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Abstract
Germanium-on-insulator (GOI) has emerged as a novel platform for Ge-based electronic and photonic applications. Discrete photonic devices, such as waveguides, photodetectors, modulators, and optical pumping lasers, have been successfully demonstrated on this platform. However, there is almost no report on the electrically injected Ge light source on the GOI platform. In this study, we present the first fabrication of vertical Ge p-i-n light-emitting diodes (LEDs) on a 150 mm GOI substrate. The high-quality Ge LED on a 150-mm diameter GOI substrate was fabricated via direct wafer bonding followed by ion implantations. As a tensile strain of 0.19% has been introduced during the GOI fabrication process resulting from the thermal mismatch, the LED devices exhibit a dominant direct bandgap transition peak near 0.785 eV (∼1580 nm) at room temperature. In sharp contrast to conventional III-V LEDs, we found that the electroluminescence (EL)/photoluminescence (PL) spectra show enhanced intensities as the temperature is raised from 300 to 450 K as a consequence of the higher occupation of the direct bandgap. The maximum enhancement in EL intensity is a factor of 140% near 1635 nm due to the improved optical confinement offered by the bottom insulator layer. This work potentially broadens the GOI’s functional variety for applications in near-infrared sensing, electronics, and photonics.
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1. Introduction
Over the past few decades, significant efforts have been undertaken to extend the reach of current silicon (Si)-based complementary metal oxide semiconductor (CMOS) electronics technology towards integrated photonics [1]. Monolithic integration of group-IV materials and devices has gained a lot of interest because they are CMOS-compatible [2]. Despite other key photonic devices such as modulators, waveguides, and detectors have been successfully integrated on Si electronics chip [3], the indirect bandgap nature of Si makes it difficult to monolithically integrate a high-efficient light source on the chip. Ge is an alternate and suitable group-IV material based on which the first transistor was invented, as was the first integrated circuit. The direct bandgap edge is just 140 meV above the indirect bandgap edge in Ge, which renders it the feasibility for light-emitting devices. Tensile strain, n-type doping, and the incorporation of Sn atoms are the three main strategies to reduce this energy separation and even to transfer indirect bandgap Ge to a direct bandgap [4–7]. Electrically injected Ge LEDs and lasers operating at room temperature were first demonstrated in nearly 2010 by combining tensile strain (0.2%) and n-type doping [8, 9]. After that, direct-band-gap electroluminescence (EL) from vertical Ge diodes with different structures has been studied extensively on Si substrates [10–13]. However, the large lattice mismatch (>4%) between Ge and Si results in a highly defective Ge/Si interface in the Ge-on-Si (GOS) wafer, which significantly degrades the device performance [14]. In addition, the small mismatch in refractive indices (n) between Ge and Si (n = 4.25 and 3.48 at 1,550 nm) gives rise to weak optical confinement in the GOS platform, which is also unfavorable for photonic devices.
In recent years, Germanium-on-insulator (GOI) has been the subject of intense research as one of the promising platforms for sensing, electronic and photonic applications [15–17]. Due to the defective region on the Ge/Si interface could be removed from the substrate fabrication process, the quality of the Ge on the GOI substrate is significantly improved compared to the GOS wafer [18]. Furthermore, the insulator layer, such as SiN, SiO2, or Al2O3 (n = 2.00, 1.44, 1.62 at 1550 nm), usually has a large n mismatch from Ge and it thus can provide better optical confinement for photonic devices. Additionally, the insulator layer also benefits the electrostatic control of photonic devices by reducing the sheet resistance and parasitic capacitance. To date, discrete photonic devices such as waveguides, photodetectors, modulators, and optical pumping lasers have been successfully demonstrated on the GOI platform [16,19,20]. Especially, thanks to the high material quality and excellent heat dissipation, extremely low-threshold optically pumped Ge-based lasing has been achieved on the GOI/GeSnOI platform via strain engineering [19,21–23]. However, to our knowledge, there are almost no reports on electrically injected Ge p−i−n light sources on GOI platforms. Jain et al. attempted to use strain methods to achieve EL of GeOI platform LEDs but only obtained a weak and insufficient time-resolved pulse peak [24].
Here, we first fabricated the vertical Ge light-emitting-diodes (LEDs) on the 150 mm GOI substrate and observed clear EL spectra under various injected currents and operating temperatures. The direct wafer bonding process has been used to fabricate the 150-mm diameter GOI wafer, which has a threading dislocation density (TDD) as low as ∼1.2 × 106 cm-2. The EL spectra of the LEDs are dominated by the direct bandgap transition peak near 0.785 eV (∼1580 nm) at room temperature. As the temperature is raised above 400 K, the EL spectra show stronger emission intensities due to the enhanced direct bandgap transition. A considerable enhancement in EL was also demonstrated compared with that on the Ge-on-Si (GOS) platform due to an insulator layer that acts as a light reflector developed below the Ge active layer.
