Article pubs.acs.org/journal/apchd5
Cite This: ACS Photonics XXXX, XXX, XXX−XXX
Electrically Injected GeSn Vertical-Cavity Surface Emitters on Siliconon-Insulator Platforms Bo-Jun Huang,† Chen-Yang Chang,† Yun-Da Hsieh,† Richard A. Soref,‡ Greg Sun,‡ Hung-Hsiang Cheng,§ and Guo-En Chang*,†
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†
Department of Mechanical Engineering, and Advanced Institute of Manufacturing with High-tech Innovations, National Chung Cheng University, Chiayi County 62102, Taiwan ‡ Department of Engineering, University of Massachusetts−Boston, Boston, Massachusetts 02125, United States § Center for Condensed Matter Sciences, and Graduate Institute of Electronics Engineering, National Taiwan University, Taipei 10617, Taiwan S Supporting Information *
ABSTRACT: An efficient electrically injected group-IV light source compatible with the complementary metal-oxidesemiconductor (CMOS) process is the holy grail for realizing functional, intelligent electronic-photonic integrated circuits for a wide range of applications. The group-IV GeSn material is considered as a promising solution for efficient light sources because its bandgap can be fundamentally transformed from indirect to direct with appropriate Sn compositions. However, an important challenge in realizing efficient electrically injected light emitters is the incorporation of an optical cavity with electrical structures. Here we demonstrate, to the best of our knowledge, the first electrically injected GeSn vertical-cavity surface emitter on the silicon-on-insulator platform. A vertical cavity employing a buried oxide layer and a deposited SiO2 top layer as reflectors is developed for enhancing the electroluminescence in the GeSn active layer. Room-temperature electroluminescence experiments reveal clear cavity-resonant modes with adequate vertical-cavity Q-factor and greatly enhanced electroluminescence. Most importantly, under electrical injection, considerably reduced optical loss in the GeSn optical cavity was found at room temperature, toward achieving electrically injected optical gain. In addition, theoretical models for evaluating the optical gain and loss are presented for estimating net optical gain. These results on our GeSn vertical-cavity surface emitter pave the way toward efficient, continuouswave electrically injected GeSn lasers operating at room temperature for electronic−photonic integrated circuits. KEYWORDS: GeSn alloys, silicon photonics, optical cavity, optical gain
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compensate for this energy difference to assist population inversion, including tensile strain and n-type doping.6−9 Combining ∼0.2% tensile strain and heavily n-type doping, optically pumped and electrically injected Ge lasers operating at room temperature have been demonstrated.10−12 However, because of the limited tensile strain, the demonstrated electrically injected Ge lasers suffer from an extremely high threshold of 280 kA/cm2, making the devices less useful for practical applications. In this route, research attention has been directed to introduce large tensile strain into Ge to further reduce ΔEΓL to lower the threshold of Ge lasers. Different approaches have been developed to introduce large tensile strain into Ge, including external stressors13 and microbridge structures based on a strain concentration mechanism.14 Recently, low-threshold optically pumped lasing has been
s the shrinkage of the feature size of silicon (Si)-based micron electronics driven by Moore’s law is approaching the ultimate physical limit, there have been tremendous efforts to broaden the functionalities of Si-based electronics for Morethan-Moore applications. Si-based electronic−photonic integrated circuits (EPICs), through the fusion of Si electronics and photonics, has been considered the leading candidate to enhance the performance by breaking the bandwidth bottleneck of the copper-based interconnections for a wide range of applications, such as fiber-optic telecommunication, Lidar, and chip-scale optical interconnection.1−3 Among the various Sibased photonic devices, the realization of efficient group-IV light emitters using Si, Ge, and their alloys faces fundamental and scientific challenges because the indirectness of the band structure prevents population inversion and thus optical gain for stimulated emission.4−6 Fortunately, while Ge is an indirect bandgap semiconductor, the energy difference between the Γand L-conduction band edges (ΔEΓL) is just 136 meV at room temperature. Tremendous efforts have been made to © XXXX American Chemical Society
