P-type Surface Research Articles

Investigation of Fermi level pinning at semipolar (11–22) p-type GaN surfaces

Schottky barrier height (SBH; ΦB) and their dependence on the work function of metals (ΦM) at semipolar (11–22) p-GaN surfaces were investigated using Schottky diodes fabricated with different metals. The SBH increased with temperature, whereas the ideality factor decreased. This behavior was explained by means of the barrier inhomogeneity model, giving the mean barrier heights of 1.93–2.05eV for different metals. The S-parameter (dΦB/dΦM) was obtained to be 0.04. This small S-parameter implies that the surface Fermi level is nearly perfectly pinned at deep-level states (caused by vacancy-related and/or Mg-induced defects) located at 1.98eV above the valence band. This finding indicates that the surface modification is essentially required for the formation of high-quality ohmic and/or Schottky contacts.

B-Doped Graphene as Catalyst To Improve Charge Rate of Lithium–Air Battery

The lithium–air battery as an energy storage technology can be used in electric vehicles due to its large energy density, while its poor rate capability limits its practical usage under large current density. According to first-principles thermodynamics calculation, we predict B-doped graphene can be a potential catalyst to improve the charge rate of lithium–air battery. The lowest-energy reaction pathway for oxygen evolution reaction (OER) is predicted as Li+ → Li+ → O2. The rate-determining step (RDS) is predicted as the O2 evolution step. B-doped graphene can reduce the RDS barrier by 0.40 eV, indicating that charge rate may be significantly improved. B-doping can increase charge transferring of Li2O2 to the substrate by 0.36 e–, which helps to activate Li–O bonds and oxidize O22– to O2. We suggest a good OER catalytic substrate that can reduce the O2 evolution barrier should show p-type surface behavior.

Effective surface passivation of p-type crystalline silicon with silicon oxides formed by light-induced anodisation

Electronic surface passivation of p-type crystalline silicon by anodic silicon dioxide (SiO2) was investigated. The anodic SiO2 was grown by light-induced anodisation (LIA) in diluted sulphuric acid at room temperature, a process that is significantly less-expensive than thermal oxidation which is widely-used in silicon solar cell fabrication. After annealing in oxygen and then forming gas at 400°C for 30min, the effective minority carrier lifetime of 3–5Ωcm, boron-doped Czochralski silicon wafers with a phosphorus-doped 80Ω/□ emitter and a LIA anodic SiO2 formed on the p-type surface was increased by two orders of magnitude to 150μs. Capacitance–voltage measurements demonstrated a very low positive charge density of 3.4×1011cm−2 and a moderate density of interface states of 6×1011eV−1cm−2. This corresponded to a silicon surface recombination velocity of 62cms−1, which is comparable with values reported for other anodic SiO2 films, which required higher temperatures and longer growth times, and significantly lower than oxides grown by chemical vapour deposition techniques. Additionally, a very low leakage current density of 3.5×10−10 and 1.6×10−9Acm−2 at 1 and −1V, respectively, was measured for LIA SiO2 suggesting its potential application as insulation layer in IBC solar cells and a barrier for potential induced degradation.

CdSe Quantum Dot-sensitized, Nanoporous p-type NiO Photocathodes for Quantum Dot-sensitized Solar Cells

CdSe quantum dots (QDs) were deposited on a p-type nanoporous NiO surface by repeating the successive ionic layer adsorption and reaction (SILAR) procedure 6, 9, 12, and 15 times. The NiO/CdSe prepared using 12 SILAR cycles, which is denoted as the NiO/CdSe(12), exhibited a larger QD size and better QD coverage on the nanoporous NiO than the NiO films with CdSe(6) and CdSe(9), leading to better light absorption over longer wavelengths and a significantly increased hole lifetime. The resulting quantum dot-sensitized solar cell (QD-SSC) exhibited significantly enhanced cell efficiency (0.35%) compared to that (0.013%) of the QD-SSC with the NiO/CdSe(6). NiO/CdSe(15), however, exhibited a lower hole lifetime and hindered ion transport as a result of the agglomeration of QDs as well as the severely blocked pores of the nanoporous NiO, causing a significantly reduced cell efficiency for QD-SSCs with NiO/CdSe(15).

