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Ion bombardment in surface passivation of GaAs
Vacuum 63 (2001) 487}490
Ion bombardment in surface passivation of GaAs Z. Synowiec* Institute of Microsystems Technology, Wroclaw University of Technology, 50-327 Wroclaw, Poland
Abstract The electrical properties of SI GaAs substrates after oxygen ion bombardment have been investigated in the temperature range of 150}373 K. It was found that the oxygen ion bombardment of poor substrates stabilised their surface electrical properties in the sense of increase of the breakdown voltage. Because of leakage current which increased when the ion dose was increased only ion bombardment for the dose range from 10 to 5;10 cm\ may have a practical signi"cance. However, the dose of ion bombardment of 10 cm\ seems to be optimal. In this case the breakdown voltages were about 50 V for a 20 m electrode separation (i.e. 25 kV/cm) and leakage sheet current densities were from 3;10\ to 5;10\ A/mm at the temperature range from 293 to 373 K. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Ion bombardment; GaAs; Surface passivation
1. Introduction The III}V compound semiconductors, such as gallium arsenide (GaAs) and indium phosphide (InP), have been long recognised for their potential applications in high speed electronic and optoelectronic circuits. The advantages of these semiconductors over silicon are high electron mobility, high saturation drift velocity, radiation hardness, fabrication of a variety of useful heterojunction structures, and potential monolithic integration of optical and electronic function. However, the most severe disadvantage is poor electronic quality of their surfaces and the metal}semiconductor and insulator}semiconductor interfaces. A good passivating "lm with suitable electrical and physical
* Fax: 48-71-328-3504. E-mail address: [email protected] (Z. Synowiec).
properties has not yet become available what has severely impeded the broader use of the materials in electronic and photonic applications. The technology of reducing the negative e!ects of poor surface and interface is referred to as passivation, the objective of which is to stabilise the properties of the surface so that it becomes immune to exposure of the device to operating ambient [1]. The presence of a large number of electronically active defect states results in poorer device performance and reliability. Therefore, preparation of surfaces with acceptable levels of electronically active defects is essential for proper functioning of many devices. Unfortunately, the fundamental knowledge of the surfaces has been very limited and the research on passivation technology, for Si and III}V compound semiconductors, has been primarily based on empirical techniques [1]. Several attempts were made to form native oxides (analogous to the SiO /Si interface), however;
0042-207X/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 1 ) 0 0 2 2 6 - 3
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Z. Synowiec / Vacuum 63 (2001) 487}490
none exhibited as a passivation layer. Although passivation of the defects may be achieved using epitaxial growth of heterojunction structures, for example AlGaAs on GaAs, these schemes are applicable only to a limited number of devices [1]. Recently, gallium arsenide grown by molecular beam epitaxy (MBE) at low-temperature (LT-GaAs) (200}3003C) has attracted much attention, because of its potential for superior surface passivation of SI GaAs but it can be also used with a limited application [2]. Oxygen ion bombardment of SI GaAs substrate presented in this paper is one of the methods of free surface passivation. In this case the passivation relies on the stabilisation of the surface electrical properties, particularly leakage current and breakdown behaviour of SI GaAs substrates.
2. Experimental procedure Ion bombardment of the semi-insulating GaAs substrates has been carried out by double energy of oxygen ions with 250 keV followed by 100 keV process to provide uniform distribution of generated defects. Ion doses in the range from 10 to 5;10 cm\ have been used. InterDigital Resistor (IDR) pattern with 2;21 contact "ngers, "nger length of 1000 m, width of 20 m and contact separation of 20 m were formed on ion bombarded SI GaAs and for comparison on SI GaAs (without ion bombardment) by alloying AuGe/Ni/Au at 4303C for 3 min. IDR pattern contacts with 0.5;10\ square of a sheet resistance allowed to measure I}V characteristics of the highly resistive samples at low temperature. These I}V characteristics were carried out in a range of temperatures from 150 to 373 K. The samples were mounted in a Gi!ord McMahon refrigerator with an air pressure of about 10 Pa (0.1 Torr) during I}V measuring at room and below room temperature but for the temperature above room temperature the samples were kept in an air-convection oven.
