|Publication number||US20080258604 A1|
|Application number||US 12/144,234|
|Publication date||23 Oct 2008|
|Filing date||23 Jun 2008|
|Priority date||25 May 2001|
|Also published as||US7390689, US20030029495|
|Publication number||12144234, 144234, US 2008/0258604 A1, US 2008/258604 A1, US 20080258604 A1, US 20080258604A1, US 2008258604 A1, US 2008258604A1, US-A1-20080258604, US-A1-2008258604, US2008/0258604A1, US2008/258604A1, US20080258604 A1, US20080258604A1, US2008258604 A1, US2008258604A1|
|Inventors||Eric Mazur, James Edward Carey, Catherine H. Crouch, Rebecca Jane Younkin, Claudia Wu|
|Original Assignee||President And Fellows Of Harvard College|
|Export Citation||BiBTeX, EndNote, RefMan|
|Referenced by (21), Classifications (22)|
|External Links: USPTO, USPTO Assignment, Espacenet|
The following application claims priority to Provisional Application Ser. No. 60/293,560 filed on May 25, 2001 by Mazur et al.
The present invention relates generally to semiconductor devices, and specifically relates to systems and methods for absorbing light, emitting current, and luminescence using microstructured silicon.
Silicon devices and methods for fabricating them are ubiquitous in modern society. This ubiquity arises because crystalline silicon is cheap, abundant, and present in a myriad of optoelectronic devices that include microprocessor chips. Because of the wide use, many efficient methods have been developed for engineering silicon into various shapes and sizes. Therefore, if both silicon and a second substance can alternatively be used to fulfil some function, it is often more advantageous to use silicon because of familiarity of use, and manufacture.
While crystalline silicon has many useful properties that make the element eminently suitable for semiconductor applications, there are desirable properties that crystalline silicon does not possess. For example, crystalline silicon has a band gap of 1.07 eV, and hence only absorbs light that is no less energetic than visible light, which limits its use in certain optoelectronic devices. In addition, the current emitting and luminescent properties of crystalline silicon also limit the use of this element in certain optoelectronic devices. Any technique that would improve the light absorption, current emission, or luminescent properties of silicon would be very welcome.
Presented herein is a system and method for enhancing the light absorption, the current emitting, and the luminescent properties of silicon. These properties are enhanced by microstructuring silicon using femtosecond laser pulses. The resultant microstructured silicon contains cone-like structures on its surface that help to enhance these properties.
In particular, a method for absorbing electromagnetic radiation to harness energy is described herein. The method includes providing a semiconductor having a plurality of cone-like structures formed thereon by laser light, the semiconductor possessing a band gap energy, Ebg. The term “light” is used herein to denote not just visible light, but all electromagnetic radiation, such as infrared light, for example. In addition to using laser light to produce the cone-like structures, the semiconductor can also be exposed to a background gas to help form the structures. The background gas can include a halogenic gas, i.e., a gas containing a halogen, such as SF6. The method for absorbing electromagnetic radiation further includes exposing the semiconductor, which can include primarily silicon, to electromagnetic radiation to allow the sample to absorb a portion thereof, the electromagnetic radiation having a frequency smaller than Ebg divided by Planck's constant. The method also includes harnessing energy from the absorbed radiation.
Thus, the method can include exposing the microstructured sample composed primarily of silicon to electromagnetic radiation to allow the sample to absorb a portion thereof, the electromagnetic radiation having a wavelength greater than about 1.05 micrometers (corresponding approximately to the 1.07 eV band gap of silicon). If the electromagnetic radiation includes sunlight, the step of harnessing can include utilizing the sample in a solar cell to convert solar energy into other types of energy, such thermal energy, electrical energy, and chemical energy. Instead, the step of harnessing can include utilizing the sample in a photodetector to convert energy of the electromagnetic radiation into an electrical signal indicative of the intensity of the electromagnetic radiation.
Also described herein is a method for emitting an electric current. The method includes providing a sample composed of primarily silicon, the sample having a plurality of cone-like structures formed thereon by laser light. In addition to using laser light to produce the cone-like structures, the semiconductor can also be exposed to a background gas to help form the structures. The background gas can include a halogenic gas, i.e., a gas containing a halogen, such as SF6. The method for emitting an electric current also includes applying an electric field to the sample thereby causing the sample to emit a current. The sample can be characterized by having a turn-on field that is less than about 10 V/μm (for example, less than about 1.4 V/μm), and a threshold field that is less than about 20 V/μm (for example, less than about 2.2 V/μm). The turn-on field is defined as the electric field (bias voltage divided by the tip-to-anode spacing) for which a current density of 0.01 μA/mm2 is produced, and the threshold field is defined as the field at which a current density of 0.1 μA/mm2 is produced. The method can be used in a microwave source, a mass spectrometer, a pressure sensor, an acoustic intensity sensor, and/or a displacement sensor.
