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Utilizing Buried Heterostructure Defined by Quantum-Well Intermixing. C. L. Walker, A. C. Bryce, Senior Member, IEEE, and J. H. Marsh, Fellow, IEEE.
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 14, NO. 10, OCTOBER 2002

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High Brightness Single-Mode Ridge Laser Utilizing Buried Heterostructure Defined by Quantum-Well Intermixing C. L. Walker, A. C. Bryce, Senior Member, IEEE, and J. H. Marsh, Fellow, IEEE

Abstract—The authors demonstrate a novel high brightness single-lateral mode ridge laser using quantum well intermixing to form a buried heterostructure. Increased discrimination between the fundamental and higher order modes can be achieved using the buried heterostructure to reduce the width of the gain section, enhancing fundamental mode operation. This allows the ridge width to be increased while maintaining fundamental mode operation, hence reducing the optical intensity at the facet and increasing the optical power before mirror degradation. Standard and novel buried heterostructure ridge lasers of 5- m width are compared; far-field beam profiles clearly show improved modal stability for the novel structure. Index Terms—Buried heterostructures, high brightness lasers, integrated optoelectronics, optical waveguides, quantum well intermixing, spatial mode stabilization.

I. INTRODUCTION

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IGH brightness semiconductor lasers operating in the fundamental lateral mode are required for applications including erbium-doped fiber amplifier (EDFA) pumping, Raman amplification, free-space communications, printing, material processing, and optical disk storage [1]. Such applications typically require fundamental mode operation up to hundreds of milliWatts. Performance limits for these lasers are generally imposed by: 1) lateral mode instability or 2) catastrophic optical mirror damage (COMD). Instability of the lateral mode is caused by spatial hole burning and the excitation of higher order modes [2]. Mirror degradation and eventual destruction of a GaAs–AlGaAs lasers is caused by facet heating due to nonradiative recombination of carriers [3], [4]. Stabilization of the lateral mode is typically achieved using a narrow laser waveguide of a few micron width, which does not support higher order lateral modes. Confining the optical mode using such a waveguide has the limitation that the output aperture is small, and consequently the optical intensity is high, resulting in COMD. Ridge waveguide and buried heterostructure lasers are typically limited in that the optical and electrical confinement are interdependent; the optical waveguide width is approximately the same as the current aperture. The device requirements to give stable fundamental mode operation and avoid COMD are disparate, limiting the power capability of such devices. Manuscript received March 21, 2002; revised May 30, 2002. The authors are with the Department of Electronics and Electrical Engineering, University of Glasgow, Glasgow G12 8QQ, Scotland, U.K. (e-mail: [email protected]). Publisher Item Identifier 10.1109/LPT.2002.802070.

In this letter, we propose a novel ridge laser structure using a buried heterostructure, which suppresses higher order lateral modes and allows wider ridges to operate in the fundamental mode, increasing the power capability before COMD. Optical confinement of the mode is defined by the ridge waveguide, and the electrical confinement by the buried heterostructure. Independent control of the optical and electrical confinement allows modification of the lateral gain profile, reducing the gain of higher order modes relative to the fundamental mode. Furthermore, the buried heterostructure should reduce carrier leakage, leading to reduced threshold currents and increased differential efficiencies [5]. The concept of decoupled optical and electrical confinement has also recently been applied to a vertical-cavity surface-emitting lasers (VCSELs) for fundamental spatial mode operation [6], and edge-emitting lasers for reduced threshold currents [7]. The buried heterostructure is created using a quantum-well intermixing (QWI) technique described previously [8]. Point defects are generated at the surface during sputter deposition of SiO ; diffusion of these point defects at elevated temperatures results in the intermixing of the wells with the barriers and a consequent increase in the bandgap energy. Higher order lateral modes are less well confined by the waveguide than the zero order mode; as the order increases the mode overlaps less with the central region of the waveguide. Narrowing the width of the gain region in the center of the waveguide can, therefore, reduce the gain of the higher order modes relative to the fundamental, enhancing stability. For example, at the center of the waveguide the fundamental mode has a maximum, whereas the first-order mode a minimum; therefore, if the gain is tightly confined to the center of the waveguide the first-order mode experiences much smaller gain than the fundamental mode, and hence is less likely to oscillate. This concept is similar to the broad-waveguide wafer design favored for expanding the vertical mode developed for high power lasers [9]; the modal overlap of the first-order vertical mode with the QWs is very small, hence unlikely to oscillate; therefore, the waveguide is usually designed just below the cutoff of the second-order mode. An alternative approach to lateral mode stabilization is to increase the loss in the regions adjacent to the waveguide; higher order modes overlap more with these regions and hence their modal loss relative to the fundamental mode is increased, reducing the likelihood of them oscillating. This approach to modal stabilization was demonstrated in 980-nm GaInAs–GaInP ridge lasers using Si

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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 14, NO. 10, OCTOBER 2002

Fig. 1. Schematic of the GaAs QW ridge waveguide laser using a buried heterostructure defined by QWI. Optical confinement is provided by the ridge, electrical confinement by the buried heterostructure. Individual control of the optical and electrical confinement allows reduced modal gain of higher order lateral modes relative to the fundamental, enhancing lateral mode stability.

