- Email: [email protected]
Transparent ZnO ohmic contact on semi-insulating GaAs
Solid-State Electronics 43 (1999) 2021±2024
Transparent ZnO ohmic contact on semi-insulating GaAs V. Palumbo a, c, A. Valentini a, c,*, A. Cola b, F.M. Quaranta b a
INFN Ð Sezione di Bari, Dipartimento Interateneo di Fisica, Via Amendola 173, 70126 Bari, Italy CNR Ð Istituto per lo Studio di Nuovi Materiali per l'Elettronica, Via Arnesano, 73100 Lecce, Italy c INFM Ð UnitaÁ di Bari, Dipartimento Interateneo di Fisica, Via Amendola 173, 70126 Bari, Italy
b
Received 4 March 1999; received in revised form 3 June 1999; accepted 5 June 1999
Abstract We present a study of the ZnO/GaAs SI junction and its possible application in radiation detectors. Ion beam sputtered ZnO ®lms obtained in a H2 environment show low resistivity and very good optical transmission in the visible range. Current±voltage measurements reveal an ohmic behaviour of the ZnO contact on semi-insulating GaAs. To carry out an evaluation of the injection properties of the ZnO contact a ZnO/GaAs/Schottky structure has been realised. Under high reverse electric ®eld (14 104 V/cm) low minority injection from the ZnO contact has been observed. The same structure used as light detector has shown an eciency of 34% for a bias of 600 V at a wavelength of 670 nm. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction Semi-insulating (SI) GaAs barrier devices are widely used as room temperature radiation and particle detectors. GaAs has demonstrated greater radiation hardness to neutrons and g rays [1] with respect to Si, and semi-insulating GaAs has the advantage of being readily available in thicknesses of several hundred mm and having a high resistivity (1107 O cm) [2]. Because of the presence of intrinsic deep donor levels introduced during the semiconductor growth process (i.e. due to EL2 defects), reverse biased SI/ GaAs/Schottky diodes have shown to have a truncated electric ®eld distribution [3]. It has also been demonstrated that when the bias voltage approaches a value corresponding to an electric ®eld near to 1104 V/cm, a linear change of the depletion layer results [4]. At sucient high reverse bias the bulk is fully
* Corresponding author. E-mail address: [email protected] (A. Valentini)
depleted and the electric ®eld reaches the ohmic contact. This is a critical condition and a good ohmic contact should inject a minority carrier current as low as possible. The most used ohmic contact on nGaAs consists of an alloyed multilayer structure as Ni/Ge/Au or In/Au [5]. The same contacts are used on SI GaAs as well [1,2,6]. The alloyed contact, obtained by evaporation or sputtering, needs a post-deposition rapid thermal annealing that provides a local n+ doping of the GaAs. Some problems aect these alloyed contacts. Thermal treatment could damage any thin ®lm structure preexisting on the opposite side of the GaAs substrate. The eective behaviour of the contact is very sensitive to the GaAs surface states (i.e. etched or oxidised surface). The metal contact is opaque to radiation in the visible range. In this work we analyse, by means of I±V and C±V characteristics, the junction obtained by growing a low resistivity ZnO ®lm on SI GaAs. Moreover we present eciency measurements at various reverse voltages.
0038-1101/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 1 1 0 1 ( 9 9 ) 0 0 1 6 9 - 0
2022
V. Palumbo et al. / Solid-State Electronics 43 (1999) 2021±2024
Fig. 1. Optical transmittance for ion beam (IB) sputtered ZnO ®lm.
We shall demonstrate that ZnO forms a transparent ohmic contact on SI GaAs with a low minority current injection under a high electric ®eld. 2. Sample preparation Semi-insulating GaAs substrates were cut from a 200 mm thick wafer with both lapped and polished surfaces. The material has a high resistivity (1108 O cm) due to a low carrier density (1106 cmÿ3). The substrates were prepared by cleaning with organic solvents and by etching in a H2SO4:H2O2:H2O=4:1:10 solution for 30 s. After successive rinsing in doubly distilled water the samples were blown dry in nitrogen and quickly loaded into the deposition chamber. We classify the samples into two series: the ZnO/ GaAs/ZnO and the ZnO/GaAs/Ti specimens. ZnO ®lms (200 nm thick) were obtained by means of ion beam sputtering technique, starting from a sintered ZnO target of purity 99.999% [7]. The Ti ®lm (80 nm thick) were prepared by means of the same deposition technique [8]. An Au thin ®lm (50 AÊ) on Ti and a small dot of Cr/ Au thin ®lm (50 AÊ/500 AÊ) on ZnO provided better external contacts. The current±voltage characteristics were measured using a Keithley 237 high voltage supply/measure unit where the sample was kept in a liquid nitrogen cryostat. Various measurements were carried out in the temperature range 200±380 K. Photo-current measurements at various values of reverse voltage were obtained with the same apparatus con®guration and by means of a laser diode beam normally incident on the ZnO side. The capacitance±voltage characteristics were measured by using a HP4284A precision LCR meter.
