Deep Ridge InGaAsP/InP Light Emitting Transistors (LET) with ~1.5μm light emissions have been fabricated and characterized. In the deep ridge LETs, all the light emissions are from the intrinsic base area, which makes them more suitable for high speed direct modulation. A collector emitter voltage (VCE) dependent output power, which has been predicted numerically, is observed experimentally for the first time and may facilitate the use of LETs in optoelectronic integrations. A novel trend of self-heating related saturation of light power with base current is also observed, which is explained by the three port operation of the device. Further, an abnormal common-emitter current-voltage (I-V) characteristic of the deep ridge LETs is shown and is attributed to the non-radiative recombination centers at the ridge side walls. With the good quality of the quantum wells, laser operation at near room temperature is achieved in the deep ridge LET with 800μm cavity length. With proper surface passivation techniques and device optimizations, performance of the deep ridge transistor based optoelectronic devices can be further enhanced greatly and ultra low power consumption which is highly desirable can be expected.
© 2014 Optical Society of America
Through introduction of multi-quantum wells (MQWs) to the base region of a transistor, efficient light emissions can be achieved from the traditional electrical device, forming transistor laser (TL)  or light emitting transistor (LET) . The novel three port optoelectronic devices have the potential for combined optical and electrical integration, because a light signal can be outputted simultaneously with a electrical signal by taking one electrical input signal . Due to the unique structure, TLs have been shown to process special characters such as gain compress when base current is larger than threshold , voltage controlled mode of operation  and abnormal temperature performance . LETs have also been shown to process superior properties. Because of the transistor structure, the distribution of injected carriers is tilted and the slowly recombining carriers are removed, resulting in a short recombination life time which is comparable with the base transit time . This leads to a 7GHz direct modulation bandwidth of LET , which is much higher than that of a light emitting diode (LED), making LET promising for application in future high speed data communication systems.
Up to now, several types of structures have been proposed for the fabrication of transistor based light emitting devices. The first is shallow ridge structure, in which MQWs are embedded in the base region. GaAs based shallow ridge TLs and LETs with ~1μm wavelength emission have been fabricated successfully [1–4, 6, 7]. The strong light absorption of the heavily doped base material on both sides of the MQW layer and the possible diffusion of dopant into the MQWs, however, damage the light emission properties of the devices seriously, making the fabricated InP based TLs work only at around −190 °C . Sato et al. have fabricated AlGaInAs/InP TLs by a buried emitter ridge structure. Though room temperature working have been realized, the fabrication process, however, is complex, needing at least three epitaxy runs and several precise etching steps [9,10].
To enhance the performance and simplify the fabrication of the device, a deep ridge structure has also been presented [5,11], for which only one epitaxy run is need. The MQW layer in such a structure is placed above the heavily p-doped base layer, and the two layers can be further separated by inserting a setback layer in between. Thus, both the optical absorption of the base material and the possible dopant diffusion into the MQWs can be decreased. With the structure, InGaAsP/InP TLs with 1.4μm emission wavelength have been demonstrated . What is more, because of the good optical and lateral current confinement of the deep ridge structure, LETs or TLs with ultralow power consumption, which are highly desirable for next generation optical interconnects, can be expected .
Although LETs with shallow ridge structure have already been reported, the kind of device is still in its early stage of development. Further study on both the fabrication process and the characterization of the LET is needed and is of both practical and fundamental concerns. In this paper, we report the fabrication and characterization of deep ridge InGaAsP/InP LETs working at around 1.5μm. Some unique properties of the device are presented and discussed. Ways to further enhance the device performance are also suggested.
2. Experimental procedure
The InGaAsP/InP LETs have been grown by an AIXTRON 3 × 2 in. close-coupled showerhead metalorganic chemical vapor deposition (MOCVD) system at 630 °C and a reactor pressure of 100 Torr. The substrates used for the devices are (001) oriented epiready s doped InP (n = 2 × 1018 cm−3). Trimethylindium (TMIn), trimethylgallium (TMGa), arsine (AsH3) and phosphine (PH3) are used as source materials. Disilane (Si2H6) and diethylzinc (DEZn) were used for n-type and p-type doping, respectively.