Fig. 2. Photoluminescence spectra showing differential blue shift achieved by intermixing. Both the suppressed and intermixed data come from test samples annealed at 875 C for 60 s, as the grown sample was not annealed. Sputtered SiO was used as dielectric for intermixing, whereas PECVD SiO suppressed the intermixing.

as the implant source to increase the free carrier absorption loss [2].

II. DEVICE DESIGN AND FABRICATION Fig. 1 is a schematic diagram of the device. The double quantum well (DQW) separate confinement heterostructure (SCH) wafer was grown by metal–organic vapor phase epitaxy (MOVPE). Two 10-nm GaAs QWs were placed at the center of an Al Ga As waveguide core, surrounded by two Al Ga As cladding layers. Precise alignment of the ridge to the buried heterostructure is difficult with standard photolithography due to the lateral and rotational errors involved, hence a novel fabrication process was developed which self-aligned the ridge and heterostructure. The fabrication sequence was as follows. First a 500-nm layer of plasma enhanced chemical vapor deposition (PECVD) SiO was deposited. Photolithography and pattern transfer using a CHF dry etch were then used to form a stripe in the PECVD SiO ; this stripe formed the intermixing suppressant cap and ridge etch mask. Deposition of 50 nm of sputtered SiO was performed to create the point defects necessary for this intermixing process; intermixing takes place in regions where the semiconductor surface is exposed to this deposition, whereas the 500-nm PECVD SiO is sufficiently thick to suppress intermixing under this cap. Annealing at 875 C for 60 s was performed to diffuse the point defects and intermix the QWs. Some lateral diffusion of point defects takes place, therefore, the buried heterostructure is narrower than the ridge waveguide, and from previous experiments within the department [10], the buried heterostructure is believed to be around 2 m. Fig. 2 shows photoluminescence (PL) spectra from the sample. Successful intermixing is demonstrated by the 48-nm differential blue-shift between the intermixed and suppressed peaks.

Fig. 3. Light–current (L–I ) and lateral far-field characteristics of 5-m-wide standard ridge waveguide laser (600-m-long uncoated facets). Although the L–I characteristic is linear, the far field is poor, indicating instability of the lateral modes, as would be expected from such a wide ridge device.

Dry etching using CHF was then used to remove the 50-nm sputtered SiO layer, leaving the stripe of 500-nm PECVD SiO , which was used as the etch mask during SiCl reactive ion etching (RIE) of the semiconductor ridge. Deposition of a 300-nm thick PECVD isolation layer was performed, followed by opening the contact window and deposition of the p contact. Wafer thinning and deposition of the n contact, followed by contact annealing and device cleaving completed the fabrication sequence. No facet coatings were used. III. DEVICE CHARACTERISTICS Fig. 3 shows the light–current ( – ) and lateral far-field characteristics of a standard 5- m-wide ridge laser. The wafer design was not optimized for low threshold operation, and facet coatings were not deposited, hence the threshold current is

WALKER et al.: HIGH BRIGHTNESS SINGLE-MODE RIDGE LASER UTILIZING BURIED HETEROSTRUCTURE

Fig. 4. L–I and lateral far-field characteristics of 5-m-wide ridge waveguide laser with buried heterostructure (600-m-long uncoated facets). Fundamental mode operation demonstrates the improved lateral mode discrimination and stability.

