GB2492771A - broadband optical device structure and method of fabrication thereof - Google Patents

broadband optical device structure and method of fabrication thereof Download PDF

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GB2492771A
GB2492771A GB1111826.2A GB201111826A GB2492771A GB 2492771 A GB2492771 A GB 2492771A GB 201111826 A GB201111826 A GB 201111826A GB 2492771 A GB2492771 A GB 2492771A
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active element
layer
quantum dots
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Richard Andrew Hogg
David Dylan Timothy Childs
Ziyang Zhang
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University of Sheffield
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L33/00—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/02—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies
    • H01L33/04—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies with a quantum effect structure or superlattice, e.g. tunnel junction
    • H01L33/06—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies with a quantum effect structure or superlattice, e.g. tunnel junction within the light emitting region, e.g. quantum confinement structure or tunnel barrier
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y20/00—Nanooptics, e.g. quantum optics or photonic crystals
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02518—Deposited layers
    • H01L21/02521—Materials
    • H01L21/02538—Group 13/15 materials
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
    • H01L29/86—Types of semiconductor device ; Multistep manufacturing processes therefor controllable only by variation of the electric current supplied, or only the electric potential applied, to one or more of the electrodes carrying the current to be rectified, amplified, oscillated or switched
    • H01L29/861—Diodes
    • H01L29/868—PIN diodes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/30—Structure or shape of the active region; Materials used for the active region
    • H01S5/34—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers

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Abstract

A broadband optical device structure 10 comprises a p-i-n semiconductor diode wherein an intrinsic region 16a, 16b of the diode comprises a plurality of quantum dots 23 which may form DWELL layers (dot-in-well) 22 and an active element 26. The active element may comprise a quantum well for example In x Ga 1-x As , a quantum wire or a bulk semiconductor material, and the active element is spectrally positioned to emit at a wavelength substantially coincident with the wavelength of emission from the second excited state of the plurality of quantum dots. The DWELL layers 22 (for example monolayers of InAs) may be separated from other DWELL layers or from the quantum well 26 by a spacer layer 24 of GaAs. The substrate may be <100> Si-doped GaAs and the diode may be produced by molecular beam epitaxy (MBE). There may also be a contact layer formed from p- AlGaAs.

Description

Broadband Optical Device Structure and Method of Fabrication Thereof (0001] This invention relates to a broadband optical device structure and a method of fabrication thereof, and in particul&, to a broadband optical device structure comprsing a plurality of quantum dots and an active element and a method of fabrication thereof.
BACKGROUND
[0002] Broadband light sources are vital components for many applications such as optical gyroscopes and sensors, optical coherence tomography, optical time domain reflectometers and wavelength division multiplexing systems [1-3].
(0003] Several techniques are known that achieve broad bandwidth, which results in a short coherence length LC proportional to A2! AA, where A is the central wavelength and z\A is the bandwidth of the emission spectra, as a short coherence length allows the devices to achieve improved sensitivity.
(0004] These known methods include using chirped quantum well (QW) structures [4] or postgrowth OW intermixing [5]. Though OWs can easily satisfy the requirement of output power, relatively narrow bandwidths of 100 nm still limit the application of the devices.
Recently, self-assembled quantum dot (QD) structures have revealed significant advantages and have attracted much attention for the realization of broadband sources utilizing their natural size and compostion fluctuations [6]. Most recently, a superluminescent light emitting diode (SLED) based on GD active materials with bandwidth -150 nm centered at -1.2 pm has been realized by using a multi-section device structure [7], and a 146 nm broadband QD-SLED centered at -1 jm has been demonstrated by using post-growth ritermixing process [8].
However, attaining broadband devices with spectral bandwidth greater than 200 nm is a significant challenge.
[0005] In a known arrangement, QW/QD structures have been used to realize the tunnel injection of electrons in an lnGaAs QD laser [9].
[0006] In another example of a prior art broadband light source, US Patent No 7,561,607 describes a quantum dot laser that operates on a quantum dot ground state optical transmission. The laser has a broadband spectrum of emission (claimed to be greater than 15 nm) and a high power output (claimed to be greater than 100 mW). In one described arrangement, a spectrally selective loss (e.g. a quantum well) is introduced into the laser resonator in order to suppress lasing on a quantum dot excited-state optical transition, thereby purportedly increasing the bandwidth of the emission spectrum.
[0007] There stifl exists the desire to provide a broadband device with improved bandwidth in comparison with the prior art. It is therefore an object of the present invention to produce a broadband optical device structure that may be used to produce a broadband spectrum of emission that overcomes at least some of the drawbacks associated with the prior art devices.
BRIEF SUMMARY OF THE DISCLOSURE
[0008] Embodiments of the present invention are defined in the appended claims.
[0009] In accordance with a first aspect of the present invention there is provided a broadband optical device structure comprising a p-i-n semiconductor diode wherein the intrinsic region oF the diode comprises a plurality of quantum dots and an active element; and wherein the active element comprises a quantum well, a quantum wire or a bulk semiconductor material, and the active element is spectrally positioned to emit at a wavelength substantiaUy coincident with the wavelength of emission from the second excited state of the plurality of quantum dots.
(0010] The structure of the present invention therefore provides a broad spectrum of emission
in comparison with prior art devices.