Electronic passivation of silicon surfaces by thin films of atomic layer deposited gallium oxide

This paper proposes the application of gallium oxide (Ga2O3) thin films to crystalline silicon solar cells. Effective passivation of n- and p-type crystalline silicon surfaces has been achieved by the application of very thin Ga2O3 films prepared by atomic layer deposition using trimethylgallium (TMGa) and ozone (O3) as the reactants. Surface recombination velocities as low as 6.1 cm/s have been recorded with films less than 4.5 nm thick. A range of deposition parameters has been explored, with growth rates of approximately 0.2 Å/cycle providing optimum passivation. The thermal activation energy for passivation of the Si-Ga2O3 interface has been found to be approximately 0.5 eV. Depassivation of the interface was observed for prolonged annealing at increased temperatures. The activation energy for depassivation was measured to be 1.9 eV.

Influence of Hf and H<sub>2</sub>O<sub>2</sub> on Morphology of Silicon Formed by Ag Assisted Chemical Etching

The Ag-assisted electroless etching of p-type silicon substrate in HF/H2O2 solution at room temperature was investigated. In this work, the effects of HF, H2O2 and their volume ratio on morphology and growth of p-type silicon substrate surface by using metal assisted etching were investigated in order to produce a highly efficient antireflecting structure. The Ag metal particles were deposited onto Si wafer by electroless deposition from a metal salt solution including HF. The experimental results show that the growth rate and morphology of the pores formed on the Ag metalized Si surfaces are strongly dependent on the volume ratio of HF and H2O2.

Diamond surface conductivity: Properties, devices, and sensors

Abstract

Effects of substrate characteristics on the passivation performance of ALD-Al2O3 thin film for high-efficiency solar cells

Atom layer deposition (ALD)-Al2O3 thin films are considered effective passivation layers for p-type silicon surfaces. A lower surface recombination rate was obtained through optimizing the deposition parameters. The effects of some of the basic substrate characteristics including material type, bulk resistivity and surface morphology on the passivation performance of ALD-Al2O3 are evaluated in this paper. Surface recombination velocities of 7.8 cm/s and 6.5 cm/s were obtained for p-type and n-type wafers without emitters, respectively. Substrates with bulk resistivity ranging from 1.5 to 4 Ω·cm were all great for such passivation films, and a higher implied Voc of 660 mV on the 3 Ω·cm substrate was achieved. A minority carrier lifetime (MCL) of nearly 10 μs higher was obtained for cells with a polished back surface compared to those with a textured surface, which indicates the necessity of the polishing process for high-efficiency solar cells. For n-type semi-finished solar cells, a lower effective front surface recombination velocity of 31.8 cm/s was acquired, implying the great potential of (ALD)-Al2O3 thin films for high-efficiency n-type solar cells.

Theoretical insights into structural–electronic relationships and relative stability of the Cu-, Al- and O-terminated CuAlO2(0001) surfaces

Geometrical, electronic, energetic and thermodynamic properties of the Cu-, Al- and O-terminated CuAlO2(0001) surfaces have been studied by first-principles density functional theory (DFT) calculations. The atomic displacements and surface relaxations of four different terminations were taken into account. According to the results of the surface electronic properties, the Cu-terminated and Al-terminated CuAlO2(0001) surfaces are metallic, and the OI and OII surfaces present the p-type semiconductor surfaces. The surface energies and thermodynamic stability of CuAlO2(0001) surfaces have been analyzed, indicating that the OI surface is the thermodynamically most stable termination.

Enhanced photoelectrochemical response of CdSe quantum dot-sensitized p-type NiO photocathodes

CdSe quantum dots (QDs) were deposited on a p-type nanoporous NiO surface by repeating the successive ionic layer adsorption and reaction (SILAR) procedure from 6 to 15 cycles. All of the CdSe QD-assembled NiO films exhibited much better p-type photoelectrochemical (PEC) performances than the bare NiO film. In particular, the NiO film with CdSe QD prepared from the 12 SILAR cycles exhibited a significantly enhanced p-type PEC response, due to an effective light absorption in the long wavelengths and the increased loading amount of the assembled QDs. However, as the SILAR cycles further increased up to 15, the QD-assembled NiO film exhibited the reduced PEC performance, despite better light absorption in the long wavelengths. This was because of the suppressed ion transport, poor charge transfer, and non-active aggregated QDs, facilitating the recombination reaction.