The epitaxial structure for the fabrication of the deep ridge LETs was grown in a single MOCVD run and is shown in Fig. 1(a), including a 500 nm Si-doped buffer layer (n = 5 × 1018 cm−3), a 200 nm undoped InP collector, a 80 nm undoped quaternary InGaAsP collector with a 1.2 μm emission wavelength (1.2Q), a 200 nm Zn-doped 1.2Q InGaAsP base layer (p = 5 × 1018 cm−3), a 10 nm undoped 1.2Q InGaAsP setback layer, an undoped active layer which consists of 5 InGaAsP QWs with 1.5 μm emission wavelength, a 80 nm undoped 1.2Q InGaAsP upper waveguide layer, a 1200 nm n-doped InP emitter/cladding layer (n = 3 × 1018 cm−3), and a 100 nm n + 1.2Q InGaAsP emitter contact layer (n = 5 × 1018 cm−3). To reduced the group III interstitials in the Si doped buffer layer, which enhance the Zn diffusion into the MQWs, a 15 minutes growth interruption was introduced before the growth of the base layer . Each QW of the active layer has a thickness of 4 nm and is sandwiched by two 8 nm undoped 1.2Q InGaAsP barriers. For comparison, shallow ridge LETs have also been grown, with the material structure shown in Fig. 1(b). Different from the deep ridge LETs, above the 80 nm undoped 1.2Q collector, the unintentionally doped QMW layer is in the middle of the 200nm P doped 1.2Q base layer.
Ridge waveguide edge emitting LETs have been fabricated. While the etching of the ridge stops at the upper base layer for shallow ridge LETs, the etching of ridge terminates in the upper part of the base layer for deep ridge LETs, as shown schematically in Figs. 1(a) and 1(b), respectively. The detailed fabrication process of the devices can be found in reference . The devices have a 2.5 μm wide emitter ridge and were alloyed onto Cu heat sinks for testing. The facets of the devices were left uncoated. An Agilent B2902A power source was used for electrical measurements providing current and bias voltage. The photoluminescence (PL) spectra of the MQWs were obtained after the InP emitter layer and the InGaAsP emitter contact were removed. The optical output power of the devices was measured by an integrating sphere. The Electroluminescence (EL) spectra of the LETs were obtained by measuring the light output from one facet of the devices with a Jobin-Yvon spectrometer.
3. Result and discussion
The PL spectra of the MQWs of the shallow ridge and deep ridge LETs are shown in Fig. 2(a). As can be seen, the PL peak intensity of the MQWs of the deep ridge LET is over 3 times larger than that of the shallow ridge LET. The full width at half maximum (FWHM) of the PL spectrum of the MQWs of the deep ridge LET is 89 nm, which is smaller than the 112 nm of the shallow ridge LET. In a shallow ridge LET structure, the MQW layer are surrounded by he heavily p doped base layer, which absorb the light from the MQWs strongly. During the growth of the InP emitter layer, the p-type dopant Zn diffuse from both sides into the MQW, degrading the material quality of the MWQ greatly. In the deep ridge LET structure, the MQW is above the base layer. Thus the effects of both the light absorption and Zn diffusion are alleviated, leading to the better optical properties. It should be noted that in the deep ridge device structure, the quality of the MQWs is still inferior to the MQWs grown on a normal N substrate, which has a narrower PL emission (60nm FWHM). The material quality of the MQWs can be increased by optimizing the thickness of the setback layer and the growth temperature of the InP emitter layer.
