Bake stability of CdTe and ZnS on HgCdTe: An x-ray photoelectron spectroscopy study

Mercury cadmium telluride (Hg_1?xCd_xTe or MCT) has been commonly used in devices for infrared (IR) detection. For the optimum performance of the device, a compatible surface-passivation technology that provides long-term stability is required. Using x-ray photoelectron spectroscopy (XPS), the present study examines the effects on Hg_0.8Cd_0.2Te passivated with CdTe and ZnS undergoing baking in vacuum at temperatures typically used for dewar bakeout. Spectra recorded as a function of depth in both cases clearly show out-diffusion of Hg from the substrate toward the surface, even before the bakeout. On baking in vacuum, dramatic changes are observed in the ZnS/MCT case with complete loss of Hg from the sample up to the tested depth of more than 1,000 Å. Compositions of the HgCdTe matrix, formed after Hg out-diffusion, before and after the bakeout are also calculated at selected depths (from 250 Å to 700 Å), which is vital information from a device point of view, as it affects the bandgap of this narrow-band semiconductor.     

A CdTe passivation process for long wavelength infrared HgCdTe photo-detectors

Passivant-Hg_1−xCd_xTe interface has been studied for the CdTe and anodic oxide (AO) passivants. The former passivation process yields five times lower surface recombination velocity than the latter process. Temperature dependence of surface recombination velocity of the CdTe/n-HgCdTe and AO/n-HgCdTe interface is analyzed. Activation energy of the surface traps for CdTe and AO-passivated wafers are estimated to be in the range of 7–10 meV. These levels are understood to be arising from Hg vacancies at the HgCdTe surface. Fixed charge density for CdTe/n-HgCdTe interface measured by CV technique is 5×10¹⁰ cm⁻², which is comparable to the epitaxially grown CdTe films. An order of magnitude improvement in responsivity and a factor of 4 increase in specific detectivity (D*) is achieved by CdTe passivation over AO passivation. This study has been conducted on photoconductive detectors to qualify the CdTe passivation process, with an ultimate aim to use it for the passivation of p-on-n and n-on-p HgCdTe photodiodes.     

Passivation effect on optical and electrical properties of molecular beam epitaxy-grown HgCdTe/CdTe/Si layers

The effects of passivation with two different passivants, ZnS and CdTe, and two different passivation techniques, physical vapor deposition (PVD) and molecular beam epitaxy (MBE), were quantified in terms of the minority carrier lifetime and extracted surface recombination velocity on both MBE-grown medium-wavelength ir (MWIR) and long-wavelength ir HgCdTe samples. A gradual increment of the minority carrier lifetime was reported as the passivation technique was changed from PVD ZnS to PVD CdTe, and finally to MBE CdTe, especially at low temperatures. A corresponding reduction in the extracted surface recombination velocity in the same order was also reported for the first time. Initial data on the 1/f noise values of as-grown MWIR samples showed a reduction of two orders of noise power after 1200-Å ZnS deposition.     

CdTe/ZnS复合钝化层对长波碲镉汞器件性能的影响研究

采用CdTe/ZnS复合钝化技术对长波HgCdTe薄膜进行表面钝化,并对钝化膜生长工艺进行了改进。采用不同钝化工艺分别制备了MIS器件和二极管器件,并进行了SEM、C-V和I-V表征分析,研究了HgCdTe/钝化层之间的界面特性及其对器件性能的影响。结果表明,钝化工艺改进后所生长的CdTe薄膜更为致密且无大的孔洞,CdTe/HgCdTe界面晶格结构有序度获得改善;采用改进的钝化工艺制备的MIS器件C-V测试曲线呈现高频特性,界面固定电荷面密度从改进前的1.671011 cm-2下降至5.691010 cm-2;采用常规钝化工艺制备的二极管器件在较高反向偏压下出现较大的表面沟道漏电流,新工艺制备的器件表面漏电现象获得了有效抑制。     

Bake stability of long-wavelength infrared HgCdTe photodiodes

The bake stability was examined for HgCdTe wafers and photodiodes with CdTe surface passivation deposited by thermal evaporation. Electrical and electrooptical measurements were performed on various long-wavelength infrared HgCdTe photodiodes prior to and after a ten-day vacuum bakeout at 80°C, similar to conditions used for preparation of tactical dewar assemblies. It was found that the bakeout process generated additional defects at the CdTe/ HgCdTe interface and degraded photodiode parameters such as zero bias impedance, dark current, and photocurrent. Annealing at 220°C under a Hg vapor pressure following the CdTe deposition suppressed the interface defect generation process during bakeout and stabilized HgCdTe photodiode performance.     

