HgCdTe photoelectric detector based on unipolar quantum well
By introducing a multi-quantum well structure and a fully n-type doping system into HgCdTe photodetectors, the problem of high dark current was solved, efficient mid-infrared and long-wave infrared detection was achieved, the process flow was simplified, and the quantum efficiency and response were improved.
Patent Information
- Application Number
- CN202510515239.0
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-04-23
- Publication Date
- 2025-09-12
AI Technical Summary
Existing mercury cadmium telluride photodetectors have high dark current in the mid-infrared and long-wave infrared bands, require lower operating temperatures, have deteriorated noise equivalent power and limited response speed, and material interface defects and carrier recombination losses limit quantum efficiency.
A multi-quantum well structure is used to replace the traditional single wide-bandgap barrier layer to construct a fully n-type doping system. Carrier selective transport is achieved through the quantum confinement effect, and the process flow is simplified, the component gradient layer is eliminated, and the band matching is optimized to reduce dark current.
It significantly reduces the dark current density to less than 1/3 of the traditional structure, increases the quantum efficiency to more than 70%, maintains high responsiveness, simplifies the device preparation process, and reduces the operating temperature requirement.
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Figure CN120640825A_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to the technical field related to infrared detectors, specifically to a high-performance unipolar quantum well type HgCdTe photodetector structure based on HgCdTe material and its unipolar working mode design, as well as a unipolar quantum well photodetector, which is suitable for high-sensitivity detection in the mid- and long-infrared range. Background Art
[0002] Mercury cadmium telluride (HgCdTe) photodetectors are widely used to detect near-infrared, mid-infrared, and far-infrared wavelengths. By adjusting the ratio of Hg to Cd, the bandgap width and cutoff wavelength can be adjusted, enabling full-band control. As an important infrared detector, HgCdTe photodetectors are used in infrared imaging, remote sensing, astronomical observation, military detection, and other fields.
[0003] The NBN (Non-Non-Negative-Resistance) HgCdTe (HgCdTe) photodetector is an infrared photodetector that enhances its performance by incorporating an NBN structure. The NBN structure was proposed by DL Stokes et al. in the late 1970s. With the further development of the concept of negative resistance, NBN detectors have gradually moved from theory to practice.
[0004] A quantum well is a device that forms a very thin layer (usually nanometer-level thickness) between semiconductor materials with different energy band gaps, so that electrons or holes are confined in a two-dimensional plane, forming a quantum effect. Through the design of the quantum well, the energy state and distribution of carriers can be adjusted, and certain undesirable physical processes, such as thermal excitation and tunneling, can be suppressed. In the device, the quantum well structure is usually formed by inserting a layer of HgCdTe material (with a smaller band gap) into a material with a larger band gap (such as CdTe or HgTe). By adjusting the thickness and composition of these materials, the behavior of carriers can be effectively controlled so that they move only within the quantum well. By regulating the device's dark current, noise equivalent power, detectivity, responsivity, etc., the device structure is optimized, which can minimize the device's dark current while ensuring high performance.
[0005] The dark current under this device structure mainly includes the following mechanisms: recombination current, tunneling current, thermal excitation current, defect current, and surface current.
[0006] I dark =I GR +I tunnel +I thremionic +I leak +I interface
[0007] The generation-recombination process is caused by defects in the semiconductor, typically due to deep-level defects in the material. This current is typically determined by temperature and defect density. Carriers in a quantum well tunnel through an energy barrier via quantum tunneling. The tunneling current is typically expressed using Fermi's golden rule. Hot electron current refers to the process of thermally excited carriers crossing a potential barrier, particularly at metal-semiconductor contacts. Leakage current is typically caused by material or interface defects, particularly those on the surface or at an interface. Defect states at interfaces or oxide layers can lead to current flow, typically due to recombination or tunneling of surface states.
[0008] The cadmium composition gradient layer in NBN-type HgCdTe photodetectors is difficult to control experimentally. By converting the barrier layer into a quantum well layer, the fabrication process is simplified, resulting in a uniform cadmium composition in both the barrier and well layers. Furthermore, due to the presence of quantum wells, detectors with an n-type-quantum-well-n-type structure can increase carrier generation and transmission efficiency through quantum effects (such as quantum tunneling), particularly within the photodetector's wavelength response range. The patented quantum well design reduces the probability of thermal excitation, while the n-type-quantum-well-n-type structure further suppresses tunneling current.
