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Abstract
We report the fabrication and characterization of a multilayer Ge quantum dot detector grown on Si1−xGex virtual substrate (x = 0.18) for photovoltaic mid-wave infrared photodetection. Detector displays an over 100% photovoltaic response enhancement as compared to a conventional Ge/Si device due to smaller hole effective mass in the SiGe barriers. A further enhancement in sensitivity is achieved by excitation of surface plasmon polariton waves in a Ge/SiGe photodetector coupled with a two-dimensional plasmonic structure. The plasmonic resonance induced photocurrent enhancement is found to be larger when the incident infrared light illuminates the detector from its substrate side. At zero bias and 90 K, the responsivity of 40 mA/W and peak detectivity of 1.4 × 1011 cm·Hz1/2/W are determined at a wavelength of 4 µm.
© 2017 Optical Society of America
1. Introduction
Photovoltaic sensors operating without external bias voltage are preferable for application in focal plane arrays with low energy consumption and power dissipation, as they have the advantage of reduced noise equivalent temperature difference [1]. At zero or low bias, the detector noise is minimal and the detectivity is limited only by the thermal fluctuations. Photovoltaic type of detection with zero bias operation has been previously reported for remotely doped Ge/Si quantum dot (QD) infrared (IR) photodetector (QDIP) structures [2, 3]. The peaked density of states in QDs benefits a number of highly important properties in the mid-IR regions where the Earth’s atmosphere has its major transmission windows and many molecules exhibit vibrational absorption resonances. The three-dimensional (3D) confinement of the QD structure provides the possibility to suppress the electron-phonon scattering due to phonon bottleneck effect [4–6] resulting in intrinsically large photoconductive gain [7], ultimately leading to low dark current and high operating temperature [8,9], thus opening a route to efficient optical sensing, thermal signature detection and control.
QDIPs based on group IV elements are of particular intererest due to their compatibility with the integrated silicon photonics platform. The most attractive feature of Ge QDs grown by strain-driven self-organization in Si matrix is that Ge/Si epitaxial growth process can be combined with both front- and back-end silicon complementary metal-oxide-semiconductor fabrication technologies [10]. Another important advantage of photodetectors based on Ge/Si QDs, compared with II–VI and III–V materials of hybrid matrices, is the match of the coefficient of thermal expansion with the silicon readout circuit. Owing to the type-II band alignment, Ge QDs form 3D potential wells only for holes, while the electrons are weakly confined in Si in the vicinity of the Si/Ge heterointerface by the tensile strain fields [11, 12]. The photocurrent of Ge/Si QDIPs is generated in the mid-wave atmospheric window (3–5 µm) and originated from the transitions between the hole states bound inside Ge QDs and continuum or quasi-bound states of the Si matrix [2, 13, 14]. The internal built-in electric field provided by charge distribution between Ge QDs and delta-doping planes in the Si barriers results in zero bias operation and a photovoltaic type of response [2].
However, p-type-doped Ge/Si QD structures exhibit the problem of a lower absorption coefficient and responsivity in the mid-IR as compared to the n -type InAs/GaAs QDIPs [15, 16] due to the large effective mass of holes in the Si valence band. The large hole mass leads to reduced bound-to-continuum transition matrix elements and low mobility of charge carriers. At zero bias, the mid-IR responsivity of ten-period Ge/Si QDIP does not exceed 5 mA/W at a temperature of T = 80 K [3]. In this work, we demonstrate an over 100% photovoltaic response enhancement by replacing Si barriers surrounding Ge QDs with a SiGe alloy (the Ge fraction is 18%) due to the smaller hole mass in the SiGe layers [17]. A further enhancement in sensitivity is achieved by excitation of surface plasmon polariton (SPP) waves in a Ge/SiGe QDIP integrated with a 2D plasmonic structure. At zero bias and T = 90 K, the responsivity of 40 mA/W is obtained at a wavelength of 4 µm.
