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III ratio on InGaAs and InP grown at low temperature by LP-MOCVD
ARTICLE IN PRESS
Journal of Crystal Growth 260 (2004) 23–27
Effects of V/III ratio on InGaAs and InP grown at low temperature by LP-MOCVD L. Jiang*, T. Lin, X. Wei, G.H. Wang, G.Z. Zhang, H.B. Zhang, X.Y. Ma National Engineering Research Center for Opto-electronic Devices, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, China Received 30 June 2003; accepted 13 August 2003 Communicated by M. Schieber
Abstract Effects of V/III ratio on heavily Si doped InGaAs and InP were studied using low pressure metalorganic chemical vapor deposition (LP-MOCVD) at a growth temperature of 550 C. In InGaAs, as the V/III ratio decreases from 256 to 64, the carrier concentration increases from 3.0 1018 to 5.8 1018 cm3; and the lattice mismatch of InGaAs to InP was observed to vary from 5.70 104 to 1.49 103. In InP, when the V/III ratio decreases from 230 to 92, the same trend as that in Si doped InGaAs was observed that the carrier concentration increases from 9.2 1018 to 1.3 1019 cm3. The change of AsH3 was found to have stronger effect on Si incorporation in InGaAs at lower growth temperature than at higher growth temperature. r 2003 Elsevier B.V. All rights reserved. PACS: 73.61.Ey; 81.05.Ea; 81.15.Gh Keywords: A1. Doping; A1. X-ray diffraction; A3. Low press. metalorganic vapor phase epitaxy; B2. Semiconducting III–V materials; B2. Semiconducting indium phosphide
1. Introduction The small bandgap energy and superior carrier transport properties of InGaAs have made it widely used in high-speed devices such as heterostructure bipolar transistors (HBTs) [1]. Usually, increasing the doping level can result in lowered base resistivity and increased operation frequency. However, the commonly used p-type dopant, Zn, *Corresponding author. Tel.: +86-10-8230-4028, ext. 239; fax: +86-10-8230-5060. E-mail address: [email protected] (L. Jiang).
has relatively high diffusion coefficient, which makes it difficult to obtain abrupt doping profile at the emitter/base junction especially when the doping level of base is high [1] and the width of base is small. An effective way to suppress the outdiffusion of Zn is to lower the growth temperature of the whole HBT structure. But new problem arises when the growth temperature is decreased. As the pyrolysis temperature of silane is relatively high, the insufficient pyrolysis of silane limits the realization of heavily doped InGaAs subcollector and InGaAs/InP emitter cap. Generally, two methods are adopted
0022-0248/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2003.08.013
ARTICLE IN PRESS L. Jiang et al. / Journal of Crystal Growth 260 (2004) 23–27
24
to solve the problem: increasing the flow rate of silane or using a new Si precursor such as disilane [2,3]. In another aspect, varying the V/III ratio may be an effective way. The effects of V/III ratio on unintentionally [4] and lightly [5] doped InGaAs have been investigated at growth temperatures of 640 C and 600 C, respectively. In the unintentionally doped InGaAs, residual carrier concentration increases almost linearly with the increase of V/III ratio [4]. However, in the lightly doped InGaAs at a doping level of 1016–1017, an adverse trend is observed that, when the V/III ratio increases from 2.5 to 81, the carrier concentration decreases from 8.2 1016 to 2.7 1016 cm3 [5]. In this investigation, the growth temperature was lowered to 550 C. The influence of V/III ratio on the carrier concentration of Si doped InGaAs and InP, and on the lattice mismatch of InGaAs to InP were studied. It should be noted that, all V/III ratios mentioned hereinafter refer to input V/III ratios that are calculated using the following equation: V=III ¼
Fhydride ðp=ðp0 pÞÞFhydrogen
ð1Þ
where Fhydride is the flow rate of hydride and Fhydrogen is the flow rate of hydrogen that passing through the MO (metalorganic precursor) bubbler, p0 is the pressure in the MO bubbler and p is the vapor pressure of that metalorganic precursor. p is calculated by this equation [6]: logðp½TorrÞ ¼ A B=T
n-type dopant source. The carrier gas was palladium-diffused hydrogen. The flow rates of metalorganic sources, the flow rate of silane and total flow rate were kept constant, and the V/III ratio was changed by varying the flow rates of AsH3 and PH3. In order to minimize the error brought by measurements, three layers were grown on a single substrate. Thus, we could make the measurement for one sample at a time, and obtain a set of relevant data. First, we grew a three-layer structure of InGaAs (Sample I). The growth conditions for each layer were almost identical except for the flow rate of AsH3. The so grown epi-wafer was subjected to Double-crystal X-ray diffraction measurement for lattice mismatch, to Electro-chemical capacitancevoltage (ECV) measurement for carrier concentration, and to secondary ion mass spectroscopy (SIMS) for Si profile. After that, an epi-wafer of InP (Sample II) that contains three layers of InP grown with different PH3 flow rates was obtained, and was similarly subjected to ECV measurement for carrier concentration and to SIMS for Si profile.
