Ion beam studies of InAs/GaAs quantum dots after annealing

(1)

Ion beam studies of InAs/GaAs quantum dots after annealing

H. Niu

a,*

, C.H. Chen

b

, H.Y. Wang

c

, S.C. Wu

b

, C.P. Lee

c

a_{Nuclear Science and Technology Development Center, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC} b_{Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC}

c_{Department of Electronics Engineering, National Chiao Tung University, Hsinchu 30013, Taiwan, ROC}

Received 22 September 2007; received in revised form 5 February 2008 Available online 29 February 2008

Abstract

The microstructure changes of self-assembled InAs/GaAs quantum dots during RTA treatment was investigated using ion channeling and photoluminescence (PL). A small blueshift of the PL emission is observed for annealing temperatures of 650–800°C and an obvious blueshift at 850°C. The yield of channeled spectra decreased as annealing temperature was increased, but the yield increased while tem-perature above 800°C in RTA. These results imply the strain of QD varied during RTA treatments. In addition, the As/Ga atomic ratio near the surface was determined from the surface peaks of the channeled spectrum.

PACS: 61.85.+p; 68.65.H

Keywords: Quantum dots; GaAs; InAs; Ion-channeling

1. Introduction

Self-assembled semiconductor quantum dots (QDs) have been used extensively in optoelectronic devices, such as semi-conductor lasers and detectors[1–3]. The physical properties of the dots are strongly dependent on the strain caused by the lattice mismatch between materials (here InAs and GaAs). Thermal annealing is a way to tune the structure parameters of QDs, such as their composition, strain and size distribu-tion[4]. The structural changes induced by thermal anneal-ing cause changes in the QD’s interband transition as well as intersublevel space energy. Generally, such a procedure leads to blueshift and narrowing in the QD’s photolumines-cence (PL) spectrum. However, analysis of QD’s structure is diﬃcult due to its ultra thin proﬁle, being capped with a GaAs layer as well as its three dimension structure.

Ion channeling is a powerful technique for probing struc-ture of thin ﬁlms. It has long been used in analyzing atomic

ordering in crystal structures and has been successfully applied for the study of strain in buried QDs. Previous reports have demonstrated that the strain and interdiﬀusion of QDs can be studied from angular scan curves in ion chan-neling measurements[5,6]. In this work, the strain relaxation of the QDs after thermal annealing was studied using RBS/ channeling aligned energy spectra in bothh1 0 0i and h1 1 0i directions.

2. Experiment

The InAs quantum dots studied in this work were grown by molecular beam epitaxy (MBE) on semi-insulating GaAs(1 0 0) substrate. Growth rates were 0.8 lm/h for GaAs and 0.056 lm/h for InAs. Arsenic pressure was 2–

3 106Torr. A buﬀer layer of 500 nm GaAs was grown

ﬁrst. Then, 0.9 nm InAs QD layers (about 2.6 monolayers)

was grown at a temperature of 520°C. All samples were

capped with a 50 nm thick GaAs layer. From the atomic force microscope (AFM) image, the density of QDs was determined to be approximately 1 1010/cm2. The

struc-ture scheme is shown in Fig. 1. Post-growth annealing

*

Corresponding author. Tel.: +886 3 5715131x35852; fax: +886 3 5717160.

E-mail address:[email protected](H. Niu).

www.elsevier.com/locate/nimb

Available online at www.sciencedirect.com

Nuclear Instruments and Methods in Physics Research B 266 (2008) 1235–1237

NIM

B

Beam Interactions with Materials & Atoms

(2)

was performed in forming gas at temperatures of 650–

850°C for 30 s. Low temperature (25 K)

photolumines-cence measurement was performed with a 488 nm argon ion laser and a 75 cm spectrometer. The signal was col-lected using an InGaAs detector with lock-in technique.

RBS/w channeling measurement was performed with

4 MeV 12C++ beam produced by the 9SDH-2 Tandem

accelerator at National Tsing Hua University. The beam divergence was less than 0.02°, deﬁned by two sets of slits 2.3 m apart. The incident particle ﬂux was about 200 nA on the target. The sample was mounted in a three-axis

goniometer with an angular resolution of <0.01°. The

scat-tering chamber was kept at a vacuum of 2 106torr.

Backscattered particles were collected by a PIPS detector at 160° laboratory angle. The energy resolution of the sys-tem was 90 keV, determined by ﬁtting the GaAs edge of the

energy spectrum, using RBS simulation code RUMP[7].

This value is significantly lower than the energy difference between the signals from In atoms and the GaAs edge in the backscattering spectrum (526 keV). The In atom depth profile can not deduced from In peaks, as shown on the

inset in Fig. 2, due to its ultra thin proﬁle and low

concentration.

3. Result and discussion

The self-assembled InAs QD growth starts with the for-mation of a two-dimensional InAs layer which completely covers the surface of the GaAs substrate. As the thickness of InAs ﬁlm exceeds the threshold value, the island-like dots are formed on top of the 2D layer, or the wetting layer. The lateral lattice constant of the fully strained wet-ting layer is constrained to be the same as the underlying GaAs lattice constant and the vertical lattice constant is thus greater than the lattice constant of GaAs matrix as a result of tetragonal distortion. As a consequence of island formation, the lattice constants of the InAs dots for both the vertical and lateral directions are expected to be similar to the values for the InAs bulk crystal. Channeled ions may be dechanneled if the continuity of the channel is disrupted at the interface between dots and layers. This provides us the opportunity to look at the thin QD layer strain change after thermal treatment.

