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Characterization of undoped n-type LPE gallium arsenide by hall, photo-hall and PL measurements
Volume
3, number
MATERIALS
9,lO
LETTERS
July 1985
CHARACTERIZATION OF UNDOPED n-TYPE LPE GALLIUM ARSENIDE BY HALL, PHOTO-HALL AND PL MEASUREMENTS Weiying SHI, Haisheng YU, Binglin ZHOU, Yuanxi ZOU (Yuanhsi CHOU), Yaocheng REN, Zhenxiu CHEN and Shaobo WANG Shanghai Institute of Metallurgy,
Academia
Sinica, 865 Chang Ning Road, Shanghai 200050, China
Received 4 April 1985
In a previous paper, a brief comment has been made on some Hall and photo-Hall measurements on several undoped n-type LPE GaAs samples grown in our institute. The experimental results are further discussed in this letter with a view to shedding more light on the scattering mechanism for LPE GaAs. Photoluminescence measurements on some additional samples are reported and discussed.
1. Introduction The lowering in room-temperature electron mobility in n-type GaAs, InAs and InP has been attributed by Weisberg and Blanc [l] to the presence of space-charge centers related with oxygen and/or copper in the case of GaAs. The concept of space-charge scattering has also been used by Kaneko et al. [2] as well as by Stringfellow [3] to account for their data on mobility in n-type Al,Gal,As. Recently however, Stringfellow [4] interpreted his mobility data, as well as those of Chandra and Eastman [S] for LPE GaAs and AIXGal+As, by the presence of central-cell scattering due to carbon atom on an arsenic site. On the other hand, Walukiewicz et al. [6] explained the abovementioned data by using the variational method instead of Mathiessen’s rule in calculations without invoking the contribution of central-cell scattering. Further experiments and discussions are, therefore, necessary to clarify this problem.
baked in a purified H, gas stream at 650~8OO’C for a given period. The substrate was usually unprotected during the baking unless stated otherwise. At the end of baking, the melt was brought into contact with the substrate at 650-730°C by rotating the horizontal quartz tube. Growth of the epilayer was started by cooling at a given rate or by turning off the power, The epilayers were grown on SI GaAs substrates to a thickness of 3.7-5.7 pm. Hall and photo-Hall measurements were performed on the epilayers by using the van der Pauw technique in the temperature range of 77-300 K. A 750 W halogen tungsten lamp and a prism monochromator were used as a monochromatic light source (+3750 A) in photo-Hall measurements. PL measurements were performed on the epilayers at 77 K.
3. Experimental results 3.1. Temperature dependence of Hall measurements
2. Experimental The LPE technique used in the present work has been described elsewhere [7]. Briefly, a rotating boat made of high-purity graphite or quartz treated by a special process [8] was used. The Ga-GaAs melt was 336
The temperature dependence of Hall measurements was studied on five undoped LPE n-GaAs samples. The results are shown in fig. 1 as a plot of p versus T. It is easily seen that crossing of the p-T curves for these LPE GaAs samples occurs, in agreement with the findings of Weisberg and Blanc [ 1] for bulk GaAs. 0 167-577x/85/$ 03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
July 1985
MATERIALS LETTERS
Volume 3, number 9,lO
6
IO4
-
8 6
Fig. 2. Relationship between (Ap/k)77~ and (An/n)77K five LPE n-GaAs samples as a result of illumination.
4
mobility resulting from light illumination below the limit of detection.
6
a IO*
2
T
4
( K)
3.2. Photo-Hall measurements at 77 K The effects of light illumination on the electron at 77 K in five undoped LPE n-GaAs samples were studied by the photo-Hall method. The results were reported in a previous paper [9] and are replotted in fig. 2 by showing the dependence of (Af_~/p)~~K on (h/n),, K. It can easily be seen that a fairly good correlation exists between these two parameters for the samples S 1-S3, which have relatively high values ofNJ , the product of the concentration of scattering centers and the cross section calculated after Conwell and Vassell [IO] using the following relations: mobility
c1SC= 2.4 X lo9 [Ns~(TM*/mo)1’2]-1,
(1)
III-4 = l//$
(2)
On the other hand, the experimental points for the S4 and S5 having a relatively low value ofN,,A lie much below the curve, indicating an increase in 77 K samples
close to or
3.3. PL measurements at 77 K
Fig. 1. p-Tcurves for 5 LPE GaAs samples: o 79-24, o 80-81, e 82-12,. 82-13, o 82-22.
