| dc.description.abstract |
<p>Quantum Dot Infrared Photodetectors (QDIPs) are important alternatives to conventional
infrared photodetectors with high potential to provide required detector performance,
such as higher temperature operation and multispectral response, due to the 3-D quantum
confinement of electrons, discrete energy levels, and intrinsic response to perpendicular
incident light due to selection rules. However, excessive dark current density, which
causes QDIPs to underperform theoretical predictions, is a limiting factor for the
advancement of QDIP technologies. The purpose of this dissertation research is to
achieve a better understanding of dopant incorporation into the active region of QDIPs,
which is directly related to dark current control and spectral response. From this
dissertation research, doping related dipole fields are found to be responsible for
excessive dark current in QDIPs. </p><p>InAs/GaAs QDIPs were grown using solid source
molecular beam epitaxy (MBE) with different doping conditions. The QDIPs were optically
characterized using photoluminescence and Fourier transform infrared (FT-IR) spectroscopy.
Devices were fabricated using standard cleanroom fabrication procedures. Dark current
and capacitance measurements were performed under different temperature to reveal
electronic properties of the materials and devices. A novel scanning capacitance microscopy
(SCM) technique was used to study the band structure and carrier concentration on
the cross section of a quantum dot (QD) heterostructure. In addition, dark current
modeling and bandstructure calculations were performed to verify and better understand
experimental results.</p><p>Two widely used QDIP doping methods with different doping
concentrations have been studied in this dissertation research, namely direct doping
in InAs QD layer, and modulation doping in the GaAs barrier above InAs QD layer. In
the SCM experiment, electron redistribution has been observed due to band-bending
in the modulation-doping region, while there is no band-bending observed in directly
doped samples. A good agreement between the calculated bandstructure and experimental
results leads to better understanding of doping in QD structures. The charge filling
process in QDs has been observed by an innovative polarization-dependent FT-IR spectroscopy.
The red-shift of QD absorbance peaks with increasing electron occupation supports
a miniband electronic configuration for high-density QD ensembles. In addition, the
FT-IR measurement indicates the existence of donor-complex (DX) defect centers in
Si-doped QDIPs. The existence of DX centers and related dipole fields have been confirmed
by dark current measurements to extract activation energies and by photocapacitance
quenching measurements. </p><p>With the understanding achieved from experimental results,
a further improved dark current model has been developed based on the previous model
originally established by Ryzhii and improved by Stiff-Roberts. In the model described
in this dissertation, two new factors have been considered. The inclusion of background
drift current originating from Si shallow donors in the low bias region results in
excellent agreement between calculated and measured dark currents at different temperatures,
which has not been achieved by previous models. A very significant effect has been
observed in that dark current leakage occurs due to the dipole field caused by doping
induced charge distribution and impact-ionized DX centers. </p><p>Last but not least,
QDIPs featuring the dipole interface doping (DID) method have been designed to reduce
the dark current density without changing the activation energy (thus detection wavelength)
of QDIPs. The DID samples involve an InAs QD layer directly-doped by Si, as well as
Be doping in the GaAs barrier on both sides of the QD layer. The experimental result
shows the dark current density has been significantly reduced by 104 times without
any significant change to the corresponding activation energy. However, the high p-type
doping in the GaAs barrier poses a challenge in that the Fermi level is reduced to
be well below the QD energy states. High p-type doping is reported to reduce the dark
current, photocurrent and the responsivity of the devices. </p><p>To conclude, it
is significant to identify to effect of Si-induced defect centers on QDIP dark currents.
The subsequent study reveals doping induced dipole fields can have significant effects
on QDIP device performance, for example, causing charge leakage from QDs and reducing
activation energy, thereby increasing dark current density. The DID approach developed
in this work is a promising approach that could help address these issues by using
controlled dipole fields to reduce dark current density without changing the minimum
detectable energy of QDIPs.</p>
|
|
