## Spectral Features Indicating the Presence of Sub-Bandgap Defects in CZT Crystal
Sub-bandgap defects in
Cadmium Zinc Telluride (CZT) crystals can significantly affect the performance of detectors, particularly in
radiation detection applications such as
gamma-ray or
X-ray detection. These defects can introduce energy states within the
bandgap of the crystal, resulting in
electron trapping,
carrier recombination, and degraded
spectral response. Detecting these defects often requires analyzing the spectral characteristics of CZT crystals, where specific features in the
photoluminescence (PL),
electrical, and
spectral response can be indicative of these sub-bandgap states.
## 1. Photoluminescence (PL) Spectroscopy
One of the most sensitive techniques to detect sub-bandgap defects in CZT crystals is
photoluminescence (PL) spectroscopy. In this technique, the crystal is excited with photons, and the emitted light is analyzed. The key spectral features to look for include:
*
Defect-related Emission Peaks:
* Sub-bandgap defects often give rise to
localized energy states in the
mid-gap region. These states can act as recombination centers for carriers, resulting in
defect-related PL peaks just below the
bandgap energy.
* For CZT, these emission peaks often appear between
1.5 eV and 1.8 eV, below the primary bandgap of the material (which typically ranges from 1.5 eV for CdTe to 1.7 eV for ZnTe).
* Peaks in the
1.45–1.7 eV range are commonly attributed to deep-level defects or impurity states, often associated with
cadmium vacancies (V\_Cd),
zinc vacancies (V\_Zn), or
tellurium antisites (Te\_Cd).
*
Red-Shifted Emissions:
* If a defect-induced state is deep within the bandgap, the emission peaks can be
red-shifted (i.e., moved towards lower energies) due to the low energy of these defect levels.
* In some cases, a
shoulder or
broad emission band in the
1.4–1.6 eV region may appear, indicating
deep acceptor levels that are typical of
defective regions.
## 2. Photoresponse Spectroscopy
In
photoresponse spectroscopy, a
photodetector made from CZT is illuminated by photons of varying energy, and the
current generated is measured as a function of wavelength or energy. Sub-bandgap defects in the crystal can alter the detector’s response:
*
Decreased Signal Efficiency: Defects in the sub-bandgap region can act as
recombination centers for charge carriers, leading to
lower efficiency in converting absorbed photons into collected charge, which is reflected in a
reduced signal output.
*
Enhanced Low-Energy Peaks: Defects within the bandgap may introduce additional
low-energy peaks or
tailing effects in the energy spectrum, particularly in detectors designed for high-energy radiation like
gamma-rays. This manifests as
broadening or
shouldering of peaks in the low-energy side of the spectrum.
*
Tailing Phenomenon: Spectral
tailing in the
low-energy region of the gamma-ray spectrum can be indicative of sub-bandgap defects, as these defects can trap carriers and delay their movement to the electrodes.
## 3. Deep-Level Transient Spectroscopy (DLTS)
Deep-Level Transient Spectroscopy (DLTS) is a highly effective technique for probing sub-bandgap defects. It involves applying a
pulse bias to a
CZT detector and measuring the transient capacitance or current response. This method is particularly sensitive to
trap states and
deep-level defects in the material. The key features include:
*
Deep-Level Trap States: DLTS can identify
energy levels within the
bandgap by analyzing the
time-dependent response to a voltage pulse. Sub-bandgap defects often correspond to
localized trap states that exhibit characteristic time constants (e.g.,
capture times) that vary based on their depth in the gap.
*
Peak Signatures: Peaks in the DLTS spectrum that appear at
lower temperatures or at
specific energies in the bandgap (typically below the conduction band edge) provide evidence for
deep-level traps. These traps often result from
vacancies (e.g.,
Cd vacancies) or
antisite defects.
## 4. X-ray and Gamma-ray Spectroscopy
When CZT detectors are exposed to
X-ray or
gamma-ray radiation, sub-bandgap defects can affect the
energy resolution and
spectral response:
*
Spectral Tailing: Defects within the bandgap can introduce
carrier trapping that leads to
tailing effects in the
energy spectrum, particularly at lower energies. These defects act as
trapping centers, delaying the response of the carriers and leading to
non-ideal spectral peaks.
*
Increased Dark Current: Sub-bandgap defects can increase the
dark current and
leakage current in CZT detectors. This can be observed as a
shift or
broadening in the energy spectrum when detectors are operating in low-light conditions.
## 5. Current-Voltage (I-V) Measurements
*
Reverse Leakage Current: Sub-bandgap defects often result in
increased leakage current in
reverse bias conditions. This increased leakage current can lead to a
shift in the energy spectrum and reduce the
signal-to-noise ratio (SNR). The I-V characteristics can reveal the presence of
deep levels by showing
non-idealities in the current-voltage relationship, particularly at higher reverse biases.
## 6. Surface and Bulk Defects
*
Surface Defects: In CZT,
surface defects can create
localized energy states within the bandgap, leading to
shallow states. These defects can affect the surface recombination of carriers and can be characterized by
surface photovoltage spectroscopy or
scanning electron microscopy (SEM) with
energy dispersive X-ray spectroscopy (EDX).
*
Bulk Defects:
Bulk defects such as
vacancies,
dislocations, and
antisites typically create
deep traps that can trap carriers for longer durations, resulting in
spectral distortions. These can be identified by
electron paramagnetic resonance (EPR) or
deep-level optical spectroscopy (DLOS).
## 7. Summary
Sub-bandgap defects in CZT crystals can be identified by the following spectral features:
*
Photoluminescence Peaks: Emission bands in the
1.4–1.7 eV range, corresponding to deep-level defects.
*
Spectral Tailing: Tailing of the energy spectrum, particularly in the
low-energy region, due to trapped carriers.
*
Defect-related Emissions: Red-shifted PL emissions indicating
mid-gap defect states.
*
Deep-Level States: Peaks in
DLTS spectra that correspond to deep traps in the bandgap.
*
Reduced Efficiency: A drop in the photoresponse and energy resolution due to recombination centers introduced by defects.
Identifying these spectral features allows for better understanding and mitigation of the impacts of defects on the performance of CZT detectors in radiation detection applications.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/blog/what-spectral-features-indicate-the-presence-of-sub-bandgap-defects-in-czt-crystal.html