ð2Þ
in which, T is the temperature in the MO bubbler and A; B are two parameters. The specific values of A and B also come from the same reference [6].
2. Experimental procedure Epitaxial growth was carried out in an AIX-200 R&D system which contains a horizontal reactor. The growth temperature was 550 C and pressure was 20 mbar. The source materials were triethylgallium (TEGa), trimethylindium (TMIn), 100% arsine (AsH3) and 100% phosphine (PH3). Silane (SiH4) diluted to 2% in hydrogen was used as the
3. Results and discussion 3.1. Effect of V/III ratio on lattice mismatch of InGaAs to InP In Sample I, the flow rates of AsH3 for the three layers were 50, 100 and 200 sccm from the bottom up. Corresponding V/III ratios are about 64, 128 and 256, respectively. Fig. 1 shows the X-ray rocking curve of Sample I, in which S represents substrate peak, while L1, L2 and L3 represent three epilayers, respectively. By carefully etching the epi-wafer using the mixture of H2SO4:H2O2:H2O=4:1:1, we found that L1 is the peak of the uppermost epilayer, L3 the peak of the lowermost one, and L2 the middle one. That is, the epilayer grown with the highest V/ III ratio has the smallest lattice constant. Generally, TEGa pyrolyses via the following two reactions: GaðC2 H5 Þ3 -dGaðC2 H5 Þ2 þ C2 H5 d
ð3Þ
ARTICLE IN PRESS L. Jiang et al. / Journal of Crystal Growth 260 (2004) 23–27
Intensity (counts/s)
Intensity (counts/s)
10000 L1
1000
100
10
1 62.5
63.0 63.5 2theta (Degree)
5.50E+018
1800
5.00E+018
1600
4.50E+018
1400
4.00E+018
1200
3.50E+018
1000
3.00E+018
800
2.50E+018 40 60 80 100 120 140 160 180 200 220 240 260 280 V/III ratio
Fig. 2. Effect of V/III ratio on the carrier concentration (m) and Si concentration (’) in InGaAs, in which, Si concentration is demonstrated by the relative intensity. The solid line is the fitting result of Si concentration using y ¼ Axc :
64.0
Fig. 1. X-ray rocking curve of Sample I.
and b-hydride elimination reaction GaðC2 H5 Þ3 -GaHðC2 H5 Þ2 þ C2 H4 :
2000
Carrier concentration (cm-3)
6.00E+018
S L3 L2
25
ð4Þ
In these two pyrolysis mechanism, the latter, that is, the b-hydride elimination mechanism dominate at higher temperatures (>650 C). At lower temperatures (such as the growth temperature of 550 C as we used), TEGa mainly pyrolyses by losing ethyl radicals [7]. AsH3 was supposed to react with ethyl radicals [8]: C2 H5 d þ AsH3 -C2 H6 þ dAsH2
ð5Þ
C2 H5 d þ dAsH2 -ðC2 H5 ÞAsH2 :
ð6Þ
Thus, at a constant flow rate of TEGa, more AsH3 can make the TEGa pyrolysing more quickly, so as to produce more Ga species at a certain period of time. As is well known, lattice constant of InGaAs reduces when Ga mole fraction in solid increases. As a result, increasing AsH3 flow rate reduces the lattice constant of the result InGaAs. 3.2. Effects of V/III ratio on Si doping 3.2.1. InGaAs Fig. 2 shows the dependence of Si doping in InGaAs on the V/III ratio. The observed phenomenon cannot be explained by the simple site model where the incorporation of Si is supposed to take place via vacant sites of group-III elements, according to which, the carrier concentration
should have increased as the V/III ratio increased. The same trend as that in Fig. 2 was observed by other researchers in Si doped GaAs [9–11] as well as in lightly Si doped InGaAs grown at a temperature higher than we used [5]. From the SIMS result, we found out that the increase in the carrier concentration should be a result of the increase in Si incorporation, which confirm the conclusion of Yoo et al. [5]. The data points coming from SIMS result were also plotted in Fig. 2. It was suggested that the dependence of Si doping on the V/III ratio indicates that the incorporation of Si is controlled by surface kinetics. In order to describe the observed trend, an empirical equation was employed [5,10]: NSi pFadop FbIII FcV expðEa =kTÞ
ð7Þ
where NSi is the Si concentration incorporated in each layer, Fdop ; FIII and FV are the precursor flow rates of the dopant, group-III element and group-V element, respectively. Ea is the activation energy. In lightly and moderately Si doped materials, NSi is approximately equal to the carrier concentration n. But in heavily Si doped materials, due to the amphoteric behavior of Si [10], Si doping is more compensated, and NSi would be greater than n. From Fig. 2, c is found to be about 0.6770.09, greater than that obtained by Yoo et al. [5]. This will be discussed in Section 3.2.3.
