EP1671377A2

EP1671377A2 - Semiconductor device for emitting light

Info

Publication number: EP1671377A2
Application number: EP04787080A
Authority: EP
Inventors: William Ted Masselink; Fariba Hatami
Original assignee: Humboldt Universitaet zu Berlin
Current assignee: Humboldt Universitaet zu Berlin
Priority date: 2003-10-02
Filing date: 2004-09-30
Publication date: 2006-06-21
Also published as: DE10347292A1; US20070210315A1; WO2005034252A3; WO2005034252A2

Abstract

The inventive semiconductor device for emitting light when a voltage is applied comprises a first (3), second (5) and third active semiconductor area (7A-7C). The conductivity of the first semiconductor area (3) is based on charge carriers of a first type of conductivity. The conductivity of the second semiconductor area (5) is based on charge carriers of a second type of conductivity whereby the charge thereof is opposite to that of the charge carriers of the first type of conductivity. The active semiconductor area (5 13) is arranged between the first and second semiconductor area (3, 5). Quantum structures (13) are embedded in the active semiconductor area (5) and are made of a semiconductor material which has a direct band gap. The quantum structures are structures whose dimension in at least one direction of expansion is small enough such that the properties of the structure can be substantially influenced by quantum mechanical processes.

Description

Semiconductor device for emitting light

The present invention relates to a semiconductor device for emitting light when a voltage is applied.

Today, semiconductor light emitting devices are key components, among others. in devices for transmitting information, in storage devices, in display devices and in lighting devices.

In contrast, semiconductor devices which light up in the visible spectral range do not provide such high light intensities. The first light-emitting diodes (LEDs) were able to provide just enough intensity to be used as display elements in early calculators and digital clocks. However, there is currently a trend towards using LEDs in the visible spectral range in areas where high light intensity is required. For example, automobile manufacturers are trying to replace more and more conventional lighting elements in cars with LEDs. Another area of application for LEDs with high light intensity is, for example, traffic lights in which highly intensively shining red, green and yellow emitters are required. But LEDs, which provide a high light intensity in the visible spectral range, are useful not only in traffic and vehicle technology, but also in information transmission. For example, in the visible spectral range, LEDs that emit highly intensively can be used for short-range data transmission via plastic fibers Find. In contrast to glass fibers, in which the maximum transmission, ie the maximum transmission for electromagnetic radiation, is in the infrared spectral range, the maximum transmission for plastic fibers is in the green spectral range, so that LEDs emitting high-intensity green light, in particular, for data transmission via plastic fibers are of interest. Both the efficiency of the radiation generation process in the semiconductor material is important for the fields of application mentioned, since this is important for the intensity of the radiation emitted, and also the wavelength of the radiation emitted.

The electrical behavior of a semiconductor material can be described with the so-called ribbon model. This means that the charge carriers of the semiconductor material have various energy ranges, the so-called energy bands, within which they can essentially assume any energy values. Different bands are often separated by a band gap, i.e. separate an energy range with energy values that the charge carriers cannot accept. When a charge carrier changes from an energetically higher energy band to an energetically lower energy band, an energy is released that corresponds to the difference between the energy values before and after the transition. The differential energy can be released in the form of light quanta (photons). A distinction is made between so-called direct and indirect band gaps. In the case of an indirect band gap, two processes must meet so that a transition between the energy bands can take place with the emission of light. Therefore, semiconductor materials with indirect bandgaps generally have a much lower efficiency in generating light than semiconductor materials with so-called direct bandgaps, in which only one process is required to emit light.

Negatively charged electrons and positively charged holes, which can essentially be imagined as "missing" electrons in an energy band, are available as charge carriers in a semiconductor material. A hole can be filled by the transition of an electron from another energy band into the energy band in which the hole is present. The process of filling up is called recombination. By introducing foreign substances, so-called dopants, into the semiconductor material, an excess of electrons or holes can be generated as charge carriers. If the electrons are overweight, the semiconductor material is referred to as n-type or n-doped; if the holes are overweight, they are called p-type or p-type charge carriers. The introduction of dopants can also be used to influence the energy levels in the semiconductor material available to the charge carriers.

Today, many commercially available LEDs are based on gallium phosphide (GaP), which is a semiconductor material with an indirect band gap. The introduction of so-called deep impurities, which can be simplified as energy levels accessible to the charge carriers outside the energy bands of the GaP, enables the production of LEDs based on GaP. Due to the indirect band gap, the efficiency of such LEDs when generating light is low. The deep impurities can be created by appropriately introducing foreign atoms such as nitrogen atoms into the GaP.

LEDs based on GaP doped with nitrogen (N), i.e. into which nitrogen is introduced as a dopant, depending on the amount of N with which it is doped, emit in the spectral range from green to yellow.

