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GaSb superlattices for photodetection in short wavelength infrared range
Infrared Physics & Technology 52 (2009) 124–126
Contents lists available at ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
InAs/GaSb superlattices for photodetection in short wavelength infrared range Jie Guo a,b,*, Zhenyu Peng b, Weiguo Sun a,b, Yingqiang Xu b,c, Zhiqiang Zhou b,c, Zhichuan Niu b,c a b c
NorthWest Polytechnical University, Xi’an, Shaanxi 710000, China Luoyang Optical Electronics Center, Luoyang, Henan 471009, China Institute of Semiconductors, CAS, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 8 October 2008 Available online 7 May 2009 Keywords: Superlattices InAs/GaSb Short wavelength Infrared detector
a b s t r a c t The first report of a short wavelength infrared detector based on type II InAs/GaSb superlattices is presented. Very short period superlattices containing InAs (2ML)/GaSb (8ML) superlattices (SLs) were grown by molecular-beam epitaxy on GaSb substrates. The photoluminescence showed a cut-off wavelength at 2.1 lm at 10 K and 2.6 lm at 300 K. Room-temperature optical transmittance spectra shows obvious absorption in InAs (2ML)/GaSb (8ML) SL in the range of 450–680 meV, i.e. 1.8–2.7 lm. The cut-off wavelength moved from 2.3 lm to 2.6 lm with temperature rising from 77 K to 300 K in photoresponse spectra. The blackbody response Rv exponentially decreased as a function of 1/T in two temperature sections (130–200 K and 230–300 K). The blackbody detectivity Dbb was beyond 1 108 cmHz1/2/W at room temperature. Ó 2009 Elsevier B.V. All rights reserved.
Short wavelength infrared (IR) detectors have attracted a great deal of attention due to their potential applications, including missiles guiding, spatial remote sensing, infrared imaging, biology molecules spectroscopy, etc. Infrared detectors based on InAs/GaSb SLs can operate in the 3–30 lm range with great spatial uniformity and small tunneling current. Furthermore, InAs/GaSb SLs provides the band structure engineering that can reduce Auger recombination rates, which are especially important for realizing operation at room temperature [1,2]. In recent years, many papers on the mid[3,4] and long-infrared wavelength infrared [5,6] InAs/GaSb SLs detectors have been reported. However, there have been few reports [7] on short wavelength (between 2.0 lm and 3.0 lm) infrared SLs detectors. In this paper, Type II SLs InAs (2ML)/GaSb (8ML) were grown by molecular-beam epitaxy on GaSb substrates. The crystal structure and optical characterization were investigated by X-ray diffraction (XRD), photoluminescence (PL) and transmission spectra. The short wavelength IR photodetectors were fabricated and measured from 77 K to room temperature. The samples were grown by a VG 80II MBE system equipped with low temperature As and Sb cells supplying As4 and Sb4. The 0.5 lm GaSb buffer were grown on the undoped GaSb substarates, followed by 200 period undoped InAs (2ML)/GaSb (8ML) SLs. The group V to III flux ratio was approximately 6 for the InAs and 5 for the GaSb growth. Another growth parameter was interface (IF) control balancing the stress in the InAs/GaSb SL. 0.17ML InSb-like IF was formed by adjusting the IF shutter sequences. In detail, after * Corresponding author. Address: NorthWest Polytechnical University, Xi’an, Shaanxi 710000, China. E-mail address: [email protected] (J. Guo). 1350-4495/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.infrared.2009.04.003
GaSb layer growth, the Ga shutter was closed while the Sb-flux continued for 4 s; Indium then was deposited without the Sb-flux, and InAs layer growth then was continued. XRD profiles covering the symmetric 004 reflection range are shown in Fig. 1. The well-resolved SL diffraction peaks up to the sixth order were observed. The period as measured by the fringes spacing of the peaks was only 31.2 Å, in good agreement with the nominal structure. It was found that the full width half maximum of the zeroth order peak was below 5000 . The mismatch between the zeroth peak and GaSb substrates (4a/a) was about 4 103 and resulted in some compressive strain in the SLs largely due to excessive InSb IFs. Compared to long period SLs, the same deviation of InSb IFs thickness for short period SLs will produce bigger relative error. So controlling the IFs of short period SLs accurately is important and challenging work. The cross-sectional transmission electron microscopy (TEM) image showed the roughness at the interfaces in the inset in Fig. 1. This phenomenon had been observed by Haugan et al. in short period InAs/GaSb SLs [8]. The exchange of As and Sb atoms at the interface is existent in InAs/GaSb SLs. The layer thickness fluctuation will only be noticeable when the InAs or GaSb layer is very thin, such as 6.1 Å for InAs in this article. Fig. 2 displays the PL spectra measured at 10 K, 150 K and room temperature. It was clear that the peak energy was around 2.1 lm and the FWHM was 25 meV at 10 K. At room temperature the peak energy was at 2.6 lm and the FWHM was 60 meV. It could be found that the peak shifted towards long wavelength. The FWHM increasing monotonously with temperature can be explained by the spreading of the energy band due to carrier-phonon interaction and band filling. The FWHM is broader than that of other reports
J. Guo et al. / Infrared Physics & Technology 52 (2009) 124–126
Fig. 1. XRD scan of a 200 repeats InAs (2ML)/GaSb (8ML) SLs (inset: the TEM images showing the InAs (dark) and GaSb (light) layers).
