HgCdTe Photodiodes Research Articles

P-on-n HgCdTe Infrared Focal-Plane Arrays: From Short-Wave to Very-Long-Wave Infrared

This paper reports on recent developments made at the DEFIR joint laboratory on fabrication of planar p-on-n arsenic (As)-ion-implanted HgCdTe photodiodes. Our infrared focal-plane arrays (IRFPAs) cover a wide spectral range, from the short-wave infrared (SWIR) to the very-long-wave infrared (VLWIR). Our planar p-on-n technology is a classical one based on ion implantation followed by diffusion and activation. The p-type doping is obtained by As implantation, and n-type indium (In) doping is achieved during the epilayer growth. Our p-on-n IRFPAs show state-of-the-art performance from the SWIR to VLWIR spectral range. Mid-wave infrared (MWIR) and long-wave infrared (LWIR) FPAs have been designed with a television (TV) format and 15 μm pixel pitch. Preliminary results of high-operating-temperature detectors obtained in the MWIR (λ c = 5.3 μm at 80 K) have shown highly promising electrooptical performance above 130 K. For space applications, imagers dedicated to low-flux detection have first been produced as TV/4 focal-plane arrays, with 15 μm pitch in the SWIR range (2 μm). Finally, TV/4 arrays with 30 μm pixel pitch have been manufactured for the VLWIR range. The measured dark current fits the “Rule 07,” with homogeneous imagers.

Numerical Analysis of Current–Voltage Characteristics of LWIR nBn and p-on-n HgCdTe Photodetectors

We performed numerical analysis of the current–voltage characteristics of long-wavelength infrared unipolar HgCdTe nBn photodetectors and compared those results with those of conventional p-on-n HgCdTe photodiodes. A computer program was applied to explain in detail the impact of the charge carrier generation and recombination processes on current densities. In our model the carrier diffusion, thermal generation–recombination, band-to-band tunneling, trap-assisted tunneling (via states located at mercury vacancies as well as dislocation cores), and impact ionization are included as potential limiting mechanisms. To validate the model, we compared the theoretical predictions with experimental data of high-quality p-on-n photodiodes published in the literature.

Issues in HgCdTe Research and Expected Progress in Infrared Detector Fabrication

The purpose of this paper is to review the trends in HgCdTe research, illustrating the discussed ideas with the latest results obtained at DEFIR (CEA-LETI and Sofradir joint laboratory). The beginning of this paper is devoted to an extended introduction to today’s issues concerning HgCdTe photodiode performance enhancement. In fact, very high-quality material is mandatory for ultrahigh performance at low temperature as well as for high noise operability at high operating temperature (HOT). Therefore, a strong effort has been carried out during the last few years for lattice-matched CdZnTe substrate and HgCdTe active layer growth improvement, leading to very large substrates and ultraflat liquid-phase epitaxy layers. The same analysis holds for diode process quality and passivation. Therefore, the photodiode process has been completely revisited in order to optimize HOT operability. Also necessary for the next generation of HOT devices, some significant progress has been made in small-pixel-pitch interconnection on silicon read-out circuits. Indeed, the first 10-μm focal-plane arrays (FPAs) have been successfully fabricated this year in the mid-wave infrared (IR) band. The latest avalanche photodiode (APD) realizations are also presented with 15-μm-pitch FPAs for passive imaging and 30-μm-pitch ultrafast arrays running at 1.5 kHz full frame rate. Linear-mode photon counting using APDs is also briefly discussed. Finally, the paper concludes on more complex structures for the third generation of IR detectors, discussing the latest achievements in dual-band FPA fabrication in various spectral bands.

