful for gas pollutant analysis. The unusual band energy diagram of type II heterojunction results in electron and holes being localized in self-consistent quantum wells on either side of the interface [4]. The importance of this interface results in some new optical, photoelectrical and magnetotransport properties [5]. So an electron channel with high
E-mail address: [email protected] (M.A. Afrailov).
mobility was found at the interface of type II GaInAsSb/InAs broken-gap heterojunctions [6], and intensive interface electroluminescence was observed due to tunneling optical transitions and recombination of spatially separated carriers [7]. Type II heterojunctions on the base of GaSb– Ga1x Inx Asy Sb1y can demonstrate two different kinds of band alignment. In a staggered type II system (x 6 0:24), either the conduction or the valence band of one semiconductor lies outside the band gap of the other material, the band offsets having the same sign. At large band offsets a broken-gap alignment will be formed in type II heterojunctions (x P 0:80), when neither the conduction band nor the valence band of the narrowgap semiconductor is located within the band gap of the wide-gap material [8]. Electrons and holes being mutually transferred through the interface, a
1350-4495/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.infrared.2003.09.001
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M.A. Afrailov / Infrared Physics & Technology 45 (2004) 169–175
high band bending is formed at the heteroboundary. It was shown that a type II broken heterojunction could behave as Schottky diode in which a wide-gap semiconductor (in P–p structure) or a narrow-gap one (in N–n structure) plays the role of metal [4,9]. Photoluminescence of the type II staggered heterostructures was studied in [10]. The present paper deals only with staggered type II heterojunctions based on quaternary Ga1x Inx Asy Sb1y (x ¼ 0:20) solid solution grown liquid phase epitaxy (LPE) by lattice-matched to GaSb substrate. The dark current–voltage characteristics of isotype N–n GaSb–GaInAsSb structures with single heterojunction were investigated and analyzed in the temperature range of 90–300 K, with the intention of identifying various dark current mechanisms dominating in different temperature and bias regions. It is hoped that the obtained results will serve for improving the material and device technology. Moreover, this paper present a new photoelectric phenomenon––the dependence of the photocurrent sign on photon energy as a function of forward bias, which have been first observed in type II heterojunctions based on the isotype GaSb–GaInAsSb structure.
characteristics––by KEITHLEY 590 C–V Analyzer. The photosensitivity spectra were measured with SPM-2 monochromator equipped with a LiF prism and with the use of a globar light source. All measurements were computer controlled.
3. Results and discussions Fig. 1 presents typical current–voltage (I–V ) characteristics of the isotype GaSb/GaInAsSb single heterostructure at several temperatures. Forward bias corresponds to a positive potential on the narrow-gap GaInAsSb side of the heterojunction. It is seen that the dark current–voltage characteristics were rectifying over the whole temperature range 90–300 K. No double saturation branches in the forward or reverse I–V characteristics were observed, which indicates a low density of surface states. The structures studied were grown lattice-matched to an accuracy better than Da=a < 103 and the expected density of surface states did not exceed 6 · 1011 cm2 . In Fig. 2 there are presented energy band diagrams of the N-GaSb/n-GaInAsSb structure at equilibrium (a), forward (b) and reverse (c) applied biases. The band offsets of the GaInx AsSb/GaSb
2. Device fabrication and measurements The structures were fabricated by LPE on n-type GaSb substrates, doped with tellurium to a carrier concentration of (5–7) · 1017 cm3 . The structures were actually single heterostructures in which a narrow-gap n0 -GaInAsSb layer 2–3 lm thick (Eg ¼ 0:52 eV at 300 K) is sandwiched between a Nþ -GaSb layer and a nþ -GaInAsSb layer of the same composition Te-doped to an electron concentration of (3–5) · 1017 cm3 . Mesa-diode samples with a sensitive area 250–300 lm in diameter were fabricated from these structures by standard photolithography. An abrupt junction was found by C–V measurements, with electron concentration 5 · 1015 cm3 in the narrow-gap n0 -GaInAsSb layer. Fabricated devices were mounted into a glass Dewar with a cold shield for detailed electrical measurements at variable temperatures. I–V characteristics of the samples were measured using a KEITHLEY 485 pA meter and capacity–voltage
Fig. 1. Current–voltage characteristics in a linear scale of Nþ GaSb/n0 -GaInAsSb isotype heterostructure at several temperatures. In the inset is the schematic diagram of the structure: (1, 5) Ohmic contacts, (2) nþ -GaInAsSb, (3) narrow-gap n0 GaInAsSb active region, (4) Nþ -GaSb substrate.
