Photoconducting properties of heavily doped black silicon (BSi) fabricated by laser-chemical etching method have been investigated. BSi is used as a photoconductor in the study of carrier transport, photo response and minority carrier lifetime of the BSi material. It is found that the BSi is a good photoconductor with enhanced absorption in NIR from the density of states from the cone like spikes on 2 D surface. Metal contacts can be easily made by inserting metal stripes onto the BSi surface due to its high conductivity. Photocurrent spectra were measured under different voltages in the wavelength range 300 nm ~ 1300 nm. From the linear relationship between the photocurrent and the voltage, the minority carrier lifetime is determined to be about 171 μs for an electron density of 7x1017 cm-3 , which is more than 17 times as long as that for n-type silicon. The experimental results demonstrates that the BSi is a suitable material for solar cells not only as absorber but also as conductor.
In this work, time-resolved microscopic photoluminescence (PL) spectra of self-assembled CdSe quantum dots (QDs) grown by Molecular Beam Epitaxy were investigated under excitation of femto-second laser. Interface-state-phonon (ISP) assisted carrier relaxation in self-assembled CdSe quantum dots are investigated by ultrafast time-resolved photoluminescence (PL). Electrons excited in barriers are found to relax into quantum dots and then have radiative recombination with holes by the mean of ISP assisted relaxation. Temperature dependence of rise time and decay time of time-resolved PL spectra are measured in the temperature range from 77 K to 286 K. The rise time decreases from 76 ps to 32 ps while the decay time first decreases then increases accordingly. The rise time shows exponential decay with the increasing of temperature. The thermal activation temperature for ISP process is deduced to be 184.9 K, corresponding to a thermal activation energy of 16 meV.
Polarization-induced electric fields in AlGaN quantum wells have important effects on avalanche breakdown of AlGaN
quantum-well photodiodes. When the polarization-induced fields within the AlGaN well layers have the same direction
as applied electric field, they can help enhance impact ionization rate and decrease threshold voltage of avalanche
breakdown of AlGaN avalanche photodiodes. However, according to previous research on avalanche breakdown of
AlGaN photodiodes, no distinct breakdown threshold was observed from current-voltage curve. Instead, a soft avalanche
breakdown was observed across applied voltage ranging from zero to a few volts while electroluminescence spectra
show a threshold of about 10 V for avalanche breakdown. In this work, by considering impact ionization of defect levels
and carrier screening effect, impact ionization coefficients are calculated as functions of applied voltage and the soft
breakdown is well explained. It is also found that strong carrier screening effect will decrease impact ionization rate in a
certain range of voltage thus affecting device performance.
Photocurrent spectra of doped black silicon (BSi) samples were investigated using metal-semiconductor-metal (MSM)
structure. The BSi samples were fabricated through femtosecond-laser doping method. Two pieces of samples were
annealed in nitrogen ambient for 30 minutes at different temperatures 350°C and 700°C. One control sample remains
without annealing. It was found that the doped black silicon samples have an electron mobility as low as 40~50 cm2/V s
but a conductivity as high as 4 ~ 5 Scm-1. The high conductivity allows making electrodes by directly contacting metal
stripes onto the black silicon surfaces. For the sample without annealing, its photocurrent spectrum covers a wavelength
range from 400 nm to 1200 nm. For the sample annealed at 350°C, no significant improvement was found except
disappearance of a defect induced photocurrent peak at 660 nm. Further annealing at 700°C, as observed for the third
sample, was found to greatly help enhance photoresponse in the wavelength range from 400 nm to 800 nm. The
photocurrent spectra under different biases were also measured. With the increasing of bias from 0 to 0.6 V, the peak
photoresponse was enhanced by about 5 times while large dark current brought in substantial noise level as well.
Carrier-screening effect in an AlGaN quantum-well avalanche photodiode was investigated. The avalanche photodiode
is a p-i-n diode consisting of three periods of Al0.1Ga0.9N/Al0.15Ga0.85N multiple quantum wells (MQWs) as the active
region. Avalanche electroluminescence (EL) spectra were measured at different reverse bias voltages. By measuring the
quantum Stark induced red-shift of the EL peak at different bias values , it was found that carrier screening decreases the
local electric field across the AlGaN quantum wells, resulting in a reduced red shift of the EL peak, while enhancing the
EL intensity. The carrier screening was found to be strong at the onset of avalanche breakdown of the diode and become
weaker with increasing of the applied electric field. This is explained by considering the strong polarization-induced
internal field in the barriers of the quantum wells. The polarization-induced electric field in the barriers has a direction
opposite to the applied field thus producing a potential barrier to block the carrier transport. This leads to accumulation
of carriers in the quantum wells, thus producing the screening effect. The direction of electric field in the barriers inverts
when the applied bias increases to be larger than an inverse threshold. Carriers can then be smoothly transported and
carrier screening disappears. Our study shows that carrier-screening effect can play an important role in III-Nitride
heterojunction devices due to the existence of strong polarization fields.
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