Ge/Si heterojunction light emitting diodes with 20-bilayers undoped or phosphorus in situ doped GeSi islands were fabricated on n+-Si(001) substrates by ultrahigh vacuum chemical vapor deposition (UHV-CVD). Enhanced room temperature photoluminescence (PL) and electroluminescence (EL) around 1.5 μm were observed from the devices with phosphorus-doped GeSi islands. Theoretical calculations indicated that the emission is from the radiative recombination in GeSi islands. The intensity enhancement of PL and EL is attributed to the sufficient supply of electrons in active layer for radiative recombination.
© 2012 OSA
Room-temperature Si-based light source is one of the most important components for Si-based photonic integration. Although many progresses have been made for silicon-based light emitting in recent years [1–4], it is still a big challenge to overcome the inefficient band-to-band radiative recombination of silicon. The confinement of charge carriers in low-dimensional Ge/Si heterostructures is promising to increase the efficiency of the radiative recombination. In the past two decades, due to their compatibility with CMOS processes, self-assembled GeSi quantum dots (QDs) or islands have been widely studied for Si-based optoelectronic device applications [5–7]. Unfortunately, the insufficient concentration of electrons for radiative recombination in GeSi islands limits its emission efficiency as the band offset at Ge/Si interface is mainly lied on valance band, which can provide good confinement for the holes [8, 9] while electrons are hard to be confined in GeSi islands layer. Many efforts had been made to increase luminescence of GeSi islands/Si(001) multilayer structure [10–13], but the radiative recombination in GeSi islands is still weak. It has been proven that impurity doping improves the light emission characteristics of QDs in III–V materials by providing suitable carriers to increase quasi-Fermi level separation for radiative recombination . However, in Ge/Si(001) system, although impurity doping has applied in the enhancement of areal density, self-organization [15, 16], and infrared photodetector [6, 17], there is only a limited investigation on the light emission enhancement.
In this work, we study the effects of phosphorus doping on the light emission of GeSi islands by fabricating light emitting diodes on in situ phosphorus-doped 20-bilayers GeSi islands multilayer structure on Si substrates. A significant enhancement on the room temperature PL and EL spectra of phosphorus-doped GeSi islands is observed. The improvement and emission mechanism is also discussed.
2. Material growth and device fabrication
The samples were grown by cold wall UHV-CVD on n+-Si(001) substrates with a resistivity of 0.02 Ω cm, using pure disilane (Si2H6) and germane (GeH4). The Si substrates were first cleaned by using an ex situ improved RCA wet-chemical cleaning recipe, and then loaded into the pre-treatment chamber. Before growing, the substrate was degassed at 300 °C in the pre-treatment chamber, and then heated up to 920 °C for 5 minutes in the growth chamber with a background pressure lower than 1 × 10−7 Pa to deoxidize. Next, a 100 nm thick undoped Si buffer layer was grown at 750 °C to obtain a flat starting surface. After a 240s growth interruption to change growth temperature, undoped or phosphorus doped (supplied by diluted PH3) 7 monolayers (ML, 1 ML = 6.27 × 1014 Ge atom cm−2) of Ge was deposited at 520 °C with a rate of 0.056 Å /s. The phosphorus doping concentration is about 1 × 1018 cm−3. After a second growth interruption to change the growth temperature, 17 nm undoped Si space was deposited at 580 °C. After a third growth interruption, the next 19-bilayers were grown in the same way. It is notable that in the middle of the multilayer structure, an extra 30 nm Si space was deposited for further smoothing the surface. 100 nm undoped Si blocking layer and 200 nm Si top contact layer with boron doping (supplied by diluted B2H6) of 1 × 1019 cm−3 were deposited at 580 °C. Samples were grown below 600 °C (except Si buffer layer) to prevent the Si-Ge interdiffusion [18, 19]. The reflection high energy electron diffraction (RHEED) system was used to in situ monitor the growth of GeSi islands and Si spacers. Scanning transmission electron microscopy (STEM) was used to study the GeSi islands’ size and growth behavior in the multilayer structure. The STEM image of the multilayer structure is shown in Fig. 1(a) . The extra 30 nm Si space can be seen cleanly. The average size of the GeSi islands is about 90 nm × 8 nm (base width × height). Details of material growth can be found in our previous work .
Circular mesa with diameters of 100 μm was then fabricated by dry etching down to the n+-Si(001) substrate by using an inductively coupled plasma etcher. After a 700 nm thick SiO2 film was deposited, metal contacts were formed with a 100 nm nickel adhesion layer and a 700 nm thick aluminum layer. The cross-sectional view of the device is shown in Fig. 1(b). The current-voltage (I-V) characteristic of the device was obtained by using Keithley 4200 semiconductor characterization system. PL and EL measurements were performed with LabRam HR 800 Raman instrumentation with InGaAs photodetector within 0.775–1 eV range at room temperature. The photodetector has a low energy cut-off at 0.775 eV, which distorts the signal smaller than 0.785 eV. The PL measurements were using a 488 nm line of Ar+ laser with a power of 1 mW or 10 mW.
