Spatially resolved photoluminescence of laterally overgrown InP on InP-coated Si substrates

j. . . . . . . .

ELSEVIER

CRYSTAL GROWTH

Journal of Crystal Growth 174 (1997) 622-629

Spatially resolved photoluminescence of laterally overgrown InP on InP-coated Si substrates Shigeya Naritsuka*, Tatau Nishinaga Department of Electric Engineering, Graduate School q['Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, To~o 113, Japan

Abstract Optical properties of InP grown by ELO (epitaxial lateral overgrowth) on InP-coated Si substrates were characterized by spatially resolved photoluminescence. Intensity of the photoluminescence spectra of these layers was found to be as strong as that of InP homoepitaxially grown on InP substrates. The luminescence spectra showed a small FWHM of 23 meV and no shift in wavelength was seen across the ELO layers. This means almost no stress exists in InP ELO layers on Si substrates. It is concluded that stress free InP has been obtained successfully on Si substrates. Simulation to evaluate stress in ELO layers was conducted and a good agreement with the experiments was obtained. Keywords: Spatial resolved photoluminescence; Heteroepitaxy; Epitaxial lateral overgrowth; Indium phosphide; Silicon;

Liquid-phase epitaxy; Stress-free; Dislocation-free; Crystalline quality improvement

1. Introduction G r o w t h of I I I - V materials on a Si substrate is important for fabrication of opto-electric integrated circuits (OEIC), which is a key technology for future telecommunications and computer applications. While m a n y studies of the growth of I I I - V materials on Si have been made the dislocation density and residual stress in these layers are still high due to the large lattice mismatch and the large difference in the thermal expansion coefficient between I I I - V materials and Si substrates. The

*Corresponding author. Fax: + 81 3 5684 3947; e-mail: [email protected].

characteristics of devices fabricated with II1-V materials grown on Si substrates are therefore still inferior to those of devices fabricated directly on I I I - V substrates. Further crystallinity improvement is necessary to realize performance and reliability levels high enough for commercial applications. Epitaxial lateral overgrowth (ELO) by liquidphase epitaxy (LPE) is known as a very promising technique to obtain atomically flat layers with few dislocations [1-7]. This technique is applicable to reduce dislocation density of II1-V materials grown on Si I-8-10]. Recently, we have demonstrated the growth of I n P layers with very low dislocation densities on InP-coated Si substrates [-11]. To use these layers for optical devices,

0022-0248/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved PII S 0 0 2 2 - 0 2 4 8 ( 9 7 ) 0 0 0 4 6 - 8

S. Naritsuka, T. Nishinaga / Journal of C~stal Growth 174 (1997) 622 629

we should show that their optical properties are also excellent. Stress is an extremely important factor in heteroepitaxial growth. Since the thermal expansion coefficients are different among InP, Si and S i O 2 , stress may remain in the ELO layer. The stress in ELO layers should be evaluated and if any, it should be decreased until it does not generate dislocations during the device operation. It is well known that stress makes peak shift and broadening of photoluminescence spectrum. Therefore, we employed spatially resolved photoluminescence (SRPL) to evaluate the residual stress and its distribution in ELO layers. In this article, the characterization of optical properties and residual stress in InP ELO layers grown on InP-coated Si substrates with SRPL are reported. The SRPL results of these layers are also compared with those of InP ELO layers homoepitaxially grown on InP substrates. These results are then compared with a stress simulation by finite element method to find the mechanism for stress releasing in ELO layers.

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ELO of InP was carried out on InP-coated Si substrates with the misorientation of 2 ° toward (1 1 0~, which were prepared at N T T Opto-electronics Laboratories [12]. The InP-coated Si substrate consists of a 2 ~m GaAs buffer layer and a 13 ~tm InP layer. The etch pit density and X-ray full-width at half-maximum (FWHM) of the InPcoated Si are about 5 x 106 c m - 2 and 65 arcsec, respectively. Prior to LPE growth, a SiO2 layer whose thickness was about 120 nm was deposited by using an organic liquid mainly consisting of SiO2 (OCD, Tokyo Ohka Kogyo Co. Ltd.) as shown in Fig. l a. A window for the line seed was opened in the SiO2 layer by a conventional photolithographic technique (Fig. lb). In the present experiment we used long line seeds in parallel arrangements, which were aligned 22 ° off from (1 1 0) direction to maximize the width of an ELO layer [7]. The width of the line seed was varied in the range between 3 and 5 ~tm and the length was 700 ~tm. Then, LPE was carried out on this

