This application claims priority benefit of Chinese patent application serial No. 201310168615.0 filed on May 9, 2013. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification.
The present invention relates in general to light-emitting device made of polar semiconductors, more particularly to light-emitting device made of group III nitride polar semiconductors with composition gradient in the active-region.
Nitride based light-emitting diodes (LEDs) have achieved fast progress in recent years. In the visible spectrum regime, InGaN LEDs are increasingly challenging traditional lighting sources such as fluorescent lamps, due to their technological and economical advantages. Currently, high-efficiency InGaN LED white light lamps with efficacy over 130 lm/watt are commercially available. In the ultraviolet (UV) regime, especially in the UVB (315 nm-280 nm)/UVC (≦280 nm) regimes, AlGaN LEDs, even though still in the technological debut stage, have already outperformed the traditional UV light sources in duration, compactness, and UV-power-density aspects. High-efficiency UVC LEDs will lead to numerous disinfection applications using the UV germicidal effect, making revolutionary advances in food safety, water treatment, and medical applications.
Currently, most UV LEDs with emissions shorter than 350 nm adopt the layer structure developed by Zhang et al (See below), which contains a c-plane sapphire as UV transparent substrate, a high-quality AlN layer coated over the substrate serving as epitaxy template, and a set of AlN/AlGaN superlattice for dislocation and strain management. The utilization of high-quality AlN template and AlN/AlGaN superlattice enables the growth of high-quality highly-conductive n-type AlGaN electron supplier layer, which injects electrons into the following AlGaN-based multiple quantum well (MQW) active-region. On the other side of the MQW active-region are an AlGaN electron-blocking layer, an AlGaN hole injection layer, a hole supplier layer and a p-type GaN layer for ohmic contact formation. The prior art AlGaN LED structures can be found in the references. (J. P. Zhang et al, Milliwatt power deep ultraviolet light-emitting diodes over sapphire with emission at 278 nm, APPLIED PHYSICS LETTERS 81, 4910 (2002); J. P. Zhang et al, Crack-free thick AlGaN grown on sapphire using AlN/AlGaN superlattices for strain management, APPLIED PHYSICS LETTERS 80, 3542 (2002)).
On the other hand, group III nitrides are polar semiconductors. This means that interface space charges are inevitably generated when forming heterostructures using nitrides, due to the discontinuity of spontaneous and piezoelectric polarizations at the heterointerface. The spontaneous and piezoelectric polarizations in nitrides have maximal values along c-direction (<0001>), and the resultant interface space charge density in GaN/InGaN and AlGaN/AlGaN c-oriented heterostructures can exceed 1013/cm2, leading to electric field larger than 1 MV/cm resulting in strong band structure distortion. Illustrated in
In the prior art, quaternary AlInGaN materials have been proposed to replace binary (AlN, GaN, and InN) and ternary (AlGaN, AlInN and InGaN) materials for heterostructure formation, owing to the flexibility of nearly independent bandgap and lattice constant adjustment in the quaternaries for a reduced polarization mismatch. (e.g.: “Quaternary AlInGaN Multiple Quantum Wells for Ultraviolet Light Emitting Diodes”, J. P. Zhang, et al, Jpn J. Appl. Phys. 40, L921-L924 (2001); U.S. Pat. No. 7,348,606). In principle, quaternary heterostructure approach can result in high quantum efficiency for MQW active-regions. However, since the optimal incorporation conditions of Al and In are not compatible with each other, it is difficult to obtain high-quality AlInGaN quaternary materials.
The present invention discloses MQW embodiments having reduced polarization field and improved quantum confinement effect, and provides ultraviolet LEDs with improved efficiency and reduced forward voltage.