Fig. 2. Current±voltage characteristics of the ZnO/GaAs/ZnO structure at opposite biases in the temperature range 240±360 K with a step of 20 K.
3. Results and discussion 3.1. ZnO ®lms properties ZnO ®lms deposited onto glass substrates show a sharp absorption edge at about 370 nm and an optical transmittance of about 90% in the 400±900 nm range (Fig. 1). The ®lms have a low resistivity (10ÿ2±10ÿ3 O cm), very stable in time [9]. Assuming that ZnO absorbs through allowed direct transition, the Moss±Burnstein shift of the absorption edge has been estimated to be 0.08±0.12 eV, leading to a free carrier density more then 4 1019 cm3 (i.e. the ZnO ®lms deposited are extrinsic). 3.2. ZnO/GaAs properties The I±V characteristics at various temperatures of the ZnO/GaAs/ZnO structure are shown in Fig. 2. The bending at low voltage and low temperature is typical of semi-insulating materials. Owing to the low carrier density, the thermodynamic equilibrium is slowly reached. This leads to a bulk memory eect that can not be ascribed to transport mechanism through the junctions. In the log±log scale all the curves have the same slope up to about 50 V. The linear behaviour implies that no current limitation mechanism is involved. The result is that ZnO forms a not rectifying contact on SI GaAs. The super linear behaviour for higher voltage is due to the presence of the typical high trap density of SI GaAs. The traps ®lled by the injected carriers do not represent any more limitation to the current ¯ow, hence a sharp increase is observed (TFL regime) [9].
V. Palumbo et al. / Solid-State Electronics 43 (1999) 2021±2024
Fig. 3. Current±voltage characteristics for the ZnO/GaAs/Ti structure with positive bias on the Ti contact (solid curves) and positive bias on ZnO contact (dotted curves) in the temperature range 240±360 K with a step of 20 K.
This is a bulk eect and can not be considered as a breakdown. The ohmic behaviour of the ZnO/GaAs junction is in contrast with a simple Anderson model [10], which assumes an ideal interface. According to this model the band diagram should present a conduction band discontinuity DEC=0.28 eV, a valence band discontinuity DEV=2.23 eV and a band bending given by qVD fGaAs ÿ fZnO 0:44 eV, where f GaAs and f ZnO are the work functions. Since the I±V characteristics do not show the presence of a potential barrier we deduce that a Fermi level pinning to the GaAs surface results from the ZnO deposition process. To evaluate the minority carrier injection from the ZnO contact, a sample with a ZnO/GaAs/Ti structure
Fig. 5. Eciency versus reverse applied voltage for the ZnO/ GaAs/Ti structure at the wavelength of 670 nm.
has been analysed. It is well known that the Ti/GaAs junction has a Schottky barrier of about 0.8 eV [8,11]. The I±V characteristics, reported in Fig. 3, show the presence of the Ti side barrier. At reverse voltage higher than 400 V the electric ®eld distribution reaches the ZnO contact and the carrier injection give rise to the current increase. In this condition the GaAs is fully depleted and the measured current is the result of the low current injected from the ZnO contact. A more accurate determination of the full depletion voltage is given by C±V measurements at various signal probe frequencies. In Fig. 4 we present 1/C 2 against the reverse voltage at the frequencies of 50 Hz and 1 MHz. At low frequencies the experimental data are ®tted by the linear dependence obtained from the Schottky model with the presence of traps [12]. The slope of this straight line is 2/qes(Nd+Nt), where es is the semiconductor dielectric constant, Nd and Nt the net ionised donor and trap densities respectively. By neglecting Nd, we calculate Nt 1 1015 cm3, which is a value comparable with those reported in the literature [13]. For a suciently high reverse voltage, the modulated charge reaches the ZnO ohmic contact and the capacitance is now given by the geometrical capacitance CS
Fig. 4. 1/C2 versus reverse applied voltage for the ZnO/GaAs/ Ti structure at dierent signal frequencies.
