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Neutron transmutation doping of liquid phase epitaxial gallium arsenide
NIOMI B
Nuclear Instruments and Methods in Physics Research B 86 (1994) 288-292 North-Holland
Neutron transmutation gallium arsenide
Beam Interactions with Materials&Atoms
doping of liquid phase epitaxial
D. Alexiev a~*, K.S.A. Butcher b, M. Edmondson a and T.L. Tansley b ‘AustralianNuclear Science and Technology Organisation, Menai, NSW 2234, Australia b Semiconductor Science and Technology Laboratories, Physics Department, Macquarie University, North Ryde, NSW 2109, Australia
Received 23 August 1993 and in revised form 17 November 1993
Neutron transmutation doping (NTD) was studied as a means of compensating p-type liquid phase epitaxial (LPE) gallium arsenide. It is shown that such epitaxial layers can be transmuted into approximately 3 x 1013 crne3 n-type material, defect free after appropriate thermal annealing.
1. Introduction
Neutron transmutation doping (NTD) of semiconductor material was first demonstrated by Cleland et al. [l]. They showed that a neutron flux can produce homogeneous distribution of dopants in silicon. This work was continued by Colin [2] and Cuevas [3] and is now routinely used in commercial doping of silicon for the production of very high-power devices. In Si, only 3oSi is used to produce phosphorus. 3oSi is one of three naturally occurring isotopes and is present in Si at a concentration of about 3.1%, and is also homogeneously distributed. Thus, when 31P is introduced by NTD it acts as a stable donor when it is situated in a substitutional lattice site and is uniformly distributed. During the NTD process the material is subject to thermal neutron fluence. A lattice atom can capture a thermal neutron and becomes an excited isotope. To reach its ground state, the isotope emits y and p particles with the recoil displacing the atom from its original location into an interstitial position. Further atomic displacements can arise due to energetic neutrons which will inevitably accompany a thermal neutron fluence. Thus, after NTD, Si has to be annealed [4] to remove the induced damaged and to relocate the 31P into substitutional sites. NTD of GaAs has received only limited attention when compared to NTD of Si. NTD doping of GaAs was first studied by Mirianashvili et al [5], who demonstrated that an efficiency of the order of lo3 times higher than NTD of Si can be achieved. Gallium ar-
* Corresponding author, tel. +61 2 717 3182, fax +61 2 717 9265.
senide contains three naturally occurring isotopes, all act in the transmutation doping process as shown in Table 1. If all the stable isotopes of Ge and Se are located on Ga and As sites, then each will act as a shallow donor. This means that NTD only has one result in GaAs: p-type material can be converted to n-type or n-type can be altered to lower resistivity n-type. Vesaghi [6] expressed the efficiency of the doping process as N,=Kc$t,
(1) where 4 is the thermal flw, n cm-’ s-l, t is the time exposed to the neutron flux, K is the NTD constant and equal to 0.16, and No is the concentration of the donor produced by the NTD process. Similarly to NTD-Si, transmuted atoms in GaAs are not in their original locations and are displaced into an interstitial position. As with Si irradiation, this defect damage can be reduced or totally removed by annealing. For bulk GaAs, annealing temperatures suggested by Vesaghi [6] are in the range of 800 to 900°C with lower temperatures for epitaxial wafers, suggesting that Table 1 Production of donor atoms in GaAs by NTD Transmutation reaction of GaAs
Capture cross Half-life section for thermal neutrons [b]
P Ga6’(n, y)Ga” -+ Ge’O 1.68 B Ga%, r)Ga’*-r Ge’* 4.86 P As’%, Y)AS’~+ Se76 4.30
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bulk GaAs is associated with a high concentration of defects and impurities which may act as binding sites for the transmuted atoms. The need for higher annealing temperatures inevitably creates an out-diffusion of As and possible contamination of the GaAs with common metallic impurities, particularly Cu, from the silica furnace tube and the silica sample pedestal within the furnace. To avoid both dissociation and contamination, the GaAs can be encapsulated in phosphosilicate glass [7] or in a Ga film [8]. Annealing of the material can also be done by rapid isothermal annealing and quenching, often termed as flash annealing in silicon wafer processing. For these reasons annealing of LPE-GaAs has to be examined as a separate entity. Thus, the goal of the present work is to counter-dope p-type LPE epitaxies with stable transmuted isotopes of Ge and Se to produce near 1013 to 1014 crnp3 carrier concentration, n-type epitaxies with no detectable deep level defects. That is epitaxial GaAs of sufficient purity for construction into nuclear radiation detectors [9]. It should be noted, however, that this material has many properties which are in contrast to semi-insulating (SI) GaAs, which itself has received much recent attention as a potential radiation detector material [lo-131. In particular the LPE material is conducting with smaller depletion regions than more highly compensated SI material. The great advantage of this LPE material is that it can be grown free of the deep level traps which degrade detector charge carrier collection efficiencies and lower spectral peak resolution. The ability to apply NTD to p-type LPE GaAs of moderate free carrier concentration is an important step in realising a higher yield of detector quality material.
