Strain-Induced Enhancement of Electroluminescence from Highly

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Strain-induced enhancement of electroluminescence from highly-strained germanium light-emitting diodes Jialin Jiang, Muyu Xue, Ching-Ying Lu, Colleen Shang Fenrich, Matthew Morea, Kai Zang, Jianfeng Gao, Ming Cheng, Yi Zhang, Theodore I. Kamins, James S. Harris, and Junqiang Sun ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b01553 • Publication Date (Web): 20 Mar 2019 Downloaded from http://pubs.acs.org on March 21, 2019

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Strain-induced enhancement of electroluminescence from highly-strained germanium light-emitting diodes Jialin Jiang,† Muyu Xue,‡,§ Ching-Ying Lu,§ Colleen S. Fenrich,‡ Matthew Morea,§ Kai Zang,§ Jianfeng Gao,† Ming Cheng,† Yi Zhang,† Theodore I. Kamins,§ James S. Harris‡,§ and Junqiang Sun†* †Wuhan

National Laboratory for Optoelectronics, School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan, 430074, China ‡Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA §Department of Electrical Engineering, Stanford University, Stanford, CA 94305, USA *Corresponding author: Junqiang Sun, E-mail: [email protected] Keywords: strain engineering, strained germanium, group IV light source, electroluminescence, LED, lateral junction Abstract The full exploration of Si-based photonic integrated circuits (PICs) is limited by the lack of an efficient light source that is compatible with the complementary metal-oxide-semiconductor (CMOS) process. Highly-strained germanium (Ge) is a promising solution as its band structure can be fundamentally altered by introducing tensile strain. However, the main challenge lies in the incorporation of an electrical structure while maintaining high strain with uniform distribution in the active region. Here we present highly-strained Ge LEDs driven by lateral pi-n junctions and report the strain-induced enhancement of electroluminescence (EL) from Ge. Raman characterization shows that 1.76% strain along the direction with relatively uniform strain distribution is achieved. The observed strain-induced red-shifts of EL spectra agree well with the theoretical prediction, revealing that the direct band gap of Ge can be tuned in the range of 0.785 eV (1580 nm) ~ 0.658 eV (1885 nm). This work offers a pathway towards a strained Ge laser with low threshold current, as well as open possibilities for new types of optoelectronics devices based on strain engineering.

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The photonic integrated circuits (PICs) technology plays a crucial role in the information industry owing to its wide applications in optical communications,1 signal processing2, 3 and sensing.4 Silicon is considered an excellent platform to implement PICs since the low-cost production and even the convergence of electronics and photonics5 can be achieved using the mature CMOS technology. During the past two decades, Si-based PICs have made tremendous progress involving the development of high performance modulators,6 photodetectors7 and a variety of passive components.8 However, the indirect band-gap nature of Si makes it challenging to realize an efficient on-chip light source, necessitating the incorporation of other luminescent materials. It has been demonstrated that the group III-V compounds, well-known gain materials for semiconductor lasers, can be heterogeneously integrated on Si platforms by wafer bonding9 or direct epitaxy.10 Although III-V on Si light sources have achieved desirable performance and seem to be a reliable solution, they are not fully compatible with the CMOS process. This limits their high-volume production capability and increases cost. On the other hand, the group IV materials Ge and germanium-tin (GeSn), which are inherently CMOS-compatible, have received considerable research interest as their indirect band structures can be converted into direct band gap with tensile strain or Sn alloying.11-15 According to the theoretical results, the indirect-to-direct crossover occurs at 5.6% strain under uniaxial stress16 or 1.7%-1.9% strain under biaxial stress17 for Ge and 6.5-11% Sn composition for GeSn alloys.13 Recently, low-temperature optically-pumped lasing from bulk GeSn13 and electroluminescence (EL) from GeSn multiple quantum wells (MQWs)15 have been demonstrated. However, the material quality needs to be further improved to realize a practical device. The early optically-pumped and electrically-driven Ge lasers were realized by combining biaxial tensile stress with an in-plane strain of ~ 0.2% and heavy n-doping.18, 19 Owing to the relatively low strain, the first demonstration of a Ge laser suffered from a threshold current density as high as 280 kA/cm2. Subsequent investigations have been focusing on introducing high tensile stress in Ge to reduce the energy difference between the direct Γ-valley and the indirect L-valleys. As a result, a larger fraction of electrons can occupy the Γ-valley where efficient direct band gap recombination occurs such that the threshold current of Ge laser can be reduced. Several strain technologies for Ge have been demonstrated, including fabricating Ge micro-structures12, 17 and depositing stressor layers.20, 21 Leveraging these strain techniques, strain-induced enhancement of photoluminescence (PL)12 and low-temperature optically pumped lasing have been demonstrated.22, 23 Although the state-of-the-art strain technologies have achieved a strain of 4.9% under uniaxial stress24 and a strain of 1.9% under biaxial stress,17 it is challenging to incorporate an electrical structure while maintaining high strain with uniform distribution which is critical for optoelectronics devices like laser, modulator and detector.25 On the other hand, while there have been demonstrations of strained Ge LEDs,25, 26 the enhancement effect of strain on EL was only confirmed at very low strain level (~0.35% biaxial strain).27 Hence, a clear experimental evidence at high strain level that can serve as a preliminary proof-of-principle for the strained Ge laser with low threshold current is in demand. In this paper, we demonstrate room-temperature EL from highly-strained Ge LEDs with a lateral p-i-n junction configuration. Up to 2.15% strain under uniaxial stress is introduced in the active region exploiting the micro-bridge structure. Raman mapping measurement indicates that uniform strain distribution can be achieved by properly designing


