Metalorganic vapour-phase epitaxial growth of red and infrared vertical-cavity surface-emitting laser diodes

Microelectronics Journal, 25 (1994) 747-755 !:::all!;

Metalorganic vapour-phase epitaxial growth of red and infrared vertical-cavity surface-emitting laser diodes M.K. Hibbs-Brenner ~, R.P. Schneider Jr. 2,

R.A. Morgan1 R.A. Walterson~ q

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J.A. Lehman I E.L. Kalweit I J.A. Lott 3 K.L. Lear 2, K.D. Choquette 2 and H. Juergensen 4 I

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tHoneyu,ell Tecllnolo~,yCenter, Bloomington, M N 55420, USA 2Sandia National Lalx,rat,,ries, Albuquerque, N M 87185-0603, USA ~nou, with the Air Force Institute of Technol,,gy, Wright-PattersonAFB, Dayton, OH 4AIXTRON Semiconductor Technoh,gies GmbH, Aadlen, Germany

Metalorganic vapour phase epitaxy (MOVPE) is used for the growth of vertical-cavity surface-emitting laser (VCSEL) diodes. MOVPE exhibits a number of important advantages over the more commonly used molecular beam epitaxial (MBE) techniques, including ease of continuous compositional and dopant grading for low-resistance ptype distributed Bragg reflectors (DBlks), higher growth rates for rapid throughput and greater versatility in choice of materials, especially with phosphorus, and dopants. Top-emitting InGaAs/A1GaAs/AIAs 950nm VCSELs exhibit the highest power conversion (wallplug) efficiencies (21%), lowest threshold voltage (1.47V), and highest single mode power (4.4 mW from an 8/ml device)

0026-2692/94/$7.00 © 1994 Elsevier Science Ltd

yet reported. GaAs/ AIGaAs/AIAs VCSELs lasing near 85(Jnm have demonstrated record low threshold voltage (1.7V) at this wavelength, and excellent unifomlity in wavelength (+1%) across a 3-inch wafer, and in threshold voltage (+1.5%), threshold current (+10%) and output power (+20%) across a 34-element array. 650nm VCSELs based on AlGalnP/AlGaAs heterostructures have been demonstrated by MOVPE only, and lase continuous wave at room temperature, with voltage thresholds between 2.5 and 3 V and maxinmm power outputs of over 0.3roW. These results establish MOVPE as a proven growth technique for this important new family of photonic devices.

747

M.K. Hibbs-Brenner et al./MOVPE growth of VCSEL diodes

1. Introduction

ertical-cavity surface-emitting laser (VCSEL) diodes offer many advantages V over conventional edge-emitting lasers, including ease of manufacture into two-dimensional arrays, single longitudinal mode operation with a tight (6-10 ° divergence), circular beam profile, potential for on-wafer testing and planar, batch style device manufacturing. Because of their inherent advantages, many expect VCSEL diodes to revolutionize the use of semiconductor lasers, displacing conventional edge-emitting lasers in a broad range of applications and allowing the development of entirely new applications. Potential applications of VCSELs include printheads for laser printers, readout of optical storage devices, displays, serial and parallel fibre/guided wave optical links, and smart pixels for free space optical interconnects. 980nm lasers provide particular advantages for smart pixels since the GaAs substrate is transparent to the emitted light. 850nm devices are compatible with current and emerging standards for fibre optic data links, and with GaAs- and Si based photodetectors. 650 nm devices are most desirable for plastic fibre data links, printing, displays and optical storage applications. The performance of these devices is a very sensitive function of the optical quality of the material, the control which can be maintained over composition, growth rate and doping, and the degree of flexibility one has in tailoring the composition and doping profile of the interface between layers in the mirror. The optical quality will affect the threshold current and power conversion efficiency. Since the emission wavelength is directly related to a product of the layer thicknesses and refractive indices, approximately 1% control over growth rate and layer composition is required. Control over growth rate and composition will also directly affect the threshold current and power conversion efficiency

