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A line-scan system to assess homogeneity of [EL2] in heat-treated LEC SI GaAs
102
Journal of Crystal Gro~th 103 (1990)102—I 05 North—Holland
A LINE-SCAN SYSTEM TO ASSESS HOMOGENEITY OF IEL2I IN HEAT-TREATED LEC SI GaAs S. CLARK Department of Electrical and Electronic Engineering. Nottingham Polytechnic, Burton Street, Nottingham NC] 4BU. UK
MR. BROZEL Department of Electrical Engineering and Electronics. Unn’ersrtr o/ Manche,iter Institute of Science and Technology. P. U Bos ‘kS. Manchester M60 1 QD, UK
and D.J. STIRLAND Allen Clark Research Centre, Plessey Research Cass’ell Ltd., Casiiell, Tost’cester, Northants. SN 12 hEQ, UK
We have developed a system which extracts near-infrared video information from a conventional absorption imaging system based around a vidicon camera. This information is processed to give EL2 concentration ([EL2]) scans on selected lines from an absorption image, using a point calibration from a spectro-photometer. In addition, the system can he used to calculate the mean deviation of [EL2] fluctuations. This mean deviation is used to measure the microscopic uniformity of [12L2] in commercial material. The system is demonstrated by assessment of “quenched” material. This material has been annealed at high temperatures >u00°C). and then rapidly cooled to room temperature. Subsequently some of this material has been conventionally annealed at 950°C followed by a slow cool. Quenched and annealed material is shown to have excellent [EL2] uniformity compared with conventional as-grown or annealed material.
1. Infroduction The evaluation of [EL2I from its characteristic near infra-red (NIR) absorption spectrum has become one of the most important assessment techniques for semi-insulating (SI) GaAs. Mainly, this is due to the key importance of EL2 in electrical compensation. combined with the difficulty of obtaming accurate [EL2] data in SI material using deep level assessment techniques such as DLTS. Hence many methods of NIR absorption assessment have been developed to investigate [EL2] distributions in GaAs ingots.
2. The near-infra-red assessment of EL2 These techniques are based on the proportionality between [EL2] in a sample and its optical 0022-0248/90/$03.50
1990
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absorption coefficient at NIR wavelengths. This was first demonstrated by Martin [1]. He used DLTS to measure [EL2] in lightly n-type material. He also produced calibration factors for the absorption coefficient, at room temperature. from which [EL2] (in the neutral charge state) can be calculated (at a wavelength of I jim an absorption coefficient of 1 cm corresponds to 7>< 10~~’ cm EL2 centres). Early work involved dual beam spectrophotometers to take point measurements of [EL2] across slices [2]. Brozel and co-workers [3,4] adapted this technique. using a simple sample scanning system. to produce moderately high spatial resolution linescans of EL2 absorption across slices. Sample transmission scans were taken at a wavelength of I jim where the absorption cross section of EL2 is high and at 2 p.m where it is negligible. The sample absorption coefficient is
Elsevier Science Publishers By. (North-Holland)
S. Clark et aL
/
Line-scan system to assess homogeneity of [EL2] in LEC SI GaAs
then calculated by taking the natural logarithm of the ratio of points on these two transmission scans and by dividing the result by the sample thickness, This method is quantitative, but its spatial resolution is not suitable for certain measurements. It cannot be used, for example, to measure microscopic (less than 100 jim) [EL2] fluctuations. Brozel and co-workers [3,4] also carried out two-dimensional imaging of EL2 absorption using a silicon vidicon television camera, The GaAs sample is illuminated by an infra-red source and viewed in transmission. They compared the results with those produced using the spectrophotometer. Unlike the spectrophotometer, the imaging system in its basic form is not monochromatic; it is sensitive to light from the bandgap of GaAs to the cut-off of the vidicon camera at about 1.1 p.m. In this region the absorption cross section of EL2 is high, resulting in high sensitivity to non-uniform distributions of EL2. To produce a monochromatic image, a narrow band-pass filter can be introduced. No qualitative differences have been seen between the image produced with or without the filter [5]. Records of the vidicon image are taken by photographing the image on the monitor. The system is capable of investigating qualitative EL2 distributions on a macroscopic or microscopic scale. Thus, the association of EL2 centres with dislocation cellular structure was clearly demonstrated in this way [6,4]. The vidicon system was soon adapted to produce full slice images [5] and later, “three-dimensional” images by tilting the specimen to produce stereo pairs [71.Recently, image processing has been used to remove the effects of variations in illumination and to improve signal to noise ratios by image integration [8—10]. Instead of recording the image with a TV camera, Kaufmann et al. [11] used a camera with infra-red sensitive film. The same group developed a mapping system based on a linear array of 256 photodiode detectors and compared the results they obtained with that of the photographic systern [12—14].Alt and co-workers [15—171used a similar system with improved resolution (using an array of 1024 photodiodes), to produce microscopic maps of EL2 absorption, illustrating cell structure.