The present invention discloses a light emitting device with improved quantum efficiency and forward voltage. Throughout the specification, the term III-nitride or nitride in general refers to metal nitride with cations selecting from group IIIA of the periodic table of the elements. That is to say, III-nitride includes AlN, GaN, InN, their ternary (AlGaN, InGaN, InAlN) and quaternary (AlInGaN) alloys. III-nitride or nitride can also include small compositions of transition metal nitride such as TiN, ZrN, HfN with molar fraction not larger than 10%. For example, III-nitride or nitride may include AlxInyGazTi(1-x-y-z)N, AlxInyGazZr(1-x-y-z)N, AlxInyGazHf(1-x-y-z)N, with (1-x-y-z)≦10%. A III-nitride layer or active-region means that the layer or active-region is made of III-nitride semiconductors.
According to one aspect of the present invention, composition grading, for example, Al-composition grading is used to modify the band structure of polar semiconductor heterostructures in order to mitigate the polarization field induced band structure distortion. Al-composition changes can result in AlInGaN bandgap width changes. For example, increasing Al-composition can enlarge the bandgap width, enabling conduction band moving upwards and valence band moving downwards. These movements can be used to mitigate the band edge tilt arising from the polarization field of polar semiconductor heterostructures. In some embodiments, composition grading is used to alleviate the band edge tilt in quantum wells and quantum barriers, leading to improved device forward voltage and quantum efficiency.
Another aspect of the present invention provides a solid-state ultraviolet light emitting device, comprising an N-type layer, a P-type layer and a light-emitting active-region sandwiched in-between the N-type and the P-type layer, and the light-emitting active-region contains at least one quantum well, embedded in quantum barriers, wherein the quantum wells and quantum barriers have composition gradients along the device epitaxy direction.
Another aspect of the present invention provides a solid-state ultraviolet light emitting device, comprising an N-type layer, a P-type layer and a light-emitting active-region sandwiched in-between the N-type and the P-type layer, and the light-emitting active-region contains at least one quantum well, embedded in quantum barriers, wherein the quantum wells and quantum barriers have opposite composition gradients along the device epitaxy direction.
Optionally, the light-emitting device is made of wurtzite group III nitrides and the main epitaxial growth plane is (0001) c-plane, meaning an epitaxy direction along [0001] direction.
Optionally, the Al-composition in the quantum wells and quantum barriers is within 1%-90%. Preferably, the Al-composition of the quantum wells is within 1%-60%, and the Al-composition of the quantum barriers is within 5%-85%.
Optionally, the composition gradients of the quantum wells and the quantum barriers are evidenced as composition linear or nonlinear changes, or abrupt changes, or stair-case changes.
Preferably, the Al-composition of the quantum wells increases along the device epitaxy direction, and Al-composition of the quantum barriers decreases along the device epitaxy direction, when the epitaxy direction is along c-direction [0001].
Preferably, the Al-composition of the quantum wells linearly increases along the device epitaxy direction with the gradient within 0.6% per nanometer to 12% per nanometer, and the Al-composition of the quantum barriers linearly decreases along the device epitaxy direction with the gradient within −0.1% per nanometer to −2% per nanometer.
Optionally, the donor concentration in the quantum wells and quantum barriers can possess linear, nonlinear, abrupt, or stair-case change along the device epitaxy direction.
Optionally, the donor concentration in the quantum barriers increases along the device epitaxy direction, when the epitaxy direction is along c-direction [0001].
Preferably, the donor concentration in the quantum barriers possesses a gradient within 1017 cm−3/nm to 1018 cm−3/nm along the device epitaxy direction.
Optionally, the donor concentration in the quantum wells decreases along the device epitaxy direction, when the epitaxy direction is along c-direction [0001].
Preferably, the donor concentration in the quantum wells possesses a gradient within −2×1018 cm−3/nm to −2×1017 cm−3/nm along the device epitaxy direction.
Furthermore, the (average) Al-composition of the quantum barriers can be less than or equal to the Al-composition of the N-type layer. Preferably, the Al-composition of the N-type layer is about 1.1 to 1.2 times of that of the quantum barriers.
The accompanying drawings, which are included to provide a further understanding of the invention and constitute a part of this application, illustrate embodiments of the invention and together with the description serve to explain the principle of the invention. Like reference numbers in the figures refer to like elements throughout, and a layer can refer to a group of layers associated with the same function.