ARTICLE IN PRESS L. Jiang et al. / Journal of Crystal Growth 260 (2004) 23–27
26
3.2.2. InP In sample II, the flow rates of PH3 for the three layers were 200, 120 and 80 sccm from the bottom up. Corresponding V/III ratios are 230, 138 and 92, respectively. Some researchers have concluded that Si doping in InP using silane [13] or disilane [3] are not dependent on group-V concentration in the gas phase, which conclusion is consistent to the suggestion that Si incorporation takes place via vacant sites of group-III elements. So c in Eq. (7) should be zero. But in a more recent research [14] using SiH4 as the dopant, Keiper found that the doping level decreased by 29% when the V/III ratio doubled. Fig. 3 shows the dependence of Si doping in InP on the V/III ratio in our experiment. It is clear that the carrier concentration and Si concentration both decrease as the V/III ratio (that is, PH3 flow rate) increases. From Fig. 3, c is found to be 0.6870.02. This matches the result of Keiper’s study. 3.2.3. Qualitative explanation and discussion Field and Ghandhi have proposed a simple model to describe the Si incorporation in GaAs from SiH4 [10]. Since the Si–H bond strength in SiH4 (about 3.3 eV) [10] is comparable to the As–H bond strength of AsH3 (about 3.1 eV) [10,12], they assume that the thermal decomposition of SiH4 is heterogeneous. They also assumed that SiH2 is the initial adsorbed species on the surface, which 4500
1.60E+019
Intensity (counts/s)
-3
Carrier concentration (cm )
1.50E+019
4000
1.40E+019 3500
1.30E+019
3000
1.20E+019 1.10E+019
2500
1.00E+019 2000
9.00E+018
1500 80
100
120
140
160 180 V/III ratio
200
220
comes from the following reaction: SiH4 þ V -SiH2ðadsÞ þ H2
ð8Þ
where V is a vacant adsorption site. Similarly, P–H bond strength of PH3 (about 3.4 eV) [12] is also approximately equal to the Si–H bond strength of SiH4. So, we can assume the decomposition mechanism of SiH4 in InP and InGaAs to be similar to that in GaAs, and Si to take the same way as that in Eq. (8) to incorporate into InP and InGaAs. Then, Field and Ghandhi proposed one equation describing the Si concentration in the solid [10]: AðTÞPSiH4 yv ðT; PÞ Eads ðT; PÞ NSi D exp ð9Þ g kT where yn ðT; PÞ is the fraction of vacant SiH2 adsorption sites and Eads ðT; PÞ is the energy barrier to the adsorption reaction, Eq. (8). They proposed that increasing AsH3 pressure in gas phase (that is, increasing the flow rate of AsH3) increases the coverage of As species, increasing Eads ðT; PÞ; thus reducing adsorption of SiH2. Based on this suggestion, we can consider that PH3 has the similar effect on Eads ðT; PÞ: This assumption explained the effects of V/III ratio on Si doped InGaAs and InP observed in our experiment and researches of other people. Now we come to discuss the difference between the values of c obtained by us and Yoo et al. [5]. According to Field and Ghandhi, Eads ðT; PÞ increases as the flow rate of AsH3 increases. At lower growth temperature (as in our research), less change in Eads can cause the same change in NSi as that at higher growth temperature (as in the research of Yoo et al.). That is to say, less change in AsH3 flow rate at lower temperature can cause the same change in NSi as that at higher temperature. This may be the reason why the value of c we obtained is greater than that obtained by Yoo et al.
