EP1073933A1

EP1073933A1 - Optically non-linear semiconductor material and a method for the production thereof

Info

Publication number: EP1073933A1
Application number: EP99913054A
Authority: EP
Inventors: Ursula Keller; Uwe Siegner; Markus Haiml
Original assignee: GigaTera AG; Time Bandwidth Products AG
Current assignee: GigaTera AG
Priority date: 1998-04-30
Filing date: 1999-04-20
Publication date: 2001-02-07
Also published as: US6551850B1; WO1999057603A1

Abstract

Essentially non-linear optical material characteristics of a semiconductor material which is epitaxially grown at low temperatures can be substantially improved by measures comprising the addition of foreign atoms and/or an additional curing. Epitaxially grown GaAs is dosed, for example at 300 DEG C, with Be in a concentration of 3 x 10<19> cm<-3>. The response time is reduced from 480 fs (curve 1.1) to 110 fs (curve 3.1) such that the absorption modulation is not reduced thereby or such that the non-saturable absorption losses do not increase. Semiconductor materials which have been subjected to at least one of the aforementioned measures during production exhibit impressionable, especially short response times and, at the same time, exhibit high absorption modulations and low non-saturable absorption losses. As a result, the semiconductor materials are excellently suited for non-linear optical applications such as optical information processing, optical communication or ultra-short pulse laser physics.

Description

- 1 -

OPTICALLY NON-LINEAR SEMICONDUCTOR MATERIAL AND METHOD

TO ITS MANUFACTURE

The invention relates to an optically nonlinear semiconductor material, a method for its production and its use.

Optically nonlinear materials are used in many applications. In optical information processing they are used e.g. B. used to switch light by means of light. In optical communication, they can be used to clean signals from disturbing noise, which arises, for example, from amplified spontaneous emission (ASE). Another area of application is laser physics, where such materials are used as saturable absorbers for passive mode coupling in laser resonators for the purpose of generating ultra-short laser pulses (in the femto- or picose customer range). The passive mode coupling can be achieved, for example, by using a mirror with saturable absorbers made of semiconductor materials (semiconductor saturable absorber miιτor, SESAM) as a resonator mirror (cf. U. Keller et al., "Semiconductor Saturable Absorber Mirrors (SESAM's) for Femtosecond to Nanosecond Pulse Generation in Solid-State Lasers ", IEEE Journal of Selected Topics in Quantum Electronics, Volume 2, No. 3, September 1996). The reflectivity of such SESAM level is higher at high intensity levels due to the fading of the absorption. A SESAM mirror typically consists of a reflective substrate, a saturable semiconductor absorber structure and, optionally, an additional reflection or anti-reflection layer.

In applications such as the above, the material properties a) response time, b) absorption modulation and c) unsaturated absorption losses play an important role (and others) and can therefore be described as key parameters. The following requirements are placed on these key parameters of optically nonlinear materials: a) the response time should be adaptable to the respective application (for example in the pico or femtosecond range); b) the absorption modulation should be high; c) the unsaturated absorption losses should be low.

So far, no material is known that ideally fulfills all these requirements at the same time. Rather, in practice it is about finding a material that meets the requirements as well as possible. A measure to improve a key parameter often results in a deterioration of another key parameter. If necessary, a compromise that is not entirely satisfactory, but acceptable, between partially opposite material properties has to be made for a certain application.

In this document, the term “response time” means the time during which the initially rapid change in the optical material - 3 -

all properties are mainly determined by trapping. In addition, the optical material properties are influenced by other, mostly slower mechanisms.

Gallium arsenide (GaAs) may be mentioned here as an example of a known optically nonlinear material. The preferred method for producing GaAs is molecular beam epitasy (MBE). Usually GaAs is grown at temperatures between 500 and 800 ° C. This normal growth provides almost ideal stoichiometric crystals with high absorption modulations, low unsaturated absorption losses, but long response times (in the range of 100 ps).

