1. Introduction

Si based light emitting devices are essential to realize all optical devices and on-chip optical communication compatible to Si CMOS technology [1]. The strained silicon germanium technology has proven to be useful for the electronic (both compressive and tensile strain for high mobility transistor) [2–6] as well as optical (tensile strain for light emission) properties [7, 8]. Among group-IV elemental semiconductors, Ge with a direct band gap of 0.8 eV, which closely matches with the wavelength for optical communication, has attracted profound interests for developing Ge based light emitters and photodetectors [8–13]. The direct band gap for Ge is only 0.136 eV above its indirect band gap. This has resulted in the strain induced electron scattering into the direct Γ valley from the indirect one (L valley), for high carrier injection or n-type doping into Ge [14]. Quite recently, there have been a number of reports on the direct band gap light emission from the Ge/Si at room temperature [15–17]. Optical properties of the tensile strained suspended Ge-on-insulator device layers [18], Ge membranes [19, 20] and Ge layer on In_xGa_1−xAs using GaAs (100) substrate [21], for higher degree of strain, has also been studied. Generally, the direct band gap emission has been reported for the p-i-n structures using tensile strained Ge. The metal–insulator–semiconductor (MIS) tunneling diodes can also be utilized for the fabrication of light emitting devices (LED), which are more compatible with ULSI process compared to the p-i-n structure [22]. Liu et al. [22,23] have demonstrated the direct band gap (no phonon) as well as indirect band gap (phonon assisted) related transition, assisted by the huge availability of momentum conserving phonons due to the presence of oxide/semiconductor interface in Si based MISLED devices.

In this paper, we report, for the first time, the electroluminescence characteristics from MIS devices based on in-plane bi-axial compressively strained Ge layer grown on the relaxed Si_0.5Ge_0.5 substrate. The temperature dependent study has shown the direct band gap emission from strained Ge with splitted heavy hole and light hole valence bands up to near room temperature (:275 K). We have achieved the electroluminescence with a very low onset current density (140 µA/cm²) for the emission peak around 0.8 eV. Interestingly, different salient features have been observed with increase in current density, the origin of which has been discussed.

2. Experimental

Compressively strained Ge films were deposited using a solid source molecular beam epitaxy (MBE) (RIBER, France, model SUPRA 32) system on fully relaxed Si_0.5Ge_0.5virtual substrates. Figure 1(a) shows the schematic diagram of the device structure of strained Ge MIS diode. The composition profile of relaxed Si_0.5Ge_0.5 graded virtual substrate substrate is presented in the Fig. 1(b). The Ge concentration in the graded layer varies from 0 to 50% over a thickness of 5.4 µm and the thickness of the constant composition Si_0.5Ge_0.5 is 1.34 µm. The background boron doping concentration in the relaxed Si_0.5Ge_0.5 layer is about 7 × 10¹⁷ cm⁻³. The MBE deposited Ge layer is nominally undoped. The pressure of the growth chamber was maintained at 5 × 10⁻¹⁰ torr using both ion and cryo-pumps. The Ge K-cell was heated up to 1150 °C with a ramp rate of 5 °C/min. The growth rate of Ge was kept fixed at 0.1 Å/s. The substrate temperature was kept low (400 °C) during the deposition of Ge layer on relaxed Si_0.5Ge_0.5/Si (001) substrate, with a view to achieve compressively strained Ge layer. Surface morphology of the samples, the atomic fraction of Si and Ge in the virtual substrate as well as in the films and the strain in the samples have been reported elsewhere [24]. The biaxial compressive strain in the epitaxial Ge film of typical thickness 2.1 nm (corresponding deposition time is 3.5 min) has a value of 0.36%, as measured by high resolution X-ray diffraction scan around Ge (004) and (224) planes. The photoluminescence characteristics of the samples were recorded using a 980 nm diode laser, a TRIAX 320 monochromator and liquid nitrogen cooled extended InGaAs detector. The measurements were done using the basic lock-in technique. MIS diode structures were fabricated using the sample with 2.1 nm Ge layer on relaxed Si_0.5Ge_0.5 substrate. A 4.0 nm thick Al₂O₃, acting as a tunneling layer, was deposited by RF sputtering at a low temperature (100 °C) to prevent the material degradation such as Ge interfacial diffusion and strain relaxation. Thick Al was deposited at the backside of the devices to make the ohmic contact. Semi-transparent circular gold dots of area 0.07 cm² were deposited on top of the samples as the gate electrode and also for the collection of emitted light from the devices. Low temperature electroluminescence (EL) spectra were recorded by a TRIAX 320 monochromator, and the extended InGaAs detector using a standard lock-in detection technique. Sample temperature was varied from 10 to 300 K using a closed cycle He cryostat.

