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A new kind of quasi-ohmic metallization in semi-insulating GaAs: Study of electrical characteristics
ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 607 (2009) 132–134
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
A new kind of quasi-ohmic metallization in semi-insulating GaAs: Study of electrical characteristics Frantisˇek Dubecky´ a, Bohumı´r Zat’ko a,, Pavel Hubı´k b, Enos Gombia c, Pavol Boha´cˇek a, Jozef Huran a, M. Seka´cˇova´ a a
´ cesta 9, SK-841 04 Bratislava, Slovakia ´ bravska Institute of Electrical Engineering, Slovak Academy of Sciences, Du ´ 10, CZ-162 00 Praha 6, Czech Republic Institute of Physics, v.v.i., Academy of Sciences of the Czech Republic, Cukrovarnicka c IMEM—CNR, Area delle Scienze, 37/A Fontanini, Parma I-43010, Italy b
a r t i c l e in f o
a b s t r a c t
Available online 28 March 2009
The present work describes current–voltage (I–V) characteristics of radiation detector structures based on semi-insulating (SI) GaAs obtained with new types of metallizations used to form quasi-ohmic contacts. Three different metals (In, Gd and Mg) having lower work function in comparison with the standard AuGeNi eutectic alloy, were used. These metals are expected to form a band bending at the M–S interface which should give rise to a blocking barrier for holes (‘‘minority’’ carriers in SI GaAs). Such contacts, which could effectively replace standard alloyed N+ ‘‘ohmic’’ contact, show an unusual electrical charge transport as deduced from the measured I–V characteristics. Pulse-height spectra of 241 Am radionuclide source detected by the structures are also reported. & 2009 Elsevier B.V. All rights reserved.
Keywords: GaAs Semi-insulating Metal–semiconductor contact Schottky barrier Work function
1. Introduction The crucial tasks relating to the radiation detector performances mainly concern the electrode metallization [1], the overall electrodes technology and topology (blocking Schottky or ohmic contacts) of the structure and finally the surface passivation. A special emphasis should be devoted to the ohmic electrode technology following the results of Alietti et al. [2]. They introduced a new ‘‘non-alloyed’’ ohmic contact using implantation, which improved performance of semi-insulating (SI) GaAs radiation detector. Another possible solution is represented by the formation of a metal–semiconductor Schottky barrier using a metal with low enough work function in comparison with the semiconductor (GaAs work function is about 4.5 eV [3]). Such a contact should block injected holes from anode and should behave as a noninjecting quasi-ohmic contact. We fabricated SI GaAs detectors using as ohmic contacts In, Mg and Gd with work functions of 4.12, 3.68 and 3.10 eV, respectively, and standard non-alloyed eutectic AuGeNi/Au as a reference contact. Our previous work [4] was devoted to the spectrometric and noise characterization of the new structures. The aim of this paper is to report on the complementary measurements and on the more detailed study of electrical
Corresponding author.
E-mail address: [email protected] (B. Zat’ko). 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.03.191
characteristics of the detector structures with the new ‘‘quasiohmic’’ metallization.