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Another approach to EL2 mapping is to use point measurements in conjunction with a scanfling apparatus. The most advanced system of this type is that developed by Dobrilla and Blakemore [18—20].It has a maximum resolution of 50 p.m and takes about 20 mm to produce an EL2 map. Many similar systems exist but with lower resolution [21—25]. Silverberg and co-workers [26] were the first to develop a computer based mapping system for producing EL2 maps from the vidicon image. The EL2 maps are obtained by processing the absorption images, taken before and after bleaching the EL2 absorption, by illuminating the sample at 77 K with white light [1,27]. A similar system for obtaining EL2 maps was later demonstrated by Fillard et al. [281. An alternative to using the wide absorption band of EL2 is to use the absorption of the zero-phonon-line (ZPL) associated with the EL2 intracentre transition. The absorption coefficient of the ZPL was recently calibrated [29,30]. In conjunction with bleaching, concentrations of both charge states of EL2 can be calculated [29,30]. Unfortunately, this technique requires high resolution FTIR spectrophotometers and liquid helium cryostats and to date has only been used for point measurements.
3. IEL2I linescan system The great flexibility of the vidicon imaging system, in combination with its high spatial resolution, has led to an interest in adapting it to obtain quantitative, microscopic, [EL2] information. The system we developed in an attempt to achieve this is described below. The block diagram is given in fig. 1. The analogue video output of the camera is converted to digital data using a fast analogue to digital (A/D) converter operating at 10 MHz. The line to be digitised is selected manually, and is highlighted on the video image by a white line. The line selector circuitry then generates an enable signal, which controls the multiplexing circuitry, and causes the digitised output to be stored in external RAM. Whilst the video output cycles through the
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S. Clark ci al.
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Line-scan system to assess homogeneity of /EL2/ in LEC .S’I GaAs
‘lid
icon LGTV
___________
Locern
no Selector
fl/S
[Thople
Elect,r~onico
I _ _____________
ton Line
el LH
0ed
I
_
t pl eoer
____
Et~
Mi~
LOOLJ
ten
S
no
Fig. 1. System block diagram.
rest of the frame, the data in the external RAM are slowly transferred into the microcomputer. They are then added to the data for the line from the previous cycle. This process is repeated 256 times in order to produce an averaged result. The averaging slows the system (it takes about 10 s to produce a line-scan), hut this is necessary to irnprove the signal-to-noise ratio to an acceptable
level. The line-scan can then he processed in the microcomputer as desired. One of the main problems with the system is the difficulty in obtaining uniform illumination. This is overcome in two ways. Firstly, the illumination optics are optimised. Secondly, a hackground scan is taken where the only non-uniformities in the scan are due to non-uniform illumina-
] ,
b
Fig. 2. (a) NIR absorption topograph of an un-annealed. undoped SI LEC sample together with (h) an [EL2] scan from the indicated bright line on the topograph.
S. Clark et al.
/ Line-scan system to assess
tion. The background scan is obtained in one of two ways: either the sample is removed or the image is defocussed. Neither of the two methods is ideal. The former tends to distort the illumination profile because of the high refractive index of GaAs (ideally the sample should be replaced with an sample of identical size and shape with uniform absorption). The second method removes macroscopic EL2 fluctuations. However, this is useful if the primary interest is in microscopic fluctuations of [EL2], as in this work. The line-scan is divided by the background scan to give the normalised absorption linescan. The EL2 scan is obtained after calibrating the normalised absorption scan using a transmission measurement at a wavelength of 1 p.m on a spectrophotometer. A second measurement is made at a wavelength of 2 ~im where the absorption due to EL2 is very small. The transmission at a wavelength of 2 p.m is assumed to be uniform across the sample. The [EL2] scan is then obtained, for a known sample thickness, from the normalised absorption scan using Martin’s calibration [1] for absorption at a wavelength of 1 p.m (given above), An EL2 “uniformity parameter” is obtained by taking the mean deviation of the [EL2] scan data, This mean deviation gives a numerical measure of the uniformity of the [EL2] scan and is primarily used in this work to measure the microscopic uniformity of [EL2] in heat treated material. In the results presented in this work, the background absorption line-scan is taken with the image defocussed. Hence, as macroscopic [EL2] fluctuations are removed, the mean deviation of the [EL2] scan data can be regarded as a microscopic uniformity parameter. In fig. 2, an example of a vidicon NIR absorption image of an as-grown SI LEC GaAs sample is given, together with an [EL2] scan taken along the bright line indicated on the absorption image. The [EL2] scan is obtained from the normalised absorption scan, shown in fig. 3, using a measurement from the sepctrophotometer and Martin’s calibration [1]. Fig. 3 shows the normalised absorption line-scan of the sample together with the linescan and the background scan (obtained by defocussing the image) from which it was derived,
homogeneity of [EL2] in LEC SI GaAs
105
non~iiseii ab~o,pi~on
-
I
-
urntsj I
SorPSon
. ‘
.
deloc~med b~ckg,o~nd
.--
..
~
—
~.
Fig. 3. Absorption, background and normalised absorption scans used to calculate the IEL2] scan in fig. 2.
4. Experimental details and results Block samples of as-grown LEC SI GaAs were annealed in evacuated, sealed ampoules, at various temperatures between 700 and 1200°C. Each was subsequently quenched by removing the ampoule rapidly from the furnace and dropping it into cold water. After slices were removed for assessment, the remaining material was resealed in the ampoule and annealed at 950 °C for 5 h followed by a conventional slow cool. In addition to this an entire 2 inch diameter LEC ingot was annealed in an evacuated, sealed ampoule and quenched. The ingot was subsequently annealed at 950°C for 5 h before being removed from the ampoule. Fig. 4 shows two representative EL2 absorption images taken from samples quenched from 1000 and 1100°C. The absorption line-scan positions are indicated by bright lines. Note that the former still shows clear cell structure and resembles asgrown material, whereas the latter appears to be virtually featureless. Images of all samples quenched from above 1100°C appear featureless both after quenching and after subsequent annealing at 950°C. Fig. 5 shows the [EL2] scans obtamed from samples quenched from 1000, 1100 and 1200°C. [EL2] for the sample quenched from 1000 °Cis similar to that of the as-grown samples. Note the pronounced drop in [EL2] and increase in [EL2] uniformity for the samples quenched from above 1100°C. [EL2] together with [EL2] mean deviations for all the heat treated samples in this work are given in table 1. Note again that quenching from 1100°C and above causes a substantial drop in [EL2}, but
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lark etal.