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the geometrical parameters of the device. The theoretical model for evaluating the effects of strain on EL is presented. The observed strain-induced enhancement of EL intensity and red shifts of emission spectra are in good agreement with the theoretical prediction. Device design and fabrication We employed a micro-bridge structure to introduce high tensile stress in Ge considering its ability to achieve relatively uniform strain compared with the SiNx stressor technique. As illustrated in Figure 1, the structure consists of a narrow bridge (hereinafter bridge) in the center connected to a pair of much wider pads on both sides. After releasing the pre-strained membrane from the substrate by undercutting the Si layer, the wider pads tend to shrink, resulting in high uniaxial tensile stress in the narrow bridge. The magnitude of the stress in the bridge can be easily tuned by adjusting the dimensions of the bridge and pads, implying that bridges with different strain levels can be fabricated using a single lithography step. In this work, we designed 4-μm-wide bridges with lengths of 5 μm and 10 μm. The i-region of the lateral p-i-n junction was designed to be in the center of the bridge where the strain distribution is relatively uniform. The lengths of the i-region for the 5-μm-long and 10-μm-long bridges are 3 μm and 5 μm, respectively. The lateral junction configuration features high design flexibility as the lithographically defined doping profile can be handily adjusted for various purpose.

Anode

p-region

n-region

Cathode

SiOx i-Ge Si SiO2 Si

Figure 1. Schematic of the strained Ge LED with lateral p-i-n junction (not to scale). 1. Ion implantations

10. SiOx partially removal

2. SiOx deposition, + RTA

9. Si wet etching

3. SiOx removal +Dry etching

8. Opening wet-etching window

4. SiOx insulation layer deposition

7. SiOx protection layer deposition

5. Contact opening

6. Metal deposition

Figure 2. Fabrication process flow of the strained Ge LEDs

Figure 2 depicts the fabrication process flow of the strained Ge LEDs. The devices were fabricated on a silicon-on-insulator (SOI) substrate that includes a 15-μm-thick top p--Si(001) layer with a resistivity of 10~50 Ω∙cm and a 1-μm-thick buried oxide layer (BOX). Nominally