M.A. Afrailov / Infrared Physics & Technology 45 (2004) 169–175
171
Fig. 2. Energy band diagram of a Nþ -GaSb/n0 -GaInAsSb isotype heterostructure in equilibrium (a); under forward (b) and reverse (c) biases.
structures with x ¼ 0:20 were found to be DEC ¼ 0:3 eV and DEV ¼ 0:1 eV [11], which are in reasonable agreement with the data of [12]. From the band diagram one finds that a barrier exists for electrons going from the GaSb to the GaInAsSb as well as for electrons going in the opposite direction. Current transport across a Nþ –n heterojunction is similar to that of a metal–semiconductor junction: diffusion, thermionic emission as well as tunneling of carriers across the barrier can occur. The dominant current transport mechanism at high-temperature region is the thermionic emission of carriers over the barrier, and the current as a function of applied bias V is given by the relation [13] Va qVa I ¼ I0 1 exp 1 ð1Þ Vi kT where I0 ¼
qAR TVi exp k
qVi kT
ð2Þ
Here R is the effective Richardson constant, Va is applied voltage, Vi is the built-in voltage, A is the
junction area and the rest symbols have their usual meanings. Whereas expression (1) is similar to that of metal–semiconductor barrier, it differs in that the temperature dependence is somewhat modified and the reverse bias current increases with voltage. Besides, the small complication is that the applied voltage is distributed between two barriers on either side of the interface. The built-in voltage for Nþ –n heterojunction with doping concentrations Nþ d and Nd is given by the relation [14] þ kT Nd Ncn DEC ln ð3Þ Vi ¼ þ q q Nd Nþ cn where Nþ cn and Ncn are the effective densities of states of the high and low doped region, respectively. 3.1. Forward current–voltage characteristics Fig. 3 shows the forward current–voltage (IF –VF ) characteristics at several temperatures. The junction current as a function of applied bias, can be written in the following empirical form: Va qVa I ¼ I0 1 exp 1 ð4Þ Vi bkT where b ¼ 1 in the case of thermionic current.
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M.A. Afrailov / Infrared Physics & Technology 45 (2004) 169–175
Amperes
1,00E-03
1,00E-05
1,00E-07 T=90 K T=160 K T=230 K
1,00E-09
T=250 K T=300 K
1,00E-11 0
100
200
300
400
500
600
Millivolts
Fig. 3. Forward bias IF –VF characteristics of isotype Nþ -GaSb/ n0 -GaInAsSb structure in semi-logarithmic scale at several temperatures.