2023
es w
where S is the Schottky contact area, w the GaAs thickness. At high frequency, the traps cannot follow the probe signal and the capacitance value is given by the last equation for every bias voltage. The fully depletion voltage is calculated at 370 V, by the intersection of the linear ®ts at low and high frequency. This ZnO/GaAs/Ti structure could be used as radi-
2024
V. Palumbo et al. / Solid-State Electronics 43 (1999) 2021±2024
ation detector if exposed from the ZnO side. The eciency should be considered as the quantum eciency, i.e. the ratio between the collected charge and the generated charge. As the ZnO contact is not an ecient hole injector, when pairs are photo-generated at a positive bias contact, the current signal is due only to the photo-generated (primary) holes, and the eciency is unity if all primary holes are collected at the negatively biased Ti contact. Figure 5 reports the eciency as a function of the applied voltage (negative on Ti) as calculated from the photo-current data due to a 670 nm diode laser light that impinges on the ZnO contact. In order to calculate the eciency, the ZnO ®lm re¯ectivity has been measured and taken into account. At voltage higher than 400±500 V the eciency rises, sharply reaching a value of 34% at 600 V. The holes, which are photo-generated in a thin GaAs layer below the ZnO contact have to be accelerated by the electric ®eld and the higher the applied voltage is, the higher is the extension of the electric ®eld towards the ZnO contact and the better is the carrier collection. If suitably cooled, the dark current can be strongly reduced and a very high photo-current/dark current ratio can be obtained even up to 800 V. 4. Conclusions In this work we have shown that a low resistivity ZnO contact on semi-insulating GaAs has ohmic behaviour. The lack of a potential barrier, as indeed expected by the Anderson model, could be ascribed to Fermi level pinning at the GaAs surface. Furthermore, the absence of a barrier involves many diculties in the characterisation of the interfacial defects responsible for the conduction mechanism. A ®rst important result is that under high electric ®eld the ZnO contact injects a low minority current. This makes the ZnO attractive as an ohmic contact on semi-insulating GaAs in comparison to an alloyed contact, which needs a thermal process.
The second result is that the ZnO/GaAs/Ti structure shows a photo-response with eciency higher than 30% at 670 nm. In addition the same structure could be used as X, g or particle detector.
Acknowledgements The authors wish to acknowledge G. Casamassima for his technical assistance in the experiments.
References [1] McGregor DS, Lindsay JT, Brannon CC, Olsen RW. Nuclear Instruments and Methods in Physics Research A 1996;380:271±5. [2] Berwick K, Brozel MR, Buttar CM, Pooni JS, Sellin PJ, Young SM. Nuclear Instruments and Methods in Physics Research A 1996;380:46±9. [3] Cola A, Reggiani L, Vasanelli L. Journal of Applied Physics 1997;81(2):997±9. [4] Castaldini A, Cavallini A, del Papa C, Alietti M, Canali C, Nava F, Lanzieri C. Scanning Microscopy 1995;9:1±6. [5] Rideout VL. Solid State Electronics 1975;18:541±50. [6] Chen JW, Ebling DG, et al. Applied Physics Letters 1997;70(20):2693±5. [7] Valentini A, Quaranta F, Penza M, Rizzi FR. Journal of Applied Physics 1993;73(3):1143±5. [8] Cola A, Lupo MG, Vasanelli L, Valentini A. Solid State Electronics 1993;36(5):785±9. [9] Eizemberg M, Hovel HJ. Journal of Applied Physics 1991;69(4):2256±63. [10] Milnes AG, Feucht DL. Heterojunctions and metalsemiconductor junctions. New York: Academic Press, 1972. [11] Cola A. Gallium arsenide and related compounds. In: Third International Workshop. World Scienti®c, 1995. p. 217. [12] Rhoderick EH, Williams RH. Metalsemiconductor contacts. Oxford: Clarendon Press, 1988. [13] Rogalla M, Chen JW, et al. Nuclear Instruments and Methods in Physics Research A 1996;380:14±7.