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undoped Ge with a thickness of 1 μm was directly grown by the two-step growth technique using a reduced-pressure chemical vapor deposition (RPCVD) system. First, a 60-nm-thick low temperature Ge buffer layer was deposited at 400 °C followed by 440 nm high quality Ge grown at 600 °C. Then, 20 min hydrogen annealing at 800 °C was carried out to reduce the surface roughness and the threading dislocation density. Next, the deposition-anneal process was repeated to achieve the targeted thickness. The threading dislocation density (TDD) in Ge is ~2×107 cm-2 which was estimated by the etch pits measurement (see Supporting Information). BF2+ implantation at an energy of 50 keV with a dose of 2×1015 cm-2 and phosphorus implantation at an energy of 60 keV with a dose of 2×1015 cm-2 were conducted to form the pand n-regions. To prevent dopant out-diffusion during annealing, a 150-nm-thick silicon oxide (SiOx) layer was deposited by plasma-enhanced chemical vapor deposition (PECVD) at 350 °C. The samples were then subjected to 650 °C rapid thermal annealing (RTA) in N2 atmosphere for 1 min to activate the dopant as well as repair the crystal damage induced by ion implantation. The electrically-active concentration of the p-type dopant after the annealing process is on the order of 1020 cm-3, estimated by four-point probe sheet resistance measurements. For the n-type dopant, the electrically-active concentration is on the order of 1019 cm-3 due to the high diffusivity of phosphorus in Ge.28, 29 After etching the SiOx using 50:1 hydrofluoric acid (HF), the micro-bridge patterns were formed by reactive ion etching (RIE) using NF3 gas. Next, 120 nm SiOx was deposited by PECVD at 350 °C for electrical insulation. Contact windows were opened using 20:1 buffered oxide etchant (BOE). After that, the contact metals (15nm Cr/180 nm Au) were deposited in an electron beam evaporation system followed by standard lift-off process. It should be noted that Ge with Cr/Au metal contacts can be etched electrochemically in tetramethylammonium hydroxide (TMAH) solution which is a conventional Si etchant and normally does not react with Ge (see Supporting Information). To eliminate the electrochemical effect, the metals were covered by 200 nm SiOx cap layer, such that the current path for the electrochemical cell was blocked. Next, the Si wet etch window was opened using 20:1 BOE. The structures were released from the substrate by etching away the underlying Si layer in 5% TMAH at 80 °C. Finally, in order to expose the metal contacts, the 200 nm SiOx cap layer was removed using 20:1 BOE. All the fabrication processes of the strained Ge LED are CMOScompatible except for the use of gold metal contact. This problem can be solved by using the germanium-on-insulator (GOI) platform since HF vapor etching can be leveraged for the undercutting process, which allows for the use of Al metal contact. Characterization methods The strain in the micro-bridge was characterized by confocal Raman spectroscopy using a commercial Horiba Labram HR Evolution System. A 532 nm laser source was focused on the sample through a 100x objective with a spot size of ~0.8 μm. The laser power was attenuated to 0.5 mW to minimize thermal effects. The back-scattered light was collected by a 100x objective and then dispersed by a high-resolution grating (1800 mm-1). The obtained Raman spectra were fitted using a Lorentz line shape. For uniaxial stress along the direction, the Raman peak shifts with respect to the relaxed Ge were translated into strain through the nonlinear relationship30 ε100=0.68Δω−0.019Δω2. For in-plane biaxial stress, we used a linear strain vs. relative Raman shift function30 ε100,010=0.23Δω.The step size of the Raman mapping is 600 nm.


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The current density-voltage characteristics of the devices were measured by a Keysight B1500A Semiconductor Device Analyzer equipped with a probe station. The EL characteristics of the strained Ge LEDs were measured using a home-built micro-electroluminescence (μ-EL) setup in a phase-sensitive lock-in detection scheme. A 1 kHz square wave with a duty cycle of 50% was generated from a function generator (Agilent 33120A) to drive the device. The EL signal was collected by a 20x objective (NA=0.4) and then dispersed by a grating monochromator with 5 nm spectrum resolution. An extended InGaAs detector cooled to -20 °C with a cut-off wavelength of ~2.4 μm was utilized to convert the dispersed light signal into electrical signal which was connected to the lock-in amplifier (Stanford Research SR830) together with the reference signal from the function generator. The system response of the μEL setup was calibrated by a tungsten halogen lamp at 2960 K. All the luminescence data presented in this paper have been corrected using the obtained system response. Theoretical model of strain-induced enhancement of EL To estimate the effects of strain on EL, we calculate the spontaneous emission rate using the joint density of state (JDOS) model. First, the band structure of strained Ge is calculated by the 8 band k∙p method (see Supporting Information for details). After obtaining the energies of band edges and effective masses, the spontaneous emission rate per unit volume per unit energy intervals can be expressed as rs (h )  nr  q 2 2   hm2 c3 eˆ  pcv r (h  Eg ) fc (k0 ) 1  fv (k0 )  0 0