For the heterojunctions under consideration, the barrier height is higher than kT =q and the depletion region in the substrate is about 106 cm wide, mechanism of current flowing in this case is mainly governed by the emission theory [15]. The comparison was theoretically calculated (Eq. (4)) and experimental current–voltage characteristics in the forward bias region for several temperatures are shown in Fig. 4. It is seen that the b-factor determined from the slope of the (IF –VF ) characteristics at high-temperatures (T P 250 K) for small forward bias region, is equal to b ¼ 1:1–1:3 and indicates the predominantly thermionic nature of the current. Indeed, small forward - bias applied to the heterojunction lowers its barriers. That is, in
the heterostructure under study the current is due to thermionic emission of electrons over the barrier and flows from GaSb to the narrow-gap semiconductor. The discrepancy from the theory for large forward bias voltages can be explained as due to the effect of series resistance. At low temperatures the tunnelling processes become dominant, because of improbability of passing over barriers. This is confirmed by large bfactor values obtained (b ¼ 2:7 and 7.7) for two linear portions of the IF –VF characteristic at T ¼ 160 K (Fig. 4) and for two linear portions of the IF –VF characteristic (b ¼ 3.6 and 10.2) at T ¼ 90 K (Fig. 3). Thus the variation of b-factor as a function of temperature can be explained by tunnelling and by distribution of the applied voltage between the two potential barriers. 3.2. Reverse current–voltage characteristics Fig. 5 shows the reverse current–voltage (IR –VR ) characteristics as a function of temperature measured for the same structures, as shown above in logarithmic scale. Reverse voltage corresponds to negative potential on the narrow-gap GaInAsSb side of the heterojunction. It is seen that at hightemperatures (T > 230 K) a lightly saturation of the reverse current was observed for small biases (V < 500 mV), indicating that in the case a thermionic emission current mechanism prevails. This current is due to thermal activation of electrons from GaInAsSb into GaSb over the barrier
1,00E+00 1 − β = 1.1 (300 K) 2 − β = 1.3 (250 K) 3 − β = 2.7 (160 K)
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I (theor., 300 K) I (exper., 300 K) I (theor., 250 K) I (exper., 250 K) I (theor., 160 K) I (exper., 160 K)
1,00E-07 1,00E-08 1,00E-09
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0
50
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150
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250
300
350
400
450
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Fig. 4. Comparison of theoretically calculated (continuous line curve) and experimental (discrete points) current–voltage characteristics in the forward bias region at several temperatures.
1,00E-09 10
100
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Fig. 5. Reverse bias IR –VR characteristics in logarithmic scale at several temperatures.
M.A. Afrailov / Infrared Physics & Technology 45 (2004) 169–175
at the heterointerface. Since, the barrier height seen from the GaInAsSb remains unchanged, the reverse current increases with bias due to electrons partly tunnelling from the GaInAsSb into GaSb. Fig. 6 shows IR versus 103 /T as a function of the reverse bias. The activation energy estimated from this figure in the high-temperature region is equal to Ea ¼ 0:288 eV. This value is very close to the barrier height DEC at the interface for electrons in GaInAsSb, which is in reasonable agreement with the data obtained in [11,12]. This is also evidence of the predominant contribution of the thermionic emission mechanism to dark current flow at high-temperature region. For large values of reverse bias the current determined by tunnelling over the whole temperature range of 90–300 K. Indeed, a large reverse bias can result in the barrier to become thin enough for significant tunnelling of electrons from GaInAsSb to GaSb. Estimations show that at low temperatures the tunnelling mechanism of the current flowing dominates, since carrier passing over the barrier becomes improbable. This is confirmed by the weak temperature dependence of the reverse current in the low temperature region. 3.3. Photoelectric properties As stated above, spatial separation of electrons and holes at the type II heterointerface leads to new quantum effects that manifest themselves in phenomenon of photocurrent amplification on N–n heteroboundary [4]. A novel photoelectric 1,00E-01 V = -0,2 V
1,00E-02
V = -0,4 V V = -0,6 V
Amperes
1,00E-03
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1,00E-04 1,00E-05 1,00E-06 1,00E-07 1,00E-08 1,00E-09 2
4
6
8
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12
1000/T, K-1
Fig. 6. The reverse current as a function of reciprocal temperature at different reverse applied bias.