125
Fig. 3. Absorption spectra of InAs (2ML)/GaSb (8ML) SLs measured at room temperature.
[9] due to the unclear interfaces such as interdiffusion, segregation and roughness in the short period SLs. The absorbance measurements were made using a Fourier transform infrared (FTIR) spectrometer in the transmission mode at room temperature. Substrate-related absorption features were eliminated by subtracting a weighted absorption spectrum of a bare substrate from the SL spectrum. This also resulted in the cancellation of free-carrier substrate absorption at wavelengths longer than the primary band gap of the superlattice. The absorption spectrum was shown in Fig. 3. The absorption coefficient was about 103 cm1 in the range of 2.0–2.7 lm. It shows the obvious absorption in the short wavelength range of in the range of 450–680 meV, according to 1.8–2.7 lm. The absorption spectrum is exceptionally clean, exhibiting a sharp band edge and a series of features associated with transitions between the lowest conduction band and the various balance subbands. In addition to the C1–HH1 transition between the lowest electron subband (C1) and the heavy-hole subband (HH1) as shown in the inset in Fig. 3. Other absorption edges in the spectra are likely due to excited inter subband transitions C1–LH1 and C1–HH2. Photoconductor detectors were fabricated by standard lithography and subsequent etching with a tartaric solution. For ohmic contacts, Ti/Au (500 Å/1500 Å) was deposited and then the con-
tacts were defined by a lift-off technique. No anti-reflection coating or surface passivation was employed. The photoresponse of the detector was performed with the FTIR system. The sample was illuminated through the front side at normal incidence under in-plane current bias using a current preamplifier. Fig. 4 shows the normalized photoresponse spectra (obtained by dividing the photocurrent of the SLs detector with that obtained using a pyroelectric detector) at different temperature. At 77 K, the interband transitions were observed mainly near 540 meV corresponding to C1–HH1. At 300 K the cut-off edge locates at 2.7 lm, corresponding to the edge in the PL and absorption spectra. The band gap Eg (defined as the point where the photoresponse drops to 10% of the maximum value) decreased from 540 meV to 470 meV, according to the cut-off wavelength from 2.3 lm to 2.6 lm with temperature rising from 77 K to 300 K. The shift of Eg to lower energy (about 0.32 meV/K) derived from the temperature-dependence of valence band offset and band overlap [8]. The band gap shift is comparable to the MWIR superlattices. The voltage responsivity (Rv) of the detectors was calculated using a blackbody test set. The detectors were under a current bias with chopper frequency of 1000 Hz. The responsivity can be calculated as
Fig. 2. Photoluminescence spectra of InAs (2ML)/GaSb (8ML) SLs recorded at 10 K, 150 K and 300 K.
Fig. 4. Photoreponse spectra of InAs (2ML)/GaSb (8ML) SLs detectors collected at different temperatures from 77 K and 300 K.
