- Email: [email protected]
Novel Sb-based materials for uncooled infrared photodetector applications
Journal of Crystal Growth 221 (2000) 444}449
Novel Sb-based materials for uncooled infrared photodetector applications J.J. Lee, M. Razeghi* Department of Electrical and Computer Engineering, Center for Quantum Devices, Northwestern University, 2225 N. Compus Drive, MLBS Room 4051, Evanston, IL 60208, USA
Abstract We have developed low-pressure metalorganic chemical vapor deposition technology for the growth of novel III}V Sb-based compounds such as InTlSb, InTlAsSb, and InSbBi. The incorporation of Tl and Bi is investigated with various characterization techniques. Preliminary infrared photodetectors based on these materials are fabricated and tested. The maximum responsivity of an In Tl Sb photodetector is about 6.64 V/W at 77 K, corresponding to a Johnson noise-limited detectivity of about 7.64;10 cm Hz /W. Photoresponse of the In Tl Sb photodetector has been extended to 11 lm at 300 K. Infrared photoresponse up to 15 lm is achieved from the InTlAsSb alloy at room temperature. We also demonstrate the uncooled InSbBi photodetector operating in the 8}12 lm range. The voltage responsivity at 10.6 lm is about 1.9 mV/W at 300 K and the corresponding Johnson-noise-limited detectivity is 1.2;10 cm Hz /W. The carrier lifetime is estimated to be 0.7 ns. 2000 Elsevier Science B.V. All rights reserved. PACS: 81.15.G; 07.57.K; 85.60.G Keywords: Metalorganic chemical vapor deposition; III}V; Infrared photodetector
There has been great interest in uncooled longwavelength infrared (LWIR) photodetectors due to the many military and civilian applications [1]. The most widely used material system has been the II}VI compound HgCdTe. However, HgCdTe has inherent problems of thermal instability and poor compositional uniformity over large areas due to the high Hg vapor pressure and the weak Hg bond, hindering the further development of infrared technology. This has intensi"ed the search for alternative infrared materials with better material quality.
* Corresponding author. Tel.: #1-847-491-7251; fax: #1847-467-1817. E-mail address: [email protected] (M. Razeghi).
As alternatives to the current LWIR material, Tl and Bi containing III}V semiconductor alloys have been investigated because relatively small incorporation of Tl and/or Bi into InSb is expected to attain 8}12 lm cuto! wavelength needed for LWIR photodetector applications [2}19]. Also, these compounds are believed to have better material quality over HgCdTe. However, relatively little progress has been made due to the solubility limit of Tl and Bi in InSb. In this paper, we report the low-pressure metalorganic chemical vapor deposition (LPMOCVD) growth and characterization of novel Sb-based III}V compounds such as InTlSb, InTlAsSb, and InSbBi. Photodetectors based on the grown materials also have been demonstrated to explore their merits for LWIR applications.
0022-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 7 3 8 - 7
J.J. Lee, M. Razeghi / Journal of Crystal Growth 221 (2000) 444}449
The epilayers are grown on semi-insulating GaAs (0 0 1) substrates using a horizontal, rfheated, LP-MOCVD reactor. Trimethylindium (TMIn), trimethylantimony (TMSb), trimethylbismuth (TMBi), and cyclopentadienylthallium (CpTl) are used as metalorganic sources for indium, antimony, bismuth, and thallium, respectively. Palladium-di!used hydrogen is used as a carrier gas with a total #ow rate of 1.5 l/min. Growth conditions such as temperature, pressure, and V/III ratio are varied to obtain good-quality "lms [18]. The crystalline quality is determined by X-ray di!raction spectra in the vicinity of the symmetric (0 0 4) re#ection using a Cu K radiation. Mattson ? Galaxy 3000 Fourier transform infrared (FTIR) spectrometer with a liquid-nitrogen-cooled cryostat is used in obtaining the infrared photoresponse. The electrical properties of the "lms are determined by Hall-e!ect measurements. Fig. 1 shows high-resolution (0 0 4) X-ray di!raction spectra of the In Tl Sb layers grown on the \V V GaAs substrates. An InSb epilayer is also grown on the GaAs substrate to provide a reference peak for the X-ray di!raction study. It can be seen that the X-ray peaks corresponding to the InSb epilayer and GaAs substrate are observed at an angle of 28.393 and 33.023, respectively. A noticeable feature appears in the X-ray di!raction spectra for the
Fig. 1. (0 0 4) X-ray di!raction spectra of the In Tl Sb epi\V V layers on GaAs substrates.
