The invention relates to a light emitting device such as a light emitting diode (LED) and/or lasers with staggered quantum wells (QWs) as the active regions.
An LED is a semiconductor diode that emits incoherent narrow-spectrum light when electrically biased in the forward direction of a p-n junction. This effect is a form of electroluminescence.
An LED typically comprises a small area source, often with extra optics added to the chip that shape its radiation pattern. Color of emitted light depends on semiconductor material composition and can be infrared, visible, or near-ultraviolet. The LED can comprise a chip of semiconducting material impregnated or doped with impurities to create the p-n junction. Charge-carriers—electrons and holes—flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level and releases energy in the form of a photon (light) causing current flow from the p-side, or anode, to the n-side, or cathode. The wavelength of the light emitted, and hence its color, depends on the band gap energy of the materials forming the p-n junction.
An optoelectronic device is based on the quantum mechanical influences of light on semiconducting materials. An optoelectronic device can include a multilayer semiconductor structure comprising a GaN layer and an active region comprising at least one QW layer of InGaN and GaN. In a typical InGaN QW, performance (luminescence efficiency) can be adversely affected by 1) defect density (threading dislocation) and 2) the existence of an electrostatic field. A high threading dislocation density leads to low radiative efficiency. Spontaneous and piezoelectric polarization of the InGaN/GaN QW can induce a built-in electrostatic field that results in significant reduction of electron-hole wavefunction overlap Γe
There is a need for an InGaN/GaN QW with reduced defect density and electron-hole wavefunction overlap Γe
The invention provides an InGaN/GaN QW with improved electron-hole wavefunction overlap Γe
In an embodiment, the invention is a method for improving light efficiency of a light emitting device, comprising: providing a semiconductor substrate; and forming on the substrate a succession of layers to provide QW comprising a staggered composition quantum well adjacent a GaN barrier layer.
In another embodiment, the invention is an optoelectronic device comprising: a multilayer semiconductor structure comprising a GaN layer and an active region, the active region comprising a staggered composition quantum well.
In another embodiment, the invention is a method for generating optical emission from an optoelectronic device, comprising: providing a GaN layer and an active region, the active region comprising a staggered composition quantum well; and exciting the active region to produce optical emission.
LED light extraction efficiency is the portion of emitted electromagnetic radiation that is transmitted usable for human vision. It is a ratio of emitted luminous flux to radiant flux. The present invention relates to an LED with improved light extraction efficiency.
An LED can comprise a chip of semiconducting material impregnated or doped with impurities to create a p-n junction. Current flows from the p-side or anode, to the n-side or cathode, but not in the reverse direction. Charge carriers—electrons and holes—flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level and releases energy in the form of a photon.
A quantum well is an LED potential well. The term “quantum well” or “QW” used herein refers to a thin-layer structure comprising alternate layers consisting of a first semiconductor layer with a thickness smaller than the de Broglie wavelength of about 200 Å to 300 Å with respect to electrons or holes and at least a second semiconductor layer with a band gap greater than that of the first semiconductor layer. A “substrate” is an underlying template or substratum can such as a sapphire template, a GaN substrate, a Si substrate, SiC substrate or ZnO substrate.
A QW structure can be formed by sandwiching a semiconductor thin layer of a narrow band gap between semiconductor layers of a large band gap. If a single semiconductor thin layer constitutes a quantum well for both electrons and holes, the quantum well is called a type I quantum well. In this case, the semiconductor layer of a narrow band gap is called a well layer, and the semiconductor layers of a large band gap are called barrier layers. A type I multi-quantum well structure can be formed by alternately laminating semiconductor layers of narrow and broad band gaps. A type II quantum well structure has a first semiconductor layer forming a quantum well for electrons, a second semiconductor layer forming a quantum well for holes formed on the first semiconductor layer and third semiconductor layers sandwiching the first and second semiconductor layers as barrier layers to the electrons and holes. A type II multi-quantum well structure can be formed by alternately laminating first semiconductor layers, second semiconductor layers and third semiconductor layers.
