1. Introduction

Superluminescent diodes (SLDs) with high output power and wide optical bandwidth performance serve as key components for applications of optical coherence tomography (OCT) [1] and low-cost fiber communication systems [2]. In traditional vertical junction (VJ) SLDs with several nonidentical multiple quantum wells (MQWs) [3,4] or quantum dots (QDs) [5,6], the electroluminescence (EL) spectrum is strongly governed by the center wavelength of the QWs or QDs near the p side due to the nonuniform distribution of injected carriers among these MQWs or QDs. Such a phenomenon also seriously limits the maximum available bandwidth of VJ SLDs. The QD-based VJ SLDs [5,6] may achieve better performance of optical bandwidth than that of VJ SLDs, which consist of MQWs, due to the fact that the QDs with nonuniform sizes and quantized states in the same plane effectively broaden the gain spectrum. By use of chirped nonuniform-size QD structures [6], state-of-the-art powerbandwidth product performances of SLDs have been demonstrated at a wavelength of around 1.2 µm. However, as compared with MQW-based SLDs, the performance of such SLDs is more sensitive to the growth conditions of material QDs, which is an important issue for the mass production of such devices. In contrast to VJ SLDs, the structure of transverse-junction stripe (TJS) lasers [7,8] can overcome the problem of nonuniform carrier distribution among different MQWs by utilizing the concentrated transverse flow of injected carriers in the active region [9,10]. Recently we designed and demonstrated TJS light-emitting diodes (LEDs) with a sequence of different emission wavelength MQWs in the active region and with tremendously wide optical bandwidths of 580 nm (1042 to 1622 nm) [9] and 150 nm (988 to 1153 nm) [10] at 1.55 µm and 1.1 µm wavelengths, respectively. In our TJS structure, the distribution of the injected carriers and the intensity of light emission among MQWs with different center wavelengths can be made more uniform due to the lateral injection of a hole from the sidewall of MQWs, which thus improves the optical bandwidth performance [9,10]. In this study we first demonstrate a TJS SLD, which has a series of different In_xGa_1-xAs/GaAs strained MQWs at the center wavelength of around 1.1 µm. This wavelength (~1 µm) is an important window for the application of OCT [11] systems in which a broadband and high-power SLD serves as the key component for improvement of system resolution [1]. In contrast to the amplified spontaneous emission (ASE) spectra of the VJ SLD, the measured center wavelength of our device is located at 1100 nm, which corresponds to the wavelength of QWs buried in the center of the active region instead of the QWs closest to the topmost p-side cladding layer. This result can be attributed to the uniform distribution of injected carriers among different wells and the highest differential gain (1100 nm) of the QWs in the active region [12].

2. Device structure and fabrication process

Figure 1 shows cross-sectional and top views of the device. The epitaxial layers are grown on the (100) surface of a semi-insulating (SI) GaAs substrate by metal-organic chemical-vapor deposition (MOCVD). The strained In_xGa_1-xAs/GaAs of well/barrier layers, which serve as the active region, have different indium mole fractions (x) of 0.17, 0.24, 0.34, and 0.34 nm and thicknesses of 8.1, 7.2, 5.4, and 9.4 nm corresponding to emission wavelengths of 950, 1000, 1050, and 1100 nm, respectively. The different-wavelengths-emission-designed MQW can effectively broaden optical gain spectrum [9,10]. The number of MQWs of each wavelength equals two. The layout and band structure of these symmetric MQWs is illustrated schematically in the inset. The 1100 nm QWs are located at the center of the active region, which is sandwiched between pairs of 400 nm Al_0.2Ga_0.8As core layers and 400 nm Al_0.3Ga_0.7As cladding layers, to achieve a separated-confinement heterostructure (SCH). The GaAs barrier layer is lightly n-doped (1×10¹⁷cm^-3) to suppress lateral disordering in undesired regions during the Zn-diffusion process [7]. Finally, a 100 nm N⁺ doped GaAs layer (>10¹⁹ cm^-3) is capped to form an n-type Ohmic contact layer.

The Zn diffusion and disordering process is adopted to create the p-type region of the transverse p-n junction [7]. A Si₃N₄ film, which serves as the diffusion mask, is used to define the desired n-type region. According to our primary measurement results, the optimum diffusion time and temperature for obtaining the best current-voltage (I–V) characteristics without significant degradation in the output optical power is around 10 m and 600°C. The optical confinements of the TJS device are realized by the fabrication of a 7° bent ridge waveguide (0.7 µm high and 5 µm wide) via selective wet chemical etching of the topmost GaAs contact layer, followed by reactive ion etching. The introduction of this 7° bending waveguide enables us to successfully eliminate the lasing and broaden the emission spectra. To minimize the leakage current in the neighboring p-n junction, each single device is etched down to the S.I. GaAs substrate. After thinning of the substrate, the final 1.2 mm long device (including the 0.6 mm 7° bending and 0.6 mm straight ridge waveguide) is carefully cleaved. No antireflection coating is deposited on the facets.

 

Fig. 1. Conceptual cross-sectional view and picture of the top-view of demonstrated TJ SLD. The x and y directions labeled in the cross-sectional view are used for far-field measurement. The layout of our epi-layer structure is shown in the inset.

