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HgCdTe photovoltaic detectors fabricated using a new junction formation technology
Microelectronics Journal Microelectronics Journal 31 (2000) 545–551 www.elsevier.com/locate/mejo
HgCdTe photovoltaic detectors fabricated using a new junction formation technology M.H. Rais, C.A. Musca, J.M. Dell*, J. Antoszewski, B.D. Nener, L. Faraone Department of Electrical and Electronic Engineering, The University of Western Australia, Nedlands, WA 6907, Australia Received 26 November 1999; accepted 30 December 1999
Abstract The current–voltage characteristics measured over a wide temperature range are reported for HgCdTe mid-wavelength infrared n-on-p photodiodes fabricated using a novel junction formation technology. The planar homojunction device junctions were formed on LPE grown vacancy doped HgCdTe using a reactive ion etching (RIE) plasma induced conversion process. The zero bias dynamic resistance–junction area product, RoA, was 4:6 × 107 V cm 2 at 80 K and is comparable to the best planar diodes reported using conventional ion implantation junction formation technology. Arrhenius plots of RoA exhibit an activation energy equal to the bandgap, Eg, and show that the diodes are diffusion limited for temperatures ⱖ135 K. A series of temperature dependent 1/f noise measurements were performed, indicating that the activation energy for 1/f noise in the region where the diodes are diffusion limited is 0.7Eg. Energies close to this value have previously been associated with Hg vacancies in HgCdTe. These results are similar to those obtained from high quality HgCdTe photodiodes fabricated using mature ion implantation technology. However, the plasma based technology used in this work is significantly less complex and does not require any high temperature annealing steps. 䉷 2000 Elsevier Science Ltd. All rights reserved. Keywords: HgCdTe; Photovoltaic; RoA product; 1/f noise; Reactive ion etching; Semiconductors
1. Introduction There are number of different noise sources in a typical infrared (IR) photodiode, some of which are fundamental, while others are strongly influenced by device processing. In particular, 1/f noise is an important device parameter for IR photodiodes, often representing the limiting factor for twodimensional staring arrays, and appears to be related to processing induced effects. For any new junction formation technology, it is therefore important to investigate 1/f noise processes to ensure that the technology is compatible with high performance devices. A detailed study of the noise behaviour of mercury cadmium telluride (HgCdTe) IR photodiodes fabricated using a novel reactive ion etching (RIE) based type conversion process for the formation of the junction has been undertaken. As part of this work, detailed dark current and 1/f noise measurements have been made. The diodes studied in this work were fabricated on vacancy doped p-type HgCdTe grown by LPE on CdZnTe. The n-onp junctions were formed by exposing the HgCdTe surface to a plasma in a parallel plate RIE reactor using a ZnS mask. * Corresponding author. Tel.: ⫹ 61-8-9380-3787; fax: ⫹ 61-8-93801095. E-mail address: [email protected] (J.M. Dell).
This results in type conversion of the HgCdTe in the exposed regions [1]. The junctions were passivated using thermally deposited ZnS. In contrast to other reported RIE type conversion processes for the formation of n–p junctions [2,3], the work presented here uses a single ZnS layer which acts as both mask and passivant. The RIE junction formation process does not damage the ZnS masking layer, and hence the ZnS mask does not have to be removed as in conventional processes. As a result the n-on-p junctions at the surface are never exposed to atmosphere. Exposure of the junction is known in increase surface leakage currents. In addition, no high temperature anneal is required after junction formation to repair the crystal lattice.
2. Excess noise in HgCdTe diodes At low frequencies, the most important source of noise in HgCdTe IR photodiodes under reverse bias is 1/f or excess noise. This noise component is distinguished from other noise types by its unique power spectral distribution that is proportional to 1/f a , with a generally close to unity. While at zero bias voltage, photodiodes display no 1/f noise [4], under slight reverse bias, a typical operating condition used to increase the dynamic resistance of
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photodiodes in HgCdTe infrared focal plane arrays (IRFPAs), 1/f noise is observed, and often limits IRFPA performance. In addition it has been shown that 1/f noise is very sensitive to processing conditions and may give an insight to both optimal process conditions and the mechanisms involved in the type conversion process. Several studies have been conducted into 1/f noise mechanisms in HgCdTe photodiodes. Tobin et al. [4] reported 1/f noise behaviour in HgCdTe photodiodes fabricated by boron implantation into bulk grown material. Their results indicated that for the devices tested 1/f noise was directly proportional to surface leakage current rather than diffusion or photocurrent. At high temperatures where the dark current was diffusion limited, the 1/f noise continued to follow the dependence of the surface generation current. The investigators suggested that the 1/f noise was a result of modulation of the surface generation current by fluctuations in the surface potential. This mechanism was also proposed by Hsu [5]. Furthermore Chung et al. [6] found the same correlation with generation–recombination (g–r) current dependence and also associated the 1/f noise current with the surface. Bajaj et al. [7,8] have reported 1/f noise results on n-on-p junctions also formed by ion implantation technology but with more mature passivation technology. In contrast to Tobin et al., the 1/f noise for their devices did not originate from a perimeter source but scaled with junction area and was therefore identified as a bulk phenomenon. The strong bias dependence observed by Bajaj et al. further localized the noise source to the depletion region since this is the region across which most of the bias voltage appears. From temperature dependent measurements they extracted an activation energy of 0.72Eg for the 1/f noise, a value that has been associated with Hg vacancies in HgCdTe [9]. At low temperatures the 1/f noise for their diodes was temperature independent, indicating that a tunnelling mechanisms was dominant. Finally they failed to get good fit to their data using Kleinpenning diffusion model [10]. Both the Tobin and Bajaj results indicate that the 1/f noise process is a sensitive parameter and an important indicator of process technology performance. An initial study of 1/f noise in n-on-p Hg1⫺xCdxTe x ⬇ 0:32 diodes fabricated using the plasma type conversion process on vacancy doped p-type HgCdTe grown by LPE on a CdZnTe substrate and passivated using ZnS has been reported [11,12]. This paper presents a more detailed and comprehensive study on the temperature dependence of 1/f noise for the temperature range 80–180 K. The 1/f noise current for the diodes presented here is not proportional to dark current, even when the dark current is diffusion dominated, and exhibits an activation energy of 0.7Eg. The results presented in this work indicate that for temperatures below ⬃120 K, the 1/f noise current becomes approximately independent of temperature. These results are similar to those reported by Bajaj et al. [8] for state-of-the-art ion implanted MWIR photodiodes.