LEDs based on GaP, which is doped with zinc oxide (ZnO), on the other hand, emit red light. Although GaP doped with ZnO has a slightly higher efficiency in generating light than GaP doped with N, the emission takes place in a spectral frequency range in which the human eye is relatively insensitive, so that the emitted light appears to be less bright , In addition, the efficiency of the light generation process in the GaP doped with ZnO decreases with increasing control current of the LED. The object of the present invention is to provide a light-emitting semiconductor device which has a high efficiency in emitting light, in particular in the visible spectral range.

This object is achieved by a light-emitting semiconductor device according to claim 1. The dependent claims contain advantageous refinements of the invention.

A semiconductor device according to the invention for emitting light when a voltage is applied comprises a first, a second and a third, active semiconductor region. The first and the second semiconductor region can in particular each comprise Al _x Ga-ι _-x P (aluminum gallium phosphide) with 0 x x 1 1. While the conductivity of the first semiconductor region is based on charge carriers of a first conductivity type, the conductivity of the second semiconductor region is based on charge carriers of a second conductivity type which have a charge opposite the charge carriers of the first conductivity type. The active semiconductor region, which in particular Al _x Gaι. _x P with 0 x x 1 1, with quantum structures embedded in the active semiconductor region, which are made of a semiconductor material that has a direct band gap. The Al _x Gaι _-x P of all semiconductor regions can also contain a small amount of arsenic (As) (up to about 50%), which is not mentioned here, but is also included in the designation Al _x Ga _1-x P should be.

Quantum structures are understood to mean structures that have a dimension in at least one direction of expansion that is so small that the properties of the structure are significantly determined by quantum mechanical processes. Examples of quantum structures are quantum dots in which all directions of expansion have small dimensions, and quantum wires in which two directions of expansion are small Have dimensions, or quantum layers (quantum wells) in which an expansion direction has small dimensions in question.

The semiconductor material from which the quantum structures are made can be, in particular, an III-V semiconductor material, ie a combination of elements of the 3rd and 5th group of the periodic table, which has a direct band gap and a lattice constant that is larger, than that of GaP. It should be noted that the lattice constant of Al _x Gaι _-x P does not depend on x and has essentially the same value as GaP. For example, InP (indium phosphide) is suitable as the Ill-V semiconductor material, but also other compounds of elements of the third group, such as indium (In), gallium (Ga) or aluminum (AI) with elements of the fifth group such as phosphorus (P), arsenic (As) or antimony (Sb) are generally suitable.

With the semiconductor structure according to the invention for emitting light when a voltage is applied, a higher efficiency in emitting light can be achieved in the visible spectral range than with light-emitting semiconductor structures according to the prior art. The reason for this is as follows:

In contrast to the GaP-based light-emitting semiconductor devices according to the prior art, the semiconductor device according to the invention makes it possible to use a direct transition between two energy bands for emitting light in the visible spectral range. The direct transition takes place in the embedded quantum structures, e.g. in InP, which has a direct band gap. As mentioned above, the efficiency of emitting light is higher in the case of a direct transition than in the case of an indirect transition, so that the efficiency of the semiconductor device according to the invention for emitting light when a voltage is applied is higher than that of prior art light-emitting semiconductor devices. In addition, the technology of GaP-based LEDs can be used to some extent when manufacturing the semiconductor device according to the invention.

In an advantageous embodiment of the semiconductor device according to the invention, the semiconductor regions are implemented in the form of semiconductor layers of a layer stack. In this case, epitaxy methods known from semiconductor technology can be used to manufacture the semiconductor device. Epitaxy processes are to be understood here to mean all processes with which a layer can be applied in an orderly manner to a crystalline substrate. Examples include molecular beam epitaxy (MBE, molecular beam epitaxy) and deposition from the gas phase (CVD, chemical vapor deposition). With the epitaxy process, the bonding of wafers used in the production of LEDs based on AlGalnP or GaP, that is, a bonding of wafers, is not necessary. The manufacture of semiconductor devices according to the invention, which are designed as LEDs, is therefore simplified compared to LEDs according to the prior art. In addition, the epitaxy process can be easily integrated into existing processes for the manufacture of semiconductor devices. In addition, the use of epitaxy methods can reduce the occurrence of defects in the semiconductor regions. Such defects would negatively affect the emission properties of the semiconductor device.

The presence of a direct transition is ensured in the semiconductor device according to the invention in particular if the quantum structures have a lateral expansion, i.e. have an extension perpendicular to the stacking direction, which is less than about 50 nm on average. In particular, the average lateral extent of the quantum structures is in the range between 10 and 30 nm.

In particular, if the InP coverage is at least 0.5 monolayers (ML), the emission takes place in the visible spectral range. A monolayer corresponds to a covering that is uniform Distributing the InP over the layer located under the quantum structures would result in an InP layer which is monatomic in the stacking direction. In particular, the InP coverage can be between 0.5 ml and approx. 10 ml, preferably between 0.5 and 8 ml, and in particular between 0.5 ml and approx. 4 ml. The color of the emitted light can be determined by a suitable choice of the covering within the specified limits.