8.00E+018 240
Fig. 3. Effect of V/III ratio on the carrier concentration (m) and Si concentration (’) in InP, in which, Si concentration is demonstrated by the relative intensity. The solid line is the fitting result of Si concentration using y ¼ Axc :
4. Conclusion We investigated the effects of V/III ratio on Si doped InGaAs and InP at a low growth tempera-
ARTICLE IN PRESS L. Jiang et al. / Journal of Crystal Growth 260 (2004) 23–27
ture of 550 C. As the V/III ratio decreases, the lattice constant of InGaAs increases, as well as the carrier concentrations in InGaAs and InP. At lower temperature, the variation of arsine flow rate has stronger effect on Si doping in InGaAs.
Acknowledgements This research was supported by the National High Technology Research and Development Program of China (863 Program). The authors thank Mr. Ma Nongnong for his help in SIMS measurements.
References [1] K. Kurishima, T. Makimoto, T. Kobayashi, T. Ishibashi, J.J. Appl. Phys. 30 (1991) L258.
27
[2] C. Blaauw, F.R. Shepherd, C.J. Miner, A.J. Springthorpe, J. Electron. Mater. 19 (1990) 1. [3] E. Woelk, H. Beneking, J. Appl. Phys. 63 (1988) 2874. [4] M. Kamada, H. Ishikawa, J. Crystal Growth 94 (1989) 849. [5] J.B. Yoo, J.S. Kim, D.H. Jang, B.H. Koak, D.K. Oh, H.M. Kim, Y.T. Lee, J. Crystal Growth 132 (1993) 43. [6] G.B. Stringfellow, Organometallic Vapor-Phase Epitaxy: Theory and Practice, Academic Press, San Diego, 1999, p. 164. [7] P.W. Lee, T.R. Omstead, D.R. Mckenna, K.F. Jensen, J. Crystal Growth 85 (1987) 165. [8] M. Mashita, S. Horiguchi, M. Shimazu, K. Kamon, M. Mihara, M. Ishii, J. Crystal Growth 77 (1986) 194. [9] S.J. Bass, J. Crystal Growth 47 (1979) 613. [10] R.J. Field, S.K. Ghandhi, J. Crystal Growth 74 (1986) 543. [11] J.M. Redwing, H. Simka, K.F. Jensen, T.F. Kuech, J. Crystal Growth 145 (1994) 397. [12] G.B. Stringfellow, Organometallic Vapor-Phase Epitaxy: Theory and Practice, Academic Press, San Diego, 1999, p. 155. [13] A.R. Clawson, T.T. Vu, D.I. Elder, J. Crystal Growth 83 (1987) 211. [14] D. Keiper, J. Crystal Growth 233 (2001) 125.
Journal of Crystal Growth 260 (2004) 23–27
Effects of V/III ratio on InGaAs and InP grown at low temperature by LP-MOCVD L. Jiang*, T. Lin, X. Wei, G.H. Wang, G.Z. Zhang, H.B. Zhang, X.Y. Ma National Engineering Research Center for Opto-electronic Devices, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, China Received 30 June 2003; accepted 13 August 2003 Communicated by M. Schieber
Abstract Effects of V/III ratio on heavily Si doped InGaAs and InP were studied using low pressure metalorganic chemical vapor deposition (LP-MOCVD) at a growth temperature of 550 C. In InGaAs, as the V/III ratio decreases from 256 to 64, the carrier concentration increases from 3.0 1018 to 5.8 1018 cm3; and the lattice mismatch of InGaAs to InP was observed to vary from 5.70 104 to 1.49 103. In InP, when the V/III ratio decreases from 230 to 92, the same trend as that in Si doped InGaAs was observed that the carrier concentration increases from 9.2 1018 to 1.3 1019 cm3. The change of AsH3 was found to have stronger effect on Si incorporation in InGaAs at lower growth temperature than at higher growth temperature. r 2003 Elsevier B.V. All rights reserved. PACS: 73.61.Ey; 81.05.Ea; 81.15.Gh Keywords: A1. Doping; A1. X-ray diffraction; A3. Low press. metalorganic vapor phase epitaxy; B2. Semiconducting III–V materials; B2. Semiconducting indium phosphide
1. Introduction The small bandgap energy and superior carrier transport properties of InGaAs have made it widely used in high-speed devices such as heterostructure bipolar transistors (HBTs) [1]. Usually, increasing the doping level can result in lowered base resistivity and increased operation frequency. However, the commonly used p-type dopant, Zn, *Corresponding author. Tel.: +86-10-8230-4028, ext. 239; fax: +86-10-8230-5060. E-mail address: [email protected] (L. Jiang).