To eliminate the disadvantage of long response times, the GaAs can also be grown at lower temperatures of approx. 180 to 500 ° C. This low-temperature process creates non-stoichiometric crystals with a high crystal defect density. The crystal defect density and thus also the low-temperature method can be determined or ascertained with the aid of near-infrared absorption (NTJ A) or magnetic circular dichroism of absorption (MCDA) (see, for example, BX Liu et al., "Mechanism responsible for the semi-msulating properties of low-temperature-grown GaAs ", Appl. Phys. Lett. 65 (23), December 5, 1994, pp. 3002 ff). In fact, the low-temperature method can be used to set short response times (in the range from subpicoseconds to several 10 ps); however, these advantages must be bought through low absorption modulations and high unsaturated absorption losses. It is an object of the invention to provide an optically nonlinear semiconductor material which at the same time has response times which can be influenced, high absorption modulations and low unsaturated absorption losses. Another object of the invention is to provide a method for producing such a material.

The object is achieved by the material according to the invention and the method according to the invention as defined in the independent patent claims.

Surprisingly, it was found that the above-mentioned key parameters a) -c) of an optically nonlinear semiconductor material grown at low temperatures, such as, for. B. Significantly improve GaAs by the following measures, ie by up to an order of magnitude: i) adding foreign atoms and / or ii) additional annealing. Semiconductor materials, in the production of which at least one of these measures has been taken, combine astonishingly favorable nonlinear-optical material properties and thus come close to optimizing the key parameters in an unprecedented way. In particular, they have a) response times which can be influenced, and at the same time b) high absorption modulations (comparable to those of normally grown semiconductor materials) and c) low unsaturated absorption losses (comparable to those of normally grown semiconductor materials). They are therefore excellently suited for non-linear-optical applications, in particular in optical information processing for the ultra-fast switching of light by means of light, in optical communication - 5 -

Cleaning of optical signals from disturbing noise or in ultrashort pulse laser physics as a saturable absorber for ultrashort pulse emitting lasers. In the latter application area, the semiconductor material according to the invention is particularly suitable for mirrors with at least one saturable absorber made of this semiconductor material.

An ITJ-V semiconductor, for example gallium arsenide (GaAs), indium gallium arsenide (InGaAs), aluminum gallium arsenide (AlGaAs) or indium gallium arsenide phosphide (InGaAsP), is preferably selected as the semiconductor material. The semiconductor material is preferably produced by means of molecular beam epitaxy (MBE). Another possible manufacturing process is gas phase deposition, in particular metal organic chemical vapor deposition (MOCVD).

In a first method variant according to the invention, a semiconductor material is produced at temperatures between 180 and 500 ° C. and mixed with foreign atoms. The foreign atoms are preferably at least one acceptor material, e.g. B. Beryllium (Be). The doping is preferably carried out during the epitaxial growth of the semiconductor material in an ultra-high vacuum chamber in the molecular beam. The impurity concentration is set via the ratio of the molar flux, e.g. from Be to Ga and As. Such doping can be done later. B. with the help of secondary ion mass spectroscopy (SIMS). Typical Be concentrations are between 10 ¹⁷ cm ^"3 and 10 ²⁰ cm ^{" 3} .

In a second method variant according to the invention, the semiconductor material is produced at temperatures between 180 and 500 ° C. and then - 6 -

send heated. The bakeout can be carried out for at least 10 minutes at temperatures between 500 and 800 ° C, or as a short-term bake (rapid thermal annealing, RTA) for, for example, 10 s at about 600 to 1000 ° C. The baking usually leads to a certain precipitation of a semiconductor component; in the case of GaAs, for example, As spheres with diameters in the nanometer range, typically between 2 and 10 nm, with a density of 10 ¹⁷ to 10 ¹⁸ cm ^"3 , (see, for example, BMR Melloch et al.," Formation of arsenic precipitates in GaAs buffer layers grown by molecular beam epitaxy at low Substrate temperatures, Appl. Phys. Lett. 57 (15), October 8, 1990, pp. 1531 ff); a larger bandwidth of the density, for example from 10 ¹⁵ to 10 ¹⁹ cm ^"3 , is also possible. If it is an As-containing HI-V semiconductor material, the baking is preferably carried out in an As atmosphere prevent or at least reduce displacement of As from the semiconductor.