2. Methods
Figure 1 shows schematically the structure of the detectors discussed in this paper. The Ge/SiGe samples were grown by solid source molecular beam epitaxy (MBE) on a (001)-oriented fully relaxed SiGe/Si substrate (i.e., virtual substrate-VS), using a Riber SIVA-21 system. The growth temperature for all epitaxial layers was 500°C. The SiGe virtual substrates were grown at a temperature of 900°C and at a pressure of 20 Torr using Reduced Pressure - Chemical Vapour Deposition [18]. Precursor gases were dichlorosilane and germane. Such a high growth temperature promoted the glide of misfit dislocations and minimized their threading density. VS represents the 2-µm-thick compositionally graded buffer Si1−xGex layer, where the Ge content is increased progressively up to the maximum value (∼ 10% Ge/µm), with the 1-µm-thick constant composition SiGe layer on the top. The resulting threading dislocation density was measured to be less than 105 cm−2. Chemical-mechanical polishing was used afterwards in order to suppress the surface roughness. The Ge content in the upper layer of VS was 18%. A 100 nm undoped SiGe buffer layer with the same Ge composition as in the virual substrate was first grown on a VS, following by the growth of a 500 nm B-doped p +-SiGe contact layer (p = 2 × 1018 cm−3). The active region of the device was composed of ten stacks of Ge QDs separated by 35-nm Si0.82Ge0.18 barriers and was sandwiched in between the 100-nm-thick intrinsic buffer and cap SiGe layers. Each Ge QD layer consisted of a nominal Ge thickness of 4.8 ML (1 monolayer (ML) ≈1.41 Å) and formed with a rate of 0.05 Å/s by self-assembling in the Stranski-Krastanov growth mode. The Ge growth was stopped just after appearance of well-defined 3D spots in the reflection high-energy electron diffraction pattern. The p -type remote doping of the dots was achieved with a boron δ-doping layer inserted 5 nm above each dot layer. The areal doping density was 6 × 1011 cm−2. Finally, a boron doped 100-nm-thick p +-SiGe top contact layer (p = 1 × 1019 cm−3) was deposited. The rms roughness of the topmost epitaxial SiGe layer was measured to ~2 nm. From scanning tunneling microscopy (STM) experiments with uncapped samples, we observed that the Ge islands have the shape of hut clusters with a typical base length of about 12 nm, and an areal density of (1.0 − 1.3) × 1011 cm−2 [Fig. 1(c)].
Fig. 1 Layer sequence of 10-period (a) Ge/SiGe QDIP and (b) Ge/Si QDIP. (c) STM image from topmost uncapped Ge layer where 4.8 ML of Ge was deposited on a relaxed SiGe layer at 500°C with a rate of 0.05 Å/s. (d) Detector top view obtained in optical microscope.
A conventional Si-based QDIP was also fabricated for comparison of device performance [Fig. 1(b)]. For this sample, all SiGe layers were replaced by pure Si layers while other growth parameters were kept to be the same. The only one exception is that the amount of deposited Ge required for elastic relaxation and formation of self-assembled Ge hut clusters on pure Si (5.0 ML) turns out to be slightly larger than that for the Ge grown on a SiGe VS (4.8 ML), in agreement with the previously published results [19]. Nevertheless, similar QD sizes and densities were determined in both Ge/SiGe and Ge/Si samples. After the MBE growth, the wafers were processed into 1.4 mm diameter circular mesa-shaped QDIPs with top and bottom gold electrodes using the standard optical lithography, plasma etching, e-beam metal deposition, and lift-off techniques [Fig. 1(d)]. A 5-nm-thick Ti film was deposited between the 50-nm-thick gold layer and the QDIP to promote the adhesion of the Au film to the Si or SiGe surface.
The normal incidence photoresponse and transmission spectra were obtained using a Bruker Vertex 70 Fourier transform infrared spectrometer with a spectral resolution of 10 cm−1 along with a SR570 low noise current preamplifier. The PC spectra were calibrated with a deuterated L-alanine doped triglycine sulfate (DLaTGS) detector. The devices were mounted in a cold finger inside a Specac cryostat with ZnSe windows. In this work, photocurrent measurements were performed at T = 90 K while transmission was recorded at room temperature.