A theoretical model for 1/f noise in homojunction diodes for regimes in which the diode current is either generation– recombination dominated or diffusion dominated has been developed by Kleinpenning [10]. For one-sided long-base diodes operating in the diffusion-dominated regime, the power spectral density of the 1/f noise is given by aH qV qV SI qIo exp ⫺1⫺ 1 4f t kT kT where a H is the Hooge parameter, t is the minority carrier lifetime in the base region of the diode, Io is the diode saturation current, and V is the applied bias. The saturation current in this regime is given by qAn2i D 1=2 2 Io Na t where D is the diffusion constant and ni is the intrinsic carrier concentration. Assuming that the Hooge parameter is relatively temperature insensitive (primarily dependent on mobility) [13], the temperature dependence of this process is dominated by the n2i term for reverse biased diodes, and should therefore exhibit an activation energy of Eg. For the g–r dominated regime, the 1/f noise power spectral density is given by ! 2 2 aH qV ⫺qV ⫺ 1 exp 3 SI qIo exp 2kT 2kT 3f tj where t j is the lifetime in the junction depletion region and the other parameters are as before. The saturation current in the g–r limited regime is given by Io qA
ni Wj tj
4
where Wj is the depletion layer width. In this work, we have attempted to fit these theoretical models to our data to allow extraction of the Hooge parameter. 3. Modelling of dark current In order to ensure that the correct Kleinpenning model is used, it is important to determine the dominant dark current mechanisms present in the diode. There are numerous mechanisms within the various regions of a photodiode that contribute to the dark current and these combine to give the I–V characteristic of the device. The dynamic resistance, Rd, as a function of applied bias was measured and fitted using three bias voltage and temperature dependent dark current mechanisms, with the three dark current mechanisms assumed to be independent. The dark current mechanisms included in the modelling were diffusion (Diff) current, g–r current (both bulk and surface), and trapassisted tunnelling (TAT). Band to band tunnelling is insignificant in these devices because of the comparatively wide bandgap material used and limited reverse bias applied in this study.
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Fig. 1. I–V characteristic at 80 K for n-on-p HgCdTe photodiode with a cut-off wavelength of 4.3 mm fabricated using the plasma induced junction formation technology.
The total dynamic resistance, Rd 2I=2V⫺1 is given by 1 1 1 1 ⫹ ⫹ : Rd Rd;Diff Rd;g–r Rd;TAT
expressed as [14].
5
Jd;g–r
3.1. Diffusion current Diffusion current is considered as the most important mechanism in n-on-p junction photodiodes and is modelled using a modified Shockley equation for an arbitrary base length, one sided (n ⫹p) diode that accounts for device geometry and interface and surface properties. The expression for diffusion current density is given by s n2i De qV ⫺1 6 exp Jd;Diff ap × q Na te kT where a p is a parameter that accounts for device geometry and surface parameters, Na is the net acceptor concentration on the p side, t e is the minority carrier lifetime and De is the minority carrier diffusion constant. 3.2. Generation–recombination current At lower temperatures the g–r current is more pronounced than diffusion current. The g–r current, due to defects and impurities located in the depletion region and that act as Shockley–Read–Hall (SRH) type g–r centers, can dominate over diffusion current despite the fact that the junction space charge layer is small. This current component is process sensitive and can be reduced by employing different growth and device fabrication processes. The g–r current (including bulk and surface contributions) can be
0 1 qV B 2 sinh C ni wo kT Ps n w kT B 2kT C ⫹ o i o Bÿ 1=2 Cf b @ Vbi ⫺ V A Vbi to AVbi Vbi 7
where Vbi is the built-in voltage, wo is depletion layer width at zero bias, so is surface recombination velocity, P is the length of the perimeter of the diode, and A is the area of the diode. The parameter t o is an effective depletion layer lifep time given by to th te where th 1=s p vth Nt and te 1=s n vth Nt ; and s p and s n are the capture cross section of the trap with density Nt. The function f(b) can be determined by recombination centre energy level (Et), and is given by f b
Z∞ 0
du u ⫹ 2bu ⫹ 1 2
8
where
⫺qV b exp 2kT
E t ⫺ Ei t ⫹ 0:5 ln h ; cosh kT te
9
V is the applied bias, and Ei is the intrinsic Fermi energy level. 3.3. Trap-assisted tunnelling current At low temperatures and under sufficiently high reverse bias tunnelling via SRH centres located within the bandgap may dominate the dark current [15–17]. This trap-assisted
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Fig. 2. RoA product versus reciprocal temperature for n-on-p Hg1⫺xCdxTe photodiode fabricated using the plasma induced junction formation technique. The junction area is 250 × 250 mm2 ; x ⬇ 0:32 and lco 80 K 4:3 mm:
tunnelling current can be written as [17]. ⫺c Jd;TAT qB exp 1=2 Vt
10
where B is a constant related to trapping centre density, and Vt Vbi ⫹ Vd is the total junction potential. The parameter " # 4:3 × 1010 Eg1=2 Eg ⫺ Et 3=2 11 c Na1=2 where Et is the trap level within the bandgap with respect to the valance band. 4. Experiment In the present study the planar n-on-p homojunction