In an embodiment, the invention relates to an optoelectronic device that includes a GaN layer and a quantum well active region. The active region can comprise at least a staggered InGaN quantum well layer, with different In-composition in the quantum well active region, surrounded by GaN barriers. The electron quantum well layers and hole quantum well layer form a first quantum well stage. The active region can include a plurality of the quantum well stages adjacent to each other having the same structure as the first quantum well stage. The structure can include a transitional layer of GaN between each quantum well stage.
A III-Nitride gain media emitting in the visible regime can be based on a type-I InGaN QW. Challenges to high performance in these InGaN QWs include 1) defect density (threading dislocation) including phase separation in high-In content InGaN QW, and 2) the existence of electrostatic fields in III-Nitride semiconductor. High threading dislocation density leads to low radiative efficiency. Spontaneous and piezoelectric polarizations induce built-in electrostatic fields resulting in significant reduction of electron-hole wavefunction overlap Γe
The present invention provides a method and structure to improve luminescence efficiency in nitride gain media by utilizing a staggered InGaN QW with a step-function In-content profile. According to Fermi's rule, electronic transition from |2 to |1 is governed by a transition matrix element via the interacting Hamiltonian value Ĥ′21, resulting in quantum mechanical transition rate
where ρf is the density of the final state. The Hamiltonian Ĥ′21 in a semiconductor can be expressed in terms of transition matrix element and envelope functions overlap acceding to |H′21|2˜|uc|ê·{right arrow over (p)}|uv|2·|Γe
The improved efficiency invention may be embodied in various types of optoelectronic devices including amplifiers, light emitting diodes, and edge emitting and surface emitting lasers that incorporate optical feedback to provide lasing action. The invention may find application in solid state lighting, solid state displays, lasers, light emitting diodes (LEDs), biomedical therapy and diagnostic devices, medical lasers, eye surgery devices and DVD lasers.
These and other features of the invention will become apparent from the drawings and following detailed discussion, which by way of example without limitation describe preferred embodiments of the invention.
The device 10 includes a QW 70 that comprises staggered GaN/InGaN layers. In this context “staggered” means a composition Xn wherein X is a quantum well component and in one quantum well layer, n is a greater step function than a layer comprising a composition for emission at a target light regime and in at least one other quantum well layer, n is a lesser step function than in the layer comprising a composition for emission at a target light regime. The value for n in one layer and in the other, provide alternating values of the center value providing a target regime. In the illustrated QW region 70, an InGaN layer can comprise multiple InGaN layers of varying In and Ga content. For example, 13-Å In0.28Ga0.72N/13-Å In0.21Ga0.29N are staggered compositions to a 27-Å In0.26Ga0.74N QW. Another example includes layers arranged in multiple periods, for example 4-periods of 7.5-Å In0.25Ga0.75N/7.5-Å In0.15Ga0.85N. In these examples, X is an In-content and the multiple Xn-containing layers provide a step function In-content profile.
In
The QW structures can be grown by III-V semiconductor MOCVD/MBE epitaxy and molecular beam epitaxy (MBE). However, for manufacturing considerations such as high-throughput, the use of metal organic chemical vapor deposition (MOCVD) growth may be preferred.
The following EXAMPLES are illustrative and should not be construed as a limitation on the scope of the claims unless a limitation is specifically recited.
The EXAMPLES are based on the following theory and calculations:
Fermi's golden rule provides a calculation for transition rate (probability of transition per unit time) from one energy eigenstate of a quantum system into a continuum of energy eigenstates, due to a perturbation. According to Fermi's golden rule, electronic transition from state |2 to |1 is governed by transition matrix elements via the perturbation Hamiltonian Ĥ′21 to provide the quantum mechanical transition rate W2→1 as follows:
where ρf is the density of final states and Ĥ′21 is expressed as a function of the transition matrix element and the envelope functions overlap. In a semiconductor, transitions occur between conduction band states and valence band states that result in the following perturbation Hamiltonian |H′21|2∝·|uc|ê·|uv|2·F2|F12, which depends on the square of transition matrix element |MT|2=|uc|ê·|uv|2 and the square of envelope electron-hole wave function overlap Γe
Therefore, radiative recombination rate and optical gain of III-Nitride active regions can be enhanced by engineering nanostructures with improved overlap.