2. Experimental A 300 pm thick LPE GaAs epitaxy grown on a conducting n-type substrate (MCP 1 x 1016 cmm3 carrier concentration) using the transient temperature growth method in a Si-0 environment, was cleaved into approximately 6 mm squares. These LPE GaAs samples were then prepared according to accepted procedures. Briefly, after the LPE GaAs samples were degreased in xylene, washed in methanol and finally rinsed in 18 Ma cm H,O, the sections were then etched one at a time in hot (- 60°C) solutions of 3H,SO, : lH,O, : lH,O for 2 minutes and quenched in 18 MR cm HzO. The resulting surfaces had a mirror finish with rounded edges; one such sample was angle lapped (Fig. 1) metallised by evaporating a number of 1 mm diameter aluminium areas on the lapped epitaxy and a gold ohmic contact on the back of the substrate. The resulting Schottky-barrier diodes were then pro-
289 Diode
1
Fig. 1. Angle lapped 300 pm n-type epitaxy on n-type substrate. filed for the net carrier concentration (N(x)) using capacitance-voltage (C-V) measurements. The standard equation given by Sze [14] was used for these calculations; i.e. N(x)
=
z
1
(2)
I’ where the symbols have their usual meanings. The plots of l/C2 vs V, from which N(n) values were calculated, revealed normal slopes without any of the flattening which occurs for SI GaAs when that material is fully depleted. For the C-V measurements capacitances were measured up to a maximum reverse bias of 10 V. At this bias a depletion width (W) of 25 km would be expected for the samples with the lowest carrier concentrations measured for this paper (i.e. 2 X 1013 cmp3>, as calculated from the equation [14] qEs
d(l/C’)/dv
(3) where again the symbols have their usual meaning. This depletion width is well below the full 300 km thickness of the epitaxial layer. Using the angle lapped sample N(x) was found to be 0.9 to 1.2 X 1015 cmp3 over the 300 pm thickness of the epitaxy; the polarity of the material being p-type. A capacitance deep level transient spectrum (DLTS) revealed that the LPE GaAs had no deep levels when measured between the temperature range of 11 to 360 K. The DLTS system used was a correlation system of the type first described by Miller [15] which has the advantage of high sensitivity being able to detect majority traps at levels of 1O-4 that of the net carrier concentration [16]. A correlation time constant (TJ of 10 ms was used for all the DLTS measurements. From the initial C-V measurements and using a published transmutation value of K = 0.16 [6], an irradiation sequence can be calculated so that the resulting transmuted N(x) will have values progressing from low p to low n-type carrier concentration. The LPE GaAs samples for neutron irradiation were then sealed in polythene pouches to exclude any surface cross contamination and submitted to the irradiation facility at HIFAR ‘r. All irradiations were done in RIG X7, #’ HIFAR is a 10 MW DID0 class research reactor, located at Lucas Heights, NSW, Australia.
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D. Alexiev et al. / Nucl. In&. and Meth. in Phys. Res. B 86 (1994) 288-292
> EfWT
Fig. 2. Spectrum of residual radioactivity produced by NTD of LPE-GaAs.
having a flux rating of 4 x 1012 ncmm2 s-i k 10%. After irradiation, a period of 14 days was allowed to elapse so that the induced radioactivity was below the international accepted standard #‘. The residual radioactivity of the NTD-LPE GaAs was measured using a 35 cm3 Ge(Li) detector. The y-ray spectrum (Fig. 2) showed remanence photo-peaks of 16As and “Ga. No contamination photo-peaks such as i9’Au (Au being a common replating impurity from HF acids) were noted [8]. Other lines noted are of K4’ and Cs13’ seen usually as background in the area locating the spectrometer facility. Since neutron generated defects can be expected in the atomic structure of the NTD-LPE GaAs, a DLTS spectrum and a capacitance versus temperature (C-T) plot were made prior to temperature annealing of the n-type transmutation doped samples. The DLTS spectrum (Fig. 3) showed a high intensity defect located at about 360 K (rc = 10 ms). The variation of capacitance with temperature for a C-T plot of the same sample showed that this trap is about 100% larger than the net carrier concentration, thus this trap could not be measured [18].
3. Thermal annealing Initial thermal annealing was attempted using a rapid thermal annealer (RTA) capable of pulsing heat, using halogen lamps for a few seconds. Temperatures in the range of 600 to 1000°C in steps of 100°C can be induced. Such RTAs are widely used in ion implanted GaAs technology [19] and it has been demonstrated by Wagner et al. [20], that it should be possible, in principle, to pulse anneal NTD GaAs. However, others [21]
X2 IAEA Safety Series no. 6 [IAEA 19851states international accepted standards for the removal and transport of radioactive packages 1171.
