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Stability of electrical parameters of Ti Schottky contacts on GaAs fabricated surfaces using different surface cleaning techniques
Vacuum/volume
Pergamon 0042-207X
(94) E0020-Y
46/number
2/pages 127 to 130/1995 Elsevier Science Ltd Printed in Great Britain 0042-207x/95 $9.50+.00
Stability of electrical parameters of Ti Schottky contacts on GaAs fabricated surfaces using different surface cleaning techniques Krishnamachar Prasad, School of Electrical Nanyang Avenue, Singapore 2263 received 23 September
& Electronic
1993 and in revised form 79 January
Engineering,
Nanyang
Technological
University,
1994
Titanium Schottky contacts were formed on n-type and p-type GaAs surfaces that were cleaned using different techniques. The Schottky contacts were subsequently aged at 200°C for times up to 1000 h and their electrical parameters were observed as a function of aging time. Although the initial properties of Schottky contacts were nearly identical before any aging, photochemically passivated surfaces exhibited least degradation in their electrical properties when compared to cleaning with other techniques. The surfaces cleaned using either H,SO, or H,PO, exhibited maximum degradation in their electrical properties.
1. Introduction Metal-semiconductor contacts form the basic building blocks of compound semiconductor technology. In the absence of a suitable stable native oxide, the possibility of metal-insulator-semiconductor (MIS) structures on GaAs are greatly reduced and the focus has been directed more at metal-semiconductor (M-S) structures. The performance of M-S contacts depends predominantly on the choice of the metal, the doping density of the semiconductor and the quality of the semiconductor surface before the deposition of the metal. The importance of semiconductor surface preparation procedures in microelectronics device technology is well recognised. In the case of GaAs device technology, the use of the lift-off technique inevitably leaves the GaAs surface contaminated prior to metal evaporation (Ohmic or Schottky). Any cleaning procedures are severely limited by the presence of photoresist after developing the patterns and before the metal evaporation. In the recent past, several procedures have been recommended to obtain contamination free GaAs surfaceslm3. The use of photochemical passivation was reported by Woodall et al’ where an aqueous solution of HCl was used in the presence of uv radiation. The surfaces after photochemical passivation were found to be ideal for the growth of epitaxial layers using molecular beam epitaxy (MBE). The technique used by Offsey et al’ involved using argon laser irradiation while the GaAs wafers were being rinsed in deionized (DI) water. The surfaces were found to be contamination free and subsequent photoluminescent spectroscopic studies showed that the Fermi level was unpinned. In a recent study by Love11 et a13, Ti Schottky contacts were formed on p-GaAs surfaces that were cleaned using either H,SO, or
H,PO+ They reported that H3P04 cleaning resulted in more ideal M-S interfaces. Although there are several techniques for cleaning GaAs surfaces before metal deposition (Schottky or Ohmic), there are no available results on the comparison of either several or all kinds of surface preparation techniques and, furthermore, the stability of such contamination free surfaces after prolonged high temperature aging. In this paper, we compare several types of surface treatments to both n-type and p-type GaAs surfaces before the Schottky metal deposition and investigate their long term stability.
