Simulation of a New Type of Oxide Confined

850 nmVCSEL

S. M. Mitani*, M. S. Alias*, and P. K. Choudhury*sufiangtmrnd.com.my

*Optoelectronic Devices Unit, Telekom Research andDevelopment Sdn Bhd.UPM-MTDC, UPM, 43400 Serdang

Selangor, Malaysia

**Faculty ofEngineering, Multimedia University63100 Cyberjaya, Selangor, Malaysia

Abstract- The VCSEL design presented in this paper cansuitably be operated in the 850 nm region of theelectromagnetic spectrum. The reflection mirrors are made ofAlGaAs layers. A better confinement of light is achieved by theintroduction of an oxide confinement layer. PIC3D software isused to finally obtain various relevant results that greatlyaffect the performance of the device; the paper reports a few ofthem.

I. INTRODUCTION

The present R&D community has shown remarkableadvancements in the design and development of oxideconfined VCSELs that can be used for their operation ondemand [1-4], which primarily include the operatingwavelength of the device. Low threshold currentrequirement and power consumption, single longitudinalmode operation, wafer-scale 2D array integration, simplepackaging, and many other advantageous features areprimarily responsible for the great attraction of such smartdevices in the present era of optical technologies. Most ofsuch devices have well-established sophisticated design andstable high-speed operation [5]. The design anddevelopment of 850 nm VCSELs are of special mention inthis respect, because such devices have reached the level ofstandard technology, and also, these are commerciallyimplemented in the area of LANs [6].The present paperreports the design and construction of an oxide confinedVCSEL that can be operated at 850 nm. Since therequirement of specific DBRs is essential for the control ofthe emitted light, in the present paper, apart from theconstruction of DBRs, the emphasis is given on thefabrication aspect too of the device. In that stream, it isexpected that suitable matching of the lattice parametersalso play an important role in the fabrication of the device.As such, a few low-doped layers are introduced just after thespacer layers in order to achieve a better matching of thelattice constants. Furthermore, the barrier reduction layersare also introduced within the DBRs, and it is expected thatthese will greatly reduce the threshold current requirement

as well as the delamination effect (which has often been aproblem during the multilayer fabrication). PIC3D softwarehas been used to analyze the device in respect of some of thegoverning parameters, and it is observed that the VCSELstructure presents an excellent performance.

II. THE VCSEL DESIGN

The structural design of the VCSEL considered in thispaper is shown in fig. 1, which is the transverse crosssectional view of the half portion of the device. Thedifferent regions of the device are as indicated in the figureitself. As usual, the n-type DBRs are located in the bottompart of the device, which are made of A10.16Gao.84As materialhaving the refractive index (RI) - 3.533. The p-type DBRsin the upper section are constructed of A10.92Gao.08Asmaterial, which has the RI value as - 2.978. Further, thereare thirty layers of n-DBRs with the uppermost two layerswith low doping and the remaining twenty eight layers withhigh doping. Also, among the twenty four layers ofp- DBRs,the lowest two layers are with low doping, whereas theremaining twenty two layers are with high doping.Al0.5GaO.5As material is used for the construction of spacerlayers, and Al0.2GaO.8As is implemented for barrier reductionlayers. The introduction of low doped DBRs just after thespacers results in proper matching of the lattice constants ofthe DBRs and the spacers - a very important aspect to beconsidered for the device fabrication. The barrier reductionlayers are implemented for the reduction of the requiredthreshold current. GaAs layers play the role of multiplequantum well (MQW). An oxide confinement layer issandwiched between the p-type spacer layer and the lowdoped p-DBR; this layer is introduced in a way so that thecentral region of the device remains transparent, and thelight remains confined within this region.