Fig. 3. DLTS spectrum of NTD n-type LPE-GaAs sample, showing an induced defect at approximately 340 K (7, = 10 ms).
found that RTA is not uniform throughout the thickness of the sample, producing dark spots associated with high dislocations when viewing the annealed samples using cathodoluminescence microscopy. Temperatures selected for annealing were of the order of 800°C for a period of 10 s duration, heated in air, using a laboratory constructed RTA. Such pulse annealing was followed by polishing the epitaxial surface, chemical cleansing and a 30 s dip etch in 3H,SO, : lH,O, : lH,O at 40°C. When examining the LPE surfaces with a microscope, it was noted that parallel structural slip lines appeared - an obvious detrimental effect to the material. This structural slip could be explained, as Third and Weinberg [21] found, by the temperature distribution through the sample being non-uniform. Further attempts at different temperatures and periods produced similar results. For these reasons, RTA was abandoned and annealing was transferred to a resistance heater with a hydrogen gas flow, as shown in Fig. 4. The following method for thermal annealing of the NTD-LPE GaAs was found to be successful and hence adopted throughout the following work. The GaAs samples were first dip etched in 3H2S04 : lH,O,: lH,O at 40°C for 10 s, Ga was then applied to the substrate of the LPE GaAs and to the inner surface of a freshly etched (1HF : 3HN0,) spectrosil silica crucible. In each case a Ga film was formed. The importance of using Ga films on contacting surfaces is to eliminate, by gettering, fast diffusers such as Cu, a common environmental contaminant when using silica and other refractory components at high temperatures. When the furnace reached set temperature, the crucible containing the samples was drawn into the hot region and removed after a predetermined period. It was found that 600°C and a period of 5 min removed all structural defects noted before with the DLTS
D. Alexiev et al. / Nucl. Instr. and Meth. in Phys. Rex B 86 (1994) 288-292
291
Fig. 4. Annealing of neutron irradiated LPE-GaAs. spectra for the samples transmuted to n-type carrier concentration. When using a lower temperature, 550°C and the same period, some trace of the electron trap remained. A temperature profile for the sample in the resistance heater is shown in Fig. 5. All NTD-LPE GaAs samples were annealed in this manner, followed by HCl etching at 60°C for 0.5 h to remove the Ga and dip etch metallisation. The C-I, relation was measured to determine the N(X) value (again using a maximum reverse bias of no greater than 10 V> and the transmutation constant for each sample, as summarised in Table 2 and Fig. 6 which shows a graph of K versus +t. The polarity of the diodes shown in Table 2 was determined by the direction of increase of the depletion layer formed beneath the Schottky barrier. The capacitance of this layer followed the relation given by Eq. (2) when the devices were placed in reverse bias. Current saturation also occurred in reverse bias. It can be noted from Fig. 6 that K varies with 4t. Errors in the irradiation facility such as neutron flux variation (rated at &lo%,> and operator timing error could account for some of this error. The variation of carrier concentration throughout the sample may account for more of the error. The thermal annealing
process itself may also produce secondary effects in this material which could account for some of the remaining error. The effect of thermal annealing on the net carrier concentration of LPE GaAs has been outlined by Alexiev et al. [22] who have shown that long term annealing for 40 h at 600°C can cause a significant change in the net carrier concentration. Although the annealing times used here were considerably shorter than that of Alexiev et al. some lattice relaxation or impurity relocation may occur on a shorter time scale introducing some error in the net carrier concentration change (AN) ascribed to the NTD process. One further potential error source exists. No DLTS of the p-type samples was performed since for our purposes (i.e. the production of high purity n-type GaAs for construction into nuclear radiation detectors) only electron traps are of consequence, this is because the devices normally constructed are Schottky barriers which are majority carrier devices. DLTS usually only 0.2
I
I
I
I
I
I
0 0 Y
z0 t; c
--
__--------------0.1 0
8 ____.._______ 0
:
. . ..______......____.....................
I 0
I
I
I
I
2
I
6
8
I 10
I
I
I
ie
14
16
sbc.
Fig. 5. Annealing
profile (temperature versus time) of neutron irradiated LPE-GaAs.
.x
~ . . . . .._.........
I
0.0
0
0
4
6
8
10 qst x
I
12
14
10'5".cm
16
16
20
-2
Fig. 6. A plot for the transmutation constant K versus neutron flux (dt) showing the average value (dashed line) and the standard deviation (dotted line).
