Contact Buffer Layer Research Articles

Bismuth as a buffer layer for metal contact with silicon carbide studied by In situ photoelectron spectroscopy

Silicon carbide (SiC) is a promising third-generation semiconductor due to its wide bandgap. However, the high Schottky barrier and metal-induced gap states (MIGS) at the metal/SiC interface present significant challenges for device fabrication, leading to high contact resistance and poor current delivery. This study proposes the use of bismuth (Bi), with its semimetallic properties and gap-state saturation effect, as a contact buffer layer to address these issues. We conducted a systematic investigation of the chemical and electronic characteristics of the Pt/Bi/4H-SiC(0001) system, fabricated via molecular beam epitaxy (MBE), using in situ X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). Our findings reveal weak bonding between the Bi buffer layer and the 4H-SiC(0001) surface, resulting in a slight downward band bending effect and the formation of a substantial dipole across the Bi/4H-SiC(0001) interface. Moreover, UPS spectra indicate a reduction in the work function of Pt/Bi/4H-SiC(0001), suggesting the potential for achieving low contact resistance. Notably, the Pt/Bi/4H-SiC(0001) system remains stable when exposed to 1.6×109 Langmuir of oxygen at room temperature, while a bare Bi buffer layer undergoes partial oxidation. These results provide a comprehensive understanding of the Pt/Bi/4H-SiC(0001) interfaces and strategies for improving metal/SiC contacts.

Effect of Cu2Te Back Surface Interfacial Layer on Cadmium Telluride Thin Film Solar Cell Performance from Numerical Analysis

Even though substantial advances made in the device configuration of the frontal layers of the superstrate cadmium telluride (CdTe) solar cell device have contributed to conversion efficiency, unresolved challenges remain in regard to controlling the self-compensation and minority carrier recombination at the back contact that limits the efficiency. In this study, a SCAPS-1D simulator was used to analyze the loss mechanism and performance limitations due to the band-bending effect upon copper chloride treatment and subsequent Cu2Te layer formation as the back contact buffer layer. The optimal energy bandgap range for the proposed back surface layer of Cu2Te is derived to be in the range of 1.1 eV to 1.3 eV for the maximum conversion efficiency, i.e., around 21.3%. Moreover, the impacts of absorber layer’s carrier concentration with respect to CdTe film thickness, bandgap, and operational temperature are analyzed. The optimized design reveals that the acceptor concentration contributes significantly to the performance of the CdTe devices, including spectral response. Consequently, the optimized thickness of the CdTe absorber layer with a Cu-based back contact is found to be 2.5 µm. Moreover, the effect of temperature ranging from 30 °C to 100 °C as the operating condition of the CdTe thin-film solar cells is addressed, which demonstrates an increasing recombination tread once the device temperature exceeds 60 °C, thus affecting the stability of the solar cells.

Fluorinated Graphene Contacts and Passivation Layer for MoS2 Field Effect Transistors (Adv. Electron. Mater. 10/2022)

MoS2 Field Effect Transistors In article number 2101370, Jangyup Son, Gwan-Hyoung Lee, and co-workers suggest a facile one-step technique utilizing fluorinated graphene as the passivation and contact buffer layer to MoS2, leading to the formation of Ohmic contacts and high carrier mobility of up to 64 cm2 V−1 s−1. The work provides a strategy for high-performance 2D semiconductor electronics.

Fluorinated Graphene Contacts and Passivation Layer for MoS2 Field Effect Transistors

AbstractRealizing a future of 2D semiconductor‐based devices requires new approaches to channel passivation and nondestructive contact engineering. Here, a facile one‐step technique is shown that simultaneously utilizes monolayer fluorinated graphene (FG) as the passivation layer and contact buffer layer to 2D semiconductor transistors. Monolayer graphene is transferred onto the MoS2, followed by fluorination by XeF2 gas exposure. Metal electrodes for source and drain are fabricated on top of FG‐covered MoS2 regions. The MoS2 transistor is perfectly passivated by insulating FG layer and, in the contacts, FG layer also acts as an efficient charge injection layer, leading to the formation of Ohmic contacts and high carrier mobility of up to 64 cm2 V−1 s−1 at room temperature. This work shows a novel strategy for simultaneous fabrication of passivation layer and low‐resistance contacts by using ultrathin functionalized graphene, which has applications for high performance 2D semiconductor integrated electronics.