[0011] In a particularly preferably embodiment, the active element is spatially arranged relative to the plurality of quantum dots so that the sum of: thermal escape of electron and hole carriers from the active element: carrier transport to the plurality of quantum dots due to drift of carriers across the active element; and capture of carriers into high energy continuum states and relaxation to lower energy states, in the plurality of quantum dots; is substantially equal to the thermal emission of carriers from the plurality of quantum dots.
[0012] In this preferable embodiment, an optimum gain is achieved.
[0013] In one preferable embodiment, each of the plurality of quantum dots comprises a dot-in-well (DWELL) structure, wherein, preferably, the plurality of DWELL structures are arranged in layers stacked on top of one another. In a particularly preferable embodiment, the structure comprises six DWELL layers. In any case, each of the DWELL layers is preferably separated from adjacent DWELL layers by a spacer layer which preferably comprise layers of GaAs.
Each spacer layer preferably has a thickness of between 25 nm and 65 nni, and is preferably nm in thickness. In one preferable embodiment, each DWELL layer comprises a layer of nM disposed between a base layer of lnGaAs and a cap layer of lnGaAs. Each DWELL layer preferably comprises between 2.5 and 3.0 rnonolayers of InAs disposed between the base layer and the cap layer. In a particularly preferable embodiment, each DWELL layer comprises 2.7 monolayers of InAs disposed between the base layer and the cap layer. The base layer is preferably between 0.8 and 1.2 nm in thickness, and is further preferably 1 urn in thickness.
The cap layer is preferably between 5 nm and 7 nm in thickness, and is further preferably 6 nm in thickness.
[0014] In one preferable embodiment, the active element comprises a lnxGal-xAs quantum well, and is preferably a lnO,34Ga0.SGAs quantum well. The quantum well preferably comprises a layer between 6 nm and 8 nm in thickness, and is further preferably 7 nm in thickness.
[0015] The p-region of the diode preferably comprises a p-AIGaAs cladding layer, which is preferably between 1.5 pm and 2.0 pm in thickness, and is further preferably 1.7 pm.
[0016] The n-region of the diode preferably comprises a n-AlGaAs cladding layer, which is preferably between 2.0 pm and 2.5 pm in thickness, and is further preferably 2.3 pm.
[0017] The structure preferably further comprises a p-doped GaAs contact layer, which, in one preferable embodiment, is arranged in contact with the p-region of the diode. In an alternative preferable embodiment, the contact layer is arranged in contact with the n-region of the diode.
[0018] In any embodiment, the structure preferably comprises a GaAs substrate layer on which the remainder of the structure is disposed. The GaAs substrate layer is preferably a <100> Si-doped GaAs layer.
[0019] Preferably, the active element is spatially separated from the plurality of quantum dots, and is further preferably spaced from the plurality of quantum dots by a spacer layer. The spacer layer separating the active element from the plurality of quantum dots is preferably a GaAs spacer layer that is, in one preferable embodiment, between 35 nm and 65 nm in thickness, and is preferably 45 nm in thickness in a particularly preferable embodiment.
[0020] In one preferable embodiment, the structure further comprises at least one additional active element that is spectrally positioned to emit at a wavelength that is shorter than the wavelength of emission from the second excited state of the plurality of quantum dots. In this preferable embodiment, a broader emission spectrum can be achieved. In a particularly preferable embodiment, the additional active element is spatially arranged relative to the first active element and the plurality of quantum dots so that the thermal escape of electron and hole carriers from the additional active element is such that the gain of the additional active element is substantially equal to the gain of the first active element and is substantially equal to the saturated gain of the ground state of the plurality of quantum dots. Thus, an optimum gain can be achieved by the structure.
[0021] In accordance with a second aspect of the present invention, there is provided a broadband optical device including a broadband optical device structure according to the first aspect of the present invention.
(0022] In accordance with a third aspect of the present invention, there is provided a method of fabricating a broadband optical device structure comprising the steps of: providing a plurahty of quantum dots; providing an active element which comprises a quantum well, a quantum wire or a bulk semiconductor material and is spectrally positioned to emit at a wavelength substantially coincident with the wavelength of emission from the second excited state of the plurality of quantum dots; forming a p-i-n semiconductor diode where the diode contains the plurality of quantum dots and the active element in the intrinsic region of the diode, wherein the active element is spectrally positioned to emit at a wavelength substantially coincident with the wavelength of emission from the second excited state of the plurality of quantum dots.
[0023] Preferably, the active element is spatially arranged relative to the plurality of quantum dots so that the sum of: thermal escape of electron and hole carriers from the active element; carrier transport to the plurality of quantum dots due to drift of carriers across the active element; and capture of carriers into high energy continuum states and relaxation to lower energy states, in the plurality of quantum dots; is substantially equal to the thermal emission of carriers from the plurality of quantum dots.
[0024] Additionally or alternatively, the method further comprises the step of providing a substrate, and wherein the p-i-n semiconductor diode is formed on the substrate. Preferably, the substrate is a <100> Si-doped GaAs layer and the p-i-n semiconductor is preferably formed by molecular beam epitaxy (MBE).