(Invited) Carrier Dynamics and Photon Management for Improvement in Quantum Efficiencies of GaN-Based Visible Light-Emitting Diodes

Data and analysis are presented employing several new methods to address carrier dynamics and photon management issues in order to improve internal quantum efficiency (IQE) and light-extraction efficiency (LEE) of GaN-based light-emitting diodes (LEDs). First, the roles of a new InAlN electron blocking layer (EBL) and hole transport among quantum wells (QWs) are discussed with a focus on the mitigation of efficiency droop at high current densities. While the origins of the efficiency droop remain to be controversial, currently suggested origins are all associated with carrier dynamics: the effect of the electron leakage being related to carrier confinement; hole transport to active region being pertinent to carrier injection and concentration in each QW, and Auger recombination becoming more dominant with increasing carrier density in each QW. The strategy for the improvement, therefore, includes (1) confining electrons in the active region as much as possible, (2) injecting holes into the active region as much as possible, (3) distributing the both carriers among QWs as uniformly as possible. In order to implement the strategies, we especially studied EBLs and hole injection and transport in the multiple QW (MQW) active region. The effects of the various EBLs on the efficiency droop was compared (Figure 1), showing the lowest efficiency droop ratio of ~18% for the LEDs with InAlN EBL due to better electron confining/blocking effect than the standard EBLs. Hole injection (into active region) and transport (across MQWs) are as important as electron confinement. Also, they are not inter-related. Uniform distributions of electrons and holes with the same concentration in each QW are a goal to effectively mitigate the efficiency droop. Whereas the uniform distribution of electrons among QWs is achieved without difficulties, that of hole is quite challenging. We employed triple-wavelength-emitting MQWs with different bandgap energies for the experimental evaluation of the hole distribution among MQWs (Figure 2), showing hole transport and resulting distribution of holes can be engineered by p-type layers. Secondly, new way of surface patterning is demonstrated. The periodic surface patterns having 2-dimensional hexagonal array were directly achieved by three-beam interference laser ablation technique without photolithography processes (Figure 3), resulting in improvement of LEE by ~20%. This new direct laser patterning also maintained ohmic behavior of contacts on p-type surface, unlike the case of the patterning using plasma dry etching.

(Invited) Spatial ALD, Deposition of Al2O3 Films at Throughputs Exceeding 3000 Wafers per Hour

During the last few years various types of ALD films have successfully been introduced in high-volume manufacturing in a number of industries. Particularly in the semiconductor industry the implementation is wide spread (high-k gate stacks, capacitor dielectrics, diffusion barriers, etc.).More recently a specific type of ALD film, Al2O3, was widely researched as a potential surface passivation layer for solar cell surfaces in the PV industry. Thin (< 10nm) Al2O3 films passivate p-type surfaces very effectively. On the one hand intrinsic negative charges in the dielectric film repel charge carriers, while on the other hand hydrogen that is present in the ALD films passivates dangling bonds at the Si/SiO2/ Al2O3 interface. The combination of both effects reduces the charge recombination losses considerably. The overall result is an increase in cell efficiency of 0.5-1% (absolute). Based on extensive testing of these films, it is to be expected that Implementation in PV manufacturing will take place in the near future.There is a large difference in film and process requirements of ALD films applied in the semiconductor and PV industries. Whereas uniformity and defects (particles) are very important in the IC industry this is much less so in PV. Much more important in the PV industry are cost and throughput. Typical numbers for both industries are a Cost of Ownership of 2-10 vs. 0.02-0.05$/wafer, respectively, and 10-50 vs 1500-2500 wfrs/hr..As ALD processes are notoriously slow, ways must be found to increase the throughputs considerably. This is done by either using batch furnaces loaded with 500-1000 wafers per batch, or in-line systems in which wafers are loaded, processed and unloaded in a continuous flow of 0.5-0.8 wafers/s.The latter type of system is based on the so-called spatial ALD process. An example is shown in the figure. In this system (Levitrack) wafers are loaded in a track in which zones with precursors TMA and H2O are spatially separated by zones of inert gas. While floating on a gas cushion, the wafers pass ALD ‘cells’ with fixed sequences of TMA and water, adding a thin layer of Al2O3 with each passage through a cell. The advantages of this approach are several: high throughput, atmospheric processing, no deposition on the inside walls of the track, no moving parts such as valves, pumps, etc.Typical results obtained with this system are: film uniformity 3-4%, life time of solar cell charge carriers > 3ms, efficiency enhancement 0.4-0.8%, CoO ~ 0.04$/wfr.In the paper more details will be provided on the economy of this type of system, as well as on film characteristics and efficiency enhancements realized in a variety of solar cell structures.

