WO2002075876A2

WO2002075876A2 - New quantum dot laser structure

Info

Publication number: WO2002075876A2
Application number: PCT/IB2002/001842
Authority: WO
Inventors: Roberto Cingolani; Massimo De Vittorio; Adriana Passaseo; Sergio De Rinaldis
Original assignee: Agilent Technologies, Inc.
Priority date: 2001-03-17
Filing date: 2002-03-18
Publication date: 2002-09-26
Also published as: GB0106709D0; WO2002075876A3; GB2373371A

Abstract

The present invention uses a p-type Gallium Arsenide substrate or an underlying p-type grown layer with the consequent inversion of the doping sequence in order to realise a n-i-p structure. This results in the heterostructure built-in electric field being opposite to the QD internal dipole electric field. As a consequence the separation of the ground state electron and hole wavefunctions is strongly reduced, resulting in a high efficiency ground state optical emission in the region of 1.4 microns.

Description

New Quantum Dot Laser Structure

The present invention concerns a new Quantum dot structure. More specifically, the present invention concerns a new quantum dot structure used in a light emitting device, such as a laser diode.

InGaAs Quantum dots (QDs) are among the most promising active layer materials for future light emitting devices operating at room temperature in the 1.3 micron spectral region, which is the wavelength region of minimum optical dispersion in conventional optical fibers. Quantum dots are zero-dimensional systems where electrons and holes are confined in a few nanometer size region. The three- dimensional confinement strongly affects the physical properties of the material, leading to discrete energy levels in the density of states and to the enhancement of the material gain. These properties can be exploited to produce in perspective, high speed, low threshold and high critical temperature (T₀) laser diodes.

In spite of their potential for the fabrication of high performance lasers, the QDs growth is a critical issue. QDs structures which emit above 1300 nm at room temperature, show a shift to shorter wavelengths (blue shift) when embedded in the intrinsic region of a conventional p-i-n laser structure.

Optoelectronic devices containing InGaAs QD structures as active layers, for 1.3 micron operations have recently been fabricated. Room temperature lasers emitting in the 1.3 μm region were recently fabricated by Molecular Beam Epitaxy (MBE) by using long cavities or by covering binary InAs QDs with an InGaAs layer. However, long-wavelength lasing in QD laser structures growth by metalorganic chemical vapor deposition (MOCVD) has never been reported so far and very few works report emission wavelength near 1.3 μm in QDs fabricated by MOCVD.

Self organised QD structures, when grown in conditions ensuring high surface atom mobility, generally result in a characteristic truncated pyramid shape and in a non uniform In distribution inside the dots. These structures are surrounded by a uniform thickness bi-dimensional layer called wetting layer. As a consequence, the hole and electron wavefunctions are spatially separated by the action of the non uniform strain field, generating an internal permanent dipole directed from the base of the dots to their apex. When QDs, with features similar as those described, , are inserted in a conventional p-i-n structure (grown on n-type substrate) the junction built-in field is parallel to the dipole field, thus further reducing the overlap of the ground state electron and hole wavefunctions. This results in a strong reduction of the oscillator strength and in a quench of the ground state optical transition. Since the ground state is depleted, the overall effect is a predominant recombination from the QDs excited states with a shift to shorter wavelengths of the spectral gain.

A similar strong effect of the built-in dipole, that prevents emission from the ground state, is not exhibited in QDs structures grown by MBE. The direction and the strength of the permanent dipole momentum depend on the geometrical shape and composition profile in the dots and, consequently it changes depending on the growth technique. Self organized growth of InGaAs QDs, generally results in a strong faceting of the structures. The piezoelectric field associated to the presence of high index surfaces, enhances the dipole strength due to the dot shape and composition profile.

Thus it is an object of the present invention to reduce the blue shift associated with know quantum dot structures.

According to the present invention there is provided a quantum dot structure comprising InGaAs quantum dots grown on a p-type Gallium Arsenide substrate or on an underlying p-type layer.

In a further aspect of the present invention, there is provided an inversion of the doping sequence in order to realise a n-i-p structure. Here the heterostructure built-in electric field is opposite to the QD internal dipole electric field. As a consequence the separation of the ground state electron and hole wavefunctions is strongly reduced, resulting in a high efficiency ground state optical emission.

Advantageously, the structure according to present invention has an optical emission is in the region of 1.25 to 1.4 microns and the blue shift has been reduced.

The structure according to the present invention has been experimentally verified through the growth and deep characterisation of both p-i-n and n-i-p heterostructures. The latter always show a ground state emission in the electroluminescence spectra. Moreover the direction and intensity of the QDs dipole electric field have been confirmed by means of photocurrent spectra recorded under external bias. The type of substrate and the consequent structures grown (p-i-n or n-i-p configuration) has a direct influence only on the dipole formation in In ₅Ga₅As/m_xGaι__xAs Quantum Dots , while it has no influence on a lateral uniform layer as the wetting layer is. This is confirmed by the fact that the wetting layer absorption wavelength shows no dependence on the growth configuration.