445
In Tl Sb "lms. The epilayers show a clear shift \V V of the X-ray di!raction peak toward a higher angle relative to that of InSb. This implies a decrease of the lattice constant (in the growth direction) of the In Tl Sb compared to the lattice constant of \V V InSb, according to Bragg's law [20]. As Tl incorporation increases, the di!raction peaks shift gradually toward a higher angle in the experimental range, indicating the gradual decrease of the lattice constants in the growth direction. The lattice constants normal to the growth direction are assessed by asymmetric +1 1 5, re#ections and found to be the same as the lattice constants in the growth direction. These results indicate that the lattice of In Tl Sb is contracted compared to that of InSb \V V due to the incorporation of Tl in InSb. This cannot be explained by the atomic size e!ect based on tetrahedral covalent radii [21], suggesting that the covalent bonding character of In Tl Sb may be \V V signi"cantly in#uenced by the incorporation of Tl. Within the framework of atomic size e!ect based on tetrahedral covalent radii, the lattice constant of In Tl Sb, a, may be expressed by the following \V V equation [22]:
r !r c #2c x ' 1# 2 , a"a '1 r #r c 2 ' 1
(1)
where a is the lattice parameter of InSb, r are '1 the Pauling values of the tetrahedral covalent radii, c and c are the sti!ness coe$cients of the InSb, and x is the Tl concentration. This model is found to be valid for isoelectronic covalent alloys such as Ga In As [22]. Fig. 2 plots the estimated lattice \V V constants of In Tl Sb as a function of Tl content \V V evaluated by Eq. (1) (solid line) together with the measured lattice constants (solid circles). In this evaluation, we used the values of r "1.44, ' r "1.47, and r "1.36 As [21] for the covalent 2 1 radii and c "6.669;10 and c "3.645; 10 dyn/cm [23] for the sti!ness coe$cients of InSb. As can be seen in this "gure, it is evident that the experimental values are not in agreement with the expected theoretical curves of Eq. (1). The Tl radius corresponding to the measured lattice constants of In Tl Sb can be estimated using Eq. (1) \V V (dashed line). The estimated value of 1.06 As is notably smaller than the tetrahedral covalent radius
446
J.J. Lee, M. Razeghi / Journal of Crystal Growth 221 (2000) 444}449
Fig. 2. The solid circles are the measured lattice constants of In Tl Sb. The solid line is the result calculated from Eq. (1) \V V using r "1.47 As . The dashed line corresponds to the lattice 2 constants calculated from Eq. (1) using r "1.06 As . 2
of Pauling (1.47 As ). This presumably indicates that there is a signi"cant change in bonding character of In Tl Sb compared with the bonding in InSb. \V V In this regard, it is worth noting that a group III heavy element Tl exhibits, in some cases, a valence that is 2 less than the group valence [24]. For example, TlI and TlBr compounds are known to have 1 instead of 3 oxidation state of Tl and they form the ionic rather than covalent bonding. This chemical property is the result of the reluctance of 6s electrons to be oxidized, which is called the inert pair e!ect. In a Tl> electronic con"guration state, the 6s lone pair of electrons is held fairly tight to the core of the atom and does not tend to participate in bond. The result of recent full-potential linear mu$n tin orbital (LMTO) calculation shows that the primarily covalent bonded sp hybrids of InSb is not favored for TlSb [2]. And it is likely that the cohesion be a!ected by the electrostatic attraction of the ions. It may thus be suggested that the lattice of In Tl Sb is contrac\V V ted compared with that of InSb due to the lowoxidation state of Tl. This is in contrast with the case of In Tl P that shows lattice dilation by the \V V incorporation of Tl [25,26]. The measured lattice
Fig. 3. The normalized spectral response of the In Tl Sb \V V photoconductors at 77 K.
