DD158198A3 - PHOTODIODE - Google Patents

Abstract

Photodiodes with high sensitivity for detection and / or intensity measurement, preferably monochromatic electromagnetic radiation of high pulse repetition frequencies, for example for use in the Lichtnachrichtenuebertragung. In particular, these are semiconductor diodes with at least one barrier layer in the form of a PN or PIN junction. The aim of the invention are photodiodes with such optical structures, by which the conversion losses of the radiation energy occurring in conventional photodiodes by incomplete absorption in the barrier layer region and at the same time the reflection losses for a given wavelength range are largely canceled. This is done essentially by the dimensioning of the optical layer thickness of the semiconductor made in accordance with the invention claims and its deposition according to the invention with an interference layer sequence.

Description

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Title: Photodiode

Field of application of the invention:

The invention relates to photodiodes with high sensitivity for the detection and / or intensity measurement, preferably monochromatic electromagnetic radiation high pulse repetition frequencies, for example, for use in the Lichtnachrichtenübertragung.

Characteristic of the Known Technical Solutions: Photodiodes for detecting and / or measuring the intensity of electromagnetic radiation are known in numerous embodiments. They usually consist of a semiconductor with at least one barrier layer in the form of a PN junction, PIN transition or Schottky junction, whose electric field serves to separate the free charge carrier pairs generated by the penetrating photons. An external electric current applied in the reverse direction generates an external current which grows monotonically with the number of absorbed photons, insofar as the energy of the penetrating photons is greater than the energy gap between the semiconductor and conduction band of the semiconductor, or insofar the penetrating electromagnetic radiation lies within the spectral range in which the semiconductor substance absorbs.

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It is known that - in a simplified way - only those electron-hole pairs for the photocurrent are effective in the field formed by the barrier layer or in the so-called "depletion zone" of the PN, PIN or Schottky structure or be formed in their immediate vicinity, since far outside this zone resulting free charge carriers recombine before their diffusion in the field of the barrier layer.

The penetration depth of the photons entering the semiconductor is therefore of great importance with regard to the sensitivity of photodiodes. In fact, if the electric field generated by the barrier layer is located at a distance from the light entry surface which is greater than the penetration depth of relatively high-energy photons or relatively short-wave radiation, these photons become responsible for the formation of the outer photocurrent

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not or only slightly effective. In contrast, if the junction field lies directly behind the entrance surface, that is to say within the range of the higher-energy photons, then the photons of lower energy which have relatively large penetration depths or the longer-wave radiation are utilized to a limited extent for the photoeffect in previously known photodiode arrangements, because the absorption spreads over a region of the semiconductor that is much larger than the depletion zone.

In the photodiodes known hitherto, the spatial coincidence of the barrier layer field or depletion zone and the absorption region is therefore not or only insufficiently realized, in particular with photon energies lying only slightly above the band gap of the semiconductor. In addition, occur in the known semiconductor photodiodes high additional losses due to reflection at the entrance surface, which by consuming

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Antireflective measures must be compensated.

Aim of the invention;

The aim of the invention are photodiodes, which have such a geometric structure and such an optical structure, that in bekanten photodiode arrays occurring by incomplete absorption in the barrier region conversion losses at a given wavelength canceled or substantially reduced. At the same time, with the arrangement according to the invention, elimination of the generally high reflection losses in the case of semiconductors is effected when the radiation enters.

Presentation of the essence the Invention;

The arrangement according to the invention consists of a semiconductor with a barrier layer in the form of a PN or PIN junction, wherein the effective penetration depth is reduced by deposition of the semiconductor with an interference layer sequence of high reflectance and by suitable dimensioning of the semiconductor thickness.

and, in this way, a much higher proportion of the radiant energy is made effective at the desired wavelength for the external electrical photocurrent.

This object is inventively achieved in that the semiconductor has an optical thickness of η * d = m ₍ \ / A

where Λ is the desired wavelength at which the absorption coefficient of the semiconductor takes the value k, η is the refractive index of the semiconductor at the wavelength / \ and m is an integer near the value 2 / fr »k. According to the invention, the interference layer sequence consists of / V / 4 layers with alternately low and high refractive indices in the case of even m and with alternately high and low refractive indices in the case of odd m.

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In this case, according to the invention, the first layer 'of the interference layer sequence can be formed in the form of an electrically conductive transparent layer and exert the function of the rear electrode.

