Jpn. J. Appl. Phys. Vol. 39 (2000) pp. 4404–4407
Part 1, No. 7B, July 2000c
The Japan Society of Applied Physics°2000

Hydrogen as the Cause of Step Bunching Formed on Vicinal GaAs(001)
Kenji H ATA1,∗ , Hidemi S HIGEKAWA1 , Takashi U EDA2 , Masahiro A KIYAMA2 and Tatsuo O KANO3
1 Institute of Applied Physics, CREST, Japan Science and Technology Corporation, University of Tsukuba, Tsukuba 305-8573, Japan
2 Oki Semiconductor Technology Laboratory, Co. Ltd, Tokyo 193, Japan
3 Institute of Industrial Science, The University of Tokyo, Tokyo 106, Japan(Received October 25, 1999; accepted for publication November 10, 1999)

In order to elucidate the cause of step bunching formed on vicinal GaAs(001) annealed in AsH3 /H2 ambient, we investigated
the surface of vicinal GaAs(001) annealed in H2 , AsH3 /N2 , AsH3 /H2 , and N2 by scanning tunneling microscope. Since stepbunches always formed on surfaces annealed with hydrogen but not on surfaces annealed in ambients without hydrogen, we
conclude that hydrogen is crucial in the formation of step bunching.
KEYWORDS: scanning tunneling microscopy; diffusion and migration; growth; surface diffusion; surface structure, gallium arsenide, stepped single-crystal surface

1. Introduction

2. Experimental

Step bunches on vicinal GaAs(001), the sizes of which areon the scale of µm, were observed on layers grown by liquid phase epitaxy.1, 2) Recently, Fukui and Saito reported theformation of another type of step bunches on layers grown bymetal-organic chemical vapor deposition (MOCVD).3) These
step bunches are small, with size on the scale of nm. Therefore, they have the potential to serve as templates for thefabrication of nanostructures.4) In order to utilize this stepbunching, the characteristics, dynamics, and formation mechanism of step bunching must be elucidated. Only with thisunderstanding, will it become possible to artificially control
the size, shape, and morphology of the step bunches at theirfull potential. However, no general consensus concerning thecharacteristics of step bunching has been reached. Even theformation mechanism remains an open question.5–7) This isbecause epitaxial growth is a very complicated process inwhich many factors influence the dynamics.
Step bunches similar to those observed on layers grown by
MOCVD form upon annealing vicinal GaAs(001) in AsH3 /H2ambient.8–10) The characteristics of these two types of stepbunches are very similar, and they are likely to form due to thesame cause. The dynamics of annealing is much simple thanthat of epitaxial growth, thus step bunching caused by annealing in AsH3 /H2 ambient will serve as a foundation to understand the nature of the step bunches which form in MOCVD
ambients. Since step bunching always occurred when vicinalGaAs(001) surfaces were annealed in AsH3 /H2 ambient fora very wide range of annealing conditions, while it usually
did not occur on vicinal GaAs(001) surfaces annealed in ultra high vacuum (UHV),11, 12) it was proposed that AsH3 /H2is the direct cause of this step bunching.10)
In this letter, we investigate the cause of step bunchingby observing the surface of vicinal GaAs(001) annealed in
H2 , AsH3 /N2 , AsH3 /H2 and N2 using a scanning tunneling microscope (STM). Under the annealing conditions employed in this research, step bunches always formed on vicinal GaAs(001) annealed in an ambient which includes hydrogen but never on substrates annealed in ambients which do
not include hydrogen, which strongly suggests that hydrogenis crucial to the formation of step bunching.

The annealing process is very similar to that of MOCVD
growth; the only difference is that no III-species is exposed,thus there is no crystal growth. Before annealing, the sample
was cleaned by H2 SO4 , followed by chemical etching in an
H2 SO4 : H2 O2 : H2 O = 4 : 1 : 1 solution. Next, the substrate
was placed on a GaAs-coated carbon susceptor (MOCVDchamber) and annealed by radiation field heating. The ratesof increase and decrease of temperature were 50◦ C and 100◦ Cper min, respectively. Substrates were Si-doped with a carrier
concentration of 4×1017 cm−3 . Dopants are shown to have noinfluence on the formation of step bunches.13) The tunnelingcurrent and voltage were in the range of 0.9 nA to 3 nA and−1.8 V to −3.0 V, respectively. STM images obtained withtunneling voltage and current in this range were very similar.
First, we annealed vicinal GaAs(100) in H2 ambient. Determining the annealing temperature involves a delicate tradeoff. If the annealing temperature is too high, As will desorb from the surface resulting in surface roughening and Ga
droplet formation. On the other hand, if the annealing temperature is too low, kinetic processes such as surface diffusion will freeze out, and the time required for step bunchingto occur will become too long to measure by ordinal experiments. Another important factor which must be considered isthe presence of the oxidized layer. Ellipsometry measurement
suggests that the surface is covered with oxidized layers of athickness around −20 Å. In order to achieve a clean surface,
we have to anneal the substrate and remove this oxidized compound. This requires an annealing temperature above 580◦ Cin UHV ambient. However, when hydrogen is exposed to thesurface, it etches the oxidized layer, and it is reported thata clean surface is obtainable at far lower annealing temperatures compared to that of UHV.14, 15) Kisker et al. have shownthat an annealing temperature of 500◦ C in hydrogen ambient is sufficient to remove the oxidized layer and to achievea clean surface.14) Taking into account these effects, the annealing temperature was determined to be in the ranged of500◦ C to 650◦ C with an annealing time of 10 to 20 min. The
flow rates of H2 /N2 and AsH3 were 4000 sccm and 40 sccm,respectively. The total pressure was 1.3 × 104 Pa, therefore
the partial pressures of H2 /N2 and AsH3 were 1.3 × 104 Pa,
and 1.3 × 102 Pa, respectively. N2 was produced by evaporating liquid nitrogen and passing the evaporated gas througha nitrogen purifier, which produces the purest nitrogen gas∗ E-mail address: corresponding author. hata@ims.tsukuba.ac.jp4404Jpn. J. Appl. Phys. Vol. 39 (2000) Pt. 1, No. 7B

