As a narrow bandgap semiconductor, the preparation of surface passivation layers on HgCdTe film epilayers is essential in the process of device fabrication. Most new infrared detectors use the mesa structure. A stable and reproducible passivation technology which meets the surface uniform cover of the high aspect ratio mesa is particularly important. Atomic layer deposition (ALD) is a new type of accurate surface thin film preparation technique, which has several characteristics such as depositing large-area uniform films, making the film thickness control at nanometer level feasible, and lower deposition temperature. ALD-ZnS film is prepared on the HgCdTe IRFPAs chip at 65°. I-V and R-V curves are similar to that of IRFPAs with CdTe thermal passivation. This shows that ALD ZnS film has a good potential application in the passivation of high aspect ratio mesa-array HgCdTe devices.
The infrared focal plane arrays detector is a multilayer structure which is mainly composed of detector chip, Si-ROIC, and fan out layer. In view of the different thermal expansion coefficients between the material layers, considerable thermal stress will be generated in this device among the cooling cycle which could lead to physical breakdown of the chip under extreme circumstances. Models of finite element analysis (FEA) were established to explore the thermal stress of HgCdTe infrared focal plane devices at low temperature. According to the characteristics of the expansion alloy, the two kinds of focal plane device structures were simulated: one is that with invar layer below the Al2O3 piece, the other is that kovar layer between the Si-ROIC and Al2O3 pieces. Both of them can reduce the thermal stress effectively, and improve the reliability of IRFPAs detector.
Etching at cryogenic temperature can reduced the etch-induced damage in HgCdTe during etch process, which is important to fabricate high performance IRFPAs (Infrared Focal Plane Arrays) detectors. The etch rates of HgCdTe were examined to be similar at different temperatures and the smoothness of the etched surface improves at cryogenic temperature using a standard process, and the etch rates of different CH4/Ar/H2 plasmas at 123K were also investigated. Addition of H2 increases the roughness of etched sidewall while has little effect on etched bottom surface roughness, and SiO2 with a contact layer of ZnS functioned well as etch mask during cryoetching under CH4/Ar/H2 plasmas.
HgCdTe is one of the dominating materials for infrared detection. To pattern this material, our group has proven the feasibility of SiO2 as a hard mask in dry etching process. In recent years, the SiO2 mask patterned by plasma with an auto-stopping layer of ZnS sandwiched between HgCdTe and SiO2 has been developed by our group. In this article, we will report the optimization of SiO2 etching on HgCdTe. The etching of SiO2 is very mature nowadays. Multiple etching recipes with deferent gas mixtures can be used. We utilized a recipe containing Ar and CHF3. With strictly controlled photolithography, the high aspect-ratio profile of SiO2 was firstly achieved on GaAs substrate. However, the same recipe could not work well on MCT because of the low thermal conductivity of HgCdTe and CdTe, resulting in overheated and deteriorated photoresist. By decreasing the temperature, the photoresist maintained its good profile. A starting table temperature around -5°C worked well enough. And a steep profile was achieved as checked by the SEM. Further decreasing of temperature introduced profile with beveled corner. The process window of the temperature is around 10°C. Reproducibility and uniformity were also confirmed for this recipe.
To fabricate various advanced structures with HgCdTe material, the Inductively Coupled Plasma
enhanced Reactive Ion Etching system is indispensable. However, due to low damage threshold and
complicated behaviors of mercury in HgCdTe, the lattice damage and induced electrical conversion is
very common. According to the diffusion model during etching period, the mercury interstitials, however,
may not diffuse deep into the material at cryogenic temperature. In this report, ICP etching of HgCdTe at
cryogenic temperature was implemented. The etching system with cryogenic assembly is provided by
Oxford Instrument. The sample table was cooled down to 123K with liquid nitrogen. The mask of SiO2
with a contact layer of ZnS functioned well at this temperature. The selectivity and etching velocity
maintained the same as reported in the etching of room temperature. Smooth and clean surfaces and
profiles were achieved with an optimized recipe.