2. Technology and experimental details The radiation detector structures were prepared from the bulk undoped SI GaAs 200 wafer grown by the vertical gradient freeze (VGF) method (producer CMK Ltd., Zˇarnovica, Slovakia) with (1 0 0) crystallographic orientation and dislocation density of about 3000 cm2. The wafer was polished by the producer from both sides down to (250710) mm. Resistivity and the Hall mobility measured by the van der Pauw method at 295 K in our laboratory give values of 1.82 107 O cm and 7062 cm2/Vs, respectively. The measured values fulfill key requirements for a ‘‘detector-grade’’ bulk SI GaAs material [5]. The wafer was segmented into four samples. Square Schottky electrodes of Ti/Pt/Au (10/40/70 nm) with four different sizes (0.75, 0.50, 0.30, 0.20 mm) were formed using photolithography masking onto one side (top) of each sample. Just before evaporation, the surface oxides were removed in a solution of HCl:H2O ¼ 1:1 at RT (300 K) for 30 s. The quasi-ohmic (back) side of samples was fully covered by a double layer of In/Au, Mg/Au or Gd/Au (50/70 nm). AuGeNi/Au (50/70 nm) eutectic alloy was deposited on the back side of the reference sample. The metal contacts were evaporated in a dry high-vacuum system. I–V characteristics of the radiation detector structures were measured at RT in the dark using an electrically shielded probe
ARTICLE IN PRESS ´ et al. / Nuclear Instruments and Methods in Physics Research A 607 (2009) 132–134 F. Dubecky
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Fig. 1. Reverse and forward I–V characteristics of SI GaAs detector structures in log–log and a log–lin scale (inset): (i) Ti/Pt/Au–AuGeNi/Au, (ii) Ti/Pt/Au–In/Au, (iii) Ti/Pt/ Au–Mg/Au, and (iv) Ti/Pt/Au–Gd/Au where reproducibility of the measurement is illustrated by plotting of I–V characteristics of 2 different detector structures.
station using a Keithley 237 source controlled by a personal computer. The reverse branches correspond to the negative bias polarity applied to the top Ti/Pt/Au electrode.
3. Results and discussion I–V characteristics of the fabricated detectors measured at RT in the dark are shown in Fig. 1. The solid straight lines correspond to the linear-ohmic transport calculated from the SI GaAs bulk conductivity measurements. It can be noted that the reverse characteristics of detectors are similar. However, some quantitative differences are clearly observable. The linear part of all characteristics in the voltage region under 0.1 V is about 1.5–4 times lower than that correspond to the calculated ohmic current. The deviation is larger for the contacts made with the new metals. Moreover, it has been observed that the first sub-linear part of the reverse characteristics between 0.1 and 10 V corresponds to the value of the saturation current Is of the thermionic field emission. The second sub-linear part at voltages over 10 V, which represents the detector operation bias region, is characterized by dominant transport mechanisms which have not been yet understood. The same is true for the measured forward characteristics. As for the forward characteristic of the reference detector with AuGeNi metallization it can be seen that, after an initial linear part, a super-linear injection region starts reaching the linear-ohmic
current limit (the solid straight line). Such a tendency towards the electrical charge injection is not observed for In, Mg and Gd electrodes. In fact, in the voltage range 0.1–10 V, the detectors prepared with these contacts show always a sub-linear behaviour, which follows the initial linear characteristic, similarly to the reverse I–V branch. Finally, for voltage values over about 100 V the forward current is almost identical for all structures. These observations are in contradiction with expectations. Indeed detectors with In, Mg and Gd contacts should exhibit an injection behaviour, i.e. faster rising of the forward current coupled with rather lower breakdown voltage in comparison with the reference AuGeNi electrode due to the lower work function of used metals. If we calculate current density considering different areas of the blocking (0.25 mm2) and quasi-ohmic (10 mm2) contacts the effect became still more evident. Hence, if the quasi-ohmic electrode operates as a blocking (forward direction) through the full area (first approach consideration) the value of the current density will be up to 40 lower compared to the reverse direction at a bias 0.1–10 V and nearly equivalent, or even lower in operation bias voltage region 4 10 V. Detectors were connected to the spectrometric readout chain consisted of charge-sensitive preamplifier based on CREMAT CR 101D, shaping amplifier ORTEC 572, high-voltage supply ORTEC 459, analog-to-digital converter ORTEC 800 with a multichannel analyser M2D. Pulse-height spectra of the 241Am radionuclide source obtained with the detectors having two different Schottky
ARTICLE IN PRESS ´ et al. / Nuclear Instruments and Methods in Physics Research A 607 (2009) 132–134 F. Dubecky
134
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the detector with In/Au metallization. Further evaluations and studies of the reported results are in progress.