Transparent ZnO ohmic contact on semi-insulating GaAs V. Palumbo a, c, A. Valentini a, c,*, A. Cola b, F.M. Quaranta b a
INFN Ð Sezione di Bari, Dipartimento Interateneo di Fisica, Via Amendola 173, 70126 Bari, Italy CNR Ð Istituto per lo Studio di Nuovi Materiali per l'Elettronica, Via Arnesano, 73100 Lecce, Italy c INFM Ð UnitaÁ di Bari, Dipartimento Interateneo di Fisica, Via Amendola 173, 70126 Bari, Italy
b
Received 4 March 1999; received in revised form 3 June 1999; accepted 5 June 1999
Abstract We present a study of the ZnO/GaAs SI junction and its possible application in radiation detectors. Ion beam sputtered ZnO ®lms obtained in a H2 environment show low resistivity and very good optical transmission in the visible range. Current±voltage measurements reveal an ohmic behaviour of the ZnO contact on semi-insulating GaAs. To carry out an evaluation of the injection properties of the ZnO contact a ZnO/GaAs/Schottky structure has been realised. Under high reverse electric ®eld (14 104 V/cm) low minority injection from the ZnO contact has been observed. The same structure used as light detector has shown an eciency of 34% for a bias of 600 V at a wavelength of 670 nm. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction Semi-insulating (SI) GaAs barrier devices are widely used as room temperature radiation and particle detectors. GaAs has demonstrated greater radiation hardness to neutrons and g rays [1] with respect to Si, and semi-insulating GaAs has the advantage of being readily available in thicknesses of several hundred mm and having a high resistivity (1107 O cm) [2]. Because of the presence of intrinsic deep donor levels introduced during the semiconductor growth process (i.e. due to EL2 defects), reverse biased SI/ GaAs/Schottky diodes have shown to have a truncated electric ®eld distribution [3]. It has also been demonstrated that when the bias voltage approaches a value corresponding to an electric ®eld near to 1104 V/cm, a linear change of the depletion layer results [4]. At sucient high reverse bias the bulk is fully
* Corresponding author. E-mail address: [email protected] (A. Valentini)
depleted and the electric ®eld reaches the ohmic contact. This is a critical condition and a good ohmic contact should inject a minority carrier current as low as possible. The most used ohmic contact on nGaAs consists of an alloyed multilayer structure as Ni/Ge/Au or In/Au [5]. The same contacts are used on SI GaAs as well [1,2,6]. The alloyed contact, obtained by evaporation or sputtering, needs a post-deposition rapid thermal annealing that provides a local n+ doping of the GaAs. Some problems aect these alloyed contacts. Thermal treatment could damage any thin ®lm structure preexisting on the opposite side of the GaAs substrate. The eective behaviour of the contact is very sensitive to the GaAs surface states (i.e. etched or oxidised surface). The metal contact is opaque to radiation in the visible range. In this work we analyse, by means of I±V and C±V characteristics, the junction obtained by growing a low resistivity ZnO ®lm on SI GaAs. Moreover we present eciency measurements at various reverse voltages.
0038-1101/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 1 1 0 1 ( 9 9 ) 0 0 1 6 9 - 0
2022
V. Palumbo et al. / Solid-State Electronics 43 (1999) 2021±2024
Fig. 1. Optical transmittance for ion beam (IB) sputtered ZnO ®lm.
We shall demonstrate that ZnO forms a transparent ohmic contact on SI GaAs with a low minority current injection under a high electric ®eld. 2. Sample preparation Semi-insulating GaAs substrates were cut from a 200 mm thick wafer with both lapped and polished surfaces. The material has a high resistivity (1108 O cm) due to a low carrier density (1106 cmÿ3). The substrates were prepared by cleaning with organic solvents and by etching in a H2SO4:H2O2:H2O=4:1:10 solution for 30 s. After successive rinsing in doubly distilled water the samples were blown dry in nitrogen and quickly loaded into the deposition chamber. We classify the samples into two series: the ZnO/ GaAs/ZnO and the ZnO/GaAs/Ti specimens. ZnO ®lms (200 nm thick) were obtained by means of ion beam sputtering technique, starting from a sintered ZnO target of purity 99.999% [7]. The Ti ®lm (80 nm thick) were prepared by means of the same deposition technique [8]. An Au thin ®lm (50 AÊ) on Ti and a small dot of Cr/ Au thin ®lm (50 AÊ/500 AÊ) on ZnO provided better external contacts. The current±voltage characteristics were measured using a Keithley 237 high voltage supply/measure unit where the sample was kept in a liquid nitrogen cryostat. Various measurements were carried out in the temperature range 200±380 K. Photo-current measurements at various values of reverse voltage were obtained with the same apparatus con®guration and by means of a laser diode beam normally incident on the ZnO side. The capacitance±voltage characteristics were measured by using a HP4284A precision LCR meter.