The electric field influences the electroluminescence (EL) emission wavelength of Ino_.5Gao_.5As/GaAs QD laser structures emitting from 1.28 μm to near 1.4 μm. By comparing photoluminescence (PL), electroluminescence (EL) and photocurrent (PC) measurements it is demonstrated that the strong blue shift observed in QD laser structures grown on n-type substrates is related to the existence of a permanent electron-hole dipole moment, directed from the base of the dots to their apex.

Advantageously and according to an aspect of the present invention, by reversing the direction of the electric field, in n-i-p laser structures (on p-type substrate or on a p- type grown layer ), the effect of the permanent dipole is strongly reduced, allowing EL emission close to the designed wavelength of 1.3 μm, up to near 1.4 μm. The electric field associated to the dipole moment is estimated to be around 150 kV/cm.

While the principle advantages and features of the invention have been described above, a greater understanding and appreciation of the invention may be obtained by referring to the drawings and detailed description of a preferred embodiment, presented by way of example only, in which;

FIGURE 1 shows a schematic of a n-i-p quantum dots laser structure according to the present invention, FIGURE 2 shows a graph of room temperature PL emission of a QD structure with In_xGa₍ι._X)As barrier of x=0.1, compared with and EL and PC spectra of the correspondent laser structure grown on n+ doped substrates (p-i-n structure), FIGURE 3 shows a graph of room temperature PL emission of a QD structure with m_xGa₍ι__X)As barrier of x=0.1, compared with and EL and PC spectra of the correspondent laser structure grown on p+ doped substrates (n-i-p structure) according to the present invention,

FIGURE 4 shows a graph of room temperature emission wavelength of the ground state and the first excited state of the PL test structures as a function of the In_xGa₍i. _X)As barrier composition for x ranging from 0 to 0.15, grown on an n-type substrate, together with the EL emission wavelength of the corresponding p-i-n laser structure and,

FIGURE 5 shows a graph of room temperature emission wavelength of the ground state and the first excited state of the PL test structures as a function of the In_xGa₍ι_ _x)As barrier composition for x ranging from 0 to 0.15, grown on a p-type substrate, together with the EL emission wavelength of the corresponding n-i-p laser structure according to the present invention.

In figure 1 the laser structure has a single QD stack. However, the active structure can present multiple stacks. In contrast, a conventional p-i-n structures simply show inverted doping sequence. The structure is grown by low pressure (MOCVD) on (100) n+ doped and p+ doped GaAs substrates. The layer sequence of the p-i-n structures is as follows: a 200 nm thick GaAs buffer layer n+ doped, a 1 μm thick lightly doped (n ≡ 1x10 cm^" ) Alo_.4Gao_.6As lower cladding layer, a 180 nm thick GaAs waveguide embedding the laser active region and a 1 μm thick p doped (p ≡ lxl 0¹⁷ cm^"3) Alo_.4Gao_.6As upper cladding layer. The structures are terminated by a 100 nm thick GaAs p+-doped cap layer. The laser active region is grown at the center of the waveguide and consists of a single Ino.₅Gao.₅As dot layer inserted between two In_xGa₍i__X)As barriers (5 nm thick). The InAs mole fraction in the barrier was varied between x=0 and x=0.15. Details about the growth of the QD layer are generally known in the art and as such only briefly described below. The samples were grown at 700 °C, except for the InGaAs dots and for the In_xGa(i__X)As barrier, which was grown at 550 °C. A second set of samples consists of n-i-p structures, grown on p+ doped substrates, for which the growth sequences was reversed with respect the p-i-n structures. PL spectroscopy was performed on test samples terminated after the growth of the intrinsic region (without the upper cladding layer) grown on both, n+ and p+ doped substrates. The PL spectra were recorded at 10K and 300K, using an Ar⁺ laser tuned at 514.4 nm. The signal was spectrally analyzed by a 1 m monochromator, and detected by a liquid nitrogen cooled Ge detector. Electroluminescence (EL) spectroscopy was carried out by supplying an AC current to the as cleaved samples. The signal coming out from the cleaved edge was spectrally analyzed by a 1 m monochromator and detected by a standard lock-in technique. Photocurrent (PC) measurements were performed connecting the samples in series to a variable load resistor and a voltage source. The PC signal was measured across the load resistor using a lock-in amplifier.