Characterization of ZnTe thin films prepared by cathodic electrodeposition as a back contact buffer layer to CdS/CdTe solar cells

A low-cost solution-processable electrochemical technique was used to deposit the Zinc telluride (ZnTe) thin films. The growth potential −0.9 V was optimized by using cyclic voltammetry. The samples were annealed at 400 °C for 20 min. to improve the crystallinity and particle size suitable for the development of CdS/CdTe solar cell devices. The as-prepared and annealed samples were characterized thoroughly with structural, optical, compositional, morphological, and electrical techniques and the results are correlated.

Enhanced current collection of CdTe solar cells in the long wavelength region by co-evaporation deposition CdSexTe1-x films

Using a gradient band gap absorption layer to ensure high short-circuit current density is one of the key factors to prepare high-efficiency CdTe solar cells. CdSexTe1-x films with different selenium compositions were prepared by vacuum co-evaporation deposition technology. The effects of composition changes on the structure, morphology and properties of the films were studied. CdSexTe1-x films were incorporated into the CdTe solar cells with the structure of Glass/SnO2:F/CdS/CdSexTe1-x/CdTe/ZnTe:Cu/Au. The results show that the structure of CdSexTe1-x films is cubic when x is 0, 0.23 or 0.41, and while x is 0.61, 0.78 or 1 the structure is hexagonal. The minimum optical band gap is 1.40 eV with x = 0.41. It is found that in the long wavelength region, the cut-off edge of quantum efficiency curve depends on not only the thickness of CdSexTe1-x film with specific Se composition, but also the whole preparation process of solar cells. After using a 70 nm CdSe0.41Te0.59 film and 70 nm ZnTe:Cu back contact buffer layer, the cut-off edge of the long wavelength region can be reduced to 1.40 eV and the short-circuit current density of the device without antireflection layer reaches up to 28.8 mA/cm2.

Determination of band alignment at the CdTe/SnTe heterojunction interface for CdTe thin-film solar cells

The Ohmic back contact for CdTe is a key issue to realize high-efficiency CdTe thin film solar cells because of the high work function of CdTe. CdTe/SnTe heterojunctions (HJs) have been implemented to address this issue which shows promising potential, but the band alignment at the HJs is unknown. The valence band offsets in MBE-grown cadmium telluride (CdTe)/tin telluride (SnTe) (111) heterostructures were measured with X-ray photoelectron spectroscopy (XPS). The XPS results show that the heterostructure has an ideal type-I band structure for CdTe solar cells applications, with a valence band offset of and a conduction band offset of , which expedites hole transport from the CdTe absorber to the hole electrode and improves the Ohmic contact for CdTe. Experimental determination of the band structure of CdTe/SnTe HJs can help improve the photovoltaic performance of CdTe thin film solar cells and facilitate the design and fabrication of CdTe/SnTe related devices. Furthermore, we inserted a SnTe back contact buffer layer into the CdTe thin film solar cells, and it was compared with the cell structure without the SnTe buffer layer. The feasibility of using SnTe as a solar cell back contact is confirmed.

CdTe thin film solar cells with copper iodide as a back contact buffer layer

In this study, CuI was employed as a back contact buffer layer which not only assisted hole transport, but also acted as an electron reflector at back contact. Quantitative analysis of the band alignment at the CdTe/CuI interface was carried out by using X-ray photoelectron spectroscopy (XPS). The results showed that the valence band offset and conduction band offset between CuI and CdTe were 0.08 and 1.63 eV, respectively. By optimizing the thickness of the CuI layer, a CdS/CdTe solar cell with a maximum efficiency of 14.5% has been fabricated. Capacitance–voltage (C–V) and impedance spectroscopy measurements were carried out and the results demonstrated that the CuI buffer layer between CdTe and metal electrode eliminated contact barrier and reduced carrier recombination at the back contact. The current–voltage (J–V) characteristics measured under different spectrum illumination conditions provided convincing evidences that CuI buffer layer acted as an electron reflector and increased the open-circuit voltage (VOC) of the CdTe solar cells. Long time device stressing test results demonstrated that CdTe solar cells with a CuI buffer layer have much better device stability compared to the cells with a Cu/Au bi-layer metal contact.