3. Results and discussion Fig. 1 shows room temperature, log I }log V plots for SI GaAs samples after ion bombardment and
Fig. 1. Room temperature dark I }< characteristics of SI GaAs substrates after ion bombardment for di!erent doses D in (cm\). For comparison the I }< plot for the substrate before ion bombardment (D"0) is also shown.
for comparison one plot for the substrates before bombardment. I means sheet current density in A/mm, i.e. measured current was converted into current per 1 mm width of investigated GaAs layers. The "gure shows a very low breakdown voltage of the SI GaAs substrate. Hasegawa et al. [3,4] also investigated such I}V characteristics of planar contact semi-insulating samples and came to the conclusion that a high density of surface states formed a surface conduction channel which dominated the conduction mechanism of the material and were responsible for the very low breakdown voltage. In the case of the SI GaAs after ion bombardment we observed a signi"cant increase of the breakdown voltages. In the samples with the high level conduction, i.e. after ion bombardment of 10 and 5;10 cm\ doses, we observed thermal catastrophic breakdown for the voltages of 80 and 5 V, respectively. However, no catastrophic breakdown was observed up to 100 V for a 20 m electrode separation in the case of 10 and 5;10 cm\ ion bombardment doses. The voltage of 100 V for a 20 m electrode separation corresponds to the electrical "eld of 50 kV/cm, i.e. two orders of magnitude higher
Z. Synowiec / Vacuum 63 (2001) 487}490
than in the SI GaAs layer. It is believed that the observed high breakdown "eld of the ion bombarded SI GaAs substrates is dominated by the thin layer (approximately 0.5 m) on the surface that is in#uenced by the bombarding ions. The explanation of these observations is that an increase of bulk defect states near the surface due to ion bombardment weakens the relative importance of the surface states of SI GaAs in the conduction mechanism and thus relaxes "eld concentration by more diversi"ed distribution of ionised trapping centres and prevents surface breakdown. The resistivity of the ion bombarded structures decreased with increasing bombarding ion dose as a result of the current density of hopping conduction increased [5]. As a result of the increase of hopping current, the samples after ion bombardment with 10 and 5;10 cm\ doses demonstrated sheet current density more than 10\ A/mm and it seems too high for device applications. Because of that in further studies we concentrated on the samples after ion bombardment with 10 and 5;10 cm\ doses. In this case the sheet current density has been below 10\ A/mm for a 20 m electrode separation and for a 10 V of bias voltage. Then I}V characteristics has been investigated as a function of measurement temperature from 150 to 373 K. Figs. 2 and 3 show the temperature dependence of I }V characteristics on a log}log scale for samples after ion bombardment with 5;10 and 10 cm\ doses, respectively. The response of the sheet current density}voltage plot before breakdown is linear at all temperatures. In the case of the samples after ion bombardment with 5;10 cm\ dose we observed catastrophic thermal breakdown when the sheet current density exceeded the value of 10\ A/mm, but the value of the breakdown voltage decreased when the temperature increased. We think that this decrease of breakdown voltage is caused by a smaller out#ow of the thermal energy from the GaAs layer when the temperature increases. In contrast to the previous case for the samples after ion bombardment with 10 cm\ we observed soft electrical breakdown above &50 V at room and above room temperatures, but at the temperature of 150 K the breakdown voltage exceeded the value of 100 V. The explanation of such phenomenon is more di$cult. The measurement of
489
Fig. 2. Temperature dependence of I }< characteristics on a log}log scale for the samples of SI GaAs substrates after ion bombardment with 5;10 cm\ dose and with 20 m of electrodes separation.
Fig. 3. Temperature dependence of I }< characteristics on a log}log scale for the samples of SI GaAs substrates after ion bombardment with 10 cm\ dose and with 20 m of the electrodes separation.
I}V characteristics at temperature of 150 K were carried out in a Gi!ord McMahon refrigerator with an air pressure of about 10 Pa (0.1 Torr) but the measurements at room and above room
490
Z. Synowiec / Vacuum 63 (2001) 487}490
temperature were carried out in an oven with an air at atmospheric pressure. Then we could presume that the air at atmospheric pressure degrades the SI GaAs surface and decreases its breakdown voltage.