3. Experimental results
Room temperature PL spectra of GeSi islands with a laser power of 1 mW are depicted in Fig. 2(a) . Compared to the undoped islands sample, an appreciable PL intensity enhancement was observed for the phosphorus-doped GeSi islands sample. Although, as an impurity, phosphorus dopant in active area (GeSi islands) should have a negative impact for its light emission, the additional electrons provided by phosphorus improve the radiative recombination of GeSi islands and induce this PL enhancement. In order to confirm this effect, three 4-bilayers samples with undoped, phosphorus-doped, and boron-doped GeSi islands were grown without Si cap layer. Boron doping in GeSi islands was to reduce the concentration of electrons in active layer. It is noteworthy that a further reduction of their PL intensity is observed for boron-doped GeSi islands . This phenomenon also suggests that lacking of electrons in GeSi islands limits its emission efficiency and phosphorus doped in GeSi islands benefits its radiative recombination.
Figure 2(b) gives the PL spectra of phosphors-doped and undoped GeSi islands at higher laser power of 10 mW. The two PL spectra have an identical shape, which indicates that doping in the GeSi islands do not change the emission mechanism of the GeSi islands. A clear blue shift is observed compared in Fig. 2(a). The PL peaks are located at around 0.82 eV and 0.83 eV with the laser power of 1 mW and 10 mW respectively. This pump power-dependent blueshift behavior can induce by band-filling effect. This is a characteristic of radiative recombination between conduction and valence band . This PL peak is from the radiative recombination of GeSi islands. The high-energy shoulder (> 0.86 eV) are attributed to the disunity size distribution of GeSi islands  and Si-Ge interdiffusion [7, 24]. Note that the valley in the PL spectra around 0.85 eV is induced by the color filter of instrumentation. The PL spectra of the 20-bilayers phosphorus-doped GeSi islands sample with 10mW laser pump power at different temperatures (30°C~60°C) are shown in Fig. 3 . The measurements were carried out under external control of the sample temperature. A redshift of PL peak energy from 0.83 eV to 0.822 eV between 30°C and 60°C is observed. Theoretically, wavelength-shift would not be observed for luminescence from defects .
In order to confirm the emission energy of GeSi islands, the radiative recombination energy of GeSi islands is given by :25], which calculated from the ratio between the integrated intensities of the Raman peaks corresponding to the Ge-Ge and Ge-Si bonds by Raman scattering measurements [26, 27]. We calculate the energy shift in islands by the well known expression:
The typical I-V characteristic of the devices is show in Fig. 4 . The device with undoped GeSi islands exhibits a lower dark current than that of doped GeSi islands device. Larger dark current in phosphorus-doped device is attributed to higher defect density induced by doping. This effect was also observed in other studies . Although phosphorus doping in GeSi islands decreases the crystal quality, the dark currents of most devices are lower than 30 nA at −1 V reverse bias, which suggest the doped sample still have very high quality. The STEM image (Fig. 1(a)) confirms this viewpoint. It is found in Fig. 1(a) that the GeSi islands have a good vertical correlation. No stacking dislocations or threading dislocations in the multilayer structure is observed.
Room temperature EL results of two devices under 1.1 V forward bias are depicted in Fig. 5(a) . The injection current density is 40 A/cm2 and 32 A/cm2 for undoped GeSi islands device and phosphorus-doped GeSi islands device, respectively. Although, the injection current density of former is a little larger than the latter, the clear EL intensity enhancement was also observed from phosphorus-doped GeSi islands device.
The shapes of the EL spectra from the two devices are almost same. Compared with broad PL spectra, the EL spectra has a narrow emission peak around 0.81 eV (a little redshift than PL peaks) with a lower high-energy shoulder, which are more agreement with the GeSi islands’ emission energy of the previous calculations. This difference between PL spectra and EL spectra is originated from the different pump mechanisms. The photon energy of pump laser is 2.54 eV, which is much larger than the bandgap of Ge and Si. Because of the hot carriers excited by optical pumping, we believe that the carriers excited by optical pumping are more discrete distribution than that of electrical pumping. This mechanism will broaden and blueshift the PL spectra. In other words, electrical pumping more efficient than the optical pumping .
The typical current dependent integrated EL intensities of the two devices are shown in Fig. 5(b). The dependence is characterized by L ~Jm, where L is the integrated EL intensity and J is the current density. The exponent m can be used to characterize the emission mechanism of the GeSi islands. When J<20 A/cm2, the exponent m is about 1.3, exhibiting a superlinear dependence. The superlinear dependence suggests that the radiative processes and the SRH processes are the most important recombination in this situation . When J>20 A/cm2, EL intensity is trend to saturation, which shows that more carriers recombination through nonradiative processes.
In summary, we had observed the enhancement of room temperature PL and EL from the samples with phosphorus-doped GeSi islands. The additional electrons provided by phosphorus for GeSi islands’ emission is responsible for the enhancement. The theoretical calculations and the superlinear relationship of current dependent integrated EL intensity indicated that the emission is from the radiative recombination in GeSi islands. Phosphorus doping in GeSi islands is an effective way to improve the GeSi islands’ emission performance.
This work was supported by National Natural Science Foundation of China (Grant No. 61036003, 61176013, 60906035, and 61177038), the National High Technology Research and Development Program of China (Grant No. 2011AA010302) and by Tsinghua National Laboratory for Information Science and Technology(TNList)Cross-discipline Foundation.
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