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3. Results and discussion

The EL spectra were measured by an optical spectrum analyzer (0.3~1.7 µm, ANDO AQ-6315A). The SLD was operated under different cw bias currents and at three different temperatures [6°C, 16°C, and RT] for power and EL spectra measurement. Figure 2(a) shows the free-space output power of the TJS SLD from the facet of the partially bending waveguide. The current-voltage characteristics are given in the inset. The measured differential resistance is around 8 Ω and is about twice that of a traditional vertical p-i-n junction SLD and similar in geometric size, possibly as a result of the larger spacing between the p-type and n-type electrodes (~20 µm versus <1µm) [7,8] in our TJS structure. The low leakage current (2nA under I–V operation) is due to the high quality of the Zn-diffusion-defined p-n junction and the good isolation between neighboring devices. As can be seen under the same 200 mA CW bias current, the output power is 9.7, 6.9, and 4.5 mW at 6°C, 16°C, and RT operation, respectively. We can clearly see that lowering the operating temperature improves the threshold current and enhances the power performance of the device significantly. The measured-bias-dependent EL optical spectra are very similar for all three operating temperatures. Figure 2(b) shows the spectra measured at 16°C. We can clearly see that under 200 mA bias current, the 3 dB bandwidth is around 20 nm.

 

Fig. 2. (a) Measured free-space output optical power under different cw bias currents and different temperatures (6°C, 16°C, and RT). The inset to (a) shows the I–V characteristic curves of the fabricated device. (b) Shows the traces of the spectra under different bias currents and a fixed temperature of 16°C.

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Figure 3(a) is the measured far-field pattern in the x and y directions, which are specified in Fig. 1, and under cw bias currents of 70 and 200 mA. As can be seen, when the bias current reaches 200 mA (the superluminescent phenomenon occurs), one can clearly see that the intensity profile become narrow in both the x and y directions. A stable spatial fundamental spot with divergence angles of 13° and 34° in x and y directions, respectively, can be obtained under cw 200 mA driving. With the bias current increasing, the characteristic of beam narrowing also accompanies the narrowing of optical spectra. Figure 3(b) shows the 3 dB bandwidths of all the measured optical spectra versus the bias current at these three different temperatures and under cw or pulse mode operations. We can clearly see that as the bias current increases, the 3 dB bandwidth of the superluminescent phenomenon that occurs near the wavelength of 1100 nm gradually narrows.

 

Fig. 3. (a) The far-field intensity profiles of TJ LED and SLD under different cw bias currents of 70 and 200 mA, respectively; (b) FWHMs of the measured spectra vs. bias current under cw and pulse mode operation at three different temperatures (6°C, 16°C, and RT).

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The observed behavior of the bias-dependent EL spectra is different from that reported for VJ SLDs with nonidentical MQWs [3,4] and QDs [5,6] in which the FWHMs of the optical spectra increase with the bias current. In these devices, the injected carriers are concentrated in the QWs near the p side due to the low mobility of injected holes. The peak of their ASE spectra is thus located at the center wavelength of these QWs. When the bias current increases, the excess carriers start to overflow to other MQWs and thus broaden the ASE spectrum. The horizontal current flow in the transverse p-i-n junction that characterizes our demonstrated SLD means that we can enjoy the advantage of a much more uniform spreading of injected carriers among different MQWs. [9,10]. As can be clearly seen in Fig. 2(b), the measured center wavelength of our ASE spectrum is located at 1100 nm, which corresponds to the wavelength of the QWs buried in the center of the active region rather than the QWs closest to the upper-most p-type contact. These measurement results can be ascribed to that with our TJS structure—the distribution of injected carriers is no longer what determines the center wavelength of the ASE spectrum. The 1100 nm QWs, which show the highest degree of strain (indium mole fraction) and differential gain among the four different MQWs (950, 1000, 1050, and 1100 nm), thus dominate the ASE spectrum. In addition, such a QW is located in the center of the active region, where the optical mode field has the highest intensity and the net optical gain can thus be increased. The narrowing of the 3 dB bandwidth with the increase in the bias current, as shown in Fig. 3(b), is usually reported for VJ SLDs with a single wavelength of MQWs, due to the appearance of the lasing phenomenon under high current injection [5,13]. Further improvement in optical bandwidth performance of the demonstrated device may be expected by optimizing the number of MQWs with different wavelengths [14].

In order to minimize thermal effects during cw power measurement, as shown in Fig. 2(a), we performed pulse measurement on our devices. Figures 4(a) and 4(b) show the peak power traces versus peak current pulse (1/100 duty cycle and 10 µs duration) for the three fixed temperatures of 6°C, 16°C, and RT, as well as the spectra measured under different pulse currents at 16°C. The 3 dB bandwidths of these measured optical spectra narrow as the bias current increases [see Fig. 3(b)]. This is similar to the behavior of the device under cw measurement. We can clearly see that under pulse measurement, thermal effects have little influence. The maximum output power at RT is improved from 5 mW to 39 mW with a 12 nm optical 3 dB bandwidth. The achieved power-bandwidth product performance of our primarily demonstrated device is comparable with that of commercial products [15] at the same center wavelength (~1.1 µm).

 

Fig. 4. (a) Measured free-space output optical power under different pulse bias currents and different temperatures (6°C, 16°C, and RT). (b) Shows the traces of the spectra under different bias currents and a fixed temperature of 16°C.

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4. Conclusion

In summary, we demonstrate a novel superluminescent diode fabricated using the transverse junction structure and a series of MQWs in a separated confinement heterostructure. Our prototype is different from the VJ white-light SLD in that the uniform distribution of injected carriers among different MQWs is improved. The occurrence of the superluminescent phenomenon is, therefore, dominated by the QWs having the highest differential gain and optical confinement factor in the active region rather than the QWs closest to the p side.