Also described herein is a device for absorbing electromagnetic radiation. The device includes a semiconductor, with band gap energy Ebg, for absorbing electromagnetic radiation of frequency less than Ebg divided by Planck's constant. The semiconductor is microstructured to have a plurality of cone-like structures formed thereon by laser light. The device can optionally have a housing for disposing the semiconductor therein or thereon. For example, the system can include a sample composed of primarily microstructured silicon for absorbing electromagnetic radiation having a wavelength greater than about 1.05 micrometers. The system can also include a load coupled to the sample to receive energy derived from the electromagnetic radiation absorbed by the sample.
Also described herein is a system for emitting an electric current. The system includes an electric field generator for producing an electric field, and a microstructured sample for emitting an electric current when exposed to the electric field. The system also can include a load for receiving the electric current and for producing a signal responsive to the received electric current. For example, the system and load can be made suitable for use in at least one of a microwave source, a mass spectrometer, a pressure sensor, an acoustic intensity sensor, and a displacement sensor.
Microstructured samples, such as those composed of primarily silicon having a plurality of cone-like structures formed thereon by laser light, have many applications, which include their use in light absorbing devices, such as solar cells, photodetectors, and other photovoltaic devices. Other applications include field emission devices, such as cathode-ray and panel displays, microwave sources, mass spectrometers, and pressure, acoustic intensity and displacement sensors. Still another application of microstructured samples is their use in luminescence devices. These applications are herein discussed with reference to example embodiments illustrated by
The sample 12 can include, for example, a silicon sample having a surface that can be microstructured by irradiating the surface with a train of 800 nm, 100 fs laser pulses in the presence of a background gas such as SF6, as described in more detail below. This process creates a quasi-ordered array of sharp conical microstructures up to fifty (50) micron high that are about 0.8 micron wide near the tip and up to ten (10) micron wide near the base. The pattern forms spontaneously without the use of masks, and forms only in the region illuminated by the laser.
According to one practice, as contemplated by
In another embodiment as contemplated by
Even for areas patterned with the smallest microstructures, only 1-2 micrometers tall, the optical absorptance over the wavelength range 250 nm <λ<2.5 μm is substantially greater than that of flat, crystalline silicon. For wavelengths between 500 nm and 1.1 μm, the absorptance for these small microstructures is as high as 0.9. The absorptance drops at the band edge, as it does for flat silicon, but even for these longer wavelengths, λ>1.1 μm, the absorptance is greater than 0.8, or more than five times the absorptance of the flat, crystalline silicon. As the heights of the structures increase, so does the optical absorptance, both below and above the band gap. For the tallest microstructures studied, with heights of 10-12 μm, the absorptance is approximately 0.9 or greater across the entire wavelength region investigated. This represents a remarkable increase in absorptance compared with flat silicon, especially for infrared wavelengths 1.1 μm<λ<2.5 μm.
Background gases other than SF6 can also be used. Referring to
The electric field generator 52 produces an electric field, which is applied to the microstructured sample 54. The applied electric field causes the microstructured sample to emit an electric current. The load receives the electric current, and produces a signal responsive to the received electric current.
The emitted field can be used in many devices, such as a display, a microwave source, a mass spectrometer, a pressure sensor, an acoustic intensity sensor, and a displacement sensor, as known to those of ordinary skill in the art. For example, the load can be suitable for use in a display. In this case, the load can include a screen containing luminescent material. When the current emitted from the microstructured sample 54 strikes a portion of the screen, the luminescent material in that portion of the screen luminesces. By judiciously striking portions of the screen, an image can be made to appear on the screen.
Field emitters are often characterized by the field needed to create measurable emission current (the “turn-on field”) and the field needed to create a substantial emission current (the “threshold field”). For the present devices, the turn-on field, defined as the electric field (bias voltage divided by the tip-to-anode spacing) for which a current density of 0.01 μA/mm2 is observed, is 1.3 V/μm. The threshold field, defined as the field at which a current density of 0.1 μA/mm2 is produced, is 2.15 V/μm.
The photocurrent properties of microstructured samples that arise from current emission can also be explored using avalanche photodiodes (APD), such as those available from Radiation Monitoring Devices, Inc., Watertown, Mass An APD is a strongly biased photodiode and is several orders of magnitudes more sensitive than ordinary photodetectors. In an APD, photocarriers produced by absorption of photons, are multiplied in an avalanche process: a high reverse bias (typically>1000 V) provides a strong electric field in the junction region. Photocarriers entering the region are accelerated in the high electric field. If the acquired energy is larger than the band gap energy, these carriers can create additional electron-hole pairs by impact ionization. The generated carriers can produce more carriers if the energy they acquire in the electric field is sufficient. The result is a cascade of carriers. Typically, one photon can create several thousand electron-hole pairs in an APD. APD measurements can be performed by Radiation Monitoring Devices, Inc.