photodiodes were formed on p-type HgCdTe grown by LPE on a CdZnTe substrate and purchased from Fermionics Corporation. The 24 mm thick epilayer had a cut-off wavelength of 4.3 mm at 80 K with a carrier concentration of Na 5:0 × 1015 cm⫺3 and a hole mobility of 430 cm 2 V ⫺1 s ⫺1 at 80 K. The 250 × 250 mm2 diodes were formed using CH4/H2 plasma induced type conversion in a parallel plate RIE reactor through windows in a ZnS mask [11,12]. The HgCdTe surface was passivated using thermally evaporated ZnS. For all 1/f noise and I–V measurements the packaged photodiodes were placed in a liquid nitrogen cooled temperature controlled cryostat with a cooled heat shield. The noise currents were measured using either a custom designed low noise transimpedence amplifier [18] or a commercial Stanford Research Instruments amplifier SR570 in combination with a HP Dynamic Signal Analyser. The custom designed amplifier had significantly better noise performance compared to the SR570 amplifier, but had limited dynamic range. The SR570 amplifier was only used for the higher temperature, higher noise measurements. Current–voltage characteristics were measured using a HP4156A semiconductor parameter analyser. 5. Results and discussion Fig. 1 shows a typical I–V characteristic at 80 K of a photodiode fabricated using the plasma induced junction formation technology. The RoA product for these devices is 4:6 × 107 V cm 2 and is comparable to the best results reported for n-on-p MWIR photodiodes fabricated on LPE grown HgCdTe using ion-implantation and annealing for the formation of the junction [8].
Fig. 3. Measured and calculated dynamic resistance versus reverse bias is shown for temperatures of: (a) 80; (b) 145; and (c) 195 K.
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Table 1 Directly determined material and device parameters x value p mp l co at 80 K Area
0.32 5 × 1015 cm⫺3 430 cm 2 V ⫺1 s ⫺1 4.3 mm 250 × 250 mm2
The measured RoA versus 1000/T is plotted along with the theoretical diffusion limited and g–r limited RoA product variation in Fig. 2. The results show that the diodes are diffusion limited down to ⬃135 K, and g–r limited at lower temperatures. Fig. 3 shows the dependence of dynamic resistance-area product (RdA) on the applied bias voltage for temperatures of 80, 145 and 195 K. The bias at which the tunnelling mechanism begins to become important is located where differential resistance is maximum. The peak in differential resistance is temperature dependent and occurs at higher reverse bias with increasing temperature. At 80 K the tunnelling mechanism starts to dominate at ⫺20 mV. Also shown in Fig. 3 are the results of fitting the dynamic resistance to the analytical expressions for the diffusion, g–r and trap-assisted tunnelling components. All the material, device, and fitting parameters used in this work are summarised in Tables 1 and 2. The minority carrier lifetime in the bulk, t e, and the a p parameter are extracted from high temperature data, and are taken as constant for T ⱕ 195 K: These parameters have only a weak effect in the region where Rd is not strongly diffusion dependent. Calculations of the g–r and trap-assisted tunnelling contributions to Rd assume a trap energy at ⬃0.7Eg. The surface related g–r contribution to Rd is characterised by so, an effective surface recombination velocity for the surface depletion region at the perimeter of the diode. This parameter shows strong temperature dependence increasing by more than an order of magnitude as the temperature changes from 195 to 80 K. The lifetime in the bulk depletion region, t o, obtained from the g–r expression shows a weaker dependence on temperature but exhibits the correct tendency of increasing with decreasing temperature. The trap-assisted tunnelling parameter B is temperature dependent with similar results to those reported in Ref. [17]. Fig. 4 shows the measured noise spectrum of the detector and amplifier combination at a reverse bias of 50 mV with Table 2 Device parameters obtained from fitting diffusion, g ⫺ r and trap-assisted tunnelling currents to measure dynamic resistance characteristics. The value for t e and a p extracted from high temperature data are 500 ns and 2.0, respectively ⫺1
⫺1
⫺2
T (K)
De (cm s )
t o (ns)
so (cm s )
B (cm
80 145 195
246.36 182.12 156.67
600 600 500
3:7 × 105 3:0 × 104 8:0 × 103
6:0 × 1014 6:0 × 1016 1:0 × 1018
2
⫺1
s )
Et 0.7Eg 0.7Eg 0.7Eg
Fig. 4. Noise spectra at ⫺50 mV of the detector and amplifier combination from a typical n-on-p HgCdTe diode with x ⬇ 0:32; p 5:0 × 1015 cm⫺3 ; and diode size of 250 × 250 mm2 :
temperature as a parameter. The 1/f detector noise at 1 Hz was extracted by subtracting the amplifier noise at 1 Hz and the detector base level noise (shot noise) determined from the measured high frequency noise values. The results of this analysis process are presented in Fig. 5 for measurements at 80, 110, 120, 130, 145, 165 and 180 K as a function of bias. The increase in 1/f noise with increasing reverse bias is significantly reduced for higher reverse biases. An attempt was made to fit the Kleinpenning diffusion model to the experimental data at each temperature. The minority carrier lifetime, t e, was extracted from the modelling of the dynamic resistance, the carrier mobility was calculated at each temperature using the expression developed by Rosbeck et al. [19], and diffusion constant was calculated using the Einstein temperature dependence relation. A reasonable fit is obtained to the Kleinpenning diffusion model at temperatures ⱖ130 K with the best fit obtained
Fig. 5. The 1/f noise current at 1 Hz various temperatures and different biases. The Kleinpenning diffusion model is fitted to the experimentally measured 1/f noise current at different temperatures for the fabricated photodiode. × T 80 K: X T 110 K; aH 1:0 × 10⫺4 : A T 120 K; aH 1:0 × 10⫺4 : P T 130 K; aH 1:7 × 10⫺4 : O T 145 K; aH 1:8 × 10⫺3 : W T 165 K; aH 2:0 × 10⫺3 : B T 180 K; aH 2:25 × 10⫺3 :
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Fig. 6. Temperature dependence of dark current at ⫺50 mV bias. The activation energy is calculated from the straight line fit to the experimental data for the fabricated diode. This activation energy obtained is close to the bandgap of the material as expected in the diffusion-limited regime.