[0009] At present, mercury cadmium telluride photodetectors are widely used in fields such as military, aerospace, remote sensing, and medical treatment.
[0010] In recent decades, most HgCdTe detectors have faced high dark currents, requiring low operating temperatures. When the operating temperature exceeds 100K, the noise equivalent power (NEP) deteriorates by more than two orders of magnitude. Furthermore, all electrons and holes participate in conduction, limiting response speed. Most importantly, quantum efficiency is limited by material interface defects and carrier recombination losses. While current quantum well structures can partially suppress dark current, the band matching between the barrier and well layers is not optimized, and the advantages of unipolar carrier transport are not fully utilized. Therefore, a new structure is urgently needed to improve the overall performance of detectors. A currently designed unipolar barrier quantum well detector has good band matching and excellent optoelectronic performance. Summary of the Invention
[0011] A high-performance quantum well-type mercury cadmium telluride photodetector structure and a unipolar quantum well photodetector were invented. The dark current was minimized by adjusting the device material thickness, material composition, and doping concentration. The device quality factor was also controlled and the device structure was optimized to ensure high performance.
[0012] NBN-type HgCdTe photodetectors are widely used in infrared detection, particularly in the mid-infrared and long-wave infrared (LWIR) ranges. Their structure consists of two N-type HgCdTe layers sandwiched with a barrier layer, typically with a large bandgap, forming a heterojunction. The device structure of an N-type unipolar quantum well photodetector primarily enhances photodetection performance by sandwiching multiple layers of narrow-bandgap material between wider-bandgap materials to form multiple quantum wells. The quantization effect of the quantum wells optimizes electron absorption characteristics, improving detector efficiency, particularly in the mid-infrared and LWIR ranges.
[0013] Compared to traditional NBN-type HgCdTe detectors, this innovation replaces the single wide-bandgap barrier layer (HgCdTe) in the traditional NBN structure with a multi-quantum well structure. This creates sub-bands through the band-folding effect and eliminates the need for a graded composition layer, greatly simplifying the process.
[0014] At the same time, the photodetector mainly adopts N-type and abandons P-type doping, avoiding the problem of reduced carrier lifetime caused by Hg vacancies in p-type HgCdTe materials. In addition, since the precise doping compensation and annealing activation steps required for pn junction preparation are not required, the device qualification rate is improved. At the same time, the quantum well layer adopts MBE in-situ doping technology to improve doping accuracy.
[0015] The invention comprises a substrate layer, an absorption layer, a quantum well layer, a contact layer and a passivation layer which are grown in sequence. BRIEF DESCRIPTION OF THE DRAWINGS
[0016] Figure 1 This is the overall structure diagram of the unipolar quantum well mercury cadmium telluride photodetector;
[0017] Figure 2 The quantum well period of the HgCdTe photodetector with a unipolar quantum well is a structural diagram;
[0018] Figure 3 The quantum well period of the HgCdTe photodetector with a unipolar quantum well is two structural diagrams;
[0019] Figure 4 The quantum well period of the HgCdTe photodetector with a unipolar quantum well is three structural diagrams;
[0020] Figure 5 It is a three-period band structure;
[0021] Figure 6 It is a band structure with four periods;
[0022] Figure 7 It is a five-period band structure;
[0023] Figure 8 It is a ten-period band structure;
[0024] Figure 9 It is the forward bias and reverse bias voltage-current diagram (dark current). DETAILED DESCRIPTION
[0025] NBN-type HgCdTe photodetectors are widely used in infrared detection, particularly in the mid-infrared and long-wave infrared (LWIR) ranges. Their structure consists of two N-type HgCdTe layers sandwiched with a barrier layer, typically with a large bandgap, forming a heterojunction. The device structure of an N-type unipolar quantum well photodetector primarily enhances photodetection performance by sandwiching multiple layers of narrow-bandgap material between wider-bandgap materials to form multiple quantum wells. The quantization effect of the quantum wells optimizes electron absorption characteristics, improving detector efficiency, particularly in the mid-infrared and LWIR ranges.