Received: December 4, 2018 Published: June 12, 2019 A
DOI: 10.1021/acsphotonics.8b01678 ACS Photonics XXXX, XXX, XXX−XXX
ACS Photonics
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observed from highly strained Ge nanowire cavities,15 but with limited operation temperatures up to 77 K. On the other hand, another promising approach that addresses the challenge of Ge’s indirect bandgap for efficient light emitters is to alloy Ge with Sn, another group-IV element. By alloying Ge with Sn, both direct and indirect bandgap energies decrease, and their energy separation is considerably reduced with increasing Sn composition. With a Sn mole fraction more than 6%, bulk GeSn can be transferred to direct bandgap materials to enable efficient direct transitions.16 Earlier predictions were made of double heterostructure and quantum-well lasers based on direct bandgap GeSn confined with SiGeSn barriers.17−19 Despite the limited equilibrium solid solubility of ∼1% Sn in Ge, recent advances in lowtemperature molecular beam epitaxy (MBE)20,21 and chemical vapor deposition (CVD)22−24 epitaxy techniques have enabled the growth of high quality GeSn layers with high Sn composition on silicon. Based on this development, enhanced photoluminescence (PL)16,23,25 from GeSn alloys have been observed. More recently, the first optically pumped GeSn Fabry−Perot waveguide laser has been demonstrated with a threshold of 325 kW−1 at cryogenic temperature of 20 K,26 showing great promise for efficient Si-based lasers for on-chip applications. Different types of optically pumped GeSn lasers have followed with lower thresholds and higher operation temperatures up to 180 K.27−29 However, to be practical for EPICs, efficient electrically injected GeSn light-emitting diodes (LEDs) or lasers operating at room temperature are highly desired and they remain the unaccomplished crucial component for EPIC applications. Although a few GeSnbased LEDs have been investigated,30−33 very little has been done to incorporate optical cavities into GeSn LEDs to enhance the light emission efficiency. In this paper, we demonstrate, to the best of knowledge, the first electrically injected GeSn vertical-cavity surface emitter grown on SOI substrates. We introduce a vertical cavity with the GeSn active layer, allowing for optical confinement and, thus, enhancing the light-emitting efficiency. The roomtemperature EL experiment demonstrates clear resonant cavity modes and considerably enhanced light-emitting efficiency compared to a GeSn LED on silicon. Analysis of optical gain suggests important absorption bleaching at room temperature in the GeSn cavity toward achieving net optical gain in the devices. In addition, optical gain and loss characteristics are discussed for achieving net optical gain in the GeSn cavity. These results pave the way toward electrically injected GeSn lasers operating at room temperature or higher for practical EPIC applications.
Figure 1. (a) 3D schematic diagram of our GeSn p−i−n heterodiode vertical-cavity surface emitter grown on a silicon-on-insulator (SOI) substrate (not to scale). (b) Schematic band diagram of the Ge0.969Sn0.031 active layer. (c) Refractive index profile and the field pattern |E|2 of the device simulated by finite-element method (FEM). The buried oxide (BOX) layer serves as the bottom reflector and the deposited SiO2 layer serves as the top reflector, creating a vertical cavity for optical confinement.
GeSn active layer is sandwiched between n- and p-Ge layers with a larger bandgap, forming a DBH structure for carrier confinement in the GeSn active region. Figure 1c shows the refractive index profile and the optical field distribution at resonant conditions calculated using the finite element method (FEM; Support Information). The buried oxide (BOX) layer and the deposited SiO2 layer serve as the top and bottom reflectors, respectively, because of their much lower refractive index (∼1.45) compared to the GeSn p−i−n structure (∼4.2). According to the simulated field, the optical field pattern shows that light indeed bounces forth and back between the top SiO2 and bottom buried oxide (BOX) layers, creating a vertical Fabry−Perot-like cavity for the GeSn active layer to enhance the light emission.