Fig. 2. Current±voltage characteristics of the ZnO/GaAs/ZnO structure at opposite biases in the temperature range 240±360 K with a step of 20 K.
3. Results and discussion 3.1. ZnO ®lms properties ZnO ®lms deposited onto glass substrates show a sharp absorption edge at about 370 nm and an optical transmittance of about 90% in the 400±900 nm range (Fig. 1). The ®lms have a low resistivity (10ÿ2±10ÿ3 O cm), very stable in time [9]. Assuming that ZnO absorbs through allowed direct transition, the Moss±Burnstein shift of the absorption edge has been estimated to be 0.08±0.12 eV, leading to a free carrier density more then 4 1019 cm3 (i.e. the ZnO ®lms deposited are extrinsic). 3.2. ZnO/GaAs properties The I±V characteristics at various temperatures of the ZnO/GaAs/ZnO structure are shown in Fig. 2. The bending at low voltage and low temperature is typical of semi-insulating materials. Owing to the low carrier density, the thermodynamic equilibrium is slowly reached. This leads to a bulk memory eect that can not be ascribed to transport mechanism through the junctions. In the log±log scale all the curves have the same slope up to about 50 V. The linear behaviour implies that no current limitation mechanism is involved. The result is that ZnO forms a not rectifying contact on SI GaAs. The super linear behaviour for higher voltage is due to the presence of the typical high trap density of SI GaAs. The traps ®lled by the injected carriers do not represent any more limitation to the current ¯ow, hence a sharp increase is observed (TFL regime) [9].
V. Palumbo et al. / Solid-State Electronics 43 (1999) 2021±2024
Fig. 3. Current±voltage characteristics for the ZnO/GaAs/Ti structure with positive bias on the Ti contact (solid curves) and positive bias on ZnO contact (dotted curves) in the temperature range 240±360 K with a step of 20 K.
This is a bulk eect and can not be considered as a breakdown. The ohmic behaviour of the ZnO/GaAs junction is in contrast with a simple Anderson model [10], which assumes an ideal interface. According to this model the band diagram should present a conduction band discontinuity DEC=0.28 eV, a valence band discontinuity DEV=2.23 eV and a band bending given by qVD fGaAs ÿ fZnO 0:44 eV, where f GaAs and f ZnO are the work functions. Since the I±V characteristics do not show the presence of a potential barrier we deduce that a Fermi level pinning to the GaAs surface results from the ZnO deposition process. To evaluate the minority carrier injection from the ZnO contact, a sample with a ZnO/GaAs/Ti structure
Fig. 5. Eciency versus reverse applied voltage for the ZnO/ GaAs/Ti structure at the wavelength of 670 nm.
has been analysed. It is well known that the Ti/GaAs junction has a Schottky barrier of about 0.8 eV [8,11]. The I±V characteristics, reported in Fig. 3, show the presence of the Ti side barrier. At reverse voltage higher than 400 V the electric ®eld distribution reaches the ZnO contact and the carrier injection give rise to the current increase. In this condition the GaAs is fully depleted and the measured current is the result of the low current injected from the ZnO contact. A more accurate determination of the full depletion voltage is given by C±V measurements at various signal probe frequencies. In Fig. 4 we present 1/C 2 against the reverse voltage at the frequencies of 50 Hz and 1 MHz. At low frequencies the experimental data are ®tted by the linear dependence obtained from the Schottky model with the presence of traps [12]. The slope of this straight line is 2/qes(Nd+Nt), where es is the semiconductor dielectric constant, Nd and Nt the net ionised donor and trap densities respectively. By neglecting Nd, we calculate Nt 1 1015 cm3, which is a value comparable with those reported in the literature [13]. For a suciently high reverse voltage, the modulated charge reaches the ZnO ohmic contact and the capacitance is now given by the geometrical capacitance CS
Fig. 4. 1/C2 versus reverse applied voltage for the ZnO/GaAs/ Ti structure at dierent signal frequencies.