Applications of the Infrared Measurement Analyzer: Hydrogenated LWIR HgCdTe Detectors

Low-cost silicon-based alternative substrates are an attractive choice for next-generation large-area high-resolution multicolor infrared (IR) detector arrays. However, the high density of dislocations formed during molecular-beam epitaxy growth of HgCdTe/CdTe/Si limits the performance of IR arrays, especially in the long-wavelength infrared (LWIR) region. Atomic hydrogen introduced by inductively coupled plasma (ICP) into HgCdTe is expected to passivate dislocations, bulk and surface defects, removing their contributions to dark current. Passivation using ICP hydrogenation can have different effects on HgCdTe photodiode performance, depending on which class of defects is being passivated. The infrared measurement analyzer (IRMA) was used to deconvolute the effects of hydrogenation on LWIR HgCdTe photodiodes through a reverse-modeling fit of the current–voltage (I–V) characteristic. This approach results in a fit with fewer false minima, low parameter error and bias, and high confidence in extracted device parameters. A description of this tool and its application to hydrogenated HgCdTe LWIR detectors is presented. Lower dark currents have been observed after hydrogenation of fully fabricated devices. Model-fits performed on a wide variety of LWIR HgCdTe photodiodes suggest that hydrogenation provides both surface and bulk quality improvements. These benefits of ICP hydrogenation have been retained over several months.

Effects of Inductively Coupled Plasma Hydrogen on Long-Wavelength Infrared HgCdTe Photodiodes

Bulk passivation of semiconductors with hydrogen continues to be investigated for its potential to improve device performance. In this work, hydrogen-only inductively coupled plasma (ICP) was used to incorporate hydrogen into long-wavelength infrared HgCdTe photodiodes grown by molecular-beam epitaxy. Fully fabricated devices exposed to ICP showed statistically significant increases in zero-bias impedance values, improved uniformity, and decreased dark currents. HgCdTe photodiodes on Si substrates passivated with amorphous ZnS exhibited reductions in shunt currents, whereas devices on CdZnTe substrates passivated with polycrystalline CdTe exhibited reduced surface leakage, suggesting that hydrogen passivates defects in bulk HgCdTe and in CdTe.

Temperature dependence characteristics of dark current for arsenic doped LWIR HgCdTe detectors

Resistance–voltage (R–V) curves of arsenic doped long-wavelength infrared (LWIR) Mercury Cadmium Telluride (HgCdTe) photodiodes were measured in the temperature range of 59–92K. The dark current characteristics of HgCdTe junction diode are presented by using a simultaneous-mode nonlinear fitting method. The observed R–V characteristics have been shown in agreement with the theoretical calculation by taking into account the contributions: (i) diffusion mechanism (Rdiff), (ii) generation–recombination mechanism (Rgr) in the depletion region, (iii) trap-assisted tunneling mechanism (Rtat), and (iv) band-to-band tunneling mechanism (Rbbt). Six characteristic parameters as function of temperature are extracted from the measured current–voltage (I–V) curves by considering the dominant current mechanisms under different bias levels. The fitted current components under different temperatures show that, as the temperature rises, the contribution to the dominant dark current component around maximum dynamic resistance range is changed from the trap-assisted tunneling and diffusion currents to the generation recombination effect. This change indicates that the dark current component may mainly be caused by the generation recombination current, which limits the performance of arsenic doped LWIR HgCdTe detectors.

Analysis of the response time in high-temperature LWIR HgCdTe photodiodes operating in non-equilibrium mode

The paper presents the theoretical investigation of the active region parameters, especially the influence of thickness and doping, on the response time and current responsivity of high-temperature long wavelength infrared HgCdTe photodiodes operating at 230K in non-equilibrium mode. Results of theoretical predictions of time constant were compared to the experimental data. The response time of the devices have been characterized using Nd:YAG laser, optical parametric generator with pulse width <25ps and fast oscilloscope with suitable transimpedance amplifier as a function of detector design, temperature and bias. The reverse bias applied to the photodiode causes Auger-suppression and improve the performances of the devices. This way the response time decreases to the value below 1ns at the good current responsivity increased to the value of about 6A/W and what is a promising parameter in view of potential telecommunication applications. Due to the series resistance of electrical connections, the response time of the devices is mainly limited by RC constant while the calculations show that the time constant of the Auger suppressed structures should be limited by the drift time of carriers.