/
Line-i, an
ii
sum
is’
as si’s s hssmogenei’ty of /E/..2J in LI/C SI GaA Table I [F.L2] and [EL2] mean deviations for all heat treated samples in present work Sample details and [EL2] Mean heat treatments
)cni ~)
deviation of[FL2]
t
Block Block Block Block Block Block Block
sample, as-grown annealed at 950°C quenched from 700°(~ quenched from 800°C’ quenched from 900°C quenched from 1000°C quenched from 1100°C
l.34x1O°’ l.62x l0’~ 1 .34X 10° l.50x 10°’ 1.43 X 100 l.50x10°’ 1.1(1 x 100
X.2x l0’~ 5.1
x
l0’~
7.9 X l0°~ 79x l0’~ 4.9 x l0”~ 4 5.2xl0’ 2.9 x 1ç1i4
Block quenched from 1150°C 1.02 X l0’~’ 1.9>< l014 Block quenched from 1180°C’ 0.70>< 10°’ j,~~ 10i4 Block quenched from 1200°C’ 0.60xl0°’ l.7x io’~ Block quenched from 1100°C and then annealed at 950°C’ 1.17>< I0’~’ 1.9 x Block quenched from 1150°C’ and then annealed at 950°C 1.23 x 10°’ 2.7 x 10’~ Block quenched from 1180°C and then annealed at 950°C 1.07>< 100 1.8 x l0’~ Block quenched from 1200°C and then annealed at 950°C’ 1.02 x 10°’ 1.7 ~ Ingot quenched from 1100°C’ 1° 1.4 ~ 10’~ and then annealed at 950°C 1.28 x10 Note: all block samples are taken from the same ingot: all anneals are of 5 h duration.
______________________________________________________
~r. I ig. 4. NI R .ibsorption topographs of samples quenched front 1000°( . I’he bright lines indicate is here [EL2] ~c,in’ 5¼crc obtained,
a significant improvement in optical transmission uniformity. Electrical assessment on this material presented elsewhere [31,32] shows that the asgrown (n-type) SI samples quenched from above 1100°C have been converted to p-type by the heat
treatment but are restored to SI (n-type) by the subsequent 950 ° C anneal. The results presented here show that subsequent annealing at 950°C causes [EL2j to return to about the as-grown concentration, but the superior [EL2] uniformity, produced by quenching. is retained. ‘
.
5. Discussion 2X1016 ~
5.1, Discussion of /EL2J linescan system
~EL21
1000°C~ 1100°C
.
.
‘
. ..
1200C0
.
The advantages and disadvantages of this systemAdvantages: are presented below.
. ~ ..
‘
.
The system can be used with any existing NIR absorption system that has a video output. Microscopic uniformity scans are produced, the —
1nIrn
Fig 5. [EL2] scans for samples quenched from 1000, 1100 and 1200°C.
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resolution is only limited by the imaging system. Measurements are taken at room temperature. —
S. Clark et al.
/
Line-scan system to assess homogeneity of [EL2] in LEC SI GaAs
A microscopic EL2 uniformity parameter can be calculated, which can be used for characterisation of commercial material. Disadvantages: The system requires a point calibration from a spectrophotometer. [EL2] results are to be regarded as semiquantitative (see below). Only [EL2] linescan results are produced whereas some other systems give complete [EL2] maps. In principle, of course, this type of system could be easily expanded to give full [EL2] maps with the same resolution as the monitor on the imaging system. This would require some modifications to the electronics and a more powerful microcomputer to handle the increased computing requirements. The use of photoquenching to calculate the sample absorption coefficient due to EL2 was deliberately avoided, because it is a slow technique, which involves measurements at temperatures below 77 K. Quenching also has other problems as removal of EL2 properties will alter the Fermi level, which may change the charge states of other absorbing defects, hence changing their absorption cross sections. The system should be regarded as semi-quantitative only, for the following reasons: (1) Non-linearity of the silicon vidicon response (this problem can be reduced by good camera selection). (2) The absorption is measured from a broad band spectrum (this could be prevented by using a band-pass filter, but this would reduce the signalto-noise ratio and hence increase the time required for averaging). (3) Problems arising from incorrect background subtraction (this effect can be reduced by optimising the normalisation procedure as described in the system description above). (4) Non-uniformities in transmitted light due to scratches, precipitates and other features (these are usually easy to spot on the absorption image and should be avoided), (5) Absorption from the other charge states of EL2, especially the hole absorption cross-section of the empty trap [33].
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(6) Any absorption contributions from other deep levels. (7) Any inaccuracy in the calibration of the absorption coefficient of EL2. The problems from (5) onwards are common to most present systems and probably involve the largest errors.
