P-layer Thickness Research Articles

Design issues of two-dimensional amorphous silicon position-sensitive detectors

The two-dimensional amorphous silicon position-sensitive detector (PSD) is usually in the form of large-area, continuous p-i- n silicon layer structure coupled with resistive layers next to the p and n Si layers. The device has many applications (e.g. light position measurements, light-pen input devices, etc.) and can be fabricated by using low-cost PECVD process. When used as a light-pen-based input device, several material-related design issues must be critically considered for achieving acceptable performance. The present work addresses three important issues, namely the spectral response of PSD, the uniformity requirement of the resistive layers, and the design of optical filter on the input side of PSD. They correspond to the signal-to-noise ratio of the device, the accuracy of light-position determination, and the integration problem with liquid crystal displays (LCD), respectively. Analytical analysis and computer simulation results draw the following important conclusions: (1) red-light-sensitive PSD can be obtained by properly tuning the thickness of p-layer and i-layer, which suppress the interference of background light when using the input device under sun light or similar illumination (2) the spot size of input light has little effect on position determination, as long as the size does not differ too much from that of required resolution. And a conservative uniformity requirement for the resistive layers can be obtained as | Δh/ h|≤4/ n with n being the required number of pixels of display and h being the film thickness (3) multi-layered filters made of oxides can be deposited on PSD to reflect non-signal light for LCD display while preserving the input-signal when the PSD is placed under a TN LCD.

High speed, low noise ultraviolet photodetectors based on GaN p-i-n and AlGaN(p)-GaN(i)-GaN(n)structures

We have investigated the spectral response of front-surface-illuminated GaN and AlGaN/GaN p-i-n ultraviolet photodetectors prepared by reactive molecular beam epitaxy on sapphire substrates. GaN homojunction p-i-n photodiodes exhibited a peaked response near the band edge. This enhanced response was absent in the AlGaN/GaN heterojunction p-i-n detectors. We analyzed the effect of p-layer thickness of the GaN p-i-n diodes on the magnitude of the peak photoresponse. The AlGaN/GaN photodiodes had a maximum zero-bias responsivity of 0.12 A/W at 364 nm, which decreased by more than 3 orders of magnitude for wavelengths longer than 390 nm. A reverse bias of −10 V raised the responsivity to 0.15 A/W without any significant increase in noise. The root-mean-square noise current in a 1 Hz bandwidth is ∼1.0 pA, corresponding to a noise-equivalent-power of ∼8.3 pW. We measured extremely fast decay times of 12 ns for the AlGaN/GaN and 29 ns for the GaN photodiodes.

Post Hydrogen Treatment Effects of Boron-doped a-SiC:H p-Layer of a-Si:H Solar Cell Using a Mercury-Sensitized Photo-Chemical Vapor Deposition Method

Post hydrogen treatment effects of boron-doped hydrogenated amorphous silicon carbide (a-SiC:H) film used as a p-layer of p/i/n type amorphous silicon based solar cells using a mercury-sensitized photochemical vapor deposition method were investigated by measuring the thickness, electrical, and optical properties of the film before and after hydrogen treatment. It was found that the boron-doped a-SiC:H film was simultaneously etched and passivated by the treatment. The performance of the cell with a hydrogen treated p-layer was improved by ∼7% due to an increase in open circuit voltage (V oc) and fill factor (F.F.), compared to that of the untreated cell, although the p-layer thickness was nearly identical in both cases. The increase in V oc and F.F. could be explained by an increase in the built-in potential due to a decrease in the film activation energy. This electrical property improvement was well explained by the passivation effect of a SiH2/SiH ratio decrease of the film calculated by FTIR spectra. These film changes by post hydrogen treatment are considered to occur in the bulk of the boron-doped a-SiC:H film.

Etching and passivation effects on boron-doped amorphous silicon carbide p layer of amorphous silicon solar cell by hydrogen treatment using a mercury-sensitized photochemical vapor deposition method

Etching and passivation effects of hydrogen treatment of boron-doped hydrogenated amorphous silicon carbide (a-SiC:H) film used as a p layer of p-i-n type amorphous silicon based solar cells using a mercury-sensitized photochemical vapor deposition method were investigated. For the hydrogen treatment of the p-layer film, longer p-layer deposition time was needed to obtain the same thickness as for no hydrogen treatment because of hydrogen etching effect. However, the cell performance was improved by ∼7% due to an increase in the open circuit voltage (Voc) and fill factor (FF) although the p-layer thickness was nearly identical in both cases. The increase in the Voc and FF could be explained by an increase in the built-in potential due to a decrease in the film activation energy. Moreover, the electrical property improvement of the film was well explained by the passivation effect of a SiH2/SiH ratio decrease and a hydrogen content increase calculated from Fourier transformed infrared absorption measurements.