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Study of electrical characteristics of a new kind of quasi-ohmic contact metallization is presented. The new contact uses metals with low work function. In, Gd and Mg were employed to form such new contacts in SI GaAs detectors. Their characteristics were compared with a standard AuGeNi metallization. It has been observed that, unlike the AuGeNi metallization, the forward current in the voltage range of 0.1–10 V is lower than that of the reverse biased small area Ti/Pt/Au Schottky barrier. However, the dominant mechanism responsible of this effect remains still unclear. The influence of the metallization on the detected 241Am pulse-height spectra are also reported. It is shown that the best value of energy resolution is observed for detectors with In/Au metallization. In order to better understand the above effects more detailed theoretical and experimental investigations will be carried out.
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Acknowledgements
1 241
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Fig. 2. Measured spectra of g-ray generated by 241Am radioisotope for square pads 0.5 0.5 mm2 (a) and 0.3 0.3 mm2 (b), respectively.
contact areas are shown in Fig. 2. As it can be seen the spectra are quite similar; however, the best value of energy resolution of 5.6 keV in FWHM of 241Am photopeak (59.5 keV) is observed for
This work was partially supported by the Slovak Grant Agency for Science through the Grant no. 2/7170/27, the Agency for Promotion of Research and Development under the Contracts nos. APVV–99–P06305 and APVV–0459–06, and by the Slovak Association EURATOM/EC and also by Grant Agency of the ASCR (Project no. IAA1010404).
References [1] [2] [3] [4] [5]
F. Dubecky´, et al., Nucl. Instr. and Meth. A 576 (2007) 87. M. Alietti, et al., Nucl. Instr. and Meth. A 362 (1995) 344. J. Massies, et al., J. Vac. Sci. Technol. 16 (1979) 1244. P. Boha´cˇek, et al., Nucl. Instr. and Meth. A 591 (2008) 105. F. Dubecky´, et al., Nucl. Instr. and Meth. A 576 (2007) 27.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
A new kind of quasi-ohmic metallization in semi-insulating GaAs: Study of electrical characteristics Frantisˇek Dubecky´ a, Bohumı´r Zat’ko a,, Pavel Hubı´k b, Enos Gombia c, Pavol Boha´cˇek a, Jozef Huran a, M. Seka´cˇova´ a a
´ cesta 9, SK-841 04 Bratislava, Slovakia ´ bravska Institute of Electrical Engineering, Slovak Academy of Sciences, Du ´ 10, CZ-162 00 Praha 6, Czech Republic Institute of Physics, v.v.i., Academy of Sciences of the Czech Republic, Cukrovarnicka c IMEM—CNR, Area delle Scienze, 37/A Fontanini, Parma I-43010, Italy b
a r t i c l e in f o
a b s t r a c t
Available online 28 March 2009
The present work describes current–voltage (I–V) characteristics of radiation detector structures based on semi-insulating (SI) GaAs obtained with new types of metallizations used to form quasi-ohmic contacts. Three different metals (In, Gd and Mg) having lower work function in comparison with the standard AuGeNi eutectic alloy, were used. These metals are expected to form a band bending at the M–S interface which should give rise to a blocking barrier for holes (‘‘minority’’ carriers in SI GaAs). Such contacts, which could effectively replace standard alloyed N+ ‘‘ohmic’’ contact, show an unusual electrical charge transport as deduced from the measured I–V characteristics. Pulse-height spectra of 241 Am radionuclide source detected by the structures are also reported. & 2009 Elsevier B.V. All rights reserved.