(1)

where nr is the refractive index. ω is the angular frequency of the photon. ε0 is the permittivity in vacuum. q and m0 are the charge and mass of electron, respectively. c is the speed of light in 2 vacuum. eˆ  pcv is the squared momentum matrix element. χ denotes different transition processes including Gamma to the first valence band (Γ-VB1) and Gamma to the second    valence band (Γ-VB2) transitions. Eg is the Γ-VB1 or Γ-VB2 energy gap.  r (h  Eg ) is 



the reduced joint density of state. f c (k0 ) and f v (k0 ) are the Fermi-Dirac functions of conduction band and valence band, respectively. As pivotal factors for the carrier distribution status, the quasi-Fermi levels Efc and Efv can be obtained by solving the following equations 

 ( E )

Ec

1  exp ( E  E fc ) / k B Tk 

ne   

L (E)

EcL

1  exp ( E  E fc ) / k B Tk 



nh  

EVB 1





EVB 2



1  exp ( E fv  E ) / k B Tk 

1  exp ( E fv  E ) / k B Tk 

(2)

dE

VB1 ( E ) VB 2 ( E )

dE

dE

(3)

dE

where ne and nh are the concentrations of electron and hole, respectively. kB is the Boltzmann constant. Tk is the temperature and is set to be 300 K. The material parameters used in the calculation are given in Table S1 of Supporting Information. Results and discussions


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The as-grown Ge is slightly biaxially tensile-stressed with an in-plane strain of ~0.16%, which was characterized by Raman spectroscopy. Figure 3a and 3b present scanning electron microscope (SEM) images of the fabricated strained Ge LED and the active region of the device, respectively. As illustrated in Figure 3c, Raman mapping measurement reveals that the iregions of the 5-μm-long and 10-μm-long bridge exhibit a relative Raman shift of 3.5 cm-1 and 2.8 cm-1, respectively. These values indicate that 2.15% and 1.76% strain were introduced in the i-regions for the 5-μm-long and 10-μm-long bridges. We found that the 10μm-long bridge has a more uniform strain distribution in the i-region compared to the 5-μmlong bridge. Empirically, the higher the length-to-width ratio is, the more uniform strain distribution can be realized. It is noteworthy that the heavy p-type doping significantly shifts the Raman peak of Ge. This is because high carrier concentration can alter the lattice deformation potential, giving rise to a lower phonon vibration frequency.31 Nevertheless, this effect is much less prominent for n-type Ge because of the low rate of intervalley scattering. The relatively low critical strain values in this study compared to the 4.9% record can be attributed to the undesirable material quality. We have fabricated bare Ge micro-bridges with the same dimensions as the LED devices from the same wafer. It turns out that the ion implantation process and the incorporation of oxide layer and metal contact barely affect critical strain values. Further improvement of the strain can be envisioned using a GOI wafer since it eliminates the defective Ge/Si interface to enable higher yield strength.24

Figure 3. Scanning electron microscope (SEM) images and strain characteristics of the fabricated devices. (a) SEM image of the strained Ge LED with a bridge width of 4 μm and a bridge length of 10 μm. Isolation trench was etched to the buried oxide (BOX) layer to eliminate the bypassing leakage current. (b) SEM image of the active region of the device. (c) Measured Raman shift maps (relative to the relaxed bulk Ge peak 301.1 cm-1) of the 5-μm-long (top) and 10-μm-long bridges (bottom) with a width of 4 μm.

Since the penetration depth of the 532 nm laser in Ge is ~43 nm, the Raman signal is dominated by the top surface of Ge. To get insight into the vertical strain distribution, 3D finite element (FE) simulations were conducted for the fabricated bridges. An initial biaxial stress with ε100,010=0.16% is set in Ge and an initial biaxial stress of 300 Mpa is set in SiOx. Fixed constraint is used for the bottom of the substrate. Other boundaries are considered to be free. Figures 4a and 4b show the simulated 3D distribution of strain for the 5-μm-long and 10-μm-long bridges, respectively. As can be seen, the 10-μm-long bridge has a more uniform strain distribution compared to the 5-μm-long bridge, which is consistent with the Raman measurements. According to the simulation results, the strain in the top surface of i-region for the 5-μm-long and 10-μm-long bridges are 2% and 1.55%, respectively. Those values are


