This issue has been discussed by Rogalski and Martyniuk (2006) and is found to be, however, much more favorable in the case of SLS than for HgCdTe composition variation in these wavelength ranges. It is obvious that the influence of thickness variation is greater in the case of LWIR material than it is for MWIR, and even larger in the case of VLWIR SLS. By looking at Figure 4, one can observe that thickness variation has a pronounced effect depending on the desired cutoff wavelength. Nowadays, GaSb wafers are available in 3″ diameter and III–V MBE growth can be achieved with <1% thickness uniformity over such wafer sizes. Of course, uniformity on a larger scale is desirable in order to grow several FPAs on the same wafer in order to lower production costs. Indeed, performance variations from pixel to pixel on a same focal plane array (FPA) introduce spatial noise and increase the noise of the system. Requirements for high-end thermal imagery systems also include very high material uniformity on areas that are of the order of tens of cm 2.
Further information on MOVPE growth of SLS materials can be found, for example, in the works of Zhang et al. However, most of the studies presented so far have been focused on solid-source MBE, which offers lower growth rates, lower growth temperatures, and an easier handling of Sb-based materials. Modern epitaxy techniques such as metal-organic vapor-phase epitaxy (MOVPE) and molecular beam epitaxy (MBE) permit the crystalline growth of very good quality SLS. Moreover, in such a stack of very thin layers, interfaces play a significant role on the material quality and optoelectrical properties, which implies a thickness and composition control of the order of the atomic monolayer thickness. Hence, the control of each layer is very critical. Krishna, in Comprehensive Semiconductor Science and Technology, 2011 6.06.3.3.1 Material uniformityĭepending on the desired cutoff wavelength, each layer of InAs/Ga(In)Sb of the SLS period ranges from 5 to 20 atomic monolayers (MLs) (about 15–60 Å). The activation energies derived from Arrhenius plots of dark current, 170, 204, and 226 meV for bias voltages + 9, −9, and 0.1 V respectively, agree with the cut-off wavelengths observed in the photoresponse curve. This QWIP remains background limited up to a detector temperature of 100 K for biases between −7.5 V and +2.5 V. Compared with the GaAs/GaInP QWIP reported in the last section, the peak wavelength increased slightly.
The photoresponse has a peak around 4 μm with broad maxima. Such asymmetry could be related to either a structural difference in the two quantum well interfaces or to dopant migration during the material growth. This arises from an asymmetric quantum well potential profile. 72 The effect of the quaternary alloy was to reduce the valence band offset relative to lattice matched GaAs and thus increase the cut-off wavelength.Ī photovoltaic effect is observed from the photoresponse curve (see Figure 1 in reference 72). In 0.29As 0.39P 0.61 QWIP was grown by LP-MOCVD with 50 periods of 30 Å wide GaAs quantum wells separated by 280 Å wide GaInAsP (Eg = 1.8 eV at T=300 K) barriers. To increase the cut-off wavelength, a lattice-matched GaAs/Ga 0.71. Razeghi, in Handbook of Infra-red Detection Technologies, 2002 p-type GaAs/GaInAsP QWIP