has relatively high diffusion coefficient, which makes it difficult to obtain abrupt doping profile at the emitter/base junction especially when the doping level of base is high [1] and the width of base is small. An effective way to suppress the outdiffusion of Zn is to lower the growth temperature of the whole HBT structure. But new problem arises when the growth temperature is decreased. As the pyrolysis temperature of silane is relatively high, the insufficient pyrolysis of silane limits the realization of heavily doped InGaAs subcollector and InGaAs/InP emitter cap. Generally, two methods are adopted
0022-0248/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2003.08.013
ARTICLE IN PRESS L. Jiang et al. / Journal of Crystal Growth 260 (2004) 23–27
24
to solve the problem: increasing the flow rate of silane or using a new Si precursor such as disilane [2,3]. In another aspect, varying the V/III ratio may be an effective way. The effects of V/III ratio on unintentionally [4] and lightly [5] doped InGaAs have been investigated at growth temperatures of 640 C and 600 C, respectively. In the unintentionally doped InGaAs, residual carrier concentration increases almost linearly with the increase of V/III ratio [4]. However, in the lightly doped InGaAs at a doping level of 1016–1017, an adverse trend is observed that, when the V/III ratio increases from 2.5 to 81, the carrier concentration decreases from 8.2 1016 to 2.7 1016 cm3 [5]. In this investigation, the growth temperature was lowered to 550 C. The influence of V/III ratio on the carrier concentration of Si doped InGaAs and InP, and on the lattice mismatch of InGaAs to InP were studied. It should be noted that, all V/III ratios mentioned hereinafter refer to input V/III ratios that are calculated using the following equation: V=III ¼
Fhydride ðp=ðp0 pÞÞFhydrogen
ð1Þ
where Fhydride is the flow rate of hydride and Fhydrogen is the flow rate of hydrogen that passing through the MO (metalorganic precursor) bubbler, p0 is the pressure in the MO bubbler and p is the vapor pressure of that metalorganic precursor. p is calculated by this equation [6]: logðp½TorrÞ ¼ A B=T
n-type dopant source. The carrier gas was palladium-diffused hydrogen. The flow rates of metalorganic sources, the flow rate of silane and total flow rate were kept constant, and the V/III ratio was changed by varying the flow rates of AsH3 and PH3. In order to minimize the error brought by measurements, three layers were grown on a single substrate. Thus, we could make the measurement for one sample at a time, and obtain a set of relevant data. First, we grew a three-layer structure of InGaAs (Sample I). The growth conditions for each layer were almost identical except for the flow rate of AsH3. The so grown epi-wafer was subjected to Double-crystal X-ray diffraction measurement for lattice mismatch, to Electro-chemical capacitancevoltage (ECV) measurement for carrier concentration, and to secondary ion mass spectroscopy (SIMS) for Si profile. After that, an epi-wafer of InP (Sample II) that contains three layers of InP grown with different PH3 flow rates was obtained, and was similarly subjected to ECV measurement for carrier concentration and to SIMS for Si profile.
ð2Þ
in which, T is the temperature in the MO bubbler and A; B are two parameters. The specific values of A and B also come from the same reference [6].