The quantum efficiency of ordinary silicon avalanche photo diodes drops rapidly towards zero for wavelengths longer than 1 μm due to the absorption edge of silicon at 1.1 μm. The quantum efficiency of a 1×1 mm2 APD pixel fabricated by Radiation Monitoring Devices™ as a function of illumination wavelength can be measured. At 1.06 μm, for example, the quantum efficiency is 22% and drops quickly to below 1% for longer wavelengths.
The microstructured sample 92 absorbs light from the light source 94. In response to absorbing the light, the microstructured sample 92 produces luminescent light having a wavelength in the range 500-800 micrometers. Such luminescent light can be used in a myriad of applications, such as the lighting of instrument dials at night, and in aesthetic or entertainment illumination systems.
The amount of luminescence produced by the microstructured silicon depends on the laser fluence, number of laser shots, and amount of annealing. Three series of samples were produced (series A, series B, series C) under the following conditions:
Series A: Variation of Laser Fluence
In series A, 4 samples were patterned using fluences of 2.5 kJ 1 m2, 5 kJ 1 m2, 10 kJ 1 m2, and 30 kJ 1 m2 respectively. The laser scan speed was 50 μm/s.
Series B: Variation of Number of Laser Shots
In series B, the average number of laser shots per unit area was changed by varying the sample translation velocity and the number of scans over the samples. The average number of laser shots per unit area decreases with increasing scanning speed and increases with increasing number of scans. The average number of laser pulses per unit surface area is roughly navg=SfN/v, where S is the laser spot size, v the scan velocity, f the repetition rate of the laser and N the number of scans.
Two samples were produced with a single scan using a laser spot size of 100 μm and a scan velocity of 100 μm/s (navg=1000) and 50 μm/s (navg=2000) respectively. Two more samples were made by scanning over the sample twice using the same laser spot size with a scan velocity of 50 μm/s (navg=4000) and 100 μm/s (navg=2000) respectively. For all samples, the fluence used was 2.5 kJ/m2.
Series C: Annealing
In series C, two sets of samples were patterned using a scan velocity of 100 μm/s. Four of these samples were produced using a fluence of 7.5 kJ/m2 (series C1) and 4 samples were produced using a fluence of 15 kJ/m2 (series C2). After patterning, one sample of each set was annealed at 1300 K in vacuum (P=10−7 Torr) for 30 minutes, 60 minutes, and 3 hours respectively. The remaining two samples were not annealed and served as reference samples. In addition, the sample from series A made with a fluence of 30 kJ/m2 and a scan speed of 50 μm/s was annealed for 5 hours at 1300 K.
Series A Results:
Rutherford Backscattering Spectrometry (RBS) reveals that the oxygen content of the samples increases with increasing laser fluence; the RBS spectra for the four samples appear in
Layer No. O/Si 1 0.81 2 0.66 3 0.53 4 0.42 5 0.33 6 0.28 7 0.21 8 0.11
Column 2 shows the atomic ratio of oxygen to silicon in the layers. Plotting the photoluminescence intensity versus surface oxygen concentration of the four samples made with different laser fluence, yields a linear dependence of the luminescence intensity on surface oxygen content.
As shown by RBS, increasing the laser fluence increases the oxygen content in the samples. This may be associated with the fact that a higher fluence causes a greater increase in the surface temperature and a larger melt depth upon laser irradiation, because more energy is deposited in the surface during the duration of the laser pulse. The diffusion coefficient of oxygen in liquid silicon increases by six orders of magnitudes compared to that of solid silicon. Therefore, with higher laser fluence, a thicker layer can be oxidized. The linear increase of the photoluminescence intensity with the oxygen content indicates that doubling the number of oxygen sites doubles the number of luminescence centers.
Series B Results:
The number of laser shots per unit surface area to which the surface is exposed, is related to the oxidation time. In conventional thermal oxidation, the surface first oxidizes by forming SiOx, with x<2, which then gradually evolves into SiO2 with increasing oxidation time. A blue-shift of the photoluminescence peak is observed with increasing number of laser shots. In comparison to thermal oxidation, the observed shift of the peak wavelength may be due to a changing stoichiometry of SiO2 with increasing number of laser pulses.
Series C Results:
Annealing changes the photoluminescence spectra drastically.
Referring now to
A programmable controller 220 is operatively connected to the translation system 180 for controllably directing the output of the pulsed laser system 120 to different, pre-selected points on the surface of the substrate 140. In the presently preferred embodiment, the translation system 180 includes a specular member 240 and a two-degree-of-freedom actuator 260 controlled by the programmable controller 220. The optics 200 move in accord, although any otter means for displacing the output of the pulsed laser system 120 relative to the substrate 140 in the processing chamber 160, such as an X, Y translation stage coupled to the processing chamber 160 and controller 220, may be employed.