In an advantageous development of the semiconductor device according to the invention, the active semiconductor region comprises a plurality of subregions which have different InP coverings. A suitable choice of the respective covering of the sub-areas can be used to produce a semiconductor device which emits quasi white light. The subregions can in particular be designed as different semiconductor layers. Alternatively, they can also differ in their lateral arrangement instead, so that they form different subregions of a common semiconductor layer.

The semiconductor device according to the invention can be configured in particular as a light-emitting diode, superluminescent diode or laser diode. In the case of the superluminescent diode or the laser diode, the semiconductor device according to the invention forms the active region of the superluminescent diode or the laser diode and the immediately adjacent regions. Superluminescent diodes and in particular laser diodes cannot be realized with the aid of the deep impurities known from the prior art.

Further features, properties and advantages of the semiconductor device according to the invention result from the following description of an exemplary embodiment of the invention with reference to the accompanying drawings.

1 schematically shows a layer stack realizing the invention. 2 shows a detail from the active semiconductor region of the semiconductor device according to the invention.

1 shows, as an exemplary embodiment of the semiconductor device according to the invention, the layer stack of a light-emitting diode, which is applied to an n-doped substrate 1. The layer stack comprises an n-doped first semiconductor layer 3, which forms a first semiconductor region, and a p-doped second semiconductor layer 5, which forms a second semiconductor region. In the present exemplary embodiment, the electrons of the n-doped first semiconductor layer 3 represent the charge carriers of the first conductivity type, whereas the holes of the p-doped second semiconductor layer 5 represent the charge carriers of the second conductivity type. Between the n-doped first semiconductor layer 3 and the p-doped second semiconductor layer 5, three undoped quantum structure layers 7A-7C are arranged, which form the active semiconductor region of the LED. Although the quantum structure layers 7A-7C are undoped in the present exemplary embodiment, in alternative embodiments of the exemplary embodiment they can also have an n-doping or a p-doping. Finally, there is a heavily p-doped contact layer 9 for electrically contacting the second semiconductor layer 5 above the second semiconductor layer 5.

It should be noted that the doping of the substrate 1, the first and second semiconductor layers 3, 5 and the contact layer 9 can also be reversed. The semiconductor structure according to the invention would then have a p-doped substrate, a p-doped first semiconductor layer 3, an n-doped second semiconductor layer 5 and an n-doped contact layer 9.

The layer thicknesses are not shown to scale in FIG. 1. While the semiconductor layer 3 has a thickness of 200 nm and the semiconductor layer 5 has a thickness of 700 nm, the three quantum structure layers 7A-7C together have a thickness of only about 9 nm and the contact layer 9 has a layer thickness of 10 nm. The substrate 1, the first semiconductor layer 3, the second semiconductor layer 5 and the contact layer 9 are designed as doped GaP layers. The substrate 1 and the first semiconductor layer 3 each contain silicon (Si) as the dopant, the Si concentration in the first semiconductor layer ³ corresponding to 5 × 10 ¹⁷ cm ^"3. The second semiconductor layer 5 and the contact layer 9, on the other hand, contain beryllium (Be) as the dopant , namely in a concentration of 5x10 ¹⁷ cm ^'3 (second semiconductor layer 5) or 1x10 ¹⁹ cm ^"3 (contact layer 9).

One of the quantum structure layers 7A-7C is shown in detail in FIG. 2. The quantum structure layer 7 comprises a GaP layer 11, in which InP islands 13 are embedded as quantum dots. The GaP layer 11 is sometimes referred to as a GaP matrix. The InP islands are based on a so-called InP wetting layer 15, which covers the entire surface of the layer located under the quantum structure layer 7 and has a thickness between 0.1 and 0.3 nm. The thickness of the GaP layer 11 is selected such that the InP islands 13 are still covered with GaP, but at most with approximately 1 nm GaP. Overall, the thickness of the quantum structure layer 7 shown in FIG. 2 is approximately 3 nm.

The lateral dimensions of the InP islands 13 averaged a maximum of approximately 50 nm. The average of the lateral dimensions is preferably in the range between 10 and 30 nm, and the coverage of the layer located under the quantum structure layer 7 by the InP is approximately 3. 5 ML, ie the InP would suffice to cover the layer underneath with about 3.5 monatomic InP layers. Approx. 1 ML of the InP is applied to the wetting layer. In the present exemplary embodiment, this covering leads to the emission of light with a wavelength of approximately 600 nm. By varying the InP covering, light-emitting diodes can be realized which emit light in the spectral range between orange and green.