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substrate in a conventional sliding boat. A schematic illustration of the ELO on InP-coated Si substrate is shown in Fig. lc. A detail of ELO by LPE has been reported elsewhere [11]. The optical properties of ELO layers were investigated by SRPL at 98K. A 4 8 8 n m line of an Ar + laser was used for the excitation. The spot size and the power density of the laser beam were about

S. Naritsuka, 72. Nishinaga / Journal o f Crystal Growth 174 (1997) 622 629

624

5 ~tm and 6.4 kW/cm 2, respectively. The diffusion length of hole in InP can be estimated 2 ,-- 3 ~tm at 100 K from the hole diffusion constant of the order of 1 cmZ/V s derived from Einstein's equation and the time constant of hole recombination of the order of 10 -~ s. As a result, the spatial resolution of the SRPL measurement is thought to be about 10 ~tm in the worst case.

3. Results and discussion 3.1. SRPL

SRPL spectra of an InP ELO layer homoepitaxially grown on an InP substrate are shown in Fig. 2 for five positions across the ELO stripe. The sample was grown from an In solution with the saturation temperature Ts, the initial supersaturation AT, the cooling rate R, the growth t i m e tg and the width of the line seeds W, respectively, of 550°C, 0°C, 0.05°C/min, 60 min and 3 ~tm. The thickness and width of the grown layer were 7 and 50 lain, respectively. It is seen that the shapes of these spectra are almost the same. The intensities of all spectra are fairly strong, which is as strong as those of InP layers directly grown on InP substra-

tes. The full-widths at half-maximum (FWHM) of these spectra are narrow and the peak wavelengths of these spectra are located around 882.5 nm. The changes of peak wavelength, F W H M and intensity across the ELO layer are shown in Fig. 3a-Fig. 3c, respectively. Peak wavelengths of the PL spectra of the ELO layer are approximately the same and take the values within 882.5 ___0.5 nm as shown in Fig. 3a. The peaks are attributed to a band edge emission because the value agrees with the wave length corresponding to the band gap of InP which is 1.407eV (881.2 nm) at 100 K [-13]. The difference in thermal expansion coefficient between InP and SiO2 might bring some stress in these layers during the cooling period after the growth. Nevertheless, the coincidence between these peak wavelengths and the band gap energy of InP suggests that almost no stress exists in the layer. Fig. 3b shows that the F W H M is as narrow as 19 meV and has uniform distribution across the stripe. The narrow F W H M confirms that the stress is very low in this layer. It was also found that the removal of SiO2 mask by chemical etching makes no change in SRPL spectra. This confirms that S i O 2 mask brings almost no stress in the ELO layer. It is noticed that the intensity of SRPL spectra decreases slightly at both edges as shown in

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Fig. 3c. This is probably because the SRPL response consists of the reflection from the interface between E L O and S i O 2 a s well as the direct emission. The component of the reflection might be decreased at both edges although the true reason for the decrease is not clear. SRPL spectra of an lnP ELO layer grown on an InP-coated Si are shown in Fig. 4 for 7 points on a line across the E L O layer. The growth was carried out from an In solution with T~. AT, R, tg and W of 5 5 0 C , 0 C , 0.08 C/min, 180 min and 5 pm, respectively. The thickness and width of the grown layer are 18 and 70 lam, respectively. The shapes of these spectra are almost the same as the case of the homoepitaxial E L O layer shown in 1-:ig. 2. The intensity of these spectra were as strong as those of the homoepitaxially grown E L O layers. Moreover, the F W H M of these spectra are as narrow as 23 meV, which are comparable to those of homoepitaxially grown E L O layers. The changes in peak wavelength, F W H M and intensity across the E L O stripe are shown in Fig. 5a Fig. 5c. The peak wavelength of the SRPL spectra of the E L O layer takes the value of 881 _+ 1 nm as shown in Fig. 5a. The small deviation of the peak wavelength from the central value indicates a high uniformity of the optical property of the layer. Fig. 5b shows that the F W H M of the SRPL spectra is as narrow as 23 meV and that the F W H M is also uniform over this layer. Furthermore, the intensity of PL spectra is strong and uniform as shown in Fig. 5c. These results indicate that the optical property of the InP ELO layers grown on InP-coated Si is excellent, which is as good as those of homoepitaxially grown ones. The optical properties of the ELO layer grown on an InP-coated Si is determined mainly by the growth condition of LPE and not by the residual stress. Moreovcr, it seems that a low dislocation density, which is as low as dislocation-free level, means the crystal quality is very high and the density of nonradiative recombination centers is also low resulting in the strong intensity of SRPL spectra. On the contrary, the PL wavelength and F W H M of an InP-coated Si substrate (i.e. uniformly grown I n P on Si) were measured at 885 nm and 38 meV, respectively. The wavelength is about 3 nm longer than that of the LPE InP layers. The shift of

