- Email: [email protected]
SiNx stacks
Accepted Manuscript Low leakage of In0.83Ga0.17As photodiode with Al2O3/SiNx stacks Jing Yang, Ming Shi, Xiumei Shao, Tao Li, Xue Li, Naiyun Tang, Haimei Gong, Ran Liu, Hengjing Tang, Zhi-Jun Qiu PII: DOI: Reference:
S1350-4495(15)00082-1 http://dx.doi.org/10.1016/j.infrared.2015.04.003 INFPHY 1768
To appear in:
Infrared Physics & Technology
Received Date:
4 March 2015
Please cite this article as: J. Yang, M. Shi, X. Shao, T. Li, X. Li, N. Tang, H. Gong, R. Liu, H. Tang, Z-J. Qiu, Low leakage of In0.83Ga0.17As photodiode with Al2O3/SiNx stacks, Infrared Physics & Technology (2015), doi: http:// dx.doi.org/10.1016/j.infrared.2015.04.003
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Low leakage of In0.83Ga0.17As photodiode with Al2O3/SiNx stacks Jing Yang a, b, Ming Shi b, c, d, Xiumei Shao b, c, Tao Li b, c, Xue Li b, c, Naiyun Tange*, Haimei Gong b, c , Ran Liu a, Hengjing Tang b, c ∗ Zhi-Jun Qiu a∗ a
State Key Laboratory of ASIC & System, School of Information Science and Technology, Fudan University, Shanghai 200433, China b State Key Laboratory of Transducer Technology, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China c Key Laboratory of Infrared Imaging Materials and Detectors, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China d University of the Chinese Academy of Sciences, Beijing 100039, China e Shanghai University of Electric Power, Shanghai 200090, China
Abstract Al2O3/SiNx stacks were used as the passivation layer to reduce the dark current of extended wavelength In0.83Ga0.17As photodiodes.
The Al2O3 and SiNx layers were
deposited sequentially by atomic layer deposition (ALD) and the inductively coupled plasma chemical vapor deposition (ICPCVD), respectively. Compared to the single passivation layer of SiNx deposited by ICPCVD, the Al2O3/SiNx stacks result in a more than 20% lower dark current of the In0.83Ga0.17As photodiodes. The substantial dark current reduction can be attributed to lower defect density at the InGaAs/Al2O3 interface and thus lower surface leakage current.
Keywords: InGaAs photodiode, leakage current, Al2O3, ALD 1. Introduction In recent years, there are growing needs for short wavelength infrared (SWIR) band detectors near 1-3µm in environmental monitoring, night vision, and mineral search[1,
∗a
Corresponding author at: School of Information Science and Technology, Fudan University, Shanghai 200433, China. Tel: +86 021 55664269. Email address: [email protected] ∗b Corresponding author at: Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China. Tel.: +86 021 25051002. Email address: [email protected] ∗e Corresponding author at: Shanghai University of Electric Power, Shanghai 200090, China. Tel.: +86 021 68029236. Email address: [email protected]
2]. Compared to other (SWIR detectors, the ternary InGaAs detectors are preferred due to the advantage of material growth, device process technology, and better performance at higher operation temperature. The InGaAs based semiconductor devices could be fabricated for any wavelength within a spectral range of 0.85-3.60 µm. Especially, the ternary In0.53Ga0.47As material lattice is matched to InP substrate with cut-off wavelength at about 1.7 µm. To extend the cut-off wavelength of the photodiode to 2.6 µm, the In content of the InGaAs alloy must be increased from 53% to about 83%, which leads to lattice mismatch of about 2.06% between the InGaAs active layer and the InP substrate and thus creates more dislocation defects and surface defects. These defects can cause larger dark current and suppress the performance of the detectors. Surface passivation is a very important way to tackle the surface defect issue for extended wavelength InxGa1-xAs (x>53%) detectors. Sulfur passivation [3] or SiNx passivation [4] has been used to improve the performance. In our previous work, SiNx deposited by ICPCVD was applied on the extended wavelength InGaAs detectors [5]. However, it was found that new damages could be introduced by the plasma during the ICPCVD deposition to the surfaces of III–V semiconductors. Recently, atomic layer deposition (ALD) was developed to grow thin Al2O3 film on III-V semiconductors [6]. It uses Al(CH3)3 (trimethylaluminum, TMA) and water as the precursors, which has the so-called self-cleaning effect [7-10] that reduces native oxides on the surfaces of III–V semiconductors. This can decrease the interfacial density of states (Dit) [10] and improve the interfacial properties. In this paper, we use ALD deposited Al2O3 as the passivation interlayer between the ICPCVD deposited SiNx layer and the III–V semiconductors. We compare the electrical behaviors of PIN mesa-type In0.83Ga0.17As photodiodes passivated by SiNx and Al2O3/SiNx stacks, respectively. The photodiodes passivated with Al2O3/SiNx stacks has shown a more than 20% improvement in dark current over the ones passivated with only SiNx.