4. Summary In summary, the surface electrical properties of SI GaAs substrates after oxygen ion bombardment for the dose range from 10 to 5;10 cm\ have been investigated as a function of measurement temperature, and the main results obtained are summarised as follows: The surface electrical properties of Cr-compensated SI GaAs were very poor. The current increased rapidly for a voltage higher than 1 V with distance between electrodes of 20 m, i.e. for electric "eld higher than 0.5 kV/cm. This value of the voltage may be insu$cient for most applications. The oxygen ion bombardment of such poor substrates stabilises their surface electrical properties in the sense of increase of the breakdown voltage. Because of leakage current between electrodes which increases when the ion dose increases only ion bombardment for the dose range from 10 to 5;10 cm\ may have a practical signi"cance. However, the dose of ion bombardment with 10 cm\ seems to be optimal. In the samples after ion bombardment for the dose of 10 cm\, the breakdown voltages were
about 50 V for a 20 m electrode separation (i.e. 25 kV/cm) at the temperature range from 293 to 373 K and leakage sheet current densities were from 3;10\ to 5;10\ A/mm at the same temperature range. The surface electrical properties of Cr-compensated SI GaAs substrates after ion bombardment imply that ion bombardment can be useful in surface passivation of GaAs.
Acknowledgements This work was partially supported by the State Committee for Scienti"c Research (Poland) under contract No. PBZ-02811.
References [1] Malhotra V, Wilmsen CW. In: Holloway PH, McGuire GE, editors. Passivation of GaAs and InP in Handbook of Compound Semiconductors, Noyes Publications: New Jersey, 1995. p. 328}69. [2] Luo JK, Thomas H, Morgan DV, Westwood D, Williams RH. Semicond Sci Technol 1994;9:2199}204. [3] Hasegawa H, Kitigawa T, Sawada T, Ohno H. Electron Letters S R 1989;65(1):215}26. [4] Hasegawa H, Kitigawa T, Sawada T, Ohno H. J Appl Phys 1984;20(13):561}2. [5] Synowiec Z. Nukleonika 1999;44(2):349}56.
Ion bombardment in surface passivation of GaAs Z. Synowiec* Institute of Microsystems Technology, Wroclaw University of Technology, 50-327 Wroclaw, Poland
Abstract The electrical properties of SI GaAs substrates after oxygen ion bombardment have been investigated in the temperature range of 150}373 K. It was found that the oxygen ion bombardment of poor substrates stabilised their surface electrical properties in the sense of increase of the breakdown voltage. Because of leakage current which increased when the ion dose was increased only ion bombardment for the dose range from 10 to 5;10 cm\ may have a practical signi"cance. However, the dose of ion bombardment of 10 cm\ seems to be optimal. In this case the breakdown voltages were about 50 V for a 20 m electrode separation (i.e. 25 kV/cm) and leakage sheet current densities were from 3;10\ to 5;10\ A/mm at the temperature range from 293 to 373 K. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Ion bombardment; GaAs; Surface passivation
1. Introduction The III}V compound semiconductors, such as gallium arsenide (GaAs) and indium phosphide (InP), have been long recognised for their potential applications in high speed electronic and optoelectronic circuits. The advantages of these semiconductors over silicon are high electron mobility, high saturation drift velocity, radiation hardness, fabrication of a variety of useful heterojunction structures, and potential monolithic integration of optical and electronic function. However, the most severe disadvantage is poor electronic quality of their surfaces and the metal}semiconductor and insulator}semiconductor interfaces. A good passivating "lm with suitable electrical and physical
* Fax: 48-71-328-3504. E-mail address: [email protected] (Z. Synowiec).
