The measured I-V curve from a ten period Si/SiGe MQW pin LED fabricated using a UHVCVD system is compared with ATLAS simulation results. A sizable silicon emission peak is observed at high current injection mode at room temperature. This phenomena can be explained as follows: because of the hetero-junction between the top silicon buffer layer and MQW, when bias increases there is a potential barrier formed due to band bending. Thus there will be a large accumulation of holes in the buffer layer. The recombination rate in this layer increases which results in increased silicon light intensity.

Introduction

The use of silicon germanium (SiGe) for optoelectronic components is highly advantageous since SiGe is compatible with Si based technologies. Improved growing techniques for heterostructures have also made the manufacturing of SiGe based devices much easier. Advantages of using SiGe for optoelectronic structures include the low defect density of the material, which enhances operation at room temperatures. Also a SiGe based device’s operating wavelength can be tuned over the range of 1.3um to 1.55um making them ideal choices for optical fiber communications. Therefore there is wide spread interest in SiGe and SiGe based devices. The device studied in this article is a ten period Si/SiGe multi quantum well (MQW) structure. ATLAS is then used to simulate the device and the simulated data is compared to the measured I-V data. In this way a more physical insight into the device operation can be obtained.

Preparation

The device studied in the article utilizes a p-i-n structure with a silicon buffer layer. The sample was grown on n-Si(001) substrates by a UHV chemical vapor deposition (UHV-CVD) system at a pressure of 510^-9 Torr at 600^oC for all of the epitaxial layers. After depositing a 25 nm undoped Si layer on the n+ substrate, the 10 periods consisting of Si/Si_0.5Ge_0.5 making up the MQW structure were grown. Each period of the MQW consists of a 3.9 nm Si_0.5Ge_0.5 well and a 3nm Si barrier. However, because of the background doping of the UHV-CVD, this region was actually lightly p-type doped (NA~1016 cm^-3) and denoted as P- region. After the growth of the MQW, a 24nm undoped Si layer was deposited. Finally, a silicon layer was deposited on top acting as the buffer layer. The top layer is heavily doped p-type (N_A=10¹⁹cm^-3) in order to form an ohmic contact.

The material parameters and the MQW module parameters used in ATLAS are as following:

mqw ww=0.0039 wb=0.003 nwell=10 nx=5 ny=240 acceptors=1e16 \
xmin=0 xmax=0.05 ymin=-0.094 ymax=-0.022 material=SiGe xcomp=0.5

material material=silicon EG300=1.12 affinity=4.05 taun0=1e-7 taup0=1e-7 ni=1e10 \
nc300=2.8e19 nv300=1.8e19 mun=1450 mup=450 vsatn=2.4e7 vsatp=1.65e7 bn=1 bp=0

material material=sige EG300=0.917 affinity=4.033 taun0=4e-11 taup0=4e-11 mso=0.1625 ni=1e12 \
nc300=2.8e19 nv300=1.8e19 vsatn=2.4e7 vsatp=1.775e7 permi=13.95 mun=101.5 mup=400

Discussion

Figure 1 shows a cross section of the device. Figure 2 shows the measured and simulated IV curves. Figure 3 shows the simulated device band diagram at zero bias. The band diagram and corresponding hole distribution for three different injection currents are shown in Figure 4. Figure 5 shows the intensity versus energy for two different injection currents. From Figure 5 we see that for low injection levels the light intensity has two peaks, one for Si and one for SiGe. Both the peaks are somewhat comparable. As the injection level is increased the peak corresponding to SiGe is considerably reduced whilst the peak corresponding to the Si material is increased. Even though excellent confinement is achieved by the quantum wells, the main component in the optical spectrum at high injection levels is not from SiGe, as expected, but from Si.

Figure 1. Sample cross section.

Figure 2. Simulated (red) and measure (green) IV curve.

Figure 3. Band diagram for the device at zero bias.

Figure 4 The simulation results of band diagram and hole distribution with different currents.

Figure 5. Compared EL intensity (a) injection current=50mA (b) injection current=250mA.

Figure 6. Electron and hole distribution.

To try to understand the phenomena simulations were performed to analyse the electron and hole distributions in the structure (shown in Figure 6). For initial injection currents, holes flow into the quantum wells and are stored in them. This in turn promotes a build up of the internal electric field at the interface of the QW / Si buffer regions. This electric field - band bending forms a barrier to hole flow. Figure 6 shows that the hole distribution in the quantum well for injection currents of 50mA and 250mA is almost the same, however in the Si buffer region the hole distribution is increased by a factor of almost five for the two injection currents. As the light emission is dependent on the radiative recombination and the radiative recombination in turn is proportional to the electron - hole product, the Si light emission increases considerably with higher injection currents whilst the SiGe light emission receives little increase.

Conclusion

We successfully simulated a MQW SiGe LED which can enhance Si light emission using ATLAS. This has enabled the underlying physical behaviour of the device to be more thoroughly understood.

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