MBE原位碲化镉钝化的碲镉汞长波光电二极管列阵

采用分子束外延(MBE)技术在表面生长碲化镉(CdTe)介质膜的p型碲镉汞(HgCdTe)材料,并通过离子注入区的光刻、暴露HgCdTe表面的窗口腐蚀、注入阻挡层硫化锌(ZnS)的生长、形成p-n结的B+注入、注入阻挡层的去除、绝缘介质膜ZnS的生长、金属化和铟柱列阵的制备等工艺,得到了原位CdTe钝化的n+-on-p...     

A comparison of gamma radiation effects on bromine- and hydrazine-treated HgCdTe photodiodes

In this study, we investigated the effects of gamma radiation on ZnS/CdTe-passivated HgCdTe photodiodes that were fabricated with one of two different surface treatments using bromine, Br₂, or hydrazine, N₂H₄. Unlike the ZnS-passivated HgCdTe photodiodes, the ZnS/CdTe-passivated HgCdTe photodiodes showed no degradation in resistance-area product at zero bias (R₀A) values after gamma irradiation of up to 1 Mrad. However, there is a significant difference between the bromine- and hydrazine-treated samples. Regardless of the dose of gamma radiation, there was little change in the forward current characteristics of the hydrazine-treated diode in comparison with the conventional bromine-treated diode. The hydrazine-treated diode showed fairly uniform R₀A values of >10⁷ Ω-cm² up to 1 Mrad of gamma irradiation, whereas the bromine-treated diode showed an abrupt change in R₀A values from ∼10⁶ Ω-cm² to ∼10⁷ Ω-cm² after gamma irradiation. Therefore, for use in a gamma radiation environment, the best combination for high-performance HgCdTe photodiodes is a ZnS/CdTe passivant that has been treated with hydrazine.     

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.     

Imaging one-dimensional and two-dimensional planar photodiode detectors fabricated by ion milling molecular beam epitaxy CdHgTe

Imaging one-dimensional (1-D) and two-dimensional (2-D) arrays of mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) planar photodiodes were fabricated by ion milling of vacancy-doped molecular beam epitaxy Cd_xHg_1−xTe layers. Sixty-four-element 1-D arrays of 26×26 μm² or 26×56 μm² diodes were processed. Zero-bias resistance-area values (R₀A) at 77 K of 4×10⁶ Θcm² at cutoff wavelength λ_CO=4.5 μm were measured, as well as high quantum efficiencies. To avoid creating a leakage current during ball bonding to the 1-D array diodes, a ZnS layer was deposited on top of the CdTe passivation layer, as well as extra electroplated Au on the bonding pads. The best measured noise equivalent temperature difference (NETD) on a LWIR array was 8 mK, with a median of 14 mK for the 42 operable diodes. The best measured NETD on a MWIR array was 18 mK. Two-D arrays showed reasonably good uniformity of R₀A and zero-bias current (I₀) values. The first 64×64 element 2-D array of 16×16 μm² MWIR diodes has been hybridized to read-out electronics and gave median NETD of 60 mK.     

Photo-enhanced native oxidation process forHg0.8Cd0.2Te photoconductors

Proposes an easy and reproducible vapor-phase photo surface treatment method to improve the device performance of the Hg_0.8 Cd_0.2Te photoconductive detector. We explore the effect of surface passivation on the electrical and optical properties of the HgCdTe photoconductor. Experimental results, including surface mobility, surface carrier concentration, metal-insulator-semiconductor leakage current, 1/f noise voltage spectrum, the 1/f knee frequency, responsivity R_λ, and specific detectivity D* for stacked photo surface treatment and ZnS or CdTe passivation layers are presented. These data are all directly related to the quality of the interface between the passivation layer and the HgCdTe substrate. We found that, by inserting a photo native oxide layer, we can shift the 1/f knee frequency, reduce the noise power spectrum, and achieve a lower surface recombination velocity S. A higher D* can also be achieved. It was also found that HgCdTe photoconductors passivated with stacked layers show improved interface properties compared to the photoconductors passivated only with a single ZnS or CdTe layer     

Au／CdTe＋ZnS／HgCdTe双层介质膜MIS器件的制备及研究

通过介质膜ZnS、CdTe薄膜材料的Ar^ 束溅射沉积研究,结合HgCdTe器件工艺,成功制备了以ZnS、CdTe双层介质膜为绝缘层的HgCdTe MIS器件;通过对器件的C－V特性实验分析,获得了CdTe/HgCdTe界面电学特性参数。实验表明：溅射沉积介质膜CdTe ZnS对HgCdTe的表面钝化已经可以满足HgCdTe红外焦麦面器件表面钝化的各项要求。     