2. Experiment The
devices
were
fabricated
on
the
PIN-type
InAlAs/InGaAs/InAlAs
hetero-structures grown on a semi-insulating InP (Fe dopant) substrates using Gas Source Molecular Beam Epitaxy (GSMBE). Scheme of the mesa PIN photodiode is shown in Fig. 1. The epitaxial material consisted of 0.6 µm Be-doped p-InAlAs cap layer with a doping concentration of about 2×10 18 cm-3, a 1.5-µm InGaAs absorbing layer (unintentionally doped) and a 2-µm Si-doped n-InAlAs buffer layer with a doping concentration of about 2×1018 cm-3. Conventional mesa structure photodiodes were fabricated by Inductively Coupled Plasma (ICP) etching, non-selective wet etching and lift-off processes. After the mesa isolation, Ti/Pt/Au ohmic contacts were formed by an e-beam evaporation system and lift-off processes. Post metallization annealing was performed in an N2 flow at 420 °C for 40 s. Two series of samples with radii of photosensitive elements of 20, 25, 30, 40, 50, 60, 75, 100 µm, respectively, were prepared. For samples A, a single SiNx passivation layer was deposited by ICPCVD (75 °C) at a low growth rate. For samples B, prior to the SiNx layer deposition, a thin layer of Al2O3 was grown by ALD using alternating pulses of TMA and H2O as precursors with a wafer temperature of 200 °C and a chamber pressure of 1 Torr. The physical thickness of the Al2O3 layer was determined to be 20 nm by ellipsometry. Photolithography, ICP etching and wet etching processes were used to fabricate the contact holes for p-type and n-type electrodes. Finally, Cr/Au (20 nm/400 nm) electrodes were formed by an e-beam evaporation system and lift-off process. The I–V characteristics were measured by Agilent B1500A Semiconductor Device Analyzer at different temperature.
Fig. 1. Scheme of the mesa PIN photodiode
3. Results and discussion The dark current is an important parameter for the photoelectric detector and should be as low as possible to improve signal to noise ratio. Fig. 2 shows the I-V curves of samples A and B with radius of 100 µm at 290 K. The results indicate that the photodiodes passivated by the Al2O3/SiNx stacks have a lower dark current than the ones with only SiNx passivation at 290 K. Fig. 3 shows the the dark current at 100-mV reverse bias as a function of the perimeter-to-area (P/A) ratio of the samples A and samples B at 290 K, respectively. 10 groups of photoelectric detectors were measured to calculate the average values of the dark current density at 100-mV reverse bias. The result indicates that both samples A and samples B show no significant dependence on the P/A ratio of the junction and this indicates that the sidewalls of both samples are well passivated. The average dark current density of samples B for each radius is about 20% lower than that of samples A.
Fig. 2. The I-V curves of the samples A, B at 290 K.
Fig. 3. The dark current density versus P/A ratio at 100-mV reverse bias and 290 K for sample A and sample B, respectively.
At small reverse bias, the dark current of extended InGaAs detectors consists of diffusion current Idif，generation–recombination current Igr, trap assisted tunneling (TAT) current Itat, ohmic current Ioh and surface recombination current Is. Using an Arrhenius thermal activation model, the relationship between dark current density J and 1/T can be expressed by [11]
∝ exp (−
)
(1)
where Ea is the activation energy, k the Boltzmann constant and T the temperature. If the activation energy equals to the bandgap (Eg=0.47 eV) of In0.83Ga0.17As, Ea=Eg, the dark
current
mechanism
is
dominated
by
diffusion
current,
whereas
generation–recombination and ohmic leakage current are dominate while Ea=Eg/2 [12], and surface recombination current is dominate for Ea=Eg/4 [12]. The dark current and dynamic zero bias resistance of the samples A and B with radius of 100 µm were measured in the temperature range of 180-290 K with a step of 10 K.
Fig. 4 displays the dark current density at 10-mV reverse bias and zero bias
resistance area product R0A in Arrhenius plot for sample A and sample B, respectively. It can be seen that, in the temperature range 220 K-290 K, the dark current mechanism is dominated by the diffuse current for both sample A (Ea=0.54 eV) and sample B (Ea=0.56 eV). In the temperature range 190 K-210 K, the dark current mechanisms are dominated by the generation–recombination and ohmic leakage current for both sample A (Ea=0.29 eV) and sample B (Ea=0.26 eV). At 290 K, R0A of samples A and B are 15.6 Ωcm2 and 21.0 Ωcm2, respectively, and increase exponentially to 2.1×10 5 Ωcm2 and 3.1×10 5 Ωcm2 when the detectors are cooled down to 190 K.