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5.2. Discussion of results
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Recent analysis of device results [34,35] mdicates that point defects have a more significant effect than dislocations on the uniformity of device parameters. The correlation sometimes observed between dislocations and device parameters is then caused by a secondary effect: dislocation gettering of the relevant point defects. If this is the case, specifying dislocation densities may be irrelevant, whereas methods of measuring point defect distributions, especially that of EL2, would be more important. A microscopic uniformity parameter of [EL2] is of particular importance as it is expected to determine the local fluctuations in FET properties. Hence we emphasise the role of heat treatments to give superior EL2 uniformity (although the dislocation density is substantially 2 in the case increased from 10~to 106 cm of the quenched ingot). The quenching results presented here have been discussed in more detail elsewhere [31,32], where it is postulated that the uniformity improvements observed are due to the dissolution of defects into the lattice at high temperature. Quenching reduces [EL2] and prevents the reformation of non-uniformities by preventing extensive point defect diffusion. Unfortunately the material is no longer SI. Annealing at 950°Crestores SI behaviour, allows the reformation of EL2 but does not allow extensive redistribution. -~
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6. Conclusions We have demonstrated that this system can be used to give line-scans of [EL2] at a resolution limited only by the imaging system. In addition a microscopic [EL2] uniformity parameter can be calculated. This can be used to assess material for
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Line-scan system to assess homogeneity of [EL2J in LEC’ SI GaA.s
commercial IC production. We have shown that SI, quenched and reannealed, material has a supenor uniformity to conventional, as-grown or annealed material using this uniformity parameter. Acknowledgements The authors gratefully acknowledge provision of samples by I. Grant. The work was supported by the Procurement Executive, Ministry of Defence, DCVD.
[141W.
Wettling and J. Windscheif, AppI. Phys. A40 (1986) 191. [151 EtC. Alt (1986) [16] H.C. Alt and and Ci. 0. Packeiser, Packeiser. J. in:Appt. Proc.Phys. 18th 60 Conf. on 2954. Solid State Devices and Materials. Tokyo, 1986, p 659. [17] H.C. Alt and H. Schink. AppI. Phys. Letters 52 (1988) 1661. [181 P. Dobrilla, iS. Blakemore and R.Y Koyama, in: SemiInsulating 111—V Materials, Proc. 3rd Conf., Kah-nee-ta, 1984, Eds. D.C. Look and J.S. Blakemore (Shiva, Nantwich, 1984) p. 282 [19] P. Dobrilla and iS. Blakemore. 1. AppI. Phys. 58 (1985)
References
208. [201 P. Dobrilla and J.S. Blakemore, J. AppI. Phys. 61 (1987) 1442. [211 D.E. Holmes, R.T. Chen, KR. Elliot and CU. Kirkpatrick. AppI. Phys. Letters 43 (1983) 305. [221D.E. Holmes and R.T. Chen. J. AppI. Phys. 55 (1984)
[1] G.M. Martin, AppI. Phys. Letters 39 (1981) 747. [2]G.M. Martin, Ci. Jacob, G. Poiblaud, A. Goltzene and C. Schwab, in: Defects and Radiation Effects in Semiconductors 1980, Proc. 11th Intern. Conf., Oiso, 1980, Inst. Phys. Conf. Ser. 59, Ed. R.R. Hasiguti (Inst. Phys., London—Bristol, 1981) p. 281. [3] MR. Brozel, I. Grant, R.M. Ware and D.J. Stirland, AppI. Phys. Letters 42 (1983) 610. [4] D.J. Stirland, I. Grant, MR. Brozel and R.M. Ware, in: Microscopy of Semiconducting Materials 1987, Proc. 5th Oxford Conf., 1987, Inst. Phys. Conf. Ser. 87, Ed. AG. Cullis (Inst. Phys., London—Bristol, 1987) p. 285. [51MR. Brozel, I. Grant, R.M. Ware, D.J. Stirland and MS. Skolnick, J. AppI. Phys. 56 (1984) 1109. [6] MS. Skotnick, M.R. Broad, L.J. Reed, I. Grant, D.J. Stirland and R.M. Ware, J. Electron. Mater. 13 (1984) 107. [7] MR. Brozel, E.J. Foulkes and D.J. Stirland, in: GaAs and Related Compounds 1984, Proc. 11th Intern. Symp., Biarritz, 1984, Inst. Phys. Conf. Ser. 74, Ed. B. de Cremoux (Inst. Phys., London—Bristol, 1985) p. 59. [8] T. Katsuma, T. Obokata, M. Nakajima and T. Fukuda, in: DRIP I, Montpellier. 1985, Ed. J.P. Fillard (Elsevier, Amsterdam, 1985) p. 149. [9] T. K.atsumata and T. Fukuda, Rev. Sci. tnstr. 57 (1986) 202. [10] M. Castagné, J. Bonnafe, F. Zhang and J.P. Fillard, in: DRIP I. Montpellier, 1985, Ed. J.P. Fillard (Elsevier, Amsterdam, 1985) p. 35 [Ill U. Kaufmann, .1. Windscheif, M. Baeumler, J. Schneider and F. KOhl, in: Semi-Insulating Ill—V Materials, Proc. 3rd Conf., Kah-nee-ta, 1984, Eds. D.C. Look and J.S. Blakemore (Shiva, Nantwich, 1984) p. 246. [12]J. Windscheif, M. Baeumler and U. Kaufmann, Appl. Phys. Letters 46 (1985) 661. [13] J. Windschejf, M. Baeum.ler and U. Kaufmann, in: DRIP I, Montpellier, 1985, Ed. J.P. Fillard (Elsevier, Amsterdam, 1985) p. 193.