properties has not yet become available what has severely impeded the broader use of the materials in electronic and photonic applications. The technology of reducing the negative e!ects of poor surface and interface is referred to as passivation, the objective of which is to stabilise the properties of the surface so that it becomes immune to exposure of the device to operating ambient [1]. The presence of a large number of electronically active defect states results in poorer device performance and reliability. Therefore, preparation of surfaces with acceptable levels of electronically active defects is essential for proper functioning of many devices. Unfortunately, the fundamental knowledge of the surfaces has been very limited and the research on passivation technology, for Si and III}V compound semiconductors, has been primarily based on empirical techniques [1]. Several attempts were made to form native oxides (analogous to the SiO /Si interface), however;
0042-207X/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 1 ) 0 0 2 2 6 - 3
488
Z. Synowiec / Vacuum 63 (2001) 487}490
none exhibited as a passivation layer. Although passivation of the defects may be achieved using epitaxial growth of heterojunction structures, for example AlGaAs on GaAs, these schemes are applicable only to a limited number of devices [1]. Recently, gallium arsenide grown by molecular beam epitaxy (MBE) at low-temperature (LT-GaAs) (200}3003C) has attracted much attention, because of its potential for superior surface passivation of SI GaAs but it can be also used with a limited application [2]. Oxygen ion bombardment of SI GaAs substrate presented in this paper is one of the methods of free surface passivation. In this case the passivation relies on the stabilisation of the surface electrical properties, particularly leakage current and breakdown behaviour of SI GaAs substrates.
2. Experimental procedure Ion bombardment of the semi-insulating GaAs substrates has been carried out by double energy of oxygen ions with 250 keV followed by 100 keV process to provide uniform distribution of generated defects. Ion doses in the range from 10 to 5;10 cm\ have been used. InterDigital Resistor (IDR) pattern with 2;21 contact "ngers, "nger length of 1000 m, width of 20 m and contact separation of 20 m were formed on ion bombarded SI GaAs and for comparison on SI GaAs (without ion bombardment) by alloying AuGe/Ni/Au at 4303C for 3 min. IDR pattern contacts with 0.5;10\ square of a sheet resistance allowed to measure I}V characteristics of the highly resistive samples at low temperature. These I}V characteristics were carried out in a range of temperatures from 150 to 373 K. The samples were mounted in a Gi!ord McMahon refrigerator with an air pressure of about 10 Pa (0.1 Torr) during I}V measuring at room and below room temperature but for the temperature above room temperature the samples were kept in an air-convection oven.
3. Results and discussion Fig. 1 shows room temperature, log I }log V plots for SI GaAs samples after ion bombardment and
Fig. 1. Room temperature dark I }< characteristics of SI GaAs substrates after ion bombardment for di!erent doses D in (cm\). For comparison the I }< plot for the substrate before ion bombardment (D"0) is also shown.
for comparison one plot for the substrates before bombardment. I means sheet current density in A/mm, i.e. measured current was converted into current per 1 mm width of investigated GaAs layers. The "gure shows a very low breakdown voltage of the SI GaAs substrate. Hasegawa et al. [3,4] also investigated such I}V characteristics of planar contact semi-insulating samples and came to the conclusion that a high density of surface states formed a surface conduction channel which dominated the conduction mechanism of the material and were responsible for the very low breakdown voltage. In the case of the SI GaAs after ion bombardment we observed a signi"cant increase of the breakdown voltages. In the samples with the high level conduction, i.e. after ion bombardment of 10 and 5;10 cm\ doses, we observed thermal catastrophic breakdown for the voltages of 80 and 5 V, respectively. However, no catastrophic breakdown was observed up to 100 V for a 20 m electrode separation in the case of 10 and 5;10 cm\ ion bombardment doses. The voltage of 100 V for a 20 m electrode separation corresponds to the electrical "eld of 50 kV/cm, i.e. two orders of magnitude higher
Z. Synowiec / Vacuum 63 (2001) 487}490
