Split-off band detectors have been demonstrated operating at or above room temperatures. However the specific
detectivity was somewhat low due to the high dark current. Increasing the barrier height can suppress the dark
current at high temperatures but results in a low responsivity due to the capture of carriers in emitters. A difference
between the heights of the barriers on the two sides of an emitter provides highly energized carriers injected into the
emitter and hence reduces the trapping effects. Three p-GaAs/AlGaAs samples with different Al fractions in the
graded barriers are used to test these effects. Due to the graded barrier, the samples have an asymmetric band
structure which makes it easier for excited carriers to travel in one direction than in the other. Therefore,
photovoltaic operation is possible due to the built-in potential under equilibrium. Preliminary results obtained from
these samples will be discussed.
Overcoming the stringent cooling requirement for the operation of most of the infrared (IR) detectors is one of the major
challenges towards capturing their full potential. Split-off (SO) transitions based detector exhibit encouraging results and
gives hope to provide a novel alternative to the conventional IR detectors operating with cryogenic aid. Recently, a
GaAs/AlGaAs SO detector operating up to 330 K in the 3-5 μm spectral region was developed. This paper presents
various design modifications including graded barrier (in place of flat barrier), and double barrier resonant structure (in
place of a single barrier) to improve the performance of these detectors. The graded barrier improves the detector
performance by reducing the space charge buildup due to the trapping of charge carriers at the emitter-barrier interface;
additionally, the model implementation on GaAs/AlGaAs based detectors also suggests that a barrier offset of 20 meV
approximately doubles the responsivity. The implementation of a double barrier resonant structure increases the escape
of holes from the SO to the light/heavy hole (LH/HH) bands by bringing the two bands into resonance and increases the
response by a factor of ~ 85. The results from our ongoing efforts to extend the concept of SO mechanism based IR
detection towards longer wavelength are also presented. This should be possible by exploiting SO absorption in
alternative material systems such as phosphides and nitrides. The successful utilization of SO mechanism can result in
the high operating temperature detectors operating in mid-IR and terahertz (THz) region.
Recently developed high operating temperature (up to 330 K) GaAs/AlGaAs detectors responding in the 3-5 μm
wavelength range and based on split-off (SO) transitions followed by escape by scattering to the light/heavy
hole(LH/HH) band or by direct quantum mixing of the states offer a viable alternative to present day detectors operating
at cryogenic temperatures. This paper presents a theoretical model to predict the response of SO detectors. The model
calculates the dark current and illuminated currents from the photoabsorption, carrier escape, and transport, explaining
the experimental response. Using this model, different strategies to improve the performance of the GaAs based SO
detectors are presented. A graded barrier improves the performance by reducing the space charge build up, and the
double barrier resonant structure by enhanced escape of holes from the SO to the light/heavy hole bands by bringing the
two bands into resonance. A detailed analysis of the effect of detector parameters on responsivity and D* is made. The
change of material system to GaN/AlGaN should extend the response to longer wavelengths (THz) as its zinc blende and
wurtzite crystal structures have SO transition energies of 20meV and 8meV respectively. Experimental measurement of
SO absorption in GaN and potential THz detector designs are discussed.
Capacitance-voltage-frequency measurements on n+-GaN/AlxGa1-xN Heterojunction Interfacial Workfunction
Internal Photoemission (HEIWIP) detectors were used to analyze the effects of Al fraction induced heterojunction
barrier and its effect on the electrical characteristics at the heterointerface. The detector's IR threshold can
be modified by changing the barrier Al concentration. A sample with an Al fraction of 0.1 shows a distinct
capacitance step and capacitance hysteresis, which is attributed to N-vacancies and/or C-donor electron trap
states located just above the Fermi level (200 meV) at the GaN/AlGaN interface, with activation energies of
149±1 and ~189 meV, respectively. A sample with an Al fraction of 0.026 showed negative capacitance and
dispersion, indicating interface electron trap states located below the Fermi level (88 meV), most likely due to
C-donor and/or N-vacancy with activation energies of 125±1 and 140±2 meV, respectively. Additional impurity
related absorption centers were identified in both samples, however these shallow Si-donor sites (~30.9±0.2 meV)
did not affect the capacitance as these states were located in the barrier layer and not in the vicinity of the Fermi
level. The Al fraction in the barrier layer was found to significantly change the positions of the interface trap
states relative to the Fermi level, resulting in the observed capacitance characteristics.
Detection of both UV and IR radiation is useful for numerous applications such as firefighting and military
sensing. At present, UV and IR dual wavelength band detection requires separate detector elements. Here
results are presented for a GaN/AlGaN single detector element capable of measuring both UV and IR response.