173
phenomenon––the dependence of the photocurrent sign on photon energy as a function of forward bias in Nþ -GaSb/n0 -GaInAsSb structures have been studied in this work. This effect is due to modulation of the barrier transparency at the interface limiting the tunnel transitions of the conduction electrons and to the localization of photoholes in the potential well at the type II interface. In this type of N–n junction a photovoltage will be produced, which may depend in sign on the photon energy. It is important to note that in our case the sign reversal of the photocurrent should not be attributed to the surface states at the interface as in [16] because the structures studied were well lattice-matched and observes were no double saturation branches on the forward or reverse I–V characteristics as opposed to results of [16]. A fairly deep (up to 100 meV) quantum well for electrons is formed at the interface in the GaInAsSb/GaSb structure on the side of narrow-gap quaternary layer (see Fig. 2). Simultaneously, a barrier for electrons arises in the conduction band at the interface. The barrier depth and width vary with the composition of solid solution and the layer doping level. If we excite the structure with light whose wavelength falls in the intrinsic absorption band of the narrow-gap semiconductor, non-equilibrium holes are generated and eventually tunnel through the barrier in the valence band. As a result they are captured by the native acceptors in the band-bending region on the GaSb side. The barrier transmission for electrons in the conduction band is extremely sensitive to the presence of non-equilibrium holes at the interface. The accumulation of these holes results in narrowing of the barrier, which augments the tunneling of conduction electrons. The spectral response of the isotype GaSb– GaInAsSb heterostructure as a function of forward biases at room temperature is presented in Fig. 7. At zero or small forward bias, the photocurrent is positive, which corresponds to photoelectrons crossing the barrier towards GaSb and to photoholes drifting and diffusing in the opposite direction. The forward bias of several millivolts applied to the structure lowers the energy barriers at the
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M.A. Afrailov / Infrared Physics & Technology 45 (2004) 169–175 120 100
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80 60 40 20 0 -20 -40
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-60 -80 -100 -120 0.6
1.1
1.6
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Fig. 7. Spectral response of a Nþ -GaSb/n0 -GaInAsSb isotype heterostructure as a function of the forward bias at room temperature.
heterojunction, and the energy bands become partly unbent (see Fig. 2b). It becomes possible for non-equilibrium electrons photogenerated in GaSb to pass to GaInAsSb layer and move further toward the positive contact. As a result the negative photocurrent appears firstly in this spectral range beginning from k 1:8 lm. At larger forward bias, electrons photogenerated in the GaInAsSb layer are no more able to cross the barrier and holes accumulate at the interface. The resulting narrowing of the barrier could enhance the dark current due to electrons coming from GaSb, which thus appears as a negative photocurrent in the spectral range k > 1:8 lm (see Fig. 7). If we apply a reverse bias of several millivolts to the structure (see Fig. 2c), photogenerated electrons in GaInAsSb and photoholes in GaSb begin to tunnel through the interface and the photocurrent is determined by flowing photoelectrons from GaInAsSb to GaSb and by photoholes flowing in opposite direction. As a result, the photocurrent becomes positive. 4. Conclusion In this letter we have considered the electrical and photoelectric properties of a single type II
heterojunction in the GaSb/GaInAsSb system with the staggered band alignment. It is shown that a type II staggered heterojunction can behave as Schottky diode and the dark current–voltage characteristics of this heterostructure are rectifying over the whole temperature range of 90–300 K. No double saturation branches in the forward or reverse I–V characteristics were observed, which indicates a low density of surface states in this heterostructure. The qualitative comparison of experimental results with theory shows that various components of current flowing (diffusion, thermal emission tunnelling) through the heterojunction make contribution to dark current flow, dominating at different temperatures. The sign reversal of the photocurrent on photon energy on forward bias has been studied. This effect is due to modulation of the barrier transparency at the interface limiting the tunnel transitions of the conduction electrons and to the localization of photoholes in the potential well at the type II interface. The sign reversal of the photocurrent on photon energy as a function of applied voltage takes place only on the forward bias. Studies of these heterostructures will provide the physical basis for the fabrication of photodetectors operating in the wavelength range of 2–5 lm, important for gas pollutant analysis (CO2 , CO), combustion diagnostics and a null detecting frequency meter operating in the near IR region at room temperature.
Acknowledgements I thank Prof. M. P. Mikhailova and Dr. Yu.P. Yakovlev who are both with the Ioffe PhysicoTechnical Institute of RAS (St. Petersburg, Russia) for discussion, encouragement.
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