dilation in In Tl P grown by gas source molecu\V V lar beam epitaxy and LP-MOCVD agrees with the theoretical results based on LMTO method, which predict stable zinc blende TlP [27]. InTlSb photoconductors are fabricated by depositing Au/Ti on the InTlSb epitaxial layers using an e-beam evaporator. The photoconductors are mounted onto copper heat sink and the electrical contact is made by an Au wire bonding. The responsivity is calibrated with the blackbody source at a temperature of 800 K and at a modulating frequency of 400 Hz. Fig. 3 shows the spectral response of the In Tl Sb (0.02)x)0.06) \V V photodetectors at 77 K, which is normalized to the peak responsivity. The cuto! wavelengths are clearly seen in the In Tl Sb photodetectors, \V V which extend to long wavelengths with increasing Tl content. The maximum responsivity of an In Tl Sb photodetector is about 6.64 V/W at 77 K (see Fig. 4), corresponding to a Johnson-noiselimited detectivity of about 7.64;10 cm Hz /W. The absolute spectral voltage responsivity of the In Tl Sb photoconductor at di!erent temper atures is shown in Fig. 5. The photoresponse cuto! wavelength shifts from 5.5 of InSb to 9 lm at 77 K and has been extended up to 11 lm at 300 K. Fig. 6 shows the normalized spectral photoresponse of the InSb, InTlSb, and InTlAsSb samples
J.J. Lee, M. Razeghi / Journal of Crystal Growth 221 (2000) 444}449
Fig. 4. The bias voltage-dependent responsivity of an In Tl Sb photodetector.
Fig. 5. The spectral voltage responsivity of an In Tl Sb photodetector at 77, 200, and 300 K.
grown on GaAs substrates. Quaternary sample (b) [(d)] has the same growth conditions with ternary (a) [(c)] except for the AsH #ux. A clear shift of the absorption cuto! wavelength to the longer wavelength up to 8.5 lm is obtained for InTlSb [(c)], which indicates the reduction of the band gap energy of InSb by Tl incorporation. Further in-
447
Fig. 6. The normalized infrared spectral photoresponse of the InSb, InTlSb, and InTlAsSb "lms at 77 K. Quaternary sample (b) [(d)] has the identical growth conditions with ternary (a) [(c)] except for the arsenic #ux.
Fig. 7. Room-temperature infrared photoresponse up to 15 lm from the InTlAsSb alloy.
crease of the cuto! wavelength is achieved by incorporating As into the InTlSb alloy. A cuto! wavelength as long as 10.8 lm is obtained from the InTlAsSb [(d)] at 77 K. Signi"cantly, room-temperature photoresponse up to 15 lm is achieved from the InTlAsSb alloy (see Fig. 7). These results
448
J.J. Lee, M. Razeghi / Journal of Crystal Growth 221 (2000) 444}449
Fig. 8. Energy-dispersive X-ray analysis (EDAX) spectrum of an InSb Bi epilayer.
Bi peak at 2.4 keV, which corresponds to a Bi M ? line (see Fig. 8). The epilayers, with Bi peak in EDAX spectra, show a clear shift of X-ray di!raction peak with respect to that of InSb, indicating the signi"cant incorporation of Bi into the crystal lattice [18]. Fig. 9 displays the measured spectral voltage responsivity of an InSb Bi photoconductor at room temperature. The spectral photoresponse could be measured up to 12 lm at room temperature. The responsivity at 10.6 lm, which is important for the application of CO laser monitoring [1], is about 1.9 mV/W at 300 K for 1.74 V bias voltage. The corresponding Johnsonnoise-limited detectivity is estimated to be about 1.2;10 cm Hz /W. The carrier lifetime of the fabricated InSbBi LWIR photodetector is estimated from the voltagedependent responsivity measurements shown in Fig. 10. The voltage responsivity R at an incident wavelength j under the bias voltage < can be expressed as [1],
qj gqk < R 1 " 1# , R " hc b ¸
(2)
where q is the electron charge, h the Planck's constant, c the light velocity, k the electron mobility,
Fig. 9. Spectral voltage responsivity of an InSb Bi photo conductor at 300 K.