for the 130 K data. At temperatures above 130 K, the diffusion model does not fit well for higher biases, even though the diode is operating in the diffusion limited dark currentregime. The Hooge parameter extracted from the fitted diffusion model is strongly temperature dependent and increases monotonically from 1:0 × 10⫺4 and 2:25 × 10⫺3 Hz⫺1 as the temperature is increased from 110 to 180 K. Figs. 6 and 7 show the temperature dependence of dark current and 1/f noise current at 1 Hz, respectively; both measured at a reverse bias of ⫺50 mV for the fabricated photodiode. For higher temperatures, the dark current gives an activation energy of Ea ⬇ 0:262 eV (a value close to the optically determined bandgap) which indicates that the diode is diffusion limited for temperatures greater than ⬃130 K for ⫺50 mV reverse bias. The activation energy for the 1/f noise at 1 Hz in this diffusion limited regime is 0.196 eV, clearly indicating that the 1/f noise is not proportional to diffusion current. The ratio of 1/f noise activation energy to the bandgap determined from the dark current activation energy is 0.7, similar to the ratio reported by Bajaj et al. [8]. This value has been correlated to trapping centres associated with Hg vacancies. The present work confirms that the 1/f noise mechanism in high quality HgCdTe diodes is not determined solely by the diffusion current. Furthermore, the result indicates that the noise source of 1/f noise is independent of the method used to form the n-on-p junction since the diodes in the present study were fabricated using a completely different process to the ion implantation techniques used in previous studies.
Fig. 7. Temperature dependence of 1/f noise current at 1 Hz and ⫺50 mV bias. The activation energy is calculated from the straight line fit to the experimental data for the fabricated diode. The value is approximately 0.7Eg.
6. Conclusions A comprehensive and systematic 1/f noise characterisation has been performed on planar HgCdTe MWIR photodiodes fabricated using a novel RIE based junction formation technology. The results of this study verify that the 1/f noise current in HgCdTe has an activation energy of ⬃0.7Eg when the diode dark current is diffusion limited, indicating that the 1/f noise is not solely related to the diffusion current. Furthermore, detailed modelling of dynamic resistance versus bias has been carried out as a function of temperature where the calculated dark current mechanisms are diffusion, g–r and trap-assisted tunnelling. The modelling has shown that the different dark currents are dominant in similar temperature ranges as observed for ion implanted devices. These results are in agreement with findings of other workers but were obtained using a completely different junction formation technology. These results show that the new RIE based junction formation technology can produce high performance HgCdTe photodiodes, while employing a simpler and potentially higher yield fabrication process.
Acknowledgements The work described herein was supported by the Australian Research Council (ARC).
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References [1] J.F. Siliquini, J.M. Dell, C.A. Musca, L. Faraone, Scanning laser microscopy of reactive ion etching induced n-type conversion in vacancy-doped p-type HgCdTe, Appl. Phys. Lett. 70 (1997) 3443– 3445. [2] J.L. Elkind, G.J. Orloff, P.B. Smith, Method for via formation and type conversion in group II and group VI materials, US Patent 5318666, 1994. [3] E. Belas, J. Franc, A. Toth, P. Moravec, R. Grill, H. Sitter, P. Ho¨schl, Type conversion of p-(HgCd)Te using H2/CH4 and Ar reactive ion etching, Semicon. Sci. Technol. 11 (1996) 1116–1120. [4] S.P. Tobin, S. Iwasa, T.J. Tredwell, 1/f noise in HgCdTe photodiodes, IEEE Trans. Electron. Devices ED-27 (1980) 43–47. [5] S.T. Hsu, Surface states related 1/f noise in MOS transistors, SolidState Electron. 13 (1970) 1451–1459. [6] H.K. Chung, M.A. Rosenberg, P.H. Zimmermann, Origin of 1/f noise observed in Hg0.7Cd0.3Te variable area photodiode arrays, J. Vac. Sci. Technol., A 3 (1985) 189–191. [7] J. Bajaj, G.M. Williams, N.H. Sheng, M. Hinnrichs, D.T. Cheung, J.P. Rode, W.E. Tennant, Excess (1/f ) noise in Hg0.7Cd0.3Te p–n junctions, J. Vac. Sci. Technol., A 3 (1985) 192–194. [8] J. Bajaj, E.R. Blazejewski, G.M. Williams, R.E. DeWames, M. Brown, Noise (1/f ) and dark currents in midwavelength infrared PACE-I HgCdTe photodiodes, J. Vac. Sci. Technol., B 10 (1992) 1617–1625. [9] C.E. Jones, V. Nair, J. Lindquist, Effects of deep-level defects in Hg1⫺xCdxTe provided by DLTS, J. Vac. Sci. Technol. 21 (1982) 187–190. [10] T.G.M. Kleinpenning, 1/f noise in p–n junction diodes, J. Vac. Sci. Technol., A 3 (1985) 176–182.
