Applied Surface Science 117/l 18 (1997) 771-775

surface science ELSEVIER

Characterization of AlGaAs/GaAs vertical-cavity surface-emitting laser diode grown on Si substrate by MOCVD

Takashi Egawa a, * , Yoshihiko Murata b, Takashi Jimbo a, Masayoshi Umeno b

a Research Center for Micro-Structure Devices, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466, Japan

b Department of Electrical and Computer Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466, Japan

Abstract

An AlGaAs/GaAs multi-quantum well vertical-cavity surface-emitting laser diode (VCSELD) has been grown on a Si substrate using metalorganic chemical vapor deposition. The VCSELD structure grown on a Si substrate consists of ten quantum well active layers and a 23-pairs of AIAs/Al,,,Gac,, As distributed Bragg reflector (DBR). The VCSELD on a Si substrate exhibited a threshold current of 82 mA and a threshold current density of 4.2 kA/cm* under continuous-wave (cw) condition at 1.50 K. Cross-sectional scanning electron microscopy observation showed quasi-periodic zigzag roughness and nonuniformity in the DBR structure. Auger-electron spectroscopy also showed compositional transitions at the heterointer- faces of DBR. A low reflectivity of the DBR on Si substrate is caused by the degraded heterointerfaces, which prevent 300

K cw operation for the VCSELD grown on Si.

Keywords: Vertical-cavity surface-emitting laser; Distributed Bragg reflector; GaAs/Si; Heterointerface; Roughness; Interdiffusion

1. Introduction

Growth and fabrication of III-V optical devices on Si substrates are very promising for the realiza-

tion of optoelectronic integrated circuits (OEIC’s) [ 1,2]. In particular, the integration of vertical-cavity

surface-emitting laser diodes (VCSELD’S) on Si has significant advantages over edge-emitting lateral- cavity laser diodes, such as the potential for wafer scale testing, high-density two-dimensional array fabrication, chip-to-chip optical interconnection, ul- trafast parallel optical information processing and the possibility of monolithic integration with other opti-

* Corresponding author. Tel.: + 81-52-7355544; fax: + Sl-52-

7355546: e-mail: egawa@mothra.elcom.nitech.ac.jp.

cal or electronic devices. However, only a few demonstrations have been reported on VCSELD grown on Si under the pulsed operation at 300 K [3,4]. We demonstrated 300 K pulsed operation for the AlGaAs/GaAs single quantum well VCSELD on Si with AlAs/GaAs distributed Bragg reflector

(DBR) [41. Heteroepitaxial growth of GaAs on Si involves a

high density of dislocation (> lo6 cm-*) and a large residual tensile stress ( _ lo9 dyn/cm*), which result from the 4.1% lattice mismatch and 250% difference in thermal expansion coefficients between GaAs and Si. In addition to this incompatibility of the material properties, the GaAs/Si exhibits a rough surface morphology [5,6]. The rough surface mor- phology is thought to affect the structure and reflec- tivity of the DBR grown subsequently on this rough

0169-4332/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved.

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772 T. Egawa et al./Applied Su$ace Science 117/118 (1997) 771-775

GaAs/Si. The high density of dislocations and the large stress cause the failure of reliable and stable lasing operation due to growth of dark-line defects [7]. Continuous-wave (cw) operation could not be achieved even at low temperature because of poor crystallinity and low reflectivity of AlAs/GaAs DBR. Enhancement of the optical gain and the re- flectivity of DBR is important for cw operation of VCSELD on Si, because the mirror loss of the VCSELD is larger than that of the conventional edge-emitting laser diode. To achieve room-tempera-

ture cw operation with low threshold current, the DBR with high reflectivity of 99% is required. In the GaAs active layer, the use of the AIAs/AlO,,Ga,,As DBR instead of the AlAs/GaAs DBR reduces the absorption loss of the n+-GaAs layers in the DBR, which results in high reflectivity of the DBR on Si. It is thus important to study the structure of the DBR

on Si substrate. In this study, we show that quasi- periodic zigzag roughness, nonuniformity of thick- ness and compositional transitions are observed in the DBR on Si substrate. We demonstrate 150 K cw operation of AlGaAs/GaAs multi-quantum well

(MQW) ~~SELD on Si substrate with 23-pair of

AlAs/Al,,Ga,, As DBR. This degraded DBR het- erointerface results in difficulties in obtaining high reflectivity and room-temperature cw operation for

the VCSELD on Si substrate.