are the "rst observation of an infrared photoresponse at such a long wavelength from the III}V quaternaries. An energy-dispersive X-ray analysis (EDAX) technique is used to investigate the incorporation of Bi into InSb. EDAX spectrum of an InSb Bi layer clearly reveals the presence of
Fig. 10. Bias voltage-dependent responsivity of InSb Bi photoconductor at room temperature.
an
J.J. Lee, M. Razeghi / Journal of Crystal Growth 221 (2000) 444}449
R the detector resistance, ¸ the detector length, " and b is the electron-to-hole mobility ratio. The measured responsivity increases linearly with the bias at low bias voltage as Eq. (2) predicts. Taking the absorption quantum e$ciency as g"0.35 from infrared transmission measurements, the carrier lifetime}mobility product calculated from the slope of the linear portion of Fig. 10 is found to be qk "3.65;10\ cm/V. Based on the measured mobility data, the carrier lifetime in the InSbBi photodetector is estimated to be about 0.7 ns at 300 K. In conclusion, we have successfully demonstrated the growth of novel Sb-based alloys such as InTlSb, InTlAsSb, and InSbBi with an LP-MOCVD technique. The assessed detector characteristics of the preliminary photodetectors suggest the feasibility of using III}V materials for the uncooled LWIR photodetector applications as an alternative to the II}VI HgCdTe detector. The authors would like to acknowledge the support and encouragement of Dr. Y.S. Park and C. Wood at the O$ce of Naval Research. References [1] A. Rogalski, Infrared Photon Detectors, SPIE Optical Engineering Press, Bellingham, 1995. [2] M. van Schilfgaarde, A. Sher, Appl. Phys. Lett. 62 (1993) 1857. [3] Y.H. Choi, C. Besikci, R. Sudharsanan, M. Razeghi, Appl. Phys. Lett. 63 (1993) 361. [4] P.T. Staveteig, Y.H. Choi, G. Labeyrie, E. Bigan, M. Razeghi, Appl. Phys. Lett. 64 (1994) 460. [5] J.D. Kim, E. Michel, S. Park, J. Xu, S. Javadpour, M. Razeghi, Appl. Phys. Lett. 69 (1996) 343.
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[6] N.H. Karam, R. Sudharsanan, T. Parodos, M.A. Dodd, J. Electron. Mater. 25 (1996) 1209. [7] C.E.C. Wood, A. Noreika, M. Francombe, J. Appl. Phys. 59 (1986) 3610. [8] K.T. Huang, R.M. Cohen, G.B. Stringfellow, J. Crystal Growth 156 (1995) 320. [9] M.J. Antonell, C.R. Abernathy, W.A. Acree, M.A. Berding, A. Sher, J. Vac. Sci. Technol. A 17 (1999) 338. [10] A.M. Jean-Louis, C. Hamon, Phys. Stat. Sol. 34 (1969) 329. [11] J.L. Zilko, J.E. Greene, Appl. Phys. Lett. 33 (1978) 254. [12] K. Oe, S. Ando, K. Sugiyama, Japan J. Appl. Phys. 20 (1981) L303. [13] Yu.A. Bratashevskii, V.M. Vasil'kov, N.A. Doroshenko, Yu.Yu. Kovalev, Sov. Phys. Semicond. 11 (1977) 1302. [14] A.J. Noreika, W.J. Takei, M.H. Francombe, C.E.C. Wood, J. Appl. Phys. 53 (1982) 4932. [15] M.A. Berding, A. Sher, A.B. Chen, W.E. Miller, J. Appl. Phys. 63 (1988) 107. [16] K.Y. Ma, Z.M. Fang, D.H. Jaw, R.M. Cohen, G.B. Stringfellow, Appl. Phys. Lett. 55 (1989) 2420. [17] S. Barnett, J. Vac. Sci. Technol. A 5 (1987) 2845. [18] J.J. Lee, J.D. Kim, M. Razeghi, Appl. Phys. Lett. 70 (1997) 3266. [19] J.J. Lee, J.D. Kim, M. Razeghi, Appl. Phys. Lett. 71 (1997) 2298. [20] B.E. Warren, X-ray Di!raction, Dover, New York, 1990. [21] J.C. Phillips, Bonds and Bands in Semiconductors, Academic Press, New York, 1973. [22] N. Chen, Y. Wang, H. He, L. Lin, Phys. Rev. B 54 (1996) 8516. [23] King-Ning Tu, J.W. Mayer, L.C. Feldman, Electronic Thin Film Science for Electrical Engineers and Materials Scientists, Macmillan Publishing Company, New York, 1992. [24] A.R. West, Solid State Chemistry and its Applications, Wiley, New York, 1984. [25] H. Asahi, K. Yamamoto, K. Iwata, S. Gonda, K. Oe, Japan J. Appl. Phys. 35 (1996) 876. [26] M. Razeghi et al., unpublished results. [27] M. van Schilfgaarde, A.