[0025] Each of the plurality of quantum dots preferably comprises a dot-in-well (DWELL) structure, wherein each DWELL structure is preferably a layer and is formed by depositing monolayers of InAs on a base layer of lnGaAs and forming a cap layer of lnGaAs on the deposited InAs. Preferably, between 2.5 and 3.0 monolayers of InAs are deposited on the base layer and/or wherein the base layer is between 0.8 nm and 1.2 nm in thickness and/or wherein the cap layer is between 5 nm and 7 nm in thickness. In a particularly preferable embodiment, 2.7 monolayers of InAs are deposited on the base layer and/ar the base layers 1 tim in thickness and/or the cap layer is 6 nm in thickness. In any embodiment, the DWELL structures are preferably grown at a temperature between 470CC and 570CC and preferably at 520CC.
Each of the DWELL layers is preferably separated from adjacent DWELL layers by a spacer layer, which preferably comprises a layer of GaAs. Each spacer layer preferably has a thickness of between 25 nm and 65 nm, and is further preferably 45 tim in thickness.
[0026] Preferably, the active element comprises a lnxGal -xAs quantum well, and is further preferably a lno.34Ga0.6GAs quantum well. The quantum well preferably comprises a layer between 6 tim and 8 nm in thickness, and is further preferably 7 nm in thickness. In any embodiment, the quantum well is preferably grown at a temperature of between 450°C and 550°C, and preferably at 500°C.
[0027] The p-region of the diode preferably comprises a p-AlGaAs cladding layer that is preferably between 1.5 pm and 2.0 pm in thickness, and is further preferably 1.7 pm in thickness.
IS [0028] The n-region of the diode preferably comprises a n-AIGaAs cladding layer that is preferably between 2.0 pm and 2.5 pm in thickness, and is further preferably 2.3 pm.
[0029] In a preferable embodiment, the method further comprises the step of providing a p-doped GaAs contact layer.
[0030] Preferably, the active element is spatially separated from the plurality of quantum dots, preferably by a spacer layer. The spacer layer separating the active element from the plurality of quantum dots is preferably a GaAs spacer layer that is preferably between 35 tim and 65 tim in thickness, and is further preferably 45 nm in thickness.
[0031] In one preferable embodiment, the method further comprises the step of providing at least one additional active element that is spectrally positioned to emit at a wavelength that is shorter than the wavelength of emission from the second excited state of the plurality of quantum dots, wherein the diode contains the at least one additional active element in the intrinsic region of the diode. The additional active element is preferably spatially arranged relative to the first active element and the plurality of quantum dots so that the thermal escape of electron and hole carriers from the additional active element is such that the gain of the additional active element is substantially equal to the gain of the first active element and is substantially equal to the saturated gain of the ground state of the plurality of quantum dots.
BRIEF DESCRIPTION OF ThE DRAWINGS
[00321 Embodiments of the invention arc further described hereinafter with reference to
B
the accompanying drawings, in which: Figure I is a graph showing modal gain and loss for a known IC quantum dot layer laser; Figures 2A and 28 are schematics diagrams each showing a broadband optical device structure according to an embodiment of the present invention, where the structure of Figure 2a has a quantum well positioned between a plurality of quantum dots and the n-side of the diode (an n-type structure), and the structure of Figure 2b has a quantum well positioned between a plurality of quantum dots and the p-side of the diode (a p-type structure); Figure 3 is a schematic diagram of a known control optical device structure which has a plurality of quantum dots but no active element; Figures 4A and 48 show the emission spectra for the n-type and p-type structures of Figures 2A and 26, respectively, at various temperatures; Figures 5A and SB show the emission spectra at 75K and 295K, respectively, for the control structure of Figure 3; Figures BA and GB show overlapping emission spectra of an n-type, p-type and control structure at 75K and 295K, respectively; Figure 7 shows an exemplary emission spectrum of an n-type structure; Figures BA and SB show the relative potentials of an n-type structure and the flow of carriers at low and high temperatures, respectively; Figures 9A and 9B show the relative potentials of a p-type structure and the flow of carriers at low and high temperatures, respectively; Figure Ia is a graph showing the relationship between electroluminescence intensity and I/kT for an n-type and p-type structure; and Figure 11 shows the band structure of a quantum well of a structure according to an aspect of the present invention.
DETAILED DESCRIPTION
[0033] Figure 1 is a graph showing the modal gain and loss for a known ten quantum dot layer laser device. As shown, the maximum spectral bandwidth of the device occurs when the gain is balanced from the 1 St excited state (ES 1) and the ground state (OS) of the quantum dots. However, when this occurs, significant absorption is exhibited at a wavelength equal to the second excited state (ES2) of the quantum dots. This absorption (or loss) is due to the increasing degeneracy of the quantum dots. These losses limit the bandwidth of the device and are undesirable for broadband devices. The present invention, in at least one aspect, seeks to produce a broadband optical device structure that can produce a broadband spectrum
that improves over known prior art arrangements.
[0034] Figures 2A and 23 each schematically show a broadband optical device structure 10 (ba, lOb) according to aspects of the present invention. The epitaxy for each structure lois based on a typical p-i-n diode structure on a substrate layer 12. Each p-i-n diode structure includes an n-side (or n-region) 14, a p-side (or p-region) IS and an intrinsic region lGa,IGb. In S the embodiments shown in Figures 2A and 2B, the structures 10 each have a contact layer 20 in contact with the p-side 18.
