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Thermal annealing effect on the structural properties of epitaxial growth of GaP on Si substrate
Author's Accepted Manuscript
Thermal annealing effect on the structural properties of epitaxial growth of GaP on Si substrate Emad H. Hussein, Shabnam Dadgostar, Fariba Hatami, W.T. Masselink
www.elsevier.com/locate/jcrysgro
PII: DOI: Reference:
S0022-0248(15)00187-6 http://dx.doi.org/10.1016/j.jcrysgro.2015.02.090 CRYS22735
To appear in:
Journal of Crystal Growth
Received date: 16 December 2014 Revised date: 24 February 2015 Accepted date: 25 February 2015 Cite this article as: Emad H. Hussein, Shabnam Dadgostar, Fariba Hatami, W.T. Masselink, Thermal annealing effect on the structural properties of epitaxial growth of GaP on Si substrate, Journal of Crystal Growth, http://dx.doi.org/10.1016/ j.jcrysgro.2015.02.090 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Thermal annealing effect on the structural properties of epitaxial growth of GaP on Si substrate Emad H. Husseina,∗, Shabnam Dadgostara , Fariba Hatamia , W. T. Masselinka a
Institut f¨ ur Physik, Mathematisch-Naturwissenschaftliche Fakult¨ at I, Humboldt Universt¨ at zu Berlin, Newtonstrasse D-15, 12489 Berlin, Germany
Abstract The effect of post-growth annealing of epitaxial gallium phosphide grown on silicon substrates using gas-source molecular-beam epitaxy is described. The epitaxial layers were grown at substrate temperatures ranging from 250 to 550 ◦
C. After optimizing the growth temperature, the prepared films were ther-
mally annealed. Two thermal annealing methods are compared: annealing at a constant temperature and step-graded annealing, in which the temperature is raised by a constant rate per unit time. The effect of the thermal annealing on the crystal structure was studied by characterizing the epilayer using reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD) measurements, and scanning-electron microscopy (SEM). After thermal annealing, the epilayer exhibited a reconstructed RHEED pattern, and its quality was much improved. Keywords: B1. GaP/Si, A1. Annealing, A1. RHEED, A1. XRD, A1. Crystal structure. ∗
On leave from the Department of Physics, College of Science, Al-Mustansiriyah University, Baghdad, Iraq. Email address: [email protected] (Emad H. Hussein)
Preprint submitted to J. crystal growth
February 28, 2015
1. Introduction The growth of III-V materials on silicon substrates is an interesting technology for the fabrication of optoelectronic devices, such as light-emitting diodes [1]. Because GaP has the smallest lattice mismatch with Si (∼0.37% at room temperature), it can be considered an appropriate buffer layer for the growth of III-V compounds on Si. There are, however, several crystalline defects associated with the growth process as a result of the lattice mismatch, the difference in thermal expansion coefficients, and the growth of zinc blende polar crystals on non-polar diamond substrate. Issues that have been reported include defects accompanying growth of GaP/Si, i.e. pits on the layer surface [2], dislocations and staking faults in the interface of the heterostructure or inside the epilayer [3, 4], and anti-phase domains (APDs) [5–7]. One approach to reduce these defects is thermally annealing the GaP layer in situ [8]. Moreover, to improve the crystal quality of the epilayer, a two-step growth process can be used. In this process a nucleated layer is grown on the substrate at a relatively low growth temperature, followed by thermal annealing and growth of a GaP epilayer at a higher growth temperature [9]. When the islands of the initial GaP layer are highly coalesced, the post-growth epilayer shows a smooth surface morphology [10]. In this paper, the effect of post-growth annealing of epitaxially GaP grown on Si substrates using gas-source molecular-beam epitaxy (GS-MBE) is reported. Since the quality of the post GaP epilayer depends on the quality of the initial layer grown on the substrate, the growth process only focuses on one-step growth, in which a GaP layer is directly grown on a desorbed Si 2
substrate at a certain growth temperature. The crystal structure was characterized using reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD) measurements, and scanning-electron microscopy (SEM). 2. Experiments Growth of GaP films on silicon substrate was carried out using a RIBER 32P GS-MBE system. P-type silicon (100) wafers with 4◦ off towards [011] direction were used as substrates. The source materials were solid gallium (Ga) evaporated at 935 ◦ C and phosphine gas (P H3 ) thermally cracked at 850 ◦ C. After chemically cleaning the substrate using a modified RCA method [11] it was loaded and preheated in the intro-chamber at 200 ◦ C for one hour. Then, the substrate was transferred into the growth chamber. Desorption of the native oxide from the Si substrate took place in the growth chamber at 930 ◦ C for 30 min, during which the RHEED pattern of the Si surface gradually changed to a (2x1) reconstructed surface [12]. The substrate temperature was then reduced to the growth temperature (TG ) between 250 and 550 ◦ C. The growth is initiated by introducing cracked P H3 gas with a flow rate of 4 sccm (calibrated for nitrogen) for 3 min to form a P2 prelayer on the substrate, after which the gallium was introduced. The prelayer enables the self-annihilation of APDs [13] and forms nucleus centers for the GaP growth [8]. As soon as the Ga was introduced, a spotty RHEED pattern of GaP islands was observed on the substrate. For all the samples the GaP layer growth rate was kept at 0.96 μm/h. The quality of the GaP film was characterized using XRD measurements in a Bede QC1a difractometer with a 004 GaAs beam-conditioning crystal. The surface morphology of the GaP 3
layer and the interface of the GaP/Si heterostructure were imaged using a JEOL JSM-6360 and Hitachi S4800 SEM. 3. Results and discussion 3.1. Growth temperature optimization The growth conditions of the GaP films on the Si substrate are listed in Table 1. Fig. 1 shows θ/2θ scans for the (004) reflection plane of GaP/Si films obtained at growth temperatures of 250 (S1), 400 (S2) and 550 ◦ C (S3). As shown in this figure, the full width at half maximum (FWHM) of the GaP peak decreases, whereas its position goes towards the Si peak as the growth temperature increases. The GaP peak of the sample S3 indicates a highly relaxed layer compared to the other samples, as we demonstrate later. Sample No.