+ l/i+ + UPsc.
for
In order to find out whether the lowering in roomtemperature electron mobility in LPE n-GaAs is caused by the presence of carbon on an arsenic site, the relative height of the CA, peak in the 77 K PL spectra of five LPE n-GaAs samples grown under similar conditions is plotted versus Ns,.J in fig. 3. It is easily seen that the five experimental points do not show a good correlation betweenN& and CA,, apparently in disagreement with the proposal of Stringfellow [4 1. I
-40 -i f u
0
“020 x
-
310 m
=
Q
0 -
0
86-
0
4
I 0.1 II AS PEAK
0
Fig. 3. Dependence of N,A peak in 77 K PL spectra.
0.2 HEIGHT
on the relative height of the CAs
337
photoinduced shrinkage of spacecharge region
(D)
no change
+
photoexcitation of electrons from valence band
(C)
-
probably small
photoneutralization of donor
(B)
+
AVIK
K
no change
+ (k//.977
(&/&)77K
a slight increase
no change
K < (An/n)77 K
< W/n)77K
(&/1~)77 K > (An/n)77 K
ti
- -
(All/r)77 K = -@n/n)77
t
K
-
(M/i.977 K = (m/n)77
and &II
t
ALc77K
h17K
Comparison between
-
(fl1)77K
Expected variation of Hall parameters
(AB) combination of (A) and (B)
photodeactivation of acceptor
(A)
Process
Table 1 Effects of illumination on Hall parameters
K
true s4, s5
not true s4, s5
not true Sl-s3 true
Sl-s3
not true s4, s.5 true Sl-s3
not true Sl-s5
not true Sl-s5
Experimental results for samples S 1- S5
significant
not as significant as (C)
significant
not significant
same as above
not as significant as (D)
not significant
not significant
s4, s5
same as above
probably not very significant by analogy with S4 and S5
Sl-s3
Relative significance of various processes
MATERIA.LS LETTERS
Volume 3, number 9,lO
July 1985
4. Discussion
5. Summary
4.1. Crossing of the p-T curves
Our observation of the crossing of the p-T curves for five LPE n-GaAs samples can hardly be explained by the calculations of Walukiewicz et al. [6] using the variational method. The non-existence of a good correlation betweenN,,A and the relative C,, peak height in the 77 K PL spectra does not seem to support the central-cell scattering mechanism proposed by Stringfellow [4] although further work would seem to be necessary to clarify this problem. The experimental results on the effect of light illumination on p77 K seem to provide an adequate support to the presence of space-charge scattering in at least three samples (Sl-S3). Further work on the probable nature of the space-charge centers involved will be reported elsewhere.
The experimental observation of the crossing of the p-T curves for bulk GaAs samples by Weisberg and Blanc [ I] and for LPE GaAs samples
by the authors
as
shown in fig. 1 can, in our opinion, hardly be explained by the calculations of Walukiewicz et al. [6] using the variational method. 4.2. C,, peak in the 77 K PL spectra The correlation between Nsd and the relative CA, peak height for give LPE n-GaAs samples seems to be not as good as expected from the view point of the central-cell scattering mechanism as shown in fig. 3. In the light of this result, some doubt may be cast, in our opinion, on the validity of the mechanism suggested by Stringfellow [4], although a more reliable method for the determination of C,, would seem to be desirable. 4.3. Effect of light illumination on 77 K electron mobility In order to shed more light on the problem, possible
illumination-induced
processes
four
are listed in
Acknowledgement The authors are grateful to Qiuxia Sun and Jian Hu for technical assistance, Liang Qi, Wenlin Tan and Ruidi Wu for Hall and PL measurements, Lifang Tan, Liying Liao, Huizu Tan and Qidong Lu for supplying the SI GaAs substrates and polycrystalline GaAs crystals as the source material for liquid-phase epitaxy.