Metalorganic chemical vapor deposition CdTe passivation of HgCdTe

CdTe epilayers are grown by metalorganic chemical vapor deposition (MOCVD) on bulk HgCdTe crystals with x ~ 0.22 grown by the traveling heater method (THM). The THM HgCdTe substrates are (111) oriented and the CdTe is grown on the Te face. The metalorganic sources are DMCd and DETe, and the growth is performed at subatmospheric pressure. Ultraviolet (UV) photon-assisted hydrogen radicals pretreatment plays a dominant role in the electrical properties of the resulting heterostructures. The requirements of a good passivation for HgCdTe photodiodes vis-a-vis the passivation features of CdTe/HgCdTe heterostructures are discussed. The effect of valence band offset and interface charges on the band diagrams of p-isotype CdTe/HgCdTe heterostructures, for typical doping levels of the bulk HgCdTe substrates and the MOCVD grown CdTe, is presented. Electrical properties of the CdTe/HgCdTe passivation are determined by capacitance-voltage and current-voltage characteristics of metal-insulator-semiconductor test devices, where the MOCVD CdTe is the insulator. It is found that the HgCdTe surface is strongly inverted and the interface charge density is of the order of 10¹²cm² when the CdTe epilayer is grown without the UV pretreatment. With the in-situ UV photon-assisted hydrogen radicals pretreatment, the HgCdTe surface is accumulated and the interface charge density is -4. 10¹¹ cm^-2.     

Processing of LPE-Grown HgCdTe for MWIR Devices Designed for High Operating Temperatures

Significant improvements of HgCdTe (MCT) detectors for the midwave infrared (MWIR) region with cutoff wavelength of about 5.2 μm at 77 K have been achieved. Optimizing the CdTe passivation proved to be a decisive step towards higher operating temperatures. The optimization was done by refining the interdiffusion process of the CdTe passivation layer with the liquid phase epitaxy-grown layer. The dark current density was reduced almost to the level of Rule 07, a common infrared detector benchmark. Additionally, improving the passivation process also decreased tunneling. These advancements also showed up in the focal-plane array (FPA) performance. A considerable reduction of the noise-equivalent temperature difference at temperatures above 130 K was attained. Based on these preliminary results, an operating temperature for these devices of more than 160 K is expected. Additionally, infrared (IR) pictures taken with a MWIR MCT-based FPA processed with the previous, slightly improved technology are presented. It is shown that good picture quality is attained at operating temperature of 140 K while retaining operability of 99.61%.     

碲镉汞物理与化学钝化界面的AES研究

采用化学和物理方法分别在Hg1-xCdxTe (MCT)表面制备了阳极氟化膜、CdTe、ZnS和类金刚石薄膜(DLC)钝化层.采用俄歇光谱(AES)和红外透射光谱(IR)研究了这些钝化层与MCT之间的界面特性.结果表明与阳极氟化膜和CdTe膜相比,ZnS和DLC膜能较好地抑制MCT组元的外扩散.ZnS层中的Zn和S 易于向MCT内部扩散,而且发现在ZnS层中有O的存在,这可能是由于ZnS易与空气中水份发生作用所致.而DLC中C向MCT内表面扩散较少.MCT表面沉积DLC薄膜后红外透过率较ZnS有明显的提高.     

Hg1-xCdxTe光伏探测器的钝化研究

用倒易二维点阵对HgCdTe光伏探测器钝化及其热处理行为进行了研究,发现测射沉积的钝化膜会引起HgCdTe的晶面弯曲,严重的会出现晶面扭曲和mosaic结构,而钝化后的热处理能改善MCT晶体的完整性,在不同的钝化介质层钝化MCT的研究中发现,ZnS钝化层在高温下并不稳定,而CdTe钝化层却能保持较高的耐温性能。     

HgCdTe-MISFET fabrication with multi-step surface passivation and quantum effects

We have used multi-step surface passivation process integrating electrochemical reduction and UV exposure with native sulfidization by H₂S gas to obtain high quality ZnS/p-HgCdTe interface. It shows very low parasitic interface charge density of the order of 10¹⁰cm⁻². The insulating ZnS layer also exhibits very high resistivity of ∼10¹² Θcm. The resulting fabricated HgCdTe-MISFETs show 2D quantum effects. Magnetoresistance measured at 1.5K displays oscillations which begin to appear above the gate voltage of 10V. They are identified as the Shubnikov-de Haas oscillations involving three electronic subbands. The magnetotransport data are quantitatively analyzed with the calculated Landau level-fan diagram and confirm the 2D subband quantization of the inversion layer at the ZnS/p-HgCdTe interface. This result demonstrates successful role of the multi-step surface passivation for realizing 2D ZnS/HgCdTe interface which will provide high quality 2DEG resevoir basis in future Hg-based narrow-gap nanostructure device applications.     