TG ◦
C
m⊥
ε⊥
C/min
(%)
10−3
Annealing ◦
a⊥ ◦
N
A
cm−2 , 108
S1
250
-
0.379
7.337
5.4516
3.99
S2
400
-
0.234
-1.375
5.4437
3.52
S3
550
-
0.187
-1.852
5.4411
1.16
S4
400
400 / 10
0.242
-1.302
5.4441
3.12
S5
400
(400-480) / 90
0.373
0.018
5.4513
1.80
Table 1. Growth conditions of GaP/Si films. TG is the growth temperature, m⊥ is the perpendicular lattice mismatch, ε⊥ is the layer perpendicular strain, a⊥ is the layer perpendicular lattice parameter and N is the dislocation density of the heterostructure.
At the lowest growth temperature (TG = 250 ◦ C), the peak is extremely broadened (FWHM = 477 sec) with lower intensity, indicating poor crystal 4
quality. On the other hand, although the FWHM value of sample S3 is the lowest, the GaP peak is closer to the Si peak, which indicates that the layer is highly relaxed on the substrate compared to the other samples. Hence, 400 ◦ C was considered as the optimal growth temperature for subsequent samples.
Fig. 1. The 004 XRD of GaP/Si samples obtained at growth temperatures of 250 ◦ C (S1), 400 ◦ C (S2) and 550 ◦ C (S3).
3.2. Thermal annealing optimization Two epitaxial films were thermally annealed under P H3 flux by two methods: annealing at a constant temperature of 400 ◦ C for 10 min (S4), and step-graded annealing, in which the temperature was raised from 400 to 480 ◦
C at a rate of 1.6 ◦ C/min for 50 min and then kept constant for 40 min
(S5). Figure 2 illustrates the change of the annealing temperature of sample S5 using the second method. During the annealing, the structure of the GaP crystal was monitored by RHEED. In the first thermal annealing method, 5
the GaP epilayer showed a spotty RHEED pattern with a rough surface, as it is seen later. This was not the case in the step-graded annealing.
Fig. 2. Step-graded thermal annealing of sample S5 at temperatures of 400 to 480 ◦ C for 90 min. The annealing temperature was raised by 8 ◦ C per 5 min up to 480 ◦ C, and then it was held constant for 40 min.
Fig. 3 shows the RHEED patterns for sample S5 at different annealing temperatures. The pattern remains spotty at the annealing temperature of 400 ◦ C, which is in agreement with observations by Yu et al [11]. Then, the RHEED pattern was reconstructed into 2x4 at 420 ◦ C (Fig. 3b). This pattern remained unchanged until a temperature of 460 ◦ C (Fig. 3c), and it became sharper at 480 ◦ C (Fig. 3d), indicating improvement in the crystal quality. A two-dimensional RHEED pattern could not be seen with the annealing.
6
Fig. 3. The RHEED patterns for the GaP epilayer of sample S5 obtained at a growth temperature of 400 ◦ C and annealing at (a) 400 ◦ C, (b) 420 ◦ C, (c) 460 ◦ C and (d) 480 ◦ C.
Figure 4 shows a comparison of the 004 XRD curves of the non-annealed film (S2) and the annealed films (S4 and S5) obtained at the same growth temperature. The perpendicular lattice mismatch m⊥ of the GaP/Si structure is calculated from the following relation: m⊥ = −Δθ cot(θs ),
(1)
where Δθ and θs are the XRD peak splitting and the substrate Bragg angle, respectively. With aS the lattice parameter of the substrate, the perpendicular lattice parameter of the epilayer a⊥ can be calculated as [14] a ⊥ = as (
sin(θs ) ), sin(θs + Δθ)
(2)
Accordingly, the GaP perpendicular layer strain ⊥ is calculated from ε⊥ =
a⊥ − 1, a◦ 7
(3)
where ao is the lattice parameter of the bulk GaP at room temperature (TR ).
Fig. 4. The 004 XRD of the non-annealed (S2) and annealed GaP/Si films (S4 and S5) grown at 400 ◦ C.
As shown in Fig. 1, higher growth temperature is correlated with the lower perpendicular lattice constant. This leads to perpendicular contraction and horizontal expansion of the epilayer, which increases with the growth temperature. In other words, the layer becomes more tensile strained perpendicularly, and hence the layer of sample S3 exhibits the biggest tensile strain (ε⊥ = −1.852 × 10−3 ). The negative sign refers to the perpendicular compression of the GaP crystal compared to the bulk GaP, which is interpreted as follows. When the temperature is raised from TR to TG the parallel lattice constant of the substrate expands, depending on its thermal expansion coefficient only. Then, the layer is grown on the substrate with a parallel lattice constant equal to that of the substrate, as long as its thickness (h). is less than or - equal to a critical value (hc ). The critical thickness is estimated 8
as 73 nm [15]. After cooling the film to TR , the residual strain of the film comprises lattice and thermal mismatches (εM and εT ), which are expressed as [16] ε = εM + εT ,
(4)
where εM = εo hhc and εo is the lattice misfit between bulks GaP and Si at room temperature. The perpendicular strain of the layer at a growth temperature can be given as (see Appendix A) ε⊥TG =
ε⊥T + hhc KΔαΔT , 1 + hhc KΔαΔT
(5)
where ε⊥T = ε⊥ + ε⊥M is calculated from XRD measurements. The value of ε⊥TG at 400 ◦ C is estimated as −7.3 × 10−4 . This suggests that the layer exhibits tensile strain and contracts perpendicularly compared to bulk GaP at that temperature (Fig. 5). Then, on cooling the film to room temperature, this strain and the strain due to lattice mismatch are relieved by the generation of dislocations at the interface. Finally, the perpendicular residual strain is decreased. That is, the layer shows a higher horizontal expansion, which results in a negative-perpendicular strain. The change in GaP lattice constants as ΔT changes causes different positions of the XRD peaks of the GaP layers.