available. The concentrations of impurities are estimated to
be lower than 1.0 ppm for H2 O, 0.05 ppm for O2 , and 0.1 ppm
for CO2 .
3. Results and Discussion
Figures 1 and 2 show sets of typical STM images of thesurfaces of vicinal GaAs(001)-[100]2◦ annealed in H2 andN2 , respectively, at 550◦ C for 20 min ((a),(b)) and 650◦ C for
10 min ((c),(d)). Surfaces exposed to N2 were taken for comparsion. Many stripes running against the miscut direction
are observed in Fig. 1. The average distance between thesestripes is greater than ∼20 nm, which is greater than 8 nm;
the step-step distance calculated from the net misorientation
angle; thus the observed stripes represent step bunches. The
facet edges of these step bunches are wavy and the surface is
not completely smooth. On the other hand, no step bunches
form on surfaces annealed in N2 , the surfaces remain roughand messy, similar to the surface just after chemical etching,as shown in Fig. 3(d).
STM images of the surface annealed in H2 ambient at650◦ C were similar to those annealed at 550◦ C. There was
no evidence of surface roughening due to As desorption. In
contrast, the roughness of the surface increased on the surface annealed in N2 at 650◦ C compared to that annealed at550◦ C. Small grains and pits observed on surfaces annealed
at 550◦ C evolved in size and the number of exposed layers

K. H ATA et al.

4405

has increased.
Subsequently, with the intention of elucidating the role ofAsH3 in the formation process of step bunching, STM observations were carried out on vicinal GaAs(001)-[100]2◦ surfaces annealed in AsH3 and N2 ambient. As shown above,N2 is unrelated to the occurrence of step bunching. It is supplied as a carrier gas to dilute the AsH3 gas. Figure 3 shows
STM images of vicinal GaAs(001)-[100]2◦ surfaces annealedat 650◦ C (a), and 700◦ C (b) for 10 minutes in N2 and AsH3 .
Step bunches have formed which are exactly the same in sizeand shape to those observed on surfaces annealed in AsH3and H2 under the same annealing conditions, as displayed inFig. 3(c).
Experimental results show that step bunches form on vicinal GaAs(001) annealed in H2 , AsH3 /H2 , and AsH3 /N2 but
not on surfaces annealed in N2 . Step bunching always occurred when hydrogen was present on the surface. Basedon these results, we propose that the presence of hydrogenis the direct cause of step bunches. In order to describe
the formation mechanism of step bunching where hydrogenis treated as the cause, we follow the model proposed inref. 10. We assume that hydrogen produced by the dissociation of either AsH3 or H2 attaches to Ga atoms that terminate the step edges and influence the kinetics of step edge Ga
atoms in the manner described in detail in ref. 10 (replaceAsHx with hydrogen). Step bunches form when hydrogen at-Fig. 1. STM images of the surface of vicinal GaAs(001)-[100]2◦ annealed in H2 ambient for 550◦ C for 20 min, (a) and (b), and 650◦ Cfor 10 min, (c) and (d). The scale is 1000 × 1000 nm for (a) and (c), 500 × 500 nm for (b) and (d).

4406

K. H ATA et al.

Jpn. J. Appl. Phys. Vol. 39 (2000) Pt. 1, No. 7B

Fig. 2. STM images of the surface of vicinal GaAs(001)-[100]2◦ annealed in N2 ambient for 550◦ C for 10 min, (a) and (b), and 550◦ Cfor 20 min, (c) and (d). The scale is 1000 × 1000 nm for (a) and (c), 500 × 500 nm for (b) and (d).