Single-phase control of CuTe thin films for CdTe solar cells

Phase control is important to obtain high-performance CdTe solar cells when copper telluride is used as a back-contact for CdTe solar cells. In this paper single-phase CuTe was mainly controlled by the Cu/Te ratio via a co-evaporation method in combination with rapid thermal processing treatments. The alloy thin films as-deposited were mainly amorphous. After thermal treatments, the films experienced the phase transformation from mixed phases of CuTe and Cu1.4Te to single phase CuTe and then to coexisting phases. Single phase CuTe thin films were p-type semiconductor compounds and had a high carrier concentration, ∼1021 cm−3. With the copper telluride buffer layer, device performance than those without the buffer layer has been obtained. Moreover, single-phase orthorhombic CuTe contact buffer layer obviously improved diode characteristics and enhanced the device performance.

CdTe thin film solar cell with NiO as a back contact buffer layer

In this work, p-type NiO thin film was applied as a buffer layer in the back contact for CdTe thin film solar cell fabrication. The NiO layers were prepared by electron beam evaporation. By optimizing NiO layer thickness in the back contact a CdTe solar cell with an efficiency of 12.17% and an open-circuit voltage Voc of 790mV has been fabricated. X-ray photoelectron spectroscopy study showed that NiO has a staggered band alignment with CdTe. Such an electronic band structure alignment allows smooth hole transport from CdTe to NiO and at the same time NiO acts as a surface field layer to reduce electron recombination at the back contact interface, thus leading to improved Voc. This study also demonstrates that by employing NiO as a buffer layer, much less Cu was needed for the CdTe solar cell fabrication and the cell device stability had been significantly enhanced compared to the CdTe solar cell with a conventional Cu/Au bimetal layer as the back contact.

A computational study on the energy bandgap engineering in performance enhancement of CdTe thin film solar cells

In this study, photovoltaic properties of CdTe thin film in the configuration of n-SnO2/n-CdS/p-CdTe/p-CdTe:Te/metal have been studied by numerical simulation software named “Analysis of Microelectronic and Photonic Structure” (AMPS-1D). A modified structure for CdTe thin film solar cell has been proposed by numerical analysis with the insertion of a back contact buffer layer (CdTe:Te). This layer can serve as a barrier that will decelerate the copper diffusion in CdTe solar cell. Four estimated energy bandgap relations versus the Tellurium (Te) concentrations and the (CdTe:Te) layer thickness have been examined thoroughly during simulation. Correlation between energy bandgap with the CdTe thin film solar cell performance has also been established.

Nickel oxide as back surface field buffer layer in CdTe thin film solar cell

In this work, we report that NiO thin film can be used as a back contact buffer layer in CdTe thin film solar cells. The NiO layer is prepared by electron beam evaporation. To optimize the thickness of the NiO thin film, we fabricate some CdTe solar cells with different NiO thickness values. A NiO/Au back contact CdTe solar cell with an efficiency of 12.17% and an open-circuit voltage Voc of 789 mV is obtained, which are comparable to those of a standard Cu/Au back contact solar cell. The X-ray photoelectron spectroscopy (XPS) is used to quantitatively characterize the band alignment at the CdTe/NiO interface. It can be seen from the band alignment that the valence band offset (EVBO) is 0.52 eV and the conduction band offset (ECBO) is 2.68 eV. The EVBO presents no energy barrier for hole to transport from CdTe to NiO. The value of ECBO indicates that NiO can act as a back surface field layer (BSF) to dramatically reduce carrier recombination in the contact region of a CdTe cell, leading to an improved Voc. The band alignment obtained from XPS measurement shows that the band alignments of NiO and CdTe are perfectly matched. However, the conductivity of NiO film is poor. The insertion of a NiO buffer layer in the back contact increases the series resistance and reduces the fill factor (FF). We propose to use Cu/NiO composite structure as a bi-layer contact to improve the conductivity of the NiO buffer layer, which at the same time can be used to dope the CdTe film surface by Cu to obtain a low resistive contact. We fabricate a cell with a contact structure of 3-nm-Cu/20-nm-NiO/Au and the cell has a Voc of 796 mV, a Jsc (short-circuit currrent) of 24.2 mA/cm2, an FF of 70.2% and an efficiency of 13.5%. In order to study the stability of the solar cell with a Cu/NiO/Au back contact, a thermal stressing test is carried out at a temperature of 80 ℃ in the air atmosphere. For the Cu/NiO/Au back contact structure solar cell, the efficiency decreases from 13.1% to 12.9% after the cell is stressed for 80 h, showing that the stability of the Cu/NiO/Au back contact cell is significantly improved compared with that of the standard Cu/Au contact cell. In summary, the experimental results obtained in this study demonstrate that NiO thin film is a promising buffer layer for manufacturing stable and high efficiency CdTe thin film solar cells.