MOCVD grown HgCdTe device structure for ambient temperature LWIR detectors

The authors report on advanced metalorganic chemical vapor deposition (MOCVD) grown HgCdTe device structure for an ambient temperature long wavelength infrared radiation (LWIR) detector application. MOCVD technology with a wide range of composition and donor/acceptor doping and without post grown annealing seems to be an excellent tool for HgCdTe heterostructure epitaxial growth structure. The N+/G/π/G/P+/G/n+ (where G denotes graded interface region) HgCdTe photovoltaic device concept of a specific barrier bandgap architecture integrated with Auger-suppression is a good solution for high operating temperature (HOT) infrared detectors. Selected problems of photodiode designing are indicated. Theoretical modelling using APSYS platform supports design and better understanding of the carrier transport mechanism in the photodiode structures. SIMS profiles allowed comparing projected and obtained structures and revealed diffusion processes in the structures. The negative differential resistance, clearly visible on current–voltage characteristics, evidences for Auger-suppression due to exclusion and extraction phenomena. It is shown that the thickness and arsenic doping of the active region influence the reverse bias dark current minimum and the response time of LWIR HOT HgCdTe photodiodes. Reverse bias highly (up to 50 times) increases responsivity that could reach 6 A/W at 300 K.

Investigations on a Multiple Mask Technique to Depress Processing-Induced Damage of ICP-Etched HgCdTe Trenches

A multiple mask technique, integrating patterned silicon dioxide (SiO2) film over patterned thick photoresist (PR) film, has been investigated as a method to perform mesa etching for device delineation and electrical isolation of mercury cadmium telluride (HgCdTe) third-generation infrared focal-plane arrays. The multiple mask technique was achieved by standard thick PR photolithography, SiO2 film deposition to cover the thick PR patterned film, and etching the SiO2 film at the bottom region after another photolithography process. The dynamic resistance in the zero-bias and low-reverse-bias regions of HgCdTe photodiode arrays isolated by inductively coupled plasma (ICP) etching with the multiple mask of patterned SiO2 and patterned thick PR film underneath was improved one- to twofold compared with a simple mask of patterned SiO2. It is suggested that the multiple mask technique is capable of maintaining high etching selectivity while reducing the side-wall processing-induced damage of ICP-etched HgCdTe trenches. The results show that the multiple mask technique is readily available and shows great promise for etching HgCdTe mesa arrays.

Principal component analysis of non-uniformity in electrical characteristics of HgCdTe photodiode arrays

We present a method of analyzing the non-uniformity in electrical characteristics of HgCdTe photodiode arrays for infrared imaging applications. We have selected dynamic resistance–voltage (R–V) characteristics for analyzing electrical behavior of HgCdTe photodiodes because the dynamic resistance at a given operating voltage directly governs the imager performance and being derivative of current–voltage (I–V) characteristics, it has little impact of the constant shifts due to stray illumination during dark measurements, relaxing the stringent requirement of perfect dark conditions to some extent for performance analysis. We have demonstrated that by using statistical analysis such as correlation of the selected signatures and their principal component analysis, we can identify the root cause of the high non-uniformity among sensor pixels in the array. The method has been implemented using theoretical I–V model of MWIR HgCdTe photodiodes, but it is generic and may be implemented on any other types of diode arrays for theoretical or experimental analysis of their non-uniformity.

Numerical investigation of Auger contributed performance loss in long wavelength infrared HgCdTe photodiodes

Through numerical simulations at 77K, it is shown that Auger suppression has a twofold effect on the sensitivity of the LWIR p on n HgCdTe sensors by decreasing the dark current and increasing the photocurrent.It has been demonstrated that Auger mechanism behaves in recombination mode along the n-type absorber layer under illumination. On the other hand, Auger behaves as a generation mechanism throughout the device under dark conditions. It is also shown that the reduction in the photocurrent due the presence of the Auger process results from the partial loss in the density of the photogenerated carriers due to Auger recombination.