9
Fig. 5. Thermal expansion of the GaP on the Si substrate at growth temperature (TG ) when the epilayer is thicker than the critical thickness. The G dashed square represents bulk GaP. aTst and aT G are the strained parallel
lattice constant of the layer and the parallel lattice constant of the GaP bulk at growth temperature. aS and aTSG are the parallel lattice constants of the substrate at room and growth temperatures, respectively.
The broadening in the GaP peak confirms the existence of dislocations with density that can be calculated by N=
β2 , 9b2
(6)
where β is the FWHM of the (004) diffraction peak in radians, and b is Burgers vector, which is equal to
a 2
[110], and a is the GaP lattice parameter. 10
Table 1 shows that the dislocation density was reduced from 3.52 ×108 cm−2 (S2) to 1.8 ×108 cm−2 (S5) at the same growth temperature by step-graded thermal annealing. Furthermore, sample S3 has lower dislocation density than sample S5. With the increase of the temperature, it is expected that the GaP grain size will increase and the dislocations will move and nucleate with each other [17]. Hence, annihilation of the dislocations is enhanced at higher temperature, so their density decreases. In addition, comparison of the XRD peaks of samples S2, S4 and S5 (Fig. 4) indicates that the highest intensity and the lowest broadened peak correlate with sample S5.
Fig. 6. The FWHM of the GaP/Si films versus growth temperature with / without thermal annealing.
A slight difference is also seen between the peaks of the non-annealed layer (S2) and the annealed layer (S4). These results clearly confirm that the GaP layer of film S5 is relaxed on the Si substrate with a lattice mismatch 11
that is very close to that of the unstrained crystal (m⊥ = 0.373 %). Figure 6 illustrates the FWHM of the (004) diffraction peaks versus the growth temperature of the samples with and without the thermal annealing. The results indicate that the layer of sample S5 has better crystalline quality than those of samples S1, S2 and S4. This can be interpreted as the step-graded thermal annealing causing a gradual coalescence of the GaP islands on the substrate, which finally led to the formation of an approximately uniform layer. In contrast, with annealing at 400 ◦ C/ 10 min, insufficient coalescence of the GaP islands could occur due to insufficient temperature, and hence no change in the RHEED pattern was seen. The carrier concentrations of the GaP layers were estimated by capacitancevoltage measurements. The epilayers were found to be n-type autodoped. For instance, the carrier concentration for sample S4 was determined to be 8.5 ×1014 cm−3 . The basis of the autodoping is interpreted as follows. Since the substrate is p-type, and the films are thermally annealed and then cooled to room temperature under phosphine flux, the autodoping may be caused by diffusion of phosphorous atoms in the GaP epilayer [3].
12
Fig. 7. The surface and interface SEM images of the GaP/Si films; S4: a,b and S5: c,d. 3.3. SEM The thermally annealed layers were further compared by SEM measurements. Fig. 7 shows the surface morphology images of the GaP epilayer and the interface of the GaP/Si heterostructure for samples S4 and S5. The nominal thickness value of the films is about 0.48 μm, as measured from SEM images. The epilayer annealed with the graded method (S5) presents a smoother surface (Fig. 7c) than that annealed at constant temperature (Fig. 7a), which confirms the high coalescence of the islands that occurred during the graded annealing.
13
4. Conclusion We have investigated the growth conditions for GaP on Si using only a one-step growth method at different growth temperatures and a growth rate of 0.96 μm/h. All of the prepared films have a nominal thickness of 480 nm. At low growth temperature, the epilayer showed poor crystal quality with a much higher dislocation density, which was confirmed by the (004) XRD measurements. The growth temperature was then optimized at 400 ◦ C. At this temperature, two samples were thermally annealed to study the effect of the annealing on the crystal structure properties. A reconstructed RHEED pattern was seen, and a two-dimensional pattern could not be observed. It is concluded that the crystal quality of the GaP epilayer can be much improved by using step-graded thermal annealing. Appendix A. Thermal strain: The layer is grown on the substrate with a parallel lattice constant equal to that of the substrate when its thickness is approximately the critical thickness (hc ). After cooling the film to TR , the residual strain is expressed as [16] ε = εM + εT ,
(A.1)
where εM and εT are the lattice and thermal mismatch strains. Also, εM = εo hhc , where h and εo are respectively the thickness of the layer and misfit strain of bulk GaP at TR . The thermal strain of the layer in Eq. ( A. 1) is given as εT = (αGaP − αSi )(TG − TR ). 14
(A.2)
The parallel lattice constant of the layer a at TR is given in terms of the parallel strain ε as a = ao (1 + ε ).
(A.3)
As shown in Fig. 8, since this lattice constant coincides with that of the substrate, the perpendicular constant will only expand according to the thermal expansion coefficient of the layer [18]. Therefore, on cooling the film from TG to TR , the parallel lattice constant can be expressed as G (1 − αSi ΔT ), a = aTst
(A.4)
G is the strained parallel lattice constant of the layer at TG . The where aTst G bulk GaP at TG exhibits parallel thermal strain εT G . Thus, aTst is given as G = aT G (1 + εT G ), aTst
(A.5)
where aT G is the parallel lattice constant of the bulk at TG , which is given as aT G = ao (1 + αGaP ΔT ).
15
(A.6)
Fig. 8. Thermal expansion of the GaP on the Si substrate at growth temperature (TG ) when the epilayer thickness is approximately the critical thickness. G The dashed square represents bulk GaP. aTst and aT G are the strained parallel
lattice constant of the layer and the parallel lattice constant of the GaP bulk at growth temperature. aS and aTSG are the parallel lattice constants of the substrate at room and growth temperature, respectively.
From the above equations, we get εT G ≈
ε−ΔαΔT . 1 + ΔαΔT
(A.7)
The perpendicular strain at TG is obtained from Eq. (A.7) as εT⊥G ≈
ε⊥ + kΔαΔT , 1 + ΔαΔT 16
(A.8)
where K = 2ν(1 − ν), and ν is the Poisson ratio of the GaP. ε⊥ is the perpendicular strain measured using XRD. When the effect of the thickness and misfit strain on the measured perpendicular strain after cooling to TR are taken into account, the thermal strain is again written as [19] h c TG (αGaP − αSi ) dT εT = h TR
(A.9)
and Eq. (A.1) can be written as ε⊥ = ε⊥M + ε⊥T .