A numerical model was developed to design a conventional (non-staggered) InGaN QW and an inventive staggered InGaN QW. The model was based on 6×6 k·p formalism for wurtzite semiconductor. Luminescence characteristics were studied by calculating spontaneous recombination rate spectra using energy dispersion relation and momentum matrix elements for both the conventional and staggered InGaN QW. Valence band states mixing, strain, and spontaneous and piezoelectric polarization-induced electric fields were taken into account in the calculations. Band parameters for III-Nitride alloys were obtained from references (S L Chuang and C S Chang, “A band-structure model of strained quantum-well wurtzite semiconductors”, Semicond. Sci. Technol., vol. 12, pp. 252-263, March 1997; S L Chuang, “Optical gain of strained wurtzite GaN quantum-well lasers”, IEEE J. Quant. Elect., vol. 32, pp. 1791-1800, October 1996; I. Vurgaftman and J. R. Meyer, “Band parameters for nitrogen-containing semiconductors,” J. Appl. Phys., vol. 94, pp. 3675-3696, September 2003; J. Piprek, Semiconductor Optoelectronic Devices Introduction to Physics and Simulation, London: Academic Press, 2003; Y. C. Yeo, T. C. Chong, M. F. Li, and W. J. Fan., “Analysis of optical gain and threshold current density of wurtzite InGaN/GaN/AlGaN quantum well lasers,” J. Appl. Phys. vol. 84, pp. 1813-1819, August 1998; J. Wu, W. Walukiewicz, W. Shan, K. M. Yu, J. W. Ager III, S. X. Li, E. E. Haller, H. Lu, and W. J. Schaff, “Temperature dependence of the fundamental band gap of InN”, J. Appl. Phys., vol. 94, pp. 4457-4460, October 2003; O. Ambacher, J. Majewski, C. Miskys, A. Link, M. Hermann, M. Eickhoff, M. Stutzmann, F. Bernardini, V. Fiorentini, V. Tilak, W. J. Schaff and L. F. Eastman, “Pyroelectric properties of Al(In)GaN/GaN hetero- and quantum well structures”, J. Phys.: Condens. Matter, vol. 14, pp. 3399-3434, March 2002). These parameters are listed in TABLE I.
GaN electron effective mass constants of 0.18 mo and 0.2 mo were used for c-axis and transverse direction, respectively. InN electron effective mass values of 0.11 mo were used for both the c-axis and transverse directions. Heavy hole effective masses were calculated following the treatment presented in Piprek, et al., supra. The ratio of conduction and valence band offsets ΔEc:ΔEv was taken as 70:30. Energy gap of the InGaN QW was calculated using a bowing parameter of 1.4 eV and an InN energy gap of 0.6405 eV. Indium concentration profile along the growth axis was incorporated into energy band lineup, with corresponding strain taken into account as band edge energy shifts. Polarization-induced electric field was manifested in the energy band bending. Spontaneous polarization Psp
P
sp
InGaN(x)=−0.042·x−0.034·(1−x)+0.037·x·(1−x),
Ppz
Spontaneous recombination rate per unit energy per unit volume rsp(ηω) was calculated assuming that the momentum-matrix element of spontaneous emission is the angular average of two TE-polarization components in the transverse plane and one TM-polarization component in the z-direction, defined as
with gspTE or TM defined as
Fermi Dirac distribution functions for electron fnc and hole fσmv were defined as
where Enc and Eσ,mv(kt) are the eigenenergies of electron and hole, respectively. Parameters Fc and Fv are carrier-density dependent quasi Fermi levels for electron and holes. These terms are related to injection carriers in the QW. Electron and hole concentrations in the QW were assumed to be equal. TE- and TM-polarized matrix elements are shown following:
for σ=L, where φn and gm are respectively conduction and valence band confined states. These confined states are the eigenvectors of a block-diagonalized six-by-six Hamiltonian matrix. The Upper and lower Hamiltonian blocks were indicated by σ=U and σ=L, respectively. Linewidth broadening with Lorentzian shape of γ=0.1 ps was used throughout the calculations. Details of the material parameters utilized in the calculation are listed in TABLE I. In these EXAMPLES, the conventional and staggered InGaN QWs were designed for emission in a particular wavelength regime with the staggered QWs optimized to give the largest wavefunction overlap (Γe