[0026] This invention relates to the field of high-performance infrared photoelectric detection technology. Specifically, it provides a unipolar HgCdTe photodetector based on a multi-quantum well structure and a method for its preparation. Addressing the technical bottlenecks of traditional NBN-type HgCdTe detectors, such as complex processes and high dark current, this invention innovatively introduces a multi-quantum well barrier structure and a unipolar doping system, significantly reducing dark current while effectively simplifying the device fabrication process.
[0027] like Figure 1 As shown, the photodetector of the present invention adopts a five-layer vertical stacking structure, including: a cadmium zinc telluride substrate layer, an n-type Hg 1-a Cd a Te absorption layer, multi-quantum well barrier layer, n-type Hg 1-b Cd b Te electrode contact layer and surface passivation layer. Compared with existing technologies, its core innovations are reflected in two aspects: first, the use of a periodic quantum well structure to replace the single-component barrier layer (B layer) in traditional NBN detectors, achieving selective carrier transport through quantum confinement effects; second, the construction of a fully n-type doping system to suppress the generation of hole current at the energy band engineering level. The synergistic effect of these two innovations enables the device to maintain high responsiveness while reducing the dark current density to less than 1 / 3 of that of traditional structures.
[0028] To meet the requirements for high sensitivity, high responsiveness, high power, and low dark current of single-stage quantum well HgCdTe photodetectors, and to ensure the rationality of device fabrication, this paper establishes a simulation model based on the physical mechanism of dark current generation in HgCdTe photodetectors. The band structure is obtained when the degeneracy factor of the valence band and conduction band is 4:2, as well as the change in cadmium composition with a cut-in depth of x = 15 microns:
[0029] Figure 2 The energy band and cadmium composition changes when the quantum well period is 3; Figure 3 The energy band and cadmium component variation when the period of the quantum well is 4; Figure 4 The energy band and cadmium component variation when the period of the quantum well is 5; Figure 5 The energy band and cadmium component variation when the period of the quantum well is 10;
[0030] Specifically, the specific composition and technical features of the structure are as follows:
[0031] First, in terms of substrate selection, this invention uses (211)-oriented cadmium telluride zinc as the growth substrate. This substrate material has a lattice constant that highly matches the HgCdTe epitaxial layer, with a mismatch degree < 0.01%, which can effectively reduce the dislocation density. Particularly, the zinc component z value is selected in the range of 4% - 6%, which can not only ensure sufficient mechanical strength but also achieve a good match of the thermal expansion coefficient with the subsequent epitaxial layer, avoiding the generation of interface stress cracks in the device during temperature cycling.
[0032] In the design of the absorption layer, Hg 1-a Cd a Te alloy material (0.18 < a < 0.21) is used as the n-type light absorption main body. By precisely controlling the Cd component a value in the narrow range of 0.18 - 0.21, the bandgap width of the material corresponds to the detection requirements of long-wave infrared (8 - 12 μm). The thickness of this layer is set to 10 - 20 μm. This range can not only ensure sufficient absorption of incident photons (absorption coefficient > 1000 cm -1 ), but also avoid carrier recombination losses caused by excessive thickness. Indium (In) is used as the n-type dopant, and the doping concentration gradient is set to (1 - 5)×10 15 cm -3 (lower layer) to (5 - 10)×10 16 cm -3 (upper layer) to form a built-in electric field to promote the directional migration of photo-generated carriers.
[0033] The core innovation layer of this invention is the multi-quantum well barrier layer, which is composed of periodically alternating Hg 1-x Cd x Te well layers (0.2 < x < 0.3) and Hg 1-y Cd y Te barrier layers (0.3 < y < 0.5). The thickness of the well layer is 5 - 30 nm, preferably 15 - 20 nm, and the Cd component x value is designed to be 0.2 - 0.3. At this time, the well depth (ΔE c= 0.15 - 0.25 eV) can effectively limit electron tunneling. The thickness of the barrier layer is controlled within 5 - 15 nm, and the Cd component y value is increased to 0.3 - 0.5 to form a high enough energy barrier to block hole migration. It should be noted that by setting the number of periods to 10 - 30 repetitions, a significant quantum confinement effect can be established with a total thickness of only 0.1 - 1.5 μm, which has obvious advantages compared with the 2 - 3 μm composition grading layer required by traditional NBN devices. All quantum well layers are doped with In to be n-type conductive (concentration 1×10 16 - 5×10 16 cm -3 ), ensuring the unipolar characteristics of carrier transport.