D. Alexiev et al. / Nucl. Instr. and Meth. in Phys. Rex B 86 (I 994) 288-292
292 Table 2 Excess donor/acceptor Sample no.
concentration
as a function of irradiated neutron dose
Start N(x) [cmw3]
(bt used
0.9 to 1.2 x 10’5 p-type same
5 x 101s 6.25 x 10”
same
7.5 x 1015
same
1.25 x 1016
same
1.875 x 1016
[ncm-*I
detects majority carrier traps, so this does leave unresolved the possibility that hole traps could have been generated during the NTD process which were not removed by the thermal anneal. Such trapping levels
would be low since our l/C2 plots were unaffected by them, however, these traps, if present, could have introduced some error into the measurement of the net carrier concentration for the p-type samples thereby affecting our calculation of K. To test the relationship between K and +t another group of LPE GaAs samples were prepared and irradiated as before but using a value of K = 0.1 from the graph to calculate required fluence ($t = 1.02 X 1Or6 ncm-2) for near compensation. After processing the irradiated samples as described above, values for N(x) = 2 to 6 X 1013 cm-3, n-type were obtained.
4. Conclusion It has been shown, for the first time, that neutron transmutation doping of p-type LPE GaAs can produce high quality material with a very low net carrier concentration. In the NTD experiments 2-l X lOI5 cmm3 p-type LPE GaAs were compensated to 2-8 x 1013 cmp3 net carrier concentration, n-type. The material, after thermal annealing had no traps detectable by DLTS analysis. It is expected that such material should
prove useful for device construction, specifically for construction of LPE GaAs radiation detectors.
References [l] J.W. Cleland, K. Lark-Horovitz and J.C. Pigg, Phys. Rev. 78 (1950) 814. [2] S. Colin, Phys. Rev. 132 (1963) 178.
Resultant N(x) [cmm3]
AN achieved [cm-‘]
K = AN/&
1 x 10’4 p-type 6~10’~ p-type 4.8 x 1014 p-type 1.5 x 10’4 n-type 7.3 x 1014 n-type
0.9 x 101s
0.18
0.94 x 10’5
0.15
5.2x 1014
0.069
1.15 x 10’5
0.09
1.73 x 10’5
0.09
131M. Cuevas, Phys. Rev. 164 (1967) 1021. [41 E.M. Lawson, P.J. Lee and A.J. Tavendale, AAEC unpublished report, AP/TN/189 (1984). [51 Sh.M. Mirianashvili and D.J. Nanobashvili, Sov. Phys. Semicond. 4 (1971) 1612. [61 M.A. Vesaghi, Phys. Rev. B. 25 (1982) 5436. [71 G. Mathur, M.L. Wheaton, J.M. Borrego and S.K. Ghandi, J. Appl. Phys. 57 (1985) 4711. PI D. Alexiev and K.S.A. Butcher, Nucl. Instr. and Meth. B 83 (1993) 430. 191 D. Alexiev and K.S.A. Butcher, Nucl. Instr. and Meth. A 317 (1992) 111. [lOI R. Bertin, S.D.‘Auria, C. Del Papa, F. Fiori, B. lisowski, V. O’Shea, P.G. Pelfer, K. Smith and A. Zichichi, Nucl. Instr. and Meth., A 294 (1990) 211. Ml D.S. McGregor, G.F. Knoll, Y. Eisen and R. Brake, IEEE Trans. Nucl. Sci. 39 (1992) 1226. WI S.P. Beaumont et al., Nucl. Instr. and Meth. A 322 (1992) 472. [131 T.J. Sumner, S.M. Grant, A. Bewick, J.P. Li, K. Smith and S.P. Beaumont, Nucl. Instr. and Meth. A 322 (1992) 514. [141 S.M. Sze, Physics of Semiconductor Devices, 2nd ed. (Wiley, New York, 19811, 248/249. [151 G.L. Miller, J.V. Ramirez and D.A.H. Robinson, J. Appl. Phys. 46 (1975) 2638. Ml J.L. Benton, J. Crystal Growth 106 (1990) 116. 1171 IAEA - Regulations for Safe Transport of Radioactive Materials, IAEA Safety Series no. 6 (IAEA, Vienna, 1985). 081 G.L. Miller, D.V. Lang and L.C. Kimmerling, Ann. Rev. Mater. Sci. 377 (1977). 1191 S.J. Pearton, K.D. Cummings and G.P. Vella-Coleiro, J. Electrochem. Sot. 132 (1985) 2743. DO1 J. Wagner, M. Rainsteriner and V. Hayde, J. Appl. Phys. 61 (1987) 3050. Dll C.E. Third and F. Weinberg, Appl. Phys. Letts. 54 (1989) 2671. WI D. Alexiev, K.S.A. Butcher, M. Edmondson and T.L. Tansley, J. Crystal Growth, in press.