For the dimensioning of the semiconductor thicknesses and the deposited interference layer sequence, the invention permits a radiation incidence directed both vertically and parallel to the barrier layer.

Embodiment:

The invention will be explained in more detail below with reference to a (not to scale) figure by an embodiment.

It is a photodiode of an amorphous silicon layer, which was produced according to one of the known Her-position technologies, wherein the barrier layer in a known manner by a PIN structure consisting of a highly doped P-region 1, an "intrinsic" area high semiconductor purity 2 and a doped N-region 3 is formed. The radiation 4 to be detected or measured of the wavelength 0.920, Um is perpendicular to the PIN interfaces. The external contacts (not shown in the figure), to which the diode blocking voltage is applied, can be formed in a known manner as edge contacts. The deposited interference layer sequence consists of four pairs of layers, wherein the individual layers with the optical layer thicknesses Λ / 4 have alternately low refractive indices 5 (n = 1.34) and high refractive indices 6 (n = 2.4); only the first, directly adjacent to the semiconductor interference layer 7, which consists of indium oxide doped tin oxide and serves as a rear electrode, has the substance of this substance refractive index of about 1.8. The entire system is located on a stable base 8, for example made of glass

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the refractive index 1.5, which exercises no essential function in the context of the invention.

Refractive index and absorption coefficient of the silicon layer are at the wavelength 0.920, Um η = 3.80

and l <= 0.01. Accordingly, the value m that serves to determine the optical thickness is m a? 2 / tf. k β 63,66 the integer value m = 64, and thus the optical thickness of the semiconductor η. d =

m 7 ^ / 4 = 64. Λ / 4 or the geometric thickness d = m fr. / 4n = 3,874, um. The P range is approximately 0.1 to 1 ".

Since the exact setting of the optimum semiconductor thickness is directly complicated because of possible slight deviations from refractive index and absorption coefficient, the reflectance at the wavelength 0.920 μm measured with the increasing layer thickness and the coating process are measured in a known manner during the coating is canceled as soon as the lowest reflection minimum is reached. If one calculates the transmittance X , the reflectance und and hence the absorption C * for the photodiode arrangement described with this exemplary embodiment. for incident radiation of the wavelength 0.920 μm, for V approximately 2% and for weniger less than 0.0001% are obtained, so that approximately "98% of the incident radiation is absorbed in the photodiode, the absorption range According to the geometric arrangement, it is only slightly larger than the barrier layer or depletion zone, and thus all the photons are incident on the incident photons, while the depletion zone realized in the described arrangement has the same order of magnitude as known from known considerations and calculations for the Processing of very high pulse repetition frequencies required

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Assuming that, for fast silicon photodiodes, the depletion zone should not exceed 4.5 by size, and therefore the semiconductor regions at a distance above about 5.5 to no longer significantly contribute to the formation of photocurrent-efficient absorption processes from the entrance surface For a conventional photodiode array with the optical constants of silicon for the vertical length 0.920, um, the incident radiation is used to only about 50% because the remaining 50% is absorbed in semiconductor regions that are substantially behind the depletion zone , With this consideration, the reflection losses of the entrance surface were not even considered. In reality, therefore, the overall yield of the incident radiation is still significantly greater if the entrance surface is not largely coated.

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Claims (3)

~ J. - 1. photodiode with high sensitivity for detection and / or intensity measurement preferably monochromatic] electromagnetic radiation, consisting of a semiconductor with a designed as a PN or PIN junction junction structure, which is deposited with a layer system of high reflectance, characterized in that -daß Semiconductor has a measured in the radiation direction optical thickness nd = ηι «λ / 4, where λ is the wavelength at which the absorption coefficient of the K) semiconductor takes the value k and the refractive index the value η and m is an integer close to the value 2 / TTk. 2. photodiode according to item 1, characterized in that the highly reflective layer system, with which the semiconductor is deposited, a directly adjacent to the semiconductor interference layer low refractive index at a straight m and an interference layer of high refractive index at odd m contains. 2 2 0 0 6 3 Invention claim: f ~ 3 · Photodiode according to item 1 and 2, characterized in that the directly adjacent to the semiconductor interference layer of the deposited highly reflective layer system consists of an electrically conductive, transparent in the wavelength substance, for example tin and / or indium oxide. For this 1 page drawings 3581