[23] B. Gouteraux, N. Visentin, B. Lent and M. Bonnet. in: GaAs and Related Compounds 1984, Proc. 11th Intern. Symp., Biarritz. 1984, Inst. Phys. Conf. Ser. 74, Ed. B. de Cremoux (Inst. Phys.. London—Bristol, 1985) p. 89. [241 N. Visentin. B. Gouteraux, M. Bonnet and B. Lent. in: DRIP I, Montpellier, 1985, Ed. J.F. Fillard (Elsevier. Amsterdam, 1985) p. 117. [25] CR. Abernathy, R. Caruso, AS. Jordan, Si. Pearton and H. Tenikin, in: DRIP II, Monterey, 1987, Ed. ER. Weher (Elsevier, Amsterdam, 1987) p. 35. [261 P. Silverberg, L. Nielson, P. Omling and L. Samuelson, in: DRIP I. Montpellier, 1985, Ed. J.P. Fillard (Elsevier. Amsterdam, 1985) p. 157. 127] MS. Skolnick, L.J. Reed and AD. Pitt. Appl. Phys. Letters 44 (1984) 447. [28]J.P. Fillard, P. Montgomery, A. Baroudi. J. Bonnafe and P. Gall, Japan. J. AppI. Phys. 27 (1988) L258. [29] M. Skowronski, J. Lagowski and H.C. Gatos. J. AppI. Phys. 56 (1986) 2451. [30] J. Lagowski, M. Bugajskt, M. Matsui and H.C. Gatos. AppI. Phys. Letters 51(1987) 511. [31] S. Clark, D.J. Stirland, MR. Brozel. M. Smith and CA. Warwick, in: Semi-Insulating Ill—V Materials. Proc. 5th Conf., Malmö, 1988, Eds. G. Grossmann and L. Ledebo (Hilger, Bristol, 1988) p. 31. [32] D.J. Stirland, P. Kidd, G.R. Booker, S. Clark, D.T.J. Hurle, MR. Brozel and I. Grant, in: Microscopy of Semiconducting Materials, 1989, Proc. 6th Oxford Conf., 1989, Inst. Phys. Conf. Ser. 100, Ed. AG. Cullis (Inst. Phys., London—Bristol, 1989). [33] P. Silverberg, P. Omling and L. Samuelson, in: Semi-Insulating Ill—V Materials, Proc. 5th Conf., Malmo, 1988. Eds. G. Grossmann and L. Ledebo (Hilger. Bristol, 1988) p. 369. [34] S. Miyazawa. K. Watanabe, J. Osaka and K. Ikuta, Rev. Physique AppI. 23 (1988) 727. [35]P. Suchet, J. Duseaux, J. Maluenda and G.M. Martin. J. AppI. Phys. 62 (1987) 1097.
3588.
Journal of Crystal Gro~th 103 (1990)102—I 05 North—Holland
A LINE-SCAN SYSTEM TO ASSESS HOMOGENEITY OF IEL2I IN HEAT-TREATED LEC SI GaAs S. CLARK Department of Electrical and Electronic Engineering. Nottingham Polytechnic, Burton Street, Nottingham NC] 4BU. UK
MR. BROZEL Department of Electrical Engineering and Electronics. Unn’ersrtr o/ Manche,iter Institute of Science and Technology. P. U Bos ‘kS. Manchester M60 1 QD, UK
and D.J. STIRLAND Allen Clark Research Centre, Plessey Research Cass’ell Ltd., Casiiell, Tost’cester, Northants. SN 12 hEQ, UK
We have developed a system which extracts near-infrared video information from a conventional absorption imaging system based around a vidicon camera. This information is processed to give EL2 concentration ([EL2]) scans on selected lines from an absorption image, using a point calibration from a spectro-photometer. In addition, the system can he used to calculate the mean deviation of [EL2] fluctuations. This mean deviation is used to measure the microscopic uniformity of [12L2] in commercial material. The system is demonstrated by assessment of “quenched” material. This material has been annealed at high temperatures >u00°C). and then rapidly cooled to room temperature. Subsequently some of this material has been conventionally annealed at 950°C followed by a slow cool. Quenched and annealed material is shown to have excellent [EL2] uniformity compared with conventional as-grown or annealed material.
1. Infroduction The evaluation of [EL2I from its characteristic near infra-red (NIR) absorption spectrum has become one of the most important assessment techniques for semi-insulating (SI) GaAs. Mainly, this is due to the key importance of EL2 in electrical compensation. combined with the difficulty of obtaming accurate [EL2] data in SI material using deep level assessment techniques such as DLTS. Hence many methods of NIR absorption assessment have been developed to investigate [EL2] distributions in GaAs ingots.