-B. Chen, S. Krishnamurthy, A. Shur, Appl. Phys. Lett. 65 (1994) 2714.
Novel Sb-based materials for uncooled infrared photodetector applications J.J. Lee, M. Razeghi* Department of Electrical and Computer Engineering, Center for Quantum Devices, Northwestern University, 2225 N. Compus Drive, MLBS Room 4051, Evanston, IL 60208, USA
Abstract We have developed low-pressure metalorganic chemical vapor deposition technology for the growth of novel III}V Sb-based compounds such as InTlSb, InTlAsSb, and InSbBi. The incorporation of Tl and Bi is investigated with various characterization techniques. Preliminary infrared photodetectors based on these materials are fabricated and tested. The maximum responsivity of an In Tl Sb photodetector is about 6.64 V/W at 77 K, corresponding to a Johnson noise-limited detectivity of about 7.64;10 cm Hz /W. Photoresponse of the In Tl Sb photodetector has been extended to 11 lm at 300 K. Infrared photoresponse up to 15 lm is achieved from the InTlAsSb alloy at room temperature. We also demonstrate the uncooled InSbBi photodetector operating in the 8}12 lm range. The voltage responsivity at 10.6 lm is about 1.9 mV/W at 300 K and the corresponding Johnson-noise-limited detectivity is 1.2;10 cm Hz /W. The carrier lifetime is estimated to be 0.7 ns. 2000 Elsevier Science B.V. All rights reserved. PACS: 81.15.G; 07.57.K; 85.60.G Keywords: Metalorganic chemical vapor deposition; III}V; Infrared photodetector
There has been great interest in uncooled longwavelength infrared (LWIR) photodetectors due to the many military and civilian applications [1]. The most widely used material system has been the II}VI compound HgCdTe. However, HgCdTe has inherent problems of thermal instability and poor compositional uniformity over large areas due to the high Hg vapor pressure and the weak Hg bond, hindering the further development of infrared technology. This has intensi"ed the search for alternative infrared materials with better material quality.
* Corresponding author. Tel.: #1-847-491-7251; fax: #1847-467-1817. E-mail address: [email protected] (M. Razeghi).
As alternatives to the current LWIR material, Tl and Bi containing III}V semiconductor alloys have been investigated because relatively small incorporation of Tl and/or Bi into InSb is expected to attain 8}12 lm cuto! wavelength needed for LWIR photodetector applications [2}19]. Also, these compounds are believed to have better material quality over HgCdTe. However, relatively little progress has been made due to the solubility limit of Tl and Bi in InSb. In this paper, we report the low-pressure metalorganic chemical vapor deposition (LPMOCVD) growth and characterization of novel Sb-based III}V compounds such as InTlSb, InTlAsSb, and InSbBi. Photodetectors based on the grown materials also have been demonstrated to explore their merits for LWIR applications.