As matters of definition, an energy band lineup calculation is a method to compute energy band edges of conduction bands and valence bands of different types of semiconductor materials. Energy band lineup is used to determine transition wavelength and electron-hole wavefunction overlap Γe
The EXAMPLES are PL/CL studies of designed 420-430 nm emitting structures, PL/CL studies of 500-505 nm emitting structures and studies of LED 455-465 nm emitting structures. In the EXAMPLES, both conventional and staggered InGaN QWs were designed for emission at a particular wavelength regime. Conventional InGaN structures were based on QW layers with thicknesses (wc1) of 25-27 Å and In-contents (xc1) of 15% (for λpeak=420-430 nm), 21% (for λpeak=455-465 nm), 26% (for λpeak=500-505 nm). Then, the staggered InGaN QW structures were optimized at each wavelength regime with improved wavefunction overlap (Γe
Experiments were conducted to compare optical properties of staggered InGaN QWs and conventional InGaN QW, with emission wavelength ˜420-430 nm. Both conventional and staggered InGaN QWs samples were grown by metalorganic chemical vapor deposition (MOCVD) on 2.5-μm thick undoped GaN (Tg ˜1080° C.) grown on c-plane sapphire, employing a low temperature 30-nm GaN buffer layer (Tg ˜535° C.). The conventional QW structure consisted of 4-periods of 25-Å In0.15Ga0.85N QW, while the staggered QW structure was formed by 4-periods of 7.5-Å In0.25Ga0.75N/7.5-Å In0.15Ga0.85N layers. The QW structures included 11-nm GaN barrier layers.
Room temperature cathodoluminescence (RT-CL) measurements were performed utilizing a 10 keV electron beam with 1 1 μA of current. CL emission wavelengths for staggered QW and conventional QW were measured as 407-nm and 417-nm, respectively. CL emission of both the staggered and conventional QWs was blue-shifted by ˜10-15 nm in comparison to those of the photoluminescence wavelength. The staggered QW exhibited a much higher CL peak and integrated total intensity of ˜3.4 and 4.2 times that of the conventional QW, as shown in
Room temperature photoluminescence (RT-PL) measurements were also performed on both samples using a 325-nm He—Cd laser. PL emission wavelengths of the staggered QW and conventional QW were measured as ˜420-nm and ˜430-nm, respectively. Comparison of RT-PL intensity indicated improvement in the peak and integrated luminescence intensity for the staggered In0.25Ga0.75N/In0.15Ga0.85N QWs by factors of ˜4.2 times and 4.4 times, respectively, in comparison to those of the conventional In0.25Ga0.75N QW. Integrated luminescence improvement in the staggered InGaN QW was not accompanied by increased linewidth, rather it was attributed to higher peak intensity at the same excitation laser power. This observation was consistent with the fact that the measurements were conducted at the same temperature employing identical optical excitation power to facilitate direct spectrum comparison. The comparable linewidths further indicated similar materials quality of both the staggered and conventional InGaN QW. PL full width at half max (FWHM) for staggered InGaN QWs was observed to be ˜14.8-nm (106.25 meV), which is narrower than the conventional InGaN QW of 17.26-nm (113.85 meV).
To demonstrate luminescence enhancement at longer wavelength, experiments were conducted comparing a 27-Å conventional In0.26Ga0.74N QW and staggered 13-Å In0.28Ga0.72N/13-Å In0.21Ga0.79N QW, emitting in the bluish-green regime λ˜500-525-nm. As shown in
To assess the staggered QW in device applications, two LED structures were realized utilizing 1) 4 periods of staggered QWs of 12-Å In0.25Ga0.75N/12-Å In0.15Ga0.85N layers, and 2) 4-periods of 27-Å conventional In0.21Ga0.79N QWs as an active region of each LED. Both structures were grown on a 2.5-μm n-GaN template (n˜3×1018 cm−3) on c-plane sapphire substrates. The acceptor level for the p-GaN layer was ˜3×1017 cm−3. Continuous wave (CW) power measurements were performed at room temperature.