(A.10)
Substituting Eq. (A.9) and Eq. (A.10) in Eq. (A.8), yields: ε⊥TG
ε⊥T + hhc KΔαΔT = 1 + hhc KΔαΔT
(A.11)
Acknowledgment The authors would like to thank Ahmed. S. Al-Haddad - from Ilmenau University and Gang Niu- from IHP Frankfurt (Oder) for performing SEM measurements. The first author is also grateful to the MOHESR/DAAD Iraqi-German scholarship program for the financial support. References [1] L. C. Chuang, F. G. Sedgwick, R. Chen, W. S. Ko, M. Moewe, K. W. Ng, T.-T. D. Tran, C. C-Hasnain, Nano Lett. 11 (2011) 385. [2] K. Yamane, T. Kobayashi, Y. Furukawa, H. Okada, H. Yonezu, A. Wakahara, J. Cryst. Growth 311 (2009) 794.
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[3] V. K. Dixit, T. Ganguli, T. K. Sharma, S. D. Singh, R. Kumar, S. Porwal, P. Tiwari, A. Ingale, S. M. Oak, J. Cryst. Growth 310 (2008) 3428. [4] K. Yamane, T. Kawai, Y. Furukawa, H. Okada, A. Wakahara, J. Cryst. Growth 312 (2010) 2179. [5] H. D¨oscher, B. Borkenhagen, G. Lilienkamp, W. Daum, T. Hannappel, Surf. Sci. Lett. 605 (2011) L38. [6] B. Kunert, I. N´emeth, S. Reinhard, K. Volz, W. Stolz, Thin Solid Films 517 (2008) 140. [7] R. Kumar, T. Ganguli, V. Chouhan, V. K. Dixit, J. Nano- Electron. Phys. 3 (1) (2011) 17. [8] V. K. Dixit, T. Ganguli, T. K. Sharma, R. Kumar, S. Porwal, V. Shukla, A. Ingale, P. Tiwari, A. K. Nath, J. Cryst. Growth 293 (2006) 5. [9] W. G. Bi, X. B. Mei, C. W. Tu, J. Cryst. Growth 164 (1996) 256. [10] T. Soga, T. Jimbo, M. Umeno, Appl. Surf. Sci. 82/83 (1994) 64. [11] X. Yu, P. S. Kuo, K. Ma, O. Levi, M. M. Fejer, J. S. Harris, J. Vac. Sci. Technol. B22 (3) (2004) 1450. [12] D. K. Goswami, B. Satpati, P. V. Satyam, B. N. Dev, Curr. Sci. 84 (7) (2003) 903. [13] Y. Furukawa, H. Yonezu, K. Ojima, K. Samonji, Y. Fujimoto, K. Momose, K. Aiki, Jpn. J. Appl. Phys., Pt. 1. 41 (2A) (2002) 528.
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[14] S. Nam, B. O, K.-S. Lee, Y. D. Choi, C.-S. Kim, J. Appl. Phys. 84 (2) (1998) 1047. [15] J. Singh, Semiconductor Optoelectronics Physics and Technology, McGrawHill, New York, 1995. [16] R. N. Jacobs, J. Markunas, J. Pellegrino, L. A. Almeida, M. Groenert, M. J-Vasquez, N. Mahadik, C. Andrews, S. B. Qadri, J. Cryst. Growth 310 (2008) 2960. [17] A. Jandl, M. T. Bulsara, E. A. Fitzgerald, J. Appl. Phys. 115 (2014) 153503. [18] P. Zaumseil, T. Schroeder, J. Phys. D: Appl. Phys. 44 (2011) 055403. [19] H. C. Jeon, J. H. Leem, Y. S. Ryu, T. W. Kang, T. W. Kim, Appl. Surf. Sci. 156 (2000) 110.
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Figure and Table Captions
Table 1. Growth conditions of GaP/Si films. TG is the growth temperature, m⊥ is the perpendicular lattice mismatch, ε⊥ is the layer perpendicular strain, a⊥ is the layer perpendicular lattice parameter and N is the dislocation density of the heterostructure.
Fig. 1. The 004 XRD of GaP/Si samples obtained at growth temperatures of 250 ◦ C (S1), 400 ◦ C (S2) and 550 ◦ C (S3)
Fig. 2. Step-graded thermal annealing of sample S5 at temperatures of 400 to 480 ◦ C for 90 min. The annealing temperature was raised by 8 ◦ C per 5 min up to 480 ◦ C, and then it was held constant for 40 min.
Fig. 3. The RHEED patterns for the GaP epilayer of sample S5 obtained at a growth temperature of 400 ◦ C and annealing at (a) 400 ◦ C, (b) 420 ◦ C, (c) 460 ◦ Cand (d) 480 ◦ C.
Fig. 4. The 004 XRD of the non-annealed (S2) and annealed GaP/Si films (S4 and S5) grown at 400 ◦ C. The 004 XRD of the non-annealed and annealed GaP/Si films (S2 and S4, S5) grown at 400 ◦ C.
Fig. 5. Thermal expansion of the GaP on the Si substrate at growth temperature (TG ) when the epilayer is thicker than the critical thickness. The G dashed square represents bulk GaP. aTst and aT G are the strained parallel
20
lattice constant of the layer and the parallel lattice constant of the GaP bulk at growth temperature. aS and aTSG are the parallel lattice constants of the substrate at room and growth temperatures, respectively.
Fig. 6. The FWHM of the GaP/Si films versus growth temperature with / without thermal annealing.
Fig. 7. The surface and interface SEM images of the GaP/Si films; S4: a,b and S5: c,d.
Fig. 8. Thermal expansion of the GaP on the Si substrate at growth temperature (TG ) when the epilayer thickness is approximately the critical thickness. G The dashed square represents bulk GaP. aTst and aT G are the strained parallel
lattice constant of the layer and the parallel lattice constant of the GaP bulk at growth temperature. aS and aTSG are the parallel lattice constants of the substrate at room and growth temperature, respectively.
