• 沒有找到結果。

As+-implanted AlGaAs oxide-confined VCSEL with enhanced oxidation rate and high performance uniformity

N/A
N/A
Protected

Academic year: 2021

Share "As+-implanted AlGaAs oxide-confined VCSEL with enhanced oxidation rate and high performance uniformity"

Copied!
3
0
0

加載中.... (立即查看全文)

全文

(1)

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 6, JUNE 2004 1423

As

+

-Implanted AlGaAs Oxide-Confined

VCSEL With Enhanced Oxidation Rate and

High Performance Uniformity

Li-Hong Laih, H. C. Kuo, Gong-Ru Lin, Member, IEEE, Lih-Wen Laih, and S. C. Wang

Abstract—We report the utilization of an As+-implanted Al-GaAs region and regrowth method to enhance and control the wet thermal oxidation rate for 850-nm oxide-confined vertical-cavity surface-emitting laser (VCSEL). The oxidation rate of the As+ -im-planted device showed a four-fold increase over the nonim-im-planted one at the As+dosage of1 1016 cm 3and the oxidation tem-perature of 400 C. 50 side-by-side As+-implanted oxide-confined VCSELs fabricated using the method achieved very uniform per-formance with a deviation in threshold current of1 th 0 2 mA and slope-efficiency of1S E 3%.

Index Terms—As+-implanted, oxide-confined, vertical-cavity surface-emitting laser (VCSEL), wet-thermal oxidation.

I. INTRODUCTION

V

ERTICAL-CAVITY surface-emitting lasers (VCSELs) have emerged as the attractive light sources for various optoelectronic applications such as optical communications, which offer several advantages over edge-emitting semicon-ductor lasers, such as low divergence circular beam, low threshold current, the possibility of one- and two-dimensional array formation, and cost-effective wafer-scale fabrication, etc. Over the past few years, the oxide-confined VCSELs have been shown to exhibit excellent performances such as low threshold current, high wall-plug efficiency, and high frequency modulation capability [1]. Up until now, the optical-confine-ment method commonly used for the VCSEL is the selective wet-oxidation of AlGaAs layers with desired oxidized aperture circulating around the VCSEL [2]–[4]. Various reports have reported about the control of wet thermal oxidation rate [5]–[8]. However, such a thermal oxidation process has a relatively slow oxidation rate, which is also difficult to control and to achieve uniformity for large area wafer. To overcome this, Reese et

al. [5] used the low-temperature-grown GaAs (LT-GaAs)

layer below the oxidation layer to enhance the wet thermal oxidation rate, while a maximum oxidation rate of 1.4 m/min

Manuscript received July 17, 2003; revised February 20, 2004. This work was supported in part by the National Science Council, R.O.C., under Contracts NSC-91-2215-E-009-030 and MOC-X92022.

L.-H. Laih is with the Institute of Electro-Optical Engineering, National Chiao-Tung University and the Millennium Communication (M-Com) Co., Ltd., Hsinchu Hsien 303, Taiwan, R.O.C.

H. C. Kuo, G.-R. Lin, and S. C. Wang are with the Institute of Electro-Op-tical Engineering, National Chiao-Tung University, Hsinchu City 300, Taiwan, R.O.C. (e-mail: hckuo@faculty.nctu.edu.tw).

L.-W. Laih is with the Department of Electronic Engineering, Ching Yun Uni-versity, Jung-Li 320, Taiwan, R.O.C.

Digital Object Identifier 10.1109/LPT.2004.827116

was achieved. The gallium vacancy defects left in LT-GaAs after annealing was considered to be responsible for the acceleration of the oxidation rate. However, the density in the LT-GaAs is difficult to be precisely controlled due to the uncertainty in growth temperature of the LT-GaAs (well below the system limit). In addition, the poor quality of the LT-GaAs material could affect the device reliability and, thus, degrade the device uniformity of VCSELs. In addition, Yoshikawa et

al. [7] demonstrated a self-stopping selective-oxidation process

to control the oxide aperture, providing an controlled oxide aperture as small as about 3 m in diameter. Chavarkar et al. [8] studied the effect of antimony (Sb) composition on the oxidation mechanism of AlAs Sb layers grown on GaAs substrate.

