Step-free surface and interface by finite area metalorganic vapor phase epitaxy

Journal of Crystal Growth 195 (1998) 459—465

Step-free surface and interface by finite area metalorganic vapor phase epitaxy T. Nishida*, T. Akasaka, Y. Yamauchi, N. Kobayashi NTT Basic Research Laboratories, 3-1 Morinosato, Wakamiya, Atsugi-shi, Kanagawa 243-0198, Japan

Abstract By using finite area metalorganic vapor phase epitaxy (FAE), atomically step-free surfaces and interfaces are obtained. Their flatness is confirmed by ex situ atomic force microscopy (AFM) and low-temperature spatially resolved photoluminescence (SR-PL) measurement. We suggest a step elimination principle by FAE, where two-dimensional (2D) nucleation is suppressed on a facet due to surface migration or desorption. We conclude that, by utilizing desorption, a step-free surface of any size can be formed as long as the size is finite. To demonstrate this, we formed an extraordinarily wide step-free GaAs(1 1 1)B surface 100 lm in diameter. Surface stoichiometry control based on in situ monitoring of surface photo-absorption (SPA) is indispensable for flatness control. We investigate the feasibility of FAE with respect to growth methods, materials, and thickness by forming GaAs/Al Ga As quantum well on patterned GaAs(1 1 1)B     substrate. We also show the potential of FAE of InP and GaN.  1998 Elsevier Science B.V. All rights reserved. Keywords: MOVPE; Facet; GaAs; Step; Nucleation; (1 1 1)B

1. Introduction The flatness of heterostructures in compound semiconductor crystals has been controlled on a nanometer scale, and these heterostructures have been applied to practical devices, such as quantum well (QW) lasers [1] and high electron mobility transistors (HEMTs) [2]. Thin layered structures at this scale are mainly formed by molecular beam epitaxy (MBE) [3] and metalorganic vapor phase

* Corresponding author. Fax: #81 462 40 4729; e-mail: [email protected].

epitaxy (MOVPE) [4]. In epitaxial growth processes, monolayer steps act as the incorporating sites. Therefore, steps exist at unintentional positions and it is principally impossible to completely suppress thickness fluctuations in planar growth. However, the suppression of thickness fluctuations in quantum structures has become more important in studies on advanced devices utilizing resonant tunnelling [3] and subband transition [5]. Further, a regular surface free from singularities, such as steps and kinks, would be a good stage for atomic structure and nonostructure fabrication. We reported on the formation of a step-free surface [6,7] and a step-free InAs monolayer QW

0022-0248/98/$ — see front matter  1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 6 2 5 - 3

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[8,9] on a GaAs(1 1 1)B substrate by selective area MOVPE. In this paper, we investigate the further possibilities of this flattening procedure with respect to size, materials, and structures.

2. Step elimination mechanism Fig. 1 shows the flattening process during selective area epitaxy (SAE) as an example of finite area epitaxy (FAE). Two-dimensional (2D) nucleation is suppressed and incorporation at steps becomes dominant when the concentrations of source materials are sufficiently small on stabilized surfaces. This results in the elimination of steps on the top facet, and a flat facet is formed on the mesa. When the facet area is sufficiently small compared with 2D nucleus density, the stacking of 2D islands does not occur and growth proceeds under the ‘‘real’’ layer-by-layer growth mode. That is, a 2D nucleus forms on the step-free top facet as shown in the middle of Fig. 1. Until the top facet becomes covered with the next monolayer island, sub-

Fig. 1. Schematic illustration of surface flattening by finite area epitaxial (FAE) growth. The left side shows the flattening due to surface migration to neighbor incorporating sites and the right side due to desorption.

sequent nucleation is suppressed, as shown in the bottom of Fig. 1. Therefore, the formation of an atomically flat surface and interface would be possible on the top facet. Because the key factor is the finiteness of growth area, this mechanism is commonly effective in FAE, such as SAE and growth on patterned substrate. In our previous reports [8,9], we showed flattening phenomena by selective area growth, and the flattening benefited from the self-termination of InAs monolayer growth on step-free GaAs surface. In that case, migration is the dominant flattening factor, as shown at the top of Fig. 1 on the left. The surface atom concentration is lowest at the edge of the growth area and is highest at the center of the growth area. When the surface atom concentration exceeds the critical concentration n at the center   of the growth area, 2D nucleation occurs. Therefore, 2D nucleation easily occurs as the growth area becomes larger. On the other hand, expanding the step-free area is easy under the desorption balance condition, as shown at the top of Fig. 1 on the right. That is, if the source material supply is balanced with its desorption from the surface, the surface atom concentration will be less than the critical concentration n everywhere within the growth area.   Therefore, as long as the growth area is finite, a step-free surface can be obtained by keeping the source supply sufficiently small. In order to practically form a very wide step-free surface, we must switch growth mode between growth initiation and 2D nucleation to layer completion by suppression of 2D nucleation. The control of n as well as source supply are effective to   switch growth mode. Of course, the critical concentration for 2D nucleation n greatly depends on   the surface chemical activity, so surface control during growth is indispensable. The surface photo-absorption (SPA) monitoring is one of the most effective in situ monitoring of surface chemical state during MOVPE growth, because it is applicable to surfaces other than those with two-fold symmetry. Fig. 2 is the reflectivity transient due to the GaAs(1 1 1)B surface phase transition [10]. The fast transient corresponds to the change from As-rich (2;2)-like to As-poor ((19;(19)-like and the slow one corresponds to