2023
es w
where S is the Schottky contact area, w the GaAs thickness. At high frequency, the traps cannot follow the probe signal and the capacitance value is given by the last equation for every bias voltage. The fully depletion voltage is calculated at 370 V, by the intersection of the linear ®ts at low and high frequency. This ZnO/GaAs/Ti structure could be used as radi-
2024
V. Palumbo et al. / Solid-State Electronics 43 (1999) 2021±2024
ation detector if exposed from the ZnO side. The eciency should be considered as the quantum eciency, i.e. the ratio between the collected charge and the generated charge. As the ZnO contact is not an ecient hole injector, when pairs are photo-generated at a positive bias contact, the current signal is due only to the photo-generated (primary) holes, and the eciency is unity if all primary holes are collected at the negatively biased Ti contact. Figure 5 reports the eciency as a function of the applied voltage (negative on Ti) as calculated from the photo-current data due to a 670 nm diode laser light that impinges on the ZnO contact. In order to calculate the eciency, the ZnO ®lm re¯ectivity has been measured and taken into account. At voltage higher than 400±500 V the eciency rises, sharply reaching a value of 34% at 600 V. The holes, which are photo-generated in a thin GaAs layer below the ZnO contact have to be accelerated by the electric ®eld and the higher the applied voltage is, the higher is the extension of the electric ®eld towards the ZnO contact and the better is the carrier collection. If suitably cooled, the dark current can be strongly reduced and a very high photo-current/dark current ratio can be obtained even up to 800 V. 4. Conclusions In this work we have shown that a low resistivity ZnO contact on semi-insulating GaAs has ohmic behaviour. The lack of a potential barrier, as indeed expected by the Anderson model, could be ascribed to Fermi level pinning at the GaAs surface. Furthermore, the absence of a barrier involves many diculties in the characterisation of the interfacial defects responsible for the conduction mechanism. A ®rst important result is that under high electric ®eld the ZnO contact injects a low minority current. This makes the ZnO attractive as an ohmic contact on semi-insulating GaAs in comparison to an alloyed contact, which needs a thermal process.
The second result is that the ZnO/GaAs/Ti structure shows a photo-response with eciency higher than 30% at 670 nm. In addition the same structure could be used as X, g or particle detector.
Acknowledgements The authors wish to acknowledge G. Casamassima for his technical assistance in the experiments.
References [1] McGregor DS, Lindsay JT, Brannon CC, Olsen RW. Nuclear Instruments and Methods in Physics Research A 1996;380:271±5. [2] Berwick K, Brozel MR, Buttar CM, Pooni JS, Sellin PJ, Young SM. Nuclear Instruments and Methods in Physics Research A 1996;380:46±9. [3] Cola A, Reggiani L, Vasanelli L. Journal of Applied Physics 1997;81(2):997±9. [4] Castaldini A, Cavallini A, del Papa C, Alietti M, Canali C, Nava F, Lanzieri C. Scanning Microscopy 1995;9:1±6. [5] Rideout VL. Solid State Electronics 1975;18:541±50. [6] Chen JW, Ebling DG, et al. Applied Physics Letters 1997;70(20):2693±5. [7] Valentini A, Quaranta F, Penza M, Rizzi FR. Journal of Applied Physics 1993;73(3):1143±5. [8] Cola A, Lupo MG, Vasanelli L, Valentini A. Solid State Electronics 1993;36(5):785±9. [9] Eizemberg M, Hovel HJ. Journal of Applied Physics 1991;69(4):2256±63. [10] Milnes AG, Feucht DL. Heterojunctions and metalsemiconductor junctions. New York: Academic Press, 1972. [11] Cola A. Gallium arsenide and related compounds. In: Third International Workshop. World Scienti®c, 1995. p. 217. [12] Rhoderick EH, Williams RH. Metalsemiconductor contacts. Oxford: Clarendon Press, 1988. [13] Rogalla M, Chen JW, et al. Nuclear Instruments and Methods in Physics Research A 1996;380:14±7.