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DEVICE DESIGN Figure 1a presents a schematic diagram of our p−i−n Ge/ GeSn/Ge double-barrier heterostrucrure (DHB) grown on a SOI substrate via a Ge virtual substrate (VS). A schematic band diagram of the GeSn active layer is depicted in Figure 1b. The p−i−n DHB structure features a GeSn active layer with reduced ΔEΓL compared to pure Ge for enhancing light emission accomplished by direct transitions. The GeSn active layer was grown on Ge VS, inducing a compressive strain which lifts the degeneracy of the heavy-hole (HH) and lighthole (LH) bands. As the result, the top valence band is the HH band and the lowest direct transition is from the Γ-conduction band to the HH band (cΓ → HH), which dominates the light emission process under current injection. In addition, the
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MATERIAL GROWTH AND CHARACTERIZATION The GeSn p−i−n DHB samples used in this study were grown using solid-source MBE. Two types of GeSn p−i−n DHB samples were grown. One is grown on a SOI substrate (labeled as GeSn-on-SOI), and the other one was grown on the silicon substrate (labeled as GeSn-on-Si) for comparison. The structural properties of the grown samples were characterized using different techniques. Figure 2a presents a cross-sectional transmission electron microscopy (XTEM) image together with the secondary ion mass spectroscopy (SIMS) atomic distribution of the Ge, Sn, B, and Sb atoms probed for the GeSn-on-SOI sample. The XTEM image clearly resolves B
DOI: 10.1021/acsphotonics.8b01678 ACS Photonics XXXX, XXX, XXX−XXX
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GeSn active layer is determined to be 3.1%, with a compressive strain of 0.454%. In contrast, the GeSn-on-Si sample has a 420 nm thick GeSn active layer with a Sn composition of 4.5% and a residual strain of −0.659%. The material characterization results of the samples are summarized in Table 1. Table 1. Summary of the Grown GeSn p−i−n Heterostructures sample
Sn composition (%)
GeSn layer thickness (nm)
strain (%)
substrate
GeSn-on-SOI GeSn-on-Si
3.1 4.5
440 420
−0.454 −0.659
SOI silicon
Electrical Characterization. The samples were then fabricated into p−i−n diodes using a standard CMOS compatible process.34 The p−i−n diodes have a circular mesa of 200 μm in diameter with a 420 nm thick SiO2 passivation layer and two ring-shaped Cr/Au metal pads with a thickness of 20/200 nm to form Ohmic contact. An optical image of the fabricated device is displayed in the inset of Figure 3. The current−voltage (I−V) characteristics of the
Figure 2. Material characterization of the grown GeSn-on-SOI sample. (a) Cross-sectional transmission microscopy (XTEM) image of the grown sample overlaid with secondary ion mass spectroscopy (SIMS) atomic distribution of various elements. (b) X-ray diffraction (XRD) ω-2θ scan for the grown sample, displaying the peaks of the top Si layer, the Ge layers, and the GeSn active layer.
Figure 3. Room-temperature current characteristics of the fabricated GeSn-on-SOI diode, showing clear rectifying behavior. The inset shows an optical image of the fabricated device.
GeSn-on-SOI devices were measured at room temperature using a Keithley 2400 SourceMeter, and the results are displayed in Figure 3. The I−V characteristics exhibit good on−off ratio of 200 at ±1 V, displaying clear rectifying behavior of the fabricated GeSn p−i−n diodes. Electroluminescence. Figure 4 presents the EL and reflectivity spectra measured at room-temperature for the GeSn-on-SOI and GeSn-on-Si devices. For the reference GeSn-on-Si diode, as shown in Figure, 4b, a single emission peak located at ∼1900 nm with a full-width-at-half-maximum (FWHM) of ∼250 nm is observed, corresponding to the direct bandgap of 650 meV. In addition, the reflectivity spectrum displays ripple features with a free spectral range (FSR) of ∼240 nm, which is attributed to the interference between the layers. On the other hand, unlike a single emission peak for the GeSn-on-Si device, the GeSn-on-SOI device exhibits several much sharper emission peaks with a smaller FWHM of ∼66 nm than that of the GeSn-on-Si light emitter. In addition, the reflectivity spectrum also displays clear oscillation features with a smaller FSR of ∼110 nm, and the reflectivity dips match the EL peaks. Thus, comparing the EL and reflectivity spectra between the GeSn-on-SOI and GeSn-on-Si devices provides direct evidence for the longitudinal resonant cavity modes in