Buffer Layer Point Contacts for CIGS Solar Cells Using Nanosphere Lithography and Atomic Layer Deposition

Point contacts provide an interesting approach for reducing the buffer layer/Cu(In,Ga)Se2 interface recombination that typically limits Cu(In,Ga)Se2 solar cell performance when nontoxic alternatives to CdS buffer layers are used. In this study, we implement a scheme to create a point contact buffer layer on Cu(In,Ga)Se 2 solar cells using a combination of atomic layer deposition and nanosphere lithography. While we showcase these buffer layers using Al2O3 as the passivating material, ZnO as the conductive material, and a silica nanosphere size of 310 nm in diameter, this scheme is general and could readily be applied for other materials and other sphere sizes. The resulting solar cells with Al2O3 and ZnO point contact buffer layers demonstrate successful application of this scheme, yielding a higher conversion efficiency ( $6.58\,\pm \,0.58\% $ ) than either of the binary buffer layers Al2O3 (0%) and ZnO ( $5.15\,\pm \,0.57\% $ ). The improvement over ZnO is mainly due to an increased open circuit voltage, which is an indication of a reduced surface recombination.

Stable CdTe solar cell with V2O5 as a back contact buffer layer

A low electric resistive and stable back contact on p-type CdTe semiconductor is crucial for the commercial employment of high efficiency CdTe thin film solar cell. In this study, V2O5 was deposited as a buffer layer between CdTe and metal electrode in the back contact of CdTe solar cells. Different back contact structures were fabricated on CdTe to study the effect of a V2O5 buffer layer on cell device performance. Both the quantitative band alignment and the device performance of the CdTe solar cells with a V2O5 buffer layer demonstrated that a much lower Schottky barrier was formed compared to the cells with an Au-only back contact. The defect states related to oxygen vacancy within the band gap of the V2O5 played a crucial role in reducing the energy barrier for hole carrier transport. Employing a back contact structure of Cu/V2O5/Cu/Au, a CdTe solar cell with an efficiency as high as 14.0% was fabricated. Long term device stressing test demonstrated that, compared to the CdTe cells with a Cu/Au back electrode, solar cells with the insertion of a V2O5 buffer layer showed much enhanced device stability.

Enhanced Performance Designs of Group-IV Light Emitting Diodes for Mid IR Photonic Applications

The optical emission properties of Ge1-y Sny and Ge1-x-y Si x Sn y LEDs on Si(100) substrates are investigated by room temperature electroluminescence (EL). In the case of Ge1-y Sn y LEDs, the basic device architecture consists of a thick (1.0-1.5 μm) highly doped (n = 1-3x1019 cm-3) n-Ge layer grown on a Si wafer to act simultaneously as a buffer and bottom contact. This is followed by the growth of the active i-Ge1-y Sn y layer (400-700 nm) with compositions that span from pure Ge (y = 0) to levels beyond the crossover to direct gap semiconductors for this alloy (y = 0.12). The device is capped by a p-Ge1-y’ Sn y’ layer (100-200 nm) grown pseudomorphically to the intrinsic layer containing y’ < y to promote carrier confinement in the active region. These devices feature a single interface between the n-Ge and i-Ge1-y Sn y layers where strain relaxation may occur to accommodate the lattice mismatch by generating defects. The EL spectra from these devices show a monotonic redshift of the direct and indirect gap signals with the separate peaks merging into a single peak as the active layer materials are tuned into the direct gap composition range through higher Sn incorporation. Additionally, analysis of the EL intensities shows evidence for a significant increase in the non-radiative recombination rate due to the generation of relaxation-induced misfit dislocations at the n-Ge/i-Ge1-y Sn y interface. The emission properties of devices with non-relaxed interfaces are then investigated by replacing the n-Ge bottom contact with a n-Ge1-y’’ Sn y’’ material containing y’’ < y such that pseudomorphic growth takes place between all three active components of the device while maintaining carrier confinement to the active layer. These devices exhibit remarkably stronger EL than the counterparts with a single defected interface. This result can be traced to at least a 3x longer non-radiative recombination lifetime in the lattice-matched devices due to the elimination of strain relaxation induced defects at both interfaces. The room temperature EL from an Esaki-like diode formed from a pn junction of heavily doped (n, p ~ 1019) Ge0.91Sn0.09 layers at the indirect to direct gap crossover grown on a Ge buffer is also presented. This device represents the viability of electrically injected Ge1-y Sn y devices for IR lasers on Si. Finally EL from Ge1-x-y Si x Sn y LEDs is discussed. These devices are grown on Si substrates through the same n-Ge bottom contact buffer layer that was applied in the Ge1-y Sn y case. The Si composition in the active intrinsic region of these devices is held nearly constant (x = 0.02-0.03) while the Sn concentration is varied between y = 0.03-0.10. These devices are then capped with p-Ge1-x’-y’ Si x’ Sn y’ layers grown pseudomorphically to the intrinsic layer to prevent the formation of strain relaxation defects. In this case, to promote carrier confinement we choose x’ > x and y’ ~ y. The EL spectra from these devices show the same redshift of the direct and indirect band gaps to lower energies due to the compositional tuning associated with Sn substitution analogous to that observed in the Ge1-y Sn y devices. This includes a merging of the direct and indirect gap peak signals for devices containing 7-10% Sn in the active layer due to the separation between the direct and indirect band gaps narrowing. This indicates that electrically pumped direct-gap Ge1-x-y Si x Sn y devices are feasible and thus may present a more thermally robust alternative to Ge1-y Sn y for similar Si-based IR photonic applications.