Fig. 4. Arrhenius plot of dark current density behavior and R0A for sample A and sample B, respectively. It is noted from Fig. 3 that the dark currents of samples B are about 20% less than
those of samples A. The main difference between the two series of samples is the interface between the III–V semiconductors (InGaAs and InAlAs) and the passivation layer, which can influence surface recombination current Is and ohmic current Ioh. Recent simulations for the dark current of extended wavelength InGaAs detectors indicate that surface recombination current of extended wavelength InGaAs detectors passivated by ICPCVD SiNx accounts only for a negligible small proportion [12]. The reduction of ohmic current may be accountable for the decrease in dark current from samples A to samples B. The ohmic current Ioh is responsible for exhibiting a shunt-like behavior and owes its origin to surface leakage currents [13] and dislocations intersecting the junction [14, 15]. The ohmic current can be calculated as follows:
=
(2)
where Rsh is the diode shunt resistance and it can be estimated from the peak position of its dynamic resistance-voltage (Rd-V) characteristic [16, 17].The peak position in Rd –V curve corresponds to the condition:
| =0
(3)
where Rd is the resultant dynamic resistance given by
1 1 1 1 1 1 = + + + + ! " " #"#
(4)
where Rdif, Rgr, Rtat, Rbtb are dynamic resistances due to diffusion current, generation–recombination current, TAT and BTB tunneling current contributions, respectively. From the peak position of the dynamic resistance-voltage (Rd-V) characteristic, trap density (Nt) can be calculated as follows: N" =
{(
& & ! & #"# ) + ( ) + ( ) } & & & 2) * +, & -. /& 1 1 1& exp (− 3 )( * + & ) * ℎ ( − " ) 42 2& 42&
(5)
3
4(2-. )&( − " )*/& B= 2)8 3/& 3)ℎ( ) 9: 9
(6)
where mv is the effective mass of carrier in the valence band, A the junction area, Eg the band gap, M the matrix element associated with the trap potential, h the Planck’s constant and Et the position of trap levels in the band gap. Since all the parameters of the above equation are known for the given material of known composition, the density of traps Nt responsible for the TAT can be calculated from Vm. Making use of Nt, Rtat can be calculated as follows:
(" " );3 =
2) * +, &-. / & 1 1& N exp (− )(1 + ) 3/& 3/& ℎ* ( − " ) " 2 "
"
(7)
where Vt=V-Vbi. Since dynamic resistance due to generation–recombination current, BTB tunneling and diffusion current contributions can be calculated from the parameters for the given material and shunt resistance Rsh is independent of applied voltage V [16], shunt resistance can be calculated from a comparison of experimental peak dynamic resistance and calculated resultant dynamic resistance due to TAT, generation–recombination current, BTB tunneling and diffusion current contributions. The diode shunt resistance can be given by = [(
1 1 1 1 1 >];3 ) − =( ) + ( ) + ( ) +( ) ! " " #"# =-
(8)
The diode shunt resistance at different temperature for the sample A and the sample B are shown in Fig. 5. The decrease of the ohmic current ∆Ioh and its proportion in the reduction of total current at certain reverse bias and same temperature can be calculated as follows:
@ = (A A−A A) (B) (C) DEFG
(9)
@ = (|B | − |C |)DEFG
(10)
P=
@ × 100% @
(11)
where Rsh(A) and Rsh(B) are the diode shunt resistance at the same temperature for the sample A and the sample B, respectively, ∆I is the reduction of total current between the two samples at the same reverse bias voltage and same temperature. The proportion of ∆Ioh in ∆I at different reverse bias and temperature is shown in Fig. 6.
Fig. 5. The diode shunt resistance at different temperature for sample A and sample B, respectively
Fig. 6. The proportion of ∆Ioh in ∆I as a function of voltage at different temperature Fig. 6 indicates that the proportion of ∆Ioh in ∆I increases with rising voltage bias and decreases as temperature increases. At relatively high voltage bias and relatively low temperature, the reduction of ohmic current plays a role in the reduction of total current. At the reverse bias of 200 mV and temperature of 230 K, ∆Ioh accounted for a large proportion of ∆I. The largest value of the proportion of ∆Ioh in ∆I is 81%.