than in the SI GaAs layer. It is believed that the observed high breakdown "eld of the ion bombarded SI GaAs substrates is dominated by the thin layer (approximately 0.5 m) on the surface that is in#uenced by the bombarding ions. The explanation of these observations is that an increase of bulk defect states near the surface due to ion bombardment weakens the relative importance of the surface states of SI GaAs in the conduction mechanism and thus relaxes "eld concentration by more diversi"ed distribution of ionised trapping centres and prevents surface breakdown. The resistivity of the ion bombarded structures decreased with increasing bombarding ion dose as a result of the current density of hopping conduction increased [5]. As a result of the increase of hopping current, the samples after ion bombardment with 10 and 5;10 cm\ doses demonstrated sheet current density more than 10\ A/mm and it seems too high for device applications. Because of that in further studies we concentrated on the samples after ion bombardment with 10 and 5;10 cm\ doses. In this case the sheet current density has been below 10\ A/mm for a 20 m electrode separation and for a 10 V of bias voltage. Then I}V characteristics has been investigated as a function of measurement temperature from 150 to 373 K. Figs. 2 and 3 show the temperature dependence of I }V characteristics on a log}log scale for samples after ion bombardment with 5;10 and 10 cm\ doses, respectively. The response of the sheet current density}voltage plot before breakdown is linear at all temperatures. In the case of the samples after ion bombardment with 5;10 cm\ dose we observed catastrophic thermal breakdown when the sheet current density exceeded the value of 10\ A/mm, but the value of the breakdown voltage decreased when the temperature increased. We think that this decrease of breakdown voltage is caused by a smaller out#ow of the thermal energy from the GaAs layer when the temperature increases. In contrast to the previous case for the samples after ion bombardment with 10 cm\ we observed soft electrical breakdown above &50 V at room and above room temperatures, but at the temperature of 150 K the breakdown voltage exceeded the value of 100 V. The explanation of such phenomenon is more di$cult. The measurement of
489
Fig. 2. Temperature dependence of I }< characteristics on a log}log scale for the samples of SI GaAs substrates after ion bombardment with 5;10 cm\ dose and with 20 m of electrodes separation.
Fig. 3. Temperature dependence of I }< characteristics on a log}log scale for the samples of SI GaAs substrates after ion bombardment with 10 cm\ dose and with 20 m of the electrodes separation.
I}V characteristics at temperature of 150 K were carried out in a Gi!ord McMahon refrigerator with an air pressure of about 10 Pa (0.1 Torr) but the measurements at room and above room
490
Z. Synowiec / Vacuum 63 (2001) 487}490
temperature were carried out in an oven with an air at atmospheric pressure. Then we could presume that the air at atmospheric pressure degrades the SI GaAs surface and decreases its breakdown voltage.
4. Summary In summary, the surface electrical properties of SI GaAs substrates after oxygen ion bombardment for the dose range from 10 to 5;10 cm\ have been investigated as a function of measurement temperature, and the main results obtained are summarised as follows: The surface electrical properties of Cr-compensated SI GaAs were very poor. The current increased rapidly for a voltage higher than 1 V with distance between electrodes of 20 m, i.e. for electric "eld higher than 0.5 kV/cm. This value of the voltage may be insu$cient for most applications. The oxygen ion bombardment of such poor substrates stabilises their surface electrical properties in the sense of increase of the breakdown voltage. Because of leakage current between electrodes which increases when the ion dose increases only ion bombardment for the dose range from 10 to 5;10 cm\ may have a practical signi"cance. However, the dose of ion bombardment with 10 cm\ seems to be optimal. In the samples after ion bombardment for the dose of 10 cm\, the breakdown voltages were
about 50 V for a 20 m electrode separation (i.e. 25 kV/cm) at the temperature range from 293 to 373 K and leakage sheet current densities were from 3;10\ to 5;10\ A/mm at the same temperature range. The surface electrical properties of Cr-compensated SI GaAs substrates after ion bombardment imply that ion bombardment can be useful in surface passivation of GaAs.
Acknowledgements This work was partially supported by the State Committee for Scienti"c Research (Poland) under contract No. PBZ-02811.
References [1] Malhotra V, Wilmsen CW. In: Holloway PH, McGuire GE, editors. Passivation of GaAs and InP in Handbook of Compound Semiconductors, Noyes Publications: New Jersey, 1995. p. 328}69. [2] Luo JK, Thomas H, Morgan DV, Westwood D, Williams RH. Semicond Sci Technol 1994;9:2199}204. [3] Hasegawa H, Kitigawa T, Sawada T, Ohno H. Electron Letters S R 1989;65(1):215}26. [4] Hasegawa H, Kitigawa T, Sawada T, Ohno H. J Appl Phys 1984;20(13):561}2. [5] Synowiec Z. Nukleonika 1999;44(2):349}56.