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slightly lower than the Raman results, which could result from the deviation of initial stresses and elastic constants used in the simulation. Figure 4c depicts the 2D strain distribution of the cross-section in the center of the bridges. The top graph and the bottom graph correspond to the 5-μm-long bridge and the 10-μm-long bridge, respectively. The strain undergoes a descent from top to bottom for the 5-μm-long bridge, while the strain distribution is uniform in the vertical direction for the 10-μm-long bridge. (a)

ε 100

ε 100

(b)

(c)

ε 100

1 μm 4 μm

Figure 4. Distribution of strain obtained by the 3D FE modeling. (a) 3D distribution of strain for the 5-μm-long bridge. (b) 3D distribution of strain for the 10-μm-long bridge. (c) 2D distribution of strain of the cross-section in the center of 5-μm-long bridge (top) and 10μm-long bridge (bottom). 240 210 180 150 120 90

100=2.15% Current density (kA/cm2)

Current density (kA/cm2)

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60 30 0 -1.5

Unsuspended

1000 100 10 1 0.1 0.01

=1.92 Rs=93 

1E-3 1E-4

1E-5 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 Voltage (V)

-1.0

-0.5

0.0

Voltage (V)

0.5

1.0

1.5

Figure 5. Current density-voltage characteristics of the strained Ge LED with 2.15% strain (red) and the unsuspended p-i-n diode (blue). The inset shows the semi-log plot of the J-V characteristics.

The current density-voltage (J-V) characteristics of the strained Ge LED under 2.15% strain with a bridge length of 5 μm and the reference p-i-n diode without undercutting the Si layer are shown in Figure 5. The inset shows the semi-log plot of the J-V curve. Typical rectifying behaviors can be observed for both groups. The strained Ge diode exhibits an on-off ratio of 105, reflecting that the crystal damage induced by the ion implantation has been well repaired during the annealing process. Note that when the forward-bias voltage exceeds 0.5 V, the current density increases linearly with the voltage because of the series resistance effect. Therefore, the J-V characteristics can be fitted using the diode model below   q(V  JARs )   J  J s exp    1    k BTk  


(4)

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where Js is the reverse saturation current density, A is the cross-section area of the junction, Rs is the series resistance, η is the ideality factor. The voltage range used for the fitting is -1.5~1.5 V. The extracted ideality factor for the strained Ge diode is 1.92, suggesting that the recombination current in the intrinsic region dominates, which is reasonable for a p-i-n junction. The series resistance that originates from the metal-semiconductor contact and the quasi-neutral region is estimated to be 93 Ω. The fitted reverse saturation current density of the strained diode is 0.875 A/cm2 which is approximately 25 times higher than those of the vertically injected Ge diodes.26, 32 This could be caused by the stronger surface recombination of the laterally injected diode. To alleviate the surface recombination and thus improve the internal quantum efficiency, an optimized surface passivation process can be employed in future work. For the unsuspended diode, traps from the Ge/Si interface act as additional recombination-generation centers, leading to a higher reverse saturation current density.26 Additionally, a relatively high ideality factor of 3.6 was observed, probably due to the existence of the Ge/Si heterojunction (see Supporting Information).33 For the same reason, Eq. 4 fails to capture the J-V characteristics of the unsuspended diode, inhibiting the accurate extraction of the series resistance. Nevertheless, the increase of the forward current density in the linear region for the highly-strained Ge diode indicates a small reduction of the series resistance. This is possibly due to the changes of dopant activation and contact resistance. Next we focus on the EL characteristics of the 10-μm-long bridges since they possess a more uniform strain distribution compared to the 5-μm-long bridge. The EL spectrum of the 2.15% strained LED with a bridge length of 5 μm is provided and discussed in the Supporting Information. Figure 6a shows the measured room-temperature EL spectra of a 10-μm-long micro-bridge with an i-region length of 5 μm under 1.76% strain. Observable EL signal starts to appear at a driving current of 0.88 mA, corresponding to a driving current density of 22 kA/cm2. The EL spectra become prominent at a driving current density higher than 46 kA/cm2. It should be noted that the relatively high current density is caused by the small crosssection of the current path since the carriers are injected longitudinally, which is different from the conventional vertically-injected LEDs. Therefore, the current density of our device should be rescaled by the factor H/L when it is compared with that of the conventional LEDs. Here, H and L represent the thickness of the bridge (1 μm) and length of the intrinsic region (5 μm), respectively. In this situation, a clear room-temperature EL signal can be obtained at a moderate current density of ~10 kA/cm2. In addition, the measured EL spectra display oscillations especially at high injection level. This can be ascribed to the optical resonance effect caused by the high refractive index contrast of the vertical layer stack. In order to evaluate the impact of the resonance effects on the EL results, the transmission spectra of the LED structures have been calculated by finite-different time-domain (FDTD) simulations (see Supporting Information). A dipole source is used in the FDTD simulations to take into account the emission-angle dependence of the transmittance. For the suspended LED, the structure comprises air/SiOx/Ge/air/SiO2/Si multiple layers (cavity 1). As shown in Figure 6b, cavity 1 displays low transmittance in the spectral window of 1750-1950 nm and the minor peaks in the transmission spectrum can account for the oscillations observed in the EL spectra. Nevertheless, the simulated transmittance of cavity 1 cannot exactly reflect the complex physics in a real case since strain, carrier injection and Joule heating would affect the refractive index of Ge. In addition, the bridge could bend when it is suspended, which leads to a deviation of the thickness