[0034] In the construction of the contact layer, the top layer uses Hg 1-b Cd b Te alloy (0.2 < b < 0.23) as the electrode contact layer. The Cd component of this layer is slightly higher than that of the absorption layer (b = a + 0.02), and an electron injection barrier is formed through the energy band offset. The thickness of the contact layer is optimized to 150 - 300 nm. Being too thin will lead to an increase in contact resistance (> 10 -4 Ω·cm 2 ), and being too thick may cause the lateral diffusion loss of photo-generated carriers. High-concentration In doping (5×10 17 - 1×10 18 cm -3 ) is used to form an ohmic contact, and at the same time, the interface state density is reduced through the composition grading design (the b value increases by 0.005 / μm along the growth direction).
[0035] In a unipolar quantum well mercury cadmium telluride (Hg 1-x Cd x Te) detector, the use of a CdTe (20 - 50 nm) / ZnTe (20 - 60 nm) double-layer passivation structure exhibits multiple synergistic optimization effects. First, the lattice mismatch between CdTe and the HgCdTe electrode contact layer (Cd component 0.2 - 0.23) and the lattice gradient transition between ZnTe and CdTe achieve atomic-level interfacial bonding, suppressing the surface dangling bond density to the order of (5×10 10 ~1×10 11 )cm -2 , significantly reducing the surface recombination rate.
[0036] In terms of device processing, this invention eliminates the complex composition-gradient layers required by traditional NBN structures. Because all layers of the multi-quantum well are n-type doped, precise pn junction alignment is unnecessary, shortening the molecular beam epitaxy (MBE) growth cycle by approximately 40%. Experimental performance of samples produced using MBE demonstrates that devices employing this structure can reduce dark current by one to two orders of magnitude at an operating temperature of 77K, while achieving quantum efficiency exceeding 70% and maintaining an excellent responsivity of >3.5A / W.
Claims
1. A mercury cadmium telluride photodetector structure based on a unipolar quantum well, characterized by: It includes a substrate layer, an absorption layer, a quantum well layer, a contact layer, and a passivation layer grown sequentially, wherein: The substrate layer is made of cadmium zinc telluride material; Hg 1-a Cd a The absorption layer of Te, with an n-type structure, doped with In, and the doping concentration is 10 15 -10 18 cm -3 , with a thickness of 10 μm - 20 μm, a Cd component value of a and 0.18 < a < 0.21, grown on the above substrate layer; The quantum well layer includes Hg 1-x Cd x Te potential well layer and Hg 1-y Cd y Te barrier layer, with a period of 10 to 30, is grown on the above Hg 1-a Cd a On the absorption layer of Te; Hg 1-x Cd x Te potential well layer with a thickness of 5 - 30 nm, Cd component value x where 0.2 < x < 0.3, of n-type structure, doped with In, doping concentration is 10 15 -10 17 cm -3 ; Hg 1-y Cd y The Te barrier layer has a thickness of 5 - 15 nm, a Cd component value of y where 0.3 < y < 0.5, is of n-type structure, doped with In, and the doping concentration is 10 15 ~10 17 cm -3 ; Hg 1-b Cd b The electrode contact layer of Te, which is an n-type structure, doped with In, and the doping concentration is 10 15 ~10 18 cm -3 , the Cd component value is b and 0.2 < b < 0.23, the thickness is 150 - 300 nm, and it is on the above-mentioned quantum well layer; The passivation layer is made of CdTe and ZnTe in sequence, with a thickness of 20 to 50 nm for CdTe and 20 to 60 nm for ZnTe. 1-b Cd b On the contact layer of Te.
2. The n-type unipolar quantum well mercury cadmium telluride photodetector device structure according to claim 1, The quantum well layer has an overall thickness of 100 nm to 1350 nm.
3. A mercury cadmium telluride photodetector based on a unipolar quantum well, characterized in that: The photodetector comprises the structure as claimed in claim 1 or 2; using target etching technology, etching is performed on the electrode contact layer, the quantum well layer, and the absorption layer, etching to part of the absorption layer to form pixel isolation, and etching is performed on each pixel.