InGaAs/InP quantum well infrared photodetector integrated on Si substrate by Mo/Au metal-assisted wafer bonding

: Integration of an InGaAs/InP quantum well infrared photodetector (QWIP) onto a Si substrate was successfully demonstrated via a metal-assisted wafer bonding (MWB) using a Mo/Au metal scheme. The Mo/Au/Mo layer, situated between the QWIP structure and the Si, has shown a well-ordered lamination. It provides a smooth surface with a roughness of about 0.8 nm, as measured by a scanning electron microscope (SEM) and atomic force microscopy (AFM). The results on crystalline quality evaluated by Raman spectroscopy and X-ray diffraction (XRD) imply that the MWB could be achieved without any measurable material degradation and residual strain. Temperature dependence of dark current revealed that there is no noticeable change in the dark current properties of the QWIP after bonding on Si, despite that the quantum wells are only 200 nm away from the bonding interface.


Introduction
Compound semiconductor based quantum well infrared photodetectors (QWIPs) have been showing a tremendous progress for infrared (IR) applications such as high resolution IR imaging from space, medical diagnosis, monitoring environment, and surveillance [1][2][3]. Since technology for growth and fabrication of this material system have been developed continually, large format arrays having a high pixel uniformity and operability can be mass produced [3]. Such focal plane arrays (FPAs) are typically connected to Si-based readout integrated circuits (ROICs) through indium bump and flip-chip bonding process, so that the hybrid integrated chip provides an integrated IR imaging sensor. However, indium bumping technology has limitation of high-throughput due to the tedious processes, and the fact that it is a die-level process. Although the indium bumps on the both sides are interconnected by flip-chip bonding, the unintentionally formed indium oxide on the surface of the bumps induces to decrease shear strength of the interconnection [4], and displacement of the indium bump by thermal cycle between liquid nitrogen and room temperature is incurred [5]. Furthermore, it is hard to implement the wafer-scale integration, which is the main bottleneck in mass-production of the non-silicon integrated IR imaging sensors.
In order to circumvent such integration issues, two principal different paths been considered. One approach is monolithic growth of compound semiconductor on Si platform (ROICs), using molecular beam epitaxy (MBE) or metalorganic chemical vapor deposition (MOCVD). They have an advantage of CMOS-compatible fabrication, and hence large format size and cost effectiveness. Only recently has it become possible to realize the competitive device performance by achieving a good quality of buffer layers on Si [6,7]. However, the intrinsic threading dislocation density of the device films, which originated from the lattice mismatch and thermal expansion coefficient difference from Si, is still an impediment to their practical use and commercialization. Besides, the high temperature process to remove the native oxide or contaminants in the growth chamber is virtually impossible to be applied to fully processed ROICs [6]. On the other hand, metal-assisted wafer bonding (MWB) has been used. It effectively utilizes an intermediate thin metal to bond each other wafers. Gold−Gold (Au−Au) bonding has been extensively studied to implement the integration of compound semiconductor based device on the Si platform by taking advantage of a low-temperature process that prevents residual stress responsible for bowing or cracks [8][9][10][11]. Moreover, shear strength of Au-Au bonding has been as high as ~20 MPa [12,13], promising a strong interconnection between the FPA and ROIC for applications in harsh environments. However, this method suffers from a propensity for deep spiking, poor controllability, and insufficient thermal stability over 300°C [14]. In order to avoid the unfavorable diffusion of Au atoms, a good diffusion barrier prior to Au deposition is highly desired. Platinum (Pt) has been typically utilized as the diffusion barrier, thanks to its excellent resistance to corrosive acids, high temperature durability, and good ohmic contact to n-type InGaAs layers. As a result, Pt/Au double layer has been successfully achieved as the intermediate metal layer for MWB of III-V based electronic and optoelectronic devices on Si [9][10][11].
In this study, we have introduced molybdenum (Mo) as the diffusion barrier of MWB and evaluated the feasibility of Mo/Au bonding layer for InGaAs/InP QWIP integrated on a Si substrate. The QWIP active layer was extremely close to the bonding interface (~200 nm), which significantly adds to the sensitivity to any damage produced by the bonding. Mo is superior to Pt in terms of its thermal properties and the contact resistance for n-InGaAs. A metal film used for MWB with higher thermal conductivity (TC) and lower coefficient of thermal expansion (CTE) causes less cracking and deformation in transferred III-V detectors into Si substrate (ROIC). TC and CTE for Mo are 139 W/mK and 5.1 × 10 −6 /K, respectively. However, Pt has a lower TC of 72 W/mK and a higher CTE of 9.1 × 10 −6 /K. Besides, ultralow contact resistivity of (1.1 ± 0.6) × 10 −8 Ω cm 2 from the Mo/n-InGaAs interface for application of transistors operating terahertz frequencies was reported in [15]. Therefore, Mo/Au metal scheme is more favorable for application to abrupt temperature change, hightemperature fabrication, and high-speed [15][16][17][18]. The quality of the bonded device film on the Si has been evaluated using scanning electron microscopy (SEM), atomic force microscopy (AFM), x-ray diffraction (XRD), and Raman spectroscopy. In addition, the dark current characteristics of the InGaAs/InP QWIP devices, before and after bonding to Si substrate, have been compared.