126
J. Guo et al. / Infrared Physics & Technology 52 (2009) 124–126
The noise Vn was measured under the dark condition. The signal-noise ratio was obtained and the blackbody detectivity Dbb can be calculated to 1 108 cmHz1/2/W at room temperature. The value is still lower than other short wavelength IR detectors such as InGaAs, HgCdTe materials. It is mainly due to two aspects. First, the 0.6 lm thick InAs/GaSb SLs is not enough to absorb the incidence light. It decreases the quantum efficiency. Second, interface roughness results in the increasing dislocations and recombination centers that have a deleterious effect on carrier mobilities and lifetime. The increased Shockley–Read–Hall recombination noise degraded the responsivity of the detectors.
Conclusions
Fig. 5. Blackbody responsivity of InAs (2ML)/GaSb (8ML) SLs detectors versus reciprocal temperature from 77 K to 300 K.
Rv ¼
V so =G arT 4 D2A Ad
ð1Þ
4r 2
where Vso is the measured voltage of the detector, G the amplification of the preamplifier, DA the blackbody aperture diameter, and r the distance between the aperture and detector. Ad the area of the sample, a the chopper coefficient, r the Stefan–Boltzmann constant, and T = 900 K is the temperature of the blackbody. A filter (kc = 1.7 lm) was placed in front of the detectors in order to remove the response of GaSb substrates. The Rv was calculated to 1.3 103 V/W at 77 K and decreased to 80 V/W at room temperature. The temperature-dependence of Rv was investigated in the range of 77–300 K as shown in Fig. 5. There was little change below 120 K. Above 120 K, the intrinsic carrier concentration from thermal activation started to dominate leading to the reduction of device quantum efficiency and responsivity. The photoresponse intensity is nearly an exponential function of the inverse temperature. The relation can be simply described
I ¼ I0 =f1 þ expðEa =kTÞg
ð2Þ
where Ea is the activation energy and k the Boltzmann constant. The experimental data was fitted using Eq. (2). There were two well-defined slopes which denoted two thermal activation process characterized by Ea = 48 meV at low temperature between 130 K and 200 K and Ea = 258 meV between 230 K and 300 K.
InAs (2ML)/GaSb (8ML) superlattices (SLs) were grown by molecular-beam epitaxy on GaSb substrates. The cut-off wavelength moved from 2.3 lm to 2.6 lm with temperature rising from 77 K to 300 K. The blackbody Rv exponentially decreased as a function of 1/T in two thermal activation process. Although the Dbb was at the low level of 108 lcmHz1/2/W due to the non-optimized structure, the results indicated the application of InAs/GaSb SLs for multiband detector covering short, middle and long wavelength. The performance will be improved by optimizing the growth condition and device structure. References [1] J.L. Johnson et al., Electrical and optical properties of infrared photodiodes using the InAs/GaInSb superlattice in heterojunctions with GaSb, J. Appl. Phys. 80 (1996) 1116–1127. [2] F. Fuchs et al., High performance InAs/GaInSb superlattice infrared photodiodes, Appl. Phys. Lett. 71 (1997) 3251–3253. [3] M. Walther et al., Growth of InAs/GaSb short-period superlattices for highresolution mid-wavelength infrared focal plane array detectors, J. Cryst. Growth 278 (2005) 156–161. [4] J.B. Rodriguez et al., Uncooled InAs/GaSb superlattice photovoltaic detector operating in the mid-wavelength infrared range, Electron. Lett. 41 (2005) 362– 363. [5] H. Mohseni et al., Growth and characterization of InAs/GaSb photoconductors for long wavelength infrared range, Appl. Phys. Lett. 77 (2000) 1572–1574. [6] G.J. Brown et al., Type-II InAs/GaSb superlattices for very long wavelength infrared detectors, Physica E 20 (2006) 471–474. [7] R.T. Hao et al., MBE growth of very short period InAs/GaSb type II superlattices on (0 0 1) GaAs substrates, J. Phys. D: Appl. Phys. 40 (2007) 6690–6693. [8] H.J. Haugan et al., Short-period InAs/GaSb type II superlattices for mid-infrared detecters, Appl. Phys. Lett. 87 (2005) 261106. [9] Y.J. Wei et al., High quality type II InAs/GaSb superlattices with cutoff wavelength 3.7 lm using interface engineering, J. Appl. Phys. 94 (2003) 4720–4722.