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of air gap. Therefore, using the calculated transmittance of cavity 1 to correct the EL spectra can give rise to artificial peaks. We found that the air/SiOx/Ge/air layer structure (cavity 2) dominates the cavity effect, as shown in Figure 6b. Meanwhile, cavity 2 has a smoother transmission spectrum compared to cavity 1. Therefore, the transmittance of cavity 2 is used to correct the measured EL results so that the artificial peaks can be avoided and the major information of the transmission spectrum can be retained. After correcting the EL spectra, major peaks located at ~1840 nm can be identified (Figure 6c). This corresponds to an energy of 0.674 eV which is slightly higher than the calculated band gap (0.658 eV) due to the carrier filling effect. The shoulder peaks appear in 1550~1650 nm can be attributed to the Γ-VB2 transitions. Other minor peaks could arise from the resonance effect of cavity 1. Device failure occurs at a forward-bias voltage higher than 2V due to Joule heating and the poor thermal conductivity of the suspended structure. To overcome the heat conduction issue, the micro-bridge structure can rest on the substrate taking advantage of the capillarity forces as long as the air gap is thin enough.22 In addition, the mechanical strength of the device can be enhanced as well, which can facilitate the fabrication process and improve the reliability of the device. It has been proved that the high strain in the narrowest region can be preserved after the “landing process” and the optical performance of the structure is desirable when Ge is attached on SiO2/Si substrate.22, 34


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(a)

1400

100=1.76% 2

EL Intensity (a.u.)

1200

22 kA/cm 2 46 kA/cm 2 74 kA/cm 2 103 kA/cm

1000 800 600 400 200 0 1500

1600

1700

1800

1900

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Wavelength (nm) (b)

1.0 Cavity 1 Cavity 2

Transmittance

0.8

0.6

0.4

0.2

0.0 1500

1600

1700

1800

1900

Wavelength (nm) (c) 5000

100=1.76%

-VB1

22 kA/cm2 46 kA/cm2 74 kA/cm2 2 103 kA/cm

4000

EL intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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3000

2000

-VB2

1000

0 1500

1600

1700

1800

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Wavelength (nm)

Figure 6. Optical characteristics of a highly-strained Ge LED under 1.76% strain with a bridge length of 10 μm and an intrinsic region length of 5 μm. (a) Measured EL spectra at various driving current densities ranging from 22 to 103 kA/cm2. (b) Transmission spectra of the vertical cavities calculated by FDTD simulations. Cavity 1 represents the air/SiOx/Ge/air/SiO2/Si layer structure. Cavity 2 represents the air/SiOx/Ge/air layer structure. (c) EL spectra corrected by the transmittance of cavity 2 at various driving current densities ranging from 22 to 103 kA/cm2.


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(a)

700 J =60 kA/cm2

EL intensity (a.u.)