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the wavelength to the longer wavelength side indicates the existence of tensile stress in the InP-coated Si, which occurs in ordinary uniform heteroepitaxial layers. In spite of this, the SRPL spectra of the InP ELO layer grown on an InP-coated Si show no peak shift, indicating that there is almost no residual stress in the layer. Almost all residual stress inherited from the InP-coated Si is thought to be released by the 3D deformation of the ELO layer or slip of the E L O layer above the SiO2 mask. We also measured SRPL spectra of an E L O layer with 7 lam wide line seed but they did not show any distinct difference from that of an ELO layer with 5 gm wide line seed. Hence, the stress does not depend strongly on the width of the line seed. A computer simulation in the next section supports the experimental result, namely, it shows very weak dependence on the width of the line seed.

3.2. Computer simulation A stress simulation was made using the finite element method. As ELO stripes have a uniform cross section in the stripe direction, we can employ a plane strain approximation. Therefore, only a two-dimensional finite element calculation is required to obtain a stress distribution in the ELO layer. In the simulation, stress is assumed to be

caused mainly by the difference in the thermal expansion coefficient between InP and Si. At growth temperature, all stress is assumed to be relaxed by the generation of misfit-dislocation at the interface between the InP-coated layer and the Si substrate. Therefore, incorporated stress is assumed to be produced during the cooling process from the growth temperature to room temperature, in other words from 550 to 0°C. Stress distributions in the ELO layers grown on an InP-coated Si substrate with the 2-50 lain wide line seeds were calculated. The thickness and the width of the ELO layers were assumed to be 20 and 50 lam, respectively. The elastic constants used in the simulation are given in Table 1. The simulated distribution of the stress in an ELO layer with a 2 lain wide line seed is shown in Fig. 6a. In the figure, the length of the arrow indicates the strength of a stress and the direction of the arrowhead indicates whether the stress is compressive or tensile. The simulation may overestimate the stress because no nucleation of dislocations was assumed during cooling stage, but the distribution of the stress is thought to be properly estimated. A large and constant tensile stress, which is as large as 7 x 10 v N/m 2, is produced in the InP-coated layer. The stress is thought to be caused by the difference in the thermal expansion coefficient

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between lnP and Si. On the other hand, the stress is drastically decreased in the ELO layer and it is even decreased as the distance from the interface between the ELO layer and the SiO2 mask is increased. This shows that there is a self-stress releasing mechanism in the ELO layer. The result also shows that the stress caused by the difference in the thermal expansion coefficient between lnP and SiO2 mask is very small when compared to the stress caused by the difference between InP and Si. These results coincide with the SRPL results which indicate a weak stress at the surface of the ELO layer. The simulated stress in E L O layers grown on InP-coated Si substrates through the line seeds of 6, 10, 20 and 50gin wide are shown in Fig. 6b Fig. 6e, respectively. The stress distributions of these E L O layers are almost the same as the above-mentioned stress distribution of the ELO layer with a 2 gm wide line seed except the areas near the line seed, where the stress distributions are influenced by the change of the line seed width. These results suggest that not only the line seed region but also the S i O 2 mask transmit the stress of the InP-coated Si substrate to the ELO layer. Therefore, the increase in the width of line seed has brought a small change in the stress distribution in the calculation. In fact, the extreme case of a 50 gm wide line seed, as shown in Fig. 6e, whose width is equal to the width of the E L O layer itself, shows almost the same distribution of the stress as that of the ELO layer with a 2 tam wide line seed/Fig. 6a). These result of the simulation coincides with the above-mentioned weak dependence of the line seed width on SRPL spectra. Now, we will discuss the mechanism of the stress release in the E L O layer. In all cases in Fig. 6, the stress decreases as the distance from the interface between the ELO layer and the SiO2 mask is increased and takes the minimum value at the top corners. This suggests that the stress is released because of the 3D structure. Hence, we simulated the influence of the ELO ratio, which is defined as the ratio of ELO width to the thickness, on tile stress in E L O layers. In the simulation, the thickness of the E L O layer was varied to change the E L O ratio while the width was kept constant (50 tam). The width of the line seed was assumed to

