1. Introduction

Optical data transmission systems are widely used in fiber-to-home systems and interconnections between computers instead of electrical wiring because of their high throughput capabilities. It is expected that they will soon be used in short-reach interconnections between boards and chips [1]. For such applications, cost effective and simple configuration light sources should be employed. Directly modulated laser diodes (LDs), which meet these requirements, however, are approaching their modulation speed operating limits, one reason being due to the damping effect [2, 3]. A transistor laser (TL), which is based on a heterojunction bipolar transistor with an active layer in the base region, is a promising candidate for high-speed light data transmission and could replace LDs [4–6]. This is primarily because the direct modulation bandwidth can be enhanced in TLs because of the fact that the carrier can be pulled from the emitter to the collector. Therefore, carrier flow to the collector occurs in the active region in the base, and this carrier injection into quantum wells (QWs) for lasing is not limited by the time it takes for diffusion to occur. As a result, faster carrier recovery times for the active region can be obtained, and hence, higher modulation bandwidths than those of conventional LDs can be expected for TLs. This is in addition to less suppression of the gain compression effect related to the carrier transport effect [7], whereas other effects such as hole burning [8] or carrier heating [9] still exist. This concept was proposed in 1985 using a GaInAsP/InP bulk crystal as the active region [10, 11]. Recently, the continuous wave (CW) operation of short wavelength GaInAs/GaAs TLs and their properties have been reported [12–18]. However, long wavelength TLs, which are important for optical communication systems, have to be developed. Experimental results, such as wafer design [19], CW operation of an AlGaInAs/InP TL at −185 °C with a threshold base current of 10 mA and a current gain of 0.14 under lasing operation [20] have already been reported for long wavelength TLs. We have reported a room temperature (RT)-CW operation with a pnp configuration with a threshold base current of 17 mA whereas it was not suitable for high-speed operation because of a heavy effective mass of holes in the base region [21].

In this paper, we report the successful development of a 1.3-µm wavelength AlGaInAs/InP TL with an npn configuration and its operation at RT.

2. Device structure and fabrication

The fabrication processes that we employed and the structure of the device that we created are shown in Fig. 1 . An AlGaInAs/InP material system was introduced into the active region because higher optical gain could be achieved and given the fact that it exhibited high temperature characteristics. A buried heterostructure (BH) was introduced to confine the carriers and light within the active region.

 

Fig. 1 Fabrication processes of AlGaInAs/InP npn-TL with BH structure.

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The fabrication process consisted of five cycles of organometallic vapor phase epitaxial (OMVPE) growth as follows. The initial wafer consisted of a 500-nm-thick n-InP buffer layer, a 130-nm-thick n-AlGaInAs layer, five strain compensated AlGaInAs QWs with 5-nm-thick wells (strain: 1.4%) and 10-nm-thick barriers (strain: −0.7%), a 100-nm-thick p-AlGaInAs layer (E_g = 1.1 eV), a 30-nm-thick p-GaInAsP protection layer (E_g = 1.2 eV), which was introduced to prevent oxidization of the Al-containing layer in air before the regrowth process, and a GaInAs cap layer. An 800-nm-high mesa stripe was formed using wet (Br₂/CH₃OH = 1:1000) etching followed by reactive ion etching (RIE) with CH₄/H₂ plasma with a 2.5-µm-wide SiO₂ mask. An ex-situ wet cleaning process using 1:40000 Br₂:CH₃OH, 1:1:40 H₂SO₄:H₂O₂:H₂O, and 1% BHF followed by in situ thermal cleaning at 650 °C in PH₃ for 45 min in the OMPVE system reactor was carried out prior to regrowth. The conditions for thermal cleaning were optimized for high-quality AlGaInAs/InP BH-LD, as described previously [22]. Then, a BH with n(100 nm)/p(200 nm)/n(300 nm)/p(400 nm)-InP current blocking layers was grown. After removing the SiO₂ mask and the GaInAs cap layer, a 100-nm-thick p-GaInAsP base layer and a 100-nm-thick n-InP collector layer were grown across the entire surface.

Next, a 6-µm-wide SiO₂ mask was formed on the stripe and a 220-nm-high mesa stripe was formed by RIE (p-InP exposed) followed by OMVPE regrowth of a 300-nm-thick p-InP, a 30-nm-thick p⁺-GaInAs base contact layer, and a 100-nm-thick n-InP layer. After removing the SiO₂ mask, a 2000-nm-thick n-InP collector layer, a 50-nm-thick n +-GaInAs collector contact layer, and an InP cap layer were re-grown. Then, the collector and base mesa were formed by RIE and wet etching. After forming the collector and base electrodes comprising Ti(25 nm)/Au(200 nm), the backside of the wafer was polished chemically to be about 100-µm thick, and an emitter electrode was formed. Finally, a laser cavity was formed by cleaving without any high reflection and anti-reflection coatings. Clear regrowth interfaces were observed without any voids, as shown in Fig. 2 .

 

Fig. 2 Cross-sectional SEM image of fabricated device.