600

100=1.76% 100=1.3%

500

Unsuspended

400 300 200 100 0 1500

1600

1700

1800

1900

2000

2100

Wavelength (nm) (b)

3500

-VB1

EL intensity (a.u.)

3000

J =60 kA/cm2 100=1.76% 100=1.3%

2500

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2000 1500

-VB1 -VB2

-VB2

1000 500 0 1500

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(c)

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Integrated EL intensity (a.u.)

Wavelength (nm)

Spontaneous emission rate (1024 cm-3s-1eV-1)

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17 13

100=1.76%

Experiment JDOS Modeling

n=2.191018 cm-3

9

100=1.3%

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100, 010=0.16%

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Figure 7. Strain-dependent EL characteristics and simulation results for the strained Ge LEDs with a bridge length of 10 μm and an intrinsic region length of 5 μm. (a) Measured EL spectra of the unsuspended diode (black) and the strained Ge diodes with 1.3% (blue) and 1.76% (red) strain at a driving current density of 60 kA/cm2. (b) Corrected EL spectra of the unsuspended diode (black) and the strained Ge diodes with 1.3% (blue) and 1.76% (red) strain at a driving current density of 60 kA/cm2. In the correction, data below the noise level were omitted. (c) Spontaneous emission spectra calculated by the JDOS model for biaxially tensile-stressed Ge with ε100,010= 0.16% (black) and uniaxially tensile-stressed Ge with ε100=1.3% (blue) and ε100=1.76% (red). The corresponding injected carrier densities are 2.25×1018 cm-3, 2.19×1018 cm-3 and 2.19×1018 cm-3, respectively. The inset shows the relationship between the integrated EL intensity and the strain along the direction.


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Strain effects on the EL characteristics were investigated through the comparison of an unsuspended diode and strained diodes under 1.3% and 1.76% strain. The dimensions of the active regions for those LEDs are identical. The bridge length was chosen to be 10 μm so that uniform strain can be achieved in the active region. Various strain values were obtained by altering the wet-etching time to control the dimensions of the suspended pads. The driving current density is 60 kA/cm2. For the unsuspended diode, a weak EL signal appears in the 15001600 nm window (Figure 7a), corresponding to the energy of direct band gap transition for biaxially-stressed Ge with ε100,010=0.16%. With the increase of strain, a red-shift of the emission wavelength can be observed, which confirms the strain-induced reduction of the direct band gap. More importantly, the EL signal is remarkably enhanced, revealing that more electrons can be injected into the Γ-valley where efficient radiative recombination occurs thanks to the reduced Γ-L energy gap. Figure 7b depicts the corrected EL spectra when the resonance effect is taken into account. In the correction, data below the noise level were omitted to avoid confusion. The transmission spectrum of the air/SiOx/Ge/Si layer structure is utilized to correct the EL spectrum of the unsuspended diode (see Supporting Information). As can be seen, the major peaks of the EL spectra for 1.3% and 1.76% strain shift to 1760 nm and 1840 nm, respectively. The EL peaks in the 1550-1650 nm window for the 1.3% and 1.76% strained LEDs can arise from the Γ-VB2 transitions. The strain-induced EL enhancement was theoretically validated by the calculation of spontaneous emission rate using the JDOS model. The injected carrier concentrations δn were estimated by the following model J / qd  A n  B n 2  C n3

(5)

where d is the length of the active region. A, B and C are the Shockley–Read–Hall (SRH) recombination coefficient, radiative recombination coefficient and Auger recombination coefficient, respectively (see Supporting Information for the details of those parameters). The calculated injected carrier concentrations for the unsuspended, 1.3% and 1.76% strained devices under a current density of 60 kA/cm2 are 2.25×1018 cm-3, 2.19×1018 cm-3and 2.19×1018 cm-3, respectively. Figure 7c shows the simulated spontaneous emission spectra for biaxially tensile-stressed Ge with ε100,010=0.16% and uniaxially tensile-stressed Ge with ε100=1.3% and ε100=1.76%. According to the simulation results, the integrated spontaneous emission rate is enhanced by factors of 3.5 and 6.2 for 1.3% and 1.76% strain, respectively, as shown in the inset of Figure 7c. The corresponding enhancement factors of integrated EL intensity deduced from the corrected EL spectra are 7 and 16, which are higher than the prediction of JDOS model. This can be explained by the strain-induced pseudo-heterostructure effect which can improve the carrier injection efficiency.35 We notice that the extracted EL peaks at 1580 nm, 1760nm and 1840 nm are in good agreement with the theoretically predicted Γ-VB1 peaks. This confirms that the Raman characterization, EL characterization and band structure calculation are self-consistent. A prerequisite for lasing in Ge is that the optical gain surpasses the free electron absorption (FEA) loss, inter-valence band absorption (IVBA) loss and optical cavity loss. Although a number of theoretical investigations have been conducted on the prediction of net optical gain in strained Ge,11, 36, 37 the IVBA was inappropriately modeled using empirical formulas which fail to capture the band structure characteristics at high strain levels. Recently, this issue was tackled by a rigorous model based on first principles and a net optical gain of 275 cm-1 for 4%-5%