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627

S. Naritsuka, T. Nishinaga /Journal of Crystal Growth 174 (1997) 622 629

628 Table 1 Elastic constants of materials

Young's modulus E Poisson's ratio v Thermal expansion coefficient Ac~

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2

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5

10

15

20

25

ELO RATIO

SiO2 ~.

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(b) -~

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Fig. 7. ELO ratio dependence of the stimulated stress at the center of the ELO surfaces.

w= lOpm

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InP Buffer Layer

(e) Fig. 6. Simulated distributions of the stress in ELO layers grown on InP-coated Si substrates. The width of the line seed are (a) 2 lam, (b) 6 lam, (c) 10 gin, (d) 20 lain and (e) 50 pm, respectively.

be 6 pm. Other parameters were the same as used in the above simulation. Fig. 7 shows the stress at the center of the E L O surface as a function of the E L O ratio. At a large E L O ratio such as 20, the stress becomes as large as 9 x 107 N / m 2, which is almost the same value of a flat InP layer grown on a Si substrate. It is seen that the reduction in the ELO ratio from 20 to 10 does not change the stress largely but a drastic decrease in the stress is realized when the E L O ratio decreases below 10. Therefore, a 3D structure of the E L O layer is important to release the stress and for this purpose the E L O ratio should be less than 10. Slip of the E L O layer on the SiO2 mask can also decrease the stress. In the above simulation a nonslip boundary condition was employed at the interface between the E L O layer and the SiO2 mask. Hence, if the slip occurs, the residual stress will be reduced depending on how large the slip can happen. It can be concluded that E L O is a very promising technique to grow III V materials on Si because not only the dislocation density, but also the residual stress can be reduced. The performance

S. Naritsuka, T. Nishinaga / Journal of Crystal Growth 174 (1997) 622 629

of the devices fabricated on Si must be improved by the use of the ELO technique.

4. Conclusions Optical properties of lnP ELO layers on InPcoated Si substrates were characterized by SRPL. Intensities of SRPL spectra of these layers are as strong as those of homoepitaxially grown InP ELO layers on InP substrates. This indicates that optical properties of these layers are comparable to those of homoepitaxially grown ones. Narrow F W H M of about 23 meV and uniform wavelength of about 881 nm of the SRPL spectra from the ELO layer grown on InP-coated Si show that almost all residual stress is released in the E g o layer and that high-quality InP layers can be obtained by using the E g o technique. Furthermore, the optical properties of these ELO layers are excellent and quite uniform, and are comparable to the optical properties of normal LPE grown InP. The stress in ELO structure obtained by using finite element method simulations shows good agreement with the experimental results. It can be said that not only dislocations but also residual stress are greatly reduced in ELO InP layers grown on InP-coated Si substrate.

Acknowledgements The authors wish to express their gratitude to Drs. H. Mori and M. Tachikawa, N T T Opto-elec-

629

tronics Laboratories, for supplying lnP-coated Si substrates. They would also like to express thanks to Dr. M. Tanaka for useful discussions. This work was supported by a Grant-in-Aid for Scientific Research on Priority Areas "Studies of InP layers grown on Si substrates by epitaxial lateral overgrowth and fabrication of long-wavelength lasers" No. 07555107 from the Ministry of Education, Science, Sports and Culture.

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