Fig. 2(a) and (b) show the RBS/w channeling spectra of the samples along theh1 0 0i and h1 0 0i directions, respec-tively. The aligned spectrum of GaAs wafer was also plot-ted as a reference. The yields of samples with InAs QDs were higher than the GaAs reference, implied some crystal distortion in the samples. Interestingly, the yields of aligned spectra in both directions did not vary monotonically with

the thermal treatment temperature. Below 750°C, the

yields decreased as temperature increased, approaching to the GaAs reference. This behavior is similar to the ion implanted samples during the annealing process, namely, solid phase epitaxial re-growth. Some defects in samples

are recovered by thermal annealing. Above 750°C, the

yields increased surprisingly with an abrupt increase

observed at 850°C. The yield increase indicated that the

channeled ions underwent dechanneling when they passed through the InAs layer. This implied that the atoms in InAs layer displaced from their lattice sites and interdiﬀused with surrounding GaAs. Furthermore, the channeled yield of

850°C annealed sample is higher than as-grown sample

alongh1 1 0i direction, but not along h1 0 0i, indicating that the vertical displacement, along growth direction, is larger than horizontal displacement. It evidences tentatively the former cubic-like InAs dots mixed with surrounding GaAs

50nm

GaAs

4nm InAs

Fig. 1. Schematic diagram of the QD samples.

Energy (KeV) 1000 1500 2000 2500 3000 3500 4000 Counts 0 2000 4000 6000 8000 10000 12000 14000 In peaks Energy (keV) 2500 2550 2600 2650 2700 2750 2800 Counts 0 20 40 60 80 100 <100> Energy (KeV) 1000 1500 2000 2500 3000 3500 4000 Counts 0 2000 4000 6000 8000 10000 12000 14000 Random x 0.5 GaAs virgin As groth 650 750 800 850 <110>

Fig. 2. RBS/w channeling spectra along the (a) 1 0 0 and (b) (1 1 0) axes of the samples.

(3)

and gradually to become InxGa1xAs with tetragonal

dis-tortion after high temperature annealing.

Fig. 3 shows 25 K PL spectra of samples before

and after annealing. Below 800°C, there is only a little

blueshift. But a signiﬁcant blueshift (70 nm) was observed

at 850°C with a narrower peak width compared to low

temperature annealing and as-grown samples. It implied that the structure and (or) the composition of QDs was changed after annealing. From the channeling result, we know that annealing causes the capping and buffer GaAs layer to restore and strain of the QDs to relax. Strain relax-ation should result in a redshift in the PL measurement due to band gap shrinkage. However, the results of PL mea-surements show a clear blueshift. The band gap energy is influenced by both strain and composition. However, we can deduce only the information on microstructure change of the QDs from dechanneling effect, but cannot gather any composition information because of the depth resolution limit in this work. Therefore, the explanation for this con-tradistinction between PL and channeling results might be attributed to the composition intermixing between the QDs and the surrounding GaAs during annealing. This is

con-sistent with the work of Fu et al. [8], who had studied

effects on interdiffusion in InGaAs/GaAs quantum dot using different capping layer. Large energy shift was also

observed at high temperature (800–850°C) annealing.

Table 1 listed the surface stoichiometry (As/Ga ratios) that calculated from Ga and As surface peaks. The As/

Ga ratio decreases to 0.48 for samples under annealing

temperature of 750°C. This is due to arsenic evaporation

on surface during the annealing process [9]. The ratio

increases to 0.8 for 850°C annealing, which may be due

to Ga atoms desertion at high temperature. 4. Conclusion

Combining the channeling results and PL measure-ments, we conclude that annealing causes InAs/GaAs QDs strain to relax and atomic intermixing to take place. A signiﬁcant blueshift and a large dechanneled yield were observed respectively in PL and channeling measurement, indicating that the cubiclike InAs dots mixed with

sur-rounding GaAs and gradually to become InxGa1xAs with

tetragonal distortion after high temperature annealing. The atomic intermixing likely plays a bigger role than the strain eﬀect in determining the photoluminescence emission spec-trum when the QDs are annealed.

Acknowledgment

This work was ﬁnancially supported by the National Science Council of Taiwan under Contract No. NSC 95-2221-E-007-169.

References

[1] N. Nuntawong, Y.C. Xin, S. Birudavolu, P.S. Wong, S. Huang, C.P. Hains, D.L. Huﬀaker, Appl. Phys. Lett. 86 (2005) 193115.

[2] H. Chen, Z. Zou, O.B. Shchekin, D.G. Deppe, Electron. Lett. 36 (2000) 1703.

[3] S.Y. Wang, S.D. Lin, H.W. Wu, C. Lee, Appl. Phys. Lett. 78 (2001) 1023.

[4] J. Zhu, S. Oktyabrsky, M.B. Huang, J. Appl. Phys. 100 (2006) 104312. [5] Hsing-Yeh Wang et al., J. Appl. Phys. 100 (2006) 103502.

[6] H. Niu et al., Nucl. Instr. and Meth. B 241 (2005) 470.

[7] M. Thompson, L. Doolittle, RBS Analysis and Simulation Package V4.0, Computer Graphic Service Ltd, 1988.

[8] L. Fu et al., Appl. Phys. Lett. 82 (2003) 2613.

[9] T.N.C. Venkatesan et al., Appl. Phys. Lett. 35 (1979) 88. Fig. 3. Photoluminescence spectra at 25 K of QDs before and after

annealing.

Table 1

The As/Ga ratio calculated from the surface peaks of RBS/w channeling spectra

Sample GaAs wafer NA 650 750 800 850 As/Ga 1 0.7 0.67 0.48 0.52 0.81 H. Niu et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1235–1237 1237