Contents lists available at ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
InAs/GaSb superlattices for photodetection in short wavelength infrared range Jie Guo a,b,*, Zhenyu Peng b, Weiguo Sun a,b, Yingqiang Xu b,c, Zhiqiang Zhou b,c, Zhichuan Niu b,c a b c
NorthWest Polytechnical University, Xi’an, Shaanxi 710000, China Luoyang Optical Electronics Center, Luoyang, Henan 471009, China Institute of Semiconductors, CAS, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 8 October 2008 Available online 7 May 2009 Keywords: Superlattices InAs/GaSb Short wavelength Infrared detector
a b s t r a c t The first report of a short wavelength infrared detector based on type II InAs/GaSb superlattices is presented. Very short period superlattices containing InAs (2ML)/GaSb (8ML) superlattices (SLs) were grown by molecular-beam epitaxy on GaSb substrates. The photoluminescence showed a cut-off wavelength at 2.1 lm at 10 K and 2.6 lm at 300 K. Room-temperature optical transmittance spectra shows obvious absorption in InAs (2ML)/GaSb (8ML) SL in the range of 450–680 meV, i.e. 1.8–2.7 lm. The cut-off wavelength moved from 2.3 lm to 2.6 lm with temperature rising from 77 K to 300 K in photoresponse spectra. The blackbody response Rv exponentially decreased as a function of 1/T in two temperature sections (130–200 K and 230–300 K). The blackbody detectivity Dbb was beyond 1 108 cmHz1/2/W at room temperature. Ó 2009 Elsevier B.V. All rights reserved.
Short wavelength infrared (IR) detectors have attracted a great deal of attention due to their potential applications, including missiles guiding, spatial remote sensing, infrared imaging, biology molecules spectroscopy, etc. Infrared detectors based on InAs/GaSb SLs can operate in the 3–30 lm range with great spatial uniformity and small tunneling current. Furthermore, InAs/GaSb SLs provides the band structure engineering that can reduce Auger recombination rates, which are especially important for realizing operation at room temperature [1,2]. In recent years, many papers on the mid[3,4] and long-infrared wavelength infrared [5,6] InAs/GaSb SLs detectors have been reported. However, there have been few reports [7] on short wavelength (between 2.0 lm and 3.0 lm) infrared SLs detectors. In this paper, Type II SLs InAs (2ML)/GaSb (8ML) were grown by molecular-beam epitaxy on GaSb substrates. The crystal structure and optical characterization were investigated by X-ray diffraction (XRD), photoluminescence (PL) and transmission spectra. The short wavelength IR photodetectors were fabricated and measured from 77 K to room temperature. The samples were grown by a VG 80II MBE system equipped with low temperature As and Sb cells supplying As4 and Sb4. The 0.5 lm GaSb buffer were grown on the undoped GaSb substarates, followed by 200 period undoped InAs (2ML)/GaSb (8ML) SLs. The group V to III flux ratio was approximately 6 for the InAs and 5 for the GaSb growth. Another growth parameter was interface (IF) control balancing the stress in the InAs/GaSb SL. 0.17ML InSb-like IF was formed by adjusting the IF shutter sequences. In detail, after * Corresponding author. Address: NorthWest Polytechnical University, Xi’an, Shaanxi 710000, China. E-mail address: [email protected] (J. Guo). 1350-4495/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.infrared.2009.04.003
GaSb layer growth, the Ga shutter was closed while the Sb-flux continued for 4 s; Indium then was deposited without the Sb-flux, and InAs layer growth then was continued. XRD profiles covering the symmetric 004 reflection range are shown in Fig. 1. The well-resolved SL diffraction peaks up to the sixth order were observed. The period as measured by the fringes spacing of the peaks was only 31.2 Å, in good agreement with the nominal structure. It was found that the full width half maximum of the zeroth order peak was below 5000 . The mismatch between the zeroth peak and GaSb substrates (4a/a) was about 4 103 and resulted in some compressive strain in the SLs largely due to excessive InSb IFs. Compared to long period SLs, the same deviation of InSb IFs thickness for short period SLs will produce bigger relative error. So controlling the IFs of short period SLs accurately is important and challenging work. The cross-sectional transmission electron microscopy (TEM) image showed the roughness at the interfaces in the inset in Fig. 1. This phenomenon had been observed by Haugan et al. in short period InAs/GaSb SLs [8]. The exchange of As and Sb atoms at the interface is existent in InAs/GaSb SLs. The layer thickness fluctuation will only be noticeable when the InAs or GaSb layer is very thin, such as 6.1 Å for InAs in this article. Fig. 2 displays the PL spectra measured at 10 K, 150 K and room temperature. It was clear that the peak energy was around 2.1 lm and the FWHM was 25 meV at 10 K. At room temperature the peak energy was at 2.6 lm and the FWHM was 60 meV. It could be found that the peak shifted towards long wavelength. The FWHM increasing monotonously with temperature can be explained by the spreading of the energy band due to carrier-phonon interaction and band filling. The FWHM is broader than that of other reports
J. Guo et al. / Infrared Physics & Technology 52 (2009) 124–126
Fig. 1. XRD scan of a 200 repeats InAs (2ML)/GaSb (8ML) SLs (inset: the TEM images showing the InAs (dark) and GaSb (light) layers).