table 1, together with the predicted effect of each on An and Ap for 77 K. For the samples S4 and S.5 there
is a significant relative increase in n77 K, but the corresponding increase in p77 K is either below the limit of detection or very small. A reasonable explanation would be that process (C), viz. photoexcitation of electrons from the valence band, may play an important role for these samples, while the other three processes may not proceed to any significant extent. Unlike the samples S4 and S5, the increase in p7, K is much higher than that in nT7 K for the samples Sl-S3. If processes (A) and (B) can be assumed to take place only to an insignificant extent for these three samples, in analogy to samples S4 and S5, a probable explanation would be that the relative significance of processes (C) and (D) could be different for the samples Sl -S3 on the one hand and S4 and SS on the other. This seems to be a reasonable suggestion in view of the relatively low value ofN,,A for samples S4 and SS in comparison with those for the other three samples. The above argument seems to provide an adequate support to the suggested presence of space-charge scattering in samples Sl-s3.
References [ll L.K. Weisberg and J. Blanc, Proceedings of the International Conference on Semiconductors. Prague (1960) p. 940. [21 K. Kaneko, M. Ayabe and N. Watanabe, Gallium Arsenide and Related Compounds, Edinburgh, In,titute of Physics Conference Series 33a (1976) 216. I31 G.B. Stringfellow, J. Appl. Phys. 50 (1979) 4178. [41 G.B. Stringfellow, Appl. Phys. Letters 36 (1980) 1. ISI A. Chandra and L.F. Eastman, Solid State Electron. 22 (1979)
645.
[61 W. Walukiewicz,
J. Lagowski and H.C. Gates, J. Appl. Phys. 52 (1981) 5853. I71 W. Shi, H. Yu, Y. Ren and Y. Zou, Semicond. Technol. No. 3 (1979) 62, in Chinese. [81 L. Tan, L. Liao, W. Tan and Y. Zou, Sci. Technol. Commun. No. 2 (1981) 57, in Chinese. [91 W. Shi, H. Yu, B. Zhou, Y. Zou, Y. Ren and Z. Chen,
Mat. Letters 2 (1984) 313. 1101 E.M. Conwell and M.O. Vassell, Phys. Rev. 166 (1968) 797.
339
3, number
MATERIALS
9,lO
LETTERS
July 1985
CHARACTERIZATION OF UNDOPED n-TYPE LPE GALLIUM ARSENIDE BY HALL, PHOTO-HALL AND PL MEASUREMENTS Weiying SHI, Haisheng YU, Binglin ZHOU, Yuanxi ZOU (Yuanhsi CHOU), Yaocheng REN, Zhenxiu CHEN and Shaobo WANG Shanghai Institute of Metallurgy,
Academia
Sinica, 865 Chang Ning Road, Shanghai 200050, China
Received 4 April 1985
In a previous paper, a brief comment has been made on some Hall and photo-Hall measurements on several undoped n-type LPE GaAs samples grown in our institute. The experimental results are further discussed in this letter with a view to shedding more light on the scattering mechanism for LPE GaAs. Photoluminescence measurements on some additional samples are reported and discussed.
1. Introduction The lowering in room-temperature electron mobility in n-type GaAs, InAs and InP has been attributed by Weisberg and Blanc [l] to the presence of space-charge centers related with oxygen and/or copper in the case of GaAs. The concept of space-charge scattering has also been used by Kaneko et al. [2] as well as by Stringfellow [3] to account for their data on mobility in n-type Al,Gal,As. Recently however, Stringfellow [4] interpreted his mobility data, as well as those of Chandra and Eastman [S] for LPE GaAs and AIXGal+As, by the presence of central-cell scattering due to carbon atom on an arsenic site. On the other hand, Walukiewicz et al. [6] explained the abovementioned data by using the variational method instead of Mathiessen’s rule in calculations without invoking the contribution of central-cell scattering. Further experiments and discussions are, therefore, necessary to clarify this problem.
baked in a purified H, gas stream at 650~8OO’C for a given period. The substrate was usually unprotected during the baking unless stated otherwise. At the end of baking, the melt was brought into contact with the substrate at 650-730°C by rotating the horizontal quartz tube. Growth of the epilayer was started by cooling at a given rate or by turning off the power, The epilayers were grown on SI GaAs substrates to a thickness of 3.7-5.7 pm. Hall and photo-Hall measurements were performed on the epilayers by using the van der Pauw technique in the temperature range of 77-300 K. A 750 W halogen tungsten lamp and a prism monochromator were used as a monochromatic light source (+3750 A) in photo-Hall measurements. PL measurements were performed on the epilayers at 77 K.