 

Fig. 1 (a) Schematic diagram of the strained Ge/Si_0.5Ge_0.5 MISLED device and (b) SIMS depth profile analysis of relaxed Si_0.5Ge_0.5 graded virtual substrate

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3. Results and discussion

Electroluminescence characteristics of strained Ge layer on relaxed Si_0.5Ge_0.5 using MIS tunneling structure have been studied. When the positive voltage is applied to the Au metal gate, the device becomes reverse-biased and the device is in forward biased for the top gate being negative. The current density as a function of gate voltage of the strained Ge film at 10 K and 300 K is shown in Fig. 2. At 10 K, the reverse bias current density of the Si/Si_0.5Ge_0.5/Ge/Al₂O₃ device is in the order of 0.1 µA/cm² upto 3.0 V, whereas it is quite high (1 mA/cm²) for the same bias at 300 K. For higher temperature, thermally generated electrons control the current in the reverse bias and the current increases with the increase of reverse bias. There is a fluctuation in the current at 10 K below 2.5 V, which might be due to carrier freeze out at a low temperature [25]. It may be noted that the electroluminescence could be achieved only under the forward bias condition at all temperatures.

 

Fig. 2 Current-voltage characteristics of the strained Ge based MIS device at 10 K and 300 K with virtual substrate being grounded

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Figure 3 shows the EL spectra of the strained Ge device at 10 K for different injection current density (J). When the current density through the device is 0.14 mA/cm², the emission onsets around the direct band-gap (0.8 eV) of compressively strained Ge layer. The EL peak intensity becomes stronger with increasing current density. This continues up to 360 mA/cm², then the decreasing trend of the peak intensity is observed and finally vanished above 710 mA/cm². When the current density is increased beyond 360 mA/cm², a new EL peak starts to originate at 1.10 eV (shown in Fig. 3), which is attributed to the TO phonon assisted transition within the Si region. This peak intensity increases with increase in injection current. It can be also observed that above the current density of 360 mA/cm², the development of a base level broad electroluminescence without any clear peak appears. The appearance of a broad peak around 0.69 eV starts only at a very high current density above 1 A/cm². With increase in current density, the peak intensity also gets enhanced. This broad EL emission might be due to the combined contribution of phonon assisted emission in Si_0.5Ge_0.5 and the dislocation related emission from relaxed layer. The dislocations within the substrate create some deep levels acting as radiative recombination centers. These kinds of defect centers giving rise to D1, D2, D3, D4 peaks in the luminescence spectra of SiGe have been well reported in the literature [26–29]. If the dislocation related deep levels merge to form a certain dislocation band within the forbidden gap of Si_0.5Ge_0.5, an electron can decay nonradiatively to this band emitting one or more TO phonons and ultimately recombine with a hole in the valence band to give rise to emission. There might be some contribution from phonon assisted Ge indirect transition as well, since in MIS device, the oxide/semiconductor interface has an effective role to provide momentum conserving phonons.

 

Fig. 3 Electroluminescence spectra as a function of current density at 10 K.

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The EL emission spectrum for low injection level (14 mA/cm²) can be deconvoluted into three EL peaks, at 0.797, 0.813 and 0.876 eV, respectively, as shown in Fig. 4. First two EL peaks at 0.797 and 0.813 eV, respectively, are attributed to the Ge direct band gap transition, the details of which will be discussed later. The third one at 0.876 eV is ascribed to the Si-Ge transverse optical (TO) phonon assisted transition within Si_0.5Ge_0.5 region.

 

Fig. 4 Typical electroluminescence spectrum (at J = 14 mA/cm²) from compressively the strained Ge MIS device at 10 K

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Figure 5(a) presents the band diagram for the MIS device structure at zero bias condition. Several groups have reported that the Ge/Si_0.5Ge_0.5 hetero-structure exhibits the type-II band alignment [30, 31]. For relaxed Si_0.5Ge_0.5 substrate, the valence band is degenerate at Γ point and the conduction band consists of six valleys (∆6 band). Since the grown Ge layer is biaxially strained, the degenerate valence band splits into heavy hole (HH) and light hole (LH) bands. Because of the compressive strain, the HH valence band in the Ge layer determines the valence band maxima, and four fold conduction (∆4) band lies at the minimum energy. Figure 5(b) presents the schematic energy band diagram of the MIS device at an accumulation (forward) bias. Two different mechanisms for light emission in MIS devices might be possible, which have been reported mainly for the Si or Ge nanocrystals. One is the unipolar tunnelling of hot electrons, which causes impact ionization leading to the light emission, and the other one is the bipolar tunnelling of electrons and holes into the same nanocrystals [32].