2. Experimental The starting materials used in this study were either n-type or ptype epitaxial GaAs grown by MBE on semi-insulating GaAs substrates. The n-type material consisted of a 1.5 pm thick n-type epitaxial layer (Si doped) of doping density lOI cm-3. Similarly, the p-type material consisted of a 1.5 pm thick p-type epitaxial layer (Be doped) of doping density lOI cmm3. The first step in the diode fabrication was the isolation of regions of the epitaxial active layers. After mesa etching, either Au-Ge/Ni (Au-Ge alloy with 88 wt% Au, 12 wt% Ge) for n-type contacts or Au-Zn (AuZn alloy with 85 wt% Au 15 wt% Zn) for p-type contacts were evaporated onto a photoresist pattern and defined using lift-off techniques. The contacts were subsequently alloyed in a furnace at 450°C for 120 s to obtain low resistance Ohmic contacts. Circular Schottky diode patterns (diameters ranging from 50 to 500 pm in 50 pm steps) were defined on the active epitaxial layers using standard photolithography. The wafers were then subjected to either photochemical passivation’, or etching using 127
Krishnamachar
Prasad:
Schottky
contacts
on GaAs surfaces
mixtures of 20: 1 H,O: H,SO, for 10 s, ref (3) or 20: 1 H,O : H,P04 for 10 s, ref (3), or 60 s dip in a 4 : 1 solution of H,O : H,O, followed by 60 s etch in a 4 : 1 solution of H,O : HCI, ref (4). Photochemical passivation was carried out in a 1 : 1 solution of 36% HCl in DI water using 254 nm uv radiation at a power density of - 500+30 PW cm-’ for 10 min. About 50 nm of Ti metal was subsequently evaporated onto the GaAs surface in an electron beam evaporator. For probing purposes, a sandwich structure of 50 nm of Pt followed by 200 nm of Au overlayer was evaporated onto the samples in the same cycle. The Schottky contacts were subsequently defined by lift-off techniques. Room temperature current-voltage (I-V) and high frequency (1 MHz) capacitance-voltage (C-V) measurements were carried on the Schottky contacts using an HP 4140B pAmmeter/dc voltage source and an HP 4275A LCR meter, respectively. Measurements were repeated on each of the ten different sized diodes and over several retitles on each wafer. The measurement data were averaged and standard deviations were calculated, in order to assess the uniformity of measurements across each wafer. High temperature aging of the Schottky contacts was carried out at 200°C for times up to 1000 h. The Schottky diode parameters were obtained using standard I-V and C-V analyses5. From the C-V measurement data, the Schottky barrier height mB, and the doping density of the semiconductor ND or NA were extracted. Similarly, from the I-V measurement data, the diode ideality factor n and the saturation current density Js were determined. Interface state density evaluation was carried out using the I-V data based on the model proposed by Tseng and Wu6. 3. Results and discussion Initial electrical measurements of both n-type and p-type Schottky contacts on all the wafers revealed near ideal diodes. The results are shown in Tables 1 and 2. From the tables, it is clear that all the diodes have nearly ideal diode characteristics, as shown by the diode ideality factor n being close to one. The barrier height 0, ranged from 0.70 to 0.73 eV for n-GaAs and from 0.72 to 0.74 for p-GaAs. Similarly, the diode saturation current Js was in the range from 300 to 3 10 nA cmm2 for n-GaAs
and from 5.5 to 5.7 PA cm-’ for p-GaAs. In all the cases, the uniformity of the data from the diodes fabricated using photochemical passivation is evident, as shown by the lowest standard deviation in their respective diode parameters. The results are true for both n-type and p-type GaAs wafers. It is interesting to compare our experimental results with that of Butcher et al’ where results on Schottky barrier height modification on GaAs, following a sulphur based etch, were reported. Their results showed that the Schottky barrier height @, from C-V measurements was largely unchanged using both aqua regia etch and H,SO, based etch before the Schottky metallization. However, the zero bias barrier height @B,0,from 1-V measurements, increased after H,SO, etch; this was shown to be due to sulphur contamination. From the value of Js in our work, it is clear that the zero bias barrier height OB,,)is largely unchanged, regardless of the cleaning/etching procedures. It should be noted that there are some basic differences between our experimental approach and that of Butcher et al. They formed the Schottky