Keywords: GaAs Semi-insulating Metal–semiconductor contact Schottky barrier Work function
1. Introduction The crucial tasks relating to the radiation detector performances mainly concern the electrode metallization [1], the overall electrodes technology and topology (blocking Schottky or ohmic contacts) of the structure and finally the surface passivation. A special emphasis should be devoted to the ohmic electrode technology following the results of Alietti et al. [2]. They introduced a new ‘‘non-alloyed’’ ohmic contact using implantation, which improved performance of semi-insulating (SI) GaAs radiation detector. Another possible solution is represented by the formation of a metal–semiconductor Schottky barrier using a metal with low enough work function in comparison with the semiconductor (GaAs work function is about 4.5 eV [3]). Such a contact should block injected holes from anode and should behave as a noninjecting quasi-ohmic contact. We fabricated SI GaAs detectors using as ohmic contacts In, Mg and Gd with work functions of 4.12, 3.68 and 3.10 eV, respectively, and standard non-alloyed eutectic AuGeNi/Au as a reference contact. Our previous work [4] was devoted to the spectrometric and noise characterization of the new structures. The aim of this paper is to report on the complementary measurements and on the more detailed study of electrical
Corresponding author.
E-mail address: [email protected] (B. Zat’ko). 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.03.191
characteristics of the detector structures with the new ‘‘quasiohmic’’ metallization.
2. Technology and experimental details The radiation detector structures were prepared from the bulk undoped SI GaAs 200 wafer grown by the vertical gradient freeze (VGF) method (producer CMK Ltd., Zˇarnovica, Slovakia) with (1 0 0) crystallographic orientation and dislocation density of about 3000 cm2. The wafer was polished by the producer from both sides down to (250710) mm. Resistivity and the Hall mobility measured by the van der Pauw method at 295 K in our laboratory give values of 1.82 107 O cm and 7062 cm2/Vs, respectively. The measured values fulfill key requirements for a ‘‘detector-grade’’ bulk SI GaAs material [5]. The wafer was segmented into four samples. Square Schottky electrodes of Ti/Pt/Au (10/40/70 nm) with four different sizes (0.75, 0.50, 0.30, 0.20 mm) were formed using photolithography masking onto one side (top) of each sample. Just before evaporation, the surface oxides were removed in a solution of HCl:H2O ¼ 1:1 at RT (300 K) for 30 s. The quasi-ohmic (back) side of samples was fully covered by a double layer of In/Au, Mg/Au or Gd/Au (50/70 nm). AuGeNi/Au (50/70 nm) eutectic alloy was deposited on the back side of the reference sample. The metal contacts were evaporated in a dry high-vacuum system. I–V characteristics of the radiation detector structures were measured at RT in the dark using an electrically shielded probe
ARTICLE IN PRESS ´ et al. / Nuclear Instruments and Methods in Physics Research A 607 (2009) 132–134 F. Dubecky
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Fig. 1. Reverse and forward I–V characteristics of SI GaAs detector structures in log–log and a log–lin scale (inset): (i) Ti/Pt/Au–AuGeNi/Au, (ii) Ti/Pt/Au–In/Au, (iii) Ti/Pt/ Au–Mg/Au, and (iv) Ti/Pt/Au–Gd/Au where reproducibility of the measurement is illustrated by plotting of I–V characteristics of 2 different detector structures.
station using a Keithley 237 source controlled by a personal computer. The reverse branches correspond to the negative bias polarity applied to the top Ti/Pt/Au electrode.