tached to step edges reduces the effective Schwoebel’s barrier of attachment (detachment) of the Ga atoms to (from)step edges. Clear evidence that hydrogen does attach to stepedges and influences step edge kinetics was found by Asahi
et al, who demonstrated hydrogen-assisted step-flow growthduring migration-enhanced epitaxy and molecular-beam epitaxy.16) When hydrogen attached to step edges reduces the effective Schwoebel’s barrier and creates a condition in which
step bunching occurs, it is conjectured that Ga atoms diffusing on the terrace are more easily incorporated into the stepedges,10) providing a situation in which step-flow growth ismore likely to take place. This result is in consistent with theobservation of enhanced step-flow growth induced by hydrogen.
There are several possible explanations for the small andrough step bunches observed on vicinal GaAs(001) annealedin H2 compared to the large step bunches observed on surfaces
annealed with AsH3 under the same annealing conditions.
The first possibility is that H2 does not react or dissociate efficiently on the surface to produce enough hydrogen atoms
to cover the step edges and to cause step bunching. Whether
a step bunch or a regular monostep array forms depends onthe degree of the coverage of hydrogen on step edges; smallcoverage of hydrogen will result in small step bunches.10) In
addition, the surface reconstruction must be considered. Usually, a GaAs(001) surface annealed in AsH3 and H2 ambient

reconstructs to the extra-As rich c(4 × 4) phase. On the other
hand, surfaces annealed in H2 ambient are reported to reconstruct to a Ga-rich c(8 × 4) or a 4 × 1 phase.14, 15) Different
surface reconstructions have different atomic configurations
around step edges and thus different step edge kinetics whichmeans different step bunches.
It is well known that surfaces of vicinal GaAs(001)substrates show regular monostep arrays when grown by
molecular-beam epitaxy or annealed in UHV11, 12) (no hydrogen in both cases). This suggests that there exists a repulsive
interaction among steps. Practically, this is natural, as there
usually exists elastic-17) and entropic-repulsive interactions18)
among steps. Our results show that it is possible to overcome
this repulsive interaction and induce a net attractive interaction among steps by exposing hydrogen, a method which maylead to a new way to artificially control step motion.
4. Conclusion
We showed that step bunching forms on vicinal GaAs(001)
not only annealed in AsH3 /H2 ambient but also annealed in
H2 and AsH3 /N2 ambient but not on surfaces annealed in N2ambient. These experimental results strongly suggest that hydrogen is the direct cause of step bunching.

1) D. L. Rode: J. Cryst. Growth 27 (1974) 313.

Jpn. J. Appl. Phys. Vol. 39 (2000) Pt. 1, No. 7B

K. H ATA et al.

4407

Fig. 3. STM images of the surface of vicinal GaAs(001)-[100]2◦ annealed in AsH3 and N2 ambient for 650◦ C (a) and 700◦ C (b) for
10 min and surface annealed in AsH3 and H2 at 700◦ C for 10 min (c) and just after chemical etching (no annealing) (d). Scale of allimages is 1000 × 1000 nm.

2) T. Kajimura, K. Aiki and J. Umeda: Appl. Phys. Lett. 30 (1977) 526.
3) T. Fukui and H. Saito: Jpn. J. Appl. Phys. 29 (1990) L483.
4) H. Saito, K. Uwai and N. Kobayashi: Jpn. J. Appl. Phys. 32 (1993)
L4400.
5) J. Ishizaki, K. Ohkuri and T. Fukui: Jpn. J. Appl. Phys. 35 (1996) 1280.
6) M. Kasu and N. Kobayashi: J. Appl. Phys. 78 (1995 )3026.
7) M. Shinohara and N. Inoue: Appl. Phys. Lett. 66 (1995) 1936.
8) K. Hata, A. Kawazu, T. Okano, T. Ueda and M. Akiyama: Appl. Phys.
Lett. 63 (1993) 1625.
9) K. Hata, H. Shigekawa, T. Ueda, M. Akiyama and T. Okano: Phys. Rev.
B 55 (1997) 7039.
10) K. Hata, H. Shigekawa, T. Ueda, M. Akiyama and T. Okano: Phys. Rev.
B 57 (1998) 4500.
11) M. D. Pashley, K. W. Haberern and J. M. Gaines: Appl. Phys. Lett. 58(1991) 406.

12) Under some annealing conditions, step bunches, and double steps appear. However, the cause discussed here is quite different. K. Pond, A.
Lorke, J. Ibbetson, V. Bressler-Hill, R. Maboudian, W. H. Weinberg,A. C. Gossard and P. M. Petroff: J. Vac. Sci. Technol. B 12 (1994)
2689.
13) T. Fukui, J. Ishizaki, S. Hara, J. Motohisa and H. Hasegawa: J. Cryst.
Growth 146 (1995) 183.
14) D. W. Kisker, P. H. Fuoss, K. L. Tokuda, G. Renaud, S. Brennan andJ. L. Kahn: Appl. Phys. Lett. 56 (1990) 2025.
15) F. J. Lamelas, P. H. Fuoss, P. Imperatori, D. W. Kisker, G. B. Stephenson
and S. Brennan: Appl. Phys. Lett. 60 (1992) 2610.
16) H. Asahi, T. Hisaka, S. G. Kim, T. Kaneko, S. J. Yu, Y. Okuno and G.
Gonda: Appl. Phys. Lett. 61 (1992) 1054.
17) J. M. Rickman and D. J. Srolovitz: Surf. Sci. 248 (1993) 211.
18) E. D. Williams and N. C. Bartelt: Science 251 (1991) 393.