diodes after aqua regia based cleaning, carried out electrical characterization, etched away the Schottky metal, carried out H,SO, based etching, formed the Schottky contact again and conducted I-V and C-V measurements. The modification of the GaAs surface as a result of the removal of Schottky metal and the subsequent H$O, etching cannot be ruled out in their work. When the diodes were aged at 200°C for times up to 1000 h, there was a gradual degradation in their electrical parameters. The diode ideality factor n did not exhibit any specific dependence with aging and remained within the range 1.0-1.2 for both ptype and n-type GaAs contacts. Conversely, the diode saturation current Js increased monotonically with aging time, regardless of surface preparation, as shown in Figure 1. From the figure, it is clear that the change in Js value, after aging, for diodes fabricated on photochemically passivated surfaces is much smaller than those formed using other techniques. In the case of Schottky barrier height, a slight decrease in the QB value was observed after 1000 h of aging, regardless of the surface preparation techniques. However, taking into account the error bands shown in Tables 1 and 2, we believe that there is no evidence for a difference in
Table 1. Electrical properties of Ti/n-GaAs Schottky diodes before any aging
Diode parameters
Photochemical passivation
HCI : HzO,
H,PQ
&SO,
& (eV) Js (nA cm-‘) No (lOI cm-‘) D,, (lOI cm-* eV_‘)
1.10+0.03 0.72 + 0.03 300+ 15 1.1 TO.1 1.75kO.04
1.15*0.05 0.72kO.06 305+25 0.9 ,O.l 1.80 k 0.05
1.09f0.08 0.72+0.08 310+29 1.1 10.2 1.60+0.10
1.19+0.11 0.70+0.10 304*35 1.2 kO.1 1.7OkO.20
Table 2. Electrical properties of Ti/p-GaAs Schottky diodes before any aging Diode parameters
Js (PA cm-*)
NA (lOI cm-‘) D,, (10” cmm2eV_‘)
128
Photochemical passivation
HCl : HZ02
H,PQ
HISO,
1.05 f 0.03 0.73 + 0.03 5.6 +0.2 1.3 +0.1 3.70+0.06
1.12&0.04 0.74 * 0.05 5.5 +0.5 1.1 kO.1 4.00&0.12
1.1450.08 0.72+0.08 5.7 +0.8 0.9 kO.2 3.8OkO.23
1.15+0.13 0.74kO.11 5.6 +l.l 1.0 +0.2 4.00*0.50
Krishnamachar
frasad:
(a)
Schottky
contacts
on GaAs surfaces
700
600
10 Aging
10
100
Aging
Time (h)
100 Time
(h)
Figure 1. The effect of 200°C aging on the diode saturation current J, of Ti Schottky contacts to (a) n-GaAs and (b) p-GaAs passivation, q-HCl : H,O,, l -H,PO, and O-H,SO,).
barrier height between the different processing methods used in this study, both before aging and after aging. The results are consistent with the observations of other workers.3. The electrical parameters of M-S contacts are governed by the quality of the M-S interface. A good M-S interface implies a diode with nearly ideal characteristics. In order to find out the quality of the M-S interface, interface state density, D,,, analysis was carried out6. The results are plotted in Figure 2, where the D,, value at E-E, = O.l5eV, E, being the intrinsic Fermi energy level in GaAs, is plotted as a function of aging time. Again, the variation in D,, is the smallest for photochemically passivated surfaces when compared with other cleaning techniques. The chemistry of reactions of various cleaning procedures used in GaAs device technology is complex. The various cleaning procedures used for GaAs surface preparation achieve the primary objective of leaving a contamination free surface. However, the stability of the GaAs surfaces is strongly dependent on the type of cleaning technique implemented, as shown in this study. For example, it is well known that photochemical passivation’ results in the formation of a film, about 5 nm thick, of elemental arsenic. In the case of other acid based cleaning techniques, it is likely that the cleaning leaves the GaAs surface with some weak bonds. Upon aging, it is believed that these bonds would break and form dangling bonds which effectively act as generation centres’ ; this would explain why the diode parameters of GaAs surfaces that were cleaned using either H,SO, or H3POd degrade
-1
10 Aging
100 Time
Figure 2. The effect of 200°C aging on the interface passivation,
q-HCl
: H20,,
l -H,PO,
(h)
1000
(a-photochemical
more rapidly than those formed on photochemically passivated surfaces. The presence of a thin layer of elemental arsenic, as a result of photochemical passivation, is expected to better stabilize the GaAs surface. Under such conditions, any active devices formed thereon would exhibit more stable and repeatable characteristics, as is evident from the parameter standard deviations observed in this work.