 

Fig. 5 Energy band diagram for the MIS tunneling device with strained Ge active layer at the (a) zero and (b) accumulation bias.

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The electroluminescence from MIS diode using strained Ge is achieved only under the forward bias (accumulation mode) of the device with a negative voltage at the top gate. If impact ionization mechanism is dominant, emission could be observed under both biasing conditions, as carrierscan be injected for both voltage polarities. On the contrary, holes can be injected into strained Ge only in forward bias, when the electroluminescence is observed. Therefore, the impact ionization mechanism is discarded, and rather the bipolar injection mechanism is considered to be responsible for the electroluminescence (EL). During forward bias, the tunnelling of injected electrons through the oxide leads to the electron-hole radiative recombination within the Ge layer giving rise to emission. The momentum conserving phonons necessary for the indirect band gap (0.876 eV) emission from Si_0.5Ge_0.5 could be compensated by the Ge layer and Al₂O₃ interface roughness scattering and the localized holes owing to the negative gate bias [23]. In this case, not only the electrons tunnel from the Al gate to the effective layer, but also the negative gate bias assists hole injection from the p-type Si substrate. This kind of bipolar injection mechanism for EL has been reported by us previously in our Ge quantum dots based MISLED devices [33–35].

We can now discuss more about the electroluminescence that originates from Ge direct band gap transition. The two peaks at 0.797 eV and 0.813 eV, shown in Fig. 4, are considered to originate due to the Γ→HH and Γ→LH transitions, respectively in strained Ge. To verify the splitting of degenerate hole bands at Γ point, the photoluminescence (PL) measurement has been carried out at 10 K on both strained and relaxed (120 nm thick Ge on relaxed Si_0.5Ge_0.5) Ge sample. Since the absorption length of the laser source is more than the strained Ge thickness, we observe the PL emission from both the surface Ge and the bulk RSG substrate. On deconvolution of PL spectrum of strained Ge sample at 10 K, shown in Fig. 6, a peak at 0.798 eV, the second one at 0.814 eV and a quite separate peak around 0.876 eV are observed. From the analysis of the area under the curve, it is observed that the integrated PL intensity of the Γ→HH is more than the Γ→LH transition. This seems reasonable because the compressive strain lifts the HH valence band above the LH one and the consequent increase in the HH population causes the Γ→HH transition to be stronger. Direct optical transition energies between the Γ conduction-band minima and the HH ($E_g^{\Gamma \to HH}$) or LH ($E_g^{\Gamma \to LH}$) valence-band maxima with and without strain have been reported using standard deformation-potential theory [36]. Using 0.36% in-plane compressive strain in our strained Ge sample, the difference between the values of $E_g^{\Gamma \to HH}$and $E_g^{\Gamma \to LH}$is found to be:20 meV, which is in excellent agreement with our experimental data.

 

Fig. 6 Photoluminescence spectrum of the strained Ge at 10 K deconvoluted into three peaks, due to Γ-HH, Γ-LH transitions in Ge and TO phonon emission in Si_0.5Ge_0.5. The corresponding PL spectrum for 120 nm relaxed Ge on virtual substrate yields two peaks as shown in the inset.

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For the relaxed Ge layer, very weak PL emission without any additional peak on the lower energy side of the 0.814 eV peak is observed, as shown in the inset of Fig. 6. This clearly indicates the occurrence of valence band splitting in compressive strained Ge film. For both cases, the effect of virtual substrate comes through PL peak at 0.876 eV due to Si-Ge TO phonon assisted transition in Si_0.5Ge_0.5 region, in agreement with the EL spectra. The PL spectrum, on the whole, corroborates the EL spectrum at lower current density with respect to all the peak positions (0.797, 0.813 and 0.876 eV) for strained Ge layer. The similarity in PL and EL spectra implies that excited states decay radiatively, independently of the carrier excitation mechanism.