748

which can be achieved. Finally, yield will be determined by the across wafer epitaxial layer uniformity characteristics and the wafer-to-wafer reproducibility. As an example of desirable device characteristics, approximate requirements for lasers to be used for data communication links include: (1) low drive current (preferably <10mA); (2) a drive voltage compatible with power supplies commonly available on boards (< 3.5 V for 5 V PECL, < 2 V for 3.3V PECL); (3) operation within specifications over a temperature range from 0 to 70°C; (4) approximately I mW of output power. In order to achieve good yield in a production environment, uniformity and reproducibility of epitaxial material and of the resulting device characteristics is of the utmost importance. It is also desirable that characteristics such as threshold current and voltage, slope efficiency, and optical output power remain as constant as possible as a function of temperature. Aside from issues of device design, the most critical element in achieving the performance characteristics just discussed is the quality and control over the epitaxial material. Metalorganic vapour phase epitaxy (MOVPE) exhibits numerous advantages compared to MBE for VCSEL growth including greater versatility in choice of materials (including the phosphides) for extension of the range of operational wavelengths into the visible and Ilk (1.3-1.5/1m), ease of compositional and dopant grading [3] for low-resistance p-type distributed Bragg reflectors (DBRs), and inherently higher growth rates and scalability for rapid throughput in a manufacturing environment. It has long been apparent that interface abruptness achievable by MOVPE is sufficient for the quantum well layers of these devices, as is evident by the use of MOVPE in the production of both InGaAs/InP and A1GaAs/ GaAs quantum well lasers. One of the primary roadblocks to the develop-

Microelectronics Journal, Vol. 25

ment of IR (850-980 nm) VCSELs has been the high resistance associated with vertical carrier transport through the many heterobarriers present in the distributed Bragg reflectors (DBKs), in particular the p-type DBR. In fact, many of the most important advances in IR. VCSEL development over the past several years have been made possible by advances in the design and growth of DBRs exhibiting low resistance, particularly methods of reducing the effects of the heterobarrier offsets between D B R layers [1,2, 13-17, 23-25]. Possibly the most critical advantage of MOVPE for the growth of VCSELs lies in the flexibility provided by the ramping of mass flow controllers to achieve a specified continuous grading of composition and/or doping throughout the D B R stacks, thus allowing optimal suppression of valence band spikes originating in the heterobarrier bandgap discontinuities between the constituent mirror layers. Continuous parabolic grading between the endpoint compositions should provide the most efficient carrier transport, and is readily accomplished using MOVPE. In spite of these potential advantages, MOVPE has received little attention for the growth ofallepitaxial VCSELs (i.e. those in which one or both of the DBRs are grown epitaxially) [4, 5], presumably because of concerns about achieving the necessary control of growth (to within 1% of target) which is essential for high-reflectivity DBRs, proper alignment of the cavity resonance, and manufacturability. However, Bragg reflector structures similar to those used in VCSELs are now used to fine tune MOVPE processes to calibrate both absolute thicknesses and lateral thickness uniformity. This technique has been used to demonstrate uniformities in growth rate and thickness of + 1 % across the entire wafer area in reactors similar to those employed in the work reported here. This technique has also been used to characterize the AIX 2000 or AIX 2400 multiwafer reactors, that yield the same quality on up to 15 x 2-inch or 5 x 4-inch wafer loads. Progress in in situ diagnostics for MOVPE

promise further enhancements in reproducibility and manufacturability.

2. Experimental All of the structures described in this work were grown in an A I X T K O N 200 (Sandia) or A I X T R O N 200/4 (Honeywell) low pressure metalorganic vapour-phase~ epitaxial growth system with a horizontal quartz tube reactor, graphite susceptor and fast-switching run-vent manifold. Sources used for the 980nm and 650 nm devices include trimethylindium (TMIn), triethylgallium (TEGa), trimethylaluminum (TMA1), arsine (ASH3) and phosphine (PH3). Dopants are Si from Si2H(, for n-type materials, C from CC14 for p-type AIGaAs alloys, and Zn and Mg from diethylzinc (DEZn) and biscyclopentadienyl magnesium (Cp2Mg), respectively, for p-type AIGalnP. Growth pressure is 80-110 mbar, and growth temperatures are typically 750-775°C. Substrates are n + GaAs oriented (100)/6 ° to < 111 > A for most devices, although some results will be presented for devices grown on n + (311)A GaAs wafers. For the 850nm devices the sources used include trimethylgallium (TMGa), TMA1, ASH3, diethyltellurin (DETe) for the n-dopant, and diethylzinc (DEZn) for the p-dopant. Growth pressure is 200mbar, growth temperature is 750°C, and substrates are n GaAs oriented (100)/2 ° to < 110 > . All devices were fabricated using proton implantation to define the lateral extent of the cavity. 3. Results and discussion 3.1 850 nm VCSELs