[0035] Each of the structures I Oa and I Gb of Figures 2A and 2B differ from each other only in the arrangement of the intrinsic region 18a,16b. However, each intrinsic region 16a,16b comprises the same components, albeit orientated differently within the structure 10 with respect to one another. Each intrinsic regcon 16a,16b comprises a series of dot-in-well (DWELL) layers 22 which each comprise a plurality of quantum dots 23. The DWELL layers 22 are spaced from one another by spacer layers 24. Additionally, each intrinsic region 16a,16b includes a quantum well 26. In each of the embodiments shown in Figures 2A and 23, the quantum well 28 is spaced from the nearest DWELL layer 22 by a spacer layer 24, however this may not be the case in all embodiments. In a preferable embodiment, the quantum well 26 is grown at a temperature of between 450°C and 550°C (preferably 500°C).
[0036] In the structure 1 Ga shown ri Figure 2A, the quantum well 26 is positioned in the intrinsic region 1 6a such that it is disposed nearer to the n-side 14 than the p-side 18. In particular, in the intrinsic region 16a, the quantum well 26 is disposed between the plurality of quantum dots 23 (in the DWELL layers 22) and the n-side 14. For this reason, this structure 1 Oa is an "n-side structure", but will be referred to hereinafter as an n-type structure 1 Oa to avoid confusion with the n-side 14 of the p-i-n diode.
[0037] In contrast, in the structure lob shown in Figure 23, the quantum well 26 is positioned in the intrinsic region 1Gb such that it is disposed nearer to the p-side 18 than the n-side 14. In particular, in the intrinsic region 16b, the quantum well 26 is disposed between the plurality of quantum dots 23 (in the DWELL layers 22) and the p-side 18. For this reason, this structure 1 Ob is a "p-side structure". but will be referred to hereinafter as a p-type structure 1Db to avoid confusion with the p-side 18 of the pSi-n diode.
(0038] In a preferred embodiment, the substrate 12 is <100> Si-doped GaAs and/or the diode is preferably produced by molecular beam epitaxy (MBE). In one embodiment, the quantum well 26 is formed from lnGaAs and in particular may be formed from lnGa1As, or, specifically, ln034Ga066As. The quantum well 26 preferably comprises a layer having a thickness TI of between 6 rim and 8 nm and, in one preferred embodiment, 11 = 7 nm.
[0039] The contact layer 20 is preferably a highly p-doped GaAs contact layer.
(0040] The p-side 18 is preferably a p-AIGaAs cladding layer that is preferably between 1.5
B
and 2.0 pm (preferably 1.7 pm) in thickness. The n-side 14 is preferably an n-AIGaAs cladding layer that is preferably between 2.0 and 2.5 pm (preferably 2.3 pm) in thickness.
[0041] In one preferred embodiment, the DWELL layers 22 are formed by depositing monolayers of InAs onto a base layer of lnGaAs, and capping the InAs with a cap layer of InGaAs. In preferred embodiments, between 2.5 and 3.0 rnonolayers of InAs are used (2.7, for example), and/or the base layer is between 0.8 and 1.2 nm in thickness (e.g. 1 nm), and/or the cap layer is between S and 7 nm (e.g. 6 nm). In a preferable embodiment, the DWELL layers 22 are grown at a temperature of between 410°C and 570°C (preferably 520°C).
[0042] The spacer layers 24 are preferably made of GaAs and/or preferably have a thickness 12 of between 25 and 65 nm (preferably 45 nm). Other alternative materials may be used for the spacer layers, however (e.g. AIGaAs).
(0043] Although the embodiments shown 10a,lOb in Figures 2A and 2B comprise a quantum well 26, alternative embodiments within the scope of the present invention may comprise other active elements" such as a quantum wire or a bulk semiconductor in place of the quantum well IS 26. In any case, the active element should be chosen so it has a higher saturated modal gain than the quantum dots and so it is spectrally positioned to emit at a wavelength that is substantially coincident with the wavelength of emission from the second excited state of the quantum dots 23. In this hybrid arrangement (of active element and quantum dots 23), the emission from the active element offsets the losses exhibited by the quantum dots 23 at wavelengths around the second excited state of the quantum dots 23 (as shown in Figure I).
[0044] In order to verify the benefits of the present invention, and demonstrate the effect of the active element, a control optical device structure 100 was produced and tested alongside the n-type ba and p-type lOb structures. The control structure 100 is shown schematically in Figure 3. The control structure 100 is identical to the n-type bOa and p-type lOb structures but for its intrinsic region 160. The intrinsic region ISO of the control structure 100 does not contain an active element, but only has a series of quantum dots in DWELL layers 22 stacked with spacer layers 24 therebetween.
[0045] Figures 4A and 4B show the measured electroluminescence intensity for a given injection current for the n-type 1 Oa and p-type 1 Ob structures, respectively, at various temperatures ranging from 75K to 295K. The spontaneous emission (SE) was collected from the surface of the structures lOa,bOb perpendicular to the active regions 16,16b.