The addition of foreign atoms and / or baking leads to a significant improvement in the key parameters of the semiconductor material. These manufacturing measures according to the invention have the effect that the response times are considerably shorter than in the case of semiconductor materials which were produced at low temperatures without doping and without baking out; nevertheless, the absorption modulation remains high and the unsaturated absorption losses are low.

The beneficial effects of Be doping and / or baking can be explained by the following model (cf. the model for undoped GaAs grown at low temperatures in, e.g., U. Siegner et al., "Ultrafast high-intensity nonlinear absorption dynamics in low-temperature grown gallium arsenide ", Appl. Phys. Lett. 69 (17), October 21, 1996, p. 2566 - 7 -

ff). The unsaturated absorption losses are attributed to a transition between neutral antisites (for example As _Ga °) in the semiconductor material (for example GaAs) and the band states which are 0.7 eV above the lower end of the conduction band. Because of the high neutral-anti- site concentration and the high density of the final states, this transition can only be saturated at very high light flux densities. The doping according to the invention and the baking out according to the invention now considerably reduce the concentration of the neutral antisites by changing the charge state of the defects or by precipitation. As a result, the transition between the neutral antisites and the conduction band can be at least partially saturated, thereby reducing the unsaturated absorption losses and increasing the absorption modulation.

The invention is explained in detail below with reference to figures. The examples listed relate to GaAs; however, the invention relates not only to this semiconductor material, but also to other semiconductor materials. Show it:

1 shows the measured standardized differential reflectivity versus the time delay of a test pulse to a short pump pulse for GaAs with different Be concentrations,

2 shows the measured standardized differential reflectivity versus the time delay of a test pulse to a short pump pulse for heated and unheated GaAs,

3 shows the measured standardized differential reflectivity versus the time delay of a test pulse to a long pump pulse for GaAs produced using different methods, - 8th -

4 measured absorption modulations versus the growth temperature for GaAs with different Be concentrations,

5 measured absorption modulations versus the growth temperature for heated and unheated GaAs,

6 measured unsaturated absorption losses compared to the growth temperature for GaAs with different Be concentrations,

7 measured non-saturable absorption losses versus the growth temperature for heated and unheated GaAs,

8 shows the measured reflectivity versus the light flux density for GaAs with different Be concentrations and

9 shows a schematic cross section through a preferred embodiment of a mirror with saturable absorbers made of semiconductor materials according to the invention.

FIG. 1 shows the measured standardized differential reflectivity versus the time delay of a test pulse to a short pump pulse with a pulse length of 15 fs and a central wavelength of 750 nm. The measurement curves relate to GaAs grown at 300 ° C. with different concentrations, namely: curve 1.1: undoped GaAs, curve 2.1: GaAs with a Be concentration of 1 • 10 ¹⁹ cm ^"3 and curve 3.1: GaAs with a Be concentration of 3 • 10 ¹⁹ cm ^{" 3} . - 9 -

Curves 1.2, 2.2 and 3.2 are compensation curves (fits) for the corresponding measured values 1.1, 2.1 and 3.1, whereby the function a + b - exp (-t / τ) was used for the compensation calculation. The time constants r calculated in this way decrease from 480 fs to 390 fs to 110 fs for the increasing Be concentrations. This measurement thus demonstrates impressively how the response time can be influenced by the Be concentration.

The same measurement is plotted in FIG. 2 for the same test pulse as in FIG. 1, but for undoped GaAs grown at 300 ° C. Curve 4.1 shows measured values for unheated GaAs, curve 5.1 measured values for GaAs, which was heated for one hour at 600 ° C after growing up. Curves 4.2 and 5.2 are the corresponding fits. FIG. 2 proves that the response time can also be influenced with baking out in the method according to the invention.