2. Material growth and characterization
The fabrication process of the vertical p-i-n structured GOI substrate is similar to that used for Ge photodetectors reported in our previous works [16,25,26]. The Ge film was grown on a 150 mm diameter Si (001) wafer by reduced pressure chemical vapor deposition (RPCVD) to produce a GOS wafer [27,28], which is followed by boron (B) ion implantation to form a p-type doped Ge layer. The insulator layers of SiO2 and SiN were then deposited on the top of the GOS substrate and the Si handle wafer, respectively. Subsequently, the GOS wafer was bonded to the handle wafer at room temperature with the sides of the insulator layers face-to-face. To enhance the bonding strength, an annealing process at 300°C was performed for 3 hours in an inert atmosphere. Grinding, wet etching, and chemical-mechanical polishing (CMP) were used orderly to remove the Si handle wafer and its contacted defective Ge layer (about 200 nm thick), leaving behind a relatively high-quality Ge layer (about 1300 nm thick) on the insulator layer (forming a GOI wafer). After that, an additional O2 annealing process was performed for 4 hours at 850°C to further reduce the TDD by two orders of magnitude [29]. The etch-pit experiments show that the Ge layer of the GOI wafer has a TDD near 1.2 × 106 cm-2 [18], which is less than the good-quality GOS wafers that usually have a TDD near 2 × 107 cm-2 [29]. Finally, the phosphorus (P) ion implantation was used to form an n-type doped Ge layer. At this moment, a GOI wafer with the Ge layer doped in a p-i-n structure was fabricated.
To analyze the bonding and crystal quality of the GOI wafer, transmission electron microscopy (TEM) was used to examine the cross-section of the GOI wafer. Figure 1(a) shows the energy dispersive X-ray spectroscopy (EDS) mapping. One can see that the Ge layer is on top of a SiO2/SiN/SiO2 insulator layer, which is on the Si substrate. The thickness of the Ge layer is 1150 nm. From the weak beam dark field (WBDF) TEM image (Fig.1b) one can hardly observe the usual misfit and threading dislocations, indicating the GOI wafer is of high quality. The B and P doping profiles in the Ge layer were measured using secondary ion mass spectrometry (SIMS) (Fig. 2). Sharp interfaces with concentration dropping can be observed in both the P and B profiles at about 180 and 1100 nm depth, respectively. The relatively thick i-Ge (∼900 nm) is deliberately designed for enhancing carrier radiative recombination. It has been demonstrated that a thicker i-Ge layer increases LED light emission due to the extended travel length of the injected carriers, ultimately enhancing the direct radiative transition [30].
3. Device fabrication and characterization
The Ge LEDs were fabricated on the GOI substrate employing the standard p-i-n photodiode process flow. Figure 3(a) shows the schematic diagram of the p-i-n Ge LED on GOI. To expose the bottom p + -Ge region, top mesas were first formed by reactive ion etching (RIE). Subsequently, a second dry etching process was implemented to develop the bottom mesa. A 400 nm SiN layer was then deposited on the sample as a passivation layer. Finally, Ti (20 nm)/Au (150 nm) was deposited on devices by electron beam evaporation followed by the etching of the contact. As an example of the fabricated device, the top view microscopic image is shown inset of Fig. 3(b).
Fig. 3. (a) 3D schematic diagram of the p-i-n Ge LED on the GOI platform; (b) I-V characteristics of the Ge LEDs. Inset shows the example microscopic image of the device.
The I-V characteristics of the Ge LEDs with a top mesa diameter of 60 µm were measured by a Keithley 2450 source meter unit (SMU). The I-V curve with the voltage from -2 to 1 V is shown in Fig. 3 (b). The LEDs show excellent rectifying behavior with an on/off ratio as high as ∼105 under ±1 V and with a dark current of 10−8 A at -1 V. The dark current density (Jdark) at -1 V is calculated to be 0.51 mA/cm2, which is among the lowest in reported values of Ge photodiode [18,26]. The low dark current is mainly due to the suppression of bulk leakage current thanks to the high crystal quality of the GOI wafer. The current-voltage characteristics of an ideal p-n junction diode are given by I$= {I_0}\left[ {\exp \left( {\frac{{\textrm{qV}}}{{\mathrm{\eta }{k_B}T}}} \right) - 1} \right]$, where I0, q, η, kB, and T are the reverse saturated current, elementary charge, ideality factor, Boltzmann constant, and absolute temperature, respectively. By fitting the slope of the lnI-V fitted line, we deduced the ideality factor η to be 1.21. This low ideality factor indicates the device is close to the ideal PN junction.
