Wavelength (um)
350K, Peak ~8.5um
Figure 1.1: The blackbody radiation spectra at 350K, 300K, and 250K. The peak intensity varies from 8.5μm to 11.5μm.
Figure 1.2: The atmosphere transmittance spectrum of infrared.
change after the absorption of infrared radiation, such as resistance, thermoelectric, and charge. Bolometers and microbolometers are based on the resistance change.
Thermocouples and thermopiles use the thermoelectric effect. In general, the thermal detector could be operated at higher temperature, but the response time is much longer due to the limitation of the thermal conductivity.
There are three types of the photon detectors, including QWIPs (or QDIPs), II-VI photodiodes, and type-II InAs/GaSb superlattice photodiodes. The operation of the photon detectors is based on the carrier transition from lower energy state (valence band) to higher energy state (conduction band) and becomes free carrier. The free carrier (electron) concentration increase causes the photo current increase. The major advantage of the photon detector is the shorter response time than thermal detector because the carrier transition process is faster than the physics property change.
However, the photon detectors must be operated at low temperature in order to reduce the thermal noise. The advantages and limitations of these three type detectors are listed in table 1.1 [8].
Detector type Advantages Limitations
QWIPs Mature III-V technology
Large, uniform FPA InAs/GaSb SLS High detectivity
High quantum efficiency
High dark current Epitaxy difficulty
The major advantages of QWIPs are the mature III-V epitaxial technology and large area uniformity, which is important for the focal plane array (FPA) application.
However, the high dark current, low quantum efficiency, and the non-normal incident Table 1.1: Advantages and limitations of different type infrared detectors.
radiation absorption limit the device performance.
1.1.3 Overview of QWIPs and QDIPs
The intersubband infrared absorption of the GaAs/AlGaAs QW was first observed by West and Eglash in 1985 [9]. The absorption peak at 8μm and 10μm was observed in the 65Å and 82 Å GaAs/AlGaAs QW. Two years later (1987), the first QWIPs device was demonstrated by Levine et al. [10]. The absorption of the 65 Å GaAs/AlGaAs QW was 10.8μm with a responsivity of 0.52 A/W. In 1991, the first 128×128 GaAs/AlGaAs QWIP FPA was fabricated [11]. In 2003, the large 640×514 four detection bands IR FPA with 99.9% of the pixels working was fabricated by JPL [12]. However, the operation temperature of QWIPs is difficult to further increase and the device is insensitive for the normal incident radiation. These drawbacks limit the application of QWIPs.
After the high quality self-assembled quantum dot (QD) epitaxial technology was carried out, the QD is assumed as potential active layer structure for the intersubband infrared detector application. Due to the three dimensional confinement of the carrier, the QDIPs are sensitive to normal incident infrared radiation. And, because of the discrete energy level of the QD, the carrier intersubband relaxation time is much longer than that of QW. The longer relaxation time would enhance the device current gain and reduce the detector noise. Thus, the QDIPs are of the great potential to surpass the disadvantages of the QWIPs [13].
The infrared absorption of the QDs had been observed by different method in different groups. Drexler et al. studied the intersubband far-infrared absorption by a capacitance spectroscopy technique coupled with a far-infrared spectrometer [14].
Phillips et al. reported the far-infrared absorption of Si-doped InAs QDs by Fourier Transform Infrared (FTIR) spectrometer [15]. Berryman et al. studied the
mid-infrared intersubband absorption in InAs QDs by photo conductivity measurement method [16]. And, Chua et al. investigated the polarization dependence of the intersubband absorption in InAs QDs by a FTIR spectrometer [17].
In 1998, several reports of the InAs/GaAs QDs as active structure intersubband photoconductive detectors were published. Phillips et al. measured the 17μm photo response signal by a FTIR and a low-noise amplifier measurement system [18].
Maimon et al. demonstrated the responsivity signals in the different polarization infrared radiation, and the carrier transition states were also studied [19]. Pan et al.
showed the response signals were from E0 to E1 (13μm) and E0 to E2 (11μm), and this device showed a peak detectivity of 1×1010 cm-Hz0.5/W at 40K [20]. Xu et al. studied the electro-optic characteristics of the QDIPs, and showed the ratio of TE (s-polarization) and TM (p-polarization) mode [21].
Except for the typical vertical QDIPs, the lateral QDIPs, which carriers transport direction is perpendicular to growth direction, had been also studied [22]. The lateral QDIPs was predicted of higher responsivity than vertical QDIPs due to the higher current gain. However, the lateral QDIPs was more difficult to fabricate the FPA, because the two electric contact pads were fabricated at the same side.
In 2001, the performance of the QDIPs was improved by inserted the high band gap material (AlxGa1-xAs) in the active region to reduce the device dark current and increase the device performance. Wang et al. demonstrated the low dark current and high detectivity (2.5×109 cm-Hz0.5/W at 77K, ~6μm) QDIPs by capping a thin Al0.2Ga0.8As layer after the QDs [23]. Chen et al. compared two different structures:
the InAs QDs surrounded by GaAs or AlAs/GaAs super lattice. The ratio of Ipc/Idark of latter structure showed more than two orders higher than former one [24]. Stiff et al.
used a single Al0.3Ga0.7As barrier as current blocking layer to reduce the dark current [25]. The device showed a detectivity of 3×109 cm-Hz0.5/W at 100K, and it could be
operated up to 150K. With the different active structure design, encouraging results have been demonstrated with operation temperatures over 200K [26-28] and even up to room temperature [29, 30]. The 640×512 pixels long wavelength (8.1μm) QDIP FPA has fabricated in 2007 by Gunapala et al [31]. The noise equivalent temperature difference is of 40mK at 60K operation temperature.
1.2 GaSb/GaAs quantum structure and GaSb growth on Silicon substrate
1.2.1 Semiconductor heterojunction
A heterojunction is the junction of two different semiconductor materials with different bandgap energy. In the ideal case, the interface is strain-free and the bandgap energy is not changed. Due to the bandgap energy, the work function, and the electron affinity difference in different material, the heterojunctions have three kinds of band alignment, including type-I, type-II and type-III [32, 33]. Fig. 1.3 shows the ideal case energy band alignment of these three kinds of heterojunction. By controlling the composition of the group-III and V in ternary or quaternary semiconductor, we can adjust the band alignment in the heterostructure device.