2. Experimental procedure Epitaxial growth was carried out in an AIX-200 R&D system which contains a horizontal reactor. The growth temperature was 550 C and pressure was 20 mbar. The source materials were triethylgallium (TEGa), trimethylindium (TMIn), 100% arsine (AsH3) and 100% phosphine (PH3). Silane (SiH4) diluted to 2% in hydrogen was used as the
3. Results and discussion 3.1. Effect of V/III ratio on lattice mismatch of InGaAs to InP In Sample I, the flow rates of AsH3 for the three layers were 50, 100 and 200 sccm from the bottom up. Corresponding V/III ratios are about 64, 128 and 256, respectively. Fig. 1 shows the X-ray rocking curve of Sample I, in which S represents substrate peak, while L1, L2 and L3 represent three epilayers, respectively. By carefully etching the epi-wafer using the mixture of H2SO4:H2O2:H2O=4:1:1, we found that L1 is the peak of the uppermost epilayer, L3 the peak of the lowermost one, and L2 the middle one. That is, the epilayer grown with the highest V/ III ratio has the smallest lattice constant. Generally, TEGa pyrolyses via the following two reactions: GaðC2 H5 Þ3 -dGaðC2 H5 Þ2 þ C2 H5 d
ð3Þ
ARTICLE IN PRESS L. Jiang et al. / Journal of Crystal Growth 260 (2004) 23–27
Intensity (counts/s)
Intensity (counts/s)
10000 L1
1000
100
10
1 62.5
63.0 63.5 2theta (Degree)
5.50E+018
1800
5.00E+018
1600
4.50E+018
1400
4.00E+018
1200
3.50E+018
1000
3.00E+018
800
2.50E+018 40 60 80 100 120 140 160 180 200 220 240 260 280 V/III ratio
Fig. 2. Effect of V/III ratio on the carrier concentration (m) and Si concentration (’) in InGaAs, in which, Si concentration is demonstrated by the relative intensity. The solid line is the fitting result of Si concentration using y ¼ Axc :
64.0
Fig. 1. X-ray rocking curve of Sample I.
and b-hydride elimination reaction GaðC2 H5 Þ3 -GaHðC2 H5 Þ2 þ C2 H4 :
2000
Carrier concentration (cm-3)
6.00E+018
S L3 L2
25
ð4Þ
In these two pyrolysis mechanism, the latter, that is, the b-hydride elimination mechanism dominate at higher temperatures (>650 C). At lower temperatures (such as the growth temperature of 550 C as we used), TEGa mainly pyrolyses by losing ethyl radicals [7]. AsH3 was supposed to react with ethyl radicals [8]: C2 H5 d þ AsH3 -C2 H6 þ dAsH2
ð5Þ
C2 H5 d þ dAsH2 -ðC2 H5 ÞAsH2 :
ð6Þ
Thus, at a constant flow rate of TEGa, more AsH3 can make the TEGa pyrolysing more quickly, so as to produce more Ga species at a certain period of time. As is well known, lattice constant of InGaAs reduces when Ga mole fraction in solid increases. As a result, increasing AsH3 flow rate reduces the lattice constant of the result InGaAs. 3.2. Effects of V/III ratio on Si doping 3.2.1. InGaAs Fig. 2 shows the dependence of Si doping in InGaAs on the V/III ratio. The observed phenomenon cannot be explained by the simple site model where the incorporation of Si is supposed to take place via vacant sites of group-III elements, according to which, the carrier concentration
should have increased as the V/III ratio increased. The same trend as that in Fig. 2 was observed by other researchers in Si doped GaAs [9–11] as well as in lightly Si doped InGaAs grown at a temperature higher than we used [5]. From the SIMS result, we found out that the increase in the carrier concentration should be a result of the increase in Si incorporation, which confirm the conclusion of Yoo et al. [5]. The data points coming from SIMS result were also plotted in Fig. 2. It was suggested that the dependence of Si doping on the V/III ratio indicates that the incorporation of Si is controlled by surface kinetics. In order to describe the observed trend, an empirical equation was employed [5,10]: NSi pFadop FbIII FcV expðEa =kTÞ
ð7Þ
where NSi is the Si concentration incorporated in each layer, Fdop ; FIII and FV are the precursor flow rates of the dopant, group-III element and group-V element, respectively. Ea is the activation energy. In lightly and moderately Si doped materials, NSi is approximately equal to the carrier concentration n. But in heavily Si doped materials, due to the amphoteric behavior of Si [10], Si doping is more compensated, and NSi would be greater than n. From Fig. 2, c is found to be about 0.6770.09, greater than that obtained by Yoo et al. [5]. This will be discussed in Section 3.2.3.