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[11] M.H. Rais, C.A. Musca, J.M. Dell, J. Antoszewski, B.D. Nener, L. Faraone, Low frequency noise in HgCdTe 3–5 micron IR photovoltaic detectors at elevated temperatures, in: IEEE Proc. Conf. Optoelectronic Microelectronic Mats and Devs, Perth, WA, 1999, pp. 369– 372. [12] M.H. Rais, C.A. Musca, J.M. Dell, J. Antoszewski, B.D. Nener, L. Faraone, 1/f noise in HgCdTe MWIR photovoltaic detectors fabricated using a Novel Junction Formation Technology, in: 29th European Solid-State Device Research Conference, ESSDERC’99, Leuven, Belgium, 13–15 September, 1999. [13] F.N. Hooge, 1/f noise sources, in: C. Claeys, G. Bosman, E. Simoen (Eds.), Part 2, Noise in Semiconductor Materials and Devices, IMEC, Leuven, Belgium, December 14–16, 1993, pp. 1–21. [14] A. Ajisawa, N. Oda, Improvements in HgCdTe diode characteristics by low temperature post-implantation annealing, J. Electron. Mater. 24 (1995) 1105–1111. [15] A. Unikovsky, Y. Nemirovsky, Trap-assisted tunneling in mercury cadmium telluride photodiodes, Appl. Phys. Lett. 61 (1992) 330–332. [16] Y. Nemirovsky, R. Fastow, M. Meyassed, A. Unikovsky, Trapping effects in HgCdTe, J. Vac. Sci. Technol., B 9 (1991) 1829–1839. [17] Y. Nemirovsky, D. Rosenfeld, R. Adar, A. Kornfeld, Tunneling and dark currents in HgCDTe photodiodes, J. Vac. Sci. Technol., A 7 (1989) 528–535. [18] R.M. Howard, Ultralow noise high gain transimpedance amplifier for characterizing the low frequency noise of infrared detectors, Rev. Sci. Instrum. 70 (1999) 1860–1867. [19] J.P. Rosbeck, R.E. Starr, S.L. Price, K.J. Riely, Background and temperature dependent current–voltage characteristic of HgCdTe photodiodes, J. Appl. Phys. 53 (1982) 6430–6440.
HgCdTe photovoltaic detectors fabricated using a new junction formation technology M.H. Rais, C.A. Musca, J.M. Dell*, J. Antoszewski, B.D. Nener, L. Faraone Department of Electrical and Electronic Engineering, The University of Western Australia, Nedlands, WA 6907, Australia Received 26 November 1999; accepted 30 December 1999
Abstract The current–voltage characteristics measured over a wide temperature range are reported for HgCdTe mid-wavelength infrared n-on-p photodiodes fabricated using a novel junction formation technology. The planar homojunction device junctions were formed on LPE grown vacancy doped HgCdTe using a reactive ion etching (RIE) plasma induced conversion process. The zero bias dynamic resistance–junction area product, RoA, was 4:6 × 107 V cm 2 at 80 K and is comparable to the best planar diodes reported using conventional ion implantation junction formation technology. Arrhenius plots of RoA exhibit an activation energy equal to the bandgap, Eg, and show that the diodes are diffusion limited for temperatures ⱖ135 K. A series of temperature dependent 1/f noise measurements were performed, indicating that the activation energy for 1/f noise in the region where the diodes are diffusion limited is 0.7Eg. Energies close to this value have previously been associated with Hg vacancies in HgCdTe. These results are similar to those obtained from high quality HgCdTe photodiodes fabricated using mature ion implantation technology. However, the plasma based technology used in this work is significantly less complex and does not require any high temperature annealing steps. 䉷 2000 Elsevier Science Ltd. All rights reserved. Keywords: HgCdTe; Photovoltaic; RoA product; 1/f noise; Reactive ion etching; Semiconductors
1. Introduction There are number of different noise sources in a typical infrared (IR) photodiode, some of which are fundamental, while others are strongly influenced by device processing. In particular, 1/f noise is an important device parameter for IR photodiodes, often representing the limiting factor for twodimensional staring arrays, and appears to be related to processing induced effects. For any new junction formation technology, it is therefore important to investigate 1/f noise processes to ensure that the technology is compatible with high performance devices. A detailed study of the noise behaviour of mercury cadmium telluride (HgCdTe) IR photodiodes fabricated using a novel reactive ion etching (RIE) based type conversion process for the formation of the junction has been undertaken. As part of this work, detailed dark current and 1/f noise measurements have been made. The diodes studied in this work were fabricated on vacancy doped p-type HgCdTe grown by LPE on CdZnTe. The n-onp junctions were formed by exposing the HgCdTe surface to a plasma in a parallel plate RIE reactor using a ZnS mask. * Corresponding author. Tel.: ⫹ 61-8-9380-3787; fax: ⫹ 61-8-93801095. E-mail address: [email protected] (J.M. Dell).
This results in type conversion of the HgCdTe in the exposed regions [1]. The junctions were passivated using thermally deposited ZnS. In contrast to other reported RIE type conversion processes for the formation of n–p junctions [2,3], the work presented here uses a single ZnS layer which acts as both mask and passivant. The RIE junction formation process does not damage the ZnS masking layer, and hence the ZnS mask does not have to be removed as in conventional processes. As a result the n-on-p junctions at the surface are never exposed to atmosphere. Exposure of the junction is known in increase surface leakage currents. In addition, no high temperature anneal is required after junction formation to repair the crystal lattice.