A new laterally conductive bridge random access memory by fully CMOS logic compatible process

This paper proposes a novel laterally conductive bridge random access memory (L-CBRAM) module using a fully CMOS logic compatible process. A contact buffer layer between the poly-Si and contact plug enables the lateral Ti-based atomic layer to provide on/off resistance ratio via bipolar operations. The proposed device reached more than 100 pulse cycles with an on/off ratio over 10 and very stable data retention under high temperature operations. These results make this Ti-based L-CBRAM cell a promising solution for advanced embedded multi-time programmable (MTP) memory applications.

Development of MoOx thin films as back contact buffer for CdTe solar cells in substrate configuration

Molybdenum oxide compounds exhibit unique electrical and optical properties depending on oxygen vacancy concentration and composition and therefore, have recently attracted a lot of attention as a hole transport layer in various devices.In this work CdTe solar cells in substrate configuration were grown with evaporated MoOx back contact buffer layers and efficiencies of up to 10% could be achieved without using Cu in the back contact processing. The buffer layer – at the CdTe/back contact interface – in the finished cell was found to consist of MoO2 phase instead of the expected MoO3 phase as observed in as-deposited or annealed MoOx layers without CdTe deposition.In order to obtain MoOx buffer layers with desired stoichiometry, MoOx thin films were deposited by radio-frequency sputtering under different growth conditions. The chemical phase, composition, microstructure and optical properties of such layers were studied for their possible use in CdTe solar cells.

CdTe/CdS thin film solar cells grown in substrate configuration

ABSTRACTThe ability to grow efficient CdTe/CdS solar cells in substrate configuration would not only allow for the use of non‐transparent and flexible substrates but also enable a better control of junction formation. Yet, the problems of barrier formation at the back contact as well as the formation of a p–n junction with reduced recombination losses have to be solved. In this work, CdTe/CdS solar cells in substrate configuration were developed, and the results on different combinations of back contact materials are presented. The Cu content in the electrical back contact was found to be a crucial parameter for the optimal CdCl2‐treatment procedure. For Cu‐free cells, two activation treatments were applied, whereas Cu‐containing cells were only treated once after the CdTe deposition. A recrystallization behavior of the CdTe layer upon its activation similar to superstrate configuration was found; however, no CdTe–CdS intermixing could be observed when the layers were treated consecutively. Remarkably high VOC and fill factor of 768 mV and 68.6%, respectively, were achieved using a combination of MoO3, Te, and Cu as back contact buffer layer resulting in 11.3% conversion efficiency. With a Cu‐free MoO3/Te buffer material, a VOC of 733 mV, a fill factor of 62.3%, and an efficiency of 10.0% were obtained. Copyright © 2012 John Wiley &amp; Sons, Ltd.