0022-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 7 3 8 - 7
J.J. Lee, M. Razeghi / Journal of Crystal Growth 221 (2000) 444}449
The epilayers are grown on semi-insulating GaAs (0 0 1) substrates using a horizontal, rfheated, LP-MOCVD reactor. Trimethylindium (TMIn), trimethylantimony (TMSb), trimethylbismuth (TMBi), and cyclopentadienylthallium (CpTl) are used as metalorganic sources for indium, antimony, bismuth, and thallium, respectively. Palladium-di!used hydrogen is used as a carrier gas with a total #ow rate of 1.5 l/min. Growth conditions such as temperature, pressure, and V/III ratio are varied to obtain good-quality "lms [18]. The crystalline quality is determined by X-ray di!raction spectra in the vicinity of the symmetric (0 0 4) re#ection using a Cu K radiation. Mattson ? Galaxy 3000 Fourier transform infrared (FTIR) spectrometer with a liquid-nitrogen-cooled cryostat is used in obtaining the infrared photoresponse. The electrical properties of the "lms are determined by Hall-e!ect measurements. Fig. 1 shows high-resolution (0 0 4) X-ray di!raction spectra of the In Tl Sb layers grown on the \V V GaAs substrates. An InSb epilayer is also grown on the GaAs substrate to provide a reference peak for the X-ray di!raction study. It can be seen that the X-ray peaks corresponding to the InSb epilayer and GaAs substrate are observed at an angle of 28.393 and 33.023, respectively. A noticeable feature appears in the X-ray di!raction spectra for the
Fig. 1. (0 0 4) X-ray di!raction spectra of the In Tl Sb epi\V V layers on GaAs substrates.
445
In Tl Sb "lms. The epilayers show a clear shift \V V of the X-ray di!raction peak toward a higher angle relative to that of InSb. This implies a decrease of the lattice constant (in the growth direction) of the In Tl Sb compared to the lattice constant of \V V InSb, according to Bragg's law [20]. As Tl incorporation increases, the di!raction peaks shift gradually toward a higher angle in the experimental range, indicating the gradual decrease of the lattice constants in the growth direction. The lattice constants normal to the growth direction are assessed by asymmetric +1 1 5, re#ections and found to be the same as the lattice constants in the growth direction. These results indicate that the lattice of In Tl Sb is contracted compared to that of InSb \V V due to the incorporation of Tl in InSb. This cannot be explained by the atomic size e!ect based on tetrahedral covalent radii [21], suggesting that the covalent bonding character of In Tl Sb may be \V V signi"cantly in#uenced by the incorporation of Tl. Within the framework of atomic size e!ect based on tetrahedral covalent radii, the lattice constant of In Tl Sb, a, may be expressed by the following \V V equation [22]:
r !r c #2c x ' 1# 2 , a"a '1 r #r c 2 ' 1
(1)
where a is the lattice parameter of InSb, r are '1 the Pauling values of the tetrahedral covalent radii, c and c are the sti!ness coe$cients of the InSb, and x is the Tl concentration. This model is found to be valid for isoelectronic covalent alloys such as Ga In As [22]. Fig. 2 plots the estimated lattice \V V constants of In Tl Sb as a function of Tl content \V V evaluated by Eq. (1) (solid line) together with the measured lattice constants (solid circles). In this evaluation, we used the values of r "1.44, ' r "1.47, and r "1.36 As [21] for the covalent 2 1 radii and c "6.669;10 and c "3.645; 10 dyn/cm [23] for the sti!ness coe$cients of InSb. As can be seen in this "gure, it is evident that the experimental values are not in agreement with the expected theoretical curves of Eq. (1). The Tl radius corresponding to the measured lattice constants of In Tl Sb can be estimated using Eq. (1) \V V (dashed line). The estimated value of 1.06 As is notably smaller than the tetrahedral covalent radius