3. Results and discussion I–V characteristics of the fabricated detectors measured at RT in the dark are shown in Fig. 1. The solid straight lines correspond to the linear-ohmic transport calculated from the SI GaAs bulk conductivity measurements. It can be noted that the reverse characteristics of detectors are similar. However, some quantitative differences are clearly observable. The linear part of all characteristics in the voltage region under 0.1 V is about 1.5–4 times lower than that correspond to the calculated ohmic current. The deviation is larger for the contacts made with the new metals. Moreover, it has been observed that the first sub-linear part of the reverse characteristics between 0.1 and 10 V corresponds to the value of the saturation current Is of the thermionic field emission. The second sub-linear part at voltages over 10 V, which represents the detector operation bias region, is characterized by dominant transport mechanisms which have not been yet understood. The same is true for the measured forward characteristics. As for the forward characteristic of the reference detector with AuGeNi metallization it can be seen that, after an initial linear part, a super-linear injection region starts reaching the linear-ohmic
current limit (the solid straight line). Such a tendency towards the electrical charge injection is not observed for In, Mg and Gd electrodes. In fact, in the voltage range 0.1–10 V, the detectors prepared with these contacts show always a sub-linear behaviour, which follows the initial linear characteristic, similarly to the reverse I–V branch. Finally, for voltage values over about 100 V the forward current is almost identical for all structures. These observations are in contradiction with expectations. Indeed detectors with In, Mg and Gd contacts should exhibit an injection behaviour, i.e. faster rising of the forward current coupled with rather lower breakdown voltage in comparison with the reference AuGeNi electrode due to the lower work function of used metals. If we calculate current density considering different areas of the blocking (0.25 mm2) and quasi-ohmic (10 mm2) contacts the effect became still more evident. Hence, if the quasi-ohmic electrode operates as a blocking (forward direction) through the full area (first approach consideration) the value of the current density will be up to 40 lower compared to the reverse direction at a bias 0.1–10 V and nearly equivalent, or even lower in operation bias voltage region 4 10 V. Detectors were connected to the spectrometric readout chain consisted of charge-sensitive preamplifier based on CREMAT CR 101D, shaping amplifier ORTEC 572, high-voltage supply ORTEC 459, analog-to-digital converter ORTEC 800 with a multichannel analyser M2D. Pulse-height spectra of the 241Am radionuclide source obtained with the detectors having two different Schottky
ARTICLE IN PRESS ´ et al. / Nuclear Instruments and Methods in Physics Research A 607 (2009) 132–134 F. Dubecky
134
Np(XL)
Counts per channel
15000
SI GaAs: Schottky Ti/Pt/Au
the detector with In/Au metallization. Further evaluations and studies of the reported results are in progress.
Log-Lin scale 1000 2
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Study of electrical characteristics of a new kind of quasi-ohmic contact metallization is presented. The new contact uses metals with low work function. In, Gd and Mg were employed to form such new contacts in SI GaAs detectors. Their characteristics were compared with a standard AuGeNi metallization. It has been observed that, unlike the AuGeNi metallization, the forward current in the voltage range of 0.1–10 V is lower than that of the reverse biased small area Ti/Pt/Au Schottky barrier. However, the dominant mechanism responsible of this effect remains still unclear. The influence of the metallization on the detected 241Am pulse-height spectra are also reported. It is shown that the best value of energy resolution is observed for detectors with In/Au metallization. In order to better understand the above effects more detailed theoretical and experimental investigations will be carried out.
In/Au
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Mg/Au
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Acknowledgements
1 241
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Fig. 2. Measured spectra of g-ray generated by 241Am radioisotope for square pads 0.5 0.5 mm2 (a) and 0.3 0.3 mm2 (b), respectively.
contact areas are shown in Fig. 2. As it can be seen the spectra are quite similar; however, the best value of energy resolution of 5.6 keV in FWHM of 241Am photopeak (59.5 keV) is observed for
This work was partially supported by the Slovak Grant Agency for Science through the Grant no. 2/7170/27, the Agency for Promotion of Research and Development under the Contracts nos. APVV–99–P06305 and APVV–0459–06, and by the Slovak Association EURATOM/EC and also by Grant Agency of the ASCR (Project no. IAA1010404).
References [1] [2] [3] [4] [5]
F. Dubecky´, et al., Nucl. Instr. and Meth. A 576 (2007) 87. M. Alietti, et al., Nucl. Instr. and Meth. A 362 (1995) 344. J. Massies, et al., J. Vac. Sci. Technol. 16 (1979) 1244. P. Boha´cˇek, et al., Nucl. Instr. and Meth. A 591 (2008) 105. F. Dubecky´, et al., Nucl. Instr. and Meth. A 576 (2007) 27.