Detection in the very long wave infrared range (LWIR, 12-15µm) using third-generation infrared focal plane array (FPAs) is essential for remote atmosphere sounding. Indeed, these wavelengths are particularly rich in information about humidity and CO2 levels and provide additional information about cloud structure and temperature profile across the atmosphere. However, the dark current characteristic and associated noise behavior of the HgCdTe photodiode in the wavelength range of 12-15µm, operating at ~77K, are very sensitive to surface passivation techniques as well as to surface material treatments. For current HgCdTe material and device technology, detection of LWIR and VLWIR energy is the subject of current research. Within this range of shrinking band-gaps in detector material, precise control of the quality of the surface passivation and treatment is of great importance. The underlying physics of dark current mechanism is theoretically investigated by using a previously developed simultaneous current extraction approach and numerical simulations.
In addition, HgCdTe electron avalanche photodiodes (e-APD) have been widely used for low-flux and high-speed application. To better understand the dark current transport and electron-avalanche mechanism of the devices and optimize the structures, we perform accurate numerical simulations of the current-voltage characteristics and multiplication factor in planar and mesa homojunction (p-i-n) HgCdTe electron-avalanche photodiodes.
The barrier cap layer (BCL) is considered to be able to absorb partially implant induced damages during ion implantation, thus its structure and property could impact the result of ion implantation. In this paper, for As ion implantation in HgCdTe, the different BCLs were deposited on the CdZnTe-based (LPE) and GaAs-based (MBE) HgCdTe epilayers, respectively. Then, the influences of thicknesses and structures of these BCLs on dopant profiles and implant damages were investigated. The as-grown BCLs include thermally evaporated (TE) ZnS, TE CdTe, electron beam evaporated (EBE) CdTe and in-situ CdTe/ZnTe grown by MBE. The SIMS profiles and TEM characterization indicate: For TE ZnS BCLs, there exists an optimized thickness to obtain the deepest As indiffusion after high temperature annealing, and the end-of-range (EOR) depth is linearly proportional to the thickness ratio of a-MCT layer/damage layer. For TE CdTe BCLs, the barrier layer induced channeling effect (BLICE) occurs to the thin BCL samples, while this effect is suppressed in the thick BCL samples. The phenomenon might be due to that the blocking effect of the layered structure inside each crystal column becomes dominate in the thick BCL samples. Additionally, the EBE CdTe BCL with layered structure can suppress effectively the BLICE effect; in the in-situ CdTe/ZnTe BCL, the short defect layer generated in the CdTe buffer layer and the amorphization of the ZnTe layer during ion implantation also play a significant role in suppressing the BLICE effect.
Thermal stability of Atomic Layer Deposition Al2O3 film on HgCdTe was investigated by Al2O3 film post-deposition annealing treatment and Metal-Insulator-Semiconductor device low-temperature baking treatment. The effectiveness of Al2O3 film was evaluated by measuring the minority carrier lifetime and capacitance versus voltage characteristics. After annealing treatment, the minority carrier lifetime of the HgCdTe sample presented a slight decrease. Furthermore, the fixed charge density and the slow charge density decreased significantly in the annealed MIS device. After baking treatment, the fixed charge density and the slow charge density of the unannealed and annealed MIS devices decreased and increased, respectively.
The nBn structure with an electron barrier sandwiched by n-type cap and absorber layers was predicted to suppress the Shockley-Read-Hall (SRH) generation-recombination processes and surface leakage. The MCT nBn structure has been studied by several groups to implement high operating temperature (HOT) device. In this report, the numerical analysis of the Hg1-xCdxTe nBn device in LWIR region (x=0.225) is performed utilizing Crosslight APSYS. The detector performance characterized by dark current, photo-current and detectivity is optimized by adjusting structural parameters such as Cd component and doping of each layer under various biases. Among the parameters, the trade-off between ΔEc and ΔEv is most intensively affected by Cd component of the barrier which was modified carefully and accomplished firstly. Furthermore, the effect of the trap density and trap energy level on the device performance is also investigated especially according to the processing techniques. At 110K, the optimized detectivity of the LWIR MCT nBn device reaches 7.5×1010 cmHz1/2/W in this report, comparable with that of the DLPH device (7.6×1010 cmHz1/2/W). The novel nBn HgCdTe structure is potentially valuable in LWIR region since the controllable p-doping issue is circumvented and passivation process is simplified.