Au Emission

active layer

2. Experimental

An AlGaAs/GaAs VCSELD structure was grown on an n+-Si substrate oriented 2” off (100) toward [I lo] using metalorganic chemical vapor deposition

(MOCVD) at atmospheric pressure. The VCSELD structure was grown on Si substrate at 750°C by the conventional two-step growth technique. Fig. 1 shows a schematic cross-sectional structure of the VCSELD grown on Si substrate. The laser structure consists of a 2.1 pm thick thermal-cycle . annealed n+-GaAs buffer layer, a 23-pairs of n+-AlAs/n+-Al, ,Ga,.,As (71 nm/60 nm) quarter-wave multi-layer DBR, a 0.29 pm thick n-Al,,Ga,,,As lower cladding layer, a 51 nm thick undoped Al,,Ga,,As lower confining layer, ten undoped 9 nm thick GaAs MQW active layers separated by 5.5 nm thick undoped Al,,Ga,,, As barrier layers, a 51 nm thick undoped Al,,Ga,,,As upper confining layer, a 0.22 ,um thick p-Al,,Ga,,,As upper cladding layer, and a 34 nm thick pf-GaAs contact layer. After the growth, the laser device was fabricated as follows: a 0.1 pm thick SiO, insulating layer was deposited on the p+-GaAs contact layer and 50 pm diameter contact windows were opened by wet chemical etching of SiO,. Next, 50 nm thick Ti/60 nm thick Au were evaporated on the pf-GaAs layer as a p-side elec- trode except for the 20 pm diameter area at the

MQW

Alo.sGao TAS, 5.5 nm

;I Fig. 1. Schematic cross-sectional structure of AlGaAs/GaAs MQW VCSELD grown on Si substrate.

T. Egawa et al./Applied Surface Science 117/118 (1997) 771-775 713

center of the 50 pm diameter contact window. A 70 nm thick semi-transparent Au was then evaporated on the p-side surface. A 50 nm thick AuSb/lSO nm thick Au was used for the n-side electrode on the n+-Si substrate. The devices were mounted junction up on a sample holder and tested. In this VCSELD on Si, the light output was measured by detecting the emitted light through the 20 pm diameter semi-

transparent Au. We have also grown 20-pairs of quarter-wave

nf-AlAs/n+-GaAs (71 nm/59 nm) DBR on Si. In order to determine the sharpness of compositional gradients at the DBR heterointerfaces, the composi- tional profile of the AlAs/GaAs DBR grown on Si was measured by Auger electron spectroscopy (AES). For comparison, a similar structure of the VCSELD

was grown on (100) GaAs substrate. The cross-sec- tional structure of the AlAs/GaAs DBR was ob- served by scanning electron microscopy (SEMI. The compositional structure was profiled by measuring the Ga-Auger peak (1070 eV>, As-Auger peak (1228 eV> and Al-Auger peak (1390 eV> intensities by sputtering with a 5-keV Ar-ion beam.

3. Results and discussion

Cross-sectional SEM micrographs, obtained with backscattered electrons, of the AlAs/GaAs DBR structures on GaAs and Si substrates are shown in Fig. 2a and b, respectively. The AlAs/GaAs DBR grown on GaAs substrate shows the interfacial sharpness with uniform and smooth heterointerfaces. As shown in Fig. 2b, however, the AlAs/GaAs DBR on Si substrate exhibits quasi-periodic zigzag roughness and nonuniformity in the AlAs and GaAs layers. The thickness of AlAs layers is from 65 to 75 nm and that of GaAs layers is from 57 to 67 nm. The surface and interfacial roughnesses are probably due to the three-dimensional growth at the initial growth stage [8,9] and defects such as the threading disloca- tions and the stacking faults [6]. The zigzag rough- ness and the interfacial roughness introduce diffrac- tion and scattering loss, and reduce the reflectivity of the DBR. The quality of the active layer grown subsequently on the rough DBR may also be de- graded.

In order to determine the sharpness of composi-

Fig. 2. Cross-sectional SEM micrographs, obtained with backscat-

tered electrons, of the VCSELD with the AIAs/GaAs DBR grown

on (a) GaAs and (b) Si substrates.

tional gradients at the AlAs/GaAs heterointerfaces, the compositional profile of the AlAs/GaAs DBR was measured by AES. Fig. 3a and b show the ratios

of the Auger signals of Ga/As and Al/As as a function of depth for the AlAs/GaAs DBRs grown on GaAs and Si substrates, respectively. The sharp square profiles were obtained in the AlAs/GaAs DBR grown on GaAs substrate. In the case of the sample grown on Si, however, the compositional transitions were seen at the AlAs/GaAs heterointer- faces. The interface width between the AlAs and

GaAs layers was defined as the distance between two points wherein the investigated Al peak height changed from 90 to 10% of its maximum. The interface widths were found to be about 4.6 nm for the AlAs/GaAs DBR on GaAs, and from 13.2 to 18.9 nm for that on Si substrate. Thus, the apparent interdiffusion across the AlAs/GaAs heterointer- faces has been observed in the AlAs/GaAs DBR on

714 T. Egawa et al./Applied Surface Science 117/118 (1997) 771-775

Si substrate. This apparent interdiffusion is probably caused by two reasons. One is due to the inter-facial roughness at the AlAs/GaAs heterointerfaces as shown in Fig. 2b. The other is due to the high density of defects ( > lo7 cm-*) and the large stress (N 10’ dyn/cm*), which result from the differences of lattice constants and thermal expansion coeffi- cients between GaAs and Si. The Al-Ga interdiffu- sion [lo] is thought to be enhanced by the high density of defects and the large stress in the AlAs/GaAs DBR on Si.