446
J.J. Lee, M. Razeghi / Journal of Crystal Growth 221 (2000) 444}449
Fig. 2. The solid circles are the measured lattice constants of In Tl Sb. The solid line is the result calculated from Eq. (1) \V V using r "1.47 As . The dashed line corresponds to the lattice 2 constants calculated from Eq. (1) using r "1.06 As . 2
of Pauling (1.47 As ). This presumably indicates that there is a signi"cant change in bonding character of In Tl Sb compared with the bonding in InSb. \V V In this regard, it is worth noting that a group III heavy element Tl exhibits, in some cases, a valence that is 2 less than the group valence [24]. For example, TlI and TlBr compounds are known to have 1 instead of 3 oxidation state of Tl and they form the ionic rather than covalent bonding. This chemical property is the result of the reluctance of 6s electrons to be oxidized, which is called the inert pair e!ect. In a Tl> electronic con"guration state, the 6s lone pair of electrons is held fairly tight to the core of the atom and does not tend to participate in bond. The result of recent full-potential linear mu$n tin orbital (LMTO) calculation shows that the primarily covalent bonded sp hybrids of InSb is not favored for TlSb [2]. And it is likely that the cohesion be a!ected by the electrostatic attraction of the ions. It may thus be suggested that the lattice of In Tl Sb is contrac\V V ted compared with that of InSb due to the lowoxidation state of Tl. This is in contrast with the case of In Tl P that shows lattice dilation by the \V V incorporation of Tl [25,26]. The measured lattice
Fig. 3. The normalized spectral response of the In Tl Sb \V V photoconductors at 77 K.
dilation in In Tl P grown by gas source molecu\V V lar beam epitaxy and LP-MOCVD agrees with the theoretical results based on LMTO method, which predict stable zinc blende TlP [27]. InTlSb photoconductors are fabricated by depositing Au/Ti on the InTlSb epitaxial layers using an e-beam evaporator. The photoconductors are mounted onto copper heat sink and the electrical contact is made by an Au wire bonding. The responsivity is calibrated with the blackbody source at a temperature of 800 K and at a modulating frequency of 400 Hz. Fig. 3 shows the spectral response of the In Tl Sb (0.02)x)0.06) \V V photodetectors at 77 K, which is normalized to the peak responsivity. The cuto! wavelengths are clearly seen in the In Tl Sb photodetectors, \V V which extend to long wavelengths with increasing Tl content. The maximum responsivity of an In Tl Sb photodetector is about 6.64 V/W at 77 K (see Fig. 4), corresponding to a Johnson-noiselimited detectivity of about 7.64;10 cm Hz /W. The absolute spectral voltage responsivity of the In Tl Sb photoconductor at di!erent temper atures is shown in Fig. 5. The photoresponse cuto! wavelength shifts from 5.5 of InSb to 9 lm at 77 K and has been extended up to 11 lm at 300 K. Fig. 6 shows the normalized spectral photoresponse of the InSb, InTlSb, and InTlAsSb samples
J.J. Lee, M. Razeghi / Journal of Crystal Growth 221 (2000) 444}449
Fig. 4. The bias voltage-dependent responsivity of an In Tl Sb photodetector.
Fig. 5. The spectral voltage responsivity of an In Tl Sb photodetector at 77, 200, and 300 K.
grown on GaAs substrates. Quaternary sample (b) [(d)] has the same growth conditions with ternary (a) [(c)] except for the AsH #ux. A clear shift of the absorption cuto! wavelength to the longer wavelength up to 8.5 lm is obtained for InTlSb [(c)], which indicates the reduction of the band gap energy of InSb by Tl incorporation. Further in-
447
Fig. 6. The normalized infrared spectral photoresponse of the InSb, InTlSb, and InTlAsSb "lms at 77 K. Quaternary sample (b) [(d)] has the identical growth conditions with ternary (a) [(c)] except for the arsenic #ux.