The integrated EL intensity at 10 K corresponding to all three transitions is plotted in Fig. 7 as a function of the current density. Clearly all three peaks have the same nature, first the peak intensity increases as a function of current density followed by a decreasing tendency. Generally, with increase in current, the direct band gap EL intensity increases due to the enhanced ratio of electron transfer from the L-valley into the Γ-valley conduction band of Ge. The reason of the anomaly at high current may be attributed to the fact that the injected carriers are not efficiently captured by the active radiative recombination sites within the Ge layer for the given bias level. So, the EL intensity decreases due to the external field assisted carrier escape from active recombination centers to the defects level positioned at higher energy [37]. The activation energies of these defect levels can be found from the temperature dependent EL measurements. The integrated EL intensity (I_EL) vs. current density (J) characteristics is considered to obey a power law nature,$I_{EL}\propto J^m$, where power exponent m accounts for the influence of defects in the light emission characteristics. For the EL peaks at 0.797 eV, 0.813 eV and 0.876 eV, ‘m’ value is found to be 0.50, 0.33 and 0.39, respectively. This sublinear behavior indicates the presence of nonradiative trap centers in MIS structure involving Ge/Si_0.5Ge_0.5/Si film.

 

Fig. 7 Integrated electroluminescence intensity as a function of current density for three different transitions: TO phonon assisted transition in Si_0.5Ge_0.5 at 0.876 eV, Γ →LH transition at 0.813 eV and Γ →HH transition at 0.797 eV are shown. The fit for the linear regime is shown to extract the exponent ‘m’.

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The evolution of EL peak due to direct optical transition in strained Ge film has been studied as a function of temperature for a low injection current density of 14 mA/cm². The deconvoluted peaks for Γ→HH and Γ→LH optical transitions are plotted in Fig. 8(a). Using linear regression fitting, the temperature coefficients of transition energy of Γ→LH is extracted to be −2.44 × 10⁻⁵ eV/K and for the Γ→HH transition, the above temperature coefficient is −2.38 × 10⁻⁵ eV/K and −5.72 × 10⁻⁵ eV/K in the temperature range of 10–50 K and 80–275 K, respectively. The temperature coefficient for the direct bandgap of bulk Ge was reported to be −4.2 × 10⁻⁴ eV/K within the 200–300 K range [38, 39]. However, the smaller value of the temperature coefficient has also been observed for the n-type tensile Ge film deposited on Si due to the interplay of temperature and strain dependent band gap variation [15]. It may be noted that the temperature coefficient of the optical transitions reported in the literature is from the PL measurements instead of EL data presented here. The full-width at half-maximum (FWHM) values of the direct band gap EL emissions are plotted as a function of temperature in Fig. 8(b). An almost constant FWHM, in the temperature range 10–100 K, reveals that the thermal equilibrium energy distribution takes place. With further increase in temperature, the FWHM is broadened, as a consequence of the electron–phonon scattering mechanism.

 

Fig. 8 (a) EL peak position due to direct gap Γ→HH and Γ→LH optical transitions in compressively strained Ge film as a function of temperature for an injection current density of 14 mA/cm² and (b) variation of FWHM with temperature

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Figure 9(a) presents the electroluminescence spectra from strained Ge film at different temperatures for an injection current density 14 mA/cm². The direct band gap EL intensity decreases with increase in temperature. This EL emission sustains up to 275 K, as shown in the inset of Fig. 9(a). The EL quenching temperature is comparable or superior to that reported for Ge quantum dots grown on patterned substrates fabricated using expensive lithography techniques [40, 41]. However, the TO phonon transition in Si_0.5Ge_0.5 at 0.876 eV is found to be quenched above 25 K. This observation makes compressively strained Ge films attractive for optical emitters at room temperature in near future.

 

Fig. 9 (a) Electroluminescence spectra of strained Ge devices for different temperatures from 10 K to 300 K at an injected current density of 14 mA/cm² and (b) Normalized direct band gap integrated EL intensity as a function of temperature to extract the activation energy.

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Previously, researchers have reported the reduction of EL intensity [9, 15] at lower temperatures for very highly doped Ge. In comparison, the EL studied here is for nominally undoped Ge. From our forward bias I-V characteristics (Fig. 2), it is clear that the forward current reduces only slightly at 10 K from 300 K, keeping the density of injected carriers almost same. On the other hand, the nonradiative recombination probability generally decreases abruptly at a lower temperature. A combination of the above may result in increased EL intensity at lower temperature in the present devices, which is usually observed in case of Ge quantum dots [33].