4. Conclusion The In0.83Ga0.17As photodiodes with two different passivation layers and various junction areas have been studied systematically. Both passivation layers suppress the P/A dependence of the photodiode current and result in a very low perimeter dark current. For detectors with 100-µm mesa radius at 190 K, the resistance area products R0A are 2.1×105 Ωcm2 and 3.1×10 5 Ωcm2 and the dark current densities at 10-mV reverse bias are 40 nA/cm2 and 30 nA/cm2 for samples without and with the Al2O3 interfacial passivation layer, respectively. The photodiodes passivated with Al2O3/SiNx stacks has a considerably lower dark current than the ones passivated with SiNx only. This can be well attributed to the lower interfacial state density for the Al2O3/SiNx passivated photodiodes. Analysis of the dark current at reverse bias
indicates that the reduction of ohmic current play a predominant role in the performance enhancement for the In0.83Ga0.17As photodiodes.
Acknowledgments This research is supported by National Key Basic Research and Development Program of China (973 Program No.2012CB619200) and National Natural Science Foundation of China (Nos. 61007067 and 61204105).
References [1] R.W.M. Hoogeveen, R.J. van der A, A.P.H. Goede. Extended wavelength InGaAs infrared (1.0-2.4 µm) detector arrays on SCIAMACHY for space-based spectrometry of the Earth atmosphere[J]. Infrared Phys. Technol. 42 (2001) 1-16. [2] Q.L. Kleipool, R.T. Jongma, A.M.S. Gloudemans, et al. In-flight proton-induced radiation damage to SCIAMACHY’s extended-wavelength InGaAs near-infrared detectors[J]. Infrared Phys. Technol. 50 (1) (2007) 30-37. [3] Z. Jin, S. Neumann, W. Prost, et al. Effects of (NH4)2S passivation on the performance of graded-base InGaAs/InP HBTs[J]. Phys. Status Solidi A. 201 (2004) 1017-1021. [4] Z. Jin, K. Uchida, S. Nozaki, et al. Passivation of InP-based HBTs[J]. Appl. Surf. Sci. 252 (21) (2006) 7664-7670. [5] P. Wei, X. Li, T. Li, et al. Surface passivation of In0.83Ga0.17As photodiode with high-quality SiN layer fabricated by ICPCVD at the lower temperature[J]. Infrared Phys. Technol. 62 (0) (2014) 13-17. [6] S.M. George. Atomic Layer Deposition: An Overview[J]. Chem. Rev. 110 (1) (2010) 111-131. [7] M. Milojevic, C.L. Hinkle, F.S. Aguirre-Tostado, et al. Half-cycle atomic layer deposition reaction studies of Al2O3 on (NH4)2S passivated GaAs(100) surfaces[J]. Applied Physics Letters. 2008, 93(25). [8] M. Milojevic, F.S. Aguirre-Tostado, C.L. Hinkle, et al. Half-cycle atomic layer deposition reaction studies of Al2O3 on In0.2Ga0.8As (100) surfaces[J]. Appl. Phys. Lett. 93 (25) (2008).
[9] C.H. Chang, Y.K. Chiou, Y.C. Chang, et al. Interfacial self-cleaning in atomic layer deposition of HfO2 gate dielectric on In0.15Ga0.85As[J]. Appl. Phys. Lett. 89 (24) (2006). [10] M.L. Huang, Y.C. Chang, C.H. Chang, et al. Surface passivation of III-V compound semiconductors using atomic-layer-deposition-grown Al2O3[J]. Appl. Phys. Lett. 87 (25) (2005). [11] A. Majumdar, K.K. Choi, J.L. Reno, et al. Two-color quantum-well infrared photodetector with voltage tunable peaks[J]. Appl. Phys. Lett. 80 (5) (2002) 707-709. [12] X. Li, H. Tang, T. Li, et al. Study on dark current of extended wavelength InGaAs detectors[C]. 2013. [13] V. Gopal. Variable-area diode data-analysis of surface and bulk effects in HgCdTe photodetector arrays[J]. Semicond. Sci. Technol. 9 (12) (1994) 2267-2271. [14] S.M. Johnson, D.R. Rhiger, J.P. Rosbeck, et al. Effect of dislocations on the electrical and
optical properties of long-wavelength
infrared
HgCdTe
photovoltaic detectors[J]. J. Vac. Sci. Technol., B. 10 (4) (1992) 1499-1506. [15] I.M. Baker, C.D. Maxey. Summary of HgCdTe 2D array technology in the U.K.[J]. J. Electron. Mater. 30 (6) (2001) 682-689. [16] V. Gopal, S.K. Singh, R.M. Mehra. Analysis of dark current contributions in mercury cadmium telluride junction diodes[J]. Infrared Phys. Technol. 43 (6) (2002) 317-326. [17] V. Gopal, S. Gupta, R.K. Bhan, et al. Modeling of dark characteristics of mercury cadmium telluride n+–p junctions[J]. Infrared Phys. Technol. 44 (2) (2003) 143-152.