different layers with very flat and sharp interfaces, which are also visible from the SIMS results. Defects near the interface between the top Si layer and the Ge VS are observed, as indicated by the yellow arrows, indicating that the Ge VS is strain relaxed. For the GeSn p−i−n structure, no apparent defects are observed, indicating that the Ge/GeSn/Ge p−i−n DHB structure is pseudomorphic to the underlying Ge VS. The SIMS results also reveal a uniform Sn distribution in the GeSn active layer, showing good material quality of the grown sample. Figure 2b shows the X-ray diffraction (XRD) ω-2θ scan of the GeSn-on-SOI sample. Three peaks are clearly resolved and they are associated with the Si layer, the Ge layers, and the GeSn active layer, respectively. From the positions of the Bragg angles (θ), the lattice constants along the growth direction (a⊥) are determined using a⊥ = 2λ/ sin θ, where λ = 0.15406 nm is the wavelength of the X-ray. The results yield a lattice constant of 0.5653 nm along the growth direction for the Ge layers, suggesting the Ge layers are fully strain relaxed. On the other hand, the lattice constant along the growth direction for the GeSn layer are determined. According to the pseudomorphic condition, the Sn composition of the C
DOI: 10.1021/acsphotonics.8b01678 ACS Photonics XXXX, XXX, XXX−XXX
ACS Photonics
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Figure 4. Room-temperature electroluminescence (EL) at an injected current density of 0.14 kA/cm2 and reflectivity spectra of the (a) GeSn-onSOI and (b) GeSn-on-Si light emitters. The EL signal of the GeSn-on-Si light emitter near 1800−1950 nm is slightly distorted because of atmospheric water absorption.
Figure 5. (a) Room temperature electroluminescence (EL) spectra of the GeSn-on-SOI device measured under different injected currents. (b) Measured EL spectrum and the mode envelopes at I = 200 mA. (c) Mode envelopes at different injected currents. The peak wavelength of the mode envelope (λp) is indicated by gray dots. (d) Peak wavelength of the mode envelope, (e) integrated EL intensity, and (f) m-factor extracted from the EL spectra as a function of injected current.
1780 nm and λ2 ∼ 1880 nm are found, labeled as peaks 1 and 2, respectively. By fitting the peaks using Gaussian functions to obtain the FHWM, quality factors (Q-factors) of 33 and 31 are obtained for peaks 1 and 2, respectively. In addition, the EL intensity increases with increased injected currents, and the emission peaks show strong mode competition behavior at different injected currents; this behavior can be explained by the GeSn direct bandgap. To extract the GeSn direct bandgap energy, Figure 5c shows the envelopes of the EL emission spectra under different injected currents, where the envelope maximum (λp) is indicated by gray dots, which represents the lowest direct bandgap (cΓ → HH) of the GeSn active layer. The envelope maximum as a function of the injected current is
the GeSn vertical cavity on SOI. In addition, despite the smaller Sn composition, the peak and integrated EL intensities of the GeSn-on-SOI devices are 6.31 and 3.7 times higher than these of the GeSn-on-Si device, highlighting the important EL enhancement owing to the cavity effect. This significant EL enhancement is attributed to the vertical cavity that provides better optical confinement and thus considerably suppresses the radiation loss. Having shown the cavity effect, next we focus on the EL characteristics of the GeSn-on-SOI device. Figure 5a shows the measured room-temperature EL spectra under different CW injected forward currents (I). Among these emission peaks, two distinguishable strongest emission peaks located at λ1 ∼ D
DOI: 10.1021/acsphotonics.8b01678 ACS Photonics XXXX, XXX, XXX−XXX
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decreases because of the BNG effect under high injection currents. Theoretical Analysis. To further study the temperature effect caused by the Joule heating effect on optical gain in the GeSn-on-SOI vertical-cavity light emitter, theoretical analysis is performed to calculate the optical gain for our GeSn-on-SOI vertical-cavity light emitter. The strained band structure was calculated using deformation potential theory and eight band k·p method,8,17,37 taking into account the temperature effect. After the calculation of strained electronic band structures, the optical gain is calculated using the Fermi’s golden rule with a Lorentzian line shape8,37 (Supporting Information). Figure 7a shows the calculated lowest direct (cΓ → HH) and indirect (L → HH) transition energies as a function of
depicted in Figure 5d. For low injected current