P-Type dopant incorporation and surface charge properties of catalyst-free GaN nanowires revealed by micro-Raman scattering and X-ray photoelectron spectroscopy.

Micro-Raman scattering and X-ray photoelectron spectroscopy were employed to investigate Mg-doped GaN nanowires. With the increase of Mg doping level, pronounced Mg-induced local vibrational modes were observed. The evolution of longitudinal optical phonon-plasmon coupled mode, together with detailed X-ray photoelectron spectroscopy studies, show that the near-surface region of nanowires can be transformed from weakly n-type to p-type with the increase of Mg doping.

Characterization of Silicon Oxides Formed by Light-Induced Anodisation for Silicon Solar Cell Surface Passivation

ABSTRACTIn this paper we report the properties of the anodic silicon dioxide film formed using light-induced anodisation (LIA) method and its potential to be used as surface passivation layer of p-type silicon surfaces of silicon solar cells. The high uniformity of the formed oxide is possibly due to the uniform drift of the positive charge carriers in the silicon to the surface being anodised. The oxide grows at higher rate than that in nitric acid, an oxide layer with thickness of 18 nm can be formed by anodising for 10 min with 15 V bias in 0.5 M sulphuric acid. After annealing in oxygen and then forming gas at 400 °C for 30 min, an average effective carrier lifetime of 120 μs was measured by quasi-steady state photoluminance on 180 μm p-type 3-5 Ohm cm Cz silicon wafers, with a value of 110 μs being measured for the same wafers passivated by a thermally-grown oxide of the same thickness. The properties of the anodic silicon dioxide layers formed by LIA have been characterized by ellipsometry, x-ray photoelectron spectroscopy, quasi-steady state photoluminance and Fourier transform infrared spectroscopy.

Electrical and Chemical Studies on Al2O3 Passivation Activation Process

Al2O3 deposited using Atomic layer Deposition (ALD) technique is known as an excellent material for p-type silicon surface passivation. However a post-deposition annealing step is needed to make the passivation effective. Thanks to coupled electrical measurements (capacitance and photoconductance) and chemical analyses (X-ray Photoelectron Spectroscopy (XPS) and Secondary Ion Mass Spectrometry (SIMS)) carried out on the same p-type Cz silicon sample, a closer explanation of this activation process is given. The presence of hydrogen and oxygen is correlated to the evolution of the electrical parameters and the minority carrier lifetime for 0 to 60min 450°C annealing.

Sputtered Hydrogenated Amorphous Silicon for Silicon Heterojunction Solar Cell Fabrication

This work shows that RF sputter-deposited hydrogenated amorphous silicon (a-Si:H) films are very effective in passivating silicon surfaces. We have previously found that sputter-deposited 45nm thick intrinsic a-Si:H provides outstanding surface passivation on n-type silicon, similar to that achieved by ‘classic’ plasma enhanced chemical vapour deposition [1]. In this paper, we show that p-type silicon surfaces can be well passivated as well, achieving effective carrier lifetimes of 1.1ms for a 1Ω cm p-type wafer, compared to 4.5ms for a 1.5Ω cm n-type sample. Next, on n-type textured surfaces reasonable passivation is also achieved. Post-deposition annealing of our samples shows that sputtered a-Si:H films can perform similarly to PECVD deposited films in terms of thermal stability. Importantly, with stacks of intrinsic and doped (n or p) amorphous silicon effective carrier lifetimes of 1.9ms and 1.6ms on 1.5Ω cm n-type wafers were obtained for i/n+ and i/p+ stacks respectively. These results underline the promise of sputter-deposited a-Si:H as an attractive alternative for heterojunction solar cell fabrication. However, dark conductivity measurements show that sputter-deposited doped a-Si:H films feature a relatively low conductivity, so far. We speculate that this may be caused by differences in microstructure compared to PECVD a-Si:H films, as suggested from the extracted optical band gap values for the respective films.