In the following contents, nitride light-emitting devices or structures are used as embodiments to elucidate the principle and spirit of the present invention. Those of ordinary skills in the field can apply the teachings in this specification and given by the following embodiments of nitride light-emitting devices or structures to II-VI semiconductor and other polar semiconductor devices or light-emitting devices without creative work.
Embodiment 1 according to the present invention is a wurtzite [0001]-oriented group III nitride ultraviolet light emitting diode, with the layered structure illustrated in
Embodiment 1 is further characterized by its MQW active-region design. The quantum wells and quantum barriers in the MQW of embodiment 1 are of non-uniform composition distribution. Preferably, they are of gradually changing compositions, more preferably, the composition of the quantum wells and quantum barriers are of opposite gradient. Illustrated in
As seen from
Preferably, as illustrated in
UV LED embodiment 1 with the MQW active-region having Al-composition distribution illustrated in
Plotted in
Embodiment 2 is similar to embodiment 1 except that electron supplier layer 40 has higher Al-composition than the average Al-composition of quantum barrier 551. Preferably, the Al-composition of layer 40 is about 1.1-1.2 times of the average Al-composition of quantum barrier 551. This arrangement exerts biaxial compressive strain to quantum barriers 551, introducing piezoelectric polarization field to compensate spontaneous polarization field within quantum barriers 551, facilitating carriers' injection into quantum wells 552 and leading to reduced device forward voltage.
Embodiment 3 distinguishes from embodiment 1 and 2 in the MQW active-region Al-composition distribution, which is illustrated in
As seen, for the quantum barrier Al-composition xAl
This is to say, at the beginning of the quantum barrier, Al-composition gradient is zero (constant Al-distribution), and for the rest of the quantum barrier the Al-composition has negative gradient (Al-decreasing). This gradient is preferably to be within −0.1% per nanometer to −2% per nanometer.
Further, for the quantum well Al-composition xAl-QW, the distribution along the epitaxy direction satisfies the following relationship.
This is to say, at the beginning of the quantum well, Al-composition gradient is zero (constant Al-distribution), and for the rest of the quantum well the Al-composition has positive gradient (Al-increasing). This gradient is preferably to be within 0.6% per nanometer to 12% per nanometer.
Embodiment 4 distinguishes from embodiment 1 in terms of the MQW active-region doping. For this embodiment, the Al-composition and donor concentration ([D]) distributions in one quantum well and two quantum barriers of the MQW active-region 55 are illustrated in
The donor can be Si or Ge. In quantum barriers 551 the donor concentration can increase from zero to 1×1019 cm −3, or from 1×1017 cm−3 to 1×1019 cm −3,along the epitaxy direction [0001]. When possessing linear change rate, the donor concentration in the quantum barriers can have a gradient of 1017 cm−3/nm to 1018 cm−3/nm along the epitaxy direction. While [D] in the quantum wells can decrease from 5×1018 cm−3 to zero, or from 1×1018 cm−3 to zero, or from 1>1018 cm−3 to 1×1017 cm−3. Preferably, it has a linear gradient of −2×1018 cm−3/nm to −2×1017 cm−3/nm along the epitaxy direction [0001].
[D] in quantum barriers 551 can also have abrupt changes, for example, in the first part of a quantum barrier (epitaxially formed firstly), [D] can be zero or 5×1017 cm−3, in the rest part, [D] can be 3×1018 cm−3 or 5×1018 cm−3. Similarly, [D] in quantum wells 552 can also have abrupt changes, for example, in the first part of a quantum well (epitaxially formed firstly), [D] can be 1×1018 cm−3 or 5×1017 cm−3, in the rest part, [D] can be 3×1017 cm−3 or zero.
This embodiment employs the built-in electric field from the donor concentration grading to mitigate the polarization field within the MQW active-region, leading to improved internal quantum efficiency and reduced device forward voltage.
The present invention has been described using exemplary embodiments. However, it is to be understood that the scope of the present invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangement or equivalents. The scope of the claims, therefore, should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and equivalents.
Number | Date | Country | Kind |
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201310168615.0 | May 2013 | CN | national |