2. The near-infra-red assessment of EL2 These techniques are based on the proportionality between [EL2] in a sample and its optical 0022-0248/90/$03.50
1990
—
absorption coefficient at NIR wavelengths. This was first demonstrated by Martin [1]. He used DLTS to measure [EL2] in lightly n-type material. He also produced calibration factors for the absorption coefficient, at room temperature. from which [EL2] (in the neutral charge state) can be calculated (at a wavelength of I jim an absorption coefficient of 1 cm corresponds to 7>< 10~~’ cm EL2 centres). Early work involved dual beam spectrophotometers to take point measurements of [EL2] across slices [2]. Brozel and co-workers [3,4] adapted this technique. using a simple sample scanning system. to produce moderately high spatial resolution linescans of EL2 absorption across slices. Sample transmission scans were taken at a wavelength of I jim where the absorption cross section of EL2 is high and at 2 p.m where it is negligible. The sample absorption coefficient is
Elsevier Science Publishers By. (North-Holland)
S. Clark et aL
/
Line-scan system to assess homogeneity of [EL2] in LEC SI GaAs
then calculated by taking the natural logarithm of the ratio of points on these two transmission scans and by dividing the result by the sample thickness, This method is quantitative, but its spatial resolution is not suitable for certain measurements. It cannot be used, for example, to measure microscopic (less than 100 jim) [EL2] fluctuations. Brozel and co-workers [3,4] also carried out two-dimensional imaging of EL2 absorption using a silicon vidicon television camera, The GaAs sample is illuminated by an infra-red source and viewed in transmission. They compared the results with those produced using the spectrophotometer. Unlike the spectrophotometer, the imaging system in its basic form is not monochromatic; it is sensitive to light from the bandgap of GaAs to the cut-off of the vidicon camera at about 1.1 p.m. In this region the absorption cross section of EL2 is high, resulting in high sensitivity to non-uniform distributions of EL2. To produce a monochromatic image, a narrow band-pass filter can be introduced. No qualitative differences have been seen between the image produced with or without the filter [5]. Records of the vidicon image are taken by photographing the image on the monitor. The system is capable of investigating qualitative EL2 distributions on a macroscopic or microscopic scale. Thus, the association of EL2 centres with dislocation cellular structure was clearly demonstrated in this way [6,4]. The vidicon system was soon adapted to produce full slice images [5] and later, “three-dimensional” images by tilting the specimen to produce stereo pairs [71.Recently, image processing has been used to remove the effects of variations in illumination and to improve signal to noise ratios by image integration [8—10]. Instead of recording the image with a TV camera, Kaufmann et al. [11] used a camera with infra-red sensitive film. The same group developed a mapping system based on a linear array of 256 photodiode detectors and compared the results they obtained with that of the photographic systern [12—14].Alt and co-workers [15—171used a similar system with improved resolution (using an array of 1024 photodiodes), to produce microscopic maps of EL2 absorption, illustrating cell structure.
103
Another approach to EL2 mapping is to use point measurements in conjunction with a scanfling apparatus. The most advanced system of this type is that developed by Dobrilla and Blakemore [18—20].It has a maximum resolution of 50 p.m and takes about 20 mm to produce an EL2 map. Many similar systems exist but with lower resolution [21—25]. Silverberg and co-workers [26] were the first to develop a computer based mapping system for producing EL2 maps from the vidicon image. The EL2 maps are obtained by processing the absorption images, taken before and after bleaching the EL2 absorption, by illuminating the sample at 77 K with white light [1,27]. A similar system for obtaining EL2 maps was later demonstrated by Fillard et al. [281. An alternative to using the wide absorption band of EL2 is to use the absorption of the zero-phonon-line (ZPL) associated with the EL2 intracentre transition. The absorption coefficient of the ZPL was recently calibrated [29,30]. In conjunction with bleaching, concentrations of both charge states of EL2 can be calculated [29,30]. Unfortunately, this technique requires high resolution FTIR spectrophotometers and liquid helium cryostats and to date has only been used for point measurements.
3. IEL2I linescan system The great flexibility of the vidicon imaging system, in combination with its high spatial resolution, has led to an interest in adapting it to obtain quantitative, microscopic, [EL2] information. The system we developed in an attempt to achieve this is described below. The block diagram is given in fig. 1. The analogue video output of the camera is converted to digital data using a fast analogue to digital (A/D) converter operating at 10 MHz. The line to be digitised is selected manually, and is highlighted on the video image by a white line. The line selector circuitry then generates an enable signal, which controls the multiplexing circuitry, and causes the digitised output to be stored in external RAM. Whilst the video output cycles through the
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S. Clark ci al.
/
Line-scan system to assess homogeneity of /EL2/ in LEC .S’I GaAs
‘lid
icon LGTV
___________
Locern
no Selector
fl/S
[Thople
Elect,r~onico
I _ _____________
ton Line
el LH
0ed
I
_
t pl eoer
____
Et~
Mi~
LOOLJ
ten
S
no
Fig. 1. System block diagram.
rest of the frame, the data in the external RAM are slowly transferred into the microcomputer. They are then added to the data for the line from the previous cycle. This process is repeated 256 times in order to produce an averaged result. The averaging slows the system (it takes about 10 s to produce a line-scan), hut this is necessary to irnprove the signal-to-noise ratio to an acceptable
level. The line-scan can then he processed in the microcomputer as desired. One of the main problems with the system is the difficulty in obtaining uniform illumination. This is overcome in two ways. Firstly, the illumination optics are optimised. Secondly, a hackground scan is taken where the only non-uniformities in the scan are due to non-uniform illumina-
] ,
b
Fig. 2. (a) NIR absorption topograph of an un-annealed. undoped SI LEC sample together with (h) an [EL2] scan from the indicated bright line on the topograph.