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Fig. 2. Reflectivity change due to As desorption on GaAs(1 1 1)B surface measured by the surface photo-absorption method [10]. There are two types of arsenic desorption. The fast change corresponds to the transition from (2;2)-like surface to ((19;(19)-like surface.

Fig. 3. (a) Flattening procedure and (b) AFM micrograph of the top surface of the selectively grown hexagonal GaAs mesa. The step-free area is as wide as 100 lm.

the change from ((19;(19)-like to high-temperature (1;1). The As-rich (2;2)-like surface is covered by arsenic trimers, as shown in the inset and very stable [10—12]. The growth rate on this

surface is very low, which results in the triangle hillrock morphology [10]. The ((19;(19)-like surface is active, and mirror growth on this surface is easy [10], which implies nucleation easily occurs.

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Fig. 4. Morphologies of top grown on patterned GaAs(1 1 1)B substrates. AFM micrographs of (a) a 3.5 lm-wide step-free GaAs surface, (b) a 2.5 lm-wide step-free Al Ga As surface are shown, (c) a 15 lm-wide GaAs top facet, which has a 4—6 lm-wide step-free     area, and (d) a 15 lm-wide AlGaAs top facet, which has a 3—4.5 lm-wide step-free area. The shapes within the top facets are schematically drawn in the insets.

3. 100 lm wide step-free GaAs surface To demonstrate the flattening principle above, we tried to flatten extraordinarily wide facets as wide as 100 lm [13]. Ten-micron-wide SiO guard 

patterns surrounding the 100 lm-wide hexagonal growth area were used as a selective mask. Fig. 3 shows the surface morphology of selectively grown area observed by atomic force microscope (AFM). We used a relatively high substrate

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temperature of 850°C to enhance the desorption of surface Ga atoms. The AsH partial pressure for  buffer growth was low which corresponds to a ((19;(19)-like condition. After growing about a 300 nm GaAs buffer layer, GaAs was grown at a sufficiently high AsH partial pressure of 40 Pa,  which corresponds to the (2;2)-like condition. In order to suppress 2D nucleation perfectly, we set the nominal growth rate at the end of the growth procedure at only 70 nm/h for 5 min and linearly decreased it to zero within the following 2 min, as shown in Fig. 3a. After the growth, the sample was cooled under a very high AsH partial pressure of  60 Pa in order to quench the surface morphology at the growth temperature. As clearly shown in Fig. 3b, the surface of the selectively grown GaAs(1 1 1)B facets is step-free as wide as 100 lm, which is larger than that previously reported [6,7] by one order of magnitude in length and two orders of magnitude in area. Such a step-free surface was obtained for smaller hexagonal mesas.

4. Step-free GaAs and AlGaAs surface on patterned substrate Next, we grew GaAs and AlGaAs layers on patterned GaAs(1 1 1)B substrates. Prior to growth, circular mesas 2—50 lm in diameter and 300 nm in height were fabricated by conventional photolithography. After a buffer GaAs layer was grown at 820°C, a quantum well with a nominally 2 nm thick GaAs well was formed [13]. In Fig. 4, we show surface morphologies obtained by the growth on a patterned substrate. To estimate the flatness of the GaAs/Al Ga As QW structure,     we investigated the surface morphology of a 1 nm thick GaAs layer on an Al Ga As barrier layer     and the surface morphology of an Al Ga As     barrier. Fig. 4a and Fig. 4b show the maximum mesa area free of atomic steps observed by AFM [13]. The equilaterally triangular step-free surfaces of the GaAs layer and of the Al Ga As barrier had     sides of 3.5 and 2.5 lm, respectively. This result implies three promising possibilities with respect to materials, thickness and growth methods: a step-

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free surface can be obtained on material other than GaAs, on the layers thicker than a monolayer, by facet growth other than SAE. The formation of a finite-area top facet is essential in achieving a step-free surface, and the potential of forming a step-free surface should be universal among finite-area facet growth methods.