Not long ago, we have also demonstrated an alternative arsenic-rich GaAs material using arsenic-ion implanting technique, which exhibits almost identical properties to the LT-GaAs layer [5]–[7]. The advantages for preparation of arsenic-rich GaAs layer by ion-implantation is its flexibility in controlling the arsenic excess density and the associated arsenic antisite defect concentration by adjusting the implanting dosage [9]. Subpicosecond carrier lifetimes and picosecond photoconductive responses of GaAs : As comparable to that of LT-GaAs were also reported [10]. After annealing, the dense As precipitates within the implanted region introduces highly resistive electrical and ultrafast optoelectronic properties [11]. These make the GaAs : As a best candidate for electric-buffer layer with extremely low leakage current. In this letter, we re-port the use of selective As -implanted buffer layer to enhance the oxidation rate of an AlGaAs layer grown upon the buffer layer. The mechanism for oxidation rate acceleration and aper-ture control of the AlGaAs layer grown on the As -implanted buffer layer is interpreted. An array of 50 VCSELs fabricated with an oxidized AlGaAs layer of accelerated oxidation rate have been shown to achieve uniform performances.

II. DEVICESTRUCTURE ANDFABRICATION

The cross-sectional schematic of As -implanted oxide-con-fined VCSEL is shown in Fig. 1. The VCSEL epitaxial layers were grown on n -type GaAs (100) 6 off toward with orien-tation of (110) substrate by a metal–organic chemical vapor deposition (MOCVD) system. The bottom distributed Bragg re-flector (DBR) consists of 35-pairs of quarter wavelength-thick n-type (Si-doped) Al Ga As–Al Ga As. The active

(2)

1424 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 6, JUNE 2004

Fig. 1. Schematic of As -implanted oxide-confined GaAs VCSEL.

Fig. 2. Nomarski microscopic photograph of the VCSEL containing oxided Al Ga As layer after oxidation for 45 min.

region has three GaAs–AlGaAs quantum wells with peak gain at 850 nm. Afterwards, a partial top DBR structure with three pairs of p-type (C-doped) Al Ga As–Al Ga As was grown for As -implantation. The aperture of As -im-plantation is m , the dosage of As is varied from to cm , and the implanted energy is 100 KeV. After implantation, the sample was annealed at 700 C for 2 h. Subsequently, 22 pairs of p-type (C-doped) Al Ga As–Al Ga As DBR structures with an ox-idation layer of 30-nm Al Ga As were grown upon the As -implanted AlGaAs layer. For comparison, a similar VCSEL wafer structure without As implanted buffer layer was also grown by MOCVD. Both MOCVD grown wafers with different structures were patterned by lithography and were mesa-etched down to n-DBR layer. Later on, both samples were oxidized in a N –H O atmosphere at various temperatures with an oxidation aperture of m . Finally, the p-type metal (Ti–Pt–Au) with a window aperture is m and the n-type metal (Au–Ge–Ni–Au) contacts were deposited and annealed sequentially. The lateral oxidation width of the Al Ga As layer grown upon the As -implanted, three periods of p-type Al Ga As–Al Ga As layers is determined using Nomarski microscopic photograph. It is seen that the color and the size of the region containing oxided Al Ga As layer are significantly changed after oxidation for 45 min, as shown in Fig. 2.

III. RESULTS ANDDISCUSSIONS

The relationship between the oxidation rate at 400 C and the As implantation dosages is depicted in Fig. 3. The oxidation rate shows a rapid increasing trend with the As dose, however, which is found to saturate at the dose beyond cm . The oxidation rate at the dosage higher than cm saturates

Fig. 3. Relationship between the As -implanted dose and the oxidation rate.

at a constant rate of about 0.5 m/min. Such an improvement in oxidation rate of the AlGaAs layer is mainly attributed to the en-hancement in the removal of the products of the oxidation reac-tion, in particular, the As-containing materials by the underlying As -implanted AlGaAs layer [5]. If removal of the As-con-taining products is a rate-limited process, the enhanced diffu-sivity of As-containing material results in an observed increase in the oxidation rate of the 30-nm Al Ga As layer. More-over, the enhanced removal of As containing products could have important consequences for the quality of the oxide–semi-conductor interface [5].