2. Excess noise in HgCdTe diodes At low frequencies, the most important source of noise in HgCdTe IR photodiodes under reverse bias is 1/f or excess noise. This noise component is distinguished from other noise types by its unique power spectral distribution that is proportional to 1/f a , with a generally close to unity. While at zero bias voltage, photodiodes display no 1/f noise [4], under slight reverse bias, a typical operating condition used to increase the dynamic resistance of
0026-2692/00/$ - see front matter 䉷 2000 Elsevier Science Ltd. All rights reserved. PII: S0026-269 2(00)00028-8
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photodiodes in HgCdTe infrared focal plane arrays (IRFPAs), 1/f noise is observed, and often limits IRFPA performance. In addition it has been shown that 1/f noise is very sensitive to processing conditions and may give an insight to both optimal process conditions and the mechanisms involved in the type conversion process. Several studies have been conducted into 1/f noise mechanisms in HgCdTe photodiodes. Tobin et al. [4] reported 1/f noise behaviour in HgCdTe photodiodes fabricated by boron implantation into bulk grown material. Their results indicated that for the devices tested 1/f noise was directly proportional to surface leakage current rather than diffusion or photocurrent. At high temperatures where the dark current was diffusion limited, the 1/f noise continued to follow the dependence of the surface generation current. The investigators suggested that the 1/f noise was a result of modulation of the surface generation current by fluctuations in the surface potential. This mechanism was also proposed by Hsu [5]. Furthermore Chung et al. [6] found the same correlation with generation–recombination (g–r) current dependence and also associated the 1/f noise current with the surface. Bajaj et al. [7,8] have reported 1/f noise results on n-on-p junctions also formed by ion implantation technology but with more mature passivation technology. In contrast to Tobin et al., the 1/f noise for their devices did not originate from a perimeter source but scaled with junction area and was therefore identified as a bulk phenomenon. The strong bias dependence observed by Bajaj et al. further localized the noise source to the depletion region since this is the region across which most of the bias voltage appears. From temperature dependent measurements they extracted an activation energy of 0.72Eg for the 1/f noise, a value that has been associated with Hg vacancies in HgCdTe [9]. At low temperatures the 1/f noise for their diodes was temperature independent, indicating that a tunnelling mechanisms was dominant. Finally they failed to get good fit to their data using Kleinpenning diffusion model [10]. Both the Tobin and Bajaj results indicate that the 1/f noise process is a sensitive parameter and an important indicator of process technology performance. An initial study of 1/f noise in n-on-p Hg1⫺xCdxTe x ⬇ 0:32 diodes fabricated using the plasma type conversion process on vacancy doped p-type HgCdTe grown by LPE on a CdZnTe substrate and passivated using ZnS has been reported [11,12]. This paper presents a more detailed and comprehensive study on the temperature dependence of 1/f noise for the temperature range 80–180 K. The 1/f noise current for the diodes presented here is not proportional to dark current, even when the dark current is diffusion dominated, and exhibits an activation energy of 0.7Eg. The results presented in this work indicate that for temperatures below ⬃120 K, the 1/f noise current becomes approximately independent of temperature. These results are similar to those reported by Bajaj et al. [8] for state-of-the-art ion implanted MWIR photodiodes.
A theoretical model for 1/f noise in homojunction diodes for regimes in which the diode current is either generation– recombination dominated or diffusion dominated has been developed by Kleinpenning [10]. For one-sided long-base diodes operating in the diffusion-dominated regime, the power spectral density of the 1/f noise is given by aH qV qV SI qIo exp ⫺1⫺ 1 4f t kT kT where a H is the Hooge parameter, t is the minority carrier lifetime in the base region of the diode, Io is the diode saturation current, and V is the applied bias. The saturation current in this regime is given by qAn2i D 1=2 2 Io Na t where D is the diffusion constant and ni is the intrinsic carrier concentration. Assuming that the Hooge parameter is relatively temperature insensitive (primarily dependent on mobility) [13], the temperature dependence of this process is dominated by the n2i term for reverse biased diodes, and should therefore exhibit an activation energy of Eg. For the g–r dominated regime, the 1/f noise power spectral density is given by ! 2 2 aH qV ⫺qV ⫺ 1 exp 3 SI qIo exp 2kT 2kT 3f tj where t j is the lifetime in the junction depletion region and the other parameters are as before. The saturation current in the g–r limited regime is given by Io qA
ni Wj tj
4
where Wj is the depletion layer width. In this work, we have attempted to fit these theoretical models to our data to allow extraction of the Hooge parameter. 3. Modelling of dark current In order to ensure that the correct Kleinpenning model is used, it is important to determine the dominant dark current mechanisms present in the diode. There are numerous mechanisms within the various regions of a photodiode that contribute to the dark current and these combine to give the I–V characteristic of the device. The dynamic resistance, Rd, as a function of applied bias was measured and fitted using three bias voltage and temperature dependent dark current mechanisms, with the three dark current mechanisms assumed to be independent. The dark current mechanisms included in the modelling were diffusion (Diff) current, g–r current (both bulk and surface), and trapassisted tunnelling (TAT). Band to band tunnelling is insignificant in these devices because of the comparatively wide bandgap material used and limited reverse bias applied in this study.
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547
Fig. 1. I–V characteristic at 80 K for n-on-p HgCdTe photodiode with a cut-off wavelength of 4.3 mm fabricated using the plasma induced junction formation technology.
The total dynamic resistance, Rd 2I=2V⫺1 is given by 1 1 1 1 ⫹ ⫹ : Rd Rd;Diff Rd;g–r Rd;TAT
expressed as [14].