EL was first measured by injecting continuous current into the Ge LED device at room temperature. A 20x objective lens was used to collect the EL from the LED surface. Then, the collected light was directed and propagated to a monochromator and finally detected by a liquid nitrogen-cooled linear InGaAs detector array. The electrical probes were used to electrically contact the devices. Figure 4(a) shows the EL of the device under different forward injection currents at room temperature. One could find that a direct band Ge peak near 0.75-0.78 eV is dominated for the EL spectra. Even if the L-valley is below the Г-valley, part of the electrons will transfer from L- to Г-valley from thermal activation. One can also observe that the EL peaks shift to low energy increasing the injunction, which arises from the Joule heating from the electrical excitation [31]. The dependence of the peak on the electric power is shown in Fig. 4(b). By linear fitting the measured point, a peak energy of 0.785 eV at zero electrical power is extracted, representing the EL peak without interference from heating. As the theoretical EL peak of a LED is determined by the Boltzmann distribution, the bandgap energy of the Ge LED could be calculated from the following equation: ${E_{gap}} = {E_{peak}} - \frac{{{k_B}T}}{2}$ . By subtracting the 1/2 thermal energy (0.0129 eV), the bandgap of the Ge LED is obtained to be 0.772 eV. Unlike the direct band energy of 0.800 eV for bulk Ge, a tensile strain of ∼0.19% could be deduced for the Ge layer on GOI according to the deformation potential theory [32]. This tensile strain value is consistent with the shift of the Raman peak (not shown) and our previous research on GOI [25,33]. This strain is a thermal strain resulting from the thermal expansion coefficient mismatch between Si and Ge. On the other hand, a prominent enhancement in EL is observed when the injected current increases, which is the consequence of the increasing amount of the injected carriers. Figure 4(c) shows the monotonic increase of the integrated EL intensities against the current density, indicating that a higher emission could be achieved by further increasing the current.
Fig. 4. (a) Room temperature EL spectra of the p-i-n Ge LED on GOI platform under different injection currents; (b) peak energy as a function of the electric power. (c) Integrated EL intensity as a function of current density.
Optoelectronic devices that can be utilized at high temperatures are essential as they reflect stability in different environments. Due to the Joule heating effect, the maximum operating temperature of devices on Si integrated circuits should reach 150 °C (423 K) [34]. However, the study of Ge LEDs on Si above 350 K is still lacking. Here, we examine the EL of the Ge LED on the GOI platform ranging from 300-450 K, shown in Fig. 5(a) and (c). Unlike traditional direct band III-V LEDs in which efficiency drops sharply at high temperatures, the EL intensity of the Ge LED is enhanced as temperature increases. Figure 5(b) shows that the integrated EL intensity increases from 270 to 340 when the temperature increases from 300 to 450 K. The carriers’ equilibrium at the band edge is according to the Fermi-Dirac distribution. The electrons have more chance to occupy the high energy Г valley at higher temperatures. Moreover, the energy difference between the L and Г valley is also reduced when temperatures increase due to volume expansion [35]. It should be noted that the previous report of the Ge LED on the GOS substrate shows that the EL intensity does not change when the temperature exceeds 200 K due to serious nonradiative recombination from the high TDD (∼ 109 cm-2) [14], the previous report of the Ge LED on the GOS substrate shows EL intensity does not change when the temperature is greater than 200 K. Thus, our EL results indicate the high-quality GOI platform is more suitable for implementing Ge LED devices with higher performance. The PL spectra (Fig. 5(c) and (d)) of the GOI sample under temperatures from 200-450 K also confirm the EL measurements. It could be observed the direct PL peak is gradually dominant when the temperature increase from 200 K, along with the disappearance of indirect emission at 300 K. Due to an increase in carriers in the Г valley, the direct emission intensity increased from 500 to 930 when the temperature was raised from 300 K to 450 K.
Fig. 5. (a) and (b) Temperature-dependent of EL under a fixed injection current of 23 mA; (b) shows the peak energy and integrated intensity extracted from (a); (c) and (d) temperature-dependent of the PL. (d) shows the peak energy and integrated intensity extracted from (c).