Fig. 7. Room-temperature infrared photoresponse up to 15 lm from the InTlAsSb alloy.
crease of the cuto! wavelength is achieved by incorporating As into the InTlSb alloy. A cuto! wavelength as long as 10.8 lm is obtained from the InTlAsSb [(d)] at 77 K. Signi"cantly, room-temperature photoresponse up to 15 lm is achieved from the InTlAsSb alloy (see Fig. 7). These results
448
J.J. Lee, M. Razeghi / Journal of Crystal Growth 221 (2000) 444}449
Fig. 8. Energy-dispersive X-ray analysis (EDAX) spectrum of an InSb Bi epilayer.
Bi peak at 2.4 keV, which corresponds to a Bi M ? line (see Fig. 8). The epilayers, with Bi peak in EDAX spectra, show a clear shift of X-ray di!raction peak with respect to that of InSb, indicating the signi"cant incorporation of Bi into the crystal lattice [18]. Fig. 9 displays the measured spectral voltage responsivity of an InSb Bi photoconductor at room temperature. The spectral photoresponse could be measured up to 12 lm at room temperature. The responsivity at 10.6 lm, which is important for the application of CO laser monitoring [1], is about 1.9 mV/W at 300 K for 1.74 V bias voltage. The corresponding Johnsonnoise-limited detectivity is estimated to be about 1.2;10 cm Hz /W. The carrier lifetime of the fabricated InSbBi LWIR photodetector is estimated from the voltagedependent responsivity measurements shown in Fig. 10. The voltage responsivity R at an incident wavelength j under the bias voltage < can be expressed as [1],
qj gqk < R 1 " 1# , R " hc b ¸
(2)
where q is the electron charge, h the Planck's constant, c the light velocity, k the electron mobility,
Fig. 9. Spectral voltage responsivity of an InSb Bi photo conductor at 300 K.
are the "rst observation of an infrared photoresponse at such a long wavelength from the III}V quaternaries. An energy-dispersive X-ray analysis (EDAX) technique is used to investigate the incorporation of Bi into InSb. EDAX spectrum of an InSb Bi layer clearly reveals the presence of
Fig. 10. Bias voltage-dependent responsivity of InSb Bi photoconductor at room temperature.
an
J.J. Lee, M. Razeghi / Journal of Crystal Growth 221 (2000) 444}449
R the detector resistance, ¸ the detector length, " and b is the electron-to-hole mobility ratio. The measured responsivity increases linearly with the bias at low bias voltage as Eq. (2) predicts. Taking the absorption quantum e$ciency as g"0.35 from infrared transmission measurements, the carrier lifetime}mobility product calculated from the slope of the linear portion of Fig. 10 is found to be qk "3.65;10\ cm/V. Based on the measured mobility data, the carrier lifetime in the InSbBi photodetector is estimated to be about 0.7 ns at 300 K. In conclusion, we have successfully demonstrated the growth of novel Sb-based alloys such as InTlSb, InTlAsSb, and InSbBi with an LP-MOCVD technique. The assessed detector characteristics of the preliminary photodetectors suggest the feasibility of using III}V materials for the uncooled LWIR photodetector applications as an alternative to the II}VI HgCdTe detector. The authors would like to acknowledge the support and encouragement of Dr. Y.S. Park and C. Wood at the O$ce of Naval Research. References [1] A. Rogalski, Infrared Photon Detectors, SPIE Optical Engineering Press, Bellingham, 1995. [2] M. van Schilfgaarde, A. Sher, Appl. Phys. Lett. 62 (1993) 1857. [3] Y.H. Choi, C. Besikci, R. Sudharsanan, M. Razeghi, Appl. Phys. Lett. 63 (1993) 361. [4] P.T. Staveteig, Y.H. Choi, G. Labeyrie, E. Bigan, M. Razeghi, Appl. Phys. Lett. 64 (1994) 460. [5] J.D. Kim, E. Michel, S. Park, J. Xu, S. Javadpour, M. Razeghi, Appl. Phys. Lett. 69 (1996) 343.
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