S. Clark et al.
/ Line-scan system to assess
tion. The background scan is obtained in one of two ways: either the sample is removed or the image is defocussed. Neither of the two methods is ideal. The former tends to distort the illumination profile because of the high refractive index of GaAs (ideally the sample should be replaced with an sample of identical size and shape with uniform absorption). The second method removes macroscopic EL2 fluctuations. However, this is useful if the primary interest is in microscopic fluctuations of [EL2], as in this work. The line-scan is divided by the background scan to give the normalised absorption linescan. The EL2 scan is obtained after calibrating the normalised absorption scan using a transmission measurement at a wavelength of 1 p.m on a spectrophotometer. A second measurement is made at a wavelength of 2 ~im where the absorption due to EL2 is very small. The transmission at a wavelength of 2 p.m is assumed to be uniform across the sample. The [EL2] scan is then obtained, for a known sample thickness, from the normalised absorption scan using Martin’s calibration [1] for absorption at a wavelength of 1 p.m (given above), An EL2 “uniformity parameter” is obtained by taking the mean deviation of the [EL2] scan data, This mean deviation gives a numerical measure of the uniformity of the [EL2] scan and is primarily used in this work to measure the microscopic uniformity of [EL2] in heat treated material. In the results presented in this work, the background absorption line-scan is taken with the image defocussed. Hence, as macroscopic [EL2] fluctuations are removed, the mean deviation of the [EL2] scan data can be regarded as a microscopic uniformity parameter. In fig. 2, an example of a vidicon NIR absorption image of an as-grown SI LEC GaAs sample is given, together with an [EL2] scan taken along the bright line indicated on the absorption image. The [EL2] scan is obtained from the normalised absorption scan, shown in fig. 3, using a measurement from the sepctrophotometer and Martin’s calibration [1]. Fig. 3 shows the normalised absorption line-scan of the sample together with the linescan and the background scan (obtained by defocussing the image) from which it was derived,
homogeneity of [EL2] in LEC SI GaAs
105
non~iiseii ab~o,pi~on
-
I
-
urntsj I
SorPSon
. ‘
.
deloc~med b~ckg,o~nd
.--
..
~
—
~.
Fig. 3. Absorption, background and normalised absorption scans used to calculate the IEL2] scan in fig. 2.
4. Experimental details and results Block samples of as-grown LEC SI GaAs were annealed in evacuated, sealed ampoules, at various temperatures between 700 and 1200°C. Each was subsequently quenched by removing the ampoule rapidly from the furnace and dropping it into cold water. After slices were removed for assessment, the remaining material was resealed in the ampoule and annealed at 950 °C for 5 h followed by a conventional slow cool. In addition to this an entire 2 inch diameter LEC ingot was annealed in an evacuated, sealed ampoule and quenched. The ingot was subsequently annealed at 950°C for 5 h before being removed from the ampoule. Fig. 4 shows two representative EL2 absorption images taken from samples quenched from 1000 and 1100°C. The absorption line-scan positions are indicated by bright lines. Note that the former still shows clear cell structure and resembles asgrown material, whereas the latter appears to be virtually featureless. Images of all samples quenched from above 1100°C appear featureless both after quenching and after subsequent annealing at 950°C. Fig. 5 shows the [EL2] scans obtamed from samples quenched from 1000, 1100 and 1200°C. [EL2] for the sample quenched from 1000 °Cis similar to that of the as-grown samples. Note the pronounced drop in [EL2] and increase in [EL2] uniformity for the samples quenched from above 1100°C. [EL2] together with [EL2] mean deviations for all the heat treated samples in this work are given in table 1. Note again that quenching from 1100°C and above causes a substantial drop in [EL2}, but
106
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lark etal.
/
Line-i, an
ii
sum
is’
as si’s s hssmogenei’ty of /E/..2J in LI/C SI GaA Table I [F.L2] and [EL2] mean deviations for all heat treated samples in present work Sample details and [EL2] Mean heat treatments
)cni ~)
deviation of[FL2]
t
Block Block Block Block Block Block Block
sample, as-grown annealed at 950°C quenched from 700°(~ quenched from 800°C’ quenched from 900°C quenched from 1000°C quenched from 1100°C
l.34x1O°’ l.62x l0’~ 1 .34X 10° l.50x 10°’ 1.43 X 100 l.50x10°’ 1.1(1 x 100
X.2x l0’~ 5.1
x
l0’~
7.9 X l0°~ 79x l0’~ 4.9 x l0”~ 4 5.2xl0’ 2.9 x 1ç1i4
Block quenched from 1150°C 1.02 X l0’~’ 1.9>< l014 Block quenched from 1180°C’ 0.70>< 10°’ j,~~ 10i4 Block quenched from 1200°C’ 0.60xl0°’ l.7x io’~ Block quenched from 1100°C and then annealed at 950°C’ 1.17>< I0’~’ 1.9 x Block quenched from 1150°C’ and then annealed at 950°C 1.23 x 10°’ 2.7 x 10’~ Block quenched from 1180°C and then annealed at 950°C 1.07>< 100 1.8 x l0’~ Block quenched from 1200°C and then annealed at 950°C’ 1.02 x 10°’ 1.7 ~ Ingot quenched from 1100°C’ 1° 1.4 ~ 10’~ and then annealed at 950°C 1.28 x10 Note: all block samples are taken from the same ingot: all anneals are of 5 h duration.