4. Conclusions A systematic study of various acid based surface cleaning procedures that are commonly used in GaAs device technology has been carried out. The results indicate that photochemical passivation of the GaAs surface is the best way of obtaining stable and contamination free GaAs surfaces. Diodes formed on photochemically passivated surfaces exhibit more uniform electrical parameters than those formed using either H,SO, or H,PO, or HCl: H,Oz solutions. Furthermore, after 1000 h of aging, the diode parameters of photochemically passivated surfaces exhibit a significantly lower degree of degradation than those of surfaces formed using any other acid based cleaning procedures.
References
IJ M Woodall,
P Oelhafen, T N Jackson, Vat Sci Technol. Bl, 795 (1983).
-1
100
IO Aging
Time
J L Freeouf
and G D Petti, /
1000
(h)
state density D,, of Ti Schottky contacts to (a) n-GaAs and (b) p-GaAs and O-H,S04). The D,, value at E-E, = 0.15 eV is shown in the figure.
(m-photochemical
129
Krjs~~arnachar
Prasad:
Schottky contacts
on GaAs surfaces
*S D Offsey, J M Woodall, A C Warren, P D Kirchner, T I Chappel and G D Petti, Appl Phys Lett, 48,475 (1986). ‘D R Lovell, T Yamamoto, M Inai, T Takebe and K Kobayishi, Japan J Appl Phys, 31, L924 (1992). 4K Prasad, L Faraone and A G Nassibian, Tkilz Solid F&E, 195, L11 (1991).
5S M Sze, Physics of Semiconductor De&es,
130
2nd Edn. John Wiley &
Sons, New York (1981). 6H H Tseng and C Y Wu, J Appl Phys, 61,299 (1987). 7K S A Butcher, D Alexiev, V W L Chin, T L Tansley, R J Egan and M Keane, Semicond Sci Technol, 8, 1451 (1993). ‘A L Fahrenbruch and R H Bube. Fun~~mensais of Solar Cells: P~ot~~Qltu~~ S&r Energy Conversion. Academic Press. London (1973).
Pergamon 0042-207X
(94) E0020-Y
46/number
2/pages 127 to 130/1995 Elsevier Science Ltd Printed in Great Britain 0042-207x/95 $9.50+.00
Stability of electrical parameters of Ti Schottky contacts on GaAs fabricated surfaces using different surface cleaning techniques Krishnamachar Prasad, School of Electrical Nanyang Avenue, Singapore 2263 received 23 September
& Electronic
1993 and in revised form 79 January
Engineering,
Nanyang
Technological
University,
1994
Titanium Schottky contacts were formed on n-type and p-type GaAs surfaces that were cleaned using different techniques. The Schottky contacts were subsequently aged at 200°C for times up to 1000 h and their electrical parameters were observed as a function of aging time. Although the initial properties of Schottky contacts were nearly identical before any aging, photochemically passivated surfaces exhibited least degradation in their electrical properties when compared to cleaning with other techniques. The surfaces cleaned using either H,SO, or H,PO, exhibited maximum degradation in their electrical properties.