Calculated spontaneous emission rate spectra for corresponding conventional and staggered InGaN QW structures are shown in
In the conventional InGaN QW structure, excited state transitions become prominent as carrier density n was increased, as evident in the multiple
Carrier density dependence on recombination rate per unit volume for the QWs was calculated as follows. First, rsp(ηω) was integrated over all frequencies to result in a spontaneous recombination rate per unit volume (Lsp) at a particular carrier density no: Lsp(no)=∫rsp(ηω)dω at n=no.
Spontaneous emission rate per unit volume (Lsp) is related to the carrier density in the QW (n) with the following phenomenological approximation: Lsp(n)≅B·n·p=B·nβ, where B is the bimolecular recombination constant for radiative transition. At sufficiently high carrier injection, electron density is typically assumed as equal to that of hole (n ∞p), or β=2 is a theoretical, upper-limit value for pure radiative transition.
A typical non-polar GaAs or InGaAs QW active region with no built-in polarization fields, has a spontaneous recombination rate per unit volume (Lsp) wherein dependence on carrier density (n) is described as Lsp˜n2. The spontaneous recombination current density (Jsp) can be expressed as: Jsp=q·dQW·Lsp, where dQw is the QW thickness. Departure of β of less than 2 in non-polar QWs indicates the presence of monomolecular non-radiative recombination.
Conventional and staggered InGaN QW structures emitting at λpeak=500-510-nm were investigated. The calculated spontaneous emission rate spectrum at this wavelength regime is shown in
Nominal calculated values of spontaneous emission spectrum at this λpeak=500-515-nm regime were close to one order of magnitude lower than that of the 420-nm regime. This may be attributed to the high In-content required to achieve long emission wavelength. The high In-content leads to high polarization field and lower wavefunction overlap (Γe
For the case of 420-430 nm emitting InGaN QW, the optimized staggered InGaN QW design consisted of 7.5-Å In0.25Ga0.75N/7.5-Å In0.15Ga0.85N (with total QW thickness of 15-Å). Utilizing 15 Å QW thickness for the conventional QW structure would have improved radiative efficiency, however the use of 15-Å In0.15Ga0.85N (similar content with conventional QW) results in a much shorter wavelength λ ˜400-nm (not λ˜420-430 nm)—which would not provide comparison in the same wavelength regime.
For the case of 500-505-nm emitting InGaN QW, both conventional and staggered QW structures utilized substantially nominally identical QW thicknesses, which were 26 Å for the staggered and 27 Å for the conventional QWs (26% In-content). The staggered InGaN QW consisted of 13-Å In0.28Ga0.72N/13-Å In0.21Ga0.79N layers with a much higher wavefunction overlap Γe
In summary, polarization band engineering via staggered InGaN QW layers leads to significant enhancement of radiative recombination rate. Improvement in peak luminescence intensity and integrated luminescence by a factor ˜3.5-4 times was experimentally demonstrated for staggered InGaN QW active regions emitting in 420-430 nm and 490-500 nm, in comparison to those of conventional InGaN QW designs. Preliminary LED devices fabricated with staggered InGaN QW emitting in λ˜455-465 nm, showed an almost order magnitude improvement in device output power.
The staggered QW layers provide significantly improved radiative recombination rate and optical gain in comparison to non staggered InGaN QW. Experimental results of light emitting diode structures utilizing staggered InGaN QW showed good agreement with theory. The staggered InGaN QW allowed polarization engineering to improve photoluminescence intensity and LED output power as a result of enhanced radiative recombination rate.
While preferred embodiments of the invention have been described, the present invention is capable of variation and modification and therefore should not be limited to the precise details of the EXAMPLES. The invention includes changes and alterations that fall within the purview of the following claims.
This application claims the benefit of U.S. Provisional Application No. 60/871,822, filed 24 Dec. 2006, which is incorporated herein by reference.
This invention was made with government support under Contracts Nos. W911NF-07-2-0064 and 07014121 respectively awarded by the Department of Defense—Army Research Lab and by the National Science Foundation. The government has certain rights in the invention.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/US07/88778 | 12/24/2007 | WO | 00 | 6/22/2009 |
Number | Date | Country | |
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60871822 | Dec 2006 | US |