Figure 1 shows a schematic drawing of a typical VCSEL structure. The structure is a Fabry-Perot cavity with two mirrors consisting of quarterwave stacks of alternating GaAs (or AIGaAs) and AlAs layers, sandwiching an active region. The emission wavelength of the laser is determined by the resonance wavelength of the Fabry-Perot cavity. For optimum performance as a function

749

M.K. Hibbs-Brenner et al./MOVPE growth of VCSEL diodes

region with a mask diameter of 20 #m. The top ohmic contact metal was used to aperture the light emitted from the device and to aid in mode control [26], with the diameter of the metal opening controlling the magnitude of the emitted light. Figure 2 illustrates the performance achieved in a device fabricated with the process just described. The figure plots drive voltage and optical output power as a function of the current passed through the device. The threshold for lasing occurs at 2.1 mA and 1.75 V. An optical output power of 1 m W is achieved at 5.25 mA and 2.13 V. The maximum power output is 3.9 roW. These device characteristics are well within those specified above for output power, drive current and voltage for data communications, and represent state-of-the-art performance for an 850 nm VCSEL.

n-contact Fig. 1. General schematic of an ion-implanted VCSEL structure.

of temperature, this resonance should be located at a wavelength 5-25 nm longer than the wavelength corresponding to the gain peak of the quantum wells. For IR devices, the number of mirror periods in each mirror typically ranges from 20 to 30. In the case of the 8 5 0 n m devices reported here, the mirrors consist of alternating A10.16Gao.~4As and AlAs layers, doped n-type in the bottom mirror and p-type in the top mirror. The composition is graded linearly between the two end-point compositions. The doping is typically le18 c m m the constant composmon layers, increasing to nominally 3 - 4 e 1 8 c m in the graded regions. The doping was further increased in the topmost layers to facilitate ohmic contact formation. The cavity thickness is 1 2 and contains three 7 nm thick GaAs quantum wells. Lateral cavity definition was carried out by proton implantation to form a circular active .

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Figure 3 illustrates the kind of unifomlity and reproducibility which can be achieved in an AIX 200/4 single wafer M O C V D reactor. The points labelled "PL wavelength from active region" correspond to the photoluminescence wavelength emitted from a sample containing GaAs quantum wells but no Fabry-Perot cavity. This shows a variation of approximately 2 nm across the radius of a 3-inch wafer. The other data points represent the emission wavelength from four VCSEL structures grown sequentially. The

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Microelectronics Journal, Vol. 25

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Fig. 5. Reflectance spectra for a 20-period Al..s(;a..sAs/ AlAs red DBP, with 0.1 2 biparabolic interface grading: (--) measured spectra from an MOVPE-grown structure; (. . . . ) calculated spectra. tion to the design o f red D B R s for the lowest possible resistance remains a requisite for efficient performance. The ability o f M O V P E to grow such complex heterostructures precisely is illustrated in Fig. 5, which shows the reflectance spectrum obtained from an M O V P E - g r o w n 20-period red DBlk with 0.1 2 parabolic continuously graded portions between 0.15-2-thick endpoints. Also shown is the calculated spectrum for this structure, taking into account the continuously graded profile. Excellent agreement between the measured and calculated spectra is observed, indicating that M O V P E is capable of growth of even such complex heterostructures with an extremely high degree o f precision. For the best possible carrier confinement and high-temperature operation, we have employed AIlnP confining layers in the active region for the red VCSELs, with a triple strained quantum well active region designed for emission at ~ 660-670 nm. T h e confining layers are 3 2 ( ~ 0 . 8 m m ) thick, on either side of a central 2 2 active region, to alleviate injection problems originating in nonlinear Mg diffusion behaviour

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near the AIInP-A1GaAs interfaces and to take advantage o f the enhanced confinement provided by the AIInP layers [191. T h e DBlks, which include 0.1 2 continuously graded parabolic segments between the end-point compositions, are doped to a nominal carrier concentration o f n, p ~ 2 x 101Scm -3. T h e p-doping is reduced to p ~ 5-10 x 1017 cm -3 over several DBlk pairs nearest the cavity to reduce free-carrier absorption losses, while in the last DBlk period it is increased to p ~ 5-10 x 1019 c m - 3 t o minimize contact and spreading resistance. Conventional gain-guided proton-implanted devices were fabricated as described previously [20]. Optical apertures o f 10-30/~m are defined in the top contact pad, and similarly sized implant apertures control the injected current path into the active region. Although the implant is placed near the optical cavity/DBlk interface on the p-side o f the junction, current spreading in the 3 2 AIInP confining layer is limited because of the relatively high resistance of p-type AIInP. The devices and associated contact pads are isolated with a mesa etch in ~ 7 5 # m squares. Continuous-wave (cw) light-current (L-I) and current-voltage (I-V) curves obtained at room temperature for representative devices are given in Fig. 6. Threshold currents as low as 1.25 mA are observed from 10/am diameter devices, while for devices ~<30pm, power output as high as 0.33 m W is measured. Threshold voltages are in the range 2 . 5 - 3 . 0 V for all devices tested, or as low as ~ 0 . 6 V above the photon energy of about 1.85 V (670 nm). O u t p u t powers in the present devices are limited to some degree by the very high reflectivity of the output-coupling DBlks, which also contributes to the low thresholds measured from the smaller devices. 3.3 980 n m VCSELs