3. Results and discussion
Figure 2(a) compares the photovoltaic response of the SiGe- and Si-based QDIPs measured at a zero bias voltage. The performance uniformity of the devices has been confirmed by several control samples randomly selected from different regions of the substrate. The incident IR light illuminates detectors from their substrate side (substrate-side illumination–SSI). Top-side illumination (TSI) produces the same results. Both QDIPs are of wide detection window and cover the whole mid-wave IR region with the cutoff wavelength of about 5.5 µm for the Ge/Si device and 6 µm for the Ge/SiGe structure. Also, for the SiGe-based detector, the photocurrent peak position shifts toward longer photon wavelength by ~0.5 µm. The observed redshift is a result of smaller valence band offset at the Ge/SiGe interface [20]. Figure 2(a) demonstrates a significant improvement of the Ge/SiGe detector sensitivity over the wavelength region from 3 to 5 µm, as compared to the Ge/Si heterostructure. The over 100% photocurrent enhancement is attributed to the smaller hole effective mass in SiGe layers, which enables more efficient light absorption and photoexcited hole transportation. To support this conclusion, we measured the photoconductive gain in both devices. From noise measurements, we established that the noise level at finite bias is dominated by a generation-recombination noise. In this case the relation between the noise current in and the gain g is , where e is the charge of an electron, Id is the dark current, and Δf is the noise bandwidth. The photoconductive gain was calculated and plotted in Figure 2(b). The detector noise characteristics were measured with an SR770 fast Fourier transform analyzer. The sample noise was obtained by subtracting the preamplifier-limited noise level from the experimental data. The dark current was tested as a function of bias (U b) by a Keithley 6430 Sub-Femtoamp Remote SourceMeter. Obviously, the gain is larger in the SiGe-based QDIP probably due to the increase of the hole mobility and hence the decrease of the hole transit time through the device.
Fig. 2 (a) Zero-bias spectral response curves and (b) voltage dependence of the photocon-ductive gain of the Ge/Si QD device and the Ge/SiGe QDIP grown on virtual substrate.
A further improvement of the detector performance was achieved with the use of plasmonic structures. Since the pioneering work of Ritchie in the 1950s [21], surface plasmon resonances in metallic nanostructures are currently being exploited for a variety of applications in visible, near-IR, and mid-IR regions [22–26]. The excitation of SPP modes offers an effective surface light trapping, enhancement of local field intensities, and thus interaction with the thin device active region. A simple plasmonic structure which can support the SPP waves is a metal film perforated with regular 2D subwavelength hole arrays (2DHAs) on top of dielectric material. 2DHAs show an extraordinary optical transmission (EOT) effect [27, 28] and have been successfully implemented to improve the performance of long-wave InAs/(In)GaAs QDIPs [29–36]. In this work, we fabricated metallic 2DHA plasmonic structure by the deposition of a 50-nm-thick Au film and formation of a periodic lattice of circular holes on top of the Ge/SiGe detector absorption region [Fig. 3(a)]. The 2DHA has the square lattice symmetry, with lattice constant a = 1.6 µm and the measured hole diameter d = 1.00 ± 0.02 µm [Fig. 3(b)]. The active QD region is from 200 to 550 nm below the sample surface. Both the plasmonic enhanced detector and the bare QDIP without the surface plasmonic structure were taken from a single die of the same wafer right next to each other.
Fig. 3 (a) Optical image of a Ge/SiGe photodetector integrated with a 2D square lattice of holes in the gold film on its top surface (plan view). A 1D periodicity along the x axis is an artifact of the image. (b) A zoomed-in scanning electron microscopy image of the square lattice of circular holes in the Au film. For this sample, the diameter of holes is d = 1.00 ± 0.02 µm and the lattice constant is a = 1.6 µm. (c) Measured and simulated transmission spectra for a 50-nm-thick perforated gold film with circular air holes on a SiGe substrate with the Ge content of 0.18. The experimental spectra were obtained for substrate-side illumination (SSI) and top-side illumination (TSI) of the device. Since the calculated transmission is independent of the direction of light incidence, only one theoretical curve is shown. The peaks are labeled with the (i, j) grating orders. (d) A schematic illustration of the simulated structure.