Previous experiments revealed that the quality of the oxide–GaAs interface beneath the LT-GaAs layer can be dra-matically improved [5], [12]. With the underlying arsenic-rich GaAs layer, the As precipitation phenomenon is less pro-nounced at the oxide–GaAs interface. The present hypothesis suggests that both the arsenic (or gallium) atoms have been diffused away from the vicinity of the oxide layer during oxidation. The LT-GaAs layer which contains high concen-trations of As precipitates and may provide an enhanced interdiffusion efficiency of the arsenic and/or gallium atoms in the AlGaAs layer. This eventually leads to the accelerated oxi-dation of AlGaAs layer and the improved oxide–GaAs interface quality. Since the faster oxidation rate of the AlGaAs layer is strongly correlated with the density in the As -implanted layer [5], [12], a further enhancement of oxidation speed due to the increasing of the defect density at higher implanted doses is, thus, expected. However, previous observations also imply a saturation of these defects at implanting dose beyond ions/cm .

The oxidation depth versus oxidation time for the VCSEL samples with As -implantation of cm and without implantation is shown Fig. 4. By measuring the oxidation depth from Nomarsky microscope photograph, most of the data at two different oxidation temperatures show approximately linear dependency. Nonetheless, the VCSEL sample with underlying As -implanted layer exhibits a saturated oxidation depth be-yond 40 m after oxidizing at 420 C. Such a saturation in oxidation rate has indicated a diffusion-limited oxidation phe-nomenon [5]. After oxidizing at a temperature of 400 C for

(3)

LAIH et al.: AS -IMPLANTED AlGaAs OXIDE-CONFINED VCSEL WITH ENHANCED OXIDATION RATE 1425

Fig. 4. Oxidation depth versus the oxidation time at 400 C and 420 C. The open markers and solid markers represent the samples with and without As implanted, respectively.

Fig. 5. L–I curves of 50 side-by-side As implanted oxide-confined GaAs VCSELs.

120 min, the lateral depths of the oxidized AlGaAs layer are 60 and 15 m for the VCSEL samples with and without an As -implanted underlying layer, respectively. This corresponds to a four-fold increase in oxidation rate of the VCSEL with an As -implanted underlying AlGaAs layer as compared to that of the nonimplanted one.

Fig. 5 shows the power–current ( – ) curves of 50 units of side-by-side VCSELs fabricated using the As -implantation as-sistant wet-selective oxidation process. The dosage and energy of As implantation were cm and 100 KeV, respec-tively. The oxidation temperature and time were 400 C and 40 min. At the driving current of 20 mA, the average output power is about 5.5 mW, the threshold current of the VCSEL is 1.2 mA, and the slope efficiency (S.E.) is 34%. The variation of threshold current and S.E. are only 0.2 mA and 3%, respec-tively. These results indicate that the VCSELs with an As -im-planted underlying layer have a better control on the oxidation depth and aperture, which results in the VCSELs with nearly identical performances. Since the As -implanted region has a faster oxidation rate than the nonimplanted region, when the ox-idation proceeds close the designed aperture (nonimplanted re-gion), the oxidation rate will significantly reduce and, hence, the tolerance of oxidation time can be released. This essentially reduces the processing failures occurring during oxidation and greatly improves the uniformity performance of the VCSELs.

With the adding of underlying AlGaAs : As layer, the increase in production yield and reduction in the fabrication cost of the VCSELs are straightforward.

IV. CONCLUSION

By adding an As -implanted underlying AlGaAs layer and using the MOCVD regrowth method, we have successfully demonstrated the enhancement and precise control the wet thermal oxidation rate of the AlGaAs layer in the 850-nm oxide-confined VCSEL. With the As dosage of

cm and the oxidation temperature of 400 C, our results reveal that the oxidation rate of the VCSELs with an As -im-planted underlying layer have a four-fold increase on the oxidation rate over the nonimplanted one. The testing on 50 units of side-by-side As -implanted and oxide-confined VCSELs fabricated using the method shows high uniformity in their overall performances. The deviations in threshold current and slope-efficiency of these VCSELs are % and %. The application of As -implanting tech-nique in fabrication of the large-area VCSEL array has been demonstrated.

REFERENCES

[1] N. Nishiyama, A. Mizutani, N. Hatori, M. Arai, F. Koyama, and K. Iga, “Single-transverse mode and stable-polarization operation under high-speed modulation of InGaAs-GaAs vertical-cavity surface-emit-ting laser grown on GaAs (311) B substrate,” IEEE Photon. Technol. Lett., vol. 10, pp. 1676–1678, Dec. 1998.

[2] D. L. Huffaker, D. G. Deppe, K. Kumar, and T. J. Roger, “Native-oxide defined ring contact for low threshold vertical-cavity lasers,” Appl. Phys. Lett., vol. 65, pp. 97–99, 1994.