5. Photoluminescence from individual thick quantum well As a feasibility study on the application to quantized state control by FAE, we tried to form a step-free quantum well sufficiently wider than the resolution of conventional optical characterization by such means as photoluminescences microscopy. In Fig. 4, AFM micrographs of (c) the top facet of a 15 lm-wide and 1 nm-thick GaAs layer on an Al Ga As barrier and (d) the top     facet of an Al Ga As barrier layer are shown.     Both have sufficiently wider step-free area than spatially resolved photoluminescence (SR-PL) measurement. We measured the PL of a nominal 2 nm-thick GaAs/Al Ga As QW grown on a 15 lm-mesa     at 4 K. Fig. 5 shows the results of the PL measurement. SR-PL spectra A, B, C, D and E in Fig. 5

Fig. 5. Spatially resolved photoluminescence spectra measured on a 15 lm-wide triangular mesa. Detection positions are indicated in the inset.

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Fig. 6. AFM micrographs of selectively grown (a) InP on an InP(1 1 1)B substrate and (b) GaN on a sapphire substrate [14]. In both cases, the top facets of selectively grown mesas are much flatter than the surface of the surrounding planar area.

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were measured at the positions shown in the inset. The differences in PL peak wavelengths are attributed to the differences in thickness of the quantum well [13] because the PL from the Al Ga As barrier layer is almost constant. This     means that flattening by finite area growth has the potential to achieve quantum well devices which researchers had given up due to monolayer heterointerface fluctuations.

surface is possible however large its size is, as long as its size is finite. To demonstrate this, we formed an extraordinarily wide step-free GaAs(1 1 1)B surface 100 lm in diameter. We have also shown the possibility of forming a step-free surface on GaAs and AlGaAs layers by FAE. The possibility of forming wide a step-free interface for GaAs/AlGaAs QWs and of application to other material such as InP and GaN, was also examined.

6. Application to other materials

Acknowledgements

As mentioned above, we showed step-free surfaces and interfaces on the GaAs (1 1 1)B substrate. But those are formed only on the GaAs (1 1 1)B substrate. However, this flattening is due to the suppression of 2D nucleation on the stable crystal facet. Therefore, it is reasonable to say that a similar mechanism might be effective in the finite area growth of other III—V materials, such as InP and GaN systems. Fig. 6 shows AFM images of selectively grown InP and GaN. The InP was grown at 76 Torr and 700°C. The growth rate and V/III ratio were 1 lm/h and 80, respectively. The growth conditions for GaN layer are reported elsewhere [14]. As shown in Fig. 6, the surfaces of the isolatedly grown mesas are much flatter than the surface of the planar area around the mesas. Further, the mesas are much lower than the planar areas. These results imply the flattening mechanism by desorption balance is also effective in growing materials other than III—V arsenide.

We would like to thank Dr. Yasuyuki Kobayashi and Dr. Seigo Ando for fruitful discussions. We are also grateful to Dr. Nobuo Matsumoto and Dr. Naoshi Uesugi for their encouragement.

7. Conclusions In summary, we suggested the step-free surface and interface can be formed by FAE, and investigated the effectiveness of a flattening mechanism due to both migration and desorption in FAE. We conclude that, by utilizing desorption, a step-free

References [1] J.P. van der Ziel, R. Dingle, R.C. Miller, W. Wiegmann, W.A. Nordland Jr., Appl. Phys. Lett. 26 (8) (1975) 463. [2] T. Mimura, S. Hiyamizu, T. Fujii, K. Nanba, Jpn. J. Appl. Phys. 19 (5) (1980) L225. [3] L.L. Chang, L. Esaki, R. Tsu, Appl. Phys. Lett. 24 (12) (1974) 593. [4] N. Holonyak Jr., R.M. Kolbas, R.D. Dupuis, P.D. Dapkus, IEEE J. Quantum Electron. QE-16 (2) (1980) 170. [5] J. Faist, F. Capasso, D.L. Sivco, C. Sirtori, A.L. Hutchinson, A.Y. Cho, Science 264 (1994) 553. [6] T. Nishida, N. Kobayashi, Appl. Phys. Lett. 69 (17) (1996) 2549. [7] T. Nishida, N. Kobayashi, Jpn. J. Appl. Phys. 36 (3) (1997) 1690. [8] T. Nishida, N. Kobayashi, Appl. Phys. Lett. 70 (20) (1997) 2726. [9] T. Nishida, N. Kobayashi, J. Electron. Mater. 26 (10) (1997) 1214. [10] T. Nishida, K. Uwai, Y. Kobayashi, Naoki Kobayashi, Jpn. J. Appl. Phys. 34 (12) (1995) 6326. [11] T. Nishida, N. Kobayashi, Jpn. J. Appl. Phys. 35 (7) (1996) L930. [12] M. Sasaki, S. Yoshida, J. Crystal Growth 356 (1996) 233. [13] T. Nishida, N. Kobayashi, Appl. Phys. Lett. 72 (22) (1998) 2487. [14] T. Akasaka, Y. Kobayashi, S. Ando, N. Kobayashi, Appl. Phys. Lett. 71 (15) (1997) 2196.