1. Introduction Metal-semiconductor contacts form the basic building blocks of compound semiconductor technology. In the absence of a suitable stable native oxide, the possibility of metal-insulator-semiconductor (MIS) structures on GaAs are greatly reduced and the focus has been directed more at metal-semiconductor (M-S) structures. The performance of M-S contacts depends predominantly on the choice of the metal, the doping density of the semiconductor and the quality of the semiconductor surface before the deposition of the metal. The importance of semiconductor surface preparation procedures in microelectronics device technology is well recognised. In the case of GaAs device technology, the use of the lift-off technique inevitably leaves the GaAs surface contaminated prior to metal evaporation (Ohmic or Schottky). Any cleaning procedures are severely limited by the presence of photoresist after developing the patterns and before the metal evaporation. In the recent past, several procedures have been recommended to obtain contamination free GaAs surfaceslm3. The use of photochemical passivation was reported by Woodall et al’ where an aqueous solution of HCl was used in the presence of uv radiation. The surfaces after photochemical passivation were found to be ideal for the growth of epitaxial layers using molecular beam epitaxy (MBE). The technique used by Offsey et al’ involved using argon laser irradiation while the GaAs wafers were being rinsed in deionized (DI) water. The surfaces were found to be contamination free and subsequent photoluminescent spectroscopic studies showed that the Fermi level was unpinned. In a recent study by Love11 et a13, Ti Schottky contacts were formed on p-GaAs surfaces that were cleaned using either H,SO, or
H,PO+ They reported that H3P04 cleaning resulted in more ideal M-S interfaces. Although there are several techniques for cleaning GaAs surfaces before metal deposition (Schottky or Ohmic), there are no available results on the comparison of either several or all kinds of surface preparation techniques and, furthermore, the stability of such contamination free surfaces after prolonged high temperature aging. In this paper, we compare several types of surface treatments to both n-type and p-type GaAs surfaces before the Schottky metal deposition and investigate their long term stability.
2. Experimental The starting materials used in this study were either n-type or ptype epitaxial GaAs grown by MBE on semi-insulating GaAs substrates. The n-type material consisted of a 1.5 pm thick n-type epitaxial layer (Si doped) of doping density lOI cm-3. Similarly, the p-type material consisted of a 1.5 pm thick p-type epitaxial layer (Be doped) of doping density lOI cmm3. The first step in the diode fabrication was the isolation of regions of the epitaxial active layers. After mesa etching, either Au-Ge/Ni (Au-Ge alloy with 88 wt% Au, 12 wt% Ge) for n-type contacts or Au-Zn (AuZn alloy with 85 wt% Au 15 wt% Zn) for p-type contacts were evaporated onto a photoresist pattern and defined using lift-off techniques. The contacts were subsequently alloyed in a furnace at 450°C for 120 s to obtain low resistance Ohmic contacts. Circular Schottky diode patterns (diameters ranging from 50 to 500 pm in 50 pm steps) were defined on the active epitaxial layers using standard photolithography. The wafers were then subjected to either photochemical passivation’, or etching using 127
Krishnamachar
Prasad:
Schottky
contacts
on GaAs surfaces
mixtures of 20: 1 H,O: H,SO, for 10 s, ref (3) or 20: 1 H,O : H,P04 for 10 s, ref (3), or 60 s dip in a 4 : 1 solution of H,O : H,O, followed by 60 s etch in a 4 : 1 solution of H,O : HCI, ref (4). Photochemical passivation was carried out in a 1 : 1 solution of 36% HCl in DI water using 254 nm uv radiation at a power density of - 500+30 PW cm-’ for 10 min. About 50 nm of Ti metal was subsequently evaporated onto the GaAs surface in an electron beam evaporator. For probing purposes, a sandwich structure of 50 nm of Pt followed by 200 nm of Au overlayer was evaporated onto the samples in the same cycle. The Schottky contacts were subsequently defined by lift-off techniques. Room temperature current-voltage (I-V) and high frequency (1 MHz) capacitance-voltage (C-V) measurements were carried on the Schottky contacts using an HP 4140B pAmmeter/dc voltage source and an HP 4275A LCR meter, respectively. Measurements were repeated on each of the ten different sized diodes and over several retitles on each wafer. The measurement data were averaged and standard deviations were calculated, in order to assess the uniformity of measurements across each wafer. High temperature aging of the Schottky contacts was carried out at 200°C for times up to 1000 h. The Schottky diode parameters were obtained using standard I-V and C-V analyses5. From the C-V measurement data, the Schottky barrier height mB, and the doping density of the semiconductor ND or NA were extracted. Similarly, from the I-V measurement data, the diode ideality factor n and the saturation current density Js were determined. Interface state density evaluation was carried out using the I-V data based on the model proposed by Tseng and Wu6. 