[0046] As seen from Figure 4A, the n-type 10 structure produces a strong emission at around 1075 nm at low temperatures (around 75K) which shifts to longer wavelengths (approximately 1100 nm) as the temperature increases. At higher temperatures (around 295K), the emission at shorter wavelengths is suppressed and the peak emission occurs at longer wavelengths (approximately 1300 nm at 295K), although a significant (relative) peak still exists at around 1125 nm.
[0047] A similar effect is exhibited by the p-type lOb structure as shown in Figure 4B. At 75K the primary peak emission occurs at around 1075 nm with some strong emission at a secondary peak around 1225 nm (which has a lower intensity than the primary peak emissioni.
As temperature increases, both primary and secondary peak emissions shift to longer wavelengths with the primary peak reduces in magnitude relative to the secondary peak. At 295K the secondary peak is greater than the primary peak and occurs at around 1290 nm.
However, some emission at shorter wavelengths (around 1100) is still exhibited.
[0048] In contrast, the emssion spectra for the control structure 100 of Figure 3 is shown in Figures SA and SB for temperatures 75K and 295K, respectively, for various injection currents.
In both emission spectra, and especially at 75K, very little emission is observed at shorter wavelengths (around 1075 nm). Indeed, both emission spectra are dominated by a large ground state emission peak and a slightly smaller peak corresponding to the first excited state of the quantum dots.
[0049] Figures 6A and SB show overlays of emission spectra from the n-type 1 oa, p-type 1 Ob and control 100 structures at a temperature of 75K and 295K, respectively. At 75K it is clear that the peak emission of the n-type 1 Ga and p-type I Ob structures do not correspond with any spectral feature of the control 100 structure. This demonstrates that the peak emissions of the n-type 1 iDa and p-type I Gb structures is not due to the second excited state (ES2) of the quantum dots 23, and is instead due to the presence of the active element (i.e. the quantum well 26, in the case of structures 1 Ga and lob). At 295K, the n-type 1 0a structure exhibits an emission peak at around 1110 nm which is not seen in the spectra of the p-type 1 Gb or control structures 100. Due to the absence of this spectral feature in the spectrum of the control 100 structure, the emission around 1110 nm in the n-type I Oa structure is attributed to the active element (quantum well 26). Since this emission is not evident in the spectrum of the p-type lOb structure, it appears that the position of the quantum well 26 (or equivalent active element) relative to the p-side and n-side is influential in determining the overall emission spectrum of the hybrid structure 10.
[0050] In preferable embodiments, it is desirable to improve the gain of the emission due to the active element. For example. Figure 7 shows an emission spectrum for an n-typo 1 Ga structure at room temperature (approximately 295K) where the emission (OW) from the quantum well 26 has be balanced so that it is similar in magnitude to the ground state emission (GS) of the quantum dots 23 and larger in magnitude than the emission (ES1) from the first excited state of the quantum dots 23. The emission from the n-type ba structure has a full-width-half-maximum (FWHM) of 250 nm centered at -1200 nm. Self heating under high injection continuous wave (CW) operation can account for the observed reduced peak intensity of GS and the red shift of all three peaks.
[0051] Figures 8 and 9 demonstrate the effect of temperature on the flow of charge carriers for n-type lOs and p-type structures, respectively. Figure BA shows the relative potentials for an n-type Wa structure at low temperature (e.g. 75K). At low temperature, electrons e tiow from the n-side 14 into the quantum well 26 and holes hflow from the p-side 18 into the quantum well 26. Due to the low temperature, the quantum dots 23 are unable to capture the holes Ii as they flow from the p-side 18 to the quantum well 26. Substantially all of the charge carriers e,h therefore flow into the quantum well 26 and, for this reason, emission from the quantum well 26 is strong. The same structure is shown in Figure 88, but at a higher temperature (e.g. 295K). At this higher temperature, electrons e are thermally liberated from the quantum well 26 and are able to flow to the quantum dots 23 where they are captured.
Additionally. the holes h are captured by the quantum dots 23 as they flow from the p-side 18 towards the quantum well 26. For this reason, emission from the quantum well 26 is generally lower at ftgher temperatures.
[0052] Similarly, for a p-type 1 Ob structure at low temperature (e.g. 75K), as shown in Figure 9A, electrons e flow freely from the n-side 14 into the quantum well 26 and holes h flow freely from the p-side 18 into the quantum well 26. However, at higher temperatures (e.g. 295K) as shown in Figure 98, electrons e are captured by the quantum dots 23 as they flow towards the quantum well 28 from the n-side 14, and holes h are thermally liberated from the quantum well 26 and flow into the quantum dots 23. Again, this phenomenon may account for reduced emission from the quantum well 26 at higher temperatures.
[0053] In a further aspect of the present invention, the active element is spatially positioned relative to the quantum dots so that the sum of: (a) thermal escape of electron and hole carriers from the active element; (b) carrier transport to the plurality of quantum dots due to drift of carriers across the active element; and (c) capture of carriers into high energy continuum states and relaxation to lower energy states, in the plurality of quantum dots; is substantially equal to (d) the emission of carrier from the plurality of quantum dots.
[0054] When balanced in this way, the gain from the quantum well 26 can approach that from the ground state of the quantum dots thereby producing a high gain, broadband spectrum, like the spectrum shown in Figure 7.