The measured peak reflectivity of the 23-pairs of AlAs/Al,,Ga,,As DBR on Si was 98.6% at 840 nm. Although a little increase of reflectivity was observed in comparison with that of the 20-pairs of

AlAs/GaAs DBR on Si [4], the reflectivity of 98.6% still remains to be low probably due to the zigzag

_I

(a) on GaAs substrate

0' 0 200 400

Depth (nm)

I

600 800

31 I

I (b) on Si substrate

I

400 Depth (nm)

600 800

Fig. 3. Auger signals of Ga/As and Al/As as a function of depth for the AlAs/GaAs DBRs grown on (a) GaAs and (b) Si sub-

strates.

20 1

0

100 K, CW

Ia=73mA

(Jti = 3.7 kA/cm*)

t

q,, = 0.8 %

1 820 840 860

40 60 Current (mA)

Fig. 4. Cw L-Z characteristic and emission spectrum of

AlGaAs/GaAs MQW VCSELD on Si substrate at 100 K.

interfacial roughness and the Al-Ga interdiffusion in the DBR on Si substrate as shown above. The top mirror had the reflectivity of 93.6% and transparency of 0.6% for the semi-transparent Au.

Fig. 4 shows the light-current (L-Z) characteris- tic and emission spectrum of the VCSELD at 100 K

in cw operation. The VCSELD on Si exhibited the threshold current (I,,) of 73 mA, the threshold cur- rent density (Jt,> of 3.7 kA/cm2, and the external differential quantum efficiency (~~1 of 0.8%. The qd of VCSELD is lower than the hd of 28% for the edge-emitting lasers on Si. This is thought to be caused by the absorption in the semi-transparent Au and the interfacial roughness of DBR. The VCSELD on Si showed the peak wavelength of 834.4 nm with the full width at half maximum (FWHM) of 39.5 nm at 0.96 X Z,, and 844 nm with the FWHM of 2.2 nm at 1.2 X Z,,. The VCSELD on Si also showed the pulsed It,, of 132.6 mA (Jth = 6.7 kA/cm*) at 300 K. We have confirmed the cw operation up to 150 K for the VCSELD on Si. The VCSELD on Si exhib- ited the cw Z, of 82 mA (.Zth = 4.2 kA/cm*) at 150 K. The cw operation was thought to be achieved by the use of the AlAs/AlGaAs DBR and MQW struc- ture.

T. Egawa et al./Applied Surface Science 117/118 (1997) 771-775 115

4. Summary

We have grown the VCSELD with the AlAs/Al- GaAs DBR on Si substrate by MOCVD, and charac- terized the DBR heterointerfaces using cross-sec- tional SEM and Auger-electron spectroscopy. We have shown that the AlAs/GaAs DBR structure has the quasi-periodic zigzag roughness, the nonuniform thickness and the compositional transitions, which

result in low reflectivity. The cw operation of the AlGaAs/GaAs MQW VCSELD on Si with the 23- pairs of AlAs/Al,,Ga,,,As DBR was demonstrated up to 150 K. The VCSELD on Si exhibited the Zth of

82 mA and the Jth of 4.2 kA/cm’ under the cw condition at 150 K. The degradation of heterointer- faces prevent the cw operation of VCSELD on Si at room temperature.

Acknowledgements

The authors wish to thank Mr. N. Nakanishi for his help in device fabrication. This work was par-

tially supported by a Grant-in-Aid for Scientific Re- search (No. 06650053) from The Ministry of Educa- tion, Science, Sports and Culture, Japan.

References

ill

[21 131

[41

151

161

[71

NJ

[91

DOI

H.K. Choi, G.W. Turner, T.H. Windhom, B.-Y. Tsaur, IEEE

Electron Device Lett. EDL-7 (1986) 500.

I. Hayashi, Jpn. J. Appl. Phys. 32 (1993) 266.

D.G. Deppe, N. Chand, J.P. van der Ziel, G.J. Zydzik, Appl.

Phys. Lett. 56 (1990) 740.

T. Egawa, Y. Hasegawa, T. Jimbo, M. Umeno, IEEE Photon.

Technol. Lett. 6 (1994) 681.

T. Egawa, T. Jimbo, M. Umeno, Appl. Phys. Lett. 61 (1992)

2923.

H. Mod, M. Tachikawa, T. Yamada, T. Sasaki, J. Cryst.

Growth 154 (1995) 23.

T. Egawa, Y. Hasegawa, T. Jimbo, M. Umeno, Appl. Phys.

Lett. 67 (1995) 2995.

T. Egawa, T. George, T. Jimbo, M. Umeno, IEEE Photon.

Technol. Lett. 6 (1994) 150.

SF. Fang, K. Adomi, S. Iyer, H. Morkoc, H. Zabel, C. Choi,

N. Otsuka, J. Appl. Phys. 68 (1990) R31.

Y.J. Li, M. Tsuchiya, P.M. Petroff, Appl. Phys. Lett. 57

( 1990) 472.