______________________________________________________
~r. I ig. 4. NI R .ibsorption topographs of samples quenched front 1000°( . I’he bright lines indicate is here [EL2] ~c,in’ 5¼crc obtained,
a significant improvement in optical transmission uniformity. Electrical assessment on this material presented elsewhere [31,32] shows that the asgrown (n-type) SI samples quenched from above 1100°C have been converted to p-type by the heat
treatment but are restored to SI (n-type) by the subsequent 950 ° C anneal. The results presented here show that subsequent annealing at 950°C causes [EL2j to return to about the as-grown concentration, but the superior [EL2] uniformity, produced by quenching. is retained. ‘
.
5. Discussion 2X1016 ~
5.1, Discussion of /EL2J linescan system
~EL21
1000°C~ 1100°C
.
.
‘
. ..
1200C0
.
The advantages and disadvantages of this systemAdvantages: are presented below.
. ~ ..
‘
.
The system can be used with any existing NIR absorption system that has a video output. Microscopic uniformity scans are produced, the —
1nIrn
Fig 5. [EL2] scans for samples quenched from 1000, 1100 and 1200°C.
—
resolution is only limited by the imaging system. Measurements are taken at room temperature. —
S. Clark et al.
/
Line-scan system to assess homogeneity of [EL2] in LEC SI GaAs
A microscopic EL2 uniformity parameter can be calculated, which can be used for characterisation of commercial material. Disadvantages: The system requires a point calibration from a spectrophotometer. [EL2] results are to be regarded as semiquantitative (see below). Only [EL2] linescan results are produced whereas some other systems give complete [EL2] maps. In principle, of course, this type of system could be easily expanded to give full [EL2] maps with the same resolution as the monitor on the imaging system. This would require some modifications to the electronics and a more powerful microcomputer to handle the increased computing requirements. The use of photoquenching to calculate the sample absorption coefficient due to EL2 was deliberately avoided, because it is a slow technique, which involves measurements at temperatures below 77 K. Quenching also has other problems as removal of EL2 properties will alter the Fermi level, which may change the charge states of other absorbing defects, hence changing their absorption cross sections. The system should be regarded as semi-quantitative only, for the following reasons: (1) Non-linearity of the silicon vidicon response (this problem can be reduced by good camera selection). (2) The absorption is measured from a broad band spectrum (this could be prevented by using a band-pass filter, but this would reduce the signalto-noise ratio and hence increase the time required for averaging). (3) Problems arising from incorrect background subtraction (this effect can be reduced by optimising the normalisation procedure as described in the system description above). (4) Non-uniformities in transmitted light due to scratches, precipitates and other features (these are usually easy to spot on the absorption image and should be avoided), (5) Absorption from the other charge states of EL2, especially the hole absorption cross-section of the empty trap [33].
107
—
(6) Any absorption contributions from other deep levels. (7) Any inaccuracy in the calibration of the absorption coefficient of EL2. The problems from (5) onwards are common to most present systems and probably involve the largest errors.
—
5.2. Discussion of results
—
—
Recent analysis of device results [34,35] mdicates that point defects have a more significant effect than dislocations on the uniformity of device parameters. The correlation sometimes observed between dislocations and device parameters is then caused by a secondary effect: dislocation gettering of the relevant point defects. If this is the case, specifying dislocation densities may be irrelevant, whereas methods of measuring point defect distributions, especially that of EL2, would be more important. A microscopic uniformity parameter of [EL2] is of particular importance as it is expected to determine the local fluctuations in FET properties. Hence we emphasise the role of heat treatments to give superior EL2 uniformity (although the dislocation density is substantially 2 in the case increased from 10~to 106 cm of the quenched ingot). The quenching results presented here have been discussed in more detail elsewhere [31,32], where it is postulated that the uniformity improvements observed are due to the dissolution of defects into the lattice at high temperature. Quenching reduces [EL2] and prevents the reformation of non-uniformities by preventing extensive point defect diffusion. Unfortunately the material is no longer SI. Annealing at 950°Crestores SI behaviour, allows the reformation of EL2 but does not allow extensive redistribution. -~
—
6. Conclusions We have demonstrated that this system can be used to give line-scans of [EL2] at a resolution limited only by the imaging system. In addition a microscopic [EL2] uniformity parameter can be calculated. This can be used to assess material for
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Line-scan system to assess homogeneity of [EL2J in LEC’ SI GaA.s
commercial IC production. We have shown that SI, quenched and reannealed, material has a supenor uniformity to conventional, as-grown or annealed material using this uniformity parameter. Acknowledgements The authors gratefully acknowledge provision of samples by I. Grant. The work was supported by the Procurement Executive, Ministry of Defence, DCVD.
[141W.
Wettling and J. Windscheif, AppI. Phys. A40 (1986) 191. [151 EtC. Alt (1986) [16] H.C. Alt and and Ci. 0. Packeiser, Packeiser. J. in:Appt. Proc.Phys. 18th 60 Conf. on 2954. Solid State Devices and Materials. Tokyo, 1986, p 659. [17] H.C. Alt and H. Schink. AppI. Phys. Letters 52 (1988) 1661. [181 P. Dobrilla, iS. Blakemore and R.Y Koyama, in: SemiInsulating 111—V Materials, Proc. 3rd Conf., Kah-nee-ta, 1984, Eds. D.C. Look and J.S. Blakemore (Shiva, Nantwich, 1984) p. 282 [19] P. Dobrilla and iS. Blakemore. 1. AppI. Phys. 58 (1985)
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3588.