3. Experimental results 3.1. Temperature dependence of Hall measurements
2. Experimental The LPE technique used in the present work has been described elsewhere [7]. Briefly, a rotating boat made of high-purity graphite or quartz treated by a special process [8] was used. The Ga-GaAs melt was 336
The temperature dependence of Hall measurements was studied on five undoped LPE n-GaAs samples. The results are shown in fig. 1 as a plot of p versus T. It is easily seen that crossing of the p-T curves for these LPE GaAs samples occurs, in agreement with the findings of Weisberg and Blanc [ 1] for bulk GaAs. 0 167-577x/85/$ 03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
July 1985
MATERIALS LETTERS
Volume 3, number 9,lO
6
IO4
-
8 6
Fig. 2. Relationship between (Ap/k)77~ and (An/n)77K five LPE n-GaAs samples as a result of illumination.
4
mobility resulting from light illumination below the limit of detection.
6
a IO*
2
T
4
( K)
3.2. Photo-Hall measurements at 77 K The effects of light illumination on the electron at 77 K in five undoped LPE n-GaAs samples were studied by the photo-Hall method. The results were reported in a previous paper [9] and are replotted in fig. 2 by showing the dependence of (Af_~/p)~~K on (h/n),, K. It can easily be seen that a fairly good correlation exists between these two parameters for the samples S 1-S3, which have relatively high values ofNJ , the product of the concentration of scattering centers and the cross section calculated after Conwell and Vassell [IO] using the following relations: mobility
c1SC= 2.4 X lo9 [Ns~(TM*/mo)1’2]-1,
(1)
III-4 = l//$
(2)
On the other hand, the experimental points for the S4 and S5 having a relatively low value ofN,,A lie much below the curve, indicating an increase in 77 K samples
close to or
3.3. PL measurements at 77 K
Fig. 1. p-Tcurves for 5 LPE GaAs samples: o 79-24, o 80-81, e 82-12,. 82-13, o 82-22.
+ l/i+ + UPsc.
for
In order to find out whether the lowering in roomtemperature electron mobility in LPE n-GaAs is caused by the presence of carbon on an arsenic site, the relative height of the CA, peak in the 77 K PL spectra of five LPE n-GaAs samples grown under similar conditions is plotted versus Ns,.J in fig. 3. It is easily seen that the five experimental points do not show a good correlation betweenN& and CA,, apparently in disagreement with the proposal of Stringfellow [4 1. I
-40 -i f u
0
“020 x
-
310 m
=
Q
0 -
0
86-
0
4
I 0.1 II AS PEAK
0
Fig. 3. Dependence of N,A peak in 77 K PL spectra.
0.2 HEIGHT
on the relative height of the CAs
337
photoinduced shrinkage of spacecharge region
(D)
no change
+
photoexcitation of electrons from valence band
(C)
-
probably small
photoneutralization of donor
(B)
+
AVIK
K
no change
+ (k//.977
(&/&)77K
a slight increase
no change
K < (An/n)77 K
< W/n)77K
(&/1~)77 K > (An/n)77 K
ti
- -
(All/r)77 K = -@n/n)77
t
K
-
(M/i.977 K = (m/n)77
and &II
t
ALc77K
h17K
Comparison between
-
(fl1)77K
Expected variation of Hall parameters
(AB) combination of (A) and (B)
photodeactivation of acceptor
(A)
Process
Table 1 Effects of illumination on Hall parameters
K
true s4, s5
not true s4, s5
not true Sl-s3 true
Sl-s3
not true s4, s.5 true Sl-s3
not true Sl-s5
not true Sl-s5
Experimental results for samples S 1- S5
significant
not as significant as (C)
significant
not significant
same as above
not as significant as (D)
not significant
not significant
s4, s5
same as above
probably not very significant by analogy with S4 and S5
Sl-s3
Relative significance of various processes
MATERIA.LS LETTERS
Volume 3, number 9,lO
July 1985
4. Discussion
5. Summary
4.1. Crossing of the p-T curves
Our observation of the crossing of the p-T curves for five LPE n-GaAs samples can hardly be explained by the calculations of Walukiewicz et al. [6] using the variational method. The non-existence of a good correlation betweenN,,A and the relative C,, peak height in the 77 K PL spectra does not seem to support the central-cell scattering mechanism proposed by Stringfellow [4] although further work would seem to be necessary to clarify this problem. The experimental results on the effect of light illumination on p77 K seem to provide an adequate support to the presence of space-charge scattering in at least three samples (Sl-S3). Further work on the probable nature of the space-charge centers involved will be reported elsewhere.