HIGHLIGHTS The ALD Al2O3 is used as passivation layer between SiNx layer and semiconductors. The photodiodes with Al2O3/SiNx stacks has shown a 20% improvement in dark current. Dark current mechanisms of photodiodes have been analyzed.
S1350-4495(15)00082-1 http://dx.doi.org/10.1016/j.infrared.2015.04.003 INFPHY 1768
To appear in:
Infrared Physics & Technology
Received Date:
4 March 2015
Please cite this article as: J. Yang, M. Shi, X. Shao, T. Li, X. Li, N. Tang, H. Gong, R. Liu, H. Tang, Z-J. Qiu, Low leakage of In0.83Ga0.17As photodiode with Al2O3/SiNx stacks, Infrared Physics & Technology (2015), doi: http:// dx.doi.org/10.1016/j.infrared.2015.04.003
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Low leakage of In0.83Ga0.17As photodiode with Al2O3/SiNx stacks Jing Yang a, b, Ming Shi b, c, d, Xiumei Shao b, c, Tao Li b, c, Xue Li b, c, Naiyun Tange*, Haimei Gong b, c , Ran Liu a, Hengjing Tang b, c ∗ Zhi-Jun Qiu a∗ a
State Key Laboratory of ASIC & System, School of Information Science and Technology, Fudan University, Shanghai 200433, China b State Key Laboratory of Transducer Technology, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China c Key Laboratory of Infrared Imaging Materials and Detectors, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China d University of the Chinese Academy of Sciences, Beijing 100039, China e Shanghai University of Electric Power, Shanghai 200090, China
Abstract Al2O3/SiNx stacks were used as the passivation layer to reduce the dark current of extended wavelength In0.83Ga0.17As photodiodes.
The Al2O3 and SiNx layers were
deposited sequentially by atomic layer deposition (ALD) and the inductively coupled plasma chemical vapor deposition (ICPCVD), respectively. Compared to the single passivation layer of SiNx deposited by ICPCVD, the Al2O3/SiNx stacks result in a more than 20% lower dark current of the In0.83Ga0.17As photodiodes. The substantial dark current reduction can be attributed to lower defect density at the InGaAs/Al2O3 interface and thus lower surface leakage current.
Keywords: InGaAs photodiode, leakage current, Al2O3, ALD 1. Introduction In recent years, there are growing needs for short wavelength infrared (SWIR) band detectors near 1-3µm in environmental monitoring, night vision, and mineral search[1,
∗a
Corresponding author at: School of Information Science and Technology, Fudan University, Shanghai 200433, China. Tel: +86 021 55664269. Email address: [email protected] ∗b Corresponding author at: Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China. Tel.: +86 021 25051002. Email address: [email protected] ∗e Corresponding author at: Shanghai University of Electric Power, Shanghai 200090, China. Tel.: +86 021 68029236. Email address: [email protected]
2]. Compared to other (SWIR detectors, the ternary InGaAs detectors are preferred due to the advantage of material growth, device process technology, and better performance at higher operation temperature. The InGaAs based semiconductor devices could be fabricated for any wavelength within a spectral range of 0.85-3.60 µm. Especially, the ternary In0.53Ga0.47As material lattice is matched to InP substrate with cut-off wavelength at about 1.7 µm. To extend the cut-off wavelength of the photodiode to 2.6 µm, the In content of the InGaAs alloy must be increased from 53% to about 83%, which leads to lattice mismatch of about 2.06% between the InGaAs active layer and the InP substrate and thus creates more dislocation defects and surface defects. These defects can cause larger dark current and suppress the performance of the detectors. Surface passivation is a very important way to tackle the surface defect issue for extended wavelength InxGa1-xAs (x>53%) detectors. Sulfur passivation [3] or SiNx passivation [4] has been used to improve the performance. In our previous work, SiNx deposited by ICPCVD was applied on the extended wavelength InGaAs detectors [5]. However, it was found that new damages could be introduced by the plasma during the ICPCVD deposition to the surfaces of III–V semiconductors. Recently, atomic layer deposition (ALD) was developed to grow thin Al2O3 film on III-V semiconductors [6]. It uses Al(CH3)3 (trimethylaluminum, TMA) and water as the precursors, which has the so-called self-cleaning effect [7-10] that reduces native oxides on the surfaces of III–V semiconductors. This can decrease the interfacial density of states (Dit) [10] and improve the interfacial properties. In this paper, we use ALD deposited Al2O3 as the passivation interlayer between the ICPCVD deposited SiNx layer and the III–V semiconductors. We compare the electrical behaviors of PIN mesa-type In0.83Ga0.17As photodiodes passivated by SiNx and Al2O3/SiNx stacks, respectively. The photodiodes passivated with Al2O3/SiNx stacks has shown a more than 20% improvement in dark current over the ones passivated with only SiNx.