In this work, a novel junction profile measurement method is proposed. A serial of junctions were fabricated by B+ implantation. Then a beveled bar which was about 10mm long and several micrometers deep was formed by carefully controlled wet-etching. The remaining depth of n region changes from the full depth that is about 5.3mm after ion implantation to zero depending on its lateral position and the slope of the etching bar. Voltage-current and Laser Beam Induced Current (LBIC) measurements were applied to determine the HgCdTe junction edge. The LBIC signal orrectification characteristic indicates the existence of a PN junction. The junction depth is extracted from the position where the PN junction disappears and the slope of the etching bar. The junction depth of intrinsic doped HgCdTe was measured, which is about 2.4μm. A significant 0.4mm thick N-region was observed. Moreover, junction depths of samples annealed for different time were also investigated. By this method, it’s possible to measure the three dimensional profile of a planar PN junction.
An innovative heterojunction photodiode structure in HgCdTe-on-Si long-wavelength (LW) infrared focal plane array (IRFPA) detector is investigated in this paper. The quantum efficiency and the photoresponse of devices have been numerically simulated, using Crosslight Technology Computer Aided Design (TCAD) software. Simulation results indicate that in contrast to the p+-on-n homojunction photodiode, the heterojunction photodiode effectively suppresses the
crosstalk between adjacent pixels and interface recombination between HgCdTe active region and
buffer layer on Si substrate. And in the range of the LW-band, the quantum efficiency of the heterojunction photodiode increases by 35.5%. Furthermore, the heterojunction photodiode acquires the narrow-band response spectrum desired in the application of the LW IRFPA detectors
as the p+-on-n homojunction photodiode with the optical filter. Finally, the smaller bulk resistance of its heavily doped N-type layer ensures the uniformity of the pixel series resistance in the large format IRFPAs.
N2 atmosphere annealing process to recover ICP etching induced damage on p type mercury vacancy HgCdTe film has been exploited in this paper. ICP etching and N2 atmosphere annealing processes were carried out on a series of Hgvacancy-doping p-type HgCdTe samples. The carrier transport and lifetime properties of these samples were characterized by Hall measurement and microwave reflectance method respectively. P-to-n electrical damage of the surface HgCdTe film induced by the ICP etching process was deduced from the polarity inversion of Hall coefficient. The carrier transport and lifetime properties were similar to those of the non-etched samples, indicating that the surface HgCdTe electrical damage was recovered partially by annealing at 210°C for 2 hours. I-V and R-V characteristics curves of photodiodes fabricated on the etched and N2 atmosphere annealing processed MCT samples were also comparable to those of photodiodes on the non-etched MCT samples in the following experiments. These results show that N2 atmosphere annealing process is a readily available and promising recovering technique for HgCdTe ICP etching induced damage.
The polarity inversion of laser beam induced current (LBIC) signal at low temperature and high
laser power density in As-doped p-type HgCdTe is investigated in this paper. It is found that the
polarity of LBIC signal reverses at 87 K compared to that at 300 K and the high laser power density is
also an important factor in inducing the LBIC signal reverse. The results demonstrate that the shape of
the LBIC signal profile is strongly dependent on the temperature of the device and the laser irradiation.
To provide a reasonable analysis for this interesting fact, a photocarrier spreading mode is presented in
this paper.