A gas manifold 280 is operatively coupled to the processing chamber 160 for controllably injecting ambient(s) into the chamber 160, and a vacuum pump 300 is operatively coupled to the processing chamber 160.
Referring now to
As shown by block 440, the silicon substrate is then mounted in the processing chamber. Preferably, the silicon substrate is mounted on a thin metal plate with conducting carbon tape.
As shown by block 460, the processing chamber is then evacuated, preferably to 10−6 Torr. When air is the predetermined background gas, it may not be necessary to evacuate the processing chamber prior to laser irradiation. As shown by block 480, the processing chamber is then filled with a predetermined background gas through the gas manifold.
To provide infrared absorbing microstructured silicon active elements in accord with the present invention, the predetermined background gas is determined to introduce states in the silicon substrate that absorb infrared energy and produce photocurrent in response thereto. Preferably, the processing chamber is filled with SF6 as background gas to provide infrared absorbing microstructured silicon active elements in accord with the present invention, although other background gases that introduce states in the silicon or other substrate that absorb infrared energy and produce photocurrent in response thereto may be employed without departing from the inventive concepts.
To provide field emitting microstructured silicon active elements in accord with the present invention, the predetermined background gas is determined to incorporate impurities into the silicon substrate that so alter its electronic structure as to enhance its field emission when exposed to an electric field. Preferably, the processing chamber is filled with SF6 as background gas to provide field emitting microstructured silicon active elements in accord with the present invention. Other background gases that incorporate impurities into the silicon or other substrate that so alter its electronic structure as to enhance its field emission when exposed to an electric field may also be employed without departing from the inventive concepts.
To provide luminescent microstructured silicon active elements in accord with the present invention, the predetermined background gas is determined to produce luminescent states in the silicon substrate. Preferably, the processing chamber is filled with any background gas containing oxygen, such as air, to provide luminescent microstructured silicon active elements in accord with the present invention. Other background gases that produce luminescent states in the silicon or other substrate may also be employed without departing from the inventive concepts.
As shown by block 500, the silicon substrate is then irradiated to produce microstructuring of the surface of the substrate. For fabricating infrared absorbing microstructured silicon active elements and for fabricating luminescent microstructured silicon active elements in accord with the present invention, any combination of laser intensity, pulse duration, number of pulses, wavelength, k-vector and polarization that produces surface microstructuring, or microscopic surface roughness, at each point of the silicon substrate, may be employed. For fabricating field emitting microstructured silicon active elements in accord with the present invention, it is presently preferred to employ that combination of laser intensity and pulse widths that produce comparatively sharp microstructured spikes on the silicon substrate. In general, it may be noted that the higher the intensity, the greater the height and sharpness of the spikes microstructured, and the shorter the pulse duration, the less is the height of the spikes microstructured. Reference may be had to an article by Her et al., entitled “Microstructuring of Silicon with Femtosecond Laser Pulses,” appearing at Appl. Phys. Lett. 73, 1673-1675 (1998), incorporated herein by reference, for a description of silicon microstructured with 800 nm 100 fs laser pulses in SF6 to give a patch of microstructured spikes fifty (50) micrometers high and from 0.8 to ten (10) microns wide. Reference may also be had to another article by Her et al. entitled “Femtosecond Laser-induced Formation of Spikes on Silicon,” appearing at Appl. Phys. A 70, 383-385 (2000), incorporated herein by reference, for a description of the laser pulse duration, intensity and other laser parameters that control the morphology, including the comparative sharpness, of the features microstructured.
As shown by block 520, the silicon substrate is then translated relative to the laser beam to microstructure another surface point selected and the process is repeated until an area of interest has been microstructured. As shown by block 540, the substrate is then removed from the processing chamber.
Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents or additions to the specific embodiments and methods described herein. For example, the microstructured sample can be coupled to a controller, such as a microprocessor, to control the operation thereof. Also, although emphasis has been placed in the above description on using laser light to produce cone-like structures on a semiconductor surface, it should be understood that other methods, such as photolithographic techniques, can also be used to produce such structures. Equivalents are intended to be encompassed by the scope of the following claims.
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|U.S. Classification||313/503, 438/20, 257/E31.014, 257/E31.13|
|International Classification||H01L31/00, H01L31/109, H01J1/62|
|Cooperative Classification||H01L31/0543, H01L31/0547, Y02E10/547, H01L31/1804, Y02E10/52, H01L31/1864, H01L31/1872, H01L31/0236, H01L31/0288|
|European Classification||H01L31/18C, H01L31/0288, H01L31/18G2, H01L31/052B, H01L31/18G4, H01L31/0236|