Experiment
The QWIP structure was grown by metal-organic chemical vapor deposition (MOCVD) on a (001) semi-insulating InP substrate as shown in Fig. 1(a). The epitaxial layers, from the bottom to the top, consist of a 200-nm-thick InP buffer layer, a 500-nm-thick Si-doped In 0.53 Ga 0.47 As, 8-periods of a 5.6-nm-thick In 0.53 Ga 0.47 As quantum wells (QWs) surrounded by a 30-nm-thick undoped InP barrier, an 100-nm-thick undoped InP, and a 50-nm-thick Sidoped In 0.53 Ga 0.47 As. A 2.8-nm-thick middle layer of the QWs contains an n-type modulation doped In 0.53 Ga 0.47 As. The calculated energy band diagram shown in Fig. 1(b) displays the conventional asymmetric n-i-n QWIP structure detecting long wavelength infrared. Figure  2(a) shows the process flow of the QWIP bonding on Si substrate using the MWB technique. The process started by a solvent cleaning to remove organics on the surface of the QWIP sample and Si substrate, it followed treating the sample with NH 4 OH and (NH 4 ) 2 S sequentially. The Mo/Au (10/10 nm) bilayer is deposited on both samples using an electronbeam evaporator. The two samples are then bonded to each other after surface activation with Ar-based plasma treatment. The MWB is conducted at 200°C and a uniaxial pressure of around 2 MPa for 1 hour. Then, the bonded sample was immersed in HCl (37%) to remove the InP substrate. After removing the substrate, the AFM image displays that the root mean square (RMS) value of the surface roughness of the bonded film is about 0.8 nm as shown in Fig. 2(b). Since the surface roughness plays an important role in achieving intimate contact at the interfaces, the measured surface roughness signifies that the Mo/Au based MWB could be attained without significant contamination and the void formation.  Figure 2(c) illustrates crosssectional scanning electron microscopy images of the bonded device after cleaving. A reverse 8-periods InGaAs/InP MQW is clearly observed and a bonding interface of Mo/Au/Mo is well arranged between the device structure and Si. The fabricated devices were mounted in a liquid nitrogen cooled cryostat equipped with in situ electrical probes. Device dark current was measured by a multimeter (34410A) connected to a low-noise current preamplifier (SR 570) at different temperatures. The temperature of the cold shield around the devices was kept the same as that of the sensor controller (LakeShore).