21
Thermal annealing effect on the structural properties of epitaxial growth of GaP on Si substrate Emad H. Hussein, Shabnam Dadgostar, Fariba Hatami, W.T. Masselink
www.elsevier.com/locate/jcrysgro
PII: DOI: Reference:
S0022-0248(15)00187-6 http://dx.doi.org/10.1016/j.jcrysgro.2015.02.090 CRYS22735
To appear in:
Journal of Crystal Growth
Received date: 16 December 2014 Revised date: 24 February 2015 Accepted date: 25 February 2015 Cite this article as: Emad H. Hussein, Shabnam Dadgostar, Fariba Hatami, W.T. Masselink, Thermal annealing effect on the structural properties of epitaxial growth of GaP on Si substrate, Journal of Crystal Growth, http://dx.doi.org/10.1016/ j.jcrysgro.2015.02.090 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Thermal annealing effect on the structural properties of epitaxial growth of GaP on Si substrate Emad H. Husseina,∗, Shabnam Dadgostara , Fariba Hatamia , W. T. Masselinka a
Institut f¨ ur Physik, Mathematisch-Naturwissenschaftliche Fakult¨ at I, Humboldt Universt¨ at zu Berlin, Newtonstrasse D-15, 12489 Berlin, Germany
Abstract The effect of post-growth annealing of epitaxial gallium phosphide grown on silicon substrates using gas-source molecular-beam epitaxy is described. The epitaxial layers were grown at substrate temperatures ranging from 250 to 550 ◦
C. After optimizing the growth temperature, the prepared films were ther-
mally annealed. Two thermal annealing methods are compared: annealing at a constant temperature and step-graded annealing, in which the temperature is raised by a constant rate per unit time. The effect of the thermal annealing on the crystal structure was studied by characterizing the epilayer using reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD) measurements, and scanning-electron microscopy (SEM). After thermal annealing, the epilayer exhibited a reconstructed RHEED pattern, and its quality was much improved. Keywords: B1. GaP/Si, A1. Annealing, A1. RHEED, A1. XRD, A1. Crystal structure. ∗
On leave from the Department of Physics, College of Science, Al-Mustansiriyah University, Baghdad, Iraq. Email address: [email protected] (Emad H. Hussein)
Preprint submitted to J. crystal growth
February 28, 2015
1. Introduction The growth of III-V materials on silicon substrates is an interesting technology for the fabrication of optoelectronic devices, such as light-emitting diodes [1]. Because GaP has the smallest lattice mismatch with Si (∼0.37% at room temperature), it can be considered an appropriate buffer layer for the growth of III-V compounds on Si. There are, however, several crystalline defects associated with the growth process as a result of the lattice mismatch, the difference in thermal expansion coefficients, and the growth of zinc blende polar crystals on non-polar diamond substrate. Issues that have been reported include defects accompanying growth of GaP/Si, i.e. pits on the layer surface [2], dislocations and staking faults in the interface of the heterostructure or inside the epilayer [3, 4], and anti-phase domains (APDs) [5–7]. One approach to reduce these defects is thermally annealing the GaP layer in situ [8]. Moreover, to improve the crystal quality of the epilayer, a two-step growth process can be used. In this process a nucleated layer is grown on the substrate at a relatively low growth temperature, followed by thermal annealing and growth of a GaP epilayer at a higher growth temperature [9]. When the islands of the initial GaP layer are highly coalesced, the post-growth epilayer shows a smooth surface morphology [10]. In this paper, the effect of post-growth annealing of epitaxially GaP grown on Si substrates using gas-source molecular-beam epitaxy (GS-MBE) is reported. Since the quality of the post GaP epilayer depends on the quality of the initial layer grown on the substrate, the growth process only focuses on one-step growth, in which a GaP layer is directly grown on a desorbed Si 2
substrate at a certain growth temperature. The crystal structure was characterized using reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD) measurements, and scanning-electron microscopy (SEM). 2. Experiments Growth of GaP films on silicon substrate was carried out using a RIBER 32P GS-MBE system. P-type silicon (100) wafers with 4◦ off towards [011] direction were used as substrates. The source materials were solid gallium (Ga) evaporated at 935 ◦ C and phosphine gas (P H3 ) thermally cracked at 850 ◦ C. After chemically cleaning the substrate using a modified RCA method [11] it was loaded and preheated in the intro-chamber at 200 ◦ C for one hour. Then, the substrate was transferred into the growth chamber. Desorption of the native oxide from the Si substrate took place in the growth chamber at 930 ◦ C for 30 min, during which the RHEED pattern of the Si surface gradually changed to a (2x1) reconstructed surface [12]. The substrate temperature was then reduced to the growth temperature (TG ) between 250 and 550 ◦ C. The growth is initiated by introducing cracked P H3 gas with a flow rate of 4 sccm (calibrated for nitrogen) for 3 min to form a P2 prelayer on the substrate, after which the gallium was introduced. The prelayer enables the self-annihilation of APDs [13] and forms nucleus centers for the GaP growth [8]. As soon as the Ga was introduced, a spotty RHEED pattern of GaP islands was observed on the substrate. For all the samples the GaP layer growth rate was kept at 0.96 μm/h. The quality of the GaP film was characterized using XRD measurements in a Bede QC1a difractometer with a 004 GaAs beam-conditioning crystal. The surface morphology of the GaP 3
layer and the interface of the GaP/Si heterostructure were imaged using a JEOL JSM-6360 and Hitachi S4800 SEM. 3. Results and discussion 3.1. Growth temperature optimization The growth conditions of the GaP films on the Si substrate are listed in Table 1. Fig. 1 shows θ/2θ scans for the (004) reflection plane of GaP/Si films obtained at growth temperatures of 250 (S1), 400 (S2) and 550 ◦ C (S3). As shown in this figure, the full width at half maximum (FWHM) of the GaP peak decreases, whereas its position goes towards the Si peak as the growth temperature increases. The GaP peak of the sample S3 indicates a highly relaxed layer compared to the other samples, as we demonstrate later. Sample No.