40 mA. Although the – characteristic is linear, the far-field beam profile is poor, indicating instability of the lateral mode. Similar lasers of width 4 m were processed in the same batch; the waveguides of these lasers were narrow enough to cutoff higher order modes, and hence they exhibited fundamental mode operation up to an output power in the region of 50 mW, beyond which they became unstable. Clearly the 5- m devices are too wide to operate in the fundamental mode; the waveguide supports higher order modes, and the excitation of these causes the far field to deteriorate. – and far-field characteristics of the novel 5- m-wide ridge laser with buried heterostructure are shown in Fig. 4. The threshold current has increased and the differential efficiency reduced slightly. Since this QWI process has demonstrated very low losses [8], this increase in threshold current is likely to be due to a combination of a reduced lateral confinement factor, and losses related to the novel fabrication scheme, though exact analysis of each contribution is difficult to attain accurately. A slight nonlinearity can be seen in the – characteristic, but this is not like the sharp kinks typical of mode instability. Most interestingly, the far-field beam profile shows dominantly single mode operation, and although a slight deterioration can be seen at 500 mA the beam still has one dominant lobe; this is proof that the buried heterostructure helps reject higher order lateral modes. IV. CONCLUSION We have successfully demonstrated a novel single lateral mode ridge waveguide laser benefiting from a self-aligned

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buried heterostructure engineered by QWI. Comparison of far-field beam profiles from the standard and buried heterostructure devices shows the improvement gained. Standard 5- m-wide devices are too wide to operate in fundamental mode; higher order modes have sufficient gain to oscillate. Although the buried heterostructure 5- m-wide ridge devices have similar optical confinement to the standard ridge devices, they benefit from improved lateral mode discrimination due to the narrower width of the gain section; higher order modes have reduced modal gain relative to the fundamental, hence insufficient gain to oscillate and cause lateral mode instability. This clearly demonstrates the advantage of using the buried heterostructure; the ridge width can be increased and the laser still operates in the fundamental mode, a clear benefit for high brightness lasers prone to COMD. Furthermore, it demonstrates the feasibility of the fabrication and QWI processes. Inclusion of facet coatings offers the possibility of reduced threshold currents and higher single-mode power from one facet. REFERENCES [1] D. F. Welch, “A brief history of high-power semiconductor lasers,” IEEE J. Select. Topics Quantum Electron., vol. 6, pp. 1470–1477, Nov.–Dec. 2000. [2] J. K. Lee, K. H. Park, D. H. Jang, H. S. Cho, E. S. Nam, K. E. Pyun, and J. Jeong, “Improvement of kink and beam steering characteristics of 0.98 m GaInAs–GaInP high-power lasers utilizing channel ion implantation,” IEEE Photon. Technol. Lett., vol. 12, pp. 140–142, Feb. 2000. [3] W. C. Tang, H. J. Rosen, P. Vettiger, and D. J. Webb, “Evidence for current-density induced heating of AlGaAs single-quantum-well laser facets,” Appl. Phys. Lett., vol. 59, pp. 1005–1007, Aug. 1991. [4] R. Puchert, A. Bärwolf, U. Menzel, A. Lau, M. Voss, and T. Elaesser, “Facet and bulk heating of GaAs/AlGaAs high-power laser arrays studied in spatially resolved emission and micro-Raman experiments,” J. Appl. Phys., vol. 80, pp. 5559–5563, Nov. 1996. [5] J. P. Reithmaier and A. Forchel, “Focused ion-beam implantation induced thermal quantum-well intermixing for monolithic optoelectronic integration,” IEEE J. Select. Topics Quantum Electron., vol. 4, pp. 595–605, July–Aug. 1998. [6] E. W. Young, K. D. Choquette, S. L. Chuang, K. M. Geib, A. J. Fischer, and A. A. Allerman, “Single-transverse-mode vertical-cavity lasers under continuous and pulsed operation,” IEEE Photon. Technol. Lett., vol. 13, pp. 927–929, Sept. 2001. [7] R. B. Swint, C. Y. Woo, A. E. Huber, S. D. Roh, J. J. Coleman, B. O. Faircloth, and M. S. Zediker, “A novel separate lateral confinement quantum-well heterostructure laser,” IEEE Photon. Technol. Lett., vol. 14, pp. 134–136, Feb. 2002. [8] S. D. McDougall, O. P. Kowalski, C. J. Hamilton, F. Camacho, B. Qiu, M. Ke, R. M. De La Rue, A. C. Bryce, and J. H. Marsh, “Monolithic integration via a universal damage enhanced quantum-well intermixing technique,” IEEE J. Select. Topics Quantum Electron., vol. 4, pp. 636–646, July–Aug. 1998. [9] D. Botez, “Design considerations and analytical approximations for high continuous-wave power, broad-waveguide diode lasers,” Appl. Phys. Lett., vol. 74, pp. 3102–3104, May 1999. [10] A. Saher Helmy, A. C. Bryce, C. N. Ironside, J. S. Aitchison, J. H. Marsh, and S. G. Ayling, “Optical diagnostics of microstructures fabricated using quantum well intermixing,” in MRS 1999 Fall Meeting, Boston, Nov. 29–Dec. 3, 1999.