Figure 2(b) shows the optical light output power vs. base current (L-IB) characteristics of LETs with a 800μm cavity length under two terminal configuration (emitter-base with floating collector). As can be seen, though the MQWs of the shallow ridge LET have worse optical properties as shown in Fig. 2(a), the light output power of the LET is slightly higher than that of the deep ridge LET. In the deep ridge LETs, the side walls of the MQW active layer are exposed. Except for SiO2 covering, no passivation process is applied in the devices reported here. There are nonradiative recombination centers at the side walls of the MQW active layer, which results in a lower light emitting efficiency of the deep ridge LETs, leading to the lower light power. The optical spectra of the LETs are shown in the inset of Fig. 2(b). The FWHM of the spectra is larger than 100nm, which is typical for light emitting devices, indicating a spontaneous light emission.
Figure 3(a) shows the L-IB curves at different collector to emitter voltage (VCE) of a deep ridge LET with a 200μm cavity length under common emitter configuration measured at 20 °C. As can be seen, for VCE = 0, the output power of the device increases with the base current in a way similar to a light emitting diode. As VCE is increased and in the range from 0.2V to 1.0V, however, the L-IB curves of the device show two distinct regions. In the first region, the output power increase with the base current with a slope efficiency which is noticeably larger than when VCE = 0. In the second region, the slope efficiency of each L-IB curve decrease abruptly to the value of the curve when VCE = 0. Both the current at which the saturation starts and the saturation power increase with VCE. A similar output power saturation behavior have been predicted in a numerical study of a transistor vertical-cavity surface-emitting laser . For VCE = 0, both the base emitter (BE) junction and the collector base (CB) junction turn on when VBE is increased to about 0.75V as shown in Fig. 3(b). The part of the base current which flows from the base to the collector dose not contribute to the light emission leading to the low slop efficiency of the L-IB curve. As VCE is increased, the turn on of the CB junction is delayed till the ends of the first regions of the L-IB curves. In the first region of the L-IB curves, the whole base current contributes to the light emission, resulting in the much higher slope efficiency. Till the end of the first region, the CB junction starts to turn on. The fraction of the base current which leads to light emission falls to be similar to that when VCE = 0V. As shown in Fig. 3(b), when the CB junction is turned on, the base to emitter voltage (VBE) is clamped, which agrees also with the numerical study . This voltage dependent output power offers the LETs a kind of voltage controlled mode, which may facilitate the use of LETs in optoelectronic integrations.
In Fig. 3(a), the effect of device self-heating on the light emission properties can also be clearly seen. Different from the reported numerical results , the heating effect speeds up the saturation of optical power, slowing down the increase of the saturation current and the saturation power with VCE gradually. As VCE is larger than 1.2V, the increase of the voltage starts to lower down the saturation power. For VCE values of 1.6V and 1.8V, the light output power saturate and then decrease as have been observed in conventional light emitting devices . What unique to the LETs, however, is that the trend of decrease of the light power slows down after the base currents are larger than about 65mA and 75mA, for the 1.6V and 1.8V curves, respectively, as indicated by the dotted lines in Fig. 3(a). This phenomena can also be attributed to the three port operation of the LETs. As can been seen from Fig. 3(b), the CB junction starts to turn on when IB reaches 65mA and 75mA, for 1.6V and 1.8V VCE, respectively. Once the CB junction is turned on, the resistance for IB is decreased from RBE to RBE × RCB/(RBE + RCB), where RBE and RCB are the resistance of the BE junction and CB junction, respectively, when they are in the turn-ON state. With a lower series resistance, the heat generation of the device is decreased, slowing down the decrease of the light power with IB.
