Mid-Infrared Photodetector Using Self-Assembled InAs Quantum Dots Embedded in Modulation-Doped GaAs Quantum Wells
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operation in the mid-infrared range has been demonstrated [2-7]. Most of the structures reported so far utilize a vertical transport through stacked self-assembled QDs. However, because of unavoidable inhomogeneity in the size and spatial alignment of the self-assembled QDs, the tunneling transport through multiple self-assembled QDs is strongly affected by the disorder. As a result, it seems to be difficult to obtain high photoconductive gains in the vertical transport structures. The photoconductive gain Gp(. of photodetectors is generally expressed as G, = I/L = ItrE/L, where p and 'r are the mobility and lifetime of photoexcited carriers, respectively, E the electric field in the conducting channel, I the drift length, and L the inter-electrode spacing [8]. The equation indicates that, to improve a detector performance, a high mobility as well as a long lifetime of photoexcited carriers are desirable. In this work, we have designed a mid-infrared (MIR) photoconductive device which utilizes self-assembled InAs QDs as "artificial impurities". A cross-sectional view and a schematic band diagram are shown in Fig. 1. The essential points of the structure shown in Fig. I are the following; 1) the size of the InAs self-assembled QDs is made small to accommodate only one bound state in the QDs by adopting relatively low temperature growth, 2) highmobility modulation-doped A1GaAs/GaAs heterointerfaces are used as the conducting channels for photoexcited carriers, and 3) a long carrier lifetime is achieved by using a large distance 147 Mat. Res. Soc. Symp. Proc. Vol. 607 0 2000 Materials Research Society
between the quantum dot layers and the heterointerfaces. The electrons are photoexcited from the ground state in the QDs to the virtual excited state above the conduction band of GaAs (bound-to-continuum operation), relax to the heterointerface, and drift laterally along the interface due to the applied electric field, thereby producing a photocurent (Fig. l(b)). The advantage of using InAs QDs as "artificial impurities" is that, since the spatial extent of the electron wavefunction in the self-assembled InAs QDs is in the order of 10 nm, the dipole matrix element for the intersubband transition for a radiation in the , = 10 p m-range is large, which is not the case for extrinsic deep impurities. Another advantage is that the InAs QDs become electrically neutral after electrons escape from them by irradiation, which is suitable for obtaining long photocarrier lifetimes.
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L= 20Am
-b-
A GaAis .-
Electric Field
0-r
,3aAs 00A
AIGaAs GaAs
LI
InAs 00
le
'3aA s
MIR radiation 0 2!'m AIGaAs GaAs buffer uo, ,•11 SI GaAs
(a) Fig. 1
,Idoped AIGaAs
VAD1-,
(b)
(a) The schematic band diagram and (b) the cross-sectional
view of the modulation-doped QDIP structure. In the following, we will show from investigation by photoluminescence and transport measurements that the InAs QDs in our sample contain only one bound state. We have observed a broad photocurrent signal in the photon energy range of 100-300 meV due t
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