With a coverage of approximately 1.8 ML or less, there are no longer InP islands. Instead, the InP forms an even layer, so that you get a quantum layer instead of quantum dots. If quantum dots are mentioned in the present exemplary embodiment, coverings under 1.8 ML should also be understood, without expressly referring to quantum layers instead of quantum dots.

In the present exemplary embodiment, three quantum structure layers 7A-7C are arranged between the first and the second semiconductor layers 3, 5. However, it is sufficient if such a quantum structure layer 7 is present. On the other hand, more than just three quantum structure layers can also be present. There are preferably three to five quantum structure layers.

Together with the quantum structure layers 7A-7C, the first and second semiconductor layers 3, 5 form a light-emitting diode. In this, electrons from the first semiconductor layer 3 and holes from the second semiconductor layer 5 enter the quantum structure layers 7A-7C at a voltage suitably applied between the contact layer 9 and the substrate 1 and generally referred to as forward voltage. A recombination of electrons and holes takes place in the quantum structure layers 7A - 7C, ie electrons fill the holes. For the electrons, this recombination represents a transition from an energetically higher energy band to an energetically lower energy band. The transition is a direct transition that essentially takes place in the quantum dots, ie in the InP. Due to the small dimensions of the InP quantum dots, the band gap in the InP is much larger than in a large-volume InP material, so that the wavelength of the light emitted during the direct transition lies in the visible spectral range. Since the band gap in the InP quantum dots, ie the minimum energy distance between the two bands, and thus the wavelength of the emitted light, depends on the InP coverage, the color of the emitted light in the Range can be varied from orange to green. Although the substrate 1, the first semiconductor layer 3, the second semiconductor layer 5 and the contact layer 9 are described as GaP layers in the exemplary embodiment described, these layers can generally be formed as AlχGa- ₁ -χP layers with 0 x x 1 1, where the values for x can vary from layer to layer. Accordingly, the quantum structures need not be made from InP. Instead, they can be formed as In _y Gaι _-y P layers with 0 <y ≤ 0.5, preferably with 0 ≤ y ≤ 0.1. Since Al _x Ga-ι _-x P is transparent in the visible spectral range, the layer structure described can in particular also be used to produce LEDs which emit vertically, ie in the stacking direction.

With the aid of suitable measures for enclosing the emitted light in the active region of the semiconductor device, for example by suitable selection of the refractive indices of the individual layers or by the provision of facets on the semiconductor structure, superluminescent diodes or coherent light emitting light can be emitted with the semiconductor device according to the invention Laser diodes are manufactured. The basic structure of superluminescent diodes and laser diodes is, for example, the books "Spontaneous Emission and Laser Oscillation in Microcavities", Edit. By Hiroyuki Yokoyama, and Kikuo Ujihara, CRC Press (1995) "and" Optoelectronics: An Introduction to Material and Devices " , Jasprit Singh, The McGraw-Hill Companies, Inc., (1996) ", to which reference is made with regard to the further configuration of the superluminescent diode according to the invention and the laser diode according to the invention.

Claims

claims

1. A semiconductor device for emitting light when a voltage is applied with - a first semiconductor region (3), the conductivity of which is based on charge carriers of a first conductivity type, - a second semiconductor region (5), whose conductivity is on the charge carriers of a second conductivity type, which is one of the charge carriers of the first conductivity type have opposite charge, and - an active semiconductor region (7A - 7C) arranged between the first semiconductor region (3) and the second semiconductor region (5), in which quantum structures (13) of a semiconductor material with a direct band gap are embedded.

2. The semiconductor device according to claim 1, wherein the first semiconductor region (3), the second semiconductor region (5) and the active semiconductor region (7A-7C) each comprise Al _x Gaι _-x P with 0 <x ≤ 1 and the quantum structures (13) are made of an Ill-V semiconductor material that has a lattice constant that is larger than that of GaP.

3. The semiconductor device of claim 2, wherein the III-V semiconductor material comprises InP.

4. The semiconductor device according to claim 1, 2 or 3, in which the semiconductor regions are implemented in the form of semiconductor layers (3, 5, 7A - 7C) of a layer stack.

5. Semiconductor device according to one of claims 1 to 4, wherein the quantum structures (13) have a lateral extent which is less than about 50 nm on average.

6. The semiconductor device according to claim 5, wherein the average lateral dimension of the quantum structures (13) is in the range between 10 and 30 nm.

7. The semiconductor device according to claim 3 and one of claims 4 to 6, wherein the InP coverage is at least 0.5 ML.

8. The semiconductor device according to claim 7, characterized in that the active semiconductor region (7a-7c) comprises a plurality of subregions which have different InP coverages.

9. LED with a semiconductor device according to one of claims 1 to 8.

10. Superluminescent diode with a semiconductor device according to one of claims 1 to 8.

11. Laser diode with a semiconductor device according to one of claims 1 to 8.