The experimental observation of the crossing of the p-T curves for bulk GaAs samples by Weisberg and Blanc [ I] and for LPE GaAs samples
by the authors
as
shown in fig. 1 can, in our opinion, hardly be explained by the calculations of Walukiewicz et al. [6] using the variational method. 4.2. C,, peak in the 77 K PL spectra The correlation between Nsd and the relative CA, peak height for give LPE n-GaAs samples seems to be not as good as expected from the view point of the central-cell scattering mechanism as shown in fig. 3. In the light of this result, some doubt may be cast, in our opinion, on the validity of the mechanism suggested by Stringfellow [4], although a more reliable method for the determination of C,, would seem to be desirable. 4.3. Effect of light illumination on 77 K electron mobility In order to shed more light on the problem, possible
illumination-induced
processes
four
are listed in
Acknowledgement The authors are grateful to Qiuxia Sun and Jian Hu for technical assistance, Liang Qi, Wenlin Tan and Ruidi Wu for Hall and PL measurements, Lifang Tan, Liying Liao, Huizu Tan and Qidong Lu for supplying the SI GaAs substrates and polycrystalline GaAs crystals as the source material for liquid-phase epitaxy.
table 1, together with the predicted effect of each on An and Ap for 77 K. For the samples S4 and S.5 there
is a significant relative increase in n77 K, but the corresponding increase in p77 K is either below the limit of detection or very small. A reasonable explanation would be that process (C), viz. photoexcitation of electrons from the valence band, may play an important role for these samples, while the other three processes may not proceed to any significant extent. Unlike the samples S4 and S5, the increase in p7, K is much higher than that in nT7 K for the samples Sl-S3. If processes (A) and (B) can be assumed to take place only to an insignificant extent for these three samples, in analogy to samples S4 and S5, a probable explanation would be that the relative significance of processes (C) and (D) could be different for the samples Sl -S3 on the one hand and S4 and SS on the other. This seems to be a reasonable suggestion in view of the relatively low value ofN,,A for samples S4 and SS in comparison with those for the other three samples. The above argument seems to provide an adequate support to the suggested presence of space-charge scattering in samples Sl-s3.
References [ll L.K. Weisberg and J. Blanc, Proceedings of the International Conference on Semiconductors. Prague (1960) p. 940. [21 K. Kaneko, M. Ayabe and N. Watanabe, Gallium Arsenide and Related Compounds, Edinburgh, In,titute of Physics Conference Series 33a (1976) 216. I31 G.B. Stringfellow, J. Appl. Phys. 50 (1979) 4178. [41 G.B. Stringfellow, Appl. Phys. Letters 36 (1980) 1. ISI A. Chandra and L.F. Eastman, Solid State Electron. 22 (1979)
645.
[61 W. Walukiewicz,
J. Lagowski and H.C. Gates, J. Appl. Phys. 52 (1981) 5853. I71 W. Shi, H. Yu, Y. Ren and Y. Zou, Semicond. Technol. No. 3 (1979) 62, in Chinese. [81 L. Tan, L. Liao, W. Tan and Y. Zou, Sci. Technol. Commun. No. 2 (1981) 57, in Chinese. [91 W. Shi, H. Yu, B. Zhou, Y. Zou, Y. Ren and Z. Chen,
Mat. Letters 2 (1984) 313. 1101 E.M. Conwell and M.O. Vassell, Phys. Rev. 166 (1968) 797.
339