1/f Noise in HgCdTe Focal-Plane Arrays

Characterization of mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) HgCdTe focal-plane arrays (FPAs) indicates that limitations on operability at elevated temperatures are due to detector dark current and excess 1/f noise. Dark-current models in HgCdTe are well established and understood; however, the same cannot be said for 1/f noise. In this paper we propose two models for separate sources of 1/f noise in HgCdTe photodiodes based upon charge fluctuations out of McWhorter-like surface traps. The two 1/f noise components are designated as (1) systemic, being associated with passivated external surfaces of the diodes, and (2) isolated defect, being, it is proposed, associated with the internal surfaces of built-in physical defects such as dislocations. The models are utilized to explain data measured on LWIR and MWIR test-diode structures, and predictions are made regarding the performance of MWIR and LWIR FPAs at elevated temperatures.

Modeling of midwavelength infrared InAs/GaSb type II superlattice detectors

The performance of midwavelength infrared type II superllatice InAs/GaSb PIN and nBn (AlGaSb barrier) photodetectors in a reverse bias voltage range and a temperature range from 77 to 240 K is described. The PIN and nBn structures are modeled by a bulk based model, i.e., type II superllatice is treated as an artificial semiconductor material where parameters describing its physical properties are extracted from the experimental data. The model assumes that position of the effective trap energy level depends on temperature, what allowed to obtain a very good fitting to the measurements. Temperature and bias dependent dark current and differential resistance area product of the both devices have been analyzed to investigate contributing mechanisms such as: diffusion, generation-recombination, band-to-band and trap-assisted tunneling that limit the electrical performance of both types of the detectors. The I−V and RA(V) product characteristics of both types of type II superllatice InAs/GaSb photodetectors were found to be dominated by diffusion and generation-recombination currents in the nearly zero-bias region. At medium values of reverse bias, the trap-assisted tunneling reveals its significance, while at higher reverse voltages—the band-to-band tunneling is decisive to I−V and RA(V) characteristics. The fitting procedure allowed to extract both generation-recombination and diffusion components of carrier lifetimes pointing out that the carrier lifetimes range from 2 to 10 ns at T=200 K . Detectivity for T=240 K and V=50 mV was estimated to be 10 9 cmHz 1/2 /W and 4×10 9 cmHz 1/2 /W for PIN and nBn detector, respectively. Finally, type II superlattice InAs/GaSb PIN and nBn structures’ performance is compared to both unipolar barrier nBn HgCdTe detector and bulk HgCdTe photodiodes operated at near-room temperature. It is shown that the performance of SL and HgCdTe photo detectors with a cut-off of about 5 μm is comparable at operation temperatures around 240 K.

Parameter determination from current–voltage characteristics of HgCdTe photodiodes in forward bias region

Current–voltage characteristics of HgCdTe photodiodes in the forward bias region have been modeled considering mechanisms including drift-diffusion current, recombination current, metal-semiconductor contact and constant series resistance. Moreover, a fitting method based on the genetic algorithm has been developed to obtain values of related physical parameters from the measured dynamic resistance–voltage curves. Fitting results of \(n^+\)-on-\(p\) planar devices with different cutoff wavelengths are presented to illustrate the model and method, which are available and promising in acquiring device parameter values and evaluating the electrode contact quality.

Analysis of the mechanisms of electron recombination in HgCdTe infrared photodiode

This paper presents an experimental study of minority carrier lifetime and recombination mechanisms in HgCdTe photodiode. The excitation light source is a wavelength-tunable pulsed infrared laser. A constant background illumination has been introduced to minimize the effect of the junction equivalent capacitor and resistance. The decay of the photo-generated voltage is recorded by a storage oscilloscope. By fitting the exponentially decay curve, the time constant has been obtained which is regarded as the photo-generated minority carrier lifetime of the HgCdTe photodiode. The experimental results show that the carrier lifetime is in the range of 18–407 ns at 77 K for the measured detectors of four Cd compositions. It was found that the Auger recombination process is more effective for low Cd composition while the radiative recombination process became more important for high composition materials. The Shockley–Read–Hall recombination processes could not be ignored for all Cd composition.