125
Fig. 3. Absorption spectra of InAs (2ML)/GaSb (8ML) SLs measured at room temperature.
[9] due to the unclear interfaces such as interdiffusion, segregation and roughness in the short period SLs. The absorbance measurements were made using a Fourier transform infrared (FTIR) spectrometer in the transmission mode at room temperature. Substrate-related absorption features were eliminated by subtracting a weighted absorption spectrum of a bare substrate from the SL spectrum. This also resulted in the cancellation of free-carrier substrate absorption at wavelengths longer than the primary band gap of the superlattice. The absorption spectrum was shown in Fig. 3. The absorption coefficient was about 103 cm1 in the range of 2.0–2.7 lm. It shows the obvious absorption in the short wavelength range of in the range of 450–680 meV, according to 1.8–2.7 lm. The absorption spectrum is exceptionally clean, exhibiting a sharp band edge and a series of features associated with transitions between the lowest conduction band and the various balance subbands. In addition to the C1–HH1 transition between the lowest electron subband (C1) and the heavy-hole subband (HH1) as shown in the inset in Fig. 3. Other absorption edges in the spectra are likely due to excited inter subband transitions C1–LH1 and C1–HH2. Photoconductor detectors were fabricated by standard lithography and subsequent etching with a tartaric solution. For ohmic contacts, Ti/Au (500 Å/1500 Å) was deposited and then the con-
tacts were defined by a lift-off technique. No anti-reflection coating or surface passivation was employed. The photoresponse of the detector was performed with the FTIR system. The sample was illuminated through the front side at normal incidence under in-plane current bias using a current preamplifier. Fig. 4 shows the normalized photoresponse spectra (obtained by dividing the photocurrent of the SLs detector with that obtained using a pyroelectric detector) at different temperature. At 77 K, the interband transitions were observed mainly near 540 meV corresponding to C1–HH1. At 300 K the cut-off edge locates at 2.7 lm, corresponding to the edge in the PL and absorption spectra. The band gap Eg (defined as the point where the photoresponse drops to 10% of the maximum value) decreased from 540 meV to 470 meV, according to the cut-off wavelength from 2.3 lm to 2.6 lm with temperature rising from 77 K to 300 K. The shift of Eg to lower energy (about 0.32 meV/K) derived from the temperature-dependence of valence band offset and band overlap [8]. The band gap shift is comparable to the MWIR superlattices. The voltage responsivity (Rv) of the detectors was calculated using a blackbody test set. The detectors were under a current bias with chopper frequency of 1000 Hz. The responsivity can be calculated as
Fig. 2. Photoluminescence spectra of InAs (2ML)/GaSb (8ML) SLs recorded at 10 K, 150 K and 300 K.
Fig. 4. Photoreponse spectra of InAs (2ML)/GaSb (8ML) SLs detectors collected at different temperatures from 77 K and 300 K.