FIG. 3 shows the measured normalized differential reflectivity versus the time delay of a test pulse to a long pump pulse with a pulse length of 100 fs and a central wavelength of 830 nm. The measurement curves relate to GaAs grown at 300 ° C., which was produced as follows: curve 6 : undoped, not heated GaAs,

Curve 7: unheated GaAs with a Be concentration of 3 · 10 ¹⁹ cm ^'3 and curve 8: undoped GaAs heated for one hour at 600 ° C. - 10 -

According to these measurements, the response times can be influenced using the method according to the invention, which includes doping and / or baking.

Figure 4 shows measured absorption modulations versus growth temperature for GaAs with different Be concentrations. Open squares mean: undoped GaAs, full circles: GaAs with a Be concentration of 1 • 10 ¹⁹ cm " ³ and full triangle: GaAs with a Be concentration of 3 • 10 ¹⁹ cm ^{" 3} .

The measurements for undoped GaAs (open squares) confirm the known fact that GaAs grown at high temperatures (≥ approx. 300 ° C) exhibits satisfactorily high absorption modulations (around 60%), at low temperatures (<approx. 300 ° C) ) grown up GaAs however not. The astonishing statement from FIG. 4 is that the Be doping does not lower the absorption modulation (cf. 300 ° C.), but rather increases it (cf. 250 ° C.). It should be noted that the response times in Be-doped material are much shorter. Undoped GaAs grown at low temperatures with comparable response times would have to be grown at approx. 200 ° C, whereby the absorption modulation drops considerably and the unsaturated absorption losses are considerably greater.

The measurements of FIG. 5 are analogous to those of FIG. 4, but for undoped GaAs, open squares: GaAs not heated and solid diamonds: GaAs heated at 600 ° C. for one hour. Here, too, the baking according to the invention does not lower the absorption modulation (cf. 300-600 ° C.), but rather increases it (cf. 200-250 ° C). - 11 -

FIG. 6 shows measured non-saturable absorption losses with respect to the growth temperature for GaAs with different Be concentrations, the same Be concentrations and symbols being used in FIG. 4. The measurements for undoped GaAs (open squares) confirm the known fact that at GaAs grown at high temperatures (> approx. 300 ° C) exhibits satisfactorily low unsaturated absorption losses (around approx. 10), but GaAs grown at low temperatures (<approx. 300 ° C) does not. The astonishing statement from FIG. 6 is that the Be doping does not increase the unsaturated absorption losses, but rather rather decreases them.

The measurements of FIG. 7 are analogous to those of FIG. 6, but for undoped GaAs, the same heating conditions and symbols being used as in FIG. 5. Here too, the heating according to the invention does not increase the unsaturated absorption losses, but rather degraded.

FIG. 8 shows the measured reflectivity versus the light flux density for GaAs with different Be concentrations, namely: curve 9: undoped GaAs,

Curve 10: GaAs with a Be concentration of 1 • 10 ¹⁹ cm ^"3 and Curve 11: GaAs with a Be concentration of 3 • 10 ¹⁹ cm ^{" 3} . Different ranges of the light flux density are discussed on the basis of curve 11. For low light flux densities (<approx. 1 μJ / cm ² ) the GaAs behaves optically linear, ie the reflectivity has a constant value R _Q ~ 40%. From approx. 1 μJ / cm ² , nonlinear optical effects begin to play a role; the reflectivity can be varied by a maximum of an absorption modulation ΔR - 45%, depending on the light flux density. With high luminous flux - 12 -

dense (> approx. 1000 μJ / cm ² ) a saturation of approx. R _s ~ 85% occurs. The non-saturable absorption losses are calculated according to R, = 100% - R _s «15%.

FIG. 9 schematically shows a preferred embodiment of a mirror 20 with saturable absorbers made of semiconductor materials according to the invention in cross section. The founders of the clear presentation do not necessarily have the thicknesses of individual elements in the correct relationship to each other. The mirror 20 consists of a reflective substrate 21 and a saturable semiconductor absorber structure 22. Optionally, an additional reflection or anti-reflection layer (not shown) could be applied to the saturable semiconductor absorber structure 22.