Finally, as expected, a red shift of the emission peak due to the band gap shrinkage is observed in EL/PL spectra when temperature increases. Figure 5(b) and (d) show that the EL/PL peak decrease from 0.755 (0.777) eV to 0.718 (0.73) eV as the temperature increases from 300 K to 450 K. The slightly low EL peak energy (∼0.1-0.2 eV) is due to the heating effect as the EL spectra were measured in a relatively high injection current (∼23 mA). The above not only demonstrates the high-temperature stability of the Ge LEDs but also indicates the enhancement of the Ge LED EL emission from increasing operation temperature.
The optical confinement of the Ge LEDs is also studied below concerning the influence of the GOI platform. An insulator layer below the Ge or GeSn photodiode usually causes resonance phenomena [33,36,37]. However, due to the relative thickness of the Ge layers (1150 nm), clear cavity-resonant modes are lacking in our EL spectra. To analyze this further, Finite-Difference Time-Domain (FDTD) simulations were performed on the Ge p-i-n structures on the GOI wafer. For simulation of the reflectivity spectrum, a transverse-electric plane wave was used as the light source and excited on the top of the Ge active layer which has a thickness of 1150 nm. The refractive indices of Ge are measured from an ellipsometer. Figure 6(a) shows the simulated reflectivity spectrum of the GOI wafer between 1200 to 2000 nm.
Fig. 6. (a) Reflectivity spectrum of the GOI wafer with a comparison of EL spectrum at low injection current; (b) calculated LED emission ratio on the GOI and GOS platform between the wavelength of 1500-1700 nm.
The reflectivity spectrum exhibits a wide ripple feature with a free spectra range (FSR) as large as ∼270 nm. As a single FSR almost contains the entire EL emission range (1500 to 1800 nm), the longitudinal resonant cavity modes in this LED could not occur. However, the insulator layer below the Ge active layer has a positive effect on the EL. Coinciding with the reflectivity valley near 1635 nm (Fig. 6(a)), the EL spectra exhibit a shoulder peak here. On the other hand, the EL intensity ratio calculated from FDTD between the GOS and GOI LEDs also indicates an enhancement of EL from the GOI platform. To simulate the emission of the LED, a transverse-electric plane wave was utilized as the light source and stimulated at the central plane of the Ge active layer. As shown in Fig. 6(b), an EL enhancement between the wavelength of 1500-1700 nm with a maximum enhancement factor of 140% near 1635 nm was demonstrated. This result is consistent with our previous work on Ge photodetectors with a thick i-Ge layer [18,26], where a resonant enhanced peak was also not observed. A decrease in the Ge layer thickness will be adopted further to achieve the vertical-cavity surface Ge emitter on the GOI platform. Furthermore, we also observed that the material quality of the GOI is superior to that of common GOS wafers, with the TDD one order of magnitude lower. Thus, the light emission efficiency should also improve due to less radiative recombination. A numerical comparison between the two platforms in terms of light emission efficiency is required and will be conducted in our future work.
4. Conclusion
In conclusion, we have presented the successful electrically injected vertical Ge p-i-n LEDs on a 150 mm GOI substrate. A 0.19% thermal tensile strain introduced from the GOI fabrication process guarantees the devices exhibited dominance of the direct bandgap transition peak at 0.772 eV at room temperature. The temperature-dependent EL spectra reveal the direct emission intensity increases when the temperature increases, in sharp contrast to the Ge LED on the GOS platform with poor crystal quality. The operating temperature as high as 450 K demonstrates the high-temperature stability and good crystal quality of the Ge LEDs. As an insulator layer is below the Ge active layer, an EL enhancement also was demonstrated compared to the GOS platform. By decreasing the Ge layer thickness, the vertical-cavity surface Ge emitters on the GOI platform are expected to be demonstrated further. As the previous low-threshold optically Ge-based lasing all are realized on the GOI platform, the present successful electrically driven Ge emitter further confirms the potential of realizing electric injection laser on the GOI platform. To further enhance the Ge LED performance on the GOI platform, tensile strain, n-type doping, or heterojunction structures will be applied in future work.
Funding
CAS Project for Young Scientists in Basic Research (YSBR-026); Key Research Program of Frontier Sciences, CAS (ZDBS-LY-JSC019); National Research Foundation Singapore (CRP Award NRF-CRP19-2017-01); Ministry of Education AcRF Tier 1 (2021-T1-002-031 (RG112/21)); Ministry of Education AcRF Tier 2 (T2EP50121-0001 (MOE-000180-01)).
Acknowledgments
The authors acknowledge Ms. Xiaohong Yang, Dr. Gang Yih Chong, and Ms. Ling Ling Ngo in Nanyang NanoFabrication Centre for the assistance in PECVD, and sputtering.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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