Nuclear Instruments and Methods in Physics Research B 86 (1994) 288-292 North-Holland
Neutron transmutation gallium arsenide
Beam Interactions with Materials&Atoms
doping of liquid phase epitaxial
D. Alexiev a~*, K.S.A. Butcher b, M. Edmondson a and T.L. Tansley b ‘AustralianNuclear Science and Technology Organisation, Menai, NSW 2234, Australia b Semiconductor Science and Technology Laboratories, Physics Department, Macquarie University, North Ryde, NSW 2109, Australia
Received 23 August 1993 and in revised form 17 November 1993
Neutron transmutation doping (NTD) was studied as a means of compensating p-type liquid phase epitaxial (LPE) gallium arsenide. It is shown that such epitaxial layers can be transmuted into approximately 3 x 1013 crne3 n-type material, defect free after appropriate thermal annealing.
1. Introduction
Neutron transmutation doping (NTD) of semiconductor material was first demonstrated by Cleland et al. [l]. They showed that a neutron flux can produce homogeneous distribution of dopants in silicon. This work was continued by Colin [2] and Cuevas [3] and is now routinely used in commercial doping of silicon for the production of very high-power devices. In Si, only 3oSi is used to produce phosphorus. 3oSi is one of three naturally occurring isotopes and is present in Si at a concentration of about 3.1%, and is also homogeneously distributed. Thus, when 31P is introduced by NTD it acts as a stable donor when it is situated in a substitutional lattice site and is uniformly distributed. During the NTD process the material is subject to thermal neutron fluence. A lattice atom can capture a thermal neutron and becomes an excited isotope. To reach its ground state, the isotope emits y and p particles with the recoil displacing the atom from its original location into an interstitial position. Further atomic displacements can arise due to energetic neutrons which will inevitably accompany a thermal neutron fluence. Thus, after NTD, Si has to be annealed [4] to remove the induced damaged and to relocate the 31P into substitutional sites. NTD of GaAs has received only limited attention when compared to NTD of Si. NTD doping of GaAs was first studied by Mirianashvili et al [5], who demonstrated that an efficiency of the order of lo3 times higher than NTD of Si can be achieved. Gallium ar-
* Corresponding author, tel. +61 2 717 3182, fax +61 2 717 9265.
senide contains three naturally occurring isotopes, all act in the transmutation doping process as shown in Table 1. If all the stable isotopes of Ge and Se are located on Ga and As sites, then each will act as a shallow donor. This means that NTD only has one result in GaAs: p-type material can be converted to n-type or n-type can be altered to lower resistivity n-type. Vesaghi [6] expressed the efficiency of the doping process as N,=Kc$t,
(1) where 4 is the thermal flw, n cm-’ s-l, t is the time exposed to the neutron flux, K is the NTD constant and equal to 0.16, and No is the concentration of the donor produced by the NTD process. Similarly to NTD-Si, transmuted atoms in GaAs are not in their original locations and are displaced into an interstitial position. As with Si irradiation, this defect damage can be reduced or totally removed by annealing. For bulk GaAs, annealing temperatures suggested by Vesaghi [6] are in the range of 800 to 900°C with lower temperatures for epitaxial wafers, suggesting that Table 1 Production of donor atoms in GaAs by NTD Transmutation reaction of GaAs
Capture cross Half-life section for thermal neutrons [b]
P Ga6’(n, y)Ga” -+ Ge’O 1.68 B Ga%, r)Ga’*-r Ge’* 4.86 P As’%, Y)AS’~+ Se76 4.30
0168-583X/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved SSDZ 0168-583X(93)E0945-D
Natural abundance
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14 h
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26h
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D. Altxiev et al. / Nucl. I&r. and Meth. in Phys. Res. B 86 (1994) 288-292
bulk GaAs is associated with a high concentration of defects and impurities which may act as binding sites for the transmuted atoms. The need for higher annealing temperatures inevitably creates an out-diffusion of As and possible contamination of the GaAs with common metallic impurities, particularly Cu, from the silica furnace tube and the silica sample pedestal within the furnace. To avoid both dissociation and contamination, the GaAs can be encapsulated in phosphosilicate glass [7] or in a Ga film [8]. Annealing of the material can also be done by rapid isothermal annealing and quenching, often termed as flash annealing in silicon wafer processing. For these reasons annealing of LPE-GaAs has to be examined as a separate entity. Thus, the goal of the present work is to counter-dope p-type LPE epitaxies with stable transmuted isotopes of Ge and Se to produce near 1013 to 1014 crnp3 carrier concentration, n-type epitaxies with no detectable deep level defects. That is epitaxial GaAs of sufficient purity for construction into nuclear radiation detectors [9]. It should be noted, however, that this material has many properties which are in contrast to semi-insulating (SI) GaAs, which itself has received much recent attention as a potential radiation detector material [lo-131. In particular the LPE material is conducting with smaller depletion regions than more highly compensated SI material. The great advantage of this LPE material is that it can be grown free of the deep level traps which degrade detector charge carrier collection efficiencies and lower spectral peak resolution. The ability to apply NTD to p-type LPE GaAs of moderate free carrier concentration is an important step in realising a higher yield of detector quality material.