The reflective substrate 21 is preferably a carrier substrate 24 provided with a Bragg structure 23, for example made of GaAs. The Bragg structure 23 is preferably formed as a stack of lambda quarter layers 25.1, ..., 25.p or 26.1, ..., 26.q made of semiconductor materials and / or dielectrics, layers 25.1, .. ., 25.p alternate lower refractive index with layers 26.1, ..., 26.q higher refractive index; typically p ~ q ~ 25. The lambda quarter layers consist, for example, of AlAs 25.1, ..., 25.p (n = 2.99 at λ = 835 nm) or of undoped, unheated GaAs 26.1, .. ., 26.q (n = 3.65 at λ = 835 nm). The saturable semiconductor absorber structure 22 of the exemplary embodiment from FIG. 9 consists of: a first AlAs layer 27.1 with a thickness of 75 nm, a first non-linear optical GaAs layer 28.1 with a thickness of 15 nm produced by the method according to the invention , a second AlAs layer 27.2 with a thickness of 15 nm and - 13 -

a second non-optical-optical GaAs layer 28.2 with a thickness of 5 nm produced by the method according to the invention. Such a mirror 20 has high reflectivities of approx. R ≥ 0.99 (slightly dependent on the incident light output). Schematically, as arrow 29, a light beam reflected on the mirror 20 is indicated.

Of course, it is possible for the person skilled in the art to construct further mirrors according to the invention with saturable absorbers if the present invention is known.

Claims

- 14 -PATENT REQUESTS

1. A method for producing an optically nonlinear semiconductor material, in which a semiconductor material is produced at temperatures between 180 and 500 ° C. and mixed with foreign atoms.

2. The method according to claim 1, characterized in that the semiconductor material with at least one acceptor material, preferably with beryllium (Be), is added.

3. The method according to claim 1 or 2, characterized in that the semiconductor material is produced by means of molecular beam epitaxy (MBE).

4. A process for producing an optically nonlinear semiconductor material, in which a semiconductor material is produced at temperatures between 180 and 500 ° C. and then baked out.

5. The method according to claim 4, characterized in that the baking is carried out for at least 10 minutes at temperatures between 500 and 800 ° C. - 15 -

6. The method according to claim 4, characterized in that the baking is carried out for a maximum of one minute at temperatures between 600 and 1000 ° C.

7. The method according to any one of claims 1-6, characterized in that an ITJ-V semiconductor is selected as the semiconductor material.

8. The method according to claim 7, characterized in that the ILT.-V semiconductor is gallium arsenide (GaAs), indium gallium arsenide (InGaAs), aluminum gallium arsenide (AlGaAs) or indium gallium arsenide phosphide (InGaAsP).

9. Semiconductor material produced by the method according to one of claims 1-8, characterized in that it is Be-doped GaAs produced at temperatures between 180 and 500 ° C.

10. Semiconductor material according to claim 9, characterized in that it has a loading concentration between 10 ¹⁷ cm ^"3 and 10 ²⁰ cm ^{" 3} .

11. Semiconductor material produced by the method according to one of claims 1-8, characterized in that it is GaAs produced and subsequently heated at temperatures between 180 and 500 ° C. - 16 -

12. Semiconductor material according to claim 11, characterized in that it contains As spheres with diameters between 2 and 10 nm with a density of 10 ¹⁵ to 10 ¹⁹ cm ^"3 .

13. Use of the semiconductor material produced by the method according to any one of claims 1-8 as a nonlinear optical material.

14. Use according to claim 13 for ultrafast switching of light by means of light, for cleaning optical signals from disturbing noise or as a saturable absorber for ultrashort pulse-emitting lasers.

15. Semiconductor material, produced by the method according to any one of claims 1-8, for use in the manufacture of products for optical information processing, optical telecommunications or ultra-pulse laser physics.

16. mirror (20) with at least one saturable absorber (28.1, 28.2) made of at least one semiconductor material, which is produced by the method according to any one of claims 1-8.