126
J. Guo et al. / Infrared Physics & Technology 52 (2009) 124–126
The noise Vn was measured under the dark condition. The signal-noise ratio was obtained and the blackbody detectivity Dbb can be calculated to 1 108 cmHz1/2/W at room temperature. The value is still lower than other short wavelength IR detectors such as InGaAs, HgCdTe materials. It is mainly due to two aspects. First, the 0.6 lm thick InAs/GaSb SLs is not enough to absorb the incidence light. It decreases the quantum efficiency. Second, interface roughness results in the increasing dislocations and recombination centers that have a deleterious effect on carrier mobilities and lifetime. The increased Shockley–Read–Hall recombination noise degraded the responsivity of the detectors.
Conclusions
Fig. 5. Blackbody responsivity of InAs (2ML)/GaSb (8ML) SLs detectors versus reciprocal temperature from 77 K to 300 K.
Rv ¼
V so =G arT 4 D2A Ad
ð1Þ
4r 2
where Vso is the measured voltage of the detector, G the amplification of the preamplifier, DA the blackbody aperture diameter, and r the distance between the aperture and detector. Ad the area of the sample, a the chopper coefficient, r the Stefan–Boltzmann constant, and T = 900 K is the temperature of the blackbody. A filter (kc = 1.7 lm) was placed in front of the detectors in order to remove the response of GaSb substrates. The Rv was calculated to 1.3 103 V/W at 77 K and decreased to 80 V/W at room temperature. The temperature-dependence of Rv was investigated in the range of 77–300 K as shown in Fig. 5. There was little change below 120 K. Above 120 K, the intrinsic carrier concentration from thermal activation started to dominate leading to the reduction of device quantum efficiency and responsivity. The photoresponse intensity is nearly an exponential function of the inverse temperature. The relation can be simply described
I ¼ I0 =f1 þ expðEa =kTÞg
ð2Þ
where Ea is the activation energy and k the Boltzmann constant. The experimental data was fitted using Eq. (2). There were two well-defined slopes which denoted two thermal activation process characterized by Ea = 48 meV at low temperature between 130 K and 200 K and Ea = 258 meV between 230 K and 300 K.
InAs (2ML)/GaSb (8ML) superlattices (SLs) were grown by molecular-beam epitaxy on GaSb substrates. The cut-off wavelength moved from 2.3 lm to 2.6 lm with temperature rising from 77 K to 300 K. The blackbody Rv exponentially decreased as a function of 1/T in two thermal activation process. Although the Dbb was at the low level of 108 lcmHz1/2/W due to the non-optimized structure, the results indicated the application of InAs/GaSb SLs for multiband detector covering short, middle and long wavelength. The performance will be improved by optimizing the growth condition and device structure. References [1] J.L. Johnson et al., Electrical and optical properties of infrared photodiodes using the InAs/GaInSb superlattice in heterojunctions with GaSb, J. Appl. Phys. 80 (1996) 1116–1127. [2] F. Fuchs et al., High performance InAs/GaInSb superlattice infrared photodiodes, Appl. Phys. Lett. 71 (1997) 3251–3253. [3] M. Walther et al., Growth of InAs/GaSb short-period superlattices for highresolution mid-wavelength infrared focal plane array detectors, J. Cryst. Growth 278 (2005) 156–161. [4] J.B. Rodriguez et al., Uncooled InAs/GaSb superlattice photovoltaic detector operating in the mid-wavelength infrared range, Electron. Lett. 41 (2005) 362– 363. [5] H. Mohseni et al., Growth and characterization of InAs/GaSb photoconductors for long wavelength infrared range, Appl. Phys. Lett. 77 (2000) 1572–1574. [6] G.J. Brown et al., Type-II InAs/GaSb superlattices for very long wavelength infrared detectors, Physica E 20 (2006) 471–474. [7] R.T. Hao et al., MBE growth of very short period InAs/GaSb type II superlattices on (0 0 1) GaAs substrates, J. Phys. D: Appl. Phys. 40 (2007) 6690–6693. [8] H.J. Haugan et al., Short-period InAs/GaSb type II superlattices for mid-infrared detecters, Appl. Phys. Lett. 87 (2005) 261106. [9] Y.J. Wei et al., High quality type II InAs/GaSb superlattices with cutoff wavelength 3.7 lm using interface engineering, J. Appl. Phys. 94 (2003) 4720–4722.