Structural and Optical Properties of a-Si:H/μc-Si:H:B Junctions in the a-Si:H-Based n-i-P Solar Cell Configuration

ABSTRACTWe have extended previous real time spectroscopie ellipsometry (RTSE) capabilities in order to investigate the effects of H2-plasma treatment of i-type hydrogenated amorphous silicon (a-Si:H) on the deposition of the overlying p-type microcrystalline silicon (μc-Si:H:B)) in the formation of an n-i-p solar cell structure. In this study, we compare in detail the nucleation and growth of p-layers by plasma-enhanced chemical vapor deposition (PECVD) from SiH4 highly diluted in H2 on the surfaces of untreated and H2-plasma treated a-Si:H i-layers. We find that for intended single-phase μc-Si:H:B p-layer PECVD under optimum conditions on an untreated i-layer surface, a wide gap (∼2.0 eV Taue gap) amorphous layer nucleates and grows in the first ∼150 Å. This layer develops uniformly to a bulk thickness of ∼150 Å, but gradually acquires a crystalline structure for thicknesses greater than the desired p-layer thickness (200 Å). In contrast, for p-layer PECVD under identical conditions on the H2-plasma treated i-layer, high-density crystalline nuclei form immediately. This conclusion is drawn on the basis of the unique optical properties of the bulk p-layer that develops on the surface of the H2-plasma treated i-layer. Specifically, an absorption onset near ∼2.5 eV is observed for a 48 Å fully-coalesced p-layer, as measured by RTSE at 200°C. For this μc-Si:H:B p-layer, the optical gap decreases by ∼0.15 eV with increasing thickness from 50 to 200 Å. This effect is attributed to a reduction in the quantum confinement energy with an increase in the average crystallite size in the film.

High electric-field amorphous silicon p-i-n diodes: Effect of the p-layer thickness

We present amorphous silicon p-i-n diodes able to sustain a reverse bias corresponding to 106 V/cm with a reasonably low leakage current. The influence of the p-layer thickness on the reverse bias current and the breakdown voltage is investigated. The high-voltage reverse current at room temperature is attributed to two different mechanisms: field enhanced thermal generation in the p-i interface region and, at the highest bias, electron injection through the p layer. Variable range hopping is also contributing to the low-temperature reverse current. Charge collection measurements after pulsed photogeneration were also performed up to the maximum voltage. No evidence for signal amplification is found, which sets a lower limit of 106 V/cm for impact ionization and avalanche phenomena.

30 × 40 cm 2 double-junction tandem cells with ZnO coated SnO 2 transparent electrodes

Fabrication technologies are presented for large area tandem solar cells. We have attained a total-area efficiency of 10.05% for a 30 × 40 cm 2 tandem submodule. For further improvement of conversion efficiency of the tandem cell, we tried to reduce the p-layer thickness. Using ZnO coated SnO 2 transparent electrode, we have reduced the p-layer thickness from 12 to 4 nm without decreasing open-circuit voltage and attained a conversion efficiency of 12.0% for a 1 cm 2 tandem cell.

Computer analysis of the role of p-layer quality, thickness, transport mechanisms, and contact barrier height in the performance of hydrogenated amorphous silicon p-i-n solar cells

The transport simulations provided by the computer program AMPS have been used to give an in-depth analysis of the role of the p-layer contact barrier height, contact transport mechanism, p-layer thickness, and p-layer quality on the performance of hydrogenated amorphous silicon p-i-n solar cells. We demonstrate for the first time that, if the contact barrier height to the p-layer is below a critical value and if tunneling through the p-layer is not important, then the performance of cells with either active or dead p-layers varies with contact barrier height regardless of p-layer thickness. We show that, even for an optimistic p-layer active doping density of 1019 cm−3, this critical barrier height is high (∼1.2 eV). Our analysis implies that one of two situations must occur in an actual a-Si:H p-i-n structure: the p-layer contact plays an important role in determining cell efficiency, or the tunneling of holes through the front contact/p-layer interface must be important. Comparison of simulated results, with and without tunneling, with experimental data suggests that tunneling is occurring in actual devices and is important in efficient structures.

