E. 參考文獻
3. Superlattice Infrared Photodetector with Grating Structure for
A. 前言
QWIPs do not absorb radiation incident normal to the surface since the light polarization must have an electric field component normal to the growth direction to be absorbed by the confined carriers. When the incoming light contains no polarization component along the growth direction, the matrix element of the interaction vanishes. As a consequence, these detectors have to be illuminated through a 450 polished facet. However, this illumination scheme limits the configuration of detectors to linear arrays and single elements. For imaging, it is necessary to couple light to two-dimension arrays of these detectors uniformly.
Some different methods can to deflect the incoming light away from the direction normal to the surface, enabling intersubband absorption, such as random reflectors, two-dimension periodic gratings, corrugated structure, microlenses and so on. In the thesis, normal light incident is successful by using periodic gratings
Surface plasmon at the interface between a metal and a dielectric material have a combined electromagnetic wave and surface charge character as shown in Fig. 1. They are transverse magnetic in character (H is in the y direction), and the generation of surface charge requires an electric field normal to the surface. This combined character also leads to the field component perpendicular to the surface being enhanced near the surface and decaying exponentially with distance away from it. The field in this perpendicular direction is said to be evanescent, reflecting the bond, non-radiative nature of surface plasmon, and prevents power from propagating away from the surface. The surface plasmon mode has the momentum mismatch problem that must be overcome in order to coupling light and surface plasmon modes together, with the surface plasmon mode always lying beyond the light line, this is, it has greater momentum than a free space photon of the same frequency.
The enhanced transmission through periodic arrays of subwavelength holes in optically thick metallic films has demonstrated. Not only is the transmission much higher than expected from classic diffraction theory, it can be greater than the percentage area occupied by the holes, implying that even the light impinging on the metal between the holes can be transmitted. For a square array of period a0 the peaks λmax the normal incidence transmittance spectral can be identified approximately from the dispersion relation, and they are given by :
2 2
max 0 m d
m d
i j a ε ε
λ + ≈ ε +ε (2.3)
where indices i and j are the scattering orders from the array.
In order to apply surface plasmon in our device, we make the grating structure on our mesa.
We will discuss the phenomenon in following sections. Fig. 2 shows an accomplished surface
structure by using Scanning Electron Microscope (SEM).
Fig. 1 The surface plasmons between a metal and a dielectric material.
(a) (b)
Fig. 2 The top view of the mesa surface pattern (a) and (b) are 3μm apertures with 6μm period.
B. 研究方法
We will divide detector structure into two parts in this section. One is the structure of wafer.
The other is the surface structure on the mesa.
The sample was grown by molecular beam epitaxy on a semi-insulating GaAs substrate. The sample structure has a 10000 Å bottom contact, a 3-period superlattice on the bottom layer, a
3000 Å barrier layer ( Al0.28Ga0.72As ), another 45-period on the top layer, and a 8500 Å Al0.32Ga0.28As on the top contact layer. Each period of the bottom superlattice consists of 65 Å GaAs well and 35 Å undoped Al0.32Ga0.28As barrier, but the well is modulated doped with 1×1018 cm-3of Si in sample. We only doped in the middle well in each three quantum well.
In order to show the design principles, we have to know the operation mechanism on our sample.
Figure 3 shows the theoretical band structure of our sample. The band structure is estimated by the transfer matrix method with taking into account the band-nonparabolicity.
In our sample, the top superlattice has two minibands. The first miniband is estimated to range between 48 meV and 60 meV. The second miniband is estimated to range between 182 meV and 242 meV. Therefore, the corresponding absorption wavelengths are 6.39
μ m to 10.16 μ m. The
bottom superlattice also has two minibands. The first miniband is estimated from 50 meV to 58meV. The second miniband is estimated to range from 189 meV to 230 meV. Consequently, the
corresponding absorption wavelengths are 6.89μ m to 9.46 μ m.
Fig. 3 The band diagram of our sample
In order to achieve optical coupling for normal incidence, we design some grating structures on the mesa. Figure 4 shows SEM image of the grating structure on the mesa. The gratings and contact consist of Au. We manufacture three different period grating structures, including 6
μ m,
9μ m, and 12 μ m. The trench width in all samples is 3 μ m.
The operational mechanisms are shown in Figure 5. Under positive bias voltage, the photoresponse corresponds to the miniband transition in the bottom SL. The photoelectrons in the second miniband of bottom SL can tunnel through the barrier due to the applied voltage on the blocking layer. Those escaped photoelectrons leave a positive field to attract electrons from bottom contact and therefore the photocurrent in the external circuit is formed. On the contrary, the photoelectrons in the top SL are drawn to top contact, and the left positive field attracts electrons from top contact. This is an internal current circulation which can not be metered by
external circuit. On the other hand, the photoresponse under negative voltage is attributed to front SL.
(a) (b)
Fig. 4 The top view of the mesa surface pattern : (a) and (b) are 3
μ m trenches with 6 μ m
period..Fig. 5 Schematic illustrations of operational mechanisms at positive and negative bias.
In order to have the electric field for perpendicular direction, the gratings on the mesa deflect the incoming light away from the direction normal to the surface. Figure 6 shows the diagram.
The samples have different open air fraction on the mesa. Their open air fractions are 25%, 33%, and 50% individually. We will discuss the three cases later.
Fig. 6 The gratings deflect the incoming light away from the direction normal to the surface.