3. Results and discussion Initial electrical measurements of both n-type and p-type Schottky contacts on all the wafers revealed near ideal diodes. The results are shown in Tables 1 and 2. From the tables, it is clear that all the diodes have nearly ideal diode characteristics, as shown by the diode ideality factor n being close to one. The barrier height 0, ranged from 0.70 to 0.73 eV for n-GaAs and from 0.72 to 0.74 for p-GaAs. Similarly, the diode saturation current Js was in the range from 300 to 3 10 nA cmm2 for n-GaAs
and from 5.5 to 5.7 PA cm-’ for p-GaAs. In all the cases, the uniformity of the data from the diodes fabricated using photochemical passivation is evident, as shown by the lowest standard deviation in their respective diode parameters. The results are true for both n-type and p-type GaAs wafers. It is interesting to compare our experimental results with that of Butcher et al’ where results on Schottky barrier height modification on GaAs, following a sulphur based etch, were reported. Their results showed that the Schottky barrier height @, from C-V measurements was largely unchanged using both aqua regia etch and H,SO, based etch before the Schottky metallization. However, the zero bias barrier height @B,0,from 1-V measurements, increased after H,SO, etch; this was shown to be due to sulphur contamination. From the value of Js in our work, it is clear that the zero bias barrier height OB,,)is largely unchanged, regardless of the cleaning/etching procedures. It should be noted that there are some basic differences between our experimental approach and that of Butcher et al. They formed the Schottky diodes after aqua regia based cleaning, carried out electrical characterization, etched away the Schottky metal, carried out H,SO, based etching, formed the Schottky contact again and conducted I-V and C-V measurements. The modification of the GaAs surface as a result of the removal of Schottky metal and the subsequent H$O, etching cannot be ruled out in their work. When the diodes were aged at 200°C for times up to 1000 h, there was a gradual degradation in their electrical parameters. The diode ideality factor n did not exhibit any specific dependence with aging and remained within the range 1.0-1.2 for both ptype and n-type GaAs contacts. Conversely, the diode saturation current Js increased monotonically with aging time, regardless of surface preparation, as shown in Figure 1. From the figure, it is clear that the change in Js value, after aging, for diodes fabricated on photochemically passivated surfaces is much smaller than those formed using other techniques. In the case of Schottky barrier height, a slight decrease in the QB value was observed after 1000 h of aging, regardless of the surface preparation techniques. However, taking into account the error bands shown in Tables 1 and 2, we believe that there is no evidence for a difference in
Table 1. Electrical properties of Ti/n-GaAs Schottky diodes before any aging
Diode parameters
Photochemical passivation
HCI : HzO,
H,PQ
&SO,
& (eV) Js (nA cm-‘) No (lOI cm-‘) D,, (lOI cm-* eV_‘)
1.10+0.03 0.72 + 0.03 300+ 15 1.1 TO.1 1.75kO.04
1.15*0.05 0.72kO.06 305+25 0.9 ,O.l 1.80 k 0.05
1.09f0.08 0.72+0.08 310+29 1.1 10.2 1.60+0.10
1.19+0.11 0.70+0.10 304*35 1.2 kO.1 1.7OkO.20
Table 2. Electrical properties of Ti/p-GaAs Schottky diodes before any aging Diode parameters
Js (PA cm-*)
NA (lOI cm-‘) D,, (10” cmm2eV_‘)
128
Photochemical passivation
HCl : HZ02
H,PQ
HISO,
1.05 f 0.03 0.73 + 0.03 5.6 +0.2 1.3 +0.1 3.70+0.06
1.12&0.04 0.74 * 0.05 5.5 +0.5 1.1 kO.1 4.00&0.12
1.1450.08 0.72+0.08 5.7 +0.8 0.9 kO.2 3.8OkO.23
1.15+0.13 0.74kO.11 5.6 +l.l 1.0 +0.2 4.00*0.50
Krishnamachar
frasad:
(a)
Schottky
contacts
on GaAs surfaces
700
600
10 Aging
10
100
Aging
Time (h)
100 Time
(h)
Figure 1. The effect of 200°C aging on the diode saturation current J, of Ti Schottky contacts to (a) n-GaAs and (b) p-GaAs passivation, q-HCl : H,O,, l -H,PO, and O-H,SO,).