TG ◦
C
m⊥
ε⊥
C/min
(%)
10−3
Annealing ◦
a⊥ ◦
N
A
cm−2 , 108
S1
250
-
0.379
7.337
5.4516
3.99
S2
400
-
0.234
-1.375
5.4437
3.52
S3
550
-
0.187
-1.852
5.4411
1.16
S4
400
400 / 10
0.242
-1.302
5.4441
3.12
S5
400
(400-480) / 90
0.373
0.018
5.4513
1.80
Table 1. Growth conditions of GaP/Si films. TG is the growth temperature, m⊥ is the perpendicular lattice mismatch, ε⊥ is the layer perpendicular strain, a⊥ is the layer perpendicular lattice parameter and N is the dislocation density of the heterostructure.
At the lowest growth temperature (TG = 250 ◦ C), the peak is extremely broadened (FWHM = 477 sec) with lower intensity, indicating poor crystal 4
quality. On the other hand, although the FWHM value of sample S3 is the lowest, the GaP peak is closer to the Si peak, which indicates that the layer is highly relaxed on the substrate compared to the other samples. Hence, 400 ◦ C was considered as the optimal growth temperature for subsequent samples.
Fig. 1. The 004 XRD of GaP/Si samples obtained at growth temperatures of 250 ◦ C (S1), 400 ◦ C (S2) and 550 ◦ C (S3).
3.2. Thermal annealing optimization Two epitaxial films were thermally annealed under P H3 flux by two methods: annealing at a constant temperature of 400 ◦ C for 10 min (S4), and step-graded annealing, in which the temperature was raised from 400 to 480 ◦
C at a rate of 1.6 ◦ C/min for 50 min and then kept constant for 40 min
(S5). Figure 2 illustrates the change of the annealing temperature of sample S5 using the second method. During the annealing, the structure of the GaP crystal was monitored by RHEED. In the first thermal annealing method, 5
the GaP epilayer showed a spotty RHEED pattern with a rough surface, as it is seen later. This was not the case in the step-graded annealing.
Fig. 2. Step-graded thermal annealing of sample S5 at temperatures of 400 to 480 ◦ C for 90 min. The annealing temperature was raised by 8 ◦ C per 5 min up to 480 ◦ C, and then it was held constant for 40 min.
Fig. 3 shows the RHEED patterns for sample S5 at different annealing temperatures. The pattern remains spotty at the annealing temperature of 400 ◦ C, which is in agreement with observations by Yu et al [11]. Then, the RHEED pattern was reconstructed into 2x4 at 420 ◦ C (Fig. 3b). This pattern remained unchanged until a temperature of 460 ◦ C (Fig. 3c), and it became sharper at 480 ◦ C (Fig. 3d), indicating improvement in the crystal quality. A two-dimensional RHEED pattern could not be seen with the annealing.
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Fig. 3. The RHEED patterns for the GaP epilayer of sample S5 obtained at a growth temperature of 400 ◦ C and annealing at (a) 400 ◦ C, (b) 420 ◦ C, (c) 460 ◦ C and (d) 480 ◦ C.
Figure 4 shows a comparison of the 004 XRD curves of the non-annealed film (S2) and the annealed films (S4 and S5) obtained at the same growth temperature. The perpendicular lattice mismatch m⊥ of the GaP/Si structure is calculated from the following relation: m⊥ = −Δθ cot(θs ),
(1)
where Δθ and θs are the XRD peak splitting and the substrate Bragg angle, respectively. With aS the lattice parameter of the substrate, the perpendicular lattice parameter of the epilayer a⊥ can be calculated as [14] a ⊥ = as (
sin(θs ) ), sin(θs + Δθ)
(2)
Accordingly, the GaP perpendicular layer strain ⊥ is calculated from ε⊥ =
a⊥ − 1, a◦ 7
(3)
where ao is the lattice parameter of the bulk GaP at room temperature (TR ).
Fig. 4. The 004 XRD of the non-annealed (S2) and annealed GaP/Si films (S4 and S5) grown at 400 ◦ C.
As shown in Fig. 1, higher growth temperature is correlated with the lower perpendicular lattice constant. This leads to perpendicular contraction and horizontal expansion of the epilayer, which increases with the growth temperature. In other words, the layer becomes more tensile strained perpendicularly, and hence the layer of sample S3 exhibits the biggest tensile strain (ε⊥ = −1.852 × 10−3 ). The negative sign refers to the perpendicular compression of the GaP crystal compared to the bulk GaP, which is interpreted as follows. When the temperature is raised from TR to TG the parallel lattice constant of the substrate expands, depending on its thermal expansion coefficient only. Then, the layer is grown on the substrate with a parallel lattice constant equal to that of the substrate, as long as its thickness (h). is less than or - equal to a critical value (hc ). The critical thickness is estimated 8
as 73 nm [15]. After cooling the film to TR , the residual strain of the film comprises lattice and thermal mismatches (εM and εT ), which are expressed as [16] ε = εM + εT ,
(4)
where εM = εo hhc and εo is the lattice misfit between bulks GaP and Si at room temperature. The perpendicular strain of the layer at a growth temperature can be given as (see Appendix A) ε⊥TG =
ε⊥T + hhc KΔαΔT , 1 + hhc KΔαΔT
(5)
where ε⊥T = ε⊥ + ε⊥M is calculated from XRD measurements. The value of ε⊥TG at 400 ◦ C is estimated as −7.3 × 10−4 . This suggests that the layer exhibits tensile strain and contracts perpendicularly compared to bulk GaP at that temperature (Fig. 5). Then, on cooling the film to room temperature, this strain and the strain due to lattice mismatch are relieved by the generation of dislocations at the interface. Finally, the perpendicular residual strain is decreased. That is, the layer shows a higher horizontal expansion, which results in a negative-perpendicular strain. The change in GaP lattice constants as ΔT changes causes different positions of the XRD peaks of the GaP layers.