Compositionally graded interface for passivation of HgCdTe photodiodes

A compositionally graded CdTe-Hg_1−xCd_xTe interface was created by deposition of CdTe on p-HgCdTe and subsequent annealing. The compositionally graded layer between CdTe and HgCdTe was formed by an interdiffusion process and was used for passivation. The composition gradient (Δx) in the interfacial region and the width of the graded region were tailored by adopting a suitable annealing procedure. The effect of process conditions on the interfacial profile and photoelectric properties such as lifetime and surface recombination velocity was studied in detail. Surface recombination velocity of the p-HgCdTe could be reduced to the level of 3,000 cm/s at 77 K, which represents very good passivation characteristics. The passivation layer formed by this method can be used for the fabrication of high performance and stable modern infrared detectors. Thus, a passivation process is developed, which is simple, effective, reproducible, and compatible with the HgCdTe device fabrication and packaging processes.     

Electrical and structural properties of epitaxial CdTe/HgCdTe interfaces

In this study, CdTe epilayers were grown by metalorganic chemical vapor deposition on epitaxial HgCdTe with the purpose of developing suitable passivation for HgCdTe photodiodes. Two types of CdTe layers were investigated. One was grown directly,in situ, immediately following the growth of HgCdTe. The second type of CdTe was grown indirectly, on top of previously grown epitaxial HgCdTe samples. In this case, the surface of the HgCdTe was exposed to ambient atmosphere, and a surface cleaning procedure was applied. The material and structural properties of the CdTe/HgCdTe interfaces were investigated using secondary ion mass spectroscopy, Auger electron spectroscopy, Rutherford back scattering, and x-ray double crystal diffractometry techniques. Electrical properties of the CdTe/HgCdTe heterostructure were determined by capacitance-voltage (C-V) characterization of Schottky barrier devices and metal insulator semiconductor devices. Also, a preliminary current-voltage characterization of n⁺ p photodiodes was performed. A theoretical model suitable for analysis of graded heterojunction devices was used for interpretation of C-V measurements.     

Surface Passivation of HgCdTe Using Low-Pressure Chemical Vapor Deposition of CdTe

CdTe passivation films have been deposited on Hg_1?x Cd _x Te (x = 0.35) samples used for infrared detectors by low-pressure chemical vapor deposition (LPCVD) and atomic layer deposition (ALD) at temperatures as low as 135°C to 170°C. ALD has been used to deposit an initially uniform starting surface before continuing the deposition using LPCVD. Favorable conformal coverage has been demonstrated on high-aspect-ratio HgCdTe structures. LPCVD deposition rates of 40 nm/h to 70 nm/h were obtained by varying the sample temperature from 135°C to 170°C. Lifetime measurements carried out at 300 K exhibited a significant improvement in minority-carrier lifetime from 0.9 μs (sample without passivation) to 4.28 μs for samples passivated at 135°C.     

Enhance the performance of quantum dot-sensitized solar cell by manganese-doped ZnS films as a passivation layer

To make quantum dot-sensitized solar cells (QDSSCs) more attractive, it is necessary to achieve higher power conversion efficiency. A novel Mn-doped ZnS has been successfully fabricated on CdS/CdSe quantum dots (QDs) by simple successive ion layer adsorption and reaction (SILAR) technique. The Mn-doped ZnS is used as a passivation layer in the QDSSCs. The performance of the QDSSCs was examined in detail using sulfide/polysulfide electrolyte with a Pt or copper sulfide (CuS) counter electrode. Here we demonstrated, the fabricated Mn-doped ZnS QDs shows an improved V_oc (0.65 V) compared to bare ZnS QDs (0.60 V). The QDSSC based on a photoanode with Mn-doped ZnS (10 wt% of Zn) shows higher J_sc (15.32 mA cm⁻²) and power conversion efficiency (4.18%) compared to the bare ZnS photoanode (2.90%) under AM 1.5 G one sun illumination. We explore the reasons for this enhancement and demonstrated that it is caused by improved passivation of the ZnS surface by Mn ions, leading to a lower recombination of photo-injected electrons with the electrolyte. The effect of Cu ions in ZnS has been investigated by UV–Vis spectra and current density–voltage analysis.