2. Experiment The
devices
were
fabricated
on
the
PIN-type
InAlAs/InGaAs/InAlAs
hetero-structures grown on a semi-insulating InP (Fe dopant) substrates using Gas Source Molecular Beam Epitaxy (GSMBE). Scheme of the mesa PIN photodiode is shown in Fig. 1. The epitaxial material consisted of 0.6 µm Be-doped p-InAlAs cap layer with a doping concentration of about 2×10 18 cm-3, a 1.5-µm InGaAs absorbing layer (unintentionally doped) and a 2-µm Si-doped n-InAlAs buffer layer with a doping concentration of about 2×1018 cm-3. Conventional mesa structure photodiodes were fabricated by Inductively Coupled Plasma (ICP) etching, non-selective wet etching and lift-off processes. After the mesa isolation, Ti/Pt/Au ohmic contacts were formed by an e-beam evaporation system and lift-off processes. Post metallization annealing was performed in an N2 flow at 420 °C for 40 s. Two series of samples with radii of photosensitive elements of 20, 25, 30, 40, 50, 60, 75, 100 µm, respectively, were prepared. For samples A, a single SiNx passivation layer was deposited by ICPCVD (75 °C) at a low growth rate. For samples B, prior to the SiNx layer deposition, a thin layer of Al2O3 was grown by ALD using alternating pulses of TMA and H2O as precursors with a wafer temperature of 200 °C and a chamber pressure of 1 Torr. The physical thickness of the Al2O3 layer was determined to be 20 nm by ellipsometry. Photolithography, ICP etching and wet etching processes were used to fabricate the contact holes for p-type and n-type electrodes. Finally, Cr/Au (20 nm/400 nm) electrodes were formed by an e-beam evaporation system and lift-off process. The I–V characteristics were measured by Agilent B1500A Semiconductor Device Analyzer at different temperature.
Fig. 1. Scheme of the mesa PIN photodiode
3. Results and discussion The dark current is an important parameter for the photoelectric detector and should be as low as possible to improve signal to noise ratio. Fig. 2 shows the I-V curves of samples A and B with radius of 100 µm at 290 K. The results indicate that the photodiodes passivated by the Al2O3/SiNx stacks have a lower dark current than the ones with only SiNx passivation at 290 K. Fig. 3 shows the the dark current at 100-mV reverse bias as a function of the perimeter-to-area (P/A) ratio of the samples A and samples B at 290 K, respectively. 10 groups of photoelectric detectors were measured to calculate the average values of the dark current density at 100-mV reverse bias. The result indicates that both samples A and samples B show no significant dependence on the P/A ratio of the junction and this indicates that the sidewalls of both samples are well passivated. The average dark current density of samples B for each radius is about 20% lower than that of samples A.
Fig. 2. The I-V curves of the samples A, B at 290 K.
Fig. 3. The dark current density versus P/A ratio at 100-mV reverse bias and 290 K for sample A and sample B, respectively.
At small reverse bias, the dark current of extended InGaAs detectors consists of diffusion current Idif，generation–recombination current Igr, trap assisted tunneling (TAT) current Itat, ohmic current Ioh and surface recombination current Is. Using an Arrhenius thermal activation model, the relationship between dark current density J and 1/T can be expressed by [11]
∝ exp (−
)
(1)
where Ea is the activation energy, k the Boltzmann constant and T the temperature. If the activation energy equals to the bandgap (Eg=0.47 eV) of In0.83Ga0.17As, Ea=Eg, the dark
current
mechanism
is
dominated
by
diffusion
current,
whereas
generation–recombination and ohmic leakage current are dominate while Ea=Eg/2 [12], and surface recombination current is dominate for Ea=Eg/4 [12]. The dark current and dynamic zero bias resistance of the samples A and B with radius of 100 µm were measured in the temperature range of 180-290 K with a step of 10 K.
Fig. 4 displays the dark current density at 10-mV reverse bias and zero bias
resistance area product R0A in Arrhenius plot for sample A and sample B, respectively. It can be seen that, in the temperature range 220 K-290 K, the dark current mechanism is dominated by the diffuse current for both sample A (Ea=0.54 eV) and sample B (Ea=0.56 eV). In the temperature range 190 K-210 K, the dark current mechanisms are dominated by the generation–recombination and ohmic leakage current for both sample A (Ea=0.29 eV) and sample B (Ea=0.26 eV). At 290 K, R0A of samples A and B are 15.6 Ωcm2 and 21.0 Ωcm2, respectively, and increase exponentially to 2.1×10 5 Ωcm2 and 3.1×10 5 Ωcm2 when the detectors are cooled down to 190 K.