Development of hydrogenated amorphous silicon sensors for high energy photon radiotherapy imaging

Measurements with a-Si:H n-i-p photodiodes with sensitive areas of approximately 0.6 mm/sup 2/ exposed to a 6 MV radiation therapy treatment beam have been performed. Such photodiodes can be configured into large two-dimensional arrays of addressable sensors suitable for real-time imaging of megavoltage treatment beams as well as other applications such as diagnostic X-ray imaging. Signal sizes per radiation pulse up to 3.9*10/sup 6/ and 69*10/sup 6/ electrons are observed when the sensors are used with cronex (CaWO/sub 4/) and lanex (gadolinium-oxysulfide) intensifying screens, respectively. For the cronex and lanex screens the size of the measured signals can be enhanced by reducing the thickness of the top p-layer, thereby reducing the attenuation of the incident light. However, this is a smaller effect for the lanex screen, which provides substantially more light signal. The variation in cronex signal size with p-layer thickness is consistent with calculations based on known attenuation coefficients.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Nonlinear analysis of surface-emitting distributed feedback lasers

An analysis of surface-emitting distributed feedback lasers under conditions above threshold is presented. The coupled-wave equations are integrated numerically using a self-consistent technique that includes the effects of gain saturation, carrier diffusion, antiguiding, and free carrier loss. The laser threshold current and slope efficiency are determined as a function of the strip length and p-layer thickness where it is found that the device efficiency can be very high. Antiguiding, which varies when the grating is mistuned with respect to the gain peak, influences the slope, threshold, and shape of the emission far field. Small variations in the longitudinal index of refraction are shown to affect the far-field intensity profile significantly.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Photovoltaically active p layers of amorphous silicon

Studies on amorphous silicon p-n diodes have shown that the p layers are photovoltaically active. Detailed quantum efficiency measurements were performed as a function of bias voltage, p-layer thickness and boron doping. The data fit a simple depletion width model in which all photogenerated carriers created inside the depletion region are collected. An additional “reverse” barrier depletion width is assumed at the interface between the front transparent oxide electrode and the p layer to explain the results. A 10-nm thick p layer typically used in p-i-n cells could collect up to 1 mA/cm 2 if the electric field in the p layer is greater than 10 5 volt/cm.

Photovoltaically active p layers of amorphous silicon

Studies on amorphous silicon p-n diodes have shown that the p layers are photovoltaically active. Jsc of up to 3.4 mA/cm2 and Voc of 780 mV have been observed. Detailed quantum efficiency measurements were performed as a function of bias voltage, p-layer thickness, and boron doping. The data fit a simple depletion width model in which all photogenerated carriers created inside the depletion region are collected. An additional ‘‘reverse’’ barrier depletion width is assumed at the interface between the front transparent oxide electrode and the p layer to explain the results. A 10-nm-thick p layer typically used in p-i-n cells could collect up to 1 mA/cm2 if the electric field in the p layer is greater than 105 V/cm.

Window Effects of Hydrogenated Amorphous Silicon Carbide in a p-i-n a-Si Solar Cell

A good valency electron controllability has been found in hydrogenated amorphous silicon carbide prepared by the plasma decomposition of [SiH4 + CH4] gas mixture. Utilizing this a-SiC: H layer as window in an a-SiC: H/a-Si: H heterojunction structure, effects of a-SiC: H on the photovoltaic performance have been investigated. A series of systematic experimental data of photovoltaic performance with parameters of methane fraction, optical gap to window side material and p-layer thickness are presented. Remarkable improvements in both short circuit current density and open circuit voltage have been seen in an a-SiC:H/a-Si: H heterojunction cell. This experimental fact implies that not only a wide gap window effect but also voltage factor due to potential profile steepened with wide dap a-SiC: H contribute to the photovoltaic performance of the a-SiC: H/a-Si: H heterojunction solar cell. It has been shown that a-SiC: H might be a useful new material for the development of low cost high efficiency solar cells.