A novel mask technique, combining high selectivity silicon dioxide patterns over high aspect-ratio
photoresist (PR) patterns has been exploited to perform mesa etching for device delineation and electrical
isolation of HgCdTe third-generation infrared focal plane arrays (IRFPAs). High-density silicon dioxide film
covering high aspect-ratio PR patterns was deposited at the temperature of 80°C and silicon dioxide film
patterns over high aspect-ratio PR patterns of HgCdTe etching samples was developed by standard
photolithography and wet chemical etch. Scanning electron microscopy (SEM) shows that the surfaces of
inductively coupled plasma (ICP) etched samples are quite clean and smooth. The etching selectivity between
the novel mask and HgCdTe of the samples is increased to above 32: 1 while the side-wall impact of etching
plasma is suppressed by the high aspect ratio patterns. These results show that the combined patterning of
silicon dioxide film and thick PR film is a readily available and promising masking technique for HgCdTe
mesa etching.
We report on the temperature-dependent extension of n-type inversion regions in mercury cadmium telluride (HgCdTe) photodiodes at low temperatures (87 K) compared to inversion regions at room temperature (300 K). Laser-beam-induced-current (LBIC) measurement techniques are used to obtain the photosensitive area extensions of n-type inversion in HgCdTe photodiodes for typical n+-on-p HgCdTe photovoltaic IR detectors. The effect of temperature on the extension of n-type conversion region is investigated by considering the sign of the LBIC signal. Theoretical results show that the hole concentration decreases in multidoped HgCdTe due to the freeze-out effect as the temperature decreases. Consequently, hole concentration is much lower than electron concentration at 87 K. The n-type inversion region extension is shown to be caused with the p-to-n type conversion.
Hydrogen-based dry plasmas, generated in inductively coupled plasma reactors have been demonstrated to be very
effective in fabricating high fill-factor mesa of Hg1-xCd xTe multi-layer hetero-structure material for infrared focal plane
array applications. To obtain reasonable dry etching process for Hg 1-xCd xTe, it is essential to investigate the physical,
chemical, and electrical characteristics of the surface. This paper explores the effect of varying the plasma process
parameters on the surface of Hg 1-xCd xTe. The surface chemical analysis was carried out using spot X-ray photoelectron
spectroscopy (XPS), the surface roughness was measured by atomic force microscopy (AFM), and p-to-n type
conversion depth was assessed by a reliable current-voltage test of a designed structure basing on material-chip
technology concept and a convenient technology of cross-section surface potential imaging (SPM). At last, Hg 1-xCd xTe
etched surfaces with roughness low and mechanical or electrical damage free were achieved.
The high-density inductively coupled plasma etching technique was applied to HgCdTe, while using the RF-powered wafer electrode to provide low plasma energy. By using a CH4/H2/N2/Ar chemistry the HgCdTe etch profiles were studied as a function of mask selectivity, chamber pressure, gas ratio and ICP power. The etch rate was found to decrease as etch depth increasing. The LBIC and I-V measurements were employed to investigate the electrical damage of HgCdTe material caused by plasma bombardment.
This paper presents the recent progress on the study of device processings at multilayer HgCdTe film for integrated two-color (SWIR/MWIR) n-p-P-P-N detector arrays. The four-layer p-P-P-N heterostructures Hg1-xCdxTe film needed to achieve two color detector arrays was grown by molecular beam epitaxy (MBE) on (211)B oriented GaAs substrates. The secondary ion mass spectroscopy (SIMS) data for the HgCdTe film was obtained. The p-type layer on top of a thin P-type potential barrier layer and the SWIR P-on-N homojunction photodiode formed in-situ during MBE growth using indium impurity doping was processed into the MWIR planar photodiode by selective B+-implantation. The preliminary 256×1 linear arrays of SWIR/MWIR HgCdTe two-color FPAs detector were then achieved by mesa isolation, side-wall passivation and contact metallization. At 78K, the average R0A values of SWIR and MWIR are 3.852×105 Wcm2 and 3.015×102 Wcm2, and the average peak detectivities Dλp* are 1.57×1011cmHz1/2/W and 5.63×1010 cmHz1/2/W respectively. The SWIR photodiode cut-off wavelength is 3.04μm and the MWIR photodiode cut-off wavelength is 5.74μm, quite consistent with the initial device design. The SWIR response spectrum of the two-color detector with a distinct fall-off at shorter wavelength regime was discussed especially.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.