2. Experimental A 300 pm thick LPE GaAs epitaxy grown on a conducting n-type substrate (MCP 1 x 1016 cmm3 carrier concentration) using the transient temperature growth method in a Si-0 environment, was cleaved into approximately 6 mm squares. These LPE GaAs samples were then prepared according to accepted procedures. Briefly, after the LPE GaAs samples were degreased in xylene, washed in methanol and finally rinsed in 18 Ma cm H,O, the sections were then etched one at a time in hot (- 60°C) solutions of 3H,SO, : lH,O, : lH,O for 2 minutes and quenched in 18 MR cm HzO. The resulting surfaces had a mirror finish with rounded edges; one such sample was angle lapped (Fig. 1) metallised by evaporating a number of 1 mm diameter aluminium areas on the lapped epitaxy and a gold ohmic contact on the back of the substrate. The resulting Schottky-barrier diodes were then pro-
289 Diode
1
Fig. 1. Angle lapped 300 pm n-type epitaxy on n-type substrate. filed for the net carrier concentration (N(x)) using capacitance-voltage (C-V) measurements. The standard equation given by Sze [14] was used for these calculations; i.e. N(x)
=
z
1
(2)
I’ where the symbols have their usual meanings. The plots of l/C2 vs V, from which N(n) values were calculated, revealed normal slopes without any of the flattening which occurs for SI GaAs when that material is fully depleted. For the C-V measurements capacitances were measured up to a maximum reverse bias of 10 V. At this bias a depletion width (W) of 25 km would be expected for the samples with the lowest carrier concentrations measured for this paper (i.e. 2 X 1013 cmp3>, as calculated from the equation [14] qEs
d(l/C’)/dv
(3) where again the symbols have their usual meaning. This depletion width is well below the full 300 km thickness of the epitaxial layer. Using the angle lapped sample N(x) was found to be 0.9 to 1.2 X 1015 cmp3 over the 300 pm thickness of the epitaxy; the polarity of the material being p-type. A capacitance deep level transient spectrum (DLTS) revealed that the LPE GaAs had no deep levels when measured between the temperature range of 11 to 360 K. The DLTS system used was a correlation system of the type first described by Miller [15] which has the advantage of high sensitivity being able to detect majority traps at levels of 1O-4 that of the net carrier concentration [16]. A correlation time constant (TJ of 10 ms was used for all the DLTS measurements. From the initial C-V measurements and using a published transmutation value of K = 0.16 [6], an irradiation sequence can be calculated so that the resulting transmuted N(x) will have values progressing from low p to low n-type carrier concentration. The LPE GaAs samples for neutron irradiation were then sealed in polythene pouches to exclude any surface cross contamination and submitted to the irradiation facility at HIFAR ‘r. All irradiations were done in RIG X7, #’ HIFAR is a 10 MW DID0 class research reactor, located at Lucas Heights, NSW, Australia.
290
D. Alexiev et al. / Nucl. In&. and Meth. in Phys. Res. B 86 (1994) 288-292
> EfWT
Fig. 2. Spectrum of residual radioactivity produced by NTD of LPE-GaAs.
having a flux rating of 4 x 1012 ncmm2 s-i k 10%. After irradiation, a period of 14 days was allowed to elapse so that the induced radioactivity was below the international accepted standard #‘. The residual radioactivity of the NTD-LPE GaAs was measured using a 35 cm3 Ge(Li) detector. The y-ray spectrum (Fig. 2) showed remanence photo-peaks of 16As and “Ga. No contamination photo-peaks such as i9’Au (Au being a common replating impurity from HF acids) were noted [8]. Other lines noted are of K4’ and Cs13’ seen usually as background in the area locating the spectrometer facility. Since neutron generated defects can be expected in the atomic structure of the NTD-LPE GaAs, a DLTS spectrum and a capacitance versus temperature (C-T) plot were made prior to temperature annealing of the n-type transmutation doped samples. The DLTS spectrum (Fig. 3) showed a high intensity defect located at about 360 K (rc = 10 ms). The variation of capacitance with temperature for a C-T plot of the same sample showed that this trap is about 100% larger than the net carrier concentration, thus this trap could not be measured [18].