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Fig. 5. Thermal expansion of the GaP on the Si substrate at growth temperature (TG ) when the epilayer is thicker than the critical thickness. The G dashed square represents bulk GaP. aTst and aT G are the strained parallel
lattice constant of the layer and the parallel lattice constant of the GaP bulk at growth temperature. aS and aTSG are the parallel lattice constants of the substrate at room and growth temperatures, respectively.
The broadening in the GaP peak confirms the existence of dislocations with density that can be calculated by N=
β2 , 9b2
(6)
where β is the FWHM of the (004) diffraction peak in radians, and b is Burgers vector, which is equal to
a 2
[110], and a is the GaP lattice parameter. 10
Table 1 shows that the dislocation density was reduced from 3.52 ×108 cm−2 (S2) to 1.8 ×108 cm−2 (S5) at the same growth temperature by step-graded thermal annealing. Furthermore, sample S3 has lower dislocation density than sample S5. With the increase of the temperature, it is expected that the GaP grain size will increase and the dislocations will move and nucleate with each other [17]. Hence, annihilation of the dislocations is enhanced at higher temperature, so their density decreases. In addition, comparison of the XRD peaks of samples S2, S4 and S5 (Fig. 4) indicates that the highest intensity and the lowest broadened peak correlate with sample S5.
Fig. 6. The FWHM of the GaP/Si films versus growth temperature with / without thermal annealing.
A slight difference is also seen between the peaks of the non-annealed layer (S2) and the annealed layer (S4). These results clearly confirm that the GaP layer of film S5 is relaxed on the Si substrate with a lattice mismatch 11
that is very close to that of the unstrained crystal (m⊥ = 0.373 %). Figure 6 illustrates the FWHM of the (004) diffraction peaks versus the growth temperature of the samples with and without the thermal annealing. The results indicate that the layer of sample S5 has better crystalline quality than those of samples S1, S2 and S4. This can be interpreted as the step-graded thermal annealing causing a gradual coalescence of the GaP islands on the substrate, which finally led to the formation of an approximately uniform layer. In contrast, with annealing at 400 ◦ C/ 10 min, insufficient coalescence of the GaP islands could occur due to insufficient temperature, and hence no change in the RHEED pattern was seen. The carrier concentrations of the GaP layers were estimated by capacitancevoltage measurements. The epilayers were found to be n-type autodoped. For instance, the carrier concentration for sample S4 was determined to be 8.5 ×1014 cm−3 . The basis of the autodoping is interpreted as follows. Since the substrate is p-type, and the films are thermally annealed and then cooled to room temperature under phosphine flux, the autodoping may be caused by diffusion of phosphorous atoms in the GaP epilayer [3].
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Fig. 7. The surface and interface SEM images of the GaP/Si films; S4: a,b and S5: c,d. 3.3. SEM The thermally annealed layers were further compared by SEM measurements. Fig. 7 shows the surface morphology images of the GaP epilayer and the interface of the GaP/Si heterostructure for samples S4 and S5. The nominal thickness value of the films is about 0.48 μm, as measured from SEM images. The epilayer annealed with the graded method (S5) presents a smoother surface (Fig. 7c) than that annealed at constant temperature (Fig. 7a), which confirms the high coalescence of the islands that occurred during the graded annealing.
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4. Conclusion We have investigated the growth conditions for GaP on Si using only a one-step growth method at different growth temperatures and a growth rate of 0.96 μm/h. All of the prepared films have a nominal thickness of 480 nm. At low growth temperature, the epilayer showed poor crystal quality with a much higher dislocation density, which was confirmed by the (004) XRD measurements. The growth temperature was then optimized at 400 ◦ C. At this temperature, two samples were thermally annealed to study the effect of the annealing on the crystal structure properties. A reconstructed RHEED pattern was seen, and a two-dimensional pattern could not be observed. It is concluded that the crystal quality of the GaP epilayer can be much improved by using step-graded thermal annealing. Appendix A. Thermal strain: The layer is grown on the substrate with a parallel lattice constant equal to that of the substrate when its thickness is approximately the critical thickness (hc ). After cooling the film to TR , the residual strain is expressed as [16] ε = εM + εT ,
(A.1)
where εM and εT are the lattice and thermal mismatch strains. Also, εM = εo hhc , where h and εo are respectively the thickness of the layer and misfit strain of bulk GaP at TR . The thermal strain of the layer in Eq. ( A. 1) is given as εT = (αGaP − αSi )(TG − TR ). 14
(A.2)
The parallel lattice constant of the layer a at TR is given in terms of the parallel strain ε as a = ao (1 + ε ).
(A.3)
As shown in Fig. 8, since this lattice constant coincides with that of the substrate, the perpendicular constant will only expand according to the thermal expansion coefficient of the layer [18]. Therefore, on cooling the film from TG to TR , the parallel lattice constant can be expressed as G (1 − αSi ΔT ), a = aTst
(A.4)
G is the strained parallel lattice constant of the layer at TG . The where aTst G bulk GaP at TG exhibits parallel thermal strain εT G . Thus, aTst is given as G = aT G (1 + εT G ), aTst
(A.5)
where aT G is the parallel lattice constant of the bulk at TG , which is given as aT G = ao (1 + αGaP ΔT ).
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(A.6)
Fig. 8. Thermal expansion of the GaP on the Si substrate at growth temperature (TG ) when the epilayer thickness is approximately the critical thickness. G The dashed square represents bulk GaP. aTst and aT G are the strained parallel
lattice constant of the layer and the parallel lattice constant of the GaP bulk at growth temperature. aS and aTSG are the parallel lattice constants of the substrate at room and growth temperature, respectively.
From the above equations, we get εT G ≈
ε−ΔαΔT . 1 + ΔαΔT
(A.7)
The perpendicular strain at TG is obtained from Eq. (A.7) as εT⊥G ≈
ε⊥ + kΔαΔT , 1 + ΔαΔT 16
(A.8)
where K = 2ν(1 − ν), and ν is the Poisson ratio of the GaP. ε⊥ is the perpendicular strain measured using XRD. When the effect of the thickness and misfit strain on the measured perpendicular strain after cooling to TR are taken into account, the thermal strain is again written as [19] h c TG (αGaP − αSi ) dT εT = h TR
(A.9)
and Eq. (A.1) can be written as ε⊥ = ε⊥M + ε⊥T .