Results and discussion
Since the strain in the crystal affects the frequency of the phonons, the strain in bonded epitaxial layers and the as-grown layers were evaluated using Raman spectroscopy. Figure  3(a) shows the Raman spectra of the samples before and after the MWB. The Raman spectra were measured in the backscattering configuration at room temperature using a 50 × objective lens with a 532-nm laser. The peaks of InAs-like and GaAs-like LO mode was observed in the both samples. However, the as-grown sample displays the InP LO phonon because the penetration depth of the excitation wavelength reaches as deep as the 100-nm-thickness InP layer of the as-grown sample. The bonded sample shows almost same Raman shift compared with the as-grown sample without any peak shift, indicating there is no perceptible strain in the boned films. The films were subsequently characterized by using XRD, to investigate their crystalline characteristics. The XRD patterns for the both samples are as shown in Fig.  3(b). The XRD spectrum of the as-grown sample displays In 0.53 Ga 0.47 As and InP diffraction peak located at 56.6° and 63.3°, respectively. The same sharp peaks of In 0.53 Ga 0.47 As and InP are also obtained in the bonded sample. Besides, the peak corresponding to Si and the peak at 61.7° due to the CuKβ radiation diffracted from the Si (400) planes are observed in the bonded film [19]. These results from the Raman and XRD spectra confirm that the MWB could be achieved without any observable material degradation and residual strain inside the film.  The measured dark current (I D ) of the bonded and the as-grown QWIP having a 200 × 200 μm 2 mesa area is plotted as a function of the applied bias voltage (V a ) at temperatures range from 78 to 160 K (see Fig. 4). The V a was applied to the top electrode formed on the 50-nmthick InGaAs for the as-grown device and on the 500-nm-thick InGaAs for the bonded device. The dark current increases with temperature due to the thermal excitation of electrons from the QWs. The I D curves of the bonded device show mirror symmetry to those of the asgrown device around the zero bias. This symmetry is visible across the whole temperature range. For example, at T = 78K, the I D of the bonded QWIP at V a = −0.5 V and the as-grown QWIP at V a = 0.5 V are 8.1 × 10 −8 and 8.5 × 10 −8 A, respectively. There is little change in the I D after and before the MWB, indicating that the strain resulted from thermal coefficient mismatch between the Si and the InGaAs is not likely to affect the performance of the bonded QWIP. The asymmetric I D -V a relationship observed in both devices is attributed to the asymmetrical In 0.53 Ga 0.47 As/InP heterointerface [20]. When the positive V a is applied to the electrode on the top of the thicker n-InGaAs contact layer of the devices, the higher I D values were recorded due to the free carriers compared with those measured in the negative V a range. We believe that the electron escape probability from the QW is larger at the thin InP barrier (50 nm) than the thicker InP barrier (100 nm) at the other side. Therefore, the electrons escaped from the QW contribute to the dark current so that the I D of the bonded QWIP was higher in the positive bias range (Inversely, the I D of the as-grown QWIP was higher in the negative bias range). Assuming that bulk and surface are two dominant paths of the I D for the bonded QWIP, the total dark current can be expressed as I D = J A × A + J P × P, where J A , A, J P , and P are the bulk leakage current density, device area, the surface leakage current density, and device perimeter. Figure 5(a) shows the I D with respect to different side length (L) of the square devices at V a = −0.5 V and T = 78K. The trace of the I D values exhibits quadratic than linear characteristics. It is found that the dominant dark current component in the bonded QWIP is bulk leakage current rather than sidewall leakage current because the I D is proportional to the A of the devices. Since there was little change in the comparison of the I D between the asgrown and the bonded QWIP, the I D -L relationship of the as-grown QWIP is also likely to show a quadratic trend. The J A taking a dominant role in the I D can be extracted through a A/P-I D /P relationship as shown in Fig. 5(b). The slope of the characteristic is proportional to the area-dependent leakage current so that the extracted J A is 2.54 × 10 −4 A/cm at V a = −0.5 V. The low dark current indicates that the active QW region in the vicinity of the bonding interface has not been damaged. Figure 6 displays the temperature dependence of the J A for the bonded device at the different bias voltages. Linear fitting to each J A value at the respective V a shows good agreement. The slope of the lines can be used for the extraction of the activation energy (E a ). The estimated E a of the bonded QWIP is 142, 137, 129, and 117 meV at the V a of −0.1, −0.3, −0.5, and −0.7 V, respectively. The lower E a was observed as negative bias voltage increased owing to the electric field-assisted tunneling of the carriers [21]. Since the dark current characteristics shows no significant differences between the asgrown and the bonded QWIP, the bonding quality conducted by the proposed MWB has shown great potential for integration of a wide variety of III-V based devices on silicon platforms.

Summary
In summary, we established the feasibility of the Mo/Au based MWB for the InGaAs/InP QWIP integrated on silicon platforms. The bonding quality was evaluated using AFM, SEM, Raman spectroscopy, and XRD. These demonstrated that the MWB can be achieved without voids, defects, and deleterious strain. In addition, the temperature dependence of the I D of the bonded QWIP on Si was remarkably consistent with the as-grown QWIP on InP. The bonded devices exhibited that the dark current is dominated by the bulk, the measured electronic activation energy is as predicted. Integration of QWIP within 200 nm of the bonded interface to silicon shows the robustness of the approach across a wide temperature range, making it a promising path toward image sensor integration.