Effects of Surface Conditions of Substrates upon Performance of a-Si Photovoltaic Cells

The growth process of a-Si film deposited onto an ITO substrate by a glow discharge reaction in SiH4 was investigated by transmission electron microscopy. The a-Si film grows to form an island structure at the first step (0–200Å-thick) of the growth process. The p-layer of conventional ITO/p-i-n photovoltaic cells has an island structure with a thickness in the range of 50 to 200Å. A multi-junction model in which a-Si photovoltaic cells consist of an ITO/p-i-n junction (hetero-face junction) and an ITO/i-n junction (hetero junction) is proposed. The radius of the island structure in the p-type a-Si is calculated with this model. The result of the calculation is in approximate agreement with that of the observation by transmission electron microscopy. The dependence of the open-circuit voltage of Glass/ITO/p-i-n photovoltaic cells upon the p-layer thickness can be explained by the multi-junction model.

Vapor-grown InGaP/GaAs solar cells

GaAs solar cells with conversion efficiencies as high as 14% at AM1 have been grown by the GaCl/AsH3 hydride technique. Thin (∼200 Å) layers of InGaP were used to passivate the GaAs top surface. We observe a 70-fold increase in photoluminescence intensity of the GaAs after passivation, which is consistent with a lowering of the GaAs surface recombination velocity from ≳106 to &amp;lt;104 cm/sec. Short-circuit current densities (Jsc) as high as 22 mA/cm2 and open-circuit voltages as high as 0.96 V were observed. (If achieved together, those values would yield an efficiency of almost 18%.) Voc was observed to increase directly with p-layer thickness (t), whereas Jsc decreased directly with t. The efficiency also increased as the zinc doping was decreased.

Selection of modes perpendicular to the junction plane in GaAs large-cavity double-heterostructure lasers

The gains and reflectivities of the transverse-electric (TE) modes of GaAs large-cavity double-heterostructure (DH) (P-p-n-N) lasers havebeen calculated in order to study mode selection perpendicular to the junction plane. The GaAs laser is modeled as a passive dielectric-slab waveguide composed of four layers whose optical constants are taken from recent literature. In lasers having a cavity thickness of a few microns, a small difference in refractive index between the p- and n-region leads to the fundamental mode being confined to the p-region. Discrimination in favor of this mode without significant penalty in threshold gain may be achieved by utilizing the mode-dependent loss in the n-layer. It is found that optimum mode discrimination in favor of the fundamental mode is governed by a suitable combination of the p-layer thickness, loss in the n-region, and the difference of refractive indices of the p- and n-layers. This analysis contrasts with an earlier analysis of the case of uniform refractive index in which it was found that the p-layer should be twice as thick as the n-layer for optimum mode discrimination. The calculated mode shapes in P-p-n-N lasers are found to be in qualitative agreement with those observed recently by Hakki, and also with earlier observations that catastrophic mirror damage in P-p-n-N lasers is confined to the p-region. Contrary to experimental observation, the model predicts that lasers having cavity thicknesses as large as 3 μm should operate stably in the fundamental mode. Possible reasons for this discrepancy are advanced. A simplification of the P-p-n-N laser, which consists of omitting the N-layer thus yielding an asymmetric double-heterostructure laser, is suggested, and the merits and drawbacks of such a design are discussed.

The present state-of-the-art of photovoltaic solar energy conversion

The present state-of-the-art in photovoltaic solar energy conversion, in particular, as applied for space vehicle power supplies is summarized. Past and future development goals are: lower weight-to-power and cost-to-power ratios, higher reliability, and increased useful lifetimes. Difficulties with photovoltaic solar energy converting devices were encountered in their classification based on exact determination of the power output and efficiency in “standard” sunlight, and by a reduction of their useful life expectancy due to radiation damage encountered in the Van Allen belts and in solar flares. Remedial actions in progress are outlined. Recent improvements introduced on the present standard device, namely, the silicon solar cell are discussed. They include application of metallic grids for the reduction of series resistance and change of the p-layer thickness for improved spectral response. Further, attempts to develop methods for preparing large area and thin film silicon solar devices are reviewed, and finally a survey is given of the progress made in the application of other semiconductor materials to solar photovoltaic energy conversion. Here the work on cadmium sulphide solar cells stands out due to the achievement of solar conversion efficiencies of up to 4 1 2 % on thin film cells.