(A.10)
Substituting Eq. (A.9) and Eq. (A.10) in Eq. (A.8), yields: ε⊥TG
ε⊥T + hhc KΔαΔT = 1 + hhc KΔαΔT
(A.11)
Acknowledgment The authors would like to thank Ahmed. S. Al-Haddad - from Ilmenau University and Gang Niu- from IHP Frankfurt (Oder) for performing SEM measurements. The first author is also grateful to the MOHESR/DAAD Iraqi-German scholarship program for the financial support. References [1] L. C. Chuang, F. G. Sedgwick, R. Chen, W. S. Ko, M. Moewe, K. W. Ng, T.-T. D. Tran, C. C-Hasnain, Nano Lett. 11 (2011) 385. [2] K. Yamane, T. Kobayashi, Y. Furukawa, H. Okada, H. Yonezu, A. Wakahara, J. Cryst. Growth 311 (2009) 794.
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[3] V. K. Dixit, T. Ganguli, T. K. Sharma, S. D. Singh, R. Kumar, S. Porwal, P. Tiwari, A. Ingale, S. M. Oak, J. Cryst. Growth 310 (2008) 3428. [4] K. Yamane, T. Kawai, Y. Furukawa, H. Okada, A. Wakahara, J. Cryst. Growth 312 (2010) 2179. [5] H. D¨oscher, B. Borkenhagen, G. Lilienkamp, W. Daum, T. Hannappel, Surf. Sci. Lett. 605 (2011) L38. [6] B. Kunert, I. N´emeth, S. Reinhard, K. Volz, W. Stolz, Thin Solid Films 517 (2008) 140. [7] R. Kumar, T. Ganguli, V. Chouhan, V. K. Dixit, J. Nano- Electron. Phys. 3 (1) (2011) 17. [8] V. K. Dixit, T. Ganguli, T. K. Sharma, R. Kumar, S. Porwal, V. Shukla, A. Ingale, P. Tiwari, A. K. Nath, J. Cryst. Growth 293 (2006) 5. [9] W. G. Bi, X. B. Mei, C. W. Tu, J. Cryst. Growth 164 (1996) 256. [10] T. Soga, T. Jimbo, M. Umeno, Appl. Surf. Sci. 82/83 (1994) 64. [11] X. Yu, P. S. Kuo, K. Ma, O. Levi, M. M. Fejer, J. S. Harris, J. Vac. Sci. Technol. B22 (3) (2004) 1450. [12] D. K. Goswami, B. Satpati, P. V. Satyam, B. N. Dev, Curr. Sci. 84 (7) (2003) 903. [13] Y. Furukawa, H. Yonezu, K. Ojima, K. Samonji, Y. Fujimoto, K. Momose, K. Aiki, Jpn. J. Appl. Phys., Pt. 1. 41 (2A) (2002) 528.
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[14] S. Nam, B. O, K.-S. Lee, Y. D. Choi, C.-S. Kim, J. Appl. Phys. 84 (2) (1998) 1047. [15] J. Singh, Semiconductor Optoelectronics Physics and Technology, McGrawHill, New York, 1995. [16] R. N. Jacobs, J. Markunas, J. Pellegrino, L. A. Almeida, M. Groenert, M. J-Vasquez, N. Mahadik, C. Andrews, S. B. Qadri, J. Cryst. Growth 310 (2008) 2960. [17] A. Jandl, M. T. Bulsara, E. A. Fitzgerald, J. Appl. Phys. 115 (2014) 153503. [18] P. Zaumseil, T. Schroeder, J. Phys. D: Appl. Phys. 44 (2011) 055403. [19] H. C. Jeon, J. H. Leem, Y. S. Ryu, T. W. Kang, T. W. Kim, Appl. Surf. Sci. 156 (2000) 110.
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Figure and Table Captions
Table 1. Growth conditions of GaP/Si films. TG is the growth temperature, m⊥ is the perpendicular lattice mismatch, ε⊥ is the layer perpendicular strain, a⊥ is the layer perpendicular lattice parameter and N is the dislocation density of the heterostructure.
Fig. 1. The 004 XRD of GaP/Si samples obtained at growth temperatures of 250 ◦ C (S1), 400 ◦ C (S2) and 550 ◦ C (S3)
Fig. 2. Step-graded thermal annealing of sample S5 at temperatures of 400 to 480 ◦ C for 90 min. The annealing temperature was raised by 8 ◦ C per 5 min up to 480 ◦ C, and then it was held constant for 40 min.
Fig. 3. The RHEED patterns for the GaP epilayer of sample S5 obtained at a growth temperature of 400 ◦ C and annealing at (a) 400 ◦ C, (b) 420 ◦ C, (c) 460 ◦ Cand (d) 480 ◦ C.
Fig. 4. The 004 XRD of the non-annealed (S2) and annealed GaP/Si films (S4 and S5) grown at 400 ◦ C. The 004 XRD of the non-annealed and annealed GaP/Si films (S2 and S4, S5) grown at 400 ◦ C.
Fig. 5. Thermal expansion of the GaP on the Si substrate at growth temperature (TG ) when the epilayer is thicker than the critical thickness. The G dashed square represents bulk GaP. aTst and aT G are the strained parallel
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lattice constant of the layer and the parallel lattice constant of the GaP bulk at growth temperature. aS and aTSG are the parallel lattice constants of the substrate at room and growth temperatures, respectively.
Fig. 6. The FWHM of the GaP/Si films versus growth temperature with / without thermal annealing.
Fig. 7. The surface and interface SEM images of the GaP/Si films; S4: a,b and S5: c,d.
Fig. 8. Thermal expansion of the GaP on the Si substrate at growth temperature (TG ) when the epilayer thickness is approximately the critical thickness. G The dashed square represents bulk GaP. aTst and aT G are the strained parallel
lattice constant of the layer and the parallel lattice constant of the GaP bulk at growth temperature. aS and aTSG are the parallel lattice constants of the substrate at room and growth temperature, respectively.
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