[0055] In other embodiments, the sum (a)÷(b)+{c) may be equal to (d)12 or 2(d) or any value in between to achieve high gain. However, maximum gain wiU be achieved when (a)i-(b)i-(c)=(d). The relationship between (a), (b), (a) and (d) is determined by the spatial arrangement between the quantum dots 23 and the active element, [0056] By plotting the measured (normalized) electroluminescence intensity of the n-type 1 Ga and p-type 1 Gb structures against 1 /KT (where k is the Boltzmann constant and T is the temperature) (as shown in Figure 10), an activation energy Eacton can be obtained from the gradient.
[0057] The measured activation energies (obtained from the gradient of Figure IC) for then-type 1 Ga and p-type 1 Oh structures are 191 meV and 135 meV, respectively. These values are in very good agreement with the electron and hole thermal activation energies determined from the band structure calculated using an effective mass approximation for this quantum well structure. Figure 11 shows the band structure of the quantum well 26 and surrounding spacer layer 24, as calculated using an effective mass approximation. As shown in Figure ii, the calculated activation energies are 190 may and 130 may, for electrons and holes, respectively, which is in very good agreement with the measured values of Figure 10.
[0058] In an alternative embodiment, the structure 10 includes at least one additional active element (a quantum well, a quantum wire, or bulk semiconductor material) that is spectrally positioned to emit at a wavelength that is shorter than the wavelength of emission from the second excited state (ES2) of the plurality of quantum dots 23. In this embodiment, the at least one additional element would be disposed within the intrinsic region 16 of the structure 10 and is preferably spatially arranged relative to the first active element (e.g. the first quantum well 26) and the plurality of quantum dots 23 so that the thermal escape of electron and hole carriers from the at least one additional active element is such that the gain of the additional active element is substantially equal to the gain of the first active element and is substantially equal to the saturated gain of the ground state (OS) of the plurality of quantum dots 23. Whilst preferably equal, the gains of the at least one additional active element, the first active element and the ground state of the plurality of quantum dots 23 may be within a factor of 2 of one another (i.e. may range from 1⁄2 the magnitude of one another to twice the magnitude of one another), to still provide a useful broadband optical structure. The addition of one or more additional active elements provides even broader emission.
[0059] In summary, the present invention relates to a broadband optical device structure comprising a p-i-n semiconductor diode wherein the intrinsic region of the diode comprises a plurality of quantum dots and an active element. The active element comprises a quantum well, a quantum wire or a bulk semiconductor material, and the active element is spectrally positioned to emit at a wavelength substantially coincident with the wavelength of emission from the second excited state of the plurality of quantum dots.
[0060] Throughout the description and claims of this specification, the words "comprise" and contain" and variations of them mean including but not limited to, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the S context otherwise requires. In particular, where the indefinite article is used. the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.
[0061] Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and!or steps are mutually exclusive, The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.
E0062] The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.
REFERENCES
[1] Bums, W.K., Chen, C., and Moeller, FtP.: Fiber-optic gyroscopes with broad-band sources', 3. Lightw. Technol., 1983, LT -1, p. 93-105 [2] Shidlovski, yR., and Wei, 3.: Superluminescent diodes for optical coherence tomography', Proc. SPIE., 2002, 4648, p. 139-147 [3] Sampson, D.D., and Holloway, W.T.: 100mW spectrally-uniform broad-band ASE for spectrum-sliced WDM systems', Electron. Lett., 1994, 30, p. 1811-1612 [4] Lin, C. and Lee, B.: extremely broadband AlGaAs /GaAs superluminescent diodes', Appl.
Phys. Lett., 71, p. 1593-1 600 [5] Ong, T.K., Yin, M., Yu, 7., Chan, Y.C., and Lam, Y.L.: High-performance quantum well intermixed superluminescent diodes', Meas. Sci. Technol., 2004, 15, p. 1591-1595 [6] ihang, Z.Y.. Hogg, PA., Lv, X.Q., and Wang, Z.G.: Self-assembled quantum dot superluminesoent Fight emitting dodes', Adv. Opt. Photonics., 2010, 2, p. 201 [7]Xin, 1.-C., Martinez, A., Saiz, T., Moseho, A.J., U, V., Nilsen, TA., Gray, AL., and Lester, L.F.: I.3-urn Quantum-Dot multisection superluminescerit diodes with extremely broad bandwidth', IEEE Photon, Technol. Lett., 2007, 19, p. 501-503 [8] Zhang, 7.'!., l-fogg, PA., Xu, B., Jin, P., and Wang. 7G.: Realization of extremely broadband quantum-dot superluminescent lght emitting diodes by rapid thermal-annealing process', Opt. Lett., 2008, 33, p. 121 0-1212 [9] Bhattachaya, P., and Ghosh, S.: Tunnel injection lnO.4Ga 0.6AsIGaAs quantum dot lasers with 15 GHz modulation bandwidth at room temperature', AppI. Phys. Lett., 2002, 80, p.3482-

Claims (3)