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3. Device characteristics

First, characterization of the device was performed under two-terinal (emitter-base) configuration with a floating collector. Pulsed current with the pulse duty of 0.1% (1 µs pulse width, 1 ms period cycle) was applied and a high threshold current of 160 mA was obtained. The reason of such high threshold may be that electrons passed through the QWs diffused into the p-GaInAsP base layer laterally and recombined with holes at the outside of QWs, which was observed previously [23]. Next, Characterization of the device was performed while it was in the common-emitter (CE) configuration. In this case, the bias was controlled by the base current I_B. Figure 3(a) shows the I_B dependence of the light output power for various collector emitter voltages (V_CE) under the CE configuration. A pulsed base current with 0.1% of the pulse duty (1 µs pulse width, 1 ms period cycle) was applied, whereas the reversal bias for the collector was a continous wave. The RT pulsed lasing operation of a 1.3-µm-wavelength AlGaInAs/InP pnp TL was achieved at a cavity length of 500 µm and a stripe width of 1.3 µm. The threshold base current was as high as I_Bth = 200 mA at V_CE = 0 because the recombination of holes and electrons occurred outside of the active regions near the base electrode. However, it decreased dramatically with increasing V_CE and reached a constant value of I_Bth = 18 mA where the threshold base current density J_Bth was 2.8 kA/cm² at around V_CE = 1 V. This is comparable to the previously reported values for pnp TLs (J_Bth = 1.9 kA/cm²), whereas a p-type base was used in this case. This phenomenon results from two reasons. One is the forward biased base-collector junction with low V_CE region. The collector voltage dependence was numerically explained previously [24]. The other is the change of the electron flow, which was injected from emitter. With high V_CE, most holes contributed to lasing because the leakage of electrons diffused into the p-GaInAsP base region laterally, which caused the electron-hole recombination at the outside of the QWs, was reduced owing to the carrier pulling effect to the collector. In such cases, the recombination current or the output power of the laser is governed by the base current. New functions such as gating can be expected for this device because the output power can be controlled independently with I_B (or base emitter voltage V_BE) by changing V_CE.

 

Fig. 3 RT pulse measurements for an npn TL with a cavity length of 500 µm and a stripe width of 1.3 µm. (a) Optical output power as a function of base current (black line) under two-terminal configuration and (red lines) common-emitter (CE) configuration for various V_CE (0.1 V steps). (b) Lasing spectrum at a bias current of two times threshold.

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As can be seen for the lasing spectrum at I_B = 2I_Bth in Fig. 3(b), the lasing wavelength was 1275 nm. Figure 4 shows the V_CE dependences for the threshold base current I_Bth and an external differential quantum efficiency for both facets η_d. Corresponding to the decrease in I_Bth, η_d was also improved and reached the linear region for V_CE > 1.5 V.

 

Fig. 4 Collector-emitter voltage dependences of threshold base current and external differential quantum efficiency estimated from Fig. 3(a).

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The current gain β is an important parameter for enhancing the modulation bandwidths of TLs. Figure 5 shows the collector current I_C dependence on the collector emitter voltage V_CE of the same device shown in Figs. 3 and 4. The base current I_B was increased with 5 mA steps under CW drive and a high current gain of β ≈6–9 was obtained. No lasing was observed in this case because this laser was operated only under the pulse condition. This means that there was an absence of current gain suppression due to stimulated emission, which has been reported previously [12]. A negative I_C was observed at V_CE = 0, which implicates the occurrence of an injection of electrons from the collector because of the forward biased collector base electrode. It reached the linear region at V_CE = 1 V for the case of I_B = 20 mA, which corresponds to the constant region of I_Bth ( = 18 mA) in Fig. 4. For the saturation region (V_CE > 1 V), the electron pulling effect in the collector was weak, which resulted in the enhancement of carrier recombination of the holes injected from the base electrode and electrons that passed through the active region. In the linear region, however, base collector reversal bias was high enough to withdraw electrons from the collector. Therefore, holes were injected effectively into the active region without them recombining with electrons in the p-GaInAsP base layer.

 

Fig. 5 Collector-emitter voltage dependence of the collector current with 5 mA steps for the base current to 50 mA under the CE configuration.

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Figure 6 shows the emitter current (I_E) dependence of the light output power and emitter base voltage (V_EB) for various base collector voltages (V_CB) under a common base (CB) configuration. A threshold emitter current of I_Eth = 150 mA was observed for V_BC = 0 V. It should be noted that carrier pulling at the collector occurred under the CB configuration (i.e., linear region) for normal bipolar transistors, and therefore, weak V_BC dependences for output power and threshold current were observed. A change in the resistance from 7 Ω to 11 Ω was observed at the threshold as indicated by the dashed line (V_CB = 1.5 V). It is evident that the reduction in the collector current was due to the stimulated emission (for carrier recombination) because of the shorter carrier recombination time for the base region compared with that for spontaneous emission. The collector current could not be measured under the CB configuration due to a lack of an experimental system. Assuming that the operation occurred within the linear region of the transistor, the collector current at the threshold condition can be estimated to be approximately 7.3-fold of I_Bth (I_Cth ≈I_Eth – I_Bth = 132 mA), which is much higher than that for a reported previously long wavelength TL at 0.14-fold of I_Bth [20]. However, a lower collector current is necessary for TLs because high injection currents cause high heat generation as well as high power consumption. From our calculations, we found that the collector current required to enhance the bandwidth was as low as ~1.8-fold of the base [25]. Therefore, by carefully designing the thickness and band gap energy of the p-GaInAsP base, RT-CW operation could be realized.

 

Fig. 6 Emitter current dependence of the output power and the emitter base voltage under a common base (CB) configuration with a 0.5 V of collector base voltage step. Dashed lines show that V_CB = 0.

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4. Conclusion

In conclusion, RT pulsed operation of a 1.3-µm wavelength npn TL was achieved for the first time by using AlGaInAs/InP quantum wells and a BH structure. A threshold base current of 18 mA was obtained in the linear region under the common-emitter configuration for the device with a stripe width of 1.3 µm and a cavity length of 500 µm. A threshold emitter current of 150 mA was obtained under a common base configuration. This means that lasing and transistor activity were achieved at the same time, which is essential for the high-speed operation of TLs.