Surface Treatments Effects for NO2 Gas Responsivity of p-Type Surface Conductive Layer on Diamond Films

ABSTRACTThe low resistance layer called p-type Surface Conductive Layer (PSCL) is formed when the nitrogen dioxide (NO2) was absorbed onto hydrogen-terminated surface, although the diamond is generally isolator. The PSCL conductance is dependent on NO2 concentration in the atmosphere, and this reaction has reversibility. The diamond having these characteristics can apply to gas sensor. In this study, the gas responsivity of PSCL was improved by surface treatment. The changes in the responsivity were evaluated from serial measurements of the conductance in the gas atmosphere. From this evaluation, it was observed that the adsorption and desorption of NO2 were faster via the surface conditions variation by treatment.

Effect of p-type multi-walled carbon nanotubes for improving hydrogen storage behaviors

In this study, the hydrogen storage behaviors of p-type multi-walled carbon nanotubes (MWNTs) were investigated through the surface modification of MWNTs by immersing them in sulfuric acid (H2SO4) and hydrogen peroxide (H2O2) at various ratios. The presence of acceptor-functional groups on the p-type MWNT surfaces was confirmed by X-ray photoelectron spectroscopy. Measurement of the zeta-potential determined the surface charge transfer and dispersion of the p-type MWMTs, and the hydrogen storage capacity was evaluated at 77K and 1bar. From the results obtained, it was found that acceptor-functional groups were introduced onto the MWNT surfaces, and the dispersion of MWNTs could be improved depending on the acid-mixed treatment conditions. The hydrogen storage was increased by acid-mixed treatments of up to 0.36wt% in the p-type MWNTs, compared with 0.18wt% in the As-received MWNTs. Consequently, the hydrogen storage capacities were greatly influenced by the acceptor-functional groups of p-type MWNT surfaces, resulting in increased electron acceptor–donor interaction at the interfaces.

Light-induced anodisation of silicon for solar cell passivation

This paper reports a new method for forming anodic oxides on silicon surfaces using the light-induced current of pn-junction solar cells to make p-type silicon surfaces anodic. The light-induced anodisation process enables anodic oxide layers as thick as 79 nm to be formed at room temperature in a faster, more uniform, and controllable manner compared to previously reported clip-based anodisation methods. Although the effective minority carrier lifetime decreased immediately after light-induced anodisation from initial values measured with an 17 nm thermally grown oxide on both wafer surfaces, the 1-sun implied open circuit voltage of wafers on which the thermally grown oxide on the p-type surface was replaced by an anodic oxide of the same thickness could be returned to its initial value of ∼635 mV (for 3–5 Ω-cm Cz silicon wafers) after a 400 °C anneal in oxygen and then forming gas. The passivation of the formed anodic oxide layers was stable for a period of 50 days providing the oxide was protected by a 75 nm thick silicon nitride capping layer.

Excellent passivation and low reflectivity with atomic layer deposited bilayer coatings for n-type silicon solar cells

Bilayer coatings deposited by atomic layer deposition are shown to simultaneously achieve excellent passivation and low reflectivity, suitable for application on the front side of high-performance n-type Si solar cells. We designed and fabricated bilayer coatings of 10nm Al2O3 followed by a top layer of either 50.5nm TiO2 or 52.5nm ZnS. The bilayers have absolute reflectivity nearly 2% lower than state-of-the-art silicon nitride anti-reflection coatings. They passivate both highly doped p-type emitter surfaces and also low-doped p-type Si. For a B-doped emitter with sheet resistance of 159Ω/sq on n-type Si, the Al2O3/TiO2 coating has a low emitter saturation current density J0,e of 38fA/cm2, while Al2O3/ZnS has 52fA/cm2. On low-doped p-type Si wafers, passivation using either coating resulted in minority carrier lifetimes above 1ms, corresponding to surface recombination velocities below 10cm/s.