Figure 4 shows the L-IB curves of two deep ridge LETs with 800μm and 200μm cavity length, respectively. The curves were measured at 20 °C and 40 °C with VCE = 1.4 V under common emitter mode. Compared to the 200μm LET, the 800μm LET has a higher maximum output power, the slop efficiency, however, is lower. Take the results at 20 °C as an example, for a cavity length of 800μm, the slop efficiency is about 0.5μW/mA (for IB smaller than 10mA) and the maximum output power is larger than 30μW. The slop efficiency and the maximum output power are about 1μW/mA and 23μW, respectively, for the 200 μm device. At 40 °C, a similar trend is observed, though both the output power and slop efficiency are lower than at 20 °C. The common-emitter current-voltage (I-V) characteristics of the two LETs measured at the two temperatures are shown in Fig. 5. VCE is swept from 0 to 1.7V and 1.5V for the 200μm and 800μm LETs, respectively. For all the measurements, IB is varied from 10mA to 60mA with an increment of 10 mA. As shown in Fig. 5, with the same IB, the collector current (IC) of the same LET is larger at 40 °C than at 20 °C, which is complementary to the temperature dependence optical power shown in Fig. 4. At a higher temperature, less electrons are recombined radiatively in the MQWs and more are collected by the collector, leading to the lower optical power and a simultaneous higher IC.
As can be seen from Fig. 4, at 20 °C, the 200μm LET has larger output power than that of the 800μm device when IB is smaller than about 50mA. The current gain (β = ΔIC/ΔIB) of the 200μm LET, however, is also larger than that of the 800μm device. For example, when IB = 30mA, the light power and β are 18μW an 1.1, 14μW and 0.42, for the LET with 200μm and 800μm cavity length, respectively. This is opposite to the change of β with the optical power at different temperatures as discussed above. A contrary trend was also observed in an early study of GaAs base LETs, in which it was found that the LET with larger aperture have higher output power and smaller β . The abnormal trend can be attributed to the non-radiative recombination centers at the side walls of the MQW active layer. A large portion of the electrons from the emitter can be consumed by these centers, decreasing the current gain of the deep ridge InP based LETs noticeably. Because the length of the emitter ridge is much larger than its width, the two LETs has nearly the same perimeter to area (P/A) ratio, which are 0.81 and 0.80 for the 200μm and 800μm LETs, respectively. The number of the non-radiative recombination centers, which is proportional to the length of the device, of the 800μm LET is, however, four times larger than that of the 200 μm LET. This leads to an increase in the ratio of the non-radiative recombination electrons to the total electrons in the 800μm LET. The resulted decrease of current gain counteracts the current gain increase caused by the lower optical output power of the 800μm LET. The effect of the non-radiative recombination can also be reflected by the higher β of the shallow ridge LETs. For example, for a 800μm shallow ridge LET at 20 °C, while the output power is similar, β = 1.4, which is over 3 times larger than that of the shallow ridge LETs under the same measurement conditions. This is because the MQWs are not exposed in the shallow ridge LETs.
With the MOCVD growth procedure as described in the experiment section, a good quality of the MQWs has been obtained. As a result, laser emission near room temperature has been realized for the device with 800μm cavity length, which is a noticeable improvement over our previous devices which work below −40 °C . The L-IB curves of the device measured at different temperatures under two terminal configuration and pulse working condition (2 μs pulse width and 1000 Hz repetition rate) are shown in Fig. 6(a). The stimulated emission spectra (not shown here) above the threshold current are similar to Ref , with a ~1.5μm emission wavelength. As shown in Fig. 6(b), the calculated characteristic temperature of the device is 31K, which is typical for a InP based laser. It is worth mentioning that, though the exposed MQW side walls are inherent to the deep ridge structure as shown in Fig. 1(a), the effects of the non-radiative recombination centers on both the optical and electrical properties of the LETs, however, can be greatly reduced by using proper passivation techniques . For example, (NH4)2S aqueous solution can be used for the passivation of the deeply-etched base-emitter junction by removing the surface native oxides , which are known to introduce nonradiative recombination defects. The passivation procedure is as follows: first, samples are dipped in the (NH4)2S solution for a specific time after base-emitter junction etching. Then, the samples are rinsed in acetone and methanol successively. Finally, the samples are blown dry by Nitrogen after washing, before transferred rapidly for further processing.