Fig. 4. Arrhenius plot of dark current density behavior and R0A for sample A and sample B, respectively. It is noted from Fig. 3 that the dark currents of samples B are about 20% less than
those of samples A. The main difference between the two series of samples is the interface between the III–V semiconductors (InGaAs and InAlAs) and the passivation layer, which can influence surface recombination current Is and ohmic current Ioh. Recent simulations for the dark current of extended wavelength InGaAs detectors indicate that surface recombination current of extended wavelength InGaAs detectors passivated by ICPCVD SiNx accounts only for a negligible small proportion [12]. The reduction of ohmic current may be accountable for the decrease in dark current from samples A to samples B. The ohmic current Ioh is responsible for exhibiting a shunt-like behavior and owes its origin to surface leakage currents [13] and dislocations intersecting the junction [14, 15]. The ohmic current can be calculated as follows:
=
(2)
where Rsh is the diode shunt resistance and it can be estimated from the peak position of its dynamic resistance-voltage (Rd-V) characteristic [16, 17].The peak position in Rd –V curve corresponds to the condition:
| =0
(3)
where Rd is the resultant dynamic resistance given by
1 1 1 1 1 1 = + + + + ! " " #"#
(4)
where Rdif, Rgr, Rtat, Rbtb are dynamic resistances due to diffusion current, generation–recombination current, TAT and BTB tunneling current contributions, respectively. From the peak position of the dynamic resistance-voltage (Rd-V) characteristic, trap density (Nt) can be calculated as follows: N" =
{(
& & ! & #"# ) + ( ) + ( ) } & & & 2) * +, & -. /& 1 1 1& exp (− 3 )( * + & ) * ℎ ( − " ) 42 2& 42&
(5)
3
4(2-. )&( − " )*/& B= 2)8 3/& 3)ℎ( ) 9: 9
(6)
where mv is the effective mass of carrier in the valence band, A the junction area, Eg the band gap, M the matrix element associated with the trap potential, h the Planck’s constant and Et the position of trap levels in the band gap. Since all the parameters of the above equation are known for the given material of known composition, the density of traps Nt responsible for the TAT can be calculated from Vm. Making use of Nt, Rtat can be calculated as follows:
(" " );3 =
2) * +, &-. / & 1 1& N exp (− )(1 + ) 3/& 3/& ℎ* ( − " ) " 2 "
"
(7)
where Vt=V-Vbi. Since dynamic resistance due to generation–recombination current, BTB tunneling and diffusion current contributions can be calculated from the parameters for the given material and shunt resistance Rsh is independent of applied voltage V [16], shunt resistance can be calculated from a comparison of experimental peak dynamic resistance and calculated resultant dynamic resistance due to TAT, generation–recombination current, BTB tunneling and diffusion current contributions. The diode shunt resistance can be given by = [(
1 1 1 1 1 >];3 ) − =( ) + ( ) + ( ) +( ) ! " " #"# =-
(8)
The diode shunt resistance at different temperature for the sample A and the sample B are shown in Fig. 5. The decrease of the ohmic current ∆Ioh and its proportion in the reduction of total current at certain reverse bias and same temperature can be calculated as follows:
@ = (A A−A A) (B) (C) DEFG
(9)
@ = (|B | − |C |)DEFG
(10)
P=
@ × 100% @
(11)
where Rsh(A) and Rsh(B) are the diode shunt resistance at the same temperature for the sample A and the sample B, respectively, ∆I is the reduction of total current between the two samples at the same reverse bias voltage and same temperature. The proportion of ∆Ioh in ∆I at different reverse bias and temperature is shown in Fig. 6.
Fig. 5. The diode shunt resistance at different temperature for sample A and sample B, respectively
Fig. 6. The proportion of ∆Ioh in ∆I as a function of voltage at different temperature Fig. 6 indicates that the proportion of ∆Ioh in ∆I increases with rising voltage bias and decreases as temperature increases. At relatively high voltage bias and relatively low temperature, the reduction of ohmic current plays a role in the reduction of total current. At the reverse bias of 200 mV and temperature of 230 K, ∆Ioh accounted for a large proportion of ∆I. The largest value of the proportion of ∆Ioh in ∆I is 81%.
4. Conclusion The In0.83Ga0.17As photodiodes with two different passivation layers and various junction areas have been studied systematically. Both passivation layers suppress the P/A dependence of the photodiode current and result in a very low perimeter dark current. For detectors with 100-µm mesa radius at 190 K, the resistance area products R0A are 2.1×105 Ωcm2 and 3.1×10 5 Ωcm2 and the dark current densities at 10-mV reverse bias are 40 nA/cm2 and 30 nA/cm2 for samples without and with the Al2O3 interfacial passivation layer, respectively. The photodiodes passivated with Al2O3/SiNx stacks has a considerably lower dark current than the ones passivated with SiNx only. This can be well attributed to the lower interfacial state density for the Al2O3/SiNx passivated photodiodes. Analysis of the dark current at reverse bias
indicates that the reduction of ohmic current play a predominant role in the performance enhancement for the In0.83Ga0.17As photodiodes.