Generation of SPPs is allowed when their momentum matches the momentum of the incident photon and the reciprocal lattice vectors characterizing the periodic modulation of the electron density in the perforated metal [27, 28]. In the first approximation, the resonance wavelengths of the square hole lattice can be calculated using:
where a is the lattice constant, and i and j are the integer numbers determining the SPP propogation direction, εm and εd are the relative permittivities of the metal and dielectric, respectively. For a = 1.6 µm, Eq. (1) predicts coupling to the (0, ±1) (or its degenerate mode (±1, 0)) fundamental SPP resonance at λ0,1 = 5.6 µm, the (1, 1) higher order plasmonic mode is expected at λ1,1 = 4.0 µm. We used frequency-dependent dielectric function of the gold from the paper of Rakić et al. [37], and refractive index of the SiGe was taken to be 3.49 [38]. Figure 3(c) shows the transmission spectra of the 2DHA plasmonic structure on a SiGe VS measured using an optical condensor system. The top- and the backside-configured plasmonic structures show approximately the same transmission. Some difference may arise from different surface roughness of the substrate backside and the epitaxial layer. The transmission spectra show two peaks at the 5.7 µm and 4.1 µm, corresponding to the (0, 1) and (1, 1) orders of the SPP waves, respectively.The SPP waves and the near-field components distribution were calculated with the 3D finite-element frequency-domain (FEFD) method [39] based commercial software Comsol Multiphysics by numerically solving the Maxwell equations [40]. We used the periodical boundary conditions that allows the plasmonic structure to cover an infinite large area. The simulated structure is depicted in Figure 3(d). It is a square hole array in a layer of Au on a SiGe crystal. The diameter of the hole is 1 µm, the 2DHA period is 1.6 µm, and the thickness of the Au layer is 50 nm. The plane-wave radiation with a circular polarization falls normally either on the top or on the backside of the QDIP. The air and SiGe regions were modeled using rectangular parallelepiped geometry with correspondent refractive indices. The grid-independence of the results has been verified by doubling the number of elements in the simulation. In Figure 3(c), we show computed transmittance spectrum for the TSI geometry. The simulated curve reproduces well the positions of the maxima and minima in the experiment transmission profiles.
Figure 4 shows the measured zero-bias responsivity spectra of the SiGe-based QDIPs with and without the 2DHA plasmonic structure for both top-side and substrate-side illumination. Compared with bare QDIPs, both plasmonic structures provide photocurrent enhancement at the plasmon resonance frequencies. The responsivity of 40 mA/W is obtained for the 2DHA-QDIP illuminated from its substrate side at λ ≃ 4 µm. Note that responsivity of the bare detectors is independent of the direction of light incidence, while the response spectra of the plasmonic QDIPs are quite different for TSI and SSI. The photocurrent enhancement ratio is plotted in Fig. 5. The peak positions in each spectrum in Fig. 5 are correlated with the EOT peaks indicated in Fig 3(c). The larger enhancement (∼ 10) is achieved in the lowest order resonance (0, 1) for the backside configured detector. An over 4 times improvement in photocurrent is obtained in the (1, 1) plasmonic mode. For a TSI direction the enhancement factor is about two times less. Light direction-dependent plasmonic enhancement has been observed in InAs/GaAs QDIPs and attributed to the more efficient SPP excitation when light impinges the active absorption layer before it reaches the 2DHA [31,32].
Fig. 4 Zero-bias spectral response of the SiGe-based QDIP with the 2DHA plasmonic structure compared to the bare QDIP for (a) TSI and (b) SSI.
Fig. 5 Photocurrent enhancement ratio for top-side and substrate-side illumination. The larger enhancement is observed for SSI.