The device performance can also be improved by optimizing the structure and material parameters of the device. In the deep ridge InGaAsP LETs, though the Zn diffusion into the MQW layer is reduced by placing the MQW layer above the heavily Zn doped base layer, there is still a large Zn concentration in the MQW layer. Despite that the optical quality of the MQWs in the deep ridge LETs is much better than the MQWs in the shallow ridge LETs, it is noticeably worse than the MQWs grown on a n InP substrate. To reduce Zn in the MQWs further, one way is to lower the growth temperature of the InP emitter. Then, it may also help to grow the material layers below and above the base layer in two separate MOCVD runs. The MOCVD growth chamber can be cleaned thoroughly before the growth of the MQWs. Thus any Zn re-evaporation from the hot surface in the chamber and incorporation into the MQWs can be avoided. It is worth pointing out that carbon is known as a dopant that has a lower diffusivity than Zn and was used in InGaAlAs/InP LETs and TL . To use carbon in InGaAsP/InP LETs, InGaAsP with emission wavelength longer than about 1.35μm should be used, because for InGaAsP with shorter than 1.35μm, carbon introduces a n type doping . A thicker setback layer can be used to reduce the light absorption from the base layer below the MQWs. The possible effects of the layer on the device performance should be considered when optimizing its thickness. With these optimizations, larger output power of deep ridge LETs and a lot lower threshold current [13, 18] of deep ridge TLs can be obtained. Related work are still ongoing.
In the deep ridge LETs, the width of the light emission MQW layer is 2.5μm which is the width of the intrinsic base. In the shallow ridge LETs, however, besides the intrinsic base area, the area that may have light emission includes also the extrinsic base area on both sides of the emitter ridge. In the shallow ridge LETs presented here, the total width of the extrinsic base is 5μm. It has been shown that the high direct modulation bandwidth of the shallow LETs or TLs is originated from the tilted charge population , which is imposed by zero base population density at the boundary of the base and the collector. Such tilted charge population should also be present in the deep ridge LETs or TLs because of the same base-collector structure beneath the MQW layer. The absence of MQWs outside the intrinsic base area may make deep ridge LETs more suitable for high speed direct modulation, because the extrinsic light emission was shown to limit the modulation bandwidth of the shallow ridge LETs noticeably .
It should be finally noted that to increase the repeatability of the fabrication process of the device, a precise control of the etching depth during the etching of the base-emitter junction is desired. By lowering the temperature of and/or diluting the solvent for etching, a lower etching speed of material and thus a better etching control can be obtained. Alternatively, dry etching followed by a wet cleaning process cab be used to control the etching thickness of the base layer more precisely.
Deep ridge InGaAsP/InP LETs with ~1.5μm light emissions have been fabricated. A VCE dependent output power is observed experimentally, which agrees well with the previous numerical prediction. A unique trend of self heating related saturation of the light power with inject current is presented. At relatively high VCE, after the saturation point, the decrease of the light power with IB slows down when the IB are larger than a certain value. This is can be explained by the three port operation of the device. An abnormal common-emitter I-V characteristic of the deep ridge LETs is then shown. It is found that while the LET with a 200μm cavity length has larger light output power than the LET with a 800μm cavity length, the current gain of the 200μm LET is also higher. This is attributed to the non-radiative recombination centers at the ridge side walls. Because of the good quality of the MQWs in the deep ridge structure, laser operation at near room temperature is achieved in the LET with 800μm cavity length. Further improvement of the performance can be obtained by using proper passivation techniques and optimizing the structure and material parameters of the device. Beside a a simple fabrication procedure, with the deep ridge structure, ultralow power consumption of LETs or TLs, which is highly disable, can be expected.
The work is supported by the National Nature Science Foundation of China (NSFC) (Grant Nos. 61274071, 61006044, 61090392), the National “863” project (Grant Nos. 2013AA014502, 2011AA010303), and the National 973 Program (Grant No. 2012CB934202).
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