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III. RESULTS AND DISCUSSION

The formation of the standing wave pattern in the VCSELdevice is shown in fig. 2. This, in turn, presents the intensityof the light generated within the MQW structure. This canalso be visualized in fig. 3, where the evolution of light ispresented in the three-dimensional form. This figure showsthe intensity variation of the output light in the x- and y-directions, where y represents the vertical position and xrepresents the lateral distance from the y-axis. We observein fig. 3 that the output light is maximum at the center (i.e.at x = 0), whereas a gradual decrease in intensity is observedwith the increase in x. This feature can also be seen in thetwo-dimensional form as shown in fig. 4.

Fig. 2 Wave intensity in the VCSEL device

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Fig. 4 2D representation of the intensitydistribution

The plot of the left/right round-trip gain with the changein wavelength is shown in fig. 5. We notice that the gain isuniform over a considerable range of wavelength, which isdesirable in order to reduce the presence of noise in theoutput light- an essential requirement for stable operationof the device at the desired operating wavelength. Theseimportant characteristics explicitly demonstrate an excellentconfinement of light in the VCSEL device.

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Single-mode operation of VCSELs has been one of theimportant requirements for a stable operation of the device[7-8]. As such, the intensity of the light must primarilyremain confined within only one operating mode. This isbecause of the fact that the presence of other modes willgreatly affect the intensity of the output light, affectingthereby the performance of the device. For the proposedVCSEL structure in this paper, the modal spectrum within arange of wavelengths is shown in fig. 6. We observe that themaximum intensity of the light is confined within the modethat is dominant at 850 nm wavelength. We also observe theexistence of a few more mode spectral peaks near 800 nmand 900 nm wavelengths. However, the modal intensities atthese wavelengths are too weak to affect the output light at850 nm- the desired operating wavelength. This reflects thepresence of single dominant mode, indicating thereby anexcellent performance of the device. Fig. 7 illustrates thevariation of the material gain with the change in operatingwavelength. We observe different lines in this figure whichcorrespond to the different values of the carrierconcentrations. A negative value of gain is the indication ofthe absorption of light. We notice that, corresponding to thecertain values of concentrations, the material gain becomespositive reducing thereby the absorption of the light by thematerial. The gain is almost completely positive at thedesired wavelength of 850 nm. As such, in order to designthe device, one must consider only those values of thecarrier concentrations which provide positive values of thematerial gain for a particular value of the operatingwavelength.

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Slope efficiency has been one of the important factorsrequired for characterizing VCSELs, and for this, theknowledge of the variation ofpower with current is essential.Figs. 8 and 9 show the variation of the left and the rightoutput power, respectively, against the injection current, forthe operation of the proposed VCSEL at the roomtemperature. We observe that the output power is almostlinear, which essentially represents wide dynamic range ofthe device. Also, the right output power is higher than theleft output power, which is due to the different number ofthe DBRs present in the top and bottom sections of theVCSEL. We observe from these figures that the thresholdcurrent is -1 mA and the slope efficiency is - 0.13 mW/mA,which will also greatly depend upon the operatingtemperature.

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IV. CONCLUSION

In the foregoing analysis, some important features, viz.generation of the intensity pattern, output gain as well as thematerial gain response to the operating wavelength, spectraldistribution of modes, threshold current requirement, andthe spectral distribution of modes, are described for a new

type of oxide confined VCSEL. The construction details ofthe device are also presented. The inference can be drawnthat the proposed VCSEL structure with the oxideconfinement layer provides an excellent performance in the850 nm of the electromagnetic spectrum. However, a very

small deviation from the proposed structure will essentiallyaffect the features of the output light.

ACKNOWLEDGMENT

This work is sponsored by Telekom Malaysia Berhadunder project number R05-0604-0. The authors of this paperwould like to express their gratitude to Mr. MohamedRazman Yahya, Head of Microelectronics &Nanotechnology Program; Dr. Abdul Fatah Awang Mat,Head of Basic Research Division; and Telekom MalaysiaResearch & Development for the technical aid and constantsupport throughout the project duration.

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