5
Jd;g–r
3.1. Diffusion current Diffusion current is considered as the most important mechanism in n-on-p junction photodiodes and is modelled using a modified Shockley equation for an arbitrary base length, one sided (n ⫹p) diode that accounts for device geometry and interface and surface properties. The expression for diffusion current density is given by s n2i De qV ⫺1 6 exp Jd;Diff ap × q Na te kT where a p is a parameter that accounts for device geometry and surface parameters, Na is the net acceptor concentration on the p side, t e is the minority carrier lifetime and De is the minority carrier diffusion constant. 3.2. Generation–recombination current At lower temperatures the g–r current is more pronounced than diffusion current. The g–r current, due to defects and impurities located in the depletion region and that act as Shockley–Read–Hall (SRH) type g–r centers, can dominate over diffusion current despite the fact that the junction space charge layer is small. This current component is process sensitive and can be reduced by employing different growth and device fabrication processes. The g–r current (including bulk and surface contributions) can be
0 1 qV B 2 sinh C ni wo kT Ps n w kT B 2kT C ⫹ o i o Bÿ 1=2 Cf b @ Vbi ⫺ V A Vbi to AVbi Vbi 7
where Vbi is the built-in voltage, wo is depletion layer width at zero bias, so is surface recombination velocity, P is the length of the perimeter of the diode, and A is the area of the diode. The parameter t o is an effective depletion layer lifep time given by to th te where th 1=s p vth Nt and te 1=s n vth Nt ; and s p and s n are the capture cross section of the trap with density Nt. The function f(b) can be determined by recombination centre energy level (Et), and is given by f b
Z∞ 0
du u ⫹ 2bu ⫹ 1 2
8
where
⫺qV b exp 2kT
E t ⫺ Ei t ⫹ 0:5 ln h ; cosh kT te
9
V is the applied bias, and Ei is the intrinsic Fermi energy level. 3.3. Trap-assisted tunnelling current At low temperatures and under sufficiently high reverse bias tunnelling via SRH centres located within the bandgap may dominate the dark current [15–17]. This trap-assisted
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Fig. 2. RoA product versus reciprocal temperature for n-on-p Hg1⫺xCdxTe photodiode fabricated using the plasma induced junction formation technique. The junction area is 250 × 250 mm2 ; x ⬇ 0:32 and lco 80 K 4:3 mm:
tunnelling current can be written as [17]. ⫺c Jd;TAT qB exp 1=2 Vt
10
where B is a constant related to trapping centre density, and Vt Vbi ⫹ Vd is the total junction potential. The parameter " # 4:3 × 1010 Eg1=2 Eg ⫺ Et 3=2 11 c Na1=2 where Et is the trap level within the bandgap with respect to the valance band. 4. Experiment In the present study the planar n-on-p homojunction
photodiodes were formed on p-type HgCdTe grown by LPE on a CdZnTe substrate and purchased from Fermionics Corporation. The 24 mm thick epilayer had a cut-off wavelength of 4.3 mm at 80 K with a carrier concentration of Na 5:0 × 1015 cm⫺3 and a hole mobility of 430 cm 2 V ⫺1 s ⫺1 at 80 K. The 250 × 250 mm2 diodes were formed using CH4/H2 plasma induced type conversion in a parallel plate RIE reactor through windows in a ZnS mask [11,12]. The HgCdTe surface was passivated using thermally evaporated ZnS. For all 1/f noise and I–V measurements the packaged photodiodes were placed in a liquid nitrogen cooled temperature controlled cryostat with a cooled heat shield. The noise currents were measured using either a custom designed low noise transimpedence amplifier [18] or a commercial Stanford Research Instruments amplifier SR570 in combination with a HP Dynamic Signal Analyser. The custom designed amplifier had significantly better noise performance compared to the SR570 amplifier, but had limited dynamic range. The SR570 amplifier was only used for the higher temperature, higher noise measurements. Current–voltage characteristics were measured using a HP4156A semiconductor parameter analyser. 5. Results and discussion Fig. 1 shows a typical I–V characteristic at 80 K of a photodiode fabricated using the plasma induced junction formation technology. The RoA product for these devices is 4:6 × 107 V cm 2 and is comparable to the best results reported for n-on-p MWIR photodiodes fabricated on LPE grown HgCdTe using ion-implantation and annealing for the formation of the junction [8].
Fig. 3. Measured and calculated dynamic resistance versus reverse bias is shown for temperatures of: (a) 80; (b) 145; and (c) 195 K.