[3] K. D. Choquette, K. L. Lear, R. P. Schneider Jr., K. M. Geib, J. J. Figiel, and R. Hull, “Fabrication and performance of selectively oxidized ver-tical-cavity lasers,” IEEE Photon. Technol. Lett., vol. 7, pp. 1237–1239, Nov. 1995.

[4] K. D. Choquette, K. M. Geob, C. I. H. Ashby, R. D. Rwesten, O. Blum, H. Q. Hou, D. M. Follstaedt, B. E. Hammons, D. Mathes, and R. Hull, “Advances in selective wet oxidtion of AlGaAs alloys,” IEEE J. Select. Topics Quantum Electron., vol. 3, no. 3, pp. 916–926, 1997.

[5] H. Reese, Y. J. Chiu, and E. Hu, “Low-temperature GaAs enhanced wet thermal oxidation of Al Ga As,” Appl. Phys. Lett., vol. 73, no. 18, pp. 2624–2626, 1998.

[6] H. Q. Jia, H. Chen, W. C. Wang, W. X. Wang, W. Li, Q. Huang, and J. Zhou, “Improved thermal stability of wet-oxidived AlAs,” Appl. Phys. Lett., vol. 80, no. 6, pp. 974–976, 2002.

[7] T. Yoshikawa, H. Saito, H. Kosaka, Y. Sugimoto, and K. Kasahara, “Self-stopping selective-oxidation process of AlAs,” Appl. Phys. Lett., vol. 72, no. 18, pp. 2310–2312, 1998.

[8] P. Chavarkar, U. K. Mishra, S. K. Mathis, and J. S. Speck, “Effect of Sb composition on lateral rates in AlAs Sb,” Appl. Phys. Lett., vol. 76, no. 10, pp. 1291–1293, 2000.

[9] G.-R. Lin, W.-C. Chen, C.-S. Chang, and C.-L. Pan, “Electrical char-acterization of arsenic-ion-implanted semi-insulating gaas by current-voltage measurement,” Appl. Phys. Lett., vol. 65, no. 25, pp. 3272–3274, 1994.

[10] F. Ganikhanov, G.-R. Lin, and C.-L. Pan, “Subpicosecond carrier life-times in arsenic-ion-implanted GaAs,” Appl. Phys. Lett., vol. 67, no. 23, pp. 3465–3467, 1995.

[11] G.-R. Lin, W.-C. Chen, C.-S. Chang, S.-C. Chao, K.-H. Wu, T. M. Hsu, W. C. Lee, and C.-L. Pan, “Material and ultrafast optoelectronic properties of furnace-annealed arsenic-ion-implanted GaAs,” IEEE J. Quantum Electron., vol. 34, no. 9, pp. 1740–1748, Sept. 1998. [12] J. C. Ferrer, Z. Liliental-Weber, H. Resse, Y. J. Chiu, and E. Hu,

“Improvement of the interface quality during thermal oxidation of Al Ga As layers due to the presence of low-temperature-grown GaAs,” Appl. Phys. Lett., vol. 77, no. 2, pp. 205–207, 2000.

數據

Fig. 3. Relationship between the As -implanted dose and the oxidation rate.
Fig. 5. L–I curves of 50 side-by-side As implanted oxide-confined GaAs VCSELs.

參考文獻

相關文件

In addition that the training quality is enhanced with the improvement of course materials, the practice program can be strengthened by hiring better instructors and adding

Then, we tested the influence of θ for the rate of convergence of Algorithm 4.1, by using this algorithm with α = 15 and four different θ to solve a test ex- ample generated as

Second, we replicate the AN+MM and use European options sampling at exercise as control variates (CV-at-exercise). Last, we also replicate the AN+MM and use

* School Survey 2017.. 1) Separate examination papers for the compulsory part of the two strands, with common questions set in Papers 1A & 1B for the common topics in

Microphone and 600 ohm line conduits shall be mechanically and electrically connected to receptacle boxes and electrically grounded to the audio system ground point.. Lines in

Adding a Vertex v. Now every vertex zl. Figure 14 makes this more precise. Analysis of the Algorithm. Using the lmc-ordering and the shift-technique, explained in Section

n Media Gateway Control Protocol Architecture and Requirements.

Biases in Pricing Continuously Monitored Options with Monte Carlo (continued).. • If all of the sampled prices are below the barrier, this sample path pays max(S(t n ) −