Figures 2 and 3 show the room temperature PL emission of the QD samples with In_xGa₍i._X)As barrier of x=0.1, grown on n+ doped and p+ doped substrates respectively. The PL emission is compared with the EL emission of the corresponding p-i-n and n-i-p structure. The ground state PL emission of the test sample in figure 2, grown on the n+ substrate, is centered at 1347 nm, with a FWHM of 23 meV; the second structure in the spectra at 1273 nm is attributed to the first excited state El, as resulted by analyzing the PL spectra with increased excitation energy. When the actual laser structure was grown, the EL emission of the p-i-n structure is shifted to lower wavelength, up to 1242 nm on this sample, at energy position nearly correspondent to the first excited state. PC measurements performed at 0 V of applied bias on the same sample, also reported in figure 2, confirm the absence of the ground state related emission, showing the lower energy absorption peak in correspondence of the first excited state. On the contrary, the samples grown on p+ doped substrates present a different behavior as shown in figure 3, were a plot the RT- PL spectrum of the test QD sample grown on p+ doped substrate having the same structure of sample shows in figure 2, compared with the EL spectrum of the corresponding n-i-p laser structure and the PC spectrum, performed on the same sample at 0 V. By inverting the structure, and consequently reversing the direction of the junction built-in field, the EL emission turn out to be nearly coincident with the correspondent ground state PL emission, centered around 1355 nm. As shown, on this sample, PC spectrum shows both, the ground state and the excited state absorption peaks.

The same experiment was repeated by systematically varying the composition of the In_xGa₍i__X)As barrier from x=0 to x=0.15 in steps of 0.05. The different In content in the barrier allows for the wavelength emission of the QD structures to be tuned from 1.28 μm up to near 1.4 μm. Figure 4 plots the emission wavelength of the ground state and the first excited state of the PL test structures grown on n+ and p+ substrates as a function of barrier composition, together with the EL emission wavelength of the corresponding p-i-n and n-i-p structures. As shown in figure 4, in all the p-i-n samples the EL lower energy emissions results nearly correspondent with the energy of the first excited states, whereas EL emission from the ground state is observed in all the structure grown on p-type substrates (figure 5 ), confirming the asymmetric behavior with the junction polarity of our QD structures.

Recently, theoretical studies confirmed by experimental evidence, have demonstrated the existence of a permanent dipole in dot structures, due to the built-in strain fields, which spatially separate the hole and electron energy state. The built-in dipole momentum lead to an asymmetric quantum confined Stark shift (QCSS) that shifts the optical transition energy of QD structures when an electric field is applied.

The results of the structure according to the present invention can be explained with the existence of a permanent electron-hole dipole in the QD structures, directed from the base of the dots to their apex. Actually, in a p-i-n structures, the junction built-in field results parallel to the dipole field, thus increasing the electron-hole spatial separation and reducing the overlap of the electron and hole wavefunctions. Under this condition the EL emission is from the excited states because of the ground state population is completely depleted. On the contrary, the excited states wavefunctions are spatially spread in the dots, experiencing a reduced effect of the strain field. The situation is completely reversed in n-i-p structures. Here the electric field associated to the n-i-p junction is antiparallel to the built-in dipole and acts to reduce the spatial separation of electron and hole, increasing the overlap of the electron and hole wavefunctions and, as a consequence, the oscillator strength of the ground state transition. The use of p-type substrates (or the introduction of an underlying p-type layer ) and the control of the In incorporation in the QD layers and/or in the barrier embedding the QDs enable a reproducible achievement of In.₅Ga_.5As/In_xGaι._xAs Quantum Dot devices with ground state emission wavelength from 1.25 μm up to 1.4 μm at room temperature.

Preferably, MOCVD is used to grow the quantum dots, in which the characteristic truncated pyramidal shape and the non uniform In distribution generate the QD internal dipole. However, other epitaxial techniques may be used, such as MBE. Under the growth conditions used to obtain high surface ad-atoms diffusion, the characteristic truncated pyramidal shape and the non uniform Indium distribution can be developed, thus generating the QD internal dipole.

As will be appreciated by those skilled in the art, various modifications may be made to the embodiment hereinbefore described without departing from the scope of the present invention.

Claims

1. A quantum dot structure comprising InGaAs quantum dots grown on a p-type GaAs substrates.

2. A structure as claimed in Claim 1, wherein the structure obtains ground state emission at 1.3 microns at room temperature.

3. A structure as claimed in Claims 1-2, wherein the structure is doped thereby enabling In₅Ga_.5As/In_xGaι._xAs quantum dots ground state emission in the spectral range between 1.25 microns and 1.4 microns.

4. A structure as claimed in Claims 1-3, wherein the content of In present in the quantum dot layers and/or in the barrier varies from 0.05 to 0.15.

5. A structure as claimed in Claims 1-4, wherein the structure in grown by an epitaxial technique producing a quantum dot faceted shape.

6. A structure as claimed in Claim 5, wherein the quantum dot is pyramid shaped.

7. A structure as claimed in Claims 1-6, wherein the structure in grown by MOCVD.

8. A structure as claimed in Claims 1-7, wherein the structure in grown by MBE.

9. A laser device comprising a structure as claimed in any preceding claim.