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Table 1 Directly determined material and device parameters x value p mp l co at 80 K Area
0.32 5 × 1015 cm⫺3 430 cm 2 V ⫺1 s ⫺1 4.3 mm 250 × 250 mm2
The measured RoA versus 1000/T is plotted along with the theoretical diffusion limited and g–r limited RoA product variation in Fig. 2. The results show that the diodes are diffusion limited down to ⬃135 K, and g–r limited at lower temperatures. Fig. 3 shows the dependence of dynamic resistance-area product (RdA) on the applied bias voltage for temperatures of 80, 145 and 195 K. The bias at which the tunnelling mechanism begins to become important is located where differential resistance is maximum. The peak in differential resistance is temperature dependent and occurs at higher reverse bias with increasing temperature. At 80 K the tunnelling mechanism starts to dominate at ⫺20 mV. Also shown in Fig. 3 are the results of fitting the dynamic resistance to the analytical expressions for the diffusion, g–r and trap-assisted tunnelling components. All the material, device, and fitting parameters used in this work are summarised in Tables 1 and 2. The minority carrier lifetime in the bulk, t e, and the a p parameter are extracted from high temperature data, and are taken as constant for T ⱕ 195 K: These parameters have only a weak effect in the region where Rd is not strongly diffusion dependent. Calculations of the g–r and trap-assisted tunnelling contributions to Rd assume a trap energy at ⬃0.7Eg. The surface related g–r contribution to Rd is characterised by so, an effective surface recombination velocity for the surface depletion region at the perimeter of the diode. This parameter shows strong temperature dependence increasing by more than an order of magnitude as the temperature changes from 195 to 80 K. The lifetime in the bulk depletion region, t o, obtained from the g–r expression shows a weaker dependence on temperature but exhibits the correct tendency of increasing with decreasing temperature. The trap-assisted tunnelling parameter B is temperature dependent with similar results to those reported in Ref. [17]. Fig. 4 shows the measured noise spectrum of the detector and amplifier combination at a reverse bias of 50 mV with Table 2 Device parameters obtained from fitting diffusion, g ⫺ r and trap-assisted tunnelling currents to measure dynamic resistance characteristics. The value for t e and a p extracted from high temperature data are 500 ns and 2.0, respectively ⫺1
⫺1
⫺2
T (K)
De (cm s )
t o (ns)
so (cm s )
B (cm
80 145 195
246.36 182.12 156.67
600 600 500
3:7 × 105 3:0 × 104 8:0 × 103
6:0 × 1014 6:0 × 1016 1:0 × 1018
2
⫺1
s )
Et 0.7Eg 0.7Eg 0.7Eg
Fig. 4. Noise spectra at ⫺50 mV of the detector and amplifier combination from a typical n-on-p HgCdTe diode with x ⬇ 0:32; p 5:0 × 1015 cm⫺3 ; and diode size of 250 × 250 mm2 :
temperature as a parameter. The 1/f detector noise at 1 Hz was extracted by subtracting the amplifier noise at 1 Hz and the detector base level noise (shot noise) determined from the measured high frequency noise values. The results of this analysis process are presented in Fig. 5 for measurements at 80, 110, 120, 130, 145, 165 and 180 K as a function of bias. The increase in 1/f noise with increasing reverse bias is significantly reduced for higher reverse biases. An attempt was made to fit the Kleinpenning diffusion model to the experimental data at each temperature. The minority carrier lifetime, t e, was extracted from the modelling of the dynamic resistance, the carrier mobility was calculated at each temperature using the expression developed by Rosbeck et al. [19], and diffusion constant was calculated using the Einstein temperature dependence relation. A reasonable fit is obtained to the Kleinpenning diffusion model at temperatures ⱖ130 K with the best fit obtained
Fig. 5. The 1/f noise current at 1 Hz various temperatures and different biases. The Kleinpenning diffusion model is fitted to the experimentally measured 1/f noise current at different temperatures for the fabricated photodiode. × T 80 K: X T 110 K; aH 1:0 × 10⫺4 : A T 120 K; aH 1:0 × 10⫺4 : P T 130 K; aH 1:7 × 10⫺4 : O T 145 K; aH 1:8 × 10⫺3 : W T 165 K; aH 2:0 × 10⫺3 : B T 180 K; aH 2:25 × 10⫺3 :
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Fig. 6. Temperature dependence of dark current at ⫺50 mV bias. The activation energy is calculated from the straight line fit to the experimental data for the fabricated diode. This activation energy obtained is close to the bandgap of the material as expected in the diffusion-limited regime.
for the 130 K data. At temperatures above 130 K, the diffusion model does not fit well for higher biases, even though the diode is operating in the diffusion limited dark currentregime. The Hooge parameter extracted from the fitted diffusion model is strongly temperature dependent and increases monotonically from 1:0 × 10⫺4 and 2:25 × 10⫺3 Hz⫺1 as the temperature is increased from 110 to 180 K. Figs. 6 and 7 show the temperature dependence of dark current and 1/f noise current at 1 Hz, respectively; both measured at a reverse bias of ⫺50 mV for the fabricated photodiode. For higher temperatures, the dark current gives an activation energy of Ea ⬇ 0:262 eV (a value close to the optically determined bandgap) which indicates that the diode is diffusion limited for temperatures greater than ⬃130 K for ⫺50 mV reverse bias. The activation energy for the 1/f noise at 1 Hz in this diffusion limited regime is 0.196 eV, clearly indicating that the 1/f noise is not proportional to diffusion current. The ratio of 1/f noise activation energy to the bandgap determined from the dark current activation energy is 0.7, similar to the ratio reported by Bajaj et al. [8]. This value has been correlated to trapping centres associated with Hg vacancies. The present work confirms that the 1/f noise mechanism in high quality HgCdTe diodes is not determined solely by the diffusion current. Furthermore, the result indicates that the noise source of 1/f noise is independent of the method used to form the n-on-p junction since the diodes in the present study were fabricated using a completely different process to the ion implantation techniques used in previous studies.
Fig. 7. Temperature dependence of 1/f noise current at 1 Hz and ⫺50 mV bias. The activation energy is calculated from the straight line fit to the experimental data for the fabricated diode. The value is approximately 0.7Eg.
6. Conclusions A comprehensive and systematic 1/f noise characterisation has been performed on planar HgCdTe MWIR photodiodes fabricated using a novel RIE based junction formation technology. The results of this study verify that the 1/f noise current in HgCdTe has an activation energy of ⬃0.7Eg when the diode dark current is diffusion limited, indicating that the 1/f noise is not solely related to the diffusion current. Furthermore, detailed modelling of dynamic resistance versus bias has been carried out as a function of temperature where the calculated dark current mechanisms are diffusion, g–r and trap-assisted tunnelling. The modelling has shown that the different dark currents are dominant in similar temperature ranges as observed for ion implanted devices. These results are in agreement with findings of other workers but were obtained using a completely different junction formation technology. These results show that the new RIE based junction formation technology can produce high performance HgCdTe photodiodes, while employing a simpler and potentially higher yield fabrication process.
Acknowledgements The work described herein was supported by the Australian Research Council (ARC).
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