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strain under uniaxial stress at room-temperature is predicted.38 Fortunately, such a strain level has proved to be realistic using the state-of-the-art strain techniques based on the GOI platform. Therefore, it can be anticipated that our configuration with further increased strain will lead to a net optical gain under electrical injection. As another essential factor for lasing, the optical feedback can be achieved by integrating distributed Bragg reflector (DBR)39 or distributed feedback (DFB) cavities into the structure. Conclusions In summary, we have demonstrated highly-strained Ge LEDs based on lateral p-i-n junctions operating at room temperature. 1.76% strain along the direction with uniform strain distribution was introduced in the active region, which can tune the direct band gap of Ge to 0.658 eV (1885 nm). In particular, strain-induced red-shift of the emission peak and enhancement of electroluminescence were observed, which are consistent with the Raman characterization and theoretical prediction. We believe that future improvement of strain and incorporation of a low-loss optical cavity hold promise for a practical CMOS-compatible laser. On the other hand, since tensile strain is expected to enhance the hole mobility of Ge,40 the proposed technique can be leveraged to improve the high-frequency performance of optoelectronics devices.41 In view of this, our study also provides opportunities for the development of novel optoelectronics devices based on strain engineering. For example, the high-speed Ge photodetectors and Franz–Keldysh electro-absorption modulators42 operating at 1580~1885 nm and even longer wavelength can be conceived. Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Etch pits measurement result and XRD result of the Ge-on-SOI sample; Effects of doping on the Raman spectra; J-V curve fitting and the explanation of high ideality factor; FDTD simulation of the resonance effect; EL results of the 5-μm-long LED with 2.15% strain; Calculation of band structure for strained Ge; Material parameters used in the ABC model; Electrochemical etching of Ge in TMAH Author contributions J. J. and J. S. conceived the initial idea of the project. J. S., T. I. K., and J. S. H. supervised the project. C. L. and M. X. performed the epitaxy. J. J. and M. X. performed the device fabrication. C. L., M. M., and K. Z. helped with the fabrication. J. J. performed the Raman characterization, J-V measurement and electroluminescence characterization. C. S. F. and M. C. helped with the Raman characterization. C. S. F., M. X., and M. M. helped with the electroluminescence characterization. J. J. and J. G. performed the simulations. Y. Z. prepared the figures. J. J. drafted the manuscript, and all co-authors contributed to and proofread the manuscript. Acknowledgements This work is supported by the National Natural Science Foundation of China under Grant No. 61435004. Work was performed in part in the nano@Stanford labs, which are supported by the National Science Foundation as part of the National Nanotechnology Coordinated Infrastructure under award ECCS-1542152. References


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For Table of Contents Use Only Manuscript title: Strain-induced enhancement of electroluminescence from highly-strained germanium light-emitting diodes Authors: Jialin Jiang, Muyu Xue, Ching-Ying Lu, Colleen S. Fenrich, Matthew Morea, Kai Zang, Jianfeng Gao, Ming Cheng, Yi Zhang, Theodore I. Kamins, James S. Harris and Junqiang Sun Description of the Table of Contents graphic: The following TOC Graphic illustrates the structure (left side) and the operating principle (right side) of our highly-strained Ge LED. Since the active region of the proposed device is highlystrained, the energy difference between the Γ and L valley is reduced. As a result, more electrons can occupy the direct valley, which enhances the radiative recombination.


Strain-Induced Enhancement of Electroluminescence from Highly

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