To confirm the interpretation of the difference in the enhancement spectra, we performed further numerical analysis. Figure 6 depicts the simulated electric field distribution in the y − z cross-section at x = 0 (center of the air hole) for a 2DHA-QDIP qstructure with a = 1.6 µm and d = 1.0 µm. The horizontal white dash lines in the figure indicate the QD active region. The light with circular polarization and a unit power density of 1 W/cm2 is incident along the z axis from the +z direction. The color scale denotes the normalized magnitude of the electric field and is the same for all panels. At resonance wavelengths the effective light trapping and field enhancement is observed near the metal-semiconductor interface. Detailed analysis revealed that in-plane fields Ey(x) are primarily localized under the circular hole, whereas the vertical Ez component is trapped by the edge of the hole. For substrate-side illumination (bottom panels in Fig. 6), the strong field region is localized near the SiGe surface and has a large overlap with the QD layers, resulting in the photoresponse enhancement. From TSI, the plasmonic resonance excitation is through the evanescent modes in the subwavelength holes (top panels in Fig. 6). Therefore the SPP field around the 2D holes is much weaker than that for a SSI direction. This qualitatively explains the reason why the photocurrent enhancement is larger when incident IR light illuminates the detector from its substrate side.
Fig. 6 Spatial near-field profiles in the y − z plane at various excitation wavelengths for a 2DHA-QDIP structure illuminated from the top (TSI) and from the bottom (SSI). The cut plane is at x = 0. The electromagnetic field is computed in a FEFD simulation, which includes the incident wave and all scattered waves. The light with circular polarization is incident along the z axis from the +z direction. The color scale denotes the normalized magnitude of the electric field and is the same for all panels. The active region of Ge quantum dots is between the white dash lines.
The thermal (Johnson) noise is expressed as , where k is the Boltzmann’s constant, T is the absolute temperature, and R is the differential resistance, which is extracted from the dark current measurements. At Ub = 0 V, the calculated thermal noise (0.8 × 10−14 A/Hz1/2) is close to the measured noise current (0.9 × 10−14 A/Hz1/2), indicating that at zero bias the dominant noise is the thermal noise. Using the photovoltaic responsivity of 40 mA/W and ith = 0.9 × 10−15 A/Hz1/2, the specific peak detectivty of 1.4 × 1011 cm ·Hz1/2/W was determined at 4 µm.
4. Conclusion
Here we report on the Ge/SiGe QDIPs with self-assembled Ge quantum dots grown on a virtual SiGe substrate. The detectors show an over 100% photovoltaic response enhancement as compared to a conventional Ge/Si device due to smaller hole effective mass in the SiGe layers. The metallic 2DHA plasmonic structure was successfully implemented on the top of QDIP to convert the incident electromagnetic radiation into the surface plasmon polariton waves and to improve further the device performance. We studied the QDIP enhancement for the top-side and substrate-side light incidence directions.The substrate-side illumination considerably increases the direct coupling of the light to the plasmonic QDIP as compared with a TSI direction and results in a two-fold increase of photocurrent. The responsivity and detectivity values for the detector are 40 mA/W and 1.4 × 1011 cm·Hz1/2/W at 90 K for zero bias operation, which are comparable or higher than n -type InAs/GaAs QDIPs [9,15] working at similar wavelengths. We expect that the performance of the SiGe-based QDIPs fabricated on virtual substrates can be improved more by further optimizing the 2DHA surface plasmonic structure. As it has been previously demonstrated [33,34,36], plasmonic enhancement is maximized for a fundamental plasmon mode (Fig. 5) and becomes smaller as the 2DHA period increases and the hole diameter is reduced. Therefore by choosing the hole spacing to locate the fundamental SPP resonance wavelength on the dominant photocurrent peak, the plasmonic structure can provide a strategy to make the device more tunable. Additional functionality can be achieved with multiple array periods, and by adjusting the diameter and the shapes of holes.
Funding
Russian Science Foundation (14-12-00931 Π).
Acknowledgments
The authors thank A. A. Shklyaev for the STM measurements.
References and links
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