T h e DBlks employed in Ilk (950 nm) VCSELs are similar to those used for the red VCSELs. In this case the requirement for transparency does

Microelectronics Journal, Vol. 25

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geneous materials system used throughout the device structure. Continuous parabolic grading was employed in the p- and n-type DBR.s, as described earlier, but in this case we used a conventional 1 2 optical cavity containing an In0.2Gao.sAs/GaAs triple strained quantum well active region and linearly graded barrier layers (from Alo.2Ga4~.sAs to Al..sGao.sAs). As in the case o f the red VCSELs, the D B R s are doped to a nominal concentration o f n, p ~ 2 × 1()~8cm-3, but reduced to p ~ 5-10 × 1017 cm -3 over several D B R pairs nearest the cavity and increased to p ~ 5 - 1 0 × 1019cm -3 over the top D B R period. Conventional top-emitting gainguided, proton-implanted VCSELs were fabricated from the material as described in greater detail elsewhere [22]. The I R VCSELs were tested under cw injection at r o o m temperature. An L-I and I-V characteristic for a 30/tin device emitting at a wavelength o f 951 n m is given in Fig. 7. The device exhibits a power-conversion efficiency o f 21%,

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not limit the AlxGal _xAs composition range which may be used, allowing higher reflectivity per interface at the risk o f increasing the resistance o f the stack by increasing the valence band offset between layers• O u r approach has been to use essentially the entire AlxGal _.,.As composition range in the D B R s (from GaAs to Alo.96Gao.I)aAs) while at the same time taking advantage o f M O V P E to incorporate continuous parabolic grading between the layers to achieve low resistance. Further details o f Ilk D B R design and characterization will be presented elsewhere [17]. Top-emitting gain-guided Ilk VCSELs were grown and fabricated in much the same manner as the red VCSELs, although the growth is considerably simplified because o f the h o m o -

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753

M.K. Hibbs-Brenner et al./MOVPE growth of VCSEL diodes

the highest yet reported for VCSELs fabricated in any geometry. The laser operates at less than 2 V for most o f the useful range, emitting in excess o f 19 m W before thermal rollover, and a similarly sized device exhibits a threshold voltage o f 1.47 V, a new standard for VCSELs. Furthermore, single-mode powers as high as 4 . 4 m W were measured from an 8/am device, also a new record for Ilk VCSELs. Further improvements in device performance are expected as similar material is fabricated into bottom-emitting structures, which benefit from improved current injection uniformity.

4. Summary W e have exploited many o f the advantages that M O V P E offers for the growth o f vertical-cavity surface-emitting laser diodes, including ease o f continuous compositional grading and greater versatility for both host and dopant materials, and established several performance milestones at both red (670nm) and Ilk (850nm, 980nm) wavelengths. These results establish M O V P E as a proven growth technique for this important new family ofphotonic devices. Critical areas for further development include in situ diagnostics for improved growth control and reproducibility, and advances in the state o f the art for wafer uniformity from several percent to several tenths o f a percent across 2 - 3 inch wafers. Such improvements in the growth process will lead to a truly manufacturable high-performance VCSEL technology.

Acknowledgements The Sandia authors acknowledge the assistance o f E.D. Jones, M.E. Warren, A. O w y o u n g and J.Y. Tsao in useful technical discussions and continued support. Part o f this work was performed at Sandia National Laboratories under D.O.E. contract No. DE-AC04-94AL85000. J.A.L. acknowledges additional support from the Air Force Institute o f Technology, W r i g h t Patterson AFB, Dayton, O H , USA. The

754

Honeywell authors acknowledge the technical contributions o f T. Akinwande, J. Novaha, S. Bounnak and T. Marta. The Honeywell effort was partially supported by the Advanced lkesearch Projects Agency (AlkPA).

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