barrier height between the different processing methods used in this study, both before aging and after aging. The results are consistent with the observations of other workers.3. The electrical parameters of M-S contacts are governed by the quality of the M-S interface. A good M-S interface implies a diode with nearly ideal characteristics. In order to find out the quality of the M-S interface, interface state density, D,,, analysis was carried out6. The results are plotted in Figure 2, where the D,, value at E-E, = O.l5eV, E, being the intrinsic Fermi energy level in GaAs, is plotted as a function of aging time. Again, the variation in D,, is the smallest for photochemically passivated surfaces when compared with other cleaning techniques. The chemistry of reactions of various cleaning procedures used in GaAs device technology is complex. The various cleaning procedures used for GaAs surface preparation achieve the primary objective of leaving a contamination free surface. However, the stability of the GaAs surfaces is strongly dependent on the type of cleaning technique implemented, as shown in this study. For example, it is well known that photochemical passivation’ results in the formation of a film, about 5 nm thick, of elemental arsenic. In the case of other acid based cleaning techniques, it is likely that the cleaning leaves the GaAs surface with some weak bonds. Upon aging, it is believed that these bonds would break and form dangling bonds which effectively act as generation centres’ ; this would explain why the diode parameters of GaAs surfaces that were cleaned using either H,SO, or H3POd degrade
-1
10 Aging
100 Time
Figure 2. The effect of 200°C aging on the interface passivation,
q-HCl
: H20,,
l -H,PO,
(h)
1000
(a-photochemical
more rapidly than those formed on photochemically passivated surfaces. The presence of a thin layer of elemental arsenic, as a result of photochemical passivation, is expected to better stabilize the GaAs surface. Under such conditions, any active devices formed thereon would exhibit more stable and repeatable characteristics, as is evident from the parameter standard deviations observed in this work.
4. Conclusions A systematic study of various acid based surface cleaning procedures that are commonly used in GaAs device technology has been carried out. The results indicate that photochemical passivation of the GaAs surface is the best way of obtaining stable and contamination free GaAs surfaces. Diodes formed on photochemically passivated surfaces exhibit more uniform electrical parameters than those formed using either H,SO, or H,PO, or HCl: H,Oz solutions. Furthermore, after 1000 h of aging, the diode parameters of photochemically passivated surfaces exhibit a significantly lower degree of degradation than those of surfaces formed using any other acid based cleaning procedures.
References
IJ M Woodall,
P Oelhafen, T N Jackson, Vat Sci Technol. Bl, 795 (1983).
-1
100
IO Aging
Time
J L Freeouf
and G D Petti, /
1000
(h)
state density D,, of Ti Schottky contacts to (a) n-GaAs and (b) p-GaAs and O-H,S04). The D,, value at E-E, = 0.15 eV is shown in the figure.
(m-photochemical
129
Krjs~~arnachar
Prasad:
Schottky contacts
on GaAs surfaces
*S D Offsey, J M Woodall, A C Warren, P D Kirchner, T I Chappel and G D Petti, Appl Phys Lett, 48,475 (1986). ‘D R Lovell, T Yamamoto, M Inai, T Takebe and K Kobayishi, Japan J Appl Phys, 31, L924 (1992). 4K Prasad, L Faraone and A G Nassibian, Tkilz Solid F&E, 195, L11 (1991).
5S M Sze, Physics of Semiconductor De&es,
130
2nd Edn. John Wiley &
Sons, New York (1981). 6H H Tseng and C Y Wu, J Appl Phys, 61,299 (1987). 7K S A Butcher, D Alexiev, V W L Chin, T L Tansley, R J Egan and M Keane, Semicond Sci Technol, 8, 1451 (1993). ‘A L Fahrenbruch and R H Bube. Fun~~mensais of Solar Cells: P~ot~~Qltu~~ S&r Energy Conversion. Academic Press. London (1973).