ARTICLE IN PRESS L. Jiang et al. / Journal of Crystal Growth 260 (2004) 23–27
26
3.2.2. InP In sample II, the flow rates of PH3 for the three layers were 200, 120 and 80 sccm from the bottom up. Corresponding V/III ratios are 230, 138 and 92, respectively. Some researchers have concluded that Si doping in InP using silane [13] or disilane [3] are not dependent on group-V concentration in the gas phase, which conclusion is consistent to the suggestion that Si incorporation takes place via vacant sites of group-III elements. So c in Eq. (7) should be zero. But in a more recent research [14] using SiH4 as the dopant, Keiper found that the doping level decreased by 29% when the V/III ratio doubled. Fig. 3 shows the dependence of Si doping in InP on the V/III ratio in our experiment. It is clear that the carrier concentration and Si concentration both decrease as the V/III ratio (that is, PH3 flow rate) increases. From Fig. 3, c is found to be 0.6870.02. This matches the result of Keiper’s study. 3.2.3. Qualitative explanation and discussion Field and Ghandhi have proposed a simple model to describe the Si incorporation in GaAs from SiH4 [10]. Since the Si–H bond strength in SiH4 (about 3.3 eV) [10] is comparable to the As–H bond strength of AsH3 (about 3.1 eV) [10,12], they assume that the thermal decomposition of SiH4 is heterogeneous. They also assumed that SiH2 is the initial adsorbed species on the surface, which 4500
1.60E+019
Intensity (counts/s)
-3
Carrier concentration (cm )
1.50E+019
4000
1.40E+019 3500
1.30E+019
3000
1.20E+019 1.10E+019
2500
1.00E+019 2000
9.00E+018
1500 80
100
120
140
160 180 V/III ratio
200
220
comes from the following reaction: SiH4 þ V -SiH2ðadsÞ þ H2
ð8Þ
where V is a vacant adsorption site. Similarly, P–H bond strength of PH3 (about 3.4 eV) [12] is also approximately equal to the Si–H bond strength of SiH4. So, we can assume the decomposition mechanism of SiH4 in InP and InGaAs to be similar to that in GaAs, and Si to take the same way as that in Eq. (8) to incorporate into InP and InGaAs. Then, Field and Ghandhi proposed one equation describing the Si concentration in the solid [10]: AðTÞPSiH4 yv ðT; PÞ Eads ðT; PÞ NSi D exp ð9Þ g kT where yn ðT; PÞ is the fraction of vacant SiH2 adsorption sites and Eads ðT; PÞ is the energy barrier to the adsorption reaction, Eq. (8). They proposed that increasing AsH3 pressure in gas phase (that is, increasing the flow rate of AsH3) increases the coverage of As species, increasing Eads ðT; PÞ; thus reducing adsorption of SiH2. Based on this suggestion, we can consider that PH3 has the similar effect on Eads ðT; PÞ: This assumption explained the effects of V/III ratio on Si doped InGaAs and InP observed in our experiment and researches of other people. Now we come to discuss the difference between the values of c obtained by us and Yoo et al. [5]. According to Field and Ghandhi, Eads ðT; PÞ increases as the flow rate of AsH3 increases. At lower growth temperature (as in our research), less change in Eads can cause the same change in NSi as that at higher growth temperature (as in the research of Yoo et al.). That is to say, less change in AsH3 flow rate at lower temperature can cause the same change in NSi as that at higher temperature. This may be the reason why the value of c we obtained is greater than that obtained by Yoo et al.
8.00E+018 240
Fig. 3. Effect of V/III ratio on the carrier concentration (m) and Si concentration (’) in InP, in which, Si concentration is demonstrated by the relative intensity. The solid line is the fitting result of Si concentration using y ¼ Axc :
4. Conclusion We investigated the effects of V/III ratio on Si doped InGaAs and InP at a low growth tempera-
ARTICLE IN PRESS L. Jiang et al. / Journal of Crystal Growth 260 (2004) 23–27
ture of 550 C. As the V/III ratio decreases, the lattice constant of InGaAs increases, as well as the carrier concentrations in InGaAs and InP. At lower temperature, the variation of arsine flow rate has stronger effect on Si doping in InGaAs.
Acknowledgements This research was supported by the National High Technology Research and Development Program of China (863 Program). The authors thank Mr. Ma Nongnong for his help in SIMS measurements.
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