Acknowledgments This research is supported by National Key Basic Research and Development Program of China (973 Program No.2012CB619200) and National Natural Science Foundation of China (Nos. 61007067 and 61204105).
References [1] R.W.M. Hoogeveen, R.J. van der A, A.P.H. Goede. Extended wavelength InGaAs infrared (1.0-2.4 µm) detector arrays on SCIAMACHY for space-based spectrometry of the Earth atmosphere[J]. Infrared Phys. Technol. 42 (2001) 1-16. [2] Q.L. Kleipool, R.T. Jongma, A.M.S. Gloudemans, et al. In-flight proton-induced radiation damage to SCIAMACHY’s extended-wavelength InGaAs near-infrared detectors[J]. Infrared Phys. Technol. 50 (1) (2007) 30-37. [3] Z. Jin, S. Neumann, W. Prost, et al. Effects of (NH4)2S passivation on the performance of graded-base InGaAs/InP HBTs[J]. Phys. Status Solidi A. 201 (2004) 1017-1021. [4] Z. Jin, K. Uchida, S. Nozaki, et al. Passivation of InP-based HBTs[J]. Appl. Surf. Sci. 252 (21) (2006) 7664-7670. [5] P. Wei, X. Li, T. Li, et al. Surface passivation of In0.83Ga0.17As photodiode with high-quality SiN layer fabricated by ICPCVD at the lower temperature[J]. Infrared Phys. Technol. 62 (0) (2014) 13-17. [6] S.M. George. Atomic Layer Deposition: An Overview[J]. Chem. Rev. 110 (1) (2010) 111-131. [7] M. Milojevic, C.L. Hinkle, F.S. Aguirre-Tostado, et al. Half-cycle atomic layer deposition reaction studies of Al2O3 on (NH4)2S passivated GaAs(100) surfaces[J]. Applied Physics Letters. 2008, 93(25). [8] M. Milojevic, F.S. Aguirre-Tostado, C.L. Hinkle, et al. Half-cycle atomic layer deposition reaction studies of Al2O3 on In0.2Ga0.8As (100) surfaces[J]. Appl. Phys. Lett. 93 (25) (2008).
[9] C.H. Chang, Y.K. Chiou, Y.C. Chang, et al. Interfacial self-cleaning in atomic layer deposition of HfO2 gate dielectric on In0.15Ga0.85As[J]. Appl. Phys. Lett. 89 (24) (2006). [10] M.L. Huang, Y.C. Chang, C.H. Chang, et al. Surface passivation of III-V compound semiconductors using atomic-layer-deposition-grown Al2O3[J]. Appl. Phys. Lett. 87 (25) (2005). [11] A. Majumdar, K.K. Choi, J.L. Reno, et al. Two-color quantum-well infrared photodetector with voltage tunable peaks[J]. Appl. Phys. Lett. 80 (5) (2002) 707-709. [12] X. Li, H. Tang, T. Li, et al. Study on dark current of extended wavelength InGaAs detectors[C]. 2013. [13] V. Gopal. Variable-area diode data-analysis of surface and bulk effects in HgCdTe photodetector arrays[J]. Semicond. Sci. Technol. 9 (12) (1994) 2267-2271. [14] S.M. Johnson, D.R. Rhiger, J.P. Rosbeck, et al. Effect of dislocations on the electrical and
optical properties of long-wavelength
infrared
HgCdTe
photovoltaic detectors[J]. J. Vac. Sci. Technol., B. 10 (4) (1992) 1499-1506. [15] I.M. Baker, C.D. Maxey. Summary of HgCdTe 2D array technology in the U.K.[J]. J. Electron. Mater. 30 (6) (2001) 682-689. [16] V. Gopal, S.K. Singh, R.M. Mehra. Analysis of dark current contributions in mercury cadmium telluride junction diodes[J]. Infrared Phys. Technol. 43 (6) (2002) 317-326. [17] V. Gopal, S. Gupta, R.K. Bhan, et al. Modeling of dark characteristics of mercury cadmium telluride n+–p junctions[J]. Infrared Phys. Technol. 44 (2) (2003) 143-152.
HIGHLIGHTS The ALD Al2O3 is used as passivation layer between SiNx layer and semiconductors. The photodiodes with Al2O3/SiNx stacks has shown a 20% improvement in dark current. Dark current mechanisms of photodiodes have been analyzed.