3. Thermal annealing Initial thermal annealing was attempted using a rapid thermal annealer (RTA) capable of pulsing heat, using halogen lamps for a few seconds. Temperatures in the range of 600 to 1000°C in steps of 100°C can be induced. Such RTAs are widely used in ion implanted GaAs technology [19] and it has been demonstrated by Wagner et al. [20], that it should be possible, in principle, to pulse anneal NTD GaAs. However, others [21]
X2 IAEA Safety Series no. 6 [IAEA 19851states international accepted standards for the removal and transport of radioactive packages 1171.
Fig. 3. DLTS spectrum of NTD n-type LPE-GaAs sample, showing an induced defect at approximately 340 K (7, = 10 ms).
found that RTA is not uniform throughout the thickness of the sample, producing dark spots associated with high dislocations when viewing the annealed samples using cathodoluminescence microscopy. Temperatures selected for annealing were of the order of 800°C for a period of 10 s duration, heated in air, using a laboratory constructed RTA. Such pulse annealing was followed by polishing the epitaxial surface, chemical cleansing and a 30 s dip etch in 3H,SO, : lH,O, : lH,O at 40°C. When examining the LPE surfaces with a microscope, it was noted that parallel structural slip lines appeared - an obvious detrimental effect to the material. This structural slip could be explained, as Third and Weinberg [21] found, by the temperature distribution through the sample being non-uniform. Further attempts at different temperatures and periods produced similar results. For these reasons, RTA was abandoned and annealing was transferred to a resistance heater with a hydrogen gas flow, as shown in Fig. 4. The following method for thermal annealing of the NTD-LPE GaAs was found to be successful and hence adopted throughout the following work. The GaAs samples were first dip etched in 3H2S04 : lH,O,: lH,O at 40°C for 10 s, Ga was then applied to the substrate of the LPE GaAs and to the inner surface of a freshly etched (1HF : 3HN0,) spectrosil silica crucible. In each case a Ga film was formed. The importance of using Ga films on contacting surfaces is to eliminate, by gettering, fast diffusers such as Cu, a common environmental contaminant when using silica and other refractory components at high temperatures. When the furnace reached set temperature, the crucible containing the samples was drawn into the hot region and removed after a predetermined period. It was found that 600°C and a period of 5 min removed all structural defects noted before with the DLTS
D. Alexiev et al. / Nucl. Instr. and Meth. in Phys. Rex B 86 (1994) 288-292
291
Fig. 4. Annealing of neutron irradiated LPE-GaAs. spectra for the samples transmuted to n-type carrier concentration. When using a lower temperature, 550°C and the same period, some trace of the electron trap remained. A temperature profile for the sample in the resistance heater is shown in Fig. 5. All NTD-LPE GaAs samples were annealed in this manner, followed by HCl etching at 60°C for 0.5 h to remove the Ga and dip etch metallisation. The C-I, relation was measured to determine the N(X) value (again using a maximum reverse bias of no greater than 10 V> and the transmutation constant for each sample, as summarised in Table 2 and Fig. 6 which shows a graph of K versus +t. The polarity of the diodes shown in Table 2 was determined by the direction of increase of the depletion layer formed beneath the Schottky barrier. The capacitance of this layer followed the relation given by Eq. (2) when the devices were placed in reverse bias. Current saturation also occurred in reverse bias. It can be noted from Fig. 6 that K varies with 4t. Errors in the irradiation facility such as neutron flux variation (rated at &lo%,> and operator timing error could account for some of this error. The variation of carrier concentration throughout the sample may account for more of the error. The thermal annealing
process itself may also produce secondary effects in this material which could account for some of the remaining error. The effect of thermal annealing on the net carrier concentration of LPE GaAs has been outlined by Alexiev et al. [22] who have shown that long term annealing for 40 h at 600°C can cause a significant change in the net carrier concentration. Although the annealing times used here were considerably shorter than that of Alexiev et al. some lattice relaxation or impurity relocation may occur on a shorter time scale introducing some error in the net carrier concentration change (AN) ascribed to the NTD process. One further potential error source exists. No DLTS of the p-type samples was performed since for our purposes (i.e. the production of high purity n-type GaAs for construction into nuclear radiation detectors) only electron traps are of consequence, this is because the devices normally constructed are Schottky barriers which are majority carrier devices. DLTS usually only 0.2
I
I
I
I
I
I
0 0 Y
z0 t; c
--
__--------------0.1 0
8 ____.._______ 0
:
. . ..______......____.....................
I 0
I
I
I
I
2
I
6
8
I 10
I
I
I
ie
14
16
sbc.
Fig. 5. Annealing
profile (temperature versus time) of neutron irradiated LPE-GaAs.
.x
~ . . . . .._.........
I
0.0
0
0
4
6
8
10 qst x
I
12
14
10'5".cm
16
16
20
-2
Fig. 6. A plot for the transmutation constant K versus neutron flux (dt) showing the average value (dashed line) and the standard deviation (dotted line).