Electrical characteristic signatures for non-uniformity analysis in HgCdTe photodiode arrays

In this paper we present a method of analyzing the performance non-uniformity of HgCdTe photodiode arrays for infrared imaging applications. For quantifying the characteristic behavior of various photodiodes, we have parametrized the dynamic resistance verses voltage signatures in such a way that the obtained signature parameters have some relevance with different physical parameters. We also estimated the sensitivity of the proposed signatures on physical parameters using statistical technique. These characteristics signatures may be used to quantify the non-uniformity of the HgCdTe photodiodes in IR imaging arrays and its analysis. The method presented here is based on theoretical calculation of MWIR HgCdTe photodiodes. However, the method is generic and may be implemented on any other type of diode arrays for theoretical or experimental analysis of their non-uniformity.

Study on the structural characteristics of HgCdTe photodiodes using laser beam-induced current

The structural characteristics of typical n+-on-p HgCdTe photodiodes have been studied by laser beam-induced current (LBIC). The dependence of LBIC on laser wavelength, junction depth and localized leakage has been presented. The diffusion length of minority carrier of p-type region is extracted by the exponential decay fitting of the curve of LBIC. It is found that the peak magnitude of LBIC and junction depth approximate to a linear relationship for practical values of device fabrication. The diffusion length monotonously increases with the junction depth. A notable shift of LBIC profile is observed when localized leakage exists. This provides a useful explain for LBIC applying to characterize the structure and process uniformity of HgCdTe infrared detectors.

Numerical Estimations of Carrier Generation–Recombination Processes and the Photon Recycling Effect in HgCdTe Heterostructure Photodiodes

An enhanced computer program has been applied to explain in detail the photon recycling effect which drastically limits the influence of radiative recombination on the performance of p-on-n HgCdTe heterostructure photodiodes. The computer program is based on a solution of the carrier transport equations, as well as the photon transport equations for semiconductor heterostructures. We distinguish photons in two energy ranges according to p+ and n region with unequal band gaps. As a result, both the distribution of thermal carrier generation and recombination rates and spatial photon density distribution in photodiode structures have been obtained. The general conclusion, similar to our earlier work concerning 3-μm n-on-p HgCdTe heterostructure photodiodes, confirms the previous assertion by Humphreys that radiative recombination does not limit HgCdTe photodiode performance.

Temperature dependence on photosensitive area extension in mercury cadmium telluride photodiodes using laser beam induced current

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.

HgCdTe photodiode arrays passivated by MBE in-situ grown CdTe film

The results of HgCdTe long-wavelength infrared n+-on-p planar photodiode arrays passivated by molecular beam epitaxy(MBE) in-situ grown CdTe film were presented in this paper.By mercury-vacancy p-type annealing,ion-implantation window exposure,ZnS ion-implantation barrier layer deposition,B+-implantation,ion-implantation barrier layer removal,ZnS dielectric film deposition,metallization and indium-bump arrays fabrication,HgCdTe long-wavelength infrared n+-on-p planar photodiode arrays using in-situ CdTe passivation was achieved from a Hg1-xCdxTe film covered with a layer of MBE in-situ grown CdTe film.Zero bias dynamic resistances of HgCdTe photodiode arrays using in-situ CdTe passivation were improved 1~2 times higher than those of non-in-situ CdTe passivation processed one,and the maximum dynamic resistances near small reverse biases were even increased by a factor of 30~40.Since their current-voltage curves were all measured at 78K,it is obvious that in-situ CdTe passivation was beneficial to suppress dark current of n+-on-p planar photodiode by optimizing the interface between the HgCdTe detector and CdTe passivation layer,and then to enhance the performance of long-wavelength infrared photodiode arrays operating at small reverse biases.