  1. <claim-text>CLAIMS1. A broadband optical device structure comprising a p-i-n semiconductor diode wherein the intrinsic region of the diode comprises a plurality of quantum dots and an S active element; and wherein the active element comprises a quantum well, a quantum wire or a bulk semiconductor material, and the active element is spectrally positioned to emit at a wavelength substantially coincident with the wavelength of emission from the second excited state of the plurality of quantum dots.</claim-text> <claim-text>2. A structure as claimed in claim 1, wherein the active element is spatially arranged relative to the plurality of quantum dots so that the sum of: thermal escape of electron and hole carriers from the active element; carrier transport to the plurality of quantum dots due to drift of carriers across the active element; and capture of carriers into high energy continuum states and relaxation to lower energy states, in the plurality of quantum dots; is substantially equal to the thermal emission of carriers from the plurality of quantum dots.</claim-text> <claim-text>3. A structure as claimed in claim 1 or 2, wherein each of the plurality of quantum dots comprises a dot-in-well (DWELL) structure.</claim-text> <claim-text>4. A structure as claimed in claim 3, wherein the plurality of DWELL structures are arranged in layers stacked on top of one another.</claim-text> <claim-text>5. A structure as claimed in claim 4, comprising six DWELL layers.</claim-text> <claim-text>6. A structure as claimed in claim 4 or 5, wherein each of the DWELL layers is separated from adjacent DWELL layers by a spacer layer.</claim-text> <claim-text>7. A structure as claimed in claim 6, wherein the spacer layers comprise layers of GaAs.</claim-text> <claim-text>8. A structure as claimed in claim 6 or?, wherein each spacer layer has a thickness of between 25 nm and 65 nm, and is preferably 45 nm in thickness.</claim-text> <claim-text>9. A structure as claimed in any of claims 4 to 8, wherein each DWELL layer comprises a layer of InAs disposed between a base layer of lnGaAs and a cap layer of lnGaAs.</claim-text> <claim-text>10. A structure as claimed in claim 9, wherein each DWELL layer comprises between
  2. 2.5 and
  3. 3.0 monolayers of InAs disposed between the base layer and the cap layer.</claim-text> <claim-text>11. A structure as claimed claim 10, wherein each DWELL layer comprises 2.7 monolayers of lnAs disposed between the base layer and the cap layer.</claim-text> <claim-text>12. A structure as claimed in any of claims B to 11, wherein the base layer is between 0.8 and 1.2 am in thickness, and is preferably 1 nm in thickness.</claim-text> <claim-text>13. A structure as claimed in any of claims 9 to 12, wherein the cap layer is between 5 rim and 7 rim in thickness, and is preferably 6 rim in thickness.</claim-text> <claim-text>14. A structure as claimed in any preceding claim, wherein the active element comprises a lnXGa(As quantum well.</claim-text> <claim-text>15. A structure as claimed in claim 14, wherein the quantum well is a ln0.Ga0.66As quantum well.</claim-text> <claim-text>16. A structure as claimed in claim 14 or 15, wherein the quantum well comprises a layer between 6 nm and 8 rim in thickness, and is preferably? nm in thickness.</claim-text> <claim-text>17. A structure as claimed in any preceding claim, wherein the p-region of the diode comprises a p-AlGaAs cladding layer.</claim-text> <claim-text>18. A structure as claimed in claim 17, wherein the p-AIGaAs cladding layer is between 1.5 pm and 2.0 pm in thickness, and is preferably 1.7 pm in thickness.</claim-text> <claim-text>19. A structure as claimed in any preceding claim, wherein the n-region of the diode comprises a n-AIGaAs cladding layer.</claim-text> <claim-text>20. A structure as claimed in claim 19, wherein the n-AIGaAs cladding layer is between 2.0 pm and 2.5 pm in thickness, and is preferably 2.3 pm.</claim-text> <claim-text>21. A structure as claimed in any preceding claim, further comprising a p-doped GaAs contact layer.</claim-text> <claim-text>22. A structure as claimed ri claim 21, wherein the contact layer is arranged in contact with the p-region of the diode.</claim-text> <claim-text>23. A structure as claimed in claim 21, wherein the contact layer is arranged in contact with the n-region of the diode.</claim-text> <claim-text>24. A structure as claimed in any preceding claim, comprising a GaAs substrate layer on which the remainder of the structure is disposed.</claim-text> <claim-text>25. A structure as claimed in claim 24, wherein the GaAs substrate layer is a <IOU> Si-doped GaAs layer.</claim-text> <claim-text>26. A structure as claimed in any preceding claim, wherein the active element is spatially separated from the pluralfty of quantum dots.</claim-text> <claim-text>27. A structure as claimed in claim 26, wherein the active element is spaced from the plurality of quantum dots by a spacer layer.</claim-text> <claim-text>28. A structure as claimed in claim 27 wherein the spacer layer separating the active element from the plurality of quantum dots is a GaAs spacer layer.</claim-text> <claim-text>29. A structure as claimed in claim 27 or 28 wherein the spacer layer separating the active element from the plurality of quantum dots is between 35 nm and 65 nm in thickness, and is preferably 45 nm in thickness.</claim-text> <claim-text>30. A structure as claimed in any preceding claim, further comprising at least one additional active element that is spectrally positioned to emit at a wavelength that is shorter than the wavelength of emission from the second excited state of the plurality of quantum dots.</claim-text> <claim-text>31. A structure as claimed in claim 30, wherein the additional active element is spatially arranged relative to the first active element and the plurality of quantum dots so that the thermal escape of electron and hole carriers from the additional active element is such that the gain of the additional active element is substantially equal to the gain of the first active element and is substantially equal to the saturated gain of the ground state of the plurality of quantum dots.</claim-text> <claim-text>32. A broadband optical device structure substantially as hereinbefore described with reference to the accompanying figures.