D. Alexiev et al. / Nucl. Instr. and Meth. in Phys. Rex B 86 (I 994) 288-292
292 Table 2 Excess donor/acceptor Sample no.
concentration
as a function of irradiated neutron dose
Start N(x) [cmw3]
(bt used
0.9 to 1.2 x 10’5 p-type same
5 x 101s 6.25 x 10”
same
7.5 x 1015
same
1.25 x 1016
same
1.875 x 1016
[ncm-*I
detects majority carrier traps, so this does leave unresolved the possibility that hole traps could have been generated during the NTD process which were not removed by the thermal anneal. Such trapping levels
would be low since our l/C2 plots were unaffected by them, however, these traps, if present, could have introduced some error into the measurement of the net carrier concentration for the p-type samples thereby affecting our calculation of K. To test the relationship between K and +t another group of LPE GaAs samples were prepared and irradiated as before but using a value of K = 0.1 from the graph to calculate required fluence ($t = 1.02 X 1Or6 ncm-2) for near compensation. After processing the irradiated samples as described above, values for N(x) = 2 to 6 X 1013 cm-3, n-type were obtained.
4. Conclusion It has been shown, for the first time, that neutron transmutation doping of p-type LPE GaAs can produce high quality material with a very low net carrier concentration. In the NTD experiments 2-l X lOI5 cmm3 p-type LPE GaAs were compensated to 2-8 x 1013 cmp3 net carrier concentration, n-type. The material, after thermal annealing had no traps detectable by DLTS analysis. It is expected that such material should
prove useful for device construction, specifically for construction of LPE GaAs radiation detectors.
References [l] J.W. Cleland, K. Lark-Horovitz and J.C. Pigg, Phys. Rev. 78 (1950) 814. [2] S. Colin, Phys. Rev. 132 (1963) 178.
Resultant N(x) [cmm3]
AN achieved [cm-‘]
K = AN/&
1 x 10’4 p-type 6~10’~ p-type 4.8 x 1014 p-type 1.5 x 10’4 n-type 7.3 x 1014 n-type
0.9 x 101s
0.18
0.94 x 10’5
0.15
5.2x 1014
0.069
1.15 x 10’5
0.09
1.73 x 10’5
0.09
131M. Cuevas, Phys. Rev. 164 (1967) 1021. [41 E.M. Lawson, P.J. Lee and A.J. Tavendale, AAEC unpublished report, AP/TN/189 (1984). [51 Sh.M. Mirianashvili and D.J. Nanobashvili, Sov. Phys. Semicond. 4 (1971) 1612. [61 M.A. Vesaghi, Phys. Rev. B. 25 (1982) 5436. [71 G. Mathur, M.L. Wheaton, J.M. Borrego and S.K. Ghandi, J. Appl. Phys. 57 (1985) 4711. PI D. Alexiev and K.S.A. Butcher, Nucl. Instr. and Meth. B 83 (1993) 430. 191 D. Alexiev and K.S.A. Butcher, Nucl. Instr. and Meth. A 317 (1992) 111. [lOI R. Bertin, S.D.‘Auria, C. Del Papa, F. Fiori, B. lisowski, V. O’Shea, P.G. Pelfer, K. Smith and A. Zichichi, Nucl. Instr. and Meth., A 294 (1990) 211. Ml D.S. McGregor, G.F. Knoll, Y. Eisen and R. Brake, IEEE Trans. Nucl. Sci. 39 (1992) 1226. WI S.P. Beaumont et al., Nucl. Instr. and Meth. A 322 (1992) 472. [131 T.J. Sumner, S.M. Grant, A. Bewick, J.P. Li, K. Smith and S.P. Beaumont, Nucl. Instr. and Meth. A 322 (1992) 514. [141 S.M. Sze, Physics of Semiconductor Devices, 2nd ed. (Wiley, New York, 19811, 248/249. [151 G.L. Miller, J.V. Ramirez and D.A.H. Robinson, J. Appl. Phys. 46 (1975) 2638. Ml J.L. Benton, J. Crystal Growth 106 (1990) 116. 1171 IAEA - Regulations for Safe Transport of Radioactive Materials, IAEA Safety Series no. 6 (IAEA, Vienna, 1985). 081 G.L. Miller, D.V. Lang and L.C. Kimmerling, Ann. Rev. Mater. Sci. 377 (1977). 1191 S.J. Pearton, K.D. Cummings and G.P. Vella-Coleiro, J. Electrochem. Sot. 132 (1985) 2743. DO1 J. Wagner, M. Rainsteriner and V. Hayde, J. Appl. Phys. 61 (1987) 3050. Dll C.E. Third and F. Weinberg, Appl. Phys. Letts. 54 (1989) 2671. WI D. Alexiev, K.S.A. Butcher, M. Edmondson and T.L. Tansley, J. Crystal Growth, in press.