The integrated EL intensity (I_EL) of the Ge direct band gap related transitions has been plotted as a function of 1/T in Fig. 9(b). Considering the temperature dependent EL intensity follows the Arrhenius type of thermal activation process, the thermal activation energies have been extracted following the relation:

The best fitted experimental data in Fig. 9(b) is obtained with the double thermal activation energy. From the fitting of EL data, E₁ and E₂ are extracted to be 3.6 and 28.0 meV, respectively. Therefore, a combination of carrier delocalization and nonradiative trap centres is considered to be responsible for the thermal quenching of EL intensity.

For Si-based light emitting devices, alternating current light modulation has been proved to be a technique to generate the data stream used for on-chip optical networks [32]. Furthermore, the application of an ac field may be useful to enhance the emission efficiency of the devices. Since the emission around 0.8 eV is useful for optical communication, ac driving as well as dc current is applied to the strained Ge MIS device to study the emission characteristics. Here the results are shown using the sinusoidal waveforms with a fixed peak-to-peak voltage (V_pp) of 2.0 V but with different offset voltages (2.5, 3.0 and 3.5 V). The range of offset values is chosen on the basis of the EL turn-on DC voltage of 2.0 V but keeping below a bias level above which the emission of 0.8 eV starts to decrease. As shown in Fig. 10, for an ac sinusoidal signal of V_pp = 2.0 V with 3.5 V dc offset, the electroluminescence intensity increases with frequency upto 5 MHz, and decreases thereafter. It indicates the enhancement in emission efficiency due to the ac modulation over the dc one.

 

Fig. 10 EL spectra of strained Ge for a sine wave ac current modulation showing the maximum intensity occurs at 5 MHz

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Figure 11(a) shows the integrated EL intensity of direct band gap (:0.8 eV) and Si_0.5Ge_0.5 TO phonon emission as a function of ac frequency for different offset values. The DC level is chosen as 3.571 V, which is the rms value of 3.5 V offset with a V_pp = 2.0 V. The integrated EL intensity is apparently flat initially. The EL intensity corresponding to this flat part generally depends on the voltage offset, and increases with increase in offset value. This is because of the fact that for different offset voltages, different regime of the I-V characteristics is traced during a modulation period. For all the offset values, the particular frequency at which the device exhibits maximum electroluminescence efficiency remains same at 5 MHz. Above 5 MHz, the EL signal decreases rapidly for all the offset values. The direct gap EL intensity is observed to be enhanced by 120% at 5 MHz for 3.5 V offset.

 

Fig. 11 Integrated EL intensity as a function of frequency for (a) Ge direct band gap transition and (b) TO phonon assisted transition in Si_0.5Ge_0.5.

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For DC bias, the charges are accumulated at the interface of the oxide and strained Ge. This results in the screening of the applied electric field by the accumulated charges, causing a decrease in the effective injection. On the other hand, when the device is driven by an ac bias, the charge accumulation is reduced causing the charge injection to be more efficient for light emission. The carriers can move long enough distances to have high probability of recombination up to a certain ac frequency. At very high frequencies (>5 MHz in our samples), the injected charges can traverse only short dissipative paths, which ultimately reduces the electron-hole recombination probability, leading to the reduction of the EL intensity [32]. The ac driven EL characteristics of TO phonon assisted transition (0.876 eV) has also been studied in relaxed Si_0.5Ge_0.5 virtual substrate, which also follow the same nature as shown in Fig. 11(b). The 0.876 eV emission peak also attains the maximum at 5 MHz, having an enhancement by 130% over the DC drive for an offset of 3.5 V.

4. Conclusion

Electroluminescence characteristics of compressively strained epitaxial Ge on virtual Si_0.5Ge_0.5 substrate using metal-insulator-semiconductor structure have been studied as a function of temperature. For low injected current density, the electroluminescence is dominated by the direct band gap optical transition from conduction band Γ valley to strained induced splitted heavy hole and light hole valence bands. A bipolar carrier injection mechanism is found to be responsible for the direct band emission from strained Ge which sustains up to a temperature of 275 K. At higher injected current density, TO phonon assisted transition in relaxed Si_0.5Ge_0.5 and Si substrate dominate in the EL spectra. Above 1A/cm², strong and broad EL emission is observed attributed to dislocation bands in relaxed Si_0.5Ge_0.5 layer. The direct band gap EL emission behavior is in excellent agreement with the photoluminescence spectra of compressively strained Ge films. The EL emission intensity could be enhanced by the ac current modulation upto 5 MHz due to more efficient charge injection in the strained layer. The study demonstrates the possibility of developing strained Ge based LED for optical communication operating at room temperature in near future.