</claim-text> <claim-text>33. A broadband optical device including a broadband optical device structure as claimed in any preceding claim.</claim-text> <claim-text>34. A method of fabricating a broadband optical device structure comprising the steps of: providing a plurality of quantum dots; providing an active element which comprises a quantum well, a quantum wire or a bulk semiconductor material and is spectrally positioned to emit at a wavelength substantially coincident with the wavelength of emission from the second excited state of the plurality of quantum dots; forming a p-i-n semiconductor diode where the diode contains the plurality of quantum dots and the active element in the intrinsic region of the diode, wherein the active element is spectrally positioned to emit at a wavelength substantially coincident with the wavelength of emission from the second exdted state of the plurality of quantum dots 35. A method according to claim 34, wherein the active element is spatially arranged relative to the plurality of quantum dots so that the sum of: thermal escape of electron and hole carriers from the active element; carrier transport to the plurality of quantum dots due to drift of carriers across the active element; and capture of carriers into high energy continuum states and relaxation to lower energy states, in the plurality of quantum dots; is substantially equal to the thermal emission ot carriers from the plurality of quantum dots.38. A method according to claim 34 or 35, further comprising the step of providing a substrate, and wherein the p-i-n semiconductor diode is formed on the substrate.37. A method according to claim 36, wherein the substrate is a <100> Si-doped GaAs layer.38. A method according to claim 37, wherein the p-i-n semiconductor is formed by molecular beam epitaxy (MBE).39. A method according to any of claims 34 to 35, wherein each of the plurality of quantum dots comprises a dot-in-well (DWELL) structure.40. A method according to claim 39, wherein each DWELL structure is a layer and is formed by depositing monolayers of InAs on a base layer of lnGaAs and forming a cap layer of lnGaAs on the deposited InAs.41. A method according to claim 40, wherein between 2.5 and 3.0 monolayers of InAs are deposited on the base layer and/or wherein the base layer is between 0.8 nm and 1.2 nm in thickness and/or wherein the cap layer is between 5 nm and 7 nm in thickness.42. A method according to claim 41, wherein 2.7 monolayers of InAs are deposited on the base layer and/or the base layer is 1 nm in thickness and/or the cap layer is 6 nm in thickness.43. A method according to any of claims 39 to 42, wherein the DWELL structures are grown at a temperature between 470°C and 570°C and preferably at 520°C.44. A method according to any of claims 34 to 43, wherein each of the DWELL layers is separated from adjacent DWELL layers by a spacer layer, which preferably comprises a layer of GaAs.45. A method according to claim 44, wherein each spacer layer has a thickness of between 25 nm and 65 mu, and is preferably 45 rim in thickness.46. A method according to any of claims 34 to 45, wherein the active element comprises a lnGaAs quantum well.47. A method according to claim 46, wherein the quantum well is a ln0.34Ga0.As quantum well.48. A method according to claim 46 or 47, wherein the quantum well comprises a layer between 6 nm and 8 nm in thickness, and is preferably 7 nm in thickness.49. A method according to any of claims 46 to 48, wherein the quantum well is grown at a temperature of between 450°C and 550°C, and preferably at 500°C.50. A method according to any of claims 34 to 49, wherein the p-region of the diode comprises a p-AlGaAs cladding layer.51. A method according to claim 50, wherein the p-AlGaAs cladding layer is between 1.5 pm and 2.0 pm ri thickness, and is preferably 1.7 pm.52. A method according to any of claims 34 to 51, wherein the n-region of the diode comprises a n-AlGaAs cladding layer.53. A method according to claim 52, wherein the n-AlGaAs cladding layer is between 2.0 pm and 2.5 pm in thickness, and is preferably 2.3 pm.54. A method according to any of claims 34 to 53, further comprising providing a p-doped GaAs contact layer.55. A method according to any of claims 34 to 54, wherein the active element is spatially separated from the plurality of quantum dots.56. A method according to claim 55, wherein the active element is spaced from the plurality of quantum dots by a spacer layer.57. A method according to claim 56 wherein the spacer layer separating the active element from the plurality of quantum dots is a GaAs spacer layer.58. A method according to claim 56 or 5/wherein the spacer layer separating the active element from the plurality of quantum dots is between 35 nm and 65 rim in thickness, and is preferably 45 nm in thickness.59. A method according to any of claims 34 to 58, further comprising the step of providing at least one additional active element that is spectrally positioned to emit at a wavelength that is shorter than the wavelength of emission from the second excited state of the plurality of quantum dots, wherein the diode contains the at least one additional active element in the intrinsic region of the diode.60. A method according to claim 59, wherein the additional active element is spatially arranged relative to the first active element and the plurality of quantum dots so that the thermal escape of electron and hole carriers from the additional active element is such that the gain of the additional active element is substantially equal to the gain of the first active element and is substantially equal to the saturated gan of the ground state of the pluralty of quantum dots.61. A method of fabricating a broadband optical device structure substant ally as hereinbefore described with reference to the accompanying figures.</claim-text>
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