The initial detector used to prove the dualband concept consists of an undoped AlGaN barrier layer between
two highly doped GaN emitter/contact layers. The UV response is due to interband absorption in the AlGaN
barrier region producing electron-hole pairs which are then swept out of the barrier by an applied electric field
and collected at the contacts. The IR response is due to free carrier absorption in the emitters and internal
photoemission over the work function at the emitter barrier interface, followed by collection at the opposite
contact. The UV threshold for the initial detector was 360 nm while the IR response was in the 8-14 micron
range. Optimization of the detector to improve response in both spectral ranges will be discussed. Designs
capable of distinguishing the simultaneously measured UV and IR by using three contacts and separate IR and
UV active regions will be presented. The same approach can be used with other material combinations to cover
additional wavelength ranges, e.g. GaAs/AlGaAs NIR-FIR dual band detectors.
A three color normal incidence quantum dots in a well (DWELL) operating in the mid-wave infrared (MWIR), long wave infrared (LWIR) and very long wave infrared (VLWIR) are reported. The peak operating wavelengths are at ~ 6 mm, ~10.5 mm and ~ 23.2 mm. We believe that the shorter wavelength response (6 mm and 10.5 mm) are due to bound-to-continuum and bound-to-bound transitions between the states in the dot and states in the well, whereas the longer wavelength response (23.2 mm) is due to intersubband transition between dot levels. A bias dependent activation energy ~ 100 meV was extracted from the Arrhenious plots of the dark currents, which is a factor of three larger than that observed in quantum well infrared photodetectors operating at comparable wavelengths.
HEterojunction Interfacial Workfunction Internal Photoemission (HEIWIP) detectors have been demonstrated with cutoff wavelengths λc up to 92 μm. One method of increasing the response in a desired range is to employ the cavity effect to create resonant maxima. Results are reported here confirming the presence of cavity enhancements in both the absorption and the detector response of HEIWIP structures at the 3λ/4 resonance. The detectors consisted of 13, 19 and 30 Be doped GaAs emitter and undoped Al0.02Ga0.98As barrier layers. Transmission and reflection spectra for multilayer GaAs/AlGaAs IR detectors in the range 2000-100 cm-1 at room temperature are presented. Comparisons with the calculated results based on free carrier absorption and interaction with optical phonons model are reported. It is shown that the absorption can be maximized by
using the resonant cavity effect. The use of the resonant cavity effect should allow the design of detectors with increased response in the desired wavelength ranges.
Results are presented for a novel HEterojunction Interfacial Workfunction Internal Photoemission (HEIWIP) far-infrared detector with a cutoff wavelength of 70 micrometers . A responsivity of 10.5 A/W and a D* of approximately 1013 cm(root)Hz/W at 20 micrometers was achieved at 4.2 K. Dark current for the detectors was 2 orders of magnitude better than for homojunction interfacial workfunction internal photoemission (HIWIP) detectors at liquid helium temperatures. Capacitance measurements show similar behavior to other infrared photodetectors such as HIWIPs and QWIPs. The overall superior characteristics of HEIWIP detectors over HIWIP and QWIP detectors at longer wavelengths are of interest for future developments in far-infrared applications.
A new method of infrared detection without any preamplifiers is presented. The method is based on well studied multiquantum well (MQW) structures and the injection mode integrate-and-fire infrared detectors using p+-n- n+ structures. A brief introduction to injection mode IR detectors (with p+-n-n+ diodes) is given. A model based on device physics to describe spontaneous pulsing in GaAs/AlGaAs quantum well structures is presented. The model includes space charge generation/recombination mechanisms such as tunneling and impact of injected hot electrons with cold well electrons, with current voltage relations leading to spontaneous pulsing in MQW structures. It is shown with the correct parameters, spontaneous pulsing could be observed in MQW structures at 300 K. The IR photoionization contribution to space charge generation is added to the model to obtain the infrared response. This response can take the form of a change in the pulse rate or pulses being generated for a subthreshold bias conditions. This gives rise to novel IR detectors operating above 77 K up to near room temperature.
We consider an iterative map derived from the device equations for a silicon p+-n-n+ diode, which simulates a biological neuron. This map has been extended to a coupled neuron circuit consisting of two of these artificial neurons connected by a filter circuit, which could be used as a single channel of a parallel asynchronous processor. The extended map output is studied under different conditions to determine the effect of various parameters on the pulsing pattern. As the control parameter is increased, fixed points (both stable and unstable) as well as a limit cycle appear. On further increase, a Hopf bifurcation is seen causing the disappearance of the limit cycle. The increasing control parameter, which is related to a decrease in the bias applied to the circuit, also causes variation in the location of the fixed points. This variation could be important in applications to neural networks. The control parameter value at which the fixed point appear and the bifurcation occurs can be varied by changing the weightage of the filter circuit. The modeling outputs, are compared with the experimental outputs.
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